Preparation of a AgCl/PbMoO4 Composite and Investigation of Its Photocatalytic Oxidative Desulfurization Performance.

PbMoO4 materials were synthesized by the glycerol and hydrothermal methods, and AgCl nanoparticles were loaded onto the surface of PbMoO4 by using the precipitation-deposition method. Finally, a AgCl/PbMoO4 photocatalyst was successfully prepared. X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and UV-vis diffuse reflectance spectroscopy (UV-vis-DRS) were used to characterize the phase composition, morphology, and light absorption characteristics of the catalyst. An n-octane solution of dibenzothiophene (DBT) was used to simulate fluid catalytic cracking to make gasoline. The photocatalytic oxidation performance of DBT under visible light was studied in terms of the type of light source as well as of the catalyst, substrate, and AgCl content. The mechanism of photocatalytic oxidation was also studied. The results show that AgCl loading causes a red shift of the absorption edge of PbMoO4, which improves the photocatalytic activity of the material. When the AgCl loading was 25.0%, the amount of catalyst was 1.5 g/L, and the visible light irradiation time was 2.0 h, the highest desulfurization rate of DBT reached 97.0%.

Introduction

With the increasing use of automobiles, automobile exhaust has become one of the main sources of air pollution.[1] SO emitted into the atmosphere after the combustion of gasoline not only causes direct harm to the human body but also contributes to acid rain, thereby causing environmental degradation and severely jeopardizing people’s lives.[2] In 2017, China fully implemented the National V Gasoline Standard, which requires the sulfur content to be reduced to 10 μg/g.[3] In China, 90% of sulfur in automotive gasoline comes from fluid catalytic cracking to make gasoline, and the main sulfur compounds are mercaptans, thioethers, disulfides, and thiophenes (Th).[4,5] Mercaptan sulfur and thioether sulfur are mainly distributed in the light fraction. The C–S bonds of these types of aliphatic sulfides are easy to break, and the lone pair electron density of the S atom is high and can be easily removed by hydrodesulfurization (HDS). Th sulfur compounds are mainly distributed in the 60–100 °C fraction and macromolecular Th and benzocyclothiophene sulfides are mainly distributed in the heavy fraction.[6] Among these compounds, Th and benzocyclothiophene compounds are difficult to remove by traditional HDS technology because of their high steric hindrance and good molecular stability.[7,8]

Therefore, the key to deep desulfurization of gasoline lies in the removal of Ths and benzothiophenes (BTs). At present, nonhydrodesulfurization technologies for desulfurization include adsorption desulfurization,[9−11] extractive desulfurization (EDS),[12−14] biological desulfurization,[15−17] oxidative desulfurization (ODS),[18−20] photocatalytic ODS,[21−24] and so on. Photocatalytic oxidation technology, because of its low cost and mild reaction conditions, can effectively remove Th sulfides that are difficult to remove by HDS technology and has become an important means to the deep desulfurization of gasoline.

The key to photocatalytic oxidation technology is photocatalysis. Semiconductor photocatalysts have obvious advantages in environmental pollution control because of their advantages of zero pollution, high performance, and degradation of most toxic and harmful organic substances.[25] Molybdate photocatalyst materials have a wide range of applications in chemical, biological, pharmaceutical, and other fields,[26] mainly because of their advantages of high stability, high specific surface energy, selectivity, strong oxidation or reduction, narrow band gap, and response in the ultraviolet and visible regions. Molybdate has been widely studied by many researchers. PbMoO4 has a tetragonal structure of scheelite type, in which each Mo6+ is surrounded by four •O2– and each Pb2+ is surrounded by eight •O2–.[27] This complex has a unique crystal structure and a unique energy band structure, with a band gap of approximately 3.20 eV. In recent years, lead molybdate not only has been widely studied in the fields of optical instrument manufacturing, photoconductivity, chemiluminescence, and so on but also shows good visible light catalytic activity. In a solution of formic acid and silver nitrate, this complex catalyzes photocatalytic hydrogen production and oxygen production[28] as well as the photocatalytic degradation of organic matter upon ultraviolet irradiation. However, the photocatalytic performance of PbMoO4 is limited by visible light absorption and the high recombination rate of photogenerated e– and h+, which greatly reduce the catalytic activity of PbMoO4. Therefore, to solve this problem, AgCl/PbMoO4 photocatalyst composites were successfully prepared by supporting AgCl nanoparticles on the surface of PbMoO4, and their photocatalytic performance for the removal of dibenzothiophene (DBT) was studied. During photocatalytic oxidation, the photocatalytic material undergoes an electronic transition after being irradiated with light to generate photogenerated electron–hole pairs. The hole (h+) can directly oxidize the organic sulfur model compound adsorbed onto the surface of the photocatalyst and react with H2O2 to generate h+, •OH, and •O2–. These active radicals play an important role in the photocatalytic oxidation system. Therefore, a radical scavenger was added to the photocatalytic oxidation reaction system, and the main active species in AgCl/PbMoO4 and the mechanism of DBT removal were investigated.

Experimental Section

Materials

pan class="Chemical">Pb(NO3)2, n>an class="Chemical">C3H8O3, (NH4)6Mo7O24·4H2O, C2H5OH, AgNO3, NaCl, BT, DBT, 4,6-dimethyldibenzothiophene (4,6-DMDBT), Th, n-octane, acetonitrile, sodium hydroxide, sodium molybdate hydrate, cetyltrimethylammonium bromide (CTAB), glacial acetic acid, and deionized water were used. All chemicals used were of analytical reagent grade.

Characterization

The morphology and the size of the catalyst were characterized using a scanning electron microscope (TESCAN VEGA3) manufactured by Thermo Fisher Scientific (United States). The structure of the catalyst was analyzed using a Rigaku Ultimate IV-type X-ray powder diffraction analyzer produced by Rigaku Corporation (Japan). The experiment used Cu Kα radiation over a scanning range of 10 < 2θ < 80 at a rate of 10°/min, and the tube voltage and current were 40 kV and 40 mA, respectively. The absorption spectrum of the catalyst was obtained using a U-3900H UV–visible diffuse reflection absorption spectrometer produced by Hitachi, Japan, with a wavelength range of 200–800 nm. The elemental composition of the catalyst surface was analyzed using an X-ray photoelectron spectrometer (Thermo ESCALAB250Xi) produced by Thermo Fisher Scientific (United States).

Catalyst Preparation

Preparation of PbMoO4 by the Glycerin Hydrothermal Method

pan class="Chemical">Pb(NO3)2 (0.001 mol) was added to 5 mL deionized n>an class="Chemical">water and 20 mL glycerin. Then, 5 mL of deionized water containing 0.001 mol of Na2MoO4·2H2O was slowly dropped into the above solution, and the mixture was stirred for 10 min and transferred to a 40 mL stainless steel autoclave. After being treated at 180 °C for 24 h, the solution was naturally cooled to room temperature. The product was washed with deionized water and anhydrous ethanol and then dried under vacuum at 60 °C for 12 h to obtain a PbMoO4 sample.

Preparation of PbMoO4 by the Hydrothermal Method

A certain amount of (NH4)6Mo7O24·4H2O was dissolved in 20 mL deionized water, and 30 mL of 0.5 mol/L CTAB solution was added; the solution was stirred for 30 min and then labeled solution A. A certain amount of Pb(NO3)2 was dispersed in 20 mL ultrapure water, mixed with solution A, and stirred for 10 min. After the pH value was adjusted to 5 and 7 with NaOH, the solution was transferred into a Teflon-sealed autoclave for the hydrothermal reaction at 180 °C for 24 h. The obtained dispersion was cooled to room temperature, washed with water and alcohol several times, and dried under vacuum at 60 °C for 12 h to obtain PbMoO4.

Preparation of AgCl/PbMoO4 with Different AgCl loadings

First, a certain amount of NaCl solid was dissolved in 25 mL deionized water, and then a certain amount of the prepared PbMoO4 sample was added. The solution was recorded as solution A, and then, solution A was sonicated for 20 min. The solution was stirred for 1 h, so that Cl– was fully adsorbed onto PbMoO4 crystallites. After that, AgNO3 (0.1 mol/L) solution with NaCl was added to solution A, and then, the solution was mixed under magnetic force for 6 h in the dark. Finally, the treated solution was centrifuged, washed with water and alcohol several times, and then dried in a vacuum drying oven at 60 °C for 12 h to obtain a white powder sample. To explore the influence of different loading amounts on the catalyst activity, seven control groups were established in this experiment; the total mass of each AgCl/PbMoO4 sample was 0.3 g, and the samples with different AgCl mass fractions were labeled 0%-AgCl/PbMoO4, 5%-AgCl/PbMoO4, 10%-AgCl/PbMoO4, 15%-AgCl/PbMoO4, 20%-AgCl/PbMoO4, 25%-AgCl/PbMoO4, and 30%-AgCl/PbMoO4.

Photocatalytic Oxidation Desulfurization Experiment

A certain amount of DBT was dissolved in 100 mL n-octane solution and mixed thoroughly to prepare a model oil solution with a sulfur content of 200 mg/L. From that solution, 30 mL was removed, which, along with an appropriate amount of catalyst and oxidant, was added to a 50 mL quartz tube. The quartz tube was placed in a photochemical reactor, which was magnetically stirred for 30 min in a dark environment to achieve adsorption–desorption equilibrium. Then, the xenon lamp was turned on, and 5 mL samples were taken every 30 min. Then, the acetonitrile fraction of the above solution was collected. Finally, the upper solution was collected, and the sulfur content was determined with a WK-2D microcoulomb analyzer. By comparing the results with the sulfur content of the original solution, the desulfurization rate was obtained, and the desulfurization effect was analyzed. The desulfurization rate was calculated according to formula 1, where η is the desulfurization rate and C0 and C are the sulfur contents of the solution before and after the reaction, respectively.

Results and Discussion

X-ray Photoelectron Spectroscopy Analysis of AgCl/PbMoO4

As shown in Figure a–d, the binding energies of different valence states of various elements in the PbMoO4 catalyst were studied by X-ray photoelectron spectroscopy (XPS) to analyze the composition and the chemical state of PbMoO4 microcrystalline particles. Figure a shows the general XPS spectrum of the PbMoO4 photocatalyst, which shows the best catalytic activity compared with other samples. The spectrum clearly shows that the photocatalyst was composed of Pb, Mo, O, and C (elements inherent in the system), and no other impurity peaks were detected. The inner layer electron binding energy of Mo 3d is shown in Figure b. The Mo 3d peaks observed at 234.9 and 231.8 eV were ascribed to the chemical valence states of Mo6+ of MoO3 and represent Mo 3d5/2 and Mo 3d3/2, respectively. The 3d bimodal split was approximately 3.0 eV.[29]Figure c shows the inner layer electron binding energy of Pb 4f. There were two characteristic peaks of Pb 4f at 138.1 and 142.9 eV, corresponding to Pb 4f7/2 and Pb 4f5/2, respectively. These two peaks were mainly attributed to PbO, which originated from the Pb2+ in PbMoO4.[30]Figure d shows a characteristic diagram of the inner electron binding energy of O 1s, which shows three peaks. The peak at 529.2 eV corresponds to the lattice oxygen of the sample. The binding energy at 530.0 eV is attributed to Mo–O, and the peak at 531.5 eV is characteristic of Mo–OH. The above analysis results were consistent with the X-ray diffraction (XRD) results.

Figure 1

XPS spectra of PbMoO4: (a) the general XPS spectrum of PbMoO4; (b) high-resolution Mo 3d spectrum; (c) high-resolution Pb 4f spectrum; and (d) high-resolution O 1s spectrum.

Xpan class="Chemical">PS spn>en>an class="Chemical">ctra of PbMoO4: (a) the general XPS spectrum of PbMoO4; (b) high-resolution Mo 3d spectrum; (c) high-resolution Pb 4f spectrum; and (d) high-resolution O 1s spectrum.

As shown in Figure a–f, to further analyze the composition and chemical states of the elements in AgCl/PbMoO4 microcrystalline particles, the binding energies of different valence states of the elements were investigated by XPS. Figure a shows the general XPS spectrum of the AgCl/PbMoO4 photocatalyst, which exhibited better catalytic activity than the other samples. It can be clearly seen from the spectrum that the photocatalyst was composed of Pb, Mo, O, Ag, Cl, and C (elements inherent in the system), and no other impurity peaks were detected. Figure b shows two characteristic absorption peaks at 367.9 and 373.9 eV attributed to the inner electron bonding energy of Ag 3d, corresponding to Ag 3d5/2 and Ag 3d3/2, respectively. The positions of these two characteristic absorption peaks were exactly consistent with Ag+, and the characteristic absorption peaks at 368.8 and 374.3 eV can be attributed to Ag0. It can be seen that a small amount of Ag metal was formed in AgCl/PbMoO4, which was consistent with the result of XRD. Moreover, as shown in Figure c, the binding energies of 198.1 and 200.1 eV correspond to Cl 2P3/2 and Cl 2p1/2, respectively, thus proving the existence of Cl–. Figure d shows the inner layer electron binding energy of Mo 3d, and the peaks of Mo 3d were at 234.9 and 231.8 eV, which represent Mo 3d5/2 and Mo 3d3/2, respectively. These peaks were attributed to the Mo6+ state of MoO3, and the split of the 3d double peaks was approximately 3.0 eV. Figure e shows a characteristic diagram of the inner electron binding energy of O 1s, which shows three peaks. The peak at 529.2 eV corresponds to the lattice oxygen of the sample. The binding energy at 530.0 eV is attributed to Mo–O, and the peak at 531.5 eV is characteristic of Mo–OH. Figure f clearly shows the inner layer electron binding energy of Pb 4f. There were two characteristic peaks of Pb 4f at 138.1 and 142.9 eV, corresponding to Pb 4f7/2 and Pb 4f5/2, respectively. These two peaks were mainly attributed to PbO, which originated from the Pb2+ of PbMoO4. The results of the abovementioned analysis were consistent with the XRD results.

Figure 2

XPS spectra of AgCl/PbMoO4: (a) the general XPS spectrum of AgCl/PbMoO4; (b) high-resolution Ag 3d spectrum; (c) high-resolution Cl 2p spectrum; (d) high-resolution Mo 3d spectrum; (e) high-resolution O 1s spectrum; and (f) high-resolution Pb 4f spectrum.

Xpan class="Chemical">PS spn>en>an class="Chemical">ctra of AgCl/PbMoO4: (a) the general XPS spectrum of AgCl/PbMoO4; (b) high-resolution Ag 3d spectrum; (c) high-resolution Cl 2p spectrum; (d) high-resolution Mo 3d spectrum; (e) high-resolution O 1s spectrum; and (f) high-resolution Pb 4f spectrum.

From Figure a–d, it can be seen that the crystallinity of each sample was excellent. The characteristic diffraction peak 2θ and d values of each sample showed a good agreement with the PbMoO4 standard card (44-1486) of the tetragonal molybdenite-type structure, and there was no impurity peak, indicating that the purity of the obtained material was relatively high. Figure e shows the XRD patterns of AgCl/PbMoO4, which contain all the diffraction peaks of the PbMoO4 photocatalyst. As seen in Figure e, the AgCl/PbMoO4 composite gives rise to diffraction peaks at 2θ = 27.80, 32.20, 46.23, and 57.44°, which could be assigned to the (111), (200), (220), and (222) crystal planes of AgCl (31-1238), indicating that AgCl and PbMoO4 were present in the AgCl/PbMoO4 sample. The reflection at 2θ = 54.8° could be assigned to the (311) crystal plane of Ag. According to the limited conditions of the control reaction to avoid light, part of the AgCl was decomposed into Ag. There were no other heteropeaks in the XRD, which indicated that the photocatalyst shows high purity. There are no obvious differences in the crystallinity and the phase composition of the catalysts prepared using different conditions and methods after analysis, but considering the principle of using the least number of chemical synthetic steps, the catalysts prepared by the glycerin method are preferred in this paper.

Figure 3

XRD patterns of PbMoO4 and AgCl/PbMoO4. (a): PbMoO4 samples prepared by the glycerin method; (b): PbMoO4 at pH = 5 (180 °C CTAB); (c): PbMoO4 at pH = 7 (180 °C CTAB); (d): PbMoO4 at pH = 7 (170 °C CTAB); and (e): AgCl/PbMoO4.

XRD patterns of pan class="Chemical">PbMoO4 and n>an class="Chemical">AgCl/PbMoO4. (a): PbMoO4 samples prepared by the glycerin method; (b): PbMoO4 at pH = 5 (180 °C CTAB); (c): PbMoO4 at pH = 7 (180 °C CTAB); (d): PbMoO4 at pH = 7 (170 °C CTAB); and (e): AgCl/PbMoO4.

Scanning Electron Microscopy Analysis of PbMoO4 and AgCl/PbMoO4

Figure a–d shows the field emission scanning electron microscopy (FESEM) images of the PbMoO4 sample (glycerin method), showing the microstructure and the morphology of the product. As shown in Figure b, the prepared products were composed of abundant PbMoO4 polyhedral crystallites and had good dispersibility. The high-magnification SEM images shown in Figure a,b show that the surface of PbMoO4 was very smooth, and no small nanoparticles were attached. Figure c,d shows the SEM photographs of the AgCl/PbMoO4 sample obtained by loading AgCl onto PbMoO4 by the precipitation–deposition method. It can be seen that there were some small nanoparticles on each crystal surface of the sample, with a size of approximately 70 nm. Combined with the XRD and XPS analysis results, it can be inferred that the small particles were the deposited AgCl nanoparticles with good dispersity on the surface of PbMoO4. After being loaded with AgCl nanoparticles, the morphology of PbMoO4 did not change.

Figure 4

(a,b) SEM images of PbMoO4 and (c,d) SEM images of AgCl/PbMoO4.

(a,b) SEM images of pan class="Chemical">PbMoO4 and (n>an class="Chemical">c,d) SEM images of AgCl/PbMoO4.

Energy-Dispersive X-ray Spectroscopy Analysis of PbMoO4 and AgCl/PbMoO4

The energy spectrum and the element content distribution curve of the PbMoO4 catalyst are shown in Figure a–e. Polyhedral PbMoO4 particles contain only Pb, Mo, and O and have obvious distributions with no other impurities. The elemental ratio of Pb, Mo, and O was 1:1:4, confirming that the PbMoO4 material with high purity was prepared (Figure e). Figure a–f shows the energy spectrum of the AgCl/PbMoO4 composites (after photocatalytic performance testing, this catalyst was the best performing group, so according to the principle of optimal performance, a series of characterization analyses were conducted for only AgCl/PbMoO4). As shown in the energy spectrum, the sample was composed of five elements: Ag, Cl, Pb, Mo, and O. The atomic ratios of these five elements were Ag/Cl = 1:1 and Pb/Mo/O = 1:1:4. This result also proved that the AgCl/PbMoO4 composite material was composed of AgCl and PbMoO4.

Figure 5

(a) Gray scale of PbMoO4. (b) O element distribution of PbMoO4. (c) Mo element distribution of PbMoO4. (d) Pb element distribution of PbMoO4. (e) EDS spectrum of PbMoO4.

Figure 6

(a) Gray scale of AgCl/PbMoO4. (b) Cl element distribution of PbMoO4. (c) Mo element distribution of PbMoO4. (d) Ag element distribution of PbMoO4. (e) Pb element distribution of PbMoO4. (f) EDS spectrum of AgCl/PbMoO4.

(a) Gray span class="Chemical">cale of n>an class="Chemical">PbMoO4. (b) O element distribution of PbMoO4. (c) Mo element distribution of PbMoO4. (d) Pb element distribution of PbMoO4. (e) EDS spectrum of PbMoO4.

(a) Gray span class="Chemical">cale of n>an class="Chemical">AgCl/PbMoO4. (b) Cl element distribution of PbMoO4. (c) Mo element distribution of PbMoO4. (d) Ag element distribution of PbMoO4. (e) Pb element distribution of PbMoO4. (f) EDS spectrum of AgCl/PbMoO4.

Transmission Electron Microscopy Analysis of AgCl/PbMoO4

To further analyze the structure and morphology of the AgCl/PbMoO4 composites, this experiment used transmission electron microscopy (TEM) and high-resolution (HR) TEM to analyze the microstructure and the interface composition of these composite materials. Figure a–d shows the TEM images of the AgCl/PbMoO4 photocatalyst. The results show that the morphology of AgCl/PbMoO4 consisted of uniform octahedral structures, and the results were consistent with the SEM results. It can be seen from the figures that some particles were distributed on the surface of the PbMoO4 structure, and the TEM results were consistent with the SEM results. More evidence for AgCl on the surface of PbMoO4 is further provided by HR-TEM (Figure f). The HR-TEM images correspond to Figure c. The HR-TEM results show clear lattice fringes with d-spacing of 0.16 and 0.28 nm belonging to the lattice fringe of the (222) and (200) planes for AgCl. The d-spacing of 0.32 nm corresponds to the (112) planes of PbMoO4. The results prove that AgCl was successfully loaded onto the surface of PbMoO4, as shown in the figures.

Figure 7

(a–e) TEM images of AgCl/PbMoO4 (f) HRTEM images of AgCl/PbMoO4.

(a–e) TEM images of pan class="Chemical">AgCl/n>an class="Chemical">PbMoO4 (f) HRTEM images of AgCl/PbMoO4.

UV–vis Diffuse Reflectance Spectroscopy Analysis of PbMoO4 and AgCl/PbMoO4

The light absorption properties of semiconductors affect the photocatalytic activity of the catalyst. Figure shows the UV diffuse absorption spectra of PbMoO4 and AgCl/PbMoO4 photocatalysts. As is shown, both materials have strong light absorption between 250 and 380 nm. The absorption edge of each sample is different; the absorption edge of PbMoO4 is at 392 nm, while that of AgCl/PbMoO4 is significantly red-shifted to 410 nm, indicating that the AgCl/PbMoO4 composite has stronger visible light absorption. The analysis shows that the AgCl/PbMoO4 composites obtained by loading AgCl particles onto PbMoO4 by precipitation–deposition improved the visible light response to some extent, which was also beneficial for the generation of electron–hole pairs and the photocatalytic activity. The relationship between the band gap and the band edge absorption of a semiconductor material is Eg = 1239.8/λg, where λg is the absorption edge. The band gaps of PbMoO4 and AgCl/PbMoO4 were calculated to be 3.16 and 3.02 eV, respectively. The recombination of AgCl and PbMoO4 reduced the band gap value of the catalyst and improved the visible light absorption capacity of the AgCl/PbMoO4 composite. In the photocatalytic process, photogenerated electrons and holes were more easily generated, and the activity of the material was also improved.

Figure 8

UV–vis-DRS spectra of PbMoO4 and AgCl/PbMoO4.

UV–vis-DRS spepan class="Chemical">ctra of n>an class="Chemical">PbMoO4 and AgCl/PbMoO4.

Conclusions

In this paper, PbMoO4 polyhedral crystallites were synthesized by glycerol and hydrothermal methods. The AgCl/PbMoO4 photocatalyst was successfully prepared by the precipitation–deposition method. The composite materials were characterized by various methods. The analysis results proved that AgCl was successfully supported on the PbMoO4 surface, and a significant redshift occurred, improving the response to visible light. Using DBT as the removal target, the photocatalytic oxidation desulfurization performance of AgCl/PbMoO4 under visible light was investigated under different reaction conditions, and the photocatalytic oxidation mechanism was explored. The experimental results show that the desulfurization performance of AgCl/PbMoO4 for different organic sulfur compounds in gasoline is DBT > 4,6-DMDBT > BT > Th. When the loading of AgCl was 25.0%, the amount of catalyst was 1.5 g/L, and the removal rate of DBT was up to 97.0% after 120 min of light irradiation, indicating that the photocatalytic activity of the composite was much higher than that of pure PbMoO4. The AgCl/PbMoO4 composite was used in the recycling experiment. After being reused five times, the removal rate of DBT by the composite catalyst was only reduced to 90.0%, which showed that the AgCl/PbMoO4 composite catalyst has good stability. At the same time, radical capture experiments show that •O2– and h+ are the main active species in the photocatalytic oxidation process.

Effect of AgCl/PbMoO4 on Photocatalytic ODS

Effect of Light Source Types

The conditions of the photocatalytic reaction exert significant influence on the catalytic activity, and the light source is an important factor. Different light sources have different distributions, so they have different effects on the photocatalytic activity of the catalyst. Figure shows the experimental results obtained with a high-pressure mercury lamp and a xenon lamp. PbMoO4 has the highest catalytic activity for DBT when a mercury lamp is used. The main reason for this finding is that in the wavelength range of 200–400 nm, the high-pressure mercury lamp has a strong spectral distribution, while the xenon lamp is very weak. The spectral distribution of sunlight in the range of 300–1000 nm is relatively uniform. The UV-DRS spectra show that in the range of 200–400 nm, PbMoO4 has strong absorption. Therefore, in this experiment, a high-pressure mercury lamp was used as the light source for the reaction. The photocatalytic activity was relatively good, and the DBT removal rate achieved was 94.0%.

Figure 9

Relationship between the desulfurization rate and the time of DBT under different light sources.

Relationship between pan class="Chemical">the den>an class="Chemical">sulfurization rate and the time of DBT under different light sources.

Effect of the Amount of Photocatalyst

To research on the desulfurization ability of AgCl/PbMoO4, the influence of the amount of catalyst used on DBT removal was explored. The analysis results are shown in Figure . First, 0, 6, 12, 18, 24, and 30 mg of the AgCl/PbMoO4 photocatalyst (concentrations of 0, 0.3, 0.6, 0.9, 1.2, and 1.5 g/L, respectively) were added to 20 mL model oil with a sulfur content of 200 mg/L. At a constant reaction time, the use of more photocatalyst resulted in a higher DBT removal rate. After reacting for 120 min, when the amount of photocatalyst was sequentially increased from 0 to 30 mg, the DBT removal rate gradually increased to 97.0%. When the amount continued to increase, the removal rate did not increase because the reaction reached a saturation state. As the amount increased, the number of active sites available for the reaction gradually increased, but when the reaction system reached a saturation state, the photocatalyst agglomerated, the number of exposed active sites decreased, and the removal effect was reduced. Therefore, the optimal amount of photocatalyst was determined to be 1.5 g/L.

Figure 10

Relationship between the desulfurization rate and the time of DBT with different catalyst amounts.

Relationship between pan class="Chemical">the den>an class="Chemical">sulfurization rate and the time of DBT with different catalyst amounts.

Effect of the Loading of AgCl

The loading of AgCl was an important factor affecting the efficiency of the photocatalytic oxidation reaction. To study the effect of AgCl loading on the desulfurization rate of DBT, 0%-AgCl/PbMoO4, 5%-AgCl/PbMoO4, 10%-AgCl/PbMoO4, 15%-AgCl/PbMoO4, 20%-AgCl/PbMoO4, 25%-AgCl/PbMoO4, and 30%-AgCl/PbMoO4 were prepared. Their removal efficiencies for DBT were studied. Figure shows that with the same reaction time, the removal efficiency of DBT increased gradually as the AgCl loading amount increased. When the loading amount of AgCl increased from 0 to 25%, the DBT removal efficacy increased from 56.0 to 97.0% because when the loading amount of AgCl was 25.0%, there were more active sites at a high loading amount of AgCl/PbMoO4 than at a low loading amount of AgCl/PbMoO4. However, an excess of AgCl (30%) reduced the removal efficiency. One possibility was that excessive loading led to serious aggregation, resulting in a decrease in the number of active sites of AgCl/PbMoO4. Another possible reason was that 25% AgCl loading on the surface of PbMoO4 had the best dispersion, so the desulfurization rate would not change as the AgCl loading amount was increased. Thus, the optimal loading amount of AgCl was determined to be 25.0% with an excellent desulfurization rate of 97.0%.

Figure 11

Relationship between the desulfurization rate and the time of DBT under different AgCl loading amounts.

Relationship between pan class="Chemical">the den>an class="Chemical">sulfurization rate and the time of DBT under different AgCl loading amounts.

Degradation Rate of Different Sulfur-Containing Model Compounds

To investigate the degradation rate of AgCl/PbMoO4 on different substrates (sulfides), BT, Th, and 4,6-DMDBT were used as new model sulfur compounds in the reaction system to prepare a new n-octane solution with a sulfur content of 200 mg/L. Under the same experimental conditions, only the type of sulfur-containing substrate was changed, and the desulfurization rate was compared. As shown in Figure , the best desulfurization rate was achieved for DBT, followed by 4,6-DMDBT, then BT, and finally Th. After 120 min of reaction, the desulfurization rate of DBT was 97.0%, while that of Th was only 53.0%. Therefore, the order of removal efficiency of various model compounds was DBT > 4,6-DMDBT > BT > Th. This conclusion may be because of the influence of steric hindrance of the methyl groups and the electron cloud density of the S atom. The electron cloud densities of the S atom in Th and BT were 5.739 and 5.696, respectively, while those of 4,6-DMDBT and DBT were 5.760 and 5.758, respectively. The S atom electron cloud of Th had the lowest density and was the most difficult to oxidize; therefore, its degradation rate was far lower than those of 4,6-DMDBT and DBT. The S electron cloud densities of 4,6-DMDBT and DBT are very similar, indicating that their activities are also similar, so it is difficult to distinguish these complexes. Therefore, the degradation rates of these two sulfides are mainly affected by the steric hindrance of the methyl group, which reduces the chance of contact between active species and S atoms. Because there is a dimethyl group in 4,6-DMDBT, the degradation rate of 4,6-DMDBT is lower than that of DBT.

Figure 12

Relationship between the desulfurization rate and the time of different sulfur model compounds.

Relationship between pan class="Chemical">the den>an class="Chemical">sulfurization rate and the time of different sulfur model compounds.

Cycling and Stability of the Catalyst

To study the cycling performance and the stability of the catalyst, the used photocatalyst was recovered, filtered, and dried for use. Under the conditions of high-pressure mercury lamp irradiation for 2.5 h, a catalyst dosage of 1.5 g/L, and 30 mL model oil with a sulfur content of 500 mg/L, the effects of photocatalyst usage on the desulfurization efficiency of the DBT model oil were investigated. The experimental results are shown in Figure . It can be seen from Figure that after five times of recycling, the desulfurization rate of photocatalytic oxidation was reduced from 97.0 to 90.0%, indicating that the catalyst still has good activity after recycling. When the catalyst was used for the sixth time, after 2.5 h of reaction, the desulfurization rate was only reduced to 90.0%, indicating that the catalyst has good stability.

Figure 13

Recycling performance of AgCl/PbMoO4.

Repan class="Chemical">cyn>an class="Chemical">cling performance of AgCl/PbMoO4.

Desulfurization Mechanism

Determination of Active Radicals

To investigate the main active species in the AgCl/PbMoO4 photocatalytic reaction system, 1 mM concentrations of the radical scavengers isopropanol (IPA), sodium oxalate (Na2C2O4), and p-benzoquinone (1,4-BQ) were used to capture h+, •OH, and •O2–, respectively. To remove the interference of DBT itself by the three radical scavengers, a group of control experiments was done. There was no direct relationship between the radical scavenger and the DBT removal efficiency, as shown in Figure . As shown in Figure a,b, after 2 h of reaction, the desulfurization rates with Na2C2O4 and 1,4-BQ as the radical scavenger decreased from 97.0 to 42.0 and 26.0%, respectively, indicating that the photocatalytic activity was inhibited. This result shows that during the photocatalytic degradation process, h+ and •O2– were generated and participated in the reaction. Figure c shows that the DBT desulfurization rate was almost unchanged after adding IPA to the reaction system for 2 h. Therefore, it can be concluded that •O2– and h+ were the main active species in this process.

Figure 14

Relationship between the desulfurization rate and time with different radicals: (a) Na2C2O4 (b) 1,4-BQ, and (c) IPA.

Relationship between pan class="Chemical">the den>an class="Chemical">sulfurization rate and time with different radicals: (a) Na2C2O4 (b) 1,4-BQ, and (c) IPA.

Study of the Mechanism of Photocatalytic Reaction

It can be seen from the abovepresented radical analysis that the main active species in the AgCl/PbMoO4 composite photocatalytic oxidation system are holes and superoxide radicals. However, because the activity of the catalyst is closely related to its band gap width, the reaction mechanism was analyzed. The Eg value of PbMoO4 is 3.16 eV, while the Eg value of AgCl is 3.25 eV.[31,32] From the empirical formulas 2 and 3, the positions of the conduction band (CB) and the valence band (VB) of AgCl and PbMoO4 were calculated. The values are listed in Table .

Table 1

Electronegativity, Band Edge, VB Position, and Band Gap Energy of AgCl and PbMoO4 Semiconductors

semiconductor	X/eV	EVB/eV	ECB/eV	Eg/eV	Ee/eV
AgCl	6.04	3.19	–0.06	3.25	4.50
PbMoO4	5.98	3.06	–0.1	3.16	4.50

Based on radical capture experiments and energy band results, a schematic diagram of the photocatalytic oxidation mechanism is shown in Figure . Because of the wide band gaps of PbMoO4 and AgCl, both can be excited by visible light. According to the VB structure characteristics of PbMoO4 and Ag/AgCl, the composite material can be transformed into Z-type Ag/AgCl/PbMoO4 under visible light irradiation. The Fermi level of metallic Ag is 0.4 eV, which is higher than the CB potential of PbMoO4 (−0.1 eV). Therefore, under the action of the interface Schottky barrier, an e– on the CB of PbMoO4 is very easy to inject into metal Ag. The potential of the AgCl VB (3.19 eV) is higher than the Fermi level of Ag metal (0.4 eV), and thus the e– on Ag metal is transferred to the AgCl VB. Ag particles can transport photogenerated electron–hole pairs, which can transfer electrons from the CB of PbMoO4 to the VB of AgCl. At the same time, photogenerated electrons can reduce the O2 adsorbed onto AgCl to give superoxide radicals. Superoxide radicals have strong oxidizing properties and can oxidize the DBT molecules on the catalyst surface. The h+ remaining on the PbMoO4 surface can oxidize DBT. Because the VB (+3.19 eV) of AgCl is more correct than that of PbMoO4 (+3.06 eV), the h+ on the VB of AgCl can be transferred to the VB of PbMoO4, where OH– can be oxidized to •OH, which has a strong oxidizing effect on DBT. According to the above analysis and the results of the radical capture experiment, after being irradiated by visible light, there were three kinds of active species in the reaction system, namely, h+, •OH, and •O2–, but the main active species were •O2– and h+.

Figure 15

Photocatalytic desulfurization mechanism of AgCl/PbMoO4.

pan class="Chemical">Photon>an class="Chemical">catalytic desulfurization mechanism of AgCl/PbMoO4.