A Catalytic Conversion

      In 1771, Italian physician Luigi Galvani was conducting a routine dissection of a frog leg when something unexpected occurred. Working at a table he had used earlier for some experiments on static electricity, Galvani picked up a metal scalpel that had accumulated charge while resting on the surface. When he touched the scalpel to the dead frog’s sciatic nerve, the leg twitched—Galvani had unintentiona lly discovered that nerves conduct electricity. Over two centuries later, biophysicist and Nobel Laureate Max Delbrück described accidents like Galvani’s as a necessary component of scientific discovery and thus coined his whimsical “Principle of Limited Sloppiness.” Defending his claim, Delbrück argued that science should be, “Sloppy enough so that unexpected things can occur, but not so sloppy that we can’t find out what happened.”

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      Chemistry graduate student Hemamala Karunadasa is continuing to investigate how her novel molybdenum-based catalyst splits water.
      Credit: Marek Jakubowski

      Fast-forward to UC Berkeley in 2009. Hemamala Karunadasa, a graduate student in the chemistry lab of Professor Jeff Long, had been conducting experiments using the heavy metal molybdenum, exploring its possibility for use as a single-molecule magnet. Because molybdenum is highly reactive, Karunadasa conducted all of her experiments in specialized tanks containing the inert gas nitrogen. However, at the end of the day as she was washing her glassware with water, she noticed a curious reaction taking place—tiny traces of her molybdenum compound, originally orange, appeared to be bubbling and turning green when exposed to the water. Karunadasa hypothesized that the compound was splitting water to produce hydrogen gas, resulting in the bubbling she observed.

      If true, Karunadasa, like Galvani, had possibly made a very fortuitous discovery by accident, for hydrogen gas produced from water has been touted as an enticing green energy source. Though ideal in theory, the technology has been tough to implement in practice because of a couple knotty issues. First, producing hydrogen requires transferring electrons efficiently to water, which often requires very expensive metals such as platinum to act as catalysts, as well as electricity to power the reaction. Second, most catalysts discovered to date can only efficiently split water free of impurities, adding another costly step to hydrogen production. Nevertheless, water splitting is The apparatus used for measuring the electrochemical parameters of the water-splitting reaction. The metallic mesh pictured is a platinum gauze, which serves as an electrode. one of the most common chemical reactions on Earth—it is the final step in photosynthesis—providing a tantalizing example of the potential to generate hydrogen from water and light if only the right catalyst could be discovered (also see “Photosynthesis,” <i>BSR</i> Spring 2009).

      Before she could begin thinking about any far-reaching applications, Karunadasa first had to verify that hydrogen was being produced and was coming from water. By labeling the oxygen in water molecules with a heavy isotope, Karunadasa, Long, and their colleagues in chemistry professor Chris Chang’s lab demonstrated that hydroxide ions (and by extension hydrogen) were indeed produced in the reaction by splitting water. This proved successful, but Karunadasa points out that she was still skeptical of the reaction’s practicality. The question was, could they get the cycle to occur again and again without depleting the molybdenum compound? “We asked ourselves: can we take this molybdenum that has lost two electrons and pump in two new ones? And if we continuously add electrons, could this molecule continuously generate hydrogen?” says Karunadasa.

      What they subsequently discovered was significant—the molybdenum compound was rare in that it could continually give up and accept up to three electrons, serving as a replenishable electron source. By using an external electron source known as a potentiostat, Karunadasa could pump the two electrons lost in the splitting reaction back into the molybdenum compound, restoring it to its original oxidation state and allowing the reaction to occur again from square one. Thus, the molybdenum compound’s role in the reaction was shown to be catalytic, making it a potentially invaluable reagent in the ongoing search for efficient energy sources.

      The apparatus used for measuring the electrochemical parameters of the water-splitting reaction. The metallic mesh pictured is a platinum gauze, which serves as an electrode.

      The apparatus used for measuring the electrochemical parameters of the water-splitting reaction. The metallic mesh pictured is a platinum gauze, which serves as an electrode.

      While not yet as efficient at splitting water as current front-runner platinum, molybdenum may be a potential boon because it is significantly cheaper than its competitors and can even split our most abundant water source—seawater. The major disadvantage right now is the energy required to replenish the compound’s lost electrons after each cycle. “A lot of the reason people have been pursuing this kind of research is that it’s clear from biology that it’s possible to do this reaction effectively. Nature does this reaction at close to zero [energy loss]. We think we can try to match that,” says Long. One way the lab is currently attempting to tackle this issue is by tinkering with the geometry of the groups of atoms surrounding the molybdenum metal to try to lower the energy requirements for catalysis. The ideal scenario, however, would be to power the system using nature—which is why Long, Chang, and Kuranadasa are collaborating with other groups to figure out a way to run the system using light energy. “If we can power this reaction with light, that would be great. After all, we’ll never run out of sunlight and seawater,” says Kuranadasa.  Because of her fortuitous discovery, Karunadasa decided to remain at Berkeley for an extra year to continue her work with the molybdenum catalyst. Currently attempting to pick apart the mechanism for the reaction (“a bit of a black box,” she admits), Karunadasa is modest about her unexpected good fortune. Describing her sudden switch from studying magnets to trying to crack the renewable energy problem, Karunadasa exclaims, “I had to learn everything from scratch—this was completely accidental!” Max Delbrück may be grinning in his grave.

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