Mechanisms of Direct and Indirect (Photo)Electrochemical Alcohol Oxidation Reactions

Abstract

Selective alcohol oxidation to carbonyl compounds is a crucial oxidative transformation in academic and industrial settings. When derived from renewable (inedible) biomass resources, this commodity-forming platform has the potential to offset—or eliminate completely—current petrochemical oxidation routes in polymer production and fine chemical synthesis. During (photo)electrochemical alcohol oxidation, protons are eliminated from the alcohol substrate that can be used for cathodic H2 generation at a cathode, representing an additional petroleum offset for chemical fuel production. Renewable solar energy can be used directly through photo(electro)chemical (PC/PEC) or indirectly with electrochemical (EC) control to drive these high value commodity- and fuel-forming redox reactions. In this thesis, catalytic and cocatalytic selective alcohol oxidation platforms are explored in acetonitrile and aqueous brine solutions. Motivated by the discovery that nitrate anions can mediate photochemical alcohol oxidation in acetonitrile, bismuth vanadate (BiVO4) photoelectrodes were used in place of cadmium sulfide (CdS) powder to test the codependency of nitrate on the light-absorbing surface for indirect alcohol oxidation. Moving from photochemical control to photoelectrochemical control separates the light-driven redox half reactions onto two separate (photo)electrode materials, allowing for a more detailed investigation on nitrate’s exclusive role(s) in this mediated process. Under photoelectrochemical control, nitrate improved the rate of benzaldehyde on BiVO4 in the presence of blue LED light (100 mW cm-2, max = 448 nm) but surprisingly nitrate was consumed in the process, serving as stoichiometric oxidant rather than a solution phase cocatalyst. Platinum electrodes were used to further explore the technique-dependent behavior of nitrate in acetonitrile toward indirect alcohol oxidation. Electrochemical nitrate oxidation led to faster rates of benzyl alcohol oxidation yet displayed a similar nitrate consumption to the PEC work on BiVO4 photoelectrodes. Rotating ring disk experiments confirm a latent role of collocal O2 reduction, where reduced oxygen species (ROS) generated near nitrate anion oxidation (during PC alcohol oxidation on CdS) can serve as an in-situ generated base; ROS deprotonate HNO3 after nitrate radical reacts with the alcohol substrate, leading to a catalytic behavior of nitrate not observed under (photo)electrochemical conditions. Unfortunately, alcohol oxidation in acetonitrile always occurred with undesired oxygen reduction at the cathode. To encourage proton reduction to form H2 product during alcohol oxidation, aqueous systems were used to explore alcohol oxidation on manganese oxide electrocatalysts. Here, we discovered a dependence on the presence of O 1s (531 eV) feature, attributed to oxygen vacancies, for observed current increase as a response to 5-HMF introduction in brine. Films with the largest relative signal intensity (a-MnOx) showed the highest current with 5-HMF present and were able to sustain > 1 mA cm-2 for 4 hours during CPC. Through MnOx surface XPS analysis and solution ICP-MS, we identify a spontaneous chemical process in which 5-HMF dissolves the Mn film. The applied bias in this solution leads to much smaller alterations to Mn AOS, solution Mn concentration and film discoloration/removal, suggesting the 1.65 V vs RHE applied introduces a new reactive pathway that protects MnOx in this electrolyte.