Abstract
NH3 synthesis under ambient conditions using the N source of N2 or NO3- and the proton source of H2O has become an attractive alternative to the conventional energy-intensive Haber-Bosch approach. Unfortunately, the slow kinetics of the multi-proton coupled electron transfer (PCET) in NH3 synthesis and the competing hydrogen evolution reaction (HER) severely restrain the activity and selectivity during NH3 synthesis. H2O molecules, as feedstock, play a crucial role in both competing HER and the hydrogenation process during NH3 synthesis. Therefore, regulating the behavior of H2O molecules is highly valuable and may remarkably contribute to the NH3 synthesis.Attributing to the "accept-donate" mechanism of electrons, highly exposed catalytic active sites, and unique quantum size effects, transition metal single-atom (TM SA) catalysts in nitrogen reduction reaction (NRR) exhibit significant potential. Nevertheless, the electrons in d-orbitals of TM SA are also conducive to forming strong metal-H bonds, which causes the competing HER to be more kinetically dominant. Therefore, we designed a novel SA molecular catalyst with the Ni-organic hydrogen bonding framework, which was further melted to prepare rGO supported Ni-N2O2 molecular catalyst (Ni-N2O2/rGO). The Ni-N2O2/rGO contains abundant SA Ni centers with high activity attributed to the distinctive electronic environment. More significantly, the O atoms near the Ni centers can bond with H2O molecules and weaken the activity of H2O molecules, thereby effectively regulating the migration of protons and suppressing the parasitic HER. Consequently, this molecular catalyst exhibits simultaneously facilitated activity and selectivity with a high NH3 yield rate of 209.7 μg h-1 mgcat.-1 and a Faradaic efficiency (FE) of 45.7% at -0.30 V vs. RHE. The design strategy of this catalyst over hydrogen bond-mediated proton transfer provides a promising approach for breaking the leverage relationship in the catalytic process.
The plasmonic metal nanoparticle is another potential novel photosensitizer in the nitrogen fixation process. The participation of high-energy hot electrons generated from the non-radiative decay of localized surface plasmons is an important mechanism for promoting catalytic processes. Additionally, we find another vital mechanism associated with the localized surface plasmon resonance (LSPR) effect significantly contributing to the NRR; that is, the LSPR-induced strong localized electric fields could weaken the intermolecular hydrogen bonds and regulate the arrangement of water molecules at the solid-liquid interface. The AuCu pentacle nanoparticles with excellent light absorption ability and the capability to generate strong localized electric fields are chosen to demonstrate this effect. Due to the promoted electron transfer at the interface by the well-ordered interfacial water, as well as the participation of high-energy hot electrons, the optimal catalyst exhibits excellent performance with an NH3 yield of 52.09 μg h-1 cm-2 and an FE of 45.82% at -0.20 V vs. RHE. The results are significant for understanding the LSPR effect in catalysis and provide a promising approach to couple light energy and electric energy to facilitate the reaction process.
Electrochemical nitrate reduction reaction (NO3RR) is also a promising alternative in NH3 generation. Nevertheless, it remains hindered by the competing HER. The mechanical energy and electric energy are coupled to promote NO3RR performance. The piezoelectric effect of electron-rich BaTiO3 with oxygen vacancies is introduced. Combining with metal particles (Ru, Pd, and Pt), the catalyst achieves a maximal FE of 95.3% and an NH3 yield rate of 6.87 mg h-1 mgcat.-1. Upon piezoelectricity, the interface between metal nanoparticles and BaTiO3 is effectively modulated from Schottky to ohmic contact, leading to unobstructed electron transfer. Abundant hydrogen radicals (·H) can then be produced from the collision between plentiful electrons and polar water molecules adsorbed on the polar surface. Such ·H can significantly facilitate the hydrogenation of reaction intermediates in NO3RR. Meanwhile, this process suppresses the Volmer-Heyrovsky step, inhibiting the HER within a wide range of external potential. This work suggests a new strategy for promoting the performance of multi-electron-involved catalytic reactions.
In summary, three strategies for regulating the behavior of interfacial water molecules in electrocatalytic NH3 synthesis were proposed, including hydrogen bonding regulation, LSPR effect, and piezoelectric regulation, which effectively inhibited the competing HER and improved the FE and yield of NH3. The strategies and results of this article can bring new inspiration to improve the efficiency of practical NH3 production under normal temperature and pressure and are expected to be applied to other catalytic systems involving multi-PCET to promote the selectivity and yield of the target product.
| Date of Award | 5 Aug 2024 |
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| Original language | English |
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| Supervisor | Chunyi ZHI (Supervisor) |