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Transition metal doped cryptomelane-type manganese oxide for low-temperature catalytic combustion of dimethyl ether
Chemical Engineering Journal 220 (2013) 320–327
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Transition metal doped cryptomelane-type manganese oxide for low-temperature catalytic combustion of dimethyl ether Ming Sun a, Lin Yu a,⇑, Fei Ye a, Guiqiang Diao a, Qian Yu a, Zhifeng Hao a, Yuying Zheng a, Lixiang Yuan b a b
School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, PR China Laboratory for Sustainable Technology, School of Chemical and Biomolecular Engineering, University of Sydney, Sydney, NSW 2006, Australia
h i g h l i g h t s " Fe, Co, Ni, Cu, Cr doped OMS-2 catalyst were prepared and applied to the DME catalytic combustion. " Cu-OMS-2 has the best activity with a T10 of 171 °C and a T90 of 180 °C. " The activity of M-OMS-2 catalyst were greatly influenced by the type of doped metals. " The activity is related with surface area, average oxidation state and redox properties.
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Article history: Received 10 July 2012 Received in revised form 14 January 2013 Accepted 18 January 2013 Available online 30 January 2013 Keywords: Transition metal Cryptomelane-type manganese oxide Dimethyl ether Catalytic combustion
a b s t r a c t The catalysts of transition metals (Fe, Co, Ni, Cu, Cr) doped cryptomelane-type manganese oxide (M-OMS-2) were prepared and investigated for the catalytic combustion of dimethyl ether (DME). The catalysts were characterized by XRD, FT-IR, BET, TEM, ICP-AES, XPS and H2-TPR techniques. The effects of surface area, the average oxidation state, and the redox properties on catalytic activities were studied. The results showed that 100% CO2 selectivity can be realized over the doped OMS-2 catalysts. The M-OMS-2 catalysts displayed the nanorod morphology with a 13 nm diameter and the length ranging from 50 to 220 nm. The Cu doped OMS-2 catalyst exhibited the best activity, achieving the light-off temperature (the temperature acquired for 10% DME conversion, T10) and full-conversion temperature (the temperature acquired for 90% DME conversion, T90) of 171 and 180 °C, respectively. DME catalytic combustion over M- OMS-2 catalysts was found to follow a Mars–van-Krevelen mechanism with the presence of redox between Mn4+/Mn3+ and Mm+/M(m 1)+ (M = Fe, Co, Ni, Cu, Cr). Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Dimethyl ether (DME) has attracted increasing attention as an environmentally benign energy carrier. Being non-toxic and chemically stable, DME has a boiling point of 248.1 K at 1 atm and a vapor pressure of 530 kPa at 298 K, which is similar to those of liquefied petroleum gas [1]. DME can be mass-produced from natural gas, crude oil, coal, waste produces and biomass [1,2]. As a renewable fuel, the homogeneous combustion of DME has been investigated extensively [3]. However, the catalytic combustion of DME is seldom reported until now. The main advantage of DME combustion by catalytic routes lie in that catalytic combustion can effectively decrease the temperature and minimize NOx and hydrocarbons emission. The catalysts for catalytic combustion are generally precious metals, mixed metal-oxide and perovskite [4]. Ishikawa and Iglesia [5–7] have conducted a comprehensive study on the DME ⇑ Corresponding author. Tel.: +86 20 39322201; fax: +86 20 39322231. E-mail address: [email protected] (L. Yu). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.01.061
combustion using noble metal (Pd, Rh, Pt) clusters, which demonstrated that Pt and Pd catalyzes DME to CO2 and H2O at 400– 600 °C. However, noble metals are rare, expensive and unsuitable for large-scale practical applications. Recently, we have reported the catalytic combustion of DME over large-surface area of hexaaluminate catalysts with outstanding stability, achieving 100% DME conversion at 320 °C for 100 h [8]. However, the light-off temperature of the hexaaluminate catalysts is relatively high, and the low-temperature activity is not satisfactory. Therefore, it is necessary to find a more efficient catalyst for DME catalytic combustion. Cryptomelane-type manganese oxide, also known as manganese oxide octahedral molecular sieve (OMS-2), has a 2 2 tunnel structure and a pore size of 0.46 nm. The OMS-2 material has many attractive properties such as abundant porous and tunnel structure, ion-exchange ability, facile evolution of lattice oxygen and reasonable thermal stability [9]. Thus, it has been exploited for the total oxidation of CO and VOCs with excellent oxidation activities [10–14]. The properties of the OMS-2 as a catalyst, such as the surface area, morphology and redox property can be altered by tuning the preparation parameters or by doping with metal
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cations. Some cations including Cu2+, Zn2+, Ni2+, Co2+, Al3+, Mg2+, Fe3+, Ti4+, Ce4+, Zr4+ or V5+ have been successfully introduced into the OMS-2 tunnel to enhance their catalytic performance [15– 20]. We have found that the Ce-doped OMS-2 catalyst showed good performance in DME combustion [21]. In this study, the transition metal (Fe, Co, Ni, Cu, Cr) doped OMS-2 catalysts have been prepared, and the influence of transition metal on the structure, redox property and catalytic performance of the OMS-2 materials for the catalytic combustion of DME were investigated. 2. Experimental
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loaded. The temperature was measured by a thermocouple placed in the catalyst bed. The catalyst was pretreated in a flow of O2/He for 1 h at 350 °C. A mixture of DME and 20 vol.% O2/He, was introduced into the reactor with a molar ratio of 1:50. The gas flow was controlled by a mass flow controller (Beijing Sevenstar). The gas hourly space velocity was approximately 10 dm3 (gcat h) 1. The reactor effluent was analyzed by online gas chromatograph (Agilent 6820) with a methyl silicone capillary column (ATSE30) and a Porapak Q packed column, connected to a flame ionization detector and thermal conductive detector, respectively. The catalytic performance at each set temperature was evaluated at 45 min intervals.
2.1. Catalyst preparation 3. Results and discussion OMS-2 was synthesized through the oxidation of manganese (II) sulphate by potassium permanganate in an acidic medium under reflux conditions, as described in a previous report [22]. A certain amount of OMS-2 was added to the nitrate salt (Fe(III), Co(II), Ni(II), Cu(II), Cr(III)) dissolved in methanol at a fixed molar ratio of Mn to the metal (10:1). The mixture was stirred for 24 h, followed by drying at 60 °C for 12 h. The catalysts were pressed and sieved to 40–60 mesh after calcination under air atmosphere at 450 °C for 2 h. The catalyst was denoted as M-OMS-2, where M stands for Fe, Co, Ni, Cu or Cr. 2.2. Characterization The phase structure of the M-OMS-2 catalysts was studied by X-ray powder diffraction (XRD, MSAL-XD2) with Cu Ka radiation at 40 kV and 25 mA. Fourier transform infrared (FT-IR) spectra were obtained from a Thermo Nicolet 380 spectrometer. The samples were mixed with KBr and well grounded before measurement. The BET surface areas and pore volumes of the catalysts were measured by a multipoint N2 adsorption–desorption method in liquid N2 ( 196 °C) with a GEMINI V 2380 surface area analyzer. Samples were treated under 200 °C in N2 atmosphere for 4 h to evacuate the physisorbed moisture before measurement. The morphology of the catalysts was investigated using a transmission electron microscopy (TEM, Philips Techai-10). The finely grounded samples were dispersed in ethanol under ultrasonic treatment. Elemental analysis of the catalysts was conducted by a coupled plasma-atomic emission spectroscopy (ICP-AES) on an ICPS-1000 IV instrument. The catalyst of 20 mg was dissolved in 5 mL of concentrated hydrochloric acid, and then diluted to 1000 mL with deionized water. X-ray photoelectron spectroscopy (XPS) was recorded on a VG MultiLab 2000 instrument equipped with an Mg Ka X-ray source. The spectra were calibrated using the carbon peak. Hydrogen temperature programmed reduction (H2-TPR) experiments were carried out in a 5 vol.% H2/Ar gas flow on an Auto Chem 2920 (Micromeritics) instruments. Catalyst sample (50 mg) was pretreated in flowing He for 0.5 h at 350 °C, followed by cooling to room temperature. The inlet total flow was 50 mL/min and the temperature was increased linearly at a rate of 10 °C/min. The pulse reaction was carried out on an Auto Chem 2920 (Micromeritics) to investigate the reactivity of active oxygen species. The Cu-OMS-2 catalyst (40–60 mesh, 200 mg) packed in a U-shaped quartz reactor was pretreated with a flow of He (30 mL/min) at 300 °C for 60 min in order to remove the reversibly and physically bound oxygen. Then the DME pulses (0.5 mL) were injected into He carrier (flow rate 30 mL min 1) over the catalyst at 190 °C. The DME and products were analyzed on-line using a Balzers Omnistar quadrupole mass spectrometer.
3.1. Evaluation of the catalytic combustion performance The result of DME combustion without catalyst is shown in Fig. S1 (Supplementary data, Fig. S1), which indicates that the homogeneous oxidation starts after 200 °C and the temperature for complete conversion is higher than 525 °C. Fig. 1 and Table 1 present the results of the catalytic activities of DME combustion over M-OMS-2 catalysts. It can be seen from Fig. 1 and Table 1 that Co, Ni and Cu doped OMS-2 catalysts exhibit higher catalytic activities compared to Cr and Fe doped OMS-2 catalysts. More importantly, Cu-OMS-2 shows the best activity with the light-off temperature (T10, the temperature acquired for 10% DME conversion) and the full-conversion temperature (T90, the temperature acquired for 90% DME conversion) at 171 and 180 °C, respectively. The temperature window from T10 to T90 over Cu-OMS-2 catalyst is less than 10 °C, and this narrow window suggests that Cu-OMS-2 catalyst has excellent low-temperature activity. Considering that the undoped OMS-2 has a T10 of 167 °C and a T90 of 213 °C [21], the T90 for DME combustion has been significantly reduced after doping with transition metals. Over the M-OMS-2 catalysts, CO2 and H2O were the only detectable products within the detection limit. 3.2. Structure and morphology of M-OMS-2 catalysts Fig. 2 illustrates the XRD patterns of the M-OMS-2 catalysts. Only peaks that belong to Q-cryptomelane (JCPDS 29-1020) with a tetragonal structure are detected, which means that the doped metals are well-dispersed in the OMS-2 structure [23,24].
2.3. Catalytic activity The DME catalytic combustion was carried out in a fixed bed continuous flow quartz reactor (8 mm i.d.) with 300 mg catalyst
Fig. 1. DME combustion activity of the M-OMS-2 catalysts.
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Table 1 BET specific surface areas, Rs, H2-TPR, XPS peak fit and T10, T90 values of M-OMS-2 catalysts. SBET (m2/g)
Fe-OMS-2 Co-OMS-2 Ni-OMS-2 Cu-OMS-2 Cr-OMS-2
58 51 55 47 46
Rsa lmol/(s m2).
0.15 0.96 1.23 1.50 0.072
T10b (°C)
178 170 167 171 185
T90b (°C)
200 185 183 180 190
H2-TPR
XPS O 1s peak fit Peak area
Peak1
Peak2
H2 consumption (mmol/g)
272 281 242 225 275
320 290 255 250 310
10.80 11.84 11.20 12.14 10.73
AOb/A(Ob+a)
Oa
Ob
52,084 62,088 45,713 48,028 10,2894
16,921 41,399 33,042 34,498 67,197
c
0.245 0.400 0.419 0.418 0.395
a Rs (specific reaction rates): the amount of reactant (DME) consumed per unit of surface area and per second at 180 °C were introduced to eliminate the effect of surface area. Rs = ((PV/RT)DME conversion)/(SBET 0.3), it was calculated based on the weight of the catalysts of 0.3 g, the reactant flow rate of 51 cm3 min 1, the flow time of 60 min, 101.3 kPa, 180 °C. Noted: virial equation,PV = ZnRT, should be applied here. However, the compressibility factor Z is very close to 1 after calculation. Therefore, an approximate value of Z = 1 is used. b T10, T90 represent the temperature acquired for 10% and 90% DME conversion, respectively. c The concentrations of the three oxygen species were calculated from the relative area of the subpeaks.
The FT-IR results are shown in Fig. 3. The peaks at around 710, 530 and 463 cm 1 are characteristic of cryptomelane and are involved in the vibrations of the MnO6 octahedral framework [12,25,26]. The peak positions for M-OMS-2 slightly shifted to higher wavelength of 712, 530, 469 cm 1 when compared to those of OMS-2. Fig. 4 shows the nitrogen adsorption–desorption isotherms of the catalysts and the specific surface areas of the catalysts are listed in Table 1. It can be seen from Table 1 that the doping of metals has little impact on the surface areas of the M-OMS-2 catalysts. The isotherms show a characteristic of type II isotherm pattern, with a hysteresis loop of type H3 in the IUPAC classification, which is usually associated with the adsorption on aggregates of particles with a layered morphology, forming slit-like pores [27]. The results are similar to those of the reported K-OMS-2 and Ti-OMS-2 [17]. The TEM images of the M-OMS-2 catalysts are shown in Fig. 5 (the image of Cr-OMS-2 is shown in Supplementary data, Fig. S2). All the M-OMS-2 catalysts show a rod-like morphology with a length ranging from 50 to 220 nm and a diameter of about 13 nm. No agglomerated particles corresponding to the doped metal oxide were found. The bulk and surface atomic ratios of K/Mn and M/Mn of the M-OMS-2 catalysts listed in Table 2 were determined by ICP-AES and XPS techniques, respectively. In general, the doping of metals increases the K/Mn atomic ratio in bulk, and decreases the K/Mn atomic ratio in surface. However, the M/Mn atomic ratios in bulk of the M-OMS-2 catalysts are higher than those in the surface
Fig. 3. FT-IR spectra of the M-OMS-2 catalysts.
except for Cr, indicating that more doped metal cations (Fe, Co, Ni, and Cu) exist in the bulk of the OMS-2 structure. This result is contrary to the V-doped OMS-2 [28]. Based on the ICP-AES results, the formula of the catalysts and the average oxidation state (AOS) of manganese oxide were listed using the same method as reported [29]. 3.3. Effect of specific surface area on the activity
Fig. 2. XRD patterns of the M-OMS-2 catalysts.
Generally, the specific surface area plays an important role in catalyst activity. Liu et al. [30] and Schurz et al. [31] reported that the activities of OMS-2 catalysts were correlated to their specific surface areas in the oxidation of cyclohexanol and benzyl alcohol. In our study, the specific surface area did not fit very well with the catalytic activity trend. For instance, the Fe-OMS-2 catalyst with the largest specific surface area (58 m2/g) showed the lowest activity, while the Cu-OMS-2 catalyst with relatively smaller specific surface area (47 m2/g) exhibited the best activity. Therefore, the specific reaction rates (Rs), namely, the amounts of DME consumed per unit of surface area and per second at 180 °C, were employed to eliminate the effect of specific surface area [32,33]. According to our previous studies, the Rs could reflect the catalytic activity trend very well [21]. As shown in Table 1, the Rs values are in the following order: Cu-OMS-2 > Ni-OMS-2 > Co-OMS-2 > Fe-OMS-2 > Cr-OMS-2, which agrees well with the catalytic activity trend. For example, the Cu-OMS-2 catalyst with the highest
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[35,36]. Herein, the AOS was calculated from the corresponding formula using the result from the ICP-AES analysis (Table 2). It can be seen from Table 2 that the AOS value of the OMS-2 catalyst (3.91) in our study is in agreement with that (3.9) of the reported OMS-2 [35], suggesting that this method is reasonable. The AOS values of the M-OMS-2 increase after doping with transition metals. The AOS values decrease in the following order: CuOMS-2 > Ni-OMS-2 > Co-OMS-2 > Cr-OMS-2 > Fe-OMS-2, which is very similar to the decreasing order of the Rs values. Xia et al. [34] observed that the higher AOS lead to higher CO oxidation rates over the Cu(Co, Ag)-OMS-2 catalysts. Santos et al. [35] also found that the OMS-2 with the highest AOS showed the best catalytic activity for the total oxidation of ethyl acetate. Therefore, it could be concluded that the AOS values of the M-OMS-2 catalysts are in correlation with their catalytic activities of DME combustion. 3.5. Effect of the redox properties on the activity
Fig. 4. Nitrogen adsorption/desorption isotherms of the M-OMS-2 catalysts (filled circles: absorption; empty circles: desorption).
Rs (1.50 lmol/(s m2).) has the best activity, and Cr-OMS-2 catalyst with the lowest Rs (0.072 lmol/(s m2)) has the lowest activity. 3.4. Effect of average oxidation state (AOS) on the activity The AOS of OMS-2 catalyst is usually determined by potentiometric titration method [34] or by the XPS analysis of Mn 3s
To further explore the relationship between the redox property and activity, XPS and H2-TPR techniques were conducted. The O 1s spectra of the M-OMS-2 catalysts are shown in Fig. 6, and the results of O 1s peak fitting are in Table 1. For all the catalysts, the O1s peak is composed of three subpeaks at around 530, 531.5 and 533 eV, which can be attributed to the lattice oxygen (Oa), the defect oxide or a hydroxyl-like group (Ob), and the adsorbed molecular water (Oc), respectively [20,37,38]. The amount of the Oa and Ob species were calculated from the relative area of the subpeaks. For each catalyst, the most abundant species is lattice oxygen (Oa), suggesting that lattice oxygen is important for the catalytic activity. The Ob species has higher mobility than the Oa species, and allow the diffusion of lattice oxygen [39,40]. To confirm the contribution of Ob species in the reaction, the area ratios of AOb/AO(b+a) were calculated (Table 1). The AOb/AO(b+a) values follow the order: Ni-OMS-2 Cu-OMS-2 > Co-OMS-2 > Cr-OMS-2 > Fe-OMS-2, which is very similar to the orders of the Rs and AOS values. Kim et al. [39] reported that a defect-oxide or
Fig. 5. TEM images of the M-OMS-2 catalysts.
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Table 2 Results of the elemental analysis of the M-OMS-2 catalysts. Sample
K/Mn (mol.) a
OMS-2 10Fe-OMS 10Co-OMS 10Ni-OMS 10Cu-OMS 10Cr-OMS a b c d
M/Mn (mol.)
Bulk
Surface
0.0892 0.0952 0.0674 0.0521 0.0303 0.0845
0.1240 0.1096 0.141 0.139 0.140 0.139
b
a
Bulk
Surface
– 0.066 0.107 0.103 0.0998 0.0971
– 0.026 0.051 0.049 0.033 0.412
Formulac
AOSd
K0.71Mn8O16 K0.71Fe0.50Mn7.50O16 K0.49Co0.77Mn7.23O16 K0.38Ni0.75Mn7.25O16 K0.22Cu0.73Mn7.27O16 K0.62Cr0.71Mn7.29O16
3.91 3.97 4.14 4.15 4.17 4.01
b
Obtained from the ICP-AES analysis. Calculated from the XPS data. Determined by the ICP-AES analysis using the same stratage as Ref. [29]. AOS of Mn: calculated by the corresponding formula, (AOS = average oxidation state).
Fig. 6. O 1s XPS spectra of the M-OMS-2 catalysts.
a hydroxyl-like group could positively participate in the catalytic activity of VOCs combustion over K, Ca and Mg promoted Mn3O4 catalysts. Wang et al. [41] found that the accessibility was more important than the concentration of catalyst lattice oxygen for ethanol and acetaldehyde oxidation. Therefore, the defect-oxide species in M-OMS-2 catalyst could contribute to the catalytic activities of DME combustion. Fig. 7 presents the H2-TPR profiles of the M-OMS-2 catalysts. Reduction of all the catalysts occurs in a temperature range of 170–350 °C. The peak position, area, and peak types vary among the catalysts (Table 1), indicating different influence of the doping metal on the OMS-2. These peaks between 200 °C and 350 °C can be ascribed to the hydrogen consumption during the formation of Mn2O3, the reduction of Mn2O3 to Mn3O4, and Mn3O4 to MnO [42]. From Fig. 7, one can readily observe that Cu-OMS-2 catalyst shows the highest reducibility with the largest H2 consumption (12.14 mol/g) and the lowest starting temperature of 170 °C, which might be caused by the reduction of Cu species [24]. Furthermore, the Fe-OMS-2 and Cr-OMS-2 catalysts show lower reducibility with the smallest H2 consumption (10.80 and 10.73 mol/g, respectively), and higher starting reduction temperature (272 °C and
275 °C, respectively). The reducibility of the M-OMS-2 catalysts decreases in the following order: Cu-OMS-2 > Ni-OMS-2 > Co-OMS-2 > Cr-OMS-2 > Fe-OMS-2. The higher reducibility indicates the higher mobility of the oxygen species in catalysts. Therefore, Cu2+ can significantly improve the oxygen mobility in OMS-2 catalyst compared with the other doping metals. The TPR results correlate fairly well with the catalytic combustion activities and the XPS results. Similar results have been reported for the silver modified OMS-2 catalysts in the CO oxidation reaction [43]. Moreover, Hernández et al. [44] also suggested that Cu could improve the reducibility of OMS-2 and enhance the catalytic activity in preferential oxidation of CO. The promotion effect of Cu on the reducibility of OMS-2 could be attributed to the formation of Cu–O–Mn bridge and the hydrogen spillover from Cu atoms to manganese oxides [24,45]. Combined with the XPS and H2-TPR results, it can be concluded that the highest catalytic activity of Cu-OMS-2 catalyst can be ascribed to the richer defect-oxide species and the higher reducibility or oxygen mobility. Benzyl alcohol oxidation over OMS-2 catalyst has been proposed to obey the Mars–van-Krevelen (MVK) mechanism [31]. Liu et al. [24] also concluded that CO oxidation over CuO/OMS-2
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Fig. 7. H2-TPR profiles of the M-OMS-2 catalysts.
catalyst follow the MVK mechanism. The catalytic combustion of VOCs (such as ethyl acetate, toluene and toluene) [40,46–48] and Diesel soot [49] over different manganese oxides are reported to proceed by a MVK mechanism. By combining the activity of M-OMS-2 catalysts and the results of H2-TPR and XPS characterizations, we propose that the catalytic combustion of DME over the
325
M-OMS-2 catalysts also follows the MVK mechanism. To prove this, we took DME pulse reactions over the Cu-OMS-2 catalyst, and the results were shown in Fig. 8. From Fig. 8a and b, it can be observed that a large quantity of CO2 and a small quantity of CO formed after the first DME pulse injection. From the second cycle, the produced CO2 and CO decreased sharply until the 12th injection. Since the reversibly and physically bound oxygen species were removed by pretreatment, only the remaining lattice oxygen species convert DME to CO2 and CO. When the lattice oxygen species were exhausted, no CO2 was produced and the DME level detected remained constant. After the first run, the catalyst was regenerated under O2/He atmosphere at 300 °C for 30 min, and then was shifted under He to blow off the physically bound oxygen, finally the second and third pulse reaction runs were performed. It can be seen from Fig. 8c and d that CO2 formed again after the oxygen was replenished. However, the quantity of CO2 decreased over the regenerated catalyst compared with that of the new catalyst. The pulse reaction confirms that DME catalytic combustion over the M-OMS-2 catalysts follows the MVK mechanism as shown in Fig. 9. Under the reaction conditions, the oxygen species of the M-OMS-2 catalysts responsible for the complete conversion of DME to CO2 and H2O could be replenished by the oxygen in the feed stream to prevent lattice collapse, and thus lead to a redox couple between Mn4+/Mn3+ and Mm+/M(m 1)+ (M = Fe, Co, Ni, Cu, Cr). The redox couple is very important to the activity. Studies on the CO oxidation over CuO/OMS-2 catalyst have shown that there exists Cu2+–O2 –Mn4+ M Cu+–h–Mn3++O2 redox couple [24,45]. In order to prove that Cu/Mn redox couple can work effectively under the reaction temperature, the following reduction–reoxidation
Fig. 8. Pulse reaction results.
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Fig. 9. Scheme of the reaction mechanism.
experiment was carried out for three times. Cu-OMS-2 catalyst was firstly reduced from 50 to 200 °C, then re-oxidized at 180 °C (T90 for Cu-OMS-2) in a 5%O2/He flow for 1 h, and finally reduced from 50 to 200 °C. The H2-TPR profiles (Supplementary data, Fig. S3) showed that the Cu-OMS-2 catalyst could be re-oxidized to its origin state after reduction. After one reduction-reoxidation cycle, the Cu-OMS-2 became easier to reduce. The doped transition metals generally enhance the catalytic activity of OMS-2, however, different transition metals result in different catalytic activities. Yin et al. [50] had reported that the reactivities and availabilities of the oxygen species in Cu2+, Ni2+, Zn2+ and Mg2+ doped todorokite were associated with the heat of formation of the corresponding oxides and their electronegativity properties. Hernández et al. [44] also attributed the better CO oxidation performance over Cu modified manganese dioxide for its higher electronegativity. The element electronegativity (Pauling Scale) follows the order: 1.91(Ni) > 1.90(Cu) > 1.88(Co) > 1.83 (Fe) > 1.66(Cr), which agrees well with the order of catalytic activities of M-OMS-2 catalysts. The higher the electronegativity, the weaker the Mn–O bond in Mn–O–M bridge, and the stronger of the mobility and reactivity of the active oxygen species. 3.6. Cu-OMS-2 stability evaluation TG-DSC, XRD and TEM techniques were used to evaluate the stability of the used Cu-OMS-2 catalyst (Supplementary data, Figs. S4–6). As shown in Fig. S4, there is no obvious mass loss or exothermal peak below 400 °C, which suggests that no carbon is deposited on Cu-OMS-2 catalyst during the reaction. The three weight loss steps and the endothermic peaks observed after 500 °C are caused by phase transformation of manganese oxide into lower oxidation state because of lattice oxygen loss [19]. As shown in Fig. S5, the characteristic XRD peaks of the Cu-OMS-2 catalyst before and after reaction are almost identical, which means that the structure and particle size of used catalyst remain the same during reaction. Fig. S6 shows the TEM image of the used Cu-OMS-2 catalyst with the short nano-rod morphology. From the TG-DSC, XRD and TEM characterization, it can be concluded that the Cu-OMS-2 catalyst has a robust stability under the reaction condition. 4. Conclusions Transition metal (Fe, Co, Ni, Cu, Cr) doped OMS-2 catalysts for DME catalytic combustion were studied. The doping of transition metals promotes the catalytic activities of OMS-2 catalysts in the reaction of DME combustion, and Cu doped OMS-2 showed the highest activity. The structure and oxidation activity of the M-OMS-2 catalysts were greatly influenced by the type of doped metals as shown by BET, TEM, H2-TPR, and XPS characterization. The enhanced activities of the M-OMS-2 i.e., the higher specific reaction rates (Rs), can be attributed to higher average oxidation state, the reducibility as well as the oxygen mobility. The Mars-van-Krevelen mechanism on the basic of the redox couple
between Mn4+/Mn3+ and Mm+/M(m proposed.
1)+
(M = Fe, Co, Ni, Cu, Cr) is
Acknowledgements This work was supported by Natural Science Foundation of Guangdong Province (10251009001000003, S2012010009680), Scientific Program of Guangdong Provence (2012A030600006), Fund of higher education of Guangdong Province (cgzhzd1104) and the 211 Project of Guangdong Province, China. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2013.01.061. References [1] C. Arcoumanis, C. Bae, R. Crookes, E. Kinoshita, The potential of di-methyl ether (DME) as an alternative fuel for compression-ignition engines: a review, Fuel 87 (2008) 1014–1030. [2] T.A. Semelsberger, R.L. Borup, H.L. Greene, Dimethyl ether (DME) as an alternative fuel, J. Power Sources 156 (2006) 497–511. [3] D.A. Good, J.S. Francisco, Atmospheric chemistry of alternative fuels and alternative chlorofluorocarbons, Chem. Rev. 103 (2003) 4999–5023. [4] T.V. Choudhary, S. Banerjee, V.R. Choudhary, Catalysts for combustion of methane and lower alkanes, Appl. Catal. A 234 (2002) 1–23. [5] A. Ishikawa, E. Iglesia, Bifunctional pathways mediated by Pt clusters and Al2O3 in the catalytic combustion of dimethyl ether, Chem. Commun. (2007) 2992– 2993. [6] A. Ishikawa, E. Iglesia, Dimethyl ether combustion catalyzed by supported Pd, Rh, and Pt clusters: site requirements and reaction pathways, J. Catal. 252 (2007) 49–56. [7] A. Ishikawa, M. Neurock, E. Iglesia, Structural requirements and reaction pathways in dimethyl ether combustion catalyzed by supported Pt clusters, J. Am. Chem. Soc. 129 (2007) 13201–13212. [8] Q. Yu, L. Yu, Y. Wang, Q. Zhang, M. Sun, Y. Huang, Y. Lu, Z. Ge, Effect of preparation methods on activation of catalysts, Chin. J. Chem. Eng. 16 (2008) 389–393. [9] S.L. Suib, Structure, porosity, and redox in porous manganese oxide octahedral layer and molecular sieve materials, J. Mater. Chem. 18 (2008). [10] V.P. Santos, M.F.R. Pereira, J.J.M. Órfão, J.L. Figueiredo, Catalytic oxidation of ethyl acetate over a cesium modified cryptomelane catalyst, Appl. Catal. B 88 (2009) 550–556. [11] I. Atribak, A. Bueno-López, A. García-García, P. Navarro, D. Frías, M. Montes, Catalytic activity for soot combustion of birnessite and cryptomelane, Appl. Catal. B 93 (2010) 267–273. [12] J. Chen, J. Li, H. Li, X. Huang, W. Shen, Facile synthesis of Ag-OMS-2 nanorods and their catalytic applications in CO oxidation, Micropor. Mesopor. Mater. 116 (2008) 586–592. [13] H. Sun, S. Chen, P. Wang, X. Quan, Catalytic oxidation of toluene over manganese oxide octahedral molecular sieves (OMS-2) synthesized by different methods, Chem. Eng. J. 178 (2011) 191–196. [14] O. Sanz, J.J. Delgado, P. Navarro, G. Arzamendi, L.M. Gandía, M. Montes, VOCs combustion catalysed by platinum supported on manganese octahedral molecular sieves, Appl. Catal. B 110 (2011) 231–237. [15] X. Chen, Y.-F. Shen, S.L. Suib, C.L. O’Young, Characterization of manganese oxide octahedral molecular sieve (M-OMS-2) materials with different metal cation dopants, Chem. Mater. 14 (2002) 940–948. [16] M. Polverejan, J.C. Villegas, S.L. Suib, Higher valency ion substitution into the manganese oxide framework, J. Am. Chem. Soc. 126 (2004) 7774–7775. [17] X. Tang, Y. Li, J. Chen, Y. Xu, W. Shen, Synthesis, characterization, and catalytic application of titanium–cryptomelane nanorods/fibers, Micropor. Mesopor. Mater. 103 (2007) 250–256.
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Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Transition metal doped cryptomelane-type manganese oxide for low-temperature catalytic combustion of dimethyl ether Ming Sun a, Lin Yu a,⇑, Fei Ye a, Guiqiang Diao a, Qian Yu a, Zhifeng Hao a, Yuying Zheng a, Lixiang Yuan b a b
School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, PR China Laboratory for Sustainable Technology, School of Chemical and Biomolecular Engineering, University of Sydney, Sydney, NSW 2006, Australia
h i g h l i g h t s " Fe, Co, Ni, Cu, Cr doped OMS-2 catalyst were prepared and applied to the DME catalytic combustion. " Cu-OMS-2 has the best activity with a T10 of 171 °C and a T90 of 180 °C. " The activity of M-OMS-2 catalyst were greatly influenced by the type of doped metals. " The activity is related with surface area, average oxidation state and redox properties.
a r t i c l e
i n f o
Article history: Received 10 July 2012 Received in revised form 14 January 2013 Accepted 18 January 2013 Available online 30 January 2013 Keywords: Transition metal Cryptomelane-type manganese oxide Dimethyl ether Catalytic combustion
a b s t r a c t The catalysts of transition metals (Fe, Co, Ni, Cu, Cr) doped cryptomelane-type manganese oxide (M-OMS-2) were prepared and investigated for the catalytic combustion of dimethyl ether (DME). The catalysts were characterized by XRD, FT-IR, BET, TEM, ICP-AES, XPS and H2-TPR techniques. The effects of surface area, the average oxidation state, and the redox properties on catalytic activities were studied. The results showed that 100% CO2 selectivity can be realized over the doped OMS-2 catalysts. The M-OMS-2 catalysts displayed the nanorod morphology with a 13 nm diameter and the length ranging from 50 to 220 nm. The Cu doped OMS-2 catalyst exhibited the best activity, achieving the light-off temperature (the temperature acquired for 10% DME conversion, T10) and full-conversion temperature (the temperature acquired for 90% DME conversion, T90) of 171 and 180 °C, respectively. DME catalytic combustion over M- OMS-2 catalysts was found to follow a Mars–van-Krevelen mechanism with the presence of redox between Mn4+/Mn3+ and Mm+/M(m 1)+ (M = Fe, Co, Ni, Cu, Cr). Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Dimethyl ether (DME) has attracted increasing attention as an environmentally benign energy carrier. Being non-toxic and chemically stable, DME has a boiling point of 248.1 K at 1 atm and a vapor pressure of 530 kPa at 298 K, which is similar to those of liquefied petroleum gas [1]. DME can be mass-produced from natural gas, crude oil, coal, waste produces and biomass [1,2]. As a renewable fuel, the homogeneous combustion of DME has been investigated extensively [3]. However, the catalytic combustion of DME is seldom reported until now. The main advantage of DME combustion by catalytic routes lie in that catalytic combustion can effectively decrease the temperature and minimize NOx and hydrocarbons emission. The catalysts for catalytic combustion are generally precious metals, mixed metal-oxide and perovskite [4]. Ishikawa and Iglesia [5–7] have conducted a comprehensive study on the DME ⇑ Corresponding author. Tel.: +86 20 39322201; fax: +86 20 39322231. E-mail address: [email protected] (L. Yu). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.01.061
combustion using noble metal (Pd, Rh, Pt) clusters, which demonstrated that Pt and Pd catalyzes DME to CO2 and H2O at 400– 600 °C. However, noble metals are rare, expensive and unsuitable for large-scale practical applications. Recently, we have reported the catalytic combustion of DME over large-surface area of hexaaluminate catalysts with outstanding stability, achieving 100% DME conversion at 320 °C for 100 h [8]. However, the light-off temperature of the hexaaluminate catalysts is relatively high, and the low-temperature activity is not satisfactory. Therefore, it is necessary to find a more efficient catalyst for DME catalytic combustion. Cryptomelane-type manganese oxide, also known as manganese oxide octahedral molecular sieve (OMS-2), has a 2 2 tunnel structure and a pore size of 0.46 nm. The OMS-2 material has many attractive properties such as abundant porous and tunnel structure, ion-exchange ability, facile evolution of lattice oxygen and reasonable thermal stability [9]. Thus, it has been exploited for the total oxidation of CO and VOCs with excellent oxidation activities [10–14]. The properties of the OMS-2 as a catalyst, such as the surface area, morphology and redox property can be altered by tuning the preparation parameters or by doping with metal
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cations. Some cations including Cu2+, Zn2+, Ni2+, Co2+, Al3+, Mg2+, Fe3+, Ti4+, Ce4+, Zr4+ or V5+ have been successfully introduced into the OMS-2 tunnel to enhance their catalytic performance [15– 20]. We have found that the Ce-doped OMS-2 catalyst showed good performance in DME combustion [21]. In this study, the transition metal (Fe, Co, Ni, Cu, Cr) doped OMS-2 catalysts have been prepared, and the influence of transition metal on the structure, redox property and catalytic performance of the OMS-2 materials for the catalytic combustion of DME were investigated. 2. Experimental
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loaded. The temperature was measured by a thermocouple placed in the catalyst bed. The catalyst was pretreated in a flow of O2/He for 1 h at 350 °C. A mixture of DME and 20 vol.% O2/He, was introduced into the reactor with a molar ratio of 1:50. The gas flow was controlled by a mass flow controller (Beijing Sevenstar). The gas hourly space velocity was approximately 10 dm3 (gcat h) 1. The reactor effluent was analyzed by online gas chromatograph (Agilent 6820) with a methyl silicone capillary column (ATSE30) and a Porapak Q packed column, connected to a flame ionization detector and thermal conductive detector, respectively. The catalytic performance at each set temperature was evaluated at 45 min intervals.
2.1. Catalyst preparation 3. Results and discussion OMS-2 was synthesized through the oxidation of manganese (II) sulphate by potassium permanganate in an acidic medium under reflux conditions, as described in a previous report [22]. A certain amount of OMS-2 was added to the nitrate salt (Fe(III), Co(II), Ni(II), Cu(II), Cr(III)) dissolved in methanol at a fixed molar ratio of Mn to the metal (10:1). The mixture was stirred for 24 h, followed by drying at 60 °C for 12 h. The catalysts were pressed and sieved to 40–60 mesh after calcination under air atmosphere at 450 °C for 2 h. The catalyst was denoted as M-OMS-2, where M stands for Fe, Co, Ni, Cu or Cr. 2.2. Characterization The phase structure of the M-OMS-2 catalysts was studied by X-ray powder diffraction (XRD, MSAL-XD2) with Cu Ka radiation at 40 kV and 25 mA. Fourier transform infrared (FT-IR) spectra were obtained from a Thermo Nicolet 380 spectrometer. The samples were mixed with KBr and well grounded before measurement. The BET surface areas and pore volumes of the catalysts were measured by a multipoint N2 adsorption–desorption method in liquid N2 ( 196 °C) with a GEMINI V 2380 surface area analyzer. Samples were treated under 200 °C in N2 atmosphere for 4 h to evacuate the physisorbed moisture before measurement. The morphology of the catalysts was investigated using a transmission electron microscopy (TEM, Philips Techai-10). The finely grounded samples were dispersed in ethanol under ultrasonic treatment. Elemental analysis of the catalysts was conducted by a coupled plasma-atomic emission spectroscopy (ICP-AES) on an ICPS-1000 IV instrument. The catalyst of 20 mg was dissolved in 5 mL of concentrated hydrochloric acid, and then diluted to 1000 mL with deionized water. X-ray photoelectron spectroscopy (XPS) was recorded on a VG MultiLab 2000 instrument equipped with an Mg Ka X-ray source. The spectra were calibrated using the carbon peak. Hydrogen temperature programmed reduction (H2-TPR) experiments were carried out in a 5 vol.% H2/Ar gas flow on an Auto Chem 2920 (Micromeritics) instruments. Catalyst sample (50 mg) was pretreated in flowing He for 0.5 h at 350 °C, followed by cooling to room temperature. The inlet total flow was 50 mL/min and the temperature was increased linearly at a rate of 10 °C/min. The pulse reaction was carried out on an Auto Chem 2920 (Micromeritics) to investigate the reactivity of active oxygen species. The Cu-OMS-2 catalyst (40–60 mesh, 200 mg) packed in a U-shaped quartz reactor was pretreated with a flow of He (30 mL/min) at 300 °C for 60 min in order to remove the reversibly and physically bound oxygen. Then the DME pulses (0.5 mL) were injected into He carrier (flow rate 30 mL min 1) over the catalyst at 190 °C. The DME and products were analyzed on-line using a Balzers Omnistar quadrupole mass spectrometer.
3.1. Evaluation of the catalytic combustion performance The result of DME combustion without catalyst is shown in Fig. S1 (Supplementary data, Fig. S1), which indicates that the homogeneous oxidation starts after 200 °C and the temperature for complete conversion is higher than 525 °C. Fig. 1 and Table 1 present the results of the catalytic activities of DME combustion over M-OMS-2 catalysts. It can be seen from Fig. 1 and Table 1 that Co, Ni and Cu doped OMS-2 catalysts exhibit higher catalytic activities compared to Cr and Fe doped OMS-2 catalysts. More importantly, Cu-OMS-2 shows the best activity with the light-off temperature (T10, the temperature acquired for 10% DME conversion) and the full-conversion temperature (T90, the temperature acquired for 90% DME conversion) at 171 and 180 °C, respectively. The temperature window from T10 to T90 over Cu-OMS-2 catalyst is less than 10 °C, and this narrow window suggests that Cu-OMS-2 catalyst has excellent low-temperature activity. Considering that the undoped OMS-2 has a T10 of 167 °C and a T90 of 213 °C [21], the T90 for DME combustion has been significantly reduced after doping with transition metals. Over the M-OMS-2 catalysts, CO2 and H2O were the only detectable products within the detection limit. 3.2. Structure and morphology of M-OMS-2 catalysts Fig. 2 illustrates the XRD patterns of the M-OMS-2 catalysts. Only peaks that belong to Q-cryptomelane (JCPDS 29-1020) with a tetragonal structure are detected, which means that the doped metals are well-dispersed in the OMS-2 structure [23,24].
2.3. Catalytic activity The DME catalytic combustion was carried out in a fixed bed continuous flow quartz reactor (8 mm i.d.) with 300 mg catalyst
Fig. 1. DME combustion activity of the M-OMS-2 catalysts.
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Table 1 BET specific surface areas, Rs, H2-TPR, XPS peak fit and T10, T90 values of M-OMS-2 catalysts. SBET (m2/g)
Fe-OMS-2 Co-OMS-2 Ni-OMS-2 Cu-OMS-2 Cr-OMS-2
58 51 55 47 46
Rsa lmol/(s m2).
0.15 0.96 1.23 1.50 0.072
T10b (°C)
178 170 167 171 185
T90b (°C)
200 185 183 180 190
H2-TPR
XPS O 1s peak fit Peak area
Peak1
Peak2
H2 consumption (mmol/g)
272 281 242 225 275
320 290 255 250 310
10.80 11.84 11.20 12.14 10.73
AOb/A(Ob+a)
Oa
Ob
52,084 62,088 45,713 48,028 10,2894
16,921 41,399 33,042 34,498 67,197
c
0.245 0.400 0.419 0.418 0.395
a Rs (specific reaction rates): the amount of reactant (DME) consumed per unit of surface area and per second at 180 °C were introduced to eliminate the effect of surface area. Rs = ((PV/RT)DME conversion)/(SBET 0.3), it was calculated based on the weight of the catalysts of 0.3 g, the reactant flow rate of 51 cm3 min 1, the flow time of 60 min, 101.3 kPa, 180 °C. Noted: virial equation,PV = ZnRT, should be applied here. However, the compressibility factor Z is very close to 1 after calculation. Therefore, an approximate value of Z = 1 is used. b T10, T90 represent the temperature acquired for 10% and 90% DME conversion, respectively. c The concentrations of the three oxygen species were calculated from the relative area of the subpeaks.
The FT-IR results are shown in Fig. 3. The peaks at around 710, 530 and 463 cm 1 are characteristic of cryptomelane and are involved in the vibrations of the MnO6 octahedral framework [12,25,26]. The peak positions for M-OMS-2 slightly shifted to higher wavelength of 712, 530, 469 cm 1 when compared to those of OMS-2. Fig. 4 shows the nitrogen adsorption–desorption isotherms of the catalysts and the specific surface areas of the catalysts are listed in Table 1. It can be seen from Table 1 that the doping of metals has little impact on the surface areas of the M-OMS-2 catalysts. The isotherms show a characteristic of type II isotherm pattern, with a hysteresis loop of type H3 in the IUPAC classification, which is usually associated with the adsorption on aggregates of particles with a layered morphology, forming slit-like pores [27]. The results are similar to those of the reported K-OMS-2 and Ti-OMS-2 [17]. The TEM images of the M-OMS-2 catalysts are shown in Fig. 5 (the image of Cr-OMS-2 is shown in Supplementary data, Fig. S2). All the M-OMS-2 catalysts show a rod-like morphology with a length ranging from 50 to 220 nm and a diameter of about 13 nm. No agglomerated particles corresponding to the doped metal oxide were found. The bulk and surface atomic ratios of K/Mn and M/Mn of the M-OMS-2 catalysts listed in Table 2 were determined by ICP-AES and XPS techniques, respectively. In general, the doping of metals increases the K/Mn atomic ratio in bulk, and decreases the K/Mn atomic ratio in surface. However, the M/Mn atomic ratios in bulk of the M-OMS-2 catalysts are higher than those in the surface
Fig. 3. FT-IR spectra of the M-OMS-2 catalysts.
except for Cr, indicating that more doped metal cations (Fe, Co, Ni, and Cu) exist in the bulk of the OMS-2 structure. This result is contrary to the V-doped OMS-2 [28]. Based on the ICP-AES results, the formula of the catalysts and the average oxidation state (AOS) of manganese oxide were listed using the same method as reported [29]. 3.3. Effect of specific surface area on the activity
Fig. 2. XRD patterns of the M-OMS-2 catalysts.
Generally, the specific surface area plays an important role in catalyst activity. Liu et al. [30] and Schurz et al. [31] reported that the activities of OMS-2 catalysts were correlated to their specific surface areas in the oxidation of cyclohexanol and benzyl alcohol. In our study, the specific surface area did not fit very well with the catalytic activity trend. For instance, the Fe-OMS-2 catalyst with the largest specific surface area (58 m2/g) showed the lowest activity, while the Cu-OMS-2 catalyst with relatively smaller specific surface area (47 m2/g) exhibited the best activity. Therefore, the specific reaction rates (Rs), namely, the amounts of DME consumed per unit of surface area and per second at 180 °C, were employed to eliminate the effect of specific surface area [32,33]. According to our previous studies, the Rs could reflect the catalytic activity trend very well [21]. As shown in Table 1, the Rs values are in the following order: Cu-OMS-2 > Ni-OMS-2 > Co-OMS-2 > Fe-OMS-2 > Cr-OMS-2, which agrees well with the catalytic activity trend. For example, the Cu-OMS-2 catalyst with the highest
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[35,36]. Herein, the AOS was calculated from the corresponding formula using the result from the ICP-AES analysis (Table 2). It can be seen from Table 2 that the AOS value of the OMS-2 catalyst (3.91) in our study is in agreement with that (3.9) of the reported OMS-2 [35], suggesting that this method is reasonable. The AOS values of the M-OMS-2 increase after doping with transition metals. The AOS values decrease in the following order: CuOMS-2 > Ni-OMS-2 > Co-OMS-2 > Cr-OMS-2 > Fe-OMS-2, which is very similar to the decreasing order of the Rs values. Xia et al. [34] observed that the higher AOS lead to higher CO oxidation rates over the Cu(Co, Ag)-OMS-2 catalysts. Santos et al. [35] also found that the OMS-2 with the highest AOS showed the best catalytic activity for the total oxidation of ethyl acetate. Therefore, it could be concluded that the AOS values of the M-OMS-2 catalysts are in correlation with their catalytic activities of DME combustion. 3.5. Effect of the redox properties on the activity
Fig. 4. Nitrogen adsorption/desorption isotherms of the M-OMS-2 catalysts (filled circles: absorption; empty circles: desorption).
Rs (1.50 lmol/(s m2).) has the best activity, and Cr-OMS-2 catalyst with the lowest Rs (0.072 lmol/(s m2)) has the lowest activity. 3.4. Effect of average oxidation state (AOS) on the activity The AOS of OMS-2 catalyst is usually determined by potentiometric titration method [34] or by the XPS analysis of Mn 3s
To further explore the relationship between the redox property and activity, XPS and H2-TPR techniques were conducted. The O 1s spectra of the M-OMS-2 catalysts are shown in Fig. 6, and the results of O 1s peak fitting are in Table 1. For all the catalysts, the O1s peak is composed of three subpeaks at around 530, 531.5 and 533 eV, which can be attributed to the lattice oxygen (Oa), the defect oxide or a hydroxyl-like group (Ob), and the adsorbed molecular water (Oc), respectively [20,37,38]. The amount of the Oa and Ob species were calculated from the relative area of the subpeaks. For each catalyst, the most abundant species is lattice oxygen (Oa), suggesting that lattice oxygen is important for the catalytic activity. The Ob species has higher mobility than the Oa species, and allow the diffusion of lattice oxygen [39,40]. To confirm the contribution of Ob species in the reaction, the area ratios of AOb/AO(b+a) were calculated (Table 1). The AOb/AO(b+a) values follow the order: Ni-OMS-2 Cu-OMS-2 > Co-OMS-2 > Cr-OMS-2 > Fe-OMS-2, which is very similar to the orders of the Rs and AOS values. Kim et al. [39] reported that a defect-oxide or
Fig. 5. TEM images of the M-OMS-2 catalysts.
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Table 2 Results of the elemental analysis of the M-OMS-2 catalysts. Sample
K/Mn (mol.) a
OMS-2 10Fe-OMS 10Co-OMS 10Ni-OMS 10Cu-OMS 10Cr-OMS a b c d
M/Mn (mol.)
Bulk
Surface
0.0892 0.0952 0.0674 0.0521 0.0303 0.0845
0.1240 0.1096 0.141 0.139 0.140 0.139
b
a
Bulk
Surface
– 0.066 0.107 0.103 0.0998 0.0971
– 0.026 0.051 0.049 0.033 0.412
Formulac
AOSd
K0.71Mn8O16 K0.71Fe0.50Mn7.50O16 K0.49Co0.77Mn7.23O16 K0.38Ni0.75Mn7.25O16 K0.22Cu0.73Mn7.27O16 K0.62Cr0.71Mn7.29O16
3.91 3.97 4.14 4.15 4.17 4.01
b
Obtained from the ICP-AES analysis. Calculated from the XPS data. Determined by the ICP-AES analysis using the same stratage as Ref. [29]. AOS of Mn: calculated by the corresponding formula, (AOS = average oxidation state).
Fig. 6. O 1s XPS spectra of the M-OMS-2 catalysts.
a hydroxyl-like group could positively participate in the catalytic activity of VOCs combustion over K, Ca and Mg promoted Mn3O4 catalysts. Wang et al. [41] found that the accessibility was more important than the concentration of catalyst lattice oxygen for ethanol and acetaldehyde oxidation. Therefore, the defect-oxide species in M-OMS-2 catalyst could contribute to the catalytic activities of DME combustion. Fig. 7 presents the H2-TPR profiles of the M-OMS-2 catalysts. Reduction of all the catalysts occurs in a temperature range of 170–350 °C. The peak position, area, and peak types vary among the catalysts (Table 1), indicating different influence of the doping metal on the OMS-2. These peaks between 200 °C and 350 °C can be ascribed to the hydrogen consumption during the formation of Mn2O3, the reduction of Mn2O3 to Mn3O4, and Mn3O4 to MnO [42]. From Fig. 7, one can readily observe that Cu-OMS-2 catalyst shows the highest reducibility with the largest H2 consumption (12.14 mol/g) and the lowest starting temperature of 170 °C, which might be caused by the reduction of Cu species [24]. Furthermore, the Fe-OMS-2 and Cr-OMS-2 catalysts show lower reducibility with the smallest H2 consumption (10.80 and 10.73 mol/g, respectively), and higher starting reduction temperature (272 °C and
275 °C, respectively). The reducibility of the M-OMS-2 catalysts decreases in the following order: Cu-OMS-2 > Ni-OMS-2 > Co-OMS-2 > Cr-OMS-2 > Fe-OMS-2. The higher reducibility indicates the higher mobility of the oxygen species in catalysts. Therefore, Cu2+ can significantly improve the oxygen mobility in OMS-2 catalyst compared with the other doping metals. The TPR results correlate fairly well with the catalytic combustion activities and the XPS results. Similar results have been reported for the silver modified OMS-2 catalysts in the CO oxidation reaction [43]. Moreover, Hernández et al. [44] also suggested that Cu could improve the reducibility of OMS-2 and enhance the catalytic activity in preferential oxidation of CO. The promotion effect of Cu on the reducibility of OMS-2 could be attributed to the formation of Cu–O–Mn bridge and the hydrogen spillover from Cu atoms to manganese oxides [24,45]. Combined with the XPS and H2-TPR results, it can be concluded that the highest catalytic activity of Cu-OMS-2 catalyst can be ascribed to the richer defect-oxide species and the higher reducibility or oxygen mobility. Benzyl alcohol oxidation over OMS-2 catalyst has been proposed to obey the Mars–van-Krevelen (MVK) mechanism [31]. Liu et al. [24] also concluded that CO oxidation over CuO/OMS-2
M. Sun et al. / Chemical Engineering Journal 220 (2013) 320–327
Fig. 7. H2-TPR profiles of the M-OMS-2 catalysts.
catalyst follow the MVK mechanism. The catalytic combustion of VOCs (such as ethyl acetate, toluene and toluene) [40,46–48] and Diesel soot [49] over different manganese oxides are reported to proceed by a MVK mechanism. By combining the activity of M-OMS-2 catalysts and the results of H2-TPR and XPS characterizations, we propose that the catalytic combustion of DME over the
325
M-OMS-2 catalysts also follows the MVK mechanism. To prove this, we took DME pulse reactions over the Cu-OMS-2 catalyst, and the results were shown in Fig. 8. From Fig. 8a and b, it can be observed that a large quantity of CO2 and a small quantity of CO formed after the first DME pulse injection. From the second cycle, the produced CO2 and CO decreased sharply until the 12th injection. Since the reversibly and physically bound oxygen species were removed by pretreatment, only the remaining lattice oxygen species convert DME to CO2 and CO. When the lattice oxygen species were exhausted, no CO2 was produced and the DME level detected remained constant. After the first run, the catalyst was regenerated under O2/He atmosphere at 300 °C for 30 min, and then was shifted under He to blow off the physically bound oxygen, finally the second and third pulse reaction runs were performed. It can be seen from Fig. 8c and d that CO2 formed again after the oxygen was replenished. However, the quantity of CO2 decreased over the regenerated catalyst compared with that of the new catalyst. The pulse reaction confirms that DME catalytic combustion over the M-OMS-2 catalysts follows the MVK mechanism as shown in Fig. 9. Under the reaction conditions, the oxygen species of the M-OMS-2 catalysts responsible for the complete conversion of DME to CO2 and H2O could be replenished by the oxygen in the feed stream to prevent lattice collapse, and thus lead to a redox couple between Mn4+/Mn3+ and Mm+/M(m 1)+ (M = Fe, Co, Ni, Cu, Cr). The redox couple is very important to the activity. Studies on the CO oxidation over CuO/OMS-2 catalyst have shown that there exists Cu2+–O2 –Mn4+ M Cu+–h–Mn3++O2 redox couple [24,45]. In order to prove that Cu/Mn redox couple can work effectively under the reaction temperature, the following reduction–reoxidation
Fig. 8. Pulse reaction results.
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Fig. 9. Scheme of the reaction mechanism.
experiment was carried out for three times. Cu-OMS-2 catalyst was firstly reduced from 50 to 200 °C, then re-oxidized at 180 °C (T90 for Cu-OMS-2) in a 5%O2/He flow for 1 h, and finally reduced from 50 to 200 °C. The H2-TPR profiles (Supplementary data, Fig. S3) showed that the Cu-OMS-2 catalyst could be re-oxidized to its origin state after reduction. After one reduction-reoxidation cycle, the Cu-OMS-2 became easier to reduce. The doped transition metals generally enhance the catalytic activity of OMS-2, however, different transition metals result in different catalytic activities. Yin et al. [50] had reported that the reactivities and availabilities of the oxygen species in Cu2+, Ni2+, Zn2+ and Mg2+ doped todorokite were associated with the heat of formation of the corresponding oxides and their electronegativity properties. Hernández et al. [44] also attributed the better CO oxidation performance over Cu modified manganese dioxide for its higher electronegativity. The element electronegativity (Pauling Scale) follows the order: 1.91(Ni) > 1.90(Cu) > 1.88(Co) > 1.83 (Fe) > 1.66(Cr), which agrees well with the order of catalytic activities of M-OMS-2 catalysts. The higher the electronegativity, the weaker the Mn–O bond in Mn–O–M bridge, and the stronger of the mobility and reactivity of the active oxygen species. 3.6. Cu-OMS-2 stability evaluation TG-DSC, XRD and TEM techniques were used to evaluate the stability of the used Cu-OMS-2 catalyst (Supplementary data, Figs. S4–6). As shown in Fig. S4, there is no obvious mass loss or exothermal peak below 400 °C, which suggests that no carbon is deposited on Cu-OMS-2 catalyst during the reaction. The three weight loss steps and the endothermic peaks observed after 500 °C are caused by phase transformation of manganese oxide into lower oxidation state because of lattice oxygen loss [19]. As shown in Fig. S5, the characteristic XRD peaks of the Cu-OMS-2 catalyst before and after reaction are almost identical, which means that the structure and particle size of used catalyst remain the same during reaction. Fig. S6 shows the TEM image of the used Cu-OMS-2 catalyst with the short nano-rod morphology. From the TG-DSC, XRD and TEM characterization, it can be concluded that the Cu-OMS-2 catalyst has a robust stability under the reaction condition. 4. Conclusions Transition metal (Fe, Co, Ni, Cu, Cr) doped OMS-2 catalysts for DME catalytic combustion were studied. The doping of transition metals promotes the catalytic activities of OMS-2 catalysts in the reaction of DME combustion, and Cu doped OMS-2 showed the highest activity. The structure and oxidation activity of the M-OMS-2 catalysts were greatly influenced by the type of doped metals as shown by BET, TEM, H2-TPR, and XPS characterization. The enhanced activities of the M-OMS-2 i.e., the higher specific reaction rates (Rs), can be attributed to higher average oxidation state, the reducibility as well as the oxygen mobility. The Mars-van-Krevelen mechanism on the basic of the redox couple
between Mn4+/Mn3+ and Mm+/M(m proposed.
1)+
(M = Fe, Co, Ni, Cu, Cr) is
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