Effect of chlorine on performance of Pd catalysts prepared via colloidal immobilization

This contribution shows the effect of residual chlorine on the catalytic performance of a Pd-based catalyst in the hydrogenation of nitrite for cleaning of drinking water. The catalyst was prepared via immobilization a colloidal Pd nanoparticles using activated carbon as support. Different amount of hydrochloric acid (HCl) was added to immobilize the Pd colloid on the carbon support, facilitating the removal of the residual stabilizer, polyvinyl alcohol (PVA), from the surface of the Pd nanoparticles (NPs). The catalysts were characterized by TEM, CO-chemisorption, XRF, N₂ physisorption, UV-vis spectroscopy, and XPS. The activity and selectivity of the catalysts were measured for nitrite hydrogenation in semi-batch operation. The results show that PVA can be removed completely at pH below 2. The residual chlorine on the catalysts can be removed by reduction in H₂/N₂ at a mild temperature, i.e. 200°C, regardless the amount of HCl used. Nevertheless, high concentration of HCl during immobilization (pH 1) causes partial Pd re-dissolution according to UV-vis spectroscopy, resulting in formation of highly dispersed Pd clusters that could not be detected with TEM. Reduction of this catalyst with high chlorine content in H₂ at 200°C is resulting in formation of relatively large Pd particles via sintering. Without pre-reduction at 200°C, residual chlorine can also be removed almost completely during the hydrogenation reaction at room temperature. The activity of the Pd catalyst is insensitive to the chlorine concentration below 30μmolL^-1 in the aqueous reaction mixture. Interestingly, the selectivity to N₂ is improved by adding chlorine to the reaction mixture, independent of the way chlorine is added, i.e. via the catalyst or added directly to the reaction solution.