Papers' summary

Modulating the Active Sites of Oxygen-Deficient TiO2 by Copper Loading for Enhanced Electrocatalytic Nitrogen Reduction to Ammonia

Wahyu Prasetyo Utomo, Hao Wu*, Yun Hau Ng*

Ammonia (NH3) is one of the most common chemicals, playing an essential role in the agriculture, plastic, textile, and pharmaceutical industries. Currently, NH3 is also being considered a hydrogen carrier due to its high energy density. Nonetheless, NH3 production still relies on the Haber-Bosch process, which requires high temperature (300-500 °C) and pressure (200-350 atm) and is also responsible for ~1.4% of global CO2 emissions. Therefore, the development of a more energy-saving and environmentally friendly method for NH3 synthesis is critical.

Electrocatalytic nitrogen reduction reaction (NRR) provides a sustainable route for NH3 synthesis. However, the high dissociation energy of N2 triple bond, the low solubility of nitrogen (N2) in water, and the poor ability of the catalysts in adsorbing and activating N2, are the major bottlenecks of this process. Modification of the catalyst surface to increase N2 adsorption and activation is, therefore, crucial. Titanium dioxide (TiO2) is a highly adaptable semiconductor material because of its long-term thermodynamic stability and non-toxicity. Recent studies have demonstrated that TiO2 with oxygen vacancies (OVs) showed promising activities for NRR. In addition to OVs, co-catalyst deposition was also reported as a promising strategy to enhance NRR activity.

Inspired by the recent progress in catalysts for NRR, Wahyu Prasetyo Utomo, supervised by Prof. Yun Hau NG and Dr. Hao WU of City University of Hong Kong, assembled the Cu nanoparticles with oxygen-deficient TiO2 (OV-TiO2), expecting the electronic interaction between Cu nanoparticles and OVs. The 2.0%Cu/OV-TiO2 catalyst delivers a prominent NH3 yield rate of 13.6 µg mgcat-1 h-1  and FE of 5.9% at -0.5 V vs. reversible hydrogen electrode (RHE) in 0.5 M Na2SO4, which are 12.3-fold and 6.5-fold higher than the pristine TiO2. Additionally, the highest FE of 17.9% for the 2.0%Cu/OV-TiO2 is achieved at -0.4 V vs. RHE. The enhanced performance is ascribed to the higher electrochemically active surface area (ECSA), promoted electron transfer, and increased electron density. Correlating with the electronic interaction between Cu nanoparticles and OV-TiO2 substrate as shown by XPS analysis, the authors concluded that the promoted electrochemical properties are originated from the strong metal-support interaction (SMSI) between Cu nanoparticles and oxygen-deficient TiO2. The SMSI effect results in electron-deficient Cu with lopsided local charge distribution, which polarizes the adsorbed N2 molecules for better activation. Comparing the NRR performance of pristine TiO2, 2.0%Cu/OV-TiO2, and 2.0%Cu/TiO2, authors also evidenced that both OVs and Cu nanoparticles also serve as the active sites for N2 adsorption and activation.