A Comparison of Sulfur- and Oxygen-Modified Mo(111) Surfaces for Methylcyclopropane Hydrogenolysis: Evidence for Participation by Subsurface Atoms in the Active Site

The catalytic hydrogenolysis of methylcyclopropane has been studied over the Mo(111) crystal plane as a function of preadsorbed atomic sulfur and oxygen coverage. The selectivities for the single hydrogenolysis products, isobutane and n-butane, and for the double hydrogenolysis products, methane, propane, and ethane, were the same for sulfur- and oxygen-modified surfaces and independent of modifier coverage. Sulfur and oxygen acted as poisons for the hydrogenolysis activity. The activity decreased linearly with both sulfur and oxygen coverage, but the slope of the activity decay with sulfur was three times that with oxygen. The zero activity intercept corresponded to ca. 6 × 1014 S atoms cm2 (one sulfur atom per unit cell) and ca. 20 × 1014 O atoms cm2 (three oxygen atoms per unit cell). Examination of structural models for the Mo(111) surface suggests that molybdenum atoms in the second or even third layers must participate in the active site for hydrogenolysis.