Catalytic Oxidation Activity of Pt₃O₄ Surfaces and Thin Films

Abstract

The catalytic oxidation activity of platinum particles in automobile catalysts is thought to originate from the presence of highly reactive superficial oxide phases which form under oxygen-rich reaction conditions. Here we study the thermodynamic stability of platinum oxide surfaces and thin films and their reactivities toward oxidation of carbon compounds by means of first-principles atomistic thermodynamics calculations and molecular dynamics simulations based on density functional theory. On the Pt(111) surface the most stable superficial oxide phase is found to be a thin layer of alpha-PtO2, which appears not to be reactive toward either methane dissociation or carbon monoxide oxidation. A PtO-like structure is most stable on the Pt(100) surface at oxygen coverages of one monolayer, while the formation of a coherent and stress-free Pt3O4 film is favored at higher coverages. Bulk Pt3O4 is found to be thermodynamically stable in a region around 900 K at atmospheric pressure. The computed net driving force for the dissociation of methane on the Pt3O4(100) surface is much larger than that on all other metallic and oxide surfaces investigated. Moreover, the enthalpy barrier for the adsorption of CO molecules on oxygen atoms of this surface is as low as 0.34 eV, and desorption of CO2 is observed to occur without any appreciable energy barrier in molecular dynamics simulations. These results, combined, indicate a high catalytic oxidation activity of Pt3O4 phases that can be relevant in the contexts of Pt-based automobile catalysts and gas sensors.

Self-limited growth of triangular PtO2nanoclusters on the Pt(111) surface

“…The islands have the α-PtO 2 (0001) overlayer structure formed by an O-Pt-O trilayer. The triangular nanoislands on the upper and lower terraces represent two mirror domains with the mirror plane perpendicular to the substrate and aligned along the [1][2][3][4][5][6][7][8][9][10] direction of the latter. Furthermore, these domains share a continuous oxygen layer formed by the lower oxygen layer of the upper terrace island and the upper oxygen layer of the lower terrace island.…”

“…PtO 2 nanoislands nucleated at the substrate step edges have a triangular shape similar to those shown in Figure 2. It is evident from Figure 4a that the Pt oxide islands produce a faceting of the Pt(111) substrate steps when the latter are not aligned along the [1][2][3][4][5][6][7][8][9][10] directions favoured for the oxide growth. In this case the substrate step edge has a zig-zag shape after formation of the oxide nanoclusters.…”

“…In this case the substrate step edge has a zig-zag shape after formation of the oxide nanoclusters. Only those regions of the step edge, which are parallel to the [1][2][3][4][5][6][7][8][9][10] Figure 4c the apparent height of the first oxygen layer was measured to be 1.0 Å and is therefore attributed to a chemisorbed oxygen layer on the surface. In turn, completed PtO 2 nanoislands have the apparent height of 3.2 Å, which further confirms the O-Pt-O trilayer structure of the oxide clusters.…”

Epitaxial oxide bilayer on Pt (001) nanofacets

“…18 Following the procedure outlined by Reuter and Scheffler, 18 the oxygen chemical potential at T and P O2 is simply related to the chemical potential at T and atmospheric pressure, with data for enthalpy and entropy of the oxygen gas obtained from the thermochemical tables. 19 Motivated by the theoretical results reported by Seriani et al, 1 we investigated the effect of strain for PtO-like structures that best describe our x-ray measurements. We adjusted the lattice parameter, starting with 3.98 Å, in increments (0%, 3%, 6%, and 9% along the [110] long nanofacet direction and 0%, −2%, and −4% along [110]) to fit the PtO lattice parameters.…”

“…Despite being active for forming and breaking chemical bonds, the platinum surface is not easy to oxidize. Yet, according to recent theoretical studies 1 platinum should preferentially oxidize below 1000 K, which implies a need for a protective surface layer. Neither submonolayer oxygen adsorption 2,3 nor loosely formed oxide layers 4 reported in studies at high oxygen pressure seem sufficient to form proper protective kinetic barriers against oxidation, especially for high-index surfaces.…”