Researchers at the USC Viterbi School of Engineering, collaborating with the US Department of Energy’s National Renewable Energy Laboratory (NREL), have developed a mild and scalable synthesis route for a molybdenum carbide nanoparticle that can convert CO₂ into fuel.

The particles can be produced at an industrial scale at a low cost, and with minimal environmental impact, providing an attractive pathway toward reducing the world’s greenhouse emissions.

Transition metal carbides (TMCs) have demonstrated outstanding potential for utilization in a wide range of catalytic applications because of their inherent multifunctionality and tunable composition. However, the harsh conditions required to prepare these materials have limited the scope of synthetic control over their physical properties.

The development of low-temperature, carburization-free routes to prepare TMCs would unlock the versatility of this class of materials, enhance our understanding of their physical properties, and enable their cost-effective production at industrial scales.

Here, we report an exceptionally mild and scalable solution-phase synthesis route to phase-pure molybdenum carbide (α-MoC_1-x) nanoparticles (NPs) in a continuous flow millifluidic reactor. We exploit the thermolytic decomposition of Mo(CO)₆ in the presence of a surface-stabilizing ligand and a high boiling point solvent to yield MoC_1−x NPs that are colloidally stable and resistant to bulk oxidation in air.

To demonstrate the utility of this synthetic route to prepare catalytically active TMC NPs, we evaluated the thermochemical CO₂ hydrogenation performance of α-MoC_1−x NPs dispersed on an inert carbon support. The α-MoC_1−x/C catalyst exhibited a 2-fold increase in both activity on a per-site basis and selectivity to C₂₊ products as compared to the bulk α-MoC_1−x analogue.

—Baddour et al.

Noah Malmstadt, professor in USC Viterbi’s Mork Family Department of Chemical Engineering and Materials Science, is one of the authors of the research, in collaboration with Frederick G. Baddour from NREL and Richard Brutchey, professor of Chemistry at USC. Their work was published in the Journal of the American Chemical Society.

Malmstadt said that the aim of the project was to capture carbon emissions from an emission source, such as a flue, and then to convert it into usable fuels, with the nanoparticles functioning as a catalyst to enable the reaction.

Basically we’re turning the carbon dioxide from carbon oxygen bonds to carbon hydrogen bonds. So, we’re turning carbon dioxide back into hydrocarbons. Hydrocarbons are basic fuel stock. You can either turn them into fuel stock chemicals such as methane or propane. Or you can use them as the basis for chemical synthesis so they can be building blocks for making more complex chemicals.

—Noah Malmstadt

Malmstadt said that until now, the process for creating the catalyst particles has been very energy intensive, making it an impractical solution for converting carbon emissions. The carbides are created using a process where they are heated to temperatures higher than 600 degrees centigrade, a process that makes it difficult to control the size of the particles, which impacts on their effectiveness as catalysts.

In contrast, the team’s discovery uses a millifluidic reactor process, a very small-scale chemical reactor system, which has a minimal environmental footprint. This means the particles can be produced at temperatures as low as 300 degrees centigrade, resulting in smaller, more uniform particles, which make them ideal for converting CO₂ to hydrocarbons.

The chemical reactor system operates in channels that are less than a millimeter across, offering advantages over traditional reactors, particularly in terms of making materials that are very uniform and very high quality, with a very high surface-area-to-mass ratio.

The solution-phase synthesis strategy presented in this work is a facile and versatile method for preparing nanostructured, stable, and readily dispersible group 6 TMCs. The methods presented herein require no reactive gases or additional thermal treatments to produce phase-pure TMCs, relying instead on the extremely mild thermolytic decomposition of metal−carbonyl precursors. Further, the continuous flow mF approach developed highlights the scalability of this synthetic strategy and demonstrates the feasibility of the production of catalytically relevant quantities of nanostructured TMCs.

The resultant dispersible NPs can be deposited on any catalyst support, and, as demonstrated here with the thermocatalytic CO₂ reduction over NP-MoC_1−x/C, their performance on a per-site basis represents a 2-fold improvement as compared to the bulk α-MoC_1−x analogue, emphasizing the superior Mo utilization of the nanostructured catalyst. In addition, the increased selectivity toward C₂₊ products over the NP- MoC_1−x/C catalyst highlights the opportunity for the controlled nanostructuring of TMCs to be employed to tune the product slate with the facile synthetic method presented.

—Baddour et al.

Resources

• Frederick G. Baddour, Emily J. Roberts, Anh T. To, Lu Wang, Susan E. Habas, Daniel A. Ruddy, Nicholas M. Bedford, Joshua Wright, Connor P. Nash, Joshua A. Schaidle, Richard L. Brutchey, and Noah Malmstadt (2020) “An Exceptionally Mild and Scalable Solution-Phase Synthesis of Molybdenum Carbide Nanoparticles for Thermocatalytic CO₂ Hydrogenation” Journal of the American Chemical Society 142 (2), 1010-1019 doi: 10.1021/jacs.9b11238