Surface chemistry of chloroiodomethane, coadsorbed with H and O, on Pt(111)

X. L. Zhou, Z. M. Liu, J. Kiss, D. W. Sloan, J. Kiss

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Abstract

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 heating. In the absence of D and O, dissociation begins with C-I bond cleavage at ∼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 H2 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 (∼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 H2(g), Cx(a), and more CCH(a). Finally, dehydrogenation of CCH(a) occurs between 550 and 700 K, releasing H2 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 CD4) 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, CH3Cl, H2, 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.

Original languageEnglish
Pages (from-to)3565-3592
Number of pages28
JournalJournal of the American Chemical Society
Volume117
Issue number12
Publication statusPublished - Mar 29 1995

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Hydrogenation
Surface chemistry
Temperature programmed desorption
Temperature
Electron Energy-Loss Spectroscopy
Dehydrogenation
Carbon
Methyl Chloride
Atoms
Photoelectron Spectroscopy
Halogens
X ray photoelectron spectroscopy
Methane
Carbon Monoxide
Oxidation
Heating
Formaldehyde
Electron energy loss spectroscopy
Reaction products
Electrons

ASJC Scopus subject areas

  • Chemistry(all)

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Surface chemistry of chloroiodomethane, coadsorbed with H and O, on Pt(111). / Zhou, X. L.; Liu, Z. M.; Kiss, J.; Sloan, D. W.; Kiss, J.

In: Journal of the American Chemical Society, Vol. 117, No. 12, 29.03.1995, p. 3565-3592.

Research output: Contribution to journalArticle

Zhou, X. L. ; Liu, Z. M. ; Kiss, J. ; Sloan, D. W. ; Kiss, J. / Surface chemistry of chloroiodomethane, coadsorbed with H and O, on Pt(111). In: Journal of the American Chemical Society. 1995 ; Vol. 117, No. 12. pp. 3565-3592.
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abstract = "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 heating. In the absence of D and O, dissociation begins with C-I bond cleavage at ∼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 H2 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 (∼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 H2(g), Cx(a), and more CCH(a). Finally, dehydrogenation of CCH(a) occurs between 550 and 700 K, releasing H2 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 CD4) 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, CH3Cl, H2, 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.",
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TY - JOUR

T1 - Surface chemistry of chloroiodomethane, coadsorbed with H and O, on Pt(111)

AU - Zhou, X. L.

AU - Liu, Z. M.

AU - Kiss, J.

AU - Sloan, D. W.

AU - Kiss, J.

PY - 1995/3/29

Y1 - 1995/3/29

N2 - 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 heating. In the absence of D and O, dissociation begins with C-I bond cleavage at ∼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 H2 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 (∼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 H2(g), Cx(a), and more CCH(a). Finally, dehydrogenation of CCH(a) occurs between 550 and 700 K, releasing H2 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 CD4) 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, CH3Cl, H2, 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.

AB - 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 heating. In the absence of D and O, dissociation begins with C-I bond cleavage at ∼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 H2 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 (∼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 H2(g), Cx(a), and more CCH(a). Finally, dehydrogenation of CCH(a) occurs between 550 and 700 K, releasing H2 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 CD4) 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, CH3Cl, H2, 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.

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