DURING CATALYTIC REACTION

Wagner first proposed the use of such galvanic cells in heterogeneous catalysis, to measure in situ the thermodynamic activity of oxygen O(a) adsorbed on metal electrodes during catalytic reactions.21 This led to the technique of solid electrolyte potentiometry (SEP).22 26... 

T1O2 and Ce02 based supports. The surface of metal crystallites deposited on such supports is decorated with O2 even during catalytic reactions. And this O2 species is A (102 to 105) times less reactive than covalently bonded O. (Chapter 11). 

We shall mainly consider, in the present chapter, non-precious transition metals, but the model can be extended to precious metals presenting an oxidation state higher than zero [10,11], such as Rhx+, Pdx+, Ptx+ and tix+. The model also applies to some oxides alone, such as ceria (Ce02) [19] or mixed oxides such as ceria-zirconia (CeZr02) able to present redox properties and oxygen vacancies during catalytic reactions. 

An alternative approach for the preparation of supported metal catalysts is based on the use of a microwave-generated plasma [27]. Several new materials prepared by this method are unlikely to be obtained by other methods. It is accepted that use of a microwave plasma results in a unique mechanism, because of the generation of a nonthermodynamic equilibrium in discharges during catalytic reactions. This can lead to significant changes in the activity and selectivity of the catalyst. 

New directions have been recently advanced in the use of IR spectroscopy for the characterization of adsorbates, including the investigation of liquid-solid interfaces in situ during catalysis. Both ATR [91,92] and RAIRS [86,93] have been recently implemented for that purpose. RAIRS has also been used for the detection of intermediates on model surfaces in situ during catalytic reactions [94-96], The ability to detect monolayers in situ under catalytic environments on small-area samples promises to advance the fundamental understanding of surface catalytic reactions. 

Markovic and Hartwig isolated and characterized the first intermediate in iridium-catalyzed allylic substitution [100]. They isolated the metalacyclic iridium-phosphor-amidite fragment containing COD and the olefinic portion ofN- l -phenylallyl)aniline, the product of the allylic substitution reaction between cinnamyl carbonate and aniline (5 in Scheme 22). This complex containing the product of allylic substitution was first detected by NMR spectroscopy during catalytic reactions. It was then isolated, prepared independently, and shown to be chemically and kinetically competent to be an intermediate in allylic substitutions. 

Reactions conducted in molten quaternary phosphonium salts require no other solvent (199). This material serves as both promoter and reaction medium. Care must be exercised in choosing the salt in such a reaction, since any decomposition could lead to products such as trialkylphosphines and alkyl halides which are expected to be deleterious to catalyst performance. Tetrabutylphosphonium bromide is reported to provide a stable catalyst medium which can be recycled (199, 200), but other related salts show evidence of thermal decomposition during catalytic reactions. Experiments in tetrabutylphosphonium acetate, for example, are found to produce large amounts of methyl and ethylene glycol acetate esters (199). 

The crystalline structure of modified zeolites determine a number of properties which are specific and favorable for catalytic reactions. The complete or partial loss of crystalline structure during catalytic reactions or regeneration is in most cases accompanied by decreased catalytic activity. Thermal stability or structural stability characteristics are therefore suitable for evaluating such catalysts or supported catalysts. 

Over the past 10 years a multitude of new techniques has been developed to permit characterization of catalyst surfaces on the atomic scale. Low-energy electron diffraction (LEED) can determine the atomic surface structure of the topmost layer of the clean catalyst or of the adsorbed intermediate (7). Auger electron spectroscopy (2) (AES) and other electron spectroscopy techniques (X-ray photoelectron, ultraviolet photoelectron, electron loss spectroscopies, etc.) can be used to determine the chemical composition of the surface with the sensitivity of 1% of a monolayer (approximately 1013 atoms/cm2). In addition to qualitative and quantitative chemical analysis of the surface layer, electron spectroscopy can also be utilized to determine the valency of surface atoms and the nature of the surface chemical bond. These are static techniques, but by using a suitable apparatus, which will be described later, one can monitor the atomic structure and composition during catalytic reactions at low pressures (< 10-4 Torr). As a result, we can determine reaction rates and product distributions in catalytic surface reactions as a function of surface structure and surface chemical composition. These relations permit the exploration of the mechanistic details of catalysis on the molecular level to optimize catalyst preparation and to build new catalyst systems by employing the knowledge gained. 

In its applications to spectra on finely divided metals DRIFTS is similar to infrared transmission. Its advantage is that catalysts can be studied in powdered form, even in situ during catalytic reactions, thus avoiding some of the diffusion-limited kinetics that are encountered when pressed catalyst disks are used. 

There is evidence that at least two different pseudoliquid phases may be present, even during catalytic reactions, and these may change reversibly with changes in the reactant partial pressures, as shown for dehydration of 2-propanol in Fig. 39 (242). A small change in reactant partial pressure led to an abrupt... 

No discussion has been devoted to the recent use of Fourier transform spectrometers rather than dispersion instruments. The ease with which the spectral data can be manipulated and background subtracted make the FT methods particularly useful for studies of surface species, particularly during catalytic reaction. Recently there has been a surge of interest in the coupling of computer subtraction techniques to conventional grating instruments. For many IR surface studies, where only limited frequency range is required, this... 

The cyclic substrate 32 and other disubstituted olefins such as 35a were oxidized in sc C02 to give the corresponding epoxides with reasonable rates (>95% conversion in less than 18 h) and excellent selectivities (>98%) under otherwise similar reaction conditions (Loeker and Leitner, 2000). It is important to note, however, that no addition of a metal catalyst was required in the supercritical reaction medium. Detailed control experiments revealed that the stainless steel of the reactor walls served as efficient initiator for the epoxidation under these conditions. Terminal olefins 35b,c were oxidized with somewhat reduced rates and either epoxidation or vinylic oxidation occurred as the major reaction pathway depending on the substrate (eq. 5.11). Apart from providing the first examples for efficient and highly selective oxidation with 02 in sc C02 (earlier attempts Birnbaum et al., 1999 Loeker et al., 1998 Wu et ah, 1997), this study points to the possible importance of wall effects during catalytic reactions in this medium (see also Christian et ah, 1999 Suppes et ah, 1989). 

It is now well established that a variety of organic molecules such as polynuclear aromatic hydrocarbons with low ionization energies act as electron donors with the formation of radical cations when adsorbed on oxide surfaces. Conversely, electron-acceptor molecules with high electron affinity interact with donor sites on oxide surfaces and are converted to anion radicals. These surface species can either be detected by their electronic spectra (90-93, 308-310) or by ESR. The ESR results have recently been reviewed by Flockhart (311). Radical cation-producing substances have only scarcely been applied as poisons in catalytic reactions. Conclusions on the nature of catalytically active sites have preferentially been drawn by qualitative comparison of the surface spin concentration and the catalytic activity as a function of, for example, the pretreatment temperature of the catalyst. Only phenothiazine has been used as a specific poison for the butene-1 isomerization on alumina [Ghorbel et al. (312)). Tetra-cyaonoethylene, on the contrary, has found wide application as a poison during catalytic reactions for the detection of active sites with basic or electron-donor character. This is probably due to the lack of other suitable acidic probe or poison molecules. 

A number of transition metals are now known147-156 to form stable dioxygen complexes, and many of these reactions are reversible. In the case of cobalt, numerous complexes have been shown to combine oxygen reversibly.157 158 Since cobalt compounds are also the most common catalysts for autoxidations, cobalt-oxygen complexes have often been implicated in chain initiation of liquid phase autoxidations. However, there is no unequivocal evidence for chain initiation of autoxidations via an oxygen activation mechanism. Theories are based on kinetic evidence alone, and many authors have failed to appreciate that conventional procedures for purifying substrate do not remove the last traces of alkyl hydroperoxides from many hydrocarbons. It is usually these trace amounts of alkyl hydroperoxide that are responsible for chain initiation during catalytic reaction with metal complexes. 

One inherent problem in the method is that spectral features are broad and may not be specific changes observed during catalytic reactions may have multiple causes, such as changes in valence of metal ions, changes in dispersion of supported species, or deposition of feed components and side products on the catalyst. In this respect, input from theoretical chemistry for interpretation of the spectra (Garbowski et al., 1972 Klokishner et al., 2002 Melsheimer et al., 2002 Jentoft et al., 2003) should be considered more frequently. 

A) of the produced catalyst can be maintained during catalytic reactions. Moreover, tests of the catalytic activity indicated a high catalytic conversion of N2O by the catalysts synthesized from the microemulsion process at a lower temperature (-400 °C) than that prepared from traditional impregnation processes. The imiform elemental distribution of RUO2 on the Si02 produced by this microemulsion process was estabhshed by XPS and SEM/hne scanning. 

This statement corresponds to a new concept in classification of polyaromatic coke formed during catalytic reactions that provides a structural basis for the traditional classifications of hard coke and soft coke (d7-(5P). 

To combine optical SFG spectroscopy with the more traditional surface analysis methods (e.g., LEED, AES, TPD, XPS), the basic requirement is to simply add IR-transparent windows (e.g., CaF2 or BaF2) to a UHV chamber. However, if SFG spectroscopy is to be carried out at high pressure or during catalytic reactions, instruments combining a EIHV surface analysis system with an SFG-compatible... 

Quantificahon greatly aids the understanding of the catalytic contributions of different vanadia species during catalytic reactions. For example, our preliminary activity test over these supported catalysts showed that the I.2V/6-AI2O3 sample exhibits better stability than higher loading catalysts for butane dehydrogenation in dilute feed [57]. The explanation is that the monovanadate species (ca 50% on the surface) dilute the polyvanadate species so that the two-dimensional coke species responsible for catalyst deactivation are less likely to form [40, 57]. 

Unsaturated residue formed during catalytic reactions that produced paraffins and olefins is the source of alkyl aromatics and nonvolatile residue. When HZSM-5 catalyst is employed, aromatic alkyl chain sizes are restricted to C4 or smaller. The pores of HZSM-5 are large enough to allow formation of small alkyl aromatics by cyclization and dehydrogenation of surface species, but formation of fused unsaturated coke precursors are inhibited. Unlike HZSM-5, larger HY pores facilitate the formation of larger nonvolatile unsaturated coke precursors. 

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