4-Nitrophenol Reduction: Probing the Putative Mechanism of the Model Reaction

Abstract

We report a reinterpretation of the reduction of 4-nitrophenol catalyzed by silver nanoparticles. Mass spectrometry and ultraviolet−visible light spectroscopy measurements support the existence of 4-nitrosophenol as a stable reaction intermediate. We propose that dissolved oxygen is consumed, both by oxidizing 4nitrosophenol (an intermediate) and reoxidizing the reduced catalyst surface, resulting in the commonly observed "induction period" in the reaction kinetics. Upon complete consumption of dissolved oxygen, subsequent reduction to 4-aminophenol can occur. A complete kinetic analysis including modeling is presented, conceptually fitting data from recent reports in the literature, as well as fitting data from our own experiments.

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.…”

Surface Electronic State Mediates Proton Transfer at Metal Nanoscale Interface for Catalytic Hydride Reduction of −NO2 to −NH2

“…It is generally believed that, the metal NPs is the active site of catalytic hydride reduction, which offers a dual role by providing its surface for the adsorption of reactants and mediating the transfer of electrons and protons to reduce the kinetic barrier for the −NO2 to −NH2 conversion (NaBH4 reducer is taken for granted as the source of hydrogen, Scheme 1a). However, the mass of solid evidences showed that, the efficiency of the catalytic 4-NP to 4-AP conversion is not only attributed to the metal NPs centers (14,15), but also to the microenvironment of the secondary coordination sphere of metal center, such as the pH and the type of solvent (16)(17)(18)(19)(20)(21)(22)(23), Very interestingly, the deuterium isotopic experiments recently confirmed that the interface water is prerequisite for catalytic reduction, and that, counter-intuitively, it is water rather than the NaBH4 reducer that provides the hydrogen for the formation of the 4-AP product (16,24), suggesting a completely new water-mediated proton transport mechanisms for the catalytic hydride reduction of −NO2 to −NH2 at metal NPs surface. In this report, with mesoporous silica nanosphere (MSNs) supported noble metal NPs as a model catalyst, we systematically study the effects of different reaction parameters on the reduction kinetics in the aqueous NaBH4 solution, mainly including the composition and valence of metals (Ag/Pd/Pt), isotopic solvent (D2O) or reducer (NaBD4), and concentration of sodium hydroxide (NaOH).…”

On the Remarkable Performance of Silver‐based Alloy Nanoparticles in 4‐Nitrophenol Catalytic Reduction

“…reported a detailed review focused on recent developments and basic concepts behind the Au NPs catalyzing the 4‐NP reduction, including a variety of stabilizers and supports. Despite the extensive research being performed on 4‐NP conversion to 4‐AP, only a few mechanistic studies of this reaction have been performed . Often, the reaction starts after an induction time (t 0 ), where no reduction takes place.…”

“…Despite the extensive research being performed on 4-NP conversion to 4-AP, only a few mechanistic studies of this reaction have been performed. [28][29][30][31][32] Often, the reaction starts after an induction time (t 0 ), where no reduction takes place. Previous reports have indicated that t 0 is associated with several factors such as the dead time for the reduction of surface oxides and dissolved oxygen, [9,33] the diffusion of BH 4 À to the surface of the NPs, and the surface restructuring of NPs before the catalytic reaction starts [5] although t 0 was not observed in several cases when the catalyst was reused.…”