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Solid state reactions of nanocrystalline Ce0.5Yb0.5O1.75 mixed oxide with high surface area silica in oxidizing and reducing atmosphere
Journal of Solid State Chemistry 192 (2012) 221–228
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Solid state reactions of nanocrystalline Ce0.5Yb0.5O1.75 mixed oxide with high surface area silica in oxidizing and reducing atmosphere Ma"gorzata A. Ma"ecka, Leszek K˛epin´ski n Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroc!aw 2, Poland
a r t i c l e i n f o
abstract
Article history: Received 16 January 2012 Received in revised form 23 March 2012 Accepted 12 April 2012 Available online 24 April 2012
The interaction of nanocrystalline Ce0.5Yb0.5O1.75 mixed oxide with a high surface amorphous silica support in an oxidizing and reducing atmosphere was studied by XRD, HRTEM, SAED, SEM and BET techniques. The Ce0.5Yb0.5O1.75–SiO2 system shows very high structural and size stability in the oxidizing atmosphere up to 1000 1C, but in hydrogen spreading of the oxide onto silica occurs at temperatures above 800 1C. In the oxidizing atmosphere stability of the mixed oxide is limited by extraction of ytterbium from the oxide driven by a tendency to form ytterbium silicates. A new polymorph of Yb silicate, isomorphic with y-Y2Si2O7 (yttrialite), has been identified in the samples containing the mixed Ce–Yb oxide. The absence of y-Yb2Si2O7 silicate in the Yb2O3–SiO2 samples treated in similar conditions indicates that Ce4 þ ions are needed to stabilize the structure. & 2012 Elsevier Inc. All rights reserved.
Keywords: Ceria based oxide Nanocrystalline Ce1 xYbxO2 SiO2 TEM Ce–Yb mixed silicate y-Yb2Si2O7
1. Introduction Nanocrystalline ceria based mixed oxides unsupported or supported on high area oxide carriers like SiO2, Al2O3 play an important role in technological applications. Doping of a g-Al2O3 support with lanthanide oxides is commonly used to stabilize the alumina against sintering and phase transformation into a-Al2O3 [1]. Ceria based mixed oxides are also active catalysts of oxidation reactions [2–4], including a soot oxidation process, which is one of the most important environmental problems in the air pollution by solid by-products of a fuel combustion [5,6]. Lanthanide oxides supported on an amorphous SiO2 catalyse a-pinene isomerization [7]. Ceria based mixed oxides are also applicable in SOFC (solid oxide fuel cell) technology as the electrolyte materials [8–10]. In a literature, numerous papers describe the thermal evolution of lanthanide oxides on oxide supports. For LnOx–SiO2 systems (where Ln ¼La–Lu) solid state reactions between the oxides were observed at elevated temperature [11–18]. In most cases the reaction occurred in the oxidizing atmosphere, except CeO2–SiO2, where reducing atmosphere was needed to enable a silicate formation [19]. For a silica rich stoichiometry (SiO2/ Ln2O3 Z2) 12 types of lanthanide disilicates — Ln2Si2O7 (A, B, C, D, E, F, G, H, I, X, K, L) were found at temperatures over 1000 1C. Three of them (A, B, C) are stable at temperature up to 1200 1C
n
Corresponding author. Fax: þ48 71 344 10 29. E-mail address: [email protected] (L. K˛epin´ski).
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for light, medium and heavy lanthanides, respectively [20–22]. X, K and L types were obtained at high pressure, high temperature conditions [23–26]. A special group of the lanthanide ‘‘disilicates’’ are I- and B-type of Ln2Si2O7 containing poly-silica tetrahedrons in the structure. I-type (better described as Ln6[Si4O13][SiO4]2) was observed for light lanthanides La [27], Ce [16], Nd [11] and Pr [18] and contains groups of four vertex sharing SiO4 tetrahedra. B-type Ln2Si2O7 (better described as Ln4[Si3O10][SiO4]) was found as a stable polymorph for medium Eu–Er lanthanides [20] and as a high pressure form for heavy lanthanides [23,28]. Its structure contains groups of three vertex sharing SiO4 tetrahedra. At a lanthanide rich stoichiometry (Ln2O3/SiO2 Z1) ortho-silicates (Ln2[SiO4]O) with two structure types A and B are stable phases [20,22]. For a medium concentration of Ln2O3 (Ln2O3/SiO2 ¼7/9), an apatite type silicate (Ln9.33[SiO4]6O2) is a stable structure [22,29,30]. Much less data exists on mixed lanthanide disilicates [31,32]. Maier et al. [32] showed that Y/Ln-mixed disilicates could be obtained at high temperatures (1300–1600 1C) by a calcination of the starting oxides (Y2O3, Ln2O3, SiO2). The crystal structures of the obtained materials [32] were in a good agreement with the phase diagram presented by Felsche for single lanthanide disilicates [20]. Only a few reports concern interactions between nanocrystalline lanthanide mixed oxides and a high surface oxide support. For Ce1 xTbxO2 x/2 and Ce1 xPrxO2 x/2 supported on an alumina formation of CeAlO3, TbAlO3 or PrAlO3 was observed in a reducing atmosphere at temperatures over 900 1C [33–35]. It is worth to emphasize that the mixed oxides used in these works were prepared
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´ ski / Journal of Solid State Chemistry 192 (2012) 221–228 M.A. Ma!ecka, L. K˛epin
by co-impregnation of the support with a solution of lanthanide nitrates what rises a question on their homogeneity [35]. In our previous paper [36], preliminary results on a structure evolution of a similar Ce0.5Yb0.5O1.75–SiO2 system upon heating in air has been reported. Afterwords it appeared, however that the acetic acid used as a dispersing agent in that work caused a partial dissolution of the nanocrystalline Ce0.5Yb0.5O1.75 and in consequence formation of a number of poorly characterised mixed oxide–silica systems. We discovered this analysing XRD pattern of a dried suspension of nanocrystalline Ce0.5Yb0.5O1.75 in aqueous solution of acetic acid, which contained additional weak, broad reflections fitting roughly those of Ce acetate. Moreover, after heating in air at 1100 1C, the sample contained two distinct oxide phases: Ce rich phase with fluorite structure and Yb rich phase with bixbyite structure. Such effect of the phase separation did not occur for Ce0.5Yb0.5O1.75 not contacted with acetic acid [37]. New, improved method of preparation of nanoCe0.5Yb0.5O1.75–SiO2 samples used in this work, utilizing cyclohexane as dispersing agent, solved this problem and enabled production of well defined, homogeneous samples. The microstructure and thermal stability of the nanocrystalline Ce0.5Yb0.5O1.75 oxide supported on SiO2 at elevated temperatures in an oxidizing and reducing atmosphere was studied by XRD, TEM, HRTEM, SAED, SEM and BET techniques. The behaviour of the system was compared with that of conventional CeO2–SiO2, Yb2O3–SiO2 and (CeO2 þYb2O3)–SiO2 prepared by an impregnation with aqueous solutions of Ln nitrates.
Yb¼1/1, (CeþYb)/Si¼1/10) sample was synthesized as follows. First nanoparticles of Ce0.5Yb0.5O1.75 were prepared by W/O (water-in-oil) microemulsion method [36] with a small addition of an oleic acid. Then, the as-prepared suspension of the oxide in the organic phase was washed with methanol to remove the surfactant. Finally, the oxide was dispersed in cyclohexane. A high surface silica (Degussa OX-130, S0 ¼ 170 m2/g) was impregnated with the oxide dispersion to get Ce0.5Yb0.5O1.75–SiO2 sample with a molar ratio SiO2:Ce0.5Yb0.5O1.75 ¼10:1. After drying at 200 1C overnight, the sample was pre-heated at 550 1C for 3 h in a static air to clean out organic residues. Such standardized sample (called ‘‘as-prepared’’) was heated at temperatures from 800 to 1100 1C in a hydrogen flow or in a static air. The phase composition and structure parameters of the samples were determined by XRD (X’Pert PRO PANalytical powder diffractometer, Cu Ka radiation) with FullProf program [38] used for display and analysis of the diffraction patterns. Morphology and microstructure was investigated by TEM (Philips CM-20 SuperTwin operating at 200 kV and providing 0.25 nm resolution). HRTEM images and SAED patterns were analysed with DigitalMicrograph program. Uniformity, chemical composition and topography of the samples was checked with FE-SEM microscope (FEI NovaNanoSEM 230) equipped with EDS spectrometer (EDAX Genesis XM4). The BET surface area of the samples was measured using a low temperature N2 adsorption in a glass apparatus equipped with a conventional high vacuum system.
2. Materials and methods
3. Results
CeO2–SiO2, Yb2O3–SiO2 (Ln/Si¼1/10) and (CeO2 þYb2O3)–SiO2 (Ce/Yb ¼1/1, (CeþYb)/Si¼ 1/10) samples were prepared by impregnating a high surface silica with an aqueous solution of cerium, ytterbium or mixed nitrates. Ce0.5Yb0.5O1.75–SiO2 (Ce/
3.1. Oxidizing atmosphere A series of spot SEM-EDX analyses were done for the ‘‘as prepared’’ Ce0.5Yb0.5O1.75-SiO2 sample. Results of the spot
Fig. 1. XRD patterns of (A) Ce0.5Yb0.5O1.75–SiO2, (B) (CeO2 þYb2O3)–SiO2 after heating in a static air and (C) Ce0.5Yb0.5O1.75–SiO2, (D) (CeO2 þYb2O3)–SiO2 after heating in H2. Additionally, XRD patterns of pure Ce0.5Yb0.5O1.75, B-Yb2Si2O7 and standard patterns of CeO2 (PDF 00-004-0593), Yb2O3 (PDF 00-041-1106), y-Y2Si2O7 (PDF 00-0321448) and A-Ce2Si2O7 (PDF 01-073-3082) are shown.
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analyses presented in Table S1 (supporting information) showed small deviations from the mean chemical composition: O ¼63.8%, Si¼33.3% Ce¼1.4%, Yb¼1.5%, what confirms a uniform distribution of the Ce0.5Yb0.5O1.75 nanocrystals over the surface. The mean composition measured by EDS is also close to the assumed chemical composition: Ce/Yb¼1/1 and (CeþYb)/Si¼ 1/10. XRD patterns of the Ce0.5Yb0.5O1.75–SiO2 sample after heat treatment in a static air at various temperatures are shown in Fig. 1(A). The pattern of the ‘‘as prepared’’ sample contains broad reflections corresponding to a nanocrystalline Ce0.5Yb0.5O1.75 oxide with a mean crystallite size of 2.4 nm (Fig. 1(A), Table 1). In accordance, TEM Dark Field imaging revealed the presence of small crystallites of the oxide well dispersed on the support (Fig. S2 — supporting information). Fig. 2(A) presents HRTEM image of a small group of crystallites and FFT pattern of one of them, which appears to be cubic Ce0.5Yb0.5O1.75 oxide in [0–11] orientation. Analysis of a number of HRTEM images gave a mean crystallite size of the oxide of 3.1 nm. The difference between XRD and HRTEM derived mean crystallite sizes is assigned to difficulties in precise measurement of the sizes of Ce0.5Yb0.5O1.75 nanocrystallites supported on the SiO2. The presence of an inhomogeneous contrast from the amorphous silica decreases a visibility of the smallest crystallites of the oxide. Heating in air up to 1000 1C caused only some growth of the mean crystallite size of Ce0.5Yb0.5O1.75 and a small increase of the lattice parameter from 0.5342 to 0.5372 nm. After heating at 1100 1C for 3 h, additional weak reflections appeared in the XRD pattern (Fig. 1(A)). Extending the heating time to 45 h caused the development of the other reflections which enabled identification of two new phases: one isostructural with y-type Y2Si2O7 disilicate stabilized by MeIIO or MeIVO2 (PDF Card no. 00-032-1448 or 04-009-6929) [39,40] and B-type Yb2Si2O7 [40].
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The behaviour of the Ce0.5Yb0.5O1.75–SiO2 was compared with that of single oxides (CeO2 and Yb2O3) as well as composite oxide (CeO2 þYb2O3) deposited on the SiO2 by impregnation with nitrate solutions (Table 1). XRD and TEM revealed that the ‘‘as prepared’’ CeO2–SiO2 and Yb2O3–SiO2 samples contained particles of a lanthanide oxide with the mean crystallite size 6.2 nm (CeO2) and 4.9 nm (Yb2O3), uniformly distributed over the silica support. A structural evolution of the CeO2–SiO2 and Yb2O3–SiO2 systems with increasing temperature in the oxidizing atmosphere was in part presented in our previous papers [16,36]. For the CeO2–SiO2 heating up to 1100 1C caused only a sintering of CeO2 particles without any chemical interaction with the silica. A reaction between ytterbia and silica was observed, however during heating of the Yb2O3–SiO2 in air. First, a spreading of Yb2O3 over the silica surface took place at 900 1C and then at 1000 1C, massive crystallization of B-Yb2Si2O7 silicate occurred. Fig. 1(B) shows XRD patterns of (CeO2 þ Yb2O3)–SiO2 heated in air. For the samples heated up to 1000 1C only reflections characteristic for a fluorite structure are present. A symmetrical shape of the reflections suggests that probably there is only one crystalline oxide phase present. The lattice parameter (0.537 nm) and the mean crystallite size (3.9 nm) of the ‘‘as prepared’’ sample, smaller than those of pure CeO2, indicate that cerium–ytterbium mixed oxide could be formed. According to our previous study [37] the lattice parameter would correspond to Ce0.7Yb0.3O1.85 composition. The remaining part of ytterbium is probably dispersed as an amorphous phase over the support. Small changes in the XRD patterns appeared after heating up to 900 1C, but a noticeable growth of the mean crystallite size and increase of the lattice parameter occurred at 1000 1C. The effect may be connected with a withdrawal of Yb from the mixed oxide and its reaction with the silica support. Prolonged treatment at 1100 1C
Table 1 Structural characteristics of the samples. Treatment
Ce0.5Yb0.5O1.75–SiO2
(CeO2 þYb2O3)–SiO2
Yb2O3–SiO2
CeO2–SiO2
550 1C/3 h air
Ce0.5Yb0.5O1.75 [aF ¼0.5342 nm]a [dav ¼ 2.4 nm] —
Mixed oxide [aF ¼0.5371 nm] [dav ¼ 3.9 nm] Mixed oxide [aF ¼0.5373 nm] [dav ¼ 4.4 nm] Mixed oxide [aF ¼0.5377 nm] [dav ¼ 4.7 nm] Mixed oxide [aF ¼0.5388 nm] [dav ¼ 5.8 nm]
Yb2O3 [aF ¼0.5219 nm]b [dav ¼4.9 nm]
CeO2 [aF ¼0.5410 nm]c [dav ¼6.2 nm] —
800 1C/3 h air
900 1C/3 h air
1000 1C/3 h air
1100 1C/3 h air
1100 1C/45 h air
800 1C/3 h H2 900 1C/3 h H2 1000 1C/3 h H2
1100 1C/3 h H2
a b c
Ce1 xYbxO2 x/2 [xE 0.4] [aF ¼0.5361 nm] [dav ¼ 3.6 nm] Ce1 xYbxO2 x/2 [xE 0.35] [aF ¼0.5372 nm] [dav ¼ 4.2 nm] Ce1 xYbxO2 x/2 [xE 0.15] [aF ¼0.5395 nm] [dav ¼ 7.7 nm] B-Yb2Si2O7 y-Yb2Si2O7:CeO2 Ce1 xYbxO2 x/2 [xE 0.05] [aF ¼0.5404 nm] [dav ¼ 14.8 nm] B-Yb2Si2O7 y-Yb2Si2O7:CeO2 — Amorphous silicates A-Ce2Si2O7 Amorphous silicates A-Ce2Si2O7 B-Yb2Si2O7 A-(Ce,Yb)2Si2O7 B-Yb2Si2O7 quartz
Yb2O3 Amorphous oxide Yb2O3 Amorphous Oxide/silicate B-Yb2Si2O7
CeO2 [aF ¼0.5417 nm] [dav ¼7.9 nm] CeO2 [aF ¼0.5418 nm] [dav ¼10.2 nm]
B-Yb2Si2O7 Mixed oxide [aF ¼0.5401 nm] [dav ¼ 11.1 nm]
CeO2 [aF ¼0.5420 nm] [dav ¼14.0 nm]
Mixed oxide [aF ¼0.5407 nm] [dav ¼ 17.0 nm] y-Yb2Si2O7:CeO2
—
—
Mixed oxide amorphous oxide Amorphous silicates Amorphous silicates A-Ce2Si2O7 B-Yb2Si2O7 A-Ce2Si2O7 B-Yb2Si2O7 quartz
Yb2O3 amorphous oxide Yb2O3 amorphous silicate B-Yb2Si2O7
Amorphous oxide/silicate Amorphous oxide/silicate I-Ce2Si2O7
B-Yb2Si2O7
I-Ce2Si2O7
lattice parameters and average crystallite sizes were obtained from XRD data by whole pattern fit method. lattice parameter normalized to a hypothetical fluorite structure: aF ¼ 1/2 a. For the reference sample (PDF 00-041-1106) lattice parameter a¼ 1.0435 nm. For the reference sample (PDF 00-004-0593) lattice parameter a¼ 0.5411 nm.
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Fig. 2. HRTEM images of Ce0.5Yb0.5O1.75–SiO2 heated at (A) 550 1C/3 h in air, (B) 900 1C/3 h in H2. FFT pattern is includes as inset in (A).
y-Yb2Si2O7 can be explained by the smaller size of Yb3 þ ion (98.5 pm) than Y3 þ ion (101.9 pm) [42]. In accordance with the XRD pattern, FTIR spectrum of the sample (Fig. 3(B)) contains strong bands assigned to quartz and weaker bands at 594, 647, 868, 919 and 973 cm 1, which are very similar to those observed for y-Y2Si2O7 by Diaz et. al [43]. HRTEM image of a crystallite of the new y-Yb2Si2O7 is presented in Fig. 3(C) together with the FFT pattern that could be indexed as a [0–11] projection of the y-Yb2Si2O7. 3.2. Reducing atmosphere
Fig. 3. (A) XRD pattern, (B) FTIR spectrum and (C) HRTEM image of the sample containing y-Yb2Si2O7.
for 45 h caused crystallization of y-Yb2Si2O7 disilicate, observed also for Ce0.5Yb0.5O1.75–SiO2. To better characterize the structure of the new silicate phase and assuming that the phase is stabilized by Ce4 þ ions, we prepared a sample with higher Yb3 þ -ions and lower Ce4 þ -ions concentration (Ce/Yb¼1/9, Ln/ Si¼1/7). Fig. 3(A) depicts XRD pattern of this sample heated at 1100 1C for 45 h in air. It appears that in addition to y-type disilicate as the main phase, the sample contains quartz and a small amount of ceria. Application of the profile matching method [38] to the pattern gave the lattice parameters of the y-Yb2Si2O7: a ¼0.7394 nm, b¼0.7987 nm, c¼0.5049 nm, beta¼111.821 and V¼0.2768 nm3, slightly different from those of y-Y2Si2O7 (a¼0.750 nm, b¼ 0.806 nm, c ¼0.502 nm, beta¼112.01 and V¼0.2814 nm3) [41]. The smaller size of the unit cell for
A different behaviour was observed during heating of the Ce0.5Yb0.5O1.75–SiO2 in the reducing atmosphere. XRD pattern of the sample heated at 900 1C in the hydrogen flow (Fig. 1(C)) shows amorphization of the mixed oxide and very weak features indicating the onset of crystallization of A-Ce2Si2O7. This agrees with the HRTEM data, where spreading of the oxide over the silica support into an amorphous phase is clearly seen (Fig. 2(B)). After heating at 1000 1C reflections of B-Yb2Si2O7 appeared in addition to those of the A-Ce2Si2O7 (Fig. 1(C)). A multiphase character of the sample is evident in HRTEM micrographs (Fig. 4), where both amorphous particles (Fig. 4(B)) and small crystallites of ytterbium and cerium silicates are present. As an example Fig. 4(C) and Fig. 4(D) show HRTEM images and FFT patterns of A-Ce2Si2O7 and B-Yb2Si2O7 silicate, respectively. Increasing the heating temperature to 1100 1C caused further crystallization of the silicates (Fig. 1(C)). A noticeable shift of the reflections characteristic for A-Ce2Si2O7 towards higher angles could be due to formation of the mixed Ce–Yb disilicate. The phase evolution in (CeO2 þYb2O3)–SiO2 due to heating in the reducing atmosphere is shown in Fig. 1(D). Already at 800 1C an extensive spreading of the oxide(s) on the silica surface occurred and the process was completed at 900 1C. At 1000 1C onset of crystallization of two silicates: A-Ce2Si2O7 and B-Yb2Si2O7 was observed, which at 1100 1C was accompanied by crystallization of quartz. For comparison a structure evolution of CeO2–SiO2 and Yb2O3–SiO2 systems in hydrogen was also studied. For CeO2–SiO2, in accordance with the previous study [16], a complete amorphization and spreading of the oxide was observed at 800 1C (see Table 1). Then, at or above 900 1C, began a crystallization of I-Ce2Si2O7, which was the only crystalline phase after heating at 1100 1C. For Yb2O3–SiO2 system, contrary to CeO2–SiO2, the effect of heating in hydrogen was very similar to that in oxygen described in the previous section. Small aggregates of Yb2O3 crystallites visible after heating at 700 1C (Fig. 5(A)),
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Fig. 4. TEM images of Ce0.5Yb0.5O1.75–SiO2 heated at 1000 1C/3 h in H2: (A) general view, (B) amorphous silicate, (C) crystallite of A-Ce2Si2O7 silicate with corresponding FFT pattern and (D) crystallite of B-Yb2Si2O7 silicate with corresponding FFT pattern.
began to react with the support at 800 1C to form irregular patches of an amorphous oxide/silicate phase (Fig. 5(B)). At 900 1C, the amorphous phase was dominant, though some crystalline Yb2O3 still could be seen (Fig. 5(C)). B-Yb2Si2O7 silicate was the only crystalline phase after heating at 1000 and 1100 1C for 3 h. Fig. 5(D) shows HRTEM image and FFT pattern of a large B-Yb2Si2O7 crystal in [ 3 1 1] orientation in the Yb2O3–SiO2 sample reduced at 1000 1C for 3 h. 3.3. Textural properties of supported oxides BET surface areas of Ce0.5Yb0.5O1.75–SiO2 and (CeO2 þYb2O3)– SiO2 after preliminary thermal treatment at 550 1C in static air, 158 and 152 m2/g, respectively are very close and slightly smaller than that of pure SiO2 (170 m2/g) (Table 2). This observation agrees with the result of porosity measurement of the SiO2 support (Fig. S3 — supporting information) showing that the support contains mostly mesopores with a wide size distribution (5–100 nm), with a very small amount of microporosity. According to the information provided by the producer, consistent with our TEM observations, the Aerosil OX-130 consists of loose aggregates of nonporous particles of SiO2, with sizes 10–20 nm [Degussa, Technical Bulletin Fine Particles Basic Characteristics of AEROSILs Fumed Silica Number 11]. The pores in the SiO2 are thus simply voids between the particles. Deposition of smaller crystallites of Ce0.5Yb0.5O1.75 at the surface of the SiO2 should not thus change much the porosity (and the surface area) of the support. Under such assumption and taking into account weight ratios of the support (0.76) and the oxide (0.24) it is possible to
estimate the specific surface area of the mixed oxide. The value of 120 m2/goxide obtained for Ce0.5Yb0.5O1.75 confirms its high dispersion, though average particle size 6.2 nm calculated from the BET data (using the formula dav [nm]¼6000/r S0, where r ¼8.04 g/cm3 is the density and S0 [m2/g] is specific surface area) is larger than mean crystallite size calculated from XRD (2.4 nm) or TEM (3.1 nm). The reason must be agglomeration of Ce0.5Yb0.5O1.75 crystallites observed by TEM. Hardly any change in BET surface area occurred after heating the samples in air at temperature up to 900 1C. At 1000 1C there was significant loss of the surface area, less pronounced for Ce0.5Yb0.5O1.75–SiO2. The loss of the surface area is mostly connected with sintering of the SiO2 support (cf. Table 2), and the difference between (CeO2 þYb2O3)–SiO2 and Ce0.5Yb0.5O1.75–SiO2 is caused by a slower rate of crystallite growth for the Ce0.5Yb0.5O1.75 (cf. Table 1). At 1100 1C there was a very rapid loss of the surface area caused by a severe sintering of the support, which is clearly seen in SEM images. Fig. 6 compares images of the Ce0.5Yb0.5O1.75–SiO2 sample heated at (A) 900 1C and (B) 1100 1C in air, where coalescence of the silica into large grains is clearly visible. Noticeably different changes of the BET surface area occurred for the samples heated in hydrogen. Already at 900 1C there was a noticeable loss of the surface area of Ce0.5Yb0.5O1.75– SiO2, which is correlated with occurrence of Ce silicate phase. At 1000 1C the amount of silicate phases is significant in both samples and thus the loss of the surface area is comparable. This result corresponds with literature data where it has been shown that the formation of La silicate in SiO2 impregnated with La nitrate promotes sintering of the support [13,17].
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Fig. 5. HRTEM images of Yb2O3–SiO2 heated at (A) 700 1C/3 h H2 (with FFT), (B) 800 1C/3 h H2, (C) 900 1C/3 h H2, (D) 1000 1C/3 h H2 (with FFT).
Table 2 BET surface area of Ce0.5Yb0.5O1.75–SiO2 and (CeO2 þ Yb2O3)–SiO2. Treatment
Ce0.5Yb0.5O1.75– SiO2 (m2/g)
CeO2 þ Yb2O3– SiO2 (m2/g)
SiO2 (m2/g)
550 1C/3 h air 900 1C/3 h air 1000 1C/3 h air 1100 1C/3 h air 900 1C/3 h H2 1000 1C/3 h H2 1100 1C/3 h H2
158 159 131 51 125 68 51
152 146 93 49 167 76 72
170 a 161b 119 71
a b
heated at 600 1C. heated at 950 1C.
4. Discussion The most important observation about thermal behaviour of nanocrystalline Ce0.5Yb0.5O1.75 supported on the high surface SiO2 is its very high structural stability in the oxidizing atmosphere. Up to 1000 1C there was only small increase of the mean crystallite size of the oxide (to 4 nm) and some, minute expansion of the lattice parameter. This behaviour differs from that of (CeO2 þYb2O3)–SiO2 prepared by co-impregnation with the mixture of nitrates. In particular greater increase of the lattice parameter and mean crystallite size for (CeO2 þ Yb2O3)–SiO2 indicate that the oxide is less stable and extraction of Yb from the oxide occurs. This effect is even more pronounced at 1100 1C, where the mean crystallite size is 50% bigger. Despite some differences, both mixed oxides are more resistant to sintering than bare CeO2
deposited on SiO2 (cf. Table 1). Lower stability of (CeO2 þYb2O3)– SiO2 as compared with Ce0.5Yb0.5O1.75–SiO2 becomes clear when longer annealing time is applied. After 45 h heating at 1100 1C formation of crystalline silicate phase is clearly seen in the former but is quite weak in the latter (cf. Fig. 1(A) and Fig. 1(B)). In both cases the silicate phase appears to be isostructural with yttriallite (y-Y2Si2O7), the structure type not observed yet for lanthanides. We assume that the y-Yb2Si2O7 observed in this work is stabilized by Ce4 þ ions, similarly as Th4 þ stabilizes y-Y2Si2O7 [40]. This assumption is validated by the results obtained for Yb2O3 on SiO2 where B-type Yb2Si2O7 silicate was formed under the same conditions. B-Yb2Si2O7 is isomorphous with B-Dy2Si2O7 and a-Y2Si2O7 [44,45] and is considered now as new, low temperature polymorph of SiO2 rich Yb silicate [40]. Discussion on the differences in stability of Ce0.5Y0.5O1.75–SiO2 and (CeO2 þYb2O3)–SiO2, must take into account possible differences in their microstructure. Analysis of XRD data for (CeO2 þYb2O3)–SiO2 revealed the presence of symmetrical reflections corresponding to ceria based mixed oxide which implies the presence of crystallites of mixed oxide with Ce0.7Yb0.3O1.85 composition. It means that certain part of ytterbium did not participate in formation of the mixed oxide and was distributed as amorphous phase at the surface of SiO2. Moreover, it is probable, that a range of mixed oxides with varying composition formed during synthesis. This conjecture is validated by results obtained by Bernal et. al. [35] who showed, using analytical STEM, that in the sample prepared by co-impregnation of alumina support with a mixture of Ce- and Pr-nitrate solutions particles with broad range of compositions were present, in spite of occurrence of the symmetrical reflections in XRD pattern. Smaller content of Yb in ceria and existence of highly dispersed Yb at the
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Fig. 6. SEM images of Ce0.5Yb0.5O1.75–SiO2 heated at (A) 900 1C/3 h and (B) 1100 1C/3 h in a static air.
Fig. 7. Schematic showing the structure evolution of the Ce0.5Yb0.5O1.75–SiO2 in reducing and oxidizing atmosphere.
silica surface explain weaker stability and higher reactivity of (CeO2 þYb2O3)–SiO2. Significant differences in phase evolution of Ce0.5Yb0.5O1.75– SiO2 and (CeO2 þYb2O3)–SiO2, were noticed in hydrogen atmosphere. For the later sample amorphization and spreading of the oxide over the SiO2 at 900 1C, followed by crystallization at 1000 1C of separate A-Ce2Si2O7 and B-Yb2Si2O7 silicates was observed. In Ce0.5Yb0.5O1.75–SiO2, on the contrary, a mixed A(Ce,Yb)2Si2O7 is first formed at 900 1C and next at 1000 1C BYb2Si2O7 begins to grow. Formation of A-type Ce2Si2O7 as a first silicate phase in the mixed oxide–silica systems contrasts with pure CeO2–SiO2 system, where I-Ce2Si2O7 disilicate occurred first at around 1000 1C [16]. We could not find in the literature information on mixed disilicates containing exclusively lanthanide ions, though there are a few works on (Y–Ln)2Si2O7 systems, where Ln¼ lanthanide [31,32,46,47,48]. Mixed (Y–Ln)2Si2O7 silicates, where Ln¼Gd, Yb, Lu, prepared by ceramic method, i.e., solid state reaction of crystalline oxides (Ln2O3, Y2O3 and SiO2) at high temperatures, were studied by Maier et al. [32]. The authors observed formation of single silicate phases over wide composition range 0 rLnr0.5 and found that the obtained compounds may be in many respects treated as simple silicates containing one ‘‘lanthanide’’ ion with mean ionic radius calculated according to the relative content of end members. Under such assumption the stability regions of the solid solutions of Y and Ln silicates are well predicted by phase diagram presented by Felsche [20]. Results of [32] correspond well with our observations. In our case shift of XRD peaks towards larger 2 Y for A-Ce2Si2O7 silicate
formed by solid state reaction of highly dispersed of Ce–Yb mixed oxide with silica indicates contraction of the lattice cell. The reason could be incorporation into the crystal lattice of Yb3 þ ions, which have smaller crystal radius (98.5 pm) than Ce3 þ ion (114.3 pm) [42]. Substitution of Yb3 þ ions for Ce3 þ in the silicate decreases the mean lanthanide radius defined as rmean ¼cCe rCe þ cYb rYb, where cCe and cYb are mole fractions, and makes the stability regions of the silicate to resemble those of heavier lanthanides [32]. This explains why instead of I-type silicate expected for CeO2–SiO2 at around 1000 1C A-type structure is formed. Andreev et. al., [48] reported IR-spectroscopic studies on Ce1 xYxSi2O7 mixed silicates obtained at 1450–1500 1C in reducing atmosphere. Similarly to our findings they observed for certain content of smaller Y3 þ ions, x 41.6, sudden change in IR spectra indicating phase transformation from high temperature G-Ce2Si2O7 into A-form, which at this temperature is a stable polymorph for lanthanides with ionic radius smaller than Ce3 þ .
5. Conclusions Nanocrystalline Ce0.50Yb0.50O1.75 mixed oxide with the mean crystallite size of 3 nm and a narrow size distribution was prepared by a microemulsion method and uniformly deposited on the high surface silica support. The thermal stability of the Ce0.5Yb0.5O1.75–SiO2 system strongly depends on the gas atmosphere (Fig. 7). The system is structurally and chemically stable in the oxidizing atmosphere up to 1000 1C, exhibiting only a small
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increase of the mean crystallite size of the oxide to 4 nm. At 1100 1C a decomposition of the mixed oxide occurred with formation of two Yb silicates with unusual structures: y-Yb2Si2O7 isomorphic with y-Y2Si2O7 (yttrialite) and B-Yb2Si2O7 isomorphic with a-Y2Si2O7 (and B-Dy2Si2O7). The y-Yb2Si2O7 does not occur in the Yb2O3–SiO2 sample treated in similar conditions and is probably stabilized by Ce4 þ ions. On the contrary, in the reducing atmosphere the Ce0.5Yb0.5O1.75 mixed oxide becomes unstable on SiO2 already at 800 1C undergoing an amorphization and spreading over the silica surface. The reason is high affinity of the reduced Ce3 þ to form a silicate phase. Thanks to a very good chemical mixing of Ce and Yb in the Ce0.5Yb0.5O1.75, the mixed A(Ce,Yb)2Si2O7 silicate was formed in this case. The nanocrystalline Ce–Yb–O mixed oxide prepared by the impregnation of the silica with the aqueous solution of nitrates appeared to be chemically inhomogeneous and more prone to sintering in the oxidizing atmosphere. In the reducing atmosphere both samples behaved in a similar way, though the impregnated sample showed the extensive crystallization of quartz at 1100 1C. The high stability of nanocrystalline Ce0.5Yb0.5O1.75–SiO2 system in air makes it very good candidate as a catalyst of oxidizing reactions proceeding even at high temperatures.
Acknowledgments The authors thank Mrs. Z. Mazurkiewicz for valuable help with preparation of the samples, Mrs. E. Bukowska for XRD, Mr. M. Ptak for FT-IR, dr J. Okal and Mrs. A Cielecka for BET measurements and dr. W. Mista for measuring porosity of the SiO2 support. This work was supported by the National Science Center (Grant No. 2011/01/B/ST5/06386).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2012.04. 020.
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Solid state reactions of nanocrystalline Ce0.5Yb0.5O1.75 mixed oxide with high surface area silica in oxidizing and reducing atmosphere Ma"gorzata A. Ma"ecka, Leszek K˛epin´ski n Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroc!aw 2, Poland
a r t i c l e i n f o
abstract
Article history: Received 16 January 2012 Received in revised form 23 March 2012 Accepted 12 April 2012 Available online 24 April 2012
The interaction of nanocrystalline Ce0.5Yb0.5O1.75 mixed oxide with a high surface amorphous silica support in an oxidizing and reducing atmosphere was studied by XRD, HRTEM, SAED, SEM and BET techniques. The Ce0.5Yb0.5O1.75–SiO2 system shows very high structural and size stability in the oxidizing atmosphere up to 1000 1C, but in hydrogen spreading of the oxide onto silica occurs at temperatures above 800 1C. In the oxidizing atmosphere stability of the mixed oxide is limited by extraction of ytterbium from the oxide driven by a tendency to form ytterbium silicates. A new polymorph of Yb silicate, isomorphic with y-Y2Si2O7 (yttrialite), has been identified in the samples containing the mixed Ce–Yb oxide. The absence of y-Yb2Si2O7 silicate in the Yb2O3–SiO2 samples treated in similar conditions indicates that Ce4 þ ions are needed to stabilize the structure. & 2012 Elsevier Inc. All rights reserved.
Keywords: Ceria based oxide Nanocrystalline Ce1 xYbxO2 SiO2 TEM Ce–Yb mixed silicate y-Yb2Si2O7
1. Introduction Nanocrystalline ceria based mixed oxides unsupported or supported on high area oxide carriers like SiO2, Al2O3 play an important role in technological applications. Doping of a g-Al2O3 support with lanthanide oxides is commonly used to stabilize the alumina against sintering and phase transformation into a-Al2O3 [1]. Ceria based mixed oxides are also active catalysts of oxidation reactions [2–4], including a soot oxidation process, which is one of the most important environmental problems in the air pollution by solid by-products of a fuel combustion [5,6]. Lanthanide oxides supported on an amorphous SiO2 catalyse a-pinene isomerization [7]. Ceria based mixed oxides are also applicable in SOFC (solid oxide fuel cell) technology as the electrolyte materials [8–10]. In a literature, numerous papers describe the thermal evolution of lanthanide oxides on oxide supports. For LnOx–SiO2 systems (where Ln ¼La–Lu) solid state reactions between the oxides were observed at elevated temperature [11–18]. In most cases the reaction occurred in the oxidizing atmosphere, except CeO2–SiO2, where reducing atmosphere was needed to enable a silicate formation [19]. For a silica rich stoichiometry (SiO2/ Ln2O3 Z2) 12 types of lanthanide disilicates — Ln2Si2O7 (A, B, C, D, E, F, G, H, I, X, K, L) were found at temperatures over 1000 1C. Three of them (A, B, C) are stable at temperature up to 1200 1C
n
Corresponding author. Fax: þ48 71 344 10 29. E-mail address: [email protected] (L. K˛epin´ski).
0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.04.020
for light, medium and heavy lanthanides, respectively [20–22]. X, K and L types were obtained at high pressure, high temperature conditions [23–26]. A special group of the lanthanide ‘‘disilicates’’ are I- and B-type of Ln2Si2O7 containing poly-silica tetrahedrons in the structure. I-type (better described as Ln6[Si4O13][SiO4]2) was observed for light lanthanides La [27], Ce [16], Nd [11] and Pr [18] and contains groups of four vertex sharing SiO4 tetrahedra. B-type Ln2Si2O7 (better described as Ln4[Si3O10][SiO4]) was found as a stable polymorph for medium Eu–Er lanthanides [20] and as a high pressure form for heavy lanthanides [23,28]. Its structure contains groups of three vertex sharing SiO4 tetrahedra. At a lanthanide rich stoichiometry (Ln2O3/SiO2 Z1) ortho-silicates (Ln2[SiO4]O) with two structure types A and B are stable phases [20,22]. For a medium concentration of Ln2O3 (Ln2O3/SiO2 ¼7/9), an apatite type silicate (Ln9.33[SiO4]6O2) is a stable structure [22,29,30]. Much less data exists on mixed lanthanide disilicates [31,32]. Maier et al. [32] showed that Y/Ln-mixed disilicates could be obtained at high temperatures (1300–1600 1C) by a calcination of the starting oxides (Y2O3, Ln2O3, SiO2). The crystal structures of the obtained materials [32] were in a good agreement with the phase diagram presented by Felsche for single lanthanide disilicates [20]. Only a few reports concern interactions between nanocrystalline lanthanide mixed oxides and a high surface oxide support. For Ce1 xTbxO2 x/2 and Ce1 xPrxO2 x/2 supported on an alumina formation of CeAlO3, TbAlO3 or PrAlO3 was observed in a reducing atmosphere at temperatures over 900 1C [33–35]. It is worth to emphasize that the mixed oxides used in these works were prepared
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by co-impregnation of the support with a solution of lanthanide nitrates what rises a question on their homogeneity [35]. In our previous paper [36], preliminary results on a structure evolution of a similar Ce0.5Yb0.5O1.75–SiO2 system upon heating in air has been reported. Afterwords it appeared, however that the acetic acid used as a dispersing agent in that work caused a partial dissolution of the nanocrystalline Ce0.5Yb0.5O1.75 and in consequence formation of a number of poorly characterised mixed oxide–silica systems. We discovered this analysing XRD pattern of a dried suspension of nanocrystalline Ce0.5Yb0.5O1.75 in aqueous solution of acetic acid, which contained additional weak, broad reflections fitting roughly those of Ce acetate. Moreover, after heating in air at 1100 1C, the sample contained two distinct oxide phases: Ce rich phase with fluorite structure and Yb rich phase with bixbyite structure. Such effect of the phase separation did not occur for Ce0.5Yb0.5O1.75 not contacted with acetic acid [37]. New, improved method of preparation of nanoCe0.5Yb0.5O1.75–SiO2 samples used in this work, utilizing cyclohexane as dispersing agent, solved this problem and enabled production of well defined, homogeneous samples. The microstructure and thermal stability of the nanocrystalline Ce0.5Yb0.5O1.75 oxide supported on SiO2 at elevated temperatures in an oxidizing and reducing atmosphere was studied by XRD, TEM, HRTEM, SAED, SEM and BET techniques. The behaviour of the system was compared with that of conventional CeO2–SiO2, Yb2O3–SiO2 and (CeO2 þYb2O3)–SiO2 prepared by an impregnation with aqueous solutions of Ln nitrates.
Yb¼1/1, (CeþYb)/Si¼1/10) sample was synthesized as follows. First nanoparticles of Ce0.5Yb0.5O1.75 were prepared by W/O (water-in-oil) microemulsion method [36] with a small addition of an oleic acid. Then, the as-prepared suspension of the oxide in the organic phase was washed with methanol to remove the surfactant. Finally, the oxide was dispersed in cyclohexane. A high surface silica (Degussa OX-130, S0 ¼ 170 m2/g) was impregnated with the oxide dispersion to get Ce0.5Yb0.5O1.75–SiO2 sample with a molar ratio SiO2:Ce0.5Yb0.5O1.75 ¼10:1. After drying at 200 1C overnight, the sample was pre-heated at 550 1C for 3 h in a static air to clean out organic residues. Such standardized sample (called ‘‘as-prepared’’) was heated at temperatures from 800 to 1100 1C in a hydrogen flow or in a static air. The phase composition and structure parameters of the samples were determined by XRD (X’Pert PRO PANalytical powder diffractometer, Cu Ka radiation) with FullProf program [38] used for display and analysis of the diffraction patterns. Morphology and microstructure was investigated by TEM (Philips CM-20 SuperTwin operating at 200 kV and providing 0.25 nm resolution). HRTEM images and SAED patterns were analysed with DigitalMicrograph program. Uniformity, chemical composition and topography of the samples was checked with FE-SEM microscope (FEI NovaNanoSEM 230) equipped with EDS spectrometer (EDAX Genesis XM4). The BET surface area of the samples was measured using a low temperature N2 adsorption in a glass apparatus equipped with a conventional high vacuum system.
2. Materials and methods
3. Results
CeO2–SiO2, Yb2O3–SiO2 (Ln/Si¼1/10) and (CeO2 þYb2O3)–SiO2 (Ce/Yb ¼1/1, (CeþYb)/Si¼ 1/10) samples were prepared by impregnating a high surface silica with an aqueous solution of cerium, ytterbium or mixed nitrates. Ce0.5Yb0.5O1.75–SiO2 (Ce/
3.1. Oxidizing atmosphere A series of spot SEM-EDX analyses were done for the ‘‘as prepared’’ Ce0.5Yb0.5O1.75-SiO2 sample. Results of the spot
Fig. 1. XRD patterns of (A) Ce0.5Yb0.5O1.75–SiO2, (B) (CeO2 þYb2O3)–SiO2 after heating in a static air and (C) Ce0.5Yb0.5O1.75–SiO2, (D) (CeO2 þYb2O3)–SiO2 after heating in H2. Additionally, XRD patterns of pure Ce0.5Yb0.5O1.75, B-Yb2Si2O7 and standard patterns of CeO2 (PDF 00-004-0593), Yb2O3 (PDF 00-041-1106), y-Y2Si2O7 (PDF 00-0321448) and A-Ce2Si2O7 (PDF 01-073-3082) are shown.
´ ski / Journal of Solid State Chemistry 192 (2012) 221–228 M.A. Ma!ecka, L. K˛epin
analyses presented in Table S1 (supporting information) showed small deviations from the mean chemical composition: O ¼63.8%, Si¼33.3% Ce¼1.4%, Yb¼1.5%, what confirms a uniform distribution of the Ce0.5Yb0.5O1.75 nanocrystals over the surface. The mean composition measured by EDS is also close to the assumed chemical composition: Ce/Yb¼1/1 and (CeþYb)/Si¼ 1/10. XRD patterns of the Ce0.5Yb0.5O1.75–SiO2 sample after heat treatment in a static air at various temperatures are shown in Fig. 1(A). The pattern of the ‘‘as prepared’’ sample contains broad reflections corresponding to a nanocrystalline Ce0.5Yb0.5O1.75 oxide with a mean crystallite size of 2.4 nm (Fig. 1(A), Table 1). In accordance, TEM Dark Field imaging revealed the presence of small crystallites of the oxide well dispersed on the support (Fig. S2 — supporting information). Fig. 2(A) presents HRTEM image of a small group of crystallites and FFT pattern of one of them, which appears to be cubic Ce0.5Yb0.5O1.75 oxide in [0–11] orientation. Analysis of a number of HRTEM images gave a mean crystallite size of the oxide of 3.1 nm. The difference between XRD and HRTEM derived mean crystallite sizes is assigned to difficulties in precise measurement of the sizes of Ce0.5Yb0.5O1.75 nanocrystallites supported on the SiO2. The presence of an inhomogeneous contrast from the amorphous silica decreases a visibility of the smallest crystallites of the oxide. Heating in air up to 1000 1C caused only some growth of the mean crystallite size of Ce0.5Yb0.5O1.75 and a small increase of the lattice parameter from 0.5342 to 0.5372 nm. After heating at 1100 1C for 3 h, additional weak reflections appeared in the XRD pattern (Fig. 1(A)). Extending the heating time to 45 h caused the development of the other reflections which enabled identification of two new phases: one isostructural with y-type Y2Si2O7 disilicate stabilized by MeIIO or MeIVO2 (PDF Card no. 00-032-1448 or 04-009-6929) [39,40] and B-type Yb2Si2O7 [40].
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The behaviour of the Ce0.5Yb0.5O1.75–SiO2 was compared with that of single oxides (CeO2 and Yb2O3) as well as composite oxide (CeO2 þYb2O3) deposited on the SiO2 by impregnation with nitrate solutions (Table 1). XRD and TEM revealed that the ‘‘as prepared’’ CeO2–SiO2 and Yb2O3–SiO2 samples contained particles of a lanthanide oxide with the mean crystallite size 6.2 nm (CeO2) and 4.9 nm (Yb2O3), uniformly distributed over the silica support. A structural evolution of the CeO2–SiO2 and Yb2O3–SiO2 systems with increasing temperature in the oxidizing atmosphere was in part presented in our previous papers [16,36]. For the CeO2–SiO2 heating up to 1100 1C caused only a sintering of CeO2 particles without any chemical interaction with the silica. A reaction between ytterbia and silica was observed, however during heating of the Yb2O3–SiO2 in air. First, a spreading of Yb2O3 over the silica surface took place at 900 1C and then at 1000 1C, massive crystallization of B-Yb2Si2O7 silicate occurred. Fig. 1(B) shows XRD patterns of (CeO2 þ Yb2O3)–SiO2 heated in air. For the samples heated up to 1000 1C only reflections characteristic for a fluorite structure are present. A symmetrical shape of the reflections suggests that probably there is only one crystalline oxide phase present. The lattice parameter (0.537 nm) and the mean crystallite size (3.9 nm) of the ‘‘as prepared’’ sample, smaller than those of pure CeO2, indicate that cerium–ytterbium mixed oxide could be formed. According to our previous study [37] the lattice parameter would correspond to Ce0.7Yb0.3O1.85 composition. The remaining part of ytterbium is probably dispersed as an amorphous phase over the support. Small changes in the XRD patterns appeared after heating up to 900 1C, but a noticeable growth of the mean crystallite size and increase of the lattice parameter occurred at 1000 1C. The effect may be connected with a withdrawal of Yb from the mixed oxide and its reaction with the silica support. Prolonged treatment at 1100 1C
Table 1 Structural characteristics of the samples. Treatment
Ce0.5Yb0.5O1.75–SiO2
(CeO2 þYb2O3)–SiO2
Yb2O3–SiO2
CeO2–SiO2
550 1C/3 h air
Ce0.5Yb0.5O1.75 [aF ¼0.5342 nm]a [dav ¼ 2.4 nm] —
Mixed oxide [aF ¼0.5371 nm] [dav ¼ 3.9 nm] Mixed oxide [aF ¼0.5373 nm] [dav ¼ 4.4 nm] Mixed oxide [aF ¼0.5377 nm] [dav ¼ 4.7 nm] Mixed oxide [aF ¼0.5388 nm] [dav ¼ 5.8 nm]
Yb2O3 [aF ¼0.5219 nm]b [dav ¼4.9 nm]
CeO2 [aF ¼0.5410 nm]c [dav ¼6.2 nm] —
800 1C/3 h air
900 1C/3 h air
1000 1C/3 h air
1100 1C/3 h air
1100 1C/45 h air
800 1C/3 h H2 900 1C/3 h H2 1000 1C/3 h H2
1100 1C/3 h H2
a b c
Ce1 xYbxO2 x/2 [xE 0.4] [aF ¼0.5361 nm] [dav ¼ 3.6 nm] Ce1 xYbxO2 x/2 [xE 0.35] [aF ¼0.5372 nm] [dav ¼ 4.2 nm] Ce1 xYbxO2 x/2 [xE 0.15] [aF ¼0.5395 nm] [dav ¼ 7.7 nm] B-Yb2Si2O7 y-Yb2Si2O7:CeO2 Ce1 xYbxO2 x/2 [xE 0.05] [aF ¼0.5404 nm] [dav ¼ 14.8 nm] B-Yb2Si2O7 y-Yb2Si2O7:CeO2 — Amorphous silicates A-Ce2Si2O7 Amorphous silicates A-Ce2Si2O7 B-Yb2Si2O7 A-(Ce,Yb)2Si2O7 B-Yb2Si2O7 quartz
Yb2O3 Amorphous oxide Yb2O3 Amorphous Oxide/silicate B-Yb2Si2O7
CeO2 [aF ¼0.5417 nm] [dav ¼7.9 nm] CeO2 [aF ¼0.5418 nm] [dav ¼10.2 nm]
B-Yb2Si2O7 Mixed oxide [aF ¼0.5401 nm] [dav ¼ 11.1 nm]
CeO2 [aF ¼0.5420 nm] [dav ¼14.0 nm]
Mixed oxide [aF ¼0.5407 nm] [dav ¼ 17.0 nm] y-Yb2Si2O7:CeO2
—
—
Mixed oxide amorphous oxide Amorphous silicates Amorphous silicates A-Ce2Si2O7 B-Yb2Si2O7 A-Ce2Si2O7 B-Yb2Si2O7 quartz
Yb2O3 amorphous oxide Yb2O3 amorphous silicate B-Yb2Si2O7
Amorphous oxide/silicate Amorphous oxide/silicate I-Ce2Si2O7
B-Yb2Si2O7
I-Ce2Si2O7
lattice parameters and average crystallite sizes were obtained from XRD data by whole pattern fit method. lattice parameter normalized to a hypothetical fluorite structure: aF ¼ 1/2 a. For the reference sample (PDF 00-041-1106) lattice parameter a¼ 1.0435 nm. For the reference sample (PDF 00-004-0593) lattice parameter a¼ 0.5411 nm.
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Fig. 2. HRTEM images of Ce0.5Yb0.5O1.75–SiO2 heated at (A) 550 1C/3 h in air, (B) 900 1C/3 h in H2. FFT pattern is includes as inset in (A).
y-Yb2Si2O7 can be explained by the smaller size of Yb3 þ ion (98.5 pm) than Y3 þ ion (101.9 pm) [42]. In accordance with the XRD pattern, FTIR spectrum of the sample (Fig. 3(B)) contains strong bands assigned to quartz and weaker bands at 594, 647, 868, 919 and 973 cm 1, which are very similar to those observed for y-Y2Si2O7 by Diaz et. al [43]. HRTEM image of a crystallite of the new y-Yb2Si2O7 is presented in Fig. 3(C) together with the FFT pattern that could be indexed as a [0–11] projection of the y-Yb2Si2O7. 3.2. Reducing atmosphere
Fig. 3. (A) XRD pattern, (B) FTIR spectrum and (C) HRTEM image of the sample containing y-Yb2Si2O7.
for 45 h caused crystallization of y-Yb2Si2O7 disilicate, observed also for Ce0.5Yb0.5O1.75–SiO2. To better characterize the structure of the new silicate phase and assuming that the phase is stabilized by Ce4 þ ions, we prepared a sample with higher Yb3 þ -ions and lower Ce4 þ -ions concentration (Ce/Yb¼1/9, Ln/ Si¼1/7). Fig. 3(A) depicts XRD pattern of this sample heated at 1100 1C for 45 h in air. It appears that in addition to y-type disilicate as the main phase, the sample contains quartz and a small amount of ceria. Application of the profile matching method [38] to the pattern gave the lattice parameters of the y-Yb2Si2O7: a ¼0.7394 nm, b¼0.7987 nm, c¼0.5049 nm, beta¼111.821 and V¼0.2768 nm3, slightly different from those of y-Y2Si2O7 (a¼0.750 nm, b¼ 0.806 nm, c ¼0.502 nm, beta¼112.01 and V¼0.2814 nm3) [41]. The smaller size of the unit cell for
A different behaviour was observed during heating of the Ce0.5Yb0.5O1.75–SiO2 in the reducing atmosphere. XRD pattern of the sample heated at 900 1C in the hydrogen flow (Fig. 1(C)) shows amorphization of the mixed oxide and very weak features indicating the onset of crystallization of A-Ce2Si2O7. This agrees with the HRTEM data, where spreading of the oxide over the silica support into an amorphous phase is clearly seen (Fig. 2(B)). After heating at 1000 1C reflections of B-Yb2Si2O7 appeared in addition to those of the A-Ce2Si2O7 (Fig. 1(C)). A multiphase character of the sample is evident in HRTEM micrographs (Fig. 4), where both amorphous particles (Fig. 4(B)) and small crystallites of ytterbium and cerium silicates are present. As an example Fig. 4(C) and Fig. 4(D) show HRTEM images and FFT patterns of A-Ce2Si2O7 and B-Yb2Si2O7 silicate, respectively. Increasing the heating temperature to 1100 1C caused further crystallization of the silicates (Fig. 1(C)). A noticeable shift of the reflections characteristic for A-Ce2Si2O7 towards higher angles could be due to formation of the mixed Ce–Yb disilicate. The phase evolution in (CeO2 þYb2O3)–SiO2 due to heating in the reducing atmosphere is shown in Fig. 1(D). Already at 800 1C an extensive spreading of the oxide(s) on the silica surface occurred and the process was completed at 900 1C. At 1000 1C onset of crystallization of two silicates: A-Ce2Si2O7 and B-Yb2Si2O7 was observed, which at 1100 1C was accompanied by crystallization of quartz. For comparison a structure evolution of CeO2–SiO2 and Yb2O3–SiO2 systems in hydrogen was also studied. For CeO2–SiO2, in accordance with the previous study [16], a complete amorphization and spreading of the oxide was observed at 800 1C (see Table 1). Then, at or above 900 1C, began a crystallization of I-Ce2Si2O7, which was the only crystalline phase after heating at 1100 1C. For Yb2O3–SiO2 system, contrary to CeO2–SiO2, the effect of heating in hydrogen was very similar to that in oxygen described in the previous section. Small aggregates of Yb2O3 crystallites visible after heating at 700 1C (Fig. 5(A)),
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Fig. 4. TEM images of Ce0.5Yb0.5O1.75–SiO2 heated at 1000 1C/3 h in H2: (A) general view, (B) amorphous silicate, (C) crystallite of A-Ce2Si2O7 silicate with corresponding FFT pattern and (D) crystallite of B-Yb2Si2O7 silicate with corresponding FFT pattern.
began to react with the support at 800 1C to form irregular patches of an amorphous oxide/silicate phase (Fig. 5(B)). At 900 1C, the amorphous phase was dominant, though some crystalline Yb2O3 still could be seen (Fig. 5(C)). B-Yb2Si2O7 silicate was the only crystalline phase after heating at 1000 and 1100 1C for 3 h. Fig. 5(D) shows HRTEM image and FFT pattern of a large B-Yb2Si2O7 crystal in [ 3 1 1] orientation in the Yb2O3–SiO2 sample reduced at 1000 1C for 3 h. 3.3. Textural properties of supported oxides BET surface areas of Ce0.5Yb0.5O1.75–SiO2 and (CeO2 þYb2O3)– SiO2 after preliminary thermal treatment at 550 1C in static air, 158 and 152 m2/g, respectively are very close and slightly smaller than that of pure SiO2 (170 m2/g) (Table 2). This observation agrees with the result of porosity measurement of the SiO2 support (Fig. S3 — supporting information) showing that the support contains mostly mesopores with a wide size distribution (5–100 nm), with a very small amount of microporosity. According to the information provided by the producer, consistent with our TEM observations, the Aerosil OX-130 consists of loose aggregates of nonporous particles of SiO2, with sizes 10–20 nm [Degussa, Technical Bulletin Fine Particles Basic Characteristics of AEROSILs Fumed Silica Number 11]. The pores in the SiO2 are thus simply voids between the particles. Deposition of smaller crystallites of Ce0.5Yb0.5O1.75 at the surface of the SiO2 should not thus change much the porosity (and the surface area) of the support. Under such assumption and taking into account weight ratios of the support (0.76) and the oxide (0.24) it is possible to
estimate the specific surface area of the mixed oxide. The value of 120 m2/goxide obtained for Ce0.5Yb0.5O1.75 confirms its high dispersion, though average particle size 6.2 nm calculated from the BET data (using the formula dav [nm]¼6000/r S0, where r ¼8.04 g/cm3 is the density and S0 [m2/g] is specific surface area) is larger than mean crystallite size calculated from XRD (2.4 nm) or TEM (3.1 nm). The reason must be agglomeration of Ce0.5Yb0.5O1.75 crystallites observed by TEM. Hardly any change in BET surface area occurred after heating the samples in air at temperature up to 900 1C. At 1000 1C there was significant loss of the surface area, less pronounced for Ce0.5Yb0.5O1.75–SiO2. The loss of the surface area is mostly connected with sintering of the SiO2 support (cf. Table 2), and the difference between (CeO2 þYb2O3)–SiO2 and Ce0.5Yb0.5O1.75–SiO2 is caused by a slower rate of crystallite growth for the Ce0.5Yb0.5O1.75 (cf. Table 1). At 1100 1C there was a very rapid loss of the surface area caused by a severe sintering of the support, which is clearly seen in SEM images. Fig. 6 compares images of the Ce0.5Yb0.5O1.75–SiO2 sample heated at (A) 900 1C and (B) 1100 1C in air, where coalescence of the silica into large grains is clearly visible. Noticeably different changes of the BET surface area occurred for the samples heated in hydrogen. Already at 900 1C there was a noticeable loss of the surface area of Ce0.5Yb0.5O1.75– SiO2, which is correlated with occurrence of Ce silicate phase. At 1000 1C the amount of silicate phases is significant in both samples and thus the loss of the surface area is comparable. This result corresponds with literature data where it has been shown that the formation of La silicate in SiO2 impregnated with La nitrate promotes sintering of the support [13,17].
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Fig. 5. HRTEM images of Yb2O3–SiO2 heated at (A) 700 1C/3 h H2 (with FFT), (B) 800 1C/3 h H2, (C) 900 1C/3 h H2, (D) 1000 1C/3 h H2 (with FFT).
Table 2 BET surface area of Ce0.5Yb0.5O1.75–SiO2 and (CeO2 þ Yb2O3)–SiO2. Treatment
Ce0.5Yb0.5O1.75– SiO2 (m2/g)
CeO2 þ Yb2O3– SiO2 (m2/g)
SiO2 (m2/g)
550 1C/3 h air 900 1C/3 h air 1000 1C/3 h air 1100 1C/3 h air 900 1C/3 h H2 1000 1C/3 h H2 1100 1C/3 h H2
158 159 131 51 125 68 51
152 146 93 49 167 76 72
170 a 161b 119 71
a b
heated at 600 1C. heated at 950 1C.
4. Discussion The most important observation about thermal behaviour of nanocrystalline Ce0.5Yb0.5O1.75 supported on the high surface SiO2 is its very high structural stability in the oxidizing atmosphere. Up to 1000 1C there was only small increase of the mean crystallite size of the oxide (to 4 nm) and some, minute expansion of the lattice parameter. This behaviour differs from that of (CeO2 þYb2O3)–SiO2 prepared by co-impregnation with the mixture of nitrates. In particular greater increase of the lattice parameter and mean crystallite size for (CeO2 þ Yb2O3)–SiO2 indicate that the oxide is less stable and extraction of Yb from the oxide occurs. This effect is even more pronounced at 1100 1C, where the mean crystallite size is 50% bigger. Despite some differences, both mixed oxides are more resistant to sintering than bare CeO2
deposited on SiO2 (cf. Table 1). Lower stability of (CeO2 þYb2O3)– SiO2 as compared with Ce0.5Yb0.5O1.75–SiO2 becomes clear when longer annealing time is applied. After 45 h heating at 1100 1C formation of crystalline silicate phase is clearly seen in the former but is quite weak in the latter (cf. Fig. 1(A) and Fig. 1(B)). In both cases the silicate phase appears to be isostructural with yttriallite (y-Y2Si2O7), the structure type not observed yet for lanthanides. We assume that the y-Yb2Si2O7 observed in this work is stabilized by Ce4 þ ions, similarly as Th4 þ stabilizes y-Y2Si2O7 [40]. This assumption is validated by the results obtained for Yb2O3 on SiO2 where B-type Yb2Si2O7 silicate was formed under the same conditions. B-Yb2Si2O7 is isomorphous with B-Dy2Si2O7 and a-Y2Si2O7 [44,45] and is considered now as new, low temperature polymorph of SiO2 rich Yb silicate [40]. Discussion on the differences in stability of Ce0.5Y0.5O1.75–SiO2 and (CeO2 þYb2O3)–SiO2, must take into account possible differences in their microstructure. Analysis of XRD data for (CeO2 þYb2O3)–SiO2 revealed the presence of symmetrical reflections corresponding to ceria based mixed oxide which implies the presence of crystallites of mixed oxide with Ce0.7Yb0.3O1.85 composition. It means that certain part of ytterbium did not participate in formation of the mixed oxide and was distributed as amorphous phase at the surface of SiO2. Moreover, it is probable, that a range of mixed oxides with varying composition formed during synthesis. This conjecture is validated by results obtained by Bernal et. al. [35] who showed, using analytical STEM, that in the sample prepared by co-impregnation of alumina support with a mixture of Ce- and Pr-nitrate solutions particles with broad range of compositions were present, in spite of occurrence of the symmetrical reflections in XRD pattern. Smaller content of Yb in ceria and existence of highly dispersed Yb at the
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Fig. 6. SEM images of Ce0.5Yb0.5O1.75–SiO2 heated at (A) 900 1C/3 h and (B) 1100 1C/3 h in a static air.
Fig. 7. Schematic showing the structure evolution of the Ce0.5Yb0.5O1.75–SiO2 in reducing and oxidizing atmosphere.
silica surface explain weaker stability and higher reactivity of (CeO2 þYb2O3)–SiO2. Significant differences in phase evolution of Ce0.5Yb0.5O1.75– SiO2 and (CeO2 þYb2O3)–SiO2, were noticed in hydrogen atmosphere. For the later sample amorphization and spreading of the oxide over the SiO2 at 900 1C, followed by crystallization at 1000 1C of separate A-Ce2Si2O7 and B-Yb2Si2O7 silicates was observed. In Ce0.5Yb0.5O1.75–SiO2, on the contrary, a mixed A(Ce,Yb)2Si2O7 is first formed at 900 1C and next at 1000 1C BYb2Si2O7 begins to grow. Formation of A-type Ce2Si2O7 as a first silicate phase in the mixed oxide–silica systems contrasts with pure CeO2–SiO2 system, where I-Ce2Si2O7 disilicate occurred first at around 1000 1C [16]. We could not find in the literature information on mixed disilicates containing exclusively lanthanide ions, though there are a few works on (Y–Ln)2Si2O7 systems, where Ln¼ lanthanide [31,32,46,47,48]. Mixed (Y–Ln)2Si2O7 silicates, where Ln¼Gd, Yb, Lu, prepared by ceramic method, i.e., solid state reaction of crystalline oxides (Ln2O3, Y2O3 and SiO2) at high temperatures, were studied by Maier et al. [32]. The authors observed formation of single silicate phases over wide composition range 0 rLnr0.5 and found that the obtained compounds may be in many respects treated as simple silicates containing one ‘‘lanthanide’’ ion with mean ionic radius calculated according to the relative content of end members. Under such assumption the stability regions of the solid solutions of Y and Ln silicates are well predicted by phase diagram presented by Felsche [20]. Results of [32] correspond well with our observations. In our case shift of XRD peaks towards larger 2 Y for A-Ce2Si2O7 silicate
formed by solid state reaction of highly dispersed of Ce–Yb mixed oxide with silica indicates contraction of the lattice cell. The reason could be incorporation into the crystal lattice of Yb3 þ ions, which have smaller crystal radius (98.5 pm) than Ce3 þ ion (114.3 pm) [42]. Substitution of Yb3 þ ions for Ce3 þ in the silicate decreases the mean lanthanide radius defined as rmean ¼cCe rCe þ cYb rYb, where cCe and cYb are mole fractions, and makes the stability regions of the silicate to resemble those of heavier lanthanides [32]. This explains why instead of I-type silicate expected for CeO2–SiO2 at around 1000 1C A-type structure is formed. Andreev et. al., [48] reported IR-spectroscopic studies on Ce1 xYxSi2O7 mixed silicates obtained at 1450–1500 1C in reducing atmosphere. Similarly to our findings they observed for certain content of smaller Y3 þ ions, x 41.6, sudden change in IR spectra indicating phase transformation from high temperature G-Ce2Si2O7 into A-form, which at this temperature is a stable polymorph for lanthanides with ionic radius smaller than Ce3 þ .
5. Conclusions Nanocrystalline Ce0.50Yb0.50O1.75 mixed oxide with the mean crystallite size of 3 nm and a narrow size distribution was prepared by a microemulsion method and uniformly deposited on the high surface silica support. The thermal stability of the Ce0.5Yb0.5O1.75–SiO2 system strongly depends on the gas atmosphere (Fig. 7). The system is structurally and chemically stable in the oxidizing atmosphere up to 1000 1C, exhibiting only a small
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increase of the mean crystallite size of the oxide to 4 nm. At 1100 1C a decomposition of the mixed oxide occurred with formation of two Yb silicates with unusual structures: y-Yb2Si2O7 isomorphic with y-Y2Si2O7 (yttrialite) and B-Yb2Si2O7 isomorphic with a-Y2Si2O7 (and B-Dy2Si2O7). The y-Yb2Si2O7 does not occur in the Yb2O3–SiO2 sample treated in similar conditions and is probably stabilized by Ce4 þ ions. On the contrary, in the reducing atmosphere the Ce0.5Yb0.5O1.75 mixed oxide becomes unstable on SiO2 already at 800 1C undergoing an amorphization and spreading over the silica surface. The reason is high affinity of the reduced Ce3 þ to form a silicate phase. Thanks to a very good chemical mixing of Ce and Yb in the Ce0.5Yb0.5O1.75, the mixed A(Ce,Yb)2Si2O7 silicate was formed in this case. The nanocrystalline Ce–Yb–O mixed oxide prepared by the impregnation of the silica with the aqueous solution of nitrates appeared to be chemically inhomogeneous and more prone to sintering in the oxidizing atmosphere. In the reducing atmosphere both samples behaved in a similar way, though the impregnated sample showed the extensive crystallization of quartz at 1100 1C. The high stability of nanocrystalline Ce0.5Yb0.5O1.75–SiO2 system in air makes it very good candidate as a catalyst of oxidizing reactions proceeding even at high temperatures.
Acknowledgments The authors thank Mrs. Z. Mazurkiewicz for valuable help with preparation of the samples, Mrs. E. Bukowska for XRD, Mr. M. Ptak for FT-IR, dr J. Okal and Mrs. A Cielecka for BET measurements and dr. W. Mista for measuring porosity of the SiO2 support. This work was supported by the National Science Center (Grant No. 2011/01/B/ST5/06386).
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2012.04. 020.
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