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2007, 111, 2830-2833 Published on Web 01/26/2007
Observation of the Reversible Phase-Transformation of r-Mn2O3 Nanocrystals during the Catalytic Combustion of Methane by in Situ Raman Spectroscopy Yi-Fan Han,* Kanaparthi Ramesh, Luwei Chen, Effendi Widjaja, Srilakshmi Chilukoti, and Fengxi Chen Institute of Chemical and Engineering Sciences, 1, Pesek Road, Jurong Island, Singapore, 627833 ReceiVed: December 18, 2006; In Final Form: January 15, 2007
Real-time in situ Raman spectroscopy is used to probe the near-surface structure of R-Mn2O3 nanocrystals during methane combustion for the first time. A surface phase-transformation from R-Mn2O3 to Mn3O4-like species, as evidenced by the formation of a single band at 648 cm-1, was observed only at or above 400 °C in the presence of pure He or in the mixture of O2 + CH4. This modification is probably due to the loss of lattice oxygen at high temperatures that leads to the surface reconstruction. Very interestingly, a reversible phase-transformation was observed with decreasing the temperature to 25 °C. The phase-change at the nearsurface is suggested to be associated with the activating C-H bond of methane.
1. Introduction Manganese oxides (MnOx) and their mixtures with other oxides (MnOx-MxOy: M ) V, Cr, Co, Fe, Cu, Zr, etc.) have been proven to be highly active, durable, and low-cost catalysts for completely oxidizing various volatile organic substances or hydrocarbons.1-11 Among them, methane combustion on MnO x catalysts has been widely studied because of its attractive potential in abating CO2 emission required by Kyoto protocol,1,12,13 for example, burning turbines fueled by natural gas. More recently, because of the increase in number of naturalgas-fueled vehicles, stricter laws are now being enforced for controlling methane emissions. Methane is a flammable gas, which is also contributing to the greenhouse effect.13 However, methane combustion on manganese oxide catalysts was a vital process in the removal of gas mixtures from coal mines 100 years ago,2 and methane nowadays is regarded as a recoverable energy.3 Furthermore, it is evident that a single phase of Mn2O3 crystallites exhibited remarkable catalytic activity toward reactions, such as the oxidation of ethylene14 and carbon monoxide15,16 and decomposition of NOx.17 Therefore, it is essential to study the variation of the surface structure of this catalyst induced by reactants, temperature, and other factors under the reaction conditions in order to understand and establish the reaction mechanism. Because of the facile phase-transformation at high temperatures, for example, >400 °C, the surface structure of MnOx in the case of the combustion of methane, which is the most difficult hydrocarbon to be oxidized among the combustibles obtained,18 is not clearly understood. To date, in situ techniques, or so-called “operando” spectroscopies, suitable for the determination of the surface structure of working catalysts directly at high temperatures are very limited. Among them, the Raman scattering technique is a useful spectroscopic tool to characterize metal oxides;19-25 in particular, it has high sensitivity even to * Corresponding author. E-mail:
[email protected].
10.1021/jp0686691 CCC: $37.00
a tiny modification of the near-surface structure, and the spectroscopy is not interfered by the reactants in the gas phase. Several studies have been conducted on the bulk and supported MnOx using laser Raman spectroscopy (LRS),26-32 indicating that the near-surface structure could possibly be modified in various catalytic reactions. However, convincible evidence is still not available because of technical limitations. To our knowledge, so far, in situ Raman spectroscopy has rarely been addressed for investigating manganese oxides under reaction conditions, especially the crystals in the nanoscale, during methane combustion, or any other oxidation reactions. In the present study, a nanostructured R-Mn2O3 catalyst with coral-like features was prepared by oxidative decomposition of MnCO3. Later, the prepared R-Mn2O3 nanocrystals were tested in the reaction of methane combustion. The near-surface structure of the working catalyst was then monitored by using Raman spectroscopy at different temperatures. 2. Experimental Section 2.1. Catalyst Preparation and Reactivity Measurements. Nanostructured R-Mn2O3 was prepared by direct thermal decomposition of MnCO3 powder (Aldrich, batch no. 13520DD) in static air. Controlled calcination of high purity MnCO3 powder was carried out at 700 °C for 5 h with a ramping rate of 1 °C/min. Activity measurements were carried out in a micro-plug-flow reactor. Prior to each experiment, the catalysts were pretreated in 20 mL/min of Ar flow at 50 °C for 2 h. The temperaturedependent activity was carried out in a stream of 0.5% CH4, 3.0% O2, and helium with a space velocity of 36 000 h-1 at atmospheric pressure. The analysis of the reactants and products were performed with an online GC (Shimadzu GC-2010) equipped with a CP-carbonBOND column. For comparison, bulk R-Mn2O3 (Aldrich, 99.999%, 4.8 m2/g) and Mn3O4 (Aldrich, 99.98%, 4.0 m2/g) with an average size of 0.5 µm were tested under the same conditions. © 2007 American Chemical Society
Letters
Figure 1. Temperature-dependent catalytic activity of methane combustion over R-Mn2O3 nanocrystals (b), bulk Mn3O4 (2), and bulk R-Mn2O3 (1)
2.2. Characterization. LRS. Raman spectra were measured with an operando setup using a Raman microscope (InVia Reflex, Renishaw) equipped with deep-depleted thermoelectrically cooled CCD array detector and a high-grade Leica microscope (long working distance objective 50×). The sample was placed into the sample holder specially designed to study catalytic reactions at high temperature and pressure (CCR1000, Linkam fitted with quartz windows). The sample was mounted on unreactive disposable ceramic fabric filters placed inside the ceramic heating element, which is capable of heating samples from ambient up to 1000 °C. The reaction conditions are the same as those for the micro-fixed reactor. The carrier gases (3.0% of O2, 0.5% of CH4 diluted in He or He only) were introduced into the catalyst stage via a high-pressure 1/16 in. gas line. The gases passed through the sample and ceramic fabric filter with the flow of ca. 50 mL/min. The flow rates of the reactant gases were controlled by a set of mass flow controllers. Raman measurements were performed on the same sample spot irradiated by a visible 514.5 nm argon ion laser. The laser power from the source is normally around 25 mW, which is very close to the power (20 mW) used previously for MnOx.32 However, it should be noted that the potential heating problem induced by the laser, which usually interferes with the acquisition of the real spectroscopy, has been controlled very carefully. Thus, the laser power was minimized as much as possible by adjusting the laser power (using density filters) and optimizing the time for scanning without sacrificing too much on Raman signals. It is estimated that only about 1-2 mW of the laser power reached the samples during the measurements. So, in situ Raman signals are noisy, and high precision of the peak wave numbers cannot be available. The signal for LRS was attenuated significantly above 500 °C. The scanning time for each Raman spectrum was ca. 360 s with spectral resolution around 1-1.3 cm-1. The temperature ramping rate is 10 °C/min when the sample was heated from 25 to 500 °C. Note that any structural change induced by the laser was ruled out by the time-dependent spectra measured in the absence of reactant and helium. XRD. X-ray diffraction patterns were obtained with a Bruker D8 diffractometer using Cu KR radiation (λ ) 1.540589 Å). The texture properties of the prepared R-Mn2O3 nanocrystals can be seen in the Supporting Information. 3. Results and Discussion Figure 1 shows that the conversion of methane over the R-Mn2O3 nanocrystals as well as the bulk R-Mn2O3 and Mn3O4 started at ca. 400 °C and increased with increasing temperature.
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Figure 2. XRD patterns for the R-Mn2O3 nanocrystals (A, fresh; B, spent) and bulk R-Mn2O3 (C). The reaction was carried out in the stream of 0.5% CH4, 3.0% O2, rest He; space velocity, 36 000 h-1 at 550 °C for 10 h.
Figure 3. Ex situ Raman spectra (normalized) of R-Mn2O3 nanocrystals as-prepared (A) and the spent catalyst (B). The reaction was carried out under the same conditions as those in Figure 2.
The complete conversion of methane is achieved at 625 °C for the R-Mn2O3 nanocrystals. Comparatively, the light-off temperature (defining the temperature point for 20% conversion of methane) is 486, 613, and 597 °C for the R-Mn2O3 nanocrystals, the bulk R-Mn2O3, and Mn3O4, respectively. However, loss of 2.0% of the initial activity (at 550 °C, 0.5% CH4, 3.0% O2, rest He) was observed after running for 24 h, an indication of high catalytic stability. Figure 2 shows that the powder XRD pattern for the nanocrystals did not change during the reaction. It is still identical to the spectrum obtained for the bulk Mn2O3, corresponding to the typical bixbyite R-Mn2O3 (JCPDS 41-1442). The average particle size was calculated by the Debye-Scherrer formula based on the main reflection peaks at (222), (440), and (622). The particle size of R-Mn2O3 nanocrystals and the surface area (BET) were ca. 50.0 nm and 19.0 m2/g, respectively. Obviously, the textural properties of R-Mn2O3 nanocrystals were little affected by the reaction. Moreover, the particle sizes evaluated from the XRD patterns for all catalysts are comparable with those visualized from SEM (see Figure S1), as displayed in Table S1. Ex situ LRS obtained from the fresh and spent catalysts under ambient conditions are shown in Figure 3. In comparison with the Raman spectroscopy obtained from the bulk Mn2O3,15,16 the bands at 307, 341, 637, and 686 cm-1 detected for the R-Mn2O3 nanocrystals may correspond to the out-of-plane bending modes of Mn2O3, asymmetric stretching of bridge oxygen species (Mn-O-Mn), and symmetric stretching of Mn2O3 groups, respectively. Actually, it is very difficult to ascribe any structure
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Letters shown in Figure 4b, also showed the same variation tendency. The band at 648 cm-1 was seen at 400 °C, and a mild growth of the peak was observed at 500 °C; comparatively, its intensity was lower than that from He flow. It should be emphasized that this phenomenon was not clearly observed for the bulk R-Mn2O3 under the same experimental conditions. In accordance with the previous Raman spectra obtained from various MnOx,15,16,22,32 the band at 648 cm-1 formed at elevated temperatures, to be close to that (655 cm-1) observed for the bulk Mn3O4,15,16 and is likely attributed to a Mn-O-Mn stretching of Mn3O4 group. The general transition of MnOx at elevated temperatures due to the loss of lattice oxygen has been demonstrated as33 900 °C
1700 °C
Mn2O3 98 Mn3O4 98 MnO
Figure 4. Temperature-dependent in situ Raman spectra (normalized) for the R-Mn2O3 nanocrystals in the presence of He (a) and in the presence of 0.5% CH4, 3.0% O2, rest He (b); ramping rate, 10 °C/min; flow rate, 50 mL/min. (A) 25 °C, (B) 100 °C, (C) 200 °C, (D) 400 °C, (E) 500 °C, (F) back to 25 °C.
modifications by these bands. Interestingly, the in situ LRS in Figure 4 showed quite different profiles, which revealed a nearsurface structure modification during methane combustion. Note that useful information can still be discerned from those spectra even though the signals from in situ experiments were remarkably attenuated because of the limitations of Raman spectroscopy, only ca. 25% of that from ex situ LRS. Temperature-dependent LRS were recorded in the stream of He and the mixture of 0.5% CH4 and 3.0% O2 in He, respectively. As shown in Figure 4a, the spectrum obtained in He at 25 °C was similar to the ex situ spectra presented in Figure 3. With increasing temperature to 400 °C, the bands at 686 and 637 cm-1, corresponding to the stretching vibrations of the bridge oxygen species, shifted to 678 and 622 cm-1, respectively, accompanying the decrease of their intensities. Moreover, a new band at 648 cm-1 appeared at 500 °C, indicating the formation of new phase. Surprisingly, the new band disappeared completely, and a spectrum corresponding to R-Mn2O3 was observed again when temperature returned to 25 °C in He flow, suggesting that this structure change is reversible. The temperature-dependent spectra obtained from methane combustion, as
The generation of bulk Mn3O4 occurs only when the calcination temperature is greater than 900 °C. However, Raman spectra in Figure 4 demonstrated that the transformation of R-Mn2O3 to Mn3O4-like species could happen at as low as 400 °C, probably because of the loss of lattice oxygen that leads to the reconstruction of the surface phase. Interestingly, the feature for R-Mn2O3 vibrations reappeared after cooling down to room temperature. It reveals that the structure change during the reaction occurs only in the region near the surface. However, XRD spectra in Figure 2 also demonstrate that there is almost no modification for the bulk structure after reaction. The reverse transformation of the phase might be caused by the diffusion of the oxygen from the sublattice or bulk to the surface. Alternatively, the supplement of the oxygen is possible from the gas phase as shown in Figure 4b, which may delay the phasetransformation. Thus, a possible explanation is that the intensity of the band at 648 cm-1 is lower in the methane combustion than in He. As for the present catalytic system, more interestingly, the temperature for the formation of a Mn3O4-like species almost coincided with the onset of methane oxidation, as illustrated by Figures 1 and 4. Generally, for manganese oxides the catalytic activity is known to depend strongly on the ability of manganese to form various oxidation states, for example, redox reaction of Mn2+/Mn3+ or Mn3+/Mn4+, and “oxygen mobility” in the oxide lattice. Therefore, the reversible phase-transformation between the R-Mn2O3 and Mn3O4-like structure during the reaction has important implications for catalysis. The lattice oxygen of MnOx catalysts is considered to be the primary active species for activating the C-H bond of hydrocarbons, especially when reaction temperature is above 400 °C,34 as illustrated by eq 1.
CHx (ads) + O2- (lattice) + 2Mn4+/3+ f CO2 (ads) + H2O (ads) + 2Mn3+/2+ (1) The results in Figure 1 showed that the activity of the R-Mn2O3 nanocrystals was considerably improved compared to those for the bulk R-Mn2O3 and Mn3O4. Except for the relatively high surface area, we argue that the higher mobility of the lattice oxygen in the former should be a primary factor that leads to the enhancement of activity because under similar conditions we did not observe any changes of the surface structure for the latter. To demonstrate this proposal, the mobility of the lattice oxygen for both catalysts has been investigated through the experiments of temperature-programmed desorption of oxygen (TPD, Figure S2). The desorption of the lattice oxygen appeared at 305 °C for the nanocrystals and 450 °C for
Letters the bulk; while the desorbing oxygen amount (by peak areas) was significantly reduced with the increasing of crystal size, about one tenth measured for the bulk compared to that for the nanocrystals. A detailed discussion about this issue will be given elsewhere. Clearly, through the real-time in situ Raman spectroscopy we are able to observe a transitional phase-change produced during the methane combustion over the R-Mn2O3 nanocrystal catalyst. To the best of our knowledge, these observations have not ever been reported so far. We believe that these preliminary, also unique, results will enlighten people to attain deep insight into MnOx-based catalysts for the oxidation of hydrocarbons. Nevertheless, the role of the Mn3O4-like species in methane combustion is still not clear. The relevant study about this question is underway. 4. Conclusions Several conclusions can be drawn from this study. It is evident that the R-Mn2O3 nanocrystals prepared by oxidative decomposition of MnCO3 have superior activity to the bulk R-Mn2O3 and Mn3O4 in methane combustion, possibly because of the high mobility of the lattice oxygen. The real-time in situ Raman spectroscopy has demonstrated that a reversible phase-transformation, forming a Mn3O4-like phase, might occur in the nearsurface region of the R-Mn2O3 nanocrystals catalyst during methane combustion. The observed phenomenon indicates that the lattice oxygen may be the primary active species as suggested previously. Acknowledgment. We thank Dr. P. K. Wong, ICES, A*Star, Singapore, for his comments on this study. Supporting Information Available: Table with particle sizes of R-Mn2O3 nanocrystals estimated by XRD and SEM, and the SEM images for the R-Mn2O3 nanocrystals before and after reaction. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zarur, A. J.; Ying, J. Y. Nature 2000, 403, 65. (2) Anderson, R. B.; Stein, K. C.; Feenan, J. J.; Hofer, L. J. E. Ind. Eng. Chem. 1961, 53, 809. (3) Rostrup-Nielson, J. R. Catal. ReV. Sci. Eng. 2004, 46, 247.
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