Donghai Mei

First-principles-based kinetic Monte Carlo simulation of nitric oxide decomposition over Pt and Rh surfaces under lean-burn conditions

First-principles-based kinetic Monte Carlo simulation was used to track the elementary surface transformations involved in the catalytic decomposition of NO over Pt(100) and Rh(100) surfaces under lean-burn operating conditions. Density functional theory (DFT) calculations were carried out to establish the structure and energetics for all reactants, intermediates and products over Pt(100) and Rh(100). Lateral interactions which arise from neighbouring adsorbates were calculated by examining changes in the binding energies as a function of coverage and different coadsorbed configurations. These data were fitted to a bond order conservation (BOC) model which is subsequently used to establish the effects of coverage within the simulation. The intrinsic activation barriers for all the elementary reaction steps in the proposed mechanism of NO reduction over Pt(100) were calculated by using DFT. These values are corrected for coverage effects by using the parametrized BOC model internally within the simulation. This enables a site-explicit kinetic Monte Carlo simulation that can follow the kinetics of NO decomposition over Pt(100) and Rh(100) in the presence of excess oxygen. The simulations are used here to model various experimental protocols including temperature programmed desorption as well as batch catalytic kinetics. The simulation results for the temperature programmed desorption and decomposition of NO over Pt(100) and Rh(100) under vacuum condition were found to be in very good agreement with experimental results. NO decomposition is strongly tied to the temporal number of sites that remain vacant. Experimental results show that Pt is active in the catalytic reaction of NO into N2 and NO2 under lean-burn conditions. The simulated reaction orders for NO and O2 were found to be +0.9 and −0.4 at 723u2009K, respectively. The simulation also indicates that there is no activity over Rh(100) since the surface becomes poisoned by oxygen.

Effects of Alloying Pd and Au on the Hydrogenation of Ethylene: An ab initio-Based Dynamic Monte Carlo Study

An ab initio-based dynamic Monte Carlo simulation was developed and used to examine the kinetics of ethylene hydrogenation over Pd and PdAu alloys. The intrinsic activation barriers, overall reaction energies and chemisorption energies were calculated from first-principles density functional theoretical calculations. Lateral interactions were modeled by fitting ab initio data to semi-empirical bond order conservation and force field models. The results indicate that the intrinsic activation barriers for ethylene hydrogenation were considerably reduced from 15 to 7-8 kcal/mol due to the intermolecular interactions that take place on the surface at higher coverages. At higher temperatures or lower partial pressures of hydrogen, ethylene decomposition paths to the formation of ethylidyne become important. Alloying the surface with Au influences the intrinsic kinetics for hydrogenation by reducing the activation barrier for hydrogenation but increasing the barriers for H2 dissociation and ethylidyne formation. This is primarily due to geometric effects that result from alloying. Electronic effects, while present, are significantly smaller. Despite its influence on specific elementary steps, Au appears to have little effect on the calculated turnover frequencies for ethane formation. There are relatively minor increases in the activation barrier from 7.0 to 7.2 to 8.0 as we move from Pd(111) to Pd 87.5% Au 12.5% to Pd 66.7% Au 33.3% respectively. The qualitative effects of Au as well as the quantitative apparent activation barriers reported here are consistent with known experimental results. Au reduces the binding energy of ethylene, which increases the surface hydrogenation activity. However, Au also reduces the number of sites that can activate hydrogen. This reduces the hydrogen surface coverage and subsequently decreases the rate of ethylene hydrogenation. These effects (the weaker metal--adsorbate bonds and the decreased hydrogen surface coverage) balance each other out whereby the addition of Au shows little effect on the simulated turnover frequency on a per Pd atom basis. The primary influence of Au therefore is to decrease the ethylene decomposition paths that lead to ethylidyne and CHx products.

From First-Principles to Catalytic Turnovers: Ethylene Hydrogenation Over Palladium

Abstract We present a first-principles-based dynamic Monte Carlo method which can be used to model the kinetics of metal catalyzed reaction systems by following the explicit atomic surface structure and individual molecular transformations. The approach uses first-principle density functional quantum chemical calculations to build a comprehensive database of adsorption energies, overall reaction energies, activation barriers, and intermolecular interaction energies. The ab initio calculated lateral interaction energies were subsequently used to develop more approximate but universal interaction models that could be used in-situ in the MC. A radial function model and a bond order conservation model were both developed. The simulation algorithm was used herein to examine ethylene hydrogenation over palladium. The results indicate that it is the repulsive interactions in the adlayer that weaken the metal-carbon and metal-hydrogen bonds thus lowering the barriers for hydrogenation from 15 for ethylene to ethyl and 14.5 for ethyl to ethane to 8.5 and 8.0xa0kcal/mol for the same steps taken at higher surface coverages. The simulation results provide a very good match against known experiment results. The simulation was subsequently used to examine both the effects of alloying and surface structure. The addition of gold decreased the overall turnover number simply because the number of sites was reduced. On a per palladium basis, however, the activity remains approximately the same. The addition of gold indirectly leads to less hydrogen on the surface since it shuts down H 2 dissociation steps. This, however, is countered by a reduction in the metal-hydrogen bond strength which helps to enhance the activity. These two features tend to balance one another out as the turnover frequencies remain nearly constant. We provide a simple cursory look at the effects of surface structure by examining the changes in the kinetics over Pd(100) and Pd(111) surfaces. The barriers over these two surfaces are 7.1 and 6.4xa0kcal/mol respectively suggesting that the chemistry is relatively structure insensitive.