An electrocatalytic method has been developed that can generate metal hydrides more cleanly and efficiently. The researchers then used the hydrides to reduce carbon dioxide to formic acid, a high value-added chemical.
“We were inspired by the efficiency of enzymes,” explains Victor Mougel of ETH Zurich in Switzerland, who led the study. In nature, many different enzymes catalyze proton-electron transfers, the type of reactions that produce hydrides. “In particular, an iron-iron hydrogenase that follows a concerted mechanism … transfers a proton and an electron together … to generate the metal hydride,” he says. And it works thanks to one of the most common catalytic nuclei in living organisms: the cubic iron-sulfur cluster. Mougel’s team kept this motif as a mediator.
“For the first time … it is possible to electrochemically generate a reactive metal hydride by tuning the properties of the iron-sulfur cluster,” says Eva Nichols, an expert in bio-inspired electrocatalysis at the University of British Columbia, Canada. “Metal hydrides are useful in reduction reactions.” Better known examples include sodium borohydride and lithium aluminum hydride. In this case, the iron-sulfur cluster promotes the electrocatalytic formation of a manganese hydride. Nichols explains that “transition metal hydrides are particularly interesting,” because researchers tune their properties by adjusting the metal center, its oxidation state, or the organic ligands surrounding it. ‘This [manganese] hydride reduces carbon dioxide into a valuable product: formate,” he says.
The key, explains Mougel, was to imitate the mechanism. Traditionally, the generation of hydrides requires the recurrent reduction of the metal center, and then the capture of a proton. These stepwise redox reactions take place “through high-energy intermediates, [which need] or strong acids or extreme negative potentials,” explains Nichols. In addition, they create very reactive metal intermediates, which bind to other products present, instead of taking protons to produce hydrides. “The manganese complex that we have used, for example, reacts with carbon dioxide almost 20 times faster,” adds Mougel. This favors the formation of carbon monoxide, but hinders the generation of hydrides.
Among the many mediators tested, the iron-sulfur cluster gave the best results in terms of efficiency and selectivity. “It’s good for a number of reasons,” explains Nichols. It shows an appropriate affinity—the perfect bond strength to hold the hydrogen atom and later transfer it to the manganese complex. “The iron-sulfur cluster is also very rigid, which helps increase the [reaction] rates,” he adds. It also rearranges and recovers its original redox state efficiently, which increases recyclability. “The iron-sulfur cluster used as a mediator regenerates very quickly, which is probably why it has evolved naturally in enzymes,” says Mougel.
‘Compared to [other] catalysts, ours offers high turnover at a very low potential,” he says. The process also has significant selectivity toward formic acid, “one of the most interesting chemicals derived from carbon dioxide,” according to Mougel. The paper also provides a protocol for better choosing mediators and metal hydrides.“These rules were probably obvious to everyone in the field of electrocatalysis, but we could test them experimentally, and now they’re verified,” he says.
Typically, electrocatalysts enable the formation of value-added products using energy from sustainable sources, such as solar and wind. Besides, ‘[this] The concerted approach relaxes reliance on harsh and dangerous chemicals,” comments Nichols. “It’s a shocking inspiration…similar processes could [catalyse] many other reductive transformations in the field of electrocatalysis.’