The gas phase in the headspace of the reaction vessel was analyzed by gas chromatography. A gas-tight syringe was used to take a sample (0.2 ml) from the vessel. Before each sampling, the gas in the syringe was replaced with the gas from the reaction vessel; 0.2 mL of gas in the headspace of the vessel was once taken and discharged. Then, 0.2 mL of gas in the vessel was newly taken and injected into a gas chromatograph.