Hydrogen–Deuterium Exchange Studies on the Decomposition of Methane over Ni/SiO2

The reaction mechanism of methane decomposition over silica-supported Ni catalyst (Ni(5 wt%)/SiO2) was investigated by performing kinetic studies of the reactions using isotopic methane (CH4, CD4, and 13CH4) and hydrogen (H2 and D2). An isotope effect between CH4 and CD4 was observed in the decomposition of methane over the Ni/SiO2 catalyst. In the decomposition of a gas mixture of CH4 and CD4 over the Ni/SiO2 catalyst, the H–D exchanged methanes were not formed. These results suggested that the first C–H bond cleavage in CH4 was the rate-determining step in the decomposition of CH4 into carbon and H2. The idea well explained the reverse isotopic effect between H2 and D2 in the hydrogenation of the carbons deposited on the catalyst back into methane. Moreover, the rate-determining step was supported by the result that only CD4 was formed in the early part of the exchange reaction between CH4 and D2 (CH4⪡D2). The mechanism of carbon deposition on the Ni/SiO2 catalyst was examined by performing successive depositions from 12CH4 and 13CH4, followed by hydrogenation of the deposited carbon with H2. The carbon atoms deposited last were found to be hydrogenated to CH4 at first, suggesting that the carbon atoms did not scramble and the amorphous parts in a carbon fiber would be hydrogenated back into CH4 catalyzed by a Ni particle present at the tip of a carbon fiber.