Aqueous nitrate reduction by Sn-Pd/kaolinite catalyst

Abstract

A new clay mineral supported Sn-Pd bimetallic catalyst is developed in this study for active nitrate reduction and selective nitrogen production. Kaolinite which is a simple eco-friendly clay mineral has been used as a support for bimetallic system. The synthesized catalyst has been characterized using X-ray diffraction (XRD), Furrier Transformation Infrared spectroscopy (FT-IR), specific surface area (BET), transmission electron microscopy with energy dispersive X-ray spectroscopy (TEM-EDX), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The catalyst was optimized under variety of conditions (i.e., promoter metal type (Sn, Cu, Ni, Mn), noble metal type (Pd, Pt Au), promoter metal concentration (0-3.4%), noble metal concentration (0-2.8%), $H_2$ flow rate (0-60ml/min), $CO_2$ flow rate (0-60ml/min) and calcination temperature ($0-550^\circ C$). Complete nitrate removal and 70% nitrogen selectivity is achieved with 2.2% Sn and 2.2% Pd at $450^\circ C$ calcination temperature with 30ml/min $H_2$ and 60ml/min $CO_2$ flow rates respectively. A high kinetic rate constant of (k = $18.16 \times 10^{-2} min^{-1}$) is also achieved with optimized Sn-Pd/kaolinite catalyst. The stability of the catalyst is checked in a continuous reactor system. During the continuous test (170 h reaction), excellent nitrate removal was observed (100 % up to 60 h) with 50% $N_2$ selectivity. However removal gradually decreased to 80% in 170 h with 40% $N_2$ selectivity. The leaching of the metals Sn, Pd was low even after 170h of reaction, confirming that catalyst had adequate amount metal particles for further $NO_3$-reduction. XPS results confirmed that the decrease in reactivity after 70 h was mainly due to the oxidation Sn.