Hydrogenation of surface carbon on alumina-supported nickel

Abstract

Four types of carbon were observed to form on an alumina-supported nickel methanation catalyst on exposure to carbon monoxide at 550 ± 50 K. In order of their reactivity toward hydrogen the carbon species may be classified as: chemisorbed carbon atoms (α), bulk nickel carbide, amorphous carbon (β), and crystalline elemental carbon. The α-phase and the initial monolayers of Ni 3C are much more reactive than the elemental forms as measured by temperature-programmed surface reaction in 100 kPa H 2. At 550 K the α- and β-carbon species formed by CO exposure populate the surface at a ratio of about 2:1. Both phases are relatively stable on heating to 600 K in a He atmosphere. At higher temperatures, slow conversion of α- and β-carbon to graphite was observed. Hydrogenation of the α state at 550 K leads to methane at a sufficiently fast rate to make it a likely intermediate in nickel-catalyzed methane synthesis from hydrogen and carbon monoxide.