Rhodium catalysts supported, hydrogen chemisorption

The role of the support on hydrogen chemisorption on supported rhodium catalysts was studied using static and frequency response techniques. In all Instances, several klnetlcally distinct H2 cheml-sorptlve sites were observed. On the basis of the kinetics, at least one site appears to sorb H2 molecularly at temperatures below 150°C, regardless of the support. At higher temperatures, a dissociative mechanism may become dominant. Inducement of the SMSI state In Rh/T102 does not significantly alter Its equilibrium H2 chemisorption. 

These results are consistent with recently reported results by Haller, et al. (10) on the reactions of CO/H2 and NHj over Rh catalysts In which no significant differences were observed between catalysts reduced at low and high temperatures (presumably "normal and SMSI) but In which Rh/S102 was found to behave differently. Thus, there appears to be some correlation between the FRC chemisorption results and the reactivity patterns of supported rhodium catalysts which we would like to believe supports the assertion that the sites at which hydrogen sorbs reversibly are those at which catalytlcally Important reactions occur, and that FRC can monitor the density and relative kinetics of these sites. 

The activities in FT reaction (expressed as turnover rates, Vt of CO transformed to hydrocarbons and oxygenates) of bulk and supported tungsten carbides are compared to that of a rhodium catalyst (3.5 wt%) supported on alumina (Table 18.6). Its dispersion (94%) has been measured by hydrogen chemisorption by assuming unity stoichiometry of adsorbed hydrogen on Rh. 

Very recently, H-NMR technique has been applied to the investigation of the hydrogen chemisorption on Rh/CeO catalysts prepared from both Rh(N03)3 and RhCla precursors (163). Because of the different chemical shift shown by the hydrogen species adsorbed on the metal and the ceria support, the evolution of the rhodium... 

Table 2 Activity and hydrogen chemisorption on supported rhodium catalysts "...

Relative activities in CO hydrogenation measured for supported rhodium catalysts are listed in Table 5-36. These experimental findings are supported by H2 chemisorption measurements and active rhodium centers. 

The influence of the support is undoubted and spillover was further confirmed by the excess of hydrogen chemisorbed by a mechanical mixture of unsupported alloy and TJ-A1203 above that calculated from the known values for the separate components. It was also observed that the chemisorption was slower on the supported than on the unsupported metal and that the greater part of the adsorbate was held reversibly no comment could be made on the possible mediation by traces of water. On the other hand, spillover from platinum-rhenium onto alumina appears to be inhibited for ratios Re/(Pt Re) > 0.6. In an infrared investigation of isocyanate complexes formed between nitric oxide and carbon monoxide, on the surface of rhodium-titania and rhodium-silica catalysts, it seems that the number of complexes exceeded the number of rhodium surface atoms.The supports have a pronounced effect on the location of the isocyanate bond and on the stability of the complexes, with some suggestion of spillover. 

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