Light-Intensity-Responsive Changes of Products in Photocatalytic Reduction of Nitrous Acid on a Cu-Doped Covalent Triazine Framework-TiO2 Hybrid.

In the design of solar-energy conversion electrochemical systems, it is important to consider that natural sunlight fluctuates. By taking nitrous acid photoreduction as an example, this study has shown that the reaction pathway, and hence the reaction products, dynamically respond to variations in light intensity. Under irradiation, the photooxidation of methanol (as sacrificial agent) on TiO2 and the reduction of HNO2 on a Cu-modified covalent triazine framework (Cu-CTF) are electrically coupled, which leads to the photoreduction of HNO2 without an external bias. The major product of the reaction changes from N2 O to NH4+ with an increase in the light intensity. The operating potential also shifts negatively (or positively) when the light intensity is increased (or decreased). These results indicate that a change in the reaction pathway is triggered by a change in the operating potential of the Cu-CTF catalyst under varying light intensity. Such a light-intensity-dependent change in the reaction pathway is particularly important in systems that use photoresponsive electrodes and where multiple products can be obtained, such as the solar-driven reduction of carbon dioxide and nitrogen oxides.