Heterogeneous Tandem Catalysis Strategy for Additive-Free\nCO<sub>2</sub> Hydrogenation into Formic Acid in Water: Crystal Plane\nEffect of Co<sub>3</sub>O<sub>4</sub> Cocatalyst

The\ntransformation of carbon dioxide (CO<sub>2</sub>) into formic\nacid (FA; HCOOH) in an aqueous phase is a promising method of realizing\nan environmentally friendly FA/CO<sub>2</sub>-mediated chemical hydrogen\nstorage/supply cycle. Despite progress in the design of catalysts\nthat operate under basic conditions, the development of efficient\ncatalysts that operate under additive-free conditions lags behind\nowing to the difficulty in activating CO<sub>2</sub> and the low solubility\nof CO<sub>2</sub> in pure water. In the present study, we present\na heterogeneous tandem catalysis strategy in which Co<sub>3</sub>O<sub>4</sub> is used as a CO<sub>2</sub> hydration cocatalyst to produce\na HCO<sub>3</sub><sup>–</sup> intermediate, in combination\nwith our previously reported PdAg/TiO<sub>2</sub> as a catalyst for\nthe hydrogenation of HCO<sub>3</sub><sup>–</sup> to afford\nFA. The turnover number based on Pd improved by a factor of more than\n8 in the presence of the Co<sub>3</sub>O<sub>4</sub> cocatalyst with\na cubic particle morphology enclosed by (100) facets. A series of\nmorphology-controlled Co<sub>3</sub>O<sub>4</sub> cocatalysts was\ninvestigated to elucidate the effect of the exposed crystal facets\n(i.e., (100), (111), or (112)) on their physicochemical properties\nand catalytic activity in FA synthesis. A systematic comparison based\non experimental and density functional theory calculations demonstrated\nthat the substantial enhancement effect of the Co<sub>3</sub>O<sub>4</sub> cubes is attributable to the in situ generation of the largest\namount of surface Co–OH groups with strong basicity originating\nfrom the exposed (100) facets. In addition, the present tandem catalytic\nsystem displayed high recyclability without exhibiting a structural\nchange or a significant loss of activity. These findings will allow\nthe rational design of an environmentally benign catalytic system\nfor the hydrogenation of CO<sub>2</sub> to FA.