First-principles studies on Pt-doped fcc MoC catalysts for low-temperature water-gas shift reaction

Cao, Daofan / D.C. ORCID: 0000-0001-7236-6747 (2024). First-principles studies on Pt-doped fcc MoC catalysts for low-temperature water-gas shift reaction. University of Birmingham. Ph.D.

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Abstract

The role of hydrogen in future net-zero energy emissions is beyond doubt, both in the transport sector and stationary power generation. As the storage and transportation of hydrogen still present a huge challenge, methanol as a hydrogen carrier and portable hydrogen production have been proposed. However, the hydrogen produced by methanol through the catalytic reaction is inevitably mixed with carbon monoxide as a by-product. To ensure the purity of the hydrogen, the by-product carbon monoxide is usually controlled by the water-gas shift reaction. Thus, the development of water-gas shift catalysts at low temperatures is very important. Recently, the Pt atom doped molybdenum carbide with a face-centred cubic structure (M1/fcc MoC) has shown great catalytic potential for the low-temperature water-gas shift reaction. However, the Pt-doped fcc MoC catalyst exhibits different crystalline facets that show different physiochemical properties. More importantly, the surfaces can evolve to different surface phase structures under the H2/H2O atmosphere due to the thermodynamic stability. Therefore, the reaction mechanism of water-gas shift at different crystal faces and the phase structures of Pt1/fcc MoC remains controversial.
This work has therefore focused on systematic mechanistic studies on low-temperature water-gas shift reaction of Pt doped fcc MoC catalysts, considering the evolution of surface phase structure and different facets, based on first-principles methods including density functional theory and ab initio thermodynamics while different reaction pathways were evaluated through turnover frequencies (TOFs) calculated by the energetic span model (ESM) which is more appropriate for the catalytic cycles. The species, including OH*, O*, and H*, were all readily formed on the (001) surface according to the reaction rate constants, but the comprehensive investigation showed that the H2O*, OH*, and O* species tended to cover the (001) surface instead of H*, justifying the importance of the evolved surface in the water-gas shift reaction. Disregarding the evolution of the surface phase structure, the WGS reaction on the Pt1/(111)-Mo surface occurred mainly through the redox mechanism, whereas on the Pt1/(001) surface, it occurred mainly through the associative mechanism. However, considering the evolution of the surface phase structure, the predicted results for the WGS reaction were quite different. The Mo‒O monolayer phase enabled redox and H2O-assisted associative mechanisms to predominate on the Pt1/(111)-Mo surface, whereas the associative mechanism still predominated on the Pt1/(001) surface.
From a macro perspective, the original (111)-Mo surface cannot compete with the original or the evolved (001) surfaces in the WGS reaction. However, with the original (111)-Mo surface evolved to the thermodynamically more stable Mo‒O monolayer phase, catalytic efficiencies of the redox route and the H2O-assisted associative route could increase sharply to a level comparable with those of (001) surfaces. Thus, the (111)-Mo and (001) surfaces exhibit similar catalytic efficiencies for the WGS reaction, although through different surface phase structures and pathways.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):