Single-Step Oxidation of Low-Concentration Methane to Methanol in the Gaseous Phase Using Ceria-Based Iridium-Copper Catalysts
Graphical Here, direct oxidation of methane to methanol under gaseous condition on iridium promoted Cu/CeO 2 catalyst, prepared using a sol-gel method, is investigated. As for proposed reaction mechanism, it is believed that the CuO−IrO 2 phase is the active sites, while CeO x matrix should act as the auxiliaries, providing active oxygen (O − , O 2 2− , O 2 ⋅ − ) during the methane oxidation reaction. The role of IrO 2 is to activate CH 4 molecule and make Cu-attached O more fragile to be released through the interaction of IrO 2 and CuO, and CuO works to help generate surface methoxyl, so as to improve the methanol yield and selectivity. Single-step conversion of methane to methanol in the gaseous phase is required for high value added application of methane and environmental protection, but it is challenging. Here, direct oxidation of methane to methanol under gaseous condition on iridium promoted Cu/CeO 2 catalyst, prepared using a sol-gel method, is investigated. The addition of iridium can effectively upgrade the redox properties and oxygen storage capacity due to intense metal interaction, stimulating a prominent catalytic performance for methane conversion to methanol on Cu−Ir@CeO 2 catalyst. Approximately 26.2 μmol/g cat methanol yield and 68 % methanol selectivity are achieved on the trimetallic 5Cu0.5Ir@CeO 2 catalyst at 550 °C in 2 h. Consequences of the analysis of XRD, SEM, HRTEM, Raman, FI-IR and XPS demonstrate that highly-dispersed Ir and Cu species are uniformly distributed on CeO 2 surface, and partial Cu or Ir atoms replace Ce 4+ in CeO 2 lattice due to the metal interaction in the colloidal structure, which can impact the catalyst's electronic properties. H 2 temperature-programmed reduction (H 2 -TPR) and CH 4 temperature-programmed desorption (CH 4 -TPD) results disclose that the unique ternary surface exhibits excellent redox properties and strong adsorption capacity for methane, which can activate the first C−H bond of methane to methyl species, and then react with OH − to form methanol. The good stability in cyclic operation is an additional attribute, rendering this type of catalyst a “front-runner” in future catalyst development for direct methane-to-methanol. This composite catalyst design provides hope for developing ternary metal-oxide catalysts for functionalization of methane.
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