A novel enzyme-based catalyst developed by UK and US researchers hints at new ways of designing catalysts for the water-gas shift reaction, an important industrial reaction in the production of high grade hydrogen.    

The water-gas shift, which uses carbon monoxide and water to produce carbon dioxide and hydrogen, has been used in industry since the 1940s. Various metal catalysts are used, including copper and platinum, but according to Fraser Armstrong at the University of Oxford, industry could do better. His team have created an unusual catalyst in which two different bacterial enzymes are stuck to tiny pieces of graphite.  

One enzyme, a CO dehydrogenase, churns out carbon dioxide, releasing electrons in the process. These electrons pass via the graphite support to a [NiFe]-hydrogenase, which uses them to turn hydrogen ions into dihydrogen gas. Based on the efficiency of the less active component - the hydrogenase - Armstrong's team have calculated that their catalyst beats industry catalysts hands down. As well as working at a higher turnover frequency, it does it under ambient conditions - industrial catalysts for the reaction operate at around 200°C.

The point of the research, is not, however, to encourage industry to turn to enzymes, says Armstrong, but rather to start thinking about how catalysts can be made that match up to enzymes' superior efficiency. 'Enzymes can catalyse reactions at orders of magnitude faster than anything that we have at the moment - they set a target for what catalysts really ought to be like,' he says. 'The value of the paper is in telling industry to re-think the whole thing.'

Although Armstrong's enzymes are too fragile for industrial scale up, he believes they provide a completely fresh design for a catalyst, where the two different halves of the reaction are catalysed by two different components and connected by an electrically conducting particle. 

'The authors have designed here a "two-site" system, each of those having a well defined role,' says Frédéric Meunier, an expert on the water-gas shift at ENSICAEN, a university based in Caen, France. He agrees scale-up of the current design would be problematic, but says the idea of decoupling the two half reactions is exciting.

Chandra Ratnasamy works on the water gas shift reaction for fuel cell applications at Süd-Chemie in Louisville, Kentucky, US. She says Armstrong's work is very elegant but presents great technical challenges for industry. 'Based on the strategy outlined in the paper a synthetic catalyst could be developed, however, to scale up would not be straightforward as it would involve a number of factors including balancing the rates of the two independent half reactions,' she says. 'It would be a challenge to design a solid catalyst wherein the oxidation/reduction functions are separated but linked together electronically rather than physically.'

Hayley Birch

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