STRUCTURE SENSITIVITY OF METHANE DISSOCIATION ON PALLADIUM SINGLE-CRYSTAL SURFACES

Palladium single crystals have been found to be active for the C-H bon d dissociation of methane in the temperature range 400-600 K, and the activities of the presently investigated Pd(lll) and Pd(311) surfaces are compared with previously studied Pd(679). Structure sensitivity is reported that spans an order of magnitude in terms of the rates in th e order Pd(111)<Pd(311)<Pd(679), while the effective activation energi es range from 32-34 kJ/mol for Pd(111) and Pd(311) to 44 kJ/mol for Pd (679). These data are analyzed with a model that involves unsymmetrica l barriers, first by constructing a potential-energy surface for Pd(11 1), in which the reaction pathway is well-simulated by the Eckart barr ier. The Eckart barrier is then shown to obey exactly the Marcus rule for exothermic or endothermic processes. This property is used in comp aring the H-CH3 dissociation on the different crystal faces, with the result that the Pd(679) surface provides a driving force of some 26 kJ /mol due to the role of defects compared to smooth planar Pd(111) surf ace and 22 kJ/mol compared to the Pd(311) surface. (C) 1997 American I nstitute of Physics.