Model Inverse Nanocluster Catalysts: The Role of Size, Shape and Composition on the Catalytic Activity of Small Metal Oxide Clusters on Metal Surfaces

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

Chemical catalysis involves a chemical substance, called a catalyst, which lowers energy costs and creates more selective product distributions by providing another pathway for the chemical reaction of interest. Catalysts are often employed to generate environmentally friendly fuels, such as hydrogen which burns cleanly to water, and are also used to produce value-added chemicals, like carbon monoxide and methanol which can be made from sustainable crop sources. Because of the importance, catalyst function is a major driving force of research in the chemistry community. Transition metals and transition metal oxides possess many of the desired chemical properties for catalysts that can activate the bonds in CO2, H2O and CH4, which are particularly promising feedstocks for a more sustainable production of fuels and value-added chemicals. These metals and their oxides can be especially active as small atomic clusters in the 0.00000005 inch or nanometer size range. Nanoclusters exhibit enhanced reactivity due to their unique geometric and electronic characteristics such as under-coordinated surface atoms, modified inter-atomic spacings and large surface to volume ratio. In this project, Drs. Buratto, Bowers and Metiu produce model catalysts of both metals and metal oxides by deposition of well-defined, atomically-precise nanoclusters from the gas phase onto metal and metal oxide supports. These model systems are then tested for their reactivity in the production of hydrogen and methanol, important as both clean-burning fuels and chemical feedstocks. The research team's unique capability to control nanocluster composition atom-by-atom provides the requisite level of detail to understand the chemistry on the atomic level and provides important insight into the development of new catalytic systems. Drs. Buratto, Bowers and Metiu and graduate students supported by this project are active in outreach to high school students in the Santa Barbara and Ventura Counties to discuss their research and its impact as well as promote science education. With funding from the Chemical Catalysis Program of the Chemistry Division, Drs. Buratto, Bowers and Metiu prepare, characterize, and test two new classes of nanoscale catalysts based on the atom-by-atom assembly of small bimetallic and metal oxide clusters having one feature in common; they have very small, isolated, well-defined, catalytically-active sites and enhanced catalytic activity. The research centers on the preparation of well-defined PdAun and PtSnm clusters supported on single crystal TiO2(110) and well-defined FexOy supported a single crystal Pt(111) in the inverse catalyst geometry. These model systems are prepared by depositing mass-selected clusters from the gas phase onto single crystal surfaces to control catalyst size and composition. Samples are then studied in ultra-high vacuum by x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) to determine composition and structure. Temperature programmed reaction (TPR) is used to probe the activity to the water gas shift reaction, CO oxidation, and methanol synthesis. Density functional theory (DFT) is used to calculate the structure of the clusters, their XPS spectrum and their chemical activity, and these data are then compared to experiment. The results are used to develop a detailed fundamental understanding of the catalytic chemistry at the atomic level that will in turn help optimize important industrial processes, and improve the performance of the existing catalysts or uncover new ones. The research groups of Drs. Buratto, Bowers and Metiu are committed to K-12 outreach and the promotion of science in general. They are incorporating their research in heterogeneous catalysis into the University of California Santa Barbara's (UCSB's )5th grade outreach program. This program brings 5th grade students from elementary schools in the Santa Barbara area to the UCSB campus for hands-on science activities. The research team is also working with the teachers to develop a lesson in catalysis that is appropriate for the 5th grade curriculum and then incorporate it into the outreach program. In addition, graduate students supported by this project are active in outreach to high school students in the Santa Barbara and Ventura Counties to discuss their research and its impact, as well as to promote science education.