Palladium-Loaded on Calcined Ulexite (Pd Nanoparticles for the Catalytic Hexavalent Chromium Reduction

Abstract

Cr(VI) and Cr (III) ions exhibit very different physicochemical and toxicity properties from each other. Due to being extremely toxic, Cr(VI) has very serious dangerous effects on biochemical systems. Short-term exposure to Cr(VI) can cause stomach ulceration and skin allergies, while long-term exposure to Cr(VI) at high doses can cause nerve tissue, kidney and liver damage, and even death. On the contrary, Cr(III) ion is non-toxic, mutagenic, and non-carcinogenic and is a trace element essential for living beings. Therefore, applying the reduction method of Cr(VI) to Cr(III) for the removal of toxic chromium (VI) from the environment is quite reasonable. In this study, Pd metal nanoparticles (Pd MNPs) were supported on calcined ulexite (CU) in an aqueous solution, reduced with sodium borohydride (NaBH4), and Pd(0)@CU catalyst was obtained as a gray powder. The physicochemical properties of the obtained Pd(0)@CU catalytic material were clarified by advanced techniques such as FTIR, SEM, FESEM-EDS, P-XRD, TEM-EDX, HRTEM, ICP-OES, TEM, UV/vis, XPS, and BET. The analysis results revealed good dispersed Pd MNPs on the calcined ulexite surface (d(mean) = 2.48 nm). Multi-point BET analyses for determining the surface areas of original ulexite, calcined ulexite (CU), and Pd-loaded calcined ulexite (Pd(0)@CU) were determined for the first time. The catalytic performance of Pd(0)@CU nanoclusters was investigated in the reaction of Cr(VI) to Cr(III) using the reducing agent of formic acid at mild conditions. The prepared Pd(0)@CU nanoparticles showed very high catalytic performance (TOF = 270 mol Cr2O72-/mol Pd min.) and stability (even after the fifth catalytic cycle, it retained > 86 percent of its initial activity) in Cr(VI) reduction to Cr(III). These results showed that Pd(0)@CU nanoclusters are excellent heterogeneous nanoclusters that can be reused in Cr(VI) reduction to Cr(III). Additionally, in this work, the kinetic studies of Pd(0)@CU nanoclusters in the catalytic reaction were carried out depending on the concentrations of substrate [dichromate, Cr2O72-], FA [formic acid, CHOOH], SF [sodium formate, CHOONa], nanocluster [Pd(0)@CU] and temperature (298-318 K). From these rich kinetic data obtained, the rate expression and activation parameters of the catalytic reduction reaction were clarified.