Density functional theory calculations were performed to investigate the poisoning effect of S on the electronic and chemical properties of Pt(111) and Pt3Ni(111) surfaces. We analyze the bonding mechanisms and energetics of S, OH, and H species separately and together to determine the effects of coadsorption and elucidate the long range and short-range sulfur poisoning effects. Although both catalysts consist entirely of Pt atoms on the surface layer, they have different potential energy surfaces due to the inhomogeneity of sublayer atoms in the alloy system. On the Pt3Ni(111) surface, the adsorption sites that allow interaction with mainly Pt sublayer atoms are more energetically favorable than those adsorption sites which are closer to Ni sublayer atoms. We find that OH bonding to the metal surfaces is electrostatic and ionic in nature, hydrogen bonding is predominantly ionic, and S bonds covalently. For both surfaces, we find that the adsorption energy decreases as follows: S > H > OH with all species bonding more strongly to the Pt(111) surface due to a higher d-band center. The difference in adsorption energy between the two surfaces is by far the strongest for S, suggesting that the alloyed surface may be more poison-resistant than the pure surface. Furthermore, we find that the decrease in adsorption energy for H and OH as a result of a nearby poison is less on the Pt3Ni(111) surface than for similar sites on the Pt(111) surface, again pointing to better poison tolerance. We show that a local d-band model cannot be used to predict ORR activity on poisoned sites, because it neglects the effects of electrostatic interaction between adsorbates and does not account for the decrease in available s and p states on shared Pt atoms, thereby strongly underestimating the range and strength of poisoning on the surface. An explicit calculation of the bonding strengths of coadsorbates shows that a single S adatom poisons both neighboring and next nearest neighboring OH and H adsorption sites, with diminishing but finite effects even farther away. Both the adsorption of sulfur and the poisoning effect of its presence are stronger on the Pt(111) surface, indicating that Pt3Ni(111) will have a higher rate of ORR in the presence of S impurities.
Read full abstract