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Reports: ND651290-ND6: Exploring Electronic-Transfer Pathways of Hot Electrons in Organically-Assisted Metal Catalysts

James P. Lewis, Ph.D., West Virginia University

Report for PRF Award #51290-ND6: The focus of the final year of our award was to design and develop Au-based nanocatalysts for energy and environmental applications. Although bulk gold is known as a very stable metal, Au nanoparticles with size smaller than 8 nm exhibit unique catalytic activity and selectivity in many reactions under mild conditions. We have investigated Au nanoparticles stabilized by organic-ligands. This category of gold nanoparticles have been treated as simplified models for Au nanoparticles. As a further step, starting from the understanding of the small Au nanoparticles (Au13 in our case),   we have designed a complex model to mimic the oxide supported Au-nanocatalyst. By studying this model, we aimed to understand the synergetic cooperation between the two components-Au nanoparticle and the oxide support. Even though significant progress has been achieved for Au/TiO2 catalyst, it is still a significant challenge to understand the microscopic mechanisms regarding the catalytic activity. The key criteria is to synthesize novel catalysts by artificially controlling the synergistic effect. Herein, we design a catalyst to enhance the complex features of nanostructural Au/TiO2. When brought into intimate contact, the positioning of the Au nanoparticles on the anatase nanostructures strongly determines the resulting nature of their interaction (shown in Figure 1). Interestingly, it turns out that Au nanoparticles attached to the edge/corner sites of anatase cooperate with the Ti species, forming potential oxygen vacancies near the attaching region.

Figure 1. (a) The optimized structures of TiO2-308, (b) Au13 nanoparticle, and (c) Au13 nanoparticle attached on TiO2-308 with the lowest adsorption energy. In structure (c), there are five Au atoms involved with Ti atoms, where the Au-Ti bonds formed after Au13 attached on the TiO2 edged/corner site are highlighted by green bonds.

By analyzing the electronic properties, we expect to understand the interactions from principle. The low-coordinated Ti atoms at corner/edged sites exhibit localized electronic states at roughly -2.8 eV, which is very close in energy to the LUMO of Au13. This energy alignment results in a strong coupling between the filled Ti states and the empty states of Au13, thus promoting the charge transfer from Ti atom to Au atom. Based on this evidence, we hypothesize that the intimate interaction between the Au atoms and the edged/cornered Ti atoms possibly create oxygen vacancies around these attaching sites. Inspired by these understanding on the synergistic effect between Au and TiO2, we synthesize Au nanocatalyst with enhanced catalytic activity by artificially controlling the position of Au nanoparticles on TiO2 support. The catalysts yield remarkable high activities in the oxidation reaction of styrene.

In summary, we have developed an efficient way to boost the activity of Au nanocatalysts. Coupling computational models with synthesis and characterization, the cooperation between Au and anatase is significantly enhanced by positioning the Au nanoparticles on the edged sites. The strong interaction between Au and the Ti species around the edged/corner sites promotes oxygen vacancies which enhance catalytic activity. Importantly, our catalyst products exhibit high catalytic activity and excellent recyclability in the oxidation of styrene. Our investigation on the interaction of Au nanoparticles positioned on the edge/corner sites of TiO2 supports offers an excellent opportunity for designing and developing highly “controlled” catalysts in the future.

This research is an interdisciplinary project, which involves both computational and experimental efforts. Due to our ACS-PRF funding, we were able to develop a competitive NSF proposal as an extension of the Au-nanocatalyst direction - “Designing Tunable Au-Based Bimetallic Nanocatalysts” CHE1434378, 9/15/2014 - 9/14/2017. In addition to three pending manuscripts, publications resulting from the award:

Wang, L., Wang, H., Zhang, W., Zhang, J., Lewis, J. P., Meng, X., Xiao, F.-S. “Aerobic homocoupling of phenylboronic acid on Mg-Al mixed-oxides-supported Au nanoparticles,” J. Catal. 298, 186-197 (2013).

J. A. Carr, H. Wang, A. Abraham, T. W. Gullion, J. P. Lewis, “L-cysteine Interaction with Au55 Nanoparticle,” J. Phys. Chem. C, 116 (49), p25816-25823 (2012).