Catalysis Communications 8 (2007) 1645–1649 www.elsevier.com/locate/catcom
Low-temperature catalytic destruction of chlorinated VOCs over cerium oxide Qiguang Dai, Xingyi Wang *, Guanzhong Lu
Lab for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, PR China Received 29 November 2006; received in revised form 11 January 2007; accepted 12 January 2007 Available online 27 January 2007
Abstract Cerium oxide was used as the catalyst for the catalytic destruction of chlorinated VOCs, and trichloroethylene catalytic destruction as a model reaction was investigated in detail. It was found that cerium oxide has a high catalytic activity for the low-temperature catalytic destruction of various chlorinated VOCs. The main products in the catalytic destruction of trichloroethylene are HCl, Cl2, CO2, and trace CO, and C2HCl is a reaction intermediate, and the presence of an excess of water (30,000 ppm) in the feed stream would inhibit the catalytic performance of CeO2 catalyst, but improve the selectivity of HCl. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Catalytic destruction; Chlorinated VOCs; Cerium oxide; Catalytic combustion
1. Introduction Chlorinated volatile organic compounds (VOCs), such as dichloromethane (DCM), chloroform (CHCl3), carbon tetrachloride (CCl4), 1,2-dichloroethane (DCE), trichloroethylene (TCE) and tetrachloroethylene (PCE) are commonly found in the industrial waste streams and also widely used in the chemical industry, including the processes of lubricants, heat-transfer ﬂuids, dry cleaning, paint stripping and degreasing operations, due to their excellent slovenly and virtual non-ﬂammability. However, their improper disposal has led to groundwater and soil contamination, even destroyed the ozone layer [1–6]. Therefore, for the destruction and/or safe disposal of chlorinated VOCs the special attentions must be paid on widely and adequately. Catalytic destruction has become an eﬀective and economically advantageous strategy for the destruction of chlorinated VOCs, due to its low oper* Corresponding authors. Tel.: +86 021 642533372; fax: +86 021 64253703. E-mail addresses: [email protected]
(X.Y. Wang), [email protected]
edu.cn (G.Z. Lu).
1566-7367/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.01.024
ating temperature (300–550 °C), low energy consumption and high potential selectivity towards the formation of harmless reaction products. Most of the previous work related to the catalytic destruction of chlorinated VOCs is focused on developing two types of catalysts, that is, the noble metal and transition metal oxide catalysts. Noble metals (Pt and Pd) catalysts are very active for the catalytic destruction of chlorinated VOCs, but they are poisoned easily by HCl and Cl2 produced from the destruction of chlorinated VOCs. Traditionally, supported transition metal oxides have been proposed as the potential substitutes for the noble metal-based catalysts . Transition metal oxides are in general less catalytic active than noble metals for the destruction of chlorinated VOCs, but they can resist the deactivation by poisoning largely . Among these catalysts, chromium-based catalysts have exhibited the highest activity for chlorinated VOCs destruction [9–11]. Nevertheless, the uses of chromium-based catalysts are restricted, owing to the formation of the extremely toxic residues (such as chromium oxychloride) at low temperature . More recently, the solid acid catalysts free metal have been proposed for the catalytic destruction of chlorinated VOCs [13–18],
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which are mainly the protonic zeolites such as H-ZSM-5, H-Y and H-MOR, alumina and alumina-based materials. However, it has been found that there is a large amount of coke deposition on the acidic catalysts, resulting in the serious deactivation of catalysts. Especially for the protonic zeolites, the formation of AlCl3 usually brings on the framework destruction of zeolites. Generally, the catalysts employed in the catalytic destruction of chlorinated VOCs have some problems, such as the deactivation due to poison, high cost or formation of undesired higher chlorinated compounds, the low catalytic activity, or the presence of chromium in the catalysts. As far as we know, the completely catalytic destruction temperatures of trichloroethylene are usually at >550 °C. Therefore, developing the high performance catalysts for eliminating chlorinated VOCs is still necessary and urgent. The rare-earth elements have been used widely in the catalysts. Especially cerium oxide has much more attracted attention in the environmental catalysis and been used an eﬀective promoter or supporting material based on its high oxygen-storage capacity (OSC) and facile redox cycle of Ce4+/Ce3+ [19–21]. As we known, supported CeO2 and CeO2 based mixed oxides are cheap, environmentally friendly and eﬃcient catalysts for the catalytic destruction of non-chlorinated VOCs [22–25], such as methane, CO, methanol and propane. They can be used in the wet oxidation processes of the organic compounds, such as the treatment of the industrial waste-waters and the removal of total organic carbon from polluted waters [26,27]. Additionally, Lin [28,29] and Wang  have found that pure CeO2 has a high catalytic activity for the catalytic wet air oxidation of phenolic wastewater and the catalytic incineration of aromatic hydrocarbons. However, CeO2 has less been considered its suitability and eﬃciency for the catalytic destruction of chlorinated VOCs. In the present work, cerium oxide was employed as the catalyst for the catalytic destruction of ﬁve kinds of the most common chlorinated solvents: DCM, CCl4, DCE, TCE and PCE, in which the catalytic destruction eﬃciency of chlorinated VOCs with diﬀerent chemical nature are investigated. In the TCE catalytic destruction used as a model reaction, the selectivity of products and the eﬀect of excess water (30,000 ppm) in the feed stream on the catalytic performance of catalyst are speciﬁcally described.
2.2. Catalysts characterization 2.2.1. Powder X-ray diﬀraction The powder X-ray diﬀraction patterns (XRD) of the samples were recorded on a Rigaku D/Max-rC powder diffractometer using Cu Ka radiation (40 kV and 100 mA). The diﬀractograms were recorded in the 2h range of 10 to 80° with a 2h step size of 0.01°. 2.2.2. Nitrogen adsorption/desorption The nitrogen adsorption and desorption isotherms were measured at 77 K on an ASAP 2400 Sorptometer in static measurement mode. The samples were outgassed at 160 °C for 4 h before the measurement. The speciﬁc surface area was calculated using the BET model. 2.2.3. Energy dispersive spectroscopy The analysis of the elements was carried out with an energy dispersive spectrometer (EDS) detector from EDAX. 2.2.4. Thermogravimetry Coke formation was evaluated with thermogravimetric analysis (TGA) using a PerkinElmer Pyris Diamond TG/ TGA Setaram instrument. The fresh and used CeO2 samples were heated from room temperature to 600 °C (heating rate of 10 °C min 1) in a N2/O2 stream. 2.2.5. Raman spectroscopy The Raman spectra were obtained on a Renishaw in Viat Reﬂex spectrometer equipped with a CCD detector at ambient temperature and moisture-free conditions. 2.2.6. X-ray photoelectron spectroscopy The XPS measurements were made on a VG ESCALAB MK II spectrometer by using Mg KR (1253.6 eV) radiation as the excitation source. The binding energy of samples was corrected by adventitious carbon (C1s) at 284.6 eV. The powder samples were pressed into self-supporting disks, loaded into a sub-chamber, and evacuated for 4 h, prior to the measurements at room temperature. 2.3. Catalytic activity measurement
2. Experimental 2.1. Catalysts preparation The CeO2 catalyst was prepared by thermal decomposition of Ce(NO3)3 Æ 6H2O (SCRC, 99.0%) at 550 °C for 5 h in air. Its BET surface area measured by nitrogen adsorption was 54.6 m2/g and crystallite size estimated from XRD was 12.1 nm. The results of XPS analysis showed that the cerium species in the cerium oxide catalyst is almost Ce4+ (that is CeO2).
The catalytic destruction reactions of chlorinated VOCs were carried out in a continuous ﬂow micro-reactor system at atmospheric pressure with a U-shaped quartz tube reactor (U 4 mm). 270 mg catalyst was placed in the bottom of reactor, and the space velocity (GHSV) was 15,000 h 1. The concentration of chlorinated VOCs was 1000 ppm, which was prepared by delivering the liquid chlorinated VOCs by a syringe pump into dry air (dried by silica gel and 5A zeolite) which was metered by a mass ﬂow controller. The injection point was electrically heated to ensure complete evaporation of chlorinated VOCs. The reaction
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temperature was measured with a thermocouple located at the catalyst bed. The eﬄuent gases were analyzed on-line by two gas chromatographs equipped with an electron capture detector (ECD) to analyze quantitatively the organic chlorinated reactants and products, and a thermal conductivity detector (TCD) to analyze quantitatively CO and CO2. The concentrations of Cl2 and HCl were analyzed by the eﬄuent stream bubbling through a 0.0125 N NaOH solution. Then, Cl2 concentration was determined by the titration with ferrous ammonium sulphate (FAS) using N,Ndiethyl-p-phenylenediamine (DPD) as an indicator . The concentration of chloride ions in the bubbled solution was determined by using a chloride ion selective electrode . Furthermore, GC-MS was used to monitor and conﬁrm the formation of by-products such as COCl2, C2HCl and C2Cl2. 3. Results and discussion The catalytic activities for ﬁve kinds of chlorinated VOCs catalytic destruction over the CeO2 catalyst as a function of temperature are shown in Fig. 1. It can be seen that CeO2 exhibits a highly catalytic destruction activity for all ﬁve kinds of chlorinated VOCs. For the destruction of diﬀerent chlorinated VOCs, the catalytic activity of CeO2 is diﬀerent and its activity order is as follows: DCM > DCE > CCl4 > TCE > PCE. T90% value (the temperature of 90% destruction of chlorinated VOCs) is 260 °C for tetrachloroethylene, and is only 160 °C for dichloromethane. This temperature is greatly lower than that of all other catalysts reported in the previous literatures [7–18], including the chromium-based catalysts [9–12]. The catalytic destructions of chloralkanes over the CeO2 catalyst are easier than that of chlorinated alkylenes. That is maybe related to the
Fig. 1. Light-oﬀ curves of the catalytic destruction of chlorinated VOCs over CeO2. (–j– dichloromethane; –h– 1,2-dichloroethane; –d– carbon tetrachloride; –s– trichloroethylene; –m– tetrachloroethylene).
inductive and p–p resonance eﬀects between chlorine atom and p electron cloud of [email protected]
double bond in chlorinated alkylenes, which shortens the bond length and increases the bond energy of C–Cl. The studies of Ramachandran and co-workers  showed that, the ﬁrst step in the catalytic decomposition of chlorinated VOCs is the splitting of ﬁrst Cl atom, so the bond energy of C–Cl directly determines the catalytic destruction eﬃciency of chlorinated VOCs. The more strong the bond energy, the more diﬃcult the catalytic destruction of chlorinated VOCs is. Compared with the bond energy (430 kJ mol 1) of C–H bond, the bond energy (340 kJ mol 1) of C–Cl is lower, therefore chlorinated VOCs are more likely to lose Cl atoms ﬁrst, and the chlorinated VOCs with the high ratio of Cl/C are destroyed easily. For the catalytic destruction of chloralkanes or chlorinated alkylenes over the CeO2 catalyst here, the conversion hierarchy of chloralkanes and chlorinated alkylenes is DCM DCE > CCl4 and TCE > PCE, respectively, which is not consistent with the results reported by Ramachandran. Herefrom, we think that, the rate determining step of the catalytic destruction of chlorinated VOCs does not simply involve the breaking of the C–Cl or C–H bond, but is related to the distribution of hydrogen and chlorine atoms across the [email protected]
double bond or C–C bond and the spatial structure of chlorinated VOCs. In this study, besides the catalytic activities for diﬀerent chlorinated VOCs over CeO2 catalyst are investigated, and the activity and selectivity of CeO2 catalyst for the catalytic combustion of trichloroethylene in dry and humid air conditions were studied. It is found that the addition of 3% (V/ V) water can inhibit the catalytic decomposition of trichloroethylene over CeO2 evidently, and the T50% values increase from 165 °C to 240 °C ( as shown in Fig. 2), which maybe be resulted from the competitive adsorption between water and TCE molecule on active sites of CeO2 catalyst. However, at above 275 °C, the selectivity of HCl
Fig. 2. The eﬀect of water vapour (30,000 ppm) in the feed on the catalytic destruction of trichloroethylene over CeO2.
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is much improved by feeding water and the production of molecular chlorine is greatly lowered. That is probably there is the competitive adsorption between water and TCE molecule on the active sites of CeO2, adsorbed H2O dissociates into adsorbed H+ and OH species and protons recombine with chloride to form adsorbed HCl, resulting in the reduction of the Cl2 formation and the increase of the HCl selectivity . In order to understand the formation of chlorinated products during the catalytic destruction of trichloroethylene, GC-MS was used to monitor the composition of the eﬄuent gases. In the outlet gases, the chlorinated by-products (such as C2HCl, C2Cl2, COCl2, C2Cl4 and CCl4) were not measured, except unreacted TCE. However, it was found that, whether under dry or humid condition, the chlorine balance of the eﬄuent gases was lower 85% at higher temperature. It can be supposed that parts of chlorine adsorb on the surface of CeO2, and then react with it to form CeCl3 and CeOCl during catalytic destruction of TCE, which results in the lower chlorine balance of the eﬄuent gases. Therefore, XRD, Raman, and XPS were employed to investigate the diﬀerence between fresh and used CeO2 catalysts. In the XRD spectra of CeO2 used (see Fig. 3), the diffraction peaks of CeCl3 and CeOCl have not been observed, which may be ascribed to the XRD technology is insensitive for the analysis of trace CeCl3 and CeOCl phases on the surface of catalyst. In the Raman spectrum of the used CeO2 catalyst, there are not the absorption bands at 177, 208 cm 1 of CeCl3 and 119, 327 cm 1 of CeOCl . The Ce3d XPS spectrum of used CeO2 is almost same as that of fresh CeO2 (Fig. 4), which suggests that the valence of Ce4+ is not changed. The above results show that the formation of CeOCl and CeCl3 on the CeO2 catalyst has not been observed after being used in the destruction reaction of trichloroethylene.
Fig. 3. XRD and EDS spectra of fresh and used CeO2 in the catalytic destruction of trichloroethylene under dry condition.
Fig. 4. XPS and Raman spectra of fresh and used CeO2 in the catalytic destruction of trichloroethylene under dry condition.
However, the analyses of EDS (shown in Fig. 3) and XPS (the Cl2p peak at around 198 eV) have conﬁrmed the presence of chlorine on the surface of used CeO2. Compared with the used fresh CeO2, used CeO2 presents the Ce3d peaks at the higher binding energy about 0.4 eV, which is maybe that chlorine-adsorbed CeO2 has the stronger electro-negativity, resulting in the transition of charge density of Ce to Cl. Therefore, it can be concluded that some Cl adsorbed on the surface of catalyst during the TCE destruction, resulting in the poor chlorine balances. The complete oxidation product (CO2) and the partial oxidation product (CO) were also detected in the outlet gases. >90% carbon in TCE was converted into CO2 and only few CO formed. Moreover, EDS and TG (as shown in Figs. 3 and 5) conﬁrm no coke formation during cata-
Fig. 5. TG and DTA analysis of fresh and used CeO2 in the catalytic destruction of trichloroethylene under dry condition.
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lytic destruction of TCE. At GHSV = 15,000 h 1, COCl2, C2Cl2 and C2HCl have not been detected by MS. However, at GHSV > 30,000 h 1, C2HCl (m/z = 60 and 62) has been detected in the outlet gases. The fact that the selectivity of C2HCl increases with an increase in the space velocity shows that, C2HCl is a series intermediate and the lower space velocity is in favor of the residence of C2HCl molecule on the catalyst to be decomposed to CO2. Therefore, for the catalytic destruction of TCE, the space velocity must be controlled suitably to avoid the formation of secondary pollutant C2HCl. 4. Conclusion In summary, it has been found that ceria has a high catalytic activity for the low-temperature catalytic destruction of various chlorinated VOCs, and the catalytic destructions of chloralkanes over the CeO2 catalyst are easier than that of chlorinated alkylenes. The presence of water vapor in the feed can inhibit the catalytic destruction of trichloroethylene over the CeO2 catalyst evidently, but the selectivity of HCl can be improved. The main products of trichloroethylene catalytic destruction over the CeO2 catalyst are HCl, Cl2, CO2 and trace CO, and C2HCl is a reaction intermediate. Acknowledgments The authors gratefully acknowledge the ﬁnancial support from National Basic Research Program of China (No. 2004CB719500) and National Natural Science Foundation of China (No. 20377012). References     
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