(392f) Kinetics of Nh Formation and Dissociation on Pt(111)

The formation and dissociation of the NH species on the Pt(111) surface has been studied experimentally with reflection absorption infrared spectroscopy (RAIRS) and theoretically with density functional theory. As shown in Fig. 1, NH is characterized by an intense and narrow peak at 3321 cm-1, which allows the NH coverage to be accurately measured with RAIRS as a function of time. This permits the kinetics of an elementary surface reaction to be measured where neither the reactants nor products desorb from the surface. The experiment is performed by first preparing a well ordered p(2x2) N layer through oxydehydrogenation of NH3, then exposing to H2 at low temperature. It is found that NH formation follows first-order kinetics with an activation energy of 0.23 eV, whereas the dissociation reaction follows second-order kinetics with an activation energy of 1.1 eV. Because NH is more stable on the surface than N and H, the dissociation kinetics are limited by the recombinative desorption of H2, which accounts for the observed reaction order. The simplicity of this reaction provides an unusually favourable case for direct comparison between experimental measurements and theoretical calculations of the rate constant for a surface reaction. Density-functional theory calculations were performed with the VASP program using a plane wave basis set and ultrasoft pseudopotentials. Rate constants were calculated based on the ratio of vibrational partition functions of the reactant and the transition state. Results indicate that the experimentally derived barrier from an Arrhenius analysis is much lower than that found in our DFT calculations using classical transition state theory. However, invoking a tunneling mechanism for NH formation readily explains this apparent discrepancy, and using an enhancement factor derived from semi-classical theory, we find very good agreement with experiment.