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)

Abstract

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.

Kinetic Analysis of 4-Nitrophenol Reduction by “Water-Soluble” Palladium Nanoparticles

“…In the case of 150 eq, the pH was 9.69, and in the case of 200 eq, the pH was 9.93. This slight change influenced the NaBH 4 hydrolysis, which was faster, leading to higher formation of reactive Pd-H at the surface of NPs as described by Grzeschik and coworkers [33]. They found that the pH value has an influence on the kinetics of 4-Nip reduction and that by decreasing the initial pH value, the reaction becomes much faster.…”

Flow-Through PolyHIPE Silver-Based Catalytic Reactor

“…To investigate the catalytic activity of the deposited nanocrystals, polymer samples treated with different AgNO 3 concentrations, exhibiting the broadest range of deposited silver, were studied (see Figure 3 b). The efficiency of the silver catalyst was tested by reduction of 4-NP to 4-AP, which is an extensively investigated process commonly used for catalyst activity tests [ 17 , 18 , 19 , 56 , 57 , 58 , 59 , 60 ]. The reaction was monitored by in-line measurement of absorbance at the column outlet ( Figure 6 a).…”

“…As the NaBH 4 concentration was in high excess [ 61 ], a solution with high pH was used [ 56 ], and oxygen was removed [ 59 , 60 ]; therefore, Equation (1) can be further simplified into pseudo-first-order reaction kinetics with an apparent rate constant [ 56 ]: for a continuous tubular reactor ( Figure 6 ), resulting in the following expression: where k’ is the apparent rate pseudo-first-order rate constant [min −1 ], k cat ’ is the apparent rate pseudo-first-order catalytic rate constant [min −1 g −1 ], A in / A out and C in / C out are the absorbance and concentration ratios of 4-NP at the inlet and outlet of the column with subtracted baseline (solution without 4-NP), F is the flow rate, V c is the column volume, ε is the column porosity, and t r is the residence time.…”

Molecular Linking Selectivity on Self-Assembled Metal-Semiconductor Nano-Hybrid Systems

“…The reaction can be monitored online via UV-Vis since in the reaction mixture the 4-NP is rapidly converted into 4-nitrophenolate (reaction pH is 9) that has a characteristic absorption at 400 nm and the product 4-aminophenol (4-AP) has an absorption at 300 nm [ 17 , 18 , 19 ].…”