Influence of sulfur on methanation over tungsten(110)

Abstract

The effect of adsorbed sulfur on the rate of methanation over W(110) is reported for a range of sulfur coverages (0.03-0.55 monolayers) and temperatures (600-750 K) for 1 Torr of CO and 100 Torr of hydrogen. The rate drops sharply for low sulfur coverages (by 40% at 0.05 monolayer) but decreases less dramatically for higher coverages. This contrasts with the data of Goodman and Kiskinova for Ni, which exhibits an order of magnitude drop for 0.05 monolayer S. Postreaction Auger spectra reveal that the sulfur-free surface for this sample is covered with both carbidic carbon and oxygen and is oxygen-rich. Increasing sulfur coverages leads to little change in the postreaction carbon level but does cause the oxygen coverage to fall substantially, to zero by 0.40 monolayer S. The activation energy for methanation is unaffected by the presence of sulfur, suggesting that the decrease in rate is caused by sulfur limiting the saturation coverages that can be attained by the reactants. This possibility motivated a thermal desorption study of CO and hydrogen adsorption on sulfur precovered surfaces. Good agreement is obtained between the changes in these saturation coverages and the decrease in methanation rate due to the presence of sulfur whenmore » small coverages of coadsorbed oxygen are present. The coverage of coadsorbed oxygen used in the thermal desorption measurements was based on the postreaction Auger spectra. These results taken as a whole suggest that the inhibiting effect of sulfur appears to be suppressed by the formation of sulfur clusters above approx. 0.05 monolayer S.« less