(80d) Density Functional Theory Based Microkinetic Modeling of Ethane Total Oxidation Over Pt(111)

The catalytic total oxidation of hydrocarbons is used in various applications such as pollution abatement, portable power generation by combining with thermoelectrics. Though the reaction is well studied experimentally, the mechanistic features of the reaction are not so well understood and limit to power law models and, in few cases, Langmuir-Hinshelwood models. Initial C-H bond scission is the rate limiting step in most of the decomposition reactions involving hydrocarbons. In oxidation also the rate limiting step is C-H bond activation at relatively high oxygen partial pressures. However, the question about whether it is by oxidative dehydrogenation or by thermal dehydrogenation needs to be answered.

In this study we developed a density functional theory (DFT) based microkinetic model for ethane total oxidation over Pt(111) under fuel lean conditions and derived a plausible mechanistic steps involved in the reaction process. The energetics used in this work are obtained using SIESTA DFT code. The microkinetic model for ethanol total oxidation is formulated with a comprehensive list of elementary reactions involving (1) thermal dehydrogenation of C1 and C2 species, (2) oxidative dehydrogenation of C1 and C2 species with the assistance of O_ad and OH_ad (3) oxidation and (4) O-H cleavage of C1 and C2 species and (5) C-C cleavage of C2 species. Water gas shift reaction (from literature), water formation and adsorption/desorption reactions of reactants and products were also included. Overall, a total of 305 elementary reactions involving 68 species (54 adsorbed and 14 gaseous species) are present in the model. Adsorbate – adsorbate interactions for key species and adsorbate – transition state interactions for key reactions steps, estimated from DFT calculations, are included in the model. The model predicts the experimental reaction orders and the apparent activation energies very well. From reaction path analysis, some portion (>20%) of the reaction mechanism is found to proceed through O insertion to partially dehydrogenated C₂ hydrocarbons. Sensitivity analysis is utilized to identify the rate determining steps (RDS). The RDS is found to change with inclusion of O coverage effects.

In summary, this study provides a better understanding of the reaction mechanism of ethane total oxidation over Pt(111) and reiterates the importance of inclusion of O coverage effects to obtain correct RDS.