Synergistic effect between MnO and CeO2 in the physical mixture: Electronic interaction and NO oxidation activity

JOURNAL OF RARE EARTHS, Vol. 31, No. 12, Dec. 2013, P. 1141

Synergistic effect between MnO and CeO2 in the physical mixture: Electronic interaction and NO oxidation activity WU Xiaodong (ਈᰧϰ)1,2,*, YU Haining (Ѣ⍋ᅕ)1, WENG Duan (㖕 ッ)1,2, LIU Shuang (߬ ⠑)1, FAN Jun (῞ ֞)3 (1. Key Laboratory of Advanced Materials of Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China; 2. Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China; 3. The Administrative Centre for China's Agenda 21, Beijing 100038, China) Received 27 June 2013; revised 3 September 2013

Abstract: MnO and CeO2 powders were mechanically mixed by a spatula and by milling to obtain loose-contact and tight-contact mixed oxides, respectively. The monoxides and their physical mixtures were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), Raman, O2 temperature-programmed desorption (O2-TPD), H2 temperature-programmed reduction (H2-TPR) and NO temperature-programmed oxidation (NO-TPO). The MnOx-CeO2 solid solutions did not form without any calcination process. The oxidation state of manganese tended to increase while the ionic valence of cerium decreased in the mixed oxides, accompanied with the formation of oxygen vacancies. This long-ranged electronic interaction occured more significantly in the tight-contact mixture of MnO and CeO2. The formation of more Mn4+ and oxygen vacancies promoted the catalytic oxidation of NO in an oxygen-rich atmosphere. Keywords: MnO-CeO2; physical mixture; electronic valence equilibrium; oxygen vacancy; NO oxidation; rare earths

MnOx-CeO2 mixed oxides have been developed as an environmental friendly and efficient low-temperature oxidation catalysts for the abatement of contaminants in both liquid and gas phases, such as NO[1,2], formaldehyde[3,4], ammonia[5], phenol[6], polyethylene glycol[7], acrylic acid[8], methane[9], ethanol[10], ethyl acetate[10], toluene[10], benzene[11] and soot[12,13]. MnOx-CeO2 mixed oxides have much higher catalytic activity than pure MnOx and CeO2 owing to a favorable synergistic effect between manganese oxide and ceria. For example, Machida et al.[1] reported that the redox of Mn ions with simultaneous oxygen equilibration with the gas phase should play a key role in facilitating the oxidative adsorption of NO. However, the nature of the interaction of CeO2 with MnOx is not well known. Most of the researches ascribed the Mn–Ce interaction to the formation of MnOx–CeO2 solid solutions. The incorporation of manganese ions into the ceria lattice can greatly improve the oxygen storage capacity of ceria as well as the oxygen mobility on the surface the mixed oxides. Based on the XPS results, the partial reduction of Mn3+ to Mn2+ is suggested to occur in the fluorite structure, and the concentration of Ce4+ in the solid solutions would increase with the Mn content increasing, which is ascribed to electron transfer from Ce to Mn[2,6,14]. Additionally, Imamura et al.[15] presented a model in which a ceria-mediated shuttle mechanism pro-

vides oxygen to MnOx at low temperature, whereas the process is reversed at high temperature. It is also observed that Mn4+ would be reduced to Mn2+ in the sol-gel-synthesized MnOx-CeO2 with the calcination temperature increasing[16]. It should be noted that the above conclusions and speculations were obtained generally based on the formation of MnOx-CeO2 solid solutions. In the present work, the physical mixtures of MnO and CeO2 were prepared by two different mixing ways to obtain different contact conditions and MnOx-CeO2 interactions. No calcinations process was introduced to avoid any influences of the oxide sintering. The structure features were characterized by XRD, BET, XPS and Raman. The surface properties and catalytic activities of the oxides were evaluated by O2-TPD, H2-TPR and NO-TPO tests. The electronic interaction between these two oxides was explored via an oxygen transfer mechanism.

1 Experimental 1.1 Catalyst preparation MnO (Aladdin, China) and CeO2 (Sigma-Aldrich, USA) were mixed by two methods with the molar ratio of Mn:Ce=15:85. The MnO-CeO2-L sample was pre-

Foundation item: Projects supported by National Natural Science Foundation of China (51072096), National Basic Research Program of China (2010CB732304) and National High-Tech Research and Development Program of China (2013AA065302) * Corresponding author: WU Xiaodong (E-mail: [email protected]; Tel.: +86-10-62792375) DOI: 10.1016/S1002-0721(12)60418-9

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pared by carefully mixing manganese oxide and ceria powders via a spatula for 2 min for loose contact conditions. The MnO-CeO2-T sample was prepared by milling in an agate mortar for 15 min for tight contact conditions. MnO2 (Aladdin, China) was also used for reference.

ent gases monitored by an infrared spectrometer (Thermo Nicolet iS10). One hundred milligrams of catalyst powders were diluted with 300 mg of silica pellets, and then were sandwiched by quartz wools in a tubular quartz reactor. The gas mixture of 1000 ppm NO/10% O2/N2 was fed at a flow rate of 500 mL/min. The reactor temperature was ramped to 650 °C at a heating rate of 10 °C/min. The maximal NO2 concentration was defined as Cm and the corresponding temperature was denoted as Tm.

1.2 Catalyst characterization The powder X-ray diffraction (XRD) patterns were determined by a diffractometer (D8 ADVANCE, Bruker, Germany) employing Cu KĮ radiation (Ȝ=0.15418 nm) operating at 40 kV and 30 mA. The X-ray diffractograms were recorded at 0.02q intervals in the range of 20qd2șd 80q with a scanning velocity of 4(q /min. The specific surface areas of the samples were measured using the N2 adsorption at –196 ºC by the four-point Brunauer-Emmett-Teller (BET) method using an automatic surface analyzer (F-Sorb 3400, Gold APP Instrument). The samples were degassed in vacuum at 200 ºC for 2 h before the test. The X-ray photoelectron spectroscopy (XPS) experiments were carried out on a PHI-Quantera SXM system equipped with a monochromatic Al KĮ X-rays under UHV (6.7×10í8 Pa). A 100 ȝm spot size was used for analysis. Sample charging during the measurement was compensated by an electron flood gun. The electron takeoff angle was 45° with respect to the sample surface. The XPS data from the regions related to the C 1s, O 1s, Mn 2p and Ce 3d core levels were recorded for each sample. The binding energies were calibrated internally by the carbon deposit C 1s binding energy (BE) at 284.8 eV. Temperature-programmed desorption with O2 (O2-TPD) was measured by an Auto Chem II Chemisorption Analyzer (Micromeritics, USA). 200 mg sample was used and pretreated with He at 300 °C for 60 min. After the sample was cooled down to 50 °C, 10%O2/He mixture gas was introduced for 60 min. After purging in He for 30 min, the reactor was heated to 900 °C in He at a rate of 10 °C/min. The Raman spectra were obtained at ambient condition on a confocal micro-Raman apparatus (Aurora J300, IDSpec, China) using Ar+ laser with a CCD detector. Pure powder supported on a sheet glass was used without any pretreatment. The laser wavelength was 632.8 nm and the exposure time was 20 s. H2 temperature-programmed reduction (H2-TPR) was conducted on a chemical adsorption instrument (AutoChem II 2920, Micromeritics, America). Fifty milligrams of the sample was sandwiched by quartz wool in a tubular quartz reactor. The reactor temperature was raised to 900 °C at a constant heating rate of 10 °C/min in 10% H2/Ar with a flow rate of 50 mL/min.

2 Results and discussion 2.1 XRD and BET Fig. 1 shows the powder XRD patterns of the monoxides and mixed oxides. In respect to MnO2, MnO presents much sharper diffraction peaks because of large particles of the raw material. Both the MnO-CeO2 mixtures present broad peaks of fluorite-like structural CeO2 phase. By milling manganese oxide and ceria powders together, MnO-CeO2-T exhibits much weaker characteristic peaks of MnO at 2ș=35.0º, 40.6º and 58.7º than MnO-CeO2-L. It has been widely accepted that the incorporation of manganese oxide into the ceria lattice would lead to the shrinkage of the ceria crystal cell, which is associated with the smaller ionic radii of Mn3+ (0.065 nm) and Mn4+ (0.053 nm) as compared to those of Ce4+ (0.097 nm) and Ce3+ (0.114 nm). However, this phenomenon is generally observed for the chemically mixed oxides prepared by the co-precipitation, sol-gel and solution combustion method[1–3,6,9,10,16–19], which is not found in the physical mixtures without any shift of the characteristic peaks of ceria. Murugan et al.[17] also did not find any change in the lattice parameter of the 20 mol.% Mn/CeO2 sample prepared by wet-impregnation of ceria. Thus, it indicates no Mn substitution in the cubic lattice of ceria in the physical mixtures, although the possibility of the formation of MnOx-CeO2 solid solutions cannot be excluded at the interface of the oxides. MnO shows a much smaller BET surface area (1.6 m2/g)

1.3 Activity measurement The NO temperature-programmed oxidation (TPO) tests were carried out in a fixed-bed reactor with the efflu-

Fig. 1 XRD patterns of the MnO (1), MnO2 (2), CeO2 (3), MnOCeO2-L (4) and MnO-CeO2-T (5) samples

WU Xiaodong et al., Synergistic effect between MnO and CeO2 in the physical mixture: Electronic interaction and …

than MnO2 (52.8 m2/g), which is consistent with its sharp diffraction peaks. Because the main content of the oxide mixtures is the high-surface-area ceria (133.7 m2/g), the BET surface areas of MnO-CeO2-L and MnO-CeO2-T are 136.9 and 140.2 m2/g, respectively. Such small variations in the BET surface area should not result in any obvious differences in the catalytic performance of the mixed oxide catalysts. 2.2 XPS Fig. 2 shows the XPS spectra of Mn 2p, Ce 3d and O 1s on the surface of two MnO-CeO2 mixtures. The fit of the Mn 2p spectra as well as its three partial components are plotted in Fig. 2(a)[20]. The binding energies of Mn 2p3/2 and Mn 2p1/2 are centered at 640.9–641.6 and 652.6–653.3 eV with the spin energy difference (~11.7 eV). The deconvolution of the Mn 2p spectra reveals two main contributions: the first peak centered at 640.5 eV and the second peak centered at 642.1 eV, which are in good agreement with the literature data reported for Mn2+ and Mn4+, respectively[21,22]. The third peak centered at 645.4 eV is suggested to be the shakeup satellite of the first peak, which originates from the charge-transfer between outer electron shell of ligand and an unfilled 3d shell of MnO during creation of core hole in the photoelectron process[20,21]. Only Mn2+ species were identified here because the BE values of Mn2+ and Mn3+ are very close to each other, which causes difficulty in unambiguous identification of the oxidation states[18]. Both Mn2+ and Mn4+ ions are found to present on the surface of two mixtures. The relative percentage of Mn4+ was estimated by the area ratio of the Mn4+ 2p3/2/Mn 2p3/2. It increases from 35.2% to 44.1% when the preparation method changes from the loose mixing to the tight mixing, implying that more Mn2+ ions are oxidized to Mn4+ on the surface of MnO-CeO2-T. As shown in Fig. 2(b), the curves of Ce 3d spectra were fitted with eight peaks corresponding to four pairs of spin–orbit doublets. The labels u, uƍƍ and uƍƍƍ are attributed to the photoelectron peaks of Ce4+ 3d3/2, and the v, vƍƍ and vƍƍƍ are assigned to the photoelectron peaks of Ce4+ 3d5/2, where the uƍ and vƍ peaks are assigned to the 3d104f0 orbit[3]. The relative percentages of the cerium species

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were semi-quantitatively estimated by the area ratios of the Ce3+ 3d5/2 (vƍ)/Ce 3d5/2 (v, vƍ, vƍƍ and vƍƍƍ). The calculated surface Ce3+/Ce ratio is larger on the tight-contact mixture (21.1%) than that on the loose-contact mixture (14.3%). It is noted that the increase in the Mnx+ oxidation state and the decrease in the Cex+ oxidation state occur simultaneously, which implies an electron interaction between these two metal oxides. The formation of Ce3+ can also prove the formation of oxygen vacancies and hereby increase the amount of the adsorbed oxygen species. The corresponding XPS spectra of O 1s on the MnO-CeO2 mixtures are shown in Fig. 2(c), and two surface oxygen species are clearly observed by deconvolution of the O 1s spectra. The binding energies of 529.4 and 531.9 eV are characteristic of the lattice oxygen (Olatt) and the surface oxygen ions with low coordination situation (Oads), respectively[4]. As shown in Table 1, different mixing ways result in remarkable varieties of relative concentration of Mn4+, Ce3+ and Oads. Obviously, the MnO-CeO2-T catalyst possesses more Mn4+ and richer surface oxygen species on the surface than MnO-CeO2-L. 2.3 O2-TPD TPD technique is useful to study oxygen species of metal oxides, characterizing the type of oxygen species and their properties. Generally, three types of oxygen species can be identified[23]. As shown in Fig. 3, the weak peak at 93 °C is related to desorption of superoxide ion O2í weakly bound to the surface of MnO2. The sharp peak at 482 °C with two shoulders at 375 and 530 °C is related to desorption of peroxide ion O22í/Oí bound to oxygen vacancies. The high-temperature peak at 770 °C is related to desorption of lattice oxygen ion O2í in MnO2. Only one weak peak, which is attributed to O22í/Oí, is observed on MnO. This desorption peak becomes much stronger on CeO2 due to the generation of oxygen vacancies. Table 1 XPS results of the MnO-CeO2 mixtures Sample

Mn4+/Mn (%)

Ce3+/Ce (%)

Oads/O (%)

MnO-CeO2-L

35.2

14.3

26.7

MnO-CeO2-T

44.1

21.1

44.1

Fig. 2 XPS spectra of Mn 2p (a), Ce 3d (b) and O 1s (c) on the MnO-CeO2-L (1) and MnO-CeO2-T (2) samples

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Fig. 3 O2-TPD curves of the MnO2 (1), MnO (2), CeO2 (3), MnO-CeO2-L (4) and MnO-CeO2-T (5) samples

It can be seen in the figure that the curve shape of MnO-CeO2-L is similar to that of CeO2, and the desorption peak at 396 °C even decreases in intensity. However, the area of the middle temperature desorption peak increases and the peak temperature decreases for MnOCeO2-T. These imply that the strong interaction between MnO and CeO2 in the tight-contact mixed oxides leads to more lattice defects and oxygen vacancies and promotes the adsorption/mobility/desorption of O22í/Oí [24]. Meanwhile, the peak at 196 °C attributed to the desorption of O2í becomes a little stronger, which is consistent with the XPS result that more surface oxygen species exist on the surface of MnO-CeO2-T than on the MnO-CeO2-L. 2.4 Raman Raman spectroscopy is sensitive to both M–O band and lattice defects, and thus is a potential tool to get additional information[25]. The creation of more Ce4+ would be accompanied with the occupation of oxygen vacancies in the ceria lattice. This is evidenced by the Raman spectra of the samples in Fig. 4. The Raman spectrum of MnO2 shows one main band at 634 cm–1 attributed to the stretching mode of the Mn–O lattice and a small band

Fig. 4 Raman spectra of the MnO (1), MnO2 (2), CeO2 (3), MnO-CeO2-L (4) and MnO-CeO2-T (5) samples

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at 298 cm–1 attributed to the deformation mode of the metal-oxygen chain of Mn–O–Mn in the ȕ-MnO2 octahedral lattice[26]. The Raman spectrum of MnO also show one main band at ca. 634 cm–1 which can be related to the electronic excitation of impurity in MnO and a shoulder at 347 cm–1 which may be from minor Mn3O4 on surface[27]. No such a band is observed for the mixed oxide samples because Raman is obviously sensitive to the crystalline symmetry. As a reference of ideal fluorite structure, CeO2 has only one Raman-active band centered at 458 cm–1 due to a vibration mode of F2g symmetry, which shifts towards lower wavenumbers in respect to the standard CeO2 band (~464 cm–1). This shift may be caused by the presence of oxygen vacancies, corresponding to stoichiometry CeO2–į [28,29]. The shift is more obvious for the MnOCeO2-T sample, whereas no further shift occurs for MnO-CeO2-L. Thus, it can be related to a more significant change of CeO2 environment by interacting with the adjacent MnO under tight contact conditions. It is consistent with the result of Li et al.[30] that the high surface oxygen concentration on the MnOx-CeO2/TiO2 catalyst can be attributed to the Mn4+-related lattice oxygen and Ce3+-related chemisorbed and weakly bonded oxygen. 2.5 H2-TPR Fig. 5 shows the H2-TPR profiles of CeO2, MnO2 and MnO-CeO2 mixtures. Pure CeO2 exhibits two general reduction peaks at 480 and 740 °C assigned to the reduction of surface oxygen of ceria and bulk oxygen of ceria, respectively. The appearance of the additional low-temperature peak at 375 °C is assigned to the reduction of the readily reducible small clusters of surface ceria and may be related to some special morphology of ceria which remains unclear[31]. No reduction peaks are observed for MnO within the temperature range investigated (not shown), indicating that MnO is the final reduction state of manganese oxide. MnO2 is reduced successively to Mn3O4 and then MnO with two distinct peaks centered at 245 and 345 °C, respectively. The total

Fig. 5 H2-TPR profiles of the MnO2 (1), CeO2 (2), MnO-CeO2L (3) and MnO-CeO2-T (4) samples

WU Xiaodong et al., Synergistic effect between MnO and CeO2 in the physical mixture: Electronic interaction and …

amount of consumed hydrogen was calculated to be 8.5 mmol/g cat., which is close to the theoretical value (11.5 mmol/g cat.) by assuming that MnO is the final reduction state[3,4]. Not all the MnO2 species are reducible due to the coarse particles adopted. The reduction behaviors of the MnO-CeO2 mixtures depend strongly on the mixing way. The MnO-CeO2-L sample presents a similar reduction profile to that of CeO2, which implies that little interaction occurred between MnO and CeO2 under loose contact conditions. MnO-CeO2-T also presents a broad peak at 410 °C and a shoulder at 740 °C, which are ascribed to the combined reduction of Mn3O4 to MnO and surface Ce4+ to Ce3+ species and bulk oxygen removal of ceria, respectively. Noticeably, an intensive reduction peak appears at 245 °C for this sample, which can be ascribed to the reduction of MnO2 to Mn3O4. In comparison with the lowtemperature peak at 245 °C over MnO2, it can be roughly estimated that about 60% of Mn2+ cations are oxidized to Mn4+ over the MnO-CeO2-L sample. This demonstrates that Mn2+ species are at least partially oxidized to high valence Mn4+ ions by interacting with ceria, and these readily reducible Mn4+ species are considered as the most active sites for catalytic oxidation reactions. Based on the above results, it can be suggested that, even without the formation of the solid solutions, the electronic interaction can occur between the long-range interacted MnO and CeO2 under tight contact conditions. However, such an interaction is quite difficult for the loose-contact mixture since Mn2+ is still the dominant species in this sample. These results are consistent with the XPS results although the oxidation states of manganese species on the catalyst surface are somewhat different from those in the bulk. 2.6 NO oxidation activity Fig. 6 compares the catalytic activities of the catalysts in terms of NO conversion as a function of reaction temperature, and Table 2 summarizes the maximal NO conversion (Cm) and the corresponding temperature (Tm) values of the catalysts. Pure MnO only shows very low NO conversions within the temperature range investigated, which should be ascribed to its poor redox behavior. MnO2 is known as an active catalyst for the oxidation of NO and shows the highest activity among the catalysts investigated. CeO2 exhibits a certain catalytic activity for the reaction due to its high oxygen mobility on the surface. It can be clearly seen that the catalytic activity of the mixed oxides depends strongly on their preparation methods. Only a slightly higher activity is obtained for MnO-CeO2-L than ceria because weak interaction occurs between manganese oxide and ceria under loose contact conditions. Comparatively, MnO-CeO2-T shows much higher catalytic activity for NO oxidation given the same Mn content, which should be ascribed to the strong in-

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Fig. 6 Evolutions of NO2 during NO-TPO tests with the samples (Reaction conditions: [NO]=1000 ppm, [O2]=10%, N2 balance, GHSV=30000 h–1) Table 2 NO oxidation activities of the monoxides and mixed oxides Sample

MnO

MnO2

CeO2

MnO-CeO2-L

MnO-CeO2-T

Cm/%

13.8

80.5

36.7

39.0

67.5

Tm/°C

485

255

375

360

310

teraction between manganese oxide and ceria under tight contact conditions. It is known that no calcination process was introduced during the preparation of the physical mixtures, and the formation of MnO-CeO2 solid solutions can be ruled out by XRD. Additionally, the textural properties of the mixed oxides would not be the determining factor for the reaction since the BET surface areas of the MnO-CeO2 mixtures are similar to each other. Thus, there should be other factors responsible for the superior catalytic activity of the MnO-CeO2-L sample. It has been evidenced by XPS, Raman, O2-TPD and H2-TPR results that more Mn4+, Ce3+ and chemisorbed oxygen in the physical mixture under tight contact conditions. Imamura[7] also reported that Ce provides oxygen to Mn to increase the valence and oxidation ability of Mn. They found that Ce helps maintain the valence state of Mn of higher than +3 when Mn/Ce was calcined below 500 °C [15]. Thus, the following reaction can be presented. MnO+CeO2˩MnO2+Ce2O3 (1) This interaction is largely dependent on the contact conditions of the physical mixture. It has been found by Zhang et al.[25] with first-principles calculations that the substitution of Mn in CeO2 lattice results in activated oxygen species due to its structural and electronic modifications. Murugan et al.[17] also found that the ratio of Mn3+ to Mn2+ ions in the MnOx-CeO2 samples follows the order of combustion synthesis>coprecipitation>wetimpregnation methods by DRUV-vis, implying the importance of contact conditions for the Mn-Ce interaction. In this work, although MnOx-CeO2 solid solutions are not formed in the physical mixtures, the electronic interac-

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tion between MnO and CeO2 is clearly observed under tight contact conditions. Such an electron interaction has been also reported for WOx-CeO2 system where the addition of W with strong electronegativity that withdraws the electron atmosphere of Ce-O[32]. A similar electron withdrawing effect of Mn on CeO2 environment appears to work in this case. This Mn-Ce electronic interaction also plays a crucial role in the NO oxidation reaction. The created Mn4+ in MnO-CeO2-T catalyst is very active for NO oxidation. The oxygen transfer from molecular oxygen to MnO2 active sites through oxygen reservoir CeO2 achieved the effective activation of molecular oxygen in the feed stream. The synergetic mechanism may be explained by the following chain of reactions (2–4)[3,33]. Furthermore, the high ratio of Ce3+/Ce can create a charge imbalance, the vacancies and unsaturated chemical bonds on the catalyst surface to promote the oxidation of NO to NO2[34]. MnO2˩Mn2O3+O* (2) Mn2O3+CeO2˩MnO2+Ce2O3 (3) Ce2O3+O2˩CeO2 (4)

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3 Conclusions Investigation on the physical mixtures of MnO and CeO2 powders revealed that the electronic interaction between manganese oxide and ceria could occur under tight contact conditions without the formation of MnOx-CeO2 solid solutions. The transformation of Mn2+ to Mn4+ and the reduction of Ce4+ to Ce3+ occurred simultaneously in the tight-contact mixture of MnO and CeO2 prepared by milling in a mortar, accompanied with the formation of high density of oxygen vacancies and chemisorbed oxygen. The obtained catalyst exhibited high catalytic activity for NO oxidation, with the maximal NO conversion of 67.5% at 310 °C. The synergistic mechanism was interpreted as a process of oxygen activation, and the oxygen transfer through the redox recycles of Mn4+/ Mn3+ and Ce4+/Ce3+.

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