The Chemical Catalysis Program in the Division of Chemistry and the Office of International Science and Engineering will support Professor Craig Hill and his coworkers at Emory University to work in conjunction with Professor Lee Cronin at the University of Glasgow in the UK to develop catalysts for the visible-light-driven reduction of water to dihydrogen and the construction of a new type of multifunctional, high-surface-area microtube composed of photosensitizer cations and polyoxometalate (POM) catalysts. UK's Engineering and Physical Sciences Research Council (EPSRC) will support the research of Professor Cronin and his coworkers. New photocathodes modified with these microtubes will be constructed for the efficient photogeneration of dihydrogen. Interactions between photosensitizer ruthenium cations and POM catalysts will be studied in both solution and the solid state. The prosecution of this research requires instruments and nano- and microfabrication infrastructure and materials characterization techniques present in the UK laboratory. Equipment and knowledge for photocatalysis and mechanistic investigations in Professor Hill's laboratory will be shared with the UK laboratory.
New materials for the photogeneration of hydrogen from water and sunlight will be developed using ruthenium photosensitizer cations and polyoxometalate catalysts. The research will train young scholars in the technical, intellectual and practical challenges associated with green energy chemistry together with the catalysis, materials, and spectroscopic aspects of solar fuels research. Investigators and students at Historically Black College and Universities and Primarily Undergraduate Institutions will be invited to attend group meetings and annual conferences.
Intellectual Merit This work targeted development of microstructures contained both the photosensitizer cation, i.e. [Ru(bpy)3]2+ for light absorption-charge separation, and a polyoxometalate (POM) water oxidation 2 H2O + hv (sun) -> O2 + 2 H2 (1) catalyst (WOC) for multi-hole charge storage and O2 evolution. Specific aims included maximizing yields of these microtubes while optimizing their (a) light-absorbing properties, (b) catalytic water oxidation activity, and (c) stability. In preliminary work, some prototype microtubes remained intact for a few turnovers oxidizing water to O2 in the presence of excess oxidants. However we found these microtubes, while elegant, were too unstable for serious development. Despite Herculean efforts, a wide range of tubes proved to be unstable to mechanical perturbation and in many cases unstable to self-oxidation (the WOC component oxidized the bpy moieties present in the tubes). No combination of photosensitizers and POMs solved these stability problems. Nonetheless these failures taught us: (1) the controlling factors in excited state partitioning (specifically factors impacting (a) oxidation of the WOC, (b) self oxidation of the bpy ligands, and most interestingly, POM WOC catalysis of bpy oxidation); and (2) synthetic approaches to form and manipulate these microstructures. The degradation of photosensitizers and proximal organic structures catalyzed by the WOCs themselves is a general feature that has largely been ignored in design and study of molecular solar-fuel generation systems to date. Although the Ru(bpy)3]2+/POM microtubes didn’t have the stability to be viable, we were able to construct similar two-component systems using a rotating ring disk electrode (RRDE) system (in collaboration with A. Bond et al., experts in this technique). These systems and technique confirmed catalytic water oxidation (direct detection of O2) and importantly, clarified another key role of [Ru(bpy)3]2+ in such multi-component structures: it mediates electron transfer to and from the WOC, whether the oxidant is a soluble species or an electrode surface. These experiments have direct implications for current studies in many other laboratories. An attempt to produce chiral self assembly of the microtubes proved unfeasible. However, in collaboration with the Liu group (University of Akron), our large homochiral organic derivatives of Fe-based POM-like units, as approximate models of the [Ru(bpy)3]2+/POM WOC microtubes, successfully self-assembled into homochiral structures. Static light scattering (SLS), dynamic light scattering (DLS) and circular dichroism facilitated characterization of these complicated product structures. Another successful joint effort modelling the photosensitizer-POM WOC microtubes involved inducing the proximity of a semiconductor photosensitizer and the POM catalyst also by controlled electrostatic immobilization. These publications demonstrated that SLS, DLS CD could be applied to complicated POM-containing functional microsystems. The products (publications, etc.) from this grant have resulted in a better understanding of how to control the self assembly of POM-containing light absorbing and catalytically water oxidizing microstructures, including electrostatic and relative polyanion:polycation size effects. Broader impacts This grant required collaboration with experts in nontraditional techniques for POM characterization because of the complexity of the POM-containing multifunctional microstructures. Collaborations led to direct training of students and postdocs in SLS and DLS (University of Akron), CD, con-focal Raman spectroscopy (GA Tech) and RRDE (Monash University, Australia). Second, these extensive inter-research group tutelage efforts helped establish two new courses at Emory University in which the PI’s students and postdocs had input: (a) energy conversion (Chemistry 729R) and (b) the interface of research, government and industry; Chemistry 468). A key component of the latter course was instruction on how to write and defend research grant proposals and was open to input by professionals in non-research sectors. The PI also brought some of the students funded by this NSF grant and some of its findings to a freshman symposium. In addition, students from the Paul Kögerler laboratory (University of Aachen) were invited to work in our laboratory and interact with the PI’s co-workers on this particular program. These visitors helped design cells for electrochemical, structural and spectroscopic investigation of the delicate microtubes targeted by this NSF grant. The cell design research provided insightful instruction for both the PI’s co-workers and the visiting students. The cells were quite successful for investigating more robust traditional solar fuel generation systems but not for the microtubes because of their instability. This effort also engaged the PI’s colleagues, Jamal Musaev for computational studies and Tim Lian for transient absorption spectroscopy; however the instability and heterogeneity of the microtubes precluded acquisition of publishable data with these two groups. Finally, this NSF-funded research led to interactions with some visiting scientists who provided useful suggestions to address our challenges, while our findings helped educate them on some technical challenges their groups are facing.
