1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 z Catalysis The Effect of Poly(vinylpyrrolidone) (PVP) on the Au Catalyzed Reduction of p–nitrophenol: The Fundamental Role of NaBH 4 . María S. Álvarez Cerimedo, [a] Lucía Gago Baronio, [a, b] Cristina E. Hoppe,* [a] and María A. Ayude* [b] The effect of polymeric ligands, like poly(vinylpyrrolidone) (PVP), on the reduction of p-nitrophenol by NaBH 4 in presence of gold nanoparticles (NPs) is systematically investigated. Distinctive trends could be identified depending on the extent of the NaBH 4 excess. At rather low NaBH 4 concentration excess, PVP had a deleterious effect, probably due to the steric hindrance of the PVP layer on the gold surface that acted partially blocking gold active sites. However, under a large excess of NaBH 4 , an enhanced catalytic performance was observed, what was attributed to the exchange of the polymer by hydride anions and other ions formed during hydrolysis of NaBH 4 . This exchange occurred without aggregation of NPs and without compromising recycling capability. Based on these results, the design of new routes for the synthesis of supported catalysts based on PVP stabilized metal colloids can be envisaged, as no additional steps would be necessary for removing the stabilizer prior to use. Introduction Reduction of p-nitrophenol (p-NP) with NaBH 4, is probably one of the most studied and referred chemical reactions for the evaluation of metallic nanostructured catalysts, in particular, those formed by noble metal nanoparticles (NPs). [1–11] This is considered a “model reaction” with several advantages for the easy evaluation of catalysts performance like, e.g. the gener- ation of only one product of reaction (p-NP), simple monitoring of the reaction evolution by spectrophotometric analysis and easy fitting to pseudo-first order kinetics. Due to these advantages, the catalytic reduction of p-NP with NaBH 4 to give p-aminophenol (p-AmP) has been extensively used for evalua- tion of the catalytic performance of both, colloidal and supported, metal NPs. [1–11] Despite these advantages, the nature and origin of some features observed during p-NP reduction are still a matter of debate and make this reaction not so well understood. First of all, the p-NP reduction kinetics has been described in terms of a Langmuir-Hinshelwood or an Eley- Rideal mechanism in different research studies. [1,2,4,9,10,12–15] Systematic changes in reactant concentrations are required to distinguish between these two models. [16] The Langmuir- Hinshelwood (L  H) model has been the more consistently supported mechanism [1,2,4,9,10,12] and postulates that adsorption of both reactants on the catalyst surface (NaBH 4 and p-NP in this case) has to occur for the reaction to proceed, yielding products that are rapidly desorbed from the surface and return to the reaction medium. A second point to consider in this reaction is the effect of the reagents mixing sequence. Only a few reports considering the effect of this variable have been recently published, even when it seems to exert a profound influence on the reaction pathway of catalytic reduction of p-NP. [7,17] The third important issue is the existence of the induction time. Different explanations have been proposed to understand the nature of this induction time that include, among the most common, (i) the time necessary for the reactants to reach the surface of the catalyst by diffusion, [18] (ii) a process of surface restructuring of the Au NPs required to make the surface active, [9,19] (iii) competitive adsorption of reactant species on the Au NPs surface [7] and iv) the presence of dissolved oxygen. [8,20] An additional challenge to elucidate the actual nature of the process taking place at the interface between the reactants and the catalyst is to determine the variation in the catalytic performance with the chemical nature, molecular weight and thickness of the capping agent used to stabilize the metallic cores. This effect cannot be analyzed individually but requires taking into account the interplay between all the reagents participating in the reaction. The influence of the nature of the ligand on the reduction rate and length of the induction time has been discussed in several papers regarding the catalytic reduction of p-NP. [11,17,19] Main effects attributed to the presence of the ligands are related with partial blocking of active sites, [11,17,21] withdrawal or injection of electron density from/to [a] Dr. M. S. Álvarez Cerimedo, L. G. Baronio, Dr. C. E. Hoppe Instituto Nacional en Ciencia y Tecnología de Materiales (INTEMA), CON- ICET, UNMdP, División Polímeros Nanoestructurados Av. Juan B. Justo 4302, B7608FDQ, Mar del Plata, Argentina E-mail: hoppe@fi.mdp.edu.ar [b] L. G. Baronio, Dr. M. A. Ayude Instituto Nacional en Ciencia y Tecnología de Materiales (INTEMA), CON- ICET, UNMdP, División Catalizadores y Superficies Av. Juan B. Justo 4302, B7608FDQ, Mar del Plata, Argentina E-mail: mayude@fi.mdp.edu.ar Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.201803250 Full Papers DOI: 10.1002/slct.201803250 608 ChemistrySelect 2019, 4,608–616 © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim