Professor Caroline C. Jarrold of Indiana University is supported by the Chemical Catalysis Program in the Division of Chemistry to study the physical, electronic and chemical properties of molybdenum-based binary oxides clusters in several charge states using gas-phase physical chemistry tools, including mass spectrometry, anion photoelectron spectroscopy, and photodissociation studies used in parallel with density functional theory calculations. The main objective is to achieve a better understanding of the structure-function relationships of the catalytically-relevant interactions of these oxides with alkanes and alcohols.
Catalysts play a critical role in many processes relevant to energy, the environment and the economy, and molybdenum-based binary oxides have been identified as promising catalytic materials for the industrially important oxidation of alkanes and alcohols. Students and postdoctoral associates involved in this research will become experienced with state-of-the-art gas phase research tools and will be trained in ethics, scientific information dissemination, community outreach, and career development.
Catalysts are used in chemical reactions that lead to the production of numerous everyday materials such as plastics, medications, and pigments, and catalysts are important in reducing pollution from automobiles and industrial sources of emissions. The importance of catalysts in the economy and environment provides strong motivation for improving or developing new catalytic materials that have lower operating temperatures and improved resilience, resulting in reduced energy consumption and waste. The overarching goal of our project was to determine the specific features that make catalysts active, to benefit efforts toward improving catalysts. Intellectual merit: Most industrial catalysts are metal oxide particles, or metal particles supported on a porous oxide material. Often, combinations of metals produce better catalyst properties relative to their pure metal counterparts. Pretreating the metal oxides to reduce the oxygen content in the metal oxides is also often required to ‘activate’ the catalysts. Because the interactions between catalysts and molecules participating in catalyzed reactions take place at the atomic level, we have taken a bottom-up approach to understanding how mixed metal combinations an variations in the oxygen content affect the arrangement of atoms and electrons, both of which influence interactions with reactant molecules. Our research project draws on experimental and computational expertise. Experimentally, we have generated and probed the electronic and atomic arrangement in small metal oxide species with a range of different metal combinations and different oxygen contents. Computationally, we have calculated a range of possible cluster structures and relative energies, which, when compared with the experimental results, allow us to draw conclusions on how the metal- and oxygen content affects the properties of the metal oxides. Principal Findings: We have completed a series of studies on two classes of mixed metal oxides. The first class involves different combinations of transition metals, which serve as models for ‘doped’ metal oxide catalysts. Based on our findings on a series of transition metal combinations, the behavior of the pure bulk metals provides the most reliable predictor of the structures formed between the mixed metal atoms with varying oxygen content. The metal with the lower reduction potential, an important and readily available quantity used in electrochemistry, will preferentially bond with oxygen atoms, leaving the other atom electron-rich, and potentially more catalytically active. The second class of mixed metal oxides combined a transition metal with aluminum, a lighter metal commonly used in the porous oxide supports, but with different properties from transition metals. These mixed metal oxides exhibited strikingly different properties from the first class of metal oxides. The aluminum atoms became singly charged cations (Al+), while the remaining transition metal and oxygens formed complex anions (MOy-, y = the number of oxygen atoms). This finding suggests that the chemical nature of the boundary between a transition metal or metal oxide particle deposited on a support material will be distinct from the support and the isolated catalyst in a predictable way. Broader Impacts: Results of these studies have deepened our understanding of the fundamental processes in binary metal oxide catalysts, and have provided new, concrete directions for optimal catalyst design. However, another important goal of this research program was to provide an inclusive research environment that cultivates new scientists who give priority to problems with relevance in energy, environment and the economy when designing and executing well-conceived experiments. Students and postdoctoral research associates who have participated in this research have gone on to pursue scientific careers in both industry and academics.
