Fabrication of spent FCC catalyst composites by loaded V2O5 and TiO2 and their comparative photocatalytic activities

The spent fluid catalytic cracking catalyst (FCC) has been loaded with different content of V2O5 and TiO2 through a modified-impregnation method. X-ray Diffraction (XRD), ultraviolet-visible spectrophotometry (UV-Vis), Scanning Electron Microscope (SEM), and Fourier Transform Infrared spectroscopy (FT-IR) are used to characterize the structure and morphology of these samples. Their photocatalytic activity was evaluated by degradation of methylene blue (MB) solution under 300 W Xenon lamp irradiation. The interplanar spacing of the zeolite Y (111) plane is affected by the amount of the loaded V2O5 on spent FCC catalyst. The (111) plane of spent FCC catalyst loaded with V2O5 and TiO2 sample is 1.404 nm, which is higher than that of the zeolite Y (1.395 nm). The amount of adsorption of MB and the photocatalytic activity for the degradation increased with increasing the interplanar spacing of the (111) plane of sample. We fabricated of spent FCC catalyst composites by loaded V2O5 and TiO2, which effectively solved the spent FCC catalyst disposal problem. The efficiency of the developed sample provides a potentially economical way of degrading MB.

In the petroleum refining industry, the fluid catalytic cracking (FCC) process is one of the most important processes [1][2][3][4][5][6][7][8] . The FCC catalyst deactivates with time and when the activity of the catalyst declines below the unacceptable level, it is usually disposed as hazardous waste 9 . The Ni, V, Fe and coke from petroleum crude oil are deposited onto the surface of the FCC catalyst particle 10,11 , and these impurities are even embedded in the zeolite Y framework of the FCC catalyst. Every year, more than 160,000 tons of spent catalyst are generated in the petrochemical industry 12 . The spent FCC catalyst has been listed as HW50 type hazardous waste, which was included in the Chinese National Hazardous Waste List (2016 version) 13 . The treatment method of the spent FCC catalyst is primarily the landfill method 14 , which cause serious environmental pollution and land consumption. The spent FCC catalyst is used in cement production and as a cement additive. It makes cement more suitable for practical applications 15 . A few works use spent FCC catalyst to prepare the potential anti-corrosive and anti-biofouling materials, which indicated high corrosion inhibition efficiency 16 .
The zeolite Y framework, the pore structure and the surface area structure for the spent FCC catalyst are almost unchanged, which still has typical useful value used to support photocatalysis, although the spent FCC catalyst loses its catalytic activity 17 . The treatment and recycling of the spent FCC catalyst through other valuable processes is attractive from environmental and economic points of view. The zeolite Y framework provides high surface area and adsorbent capacity. In recent years, the spent FCC catalyst as supporter for TiO 2 is utilized for the photodegradation of dye solution 18 . TiO 2 is activated by UV irradiation due to its large band gap (3.2 eV) 19 . V 2 O 5 has band edges at E CB = 0.47 eV and E VB = 2.73 eV, which can match well with TiO 2 (E CB = −0.1 eV, E VB = 3.1 eV) to form a photocatalytic system 20,21 . These compounds display a high photocatalytic performance for degradation of the dye solution. The advantage of photocatalytic degradation of MB by use the spent FCC catalyst to supported TiO 2 and V 2 O 5 was the modulation of properties of the semiconductors by changing the valance and conduction bands in order to be activated under light irradiation 22 . It can induce a synergistic effect; they interact to generate a composite material with a favorably high surface area and high catalytic activity 23 .
In this work, the V/FCC, Ti/FCC, V-Ti/FCC and V-Ti-2/FCC samples are prepared by a modified-impregnation method. The samples are characterized by XRD, SEM, UV-Vis and FT-IR. The photocatalytic performance and adsorptive behavior of these samples are tested. The activity of the obtained V/FCC, Ti/FCC, V-Ti/FCC and V-Ti-2/ FCC samples are evaluated by studying the degradation of MB solution. The degradation of MB by use of V/FCC, Ti/FCC, V-Ti/FCC and V-Ti-2/FCC samples are ~26%, ~36%, ~75% and ~96%, respectively. These results indicated that V-Ti-2/FCC composites exhibited good photocatalytic activity.

Result and Discussions
Characterization of prepared photocatalysts. The V/FCC, Ti/FCC, V-Ti/FCC and V-Ti-2/FCC samples are rationally designed and fabricated by a modified-impregnation method, which can be directly employed as photocatalysts for MB solution degradation. Figure 1 shows the schematic illustration of the spent FCC catalyst, which are loaded by V 2 O 5 and TiO 2 . This schematic illustration can be helpful in understanding photocatalytic activities of these samples. Figure 1a shows the (111) plane of the spent FCC catalyst (zeolite Y). The interplanar spacing of the zeolite Y is 1.395 nm. Figure 1b shows the (111) plane of the Ti/FCC sample, with interplanar spacing at 1.399 nm. Figure 1c shows the (111) plane of the V-Ti-2/FCC sample, with interplanar spacing at 1.404 nm. It is clear to see that the interplanar spacing is increased, once the different content of V 2 O 5 and TiO 2 is loaded on spent FCC catalyst. As shown in the XRD patterns ( Fig. 2d-e), there are no reflections of the (111) planes of the V-Ti/FCC and V/FCC samples; the (111) plane of V-Ti/FCC and V/FCC sample have been collapsed. V 2 O 5 attacks the zeolite Y, which make the interplanar spacing change 24 . The interplanar spacing of the (111) plane of zeolite Y can be changed during this modified-impregnation process. The V-Ti-2/FCC sample has the highest interplanar spacing of the (111) plane, indication that the V-Ti-2/FCC sample can adsorb more methylene blue.
The XRD patterns (2θ range from 5° to 80°) of the spent FCC catalyst, Ti/FCC, V-Ti-2/FCC, V-Ti/FCC and V/FCC samples are shown in Fig. 2. The diffraction peaks located at 6.3°, 10.3°, 12.1° and 15.91° are the characteristic peaks of the spent FCC catalyst sample (Fig. 2a), which match well with the standard zeolite Y phase (JPDS card No. 75-1551). These diffraction peaks correspond to the reflections of the (111), (220), (311) and (331) planes of the zeolite Y arrays, respectively. In Fig. 2b, the four diffraction peaks located at 6.3°, 10.3°, 12.1° and 15.9° are the characteristic peaks of the FCC, which correspond to the (111), (220), (311) and (331) planes of the standard zeolite Y phase (JPDS card No. 75-1551), respectively. One diffraction peak located at 26.5°is the characteristic peak of the TiO 2 , which correspond to the (101) plane of the standard TiO 2 phase (JPDS card No. 73-1764). In Fig. 2c, four diffraction peaks located at 6.3°, 10. Specific surface areas generally influence the adsorption and catalytic performance of photocatalysts. Nitrogen adsorption-desorption isotherms and corresponding pore size distribution curves of spent FCC catalyst, V/FCC, Ti/FCC, V-Ti/FCC and V-Ti-2/FCC samples were shown in Fig. 3a-e, respectively. The isotherms of all samples were of type-IV with a H3 hysteresis loop in the relative pressure (P/P0) indicating the presence of a macroporous structure which were representative of mesoporous materials according to the IUPAC classification. The pore-size distribution of spent FCC catalyst, V/FCC, Ti/FCC, V-Ti/FCC and V-Ti-2/FCC samples was largest  Fig. 4a-c. The low-magnification SEM image shows the morphology of the spent FCC catalyst surface. The high-magnification SEM images of the spent FCC catalyst sample shows the surface, which appears to break and collapse 10 . The framework (large specific surface area and pore volume) of the zeolite Y is basically intact, which is favorable to support the nanostructure of TiO 2 and V 2 O 5 .
The Ti/FCC sample is obtained by loaded TiO 2 on the spent FCC catalyst. Figure 4d-f show the SEM images of the Ti/FCC sample. The low-magnification SEM image (Fig. 4d) of the Ti/FCC sample reveals that the smooth surface of the spent FCC catalyst is covered with TiO 2 particles. From the SEM images (Fig. 4e), it is evident that the surface of spent FCC sample is covered with TiO 2 particles. They are either deposited on the surface of the spent FCC sample. The high-magnification SEM image (Fig. 4f) of the Ti/FCC sample reveals that the TiO 2 tightly adhered to the spent FCC catalyst to form the heterojunction structure.
The different morphological V-Ti-2/FCC sample images are obtained by loaded TiO 2 and V 2 O 5 particles on the surface of the spent FCC catalyst.    Figure 7 shows the illustration of interparticle electron transfer behavior. The conduction band (CB) energy position of TiO 2 is −0.1 eV. The CB energy of V 2 O 5 is 0.47 eV. When the V 2 O 5 and TiO 2 form a heterostructure, the Fermi energy of the two materials has to be the same 32,35-37 . As a result, the CB and VB of V 2 O 5 move upward. In the photocatalysis reaction process, V 2 O 5 and TiO 2 form a heterostructure. The photo-generated electrons in CB of V 2 O 5 migrate to TiO 2 while the photo-generated holes transfer from the VB of TiO 2 to that of V 2 O 5 . The photo-induced electrons could rapidly transfer from the CB of the V 2 O 5 to that of the TiO 2 , which significantly promotoes the separation of photo-induced electrons and holes.
Evaluation of photocatalytic activity. The photocatalytic activity of the V-Ti-2/FCC sample is analyzed by photocatalytic degradation of the MB solution. From Fig. 8, the peak at ~664 nm is the absorption characteristic of the MB. With the photocatalytic time extending, the absorption peak intensity of the MB decreases rapidly. It is found that the concentration of MB rapidly decreases with the increase of irradiation time. The color of the MB solution almost disappears after 120 min, which suggests the complete destruction of the methylene blue conjugated structure. The inset in Fig. 7 shows the photographs of the MB solution: 0, 20, 40, 60, 80, 100 and 120 min, which corresponds with (a), (b), (c), (d), (e), (f) and (g), respectively. It can be seen that 96% of the MB solution has already been degraded by the V-Ti-2/FCC sample, after 120 min of irradiation.
The V-Ti-2/FCC sample generated hole (h + ) and electrons (e − ) under the light irradiation 38 . The detail process can be expressed as follows 1 and 2: The photocatalytic degradation performances of the spent FCC catalyst, V/FCC, Ti/FCC, V-Ti/FCC and V-Ti-2/FCC samples in the degradation of MB solution are monitored. Figure 9a shows the degradation results of these samples. The activity of the spent FCC catalyst, V/FCC, Ti/FCC, V-Ti/FCC and V-Ti-2/FCC samples are evaluated. The absorption spectra in the presence of the as-prepared samples indicate that the spent FCC catalyst, V/FCC, Ti/FCC, V-Ti/FCC and V-Ti-2/FCC samples adsorbed ~3%, ~3%, ~7%, ~11% and ~14% of MB, respectively. The dye remaining percentage for the presence of the spent FCC catalyst, V/FCC, Ti/FCC, V-Ti/ FCC and V-Ti-2/FCC samples are ~19%, ~26%, ~36%, ~75% and ~96%, respectively, after light irradiation for 120 min. The optimal decomposition ratio of MB over the V-Ti-2/FCC sample is 96% after light irradiation for 120 min. In contrast, under the same conditions, the MB is degraded only about 20% over the V-Ti/FCC sample. In all samples, the V-Ti-2/FCC sample displays a higher photocatalytic activity than other samples. All the above photocatalytic results indicate that the V-Ti-2/FCC photocatalyst displays excellent photocatalytic performance.
The stable reusability of V-Ti-2/FCC sample was investigated by a four-cycle degradation of methylene blue under identical conditions. The reusability is vital factor influencing practical application of V-Ti-2/FCC sample in dye wastewater treatment. As shown in Fig. 9b, after four cycle experiments in light irradiation, the photocatalytic degradation efficiency barely changes, with the degradation rate of about 89% during the four experiment. These results demonstrate the feasibility of recovery reuse of the V-Ti-2/FCC sample. Figure S3 shows the performance of the recycled V-Ti-2/FCC sample for the methylene blue photocatalytic degradation. It was found that the methylene blue degradation efficiency of V-Ti-2/FCC was still up to ~96%, ~95%, ~95% and ~89%, respectively.

conclusions
In summary, we have successfully fabricated the spent FCC catalyst composites by loaded V 2 O 5 and TiO 2 via a modified-impregnation method, which are applied as photocatalysts to degrade the methylene blue solution. The photo-induced electrons from the CB of V 2 O 5 rapidly transfer to the CB of the TiO 2 during the photocatalytic reaction process, which promotes the separation of photo-induced electrons on the holes. The V-Ti-2/FCC sample can significantly improve the photocatalytic activity via increasing the number of photoinduced charge carriers at the interface of the V 2 O 5 and TiO 2 structure. The optimal decomposition ratio of methylene blue over V-Ti-2/FCC sample is 96% under light irradiation for 120 min. The degradation ratios of methylene blue over the V/FCC, Ti/FCC and V-Ti/FCC samples were ~26%, ~36% and ~75%, respectively. This investigation may provide guidance for the treatment of the spent FCC catalysts and these spent FCC catalyst composites can serve as potential photocatalytic materials, which indicates their potential application in the field of dye solution degradation.

experimental Section
Fabrication of composites. The spent FCC catalyst composites have been fabricated with different content of V 2 O 5 and TiO 2 , using the spent FCC catalyst as support. The first one was fabricated by V 2 O 5 , where 2 g of spent FCC catalyst powder was impregnated with 0.2 g of V 2 O 5 and 3 mL H 2 O 2 . The mixture was grinded for 2 h. The resultant sample was denoted as V/FCC. The second one was fabricated by tetrabutyl titanate impregnation, and the fabrication process is the same to that of the first one except for the addition of 2 mL of tetrabutyl titanate. The resultant sample was denoted as Ti/FCC. The third one was fabricated by V 2 O 5 and tetrabutyl titanate, and the fabrication process is the same to that of the first one except for the addition of 0.1 g of V 2 O 5 and 1 mL tetrabutyl titanate. The resultant sample was denoted as V-Ti/FCC. The fourth one was fabricated by V 2 O 5 and tetrabutyl titanate, and the fabrication process is the same to that of the first one except for the addition of 0.02 g of V 2 O 5 and 0.18 mL tetrabutyl titanate. The resultant sample was denoted as V-Ti-2/FCC. All the samples were calcined at 800 °C for 4 hours.
Characterizations. The crystalline structure of these samples were characterized by the powder XRD (Rigaku RAD-3C). Data was collected in the Cu Kα, λ = 1.5405 Å, 35 kV, 20 mA, 2-Theta angles (5°-80°), and the scan rate was 10° min −1 . The morphology of these samples was measured using the scanning electron microscope with a JEOL S-4800 of FE-SEM under the condition of 3.0 kV operating voltage. Gas adsorption behavior and Brunauer-Emmett-Telle (BET) surface area of the samples were determined using a Micrometrics (Tristar, 3000) instrument in nitrogen atmosphere. Diffuse Reflectance UV-vis was recorded, with select wavelength coverage from 200 nm to 800 nm. FT-IR spectra were recorded by a spectrometer (Thermo Scientific TM Nicolet is 5 FT-IR Spectrometer) at room temperature in the region of 400-4000 cm −1 . The concentrations of the methylene blue relative to the photocatalysis time were tested by UV-Vis spectrophotometer.
Photocatalytic tests. Methylene blue was used as a model to investigate the adsorption and photocatalytic activities of these samples (FCC, V/FCC, Ti/FCC, V-Ti/FCC and V-Ti-2/FCC). The concentration of MB solution was determined by UV-vis spectrophotometer at 20 min. The photocatalytic degradations of these catalysts for methylene blue solution were performed in a 1000 mL reactor. The solution was irradiated using a Xenon lamp (300 W) equipped with a filter cutoff of 400 nm.
The adsorption-desorption equilibrium was determined by measuring the dye concentration for 2 h. There was no light irradiation during the time that 0.2 g of catalyst was added in a 200 mL MB solution. It was stirred in darkness to obtain a good dispersion and reach adsorption-desorption equilibrium between the methylene blue and the catalyst surface. Then the solution was irradiated under a 300 W Xenon lamp for 2 h. The concentration of the remaining MB solution was analyzed every 20 min. The photodegradation efficiency (A) was given by formula (3).
The C 0 is the initial methylene blue solution concentration, and the C is the concentration of methylene blue solution.