Abstract
In this study, the temperature-dependent activity of Au/AC nanocatalysts in redox catalytic reactions was investigated. To this end, a series of colloidal gold catalysts supported on activated carbon and titania were prepared by the sol immobilization method employing polyvinyl alcohol as a polymeric stabilizer at different hydrolysis degrees. The as-synthesized materials were widely characterized by spectroscopic analysis (XPS, XRD, and ATR-IR) as well as TEM microscopy and DLS/ELS measurements. Furthermore, 5-hydroxymethylfurfural (HMF) oxidation and 4-nitrophenol (4-NP) reduction were chosen to investigate the catalytic activity as a model reaction for biomass valorization and wastewater remediation. In particular, by fitting the hydrolysis degree with the kinetic data, volcano plots were obtained for both reactions, in which the maximum of the curves was represented relative to hydrolysis intermediate values. However, a comparison of the catalytic performance of the sample Au/AC_PVA-99 (hydrolysis degree of the polymer is 99%) in the two reactions showed a different catalytic behavior, probably due to the detachment of polymer derived from the different reaction temperature chosen between the two reactions. For this reason, several tests were carried out to investigate deeper the observed catalytic trend, focusing on studying the effect of the reaction temperature as well as the effect of support (metal–support interaction) by immobilizing Au colloidal nanoparticles on commercial titania. The kinetic data, combined with the characterization carried out on the catalysts, confirmed that changing the reaction conditions, the PVA behavior on the surface of the catalysts, and, therefore, the reaction outcome, is modified.
Highlights
In recent years, inorganic nanostructured materials have emerged as suitable heterogeneous catalysts for the industrial-scale production of value-added chemicals [1–5].a specific design for high-performing nanocatalysts resulted in more complex structures due to a large number of variables such as catalytic environment and the dynamic mass- and energy-transport processes at the solid–liquid or solid–gas interfaces [6–8].A more organic approach that includes the multifaceted role of the support along with the tunability of nanoparticles (NPs) themselves offers new strategies for the design and performance optimization of heterogeneous nanocatalyst systems [9–11]
From the the catalytic microscopy analysis (TEM) analysis, it was evident that the presence even of a minor quantity of hydroxyl groups in polyvinyl alcohol (PVA) could lead to an increase in the average aqueous solution of NaBH4 (Au) particle size
4-NP reduction and HMF oxidation were employed to evaluate the influence of polymeric stabilizers and reaction temperature
Summary
A more organic approach that includes the multifaceted role of the support along with the tunability of nanoparticles (NPs) themselves offers new strategies for the design and performance optimization of heterogeneous nanocatalyst systems [9–11]. Shafer et al showed the possibility to perform enantioselective hydrogenation modifying the structure of polymers used in supported Pd catalysts [34] In this topic, polymeric stabilizers used in the synthesis of nanostructured materials can play significant roles in controlling catalytic activity and catalyst stability and could be offered as a “toolbox” for designing highly effective and stable catalysts. Several researchers have focused on supported catalysts based on noble metals (Pd, Pt, Ru, or Au) or their combination (bimetallic systems) for the conversion of biomass and derivatives due to their role as potential alternatives to the traditional fossil sources to produce fuels and chemicals [35–39].
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