The NiOOH electrode is commonly used in electrochemical
alcohol
oxidations. Yet understanding the reaction mechanism is far from trivial.
In many cases, the difficulty lies in the decoupling of the overlapping
influence of chemical and electrochemical factors that not only govern
the reaction pathway but also the crystal structure of thein situformed oxyhydroxide. Here, we use a different approach
to understand this system: we start with synthesizing pure forms of
the two oxyhydroxides, β-NiOOH and γ-NiOOH. Then, using
the oxidative dehydrogenation of three typical alcohols as the model
reactions, we examine the reactivity and selectivity of each oxyhydroxide.
While solvent has a clear effect on the reaction rate of β-NiOOH,
the observed selectivity was found to be unaffected and remained over
95% for the dehydrogenation of both primary and secondary alcohols
to aldehydes and ketones, respectively. Yet, high concentration of
OH–in aqueous solvent promoted the preferential
conversion of benzyl alcohol to benzoic acid. Thus, the formation
of carboxylic compounds in the electrochemical oxidation without alkaline
electrolyte is more likely to follow the direct electrochemical oxidation
pathway. Overoxidation of NiOOH from the β- to γ-phase
will affect the selectivity but not the reactivity with a sustained
>95% conversion. The mechanistic examinations comprising kinetic
isotope
effects, Hammett analysis, and spin trapping studies reveal that benzyl
alcohol is oxidatively dehydrogenated to benzaldehydeviatwo consecutive hydrogen atom transfer steps. This work offers the
unique oxidative and catalytic properties of NiOOH in alcohol oxidation
reactions, shedding light on the mechanistic understanding of the
electrochemical alcohol conversion using NiOOH-based electrodes.