The synthesis of methanol from carbon dioxide can be dramatically improved by changing the catalyst support from zirconia to hafnia. Hafnia’s monoclinic crystal structure and insulating electronic properties efficiently stabilize the individual atoms of the active indium catalyst, increasing activity by up to 70% and substantially reducing the required loading of this expensive metal. These findings demonstrate the importance of carrier engineering and open the door to an alternative approach to catalyst design, the authors say.
Green methanol, produced through the hydrogenation of carbon dioxide, is an important step in the decarbonization of chemical and fuel production. Current commercial catalysts are based on copper and zinc, but offer modest selectivity and often suffer irreversible deactivation by water. In the last 10 years, indium oxides have become a promising alternative, particularly when embedded in a monoclinic zirconia support. Several hypotheses have attempted to explain the origin of this promotional effect, but the precise mechanism remains elusive, hampering efforts to reproduce or improve this activity.
Other zirconium oxide crystal structures produce a much smaller increase in catalytic activity and Javier Pérez-Ramirez and Sharon Mitchell Therefore, at ETH Zurich they proposed that it was specifically the monoclinic form that improved catalytic performance. However, a quick search of the materials database revealed only one accessible analog: hafnia. Although typically considered catalytically inert, hafnia shares many properties with zirconia (in particular, a wide bandgap and high dielectric constant) that Pérez-Ramírez and Mitchell suspected would be crucial to the auxiliary catalytic effect of the oxides.
The team prepared a series of nanostructured indium hafnium oxide catalysts and compared their hydrogenation performance with the zirconium benchmark. The alternative support exceeded this standard across the board, with the most significant improvement (a 70% increase in indium utilization) being achieved for the single-atom catalyst loading.
These findings supported the initial hypothesis that the monoclinic structure and corresponding electronic properties were responsible for the observed increase in activity and the team then performed a series of computational analyzes to further corroborate this theory.
Their analysis suggests that the monoclinic structure promotes the formation of oxygen vacancies around the indium center, providing an accessible pocket in which the reaction can occur. The insulating properties of the oxides also help ensure that the reaction remains localized around these active sites. “Materials with a wide bandgap do not transfer electrons, so they can store protons and hydrides on the surface efficiently,” explains Pérez-Ramírez. “Indium helps split hydrogen and the support stabilizes these charged hydrogen species very well.” Meanwhile, the hydroxylated surface of hafnium oxide further stabilizes the indium atoms in their most active form, increasing the life of the catalyst.
«Indeed, zirconia and hafnia have properties that go in the same direction. But in the case of hafnia, the bandgap is slightly larger, the dielectric constant is slightly larger, and it is also more stable in the monoclinic network,” says Mitchell. “All of these factors combined mean that indium utilization is indeed increasing.” This effect even worked with other methanol synthesis catalysts, and both the zinc and gallium systems experienced large increases in activity, relative to the bulk material.
The team’s detailed analysis, combined with the broader implications of the carrier’s design, was especially impressive. Kelly Kousisustainable catalysis researcher at the University of Surrey. «The authors do not limit their findings to a single catalyst formulation. “This really reinforces the importance of the research and the broader applicability of their design strategy,” he says. “More importantly, the work appears to represent more than just an incremental improvement of a catalyst, making it an important step forward for heterogeneous catalysis as a whole.”
While the high cost of hafnium oxide means it is unlikely to ever become a commercial carrier, the real impact and focus of future work will be on the intentional design of carrier structures with tailored electronic properties, says Pérez-Ramírez. ‘I think we have to elevate support engineering to the same level as active metal. It is clearly a cooperative mechanism and the carriers are much cheaper than the elements used in catalysis.’