Light-assistance in nitrogen fixation to ammonia by highly dispersed Cs-promoted Ru clusters supported on ZrO2

Light-assisted N2 fixation to NH3 under mild conditions is attracting massive attention to circumvent energy crisis and global warming. Herein, highly dispersed Cs-decorated Ru sub-nanometric clusters supported on ZrO2 NPs have been prepared by means of MOF-templated transformation method. The obtained catalyst exhibited N2 hydrogenation activity of 1.6 mmolNH3 gcat−1 h−1 in the dark (350 °C, 0.1 MPa). Remarkably this value increased by over 300 %, reaching 5.1 mmolNH3 gcat−1 h−1 (204 mmolNH3 gRu−1 h−1) under 1 Sun power illumination (1080 W/m2) The influence of the incident light wavelength has been evaluated, revealing 81 % and 213 % enhancement in the NIR and visible region, respectively. DFT calculations were carried out to understand the static adsorption states of Ru sub-nanoclusters. Mechanistic studies have confirmed the co-existence of a photothermal and a nonthermal-hot electron mechanism. XPS, PXRD and FTIR analysis have determined that the Cs species block surface Zr4+ acidity, increasing the basicity of the ZrO2 support. Moreover, partially reduced Csδ+ (0 <δ < 1) species surrounding the Ru active sites are donating electron density to the adjacent Ru sites, favoring N2 adsorption and subsequent activation. Finally, this photocatalyst has shown an extended stability for 100 h irradiation under continuous flow.