Molybdenum Oxide, Oxycarbide, and Carbide: Controlling the Dynamic Composition, Size, and Catalytic Activity of Zeolite-Supported Nanostructures

摘要

Molybdenum nanostructures are promising catalysts for a single-step methane conversion into aromatic hydrocarbons and for multiple other hydrocarbon reactions. Dynamic transformations of catalytic zeolite-supported Mo structures from an oxide to an oxycarbide and then to a carbide in methane dehydroaromatization were studied with in situ X-ray absorption spectroscopic measurements and density functional theory calculations. After a treatment in the presence of gas-phase oxygen, Mo is present in the form of isolated oxide structures. Under reaction conditions with methane, these initial oxide structures become carbided and catalyze methane conversion to aromatic hydrocarbons. As the reaction progresses, the Mo carbide structures agglomerate and accumulate excess carbon, leading to catalyst deactivation. Although the initial oxide structures and catalytic activity can be restored by reversing the agglomeration and coking with periodic oxygen regeneration treatments, it is desirable to continuously control the dynamic composition, size and catalytic activity of Mo structures under hydrocarbon reaction conditions. This objective can be accomplished by co-feeding an oxygen-containing molecule, such as CO2, that transforms Mo structures into an oxycarbide and slows the undesirable agglomeration and coking. It is, therefore, preferable for preserving catalytic activity to prevent full carburization of Mo and, instead, maintain the Mo structures in an oxycarbide form, an intermediate between an oxide and a carbide.