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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry