Aldehyde or Carboxylic Acid: Defining the Selectivity of Electrochemical Oxidation of Aliphatic Alcohols by Tuning the Surface Phase of NiO(OH)

Abstract

Nickel oxyhydroxide (NiO(OH)) is the catalytically active species during electrochemical alcohol oxidation at nickel electrodes. Herein, we demonstrate that the adopted phase structure of NiO(OH) (β- and γ-NiO(OH)) can dramatically influence the catalytic properties of nickel electrodes. We show that β-NiO(OH) oxidizes aliphatic alcohols preferentially to aldehydes and that carboxylates are formed at γ-NiO(OH). By combining the effect of the NiO(OH) phase with reagent effects (electrolyte pH, alcohol concentration, and trace amounts of Fe), we achieve aldehyde formation with ∼90% selectivity or carboxylate formation with ∼85% selectivity. Furthermore, we perform an in situ Raman spectroelectrochemical study to elucidate the reaction conditions that favor the formation of β- and γ-NiO(OH) phases, respectively. Freshly deposited nickel electrodes form γ-NiO(OH) phases upon oxidation, whereas an electrochemical conditioning protocol can be employed to transform γ- to β-NiO(OH). Finally, we show that the efficiency of this transformation depends on the precise reaction conditions and the precursor used. Hence, we showcase that the exact surface structure of the phase formed in situ on top of transition metal (oxides) affects the product distribution of electrochemical reactions. Moreover, we highlight how the “history” of these catalysts affects their performance via an intricate interplay among the reaction environment, pre-treatment, and surface structure.