The low-temperature adsorption of methyl chloride and methyl iodide on silica-supported Pt catalysts has been investigated by transmission Fourier transform infrared spectroscopy. The IR data show that methyl chloride and methyl iodide dissociate at low temperatures (near 200 K) to form an adsorbed hydrocarbon fragment on the surface, identified as methyl groups. Methyl groups are characterized by a single infrared absorption band near 2965 cm-1. Methyl groups react with hydrogen to form gas-phase methane as the sample is warmed between 200 and 473 K. Reaction of approximately 10 Torr of methyl chloride at 473 K over a Pt/SiO2 catalyst shows that only 20% of the methyl chloride decomposes to form gas-phase methane and hydrogen chloride in the absence of hydrogen. However, in the presence of an equal amount of hydrogen, all of the methyl chloride is converted to methane and hydrogen chloride. In contrast to the quantitative conversion of methyl chloride, less than 10% of the initial 10 Torr of methyl iodide forms methane at 473 K and no hydrogen iodide forms in the presence or absence of hydrogen gas. Although the activation barrier for C-Cl bond dissociation in adsorbed methyl chloride is higher than the barrier for C-I bond dissociation in adsorbed methyl iodide, the lower energy barrier for removal of adsorbed chlorine compared to adsorbed iodine is the cause of the higher catalytic activity of Pt/SiO2 toward methyl chloride decomposition. In addition to the thermal decomposition of CH3Cl, we have investigated the possibility of using solar radiation for the decomposition of CH3Cl on Pt/SiO2. The results for the photo-assisted decomposition of CH3Cl adsorbed on Pt/SiO2 are presented and discussed.
Bibliographical noteFunding Information:
Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. The authors also gratefully acknowledge the National FIG. 12. Left: Reaction energetics for the reaction CH3X C H2 ! CH4 C HX (where X D Cl and I). Right: Reaction energetics for the reaction CH3I C Pt ! CH4 C Pt–I: See text for further details.
Science Foundation (Grant CHE-9300808) for support of this research. The authors would like to thank Dr. Mauro Briceno and Leonardo Martinez for the TEM measurements.