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Influence of Mn doping on the performance of CuO-CeO2 catalysts for selective oxidation of CO in hydrogen-rich streams
Applied Catalysis A: General 381 (2010) 261–266
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Influence of Mn doping on the performance of CuO-CeO2 catalysts for selective oxidation of CO in hydrogen-rich streams Jing Li a , Pengfei Zhu a,b , Shufeng Zuo a,c , Qinqin Huang a , Renxian Zhou a,∗ a b c
Institute of Catalysis, Zhejiang University, Hangzhou 310028, PR China School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, PR China Institute of Applied Chemistry, Shaoxing University, Shaoxing 312000, PR China
a r t i c l e
i n f o
Article history: Received 1 December 2009 Received in revised form 26 March 2010 Accepted 9 April 2010 Available online 24 April 2010 Keywords: Mn doping CuO-CeO2 catalysts Selective oxidation of CO Hydrogen-rich streams Oxygen vacancies
a b s t r a c t CuO-CeO2 catalysts with different amounts of Mn doping were prepared by a hydrothermal method and were investigated by means of X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), temperatureprogrammed reduction (H2 -TPR), X-ray photoelectron spectroscopy (XPS) and UV-Raman. The results show that MnOx -CuO-CeO2 catalyst with Mn:Cu = 1:5 exhibits the best catalytic activity and the broadest operating temperature “window” for the high conversion (CO conversion > 99.0%, 110–140 ◦ C) in the selective oxidation of CO in hydrogen-rich streams. Doping an appropriate amount of Mn in CuO-CeO2 catalyst is beneficial to the formation of a more stable solid solution with a larger surface area and smaller particle size, and the redox properties of the catalysts are also enhanced, which improves the selective oxidation performance of CO in hydrogen-rich streams. Crown © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Recently, polymer electrolyte membrane fuel cells (PEMFC) have been attracting much attention in the applications to electric vehicles or residential power-generation by virtue of their many merits, such as low operating temperature, high power density, long work time, and rapid start up [1,2]. Hydrogen is the ideal fuel because the only reaction product is H2 O [3]. Owing to the instability of pure hydrogen, the primary fuel source of PEMFC is hydrogen-rich streams made from reforming hydrocarbon. Nevertheless, such hydrogen-rich streams often contain 0.3–1% of CO that can poison the Pt anode catalyst in the H2 -PEMFC anode [4]. Thus, CO must be removed to avoid poisoning the anode of fuel cells [5,6]. Among the present available methods, selective oxidation of CO in the hydrogen-rich streams is regarded as one of the promising and cost-effective methods to achieve tolerable CO concentrations (below 10 ppm) [3,7,8]. Catalysts for this reaction request having a compromise between catalytic performance and catalyst cost. Precious metal catalysts such as Au, Pt, Rh, Ru often possess excellent catalytic performance in this reaction [9–14]. However, the high cost of precious metals has encouraged researchers around the world to look for alternative catalysts.
∗ Corresponding author. Tel.: +86 571 88273290; fax: +86 571 88273283. E-mail address: [email protected] (R. Zhou).
In the last decades, as a promising substitute for precious metal catalysts in the selective oxidation of CO in excess H2 , the CuOCeO2 mixed oxide catalysts have received considerable attention [15,16]. They are more active and selective than Pt-based catalysts at lower reaction temperatures [17]. Avgouropoulos et al. [15] reported that CuO-CeO2 catalysts had the same activity as that of Pt-Al2 O3 catalyst due to the combined effect of CuO and CeO2 . Liu and Stephanopoulos [18,19] reported that better activity in selective oxidation of CO was available with CuO-CeO2 catalysts than with Pt-based catalysts because of strong interactions between the highly dispersed CuO on the surface of CeO2 support and the formation of Cu–Ce–O solid solution. However, the CuO-CeO2 catalysts have a serious disadvantage that CO conversion is greater than 99% only in a very narrow operating temperature “window” (just 5–20 ◦ C) and their activity for selective oxidation of CO in a hydrogen-rich stream will be deactivated if H2 O and CO2 exist in the reaction gas [20–24]. Manganese–copper mixed oxides have been widely employed as catalysts for total oxidation reactions, and they are very active for the abatement of CO [25–27]. For example, Hopcalite, based on manganese–copper mixed oxide, is a well-known example of a catalyst for CO oxidation [28,29]. In this paper, the influence of Mn doping on the performance of CuO-CeO2 catalyst for the selective oxidation of CO in hydrogen-rich streams is investigated. The physical and chemical properties of the MnOx -CuO-CeO2 catalysts with different amounts of Mn are characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), Temperature-programmed
0926-860X/$ – see front matter. Crown © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.04.020
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Fig. 1. The catalytic performance of MnOx -CuO-CeO2 catalysts prepared with different amounts of Mn.
reduction (H2 -TPR), UV-Raman and X-ray photoelectron spectroscopy (XPS) techniques in order to gain some insight into the potentially important parameters involving the interactions between CuO and CeO2 , which may be responsible for increasingly selective oxidation performance.
2. Experiment 2.1. Preparation of catalysts The MnOx -CuO-CeO2 catalysts with different Mn:Cu molar ratios (Mn:Cu = 0:1, 1:10, 1:7.5, 1:5, 1:2.5, and Cu/(CuO + CeO2 )wt% was always 5.0 wt%) were prepared by a hydrothermal method. NH3 ·H2 O was added to the mixed ethanol solution of Ce(NO3 )3 ·6H2 O, Cu(NO3 )2 ·3H2 O, Mn(NO3 )2 and cetyltrimethyl ammonium bromide (CTAB; CTAB:Ce molar ratio was 1:1) with constant stirring. The pH value of the mixture was adjusted to 9.0, and then the solution was aged at 100 ◦ C for 1 h in a stainless steal autoclave (As the average crystallite size of CeO2 was the smallest and the dispersion of copper species was not affected with aging time of 1 h). The precipitate was separated by filtration, dried at 110 ◦ C and then calcined at 500 ◦ C for 2 h in air. Then catalysts were obtained with Cu content of 5.0 wt% and the Mn:Cu ratios are 0:1, 1:10, 1:7.5, 1:5, 1:2.5, respectively. 2.2. Characterization of catalysts Powder XRD patterns were recorded on a Rigaku D/Max 2550 PC powder diffractometer using nickel-filtered Cu K␣ radiation in the range of 20◦ ≤ 2 ≤ 80◦ . The X-ray tube was operated at 40 kV and 300 mA. The average crystallite size was estimated from the line broadening of the most intense XRD reflections using the Scherrer formula [30]. XPS measurements were recorded with a PHI5000c spectrometer at 1486.6 eV and 12.5 kV using Al K␣ radiation. The samples were pressed into thin discs and mounted on a sample rod placed in a pretreatment chamber. The spectra of Cu 2p3/2 , Ce 3d, O 1s and Mn 2p3/2 levels were recorded. All the binding energy (BE) values were calibrated using the C 1s peak at 284.8 eV. H2 -TPR analysis was carried out in a quartz fixed-bed microreactor (i.d. = 6 mm). Prior to each H2 -TPR measurement, 50 mg of fresh catalyst was pretreated under a helium flow at 300 ◦ C for 0.5 h and then cooled to room temperature. Then the sample was exposed to a flow (40 mL min−1 ) of 5 vol% H2 /Ar and ramped to 700 ◦ C with a heating rate of 10 ◦ C/min. The amount of H2 consumption during the reduction was measured by TCD. The instrument was calibrated by the quantitative reduction of CuO to metallic copper.
BET surface area of the samples was obtained from N2 adsorption isotherms at −195.8 ◦ C using a Tristar II 3020 apparatus of Micromeritics Company. Prior to adsorption measurements, the samples were outgassed at 250 ◦ C under vacuum for 4 h. UV-Raman spectra were recorded on a UV-HR Raman spectrograph with a He–Gd laser of 325 nm excitation wavelength. The mixed oxides were in powder form to avoid diffusion problems. The spectral resolution was 4 cm−1 , and the spectra acquisition consisted of two accumulations of 30 s for each sample. A frequency range of 100–1000 cm−1 was observed. 2.3. Catalytic performance tests The catalytic performance tests for selective oxidation of CO were carried out in a fixed-bed micro-reactor (quartz glass, i.d. = 6 mm) at atmospheric pressure. One hundred mg of catalyst was used in the tests, which was diluted with inert ␣-alumina particles of the same mesh (60–80) with a mass ratio of 2:1. The gas composition (total flow rate: 100 ml/min) was 50% H2 , 1.0% O2 , 1.0% CO and Ar in balance, and the space velocity was 60,000 ml g−1 h−1 . Before activity tests, catalysts were pretreated in oxygen at 150 ◦ C for 0.5 h. The effluent gases were measured with an on-line gas chromatograph equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). H2 and O2 were separated by a carbon molecular sieve (TDX-01) column and detected by TCD. CO and CO2 were separated by a carbon molecular sieve (TDX-01) column, and converted to methane by a methanation reactor and analyzed by FID. The detection limit of FID for CO is less than 3 ppm. Taking CO2 in the feedstock into consideration, one can evaluate the CO conversion as follows: CO conversion =
[CO]in − [CO]out × 100(%) [CO]in
The selectivity was evaluated from the oxygen mass balance as follows: selectivity =
0.5([CO]in − [CO]out ) × 100(%) [O2 ]in − [O2 ]out
“In” and “out” as subscripts mean inlet and outlet gaseous stream. 3. Results and discussion 3.1. Catalytic behavior of catalysts for PROX reaction The catalytic performance of MnOx -CuO-CeO2 catalysts with different amounts of Mn for the selective oxidation of CO in hydrogen-rich streams is shown in Fig. 1. After doping Mn, the catalytic activity values of catalysts for the CO oxidation are higher,
J. Li et al. / Applied Catalysis A: General 381 (2010) 261–266
Fig. 2. XRD patterns of MnOx -CuO-CeO2 catalysts prepared with different amounts of Mn.
and the operating temperature “window” for the high conversion (CO conversion > 99.0%) is broadened, from 20 ◦ C (120–130 ◦ C) to 40 ◦ C (110–140 ◦ C). The catalytic activity for CO oxidation increases with increasing Mn content, while the catalyst with Mn:Cu = 1:5 is found to be the most active one for the catalytic oxidation of CO, which indicates that an appropriate amount of Mn doping is advantageous to enhance catalyst activity for the selective oxidation of CO in hydrogen-rich streams. From Fig. 1, it also can be seen that with increasing reaction temperatures selectivity of O2 –CO oxidation reaction exhibits a contrary trend to CO conversion. Selectivity of O2 –CO oxidation reaction is 100% at reaction temperatures below 110 ◦ C, and it decreases with the increase of reaction temperature when the reaction temperature is higher than 110 ◦ C, which is due to the strong competition for oxygen between hydrogen and carbon monoxide in the absence of reactant activation limitations. 3.2. Structural studies of catalysts XRD patterns of MnOx -CuO-CeO2 catalysts with different amounts of Mn are given in Fig. 2. All the samples present the characteristic peaks of fluorite-type oxide structure of CeO2 . No diffraction peaks of copper or manganese oxides, except for Table 1 Characteristics of MnOx -CuO-CeO2 catalysts prepared with different amounts of Mn and comparison of their activity in CO oxidation. Catalysts
Cell parameter (nm)
D(1 1 1)a (nm)
SBET (m2 ·g−1 )
T50 b (◦ C)
Mn:Cu = 1:2.5 Mn:Cu = 1:5 Mn:Cu = 1:7.5 Mn:Cu = 1:10 Mn:Cu = 0:1
0.5412 0.5410 0.5412
12.5 10.8 13.0
0.5413
9.6
47 68 60 58 52
81 74 75 84 82
a b
From line broadening of CeO2 (1 1 1) peak. W/F = 0.06 g s cm−3
263
Fig. 3. H2 -TPR profiles of MnOx -CuO-CeO2 catalysts prepared with different amounts of Mn.
Mn:Cu = 1:2.5, are found in the diffraction patterns of the samples. This may be explained as follows: (I) fine dispersion of CuO on the surface of ceria; (II) formation of a solid solution or mixed oxide with Mn ions doped into the CuO-CeO2 mixed oxides framework; and (III) high dispersion or poor crystallization of MnOx doped CuO-CeO2 samples [31]. But some diffraction peaks of CuO crystal phases at 2 = 35.6◦ and 2 =38.7◦ are observed in the catalyst with Mn:Cu = 1:2.5, which indicates that excessive levels of Mn may affect the dispersion of CuO and may lead to aggregation of copper species on the surface of cerium oxide. Aggregation of copper species on the surface of cerium oxide may also be disadvantageous to the selective oxidation of CO in hydrogen-rich streams, as part of the surface active sites are covered by it [32]. In addition, the diffraction peak width is broadened after Mn doping, indicating the occurrence of more defective cerianite lattice, lower crystallinity, and smaller particle size [33]. The BET specific surface areas, cell parameters and crystallite sizes of the catalysts are listed in Table 1. It is found that the Mn doping induces cell volume contraction of the ceria lattice, which is consistent with the results reported by Liang et al. [34]. The ionic radii of Mn4+ , Mn3+ and Mn2+ are 0.056, 0.062 and 0.067 nm, respectively; these radii are all smaller than those of Cu2+ (0.073 nm), Cu+ (0.077 nm) and Ce4+ (0.097 nm). So the decrease in cell parameter of Mn-doped CuO-CeO2 catalysts may be due to the formation of MnOx –CuO–CeO2 solid solutions. The cell parameter and crystallite size of the sample with Mn:Cu = 1:5 are smaller than those of other catalysts, exhibiting the biggest SBET and the highest catalytic activity. From what is mentioned above we can reach the conclusion that addition of an appropriate amount of Mn to CuO-CeO2 catalyst would increase the number of Mnx+ and Cu2+ to substitute Ce4+ in the ceria lattice to achieve the greatest degree. Then oxygen vacancy density and oxygen mobility would be increased with the distortion of CeO2 lattice, which are favorable for redox prop-
Table 2 H2 consumption and temperature of H2 -TPR peaks for MnOx -CuO-CeO2 catalysts prepared with different amounts of Mn. Catalysts
Mn:Cu = 0:1 Mn:Cu = 1:10 Mn:Cu = 1:7.5 Mn:Cu = 1:5 Mn:Cu = 1:2.5
␣ Peak
 Peak ◦
␥ Peak ◦
H2 cons (mol/gcat )
Peak temp ( C)
H2 cons (mol/gcat )
Peak temp ( C)
H2 cons (mol/gcat )
Peak temp (◦ C)
218 225 235 282 240
141 146 144 144 147
606 580 556 545 537
192 181 179 171 183
224 288 290 299 310
257 250 245 245 252
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Fig. 4. XPS spectra of O 1s (a), Mn 2p (b), Cu 2p3/2 (c) for MnOx -CuO-CeO2 catalysts prepared with different amounts of Mn.
erties of mixed oxides and activity for selective oxidation of CO in hydrogen-rich streams [35]. 3.3. Redox property of catalysts Fig. 3 shows H2 -TPR profiles of MnOx -CuO-CeO2 catalysts with different amounts of Mn, and Table 2 lists the reduction temperature and hydrogen consumption of all peaks. In our previous studies [32], we have found that three reduction peaks appeared in the profile of CuO-CeO2 catalysts prepared by a hydrothermal method at a temperature of 100–300 ◦ C. The low-temperature peak at about 141 ◦ C (peak ␣) is attributed to the reduction of the CuOx species which strongly interact with the ceria, while the high-temperature peak at about 257 ◦ C (peak ␥) is attributed to the reduction of Cu+ species regarded as the active centers of CO adsorption over the CuO-CeO2 catalysts [36,37]. In addition, an overlapping reduction peak (peak , at about 192 ◦ C) between peak ␣ and peak ␥ is attributed to the reduction of dispersed CuOx species on the surface of CeO2 , which includes isolated Cu2+ ions that weakly interact with CeO2 and the two- and three-dimensional copper clusters in small size. From Fig. 3 and Table 2, one can see that the reduction temperature of peak ␣ slightly shifts to higher temperature and its hydrogen consumption is obviously increased with the increase of manganese content. The hydrogen consumption of peak ␣ in the catalyst with Mn:Cu = 1:5 is the largest, reaching 282 mol/gcat, on increase of 30% compared with the catalyst without Mn (Mn:Cu = 0:1). According to the literature [38,39], coexistence of copper ions and manganese ions can be mutually promoting each other’s reduction, but the overlapping peaks of their reduction are difficult to distinguish. This result shows that there may be more Mn4+ reduced to Mn3+ at lower temperature. However, manganese presents a lower reducibility than copper due to the more negative free energy of formation of manganese oxides in comparison to that of copper oxide [40,41], so the reduction temperature of peak ␣ in the catalysts doped with manganese shifts slightly to the higher temperature. However, the reduction temperature of peak ␥ is gradually decreased with the increase of Mn content, while the one with Mn:Cu = 1:5 shows the lowest temperature of peak ␥. With the increase of Mn content, its hydrogen consumption increases obviously. The XRD results above have also showed that the existence of a small amount of Mn could increase the number of copper ions and manganese ions entering into the ceria lattice so as to strengthen their interaction. Therefore, the number of Cu+ species would be increased and its reducibility is also improved with the increasing Mn content when the Mn:Cu molar ratio is lower than 1:5. But when an excessive amount of Mn (Mn:Cu = 1:2.5) is added, the reduction temperature of peak ␥ increases significantly again because of the
appearance of CuO crystal phase. Therefore, the peak ␥ in the catalyst with Mn:Cu = 1:2.5 should be attributed to the reduction of a mixture of CuO crystal phase and Cu+ species. In addition, with Mn content increasing the hydrogen consumption of peak  has a decreasing trend accompanied with the changes in peak ␣ and peak ␥. In order to obtain information of oxidation states of the surface elements, we analyzed XPS spectra of the O 1 s, Mn 2p and Cu 2p (Fig. 4(a–c)), and we list the data about surface MnOx and CuOx species derived by XPS in Table 3. From Fig. 4(c) and Table 3, it can be seen that the peak of Cu 2p3/2 is centered at 932.7 and 933.9 eV in the MnOx -CuO-CeO2 catalysts. According to the literature [42,43], the binding energy of Cu 2p3/2 of copper cations in CuO and Cu2 O are greater than 933.1 eV and 932.2–933.1 eV, respectively. The values indicate that active sites of Cu+ exist in the sample and their number increases with Mn doping. But there is an obvious difference in Cu content on the surface, which is 19.0, 26.8 and 23.8 at.% for the catalysts with Mn:Cu = 1:5, 1:2.5 and 0:1, respectively. This indicates that doping a small amount of Mn may cause more copper oxide to enter the ceria lattice, forming a more stable solid solution; however, the presence of excessive Mn can affect the dispersion of copper oxide, separating some copper oxide from solid solution. And then the separated copper oxide would migrate to the surface of cerium oxide. Generally, better catalytic activity can be obtained with more stable Cu–Ce–O solid solutions. Fig. 4(b) shows the Mn 2p region spectra for the samples. Mn occurs in 2+ (640.9 eV), 3+ (641.8 eV) and 4+ (642.5 eV) oxidation states in cerium-free sample reported in the literature [32], so the Mn 2p3/2 region is presented as deconvoluted into all its components and its relative ion ratios are listed in Table 3. It can be seen that the actual ratio of manganese to copper on the surface of the catalyst with Mn:Cu = 1:5 is 0.37, higher than the theoretical ratio 0.2, with Mn existing mainly as Mn4+ oxidation state; however, the actual ratio of manganese to copper on the surface of catalyst with Mn:Cu = 1:2.5 is 0.3, lower than the theoretical ratio 0.4, with Mn existing mainly as inactive Mn2+ . The redox property of the catalyst may be enhanced by a large number of Mn4+ ions in high valence state existing on the surface, which is easily reduced.
Table 3 Surface of the Cu content and composition of Mn species derived by XPS. Sample
Mn:Cu = 1:2.5 Mn:Cu = 1:5 Mn:Cu = 0:1
Mn/Cu
0.30 0.37 0
Cu/(Cu + Ce + Mn) (at.%)
26.8 19.0 23.8
Mn 2p3/2 (at.%) Mn2+
Mn3+
Mn4+
61.6 22.7
13.5 5.7
25.3 71.6
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265
Fig. 6. Stability test for CO oxidation with time-on-stream over the MnOx -CuO-CeO2 catalyst with Mn:Cu = 1:5 at 110 ◦ C.
as A584 /A462 ). And the more the relative concentration of oxygen vacancies, the better the performance of the catalyst. From Fig. 5(b), one can see that the relative amount of oxygen vacancies of catalyst with Mn:Cu = 1:5 is bigger than those for other catalysts. The reaction mechanism of CO oxidation on CeO2 is considered to be a redox reaction involving lattice oxygen and oxygen vacancies. Based on the results of both XPS and UV-Raman, one can deduce that the increasing of the lattice oxygen and oxygen vacancies will be conducive to CO oxidation. 3.4. Stability test of catalyst
Fig. 5. UV-Raman profiles (a) and the A584 /A462 ratio (b) of MnOx -CuO-CeO2 catalysts prepared with different amounts of Mn.
This is in agreement with the previous results of H2 -TPR. In addition, a metal oxide catalyst possessing numerous oxidation states to facilitate electron transfer processes will be an efficient catalyst for oxidation of CO [44]. To investigate the oxygen species on the surface, we used the O 1s spectra as displayed in Fig. 4(a). The O 1s spectra of catalysts are fitted with two peak contributions. The main peak with a BE of about 529.8 eV is attributed to the lattice oxygen of metal oxide, while a shoulder peak at about 532.3 eV belongs most likely to adsorbed oxygen or oxygen in hydroxyl-like groups [35]. Compared with the Mn:Cu = 1:2.5 catalyst, the main peak intensity of the catalyst with Mn:Cu = 1:5 is stronger and the shoulder peak intensity is weaker. Such results indicates that the lattice oxygen amount of catalyst with Mn:Cu = 1:5 is more than that of the catalyst with Mn:Cu = 1:2.5 and that a part of adsorbed oxygen transforms into lattice oxygen. The selective oxidation of CO on metal oxides conforms to the following mechanism: (1) CO + Olatt. → CO2 + Ovac. , (2) O2 + 2Ovac. → 2Olatt. ; formula (1) is the rate controlling step of the above-mentioned redox reaction. Obviously, the increasing of lattice oxygen will be favorable to the selective oxidation of CO. Fig. 5 shows the profiles of UV-Raman (a) and the A584 /A462 ratio (b) of catalysts with different amounts of Mn. According to the literature [34], the relative concentration of oxygen vacancies can be represented by the area ratio of peaks 584 and 462 cm−1 (noted
A long-term stability test for the selective oxidation of CO in hydrogen-rich streams over the catalyst with Mn:Cu = 1:5 was performed, the CO conversion and the selectivity of O2 –CO oxidation reaction with time at reaction atmosphere are shown in Fig. 6. The results show that CO conversion remained at about 99.3% at the reaction temperature of 110 ◦ C and no obvious deactivation was observed for up to 50 h, although the selectivity of O2 –CO oxidation reaction slightly decreased, indicating that catalytic performance of the catalyst with Mn:Cu = 1:5 for CO oxidation in hydrogen-rich streams is well sustained. 4. Conclusion In this study, a series of CuO-CeO2 catalysts doped with different amounts of Mn have been prepared by a hydrothermal method and characterized by XRD, BET, H2 -TPR, XPS and UV-Raman techniques. The catalytic performance of CuO-CeO2 catalysts for the selective oxidation of CO in hydrogen-rich streams are strongly increased by doping a small amount of Mn and, particularly, the catalyst with Mn:Cu = 1:5 shows the best catalytic activity. The CO conversion can reach 99.8% at 120 ◦ C, the operating temperature “window” for the CO conversion > 99% is from 110 to 140 ◦ C. An important reason is that a more stable solid solution with a larger surface area and smaller particle size is formed after doping Mn. The formation of a more stable solid solution enhances the redox properties and the interactions among the various components, and the higher surface area and smaller particle size can afford more unsaturated coordination sites exposed to gas molecules and thus increase the catalytic activity. Furthermore, if we considering the reaction mechanism of CO oxidation on CeO2 as a redox reaction involving lattice oxygen and oxygen vacancies, the increased number of lattice oxygen and oxygen vacancies must be one of reasons
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for the observed high catalytic performance. Our results show that CuO-CeO2 catalysts doped with an appropriate content of Mn is favorable for CO oxidation. Thus, this type of catalyst is encouraging for the selective oxidation of CO in hydrogen-rich streams for the broader operating temperature “window” of the high conversion at lower temperature.
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Acknowledgements
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We gratefully acknowledge the financial support from the Ministry of Science and Technology of China (no. 2004 CB 719504).
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Influence of Mn doping on the performance of CuO-CeO2 catalysts for selective oxidation of CO in hydrogen-rich streams Jing Li a , Pengfei Zhu a,b , Shufeng Zuo a,c , Qinqin Huang a , Renxian Zhou a,∗ a b c
Institute of Catalysis, Zhejiang University, Hangzhou 310028, PR China School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu 610500, PR China Institute of Applied Chemistry, Shaoxing University, Shaoxing 312000, PR China
a r t i c l e
i n f o
Article history: Received 1 December 2009 Received in revised form 26 March 2010 Accepted 9 April 2010 Available online 24 April 2010 Keywords: Mn doping CuO-CeO2 catalysts Selective oxidation of CO Hydrogen-rich streams Oxygen vacancies
a b s t r a c t CuO-CeO2 catalysts with different amounts of Mn doping were prepared by a hydrothermal method and were investigated by means of X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), temperatureprogrammed reduction (H2 -TPR), X-ray photoelectron spectroscopy (XPS) and UV-Raman. The results show that MnOx -CuO-CeO2 catalyst with Mn:Cu = 1:5 exhibits the best catalytic activity and the broadest operating temperature “window” for the high conversion (CO conversion > 99.0%, 110–140 ◦ C) in the selective oxidation of CO in hydrogen-rich streams. Doping an appropriate amount of Mn in CuO-CeO2 catalyst is beneficial to the formation of a more stable solid solution with a larger surface area and smaller particle size, and the redox properties of the catalysts are also enhanced, which improves the selective oxidation performance of CO in hydrogen-rich streams. Crown © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Recently, polymer electrolyte membrane fuel cells (PEMFC) have been attracting much attention in the applications to electric vehicles or residential power-generation by virtue of their many merits, such as low operating temperature, high power density, long work time, and rapid start up [1,2]. Hydrogen is the ideal fuel because the only reaction product is H2 O [3]. Owing to the instability of pure hydrogen, the primary fuel source of PEMFC is hydrogen-rich streams made from reforming hydrocarbon. Nevertheless, such hydrogen-rich streams often contain 0.3–1% of CO that can poison the Pt anode catalyst in the H2 -PEMFC anode [4]. Thus, CO must be removed to avoid poisoning the anode of fuel cells [5,6]. Among the present available methods, selective oxidation of CO in the hydrogen-rich streams is regarded as one of the promising and cost-effective methods to achieve tolerable CO concentrations (below 10 ppm) [3,7,8]. Catalysts for this reaction request having a compromise between catalytic performance and catalyst cost. Precious metal catalysts such as Au, Pt, Rh, Ru often possess excellent catalytic performance in this reaction [9–14]. However, the high cost of precious metals has encouraged researchers around the world to look for alternative catalysts.
∗ Corresponding author. Tel.: +86 571 88273290; fax: +86 571 88273283. E-mail address: [email protected] (R. Zhou).
In the last decades, as a promising substitute for precious metal catalysts in the selective oxidation of CO in excess H2 , the CuOCeO2 mixed oxide catalysts have received considerable attention [15,16]. They are more active and selective than Pt-based catalysts at lower reaction temperatures [17]. Avgouropoulos et al. [15] reported that CuO-CeO2 catalysts had the same activity as that of Pt-Al2 O3 catalyst due to the combined effect of CuO and CeO2 . Liu and Stephanopoulos [18,19] reported that better activity in selective oxidation of CO was available with CuO-CeO2 catalysts than with Pt-based catalysts because of strong interactions between the highly dispersed CuO on the surface of CeO2 support and the formation of Cu–Ce–O solid solution. However, the CuO-CeO2 catalysts have a serious disadvantage that CO conversion is greater than 99% only in a very narrow operating temperature “window” (just 5–20 ◦ C) and their activity for selective oxidation of CO in a hydrogen-rich stream will be deactivated if H2 O and CO2 exist in the reaction gas [20–24]. Manganese–copper mixed oxides have been widely employed as catalysts for total oxidation reactions, and they are very active for the abatement of CO [25–27]. For example, Hopcalite, based on manganese–copper mixed oxide, is a well-known example of a catalyst for CO oxidation [28,29]. In this paper, the influence of Mn doping on the performance of CuO-CeO2 catalyst for the selective oxidation of CO in hydrogen-rich streams is investigated. The physical and chemical properties of the MnOx -CuO-CeO2 catalysts with different amounts of Mn are characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), Temperature-programmed
0926-860X/$ – see front matter. Crown © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.04.020
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Fig. 1. The catalytic performance of MnOx -CuO-CeO2 catalysts prepared with different amounts of Mn.
reduction (H2 -TPR), UV-Raman and X-ray photoelectron spectroscopy (XPS) techniques in order to gain some insight into the potentially important parameters involving the interactions between CuO and CeO2 , which may be responsible for increasingly selective oxidation performance.
2. Experiment 2.1. Preparation of catalysts The MnOx -CuO-CeO2 catalysts with different Mn:Cu molar ratios (Mn:Cu = 0:1, 1:10, 1:7.5, 1:5, 1:2.5, and Cu/(CuO + CeO2 )wt% was always 5.0 wt%) were prepared by a hydrothermal method. NH3 ·H2 O was added to the mixed ethanol solution of Ce(NO3 )3 ·6H2 O, Cu(NO3 )2 ·3H2 O, Mn(NO3 )2 and cetyltrimethyl ammonium bromide (CTAB; CTAB:Ce molar ratio was 1:1) with constant stirring. The pH value of the mixture was adjusted to 9.0, and then the solution was aged at 100 ◦ C for 1 h in a stainless steal autoclave (As the average crystallite size of CeO2 was the smallest and the dispersion of copper species was not affected with aging time of 1 h). The precipitate was separated by filtration, dried at 110 ◦ C and then calcined at 500 ◦ C for 2 h in air. Then catalysts were obtained with Cu content of 5.0 wt% and the Mn:Cu ratios are 0:1, 1:10, 1:7.5, 1:5, 1:2.5, respectively. 2.2. Characterization of catalysts Powder XRD patterns were recorded on a Rigaku D/Max 2550 PC powder diffractometer using nickel-filtered Cu K␣ radiation in the range of 20◦ ≤ 2 ≤ 80◦ . The X-ray tube was operated at 40 kV and 300 mA. The average crystallite size was estimated from the line broadening of the most intense XRD reflections using the Scherrer formula [30]. XPS measurements were recorded with a PHI5000c spectrometer at 1486.6 eV and 12.5 kV using Al K␣ radiation. The samples were pressed into thin discs and mounted on a sample rod placed in a pretreatment chamber. The spectra of Cu 2p3/2 , Ce 3d, O 1s and Mn 2p3/2 levels were recorded. All the binding energy (BE) values were calibrated using the C 1s peak at 284.8 eV. H2 -TPR analysis was carried out in a quartz fixed-bed microreactor (i.d. = 6 mm). Prior to each H2 -TPR measurement, 50 mg of fresh catalyst was pretreated under a helium flow at 300 ◦ C for 0.5 h and then cooled to room temperature. Then the sample was exposed to a flow (40 mL min−1 ) of 5 vol% H2 /Ar and ramped to 700 ◦ C with a heating rate of 10 ◦ C/min. The amount of H2 consumption during the reduction was measured by TCD. The instrument was calibrated by the quantitative reduction of CuO to metallic copper.
BET surface area of the samples was obtained from N2 adsorption isotherms at −195.8 ◦ C using a Tristar II 3020 apparatus of Micromeritics Company. Prior to adsorption measurements, the samples were outgassed at 250 ◦ C under vacuum for 4 h. UV-Raman spectra were recorded on a UV-HR Raman spectrograph with a He–Gd laser of 325 nm excitation wavelength. The mixed oxides were in powder form to avoid diffusion problems. The spectral resolution was 4 cm−1 , and the spectra acquisition consisted of two accumulations of 30 s for each sample. A frequency range of 100–1000 cm−1 was observed. 2.3. Catalytic performance tests The catalytic performance tests for selective oxidation of CO were carried out in a fixed-bed micro-reactor (quartz glass, i.d. = 6 mm) at atmospheric pressure. One hundred mg of catalyst was used in the tests, which was diluted with inert ␣-alumina particles of the same mesh (60–80) with a mass ratio of 2:1. The gas composition (total flow rate: 100 ml/min) was 50% H2 , 1.0% O2 , 1.0% CO and Ar in balance, and the space velocity was 60,000 ml g−1 h−1 . Before activity tests, catalysts were pretreated in oxygen at 150 ◦ C for 0.5 h. The effluent gases were measured with an on-line gas chromatograph equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). H2 and O2 were separated by a carbon molecular sieve (TDX-01) column and detected by TCD. CO and CO2 were separated by a carbon molecular sieve (TDX-01) column, and converted to methane by a methanation reactor and analyzed by FID. The detection limit of FID for CO is less than 3 ppm. Taking CO2 in the feedstock into consideration, one can evaluate the CO conversion as follows: CO conversion =
[CO]in − [CO]out × 100(%) [CO]in
The selectivity was evaluated from the oxygen mass balance as follows: selectivity =
0.5([CO]in − [CO]out ) × 100(%) [O2 ]in − [O2 ]out
“In” and “out” as subscripts mean inlet and outlet gaseous stream. 3. Results and discussion 3.1. Catalytic behavior of catalysts for PROX reaction The catalytic performance of MnOx -CuO-CeO2 catalysts with different amounts of Mn for the selective oxidation of CO in hydrogen-rich streams is shown in Fig. 1. After doping Mn, the catalytic activity values of catalysts for the CO oxidation are higher,
J. Li et al. / Applied Catalysis A: General 381 (2010) 261–266
Fig. 2. XRD patterns of MnOx -CuO-CeO2 catalysts prepared with different amounts of Mn.
and the operating temperature “window” for the high conversion (CO conversion > 99.0%) is broadened, from 20 ◦ C (120–130 ◦ C) to 40 ◦ C (110–140 ◦ C). The catalytic activity for CO oxidation increases with increasing Mn content, while the catalyst with Mn:Cu = 1:5 is found to be the most active one for the catalytic oxidation of CO, which indicates that an appropriate amount of Mn doping is advantageous to enhance catalyst activity for the selective oxidation of CO in hydrogen-rich streams. From Fig. 1, it also can be seen that with increasing reaction temperatures selectivity of O2 –CO oxidation reaction exhibits a contrary trend to CO conversion. Selectivity of O2 –CO oxidation reaction is 100% at reaction temperatures below 110 ◦ C, and it decreases with the increase of reaction temperature when the reaction temperature is higher than 110 ◦ C, which is due to the strong competition for oxygen between hydrogen and carbon monoxide in the absence of reactant activation limitations. 3.2. Structural studies of catalysts XRD patterns of MnOx -CuO-CeO2 catalysts with different amounts of Mn are given in Fig. 2. All the samples present the characteristic peaks of fluorite-type oxide structure of CeO2 . No diffraction peaks of copper or manganese oxides, except for Table 1 Characteristics of MnOx -CuO-CeO2 catalysts prepared with different amounts of Mn and comparison of their activity in CO oxidation. Catalysts
Cell parameter (nm)
D(1 1 1)a (nm)
SBET (m2 ·g−1 )
T50 b (◦ C)
Mn:Cu = 1:2.5 Mn:Cu = 1:5 Mn:Cu = 1:7.5 Mn:Cu = 1:10 Mn:Cu = 0:1
0.5412 0.5410 0.5412
12.5 10.8 13.0
0.5413
9.6
47 68 60 58 52
81 74 75 84 82
a b
From line broadening of CeO2 (1 1 1) peak. W/F = 0.06 g s cm−3
263
Fig. 3. H2 -TPR profiles of MnOx -CuO-CeO2 catalysts prepared with different amounts of Mn.
Mn:Cu = 1:2.5, are found in the diffraction patterns of the samples. This may be explained as follows: (I) fine dispersion of CuO on the surface of ceria; (II) formation of a solid solution or mixed oxide with Mn ions doped into the CuO-CeO2 mixed oxides framework; and (III) high dispersion or poor crystallization of MnOx doped CuO-CeO2 samples [31]. But some diffraction peaks of CuO crystal phases at 2 = 35.6◦ and 2 =38.7◦ are observed in the catalyst with Mn:Cu = 1:2.5, which indicates that excessive levels of Mn may affect the dispersion of CuO and may lead to aggregation of copper species on the surface of cerium oxide. Aggregation of copper species on the surface of cerium oxide may also be disadvantageous to the selective oxidation of CO in hydrogen-rich streams, as part of the surface active sites are covered by it [32]. In addition, the diffraction peak width is broadened after Mn doping, indicating the occurrence of more defective cerianite lattice, lower crystallinity, and smaller particle size [33]. The BET specific surface areas, cell parameters and crystallite sizes of the catalysts are listed in Table 1. It is found that the Mn doping induces cell volume contraction of the ceria lattice, which is consistent with the results reported by Liang et al. [34]. The ionic radii of Mn4+ , Mn3+ and Mn2+ are 0.056, 0.062 and 0.067 nm, respectively; these radii are all smaller than those of Cu2+ (0.073 nm), Cu+ (0.077 nm) and Ce4+ (0.097 nm). So the decrease in cell parameter of Mn-doped CuO-CeO2 catalysts may be due to the formation of MnOx –CuO–CeO2 solid solutions. The cell parameter and crystallite size of the sample with Mn:Cu = 1:5 are smaller than those of other catalysts, exhibiting the biggest SBET and the highest catalytic activity. From what is mentioned above we can reach the conclusion that addition of an appropriate amount of Mn to CuO-CeO2 catalyst would increase the number of Mnx+ and Cu2+ to substitute Ce4+ in the ceria lattice to achieve the greatest degree. Then oxygen vacancy density and oxygen mobility would be increased with the distortion of CeO2 lattice, which are favorable for redox prop-
Table 2 H2 consumption and temperature of H2 -TPR peaks for MnOx -CuO-CeO2 catalysts prepared with different amounts of Mn. Catalysts
Mn:Cu = 0:1 Mn:Cu = 1:10 Mn:Cu = 1:7.5 Mn:Cu = 1:5 Mn:Cu = 1:2.5
␣ Peak
 Peak ◦
␥ Peak ◦
H2 cons (mol/gcat )
Peak temp ( C)
H2 cons (mol/gcat )
Peak temp ( C)
H2 cons (mol/gcat )
Peak temp (◦ C)
218 225 235 282 240
141 146 144 144 147
606 580 556 545 537
192 181 179 171 183
224 288 290 299 310
257 250 245 245 252
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Fig. 4. XPS spectra of O 1s (a), Mn 2p (b), Cu 2p3/2 (c) for MnOx -CuO-CeO2 catalysts prepared with different amounts of Mn.
erties of mixed oxides and activity for selective oxidation of CO in hydrogen-rich streams [35]. 3.3. Redox property of catalysts Fig. 3 shows H2 -TPR profiles of MnOx -CuO-CeO2 catalysts with different amounts of Mn, and Table 2 lists the reduction temperature and hydrogen consumption of all peaks. In our previous studies [32], we have found that three reduction peaks appeared in the profile of CuO-CeO2 catalysts prepared by a hydrothermal method at a temperature of 100–300 ◦ C. The low-temperature peak at about 141 ◦ C (peak ␣) is attributed to the reduction of the CuOx species which strongly interact with the ceria, while the high-temperature peak at about 257 ◦ C (peak ␥) is attributed to the reduction of Cu+ species regarded as the active centers of CO adsorption over the CuO-CeO2 catalysts [36,37]. In addition, an overlapping reduction peak (peak , at about 192 ◦ C) between peak ␣ and peak ␥ is attributed to the reduction of dispersed CuOx species on the surface of CeO2 , which includes isolated Cu2+ ions that weakly interact with CeO2 and the two- and three-dimensional copper clusters in small size. From Fig. 3 and Table 2, one can see that the reduction temperature of peak ␣ slightly shifts to higher temperature and its hydrogen consumption is obviously increased with the increase of manganese content. The hydrogen consumption of peak ␣ in the catalyst with Mn:Cu = 1:5 is the largest, reaching 282 mol/gcat, on increase of 30% compared with the catalyst without Mn (Mn:Cu = 0:1). According to the literature [38,39], coexistence of copper ions and manganese ions can be mutually promoting each other’s reduction, but the overlapping peaks of their reduction are difficult to distinguish. This result shows that there may be more Mn4+ reduced to Mn3+ at lower temperature. However, manganese presents a lower reducibility than copper due to the more negative free energy of formation of manganese oxides in comparison to that of copper oxide [40,41], so the reduction temperature of peak ␣ in the catalysts doped with manganese shifts slightly to the higher temperature. However, the reduction temperature of peak ␥ is gradually decreased with the increase of Mn content, while the one with Mn:Cu = 1:5 shows the lowest temperature of peak ␥. With the increase of Mn content, its hydrogen consumption increases obviously. The XRD results above have also showed that the existence of a small amount of Mn could increase the number of copper ions and manganese ions entering into the ceria lattice so as to strengthen their interaction. Therefore, the number of Cu+ species would be increased and its reducibility is also improved with the increasing Mn content when the Mn:Cu molar ratio is lower than 1:5. But when an excessive amount of Mn (Mn:Cu = 1:2.5) is added, the reduction temperature of peak ␥ increases significantly again because of the
appearance of CuO crystal phase. Therefore, the peak ␥ in the catalyst with Mn:Cu = 1:2.5 should be attributed to the reduction of a mixture of CuO crystal phase and Cu+ species. In addition, with Mn content increasing the hydrogen consumption of peak  has a decreasing trend accompanied with the changes in peak ␣ and peak ␥. In order to obtain information of oxidation states of the surface elements, we analyzed XPS spectra of the O 1 s, Mn 2p and Cu 2p (Fig. 4(a–c)), and we list the data about surface MnOx and CuOx species derived by XPS in Table 3. From Fig. 4(c) and Table 3, it can be seen that the peak of Cu 2p3/2 is centered at 932.7 and 933.9 eV in the MnOx -CuO-CeO2 catalysts. According to the literature [42,43], the binding energy of Cu 2p3/2 of copper cations in CuO and Cu2 O are greater than 933.1 eV and 932.2–933.1 eV, respectively. The values indicate that active sites of Cu+ exist in the sample and their number increases with Mn doping. But there is an obvious difference in Cu content on the surface, which is 19.0, 26.8 and 23.8 at.% for the catalysts with Mn:Cu = 1:5, 1:2.5 and 0:1, respectively. This indicates that doping a small amount of Mn may cause more copper oxide to enter the ceria lattice, forming a more stable solid solution; however, the presence of excessive Mn can affect the dispersion of copper oxide, separating some copper oxide from solid solution. And then the separated copper oxide would migrate to the surface of cerium oxide. Generally, better catalytic activity can be obtained with more stable Cu–Ce–O solid solutions. Fig. 4(b) shows the Mn 2p region spectra for the samples. Mn occurs in 2+ (640.9 eV), 3+ (641.8 eV) and 4+ (642.5 eV) oxidation states in cerium-free sample reported in the literature [32], so the Mn 2p3/2 region is presented as deconvoluted into all its components and its relative ion ratios are listed in Table 3. It can be seen that the actual ratio of manganese to copper on the surface of the catalyst with Mn:Cu = 1:5 is 0.37, higher than the theoretical ratio 0.2, with Mn existing mainly as Mn4+ oxidation state; however, the actual ratio of manganese to copper on the surface of catalyst with Mn:Cu = 1:2.5 is 0.3, lower than the theoretical ratio 0.4, with Mn existing mainly as inactive Mn2+ . The redox property of the catalyst may be enhanced by a large number of Mn4+ ions in high valence state existing on the surface, which is easily reduced.
Table 3 Surface of the Cu content and composition of Mn species derived by XPS. Sample
Mn:Cu = 1:2.5 Mn:Cu = 1:5 Mn:Cu = 0:1
Mn/Cu
0.30 0.37 0
Cu/(Cu + Ce + Mn) (at.%)
26.8 19.0 23.8
Mn 2p3/2 (at.%) Mn2+
Mn3+
Mn4+
61.6 22.7
13.5 5.7
25.3 71.6
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265
Fig. 6. Stability test for CO oxidation with time-on-stream over the MnOx -CuO-CeO2 catalyst with Mn:Cu = 1:5 at 110 ◦ C.
as A584 /A462 ). And the more the relative concentration of oxygen vacancies, the better the performance of the catalyst. From Fig. 5(b), one can see that the relative amount of oxygen vacancies of catalyst with Mn:Cu = 1:5 is bigger than those for other catalysts. The reaction mechanism of CO oxidation on CeO2 is considered to be a redox reaction involving lattice oxygen and oxygen vacancies. Based on the results of both XPS and UV-Raman, one can deduce that the increasing of the lattice oxygen and oxygen vacancies will be conducive to CO oxidation. 3.4. Stability test of catalyst
Fig. 5. UV-Raman profiles (a) and the A584 /A462 ratio (b) of MnOx -CuO-CeO2 catalysts prepared with different amounts of Mn.
This is in agreement with the previous results of H2 -TPR. In addition, a metal oxide catalyst possessing numerous oxidation states to facilitate electron transfer processes will be an efficient catalyst for oxidation of CO [44]. To investigate the oxygen species on the surface, we used the O 1s spectra as displayed in Fig. 4(a). The O 1s spectra of catalysts are fitted with two peak contributions. The main peak with a BE of about 529.8 eV is attributed to the lattice oxygen of metal oxide, while a shoulder peak at about 532.3 eV belongs most likely to adsorbed oxygen or oxygen in hydroxyl-like groups [35]. Compared with the Mn:Cu = 1:2.5 catalyst, the main peak intensity of the catalyst with Mn:Cu = 1:5 is stronger and the shoulder peak intensity is weaker. Such results indicates that the lattice oxygen amount of catalyst with Mn:Cu = 1:5 is more than that of the catalyst with Mn:Cu = 1:2.5 and that a part of adsorbed oxygen transforms into lattice oxygen. The selective oxidation of CO on metal oxides conforms to the following mechanism: (1) CO + Olatt. → CO2 + Ovac. , (2) O2 + 2Ovac. → 2Olatt. ; formula (1) is the rate controlling step of the above-mentioned redox reaction. Obviously, the increasing of lattice oxygen will be favorable to the selective oxidation of CO. Fig. 5 shows the profiles of UV-Raman (a) and the A584 /A462 ratio (b) of catalysts with different amounts of Mn. According to the literature [34], the relative concentration of oxygen vacancies can be represented by the area ratio of peaks 584 and 462 cm−1 (noted
A long-term stability test for the selective oxidation of CO in hydrogen-rich streams over the catalyst with Mn:Cu = 1:5 was performed, the CO conversion and the selectivity of O2 –CO oxidation reaction with time at reaction atmosphere are shown in Fig. 6. The results show that CO conversion remained at about 99.3% at the reaction temperature of 110 ◦ C and no obvious deactivation was observed for up to 50 h, although the selectivity of O2 –CO oxidation reaction slightly decreased, indicating that catalytic performance of the catalyst with Mn:Cu = 1:5 for CO oxidation in hydrogen-rich streams is well sustained. 4. Conclusion In this study, a series of CuO-CeO2 catalysts doped with different amounts of Mn have been prepared by a hydrothermal method and characterized by XRD, BET, H2 -TPR, XPS and UV-Raman techniques. The catalytic performance of CuO-CeO2 catalysts for the selective oxidation of CO in hydrogen-rich streams are strongly increased by doping a small amount of Mn and, particularly, the catalyst with Mn:Cu = 1:5 shows the best catalytic activity. The CO conversion can reach 99.8% at 120 ◦ C, the operating temperature “window” for the CO conversion > 99% is from 110 to 140 ◦ C. An important reason is that a more stable solid solution with a larger surface area and smaller particle size is formed after doping Mn. The formation of a more stable solid solution enhances the redox properties and the interactions among the various components, and the higher surface area and smaller particle size can afford more unsaturated coordination sites exposed to gas molecules and thus increase the catalytic activity. Furthermore, if we considering the reaction mechanism of CO oxidation on CeO2 as a redox reaction involving lattice oxygen and oxygen vacancies, the increased number of lattice oxygen and oxygen vacancies must be one of reasons
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for the observed high catalytic performance. Our results show that CuO-CeO2 catalysts doped with an appropriate content of Mn is favorable for CO oxidation. Thus, this type of catalyst is encouraging for the selective oxidation of CO in hydrogen-rich streams for the broader operating temperature “window” of the high conversion at lower temperature.
[17] [18] [19] [20] [21]
Acknowledgements
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We gratefully acknowledge the financial support from the Ministry of Science and Technology of China (no. 2004 CB 719504).
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