Operando surface spectroscopy—placing catalytic solids at work under the spotlight

Heterogeneous catalysis is involved in the vast majority of industrial chemical processes performed nowadays, and an increased understanding of catalytic reactions is of the utmost relevance to develop a sustainable and cleaner technology. In order to make new (or improved) catalytic solids, an increased comprehension of the working principles of heterogeneous catalysis is a prime requirement. Towards that endeavour, many rewarding efforts have been made in the last decades to: (a) determine the nature and distribution of the catalytically active sites present on the surface of the catalyst, (b) unravel the underlying catalytic mechanisms, and (c) reveal the close relationship that exists between the structure (and composition) of the heterogeneous catalysts and their most relevant characteristics, which include activity, selectivity and stability. In this scientific endeavour, characterization techniques play a pivotal role and spectroscopic methods, in particular, have become the key tool to characterize catalytic solids. Spectroscopy (from the Latin spectrum and the Greek skopein meaning ‘to examine’) comprises a large group of techniques that, taken either individually or in several judicious combinations, can profitably be used to determine the nature, quantity, structure and environment of atoms, ions and molecules. Most spectroscopic methods currently used in the field of heterogeneous catalysis are based on the analysis of the interaction of the catalyst with electromagnetic radiation ranging from radio and microwaves through to infrared, visible and ultraviolet light and finally to X-rays.

However, characterization of the catalyst (or catalytic precursor) itself constitutes only a small step towards understanding catalytic processes. In the end, one wishes to obtain detailed information on the catalytic solids under technologically relevant working conditions, which can bring about very significant changes to the catalytically active sites. For this purpose, in situ spectroscopic cells were developed that enable the researcher to investigate the physicochemical changes taking place in the catalyst while working at even a high temperature and pressure of the substrate in a gas or liquid phase. In situ conveys the meaning of catalyst characterization at its working place, in contrast to ex-situ measurements. Preferably, the in situ characterization approach should be complemented with simultaneous measurement of catalytic activity and selectivity, which can be accomplished by coupling the in situ spectroscopic cell with (for instance) a mass spectrometer or a chromatography system; the wealth of data thus obtained is most useful for analysing structure–performance relationships of the catalysts at work. And this is how operando (meaning ‘at work’) spectroscopy was born. In other words, operando spectroscopy can be regarded as being a very important class of a much broader group of in situ catalyst characterization techniques. Since it provides detailed information on the surfaces of catalytically active materials at work, operando surface spectroscopy seems to be the proper name for this research field.

Besides studying catalytic processes relevant to the chemical industry, operando surface spectroscopy has also recently been used to gain increasing insight in other related fields, such as the study of catalytic processes in fuel cells (related to clean energy production), gas sensing and gas–solid reactivity in the broader context. In view of the increasing momentum and interest that operando surface spectroscopy is gaining, in our opinion, these are the main reasons why we think that the set of articles focused on this subject and collected together in this Physical Chemistry Chemical Physics (PCCP) themed issue comes at a very appropriate time. Moreover, it illustrates the significant progress made in the last ten years in this field of scientific research. Nine years ago, PCCP published a collection of articles in a themed issue entitled “Operando Spectroscopy: Fundamental and technical aspects of spectroscopy of catalysts under working conditions” (Phys. Chem. Chem. Phys., 2003, 5(20)). That collection compiled several first-of-its kind operando spectroscopy contributions presented at the 1st International Congress on Operando Spectroscopy held in Lunteren (The Netherlands, 2003). Since then, two other successful symposia have been organized in Toledo (Spain, 2006) and Rostock-Warnemünde (Germany, 2009). The 4th edition of this successful series of congresses takes place in Upton (New York, USA) from April 29th to May 3rd, 2012. The general trends in the operando spectroscopy of catalytic solids for the period 2003–2012 include single particle (or molecule) detection, super-resolution imaging (down to the nanometer scale), spatio-temporal resolution, 3-D imaging, selective catalyst staining and the integration of spectroscopy with several electron microscopy or scanning probe methods. Examples can be found in a recent themed issue in Chemical Society Reviews (Chem. Soc. Rev., 2010, 39(12)).

The articles compiled in the collection presented herein illustrate some recent developments on both operando surface spectroscopy and in situ spectroscopic methods; among them the design of improved operando cells and the use of operando surface spectroscopy for detailed studies of technologically relevant catalytic chemical processes.

J. A. van Bokhoven et al. (DOI: 10.1039/c1cp21933b) describe how a commercial autoclave can be converted into an operando cell by inserting an X-ray transparent window and an attenuated total reflection FTIR (ATR-FTIR) probe. They demonstrated the performance of the cell in an operando study of the hydrogenation of nitrobenzene in a water solution, at 10 bar and 120 °C, over an Au/CeO₂ catalyst. High-energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD XAS) was used to monitor the oxidation state of gold during the nitrobenzene reduction process, while simultaneously measuring the concentration of an azobenzene intermediate species by ATR-FTIR spectroscopy at a time resolution of 1 min. The authors have thus showed the potential of combining HERFD XAS and ATR-FTIR to establish the structure–performance relationships in liquid phase reactions. M. Daturi et al. (DOI: 10.1039/c1cp22629k) give a detailed account of an innovative operando transmission FTIR cell ingeniously designed for real-time studies of monolithic catalysts working under industrially relevant conditions. Easy coupling to a quadrupole mass spectrometer facilitates the simultaneous (time-resolved) analysis of evolved gases. Since, for technical reasons, industrial catalysts are often shaped in the form of monoliths (or extrudates), this operando cell gives a substantial advantage over more standard cells, which can only handle powdered catalysts (or thin wafers) and cannot match such parameters as mass and heat transfer rates, fluid dynamics and effective activity constants relevant to monoliths. As a performance test the authors studied the selective catalytic reduction (SCR) of NO_x with ammonia over an extruded (square channeled) V_x–WO_y–TiO₂/SiMgO_z catalytic monolith, showing that the operando cell duly represents the fluid dynamics of the real SCR catalyst while providing relevant new insight into the catalytic mechanisms and the nature of the reaction intermediates. J. T. Miller et al. (DOI: 10.1039/c1cp22992c) describe an operando X-ray absorption spectroscopy (XAS) plug flow reactor built using commercial vitreous carbon tubes, which have significant advantages (among them higher temperature and pressure resistance) over those made of borosilicate glass or polyimide while preserving a high X-ray transmittance. Using this operando reactor, they studied the SCR of NO_x by NH₃ on several Cu-exchanged zeolites, as well as on Cu/SAPO-34. Simultaneous XANES, EXAFS and kinetic measurements led the authors to propose that, under steady state standard SCR conditions, NO_x reduction involves a redox mechanism where the amount of Cu^I is determined by the relative rates at which Cu^I oxidation and Cu^II reduction proceed, which was found to be independent of the zeolite structure type. Another highly relevant insight comes from comparison of the XAS spectra obtained using the operando cell with those obtained using a standard in situ cell, which (for reason discussed by the authors) show significant differences regarding the oxidation state of Cu; thus alerting us to the fact that (because of significant deviation from the realistic reaction conditions) XAS results obtained with in situ cells can be misleading. C. H. F. Peden et al. (DOI: 10.1039/c1cp22692d) describe a large volume rotor engineered as a reactor for the in situ MAS-NMR spectroscopic studies of catalytic chemical processes under a constant flow (CF). The large reactor volume allows for the enhanced sensitivity needed to collect ¹³C MAS-NMR spectra at a sub-minute time scale without using ¹³C enriched reactants, a feature which facilitates the study of processes involving organic molecules. The reactants (in a carrier gas) are injected at one end of the rotor, while the other end is connected to a vacuum pump that creates a pressure gradient, which facilitates reactant flow through the solid catalyst bed. The performance of this reactor was demonstrated by in situ¹H MAS-NMR studies of the thermal dehydration (activation) of a silicalite supported H₃PW₁₂O₄₀·nH₂O catalyst precursor, followed by CF-MAS-NMR (¹H and ¹³C) studies on the reaction dynamics during catalytic dehydration of 2-butanol at 73 K. The high quality of the CF-MAS-NMR spectra obtained facilitated detection of both the reaction products and the reaction intermediates, as well as the interaction modes of the reactants with the catalyst active sites.

The contribution by E. Mikolajska et al. (DOI: 10.1039/c1cp22608h) deals with a detailed operando GC–Raman study on an alumina-supported vanadium phosphate catalyst for the propane ammoxidation reaction. The investigation is mainly centred on the evolution of the Raman bands associated with highly dispersed vanadium phosphate phases upon reaction. The adopted methodology allows us to follow the phase transformation during the reaction and to conclude that dispersed V⁵⁺ is selective to acetonitrile while V⁴⁺ promotes selectivity to acrylonitrile. F. C. Meunier et al. (DOI: 10.1039/c1cp22620g) used operando diffuse reflectance spectroscopy (DRIFTS) to investigate the effect of CO₂ (which is frequently found, in trace amounts, in H₂ feeds) on alumina-supported rhodium catalysts during toluene hydrogenation (at 1 bar and 348 K). Besides carbonate-like species on the alumina support, CO₂ was found to give rise to both linear and bridged rhodium carbonyls. However, these Rh carbonyls did not cause complete deactivation of the Rh/Al₂O₃ catalyst, which maintains about 50% of its initial toluene conversion rate even after prolonged operation under a toluene/H₂/CO₂ feed. Detailed analysis of the DRIFTS spectra showed that the CO ligand of the linear Rh–CO species can be displaced by adsorbed toluene, thus preserving hydrogenation activity in the catalyst. By contrast a Pt/Al₂O₃ catalyst (investigated under the same conditions as Rh/Al₂O₃) was found to be completely and irreversibly poisoned by CO₂, which gives rise to strongly bound Pt carbonyl species. G. Tsilomelekis and S. Boghosian (DOI: 10.1039/c1cp22586c) demonstrate the use of Raman spectroscopy under operando conditions, together with parallel DRIFT measurements, to study the transformation of molybdenum oxide species supported on anatase during the oxidative dehydrogenation of ethane. The fundamental stretching Raman modes of Mo=O mono-oxo species and the corresponding IR spectra of the Mo=O overtones were used to monitor the evolution of surface species during the reaction. The authors show that the vibrational properties of the supported phase respond to the changes occurring in the gas phase. A consistent reaction mechanism accounting for the observed spectroscopic results was proposed. U. Bentrup et al. (DOI: 10.1039/c1cp23361k) report on a detailed investigation of dimethyl carbonate (DMC) formation by the oxidative carbonylation of methanol, catalysed by a highly loaded Cu–Y zeolite. A judicious combination of operando DRIFTS with a simultaneous steady state isotopic transient kinetic analysis (SSITKA) and mass spectrometry allowed the authors to obtain precise information on the mechanisms of the whole reaction process, thus demonstrating the potential of the operando SSITKA–DRIFTS–MS technique to discriminate between reaction intermediates and spectator chemical species, as well as between unselective and selective reaction pathways. The study lead to the conclusion that appropriate fine tuning of the reaction temperature and the oxygen content of the feed are key factors for obtaining a high DMC selectivity. P. Pietrzyk et al. (DOI: 10.1039/c1cp23038g) used DRIFT spectroscopy under operando conditions coupled with QMS/GC as main tools to investigate NO reduction by propene over a Co–BEA zeolite. The characterization of the catalyst was performed by a combination of IR and EPR spectroscopy. The authors show that the SCR process is initiated by the chemisorption of NO on Co sites with formation of dinitrosyls. These dinitrosyls are then partially oxidized to nitrates and nitrites and partially decomposed with attendant formation of N₂O. It is shown that cyanide, isocyanate and propene oxygenate species are reaction intermediates. A. Martinez-Arias et al. (DOI: 10.1039/c1cp23298c) report on the redox behaviour of a CuO/Ce_1-x/Tb_xO_2-σ catalyst during CO oxidation, as investigated using operando DRIFT spectroscopy. The investigation was based (mainly) on the observed evolution of the C–O stretching band of Cu^I carbonyl species formed during the reaction. Operando DRIFT measurements revealed that formation of Cu^I carbonyls is greatly influenced by the presence of terbium oxide. E. Groppo et al. (DOI: 10.1039/c2cp23269c) investigated ethylene polymerization on a SiH₄ modified Phillips catalyst by operando FTIR spectroscopy. They found that the polymerization rate was increased by a factor of 7 (as compared to that shown by the unmodified catalyst). Moreover, they observed formation of branched polymers, which suggests that α-olefins were initially formed. The detection of elusive α-olefin intermediates, which have a very low concentration under reaction conditions, was made possible by temperature and pressure programmed FTIR spectroscopy. A low temperature and ethylene pressure facilitate detection of α-olefin intermediates by reducing the chain growth rate.

In situ FTIR spectroscopy was used by Hadjiivanov et al. (DOI: 10.1039/c1cp22616a) to test CO isotopic scrambling (¹²C¹⁶O + ¹³C¹⁸O → ¹²C¹⁸O + ¹³C¹⁶O) on a faujasite-type Ag–X zeolite containing some metallic (zero-valent) silver. They found that scrambling does take place, even at a temperature as low as 100 K, and involves Ag⁰ nanoparticles and surface oxygen species. By contrast, no isotopic scrambling was found when an Ag–Y zeolite containing no Ag⁰ was tested. M. Nomura et al. (DOI: 10.1039/c1cp22466b) used in situ time-resolved X-ray absorption fine structure spectroscopy (XAFS) to investigate the redox dynamics of ZnO-supported Pd nanoparticles during reduction under hydrogen at 673 K, followed by oxidation (under oxygen) at the same temperature. Analysis of the obtained spectra (XANES, EXAFS and dispersive XAFS) showed that both processes, reduction and oxidation, consist of (at least) two reaction steps. Under reduction conditions, the first step is the reduction of PdO to Pd⁰, followed by ZnO reduction and the formation of metallic Pd–Zn nanoparticles. The Zn atoms of these nanoparticles are the first ones to become oxidized (to ZnO), leaving free Pd atoms or Pd-rich nanoparticles, which are then gradually oxidized to PdO. Approximate rate constants for each reaction step were obtained from the corresponding time-resolved spectra. L. Barrio et al. (DOI: 10.1039/c1cp22509j) used a combination of time-resolved in situ X-ray diffraction and XANES to investigate the activity of TiO₂-supported platinum–ceria catalysts in the water–gas shift reaction. They found that catalysts having a CeO₂/TiO₂ ratio of 6% had a higher activity than those having a larger CeO₂/TiO₂ ratio (15%). These results were explained on account of the observed enhanced reducibility of the catalyst having the smaller ceria content, which facilitates formation of smaller (and hence more reactive) CeO₂ nanoparticles. According to the authors, the active phase of the catalyst consists of metallic platinum in close contact with partially reduced ceria. Aramburu et al. (DOI: 10.1039/c2cp22848c) report on the use of scanning transmission X-ray microscopy (STXM) in combination with a styrene oligomerization reaction to chemically image changes in Brønsted acidity within ZSM-5 zeolite aggregates. They found that STXM, a powerful nanoscale chemical imaging method recently introduced in the field of heterogeneous catalysis, allowed revelation of the distinctive differences within the ZSM-5 material when it was in its calcined or steamed form. Steaming leads to changes in styrene oligomerization product distribution. In this way, the nanoscale Brønsted acidity differences within the zeolite aggregates could be visualized; the core of the particles had a different acidity than that of the outer shell.

In summary, the broad range of articles presented in this collection should give a flavour of both the current trends and the recent developments in operando surface spectroscopy and, hopefully, they will provide a wide perspective of the field for the non-specialist reader and a wealth of valuable information for experts. We wish to thank our colleagues, who kindly contributed their fine articles, and the many referees who very generously gave their time and expertise. Finally, we sincerely thank the staff of the Journal and, in particular, Amaya Camara-Campos, Jane Hordern and Lois Alexander for their most efficient support at the different stages of the issue’s development—from conception to editing.

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