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palygorskite catalysts
CLAY-03566; No of Pages 6 Applied Clay Science xxx (2015) xxx–xxx
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Room-temperature CO oxidation over calcined Pd–Cu/ palygorskite catalysts Yongzhao Wang ⁎, Jing Shi, Ruifang Wu, Xiao Li, Yongxiang Zhao School of Chemistry and Chemical Engineering, Engineering Research Center of Ministry of Education for Fine Chemicals, Shanxi University, Shanxi, Taiyuan, 030006, China
a r t i c l e
i n f o
Article history: Received 28 March 2015 Received in revised form 22 August 2015 Accepted 24 August 2015 Available online xxxx Keywords: Pd-Cu/palygorskite CO oxidation Room temperature Calcined
a b s t r a c t Pd–Cu/palygorskite catalysts without calcination (U-PdCu/Pal) and with calcination (C-PdCu/Pal) were prepared via an impregnation method. These catalysts' catalytic performances for CO oxidation were studied. CO conversion can reach 100% at room temperature under conditions of 0.5 vol.% CO and 3.3 vol.% H2O in the feed gas over C-PdCu/Pal, which is considerably higher than that over U-PdCu/Pal. The XRD results suggest that the Cu species exists as a Cu(OH)Cl phase with a crystallite size of approximately 30 nm on U-PdCu/Pal, while the crystallite size of the Cu(OH)Cl phase on C-PdCu/Pal is significantly smaller. The results of TPR and IR indicate that there is an enhanced interaction resulting from the calcination, which promotes the reduction of C-PdCu/Pal and enhances the catalytic activity. © 2015 Elsevier B.V. All rights reserved.
1. Introduction CO is a toxic component of industrial emissions and exhaust fumes from automobiles. CO causes harmful effects to living beings and the environment. Thus, low-temperature CO oxidation has attracted tremendous interest from researchers because of its importance in practical applications, such as air purification, CO gas sensors, gas masks, lowering automotive emissions and catalysis (Schryer et al., 1991; Yuan et al., 1997; Chen and Goodman, 2004). In recent decades, many types of catalysts have been prepared for low-temperature CO oxidation, including noble metal catalysts, metal oxide catalysts, and supported Wacker catalysts (Yoon and Cocke, 1988; Zou et al., 2007; Hou et al., 2008; Xie et al., 2009; Jia et al., 2010; Rao et al., 2010; Royer and Duprez, 2011; Grünert et al., 2014; Kuo et al., 2014; Du et al., 2015; Wang et al., 2015a). The supported Wacker catalyst, in which an aqueous solution of PdCl2–CuCl2 or PdCl2–CuCl2–Cu(NO3)2 is loaded into the pores of high specific surface area supports without further calcination, is wellknown to accelerate room-temperature CO oxidation. Compared with metal oxides and supported gold-based catalysts, the supported Wacker catalyst is stable in the presence of halogen impurities and water vapor (Dyakonov, 2003; Wang et al., 2010). The characteristics of the support are critical parameters for the formation and stabilization of the active Cu species phase, which plays an important role in the catalytic performance. Alumina and active carbon are effective supports, but aggregation and transformation of the active Cu species phase occurs during ⁎ Corresponding author. E-mail address: [email protected] (Y. Wang).
the long-time reaction process (Lee et al., 1999; Shen et al., 2011). Because the nature of the support is of crucial importance to the catalytic performance, the exploration of new catalyst systems with less expensive and more easily available supports remains a challenge. Palygorskite (Pal), a hydrated magnesium aluminum silicate mineral that commonly has a lath or fibrous morphology, is characterized by a porous crystallite structure containing tetrahedral layers alloyed together along longitudinal sideline chains (Yan et al., 2012; Liu et al., 2013). Due to its unique structural and textural properties, palygorskite has been widely used as an adsorbent, adhesive, catalyst and catalyst support (Cao et al., 2008; Huo and Yang, 2010; Zhou, 2011; He and Yang, 2013; Papoulis et al., 2013; Zhou and Keeling, 2013; Chen et al., 2014; He et al., 2014; Jiang et al., 2014; Wu et al., 2014a; Zhang et al., 2014; Guo et al., 2015; Wang et al., 2015b; Zhang et al., 2015; Zhou et al., 2015). Cao et al. (2008) prepared CuO catalysts supported on palygorskite with different Cu contents and found that 16 wt.%-CuO/ Pal can convert completely CO at 120 °C, which is comparable to the activities of CuO-based catalysts supported on Fe2O3 and TiO2. He and Yang (2013) prepared a catalyst with coexisting Au and Au2O3 on the surface of palygorskite, and the catalytic activity of o-DCB (o-dichlorobenzene) oxidation was found to reach 99% at a Au / (Au + Au2O3) molar ratio of 0.83, three times that without the palygorskite support. The supports of the supported Wacker catalyst include active carbon, Al2O3 and molecular sieve, but to the best of our knowledge, the use of palygorskite as the support has not been reported. In addition, most supported Wacker catalysts are not calcined but are only dried during the process of preparation due to the poor stability of the active Cu species. In this work, palygorskite was used as a support of Pd–Cu catalysts for room-temperature CO oxidation. Compared with the catalyst
http://dx.doi.org/10.1016/j.clay.2015.08.034 0169-1317/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: Wang, Y., et al., Room-temperature CO oxidation over calcined Pd–Cu/palygorskite catalysts, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.034
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without calcination, the calcined catalyst showed excellent catalytic activity, suggesting that it is a potential candidate for use in environmental catalyst systems. 2. Experimental 2.1. Materials
flow rate of 30 mL/min. No pretreatment was applied before the catalytic activity test. A quantitative analysis of CO was performed with an online gas chromatograph equipped with a 3 m column packed with carbon molecular sieve, a methanator and a flame ionization detector (FID). To enhance the sensitivity of the detection, CO and CO2 were converted to CH4 by the methanator at 360 °C before entering into the FID. The activity was expressed by CO conversion, which was calculated according to.
Palygorskite was obtained from Anhui Mingmei MinChem Co., Ltd. (Hefei, China), and used without further purification. Palygorskite's primary chemical components are SiO2, Al2O3, and MgO, and it also contains small amounts of Fe2O3 and Mn3O4. All of the chemicals were of analytical grade and purchased from Shanghai Chemical Reagent Co., Ltd. (Shanghai, China).
where X is the CO conversion, COinlet represents the initial CO concentration in the inlet, and COoutlet represents the CO concentration in the outlet.
2.2. Catalyst preparation
3. Results and discussion
The PdCu/Pal catalysts were prepared by an impregnation method using metal precursors of PdCl2, CuCl2·2H2O and Cu(NO3)2·3H2O to yield a catalyst with 0.2 wt.% Pd and 12 wt.% Cu and a molar ratio of CuCl2 to Cu(NO3)2 of 2. After impregnation and drying at 80 °C for 3 h and 140 °C for 3 h, the obtained catalyst was labeled as U-PdCu/Pal. A part of the U-PdCu/Pal was calcined at 300 °C for 3 h, and the obtained catalyst was labeled as C-PdCu/Pal.
3.1. Catalyst characterization
2.3. Catalyst characterization X-ray powder diffraction (XRD) measurements were performed to verify the species present in the catalysts. XRD patterns of the samples were recorded using a Bruker D8-Advance X-ray diffractometer with a target of Cu Kα, operated at 40 kV and 40 mA with a scanning speed of 2.4°/min and a scanning angle (2θ) range of 5–80°. N2 adsorption–desorption (N2 sorption) isotherms were determined using a Micromeritics ASAP 2020 special surface area and porosity analyzer at −196 °C. Prior to the sorption experiments, the samples were degassed under vacuum at 120 °C for 12 h. The specific surface area (SBET) was determined from the nitrogen adsorption isotherm. Fourier transform infrared (FT-IR) spectra of the samples were obtained with a Bruker TENSOR 27 spectrometer in the range of 400 to 4000 cm−1 with a resolution of 4 cm−1. Also, 1 mg of each powder sample was diluted in 100 mg of vacuum-dried IR-grade KBr. Transmission electron microscopy (TEM) measurements were carried out by a JEM-2100 model TEM operated at 200 kV. Samples for TEM were dispersed by sonication in ethanol followed by deposition of the dispersion onto a standard Cu grid covered with a porous carbon film. H2 temperature-programmed reduction (H2-TPR) experiments were performed on a Micromeritics Autochem II 2920 Chemisorb instrument. 5 vol.% H2/N2 flow passed through 30 mg of catalyst (40– 60 mesh) at a flow rate of 20 mL/min. The temperature was ramped from room temperature to 700 °C at a rate of 10 °C/min. The consumption of H2 was detected using a thermal conductivity detector (TCD), and the end gas was detected by an on-line quadrupole mass spectrometer (HIDEN HPR-20 QIC) with a minimum dwell time of 3 ms.
Xð%Þ ¼ ðCOinlet –COoutlet Þ=COinlet 100;
3.1.1. XRD analysis The XRD patterns of U-PdCu/Pal and C-PdCu/Pal are given in Fig. 1. For comparison, the XRD patterns of palygorskite without and with calcination at 300 °C (Pal-300) are also given in Fig. 1. The reflections at 2θ = 8.4° (d = 1.053 nm, 110), 2θ = 13.9° (d = 0.637 nm, 200), 2θ = 16.5° (d = 0.537 nm, 130), 2θ = 19.8° (d = 0.448 nm, 040), 2θ = 20.8° (d = 0.427 nm, 121), 2θ = 24.2° (d = 0.368 nm, 221), 2θ = 27.3° (d = 0.327 nm, 400) and 2θ = 35.1° (d = 0.256 nm, 161) were identified as palygorskite (JCPDS, No. 31–0783). The typical reflection at 8.41° corresponds to the (110) of palygorskite, which is attributed to the basal plane of the palygorskite structure, and the high intensity of the reflection indicates that the palygorskite presents a high degree of crystallinity. Compared with the reflections at 8.41° of U-PdCu/Pal, Pal300 and palygorskite, that of C-PdCu/Pal is lower in intensity, indicating that the degree of crystallinity is reduced. There may be two reasons for this, one the dehydroxylation of palygorskite after calcination and the other the interaction between the Cu and Pd species and the palygorskite, which are also in agreement with the IR and TPR results (as will be presented shortly). The reflection scanned at 26.7° (d = 0.334 nm, 101) could be ascribed to a quartz impurity, and this reflection is also present in the catalysts. The XRD reflections of the Cu species on U-PdCu/Pal are consistent with the data of the JCPDS file of Cu(OH)Cl (JCPDS, No. 23–1063), which are different from the results reported on other supports, such as active carbon and Al2O3 (Park and Lee, 2000; Shen et al., 2010).
2.4. Catalytic activity test Measurements of the catalytic performance for low-temperature CO oxidation were performed in a flow-through laboratory microreactor under atmospheric pressure. The microreactor was an 8 mm i.d. quartz u-tube, and a thermocouple was set into the catalyst bed to measure the temperature. The samples were sieved to 40–60 mesh such that the temperature gradients and pressure drop over the catalyst bed were negligible. 300 mg of catalyst was used for each test. The feed gas, adjusted by mass flow controllers and comprising 0.5 vol.% CO, 3.3 vol.% H2O and the balance air, passed through the catalyst bed at a total
Fig. 1. XRD patterns of the PdCu/Pal catalysts without calcination and with calcination at 300 °C and palygorskite.
Please cite this article as: Wang, Y., et al., Room-temperature CO oxidation over calcined Pd–Cu/palygorskite catalysts, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.034
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Shen et al. (2010) reported that there were only reflections of γCu2Cl(OH)3 and the support on Pd–Cu–Clx/Al2O3 catalysts. Park and Lee (2000) also reported that the Cu species existed as Cu2Cl(OH)3 on PdCl2–CuCl2/C. The other reflections on the XRD pattern of U-PdCu/Pal correspond to palygorskite. Apparently, the Cu species on C-PdCu/Pal possesses the same phase as that on U-PdCu/Pal, and no other reflections of the Cu species are detected. However, the characteristic Cu(OH)Cl reflections decrease in intensity and become obviously broadened on C-PdCu/Pal, indicating that the Cu species possesses a higher dispersion degree. According to the Scherrer equation, the crystallite sizes of Cu(OH)Cl on U-PdCu/Pal and C-PdCu/Pal are 30.8 nm and 13.6 nm, respectively, which are in agreement with the TEM observations (that will be presented shortly). Hydrogen bonding between the hydroxyls of both the Cu species and palygorskite may be responsible for the stability of Cu(OH)Cl. The reflection of the Pd species cannot be observed in the two samples, which indicates that the Pd species exists as a highly dispersed state or below the detection limit of XRD. Therefore, the differences in the palladium species between U-PdCu/Pal and C-PdCu/Pal need to be investigated further. 3.1.2. N2 sorption analysis The nitrogen adsorption–desorption isotherms of PdCu/Pal without and with calcination are given in Fig. 2. The isotherms of the two samples are of classical type III according to the IUPAC classification. A very strong increase in the nitrogen-adsorbed volume can be observed at relative pressures greater than 0.85, which is characteristic of the presence of an appreciable amount of macropores. The specific surface area (SBET) and pore volume of the palygorskite are 107 m2/g and 0.31 cm3/g, respectively. After impregnation, the SBET and pore volume of U-PdCu/Pal decline drastically due to the deposition of Cu and Pd species nanoparticles on the surface and in the small pores of palygorskite to only 24 m2/g and 0.13 cm3/g. However, the average pore diameter increases from 6.5 to 20.8 nm. Calcination treatment has a great influence on the pore structure of C-PdCu/Pal, and SBET increases from 24 to 86 m2/g after calcination at 300 °C. This result suggests that C-PdCu/Pal has a much higher SBET and larger pore volume than U-PdCu/Pal, which should have a significant impact on their catalytic performance.
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3567 cm−1, corresponding to the stretching vibrations of –OH units in the coordinated water and bound water, respectively (Yang et al., 2010). A broad band is observed at 3500 to 3300 cm−1, which may be caused by stretching vibrations of –OH in the zeolitic water and surface-physical adsorbed water. An asymmetric band centered at 1639 cm−1 in all the samples is ascribed to the –OH bending mode of the above-mentioned water molecules. The bands approximately 1195, 1026 and 989 cm−1 are associated with the Si–O vibrations and the in-layer Si–O bands (He and Yang, 2013; Wu et al., 2014). The band at 798 cm− 1 confirms the presence of quartz, and the band at 473 cm−1 is assigned to Si–O–Si deformation vibrations, which is consistent with the XRD results. The bands of U-PdCu/Pal are similar to those of palygorskite, but a new band at 1379 cm− 1 is observed, which originates from the nitrate radical in the Cu precursor. After calcination at 300 °C, the bands at 3631 and 3567 cm−1 significantly decrease, indicating the partial removal of the coordinated water and bound water. The two bands of C-PdCu/Pal at 3464 and 3343 cm−1 are distinct and are ascribed to the stretching vibrations of the –OH units of Cu(OH)Cl (Wei et al., 2013). These bands are not clear in UPdCu/Pal because of the overlap with the –OH in the zeolitic water and surface-physical adsorbed water. The band at 1379 cm−1 nearly disappears in the C-PdCu/Pal, indicating the complete decomposition of the nitrate radical after calcination. Compared with that in palygorskite and U-PdCu/Pal, the relative intensity of the band at 534 cm−1 decreases significantly in C-PdCu/Pal, indicating that the structure of the support changes. The changes in the support structure might be due to the partial loss of coordinated water and bound water and the enhanced interaction between the Cu and Pd species and the support, which are also consistent with the results of XRD.
3.1.3. FT-IR analysis The samples were characterized by FT-IR to understand the structure and the interaction between the Cu species and palygorskite. The FT-IR spectra between 4000 and 400 cm−1 of the PdCu/Pal catalysts without and with calcination and the support are shown in Fig. 3. The FT-IR spectrum of palygorskite shows two bands at 3631 and
3.1.4. TEM analysis The representative TEM micrographs of palygorskite and the PdCu/ Pal catalysts are shown in Fig. 4a–c. The typical TEM image of palygorskite exhibits bundles of a fibrous structure (Fig. 4a). The fibers of palygorskite have smooth surfaces with an average diameter of approximately 20 to 40 nm, where a visible boundary appears between the straight fibers. For the PdCu/Pal catalyst calcined at 300 °C, there are numerous nanoparticles attached along the fibers, displaying a narrow particle distribution, and the majority of the particles have a size less than 15 nm, as shown in Fig. 4c. However, the nanoparticles on the PdCu/Pal catalyst without calcination clearly aggregate, and the particle size is approximately 26 to 33 nm (Fig. 4b), much larger than that of the nanoparticles on C-PdCu/Pal. The nanoparticles on C-PdCu/Pal possess higher dispersion than those on U-PdCu/Pal, which is consistent with the results of XRD.
Fig. 2. N2 sorption isotherms of the PdCu/Pal catalysts without calcination and with calcination at 300 °C and palygorskite.
Fig. 3. FT-IR spectra of the PdCu/Pal catalysts without calcination and with calcination at 300 °C and palygorskite.
Please cite this article as: Wang, Y., et al., Room-temperature CO oxidation over calcined Pd–Cu/palygorskite catalysts, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.034
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Fig. 4. TEM images of the PdCu/Pal catalysts without calcination (b) and with calcination at 300 °C (c) and palygorskite (a).
3.1.5. H2-TPR analysis The TPR profiles of the PdCu/Pal catalysts and palygorskite are given in Fig. 5. There is no obvious reduction peak on the TPR profile of palygorskite, and only a very broad and short peak from 500 to 650 °C can be detected, which is ascribed to the reduction of a small quantity of metal ions contained in the palygorskite. The TPR profile of the UPdCu/Pal catalyst shows three reduction peaks, namely, α2, β2 and γ peaks. The α2 peak with a Tmax at 297 °C is attributed to the coreduction of the Pd and dispersed Cu species. The β2 peak at 315 °C can be ascribed to the reduction of the crystallized Cu species contacting with the Pd species, and the γ peak at 375 °C should be the reduction of a small quantity of isolated copper species that do not interact with the Pd species. In addition, the mass spectrometry results demonstrate that the α2 and β2 peaks may include the reduction of the nitrate radicals contained in U-PdCu/Pal due to the emergence of the NO signal (not given here) coinciding with the reduction peaks of α2 and β2. There are two peaks in the TPR profile of the C-PdCu/Pal catalyst, α1 and β1. Compared with those of U-PdCu/Pal, the reduction peaks of CPdCu/Pal become wide and short, indicating high dispersion. In addition, the temperatures of the peaks shift to significantly lower values, with Tmax at 237 °C and 306 °C, respectively. This indicates the easier reduction of the Pd and Cu species and the strong interaction between them after calcination (Shen et al., 2010). Calcination promotes the dispersion of the Cu species and enhances the interaction of the Cu and Pd species, thus facilitating the reduction of the C-PdCu/Pal catalyst. Moreover, unlike the TPR profile of Pd–Cu/Al2O3 (Wang et al., 2014), the PdCu/Pal catalysts lack a negative peak at 100 °C ascribed to the decomposition of Pd β-hydride, which may be due to their low Pd content.
Fig. 5. H2-TPR profiles of the PdCu/Pal catalysts without calcination and with calcination at 300 °C and palygorskite.
3.2. Catalytic performance for room-temperature CO oxidation The catalytic activities of the PdCu/Pal catalysts and the palygorskite support for room-temperature CO oxidation with 3.3 vol.% water vapor in the feed gas are given in Fig. 6. CO oxidation occurs at room temperature over both of the catalysts. The CO conversion over U-PdCu/Pal can only be maintained at approximately 30%. Under the same reaction conditions, there is an induction period over the C-PdCu/Pal catalyst before the CO conversion reaches a steady state. The initial CO conversion over C-PdCu/Pal is lower than that over U-PdCu/Pal, but it gradually increases from approximately 20% to above 90% with the increase of the reaction time from 5 to 80 min. The presence of water is essential for the catalytic activity of the supported Wacker catalyst, as water molecules are involved in the catalytic cycle. C-PdCu/Pal exhibits poor catalytic activity without H2O in the feed. The induction period of the C-PdCu/Pal catalyst may be due to the lack of water molecules on its surface or in the pores after calcination. However, the characteristics of C-PdCu/Pal, with its high dispersion of Cu(OH)Cl and enhanced interaction between the Cu and Pd species, result in a higher catalytic activity for CO oxidation. Zhang et al. (2007) reported that the Cu(OH)Cl species showed better catalytic performance than γ-Cu2(OH)3Cl in the oxidative carbonylation of ethanol to diethyl carbonate because the Cu(OH)Cl was more efficient in regenerating Pd0 to Pd2+ and reducing Cu2+ to Cu+. The effect of water vapor pretreatment on the catalytic activity of CPdCu/Pal was also investigated to reveal the relationship between the catalytic activity and reaction time (Fig. 7). The water vapor pretreatment was performed by allowing only air into a water bubbler
Fig. 6. The catalytic activity of the PdCu/Pal catalysts without calcination and with calcination at 300 °C and palygorskite (reaction conditions: CO 0.5 vol.%, O2 14.4 vol.%, H2O 3.3 vol.%, room temperature, GHSV 5000 h−1).
Please cite this article as: Wang, Y., et al., Room-temperature CO oxidation over calcined Pd–Cu/palygorskite catalysts, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.034
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the room-temperature CO oxidation was used as model reaction to investigate the catalytic activity. C-PdCu/Pal exhibits much higher catalytic activity than U-PdCu/Pal, and CO can be completely converted over CPdCu/Pal for more than 20 h at ambient temperature under the conditions of 0.5 vol.% CO and 3.3 vol.% H2O in the feed gas. Calcination does not affect the type of Cu species phase but leads to an increase in the specific surface area. Calcination also promotes the dispersion of the Cu species and enhances the interaction between the Cu and Pd species and the support, making them more easily reduced. The promoted catalytic activity of C-PdCu/Pal can be ascribed to the enhanced interaction and reduction, as well as the superior dispersion of the Cu species. Palygorskite is much cheaper and more easily available than other supports and the loading of the noble metal Pd is low, so PdCu/Pal catalysts can have a significant effect in applications of room-temperature CO oxidation.
Acknowledgments Fig. 7. Effect of water vapor pretreatment time on the catalytic activities of C-PdCu/Pal (reaction conditions: CO 0.5 vol.%, O2 14.4 vol.%, H2O 3.3 vol.%, room temperature, GHSV 5000 h−1).
maintained at 25 °C and then passing it through the catalyst bed for different times. The samples without water vapor pretreatment and those that were pretreated for 90 min and 120 min were labeled PdCu/Pal-0, PdCu/Pal-90 and PdCu/Pal-120, respectively. The CO conversion over all the samples except for PdCu/Pal-120 increases with the reaction time, and the pretreatment time has a strong influence on the catalytic performance. Not only is the initial catalytic activity significantly enhanced as the pretreatment time is prolonged, but the CO conversion also increases more rapidly. The initial CO conversion could reach nearly 60% over PdCu/Pal-90. Although the initial activity of PdCu/Pal-120 is the highest, the CO conversion gradually decreases as the reaction time increasing. The C-PdCu/Pal catalyst was also selected as a representative to investigate the stability (Fig. 8). The reaction temperature was maintained at ambient temperature, and the feed gas contained 3.3 vol.% water vapor. The catalytic activity changes very slightly and maintains a high activity for more than 20 h during the period of reaction. This demonstrates that the calcined PdCu/Pal catalyst is very active and stable for room-temperature CO oxidation. 4. Conclusions Pd–Cu/Pal catalysts without calcination (U-PdCu/Pal) and with calcination (C-PdCu/Pal) were prepared via an impregnation method, and
Fig. 8. CO conversion over C-PdCu/Pal catalyst at room temperature as a function of reaction time (reaction conditions: CO 0.5 vol.%, O2 14.4 vol.%, H2O 3.3 vol.%, GHSV 5000 h−1).
The authors gratefully acknowledge the financial support from the International S&T Cooperation Program of China (2013DFA40460), the National Natural Science Foundation of China (21073114), and the Shanxi Youth S&T Foundation (2010021008-3).
References Cao, J.L., Shao, G.S., Wang, Y., Liu, Y.P., Yuan, Z.Y., 2008. CuO catalysts supported on attapulgite clay for low-temperature CO oxidation. Catal. Commun. 9, 2555–2559. Chen, D.M., Du, Y., Zhu, H.L., Deng, Y.X., 2014. Synthesis and characterization of a microfibrous TiO2–CdS/palygorskite nanostructured material with enhanced visiblelight photocatalytic activity. Appl. Clay Sci. 87, 285–291. Chen, M.S., Goodman, D.W., 2004. The structure of catalytically active Au on titania. Science 306, 252–255. Du, X.X., Li, H.Y., Yu, J., Xiao, X.Z., Shi, Z.P., Mao, D.S., Lu, G.Z., 2015. Realization of a highly effective Pd–Cu–Clx/Al2O3 catalyst for low temperature CO oxidation by presynthesizing the active copper phase of Cu2(OH)3Cl. Catal. Sci. Technol. 5, 3970–3979. Dyakonov, A.J., 2003. Abatement of CO from relatively simple and complex mixtures: II. oxidation on Pd–Cu/C catalysts. Appl. Catal. B 45, 257–267. Grünert, W., Groβmann, D., Noei, H., Pohl, M., Sinev, I., Toni, A.D., Wang, Y.M., Muhler, M., 2014. Low-temperature oxidation of carbon monoxide with gold(III) ions supported on titanium oxide. Angew. Chem. Int. Ed. 53, 3245–3249. Guo, H.J., Zhang, H.R., Peng, F., Yang, H.J., Xiong, L., Huang, C., Wang, C., Chen, X.D., Ma, L.L., 2015. Mixed alcohols synthesis from syngas over activated palygorskite supported Cu–Fe–Co based catalysts. Appl. Clay Sci. 111, 83–89. He, X., Fu, L.J., Yang, H.M., 2014. Insight into the nature of Au–Au2O3 functionalized palygorskite. Appl. Clay Sci. 100, 118–122. He, X., Yang, H.M., 2013. Au nanoparticles assembled on palygorskite: enhanced catalytic property and Au–Au2O3 coexistence. J. Mol. Catal. A 379, 219–224. Hou, X.D., Wang, Y.Z., Zhao, Y.X., 2008. Effect of CeO2 doping on structure and catalytic performance of Co3O4 catalyst for low-temperature CO oxidation. Catal. Lett. 123, 321–326. Huo, C.L., Yang, H.M., 2010. Synthesis and characterization of ZnO/palygorskite. Appl. Clay Sci. 50, 362–366. Jia, C.J., Liu, Y., Bongard, H., Schüth, F., 2010. Very low temperature CO oxidation over colloidally deposited gold nanoparticles on Mg(OH)2 and MgO. J. Am. Chem. Soc. 132, 1520–1522. Jiang, J.L., Xu, Y., Duanmu, C.S., Gu, X., Chen, J., 2014. Preparation and catalytic properties of sulfonated carbon–palygorskite solid acid catalyst. Appl. Clay Sci. 95, 260–264. Kuo, C.H., Li, W.K., Song, W.Q., Luo, Z., Poyraz, A.S., Guo, Y., Ma, A.W.K., Suib, S.L., He, J., 2014. Facile synthesis of Co3O4@CNT with high catalytic activity for CO oxidation under moisture-rich conditions. ACS Appl. Mater. Interfaces 6, 11311–11317. Lee, J.S., Park, E.D., Song, B.J., 1999. Process development for low temperature CO oxidation in the presence of water and halogen compounds. Catal. Today 54, 57–64. Liu, H.B., Chen, T.H., Chang, D.Y., Qing, C.S., Kong, J.D., Chen, D., Xie, J.J., Frost, R.Y., 2013. Effect of rehydration on structure and surface properties of thermally treated palygorskite. J. Colloid Interface Sci. 393, 87–91. Papoulis, D., Komarneni, S., Panagiotaras, D., Nikolopoulou, A., Christoforidis, K.C., Fernández-Garcia, M., Li, H.H., Shu, Y., Sato, T., 2013. Palygorskite–TiO2 nanocomposites: part 2. photocatalytic activities indecomposing air and organic pollutants. Appl. Clay Sci. 83–84, 198–202. Park, E.D., Lee, J.S., 2000. Effect of surface treatment of the support on CO oxidation over carbon-supported Wacker-type catalysts. J. Catal. 193, 5–15. Rao, K.N., Bharah, P., Thrimurthulu, G., Reddy, B.M., 2010. Supported copper-ceria catalysts for low temperature CO oxidation. Catal. Commun. 11, 863–866. Royer, S., Duprez, D., 2011. Catalytic oxidation of carbon monoxide over transition metal oxides. ChemCatChem 3, 24–65. Schryer, D.R., Upchurch, B.T., Sidney, B.D., Brown, K.G., Hoflund, G.B., Herz, R.K., 1991. A proposed mechanism for Pt/SnOx-catalyzed CO oxidation. 130, pp. 314–317.
Please cite this article as: Wang, Y., et al., Room-temperature CO oxidation over calcined Pd–Cu/palygorskite catalysts, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.034
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Shen, Y.X., Guo, Y., Wang, L., Wang, Y.Q., Guo, Y.L., Gong, X.Q., Lu, G.Z., 2011. The stability and deactivation of Pd–Cu–Clx/Al2O3 catalyst for low temperature CO oxidation: an effect of moisture. Catal. Sci. Technol. 1, 1202–1207. Shen, Y.X., Lu, G.Z., Guo, Y., Wang, Y.Q., 2010. A synthesis of high-efficiency Pd–Cu–Clx/ Al2O3 catalyst for low temperature CO oxidation. Chem. Commun. 46, 8433–8435. Wang, F., Zhang, J., Liu, C., Liu, J.H., 2015b. Pd–palygorskite catalysts: preparation, characterization and catalytic performance for the oxidation of styrene. Appl. Clay Sci. 105106, 150–155. Wang, F.G., Zhao, K.F., Zhang, H.J., Dong, Y.M., Wang, T., He, D.N., 2014. Low temperature CO catalytic oxidation over supported Pd–Cu catalysts calcined at different temperatures. Chem. Eng. J. 242, 10–18. Wang, L., Zhou, Y.B., Liu, Q.F., Guo, Y., Lu, G.Z., 2010. Effect of surface properties of activated carbon on CO oxidation over supported Wacker-type catalysts. Catal. Today 153, 184–188. Wang, Y.Z., Fan, L.Y., Shi, J., Li, X., Zhao, Y.X., 2015a. Effect of preparation method on the catalytic activities of Pd–Cu/APT catalysts for low-temperature CO oxidation. Catal. Lett. 145, 1429–1435. Wei, W., Gao, P., Xie, J.M., Zong, S.K., Cui, H.L., Yue, X.J., 2013. Uniform Cu2Cl(OH)3 hierarchical microspheres: a novel adsorbent for methylene blue adsorptive removal from aqueous solution. J. Solid State Chem. 204, 305–313. Wu, L.M., Tong, D.S., Zhao, L.Z., Yu, W.H., Zhou, C.H., Wang, H., 2014a. Fourier transform infrared spectroscopy analysis for hydrothermal transformation of microcrystalline cellulose on montmorillonite. Appl. Clay Sci. 95, 74–82. Wu, X.P., Gao, P., Zhang, X.L., Jin, G.P., Xu, Y.Q., Wu, Y.C., 2014b. Synthesis of clay/carbon adsorbent through hydrothermal carbonization of cellulose on palygorskite. Appl. Clay Sci. 95, 60–66. Xie, X.W., Li, Y., Liu, Z.Q., Haruta, M., Shen, W.J., 2009. Low-temperature oxidation of CO catalysed by Co3O4 nanorods. Nature 458, 746–749.
Yan, W.C., Liu, D., Tan, D.Y., Yuan, P., Chen, M., 2012. FTIR spectroscopy study of the structure changes of palygorskite under heating. Spectrochim. Acta A 97, 1052–1057. Yang, H.M., Tang, A.D., Ouyang, J., Li, M., Mann, S., 2010. From natural attapulgite to mesoporous materials: methodology, characterization and structural evolution. J. Phys. Chem. B 114, 2390–2398. Yoon, C., Cocke, D.L., 1988. The design and preparation of planar models of oxidation catalysts: I. Hopcalite. J. Catal. 113, 267–280. Yuan, Y.Z., Kozlova, A.P., Asakura, K., Wan, H.L., Tsai, K., Iwasawa, Y., 1997. Supported Au catalysts prepared from Au phosphine complexes and as-precipitated metal hydroxides: characterization and low-temperature CO oxidation. J. Catal. 170, 191–199. Zhang, H.R., Yang, H.J., Guo, H.J., Yang, J., Xiong, L., Huang, C., Chen, X.D., Ma, L.L., Chen, Y., 2014. Preparation and catalytic properties of sulfonated carbon–palygorskite solid acid catalyst. Appl. Clay Sci. 95, 260–264. Zhang, Z., Ma, X.B., Zhang, P.B., Li, Y.M., Wang, S.P., 2007. Effect of treatment temperature on the crystal structure of activated carbon supported CuCl2–PdCl2 catalysts in the oxidative carbonylation of ethanol to diethyl carbonate. J. Mol. Catal. A 266, 202–206. Zhang, Z.F., Wang, W.B., Wang, A.Q., 2015. Effects of solvothermal process on the physicochemical and adsorption characteristics of palygorskite. Appl. Clay Sci. 107, 230–237. Zhou, C.H., 2011. An overview on strategies towards clay-based designer catalysts for green and sustainable catalysis. Appl. Clay Sci. 52, 97–105. Zhou, C.H., Keeling, J., 2013. Fundamental and applied research on clay minerals: from climate and environment to nanotechnology. Appl. Clay Sci. 74, 3–9. Zhou, S.H., Xue, A.L., Zhang, Y., Li, M.S., Li, K., Zhao, Y.J., Xing, W.H., 2015. Novel polyamidoamine dendrimer-functionalized palygorskite adsorbents with high adsorption capacity for Pb2+ and reactive dyes. Appl. Clay Sci. 107, 220–229. Zou, X.H., Qi, S.X., Suo, Z.H., An, L.D., Li, F., 2007. Activity and deactivation of Au/Al2O3 catalyst for low-temperature CO oxidation. Catal. Commun. 8, 784–788.
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Room-temperature CO oxidation over calcined Pd–Cu/ palygorskite catalysts Yongzhao Wang ⁎, Jing Shi, Ruifang Wu, Xiao Li, Yongxiang Zhao School of Chemistry and Chemical Engineering, Engineering Research Center of Ministry of Education for Fine Chemicals, Shanxi University, Shanxi, Taiyuan, 030006, China
a r t i c l e
i n f o
Article history: Received 28 March 2015 Received in revised form 22 August 2015 Accepted 24 August 2015 Available online xxxx Keywords: Pd-Cu/palygorskite CO oxidation Room temperature Calcined
a b s t r a c t Pd–Cu/palygorskite catalysts without calcination (U-PdCu/Pal) and with calcination (C-PdCu/Pal) were prepared via an impregnation method. These catalysts' catalytic performances for CO oxidation were studied. CO conversion can reach 100% at room temperature under conditions of 0.5 vol.% CO and 3.3 vol.% H2O in the feed gas over C-PdCu/Pal, which is considerably higher than that over U-PdCu/Pal. The XRD results suggest that the Cu species exists as a Cu(OH)Cl phase with a crystallite size of approximately 30 nm on U-PdCu/Pal, while the crystallite size of the Cu(OH)Cl phase on C-PdCu/Pal is significantly smaller. The results of TPR and IR indicate that there is an enhanced interaction resulting from the calcination, which promotes the reduction of C-PdCu/Pal and enhances the catalytic activity. © 2015 Elsevier B.V. All rights reserved.
1. Introduction CO is a toxic component of industrial emissions and exhaust fumes from automobiles. CO causes harmful effects to living beings and the environment. Thus, low-temperature CO oxidation has attracted tremendous interest from researchers because of its importance in practical applications, such as air purification, CO gas sensors, gas masks, lowering automotive emissions and catalysis (Schryer et al., 1991; Yuan et al., 1997; Chen and Goodman, 2004). In recent decades, many types of catalysts have been prepared for low-temperature CO oxidation, including noble metal catalysts, metal oxide catalysts, and supported Wacker catalysts (Yoon and Cocke, 1988; Zou et al., 2007; Hou et al., 2008; Xie et al., 2009; Jia et al., 2010; Rao et al., 2010; Royer and Duprez, 2011; Grünert et al., 2014; Kuo et al., 2014; Du et al., 2015; Wang et al., 2015a). The supported Wacker catalyst, in which an aqueous solution of PdCl2–CuCl2 or PdCl2–CuCl2–Cu(NO3)2 is loaded into the pores of high specific surface area supports without further calcination, is wellknown to accelerate room-temperature CO oxidation. Compared with metal oxides and supported gold-based catalysts, the supported Wacker catalyst is stable in the presence of halogen impurities and water vapor (Dyakonov, 2003; Wang et al., 2010). The characteristics of the support are critical parameters for the formation and stabilization of the active Cu species phase, which plays an important role in the catalytic performance. Alumina and active carbon are effective supports, but aggregation and transformation of the active Cu species phase occurs during ⁎ Corresponding author. E-mail address: [email protected] (Y. Wang).
the long-time reaction process (Lee et al., 1999; Shen et al., 2011). Because the nature of the support is of crucial importance to the catalytic performance, the exploration of new catalyst systems with less expensive and more easily available supports remains a challenge. Palygorskite (Pal), a hydrated magnesium aluminum silicate mineral that commonly has a lath or fibrous morphology, is characterized by a porous crystallite structure containing tetrahedral layers alloyed together along longitudinal sideline chains (Yan et al., 2012; Liu et al., 2013). Due to its unique structural and textural properties, palygorskite has been widely used as an adsorbent, adhesive, catalyst and catalyst support (Cao et al., 2008; Huo and Yang, 2010; Zhou, 2011; He and Yang, 2013; Papoulis et al., 2013; Zhou and Keeling, 2013; Chen et al., 2014; He et al., 2014; Jiang et al., 2014; Wu et al., 2014a; Zhang et al., 2014; Guo et al., 2015; Wang et al., 2015b; Zhang et al., 2015; Zhou et al., 2015). Cao et al. (2008) prepared CuO catalysts supported on palygorskite with different Cu contents and found that 16 wt.%-CuO/ Pal can convert completely CO at 120 °C, which is comparable to the activities of CuO-based catalysts supported on Fe2O3 and TiO2. He and Yang (2013) prepared a catalyst with coexisting Au and Au2O3 on the surface of palygorskite, and the catalytic activity of o-DCB (o-dichlorobenzene) oxidation was found to reach 99% at a Au / (Au + Au2O3) molar ratio of 0.83, three times that without the palygorskite support. The supports of the supported Wacker catalyst include active carbon, Al2O3 and molecular sieve, but to the best of our knowledge, the use of palygorskite as the support has not been reported. In addition, most supported Wacker catalysts are not calcined but are only dried during the process of preparation due to the poor stability of the active Cu species. In this work, palygorskite was used as a support of Pd–Cu catalysts for room-temperature CO oxidation. Compared with the catalyst
http://dx.doi.org/10.1016/j.clay.2015.08.034 0169-1317/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: Wang, Y., et al., Room-temperature CO oxidation over calcined Pd–Cu/palygorskite catalysts, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.034
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without calcination, the calcined catalyst showed excellent catalytic activity, suggesting that it is a potential candidate for use in environmental catalyst systems. 2. Experimental 2.1. Materials
flow rate of 30 mL/min. No pretreatment was applied before the catalytic activity test. A quantitative analysis of CO was performed with an online gas chromatograph equipped with a 3 m column packed with carbon molecular sieve, a methanator and a flame ionization detector (FID). To enhance the sensitivity of the detection, CO and CO2 were converted to CH4 by the methanator at 360 °C before entering into the FID. The activity was expressed by CO conversion, which was calculated according to.
Palygorskite was obtained from Anhui Mingmei MinChem Co., Ltd. (Hefei, China), and used without further purification. Palygorskite's primary chemical components are SiO2, Al2O3, and MgO, and it also contains small amounts of Fe2O3 and Mn3O4. All of the chemicals were of analytical grade and purchased from Shanghai Chemical Reagent Co., Ltd. (Shanghai, China).
where X is the CO conversion, COinlet represents the initial CO concentration in the inlet, and COoutlet represents the CO concentration in the outlet.
2.2. Catalyst preparation
3. Results and discussion
The PdCu/Pal catalysts were prepared by an impregnation method using metal precursors of PdCl2, CuCl2·2H2O and Cu(NO3)2·3H2O to yield a catalyst with 0.2 wt.% Pd and 12 wt.% Cu and a molar ratio of CuCl2 to Cu(NO3)2 of 2. After impregnation and drying at 80 °C for 3 h and 140 °C for 3 h, the obtained catalyst was labeled as U-PdCu/Pal. A part of the U-PdCu/Pal was calcined at 300 °C for 3 h, and the obtained catalyst was labeled as C-PdCu/Pal.
3.1. Catalyst characterization
2.3. Catalyst characterization X-ray powder diffraction (XRD) measurements were performed to verify the species present in the catalysts. XRD patterns of the samples were recorded using a Bruker D8-Advance X-ray diffractometer with a target of Cu Kα, operated at 40 kV and 40 mA with a scanning speed of 2.4°/min and a scanning angle (2θ) range of 5–80°. N2 adsorption–desorption (N2 sorption) isotherms were determined using a Micromeritics ASAP 2020 special surface area and porosity analyzer at −196 °C. Prior to the sorption experiments, the samples were degassed under vacuum at 120 °C for 12 h. The specific surface area (SBET) was determined from the nitrogen adsorption isotherm. Fourier transform infrared (FT-IR) spectra of the samples were obtained with a Bruker TENSOR 27 spectrometer in the range of 400 to 4000 cm−1 with a resolution of 4 cm−1. Also, 1 mg of each powder sample was diluted in 100 mg of vacuum-dried IR-grade KBr. Transmission electron microscopy (TEM) measurements were carried out by a JEM-2100 model TEM operated at 200 kV. Samples for TEM were dispersed by sonication in ethanol followed by deposition of the dispersion onto a standard Cu grid covered with a porous carbon film. H2 temperature-programmed reduction (H2-TPR) experiments were performed on a Micromeritics Autochem II 2920 Chemisorb instrument. 5 vol.% H2/N2 flow passed through 30 mg of catalyst (40– 60 mesh) at a flow rate of 20 mL/min. The temperature was ramped from room temperature to 700 °C at a rate of 10 °C/min. The consumption of H2 was detected using a thermal conductivity detector (TCD), and the end gas was detected by an on-line quadrupole mass spectrometer (HIDEN HPR-20 QIC) with a minimum dwell time of 3 ms.
Xð%Þ ¼ ðCOinlet –COoutlet Þ=COinlet 100;
3.1.1. XRD analysis The XRD patterns of U-PdCu/Pal and C-PdCu/Pal are given in Fig. 1. For comparison, the XRD patterns of palygorskite without and with calcination at 300 °C (Pal-300) are also given in Fig. 1. The reflections at 2θ = 8.4° (d = 1.053 nm, 110), 2θ = 13.9° (d = 0.637 nm, 200), 2θ = 16.5° (d = 0.537 nm, 130), 2θ = 19.8° (d = 0.448 nm, 040), 2θ = 20.8° (d = 0.427 nm, 121), 2θ = 24.2° (d = 0.368 nm, 221), 2θ = 27.3° (d = 0.327 nm, 400) and 2θ = 35.1° (d = 0.256 nm, 161) were identified as palygorskite (JCPDS, No. 31–0783). The typical reflection at 8.41° corresponds to the (110) of palygorskite, which is attributed to the basal plane of the palygorskite structure, and the high intensity of the reflection indicates that the palygorskite presents a high degree of crystallinity. Compared with the reflections at 8.41° of U-PdCu/Pal, Pal300 and palygorskite, that of C-PdCu/Pal is lower in intensity, indicating that the degree of crystallinity is reduced. There may be two reasons for this, one the dehydroxylation of palygorskite after calcination and the other the interaction between the Cu and Pd species and the palygorskite, which are also in agreement with the IR and TPR results (as will be presented shortly). The reflection scanned at 26.7° (d = 0.334 nm, 101) could be ascribed to a quartz impurity, and this reflection is also present in the catalysts. The XRD reflections of the Cu species on U-PdCu/Pal are consistent with the data of the JCPDS file of Cu(OH)Cl (JCPDS, No. 23–1063), which are different from the results reported on other supports, such as active carbon and Al2O3 (Park and Lee, 2000; Shen et al., 2010).
2.4. Catalytic activity test Measurements of the catalytic performance for low-temperature CO oxidation were performed in a flow-through laboratory microreactor under atmospheric pressure. The microreactor was an 8 mm i.d. quartz u-tube, and a thermocouple was set into the catalyst bed to measure the temperature. The samples were sieved to 40–60 mesh such that the temperature gradients and pressure drop over the catalyst bed were negligible. 300 mg of catalyst was used for each test. The feed gas, adjusted by mass flow controllers and comprising 0.5 vol.% CO, 3.3 vol.% H2O and the balance air, passed through the catalyst bed at a total
Fig. 1. XRD patterns of the PdCu/Pal catalysts without calcination and with calcination at 300 °C and palygorskite.
Please cite this article as: Wang, Y., et al., Room-temperature CO oxidation over calcined Pd–Cu/palygorskite catalysts, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.034
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Shen et al. (2010) reported that there were only reflections of γCu2Cl(OH)3 and the support on Pd–Cu–Clx/Al2O3 catalysts. Park and Lee (2000) also reported that the Cu species existed as Cu2Cl(OH)3 on PdCl2–CuCl2/C. The other reflections on the XRD pattern of U-PdCu/Pal correspond to palygorskite. Apparently, the Cu species on C-PdCu/Pal possesses the same phase as that on U-PdCu/Pal, and no other reflections of the Cu species are detected. However, the characteristic Cu(OH)Cl reflections decrease in intensity and become obviously broadened on C-PdCu/Pal, indicating that the Cu species possesses a higher dispersion degree. According to the Scherrer equation, the crystallite sizes of Cu(OH)Cl on U-PdCu/Pal and C-PdCu/Pal are 30.8 nm and 13.6 nm, respectively, which are in agreement with the TEM observations (that will be presented shortly). Hydrogen bonding between the hydroxyls of both the Cu species and palygorskite may be responsible for the stability of Cu(OH)Cl. The reflection of the Pd species cannot be observed in the two samples, which indicates that the Pd species exists as a highly dispersed state or below the detection limit of XRD. Therefore, the differences in the palladium species between U-PdCu/Pal and C-PdCu/Pal need to be investigated further. 3.1.2. N2 sorption analysis The nitrogen adsorption–desorption isotherms of PdCu/Pal without and with calcination are given in Fig. 2. The isotherms of the two samples are of classical type III according to the IUPAC classification. A very strong increase in the nitrogen-adsorbed volume can be observed at relative pressures greater than 0.85, which is characteristic of the presence of an appreciable amount of macropores. The specific surface area (SBET) and pore volume of the palygorskite are 107 m2/g and 0.31 cm3/g, respectively. After impregnation, the SBET and pore volume of U-PdCu/Pal decline drastically due to the deposition of Cu and Pd species nanoparticles on the surface and in the small pores of palygorskite to only 24 m2/g and 0.13 cm3/g. However, the average pore diameter increases from 6.5 to 20.8 nm. Calcination treatment has a great influence on the pore structure of C-PdCu/Pal, and SBET increases from 24 to 86 m2/g after calcination at 300 °C. This result suggests that C-PdCu/Pal has a much higher SBET and larger pore volume than U-PdCu/Pal, which should have a significant impact on their catalytic performance.
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3567 cm−1, corresponding to the stretching vibrations of –OH units in the coordinated water and bound water, respectively (Yang et al., 2010). A broad band is observed at 3500 to 3300 cm−1, which may be caused by stretching vibrations of –OH in the zeolitic water and surface-physical adsorbed water. An asymmetric band centered at 1639 cm−1 in all the samples is ascribed to the –OH bending mode of the above-mentioned water molecules. The bands approximately 1195, 1026 and 989 cm−1 are associated with the Si–O vibrations and the in-layer Si–O bands (He and Yang, 2013; Wu et al., 2014). The band at 798 cm− 1 confirms the presence of quartz, and the band at 473 cm−1 is assigned to Si–O–Si deformation vibrations, which is consistent with the XRD results. The bands of U-PdCu/Pal are similar to those of palygorskite, but a new band at 1379 cm− 1 is observed, which originates from the nitrate radical in the Cu precursor. After calcination at 300 °C, the bands at 3631 and 3567 cm−1 significantly decrease, indicating the partial removal of the coordinated water and bound water. The two bands of C-PdCu/Pal at 3464 and 3343 cm−1 are distinct and are ascribed to the stretching vibrations of the –OH units of Cu(OH)Cl (Wei et al., 2013). These bands are not clear in UPdCu/Pal because of the overlap with the –OH in the zeolitic water and surface-physical adsorbed water. The band at 1379 cm−1 nearly disappears in the C-PdCu/Pal, indicating the complete decomposition of the nitrate radical after calcination. Compared with that in palygorskite and U-PdCu/Pal, the relative intensity of the band at 534 cm−1 decreases significantly in C-PdCu/Pal, indicating that the structure of the support changes. The changes in the support structure might be due to the partial loss of coordinated water and bound water and the enhanced interaction between the Cu and Pd species and the support, which are also consistent with the results of XRD.
3.1.3. FT-IR analysis The samples were characterized by FT-IR to understand the structure and the interaction between the Cu species and palygorskite. The FT-IR spectra between 4000 and 400 cm−1 of the PdCu/Pal catalysts without and with calcination and the support are shown in Fig. 3. The FT-IR spectrum of palygorskite shows two bands at 3631 and
3.1.4. TEM analysis The representative TEM micrographs of palygorskite and the PdCu/ Pal catalysts are shown in Fig. 4a–c. The typical TEM image of palygorskite exhibits bundles of a fibrous structure (Fig. 4a). The fibers of palygorskite have smooth surfaces with an average diameter of approximately 20 to 40 nm, where a visible boundary appears between the straight fibers. For the PdCu/Pal catalyst calcined at 300 °C, there are numerous nanoparticles attached along the fibers, displaying a narrow particle distribution, and the majority of the particles have a size less than 15 nm, as shown in Fig. 4c. However, the nanoparticles on the PdCu/Pal catalyst without calcination clearly aggregate, and the particle size is approximately 26 to 33 nm (Fig. 4b), much larger than that of the nanoparticles on C-PdCu/Pal. The nanoparticles on C-PdCu/Pal possess higher dispersion than those on U-PdCu/Pal, which is consistent with the results of XRD.
Fig. 2. N2 sorption isotherms of the PdCu/Pal catalysts without calcination and with calcination at 300 °C and palygorskite.
Fig. 3. FT-IR spectra of the PdCu/Pal catalysts without calcination and with calcination at 300 °C and palygorskite.
Please cite this article as: Wang, Y., et al., Room-temperature CO oxidation over calcined Pd–Cu/palygorskite catalysts, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.034
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Fig. 4. TEM images of the PdCu/Pal catalysts without calcination (b) and with calcination at 300 °C (c) and palygorskite (a).
3.1.5. H2-TPR analysis The TPR profiles of the PdCu/Pal catalysts and palygorskite are given in Fig. 5. There is no obvious reduction peak on the TPR profile of palygorskite, and only a very broad and short peak from 500 to 650 °C can be detected, which is ascribed to the reduction of a small quantity of metal ions contained in the palygorskite. The TPR profile of the UPdCu/Pal catalyst shows three reduction peaks, namely, α2, β2 and γ peaks. The α2 peak with a Tmax at 297 °C is attributed to the coreduction of the Pd and dispersed Cu species. The β2 peak at 315 °C can be ascribed to the reduction of the crystallized Cu species contacting with the Pd species, and the γ peak at 375 °C should be the reduction of a small quantity of isolated copper species that do not interact with the Pd species. In addition, the mass spectrometry results demonstrate that the α2 and β2 peaks may include the reduction of the nitrate radicals contained in U-PdCu/Pal due to the emergence of the NO signal (not given here) coinciding with the reduction peaks of α2 and β2. There are two peaks in the TPR profile of the C-PdCu/Pal catalyst, α1 and β1. Compared with those of U-PdCu/Pal, the reduction peaks of CPdCu/Pal become wide and short, indicating high dispersion. In addition, the temperatures of the peaks shift to significantly lower values, with Tmax at 237 °C and 306 °C, respectively. This indicates the easier reduction of the Pd and Cu species and the strong interaction between them after calcination (Shen et al., 2010). Calcination promotes the dispersion of the Cu species and enhances the interaction of the Cu and Pd species, thus facilitating the reduction of the C-PdCu/Pal catalyst. Moreover, unlike the TPR profile of Pd–Cu/Al2O3 (Wang et al., 2014), the PdCu/Pal catalysts lack a negative peak at 100 °C ascribed to the decomposition of Pd β-hydride, which may be due to their low Pd content.
Fig. 5. H2-TPR profiles of the PdCu/Pal catalysts without calcination and with calcination at 300 °C and palygorskite.
3.2. Catalytic performance for room-temperature CO oxidation The catalytic activities of the PdCu/Pal catalysts and the palygorskite support for room-temperature CO oxidation with 3.3 vol.% water vapor in the feed gas are given in Fig. 6. CO oxidation occurs at room temperature over both of the catalysts. The CO conversion over U-PdCu/Pal can only be maintained at approximately 30%. Under the same reaction conditions, there is an induction period over the C-PdCu/Pal catalyst before the CO conversion reaches a steady state. The initial CO conversion over C-PdCu/Pal is lower than that over U-PdCu/Pal, but it gradually increases from approximately 20% to above 90% with the increase of the reaction time from 5 to 80 min. The presence of water is essential for the catalytic activity of the supported Wacker catalyst, as water molecules are involved in the catalytic cycle. C-PdCu/Pal exhibits poor catalytic activity without H2O in the feed. The induction period of the C-PdCu/Pal catalyst may be due to the lack of water molecules on its surface or in the pores after calcination. However, the characteristics of C-PdCu/Pal, with its high dispersion of Cu(OH)Cl and enhanced interaction between the Cu and Pd species, result in a higher catalytic activity for CO oxidation. Zhang et al. (2007) reported that the Cu(OH)Cl species showed better catalytic performance than γ-Cu2(OH)3Cl in the oxidative carbonylation of ethanol to diethyl carbonate because the Cu(OH)Cl was more efficient in regenerating Pd0 to Pd2+ and reducing Cu2+ to Cu+. The effect of water vapor pretreatment on the catalytic activity of CPdCu/Pal was also investigated to reveal the relationship between the catalytic activity and reaction time (Fig. 7). The water vapor pretreatment was performed by allowing only air into a water bubbler
Fig. 6. The catalytic activity of the PdCu/Pal catalysts without calcination and with calcination at 300 °C and palygorskite (reaction conditions: CO 0.5 vol.%, O2 14.4 vol.%, H2O 3.3 vol.%, room temperature, GHSV 5000 h−1).
Please cite this article as: Wang, Y., et al., Room-temperature CO oxidation over calcined Pd–Cu/palygorskite catalysts, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.034
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the room-temperature CO oxidation was used as model reaction to investigate the catalytic activity. C-PdCu/Pal exhibits much higher catalytic activity than U-PdCu/Pal, and CO can be completely converted over CPdCu/Pal for more than 20 h at ambient temperature under the conditions of 0.5 vol.% CO and 3.3 vol.% H2O in the feed gas. Calcination does not affect the type of Cu species phase but leads to an increase in the specific surface area. Calcination also promotes the dispersion of the Cu species and enhances the interaction between the Cu and Pd species and the support, making them more easily reduced. The promoted catalytic activity of C-PdCu/Pal can be ascribed to the enhanced interaction and reduction, as well as the superior dispersion of the Cu species. Palygorskite is much cheaper and more easily available than other supports and the loading of the noble metal Pd is low, so PdCu/Pal catalysts can have a significant effect in applications of room-temperature CO oxidation.
Acknowledgments Fig. 7. Effect of water vapor pretreatment time on the catalytic activities of C-PdCu/Pal (reaction conditions: CO 0.5 vol.%, O2 14.4 vol.%, H2O 3.3 vol.%, room temperature, GHSV 5000 h−1).
maintained at 25 °C and then passing it through the catalyst bed for different times. The samples without water vapor pretreatment and those that were pretreated for 90 min and 120 min were labeled PdCu/Pal-0, PdCu/Pal-90 and PdCu/Pal-120, respectively. The CO conversion over all the samples except for PdCu/Pal-120 increases with the reaction time, and the pretreatment time has a strong influence on the catalytic performance. Not only is the initial catalytic activity significantly enhanced as the pretreatment time is prolonged, but the CO conversion also increases more rapidly. The initial CO conversion could reach nearly 60% over PdCu/Pal-90. Although the initial activity of PdCu/Pal-120 is the highest, the CO conversion gradually decreases as the reaction time increasing. The C-PdCu/Pal catalyst was also selected as a representative to investigate the stability (Fig. 8). The reaction temperature was maintained at ambient temperature, and the feed gas contained 3.3 vol.% water vapor. The catalytic activity changes very slightly and maintains a high activity for more than 20 h during the period of reaction. This demonstrates that the calcined PdCu/Pal catalyst is very active and stable for room-temperature CO oxidation. 4. Conclusions Pd–Cu/Pal catalysts without calcination (U-PdCu/Pal) and with calcination (C-PdCu/Pal) were prepared via an impregnation method, and
Fig. 8. CO conversion over C-PdCu/Pal catalyst at room temperature as a function of reaction time (reaction conditions: CO 0.5 vol.%, O2 14.4 vol.%, H2O 3.3 vol.%, GHSV 5000 h−1).
The authors gratefully acknowledge the financial support from the International S&T Cooperation Program of China (2013DFA40460), the National Natural Science Foundation of China (21073114), and the Shanxi Youth S&T Foundation (2010021008-3).
References Cao, J.L., Shao, G.S., Wang, Y., Liu, Y.P., Yuan, Z.Y., 2008. CuO catalysts supported on attapulgite clay for low-temperature CO oxidation. Catal. Commun. 9, 2555–2559. Chen, D.M., Du, Y., Zhu, H.L., Deng, Y.X., 2014. Synthesis and characterization of a microfibrous TiO2–CdS/palygorskite nanostructured material with enhanced visiblelight photocatalytic activity. Appl. Clay Sci. 87, 285–291. Chen, M.S., Goodman, D.W., 2004. The structure of catalytically active Au on titania. Science 306, 252–255. Du, X.X., Li, H.Y., Yu, J., Xiao, X.Z., Shi, Z.P., Mao, D.S., Lu, G.Z., 2015. Realization of a highly effective Pd–Cu–Clx/Al2O3 catalyst for low temperature CO oxidation by presynthesizing the active copper phase of Cu2(OH)3Cl. Catal. Sci. Technol. 5, 3970–3979. Dyakonov, A.J., 2003. Abatement of CO from relatively simple and complex mixtures: II. oxidation on Pd–Cu/C catalysts. Appl. Catal. B 45, 257–267. Grünert, W., Groβmann, D., Noei, H., Pohl, M., Sinev, I., Toni, A.D., Wang, Y.M., Muhler, M., 2014. Low-temperature oxidation of carbon monoxide with gold(III) ions supported on titanium oxide. Angew. Chem. Int. Ed. 53, 3245–3249. Guo, H.J., Zhang, H.R., Peng, F., Yang, H.J., Xiong, L., Huang, C., Wang, C., Chen, X.D., Ma, L.L., 2015. Mixed alcohols synthesis from syngas over activated palygorskite supported Cu–Fe–Co based catalysts. Appl. Clay Sci. 111, 83–89. He, X., Fu, L.J., Yang, H.M., 2014. Insight into the nature of Au–Au2O3 functionalized palygorskite. Appl. Clay Sci. 100, 118–122. He, X., Yang, H.M., 2013. Au nanoparticles assembled on palygorskite: enhanced catalytic property and Au–Au2O3 coexistence. J. Mol. Catal. A 379, 219–224. Hou, X.D., Wang, Y.Z., Zhao, Y.X., 2008. Effect of CeO2 doping on structure and catalytic performance of Co3O4 catalyst for low-temperature CO oxidation. Catal. Lett. 123, 321–326. Huo, C.L., Yang, H.M., 2010. Synthesis and characterization of ZnO/palygorskite. Appl. Clay Sci. 50, 362–366. Jia, C.J., Liu, Y., Bongard, H., Schüth, F., 2010. Very low temperature CO oxidation over colloidally deposited gold nanoparticles on Mg(OH)2 and MgO. J. Am. Chem. Soc. 132, 1520–1522. Jiang, J.L., Xu, Y., Duanmu, C.S., Gu, X., Chen, J., 2014. Preparation and catalytic properties of sulfonated carbon–palygorskite solid acid catalyst. Appl. Clay Sci. 95, 260–264. Kuo, C.H., Li, W.K., Song, W.Q., Luo, Z., Poyraz, A.S., Guo, Y., Ma, A.W.K., Suib, S.L., He, J., 2014. Facile synthesis of Co3O4@CNT with high catalytic activity for CO oxidation under moisture-rich conditions. ACS Appl. Mater. Interfaces 6, 11311–11317. Lee, J.S., Park, E.D., Song, B.J., 1999. Process development for low temperature CO oxidation in the presence of water and halogen compounds. Catal. Today 54, 57–64. Liu, H.B., Chen, T.H., Chang, D.Y., Qing, C.S., Kong, J.D., Chen, D., Xie, J.J., Frost, R.Y., 2013. Effect of rehydration on structure and surface properties of thermally treated palygorskite. J. Colloid Interface Sci. 393, 87–91. Papoulis, D., Komarneni, S., Panagiotaras, D., Nikolopoulou, A., Christoforidis, K.C., Fernández-Garcia, M., Li, H.H., Shu, Y., Sato, T., 2013. Palygorskite–TiO2 nanocomposites: part 2. photocatalytic activities indecomposing air and organic pollutants. Appl. Clay Sci. 83–84, 198–202. Park, E.D., Lee, J.S., 2000. Effect of surface treatment of the support on CO oxidation over carbon-supported Wacker-type catalysts. J. Catal. 193, 5–15. Rao, K.N., Bharah, P., Thrimurthulu, G., Reddy, B.M., 2010. Supported copper-ceria catalysts for low temperature CO oxidation. Catal. Commun. 11, 863–866. Royer, S., Duprez, D., 2011. Catalytic oxidation of carbon monoxide over transition metal oxides. ChemCatChem 3, 24–65. Schryer, D.R., Upchurch, B.T., Sidney, B.D., Brown, K.G., Hoflund, G.B., Herz, R.K., 1991. A proposed mechanism for Pt/SnOx-catalyzed CO oxidation. 130, pp. 314–317.
Please cite this article as: Wang, Y., et al., Room-temperature CO oxidation over calcined Pd–Cu/palygorskite catalysts, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.034
6
Y. Wang et al. / Applied Clay Science xxx (2015) xxx–xxx
Shen, Y.X., Guo, Y., Wang, L., Wang, Y.Q., Guo, Y.L., Gong, X.Q., Lu, G.Z., 2011. The stability and deactivation of Pd–Cu–Clx/Al2O3 catalyst for low temperature CO oxidation: an effect of moisture. Catal. Sci. Technol. 1, 1202–1207. Shen, Y.X., Lu, G.Z., Guo, Y., Wang, Y.Q., 2010. A synthesis of high-efficiency Pd–Cu–Clx/ Al2O3 catalyst for low temperature CO oxidation. Chem. Commun. 46, 8433–8435. Wang, F., Zhang, J., Liu, C., Liu, J.H., 2015b. Pd–palygorskite catalysts: preparation, characterization and catalytic performance for the oxidation of styrene. Appl. Clay Sci. 105106, 150–155. Wang, F.G., Zhao, K.F., Zhang, H.J., Dong, Y.M., Wang, T., He, D.N., 2014. Low temperature CO catalytic oxidation over supported Pd–Cu catalysts calcined at different temperatures. Chem. Eng. J. 242, 10–18. Wang, L., Zhou, Y.B., Liu, Q.F., Guo, Y., Lu, G.Z., 2010. Effect of surface properties of activated carbon on CO oxidation over supported Wacker-type catalysts. Catal. Today 153, 184–188. Wang, Y.Z., Fan, L.Y., Shi, J., Li, X., Zhao, Y.X., 2015a. Effect of preparation method on the catalytic activities of Pd–Cu/APT catalysts for low-temperature CO oxidation. Catal. Lett. 145, 1429–1435. Wei, W., Gao, P., Xie, J.M., Zong, S.K., Cui, H.L., Yue, X.J., 2013. Uniform Cu2Cl(OH)3 hierarchical microspheres: a novel adsorbent for methylene blue adsorptive removal from aqueous solution. J. Solid State Chem. 204, 305–313. Wu, L.M., Tong, D.S., Zhao, L.Z., Yu, W.H., Zhou, C.H., Wang, H., 2014a. Fourier transform infrared spectroscopy analysis for hydrothermal transformation of microcrystalline cellulose on montmorillonite. Appl. Clay Sci. 95, 74–82. Wu, X.P., Gao, P., Zhang, X.L., Jin, G.P., Xu, Y.Q., Wu, Y.C., 2014b. Synthesis of clay/carbon adsorbent through hydrothermal carbonization of cellulose on palygorskite. Appl. Clay Sci. 95, 60–66. Xie, X.W., Li, Y., Liu, Z.Q., Haruta, M., Shen, W.J., 2009. Low-temperature oxidation of CO catalysed by Co3O4 nanorods. Nature 458, 746–749.
Yan, W.C., Liu, D., Tan, D.Y., Yuan, P., Chen, M., 2012. FTIR spectroscopy study of the structure changes of palygorskite under heating. Spectrochim. Acta A 97, 1052–1057. Yang, H.M., Tang, A.D., Ouyang, J., Li, M., Mann, S., 2010. From natural attapulgite to mesoporous materials: methodology, characterization and structural evolution. J. Phys. Chem. B 114, 2390–2398. Yoon, C., Cocke, D.L., 1988. The design and preparation of planar models of oxidation catalysts: I. Hopcalite. J. Catal. 113, 267–280. Yuan, Y.Z., Kozlova, A.P., Asakura, K., Wan, H.L., Tsai, K., Iwasawa, Y., 1997. Supported Au catalysts prepared from Au phosphine complexes and as-precipitated metal hydroxides: characterization and low-temperature CO oxidation. J. Catal. 170, 191–199. Zhang, H.R., Yang, H.J., Guo, H.J., Yang, J., Xiong, L., Huang, C., Chen, X.D., Ma, L.L., Chen, Y., 2014. Preparation and catalytic properties of sulfonated carbon–palygorskite solid acid catalyst. Appl. Clay Sci. 95, 260–264. Zhang, Z., Ma, X.B., Zhang, P.B., Li, Y.M., Wang, S.P., 2007. Effect of treatment temperature on the crystal structure of activated carbon supported CuCl2–PdCl2 catalysts in the oxidative carbonylation of ethanol to diethyl carbonate. J. Mol. Catal. A 266, 202–206. Zhang, Z.F., Wang, W.B., Wang, A.Q., 2015. Effects of solvothermal process on the physicochemical and adsorption characteristics of palygorskite. Appl. Clay Sci. 107, 230–237. Zhou, C.H., 2011. An overview on strategies towards clay-based designer catalysts for green and sustainable catalysis. Appl. Clay Sci. 52, 97–105. Zhou, C.H., Keeling, J., 2013. Fundamental and applied research on clay minerals: from climate and environment to nanotechnology. Appl. Clay Sci. 74, 3–9. Zhou, S.H., Xue, A.L., Zhang, Y., Li, M.S., Li, K., Zhao, Y.J., Xing, W.H., 2015. Novel polyamidoamine dendrimer-functionalized palygorskite adsorbents with high adsorption capacity for Pb2+ and reactive dyes. Appl. Clay Sci. 107, 220–229. Zou, X.H., Qi, S.X., Suo, Z.H., An, L.D., Li, F., 2007. Activity and deactivation of Au/Al2O3 catalyst for low-temperature CO oxidation. Catal. Commun. 8, 784–788.
Please cite this article as: Wang, Y., et al., Room-temperature CO oxidation over calcined Pd–Cu/palygorskite catalysts, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.034