As the name suggests, the oxidative coupling of methane (OCM) relies on catalyst oxygen to drive the selective formation of C2 (ethane and ethylene) product molecules. Understanding the nature of different types of oxygen species is particularly challenging in complex, mixed metal oxide catalysts at industrially relevant temperatures. Using Temporal Analysis of Products (TAP) pulse response experiments in a pump/probe fashion, two distinct forms of surface oxygen are resolved on the Mn-Na2WO4/SiO2 catalyst. A weakly bound species in a dioxygen form (i.e., O22- or O2-) is found to be the key ingredient for ethane formation and can be used to predict catalyst performance under steady-state conditions. This species is constantly formed/depleted under process conditions but using transient methods, Figure 1, a surface lifetime of 3 s is resolved, as well as its role in competitive pathways for methyl radical and CO formation. Oxygen attributed to the catalyst lattice is also detected in a singular form (i.e., O_) which activates methane as well, but predominantly forms CO2. In another class of materials, Ruddlesden-Popper A2BO4 type oxides, substitutions of the A and B sites lead to changes in the distribution of different active oxygen species and notably, these materials indicate a surface-driven coupling process for C2 formation. This is distinct from the commonly studied Mn-Na2WO4/SiO2 catalyst where TAP experiments varying between transport regimes with and without gas phase collisions indicate this catalyst relies on gas phase coupling of methyl radicals released from the surface.

Resolving critical oxygen properties of catalysts used in the oxidative coupling of methane

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