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

Using temperature programmed desorption (TPD), predosed oxygen TPD (PO TPD), 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 on to clean, D-covered and O-covered Pt(111). At or below 100 K, ClCH2I a dsorbs molecularly on all these surfaces. While ClCH2I in physisorbed multilayers desorbs reversibly, a significant portion in the first mon olayer dissociates during hearing. In the absence of D and O, dissocia tion begins with C-I bond cleavage at similar to 150 K. Once the C-I b ond breaks, several competitive reactions take place below 260 K: (1) hydrogenation of CH2Cl(a) to form CH3Cl(g) beginning near 150 K, (2) C l-CH2(a) bond cleavage to form Cl(a) and CH2(a) above 170 K, (3) dehyd rogenation of CH2(a) to CH(a) beginning near 180 K and increasing rapi dly above 200 K, (4) hydrogenation of CH2(a) to CH4(g) above 170 K, an d (5) HCl and HZ formation and desorption above 200 K. At 260 K, the s urface species are identified as I(a), CH(a), Cl(a), and a small quant ity (similar to 0.02 ML) of CH2(a). The remaining CH2(a) reacts with i tself and Cl(a) to form CH4(g), HCl(g), and CH(a) at 360 K. Cl(a) remn ants react with CH(a) at 415 K, producing HCl(g) and CCH(a). The resid ual 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 th e amount of ClCH2I dissociating, and suppress the complete decompositi on to carbon for those ClCH2I molecules that do dissociate. In TPD wit h coadsorbed D, besides the addition products (i.e., CH3D, CH2D2 and C H2DCl), there are also H-D exchange products for methane (i.e., CHD3 a nd CD3) but not for methyl chloride (i.e., no CHD2Cl and CD3Cl). Coads orbed O atoms attenuate slightly the dissociation of ClCH2I, but stren gthen its bonding with the surface. With increasing O coverage, the yi elds of CH4, CH3CI, H-2, and HCl (reaction products found in the absen ce of O(a)) decrease; other reaction productts, H2O, CO2, CO, CH2O, an d CH2Cl2, appear and increase. To our knowledge, this is the first rep ort 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.