On the Overlooked Critical Role of the pH Value on the Kinetics of the 4-Nitrophenol NaBH4-Reduction Catalyzed by Noble-Metal Nanoparticles (Pt, Pd, and Au)

The reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with sodium borohydride (NaBH4) in aqueous media is by far the most widely used reaction for testing the catalytic performance of a large variety of metal nanoparticles (MNPs) using UV/vis absorption spectroscopy. Huge differences in the kinetic rate constant of the catalytic 4-NP to 4-AP conversion as a function of material composition, size, and morphology have been reported by many different groups; so far, these have been exclusively attributed to the catalytic activity of the corresponding MNPs. In this study, we study the overlooked critical role of the pH value on the kinetics of this important reaction. We observe a strong pH dependence of both rate constant and reaction order, which we attribute to the pH-dependent hydrolysis of the reducing agent NaBH4 in water. In each hydrolysis step, molecular hydrogen as a second reducing agent is produced. Overall, two competing pathways result in: slow hydride versus fast hydrogen reduction. Kinetic simulations based on a model including all relevant species are capable of quantitatively describing all experimental results for three different noble-metal nanoparticle catalysts (Au, Pt, and Pd). Most importantly, for a fair interlaboratory comparison of the catalytic performances of MNPs, we recommend to report rate constants of the 4-NP to 4-AP conversion at pH 13 by strict pH control using a strong base such as NaOH since at pH 13 the hydrolysis of NaBH4 in aqueous solution is slowed down significantly.