COMPUTATIONAL STUDY OF SURFACE-SEGREGATED PT ALLOY CATALYSTS FOR OXYGEN REDUCTION REACTION

Abstract

In this thesis two research objectives have been accomplished using computational simulation techniques. (1) The surface segregation phenomena in the surfaces of (111), unreconstructed (110) and reconstructed (1x2) missing row (110) surfaces of Pt-Ni and Pt-Co disordered alloys have been accurately predicted using Monte Carlo (MC) simulation method, and (2) the configuration and energy of the adsorption of O, O2, OH, and H2O molecules which are presented in oxygen reduction reaction (ORR) on the surface of pure Pt and surface-segregated Pt-binary alloys (i.e., Pt-Ni, Pt-Co and Pt-Fe) have been determined using density functional theory (DFT) calculations. This thesis yields some guiding principles for designing novel catalysts for proton exchange membrane fuel cells.

The Pt concentration profiles of the surfaces of Pt-Ni and Pt-Co alloys were attained from the MC simulations in which the system energy was evaluated through the developed modified embedded atom method (MEAM) for Pt-Ni and Pt-Co alloys. It was found from our simulations that the Pt atoms strongly segregate to the outermost layer and the Ni atoms segregate to the second sub-layer in the (111) surface of both Pt-Ni and Pt-Co alloys. When Pt concentration is higher than 75 at.%, pure Pt top layer could be formed in the outermost layer (111) surface of both alloys. Moreover, segregation reversal phenomenon (Ni atoms segregating to the outermost layer while Pt atoms to the second sub-layer) was observed in our MC simulations of unreconstructed (110) surface of Pt-Ni alloys. In contrast, a Pt enriched outermost surface layer was found in a Pt-Ni reconstructed (1x2) missing row (110) surface. Our MC simulation results agree well with published experimental observations.

In addition, adsorption of atomic and molecular oxygen, water and hydroxyl on the (111) and (100) surfaces of pure Pt and Pt-based alloys (Pt-Ni, Pt-Co and Pt-Fe) were studied using spin DFT method and assuming a coverage of 0.25 monolayer. Both the optimized configurations and the corresponding adsorption energies for each species were obtained in this study. In particular, we elucidated the influence of the adsorption energies of atomic oxygen and OH on the activity for ORR on Pt binary alloy catalysts in acidic environment. The calculated adsorption energies of atomic oxygen on the (111) surfaces of pure Pt, Pt-Ni, Pt-Co and Pt-Fe are -3.967 eV, -3.502 eV, -3.378 eV and -3.191 eV, respectively. The calculated adsorption energies of hydroxyl on the (111) surfaces of pure Pt, Pt-Ni, Pt-Co and Pt-Fe are -2.384 eV, -2.153 eV, -2.217 eV and -2.098 eV, respectively. The interaction between the adsorbed atomic and hydroxyl and the corresponding (111) surface becomes weaker for the surface-segregated alloys compared to pure Pt catalyst. The same results were obtained for the (100) surfaces.