Researchers Develop Bacterial Enzyme-Based Catalyst for Water-Gas Shift Reaction at Ambient Conditions; New Thinking About Catalyst Design
|Researchers used coupled enzymes for the WGS reaction at ambient temperature. Source: ACS. Click to enlarge.|
A team of researchers from the UK and US have developed a coupled bacterial enzyme-based catalyst for the important water-gas shift reaction (WGS) for the production of hydrogen from syngas. A paper on the work was published online in the Journal of the American Chemical Society on 15 September.
The water-gas shift (WGS) reaction for the production of hydrogen from carbon monoxide and water (CO + H2O ↔ CO2 + H2) typically requires high temperatures typically in excess of 200 °C and a metal catalyst. The team, led by Fraser Armstrong at Oxford, separated the WGS process into two half-cell electrochemical reactions (H+ reduction and CO oxidation), catalyzed by bacterial enzymes attached to a conducting particle.
The H+ reduction reaction is catalyzed by a hydrogenase, Hyd-2, from Escherichia coli, and CO oxidation is catalyzed by a carbon monoxide dehydrogenase (CODH I) from Carboxydothermus hydrogenoformans. This results in a highly efficient heterogeneous catalyst with a turnover frequency, at 30 °C, of at least 2.5 s-1 per minimum functional unit (a CODH/Hyd-2 pair) which is comparable to conventional high-temperature catalysts.
The point, said the researchers, was not to develop a replacement for robust industrial WGS catalysts, but to spur thinking about catalyst design that could match the efficiency of biological enzymes.
The experiments are unlikely to have direct relevance for energy technology as they use tiny amounts of fragile enzymes rather than robust synthetic catalysts that could be scaled up indefinitely. Even so, the study demonstrates some interesting alternative concepts for catalysis and highlights the wide gap between redox enzymes and synthetic catalysts in terms of both rates and efficiency.
Coupling via electronically conducting particles enables a catalytic redox reaction to be separated into two half-reactions having lower activation energies than the entire reaction at a single site. The H+/H2 and CO2/CO redox couples are both reversible at electrodes modified with hydrogenases and CODH, respectively, whereas the best that chemistry can currently offer are Pt metal catalysts for the hydrogen system; however, Pt is incompatible with CO and could not be used here.
Finally, the electrochemical reversibility of the CO2/CO couple catalyzed by CODH is not only a key requirement for the WGS particle catalyst but also highly inspirational in view of demonstrating the feasibility of efficient CO2/CO electrochemical cycling with CO serving as an energy store.—Lazarus et al. 2009
Oliver Lazarus, Thomas W. Woolerton, Alison Parkin, Michael J. Lukey, Erwin Reisner, Javier Seravalli, Elizabeth Pierce, Stephen W. Ragsdale, Frank Sargent and Fraser A. Armstrong (2009) Water-Gas Shift Reaction Catalyzed by Redox Enzymes on Conducting Graphite Platelets. J. Am. Chem. Soc., Article ASAP doi: 10.1021/ja905797w
Enzymes inspire new catalyst design for hydrogen production (Chemistry World)