Preface: recent advances in the in-situ characterization of heterogeneous catalysts

Heterogeneous catalysis is a fascinating, but complex multidisciplinary science, which is core business to our energy, automotive, chemical and pharmaceutical industries as most chemical reactions are catalyzed by at least one material containing a multitude of often distinct catalytic functionalities. Although progress has been made in our understanding of the working principles and dynamics of catalytic solids, their genesis and mechanistic functioning under realistic reaction conditions—as schematically visualized in Fig. 1—represent still very important scientific challenges to both academia and industrial scientists. The prospect of rational design of materials with specific active sites to selectively catalyze a chemical reaction in high yield is very attractive. First of all, it represents the most economical and sustainable way of producing our daily life transportation fuels, materials and chemicals, but maybe as crucial—from a pure fundamental point of view—it is important to have insight into the interfacial chemistry taking place between the gas or liquid phase and the catalytic surface at e.g. elevated temperatures and pressures. Such knowledge may also be very valuable to other research areas, e.g. sensor technology and geochemistry.

Fig. 1 In-situ characterization approach to elucidate the intricate chemistry taking place during (a) catalyst synthesis and/or assembly; (b) catalytic reaction and (c) molecular diffusion.

Deep mechanistic insight into the fundamentals of heterogeneous catalysis can only be acquired by using advanced characterization methods as well as proper in-situ reaction cells and related measurement protocols. This is the field of in-situ or operando characterization. Interestingly, we have seen especially in the last decade a tremendous increase in the number of manuscripts dealing with the use of in-situ and operando catalyst characterization, including the introduction of novel methods and related measurement approaches. Fig. 2 outlines the three research areas, which have received the most attention in recent years; i.e., (a) the realization of time-resolved combined spectroscopy under more realistic reaction conditions; (b) the development of in-situ tools to characterize catalytic transformations in the liquid phase; and (c) the development of space-resolved and tomographic characterization approaches, ultimately allowing to perform single molecule-single active site/catalyst particle in-situ studies.

Fig. 2 Three research areas which have received the most attention in recent years in the area of in-situ or operando catalyst characterization: (a) combined time-resolved spectroscopy under more realistic reaction conditions; (b) spectroscopy in the liquid phase, including water and (c) single molecule-single active site/catalyst particle spectroscopy.

In view of all these aspects, I felt it was the right time to put together a dedicated issue for Chemical Society Reviews on this very important scientific subject. With the view of trying to represent as much as possible of a wide variety of in-situ characterization methods as applied on a range of catalytic materials, I have collected a series of 28 review articles covering the advantages, limitations, challenges and future possibilities of in-situ characterization techniques for elucidating the genesis and working principles of heterogeneous catalysts. Special emphasis will be on recent advances as many of the authors of this collection of articles also present their latest results. In this respect, this themed issue of Chemical Society Reviews can be seen as a valuable addition to two edited books on in-situ characterization of catalytic solids (J. F. Haw, In-situ Spectroscopy of Heterogeneous Catalysis, Wiley-VCH, 2002, and B. M. Weckhuysen, In-situ Spectroscopy of Catalysts, American Scientific Publishers, 2004) and a compilation of general review articles, which appeared in the period 2006–2009 in the book series Advances in Catalysis (vol. 50–52).

In this dedicated issue, reviews on general aspects of in-situ catalyst characterization are included, which may help researchers entering this field to familiarize themselves with some important aspects and recent trends. Bentrup (DOI: 10.1039/b919711g) nicely illustrates the potential of combining in-situ characterization methods in one experimental set-up. In this manner, complementary information can be directly compared leading to a more unified picture of the phenomena taking place during a catalytic process. Meunier (DOI: 10.1039/b919705m) focuses on the importance of verifying if an in-situ spectroscopic measurement has sufficient validity to make conclusions about a potential reaction mechanism and highlights the potential pitfalls in performing in-situ characterization experiments and makes recommendations for designing proper operando spectroscopic cells. Foster and Lobo (DOI: 10.1039/c0cs00016g) build further on these ideas and show with some case studies how one can identify reaction intermediates and catalytic active sites by using the in-situ catalyst characterization approach.

In the arena of catalyst synthesis, three review articles focus on the in-situ characterization methods for elucidating the intricate chemistry of the synthesis of porous crystalline materials, including zeolites and metal organic frameworks (MOFs). O'Brien, Beale and Weckhuysen (DOI: 10.1039/c0cs00088d) in their review focus on the use of synchrotron-based characterization methods for understanding the principles of porous catalytic solids synthesis, while Fan, Feng and Li (DOI: 10.1039/c0cs00012d) convincingly illustrate the power of in-situ UV Raman spectroscopy for studying the synthesis mechanism and assembly processes of molecular sieves. In a third review, Aerts, Kirschhock and Martens (DOI: 10.1039/b919704b) present a comprehensive review article on the spectroscopic methods currently available to probe zeolite synthesis processes.

Magnetic resonance methods, including NMR, MRI and EPR, are nowadays more and more used to elucidate the organic transformations taking place within catalytic solids as well as the dynamics of transport phenomena and catalyst preparation processes in e.g. a reactor or catalyst pellet. Four review articles are focusing on these aspects of heterogeneous catalysis research. Blasco (DOI: 10.1039/c0cs00033g) and Ivanova and Kolyagin (DOI: 10.1039/c0cs00011f) highlight the use of in-situ NMR for elucidating the reaction mechanisms of heterogeneous catalysts with special emphasis on zeolite materials. Brückner (DOI: 10.1039/b919541f) nicely illustrates the power of in-situ EPR for developing structure–activity relationships for various catalytic solids. Finally, Lysova and Koptyug (DOI: 10.1039/b919540h) summarize the intriguing space-resolved findings one can obtain by using MRI for investigating heterogeneous catalysts.

One of the most used methods for in-situ catalyst characterization is vibrational spectroscopy, including both IR and Raman methods. This themed issue contains eight review articles which highlight the power of vibrational techniques and related methods. Whereas Wachs and Roberts (DOI: 10.1039/c0cs00145g) highlight in-situ and operando Raman characterization studies of supported metal oxide and bulk metal oxide catalysts, Kim, Kosuda, Van Duyne and Stair (DOI: 10.1039/c0cs00044b) emphasize in their review the potential of resonance Raman, and surface- and tip-enhanced Raman spectroscopy for studying catalytic solids and heterogeneous catalytic reactions. Two reviews by Lamberti, Zecchina, Groppo and Bordiga (DOI: 10.1039/c0cs00117a) and by Vimont, Thibault-Starzyk and Daturi (DOI: 10.1039/b919543m) discuss the advances in the use of IR spectroscopy to give insight into the active sites, reaction intermediates and plausible reaction mechanisms of catalytic solids. As many catalytic reactions are taking place at the liquid–solid interface, attenuated total reflection (ATR)-IR spectroscopy offers many new opportunities for elucidating reaction mechanisms and surface intermediates in a wide variety of catalytic processes. This emerging topic has been reviewed by Mojet, Ebbesen and Lefferts (DOI: 10.1039/c0cs00014k) and Andanson and Baiker (DOI: 10.1039/b919544k). Another vibrational spectroscopy review article focuses on the catalyst chemical imaging capabilities of Raman and IR spectroscopy and is authored by Stavitski and Weckhuysen (DOI: 10.1039/c0cs00064g). Finally, Chmelik and Karger (DOI: 10.1039/c0cs00100g) present a review article in which amongst other methods interference and IR microscopies are used to investigate molecular diffusion phenomena in porous catalytic solids, including zeolites and MOFs.

Other important in-situ methods are based on electronic spectroscopy and include UV-Vis and fluorescence spectroscopy. In recent years we have seen important steps forward in developing micro- and nano-spectroscopy approaches for investigating catalytic solids at work. This has led to in-situ single particle and single molecule studies revealing fascinating new insights in molecular diffusion processes and the functioning of heterogeneous catalysts. Three review articles focus on the use of fluorescence microscopy for elucidating catalytic reactivity. De Cremer, Sels, De Vos, Hofkens and Roeffaers (DOI: 10.1039/c0cs00047g) present a comprehensive review article on the latest developments in this field of research, while Tachikawa and Majima (DOI: 10.1039/b919698f) focus on single molecule-single particle fluorescence imaging of photocatalytic processes. A third fluorescence microscopy review article on catalyst reactivity is by Peng Chen, Zhou, Shen, Andoy, Choudhary, Han, Liu and Meng (DOI: 10.1039/b909052p). These authors have summarized the recent progress in single molecule-single particle fluorescence nanoscopy. Another fluorescence microscopy review features the visualization of diffusion processes at the nanoscale and is authored by Michaelis and Bräuchle (DOI: 10.1039/c0cs00107d). Schoonheydt (DOI: 10.1039/c0cs00080a) has reviewed the use of UV-Vis spectroscopy and microscopy. The latter in-situ method is fully complementary with fluorescence microscopy and is now more and more used in a combined fashion.

Finally, there are five review articles, which are mainly focused on the use of X-ray spectroscopy of heterogeneous catalysts. It nicely illustrates the increasing importance and availability of suitable synchrotron beamlines for doing in-situ catalyst characterization work. Singh, Lamberti and van Bokhoven (DOI: 10.1039/c0cs00054j) summarize the recent advances in X-ray absorption and emission spectroscopy for elucidating the catalytic chemistry of supported metal catalyst materials, while Bordiga, Bonino, Lillerud and Lamberti (DOI: 10.1039/c0cs00082e) show the potential of X-ray absorption spectroscopy to understand the structure and reactivity of MOF materials. Hard and soft X-ray microscopy and tomography and their use in characterizing catalytic solids are reviewed by Grunwaldt and Schroer (DOI: 10.1039/c0cs00036a) and by Beale, Jacques and Weckhuysen (DOI: 10.1039/c0cs00089b). Special emphasis is placed in these review articles on the developments of in-situ X-ray nanoscopy of catalytic solids as a powerful tool, fully complementary to the above mentioned in-situ fluorescence nanoscopy techniques. Newton and van Beek (DOI: 10.1039/b919689g) present a state-of-the-art overview of what is currently possible when combining synchrotron-based X-ray methods with vibrational spectroscopy techniques and can be seen as a more particular view of combining in-situ characterization methods as discussed in the Bentrup review article (DOI: 10.1039/b919711g).

I would like to take the opportunity to thank all the authors and referees for their valuable time in composing and reviewing the excellent review articles giving the reader different perspectives on this important scientific discipline within heterogeneous catalysis research. Finally, I would like to thank the Editor, Dr Robert Eagling, and staff of Chemical Society Reviews for giving me the opportunity to assemble this themed issue on in-situ characterization of catalytic solids. It was a real pleasure working together with them on reading and selecting the 28 review articles assembled in this themed issue.

Footnote

† Part of the themed issue covering recent advances in the in-situ characterization of heterogeneous catalysts.

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