(469d) A First Principles Study of the Poisoning Effects of Sulfur on Pt3Co(111)

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.