Catalytic Abatement of Nitrous Oxide Coupled with Ethane Oxydehydrogenation over Mesoporous Cr / Al 2 O 3 Catalyst

Waste nitrous oxide (N2O) was utilized as an oxidant for ethane oxydehydrogenation reaction at the temperature range from 450 ◦C to 700 ◦C over the mesoporous Cr/Al2O3 catalyst synthesized via the one-pot evaporation-induced self-assembly (EISA) method. The catalyst was characterized by X-ray diffraction, transmission electron microscopy, and nitrogen adsorption-desorption analysis. The obtained mesoporous material with favorable textural property and advantageous thermal stability was investigated as the catalyst for ethane oxydehydrogenation. It was found that the utilization of N2O as an oxidant for the oxydehydrogenation reaction of ethane resulted in simultaneous and complete N2O abatement. Moreover, the catalytic conversion of C2H6 to C2H4 was increased from 18% to 43% as the temperature increased from 450 ◦C to 700 ◦C. The increased N2O concentration from 5 vol % to 20 vol % resulted in an increased ethane conversion but decreased ethylene selectivity because the nonselective reactions occurred. Ethane was converted into ethylene with approximately 51% selectivity and 22% yield at 700 ◦C and N2O concentration of 10%. After a catalytic steady state was reached, no obvious decline was observed during a 15 h evaluation period.


Introduction
Nitrous oxide (N 2 O), a waste by-product and an environmental pollutant generated mainly from anthropogenic emissions and industrial processes such as adipic acid and nitric acid plants, has more than 300 times greater potential than CO 2 to cause global warming and ozone depletion [1].The abatement of N 2 O has thus been a topic of environmental relevance.The concentration of N 2 O for environmental catalytic after-treatment usually varies from 0 vol % to 50 vol %, and the N 2 O abatement in adipic acid production has been most effectively tackled due to the high concentration and limited point source [2].Among various commercial N 2 O removal technologies, the catalytic or thermal decomposition of N 2 O was widely applied.Based on the conversion of N 2 O to N 2 and O 2 reaction, the oxygen species characterized with different reactivity and different thermal stability [3,4] were formed, making N 2 O an excellent optional oxidant for many oxidation reactions.Solutia Inc. employed N 2 O as an oxidant, and this technology has recently been commercialized to produce phenol from benzene [5,6].Kondratenko et al. [7] and Novoveska et al. [8] also used N 2 O as an oxidant for the production of propene from oxidative dehydrogenation of propane, and it seems to be one of the promising and most elegant methods for the transformation of light paraffins to olefins.In this regard, a process that combines the N 2 O abatement with the simultaneous production of valuable chemical fuels would be environmentally and economically attractive, and also reveals a new perspective for the utilization of light paraffins.
Catalytic dehydrogenation of light alkanes is of important significance since it upgrades low cost paraffins feedstock into valuable olefins product [9][10][11].Nowadays, ethane dehydrogenation has become a crucial way to meet the increasing demand of ethylene, which can be used to synthesize polymers, ethane oxide, and many other basic and intermediate products [12].Supported chromium oxide catalysts were found to be effective for alkane dehydrogenation, among which a Cr/Al 2 O 3 catalyst has been used for years on a commercial scale [9][10][11].Former experimental and theoretical studies reported that the carbon deposit was identified as the main source for the deactivation of supported chromia catalysts during alkane dehydrogenation reaction [13,14].Sullivan et al. [15] investigated the process of coke deposition over chromia-alumina during propane dehydrogenation using in-situ UV Raman spectroscopy analysis.They observed the polynuclear aromatic hydrocarbons and polyenes when reaction temperature was relatively low (400 • C), and the conjugated olefins when increasing the temperature to >500 • C. Nowadays, there is an increasing body of evidence that the introduction of mesoporosity into the catalyst material could be a way to solve the deactivation of the catalyst due to carbonaceous deposits [16][17][18][19].Hartmann et al. [17], Schwieger et al. [18], and Su et al. [19] introduced a substantial amount of mesopores into the microporous structured zeolite of Fe/ZSM-5, and an improved catalyst longevity was obtained during the oxidation of benzene to phenol process.
In this contribution, a highly active and stable chromia-alumina catalyst was proposed, and the textural properties were determined.A well-developed mesoporous structure and a relatively high surface area of the present material were presented.In addition, nitrous oxide was utilized as an oxidant for the ethane oxydehydrogenation reaction in the presence of mesoporous chromia-alumina catalyst.Our concept is based on the combination of the exothermic N 2 O decomposition with the thermal dehydrogenation of ethane, with the aim to completely remove the N 2 O and simultaneously the production of C 2 H 4 from C 2 H 6 .

Results
The small angle X-ray diffraction is a useful tool for studying the ordered mesoporous material and has been widely used to access the structure of a sample [20,21].Figure 1 displays the small angle XRD pattern of the as-prepared Cr/Al 2 O 3 .A strong [1 0 0] diffraction peak around 1.1 • was presented, indicating the formation of p6mm two-dimensional hexagonal ordered mesoporous structure.The wide-angle XRD pattern of Cr/Al 2 O 3 is shown in the inset of Figure 1.Two broad peaks were obtained at the ranges of 10 • -40 • and 50 • -80 • , respectively, illustrating the formation of the mesoporous γ-Al 2 O 3 phase.It should be pointed out that no apparent XRD signal of a chromium-based compound was detected, indicating the high dispersion of chromium species among the mesoporous skeleton of the γ-Al 2 O 3 material.The obtained long-range ordered mesoporous structure was further confirmed by the TEM results, as expressly displayed in Figure 2. The alignment of cylindrical pores were distinctly observed along [1 1 0] direction (Figure 2a,b), and the hexagonal arrangement of pores were typical highly ordered along [0 0 1] direction (Figure 2b).It is noteworthy that no additional isolated chromium-based particle was detected over the entire network, and the introduction of Cr via the one-pot EISA method did not destroy the mesoporous structure of alumina, which is consistent with the XRD characterization results.The combination of the XRD and TEM results convinced us that the ordered mesoporous Cr/Al 2 O 3 was easily obtained and the mesoporous framework was successfully preserved after 700 • C calcination, demonstrating good thermal stability.The formation of mesoporous Cr/Al2O3 was also supported by nitrogen adsorption-desorption analysis.Figure 3 shows the nitrogen adsorption and desorption isotherms as well as the pore size distribution as an inset.As shown in Figure 3, the sample calcined at 700 °C gave type IV curve with H1-shaped hysteresis loop, which was the typical feature of ordered mesoporous materials with uniform mesopores [22,23] among the framework of the Cr/Al2O3 material.The deduction of H1shaped hysteresis loops in Figure 3 further confirmed the cylindrical pores along [1 1 0] in TEM analysis.Furthermore, after being calcined at 700 °C, Cr/Al2O3 had a large BET surface area of 205 m 2 g −1 and a pore volume of 0.34 cm 3 g −1 .In addition, the pore size distribution exhibited in the inset of Figure 3 was extremely narrow, and the position of the peak was located in 5.2 nm.The large surface area and narrow pore size distribution of this ordered mesoporous alumina-based material, in combination with the advantageous thermal stability, enhance its potential applications in catalysis.The formation of mesoporous Cr/Al2O3 was also supported by nitrogen adsorption-desorption analysis.Figure 3 shows the nitrogen adsorption and desorption isotherms as well as the pore size distribution as an inset.As shown in Figure 3, the sample calcined at 700 °C gave type IV curve with H1-shaped hysteresis loop, which was the typical feature of ordered mesoporous materials with uniform mesopores [22,23] among the framework of the Cr/Al2O3 material.The deduction of H1shaped hysteresis loops in Figure 3 further confirmed the cylindrical pores along [1 1 0] in TEM analysis.Furthermore, after being calcined at 700 °C, Cr/Al2O3 had a large BET surface area of 205 m 2 g −1 and a pore volume of 0.34 cm 3 g −1 .In addition, the pore size distribution exhibited in the inset of Figure 3 was extremely narrow, and the position of the peak was located in 5.2 nm.The large surface area and narrow pore size distribution of this ordered mesoporous alumina-based material, in combination with the advantageous thermal stability, enhance its potential applications in catalysis.The formation of mesoporous Cr/Al 2 O 3 was also supported by nitrogen adsorption-desorption analysis.Figure 3 shows the nitrogen adsorption and desorption isotherms as well as the pore size distribution as an inset.As shown in Figure 3, the sample calcined at 700 • C gave type IV curve with H1-shaped hysteresis loop, which was the typical feature of ordered mesoporous materials with uniform mesopores [22,23] among the framework of the Cr/Al 2 O 3 material.The deduction of H1-shaped hysteresis loops in Figure 3 further confirmed the cylindrical pores along [1 1 0] in TEM analysis.Furthermore, after being calcined at 700 • C, Cr/Al 2 O 3 had a large BET surface area of 205 m 2 g −1 and a pore volume of 0.34 cm 3 g −1 .In addition, the pore size distribution exhibited in the inset of Figure 3 was extremely narrow, and the position of the peak was located in 5.2 nm.The large surface area and narrow pore size distribution of this ordered mesoporous alumina-based material, in combination with the advantageous thermal stability, enhance its potential applications in catalysis.The XPS spectrum for fresh Cr/Al2O3 catalyst in the Cr 2p binding energy region is shown in Figure 4.The experimental curve was fitted with two characteristic peaks centered at 577.1 and 579.5 eV, typical of the corresponding Cr 3+ and Cr 6+ species [24,25], respectively.This indicated the presence of chromium with two oxidation states, and the calculated Cr 6+ /Cr 3+ ratio was 2.23, demonstrating that most chromium species existed as Cr 6+ in the fresh Cr/Al2O3 catalyst.The catalytic performance of the mesoporous Cr/Al2O3 material in N2O abatement with simultaneous C2H4 production from the oxydehydrogenation of C2H6 at various temperatures is shown in Figure 5.It is seen that the N2O conversion (XN2O) to N2 was completed (100%) in the whole temperature range, simultaneously the catalytic conversion of C2H6 (XC2H6) to C2H4 was increased from 18% to 43% with the increasing temperature from 450 °C to 700 °C.This catalytic activity was higher than that of the bulk Cr2O3 (<3% ethane conversion at 650 °C) and the mesoporous silicasupported chromium oxide catalyst (8.2%-18.7%ethane conversion) for ethane dehydrogenation reported by Sayari et al. [14].For ethane oxydehydrogenation using N2O as an oxidant, Held et al.The XPS spectrum for fresh Cr/Al 2 O 3 catalyst in the Cr 2p binding energy region is shown in Figure 4.The experimental curve was fitted with two characteristic peaks centered at 577.1 and 579.5 eV, typical of the corresponding Cr 3+ and Cr 6+ species [24,25], respectively.This indicated the presence of chromium with two oxidation states, and the calculated Cr 6+ /Cr 3+ ratio was 2.23, demonstrating that most chromium species existed as Cr 6+ in the fresh Cr/Al 2 O 3 catalyst.The XPS spectrum for fresh Cr/Al2O3 catalyst in the Cr 2p binding energy region is shown in Figure 4.The experimental curve was fitted with two characteristic peaks centered at 577.1 and 579.5 eV, typical of the corresponding Cr 3+ and Cr 6+ species [24,25], respectively.This indicated the presence of chromium with two oxidation states, and the calculated Cr 6+ /Cr 3+ ratio was 2.23, demonstrating that most chromium species existed as Cr 6+ in the fresh Cr/Al2O3 catalyst.The catalytic performance of the mesoporous Cr/Al2O3 material in N2O abatement with simultaneous C2H4 production from the oxydehydrogenation of C2H6 at various temperatures is shown in Figure 5.It is seen that the N2O conversion (XN2O) to N2 was completed (100%) in the whole temperature range, simultaneously the catalytic conversion of C2H6 (XC2H6) to C2H4 was increased from 18% to 43% with the increasing temperature from 450 °C to 700 °C.This catalytic activity was higher than that of the bulk Cr2O3 (<3% ethane conversion at 650 °C) and the mesoporous silicasupported chromium oxide catalyst (8.2%-18.7%ethane conversion) for ethane dehydrogenation reported by Sayari et al. [14].For ethane oxydehydrogenation using N2O as an oxidant, Held et al.The catalytic performance of the mesoporous Cr/Al 2 O 3 material in N 2 O abatement with simultaneous C 2 H 4 production from the oxydehydrogenation of C 2 H 6 at various temperatures is shown in Figure 5.It is seen that the N 2 O conversion (X N2O ) to N 2 was completed (100%) in the whole temperature range, simultaneously the catalytic conversion of C 2 H 6 (X C2H6 ) to C 2 H 4 was increased from 18% to 43% with the increasing temperature from 450 • C to 700 • C.This catalytic activity was higher than that of the bulk Cr 2 O 3 (<3% ethane conversion at 650 • C) and the mesoporous silica-supported chromium oxide catalyst (8.2%-18.7%ethane conversion) for ethane dehydrogenation reported by Sayari et al. [14].For ethane oxydehydrogenation using N 2 O as an oxidant, Held et al. [26] investigated the catalytic properties over iron modified different zeolite (ZSM-5, H-Y, mordenite) catalysts.The comparison results indicated that the catalytic performance of mesoporous Cr/Al 2 O 3 herein was lower than that of Fe/ZSM-5, whereas it was superior to the iron modified faujasite and mordenite catalysts.
In the reaction medium, N 2 O was expected to be partly utilized for ethane oxydehydrogenation or for direct decomposition to N 2 and O 2 .In the present work, no O 2 signal was recorded, indicating that all the oxygen species were involved in the reactions.According to the investigations from Pérez-Ramirez et al. [27] and Wang et al. [28], the decomposition of N 2 O over the active site led to the formation of reactive oxygen species, the removal of which from the surface of catalyst is a rate determining step.When C 2 H 6 is used as a reductant, the generated oxygen species participate in oxidation of C 2 H 6 , one could herein conclude that the Cr/Al 2 O 3 catalyst should be responsible not only for N 2 O decomposition but also for the ethane oxydehydrogenation.Moreover, a distinctly higher N 2 O conversion than oxydehydrogenation occurred, which can be ascribed to the nonselective heterogeneous reactions of a portion of oxygen with C 2 H 6 /C 2 H 4 to form CO x during ethane oxydehydrogenation reaction.This is also consistent with the observations by Held et al. [26].Furthermore, the ethylene selectivity was reaction temperature depended.As shown in Figure 5, only 4%-17% of C 2 H 4 selectivity (S C2H4 ) was observed at 450-550 • C, while higher C 2 H 4 selectivity was obtained with increasing operating temperature to 600-700 • C. For example, ethylene selectivity of 51% with 22% ethylene yield (Y C2H4 ) was achieved at 700 • C. Considering the complete N 2 O conversion in the whole temperature range, the ratio of generated oxygen to reaction intermediates (activated C 2 H 6 ) over the catalyst surface was temperature-dependent.At lower temperatures (450-550 • C), the significantly higher ratio of generated oxygen to reaction intermediates led to nonselective oxidation of C 2 H 6 /C 2 H 4 to CO x , and low ethylene selectivity was obtained.At higher temperatures (600-700 • C), more ethane was activated over the catalyst surface that decreased the ratio of generated oxygen to intermediates, and further increased the ethylene selectivity.In addition, more oxygen species that can selectively convert ethane to ethylene were probably produced at higher temperatures.However, future work should be conducted to gain further understanding of this system.The structure properties of the spent material after catalytic characterization (Figure 5) were characterized in Figure 6 to estimate the thermal stability of mesoporous chromia-alumina catalyst.The XRD pattern (Figure 6a) showed two broad peaks typical of γ-Al2O3 phase, and no obvious chromium-based peak was observed, which is similar to the corresponding fresh sample in Figure 1.Moreover, as depicted in Figure 6b, the highly ordered mesoporous framework was found to be comparable with the fresh catalyst displayed in Figure 2.These findings indicated that the spent catalyst retained its structure properties even after the reaction from 450 °C to 700 °C, further confirming the good thermal stability of mesoporous Cr/Al2O3 catalyst.
The XPS spectrum for the spent catalyst shown in Figure 7 revealed two peaks centered at 577.2 and 579.6 eV due to the presence of both Cr 3+ and Cr 6+ species [24,25].However, a decreased Cr 6+ /Cr 3+ ratio of 0.12 was calculated.These results suggested that a Cr 6+ /Cr 3+ redox cycle was actively involved in the ethane oxydehydrogenation using N2O as an oxidant, and most of the active Cr 6+ species (Figure 4) converted to the lower oxidant state Cr 3+ during the reaction.The XPS spectrum for the spent catalyst shown in Figure ?? revealed two peaks centered at 577.2 and 579.6 eV due to the presence of both Cr 3+ and Cr 6+ species [? ?].However, a decreased Cr 6+ /Cr 3+ ratio of 0.12 was calculated.These results suggested that a Cr 6+ /Cr 3+ redox cycle was actively involved in the ethane oxydehydrogenation using N 2 O as an oxidant, and most of the active Cr 6+ species (Figure ??) converted to the lower oxidant state Cr 3+ during the reaction.The structure properties of the spent material after catalytic characterization (Figure 5) were characterized in Figure 6 to estimate the thermal stability of mesoporous chromia-alumina catalyst.The XRD pattern (Figure 6a) showed two broad peaks typical of γ-Al 2 O 3 phase, and no obvious chromium-based peak was observed, which is similar to the corresponding fresh sample in Figure 1.Moreover, as depicted in Figure 6b, the highly ordered mesoporous framework was found to be comparable with the fresh catalyst displayed in Figure 2.These findings indicated that the spent catalyst retained its structure properties even after the reaction from 450 • C to 700 • C, further confirming the good thermal stability of mesoporous Cr/Al 2 O 3 catalyst.
The XPS spectrum for the spent catalyst shown in Figure 7 revealed two peaks centered at 577.2 and 579.6 eV due to the presence of both Cr 3+ and Cr 6+ species [24,25].However, a decreased Cr 6+ /Cr 3+ ratio of 0.12 was calculated.These results suggested that a Cr 6+ /Cr 3+ redox cycle was actively involved in the ethane oxydehydrogenation using N 2 O as an oxidant, and most of the active Cr 6+ species (Figure 4) converted to the lower oxidant state Cr 3+ during the reaction.The ethane dehydrogenation reaction was investigated over the Cr/Al2O3 catalyst at 700 °C in the absence of N2O, and a dramatic activity decay was observed upon extending the operation time (not shown), which can be attributed to the coke deposition during the reaction.In order to suppress the coke formation, N2O was employed as an oxidant for ethane oxydehydrogenation in the present study.The influence of N2O concentration on the ethane oxydehydrogenation reaction at 700 °C is displayed in Figure 8.It was found that the C2H6 conversion increased from 31% to 54% when changing the N2O concentration from 5 vol % to 20 vol %, which can be ascribed to the formation of more oxygen species from N2O decomposition in the case of a higher N2O concentration.In contrast, a decreased selectivity of ethylene was observed since the higher ratio of oxygen species favored the nonselective reactions (C2H6/C2H4 oxidation to COx).Therefore, a higher ethane conversion was always compromised with a lower ethylene selectivity.In this regard, the fluctuation of ethylene yield (17.8%-20.1%)was not distinct at different N2O concentrations, as shown in Figure 8.The ethane dehydrogenation reaction was investigated over the Cr/Al 2 O 3 catalyst at 700 • C in the absence of N 2 O, and a dramatic activity decay was observed upon extending the operation time (not shown), which can be attributed to the coke deposition during the reaction.In order to suppress the coke formation, N 2 O was employed as an oxidant for ethane oxydehydrogenation in the present study.The influence of N 2 O concentration on the ethane oxydehydrogenation reaction at 700 • C is displayed in Figure 8.It was found that the C 2 H 6 conversion increased from 31% to 54% when changing the N 2 O concentration from 5 vol % to 20 vol %, which can be ascribed to the formation of more oxygen species from N 2 O decomposition in the case of a higher N 2 O concentration.In contrast, a decreased selectivity of ethylene was observed since the higher ratio of oxygen species favored the nonselective reactions (C 2 H 6 /C 2 H 4 oxidation to CO x ).Therefore, a higher ethane conversion was always compromised with a lower ethylene selectivity.In this regard, the fluctuation of ethylene yield (17.8%-20.1%)was not distinct at different N 2 O concentrations, as shown in Figure 8. Figure 9 exemplifies the profiles of the C2H6 conversion, and the selectivity and yield of C2H4 product as a function of time-on-stream over mesoporous Cr/Al2O3 catalyst.Before analysis, the catalyst was run under the feed gas with 10 vol % C2H6 and 5 vol % N2O at a flow rate of 25 mL min −1 at 700 °C for about 6 h to reach a steady state of the catalytic performance.Compared to the initial data after 40 min of feed gas treatment (Figure 8), it was found that the catalytic activity drops to a Figure 9 exemplifies the profiles of the C 2 H 6 conversion, and the selectivity and yield of C 2 H 4 product as a function of time-on-stream over mesoporous Cr/Al 2 O 3 catalyst.Before analysis, the catalyst was run under the feed gas with 10 vol % C 2 H 6 and 5 vol % N 2 O at a flow rate of 25 mL min −1 at 700 • C for about 6 h to reach a steady state of the catalytic performance.Compared to the initial data after 40 min of feed gas treatment (Figure 8), it was found that the catalytic activity drops to a certain degree during 6 h testing, after which no noticeable deactivation of the catalyst was observed during a 15 h evaluation period.It gives an ethane conversion of approximately 30%, a selectivity for ethylene of 58%, and an ethylene yield of 17%, demonstrating the good stability of the mesoporous Cr/Al 2 O 3 catalyst, which is a key parameter regarding to the catalyst development.In view of the above results, the process of using N 2 O as an oxidant for ethane oxydehydrogenation in the presence of mesoporous Cr/Al 2 O 3 catalyst may be considered not only as a useful way to functionalize light paraffin but also as an effective procedure for N 2 O abatement.Figure 9 exemplifies the profiles of the C2H6 conversion, and the selectivity and yield of C2H4 product as a function of time-on-stream over mesoporous Cr/Al2O3 catalyst.Before analysis, the catalyst was run under the feed gas with 10 vol % C2H6 and 5 vol % N2O at a flow rate of 25 mL min −1 at 700 °C for about 6 h to reach a steady state of the catalytic performance.Compared to the initial data after 40 min of feed gas treatment (Figure 8), it was found that the catalytic activity drops to a certain degree during 6 h testing, after which no noticeable deactivation of the catalyst was observed during a 15 h evaluation period.It gives an ethane conversion of approximately 30%, a selectivity for ethylene of 58%, and an ethylene yield of 17%, demonstrating the good stability of the mesoporous Cr/Al2O3 catalyst, which is a key parameter regarding to the catalyst development.In view of the above results, the process of using N2O as an oxidant for ethane oxydehydrogenation in the presence of mesoporous Cr/Al2O3 catalyst may be considered not only as a useful way to functionalize light paraffin but also as an effective procedure for N2O abatement.

Catalyst Preparation
The mesoporous Cr/Al 2 O 3 with 5% molar fraction of chromium was prepared via the evaporation-induced self-assembly (EISA) route as reported [29][30][31].Here, the (EO) 20 (PO) 70 (EO) 20 triblock copolymer (Pluronic P123 ) was used as a structure directing agent.Typically, Pluronic P123 (5.0 g, Sigma-Aldrich Co. LLC., St. Louis, MO, USA) was dissolved in 100 mL of anhydrous ethanol at room temperature.After small vortex stirring for 4 h, 8.0 mL of 67 wt % HNO 3 (Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) and 2.5 mmol of chromium nitrate nonahydrate (Cr(NO 3 ) 3 •9H 2 O, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China, ≥99%), and 47.5 mmol aluminum isopropoxide (Sigma-Aldrich Co. LLC., St. Louis, MO, USA, 98%) were added into the above solution.The total amount of metal species (50 mmol for 5.0 g of P123) was kept constant.After vigorous stirring at room temperature for about 12 h, solvent evaporation was carried out at 60 • C for about 48 h in a drying oven.The resultant solid was homogenized by grinding, and calcined under air atmosphere for 4 h at 400 • C in a muffle furnace (heating rate 1 • C min −1 ).The above sample was further treated at 700 • C for 4 h (3 • C min −1 ramping rate), and the final Cr 2 O 3 -Al 2 O 3 catalyst obtained was denoted as Cr/Al 2 O 3 in present description.

Catalyst Characterization
Powder X-ray diffraction (XRD) images of the sample were collected at room temperature by a Bruker D8 ADVANCE diffractometer using a Cu Kα radiation source.The small angle XRD pattern was collected at the ranges from 0.6 • to 4.4 • , and the wide angle one was from 10 • to 80 • .Transmission electron microscopy (TEM) photos were recorded on a Hitachi H-7650 transmission electron microscope with an EDS detector with a 100 kV working voltage.Nitrogen adsorption-desorption isotherms were obtained with an Autosorb iQ instrument at −196 • C. Prior to the characterization, the sample was degassed under vacuum (1 × 10 −5 Torr) at 200 • C for about 6 h.The surface area was calculated using the Brunauer-Emmett-Teller (BET) method.The Barrett-Joyner-Halenda (BJH) method was utilized to calculate the pore size distribution with Kruk-Jaroniec-Sayari (KJS) correction [22].The pore volume was calculated based on the volume of liquid nitrogen adsorbed at approximately p/p 0 = 1.The X-ray photoelectron spectra (XPS) were collected on an ESCALAB MKII spectrometer with a Mg Kα radiation (hv = 1253.6eV).

Activity Test
The catalytic activity on ethane oxidative dehydrogenation using N 2 O as an oxidant was estimated in a fixed-bed corundum reactor under atmospheric pressure.A quantity of 250 mg of a catalyst was used as the prepared powder and was fixed in the middle section of the reactor.Before the reaction, the catalyst was pretreated at 450 • C with flowing helium for 30 min.A reacting feed gas at a flow rate of 25 mL min −1 containing 10 vol % of C 2 H 6 , 5-20 vol % of N 2 O, and balance He was passed through the catalyst bed.The mass flow controllers (Bronkhorst, The Netherlands) were used to control all the gas flows.The composition of outlet gas was analyzed online with an Agilent 7890B gas chromatograph equipped with a thermal conductivity detector (TCD).Before the analysis of outlet gas, the reaction proceeded for 40 min at each temperature or each gas concentration.The total gas flow rate at outlet (F out ) was calculated by using N as an internal standard.N 2 O conversion, ethane conversion, and selectivity and yield of the product were calculated by the following equations:

Conclusions
(1) Ordered mesoporous Cr/Al 2 O 3 composite oxide was facilely synthesized via an evaporationinduced self-assembly strategy.Characterization results revealed that the obtained material possessed excellent textual properties and thermal stabilities.(2) Ordered mesoporous Cr/Al 2 O 3 was utilized as the catalyst for ethane oxydehydrogenation using N 2 O as an oxidant.This mesoporous catalyst displayed a prominent activity for N 2 O abatement with 100% conversion over the whole temperature range.Moreover, an improved catalytic property for C 2 H 6 oxydehydrogenation was exhibited with the increased reaction temperatures.A maximal per pass C 2 H 6 conversion of 43% and C 2 H 4 yield of 22% were obtained at 700 • C.
(3) When feeding N 2 O from the concentration of 5 vol % to 20 vol %, a higher ratio of generated oxygen was provided, and an accordingly significant enhancement of C 2 H 6 conversion and declination of C 2 H 4 selectivity can be detected.(4) The mesoporous Cr/Al 2 O 3 catalyst was successfully operated at 700 • C for 15 h with fairly stable performance.