In situ Observation of Porosity Formation in Porous Single‐crystalline TiO2 Monolith for Enhanced and Stable Catalytic CO Oxidation

Abstract Introducing pores in single crystals creates a new type of porous materials that incorporate porosity and structural coherence. Herein, we use in situ transmission electron microscopy to disclose the porosity formation by converting KTiOPO 4 (KTP) single crystals into porous single‐crystalline (PSC) TiO 2 monoliths in a solid‐solid transformation. The isolated crystalline nuclei of TiO 2 clusters with identical lattice orientation on KTP surface moves TiO 2 /KTP interface toward mother phase for growing PSC TiO 2 monoliths. The relative density in PSC TiO 2 monoliths dominates porosity while the macroscopic dimensions remain unchanged in the transformation. The single‐crystalline nature of porous architecture stabilizes oxygen vacancy to activate lattice oxygen while the three‐dimensional percolation enhances species diffusion. PSC TiO 2 monoliths with deposited Pt clusters show enhanced and stable catalytic CO oxidation in air at ∼75 °C for 200 hours of operation.