Tuning the selectivity for ring-opening reactions of methylcyclopentane over Pt catalysts: A mechanistic study from first-principles calculations

Using density functional calculations, we studied the conversion of methylcyclopentane to its ring-opening products: branched hexanes [2-methylpentane (2MP), 3-methylpentane (3MP)], as well as unbranched n-hexane (nHx). We employed flat Pt(1 1 1) and stepped Pt(2 1 1) to describe terrace-rich large and defect-rich small Pt particles, respectively. On Pt(1 11), the barriers of all elementary steps for the paths leading to branched hexanes lie below 90 kJ mol(-1), while the formation of nHx features a barrier of 116 kJ mol(-1) in its C-C bond scission step. This higher barrier impedes the formation of nHx on Pt(1 1 1) and thus rationalizes the experimental observations that terrace-rich large Pt particles selectively produce branched hexanes. However, on Pt(211), the barrier of C-C scission for the formation of nHx decreases to 94 kJ mol(-1), thus implying enhanced formation of nHx over the defects, in agreement with the essentially statistical product distribution observed with defect-rich small Pt particles. (C) 2011 Elsevier Inc. All rights reserved.