Multiparameter optimization of non-thermal plasma-driven synthesis of carbohydrate-stabilized rhenium nanoparticles towards enhancement of their catalytical activity for reduction of nitroaromatic compounds

Abstract

This study aims to optimize the synthesis of carbohydrate-stabilized rhenium nanoparticles (ReNPs) utilizing cold atmospheric pressure plasma (CAPP), specifically direct current atmospheric pressure glow discharge (dc-APGD), hypothesizing that controlling synthesis parameters will enhance catalytic efficiency in the reduction of nitroaromatic compounds (NACs). The dc-APGD system operated in flowing liquid cathode (FLC-dc-APGD) and flowing liquid anode (FLA-dc-APGD) modes. Design of experiments (DOE) and response surface methodology (RSM) were employed for the first time to optimize ReNPs synthesis by adjustment of the concentration of ReO4- in ReNPs precursor solution, its flow rate, as well as the discharge current. ReNPs were characterized using high-resolution transmission electron microscopy (HRTEM), dynamic light scattering (DLS), and X-Ray photoelectron spectroscopy (XPS). In turn, optical emission spectroscopy (OES) was employed for the identification of reactive species generated in the plasma phase that contributed to the synthesis of ReNPs. The resultant sizes, surface charges, and phase compositions were linked with synthesis parameters and then with catalytic activities revealed by the variety of ReNPs. HRTEM photomicrographs showed spherical and spherical-like morphologies of the obtained ReNPs synthesized using FLC-dc-APGD and FLA-dc-APGD modes with the actual sizes of 7.28 ± 2.90 nm and 3.24 ± 0.57 nm, respectively. The optimized conditions yielded ReNPs of exceptional catalytic activity, that reduced 94.9 % (FLC-dc-APGD) and 90.5 % (FLA-dc-APGD) of 4-nitrophenol (4-NP) with the apparent rate constants (k1) 0.269 and 0.107 min−1, respectively. Further, the conversions of 90 % were achieved over FLC-dc-APGD-based ReNPs for nitrobenzene (NB), 2,4,6-trinitrophenol (2,4,6-TNP), 2,4-dinitrophenol (2,4-DNP), and 4-nitroaniline (4-NA). In these cases, the k1 values were 0.235, 0.174, 0.070, 0.001 min−1, respectively. The results allowed to find a correlation between ReNP size distribution, oxidation state, and catalytic activity that suggested a potential for tailored Re nanocatalysts in environmental remediation.