Reduction of nitrate by bimetallic catalysts

Abstract

Catalytic nitrate reduction by supported bimetallic catalysts is a promising and emerging technology for ecofriendly denitrification of surface and groundwater. In this PhD dissertation, catalytic nitrate removal was investigated using newly developed bimetallic catalysts supported by nano-scaled zero-valent iron (NZVI), nano crystalline ZSM-5 zeolite (NZSM-5), nano crystalline beta (NBeta) zeolite and red mud (RM). Nitrate reduction by Cu-Pd-NZVI catalyst in a continuous reactor system showed a complete nitrate removal. Control experiments showed that Cu, Pd and proper $H_2$ supply are essential parameters for catalyst stability and sustainable nitrate (30 mg/L $NO_{3}-_{-}N$) reduction in continuous mode. The removal efficiency (100%) and nitrogen gas ($N_2$) selectivity (48%) was further enhanced by optimization of operational parameters. At optimized conditions, excellent removal was observed (> 91% in 24 h) with 42-60% $N_2$ selectivity when catalyst was tested for longevity and stability. However, a gradually decrease in nitrate removal efficiency (to 13% in 200 h) along with increase in nitrite selectivity was observed over the course of time. The reason of catalyst deactivation was investigated by X-Ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analysis which revealed that NZVI and the Cu(0) oxidized after continuous denitrification. This indicates that loss of NZVI reductive capacity and the oxidation of Cu(0) to Cu(I) and Cu(II) deactivates catalytic efficiency of Cu-Pd-NZVI during continuous nitrate reduction. The deactivation of NZVI supported Cu-Pd bimetallic catalyst showed that NZVI is not a proper support material for sustainable nitrate reduction. Hence, a new and stable bimetallic catalyst was developed using an environmentally benign zeolite named as “nanocrystalline ZSM-5 (NZSM-5)”. The NZSM-5 zeolite was synthesized in lab by hydrothermal process. The NZSM-5 supported bimetallic catalyst was optimized for promoter metal type (Sn, Cu, Ag, Ni), noble metal type (Pd, Pt, Au), promoter metal concentration (0-3.4 wt.%), noble metal concentration (0-2.8 wt.%), calcination temperature ($0-550 \circ C$), $H_2$ flow rate (0-60 mL/min), and $CO_2$ flow rate (0-60 mL/min). Complete $NO_3$ - removal with highest $N_2$ selectivity (91%) was achieved at optimized condition i.e. 1%Sn-1.6%Pd-NZSM-5, initial $NO_3$- concentration: 30 mg/L $NO_3-N$, calcination temperature: $350 \circ C$, $H_2$ flow rate: 30 mL/min, and $CO_2$ flow rate: 60 mL/min in 60 minutes. The estimated kinetic rate constant (k) and catalyst loading normalized rate constant (K’) by 1%Sn-1.6%Pd-NZSM-5 is $16.4×10-2 min^{-1}$ and $65.6×10^{-2} min^{-1} g_{cat}^{-1}$ respectively, one of the fastest $NO_3$- degradation kinetics among other reported catalysts. The 1%Sn-1.6%Pd-NZSM-5 also showed remarkable $NO_3$- removal (100%) and >80% $N_2$ selectivity up to five reaction cycles. The reaction kinetics decelerated to $4.36×10^{-2} min^{-1}$ over five cycles but are still comparable to those reported in the literature for fresh catalysts. It was confirmed by characterization tests that the catalyst was chemically stable during recycling and that the decrease in the reactivity was due mainly to the sintering of metallic nano particles during regeneration process. High reactivity by 1% Sn-1.6%-NZSM-5 showed that the catalytic activity can be enhanced by employing stable support surface with higher surface area. Nano crystalline beta zeolite (NBeta) is another zeolite having higher surface area than NZSM-5 with good geometrical feature. NBeta was synthesized and used to develop novel Sn-Pd-NBeta, Cu-Pd-NBeta and In-Pd-NBeta bimetallic catalysts for complete and selective reduction of nitrate into environmental friendly nitrogen. The synthesis of NBeta support and bimetallic catalysts was confirmed by XRD and Fourier transformation infra-red spectroscopy (FT-IR). Morphology of NBeta support and dispersion was confirmed by TEM and scanning electron microscopy (SEM) with energy dispersive X-ray (EDX), which revealed that newly synthesized NBeta consisted of individually separated cubical shaped crystals and that the metals were well dispersed in uniform and closely packed ensembles. All catalysts successfully achieved complete $NO^3$- and $NO^2$- removal. Sn-Pd-NBeta showed highest reduction kinetics ($k = 19.09×10^{-2}×min^{-1}$) followed by In-Pd-NBeta ($k = 2.89×10^{-2}×min^{-1}$) and Cu-Pd-NBeta $(k = 2.31×10^{-2}×min^{-1}$). However, the $N_2$ selectivity was in the order of Cu-Pd-NBeta (92.68%)>In-Pd-NBeta (82.93%)>Sn- Pd-NBeta (80.80%). Characterization of catalysts revealed that highest $N_2$ selectivity by Cu-Pd-NBeta was due to suppression of deep hydrogenation of Pd by Cu. The NBeta supported Cu-Pd, In-Pd and Sn-Pd catalysts also showed a complete $NO_3$- reduction with consistent $N_2$ selectivity in successive and regenerated cycles. The use of red mud as support material was evaluated in this study to develop novel bimetallic catalyst for highly reactive and selective nitrate removal. The catalyst was firstly optimized for promoter metal i.e. Sn, Cu, In, Zn and then for noble metal i.e. Pd, Pt and Au. Sn-Pd-red mud and In-Pd-red mud catalysts achieved 100% nitrate removal as compared to Cu-Pd-red mud, Zn-Pd-red mud, Sn-Pt red mud and Sn-Au-red mud which could remove only 78%, 64%, 35% and 17% nitrate, respectively. Sn-Pd-red mud showed highest kinetics (k= 11.57×10？2 min？1, catalyst loading normalized rate constant $K’= 46.28×10^{-2} min^{-1}.g){cat}^{-1}$

and Pd loading normalized rate constant $K”= 578.5 ×10^{-2} L.min^{-1}.g_{cat}^{-1}$) as compared to In-Pd-red mud ($k = 2.27×10^{-2} min^{-1}$, $K’= 9.08×10^{-2} min^{-1}.g_{cat}^{-1}$

and Pd loading normalized rate constant $K”= 113.5 ×10^{-2} L.min^{-1}.g_{cat}^{-1}$). The characterization results confirmed that high reduction kinetics of Sn-Pd-red mud was due to 1) its higher affinity towards nitrate, 2) lower alloying effect of Sn on Pd, 3) higher reduction potential of Sn and 4) higher $H_2$ activation by Pd, as compared to In-Pd-red mud, Cu-Pd-red mud, Zn-Pd-red mud, Sn-Au-red mud and Sn-Pt-red mud. A 100% nitrate removal with >88% nitrogen $N_2$ selectivity with consistent kinetics ($k = 10.87±0.48×10^{-2} min^{-1}$

$K’= 43.5±1.9×10^{-2} min^{-1}.g_{cat}^{-1}$

$K” = 543.68±23.96×10^{-2} L.min^{-1}.g_{cat}^{-1}$) was also achieved by Sn-Pd-red mud catalyst over 6 successive and 5 repeated cycles. The characterization results revealed that the kinetic and mechanistic stability in successive and repeated used was due to formation of $CaFe_2O_4$ as a result of interaction between CaO and $Fe_2O_3$ during catalyst synthesis. However, such interaction between CaO, $TiO_2$ and $Al_2O_3$ was not detected.