Photocatalytic degradation of methylene blue with spent FCC catalyst loaded with ferric oxide and titanium dioxide

The spent fluid catalytic cracking (FCC) catalyst has been loaded with ferric oxide (Fe2O3) and titanium dioxide (TiO2). Fe-Ti/SF composite (loaded with 5 wt% TiO2 and 5 wt% Fe2O3), Fe/SF composite (loaded with10 wt% Fe2O3) and Ti/SF composite (loaded with 10 wt% TiO2) have been fabricated via a modified-impregnation method. The band gaps of the Fe-Ti/SF, Fe/SF and Ti/SF composites (evaluated by the energy versus [F(R∞)hv]n) are 2.23, 1.98 and 3.0 eV, respectively. Electrochemical impedance spectroscopy shows that the Fe-Ti/SF has lower electron transfer resistance, it has the small charge transfer resistance and fast charge transfer rate. The interparticle electrons transfer between the Fe2O3 and TiO2, which can improve the separation of the photo-electrons and holes. The holes transfer from valence band of TiO2 to the valence band of Fe2O3, which can provide more active sites around the adsorbed molecules. The methylene blue degradation efficiencies (with the Fe-Ti/SF, Fe/SF and Ti/SF composites) are ~ 94.2%, ~ 22.3% and ~ 54.0% in 120 min, respectively. This work reveals that the spent FCC catalyst as supporter can be loaded with Fe2O3 and TiO2. This composite is highly suitable for degradation of methylene blue, which can provide a potential method to dispose the spent FCC catalyst in industry.

In the oil refinery, the fluid catalytic cracking (FCC) is an important secondary conversion procrss [1][2][3][4] . The crude oil can be converted into the valuable small molecules products, which is an essential process for gasoline production 5 . In the FCC process, the catalytic activity of FCC catalyst decreases after several cycles. Metals (V, Ni, and Fe) accumulation occurs via deposition and incorporation into the FCC catalyst body 6,7 . There are about 840,000 t spent FCC catalyst consumed in the world every year and it is anticipated annual increase of 5% 8,9 . The spent FCC catalyst are mainly treated via landfill 10 . Some researcher explore the coating of spent FCC catalyst as anticorrosive and antimicrobial material, other researcher uses the spent FCC catalyst to recover the precious metal and rare earth, and use spent FCC catalyst as admixtures in mortar and concrete production [11][12][13] . Zeolite Y is the main components of the FCC catalyst 14 . When the FCC catalyst is deactivated, the pore volume of catalyst is almost intact, which still can be used as a carrier 15 .
Titanium dioxide (TiO 2 ) has been extensively investigated for photocatalytic reaction due to its chemical stability and lack of toxicity [16][17][18][19] . TiO 2 is a wide band gap semiconductor material (3.2 eV) [20][21][22][23] . In photocatalytic reaction, the charge carrier of the TiO 2 is fast recombinated, which can be prevented effectively by the heterogeneous structures (with the narrow band gap semiconductor materials) 24 www.nature.com/scientificreports/ this work is that it provides a new way to treat spent FCC catalyst. At present, the spent FCC catalyst are mainly treated by landfill. This method caused severe land pollution and polluted groundwater. The novelty of this work is the introduction spent FCC catalyst as supporter. These composites fabricated via this modified-impregnation method can provide an effective route to dispose the spent FCC catalyst in industry.

Materials and methods
Fabrication of the composites. In a typical experimental procedure, the spent FCC catalyst was loaded with titanium dioxide (5 wt% TiO 2 ) and ferric oxide ( Characterizations. The structures of the fabricated composites were tested by power XRD (Rigaku RAD-3C, Cu Kα radiation, 10° min −1 , 2-Theta range 5°-80°). These composites morphology was analyzed by a Scanning Electron Microscope (SEM JEOL S-4800). Fourier transformation infrared (FT-IR) was tested using a spectrometer at a wavenumber covering the range of 400-4,000 cm −1 . To measure band gap of these composites, UV-Vis light absorption was recorded on the Diffuse Reflectance spectrophotometer UV-vis. Nitrogen adsorption desorption tests were executed on a Micrometrics (Tristar 3000) instrument. Transmission electron microscope (TEM, JEOL LED JSM-6700F microscope, Japan) was used to investigate microstructure. Electrochemical impedance spectra (EIS) were tested via using a CHI660D workstation at room temperature. The electrochemical performances of the Fe-Ti/SF, Fe/SF and Ti/SF were analyzed. The working electrode were Fe-Ti/SF, Fe/SF and Ti/SF, respectively. The counter electrode was the Pt plate electrode and reference electrode were the calomel electrode. The electrolyte of the EIS measurements was the Na 2 SO 4 aqueous solution (0.2 mol L −1 ).

Photocatalytic tests.
The adsorption and photocatalytic performance of the Fe-Ti/SF, Fe/SF and Ti/SF were tested by photocatalysis degradation of MB. In the experiment, 0.2 g of fabricated composite were added into 200 mL of methylene blue (10 ppm). The photocatalytic tests were driven by irradiation with the 300 W Xenon lamp for 120 min. Before the light irradiation, the reaction system was stirred for 40 min in darkness to ensure the adsorption-desorption equilibrium. The methylene blue was taken at given time interval (20 min). The solutions were centrifuged under 4,000 rpm before analysis.  In Fig. 5, the macropores properties of the spent FCC catalyst, Fe/SF, Fe-Ti/SF and Ti/SF are analyzed. These insets show the pore diameter of these samples. All the samples show the type-II isotherms, which are corresponded of macropores materials (IUPAC classification). The isotherms of these samples exhibit H3 hysteresis loops associated with the presence of macropores. The pore diameter of the spent FCC catalyst, Fe/SF, Fe-Ti/ SF and Ti/SF are 5.6, 5.8, 5.9 and 5.8 nm, respectively. It shows that these samples are macroporous. The surface   The electron transfer efficiency can be measured by the electrochemical impedance spectroscopy (EIS). As shown in Fig. 7, the radius (Nyquist plot) of the Fe-Ti/SF is much smaller than those of Fe/SF and Ti/SF, which shows that Fe-Ti/SF has lower electron transfer resistance. The heterostructure of the Fe 2 O 3 and TiO 2 enhances the separation of the electrons and holes.

Results and discussion
In Fig. 8, the Fourier transform infrared spectroscopy (FT-IR) spectra of the spent FCC catalyst, Fe/SF, Fe-Ti/ SF and Ti/SF are measured. The peak at 3,425 cm −1 corresponds to the absorption the stretching vibration of O-H group stretching. The peak at 1645 cm −1 corresponds to the bending vibration absorption of the O-H 32 . In Fig. 8a-d,    In Fig. 9, the photocatalytic performance of the Fe-Ti/SF is evaluated by the methylene blue degradation experiment. With increase of the reaction time, the methylene blue characteristic peak is gradually declined, which indicates concentration of methylene blue is gradually declined. The photocatalytic degradation efficiency of the methylene blue is ~ 94.2%. The inset of Fig. 9 shows the color change of the methylene blue. Figure. S5 shows the degradation of methylene blue with the Fe-Ti/SF, Fe/SF and Ti/SF composites. The methylene blue degradation efficiencies (with Fe-Ti/SF, Fe/SF and Ti/SF composites) are ~ 94.2%, ~ 22.3% and ~ 54.0% in 120 min, respectively. The results show that the Fe-Ti/SF composite has the highest photocatalytic activity. Supplementary Figure S6 shows the photocatalytic degradation efficiency of methylene blue with Fe-Ti/SF, Fe 2 O 3 -TiO 2 , Fe 2 O 3 and TiO 2 .
As shown in Fig. 10, the recycling experiments of the Fe-Ti/SF composite are implemented by the methylene blue degradation, which evaluates the stability of this Fe-Ti/SF composite. After the fourth cycle reaction, the photocatalytic activity of the Fe-Ti/SF composite is not significant loss. The degradation efficiency of the methylene blue with Fe-Ti/SF composite decreases to 84.0% from the pristine degradation efficiency (94.2%). These results indicate that the Fe-Ti/SF exhibit a relatively stable photocatalytic performance.  This modified-impregnation method, the spent FCC catalyst is used as the photocatalyst supports, which provides a potential method to dispose the spent FCC catalyst in the areas of environment protection.