Selectivity and Sustainability of a Pd-In/γ-Al₂O₃ Catalyst in a Packed-Bed Reactor: The Effect of Solution Composition

This study tested the selectivity and sustainability of an alumina-supported Pd-In bimetallic catalyst for nitrate reduction with H₂ in a continuous-flow packed-bed reactor in the presence of: (i) dissolved oxygen (DO), an alternative electron acceptor to nitrate, (ii) variable NO₃ ⁻:H₂ influent loadings, and (iii) the presence of a known foulant, sulfide. The sustainability of the catalyst was promising, as the catalyst was found to be stable under all conditions tested with respect to metal leaching. The presence of DO at concentrations typical of treatment conditions will increase H₂ demand for NO₃ ⁻ reduction, but has no negative impact on the selectivity of the catalyst. Under optimal conditions, i.e., a pH of 5.0 and a high NO₃ ⁻:H₂ influent loading, low NH₃ selectivity (5%) was achieved for extended periods (36 days), resulting in sustained levels of NH₃ that approached the European legal limit. The biggest challenge to the sustainability of the catalyst was the addition of sulfide, that initially increased NH₃ selectivity and ultimately resulted in complete deactivation of the catalyst. Further work is required to identify regeneration methods to restore sulfide-fouled catalyst activity and selectivity; however, the most effective use would be to remove sulfide prior to catalytic treatment.