Quantum Dynamics of Dissociative Chemisorption ofH2 on the Stepped Cu(211) Surface

摘要

Reactions on stepped surfaces are relevant to heterogeneous catalysis, in which a reaction often takes place at the edges of nanoparticles where the edges resemble steps on single-crystal stepped surfaces. Previous results on H2 + Cu(211) showed that, in this system, steps do not enhance the reactivity and raised the question of whether this effect could be, in any way, related to the neglect of quantum dynamical effects in the theory. To investigate this, we present full quantum dynamical molecular beam simulations of sticking of H2 on Cu(211), in which all important rovibrational states populated in a molecular beam experiment are taken into account. We find that the reaction of H2 with Cu(211) is very well described with quasi-classical dynamics when simulating molecular beam sticking experiments, in which averaging takes place over a large number of rovibrational states and over translational energy distributions. Our results show that the stepped Cu(211) surface is distinct from its component Cu(111) terraces and Cu(100) steps and cannot be described as a combination of its component parts with respect to the reactiondynamics when considering the orientational dependence. Specifically,we present evidence that, at translational energies close to the reactionthreshold, vibrationally excited molecules show a negative rotationalquadrupole alignment parameter on Cu(211), which is not found on Cu(111)and Cu(100). The effect arises because these molecules react witha site-specific reaction mechanism at the step, that is, inelasticrotational enhancement, which is only effective for molecules witha small absolute value of the magnetic rotation quantum number. Froma comparison to recent associative desorption experiments as wellas Born–Oppenheimer molecular dynamics calculations, it followsthat the effects of surface atom motion and electron–hole pairexcitation on the reactivity fall within chemical accuracy, that is,modeling these effect shifts extracted reaction probability curvesby less than 1 kcal/mol translational energy. We found no evidencein our fully state-resolved calculations for the “slow”reaction channel that was recently reported for associative desorptionof H2 from Cu(111) and Cu(211), but our results for thefast channel are in good agreement with the experiments on H2 + Cu(211).