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

M. S. Touvelle*, P. C. Stair

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

5 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)556-568
Number of pages13
JournalJournal of Catalysis
Volume130
Issue number2
DOIs
StatePublished - Aug 1991

Funding

The authors are grateful for helpful discussions with Professors Robert L. Burwell, Jr. and Herman Pines. This material is based upon work supported by the National Science Foundation under Award No. CHE-8821781.

ASJC Scopus subject areas

  • Catalysis
  • Physical and Theoretical Chemistry

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