Removing excess nitrate (NO3 –) from waste streams has become a significant environmental and health topic. However, realizing highly selective NO3 – conversion toward N2, primarily via electrocatalytic conversions, has proven challenging, largely because of the kinetically uncontrollable NO3 –-to-NO2 – pathway and unfavorable N–N coupling. Herein, we discovered unique and ultra-high electrocatalytic NO3 –-to-NO2 –activity on oxide-derived silver (OD-Ag). Up to 98% selectivity and 95% Faradaic efficiency (FE) of NO2 – were observed and maintained under a wide potential window. Benefiting from the superior NO3 –-to-NO2 –activity, further reduction of accumulated NO2 – to NH4 + was well regulated by the cathodic potential and achieved an NH4 + FE of 89%, indicating a tunable selectivity to the key nitrate reduction products (NO2 – or NH4 +) on OD-Ag. Density functional theory computations provided insights into the unique NO2 – selectivity on Ag electrodes compared with Cu, showing the critical role of a proton-assisted mechanism. Based on the ultra-high NO3 –-to-NO2 – activity on OD-Ag, we designed a novel electrocatalytic–catalytic combined process for denitrifying real-world NO3 –-containing agricultural wastewater, leading to 95+% of NO3 – conversion to N2 with minimal NOX gases. In addition to the wastewater treatment process to N2 and the electrochemical synthesis of NH3, NO2 – derived from electrocatalytic NO3 – conversion can serve as a reactive platform for the distributed production of various nitrogen products.
Condensed phase reactions have recently attracted increased interest, but principles for efficiently screening and designing catalyst materials through computations are lacking. In this study, we examine the applicability of energy correlations between adsorbed surface species, which have been instrumental in accelerating the computational design of catalyst materials in gas-phase contexts, in various representations of a condensed phase reaction environment. We perform detailed density functional theory calculations of the adsorption of atomic and molecular species in the presence of various representations of solvent species. Our results show that the well-known scaling in the gas phase context is preserved, with scaling slopes unaffected by the adsorbate-liquid interactions. Moreover, these results hold when changing surface structure, solvent identity, and even in highly disordered environments. We envision the establishment of an energy scaling framework for condensed phase reactions to accelerate catalyst discovery and design in those contexts.
In our study, an exceptionally high selectivity of the electrocatalytic nitrate-to-nitrite transformation was discovered on Ag surfaces, among eighteen metals screened. It was demonstrated that this electrocatalytic step on oxide-derived Ag (OD-Ag), which possesses extended surface area (13 times) and enhanced specific activity (3 times) relative to Ag foil, can be coupled with a catalytic nitrite-to-dinitrogen step on a Pd catalyst using renewable hydrogen generated on-site by a water electrolyzer. We thereby proposed and demonstrated a combined electrocatalytic-catalytic process as an alternative strategy for innovative nitrate removal from agricultural wastewater with high selectivity of >95%. With future research and development, the combined process may hold the potential of tackling the ever-increasing nitrate pollution in water bodies to address its linked environmental and health issues. Strategically returning reactive nitrogen from wastewater back to the atmosphere in the inert form, this combined process is well-positioned to help close the global nitrogen cycle, one of the grand engineering challenges in the 21st century. In parallel with the applications in wastewater treatment, the Ag-based electrocatalytic nitrate-to-nitrate conversion with ultrahigh selectivity may be widely employed for designing cost-effective and energy-efficient syntheses of various nitrogen-based compounds in a distributed manufacturing fashion. The kinetics studies and computational insights could also be beneficial to advancing nitrogen-centric electrochemistry, materials science, and technologies. Figure 1
Removing excessive nitrate (NO3 −) from wastewater has increasingly become an important research topic in light of the growing concerns over the related environmental problems and health issues. In particular, catalytic/electrocatalytic approaches are attractive for NO3 − removal, because NO3 − from wastewater can be converted to N2 and released back to the atmosphere using renewable H2 or electricity, closing the loop of the global N cycle. However, achieving high product selectivity towards the desirable N2 has proven challenging in the direct NO3 −-to-N2 reaction. In this presentation, we will report our finding on unique and ultra-high electrochemical NO3 −-to-NO2 −activity on an oxide-derived silver electrode (OD-Ag). Up to 98% selectivity and 95% faradaic efficiency of NO2 − were observed and maintained under a wide potential window. Benefiting from overcoming the rate-determining barrier of NO3 −-to-NO2 −during nitrate reduction, further reduction of accumulated NO2 − to NH4 + can be well regulated by the cathodic potential on OD-Ag to achieve a faradaic efficiency of 89%. These indicated the potential controllable pathway to the key nitrate reduction products (NO2 −or NH4 +) on OD-Ag. DFT computations provided insights into the unique NO2 −selectivity on Ag electrodes compared with Cu, showing the critical role of a proton-assisted mechanism. Based on the ultra-high NO3 −-to-NO2 −activity on OD-Ag, we designed a novel electrocatalytic-catalytic combined process for denitrifying real-world NO3 −-containing agricultural wastewater, leading to 95+% of NO3 − conversion to N2 with minimal NOX gases. In addition to the wastewater treatment process to N2 and electrochemical synthesis of NH3, NO2 − derived from electrocatalytic NO3 − conversion can serve as a reactive platform for distributed production of various nitrogen products. Our new research progress along this direction will be briefly presented.
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