Comparison of the Surface and Subsurface Oxygen Reactivity and Dynamics with CO Adsorbed on Rh(111)

The translational and angular intensity dependence of CO2 produced from the reaction of CO with an initial oxygen coverage of 0.5 ML <= Theta(O) <= 1.6 ML on Rh(111) at a surface temperature of 450 K was measured. This range of coverages goes from only surface absorbed O to both adsorbed and selvedge absorbed O. The measured CO2 translational energy is independent of Theta(O) and is larger than steady-state experiments [Colonell et al. J. Chem. Phys. 1995, 103, 6677], which were done at lower O coverages, but follows the trend that higher Theta(O) leads to faster CO2. The angular dependence of the intensity is well-described by a sharply peaked function of the form I = a cos(n)(theta) + (1 - a) cos(theta), with a = 0.6 and n = 8.6. Again, this was consistent with the previous steady-state results. These observations point to a similar reaction geometry regardless of Theta(O); the measured dynamical parameters are independent of whether or not some of the O is subsurface and suggest that absorbed oxygen in the selvedge acts only as a reservoir for replenishing reagent as it is consumed.