Al‐Substituted Co₃O₄ Cocatalyst with Improved Durability for Tandem‐Type Base‐Free CO₂ Hydrogenation to Formic Acid in Water

The efficient hydrogenation of carbon dioxide (CO 2 ) into formic acid (FA; HCOOH) is challenging, but is required to ensure efficient CO 2 utilization and the realization of FA/CO 2 ‐mediated hydrogen energy cycle. Although catalysts that function under basic conditions have been successfully developed, materials capable of operating without additives in pure water are not readily available. The authors previously reported a heterogeneous tandem system for this reaction in which Co 3 O 4 acts as a CO 2 hydration cocatalyst to produce HCO 3 − in conjunction with PdAg/TiO 2 , which serves to promote the hydrogenation of HCO 3 − to afford FA. The present work used Al‐substituted Co 3 O 4 as an alternative cocatalyst, with the aim of improving catalyst durability during CO 2 hydrogenation to produce FA. A volcano‐type activity trend was observed as the extent of Al substitution was varied, with CoCo 8/5 Al 2/5 O 4 giving the maximum turnover number. This composite catalyst system was shown to maintain a high level of activity during reuse. Characterizations also suggested that Al‐substituted Co 3 O 4 is more resistant to reduction than pristine Co 3 O 4 . Density functional theory calculations provided further evidence that the substitution of Al at octahedral Co sites induces the localization of electrons on neighbouring O atoms, which strengthens Co─O bonds and inhibits reduction of the catalyst.