Study of Catalytic Combustion of Chlorobenzene and Temperature Programmed Reactions over CrCeOx/AlFe Pillared Clay Catalysts

In this study, both AlFe composite pillaring agents and AlFe pillared clays (AlFe-PILC) were synthesized via a facile process developed by our group, after which mixed Cr and Ce precursors were impregnated on AlFe-PILC. Catalytic combustion of organic pollutant chlorobenzene (CB) on CrCe/AlFe-PILC catalysts were systematically studied. AlFe-PILC displayed very high thermal stability and large BET surface area (SBET). After 4 h of calcination at 550 °C, the basal spacing (d001) and SBET of AlFe-PILC was still maintained at 1.91 nm and 318 m2/g, respectively. Large SBET and d001-value, along with the strong interaction between the carrier and active components, improved the adsorption/desorption of CB and O2. When the desorption temperatures of CB and O2 got closer to the CB combustion temperature, the CB conversion could be increased to a higher level. CB combustion on CrCe/AlFe-PILC catalyst was determined using a Langmuir–Hinshelwood mechanism. Adsorption/desorption/oxidation properties were critical to design highly efficient catalysts for CB degradation. Besides, CrCe/AlFe-PILC also displayed good durability for CB combustion, whether in a humid environment or in the presence of volatile organic compound (VOC), making the catalyst an excellent material for eliminating chlorinated VOCs.


Introduction
Chlorinated volatile organic compounds (CVOCs) are considered to be very harmful to the environment, not only a direct harm on human health but also destroy the ozone layer [1,2]. Today, the major industrial processes for CVOCs elimination involve direct combustion at very high temperatures (above 850 • C). This is a fairly expensive process and produces highly toxic byproducts or intermediates by incomplete combustion, such as dioxins, Cl 2 , and CO [2,3]. The low operating temperatures (<500 • C) and high selectivity into harmless product, make catalytic combustion an attractive option [4][5][6].
Due to the high toxicity of dioxins and the need for laboratory safety, model reagents, such as chlorobenzene (CB), are used to predict destruction behavior of dioxins on different catalysts [7,8].
Vanadia-based catalysts [9,10], precious metals (Pt, Pd, Ru) supported on zeolites [11,12], and various oxides [13,14] are employed for the catalytic combustion process. However, these catalysts often have some disadvantages of relative low catalytic performance, rapid deactivation caused by coking or chlorine poisoning, high price, and the formation of polychlorinated benzene [14]. Transition

Characterization
The samples were characterized using X-ray diffraction (XRD) (PANalytical, Almelo, Netherlands) for d 001 value and phase composition. The S BET , mesopore area (A mes ), V p , micropore volume (V mic ), and pore size distribution of the samples were determined via N 2 adsorption isotherms using a TristarII 3020 apparatus (Micromeritics Company, Atlanta, GA, USA). High-resolution transmission electron microscopy (HRTEM) on a JEM-2100F (JEOL, Valley, Japan) was employed to get the catalyst morphology and particle size. The chemical compositions of the catalysts were determined with energy dispersive X-ray spectroscopy (EDS) using an Oxford INCA instrument (Oxford Instruments, Warrington, UK). All the characterization methods for the samples have been reported and detailed in our previous research [30][31][32].

Catalytic Performance Tests and Temperature-Programmed Reactions
The activity of the catalysts was evaluated in a WFS-3010 microreactor (Xianquan, Tianjin, China). The degradation products were detected by mass spectrometry (MS, QGA, Hiden, U.K.). No byproduct other than H 2 O, CO 2 , and HCl was detected. Thus, the conversion was calculated based on CB consumption [31]. To further study the "mixture effect" of the feed gases, 1% (v/v) water vapor and 100 ppm toluene were also introduced. Besides, the durability of CrCe (5:1)/AlFe-PILC for the catalytic combustion of CB was investigated at a CB concentration of 500 ppm and gas hourly space velocity (GHSV) of 25,000 h −1 .
H 2 temperature-programmed reduction (H 2 -TPR) was conducted on a CHEMBET-3000 instrument (Quantachrome, Boynton Beach, FL, USA) to evaluate the reducibility of the catalysts. The sample (50 mg) was pre-treated in air at 300 • C for 0.5 h, and then the temperature was reduced to 100 • C. The flow rate of the reductive gas (5 vol.% H 2 /Ar, purified by deoxidizer and silica gel) was 40 mL/min, and the reaction temperature was elevated by 7.5 • C/min. The H 2 uptake was determined using a thermal conductivity detector (TCD) detector (Shimadzu, GC-14C, Kyoto, Japan), and the H 2 O produced was absorbed using 5 Å zeolite [32].
Temperature-programmed desorption (TPD) and temperature-programmed surface reaction (TPSR) measurement were carried out in the same equipment as the catalytic performance tests to determine the adsorption capacity and the relationship between desorption performance and catalytic combustion properties [30,32]. Prior to the measurement, 350 mg catalyst was pre-treated in Ar (99.99%) at 300 • C for 30 min, then the temperature was decreased to 50 • C. Adsorption gas (40 mL/min) was a mixture of Ar (99.99%) and CB (about 500 ppm). The quantitative amounts were estimated by integrating the desorption curve. After the adsorption reached an equilibrium (CB concentration in the effluent gas was monitored using Gas Chromatography-Mass Spectrometry (GC-MS), the catalysts were purged by Ar (99.99%) for a period of time at 50 • C until CB concentration to constant. Then, the desorption and catalytic properties of the catalysts were measured from 50 to 500 • C with a heating rate of 7.5 • C/ min in 20 vol.%O 2 /80 vol.%Ar (without CB). The reactants and products (such as CB (m/z =112), CO 2 (44), H 2 O (18), Cl 2 (71), and HCl (36.5) were analyzed with an on-line MS apparatus.
O 2 temperature-programmed desorption (O 2 -TPD) was also performed using the same apparatus. The catalyst (350 mg) was firstly treated in 10 vol.%O 2 /90 vol.%Ar at 300 • C for 0.5 h. After the temperature was slowly cooled down to room temperature, followed by an Ar purge (40 mL/min) for 30 min, the sample was heated from 50 to 900 • C with a heating rate of 7.5 • C/min in Ar flow. The signal of desorbed oxygen was monitored by the MS.

Material Textural Properties
3.1.1. XRD Analysis Figure 1 presented the XRD patterns (2θ: 10-80 • ) of the Cr/Ce catalysts supported by Na-Mt and AlFe-PILC. The diffraction peaks belonging to cristobalite and quartz appear at 19.8 • and 26.7 • (2θ), respectively [33]. Cristobalite and quartz are two of the main components of montmorillonite. They have the characteristics of high temperature resistance, which can ensure the stability of the catalysts in a high temperature gas-solid continuous reaction. Therefore, they play an important role in catalyst components. The diffraction peaks of Fe 2 O 3 appeared in all the AlFe-PILC based catalysts because the amount of Fe 2 O 3 increased after AlFe pillaring process. The diffraction peaks of CeO 2 appeared in Ce/AlFe-PILC catalyst. Compared with Cr/Na-Mt, the diffraction peak intensity of Cr 2 O 3 in Cr/AlFe-PILC clearly decreased, and the result showed that the dispersion of Cr 2 O 3 particles on AlFe-PILC was greatly improved. After adding Ce, the diffraction intensity of Cr 2 O 3 for CrCe(5:1)/AlFe-PILC further decreased. On the one hand, the addition of Ce was beneficial to the dispersion of Cr 2 O 3 , and on the other hand, it may have been due to the reduction of Cr 2 O 3 content. Notably, the CeO 2 diffraction peaks were not found in CrCe(5:1)/AlFe-PILC, possibly because the small amount of CeO 2 was highly dispersed on AlFe-PILC.  Table 1 summarizes the textural properties of samples. SBET and Ames of Na-Mt were only 51 and 41 m 2 /g, and the values of Vp and Vmic were 0.076 and 0.0043 cm 3 /g, respectively. AlFe-PILC's SBET and Vp reached 318 m 2 /g and 0.195 cm 3 /g, respectively, indicating that the formed composite AlFe polycation was relatively large and thus the clay layers were further stripped to form more porous structures. The Vmic of AlFe-PILC was 0.077 cm 3 /g, and it was about 39.5% in Vp. Compared with the Na-Mt and AlFe-PILC support, the supported Cr or CrCe catalysts exhibited lower SBET and Vp values, indicating that some of the Cr and Ce ions migrated into the pores and clay layers, and thus blocked  Table 1 summarizes the textural properties of samples. S BET and A mes of Na-Mt were only 51 and 41 m 2 /g, and the values of V p and V mic were 0.076 and 0.0043 cm 3 /g, respectively. AlFe-PILC's S BET and V p reached 318 m 2 /g and 0.195 cm 3 /g, respectively, indicating that the formed composite AlFe polycation was relatively large and thus the clay layers were further stripped to form more porous structures. The V mic of AlFe-PILC was 0.077 cm 3 /g, and it was about 39.5% in V p . Compared with the Na-Mt and AlFe-PILC support, the supported Cr or CrCe catalysts exhibited lower S BET and V p values, indicating that some of the Cr and Ce ions migrated into the pores and clay layers, and thus blocked some of the pores. It was worth noting that a large number of micro-mesoporosity in AlFe-PILC support was favorable for good dispersion of active species and rapid diffusion of reactants, thus significantly enhanced their catalytic activity of various reactions. In Figure 2a, N 2 adsorption/desorption isotherms for all the materials were type IV, while its type H3 adsorption-desorption hysteresis appeared at P/P 0 above 0.45, indicating that the material had a mesoporous structure and the pores in the material were slit pores formed by layer-like structures. The adsorption amount of Na-Mt was low; however, AlFe-PILC had a pronounced increase in adsorption because more pores were formed by AlFe polyoxycations. The addition of Cr 2 O 3 and CeO 2 to Na-Mt and AlFe-PILC decreased N 2 adsorption capacity and thus pore volume, indicating the doped cations entered and/or blocked the pores of Na-Mt and AlFe-PILC. In Figure 2b, the average mesoporous diameters of AlFe-PILC materials were distributed in a narrow range of 3.96 nm and were wider than the pore-diameter distribution range of Na-Mt (3.10 nm), confirming the pore size was increased after pillaring. The average pore size of CrCe(5:1)/AlFe-PILC was in a narrow region of approximately 3.65 nm. The stability of AlFe-PILC support was good and the mesoporous structure was not destroyed.  In Figure 2a, N2 adsorption/desorption isotherms for all the materials were type IV, while its type H3 adsorption-desorption hysteresis appeared at P/P0 above 0.45, indicating that the material had a mesoporous structure and the pores in the material were slit pores formed by layer-like structures. The adsorption amount of Na-Mt was low; however, AlFe-PILC had a pronounced increase in adsorption because more pores were formed by AlFe polyoxycations. The addition of Cr2O3 and CeO2 to Na-Mt and AlFe-PILC decreased N2 adsorption capacity and thus pore volume, indicating the doped cations entered and/or blocked the pores of Na-Mt and AlFe-PILC. In Figure 2b, the average mesoporous diameters of AlFe-PILC materials were distributed in a narrow range of 3.96 nm and were wider than the pore-diameter distribution range of Na-Mt (3.10 nm), confirming the pore size was increased after pillaring. The average pore size of CrCe(5:1)/AlFe-PILC was in a narrow region of approximately 3.65 nm. The stability of AlFe-PILC support was good and the mesoporous structure was not destroyed.  Figure 3 shows HRTEM picture and the EDS spectra of CrCe(5:1)/AlFe-PILC. It can be seen that CrCe(5:1)/AlFe-PILC had a layered structure and the active particles (5-10 nm in size) were uniformly distributed throughout the support, and the layered structure of AlFe-PILC was not damaged after loading active ingredients. Al, Fe, Cr, Ce, O, and other elements were identified in the EDS spectra, which confirmed that the active species (Cr and Ce) were successfully loaded on the surface of AlFe-PILC. The results indicated that AlFe-PILC was a good support for highly dispersed active species. All these properties were conducive to improving the catalytic degradation of CB.  Figure 3 shows HRTEM picture and the EDS spectra of CrCe(5:1)/AlFe-PILC. It can be seen that CrCe(5:1)/AlFe-PILC had a layered structure and the active particles (5-10 nm in size) were uniformly distributed throughout the support, and the layered structure of AlFe-PILC was not damaged after loading active ingredients. Al, Fe, Cr, Ce, O, and other elements were identified in the EDS spectra, which confirmed that the active species (Cr and Ce) were successfully loaded on the surface of AlFe-PILC. The results indicated that AlFe-PILC was a good support for highly dispersed active species. All these properties were conducive to improving the catalytic degradation of CB.   Figure 4 presents the conversions of CB combustion on various catalysts. In Figure 4a, Cr/Na-Mt exhibited poor performance and did not fully convert CB until 460 °C. Cr/AlFe-PILC caused complete degradation of CB at 320 °C, about 140 °C lower than the degradation temperature of CB  Figure 4 presents the conversions of CB combustion on various catalysts. In Figure 4a, Cr/Na-Mt exhibited poor performance and did not fully convert CB until 460 • C. Cr/AlFe-PILC caused complete degradation of CB at 320 • C, about 140 • C lower than the degradation temperature of CB required for Cr/Na-Mt. The conversion of Ce/AlFe-PILC was negligibly low, and it was 87%, even at a reaction temperature of 500 • C. Ceria doping significantly improved the catalytic activities of Cr/Na-Mt and Cr/AlFe-PILC. In addition, the molar ratios of Cr/Ce (2.5, 5, 7.5, and 10) had an effect on the catalytic performance of CrCe/AlFe-PILC (Figure 4b). The catalysts exhibited a lower performance when the Cr/Ce ratio was less than 5, possibly indicating Cr 2 O 3 was the active species and CeO 2 acted as an assistant. The catalyst performance decreased when the Cr/Ce molar ratio was larger than 5, possibly because less oxygen vacancies existed with a relatively lower amount of CeO 2 . Therefore, the content of CeO 2 was one of the key factors to improving the performance of CrCe/AlFe-PILC. In particular, CrCe(5:1)/AlFe-PILC had the highest catalytic performance and could completely degrade CB at about 290 • C. No Cl 2 or other byproducts were detected, showing that the catalyst had good selectivity for HCl without producing secondary pollution. Figure 5 shows the curves of CB over CrCe(5:1)/AlFe-PILC in the continuous reaction process. There was no significant drop for catalytic activities within 1000 h tests, suggesting that the CrCe(5:1)/AlFe-PILC catalyst was durable. Moreover, this catalyst also displayed good catalytic performances in the presence of 100 ppm toluene or 1% water vapor, further indicating its high potential for industrial application.

CB Combustion and Durability Test
on the catalytic performance of CrCe/AlFe-PILC (Figure 4b). The catalysts exhibited a lower performance when the Cr/Ce ratio was less than 5, possibly indicating Cr2O3 was the active species and CeO2 acted as an assistant. The catalyst performance decreased when the Cr/Ce molar ratio was larger than 5, possibly because less oxygen vacancies existed with a relatively lower amount of CeO2. Therefore, the content of CeO2 was one of the key factors to improving the performance of CrCe/AlFe-PILC. In particular, CrCe(5:1)/AlFe-PILC had the highest catalytic performance and could completely degrade CB at about 290 °C. No Cl2 or other byproducts were detected, showing that the catalyst had good selectivity for HCl without producing secondary pollution.   Figure 5 shows the curves of CB over CrCe(5:1)/AlFe-PILC in the continuous reaction process. There was no significant drop for catalytic activities within 1000 h tests, suggesting that the CrCe(5:1)/AlFe-PILC catalyst was durable. Moreover, this catalyst also displayed good catalytic performances in the presence of 100 ppm toluene or 1% water vapor, further indicating its high potential for industrial application.   Figure 6 presents the effect of CB inlet concentrations on the catalytic performance of CrCe(5:1)/AlFe-PILC. The change of its inlet concentration had a great influence on CB conversion from 500 to 2500 ppm. Furthermore, when the inlet concentration was in the range of 500 to 1500 ppm, CB conversion increased appreciably. This was primarily because low concentration CB only provided a small amount for chemisorbed CB on catalyst active sites and could act as the controlling factor of the reaction. However, as the concentration of CB continued to increase, CB conversion  Figure 6 presents the effect of CB inlet concentrations on the catalytic performance of CrCe(5:1)/AlFe-PILC. The change of its inlet concentration had a great influence on CB conversion from 500 to 2500 ppm. Furthermore, when the inlet concentration was in the range of 500 to 1500 ppm, CB conversion increased appreciably. This was primarily because low concentration CB only provided a small amount for chemisorbed CB on catalyst active sites and could act as the controlling factor of the reaction. However, as the concentration of CB continued to increase, CB conversion decreased until it was completely prohibited, which may be related to chemisorbed oxygen on the catalyst active sites becoming the reaction controlling factor [34]. The result indicated that CB degradation combustion proceeds via a Langmuir-Hinshelwood (L-H) mechanism, and this catalyst could be used for removing CB waste gases with a wide range of concentrations.  Figure 7 shows the effect of GHSV on the CB catalytic combustion activities over CrCe(5:1)/AlFe-PILC. GHSV is the gas hourly space velocity. To calculate this parameter, the flow rate of feed gas (involved inert and main components) can be adjusted. Then, GHSV is the ratio of gas flow rate in standard conditions to the volume of the catalyst. Increasing GHSV slightly decreased the catalytic performance, indicating that this catalyst was highly effective for CB destruction in different reaction conditions. The catalyst active sites were already fully occupied, even with the lowest GHSV used in this work, and more reactant molecules provided by high GHSV could not be chemisorbed and reacted. Thus, high temperature was required to obtain the same conversion with high GHSV.  Figure 7 shows the effect of GHSV on the CB catalytic combustion activities over CrCe(5:1)/AlFe-PILC. GHSV is the gas hourly space velocity. To calculate this parameter, the flow rate of feed gas (involved inert and main components) can be adjusted. Then, GHSV is the ratio of gas flow rate in standard conditions to the volume of the catalyst. Increasing GHSV slightly decreased the catalytic performance, indicating that this catalyst was highly effective for CB destruction in different reaction conditions. The catalyst active sites were already fully occupied, even with the lowest GHSV used in this work, and more reactant molecules provided by high GHSV could not be chemisorbed and reacted. Thus, high temperature was required to obtain the same conversion with high GHSV.

H2-TPR Analysis
H2-TPR profiles of the catalysts are shown in Figure 8. The reduction of Fe2O3 species was obvious in all the catalysts (γ peak), which was from the relatively high contents of Fe2O3 (4.45% in the original clay) [35,36]. Compared with the γ peak area from Cr/Na-Mt, the areas from Cr/AlFe-PILC and CrCe(5:1)/AlFe-PILC increased, revealing that more iron oxide species were formed as Fe2O3 pillars in AlFe pillaring. In the case of Ce/Na-Mt, it was beneficial for the reduction of surface and bulk CeO2 to have two reduction peaks at 541 and 745 °C. There were two reduction peaks below 650 °C in the Na-Mt and AlFe-PILC-supported Cr catalysts, which indicated that peaks α1 and α2 were the reduction peaks of the surface and inside Cr2O3, respectively. For CrCe(5:1)/AlFe-PILC, peaks α1 and α2 were divided into two or three peaks, which suggested the better-dispersed Cr2O3 on the AlFe-PILC support. Compared with Cr/Na-Mt, the reduction peaks of CrCe(5:1)/AlFe-PILC systematically shifted to lower temperatures, indicating CeO2 improved the reducibility of Cr2O3 by increasing Cr2O3 dispersion and lattice oxygen mobility. The peak β2 of CrCe(5:1)/AlFe-PILC at 588 °C was the reduction peak of bulk CeO2, and the peak of surface CeO2 overlapped with peak α2. The CeO2 reduction peak was shifted toward lower temperatures compared with that from Ce/Na-Mt. This shift occurred because Cr2O3 underwent a stronger oxidation process and could be more easily reduced, allowing it to interact with CeO2 to produce a reduction peak at a lower temperature. It suggested that the interaction between Cr2O3 and CeO2 species weakened the Ce-O bond and promoted the reduction of CeO2. The α peak temperatures followed: Cr/Na-Mt > Cr/AlFe-PILC > CrCe(5:1)/AlFe-PILC. The results indicated that the interaction between Cr2O3 and CeO2 species could improve the mobility of oxygen species in the catalysts, thus improving the reduction of both Cr2O3 and CeO2 species.

H 2 -TPR Analysis
H 2 -TPR profiles of the catalysts are shown in Figure 8. The reduction of Fe 2 O 3 species was obvious in all the catalysts (γ peak), which was from the relatively high contents of Fe 2 O 3 (4.45% in the original clay) [35,36]. Compared with the γ peak area from Cr/Na-Mt, the areas from Cr/AlFe-PILC and CrCe(5:1)/AlFe-PILC increased, revealing that more iron oxide species were formed as Fe 2 O 3 pillars in AlFe pillaring. In the case of Ce/Na-Mt, it was beneficial for the reduction of surface and bulk CeO 2 to have two reduction peaks at 541 and 745 • C. There were two reduction peaks below 650 • C in the Na-Mt and AlFe-PILC-supported Cr catalysts, which indicated that peaks α 1 and α 2 were the reduction peaks of the surface and inside Cr 2 O 3 , respectively. For CrCe(5:1)/AlFe-PILC, peaks α 1 and α 2 were divided into two or three peaks, which suggested the better-dispersed Cr 2 O 3 on the AlFe-PILC support. Compared with Cr/Na-Mt, the reduction peaks of CrCe(5:1)/AlFe-PILC systematically shifted to lower temperatures, indicating CeO 2 improved the reducibility of Cr 2 O 3 by increasing Cr 2 O 3 dispersion and lattice oxygen mobility. The peak β 2 of CrCe(5:1)/AlFe-PILC at 588 • C was the reduction peak of bulk CeO 2 , and the peak of surface CeO 2 overlapped with peak α 2 . The CeO 2 reduction peak was shifted toward lower temperatures compared with that from Ce/Na-Mt. This shift occurred because Cr 2 O 3 underwent a stronger oxidation process and could be more easily reduced, allowing it to interact with CeO 2 to produce a reduction peak at a lower temperature. It suggested that the interaction between Cr 2 O 3 and CeO 2 species weakened the Ce-O bond and promoted the reduction of CeO 2. The α peak temperatures followed: Cr/Na-Mt > Cr/AlFe-PILC > CrCe(5:1)/AlFe-PILC. The results indicated that the interaction between Cr 2 O 3 and CeO 2 species could improve the mobility of oxygen species in the catalysts, thus improving the reduction of both Cr 2 O 3 and CeO 2 species.

TPD and TPSR Analysis
The adsorption/desorption of CB, catalytic combustion behavior, and the evolution of the main products (CO 2 , H 2 O, and HCl) over the catalysts were investigated using CB-TPD/TPSR techniques ( Figure 9). As it was mentioned previously, the CB combustion on CrCe(5:1)/AlFe-PILC catalyst proceeded via an L-H mechanism, where the adsorption of reactants on the catalyst active sites was a critical step. In Figure 9a, CrCe(5:1)/Na-Mt and CrCe(5:1)/AlFe-PILC showed different CB adsorption capacities. The CB absorption capacities of CrCe(5:1)/AlFe-PILC (44.8 µmol/g) was obviously stronger than CrCe(5:1)/Na-Mt (7.9 µmol/g) by integrating over the absorption spectra. The above results fully proved that clay materials with larger S BET , V p , and d 001 -value favor CB adsorption. In Figure 9b, the temperature of CB desorption peaked for CrCe(5:1)/Na-Mt and CrCe(5:1)/AlFe-PILC were 145 • C and 198 • C, respectively, indicating that the interaction of CB and CrCe(5:1)/AlFe-PILC was stronger than with CrCe(5:1)/Na-Mt. Therefore, CB could remain inside the pores or outside the surface of CrCe(5:1)/AlFe-PILC for a longer time, being conducive to the adsorption and catalytic degradation of CB. The results indicated that improved structure and the strong interaction between CrCe mixed oxides with AlFe-PILC enhanced the adsorption of CB. As shown in Figure 9c,d, CB desorption was accompanied with CB combustion under O2/Ar, and the adsorbed CB species reacted with lattice O from CrCeOx to form CO2, H2O, and HCl. CB was completely reacted over CrCe(5:1)/AlFe-PILC at about 300 °C, while it needed about 440 °C on CrCe(5:1)/Na-Mt. CO2, H2O, and HCl were detected, but CO and Cl2 were not detected, indicating that the catalysts in the study had high selectivity to HCl and CO2 formation. It was notable that the peak temperature of the products for CrCe(5:1)/Na-Mt was at 413 °C, which was much higher than that of CB desorption peak temperature (145 °C). However, the peak temperature of product for CrCe(5:1)/AlFe-PILC was at 275 °C, which was close to that of CB desorption (198 °C). This phenomenon can explain why CrCe(5:1)/AlFe-PILC had the highest CB degradation activities compared to other catalysts in this work. The larger overlapped region between CB desorption and catalytic combustion, the better the catalytic performance. Therefore, tuning the CB adsorption and catalytic properties was a key to designing an efficient catalyst for CB catalytic combustion.
In order to find out the relationship between the oxygen species absorbed on the catalyst surface and the catalytic properties, O2-TPD were investigated from 50 to 900 °C . The O2-TPD plots for Cr and CrCe metal oxide catalysts consisted of oxygen desorption regions shown in Figure 10. There were three types of desorption peaks, the α desorption peak, the β desorption peak, and the γ desorption peak. Furthermore, these three peaks could be assigned to superoxide ion O2 − , peroxide ion O2 2− /O − , and lattice oxygen ion O 2− , respectively [37,38]. Increasing the temperature is beneficial to increase the rate of desorption and transformation of superoxide species into O2 2− , O − , and Olattice 2− [39]. It can be seen that the α and β desorption peaks follow: CrCe(5:1)/AlFe-PILC < Cr/AlFe-PILC < CrCe(5:1)/Na-Mt < Cr/Na-Mt, which was in good agreement with the aforementioned catalytic performance of CB combustion. It was worth mentioning that the total amount of surface-active oxygen species, in terms of the sum of α and β desorption areas, follows the same sequence of the peaks. It can be observed from the γ desorption peak that adding CeO2 increased the desorption area As shown in Figure 9c,d, CB desorption was accompanied with CB combustion under O 2 /Ar, and the adsorbed CB species reacted with lattice O from CrCeO x to form CO 2 , H 2 O, and HCl. CB was completely reacted over CrCe(5:1)/AlFe-PILC at about 300 • C, while it needed about 440 • C on CrCe(5:1)/Na-Mt. CO 2 , H 2 O, and HCl were detected, but CO and Cl 2 were not detected, indicating that the catalysts in the study had high selectivity to HCl and CO 2 formation. It was notable that the peak temperature of the products for CrCe(5:1)/Na-Mt was at 413 • C, which was much higher than that of CB desorption peak temperature (145 • C). However, the peak temperature of product for CrCe(5:1)/AlFe-PILC was at 275 • C, which was close to that of CB desorption (198 • C). This phenomenon can explain why CrCe(5:1)/AlFe-PILC had the highest CB degradation activities compared to other catalysts in this work. The larger overlapped region between CB desorption and catalytic combustion, the better the catalytic performance. Therefore, tuning the CB adsorption and catalytic properties was a key to designing an efficient catalyst for CB catalytic combustion.
In order to find out the relationship between the oxygen species absorbed on the catalyst surface and the catalytic properties, O 2 -TPD were investigated from 50 to 900 • C. The O 2 -TPD plots for Cr and CrCe metal oxide catalysts consisted of oxygen desorption regions shown in Figure 10. There were three types of desorption peaks, the α desorption peak, the β desorption peak, and the γ desorption peak. Furthermore, these three peaks could be assigned to superoxide ion O 2 − , peroxide ion O 2 2− /O − , and lattice oxygen ion O 2− , respectively [37,38]. Increasing the temperature is beneficial to increase the rate of desorption and transformation of superoxide species into O 2 2− , O − , and O lattice 2− [39].
It can be seen that the α and β desorption peaks follow: CrCe(5:1)/AlFe-PILC < Cr/AlFe-PILC < CrCe(5:1)/Na-Mt < Cr/Na-Mt, which was in good agreement with the aforementioned catalytic performance of CB combustion. It was worth mentioning that the total amount of surface-active oxygen species, in terms of the sum of α and β desorption areas, follows the same sequence of the peaks. It can be observed from the γ desorption peak that adding CeO 2 increased the desorption area of lattice oxygen ion O 2− compared with the non-doped catalyst. Thus CrCe(5:1)/AlFe-PILC exhibited the highest oxidation performance since electrophilic O ads (O 2 − , O 2 2− , O − ) played a critical role in the complete oxidation of organic compounds [40].

Conclusions
In this paper, AlFe-PILC supported CrCe mixed oxides are synthesized and used for adsorption/desorption and catalytic combustion of CB. A series of characterization methods were used to investigate the structure and redox properties of these materials, including HRTEM-EDS, H2-TPR, TPD/TPSR, and O2-TPD. Comparing the results of the SBET, Vp, and d001-value, AlFe-PILC performed better than Na-Mt. Without doubt, AlFe-PILC synthesized in this study constituted a class of porous materials with excellent properties. A large number of micro-mesoporosity and Ce was added to the AlFe-PILC to optimize its structure and improve the dispersion of Cr2O3 particles on the AlFe-PILC. XRD analysis and HRTEM images clearly revealed the stable layered structure with the d001 value ≈1.91 nm and well-dispersed active species in AlFe-PILC. The addition of Ce and optimized structure of support greatly improved the oxidative property of Cr2O3. CB-TPD experiments reveal that the optimized structure coupled with the strong interaction between CrCe metal oxides and AlFe-PILC enhanced CB adsorption capacity and adsorption strength. CB-TPSR results showed that the larger the overlapped region between CB desorption and the catalytic combustion, the better the catalytic performance. In particular, CrCe(5:1)/AlFe-PILC show an excellent catalytic property, and stability was due to the lower temperature of completely degraded CB (approximately 290 °C) and the conversion remained stable for 1000 h. CB catalytic combustion on CrCe/AlFe-PILC catalyst was via a Langmuir-Hinshelwood mechanism, and adjusting adsorption/desorption properties was one

Conclusions
In this paper, AlFe-PILC supported CrCe mixed oxides are synthesized and used for adsorption/desorption and catalytic combustion of CB. A series of characterization methods were used to investigate the structure and redox properties of these materials, including HRTEM-EDS, H 2 -TPR, TPD/TPSR, and O 2 -TPD. Comparing the results of the S BET , V p , and d 001 -value, AlFe-PILC performed better than Na-Mt. Without doubt, AlFe-PILC synthesized in this study constituted a class of porous materials with excellent properties. A large number of micro-mesoporosity and Ce was added to the AlFe-PILC to optimize its structure and improve the dispersion of Cr 2 O 3 particles on the AlFe-PILC. XRD analysis and HRTEM images clearly revealed the stable layered structure with the d 001 value ≈1.91 nm and well-dispersed active species in AlFe-PILC. The addition of Ce and optimized structure of support greatly improved the oxidative property of Cr 2 O 3 . CB-TPD experiments reveal that the optimized structure coupled with the strong interaction between CrCe metal oxides and AlFe-PILC enhanced CB adsorption capacity and adsorption strength. CB-TPSR results showed that the larger the overlapped region between CB desorption and the catalytic combustion, the better the catalytic performance. In particular, CrCe(5:1)/AlFe-PILC show an excellent catalytic property, and stability was due to the lower temperature of completely degraded CB (approximately 290 • C) and the conversion remained stable for 1000 h. CB catalytic combustion on CrCe/AlFe-PILC catalyst was via a Langmuir-Hinshelwood mechanism, and adjusting adsorption/desorption properties was one of the most important factors for designing efficient catalysts. CrCe/AlFe-PILC also exhibited good durability for CB destruction, both in the humid condition and in the presence of toluene; therefore, this catalyst deserves wide attention and it is a potential prospect for industrial application.