Unraveling Two Pathways for Electrochemical Alcohol and Aldehyde Oxidation on NiOOH

Abstract

Selective oxidation of alcohols to their corresponding aldehyde or carboxylic acid is one of the most important classes of organic synthesis reactions. In addition, electrochemical alcohol oxidation is considered a viable anode reaction that can be paired with H 2 evolution or other reductive fuel production reactions in electrochemical and photoelectrochemical cells. NiOOH, a material that has been extensively studied as an oxygen evolution catalyst, is among the most promising electrocatalysts for selective alcohol oxidation. Electrochemical alcohol oxidation by NiOOH has been understood since the 1970s to proceed through a hydrogen atom transfer to NiOOH. In this study, we establish that there is a second, more dominant general alcohol oxidation pathway on NiOOH enabled at more positive potentials. Using a three-step electrochemical procedure we developed, we deconvoluted the currents corresponding to these two pathways for various alcohols and aldehydes. The results show that alcohols and aldehydes have a distinct difference in their respective preferences for the two oxidation pathways. Our three-step electrochemical procedure also allowed us to evaluate the Ni valence involved with the different oxidation pathways to elucidate their mechanistic differences. Using these experimental results coupled with a computational investigation, we propose that the new pathway entails hydride transfer from the substrate to Ni 4+ sites in NiOOH. This study offers an essential foundation to understand various oxidative electrochemical dehydrogenation reactions on oxide and hydroxide-based catalytic electrodes.

Understanding the Roles of Electrogenerated Co3+ and Co4+ in Selectivity‐Tuned 5‐Hydroxymethylfurfural Oxidation

“…While in the case of Co 4+ , the oxidation current quickly increases after the onset of Co 4+ formation, suggesting the consumption rate of Co 4+ well surpasses its generation rate and therefore increasing quantity of M 4+ is generated and participating in the HMF oxidation. Based on the distinctive feature of LSV in Co 3+ and Co 4+ ‐mediated range, it can therefore be concluded that the RDS for Co 4+ ‐mediated HMF oxidation is the electrochemical regeneration of catalytically active Co 4+ species [6] …”

Understanding the Roles of Electrogenerated Co3+ and Co4+ in Selectivity‐Tuned 5‐Hydroxymethylfurfural Oxidation

“…In previously reported HMF oxidation catalyzed by Ni-based electrocatalyst, the Ni 3+ / Ni 2+ redox couple shows no selectivity towards the oxidation of hydroxyl and formyl groups,therefore producing FDCA as the dominant product. [6,19] To emphasize the oxidation-state dependent selectivity of electrogenerated Co species,b enchmark NiO x H y electrode was prepared following the same electrodeposition protocol and product selectivity towards HMF oxidation was examined under various applied potentials (1.4 V, 1.45 Vand 1.5 Vvs. RHE).…”

Unraveling the role of oxygen vacancy in the electrooxidation of 5-hydroxymethyfurfural on Co3O4

“…1i). 20 Therefore, it is of great signi cance for catalysts to have the optimal OHand organic adsorption capability. The DFT calculation was conducted to con rm the vital role of Vo in HMFOR.…”

“…15,16,17 OH ad is the crucial intermediate for the oxygen evolution reaction (OER) and alcohol molecules catalytic cycle, which has been widely recognized the reaction mechanism. 18,19,20 In ethanol oxidation, the H* derived from the dehydrogenation of hydroxyl groups is combined with OH ad in the alkaline solution. 20,21 Since the HMFOR process includes the oxidation of hydroxyl and aldehyde groups, different adsorption energies and sites of OHon catalysts signi cantly in uence the HMFOR activity.…”

“…18,19,20 In ethanol oxidation, the H* derived from the dehydrogenation of hydroxyl groups is combined with OH ad in the alkaline solution. 20,21 Since the HMFOR process includes the oxidation of hydroxyl and aldehyde groups, different adsorption energies and sites of OHon catalysts signi cantly in uence the HMFOR activity. Moreover, unlike OER, for which only OHadsorbed is involved, the competitive adsorption between OHand HMF molecules occurs on the limited reaction sites for HMFOR, leading to less e cient catalysis.…”