Testing the pairwise additive potential approximation using DFT: coadsorption of CO and N on Rh(100)

The interaction between adsorbates is a key issue in surface science, because these interactions can influence strongly the properties of chemisorbed species with consequences for the thermodn. and kinetics of surface processes. The simplest representation of adsorbate-adsorbate interactions is based on the assumption that all interactions are pairwise additive. This approach has been satisfactorily used in the modeling of temp.-programmed desorption (TPD) spectra using both continuum and Monte Carlo methods. However, the energies estd. within the pairwise approxn. have never been compared to the energies calcd. using d. functional theory (DFT) methods. We demonstrate that the pairwise additive potential approxn. is indeed a good representation of the adsorbate-adsorbate interactions, and that we do not need to include three-body interactions or higher-order terms to est. the perturbation of the adsorption energy of an adsorbate by the presence of other coadsorbates. Moreover, we show for the first time how DFT can be used to explain the desorption features that one finds in TPD expts., thus linking the TPD desorption features with actual microscopic configurations. [on SciFinder (R)

Structure sensitivity of silver-catalyzed ethylene epoxidation

\u3cp\u3eThe influence of particle size (20-200 nm) of Ag/α-Al \u3csub\u3e2\u3c/sub\u3eO \u3csub\u3e3\u3c/sub\u3e catalysts for epoxidation of ethylene to ethylene oxide (EO) under industrial conditions was investigated. Small silver particles up to 40 nm are predominantly monocrystalline and show a decreasing weight-normalized reaction rate with increasing particle size. Particles larger than 50 nm consist of multiple silver crystallites with a much smaller domain size between 25 and 30 nm. For these polycrystalline silver particles, the weight-normalized reaction rate is independent of particle size. The ethylene conversion rate normalized to the external surface area increases when the silver particles become larger. We attribute this to a specific role of the grain boundaries between silver crystallites in supplying oxygen atoms to the external surface. Oxygen is likely activated at defects of an otherwise low-reactivity silver surface (for oxygen adsorption) followed by diffusion along grain boundaries, dissolution in the bulk, and diffusion to the external surface, where oxygen atoms react with ethylene. The reaction rate normalized to the surface area of the first outer shell of crystallites making up silver particles is independent of size for polycrystalline particles. A higher reaction pressure benefits ethylene oxidation rate and EO selectivity due to a higher oxygen coverage. Adding chlorine further improves the EO selectivity through modification of the active surface. The same particle size dependences are observed at 1 bar and at 20 bar without and with chlorine. The main finding of our work is that for large enough particles the ethylene oxidation rate normalized to the silver weight is independent of size. In addition to the size-independent weight-based activity, the preference for larger particles in industrial catalysts can be attributed to the high silver loadings used to obtain larger silver particles. The resulting high coverage of the α-Al \u3csub\u3e2\u3c/sub\u3eO \u3csub\u3e3\u3c/sub\u3e support with silver decreases undesired consecutive reactions of EO on its hydroxyl groups. \u3c/p\u3

Lateral interactions and multi-isotherms : nitrogen recombination from Rh(111)

Lateral adsorbate-adsorbate interactions result in variation of the desorption rate constants with coverage. This effect can be studied in great detail from the shape of a multi-isotherm. To produce the multi-isotherm, the temperature is increased in a (semi)stepwise fashion to some temperature, followed by maintaining this temperature for a prolonged time. Then, the temperature is stepped to a higher value and held constant at this new temperature. This cycle is continued until all of the adsorbates have desorbed. Using a detailed kinetic Monte Carlo model and an optimization algorithm based on Evolutionary Strategy, we are able to reproduce the shape of the experimentally measured multi-isotherm of nitrogen on Rh(111) and obtain the lateral interactions between the nitrogen atoms