Kinetics of NH Formation and Dissociation on Pt(111)

• Published in: Journal of physical chemistry. C Vol. 111; no. 19; pp. 7127 - 7136
• Main Authors: Mudiyanselage, Kumudu, Trenary, Michael, Meyer, Randall J
• Format: Journal Article
• Language: English
• Published: American Chemical Society 17-05-2007
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp068907s

The kinetics of formation and dissociation of the NH species on the Pt(111) surface has been studied experimentally with time-resolved reflection absorption infrared spectroscopy (RAIRS) and theoretically with density functional theory (DFT). An intense and narrow peak at 3321 cm-1 due to the NH stretch vibration characterizes the NH species. This RAIRS peak enables the NH coverage to be accurately measured as a function of time during the formation and dissociation reactions. The experiment is performed by first preparing a well-ordered p(2 × 2)-N layer through oxydehydrogenation of NH3, then exposing the p(2 × 2)-N layer to H2 at low temperature. It is found that NH formation follows first-order kinetics, whereas the dissociation reaction follows second-order kinetics. Because NH is more stable on the surface than N and H, the dissociation rate is limited by the recombinative desorption of H2, which accounts for the observed reaction order. The experimental rate constants were compared with the corresponding theoretical values obtained with DFT using a plane wave basis set and ultrasoft pseudopotentials. Rate constants were calculated based on the ratio of vibrational partition functions of the transition state and the reactants. From the DFT calculations, the activation barrier for NH formation is found to increase strongly with increasing NH coverage. This may result in a compensation effect in the experimental parameters derived from an Arrhenius plot in which a low apparent activation energy is compensated by a low pre-exponential factor. Although a semiclassical theory indicates that quantum mechanical tunneling may play a role in the kinetics of NH formation, the observation of similar rates for NH and ND formation conclusively rules out any significant contribution from tunneling to the rates.