Ring-Opening Reactions of Methylcyclopentane over Metal Catalysts, M = Pt, Rh, Ir, and Pd: A Mechanistic Study from First-Principles Calculations

摘要

Using density functional calculations we studied the conversion of methylcyclopentane to its various ring-opening products, branched and unbranched hexanes, that is, 2-methylpentane and 3- methylpentane, as well as n-hexane. We examined four metal catalysts, M = Pt, Rh, Ir, and Pd, using slab models of flat M(111) and stepped M(211) surfaces, to describe terrace-rich large and defect-rich small M particles, respectively. As C-H bond activation and formation is rather independent of the particle structure, we focused on C-C bond scission which is expected to be structure sensitive. The barriers of C-C bond scission indeed vary from ~20 kJ mol~(-1) to ~140 kJ mol~(-1) on various sites of these metal surfaces. In general, lower activation energies were calculated for Rh and Ir surfaces, in agreement with the higher experimental activity of these two metals compared to Pt and Pd. From the calculated C-C bond breaking barriers, we were able to rationalize the selectivity toward different ring-opening products, as observed in experiments over the metal catalysts studied.