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Total oxidation of ethanol and propane over Mn-Cu mixed oxide catalysts
Applied Catalysis B: Environmental 67 (2006) 229–236 www.elsevier.com/locate/apcatb
Total oxidation of ethanol and propane over Mn-Cu mixed oxide catalysts Marı´a Roxana Morales, Bibiana P. Barbero, Luis E. Cadu´s * Instituto de Investigaciones en Tecnologı´a Quı´mica (INTEQUI), Universidad Nacional de San Luis, CONICET, Chacabuco y Pedernera, 5700 San Luis, Argentina Received 21 March 2006; received in revised form 24 April 2006; accepted 4 May 2006 Available online 21 June 2006
Abstract Mn-Cu mixed oxides were prepared by co-precipitation varying the aging time for 4, 18 and 24 h. The catalytic performance in propane and ethanol total oxidation on these samples was better than on Mn2O3 and CuO pure oxides. The increase of the aging time enhanced the activity and the selectivity to CO2. The nature and disposition of the phases forming the catalytic system as well as the effect of the precipitated aging time was determined by means of specific surface area measurements, X-ray diffractometry (XRD), infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and temperature programmed desorption of oxygen (O2-TPD). The catalytic behaviour seems related to the existence of a Cu1.5Mn1.5O4 mixed phase and the easier reducibility of the catalysts. # 2006 Elsevier B.V. All rights reserved. Keywords: Manganese; Copper; Mixed oxide; Catalytic combustion; Propane; Ethanol
1. Introduction When large gas volumes have to be treated, catalytic deep oxidation has to be performed at very high space velocity requiring a very active catalyst. Also, there is an additional difficulty in catalytic VOC removal because the stream generally contains many organic compounds of very different chemical nature [1]. In printing industry of flexible materials, the gaseous effluent contains a simple mixture, i.e. only ethanol and ethyl acetate. Seventy-five percent [2] of the catalysts used for VOC destruction are precious metal catalysts, generally supposed to be more active than metal oxide catalysts [3]. In recent years, many efforts were directed towards the design of catalytic materials based on mixed transition-metal oxides as a replacement for noble metal catalysts. These materials had applications in oxidation, combustion processes, fine chemical synthesis and chemical waste and emission control. Some of them exhibited high selectivity and resistance to poisons, such as halogens and sulphur [4]. For example, Lahousse et al. [5]
* Corresponding author. Tel.: +54 2652 426711; fax: +54 2652 426711. E-mail address: [email protected] (L.E. Cadu´s). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.05.006
have compared the activity of g-MnO2 and Pt/TiO2 catalysts. They have shown, for instance, that 90% of 250–300 ppm of benzene and of methanol is removed at 533 K with metal oxides. Their general conclusion is that metal oxides are not necessarily less active than noble metals. Manganese oxides are reported to be among the most efficient transition-metal compounds in catalytic combustion [1–4] and they are considered environment-friendly materials. In recent studies, different polymorphs of MnO2 [5] and Mn3O4 [6] were found to be active and stable catalysts for the combustion of organic compounds at temperatures in the order of 373–773 K. However, below 1173 K, Mn3O4 is a metastable phase in air [7,8], while MnO2 phase can only be stable at very high oxygen pressures; so, both compounds tend to transform into the stable form a-Mn2O3 in air in the 373–1173 K range at atmospheric pressure. Combinations of MnOx with other oxides deposited on a high surface area support, and used as catalysts in oxidation processes exhibit different catalytic activity as compared to a single component catalyst. The interaction of manganese with the support as well as with other components constituting the catalyst has a significant influence on the structure definition [4]. Increased dispersion of MnOx on the support surface led to significant increases in catalytic activity. This was attributed to
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a higher catalyst surface area being exposed to the reaction feed [4]. In this work, manganese and copper mixed oxides were prepared with two purposes. First, the use of manganese oxide capacity to adsorb and activate oxygen and second, the incorporation of copper to improve and stabilize active phases. Then, the preparation parameters were carefully controlled to achieve an optimal extension of the solid state reaction. 2. Experimental 2.1. Catalyst preparation The catalysts were synthesized by the co-precipitation method [9]. Aqueous solutions of Cu(NO3)24H2O (0.25 M) and Mn(NO3)24H2O (0.25 M) were prepared. The solution of copper nitrate was mixed with manganese nitrate solution and agitated for 5 min. The Mn/Cu atomic ratio was equal to 1. Aqueous solution of Na2CO3 (0.25 M) was added to mixed solution agitating continuously and maintaining the temperature within the range of 298–303 K. The pH of the solution was measured during the co-precipitation process reaching a final pH of 8.4. This operation did not take more than 15 min. The obtained precipitate was divided into three aliquots and aged for 4, 18 and 24 h. The precipitates were filtered and washed with distilled water until the presence of Na+ ions in the washed water was not detected. Washing conditions were extreme, since Na+ remains in the precipitate could act as catalyst poison [10]. Then, precipitates were dried at 393 K for 12 h obtaining the precursors. They were calcined increasing slowly the temperature up to 523 K and keeping this temperature for 2 h. Finally, the temperature was raised at 773 K and kept for 3 h. The resulting catalysts were denoted as MnCux where x was the aging time.
2.2.4. X-ray photoelectron spectroscopy (XPS) A spectrometer from Surface Science Instruments SSX 100/ 206 photoelectron with a monochromatised microfocused Al X-ray source was employed. Spectra were registered after the samples were purged at room temperature under vacuum. The residual pressure in the analysis chamber during the analysis was about 106 Pa. Flood gun energy was adjusted at 10 eV. Survey spectrum and C1s, O1s, Mn2p, Cu and C1s spectra were recorded again to check the stability of charge compensation as a function of time. Data treatment was performed with regard to C–(C, H) component of the C1s peak fixed at 284.8 eV. Atomic ratios were calculated from atomic sensitivity factors provided by the manufacturer. 2.2.5. Infrared spectroscopy (FT-IR) FT-IR spectra were registered by using a Nicolet Protege´ 460 spectrometer in KBr pellets. The spectra were the result of averaging 32 scans obtained at room temperature in a wavelength ranging from 4000 to 225 cm1. 2.2.6. Oxygen temperature programmed desorption (O2-TPD) O2-TPD experiments were performed in a quartz reactor using a TCD as detector. In each analysis, 500-mg samples were pre-treated with helium gas increasing the temperature from room temperature up to 973 K at 10 K min1. The samples were oxidized with a 20% O2/He mixture at a total flow rate of 30 ml min1 at 973 K for 30 min. Then, they were cooled down to room temperature in the oxidizing mixture and flushed by a stream of purified He for 30 min. The desorption was carried out in the same conditions as the pre-treatment, maintaining the temperature at 973 K until the TCD signal returned to baseline.
2.2. Characterization
2.3. Catalytic evaluation
2.2.1. BET specific surface area The specific surface area of catalysts was calculated by the BET method from the nitrogen adsorption isotherms obtained at 77 K on samples outgassed at 423 K using a Micromeritics Accusorb 2100E apparatus.
The catalysts were evaluated in total oxidation of propane and ethanol using a quartz reactor of fixed bed. The sample (300 mg) was diluted with 1.5 g glass. The reaction mixtures (100 ml min1) were C3H8:O2:He = 2:20:78 or C2H5OH:O2: He = 1:20.8:78.2. The temperature of reaction, coaxial measurement with a thermocouple, was increased from 398 K to total conversion of propane or ethanol, at intervals of 20 K. The reagents and products of reaction were analysed by gas chromatography on line using a Shimadzu GC9A chromatograph equipped with a Porapak T column for ethanol and Porapak Q column for propane and thermal conductivity detector.
2.2.2. X-ray diffraction (XRD) XRD patterns were obtained by using a Rigaku diffractometer operated at 30 kV and 25 mA by employing V-filtered Cr Ka radiation (l = 0.2291 nm). The crystalline phases were identified by reference to powder diffraction data (JCPDSICDD). 2.2.3. Temperature programmed reduction (TPR) The TPR was performed in a quartz U-type tubular reactor using a TCD as detector. A 20-mg sample was used. The reducing gas was a mixture of 5 vol.% H2/N2 at a total flow rate of 30 ml min1. The temperature was increased at a rate of 10 K min1 from room temperature to 973 K; then it was kept constant at 973 K until the signal of hydrogen consumption returned to the initial values.
3. Results and discussion The catalytic performance of MnCux catalysts was evaluated in total oxidation of propane and ethanol. These compounds were chosen as probe molecules since according to Tchenor and Palazzolo’s report [11], alkanes and alcohols are the difficulty extremes of oxidation. They studied the nonhalogenated VOC combustion and found that alcohols were the
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Table 1 Specific surface areas by BET method (SBET), surface atomic ratios by XPS and mmol of consumed H2 during TPR experiment, from Tonset up to 473 K Catalysts
SBET (m2 g1)
Atomic ratio Mn/Cu
mmol of consumed H2
MnCu4 MnCu18 MnCu24
18.9 20.4 21.7
1.2 4.2 3.8
48.8 38.2 54.4
CuO4 CuO18 CuO24 Mn2O34 Mn2O318 Mn2O324
5.6 7.6 8.6 15.4 15.4 19.9
7.7
most easily destroyed, followed by aldehydes, aromatics, ketones, acetates and alkanes. The results of catalytic activity in propane and ethanol oxidation are shown in Figs. 1 and 2, respectively. As it is expected, the propane combustion occurs at a higher temperature than the ethanol combustion. In both reactions, MnCux catalysts are more active than Mn2O3 and CuO pure oxides. In the case of propane oxidation, the total conversion on MnCux catalysts is achieved at 663 K, that is 50–70 K lower than on Mn2O3 and around 30–50 K less than on CuO. Baldi et al. [6] reported a Tonset (temperature at which the reaction starts) on Mn2O3, 50 K higher than on our MnCux catalysts for propane oxidation. Taking into account that VOCs catalytic combustion process operated as recuperative form requires the heating of great amounts of air up to the reaction temperature, a difference of 50 K becomes significant. The results on MnCux catalysts indicate that the activity increases with the increase of the aging time. This difference is more appreciable in propane oxidation, and it could be due to the slight increase of specific surface area obtained by the aging time increasing (Table 1). However, when the specific conversion (defined as propane conversion per surface area unit) is considered, the activity difference is still observable
below 573 K (Fig. 3). Thereby, it can be claimed that the most active catalyst is MnCu24. Differently from propane combustion where the only products are CO2 and H2O, ethanol combustion produces acetaldehyde as reaction intermediate (Fig. 4). It is a wellknown fact that alcohol is oxidized quite selectively to the corresponding aldehyde, and only when alcohol conversion approaches 100%, their total oxidation occurs. Taking into account that acetaldehyde is a more noxious volatile organic compound than ethanol, the increase of the selectivity to CO2 with the increase of the aging time is a very important effect. Thus, the most convenient catalyst for the ethanol combustion is MnCu24. The apparent activation energies for propane oxidation were calculated from an Arrhenius plot. The experimental data were obtained operating the reactor in differential mode (propane conversion < 10%). The obtained values, Ea = 17– 19 kcal mol1, fall within the usual range measured for propane catalytic combustion, Ea = 17–23 kcal mol1 [12,13]. These values show that the kinetic regime is chemical. With the aim of knowing why the aging-time increase improves the catalytic performance of MnCux catalysts, a wide characterization of them was carried out. Likewise, pure oxides of copper and manganese were prepared as reference samples following the same synthesis method used for the mixed catalysts. The samples obtained from the solutions containing
Fig. 2. Catalytic activity in ethanol total oxidation.
Fig. 3. Specific conversion of propane as a function of reaction temperature.
Fig. 1. Catalytic activity in propane total oxidation.
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Fig. 4. Selectivities to acetaldehyde (dotted lines) and CO2 (solid lines).
only copper or manganese nitrate were CuO (JCPDS 41-254) and Mn2O3 (JCPDS 41-1442) pure oxides, respectively. The nature of phases was not modified with aging time in any of the cases, but the texture was affected. As shown in Table 1, the specific surface areas increase with the aging-time increase. The surface areas of MnCux catalysts (Table 1) are about 20 m2 g1 exhibiting a slight increase with the aging time. These values are higher than those obtained for CuO (<9 m2 g1) and Mn2O3 (16 m2 g1) pure oxides. In consequence, it is expected that MnCux catalysts are more complex than the simple oxides mixture. The co-precipitation method used for the synthesis of catalysts provides a high interdispersion of copper and manganese metallic elements and thus, the synthesis conditions can produce multiple arrangements such as solid solutions, mixed compounds and/or the disposition of a phase on another one. With the help of the XRD and FT-IR results, we will describe the phase composition of the obtained catalysts. The results of XRD (Fig. 5) show the diffraction lines of Mn2O3 (JCPDS 41-1442), CuO (JCPDS 41-254) and Cu1.5Mn1.5O4 (JCPDS 35-1172). Many published studies on the Cu-Mn catalytic system report the formation of mixed phases [14–16]. The nature of the phases found depends strongly on the synthesis conditions, and thus, the results exhibit a wide dispersion. Hutchings et al. [17] found that the catalyst obtained by co-precipitation (pH 8.3, 80 8C, 12 h) with a molar ratio Cu/Mn = 1/1, and calcined at 773 K, constituted a CuMnxOy mixed phase and CuO. Only when the ratio Cu/Mn was lower than 1, the XRD results showed the Cu1.4Mn1.6O4 and Mn2O3 phases with a high background of diffraction indicating amorphous phases. At higher calcination temperature, the mixed phase was enriched in manganese and the composition tended to CuMn2O4 hopcalite. In our catalysts, the increase of aging time does not modify the nature of the present phases, although a variation in the relative intensity of diffraction lines is observed. As the aging time increases, the diffraction lines intensity of Cu1.5Mn1.5O4 increases and that of CuO decreases. This variation might indicate that a longer aging time favours the formation of a mixed phase or that there is a change of the crystallinity of the different phases forming the solid system.
Fig. 5. XRD of Mn2O324, CuO24 and MnCux catalysts.
The results of FT-IR help to explain these observations. Fig. 6 shows the spectra of MnCux catalysts together with those of CuO and Mn2O3 pure oxides included as reference. The bands exhibited by MnCux catalysts correspond to those of the pure oxides and only one additional band is observed at 665 cm1. This band would correspond to a particular bond of the Cu1.5Mn1.5O4 mixed phase. Analyzing the region between 560 and 640 cm1 (see inset in Fig. 6), MnCu4 presents a broad band at 580 cm1 which coincides with that of CuO at 582 cm1, and it would appear broader by the contribution of the bands at 575 and 605 cm1 of Mn2O3. As aging time increases, the broad band splits and becomes similar to Mn2O3 spectrum being CuO contribution barely perceptible. This observation is consistent with the results of XRD, and considering that IR spectroscopy does not depend on the sample crystallinity, we may conclude that the aging-time increase favours the formation of Cu1.5Mn1.5O4 mixed phase and decreases the CuO amount. Furthermore, the used calcination temperature limits the progress of the solid state reaction towards the hopcalite formation. Although CuMn2O4 hopcalite has proved to be an excellent catalyst in total oxidation of CO [15,18–20] as well as in total combustion of VOCs [19], Mc Cabe and Mitchell [21] have shown that the hopcalite is deactivated. On the other hand, Hutchings et al. [17] have reported that phases of manganese copper mixed oxides in conjunction with CuO are less active than mixed oxide phases in conjunction with Mn2O3 in CO oxidation. Thus, the conformation achieved for MnCux catalysts would contribute
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Fig. 6. FT-IR of MnCux catalysts and pure oxides.
to favour the catalytic performance. To know if MnCux catalysts would deactivate, hydro treatment experiments were made as follows: the experiment consisted of measuring the catalytic activity at 438 and 458 K using a reactive mixture C2H5OH:O2:He = 1:20.8:78.2 and a total flow of 100 ml min1. 1. Then, the catalyst was exposed to extreme conditions during 5 h and finally the catalytic activity was measured at the initial conditions. The extreme working conditions were: (i) reaction temperature = 623 K. This temperature is higher than the one required for 100% conversion. According to the calculus made in the reactor design, that would be the temperature of the combustion gases at the end of a fixed bed reactor with the concentration and flows involved in this study and from which the operative conditions have been selected; (ii) gaseous flow composition CO2:H2O:He = 2:3:95 and total flow of 100 ml min1. This composition was chosen assuming complete conversion from the reactive mixture to water and carbon dioxide. After hydro treatment MnCu24 catalyst did not show any appreciable change in the conversion of ethanol value, at 450 K X = 50 and 52% before and after hydro treatment, respectively. In this way it is confirmed that, MnCux catalysts did not show deactivation so MnCux compared with hopcalite is a stable catalyst. The higher catalytic activity of MnCux catalysts in comparison with pure oxides seems related to the existence of Cu1.5Mn1.5O4 mixed oxide. However, from the catalysis viewpoint, to know the constitution of the system answers only partially the most important question about how these phases are disposed and how this fact affects the catalytic performance. Zaki and Kappenstein [22] had performed bulk and surface analyses by X-ray photoelectron spectroscopy and diffracto-
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metry, to study variously composed, synthetic MnOx. They revealed that the surface composition is quite in proportion with that of the bulk. They showed that the surface oxidation state and homogeneity are critically controlled by the phase composition of the bulk. Therefore, the formation of a bulk dominated by mixed-valent Mn-oxide phases, Mn3O4, Mn5O8 or KMn8O16 results in surfaces exposing manganese sites in different oxidation states. Manganese oxide catalytic properties are attributed to the variability of the manganese oxidation state allowing the formation of oxides with several stoichiometries (MnO2, Mn2O3, Mn3O4 or Mn5O8), and to their oxygen storage capacity in the crystalline lattice [23]. The d–d electron exchange interactions between closely coupled pairs of manganese atoms in different oxidation states facilitate the electronic mobility across the surface [24,25]. The surface abundance of such electron interacting couples of metal sites has shown to optimise the redox catalytic activity of manganese oxides [26]. It is worthwhile noting, however, that such surface electron exchange interactions do not necessarily involve the bulk. In fact, this is probably the case reported by Kapteijn et al. [27] and Imamura et al. [28]. They observe that when MnO2 is placed in the proximity of La2O3 or CeO2, the oxygen mobility from MnOx structure is strongly affected. May be a small amount of a mixed oxide – difficult to detect – is formed. In the catalysts studied in this work, the surface electron exchange interactions may occur between Mn3+ and Mn4+ existing in the Cu1.5Mn1.5O4 mixed phase. The redox characteristics of MnCux catalysts were studied by means of TPR experiments. The reduction profile of our Mn2O3 (Fig. 7)
Fig. 7. TPR curves of MnCux catalysts and Mn2O324 and CuO24 pure oxides.
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exhibits two well-defined peaks with the maxima at 647 K and 750 K in agreement with the TPR curves reported for bulk MnOx catalysts [20,29]. The reduction sequence is probably MnO2 or Mn2O3 ! Mn3O4 ! MnO [1,30]. The reduction of MnO to metallic manganese has not been observed even up to 1223 K, because of its larger negative value of reduction potential [30,31]. However, Craciun et al. [4] presented a three-step reduction profile for unsupported MnOx. The first two peaks – at ca. 600 and 690 K – may be attributed to the two-step reduction of MnO2 (MnO2 ! Mn2O3 ! Mn3O4) and the third peak – at 783 K – represents the complete reduction of Mn3O4 to MnO. The first step (MnO2 ! Mn2O3) corresponding to a very small peak may be misunderstood in most of the cases [32]. The TPR profiles of MnCux catalysts are very different from that of Mn2O3 and CuO pure oxides (Fig. 7). The reduction of MnCux catalysts starts at a lower temperature than pure Mn2O3. In Cu1.5Mn1.5O4 mixed phase, part of the manganese ions are Mn4+. Tanaka et al. [16] reported that the mixed oxide has Cu+2 and Cu+1 and, therefore, there would be more Mn+4 than the nominal amount. These ions reduce at a lower temperature than Mn3+ ions; thus, the reduction beginning at a lower temperature could be due to the existence of Mn4+ ions in MnCux catalysts. Furthermore, the reduction of mixed oxides is completed at a lower temperature than Mn2O3. Copper could have a promoting effect on the manganese reduction. Perhaps, it may generate reducing spill-over species causing a similar effect to that observed by Ferradon et al. [32] in catalysts of manganese promoted by noble metals. Another possible explanation for the easier manganese reduction in MnCux catalysts in comparison with bulk Mn2O3 is the existence of structural defects associated to oxygen vacancies or a high dispersion of Mn2O3 on the mixed phase. It is a wellknown fact that supported manganese oxide is reduced at a lower temperature than bulk Mn2O3 [20]. The calcination temperature employed during the preparation of MnCux catalysts was higher than Tamman temperature of Mn2O3 (676 K). Thus, it is expected that Mn2O3 detected by XRD is dispersed on the Cu1.5Mn1.5O4 mixed oxide in MnCux catalysts. With the objective of studying the evolution of solid system, and to obtain more information about the probable synergy of the system, mechanical mixtures of pure oxides were prepared. First, adequate amounts of Mn2O3 and CuO to obtain a ratio Mn:Cu = 1:1 were ground in a mortar and then, dispersed in npentane and agitated by ultrasound for 35 min. The mixture was dried in vacuum oven and calcined up to 773 K with the same heating program used for MnCux catalysts but keeping the soak at constant temperature (773 K) for 24 or 48 h. The mechanical mixtures were denoted as MnCuxMM, where x is the calcination time in hours. The XRD results of all mechanical mixtures showed the diffraction lines corresponding to Mn2O3 and CuO pure oxides whereas the mixed phase was not observed. Then, the mechanical mixtures were reduced at programmed temperature (Fig. 8) to obtain information about the disposition and/or interaction between both phases. As expected, the reduction curve of MnCu0hMM was similar to the sum of reduction curves of pure oxides. In contrast, the reduction curves of the calcined mechanical
Fig. 8. TPR curves of Mn2O324, CuO24 and mechanical mixtures.
mixtures exhibit a first peak with the maximum at a temperature similar to that of CuO and other peaks at lower reduction temperatures than bulk Mn2O3. Evidently, the calcination process of the mechanical mixtures causes an important modification of the Mn2O3 phase. Probably, Mn2O3 migrates on the CuO surface achieving a high dispersion that facilitates its reduction. The stable state of dispersed Mn2O3 would be reached before 24 h of calcination since no relevant changes are observed after a longer calcination (48 h). To verify how easily migration process occurs, the MnCu0hMM sample was heated up to 773 K in He flow, and after cooling it down to room temperature, temperature programmed reduction was carried out. The obtained curve was quite similar to that of calcined samples. This indicates that the dispersion of Mn2O3 occurs very quickly when the sample is heated up to 773 K. However, the absence of mixed phase formation even after calcining the mechanical mixture at 773 K for 48 h, suggests that the solid state reaction requires a higher interdispersion of metallic elements such as that resulting from the preparation by the co-precipitation method. The surface migration of Mn2O3 may also occur in MnCux catalysts in which that phase has been detected by XRD. The results of surface composition corroborate this assumption. The atomic ratio Mn/Cu obtained by XPS (Table 1) is higher than 1 for all the samples indicating a surface enriched in manganese. Moreover, the ratio increases with the aging time reaching a value of approximately 4. This confirms that Mn2O3 tends to cover the surface of catalysts (by migration), which is favoured by the increase of the aging time.
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Another factor that can modify the reducibility and that has great influence on the catalytic performance is the existence of oxygen vacancies in the crystalline lattice of the oxides. In MnCux catalysts, Mn2O3 forms crystals that are detected by XRD, and the particle size can be calculated by means of ˚ for all MnCux Scherrer equation. The result was 342.6 0.3 A catalysts. Although this value is smaller than that obtained for ˚ ), it is considerably big as to present bulk Mn2O3 (397.1 A structural defects associated with oxygen vacancies. A useful technique to study oxygen vacancies is temperature programmed desorption of oxygen. Trawczynski et al. [20] observed the desorption of oxygen at high temperature in supported manganese oxides, with a first peak at a high temperature between 797 and 863 K. They assigned those high temperature peaks in TPD curve of supported MnOx catalysts to defective MnOx species. Likewise, Radha Krishnam et al. [33] studied O2-TPD of manganese catalysts (3 wt% Mn) supported on Al2O3, ZrO2, TiO2 and SiO2 supports from acetate precursors. They stated a considerably higher desorption from very dispersed defective MnOx species. The O2-TPD curves of our MnCux catalysts (Fig. 9) also show a signal of desorption at high temperature (maximum at 720 K) which we may associate to defective MnOx species. Furthermore, a desorption signal in the 450–620 K range is detected for all MnCux catalysts. This signal may be assigned to oxygen species adsorbed on surface oxygen vacancies. These oxygen species are very reactive and they can improve notably the catalytic activity considering the reaction temperature for propane and ethanol combustion.
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Fig. 10. Mn2p XPS spectra of MnCux catalysts.
During TPR experiment, it is expected that hydrogen easily reacts with the adsorbed oxygen species. This may be observed at the beginning of the TPR curve but with a very low intensity. If we consider the first portion of this TPR curve as the hydrogen consumption due to the ‘‘titration’’ of adsorbed oxygen species, they may be relatively quantified. With this aim, TPR experiments were carried out using a greater sample amount (100 mg), and the area under the curve between room temperature and 473 K was calculated by integration. Then, the hydrogen consumption was estimated from the calibration of the TCD response by means of the injection of known amounts of nitrogen on the reducing gaseous mixture (H2/N2). The results are presented in Table 1, as micromoles of consumed H2. The values for all MnCux catalysts are significantly higher than for Mn2O3, corroborating the O2-TPD results. All binding energies from XPS, of the Mn2p3/2 peak for MnCu18 and MnCu24 fall in a very narrow range (641.3– 641.5 eV), Fig. 10. This value is in the range of that reported for Mn2O3 (641.3–641.7 eV) [32,34,35]. Mn2p3/2 binding energy from MnCu4, 641.7 eV, shows a small chemical shift. As it can be observed in Table 1, Mn/Cu atomic ratio was 4.2 and 3.8 for MnCu18 and MnCu24, respectively, indicating that Cu1.5Mn1.5O4 phase is covered by manganese oxide. These results explain why Mn2p from XPS corresponds predominantly to Mn3+. The easier reducibility of MnCux catalysts with regard to Mn2O3 pure oxide may be due to a high dispersion of Mn2O3 on the Cu1.5Mn1.5O4 mixed phase and the existence of oxygen vacancies in the lattice. All these factors, without discarding the contribution of Mn4+ present in Cu1.5Mn1.5O4, are able to improve the catalytic activity of manganese oxides in combustion reactions. 4. Conclusions
Fig. 9. O2-DTP of MnCux catalysts and Mn2O324.
The co-precipitation method used for the synthesis of MnCux mixed oxides is adequate to obtain catalysts with excellent catalytic performance in combustion reactions. MnCux catalysts show a catalytic activity higher than our bulk Mn2O3 (prepared by the same method as MnCux catalysts) and manganese oxides reported in the literature. Furthermore, the increase of the aging
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Total oxidation of ethanol and propane over Mn-Cu mixed oxide catalysts Marı´a Roxana Morales, Bibiana P. Barbero, Luis E. Cadu´s * Instituto de Investigaciones en Tecnologı´a Quı´mica (INTEQUI), Universidad Nacional de San Luis, CONICET, Chacabuco y Pedernera, 5700 San Luis, Argentina Received 21 March 2006; received in revised form 24 April 2006; accepted 4 May 2006 Available online 21 June 2006
Abstract Mn-Cu mixed oxides were prepared by co-precipitation varying the aging time for 4, 18 and 24 h. The catalytic performance in propane and ethanol total oxidation on these samples was better than on Mn2O3 and CuO pure oxides. The increase of the aging time enhanced the activity and the selectivity to CO2. The nature and disposition of the phases forming the catalytic system as well as the effect of the precipitated aging time was determined by means of specific surface area measurements, X-ray diffractometry (XRD), infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and temperature programmed desorption of oxygen (O2-TPD). The catalytic behaviour seems related to the existence of a Cu1.5Mn1.5O4 mixed phase and the easier reducibility of the catalysts. # 2006 Elsevier B.V. All rights reserved. Keywords: Manganese; Copper; Mixed oxide; Catalytic combustion; Propane; Ethanol
1. Introduction When large gas volumes have to be treated, catalytic deep oxidation has to be performed at very high space velocity requiring a very active catalyst. Also, there is an additional difficulty in catalytic VOC removal because the stream generally contains many organic compounds of very different chemical nature [1]. In printing industry of flexible materials, the gaseous effluent contains a simple mixture, i.e. only ethanol and ethyl acetate. Seventy-five percent [2] of the catalysts used for VOC destruction are precious metal catalysts, generally supposed to be more active than metal oxide catalysts [3]. In recent years, many efforts were directed towards the design of catalytic materials based on mixed transition-metal oxides as a replacement for noble metal catalysts. These materials had applications in oxidation, combustion processes, fine chemical synthesis and chemical waste and emission control. Some of them exhibited high selectivity and resistance to poisons, such as halogens and sulphur [4]. For example, Lahousse et al. [5]
* Corresponding author. Tel.: +54 2652 426711; fax: +54 2652 426711. E-mail address: [email protected] (L.E. Cadu´s). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.05.006
have compared the activity of g-MnO2 and Pt/TiO2 catalysts. They have shown, for instance, that 90% of 250–300 ppm of benzene and of methanol is removed at 533 K with metal oxides. Their general conclusion is that metal oxides are not necessarily less active than noble metals. Manganese oxides are reported to be among the most efficient transition-metal compounds in catalytic combustion [1–4] and they are considered environment-friendly materials. In recent studies, different polymorphs of MnO2 [5] and Mn3O4 [6] were found to be active and stable catalysts for the combustion of organic compounds at temperatures in the order of 373–773 K. However, below 1173 K, Mn3O4 is a metastable phase in air [7,8], while MnO2 phase can only be stable at very high oxygen pressures; so, both compounds tend to transform into the stable form a-Mn2O3 in air in the 373–1173 K range at atmospheric pressure. Combinations of MnOx with other oxides deposited on a high surface area support, and used as catalysts in oxidation processes exhibit different catalytic activity as compared to a single component catalyst. The interaction of manganese with the support as well as with other components constituting the catalyst has a significant influence on the structure definition [4]. Increased dispersion of MnOx on the support surface led to significant increases in catalytic activity. This was attributed to
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a higher catalyst surface area being exposed to the reaction feed [4]. In this work, manganese and copper mixed oxides were prepared with two purposes. First, the use of manganese oxide capacity to adsorb and activate oxygen and second, the incorporation of copper to improve and stabilize active phases. Then, the preparation parameters were carefully controlled to achieve an optimal extension of the solid state reaction. 2. Experimental 2.1. Catalyst preparation The catalysts were synthesized by the co-precipitation method [9]. Aqueous solutions of Cu(NO3)24H2O (0.25 M) and Mn(NO3)24H2O (0.25 M) were prepared. The solution of copper nitrate was mixed with manganese nitrate solution and agitated for 5 min. The Mn/Cu atomic ratio was equal to 1. Aqueous solution of Na2CO3 (0.25 M) was added to mixed solution agitating continuously and maintaining the temperature within the range of 298–303 K. The pH of the solution was measured during the co-precipitation process reaching a final pH of 8.4. This operation did not take more than 15 min. The obtained precipitate was divided into three aliquots and aged for 4, 18 and 24 h. The precipitates were filtered and washed with distilled water until the presence of Na+ ions in the washed water was not detected. Washing conditions were extreme, since Na+ remains in the precipitate could act as catalyst poison [10]. Then, precipitates were dried at 393 K for 12 h obtaining the precursors. They were calcined increasing slowly the temperature up to 523 K and keeping this temperature for 2 h. Finally, the temperature was raised at 773 K and kept for 3 h. The resulting catalysts were denoted as MnCux where x was the aging time.
2.2.4. X-ray photoelectron spectroscopy (XPS) A spectrometer from Surface Science Instruments SSX 100/ 206 photoelectron with a monochromatised microfocused Al X-ray source was employed. Spectra were registered after the samples were purged at room temperature under vacuum. The residual pressure in the analysis chamber during the analysis was about 106 Pa. Flood gun energy was adjusted at 10 eV. Survey spectrum and C1s, O1s, Mn2p, Cu and C1s spectra were recorded again to check the stability of charge compensation as a function of time. Data treatment was performed with regard to C–(C, H) component of the C1s peak fixed at 284.8 eV. Atomic ratios were calculated from atomic sensitivity factors provided by the manufacturer. 2.2.5. Infrared spectroscopy (FT-IR) FT-IR spectra were registered by using a Nicolet Protege´ 460 spectrometer in KBr pellets. The spectra were the result of averaging 32 scans obtained at room temperature in a wavelength ranging from 4000 to 225 cm1. 2.2.6. Oxygen temperature programmed desorption (O2-TPD) O2-TPD experiments were performed in a quartz reactor using a TCD as detector. In each analysis, 500-mg samples were pre-treated with helium gas increasing the temperature from room temperature up to 973 K at 10 K min1. The samples were oxidized with a 20% O2/He mixture at a total flow rate of 30 ml min1 at 973 K for 30 min. Then, they were cooled down to room temperature in the oxidizing mixture and flushed by a stream of purified He for 30 min. The desorption was carried out in the same conditions as the pre-treatment, maintaining the temperature at 973 K until the TCD signal returned to baseline.
2.2. Characterization
2.3. Catalytic evaluation
2.2.1. BET specific surface area The specific surface area of catalysts was calculated by the BET method from the nitrogen adsorption isotherms obtained at 77 K on samples outgassed at 423 K using a Micromeritics Accusorb 2100E apparatus.
The catalysts were evaluated in total oxidation of propane and ethanol using a quartz reactor of fixed bed. The sample (300 mg) was diluted with 1.5 g glass. The reaction mixtures (100 ml min1) were C3H8:O2:He = 2:20:78 or C2H5OH:O2: He = 1:20.8:78.2. The temperature of reaction, coaxial measurement with a thermocouple, was increased from 398 K to total conversion of propane or ethanol, at intervals of 20 K. The reagents and products of reaction were analysed by gas chromatography on line using a Shimadzu GC9A chromatograph equipped with a Porapak T column for ethanol and Porapak Q column for propane and thermal conductivity detector.
2.2.2. X-ray diffraction (XRD) XRD patterns were obtained by using a Rigaku diffractometer operated at 30 kV and 25 mA by employing V-filtered Cr Ka radiation (l = 0.2291 nm). The crystalline phases were identified by reference to powder diffraction data (JCPDSICDD). 2.2.3. Temperature programmed reduction (TPR) The TPR was performed in a quartz U-type tubular reactor using a TCD as detector. A 20-mg sample was used. The reducing gas was a mixture of 5 vol.% H2/N2 at a total flow rate of 30 ml min1. The temperature was increased at a rate of 10 K min1 from room temperature to 973 K; then it was kept constant at 973 K until the signal of hydrogen consumption returned to the initial values.
3. Results and discussion The catalytic performance of MnCux catalysts was evaluated in total oxidation of propane and ethanol. These compounds were chosen as probe molecules since according to Tchenor and Palazzolo’s report [11], alkanes and alcohols are the difficulty extremes of oxidation. They studied the nonhalogenated VOC combustion and found that alcohols were the
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Table 1 Specific surface areas by BET method (SBET), surface atomic ratios by XPS and mmol of consumed H2 during TPR experiment, from Tonset up to 473 K Catalysts
SBET (m2 g1)
Atomic ratio Mn/Cu
mmol of consumed H2
MnCu4 MnCu18 MnCu24
18.9 20.4 21.7
1.2 4.2 3.8
48.8 38.2 54.4
CuO4 CuO18 CuO24 Mn2O34 Mn2O318 Mn2O324
5.6 7.6 8.6 15.4 15.4 19.9
7.7
most easily destroyed, followed by aldehydes, aromatics, ketones, acetates and alkanes. The results of catalytic activity in propane and ethanol oxidation are shown in Figs. 1 and 2, respectively. As it is expected, the propane combustion occurs at a higher temperature than the ethanol combustion. In both reactions, MnCux catalysts are more active than Mn2O3 and CuO pure oxides. In the case of propane oxidation, the total conversion on MnCux catalysts is achieved at 663 K, that is 50–70 K lower than on Mn2O3 and around 30–50 K less than on CuO. Baldi et al. [6] reported a Tonset (temperature at which the reaction starts) on Mn2O3, 50 K higher than on our MnCux catalysts for propane oxidation. Taking into account that VOCs catalytic combustion process operated as recuperative form requires the heating of great amounts of air up to the reaction temperature, a difference of 50 K becomes significant. The results on MnCux catalysts indicate that the activity increases with the increase of the aging time. This difference is more appreciable in propane oxidation, and it could be due to the slight increase of specific surface area obtained by the aging time increasing (Table 1). However, when the specific conversion (defined as propane conversion per surface area unit) is considered, the activity difference is still observable
below 573 K (Fig. 3). Thereby, it can be claimed that the most active catalyst is MnCu24. Differently from propane combustion where the only products are CO2 and H2O, ethanol combustion produces acetaldehyde as reaction intermediate (Fig. 4). It is a wellknown fact that alcohol is oxidized quite selectively to the corresponding aldehyde, and only when alcohol conversion approaches 100%, their total oxidation occurs. Taking into account that acetaldehyde is a more noxious volatile organic compound than ethanol, the increase of the selectivity to CO2 with the increase of the aging time is a very important effect. Thus, the most convenient catalyst for the ethanol combustion is MnCu24. The apparent activation energies for propane oxidation were calculated from an Arrhenius plot. The experimental data were obtained operating the reactor in differential mode (propane conversion < 10%). The obtained values, Ea = 17– 19 kcal mol1, fall within the usual range measured for propane catalytic combustion, Ea = 17–23 kcal mol1 [12,13]. These values show that the kinetic regime is chemical. With the aim of knowing why the aging-time increase improves the catalytic performance of MnCux catalysts, a wide characterization of them was carried out. Likewise, pure oxides of copper and manganese were prepared as reference samples following the same synthesis method used for the mixed catalysts. The samples obtained from the solutions containing
Fig. 2. Catalytic activity in ethanol total oxidation.
Fig. 3. Specific conversion of propane as a function of reaction temperature.
Fig. 1. Catalytic activity in propane total oxidation.
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Fig. 4. Selectivities to acetaldehyde (dotted lines) and CO2 (solid lines).
only copper or manganese nitrate were CuO (JCPDS 41-254) and Mn2O3 (JCPDS 41-1442) pure oxides, respectively. The nature of phases was not modified with aging time in any of the cases, but the texture was affected. As shown in Table 1, the specific surface areas increase with the aging-time increase. The surface areas of MnCux catalysts (Table 1) are about 20 m2 g1 exhibiting a slight increase with the aging time. These values are higher than those obtained for CuO (<9 m2 g1) and Mn2O3 (16 m2 g1) pure oxides. In consequence, it is expected that MnCux catalysts are more complex than the simple oxides mixture. The co-precipitation method used for the synthesis of catalysts provides a high interdispersion of copper and manganese metallic elements and thus, the synthesis conditions can produce multiple arrangements such as solid solutions, mixed compounds and/or the disposition of a phase on another one. With the help of the XRD and FT-IR results, we will describe the phase composition of the obtained catalysts. The results of XRD (Fig. 5) show the diffraction lines of Mn2O3 (JCPDS 41-1442), CuO (JCPDS 41-254) and Cu1.5Mn1.5O4 (JCPDS 35-1172). Many published studies on the Cu-Mn catalytic system report the formation of mixed phases [14–16]. The nature of the phases found depends strongly on the synthesis conditions, and thus, the results exhibit a wide dispersion. Hutchings et al. [17] found that the catalyst obtained by co-precipitation (pH 8.3, 80 8C, 12 h) with a molar ratio Cu/Mn = 1/1, and calcined at 773 K, constituted a CuMnxOy mixed phase and CuO. Only when the ratio Cu/Mn was lower than 1, the XRD results showed the Cu1.4Mn1.6O4 and Mn2O3 phases with a high background of diffraction indicating amorphous phases. At higher calcination temperature, the mixed phase was enriched in manganese and the composition tended to CuMn2O4 hopcalite. In our catalysts, the increase of aging time does not modify the nature of the present phases, although a variation in the relative intensity of diffraction lines is observed. As the aging time increases, the diffraction lines intensity of Cu1.5Mn1.5O4 increases and that of CuO decreases. This variation might indicate that a longer aging time favours the formation of a mixed phase or that there is a change of the crystallinity of the different phases forming the solid system.
Fig. 5. XRD of Mn2O324, CuO24 and MnCux catalysts.
The results of FT-IR help to explain these observations. Fig. 6 shows the spectra of MnCux catalysts together with those of CuO and Mn2O3 pure oxides included as reference. The bands exhibited by MnCux catalysts correspond to those of the pure oxides and only one additional band is observed at 665 cm1. This band would correspond to a particular bond of the Cu1.5Mn1.5O4 mixed phase. Analyzing the region between 560 and 640 cm1 (see inset in Fig. 6), MnCu4 presents a broad band at 580 cm1 which coincides with that of CuO at 582 cm1, and it would appear broader by the contribution of the bands at 575 and 605 cm1 of Mn2O3. As aging time increases, the broad band splits and becomes similar to Mn2O3 spectrum being CuO contribution barely perceptible. This observation is consistent with the results of XRD, and considering that IR spectroscopy does not depend on the sample crystallinity, we may conclude that the aging-time increase favours the formation of Cu1.5Mn1.5O4 mixed phase and decreases the CuO amount. Furthermore, the used calcination temperature limits the progress of the solid state reaction towards the hopcalite formation. Although CuMn2O4 hopcalite has proved to be an excellent catalyst in total oxidation of CO [15,18–20] as well as in total combustion of VOCs [19], Mc Cabe and Mitchell [21] have shown that the hopcalite is deactivated. On the other hand, Hutchings et al. [17] have reported that phases of manganese copper mixed oxides in conjunction with CuO are less active than mixed oxide phases in conjunction with Mn2O3 in CO oxidation. Thus, the conformation achieved for MnCux catalysts would contribute
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Fig. 6. FT-IR of MnCux catalysts and pure oxides.
to favour the catalytic performance. To know if MnCux catalysts would deactivate, hydro treatment experiments were made as follows: the experiment consisted of measuring the catalytic activity at 438 and 458 K using a reactive mixture C2H5OH:O2:He = 1:20.8:78.2 and a total flow of 100 ml min1. 1. Then, the catalyst was exposed to extreme conditions during 5 h and finally the catalytic activity was measured at the initial conditions. The extreme working conditions were: (i) reaction temperature = 623 K. This temperature is higher than the one required for 100% conversion. According to the calculus made in the reactor design, that would be the temperature of the combustion gases at the end of a fixed bed reactor with the concentration and flows involved in this study and from which the operative conditions have been selected; (ii) gaseous flow composition CO2:H2O:He = 2:3:95 and total flow of 100 ml min1. This composition was chosen assuming complete conversion from the reactive mixture to water and carbon dioxide. After hydro treatment MnCu24 catalyst did not show any appreciable change in the conversion of ethanol value, at 450 K X = 50 and 52% before and after hydro treatment, respectively. In this way it is confirmed that, MnCux catalysts did not show deactivation so MnCux compared with hopcalite is a stable catalyst. The higher catalytic activity of MnCux catalysts in comparison with pure oxides seems related to the existence of Cu1.5Mn1.5O4 mixed oxide. However, from the catalysis viewpoint, to know the constitution of the system answers only partially the most important question about how these phases are disposed and how this fact affects the catalytic performance. Zaki and Kappenstein [22] had performed bulk and surface analyses by X-ray photoelectron spectroscopy and diffracto-
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metry, to study variously composed, synthetic MnOx. They revealed that the surface composition is quite in proportion with that of the bulk. They showed that the surface oxidation state and homogeneity are critically controlled by the phase composition of the bulk. Therefore, the formation of a bulk dominated by mixed-valent Mn-oxide phases, Mn3O4, Mn5O8 or KMn8O16 results in surfaces exposing manganese sites in different oxidation states. Manganese oxide catalytic properties are attributed to the variability of the manganese oxidation state allowing the formation of oxides with several stoichiometries (MnO2, Mn2O3, Mn3O4 or Mn5O8), and to their oxygen storage capacity in the crystalline lattice [23]. The d–d electron exchange interactions between closely coupled pairs of manganese atoms in different oxidation states facilitate the electronic mobility across the surface [24,25]. The surface abundance of such electron interacting couples of metal sites has shown to optimise the redox catalytic activity of manganese oxides [26]. It is worthwhile noting, however, that such surface electron exchange interactions do not necessarily involve the bulk. In fact, this is probably the case reported by Kapteijn et al. [27] and Imamura et al. [28]. They observe that when MnO2 is placed in the proximity of La2O3 or CeO2, the oxygen mobility from MnOx structure is strongly affected. May be a small amount of a mixed oxide – difficult to detect – is formed. In the catalysts studied in this work, the surface electron exchange interactions may occur between Mn3+ and Mn4+ existing in the Cu1.5Mn1.5O4 mixed phase. The redox characteristics of MnCux catalysts were studied by means of TPR experiments. The reduction profile of our Mn2O3 (Fig. 7)
Fig. 7. TPR curves of MnCux catalysts and Mn2O324 and CuO24 pure oxides.
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exhibits two well-defined peaks with the maxima at 647 K and 750 K in agreement with the TPR curves reported for bulk MnOx catalysts [20,29]. The reduction sequence is probably MnO2 or Mn2O3 ! Mn3O4 ! MnO [1,30]. The reduction of MnO to metallic manganese has not been observed even up to 1223 K, because of its larger negative value of reduction potential [30,31]. However, Craciun et al. [4] presented a three-step reduction profile for unsupported MnOx. The first two peaks – at ca. 600 and 690 K – may be attributed to the two-step reduction of MnO2 (MnO2 ! Mn2O3 ! Mn3O4) and the third peak – at 783 K – represents the complete reduction of Mn3O4 to MnO. The first step (MnO2 ! Mn2O3) corresponding to a very small peak may be misunderstood in most of the cases [32]. The TPR profiles of MnCux catalysts are very different from that of Mn2O3 and CuO pure oxides (Fig. 7). The reduction of MnCux catalysts starts at a lower temperature than pure Mn2O3. In Cu1.5Mn1.5O4 mixed phase, part of the manganese ions are Mn4+. Tanaka et al. [16] reported that the mixed oxide has Cu+2 and Cu+1 and, therefore, there would be more Mn+4 than the nominal amount. These ions reduce at a lower temperature than Mn3+ ions; thus, the reduction beginning at a lower temperature could be due to the existence of Mn4+ ions in MnCux catalysts. Furthermore, the reduction of mixed oxides is completed at a lower temperature than Mn2O3. Copper could have a promoting effect on the manganese reduction. Perhaps, it may generate reducing spill-over species causing a similar effect to that observed by Ferradon et al. [32] in catalysts of manganese promoted by noble metals. Another possible explanation for the easier manganese reduction in MnCux catalysts in comparison with bulk Mn2O3 is the existence of structural defects associated to oxygen vacancies or a high dispersion of Mn2O3 on the mixed phase. It is a wellknown fact that supported manganese oxide is reduced at a lower temperature than bulk Mn2O3 [20]. The calcination temperature employed during the preparation of MnCux catalysts was higher than Tamman temperature of Mn2O3 (676 K). Thus, it is expected that Mn2O3 detected by XRD is dispersed on the Cu1.5Mn1.5O4 mixed oxide in MnCux catalysts. With the objective of studying the evolution of solid system, and to obtain more information about the probable synergy of the system, mechanical mixtures of pure oxides were prepared. First, adequate amounts of Mn2O3 and CuO to obtain a ratio Mn:Cu = 1:1 were ground in a mortar and then, dispersed in npentane and agitated by ultrasound for 35 min. The mixture was dried in vacuum oven and calcined up to 773 K with the same heating program used for MnCux catalysts but keeping the soak at constant temperature (773 K) for 24 or 48 h. The mechanical mixtures were denoted as MnCuxMM, where x is the calcination time in hours. The XRD results of all mechanical mixtures showed the diffraction lines corresponding to Mn2O3 and CuO pure oxides whereas the mixed phase was not observed. Then, the mechanical mixtures were reduced at programmed temperature (Fig. 8) to obtain information about the disposition and/or interaction between both phases. As expected, the reduction curve of MnCu0hMM was similar to the sum of reduction curves of pure oxides. In contrast, the reduction curves of the calcined mechanical
Fig. 8. TPR curves of Mn2O324, CuO24 and mechanical mixtures.
mixtures exhibit a first peak with the maximum at a temperature similar to that of CuO and other peaks at lower reduction temperatures than bulk Mn2O3. Evidently, the calcination process of the mechanical mixtures causes an important modification of the Mn2O3 phase. Probably, Mn2O3 migrates on the CuO surface achieving a high dispersion that facilitates its reduction. The stable state of dispersed Mn2O3 would be reached before 24 h of calcination since no relevant changes are observed after a longer calcination (48 h). To verify how easily migration process occurs, the MnCu0hMM sample was heated up to 773 K in He flow, and after cooling it down to room temperature, temperature programmed reduction was carried out. The obtained curve was quite similar to that of calcined samples. This indicates that the dispersion of Mn2O3 occurs very quickly when the sample is heated up to 773 K. However, the absence of mixed phase formation even after calcining the mechanical mixture at 773 K for 48 h, suggests that the solid state reaction requires a higher interdispersion of metallic elements such as that resulting from the preparation by the co-precipitation method. The surface migration of Mn2O3 may also occur in MnCux catalysts in which that phase has been detected by XRD. The results of surface composition corroborate this assumption. The atomic ratio Mn/Cu obtained by XPS (Table 1) is higher than 1 for all the samples indicating a surface enriched in manganese. Moreover, the ratio increases with the aging time reaching a value of approximately 4. This confirms that Mn2O3 tends to cover the surface of catalysts (by migration), which is favoured by the increase of the aging time.
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Another factor that can modify the reducibility and that has great influence on the catalytic performance is the existence of oxygen vacancies in the crystalline lattice of the oxides. In MnCux catalysts, Mn2O3 forms crystals that are detected by XRD, and the particle size can be calculated by means of ˚ for all MnCux Scherrer equation. The result was 342.6 0.3 A catalysts. Although this value is smaller than that obtained for ˚ ), it is considerably big as to present bulk Mn2O3 (397.1 A structural defects associated with oxygen vacancies. A useful technique to study oxygen vacancies is temperature programmed desorption of oxygen. Trawczynski et al. [20] observed the desorption of oxygen at high temperature in supported manganese oxides, with a first peak at a high temperature between 797 and 863 K. They assigned those high temperature peaks in TPD curve of supported MnOx catalysts to defective MnOx species. Likewise, Radha Krishnam et al. [33] studied O2-TPD of manganese catalysts (3 wt% Mn) supported on Al2O3, ZrO2, TiO2 and SiO2 supports from acetate precursors. They stated a considerably higher desorption from very dispersed defective MnOx species. The O2-TPD curves of our MnCux catalysts (Fig. 9) also show a signal of desorption at high temperature (maximum at 720 K) which we may associate to defective MnOx species. Furthermore, a desorption signal in the 450–620 K range is detected for all MnCux catalysts. This signal may be assigned to oxygen species adsorbed on surface oxygen vacancies. These oxygen species are very reactive and they can improve notably the catalytic activity considering the reaction temperature for propane and ethanol combustion.
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Fig. 10. Mn2p XPS spectra of MnCux catalysts.
During TPR experiment, it is expected that hydrogen easily reacts with the adsorbed oxygen species. This may be observed at the beginning of the TPR curve but with a very low intensity. If we consider the first portion of this TPR curve as the hydrogen consumption due to the ‘‘titration’’ of adsorbed oxygen species, they may be relatively quantified. With this aim, TPR experiments were carried out using a greater sample amount (100 mg), and the area under the curve between room temperature and 473 K was calculated by integration. Then, the hydrogen consumption was estimated from the calibration of the TCD response by means of the injection of known amounts of nitrogen on the reducing gaseous mixture (H2/N2). The results are presented in Table 1, as micromoles of consumed H2. The values for all MnCux catalysts are significantly higher than for Mn2O3, corroborating the O2-TPD results. All binding energies from XPS, of the Mn2p3/2 peak for MnCu18 and MnCu24 fall in a very narrow range (641.3– 641.5 eV), Fig. 10. This value is in the range of that reported for Mn2O3 (641.3–641.7 eV) [32,34,35]. Mn2p3/2 binding energy from MnCu4, 641.7 eV, shows a small chemical shift. As it can be observed in Table 1, Mn/Cu atomic ratio was 4.2 and 3.8 for MnCu18 and MnCu24, respectively, indicating that Cu1.5Mn1.5O4 phase is covered by manganese oxide. These results explain why Mn2p from XPS corresponds predominantly to Mn3+. The easier reducibility of MnCux catalysts with regard to Mn2O3 pure oxide may be due to a high dispersion of Mn2O3 on the Cu1.5Mn1.5O4 mixed phase and the existence of oxygen vacancies in the lattice. All these factors, without discarding the contribution of Mn4+ present in Cu1.5Mn1.5O4, are able to improve the catalytic activity of manganese oxides in combustion reactions. 4. Conclusions
Fig. 9. O2-DTP of MnCux catalysts and Mn2O324.
The co-precipitation method used for the synthesis of MnCux mixed oxides is adequate to obtain catalysts with excellent catalytic performance in combustion reactions. MnCux catalysts show a catalytic activity higher than our bulk Mn2O3 (prepared by the same method as MnCux catalysts) and manganese oxides reported in the literature. Furthermore, the increase of the aging
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