Formation of surface carbon and methanation catalysis on alumina supported nickel. [21 refs. ; Ni/Al/sub 2/O/sub 3/; CH/sub 4/ formed at and above 375/sup 0/K]

The mechanism of catalytic methanation on an alumina-supported nickel catalyst (25 wt percent Ni) has been examined by means of (1) kinetic studies with a pulse microreactor, (2) surface studies with Auger electron spectroscopy (AES), and (3) temperature programmed surface reaction kinetics (TPSR) at the gas/solid interface. During exposure of the catalyst to pulses of CO (diluted with He) at elevated temperatures (greater than 450/sup 0/K), first-order formation of surface carbon was observed. The surface carbon so formed exhibited high reactivity towards hydrogen, being converted quantitatively to methane on exposure to a pulse of H/sub 2/. The Auger electron spectra indicated that the surface carbon was bonded to the Ni as a carbidic-type surface species which converted to graphitic carbon of very low reactivity towards hydrogen at temperatures above 675/sup 0/K. The kinetics of methane formation from surface carbon and gaseous H/sub 2/ were determined by means of TPSR. The formation of CH/sub 4/ is observed at temperatures as low as 375/sup 0/K. The rates of methane formation follow first-order kinetics with respect to surface carbon. The activation energy for this process is 17.6 kcal/mol. The experimental data are analyzed in terms of a catalytic methanation mechanism in which dissociative more » adsorption of carbon monoxide and hydrogen is followed by reaction of surface carbon and surface oxygen with hydrogen atoms to yield methane and water. « less