Due to the ease of obtaining reliable kinetics, the catalytic reduction of 4-nitrophenol (4NP) is continually exploited as a benchmark reaction for comparing heterogenous catalyst activity. Concomitantly, 4NP is an anthropogenic water pollutant, and significant research has gone into remediation strategies – including catalytic reduction. However, even though there is a continuous presence of this reaction in the current literature, its mechanism is not fully understood. Moreover, while previous studies have tentatively proposed noble metal nanoparticle mediated pathways, reaction mechanisms promoted by metal oxide nanoparticles are rarely studied. The latter is particularly important as recent literature has shown remarkable noble metal like activity from Earth abundant metal oxides, and furthermore, any feasible catalytic environmental remediation strategy is far more likely to involve low cost metal oxides, as opposed to precious metal systems. Herein, we present a study of the mechanism of 4NP reduction with copper(II) oxide. Copper oxide particles synthesized through a simple aqueous co-precipitation method were separated and subjected to multiple post-synthesis treatments. These included sonication under different NaBH4 solutions and vibratory ball-milling to induce a partial reduction and surface defects respectfully. These materials were then compared for the reduction of 4NP to highlight the impact of copper oxidation state and influence of defects on the catalytic efficiency. Post-mortem analysis of the materials is used to study the impact of the reaction on the catalyst surface. Finally, analysis through NMR spectroscopy and in-situ dissolved oxygen probing helps elucidate the probable mechanism and impacting factors for metal oxide catalyzed 4NP reduction. Understanding this mechanism can lead to informed future catalyst design.

Probing the mechanism and structure-activity relationships in the CuOx catalysed reduction of 4-nitrophenol

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