Catalytic Combustion of Dichloromethane over HZSM-5-Supported

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Catalytic Combustion of DCM over HZSM-5 Supported Typical Transition Metal (Cr, Fe and Cu) Oxides Catalysts: A Stability Study Jie Su, Yue Liu, Weiyuan Yao, and Zhongbiao Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04702 • Publication Date (Web): 21 Jul 2016 Downloaded from http://pubs.acs.org on July 25, 2016

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The Journal of Physical Chemistry

Catalytic Combustion of DCM over HZSM-5 Supported Typical Transition Metal (Cr, Fe and Cu) Oxides Catalysts: A Stability Study Jie Su1, Yue Liu1*, Weiyuan Yao1 and Zhongbiao Wu1,2 1

Department of Environmental Engineering, Zhejiang University, 866 Yuhangtang

Road, Hangzhou, 310058, P. R. China. 2

Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace

Flue Gas Pollution Control, 866 Yuhangtang Road, Hangzhou, 310058, P. R. China.

ACS Paragon Plus Environment


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ABSTRACT: In this paper, three kinds of HZSM-5 supported transition metal (Cr, Fe and Cu) oxides catalysts were prepared by wet impregnation method and their stability performances for catalytic combustion of dichloromethane (DCM) were investigated. Different behaviors were observed for these three catalysts during 300 min catalytic reaction running at 320 ºC. It was found that Cr-O/HZSM-5 catalyst showed well catalytic stability while both Fe-O/HZSM-5 and Cu-O/HZSM-5 suffered obvious deactivation. Characterizations by using XRD, BET, XPS, O2-TG, O2-TP-MS, NH3-IR and TPSR techniques were then carried out to disclose the deactivation mechanisms. The results revealed that the main cause of the deactivation over Fe-O/HZSM-5 catalyst was coke formation, which could be mainly attributed to its lower deep oxidation capacity of the intermediate products, i.e. the methoxy groups. And it could be also obtained that Cu-O/HZSM-5 catalyst was severely poisoned by chlorine species owing to the formation of stable Cu(OH)Cl species. Based on the results above, it could be concluded that the close proximity and synergy between acidic sites and active oxygen species was crucial to avoid coke deposition during CVOCs catalytic oxidation process.


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1. INTRODUCTION Chlorinated volatile organic compounds (CVOCs), released into the atmosphere from a wide range of industrial processes, have attracted considerable attentions due to their toxicity, high stability, and risks on environmental and human health. Over the past decades, environmental regulations of CVOCs emissions in the industrialized countries have become increasingly rigorous. As such, it is highly desirable to control such kind of pollutants and several methods have been profoundly developed, such as absorption, adsorption, biofiltration and combustion/catalytic combustion1-2. Among which, the catalytic combustion of CVOCs into CO, CO2, HCl/Cl2, and H2O is considered as one of the most potential methods due to its efficient performances at low temperature and little production of secondary pollution. Transition metal oxides and zeolite based materials have been studied as promising candidates of catalysts in CVOCs oxidation process for their superior properties. The former ones are known for their abundant active oxygen species, which can highly facilitate the deep oxidation of the VOCs3-6. But the poor acidity limits the adsorption of CVOCs and the crack of C-Cl bond, which inhibits the activity of the catalysts to a certain extend. Moreover, the stabilities of the catalysts are also challenged for their susceptibilities to chlorine species7. As for zeolite based catalysts, it has been widely reported that H-type zeolites showed a good catalytic performance for CVOCs decomposition, mainly owing to their abundant strong acidic sites8-10. However, the poor oxidizing ability of these materials resulted in coke



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deposition on the surface by the incomplete oxidation of intermediate products, which would induce a rapid deactivation of the catalysts 11-12. By considering the above-mentioned aspects of transition metal oxides and H-type zeolites, some investigations had been motivated to study reaction behaviors of CVOCs catalytic combustion over the transition metal oxides doped protonated zeolites in order to achieve the synergy advantages. The whole destruction process (adsorption of CVOCs, the rupture of C–Cl bonds and deep oxidation of adsorbed intermediates) on those catalysts could be promoted, since the dissociated adsorption of CVOCs mainly depended on surface acidity and further oxidation of intermediates was greatly affected by the redox properties of surface active oxygen species13-14. However, the deactivations of transition metal oxides doped zeolite catalysts were still widely observed for CVOCs catalytic combustion. For instance, it was reported by Chatterjee et al.15 that the Co modified Y-zeolite catalysts had lost almost 70 % activity after a 1000 min trichloroethylene catalytic combustion reaction. Halász et al.’s work

16

also showed the stabilities were not satisfied during the catalytic

oxidation of chlorobenzene over transition metal containing zeolites. Nevertheless, few studies had emphasized on the intrinsic mechanisms of these deactivation effects. Actually, we have investigated the stabilities of three typical transition metal (Cr, Fe, Cu) oxides doped HZSM-5 samples prepared via a wet impregnation method for DCM catalytic combustion in this paper. And totally different stability performances for these samples were observed. Thus, the main purpose of this work focused on revealing the inherent mechanisms of such different reaction behaviors, which might


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provide some guidelines for the preparation of highly durable catalysts. The emphasis contents were the characterizations regarding the physical/chemical properties of the fresh and used catalysts, as well as the transient responses gained from TPSR experiments during DCM catalytic oxidation reaction. Additionally, the relative deactivation causes and reaction mechanisms were discussed based on the characterization results. 2. EXPERIMENTAL SECTION 2.1. Catalyst preparation HZSM-5 zeolite was purchased from Tianjin Yuanli Chemical Co., Ltd (China), whose molar ratio of SiO2/Al2O3 was 30. Cr(NO3)3·9H2O (AR), Fe(NO3)3·9H2O (AR) and Cu(NO3)2·3H2O (AR) were bought from Shanghai Aladdin Bio-Chem Technology Co., Ltd (China). The catalysts were prepared by a wet precipitation method. 5g HZSM-5 support and certain amount of transition metal nitrates precursor (mass ratio of HZSM-5: transition metal oxide = 5:1) were added into 25 ml deionized water. Then the solution was rigorously stirred for 4 h. After drying at 80 °C for 12 h and then annealing at 400 °C for 4 h, final samples were obtained for further analysis and evaluation. The fresh catalysts were designated as Cr-O/HZSM-5(f), Fe-O/HZSM-5(f) and Cu-O/HZSM-5(f), and used catalysts after 300 min catalytic reaction at 320 °C as Cr-O/HZSM-5(u), Fe-O/HZSM-5(u) and Cu-O/HZSM-5(u). 2.2. Catalyst characterization The analysis of the crystal phases was performed on an X-ray diffractometer (XRD: model D/max RA, Riga ku Co., Japan; Cu Kα radiation, 0.15418 nm), at 40



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kV and 50 mA in a scanning range of 10-80°. The textural properties of the samples were determined by N2 physisorption at liquid nitrogen temperature using a nitrogen adsorption apparatus (Beijing JWGB Sci. &Tech. Co., Ltd, China). All the samples were degassed under vacuum at 200 °C for 2 h prior to measurement. The morphology analysis of samples was carried out on SU8010 FE-SEM operated at an accelerating voltage of 3 kV and the distribution of elements in bulk was detected by EDS mapping. The surface atomic states of the catalysts were analyzed by using X-ray photoelectron spectroscopy (XPS: Thermo ESCALAB 250, USA) with Al Kα X-ray (hµ= 1486.6 eV) radiation at 150 W. The shift of the binding energy was corrected using the C1s level at 284.6 eV as an internal standard. O2-TP-MS experiment was carried out on a custom-made setup (TP-5079, Tianjin Xianquan Co., Ltd, China), connected with a mass spectrometer (MS). 50 mg catalyst was pre-treated in pure He gas flow at 200 °C for 1 h. After cooling to room temperature, the process was carried out by heating the catalyst in 5% O2/He from 100 to 900 °C with a heating rate of 10 °C/min. The signals of CH2Cl2, CH3Cl, HCl, Cl2, CO and CO2 were recorded by a MS detector (HIDEN QGA, UK). TG tests were measured through a Thermogravimetric Analyzer (TGA, STA1500, Polymer Labs, USA). The catalysts were heated from 30 to 800 °C at a heating rate of 10 °C/min in a pure flow (20 ml/min) of O2. Acidity measurement was carried on Bruker tensor 27 FTIR spectrometers


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(Germany) with ZnSe windows. In the DRIFT cell, catalysts were pretreated at 300 °C for 1 h in a pure He environment. Then the spectra were recorded with flowing He after the adsorption of 1000 ppm NH3 (balance He) for 30 min at 200 °C. DCM adsorptions and its temperature-programmed surface reactions (TPSR) over the catalysts were performed in a fixed-bed micro-reactor (quartz glass, 6 mm i.d.). 50 mg catalyst was firstly pretreated in He flow (50 ml/min) at 400 °C for 0.5 h. After cooled down to 50 °C, a gas flow (50 ml/min) containing 1000 ppm DCM, 10 % O2 and balance He was introduced into the reactor. After saturated, the catalyst was then heated from 50 to 320 °C at a ramping rate of 5 °C/min and kept at 320 °C for 2 h. The signals of CH2Cl2, the intermediate CH3Cl and the products (CO2, CO, HCl, and Cl2) were recorded on-line by a MS detector (HIDEN QGA, UK). 2.3. Evaluation of stability The stability tests of the catalysts were performed in a fixed-bed micro-reactor (quartz glass, 8 mm i.d.) at atmospheric pressure under the following conditions: the catalysts were exposed to a typical DCM stream ([DCM] = ~1000 ppm, [O2] =10 vol. % and balance N2) at 320°C for 300 min, respectively. And the total flow rate was 250 ml/min with a gas hourly space velocity (GHSV) at 15, 000 h-1. The CH2Cl2 concentration in gas stream was monitored on line by a gas chromatograph equipped with a flame ionization detector (FID). The conversion of DCM was defined based on the following reaction: DCM conversion =

(CH Cl ) − (CH Cl ) (CH Cl )

3. RESULTS AND DISCUSSION



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3.1 The stability measurement Figure 1 showed the stability performances of Cr-O/HZSM-5, Fe-O/HZSM-5 and Cu-O/HZSM-5 catalysts for catalytic oxidation of DCM at 320 ºC for 300 min. It could be found that Cu-O/HZSM-5 exhibited the highest initial DCM conversion in the first 40 min and then dropped sharply from around 100 % to 41 % within 300 min, especially during the first 100 min. As for Fe-O/HZSM-5, a continuous deactivation was observed. And the conversion of DCM decreased to 60 % within 300 min at 320 º

C. According to this trend, the conversion rate was likely to be undermined more

seriously as the reaction further running on. Different from Fe-O/HZSM-5 and Cu-O/HZSM-5 catalysts, DCM conversion over Cr-O/HZSM-5 remained stable at ca. 70 %, although the initial activity was lower than the other two catalysts. Such different behaviors in DCM catalytic oxidation over the three catalysts might be associated with their different physiochemical properties. Further characterizations on fresh and used catalysts were carried out to disclose the related mechanisms involved. 3.2 Characterizations 3.2.1 Morphology and textural properties As mentioned in the introduction section, two main possible causes (coke deposition and chlorine poisoning) could lead to the deactivation in CVOCs catalytic combustion, both of which could induce great changes on the micro-morphology and textural properties of the catalysts. The textural properties of HZSM-5 supported catalysts were provided in Table 1. It could be clearly seen that the surface area and pore structure did have great changes for Fe-O/HZSM-5 and Cu-O/HZSM-5 catalysts


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after 300 min reaction running, which fitted well with the results in Figure 1 that both catalysts suffered severe deactivation. The BET surface area of Fe-O/HZSM-5 decreased from 310.4 m2/g to 266.4 m2/g and pore volume from 0.220 cm3/g to 0.189 cm3/g. While for Cu-O/HZSM-5, more than half of its surface area was lost (from 251.4 m2/g to 122.5 m2/g) after being used. In addition, the pore volume was reduced from 0.168 cm3/g to 0.114 cm3/g. Notably, Cr-O/HZSM-5 sample almost maintained its micro-structure with only slight differences in surface area and pore structure after 300 min catalytic reaction. These changes in textural properties of catalysts after reaction were in good accordance with the results on morphologies of the catalysts before and after reaction provided in Figure 2. It could be observed that all the three fresh samples were characterized as a bulk structure. The morphologies of Cr-O/HZSM-5 did not change obviously after reaction. And a large amount of small particles could be found covering on the surface of Fe-O/HZSM-5 after reaction. As for Cu-O/HZSM-5, the micro-structure had been destroyed to some extent besides the deposit of small particles, which indicated the catalyst had been chemically poisoned. 3.2.2. O2-TG and O2-TP-MS Generally, the surface deposited coke or chlorine species formed during the CVOCs catalytic oxidation process could be removed by being treated with oxygen at high temperature. Therefore, the impurities accumulated on used catalysts were analyzed by O2-TG (Figure 3). The fresh samples were also measured for the sake of comparison. In the range of 120-200 ºC, a ca. 6 % weight loss of all the fresh catalysts might be ascribed to desorption of the physically adsorbed H2O 17. As the temperature



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rising, both fresh and used Cr-O/HZSM-5 catalysts presented the same trend of weight loss, which was in accord with BET and SEM results. As for the other two catalysts, there existed obvious gap of the TG curves between fresh and used catalyst. Especially for Cu-O/HZSM-5, the weight loss gap would reach up to 6%. O2-TP-MS characterizations of the used catalysts were then performed to further determine the specific gas component generated by heat-treatment with O2. The possible gas products such as CO, CO2, CH3Cl and HCl in the outlet stream were measured (no Cl2 was detected) as shown in Figure 4. As expected, it could be seen that all the detectable substances from Cr-O/HZSM-5(u) were far less than those from the other two samples, which might be not high enough to be observed in O2-TG. Particularly, much more chlorine-containing substances, CH3Cl and HCl, were detected on Cu-O/HZSM-5(u) comparing to other two catalysts. Moreover, for Cr-O/HZSM-5(u) and Fe-O/HZSM-5 (u), all HCl was released below 450 ºC. However, as for Cu-O/HZSM-5(u), vast HCl emission was found at the temperature higher than 500 ºC. It implied that chlorine might chemically combine with Cu-O/HZSM-5 catalyst tightly. As a results, the textural properties and morphology were changed greatly (see Table 1 and Figure 2), leading to its most serious deactivation in DCM catalytic combustion. 3.2.3. XPS measurements As mentioned above, the Cl species would accumulate on the used catalysts, especially on Cu-O/HZSM-5(u). A lot of studies18-21 on non-noble metal catalysts had reported that the adsorbed Cl species generating from the rupture of C-Cl bonds could


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cover the active sites of the catalyst and do harm to the CVOCs oxidation. In this work, the surface contents of Cl accumulated were analyzed by XPS measurements during the DCM catalytic oxidation reaction, where small amount of the catalyst were taken out from the reactor at different reaction stages for sampling. Figure 5 showed the chlorine atomic content with the reaction time for three catalysts. It could be observed that the surface Cl content of Cu-O/HZSM-5 increased very rapidly with the reaction going on as expected and reached a highest value (around 5.4 %). And then, a drop of chlorine content (decreased to about 3.5 %) could be found at 300 min, which was probably due to the experimental error from discrete sampling as the chlorine accumulation on the catalysts could not disperse evenly during the catalytic reaction. The surface Cl content of Fe-O/HZSM-5 gradually increased with reaction time and approached around 1.5 %. However, the chlorine atomic content on the surface of Cr-O/HZSM-5 remained at lowest level, less than 0.3 % after 300 min reaction running on. In a word, the chlorine accumulation on Cu-O/HZSM-5 was much more serious compared to the other two samples. Further analysis on the states of the transit metals were also conducted by XPS measurements and the XPS spectra of Cr 2p, Fe 2p and Cu 2p were presented in Figure 6. For Cr-O/HZSM-5 catalyst, the Cr 2p region consisted of two main peaks with binding energies at about 586.2 eV and 576.5 eV for Cr 2p1/2 and Cr 2p3/2, respectively, which were the characteristic of a mixed-valence chromium system (Cr3+ and Cr6+)22. After the catalytic reaction, the two peaks did not show detectable shift. However, as for Cu-O/ HZSM-5, all the Cu 2p peaks exhibited an obvious shift



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to the higher binding region as the reaction going on, indicating that chemical states of Cu species had been greatly changed. Furthermore, a Fe 2p3/2 peak at 710.4 eV corresponding to a mixed state of Fe2+ and Fe3+ 23 could also be found spreading to higher binding energy range after reaction. While the variations were much slighter compared with those in Cu 2p peaks. All these results could give support on the strong interactions

between

Cu

components

and

chlorine.

And

the

SEM-EDS

characterization results (Figure S1) also confirmed that chlorine mainly accumulated on the sites of Cu related species, rather than HZSM-5 support. 3.2.4. XRD Figure 7 showed the XRD patterns of the fresh and used HZSM-5-supported Cr, Fe and Cu oxides. For three fresh samples, it was observed that all of the identified diffraction peaks could be ascribed to the mixed phase of HZSM-5 and the related doped oxides (Cr2O3, Fe2O3 and CuO)7, 24-25. For used Cr-O/HZSM-5 catalysts and Fe-O/HZSM-5, no phase transformations were observed in XRD patterns comparing to the fresh samples. It was worth noting that there were several new diffraction peaks (2θ =16.0，32.4，39.9，40.2，57.2º) emerged for Cu-O/HZSM-5(s) after 300 min reaction, as marked in Figure 7(c). The identical diffraction peaks could be also observed for HCl pretreated Cu-O/HZSM-5 samples (see Figure S2). These new diffraction peaks herein could be assigned to Cu-OH-Cl compounds (copper oxychloride species) according to the identification of the same XRD peaks in the previous work26. Similar results had been reported by Ahmad et al.27 that additional sharp diffraction peaks in aged catalyst could be found for the catalytic combustion of


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TCE on Cr-Cu/modified H-ZSM-5 catalyst, which were attributed by the authors to the formation of metal chlorides (or oxychlorides). 3.2.5. TPSR In order to obtain more insight onto the deactivation process of the catalytic combustion, TPSR experiments were carried out and the concentration variations of DCM, CH3Cl, CO2, CO, HCl and Cl2 in the outlet stream were shown in Figure 8 (TP stage of 50-320 ºC in left part and continuous reaction stage at 320 ºC in right part). The lowest outlet DCM concentration could be observed over Cu-O/HZSM-5 during the TP stage, and then the outlet DCM concentration increased rapidly at 320 ºC, which was well agreed with its highest initial DCM conversion and most serious deactivation as showed in Figure 1. It could be also seen that less HCl and Cl2 were released over Cu-O/HZSM-5, indicating its higher chlorine accumulation comparing to the other two samples. For Cr-O/HZSM-5 catalyst, the concentrations of all recorded substances maintained stably during the second stage with 120 min reaction running at 320 ºC, further verifying its good catalytic stability. Furthermore, a great amount of CO2 was produced at around 200 ºC over Cu-O/HZSM-5, revealing its highest deep oxidation capacity, which was in good agreement with the H2-TPR results (see Figure S3) that Cu-O/HZSM-5 was of the strongest reducibility. And it could be also found that Fe-O/HZSM-5 catalyst showed the lowest CO2 production amount. Interestingly, relative huge CH3Cl were released over Fe-O/HZSM-5 catalyst while the outlet CH3Cl concentration of the other two samples remained at very low level. These facts demonstrated that Fe-O/HZSM-5 catalyst were short of deep



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oxidation ability, where part of DCM was just incompletely oxidized into CH3Cl as reported by many researchers28-30. 3.2.6. Acidity property As we know, acidic properties play an important role in CVOCs catalytic combustion, especially in the rupture of C–Cl bonds and the dissociative adsorption of Cl atom29. Previous studies had reported that acidic properties of catalyst also strongly affected coke formation on catalysts’ surface and that the coke deposition rate would be enhanced with increased surface acidity31-33. As such, the acidity of the three samples was evaluated via DRIFT investigation of NH3 adsorption as shown in Figure 9. For all the samples, the bands in the range of 1850–1640 cm-1 and 1500-1400 cm-1 could be attributed to asymmetric and symmetric bending vibrations of NH4+ species on Brönsted acidic (denoted as B acidic) sites34-35. The peaks at (1613, 1282, 1172 cm-1) over Cu-O/HZSM-5, (1564, 1282, 1162 cm-1) over Fe-O/HZSM-5 and (1278, 1164cm-1) over Cr-O/HZSM-5 could be ascribed to NH3 adsorbed on Lewis acidic (denoted as L acidic) sites34, 36-37. Our results indicated that the acidity properties of all the three samples were quite different. On fresh Cr-O/HZSM-5, two moderate bands and one very weak peak could be observed, corresponding to NH3 adsorbed on L acidic and B acidic sites, respectively. The L acidic sites on Cu-O/HZSM-5 were found more abundant than those on Cr-O/HZSM-5, whereas few B acidic sites could be detected. As for Fe-O/HZSM-5, it appeared that the bands of B acidic sites were almost retained compared to the pure HZSM-5 (see Figure S4) and those of the L acidic sites were much less than Cu-O/HZSM-5. Based on results of


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this study, we could speculate that after the loading of Cr oxide, the inherent B acidic sites on HZSM-5 support were weakened and even disappeared, probably through a process of ion exchange. Meanwhile, a majority of B acidic sites on HZSM-5 support were also neutralized on Cu-O/HZSM-5 catalyst. However, Fe-O/HZSM-5 sample possessed a similar B acidity with the support, indicating less interactions between doped Fe oxides and HZSM-5 support than the other two catalysts. 3.3. Discussion on deactivation mechanism By summarizing the above results, it was obtained that HZSM-5 supported Cr, Fe and Cu oxides showed totally different stabilities for the catalytic combustion of DCM. Cr-O/HZSM-5 showed the best stability performance, while both Fe-O/HZSM-5 and Cu-O/HZSM-5 catalysts suffered a certain degree of deactivation. A series of characterizations results confirmed that the physiochemical properties of Cr-O/HZSM-5 catalyst could be maintained during the catalytic reaction, neither chlorine

poisoning

nor

coke

deposition

being

detected.

Meanwhile,

the

Cu-O/HZSM-5 catalyst suffered severe chlorine poisoning due to the formation of stable copper oxychloride [Cu(OH)Cl] species. Both of the XRD and XPS results demonstrated that the Cu-O structures were greatly corroded due to the strong affinity between Cu ion and chlorine species. According to the Hard-Soft-Acid-Base (HSAB) principle38-39, Cr3+ and Fe3+ are hard acids while Cu2+ belongs to borderline bases, which is inclined to bind to Cl-, a borderline base. Therefore, the Cu-O bond was more easily broken up and formed a new bond as Cu-Cl, which would interrupt the oxygen rehearing process of catalytic oxidation and result in the deactivation of the



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whole catalyst. According to the SEM and XRD results, the crystal structure of Fe-O/HZSM-5 catalyst did not show obvious changes after catalytic oxidation, though its XRD peak intensity slightly weakened. And XPS and O2-TP-MS results also confirmed the chlorine accumulation on Fe-O/HZSM-5 was less serious. Thus, the most possible reason for the deactivation of Fe-O/HZSM-5 catalyst could be the coke deposition (small particles observed in Figure 2). Actually, coke deposition might also lead to the decrease in the intensity of XRD peaks40. It was widely known that the coke deposition during VOCs catalytic oxidation process was attributed to the polymerization of the hydrogen deficient intermediates41-42. And the DCM catalytic combustion pathway over metal oxide catalysts had been proposed in several works21, 29-30

. The DCM molecules initially adsorbed on the acidic sites of the catalyst and a

following dehydrochlorination of adsorbed species would occur with the rupture of C–Cl bonds, resulting in the formation of bidentate methoxy and formaldehyde ad-species. After that, the bidentate methoxy and formaldehyde ad-species could transfer into formic and methoxy groups via a cannizzaro type disproportionation reaction. Finally, these intermediates would be further oxidized into the final products. It was speculated that the methoxy groups herein was very important to coke formation, since the formic species were more easily decomposed into CO or be oxidized to CO2 by active oxygen species. A series of methoxy groups were supposed to be merged and form olefin species via hydrogen shift reaction in the absence of active oxygen species and leave the hydroxyl groups. And olefin species were


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considered as the coke formation precursor on acid catalyst11-12. The highest amount of CH3Cl produced over Fe-O/HZSM-5 catalyst would partially support this assumption (see TPSR results in Figure 8), where the CH3Cl were proved by Brink et al.29 as a reaction product between surface methoxy groups and HCl. The proposed mechanism of CH3Cl and coke formation were provided in Scheme 1. Therefore, the deep oxidation of methoxy groups is the key point to avoid coke deposition on the catalysts. Based on the severe coke deposition on Fe-O/HZSM-5 catalyst, the relative low deep oxidation capacity of surface ad-species should be the main cause. As we stated in surface acidity analysis, the B acidic sites were retained mostly on Fe-O/HSM-5 samples compared to the HZSM-5 support. Thus, a majority of ferrous ions were not ion-exchanged with the protons on HZSM-5 during wet impregnation procedure, indicating the isolation of the active oxygen species on ferrous oxides from acidic sites. And the as-formed intermediates on the acidic sites after DCM adsorption and dechlorination could not be further oxidized in time, leading to the coke formation. Actually, similar coke deposition could also be found for DCM catalytic oxidation on HZSM-5 support (see Figure S5). One might argue that coke formation was mainly the results of strong acidity of Fe-O/HSM-5 catalyst as suggested by many researchers31, and little coke deposition on Cr-O/HSM-5 samples just could be attributed to its very weak surface acidity. We have also prepared a new Cr-O/HZSM-5 sample with lower loading content (1 wt.%) and the relative surface acidity characterization (see Figure S6) showed that the new sample possessed both abundant B and L acidic sites. Further stability test results were given



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in Figure S7, and it was obtained that the new sample also exhibited quite good stability with even higher DCM conversion due to the increased surface acidity. According to the discussion above, it could be concluded that synergic effect between acidic sites and active oxygen species was crucial to the stability performance of CVOCs catalytic combustion. 4. CONCLUSIONS In this paper, the stabilities of low temperature catalytic combustion of DCM over three typical transition metal (Cr, Fe and Cu) oxides doped HZSM-5 catalysts were investigated. As a result, very different stability performances were observed for these three samples. Cr-O/HZSM-5 catalyst showed the best stability performance, whose physiochemical properties after 300 min reaction running were well-preserved. Both Fe-O/HZSM-5 and Cu-O/HZSM-5 catalysts suffered deactivation to a certain extent. A severe chlorine accumulation on Cu-O/HZSM-5 samples was observed due to the strong affinity between copper ion and chlorine atom, leading to the formation of bulk Cu(OH)Cl species. And this new phase generated would inhibit the oxygen rehearing process, thereby resulting in the deactivation of Cu-O/HZSM-5 catalyst. For Fe-O/HZSM-5 catalyst, the coke deposition was considered as the main reason of the deactivation. The serious coke deposit on Fe-O/HZSM-5 catalyst could be attributed to the lack of deep oxidation ability of methoxy groups, one of the intermediates for DCM catalytic combustion. Unlike Cr-O/HZSM-5 and Cu-O/HZSM-5 catalysts, the interaction between doped ferric oxides and support was not strong enough for Fe-O/HZSM-5 catalyst, resulting in the isolation of active oxygen species from


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surface acidic sites. Therefore, the adsorbed intermediates on acidic sites from DCM chemical adsorption could not be further oxidized in time and led to the formation of coke. In a word, the synergic effect between acidic sites and active oxygen species is necessary to CVOCs catalytic oxidation process with highly activity and stability. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXX. EDS cartography for used Cu-O/HZSM-5 catalyst: Cu(a), Cl(b), Al(c) and Si(d) elements, XRD patterns of samples after being 180 min treated by 1000 ppm HCl , H2-TPR profiles of catalysts: fresh samples, samples after being used for 300 min, NH3-IR profile of HZSM-5, the stability performances of HZSM-5 at 320 ºC and SEM image of used HZSM-5 (insert figure), NH3-IR profile of 1 wt. % Cr-O/HZSM-5, and the stability performances of 1 wt.% Cr-O/HZSM-5 at 320 º

C.

AUTHOR INFORMATION Corresponding Author *Tel.: +86 571 87953088; Fax: +86 571 87953088; E-mail: [email protected] Notes The authors declare no competing financial interest.



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ACKNOWLEDGMENTS The authors acknowledge for the financial support of National Key Technology Support Program (2014BAC22B06) and the Program for Zhejiang Leading Team of S&T Innovation (No. 2013TD07).


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TABLE，，FIGURE AND SCHEME CAPTIONS Table 1 Surface area and pore structure of fresh and used catalysts Figure 1 The stability performances of catalysts at 320 ºC. ([DCM] = 1000 ppm, [O2] = 10 vol.%, N2 balance, GHSV = 15, 000 h-1) Figure 2 SEM images of Cr-O/HZSM-5 fresh (A) and used (B), Fe-O/HZSM-5 fresh (C) and used (D), Cu-O/HZSM-5 fresh (E) and used (F) Figure 3 O2-TG curves of fresh and used catalysts Figure 4 MS signals of O2-TP-MS profiles for the used catalysts: CO (A), CO2 (B), CH3Cl (C) and HCl (D) of HZSM-5 supported Cr(a), Fe(b), Cu(c) oxides Figure 5 The surface chlorine atomic content determined by XPS with the reaction time Figure 6 XPS spectra of Cr 2p (a), Fe 2p (b), and Cu 2p (c) Figure 7 XRD patterns of the HZSM-5-supported Cr oxides (a), Fe oxides (b) and Cu oxides (c) fresh and after 300 min reaction on stream Figure 8 TPSR profiles of DCM decomposition over HZSM-5 supported Cr, Fe and Cu oxides Figure 9 NH3 adsorption IR profiles of HZSM-5 supported Cr (a), Fe (b) and Cu (c) oxides catalysts Scheme 1 Mechanism of CH3Cl and coke formation


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Table 1 Surface area and pore structure of fresh and used catalysts samples

） BET （m2/g）

Pore volume (cm3/g)

Cr-O/HZSM-5

fresh

262.5

0.180

Cr-O/HZSM-5

used

266.0

0.169

Fe-O/HZSM-5

fresh

310.4

0.220

Fe-O/HZSM-5

used

266.4

0.189

Cu-O/HZSM-5

fresh

251.4

0.168

Cu-O/HZSM-5

used

122.5

0.114



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Figure 1 The stability performances of catalysts at 320 ºC. ([DCM] = 1000 ppm, [O2] = 10 vol.%, N2 balance, GHSV = 15, 000 h-1)


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Figure 2 SEM images of Cr-O/HZSM-5 fresh (A) and used (B), Fe-O/HZSM-5 fresh (C) and used (D), Cu-O/HZSM-5 fresh (E) and used (F)



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Figure 3 O2-TG curves of fresh and used catalysts


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Figure 4 MS Signals of O2-TP-MS profiles for the used catalysts: CO (A), CO2 (B), CH3Cl (C) and HCl (D) of HZSM-5 supported Cr(a), Fe(b), Cu(c) oxides



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Figure 5 The surface chlorine atomic content determined by XPS with the reaction time


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Figure 6 XPS spectra of Cr 2p (a), Fe 2p (b), and Cu 2p (c)



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Figure 7 XRD patterns of the HZSM-5-supported Cr oxides (a), Fe oxides (b) and Cu oxides (c) fresh and after 300 min reaction on stream


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Figure 8 TPSR profiles of DCM decomposition over HZSM-5 supported Cr, Fe and Cu oxides



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Figure 9 NH3 adsorption IR profiles of HZSM-5 supported Cr (a), Fe (b) and Cu (c) oxides catalysts


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Scheme 1 Mechanism of CH3Cl and coke formation



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106x106mm (220 x 220 DPI)


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Catalytic Combustion of Dichloromethane over HZSM-5-Supported

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