Chemistry Department Colloquium

To realize viable (fast, stable and cost effective) devices or nanostructures for the production of solar fuels, the research community must improve the core operating units and their integrated (device) formulations. These operating units involve (a) light absorption with charge separation,(b) multi-electron-reduction catalysts (CO2 and H2O as primary substrates) and (c) water oxidation catalysts (WOCs). Our multi-group team at Emory University is tackling these challenges by new approaches but building on many informative complementary studies in the literature. Most of the systems we are developing contain or are based on soluble, tunable transition metal oxide cluster anions (polyoxometalates or “POMs”). We have developed POM WOCs that are carbon-free (thus resistant to oxidative degradation), stable to hydrolysis (the pH range of stability depends on the POM and is controllable), and stable to heat. The first three WOCs of this class, two based on POM-stabilized Ru4O4 units1-3 structurally reminiscent of the OEC (Mn4CaO4) unit, and one based on a Co4O4 unit,4 are very fast, but we will present the systematic development of a new POM WOC that is more hydrolytically stable in basic aqueous medium (the desirable one for practical water oxidation) and turns over far faster than any synthetic catalyst reported to date (up to 1000/s, depending on conditions). We will also describe a multi-manganese-containing POM catalyst for multielectron reductions (H2O to H2 and CO2 to CO). It shares the many benefits of our POM WOCs, including its compatibility with water, but on the negative side, it’s slow without help (potential from biased anode or CT excited states of photosensitizer systems). We will conclude with new types of carbon-free photosensitizers5 and brief note of dyads and triads contained these new types of catalysts.