In Silico Search For Alloy/Metal Catalysts For Methane Reforming

The development of catalysis is of fatal importance to modern and future society. It is the foundation of modern chemical industry. Raw materials such as petroleum and natural gas can be readily converted to chemicals by catalysis, supplying daily needs of transportation, food production and medical treatment for the community. Nowadays, the development of catalytic science has become the main power for the improvement of commercialized chemical processes, and it will be, in the future, the base stone of changing energy infrastructure, utilizing sustainable energy sources such as solar, wind and biomass. Some existing technology problems are also closely related to the catalysis science, e.g., energy saving and consumption reduction requirement of hydrogen production in the hydrocarbon steam reforming process for ammonia synthesis; Ni-based catalyst deactivation in the methane dry reforming process for high CO/H2synthesis gas production as the feedstock for further Fisher-Tropsh synthesis. However, due to the complexity of catalytic reaction process, the development of active, selective and stable catalysts still faces many scientific challenges.In order to design catalysts rationally, the fundamental understanding of interactions between adsorbates, catalyst surfaces, and the transition states of the elementary reactions are essential. Recent advances in computational chemistry, particularly density functional theory (DFT) made it possible to investigate reactions "in silicd". In this paper, a method based on DFT for screening for active, selective, stable and inexpensive catalyst for a large range of heterogeneous catalyst is presented. Steam and dry reforming of methane reactions were used as two model reactions to demonstrate this method.The main content of this thesis is as follows:(1) Using Ni catalyst as an example, surface structure sensitivity for methane decomposition was investigated on close packed Ni(111) and stepped Ni(211) facets. The adsorption energies of related adsorbates, the activation energies for all the elementary reactions of methane decomposition were calculated. In order to study the effect of second metal on the catalytic properties of the surface alloy, Ni(111)and Ni(211) surfaces with different silver coverages (1/4monolayer,1/9monolayer) were constructed. The energetics of the intermediates for methane steam reforming were calculated and compared with the ones on pure Ni surfaces. Coking mechanism wasinvestigated by comparing the activation barrier for C-0and C-C on Ni and Ag/Ni systems. Equilibrium shape of the catalyst crystalline was estimated by employing Wulff construction. Proportion of each surface was estimated and the effect of introduced silver was discussed.(2) Scaling relations between adsorption energies of the adsorbates and the adsorption energies of carbon and oxygen were investigated. Scaling relations between transition state energies and final state energies were also studied in both methane steam and dry reforming systems. Then the scaling relations were incorporated to a microkinetic model, which results in a two descriptor based volcano shaped rate plot. The volcano plot was further used as the check board for screening for active and selective alloy catalysts.(3) Adsorbate-adsorbate interactions were investigated on stepped (211) surfaces premise to mean field theory. Cross interaction parameters between different adsorbates and different adsorption sites were parameterized in a piecewise adsorbate-adsorbate interaction model. The adsorbate-adsorbate model was further incorporated in a microkinetic model to calculate differential adsorption energies to obtain reasonable coverages for the steam and dry methane reforming systems.(4) Descriptor-based volcano rate plots for methane steam and dry reforming were constructed that combines a microkinetic model with scaling relations while taking adsorbate-adsorbate interactions into consideration.(5) Using rates obtained by the comprehensive microkinetic model for different reaction conditions, screen thousands of transition metal and alloys by using carbon and oxygen as the two descriptors. Combing filters that consider stability and cost enables the screening for novel leads for active, stable, and inexpensive alloy catalysts.