State-resolved reactivity of CH4 on Pt(110)-(1 x 2): The role of surface orientation and impact site

The reactivity of methane (CH4) on Pt(110)-(1×2) has been studied by quantum state-resolved surface reactivity measurements. Ground state reaction probabilities, S0(v=0) ~ S0(laser-off), as well as state-resolved reaction probabilities S0(2v3), for CH4 excited to the first overtone of the antisymmetric C-H stretch (2v3) have been measured at incident translational energies in the range of 4 – 64 kJ/mol. We observe S0(2v3) to be up to 3 orders of magnitude higher than S0(v=0), demonstrating significant vibrational activation of CH4 dissociation on Pt(110)-(1×2) by 2v3 excitation. Furthermore, we have explored the azimuthal and polar incident angle dependence of S0(2v3) and S0(v=0) for a fixed incident translational energy Et = 32 kJ/mol. For incidence perpendicular to the missing row direction on Pt(110)-(1×2) and polar angles theta > 40°, shadowing effects prevent the incident CH4 molecules from impinging into the trough sites. Comparison of this polar angle dependence with reactivity data for incidence parallel to the missing rows yields state-resolved site specific reactivity information consistent with a Pt(110)-(1×2) reactivity that is dominated by top layer Pt atoms located at the ridge sites. A comparison of S0(v=0) measured on Pt(110)-(1×2) and Pt(111) yields a lower average barrier for Pt(110)-(1×2) by 13.7 +/- 2.0 kJ/mol.