We developed Sn0.5Pd/Al2O3 showing high activity and high selectivity to gaseous products towards the hydrogenation of NO3- with H2 in aqueous NO3- solutions via precise control of the Sn/Pd molar ratio. For the catalyst with the optimum Sn/Pd molar ratio, the surface concentration of adsorbed nitrogen on the Pd sites is thought to be high and that of the adsorbed hydrogen on the Pd sites is thought to be low. This is due to the abundant supply of NO2- for the Pd sites and the prompt consumption of adsorbed H on the Pd sites for reduction of the oxidized Sn sites formed by the reduction of NO3-, respectively, leading to the high selectivity to gaseous products. Although the catalytic performance of Sn0.5Pd/Al2O3, like that of Cu0.5Pd/Al2O3, was lower in groundwater, the decrease for Sn0.5Pd/Al2O3 was less than that for Cu0.5Pd/Al2O3. Nitrate in the groundwater polluted with 0.4 mmol dm-3 (250 cm3) of NO3- was completely reduced at 298 K in 24 h with a selectivity for gaseous products of around 90% in the presence of 10 mg of Sn0.5Pd/Al2O3, whereas it took 60 h in the presence of Cu0.5Pd/Al2O3, and the selectivity for gaseous products was around 75%. However, both catalysts showed comparable high activity and high selectivity in aqueous NO3- solutions. When reactions were performed in aqueous NO3- solutions containing other anions (Cl-, SO42-, and SiOxn-) present in the groundwater, Cl- had the largest negative impact but it had a smaller impact in the presence of Sn0.5Pd/Al2O3 than it did in the presence of Cu0.5Pd/Al2O3. On the basis of adsorption isotherms for Cl- and kinetics analysis of the hydrogenation of NO3-, it was concluded that Sn0.5Pd/Al2O3 had less affinity for Cl- and a strong affinity for NO3- on the Sn sites, leading to its superior catalytic performance in groundwater.
"Determination of acid site location in dealuminated MCM-68 by Al-27 MQMAS NMR and FT-IR spectroscopy with probe molecules"