Kinetic modeling and first principles study of the water -gas shift and methanation reaction on group VIII metal catalysts

Abhijit A Phatak, Purdue University

Abstract

In this research kinetic modeling and first principles study of the water-gas shift (WGS) and methanation reactions on group VIII metal catalysts were carried out in an effort to develop fundamental mechanistic understanding. These tools were combined with experimental studies for quantitative modeling of WGS over Pt/Al2O3 catalysts. Self-consistent, periodic slab density functional theory DFT-GGA calculations were used to investigate the low temperature WGS on the close packed surface (111) of five transition metals (Cu, Au, Pt, Pd and NO and based on these calculations thermodynamic potential energy surfaces were generated. Detailed minimum energy paths are computed for the elementary steps on these metal surfaces. Two main redox-type mechanisms were outlined and it is shown that on metals such as Cu, Pt, and Pd, the carboxyl mechanism provides a competitive reaction pathway for WGS. In the carboxyl pathway, water dissociation step has the highest activation energy barrier. Understanding this mechanism can have implications towards designing a better WGS catalyst. Using the DFT-derived parameters such as activation energies, binding strengths, and pre-exponential factors as initial guesses, a detailed mean-field kinetic model was developed. This model was employed to explain the experimental kinetic data on Pt/Al2O3 catalyst under conditions relevant to fuel reforming for fuel cell applications. Both the redox-type mechanisms can explain the experimental kinetic data quantitatively. For accurate kinetic modeling of experimental data, it is crucial to account for coverage-dependence of CO binding strength. Failure to account for this dependence resulted into prediction of CO coverage much different than found experimentally. Optimal estimates of kinetic parameters were used to predict the CO coverage under spectroscopic experimental conditions. Redox-type mechanisms can explain the FTIR data qualitatively. Detailed mechanisms of methanation steps were also studied using DFT on the most close-packed surfaces of Pt, Pd, and Ni. The thermochemistry and activation energies for the various elementary steps such as CO dissociation and subsequent hydrogenation of C to methane were studied. The experimental sequence for increasing methanation activity was confirmed to be Pt

Degree

Ph.D.

Advisors

Ribeiro, Purdue University.

Subject Area

Chemistry|Chemical engineering

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