Researchers from the University of New South Wales (UNSW), together with international colleagues, have demonstrated a process that could give a second life to glycerol, a cheap byproduct of biodiesel production. The work was published in the journal Applied Catalysis B: Environment and Energy.
In biodiesel production, about 100 kg of glycerol is generated for every tonne of fuel. In raw form it costs only about 0.11 US dollars per kilogram. However, it can be used to produce more valuable chemicals, such as glyceraldehyde and dihydroxyacetone, which are used in the chemical and pharmaceutical industries and cost about 75 and 150 dollars per kilogram respectively.
The scientists studied so-called photoreforming, a process in which light energy drives chemical reactions while simultaneously producing hydrogen and converting organic feedstock into useful products. To do this, they created a photocatalyst based on zinc indium sulfide, adding small amounts of nickel and cobalt to it.
Under visible light, the modified catalyst released hydrogen about 10 times faster than the original material. In addition, it changed the course of the reaction: the share of glycerol converted into glyceraldehyde and dihydroxyacetone increased noticeably. In tests simulating sunlight, simultaneous formation of hydrogen and useful organic products was observed, and the hydrogen evolution rate was comparable to other solar glycerol photoreforming systems described in the literature.
To understand why the addition of nickel and cobalt changes the reaction, the researchers observed the catalyst directly during its operation. At a synchrotron in Australia, they used X-ray absorption spectroscopy (XAS) and its fast variant FE-XAS, which makes it possible to track changes in the catalyst much faster than conventional XAS. It turned out that under light, nickel and cobalt transition from predominantly positively charged ions to more reduced states associated with the active form of the catalyst. These changes were reversible: they disappeared when the light was switched off, and repeated switching on and off was reproducible. Control measurements in the dark confirmed that the transformations were caused specifically by light and not by the X-ray beam.
In addition, using infrared microspectroscopy on another synchrotron beamline, the scientists monitored molecules on the catalyst surface. Together, these methods made it possible to see which sites are responsible for different reaction pathways. It was found that the zinc indium sulfide surface on its own mainly directs glycerol toward the formation of hydroxyacetone, while the addition of nickel and cobalt enhances hydrogen release and shifts the reaction toward glyceraldehyde and dihydroxyacetone.
According to Dr Denni Gunawan of UNSW, the value of the approach lies not only in producing hydrogen but also in understanding how to control the chemistry of the process, directing residual carbon into useful products rather than waste. The researchers note that the method could be extended to other biomass feedstocks, but for now it is only a laboratory demonstration, not a ready industrial technology.