Defective TiO2- x for High-Performance Electrocatalytic NO Reduction toward Ambient NH3 Production

Synthesis of green ammonia (NH₃ ) via electrolysis of nitric oxide (NO) is extraordinarily sustainable, but multielectron/proton-involved hydrogenation steps as well as low concentrations of NO can lead to poor activities and selectivities of electrocatalysts. Herein, it is reported that oxygen-defective TiO₂ nanoarray supported on Ti plate (TiO_{2- x} /TP) behaves as an efficient catalyst for NO reduction to NH₃ . In 0.2 m phosphate-buffered electrolyte, such TiO_{2- x} /TP shows competitive electrocatalytic NH₃ synthesis activity with a maximum NH₃ yield of 1233.2 µg h^-1  cm^-2 and Faradaic efficiency of 92.5%. Density functional theory calculations further thermodynamically faster NO deoxygenation and protonation processes on TiO_{2- x} (101) compared to perfect TiO₂ (101). And the low energy barrier of 0.7 eV on TiO_{2- x} (101) for the potential-determining step further highlights the greatly improved intrinsic activity. In addition, a Zn-NO battery is fabricated with TiO_{2- x} /TP and Zn plate to obtain an NH₃ yield of 241.7 µg h^-1  cm^-2 while providing a peak power density of 0.84 mW cm^-2 .

Steering from electrochemical denitrification to ammonia synthesis

The removal of nitric oxide is an important environmental issue, as well as a necessary prerequisite for achieving high efficiency of CO2 electroreduction. To this end, the electrocatalytic denitrification is a sustainable route. Herein, we employ reaction phase diagram to analyze the evolution of reaction mechanisms over varying catalysts and study the potential/pH effects over Pd and Cu. We find the low N2 selectivity compared to N2O production, consistent with a set of experiments, is limited fundamentally by two factors. The N2OH* binding is relatively weak over transition metals, resulting in the low rate of as-produced N2O* protonation. The strong correlation of OH* and O* binding energies limits the route of N2O* dissociation. Although the experimental conditions of varying potential, pH and NO pressures can tune the selectivity slightly, which are insufficient to promote N2 selectivity beyond N2O and NH3. A possible solution is to design catalysts with exceptions to break the scaling characters of energies. Alternatively, we propose a reverse route with the target of decentralized ammonia synthesis.

Electrochemical Reduction of Nitric Oxide with 1.7% Solar-to-Ammonia Efficiency Over Nanostructured Core-Shell Catalyst at Low Overpotentials

Transition metals have been recognized as excellent and efficient catalysts for the electrochemical nitric oxide reduction reaction (NORR) to value-added chemicals. In this work, a class of core-shell electrocatalysts that utilize nickel nanoparticles in the core and nitrogen-doped porous carbon architecture in the shell (Ni@NC) for the efficient electroreduction of NO to ammonia (NH3 ) is reported. In Ni@NC, the NC prevents the dissolution of Ni nanoparticles and ensures the long-term stability of the catalyst. The Ni nanoparticles involve in the catalytic reduction of NO to NH3 during electrolysis. As a result, the Ni@NC achieves a faradaic efficiency (FE) of 72.3% at 0.16 VRHE . The full-cell electrolyzer is constructed by coupling Ni@NC as cathode for NORR and RuO2 as an anode for oxygen evolution reaction (OER), which delivers a stable performance over 20 cycles at 1.5 V. While integrating this setup with a PV-electrolyzer cell, and it demonstrates an appreciable FE of >50%. Thus, the results exemplify that the core-shell catalyst based electrolyzer is a promising approach for the stable NO to NH3 electroconversion.

High-efficiency NO electroreduction to NH3 over honeycomb carbon nanofiber at ambient conditions

Electrocatalytic NO reduction is a promising technology for ambient NO removal with simultaneous production of highly value-added NH₃. Herein, we report that honeycomb carbon nanofiber coated on carbon paper acts as an efficient metal-free catalyst for ambient electroreduction of NO to NH₃. In 0.2 M Na₂SO₄ solution, such catalyst achieves an NH₃ yield of 22.35 μmol h^-1 cm^-2 with a high Faradaic efficiency of up to 88.33%. Impressively, it also shows excellent stability for 10-h continuous electrolysis. Theoretical calculations reveal that the most active center of functional groups is -OH group for NO reduction with a low energy barrier (ΔG of 0.29 eV) for the potential-determining step (*NO + H → *HNO).

Efficient nitric oxide electroreduction toward ambient ammonia synthesis catalyzed by a CoP nanoarray

The ever-increasing anthropic NO emission from fossil fuel combustion has resulted in a series of severe environmental issues. Ambient electrocatalytic NO reduction has emerged as a promising route for sustainable...

High-performance NH3 production via NO electroreduction over a NiO nanosheet array

Electrocatalytic NO reduction controls NO emission and produces NH₃ under ambient conditions. Herein, a NiO nanosheet array on titanium mesh is proposed as a highly active and selective electrocatalyst for NO reduction, attaining a faradaic efficiency of up to 90% with a NH₃ yield of 2130 μg h^-1 cm^-2. Its aqueous Zn-NO battery can generate electricity with a power density of 0.88 mW cm^-2 and simultaneously offer an NH₃ yield of 228 μg h^-1 cm^-2. The NO electroreduction mechanism on NiO is revealed using theoretical calculations.

High-Performance Electrochemical NO Reduction into NH3 by MoS2 Nanosheet

Electrochemical reduction of NO not only offers an attractive alternative to the Haber-Bosch process for ambient NH₃ production but mitigates the human-caused unbalance of nitrogen cycle. Herein, we report that MoS₂ nanosheet on graphite felt (MoS₂ /GF) acts as an efficient and robust 3D electrocatalyst for NO-to-NH₃ conversion. In acidic electrolyte, such MoS₂ /GF attains a maximal Faradaic efficiency of 76.6 % and a large NH₃ yield of up to 99.6 μmol cm^-2  h^-1 . Using MoS₂ nanosheet-loaded carbon paper as the cathode, a proof-of-concept device of Zn-NO battery was assembled to deliver a discharge power density of 1.04 mW cm^-2 and an NH₃ yield of 411.8 μg h^-1  mg_cat. ^-1 . Calculations reveal that the positively charged Mo-edge sites facilitate NO adsorption/activation via an acceptance-donation mechanism and disfavor the binding of protons and the coupling of N-N bond.

Unveiling Potential Dependence in NO Electroreduction to Ammonia

Recently, electrochemical NO reduction (eNORR) to ammonia has attracted enormous research interests due to the dual benefits in ammonia synthesis and denitrification fields. Herein, taking Ag as a model catalyst, we have developed a microkinetic model to rationalize the general selectivity trend of eNORR with varying potential, which has been observed widely in experiments, but not understood well. The model reproduces experiments well, quantitatively describing the selectivity turnover from N₂O to NH₃ and from NH₃ to H₂ with more negative potential. The first turnover of selectivity is due to the thermochemical coupling of two NO* limiting the N₂O production. The second turnover is attributed to the larger transfer coefficient (β) of HER than NH₃ production. This work reveals how electrode potential regulate the selectivity of eNORR, which is also beneficial to understand the commonly increasing HER selectivity with the decrease of potential in some other electroreduction reactions such as CO₂ reduction.

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

To create active sites on surfaces, the identification of structural features that could confine the local-defect structure in the lattice is required. Porous nitride single crystals, combining the advantages of porosity and structural coherence, provide the possibility to create coordinatively unsaturated metal-nitrogen active sites confined on surfaces. For the first time, ordered active sites and tailor the atomically resolved Fe-N and Co-N local structures are created through control of the unsaturated nitrogen coordination at twisted surfaces in porous single-crystalline Fe_n N (n=2-4) and Co_n N (n=1-3) nanocubes. The precise tailoring of the electronic structures of these coordinatively unsaturated active sites therefore engineer the catalytic activity. Optimum electrocatalysis performances are observed with the porous Fe₄ N and Co₃ N nanocubes with highly unsaturated nitrogen coordination for selective nitrate reduction to ammonia and nitrobenzene amination to aminobenzene, while the structural coherence of these porous nitride single crystals delivers excellent durability.