Principal Investigator Yogesh Surendranath
All artificial energy conversion catalysts fall into two broad categories: heterogeneous and molecular. Heterogeneous electrocatalysts are typically metallic extended solids that bind and transform substrates at surface-exposed active sites. The delocalized band structures of metallic solids and the strong coupling between these band states and surface active sites ensures that electron transfer is concerted with substrate binding and activation (above, right), enabling more efficient catalysis. However, metal electrode surfaces are inherently dynamic, displaying a distribution of surface structures that are recalcitrant to systematic modification to tune catalytic efficiency and/or selectivity. In contrast, for molecular catalysts, the local bonding environment around the metal active site can be tuned synthetically, enabling exquisite control over the energy landscape of a reaction. However, molecular catalysts display localized electronic states that are decoupled from the electrode surface leading, in most cases, to step-wise electron transfer and substrate activation that proceeds through a thermodynamically unfavorable intermediate. By exploiting the native surface chemistry of carbon, we have developed a versatile new class of graphite-conjugated catalysts (GCCs) that feature a conjugated pyrazine linkage between a molecular fragment and the band states of graphite.
The strong coupling afforded by this linkage simultaneously permits synthetic tunability of and concerted electron flow to interfacial active sites. We have shown that GCCs are active catalysts for the reduction of molecular oxygen, a key reaction in fuel cells, and that their activity can be modulated systematically by varying the substitution pattern of the appended unit. By synthesizing GCCs containing transition metal centers, we have developed highly active and selective heterogeneous catalysts for the reduction of carbon dioxide to carbon monoxide (left). We are currently investigating the mechanisms of inner-sphere charge transfer at these unique molecular interfaces and developing tailored GCCs for a wide variety of electrochemical and thermal reactions.