Professor Bert D. Chandler of Trinity University is supported by the Chemical Catalysis Program in the Division of Chemistry to develop new synthetic techniques for the preparation of solid oxide supported bimetallic nanoparticle catalysts and study reaction mechanisms over these heterogeneous catalysts. The bimetallic nanoparticles will be prepared in solution, using a dendrimer encapsulation synthetic route and then deposited onto high surface area support. Two kinetics-based analytical techniques will be employed to probe and evaluate the catalytic activity of the bimetallic nanoparticles: Selective inhibition will be used to interrogate the CO oxidation active sites and Hammett relationships for aerobic benzyl alcohol oxidation will be used to probe changes in nanoparticle surface charge.
Supported bimetallic nanoparticles are an important class of heterogeneous catalysts that are employed throughout the chemical industry. A fundamental understanding of the interplay between metal functions in bimetallic catalysts will informs the design of new catalysts with less reliance on expensive platinum group metals. Integrating undergraduate students in a primarily undergraduate institution into this research project early in their chemical career will motivate them to pursue this line of research and consequently enhance the chemical catalysis workforce.
Supported metal nanoparticle catalysts, which consist of a few hundred atoms of a reactive dispersed on an oxide carrier, are widely used in industrial chemistry, especially in petroleum refining, bulk chemicals production, synthesis of pharmaceuticals, and emissions control. This NSF funded grant supported research to help us better understand how these catalysts work, so that we may ultimately improve them and use them to do new chemistries. Improvements in this area can lead to substantial cost savings, providing cheaper, cleaner methods of producing many of the materials that we use every day. Our work focused on studying gold catalysts. For years, gold was considered to be an inert metal, meaning that it doesn’t react with very many things. However, about 30 years ago a Japanese scientist discovered that Au particles consisting of a few hundred atoms and supported on certain oxides are extremely active for oxidizing CO to CO2. Ever since then, scientists around the world have been trying to figure out how gold is able to do this interesting chemistry. Part of the problem is that nanoparticles, while small, are complicated. The metal atoms on the surface take on different arrangements, and only some of them are able to do the CO oxidation chemistry. We wanted to get an idea as to how many surface Au atoms are involved in the reaction. To explore this, we added small amounts of NaBr to some Au catalysts. Bromide forms strong bonds to Au; the idea is that the NaBr will bind to the most reactive Au atoms, slowing down and eventually stopping the catalysis (see Image). The image shows two separate experiments on each poisoned; the average is plotted in red circles. As you can see from the x-intercept in Figure 1, about 11% of the total Au in the sample is active for the reaction. Knowing this allows us to compare different catalysts – we can tell if one catalyst is more reactive because it has a greater number of reaction centers (larger x-intercept) or if there is something chemically different about the reaction centers. Developing this technique (we call it a kinetic titration or controlled poisoning experiment) was a major part of this research. These experiments, and some associated work that involved studying how the reaction rate was affected by how we treated the material prior to catalysis experiments, allowed us to better understand exactly how Au nanoparticles perform the CO oxidation reaction. We found that, when CO reacts with O2, surface carbonates quickly accumulate on the catalyst, and that this poisons the reaction. Interestingly, it lowers the reactivity without blocking the gold sites (like NaBr does), which confused us for quite a while. Eventually, after doing a lot of additional experiments to see how the catalyst activity changed when we changed the reaction conditions, we were able to figure out that water was critical to the reaction. With the help of some computer modeling from our friends at the University of Houston, we were able to show that O2 binding occurs with a simultaneous proton transfer from water to O2. This results in the formation of Au-OOH, which is a superoxide molecule bound to Au. Superoxide is a very reactive form of O2 – even more reactive than peroxide, which is commonly found in drug stores. Although this isn’t what we originally started out trying to do, understanding this reaction is important. Scientifically, we think we have solved a puzzle that has been troubling scientists for more than a quarter century. More importantly, it allows us to think of new ways to use these catalysts, and we are now looking for ways to extend the basic reaction chemistry to other more important industrial reactions.
