Hydrogenolysis of methylcyclopropane on oxygen-modified Mo(111): The appearance of acid sites

Catalytic evidence for oxide formation on the Mo(111) surface has been obtained using methylcyclopropane (MCP) hydrogenolysis as a probe reaction. These results have been correlated with physical measurements made on the O/Mo system. Oxide formation on Mo has been studied previously by monitoring surface polarizability using XPS of physisorbed xenon. The results from this study concluded that oxide begins to form at 1 × 10₁₅ atoms/cm² and is essentially completely formed by 1.4 × 10₁₅ atoms/cm₂. Catalytic hydrogenolysis of methylcyclopropane to i-C₄H₁₀, n-C₄H₁₀, C₃H₈, C₂H₆, and CH₄ has been investigated over initially clean Mo(111) surfaces, surfaces chemically modified by oxygen coverages of 0-2 × 1015 atoms/cm² and Mo0₂. Catalyst preparation was performed in UHV with surface characterization by LEED and Auger. Hydrogenolysis at 1 atm pressure (5 Torr MCP, 755 Torr H) was monitored by gas chromatography. MCP can undergo single hydrogenolysis by either a metallic or a Lewis/Brønsted acid mechanism depending upon the chemical state of Mo. The metallic function of the catalyst dominated at oxygen coverages of <1 × 10₁₅ atoms/cm² to form i-C₄H₁₀. In addition, the product distribution (60% i-C₄H₁₀, 11.5% n-C₄H₁₀, 14% C₃H₈, 14% CH₄, 0.5% C₂H₆) remained constant up to an oxygen coverage of 1 × 10¹⁵ atoms/cm². At oxygen coverages >1 × 1015 atoms/cm², the i-C₄H₁₀, rate fell to zero. At a coverage of 1.5 × 10₁₅ atoms /cm², the acidic function of the catalyst began to dominate, and the n-C₄H₁₀ rate increased by a factor of five. At this coverage n-C₄H₁₀ was the sole single hydrogenolysis product. No double hydrogenolysis occurred on surfaces with high O coverage. Thus the catalytic results, which indicated Lewis/Bronsted acid sites were formed at an O coverage of 1.5 × 10₁₅ atoms/cm², agreed well with the XPS measurements which have shown that oxide was completely formed at 1.4 × 10₁₅ atoms /cm².