The Adsorption of Simple Molecules on Supported Rhodium Catalysts

[摘要] The adsorption of a number of molecules relevant to the hydrogenation of carbon monoxide to organic oxygenate products over various supported rhodium catalysts has been investigated, under both static and pulsed flow conditions, using radioactive and stable isotope tracer techniques, temperature-programmed desorption and Fourier transform infra red spectoscopy. The catalysts used were 2%(w/w) and 5%(w/w) rhodium on silica, 2%(w/w) rhodium on alumina and 2%(w/w) rhodium on molybdenum trioxide. Temperature-programmed reduction techniques have also been used to characterise the catalysts. Determination of the [14-C]carbon monoxide and [14-C]carbon dioxide adsorption isotherms, under static conditions, showed that the amounts of each adsorbed were dependent on both the adsorbate and the support. Thus, each catalyst adsorbed considerably more carbon monoxide than carbon dioxide. However, whilst the carbon monoxide would undergo complete exchange at ambient temperature, no such exchange was found with adsorbed carbon dioxide. The average heat of adsorption of carbon monoxide was found to be support independent with a value of -93+/-6 kJ mol-1. The amounts of carbon monoxide adsorbed by each catalyst decreased in the order alumina > silica > molybdenum oxide. Exposure of [14-C]carbon monoxide precovered surfaces to either hydrogen or oxygen in the presence of gas phase [14-C]carbon monoxide resulted in an increase in surface radioactivity. Similar measurements with [14-C]carbon dioxide showed that the presence of hydrogen had little effect, the presence of oxygen dramatically reduced the amount of carbon dioxide adsorbed. Examination of the effects of preadsorbed carbon monoxide on the adsorption of carbon dioxide, and vice versa, showed that whilst preadsorbed carbon dioxide reduced the adsorption capacity of each catalyst for carbon monoxide, probably through a site-blocking mechanism, preadsorption of carbon monoxide increased the extent of adsorption carbon dioxide on both the silica- and molybdenum trioxide supported catalysts, but decreased the amount of carbon dioxide adsorbed on the alumina-supported catalyst. Pulsed flow adsorption measurements gave similar results to those obtained under static conditions, except that the amounts adsorbed were less due to the removal of weakly adsorbed species by the flow gas. Determinations of the amounts of carbon monoxide and carbon dioxide adsorbed at a variety of temperatures leads to the conclusion that adsorption of the dioxide is an activated process, whilst that of the monoxide proceeds with minumum activation energy. Adsorption of a 1:1 mixture of 13C16O and -12C18O showed that scrambling of the adsorbed species only occurred at elevated temperatures. Evidence is presented to show that this scrambling occurs by a concerted mechanism, rather than via dissociation, whilst the formation of small amounts of carbon dioxide at the elevated temperatures probably arises from a water gas shift reaction involving water retained by the catalyst following reduction and activation. A surprising feature to emerge from the temperature-programmed desorption studies was that whilst some carbon monoxide is still retained by the catalyst at 593K, this material would readily undergo isotopic exchange with gas phase carbon monoxide. FTIR studies of the adsorption of carbon monoxide on the rhodium-silica and rhodium-alumina catalysts showed the presence of bands ascribable to linear, bridged and gem adsorbed species on the reduced metal. An additional band ascribed to carbon monoxide adsorbed on a partially reduced rhodium site was also observed. No satisfactory measurements could be made, even in diffuse reflectance, with the rhodium-molybdenum trioxide catalysts, due to the extremely high absorbance of the catalyst samples. The effect of increasing temperature on the spectra of the adsorbed carbon monoxide was to increase the intensity of the bands due to the gem-adsorbed species, whilst the intensity of the linear band simultaneously decreased. This is taken to indicate that the presence of the adsorbed carbon monoxide causes a restructuring of the metal surface, and thereby an increase in the number of isolated rhodium sites on which the gem-dicarbonyl species are adsorbed, as the temperature is increased. The FTIR spectra of adsorbed carbon dioxide on rhodium-silica and rhodium-alumina revealed bands due to both surface carbonates and adsorbed carbon monoxide, indicating that at least some of the dioxide is dissociatively adsorbed on the surface. Investigations of the adsorption of methanol, ethanol and ethanal by infra red spectroscopy showed that, on both rhodium-silica and rhodium-alumina catalysts, decomposition to carbon monoxide occurred at ambient temperature. The extent of this decomposition, which was support dependent, was promoted by increase in temperature and by the presence of hydrogen. Bands due to the presence of surface hydrocarbonaceous species were also observed.

[发布机构] University:University of Glasgow

[关键词] Inorganic chemistry, Molecular chemistry