In the lab, sunlight turns biodiesel waste into hydrogen

Under the beam of an X-ray instrument at the Australian Synchrotron, a catalyst powder sits in front of the light that drives it. When the light goes on, the nickel and cobalt atoms on its surface change their chemical state; when it goes off, they change back. That back-and-forth, observed in less than one second, is the heart of a new study led by researchers at the University of New South Wales (UNSW).

The process is called photoreforming. Light energy drives a reaction that splits a feedstock into hydrogen and other products at the same time. The team chose glycerol, the byproduct of biodiesel production, which the growing biodiesel industry makes in large quantities, about 100 kilograms for every ton of biodiesel. Crude glycerol can be worth as little as US$0.11 per kilogram. Turning it into glyceraldehyde and dihydroxyacetone, used in the chemical and pharmaceutical industries, changes that value. Those two products are valued at approximately US$75 and US$150 per kilogram.

The photocatalyst is made of zinc indium sulfide, modified with very small amounts of nickel and cobalt. Under visible light, the nickel-cobalt cocatalyst raised the hydrogen evolution rate by approximately 10 times compared with the unmodified material. It also changed where the glycerol went. The unmodified surface mostly favored a pathway producing hydroxyacetone; with nickel and cobalt added, more of the glycerol followed the routes leading to glyceraldehyde and dihydroxyacetone.

The mechanism was the hardest part to see. Using in situ X-ray absorption spectroscopy (XAS), which reads the chemical state of a metal while the material is working, ANSTO principal scientist Bernt Johannessen and colleagues watched the nickel and cobalt sites. Their starting state was mostly positively charged ions; under light they became more reduced, the form associated with the active catalyst. Dark control measurements showed the change was driven by light, not by the X-ray beam. A second technique, infrared microspectroscopy, followed the molecules on the surface and helped identify the intermediates along the way.

For biodiesel producers, the value of the approach depends on what it could eventually do with their leftover glycerol, and the study does not answer that. It gives no price for the process, no capacity figure and no date for a demonstration outside the lab, and its hydrogen figures come from simulated sunlight. What it does establish is a method for seeing how the catalyst works, which the team says is what makes it possible to design catalysts that steer the reaction toward chosen products. Any move from this bench result to a plant would start with that design work.