Nitrate hydrogenation by microtubular CNT-made catalytic membrane contactor

Abstract

Microtubular CNT-made catalytic membrane contactor is proposed for heterogeneous three-phase (i.e. solid-liquid-gas) catalytic processes aimed at hydrogenation and oxygenation of aqueous pollutants. The contactor comprises a mesoporous CNT-made microtube with an internal diameter of ≈ 1.5 mm, wall thickness of ≈ 0.2 mm, pore diameter of ≈30 nm, loaded with a nano-scale metal catalyst. In this study the concept of CNT-made contactors was proved for hydrogenation of nitrate ions. The CNT-made microtubes were fabricated by infiltration of CNT suspension through the polypropylene microfiltration membrane. The CNT-made microtubes were loaded with Pd-Cu catalysts using the wet-impregnation method followed by reduction by H2 gas or sodium borohydride. The catalytic contactors were investigated for nitrate reduction at varied pH (5–9) and H2 pressure (0.5–2.5 bar) in pure KNO3 solutions and groundwater. Lower pH and higher H2 promoted higher nitrate reduction rates but lower selectivity towards the nitrogen gas. The highest catalytic activity of 2.6 mgN/min/gPd was obtained at pH5.0 and H2 gauge pressure of 0.5 bar. The highest selectivity (79%) towards nitrogen gas was achieved at pH7.0 and H2 pressure of 0.1 bar. Hydrogen utilization efficiency of 93.4% (±7.6%) was obtained. Operation in groundwater resulted in lower selectivity to nitrogen than in experiments conducted on KNO3 solutions in deionized water.