The six-electron reduction of carbon dioxide to form methanol has been known to occur on III-V semiconductor electrode interfaces for an extended time. However, the early reports by Halman and Frese that this reaction preceded on p-GaP and p-GaAs photocathodes also indicated limited reaction stability, low faradaic efficiency and high overpotentials. More recently, we reported that the addition of an aromatic amine such pyridine to the electrolyte stabilized the system and dramatically lowered the overpotential. Our studies indicate that it is the protonated form of the aromatic amine that is responsible for the observed electrocatalysis. Further, specific surface interactions appear to be critical to the initial reduction processes. To understand the nature of these interactions we have undertaken three classes of experimental studies. In the first instance we have utilized single crystal GaP electrodes with different faces oriented toward the electrolyte. Theoretical studies by the Carter group suggest that the [110] surface may have a surface configuration that is highly favorable for reduction of pyridinium to form a reactive hydride, which can reduce CO2. In addition to testing this suggestion by varying the reacting surface, we are also probe the system using a variety of substituted pyridines. Our second approach to this problem involves the use of platinum electrodes to probe the details of pyridinium- platinum surface interactions. While anticipated to be different that pyridinium-GaP interactions, these studies provide information on the breadth and nature of surface interactions in a most general sense. Work carried out in collaboration with the Batista group strongly suggest the formation of a surface Pt-H species via reduction of pyridinium, prior to reduction of CO2. However, the work also implicates a second pyridine-CO2 interaction that is critical to the reduction process. Our third approach to this problem involves the synthesis of novel non-oxide p-type semiconductors based on the CIGS structure, which provides great stoichiometric variability. In these studies we control both the bulk stoichiometry and surface orientation (using single crystal samples) to probe the electrochemical reaction dynamics.

Aromatic amine catalyzed photoelectrochemical reduction of CO2: What is the surface doing?

251st National Meeting (2016)

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