Abstract
Formic acid is considered as one of the most promising liquid organic hydrogen carriers (LOHC). Its catalytic dehydrogenation processes generally suffer from low activity, low reaction selectivity, low stability of the catalysts and/or the use of noble-metal based catalysts. Herein, we report a highly selective, efficient, and noble metal-free photocatalyst for the dehydrogenation of formic acid. This catalyst is built from a post-metalation of a carboxylic-functionalized Zr-MOF by copper, namely UiO-66(COOH)2-Cu. The visible light driven photocatalytic dehydrogenation process through the release of hydrogen and carbon dioxide is monitored in real-time via operando FTIR and reveals almost 100% of selectivity with high stability (over 3 days) and conversion yield exceeding 60% (around 5 mmol.g-1 cat.h-1) at ambient conditions. These performance indicators make UiO-66(COOH)2-Cu among the top photocatalysts for formic acid dehydrogenation. Interestingly, the as-prepared UiO-66(COOH)2-Cu hetero-nanostructure was found moderately active under solar irradiation during an induction phase, whereupon it undergoes an in-situ restructuring process through intra-framework cross-linking with the formation of the anhydride analogue structure, UiO-66(COO)2-Cu, and nano-clustering of highly active and stable copper sites as evidenced by the operando studies coupled with steady state isotopic transient kinetic (SSITKA) experiment, TEM, XPS analysis and DFT calculations. Beyond revealing outstanding catalytic performance for UiO-66(COO)2-Cu, this work delivers an in-depth understanding of the photocatalytic reaction mechanism implying an evolutive behavior of the post-metalated copper as well as the MOF framework over the reaction. These key findings pave the ways towards the engineering of new simple and efficient catalysts for photocatalytic dehydrogenation of formic acid.