LAUSR

As the main component of natural gas, methane is a well-established and widely available feedstock to produce several important commodity chemicals including methanol, hydrogen, ammonia, and formaldehyde.[1] With the recent discoveries of vast shale gas reserves, the energy landscape of the United States is shifting toward an increased prominence of natural gas in the feedstock portfolio.[2] Despite being widely deployed, methane conversion processes are bound by the intrinsic chemical inertness of its CH bonds.[3] For example, liquid fuel production from methane can be accomplished through steam reforming followed by Fischer–Tropsch synthesis (Figure 1a).[4] Steam-methane reforming (CH4 + H2O ⇌ CO + 3 H2) is an endothermic (≈200 kJ mol−1) process requiring elevated temperatures (700–1100 °C) and is typically catalyzed by alumina-supported nickel catalysts.[4] Although lowering the pressure increases the thermodynamic driving force of steam-methane reforming reaction, high pressures (typically above 10 bar) are usually applied to accelerate the reaction.[4] Industrially, the thermal energy requirement for this reaction is typically met through the combustion of methane, which effectively lowers the feedstock efficiency of the process. Indeed, about 25–35% of the total methane feed is burned for heat[5] instead of producing the desired syngas product as a feedstock of subsequent Fischer–Tropsch synthesis. Furthermore, a large number of unit operations and centralized infrastructure are required to accommodate the circuitous energetic pathways, thus hindering their implementation on a small scale. For this reason, at remote oil fields lacking the infrastructure to store, transport, or utilize the produced natural gas, methane is typically burned as flare gas, resulting in a total waste of 140 billion m3 methane every year, equivalent to a market cost of ≈$20 billion per year.[6] To address this problem, an attractive solution is to convert methane to liquid fuels, such as methanol, via selective partial oxidation using existing infrastructure that can be easily deployed and mobilized in remote oil fields.[7] Such methane-to-liquid fuels technology will significantly improve the volumetric energy The direct partial oxidation of methane to methanol promises an energyefficient and environmental-friendly utilization of natural gas. Unfortunately, current technologies confront a grand challenge in catalysis, particularly in the context of distributed sources. Research has been focused on the design of homogenous and heterogenous catalysts to improve the activation of methane under thermal and electrochemical conditions. However, the intrinsic relationship between thermal and electrochemical systems has not been exploited yet. This review intends to bridge the studies of thermal and electrochemical catalysts, in both homogenous and heterogenous systems, for methane activation from a mechanistic point of view. It is expected to provide a framework to rationalize the design of electrocatalysts beyond the state of art. First, methane activation systems reported previously are reviewed and classified into two basic mechanisms: dehydrogenation and deprotonation. Based on the mechanism types, activity and selectivity descriptors are defined to understand the performance of current catalysts and guide the design of future catalysts. Moreover, methods to enhance the activity and selectivity are discussed to emphasize the unique advantage of electrocatalysis in overcoming the limitations of traditional thermal catalysis. Finally, immense opportunities and challenges for catalyst design are discussed by unifying thermal and electrochemical catalysis.