Low-Temperature Catalytic Oxidative Coupling of Methane over Ce-W-O System Catalysts in the Electric Field

Direct catalytic conversion of methane to ethylene by oxidative coupling of methane (OCM) is attracted our attention. However, OCM reaction demands high temperatures over 973 K because of the stable structure of methane. At such high temperature, non-selective oxidation cannot be avoided, which greatly reduces C₂ hydrocarbon selectivity. To solve this problem, we adopted a novel catalytic system, a catalytic reaction in an electric field, to activate methane at low temperature, such as 423 K. We previously reported that CeO₂ supported tetrabutylammonium salt of Keggin-type phosphotungstate (TBA-PW₁₂O₄₀/CeO₂) showed high OCM activity in the electric field even at 423 K. The purpose of this report is to investigate active site structure for OCM reaction in the electric field at low temperature, and to evaluate contribution of an electric field to OCM activity.TBA-PW₁₂O₄₀/CeO₂ after reaction in the electric filed was characterized with Raman spectroscopy. It showed the peaks of TBA-PW₁₂O₄₀ disappeared and the peaks of Ce₂(WO₄)₃ and WO₃ appeared. Therefore, catalytic activity tests of Ce-W-O system catalysts were conducted in the electric field. From the activity tests, Ce₂(WO₄)₃/CeO₂, Ce₂(WO₄)₃ and WO₃/CeO₂ showed OCM activity while WO₃ did not showed OCM activity. Then, the catalysts before and after reaction were characterized by Raman spectroscopy. The Ce₂(WO₄)₃/CeO₂, Ce₂(WO₄)₃ and WO₃/CeO₂ catalysts after reaction had the Ce₂(WO₄)₃ structure, however the WO₃ catalyst after reaction retained the WO₃ structure. In accordance with catalyst activity tests and Raman spectroscopy, Ce₂(WO₄)₃ was active site structure for OCM in the electric field. In-situ Raman spectroscopy revealed that Ce₂(WO₄)₃/CeO₂ structure was distorted and W-O bond weakened by application of the electric field. Consequently, it was suggested that the distorted W-O bond contributed to OCM activity in the electric field.