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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Electrocatalytic Reduction of Nitrate to Nitrite and Ammonia over Oxide-Derived Silver Electrodes
The nitrogen cycle plays a key role biological, energy, environment, and industrial processes. Breaking natural nitrogen cycle is leading to accumulation of reactive nitrogen chemicals in water and atmosphere, therefore, better management of N-cycle has emerged as an urgent research need in energy and environmental science. 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. Importantly, NO2− derived from nitrate may serve as a crucial reactive platform for distributed production of various nitrogen products, such as NO, NH2OH, NH3, and urea.
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- Award ID(s):
- 2036944
- NSF-PAR ID:
- 10302838
- Date Published:
- Journal Name:
- 262nd ACS National Meeting
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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Biomass is abundant, inexpensive and renewable, therefore, it is highly expected to play a significant role in our future energy and chemical landscapes. The US DOE has identified furanic compounds (furfural and 5-(hydroxymethyl)furfural (HMF)) as the top platform building-block chemicals that can be readily derived from biomass sources [1]. Electrocatalytic conversion of these furanic compounds is an emerging route for the sustainable production of valuable chemicals [2, 3]. In my presentation, I will first present our recent mechanistic study of electrochemical reduction of furfural through a combination of voltammetry, preparative electrolysis, thiol-electrode modifications, and kinetic isotope studies [4]. It is demonstrated that two distinct mechanisms are operable on metallic Cu electrodes in acidic electrolytes: (i) electrocatalytic hydrogenation (ECH) and (ii) direct electroreduction. The contributions of each mechanism to the observed product distribution are clarified by evaluating the requirement for direct chemical interactions with the electrode surface and the role of adsorbed hydrogen. Further analysis reveals that hydrogenation and hydrogenolysis products are generated by parallel ECH pathways. Understanding the underlying mechanisms enables the manipulation of furfural reduction by rationally tuning the electrode potential, electrolyte pH, and furfural concentration to promote selective formation of important bio-based polymer precursors and fuels. Next, I will present electrocatalytic conversion of HMF to two biobased monomers in an H-type electrochemical cell [5]. HMF reduction (hydrogenation) to 2,5-bis(hydroxymethyl)furan (BHMF) was achieved under mild electrolyte conditions and ambient temperature using a Ag/C cathode. Meanwhile, HMF oxidation to 2,5-furandicarboxylic acid (FDCA) with ~100% efficiency was facilitated under the same conditions by a homogeneous nitroxyl radical redox mediator, together with an inexpensive carbon felt anode. The selectivity and efficiency for Ag-catalyzed BHMF formation were sensitive to cathode potential, owing to competition from HMF hydrodimerization reactions and water reduction (hydrogen evolution). Moreover, the carbon support of Ag/C was active for HMF reduction and contributed to undesired dimer/oligomer formation at strongly cathodic potentials. As a result, high BHMF selectivity and efficiency were only achieved within a narrow potential range near –1.3 V. Fortunately, the selectivity of redox-mediated HMF oxidation was insensitive to anode potential, thus allowing HMF hydrogenation and oxidation half reactions to be performed together in a single cathode-potential-controlled cell. Electrocatalytic HMF conversion in a paired cell achieved high molar yields of BHMF and FDCA, and nearly doubled electron efficiency compared to the unpaired cell. Finally, I will briefly introduce our recent work on the development of a three-electrode flow cell with an oxide-derived Ag (OD-Ag) cathode and catbon felt anode for paring elecatalytic oxidation and reduction of HMF. The flow cell has a remarkable cell voltage reduction from ~7.5 V to ~2.0 V by transferring the electrolysis from the H-type cell to the flow cell [6]. This represents a more than fourfold increase in the energy efficiency at the 10 mA operation. A combined faradaic efficiency of 163% was obtained to BHMF and FDCA. Alternatively, the anodic hydrogen oxidation reaction on platinum further reduced the cell voltage to only ~0.85 V at 10 mA operation. These paired processes have shown potential for integrating renewable electricity and carbon for distributed chemical manufacturing in the future.more » « less
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null (Ed.)The increased environmental abundance of anthropogenic reactive nitrogen species (Nr = ammonium [NH4+], nitrite [NO2−] and nitrate [NO3−]) may increase atmospheric nitrous oxide (N2O) concentrations, and thus global warming and stratospheric ozone depletion. Nitrogen cycling and N2O production, reduction, and emissions could be amplified in carbonate karst aquifers because of their extensive global range, susceptibility to nitrogen contamination, and groundwater-surface water mixing that varies redox conditions of the aquifer. The magnitude of N2O cycling in karst aquifers is poorly known, however, and thus we sampled thirteen springs discharging from the karstic Upper Floridan Aquifer (UFA) to evaluate N2O cycling. The springs can be separated into three groups based on variations in subsurface residence times, differences in surface–groundwater interactions, and variable dissolved organic carbon (DOC) and dissolved oxygen (DO) concentrations. These springs are oxic to sub-oxic and have NO3− concentrations that range from < 0.1 to 4.2 mg N-NO3−/L and DOC concentrations that range from < 0.1 to 50 mg C/L. Maximum spring water N2O concentrations are 3.85 μg N-N2O/L or ~ 12 times greater than water equilibrated with atmospheric N2O. The highest N2O concentrations correspond with the lowest NO3− concentrations. Where recharge water has residence times of a few days, partial denitrification to N2O occurs, while complete denitrification to N2 is more prominent in springs with longer subsurface residence times. Springs with short residence times have groundwater emission factors greater than the global average of 0.0060, reflecting N2O production, whereas springs with residence times of months to years have groundwater emission factors less than the global average. These findings imply that N2O cycling in karst aquifers depends on DOC and DO concentrations in recharged surface water and subsequent time available for N processing in the subsurface.more » « less
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null (Ed.)The increased environmental abundance of anthropogenic reactive nitrogen species (Nr = ammonium [NH4+], nitrite [NO2 ] and nitrate [NO3 ]) may increase atmospheric nitrous oxide (N2O) concentrations, and thus global warming and stratospheric ozone depletion. Nitrogen cycling and N2O production, reduction, and emissions could be amplified in carbonate karst aquifers because of their extensive global range, susceptibility to nitrogen contamination, and groundwater-surface water mixing that varies redox conditions of the aquifer. The magnitude of N2O cycling in karst aquifers is poorly known, however, and thus we sampled thirteen springs discharging from the karstic Upper Floridan Aquifer (UFA) to evaluate N2O cycling. The springs can be separated into three groups based on variations in subsurface residence times, differences in surface–groundwater interactions, and variable dissolved organic carbon (DOC) and dissolved oxygen (DO) concentrations. These springs are oxic to sub-oxic and have NO3 concentrations that range from < 0.1 to 4.2 mg N-NO3 /L and DOC concentrations that range from < 0.1 to 50 mg C/L. Maximum spring water N2O concentrations are 3.85 μg N-N2O/L or ~ 12 times greater than water equilibrated with atmospheric N2O. The highest N2O concentrations correspond with the lowest NO3 concentrations. Where recharge water has residence times of a few days, partial denitrification to N2O occurs, while complete denitrification to N2 is more prominent in springs with longer subsurface residence times. Springs with short residence times have groundwater emission factors greater than the global average of 0.0060, reflecting N2O production, whereas springs with residence times of months to years have groundwater emission factors less than the global average. These findings imply that N2O cycling in karst aquifers depends on DOC and DO concentrations in recharged surface water and subsequent time available for N processing in the subsurface.more » « less
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