Macroscopic Hexagonal CoO Tubes Derived from Controllable Two-Dimensional Metal-Organic Layer Single Crystals: Formation Mechanism and Catalytic Activity.

Macroscopic CoO hexagonal tubes were successfully synthesized using hollow two-dimensional (2D) MOL (metal-organic layer) single crystals as sacrificial templates. The hollow 2D MOL single crystals were prepared under hydrothermal conditions with acetonitrile (MeCN) as an interference agent. The formation of hollow-structured 2D MOL single crystals was tracked by time-dependent experiments, and two simultaneous paths-namely, the crystal-to-crystal transformation in solution and the dissolution + migration (toward the external surface) of inner crystallites-were identified as playing a key role in the formation of the unique hollow structure. The calculated change in Gibbs free energy (Δ = -1.18 eV) indicated that the crystal-to-crystal transformation was spontaneous at 393 K. Further addition of MeCN as an interference agent eventually leads to the formation of macroscopic hexagonal tubes. Among all of the as-synthesized CoO, Co-MeCN-O with a hexagonal tube morphology exhibited the best catalytic performance in toluene oxidation, it achieved a toluene conversion of 90% () at ∼227 °C (a space velocity of 60 000 mL g h) and the activity energy () is 69.5 kJ mol. A series of characterizations were performed to investigate the structure-activity correlation. It was found that there are more structure defects, more adsorbed surface oxygen species, more surface Co species, and higher reducibility at low temperatures on the Co-MeCN-O than on other CoO samples; these factors are responsible for its excellent catalytic performance. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) characterization showed that, when there is no oxygen in the atmosphere, the lattice oxygen may be involved in the activation of toluene, and the gas-phase oxygen replenished by the oxygen vacancies was essential for the total oxidation of toluene on the surface of the Co-MeCN-O catalysts, it also proves the importance of oxygen vacancies. Moreover, for the Co-MeCN-O catalysts, no obvious decrease in catalytic performance was observed after 120 h at 220 °C and it is still stable after cycling tests, which indicates that it exhibits excellent stability for toluene oxidation. This study sheds lights on the controllable synthesis of macroporous-microporous materials in single-crystalline form without an external template, and, thus, it may serve as a reference for future design and synthesis of hollow porous materials with outstanding catalytic performance.