Abstract
This paper examines the thermodynamics of PtO2 stripes formed as intermediates of Pt(111) surface oxidation as a function of the degree of dilation parallel to the stripes, using density functional theory and atomistic thermodynamics. Internal energy calculations predict 7/8 and 8/9 stripe structures to dominate at standard temperature and pressure, which contain 7 or 8 elevated PtO2 units per 8 or 9 supporting surface Pt atoms, respectively. Moreover, we found a thermodynamic optimum with respect to mean in-stripe Pt-Pt spacing close to that of α-PtO2. Vibrational zero point energies, including bulk layer contributions, make a small but significant contribution to the stripe free energies, leading to the 6/7 stripe being most stable, although the 7/8 structure is still close in free energy. These findings correspond closely to experimental observations, providing insight into the driving force for oxide stripe formation and structure as the initial intermediate of platinum surface oxidation, and aiding our understanding of platinum catalysts and surface roughening under oxidative conditions.
Highlights
Surface oxidation, which happens both in gas-phase catalysis and in electrochemical environments, impacts profoundly the selectivity, activity and stability ofcatalysts.[1,2] A commonly-used electrocatalyst material, platinum, has long been known to undergo surface oxidation during catalyst operation.[1,2,3,4] Whether as an electrocatalyst or as a gas-phase heterogeneous catalyst, the metal surface is exposed to oxygen species, which may lead to surface reconstruction, or in the case of aqueous media, dissolution.[5]
At low oxygen coverages generated under ultrahigh vacuum (UHV) conditions, Devarajan et al observed this on-surface adsorption of oxygen in fcc hollow sites using temperature programmed desorption and scanning-tunneling microscopy (STM) experiments.[12]
Density functional theory (DFT) calculations by Hawkins et al and force field simulations by Farkas et al corroborated part of these findings by finding stripe segments at coverages equivalent to 0.75 ML and stable PtO2 stripes for 1 ML coverage; all these structures were supported by subsurface oxygen.[13,14]
Summary
Surface oxidation, which happens both in gas-phase catalysis and in electrochemical environments, impacts profoundly the selectivity, activity and stability of (electro)catalysts.[1,2] A commonly-used electrocatalyst material, platinum, has long been known to undergo surface oxidation during catalyst operation.[1,2,3,4] Whether as an electrocatalyst or as a gas-phase heterogeneous catalyst, the metal surface is exposed to oxygen species, which may lead to surface reconstruction, or in the case of aqueous media, dissolution.[5]. Platinum atoms are expelled from the pristine surface domain on which these stripes are formed, an effect not taken into account by Hawkins et al.[13] This may in turn aid the formation of serpentine islands otherwise attributed to adsorbate-induced strain by Van Spronsen et al Most strikingly, Van Spronsen et al.’s results indicate that subsurface oxygen is not necessary to stabilize PtO2 surface structures under (near-)standard oxidizing conditions. Apart from their role in gas-phase Pt oxidation, PtO2 stripes may be first intermediates in the electrochemical oxidation of Pt electrodes. We compare various stripe PtO2/Pt(displaced) ratios, calculate their energies as a function of oxygen chemical potential and electrochemical potential, and discuss their similarity to the ordering of the spokewheel segments, and how these energies are related to PtO2 geometries and bulk platinum properties
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