Support induced influence on the reactivity and selectivity of nitrate reduction by Sn-Pd bimetallic catalysts

Abstract

In this study, two different materials i.e., naturally occurring kaolinite (Al-Si containing structured clay) and industrially produced red mud (mixture of metal oxides) were used as support surfaces for Sn-Pd bimetallic catalysts to investigate the effects of support on catalytic nitrate (NO3−) reduction. Under the same experimental conditions, the Sn-Pd-kaolinite reduced NO3− at a higher rate (k = 15.46 × 10−2 min−1) and lower N2 (70 %) selectivity compared to that of Sn-Pd-red mud, which exhibited slower reduction kinetics (k = 9.16 × 10−2 min−1) but higher N2 selectivity (85 %). We found that support material with a relatively low surface area (i.e., red mud) provided optimal Sn-Pd proximity at relatively low Sn:Pd ratios, but suffered from metallic overlap at high Sn:Pd ratios. Temperature program reduction (TPR) analysis showed that the kaolinite interaction with the Sn-Pd ensembles maintained a higher H2 activation capacity, which resulted in fast reduction kinetics but low N2 selectivity. However, the red mud support exhibited a strong alloying effect toward the Sn-Pd ensembles and regulated the H2 capacity of the Pd to suppress the reduction kinetics and NH4+ selectivity. The NO3− removal and byproduct selectivity was sustainable over a wide range of CO2:H2 ratios. Sn-Pd-red mud showed a 100 % NO3− removal and ∼85 % N2 selectivity at CO2:H2 ≥ 0.6, while complete NO3− removal was obtained by Sn-Pd-kaolinite at CO2:H2 ≥ 1. A groundwater NO3− removal experiment showed that the reactivity of the Sn-Pd-red mud was stable toward fouling by impurities.