(409c) The Chemistry of Model Catalyst-Electrode Interfaces in Photoelectrocatalytic CO2 Reduction Systems for Solar Fuels

The chemistry of semiconductor-catalyst interfaces relevant to the production of solar fuels by photoelectrocatalytic CO₂ reduction is explored by analysis of model systems.  Photoelectrocatalytic CO₂ reduction is an electrochemical process, but assessing reaction mechanisms using electrochemistry techniques alone is challenging because of the complex surface chemistry involved in fuel synthesis reactions.  These reactions are typically associated with a number of competing pathways, which yields a distribution of products and low overall efficiency for the intended energy-rich chemical. Control of product selectivity requires catalyst activation by management of electron/proton transfers as well as engineering of active chemical sites.  We wish to understand the role that the photoelectrode surface plays in achieving high selectivity in heterogeneous catalyzed electrochemical reduction of CO₂ to fuels.

We use an experimental surface science approach to fabricate well-defined electrode-adsorbate systems in ultrahigh vacuum. We use single crystal surfaces to analyze surface chemistry independent of complicating factors such as grain boundaries and morphology. Surface-sensitive core-level and vibrational spectroscopy measurements on the model systems are used to supplement electrochemistry investigations, and generate unique information describing the role of the electrode surface in CO₂ reduction. Conclusions from experimental results on these well-defined systems are supported by calculations using density functional theory.

Much of the experiments presented are designed to explain the role of the semiconductor electrode surface in the high selectivity observed in pyridine and other N-heterocycle-catalyzed CO₂ reduction systems. These systems have proven to selectively reduce CO₂ to methanol using only solar energy and water.  We examine the preferential adsorption sites, bonding interactions, and reactivities of these molecular catalysts and other relevant adsorbates on the surfaces of photoactive electrode materials.  Specifically we use ambient pressure photoemission spectroscopy (APPES), scanning tunneling microscopy (STM), and vibrational spectroscopies including infrared reflection-absorption spectroscopy (IRAS) and high resolution electron energy loss spectroscopy (HREELS) to study the interaction of water and CO₂ reduction catalysts with the surfaces of III-V semiconductors.  The chemistry of adsorbed water is important for aqueous photoelectrocatalytic systems because of the role that adsorbed negatively charged hydride and hydroxide can play in activating adsorbed molecular catalysts for subsequent electron/proton transfers.

This work assists in generating a molecular-level understanding of the heterogeneous processes important to the reaction mechanisms of efficient photoelectrocatalytic fuel generation.