DFT-based Kinetic Monte Carlo simulations of oxidation reactions over the RuO2(110) model catalyst surface

摘要

Within this dissertation, parameter sets for the Kinetic Monte Carlo KMC) simulation of the CO and HCl oxidation reactions were derived using density functional theory (DFT) and a cluster expansion. The parameter sets include activation energies and lateral interactions. The simulations were conducted over a wide range of reaction conditions and the obtained results were compared to experimental data and results from previous simulation models. In the case of the CO oxidation the comparison of KMC-simulated temperature-programmed desorption spectra to the corresponding experimental spectra reveals flaws in the DFT calculations, resulting in overbinding of COot and Oot and underbinding of CObr. The kinetics of the reaction under ultrahigh vacuum conditions are well-reproduced by the simulation only at near-stoichiometric gas feeds. Under strongly oxidizing and strongly reducing conditions the catalyst activity is underestimated. The apparent activation energy obtained at pressures in the millibar range (85-91 kJ/mol) agrees very well with the experimental value (75-85 kJ/mol). Because the present simulation model properly accounts for how the adsorption energy of CObr depends on neighboring species, it outperforms the previous experiment-based simulation model at higher temperatures. Additionally, the simulations reveal important information about the effect of lateral interactions in the kinetics of the CO oxidation over RuO2(110).The simulation results for the HCl oxidation over RuO2(110) agree very well with the experimental results overall. Only the reaction order in Cl2 is not well-described by the model. The discrepancy seems to be of mechanistic origin because even semi-empirically adjusting the adsorption energies of Oot and Clot to account for over- and underbinding was not able to completely correct the deviations.Under conditions where the simulations agree well with the experiments, more insight into the surface processes can be obtained from the simulation results. This includes atomistic explanations for the observed reaction orders and the rate-determining step under different conditions. O2 adsorption, as previously proposed from experiments, was confirmed as the rate-determining step for HCl-rich gas feeds. For oxidizing conditions, the rate-determining step changes to HCl adsorption and the OHbr + Oot hydrogen transfer, indicating inhibited H2O formation. The basicity of the catalyst surface has turned out to be an important factor for the catalyst activity. The basicity was found to dynamically adapt to the reaction conditions: through surface chlorination, the the Obr-H bond is weakened. This is very important because the strength of the Obr-H bond strongly determines the OHbr + Oot/Obr + OHot equilibrium. Shifting this equilibrium toward OHot promotes H2O formation. From a spatially resolved simulation analysis it can be concluded that the optimal bridge chlorination degree for H2O formation is 1/3.The apparent activation energy is overestimated by the KMC model, which can be traced back to shortcomings of the description of the reactor model. From conversion-dependent calculations it was estimated that the correct apparent activation energy should be obtainable by properly accounting for different conversion levels at different temperatures.