CHEMICAL ENGINEERS from MIT, US have developed catalysts with a metal carbide core coated in metallic nanoparticles that has a higher catalytic activity than metal core catalysts and uses just a tenth of the rare metals.
Since only the surface of catalytic particles is involved in accelerating a reaction, substituting the bulk of the particle with an inexpensive core can lead to drastic reductions in the use of expensive noble metals without sacrificing performance.
To this end, the team added a coating of platinum and ruthenium – measuring just a few atoms thick – over a ceramic lattice core made of cheaper transition metal carbide.
Carbides were chosen as the core material because they are resistant to corrosion and clustering, they can also be made from abundant, cheaper metals including tungsten and titanium that will not react with the precious metal coating.
Engineering the materials for the core and the coatings have been difficult in the past as metal carbides require high temperatures to force carbon into the metal lattice, which leads to particle clumping and surfaces contaminated with excess carbon layers. The noble metals for the coating do not bond easily with other materials and are reluctant to form a stable shell structure around the core.
To overcome this problem, the team encapsulated the coating and core material precursors into a silica template which keeps them close together during heat treatment, making them self-assemble into the respective core and coating structures. The silica template is then dissolved with acid treatment at room temperature.
Sean Hunt, chemical engineering doctoral student at MIT, said: “We found that the self-assembly process is very general. The reluctance of noble metals to bind to other materials means we could self-assemble incredibly complex catalytic designs with multiple precious metal elements present in the shell and multiple inexpensive elements present in the carbide core.”
The team said the new catalyst was put through 10,000 electrochemical cycles in a hydrogen fuel cell and performed 10 times better than conventional catalysts over the same number of cycles.
The team attribute the enhanced performance to its resistance to carbon monoxide poisoning. Traditional fuel cell catalysts can tolerate around 10 ppm of CO, while the team found the core-shell catalysts could withstand up to 1,000 ppm.
Yuriy Román-Leshkov, associate professor of chemical engineering at MIT, said: “[CO] can drastically curtail the performance of conventional catalysts by bonding to their surface and blocking further interaction, but on the core-shell catalysts, the carbon monoxide detaches more easily.”
The team also found the ceramic core structure was stable at higher temperatures while also remaining resistant to particle clumping. At high temperatures, other shell structures dissolved into the core over time, whereas the noble metal shell is insoluble in carbide cores.
The team say the design is still undergoing preliminary testing, however are hopeful that the principle can eventually be applied to commercial fuel-cell technology. The team are currently working with MIT's Translational Fellows Program to identify the potential markets the materials will be useful for.
Science, DOI: 10.1126/science.aad8471
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