Highly reactive and selective Sn-Pd bimetallic catalyst supported by nanocrystalline ZSM-5 for aqueous nitrate reduction

Abstract

A new bimetallic catalyst supported by environmentally benign nanocrystalline ZSM-5 (NZSM-5), was developed to reduce nitrate completely and selectively to nitrogen gas without producing nitrite. The catalyst was optimized by use under a variety of conditions (i.e., promoter metal type (Sn, Cu, Ag, Ni)), noble metal type (Pd, Pt, Au), promoter metal concentration (0–3.4wt%), noble metal concentration (0–2.8wt%), catalyst calcination temperature (0–550°C), H2 flow rate (0–60mL/min), and CO2 flow rate (0–60mL/min). Complete nitrate removal with the highest nitrogen selectivity (91%) was achieved using 1%Sn-1.6%Pd-NZSM-5 catalyst under optimized conditions that included: initial nitrate concentration: 30mg/L NO3-N; calcination temperature: 350°C; H2 flow rate: 30mL/min; and CO2 flow rate: 60mL/min for 60min. The estimated kinetic rate constant of the catalyst is 16.40×10−2min−1, the catalyst-loading normalized rate constant is 65.60×10−2min−1gcat−1, while Pd-loading normalized rate constant is 410×10−2L/mingPd−1. The catalyst showed remarkable nitrate removal (100%) and nitrogen selectivity (>88%) for up to five successive reactions with consistent kinetics. A 100% nitrate removal and >81% nitrogen selectivity was also achieved by the catalyst for five repeated cycles. However, the kinetics gradually slowed down to 4.36×10−2min−1 over five repeated cycles, (still superior to fresh catalysts already reported in the literature). Characterization tests confirmed that the used catalyst was chemically stable, and that the decrease in its reactivity was due mainly to the sintering of metallic nano particles during the regeneration process.