Kinetic and adsorption studies on the hydrogenation of nitrate and nitrite in water using Pd-Cu on active carbon support

Hydrogenations of nitrate (NO3−) and nitrite (NO2−) ions with H2 at 333 K were studied using a bimetallic catalyst, Pd (5 wt.%)-Cu (0.6 wt.%), which was supported on active carbon. Reactions of NO3− proceeded rapidly with high N2-selectivity using a H2 gas flow with the presence of CO2 (50% (v/v)). Subsequently, kinetics for the hydrogenations of NO3− and NO2− were systematically examined using a gas–liquid flow system over the bimetallic catalyst. Kinetic equations, v=k[NO3−]0.2PH20.2 and v=k′[NO2−]1.0PH20.3, were obtained for the hydrogenations of NO3− and NO2−, respectively. Adsorption studies revealed that: (a) within the wide pH range (2–10), the amount of NO3− adsorbed on the metallic sites at 298 K was small; (b) considerable amounts of NO2− were adsorbed on the metallic sites at neutral pH values; and (c) adsorption amounts of NO2− decreased as the pH value increased. From the kinetic and adsorption studies, it can be concluded that: (a) NO3− is strongly adsorbed and reduced on specific sites, such as Pd-Cu bimetallic sites; and (b) although NO2− is adsorbed on Pd and Pd-Cu, the adsorption of NO2− at high pH values is retarded by OH− ions during the reaction. It should be emphasized that high N2-selectivity was retained even at low NO3− concentrations by decreasing the H2 pressure, without significantly decreasing the activity.