Active Solvent Hydrogen-Enhanced p-Nitrophenol Reduction Using Heterogeneous Silver Nanocatalysts@Surface Functionalized Multiwalled Carbon Nanotubes

Abstract

Solvent effects in aqueous media for the fast heterogeneous catalytic reduction of p-nitrophenol were studied using two silver nanocatalysts. Binary and ternary silver nanocatalysts (BNC and TNC) utilize functionalized multiwalled carbon nanotubes (MWCNTs) and polythiophene-functionalized MWCNT nanocomposites as hosts for silver nanoparticles, respectively. The catalytic reduction kinetics were measured by UV–visible absorption spectroscopy by observing the decrease of the sodium nitrophenolate ion peak at 400 nm. The first-order rate constants (k) obtained to reduce p-nitrophenol (1.0 × 10–4 M) in an aqueous medium using TNC and BNC by taking optimum nanocatalytic concentrations were 0.0134 and 0.0364 s–1, with the activity factor 113.34 and 303.34 s–1 g–1, respectively. The role played by each component of BNC and TNC nanocomposites was determined using control experiments. UV–visible (UV–vis) absorption spectroscopy, powder X-ray diffraction studies, and X-ray photoelectron spectroscopy (XPS) analysis on recycled catalysts have revealed no substantial deactivation in catalytical activity or loss of silver nanoparticles for recycled TNC up to the fifth cycle and for recycled BNC up to the sixth cycle. Field emission scanning electron microscopy (FE-SEM) analysis manifested the crystalline byproducts of sodium metaborate hydrates which could also decelerate catalytic activity in higher cycles. The relative rates of catalytic hydrogenation in different solvent–water mixtures indicated that a 10% glycerol–water mixture is a green and nontoxic solvent combination for accelerating the catalytic reduction of p-nitrophenol. The quantity of catalyst and sodium borohydride was minimized by employing a 10% glycerol–water mixture. The highest activity factor of 936.50 s–1 g–1 was achieved using the BNC nanocatalyst (0.005 mg/mL) with the P-NP to NaBH4 molar ratio of 1:200. The catalytic reduction of p-nitrophenol in different solvent–water mixtures revealed that the reduction rate directly depends on the active hydrogen present in the solvent molecules. The industrial-scale reduction of concentrated p-nitrophenol (1.0 × 10–1 M) was achieved by the addition of NaBH4 (25 times higher molar equivalent to P-NP) using a relatively low catalyst concentration (BNC-0.18) in a reasonable reaction time of 1 h. The ternary nanocatalyst (TNC) has shown stable dispersion in water than BNC, which is suitable as an excellent antibacterial agent for Escherichia coli bacteria cultured in water-based nutrient broth at a low concentration (10 μg/mL).