Evaluation and characterization of Mn–Cu mixed oxide catalysts for ethanol total oxidation: Influence of copper content

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Fuel 87 (2008) 1177–1186 www.fuelfirst.com

Evaluation and characterization of Mn–Cu mixed oxide catalysts for ethanol total oxidation: Influence of copper content Marı´a Roxana Morales, Bibiana P. Barbero, Luis E. Cadu´s

*

Instituto de Investigaciones en Tecnologı´a Quı´mica (INTEQUI), UNSL – CONICET, Casilla de correo 290, 5700 San Luis, Argentina Received 26 February 2007; received in revised form 6 July 2007; accepted 12 July 2007 Available online 7 August 2007

Abstract Mixed oxides of manganese and copper with different wt% of copper have been prepared and evaluated in ethanol combustion. The co-precipitation method used for the synthesis of MnxCuy mixed oxides is adequate to obtain catalysts with excellent catalytic performance in combustion reactions. Catalysts were characterized by means of XRD, FT-IR, TPR and O2-TPD. A small amount of copper prevents manganese oxide reaching a crystalline structure. This poor crystalline structure of manganese oxide may improve the existence of oxygen vacancies giving a best performance in ethanol combustion to CO2. When the copper content increases, an extent of solid state reaction between Cu and Mn is favored and the partial oxidation of ethanol becomes more important. The incorporation of manganese into incomplete spinel structure diminishes CO2 yield. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Mixed oxides; Manganese; Copper; Ethanol combustion; VOCs

1. Introduction Thermal combustion is the most extended method for abatement of volatile organic compounds (VOCs) but it is not a feasible process as it operates at a high temperature, usually above 1273 K. It requires additional fuel, the use of temperature-resistant materials, and generates noxious byproducts (NOx). Catalytic combustion appears to be the most promising solution for elimination of VOCs when they are in low concentrations. Catalytic combustion offers environmental advantages, since it operates at much lower temperatures avoiding formation of nitrogen oxides. Several chemicals as well as printing processes emit volatile organic compounds (VOCs) such as ethyl acetate and ethanol. In fact, ethyl acetate and ethanol are the dominating VOCs in some segments of the printing industry. In addition, the complete oxidation of ethanol has mainly been studied with regard to the control of the emissions from *

Corresponding author. Tel./fax: +54 2652 426711. E-mail address: [email protected] (L.E. Cadu´s).

0016-2361/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.07.015

ethanol-fuelled vehicles [1]. Base metal oxides like Cu, Mn, Cr and supported noble metal catalysts (Pt, Pd) have been used so far for catalytic combustion of VOCs [2]. The high cost of noble metals has increased the interest in substitution, since transition metal oxides may fulfill the requirements. These transition metals can be considered environmental friendly. Cu–Mn–O catalysts were found to be superior to Cu–Ce–O catalysts when the reaction of reforming of methanol was used. Both catalysts were prepared with the same method (urea–nitrate combustion). The results show that formation of the spinel CuxMn3xO4 phase in the oxidized catalysts is responsible for the high activity [3]. Mixed oxides with perovskite (LaMnO3) or spinel structures (CuMn2O4) have also been found effective for the same end [4]. Manganese and copper based catalysts have been studied in total oxidation reaction of CO [3,5,6] and several VOCs combustion [7]. Combinations of MnOx, with other oxides deposited on a high surface area support and used as catalysts in oxidation processes exhibit differential catalytic activity as compared to a single component catalyst. The interaction of manganese with the support as

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well as with other components present in the catalyst significantly influences its bulk and surface structure [8–11]. Increased dispersion of MnOx on the support surface leads to significant increases in catalytic activity. This is attributed to higher catalyst surface area being exposed to the reaction feed [7,12,13]. The co-precipitation method and calcination temperature have led to a stable MnOx and a surface arrangement with excellent catalytic performance in both ethanol and propane combustion [14]. Both molecules of VOCs represent the extreme in the combustion difficulty of a great number of hydrocarbons [15]. Mc Cabe and Mitchell [1] have found that a commercial hopcalite catalyst exhibits comparable activity to Pt/Al2O3 in ethanol and acetaldehyde combustion. CuMn2O4 exhibited excellent activity but under reductive reaction conditions at high temperature led to aggregation of Cu and catalyst deactivation. Tanaka et al. [16,17] previously reported that CuMn2O4 spinel prepared by the citric acid complex method, decomposes to CuxMn3xO4 (x = 1.4–1.6) and Mn2O3 and/or Mn3O4 at high temperatures (1173 K). Hutching et al. [18] studied Mn–Cu catalysts prepared by the co-precipitation method. They observed that the catalyst containing the incomplete Mn1.5Cu1.5O4 spinel with Mn2O3 were more active in the oxidation of CO at room temperature than the catalyst which contain the same spinel with CuO. In previous studies [14], the nature and disposition of the phases forming the catalytic system, Mn:Cu = 1:1, as well as the effect of the precipitated aging time was determined. The increase of the aging time increases the catalytic activity and the selectivity to CO2 in the ethanol combustion. It also was observed that the structural defects associated with oxygen vacancies and the Mn2O3 high dispersion on the catalytic surface (deduced from the results of XPS and O2-DTP) facilitate the catalyst reducibility. This explains the enhancement of catalytic activity. Considering the excellent results obtained in the ethanol and propane oxidation on Mn:Cu = 1:1 catalysts [14], mixed oxides of manganese and copper with different wt% of copper have been prepared. Different Mn/Cu atomic ratios lead to a diverse arrangement of phases. The aim of this work is to study the role of the Cu concentration in the catalyst definition, and to correlate it with oxidation catalytic activity. 2. Experimental 2.1. Catalysts preparation The catalysts were synthesized by the co-precipitation method [19]. Aqueous solution of copper nitrate (0.25 M) (Cu(NO3) Æ 5H2O Riedel de Hae¨n, P.A. 99%) was mixed with manganese nitrate solution (0.25 M) (Mn(NO3)2 Æ 4H2O Merck, P.A. 99%) and agitated for 5 min. Aqueous solution of Na2CO3 (0.25 M) (Na2CO3 Merck, P.A. 99%) was added to mixed solution agitating constantly and

maintaining the temperature within the range of 298– 303 K. The pH of solution was measured during the coprecipitation process reaching a final pH of 8.4. The obtained precipitate was aged for 24 h. Then, it was filtered and washed with distilled water until the presence of Na+ ions in the washed water was in the 6–12 lS (micro Siemens) range. After that, the precipitate was dried in air at 393 K for 12 h obtaining the precursors. They were calcined in air increasing slowly the temperature at b = 0.8 K min1 up to 523 K and keeping this temperature for 2 h. Finally, the temperature was raised at b = 0.8 K min1 up to 773 K and kept it for 3 h. The resulting catalysts were denoted as MnxCuy where x was the wt% of manganese divided by 10 (9, 7 and 2) and y was the wt% of copper divided by 10 (1, 3 and 8). Atomic percentage is closed to wt% due to a similar atomic weight of Mn and Cu (55 and 63.5 for Mn and Cu, respectively). 2.2. Atomic absorption spectroscopy (AAS) Chemical composition was determined by atomic absorption spectroscopy. Each 0.01 g sample was dissolved using 40 ml of HCl and a total volume of 250 ml was reached with distilled water. The measurements were carried out by using a Varian AA50 equipment. 2.3. 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 out gassed at 423 K using a Micromeritics Accusorb 2100E apparatus. 2.4. 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(k = 0.2291 nm) to avoid fluorescent effects. The crystalline phases were identified by reference to powder diffraction data (PDF). 2.5. 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 in N2 (Air Liquide, H2 5% N2 QS), 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 at 973 K until the signal of hydrogen consumption returned to its initial values. 2.6. Reduction–oxidation cycles With MnxCuy catalyst, 20 mg were reduced using 5% H2/N2 (Air Liquide, H2 5% N2 QS) reducing mixture at a

M.R. Morales et al. / Fuel 87 (2008) 1177–1186

total flow rate of 50 ml min1. The temperature during this step was increased from 323 K to 623 K at 5 K min1. After cooling down at 350 K, the sample was passed by purified helium (24 ml min1) for 15 min. After that, the oxidation with a 20% O2/He oxidizing mixture (6 ml min1) was made for 30 min at 623 K. Then the sample was cooled in an oxidizing mixture flow to 323 K. When this temperature was reached and helium flow was passed, the reduction–oxidation step was repeated. The catalysts with two reduction–oxidation cycles were named MnxCuy 2cycles.

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2.9. Catalytic test The catalysts were evaluated in total oxidation of ethanol using a quartz reactor of fixed bed (12 mm i.d., 138 mm large). The catalyst powder was pressed, crushed and sieved to a size of 35–50 mesh (0.3–0.5 mm) for the catalytic evaluation. The sample (300 mg) was diluted with 1.5 g glass. The reaction mixture (100 ml min1) was C2H5OH:O2:He = 1:20.8:78.2. The temperature of reaction, coaxial measurement with a thermocouple, was increased from 373 K to total conversion of ethanol, at intervals of 20 K. The data obtained at each temperature were the average of three steady-state measurements. The reactants and reaction products were alternately analysed on-line by a Shimadzu GC9A chromatograph equipped with a Porapak T column and thermal conductivity detector. The conversion of ethanol, X%, is defined as the percentage of ethanol feed that is reacted. In the text, T10, T50 and T90 refer to the reaction temperatures that correspond to 10%, 50% and 90% of ethanol conversion, respectively and TY100 is the temperature at which 100% yield to CO2 is achieved.

2.7. FT-IR spectroscopy (FT-IR) FT-IR spectra were registered by using a Nicolet Protege´ 460 spectrometer in KBr pellets. The samples were diluted to 1 wt% with KBr. The spectra were the result of averaging 32 scans obtained at room temperature in a wavelength ranging from 4000 to 225 cm1. 2.8. Temperature programmed desorption of O2 (O2-TPD) O2-TPD experiments were performed in a quartz reactor (12 mm i.d., 138 mm large) using a TCD as detector. In each analysis, 500-mg samples were pre-treated with helium (Air Liquide, 99,999%) gas increasing the temperature at 10 K min1 from room temperature up to 773 K. The samples were oxidized with a 20 vol.% O2/He (O2, Air Liquide, 99,999%) mixture at a total flow rate of 30 ml min1 at 773 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 773 K until the TCD signal returned to baseline.

3. Results 3.1. Catalytic activity The results of catalytic activity in ethanol oxidation are shown in Figs. 1–3 and Table 1. MnxCuy catalysts are more active than Mn2O3 and CuO pure oxides. T10 decreases as the copper content increases. Mn9Cu1 catalyst has a lower T50 than Mn7Cu3 catalyst and a similar T90 to Mn2Cu8 catalyst. Table 1 shows TY100 (temperature at which the yield to CO2 reaches 100%) for all catalysts – MnxCuy and MnxCuy 2cycles. MnxCuy catalysts show a lower TY100

100 90

Conversion of ethanol (%)

80

Mn9Cu1 Mn7Cu3 Mn2Cu8 Mn 2 O 3 Mn CuO

70 60 50 40 30 20 10 0 375

395

415

435

455

475

495

Reaction Temperature (K) Fig. 1. Catalytic activity in ethanol total oxidation.

515

535

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M.R. Morales et al. / Fuel 87 (2008) 1177–1186 90 Mn9Cu1 Mn7Cu3 Mn2Cu8 Mn2O 3 CuO

80

Yield to Acetaldehyde (%)

70

60

50

40

30

20

10

0 350

370

390

410

430

450

470

490

510

530

550

Reaction Temperature (K) Fig. 2. Yield to acetaldehyde as a function of the reaction temperature.

100 90 80

Mn9Cu1 Mn7Cu3 Mn2Cu8 Mn 2O3 CuO

Yield to CO2 (%)

70 60 50 40 30 20 10 0 375

395

415

435

455

475

495

515

535

Reaction Temperature (K) Fig. 3. Yield to CO2 as a function of the reaction temperature.

than Mn2O3, being the Mn9Cu1 catalyst which exhibits the lowest TY100. 3.2. Atomic absorption spectroscopy (AAS) Table 2 shows the AAS results corresponding to wt% of manganese and copper in the catalysts. It can be observed that the experimental results are similar to the nominal values.

3.3. BET specific surface area BET specific surface areas are shown in Table 2. Mn9Cu1 and Mn7Cu3 catalysts have almost the same specific surface area (30 m2g1) while Mn2Cu8 catalyst has 43 m2g1. As it is shown, all MnxCuy catalysts have a specific surface area higher than the average that would be obtained from the specific surface areas of manganese and copper pure oxides.

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Table 1 Temperature at which the yield to CO2 reaches 100% (TY100) and XRD phases of all catalysts, MnxCuy, MnxCuy 2cycles and pure oxides Catalysts

TY100 (K)

XRD phases

CuO Mn2O3 Mn9Cu1

547 ± 2 518 ± 2 481 ± 2

Mn7Cu3 Mn2Cu8 Mn2O3 2cycles Mn9Cu1 2cycles Mn7Cu3 2cycles Mn2Cu8 2cycles

503 ± 2 493 ± 2 500 ± 2 497 ± 2

CuO Mn2O3 Mn2O3, Mn3O4, Na2Mn5O10, Mn1.5Cu1.5O4 Mn2O3, Mn1.5Cu1.5O4 CuO, Mn1.5Cu1.5O4 Mn5O8 Mn5O8, Mn1.5Cu1.5O4

508 ± 2

Mn2O3, Mn1.5Cu1.5O4

521 ± 2

MnxCuyO4, CuO

Table 2 BET specific surface areas and wt% of manganese and copper of MnxCuy, Mn2O3 and CuO catalysts Catalysts

SBET (m2g1)

Mn (wt%)

Cu (wt%)

Mn2O3 CuO Mn9Cu1 Mn7Cu3 Mn2Cu8

19.3 21.7 33.5 30.9 43.3

100 0 85.8 68.4 23.3

0 100 14.2 31.6 76.7

3.4. X-ray diffraction (XRD) 3.4.1. MnxCuy catalysts Pure oxides prepared as references showed Mn2O3 (PDF 41-1442) and CuO (PDF 41-254) phases, respectively. In Fig. 4, the XRD results for MnxCuy catalysts are presented. A more complex XRD pattern is obtained for Mn9Cu1 catalyst. Mn2O3 (PDF 41-1442) and Mn3O4 (PDF 24-734) and two mixed phases, Na2Mn5O10 (PDF 27-749) and Mn1.5Cu1.5O4 (PDF 35-1172) are observed. Mn9Cu1, Mn2O3 and Mn7Cu3 catalysts have almost the ˚ , respectively). same particle size (333, 296, 338 A Mn7Cu3 catalyst shows Mn2O3 (PDF 41-1442) and Mn1.5Cu1.5O4 (PDF 35-1172) phases while Mn2Cu8 shows CuO (PDF 41-254) and Mn1.5Cu1.5O4 (PDF 35-1172) phases. 3.4.2. MnxCuy 2cycles catalysts Pure manganese oxide only shows Mn5O8 (PDF 201218) phases. For Mn9Cu1, the identified phases were Mn5O8 (PDF 20-1218) and Mn1.5Cu1.5O4 (PDF 35-1172). For Mn7Cu3, the same phases are detected but the diffraction lines corresponding to Mn2O3 are less intense and those of Mn1.5Cu1.5O4, are more intense in comparison with Mn9Cu1. Mn2Cu8 shows the patterns of CuO (PDF 41-254) and a mixed phase MnxCuyO4. The lines do not correspond with Mn1.5Cu1.5O4 (PDF 35-1172) or

Fig. 4. XRD of MnxCuy catalysts.

Mn1.6Cu1.4O4(PDF 35-1030). There is probably an intermediate phase. 3.5. FT-IR spectroscopy (FT-IR) In Fig. 5, FT-IR skeletal spectra of samples under study are reported. Mn2O3 shows three well-defined bands at 672, 573 and 524 cm1, and two broad bands at 599 and 499 cm1. White et al. [20] observe the same bands. Those bands correspond to MO lattice vibrations of mineral grade a-Mn2O3. CuO shows three bands at 583, 534 and 480 cm1 that correspond to Cu–O bonds in CuO monoclinic structure [21]. Mn9Cu1 catalyst shows two well-defined bands at 613 and 518 cm1 that correspond Mn3O4 [22] – one broad band at 438 cm1 characteristic of Mn2O3 [21] and other band of lower intensity at 347 cm1. Two bands at 504 cm1 and 440 cm1 that should correspond to Mn1.5Cu1.5O4 were also observed. Mn7Cu3 catalyst shows a shoulder at 663 cm1, two bands at 597 and 575 cm1 and other band at 526 cm1

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Mn2O3

Mn2Cu8

CuO

TCD Signal (a.u.)

Absorbance

Mn7Cu3

Mn2Cu8

Mn7Cu3

Mn9Cu1

Mn9Cu1

CuO

Mn2O3

1200

1100

1000

900

800

700

600

500

400

300

v (cm-1)

Fig. 5. FT-IR of MnxCuy catalysts and pure oxides.

that correspond to Mn2O3 [21]. The shoulder at 477 cm1 may indicate the presence of CuO. Mn2Cu8 catalyst shows the characteristic bands of CuO, two bands at 504 cm1 and 440 cm1 that would correspond to Mn1.5Cu1.5O4. 3.6. Temperature programmed desorption of O2 (O2-TPD) Fig. 6 shows O2-TPD curves for Mn and Cu pure oxides and MnxCuy catalysts. Two desorption signals in the 450– 640 K range are detected for all MnxCuy catalysts. Mn9Cu1 catalyst also shows a signal of desorption at high temperature which was associated with defective MnOx species. 3.7. Temperature programmed reduction (TPR) In Fig. 7, the TPR profiles of all MnxCuy catalysts are compared with the TPR profiles of pure manganese and copper oxides as references. Manganese oxide shows two peaks at 596 and 690 K, and a weak reduction signal at low temperatures. The reduction signals observed for this oxide may be explained

300

350

400

450

500

550

600

650

700

750

Temperature (K)

Fig. 6. O2-TPD of MnxCuy catalysts and pure oxides.

by the following reduction steps: from Mn4+ to Mn3+ at low temperatures, and the two more intense peaks are attributed to Mn3+ to Mn3+–Mn2+ and then, from Mn3+–Mn2+ to Mn2+ (MnO2 ! Mn2O3 ! Mn3O4 ! MnO). The on-set reduction is about at 473 K. CuO shows a one-step reduction at 532 K. This comes from CuO reduction to Cu0 [23]. Mn9Cu1 catalyst shows a peak at 543 K together with a shoulder at 513 K. At low temperatures, weak reduction signals are observed. The on-set reduction is about at 459 K. Mn7Cu3 catalyst shows a weak signal at low temperature and a well-defined intense peak at 520 K. The on-set reduction is at almost 441 K. Mn2Cu8 catalyst shows two small peaks at 403 and 451 K, and a well-defined and intense peak at 491 K. The on-set reduction is around 367 K. 4. Discussion The results of catalytic activity in ethanol oxidation are shown in Fig. 1. MnxCuy catalysts have a very low Ton set (temperature at which the reaction starts), 371–383 K and

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TCD Signal (a.u.)

Mn2O3

CuO

Mn2Cu8

Mn7Cu3

Mn9Cu1

300

350

400

450

500

550

600

650

700

750

800

Temperature (K)

Fig. 7. TPR curves of MnxCuy catalysts and pure oxides.

achieve the total conversion at 463–483 K. The manganese oxide taken as reference shows a Ton set 20–30 K higher that MnxCuy catalysts and achieve the total conversion at 493 K. On the basis of the T50, all MnxCuy catalysts were more active than Mn2O3. The results on MnxCuy catalysts indicate that, when T10 is considered, the activity increases with the increase of the copper content. T50 and T90 data do not show the same tendency. One of the drawbacks of the complete oxidation of ethanol is the formation of partial oxidation products (e.g. acetaldehyde), which are more harmful than the original organic compounds. It is important to be aware of the risk of the formation of partial oxidation products in catalytic incineration. Acetaldehyde was detected as partial oxidation product on the MnxCuy catalysts. Acetaldehyde is completely oxidized to CO2 and H2O at a temperature 10–20 K higher than the temperature of 100% ethanol conversion. The yield of acetaldehyde showed the same tendency that the ethanol conversion indicating that the same reaction mechanism occur on all the catalysts (Fig. 2). To compare the performance of catalysts, CO2 yield becomes a parameter more important than the conversion of ethanol. Fig. 3 shows the yield to CO2 of MnxCuy, Mn2O3 and CuO catalysts. The yield to CO2 diminishes in the following order: Mn9Cu1 > Mn2Cu8 > Mn7Cu3 > Mn2O3 > -

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CuO. All MnxCuy catalysts are more selective to CO2 than Mn2O3. Mn9Cu1 catalyst shows the best performance, from the point of view of the catalytic activity and the yield to CO2. The great difference between the TY100 (temperature at which the 100% of yield to CO2 is achieved) for Mn2O3 and Mn9Cu1 (from 518 to 481 K, respectively), indicates the importance of studying the role of copper in these catalysts. An adequate comparison is only possible on the basis of the knowledge of the phase composition. The co-precipitation method used for the synthesis of catalysts provides a high inter-dispersion of copper and manganese metallic elements and thus, the synthesis conditions can produce multiple arrangements such as solid solutions, mixed compounds and/or the arrangement of a phase over another one. With the help of the XRD and FT-IR results, it is possible to describe the phase composition of the obtained catalysts. The results of XRD (Fig. 4 and Table 1) show that Cu1.5Mn1.5O4 (PDF 35-1172) incomplete spinel phase is formed in all MnxCuy catalysts. The intensity of Cu1.5Mn1.5O4 diffraction lines increases as the copper content increases. In the Mn2Cu8 catalyst the CuO lines are also detected and, as any manganese oxide line is detected by XRD, it can be inferred that all manganese is incorporated into the spinel phase. According to Zener [24], simple and mixed Mn-oxides with Mn atoms in different oxidation
II IV states, for example Mn O Mn Mn O and Mn3O4 8 5 8 2 3
, may establish the necessary electronMnII MnIII O 4 3 mobile environment for optimal surface redox reaction. Mn4+ ions reduce at a lower temperature than Mn3+ ions, thus, the reduction beginning at a lower temperature may be due to the existence of Mn4+ ions in MnxCuy catalysts. The reduction begins at a lower temperature with the increasing of copper content. It is probable that copper has a promoting effect on the manganese reduction. Perhaps, it generates reducing spill-over species causing a similar effect to that observed by Ferradon et al. [23] in catalysts of manganese promoted by noble metals, where they are reduced at lower temperature than the MnOx catalyst. Manganese oxides were only detected in Mn9Cu1 and Mn7Cu3 catalysts. XRD results (Table 1) show that the average oxidation state of manganese in manganese oxides increases reaching 3+. Mn2O3 is the most stable of manganese oxide under the atmosphere of reaction. FT-IR results (Fig. 5) corroborate what is observed by XRD. The most intense vibrational mode of Mn5O8 was not detected. It was stated that the catalytic activity was enhanced when the couple Mn3+–Mn4+ was present in the structure of the oxides [25]. By theoretical studies, Lamaita et al. [26] have established that there are two possible adsorption–oxidation sites of C2H5OH on the Nsutite phase; are type corresponds to the OH groups formed from Mn4+ vacancies, where ethanol could be oxidized to CO2, and the other site is the terminal oxygen of the pirolusite lattice where ethanol could be partially oxidized to acetaldehyde,

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which, in turn, could be oxidized to CO2. Working on Mn– Cu–O catalytic system, Buciuman et al. [5] have concluded that the oxidation reactions over CuO follow a redox mechanism using lattice oxygen, while over Mn2O3 the mechanism is associative, involving adsorbed oxygen species. In our catalysts, both, Mn4+ vacancies as well as vacancies from structural defects associated with a poor crystallinity of the oxide can provide adsorbing centers for oxygen. Mn3+–Mn4+ pairs are present in the Cu1.5Mn1.5O4 incomplete spinel. The concentration of Mn3+–Mn4+ pairs increases as follows: Mn9Cu1 < Mn7Cu3 ffi Mn2Cu8. The existence of different arrangements of phases in each catalyst makes the interpretation of the results more difficult. Stability of the catalysts is a very important parameter [27]. In the catalysts containing manganese, the change in the number of oxidation is an instability factor. Therefore, the improvement in the activity will be understood by the analysis of the catalytic performance in terms of the oxidation state of manganese and the phases composition will allow to understand the improvement in the activity. Stobbe et al. [28] observe a re-oxidation after reduction in CH4 reforming. MnO became Mn2O3 at 823 K and Mn3O4 at 1073 K, which corresponds to a lower Mnvalency than expected thermodynamically. MnxCuy catalysts were treated by reduction–oxidation cycles (MnxCuy 2cycles) just to obtain a comparable composition of phases. After the reduction–oxidation treatments were performed, the catalytic activity was measured in the standard conditions described in Section 2. The results of XRD (Table 1) show the diffraction lines of Mn5O8 (PDF 20-1218) for Mn2O3 2cycles, Mn5O8 (PDF 20-1218) and Mn1.5Cu1.5O4 (PDF 35-1172) for Mn9Cu1 2cycles and Mn7Cu3 2cycles, and CuO (PDF 41-254) and Mn1.5Cu1.5O4 (PDF 35-1172) for Mn2Cu8 2cycles. Mn2Cu8 2cycles catalyst shows that XRD lines of the spinel phase shift to 2h degree lower than in Mn2Cu8 catalyst. Mn1.6Cu1.4O4 phase shows similar lines of diffraction to Mn1.5Cu1.5O4 but they shift towards lower than 2h degree (Fig. 8). Thus, XRD results would indicate that manganese is incorporated into the spinel structure. As a consequence, in Mn2Cu8 2cycles, Mn3+–Mn4+ pair concentration slightly decreases. Table 1 shows TY100 of MnxCuy and Mn2O3 catalysts before and after treatment in reduction–oxidation cycles. Taking into account the phase composition, the comparison of TY100 may aid to clarify the role of each phase. Mn2O3 has a TY100 21 K higher than Mn9Cu1 2cycles. Mn2Cu8 has a TY100, 25 K lower than Mn2O3, while Mn2Cu8 2cycles has a similar TY100 to Mn2O3. However, the most relevant data is that in Mn9Cu1 2cycles catalyst, manganese has increased the average oxidation number until 3+. This observation explains the difference in TY100 between Mn9Cu1 2cycles and Mn2O3. On the contrary, it was established that in Mn2Cu8, the treatment in reduction–oxidation cycles favors the incorporation of manganese into the spinel structure. Thus, even if it is not detected by XRD, manga-

35-1030 Cu1.4Mn1.6O4 ......... 35-1172 Cu1.5Mn1.5O4 Mn2 Cu82 cycles

Mn2 Cu8 50

52

54

56

58

60

2θ

Mn2Cu8 2cycles

Mn2Cu8

20

40

60

80

100

120

2θ Fig. 8. XRD of Mn2Cu8 and Mn2Cu8 2cycles.

nese oxide phase may exist in a small quantity. With this information, it is possible to discriminate the role of the manganese between the manganese oxide and the incomplete spinel. It is suggested that the existence of the resulting redox system Cu2+ + Mn3+ M Cu+ + Mn4+ is responsible for the reactivation of the catalyst containing Mn and Cu, and for that reason crucial to the high oxidative performance of these materials [29]. In these catalysts, Mn3+–Mn4+ pairs are important active centers for total combustion but there are other reasons that explain their excellent catalytic performance. The increase of the manganese oxidation number does not improve the yield to CO2. However, at similar average oxidation number of manganese, copper improves the catalytic performance. Those results indicate that the behavior of MnxCuy catalysts when the copper content varies should be otherwise explained. Hence, the way the manganese oxide is arranged becomes an important factor. It seems that the incomplete spinel favors the reaction of partial oxidation. A useful technique to study oxygen vacancies is temperature programmed desorption of oxygen. In O2-TPD curves, two desorption peaks are usually observed: a- and b-oxygen [30], which correspond to surface- and bulk-oxygen release, respectively. The a-oxygen (Ta) is adsorbed on the surface oxygen vacancies and it is released at low temperature (< 400 °C). The b-oxygen (Tb) is observed at higher temperature (>400 °C) and it comes from the catalyst bulk. Consequently, the a-oxygen participates in the suprafacial reaction mechanism while b-oxygen is relevant when a high oxygen mobility in the catalyst framework is necessary to complete a Mars-van Krevelen redox cycle. CuO and Mn2O3 pure oxides do not show the same characteristics of oxygen desorption showed by MnxCuy catalysts (Fig. 6). Oxygen TPD shows Ta and Tb (653 K) signals similar in all MnxCuy catalysts (Fig. 6). The intensity of both

M.R. Morales et al. / Fuel 87 (2008) 1177–1186

peaks varies slightly. The amount of oxygen desorbed varies with the copper content. Above 723 K, oxygen desorption gives the following order: Mn9Cu1  Mn7Cu3 ffi Mn2Cu8. Catalysts are calcined at 773 K and then, results from TPD are taken up to 773 K. According to Trawczynski et al. [6], desorption of oxygen at high temperature may be assigned to defective MnOx species in a poorly crystalline structure. Chabre and Pannetier [31] have reported about 14 modifications of c-MnO2. This solid is characterized by three types of defects: (a) De-Wolff disorder the intergrowth of ramsdellite and pyrolusite, (b) micro twinning and (c) point defects such as Mn4+ vacancies and Mn3+ cations. They have shown that micro twinning and De-Wolff disorders are responsible for a poor crystalline structure and the c-Mn2O3 electrochemical properties. XRD results shown in this work are in agreement with those observed by Trawczynski et al. [6]. Mac Lean et al. [32] and Ruestchi and Giovanoli [33] have proved that the structural and chemical defects are responsible for the electrochemical properties by the H+ insertion and for increasing the Fermi level energy. According to Volkshtein electronic theory [34], these properties make these oxides interesting from the catalytic point of view, due to their high electrical conductivity. It has been proposed [35] that the existence of oxygen vacancies may improve the catalytic performance of the catalyst. Defective MnOx species highly dispersed, may lead to these oxygen vacancies. Mn9Cu1 catalyst is more amorphous than Mn7Cu3 catalyst (Fig. 9). These results are in agreement with Stobbe et al. [28] and Shaheen and Selim [36], who have reported that the oxides with high degree of crystallinity had a pronounced decrease in the catalytic activity. A small amount of copper prevents manganese oxide reaching a crystalline structure. Thus, on MnxCuy catalysts, a little amount of copper improves the catalytic performance and favors a poor crystalline manganese oxide which contains the Cu1.5Mn1.5O4

Fig. 9. XRD of Mn9Cu1and Mn9Cu1 2cycles.

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incomplete spinel phase. These results are in line with those obtained with manganese oxide supported on alumina [37]. In a previous work [14], it was demonstrated that MnCu catalysts did not show an appreciable change in the conversion of ethanol value after hydrotreatment. Thus, it was confirmed that MnCu catalysts did not show deactivation, so we can expect that MnxCuy catalysts are stable catalysts. 5. Conclusions The co-precipitation method used for the synthesis of MnxCuy mixed oxides is adequate to obtain catalysts with excellent catalytic performance in combustion reactions. At similar average oxidation number of manganese, the presence of copper improves the catalytic performance in total oxidation. A small amount of copper prevents manganese oxide reaching a crystalline structure. This poor crystalline structure of manganese oxide may improve the existence of oxygen vacancies giving a best performance in ethanol combustion to CO2. When copper content increases, an extent of solid state reaction between Cu and Mn is favored and the partial oxidation of ethanol becomes more important. The incorporation of manganese into incomplete spinel structure decreases the CO2 yield. Acknowledgement The financial support from CONICET, Universidad Nacional de San Luis, ANPCyT (Argentina) is gratefully acknowledged. References [1] Mc Cabe RW, Mitchell PJ. Ind Eng Chem Prod Res Dev 1984;23:196–202. [2] Noordally E, Richmond JR, Tahir SF. Catal Today 1993;17:359–66. [3] Papavasiliou J, Avgouropoulos G, Ioannides T. Catal Commun 2005;6:497–501. [4] Lintz HG, Wittstock K. Catal Today 1996;29:457–61. [5] Buciuman TC, Patcas F, Halin T. Chem Eng Process 1999;38:563–9. [6] Trawczynski J, Bielak B, Mista W. Appl Catal B 2005;55:277–85. [7] Larsson Per-Olof, Andersson A. Appl Catal B 2000;24:175–92. [8] Wollner A, Lange F, Schmetz H, Knoezinger H. Appl Catal 1993;94:181–203. [9] Craciun R, Dulamita N. Catal Lett 1997;46:229–34. [10] Kapteijn F, van Langeveld D, Moulijn JA, Andreini A, Vuurman MA, Turek AM, et al. J Catal 1994;150:94–104. [11] Craciun R. Catal Lett 1998;55:25–31. [12] Strohmeier BR, Hercules DM. J Phys Chem 1984;88:4922–9. [13] Craciun R. Doctoral thesis, Michigan State University, East Lansing, USA, CA 148607l, 1997. [14] Morales MR, Barbero BP, Cadus LE. Appl Catal B 2006;67:229–36. [15] Tichenor BA, Palazzolo MA. Environ Prog 1987;6:172–6. [16] Tanaka Y, Takeguchi T, Kikuchi R, Eguchi K. Appl Catal A 2006;279:59–66. [17] Tanaka Y, Kikuchi R, Takeguchi T, Eguchi K. Appl Catal B 2005;57:211–22.

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M.R. Morales et al. / Fuel 87 (2008) 1177–1186

[18] Hutchings GJ, Mirzaei AA, Joyner RW, Siddiqui MRH, Taylor SH. Appl Catal A 1998;166:143–52. [19] Mirzaei AA, Shaterian HR, Habibi M, Hutchings GJ, Taylor SH. Appl Catal A 2003;253:499–508. [20] White WB, Keramidas VG. Spectrochim Acta A 1972;28:501–9. [21] Nagase K, Zheng Y, Kodama Y, Kakuta J. J Catal 1999;187: 123–30. [22] Preudhomme J, Tarte P. Spectrochim Acta A 1971;27:961–8. [23] Ferradon M, Carno J, Jaras S, Bjornbon E. Appl Catal A 1999;180:141–51. [24] Zener C. Phys Rev Series II 1951;81:440–4; Zener C. Phys Rev Series II 1951;82:403–5; Zener C. Phys Rev Series II 1951;83:299–301; Zener C. Phys Rev Series II 1952;85:324–8. [25] Figueroa SJA, Requejo FG, Lede EJ, Lamaita L, Peluso MA, Sambeth JE. Catal Today 2005;107–108:849855. [26] Lamaita L, Peluso MA, Sambeth JE, Thomas HJ. Appl Catal B 2005;61:114–9.

[27] Lahousse C, Bernieer A, Grange P, Delmon B, Papaefthimiou P, Ioannides T, et al. J Catal 1998;178:214–25. [28] Stobbe ER, de Boer BA, Geus JW. Catal Today 1999;47:161–7. [29] Kra¨mer M, Schmidt T, Sto¨we K, Maier WF. Appl Catal A 2006;302:257–63. [30] Seiyama T, Yamazoe N, Eguchi K. Ind Eng Chem Prod Res Dev 1985;24:19–24. [31] Chabre Y, Pannetier J. Prog Solid State Chem 1995;23:1–130. [32] Mac Lean L, Poinsignon C, Amarilla J, Le Dars F, Strobel P. J Mater Chem 1995;5:1183–9. [33] Ruestchi P, Giovanoli R. J Electrochem Soc 1988;135(11):2663–9. [34] Volkenshtein F. The electronic theory of catalysis on semiconductors. New York: Pergamon Press; 1963. [35] Baldi M, Sanchez Escribano V, Gallardo Amores JM, Milella F, Busca G. Appl Catal B 1998;17:L175–82. [36] Shaheen W, Selim M. Thermochim Acta 1998;332:117–28. [37] Kaptjein F, Singoredjo L, van Driel M, Andreı¨ni A, Moulijn JA, Ramis G, et al. J Catal 1994;150:105–16.