Light-Assistance in Nitrogen Fixation to Ammonia by Highly Dispersed Cs-Promoted Ru Clusters Supported on Zro2

Abstract

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 oC, 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, with an estimated quantum efficiency of 39%. 81% and 213% enhancement in the light-assisted process derives from the NIR and visible region, respectively. 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+ species donate electron density to the adjacent Ru sites, favoring N2 adsorption. This photocatalyst has shown an extended stability for 100 h irradiation under continuous flow.

Keywords: metal clusters, photocatalysis, N2 fixation, surface plasmonic resonance effects, hot electrons injection