Hydrogen adsorption on model tungsten carbide surfaces

The potential of tungsten carbide to replace platinum as an anode electrocatalyst has attracted considerable attention due to its activity towards hydrogen oxidation, passivity in acidic electrolyte, and resistance to carbon monoxide poisoning. As a first step towards understanding the mechanisms underlying these qualities, we have performed density functional calculations on slab models of the tungsten carbide surface. We have calculated the adsorption energies of hydrogen on the different available bonding sites, and made a comparison to known values for platinum. The variation of bond strength with surface coverage has been investigated, and used to construct model surface Pourbaix diagrams. The nature and structure of the passivated surface is still an open question. It is possible that the tungsten dissolves out of the top-most surface layers, leaving a surface enriched in carbon. We have studied the effect of additional layers of carbon on the adsorption behaviour of hydrogen. While the most stable surface in its bare form provides binding sites that adsorb carbon with an energy of around 4.0 eV, about 60% higher than the adsorption energy of hydrogen on platinum, a carbonaceous tungsten carbide surface provides a maximum binding energy of 2.49–2.69 eV, much closer to the optimum value for efficient catalysis.