Chloride Anions Unexpectedly Promote Metal-Catalyzed Nitrate Hydrogenation

Abstract

Nitrate pollution of water, primarily due to widespread fertilizer use, is not only toxic to humans, but also leads to eutrophication. Catalytic destruction of nitrate using catalysts such as InPd has been extensively studied because of their high catalytic activity. However, their application in the treatment of real water remains largely underexplored still, as it is commonly assumed that cosolutes like chloride inhibit activity through competitive adsorption or irreversible poisoning. In this study, we found that chloride unexpectedly promotes NO3– reduction over InPd and InRh catalysts. NaCl concentrations (up to 2 M) had a volcano-shaped trend in InPd/C catalyst activity and increased InRh/C activity by 4-fold. In aqua surface-enhanced Raman spectroscopy using CO as a probe molecule showed that chloride has an electronic effect on catalysis, weakening the N–O bond of the crucial NO* reaction intermediate. Challenged with a simulated brine (ionic strength of 2.68 M) with 5000 mg-NO3–/L (1130 mg-N/L), both catalysts showed comparably high reduction activity. InRh/C was more selective to ammonium than InPd/C (90% vs 16% at 25% NO3– conversion). This study demonstrates how salt species can alter intermediate-catalyst interactions to surprisingly improve activity, which widens the prospects of hydrogenation catalysis for treating real water.