A first principles analysis of CO oxidation over Pt and Pt _66.7%Ru_33.3% (111) surfaces

Nonlocal gradient corrected DFT slab calculations were carried out to determine the overall reaction energies along with the barriers for the activation of water and the oxidation of CO over well-defined Pt(111) and PtRu(111) surfaces. We attempt to elucidate the features that control the bifunctional mechanism proposed for the oxidation of CO in solution. The addition of Ru to Pt along with the presence of solution helps to enhance the elementary steps that comprise the bifunctional mechanism. The activation of water over Pt(111) in the vapor phase is energetically unfavorable with an activation barrier of +142 kJ/mol and overall heat of reaction of +53 kJ/mol. Water will desorb before it will react on Pt(111). The presence of solution reduces the barrier to about 75 kJ/mol. The addition of Ru lowers the barrier for water activation in the vapor phase. Alloying Pt with Ru lowers the barriers for the homolytic as well as the heterolytic activation of water in solution. The presence of solution, however markedly favors the heterolytic activation which leads to the formation of an adsorbed OH* surface intermediate, a proton which migrates into solution and an electron. The energetics for this path over the Pt_66.7%Ru_33.3%(111) surface were calculated to be quite favorable with an activation barrier of +27 kJ/mol and an overall energy of reaction of +26 kJ/mol. Ab initio molecular dynamics results indicate that water can be activated over PtRu at 300 K. The resulting surface Ru-OH group that forms induces the adsorption and subsequent activation of water at a neighboring Pt site. The surface hydroxyl intermediate continues to diffuse across the surface via a sequence of proton transfer steps. The barrier for the subsequent oxidation of adsorbed CO by surface OH groups over Pt(111) in the vapor phase is 86 kJ/mol with an overall reaction energy of 21 kJ/mol. The barrier is reduced to 71 kJ/mol when carried out over the Pt _66.7%Ru_33.3% alloy in the vapor phase. The barrier for the homolytic disproportionation of CO and OH in solution (over Pt _66.7%Ru_33.3%) is 90 kJ/mol with an overall reaction energy of +24 kJ/mol. The heterolytic path which involves the oxidation of CO by OH to form CO₂+H⁺(aq)+e- is more favorable than the homolytic path. The activation barrier for the heterolytic path over Pt _66.7%Ru_33.3% is +60 kJ/mol with an overall reaction of -6 kJ/mol (exothermic). These results indicate that at potentials which are less than or equal to the potential of zero total charge, water activation over Pt(111) may be difficult. CO oxidation, however is more likely limiting over the Pt_66.7%Ru_33.3% alloy at these potentials. These results only hold for the ideal conditions studied herein including ideal single crystal surfaces, an ideal liquid water solution phase and the absence of an applied potential.