Xl. Zhou et al., SURFACE-CHEMISTRY OF CHLOROIODOMETHANE, COADSORBED WITH H AND O, ON PT(111), Journal of the American Chemical Society, 117(12), 1995, pp. 3565-3592
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.