The development of an efficient heterogeneous catalyst for storing H 2 into CO 2 and releasing it from the produced formic acid, when needed, is a crucial target for overcoming some intrinsic criticalities of green hydrogen exploitation, such as high flammability, low density, and handling. Herein, we report an efficient heterogeneous catalyst for both reactions prepared by immobilizing a molecular iridium organometallic catalyst onto a high-surface mesoporous silica, through a sol–gel methodology. The presence of tailored single-metal catalytic sites, derived by a suitable choice of ligands with desired steric and electronic characteristics, in combination with optimized support features, makes the immobilized catalyst highly active. Furthermore, the information derived from multinuclear DNP-enhanced NMR spectroscopy, elemental analysis, and Ir L 3 -edge XAS indicates the formation of cationic iridium sites. It is quite remarkable to note that the immobilized catalyst shows essentially the same catalytic activity as its molecular analogue in the hydrogenation of CO 2 . In the reverse reaction of HCOOH dehydrogenation, it is approximately twice less active but has no induction period.

A stringent comparison between two pairs of molecular/immobilized water oxidation catalysts ([Cp*Ir(Me‐pica)Cl], 1, versus 1_SiO2, Me‐pica=κ2‐N‐methyl‐picolinamide; [Cp*Ir(pysa)NO3], 2, versus 2_SiO2, pysa=κ2‐pyridine‐2‐sulfonamide]) reveals distinctive catalytic trends. While the molecular compound 1 exhibits a substantial higher activity than the analogous immobilized system 1_SiO2, under all the experimental conditions explored, the contrary is found with 2 that is far less active than its immobilized counterpart 2_SiO2. This is explained by the unique tendency of 2 to form dimeric complexes [Cp*Ir‐(κ2‐μ2‐Hpysa)(κ2‐μ2‐pysa)IrCp*], 2 a, in phosphate buffered solution at pH 7, and [Cp*Ir‐(κ2‐μ2‐Hpysa)2IrCp*], 2 b, in water. 2 a and 2 b have been completely characterized in solution by multinuclear and multidimensional NMR spectroscopy. They have been also isolated as single crystals and their structure in solid state determined by X‐Ray diffractometry. 2 a and 2 b appear to be off‐cycle species, whose formation is detrimental for water oxidation activity, as indicated by the observation of a long induction period when 2 a is used as catalytic precursor. In addition, TOF versus ΔE (E−E0=−RT/nF ln([IO4−]/[IO3−]) trends for the first two runs do not overlap for catalyst 2 and TOFmax is remarkably higher in the second run upon the addition of fresh NaIO4. In the immobilized system 2_SiO2 the detrimental associative processes are likely inhibited leading to an activity higher than that of 2.