Dissociation rates of H2 on a Ni(100) surface: the role of the physisorbed state

Abstract

The dissociation and recombination rates of physisorbed H₂, and the total dissociation rate of gas phase H₂ on the rigid Ni(100) surface, as well as the corresponding kinetic isotope effects, are calculated by using the quantum instanton method, together with path integral Monte Carlo and adaptive umbrella sampling techniques. Both the dissociation and recombination rates of physisorbed H₂ are dramatically enhanced by the quantum motions of H₂ at low temperatures, for instance, the quantum rates are 43 and 7.5 times larger than the classical ones at 200 K, respectively. For the dissociation of gas phase H₂, at high temperatures, the H₂ can fly over the physisorbed state and dissociate directly, however, at low temperatures, the H₂ is first physisorbed and then dissociates under steady state approximation. The total dissociation rate of gas phase H₂ can be expressed as a combination of the direct and steady state dissociation rates. It has the form of an inverted bell with a minimum value at about 400 K, and detailed analysis shows that the dissociation of gas phase H₂ is dominated by a steady state process below 400 K, however, both the steady state and direct processes are important above 400 K. The calculated kinetic isotope effects reveal that H₂ always has larger rates than D₂ no matter which dissociative process they undergo.