Frontiers of water oxidation: the quest for true catalysts

J. Li , R. Güttinger , R. Moré , F. Song , W. Wan and G. R. Patzke *
University of Zurich, Department of Chemistry, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland. E-mail: greta.patzke@chem.uzh.ch

Received 9th May 2017

First published on 26th July 2017


Development of efficient and economic water oxidation catalysts (WOCs) remains a crucial bottleneck on the way to artificial photosynthesis applications. Over the past few decades, WOC research has turned into a fascinating interdisciplinary field that ranges from bio-inspired molecular design over nanomaterials and thin films to solid materials tuning. Under the umbrella of WOC optimization, advanced in situ/operando analytical techniques are being developed as increasingly powerful tools to elucidate the controversial discussions about the molecular or nanoscale nature of many WOCs. More and more of these approaches also enable the monitoring of possible key intermediates as an essential prerequisite for proposing catalytic mechanisms. This review is organized in three main parts: first, recent highlights outline frontiers in WOC development, such as the benefits of connecting molecular WOCs with solids along with the introduction of molecular concepts into heterogeneous WOC research. Next, a brief overview of emerging in situ/operando approaches demonstrates new options for monitoring WOC transformations. Finally, selected monitoring studies over the entire WOC dimensionality spectrum illustrate interesting cases of catalytic border crossings as new input for WOC construction.


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J. Li

Jingguo Li is a PhD student at the University of Zurich in the group of Prof. Greta R. Patzke. He obtained his BSc from Henan University in 2010, and his MSc degree from the University of Science and Technology of China in 2013. Afterwards, he worked as a Research Engineer in the National University of Singapore developing polymeric thin films for water purification applications. Then he moved to UZH in 2016 focusing on photo(electrochemical) water oxidation research.

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R. Güttinger

Robin Güttinger studied chemistry at the University of Zurich and received his MSc in 2014 under the supervision of Prof. Greta R. Patzke focusing on polyoxometalates (POMs) and cubanes for photocatalytic water splitting. After an internship in industry, he returned to the group of Greta R. Patzke. His present research focus is on synthesis, characterization and magnetic behavior of new 3d–4f and 4f POMs, along with POM photocatalysts for water oxidation.

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R. Moré

René Moré obtained his MSc in Physical Chemistry from the Georg-August University of Göttingen. He then received his PhD degree at the Max-Planck-Institute for Biophysical Chemistry under the supervision of Prof. Simone Techert while investigating the structure dynamics of organic semiconductors applying synchrotron-based techniques. In February 2013 he joined the team of Prof. Greta R. Patzke at the University of Zurich as a postdoctoral researcher, where he applies his synchrotron expertise on artificial photosynthesis research.

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F. Song

Fangyuan Song received his BSc in Applied Chemistry from Wenzhou University in 2010 and his MSc in Physical Chemistry from Lanzhou University in 2013. Then he joined Prof. Greta R. Patzke's group to pursue his PhD degree at the University of Zurich. His research focuses on the syntheses and development of new cubane-type molecular water oxidation catalysts with special emphasis on their photocatalytic performance optimization and stability under operational conditions.

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W. Wan

Wenchao Wan was born in 1989 in China, and studied Materials Chemistry at Southwest Petroleum University (Chengdu City), where he received his MSc in 2016 under the supervision of Prof. Ying Zhou working on graphene aerogels for environmental applications. He is currently working towards his PhD at the University of Zurich under the supervision of Prof. Greta R. Patzke. His current research interests are the design and synthesis of graphene-based 2D catalytic materials for water splitting.

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G. R. Patzke

Greta R. Patzke received her PhD from the University of Hannover and her venia legendi at ETH Zurich with Prof. Reinhard Nesper. She then moved to the University of Zurich, where she was promoted to Full Professor in 2016. Her research interests were first focused on the hydrothermal synthesis and monitoring of nanomaterials for environmental applications, along with the structural and catalytic properties of POMs. She is now a board member of the UZH Research Priority Program “Light to Chemical Energy Conversion”, where she investigates a wide range of molecular, nanostructured and solid state transition metal-based catalysts for water oxidation.


Why artificial photosynthesis?

Artificial photosynthesis (AP), namely the sunlight-driven splitting of water into hydrogen and oxygen, is among the most dynamic and multidisciplinary contemporary research areas.1–4 The spectrum of AP investigations ranges from biological studies of Nature's photosystems and their mechanisms5–7 over design and pathways of water reduction and oxidation catalysts (WRCs and WOCs)8–11 to application-oriented techno-economic analyses.12,13 While the technological vision of covering humanity's energy needs from sunlight and water as the planet's most abundant resources keeps inspiring worldwide efforts and forefront joint centers,2,14 the fundamental aspects of AP are deeply connected with eminent challenges of current general catalysis research.15,16 A major roadblock on the way to applied AP is the development of efficient WOCs to widen the bottleneck of the uphill four-electron-transfer reaction of oxygen formation:17,18
2H2O → 4H+ + 4e + O2E° = 1.23 V vs. NHE

Splendid design approaches are developed to tackle this task both at the molecular and the heterogeneous level. However, the differentiation between interconversions of these catalyst types under operando conditions is not always clear.19,20 Although AP researchers share this problem with the general catalysis community, they often face even more complex multi-component systems, demanding reaction conditions, and possible side processes, e.g. while investigating water oxidation under photocatalytic conditions in the presence of buffers and sacrificial electron acceptors.21

The complex task of understanding the still elusive mechanisms of the demanding four electron transfer processes in water oxidation catalysis is further complicated through stability issues of WOCs and their frequently Janus-faced molecular vs. (nano)particular character over longer operational periods. In the following, we first outline how this complex interplay adds new perspectives to the conventional concepts of molecular and heterogeneous catalysts. The second part of the review illustrates how the distinction between molecular and heterogeneous catalysts inspires new analytical techniques for the specific in situ/operando monitoring of water splitting. Finally, recent highlights demonstrate how these techniques promote the understanding of reaction and transformation mechanisms over the entire WOC dimensionality range from classic heterogeneous systems to molecular catalysts. A critical evaluation of the actual insight emerging from new monitoring techniques requires a flexible and contemporary concept of the “true catalyst” which may in fact undergo dynamic changes over several cycles (Fig. 1).


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Fig. 1 The evolution of catalytic concepts over the past few decades from molecular catalysts towards complex multi-component systems. Reproduced from ref. 22 with permission from Elsevier, copyright 2017.

1. Part I: Molecular and heterogeneous WOCs – two different worlds?

1.1. Molecular WOCs: valuable and vulnerable

Conceptual changes in catalysis. Over the past few decades of catalysis and artificial photosynthesis research, the traditional concepts of molecular and heterogeneous catalysis have been continuously challenged.23–28 This debate is an ongoing incentive to develop cutting-edge monitoring techniques for catalytic mechanisms and the involved active species, and water splitting research greatly benefits from this progress.29–35 This steadily increasing level of insight into catalytic processes has brought forward an evolutionary concept of catalyst transformations, ranging from molecules over nanoparticles and leaching-derived catalysts to complex “cocktail” catalysts (Fig. 1).22 Tracking such possible transformations of catalytically active species across the classic categories is the first indispensable step to strategic catalyst design.

Such paradigm shifts strongly affect the dynamic field of WOC research. In the following sections, we start out with state of the art molecular WOC research. This discussion is complemented with new views of the stability and transformations of heterogeneous WOCs. Here we prefer the unifying term “molecular” over “homogeneous”, because not all molecular WOCs represent truly homogeneous systems, as outlined in detail below, e.g. for immobilized molecular catalysts or their colloidal aggregates.

Understanding and preserving molecular WOCs. Although many pathways of deactivation, degradation, and inhibition have been reported for molecular catalysts,5 the high efficiency and selectivity of stable molecular WOCs remains outstanding.18,27 While WOC construction has been pursued for almost five decades,36 some widely applied photochemical and chemical oxidation assessment protocols still involve strong oxidative reagents, such as persulfate, Ce(IV), and NaIO4etc., that challenge the stability of many molecular WOCs.21 For the fundamental understanding of molecular WOCs and basic design guidelines, reliable analytical options are required to identify the presence of possible leaching- and decomposition-based active heterogeneous species.20,37–42 Whereas operando studies on heterogeneous WOCs often require ultrahigh vacuum (UHV) conditions,11 molecular WOCs are more accessible targets for mechanistic as well as for computational studies, provided that their stability is guaranteed under operational conditions.43–47 In order to circumvent attack and decomposition of organic WOC ligands in the presence of strong oxidants, all-inorganic polyoxometalates (POMs) are flexible alternatives at the interface of molecular and oxide chemistry as further discussed below.19,48,49
Despite all challenges – incentives for molecular WOCs. The {CaMn4O5} oxygen evolving core (OEC) of photosystem II is Nature's answer to the challenge of water oxidation,50,51 and it remains a major driving force for bio-inspired WOC construction.52–54 In addition to the fundamental merit of benefiting from natural strategies through synthetic OEC mimics,55,56 molecular WOCs in general offer quite direct opportunities to widen the water oxidation bottleneck through mechanistic insights.58,59 This is illustrated with selected topics here.

To start, targeted ligand engineering is the most effective toolbox for molecular WOC development. In particular, Ru(bda)(pic)2 (bda = 2,2′-bipyridine-6,6′-dicarboxylic acid; pic = picoline) and its derivatives keep attracting intense research interest due to their high catalytic oxygen evolution activity with CeIV as a sacrificial oxidant in acidic media.60–62 Furthermore, studies of their O–O formation mechanism through kinetic measurements revealed the presence of a radical coupling pathway.63,64 Surprisingly, a RuIII–O–RuIV–O–RuIII trimer was later identified while operating Ru(bda)(pic)2 in neutral phosphate buffer, and this initial trimer underwent an oxidative deactivation under operational conditions. This result identified the elimination of trimerization processes for Ru(bda)(pic)2 through tuning the pH of the reaction system as a rational strategy for preserving its catalytic activity.65 Very recently, a water nucleophilic attack pathway was identified upon replacement of the doubly charged bda ligand with singly charged tpc (2,2′:6′,2′′-terpyridine-6-dicarboxylic acid), which highlights the key role of ligand-based electronic effects in water oxidation pathways.66

Co-based WOCs are among the most widely studied noble-metal free systems, and recent studies on a bis-μ-hydroxo-Co(III)–TPA dinuclear photocatalyst (TPA = tris(2-pyridylmethyl)amine) unified both computational and experimental results into a consistent mechanistic model for O–O bond formation. After transformation of the initial WOC into an intermediate bis-μ-oxyl dinuclear Co(III) complex via a Co(III)2(μ-O˙)2 species, the dinuclear core affords O2via an intramolecular radical coupling mechanism that was derived from isotope labeling experiments (Fig. 2).57


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Fig. 2 Proposed oxygen evolution mechanism of the bis-μ-hydroxo-Co(III)–TPA WOC. Reproduced from ref. 57 with permission from American Chemical Society, copyright 2016.

At the computational level, a concise study on mononuclear Ru-based WOCs recently addressed the fundamental question why there is a barrier at all for the coupling process of two radicals during water oxidation.67 More widely applicable guidelines for the construction of WOCs beyond the studied systems were then suggested, including high spin density at the oxo centers, appropriate steric features for the coupling conformation, and balanced hydrophilic/-phobic properties.67

Furthermore, the high potential of economic Cu-containing WOCs was underscored by the novel and robust [Cu2(BPMAN)(μ-OH)]3+ (BPMAN = 2,7-[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine) oxygen evolution electrocatalyst, which efficiently operates in neutral aqueous media.68 It served as an excellent platform for density functional theory (DFT) calculations which support a direct intramolecular coupling mechanism towards O–O bond formation.68 Many of these molecular pathways readily proceed without the need for high-oxidation-state intermediates.

However, maintaining full analytical control over all possible interconversion mechanisms between molecular WOCs and possible secondary nanoparticles in solution often remains an unsurmountable task.69,70 Recent immobilization approaches combining the best of both worlds, namely molecular WOC ‘guests’ and heterogeneous ‘hosts’, are discussed in the following section.71,72

1.2. New perspectives for stabilizing molecular WOCs

Molecular WOCs and heterogeneous surfaces – a perfect match?. Molecular WOCs with manifold noble and non-noble metal centers as active sites offer very flexible design options. Strategies such as systematic ligand engineering, metal core tuning, or tailored architectures have brought forward a large number of molecular WOCs with very impressive activity.18,73,74 However, recent studies revealed that a considerable fraction of these molecular WOCs undergo changes under homogeneous catalytic conditions in the presence of chemical oxidants or sacrificial reagents.75

In search of new stabilization strategies, immobilizing molecular WOCs on solid surfaces keeps attracting interest.76–78 Synergistic effects could be achieved through stabilizing molecular WOCs while promoting solid substrate properties. Molecular WOC loading plays a significant role in facilitating electrolysis and photoelectrolysis through lowering the water oxidation over-potential.72,79 Well-designed molecular WOCs can stabilize high energy intermediates of the water oxidation process, thus lowering the energy barrier in favor of O–O bond formation.

Combination strategies: pros and cons. Prior to immobilization, two major requirements must be considered. First, facile electron transfer at the interface between the molecular WOC and the electrode largely determines the device efficiency. Second, long-time persistence of the immobilization is crucial to prevent leaching and activity loss of molecular WOCs induced by their local oxidative operational environments. Once molecular WOCs have been loaded, it is very important to quantify their interfacial electron transfer kinetics, the durability of their immobilization interactions and – last but not least – to check on their integrity. These factors are also decisive for implementing loaded molecular WOCs in future catalytic devices.

Immobilization strategies can be categorized based on three different mechanisms, namely covalent bonding,80–82 physical interactions,83,84 and direct encapsulation.85,86 In order to ensure a strong immobilization, covalent attachment through anchoring groups such as carboxylic acid87 and phosphonate acid groups88 is the most widely applied method. Given their excellent electron withdrawing abilities, these anchoring groups generally act as electron acceptors through metal–ligand bridging.89,90

A binuclear Ir-WOC, [Ir(pyalc)(H2O)2(μ-O)]22+ (pyalc = 2-(2′pyridyl)-2-propanolate), was immobilized onto a nanostructured indium tin oxide (ITO) surface by chemical esterification (Fig. 3). In addition to unchanged cyclic voltammetry (CV) features, X-ray photoelectron spectroscopy (XPS) measurements indicated the persistence of the pyalc ligand after 16 h of electrolysis, thereby confirming the durable immobilization and preserved molecular nature of the Ir-WOCs.91 Most recently, carboxylate anchoring connections were found to be dependent on the applied bias when attaching mononuclear Ru-WOCs [(μ-bbp){Ru(py)2}2(μ-X)]2+ (bbp = 3,5-bis(dipyridyl)pyrazolate; X = CO3H) on mesoporous ITO surfaces. Loosely attached or physisorbed molecular species were removed from the surface at lower applied potentials (1.1 V vs. RHE, reversible hydrogen electrode), and leaching was gradually pronounced at higher oxidative potentials, resulting in significant loss of the attached molecular WOC [(μ-bbp){Ru(py)2}2(μ-X)]2+ at 1.9 V vs. RHE. Interestingly, molecular [(μ-bbp){Ru(py)2}2(μ-X)]2+ itself remained stable, because no significant changes in the binding energies of the complex species and the Ru[thin space (1/6-em)]:[thin space (1/6-em)]N ratio were evident from XPS data.92 This work outlines the limitations of anchoring groups operating at higher applied bias. Other issues associated with covalent immobilization techniques are the frequently complex synthetic procedures involved and the stability of metal centers in the presence of electron-withdrawing groups.


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Fig. 3 Oxidation of the [Cp*Ir(pyalc)OH] precursor (left) to form [Ir(pyalc)(H2O)2(μ-O)]22+ WOC (middle), followed by heterogenization at room temperature (right). Reproduced from ref. 91 with permission from Nature Publishing Group, copyright 2015.

Immobilization through physical adsorption, such as hydrophobic93–95 and π–π interactions,96 is another widely adopted technique.97 In a previous study, ITO electrodes and mononuclear Ru(bpa)(pic)2 (bpa = 2,2-bipyridine-6,6-dicarboxylic acid; pic = 4-picoline) were first modified with multi-walled carbon nanotubes (MWCNTs) and pyrene moieties, respectively. Through exploiting noncovalent π–π stacking interactions between MWCNTs and pyrene moieties, remarkable electrocatalytic activity was achieved. The loaded electrodes showed unchanged redox behavior over 10 h of electrolysis at 1.4 V vs. NHE, indicating the stable molecular nature of Ru(bpa)(pic)2 on the electrode surface. A slight drop in the catalytic current during the electrolysis process was ascribed to the partial desorption of MWCNTs into the electrolyte.98 Recently, a library of molecular Co-WOC complexes containing tris((2-benzimidazolymethyl)amine) ligands with tailored hydrophobicity was employed for a systematic study of their stability during immobilization on fluorine doped tin oxide (FTO) surfaces (Fig. 4). The results indicated that the stability of both the immobilization process and the molecular Co-WOCs significantly depends on the hydrophobicity of the coordinating ligands. Employment of the less hydrophobic ligand tris(2-benzimidazolylmethyl)amine, for example, leads to a rapid drop of the catalytic current density to a negligible level during controlled potential electrolysis (CPE), which indicates desorption or chemical changes of the immobilized molecular WOCs. Interestingly, substitution of the coordinating ligand with longer alkyl chains or fluorinated groups leads to non-detectable catalytic current density changes, which strongly points to stability of both the immobilization process and the corresponding molecular Co-WOC.99 This work highlights how significantly the strength of the physical interaction of molecular WOCs with the solid surface determines the durability of immobilization.


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Fig. 4 (A) Library of the tris-benzimidazole (BimH)3 Co complexes and their N-alkylated variants (BimR)3, with substituents CnR (chain of n methylene groups terminated with R; F8 = (CF2)7CF3; PA = P(O)(OH)2). (B) Controlled potential electrolysis (CPE) traces at 1.91 V vs. RHE for selected cobalt complexes. Reproduced from ref. 99 with permission from American Chemical Society, copyright 2016.

However, modifying molecular WOCs with specifically designed organic moieties is not always convenient for general applications. More straightforward immobilization methods are, for example, direct encapsulation by some polymeric materials100 and electrostatic interactions.101,102 In this context, Nafion-assisted immobilization of molecular WOCs was introduced as a robust process for generating better electrodes/photoelectrodes. However, some leaching of molecular WOCs remains inevitable due to the intrinsic swelling of Nafion polymers under operational conditions.103

Last but not least, the combination of molecular WOCs with mesoporous architectures is a promising strategy for maximizing their catalytic performance. Mesoporous nanomaterials, such as silica,104,105 carbon nitride,106 and metal–organic frameworks (MOFs)107–109 are frequently selected as substrates due to their high porosity and large surface areas. In a recent example, the molecular Ru WOC [Ru(tpy)(dcbpy)(OH2)](ClO4)2 (tpy = 2,2′:6′,2′′-terpyridine, dcbpy = 2,2′-bipyridine-5,5′-dicarboxylic acid) was incorporated into MOFs grown on FTO by ligand exchange approaches, and the molecular WOC remained unchanged even after hours of catalytic operation.109 The wide spectrum of available MOF and molecular WOC types holds great potential for further optimization progress.

In summary, the last two decades have witnessed significant progress in the immobilization of molecular WOCs on solid surfaces, e.g. through synergistic effects or the enhancement of device-related catalytic properties.72 The above-mentioned approaches can now be further developed towards device fabrication with respect to stability of the molecular components, especially through more robust covalent bonding systems, employment of multi-anchoring groups in a single WOC molecule, and through constructing moieties for stronger physical interactions. Novel techniques to check on molecular WOC integrity are discussed in Part II. In the following, POMs are briefly presented as interesting all-inorganic alternative to molecular WOCs.

1.3. POMs as all-inorganic WOCs: a promise of stability?

Design options for POMs. POMs are attractive WOCs, because they combine the advantages of both heterogeneous catalysts (such as robustness and recovery) and homogeneous catalysts (facile characterisation and spectroscopic monitoring). They can undergo rapid, reversible, and stepwise multielectron-transfer reactions while maintaining their structure. Due to their highly negative charges, POMs are easy to immobilize on a variety of electroactive surfaces.102,111,112

Among the growing number of homogeneous POM WOCs that have been developed recently, ruthenium- and cobalt-based catalysts have attracted most research attention. [Ru4O4(OH)2(H2O)4(γ-SiW10O36)2]10− (Ru4POM) was introduced in different studies in 2008, and its activity and stability in the presence of Ru(bpy)32+/S2O82− and Ce(IV) oxidants were confirmed by various spectroscopic methods (UV/Vis, FT-IR and Raman) and aging experiments.113,114 From 2010 onwards, the first cobalt based POM WOC, [Co4(H2O)2(α-PW9O34)2]10− (Co4POM), became a prominent target, and the oxygen evolution activity of Co4POM was further improved through phosphorus substitution by vanadium.115 This underscores a significant influence of POM heteroatoms on the catalytically active metal ions in the central belt structure.

Recently, many OEC-related cubane moieties have been reported which are embedded into oxidative and thermally stable POM ligands to eliminate the well-known stability problems of organic ligands in catalytic media.114,116,117

A particularly close mimic of the OEC is the tetramanganese-substituted tungstosilicate [MnIII3MnIVO3(CH3COO)3(SiW9O34)]6− with its mixed-valent {Mn4} (III,III,III,IV) core. The combination of the inorganic POM with organic carboxylate ligands provides a stable and flexible environment for the subsequent one-electron oxidation steps of the catalytic core.117 Very recently, the new Mn3POM [Mn3(H2O)3(SbW9O33)2]12− corroborated the key role of the ligand heteroatom for WOC activity optimization, because maximum O2 evolution was found for the Sb derivative, followed by P and As.118

Lately, an octanuclear cobalt complex [(A-α-SiW9O34)2Co8(OH)6(H2O)2(CO3)3]16− (Co8POM) with a bioinspired double cubane CoII4O3 core was reported, where two of the cobalt atoms are coordinated to terminal aqua ligands (Fig. 5).110 The stabilization of the two tetracobalt cores by two all-inorganic tungstosilicates provides oxidative stability, while carbonate ligands introduce flexibility and hydrolytic stability. Co8POM thus has displayed the highest TON for water oxidation to date among the growing number of bioinspired transition metal WOCs. Moreover, a unique hepta-nuclear cobalt–arsenic core resembling a “fused” double-quasi-cubane emerged as the first cobalt–arsenic core in POM WOCs.119


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Fig. 5 [(A-α-SiW9O34)2Co8(OH)6(H2O)2(CO3)3]16− (Co8POM) with a bio-inspired double cubane {CoII4O3} core. Reproduced from ref. 110 with permission from John Wiley and Sons, copyright 2015.

Rapid progress was also made for Ni-POM WOCs with the discovery of three large Ni clusters, ranging from {Ni12} to {Ni25} cores, which are connected by inorganic linkers and are encapsulated by lacunary {SiW9O34} Keggin units. The nickel centers are arranged in a {Ni4O4} cubane fashion bearing resemblance to the OEC motif. All three clusters operate as homogeneous catalysts with high photocatalytic activities (Fig. 6).120 Soon afterwards, a small POM-based nickel cluster was reported that consists of a central {Ni4O2} unit sandwiched by two {SiW10O36} Keggin units. This cluster has shown the best visible-light-driven O2 evolution activity among the Ni-POM WOCs to date.121


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Fig. 6 Synthetic route to obtain the Ni-POM WOCs (a) {Ni12}, (c) {Ni13}, and (d) {Ni25}; (b) synthetic procedure (WO6, teal octahedra; PO4, pink tetrahedra; SiO4, orange tetrahedra; C, black spheres; O, red spheres; Ni, limes spheres). Reproduced from ref. 120 with permission from American Chemical Society, copyright 2015.

In 2017, Co-POM WOCs were also expanded towards higher nuclearities with a detailed investigation on {Co9}, {Co15} and {Co16} cores.122 They all display high WOC activities (Co15 ∼ Co16 > Co9), and laser flash photolysis showed a constant value of the electron transfer kinetics that rules out a progressive leaching of Co(II) aqua ions. In particular, {Co15} with its satellite cobalt sites on the outer POM surface exhibits a correlation of a high number of open coordination sites with enhanced catalytic activity. The high nuclearity POM [Co9(H2O)6(OH)3(HPO4)2(PW9O34)3]16− has also shown increased stability at neutral pH and could be incorporated into a solid state carbon paste matrix to promote heterogeneous electrocatalytic water electrolysis.123

Specific stability studies of POM WOCs are discussed in Section 3.3.

Immobilization options for POMs. POMs can be immobilized on a variety of materials like porous carbons112 and cationic silica nanoparticles.124,125 These strategies cross the border between homogeneous and heterogeneous catalysts in order to improve POM stability and their application options in electrocatalytic water oxidation.102,112 POM derivatives are generally attractive targets for devices based on electrostatic anchoring.126

A breakthrough study was the immobilization of Ru4POM on carbon nanotubes, which notably enhanced its catalytic efficiency.102 A few years later, Ru4POM was immobilized onto glassy carbon electrodes and ITO coated glass slides through the employment of a conducting polypyrrole matrix and layer-by-layer techniques. The Ru4POM doped polypyrrole films showed stable redox behaviour in the pH range 2–5 and enhanced electrocatalytic potential towards water oxidation.101 As the working concentrations for most POM WOCs are too low upon their direct immobilization, a new method was derived to increase the concentration of the semiconductor metal oxides (SMO) and electrode-immobilized WOCs in order to analyze the integrity of the POMs with spectroscopic methods.126 After treating the SMO with a reactive silyl derivative containing a quaternary ammonium side chain, the highly charged POM binds quickly to this cationic surface in solution through robust electrostatic linkages. Even a linker-free attachment of Co4POM to nitrogen-doped carbon nanotubes (NCNTs) was reported.127 Compared to previous studies on Ru4POM/graphene hybrids, the emerging Co4POM/NCNT shows a high catalytic performance without additional oxidants, comparable with Ru based POM/CNT composites.

MOFs are another widely investigated platform for composite heterogeneous WOCs and other functional materials.128 Lately, POMs have been employed as well to develop functionalized MOFs.129–131 In a recent study, the incorporation of a Keggin-type POM into a MOF host led to significantly improved photocatalytic activity in water oxidation compared to the pristine MOF-derived nanostructure, and further progress in this interdisciplinary field is rapid.132,133 Once the stability of POMs has been established under the applied WOC assay conditions, they are promising candidates for device construction, thereby complementing molecular WOCs.

1.4. Dynamic concepts for heterogeneous WOCs

In terms of maintenance and consumer-friendly stability timescales, heterogeneous catalysts still appear to be the first choice for future artificial photosynthesis technologies.134–136 However, the higher stability of solids comes at the price of more demanding in situ/operando setups for gathering information on their rate determining steps (RDS) towards proposing overall mechanisms. Along the lines of new “cocktail” catalyst concepts (Fig. 1),22 the spectrum of heterogeneous catalysts is continuously expanded from bulk materials towards thin films, nanoclusters, and even molecular species, which mediate the self-healing processes of solids.28,137 Some cross-over studies illustrate the manifold options for heterogeneous WOC design from long-range structural tuning to the construction of their molecular mimics.
Amorphous catalysts between molecules and solids. Ever since the breakthrough discovery of the remarkably robust and efficient amorphous Co–phosphate based WOCs (referred to briefly as CoCat or CoPi etc.),137 their self-repair and oxygen evolution reaction (OER) mechanisms have been investigated intensely.139 Recently, the solid–molecule interface of CoCats was studied with a combination of in situ X-ray absorption spectroscopy (XAS) and electrocatalysis (Fig. 7).138 Oxygen evolution emerges from an equilibrium of three structural motifs containing either Co(II)/Co(II), Co(III)/Co(III), or Co(III)/Co(IV) moieties. O–O bond formation was postulated to arise from an encounter of a minimum of two Co(IV) sites within the Cox(μ-O)y(μ-OH)z fragments. Dynamic Co-oxide fragments likely contain these reactive moieties within a “sea” of other system components, thus bearing resemblance to hydrated oxides with molecular features.138 Future studies into their O–O bond formation steps will pave the way to alternative bio-inspired optimization strategies beyond the traditional structural engineering of solids.
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Fig. 7 Interconversion of structural motifs in CoCat at the margins of Co–oxo fragments estimated to contain 9–16 cobalt centers. Reproduced from ref. 138 with permission from Royal Society of Chemistry, copyright 2015.
Thin films between solids and nanomaterials. Electrocatalytic water oxidation is increasingly focusing on ultra-thin film catalysts, which circumvent many issues of bulk materials, such as electrocatalyst conductivity, extensive porous structuring, interfacial contact properties and depletion regions, and – last but not least – difficulties in performance standardization.140–142 Recent studies on CoPi thin films corroborate the Janus-nature of this key amorphous material through ascribing the oxygen evolution process to molecular sites which are supported by bulk-mediated charge transport processes.143 The fraction of active Co(III)/Co(IV) couples was found to be less than 10% of the total cobalt centers, and phosphate buffer diffusion was identified as the limiting factor, albeit depending on the effective film thickness (Fig. 8).143
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Fig. 8 CoPi thin films displaying different characteristics during electrocatalysis. Reproduced from ref. 143 with permission from American Chemical Society, copyright 2016.

Highly active Ni/Fe(oxy)hydroxide electrocatalysts of the layered double hydroxide (LDH) type are another widely studied model system for structure–activity relationships ranging from the bulk to the ultra-thin film scale.144 The crucial role of the Fe centers was recently investigated through correlating the optical properties derived from in situ spectro-electrochemical studies on Ni- and Ni/Fe-(oxy)hydroxide thin films with electronic structure calculations (Fig. 9). The superior conductivity of the mixed metal system is likely to facilitate the formation of Fe(IV) centers as active sites.145 Their precise location at the surface, defect or bulk sites, respectively, is an important activity tuning parameter and thus highlights the increasing role of local structural engineering in solid state WOC optimization. Interestingly, the recently discovered Ni/V LDH monolayer electrocatalyst excels through comparable catalytic water oxidation activity to the hitherto best-performing Ni/Fe LDHs in alkaline media.146 Another breakthrough discovery of a highly active hierarchically structured spherical Fe/V hydroxide/oxide composite electrocatalyst amplifies the WOC application options for efficient earth-abundant oxide materials.147


image file: c7cs00306d-f9.tif
Fig. 9 (A) Single-layer NiFe oxyhydroxide; (B) top view of pristine and (C) NiFe oxyhydroxide doped with 25% Fe, Ni3Fe1O8H4; Ni (green), O (red), H (gray), and Fe (blue). Reproduced from ref. 145 with permission from United States National Academy of Sciences, copyright 2017.
Nanoclusters – beyond surface enhancement. Nanoparticle-sized WOCs, namely NiO6 clusters with a few nm diameters, have been newly combined with amino acids as molecular building blocks to enhance the generally low current density of conventional transition metal oxide nanocatalysts.148Operando XAS measurements (cf. Part II for details) demonstrated that the interaction of the nanoclusters with intact amino acids notably enhances the electrodeposition of active nickel centers on the electrode surface. This bio-inspired molecular-nanoparticle composite strategy paves the way to the convenient optimization of Co- and Mn-oxide electrocatalysts and related materials.149

Nanocluster WOCs are also capable of altering the mechanistic pathways of their bulk oxide counterparts. Monodispersed 10 nm-sized MnO nanoparticles on FTO electrodes with high activity at neutral pH were subjected to in situ XAS studies in combination with EPR and further spectroscopic methods to monitor the formation of catalytically active Mn(IV)[double bond, length as m-dash]O species. Interestingly, the RDS of the nanoparticles was found to differ notably from conventional solid state Mn-based catalysts: the active species arose from a concerted chemical proton coupled electron transfer (PCET), so that the bulk RDS of one-electron oxidation from Mn(II) to Mn(III) was no longer rate determining (Fig. 10).149


image file: c7cs00306d-f10.tif
Fig. 10 Proposed mechanism for conventional bulk Mn-oxide WOCs (top) vs. different RDS in 10 nm MnO nanoparticles. Reproduced from ref. 149 with permission from American Chemical Society, copyright 2017.

Furthermore, nanoclusters enable direct insight into surface processes via advanced microscopy techniques. Cobalt oxide nanoclusters on single Au(111) crystals were employed as models for β-CoOOH layers to investigate H2O dissociation on the respective basal plane and edge sites.150 An advanced combination of X-ray photoelectron spectroscopy and in situ scanning tunneling microscopy (STM) backed with computational modelling clearly identified the edge sites as preferable locations for oxygen evolution, while the basal plane sites exhibited different interactions with water molecules.150

Molecules vs. solids – active sites and benchmarking. The key role of Co edge-sites for oxygen evolution via oxo–oxo coupling was corroborated for 18O labeled amorphous CoCats through differential electrochemical mass spectrometry (DEMS) in H216O.151 In parallel, a dinuclear cobalt complex equipped with a Co2(OH)4 edge site motif stabilized by the DPEN (dipyridylethane naphthyridine) ligand was constructed as its molecular mimic and subjected to water and anion exchange experiments.151

All of these advanced WOC design efforts lead to a growing need for generally applicable and convenient benchmarking protocols. Here, stable molecular WOCs offer the distinct practical advantage of more straightforward benchmarking (i.e. well-defined atomistic TON and TOF values) compared to solids. Over the past years, general workflows for benchmarking electrochemical systems for the oxygen evolution reaction were developed.152,153 To date, comparable standards have not yet been fully established for photocatalytic oxygen evolution. The most widely accepted current practices for comparing different heterogeneous WOCs under photocatalytic conditions correlate their activity with the oxygen evolution yields based on the sacrificial electron acceptor.75 However, standardizations in terms of the actual number of catalytic sites would be preferable. Classic nanoparticle design in heterogeneous catalysis generally facilitates this, as demonstrated in a model study comparing Co3O4 nanobelts and nanocubes with respect to their activity in CO oxidation. Co3O4 nanobelts exposing their {011} planes showed higher activity than nanocubes with exposed {001} planes.154 WOC modeling and design can directly benefit from such structure–activity relationships as an alternative to in situ investigations.

Atomistic benchmarking and mechanistic studies of amorphous Co-catalysts, however, remain demanding. A recent innovative study on colloidal Co-oxide nanoparticles (stabilized by methylene diphosphate, M2P) addressed both issues through first turnover analysis of water oxidation with [Ru(bpy)3]3+ as a chemical oxidant.155 The O–O bond formation mechanism was explored by time-resolved 18O-labeling isotope-ratio membrane inlet mass-spectrometry (MIMS). Intramolecular coupling of two adjacent Co(IV) centers was found to prevail, in line with the aforementioned studies. Moreover, this method permits the determination of the number of catalytically active sites in the amorphous cobalt oxide (Fig. 11).155


image file: c7cs00306d-f11.tif
Fig. 11 Mechanistic model for oxygen evolution of colloidal M2P stabilized Co-oxide nanoparticles. Reproduced from ref. 155 with permission from Royal Society of Chemistry, copyright 2015.

The above examples outline the major role of new analytical techniques in advanced WOC research. Part II provides an overview of synchrotron-based methods, in-house operando approaches and new aspects of day-to-day analytical techniques.

2. Part II: Monitoring the interface between molecules and solids with new techniques

2.1. Cutting-edge monitoring techniques beyond in-house facilities

2.1.1 X-ray absorption spectroscopy and related techniques. X-ray absorption fine structure (XAFS) spectroscopy is widely used to study the electronic structure and the local geometries of the respective absorber atoms in homogeneous and heterogeneous catalysts under working conditions.35,156,157 XAFS experiments on 3d metals (Sc–Zn) are generally probed at the K-edge, because the edge positions of these elements above 4 keV render UHV conditions unnecessary. X-ray absorption spectra consist of two regions (see Fig. 12). The first region is located within some eVs around the absorption edge and is referred to as the X-ray absorption near edge structure (XANES). Approx. 20–30 eV above the edge, the extended X-ray absorption fine structure (EXAFS) sets in.
image file: c7cs00306d-f12.tif
Fig. 12 Schematic representation of a K-edge spectrum; blue: XANES region, red: EXAFS region.

XANES probes the unoccupied orbitals, and it is strongly sensitive to the valence state and the coordination geometry of the absorbing atom. The valence state of the absorbing atom can be determined from the edge position. In some cases a pre-edge feature is visible, caused by the symmetry forbidden transition of a 1s electron into a 3d orbital. As its intensity is strongly enhanced for broken symmetry (e.g. tetrahedral coordination), the pre-edge feature is a strong indicator of the symmetry features of a compound.35,158

The EXAFS region, where the photoelectron is excited into the continuum,35 provides information on the local geometric structure (such as the number of neighboring atoms, distances, and disorder), mainly based on the scattering of the photoelectron by neighboring atoms. EXAFS is widely used to investigate the local geometries of the manganese centers in the {CaMn4O5} OEC.159–161 In an illustrative study, EXAFS was employed to evaluate 12 different structural models for the OEC obtained from protein crystallography.161 Another classic example for the application of EXAFS to elucidate structural motifs of complex targets is the investigation of amorphous CoPi-type WOCs.162 XAFS was furthermore used to identify structure–activity correlations in α-CoOOH nanosheets used for electrochemical water oxidation.163 The in situ formation of γ-CoOOH was indicated by the presence of undercoordinated CoO6−x sites with an average valence state of 3.3. Moreover, XAFS measurements are an excellent tool to check on the post-catalytic structural integrity of any WOC type. For example, the cobalt-based POM catalyst Na10[Co4(H2O)2(PW9O34)2] was shown to remain structurally intact after chemical oxidation with [Ru(bpy)3]3+ through XAFS analyses.164

K-edge XAFS of the 3d metals generally probes bulk states, while surface states are clearly of more immediate interest for the understanding of heterogeneous catalysts. Recording EXAFS data in electron yield mode permits surface sensitive characterization of the local structure.35 This method was applied to identify electrochemically induced changes in the local surface structures of perovskite-type WOCs. Here, an increased fraction of edge-sharing octahedra was observed for SrCo0.8Fe0.2O3−δ and Ba0.5Sr0.5Co0.8Fe0.2O3−δ after cycling at 1.1 V and 1.7 V vs. RHE in KOH.165

To date, genuine operando XAFS studies on WOCs have been increasingly explored. A straightforward example of operando XAFS WOC monitoring compared the photodeposition of Co–Pi and Co–Bi WOCs.166 The deposition of Co–Bi on SrTiO3 photoelectrodes was found to generate a higher number of catalytically active sites. In situ XAS was furthermore used to identify the catalytically active sites in a bifunctional manganese oxide (MnOx) catalyst with high activity for both oxygen reduction reaction (ORR) and OER.167 At a potential relevant for ORR (0.7 V vs. RHE) a disordered MnII,III,III3O4 phase was found. Under OER conditions (1.8 V vs. RHE) the film consists of 80% mixed MnIII,IV oxide and 20% structurally unchanged Mn3O4. Furthermore, in situ XANES experiments in fluorescence mode with different MnOx film thicknesses indicated the presence of porous structures with higher accessibility of the catalytic centers throughout the films.

The development of new experimental endstations enables even deeper insight into the electronic structure of transition metal compounds.156,168–170 Their main features are wavelength dispersive spectrometers with spectral resolution below the natural spectral broadening.156,171

XAFS detection in fluorescence mode using energy dispersive detectors (e.g. 5-element detector) was originally developed for experiments on samples with a low concentration of absorbing atoms. When applying wavelength dispersive spectrometers the signal to noise ratio is improved as well as the spectral resolution for fluorescence detection.172 The use of spectral resolution below the natural spectral broadening permits high energy resolution fluorescence detection (HERFD) XANES, which does not only offer optimal signal to noise ratios, but also significantly sharpened absorption features.156,172

While XAFS probes the unoccupied states, non-resonant X-ray emission spectroscopy (XES) probes the occupied electronic states.156,168 Here the sample is excited with monochromatic X-rays with an energy above the absorption edge and the energy of the outgoing X-ray photons is scanned. XES is able to detect the protonation state of ligands (OHversus H2O) as well as the substitution of a ligand atom by elements with a neighboring atomic number.170

In resonant inelastic X-ray scattering (RIXS) experiments, the energy of both incoming and outgoing signals is scanned, thus providing two-dimensional spectra (see Fig. 13).156,168,169 The energy of incoming X-rays (Ω) is tuned in the region of pre-edge absorption. In a secondary process, the 1s hole can be either filled by a 2p or 3p electron, followed by the emission of a Kα photon or a Kβ photon, respectively. The final state in this two-photon process is a virtual L- or M-edge, depending on the monitoring of the Kα or Kβ emission, respectively. Therefore, RIXS provides access to L- and M-edge related spectra of 3d metals while circumventing UHV conditions.


image file: c7cs00306d-f13.tif
Fig. 13 Left: RIXS spectrum of Mn(II)O with the conventional pre-edge XANES spectrum shown for comparison (black line). The two strong peaks represent 2p3/2 and 2p1/2 spin–orbit split levels. In the energy transfer direction, they correspond to the L3 and L2 edge-like features, respectively. Right: 1s2p RIXS energy diagram; Mn(II) corresponds to a 1s23d5 ground state. Reproduced from ref. 168 with permission from Springer, copyright 2009.

Recently, RIXS experiments on a molecular {Co(III)4O4} cubane WOC were employed to determine the contribution of Co(IV) centers to the OER activity against a spectroscopically active Co(III) background.169 RIXS provided not only new options to directly probe Co(IV) centers, but also insight into oxo-mediated metal–metal interactions across the cubane.

2.1.2 In situ APXPS techniques. XPS is widely used to investigate the elemental composition as well as the chemical oxidation and electronic states of a given material. The absorption of an X-ray photon is followed by the excitation of a photoelectron into the continuum. Due to the short mean free path of the photoelectron in condensed matter, only the top few nanometers of a sample are probed with XPS.

Therefore, standard XPS techniques are limited to UHV conditions. Ambient pressure XPS (APXPS) is still mainly a synchrotron-based technique, which overcomes these limitations and permits measurements in the Torr range and above.32,173–175 A typical experimental setup is shown in Fig. 14, where differential pumping stages are applied between the sample chamber and the analyzer, which is kept under UHV conditions. This method permits the maintenance of a pressure difference of several orders of magnitude. The maximum pressure in the sample chamber depends mainly on the size of the entrance aperture and on the capacity of the differential pumping stages.173


image file: c7cs00306d-f14.tif
Fig. 14 Schematic representation of an APXPS setup using a set of differentially pumped electrostatic lenses and apertures to separate the sample chamber from the analyzer under UHV conditions. Reproduced from ref. 173 with permission from American Chemical Society, copyright 2015.

Operando APXPS has been conducted on a wide range of systems covering solid–liquid interfaces,176 catalysts,177–180 and fuel cells.181 In a corresponding WOC-related study, in situ XPS experiments on Pr1−xCaxMnO3 (PCMO) revealed new insight into the electrochemical activation pathways. Additional peaks emerged in the Ca 2p and O 1s spectra,178 and the former were explained with the formation of Ca(OH)2 and CaO surface species. The appearance of broad peaks in the O 1s spectrum was assigned to weakly surface bound oxygen species.

2.1.3 Advanced scattering techniques. For WOCs with nanocrystalline, highly disordered or even amorphous structures, traditional techniques based on Bragg diffraction analysis have fallen short of providing atomic scale structural information.183–185 Consequently, atomic pair distribution function analysis (PDF) has emerged as a technique that provides specific information on interatomic distances.186 X-ray scattering based PDF for elucidating the domain size and structure of WOCs complements XAFS studies by covering the full range of atom–atom distances.182

As PDF techniques are based on the probability of finding two atoms at a given interatomic distance (Fig. 15), they can be applied to analyze a variety of materials ranging from crystalline and amorphous compounds over solid nanoclusters and thin films to molecules.182


image file: c7cs00306d-f15.tif
Fig. 15 Schematic illustration of PDF measurements using high-energy X-rays. Reproduced from ref. 182 with permission from John Wiley and Sons, copyright 2013.

In a recent case study on WOCs, a heterogeneous amorphous iridium oxide-based film was characterized by PDF analysis, and a model of Ir5O22 clusters based on rutile lattice moieties was constructed. The results suggested the presence of Ir(μ-O)3Ir or distorted Ir(μ-O)2Ir substructures.187 More recently, the structure of the molecular Ir-WOC [Cp*Ir(pyalc)OH] in a characteristic blue solution was uncovered through the combination of high energy X-ray scattering (HEXS) and EXAFS measurements with DFT-based simulations of the experimental data. New Ir(IV) dimer models with mono-μ-O cores and terminal anionic ligands were proposed to undergo oxygen evolution pathways bearing strong resemblance to the intensely investigated Ru blue dimer.188

Concerning heterogeneous WOCs, recent PDF studies focused on elucidating the domain size and structure of cobalt oxide films prepared by electrochemical deposition. The results demonstrated that atom pair correlations persist up distances about 13 Å, and the PDF data could be fitted with lattice domains of 13–14 Co atoms containing edge-sharing CoO6 octahedra with distorted geometries at the domain edges.189 Such results are important for highly sought-after correlations between high catalytic activities and certain structural elements. Further PDF investigations comparing cobalt oxide films in borate and phosphate solutions show an electrolyte dependence in the intermediate-range structure of the WOCs, thus connecting molecular chemistry and concepts of extended solids.190

Neutron scattering and small-angle X-ray scattering (SAXS) are other important techniques that have been widely employed to study catalytic reactions. The former is well known for its superior detection of light elements and bears great potential for monitoring water oxidation processes.191,192 SAXS is an effective method for studying nanoscale structures with respect to defects and crystallinity.193 Even though SAXS is well established as an in house-method, synchrotron based operando studies still offer special opportunities. They provide access to nm range structural information in both solid and liquid phases, thus opening up new possibilities for studying WOCs with amorphous and defect-rich structures.194,195

Although the above cutting-edge monitoring techniques represent the most reliable current methods to obtain information about catalyst structures with respect to valence states and local coordination environments, the complexity of many catalytic systems under consideration may still give rise to ambiguous conclusions. This applies especially when the target compounds are affected by intrinsic methodological limitations (such as difficulties in distinguishing scattering centers with related atomic numbers by EXAFS). Even such advanced approaches may thus fail to detect minor differences at the molecular level, in particular when the precise concentration and initial composition of such dynamic molecular species remain unknown.

Next, recent progress on new in-house monitoring techniques for use on an everyday scale is briefly covered.

2.2. In-house monitoring innovations

It goes without saying that the opportunities of gathering structural information at the molecular level with in house monitoring techniques are not directly comparable to the above-mentioned forefront approaches. They emerge as more and more powerful tools to collect the fundamental input for advanced follow-up analyses with respect to ligand loss, nanoparticle formation, surface alteration processes etc. as fundamental prerequisites for efficient follow-up monitoring.
2.2.1 In situ TEM methods. Transmission electron microscopy (TEM) provides access to structural investigations with atomic resolution. However, the short free mean path of the electrons limits standard TEM techniques to UHV conditions. TEM setups are frequently combined with different analytical techniques, such as electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDS).196,197

In contrast to conventional TEM, environmental TEM (ETEM) allows studies in a controlled gas or liquid environment.196–200 ETEM can be realized using two different approaches,196,200 with one of them based on differential pumping stages.199 Here, the sample is separated under a controlled gas atmosphere by differentially pumped apertures from the microscope column which is kept under UHV conditions. In the other approach, the sample is placed in a closed cell with thin windows which are transparent for electrons (e.g. Si3N4). This approach permits experiments in liquid environments that are of wide current interest, e.g. for life science applications196 or electrochemical measurements in liquid media.198Operando ETEM using a setup with differentially pumped apertures was used to study structural changes of anatase surfaces during photocatalytic water splitting.201 Surface amorphization was observed in the presence of water vapor and UV light. The amorphous oxide layer had a thickness of 1–2 monolayers, contained Ti3+ centers and did not grow over time.

In other ETEM studies on heterogeneous water splitting catalysts, the oxygen evolution activity of doped manganite perovskite (Pr0.7Ca0.3MnO3, PCMO) was investigated in He/H2O/SiH4 gas mixtures (Fig. 16).197,202 The electron beam induced OER was monitored by the growth of an amorphous SiO2−y layer on top of the catalyst. In the electron induced OER, holes are generated in the PCMO due to the formation of secondary electrons.197


image file: c7cs00306d-f16.tif
Fig. 16 ETEM study on Pr0.68Ca0.32MnO3 under water oxidation conditions: (a) UHV image of the pristine WOC surface, (b) WOC in H2O/SiH4 after 20 s and (c) after 70 s of electron beam stimulation. Reproduced from ref. 202 with permission from John Wiley and Sons, copyright 2012.

SiH4 reacts with the O2 generated on the PCMO surface to form amorphous SiO2−y. Systematic reference studies were performed to exclude SiO2−y layer formation by the direct reaction of SiH4 with H2O. Additionally, local EELS measurements showed the formation of Mn2+ at the surface, in line with earlier in situ XANES studies.178

Another perovskite-type WOC, namely Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF), displays structural oscillations during water oxidation monitored by ETEM,203 which were explained through oxygen bubble formation. The results were compared with other perovskite materials showing only minor oscillations in the case of SrCoO3−δ (SCO), and none for La0.5Sr0.5CoO3−δ (LSC) and LaCoO3 (LCO). A new correlation of structural oscillations and O 2p-band energy thus emerged from direct microscopic insight.

2.2.2 Progress in selected operando spectroscopy techniques for WOC monitoring. In situ/operando monitoring techniques permit the tracking of structural changes of species in catalytic systems over a wide range of reaction conditions, such as temperature, concentrations, or pH.204–206 Time-resolved analysis of the reaction products is equally important, so that more and more true operando spectroscopy techniques have been developed for the online process analysis of structural features and products (cf. typical setup in Fig. 17).207
image file: c7cs00306d-f17.tif
Fig. 17 Characteristic setup of an operando spectroscopy monitoring system. Reproduced from ref. 207 with permission from Elsevier, copyright 2017.

FT-IR spectroscopy is among the most frequently used operando techniques in search of WOC intermediates under reaction conditions with temporal resolutions in the millisecond range. As shown in Fig. 18, one or few spectra are collected during the light pulse, followed by any number of desired spectra after termination of the pulse.208 Earlier applications of this technique on water oxidation processes were focused on solid semiconductors, such as TiO2.209,210 Recently, operando FT-IR investigations of α-Fe2O3 demonstrated that a FeIV[double bond, length as m-dash]O intermediate is formed after the first hole-transfer process, which is an important insight to understand high performance semiconductor electrodes.211 Concerning heterogeneous catalysis with noble metals, recent rapid-scan infrared spectroscopy studies on IrO2 nanoclusters identified IrOOH intermediates on the catalyst surface.212 Related studies on Co3O4 nanoclusters brought forward new insight into the different roles of a surface superoxide intermediate (absorbing at 1013 cm−1) and a proposed Co(IV)[double bond, length as m-dash]O intermediate in the water oxidation mechanism.213 Proceeding from nanocluster to thin film monitoring, the combination of operando electrochemical impedance and XAS led to the successful observation of a CoOOH intermediate during electrochemical water oxidation with Co3O4 films.214 Growing insight into possible intermediates is vital for rendering WOC synthesis more targeted. Therefore, other operando techniques are continuously tuned for monitoring WOC processes, as illustrated by a study detecting intermediate Fe4+ species through Mössbauer spectroscopy on nickel-iron (oxy)hydroxides.215 Recently, a first rate law analysis of water oxidation on hematite was reported, which demonstrates that water oxidation can be launched with surface hole densities higher than 1 hole per nm2.216 Inspired by this work, operando XAS was performed on α-Fe2O3/IrOx mediated water oxidation reactions,217,218 indicating the presence of iridium hydroxo (Ir-OH) surface species during the reaction. We only briefly refer to the growing literature219,220 on the use of Raman and electron paramagnetic resonance (EPR) spectroscopy techniques, and further progress can be expected for the near future.221–224


image file: c7cs00306d-f18.tif
Fig. 18 Temporally resolved rapid-scan FT-IR spectroscopy scheme as used for the investigation of photocatalysts. Reproduced from ref. 208 with permission from Elsevier, copyright 2016.
2.2.3 New lab scale tabletop extreme ultraviolet (XUV) technologies. Another open challenge in the analysis of heterogeneous catalysts is the spatially resolved compositional tracking of their nm scale (sub)surface and bulk onset zones of leaching processes. Even rather stable perovskites, for example, have displayed such post-catalytic instabilities in the (sub)surface region.165 HRTEM and XPS techniques are limited with respect to element quantification options, while conventional laser ablation – mass spectroscopy (LA-MS) couplings mainly access micrometer scales.225 Over the past few years, microanalysis techniques based on XUV laser and soft X-ray techniques have been developed to transfer beamline technologies to labs and to couple them with other techniques.226–229 The most promising immediate perspective for WOC research are EXAFS and NEXAFS measurements with tabletop devices, such as recently demonstrated for in-house O K-edge measurements of Pr0.7Ca0.3MnO3.230 To date, lab-scale XUV setups have been mostly based on high harmonics (HHG) and plasma sources. Earlier studies have been conducted on charge transfer processes in Co3O4 using femtosecond pulses,231 and lifetimes of charge carriers in liquid environments were tracked for Co3O4 in methanol as well.232 In 2016, a pioneering study on high harmonics based tabletop XANES at the metal M2,3 edge was conducted on Co(II)-, Co(III)- and Fe(II) model complexes bearing resemblance to characteristic molecular WOC architectures. Information on the ligand field sensitivity and oxidation states of their metal centers was conveniently obtained without synchrotron sources.233 However, HHG based approaches appear less suitable for nm-resolved ablation techniques due to their large chromatic aberration and weak in-band photon fluxes. Current activities aim to tap the full opportunities of XUV technologies for all-in-one lab devices providing comprehensive information on chemical composition, oxidation states, and chemical environments of transition metal WOC centers at the 10–100 nm subsurface level.229
2.2.4 Lab scale options for tracking catalyst conversions. In addition to studying the integrity of molecular WOCs with the above-mentioned sophisticated techniques, the informed application of classic spectroscopic, electroanalytical and separation methods can be quite helpful for basic distinctions between molecular and heterogeneous water oxidation processes. In the following, we focus on some new developments and practical aspects.

Dynamic light scattering (DLS) has turned into the method of choice to detect whether molecular WOCs decompose to metal oxide nanoparticles during OER. However, DLS instruments were primarily designed for quantifying the size distribution of large amounts of nanoparticles – and not for proving the presence/absence of nanoparticles in their own right. The high sensitivity of DLS setups to any nanoparticles existing in the solution may give rise to false positive results arising from very small background impurities or ubiquitous environmental nanoparticles (dust, fibers…). However, more substantial amounts of nanoparticles are usually formed from the decomposition of molecular WOCs, so that DLS size distributions should be carefully checked for genuine positive results. Count rate, correlation function curve, and the measurement times for each run are other key parameters (Fig. 19).


image file: c7cs00306d-f19.tif
Fig. 19 High (right) and low quality (left) correlation function curves representing large and negligible amounts of CoOx nanoparticles with count rates of 186.2 and 3.1 kcps, respectively.

Widely applied photocatalytic assays, for example, often give rise to small amounts of insoluble [Ru(bpy)3]2+–S2O82− precipitates while mixing the components, which persist throughout tests and can lead to high background DLS counts. Preliminary filtration of test solutions may remove such impurities along with significant nanoparticles from decomposition processes.

We therefore suggest the following rapid and reliable sequence of photocatalytic activity recycling tests and DLS measurements: First, a post-catalytic molecular WOC solution is filtered, and the pH is carefully readjusted to the original value. Next, the recycling test is launched by adding the same quantity of Na2S2O8 as in the first run. Strong indications for an intact molecular WOC are (a) that no nanoparticles are found in DLS measurements of the recycled solution without filtration (Fig. 20), and (b) that no or only negligible activity losses in O2 evolution are observed from the initial to the recycling test. Furthermore, neither O2 evolution nor nanoparticles were observed in post-catalysis recycling test solutions of reference catalyst equivalents, e.g. Co(OAc)2, as a reference for the complete decomposition of a molecular WOC (Fig. 20, right). In situ formed CoOx nanoparticles were completely removed by filtration after the first run, and complete WOC decomposition would afford analogous results.


image file: c7cs00306d-f20.tif
Fig. 20 Clark electrode kinetics of visible light-driven water oxidation of the filtered post-reaction solutions catalyzed by 100 μM of a representative molecular WOC (left) and 400 μM Co(OAc)2 (right).

Mass spectrometry has been employed as a useful tool for characterizing remnant molecular WOCs after catalytic tests.234,235 However, this method is not suitable for the widespread assays employing [Ru(bpy)3]2+ as a photosensitizer or [Ru(bpy)3]3+ as a chemical oxidant, because the high ratios of Ru-complex to WOC strongly interfere with the catalyst detection. As outlined below for the general limitations of monitoring techniques, circumventing this issue just by applying higher catalyst concentrations is not a preferable option. We thus recommend a stepwise lypophilization–extraction procedure for operational catalyst concentrations. First, different solvents are to be screened for selective dissolution of [Ru(bpy)3]Cl2 and related Ru-containing compounds (e.g. butanol dissolves [Ru(bpy)3]Cl2, but not some cluster-type WOCs). This solvent is then applied to extract [Ru(bpy)3]Cl2 from the solid phase of a lyophilized post-photocatalytic solution. Application of a second catalyst-selective solvent for extraction of the residual molecular compounds then affords suitable test solutions for MS characterization with minimal residual Ru-contents.

High-performance liquid chromatography (HPLC) is another effective method to separate catalytic components from the reaction mixture. In our recent studies, we used a size selective column to analyze a post-photocatalysis solution of the cubane-type [Co3Er(hmp)4(OAc)5(H2O)2] WOC (Fig. 21).236 The cubane catalyst was successfully separated, and its integrity was determined without detecting any Ln3+ and Co2+ derivatives or the free ligand. In order to prevent the structural transformation of molecular WOCs during HPLC measurements in the mobile phase, e.g. through the influence of pH and buffer materials, optimization with respect to maximum resemblance to the buffered test media is vital.


image file: c7cs00306d-f21.tif
Fig. 21 Left: Structure of [Co3Er(hmp)4(OAc)5(H2O)2] (dark blue, Co; green, Er; blue, N; grey, C; red, O; white, H). Right: HPLC measurements of [Co3Er(hmp)4(OAc)5(H2O)2] photocatalyzed post-water oxidation solution and reference compounds.

Inductively coupled plasma mass spectrometry (ICP-MS) analyses of molecular WOCs also require their efficient post-catalytic separation. The selective removal of the POM WOC [Co4(H2O)2(PW9O34)2]10− from post-catalytic solutions through extraction with a toluene solution of tetra-n-heptylammonium nitrate is an instructive example.237 Very small amounts of residual Co-containing substances after extraction were determined by ICP-MS in the aqueous phase, and they showed no further activity in photocatalytic water oxidation. As the large applied quantities of [Ru(bpy)3]2+ and its derivatives can influence the ICP-MS accuracy, they have to be removed first. We applied methanol to dissolve these Ru-containing substances from the lyophilized solid of the post-catalytic solution of the cubane WOC [Co4(hmp)4(OAc)4(H2O)2], along with the molecular catalyst.238 A very small amount of Co species displaying no more catalytic activity was quantified by ICP-MS analyses of the methanol washed residual solid.

Moreover, numerous protocols are available to examine the stability of molecular WOCs with electrochemical methods. A direct indication of stability is the reproducibility of CV scans in the respective buffer solution over time. Possible decompositions of molecular catalysts into metal oxide particles can be identified by differences between CV scans of a thoroughly rinsed working electrode after normal tests and the respective background CV curves of catalyst-free buffer solutions. Catalytic oxidative linear-sweep voltammetry was further developed to quantify the degree of decomposition in unstable molecular WOCs based on calibration curves for the anodic currents of various Mn+ concentrations mimicking complete decomposition of the initial molecular WOCs with M representing the active site.239 The decomposition extent of the initial WOC can then be quantified through measuring its anodic current intensity at the same overpotential.

In fact, the differentiation of specific catalytic intermediates from the entire catalytic system and their targeted monitoring often remain even beyond the capability of the most advanced operando processes. These difficulties associated with the selective observation of active species arise both from intrinsic features of the catalytic system and from the technological monitoring facilities. Tracking a single active species in the presence of others often remains an ambiguous task, and many ex situ monitoring techniques may mainly capture dormant species rather than the catalytically active intermediates, thus rendering direct conclusions on the actual catalytic cycle very difficult.

This applies even more for post-catalytic characterization, which relies on the separation of the catalyst from the catalytic system as a common practice. Conveniently, the separation of reaction products is not necessary in the case of water oxidation reactions. However, the separation of possibly different catalytic species remains challenging to execute, so that the conclusions drawn from such measurements may not be representative for the entire system at all. A “classic” scenario involves the formation of active heterogeneous catalysts from a molecular precatalyst. Even though the former may be removed through filtration, the latter may still repeatedly give rise to fresh heterogeneous catalyst species in situ. In the particular case of water oxidation, the frequently complex reaction conditions, namely the presence of photosensitizers and sacrificial oxidants, further restrict the possibility of concluding on catalytic mechanisms from standard analytical methods.

Nevertheless, selected recent examples demonstrate that tracking specific catalytic species/features with advanced operando techniques under conditions close to actual settings can be realized, such as the identification of FeIV[double bond, length as m-dash]O and CoIV[double bond, length as m-dash]O intermediates through operando FT-IR spectroscopy.211,213 These challenges associated with elucidating entire catalytic cycles should be kept in mind before assuming that their complete understanding is always an option in advanced catalysis research. On the other hand, the identification of possible key intermediates can be very helpful to develop sound working hypotheses for further studies. In addition, the systematic coordination of these monitoring techniques promotes the understanding of catalytic behaviour of WOCs from various perspectives. Therefore, the design of unambiguous catalytic test systems and the advancement of probing and selective monitoring strategies are current key tasks on the way to comprehensive insight into WOC mechanisms.

3. Part III: The dimensionality spectrum of WOCs

It goes without saying that modern in situ/operando techniques go hand in hand with the advancement of computational methods for WOC modeling. They are documented in many recent studies in their own right to which we kindly refer at this point.240,241 In this concluding part, we continue with selected highlights covering the differentiation strategies between molecular and heterogeneous species over the entire spectrum of WOC dimensionalities.

3.1. Transformation pathways of molecular WOCs

As outlined above, molecular WOCs may undergo (ir)reversible transformations under various operational conditions. Deconstruction of molecular WOCs starts predominantly with dissociation and/or oxidation of the coordinated ligands, followed by a series of other transformations, which depend on the applied method for water oxidation.26 We therefore discuss dissociation and oxidation processes as the two major initial ligand transformation pathways in more detail.

Ideally, metal centers of molecular WOCs chelated by ligands with strong N/O electron donor atoms should display rather small dissociation constants. However, catalytic parameters in solution can shift dissociation constants to a non-negligible level, such as solvent polarity, pH, competing active binding species etc. Ligand dissociation in WOCs was reported early on, e.g. for the molecular Mn-WOC [(H2O)(terpy)Mn(μ-O)2Mn(H2O)(terpy)]+ (terpy = 2,2′:6,2′′-terpyridine), where the loss of terpy ligands and permanganate formation were highly dependent on the ratio between catalyst and oxygen-atom transfer reagents (HSO5 or OCl, referred to as XO). For ratios of XO to catalyst above 500, the molecular catalyst was mainly converted to permanganate.242

The hydrolytic dissociation of ligands can also be influenced by buffers and other solution components. Phosphate buffer, for example, promoted the hydrolysis of Fe–TAML (TAML = tetra-amido macrocyclic ligand) through active binding.243 The introduction of non-covalent interactions through an axial isoquinoline ligand in molecular WOCs [Ru(bda)(isoq)2] (bda = 2,2′-bipyridine-6,6′-dicarboxylic acid; isoq = isoquinoline) has enabled an extended π-system (Fig. 22), which contributes to the high activity (TOF > 300 s−1) when using CeIV as an oxidant, and a radical coupling pathway for O–O bond formation was discussed previously.64 However, the dissociation of this axial ligand could not be avoided in the long term. In a follow-up study, a stable phthalazine axial ligand against dissociation was identified using DFT calculations.61 Similarly, the dissociation of one pyridine group from the molecular catalyst [FeII(Py5OH)(CH3OH)](ClO4)2 (Py5 = 2,6-bis[methoxydi-(pyridine-2-yl)methyl]pyridine) was identified and further confirmed with DFT calculations.244 Thus, increasing the binding affinity of the ligand to the metal centers, and thereby the energy barrier for dissociation, is a crucial design strategy.


image file: c7cs00306d-f22.tif
Fig. 22 Modeling of the encounter of two [Ru(bda)(isoq)2] (isoq = isoquinoline) molecules ([O1⋯O2] = 3.22 Å). Matching isoquinolines are nearly parallel at a relative distance and geometrical arrangement indicative of a stabilizing stacking interaction. Reproduced from ref. 64 with permission from Nature Publishing Group, copyright 2012.

Studies on ligand oxidation starting from a mononuclear cyclometalated Ir-WOC ([Ir(ppy)2(H2O)2]+ (ppy = 2-phenylphridine)) brought forward trace amounts of CO2 through mass spectroscopy in the presence of CeIV as an oxidant, indicating at least partial oxidation of the ligand framework.245 Later, systematic investigations were carried out to evaluate relationships between ligand substitution and oxidative stability. Starting from a mononuclear Ru–Hbpp (Hbpp = 2,2′-(1H-pyrazole-3,5-diyl)dipyridine) WOC, CO2 was detected during O2 evolution when the Hbpp ligand was substituted with a phenol moiety, which was supposedly first oxidized to quinone, followed by steps towards CO2 evolution.47 Next, a series of mononuclear Ru–NHC (NHC = N-heterocyclic carbene) WOCs with different N-substituents were prepared and compared, and ligand oxidation was preferably observed for bulkier N-substituents.246 Similarly, the influence of substituted moieties on the WOC stability was also demonstrated for the tetranuclear WOC {[Ru2(trpy)2(L)]2(μ-(bpp)2-μ-xyl)}n+ (trpy = 2,2′:6′,2′′-terpyridine, L = Cl or OAc, n = 4; L = (H2O)2, n = 6; bpp = 3,5-bis(pyridyl)pyrazolate) and the dinuclear Ru-WOC {[Ru2(trpy)2(L)](μ-bpp-bz)}n+. The amount of generated CO2 was significantly influenced by ligand substitution patterns (degree of accessibility) as well as by the catalyst concentration.247 Therefore, ligand modifications are the most direct step to enhance molecular WOC stability, prior to the aforementioned immobilization and confinement approaches. Applying optimized operational water oxidation conditions for a specific molecular architecture is equally crucial.

Even with the help of in situ/operando techniques (cf. Part II), identifying the final transformation product of a given molecular WOC often remains impossible due to the complexity of the applied multi-component reaction systems.27 The newly formed species can span a wide size range from bi/multi nuclear species over nanoclusters or -particles to visible precipitates in the long term.69

3.2. Monitoring of thin film and nanocluster WOCs

Controlled transitions from molecular precursors to thin film WOCs. The growing number of in-depth investigations into the decomposition of molecular (pre-)catalysts has opened up new options for their targeted use as precursors. [MMn3O4] (M = Ni, Co) cubanes, for example, are not only outstanding synthetic OEC analogues, but also interesting precursors for electrocatalysts.248 Drop casting on standard electrode materials followed by heating to 400 °C afforded efficient mixed-metal electrocatalysts which were found to maintain their characteristic cuboidal structure elements through XAS investigations.

Organic cobalt complexes are convenient sources for the direct deposition of electrocatalytic CoOx films, especially cobaloximes,249 and, most recently, Co–salen complexes.250 In a new study, an interestingly strong dependence of the resulting electrocatalytic oxygen evolution activity on the precise structure of the coordinating salen ligand was found. This established a strong link between molecular WOC construction and precursor design for heterogeneous catalysts.

An advanced study on the transformation of Co–porphyrin complexes into thin film electrocatalysts illustrates that tracking such processes can reach the limits of current analytical repertoires. The Co-complex WOCs showed high activity on FTO electrodes in borate buffer. Remarkably, an entire arsenal of conventional techniques (such as SEM, conventional XPS, and other spectroscopies) was insufficient to detect the presence of active CoOx moieties in the deposited ultra-thin films. They could only be identified by means of synchrotron-based photoelectron spectroscopy techniques with adjusted photon energies (SOXPES and HAXPES) (Fig. 23).251


image file: c7cs00306d-f23.tif
Fig. 23 Co 2p core level spectra recorded with a photon energy of 2100 eV (top) and 1000 eV (bottom) of CoTPP (5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(II)) on FTO, before electrolysis (top), after electrolysis (middle), after electrolysis and washed with acetone (bottom). Reproduced from ref. 251 with permission from American Chemical Society, copyright 2017.

In another recent work, the precise transformation of nickel foil with tris-n-octylphosphine (TOP) into nickel phosphide was tracked with in situ APXPS. Phosphide formation already set in at an unusually low temperature of 150 °C, accompanied by the emergence of sp2 carbon on the surface. These results can both facilitate economic catalyst production and indicate the possible contribution of active nonmetallic species.252

Unique insights from thin film WOC monitoring. Thin film monitoring can notably promote the understanding and quantitative assessment of WOCs, as recently reviewed for the dynamic field of Ni/Fe (oxy)hydroxides. Quartz crystal microbalance (QCM) techniques were recommended as convenient tools for tracking film loading and dissolution processes, while in situ electrochemical measurements are being refined to understand the charge flow from active sites to the support electrode.253

Along these lines, electrochemical operando APXPS monitoring just above the onset of oxygen evolution processes was further developed for 7 nm thin electrodeposited Ni–Fe films. The as prepared films containing 50% Fe underwent further oxidation of their partially metallic centers to Fe3+ and Ni2+/3+, respectively, after prolonged electrochemical redox cycling (Fig. 24). The application of planar electrodes first paved the way to APXPS monitoring, and novel approaches towards investigating at higher application-related current densities were discussed.254


image file: c7cs00306d-f24.tif
Fig. 24 (a) Schematic illustration of the thin film electrochemical cell and the sample configuration; (b) CV of an electrodeposited Ni–Fe electrocatalyst (50% Fe) layer with 7 nm thickness on an Au substrate. Reproduced from ref. 254 with permission from American Chemical Society, copyright 2016.

Finally, thin film studies open up unique access to reactive centers with little need for demanding in situ techniques, which sets them apart from other WOC types. Mixed Ni–Ir thin film WOCs were recently reported to exhibit an almost 20-fold mass based activity enhancement of Ir sites compared to classic bulk materials. Furthermore, a reactivity model based on accessible surface hydroxyl groups as reactivity descriptors was corroborated with a combination of XPS, XAS, and other spectroscopic/electrochemical techniques. The results agree with DFT models and confront the widely accepted paradigm that activity and stability often go hand in hand for WOCs.255

In situ studies of nanocluster WOCs. Proceeding from thin film to nanocluster monitoring, the latter were efficiently investigated with in situ spectroscopic techniques. As briefly mentioned above, rapid-scan FT-IR spectroscopy of iridium oxide nanocluster WOCs in the presence of 18O labeled/deuterated water led to the discovery of the very first hydroperoxide surface intermediate in IrOx catalysts that suggested the mediation of photocatalytic O–O bond formation through the reaction of Ir(V)–oxo intermediates with water or hydroxo groups.212 Many other works on Ir- and Co-oxide WOCs further promoted the unique potential of this flexible methodology for identifying crucial intermediates and their lifetimes.257,258

In the case of Co-oxide nanoparticles, their advantageously high surface area is combined with bifunctional activity for both oxygen evolution and oxygen reduction processes. This interesting Janus character was recently correlated with specific size ranges through in situ XAS studies on 3–7 nm CoOx nanoparticles (Fig. 25). OER activity was promoted for smaller particles, while the oxygen reduction activity remained rather independent of the particle size.256


image file: c7cs00306d-f25.tif
Fig. 25 In situ XAS studies of nanocluster Co-oxide WOCs combined with HRTEM to monitor the phase transition from CoO to Co3O4. Reproduced from ref. 256 with permission from American Chemical Society, copyright 2016.

Furthermore, in situ APXPS is applied on more and more nanoparticle-sized systems, and one breakthrough study on nanosized IrOx electrocatalysts revealed the intermediate formation of Ir(V), suggesting a single site mechanism via OOH intermediates,180 which is consistent with the above rapid-scan FT-IR studies.212

3.3. Polyoxometalate WOC pathways: between nanoclusters and molecules

Polyoxometalates (POMs) cover an interesting size range between nanoparticles and molecules. Although POMs offer all-inorganic stability advantages, their rigorous characterization throughout the catalytic process is mandatory to check on their possible role as precatalysts. Speciation studies of the initial POM motif under photo- and electrocatalytic conditions are crucial, but they remain demanding. Several examples of initially homogeneous Co- and Mn-WOCs, however, demonstrated their transformation into highly active heterogeneous oxides through transition metal leaching into solution (e.g. CoOx, MnOx).259,260

The low catalyst concentration in standard POM-WOC protocols poses specific analytical challenges for the identification and monitoring of true active species, as outlined in Part II above. Furthermore, their anionic charges give rise to strong electrostatic interactions, e.g. with buffer solution components and especially with the most widely used photosensitizer [Ru(bpy)3]2+.261,262 The formation of colloidal POM-photosensitizer precipitates crosses the border between homogeneous and heterogeneous systems. While in situ approaches in principle offer superior insight over classic spectroscopic methods (see Part II), the μM range POM catalyst concentrations in a multi-component reaction mixture often fall below the respective signal-to-noise ratios. In order to circumvent this problem by isolating POMs from the post-catalytic media for further analyses, extraction techniques have been developed, as outlined above for [Co4(H2O)2(PW9O34)2]10−.237

Often, the informed use of multiple techniques is required to identify concealed oxide nanoparticles in POM-WOCs and related systems.264–266 We have outlined the pros and cons of DLS standard methods above, and these detection limits of DLS apply for photocatalytic POM-WOCs in particular.115,267,268

Especially the above-mentioned precipitates of POMs and photosensitizers, namely [Ru(bpy)3]2+, may interfere with the DLS detection of nanoparticles.261 Recently, a quantitative procedure for such colloid detection by using syringe filtration and UV/Vis spectroscopy was developed (Fig. 26). An increase in the ionic strength of the solution can prevent colloid formation in selected systems.263


image file: c7cs00306d-f26.tif
Fig. 26 (a) Negative filter test of a Co4POM/PS-free solution; (b) positive filter test showing the bright orange Co4POM/PS colloid particles; (c) SEM of the Co4POM/PS particles on the PTFE filter. Reproduced from ref. 263 with permission from John Wiley and Sons, copyright 2016.

The extensively studied [Co4(H2O)2(α-PW9O34)2]10− WOC raised a most intense discussion about the nature of the true operational catalyst under different water oxidation conditions.264,269–271 Its primary stability in the [Ru(bpy)3]2+/S2O82− assay was demonstrated by UV/Vis and 31P NMR spectroscopy at pH 8. Furthermore, addition of 2,2′-bipyridine as a scavenger for aqueous Co(II) led to a decrease in O2 yield from 67% to 48%, which indicates that a small fraction of Co4POM is transformed either into a different POM fragment or into heterogeneous CoOx. Complementary stability tests were performed under electrochemical conditions, and linear sweep voltammetry showed a water-oxidation active CoOx deposition on the electrode.239 On the other hand, extraction methods of Co4POM showed its genuine molecular activity in [Ru(bpy)3]2+/S2O82− photocatalytic assays.237,272 The vanadium analogue [Co4V2W18O68]10− was in the focus of related investigations.272,273

Kinetic studies are a mandatory complement to spectroscopic tests, and the investigation of Co4POM in 2014 showed that the rate law for oxygen evolution of the true catalyst is different from the initial POM and from CoOx, thus suggesting that the atomic composition of the true catalyst still remains unknown.269

More and more advanced monitoring techniques are tailored for POM-WOC systems. In 2015, SAXS was first applied for speciation and stability studies of Co-POMs under catalytic reaction conditions.274 Studies on the [Co9(H2O)6(OH)3(HPO4)2(PW9O34)3]16− (Co9) WOC showed the strong influence of pH, buffer type, and buffer concentration on the stability of POMs.122,274,275 In combination with PDF measurements, the structural stability of Co9 throughout the reaction in the absence of phosphate buffer was demonstrated. However, high concentrations of phosphate in solution can precipitate with free Co(II) ions to form cobalt phosphate. SAXS was furthermore used to investigate Ru4POM/PS aggregates.276 The observed low intermolecular Ru4POM/PS distances supported the measured fast photoinduced electron transfer processes. All in all, SAXS and PDF are a powerful analytical combination to monitor many WOCs under water oxidation conditions at low concentrations, i.e. close to standard ex situ systems.

Besides their monitoring, the critical evaluation and improvement of current water oxidation assays are vital, especially with respect to the development of stable photosensitizers. The manifold parameters of the widely applied [Ru(bpy)3]2+/S2O82− system influence the final O2 yield significantly. The sulfate radical SO4˙, which is formed upon oxidative quenching of the excited photosensitizer by S2O82− to produce the oxidized photosensitizer [Ru(bpy)3]2+, is a strongly oxidizing species that can lead to the decomposition of both catalysts and photosensitizers.277 A recent study on different photocatalyst types in the [Ru(bpy)3]2+/S2O82− assay furthermore corroborated the influence of the photosensitizer on the photocatalytic system and on the final stability as well as on the TON.278

3.4. In situ/operando investigations of molecular WOCs

Proceeding from POMs to small molecular WOCs, we discuss some representative insight into their catalytic pathways.

Molecular mimics of the OEC are fascinating bio-inspired targets, and the cubic manganese cluster [Mn4O4L6]+ (L = (MeOPh)2PO2) first showed very promising water oxidation activity when loaded on Nafion-covered electrodes.279In situ XAS studies of electrocatalysis with [Mn4O4L6]+ loaded Nafion-electrodes subsequently revealed the decomposition of the cubane-type catalyst and the formation of a disordered Mn(III/IV) oxide after reoxidation. The latter species was identified as the true catalyst during prolonged water oxidation catalysis. In particular, the K-edge XANES spectrum of [Mn4O4L6]+ was shifted towards lower energy in the range of Mn(II) (Fig. 27). Moreover, the K-edge XANES and EXAFS spectra were not recovered to the initial state of [Mn4O4L6]+ after electro-oxidation, as observed for [(bipy)2Mn(O)2Mn(bipy)2]3+ and Mn2+ in Nafion. Comparison of the EXAFS data obtained for the above electro-oxidized materials with a wider range of Mn oxide references finally strongly suggested the formation of a disordered Mn(III/IV)-oxide phase with an average Mn oxidation state of 3.75–3.85 during electro-oxidation.35,280 This fundamental work paves the way to manifold advanced stability studies on molecular WOCs over the coming years.


image file: c7cs00306d-f27.tif
Fig. 27 (a) Mn K-edge XANES data of [Mn4O4L6]+ in acetonitrile (green), after loading in Nafion (blue) and after electro-oxidation of the Nafion-loaded cluster; (b) pre-edge regions of the above spectra. Reproduced from ref. 280 with permission from Nature Publishing Group, copyright 2011.

Informed application of in-house ESI-MS on molecular WOCs can provide important insight for further stability tests. In a recent study on molecular Co-salophen WOCs, their stabilities were investigated with ESI-MS before and after photocatalytic water oxidation.235 In a post-photocatalytic solution at pH 9.0 (0.1 M phosphate buffer), the characteristic reference m/z peaks of each catalyst recorded under dark conditions disappeared, and no m/z peaks of other Co complexes, free ligands, or Co(II) aqua species appeared. This strongly suggests the decomposition of Co salophen complexes into CoOx during photocatalysis. In contrast, the relative m/z peaks of each catalyst were maintained after photocatalysis when the reaction system was changed to phosphate buffer (0.20 mM, pH 7.0). As the remaining quantities of molecular catalysts are not evident from ESI-MS, a combination of complementary characterization methods, including CV, time-dependent DLS, SEM, and EDX, was applied to establish catalyst stability in phosphate buffer.

In situ EPR results are not always straightforward to interpret either, but they can bring forward crucial insights into specific reaction steps of WOCs. For example, the mononuclear Ru complex [Ru(bpy)(tpy)I]I was reported to be a more efficient WOC than its parent compound [Ru(bpy)(tpy)(H2O)]+. A model for accommodating water as the seventh ligand of a hepta-coordinated species available for PCET was proposed to explain this phenomenon.281 However, in situ EPR studies indicated that [Ru(bpy)(tpy)I]I underwent partial decomposition during water oxidation to form [Ru(bpy)(tpy)(H2O)]2+.282 An EPR silent Ru species emerged from oxidation of [Ru(bpy)(tpy)I]I with 1 eq. Ce(IV), which differs from the observation that the complete oxidation of [Ru(bpy)(tpy)Cl]+ and [Ru(bpy)(tpy)(H2O)]2+ with 1 eq. Ce(IV) leads to their corresponding Ru(III) species.283 After oxidation with 5 eq. Ce(IV), a species having the same g-tensor in common with [Ru(III)(bpy)(tpy)(H2O)]3+ was detected by EPR with maximum intensity (Fig. 28).282 These differences between full oxidation to their respective Ru(III) species for [Ru(bpy)(tpy)I]I on the one hand vs. [Ru(bpy)(tpy)Cl]Cl and [Ru(bpy)(tpy)(H2O)]2+ on the other indicate the oxidation of the Ru–I bond and the subsequent formation of other oxidized iodine species, which was confirmed by additional X-ray absorption spectroscopy studies.


image file: c7cs00306d-f28.tif
Fig. 28 X-band EPR of [Ru(bpy)(tpy)I]+ oxidized with 1–7 eq. Ce(IV), followed by freezing, vs. [Ru(bpy)(tpy)(H2O)]3+ (black dashed line), which was obtained by oxidation of [Ru(bpy)(tpy)(H2O)]2+ with 1 eq. Ce(IV). Reproduced from ref. 282 with permission from Elsevier, copyright 2015.

4. Conclusions

Over the last decade, the development of molecular, nanoscale, and solid state WOCs has proceeded rapidly. The combination of concepts from bio-inorganic chemistry, nanoscience, and solid state engineering turns WOC research into a unique interdisciplinary hotspot. All of these research lines share the need for fundamental mechanistic understanding, and recent in situ/operando monitoring techniques for water oxidation processes will further connect them. Advanced computational methods are an integral part of this endeavor. More and more emerging insights into common reaction steps and pathways relate molecular WOCs to nanostructured solids beyond the classic long-range order materials. In parallel, enhanced classic analytical methods show that an increasing number of molecular WOC systems in fact operate at the interface of heterogeneous catalysis. In terms of fundamental research, this is a chance to reconsider traditional division lines in search of overarching WOC design concepts. We are not yet capable of running artificial photosynthesis devices in our everyday surroundings with the efficiency of Nature. The commercial solutions for marketable devices may as well lie in empirical optimization, as many decades of successful heterogeneous catalyst applications have shown. Independent of technological implementations, understanding and comparing the structure–activity relationships of all WOC types from bio-mimetic cubanes to highly crystalline solids is an ongoing intellectual endeavor in its own right that will keep fueling progress in catalysis.

Acknowledgements

This work was supported by the University Research Priority Program (URPP) for Solar Light to Chemical Energy Conversion (LightChEC) and by the Swiss National Science Foundation (Sinergia Grant No. CRSII2_160801/1). W. Wan is grateful to the Chinese Scholarship Council for a PhD fellowship.

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