Activity of MnOx–CeO2 catalysts in combustion of low concentrated methane

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Activity of MnOx –CeO2 catalysts in combustion of low concentrated methane M.M. Fiuk, A. Adamski ∗ Faculty of Chemistry of the Jagiellonian University, Ingardena 3, PL 30-060 Krakow, Poland

a r t i c l e

i n f o

Article history: Received 2 November 2014 Received in revised form 8 January 2015 Accepted 28 January 2015 Available online xxx Keywords: MnOx –CeO2 binary oxides Environmental catalysis Lean methane combustion Structural and spectroscopic characterization Heterogeneity Reducibility

a b s t r a c t Due to its warming potential over 20 times higher than that of CO2 , methane is classified as one of the most dangerous pollutants. Removal of methane emitted particularly from low calorific sources, such as coal mine ventilation air streams or dumping grounds, falls thus in line with the most important challenges of environmental catalysis. Main goal of this work was to verify the efficiency and to understand the catalytic behavior of a series of coprecipitated MnOx –CeO2 mixed oxide samples, containing nominally 30–80 mol% of MnO2 . A plethora of structural (XRD, RS), textural (BET), and spectroscopic (EPR, XPS, and RS) methods have been applied for sample characterization. Catalytic activity has been determined by TPSR in the temperature range from 100 to 700 C. It was shown that catalytic activity of the investigated MnOx –CeO2 samples is strongly related to their structural heterogeneity and reducibility of various manganese centers coexisting within the surface MnOx clusters interacting with nanocrystalline CeO2 . © 2015 Elsevier B.V. All rights reserved.

1. Introduction Efficient reduction of greenhouse gases (GHG) emitted from both stationary and mobile sources belongs to main environmental challenges over last few decades [1,2]. Based on its warming potential, over 20 times higher than that of CO2 , methane is classified as one of the most dangerous atmospheric pollutants and unfortunately its emission from anthropogenic sources is still growing [1,3,4]. Methane emitted from low calorific sources, such as coal mine ventilation air streams, dumping grounds, or rice paddies, contributes to the total anthropogenic emission in relatively high extent of about 27–34% [4–6]. Catalytic removal of such diluted methane falls thus in line with the most important trends in environmental catalysis. Unfortunately, due to high chemical stability of CH4 molecules, methane concentrations of only a few % and relatively large gas flows, this task is rather demanding. Despite many efforts focused on elucidation of the molecular mechanism of C–H bond activation in light hydrocarbon molecules or directly on elaboration of a promising catalytic system [7–10], the problem of an efficient and stable catalyst permitting to oxidize lean methane at temperatures of around 400 ◦ C is still unsolved. Two groups of catalysts have been extensively studied in CH4

∗ Corresponding author. E-mail address: [email protected] (A. Adamski).

combustion context: oxide-supported noble metals (mainly Pt and Pd) and bare and supported oxides. Even if noble metal-based catalysts exhibit per site higher activity in comparison to oxide catalysts, they are more expensive and their thermal stability is not satisfactory even in relatively low temperature windows, i.e. below 600 ◦ C [9,11,12]. Moreover, influence of water (and sulfur at higher temperatures) can be a serious drawback to the efficient use of noble metal-based catalysts for methane emission abatement. Oxide catalysts can thus be a more reasonable alternative to the noble-metal-containing catalysts. Among the most frequently investigated single oxides, tested in the recent years in methane combustion, Co3 O4 , CuO, Mn2 O3 , Cr2 O3 , NiO, and CeO2 can be considered as the most frequently studied [13,14], whereas binary oxide systems, like MnOx –CeO2 , CoO–MnOx , ZrO2 –CeO2 , CuO–MnOx belong to the most interesting [14–16]. Finally, the supported oxide systems, including Co3 O4 /CeO2 , CuO/ZnAl2 O3 , and CoCr2 O4 /CeO2 [17,18] also attracted wide scientific interest. As it can be inferred from the above list, catalysts containing such elements as manganese and cerium belong to the most promising. It was the main reason why the MnOx –CeO2 system was chosen also for our studies. However, it should be stressed that further progress in research on catalyst design is practically impossible without better understanding the molecular pathways of activation of CH4 molecules. Manganese and cerium oxides are well known as redox catalysts of structural properties strongly dependent on preparation procedures and reaction conditions. Synthesis route,

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Please cite this article in press as: M.M. Fiuk, A. Adamski, Activity of MnOx –CeO2 catalysts in combustion of low concentrated methane, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.01.029

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active phase concentration, and thermal treatment conditions are the main factors determining surface architecture of such catalysts. In turn, the population of various surface oxospecies is directly responsible for catalyst selectivity. Our main goal was thus to propose a composition of the MnOx –CeO2 catalyst optimal for lean methane combustion starting from determination of parameters, which remain essential for the methane activation process. Identification of the most crucial parameters influencing methane combustion at low concentrations and elucidation of their intimate role may be helpful for more rational design of a catalyst optimal for practical applications. 2. Experimental 2.1. Synthesis of the binary MnOx –CeO2 oxides A series of four binary oxide MnOx –CeO2 samples has been synthesized by coprecipitation, according to the procedure described in [15]. In all cases the concentrated solutions of (NH4 )2 Ce(NO3 )6 (Aldrich, 99%) and Mn(NO3 )2 ·4H2 O (Lachner, 97%) were used as precursors. Four catalysts containing nominally 30, 50, 70, and 80 mol% of MnO2 have been prepared using 2 M NaOH solution as a precipitating agent. The precipitant has been added dropwise to the solutions of precursors at a temperature of about 70 ◦ C until pH was equal to 10. Prior to filtration and washing with distilled water, all precipitates have been aged under stirring in the mother liquor for 12 h in the temperature range of 75–85 ◦ C. Washing has been repeated several times until negative result of the flame test for sodium ions was achieved. All samples were dried in 100 ◦ C and calcined in 600 ◦ C for 6 h. According to the nominal MnO2 content in the binary MnOx –CeO2 samples, they will be labeled hereafter as: Mn30, Mn50, Mn70, and Mn80, where the number stands for molar percentage of MnO2 . 2.2. Characterization techniques X-ray diffraction was performed using X’pert Pro diffractometer equipped with PW3050/60 goniometer with a CuK␣ lamp ˚ Anode voltage and anode current values were set ( = 1.54060 A). to 40 kV and 30 mA, respectively. Diffractograms were collected in 2 angles in the range of 4–70◦ with steps of 0.026 ◦ C. All diffraction patterns were indexed, based on the data collected in the PDF database [19]. UV-vis DR spectra were recorded on a Perkin Elmer Lambda 12 spectrophotometer, equipped with an integrating sphere with a diameter of 5 cm. Samples were pounded with BaSO4 (also used as a reference sample) in an agate mortar. Spectra were recorded with a resolution of 0.5 nm in the wavelength range of 200–900 nm. Recorded reflectance was recalculated by the Kubelka–Munk function. The results were baseline-corrected and normalized relative to the maximum of the spectrum, smoothed and deconvoluted. XPS spectra of the coprecipitated catalysts were recorded at room temperature under ultra-high vacuum (UHV) using a PREVAC surface analysis system. As a source of monochromatic radiation an AlK␣ lamp (1486.6 eV) was used. When recording a spectrum, a semispherical Scienta R3000 photoelectron energy analyzer was used. The spectra were measured with a pass energy of 100 eV. Ranges characteristic of the Ce 3d (BE = 875–925 eV), Mn 2p (BE = 635–665 eV), and O 2p (BE = 525–540 eV) were cut of the entire range of the collected spectrum. Spectra were deconvoluted in order to determine the speciation of various manganese ions in samples. In order to measure the Raman spectra, a Renishaw inVia Raman Microscope equipped with a laser of excitation wavelength of  = 785 nm was used. RS spectra were measured between 150 and

950 cm−1 with a laser power of 0.05%. To reduce the signal to noise ratio in each case 18 accumulations were collected. Measured spectra have been subjected to the baseline correction and normalization with respect to the intensity of the most intensive signal in the spectrum. CW-EPR spectra were recorded at −196 ◦ C using an E-500-10/12 ELEXSYS spectrometer with the field modulation of 100 kH. The spectra were subjected to the normalization with respect to the difference between the least and the most intensive signal in the spectrum. In order to determine the integral intensity of the EPR spectra, the double integration was performed. Specific surface areas were determined by the BET method using a Quantasorb Junior (Ankersmit) sorptometer. The samples were outgassed in a nitrogen atmosphere at 250 ◦ C. Nitrogen adsorption was performed at −196 C. The measured specific surface areas were in the range of 46–78 m2 /g. Catalytic activity tests were performed in temperatureprogrammed surface reaction mode (TPSR) using a custom-built flow reactor equipped with the QMS detector. The catalysts sieve fractions with a grain diameter of 200–300 ␮m was used for measurements. In each case the catalyst bed was subjected to the stream of 5% O2 /He (20 ml/min) at 400 ◦ C for 40 min, and then in a stream of helium (30 ml/min) at 400 ◦ C for 30 min. The proper TPSR measurement was accomplished using the stream of 5% O2 /He (20 ml/min) and 1% CH4 /He (20 ml/min) in the temperature range of 20–920 ◦ C with a heating rate of 10 ◦ C/min. Optimized flow rate equivalent to weight hourly space velocity (WHSV) of 0.03 h−1 was kept constant, permitting us to compare the results obtained for various catalysts tested in lean methane combustion. 3. Results and discussion 3.1. Catalytic activity For all investigated samples only CO2 and water were detected as reaction products. Fig. 1 shows the conversion curves of catalytic combustion of lean methane on the investigated binary MnOx –CeO2 oxide catalysts. All samples were active in the studied reaction, reaching 100% conversion at temperatures above 550 ◦ C. Based on the determined values of T50 temperatures it can be stated that up to 70 mol% of MnO2 , the catalytic activity progressively increases with growing manganese content in the investigated samples. The catalyst containing nominally 70 mol% of MnO2 exhibited the best catalytic performance (T50 = ∼435 ◦ C). Further increase of the nominal content of manganese to 80 mol% resulted in a remarkable decrease in the catalytic activity (T50 = ∼460 ◦ C). Comparing our results to those reported by other authors who studied methane oxidation over manganese- and ceria-based catalysts, it can be concluded that our samples showed quite satisfactory activity, taking into account 1% methane concentration in our reaction

Fig. 1. Catalytic activity (TPSR) of the investigated samples in lean methane combustion.

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Fig. 2. Diffraction patterns (A) and RS spectra (B) of the coprecipitated MnOx –CeO2 binary oxide catalysts: (a) Mn30, (b) Mn50, (c) Mn70, and (d) Mn80.

mixture, typical of the low calorific sources. In the majority of reported cases T50 temperatures were in the region of 450–585 ◦ C (regardless of the preparation procedure) at methane concentrations of 20% and of about 1%, respectively [15,16,20,21]. It is also noteworthy that manganese content in the cited cases was from 50 to 90 mol%, i.e. it was comparable or higher to that in our best Mn70 sample. Single cerium and magnesium oxides prepared in a similar way and tested earlier in our laboratory in lean methane combustion showed distinctly lower activities in comparison to those reported above for our MnOx –CeO2 samples. The determined T50 temperatures were ca. 560 ◦ C for MnOx and ca. 575 ◦ C for CeO2 . As reported by Palmqvist et al. [21], the catalytic activity of pure ceria in lean methane combustion is rather weak, T50 = 607 ◦ C (at methane content in reaction mixture equal to 1.4%). It is thus quite clear that the presence of manganese oxide in catalyst composition and its interaction with CeO2 were beneficial for the observed catalytic behavior. On the other hand, no correlation between catalytic activity and changes in specific surface area of the investigated samples, passing through a maximum for the Mn50 catalyst, was observed. This suggests a predominant role of active phase speciation at determining catalytic performance. 3.2. Structural studies To elucidate catalytic results presented above, a detailed structural characteristic study has been performed by both XRD and RS (Fig. 2A,B). The latter method is much more sensitive to the localization of light elements, like oxygen, and can sometimes provide more reliable structural information in comparison to XRD [22]. The presence of the crystalline CeO2 , giving rise to the maxima at 2 ≈ 29, 33, 47.5, 56.5, and 59◦ attributed to 111, 200, 220, 113, and 222 Bragg reflections, respectively [23], was confirmed in all recorded X-ray diffraction patterns (Fig. 2A). Just traces of the Mn5 O8 mixed-valence phase can be observed in the case of samples containing nominally 30 or 50 mol% of MnO2 (Fig. 2Aa and b) [24]. Most probably at relatively low MnO2 contents (below 50 mol%) manganese(IV) ions can easily be dispersed and stabilized within the CeO2 lattice, forming corresponding Ce1–x Mnx O2 solid solutions. The effective Mn4+ radius is equal to 0.81 A˚ and is only slightly ˚ [25]. As the Mn30 and Mn50 samsmaller than that of Ce4+ (0.92 A) ples exhibited higher catalytic activity than that observed for single CeO2 or MnOx oxides in the tested temperature range, it is thus

quite obvious that the Ce1–x Mnx O2 solid solutions can contribute to the active components occurring in the MnOx –CeO2 catalysts, as postulated by Palmqvist et al. [21]. The catalytic activity of Mn30 and Mn50 samples was however lower than those determined for the catalysts of higher Mn loadings, where the segregated ␥-Mn2 O3 and Mn3 O4 phases accompany Mn5 O8 , giving rise to the characteristic features, identified in the reported XRD patterns (Fig. 2Ac and d) [24]. It is noteworthy that Mn2 O3 seems to be an intermediate structure, appearing in the case of Mn70 only, and being finally transformed into higher oxides Mn3 O4 and/or Mn5 O8 . On the other hand, the lack of the ␥-Mn2 O3 phase could explain the decreasing of the catalytic activity observed in the case of the Mn80 sample. Generally, analyzing phase composition of the catalysts investigated by XRD, it can be concluded that an increase in the nominal content of manganese in MnOx –CeO2 samples strongly favored their structural heterogeneity. According to the results presented above, it can be postulated that the most active phases in combustion of lean methane are Mn5 O8 and ␥-Mn2 O3 in the presence of cerium(IV) oxide. As it was stated above, the Ce1–x Mnx O2 solid solutions can also contribute to the active phase, but they are not considered as the most active species in lean methane combustion. In all collected Raman spectra (Fig. 2B) characteristic bands assigned to F2g -breathing vibrations of CeO2 , contributing to the band at about 463 cm−1 , and Ag -vibrations of Mn5 O8 , giving rise to the strong band at 647 cm−1 , can be identified. Other bands of the latter phase cannot be unambiguously attributed based on the accessible literature data [26,27]. Only in the Raman spectrum of the Mn80 sample (Fig. 2Bd), besides the previously described bands, the new band appeared at lower Raman shifts. The low intensity band at around 362 cm−1 can be attributed to the Mn3 O4 phase, previously suggested also by our XRD results [28]. In the case of the Mn70 sample (Fig. 2Bc), the bands characteristic of ␥-Mn2 O3 and Mn3 O4 phases are not visible in the corresponding RS spectrum because of their overlapping with more intense bands arising from Mn5 O8 , predominantly Mn-containing surface phase. 3.3. Status of manganese and cerium centers It is expected that structural heterogeneity revealed by XRD and RS should also be reflected in manganese speciation at the nanoscale level, which is much more important for catalytic properties. It was shown in the previous section that various manganese

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Fig. 3. Recorded at liquid nitrogen temperature EPR spectra of the Mn70 catalyst measured before and after catalytic test (A), integral intensities of the EPR spectra collected before and after TPSR for all samples (B), the intensity ratio between EPR spectra recorded for samples after (IAFTER ) and before (IBEFORE ) the catalytic test, compared to the corresponding T50 temperatures (C).

oxide phases were identified on the catalyst surface, dependent on the nominal manganese content. EPR, XPS, and UV-vis spectroscopic techniques have been applied to provide deeper insight into the status of manganese centers in the investigated samples. All EPR spectra recorded both before and after the catalytic test confirm the presence of isolated or weakly magnetically interacting Mn2+ (3d5 ) centers (Fig. 3A), giving rise to the slightly anisotropic signals around gav = 1.990–1.998 with a characteristic six-line hyperfine structure (Aav = 89–90 G), originating from interaction of an unpaired electron with isolated 55 Mn nuclei (I = 5/2, nat. abund. 100%) [29,30]. The presence of Mn2+ in all investigated samples confirmed partial reduction of manganese centers. Due to fast relaxation processes, higher oxidation states of manganese, similar to Ce3+ centers, are hardly visible in EPR at liquid nitrogen temperature and practically do not contribute in the analyzed spectra. The presence of Ce3+ was however independently confirmed in all investigated samples by UV-visible DR spectra (not shown here). The broad absorption band appearing in the range of 250–400 nm was attributed to charge transfer transitions associated with the presence of surface Ce4+/3+ –O2− forms. The absorption maxima below 400 nm are associated with charge transfer Ce4+ ←O2− , occurring in the case of surface low-coordinated cerium ions, whereas those at lower wavelengths (200–220 nm) can be connected to the corresponding CT transitions within Ce3+ ←O2− couples. Location of maximum absorption varied in the range of 310–350 nm and was slightly higher than that observed in the case of pure CeO2 [30–32], confirming interaction between CeO2 and manganese ions within the MnOx –CeO2 system leading to the formation of Ce1–x Mnx O2 solid solutions postulated earlier. Another broad band at around 455–489 nm can be attributed to d–d transitions, 5 T2 ← 5 E2 of Mn3+ ions (3d4 configuration), occurring in small MnOx clusters with octahedral coordination of Mn centers [32–35]. It partially overlaps with the CT bands described above. The symmetry disorder around Mn3+ ions in previously described oxide clusters leads to the presence of additional absorption bands centered about 370 and 785 nm [33]. In turn, d–d transition (6 A1 ← 4 T2 ), giving rise to the characteristic band assigned to Mn2+ ions, located at about 600–650 nm, is a spin-forbidden transition [32]. Thus it has relatively low extinction coefficient and therefore was not observed in the recorded UV-vis DR spectra. According to Ivanova [35], Mn(IV) centers in MnO2 contribute to a broad absorption band with a maximum at around 200 nm. In the case of our spectra, it was attributed to the lowest lying band in the range of 262–316 nm. The presence of aliovalent Mn states was also confirmed by XPS spectra in all analyzed samples (Table 1A). Typical XPS results

recorded for the most active and most heterogeneous Mn70 catalyst are shown in Fig. 4. Windows characteristic of O1s , Mn2p , and Ce3d are presented. The detailed inspection of the Mn2p doublet at 640–600 eV suggests the presence of manganese in the oxidation states of II, III, and IV at the surface of the investigated Mn70 catalyst [36,37]. Simultaneously, the abundance of manganese(II) decreased with increasing the nominal content of Mn in the catalysts, while the content of superficial Mn(III) increased. This demonstrates that the increase in the nominal content of manganese in the investigated catalysts favors the reduction of Mn(IV) to Mn(III) ions. Analyzing the XPS spectra of the examined MnOx –CeO2 oxide catalysts, showed the presence of various signals associated with inequivalent surface oxygen forms reflecting different speciation of this element (Table 1B). Oxygen species have been identified as: network oxygen (OI ), responsible for the band at BE of 528–529 eV and hydroxyl/defect oxygen (OII ) contributing to the band at BE of 530–531 eV and oxygen associated with water chemisorbed on the surface of the catalyst (OIII ), giving rise to low intensity band at about 535 eV [36]. It can be confirmed that in the case of the Mn70 and Mn80 catalysts, which were active in the oxidation of methane, its surface is characterized by the highest contributions of OII defect oxygen as evidenced by the highest ratio of OII /OI components equal to 1.17 and 1.32, respectively, whereas in the case of the less homogeneous and less reactive sample Mn30 this ratio was equal to 0.53. Most probably defect oxygen remains in relation to the aliovalent manganese centers stabilized within small surface oxoclusters strongly interacting with ceria.

Table 1A Content of manganese at various oxidation states determined from XPS spectra of MnOx –CeO2 catalysts. Sample

Mn(II) (640 eV)/%

Mn(III) (641 eV)/%

Mn(IV) (642 eV)/%

Mn30 Mn50 Mn70 Mn80

76 60 41 37

16 34 41 38

8 6 18 25

Table 1B Content of various forms of oxygen determined from XPS spectra of MnOx –CeO2 catalysts. Sample Mn30 Mn50 Mn70 Mn80

OI (528–529 eV)/%

OII (530–531 eV)/%

OII /OI

65 71 46 43

35 29 54 57

0.53 0.41 1.17 1.32

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coprecipitation with 2 M NaOH, confirmed the efficiency of the hybrid MnOx –CeO2 oxide systems of the enhanced structural and functional heterogeneity as catalysts for total combustion of CH4 at concentrations as low as 1 vol%. T50 conversions obtained for the most active Mn70 catalysts were about 435 ◦ C. It was shown that the catalytic activity mainly correlates with three parameters: (i) enhanced structural heterogeneity, (ii) the presence of defect oxygen, and (iii) a relatively easy passage between various oxidation states of Mn centers within small MnOx clusters coexisting with nanocrystalline CeO2 . Aliovalent Mn5 O8 and Mn2 O3 oxide phases undergoing reversible reduction during their interaction with methane can be considered as the most active phases. Acknowledgements Work initialized in the years 2010–2013 in the framework of the international EIT KIC InnoEnergy SECoal project and continued within the NCN project No. 2011/03/B/ST5/01564. M.S. Marek Drozdek from the Faculty of Chemistry of the Jagiellonian University is greatly acknowledged for recording the XPS spectra. References

Fig. 4. Typical XPS results–characteristic windows for the Mn70 sample.

As it was shown in Fig. 3A and B, the integral intensity of the EPR spectra measured after the catalytic cycle distinctly increases. The observed increase is the highest (almost by 200%) in the case of the most active (and the most heterogeneous) Mn70 sample. In the other cases it was slightly lower (75–115%). Reduction ability can thus be directly correlated to the catalytic activity of the MnOx –CeO2 samples in methane combustion (Fig. 3C). The probable scheme of the redox changes occurring during an individual catalytic cycle is the following: interaction with methane leads to the partial reduction of Mn(III) and Mn(IV) centers existing within various oxomanganese phases. Simultaneously, CH4 molecules can be oxidatively activated. In the next step oxygen from the gas phase regenerates the reduced manganese centers. On the other hand, taking into account the structural information provided in the previous sections, the reducibility correlates also to surface heterogeneity and is the highest for the most heterogeneous Mn70 sample. In the case of the most abundant Mn5 O8 and Mn2 O3 phases coexisting with crystalline CeO2 and defect oxygen, the passage between various oxidation states of manganese seems to be the easiest. 4. Conclusions The results obtained for a series of the binary MnOx –CeO2 catalysts, containing nominally 30–80 mol% of MnO2 , synthesized by

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