Catalytic reduction of nitrate in water over alumina-supported nickel catalyst toward purification of polluted groundwater

Pollution of groundwater with NO3- is a serious problem in the world. While catalytic reduction of NO3- over Pd-bimetallic catalysts including Cu-Pd and Sn-Pd is a promising method for purification of the groundwater, the use of precious metal is a major obstacle for practical applications. In the present study, we applied Ni/Al2O3 for the catalytic reduction of NO3- and compared the catalytic performance with that of unsupported Ni catalyst. The reaction rate over 5 wt.% Ni/Al2O3 was about 5 times higher than that of the unsupported Ni catalyst, based on unit weight of catalyst. While the unsupported Ni catalyst was completely deactivated in low partial pressure of H-2 (=0.75 atm) and high concentration of NO3- (= 800 ppm), Ni/Al2O3 was still active even under less reductive conditions ([NO3-](0)= 800 ppm and P(H-2) = 0.5 atm). The unsupported Ni catalyst had the Ni-0 particles formed by the reduction of NiO with H-2 at 310-420 degrees C. On the other hand, 5 wt.% Ni/Al2O3 possessed the Ni-0 particles formed from NiAl2O4 on Al2O3 by the reduction with H-2 above 450 degrees C. It is plausible that those Ni-0 particles had different properties, giving different catalytic properties. The Ni loadings for Ni/Al2O3 had a significant impact on the catalytic properties. The reaction orders with respect to both NO3- and H-2 were 0.8 for 5 wt.% Ni/Al2O3, while those were 0 and -0.2, respectively, for 10 wt.% Ni/Al2O3. On 10 wt.% Ni/Al2O3, there were two kinds of the Ni degrees particles, which were formed by low (310-420 degrees C) and high (above 450 degrees C) temperature H-2 reductions. Unlike the Pd-bimetallic catalysts, the reduction of NO3- over Ni/Al2O3 did not proceed through NO2-.