Scientists overcome NH3 synthesis shortcomings

NH₃ is not only the key chemical raw material for the industry but also a carbon-free fuel and mobile carrier of renewable energy in the future. So far, industrial NH₃ synthesis is still dominated by the traditional Haber-Bosch reaction, which requires high temperatures of 300–500 ºC and pressures of 200–300 atm.

To overcome these shortcomings, Nanba et al. have designed the NO-CO-H₂O reaction system. In this reaction, NO was used as the raw material and reduced to NH₃ by H₂O and toxic gases CO. The reaction equation is as follows: NO + 2.5CO + 1.5H₂O → NH₃ + 2.5CO₂ (ΔH_298.15K= -414.86 kJ·mol^-1)

Recently, their research found that the NO-CO-H₂O reaction can be approximately decomposed into a series reaction of WGSR of CO-H₂O and reduction of NO-H₂. The results were published in the Chinese Journal of Catalysis.

When the incident photon frequency of incident light matches the vibration frequency of noble metal nanoparticles, the nanoparticles have strong absorption of photon energy and local surface plasmon resonance (LSPR) occurs. The metal with the LSPR effect can excite high-energy hot electrons and holes, which helps to activate the reactants, thus reducing the reaction energy and increasing the reaction rate. As a rare non-noble metal with an LSPR effect, Cu has been widely used in CO hydrogenation reactions.

Cu/CeO₂ was prepared by a simple wet impregnation method and the reactivity of NO reduction to NH₃ by CO in a photothermal synergistic system was studied. As expected, high activity was obtained over Cu/CeO₂ under visible light irradiation. The LSPR effect of Cu nanoparticles can increase the NH₃ yield under mild conditions.

Recently, a research team led by Prof. Wenxin Dai from Fuzhou University, China, reported a photothermal catalytic system comprising Cu/CeO₂ that was applied to the reaction between NO, CO and H₂O for the production of NH₃ under visible-light irradiation. High NO conversion (94.4%) and NH₃ selectivity (66.5%) were achieved over Cu/CeO₂ in the presence of H₂O at 210°C. Visible light further improved the conversion of NO (97.7%) and selectivity for NH₃ (69.1%).

The quasi-situ EPR and in-situ DRIFTS results indicated that CO initially reacts with H₂O to form an HCO₃^* intermediate, which then decomposes into CO₂ and activated H^*. Finally, NO reacts with activated H^* to produce NH₃. The localized surface plasmon resonance effect of Cu nanoparticles induced by visible light promotes the decomposition of HCO₃^* to CO₂ and H^*, while regenerating oxygen vacancies (OVs, H₂O activation sites) at the CeO₂ sites, resulting in enhanced NH₃ production.