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 2 SO 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.
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