Researchers at Northwestern University, Evanston, Illinois, have developed a stable molybdenum carbide catalyst for the high-temperature reverse water-gas shift (RWGS) reaction, a building block in many chemical processes.
Transition metal carbides, particularly molybdenum carbides, show promise in such carbon dioxide (CO2) reduction reactions because their components are earth-abundant and can catalyze some chemical transformations with noble metal-like behavior.
However, until now, their use has been hampered by a complex, multi-step synthesis route that requires noble metals and a highly controlled crystallization phase. They also suffer from poor selectivity, poor stability under reaction conditions and the need for additional constituents that may sinter upon exposure to high temperatures.
The Northwestern team has now synthesized molybdenum carbide via what they describe as a facile, inexpensive and scalable route that uses household sugar as the carbon source. Essentially, they carburized ammonium molybdate tetrahydrate with the sugar at 500°C under a hydrogen atmosphere for two days.
The catalyst achieves 100% carbon monoxide (CO) selectivity for the high-temperature RWGS reaction at 300-600°C with H2/CO2 ratios between 0.5 and 4. It also maintains 100% of its initial activity and CO selectivity for at least 500 hours under harsh conditions (600°C, 0.1 MPa).
“Collectively, these factors stand this material out as one of the top candidates for CO2 conversion,” said Milad Khoshooei, a postdoctoral fellow at Northwestern's Weinberg College of Arts and Sciences who co-led the work, details of which are published in a recent issue of Science.
Khoshooei says some of the paper’s other authors are now planning a large-scale demonstration of the synthesis of this material.
The rest of the team intends to build on its experience developing metal-organic frameworks (MOFs) for carbon capture and sequestration. They believe the new catalyst could work together with MOFs in a tandem system using the two distinct materials for two sequential steps.
“Integrating a carbon capture system into a conversion system is highly promising but, of course, requires a thorough assessment of process design,” he added.
Nevertheless, Khoshooei believes it could still help answer the question of what to do with captured CO2.
"Right now, the plan is to sequester it underground. But underground reservoirs must meet many requirements to safely and permanently store CO2. We wanted to design a more universal solution that can be used anywhere while adding economic value," he concluded.
