Combustion of Methane in Lean Mixtures over Bulk Transition-Metal

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Combustion of Methane in Lean Mixtures over Bulk Transition-Metal Oxides: Evaluation of the Activity and Self-Deactivation Jose´ R. Paredes, Eva Dıáz, Fernando V. Dıéz, and Salvador Ordoń˜ez* Department of Chemical Engineering and EnVironmental Technology, UniVersity of OViedo, Juliań ClaVerıá s/n, 33006 OViedo, Spain ReceiVed August 26, 2008. ReVised Manuscript ReceiVed October 20, 2008

Different bulk transition-metal oxides (NiO, CuO, Mn2O3, Cr2O3, and Co3O4) were prepared, by precipitation from nitrate precursors, and tested for the combustion of methane-air lean mixtures (1000-5000 ppmv of CH4). Catalyst performances were compared in terms of both intrinsic activity and resistance to self-deactivation. Methane combustion experiments were carried out at ambient pressure and a space velocity of 62 h-1 [weight hourly space velocity (WHSV)]. The activity of the studied catalysts (determined by the recording of light-off curves in the interval of 250-600 °C) decreases in the order: Co3O4 > Mn2O2 > Cr2O3 > CuO > NiO. However, deactivation studies, carried out both at constant temperature (620 °C) and in hysteresis experiments, reveal that Mn2O3 is the most stable catalyst.

1. Introduction Methane is the most abundant organic constituent in Earth’s atmosphere, and its content has been increasing sharply since 1900, with growing concern about its high global-warming potential (23 times that of CO2).1 Is has been estimated that one-third of the total CH4 was derived from punctual sources, such as mine emissions, coke ovens, livestock management, wastewater treatment plants, etc. Hence, if a part of this methane can be captured and channeled into power generation, there would result in a benefit for the environment and a significant gain in hydrocarbon utility.2 In this way, catalytic combustion of lean air-methane mixtures has become an interesting energetic alternative, by using catalytic gas turbines and other advanced reactors.3,4 On the other hand, methane is often chosen as a combustion model compound, because it is more difficult to oxidize than most of the other hydrocarbons. Therefore, higher temperatures are needed, and the stability of the catalysts at these conditions could be a crucial question.5 Although it is widely accepted that Pd catalysts are the most active catalysts,6 they present important drawbacks: they are not stable at the temperatures needed for this reaction7 (although several supports, such as ZrO2, can present enhanced thermal resistance); they are prone to sulfur-poisoning;8 and they are * To whom correspondence should be addressed. Telephone: +34-985103-437. Fax: +34-985-103-434. E-mail: [email protected]. (1) Litto, R.; Hayes, R. E.; Liu, B. J. EnViron. Manage. 2007, 84, 347– 361. (2) Bauer, S. H.; Javanovic, S.; Yu, C.-L.; Cheng, H.-Z. Energy Fuels 1997, 11, 1204–1218. (3) Cocchi, S.; Nutini, G.; Spencer, M. J.; Nickolas, S. G. Catal. Today 2006, 117, 419–426. (4) Hevia, M. A. G.; Fissore, D.; Ordoń˜ez, S.; Dıéz, F. V.; Barresi, A. A. Chem. Eng. J. 2007, 131, 343–349. (5) Zwinkels, M. F. M.; Jaras, S. G.; Menon, P. G.; Griffin, T. A. Catal. ReV. Sci. Eng. 1993, 35, 319–358. (6) Ge´lin, P.; Primet, M. Appl. Catal., B 2002, 39, 1–37. (7) Escandoń, L. S.; Ordoń˜ez, S.; Vega, A.; Dıéz, F. V. Chemosphere 2005, 58, 9–17. (8) Hurtado, P.; Ordoń˜ez, S.; Sastre, H.; Dıéz, F. V. Appl. Catal., B 2004, 47, 85–93.

expensive. On the other hand, transition-metal oxides have also been proposed as catalysts for methane deep oxidation. Although these materials are considered less active than Pd catalysts, they are relatively inexpensive and are considered stable at higher temperatures than noble metal catalysts,9 as well as more resistant to sulfur poisoning.10 Although most of the works presented in the literature deal with catalytic materials based on perovskites,11,12 spinels,13 or hydrotalcites,14 single-oxide catalysts (such as copper, manganese, cobalt, nickel, or iron oxides) could be an interesting alternative because of their low cost and simplicity. At this point, several works studied the catalytic combustion of methane and light hydrocarbons over supported oxides of Co,15 Cu,16 or Mn.17 In all of these cases, the tested materials were active but their thermal stability was not good. In addition, transition metals could react with common supports (such alumina- or silicabased supports), yielding to a loss of the active phase because of the formation of complex oxides with spinel structure. Different solutions are proposed in the literature for overcoming this problem, such as the development of specific supports (9) Choudhary, T. V.; Banerjee, S.; Choudhary, V. R. Appl. Catal., A 2002, 234, 1–23. (10) Ordoń˜ez, S.; Paredes, J. R.; Dıéz, F. V. Appl. Catal., A 2008, 341, 174–180. (11) Daturi, M.; Busca, G.; Groppi, G.; Forzatti, P. Appl. Catal., B 1997, 12, 325–337. (12) Cimino, S.; Lisi, L.; Pirone, R.; Russo, G.; Turco, M. Catal. Today 2000, 59, 19–31. (13) Marti, P. E.; Maciejewski, M.; Baiker, A. Appl. Catal., B 1994, 4, 225–235. (14) Cheng, J.; Yu, J.; Wang, X.; Li, L.; Li, J.; Hao, Z. Energy Fuels 2008, 22, 2131–2137. (15) Milt, V. G.; Ulla, M. A.; Lombardo, E. A. Catal. Lett. 2000, 65, 67–73. (16) Marion, M. C.; Garbowski, E.; Primet, M. J. Chem. Soc., Faraday Trans. 1991, 87, 1795–1800. (17) Baldi, M.; Finocchio, E.; Milella, F.; Busca, G. Appl. Catal., B 1998, 16, 43–51.

10.1021/ef800704e CCC: $40.75  2009 American Chemical Society Published on Web 11/26/2008

Combustion of Methane in Lean Mixtures

(spinels and perovskites) or working with solid solutions (for example, with cobalt-magnesium oxide).12,18 An easier possibility is to work with bulk metal oxides. Therefore, in a previous work, we have studied the performance of bulk Fe2O3 for methane oxidation, with a very stable behavior of this material being observed19 with higher activities than other complex materials found in the literature, such as perovskites, hexaaluminates, or spinels. We have also appointed in this work that it is possible to prepare these materials from waste materials (for example, red mud for Fe2O3), leading to inexpensive catalysts. At this point, although there are few works comparing the performance of bulk and supported oxides,20 it is suggested that both the behavior and reactivity are very different. Therefore, supported catalysts are more active but more prone to deactivation. The performance of LaAlO3-supported metal oxide catalysts was compared by McCarty et al.,21 reporting that the activity decreases in the order Co3O4 > CuO > NiO > Mn2O3 > Cr2O3. However, there are not, to the best of our knowledge, systematic comparisons about the reactivity and stability on stream of these bulk oxides. Therefore, the scope of this work is to carry out a systematic comparison of the performance of different bulk transition-metal oxides for the catalytic combustion of methane in lean methane-air mixtures. Materials will be compared in terms of their intrinsic activity and their resistance to thermal aging in methane combustion. 2. Experimental Section 2.1. Catalysts Preparation. Bulk Cr, Mn, and Co oxides were synthesized by adding dropwise ammonium hydroxide solution (30%, w/w) to a saturated aqueous solution of the corresponding metal nitrates [Co(NO3)2 · 6H2O, Mn(NO3)2 · 4H2O, and Cr(NO3)3 · 9H2O] until pH 8 is reached. The resulting precipitate was filtered, washed with distilled water, dried at 100 °C overnight, and calcined at 650 °C for 2 h. This method is proposed in the literature because of the high solubility of the metal nitrates and the total removal of the ammonium nitrate formed in the precipitation during the washing and calcinations.22 In the case of Ni and Cu oxides [prepared from Cu(NO3)2 · 3H2O and Ni(NO3)2 · 6H2O], this method is not applicable because of the formation of soluble ammonia complexes. The precipitation step was accomplished by addition of 0.1 M solution of NaOH, whereas the washing of the resulting material was more intensive to improve the sodium removal. Direct calcination of the precursors was discarded because it leads to glassy materials difficult to manipulate and with low surface area and activity. 2.2. Reaction Studies. Combustion experiments were carried out in a continuous packed bed reactor. The reactor consisted of a stainless-steel cylinder of 400 mm in length and 9 mm in internal diameter. Catalyst (2.0 g), crushed and sieved to 250-315 µm and diluted with R-alumina (Janssen), was placed in the middle part of the reactor, while the upper and lower parts were filled with glass balls (1 mm in diameter). The reactor was placed inside an electric furnace, with the temperature being controlled by a PID controller (Honeywell) connected to a thermocouple situated inside the reactor, which monitored the reaction temperature. The system was provided (18) Ji, S.-F.; Xiao, T.-C.; Wang, H.-T.; Flahaut, E.; Coleman, K. S.; Green, M. L. H. Catal. Lett. 2001, 75, 65–71. (19) Paredes, J. R.; Ordoń˜ez, S.; Vega, A.; Dıéz, F. V. Appl. Catal., B 2004, 47, 37–45. (20) McCarty, J. G.; Wise, H. Catal. Today 1990, 8, 231–248. (21) McCarty, J. G.; Gusman, M.; Lowe, D. M.; Hildenbrand, D. L.; Lau, K. N. Catal. Today 1999, 47, 5–17. (22) Campanati, M.; Fornasari, G.; Vaccari, A. Catal. Today 2003, 77, 299–314.

Energy & Fuels, Vol. 23, 2009 87 Table 1. Textural Parameters of the Fresh Metal Oxides Tested in this Work SBET (m2 g-1) Vpores (cm3 g-1) average diameter (nm)

Cr2O3

NiO

Mn2O3

Co3O4

CuO

26 0.12 15.1

5 0.02 16.7

14 0.09 19.6

21 0.09 11.5

5 0.03 12.8

with five additional thermocouples that measured the reactor wall temperature at different positions. The reactor was fed with 1.6 L min-1 (STP) of a mixture of N-50 synthetic air and 2.5% (vol) methane in N-50 synthetic air (Air Liquid), resulting in methane concentrations in the interval of 1000-4000 ppm and a weight hourly space velocity (WHSV) of 62 h-1. Flow rates were controlled by mass flow controllers (Brooks 5850 TR). Analyses were performed by gas chromatography (Hewlett-Packard HP 5890 Series II). Methane in the inlet and outlet streams was analyzed using a 30 m fused silica capillary column with apolar stationary phase SE-30 and a flame ionization detector (FID). CO and CO2 were analyzed using HayeSep N 80/100 and molecular sieve 45/60 columns connected in series and a thermal conductivity detector (TCD). Neither CO, partial oxidation, nor cracking byproduct was detected in any experiment, with the carbon mass balance fitting in all of the cases within 2%. Methane conversions were calculated from both outlet methane and CO2 concentrations, with both values being very close in all of the cases. 2.3. Catalyst Characterization. The catalyst pore size distribution and surface area were measured by nitrogen adsorption at -196 °C with a Micromeritics ASAP 2000 surface analyzer, considering a value of 0.164 nm2 for the cross-section of the nitrogen molecule. Analyses by scanning electron microscopy (SEM) were performed using a Jeol JSM-6100 microscope; the sample was deposited on a standard aluminum SEM holder and gold-coated. Powder X-ray diffraction patterns were obtained with a D-5000 Siemens diffractometer, using nickel-filtered Cu KR as monochromatic X-ray radiation. The patterns were recorded over a range of 2θ angles from 20° to 70°, and crystalline phases were identified using JCPDS files. Temperature-programmed desorption (TPD) and reduction (TPR) were carried out in a Micromeritics TPD/TPR 2900 apparatus connected to a TCD or a MS detector (Gaslab-300). CH4-TPD and O2-TPD experiments were carried out saturating the catalyst samples. According to this method, catalyst samples (40 mg) were cleaned in air at 550 °C, cooled in He to room temperature, and saturated with the probe gas at room temperature by injecting methane pulses (2 per min during 0.5 h). After this, the samples were purged with helium for 30 min at room temperature and the TPD experiments were started, monitoring the peaks of O2, CO, CO2, CH4, CH2O, and CH2O2. The selected fragments correspond to the following m/z ratios: O2 (32), CO (28), CO2 (44), CH4 (16), CH2O (30), and CH2O2 (46). Fragments were selected considering both the intensity and the specificity of the signal. For the TPR experiment, samples of about 40 mg were heated from 50 to 950 °C at 10 °C min-1 in a stream of 10% H2/90% Ar, with a flow rate of 25 cm3 min-1. Gases used in temperature-programmed experiments were supplied by Air Products with a purity of 99.999%.

3. Results and Discussion 3.1. Characterization of the Fresh Catalysts. Nitrogen physisorption studies reveal that all of the metal oxides are mesoporous, because the isotherms show hysteresis loops and correspond to type IV, according to the classification of Brunauer et al.23 The values of surface area, pore volume, and average pore diameter are given in Table 1. The surface area of cobalt and chromium oxide is markedly higher than that corresponding to Ni and Cu oxides. This result suggests that the precipitation with sodium hydroxide leads to a less porous (23) Brunauer, S.; Deming, L. S.; Deming, W. E.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723–1732.

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Figure 2. H2-TPR curves of the fresh catalysts: (a) Cr2O3, (b) NiO, (c) Mn2O3, (d) Co3O4, and (e) CuO. Figure 1. XRD patterns for fresh catalysts: (a) CrOx, (b) NiOx, (c) MnOx, (d) CoOx, and (e) CuOx. Crystallographic phases: eskolaite -RCr2O3- (E), NiO (N), bixbyite -R-Mn2O3- (B), cobalt-spinel structure -Co3O4- (S), and tenorite -CuO- (T).

material. This observation is in a good agreement with the behavior observed for other materials, for which sodium, even at very low concentrations, was proven to be a sintering agent for this kind of material,24,25 even at trace amounts. SEM analysis of the prepared catalysts are consistent with the nitrogen physisorption results, because Co and Cr oxides present agglomerates of particles of about 0.2 µm, whereas Mn, Ni, and Cu oxides present agglomerates of particles of 0.4, 0.5, and 0.64-1.12 µm, respectively. Concerning the crystallographic characterization, X-ray diffraction (XRD) patterns (Figure 1) show in all of the cases wellcrystallized solid phases. A chromium oxide catalyst shows pure hexagonal eskolaite phase (rhombohedrally crystallized R-Cr2O3), observing a perfect superposition with the JCPDS 85-0869 card. The nickel oxide material displays sharp and intense peaks corresponding to a NaCl-type nickel oxide structure, with the diffraction peaks being identified as NiO (JCPDS 04-0850). The XRD spectrum of the manganese oxide catalyst presents a structure corresponding to crystalline R-Mn2O3 species (bixbyite JCPDS 31-825), whereas the cobalt oxide (Co3O4) presents a cobalt spinel structure (JCPDS 801541), and tenorite crystalline phase, CuO (JCPDS 05-0661), is detected in the case of the copper oxide catalyst. TPR experiments (Figure 2) confirm the presence of only one phase. The reduction profiles of Cu and Ni correspond to the one-step reduction from the parent oxide to the reduced metal.26-29 The Co is an asymmetric peak, corresponding to a two-step reduction pathway, with CoO being the intermediate

product.30 In the case of Mn oxide, two successive peaks were observed: the first one is associated with the formation of Mn2O3, and the second one is associated with the formation of MnO.31 According to Ferrandon and Bjo¨rnbom,29 the full reduction of manganese oxides occurs at temperatures above 1200 °C. Chromium oxide does not present appreciable reduction peaks (just slight peaks around 300-400 °C, which can correspond to the reduction of surface CrVI species32). According to Kapteijn et al.,33 the reduction of Cr3+ is not thermodynamically possible in the typical operation range of a TPR experiment. Oxygen release profiles for the O2-TPD of the studied catalysts are shown in Figure 3. Two different behaviors have been found. Cu, Mn, and Co oxides present sharp O2 release peaks at relatively high temperatures (861, 793, and 833 °C, respectively). For the Cr and Ni oxides, O2 release peaks are broader, with lower released amounts and the maxima taking place at lower temperatures (691 and 605 °C, respectively). According to the literature, the former materials release reticular oxygen (also known as β-oxygen), whereas Cr and Ni release surface chemisorbed oxygen (also known as R-oxygen). Although, in the literature,34 it is stated that reticular O2 is mainly related to partial oxidation reactions and chemisorbed oxygen is active for deep oxidation reactions, in none of our experiments was partial oxidation products observed, whereas, as we will describe later, this β-oxygen seems active for total oxidation. Concerning the CH4-TPDs, the only released products observed during the experiment were O2 and CO2. Methane releases have not been observed in any case, suggesting that very little amounts of methane are adsorbed, with the reticular and chemisorbed oxygen of the catalysts being enough for its complete oxidation. This behavior is in good agreement with the observations of Finocchio et al.35 for the interaction of methane with spinels. These authors characterize by Fourier

´ lvarez, J.; Ordoń˜ez, S.; Rosal, R.; Sastre, H.; Dıéz, F. V. Appl. (24) A Catal., A 1999, 180, 399–409. (25) Ordoń˜ez, S.; Sastre, H.; Dıéz, F. V. Appl. Catal., B 2001, 29, 263– 273. (26) Mile, B.; Stirling, D.; Zammitt, M.; Lovell, A.; Webb, M. J. Catal. 1988, 114, 217–229. (27) Fierro, G.; Lojacono, M.; Inversi, M.; Porta, P.; Lavecchia, R.; Cioci, F. J. Catal. 1994, 148, 709–721. (28) Falconer, J. L.; Cordi, E. M.; O’Neill, P. J. Appl. Catal., B 1997, 14, 23–36. (29) Ferrandon, M.; Bjo¨rnbom, E. J. Catal. 2001, 200, 148–159.

(30) Dıáz, E.; Ordoń˜ez, S.; Vega, A.; Coca, J. Appl. Catal., B 2005, 56, 313–322. (31) Cimino, S.; Colonna, S.; De Rossi, S.; Faticanti, M.; Lisi, L.; Pettiti, I.; Porta, P. J. Catal. 2002, 205, 309–317. (32) Wilson, P.; Rao, P. M.; Viswanath, R. P. Thermochim. Acta 2003, 399, 109–120. (33) Kapteijn, F.; Marin, G. B.; Moulijn, J. A. Stud. Surf. Sci. Catal. 1999, 123, 375–431. (34) Morales, M. R.; Barbero, B. P.; Cadu´s, L. E. Appl. Catal., B 2007, 74, 1–10. (35) Finocchio, E.; Busca, G.; Loranzelli, V.; Willey, R. J. J. Catal. 1995, 151, 204–215.


Energy & Fuels, Vol. 23, 2009 89

Figure 5. Light-off curves for the combustion of 2000 ppmv of methane over Cr2O3 (9), NiO (4), Mn2O3 (O), Co3O4 (b), and CuO (2) expressed as methane conversion versus temperature.

Figure 3. O2-TPD curves of the fresh catalysts: (a) Cr2O3, (b) NiO, (c) Mn2O3, (d) Co3O4, and (e) CuO.

Table 2. Light-Off Curves for the Combustion of 2000 ppmv of Methane over the Studied Oxides Expressed as Moles of Methane Converted by Hour and Atom of Metal and Micromoles of Methane Converted by Hour and m2 of Surface Area, at 350 and 550 °C mol of methane converted h-1 metal atom-1 Cr2O3 NiO Mn2O3 Co3O4 CuO

Figure 4. CO2 profiles obtained in the CH4-TPD of the catalysts: (a) Cr2O3, (b) NiO, (c) Mn2O3, (d) Co3O4, and (e) CuO.

transform infrared spectroscopy (FTIR) that the spinel contacted with methane at 500 °C, observing the presence of carbonate and bicarbonate species in the solid, formed from methane oxidation.35 With regard to intermediate products, only trace amounts of formaldehyde were observed in the CH4-TPD of NiO. The CO2 profiles obtained in the CH4-TPD profiles of the studied catalysts are depicted in Figure 4. If Figures 3 and 4 are compared, different behaviors are observed. In the case of CuO, no significant CO2 releases were observed. This fact leads us to think that, for this metal oxide, the reticular O2 is not active for methane combustion. However, both cobalt and, more markedly, Mn oxide present CO2 releases at the same temperature at which O2 releases are observed. For these two catalysts, the O2 releases in the CH4-saturated samples take place at lower temperature (about 100 °C) than in the case of TPO experiments. Thermal effects associated with the combustion of the adsorbed methane or difference in oxygen chemical potentials could be the cause of this behavior. The overlapping

µmol of methane converted h-1 m-2

350 °C

550 °C

350 °C

550 °C

0.01 0.00 0.01 0.09 0.01

0.28 0.09 0.29 0.34 0.23

6.4 1.3 13 58 26

147 256 294 204 660

of the CO2 and O2 peaks for these catalysts suggests that reticular oxygen of both oxides can fully oxidize the adsorbed methane. Concerning Ni and Cr oxides, although both show very similar O2 release profiles in the O2-TPD experiment, their behavior is completely different in the CO2 release profiles. Cr oxide shows a release of CO2 at low temperature, which is not observed in the case of Ni oxide. This result suggests that, in the case of Ni, either the chemisorbed oxygen observed in the O2-TPD does not participate in methane oxidation or the oxide does not chemisorb significant amounts of methane. 3.2. Activity Tests. Catalysts activity was studied by recording light-off curves for the combustion of 2000 ppmv of methane in air using 2 g of catalyst, at temperatures from 250 to 650 °C and under a temperature ramp of 2 °C min-1. Experiments were carried out at a space time of 230 g h mol-1 of CH4 (WHSV ) 62 h-1) Preliminary blank experiments showed that no homogeneous or catalyzed by reactor wall reaction took place below 750 °C, with its contribution being negligible at the conditions of the present work. Results are shown in Figure 5 and Table 2. Experimental results show that the cobalt catalyst is the most active, followed by Mn and Cr catalysts (with similar activities), with Cu and Ni oxides being the least active. No significant differences in these trends are observed comparing conversion and activity per atom of metal. If activity is compared in terms of moles converted by surface area and looking at the kinetically controlled region, it is observed that Co oxide is the most active, followed by Cu, Mn, Cr, and Ni. This result suggests that the poor performance of CuO could be also related to the low exposed metal surface, whereas Ni seems to be the least active metal oxide for this reaction. If the activity of these catalysts is compared to the activity of bulk iron oxide (Fe2O3) studied in

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previous work,19 it is observed that this catalyst presents an intermediate behavior between manganese and chromium oxides (T50 of about 450 °C). If the activity curves are compared to the O2-TPD experiments, it is observed that relative activity per unit of surface area follows the same trend as the total amount of released oxygen, especially in the case of the catalysts releasing O2 at the highest temperatures (Cu, Co, and Mn). The activity trends also correlate with the results of the CH4-TPD. The two least active catalysts are those without CO2 release (Ni and Cu). Therefore, it seems that they are not able to either adsorb or activate the methane molecule. 3.3. Catalyst Stability. Two different experiments were carried out to check the catalytic stability of the solids. In the first experiment, methane conversion was measured at successively increasing (from 300 °C up to 625 °C) and decreasing (from 625 °C up to 300 °C) temperatures with a slope of 2 °C min-1 in both cases (hysteresis experiments). In the second experiment, catalysts were kept at constant temperature (625 °C). In both experiments, space time and methane concentrations were used in the previous section. Both experiments are complementary when evaluating catalyst deactivation, because most of the effects leading to catalyst deactivation (especially physical causes, as sintering) present a strong dependence upon temperature variations. The results of the hysteresis experiments are shown in Figure 6. The behavior of the different metal oxides tested is quite different: Cr, Ni, and Mn catalysts show negligible deactivation during the experiment (∆T50 ) 1, 2, and 4 °C, respectively), whereas Co and Cu are markedly deactivated during this experiment (∆T50 ) 25 and 65 °C, respectively). The results obtained in the experiments carried out at constant temperature (600 °C) are shown in Figure 7. This temperature was selected to ensure working at the highest temperature but without total conversion. Stability trends are in good agreement with those observed in the previous experiments for most of the studied metals. In both kinds of experiments, Mn oxide seems to be the most stable, whereas Co oxides show less stability and Cu oxides show the worst deactivation behavior. The major discrepancies between these two experiments were found for Ni and Cr oxide catalysts. For these materials, hysteresis experiments reveal the almost total absence of deactivation, whereas the deactivation curve shows a linear deactivation. Concerning Fe2O3 catalysts,19 it presents high stability in the hysteresis experiments, whereas a 10% activity loss at 600 °C is observed. To explain the deactivation behavior of the catalysts, metal oxides were characterized after the deactivation experiment. Morphological parameters are summarized in Table 3. In general, a decrease in the surface area was observed for all of the samples, except in the case of NiO, whereas Mn is the oxide with the highest decrease in surface area (71% lower than that corresponding to the fresh catalysts), followed by Co (57%), Cr (46%), and Cu (40%) oxides. Likewise, SEM images of the used catalysts show an important agglomeration of the particles, an effect that is especially marked for the Co oxide (Figure 8). Thus, deactivation behavior seems to not be correlated with changes in the porous structure. XRD analyses of the aged samples show the presence of the same crystallographic phases as the fresh material, with no new phases being observed. A qualitative estimation of the relative crystallinity based on the determination of the intensities of the main diffraction peaks and assuming as the reference value the starting material is also reported in Table 3. Cobalt oxide is

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Figure 6. Hysteresis cycles for (a) Cr2O3 (0), (b) NiO (4) and Mn2O3 (O), and (c) Co3O4 (]) and CuO (×, for increasing temperature and /, for decreasing temperature). Open symbols correspond to increasing temperatures, and filled symbols correspond to decreasing temperatures.

the catalyst that bears the highest crystallinity increase (121%), followed by Cr, Ni, and Mn oxides. These increments suggest the evolution of the active phase to larger crystal sizes, leading to a lower catalytically active surface. Although the catalyst preparation includes calcining for 2 h at 650 °C, the time on stream at 600 °C could favor the development of the crystalline structure of most of the metal oxides because of two reasons: the relatively slow kinetics of the formation of these crystals and the fact that the surface temperature on the catalysts could exceed 650 °C during methane combustion. When the O2-TPD of the used catalysts (Figure 9) is compared to that the fresh ones, it can be observed that the most active catalysts (Co and Mn oxides) present the same pattern as the corresponding fresh catalysts, suggesting that the aging does not affect the chemical state of the oxygen in the catalyst. In the case of CuO, the amount of O2 released is



Figure 7. Methane conversion versus the time on stream at 600 °C for the different catalysts: Cr2O3 (9), NiO (4), Mn2O3 (O), Co3O4 (]), and CuO (2). Reaction conditions: 2000 ppm of CH4 in air, WHSV ) 62 h-1. Table 3. Textural and Crystalline Parameters of the Metal Oxides Tested in this Work after 60 h on Stream for the Combustion of 2000 ppm of CH4 at 600 °C (m2

g-1)

SBET Vpores (cm3 g-1) average diameter (nm) crystallinity variation (%)

Cr2O3

NiO

Mn2O3

Co3O4

CuO

14 0.14 28 67

5 0.02 16 28

4 0.01 9.0 16

9 0.06 24 121

3 0.04 14 -8

markedly lower and displaced to low temperatures, whereas in the case of Cr2O3, the shape and amount released is similar but released at higher temperatures. These results suggest that the aging of the catalysts modifies the oxygen chemistry in the catalysts for Ni, Cr, and Cu oxide catalysts. Therefore, catalyst self-deactivation is caused by both the increase of material crystallinity (markedly important in the cases of Co3O4) and changes in the nature and availability of reticular oxygen (as in the case of Cu, Ni, and Cr). 3.4. Kinetic Studies. To study the kinetics of the combustion of methane over the studied catalysts, light-off curves were recorded for different methane inlet concentrations (1000, 2000, and 4000 ppmv), keeping the total flow rate (1.6 L min-1 (STP)) and catalyst weight (2.0 g) constant. The results corresponding to conversions below 20% are represented in Figure 10. The absence of mass-transfer limitations at reaction conditions was verified both experimentally and theoretically. Therefore, the external diffusion mass-transfer resistance was evaluated theoretically by estimating the Damko¨hler number, and it was found that, even in the most unfavorable conditions, its effect was negligible. The influence of internal mass transfer was evaluated using the criterion of Weisz, according to which internal masstransfer effects can be neglected for values of the dimensionless Weisz number lower than 0.1. In our case, the Weisz number was 1 × 10-4 in the most unfavorable case. These assumptions were confirmed experimentally, because the same conversion was obtained in tests at different total flow rates (at fixed space time) and different particle diameters. Thermal effects are also considered as negligible for the Weisz number below 0.01.36 Concerning the reactor model, the literature bypass and axial dispersion effects can be neglected for a reactor diameter/ particle diameter ratio higher than 10 and a catalytic bed length/ particle diameter ratio higher than 50.36 As in the present work, the values of these ratios are 26 and 131, respectively, and the plug flow in the reactor can be assumed. (36) Kapteijn, F.; Moulijn, J. A.; Tarfaoui, A. Stud. Surf. Sci. Catal. 1999, 123, 525–541.

Figure 8. SEM micrographs of Co3O4 oxides: (a) fresh catalyst and (b) after 60 h on methane stream.

Figure 9. O2-TPD curves of the catalysts after 60 h on stream at 600 °C: (a) Cr2O3, (b) NiO, (c) Mn2O3, (d) Co3O4, and (e) CuO.

In general, an increase in the concentration of methane leads to lower conversions. Although many works in the literature assume first-order behavior,37 results observed demonstrate that

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Figure 10. Light-off curves for different methane inlet concentrations: 1000 ppmv (0), 2000 ppmv (O), and 4000 ppmv (4) for the different catalysts tested in this work: (a) Cr2O3, (b) NiO, (c) Mn2O3, (d) Co3O4, and (e) CuO. Lines correspond to the power-law model predictions.

this assumption is not valid for metal oxide catalysts (in such cases, methane conversion will be not dependent upon the initial concentration). The oxidation of hydrocarbons over metal oxide catalysts has been widely studied in the literature,5,38,39 with power-law models being accurate enough most of the time. Therefore, considering that the oxygen is in great excess and its concentration remains practically constant in all of the experiments, the kinetic law used will be (-rA) ) kpAn

(1)

If the reactor is considered as an integral plug flow reactor, the integrated kinetic equation is

[

xA ) 1 - 1 -

kpA0nW(1 - n) FA0

]

1 1-n

(2)

where xA is the methane conversion, k is the kinetic constant, pA0 is the inlet methane partial pressure, n is the reaction order for methane, and WFA0-1 is the space time. The kinetic constant is assumed to follow an Arrhenius dependence with temperature. The parameters obtained together with the correlation coefficients are also listed in Table 4, and the quality of the fitting can be observed in Figure 10. If the values of the parameters are compared to those reported in the literature for similar materials, summarized by Golodets,38 it was observed that Mn2O3 has the same behavior in our work as that reported in the literature (EA ) 79 kJ mol-1 and a ) 0.7), whereas the other materials have a rather different behavior (EA ) 75 kJ mol-1 and a ) 0.9 for Co3O4; EA ) 104 kJ mol-1 and a ) 0.5 (37) Hurtado, P.; Ordoń˜ez, S.; Sastre, H.; Dıéz, F. V. Appl. Catal., B 2004, 51, 229–238. (38) Golodets, G. I. Heterogeneous Catalytic Reactions InVolVing Molecular Oxygen; Elsevier: Amsterdam, The Netherlands, 1983. (39) Spivey, J. J. Ind. Eng. Chem. Res. 1987, 26, 2165–2180.



Table 4. Values of the Kinetic Parameters for the Oxidation of Methane over the Studied Catalysts k0 (mol g-1 min-1 bar-1) EA (kJ mol-1) a R

Cr2O3

NiO

Mn2O3

Co3O4

CuO

4.48 × 10-6 81.2 0.06 0.995

5.19 × 10-6 107.5 0.04 0.986

3.84 × 10-4 71.3 0.77 0.995

7.79 × 10-5 80.2 0.51 0.992

2.65 × 10-5 69.8 0.38 0.990

for NiO; EA ) 113 kJ mol-1 and a ) 1 for Cr2O3; and EA ) 104 kJ mol-1 and a ) 0.6 for NiO). 4. Conclusions Studied single-metal-oxide catalysts (Cr2O3, Mn2O3, CuO, NiO, and Co3O4) are active catalysts for methane combustion, although they show very different performance. Thus, the following reactivity order has been established: Co3O4 > Mn2O3 > Cr2O3 > CuO > NiO. The reactivity trends are related to both the chemical properties of the oxide and the surface area. Concerning the stability of the catalysts, the Mn2O3 catalyst shows the best behavior, with no deactivation being observed either in hysteresis exeperiments or after 60 h on stream at 600

°C. Thermal stability of the other studied materials is markedly lower. Main deactivation causes were observed to be the thermal sintering of the active phase (observed by XRD) and changes in the properties of the reticular oxygen of the metal oxides. The kinetics of the methane oxidation in the concentration interval of 1000-5000 ppm follows a power-law kinetic model fairly well, with reaction orders between 0.06 and 0.77. Acknowledgment. This work has been supported by the European Commission (Contract ENV4-CT97-0599) and the Regional Asturian Government (Contract PC-CIS01-30). EF800704E

Combustion of Methane in Lean Mixtures over Bulk Transition-Metal

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