- Email: [email protected]
The effect of Ce on catalytic decomposition of chlorinated methane over RuOx catalysts
Applied Catalysis A: General 470 (2014) 442–450
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
The effect of Ce on catalytic decomposition of chlorinated methane over RuOx catalysts Le Ran, Zhengyi Wang, Xingyi Wang ∗ Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 6 July 2013 Received in revised form 30 October 2013 Accepted 4 November 2013 Available online 10 November 2013 Keywords: Dichloromethane CeOx –Al2 O3 RuO2 Catalytic combustion Chlorine
a b s t r a c t Ru oxide catalysts supported on Ce-Al2 O3 with various Ce contents were prepared by wet impregnation with RuCl3 aqueous solution and characterized by XRD, N2 adsorption, TEM/HRTEM, H2 -TPR, Raman and NH3 -TPD. The catalytic combustion of dichloromethane (DCM) was investigated over the Ru/Ce-Al2 O3 catalysts at first time. The Ru/Ce-Al2 O3 catalysts present an outstanding catalytic activity. Ru/7%Ce-Al2 O3 catalyst is most active, and DCM is converted completely at 270 ◦ C. At 250 ◦ C, the conversion of DCM over the Ru/Ce-Al2 O3 catalysts with 3% or higher Ce contents keeps constant within 1000 h. High stable activity of the Ru/Ce-Al2 O3 catalysts can be ascribed to the synergistic effect of acidity and high oxygen mobility.
1. Introduction Chlorinated volatile organic compounds (CVOCs) are hazardous pollutants that are considered as the most harmful organic contaminants due to their acute toxicity and strong bioaccumulation potential [1]. The safe disposal of alkyl chloride pollutants, such as CH2 Cl2 (DCM), CHCl CCl2 (TCE) and CH2 Cl CH2 Cl (DCE), has acquired great importance with the ever increasing concern for environmental protections [2]. Among various available detoxification techniques, the catalytic oxidation is an interesting one which can be efficiently performed within the temperature range from 250 to 550 ◦ C converting so dilute pollutants that otherwise cannot be thermally combusted without consumption of fuel. Most studies of the catalysts used in the catalytic oxidation of CVOCs have reported on the three types of catalysts based on noble metals, transition metals and zeolites [3–5]. At present, the problems such as the formation of polychlorinated compounds during treatment process and deactivation of catalysts due to the adsorption of Cl species on the active sites remain yet to be solved. Therefore, the development of catalysts having high performance in converting CVOCs is of great significance. The catalytic oxidation of DCM, as a model of alkyl chloride pollutants, has been investigated widely over Pt catalysts supported on acidic materials [6–8], especially on ␥-alumina. DCM can react readily with hydroxyl groups of catalysts to form the reactive
∗ Corresponding author. Tel.: +86 21 64253372; fax: +86 21 64253372. E-mail address: [email protected] (X. Wang). 0926-860X/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.apcata.2013.11.006
© 2013 Published by Elsevier B.V.
intermediates which can be oxidized to carbon dioxide [7,8]. It was often considered that the activity for catalytic combustion of DCM was related to the synergy between acidic role and oxidization [9,10]. Though Pt catalysts were more preferentially acceptable than other catalysts, the significant deactivation was observed at an initial stage of reaction due to the interaction of DCM with hydroxyl groups [7]. As found in our previous work, Cl species produced during the CVOC oxidation strongly adsorbed on the active sites and hence retarded the accesses of reactants [11]. The removal of Cl species was often rate-controlling step in many cases of CVOC catalytic combustion [11,12]. Raising temperature will promote the removal of Cl species via the desorption of Cl species or the oxidation of Cl species (Deacon reaction) [13]. However, it is desirable to develop more active catalytic materials which can enhance the removal of Cl species at low temperature. CeO2 presented the high activity for catalytic combustion of various CVOCs, which was attributed to the high ability for dissociating C Cl bonds, high oxygen mobility and abundant oxygen defects. Recently, it has been reported that the addition of Ce element promoted for DCM decomposition over Pt/Al2 O3 catalyst [14]. Zhou et al. found that Cr–Ce–USY zeolite catalysts possessed high activity for catalytic decomposition of DCM, TCE and DCE [15]. In our previous studies, the introduction of Ce element can improve evidently both the activity and stability of transition metal oxide (TMO) catalysts in the catalytic combustion of chlorobenene [11,12,16]. The high stability of ceria-doped TMO catalysts derives from the fact that the adsorbed inorganic chlorine species or dissociated Cl can be removed rapidly from the surface or active sites via the formation and reoxidization of intermediate TMOy Clx or TMClx .
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
As known, Ru-based catalysts have been employed for commercialized chlorine production via HCl oxidation, due to its extraordinary stability and easier Cl2 evolution [13,17]. Additionally, RuO2 has received considerable attention as a heterogeneous catalyst used in oxidation of various compounds, such as carbon monoxide, soot, methane and propane [18–21]. Recently, Ru/Al2 O3 catalyst was used in the catalytic combustion of TCE and presented high activity [22]. Our recent work showed that the high activity and stability of Ru/CeO2 for catalytic combustion of chlorinated aromatics were related to its ability for removal of Cl species from the surface of catalyst via Deacon reaction [16,23]. Additionally, we also found that Ru/TiO2 presented high activity for the combustion of DCM, due to its high removal of Cl species [24]. In this work, we used Ce-doped Al2 O3 as supports, and prepared Ru catalysts by wet impregnation with RuCl3 aqueous solution, and investigated in details the effect of Ce on catalytic decomposition of chlorinated methane over Ru/Al2 O3 catalysts through characterization, reaction kinetics and auxiliary experiments in the several aspects such as the synergistic effect of high acidity, high oxygen mobility of CeOx and the stability of RuOx exposed to Cl species in the catalytic decomposition of DCM. 2. Experimental 2.1. Catalyst preparation Ce-Al2 O3 supports were prepared by a co-precipitation method. A detailed process was as follows: the ammonia and an aqueous solution of Al(NO3 )3 ·9H2 O were added dropwise to an aqueous solution of Ce(NO3 )3 ·6H2 O under vigorous stirring. The pH value of solution was controlled at 8–8.5. The precipitates were aged for 12 h, washed with distilled water, then dried at 100 ◦ C for 12 h and finally calcined in air at 550 ◦ C for 4 h. The synthesized CeAl2 O3 samples were impregnated with 0.198 M (0.02 gRu mL−1 ) RuCl3 aqueous solution at 25 ◦ C for 12 h, dried at 100 ◦ C for 12 h in a vacuum oven, and finally calcined in air at 550 ◦ C for 4 h in a tubular furnace. Thus 1 wt% Ru/Ce-Al2 O3 catalysts with Ce content of 0% (pure Al2 O3 ), 1%, 3%, 5%, 7% and 10%, as determined by X-ray Fluorescence Spectrometer (XRF), were obtained. 2.2. Catalysts characterization The powder X-ray diffraction patterns (XRD) of the samples were recorded on a Rigaku D/Max-rC powder diffractometer using Cu K␣ radiation (40 kV and 100 mA). The diffractograms were recorded within the 2 range of 10–80◦ with a 2 step size of 0.01◦ and a step time of 10 s. The nitrogen adsorption and desorption isotherms were measured at −196 ◦ C on an ASAP 2400 system 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. Ru content was determined by XRF using a Shimadzu (XRF-1800) wavelength dispersive X-ray fluorescence spectrometer. Samples were prepared in the form of uniform tablets (20 mm of diameter) by pressing (30 MPa) the powder of catalyst. Transmission electron microscopy (TEM) analysis was carried out using a JEM-2010F operated by an accelerating voltage of 200 kV. H2 -temperature programming reduction (H2 -TPR) was investigated by heating catalysts (100 mg) in H2 (5 vol.%)/Ar flow (30 mL min−1 ) at a heating rate of 10 ◦ C min−1 from 50 to 750 ◦ C. The hydrogen consumption was monitored by thermo-conductivity detector. Before the H2 -TPR analyses, the samples were heated for 60 min in Ar flow at 300 ◦ C. The Raman spectra were obtained on a Renishaw in Viat + Reflex spectrometer equipped with a CCD detector at ambient temperature and moisture-free conditions. The emission line at 514.5 nm from an Ar+ ion laser (Spectra Physics)
443
was focused, analyzing spot about 1 mm, on the sample under the microscope. 2.3. Activity evaluation The activity and stability of catalysts were tested at atmospheric pressure in a continuous flow micro-reactor made of a quartz tube with inner diameter of 4 mm. 0.3 mL catalyst (0.185 g) as the reaction bed was packed. The feeding flow rate to the reactor was set at 50 cm3 min−1 and the gas hourly space velocity (GHSV) was maintained at 10,000 h−1 . Feed stream to the reactor was prepared by delivering liquid DCM with a syringe pump into dry gas mixture composed of a given concentration of O2 and N2 balance, which were metered by a mass flow controller, controlling DCM concentration in the feed stream at 750 ppm or 1500 ppm. The injection part was electrically heated to ensure complete evaporation of DCM. The reactor temperature was measured with a thermocouple located just at the exit of the micro-reactor. The temperature was raised by steps from 150 to 450 ◦ C. The effluent gases were analyzed on-line at a given temperature by using three gas chromatographs (GC), two equipped with FID and ECD, respectively, for the quantitative analyses of organic chlorinated compounds, and the other one with TCD for CO and CO2 . The concentrations of Cl2 and HCl were analyzed by the effluent stream bubbling through a 0.0125 N NaOH solution, and chlorine concentration was then determined by the titration with ferrous ammonium sulphate using N,N-diethyl-p-phenylenediamine as an indicator [25]. The concentration of chloride ions in the bubbled solution was determined by using a chloride ion selective electrode [26]. 3. Results and discussion 3.1. Characterization of catalysts Ru loading in RuOx catalysts, determined by XRF, is close to the nominal value of 1 wt% (Table 1), indicating that the impregnation of ruthenium with RuCl3 aqueous solution onto the Al2 O3 or CeAl2 O3 supports is effective. Fig. 1 shows the results of XRD analyses. On the XRD patterns of Al2 O3 and Ce-Al2 O3 supports, three broad diffraction peaks appearing at 36.9◦ , 45.9◦ and 66.6◦ can be ascribed to spinal phase (␥-Al2 O3 , PDF #50-0741). With the increase of Ce content, there appear four weak peaks at 28.6◦ , 33.3◦ , 47.5◦ and 56.5◦ , ascribed to CeO2 cerianite with a fluorite-like structure (PDF #43-1002). A new weak and broad peak appears at 60.1◦ , probably a reflection from CeAlO3 (PDF #28-0260) (the other two peaks at 33.6◦ and 48.3◦ overlap with the reflections at 33.3◦ and 47.5◦ from CeO2 ). The diffraction peaks of RuO2 appear at 28.0◦ , 35.0◦ and 54.2◦ for all Ru/Ce-Al2 O3 catalysts [27]. According to the Scherrer equation applied to 1 1 0 reflection of RuO2 , the size of RuO2 particles is estimated to be 11–20 nm, dependent on Ce content (Table 1). The average size of RuO2 particles is estimated to be 20.9 nm for Ru/Al2 O3 catalyst, while for Ru/7%Ce-Al2 O3 catalyst, ∼11 nm. The decrease in lattice parameters of CeO2 to some extent (Table 1) can be ascribed to the entrance of some Ru species into the CeO2 cerianite [23]. The disappearance of diffraction peak at 60.1◦ indicates that the interaction of RuOx with Ce3+ –O–Al. The size of CeO2 particles of Ru/Ce-Al2 O3 catalysts increases to 6.3–9.3 nm from 5.0–7.6 nm for Ce-Al2 O3 samples. Moreover, the BET area of samples increases gradually with the increase of Ce content in the range of 1–10%, confirming that the addition of Ce can promote ␥-Al2 O3 dispersion. Fig. 2(a)–(e) shows TEM images of the Ce-Al2 O3 and Ru/Ce-Al2 O3 samples. For 1%Ce-Al2 O3 (Fig. 2(a)) and 7%Ce-Al2 O3 (Fig. 2(b)) supports, nanoparticles have irregular shape. HRTEM (Fig. 2(f) and (g)) confirms the existence of 5–6 nm CeO2 particles in 7%Ce-Al2 O3 and Ru/7%Ce-Al2 O3 samples, which is in good agreement with the
444
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
Table 1 Surface and structural properties of Ru/Ce-Al2 O3 catalysts with various Ce content. Catalysts
Ru loading a (wt%)
SBET (m2 /g)
LCeO2 b (nm)
NCeO2 c (nm)
NRuO2 d (nm)
Al2 O3 1%Ce-Al2 O3 3%Ce-Al2 O3 5%Ce-Al2 O3 7%Ce-Al2 O3 10%Ce-Al2 O3 Ru/Al2 O3 Ru/1%Ce-Al2 O3 Ru/3%Ce-Al2 O3 Ru/5%Ce-Al2 O3 Ru/7%Ce-Al2 O3 Ru/10%Ce-Al2 O3
– – – – – – 0.975 0.968 0.961 0.958 0.919 0.908
171 178 181 194 200 198 175 184 181 196 209 190
– ND ND 5.415 5.416 5.413 – ND ND 5.405 5.407 5.401
– NDe ND 5.0 5.8 7.6 – ND ND 6.3 7.1 9.3
– – – – – – 20.9 20.8 19.7 17.6 11 ND
a b c d e
H2 consumption (mol gcat ) Actual
Theoretical
Actual/theoretical
– 3 10 35 101 190 376 431 679 1004 1397 –
– 36 107 179 250 357 193 229 300 372 443 –
– 0.09 0.10 0.20 0.40 0.53 1.96 1.88 2.28 2.70 3.20 –
Determined by XRF. CeO2 lattice parameters calculated from XRD results. CeO2 crystal size. RuO2 crystal size, calculated by the Scherrer equation based on XRD results. ND, not be detected.
average mean diameter estimated based on the XRD data (5–7 nm). CeO2 mainly exposes the (1 1 1) plane with the distance of 0.31 nm. Very thin particles with lattice fringes of distance about 0.32 nm (Fig. 2(g)) can be observed, corresponding to the 1 1 0 lattice plane of the tetragonal RuO2 . Also, the particle size of RuO2 is estimated to be 6–9 nm, similar to the results obtained by XRD analyses. The possible catalyst structures were investigated by Raman spectroscopy technique (Fig. 3). For pure CeOx sample, the band appearing at 460 cm−1 can be attributed to the mode (F2g) of fluorite-like structure [28]. As known, fluorite-structure is a cubic structure (fcc), in which cations are located on the corners and in the face centers while oxygen atoms are located at the tetrahedral sites. The spectra of F2g band are dominated by oxygen lattice vibrations and are sensitive to crystalline symmetry [29]. For 1%Ce-Al2 O3 sample, the F2g band was not observed, indicating
Fig. 1. XRD patterns of Ce-Al2 O3 and Ru/Ce-Al2 O3 catalysts with various Ce contents; 1 – Al2 O3 , 2 – 1%Ce-Al2 O3 , 3 – 3%Ce-Al2 O3 , 4 – 5%Ce-Al2 O3 , 5 – 7%CeAl2 O3 , 6 – 10%Ce-Al2 O3 , 7 – Ru/Al2 O3 , 8 – Ru/1%Ce-Al2 O3 , 9 – Ru/3%Ce-Al2 O3 , 10 – Ru/5%Ce-Al2 O3 , 11 – Ru/7%Ce-Al2 O3 and 12 – Ru/10%Ce-Al2 O3 .
that the interaction between Al2 O3 and Ce species must deform the fluorite-like structure. However, this band appears at 460 cm−1 for 3%Ce-Al2 O3 sample and becomes strong with the increase in Ce content, due to the improvement of CeO2 crystalline symmetry. As reported in the literature, Eg, A1g, and B2g modes of RuO2 are located at about 523, 646, and 710 cm−1 , respectively [30]. For Ru/Ce-Al2 O3 samples, there appear two weak and broad bands at 520 and 630 cm−1 , which can be attributed to Eg and A1g modes. The band corresponding to B2g modes can not be observed, indicating the existence of interaction between RuO2 and CeOx . Moreover, the intensity of F2g band decreases to a significant extent, which further confirms the interaction between CeO2 and RuO2 . Probably, the entrance of Ru into CeO2 lattice results in the decrease of CeO2 symmetry, especially for the samples containing low Ce content. The results of TPR analyses for catalysts are shown in Fig. 4(a) and (b). The reduction of pure CeO2 occurs at 437 and 522 ◦ C in the range of experimental temperature (Fig. 4(a)), associated with the reduction of surface Ce4+ ions from small and large size of CeO2 particles. [31]. For the Ce-Al2 O3 samples, the peak at 437 ◦ C becomes strong gradually with the increase in Ce content. Another peak appearing at higher temperature can be assigned to the reduction of Ce4+ species strongly interacted with Al2 O3 for the samples with low Ce content, and the reduction of surface Ce4+ species from large CeO2 particles for the samples with high Ce content (Fig. 4(a)). H2 consumption (Table 1) shows that the actual values are much lower than theoretical values (the amount of hydrogen required for the reduction was calculated on the basis of 2CeO2 + H2 → Ce2 O3 + H2 O), probably owing to the formation of Ce3+ –O–Al and the reduction of Ce4+ in bulk at higher temperature beyond the experimental range (for the samples with higher Ce content). For Ru/Al2 O3 sample, the reduction of RuO2 to Ru0 , as shown in Fig. 4(b), occurs at 176 ◦ C. With the increase in Ce content, the initial reduction of Ru4+ shifts to low temperature gradually and finally drops to 74 ◦ C for Ru/CeO2 sample. This phenomenon indicates that the reduction of Ru species is promoted by a high degree of coordinative unsaturation and, perhaps, by neighboring Ce4+ ions, which, in turn, undergo reduction to Ce3+ species. Thus, the reduction peak area below 300 ◦ C increases with Ce content increase, indicating the oxygen mobility of CeO2 can be promoted by Ru incorporation. H2 consumptions of the Ru/Ce-Al2 O3 catalysts are much higher than the theoretical values calculated on the basis of RuO2 + 2H2 → Ru0 + 2H2 O, probably due to spilling over hydrogen from Ru to the supports [32]. The ratios of actual value to theoretical value increase significantly with Ce content increase. Additionally,
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
445
Fig. 2. TEM images of Ce-Al2 O3 and Ru/Ce-Al2 O3 catalysts: (a) 1%Ce-Al2 O3 , (b) 7%Ce-Al2 O3 , (c) Ru/Al2 O3 , (d) Ru/1%Ce-Al2 O3 and (e) Ru/7%Ce-Al2 O3 ; HRTEM images of 7%Ce-Al2 O3 (f) and Ru/7%Ce-Al2 O3 (g).
the reduction peak at about 530 ◦ C disappears, implying that the sites of supporting RuO2 is closed to these Ce species that can be reduced readily by the dissociated H atoms on Ru species at lower temperature. The results of NH3 -TPD analyses for catalysts are shown in Table 2. For pure Al2 O3 sample, the amount of acidity is estimated to be 294 mol/g (Table 2). For the Ce-Al2 O3 catalysts, the amount of acidic sites increases from 340 mol/g for 3%Ce-Al2 O3 to 375 mol/g for 7%Ce-Al2 O3 . The increased new acidic sites could result from Ce species highly dispersed on Al2 O3 surface. When Ce content was too high, such as 10%, the acidity decreases to a significant extent, due to the coverage of CeO2 over Al2 O3 surface. In
Fig. 3. Raman spectra of catalysts; 1 – 1%Ce-Al2 O3 , 2 – 3%Ce-Al2 O3 , 3 – 5%Ce-Al2 O3 , 4 – 7%Ce-Al2 O3 , 5 – 10%Ce-Al2 O3 , 6 – Ru/1%Ce-Al2 O3 , 7 – Ru/3%Ce-Al2 O3 , 8 – Ru/5%CeAl2 O3 , 9 – Ru/7%Ce-Al2 O3 , 10 – Ru/10%Ce-Al2 O3 and 11 – CeO2 .
the case of supported RuO2 catalysts, the amount of acidic sites increases, probably due to the presence of residual Cl− species in the catalyst because of the usage of RuCl3 precursor. 3.2. Catalytic activity Under the reaction condition of 750 ppm DCM, 20% O2 , N2 balance and GHSV = 10,000 h−1 , DCM conversion over the fresh catalysts as the function of temperature is shown in Fig. 5(a), while T50% and T90% (the temperature needed for 50% and 90% conversion) as a function of Ce content, shown in Fig. 5(b). It can be seen in Fig. 5(a) that pure Al2 O3 support seems active for DCM decomposition with T50% and T90% of 304 and 341 ◦ C. With the incorporation of Ce, the conversion curves shifts significantly to low temperature. T90% decreases from 329 ◦ C for 1%Ce-Al2 O3 catalyst to 310 ◦ C for 7%CeAl2 O3 . Ru/Ce-Al2 O3 catalysts with the Ce contents of 1–7% possess higher activity and T90% drops in the range of 260–297 ◦ C (Fig. 5(b)). DCM conversion reached the complete up to 340 ◦ C over Pt/Al2 O3 [6] and to 400 ◦ C over Pd supported catalysts [33]. Obviously, Ru based catalysts are more active than other noble metal oxide catalysts for DCM decomposition. It is interesting to find that the change trend of T50% and T90% with the increase of Ce is consistent with that of the temperatures at which Ru4+ can be reduced at initial time, indicating that the reducibility of catalysts is responsible for the activity (Fig. 5(b)). On the other hand, T50% and T90% is also related to the acidic amount of catalysts. As reported previously, the acidity is favorable for activating C Cl bond [6–8]. Although Ru/10%CeAl2 O3 possesses the best oxygen mobility, its amount of acidic sites is lest. As a result, the activity of Ru/10%Ce-Al2 O3 catalyst decreases significantly. Fig. 6 presents the stability of catalysts in DCM catalytic decomposition under reaction condition as the above mentioned. Based on the analyses of all component including reactants and products in the effluent, chlorine uptake on the surface of catalysts treated in reaction feed at 250 and 270 ◦ C for 1000 min is quantitatively determined by means of Cl species balance and shown in Table 2 (TPSR on the above treated catalysts indicated that no adsorption of organic chlorinated compound deposits). At 250 and 270 ◦ C, DCM conversion on Ru/Al2 O3 catalyst decreases within the first 400 min rapidly from 45% and 61% to zero, indicating that the stability of Ru/Al2 O3
446
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
Table 2 The catalytic performance of Ce-Al2 O3 and Ru/Ce-Al2 O3 catalysts with various Ce content. Samples
NH3 uptake (mol/g)
Al2 O3 1%Ce-Al2 O3 3%Ce-Al2 O3 7%Ce-Al2 O3 Ru/Al2 O3 Ru/1%Ce-Al2 O3 Ru/3%Ce-Al2 O3 Ru/7%Ce-Al2 O3
294 318 340 375 350 381 396 470
a b c d
Cl uptakea (mmol/g−1 cat ) 250 ◦ C
270 ◦ C
1.77 – – – 1.46 1.16 0.81 0.68
1.65 – – – 0.87d 0.83 0.64 0.51
Clb (at%)
– – – – 0.58 0.52 0.38 0.35
T50 /T90 c (◦ C) CHCl3
CH3 Cl
262/301 262/297 265/295 262/301 258/295 250/281 226/267 205/245
305/362 397/348 285/325 272/313 260/292 259/281 246/278 235/275
Cl uptake, determined by subtraction of the amount of HCl and Cl2 measured at the reactor outlet from the total chlorine amount. Determined by XPS for the used catalysts for 1000 min at 270 ◦ C. Gas composition of feed: 750 ppm chlorinated methane, 20% O2 and N2 balance; GHSV = 10,000 h−1 . Obtained at 285 ◦ C.
catalyst cannot be improved essentially compared with pure Al2 O3 . Moreover, no inorganic chlorine compound can be detected in the effluent during the deactivation process, implying that the Cl species deposited on the active sites on Ru/Al2 O3 catalyst and probably resulted in the deactivation of Ru/Al2 O3 catalyst. The chlorine uptake of Ru/Al2 O3 catalyst at 250 ◦ C is estimated to be 1.46 mmol Cl/gcat . By raising temperature to 285 ◦ C, substantial decrease in DCM conversion is not observed, and the conversion can be maintained at about 65% for 1000 min. The corresponding chlorine uptake decreases to 0.68 mmol Cl/gcat , indicating the removal of Cl species is promoted by raising temperature. The stability of Al2 O3 in DCM decomposition was studied widely, and its deactivation was ascribed mainly to the consumption of hydroxyl group [6]. For the Ru/Ce-Al2 O3 catalysts, the stability at 250 ◦ C increases with Ce content. The activity of Ru/1%Ce-Al2 O3 catalyst cannot be maintained at 250 ◦ C, similar to the case of Ru/Al2 O3 catalyst. However, raising Ce content to 3%, the conversion keeps at 49–52% within 1000 min. This phenomenon is consistent with previously reported results that Ce catalysts supported on acidic zeolites presented good stable activity for the CVOCs catalytic combustion [34]. On Ru/7%CeAl2 O3 catalyst, a higher stable activity with DCM conversion of 80% can be reached at 250 ◦ C. The chlorine uptakes of Ru/1%CeAl2 O3 , Ru/3%Ce-Al2 O3 and Ru/7%Ce-Al2 O3 catalysts used at 250 ◦ C
for 1000 min are estimated to be 1.16, 0.81 and 0.68 mmol Cl/gcat , respectively. Raising reaction temperature to 270 ◦ C, the chlorine uptakes of Ru/1%Ce-Al2 O3 , Ru/3%Ce-Al2 O3 and Ru/7%Ce-Al2 O3 catalysts decreased by 20–30% and their corresponding conversions keep at 52%, 81% and 96%, respectively. Ru/7%Ce-Al2 O3 catalyst in particular was tested at 270 ◦ C for another 1000 h with the conversion as high as 96%. Therefore, the removal of these Cl species is a key step to improve the stability of catalysts. The analyses of containing-carbon product are shown in Fig. 7(a)–(c). CO2 is main containing-carbon product over the Ru/Ce-Al2 O3 catalysts, which is in coincident with the results obtained during DCM decomposition over noble metal oxides supported on acidic materials [6,7]. The selectivity for CO2 increases with the increase in temperature and reaches almost 100% at 325 ◦ C at which no other containing-carbon product can be detected. CH3 Cl can be detected at different level for all catalysts (Fig. 7(b)). The amount order of CH3 Cl is produced: Al2 O3 > Ce-Al2 O3 > Ru/Al2 O3 > Ru/Ce-Al2 O3 , lined with that of Cl uptake (Table 2). The formation of CH3 Cl should be promoted by chlorination. Over Ru/7%Ce-Al2 O3 catalyst with low Cl uptake, the lowest amount (<8 ppm) of CH3 Cl is formed, while up to 45 ppm CH3 Cl is obtained over Ru/Al2 O3 catalyst with high Cl uptake. Al2 O3 having highest Cl uptake (1.77 mmol g−1 ) presents 50% selectivity
Fig. 4. H2 -TPR profiles of Ce-Al2 O3 (a) and Ru/Ce-Al2 O3 (b) with various Ce contents; (a): 1 – 1%Ce-Al2 O3 , 2 – 3%Ce-Al2 O3 , 3 – 5%Ce-Al2 O3 , 4 – 7%Ce-Al2 O3 , 5 – 10%Ce-Al2 O3 , and 6 – CeO2 ; (b): 1 – Ru/Al2 O3 , 2 – Ru/1%Ce-Al2 O3 , 3 – Ru/3%Ce-Al2 O3 , 4 – Ru/5%Ce-Al2 O3 , 5 – Ru/7%Ce-Al2 O3 , and 6 – Ru/CeO2 .
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
447
Fig. 6. DCM conversion with time on stream over Ru/Ce-Al2 O3 catalysts with various Ce contents at different temperatures; gas composition: 750 ppm DCM, 20% O2 and N2 balance; GHSV = 10,000 h−1 .
The activity order for the catalytic combustion of chlorinated methane over the Ru/Ce-Al2 O3 catalysts is CHCl3 > CH3 Cl > CH2 Cl2 . 3.3. TPSR study
Fig. 5. The conversion curves of DCM over catalysts with various Ce contents (a) and T50% , T90% , the initial reduction temperature (TR ) and the acidic amount of Ru/CeAl2 O3 catalysts as functions of Ce contents (b); gas composition: 750 ppm DCM, 20% O2 and N2 balance; GHSV = 10,000 h−1 .
for CH3 Cl·CHCl3 , as a product of chlorination, is detected at higher than 260 ◦ C. Although the CHCl3 amount is trace, it is much higher over Ru/Al2 O3 catalyst than those over the Ru/Ce-Al2 O3 catalysts (Fig. 7(c)), of which Ru/7%Ce-Al2 O3 catalyst presents the lowest amount of CHCl3 , <1 ppm. In order to understand these phenomena, the activity of catalysts for CH3 Cl and CHCl3 catalytic combustion was investigated and the corresponding T50% and T90% are listed in Table 2. It can be seen that all Ru catalysts under study are highly active in CH3 Cl and CHCl3 decomposition, especially for the Ru/CeAl2 O3 catalysts. CH3 Cl and CHCl3 will be oxidized quickly if formed.
In order to investigate the process involving Cl species, the feed containing DCM of 2000 ppm and oxygen of 20% pass through Ru/Al2 O3 and Ru/7%Ce-Al2 O3 catalysts and the effluent was monitored by mass spectroscopy within a given range of temperature (Fig. 8(a) and (b)). It can be seen in Fig. 8(a) that for Ru/Al2 O3 catalyst, the evolvement of CO2 occurs at 223 ◦ C, and increases with temperature. At 300 ◦ C, this signal keeps constant. The signals of HCl and Cl2 appear at 300 ◦ C at the same time. This phenomenon can be explained by the fact that HCl produced from acidic sites transfers toward the active sties for Deacon reaction (HCl + O2 → Cl2 + H2 O) on RuOx and is oxidized into Cl2 (thus, H2 O is formed). The difference between the temperatures of appearing CO2 and inorganic Cl species indicates the strong adsorption of these Cl species. Thus, the evolvement of HCl and Cl2 can still be observed when DCM in the feed is cut off. For Ru/7%Ce-Al2 O3 catalyst, the signals of HCl and Cl2 can be detected at lower temperature (Fig. 8(b)). Moreover, the above mentioned temperature difference is smaller, compared with the case of Ru/Al2 O3 catalyst. Obviously, the incorporation of
Fig. 7. The product distributions during DCM decomposition over Ru/Ce-Al2 O3 catalysts with various Ce contents; (a) CO2 , (b) CH3 Cl and (c) CHCl3 ; gas composition: 750 ppm DCM, 20% O2 and N2 balance; GHSV = 10,000 h−1 .
448
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
Fig. 8. The mass spectroscopy of reactants and products during TPSR; (a) Ru/Al2 O3 , (b) Ru/7%Ce-Al2 O3 ; gas composition: 2000 ppm DCM, 20% O2 and N2 balance; GHSV = 10,000 h−1 .
Fig. 9. The conversion curves of DCM decomposition over Ru/7%Ce-Al2 O3 and Ru/Al2 O3 catalysts under various oxygen concentrations at GHSV = 10,000 h−1 ; (a) 750 ppm DCM; (b) 1500 ppm DCM.
Ce into Al2 O3 decreases the strength of Cl adsorption on active sites so that the escape of HCl species can occur at lower temperature.
750 or 1500 ppm DCM, the rate increases within 0.2–20% [O] by 0.6 mol/min gcat . The rate at 3% [O] or higher is much lower than those obtained over Ru/7%Ce-Al2 O3 catalyst. These phenomena indicate that the activity of Ru/Al2 O3 catalyst is not quite sensitive toward the change of active oxygen species on RuOx . In other words,
3.4. Kinetics consideration In order to understand the kinetics of DCM decomposition, the effect of oxygen concentration ([O]) on conversion was investigated on Ru/7%Ce-Al2 O3 and Ru/Al2 O3 catalysts under 750 and 1500 ppm DCM, and the results are shown in Fig. 9(a) and (b). For Ru/7%Ce-Al2 O3 catalyst, significant shifts of conversion curves to low temperature were observed as [O] increases. This shift occurs to a much smaller extent for Ru/Al2 O3 catalyst. Moreover, at a given temperature, with the increase in [O] the conversion under at 1500 ppm DCM increases more than that at 750 ppm DCM. Here, we calculate the reaction rates at 250 ◦ C (Fig. 10) and obtain a considerable linear relation of rate with [O] within 0.2–3% [O] at 750 ppm DCM. For Ru/7%Ce-Al2 O3 catalyst, the rate increases from 1.4 mol/min gcat at 0.2% [O] up to 4.77 mol/min gcat at 3% [O], indicating that the process involving oxygen may be slow step. Moreover, raising DCM concentration up to 1500 ppm, a more quick increase in rate within 3–20% [O] than that at 750 ppm DCM can be observed. This larger dependence of rate on [O] can be explained by the fact that Ru/7%Ce-Al2 O3 catalyst possesses high oxygen mobility and the active oxygen effectively increases with gas [O] so as to promote the removal of Cl species from the surface of Ru/7%Ce-Al2 O3 catalyst [11,12]. In the case of Ru/Al2 O3 catalyst, the dependence of rate or conversion on [O] is not strong. At either
Fig. 10. The effect of oxygen concentration on DCM decomposition at 250 ◦ C over Ru/7%Ce-Al2 O3 and Ru/Al2 O3 catalysts at GHSV = 10,000 h−1 ; the data within bracket are the conversion.
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
oxygen mobility is low for Ru/Al2 O3 catalyst without Ce species. As known from the stability and TPSR tests, the evolvement of HCl from Ru/Al2 O3 at 250 ◦ C is difficult, which can be related to low oxygen mobility of Ru/Al2 O3 catalyst. To obtain more insight into the promotion function by CeO2 , the physico-chemical properties of different catalysts were characterized. The results show that after being used for 1000 min at 270 ◦ C for the combustion of DCM (750 ppm DCM, 20% O2 and N2 balance), the specific surface area of all catalysts under study rarely change, no phase transformations are observed in their XRD patterns and the obvious deposit of coke on the catalyst surface was not detected by TG techniques. The change of XPS spectra of species (Ce 3d, Ru 3d, Al 2p and O 1s) on the used catalyst surface was not observed. However, Cl 2p peak appearing at around 198 eV on XPS spectra and EDS results (not shown) confirm the presence of chlorine species on all of the used catalysts. The amount of deposited Cl decreases from 0.58 at.% for Ru/Al2 O3 catalyst to 0.35 at.% for Ru/7%Ce-Al2 O3 catalyst, estimated by XPS. This result is consistent with the order of Cl uptake over various catalysts in Table 2. Considering no changes of binding energy for various species of the used catalysts (see Support Information), therefore, the deactivation of catalysts is related to the strong adsorption of HCl or Cl2 produced from the decomposition of DCM. Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata. 2013.11.006. From the above results, it can be concluded that oxygen species can efficiently react with the Cl species that adsorbed with moderate strength on the surface. For Ru/Al2 O3 catalyst or Ru/Ce-Al2 O3 catalysts with low amount of Ce, surface oxygen migrates to a smaller extent and the adsorption of Cl species is strong. So, these catalysts need high temperature to remove the adsorbed Cl species, and present a low stable activity. There are more active oxygen species for Ru/7%Ce-Al2 O3 catalyst to remove the Cl species adsorbed on the surface of catalysts during the reaction, so that more active sites can be accessible. Therefore, the stability of Ru/7%Ce-Al2 O3 catalyst in the catalytic combustion of DCM is promoted at 250 ◦ C, due to a larger decrease in chlorine species deposit. 3.5. The effect of water on DCM decomposition Because of water detected in the exhaust gas, it is one part of the work to investigate the effect of water on the activity of Ru/7%Ce-Al2 O3 and Ru/Al2 O3 catalysts for DCM decomposition (Fig. 11). In the presence of 3% water, the low conversion section on Ru/7%Ce-Al2 O3 catalyst shifts significantly to high temperature. The temperature needed for 15% conversion increases from 162 ◦ C to 202 ◦ C. At higher than 250 ◦ C, the conversion curve becomes approaching to that without water. At low temperature, water is dissociated probably on CeO2 species into H+ and OH− species so as to retard the transfer of active oxygen. This inhibition was observed during the combustion of trichloroethylene over cerium oxide in wet air [35]. For Ru/Al2 O3 catalyst, where the inhibition of 3% water on DCM decomposition is observed at low temperature only to a smaller extent. With raising temperature, the adsorption of water becomes difficult, and the effect of water becomes weak. Product analyses show that CO2 was main carbon-containing product, and the evolvement of HCl was observed almost at the same time as DCM was converted, indicating that Cl deposition was inhibited by water. Cl2 was not detected in the effluent during reaction, because Deacon reaction cannot proceed in the presence of 3 wt % water. CH3 Cl and CHCl3 are detected only in the reaction over Ru/Al2 O3 catalyst, with a largest amount as 5 ppm, directly verifying that the formation of CH3 Cl and CHCl3 is related to Cl deposition. At 250 ◦ C, the stability of Ru/7%Ce-Al2 O3 and Ru/Al2 O3 catalysts can be maintained (Fig. 11 insert), further confirming that the removal of Cl
449
Fig. 11. The effect of water on the activity of Ru/Al2 O3 and Ru/7%Ce-Al2 O3 catalysts; insert: the stability at 250 ◦ C in wet feed; wet: 3% H2 O, dry: absence of water; gas composition: 750 ppm DCM, 20% O2 and N2 balance; GHSV = 10,000 h−1 .
species from active sites is critical to promote the stability of Ru catalysts. 4. Conclusions Ru oxide catalysts supported on Ce-Al2 O3 and Al2 O3 were prepared by wet impregnation with RuCl3 aqueous solution and tested in catalytic combustion of DCM. The characterization of XRD, N2 adsorption, TEM/HRTEM, H2 -TPR, Raman and NH3 -TPD shows the existence of interaction between Ce species and Al2 O3 . As a result, the acidity increases to some extent. The reducibility of Ru/CeAl2 O3 is greatly increased by the addition of Ce. During the catalytic decomposition of DCM, Ru/Ce-Al2 O3 catalysts present an outstanding stable activity. The measurement of chlorine uptake shows the Cl deposition is responsible for the deactivation of Ru/Al2 O3 catalyst. High stable activity of Ru/Ce-Al2 O3 catalysts (1000 h, higher than 96% conversion at 270 ◦ C for Ru/7%Ce-Al2 O3 catalyst) can be ascribed to the synergistic effect of high acidity, high oxygen mobility of CeOx and the stability of RuOx exposed to Cl species. Therefore, Ru/Ce-Al2 O3 catalysts will become useful in the practice of removal of alkyl chlorides. Acknowledgements We would like to acknowledge the financial support from National Basic Research Program of China (No. 2010CB732300, 2011AA03A406) and National Natural Science Foundation of China (No. 21277047) and Commission of Science and Technology of Shanghai Municipality (11JC1402900). References [1] M.J. Morra, V. Borek, J. Koolpe, J. Environ. Qual. 29 (2000) 706–715. [2] F. Alonso, I.P. Beletskaya, M. Yus, Chem. Rev. 102 (2002) 4009–4091. [3] M. Taralunga, B. Innocent, J. Mijoin, P. Magnoux, Appl. Catal. B: Environ. 75 (2007) 139–146. [4] F.F. Bertinchamps, C. Gregoire, E.M. Gaigneaux, Appl. Catal. B: Environ. 66 (2006) 1–9. [5] M. Taralunga, J. Mijoin, P. Magnoux, Catal. Commun. 7 (2006) 115–121. [6] I. Maupin, L. Pinard, J. Mijoin, P. Magnoux, J. Catal. 291 (2012) 104–109. [7] D.M. Papenmeier, J.A. Rossin, Ind. Eng. Chem. Res. 33 (1994) 3094–3103. [8] R.W. van den Brink, P. Mulder, R. Louw, G. Sinquin, C. Petit, J.P. Hindermann, J. Catal. 180 (1998) 153–160. [9] R. López-Fonseca, B. de Rivas, J.I. Gutiérrez-Ortiz, A. Aranzabal, J.R. GonzálezVelasco, Appl. Catal. B: Environ. 41 (2003) 31–42. [10] P.S. Chintawar, H.L. Greene, Appl. Catal. B: Environ. 13 (1997) 81–92.
450
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
[11] Y. Dai, X.Y. Wang, Q.G. Dai, D. Li, Appl. Catal. B: Environ. 111–112 (2012) 141–149. [12] Y. Dai, X.Y. Wang, D. li, Q.G. Dai, J. Hazard. Mater. 188 (2011) 132–139. [13] N. López, J. Gómez-Segura, R.P. Marín, J. Pérez-Ramírez, J. Catal. 255 (2008) 29–39. [14] Q.-Y. Chen, N. Li, M.-F. Luo, J.-Q. Lu, Appl. Catal. B: Environ. 127 (2012) 159–166. [15] Q.Q. Huang, X.M. Xue, R.X. Zhou, J. Hazard. Mater. 183 (2010) 694–700. [16] Q.G. Dai, S.X. Bai, Z.Y. Wang, X.Y. Wang, G.Z. Lu, Appl. Catal. B: Environ. 126 (2012) 64–75. [17] S. Zweidinger, J.P. Hofmann, O. Balmes, E. Lundgren, H. Over, J. Catal. 272 (2010) 169–175. [18] H. Over, Y.D. Kim, A.P. Seitsonen, S. Wendt, E. Lundgren, M. Schmid, P. Varga, A. Morgante, G. Ertl, Science 287 (2000) 1474–1476. [19] K. Villani, C.E.A. Kirschhock, D. Liang, G. Van Tendeloo, J.A. Martens, Angew. Chem. Int. Ed. 45 (2006) 3106–3109. [20] I. Balint, A. Miyazaki, K.-I. Aika, J. Catal. 220 (2003) 74–83. [21] J. Okal, M. Zawadzki, W. Tylus, Appl. Catal. B: Environ. 101 (2011) 548–559. ˜ [22] B. Miranda, E. Díaz, S. Ordónez, F.V. Díez, Catal. Commun. 7 (2006) 945–949. [23] Q.G. Dai, S.X. Bai, Z.Y. Wang, X.Y. Wang, G.Z. Lu, Appl. Catal. B: Environ. 129 (2013) 580–588.
[24] L. Ran, Z. Qin, Z.Y. Wang, X.Y. Wang, Q.G. Dai, Catal. Commun. 37 (2013) 5–8. [25] J.R. González-Velasco, A. Aranzabal, R. López-Fonseca, R. Ferret, J.A. GonzálezMarcos, Appl. Catal. B: Environ. 24 (2000) 33–43. [26] R. López-Fonseca, A. Aranzabal, J.I. Gutiérrez-Ortiz, J.I. Álvarez-Uriarte, J.R. González-Velasco, Appl. Catal. B: Environ. 30 (2001) 303–313. [27] J. Okal, M. Zawadzki, Appl. Catal. B: Environ. 89 (2009) 22–32. [28] B. Zhang, D. Li, X. Wang, Catal. Today 158 (2010) 348–353. [29] M.F. Luo, Z.L. Yan, L.Y. Jin, J. Mol. Catal. A: Chem. 260 (2006) 157. [30] S.Y. Mar, C.S. Chen, Y.S. Huang, K.K. Tiong, Appl. Surf. Sci. 90 (1995) 497–504. [31] F. Arena, G. Trunfio, J. Negro, B. Fazio, L. Spadaro, Chem. Mater. 19 (2007) 2269–2276. [32] X.Y. Wang, J. Ni, B.Y. Lin, R. Wang, J.X. Lin, K.M. Wei, Catal. Commun. 12 (2010) 251–254. [33] R. López-Fonseca, J.I. Gutiérrez-Ortiz, J.R. González-Velasco, Appl. Catal. B: Environ. 271 (2004) 39–46. [34] Q.Q. Huang, Z.H. Meng, R.X. Zhou, Appl. Catal B: Environ. 115–116 (2012) 179–189. [35] Q.G. Dai, X.Y. Wang, G.Z. Lu, Appl. Catal. B: Environ. 81 (2008) 192–202.
Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
The effect of Ce on catalytic decomposition of chlorinated methane over RuOx catalysts Le Ran, Zhengyi Wang, Xingyi Wang ∗ Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 6 July 2013 Received in revised form 30 October 2013 Accepted 4 November 2013 Available online 10 November 2013 Keywords: Dichloromethane CeOx –Al2 O3 RuO2 Catalytic combustion Chlorine
a b s t r a c t Ru oxide catalysts supported on Ce-Al2 O3 with various Ce contents were prepared by wet impregnation with RuCl3 aqueous solution and characterized by XRD, N2 adsorption, TEM/HRTEM, H2 -TPR, Raman and NH3 -TPD. The catalytic combustion of dichloromethane (DCM) was investigated over the Ru/Ce-Al2 O3 catalysts at first time. The Ru/Ce-Al2 O3 catalysts present an outstanding catalytic activity. Ru/7%Ce-Al2 O3 catalyst is most active, and DCM is converted completely at 270 ◦ C. At 250 ◦ C, the conversion of DCM over the Ru/Ce-Al2 O3 catalysts with 3% or higher Ce contents keeps constant within 1000 h. High stable activity of the Ru/Ce-Al2 O3 catalysts can be ascribed to the synergistic effect of acidity and high oxygen mobility.
1. Introduction Chlorinated volatile organic compounds (CVOCs) are hazardous pollutants that are considered as the most harmful organic contaminants due to their acute toxicity and strong bioaccumulation potential [1]. The safe disposal of alkyl chloride pollutants, such as CH2 Cl2 (DCM), CHCl CCl2 (TCE) and CH2 Cl CH2 Cl (DCE), has acquired great importance with the ever increasing concern for environmental protections [2]. Among various available detoxification techniques, the catalytic oxidation is an interesting one which can be efficiently performed within the temperature range from 250 to 550 ◦ C converting so dilute pollutants that otherwise cannot be thermally combusted without consumption of fuel. Most studies of the catalysts used in the catalytic oxidation of CVOCs have reported on the three types of catalysts based on noble metals, transition metals and zeolites [3–5]. At present, the problems such as the formation of polychlorinated compounds during treatment process and deactivation of catalysts due to the adsorption of Cl species on the active sites remain yet to be solved. Therefore, the development of catalysts having high performance in converting CVOCs is of great significance. The catalytic oxidation of DCM, as a model of alkyl chloride pollutants, has been investigated widely over Pt catalysts supported on acidic materials [6–8], especially on ␥-alumina. DCM can react readily with hydroxyl groups of catalysts to form the reactive
∗ Corresponding author. Tel.: +86 21 64253372; fax: +86 21 64253372. E-mail address: [email protected] (X. Wang). 0926-860X/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.apcata.2013.11.006
© 2013 Published by Elsevier B.V.
intermediates which can be oxidized to carbon dioxide [7,8]. It was often considered that the activity for catalytic combustion of DCM was related to the synergy between acidic role and oxidization [9,10]. Though Pt catalysts were more preferentially acceptable than other catalysts, the significant deactivation was observed at an initial stage of reaction due to the interaction of DCM with hydroxyl groups [7]. As found in our previous work, Cl species produced during the CVOC oxidation strongly adsorbed on the active sites and hence retarded the accesses of reactants [11]. The removal of Cl species was often rate-controlling step in many cases of CVOC catalytic combustion [11,12]. Raising temperature will promote the removal of Cl species via the desorption of Cl species or the oxidation of Cl species (Deacon reaction) [13]. However, it is desirable to develop more active catalytic materials which can enhance the removal of Cl species at low temperature. CeO2 presented the high activity for catalytic combustion of various CVOCs, which was attributed to the high ability for dissociating C Cl bonds, high oxygen mobility and abundant oxygen defects. Recently, it has been reported that the addition of Ce element promoted for DCM decomposition over Pt/Al2 O3 catalyst [14]. Zhou et al. found that Cr–Ce–USY zeolite catalysts possessed high activity for catalytic decomposition of DCM, TCE and DCE [15]. In our previous studies, the introduction of Ce element can improve evidently both the activity and stability of transition metal oxide (TMO) catalysts in the catalytic combustion of chlorobenene [11,12,16]. The high stability of ceria-doped TMO catalysts derives from the fact that the adsorbed inorganic chlorine species or dissociated Cl can be removed rapidly from the surface or active sites via the formation and reoxidization of intermediate TMOy Clx or TMClx .
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
As known, Ru-based catalysts have been employed for commercialized chlorine production via HCl oxidation, due to its extraordinary stability and easier Cl2 evolution [13,17]. Additionally, RuO2 has received considerable attention as a heterogeneous catalyst used in oxidation of various compounds, such as carbon monoxide, soot, methane and propane [18–21]. Recently, Ru/Al2 O3 catalyst was used in the catalytic combustion of TCE and presented high activity [22]. Our recent work showed that the high activity and stability of Ru/CeO2 for catalytic combustion of chlorinated aromatics were related to its ability for removal of Cl species from the surface of catalyst via Deacon reaction [16,23]. Additionally, we also found that Ru/TiO2 presented high activity for the combustion of DCM, due to its high removal of Cl species [24]. In this work, we used Ce-doped Al2 O3 as supports, and prepared Ru catalysts by wet impregnation with RuCl3 aqueous solution, and investigated in details the effect of Ce on catalytic decomposition of chlorinated methane over Ru/Al2 O3 catalysts through characterization, reaction kinetics and auxiliary experiments in the several aspects such as the synergistic effect of high acidity, high oxygen mobility of CeOx and the stability of RuOx exposed to Cl species in the catalytic decomposition of DCM. 2. Experimental 2.1. Catalyst preparation Ce-Al2 O3 supports were prepared by a co-precipitation method. A detailed process was as follows: the ammonia and an aqueous solution of Al(NO3 )3 ·9H2 O were added dropwise to an aqueous solution of Ce(NO3 )3 ·6H2 O under vigorous stirring. The pH value of solution was controlled at 8–8.5. The precipitates were aged for 12 h, washed with distilled water, then dried at 100 ◦ C for 12 h and finally calcined in air at 550 ◦ C for 4 h. The synthesized CeAl2 O3 samples were impregnated with 0.198 M (0.02 gRu mL−1 ) RuCl3 aqueous solution at 25 ◦ C for 12 h, dried at 100 ◦ C for 12 h in a vacuum oven, and finally calcined in air at 550 ◦ C for 4 h in a tubular furnace. Thus 1 wt% Ru/Ce-Al2 O3 catalysts with Ce content of 0% (pure Al2 O3 ), 1%, 3%, 5%, 7% and 10%, as determined by X-ray Fluorescence Spectrometer (XRF), were obtained. 2.2. Catalysts characterization The powder X-ray diffraction patterns (XRD) of the samples were recorded on a Rigaku D/Max-rC powder diffractometer using Cu K␣ radiation (40 kV and 100 mA). The diffractograms were recorded within the 2 range of 10–80◦ with a 2 step size of 0.01◦ and a step time of 10 s. The nitrogen adsorption and desorption isotherms were measured at −196 ◦ C on an ASAP 2400 system 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. Ru content was determined by XRF using a Shimadzu (XRF-1800) wavelength dispersive X-ray fluorescence spectrometer. Samples were prepared in the form of uniform tablets (20 mm of diameter) by pressing (30 MPa) the powder of catalyst. Transmission electron microscopy (TEM) analysis was carried out using a JEM-2010F operated by an accelerating voltage of 200 kV. H2 -temperature programming reduction (H2 -TPR) was investigated by heating catalysts (100 mg) in H2 (5 vol.%)/Ar flow (30 mL min−1 ) at a heating rate of 10 ◦ C min−1 from 50 to 750 ◦ C. The hydrogen consumption was monitored by thermo-conductivity detector. Before the H2 -TPR analyses, the samples were heated for 60 min in Ar flow at 300 ◦ C. The Raman spectra were obtained on a Renishaw in Viat + Reflex spectrometer equipped with a CCD detector at ambient temperature and moisture-free conditions. The emission line at 514.5 nm from an Ar+ ion laser (Spectra Physics)
443
was focused, analyzing spot about 1 mm, on the sample under the microscope. 2.3. Activity evaluation The activity and stability of catalysts were tested at atmospheric pressure in a continuous flow micro-reactor made of a quartz tube with inner diameter of 4 mm. 0.3 mL catalyst (0.185 g) as the reaction bed was packed. The feeding flow rate to the reactor was set at 50 cm3 min−1 and the gas hourly space velocity (GHSV) was maintained at 10,000 h−1 . Feed stream to the reactor was prepared by delivering liquid DCM with a syringe pump into dry gas mixture composed of a given concentration of O2 and N2 balance, which were metered by a mass flow controller, controlling DCM concentration in the feed stream at 750 ppm or 1500 ppm. The injection part was electrically heated to ensure complete evaporation of DCM. The reactor temperature was measured with a thermocouple located just at the exit of the micro-reactor. The temperature was raised by steps from 150 to 450 ◦ C. The effluent gases were analyzed on-line at a given temperature by using three gas chromatographs (GC), two equipped with FID and ECD, respectively, for the quantitative analyses of organic chlorinated compounds, and the other one with TCD for CO and CO2 . The concentrations of Cl2 and HCl were analyzed by the effluent stream bubbling through a 0.0125 N NaOH solution, and chlorine concentration was then determined by the titration with ferrous ammonium sulphate using N,N-diethyl-p-phenylenediamine as an indicator [25]. The concentration of chloride ions in the bubbled solution was determined by using a chloride ion selective electrode [26]. 3. Results and discussion 3.1. Characterization of catalysts Ru loading in RuOx catalysts, determined by XRF, is close to the nominal value of 1 wt% (Table 1), indicating that the impregnation of ruthenium with RuCl3 aqueous solution onto the Al2 O3 or CeAl2 O3 supports is effective. Fig. 1 shows the results of XRD analyses. On the XRD patterns of Al2 O3 and Ce-Al2 O3 supports, three broad diffraction peaks appearing at 36.9◦ , 45.9◦ and 66.6◦ can be ascribed to spinal phase (␥-Al2 O3 , PDF #50-0741). With the increase of Ce content, there appear four weak peaks at 28.6◦ , 33.3◦ , 47.5◦ and 56.5◦ , ascribed to CeO2 cerianite with a fluorite-like structure (PDF #43-1002). A new weak and broad peak appears at 60.1◦ , probably a reflection from CeAlO3 (PDF #28-0260) (the other two peaks at 33.6◦ and 48.3◦ overlap with the reflections at 33.3◦ and 47.5◦ from CeO2 ). The diffraction peaks of RuO2 appear at 28.0◦ , 35.0◦ and 54.2◦ for all Ru/Ce-Al2 O3 catalysts [27]. According to the Scherrer equation applied to 1 1 0 reflection of RuO2 , the size of RuO2 particles is estimated to be 11–20 nm, dependent on Ce content (Table 1). The average size of RuO2 particles is estimated to be 20.9 nm for Ru/Al2 O3 catalyst, while for Ru/7%Ce-Al2 O3 catalyst, ∼11 nm. The decrease in lattice parameters of CeO2 to some extent (Table 1) can be ascribed to the entrance of some Ru species into the CeO2 cerianite [23]. The disappearance of diffraction peak at 60.1◦ indicates that the interaction of RuOx with Ce3+ –O–Al. The size of CeO2 particles of Ru/Ce-Al2 O3 catalysts increases to 6.3–9.3 nm from 5.0–7.6 nm for Ce-Al2 O3 samples. Moreover, the BET area of samples increases gradually with the increase of Ce content in the range of 1–10%, confirming that the addition of Ce can promote ␥-Al2 O3 dispersion. Fig. 2(a)–(e) shows TEM images of the Ce-Al2 O3 and Ru/Ce-Al2 O3 samples. For 1%Ce-Al2 O3 (Fig. 2(a)) and 7%Ce-Al2 O3 (Fig. 2(b)) supports, nanoparticles have irregular shape. HRTEM (Fig. 2(f) and (g)) confirms the existence of 5–6 nm CeO2 particles in 7%Ce-Al2 O3 and Ru/7%Ce-Al2 O3 samples, which is in good agreement with the
444
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
Table 1 Surface and structural properties of Ru/Ce-Al2 O3 catalysts with various Ce content. Catalysts
Ru loading a (wt%)
SBET (m2 /g)
LCeO2 b (nm)
NCeO2 c (nm)
NRuO2 d (nm)
Al2 O3 1%Ce-Al2 O3 3%Ce-Al2 O3 5%Ce-Al2 O3 7%Ce-Al2 O3 10%Ce-Al2 O3 Ru/Al2 O3 Ru/1%Ce-Al2 O3 Ru/3%Ce-Al2 O3 Ru/5%Ce-Al2 O3 Ru/7%Ce-Al2 O3 Ru/10%Ce-Al2 O3
– – – – – – 0.975 0.968 0.961 0.958 0.919 0.908
171 178 181 194 200 198 175 184 181 196 209 190
– ND ND 5.415 5.416 5.413 – ND ND 5.405 5.407 5.401
– NDe ND 5.0 5.8 7.6 – ND ND 6.3 7.1 9.3
– – – – – – 20.9 20.8 19.7 17.6 11 ND
a b c d e
H2 consumption (mol gcat ) Actual
Theoretical
Actual/theoretical
– 3 10 35 101 190 376 431 679 1004 1397 –
– 36 107 179 250 357 193 229 300 372 443 –
– 0.09 0.10 0.20 0.40 0.53 1.96 1.88 2.28 2.70 3.20 –
Determined by XRF. CeO2 lattice parameters calculated from XRD results. CeO2 crystal size. RuO2 crystal size, calculated by the Scherrer equation based on XRD results. ND, not be detected.
average mean diameter estimated based on the XRD data (5–7 nm). CeO2 mainly exposes the (1 1 1) plane with the distance of 0.31 nm. Very thin particles with lattice fringes of distance about 0.32 nm (Fig. 2(g)) can be observed, corresponding to the 1 1 0 lattice plane of the tetragonal RuO2 . Also, the particle size of RuO2 is estimated to be 6–9 nm, similar to the results obtained by XRD analyses. The possible catalyst structures were investigated by Raman spectroscopy technique (Fig. 3). For pure CeOx sample, the band appearing at 460 cm−1 can be attributed to the mode (F2g) of fluorite-like structure [28]. As known, fluorite-structure is a cubic structure (fcc), in which cations are located on the corners and in the face centers while oxygen atoms are located at the tetrahedral sites. The spectra of F2g band are dominated by oxygen lattice vibrations and are sensitive to crystalline symmetry [29]. For 1%Ce-Al2 O3 sample, the F2g band was not observed, indicating
Fig. 1. XRD patterns of Ce-Al2 O3 and Ru/Ce-Al2 O3 catalysts with various Ce contents; 1 – Al2 O3 , 2 – 1%Ce-Al2 O3 , 3 – 3%Ce-Al2 O3 , 4 – 5%Ce-Al2 O3 , 5 – 7%CeAl2 O3 , 6 – 10%Ce-Al2 O3 , 7 – Ru/Al2 O3 , 8 – Ru/1%Ce-Al2 O3 , 9 – Ru/3%Ce-Al2 O3 , 10 – Ru/5%Ce-Al2 O3 , 11 – Ru/7%Ce-Al2 O3 and 12 – Ru/10%Ce-Al2 O3 .
that the interaction between Al2 O3 and Ce species must deform the fluorite-like structure. However, this band appears at 460 cm−1 for 3%Ce-Al2 O3 sample and becomes strong with the increase in Ce content, due to the improvement of CeO2 crystalline symmetry. As reported in the literature, Eg, A1g, and B2g modes of RuO2 are located at about 523, 646, and 710 cm−1 , respectively [30]. For Ru/Ce-Al2 O3 samples, there appear two weak and broad bands at 520 and 630 cm−1 , which can be attributed to Eg and A1g modes. The band corresponding to B2g modes can not be observed, indicating the existence of interaction between RuO2 and CeOx . Moreover, the intensity of F2g band decreases to a significant extent, which further confirms the interaction between CeO2 and RuO2 . Probably, the entrance of Ru into CeO2 lattice results in the decrease of CeO2 symmetry, especially for the samples containing low Ce content. The results of TPR analyses for catalysts are shown in Fig. 4(a) and (b). The reduction of pure CeO2 occurs at 437 and 522 ◦ C in the range of experimental temperature (Fig. 4(a)), associated with the reduction of surface Ce4+ ions from small and large size of CeO2 particles. [31]. For the Ce-Al2 O3 samples, the peak at 437 ◦ C becomes strong gradually with the increase in Ce content. Another peak appearing at higher temperature can be assigned to the reduction of Ce4+ species strongly interacted with Al2 O3 for the samples with low Ce content, and the reduction of surface Ce4+ species from large CeO2 particles for the samples with high Ce content (Fig. 4(a)). H2 consumption (Table 1) shows that the actual values are much lower than theoretical values (the amount of hydrogen required for the reduction was calculated on the basis of 2CeO2 + H2 → Ce2 O3 + H2 O), probably owing to the formation of Ce3+ –O–Al and the reduction of Ce4+ in bulk at higher temperature beyond the experimental range (for the samples with higher Ce content). For Ru/Al2 O3 sample, the reduction of RuO2 to Ru0 , as shown in Fig. 4(b), occurs at 176 ◦ C. With the increase in Ce content, the initial reduction of Ru4+ shifts to low temperature gradually and finally drops to 74 ◦ C for Ru/CeO2 sample. This phenomenon indicates that the reduction of Ru species is promoted by a high degree of coordinative unsaturation and, perhaps, by neighboring Ce4+ ions, which, in turn, undergo reduction to Ce3+ species. Thus, the reduction peak area below 300 ◦ C increases with Ce content increase, indicating the oxygen mobility of CeO2 can be promoted by Ru incorporation. H2 consumptions of the Ru/Ce-Al2 O3 catalysts are much higher than the theoretical values calculated on the basis of RuO2 + 2H2 → Ru0 + 2H2 O, probably due to spilling over hydrogen from Ru to the supports [32]. The ratios of actual value to theoretical value increase significantly with Ce content increase. Additionally,
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
445
Fig. 2. TEM images of Ce-Al2 O3 and Ru/Ce-Al2 O3 catalysts: (a) 1%Ce-Al2 O3 , (b) 7%Ce-Al2 O3 , (c) Ru/Al2 O3 , (d) Ru/1%Ce-Al2 O3 and (e) Ru/7%Ce-Al2 O3 ; HRTEM images of 7%Ce-Al2 O3 (f) and Ru/7%Ce-Al2 O3 (g).
the reduction peak at about 530 ◦ C disappears, implying that the sites of supporting RuO2 is closed to these Ce species that can be reduced readily by the dissociated H atoms on Ru species at lower temperature. The results of NH3 -TPD analyses for catalysts are shown in Table 2. For pure Al2 O3 sample, the amount of acidity is estimated to be 294 mol/g (Table 2). For the Ce-Al2 O3 catalysts, the amount of acidic sites increases from 340 mol/g for 3%Ce-Al2 O3 to 375 mol/g for 7%Ce-Al2 O3 . The increased new acidic sites could result from Ce species highly dispersed on Al2 O3 surface. When Ce content was too high, such as 10%, the acidity decreases to a significant extent, due to the coverage of CeO2 over Al2 O3 surface. In
Fig. 3. Raman spectra of catalysts; 1 – 1%Ce-Al2 O3 , 2 – 3%Ce-Al2 O3 , 3 – 5%Ce-Al2 O3 , 4 – 7%Ce-Al2 O3 , 5 – 10%Ce-Al2 O3 , 6 – Ru/1%Ce-Al2 O3 , 7 – Ru/3%Ce-Al2 O3 , 8 – Ru/5%CeAl2 O3 , 9 – Ru/7%Ce-Al2 O3 , 10 – Ru/10%Ce-Al2 O3 and 11 – CeO2 .
the case of supported RuO2 catalysts, the amount of acidic sites increases, probably due to the presence of residual Cl− species in the catalyst because of the usage of RuCl3 precursor. 3.2. Catalytic activity Under the reaction condition of 750 ppm DCM, 20% O2 , N2 balance and GHSV = 10,000 h−1 , DCM conversion over the fresh catalysts as the function of temperature is shown in Fig. 5(a), while T50% and T90% (the temperature needed for 50% and 90% conversion) as a function of Ce content, shown in Fig. 5(b). It can be seen in Fig. 5(a) that pure Al2 O3 support seems active for DCM decomposition with T50% and T90% of 304 and 341 ◦ C. With the incorporation of Ce, the conversion curves shifts significantly to low temperature. T90% decreases from 329 ◦ C for 1%Ce-Al2 O3 catalyst to 310 ◦ C for 7%CeAl2 O3 . Ru/Ce-Al2 O3 catalysts with the Ce contents of 1–7% possess higher activity and T90% drops in the range of 260–297 ◦ C (Fig. 5(b)). DCM conversion reached the complete up to 340 ◦ C over Pt/Al2 O3 [6] and to 400 ◦ C over Pd supported catalysts [33]. Obviously, Ru based catalysts are more active than other noble metal oxide catalysts for DCM decomposition. It is interesting to find that the change trend of T50% and T90% with the increase of Ce is consistent with that of the temperatures at which Ru4+ can be reduced at initial time, indicating that the reducibility of catalysts is responsible for the activity (Fig. 5(b)). On the other hand, T50% and T90% is also related to the acidic amount of catalysts. As reported previously, the acidity is favorable for activating C Cl bond [6–8]. Although Ru/10%CeAl2 O3 possesses the best oxygen mobility, its amount of acidic sites is lest. As a result, the activity of Ru/10%Ce-Al2 O3 catalyst decreases significantly. Fig. 6 presents the stability of catalysts in DCM catalytic decomposition under reaction condition as the above mentioned. Based on the analyses of all component including reactants and products in the effluent, chlorine uptake on the surface of catalysts treated in reaction feed at 250 and 270 ◦ C for 1000 min is quantitatively determined by means of Cl species balance and shown in Table 2 (TPSR on the above treated catalysts indicated that no adsorption of organic chlorinated compound deposits). At 250 and 270 ◦ C, DCM conversion on Ru/Al2 O3 catalyst decreases within the first 400 min rapidly from 45% and 61% to zero, indicating that the stability of Ru/Al2 O3
446
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
Table 2 The catalytic performance of Ce-Al2 O3 and Ru/Ce-Al2 O3 catalysts with various Ce content. Samples
NH3 uptake (mol/g)
Al2 O3 1%Ce-Al2 O3 3%Ce-Al2 O3 7%Ce-Al2 O3 Ru/Al2 O3 Ru/1%Ce-Al2 O3 Ru/3%Ce-Al2 O3 Ru/7%Ce-Al2 O3
294 318 340 375 350 381 396 470
a b c d
Cl uptakea (mmol/g−1 cat ) 250 ◦ C
270 ◦ C
1.77 – – – 1.46 1.16 0.81 0.68
1.65 – – – 0.87d 0.83 0.64 0.51
Clb (at%)
– – – – 0.58 0.52 0.38 0.35
T50 /T90 c (◦ C) CHCl3
CH3 Cl
262/301 262/297 265/295 262/301 258/295 250/281 226/267 205/245
305/362 397/348 285/325 272/313 260/292 259/281 246/278 235/275
Cl uptake, determined by subtraction of the amount of HCl and Cl2 measured at the reactor outlet from the total chlorine amount. Determined by XPS for the used catalysts for 1000 min at 270 ◦ C. Gas composition of feed: 750 ppm chlorinated methane, 20% O2 and N2 balance; GHSV = 10,000 h−1 . Obtained at 285 ◦ C.
catalyst cannot be improved essentially compared with pure Al2 O3 . Moreover, no inorganic chlorine compound can be detected in the effluent during the deactivation process, implying that the Cl species deposited on the active sites on Ru/Al2 O3 catalyst and probably resulted in the deactivation of Ru/Al2 O3 catalyst. The chlorine uptake of Ru/Al2 O3 catalyst at 250 ◦ C is estimated to be 1.46 mmol Cl/gcat . By raising temperature to 285 ◦ C, substantial decrease in DCM conversion is not observed, and the conversion can be maintained at about 65% for 1000 min. The corresponding chlorine uptake decreases to 0.68 mmol Cl/gcat , indicating the removal of Cl species is promoted by raising temperature. The stability of Al2 O3 in DCM decomposition was studied widely, and its deactivation was ascribed mainly to the consumption of hydroxyl group [6]. For the Ru/Ce-Al2 O3 catalysts, the stability at 250 ◦ C increases with Ce content. The activity of Ru/1%Ce-Al2 O3 catalyst cannot be maintained at 250 ◦ C, similar to the case of Ru/Al2 O3 catalyst. However, raising Ce content to 3%, the conversion keeps at 49–52% within 1000 min. This phenomenon is consistent with previously reported results that Ce catalysts supported on acidic zeolites presented good stable activity for the CVOCs catalytic combustion [34]. On Ru/7%CeAl2 O3 catalyst, a higher stable activity with DCM conversion of 80% can be reached at 250 ◦ C. The chlorine uptakes of Ru/1%CeAl2 O3 , Ru/3%Ce-Al2 O3 and Ru/7%Ce-Al2 O3 catalysts used at 250 ◦ C
for 1000 min are estimated to be 1.16, 0.81 and 0.68 mmol Cl/gcat , respectively. Raising reaction temperature to 270 ◦ C, the chlorine uptakes of Ru/1%Ce-Al2 O3 , Ru/3%Ce-Al2 O3 and Ru/7%Ce-Al2 O3 catalysts decreased by 20–30% and their corresponding conversions keep at 52%, 81% and 96%, respectively. Ru/7%Ce-Al2 O3 catalyst in particular was tested at 270 ◦ C for another 1000 h with the conversion as high as 96%. Therefore, the removal of these Cl species is a key step to improve the stability of catalysts. The analyses of containing-carbon product are shown in Fig. 7(a)–(c). CO2 is main containing-carbon product over the Ru/Ce-Al2 O3 catalysts, which is in coincident with the results obtained during DCM decomposition over noble metal oxides supported on acidic materials [6,7]. The selectivity for CO2 increases with the increase in temperature and reaches almost 100% at 325 ◦ C at which no other containing-carbon product can be detected. CH3 Cl can be detected at different level for all catalysts (Fig. 7(b)). The amount order of CH3 Cl is produced: Al2 O3 > Ce-Al2 O3 > Ru/Al2 O3 > Ru/Ce-Al2 O3 , lined with that of Cl uptake (Table 2). The formation of CH3 Cl should be promoted by chlorination. Over Ru/7%Ce-Al2 O3 catalyst with low Cl uptake, the lowest amount (<8 ppm) of CH3 Cl is formed, while up to 45 ppm CH3 Cl is obtained over Ru/Al2 O3 catalyst with high Cl uptake. Al2 O3 having highest Cl uptake (1.77 mmol g−1 ) presents 50% selectivity
Fig. 4. H2 -TPR profiles of Ce-Al2 O3 (a) and Ru/Ce-Al2 O3 (b) with various Ce contents; (a): 1 – 1%Ce-Al2 O3 , 2 – 3%Ce-Al2 O3 , 3 – 5%Ce-Al2 O3 , 4 – 7%Ce-Al2 O3 , 5 – 10%Ce-Al2 O3 , and 6 – CeO2 ; (b): 1 – Ru/Al2 O3 , 2 – Ru/1%Ce-Al2 O3 , 3 – Ru/3%Ce-Al2 O3 , 4 – Ru/5%Ce-Al2 O3 , 5 – Ru/7%Ce-Al2 O3 , and 6 – Ru/CeO2 .
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
447
Fig. 6. DCM conversion with time on stream over Ru/Ce-Al2 O3 catalysts with various Ce contents at different temperatures; gas composition: 750 ppm DCM, 20% O2 and N2 balance; GHSV = 10,000 h−1 .
The activity order for the catalytic combustion of chlorinated methane over the Ru/Ce-Al2 O3 catalysts is CHCl3 > CH3 Cl > CH2 Cl2 . 3.3. TPSR study
Fig. 5. The conversion curves of DCM over catalysts with various Ce contents (a) and T50% , T90% , the initial reduction temperature (TR ) and the acidic amount of Ru/CeAl2 O3 catalysts as functions of Ce contents (b); gas composition: 750 ppm DCM, 20% O2 and N2 balance; GHSV = 10,000 h−1 .
for CH3 Cl·CHCl3 , as a product of chlorination, is detected at higher than 260 ◦ C. Although the CHCl3 amount is trace, it is much higher over Ru/Al2 O3 catalyst than those over the Ru/Ce-Al2 O3 catalysts (Fig. 7(c)), of which Ru/7%Ce-Al2 O3 catalyst presents the lowest amount of CHCl3 , <1 ppm. In order to understand these phenomena, the activity of catalysts for CH3 Cl and CHCl3 catalytic combustion was investigated and the corresponding T50% and T90% are listed in Table 2. It can be seen that all Ru catalysts under study are highly active in CH3 Cl and CHCl3 decomposition, especially for the Ru/CeAl2 O3 catalysts. CH3 Cl and CHCl3 will be oxidized quickly if formed.
In order to investigate the process involving Cl species, the feed containing DCM of 2000 ppm and oxygen of 20% pass through Ru/Al2 O3 and Ru/7%Ce-Al2 O3 catalysts and the effluent was monitored by mass spectroscopy within a given range of temperature (Fig. 8(a) and (b)). It can be seen in Fig. 8(a) that for Ru/Al2 O3 catalyst, the evolvement of CO2 occurs at 223 ◦ C, and increases with temperature. At 300 ◦ C, this signal keeps constant. The signals of HCl and Cl2 appear at 300 ◦ C at the same time. This phenomenon can be explained by the fact that HCl produced from acidic sites transfers toward the active sties for Deacon reaction (HCl + O2 → Cl2 + H2 O) on RuOx and is oxidized into Cl2 (thus, H2 O is formed). The difference between the temperatures of appearing CO2 and inorganic Cl species indicates the strong adsorption of these Cl species. Thus, the evolvement of HCl and Cl2 can still be observed when DCM in the feed is cut off. For Ru/7%Ce-Al2 O3 catalyst, the signals of HCl and Cl2 can be detected at lower temperature (Fig. 8(b)). Moreover, the above mentioned temperature difference is smaller, compared with the case of Ru/Al2 O3 catalyst. Obviously, the incorporation of
Fig. 7. The product distributions during DCM decomposition over Ru/Ce-Al2 O3 catalysts with various Ce contents; (a) CO2 , (b) CH3 Cl and (c) CHCl3 ; gas composition: 750 ppm DCM, 20% O2 and N2 balance; GHSV = 10,000 h−1 .
448
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
Fig. 8. The mass spectroscopy of reactants and products during TPSR; (a) Ru/Al2 O3 , (b) Ru/7%Ce-Al2 O3 ; gas composition: 2000 ppm DCM, 20% O2 and N2 balance; GHSV = 10,000 h−1 .
Fig. 9. The conversion curves of DCM decomposition over Ru/7%Ce-Al2 O3 and Ru/Al2 O3 catalysts under various oxygen concentrations at GHSV = 10,000 h−1 ; (a) 750 ppm DCM; (b) 1500 ppm DCM.
Ce into Al2 O3 decreases the strength of Cl adsorption on active sites so that the escape of HCl species can occur at lower temperature.
750 or 1500 ppm DCM, the rate increases within 0.2–20% [O] by 0.6 mol/min gcat . The rate at 3% [O] or higher is much lower than those obtained over Ru/7%Ce-Al2 O3 catalyst. These phenomena indicate that the activity of Ru/Al2 O3 catalyst is not quite sensitive toward the change of active oxygen species on RuOx . In other words,
3.4. Kinetics consideration In order to understand the kinetics of DCM decomposition, the effect of oxygen concentration ([O]) on conversion was investigated on Ru/7%Ce-Al2 O3 and Ru/Al2 O3 catalysts under 750 and 1500 ppm DCM, and the results are shown in Fig. 9(a) and (b). For Ru/7%Ce-Al2 O3 catalyst, significant shifts of conversion curves to low temperature were observed as [O] increases. This shift occurs to a much smaller extent for Ru/Al2 O3 catalyst. Moreover, at a given temperature, with the increase in [O] the conversion under at 1500 ppm DCM increases more than that at 750 ppm DCM. Here, we calculate the reaction rates at 250 ◦ C (Fig. 10) and obtain a considerable linear relation of rate with [O] within 0.2–3% [O] at 750 ppm DCM. For Ru/7%Ce-Al2 O3 catalyst, the rate increases from 1.4 mol/min gcat at 0.2% [O] up to 4.77 mol/min gcat at 3% [O], indicating that the process involving oxygen may be slow step. Moreover, raising DCM concentration up to 1500 ppm, a more quick increase in rate within 3–20% [O] than that at 750 ppm DCM can be observed. This larger dependence of rate on [O] can be explained by the fact that Ru/7%Ce-Al2 O3 catalyst possesses high oxygen mobility and the active oxygen effectively increases with gas [O] so as to promote the removal of Cl species from the surface of Ru/7%Ce-Al2 O3 catalyst [11,12]. In the case of Ru/Al2 O3 catalyst, the dependence of rate or conversion on [O] is not strong. At either
Fig. 10. The effect of oxygen concentration on DCM decomposition at 250 ◦ C over Ru/7%Ce-Al2 O3 and Ru/Al2 O3 catalysts at GHSV = 10,000 h−1 ; the data within bracket are the conversion.
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
oxygen mobility is low for Ru/Al2 O3 catalyst without Ce species. As known from the stability and TPSR tests, the evolvement of HCl from Ru/Al2 O3 at 250 ◦ C is difficult, which can be related to low oxygen mobility of Ru/Al2 O3 catalyst. To obtain more insight into the promotion function by CeO2 , the physico-chemical properties of different catalysts were characterized. The results show that after being used for 1000 min at 270 ◦ C for the combustion of DCM (750 ppm DCM, 20% O2 and N2 balance), the specific surface area of all catalysts under study rarely change, no phase transformations are observed in their XRD patterns and the obvious deposit of coke on the catalyst surface was not detected by TG techniques. The change of XPS spectra of species (Ce 3d, Ru 3d, Al 2p and O 1s) on the used catalyst surface was not observed. However, Cl 2p peak appearing at around 198 eV on XPS spectra and EDS results (not shown) confirm the presence of chlorine species on all of the used catalysts. The amount of deposited Cl decreases from 0.58 at.% for Ru/Al2 O3 catalyst to 0.35 at.% for Ru/7%Ce-Al2 O3 catalyst, estimated by XPS. This result is consistent with the order of Cl uptake over various catalysts in Table 2. Considering no changes of binding energy for various species of the used catalysts (see Support Information), therefore, the deactivation of catalysts is related to the strong adsorption of HCl or Cl2 produced from the decomposition of DCM. Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata. 2013.11.006. From the above results, it can be concluded that oxygen species can efficiently react with the Cl species that adsorbed with moderate strength on the surface. For Ru/Al2 O3 catalyst or Ru/Ce-Al2 O3 catalysts with low amount of Ce, surface oxygen migrates to a smaller extent and the adsorption of Cl species is strong. So, these catalysts need high temperature to remove the adsorbed Cl species, and present a low stable activity. There are more active oxygen species for Ru/7%Ce-Al2 O3 catalyst to remove the Cl species adsorbed on the surface of catalysts during the reaction, so that more active sites can be accessible. Therefore, the stability of Ru/7%Ce-Al2 O3 catalyst in the catalytic combustion of DCM is promoted at 250 ◦ C, due to a larger decrease in chlorine species deposit. 3.5. The effect of water on DCM decomposition Because of water detected in the exhaust gas, it is one part of the work to investigate the effect of water on the activity of Ru/7%Ce-Al2 O3 and Ru/Al2 O3 catalysts for DCM decomposition (Fig. 11). In the presence of 3% water, the low conversion section on Ru/7%Ce-Al2 O3 catalyst shifts significantly to high temperature. The temperature needed for 15% conversion increases from 162 ◦ C to 202 ◦ C. At higher than 250 ◦ C, the conversion curve becomes approaching to that without water. At low temperature, water is dissociated probably on CeO2 species into H+ and OH− species so as to retard the transfer of active oxygen. This inhibition was observed during the combustion of trichloroethylene over cerium oxide in wet air [35]. For Ru/Al2 O3 catalyst, where the inhibition of 3% water on DCM decomposition is observed at low temperature only to a smaller extent. With raising temperature, the adsorption of water becomes difficult, and the effect of water becomes weak. Product analyses show that CO2 was main carbon-containing product, and the evolvement of HCl was observed almost at the same time as DCM was converted, indicating that Cl deposition was inhibited by water. Cl2 was not detected in the effluent during reaction, because Deacon reaction cannot proceed in the presence of 3 wt % water. CH3 Cl and CHCl3 are detected only in the reaction over Ru/Al2 O3 catalyst, with a largest amount as 5 ppm, directly verifying that the formation of CH3 Cl and CHCl3 is related to Cl deposition. At 250 ◦ C, the stability of Ru/7%Ce-Al2 O3 and Ru/Al2 O3 catalysts can be maintained (Fig. 11 insert), further confirming that the removal of Cl
449
Fig. 11. The effect of water on the activity of Ru/Al2 O3 and Ru/7%Ce-Al2 O3 catalysts; insert: the stability at 250 ◦ C in wet feed; wet: 3% H2 O, dry: absence of water; gas composition: 750 ppm DCM, 20% O2 and N2 balance; GHSV = 10,000 h−1 .
species from active sites is critical to promote the stability of Ru catalysts. 4. Conclusions Ru oxide catalysts supported on Ce-Al2 O3 and Al2 O3 were prepared by wet impregnation with RuCl3 aqueous solution and tested in catalytic combustion of DCM. The characterization of XRD, N2 adsorption, TEM/HRTEM, H2 -TPR, Raman and NH3 -TPD shows the existence of interaction between Ce species and Al2 O3 . As a result, the acidity increases to some extent. The reducibility of Ru/CeAl2 O3 is greatly increased by the addition of Ce. During the catalytic decomposition of DCM, Ru/Ce-Al2 O3 catalysts present an outstanding stable activity. The measurement of chlorine uptake shows the Cl deposition is responsible for the deactivation of Ru/Al2 O3 catalyst. High stable activity of Ru/Ce-Al2 O3 catalysts (1000 h, higher than 96% conversion at 270 ◦ C for Ru/7%Ce-Al2 O3 catalyst) can be ascribed to the synergistic effect of high acidity, high oxygen mobility of CeOx and the stability of RuOx exposed to Cl species. Therefore, Ru/Ce-Al2 O3 catalysts will become useful in the practice of removal of alkyl chlorides. Acknowledgements We would like to acknowledge the financial support from National Basic Research Program of China (No. 2010CB732300, 2011AA03A406) and National Natural Science Foundation of China (No. 21277047) and Commission of Science and Technology of Shanghai Municipality (11JC1402900). References [1] M.J. Morra, V. Borek, J. Koolpe, J. Environ. Qual. 29 (2000) 706–715. [2] F. Alonso, I.P. Beletskaya, M. Yus, Chem. Rev. 102 (2002) 4009–4091. [3] M. Taralunga, B. Innocent, J. Mijoin, P. Magnoux, Appl. Catal. B: Environ. 75 (2007) 139–146. [4] F.F. Bertinchamps, C. Gregoire, E.M. Gaigneaux, Appl. Catal. B: Environ. 66 (2006) 1–9. [5] M. Taralunga, J. Mijoin, P. Magnoux, Catal. Commun. 7 (2006) 115–121. [6] I. Maupin, L. Pinard, J. Mijoin, P. Magnoux, J. Catal. 291 (2012) 104–109. [7] D.M. Papenmeier, J.A. Rossin, Ind. Eng. Chem. Res. 33 (1994) 3094–3103. [8] R.W. van den Brink, P. Mulder, R. Louw, G. Sinquin, C. Petit, J.P. Hindermann, J. Catal. 180 (1998) 153–160. [9] R. López-Fonseca, B. de Rivas, J.I. Gutiérrez-Ortiz, A. Aranzabal, J.R. GonzálezVelasco, Appl. Catal. B: Environ. 41 (2003) 31–42. [10] P.S. Chintawar, H.L. Greene, Appl. Catal. B: Environ. 13 (1997) 81–92.
450
L. Ran et al. / Applied Catalysis A: General 470 (2014) 442–450
[11] Y. Dai, X.Y. Wang, Q.G. Dai, D. Li, Appl. Catal. B: Environ. 111–112 (2012) 141–149. [12] Y. Dai, X.Y. Wang, D. li, Q.G. Dai, J. Hazard. Mater. 188 (2011) 132–139. [13] N. López, J. Gómez-Segura, R.P. Marín, J. Pérez-Ramírez, J. Catal. 255 (2008) 29–39. [14] Q.-Y. Chen, N. Li, M.-F. Luo, J.-Q. Lu, Appl. Catal. B: Environ. 127 (2012) 159–166. [15] Q.Q. Huang, X.M. Xue, R.X. Zhou, J. Hazard. Mater. 183 (2010) 694–700. [16] Q.G. Dai, S.X. Bai, Z.Y. Wang, X.Y. Wang, G.Z. Lu, Appl. Catal. B: Environ. 126 (2012) 64–75. [17] S. Zweidinger, J.P. Hofmann, O. Balmes, E. Lundgren, H. Over, J. Catal. 272 (2010) 169–175. [18] H. Over, Y.D. Kim, A.P. Seitsonen, S. Wendt, E. Lundgren, M. Schmid, P. Varga, A. Morgante, G. Ertl, Science 287 (2000) 1474–1476. [19] K. Villani, C.E.A. Kirschhock, D. Liang, G. Van Tendeloo, J.A. Martens, Angew. Chem. Int. Ed. 45 (2006) 3106–3109. [20] I. Balint, A. Miyazaki, K.-I. Aika, J. Catal. 220 (2003) 74–83. [21] J. Okal, M. Zawadzki, W. Tylus, Appl. Catal. B: Environ. 101 (2011) 548–559. ˜ [22] B. Miranda, E. Díaz, S. Ordónez, F.V. Díez, Catal. Commun. 7 (2006) 945–949. [23] Q.G. Dai, S.X. Bai, Z.Y. Wang, X.Y. Wang, G.Z. Lu, Appl. Catal. B: Environ. 129 (2013) 580–588.
[24] L. Ran, Z. Qin, Z.Y. Wang, X.Y. Wang, Q.G. Dai, Catal. Commun. 37 (2013) 5–8. [25] J.R. González-Velasco, A. Aranzabal, R. López-Fonseca, R. Ferret, J.A. GonzálezMarcos, Appl. Catal. B: Environ. 24 (2000) 33–43. [26] R. López-Fonseca, A. Aranzabal, J.I. Gutiérrez-Ortiz, J.I. Álvarez-Uriarte, J.R. González-Velasco, Appl. Catal. B: Environ. 30 (2001) 303–313. [27] J. Okal, M. Zawadzki, Appl. Catal. B: Environ. 89 (2009) 22–32. [28] B. Zhang, D. Li, X. Wang, Catal. Today 158 (2010) 348–353. [29] M.F. Luo, Z.L. Yan, L.Y. Jin, J. Mol. Catal. A: Chem. 260 (2006) 157. [30] S.Y. Mar, C.S. Chen, Y.S. Huang, K.K. Tiong, Appl. Surf. Sci. 90 (1995) 497–504. [31] F. Arena, G. Trunfio, J. Negro, B. Fazio, L. Spadaro, Chem. Mater. 19 (2007) 2269–2276. [32] X.Y. Wang, J. Ni, B.Y. Lin, R. Wang, J.X. Lin, K.M. Wei, Catal. Commun. 12 (2010) 251–254. [33] R. López-Fonseca, J.I. Gutiérrez-Ortiz, J.R. González-Velasco, Appl. Catal. B: Environ. 271 (2004) 39–46. [34] Q.Q. Huang, Z.H. Meng, R.X. Zhou, Appl. Catal B: Environ. 115–116 (2012) 179–189. [35] Q.G. Dai, X.Y. Wang, G.Z. Lu, Appl. Catal. B: Environ. 81 (2008) 192–202.