Probing the Catalytic Center of TiO _2/SO _4 ~(2-) Solid Superacid Catalyst by X-Band In Situ High-Temperature EPR Spectroscopy

摘要

Paramagnetic centers of the solid superacid catalyst in the sulfated TiO _2 are prone to the electron paramagnetic resonance (EPR) spectroscopy. The induction of the catalytic-active sites in TiO _2 powder presubmerged in H _2SO _4 solution as a function of the calcinated temperature of 293-873 K is investigated by X-band in situ continuous-wave EPR measurements. Sulfated-acid sites composed of the Ti ~(3+) ion are formed upon calcination. The overall experimental results show that the population of these sites goes uphill with the elevating temperature, reaches a maximum at ~623 K and decreases afterward to close zero. During the process, the decomposition of the TiO _2/SO _4 ~(2-) leads to the formation of Ti ~(3+) species and then to the increasing EPR signal amplitude, and the consecutive decomposition of the sulfur at higher temperature (<650 K) to the diminishing signal. The X-ray diffraction indicates that the introduction of SO _4 ~(2-) stabilizes the geometric structure in the anatase phase.