The Intrinsic Activation Energy as a Guide to Mechanisms of Reactions on Solid Surface
Through the Polanyi Relationship, the intrinsic activation energies calculated from gas phase reactions have been shown to provide practical information about the kinetics and mechanisms of surface reactions. Ab initio calculations showed that reaction pathways which involve C-C or C-O bond scission have 30 kcal/mole higher intrinsic activation energy than reactions that involve C-H or O-H bond scission. It was also established that the intrinsic barrier for a certain reaction mechanism stays constant over a reaction series across a wide range of Taft parameter. Based on the above premise, simple reactions on transition metal surfaces were found to follow the predicted mechanisms. Bockris method for calculating the transfer coefficient was shown to closely match the definition of transition state position. However, Marcus and Miller equations were found to be inferior in predicting the position of transition state. TPD experiments of methanol oxidation on (2 x 1)Pt(110) have demonstrated that intrinsic barrier for surface reactions is in fact similar to that of gas phase reactions.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign