Field emission study of VIII transition metals: III. Adsorption of ethylene and acetylene on Pt

Abstract

Field emission microscope has been used to study the adsorption and decomposition of ethylene and acetylene on platinum surface. Adsorption of ethylene reduced the work function by 0.65 eV. The heat treatment of ethylene-covered surface at 480 K increased relatively the emission from the stepped regions about the (211) planes, suggesting that some polymerization reactions of adsorbed ethylene occur. Exposure to acetylene decreased the work function by 1.05 eV. The adsorption of acetylene was suggested to be mainly non-dissociative. On heating the ethylene- or acetylene-covered surface, the decomposition involving the breaking of C C and C H bonds was observed at 480–830 K, accompanying an increase of the work function. Carbon left after dehydrogenation formed a surface carbide at around 830 K. At higher temperatures, the carbide carbon aggregated into graphite crystallites. The crystallites concentrated toward the (110) planes above 1100 K and finally diffused into the bulk at 1300 K. The formation of platinum carbide near the surface layer was suggested to form a new catalytic site on the (110) plane. The obtained results were compared with the previous results of the Ni and Pd emitters to discuss the catalytic behavior for the hydrogenation of hydrocarbons on these metals.