Insights into Nitrate Reduction over Indium-Decorated Palladium Nanoparticle Catalysts

Nitrate (NO3-) is an ubiquitous groundwater contaminant and is detrimental to human health. Bimetallic palladium-based catalysts have been found to be promising for treating nitrate (and nitrite, NO2-) contaminated waters. Those containing indium (In) are unusually active, but the mechanistic explanation for catalyst performance remains largely unproven. We report that In deposited on Pd nanoparticles (NPs) ("In-on-Pd NPs") shows room-temperature nitrate catalytic reduction activity that varies with volcano-shape dependence on In surface coverage. The most active catalyst had an In surface coverage of 40%, with a pseudo-first order normalized rate constant of k(cat) similar to 7.6 L g(surface-metal)(-1) min(-1), whereas monometallic Pd NPs and In2O3 have nondetectible activity for nitrate reduction. X-ray absorption spectroscopy (XAS) results indicated that In is in oxidized form in the as-synthesized catalyst; it reduces to zerovalent metal in the presence of H-2 and reoxidizes following NO3- contact. Selectivity in excess of 95% to nontoxic N-2 was observed for all the catalysts. Density functional theory (DFT) simulations suggest that submonolayer coverage amounts of metallic In provide strong binding sites for nitrate adsorption and they lower the activation barrier for the nitrate-to-nitrite reduction step. This improved understanding of the In active site expands the prospects of improved denitrification using metal-on-metal catalysts.