In situ Observation of Porosity Formation in Porous Single-crystalline TiO 2 Monolith for Enhanced and Stable Catalytic CO Oxidation.

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.