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Structural and catalytic aspects of sol–gel derived copper manganese oxides as low-temperature CO oxidation catalyst
Applied Catalysis A: General 302 (2006) 257–263 www.elsevier.com/locate/apcata
Structural and catalytic aspects of sol–gel derived copper manganese oxides as low-temperature CO oxidation catalyst M. Kra¨mer, T. Schmidt, K. Sto¨we, W.F. Maier * Institut fu¨r Technische Chemie, Universita¨t des Saarlandes, Postfach 151150, D-66041 Saarbru¨cken, Germany Received 8 September 2005; received in revised form 22 December 2005; accepted 17 January 2006 Available online 3 March 2006
Abstract The influence of the preparation method on structural properties and on the catalytic activity of sol–gel derived binary copper manganese oxides for CO oxidation in dry air at room temperature has been investigated. The catalysts were characterised by means of BET and X-ray diffraction. All results were compared with a commercial Hopcalite sample. Subsequent conventional optimisation of the most active sample – prepared by the socalled ethylene–glycol method (EG I) – including compositional and calcination temperature variation revealed Cu20Mn80Ox (EG I) calcined at 400 8C as very active catalyst. Sintering processes induced by the crystallisation of the amorphous material above 400 8C cause a dramatic decrease of the catalysts’ surface area accompanied by deactivation. XPS data recordings were used to investigate the oxidation states of the metals in this catalyst and in the reference material. In conclusion a combination of high surface area, amorphous state and the presence of Cu2+ and Mn3+ was found to be essential for the high catalytic activity of the binary copper manganese oxides. The reactivity of the best catalyst was tested under application-relevant conditions, i.e. in moist air and at low-temperature fuel cell conditions. # 2006 Elsevier B.V. All rights reserved. Keywords: Copper manganese oxide; Low-temperature CO oxidation; Catalyst; Sol–gel
1. Introduction The catalytic oxidation of CO has long been established for exhaust gas cleaning, e.g. in automobile or industrial emission control. Operation at low temperature is required for application in Proton Exchange membrane fuel cells and breathing protection. In this context Haruta et al. demonstrated that supported gold catalysts are active even at temperatures below 0 8C [1,2]. Further examples of catalysts for the low temperature CO oxidation are cobalt-based oxides [3,4] or the copper/manganese/oxygen system, summarised under the trade name Hopcalite [5–7], which is still – due to its low cost – the catalyst used for respiratory protection. Since its discovery at the beginning of the last century, the Hopcalite system has been the matter of many intensive studies. In spite of these huge efforts only little is known about the origin for its high catalytic activity, the active sites and the mechanism of oxidation. Very active in the amorphous state, Hopcalite loses activity at higher temperatures where
* Corresponding author. Tel.: +49 681 302 2582; fax: +49 681 302 2343. E-mail address: [email protected] (W.F. Maier). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.01.018
crystallisation of CuMn2O4 takes place. This partially inverse spinel AB2O4 phase, which can be described as (Cua+Mn1 a2+)A(Cu1 a2+Mna3+Mn4+)BO42 , is characterised by the coexistence of differently charged copper and manganese ions in tetrahedral (A sites) as well as octahedral (B site) sublattices [8]. It is suggested that the existence of the resulting redox system Cu2+ + Mn3+ ? Cu+ + Mn4+ is responsible for the reactivation of the catalyst and for that reason crucial to the high oxidative performance of these materials [9,10]. Nevertheless, this assumption does not comply with the observation of deactivation during crystallisation of the spinel phase. For that case, the influence of surface inhibitors [11], a spillover model with manganese oxide acting as oxygen donator and copper oxide as oxygen acceptor [12] and shifts in the predominantly existing redox couples [10,11] have been discussed in the literature. It is generally accepted that an enhancement of the Cu+ and Mn4+ concentrations is resulting in a deactivation of the material [10,11]. The starting point of our study was the search of new noble metal free catalysts for the oxidation of CO at low temperatures with high-throughput techniques based on emissivitycorrected IR-thermography [13]. Due to the highly exothermic character of this reaction ( 283.2 KJ/mol) and the lack of side
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or secondary processes under oxidative conditions, this method, which allows the rapid examination of catalytic activities of potential catalysts by recording relative heats of reaction in a highly parallel approach, is highly appropriate for the study of the CO oxidation. The automated syntheses of the materials libraries were performed with the help of a pipetting robot. Starting with alcoholic solutions of eight transition metals, cerium and aluminium (as nitrates, propionates or alkoxides) primary, binary, ternary and quaternary mixed metal oxides (MMOs) were synthesised using sol–gel methods. The calcined samples were transferred manually into the 207 wells of slate libraries and screened for CO oxidation at 50 8C. However, the examination of more than 2000 diverse MMOs did not result in significant new findings: the cobalt-manganese- and copper-manganese-based oxides were identified as the catalytically most active samples. Since the cobalt-based oxides had already been optimised by compositional variation and doping [3,14], we focussed on the examination of the copper/manganese/oxygen system. Conventional experiments evinced that no satisfying reproducibility could be found by scaling-up the recipe applied in the high-throughput experiments leading us to adopt various recipes for the preparation of cobalt- and cerium-based oxides on the copper manganese oxide. In contrast to the traditional coprecipitation synthesis, preparation of these binary oxides was carried out, similarly to the high-throughput experiments, by sol–gel methods. These methods are well established for the preparation of microporous or mesoporous materials with a homogeneous distribution of different elements and a high-surface area [15]. Especially the high-surface area in the case of the copper manganese oxides is supposed to be important for the catalytic performance [16]. Additionally, this method allows syntheses of copper manganese oxides at mild temperatures resulting in the thermodynamically unfavourable but catalytically more interesting Cu2+/Mn3+-system [10]. In this study, different preparation methods for copper manganese oxides were investigated and their influence on the catalytic and structural properties was examined. The results are compared with those of a commercial Hopcalite sample. 2. Experimental 2.1. Preparation of catalysts The copper manganese oxides with the nominal copper/ manganese atomic ratio = 20/80 were prepared by various sol–gel-syntheses (samples 1–5). Samples with a copper/ manganese atomic ratios of 10/90, 15/85, 20/80, 30/70, 40/60 and 50/50 were synthesised following the ethylene–glycol method (according to sample 3). The following metal precursors were used: Cu(NO3)23H2O (Fluka, purum p.a.) and Mn(NO3)24H2O (Merck, analysis grade) or Cu(C2H5COO)2 (Avocado, 98%) and Mn(C2H5COO)2, which was prepared from the respective metal nitrate according to Ref. [17]. Aging, drying and calcination were performed under static air in all cases.
2.1.1. Sample 1: the pentanone method (Pn) 1 M solution of the metal nitrates in methanol was stirred at 25 8C for 1 h together with three equivalents of complexing agent (4-hydroxy-4-methyl-pentanone) and propionic acid in catalytic quantities. After aging for 24 h at room temperature, the samples were dried at 40 8C and calcined at 300 8C for 5 h. 2.1.2. Sample 2: the propionic acid method (PA) The samples were prepared analogously to the cobalt-based oxides of Ref. [18]. The metal propionates were dissolved in hot propionic acid and stirred for 0.5 h at 150 8C before being aged at 25 8C for 4 days. The samples were calcined at 300 8C for 5 h. 2.1.3. Samples 3–5: the ethylene–glycol method (EG) The preparations were carried out in accordance to a process for the synthesis of ceramic thin films [19] by dissolving the metal nitrates in a mixture of ethylene–glycol, water and concentrated nitric acid. The subsequent drying procedure for the different MMOs was varied. For the formation of sample 3 (EG I), the solution was stirred for 2 h under reflux at 105 8C before drying at the same temperature for an additional 16 h. Sample 4 (EG II) was dried during 1 h, while sample 5 (EG III) was stirred at 65 8C for 5 d. The calcination was carried at temperatures of 400 8C for 5 h. Additionally, different catalysts were prepared according to sample 3 by varying the calcination temperature between 350 and 500 8C. This is indicated by giving the acronym for the synthesis method together with the calcination temperature (in 8C), e.g. EG I_350. As reference material a commercial Hopcalite was used. To generate a crystalline mixed-valence reference material for XPS analysis, a physical mixture of CuO (Merck, analysis grade) and Mn2O3 (Alfa Aesar, 98%) (nominal copper/ manganese atomic ratio = 20/80) was pressed into a pellet and annealed at 1000 8C for 7 d according to conventional solid state synthesis principles [Cu20Mn80Ox (1000)]. 2.2. Catalytic activity The catalytic tests were performed in a fixed-bed flow reactor with 100 mg of the catalyst (particle size 100–200 mm) at 25 8C. After pre-treatment for 1 h in synthetic air followed by 0.5 h in nitrogen at 200 8C, the reaction was started at the total flow rate of 30,000 ml h 1 g 1 (1 vol.% CO in synthetic air) controlled by mass flow controllers. Experiments with moist air were carried out analogously by passing the reactant gas through a washing bottle filled with water prior to the reactor. The relative humidity under these conditions was determined by means of a hygrometer. Tests near low-temperature fuel cell conditions were carried out under identical reaction conditions using a different feed gas composition (1 vol.% CO, 4.88 vol.% synthetic air, 20 vol.% CO2 in hydrogen) and varying the reaction temperature between 60 and 120 8C. The products were analyzed by IR gas sensors (GfG mbH, customised production). The reliability of the gas-phase analysis was controlled by GC measurements (Micro-GC model CP 4900, ˚ with He as carrier Varian, micro-channel molecular sieve 5 A
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gas) which was also used for the analysis of the experiments in hydrogen-rich gas mixtures. The selectivity was calculated based on the converted CO and oxygen.
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different metal cations [19] the drying procedure was adopted to the different hydrolysis rates of the precursor complexes resulting in variations of phase compositions and crystallinity. The so-synthesised oxides were tested for CO oxidation at 25 8C and characterised by BET and powder XRD measurements. The results are compared with the corresponding values of the commercial Hopcalite sample in Table 1. A significant influence of the preparation method on the properties of the different samples is evident. Compared with the CO conversion of the reference catalyst, the performances of the catalysts of identical composition synthesised by different methods varied between completely inactive and highly active. While the chemical nature of the employed metal precursor, the complexing agent and the drying procedure of the ethylene–glycol method are strongly affecting the microstructure of the materials, the oxidation activity shows a significant correlation only with the surface area of the respective MMO. This is in agreement with Hutchings et al. who pointed out that the surface area is of importance for the catalytic performance of copper manganese oxides [16]. In addition to the activity variation huge differences in the phase composition and crystallinity of the variously synthesised oxides were discovered by X-ray diffraction. Cu20Mn80Ox (EG I_400) as well as the reference catalyst showed no distinct diffraction peaks at ambient temperature, which can be unambiguously assigned to discrete compounds described in the literature. The XRD pattern of all other, less active Cu20Mn80Ox samples revealed diffraction peaks corresponding to Mn2O3 (ICPDS 78-390), Mn5O8 (ICPDS 72-1427), MnO2 (ICPDS 82-2169) Cu1+xMn2 xO4 (ICPDS 70-260, 711144, 71-1145) or CuO (ICPDS 72-629), and a surprising crystallinity considering especially the lower calcination temperatures of 300 8C. Elemental analyses revealed small amounts of carbon in all samples (see Table 1) whereas a thermal treatment at higher temperatures was generally observed to reduce the carbon contents. The higher amount of carbon in Cu20Mn80Ox (Pn) could be responsible for the very low catalytic activity despite its relatively high surface area. Nevertheless, the origin of the low catalytic activity for CO conversion of the EG III_400 is not known. The desired reproducibility of the most active Cu20Mn80Ox sample (EG I_400) could be confirmed several times.
2.3. Characterisation The total BET surface area was measured with a Carlo Erba Sorptomatic 1990. CHN-elemental analyses were performed with a Leco elemental analyzer CHN-900. The powder X-ray diffraction (XRD) patterns were obtained with a Huber G670 ˚ ). Guinier camera with Cu Ka1-radiation (l = 1.54056 A Temperature-resolved XRD measurements were carried out under static air in an Anton-Paar HTK1200 chamber attached to a Siemens D5000 diffractometer with Cu Ka1-radiation and a heating rate of 1 8C/min. X-ray photoelectron spectra were recorded by a VG ESCALAB MK II spectrometer using Mg Ka radiation which is designed for angular resolved XPS, UPS and EELS as described in Ref. [20–22]. 3. Results and discussion 3.1. Influence of the preparation method It is well known that the structural and catalytic properties of copper manganese mixed oxide catalysts often vary significantly with the preparation method [23,24]. Since irreproducible activities are not acceptable, we assumed that they resulted from the preparation procedure itself and searched for alternative, more reproducible synthesis methods for catalytically active copper manganese oxides. Several modified sol–gel methods have been tested for the preparation of Cu20Mn80Ox. The atomic ratio was selected because all prior experiments point to a maximum of catalytic activity at or close to this composition. The synthesis methods studied differ by the metal precursors employed, the solvent and the complexing agent. In the pentanone method (Pn), 4hydroxy-4-methyl-pentanone was used as complexing agent whereas the metal propionates were dissolved in hot propionic acid in the PA method. For the ethylene–glycol method (EG), where the formation of polymeric precursors consisting of ethylene–glycol and its oxidation product oxalic acid is believed to be crucial for a homogeneous distribution of the
Table 1 CO conversion after 1 h at 25 8C (WHSV = 30,000 ml g 1 h 1, 1 vol.% CO in synthetic air), BET surface area, carbon content and crystalline phases identified by XRD of the variously prepared oxides with a Cu/Mn atomic ratio = 20/80 and of the reference material Catalyst
CO conversion after 1 h (%)
BET surface area (m2/g)
Carbon content (wt.%)
Crystalline state
Pn PA EG I_400 EG II_400 EG III_400 EG I_350 EG I_450 EG I_500 Hopcalite
4 11 61 26 1 52 20 17 43
93 51 160 68 49 185 67 55 174
0.51 0.26 0.18 0.13 0.21 0.37 0.19 0.04 0.13
Mn5O8 Mn5O8 Nearly amorphous Weak peaks corresponding to Mn2O3, MnO2 and CuO Mn2O3, Mn5O8, Cu1+xMn2 xO4 Amorphous Mn2O3, Cu1+xMn2 xO4 Mn2O3, Cu1+xMn2 xO4 Amorphous
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3.2. Influence of the calcination temperature and of the composition For further optimisation of the copper manganese catalyst, the effect of compositional variations and changes in calcination temperatures on the catalytic activity of the EG I_400 catalyst was examined. As shown in Fig. 1 the samples calcined at 400 8C showed the highest CO conversions of all catalysts. Treatments at higher temperatures resulted in significant deactivations and the activity of the samples annealed at 350 8C was also lower than that of the catalysts treated at 400 8C. The latter effect could be explained by a higher carbon content of the samples. In accordance to all prior experiments on non-commercial catalysts, but also in accordance to the commercial catalyst studied by Rice et al. [25], the best composition for all MMOs calcined at different temperatures is still at the original copper/manganese atomic ratio of 20/80. Standardless elemental analysis of the Hopcalite reference by X-ray fluorescence spectroscopy pointed to a copper/ manganese atomic ratio of 40/60 with small amounts of calcium. XPS analysis revealed also significant nitrogen impurities. Other authors observed activity maxima for the Hopcalite catalyst at a copper/manganese atomic ratio of 10/ 90 and of 50/50 [23,26]. Further fine tuning of the composition of our most active system (EG I_400) confirmed the highest activity for the copper/manganese = 20/80 composition, but it also documents a rather weak dependence of the catalytic activity on the copper content (see Fig. 2). The soprepared oxides showed in the range of Cu15Mn85Ox– Cu40Mn60Ox higher CO conversions than the commercial reference catalyst. Lower copper contents caused a significant decrease of the catalyst’s activity. Note that the surface areas for all these samples treated at 400 8C were comparable. It is well-known that crystallisation of amorphous Hopcalite catalysts causes vast sintering leading to a decrease of the surface area and thus of the catalytic activity [11,27]. In our
Fig. 1. The influence of composition and calcination temperature (Tc) on the catalytic activity for the oxidation of CO with synthetic air at 25 8C of the copper manganese oxides (EG I). Reaction conditions: 100 mg (100–200 mm) catalyst, flow rate 50 ml/min, feed gas: 1 vol.% CO in synthetic air.
Fig. 2. Composition dependency of the catalytic activity for the oxidation of CO with synthetic air at 25 8C (10/90 = copper/manganese atomic ratio, 100 mg (100–200 mm) catalyst, 50 ml/min total flow, 1 vol.% CO in synthetic air).
case the amorphous to spinel transition was taking place at relatively low temperatures between 400 and 450 8C. While the samples treated at 350 or 400 8C revealed surface areas close to that of the Hopcalite, the surface area decreased by a factor of 3 after thermal treatments above that temperature (see Table 1). In contrast to Vepreˆk et al., who found an activity decrease by a factor of 20 with a surface area decrease by a factor of 5 [11], in our case the loss of activity correlated with the decrease in the area. Fig. 3 shows the influence of a further heat treatment on the crystallinity of Cu20Mn80Ox (EG I_400). Amorphous or poorly crystalline in the calcined state, the formation of copper manganese spinel (JCPDS No. 70-0260) and of Mn2O3 (JCPDS No. 73-1826) was induced by subsequent heating. The same crystalline phases could be identified in the mixed oxides calcined at 450 and 500 8C, respectively, whereas the relative amounts of these two phases varied with the total composition of the samples. Compared to EG I_400 the much less active Cu20Mn80Ox catalysts derived by different synthesis methods showed all various crystalline oxides (see Table 1). This indicates that surface and/or structural properties are strongly affecting the catalytic activity of these binary mixed oxides. Furthermore X-ray photoelectron spectroscopy was performed to acquire detailed information on the oxidation states of both metals. In the literature the existence of the Cu2+ + Mn3+ ? Cu+ + Mn4+ resonance system is postulated as one of the origins of the high catalytic performance of the Hopcalite catalyst [9–11]. XPS surface analysis performed by Porta et al. revealed that the existence of this redox system is closely connected to the cubic spinel (Cua+Mn1 a2+)A(Cu1 a2+Mna3+Mn4+)BO42 [28] which was confirmed by other authors [10,11]. Vepreˆk et al. analysed the thermal activation and deactivation of a commercial Hopcalite catalyst [11]. X-ray amorphous in the activated form, thermal treatments above 550 8C caused the crystallisation of the cubic spinel phase whereas the oxidation states of both metals changed from Cu2+/Mn3+ in the active sample to a mixture of Cu+/Cu2+ and of Mn3+/Mn4+ in the less active, crystallised one. The authors concluded that deactivation and change in the
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Table 2 XPS binding energies of Cu20Mn80Ox (EG I_400) and Hopcalite for characteristic signals
Cu20Mn80Ox (EG I_400) Hopcalite Cu20Mn80Ox (1000)
Cu-2p3/2 (eV)
Mn-3s (eV)
Mn-2p3/2 (eV)
933.2 933.2 932.6, 930.2
83.2 83.6 83.0
641.2 641.4 641.0
oxidation states from Cu2+ to Cu+ and from Mn3+ to Mn4+ are interrelated. Fortunato et al. approved these results by preparing two cubic CuMn2O4 spinels of identical overall composition but with varying Cu+/Cu2+ ratios due to different synthesis methods [10]. In their study the catalyst with lower Cu+ concentration was manifestly more active than the sample with a high content of univalent copper. In accordance with these results only Cu2+ and Mn3+ could be found in both Cu20Mn80Ox (EG I_400) and the reference catalyst by XPS analyses. The unsplit 2p3/2 part of the Cu-2p signal with a binding energy of Cu-2p3/2 = 933.2 eV indicates the existence of Cu2+ (Fig. 4, Table 2). Considering on the other hand manganese, the Mn-3s multiplet splitting was found by Wertheim et al. to decrease from MnO (6.05 0.04 eV) to Mn2O3 (5.50 0.10 eV) and MnO2 (4.58 0.06 eV) [29]. In
agreement with their results the oxidation state of manganese in Cu20Mn80Ox (EG I_400) could be determined as Mn3+ (splitting 5.4 eV, see Fig. 5). Due to the lower manganese content in the commercial sample, this multiplet splitting could not be used to unambiguously deduce the oxidation state in the Hopcalite catalyst. Comparisons of the Mn-2p spectra of both catalysts seem to indicate the same oxidation state of manganese in both Cu20Mn80Ox (EG I_400) and Hopcalite. A further Cu20Mn80Ox sample was prepared by a conventional solid state synthesis method. XRD measurements revealed diffraction peaks corresponding to Cu1+xMn2 xO4, Mn2O3 and Mn3O4. XPS surface analysis of this sample confirmed former results revealing that the Cu2+ + Mn3+ ? Cu+ + Mn4+ resonance system plays an important role in the partially inversed (Cua+Mn1 a2+)A(Cu1 a2+Mna3+Mn4+)BO42 spinel phase [10,11,28]. This can be easily seen in Fig. 4, where both split Cu-2p spin-orbit components indicate the existence of Cu+ as well as Cu2+ (e.g. BE of Cu-2p3/2 = 930.2 and 932.6 eV, respectively). On the other hand the identification of Mn2+, Mn3+ and Mn4+ is very difficult especially due to the high fraction of Mn3O4. Mn-3s multiplet splitting (DE = 5.8 eV) as well as Mn-2p3/2 binding energy of 641.0 eV (Mn2+: 640.2 eV [30], Mn3+: 641.5 eVand Mn4+: 643.0 eV [31]) point at mixedvalent di- and trivalent manganese ions. It should be mentioned that the Mn-3s spectra of the high temperature treated copper manganese oxide (see Fig. 5(C)) shows a distinct change of the relative intensities of the main band to the satellite compared to the respective spectra of Cu20Mn80Ox (EG I_400). The reason could be an overlap of two duplets
Fig. 4. Cu-2p XPS spectra of Cu20Mn80Ox (EG I_400) (a), of the commercial Hopcalite sample (b) and of Cu20Mn80Ox (1000) (c).
Fig. 5. Mn-3s and Cu-3p XPS spectra of Cu20Mn80Ox (EG I_400) (a), of the commercial Hopcalite sample (b) and of Cu20Mn80Ox (1000) (c).
Fig. 3. XRD patterns of Cu20Mn80Ox (EG I_400) as a function of temperature.
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Fig. 6. Deactivation of Cu20Mn80Ox (EG I_400), Pt1Cu20Mn79Ox (EG I_400) and Hopcalite during the oxidation of CO in moist air at 25 8C. Reaction conditions: 100 mg (100–200 mm) catalyst, flow rate 50 ml/min, feed gas: 1 vol.% CO in synthetic air; humidity: 84%.
which are caused by a mixture of Cu1+xMn2 xO4 and Mn3O4. More detailed investigations to elucidate this phenomenon are currently in progress. Table 2 summarises the corresponding XPS binding energies. 3.3. Catalytic tests with humid air and near PEM fuel cell conditions Modified sol–gel derived Cu20Mn80Ox (EG I_400) could be identified as very active catalyst for the oxidation of CO at room temperature in dry air. For respiratory and/or fuel cell applications catalytic stability in moist air is essential. Commercial Hopcalite is well known to deactivate under these conditions within a few minutes [5,7]. For that task a positive effect of noble metals like palladium [32] or gold [33] was found. Maier and Saalfrank identified a significant improve-
Fig. 7. Selective oxidation in hydrogen rich atmosphere at different temperatures on Cu20Mn80Ox (EG I_400). Reaction conditions: 100 mg (100–200 mm) catalyst, flow rate 50 ml/min, feed gas: 1 vol.% CO, 4.88 vol.% synthetic air; 20% CO2 in H2. Selectivity: 80%.
ment of moisture resistance by doping their cobalt-based mixed oxide with a small amount of platinum [34]. As can be seen from Fig. 6, Cu20Mn80Ox (EG I_400), Pt1Cu20Mn79Ox (EG I_400), and the reference material deactivate after a few minutes in moist air (humidity: 84%). In agreement with prior results where an impregnation of the reference material with 0.5 wt.% of platinum did not improve its moisture resistance, our attempts to improve the EG I_400 catalysts by platinum addition failed [18]. For application in low-temperature fuel cells the selective oxidation of CO in hydrogen rich atmosphere is also of importance. Several noble metal containing materials are described to catalyse this reaction with high selectivities [35,36]. Furthermore CuO–CeO2 as noble-metal free catalyst showed CO conversions of 80% at nearly 100% selectivity [37]. In this context Cu20Mn80Ox (EG I_400) was investigated for CO oxidation in hydrogen rich feed gas. Fig. 7 shows the effect of temperature on the activity of the catalyst which was observed to increase with temperature while the selectivity of CO oxidation over H2 oxidation was higher than 80% at all temperatures. 4. Conclusion The strong influence of the preparation method on the structure and the catalytic performance for the oxidation of CO at room temperature of sol–gel derived copper manganese oxides were investigated. The nature of the metal precursors, solvents, complexing agents as well as the drying procedure were found to affect the catalytic performance significantly. BET surface area and X-ray diffraction showed that the best catalyst, prepared with the help of the ethylene–glycol method (EG I), was nearly amorphous with a high surface area comparable to the commercial Hopcalite sample. For further optimisation of this catalyst, the influence of the copper content and of the calcination temperature on the catalytic activity was examined. The optimal copper/ manganese ratio was found to be 20/80 in all cases. Catalysts calcined at 400 8C were most active whereas sintering processes – induced by the crystallisation of Mn2O3 and the spinel Cu1+xMn2 xO4 – at higher temperatures resulted in a drastically decrease of the surface area and the catalytic performance. It was surprising to find that a copper content of 15 through 40 at.% did not affect the catalytic performance significantly. Due to the low crystallinity of the most active materials the amorphous mixed oxide may represent the catalytically most active phase. The oxidation states of copper and manganese in both Cu20Mn80Ox (EG I_400) and Hopcalite were found to be Cu2+ and Mn3+ by means of X-ray photoelectron spectroscopy. Final experiments were performed to test a potential application of this catalyst in breathing protection and/or for proton exchange membrane fuel cells PEMFC. In moist air, our catalyst deactivated to a similar degree as the reference catalyst does. In the tests near low temperature fuel cell conditions (in hydrogen rich atmosphere), Cu20Mn80Ox (EG I_400) showed relatively high selectivities for CO.
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Acknowledgements The authors thank Dr. F. Mu¨ller, UdS, for performing the XPS measurements as well as Dr. H. Natter, UdS, for recording the temperature resolved XRD patterns. References [1] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon, J. Catal. 144 (1993) 175. [2] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301. [3] J.W. Saalfrank, W.F. Maier, Angew. Chem. Int. Ed. 43 (2004) 2028. [4] J. Jansson, J. Catal. 194 (2000) 55. [5] A.B. Lamb, W.C. Bray, J.C. Frazer, J. Ind. Eng. Chem. 12 (1920) 213. [6] H.A. Jones, H.S. Taylor, J. Phys. Chem. 27 (1923) 623. [7] G.-M. Schwab, G. Drikos, Z. Phys. Chem. 185 (1940) 405. [8] A. Waskowska, L. Gerward, J.S. Olsen, S. Steenstrup, E. Talik, J. Phys.: Condens. Matter 13 (2001) 2549. [9] G.-M. Schwab, S.B. Kanungo, Z. Phys. Chem. 107 (1977) 109. [10] G. Fortunato, H.R. Oswald, A. Reller, J. Mater. Chem. 11 (2001) 905. [11] S. Vepreˆk, L. Cocke, S. Kehl, H.R. Oswald, J. Catal. 100 (1986) 250. [12] F.C. Buciuman, F. Patcas, T. Hahn, Chem. Eng. Process. 38 (1999) 563. [13] A. Holzwarth, H.-W. Schmidt, W.F. Maier, Angew. Chem. Int. Ed. 37 (1998) 2644. [14] J.W. Saalfrank, W.F. Maier, C. R. Chim. 7 (2004) 483. [15] J. Scheidtmann, P.-A. Weiß, W.F. Maier, Appl. Catal. A 222 (2001) 79. [16] G.J. Hutchings, A.A. Mirzaei, R.W. Joyner, M.R.H. Siddiqui, S.H. Taylor, Appl. Catal. A 166 (1998) 143. [17] G. Spinolo, S. Ardizzone, S.J. Trasatti, Electroanal. Chem. 423 (1997) 49.
263
[18] J.W. Saalfrank, PhD thesis, Universita¨t des Saarlandes, 2003. [19] C.C. Chen, M.M. Nasrallah, H.U. Anderson, J. Electrochem. Soc. 140 (1993) 3555. [20] F. Mu¨ller, P. Steiner, T. Straub, D. Reinicke, R. de Masi, S. Hu¨fner, Surf. Sci. 442 (1999) 485. [21] F. Mu¨ller, R. de Masi, P. Steiner, D. Reinicke, M. Stadtfeld, S. Hu¨fner, Surf. Sci. 459 (2000) 161. [22] F. Mu¨ller, R. de Masi, D. Reinicke, P. Steiner, S. Hu¨fner, K. Sto¨we, Surf. Sci. 520 (2002) 158. [23] S.B. Kanungo, J. Catal. 58 (1979) 419. [24] G.J. Hutchings, A.A. Mirzaei, R.W. Joyner, M.R.H. Siddiqui, S.H. Taylor, Catal. Lett. 42 (1996) 21. [25] J.K. Rice, J.R. Wyatt, L. Pasternack, Appl. Catal. B 24 (2000) 107. [26] D. Dollimore, K.H. Tonge, J. Chem. Soc. A (1970) 1728. [27] P.A. Wright, S. Natarajan, J.M. Thomas, Chem. Mater. 4 (1992) 1053. [28] P. Porta, G. Moretti, M. Musicanti, A. Nardella, Solid State Ionics 63–65 (1993) 257. [29] G.K. Wertheim, S. Hu¨fner, H.J. Guggenheim, Phys. Rev. B 7 (1973) 556. [30] B. Hermsmeier, J. Osterwalder, D.J. Friedman, B. Sinkovic, T. Tran, C.S. Fadley, Phys. Rev. B 42 (1990) 11895. [31] V.R. Galakhov, M.A. Korotin, N.A. Ovechkina, E.Z. Kurmaev, V.S. Gorshkov, D.G. Kellerman, S. Bartkowski, M. Neumann, Eur. Phys. J. B 14 (2000) 281. [32] M.N. Desai, J.B. Butt, J.S. Dranoff, J. Catal. 79 (1983) 95. [33] M. Date, M. Haruta, J. Catal. 201 (2001) 221. [34] W.F. Maier, J. Saalfrank, Chem. Eng. Sci. 59 (2004) 4673. [35] O. Korotkikh, R. Farrauto, Catal. Today 62 (2000) 249. [36] R.J.H. Grisel, B.E. Nieuwenhuys, J. Catal. 199 (2001) 48. [37] G. Avgouropoulos, T. Ioannides, H.K. Matralis, J. Batista, S. Hocevar, Catal. Lett. 73 (2001) 33.
Structural and catalytic aspects of sol–gel derived copper manganese oxides as low-temperature CO oxidation catalyst M. Kra¨mer, T. Schmidt, K. Sto¨we, W.F. Maier * Institut fu¨r Technische Chemie, Universita¨t des Saarlandes, Postfach 151150, D-66041 Saarbru¨cken, Germany Received 8 September 2005; received in revised form 22 December 2005; accepted 17 January 2006 Available online 3 March 2006
Abstract The influence of the preparation method on structural properties and on the catalytic activity of sol–gel derived binary copper manganese oxides for CO oxidation in dry air at room temperature has been investigated. The catalysts were characterised by means of BET and X-ray diffraction. All results were compared with a commercial Hopcalite sample. Subsequent conventional optimisation of the most active sample – prepared by the socalled ethylene–glycol method (EG I) – including compositional and calcination temperature variation revealed Cu20Mn80Ox (EG I) calcined at 400 8C as very active catalyst. Sintering processes induced by the crystallisation of the amorphous material above 400 8C cause a dramatic decrease of the catalysts’ surface area accompanied by deactivation. XPS data recordings were used to investigate the oxidation states of the metals in this catalyst and in the reference material. In conclusion a combination of high surface area, amorphous state and the presence of Cu2+ and Mn3+ was found to be essential for the high catalytic activity of the binary copper manganese oxides. The reactivity of the best catalyst was tested under application-relevant conditions, i.e. in moist air and at low-temperature fuel cell conditions. # 2006 Elsevier B.V. All rights reserved. Keywords: Copper manganese oxide; Low-temperature CO oxidation; Catalyst; Sol–gel
1. Introduction The catalytic oxidation of CO has long been established for exhaust gas cleaning, e.g. in automobile or industrial emission control. Operation at low temperature is required for application in Proton Exchange membrane fuel cells and breathing protection. In this context Haruta et al. demonstrated that supported gold catalysts are active even at temperatures below 0 8C [1,2]. Further examples of catalysts for the low temperature CO oxidation are cobalt-based oxides [3,4] or the copper/manganese/oxygen system, summarised under the trade name Hopcalite [5–7], which is still – due to its low cost – the catalyst used for respiratory protection. Since its discovery at the beginning of the last century, the Hopcalite system has been the matter of many intensive studies. In spite of these huge efforts only little is known about the origin for its high catalytic activity, the active sites and the mechanism of oxidation. Very active in the amorphous state, Hopcalite loses activity at higher temperatures where
* Corresponding author. Tel.: +49 681 302 2582; fax: +49 681 302 2343. E-mail address: [email protected] (W.F. Maier). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.01.018
crystallisation of CuMn2O4 takes place. This partially inverse spinel AB2O4 phase, which can be described as (Cua+Mn1 a2+)A(Cu1 a2+Mna3+Mn4+)BO42 , is characterised by the coexistence of differently charged copper and manganese ions in tetrahedral (A sites) as well as octahedral (B site) sublattices [8]. It is suggested that the existence of the resulting redox system Cu2+ + Mn3+ ? Cu+ + Mn4+ is responsible for the reactivation of the catalyst and for that reason crucial to the high oxidative performance of these materials [9,10]. Nevertheless, this assumption does not comply with the observation of deactivation during crystallisation of the spinel phase. For that case, the influence of surface inhibitors [11], a spillover model with manganese oxide acting as oxygen donator and copper oxide as oxygen acceptor [12] and shifts in the predominantly existing redox couples [10,11] have been discussed in the literature. It is generally accepted that an enhancement of the Cu+ and Mn4+ concentrations is resulting in a deactivation of the material [10,11]. The starting point of our study was the search of new noble metal free catalysts for the oxidation of CO at low temperatures with high-throughput techniques based on emissivitycorrected IR-thermography [13]. Due to the highly exothermic character of this reaction ( 283.2 KJ/mol) and the lack of side
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or secondary processes under oxidative conditions, this method, which allows the rapid examination of catalytic activities of potential catalysts by recording relative heats of reaction in a highly parallel approach, is highly appropriate for the study of the CO oxidation. The automated syntheses of the materials libraries were performed with the help of a pipetting robot. Starting with alcoholic solutions of eight transition metals, cerium and aluminium (as nitrates, propionates or alkoxides) primary, binary, ternary and quaternary mixed metal oxides (MMOs) were synthesised using sol–gel methods. The calcined samples were transferred manually into the 207 wells of slate libraries and screened for CO oxidation at 50 8C. However, the examination of more than 2000 diverse MMOs did not result in significant new findings: the cobalt-manganese- and copper-manganese-based oxides were identified as the catalytically most active samples. Since the cobalt-based oxides had already been optimised by compositional variation and doping [3,14], we focussed on the examination of the copper/manganese/oxygen system. Conventional experiments evinced that no satisfying reproducibility could be found by scaling-up the recipe applied in the high-throughput experiments leading us to adopt various recipes for the preparation of cobalt- and cerium-based oxides on the copper manganese oxide. In contrast to the traditional coprecipitation synthesis, preparation of these binary oxides was carried out, similarly to the high-throughput experiments, by sol–gel methods. These methods are well established for the preparation of microporous or mesoporous materials with a homogeneous distribution of different elements and a high-surface area [15]. Especially the high-surface area in the case of the copper manganese oxides is supposed to be important for the catalytic performance [16]. Additionally, this method allows syntheses of copper manganese oxides at mild temperatures resulting in the thermodynamically unfavourable but catalytically more interesting Cu2+/Mn3+-system [10]. In this study, different preparation methods for copper manganese oxides were investigated and their influence on the catalytic and structural properties was examined. The results are compared with those of a commercial Hopcalite sample. 2. Experimental 2.1. Preparation of catalysts The copper manganese oxides with the nominal copper/ manganese atomic ratio = 20/80 were prepared by various sol–gel-syntheses (samples 1–5). Samples with a copper/ manganese atomic ratios of 10/90, 15/85, 20/80, 30/70, 40/60 and 50/50 were synthesised following the ethylene–glycol method (according to sample 3). The following metal precursors were used: Cu(NO3)23H2O (Fluka, purum p.a.) and Mn(NO3)24H2O (Merck, analysis grade) or Cu(C2H5COO)2 (Avocado, 98%) and Mn(C2H5COO)2, which was prepared from the respective metal nitrate according to Ref. [17]. Aging, drying and calcination were performed under static air in all cases.
2.1.1. Sample 1: the pentanone method (Pn) 1 M solution of the metal nitrates in methanol was stirred at 25 8C for 1 h together with three equivalents of complexing agent (4-hydroxy-4-methyl-pentanone) and propionic acid in catalytic quantities. After aging for 24 h at room temperature, the samples were dried at 40 8C and calcined at 300 8C for 5 h. 2.1.2. Sample 2: the propionic acid method (PA) The samples were prepared analogously to the cobalt-based oxides of Ref. [18]. The metal propionates were dissolved in hot propionic acid and stirred for 0.5 h at 150 8C before being aged at 25 8C for 4 days. The samples were calcined at 300 8C for 5 h. 2.1.3. Samples 3–5: the ethylene–glycol method (EG) The preparations were carried out in accordance to a process for the synthesis of ceramic thin films [19] by dissolving the metal nitrates in a mixture of ethylene–glycol, water and concentrated nitric acid. The subsequent drying procedure for the different MMOs was varied. For the formation of sample 3 (EG I), the solution was stirred for 2 h under reflux at 105 8C before drying at the same temperature for an additional 16 h. Sample 4 (EG II) was dried during 1 h, while sample 5 (EG III) was stirred at 65 8C for 5 d. The calcination was carried at temperatures of 400 8C for 5 h. Additionally, different catalysts were prepared according to sample 3 by varying the calcination temperature between 350 and 500 8C. This is indicated by giving the acronym for the synthesis method together with the calcination temperature (in 8C), e.g. EG I_350. As reference material a commercial Hopcalite was used. To generate a crystalline mixed-valence reference material for XPS analysis, a physical mixture of CuO (Merck, analysis grade) and Mn2O3 (Alfa Aesar, 98%) (nominal copper/ manganese atomic ratio = 20/80) was pressed into a pellet and annealed at 1000 8C for 7 d according to conventional solid state synthesis principles [Cu20Mn80Ox (1000)]. 2.2. Catalytic activity The catalytic tests were performed in a fixed-bed flow reactor with 100 mg of the catalyst (particle size 100–200 mm) at 25 8C. After pre-treatment for 1 h in synthetic air followed by 0.5 h in nitrogen at 200 8C, the reaction was started at the total flow rate of 30,000 ml h 1 g 1 (1 vol.% CO in synthetic air) controlled by mass flow controllers. Experiments with moist air were carried out analogously by passing the reactant gas through a washing bottle filled with water prior to the reactor. The relative humidity under these conditions was determined by means of a hygrometer. Tests near low-temperature fuel cell conditions were carried out under identical reaction conditions using a different feed gas composition (1 vol.% CO, 4.88 vol.% synthetic air, 20 vol.% CO2 in hydrogen) and varying the reaction temperature between 60 and 120 8C. The products were analyzed by IR gas sensors (GfG mbH, customised production). The reliability of the gas-phase analysis was controlled by GC measurements (Micro-GC model CP 4900, ˚ with He as carrier Varian, micro-channel molecular sieve 5 A
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gas) which was also used for the analysis of the experiments in hydrogen-rich gas mixtures. The selectivity was calculated based on the converted CO and oxygen.
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different metal cations [19] the drying procedure was adopted to the different hydrolysis rates of the precursor complexes resulting in variations of phase compositions and crystallinity. The so-synthesised oxides were tested for CO oxidation at 25 8C and characterised by BET and powder XRD measurements. The results are compared with the corresponding values of the commercial Hopcalite sample in Table 1. A significant influence of the preparation method on the properties of the different samples is evident. Compared with the CO conversion of the reference catalyst, the performances of the catalysts of identical composition synthesised by different methods varied between completely inactive and highly active. While the chemical nature of the employed metal precursor, the complexing agent and the drying procedure of the ethylene–glycol method are strongly affecting the microstructure of the materials, the oxidation activity shows a significant correlation only with the surface area of the respective MMO. This is in agreement with Hutchings et al. who pointed out that the surface area is of importance for the catalytic performance of copper manganese oxides [16]. In addition to the activity variation huge differences in the phase composition and crystallinity of the variously synthesised oxides were discovered by X-ray diffraction. Cu20Mn80Ox (EG I_400) as well as the reference catalyst showed no distinct diffraction peaks at ambient temperature, which can be unambiguously assigned to discrete compounds described in the literature. The XRD pattern of all other, less active Cu20Mn80Ox samples revealed diffraction peaks corresponding to Mn2O3 (ICPDS 78-390), Mn5O8 (ICPDS 72-1427), MnO2 (ICPDS 82-2169) Cu1+xMn2 xO4 (ICPDS 70-260, 711144, 71-1145) or CuO (ICPDS 72-629), and a surprising crystallinity considering especially the lower calcination temperatures of 300 8C. Elemental analyses revealed small amounts of carbon in all samples (see Table 1) whereas a thermal treatment at higher temperatures was generally observed to reduce the carbon contents. The higher amount of carbon in Cu20Mn80Ox (Pn) could be responsible for the very low catalytic activity despite its relatively high surface area. Nevertheless, the origin of the low catalytic activity for CO conversion of the EG III_400 is not known. The desired reproducibility of the most active Cu20Mn80Ox sample (EG I_400) could be confirmed several times.
2.3. Characterisation The total BET surface area was measured with a Carlo Erba Sorptomatic 1990. CHN-elemental analyses were performed with a Leco elemental analyzer CHN-900. The powder X-ray diffraction (XRD) patterns were obtained with a Huber G670 ˚ ). Guinier camera with Cu Ka1-radiation (l = 1.54056 A Temperature-resolved XRD measurements were carried out under static air in an Anton-Paar HTK1200 chamber attached to a Siemens D5000 diffractometer with Cu Ka1-radiation and a heating rate of 1 8C/min. X-ray photoelectron spectra were recorded by a VG ESCALAB MK II spectrometer using Mg Ka radiation which is designed for angular resolved XPS, UPS and EELS as described in Ref. [20–22]. 3. Results and discussion 3.1. Influence of the preparation method It is well known that the structural and catalytic properties of copper manganese mixed oxide catalysts often vary significantly with the preparation method [23,24]. Since irreproducible activities are not acceptable, we assumed that they resulted from the preparation procedure itself and searched for alternative, more reproducible synthesis methods for catalytically active copper manganese oxides. Several modified sol–gel methods have been tested for the preparation of Cu20Mn80Ox. The atomic ratio was selected because all prior experiments point to a maximum of catalytic activity at or close to this composition. The synthesis methods studied differ by the metal precursors employed, the solvent and the complexing agent. In the pentanone method (Pn), 4hydroxy-4-methyl-pentanone was used as complexing agent whereas the metal propionates were dissolved in hot propionic acid in the PA method. For the ethylene–glycol method (EG), where the formation of polymeric precursors consisting of ethylene–glycol and its oxidation product oxalic acid is believed to be crucial for a homogeneous distribution of the
Table 1 CO conversion after 1 h at 25 8C (WHSV = 30,000 ml g 1 h 1, 1 vol.% CO in synthetic air), BET surface area, carbon content and crystalline phases identified by XRD of the variously prepared oxides with a Cu/Mn atomic ratio = 20/80 and of the reference material Catalyst
CO conversion after 1 h (%)
BET surface area (m2/g)
Carbon content (wt.%)
Crystalline state
Pn PA EG I_400 EG II_400 EG III_400 EG I_350 EG I_450 EG I_500 Hopcalite
4 11 61 26 1 52 20 17 43
93 51 160 68 49 185 67 55 174
0.51 0.26 0.18 0.13 0.21 0.37 0.19 0.04 0.13
Mn5O8 Mn5O8 Nearly amorphous Weak peaks corresponding to Mn2O3, MnO2 and CuO Mn2O3, Mn5O8, Cu1+xMn2 xO4 Amorphous Mn2O3, Cu1+xMn2 xO4 Mn2O3, Cu1+xMn2 xO4 Amorphous
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3.2. Influence of the calcination temperature and of the composition For further optimisation of the copper manganese catalyst, the effect of compositional variations and changes in calcination temperatures on the catalytic activity of the EG I_400 catalyst was examined. As shown in Fig. 1 the samples calcined at 400 8C showed the highest CO conversions of all catalysts. Treatments at higher temperatures resulted in significant deactivations and the activity of the samples annealed at 350 8C was also lower than that of the catalysts treated at 400 8C. The latter effect could be explained by a higher carbon content of the samples. In accordance to all prior experiments on non-commercial catalysts, but also in accordance to the commercial catalyst studied by Rice et al. [25], the best composition for all MMOs calcined at different temperatures is still at the original copper/manganese atomic ratio of 20/80. Standardless elemental analysis of the Hopcalite reference by X-ray fluorescence spectroscopy pointed to a copper/ manganese atomic ratio of 40/60 with small amounts of calcium. XPS analysis revealed also significant nitrogen impurities. Other authors observed activity maxima for the Hopcalite catalyst at a copper/manganese atomic ratio of 10/ 90 and of 50/50 [23,26]. Further fine tuning of the composition of our most active system (EG I_400) confirmed the highest activity for the copper/manganese = 20/80 composition, but it also documents a rather weak dependence of the catalytic activity on the copper content (see Fig. 2). The soprepared oxides showed in the range of Cu15Mn85Ox– Cu40Mn60Ox higher CO conversions than the commercial reference catalyst. Lower copper contents caused a significant decrease of the catalyst’s activity. Note that the surface areas for all these samples treated at 400 8C were comparable. It is well-known that crystallisation of amorphous Hopcalite catalysts causes vast sintering leading to a decrease of the surface area and thus of the catalytic activity [11,27]. In our
Fig. 1. The influence of composition and calcination temperature (Tc) on the catalytic activity for the oxidation of CO with synthetic air at 25 8C of the copper manganese oxides (EG I). Reaction conditions: 100 mg (100–200 mm) catalyst, flow rate 50 ml/min, feed gas: 1 vol.% CO in synthetic air.
Fig. 2. Composition dependency of the catalytic activity for the oxidation of CO with synthetic air at 25 8C (10/90 = copper/manganese atomic ratio, 100 mg (100–200 mm) catalyst, 50 ml/min total flow, 1 vol.% CO in synthetic air).
case the amorphous to spinel transition was taking place at relatively low temperatures between 400 and 450 8C. While the samples treated at 350 or 400 8C revealed surface areas close to that of the Hopcalite, the surface area decreased by a factor of 3 after thermal treatments above that temperature (see Table 1). In contrast to Vepreˆk et al., who found an activity decrease by a factor of 20 with a surface area decrease by a factor of 5 [11], in our case the loss of activity correlated with the decrease in the area. Fig. 3 shows the influence of a further heat treatment on the crystallinity of Cu20Mn80Ox (EG I_400). Amorphous or poorly crystalline in the calcined state, the formation of copper manganese spinel (JCPDS No. 70-0260) and of Mn2O3 (JCPDS No. 73-1826) was induced by subsequent heating. The same crystalline phases could be identified in the mixed oxides calcined at 450 and 500 8C, respectively, whereas the relative amounts of these two phases varied with the total composition of the samples. Compared to EG I_400 the much less active Cu20Mn80Ox catalysts derived by different synthesis methods showed all various crystalline oxides (see Table 1). This indicates that surface and/or structural properties are strongly affecting the catalytic activity of these binary mixed oxides. Furthermore X-ray photoelectron spectroscopy was performed to acquire detailed information on the oxidation states of both metals. In the literature the existence of the Cu2+ + Mn3+ ? Cu+ + Mn4+ resonance system is postulated as one of the origins of the high catalytic performance of the Hopcalite catalyst [9–11]. XPS surface analysis performed by Porta et al. revealed that the existence of this redox system is closely connected to the cubic spinel (Cua+Mn1 a2+)A(Cu1 a2+Mna3+Mn4+)BO42 [28] which was confirmed by other authors [10,11]. Vepreˆk et al. analysed the thermal activation and deactivation of a commercial Hopcalite catalyst [11]. X-ray amorphous in the activated form, thermal treatments above 550 8C caused the crystallisation of the cubic spinel phase whereas the oxidation states of both metals changed from Cu2+/Mn3+ in the active sample to a mixture of Cu+/Cu2+ and of Mn3+/Mn4+ in the less active, crystallised one. The authors concluded that deactivation and change in the
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Table 2 XPS binding energies of Cu20Mn80Ox (EG I_400) and Hopcalite for characteristic signals
Cu20Mn80Ox (EG I_400) Hopcalite Cu20Mn80Ox (1000)
Cu-2p3/2 (eV)
Mn-3s (eV)
Mn-2p3/2 (eV)
933.2 933.2 932.6, 930.2
83.2 83.6 83.0
641.2 641.4 641.0
oxidation states from Cu2+ to Cu+ and from Mn3+ to Mn4+ are interrelated. Fortunato et al. approved these results by preparing two cubic CuMn2O4 spinels of identical overall composition but with varying Cu+/Cu2+ ratios due to different synthesis methods [10]. In their study the catalyst with lower Cu+ concentration was manifestly more active than the sample with a high content of univalent copper. In accordance with these results only Cu2+ and Mn3+ could be found in both Cu20Mn80Ox (EG I_400) and the reference catalyst by XPS analyses. The unsplit 2p3/2 part of the Cu-2p signal with a binding energy of Cu-2p3/2 = 933.2 eV indicates the existence of Cu2+ (Fig. 4, Table 2). Considering on the other hand manganese, the Mn-3s multiplet splitting was found by Wertheim et al. to decrease from MnO (6.05 0.04 eV) to Mn2O3 (5.50 0.10 eV) and MnO2 (4.58 0.06 eV) [29]. In
agreement with their results the oxidation state of manganese in Cu20Mn80Ox (EG I_400) could be determined as Mn3+ (splitting 5.4 eV, see Fig. 5). Due to the lower manganese content in the commercial sample, this multiplet splitting could not be used to unambiguously deduce the oxidation state in the Hopcalite catalyst. Comparisons of the Mn-2p spectra of both catalysts seem to indicate the same oxidation state of manganese in both Cu20Mn80Ox (EG I_400) and Hopcalite. A further Cu20Mn80Ox sample was prepared by a conventional solid state synthesis method. XRD measurements revealed diffraction peaks corresponding to Cu1+xMn2 xO4, Mn2O3 and Mn3O4. XPS surface analysis of this sample confirmed former results revealing that the Cu2+ + Mn3+ ? Cu+ + Mn4+ resonance system plays an important role in the partially inversed (Cua+Mn1 a2+)A(Cu1 a2+Mna3+Mn4+)BO42 spinel phase [10,11,28]. This can be easily seen in Fig. 4, where both split Cu-2p spin-orbit components indicate the existence of Cu+ as well as Cu2+ (e.g. BE of Cu-2p3/2 = 930.2 and 932.6 eV, respectively). On the other hand the identification of Mn2+, Mn3+ and Mn4+ is very difficult especially due to the high fraction of Mn3O4. Mn-3s multiplet splitting (DE = 5.8 eV) as well as Mn-2p3/2 binding energy of 641.0 eV (Mn2+: 640.2 eV [30], Mn3+: 641.5 eVand Mn4+: 643.0 eV [31]) point at mixedvalent di- and trivalent manganese ions. It should be mentioned that the Mn-3s spectra of the high temperature treated copper manganese oxide (see Fig. 5(C)) shows a distinct change of the relative intensities of the main band to the satellite compared to the respective spectra of Cu20Mn80Ox (EG I_400). The reason could be an overlap of two duplets
Fig. 4. Cu-2p XPS spectra of Cu20Mn80Ox (EG I_400) (a), of the commercial Hopcalite sample (b) and of Cu20Mn80Ox (1000) (c).
Fig. 5. Mn-3s and Cu-3p XPS spectra of Cu20Mn80Ox (EG I_400) (a), of the commercial Hopcalite sample (b) and of Cu20Mn80Ox (1000) (c).
Fig. 3. XRD patterns of Cu20Mn80Ox (EG I_400) as a function of temperature.
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Fig. 6. Deactivation of Cu20Mn80Ox (EG I_400), Pt1Cu20Mn79Ox (EG I_400) and Hopcalite during the oxidation of CO in moist air at 25 8C. Reaction conditions: 100 mg (100–200 mm) catalyst, flow rate 50 ml/min, feed gas: 1 vol.% CO in synthetic air; humidity: 84%.
which are caused by a mixture of Cu1+xMn2 xO4 and Mn3O4. More detailed investigations to elucidate this phenomenon are currently in progress. Table 2 summarises the corresponding XPS binding energies. 3.3. Catalytic tests with humid air and near PEM fuel cell conditions Modified sol–gel derived Cu20Mn80Ox (EG I_400) could be identified as very active catalyst for the oxidation of CO at room temperature in dry air. For respiratory and/or fuel cell applications catalytic stability in moist air is essential. Commercial Hopcalite is well known to deactivate under these conditions within a few minutes [5,7]. For that task a positive effect of noble metals like palladium [32] or gold [33] was found. Maier and Saalfrank identified a significant improve-
Fig. 7. Selective oxidation in hydrogen rich atmosphere at different temperatures on Cu20Mn80Ox (EG I_400). Reaction conditions: 100 mg (100–200 mm) catalyst, flow rate 50 ml/min, feed gas: 1 vol.% CO, 4.88 vol.% synthetic air; 20% CO2 in H2. Selectivity: 80%.
ment of moisture resistance by doping their cobalt-based mixed oxide with a small amount of platinum [34]. As can be seen from Fig. 6, Cu20Mn80Ox (EG I_400), Pt1Cu20Mn79Ox (EG I_400), and the reference material deactivate after a few minutes in moist air (humidity: 84%). In agreement with prior results where an impregnation of the reference material with 0.5 wt.% of platinum did not improve its moisture resistance, our attempts to improve the EG I_400 catalysts by platinum addition failed [18]. For application in low-temperature fuel cells the selective oxidation of CO in hydrogen rich atmosphere is also of importance. Several noble metal containing materials are described to catalyse this reaction with high selectivities [35,36]. Furthermore CuO–CeO2 as noble-metal free catalyst showed CO conversions of 80% at nearly 100% selectivity [37]. In this context Cu20Mn80Ox (EG I_400) was investigated for CO oxidation in hydrogen rich feed gas. Fig. 7 shows the effect of temperature on the activity of the catalyst which was observed to increase with temperature while the selectivity of CO oxidation over H2 oxidation was higher than 80% at all temperatures. 4. Conclusion The strong influence of the preparation method on the structure and the catalytic performance for the oxidation of CO at room temperature of sol–gel derived copper manganese oxides were investigated. The nature of the metal precursors, solvents, complexing agents as well as the drying procedure were found to affect the catalytic performance significantly. BET surface area and X-ray diffraction showed that the best catalyst, prepared with the help of the ethylene–glycol method (EG I), was nearly amorphous with a high surface area comparable to the commercial Hopcalite sample. For further optimisation of this catalyst, the influence of the copper content and of the calcination temperature on the catalytic activity was examined. The optimal copper/ manganese ratio was found to be 20/80 in all cases. Catalysts calcined at 400 8C were most active whereas sintering processes – induced by the crystallisation of Mn2O3 and the spinel Cu1+xMn2 xO4 – at higher temperatures resulted in a drastically decrease of the surface area and the catalytic performance. It was surprising to find that a copper content of 15 through 40 at.% did not affect the catalytic performance significantly. Due to the low crystallinity of the most active materials the amorphous mixed oxide may represent the catalytically most active phase. The oxidation states of copper and manganese in both Cu20Mn80Ox (EG I_400) and Hopcalite were found to be Cu2+ and Mn3+ by means of X-ray photoelectron spectroscopy. Final experiments were performed to test a potential application of this catalyst in breathing protection and/or for proton exchange membrane fuel cells PEMFC. In moist air, our catalyst deactivated to a similar degree as the reference catalyst does. In the tests near low temperature fuel cell conditions (in hydrogen rich atmosphere), Cu20Mn80Ox (EG I_400) showed relatively high selectivities for CO.
M. Kra¨mer et al. / Applied Catalysis A: General 302 (2006) 257–263
Acknowledgements The authors thank Dr. F. Mu¨ller, UdS, for performing the XPS measurements as well as Dr. H. Natter, UdS, for recording the temperature resolved XRD patterns. References [1] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon, J. Catal. 144 (1993) 175. [2] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J. Catal. 115 (1989) 301. [3] J.W. Saalfrank, W.F. Maier, Angew. Chem. Int. Ed. 43 (2004) 2028. [4] J. Jansson, J. Catal. 194 (2000) 55. [5] A.B. Lamb, W.C. Bray, J.C. Frazer, J. Ind. Eng. Chem. 12 (1920) 213. [6] H.A. Jones, H.S. Taylor, J. Phys. Chem. 27 (1923) 623. [7] G.-M. Schwab, G. Drikos, Z. Phys. Chem. 185 (1940) 405. [8] A. Waskowska, L. Gerward, J.S. Olsen, S. Steenstrup, E. Talik, J. Phys.: Condens. Matter 13 (2001) 2549. [9] G.-M. Schwab, S.B. Kanungo, Z. Phys. Chem. 107 (1977) 109. [10] G. Fortunato, H.R. Oswald, A. Reller, J. Mater. Chem. 11 (2001) 905. [11] S. Vepreˆk, L. Cocke, S. Kehl, H.R. Oswald, J. Catal. 100 (1986) 250. [12] F.C. Buciuman, F. Patcas, T. Hahn, Chem. Eng. Process. 38 (1999) 563. [13] A. Holzwarth, H.-W. Schmidt, W.F. Maier, Angew. Chem. Int. Ed. 37 (1998) 2644. [14] J.W. Saalfrank, W.F. Maier, C. R. Chim. 7 (2004) 483. [15] J. Scheidtmann, P.-A. Weiß, W.F. Maier, Appl. Catal. A 222 (2001) 79. [16] G.J. Hutchings, A.A. Mirzaei, R.W. Joyner, M.R.H. Siddiqui, S.H. Taylor, Appl. Catal. A 166 (1998) 143. [17] G. Spinolo, S. Ardizzone, S.J. Trasatti, Electroanal. Chem. 423 (1997) 49.
263
[18] J.W. Saalfrank, PhD thesis, Universita¨t des Saarlandes, 2003. [19] C.C. Chen, M.M. Nasrallah, H.U. Anderson, J. Electrochem. Soc. 140 (1993) 3555. [20] F. Mu¨ller, P. Steiner, T. Straub, D. Reinicke, R. de Masi, S. Hu¨fner, Surf. Sci. 442 (1999) 485. [21] F. Mu¨ller, R. de Masi, P. Steiner, D. Reinicke, M. Stadtfeld, S. Hu¨fner, Surf. Sci. 459 (2000) 161. [22] F. Mu¨ller, R. de Masi, D. Reinicke, P. Steiner, S. Hu¨fner, K. Sto¨we, Surf. Sci. 520 (2002) 158. [23] S.B. Kanungo, J. Catal. 58 (1979) 419. [24] G.J. Hutchings, A.A. Mirzaei, R.W. Joyner, M.R.H. Siddiqui, S.H. Taylor, Catal. Lett. 42 (1996) 21. [25] J.K. Rice, J.R. Wyatt, L. Pasternack, Appl. Catal. B 24 (2000) 107. [26] D. Dollimore, K.H. Tonge, J. Chem. Soc. A (1970) 1728. [27] P.A. Wright, S. Natarajan, J.M. Thomas, Chem. Mater. 4 (1992) 1053. [28] P. Porta, G. Moretti, M. Musicanti, A. Nardella, Solid State Ionics 63–65 (1993) 257. [29] G.K. Wertheim, S. Hu¨fner, H.J. Guggenheim, Phys. Rev. B 7 (1973) 556. [30] B. Hermsmeier, J. Osterwalder, D.J. Friedman, B. Sinkovic, T. Tran, C.S. Fadley, Phys. Rev. B 42 (1990) 11895. [31] V.R. Galakhov, M.A. Korotin, N.A. Ovechkina, E.Z. Kurmaev, V.S. Gorshkov, D.G. Kellerman, S. Bartkowski, M. Neumann, Eur. Phys. J. B 14 (2000) 281. [32] M.N. Desai, J.B. Butt, J.S. Dranoff, J. Catal. 79 (1983) 95. [33] M. Date, M. Haruta, J. Catal. 201 (2001) 221. [34] W.F. Maier, J. Saalfrank, Chem. Eng. Sci. 59 (2004) 4673. [35] O. Korotkikh, R. Farrauto, Catal. Today 62 (2000) 249. [36] R.J.H. Grisel, B.E. Nieuwenhuys, J. Catal. 199 (2001) 48. [37] G. Avgouropoulos, T. Ioannides, H.K. Matralis, J. Batista, S. Hocevar, Catal. Lett. 73 (2001) 33.