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New research solves debate over water-gas shift reaction mechanism

By Scott Jenkins |

The water-gas shift reaction (WGS; in which water vapor is reacted with carbon monoxide over a copper-chromium-iron-oxide catalyst to yield hydrogen and CO2) is of paramount importance to the chemical industry, as it is a primary route to producing H2 for a host of industrial applications in petroleum refining, ammonia production, metal production, food production and others. Recently published research resolves uncertainty over the mechanism of the WGS reaction, and thereby aids efforts to design and synthesize improve catalysts for the reaction.

A team of scientists at Oak Ridge National Laboratory (ORNL; Oak Ridge, Tenn.; www.ornl.gov) used a host of in-situ characterization techniques, including neutron vibrational spectroscopy, infrared spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy, as well as computational methods, to examine the surface of the CuCrFeOx catalyst under real-world WGS conditions and to identify the intermediate species for the reaction. In the past, two possible reaction mechanisms for the WGS have been debated — a “redox” mechanism involving the participation of atomic oxygen from the catalyst, and an “associative” mechanism, proceeding via a surface formate-like intermediate, the researchers explain. “The answer is important because it helps us identify the critical point in the reaction where hydrogen is generated,” said ORNL researcher and lead author Felipe Polo-Garzon.

Because of changes in the catalyst that occur during the reaction, characterizing the material is difficult. Limitations in the measurement techniques left unanswered questions about how the surface chemistry changes during the reaction mechanism. The VISION beam line at the ORNL Spallation Neutron Source, a Dept. of Energy user facility, helped overcome the previous limitations and elucidate the actual WGS mechanism in conjunction with other techniques. The combined spectroscopic and kinetic evidence collected by the team shows that the reaction proceeds via the “redox” mechanism. According to the researchers’ data, at high-temperature conditions, the catalyst loses oxygen atoms to make room for water molecules that dissociate and give off pure H2.

The new mechanistic information opens pathways for the design and synthesis of new catalyst structures that could improve the cost and efficiency of large-scale H2 production.

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