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Cerium(III) N,N-dibutylcarbamate as precursor to nanocrystalline cerium dioxide
Accepted Manuscript Cerium(III) N,N-dibutylcarbamate as precursor to nanocrystalline cerium dioxide Daniela Belli Dell’Amico, Massimo De Sanctis, Randa Ishak, Sara Dolci, Luca Labella, Marco Lezzerini, Fabio Marchetti PII: DOI: Reference:
S0277-5387(15)00353-8 http://dx.doi.org/10.1016/j.poly.2015.06.037 POLY 11384
To appear in:
Polyhedron
Received Date: Accepted Date:
24 April 2015 30 June 2015
Please cite this article as: D.B. Dell’Amico, M. De Sanctis, R. Ishak, S. Dolci, L. Labella, M. Lezzerini, F. Marchetti, Cerium(III) N,N-dibutylcarbamate as precursor to nanocrystalline cerium dioxide, Polyhedron (2015), doi: http:// dx.doi.org/10.1016/j.poly.2015.06.037
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Labella Luca
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Cerium(III) N,N-dibutylcarbamate as precursor to nanocrystalline cerium dioxide Daniela Belli Dell’Amicoa, Massimo De Sanctisb, Randa Ishakb, Sara Dolcia, Luca Labellaa*, Marco Lezzerinic, Fabio Marchettia a
Dipartimento di Chimica e Chimica Industriale, via G. Moruzzi 13, Pisa, 56124-Italy Dipartimento di Ingegneria Civile e Industriale, Largo Lucio Lazzarino 2, Pisa 56126-Italy c Dipartimento di Scienze della Terra, via S. Maria 53, Pisa 56126-Italy b
ABSTRACT Nanostructured ceria has been prepared in mild conditions by hydrolysis of the N,Ndibutylcarbamato complex of cerium(III), [Ce(O2CNBu2)3], obtained by lanthanide extraction from aqueous solution into heptane containing CO2 saturated NHBu2. In dependence of the hydrolysis conditions, cerium(III) carbonates or hydroxocarbonate (characterized by IR, XRD and metal content) were obtained. Thermal treatment of these products at relatively low temperature (200 °C) yielded CeO2 in the form of a light yellow powder. The structure and morphology of the samples were studied by TEM, FESEM and XRD studies. The medium crystallite dimensions, estimated by TEM, appear to be of a few nanometers. Ceria with smaller medium crystallite dimensions was obtained by increasing the amine concentration in the course of the hydrolysis process. Keywords: ceria; carbamato complex; hydrolysis; nanoparticles; cerium carbonate 1.
Introduction
Over the last 20 years, there has been an increasing interest towards finely divided ceramics [1]. Among them, materials based on cerium dioxide has turned out to be particularly versatile. Cerium dioxide has the fluorite crystal structure where each cerium is coordinated by eight oxygen atoms. Defects due to oxygen vacancies produce cerium oxides of variable composition, CeO 2-x, usually called ceria. Ceria is an important material in several fields of chemistry and technology [2], especially in the area of catalysis [3], where it is used either as a catalyst or as an active catalyst support. The presence of oxygen vacancies, which formally corresponds to partial reduction of cerium(IV) to cerium(III), is essential in many catalytic processes which involve oxygen and/or electron migration [4]. On the other hand, oxygen vacancies, which are principally localized on the crystal surface, are related to the preparation method of the material and to the particle size, two 1
aspects which are strictly interconnected [4c, 5]. For this reason many efforts have been devoted to 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
the preparation of nanostructured ceria showing high surface area and high concentration of oxygen vacancies, both excellent premises of catalytic efficiency. The most frequently encountered methods are usually based on solution routes and often require the thermal treatment of an intermediate product, for instance carbonate, hydroxycarbonate or hydroxide of cerium(III), in many cases obtained starting from aqueous solutions of cerium(III) salts. Surfactants or capping agents are used to control the particle growth [5, 6]. Typical precursors are cerium(III) or (IV) nitrates or cerium(IV) sulfates, which are hydrolyzed in water or methanol usually by addition of a base (NaOH, NH3, hexamethylenetetraamine, sodium acetate) [5, 6 and references therein]. The use of cerium(III) acetate or acetylacetonate is also reported [7]. An hydrolysis carried out in ethylenediamine (acting as solvent, base and capping agent) starting from Ce(NO3)3·6H2O [8] deserves to be mentioned. We have reckoned that easily hydrolysable molecular cerium products soluble in non-polar solvents could be suitable precursors to ceria. They allow to carry out hydrolysis with low concentration of water. A few examples are reported where derivatives of cerium(III) or (IV) are used as precursors by operating in a non-polar solvent and usually they concern non-hydrolytic processes. For instance CeO2-x has been synthesized at 325 °C in trioctylphosphinoxide as solvent starting from a mixture of cerium(III) chloride and cerium(IV) alkoxide [9]. Other examples concern the use of cerium(III) oleate which is thermally decomposed in solvents with high boiling points [10], or the thermolysis of cerium benzoylacetonate in oleic acid/oleylamine [11] or the reaction of Ce(NO3)3·6H2O with diphenyl ether in oleylamine at 90 °C followed by thermal treatment at 320 °C [12]. Our previous experience with metal N,N-dialkylcarbamato complexes [13], [M(O2CNR2)n], suggests that they can be excellent precursors for the preparation of oxides or carbonates. Hydrolysis of N,N-dialkylcarbamato metal complexes involves the release of carbon dioxide and amine with the formation of oxo-, hydroxo- or carbonate complexes in dependence of the metal center. As represented in Fig. 1, the water protons can attack the oxygen atom of a carbamato ligand inducing the formation of a M-OH group and the release of CO2 and amine. Then, the M-OH group can attack another carbamato ligand of a second complex, directly (Fig. 1, route A) or after the formal insertion of a carbon dioxide molecule (Fig. 1, route B). In the former case the final product will be a metal μ-oxocarbamate [14] whereas in the latter a metal carbonate-carbamate [15].
2
O
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
R C
O
M O
HOOCNR2
M(O2CNR2)n
CO2 + HNR2
N
A R
M
O M
M - CO2, - HNR2
OH
H
O
O
B CO2
H
M(O2CNR2)n M
O
OH
M
O
O
M
- CO2, - HNR2
Fig. 1. N,N-dialkylcarbamato metal complex hydrolysis
Just for their easy hydrolysis, formation of N,N-dialkylcarbamato metal complexes normally requires strictly anhydrous conditions. A well-established synthetic route to N,N-dialkylcarbamato metal complexes starts from the anhydrous metal halide to be reacted with the dialkylamine/carbon dioxide system [16]. Recently, a fast synthetic route to prepare lanthanide carbamato derivatives avoiding tedious dehydration procedures has been developed: neodymium(III), samarium(III), europium(III) and terbium(III) were conveniently extracted from aqueous solution of their salts into heptane as N,N-dibutylcarbamato complexes [17]. Indeed, a substantially quantitative transfer of metal ions from an aqueous solution into a hydrocarbon promptly occurs using an organic phase containing lipophilic dialkylamines under carbon dioxide. This simple and effective procedure is based on the NHR2/CO2 equilibrium established in hydrocarbon solvents when a secondary amine solution is saturated with CO2 (eq. 1). [NH2R2][O2CNR2] (solv) 2 NHR2(solv) + CO2(solv)
(1)
To apply successfully this preparative method, the metal center has to be labile in order to exchange quickly the metal-coordinated water. During the extraction, the N,N-dialkylcarbamato ligand formed in situ in the organic phase gets in contact with the aqueous phase containing the metal ions and replaces the metal-coordinated water forming metal carbamato complexes. As a part of a project aimed at exploiting the easily hydrolysable N,N-dialkylcarbamato metal complexes, [M(O2CNR2)n], to produce finely divided oxides or carbonates in mild conditions, we have tried the preparation of ceria, starting from [Ce(O2CNBu2)3]. We were also driven by the interest, related to a parallel project, in testing CeO2 doped with noble metals as catalyst in the hydrogen peroxide decomposition [18]. This paper reports the convenient preparation of N,N-dibutylcarbamato complex of cerium(III) by extraction and its successful use as precursor of nanostructured ceria.
3
2. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
2.1
Experimental Materials and methods
All reactions and manipulations were carried out under a controlled atmosphere (nitrogen or carbon dioxide), using standard Schlenk techniques, unless otherwise stated. The reaction vessels were oven dried at 130 °C prior to use, evacuated (10-2 mmHg) and then filled with nitrogen. Solvents were freshly distilled from the suitable drying agents under nitrogen, unless otherwise stated. Water was degassed prior to use. Commercial cerium chloride [CeCl3·7H2O (Carlo Erba)] was used without further purification. The Infrared (IR) spectra were measured at 25 °C, between 3000 and 600 cm−1, with a Spectrum One FT-IR Perkin Elmer Spectrometer, equipped with an ATR sampling accessory. The remaining amount of cerium in the aqueous solution after the extraction and the metal content of the products were determined by titration using EDTA (0.0100 M) as a standard solution with xylenol orange (sodium salt) as an indicator [19]. The metal content of the products was determined also by sample calcination at 850 °C. The carbon dioxide content of the metal carbamates was measured by gas-volumetric determination upon the sample decomposition with 20% sulphuric acid. The gas volumetric apparatus was substantially similar to that previously described by Cotton and Calderazzo [20]. C, H, N elemental analyses of the cerium N,N-dibutylcarbamato complex are not reported because the sensitivity of the compound to moisture prevented the acquisition of reliable results. The amount of the total volatile components was determined by a simultaneous thermal analyzer, which combines a sensitive balance for use in thermogravimetric analysis (TG), with a heat-flux differential scanning calorimetry (DSC) for simultaneous TG-DSC analysis. Open alumina crucibles and heating rate of 10°C/min, under 30 ml/min nitrogen gas flow, were used. The simultaneous application of TG and DSC were obtained on ~25 mg of sample, dried at room temperature, at 40° or at C 80°C as reported. The powder diffraction patterns (XRD) have been collected in the 2 ranges 4-100° for C1, 4-60° for C2 and 15-100° for C3 by steps of 0.02° using a Philips PW1050/25 Bragg-Brentano diffractometer. The measurements have been performed at room temperature using a Cu source powered by 40 kV and 20 mA and a graphite monochromator on the secondary beam (K, = 1.54184 Å). The diffracted beam has been measured using a scintillation counter integrating for 4 s/step. Field Emission Scanning Electron Microscopy (FESEM) was carried out by means of a Zeiss Merlin microscope (equipped with a GEMINI II column and an EDS detector) in order to study the morphology of the crystal aggregates of the materials. 4
The morphology and dimension of oxide particles have been observed using a Philips CM12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
transmission electron microscope (TEM) operating with an acceleration voltage of 120 kV. Oxide particles were suspended in ethanol which was ultrasonically stirred in order to improve particle separation. Samples were collected from the suspension using a carbon coated copper mesh. XPS experiments were run on a Perkin–Elmer 5600-ci spectrometer using non-monochromatized Al K radiation (1486.6 eV). The sample analysis area was 800 μm in diameter, and the working pressure was lower than 10-9 mbar. The spectrometer was calibrated by assuming the binding energy (BE) of the Au 4f7/2 line at 83.9 eV with respect to the Fermi level. The standard deviation for the BEs values was ± 0.2 eV. Survey scans were obtained in the 0–1300 eV range. Detailed scans were recorded for the Ce3d region. The sample was mounted on steel holders and introduced directly in the fast-entry lock system of the XPS analytical chamber.
2.2
Preparation of N,N-dibutylcarbamato complex of cerium(III) [Ce(O2CNBu2)3] n
A solution of dibutylamine (66 mmol) in heptane (50 mL) was saturated with CO 2 and cooled at 0°C. In the meantime, 20 mL of a 0.33 M aqueous solution of CeCl3 (6.6 mmol) was prepared under nitrogen atmosphere, using deoxygenated water. By operating under carbon dioxide and at about 0 °C, the solution of [NH2But2][O2CNBu2] in heptane was placed in a 250 mL separatory funnel and then the aqueous solution of CeCl3 was added. The two immiscible solutions were shaken vigorously together for a few seconds, the aqueous layer was rapidly removed and the heptane solution was dried in vacuo. The resulting light yellow oil was purified from the excess of amine by stripping, then the oily residue was dissolved in few milliliters of anhydrous heptane and dried in vacuo (1.0 ×10−3 Torr). The procedure was repeated three times. A colorless solid was obtained (2.75 g, 63.5 % yield). The product is soluble in hydrocarbons and sensitive to moisture. Anal. Calcd for C27H54N3CeO6: CO2, 20.1 %; Ce, 21.3 %. Found: CO2, 20.4 %; Ce: 21.3 %. IR ATR (cm1): 2957 m, 2931 m, 2872 w, 1520 s, 1482 s, 1423 s, 1375 m, 1314 s, 1262 s, 1200 w, 1111 w, 1082 w, 1008 w, 944 w, 902 w, 857 w, 802 m, 733 w, 658 m. 2.3
Preparation of cerium carbonato- or hydroxo-carbonato derivatives
A solution of water in THF (66 mL, volume ratio H2O/THF = 1/5) was dropped in 30 minutes into a solution of cerium carbamate, Ce(O2CNBu2)3, (7.8 g, 12 mmol; H2O/Ce molar ratio ~ 50) in toluene (100 mL) under vigorous stirring. A suspension of a finely divided white solid was obtained. The slurry was stirred for about 3 hours and then let stand for a night. After decantation, the liquid phase was removed and the waxy residue was washed for three times with a mixture of THF/toluene (volume ratio 1:2; 75 mL). Finally, the solid was dried in vacuo (1.0 ×10−3 Torr). Different cerium 5
carbonato- or hydroxo-carbonato derivatives were obtained in dependence of the conditions. When 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
the hydrolysis took place at room temperature, the product analyzed as Ce2(CO3)3·6.5 H2O (sample C1), if the following work-up was carried out at room temperature or as Ce2(CO3)3·4 H2O (sample C2) if it was carried out at 40 °C. When both the controlled hydrolysis and the work-up took place at 70 °C, the product analyzed as Ce(OH)CO3 (sample C3). C1: colorless solid (3.1 g, 86.9 % yield). Anal. Calcd for Ce2(CO3)3·7 H2O: Ce, 47.8 %. Calcd for Ce2(CO3)3·6 H2O: Ce, 49.3 %. Found: Ce, 48.4 %. IR ATR (cm1): 3152 w, br, 2962 w, 1634 sh, 1472 s, 1368 s,1338 s, 1260 m, 1077 w, 1017 w, 874 w, 848 m, 796 m, 747 m, 677 m. C2: colorless solid (1.80 g, 56.5 % yield). Anal. Calcd for Ce2(CO3)3·4 H2O: Ce, 52.6 %. Found: Ce, 51,8 %. IR ATR (cm1): 3208 br, w, 2962 w, 2793 w, 1635 sh, 1456 s, 1360 s, 1078 w, 940 w, 871 w, 846 m, 747 m, 677m. C3: colorless solid (1.94 g, 74.5 % yield). Anal. Calcd for Ce(OH)CO3: Ce, 64.5 %. Found: Ce, 67.1 %. IR ATR (cm1): 3460 w, 1487 s, 1416 s, 1335 sh, 1075 w, 857 m, 804 m, 724 m, 696 m. 2.4
Preparation of cerium carbonate with addition of NHBu2
Ce(O2CNBu2)3 (about 12 mmol) was prepared by extraction following the procedure reported in [17] but without the final purification from the excess of amine by stripping. The light yellow oily residue, obtained by drying in vacuo the heptane extract, was dissolved in heptane (70 mL) and anhydrous dibutylamine was added (10 mL, 59.3 mmol). A solution of water in THF (51 mL, H2O/THF volume ratio = 1/3.63, H2O/Ce molar ratio ~ 50) was dropped in 30 minutes into the heptane solution at room temperature. The resulting gelatinous light orange suspension was stirred for about 3 h at room temperature. During the stirring the viscosity of the suspension increased up to obtain a yellow-brown paste. The paste was dried at reduced pressure (1.0 ×10−3 Torr) and accurately washed with a mixture of THF/toluene (volume ratio 1:2; 75 mL). The procedure was repeated twice. Finally, the solid was dried for 7 h in vacuo and a yellow powder was obtained. C4: yellow solid (2.29 g, 65.2 % yield). Anal. Calcd for Ce2(CO3)3·6.5 H2O: 48.5. Found: Ce, 47.8 %. IR ATR (cm1): 3260 w, br, 2961 w, 2931 w, 2870 w, 1634 w, sh, 1471 s, 1372 s, 1326 s, sh, 1259 m, 1077 w, 1011 w, 842 m, 789 m, 733 m, 679 m.
6
2.5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
Preparation of cerium dioxide
All the carbonato derivatives (C1- C4) were heated at 200 °C for 8 hours in air producing ceria. The weigh loss was checked and corresponded to the expected value. The samples were labelled O1-O3 (light yellow) and O4 (yellow), respectively.
3.
Results and discussion Cerium(III) extraction from aqueous solution into heptane via the NHBu2/CO2 system has been
carried out as sketched in Figure 2. The recovery of the metal complex from the organic phase has been performed under nitrogen atmosphere to avoid the possible oxidation of cerium(III) to cerium(IV) in air and the obtainment of a mixture of compounds. Indeed, it is known that the tetranuclear N,N-di-iso-propylcarbamato complex of cerium(III), [Ce4(O2CNiPr2)12] reacts with dioxygen yielding the tetranuclear 3-oxo-cerium(IV) derivative [Ce4(3-O)2(O2CNiPr2)12] [21]. Nevertheless, a gas-volumetric experiment carried out on our compound under O2 atmosphere did not show any oxygen absorption. Moreover, XPS analysis revealed that cerium is present as Ce(III) only, as inferred by the absence of the satellite at high energy (≈ 918 eV). The colorless [Ce(O2CNBu2)3] showed an IR spectrum essentially superimposable with the ones of the known neodymium, europium and terbium analogous complexes [17]. Furthermore, the metal and CO2 contents are in excellent agreement with the calculated ones.
[Ce(O2CNBu2)3]
[NH2Bu2][O2CNBu2]
CO2 + 2 NHBu2
Organic phase Aqueous phase
3 [NH2Bu2][O2CNBu2] + CeCl3
3 [NH2Bu2]Cl + [Ce(O2CNBu2)3]
Fig. 2. Scheme of cerium (III) extraction from the aqueous phase by the NHBu 2/CO2 system
The hydrolysis of [Ce(O2CNBu2)3] has been carried out by adding dropwise a solution of water in THF into a metal complex solution in toluene. A cerium carbonate with the composition Ce2(CO3)3·6.5 H2O (sample C1) was obtained working at room temperature. A carbonate with only four water molecules of hydration Ce2(CO3)3·4 H2O (sample C2) was obtained carrying out the work-up at 40°C. Finally, an hydroxycarbonate of formula Ce(OH)CO3 (sample C3) was obtained when both the hydrolysis and the work-up were conducted at 70°C.
7
The XRD pattern of C1, shown in Fig. 3, suggests the presence of a single crystalline phase. A 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
search on the JCPDS data base [22] showed that it corresponds to the pattern of the mineral lanthanite-(Ce), [23] (LaCe)(CO3)3 ∙ 8 H2O, JCPDS 1-083-1211. The cell parameters obtained by refining the diffraction pattern with the Rietveld method were a = 9.5373(4), b = 16.9719(8) c = 8.9506(4) Å, with a cell volume of 1448.7 Å3, only 9 Å3 greater than that reported for lanthanite-(Ce). The lower water content found in the analysis should be attributed to the presence in the sample of an amorphous fraction of an anhydrous cerium carbonate. To test this hypothesis a XRD pattern of the sample added with 16.2% by weight of corundum powder was recorded. Refining the phase composition of this new sample with the Rietveld method a content of 18.2 % by weight was obtained. This result suggests an amorphous content of 11 % in the original sample, and may explain the difference in the mean water content. An analogous conclusion has been suggested by TG analysis (Table 1), where the measured mass loss (40.2 wt %) in the temperature range 20-1000°C corresponds to Ce2(CO3)3 ∙ 6 . 3 H2O. Both the elemental analysis and the XRD pattern (Fig. 4) suggest that sample C2 corresponds to the pure cerium term of the calkinsite-(Ce) mineral [24], (La,Ce)2(CO3)3 ∙ 4 H2O, JCPDS 006-0076, having the composition Ce2(CO3)3 ∙ 4 H2O. The mass loss in the temperature range 20-1000°C measured by TG analysis (Table 1) is in good agreement with the reported composition (35.8 vs. 35.3 wt %). It is presumable that in this case an amorphous phase is not present. The XRD pattern of sample C3, Fig. 5, besides broad bands due to the presence of a minor amount of the cubic phase CeO2, shows sharp reflections compatible with the mineral ancylite-(Ce) [25],(Ln)x(Ca,Sr)2x(CO3)2 (OH)x ∙ (2x) H2O, JCPDS 029-0384 [25b]. The crystal structure of this mineral shows the lanthanide and the alkali earth atoms disorderly distributed on the same site and the OH group and water molecule both occupying another common site. Our compound represents the limit composition with x = 2. Its formula, confirmed by the elemental analysis, may be written either Ce2(CO3)2O ∙ H2O or Ce(CO3)OH. The TG/DSC analysis (Table 1) on the sample shows only one endothermic peak between RT and 300 C with a mass loss to CeO2 of 20.6 wt %, a value very close to the theoretical one (20.7 wt %). Both Ce2(CO3)2O ∙ H2O or Ce(CO3)OH are reported to have almost the same diffraction patterns in the JCPDS database with the numbers 044-0617 and 0410013, respectively. We are inclined to adopt the second one taking into account the sharing of the same crystallographic site by OH and water groups and the results of the TG-DSC analysis. A Rietveld refinement of our pattern gave cell parameters a = 5.0149(2) b = 8.5613(4), c = 7.3344(4) Å with a unit cell volume of 315 Å3, the same reported for 041-0013.
8
Arbitrary units
0
10
20
30
40
50
60
2 (deg)
Arbitrary units
Fig. 3. XRD pattern of Ce-carbonate obtained by hydrolysis of the carbamate at RT (black). XRD pattern calculated from the single crystal data of lanthanite-(Ce) [23] (red).
0
10
20
30
40
50
60
2 (deg)
Fig. 4. XRD pattern of Ce-carbonate obtained by hydrolysis of the carbamate at 40 °C (black). XRD peaks of calkinsite-(Ce) [24] reported in JCPDS 006-0076.
Arbitrary units
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
0
10
20
30
40
50
60
2 (deg)
Fig. 5. XRD pattern of Ce-carbonate obtained by hydrolysis of the carbamate at 70 °C (black). XRD pattern calculated from the single crystal data of ancylite-(Ce) [25a] (red).
Table 1 TG-DSC analysis of the cerium carbonates C1-C3 Sample
Mass loss (%) per temperature range (°C) 20-120
20-200
200-500
500-1000
20-1000
C1
12.52
21.97
18.21
0.05
40.23
C2
6.52
14.04
21.25
0.56
35.85
C3
1.60
2.44
17.92
0.24
20.60
9
TG-DSC analysis has not revealed any substantial difference among the decarboxylation step profiles of carbonato derivatives obtained at different temperatures, as on the other hand reported in the literature [26]. Thus, we decided to carry out the hydrolysis at room temperature, moving our attention to vary a different parameter. In the hydrolysis herein described an endogenous production of dibutylamine occurs (Figure 1), the amine/Ce molar ratio being 3 at the end of the reaction. Finely divided ceramics are often prepared in the presence of surface capping agents and alkylamines are used with this role, sometimes in high concentration, also acting as solvent [7, 27]. We have then increased the amine/Ce molar ratio carrying out the hydrolysis step with exogenous addition of NHBu2. By operating in the same conditions as for C1, i. e. at room temperature and in about 30 minutes, a gelatinous light orange suspension was obtained whose workup produced a yellow powder, C4, with the same composition of C1. The thermal treatment of all samples C1-C4 at 200 °C for 8 hours produced ceria in the form of light yellow (O1-O3) or yellow (O4) powders. All the oxides were characterized by IR, XRD and metal content. Fig. 6 compares the XRD patterns of the O1-O4 samples. The broad reflections observed in the O1 pattern suggest a product with small crystallite size. The crystallite size calculated by the Sherrer formula results 46 Å. By heating this sample at 850 °C for 8 h, we observe a growth of crystal size as shown by the O1-850 pattern. The FWHM of the highest reflection at 2 28.56° decreases from 1.319° to 0.231° corresponding to a growth of crystal size up to an estimated value of 430 Å. The O2 and O3 patterns in Fig. 6 suggest small crystallite size in O2 and O3, but not as small as in O1, the FWHM of the main reflection being 1.269° for O1 and 1.026° for O2 and O3, corresponding to crystallite size 50 and 60 Å, respectively. The finest crystallite size, 41 Å, was observed in the sample O4 which shows the main reflection FWHM of 1.861°.
Arbitrary units
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
O1-850 O4 O3 O2 O1 20
25
30
35
40
45
50
55
60
2 (deg)
Fig. 6. XRD patterns of CeO2 obtained in different conditions.
10
The morphologies and dimensions of samples O1 and O4 are compared using FESEM and TEM 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
analyses. In Fig. 7 the FESEM images are reported: the upper micrographs of O1 show tabular aggregates (Fig. 7a) with dimensions ranging from 0.1 to 10 m. These particles are made by small grains, about 20 nm wide (Fig. 7b). The lower images of O4 show irregular shaped particles (Fig. 7c) with dimensions in the range 0.4 – 10 m. The particles exhibit multiple secondary microcracks on external surfaces, suggesting crushing from agglomerates of larger dimensions. At higher magnifications (Fig. 7d) their external surfaces appear very irregular, studded by microcavities. The clustering of the grains does not allow to estimate the real sizes, but the emerging tips suggest very fine dimensions. The TEM examination (Fig. 8) shows the typical appearance of dispersed particles of O4. Only peripheral regions were transparent to electrons and allowed examination of structural details. It can be observed the presence of crystals of nanometric dimensions, which can be clearly distinguished each other changing the local image contrast. Fig. 9a is a higher magnification image of clustered nanoparticles, which appear below 5 nm in dimension. Nanocrystals appear embedded in a matrix exhibiting a faint image contrast. Fig. 9b is the ring diffraction pattern arising from clustered nanoparticles, which has been congruently indexed as arising from randomly oriented cerium oxide microcrystals, cubic in structure with a = 0.514 nm. Rings numbered in the figure are indexed in Table 2. A TEM study on samples of O1 reveals the presence of more massive particles with a shape similar to that of Fig. 7a. The rare regions which are transparent to electrons show a superposition of tabular forms generating a ring diffraction pattern.
11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
Fig. 7. Electron micrographs (FESEM) from experimental powders: a) low and b) high magnification images of O1; c) low and d) high magnification images of O4. Examples of secondary cracking of particles of O4 are arrowed.
Fig. 8. Electron micrograph of O4 (TEM, 200K) showing constitution of particle peripheral regions transparent to electrons.
12
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
Fig. 9. TEM study of O4: a) BF high magnification (340 K) image and b) ring diffraction patterns arising from randomly oriented microcrystals. Rings numbered in the figure are indexed in Table 2.
Table 2 Indexing of the ring diffraction pattern of O4 shown in Fig. 9b, congruent with the formation of CeO2 oxide particles, cubic with a = 0.541 nm. Ring
d (CeO2) theoretical nm
d measured nm
Relative Intensity
hkl
1
0. 312
0.31
100
111
2
0.270
0.271
27
200
3
0.191
0.191
45
220
4
0.163
0.162
33
311
As mentioned in the Introduction, many synthetic routes are present in the literature for nanocrystalline ceria [5-12]. Among the preparations based on liquid-phase, syntheses concerning hydrolysis of cerium(III) or cerium(IV) salts are widespread. The size of the resulting ceria nanoparticles typically ranges from 1–2 to 50 nm. The particle grow dinamics can be influenced by slight modifications of the synthesis conditions, like the presence of a base, the molar ratio of reactants, the cerium salt concentration, the temperature, the reaction time, the presence of capping agents. Relevant to our results, the hydrolysis of cerium(III) nitrate in the presence of urea at 85°С has been reported [6d] to yield Ce2O(CO3)2·H2O as the only product whose subsequent thermal treatment afforded nanocrystalline ceria with crystallite size within 6–40 nm in dependence of the calcining temperature (300–900°С). In our case, the use of [Ce(O2CNBu2)3]n as precursor has allowed to carry out the hydrolyses in a non-polar solvent and in mild conditions. The study of the hydrolysis products reveals that at room temperature the formation of the hydrated sesqui-carbonate, Ce2(CO3)3·nH2O, is reproduced (C1, C2, C4). On the other hand, Ce(CO3)OH (C3) is formed at 70 °C, in agreement with the result 13
obtained [6d] by conducting the hydrolysis of cerium(III) nitrate at 85 °C in the presence of urea as 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
source of CO2. Furthermore, ceria was obtained by decarboxylation of the samples C1-C4 at a relatively low temperature (200 °C). XRD data have revealed that, although the crystallite dimensions slightly decrease in the order O2~O3>O1>O4, they are all similar, and only slightly smaller than the ones of the ceria crystallites obtained by thermal decomposition (300°C) of the cerium(III) basic carbonate reported in reference 6d. Focusing our attention on O1 and O4, their FESEM and TEM images show aggregates with different morphologies and dimensions, with an estimate of the single crystallite size of about 20 and 5 nm, respectively. These data strengthens the hypothesis that the amine concentration in the course of the hydrolysis can be used to tune the particle dimensions.
4.
Conclusions
We have prepared the N,N-dibutylcarbamato complex of cerium(III), [Ce(O2CNBu2)3], by exploiting a simple and rapid method which allows to start from aqueous solutions of cerium(III) salts. The hydrocarbon soluble complex has been used as a convenient precursor to ceria. Its prompt hydrolysis in non polar solvents at room temperature yields finely divided hydrated cerium(III) carbonate whose treatment at 200 °C in air affords nanostructured ceria. Good reproducibility of the formation of both cerium(III) carbonate and ceria in such a mild conditions has been observed. A higher NHBu2/Ce molar ratio in the course of the hydrolysis step causes a decrease of the crystallite size of the final ceria. The synthetic method here reported could be extended to the preparation of nanostructured doped ceria or mixed oxides starting from solutions of neutral complexes in non-polar solvents. Studies concerning this topic are in course in our laboratories.
Acknowledgments The present work has been co-founded by the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n°313271. The authors wish to acknowledge the Dr. Silvia Andreoli of the Department of Applied Sciences and Technology of the Polytechnic University of Torino (Italy) for the acquisition of FESEM images. Thanks are due to Prof. Natale Perchiazzi of the Department of Earth Sciences of the University of Pisa for useful discussions about the results of the Rietveld refinements. We are
14
grateful to Dr. Lidia Armelao (CNR IENI and INSTM, Department of Chemical Sciences, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
University of Padova) for the XPS measurements.
References [1] Nanostructured Materials and nanotechnology VII (37th Intern. Conf. Adv. Ceramics and Composites, Daytona Beach, FL, 2013) S. Mthur, F. Hernandez-Ramirez Editors in Ceram. Eng. Sci. Proc. 34 (2014). [2] (a) G. Jacobs, L. Williams, U. Graham, D. Sparks, B. H. Davis, J. Phys. Chem. B 107 (2003) 10398. (b) K. Sohlberg, S. T. Pantelides, S. J. Pennycook, J. Am. Chem. Soc. 123 (2001) 6609. (c) F. Goubin, X. Rocquefelte, M. H. Whangbo, Y. Montardi, R. Brec, S. Jobic, Chem. Mat. 16 (2004) 662. (d) D. G. Shchukin, R. A. Caruso, Chem. Mat. 16 (2004) 2287. [3] (a) A. Trovarelli, Catalysis Reviews - Science and Engineering 38 (1996), 439. (b) A. Trovarelli, C. de Leitenburg, M. Boaro, G. Dolcetti, Catalysis Today 50 (1999) 353. (c) W.D. Michalak, G.A. Somorjai, Top. Catal. 56 (2013) 1611. [4] (a) F. Esch, S. Fabris, L. Zhou, T. Montini, C. Africh, P. Fornasiero, G. Comelli, R. Rosei, Science, 309 (2005) 752. (b) Z. Wu, M. Li, J. Howe, H.M. Meyer III, S.H. Overbury, Langmuir 26 (2010) 16595. (c) Z.-A. Qiao, Z. Wu, S. Dai, ChemSusChem 6 (2013) 1821. (d) S. Gangopadhyay, D. D. Frolov, A. E. Masunov, S. Seal, J. Alloys Comp. 584 (2014) 199. [5] (a) D. Zang, X. Du, L. Shi, R. Gao, Dalton Trans. 41 (2012) 14455. (b) W. Feng, L.-D. Sun, Y.-W. Zhang, C.-H. Yan Coord. Chem. Rev. 254 (2010) 1038. (c) M. Melchionna, P. Fornasiero, Materials Today 17 (2014) 349. [6] (a) V.K. Ivanov, O.S. Polezhaeva, Y.D. Tret'Yakov, Russ. J. Gen. Chem., 80 (2010) 604. (b) V.K. Ivanov, A.E. Baranchikov, O. S. Polezhaeva, G. P. Kopitsa, Yu. D. Tret’yakov, Russ. J. Inorg. Chem., 55 (2010) 325. (c) M.L. Dos Santos, R.C. Lima, C.S. Riccardi, R.L. Tranquilin, P.R. Bueno, J.A. Varela, E. Longo, Materials Letters 62 (2008) 4509. (d) M. Hirano, E. Kato, J. Mater. Sci. Lett. 18 (1999) 403. (e) F. Hrizi,H. Dhaouadin, F. Touati, Ceram. Int. 40 (2014) 25. [7] I. V. Zagaynov, S. V. Kutsev, Appl. Nanosci. 4 (2014) 339. [8] S. Kar, C. Patel, S. Santra, J. Phys. Chem. C 113 (2009) 4862. [9] S. W. Depner, K. R. Kort, C. Jaye, D. A. Fischer, S. Banerjee, J. Phys. Chem. C, 113 (2009) 14126. [10] H. Gu, M. D. Soucek, Chem. Mater. 19 (2007) 1103. [11] R. Si, Y.-W. Zhang, L.-P. You, C.-H. Yan, Angew. Chem. Int. Ed., 44 (2005) 3256. [12] T. Yu, J. Joo, Y. Il Park, T. Hyeon, Angew. Chem. Int. Ed. 44 (2005) 7411. [13] D. Belli Dell’Amico, F. Calderazzo, L. Labella, F. Marchetti, G. Pampaloni, Inorg. Chem. Commun. 5 (2002) 848. [14] (a) D. Belli Dell'Amico, C. Bradicich, F. Calderazzo, A. Guarini, L. Labella, F. Marchetti, A. Tomei, Inorg. Chem. 41 (2002) 2814. (b) D. Belli Dell’Amico, F. Calderazzo, L. Costa, E, Franchi, L. Gini, L. Labella, F. Marchetti, J. of Molec. Struct. 890 (2008) 295. (c) D. Belli Dell’Amico, D. Boschi, F. Calderazzo, S. Ianelli, L. Labella, F. Marchetti, G. Pelizzi, E. G. F. Quadrelli, Inorg. Chim. Acta, 300-302 (2000) 882. (d) U. Abram, D. Belli Dell'Amico, F. Calderazzo, S. Kaskel, L. Labella, F. Marchetti, R. Rovai, J. Strähle, Chem. Commun., (1997) 1941. [15] (a) U. Abram, D. Belli Dell’Amico, F. Calderazzo, L. Marchetti. J. Strähle, J. Chem. Soc., Dalton Trans., (1999) 4093. (b) D. Belli Dell’Amico, F. Calderazzo, L. Labella, F. Marchetti, J. Organomet. Chem. 596 (2000) 144. (c) D. Belli Dell’Amico, F. Calderazzo, L. Labella, F. Marchetti, M. Martini, I. Mazzoncini, Comptes Rendus Chimie, 7 (2004) 877. [16] (a) D. Belli Dell' Amico, F. Calderazzo, L. Labella, F. Marchetti, G. Pampaloni, Chem. Rev. 103 (2003) 3857. (b) U. Baisch, D. Belli Dell’Amico, F. Calderazzo, L. Labella, F. Marchetti, A. Merigo, Eur. J. Inorg. Chem. (2004) 1219. [17] L. Armelao, D. Belli Dell’Amico, P. Biagini, G. Bottaro, S. Chiaberge, P. Falvo, L. Labella, F. Marchetti, S. Samaritani, Inorg. Chem. 53 (2014) 4861 [18] S. Dolci, D. Belli Dell’Amico, A. Pasini, L. Torre, G. Pace, D. Valentini, J. Propuls. Power (2015) accepted for publication. [19] L. Yu, D. Chen, J. Li, P. G. Yang, J. Org. Chem. , 62 (1997) 3575. [20] F. Calderazzo, F. A. Cotton, Inorg. Chem. 1 (1962) 30. [21] U. Baisch, D. Belli Dell’Amico, F. Calderazzo, L. Labella, F. Marchetti, D. Vitali, J. Mol. Cat. A 204–205 (2003) 259. [22] J. Faber, T. Fawcett, Acta Cryst. B58 (2002) 325. [23] A. dal Negro, G. Rossi, V. Tazzoli, Am. Mineral. 62 (1977) 142. [24] W. T. Pecora, J. H. Kerr, Am. Mineral. 38 (1953) 1169. [25] (a) A. dal Negro, G. Rossi, V. Tazzoli, Am. Mineral. 60 (1975) 280. (b) A. Sabina, R. Traill, Geol. Surv. Canada, 60 (1960) 4. [26] H. Wakita, S. Kinoshita, J. Chem. Soc. Japan 52 (1979) 428. [27] (a) J. S. Elias, M. Risch, L. Giordano, A. N. Mansour, Y. Shao-Horn, J. Am. Chem. Soc. 136 (2014) 17193. (b) I S. Kar, C. Patel, S. Santra, J. Phys. Chem. C 113 (2009) 4862.
15
Cerium(III) N,N-dibutylcarbamate as precursor to nanocrystalline cerium dioxide
Pages 000-000 Daniela Belli Dell’Amico, Massimo De Sanctis,Randa Ishak, Sara Dolci, Luca Labella, Marco Lezzerini, Fabio Marchetti
Graphical abstract Nanostructured ceria has been prepared in mild conditions by hydrolysis of the N,Ndibutylcarbamato complex of cerium(III), [Ce(O2CNBu2)3], obtained by lanthanide extraction from aqueous solution into heptane containing CO2 saturated NHBu2. The structure and morphology of the samples were studied by TEM, FESEM and XRD studies.
S0277-5387(15)00353-8 http://dx.doi.org/10.1016/j.poly.2015.06.037 POLY 11384
To appear in:
Polyhedron
Received Date: Accepted Date:
24 April 2015 30 June 2015
Please cite this article as: D.B. Dell’Amico, M. De Sanctis, R. Ishak, S. Dolci, L. Labella, M. Lezzerini, F. Marchetti, Cerium(III) N,N-dibutylcarbamate as precursor to nanocrystalline cerium dioxide, Polyhedron (2015), doi: http:// dx.doi.org/10.1016/j.poly.2015.06.037
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Labella Luca
+390502219206
Via Moruzzi 13, Pisa 56124-Italy [email protected]
Cerium(III) N,N-dibutylcarbamate as precursor to nanocrystalline cerium dioxide Daniela Belli Dell’Amicoa, Massimo De Sanctisb, Randa Ishakb, Sara Dolcia, Luca Labellaa*, Marco Lezzerinic, Fabio Marchettia a
Dipartimento di Chimica e Chimica Industriale, via G. Moruzzi 13, Pisa, 56124-Italy Dipartimento di Ingegneria Civile e Industriale, Largo Lucio Lazzarino 2, Pisa 56126-Italy c Dipartimento di Scienze della Terra, via S. Maria 53, Pisa 56126-Italy b
ABSTRACT Nanostructured ceria has been prepared in mild conditions by hydrolysis of the N,Ndibutylcarbamato complex of cerium(III), [Ce(O2CNBu2)3], obtained by lanthanide extraction from aqueous solution into heptane containing CO2 saturated NHBu2. In dependence of the hydrolysis conditions, cerium(III) carbonates or hydroxocarbonate (characterized by IR, XRD and metal content) were obtained. Thermal treatment of these products at relatively low temperature (200 °C) yielded CeO2 in the form of a light yellow powder. The structure and morphology of the samples were studied by TEM, FESEM and XRD studies. The medium crystallite dimensions, estimated by TEM, appear to be of a few nanometers. Ceria with smaller medium crystallite dimensions was obtained by increasing the amine concentration in the course of the hydrolysis process. Keywords: ceria; carbamato complex; hydrolysis; nanoparticles; cerium carbonate 1.
Introduction
Over the last 20 years, there has been an increasing interest towards finely divided ceramics [1]. Among them, materials based on cerium dioxide has turned out to be particularly versatile. Cerium dioxide has the fluorite crystal structure where each cerium is coordinated by eight oxygen atoms. Defects due to oxygen vacancies produce cerium oxides of variable composition, CeO 2-x, usually called ceria. Ceria is an important material in several fields of chemistry and technology [2], especially in the area of catalysis [3], where it is used either as a catalyst or as an active catalyst support. The presence of oxygen vacancies, which formally corresponds to partial reduction of cerium(IV) to cerium(III), is essential in many catalytic processes which involve oxygen and/or electron migration [4]. On the other hand, oxygen vacancies, which are principally localized on the crystal surface, are related to the preparation method of the material and to the particle size, two 1
aspects which are strictly interconnected [4c, 5]. For this reason many efforts have been devoted to 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
the preparation of nanostructured ceria showing high surface area and high concentration of oxygen vacancies, both excellent premises of catalytic efficiency. The most frequently encountered methods are usually based on solution routes and often require the thermal treatment of an intermediate product, for instance carbonate, hydroxycarbonate or hydroxide of cerium(III), in many cases obtained starting from aqueous solutions of cerium(III) salts. Surfactants or capping agents are used to control the particle growth [5, 6]. Typical precursors are cerium(III) or (IV) nitrates or cerium(IV) sulfates, which are hydrolyzed in water or methanol usually by addition of a base (NaOH, NH3, hexamethylenetetraamine, sodium acetate) [5, 6 and references therein]. The use of cerium(III) acetate or acetylacetonate is also reported [7]. An hydrolysis carried out in ethylenediamine (acting as solvent, base and capping agent) starting from Ce(NO3)3·6H2O [8] deserves to be mentioned. We have reckoned that easily hydrolysable molecular cerium products soluble in non-polar solvents could be suitable precursors to ceria. They allow to carry out hydrolysis with low concentration of water. A few examples are reported where derivatives of cerium(III) or (IV) are used as precursors by operating in a non-polar solvent and usually they concern non-hydrolytic processes. For instance CeO2-x has been synthesized at 325 °C in trioctylphosphinoxide as solvent starting from a mixture of cerium(III) chloride and cerium(IV) alkoxide [9]. Other examples concern the use of cerium(III) oleate which is thermally decomposed in solvents with high boiling points [10], or the thermolysis of cerium benzoylacetonate in oleic acid/oleylamine [11] or the reaction of Ce(NO3)3·6H2O with diphenyl ether in oleylamine at 90 °C followed by thermal treatment at 320 °C [12]. Our previous experience with metal N,N-dialkylcarbamato complexes [13], [M(O2CNR2)n], suggests that they can be excellent precursors for the preparation of oxides or carbonates. Hydrolysis of N,N-dialkylcarbamato metal complexes involves the release of carbon dioxide and amine with the formation of oxo-, hydroxo- or carbonate complexes in dependence of the metal center. As represented in Fig. 1, the water protons can attack the oxygen atom of a carbamato ligand inducing the formation of a M-OH group and the release of CO2 and amine. Then, the M-OH group can attack another carbamato ligand of a second complex, directly (Fig. 1, route A) or after the formal insertion of a carbon dioxide molecule (Fig. 1, route B). In the former case the final product will be a metal μ-oxocarbamate [14] whereas in the latter a metal carbonate-carbamate [15].
2
O
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
R C
O
M O
HOOCNR2
M(O2CNR2)n
CO2 + HNR2
N
A R
M
O M
M - CO2, - HNR2
OH
H
O
O
B CO2
H
M(O2CNR2)n M
O
OH
M
O
O
M
- CO2, - HNR2
Fig. 1. N,N-dialkylcarbamato metal complex hydrolysis
Just for their easy hydrolysis, formation of N,N-dialkylcarbamato metal complexes normally requires strictly anhydrous conditions. A well-established synthetic route to N,N-dialkylcarbamato metal complexes starts from the anhydrous metal halide to be reacted with the dialkylamine/carbon dioxide system [16]. Recently, a fast synthetic route to prepare lanthanide carbamato derivatives avoiding tedious dehydration procedures has been developed: neodymium(III), samarium(III), europium(III) and terbium(III) were conveniently extracted from aqueous solution of their salts into heptane as N,N-dibutylcarbamato complexes [17]. Indeed, a substantially quantitative transfer of metal ions from an aqueous solution into a hydrocarbon promptly occurs using an organic phase containing lipophilic dialkylamines under carbon dioxide. This simple and effective procedure is based on the NHR2/CO2 equilibrium established in hydrocarbon solvents when a secondary amine solution is saturated with CO2 (eq. 1). [NH2R2][O2CNR2] (solv) 2 NHR2(solv) + CO2(solv)
(1)
To apply successfully this preparative method, the metal center has to be labile in order to exchange quickly the metal-coordinated water. During the extraction, the N,N-dialkylcarbamato ligand formed in situ in the organic phase gets in contact with the aqueous phase containing the metal ions and replaces the metal-coordinated water forming metal carbamato complexes. As a part of a project aimed at exploiting the easily hydrolysable N,N-dialkylcarbamato metal complexes, [M(O2CNR2)n], to produce finely divided oxides or carbonates in mild conditions, we have tried the preparation of ceria, starting from [Ce(O2CNBu2)3]. We were also driven by the interest, related to a parallel project, in testing CeO2 doped with noble metals as catalyst in the hydrogen peroxide decomposition [18]. This paper reports the convenient preparation of N,N-dibutylcarbamato complex of cerium(III) by extraction and its successful use as precursor of nanostructured ceria.
3
2. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
2.1
Experimental Materials and methods
All reactions and manipulations were carried out under a controlled atmosphere (nitrogen or carbon dioxide), using standard Schlenk techniques, unless otherwise stated. The reaction vessels were oven dried at 130 °C prior to use, evacuated (10-2 mmHg) and then filled with nitrogen. Solvents were freshly distilled from the suitable drying agents under nitrogen, unless otherwise stated. Water was degassed prior to use. Commercial cerium chloride [CeCl3·7H2O (Carlo Erba)] was used without further purification. The Infrared (IR) spectra were measured at 25 °C, between 3000 and 600 cm−1, with a Spectrum One FT-IR Perkin Elmer Spectrometer, equipped with an ATR sampling accessory. The remaining amount of cerium in the aqueous solution after the extraction and the metal content of the products were determined by titration using EDTA (0.0100 M) as a standard solution with xylenol orange (sodium salt) as an indicator [19]. The metal content of the products was determined also by sample calcination at 850 °C. The carbon dioxide content of the metal carbamates was measured by gas-volumetric determination upon the sample decomposition with 20% sulphuric acid. The gas volumetric apparatus was substantially similar to that previously described by Cotton and Calderazzo [20]. C, H, N elemental analyses of the cerium N,N-dibutylcarbamato complex are not reported because the sensitivity of the compound to moisture prevented the acquisition of reliable results. The amount of the total volatile components was determined by a simultaneous thermal analyzer, which combines a sensitive balance for use in thermogravimetric analysis (TG), with a heat-flux differential scanning calorimetry (DSC) for simultaneous TG-DSC analysis. Open alumina crucibles and heating rate of 10°C/min, under 30 ml/min nitrogen gas flow, were used. The simultaneous application of TG and DSC were obtained on ~25 mg of sample, dried at room temperature, at 40° or at C 80°C as reported. The powder diffraction patterns (XRD) have been collected in the 2 ranges 4-100° for C1, 4-60° for C2 and 15-100° for C3 by steps of 0.02° using a Philips PW1050/25 Bragg-Brentano diffractometer. The measurements have been performed at room temperature using a Cu source powered by 40 kV and 20 mA and a graphite monochromator on the secondary beam (K, = 1.54184 Å). The diffracted beam has been measured using a scintillation counter integrating for 4 s/step. Field Emission Scanning Electron Microscopy (FESEM) was carried out by means of a Zeiss Merlin microscope (equipped with a GEMINI II column and an EDS detector) in order to study the morphology of the crystal aggregates of the materials. 4
The morphology and dimension of oxide particles have been observed using a Philips CM12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
transmission electron microscope (TEM) operating with an acceleration voltage of 120 kV. Oxide particles were suspended in ethanol which was ultrasonically stirred in order to improve particle separation. Samples were collected from the suspension using a carbon coated copper mesh. XPS experiments were run on a Perkin–Elmer 5600-ci spectrometer using non-monochromatized Al K radiation (1486.6 eV). The sample analysis area was 800 μm in diameter, and the working pressure was lower than 10-9 mbar. The spectrometer was calibrated by assuming the binding energy (BE) of the Au 4f7/2 line at 83.9 eV with respect to the Fermi level. The standard deviation for the BEs values was ± 0.2 eV. Survey scans were obtained in the 0–1300 eV range. Detailed scans were recorded for the Ce3d region. The sample was mounted on steel holders and introduced directly in the fast-entry lock system of the XPS analytical chamber.
2.2
Preparation of N,N-dibutylcarbamato complex of cerium(III) [Ce(O2CNBu2)3] n
A solution of dibutylamine (66 mmol) in heptane (50 mL) was saturated with CO 2 and cooled at 0°C. In the meantime, 20 mL of a 0.33 M aqueous solution of CeCl3 (6.6 mmol) was prepared under nitrogen atmosphere, using deoxygenated water. By operating under carbon dioxide and at about 0 °C, the solution of [NH2But2][O2CNBu2] in heptane was placed in a 250 mL separatory funnel and then the aqueous solution of CeCl3 was added. The two immiscible solutions were shaken vigorously together for a few seconds, the aqueous layer was rapidly removed and the heptane solution was dried in vacuo. The resulting light yellow oil was purified from the excess of amine by stripping, then the oily residue was dissolved in few milliliters of anhydrous heptane and dried in vacuo (1.0 ×10−3 Torr). The procedure was repeated three times. A colorless solid was obtained (2.75 g, 63.5 % yield). The product is soluble in hydrocarbons and sensitive to moisture. Anal. Calcd for C27H54N3CeO6: CO2, 20.1 %; Ce, 21.3 %. Found: CO2, 20.4 %; Ce: 21.3 %. IR ATR (cm1): 2957 m, 2931 m, 2872 w, 1520 s, 1482 s, 1423 s, 1375 m, 1314 s, 1262 s, 1200 w, 1111 w, 1082 w, 1008 w, 944 w, 902 w, 857 w, 802 m, 733 w, 658 m. 2.3
Preparation of cerium carbonato- or hydroxo-carbonato derivatives
A solution of water in THF (66 mL, volume ratio H2O/THF = 1/5) was dropped in 30 minutes into a solution of cerium carbamate, Ce(O2CNBu2)3, (7.8 g, 12 mmol; H2O/Ce molar ratio ~ 50) in toluene (100 mL) under vigorous stirring. A suspension of a finely divided white solid was obtained. The slurry was stirred for about 3 hours and then let stand for a night. After decantation, the liquid phase was removed and the waxy residue was washed for three times with a mixture of THF/toluene (volume ratio 1:2; 75 mL). Finally, the solid was dried in vacuo (1.0 ×10−3 Torr). Different cerium 5
carbonato- or hydroxo-carbonato derivatives were obtained in dependence of the conditions. When 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
the hydrolysis took place at room temperature, the product analyzed as Ce2(CO3)3·6.5 H2O (sample C1), if the following work-up was carried out at room temperature or as Ce2(CO3)3·4 H2O (sample C2) if it was carried out at 40 °C. When both the controlled hydrolysis and the work-up took place at 70 °C, the product analyzed as Ce(OH)CO3 (sample C3). C1: colorless solid (3.1 g, 86.9 % yield). Anal. Calcd for Ce2(CO3)3·7 H2O: Ce, 47.8 %. Calcd for Ce2(CO3)3·6 H2O: Ce, 49.3 %. Found: Ce, 48.4 %. IR ATR (cm1): 3152 w, br, 2962 w, 1634 sh, 1472 s, 1368 s,1338 s, 1260 m, 1077 w, 1017 w, 874 w, 848 m, 796 m, 747 m, 677 m. C2: colorless solid (1.80 g, 56.5 % yield). Anal. Calcd for Ce2(CO3)3·4 H2O: Ce, 52.6 %. Found: Ce, 51,8 %. IR ATR (cm1): 3208 br, w, 2962 w, 2793 w, 1635 sh, 1456 s, 1360 s, 1078 w, 940 w, 871 w, 846 m, 747 m, 677m. C3: colorless solid (1.94 g, 74.5 % yield). Anal. Calcd for Ce(OH)CO3: Ce, 64.5 %. Found: Ce, 67.1 %. IR ATR (cm1): 3460 w, 1487 s, 1416 s, 1335 sh, 1075 w, 857 m, 804 m, 724 m, 696 m. 2.4
Preparation of cerium carbonate with addition of NHBu2
Ce(O2CNBu2)3 (about 12 mmol) was prepared by extraction following the procedure reported in [17] but without the final purification from the excess of amine by stripping. The light yellow oily residue, obtained by drying in vacuo the heptane extract, was dissolved in heptane (70 mL) and anhydrous dibutylamine was added (10 mL, 59.3 mmol). A solution of water in THF (51 mL, H2O/THF volume ratio = 1/3.63, H2O/Ce molar ratio ~ 50) was dropped in 30 minutes into the heptane solution at room temperature. The resulting gelatinous light orange suspension was stirred for about 3 h at room temperature. During the stirring the viscosity of the suspension increased up to obtain a yellow-brown paste. The paste was dried at reduced pressure (1.0 ×10−3 Torr) and accurately washed with a mixture of THF/toluene (volume ratio 1:2; 75 mL). The procedure was repeated twice. Finally, the solid was dried for 7 h in vacuo and a yellow powder was obtained. C4: yellow solid (2.29 g, 65.2 % yield). Anal. Calcd for Ce2(CO3)3·6.5 H2O: 48.5. Found: Ce, 47.8 %. IR ATR (cm1): 3260 w, br, 2961 w, 2931 w, 2870 w, 1634 w, sh, 1471 s, 1372 s, 1326 s, sh, 1259 m, 1077 w, 1011 w, 842 m, 789 m, 733 m, 679 m.
6
2.5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
Preparation of cerium dioxide
All the carbonato derivatives (C1- C4) were heated at 200 °C for 8 hours in air producing ceria. The weigh loss was checked and corresponded to the expected value. The samples were labelled O1-O3 (light yellow) and O4 (yellow), respectively.
3.
Results and discussion Cerium(III) extraction from aqueous solution into heptane via the NHBu2/CO2 system has been
carried out as sketched in Figure 2. The recovery of the metal complex from the organic phase has been performed under nitrogen atmosphere to avoid the possible oxidation of cerium(III) to cerium(IV) in air and the obtainment of a mixture of compounds. Indeed, it is known that the tetranuclear N,N-di-iso-propylcarbamato complex of cerium(III), [Ce4(O2CNiPr2)12] reacts with dioxygen yielding the tetranuclear 3-oxo-cerium(IV) derivative [Ce4(3-O)2(O2CNiPr2)12] [21]. Nevertheless, a gas-volumetric experiment carried out on our compound under O2 atmosphere did not show any oxygen absorption. Moreover, XPS analysis revealed that cerium is present as Ce(III) only, as inferred by the absence of the satellite at high energy (≈ 918 eV). The colorless [Ce(O2CNBu2)3] showed an IR spectrum essentially superimposable with the ones of the known neodymium, europium and terbium analogous complexes [17]. Furthermore, the metal and CO2 contents are in excellent agreement with the calculated ones.
[Ce(O2CNBu2)3]
[NH2Bu2][O2CNBu2]
CO2 + 2 NHBu2
Organic phase Aqueous phase
3 [NH2Bu2][O2CNBu2] + CeCl3
3 [NH2Bu2]Cl + [Ce(O2CNBu2)3]
Fig. 2. Scheme of cerium (III) extraction from the aqueous phase by the NHBu 2/CO2 system
The hydrolysis of [Ce(O2CNBu2)3] has been carried out by adding dropwise a solution of water in THF into a metal complex solution in toluene. A cerium carbonate with the composition Ce2(CO3)3·6.5 H2O (sample C1) was obtained working at room temperature. A carbonate with only four water molecules of hydration Ce2(CO3)3·4 H2O (sample C2) was obtained carrying out the work-up at 40°C. Finally, an hydroxycarbonate of formula Ce(OH)CO3 (sample C3) was obtained when both the hydrolysis and the work-up were conducted at 70°C.
7
The XRD pattern of C1, shown in Fig. 3, suggests the presence of a single crystalline phase. A 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
search on the JCPDS data base [22] showed that it corresponds to the pattern of the mineral lanthanite-(Ce), [23] (LaCe)(CO3)3 ∙ 8 H2O, JCPDS 1-083-1211. The cell parameters obtained by refining the diffraction pattern with the Rietveld method were a = 9.5373(4), b = 16.9719(8) c = 8.9506(4) Å, with a cell volume of 1448.7 Å3, only 9 Å3 greater than that reported for lanthanite-(Ce). The lower water content found in the analysis should be attributed to the presence in the sample of an amorphous fraction of an anhydrous cerium carbonate. To test this hypothesis a XRD pattern of the sample added with 16.2% by weight of corundum powder was recorded. Refining the phase composition of this new sample with the Rietveld method a content of 18.2 % by weight was obtained. This result suggests an amorphous content of 11 % in the original sample, and may explain the difference in the mean water content. An analogous conclusion has been suggested by TG analysis (Table 1), where the measured mass loss (40.2 wt %) in the temperature range 20-1000°C corresponds to Ce2(CO3)3 ∙ 6 . 3 H2O. Both the elemental analysis and the XRD pattern (Fig. 4) suggest that sample C2 corresponds to the pure cerium term of the calkinsite-(Ce) mineral [24], (La,Ce)2(CO3)3 ∙ 4 H2O, JCPDS 006-0076, having the composition Ce2(CO3)3 ∙ 4 H2O. The mass loss in the temperature range 20-1000°C measured by TG analysis (Table 1) is in good agreement with the reported composition (35.8 vs. 35.3 wt %). It is presumable that in this case an amorphous phase is not present. The XRD pattern of sample C3, Fig. 5, besides broad bands due to the presence of a minor amount of the cubic phase CeO2, shows sharp reflections compatible with the mineral ancylite-(Ce) [25],(Ln)x(Ca,Sr)2x(CO3)2 (OH)x ∙ (2x) H2O, JCPDS 029-0384 [25b]. The crystal structure of this mineral shows the lanthanide and the alkali earth atoms disorderly distributed on the same site and the OH group and water molecule both occupying another common site. Our compound represents the limit composition with x = 2. Its formula, confirmed by the elemental analysis, may be written either Ce2(CO3)2O ∙ H2O or Ce(CO3)OH. The TG/DSC analysis (Table 1) on the sample shows only one endothermic peak between RT and 300 C with a mass loss to CeO2 of 20.6 wt %, a value very close to the theoretical one (20.7 wt %). Both Ce2(CO3)2O ∙ H2O or Ce(CO3)OH are reported to have almost the same diffraction patterns in the JCPDS database with the numbers 044-0617 and 0410013, respectively. We are inclined to adopt the second one taking into account the sharing of the same crystallographic site by OH and water groups and the results of the TG-DSC analysis. A Rietveld refinement of our pattern gave cell parameters a = 5.0149(2) b = 8.5613(4), c = 7.3344(4) Å with a unit cell volume of 315 Å3, the same reported for 041-0013.
8
Arbitrary units
0
10
20
30
40
50
60
2 (deg)
Arbitrary units
Fig. 3. XRD pattern of Ce-carbonate obtained by hydrolysis of the carbamate at RT (black). XRD pattern calculated from the single crystal data of lanthanite-(Ce) [23] (red).
0
10
20
30
40
50
60
2 (deg)
Fig. 4. XRD pattern of Ce-carbonate obtained by hydrolysis of the carbamate at 40 °C (black). XRD peaks of calkinsite-(Ce) [24] reported in JCPDS 006-0076.
Arbitrary units
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
0
10
20
30
40
50
60
2 (deg)
Fig. 5. XRD pattern of Ce-carbonate obtained by hydrolysis of the carbamate at 70 °C (black). XRD pattern calculated from the single crystal data of ancylite-(Ce) [25a] (red).
Table 1 TG-DSC analysis of the cerium carbonates C1-C3 Sample
Mass loss (%) per temperature range (°C) 20-120
20-200
200-500
500-1000
20-1000
C1
12.52
21.97
18.21
0.05
40.23
C2
6.52
14.04
21.25
0.56
35.85
C3
1.60
2.44
17.92
0.24
20.60
9
TG-DSC analysis has not revealed any substantial difference among the decarboxylation step profiles of carbonato derivatives obtained at different temperatures, as on the other hand reported in the literature [26]. Thus, we decided to carry out the hydrolysis at room temperature, moving our attention to vary a different parameter. In the hydrolysis herein described an endogenous production of dibutylamine occurs (Figure 1), the amine/Ce molar ratio being 3 at the end of the reaction. Finely divided ceramics are often prepared in the presence of surface capping agents and alkylamines are used with this role, sometimes in high concentration, also acting as solvent [7, 27]. We have then increased the amine/Ce molar ratio carrying out the hydrolysis step with exogenous addition of NHBu2. By operating in the same conditions as for C1, i. e. at room temperature and in about 30 minutes, a gelatinous light orange suspension was obtained whose workup produced a yellow powder, C4, with the same composition of C1. The thermal treatment of all samples C1-C4 at 200 °C for 8 hours produced ceria in the form of light yellow (O1-O3) or yellow (O4) powders. All the oxides were characterized by IR, XRD and metal content. Fig. 6 compares the XRD patterns of the O1-O4 samples. The broad reflections observed in the O1 pattern suggest a product with small crystallite size. The crystallite size calculated by the Sherrer formula results 46 Å. By heating this sample at 850 °C for 8 h, we observe a growth of crystal size as shown by the O1-850 pattern. The FWHM of the highest reflection at 2 28.56° decreases from 1.319° to 0.231° corresponding to a growth of crystal size up to an estimated value of 430 Å. The O2 and O3 patterns in Fig. 6 suggest small crystallite size in O2 and O3, but not as small as in O1, the FWHM of the main reflection being 1.269° for O1 and 1.026° for O2 and O3, corresponding to crystallite size 50 and 60 Å, respectively. The finest crystallite size, 41 Å, was observed in the sample O4 which shows the main reflection FWHM of 1.861°.
Arbitrary units
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
O1-850 O4 O3 O2 O1 20
25
30
35
40
45
50
55
60
2 (deg)
Fig. 6. XRD patterns of CeO2 obtained in different conditions.
10
The morphologies and dimensions of samples O1 and O4 are compared using FESEM and TEM 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
analyses. In Fig. 7 the FESEM images are reported: the upper micrographs of O1 show tabular aggregates (Fig. 7a) with dimensions ranging from 0.1 to 10 m. These particles are made by small grains, about 20 nm wide (Fig. 7b). The lower images of O4 show irregular shaped particles (Fig. 7c) with dimensions in the range 0.4 – 10 m. The particles exhibit multiple secondary microcracks on external surfaces, suggesting crushing from agglomerates of larger dimensions. At higher magnifications (Fig. 7d) their external surfaces appear very irregular, studded by microcavities. The clustering of the grains does not allow to estimate the real sizes, but the emerging tips suggest very fine dimensions. The TEM examination (Fig. 8) shows the typical appearance of dispersed particles of O4. Only peripheral regions were transparent to electrons and allowed examination of structural details. It can be observed the presence of crystals of nanometric dimensions, which can be clearly distinguished each other changing the local image contrast. Fig. 9a is a higher magnification image of clustered nanoparticles, which appear below 5 nm in dimension. Nanocrystals appear embedded in a matrix exhibiting a faint image contrast. Fig. 9b is the ring diffraction pattern arising from clustered nanoparticles, which has been congruently indexed as arising from randomly oriented cerium oxide microcrystals, cubic in structure with a = 0.514 nm. Rings numbered in the figure are indexed in Table 2. A TEM study on samples of O1 reveals the presence of more massive particles with a shape similar to that of Fig. 7a. The rare regions which are transparent to electrons show a superposition of tabular forms generating a ring diffraction pattern.
11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
Fig. 7. Electron micrographs (FESEM) from experimental powders: a) low and b) high magnification images of O1; c) low and d) high magnification images of O4. Examples of secondary cracking of particles of O4 are arrowed.
Fig. 8. Electron micrograph of O4 (TEM, 200K) showing constitution of particle peripheral regions transparent to electrons.
12
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
Fig. 9. TEM study of O4: a) BF high magnification (340 K) image and b) ring diffraction patterns arising from randomly oriented microcrystals. Rings numbered in the figure are indexed in Table 2.
Table 2 Indexing of the ring diffraction pattern of O4 shown in Fig. 9b, congruent with the formation of CeO2 oxide particles, cubic with a = 0.541 nm. Ring
d (CeO2) theoretical nm
d measured nm
Relative Intensity
hkl
1
0. 312
0.31
100
111
2
0.270
0.271
27
200
3
0.191
0.191
45
220
4
0.163
0.162
33
311
As mentioned in the Introduction, many synthetic routes are present in the literature for nanocrystalline ceria [5-12]. Among the preparations based on liquid-phase, syntheses concerning hydrolysis of cerium(III) or cerium(IV) salts are widespread. The size of the resulting ceria nanoparticles typically ranges from 1–2 to 50 nm. The particle grow dinamics can be influenced by slight modifications of the synthesis conditions, like the presence of a base, the molar ratio of reactants, the cerium salt concentration, the temperature, the reaction time, the presence of capping agents. Relevant to our results, the hydrolysis of cerium(III) nitrate in the presence of urea at 85°С has been reported [6d] to yield Ce2O(CO3)2·H2O as the only product whose subsequent thermal treatment afforded nanocrystalline ceria with crystallite size within 6–40 nm in dependence of the calcining temperature (300–900°С). In our case, the use of [Ce(O2CNBu2)3]n as precursor has allowed to carry out the hydrolyses in a non-polar solvent and in mild conditions. The study of the hydrolysis products reveals that at room temperature the formation of the hydrated sesqui-carbonate, Ce2(CO3)3·nH2O, is reproduced (C1, C2, C4). On the other hand, Ce(CO3)OH (C3) is formed at 70 °C, in agreement with the result 13
obtained [6d] by conducting the hydrolysis of cerium(III) nitrate at 85 °C in the presence of urea as 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
source of CO2. Furthermore, ceria was obtained by decarboxylation of the samples C1-C4 at a relatively low temperature (200 °C). XRD data have revealed that, although the crystallite dimensions slightly decrease in the order O2~O3>O1>O4, they are all similar, and only slightly smaller than the ones of the ceria crystallites obtained by thermal decomposition (300°C) of the cerium(III) basic carbonate reported in reference 6d. Focusing our attention on O1 and O4, their FESEM and TEM images show aggregates with different morphologies and dimensions, with an estimate of the single crystallite size of about 20 and 5 nm, respectively. These data strengthens the hypothesis that the amine concentration in the course of the hydrolysis can be used to tune the particle dimensions.
4.
Conclusions
We have prepared the N,N-dibutylcarbamato complex of cerium(III), [Ce(O2CNBu2)3], by exploiting a simple and rapid method which allows to start from aqueous solutions of cerium(III) salts. The hydrocarbon soluble complex has been used as a convenient precursor to ceria. Its prompt hydrolysis in non polar solvents at room temperature yields finely divided hydrated cerium(III) carbonate whose treatment at 200 °C in air affords nanostructured ceria. Good reproducibility of the formation of both cerium(III) carbonate and ceria in such a mild conditions has been observed. A higher NHBu2/Ce molar ratio in the course of the hydrolysis step causes a decrease of the crystallite size of the final ceria. The synthetic method here reported could be extended to the preparation of nanostructured doped ceria or mixed oxides starting from solutions of neutral complexes in non-polar solvents. Studies concerning this topic are in course in our laboratories.
Acknowledgments The present work has been co-founded by the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n°313271. The authors wish to acknowledge the Dr. Silvia Andreoli of the Department of Applied Sciences and Technology of the Polytechnic University of Torino (Italy) for the acquisition of FESEM images. Thanks are due to Prof. Natale Perchiazzi of the Department of Earth Sciences of the University of Pisa for useful discussions about the results of the Rietveld refinements. We are
14
grateful to Dr. Lidia Armelao (CNR IENI and INSTM, Department of Chemical Sciences, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
University of Padova) for the XPS measurements.
References [1] Nanostructured Materials and nanotechnology VII (37th Intern. Conf. Adv. Ceramics and Composites, Daytona Beach, FL, 2013) S. Mthur, F. Hernandez-Ramirez Editors in Ceram. Eng. Sci. Proc. 34 (2014). [2] (a) G. Jacobs, L. Williams, U. Graham, D. Sparks, B. H. Davis, J. Phys. Chem. B 107 (2003) 10398. (b) K. Sohlberg, S. T. Pantelides, S. J. Pennycook, J. Am. Chem. Soc. 123 (2001) 6609. (c) F. Goubin, X. Rocquefelte, M. H. Whangbo, Y. Montardi, R. Brec, S. Jobic, Chem. Mat. 16 (2004) 662. (d) D. G. Shchukin, R. A. Caruso, Chem. Mat. 16 (2004) 2287. [3] (a) A. Trovarelli, Catalysis Reviews - Science and Engineering 38 (1996), 439. (b) A. Trovarelli, C. de Leitenburg, M. Boaro, G. Dolcetti, Catalysis Today 50 (1999) 353. (c) W.D. Michalak, G.A. Somorjai, Top. Catal. 56 (2013) 1611. [4] (a) F. Esch, S. Fabris, L. Zhou, T. Montini, C. Africh, P. Fornasiero, G. Comelli, R. Rosei, Science, 309 (2005) 752. (b) Z. Wu, M. Li, J. Howe, H.M. Meyer III, S.H. Overbury, Langmuir 26 (2010) 16595. (c) Z.-A. Qiao, Z. Wu, S. Dai, ChemSusChem 6 (2013) 1821. (d) S. Gangopadhyay, D. D. Frolov, A. E. Masunov, S. Seal, J. Alloys Comp. 584 (2014) 199. [5] (a) D. Zang, X. Du, L. Shi, R. Gao, Dalton Trans. 41 (2012) 14455. (b) W. Feng, L.-D. Sun, Y.-W. Zhang, C.-H. Yan Coord. Chem. Rev. 254 (2010) 1038. (c) M. Melchionna, P. Fornasiero, Materials Today 17 (2014) 349. [6] (a) V.K. Ivanov, O.S. Polezhaeva, Y.D. Tret'Yakov, Russ. J. Gen. Chem., 80 (2010) 604. (b) V.K. Ivanov, A.E. Baranchikov, O. S. Polezhaeva, G. P. Kopitsa, Yu. D. Tret’yakov, Russ. J. Inorg. Chem., 55 (2010) 325. (c) M.L. Dos Santos, R.C. Lima, C.S. Riccardi, R.L. Tranquilin, P.R. Bueno, J.A. Varela, E. Longo, Materials Letters 62 (2008) 4509. (d) M. Hirano, E. Kato, J. Mater. Sci. Lett. 18 (1999) 403. (e) F. Hrizi,H. Dhaouadin, F. Touati, Ceram. Int. 40 (2014) 25. [7] I. V. Zagaynov, S. V. Kutsev, Appl. Nanosci. 4 (2014) 339. [8] S. Kar, C. Patel, S. Santra, J. Phys. Chem. C 113 (2009) 4862. [9] S. W. Depner, K. R. Kort, C. Jaye, D. A. Fischer, S. Banerjee, J. Phys. Chem. C, 113 (2009) 14126. [10] H. Gu, M. D. Soucek, Chem. Mater. 19 (2007) 1103. [11] R. Si, Y.-W. Zhang, L.-P. You, C.-H. Yan, Angew. Chem. Int. Ed., 44 (2005) 3256. [12] T. Yu, J. Joo, Y. Il Park, T. Hyeon, Angew. Chem. Int. Ed. 44 (2005) 7411. [13] D. Belli Dell’Amico, F. Calderazzo, L. Labella, F. Marchetti, G. Pampaloni, Inorg. Chem. Commun. 5 (2002) 848. [14] (a) D. Belli Dell'Amico, C. Bradicich, F. Calderazzo, A. Guarini, L. Labella, F. Marchetti, A. Tomei, Inorg. Chem. 41 (2002) 2814. (b) D. Belli Dell’Amico, F. Calderazzo, L. Costa, E, Franchi, L. Gini, L. Labella, F. Marchetti, J. of Molec. Struct. 890 (2008) 295. (c) D. Belli Dell’Amico, D. Boschi, F. Calderazzo, S. Ianelli, L. Labella, F. Marchetti, G. Pelizzi, E. G. F. Quadrelli, Inorg. Chim. Acta, 300-302 (2000) 882. (d) U. Abram, D. Belli Dell'Amico, F. Calderazzo, S. Kaskel, L. Labella, F. Marchetti, R. Rovai, J. Strähle, Chem. Commun., (1997) 1941. [15] (a) U. Abram, D. Belli Dell’Amico, F. Calderazzo, L. Marchetti. J. Strähle, J. Chem. Soc., Dalton Trans., (1999) 4093. (b) D. Belli Dell’Amico, F. Calderazzo, L. Labella, F. Marchetti, J. Organomet. Chem. 596 (2000) 144. (c) D. Belli Dell’Amico, F. Calderazzo, L. Labella, F. Marchetti, M. Martini, I. Mazzoncini, Comptes Rendus Chimie, 7 (2004) 877. [16] (a) D. Belli Dell' Amico, F. Calderazzo, L. Labella, F. Marchetti, G. Pampaloni, Chem. Rev. 103 (2003) 3857. (b) U. Baisch, D. Belli Dell’Amico, F. Calderazzo, L. Labella, F. Marchetti, A. Merigo, Eur. J. Inorg. Chem. (2004) 1219. [17] L. Armelao, D. Belli Dell’Amico, P. Biagini, G. Bottaro, S. Chiaberge, P. Falvo, L. Labella, F. Marchetti, S. Samaritani, Inorg. Chem. 53 (2014) 4861 [18] S. Dolci, D. Belli Dell’Amico, A. Pasini, L. Torre, G. Pace, D. Valentini, J. Propuls. Power (2015) accepted for publication. [19] L. Yu, D. Chen, J. Li, P. G. Yang, J. Org. Chem. , 62 (1997) 3575. [20] F. Calderazzo, F. A. Cotton, Inorg. Chem. 1 (1962) 30. [21] U. Baisch, D. Belli Dell’Amico, F. Calderazzo, L. Labella, F. Marchetti, D. Vitali, J. Mol. Cat. A 204–205 (2003) 259. [22] J. Faber, T. Fawcett, Acta Cryst. B58 (2002) 325. [23] A. dal Negro, G. Rossi, V. Tazzoli, Am. Mineral. 62 (1977) 142. [24] W. T. Pecora, J. H. Kerr, Am. Mineral. 38 (1953) 1169. [25] (a) A. dal Negro, G. Rossi, V. Tazzoli, Am. Mineral. 60 (1975) 280. (b) A. Sabina, R. Traill, Geol. Surv. Canada, 60 (1960) 4. [26] H. Wakita, S. Kinoshita, J. Chem. Soc. Japan 52 (1979) 428. [27] (a) J. S. Elias, M. Risch, L. Giordano, A. N. Mansour, Y. Shao-Horn, J. Am. Chem. Soc. 136 (2014) 17193. (b) I S. Kar, C. Patel, S. Santra, J. Phys. Chem. C 113 (2009) 4862.
15
Cerium(III) N,N-dibutylcarbamate as precursor to nanocrystalline cerium dioxide
Pages 000-000 Daniela Belli Dell’Amico, Massimo De Sanctis,Randa Ishak, Sara Dolci, Luca Labella, Marco Lezzerini, Fabio Marchetti
Graphical abstract Nanostructured ceria has been prepared in mild conditions by hydrolysis of the N,Ndibutylcarbamato complex of cerium(III), [Ce(O2CNBu2)3], obtained by lanthanide extraction from aqueous solution into heptane containing CO2 saturated NHBu2. The structure and morphology of the samples were studied by TEM, FESEM and XRD studies.