Review on Using Molybdenum Carbides for the Thermal Catalysis of CO2 Hydrogenation to Produce High-Value-Added Chemicals and Fuels

CO2 hydrogenation is critical to producing high-value-added carbon-based chemicals and fuels to achieving both hydrogen energy storage and CO2 utilization. Examples of CO2 hydrogenation include methanation (Sabatier process) to produce methane, reverse water-gas shift reaction (RWGS) to generate CO, methanol synthesis for the methanol economy, and CO2 direct Fischer-Tropsch (CO2-FT) reaction to produce olefins. The precious metal catalysts used in these reactions are efficient but too expensive to be used on a large scale. Further, although some non-precious metal catalysts can be used for these hydrogenation reactions, they suffer from deactivation during long-term processes. Over the past few decades, molybdenum carbides, which are transition metal carbides (TMCs), have attracted significant attention owing to their low cost and similar catalytic performance to precious metal catalysts in CO2 hydrogenation reactions. Recently, two-dimensional molybdenum carbide MXenes have shown impressive activity in CO2 hydrogenation reactions. Owing to the presence of carbon, the MXene lattice is expanded; this leads to an increase in valence electrons and endows the two-dimensional molybdenum carbide-based catalyst with different properties than metallic Mo. The two-dimensional molybdenum carbide-based materials can be prepared by temperature-programmed carburization, selective etching, mechanical alloying synthesis, chemical vapor deposition, in situ thermal carburization, and solution-phase synthesis methods. Thus far, a host of studies have been performed on CO2 conversion with molybdenum carbide-based materials, which show promising activity during CO2 conversion and selectivity towards target products. Both alpha-MoC1-x and beta-MoCy have shown outstanding thermocatalytic activity, product selectivity, and reaction stability during CO2 hydrogenation to CO at 300-600 degrees C. In addition, molybdenum carbide-based materials were also found to be an interesting catalyst for direct CO2 Fischer-Tropsch synthesis. The application potential of the molybdenum carbide-based materials could be further tuned by changing the C/Mo ratio in the bulk molybdenum carbide, strengthening the interactions between molybdenum carbide and the supported metal, and tailoring the interface structure of the materials. However, the thermal catalytic CO2 conversion based on molybdenum carbide-based materials is still in its infancy. This paper summarizes the progress toward molybdenum carbide catalysis of CO2 hydrogenation process for producing high-value-added chemicals and fuels. Furthermore, the challenges and opportunities for molybdenum carbide materials as catalysts for CO2 hydrogenation are discussed to provide insights for future development in this emerging field.