The Competition between 4-Nitrophenol Reduction and BH₄⁻ Hydrolysis on Metal Nanoparticle Catalysts

Assessing competitive environmental catalytic reduction processes via NaBH₄ is essential, as BH₄⁻ is both an energy carrier (as H₂) and a reducing agent. A comprehensive catalytic study of the competition between the borohydride hydrolysis reaction (BHR, releasing H₂) and 4-nitrophenol reduction via BH₄⁻ on M⁰- and M/M′ (alloy)-nanoparticle catalysts is reported. The results reveal an inverse correlation between the catalytic efficiency for BH₄⁻ hydrolysis and 4-nitrophenol reduction, indicating that catalysts performing well in one process exhibit lower activity in the other. Plausible catalytic mechanisms are discussed, focusing on the impact of reaction products such as 4-aminophenol and borate on the rate and yield of BH₄⁻ hydrolysis. The investigated catalysts were Ag⁰, Au⁰, Pt⁰, and Ag/Pt-alloy nanoparticles synthesized without any added stabilizer. Notably, the observed rate constants for the 4-nitrophenol reduction on Ag⁰, Ag-Pt (9:1), and Au⁰ are significantly higher than the corresponding rate constants for BH₄⁻ hydrolysis, suggesting that most reductions do not proceed through surface-adsorbed hydrogen atoms, as observed for Pt⁰ nanoparticles. This research emphasizes the conflicting nature of BH₄⁻ hydrolysis and reduction processes, provides insights for designing improved catalysts for competitive reactions, and sheds light on the catalyst properties required for each specific process.