The effect of iodine on the reactivity of H2CO formed in CH2 oxidation on Pd(1 0 0)

The formation of H2CO in the partial oxidation reaction of CH2 species and the adsorption of H2CO on clean and iodine covered Pd(1 0 0) were investigated by means of temperature-programmed desorption (TPD) and photoelectron spectroscopy (UPS). Adsorbed CH2 was produced by thermal dissociation of CH2I2. Besides the stabilization of parent molecules by coadsorbed oxygen, the CH2 fragments selectively oxidized by preadsorbed oxygen to H2CO in a reaction-limited process. The amount of formaldehyde saturates at θO=0.2, while the total oxidation products (CO2, H2O) monotonously increased with the increase of θO. In separate experiments the surface chemistry of H2CO was investigated on clean and iodine covered surfaces. The decomposition of H2CO was blocked by preadsorbed I. These feature is interpreted by site blocking, by weakening of the H2CO-Pd bond and by decreasing the bond energy between Pd and O by 50 KJ/mol. The results suggest that the electronegative additives promote the selective oxidation of CH2 species on Pd(1 0 0) surface.