No abstract

Nitric acid is widely applied in agriculture and industry. The present manufacturing process via a combination of the Haber–Bosch process and the Ostwald oxidation process is accompanied by massive energy consumption and greenhouse gas emissions. The direct electrocatalytic nitrogen oxidation reaction (NOR) to nitric acid is a promising alternative, especially when it is driven by renewable energy sources. The standardization of performance evaluation is the prerequisite for the design and synthesis of efficient electrocatalysts for NOR. In this context, this Minireview first discusses the history of the development of HNO3 manufacturing and the possible reaction mechanisms for electrocatalytic NOR. Then, a strict protocol for electrochemical NOR experiments is recommended. Finally, general research targets associated with techno‐economic analysis, challenges, and prospects for NOR are summarized for future studies.

Formic acid (HCOOH) can be exclusively prepared through CO2 electroreduction at an industrial current density (0.5 A cm–2). However, the global annual demand for formic acid is only ∼1 million tons, far less than the current CO2 emission scale. The exploration of an economical and green approach to upgrading CO2-derived formic acid is significant. Here, we report an electrochemical process to convert formic acid and nitrite into high-valued formamide over a copper catalyst under ambient conditions, which offers the selectivity from formic acid to formamide up to 90.0%. Isotope-labeled in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy and quasi in situ electron paramagnetic resonance results reveal the key C–N bond formation through coupling *CHO and *NH2 intermediates. This work offers an electrochemical strategy to upgrade CO2-derived formic acid into high-value formamide.

The carbon–carbon (C–C) bond formation is essential for the electroconversion of CO2 into high-energy-density C2+ products, and the precise coupling pathways remain controversial. Although recent computational investigations have proposed that the OC–COH coupling pathway is more favorable in specific reaction conditions than the well-known CO dimerization pathway, the experimental evidence is still lacking, partly due to the separated catalyst design and mechanistic/spectroscopic exploration. Here, we employ density functional theory calculations to show that on low-coordinated copper sites, the *CO bindings are strengthened, and the adsorbed *CO coupling with their hydrogenation species, *COH, receives precedence over CO dimerization. Experimentally, we construct a fragmented Cu catalyst with abundant low-coordinated sites, exhibiting a 77.8% Faradaic efficiency for C2+ products at 300 mA cm–2. With a suite of in situ spectroscopic studies, we capture an *OCCOH intermediate on the fragmented Cu surfaces, providing direct evidence to support the OC–COH coupling pathway. The mechanistic insights of this research elucidate how to design materials in favor of OC–COH coupling toward efficient C2+ production from CO2 reduction.

Electrocatalytic nitrogen oxidation to nitrate is a promising alternative to the conventional nitrate synthesis industry, which is accompanied by huge energy consumption and greenhouse gas emission. However, breaking the NN triple bond (941 kJ·mol–1) in nitrogen is still challenging, and thus, the development of efficient electrocatalysts with established reaction pathways is highly required. Herein, a series of Ru-doped Pd materials are prepared, and the optimized Pd0.9Ru0.1 sample exhibits superior performance for electrocatalytic nitrogen oxidation into nitrate, greatly outperforming pure Pd and Ru samples. The 15N isotope-labeling studies and other characterizations results indicate that the produced nitrate originates from N2 electrooxidation. Electrochemical in situ Raman spectra reveal the formed Pd0.9Ru0.1O2 on the surface serves as the active species. Electrochemical in situ Fourier transform infrared spectroscopy and online differential electrochemical mass spectrometry experimentally unveil the reaction pathway of nitrogen electrooxidation on Pd0.9Ru0.1O2. The combined results of experiments and theoretical simulations reveal Ru doping not only promotes the formation of more active species but also changes the potential-limiting step with a lower energy barrier.