Theoretical investigations on the potential-induced formation of Pt-oxide surfaces

Abstract

Using the extended ab initio atomistic thermodynamics approach together with density functional theory calculations the interfacial structure and composition of Pt-electrodes in electrochemical environments at elevated electrode potentials was studied. Focusing on the electrode potential region, at which the oxide-formation occurs, the bulk systems and all lowindex surfaces of α-PtO 2 , β-PtO 2 , and PtO bulk-oxides were calculated. On the basis of the bulk-oxide formation energies we first deduced the stability ranges at which the bulk-oxides are the thermodynamically stable phases. In agreement with experimental observations, we find that at experimental temperature and pressure conditions α-PtO 2 and β-PtO 2 bulk-oxides are stable above 1.2 V, while PtO requires ∆φ >1.3 V. Afterwards the corresponding (p,T,φ)-phase diagrams of surface structures were obtained, showing a preference for α-PtO 2 (001), β-PtO 2 (110), and PtO(100) respectively, having bulk-like compositions even on their surfaces. However, in case of thin oxide layers a PtO composition might also be present.

<i>In</i>-<i>Situ</i> XAFS Characterization of PtPd Nanoparticles Synthesized by Galvanic Replacement

“…As already stated, above 0.85 V, the XAFS in-situ measurements suggest the presence of Pt-O bonds. Up to 1.00 V, the amount of oxide is increasing accompanied by a high degree of surface oxidation [30], the relative concentration of the oxide determined using the Varimax method for our experimental data analysis is close to 1 while it is 0 for the metal.…”

Electrochemical surface faceting of

“…As described in Refs. [30,31] an exact evaluation of the interfacial free energies is in principle possible, but requires a self-consistent modeling of the entire interfacial region, which might range up to several 100 Å. Since this is beyond nowadays capabilities of ab initio approaches, we reduce our model to the electrode and the adlayer only and assume a constant influence of the electrolyte, allowing us to neglect its presence when studying relative stabilities only.…”

“…[30] for further details). Of course, a similar expression can be obtained for the nitrogen-induced facet formations, where each N 3− ion originates from ammonia as electrolyte.…”

Ab Initio Atomistic Thermodynamics for Fuel Cell Catalysis

“…The expression in the square brackets is exactly the formation energy of the Pt bulk oxide out of bulk platinum and molecular O 2 , which can easily be evaluated by DFT calculations [Jacob, 2007b]. Together with the experimental value for DG H 2 O O 2 , we can finally formulate the following stability ranges at which the three known Pt bulk oxides are thermodynamically stable: † a-PtO 2 : þ1:74 eV À 2eDf , Dm H 2 O , 0 eV † b-PtO 2 : þ1:74 eV À 2eDf , Dm H 2 O , 0 eV † PtO: þ1:97 eV À 2eDf , Dm H 2 O , 0 eV (5:26)…”

“…In order to evaluate the structure of the electrode surface under high positive electrode potentials, we used the calculated DF T energies for all different slabs of low index Pt bulk oxide surfaces [Jacob, 2007b], together with (5.31), to obtain the corresponding (T, a, Df) phase diagrams for the two very stable Pt bulk oxides, a-PtO 2 and b-PtO 2 . Similar to Fig.…”