Catalysis by mixed oxide perovskites. I. Hydrogenolysis of ethylene and ethane on LaCoO3.
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概要
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Catalysis by a mixed oxide with a perovskite structure, LaCoO<SUB>3</SUB>, was studied for the hydrogenolysis of ethylene and ethane at temperatures 300–600 K using the hydrogen-excess mixtures. The high and stable activity was obtained by evacuating LaCoO<SUB>3</SUB> at 600 K. Ethylene hydrogenolysis above 420 K proceeded consecutively <I>via</I> the intermediate, ethane, to produce methane, whereas only the hydrogenation to form ethane occurred below 420 K. Different temperature dependence of the rate was observed, <I>i.e.</I>, the activation energy of −19.3 kJ·mol<SUP>−1</SUP> in the range 420–600 K and 33.6 kJ·mol<SUP>−1</SUP> in the range 300–420 K. The mechanistic analysis and tracer study using D<SUB>2</SUB> showed that the slow step of the hydrogenation shifts, with increasing temperature, from the adsorption of hydrogen to the surface reaction between hydrogen atom and ethyl radical. The hydrogenolysis of ethane produced only methane and no ethylene was observed in gas phase; the reaction orders with respect to hydrogen and ethane pressures were, respectively, −0.5 and 1.0. In the reaction with D<SUB>2</SUB>, methane [D<SUB>4</SUB>] was the main product and an equilibrium among the formed H<SUB>2</SUB>, HD, and D<SUB>2</SUB> was quickly established. The adsorption of ethane to undergo the rupture of the carbon-carbon bond and to form monocarbon species was proposed to be a slow step. The catalytic activities of the component oxides, La<SUB>2</SUB>O<SUB>3</SUB> and Co<SUB>2</SUB>O<SUB>3</SUB>, were also examined for a comparison. The structure and electronic states of LaCoO<SUB>3</SUB> were investigated by means of X-ray diffraction and X-ray photoelectron spectroscopy in connection with the catalytic activity. It was proposed that the hydrogenation proceeded mainly on a pair of La<SUP>3+</SUP> and O<SUP>2−</SUP> ions, whereas the hydrogenolysis was accelerated by the Co<SUP>3+</SUP> ion; the (110) plane is suggested to be the most favorable for the catalysis.
- 公益社団法人 日本化学会の論文
著者
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INOUE Yasunobu
Department of Chemistry, Nagaoka University of Technology
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Yasumori Iwao
Department Of Applied Chemistry Faculty Of Engineering Kanagawa University
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Ichimura Kenji
Department of Chemistry, Tokyo Institute of Technology
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