Atomic surface concentration ratios catalysts

For heterogeneous reactions involving fluid and solid phases, the areal rate is a good choice. However, the catalysts (solid phase) can have the same surface area but different concentrations of active sites (atomic configuration on the catalyst capable of catalyzing the reaction). Thus, a definition of the rate based on the number of active sites appears to be the best choice. The turnover frequency or rate of turnover is the number of times the catalytic cycle is completed (or tumed-over) per catalytic site (active site) per time for a reaction at a given temperature, pressure, reactant ratio, and extent of reaction. Thus, the turnover frequency is ... 

Taking into account the molecular sizes of the adsorbed compounds, the extents of adsorption, calculated as molar ratios of them to surface Pd atom, seems to be too large for most of the catalysts. Therefore, it is considered that the spillover from Pd onto support takes place with both compounds. The amounts concentrated on the surface of surrounding support will influence the surface concentration on Pd, thus affecting the hydrogenation on Pd surface. These preliminary results of adsorption measurements suggest that a support which can adsorb both modifier and substrate is favorable. In other words, a support with appreciable amounts of both acidic and basic sites, such as Ti02, seems to be preferable. 

The higher values of this ratio for the hydrophobic catalyst suggested that the hydrophobicity of the catalyst may accelerate the desorption of water from the catalyst surface and thus accelerate the forward reaction (step 4 in the Mars-van Krevelan mechanism). This may result in a reduced catalyst surface, which has previously been determined to favour high activity. A similar situation exists in the oxidation of CO on pure platinum metal [18], such that the surface concentration of CO decreases as the reaction temperature increases. Because the carbon atom of CO is known to adsorb to the platinum surface, a similar adsorption mechanism may exist for structurally similar hydrocarbons. This postulate is consistent with the mechanism employed. 

Figure 12.13. X-ray photoelectron spectroscopic analysis of the surface of V/Ti catalysts atomic ratios of fresh, calcined, used and aged catalysts indicating a change in the surface concentration of the active component.

In most cases, XPS has been u.sed to acquire quantitative data concerning catalyst surfaces. Quantitative data of interest consist of elemental concentration ratios, dispersion of the catalytically active component, or promoter, in the case of supported catalysts, and the coverage of the support by the promoter. For a given loading and support surface area dispersion and coverage are not independent parameters. Dispersion itself is defined as the ratio of surface promoter atoms to total number of promoter atoms. It is correlated with the catalyst activity. 

The differential heat of adsorption for Ni-Pt/Alnf was reduced to 111.28 kJ mol , which was 11.45 and 5.51 kJ mol lower than Pt/Alnf and Ni/Alnf, respectively. A similar result was obtained for the Ni-Pd bimetallic catalyst. This was interesting because the atomic ratio of PtiNi and Pd Ni was only 1 33 and 1 18, hence even small amount of surface concentration of solute in the alloy catalysts is sufficient to significantly alter the properties of constituent monometallic catalysts. 

Since ruthenium and rhodium are neighboring elements in the periodic table, a closer comparison of the properties of ruthenium-copper and rhodium-copper clusters is of interest (17). When we compare EXAFS results on rhodium-copper and ruthenium-copper catalysts in which the Cu/Rh and Cu/Ru atomic ratios are both equal to one, we find some differences which can be related to the differences in miscibility of copper with ruthenium and rhodium. The extent of concentration of copper at the surface appears to be lower for the rhodium-copper clusters than for the ruthenium-copper clusters, as evidenced by the fact that rhodium exhibits a greater tendency than ruthenium to be coordinated to copper atoms in such clusters. The rhodium-copper clusters presumably contain some of the copper atoms in the interior of the clusters. 

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