Insights into Active Sites and Mechanisms of Benzyl Alcohol Oxidation on Nickel–Iron Oxyhydroxide Electrodes

Abstract

The electrochemical oxidation of bio-derived molecules has recently garnered interest for its potential in opening electrified synthetic pathways toward value-added products. Herein, we investigate the electrochemical conversion of benzyl alcohol (BA) to benzaldehyde and benzoate on nickel–iron (Fe ∼ 7–18%) electrodes as a model system to understand reaction mechanisms and environmental conditions that can transform these molecules. Our results indicate a strong correlation between benzyl alcohol oxidation (BAO) onset potentials and Ni(II/III) redox peak positions, highlighting the potential role that lower oxidation states of nickel, i.e., Ni3+, can play in BAO catalysis. Our work on the Ni2+/3+ system complements mechanisms that involve higher oxidation states of Ni as reported by others. We note that the Ni redox position and thus BAO onset is impacted by Fe incorporation during electrochemistry from unpurified electrolytes, which can resemble standard reactor operating conditions. We perform a systematic computational investigation into BAO and provide density functional theory (DFT) insights into how the redox mechanism has been such a prominent focus of alcohol oxidations. This includes the mode of BA adsorption and the nature of the adsorption site; upon conversion of the Ni2+ surface to active Ni3+ via hydroxyl deprotonation, BAO is thermodynamically downhill. Our DFT study also introduces the possibility of a vacancy-driven mechanism, though expected to be less prevalent during catalysis than the redox mechanism for a Ni3+ surface. Through the systematic investigation of experimental reaction conditions and computational free energy thermodynamics, we have gained valuable insights into BAO reaction mechanisms that inform catalytic activity. Our study opens avenues for further design and development of catalyst active sites for the oxidation of related organic molecules.

Oxygen Vacancies Alter Methanol Oxidation Pathways on NiOOH

“…Note that the O-vacancy should increase the electron density at the carbon atom of surface species, facilitating the OH – insertion relative to the deeper dehydrogenation. This is similar to the influence of O-vacancy on mechanism of benzylic alcohol electrooxidation over Ni-based catalysts reported by Wei et al …”

“…Note that the O-vacancy should increase the electron density at the carbon atom of surface species, facilitating the OH − insertion relative to the deeper dehydrogenation. This is similar to the influence of O-vacancy on mechanism of benzylic alcohol electrooxidation over Ni-based catalysts reported by Wei et al 64 Next, we discuss the selectivity of the formate surface intermediate (i.e., HCOO*) compared to CO*. Figure 9 shows that the thermodynamics of CH(OH)O* deprotonation to form HCOO* is favorable and becomes even more facile at positive potentials.…”

Water electrolysis for hydrogen production: from hybrid systems to self-powered/catalyzed devices

“…17g), confirming the formation of Co-containing nickel oxyhydroxide (Co-NiOOH) as the real active species. Co/Ni-based catalysts are commonly used for Ph-CH 2 OH oxidation, such as Mo-Ni alloy nanoparticle-modified MoO 2 , 243 h-Ni(OH) 2 , 344 Ni 2 P/NF, 350 NiCo 2 O 4 nanosheets, 351 b-NiOOH, 352 2D Ni-based MOFs, 353 plasma modified nickel foam, 354 Ni(OH) 2 nanosheet/Ni foam, 355 NiCo(OOH) x nanosheets, 356 Ni-Fe thin films, 357 Co-Ni LDH, 358 and Co 3 O 4 NPs/Ti. 359 The electrochemical synthesis reactions discussed earlier exclusively yield hydrogen at the cathode, while no gas is generated at the anode.…”

Deciphering the Role of Nickel in Electrochemical Organic Oxidation Reactions

“…[45,68,85,105,143] To date, understanding the interconnection between the active phase (of the anode material) and the molecular structure, geometry, or stereochemical orientation of functional groups in the organic reactant is merely investigated. It is crucial to emphasize that the nature of the catalytic phase alone does not solely determine conversion efficiency and product selectivity; several other factors such as the pKa of the reactant, local pH, electronic factors, orientation of organic molecules (and related intermediates) during adsorption on the catalyst surface, mass transportation, [2,70,102] may exert greater influence. [131] Predicting the significance of a parameter is fraught with uncertainty.…”