Reactive atmospheres catalysts characterization

With the ability to obtain information about the concentrations of various types of metal surface sites in complex metal nanocluster catalysts, HRTEM provides new opportunities to include nanoparticle structure and dynamics into fundamental descriptions of the catalyst properties. This chapter is a survey of recent HRTEM investigations that illustrate the possibilities for characterization of catalysts in the functioning state. This chapter is not intended to be a comprehensive review of the applications of TEM to characterize catalysts in reactive atmospheres such reviews are available elsewhere (e.g., 1,8,9 )). Rather, the aim here is to demonstrate the future potential of the technique used in combination with surface science techniques, density functional theory (DFT), other characterization techniques, and catalyst testing. 

VI. Examples of HRTEM Characterization of Catalysts in Reactive Atmospheres... 

Bare SR, Ressler T. Characterization of catalysts in reactive atmospheres by X-ray absorption spectroscopy. In Advances in Catalysis. 2009 52 339. 

This chapter is largely concerned with the characterization by PL of various types of solids under inert and reactive atmospheres, and at low temperatures and pressures (i.e., under "spectroscopic" conditions). In contrast, to the best of our knowledge, there has been no report so far of PL spectra of a catalyst under working conditions (i.e., in the presence of flowing reactants at pressures and temperatures typically involved in catalysis experiments in the laboratory). 

Often it is essential to characterize the formation of a catalytically active state of a highly dispersed phase by XRD under reactive atmospheres. Investigations of reduction and calcination processes, for example (Thomas and Sankar, 2001 Sankar et al., 1991), guided the determination of recipes for catalyst preparation (Gunter et al., 2001d Kirilenko et al., 2005 Ressler et al., 2001, 2002 Wienold et al., 2003) in a complex parameter space. 

Nonetheless, characterization of catalysts in reactive atmospheres by XRD is a powerful method for obtaining the basic information needed to determine structure-activity correlations. Many phenomena of structural deactivation, either by sintering or recrystallization, are accessible by XRD under reaction conditions. The practice of approximating reacting atmospheres by simple-to-handle proxies (hydrogen for hydrocarbons or dry gases instead of steam-loaded feeds) has to be abandoned. 

A principal advantage of all peak deconvolution methods is their applicability, without modification, to both conventional XRD and that characterizing catalysts in reactive atmospheres. Thus, particle morphology data determined in conventional XRD experiments are applicable to... 

In summary, the compilation of relevant case studies shows that XRD of working catalysts is a widely applicable technique. It gives rich and useful information about synthesis and activation of catalysts as well as deactivation by structural transformations. The pertinent question about the structure of the active sites is not accessible directly by this method despite such claims in the literature. It must be pointed out that this shortcoming of a technique involving characterization of samples in reactive atmospheres is common to all methods when one is concerned with high-performance catalysts in which the active sites are a small fraction of the active surface. Model systems do a better job in this respect, provided that they are active for the reaction of interest and not only in proxy reactions. 

A major reason why XAFS spectroscopy has become a critically useful probe of catalyst structure is the fact that it is easily adapted to characterization of samples in reactive atmospheres. The X-ray photons are sufficiently penetrating that absorption by the reaction medium is minimal. Moreover, the use of X-ray- transparent windows on the catalytic reaction cell allows the structure of the catalyst to be probed at reaction temperature and pressure. For example, the catalyst may be in a reaction cell, with feed flowing over it, and normal online analytical tools (gas chromatography, residual gas analysis, Fourier transform (FT) infrared spectroscopy, or others) can be used to monitor the products while at the same time the interaction of the X-rays with the catalyst can be used to determine critical information about the electronic and geometric structure of the catalyst. 

XAS, and particularly its application to catalysis, has been the subject of several previous reviews and books. In 1988, Koningsberger and Prins published the book "X-ray absorption principles, applications, techniques of EXAFS, SEXAFS and XANES" (Koningsberger and Prins, 1988). In this monograph there is a thorough description of the technique together with a chapter on its application to catalysis. Iwasawa in 1996 published "XAFS for catalysts and surfaces" (Iwasawa, 1996), which focused solely on XAFS spectroscopy as applied to catalyst characterization. This volume includes a chapter by Bazin, Dexpert, and Lynch about measurements of catalysts in reactive atmospheres, and several other chapters allude to examples of such characterization. Recently a book entitled In situ Spectroscopy of Catalysts" (Weckhuysen, 2004) was published that contains three chapters focused on XAFS of catalysts in reactive atmospheres one on XANES, one on EXAFS, and one on time-resolved XAFS. 

We begin with a summary of the importance of XAFS spectroscopy in catalyst characterization science, using examples from the literature to illustrate each point. This introduction to the field includes all types of XAFS spectroscopy of catalysts in reactive atmospheres and is not restricted to investigations in which activity data were measured simultaneously with catalyst performance. This section is meant to familiarize the reader with the types of relevant information that can be provided by XAFS data. 

IMPORTANCE OF XAFS SPECTROSCOPY TO CATALYST CHARACTERIZATION IN REACTIVE ATMOSPHERES... 

XAS can be used in several different ways to determine local structural information about catalysts in reactive atmospheres. This structural information may be static or dynamic it may be geometric or electronic. The depth of information that can be ascertained is often dependent upon the type of catalyst, for example, supported metal nanoclusters versus bulk or surface oxides. It may also be controlled by some property of the catalyst, for example, the concentration of the element in the catalyst that is being investigated. In this section a few examples are provided to highlight the importance and relevance of XAFS in catalyst characterization. The examples are focused on (1) structural information characterizing samples in reactive atmospheres, (2) transformation of one species to another, (3) oxidation state determination, (4) determination of supported metal cluster size and shape, and (5) electronic structure. These examples illustrate the type of information that can be learned about the catalyst from XAFS spectroscopy. 

Often it is not possible to determine such information by any other characterization method. This fact, combined with the elemental specificity of the method, the fact that the edge position can be determined accurately, and the high intensity of the edge features, has made this use of XANES popular for characterization of changes in catalysts in reactive atmospheres. There are now many reported examples of changes in oxidation state of an element as a function of reduction temperature determined by XANES (for some recent examples see Becker et al., 2007 Gamarra et al., 2007 Haider et al., 2007 Jentoft et al., 2005 Martinez-Arias et al., 2007 Reed et al., 2006 Safonova et al., 2006 Saib et al., 2006 Silversmit et al., 2006). 

Another approach is to use focused X-ray beams in the scanning mode. This technique will require specialized focusing optics and a fast monochromator. Such a nanoprobe beam line is currently being developed at the APS and is expected to have the spatial resolution of several tens of nanometers. At the proposed NSLS-II synchrotron, a beam line with a spatial resolution of ten nanometers is planned. With the development of these beam lines, it is expected that spatial imaging will be available for characterization of catalysts, and of course the hope is to do this with catalysts in reactive atmospheres. 

The future trends in XAFS spectroscopy relevant to characterization of catalysts in reactive atmospheres will thus be a combination of gm-ns time-resolved XAFS spectroscopy, time-resolved and spatially resolved XAFS spectroscopy, and state-resolved XAFS observations of the local structures of working catalysts. These more precise and definitive measurements, when coupled with advances in theory, will lead to more reliable structural analysis of catalysts and the ability to definitively resolve the structures in mixed-phase catalysts. It is indeed an exciting and continuously evolving field. 

The theoretical basis of Mossbauer spectroscopy as well as its applications to catalyst characterization was reviewed in Advances in Catalysis in 1989 (2). This thorough article summarizes the physical basis of the technique and significant contributions to the characterization of solid catalysts. Since 1989, Mossbauer spectroscopy has not underwent major developments, and its applications to catalysis have been largely limited to catalysts that were not in reactive atmospheres, notwithstanding the impressive advances that have been made with other techniques in characterizing catalysts under working conditions. 

Since that time, Mossbauer spectroscopy has been used widely to characterize catalysts in reactive atmospheres, leading to continuous progress in the understanding of structure/catalytic property relationships (180-194). 

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