Per Stoltze

Surface electronic structure and reactivity of transition and noble metals

We present self-consistent density functional calculations using the LMTO-ASA method of the variations in the surface electronic structure for pseudomorfic overlayers and impurities of Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, and Au on the other metals. Knowledge of these variations is of importance in understanding trends in the reactivity of metal surfaces. A simple model is presented which gives a description of the overall trends in the self-consistently calculated results.

Making gold less noble

Self‐consistent density functional calculations for the adsorption of O and CO on flat and stepped Au(111) surfaces are used to investigate effects which may increase the reactivity of Au. We find that the adsorption energy does not depend on the number of Au layers if there are more than two layers. Steps are found to bind considerably stronger than the (111) terraces, and an expansive strain has the same effect. On this basis we suggest that the unusually large catalytic activity of highly‐dispersed Au particles may in part be due to high step densities on the small particles and/or strain effects due to the mismatch at the Au–support interface.

A kinetic model of the water gas shift reaction

A kinetic model of the water gas shift reaction based on a description of its elementary steps at the atomic level is presented. Input data for elementary steps are taken from available single crystal studies. The model is successfully tested against kinetic data for a working Cu-based catalyst. Expressions are derived for the activation energy and reaction orders.

Methanol synthesis on Cu(100) from a binary gas mixture of CO2 and H2

The rate of methanol synthesis over a Cu(100) single crystal from a 1 ∶ 1 mixture of CO2 and H2 has been measured at a total pressure of 2 bar and a temperature range of 483–563 K. At these conditions the apparent activation energy is determined to be 69 kJ mol−1, and at 543 K the turnover rate is 2.7 × 10−4 (site s)−1. A kinetic model for the methanol synthesis is presented. Predictions from this model are in good agreement with the rates of methanol synthesis observed on real catalysts at industrial conditions.

Microkinetic simulation of catalytic reactions

Abstract The study of the kinetics of heterogeneous catalyzed reactions consists of three different aspects: kinetics studies for design purposes, kinetics studies of mechanistic details and kinetics as a consequence of a reaction mechanism. The latter aspect is clearly the least explored of the three and it will not become routine until the development and analysis of microkinetic models is automated. Based on a survey of existing microkinetic models of heterogeneous catalytic reactions, three classes of Langmuir–Hinshelwood (LH) mechanisms are shown to be suitable for microkinetic modeling. The first model consists of an LH mechanism with the quasi-equilibrium approximation and a single, rate-limiting step. For this model detailed kinetic studies are possible and include the analytic determination of reaction rate, surface coverages, reaction orders and activation energy. The second model consists of an LH mechanism with the steady-state (SS) approximation. This model allows the treatment of some kinetic phenomena, which cannot be treated by Model 1. However, the treatment of Model 2 is much more difficult and the data which may be determined analytically from Model 2 are the reaction rate, the surface coverages and the degree of rate limitation. The third model is kinetic Monte Carlo (KMC) simulations which allow the modeling of even more mechanistic details, such as surface diffusion and adsorbate–adsorbate interactions. Analysis of this class of models are limited to numerical simulation. Two appendices present the implementation of a Runge–Kutta (RK) integration of the conversion through an isothermal plug-flow reactor and a KMC simulation of a reaction rate, respectively.

A semi-empirical effective medium theory for metals and alloys

A detailed derivation of the simplest form of the effective medium theory for bonding in metallic systems is presented, and parameters for the fcc metals Ni, Pd, Pt, Cu, Ag and Au are given. The derivation of parameters is discussed in detail to show how new parameterizations can be made. The method and the parameterization is tested for a number of surface and bulk problems. In particular we present calculations of the energetics of metal atoms deposited on metal surfaces. The calculated energies include heats of adsorption, energies of overlayers, both pseudomorphic and relaxed, as well as energies of atoms alloyed into the first surface layer.

The stability of the hydroxylated (0001) surface of α-Al2O3

Self-consistent density functional calculations of the hydroxylated (0001) corundum surfaces are presented. It is demonstrated that the hydroxylated surfaces are the most stable under most, but not all, conditions. Hydroxylation significantly lowers the surface free energy of α-alumina. The stability of the hydrated surface resolves the discrepancies between the morphology of the α-alumina (0001) surface observed under ultra-high vacuum, and at ambient conditions. A method for the calculation of the equilibrium surface stoichiometry is proposed. The proposed approach provides a valuable connection between theoretical calculations and experiments with metal oxides.

Formate synthesis on Cu(100)

Abstract Formate has been synthesized from a high pressure CO 2 :H 2 gas-mixture on a well defined Cu(100) single crystal, and the important kinetic parameters have been determined. The formate was synthesized in a high pressure cell incorporated into the UHV system and subsequently studied by XPS, HREELS, and TPD. The formate synthesis was studied over a pressure range of 0.5-5.0 bar and a temperature range of 323–363 K. The rate of synthesis was found to be dependent on gas composition (CO 2 : H 2 ) and weakly dependent on total pressure. At 363 K, the initial rate of formate synthesis on the Cu(100) surface in a 70:30 (CO 2 :H 2 ) gas-mixture at 2.3 bar was found to be (4.8± 0.6) × 10 -4 formate molecules per copper atom per second. The overall activation energy for the formate synthesis on the clean surface was found to be 55.6 ± 8.0 kJ mol -1 . A kinetic model for formate synthesis is presented which reproduces the experimental results for varying synthesis conditions. The model suggests that the rate limiting step in the synthesis is the reaction of adsorbed atomic hydrogen with adsorbed CO 2 forming the surface formate.

Anisotropic corner diffusion as origin for dendritic growth on hexagonal substrates

Ag aggregation on Ag(111), Pt(111), and 1 ML Ag pseudomorphically grown on Pt(111), has been studied with variable temperature STM. These systems all have in common that dendritic patterns with trigonal symmetry rather than randomly ramified aggregates, which would be expected for a simple hit and stick mechanism, form. Dendrites are characterized by preferential growth in the [ 2]-directions, i.e., perpendicular to A-steps. The key process for their formation has been found to be diffusion of one-fold comer atoms towards neighboring steps. Calculations with the effective medium theory show that this relaxation is highly asymmetric with respect to the two different kinds of close-packed steps. It leads to dendritic growth as verified by kinetic Monte-Carlo simulations which agree well with experiment.

Monte Carlo simulations of adsorption-induced segregation

Abstract Through the use of Monte Carlo simulations we study the effect of adsorption-induced segregation. From the bulk composition, degree of dispersion and the partial pressure of the gas phase species we calculate the surface composition of bimetallic alloys. We show that both segregation and adsorption are well-described within the method. It is shown that adsorption of CO and O2 on a PtRu alloy increases the concentration of Ru in the surface. Furthermore we present a database of CO adsorption energies collected from the literature.