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Surface composition and catalytic activity of La-Fe mixed oxides for methane oxidation
Applied Surface Science 351 (2015) 709–714
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Surface composition and catalytic activity of La-Fe mixed oxides for methane oxidation Fengxiang Liu a , Zhanping Li b , Hongwei Ma a , Zhiming Gao a,∗ a b
School of Chemistry, Beijing Institute of Technology, Liangxiang East Road, Beijing 102488, China Analysis Center, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 16 February 2015 Received in revised form 24 May 2015 Accepted 31 May 2015 Available online 6 June 2015 Keywords: Non-stoichiometric perovskite XPS XRD Ironic oxide Methane
a b s t r a c t Four La-Fe oxide samples with La/Fe atomic ratio y = 1.02 ∼ 0.68 (denoted as LayFe) were prepared by the citrate method. The samples had a decreased specific surface area with the La/Fe atomic ratio decreasing. XRD pattern proved that the sample La0.94 Fe is single phase perovskite La0.94 FeO3−d . Phase composition of the samples was estimated by the Rietveld refinement method. XPS analyses indicate that La3+ ions are enriched on surface of crystallites for all the samples, and surface carbonate ions are relatively abundant on the samples La1.02 Fe and La0.94 Fe. Catalytic activity for methane oxidation per unit surface area of the samples is in the order of La0.68 Fe > La0.76 Fe > La0.94 Fe > La1.02 Fe both in the presence and in the absence of gaseous oxygen. A reason for this order would be the higher concentration of Fe3+ ion on the surface of the samples La0.68 Fe and La0.76 Fe. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Perovskite-type oxide has a formula ABO3 , where usually La3+ ions occupy at the lattice A-sites and ions of a transition metal (Mn, Fe, Co etc.) occupy at the lattice B-sites [1]. When La3+ ions were substituted partially by other metallic ions at a lower valence state such as Ca2+ and Sr2+ ions, lattice oxygen vacancy would be formed together with a part of transition metal ions changing into a higher valence state in the crystal structure to maintain electrical balance [1–5]. Oxygen in gas phase can adsorb at the surface lattice oxygen vacancy, forming adsorbed oxygen species either neutral or charged [1,3,6]. Charged adsorbed oxygen species are formed only when there are electrons that can be drawn from neighboring transition metal ions [1,3,6]. Surface metallic ions are also adsorbed by or bound with oxygen species or hydroxyls or carbonate ions due to exposure to the air atmosphere in the preparation process of the samples. Perovskite-type oxides are effective catalysts for catalytic combustion of hydrocarbons and purification of exhaust gas of vehicles [1–8]. It is reported that the perovskite La0.6 Sr0.4 MnO3 can have a high catalytic activity similar to Pt/Al2 O3 for methane combustion [8], and the perovskite LaFe0.95 Pd0.05 O3 shows a higher durability than Pd/Al2 O3 catalyst in aging in engine exhaust [7].
∗ Corresponding author. Tel.: +86 10 81739075; fax: +86 10 81739075. E-mail address: [email protected] (Z. Gao). http://dx.doi.org/10.1016/j.apsusc.2015.05.189 0169-4332/© 2015 Elsevier B.V. All rights reserved.
The supported noble metal catalysts really can have high catalytic activities even below 350 ◦ C, but also have a sintering tendency above 500 ◦ C which is not good for stability [3]. Oxidation reaction on surface of metal oxide catalyst is generally believed to proceed on suprafacial mechanism or intrafacial mechanism or the mixed mechanism of the suprafacial mechanism with the intrafacial mechanism [1]. Suprafacial mechanism features with surface adsorbed oxygen species acting as catalytically active species, and operates at relatively low reaction temperature. Intrafacial mechanism goes on with participation of surface lattice oxygen as catalytically active species at high reaction temperatures that allow surface lattice oxygen to release feasibly. In most cases, the mixed mechanism takes effect. Arai et al. estimated the respective contribution of surface adsorbed oxygen species and surface lattice oxygen for methane oxidation at reaction temperatures 450 ∼ 650 ◦ C, and argued that the contribution of surface lattice oxygen increased with reaction temperature increasing [8]. When charged oxygen species, including surface lattice oxygen and charged surface adsorbed oxygen, are catalytically active species, its consumption and regeneration is accompanied by cycling of valence state of neighboring transition metal ions such as B3+ ↔ B4+ and/or B2+ ↔ B3+ [1,3,6]. The ease at the cycling of valence state is dependent on kind of transition metal. Hence, kind of transition metal ions at the B-sites influences catalytic activity of perovskitetype oxides greatly [6,8]. There are many papers discussing effect of partial substitution of La3+ ions by Ca2+ or Sr2+ ions etc. [1–5,8–10]. Only a few
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of papers are published on preparation and characterizations of perovskite-type Lax FeO3−d oxides with La3+ deficiency (x < 1) to our knowledge [6,11,12]. In the present work, a series of La-Fe oxides with La/Fe atomic ratio = 1.02 ∼ 0.68 (measured by XRF) were prepared. Phase composition and surface elemental composition of the samples, and especially the elemental composition at the different Ar+ -etching depths for the sample La0.94 Fe, were analyzed. Catalytic activity for methane oxidation of the samples was discussed in view of the difference in surface elemental composition of the samples. 2. Experimental 2.1. Preparation of La-Fe oxides Citrate method was adopted to prepare La-Fe mixed oxides in the present work, since citric acid were frequently used as complexing agent to produce uniform precursor of two kinds of metallic ions [2–5,9,10,13]. At first, an aqueous solution of mixture of lanthanum nitrate and iron nitrate (Guoyao Chemicals, China) was added dropwise into an aqueous citric acid solution (the molar ratio of the citric acid to the total metallic ions was fixed at 0.5) under rigorous stirring, and stirring was kept for 30 min after the addition completed. Then, the citrate solution was heated in a rotary evaporator at 80 ◦ C and vacuum degree of 0.08 MPa for 30 min, where a viscous liquid was obtained. The viscous liquid was dried in an oven at 80 ◦ C for 5 h and subsequently at 110 ◦ C for 2 h. At last, the dried sample was calcined in a muffle furnace at 700 ◦ C for 5 h. A series of samples were prepared by this way with atomic ratio of La/Fe at 1.02, 0.94, 0.76 and 0.68 (measured by X-ray fluorescence spectroscopy, XRF1800, Shimadzu), respectively. These samples are thus denoted as LayFe, in which the variable y is the atomic ratio of La/Fe of the samples. In addition, ICP-AES method was also used to determine atomic ratio of La/Fe for the four samples. However, unfortunately the samples La0.76 Fe and La0.68 Fe were not completely dissolved in acid solution due to the presence of Fe2 O3 phase in a notable amount (see XRD patterns in Fig. 1). The samples La1.02 Fe and La0.94 Fe were dissolved completely in acid solution and showed a value of La/Fe atomic ratio at 1.09 and 0.96, respectively, which are similar to those given by XRF measurements. 2.2. Characterizations Phase identification of the samples was performed on an X-ray diffractometer (D8 Advance, Bruker) with Cu K˛ irradiation at 40 kV
and 40 mA. The Rietveld refinement method implemented in the TOPAS software package was used to estimate phase composition of the samples and crystallite size of the perovskite phase. Specific surface area (SSA) of the samples was measured on a SSA analyzer (JW-DA, Beijing JWGB, China). At first each the samples was degassed at 150 ◦ C for 1 h in high vacuum, and then allowed to adsorb N2 at liquid nitrogen temperature (−196 ◦ C) under a relative pressure of p/p0 = 0.06 ∼ 0.30. The BET equation was used to calculate SSA value. Reducibility of the samples was measured by temperature programmed reduction (TPR) technique on a TPR instrument (PX200, Tianjin Pengxiang, China) equipped with a thermal conductivity detector (TCD), where a reducing gas of 10%H2 /Ar was set at 40 ml/min, temperature was ramped to 700 ◦ C at 10 ◦ C/min, and sample powder was fixed at 50.0 mg at each run. Surface elemental composition of the samples was analyzed on an X-ray photoelectron spectrometer (PHI Quantera) with monochromatized Al K␣ radiation. The base pressure of the instrument was 2 × 10−9 Torr. Ar+ etching (1 keV) was applied for the sample La0.94 Fe after the surface composition analysis in order to obtain elemental composition at the different (average) depths of 5 nm, 10 nm, 20 nm and 40 nm beneath the surface of crystallites for the sample. Curve-fitting for the XPS peaks, calibrated with the binding energy of adventitious carbon C1s = 284.8 eV, was performed with a Gaussian–Lorentzian profile. FT-IR spectra were recorded in the wavenumber range of 400–4000 cm−1 for the samples on an IR instrument (Thermo Fisher Scientific Nicolet iS10), where the thin disks for measurements were obtained by pressing mixture of each the samples with KBr powder. 2.3. Catalytic activity evaluations Catalytic activity for methane oxidation of the La-Fe oxides as catalysts (200 mg granules in 40–60 mesh) was evaluated in a continuous flow fixed-bed quartz tube reactor (8 mm i.d.) at reaction temperature of 600 ◦ C and atmospheric pressure. Total flow rate of the feed gas was 200 ml/min, in which methane gas was 2.0 vol%, oxygen gas 16.8 vol%, nitrogen gas as balance gas. Analyses of reaction products were carried out after 1 h stabilization of the reaction on a gas chromatograph (Shanghai Kechuang, China). Carbon dioxide was the sole carbon-containing product. Pulse reaction of methane in the absence of gaseous oxygen over the La-Fe oxides (50.0 mg powder) was performed in a quartz tube reactor (4 mm i.d.) at reaction temperature of 600 ◦ C. The reactor was connected between pressure valve and injector of a gas chromatograph (Beijing Jiafeng, China). Pressure of carrier gas (N2 ) of the gas chromatograph was set at 0.06 MPa, so the pressure on the La-Fe oxide powder was actually 0.16 MPa. Pulse operation started after 30 min stabilization of each the samples at the reaction temperature. Pulse dose of 10 vol% CH4 /N2 gas into the reactor was set at 0.2 ml, and the pulse was repeated 10 times with a fixed time interval of 15 min between any two successive pulses. Carbon dioxide was the sole carbon-containing product. It is observed that methane conversion decreased as pulse number increased for every sample. So the average value of methane conversion in the 10 times pulses is used to compare the catalytic activity of the four La-Fe samples for methane oxidation in the absence of gaseous oxygen. 3. Results and discussion 3.1. Phase composition and crystallite size
Fig. 1. XRD patterns of the four La-Fe oxide samples.
Fig. 1 shows XRD patterns of the four La-Fe oxide samples. All samples display clearly characteristic peaks of perovskite phase
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Table 1 Phase composition, specific surface area of the four La-Fe oxide samples and the crystallite size of the perovskite phase. Sample
Phase composition by Rietveld method (wt%)
Phase composition on mass balance* (wt%)
Specific surface area (m2 /g)
La1.02 Fe
LaFeO3 97.5 La2 O3 1.4 La(OH)3 1.1
Equivalent to the left
11.3
67
La0.94 Fe
Lax FeO3−d 100
LaFeO3 97.9 Fe2 O3 2.1
6.8
90
La0.76 Fe
Lax FeO3−d 95.9 Fe2 O3 4.1
LaFeO3 90.6 Fe2 O3 9.4
<1.5
220
La0.68 Fe
Lax FeO3−d 92.5 Fe2 O3 7.5
LaFeO3 86.6 Fe2 O3 13.4
<1
323
*
Crystallite size of perovskite phase (nm)
Based on the La/Fe atomic ratios measured by XRF.
(JCPDS 74-2203). Minor peaks of La2 O3 phase and La(OH)3 phase are also observed for the sample La1.02 Fe. Presence of La(OH)3 is a result of La2 O3 absorbing moisture from air during the storage period. Table 1 lists phase composition of the sample La1.02 Fe estimated by the Rietveld method. The phase composition gives a value of La/Fe atomic ratio of 1.02, equal to that measured by XRF. For the sample La0.94 Fe, only XRD peaks of perovskite phase can be observed. Faye et al. reported similarly their samples La0.94 Fe and La0.83 Fe both being of single perovskite phase according to XRD patterns, where the La/Fe atomic ratios of 0.94 and 0.83 were determined by ICP method [6]. For the other two samples La0.76 Fe and La0.68 Fe, XRD peaks of Fe2 O3 phase are observed in Fig. 1 besides the perovskite phase, and the XRD peaks of Fe2 O3 phase become stronger as La/Fe atomic ratio decreases. This is also similar to the report [6]. As given in Table 1, weight percentage of Fe2 O3 phase estimated by the Rietveld method is 4.1 wt% for the sample La0.76 Fe, and 7.5 wt% for the sample La0.68 Fe. These values are lower than those calculated on mass balance (i.e., the La/Fe atomic ratio measured by XRF). Phase composition based on the mass balance is calculated by assuming that the samples are composed of LaFeO3 phase and Fe2 O3 phase (see Table 1). About the difference in phase composition obtained by the Rietveld method and the mass balance, an explanation is proposed by Delmastro et al. [11]. They synthesized La(1 − n) FeO(3 − 1.5n) perovskites with n = 0, 0.1, 0.2 and 0.3 (nominal value of the fed chemicals) at calcination temperature of 600 ◦ C and confirmed that all the oxides are monophasic by XRD patterns. But, for the sample with n = 0.3, Moessbauer spectroscopy detected presence of ironic oxide [11]. It is proposed that the excess ironic oxide in the n > 0 samples corresponds to presence of Fe–O polyhedra at the crystallite surface, and in this way the decrease of La/Fe atomic ratio does not affect the interior of the structure with a regular perovskite lattice [11]. They further confirmed by increasing calcination temperature from 600 ◦ C to 1200 ◦ C that XRD peaks of Fe2 O3 phase first appear at 800 ◦ C and increase their intensity at higher temperatures [11]. In a paper by Faye et al., it is claimed that the decrease of La/Fe atomic ratio does not result in formation of cationic vacancies at La3+ lattice sites in perovskite structure [6]. Similarly, in the present work, no evident change in lattice parameter of the perovskite phase could be found in comparison of the samples LayFe (y = 0.94, 0.76, 0.68) with the sample La1.02 Fe. Besides, Belessi et al. estimated phase compositions for their samples La1 − m FeO3 (nominal value m = 0.10 ∼ 0.35) by use of the Rietveld method and Moessbauer spectra, respectively, and found that the Fe2 O3 content derived from Moessbauer spectra is much higher than that fitted by the Rietveld method from XRD patterns [12]. They explained the difference by assuming that a Fe2 O3 core was covered by a LaFeO3 shell and thus only a part of Fe2 O3 was detected by X-ray. In summary, by referring to the literatures mentioned above, it seems proper to believe that the Lax FeO3−d in Table 1 is a Fe3+ -excess perovskite without cationic vacancies
Fig. 2. TPR curves of the four La-Fe oxide samples.
present in an appreciable number sufficiently to be detectable by XRD. As a consequence, only a part of Fe3+ ions in the total excess Fe3+ ions can form Fe2 O3 phase which is detected by XRD. Fig. 2 shows TPR curves of the four La-Fe oxide samples. The sample La1.02 Fe exhibits a small TPR peak due to the low reducibility of LaFeO3 [14,15]. The sample La0.94 Fe seems to have a very low and wide TPR peak, indicating the excess Fe3+ ions in the Lax FeO3−d are not easily reduced similarly to the normal Fe3+ ions at the B-sites of perovskite structure, in agreement with the proposal by Delmastro et al. [11]. The samples La0.76 Fe and La0.68 Fe both show a large TPR peak due to the reduction of the increased amount of Fe2 O3 phase formed in the two samples [6]. Furthermore, pure Fe2 O3 sample was prepared in the same way and TPR peak area per mg Fe2 O3 was obtained. It is not seen that reduction of Fe2 O3 phase could have promoted reduction of LaFeO3 phase evidently according to TPR peak area. In contrast, it is reported that metallic Ru formed earlier could promote the subsequent reduction of Ni2+ into metallic Ni in H2 gas [16]. And the similar effect would exist in reduction process of the supported bimetallic (Ni-Co, Ni-Fe, Co-Fe) oxides catalysts [17]. Specific surface area (SSA) of the La-Fe oxide samples is listed in Table 1. It is seen that the SSA value decreases with the La/Fe atomic ratio decreasing. At the low La/Fe atomic ratios of 0.76 and 0.68, the SSA values are too small to be measured accurately. In consistence with the SSA value, crystallite size of the perovskite phase estimated by the Rietveld method increases with the La/Fe atomic ratio decreasing (see Table 1). By correlating the crystallite size with the La/Fe atomic ratio, one could deduce that La2 O3 crystallites in the sample La1.02 Fe have an effect to isolate perovskite crystallites
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Table 2 Surface elemental composition of the four La-Fe oxide samples. Sample
O (at%)
La (at%)
Fe (at%)
La/Fe atomic ratio
O/(La + Fe) atomic ratio
ONL /OL atomic ratio
La1.02 Fe La0.94 Fe La0.76 Fe La0.68 Fe
77.3 77.2 71.5 73.7
17.0 16.7 15.1 13.6
5.7 6.1 13.4 12.8
3.00 2.73 1.12 1.06
3.40 3.39 2.51 2.80
1.82 1.62 0.54 0.56
and thus result in a small crystallite size for the perovskite phase; whereas, in the other three samples, the excess ironic oxide at the surface of the perovskite crystallites as proposed by Delmastro et al. [11], has an effect to link perovskite crystallites together and lead to a large size of crystallites at final for the perovskite phase. 3.2. Elemental composition at the surface XPS spectra were recorded for the four La-Fe oxide samples and the surface elemental composition excluding carbon element is listed in Table 2. It is clear that surface La/Fe atomic ratio is much higher than that of the bulk phase measured by XRF for each the samples, indicating that La3+ ions are enriched on the surface of the samples. This is in agreement with the literatures [6,10,15]. Fig. 3 displays XPS spectra of C1s of the samples. All the samples have a peak at binding energy of 284.8 eV, characteristic of C1s core level of hydrocarbons (i.e., contaminated carbon come from ambience) adsorbed on the samples. For the samples La1.02 Fe and La0.94 Fe, there is also an obvious peak at 289.5 eV, characteristic of C1s core level of carbonate ion (CO3 2− ) [15]. This is due to the fact that surface La3+ ions are prone to adsorb CO2 from the air atmosphere and form surface carbonate ions, in agreement with the literature [6]. Fig. 4 displays XPS spectra of O1s of the samples. The peak at ca. 529.6 eV is generated from surface lattice oxygen (O2− ), and the peak at ca. 531.5 eV can be assigned to O1s core level of surface adsorbed oxygen species, surface hydroxyls and surface carbonate ions [1,15]. In comparison to the samples La0.76 Fe and La0.68 Fe, the samples La1.02 Fe and La0.94 Fe both show a stronger peak at ca. 531.5 eV, which is due to the presence of the relatively abundant carbonate ion on the surface of these two samples in connection with Fig. 3. As a result, the atomic ratio of ONL /OL is quite larger for the samples La1.02 Fe and La0.94 Fe as well, where ONL denotes surface non-lattice oxygen species including surface adsorbed oxygen species, surface hydroxyls and surface carbonate ions (see Table 2). The total surface oxygen (O) is a sum of the surface lattice oxygen
Fig. 4. XPS spectra of O1s of the four La-Fe oxide samples.
Fig. 5. XPS spectra of Fe2p of the four La-Fe oxide samples.
(OL ) and the surface non-lattice oxygen (ONL ). Fig. 5 displays XPS spectra of Fe2p of the samples. It is seen that the samples La0.76 Fe and La0.68 Fe have a slightly higher binding energy than the samples La1.02 Fe and La0.94 Fe. This is in agreement with the phase composition estimated by the Rietveld method in Table 1, where the samples La0.76 Fe and La0.68 Fe contain Fe2 O3 phase and the samples La1.02 Fe and La0.94 Fe do not. It is reported that the Fe2p3/2 binding energy is 710.8 eV for Fe3+ in Fe2 O3 [18] and 710.3 eV for Fe3+ in LaFeO3 [19]. Similarly to the report [6], no definite difference can be recognized from XPS spectra of La3d (not shown) for the four samples. Presence of carbonate ion on surface of the four samples was also confirmed by IR measurement. Fig. 6 shows FT-IR spectra of the four samples. The band at 1384 cm−1 is a clear indication of presence of metal carbonates [20–22], and the band at 1113 cm−1 can also be assigned to carbonate ions [20]. The bands at 3455 cm−1 and 1638 cm−1 are attributed to (OH) and ı(H2 O) [20,23]. The band at 566 cm−1 is characteristic of Fe–O stretching vibration [20–23]. 3.3. Elemental composition at the different Ar+ -etching depths
Fig. 3. XPS spectra of C1s of the four La-Fe oxide samples.
Elemental composition at the different Ar+ -etching depths of the sample La0.94 Fe is listed in Table 3. It is clear that as the depth increases, the La/Fe atomic ratio decreases. This is because La3+ ions
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Fig. 7. XPS spectra of C1s at the different Ar+ -etching depths of the sample La0.94 Fe.
Fig. 6. FT-IR spectra of the four La-Fe oxide samples. Table 3 Elemental composition at the different Ar+ -etching depths of the sample La0.94 Fe. Depth (nm)
O (at%)
La (at%)
Fe (at%)
La/Fe atomic ratio
O/(La + Fe) atomic ratio
ONL /OL atomic ratio
0 5 10 20 40
77.2 68.5 68.0 67.2 65.3
16.7 22.2 21.7 21.7 22.4
6.1 9.3 10.3 11.1 12.3
2.73 2.38 2.11 1.95 1.83
3.39 2.18 2.13 2.05 1.88
1.62 0.49 0.38 0.28 0.22
are enriched on the surface of the sample and more information of the interior of the crystallites is discovered as the depth increases. It should be noted that the results in Table 3 are statistically averaged values contributed by the interior and the remaining surface of the crystallites, because sizes of the crystallites are not totally uniform and also the boundary of the exposed plane of an etched crystallite is still providing surface information of the crystallite. For this reason, the XPS etching technique is indeed not able to provide completely the information of the interior of crystallites. It is also seen in Table 3 that the O/(La + Fe) atomic ratio and the ONL /OL atomic ratio both decrease as the depth increases, especially the decrease of the ONL /OL atomic ratio is large, indicating the surface carbonate ions were readily etched off once the etching started. The XPS spectra of C1s and O1s evolved with the depth increasing are shown in Figs. 7 and 8, respectively. It is clear that the peaks at the higher binding energy corresponding to carbonate ions are decreased as the depth increases. Fig. 9 shows XPS spectra of Fe2p evolved with the depth increasing. A shoulder peak at ca. 706 eV appeared after the etching depth more than 5 nm, indicating metallic Fe0 formed gradually during the etching process. This is similar to the reports that Ar+ etching caused samples to be partially reduced [24,25].
Fig. 8. XPS spectra of O1s at the different Ar+ -etching depths of the sample La0.94 Fe.
3.4. Catalytic activity for methane oxidation Catalytic activity for methane oxidation of the four La-Fe oxide samples as catalysts is compared in Fig. 10 at reaction temperature of 600 ◦ C. The high reaction temperature was chosen to diminish effect of surface carbonate ions. It is seen that catalytic activity for methane conversion is in the order of sample La1.02 Fe > La0.94 Fe > La0.76 Fe > La0.68 Fe at the fixed sample mass both in the presence and in the absence of gaseous oxygen. Faye et al. reported an order in methane conversion of sample La0.94 Fe > La0.83 Fe > La0.76 Fe > La0.64 Fe > La0.55 Fe (the atomic ratio of
Fig. 9. XPS spectra of Fe2p at the different Ar+ -etching depths of the sample La0.94 Fe.
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samples Lay Fe (y = 0.94, 0.76, 0.68), even though the feed La/Fe atomic ratio is less than unity, La3+ ions are enriched on the surface of the samples. The excess ironic oxide at the surface of the perovskite crystallites has an effect to link perovskite crystallites together and lead to a large size of crystallites for the perovskite phase at final. The samples La1.02 Fe and La0.94 Fe had relatively abundant surface carbonate ions due to the higher concentrations of La3+ ions on the surfaces. Catalytic activity for methane oxidation per unit surface area of the samples is in the order of La0.68 Fe > La0.76 Fe > La0.94 Fe > La1.02 Fe both in the presence and in the absence of gaseous oxygen. A reason for this order would be the higher concentration of Fe3+ ion on the surfaces of the samples La0.68 Fe and La0.76 Fe. Acknowledgment Fig. 10. Methane conversion as a function of La/Fe atomic ratio at reaction temperature of 600 ◦ C in the presence (empty circle, to the left Y axis) or absence (solid triangle, to the right Y axis) of gaseous oxygen.
This work was financially supported by the National Natural Science Foundation of China (Grant No. 21171020). References
La/Fe was measured by ICP) in the presence of gaseous oxygen [6]. Delmastro et al. reported an order in methane conversion of sample La0.9 Fe > La0.8 Fe > La0.7 Fe > La1.0 Fe (the atomic ratio of La/Fe was nominal value of the fed chemicals) in the presence of gaseous oxygen [11]. A reason for the difference in activity order in the literatures [6,11] should be the different preparation procedures adopted by the respective research group. In comparison of Fig. 10 with Table 1, it is seen that larger specific surface area is more favorable for methane conversion under the condition of the fixed catalyst mass. Therefore, specific catalytic activity, i.e., methane converted on unit surface area of a catalyst sample, needs to be compared for the four samples. For this purpose, specific surface area is assumed to be 1.5 m2 /g for the sample La0.76 Fe, and 1.0 m2 /g for the sample La0.68 Fe (see Table 1). It is found that the specific catalytic activity is in the order of sample La0.68 Fe > La0.76 Fe > La0.94 Fe > La1.02 Fe both in the presence and in the absence of gaseous oxygen. A reason for this order would be the higher concentration of Fe3+ ion on the surface of the samples La0.68 Fe and La0.76 Fe (see Table 2). This is because transition metal ions at the B-sites affect catalytic activity greatly [6,8]. Besides, chemical binding structure in the surface layer of the samples must have an effect on the catalytic activity, but it is not clear at an atomic level. 4. Conclusions The sample La0.94 Fe is single phase perovskite La0.94 FeO3−d . As the feed La/Fe atomic ratio further decreased, Fe2 O3 phase formed and specific surface area of the samples also decreased. For the
[1] T. Seiyama, Catal. Rev. – Sci. Eng. 34 (1992) 281–300. [2] P. Ciambelli, S. Cimino, L. Lisi, M. Faticanti, G. Minelli, I. Pettiti, P. Porta, Appl. Catal. B33 (2001) 193–203. [3] Z. Gao, R. Wang, Appl. Catal. B98 (2010) 147–153. [4] G. Pecchi, P. Reyes, R. Zamora, C. Campos, L.E. Cadus, B.P. Barbero, Catal. Today 133–135 (2008) 420–427. [5] B.P. Barbero, J.A. Gamboa, L.E. Cadus, Appl. Catal. B65 (2006) 21–30. [6] J. Faye, A. Baylet, M. Trentesaux, S. Royer, F. Dumeignil, D. Duprez, S. Valange, J.-M. Tatibouet, Appl. Catal. B126 (2012) 134–143. [7] H. Tanaka, M. Uenishi, M. Taniguchi, I. Tan, K. Narita, M. Kimura, K. Kaneko, Y. Nishihata, J. Mizuki, Catal. Today 117 (2006) 321–328. [8] H. Arai, T. Yamada, K. Eguchi, T. Seiyama, Appl. Catal. 26 (1986) 265–276. [9] G. Pecchi, M.G. Jiliberto, A. Buljan, E.J. Delgado, Solid State Ionics 187 (2011) 27–32. [10] C-H. Wang, C-L. Chen, H-S. Weng, Chemosphere 57 (2004) 1131–1138. [11] A. Delmastro, D. Mazza, S. Ronchetti, M. Vallino, R. Spinicci, P. Brovetto, M. Salis, Mater. Sci. Eng. B79 (2001) 140–145. [12] V.C. Belessi, P.N. Trikalitis, A.K. Ladavos, T.V. Bakas, P.J. Pomonis, Appl. Catal. A177 (1999) 53–68. [13] M. Pudukudy, Z. Yaakob, B. Narayanan, J. Cluster Sci. 25 (2014) 1599–1614. [14] N. Escalona, S. Fuentealba, G. Pecchi, Appl. Catal. A381 (2010) 253–260. [15] M.R. Goldwasser, M.E. Rivas, M.L. Lugo, E. Pietri, J. Perez-Zurita, M.L. Cubeiro, A. Griboval-Constant, G. Leclercq, Catal. Today 107–108 (2005) 106–113. [16] M. Kimura, T. Miyao, S. Komori, A. Chen, K. Higashiyama, H. Yamashita, M. Watanabe, Appl. Catal. A 379 (2010) 182–187. [17] M. Pudukudy, Z. Yaakob, Z.S. Akmal, Appl. Surf. Sci. 330 (2015) 418–430. [18] N.S. Mclntyre, D.G. Zetaruk, Anal. Chem. 49 (1977) 1521–1529. [19] S. Petrovic, A. Terlecki-Baricevic, Lj. Karanovic, P. Kirilov-Stefanov, M. Zdujic, V. Dondur, D. Paneva, I. Mitov, V. Rakic, Appl. Catal. B79 (2008) 186–198. [20] J. Feng, T. Liu, Y. Xu, J. Zhao, Y. He, Ceram. Int. 37 (2011) 1203–1207. [21] R. Abazari, S. Sanati, L.A. Saghatforoush, Mater. Sci. Semicond. Process. 25 (2014) 301–306. [22] R. Abazari, S. Sanati, Superlattices Microstruct. 64 (2013) 148–157. [23] J. Ding, X. Lu, H. Shu, J. Xie, H. Zhang, Mater. Sci. Eng. B 171 (2010) 31–34. [24] D. Gamarra, A. Hornes, Zs. Koppany, Z. Schay, G. Munuera, J. Soria, A. MartinezArias, J. Power Sources 169 (2007) 110–116. [25] S. Hashimoto, A. Tanaka, Surf. Interface Anal. 34 (2002) 262–265.
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Surface composition and catalytic activity of La-Fe mixed oxides for methane oxidation Fengxiang Liu a , Zhanping Li b , Hongwei Ma a , Zhiming Gao a,∗ a b
School of Chemistry, Beijing Institute of Technology, Liangxiang East Road, Beijing 102488, China Analysis Center, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 16 February 2015 Received in revised form 24 May 2015 Accepted 31 May 2015 Available online 6 June 2015 Keywords: Non-stoichiometric perovskite XPS XRD Ironic oxide Methane
a b s t r a c t Four La-Fe oxide samples with La/Fe atomic ratio y = 1.02 ∼ 0.68 (denoted as LayFe) were prepared by the citrate method. The samples had a decreased specific surface area with the La/Fe atomic ratio decreasing. XRD pattern proved that the sample La0.94 Fe is single phase perovskite La0.94 FeO3−d . Phase composition of the samples was estimated by the Rietveld refinement method. XPS analyses indicate that La3+ ions are enriched on surface of crystallites for all the samples, and surface carbonate ions are relatively abundant on the samples La1.02 Fe and La0.94 Fe. Catalytic activity for methane oxidation per unit surface area of the samples is in the order of La0.68 Fe > La0.76 Fe > La0.94 Fe > La1.02 Fe both in the presence and in the absence of gaseous oxygen. A reason for this order would be the higher concentration of Fe3+ ion on the surface of the samples La0.68 Fe and La0.76 Fe. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Perovskite-type oxide has a formula ABO3 , where usually La3+ ions occupy at the lattice A-sites and ions of a transition metal (Mn, Fe, Co etc.) occupy at the lattice B-sites [1]. When La3+ ions were substituted partially by other metallic ions at a lower valence state such as Ca2+ and Sr2+ ions, lattice oxygen vacancy would be formed together with a part of transition metal ions changing into a higher valence state in the crystal structure to maintain electrical balance [1–5]. Oxygen in gas phase can adsorb at the surface lattice oxygen vacancy, forming adsorbed oxygen species either neutral or charged [1,3,6]. Charged adsorbed oxygen species are formed only when there are electrons that can be drawn from neighboring transition metal ions [1,3,6]. Surface metallic ions are also adsorbed by or bound with oxygen species or hydroxyls or carbonate ions due to exposure to the air atmosphere in the preparation process of the samples. Perovskite-type oxides are effective catalysts for catalytic combustion of hydrocarbons and purification of exhaust gas of vehicles [1–8]. It is reported that the perovskite La0.6 Sr0.4 MnO3 can have a high catalytic activity similar to Pt/Al2 O3 for methane combustion [8], and the perovskite LaFe0.95 Pd0.05 O3 shows a higher durability than Pd/Al2 O3 catalyst in aging in engine exhaust [7].
∗ Corresponding author. Tel.: +86 10 81739075; fax: +86 10 81739075. E-mail address: [email protected] (Z. Gao). http://dx.doi.org/10.1016/j.apsusc.2015.05.189 0169-4332/© 2015 Elsevier B.V. All rights reserved.
The supported noble metal catalysts really can have high catalytic activities even below 350 ◦ C, but also have a sintering tendency above 500 ◦ C which is not good for stability [3]. Oxidation reaction on surface of metal oxide catalyst is generally believed to proceed on suprafacial mechanism or intrafacial mechanism or the mixed mechanism of the suprafacial mechanism with the intrafacial mechanism [1]. Suprafacial mechanism features with surface adsorbed oxygen species acting as catalytically active species, and operates at relatively low reaction temperature. Intrafacial mechanism goes on with participation of surface lattice oxygen as catalytically active species at high reaction temperatures that allow surface lattice oxygen to release feasibly. In most cases, the mixed mechanism takes effect. Arai et al. estimated the respective contribution of surface adsorbed oxygen species and surface lattice oxygen for methane oxidation at reaction temperatures 450 ∼ 650 ◦ C, and argued that the contribution of surface lattice oxygen increased with reaction temperature increasing [8]. When charged oxygen species, including surface lattice oxygen and charged surface adsorbed oxygen, are catalytically active species, its consumption and regeneration is accompanied by cycling of valence state of neighboring transition metal ions such as B3+ ↔ B4+ and/or B2+ ↔ B3+ [1,3,6]. The ease at the cycling of valence state is dependent on kind of transition metal. Hence, kind of transition metal ions at the B-sites influences catalytic activity of perovskitetype oxides greatly [6,8]. There are many papers discussing effect of partial substitution of La3+ ions by Ca2+ or Sr2+ ions etc. [1–5,8–10]. Only a few
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of papers are published on preparation and characterizations of perovskite-type Lax FeO3−d oxides with La3+ deficiency (x < 1) to our knowledge [6,11,12]. In the present work, a series of La-Fe oxides with La/Fe atomic ratio = 1.02 ∼ 0.68 (measured by XRF) were prepared. Phase composition and surface elemental composition of the samples, and especially the elemental composition at the different Ar+ -etching depths for the sample La0.94 Fe, were analyzed. Catalytic activity for methane oxidation of the samples was discussed in view of the difference in surface elemental composition of the samples. 2. Experimental 2.1. Preparation of La-Fe oxides Citrate method was adopted to prepare La-Fe mixed oxides in the present work, since citric acid were frequently used as complexing agent to produce uniform precursor of two kinds of metallic ions [2–5,9,10,13]. At first, an aqueous solution of mixture of lanthanum nitrate and iron nitrate (Guoyao Chemicals, China) was added dropwise into an aqueous citric acid solution (the molar ratio of the citric acid to the total metallic ions was fixed at 0.5) under rigorous stirring, and stirring was kept for 30 min after the addition completed. Then, the citrate solution was heated in a rotary evaporator at 80 ◦ C and vacuum degree of 0.08 MPa for 30 min, where a viscous liquid was obtained. The viscous liquid was dried in an oven at 80 ◦ C for 5 h and subsequently at 110 ◦ C for 2 h. At last, the dried sample was calcined in a muffle furnace at 700 ◦ C for 5 h. A series of samples were prepared by this way with atomic ratio of La/Fe at 1.02, 0.94, 0.76 and 0.68 (measured by X-ray fluorescence spectroscopy, XRF1800, Shimadzu), respectively. These samples are thus denoted as LayFe, in which the variable y is the atomic ratio of La/Fe of the samples. In addition, ICP-AES method was also used to determine atomic ratio of La/Fe for the four samples. However, unfortunately the samples La0.76 Fe and La0.68 Fe were not completely dissolved in acid solution due to the presence of Fe2 O3 phase in a notable amount (see XRD patterns in Fig. 1). The samples La1.02 Fe and La0.94 Fe were dissolved completely in acid solution and showed a value of La/Fe atomic ratio at 1.09 and 0.96, respectively, which are similar to those given by XRF measurements. 2.2. Characterizations Phase identification of the samples was performed on an X-ray diffractometer (D8 Advance, Bruker) with Cu K˛ irradiation at 40 kV
and 40 mA. The Rietveld refinement method implemented in the TOPAS software package was used to estimate phase composition of the samples and crystallite size of the perovskite phase. Specific surface area (SSA) of the samples was measured on a SSA analyzer (JW-DA, Beijing JWGB, China). At first each the samples was degassed at 150 ◦ C for 1 h in high vacuum, and then allowed to adsorb N2 at liquid nitrogen temperature (−196 ◦ C) under a relative pressure of p/p0 = 0.06 ∼ 0.30. The BET equation was used to calculate SSA value. Reducibility of the samples was measured by temperature programmed reduction (TPR) technique on a TPR instrument (PX200, Tianjin Pengxiang, China) equipped with a thermal conductivity detector (TCD), where a reducing gas of 10%H2 /Ar was set at 40 ml/min, temperature was ramped to 700 ◦ C at 10 ◦ C/min, and sample powder was fixed at 50.0 mg at each run. Surface elemental composition of the samples was analyzed on an X-ray photoelectron spectrometer (PHI Quantera) with monochromatized Al K␣ radiation. The base pressure of the instrument was 2 × 10−9 Torr. Ar+ etching (1 keV) was applied for the sample La0.94 Fe after the surface composition analysis in order to obtain elemental composition at the different (average) depths of 5 nm, 10 nm, 20 nm and 40 nm beneath the surface of crystallites for the sample. Curve-fitting for the XPS peaks, calibrated with the binding energy of adventitious carbon C1s = 284.8 eV, was performed with a Gaussian–Lorentzian profile. FT-IR spectra were recorded in the wavenumber range of 400–4000 cm−1 for the samples on an IR instrument (Thermo Fisher Scientific Nicolet iS10), where the thin disks for measurements were obtained by pressing mixture of each the samples with KBr powder. 2.3. Catalytic activity evaluations Catalytic activity for methane oxidation of the La-Fe oxides as catalysts (200 mg granules in 40–60 mesh) was evaluated in a continuous flow fixed-bed quartz tube reactor (8 mm i.d.) at reaction temperature of 600 ◦ C and atmospheric pressure. Total flow rate of the feed gas was 200 ml/min, in which methane gas was 2.0 vol%, oxygen gas 16.8 vol%, nitrogen gas as balance gas. Analyses of reaction products were carried out after 1 h stabilization of the reaction on a gas chromatograph (Shanghai Kechuang, China). Carbon dioxide was the sole carbon-containing product. Pulse reaction of methane in the absence of gaseous oxygen over the La-Fe oxides (50.0 mg powder) was performed in a quartz tube reactor (4 mm i.d.) at reaction temperature of 600 ◦ C. The reactor was connected between pressure valve and injector of a gas chromatograph (Beijing Jiafeng, China). Pressure of carrier gas (N2 ) of the gas chromatograph was set at 0.06 MPa, so the pressure on the La-Fe oxide powder was actually 0.16 MPa. Pulse operation started after 30 min stabilization of each the samples at the reaction temperature. Pulse dose of 10 vol% CH4 /N2 gas into the reactor was set at 0.2 ml, and the pulse was repeated 10 times with a fixed time interval of 15 min between any two successive pulses. Carbon dioxide was the sole carbon-containing product. It is observed that methane conversion decreased as pulse number increased for every sample. So the average value of methane conversion in the 10 times pulses is used to compare the catalytic activity of the four La-Fe samples for methane oxidation in the absence of gaseous oxygen. 3. Results and discussion 3.1. Phase composition and crystallite size
Fig. 1. XRD patterns of the four La-Fe oxide samples.
Fig. 1 shows XRD patterns of the four La-Fe oxide samples. All samples display clearly characteristic peaks of perovskite phase
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Table 1 Phase composition, specific surface area of the four La-Fe oxide samples and the crystallite size of the perovskite phase. Sample
Phase composition by Rietveld method (wt%)
Phase composition on mass balance* (wt%)
Specific surface area (m2 /g)
La1.02 Fe
LaFeO3 97.5 La2 O3 1.4 La(OH)3 1.1
Equivalent to the left
11.3
67
La0.94 Fe
Lax FeO3−d 100
LaFeO3 97.9 Fe2 O3 2.1
6.8
90
La0.76 Fe
Lax FeO3−d 95.9 Fe2 O3 4.1
LaFeO3 90.6 Fe2 O3 9.4
<1.5
220
La0.68 Fe
Lax FeO3−d 92.5 Fe2 O3 7.5
LaFeO3 86.6 Fe2 O3 13.4
<1
323
*
Crystallite size of perovskite phase (nm)
Based on the La/Fe atomic ratios measured by XRF.
(JCPDS 74-2203). Minor peaks of La2 O3 phase and La(OH)3 phase are also observed for the sample La1.02 Fe. Presence of La(OH)3 is a result of La2 O3 absorbing moisture from air during the storage period. Table 1 lists phase composition of the sample La1.02 Fe estimated by the Rietveld method. The phase composition gives a value of La/Fe atomic ratio of 1.02, equal to that measured by XRF. For the sample La0.94 Fe, only XRD peaks of perovskite phase can be observed. Faye et al. reported similarly their samples La0.94 Fe and La0.83 Fe both being of single perovskite phase according to XRD patterns, where the La/Fe atomic ratios of 0.94 and 0.83 were determined by ICP method [6]. For the other two samples La0.76 Fe and La0.68 Fe, XRD peaks of Fe2 O3 phase are observed in Fig. 1 besides the perovskite phase, and the XRD peaks of Fe2 O3 phase become stronger as La/Fe atomic ratio decreases. This is also similar to the report [6]. As given in Table 1, weight percentage of Fe2 O3 phase estimated by the Rietveld method is 4.1 wt% for the sample La0.76 Fe, and 7.5 wt% for the sample La0.68 Fe. These values are lower than those calculated on mass balance (i.e., the La/Fe atomic ratio measured by XRF). Phase composition based on the mass balance is calculated by assuming that the samples are composed of LaFeO3 phase and Fe2 O3 phase (see Table 1). About the difference in phase composition obtained by the Rietveld method and the mass balance, an explanation is proposed by Delmastro et al. [11]. They synthesized La(1 − n) FeO(3 − 1.5n) perovskites with n = 0, 0.1, 0.2 and 0.3 (nominal value of the fed chemicals) at calcination temperature of 600 ◦ C and confirmed that all the oxides are monophasic by XRD patterns. But, for the sample with n = 0.3, Moessbauer spectroscopy detected presence of ironic oxide [11]. It is proposed that the excess ironic oxide in the n > 0 samples corresponds to presence of Fe–O polyhedra at the crystallite surface, and in this way the decrease of La/Fe atomic ratio does not affect the interior of the structure with a regular perovskite lattice [11]. They further confirmed by increasing calcination temperature from 600 ◦ C to 1200 ◦ C that XRD peaks of Fe2 O3 phase first appear at 800 ◦ C and increase their intensity at higher temperatures [11]. In a paper by Faye et al., it is claimed that the decrease of La/Fe atomic ratio does not result in formation of cationic vacancies at La3+ lattice sites in perovskite structure [6]. Similarly, in the present work, no evident change in lattice parameter of the perovskite phase could be found in comparison of the samples LayFe (y = 0.94, 0.76, 0.68) with the sample La1.02 Fe. Besides, Belessi et al. estimated phase compositions for their samples La1 − m FeO3 (nominal value m = 0.10 ∼ 0.35) by use of the Rietveld method and Moessbauer spectra, respectively, and found that the Fe2 O3 content derived from Moessbauer spectra is much higher than that fitted by the Rietveld method from XRD patterns [12]. They explained the difference by assuming that a Fe2 O3 core was covered by a LaFeO3 shell and thus only a part of Fe2 O3 was detected by X-ray. In summary, by referring to the literatures mentioned above, it seems proper to believe that the Lax FeO3−d in Table 1 is a Fe3+ -excess perovskite without cationic vacancies
Fig. 2. TPR curves of the four La-Fe oxide samples.
present in an appreciable number sufficiently to be detectable by XRD. As a consequence, only a part of Fe3+ ions in the total excess Fe3+ ions can form Fe2 O3 phase which is detected by XRD. Fig. 2 shows TPR curves of the four La-Fe oxide samples. The sample La1.02 Fe exhibits a small TPR peak due to the low reducibility of LaFeO3 [14,15]. The sample La0.94 Fe seems to have a very low and wide TPR peak, indicating the excess Fe3+ ions in the Lax FeO3−d are not easily reduced similarly to the normal Fe3+ ions at the B-sites of perovskite structure, in agreement with the proposal by Delmastro et al. [11]. The samples La0.76 Fe and La0.68 Fe both show a large TPR peak due to the reduction of the increased amount of Fe2 O3 phase formed in the two samples [6]. Furthermore, pure Fe2 O3 sample was prepared in the same way and TPR peak area per mg Fe2 O3 was obtained. It is not seen that reduction of Fe2 O3 phase could have promoted reduction of LaFeO3 phase evidently according to TPR peak area. In contrast, it is reported that metallic Ru formed earlier could promote the subsequent reduction of Ni2+ into metallic Ni in H2 gas [16]. And the similar effect would exist in reduction process of the supported bimetallic (Ni-Co, Ni-Fe, Co-Fe) oxides catalysts [17]. Specific surface area (SSA) of the La-Fe oxide samples is listed in Table 1. It is seen that the SSA value decreases with the La/Fe atomic ratio decreasing. At the low La/Fe atomic ratios of 0.76 and 0.68, the SSA values are too small to be measured accurately. In consistence with the SSA value, crystallite size of the perovskite phase estimated by the Rietveld method increases with the La/Fe atomic ratio decreasing (see Table 1). By correlating the crystallite size with the La/Fe atomic ratio, one could deduce that La2 O3 crystallites in the sample La1.02 Fe have an effect to isolate perovskite crystallites
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Table 2 Surface elemental composition of the four La-Fe oxide samples. Sample
O (at%)
La (at%)
Fe (at%)
La/Fe atomic ratio
O/(La + Fe) atomic ratio
ONL /OL atomic ratio
La1.02 Fe La0.94 Fe La0.76 Fe La0.68 Fe
77.3 77.2 71.5 73.7
17.0 16.7 15.1 13.6
5.7 6.1 13.4 12.8
3.00 2.73 1.12 1.06
3.40 3.39 2.51 2.80
1.82 1.62 0.54 0.56
and thus result in a small crystallite size for the perovskite phase; whereas, in the other three samples, the excess ironic oxide at the surface of the perovskite crystallites as proposed by Delmastro et al. [11], has an effect to link perovskite crystallites together and lead to a large size of crystallites at final for the perovskite phase. 3.2. Elemental composition at the surface XPS spectra were recorded for the four La-Fe oxide samples and the surface elemental composition excluding carbon element is listed in Table 2. It is clear that surface La/Fe atomic ratio is much higher than that of the bulk phase measured by XRF for each the samples, indicating that La3+ ions are enriched on the surface of the samples. This is in agreement with the literatures [6,10,15]. Fig. 3 displays XPS spectra of C1s of the samples. All the samples have a peak at binding energy of 284.8 eV, characteristic of C1s core level of hydrocarbons (i.e., contaminated carbon come from ambience) adsorbed on the samples. For the samples La1.02 Fe and La0.94 Fe, there is also an obvious peak at 289.5 eV, characteristic of C1s core level of carbonate ion (CO3 2− ) [15]. This is due to the fact that surface La3+ ions are prone to adsorb CO2 from the air atmosphere and form surface carbonate ions, in agreement with the literature [6]. Fig. 4 displays XPS spectra of O1s of the samples. The peak at ca. 529.6 eV is generated from surface lattice oxygen (O2− ), and the peak at ca. 531.5 eV can be assigned to O1s core level of surface adsorbed oxygen species, surface hydroxyls and surface carbonate ions [1,15]. In comparison to the samples La0.76 Fe and La0.68 Fe, the samples La1.02 Fe and La0.94 Fe both show a stronger peak at ca. 531.5 eV, which is due to the presence of the relatively abundant carbonate ion on the surface of these two samples in connection with Fig. 3. As a result, the atomic ratio of ONL /OL is quite larger for the samples La1.02 Fe and La0.94 Fe as well, where ONL denotes surface non-lattice oxygen species including surface adsorbed oxygen species, surface hydroxyls and surface carbonate ions (see Table 2). The total surface oxygen (O) is a sum of the surface lattice oxygen
Fig. 4. XPS spectra of O1s of the four La-Fe oxide samples.
Fig. 5. XPS spectra of Fe2p of the four La-Fe oxide samples.
(OL ) and the surface non-lattice oxygen (ONL ). Fig. 5 displays XPS spectra of Fe2p of the samples. It is seen that the samples La0.76 Fe and La0.68 Fe have a slightly higher binding energy than the samples La1.02 Fe and La0.94 Fe. This is in agreement with the phase composition estimated by the Rietveld method in Table 1, where the samples La0.76 Fe and La0.68 Fe contain Fe2 O3 phase and the samples La1.02 Fe and La0.94 Fe do not. It is reported that the Fe2p3/2 binding energy is 710.8 eV for Fe3+ in Fe2 O3 [18] and 710.3 eV for Fe3+ in LaFeO3 [19]. Similarly to the report [6], no definite difference can be recognized from XPS spectra of La3d (not shown) for the four samples. Presence of carbonate ion on surface of the four samples was also confirmed by IR measurement. Fig. 6 shows FT-IR spectra of the four samples. The band at 1384 cm−1 is a clear indication of presence of metal carbonates [20–22], and the band at 1113 cm−1 can also be assigned to carbonate ions [20]. The bands at 3455 cm−1 and 1638 cm−1 are attributed to (OH) and ı(H2 O) [20,23]. The band at 566 cm−1 is characteristic of Fe–O stretching vibration [20–23]. 3.3. Elemental composition at the different Ar+ -etching depths
Fig. 3. XPS spectra of C1s of the four La-Fe oxide samples.
Elemental composition at the different Ar+ -etching depths of the sample La0.94 Fe is listed in Table 3. It is clear that as the depth increases, the La/Fe atomic ratio decreases. This is because La3+ ions
F. Liu et al. / Applied Surface Science 351 (2015) 709–714
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Fig. 7. XPS spectra of C1s at the different Ar+ -etching depths of the sample La0.94 Fe.
Fig. 6. FT-IR spectra of the four La-Fe oxide samples. Table 3 Elemental composition at the different Ar+ -etching depths of the sample La0.94 Fe. Depth (nm)
O (at%)
La (at%)
Fe (at%)
La/Fe atomic ratio
O/(La + Fe) atomic ratio
ONL /OL atomic ratio
0 5 10 20 40
77.2 68.5 68.0 67.2 65.3
16.7 22.2 21.7 21.7 22.4
6.1 9.3 10.3 11.1 12.3
2.73 2.38 2.11 1.95 1.83
3.39 2.18 2.13 2.05 1.88
1.62 0.49 0.38 0.28 0.22
are enriched on the surface of the sample and more information of the interior of the crystallites is discovered as the depth increases. It should be noted that the results in Table 3 are statistically averaged values contributed by the interior and the remaining surface of the crystallites, because sizes of the crystallites are not totally uniform and also the boundary of the exposed plane of an etched crystallite is still providing surface information of the crystallite. For this reason, the XPS etching technique is indeed not able to provide completely the information of the interior of crystallites. It is also seen in Table 3 that the O/(La + Fe) atomic ratio and the ONL /OL atomic ratio both decrease as the depth increases, especially the decrease of the ONL /OL atomic ratio is large, indicating the surface carbonate ions were readily etched off once the etching started. The XPS spectra of C1s and O1s evolved with the depth increasing are shown in Figs. 7 and 8, respectively. It is clear that the peaks at the higher binding energy corresponding to carbonate ions are decreased as the depth increases. Fig. 9 shows XPS spectra of Fe2p evolved with the depth increasing. A shoulder peak at ca. 706 eV appeared after the etching depth more than 5 nm, indicating metallic Fe0 formed gradually during the etching process. This is similar to the reports that Ar+ etching caused samples to be partially reduced [24,25].
Fig. 8. XPS spectra of O1s at the different Ar+ -etching depths of the sample La0.94 Fe.
3.4. Catalytic activity for methane oxidation Catalytic activity for methane oxidation of the four La-Fe oxide samples as catalysts is compared in Fig. 10 at reaction temperature of 600 ◦ C. The high reaction temperature was chosen to diminish effect of surface carbonate ions. It is seen that catalytic activity for methane conversion is in the order of sample La1.02 Fe > La0.94 Fe > La0.76 Fe > La0.68 Fe at the fixed sample mass both in the presence and in the absence of gaseous oxygen. Faye et al. reported an order in methane conversion of sample La0.94 Fe > La0.83 Fe > La0.76 Fe > La0.64 Fe > La0.55 Fe (the atomic ratio of
Fig. 9. XPS spectra of Fe2p at the different Ar+ -etching depths of the sample La0.94 Fe.
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samples Lay Fe (y = 0.94, 0.76, 0.68), even though the feed La/Fe atomic ratio is less than unity, La3+ ions are enriched on the surface of the samples. The excess ironic oxide at the surface of the perovskite crystallites has an effect to link perovskite crystallites together and lead to a large size of crystallites for the perovskite phase at final. The samples La1.02 Fe and La0.94 Fe had relatively abundant surface carbonate ions due to the higher concentrations of La3+ ions on the surfaces. Catalytic activity for methane oxidation per unit surface area of the samples is in the order of La0.68 Fe > La0.76 Fe > La0.94 Fe > La1.02 Fe both in the presence and in the absence of gaseous oxygen. A reason for this order would be the higher concentration of Fe3+ ion on the surfaces of the samples La0.68 Fe and La0.76 Fe. Acknowledgment Fig. 10. Methane conversion as a function of La/Fe atomic ratio at reaction temperature of 600 ◦ C in the presence (empty circle, to the left Y axis) or absence (solid triangle, to the right Y axis) of gaseous oxygen.
This work was financially supported by the National Natural Science Foundation of China (Grant No. 21171020). References
La/Fe was measured by ICP) in the presence of gaseous oxygen [6]. Delmastro et al. reported an order in methane conversion of sample La0.9 Fe > La0.8 Fe > La0.7 Fe > La1.0 Fe (the atomic ratio of La/Fe was nominal value of the fed chemicals) in the presence of gaseous oxygen [11]. A reason for the difference in activity order in the literatures [6,11] should be the different preparation procedures adopted by the respective research group. In comparison of Fig. 10 with Table 1, it is seen that larger specific surface area is more favorable for methane conversion under the condition of the fixed catalyst mass. Therefore, specific catalytic activity, i.e., methane converted on unit surface area of a catalyst sample, needs to be compared for the four samples. For this purpose, specific surface area is assumed to be 1.5 m2 /g for the sample La0.76 Fe, and 1.0 m2 /g for the sample La0.68 Fe (see Table 1). It is found that the specific catalytic activity is in the order of sample La0.68 Fe > La0.76 Fe > La0.94 Fe > La1.02 Fe both in the presence and in the absence of gaseous oxygen. A reason for this order would be the higher concentration of Fe3+ ion on the surface of the samples La0.68 Fe and La0.76 Fe (see Table 2). This is because transition metal ions at the B-sites affect catalytic activity greatly [6,8]. Besides, chemical binding structure in the surface layer of the samples must have an effect on the catalytic activity, but it is not clear at an atomic level. 4. Conclusions The sample La0.94 Fe is single phase perovskite La0.94 FeO3−d . As the feed La/Fe atomic ratio further decreased, Fe2 O3 phase formed and specific surface area of the samples also decreased. For the
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