Professor Jason Weaver of the University of Florida in the US and Professor Marcus Bäumer of the University of Bremen in Germany seek to advance the basic understanding of the selectivity of rare earth oxides (REOs) toward partial vs. complete oxidation reactions and to explore methods for tuning the catalytic selectivity of REOs through alkali-doping and nanoscale size effects. This research will employ a combined approach that involves model studies of pure and alkali-doped REO thin films in ultrahigh vacuum (UHV) and characterization of REO nanoparticles under more realistic reactive conditions, with the aim of establishing clear bridges in understanding across the materials and pressure divides. The involvement of multiple investigators with broad experience in surface science and catalysis research will enable the application of a large suite of analysis methods for characterization of materials and surface chemistries in ultrahigh vacuum (UHV) and ambient conditions, including the use of novel, plasma-based methods for growing oxides in UHV for fundamental studies.
The Chemical Catalysis Program in the Chemistry Division supports Professor Jason Weaver of the University of Texas- Austin in the US, whose research will provide new insights for improving the performance of REO catalysts used for the oxidative coupling of methane (OCM). The German funding agency, DFG, will support the work of the international collaborator, Professor Marcus Bäumer of the University of Bremen. The development of industrially-feasible routes for directly transforming methane to higher hydrocarbons could have significant economic and environmental benefits. The proposed project provides a unique opportunity to enhance the educational experiences of the students and principle investigators (PIs). The students will gain a much broader set of technical skills than they would by working at a single institution, and they will develop strong communication skills from the exchange of ideas among the research groups. Through involvement in a minority student mentoring program, Professor Weaver also plans to recruit undergraduate students from underrepresented groups to work on this project. One of the co-PIs will continue her work directing a mentoring program for female graduate students in chemical engineering.
Rare earth oxides (REOs) exhibit favorable catalytic performanc for a diverse set of chemical transformations, including both partial and complete oxidation reactions. However, with ceria as a general exception, mechanistic details of REO surface chemistry and the factors determining catalytic selectivity are understood only to a limited extent. In this project, we have been investigating the physical and chemical properties of model REO thin films and high surface-area catalysts, with the main goal to advance the fundamental understanding of the factors which determine the selectivity of REOs in promoting partial vs. complete oxidation chemistry. Of particular interest has been to develop REO catalysts that exhibit both high activity and selectivity towad the oxidative coupling of methane (OCM), which is a process by which to directly transform methane to more valuable C2 hydrocarbons. More specifically, ethylene is the desired product in the OCM as it is one of the most important hydrocarbon feedstocks in industry and is used in the production of plastics, detergents and ethylene glycol. We focused our studies on the reactivity of samaria and terbium oxides as these materials represent examples of effectively irreducible vs. reducible REOs, respectively, and hence propensity for catalyzing partial vs. complete oxidation reactions. In addition, our project has involved studies of the growth and surface chemistry of model REO thin films using diagnostic tools of ultrahigh vacuum (UHV) surface science as well as the synthesis of REO nanocatalysts and characterization of the catalytic peformance of these materials for promoting the OCUM under commercially-relevant conditions. Our surface science studies demonstrate that both samaria and terbia can be grown as high-quality thin films by deposition onto a Pt(111) substrate in UHV. We have reported detailed characterization of the surface structures of these REO films, and thereby obtained information that is essential for developing an atomic-level description of the REO surface reactivity. We have also found that Tb2O3 films can be completely oxidized to TbO2 in UHV by exposure to plasma-activated oxygen beams, and have also identified a surprisingly, weakly-bound state of oxygen on these surfaces. These findings demonstrate the capability to prepare higher Tb oxides for model studies of TbOx surface chemistry, and provide information about various states of oxygen that can contribute to oxidation catalysis on terbia surfaces. Lastly, we have also found that the samaria and terbia thin films are reactive toward methanol under UHV conditions, and find that the surface reactivity toward methanol depends sensitiviely on the oxidation state of the terbia and samaria surfaces. We have synthesized numerous Al2O3 or MgO-supported and unsupported samaria and terbia nanocatalysts using several methods, and examined their performance in the OCM reaction under different conditions. We have also explored the effects of alkali-metal doping on the behavior of REO catalysts in the OCM. As expected, we find that pure samaria is more selective than terbia toward promoting the OCM, since pure terbia tends to promote the complete oxidation of methane. Of particuarl importance is our finding that doping terbia with lithium significantly improves the catalytic selectivity toward the OCM. In fact, our Li-doped terbia catalysts exhibit higher activity toward the OCM than pure samaria and approaches the behavior of the best performing OCM catalysts reported to date. Overall, our project has provided new insights about the reactivity and catalytic behavior of model samaria and terbia surfaces that can aid in the rational design of REO catalysts for promoting oxidation chemistry.
