7822
J. Phys. Chem. C 2010, 114, 7822–7830
Poisoning of Pt3Co Electrodes: A Combined Experimental and DFT Study D. Pillay,*,† M. D. Johannes,† Y. Garsany,‡ and K. E. Swider-Lyons‡ Center for Computational Material Science, Code 6393, and AlternatiVe Energy Section, Chemistry DiVision, Code 6113 NaVal Research Laboratory, Washington, DC 20375 ReceiVed: July 17, 2009; ReVised Manuscript ReceiVed: December 15, 2009
Density functional theory calculations and rotating ring disk electrode experiments were performed to investigate the poisoning effects of sulfur species on the catalytic properties of elemental Pt and Pt3Co alloy surfaces. Experimental data indicates that there is a positive shift in the oxidation overpotential of Pt3Co accompanied by less oxidation/reduction cycles necessary in rotating ring disk electrode experiments (RRDE) in order to remove most of the sulfur species. Our theoretical calculations suggest that OH clustering is substantially reduced on the Pt3C(111) surface irrespective of the presence of Co atoms versus Pt(111). While the presence of Co does enhance adsorption of electronegative atoms/molecules on neighboring Pt sites, once Co atoms are oxidized or a Co-S bond is formed, they serve as a pin for the poison and subsequently reduce bonding of additional electronegative atoms/molecules at nearby sites. Additionally, our calculations indicate that a combination of effects due to less Pt3Co surface oxidation, more weakly adsorbed S species, and lower reaction barriers for SO2 oxidation on Pt3Co versus Pt subsequently leads to easier cleaning of the surface. 1. Introduction Tailoring the unique catalytic properties of metal alloys has recently become an area of intense research in the fuel cell community.1,2 Fuel cells often operate in nonideal environments that contain contaminants which can cause catalysts to become inactive over time. It is therefore important to understand not only how the catalytic reaction proceeds on various surface types, but also how contaminants affect these reactions and whether alloying is beneficial or detrimental for suppressing the poisoning aspects of harmful accidental adsorbates. One prevalent and damaging species of contaminant is sulfur, found in hydrocarbon fuels, in vulcan carbon which is often used as a support for catalyst surfaces/particles, and in air.3-7 The interaction between sulfur and the reactants necessary for catalysis to occur varies depending on the chemical composition of the catalyst surface, suggesting that alloys with optimal sulfur tolerance could be developed. It is already known that the oxygen reduction reaction (ORR), which proceeds as O2 + 4H+ + 4e- f 2H2O, is the major barrier in the efficiency of polymer electrolyte membrane fuel cells (PEMFC) and that alloying traditional Pt catalysts with 3d transition metals, such as Ni or Co, raises the activity toward the ORR.8-10 This result is consistent even though varying synthesis methodologies and environments can produce differing surface compositions.8 Intriguingly, even for surfaces containing transition metal atoms, less hydroxyl and bisulfate anions are adsorbed on the Pt sites of the alloys, Pt3Co or Pt3Ni, than on pure Pt8 which may indicate that detrimental oxidation as well as sulfur poisoning are simultaneously decreased for alloys. Sulfur contaminants are present at different oxidation states within the PEMFC and the particular oxidation state of an adsorbate is largely dependent on the operation voltage of the fuel cell. In rotating ring disk electrode experiments (RRDE), * Corresponding author. E-mail:
[email protected]. Tel.: (202)404-4404. Fax: (202)-404-7546. † Center for Computational Material Science. ‡ Alternative Energy Section.
10.1021/jp906778k
a sulfur coverage as low as 1.2% on active Pt sites reduces the mass activity toward the ORR by 33%.11 There is some controversy surrounding the exact structure and composition of Pt3Co nanoparticle catalysts in the electrochemical environment. A disadvantage of these alloy catalysts is that many of the first row transition metals dissolve in the acidic environment and can damage the membrane.13,14 Indeed, it has been found that for thin film electrodes formed of Pt-Ni, Pt-Co, and Pt-Fe there are no 3d transition metal atoms on the surface indicating that the leaching out process has occurred.15 For instance, cyclovoltammetry data indicates that the number of catalytically active Pt surface atoms vary in PtNi and PtCo catalysts. Approximately 70% and 58% of the atoms on the surface of PtNi and PtCo, respectively, are Pt, indicating that Ni is more likely to dissolve. Additionally, EXAFS studies by Teliska et al.16 confirm that Pt-Ni and Pt-Co nanoparticles have 3d transition metal atoms on the surface and the activity between the two are also likely to differ due to variations in morphology. The presence of Co atoms on the surface of PtxCo has been further substantiated by other groups;17,8,18 however, the controversy over whether or not this atom exists on the catalyst surface is yet to be fully resolved. In this work, we investigate S and OH adsorption on alloyed Pt3Co and unalloyed Pt using a combination of experimental and theoretical methods. We experimentally evaluate the tolerance of Pt and Pt3Co nanoparticles supported on vulcan carbon (VC) toward sulfur species using a combination of RRDE experiments and cyclic voltammetry. Catalyst electrodes were poisoned in an O2-free electrolyte with SO2 at 0.65 V and then transferred to a clean electrolyte for evaluation. With this electrochemical method, we can estimate sulfur coverage as a function of sulfur concentration in the initial solution and discern its impact on the ORR kinetics and the completeness of the 4-electron reaction. We also remove the sulfur from the Pt catalyst surface through its oxidation to a water-soluble sulfate at hcp > bridge > atop. 3.3. Correspondence with Experiment. It is clear from our calculations that the Pt-skin alloy surface has overall weaker adsorptive properties than a pure Pt(111) surface for both OH and S adatoms. The Ll2 surface, on the other hand, shows stronger adsorption than Pt(111) at all sites for OH due to active participation of Co surface atoms in bonding even at sites solely formed by Pt atoms. S adsorption on Ll2 surfaces is stronger than on the Pt(111) surface at all atop sites, the B3 site, and all
J. Phys. Chem. C, Vol. 114, No. 17, 2010 7827
Figure 10. OH adsorption energy as a function of coverage on Pt(111), Ll2, and Pt-skin surfaces.
other sites in which S directly bonds to Co. This seems hard to reconcile with the experimental observation of a decrease in bisulfate anion adsorption and an accompanying upward shift in reversible potential for forming OH on both the Ll2 and Ptskin surfaces.8 We find that by understanding the interaction between coadsorbed OH molecules and between OH and S we can form a consistent explanation. We find that two operative mechanisms are at work: the first is “clustering” of OH due to lateral interaction and the second is protective oxidation. We discuss both of these below. Clustering occurs due to hydrogen bonding between coadsorbed OH adatoms. In general the bonding strength of an OH adsorbate remains approximately constant at the atop and bridge sites with uniform coverage beneath (
