No abstract

At present, much remains unknown about the effect surface phonons and surface temperature may have on the reactivity of molecules at surfaces. Here, this problem is addressed for the dissociation of H2 on copper, which is a benchmark system for activated dissociative chemisorption on a metal surface. Ab Initio Molecular Dynamics (AIMD) calculations, quantum dynamics calculations using a static surface model, and experiments are reported and compared on the effects of surface temperature (T s) on the initial state-selected reaction of (v = 0, j = 8) and (v = 1, j = 4) H2 scattering from Cu(100) and the orientational dependence of this process, at T s = 1030 K. In the theory, the specific reaction parameter approach to density functional theory (SRP-DFT) was used. The rotational quadrupole alignment parameters computed for H2 reacting on the hot Cu(100) surface (1030 K) are smaller than the values computed with a static surface model. The initial state-selected reaction probabilities computed with AIMD for the hot surface are shifted to lower energies, by 40–60 meV, and broadened with respect to static surface quantum dynamics results. The rotational quadrupole alignment parameters computed with AIMD are in good agreement with experiment if the experimental results are shifted to lower energies by 100–150 meV. The AIMD average desorption energies underestimate the experimental results by 150–180 meV. Our study shows that the H2 + Cu(100) system presents a useful benchmark for the simultaneously accurate description of dissociative chemisorption and surface thermal effects on reaction because surface temperature effects on the (100) surface are much more pronounced than on the Cu(111) surface, while the (100) face does not yet show surface reconstruction at temperatures of interest to associative desorption experiments.

We present density functional theory calculations on a NaAlH4 cluster with two titanium atoms added. The two titanium atoms were adsorbed on the (001) surface of NaAlH4 as a Ti dimer or as two separate atoms. Various absorption sites inside the cluster were investigated, either by placing the Ti atoms in interstitial sites or by exchanging them with Na and Al atoms. The results imply that Ti is more stable in the subsurface region of the cluster than on the surface and that exchange with Na is preferred. Almost equally stable is the exchange with one Na and one Al, as long as the resulting structure contains a direct Ti−Ti bond. The calculations also show that when considering adsorption on the surface only, Ti prefers to adsorb as atomic Ti rather than as a Ti2. In this case, the Ti atoms adsorb above Na sites, with the Na atoms being displaced toward the subsurface region. A zipper model is proposed for explaining the enhanced kinetics due to Ti.