New catalytic materials for energy and chemistry in transition

Jiří Čejka

Jiří Čejka is a senior research fellow at the J. Heyrovský Institute of Physical Chemistry, Czech Academy of Sciences and a lecturer at the Faculty of Science, Charles University, Prague. In 2005, he chaired the 3rd FEZA Conference on Zeolites in Prague. His research interests range from the synthesis of zeolites, mesoporous and novel nano-structured materials, to the physical chemistry of sorption and catalysis including the role of porous catalysts in transformations of hydrocarbons and of their derivatives. He has co-authored more than 315 research papers and is the co-editor of 6 books. His current h-index is 53, with more than 9400 citations.

Petr Nachtigall

Petr Nachtigall became a full professor at the Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University, Prague, following his PhD at the University of Pittsburgh and a research position at the Academy of Sciences of the Czech Republic. His research is focused on the theoretical investigation of surface properties of solids, primarily related to gas adsorption and catalytic processes involving microporous and nano-structured materials. He has coauthored over 150 research papers and his h-index is over 40, with more than 5000 citations.

Gabriele Centi

Gabriele Centi is a full professor of Industrial Chemistry at the University of Messina (Italy), President of the International Association of Catalysis Societies (IACS) and of the European Research Institute of Catalysis (ERIC aisbl). He was the coordinator of the Network of Excellence on Catalysis IDECAT and of several EU projects. He is the Chair of the editorial board of ChemSusChem, co-editor-in-chief of the Journal of Energy Chemistry (Elsevier) and of various Book Series. He was the Chairperson of Europacat 2017 in Florence, Italy. He has published several reviews on catalysis and on green energy, and he has authored over 450 scientific publications. His current h-index (Google Scholar) is 78, with over 22 500 citations.

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Why this themed issue

Heterogeneous catalysis plays a pivotal role in chemical industry and refinery production, which employ catalysts in numerous industrial processes. Heterogeneous catalysis is also undergoing a significant transition phase, driven by a combination of pull and push factors. On one hand (pull factors), new objectives have been established in order to meet sustainability criteria, especially due to a new scenario in the chemistry and energy sectors, which will be discussed below. On the other hand (push factors), new tools have been developed to further understand synthesis procedures and catalyst properties. Thus, catalysis is rapidly evolving, as outlined in the recent “Science and Technology Roadmap of Catalysis for Europe”.¹

In this context, the targets of catalysis science and technology must be reassessed to define new research directions and to focus on open problems, particularly in refinery production, which is primarily based on catalysts involved in these industrial processes. By combining theoretical, in situ and operando approaches and by creating new reactor types, among other strategies, we will be able to unlock the potential of new materials and to establish new catalyst synthesis methods. Ultimately, these new guidelines will ensure the balanced development of this dynamic field of research.

Although several reviews on the pivotal role of catalysis in chemical industry have been previously published, the goal of this themed issue is to bring together the leading researchers in this field to address, from their different perspectives and expertise, common issues: defining the current status, bridging key gaps in knowledge and exploring new possibilities to guide the future of catalysis. For this purpose, this issue covers the most important aspects of recent developments in heterogeneous catalysis, including the identification of critical challenges in the synthesis of different types of catalysts, their detailed characterisation, the theoretical description of catalyst properties and, last but not least, the real word of catalytic transformations. Thus, this themed issue is highly multidisciplinary, encompassing a wide range of approaches, yet describing in depth the characteristics of catalysis as a core interdisciplinary science at the interface between fundamental and applied research focused on sustainable development.

Following the editorial board recommendations, each contribution of this themed issue uniquely emphasizes “collaborative aspects” for successful progress in the respective field by pairing subject-matter experts who rarely collaborate. The aim is to foster a more critical analysis of specific topics from different backgrounds, viewpoints and approaches, thereby overcoming the limitations most commonly found in reviews: the inability to go beyond narrow personal perspectives and the lack of a critical analysis of the state-of-the-art. Hence, we have stimulated synergy and cross-fertilisation in each subject discussed in this themed issue. This added value has created something unique, and we strongly encourage reading all contributions, not only those on your specific research interests, because this themed issue provides a new overview of and outlook for catalysis rarely found elsewhere. Therefore, this themed issue should be on the desks of researchers, company managers and senior scientists keen on finding new research and business opportunities in an ever-changing production scenario and of decision-makers aiming to understand new directions towards a low-carbon and sustainable society.

New scenario: chemistry and energy in transition

The last major transition in chemistry occurred approximately 60 years ago with the introduction of petrochemistry based on olefins and aromatics, which has remained in use to the present day. Most petrochemical processes are based on technologies developed then, albeit with improvements necessary to maintain production competitiveness. For example, the introduction of zeolites and their discovery and successful implementation has led to the substitution of harmful aluminium chloride and has supported phosphoric acid catalysts for petrochemical processes.² Petrochemistry production is characterised by (i) a nearly exclusive use of fossil resources both as raw materials and for heat production driving chemical transformations and separation processes, (ii) the concept of scale economy, e.g., production costs do not increase linearly but rather exponentially (typically with an exponent of approximately 0.7) with plant size, thus leading to large-volume plants, and (iii) site integration, e.g., the creation of large petrochemical sites with multiple plants.³

Currently, this business model for industrial chemical production is facing many limitations regarding sustainability and local environmental impact, flexibility to variable demands, costs and restrictions in the transport of chemicals, and social acceptability, among other issues.

Until recently, the industry had focused on building larger plants, for example, for urea production (urea is perhaps the largest volume chemical and the basis of fertilisers), which expanded from standard to mega and then to jumbo plants (0.3–0.5, 1.0–1.3 and 3.5–4.2 Mt y⁻¹ in a single line, respectively).⁴

Now, on the contrary, the possibility of distributed production, at a regional level, is under consideration. New plants with sizes one-to-two orders of magnitude smaller than standard plants are being designed, albeit using novel technologies efficient at a small scale and based on the use of alternative raw materials.⁵ In fact, most urea is actually produced from methane, which is the source of hydrogen necessary for N₂ reduction to NH₃, one of the key reactions in multistep and energy-intensive urea production.⁶ The concept of distributed production is primarily based on the use of local resources, for example, municipal waste, as C-source and as renewable energy.⁷

In the future, fully distributed production may be possible by directly using N₂, H₂O and CO₂ in an electrocatalytic process for on-site fertiliser production. Approximately 5 m² of photovoltaic panels could ideally produce fertilisers for 1 hectare, with a cost-competitive advantage,⁵ although electrocatalysts must still be developed to implement this alternative.

The following elements are crucial for determining the future of chemical production:

– Ongoing changes in society – individuals are increasingly becoming both consumers and producers (they use and produce electrical energy, for example) – require changes in business models, including chemical production;

– Increasing social awareness of environmental impacts – and consequent opposition to large-scale plants integrated within large production sites; in these cases, the environmental impact surpasses the self-depuration capacity, progressively degrading the environment and adversely affecting biodiversity.

Reducing greenhouse gas (GHG) emissions is another key goal of this shift in industrial chemistry models. However, the main opponents to environmental policies advocate that economics should determine the shift from fossil to renewable resources because fossil fuels remain less expensive than alternative energy sources. Yet, the latest evidence no longer supports this claim. The recent report by the International Renewable Energy Agency⁸ shows that renewable energy will soon become less expensive than fossil fuels. Considering that most (up to 70–80%) of fossil fuels are used in petrochemistry to provide energy for chemical transformation, transitioning from fossil fuels to renewable resources is motivated by industrial competitiveness. This opens up the possibility to use renewable energy in combination with local C-resources (CO₂ and waste, for example) to develop a new model of chemical production.⁹

There are several other motivations for this transition: ensuring supply security, supporting local development, decoupling chemical production from the high volatility of fossil fuel prices and from fossil fuel monopolies and integrating chemical production with bioeconomy (CO₂ as the C-source should be derived from bio-production, fermentation processes or biogas, for example), in addition to geopolitical reasons. All these motivations are driving the gradual transition from fossil fuels to alternative resources in energy and chemistry.

This transition is often questioned. Sceptical pundits predict that such changes, if any, will only occur in the long run. Nevertheless, the history of chemical production shows that, as briefly mentioned above, such a transition can instead be fast. Furthermore, transitions occur when most investments are shifted towards a new direction. Accordingly, already in 2017, most new investments were made in renewable energy rather than in fossil fuels.¹⁰ In chemistry, new technologies must still be developed to allow this transition, but this shift should be fast, depending on the R&D intensity in designing and implementing the necessary processes and technologies. Hence, we predict that this change will occur within the next decades.

Electrification is one of the drivers of this transition in chemistry from thermal energy (largely derived from fossil fuels) to the direct use of electrical energy for chemical transformations on an industrial scale.¹¹ This will require fully redesigning catalysts for selective activity using electrons under reaction conditions typically milder than thermal catalysis.^12,13 The reaction mechanisms are necessarily different and, therefore, the methods used to study catalysts should be adapted or changed. For example, new methodological approaches are required for constructing theoretical models of electrocatalysis because the models previously developed for conventional catalysis are no longer suitable.

(Electro)catalysts should be conductive and thus conventional supports, including alumina or silica, are ineffective. Conversely, traditional electrochemical approaches in electrode chemistry are unsuitable due to new requirements, such as productivity per electrode area and selectivity. New approaches are required to achieve selective conversions and high reaction rates. Electrocatalysis is not merely an extension of traditional electrochemistry but requires developing new methods and materials to exploit its potential role in the future of applied chemistry.

Concurrently, abrupt changes should be avoided to ensure a smooth but effective transition. Thus, technologies must be first developed to demonstrate the feasibility of new solutions, albeit synergistically with the use of old technologies. Simultaneously, the scientific bases necessary for medium- and long-term technologies must also be developed. Therefore, this transitional period requires properly identifying both new research directions and knowledge gaps or open problems to consolidate the shift from fossil fuels to renewable energy.

To adequately address these subjects, despite the limited number of contributions, we have organised this themed issue into two main sections highlighting key tools for knowledge-based catalyst design: (i) tailor-made synthesis for advanced nano-architectures and catalysts and (ii) operando catalysis. These two sections are complemented by a third area focused on new concepts and opportunities from fundamental to applied perspectives.

Tailor-made synthesis for advanced nano-architectures and catalysts

The optimisation of synthetic protocols and the discovery of new synthetic approaches are of the utmost importance for the development of new materials. While most synthetic procedures are usually based on the trial-and-error principle, recently developed knowledge-based approaches or even theoretical predictions have been used to provide further scientific basis for the development of new catalysts. The portfolio of new catalysts is increasing substantially, bringing new advantages in performance and stability, e.g., novel two-dimensional systems,¹⁴ albeit without actual practical implications yet.

In their review, Lee and his team (DOI: 10.1039/C8CS00336J) focus on clean and sustainable energy conversion systems, which primarily require the development of highly efficient catalysts with maximum performance. They discuss hollow nanostructures, including nanocages and nanoframes, identifying them as promising electrocatalysts. The authors describe recent advances in synthetic methods for noble metal-based hollow nanostructures based on thermodynamic and kinetic approaches with electrocatalytic applications of these materials in ORR, OER, and HER reactions. In addition to introducing synthetic strategies for hollow structures, the authors suggest strategies to facilitate the development of hollow nanostructure-based catalysts for energy applications.

The Czech and Spanish team, Přech, Pizarro, Serrano and Čejka (DOI: 10.1039/C8CS00370J) review recent developments in three-dimensional, hierarchical and nano-zeolites, and in their two-dimensional analogues. While zeolites remain the most important industrial heterogeneous catalysts, with numerous applications, their limited pore sizes prevent the penetration of sterically demanding molecules into their channel systems and into active sites. This review highlights the synthesis, properties and catalytic potential of three-dimensional zeolites, followed by a discussion on hierarchical zeolites combining micro- and mesoporosity. Two-dimensional zeolite analogues have been recently prepared using bottom-up and top-down approaches. Their synthetic routes and catalytic performance are also assessed herein.

Understanding catalysis in operation

In addition to major experimental breakthroughs, to the enhanced resolution of many characterisation techniques and to advances in operando techniques, significant progress has been made in material modelling. Constant hardware and software improvements have allowed researchers to study specific systems using models significantly more realistic than their predecessors. Consequently, the synergy between experimental and theoretical studies on heterogeneous catalysis has clearly increased in the last decade, and the gap between experimental and theoretical investigations has been mostly minimised (or even bridged). In recent years, the main advances in catalysis included the integration of a priori theoretical modelling with in situ/operando studies on reaction mechanisms, the preparation of nanocatalysts and the development of advanced micro-kinetics. Computational models of catalytic processes at various length and time scales are crucial for such approaches. Moreover, current theoretical tools are able to guide research in key new areas such as materials for solar fuels and chemicals.

The transition from descriptive computational models to in silico design in heterogeneous catalysis is discussed in detail by Nachtigall and Pidko and their coworkers (DOI: 10.1039/C8CS00398J). Similarly to experimental operando studies, computational studies must focus on catalytic systems mimicking experimentally relevant conditions. New methodological developments, designed to minimize or bridge the gap between 0 K/UHV and operando conditions, include (i) global optimisation techniques, (ii) ab initio constrained thermodynamics, (iii) biased molecular dynamics, (iv) microkinetic models of the reaction network and (v) machine learning approaches. The importance of this transition is highlighted, discussing how the molecular analysis of catalytic sites and associated reaction mechanisms changes when chemical environment, pressure and temperature effects are correctly accounted for in molecular simulations.

In their combined experimental and theoretical review, Pacchioni and Freund (DOI: 10.1039/C8CS00152A) focus on the control of the charge state of supported nanoparticles in catalysis. They discuss the importance of model systems (where experimental and theoretical data can be effectively combined) for identifying working principles in catalysis and for developing concepts applicable to the design of new catalytic materials. The authors exemplify the use of model systems towards further understanding and controlling charge transfer at the interface between supported metal nanoparticles and oxide surfaces. First, they review the nature of the support and differences at the metal/support interface from wide-gap oxides and from reducible oxide semiconductors. Then, they address the role of oxide nanostructuring, defectiveness, and doping. Lastly, they analyse the relationship between model systems and real catalysts in the final section of the manuscript.

The contribution by Li and coworkers (DOI: 10.1039/C8CS00320C) focuses on imaging photogenerated charge carriers on photocatalyst surfaces and interfaces. The optimisation of photocatalytic solar energy conversion efficiency requires understanding the photogenerated charge separation at both nano- and micrometer scales. Spatially resolved surface photovoltage (SPV) techniques, discussed in detail in this review, make it possible to directly image localised charge separation at surfaces or interfaces and, thus, they provide a deep insight into photocatalysis. The authors discuss in detail both theoretical and experimental aspects of SPV, also highlighting future perspectives of further developments towards operando techniques.

New concepts and opportunities from fundamental to applied perspectives

Accelerating this transition in energy and chemistry requires putting more effort into developing new concepts and approaches in heterogeneous catalysis. Even new areas such as water splitting and CO₂ conversion often lack originality and overlook crucial aspects. Knowledge on synthesis, modelling and characterisation must be creatively applied to catalyst design from new perspectives. The reviews included in this section highlight the most exciting developments in this direction.

Three reviews are dedicated to the topic of increasing the relevance of photocatalysis. Yamashita and colleagues, in collaboration with Che (DOI: 10.1039/C8CS00341F), examine the possibilities offered by single-site and nano-confined photocatalysts (particularly based on Ti), in contrast to conventional photocatalysts based on semiconductors. In their review, the authors discuss the opportunities provided by photoactive functional materials prepared by taking advantage of the empty spaces of porous ordered materials. These properties are greatly affected by the coordination structure and by the reaction field environment (hydrophilicity/hydrophobicity and electrostatic fields, among other factors) near the active species. Those materials open new possibilities for unique activity and selectivity, in contrast to conventional semiconducting photocatalysts. By combining the features of photocatalysts and porous materials, new hybrid materials could be developed for applications in environmental purification and energy conversion.

In the joint Spain–China–India contribution by Garcia, Li and Dhakshinamoorthy (DOI: 10.1039/C8CS00256H), the authors discuss catalysis and photocatalysis by metal organic frameworks (MOFs). While these materials have unique features as solid catalysts, their full potential remains untapped in practical applications. This review covers recent strategies implemented to further tailor the nature and catalytic behaviour of active sites, either by adapting synthetic conditions or by introducing post-synthetic modifications. The authors highlight the synergy resulting from the presence of different types of active sites. Particularly in photocatalysis, MOFs have been used as new photocatalytic materials for challenging reactions, such as hydrogen evolution and photocatalytic CO₂ reduction.

In the joint China–Qatar–Saudi Arabia contribution, Zhao et al. (DOI: 10.1039/C8CS00443A) examine the fascinating new area of core–shell structured nanomaterials used to integrate multiple components into a functional system. The authors focus on TiO₂ nanomaterials for solar energy. In contrast to single TiO₂ counterparts, these unique, core–shell structures provide new and improved physical and chemical properties, which have resulted in major advances in a number of applications. Substantial progress has been made when combining TiO₂ shells with functional cores or surface/interface engineering. The applications range from photodegradation and photosplitting to the use of core–shell structured TiO₂ with magnetic, enhanced optical and new electronic properties for light-induced selective chemical transformations in future photo-derived catalysis.

In their review, Bordiga, Beato and Svelle et al. (DOI: 10.1039/C8CS00373D) focus on the key research area of copper-exchanged zeolites with chabazite (CHA) topology for NO_x selective reduction with ammonia (particularly for low-temperature diesel selective catalytic reduction – SCR) and for other redox reactions. The authors highlight the use of multi-technique approaches and operando methods to understand the structural dynamics of Cu-species and their dependence on environmental conditions. Studies have shown that the active site of the low-temperature NH₃-SCR is a mobile Cu-molecular entity that “lives in symbiosis” with an inorganic solid framework.

In their joint contribution, Basset, Psaro and colleagues (DOI: 10.1039/C8CS00356D) provide an overview of surface organometallic chemistry (SOMC) applications in heterogeneous catalysis. The use of well-defined surface organometallic fragments (SOMFs) or surface coordination fragments (SCFs) combined with a “catalysis by design” strategy improves catalyst performances. The examples discussed in this review include industrially relevant processes crucial for sustainable energy production, such as alkane and CO₂ conversion.

Sels, Parvulescu and colleagues discuss novel catalysts for sustainable biomass-based chemical production (DOI: 10.1039/C8CS00410B). For efficient biomass transformation into value-added chemicals and high-energy density fuels, potential cascade chemical processes must be developed using functionalised heterogeneous catalysts. Carbon materials, metal–organic frameworks, solid phase ionic liquids, and magnetic iron oxides offer unique possibilities to accommodate adequate amounts of acid–base and redox functional species, thereby enabling various biomass conversion reactions in a one-pot reaction. This review is a comprehensive account of the most significant advances in this area, including progress in tailoring the immobilisation of desirable functional groups at particular sites and in improving sustainability in novel bio-refinery processes.

Metal-free nanocarbons are a new group of catalysts with several applications in heterogeneous, electro- and photo-catalysis. In their joint China–Germany–Italy contribution, Su, Lin and Perathoner and colleagues (DOI: 10.1039/C8CS00684A) discuss the specific example of hybrid sp²/sp³ nanocarbons, in particular sp³-hybridised ultra-dispersed nanodiamonds and derivative materials, such as sp³/sp²-hybridised bucky nanodiamonds and sp²-hybridised onion-like carbon. The authors critically analyse the state-of-the-art of the properties and reactivity of these materials, focusing on the reaction mechanisms and on the nature of the active sites. In addition, they provide a longer-term perspective on how these nanomaterials open new possibilities for the advanced design of unconventional catalysts. The charge density of carbon atoms or C–C bonds and other properties such as the formation of semiconducting areas can be controlled by nano-engineering the defects and curvature strain characteristics of the nanocarbons and by introducing heteroatoms, with synergic effects between these aspects. These new and fascinating properties can be used to develop novel catalysts to meet the challenging demands of energy and chemistry fields in transition.

Final remarks

Although not all relevant aspects related to catalysis for energy and chemistry fields in transition can be addressed in a single themed issue, we believe we have provided a comprehensive overview of new possibilities to prepare tailored materials, thanks to advances in catalysis, also addressing novel challenges to catalysis resulting from this transition. We would like to express our sincere gratitude to all authors for their timely contributions to this themed issue, especially considering the additional effort we asked from paired subject-matter experts who rarely collaborate.

Catalysis is the core and essential factor for the success of this transition in the fields of energy and chemistry. We do hope that the next decade will witness the exciting developments outlined here. Finally, we would like to thank all referees and editorial staff members of Chemical Society Reviews for their strong support in preparing this issue.

References

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