Plasma-Catalytic Steam Reforming Of Methane Over Cu-based Catalysts For Production Of Methanol

Methane will become one of the major carbon resources to support human society in the future.Industrial production of methanol from methane is achieved by a two-step process,leading to high energy consumption and high costs.In this paper,a one-step methane steam reforming to methanol has been achieved by plasma.The coupling of plasma and Cu-based catalysts can further enhance the methane conversion and methanol selectivity.The CH₄/H₂O/Ar plasma reaction conditions,and experimental laws for coupling plasma with Cu-based catalysts were systematically investigated.Furthermore,reaction mechanisms were proposed based on experimental results.The conclusions are shown as follows:The CH₄/H₂O/Ar plasma can be directly used for CH₃OH synthesis.The effect of reaction conditions such as CH₄/H₂O molar ratio and reaction temperature were systematically investigated and the best reaction results were obtained corresponding to a temperature of 170°C and a CH₄/H₂O molar ratio of 1:4.Twelve metal active components were loaded onto Si O₂ support for the reaction,and the Cu/Si O₂ catalyst coupled with plasma corresponded to the highest total alcohol selectivity.Under the optimized reaction conditions,6.9%methane conversion has been achieved with 58.8%methanol selectivity.The characterization results of Cu/Si O₂ catalysts before and after the reaction show that the highly dispersed Cu²⁺species may be the most favorable active centers for promoting CH₄conversion and CH₃OH generation.A series of Cu-based zeolite catalysts were prepared by ion exchange method,and their plasma catalytic performance for methane steam reforming was investigated.It was found that Cu/MOR had the best catalytic performance.Under the optimal reaction conditions,the selectivity of CH₃OH reached 82%with 4.4%CH₄conversion.The catalyst characterization results（XRD,H₂-TPR,XPS,UV-Vis）proved that well-dispersed Cu²⁺was the active center for catalyzing the steam reforming of CH₄ to CH₃OH and H₂,while bulk Cu O particles caused the formation of C₂H₆ and CO.The excellent catalytic performance of the Cu/MOR catalyst is related to the moderate acid content and acid strength of the MOR zeolite,which is beneficial to both the formation of Cu-O-Cu active sites and the resistance to carbon deposition.The CH₄/H₂O/Ar plasma coupled with the Cu/MOR IE-4 catalyst exhibited the best synergistic effect,but the Cu/MOR exhibited slow deactivation during the stability test.Through XPS and UV-Vis characterization,it is found that CH₄ can reduce Cu²⁺to Cu⁺in CH₄/H₂O/Ar plasma,while H₂O can oxidize Cu⁺to Cu²⁺,which proves that the reaction is a catalytic cycle process.The insufficient oxidizing ability of H₂O leads to the gradual reduction of Cu²⁺to Cu⁺,which is the main reason for the slow deactivation of Cu/MOR catalysts.However,the calcination treatment can maintain the high activity and high selectivity of the Cu/MOR catalyst.The EPR experiments demonstrated that H₂O could dissociate to form OH radicals under plasma conditions.D₂O and H₂¹⁸O isotope tracer experiments show that H₂O dissociates and participates in the reaction,and the O element in the CH₃OH molecule comes from the H₂O molecule.The OES diagnosis results show that the copper active centers on the surface of Cu/MOR catalysts have strong ability of adsorbing radical species such as CH₃ and OH.Based on the above results,a possible reaction mechanism of plasma catalytic methane steam reforming to CH₃OH and H₂ is proposed.Compared with the literature results,this thesis can continuously catalyze the reaction of CH₄and H₂O to CH₃OH and H₂,realizing the catalytic cycle,and significantly improve the CH₄conversion and CH₃OH yield.