The hydrogenation of CO and CO 2 over polycrystalline rhodium: Correlation of surface composition, kinetics and product distributions

Abstract

Rhodium, in the form of a small surface area (~1 cm 2) polycrystalline foil was used to study the COH 2 and CO 2H 2 reactions at low (~10 −4 Torr) and at high (700 Torr) pressures in the same apparatus. Reaction rates and product distributions were monitored with a mass spectrometer and a gas chromatograph, respectively, and the surface composition was determined by Auger electron spectroscopy. The various binding states of CO were studied by thermal desorption. Under reaction conditions (250–350 °C, 700 Torr) the surface is covered with a catalytically active carbonaceous deposit while some oxygen is located below the surface. No surface oxygen was detectable after reaction as the removal of chemisorbed species by either the COO 2 or H 2O 2 reactions was rapid at low temperatures. Carbon monoxide was found to adsorb in molecular form on clean Rh surfaces, but partly dissociated on surfaces pretreated in CO, or by heating in the presence of gaseous CO. Co-adsorption of H 2/CO mixtures at low pressures increased the amount of molecular CO but no changes in CO binding energy were observed. At low pressures (10 −4 Torr) the equilibrium hydrocarbon partial pressures from the COH 2 and CO 2H 2 reactions are too low to be detectable. At high pressures (700 Torr), the reaction rates, activation energy (24 ± 3 kcal) and product distributions from the small surface area Rh foil are nearly identical to that obtained on a dispersed rhodium catalyst. The CO 2H 2 reaction produces methane exclusively and with a lower activation energy (16 ± 2 kcal) than for the COH 2 reaction. Pretreatment of the clean surface changes the product distribution and the rates in both reactions. Pretreatment with acetylene deposits surface carbon and reduces methanation, but not chain growth in the COH 2 reaction (a higher percentage of C 2 and C 3 products). Pretreating with oxygen dissolves oxygen in the bulk and increases the methanation rate by up to a factor of five over the clean surface. It appears that active rhodiumcarbon-oxygen complexes form at the surface and rehydrogenate to yield the various products.