Trinity University chemists team with University of Houston engineers to unlock the role of water and gold in catalytic oxidation
by Susie P. Gonzalez
What do gold, water, shale gas, and researchers from two Texas universities have in common? When mixed together, they can begin to explain how to possibly make clean hydrogen from a growing South Texas and U.S. natural resource.
New drilling techniques have made large deposits of shale oil and natural gas available throughout the U.S. Large amounts of this shale gas can be converted into hydrogen gas, an important industrial chemical as well as a potential fuel. However, the hydrogen comes along with carbon monoxide that must be removed before the hydrogen can be used.
The best way to remove the carbon monoxide is to burn it with a little oxygen – a tricky technique that requires finding a way to selectively burn the carbon monoxide (CO) without burning the hydrogen.
This is where the Texas researchers come in. Chemists at Trinity University in San Antonio teamed with engineers from the University of Houston in using tiny gold nanoparticles and water to develop a new model for understanding the catalytic oxidation of carbon monoxide.
Bulk gold is a global commodity and currency because it is a fairly inert metal – it doesn’t readily react with other elements or compounds, especially oxygen. But when gold is divided into very small particles – particles consisting of a few hundred to thousands of atoms – many of its properties (color, for example) change. Twenty five years ago, Japanese chemist Masatake Haruta discovered that, when deposited onto certain metal oxide supports, these gold nanoparticles are outstanding catalysts for oxidizing, or burning, carbon monoxide at room temperature.
Since that discovery, the scientific community has worked to understand how gold nanoparticles catalyze CO oxidation. For more than a decade, researchers have known that water improved the catalyst, but no one understood how water promoted the reaction – until now.
Publishing their work in the Sept. 5 edition of the journal Science and at the Science Express website, Trinity University chemistry professors Bert Chandler and Chris Pursell along with University of Houston chemical engineering professor Lars Grabow provide a new, complete mechanism for this important yet elusive reaction.
“We knew gold was the best room temperature CO oxidation catalyst, but it turns out that gold by itself doesn’t catalyze the reaction very well at all,” Chandler said. “We needed to include just a little bit of water to really make the reaction go.”
Still, questions remained for the Trinity group regarding the molecular details of the reaction mechanism. Chandler decided to reach out to Grabow for help with computational work. The two met several years ago when Chandler was on academic leave at the Danish Technical University, and Grabow was a postdoctoral fellow there. They remained in touch and fell naturally into a collaboration once Grabow joined the UH faculty.
Grabow and graduate student Hieu Doan began work on a computational model to better understand the details of how oxygen, CO, and water react on the catalyst surface. The group helped explain the reaction mechanism and the UH model provided a molecular understanding of where the key proton transfer occurred during the catalysis, Chandler said
“It took all of us to make it happen – Johnny's careful experiments and determination to make things work, previous work by Dr. Pursell and myself using biochemistry kinetics to think differently about this system and the catalysis results, Dr. Grabow and Hieu building the most complete computational model of the system, and our suggestions for what steps to examine in detail,” Chandler said.
The work, which was funded by the National Science Foundation, opens the door to the potential of using this carbon monoxide oxidation process in making hydrogen from many sources, including petroleum and natural gas from underground shale formations and biomass.
“We were able to bridge the gaps between the traditional supported catalyst researchers and the surface science and computational communities,” Chandler said. “We knew water helped the reaction but didn’t fully understand its role. Now that we know water is a co-catalyst we can control the reaction more effectively. As we look to the future, we hope to be able to speed up the CO oxidation reaction in the presence of hydrogen to make this a viable system for producing clean hydrogen.”
Susie Gonzalez, director of public and media relations at Trinity University, can be reached at susie [dot] gonzalez [at] trinity [dot] edu.