Catalyst vibrational spectroscopy

Vibrational Spectroscopy. Infrared absorption spectra may be obtained using convention IR or FTIR instrumentation the catalyst may be present as a compressed disk, allowing transmission spectroscopy. If the surface area is high, there can be enough chemisorbed species for their spectra to be recorded. This approach is widely used to follow actual catalyzed reactions see, for example. Refs. 26 (metal oxide catalysts) and 27 (zeolitic catalysts). Diffuse reflectance infrared reflection spectroscopy (DRIFT S) may be used on films [e.g.. Ref. 28—Si02 films on Mo(llO)]. Laser Raman spectroscopy (e.g.. Refs. 29, 30) and infrared emission spectroscopy may give greater detail [31]. 

Le Bourdon, G., Adar, F., Moreau, M. et al. (2003) In situ characterization by Raman and IR vibrational spectroscopies on a single instrument deNO, reaction over a Pd/y-Al203 catalyst, Phys. Chem. Chem. Phys., 5, 4441. 

The book has been written as an introductory text, not as an exhaustive review. It is meant for students at the start of their Ph.D. projects and for anyone else who needs a concise introduction to catalyst characterization. Each chapter describes the physical background and principles of a technique, a few recent applications to illustrate the type of information that can be obtained, and an evaluation of possibilities and limitations. A chapter on case studies highlights a few important catalyst systems and illustrates how powerful combinations of techniques are. The appendix on the surface theory of metals and on chemical bonding at surfaces is included to provide better insight in the results of photoemission, vibrational spectroscopy and thermal desorption. 

Carbon monoxide on metals forms the best-studied adsorption system in vibrational spectroscopy. The strong dipole associated with the C-O bond makes this molecule a particularly easy one to study. Moreover, the C-0 stretch frequency is very informative about the direct environment of the molecule. The metal-carbon bond, however, falling at frequencies between 300 and 500 cm1, is more difficult to measure with infrared spectroscopy. First, its detection requires special optical parts made of Csl, but even with suitable equipment the peak may be invisible because of absorption by the catalyst support. In reflection experiments on single crystal surfaces the metal-carbon peak is difficult to obtain because of the low sensitivity of RAIRS at low frequencies [12,13], EELS, on the other hand, has no difficulty in detecting the metal-carbon bond, as we shall see later on. 

The adsorption of CO on Pt is perhaps the most throughly studied system using vibrational spectroscopy. Studies have been made using both supported catalysts (2-5) and single crystals (5-10). Sample environments have included gas phase, vacuum, and aqueous solution (11-13). The similarities between many of these results have led to a remarkably unified understanding of CO adsorption phenomena in all three environments. Features which are relevant to further studies of the metal/electrolyte interface are summarized briefly ... 

Vibrational spectroscopy techniques are quite suitable for in situ characterization of catalysts. Especially infrared spectroscopy has been used extensively for characterization of the electrode/solution interphases, adsorbed species and their dependence on the electrode potential.33,34 Raman spectroscopy has been used to a lesser extent in characterizing non-precious metal ORR catalysts, most of the studies being related to characterization of the carbon structures.35 A review of the challenges and applications associated with in situ Raman Spectroscopy at metal electrodes has been provided by Pettinger.36... 

Spectroscopic developments have accelerated advances in the field of catalysis. This volume analyzes the impact on catalyst structure and reactivity of EXAFS, SIMS, MSssbauer, magic-angle spinning NMR (MASNMR), and electron-energy-loss vibrational spectroscopy. Many of these techniques are combined with other analytical tools such as thermal decomposition and temperature-programmed reactions. 

Vibrational spectroscopy of adsorbed probe molecules is one of the most powerful tools to assess the acidic properties of catalysts. Acidity studies of dealumi-nated Y zeolites (main active component of FCC catalysts) or other zeolitic catalysts are reported using mostly Fourier Transform Infrared Spectroscopy (FTIR) with CO adsorption at 77 K or FTIR-pyridine/substituted pyridines adsorption at 425 K [22-26]. FTIR acidity studies of commercial FCC catalysts are even more scarce... 

The iron molybdenum oxide catalyst was structurally characterized by XRD and vibrational spectroscopy (IR and Raman). 

Our article has concentrated on the relationships between vibrational spectra and the structures of hydrocarbon species adsorbed on metals. Some aspects of reactivities have also been covered, such as the thermal evolution of species on single-crystal surfaces under the UHV conditions necessary for VEELS, the most widely used technique. Wider aspects of reactivity include the important subject of catalytic activity. In catalytic studies, vibrational spectroscopy can also play an important role, but in smaller proportion than in the study of chemisorption. For this reason, it would not be appropriate for us to cover a large fraction of such work in this article. Furthermore, an excellent outline of this broader subject has recently been presented by Zaera (362). Instead, we present a summary account of the kinetic aspects of perhaps the most studied system, namely, the interreactions of ethene and related C2 species, and their hydrogenations, on platinum surfaces. We consider such reactions occurring on both single-crystal faces and metal oxide-supported finely divided catalysts. 

We summarize below the types of information obtainable from the substantial number of physical methods, alternative to vibrational spectroscopy, available for investigating the structures of species on metal surfaces. A minority of these, to which we give greater attention, are applicable to work on finely divided metal catalysts. Many of the others provide direction-dependent information and are most effective when applied to adsorption studies on flat single-crystal surfaces. 

Although not exhaustive, the above summary of experiments with hydrogen chemisorbed on transition-metals serves to illustrate how neutron vibrational spectroscopy is performed with catalytic substrates and the methods used to analyze the inelastic neutron spectra. In concluding this section we note that the technique can be extended to supported catalysts such as in recent experiments with hydrogen adsorbed on both MoS and alumina supported MoSp (38). Also, as another indication of the variety of systems which can be studied, we note earlier experiments with ethylene (39) and acetylene (40) adsorbed on silver exchanged 13X zeolites. "Tn this work, deuteration of the molecules was helpful in identifying the surface vibratory modes on these ionic substrates of greater complexity. 

This appendix begins with a brief introduction to the physics of metal surfaces. We limit ourselves to those properties of surfaces that play a role in catalysis or in catalyst characterization. The second part includes an introduction to the theory of chemisorption, and is intended to serve as a theoretical background for the chapters on vibrational spectroscopy, photoemission, and the case study on promoter effects. General textbooks on the physics and chemistry of surfaces are listed in [1-8]. 

Vibrational spectroscopies were then applied, notably infrared (IR) and Raman. The IR spectra from the spent catalysts were measured with the aid of the KBr disk technique. They showed a C-H stretch band at approximately 3050 cm-1 due to H bound to aromatic (or unsaturated) carbon (H ) and bands at 2960, 2925 and 2865 cm-1 due to H bound to saturated carbon (Hsal). The Hat/Hsat ratio was determined from the intensities of these bands. 

In-situ IR measurements overcome the problems mentioned for in situ NMR spectroscopy. The information that we obtain from vibrational spectroscopy is far less detailed, however, than that from NMR. The concentration of the catalyst may be equal to the one used in practical catalytic systems. Secondly, autoclaves have been equipped with IR cells, either as flow cells or via real in-situ monitoring in the Moser cell (see below), which allows one to work with gaseous reactants. In the following we will mention a (very) few examples of complexes that may be intermediates in the hydroformylation reaction observed with these two techniques. 

Spallation sources have notable advantages over reactors for vibrational spectroscopy. ISIS (Chilton, UK) will double in size by 2007 with the construction of a second target station. This is optimized for neutrons at energies below 200 cm and so will broaden the opportunities for investigations of the low energy modes of much larger molecules and dihydrogen on catalyst surfaces. 

This section is a summary of recent high-pressure investigations of model catalysts. The focus is on vibrational spectroscopy but, when data are available. 

Volume 50 of Advances in Catalysis, published in 2006, was the hrst of a set of three focused on physical characterization of solid catalysts in the functioning state. This volume is the second in the set. The hrst four chapters are devoted to vibrational spectroscopies, including Fourier transform infrared (Lamberti et al.), ultraviolet Raman (Stair), inelastic neutron scattering (Albers and Parker), and infrared-visible sum frequency generation and polarization-modulation infrared rehection absorption (Rupprechter). Additional chapters deal with electron paramagnetic resonance (EPR) (Bruckner) and Mossbauer spectroscopies (Millet) and oscillating microbalance catalytic reactors (Chen et al.). 

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