Enhanced Nitrogen Reduction to Ammonia by Surface- and Defect-Engineered Co-catalyst-Modified Perovskite Catalysts under Ambient Conditions and Their Charge Carrier Dynamics

Abstract

An electrocatalytic nitrogen reduction reaction is considered a potential approach for green ammonia productiona zero-carbon fertilizer, fuel, and energy storage for renewable energy. To harness the synergistic properties of perovskites, the inherent dipole moment due to their non-centrosymmetric structure (that facilitates better charge separation), oxygen vacancies, and the presence of Ni metal sites that permit activation and reduction of N2 efficiently, the NiTiO3-based nanoelectrocatalysts have been synthesized. Further, these catalysts have been modified with ultra-small metal nanocrystal co-catalysts to form heterointerfaces that not only aid to improve the charge separation but also activate N2 and lower overpotential requirements. The appearance of peaks corresponding to (012), (104), (110), (11–3), (024), (11–6), (018), (027), and (300) confirms the formation of rhombohedral NiTiO3. The shift in the XRD peak corresponding to the (104) plane to a smaller 2θ value and peak shifting and widening of Raman spectra imply the lattice distortion that signifies the formation of Pd–NiTiO3 and Pt–NiTiO3 heterojunction electrocatalysts with the loadings of 0.4 and 0.3 wt % of Pd and Pt, respectively, as confirmed by ICP-OES analysis. The detailed XPS analysis reveals the presence of Pd (0), Pd (II), and Pt (0), Pt (II) in respective electrocatalysts. The appearance of XPS peaks at 528.7 and 531.1 eV suggests the presence of oxidative oxygen species (O2–/O–) and the presence of oxygen defects due to oxygen vacancy. The detailed nitrogen reduction (NRR) investigation exhibits a 5-fold enhancement in ammonia yield rate (∼14.28 μg h–1 mg–1 at −0.003 V vs RHE), a faradic efficiency of 27% (at 0.097 V vs RHE) for Pd–NiTiO3 electrocatalysts than that for bare NiTiO3 (3.08 μg h–1 mg–1), and 9-folds higher than that of the activity shown by the commercial TiO2 (P25) (1.52 μg h–1mg–1). The formation of ammonia was further confirmed by using isotopic nitrogen as the feeding gas. Furthermore, the highest NRR is observed at lower cathodic potential (−0.003 V vs RHE) in the case of the Pd–NiTiO3 electrocatalyst than that of the Pt–NiTiO3 electrocatalyst (−0.203 V vs RHE), implying significantly reduced overpotential requirement. Such enhanced NRR activity with lower overpotential requirement in the case of the Pd–NiTiO3 electrocatalyst is due to efficient charge separation as shown by the semicircle Nyquist plot, decreased photoluminescence emission intensity, shorter average lifetime (∼29 ns) of excitons, appropriate band bending, and improved activation of N2 by the oxygen vacancies and heterointerface formed between Pd nanocrystals and NiTiO3. Furthermore, no change is observed in the current density, after stabilization in the initial few seconds, even up to 2 h, which signifies that these electrocatalysts are stable. The structural and morphological integrity of the optimized catalyst remained even after the nitrogen reduction reactions, as revealed by no significant change observed in FESEM, elemental mapping, and EDS ana...

Mechanistic Insight into the Defect-Engineered White Light Emission from the Single-Phase Orthovanadate Phosphor Synthesized by a Facile Rapid Microwave-Assisted Synthesis

“…The high-resolution O 1s spectrum exhibits four peaks at 528.7, 530.5, 532.4, and 534.4 eV (Figure c,f). The peak at 528.7 eV represents the oxygen surrounding vanadium in VO 4 3– clusters, whereas the peaks at 530.5 and 532.4 eV correspond to the oxygen vacancies, chemisorbed oxygen, and hydroxyl groups on the surface, respectively, which are evident from the FTIR spectra and are due to the water oxygen. ,, In BOV-5, the concentration of oxygen vacancies is less than that of BOV-60, which is confirmed by the less-intense oxygen vacancy peak compared to that observed for BOV-60. High-resolution Ba 3d spectra (Figure a,d) exhibit two peaks that denote Ba 3d 3/2 and Ba 3d 5/2 .…”

Fe-Single-Atom Catalysts on Nitrogen-Doped Carbon Nanosheets for Electrochemical Conversion of Nitrogen to Ammonia

“…Such superior diffusion characteristics are necessary to enhance the catalytic performance of an electrocatalyst, especially in complex reactions such as the electrochemical NRR. Therefore, our findings provide valuable insights for the design and development of highly efficient electrocatalysts for NRR and related applications , …”

“…Such superior diffusion characteristics are necessary to enhance the catalytic performance of an electrocatalyst, especially in complex reactions such as the electrochemical NRR. Therefore, our findings provide valuable insights for the design and development of highly efficient electrocatalysts for NRR and related applications 50,51 The stability of the catalyst is a crucial feature that determines its potential for large-scale industrial use. Therefore, the stability of the catalyst was tested over 15 h at a potential of −0.1 V vs RHE, Figure S13.…”

Efficient Photocatalytic Reduction of Hexavalent Chromium by BiVO4-Decorated MXene Photocatalysts and Their Charge Carrier Dynamics

“…Nonetheless, MXene (Ti 3 C 2 T x ) efficiently separates e – /h + , thereby restricting charge recombination. On the other hand, BiVO 4 coupled with MXene (Ti 3 C 2 T x ) acts synergistically, owing to which there is a strong interfacial electronic coupling that aids in faster charge transfer. − The superoxide (O 2 •– ) and ( • OH) hydroxyl radical species are produced by the holes and electrons, respectively. • OH is mainly generated by the oxidation of h + to water, whereas O 2 •– is produced by the reduction of O 2 by electrons.…”