Cyclopropane hydrogenolysis on clean and chemically modified Mo(100) surfaces

Catalytic hydrogenolysis of cyclopropane (CP) to propane, ethane, and methane has been investigated over initially clean Mo(100) single-crystal surfaces and over surfaces chemically modified by adsorbed carbon, oxygen, and sulfur. Catalyst preparation was performed in ultrahigh vacuum with surface characterization by LEED and Auger. Catalytic hydrogenolysis under 1 atm pressure was monitored by gas chromatography. Initially clean Mo(100) surfaces and surfaces modified by 1 ML of atomic carbon or oxygen are active for single hydrogenolysis to propane and double hydrogenolysis to ethane and methane. The Mo(100) surface with 0.8 ML of sulfur is completely inactive. The reaction is first order in hydrogen and zero order in CP. The activation energy, determined from Arrhenius plots over the temperature range 300-523 K, is 37 ± 4 kJ/mol and is the same for all active surfaces. The product distribution (ca. 17% CH₄, 17% C₂H₆, and 66% C₃H₈) is also the same for all active surfaces and is independent of temperature. The initially clean Mo(100) surface is ca. five times more active than either the carbon- or oxygen-modified surfaces which have approximately the same activity. Since the selectivity of CP hydrogenolysis has previously been shown to be sensitive to Mo chemical state, the data indicate that the same catalytic site is operative on all active surfaces. Comparison to literature data for supported Mo catalysts indicates that the observed selectivity is characteristic of low-valent Mo rather than of the oxide or carbide. Prior knowledge that adsorbed carbon and oxygen block fourfold hollow sites on the (100) crystal plane coupled with the higher activity observed for initially clean Mo(100) suggests (1) that the active sites for hydrogenolysis are open fourfold hollows and (2) that the open hollow sites are present as defects on the carbon- or oxygen-modified surfaces. The catalysts are inactive for hydrogenolysis of propane, indicating that CH₄ and C₂H₆ are primary products. Likewise, propene is not an intermediate since propene conversion in H₂ is exclusively to propane and metathesis products. The absence of the C₄ products of metathesis from cyclopropane suggests that hydrogenolysis does not proceed via a metallacyclobutane surface intermediate. Therefore, the most likely mechanism involves a 1,3-diadsorbed surface intermediate.