Sulfur contaminants play a significant role in modifying the performance of transition metal fuel cell cathodes. Sulfur blocks relevant reactants from adsorbing on the cathode side of the fuel cell thereby reducing the oxidation reduction reaction (ORR). The ORR proceeds as 2H⁺ + 2e^- + ½ O₂  ->  H₂O. The most common catalyst being used in polymer electrolyte fuel cell cathodes is Pt. Recently however it has been found that using Pt alloys such as Pt₃Ni and Pt₃Co can increase the fuel cell cathode efficiency versus the pure Pt catalyst. These catalysts increase the overall fuel cell efficiency by increasing reaction rate of the ORR.

Despite the rate increase by using alternative Pt alloys these systems are still subject to sulfur contaminant exposure. Indeed, experimental and theoretical works have investigated a variety of ways to remove sulfur. The most common is hydrogenation of S to H₂S. Studies have found however that sulfur is extremely resistant to hydrogenation on a variety of transition metal surfaces. More specific to an alloy replacement for Pt a recent theoretical study investigated the hydrogenation process of S on a Pt₃Ni(111) surface. This study found that although the barriers for hydrogenation are decreased versus Pt(111) they were still significantly high and were unlikely to occur around room temperature. Similar to Pt₃Ni(111), Pt₃Co(111) surfaces have shown a higher resistance to sulfur poisons than Pt(111).  A variety of surfaces with varying efficiencies towards the ORR can exist for Pt₃Co(111) depending on preparation conditions. If the Pt₃Co(111) sample is annealed this results in a "Pt-skin" (a layer of pure Pt atoms) on top of a subsurface enriched with Co. However if the sample is not annealed it has a Ll₂-Cu₃Au type structure where Co atoms are present on the surface.

Here we compare the interaction between OH and S on both Pt₃Co(111) with and without the "Pt skin" as well as the Pt(111) surface. We find that on the Pt₃Co(111)-Ll₂ surface that those sites closest to Co surface atoms are the most favorable for OH and S adsorption. However, once these sites are "poisoned" additional adsorption of S species is significantly weakened. These results agree well with experimental observations that find initially S species are easier to remove from Pt₃Co(111) versus Pt(111) surface. In addition to adsorption properties of OH and S we also investigate the geometric and electronic properties of the Pt₃Co(111) with and without the Pt-skin as well as the effect of S interaction with OH adsorption.