Insights into Nitrate Reduction over Indium-Decorated Palladium Nanoparticle Catalysts

Abstract

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 ∼ 7.6 L gsurface-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 H2 and reoxidizes following NO3 − contact. Selectivity in excess of 95% to nontoxic N2 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.

Coordinatively Unsaturated Metal–Nitrogen Active Sites at Twisted Surfaces in Metallic Porous Nitride Single Crystals Delivering Enhanced Electrocatalysis Activity

“…The Gibbs free energy changes for the NO 3 À reduction progress can be obtained by the following Equations (1)- (7), where *r epresents the active site: [19][20][21][22][23][24] NO 3 1 ðIÞþ* ! NO 3 * ð1Þ…”

Mild and Selective Hydrogenation of Nitrate to Ammonia in the Absence of Noble Metals

“…Multiple "bimetallic" catalysts for this transformation exist; each consists of a mixture of two metal particles, at least one of which is made of a scarce and expensive noble metal-Ru, Pd, or Pt-which decreases their potential utility and prevents their large-scale deployment. 18,19,20,21,22 A few reports with base metals exist. 23,24,25,26,27 Compounding this problem, some nitrate reduction methods suffer from limited activity and/or poor yields.…”

Conducting Polymer–TiO2Hybrid Materials: Application in the Removal of Nitrates from Water

“…Numerous investigations and discussions have been performed to clarify the reaction intermediates. [20][21][22] They have demonstrated that nitrite ions are reduced on the surface of the noble metal to adsorbed NO as the key intermediate in the generation of nitrogen (N 2 ) and ammonium (NH 4 + ). Two different pathways for the reduction of NO to NH 4 + have been proposed: NO dissociation and NO hydrogenation.…”