SURFACE-CHEMISTRY OF CHLOROIODOMETHANE, COADSORBED WITH H AND O, ON PT(111)

Using temperature programmed desorption (TPD), predosed oxygen TPD (POTPD), high-resolution electron energy loss spectroscopy (HREELS), and Auger electron and X-ray photoelectron spectroscopy (AES and XPS), we have investigated the chemistry of chloroiodomethane (ClCH2I) dosed onto clean, D-covered and O-covered Pt(111). At or below 100 K, ClCH2I adsorbs molecularly on all these surfaces. While ClCH2I in physisorbed multilayers desorbs reversibly, a significant portion in the first monolayer dissociates during hearing. In the absence of D and O, dissociation begins with C-I bond cleavage at similar to 150 K. Once the C-I bond breaks, several competitive reactions take place below 260 K: (1) hydrogenation of CH2Cl(a) to form CH3Cl(g) beginning near 150 K, (2) Cl-CH2(a) bond cleavage to form Cl(a) and CH2(a) above 170 K, (3) dehydrogenation of CH2(a) to CH(a) beginning near 180 K and increasing rapidly above 200 K, (4) hydrogenation of CH2(a) to CH4(g) above 170 K, and (5) HCl and HZ formation and desorption above 200 K. At 260 K, the surface species are identified as I(a), CH(a), Cl(a), and a small quantity (similar to 0.02 ML) of CH2(a). The remaining CH2(a) reacts with itself and Cl(a) to form CH4(g), HCl(g), and CH(a) at 360 K. Cl(a) remnants react with CH(a) at 415 K, producing HCl(g) and CCH(a). The residual CH(a) fragments react at 520 K, yielding H-2(g), C-x(a), and more CCH(a). Finally, dehydrogenation of CCH(a) occurs between 550 and 700 K, releasing H-2 and leaving carbon, presumably clustered. Coadsorbed D atoms weaken the bonding between ClCH2I and the surface, decrease the amount of ClCH2I dissociating, and suppress the complete decomposition to carbon for those ClCH2I molecules that do dissociate. In TPD with coadsorbed D, besides the addition products (i.e., CH3D, CH2D2 and CH2DCl), there are also H-D exchange products for methane (i.e., CHD3 and CD3) but not for methyl chloride (i.e., no CHD2Cl and CD3Cl). Coadsorbed O atoms attenuate slightly the dissociation of ClCH2I, but strengthen its bonding with the surface. With increasing O coverage, the yields of CH4, CH3CI, H-2, and HCl (reaction products found in the absence of O(a)) decrease; other reaction productts, H2O, CO2, CO, CH2O, and CH2Cl2, appear and increase. To our knowledge, this is the first report of formaldehyde produced by the oxidation of a CH2 precursor on Pt(111). Reaction paths are discussed, as are the effects of coadsorbed halogen atoms on hydrogenation, C-C coupling, and oxidation of CH2.