Sites for Chemisorption

The characteristics of the support are well known for playing a sigiuficant role in determiiung catalytic performance, as already seen from the examples given above. Studies on the applicability of H-type zeolites (H-Y, H-ZSM5, H-MOR) as potential catalysts for cMorocarbon destruction demonstrated that acid sites (mainly Br0nsted-type sites) acted as efficient chemisorption sites for chlorinated molecules. The order of activity for TCE oxidation was found to... 

Different types of chemisorption sites may be observed, each with a characteristic A value. Several adsorbed states appear to exist for CO chemisorbed on tungsten, as noted. These states of chemisorption probably have to do with different types of chemisorption bonding, maybe involving different types of surface sites. Much of the evidence has come initially from desorption studies, discussed immediately following. 

Surface heterogeneity may merely be a reflection of different types of chemisorption and chemisorption sites, as in the examples of Figs. XVIII-9 and XVIII-10. The presence of various crystal planes, as in powders, leads to heterogeneous adsorption behavior the effect may vary with particle size, as in the case of O2 on Pd [107]. Heterogeneity may be deliberate many catalysts consist of combinations of active surfaces, such as bimetallic alloys. In this last case, the surface properties may be intermediate between those of the pure metals (but one component may be in surface excess as with any solution) or they may be distinctly different. In this last case, one speaks of various effects ensemble, dilution, ligand, and kinetic (see Ref. 108 for details). 

With certain types of catalysts it is easy to postulate that more than one type of chemisorption site may exist on the solid surface. For example, in the case of metal oxide catalysts, one might speculate that certain species could chemisorb by interaction with metal atoms at the surface, while other species could interact with surface oxygpn atoms. Consider the possibility that species A adsorbed on one type of site will react with species B adsorbed on a second type of site according to the following reaction. 

Table 2.1 shows the top site chemisorption energy for the adsorption of CO onto the Pt/Oj-a-alumina system as a function of metal layers, relative to Pt (111). For the monolayer of metal on the surface there is an enhancement of the CO top site binding energy relative to Pt (111). On the other hand, the second layer of Pt/Ox shows a dramatic decrease in the top site chemisorption energy. For three layers of Pt on this surface, the chemisorption energy oscillates above the Pt (111) energy, eventually returning to the Pt (111) value for n > A. 

FIGURE 9.2. Surface topography of (111) face of bcc metal with probable sites for dissociative chemisorption of N2 on bcc Fe. 

The adsorption site, i.e. the chemisorption position of the adatoms on (within, below) the substrate surface, thanks to the polarisation dependence of SEXAFS. Often a unique assignment can be derived from the analysis of both polarisation dependent bond lengths and relative coordination numbers. The relative, polarisation dependent, amplitudes of the EXAFS oscillations indicate without ambiguity the chemisorption position if such position is the same for all adsorbed atoms. More than one chemisorption site could be present at a time (surface defect sites or just several of the ideal surface sites). If the relative population of the chemisorption sites is of the same order of magnitude, then the analysis of the data becomes difficult, or just impossible. 

Autocatalytic decomposition of [PtMe2(COD)] on platinum black, under dihydrogen has already been pointed out in Uquid solution [44]. Indeed, the platinum atoms present in the starting complex are incorporated into the surface of the solid platinum catalyst, thus becoming the reactive sites for further cycles of chemisorption and reaction (Scheme 1). 

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