The Competition between 4-Nitrophenol Reduction and BH(4)(−) Hydrolysis on Metal Nanoparticle Catalysts

Assessing competitive environmental catalytic reduction processes via NaBH(4) is essential, as BH(4)(−) is both an energy carrier (as H(2)) and a reducing agent. A comprehensive catalytic study of the competition between the borohydride hydrolysis reaction (BHR, releasing H(2)) and 4-nitrophenol reduction via BH(4)(−) on M(0)- and M/M′ (alloy)-nanoparticle catalysts is reported. The results reveal an inverse correlation between the catalytic efficiency for BH(4)(−) 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 BH(4)(−) hydrolysis. The investigated catalysts were Ag(0), Au(0), Pt(0), and Ag/Pt-alloy nanoparticles synthesized without any added stabilizer. Notably, the observed rate constants for the 4-nitrophenol reduction on Ag(0), Ag-Pt (9:1), and Au(0) are significantly higher than the corresponding rate constants for BH(4)(−) hydrolysis, suggesting that most reductions do not proceed through surface-adsorbed hydrogen atoms, as observed for Pt(0) nanoparticles. This research emphasizes the conflicting nature of BH(4)(−) 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.