Local structure of active sites

The effect of the addition of citric acid on the photocatalytic reduction of hazardous Cr(VI) to less hazardous Cr(III)) with titania catalysts has been studied by means of in situ EPR of chromium species by Meichtry et al. [52] using titania P25 as photocatalyst. Reduction experiments of Cr(VI) solution were performed under near-UV (366 nm) irradiation under acidic conditions (pH 2) with bubbling air. It is found that the addition of citric acid fadhtates Cr(VI) reduction with a stepwise reduction of the chromate CrO/ (V)) via formation of Cr(V) and Cr( IV) and finally Cr(III) spedes observed. In the absence of dtric add, a cycHng between the different valence states of chromium occurs because of reduction and reoxidation processes by OH radicals. The maximum rate (fivefold increase) of Cr(VI) reduction is achieved at an initial citric add/Cr(VI) molar ratio of 1.25. Citric acid is oxidized to its anionic radical by electron abstraction of surface-trapped holes Cit + h+ j, - Cit . 

Cr(VI) reduction takes place through Cr(V) spedes, readily complexed by citrate and detected by EPR spectroscopy. Quantitative EPR determinations indicate that an important fraction (nearly 15%) of the reduced Cr(VI) is transformed to Cr(V)-Cit, which also undergoes a photocatalytic transformation. 

XPS was used to identify surface spedes and changes in coordination or valency during illumination or photocatalysis. 

Pt/Ti02 in the (a) dark, after (b) 0.5, and (c) 3 h of UV irradiation. Reproduced with permission from Elsevier [53]. 

Moreover, the PtO particles could be reduced by electrons and successively corroded by HNOj to form Pt + ions. Nitric acid is one of the products of photocatalytic oxidation of NO. In parallel, NO could be adsorbed on Pt species, forming more stable Pt +-NO nitrosyls. The latter could inhibit the photocatalytic oxidation of NO to NO2. 

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The ESR data shows that both the number of centers and the local structure of active sites associated with Cu isolated ions are not changed noticeably at T < 500°C as a result of cobalt introduction. At the same time, catalytic testing shows a 3-fold rise in oxidative activity of bi-cationic sample (Fig. 1) demonstrating an increase either in the number of sites or in the intrinsic activity of catalytic centers. The effect can be explained only in assumption of the high dispersion of cobalt ions in microporous matrix it is difficult to imagine a considerable contribution from the big particles of cobalt oxide on the outer surface of zeolitic crystals. 

Local Structures of Active Sites on Mo-MCM-41 Mesoporous Molecular Sieves and their Photocatalytic Reactivity for the Decomposition of NOj... 

The importance of XAS spectroscopy in oxidation catalysis research results from its abiUty to provide information on the local structure of active redox sites under reaction-Uke conditions, which can often hardly be obtained by other methods, such as XRD which is restricted to crystaUine material, EPR spectroscopy which only detects species with unpaired electrons, or UV-vis diffuse reflectance and Raman spectroscopy which are less sensitive to strongly absorbing TMI in reduced valence states. Thus, in situ EXAFS has been widely used to determine the local environment and redox behaviour of TMI in nanoporous oxides. A typical example is discussed in 19.3.2.2a for Fe-ZSM-5 zeolites. Another major application is the determination of size and shape of supported metal clusters, especially for particles smaller than... 

The Mossbauer effect, although not a substitute for other analytical methods such as x-ray diffraction, can be used to obtain several kinds of structural information about solids. In favorable cases, it is possible to obtain rather detailed information about the electronic configuration of atoms and the local symmetry of their sites by measuring the isomer shift and quadrupole splitting. If more than one valence state of a given atom is present, a semiquantitative determination of the amount of each kind is possible. In solid solutions, the amount of local or long range order can be estimated, and in certain defect structures the relation between the active atoms and the defects can be studied. 

It is now considered, by most groups working in this area, that vanadyl pyrophosphate (VO)2P207 is the central phase of the Vanadium Phosphate system for butane oxidation to maleic anhydride (7 ). However the local structure of the catalytic sites is still a subject of discussion since, up to now, it has not been possible to study the characteristics of the catalyst under reaction conditions. Correlations have been attempted between catalytic performances obtained at variable temperature (380-430 C) in steady state conditions and physicochemical characterization obtained at room temperature after the catalytic test, sometimes after some deactivation of the catalyst. As a consequence, this has led to some confusion as to the nature of the active phase and of the effective sites. (VO)2P207, V (IV) is mainly detected by X-Ray Diffraction. 

Nature of Active Sites. There is no apparent correlation between the increase of catalytic activity and a modification of the electronic structure of nickel oxide, since the electrical properties of both catalysts are identical. It is probable that local modifications of the nickel oxide surface are responsible for the change of its activity and of the reaction mechanism. It should be possible to associate these structural modification with local modifications of the height of the Fermi level, but it would be difficult to explain the results by the electronic theory of catalysis which considers only collective electrons or holes. A discussion based only on the influence of surface defects seems, therefore, to be more straightforward. 

In the development of meta oxiae photocata-ysts with high and stable photocatalytic activity for water decomposition, the establishment of a correlation betweer photocatalytically active sites and metal oxide structures is desirable. In particular, it is important to see how the local structures of metal oxides are associated with the essential steps such as photoexcitation, the transfei of excited charges to the surface, and reduction/oxidation of adsorbec reactants. This chapter deals with photolysis of water by titanates with tunnel structures. The roles of tunnel-related local structures ir the photocatalysis and of Ru02 promoters loaded on the titanates are presented. 

Thus, the change in oxidation state that would alter the spin is either essential to cater for spin-forbidden transitions or should not occur, to let a spin-allowed reaction to occur rapidly. This principle of specificity was identified in a large array of metal-oxo cluster reactions with small molecules [53, 54] and should be transferable to real catalyst under the concept of local electronic structures at active sites. 

Figure 1 summarizes the main differences and objectives between the major preparation strategies. A collection of the major individual reaction steps for the synthesis of unsupported catalysts can be found in Table 1. One fundamental insight from this rather schematic comparison is that differences in the reaction kinetics of the synthesis of a given material will lead to different mesoscopic and macroscopic structures which considerably affect the catalytic performance. It is necessary to control these analytically difficult-to-describc parameters with much the same precision as the atomic arrangement or the local electronic structure. Whereas these latter parameters influence the nature of the active site, it is the mcso/macrostructure which controls the distribution and abundance of active sites on a given material. It is necessary in certain cases to apply the costly method of fusion as there is no other way to... 

The periodic approach is not the only one available for atomistic simulations of these materials and we should first mention that much progress has been made in the application of molecular quantum chemical methods using cluster representations of the local structure of oxide materials [1, 2], More recently, this has given way to mixed quantum mechanics/molecular mechanics (QM/MM) calculations. In QM/MM simulations the important region, the active site for catalysis, is represented at a quantum chemical level while the influence of its environment, the extended solid, is represented using the computationally less-demanding atomistic force field approach. This allows complex structures such as metal particles supported on oxides to be tackled [3]. 

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