Chemists prove hydrogen catalyst mechanism

Hydrogen fuel cells can power cars and other vehicles, but generating power from water has some challenges. Platinum is too expensive to use as a catalyst for the reaction, and until recently, the mechanism of the cheaper cobalt-based catalysts remained a mystery.

Chemists at California Institute of Technology demonstrated support for the mechanism of cobalt-based catalyst, supporting an explanation for how the catalysts interact with water to split it into hydrogen and oxygen. Detailing their results in an article published this month in the Proceedings of the National Academies of Science, the group has managed to slow the reaction by modifying the catalyst and to gain insight into which mechanism accurately described how the catalyst worked.

In order to improve upon a chemical reaction, especially when that improvement comes from a catalyst, scientists need to know how the catalyst interacts with the various components of the reaction. For this cobalt-based catalyst, several mechanisms had been proposed to describe what was going on between the catalyst, water and other compounds that would exist until hydrogen and oxygen gases formed. The mechanism that won out came from Jillian Dempsey, a former graduate student of the article’s senior author, Harry Gray.

According to the article’s lead author, postdoctorate scholar Smaranda Marinescu, having the mechanism will help chemists improve the performance of cobalt-based catalysts. “The elucidation of the mechanism will allow us to design ligand that accepts that extra electron to facilitate the formation of the active species,” Marinescu wrote in an email to The News-Letter.

Dempsey’s mechanism points to a reaction intermediate comprised of cobalt(II)-hydride, whereas others have suggested that the intermediate was cobalt(III)-hydride, differing in the addition of an electron. Usually reaction intermediates are too unstable to exist for very long, and chemists have a hard time observing them. In order to slow down the reaction, Marinescu added tripodal phosphine ligands to the catalyst, allowing her to track the reaction using Nuclear Magnetic Resonance (NMR).

“NMR is a great technique to characterize diamagnetic hydride species,” she wrote. “The 1H NMR spectrum provides clear evidence for the hydride signal. Moreover, NMR allows for measuring the relative intensity of the hydride peak, and therefore the reaction progress can be monitored.”

With this mechanism, researchers can work on modifying cobalt catalysts to improve their performance or can work with iron-based catalysts, another low-cost material that can make economic generation of hydrogen gas a more possible part of the future energy landscape.

These catalysts can someday harness the power of solar energy to split water into hydrogen and oxygen gases, with hydrogen having the potential to be a major fuel source for transportation. Like other reactions, the cobalt-based catalysts help lower the activation barrier, the energy needed for a reaction to proceed. Marinescu adds that the hydrogen gas is insoluble and that its evaporation from water further helps to drive the reaction.

Another hurdle for these catalysts lies in their aerobic sensitivity, which can cause them to fail as the reaction proceeds and more oxygen and hydrogen gases are formed. Marinescu explains that this problem comes from the sensitivity of the hydride intermediates in the reaction to oxygen. Another group from Cambridge University found a way around this problem, publishing last month the details of their catalyst that does work under aerobic conditions, but the stability of the catalyst remains a major hurdle for them.

Researchers have also explored the use of enzymes, proteins that can catalyze reactions, as another potential tool for generating hydrogen gas. While algae and bacteria have developed these enzymes and researchers used them as a starting point in developing biologically-based catalysts for industrial production, Marinescu explained that these enzymes are much larger than their molecular counterparts and are easily inactivated by oxygen.

Presently, the major source of hydrogen gas production has been from fossil fuels and natural gas. Its incorporation into current vehicles is limited, and California is home to the country’s only operating hydrogen fuel stations. Carmakers have come out with hydrogen-fueled models in various stages of design and testing, but incorporation into the general market will be a few years away as will solar generated hydrogen.