STRUCTURE-PROPERTY RELATIONSHIPS IN ALLOY CATALYSTS AS DETERMINED USING AB-INITIO METHODS

The cluster expansion approach, with the training data calculated by density functional theory (DFT) calculations, is used to investigate the structure-property relationships of Pt3Ni alloy catalysts for oxygen reduction reaction (ORR) at atomic level so as to enable the rational design of catalysts with desired catalytic activity and stability by tailoring surface structures. Firstly, a cluster expansion built on extended surfaces is developed to construct a direct bridge between the atomic structure and ORR catalytic activity of Pt-Ni alloy catalysts at a variety of compositions and chemical environments. Thermodynamically stable Pt3Ni(111) extended surfaces are predicted to have substantial sub-surface disorder, and as a result the ORR activity of different surface sites varies by approximately three orders of magnitude. Using this model, I identify a Pt3Ni(111) surface with a multi-layer Pt skin that is predicted to maximize catalytic activity. Secondly, a cluster expansion built on nanoparticles has been developed to investigate the structure and stability of Pt-Ni and Pt-Ni-Mo nanoparticles in both vacuum and oxidizing conditions. For Pt-Ni nanoparticles, my calculations indicate that the preferential oxidation of edge sites makes (111) facet sites more vulnerable to dissolution than edge sites. Furthermore, I predict that for Mo-doped Pt3Ni nanoparticles with only a small amount of Mo doping (around 1.6% molar), the equilibrium concentration of Ni atoms on the particle surface is greatly reduced, limiting the rate at which Ni atoms dissolve from the particles. Mo doping also increases Pt/Ni vacancy formation energies in the surface layer, which further stabilizes the nanoparticles against Ni dissolution and may help preserve the nanoparticle shape. Finally, the thermodynamics of CO2/CO reduction on four different copper (Cu) facets, Cu(211), Cu(110), Cu(100) and Cu(111), are investigated by building the free energy diagrams for CO2/CO reduction to C1 and C2 products. For two possible reaction mechanisms of C-C coupling, CHO-CHO coupling and CO-CO coupling mechanisms, the Cu(110) facet is identified to be the surface with the lowest onset potential (about -0.30 V), which is in agreement with the most recent experimental observation of high catalytic activities and selectivity on highly dense Cu nanowires