California Institute of Technology (Caltech) scientists have determined the dominant catalytic mechanism for cobalt catalysts. The idea is to identify and understand the working mechanism inside water splitting catalysts. The drive is worldwide by scientists and engineers to find better catalysts powered by solar energy to run fuel cells. A water splitting catalyst system would split water during daylight hours, generating hydrogen that could then be stored and used later to produce water and electricity.
The available catalysts run from the very good platinum catalysts, but platinum is too rare and expensive to scale up for commercial use.
Several cobalt and nickel catalysts have been suggested as cheaper alternatives, but there remains plenty of room for improvement.
Until yesterday no one had announced a definitive description of how a cobalt catalyst works. Without the mechanism it’s a matter of hit or miss to methodically design and construct improved catalysts.
The Caltech research findings make building a roadmap to the development of better catalysts – even suggesting a route to the development of catalysts based on iron, an element that is plentiful and cheap and could offer part of the answer to world energy supplies.
Harry Gray, the Arnold O. Beckman Professor of Chemistry at Caltech and senior author of a paper that describes the findings said, “We’ve worked out this mechanism, and now we know what to do to make a really great catalyst out of something that’s really cheap as dirt. This work has completely changed our thinking about which catalyst designs to pursue.” The paper is in the current issue of the Proceedings of the National Academy of Sciences.
Understanding the performance of man-made catalysts has been a major problem to improving the chemical pathway that such catalysts follow in the production of hydrogen. In any multistep manufacturing project, chemists need to know what is involved in each reaction that takes place – what goes in, what changes take place, and what comes out in order to maximize efficiency and yield.
To date three mechanisms have been suggested for how the cobalt catalysts help make hydrogen. One is proposed by a French team, another suggested more recently by a former graduate student in Gray’s group, Jillian Dempsey. But Caltech”s Gray led team has managed to prove definitively which mechanisms actually occur or whether one was dominant.
Water splitting reactions proceed so quickly that it is difficult to identify the chemical intermediates that provide evidence of the reactions taking place. The cobalt catalysts are complexes that involve the metal bound to many different functional groups, or ligands.
For the Caltech study paper postdoctoral scholar Smaranda Marinescu was able to add a set of ligands to cobalt, making the reaction slow down to the point where the researchers could actually observe the key intermediate using nuclear magnetic resonance (NMR) spectroscopy.
Professor Gray explains, “Once we could see that key intermediate by NMR and other methods, we were able to look at how it reacted in real time.”
They saw that Jillian Dempsey’s mechanism is the predominant pathway that these catalysts use to generate hydrogen. The mechanism involves a key reactive intermediate gaining an extra electron, forming a compound called cobalt(II)-hydride, which turns out to be the mechanism’s active species.
The guessing is over, in previous paper published at the National Academy of Sciences, work by Gray and lead author Carolyn Valdez suggested that the Dempsey’s mechanism was the most likely explanation for the detected levels of activity.
Gray illustrates the situation now with, “We now know that you have to put another electron into cobalt catalysts in order to get hydrogen evolution. Now we have to start looking at designs with ligands that can accept that extra electron or those that can make atomic cobalt, which already has the extra electron.” Gray’s group is already working on this new approach.
The very interesting thing is these results give his group the information they need to develop an extremely active iron catalyst, and that will be their next big focus.
Gray is understandably robust in his position, “We know now how to make a great catalyst,” he said. “That’s the bottom line.”
There are still a lot of question to answer, catalyst production costs, lifetime and other matters that precede a working prototype.
A daytime fuel production to hydrogen is a desirable path to storage for use a bit later. Perhaps they have produced a clue in finding better cheaper catalysts for the fuel cell.