Search

Abstract The reduction of bromate to bromide was successfully achieved by bimetallic catalysts with NZVI support in continuous-flow reactors. The stability of NZVI-supported bimetallic catalysts was enhanced by decelerating the iron corrosion and sequential rapid passivation of the iron-Cu-Pd ensembles under optimized reaction conditions. Thus >99% bromate removal can be continuously achieved for 11 h. The lifetime of the bimetallic catalyst was further enhanced and tested under different hydraulic retention time, catalyst loading, and initial bromate concentrations. At the optimized operation conditions, the catalyst showed a complete bromate reduction by 24 h and then the reactivity slowly decreased to 20% over the next 100 h. X-ray diffraction and X-ray photoelectron spectroscopy showed that the reactive NZVI support was oxidized to Fe(II) and Fe(III) along with Cu(0) oxidation to CuO, while the oxidation state of Pd did not change. Therefore, bromate reduction occurred on the surface of reactive NZVI support and Cu(0) particle, while Pd played a role as a hydrogenation catalyst that prolonged the lifetime of the bimetallic catalyst. Graphical abstract Image 1 Highlights • An enhanced continuous reduction of BrO 3 − to Br− was achieved by the catalysts. • BrO 3 − reduction stability was in the order of NZVI-Cu-Pd> NZVI-Cu>NZVI-Pd. • Iron corrosion and sequential rapid passivation of the catalyst were decelerated. • Complete BrO 3 − removal was achieved for 24 h at optimized conditions. [ABSTRACT FROM AUTHOR]

Experiments were conducted to investigate the effect of experimental factors (types of promotor and noble metals, H2injection, and suspension pH) on catalytic nitrate reduction by bimetallic catalysts supported by nanoscale zero-valent iron (NZVI). NZVI without H2injection showed 71% of nitrate reduction in 1 h. Cu/NZVI showed the almost complete nitrate reduction (96%) in 1 h, while 67% of nitrate was reduced by Ni/NZVI. The presence of noble metals (Pd and Pt) on Cu/NZVI without H2injection resulted in the decrease of removal efficiency to 89% and 84%, respectively, due probably to the electron loss of NZVI for formation of metallic Pd and Pt. H2injection into Cu-Pd/NZVI suspension significantly improved both catalytic nitrate reduction (>97% in 30 min) and N2selectivity (18%), indicating that adsorbed H on active Pd sites played an important role for the enhanced nitrate reduction and N2selectivity. The rapid passivation of NZVI surface resulted in a dramatic decrease in nitrate reduction (79–28%) with an increase in N2selectivity (8–66%) as the suspension pH increased from 8 to 10. [ABSTRACT FROM PUBLISHER]

• Sn-Pd-kaolinite showed faster NO 3 − reduction kinetics. • Sn-Pd-red mud showed higher N 2 selectivity. • A low Sn:Pd allowed their optimal proximity on the red mud support. • Sn-Pd-red mud showed stable kinetics and selectivity over a wide range of CO 2 :H 2. • The reactivity of Sn-Pd-kaolinite was strongly affected by groundwater impurities. 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 (NO 3 −) reduction. Under the same experimental conditions, the Sn-Pd-kaolinite reduced NO 3 − at a higher rate (k = 15.46 × 10−2 min−1) and lower N 2 (70 %) selectivity compared to that of Sn-Pd-red mud, which exhibited slower reduction kinetics (k = 9.16 × 10−2 min−1) but higher N 2 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 H 2 activation capacity, which resulted in fast reduction kinetics but low N 2 selectivity. However, the red mud support exhibited a strong alloying effect toward the Sn-Pd ensembles and regulated the H 2 capacity of the Pd to suppress the reduction kinetics and NH 4 + selectivity. The NO 3 − removal and byproduct selectivity was sustainable over a wide range of CO 2 :H 2 ratios. Sn-Pd-red mud showed a 100 % NO 3 − removal and ∼85 % N 2 selectivity at CO 2 :H 2 ≥ 0.6, while complete NO 3 − removal was obtained by Sn-Pd-kaolinite at CO 2 :H 2 ≥ 1. A groundwater NO 3 − removal experiment showed that the reactivity of the Sn-Pd-red mud was stable toward fouling by impurities. [ABSTRACT FROM AUTHOR]