Prospects for using light to generate oxygen from water look sunnier because of a new inorganic catalyst. The material combines stability and selectivity with by far the highest rate of oxygen output, claim its developers at Emory University, Atlanta, and the Paris Institute of Molecular Chemistry, Paris. Laboratory experiments achieved a turnover number of 75 and a 64% oxygen yield.
Rapid Reaction
Figure 1. Cobalt-based material generates oxygen much faster than other homogeneous catalysts. Source: Emory University.The cobalt-based water oxidation catalyst (WOC) "has really upped the standard from other known homogeneous WOCs," says Craig Hill of Emory's department of chemistry, whose laboratory headed development efforts. "It's like a home run compared to a base hit," he contends.Traditional homogeneous WOCs contain an oxidizable organic structure and so suffer from instability under reaction conditions, note the researchers in a recent paper in Science. Their catalyst, which is free of carbon and consists of a Co4O4 core stabilized by polytungstate ligands, avoids such a problem. Moreover, cobalt is cheap and abundant, they add.Now the researchers are working to unravel the detailed mechanisms of water oxidation by the catalyst, notes Hill. They also are examining polyoxometalate (POM) stabilized molecules involving other metals and new POM ligands as potential WOCs. "We have looked into many already," he says. "We are examining manganese because that's the element nature uses in biological water oxidation/oxygen evolution."Such catalysts also suit oxidation of organic compounds, he adds.The key remaining challenge is achieving "still faster rates if possible while maintaining very high stability and selectively to O2 versus energy-wasting H2O2 and other intermediate species," notes Hill. "We think some multi-cobalt POMs might be even faster."