Characteristics of copper ion exchanged mordenite catalyst deactivated by HCl for the reduction of NOx with NH3

Applied Catalysis B: Environmental 64 (2006) 42–50 www.elsevier.com/locate/apcatb

Characteristics of copper ion exchanged mordenite catalyst deactivated by HCl for the reduction of NOx with NH3 Jin Woo Choung, In-Sik Nam * Department of Chemical Engineering, School of Environmental Engineering, Pohang University of Science and Technology (POSTECH), San 31 Hyoja-Dong, Pohang 790-784, Republic of Korea Received 12 July 2005; received in revised form 18 October 2005; accepted 24 October 2005 Available online 2 December 2005

Abstract The deactivation characteristics of copper ion exchanged mordenite type zeolite catalyst (CuHM) by HCl for the reduction of NOx, particularly from an incinerator with NH3, have been investigated over a fixed bed flow reactor system. X-ray absorption near edge spectroscopy (XANES), extended X-ray absorption fine structure (EXAFS), synchrotron radiation X-ray diffraction (SR-XRD), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD) have been employed to illustrate the deactivation mechanism of CuHM by HCl due to the formation of the deactivation precursor through the reaction between HCl and Cu on the catalyst surface. Cu2Cl(OH)3 has been identified as the precursor for the present reaction system and can also be converted to CuCl2
2H2O during the course of the reaction, particularly at high reaction temperature. XANES and EXAFS analyses reveal that the oxidation state of Cu ion on CuHM catalyst is mainly divalent, regardless of the degree of the catalyst deactivation. However, XPS study provides evidence for an increase of the content of Cu(I) ion on the surface of the catalyst deactivated at 450 8C for 110 h. Moreover, CuCl2
2H2O that forms on the catalyst surface during the course of the reaction evaporates at a reaction temperature higher than 350 8C. It eventually converts to CuCl and then it may evaporate to the bulk gas stream and/or introduce a copper ion onto the zeolite surface again through solid-state ion exchange. # 2005 Elsevier B.V. All rights reserved. Keywords: Selective catalytic reduction of NOx; Deactivation of CuHM by HCl; Cu content; XANES; EXAFS; XPS; SR-XRD; TPD

1. Introduction A flue gas stream treated by SCR process normally contains significant amounts of SOx, chlorinated compound as well as NOx to be removed. In particular, the emission of hydrogen chloride (HCl) is commonly observed in municipal and industrial waste incinerators where the SCR process has been employed to remove NO by NH3, mainly due to the combustion of halogenated organic wastes [1]. A serious deactivation of V2O5/TiO2 catalyst commonly employed as a commercial catalyst to remove NO from the incinerator, had been reported due to the formation of volatile vanadium chlorides including VCl4 and VCl2 and NH4Cl on the catalyst surface as a deactivation precursor during the course of SCR reaction [2,3]. However, none of the previous reports did not understand the cause of the formation of the precursor and its mechanism to

* Corresponding author. Tel.: +82 54 279 2264; fax: +82 54 279 8299. E-mail address: [email protected] (I.-S. Nam). 0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2005.10.026

ultimately develop a HCl tolerable SCR catalyst. Although CuHM catalyst exhibits high NO removal activity by NH3, its stability is still suspicious in the acidic environment of the flue gas [4–6]. For the effect of SO2 on the catalytic activity, Ham et al. [4] observed that the deactivation of the catalyst by SO2 for NH3 SCR process is mainly attributed to the filling and blocking of the pores located within the catalyst by ammonium sulfate formed during the course of the reaction. However, few studies have been done on the stability and the deactivation of deNOx catalyst by HCl, which can be frequently encountered for an SCR process to remove NO from incinerators [7]. The previous study [8] speculated that the primary cause for the deactivation of CuHM catalyst by HCl was not the alteration of the states of Cu ion or the destruction of mordenite structure but the loss of Cu ions from the catalyst. The detailed deactivation mechanism, however, over CuHM catalyst has not been established yet. For the deactivation of cation-exchanged zeolite by HCl, the decationization of the catalyst has not been systematically examined, particularly over transition metal-exchanged zeolite.

J.W. Choung, I.-S. Nam / Applied Catalysis B: Environmental 64 (2006) 42–50

However, Barrer and co-workers [5,6] observed that HCl easily reacted with the cation on alkali-metal contained zeolite for its transformation to hydrogen type zeolite through a typical ion exchange procedure. The framework structure of zeolite containing Si/Al > 2.5 can be essentially maintained at a temperature up to 600 8C [6]. It can be easily understood by the combination of exchange and HCl sorption as described in Eq. (1), where Z denotes the anionic framework and M is the univalent exchanging ion [6]: xHClðgÞ þ MZ $ MZxHClðaÞ $ Mð1yÞ Hy Zðx  yÞHClðaÞ þ yMClðo;iÞ ;

(1)

Table 1 Chemical composition of the bulk and surface of the catalysts Catalyst

2. Experimental 2.1. Catalyst preparation The parent catalyst is sodium type mordenite (NaM) from PQ Corporation (Zeolon 900Na) in the form of a 1/16 in. pellet. The HM was obtained by exchanging Na ion with NH4+ in aqueous NH4NO3 solution at 93 8C, followed by drying at 110 8C for 12 h, and then calcining in air at 500 8C for 5 h. The amount of remaining Na was less than 0.21 wt.% in the catalyst after the calcinations. CuHM catalysts containing a variety of copper loadings were obtained from NH4+-form of mordenite with a cupric nitrate solution through varying exchange time and the concentration of the solution, and then following the drying and calcinations as discussed [11]. Henceforth they are abbreviated as Cu(X)HM (X denotes the degree of copper ion exchange). The physicochemical properties of the catalysts employed in the present study are listed in Table 1. 2.2. Catalyst characterization The chemical composition of the prepared catalysts was examined by inductively coupled plasma-optical emission

Bulk composition by ICP (molar ratio)

a

Cu(23)HMF Cu(23)HMDb Cu(33)HMF Cu(33)HMD Cu(50)HMF Cu(50)HMD Cu(77)HMF Cu(77)HMD a b

where (a) and (g) represent physically adsorbed and gaseous bulk HCl, while (o) and (i) denote the outside and inside of zeolite crystal. Ozin et al. [9,10] also reported that the dipolar nature of HCl demanding the halide end of the molecule would preferentially interact with the Na+ cation, while the proton end interacts with the framework oxygen in a zeolite such as NaY. In the present study, the characteristics of Cu ion on CuHM catalyst deactivated by HCl for the reduction of NO by NH3 have been systematically examined to elucidate the deactivation mechanism of the catalyst. Synchrotron X-ray diffraction studies were performed to investigate the nature of copper ion during the course of the reaction. In order to explore the relationship between the local structure of copper species and the degree of the catalyst deactivation, the XAFS including EXAFS and XANES, which may provide the local coordination geometry of the active reaction sites on the catalyst and their coordination number, has been analyzed to confirm the evaporation of the metal and its mechanism. XPS has been also employed to determine the state of copper and its content on the catalyst surface.

43

Surface composition by XPS (atomic ratio)

Cu/Si

Cu/Al

Si/Al

Cu/Si

Cu/Al

Si/Al

0.019 0.008 0.028 0.016 0.042 0.030 0.064 0.040

0.116 0.047 0.167 0.097 0.250 0.176 0.387 0.232

5.97 5.83 6.05 6.03 5.90 5.81 5.79 5.80

0.008 0.020 0.010 0.027 0.036 0.038 0.068 0.041

0.047 0.115 0.058 0.164 0.208 0.219 0.402 0.237

5.96 5.85 6.01 5.98 5.83 5.81 5.90 5.80

F: fresh catalyst. D: deactivated catalyst at 450 8C for 110 h.

spectroscopy (ICP-Flame-EOP) from Spectro Co. To identify the chemical state of copper ion and chloride on the catalyst surface, XPS spectra of the catalyst samples were obtained by using a VG ESCALAB 220i equipped with Mg anode. Their binding energies were corrected by the C 1s line at 284.6 eV. The full widths of the peak at the half maximum (FWHM) were allowed to adjust for attaining the best fitting. In order to minimize the photo-reduction of copper species on the catalyst surface during XPS experiments, all the catalyst samples were analyzed within a short period of time. The adsorption site of HCl on the catalyst surface was examined by using TPD of HCl under atmospheric pressure. About 0.2 g of sample charged into a quartz tube was pretreated in situ in flowing 99.99% He at 500 8C for 2 h, exposed to HCl, a probe molecule for the TPD test, at 200 8C for 1 h, and then flushed with He to remove the remainder of the adsorbate in the gas phase and that physisorbed on the catalyst surface. The TPD experiment was performed from 200 to 800 8C at a heating rate of 10 8C/min and a flow rate of He of 50 cm3/min. Desorbed molecules were examined by a gas chromatograph with thermal conductivity detector (HP5890) and the down stream of the TCD was monitored with a PFEIFFER quadrupole mass spectrometer (QMG 422). Synchrotron radiation X-ray diffraction (SR-XRD) measurements of CuHM catalysts before and after deactivation were performed in 8C2 high resolution powder diffraction (HRPD) beam line of the Pohang Acceleratory Laboratory (PAL) in Pohang, Korea with a vertical scan diffractometer. The incident X-rays were vertically collimated by a mirror, and ˚ by a doublemonochromatized to the wavelength of 1.5425 A crystal Si (1 1 1) monochromator. The X-ray absorption fine structure (XAFS) spectra were investigated at the EXAFS facility beam line, 3C1 of PAL. The ring was operated at 2.5 GeV with 200 mA electron current and 1% coupling. The spectra were measured with a Si (1 1 1) channel-cut monochromator, of which energy resolution of DE/ E = 2  104 remained constant at the Cu K-edge (8979 eV). The catalyst samples were uniformly spread between the adhesive tapes to obtain an optimal absorption jump. All the

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data were recorded in a transmission mode at room temperature, and the intensities of the incident and transmitted beams were measured in 75% N2 and 25% Ar, and 100% N2filled ionization chambers, respectively. Cu foil, CuO, Cu2O, CuCl, and CuCl2
2H2O were used as reference compounds. The data analyses for EXAFS were carried out by the UWXAFS 3.0 package and FEFF8 code licensed from the University of Washington [12]. The standard analysis procedure was described elsewhere in detail [13–15]. The resulting EXAFS spectra were weighted by k3 in order to compensate for the attenuation of EXAFS amplitude at high k and then Fourier ˚ 1  k  12.0 A ˚ 1 with transformed in the range of 2.5 A 1 ˚ Hanning function of dk = 1 A . 2.3. Reaction system The deactivation of CuHM catalyst by HCl has been examined for removing NO with NH3 over a packed-bed flow reactor system similar to one described elsewhere [4,11]. One gram of 20/30 mesh size catalyst was charged into a 3/8 in. inconel tube reactor to reduce the corrosion of the reactor by HCl and pretreated at 500 8C for 2 h in air flow. The deactivation experiment was accomplished in an accelerated test mode [8]. A 20,000 ppm of the feed concentration of HCl was employed to rapidly identify the trends of the deactivation and the stability of the catalyst with respect to the reaction temperatures, particularly at 450 8C, which is the practical operating temperature of an SCR reactor in an industrial waste incinerator. NO removal activity of the catalyst was examined under the composition of the feed gas stream of 500 ppm NO, 500 ppm NH3, 5% O2, and 5% H2O at the reactor space velocity of 150,000 h1, after the termination of the injection of HCl to the reactor during 110 h of the operating time in order to protect the analytical apparatus. The range of the reaction temperature employed for examining the catalytic activity after deactivation was always less than the deactivation temperature to prevent the deactivated catalyst from its regeneration to clearly discriminate the alteration of the catalytic activity upon the deactivation of the catalyst. 3. Results and discussion 3.1. Accelerated deactivation test Fig. 1 shows the alteration of NO removal activity of CuHM catalysts containing a variety of Cu ion exchange levels by HCl. The catalytic activity of NO reduction by NH3 was significantly enhanced by adding copper ions to mordenite type zeolite and strongly depended on the content of exchanged copper ion into the catalyst. The copper contents of CuHM catalysts employed in the present study are listed in Table 1. Although the activity is nearly proportional to the exchange level of copper ion up to 50%, its activity begins to decrease at a reaction temperature above 350 8C when the level reaches to ca. 77%, as depicted in Fig. 1. Moreover, in order to identify the effect of the copper content on the deactivation of CuHM by HCl, an accelerated

Fig. 1. Alteration of NO removal activity over CuHM catalysts due to the catalyst deactivation by HCl with respect to copper content.

deactivation test program was employed under severe reactor operating conditions containing a high feed concentration of HCl, 20,000 ppm. Such a consideration may rapidly discriminate the degree of the catalyst deactivation by the content of copper within a short period of time as shown in Fig. 1. It simply reveals that the HCl tolerance of the catalyst also strongly depends on the copper loading of CuHM catalyst employed in the present study. The copper content of the series of the catalyst was measured before and after the deactivation test as listed in Table 1. The loss and migration of copper ion from the catalyst upon the deactivation were observed by the analysis of the content of copper in bulk catalyst by ICP and on the catalyst surface by XPS. The Cu/Al ratio in bulk catalyst decreases as the catalyst deactivation proceeds, while for fresh catalyst the copper content in the bulk is higher than that on the surface of the catalyst besides for Cu(77)HM catalyst. Also, the Cu/Si ratio of the catalysts increased on the surface as the catalyst deactivation proceeded at 450 8C, while the ratio decreased in the bulk [8]. However, no major variation of the Si/Al ratio of the catalyst has been observed upon the catalyst deactivation by HCl. On the basis of this observation, it can be anticipated that the catalyst deactivation is mainly due to the loss of copper ion, particularly from the surface of the catalyst. Comparing NO conversions of the fresh and deactivated catalysts, the NO removal activity of the deactivated catalysts is basically inferior to that of the fresh counterpart containing the similar content of copper, such as Cu(23)HMF and Cu(33)HMD, Cu(33)HMF and Cu(50)HMD, and Cu(50)HMF and Cu(77)HMD. The alteration of the state and/or phase of the copper ion within the framework of mordenite structure may be an additional cause of the catalyst deactivation as well as the evaporation of copper from the surface of the catalyst.

J.W. Choung, I.-S. Nam / Applied Catalysis B: Environmental 64 (2006) 42–50

3.2. HCl TPD for mordenite catalysts TPD profiles of HCl for the series of CuHM catalysts can be observed in Fig. 2. When the TPD experiment was performed without the admission of a probe molecule, nothing was detected at a ramping temperature up to 800 8C. HCl was admitted at 200 8C and allowed to be desorbed. Two distinctive TPD peaks have been observed from 400 to 670 8C, regardless of the catalyst copper loadings. The intensities of two peaks gradually increased with respect to the copper content of the catalysts. The more copper loaded on the catalyst, the more HCl adsorbed on CuHM catalyst. This may reveal that the adsorption site of HCl on the catalyst is mainly copper ion on the catalyst surface and HCl may directly attack copper ions, since HCl hardly adsorbs on HM catalyst without copper, as also depicted in Fig. 2. 3.3. Bulk and surface composition of Cu for CuHM catalysts The most important aim of the present study might be the elucidation of the deactivation mechanism of CuHM catalyst by HCl. To achieve this goal, the highest copper loaded catalyst, Cu(77)HM was systematically examined to clearly discriminate the catalyst deactivation upon the alteration of the catalyst characteristics. The XPS patterns of Cl 2p for Cu(77)HM catalyst with respect to the deactivation temperature have been observed, and the intensity of the Cl 2p peak anticipated at 198.6 eV for Cl 2p3/2 and 200.4 eV for Cl 2p1/2 decreases [16], as the deactivation temperature increases from 150 to 450 8C. It can be induced from the results listed in Table 2. The chemical composition of the bulk and surface of Cu(77)HM catalysts with respect to the deactivation temperature is listed in Table 2. The loss of copper from the catalyst

45

Table 2 Chemical composition of the bulk and surface of Cu(77)HM with respect to deactivation temperature Catalyst

Cu(77)HMF Cu(77)HMD150a Cu(77)HMD250b Cu(77)HMD350c Cu(77)HMD450d a b c d

Catalyst Catalyst Catalyst Catalyst

ICP (molar ratio)

XPS (atomic ratio)

Cu/Si

Cu/Al

Cu/Si

Cu/Al

Cl/Al

Cl/Cu

0.064 0.065 0.065 0.040 0.040

0.387 0.378 0.385 0.236 0.232

0.068 0.066 0.040 0.021 0.041

0.402 0.393 0.243 0.121 0.237

0.0 1.096 0.464 0.218 0.020

0.0 2.79 1.91 1.81 0.08

deactivated deactivated deactivated deactivated

by by by by

HCl HCl HCl HCl

at at at at

150 8C 250 8C 350 8C 450 8C

for for for for

110 h. 110 h. 110 h. 110 h.

begins at 350 8C. However, the ratio of Cu/Al on the catalyst surface decreases from 0.402 to 0.393, 0.243 and 0.121 as the deactivation temperature increases from 150 to 350 8C. At 450 8C, the ratio, however, increases probably due to the migration of copper ion from the bulk catalyst to the surface of the catalyst. Of course, the ratio of Cu/Al on the surface of Cu(77)HMD450 is smaller than that for the fresh catalyst. Moreover, the Cl/Cu ratio on the catalyst surface after the deactivation test is 1.91 for 250 8C and 1.81 for 350 8C. It significantly decreases for the catalyst deactivated at 450 8C mainly due to the evaporation of copper chloride compound from the catalyst surface. It can be anticipated that the loss of copper occurs from the surface of the catalyst, the copper content of the deactivated catalyst depends on the deactivation temperature, the copper ion may even migrate from the bulk of the catalyst to the surface, and the copper may be interacting with the chlorine atoms on the catalyst surface upon the catalyst deactivation by HCl. The main cause of the catalyst deactivation is definitely the alteration of the state and content of copper ion on the surface of the catalyst. 3.4. Chemical state of copper ion on the surface of CuHM catalysts

Fig. 2. HCl TPD spectra of: (a) HM, (b) Cu(23)HMF, (c) Cu(33)HMF, (d) Cu(50)HMF, and (e) Cu(77)HMF.

Two distinctive XPS spectra of Cu 2p3/2 for Cu(77)HM catalyst with respect to the deactivation temperature are shown in Fig. 3. The characteristics of divalent copper, Cu(II) can be confirmed by the Cu 2p3/2 binding energy within the range of 933–936 eV and the shake-up satellite peak. Monovalent copper can be assigned to the binding energy of the XPS peaks ranging from 932 to 933 eV without any shake-up satellite peaks. By the deconvolution of the Cu 2p3/2 spectra based upon the previous studies [17–22], two peaks within the ranges of 934.41–934.95 eV for Cu(II) and 932.62–932.78 eV for Cu(I) can be anticipated. Although the copper in the present catalytic system mainly exists in a form of Cu(II), regardless of their content, monovalent copper, the presence of Cu(I) has also been observed on the surface of Cu(77)HM catalyst as listed in Table 3 by the deconvolution of Cu 2p3/2 peaks in Fig. 3. The XPS spectra of Cu(77)HM catalysts contain a strong Cu 2p3/2 shake-up satellite peak at 943.6 eV and their intensities

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J.W. Choung, I.-S. Nam / Applied Catalysis B: Environmental 64 (2006) 42–50

Fig. 3. Cu 2p3/2 XPS spectra of Cu(77)HM catalysts with respect to the deactivation temperature: (a) Cu(77)HMD450, (b) Cu(77)HMD350, (c) Cu(77)HMD250, (d) Cu(77)HMD150, and (e) Cu(77)HMF.

decrease as the deactivation temperature increases. However, the intensity of the Cu 2p3/2 peak at ca. 932.7 eV increases as shown in Fig. 3. It may indicate an increase of monovalent copper on the catalyst surface. The ratio of Cu(I)/Cu(II) Cu 2p3/ 2 estimated from XPS observation has been listed in Table 3. The ratio of the copper intensities representing the relative amounts of Cu(I) and Cu(II) on the catalyst surface increases as the deactivation proceeds. It reveals that the catalyst deactivation of CuHM by HCl alters the state of copper ion to form Cu(I)-ion on the surface of the catalyst. Note that both Cu(I) and Cu metal containing similar Cu 2p3/2 binding energy, hardly comprise their Cu 2p3/2 shake-up satellite peaks. No Cu(I) has been observed on the surface of Cu(33)HMF and Cu(50)HMF catalysts, but it exists for their deactivated counterpart catalysts. 3.5. Identification of deactivation precursor by SR X-ray diffraction The phase of the copper on the catalyst deactivated by HCl was examined by synchrotron radiation X-ray diffraction (SRTable 3 Deconvolution of XPS Cu 2p peaks of various catalysts Catalyst

Cu(33)HMF Cu(50)HMF Cu(77)HMF Cu(77)HMD150 Cu(77)HMD250 Cu(77)HMD350 Cu(77)HMD450

Intensity ratio

Binding energy, Cu 2p3/2 (eV) Cu(I)

Cu(II)

Cu(I)/Cu(II)

– – 932.69 932.68 932.59 932.62 932.78

934.41 934.47 934.82 934.42 934.95 934.41 934.69

– – 0.21 0.24 0.34 0.85 0.89

XRD). No significant distinction can be observed among the XRD patterns of HM, Cu(77)HMF and Cu(77)HMD catalysts, as shown in Fig. 4. The crystallinities of the catalysts are quite similar to each other. It also shows that the mordenite structure is hardly altered by deactivation with HCl. However, new copper phases due to the formation of Cu2Cl(OH)3 (JCPDS file no. 25-1427) and CuCl2
2H2O (JCPDS file no. 33-451) appear and disappear with respect to the deactivation temperatures employed in the present study. Kim et al. [23] and Lee et al. [24] reported that most of CuCl2
2H2O supported on alumina or carbon transformed into Cu2Cl(OH)3 at room temperature and CuCl2
2H2O supported on silica converted to CuCl and Cu2Cl(OH)3 during the course of CO oxidation. For Cu(77)HMD150 deactivated by HCl at 150 8C for 110 h, the XRD pattern shows the formation of the copper phase of Cu2Cl(OH)3 on its surface. The pattern of Cu(77)HMD250 indicates the formation of CuCl2
2H2O as well as Cu2Cl(OH)3 on the surface of the deactivated catalyst. Most of Cu–Cl compound for Cu(77)HMD350 exists in a compound of CuCl2
2H2O. However, the pattern of Cu(77)HMD450 is basically similar to that of Cu(77)HMF without the formation of any new phase of copper discussed. Xiao et al. also observed that the characteristic XRD peaks assigned to CuCl2
2H2O completely disappeared when the mechanical mixture of CuCl2
2H2O and NaZSM-5 was heated at 400 8C for 24 h [25]. The peak intensity of XRD pattern assigned to Cu2Cl(OH)3 and CuCl2
2H2O, formed on the catalyst surface due to the deactivation by HCl, reveals a maximum when they were deactivated at 150 and 250 8C of the deactivation temperature, respectively. It simply indicates that the copper ion on the catalyst surface interacting with hydrogen chloride during the course of deactivation process forms Cu2Cl(OH)3, and it converts to CuCl2
2H2O depending upon the deactivation temperature [26]. Based on the present SR-XRD observation for the general feature of the phase and state of copper on the surface of Cu(77)HM catalyst upon the catalyst deactivation by HCl along with the XPS results, the deactivation mechanism can be anticipated as follows: CuCl2
2H2O formed during the course of the deactivation decomposes into CuCl through CuCl2 at 350 8C, and a part of copper may be introduced onto the zeolite structure via solid-state ion exchange, again resulting in the increase of Cu/Al on the catalyst surface. It has been commonly recognized that CuCl2
2H2O transfers into CuCl2 at ca. 100 8C; anhydrous CuCl2 is stable up to 200 8C and begins to decompose to CuCl, and the evaporation of CuCl from the catalyst surface occurs in the range of the reaction temperature higher than 320 8C [26–28]. 3.6. Phase of copper ion by X-ray absorption near edge structure (XANES) The XANES, the fine structure near the rising portion of an absorption spectrum, contains information on the chemical environment and bond of the surrounding atoms absorbing X-ray. The Cu K-edge XANES of the copper compounds including CuO, Cu2O, CuCl2
2H2O, CuCl, and Cu foil

J.W. Choung, I.-S. Nam / Applied Catalysis B: Environmental 64 (2006) 42–50

47

Fig. 4. XRD patterns of CuHM catalysts: (a) Cu(77)HMF, (b) Cu(77)HMD450, (c) Cu(77)HMD350, (d) Cu(77)HMD250, (e) Cu(77)HMD150, and (f) HM.

possibly formed during the course of the reaction, has been examined in Fig. 5. The presence of a sharp peak at 8981 eV for Cu2O may reflect the existence of Cu(I) compound in a form of trigonal or distorted tetrahedral geometry allowing s–p mixing [29]. Avery weak peak below the edge at 8977 eV for CuO may represent the quadruple allowing 1s ! 3d transition, which may be indicative for the formation of a Cu(II) compound. Note that there is no 3d hole in the Cu(0) or Cu(I) compound. The 1s ! 4s transition peak for Cu(II) compounds appears at 8985 eV as a small shoulder. The XANES for the reference sample, CuCl2
2H2O also shows a presence of the pre-edge peak, indicating that Cu is existing in a form of Cu(II). Cu K-edge XANES spectra for CuHM catalysts with respect to the amounts of copper ions exchanged can also be observed in Fig. 5. The XANES spectra of CuHM catalysts are quite distinctive compared to the spectra of the reference samples. The peak at 8977 eV is commonly observed for all the catalysts examined in the present study, revealing an indication for the dominant existence of divalent copper species, Cu(II) on the catalyst. For fresh CuHM catalysts, the 1s ! 4s transition peak, A appears at 8986 eV and the 1s ! 4p absorption peak, B becomes broader as the copper content of the catalyst increases, suggesting somewhat irregular coordination geometry of Cu(II) compound on CuHM catalyst [8,24,29–33]. The edge position energy of the catalysts was also determined to be 8986.96, 8986.88, 8986.69, and 8986.05 eV for Cu(23)HMF, Cu(33)HMF, Cu(50)HMF, and Cu(77)HMF, respectively. It is noted that the edge position for Cu(77)HMF shifts toward lower energy than that of the others, revealing a decrease of the oxidation state of the absorbing atom. This may indicate the existence of Cu(I) on the surface of Cu(77)HMF catalyst as revealed by XPS.

Fig. 5. XANES spectra of Cu K-edge for: (a) CuCl, (b) CuCl2
2H2O, (c) Cu2O, (d) CuO, (e) Cu foil, (f) Cu(23)HMF, (g) Cu(33)HMF, (h) Cu(50)HMF, and (i) Cu(77)HMF.

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Fig. 6 shows the XANES spectra of Cu(77)HM catalysts with respect to the deactivation temperature. The peak at 8977 eV is commonly observed again for the catalysts examined in the present study, representing the dominant existence of divalent copper species on the catalyst. On the other hand, the main peak at 8996 eV becomes broader and then shows multiple maxima around 8993 and 8996 eVof the energy as the deactivation temperature decreases. In particular, the XANES spectra of the catalyst deactivated at 150 8C are quite different from those of the other catalysts and the reference samples. If all Cu on the catalyst exists in the identical atomic environment mainly containing O and Cl ligands, a single maximum of XANES is expected. Note that the origin of the multiple peaks may also be due to the presence of the metal atomic copper. Indeed, the phase of Cu on Cu(77)HMD150 may be identified as Cu2Cl(OH)3 [26,34], as also discussed by SR-XRD and isolated Cu(II) ions. To further examine the state of copper upon the catalyst deactivation, Fig. 7 compares the first-derivative of XANES spectra for the catalysts before and after deactivation. It also shows the existence of three kinds of copper compounds on the catalyst as confirmed in the present study.

Fig. 7. First-derivative of XANES region for CuHM catalysts and reference samples: (a) Cu(77)HMF, (b) Cu(77)HMD450, (c) Cu(77)HMD350, (d) Cu(77)HMD250, (e) Cu(77)HMD150, (f) CuO, and (g) CuCl2
2H2O.

3.7. Deactivation mechanism Fig. 8 shows the Fourier transform (FT) of k3-weighted EXAFS oscillation with respect to the Cu loadings of the fresh CuHM catalysts at room temperature along with EXAFS data for the reference samples. The catalysts commonly contain a ˚ of the atomic distance without any distinctive peak at 1.54 A phase-shift correction. The absence of a Cu–Cu peak in the EXAFS spectrum may suggest that the copper species on the catalyst is existing in a form of mononuclear complex [30]. Contrasting FT EXAFS spectra of the catalysts to that of the

Fig. 6. XANES spectra of Cu K-edge for Cu(77)HM catalysts with respect to the deactivation temperature: (a) Cu(77)HMF, (b) Cu(77)HMD450, (c) Cu(77)HMD350, (d) Cu(77)HMD250, (e) Cu(77)HMD150, (f) CuO, and (g) CuCl2
2H2O.

Fig. 8. FT of k3-weighted Cu K-edge EXAFS for CuHM catalysts and reference samples: (a) CuCl, (b) CuCl2
2H2O, (c) Cu2O, (d) CuO, (e) Cu foil, (f) Cu(23)HMF, (g) Cu(33)HMF, (h) Cu(50)HMF, and (i) Cu(77)HMF.

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Table 4 EXFAS fitting results of the Cu K-edge for the fresh and deactivated CuHM catalysts Catalyst

Shell

C.N.a

˚ )b R (A

s2c

R-factord

Cu(23)HMF Cu(33)HMF Cu(50)HMF Cu(77)HMF Cu(77)HMD450 Cu(77)HMD350

Cu–O Cu–O Cu–O Cu–O Cu–O Cu–O Cu–Cl Cu–O Cu–Cl Cu–O Cu–Cl

4.23 4.00 3.95 3.82 3.81 3.12 1.05 2.74 1.83 2.11 3.05

1.953 1.954 1.949 1.950 1.946 1.962 2.289 1.973 2.279 1.985 2.274

0.00523 0.00521 0.00491 0.00601 0.00408 0.00424 0.00035 0.00486 0.00078 0.00440 0.00289

0.00218 0.00674 0.00595 0.00743 0.00272 0.00558

Cu(77)HMD250 Cu(77)HMD150 a b c d

Fig. 9. FT of k3-weighted Cu K-edge EXAFS for CuHM catalysts and reference samples: (a) CuCl2
2H2O, (b) CuO, (c) Cu(77)HMD150, (d) Cu(77)HMD250, (e) Cu(77)HMD350, (f) Cu(77)HMD450, and (g) Cu(77)HMF.

˚ may reveal the nearest reference samples, the peak at 1.54 A neighbor oxygen for Cu–O [30]. The Fourier transform of Cu K-edge k3-weighted EXAFS for the catalysts deactivated at a variety of the deactivation temperatures and the most possible references screened from ˚ Fig. 8 can be observed in Fig. 9. The peak intensity at 1.54 A ˚ decreases, although that at 1.87 A becomes evident as the deactivation temperature decreases. It has been also observed in the XANES results as mentioned. This tendency can be well correlated with the coordination number of the surrounding atoms of copper ion. Based upon the EXAFS spectra for CuO and hydrated CuCl2 as a reference, the first and second major ˚ without the phase-shift peaks at around 1.54 and 1.87 A

0.00634 0.00640

Coordination number. Interatomic distance. The Debye–Wallar factor. The goodness-of-fit parameter.

correction may reveal the nearest neighboring oxygen and chlorine atoms to copper, respectively. The simulation results obtained by the curve-fitting analyses of the EXAFS spectra are listed in Table 4. The quality of fitting on the basis of the parameters listed in Table 4, particularly represented by R-factor (the squared deviation of experimental data from the model spectrum) is generally acceptable [15]. The coordination number of CuHM catalyst gradually decreases as the copper loading of the catalyst increases. However, no correlation of the Cu–O bond lengths with the number has been observed. The catalysts prepared in the present study mainly contain divalent copper species on their surface. For the deactivated catalysts, the main peak in the ˚ radial distribution function (radial distance range of 1.2–2.3 A for Cu) is evident from the rest of the spectrum. The coordination numbers for Cu–O and Cu–Cl decrease from 3.81 to 2.11 and increase from 0 to 3.05, respectively, as the

Fig. 10. A schematic diagram for the deactivation mechanism of CuHM catalyst by HCl.

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J.W. Choung, I.-S. Nam / Applied Catalysis B: Environmental 64 (2006) 42–50

deactivation temperature decreases. On the contrary, the Cu–O bond length varied from 1.946 to 1.985 with respect to the temperature. The fitting parameters of Cu(77)HMD450 are similar to that of the fresh counterpart as listed in Table 4, indicating that no change of Cu species on the catalyst surface is expected when the catalyst is deactivated at 450 8C. It may reveal that the copper state on the catalysts deactivated by HCl predominantly exists in the form of the divalent copper species, as confirmed by the weak XANES peak at 8977 eV shown in Fig. 6. Moreover, the mean contents of the chlorine, which may be represented by the coordination number of Cu–Cl listed in Table 4, of the catalysts deactivated at 150, 250, 350 and 450 8C are 3.05, 1.83, 1.05, and 0, respectively. They are also quite consistent with the Cl/Cu ratios for CuHMD150, -D250, -D350, and -D450 catalysts estimated from XPS observation as a function of the ratios, 2.79, 1.91, 1.81, and 0.08, respectively as listed in Table 2. This may simply indicate that the evaporation of the copper compounds formed during the course of the catalyst deactivation occurs from the surface of the catalyst. The higher the deactivation temperature, the less chlorine has been observed on the catalyst surface. In addition, the more chlorine deposits on the catalyst surface than that in the bulk catalyst and the copper interacts with two chlorine atoms during the course of the catalyst deactivation, particularly at 350 8C. Based upon this observation, the deactivation mechanism of CuHM catalyst by HCl for the reduction of NO with NH3 has been schematically illustrated in Fig. 10. 4. Conclusions In the present study, the deactivation of CuHM catalyst by HCl for the reduction of NOx with NH3 is mainly due to the alteration of the state and/or phase of the copper ion within the framework of mordenite structure as well as the loss of Cu ion from the catalyst surface. The HCl tolerance of CuHM catalyst can be improved by increasing the Cu content of the catalyst and decreasing the concentration of copper on the catalyst surface. HCl TPD reveals that the adsorption site of hydrogen chloride is Cu(II) compound. The SR-XRD directly depicts that the deactivation precursor for CuHM by HCl, Cu2Cl(OH)3, forms through the reaction between HCl and copper ion on the catalyst and Cu2Cl(OH)3 is also converted to CuCl2
2H2O by increasing the reaction temperature. Moreover, XPS study provides evidence for an increase of the content of Cu(I) ion on the surface of CuHM catalyst as the deactivation proceeds. On the other hand, from the XAFS study by using XANES and EXAFS techniques, the copper on the catalyst deactivated at a variety of reaction temperatures mainly exists in the form of divalent copper species. It clearly provides evidence for the interaction between HCl and copper ions on the catalyst, and the chlorine content on the surface of deactivated catalysts compared to that on the bulk catalyst increases as the deactivation temperature increases.

The deactivation of CuHM catalyst by HCl is mainly due to the formation of Cu2Cl(OH)3 through the reaction between HCl and Cu ion of the catalyst. Cu2Cl(OH)3 is the deactivation precursor for the present reaction system and can also be converted to CuCl2
2H2O during the course of the catalyst deactivation. Moreover, CuCl2
2H2O formed evaporated at a reaction temperature higher than 350 8C. It may also transfer to CuCl and then may evaporate to the bulk gas stream and/or introduce the Cu(I) ion onto the catalyst again by a means of solid-state ion exchange. References [1] R. Yan, T. Chin, D.T. Liang, K. Laursen, W.Y. Ong, K. Yao, J.H. Tay, Environ. Sci. Technol. 35 (2003) 2556. [2] J.P. Chen, M.A. Buzanowski, R.T. Yang, J.E. Cicha-nowicz, J. Air Waste Manage. Assoc. 40 (1990) 1403. [3] L. Lisi, G. Lasorella, S. Malloggi, G. Russo, Appl. Catal. B 50 (2004) 251. [4] S.W. Ham, H. Choi, I.-S. Nam, Y.G. Kim, Catal. Today 11 (1992) 611. [5] R.M. Barrer, E.A.D. White, J. Chem. Soc. (1952) 1561. [6] R.M. Barrer, A.G. Kanellopoulos, J. Chem. Soc. (A) (1970) 765. [7] U. Shigeo, H. Hiroshi, Ind. Eng. Chem. Proc. Res. Dev. 22 (1983) 144. [8] G.G. Park, H.J. Che, I.-S. Nam, J.W. Choung, K.H. Choi, Micropor. Mesopor. Mater. 48 (2001) 337. [9] G.A. Ozin, S. Ozkar, G.D. Stucky, J. Phys. Chem. 94 (1990) 7562. [10] G.A. Ozin, S. Ozkar, L. McMurray, J. Phys. Chem. 94 (1990) 8289. [11] E.-Y. Choi, I.-S. Nam, Y.G. Kim, J. Catal. 161 (1996) 597. [12] J.J. Rehr, J. Mustre de Leon, S.I. Zabinsky, R.C. Albers, J. Am. Chem. Soc. 113 (1991) 5135. [13] B.K. Teo, EXAFS: Basic Principles and Data Analysis, Springer-Verlag, Berlin, 1986. [14] D.C. Koningsberger, R. Prins, X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES, Wiley/Interscience, New York, 1988, pp. 211–253. [15] P.A. O’Day, J.J. Rehr, S.I. Zabinsky, G.E. Brown Jr., J. Am. Chem. Soc. 116 (1994) 2938. [16] J. Xie, M. Huang, S. Kaliaguine, Appl. Surf. Sci. 115 (1997) 157. [17] B.A. Sexton, T.D. Smith, J.V. Sanders, J. Electron Spectrosc. 35 (1985) 27. [18] W. Gru¨nert, N.W. Hayes, R.W. Joyner, E.S. Shapiro, M.R.H. Siddiqui, G.N. Baeva, J. Phys. Chem. 98 (1994) 10832. [19] H. Raaf, N. Schwentner, Appl. Surf. Sci. 174 (2001) 13. [20] G. Schoen, Surf. Sci. 35 (1973) 96. [21] T. Robert, M. Bartel, G. Offergeld, Surf. Sci. 33 (1972) 123. [22] V.K. Kaushik, Spectrochim. Acta B 44 (1989) 581. [23] K.D. Kim, I.-S. Nam, J.S. Chung, J.S. Lee, S.G. Ryu, Y.S. Yang, Appl. Catal. B 5 (1994) 103. [24] J.S. Lee, S.H. Choi, K.D. Kim, M. Nomura, Appl. Catal. B 7 (1996) 199. [25] F.-S. Xiao, W. Zhang, M. Jia, Y. Yu, C.F. Guoxingbatu, S. Zheng, S. Qiu, R. Xu, Catal. Today 50 (1999) 117. [26] G. Leofanti, M. Padovan, M. Garilli, D. Carmello, G.L. Marra, A. Zecchina, G. Spoto, S. Bordiga, C. Lamberti, J. Catal. 189 (2000) 105. [27] J.A. Conesa, A. Fullana, R. Font, J. Anal. Appl. Pyrol. 58/59 (2001) 553. [28] D. Verhulst, A. Buekens, P.J. Spencer, G. Eriksson, Environ. Sci. Technol. 30 (1996) 50. [29] J.M. Brown, L. Powers, B. Kincaid, J.A. Larrabee, T.G. Spiro, J. Am. Chem. Soc. 102 (1980) 4210. [30] E.Y. Choi, I.-S. Nam, Y.G. Kim, J.S. Chung, J.S. Lee, M. Nomura, J. Mol. Catal. 69 (1991) 247. [31] M.H. Kim, I.-S. Nam, Y.G. Kim, Appl. Catal. B 12 (1997) 125. [32] N. Kosugi, H. Kuroda, Chem. Phys. 103 (1986) 101. [33] M. Nomura, A. Kazusuka, N. Ukisu, N. Kakuta, J. Chem. Soc., Faraday Trans. 1 83 (1987) 2635. [34] M.E. Fleet, Acta Crystallogr. B 31 (1975) 183.