WO2005080271A2

WO2005080271A2 - Process for the production of nanocrystalline oxides and mixed oxides of the lanthanides group, obtained products and their use

Info

Publication number: WO2005080271A2
Application number: PCT/EP2005/050709
Authority: WO
Inventors: Silvia Licoccia; Enrico Traversa; Alberto Rainer; Vincenzo Esposito; Chavalit Trakanprapai
Original assignee: Universita' Degli Studi Di Roma 'tor Vergata'
Priority date: 2004-02-19
Filing date: 2005-02-17
Publication date: 2005-09-01
Also published as: WO2005080271A3; ITRM20040089A1

Abstract

The present invention relates to a process for the production of nanocrystalline oxides and mixed oxides of the lanthanides group, obtained products and their use.

Description

PROCESS FOR THE PRODUCTION OF NANOCRYSTALLINE OXIDES AND MIXED OXIDES OF THE LANTHANIDES GROUP, OBTAINED PRODUCTS AND THEIR USE DESCRIPTION The present invention relates to a process for the production of nanocrystalline oxides and mixed oxides of the lanthanides group, obtained products and their use. Ceria (cerium oxide) has been recently used for applications requiring ionic conductivity at low temperatures, such as for instance the manufacture of solid oxide fuel cells, and other electrochemical applications. An important property required to use these materials in such applications is nanocrystallinity, i.e. to have crystallites which dimensions are in the range 5- 100 nm; most relevant is also to obtain such property at low temperatures since low temperature processing achieves materials having smaller dimensions and suitable for more flexible uses. Acta Materialia [49(2001) 419-426 Li and co- authors] , discloses a process to produce nanocrystalline ceria by the reaction of hydrated cerium nitrate and a secondary amine in an aliphatic alcohol at temperature higher than 500°C. The main disadvantage of such process is the operation at relatively high temperatures which influences the possibility of achieving materials having low dimensions in the subsequent processing. In the state of the art, there was the need to develop processes to obtain nanocrystalline oxides of all the elements of the lanthanides group and that the processes could be carried out at room temperature. There was also the need to develop processes for the production of mixed oxides of the elements of the lanthanides group. It has now surprisingly been found that a process where a hydrated nitrate of one element or more than one element of the lanthanide group is reacted with a tertiary amine and that includes an "ageing" step, i.e. a sufficiently long period of time where the nitrate completely reacts with the amine, allows to achieve nanocrystalline oxides at room temperature. Furthermore, the same process allows to obtain mixed oxides of several elements of the lanthanides group and this was neither disclosed or deducible in the state of the art document above cited. It is therefore object of the present invention a process for the production of nanocrystalline oxides and mixed oxides of elements of the lanthanides group having general formula I

(XO _m (YOp)_r wherein X and Y are different elements of the lanthanides groups (Ce, Pr, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb e La) and n, m, p and r are the stoichiometric indexes of the specific oxide or mixed oxide, one of the indexes m or r possibly being equal to zero, comprising the following steps: a) preparation of a solution either of the element hydrated nitrate or of each element of the hydrated nitrate of the oxide or respectively of the mixed oxide to be obtained in a C1-C₅ aliphatic alcohol either linear or branched to obtain a 1 — 0.01 M solution; b) preparation, in inert gas atmosphere, of a solution of a tertiary amine in a C₁-C₅ aliphatic alcohol either linear or branched to obtain a '5-0.5 M solution; c) addition of the solution obtained in step a) to the solution obtained in step b) under stirring and progress of the reaction until complete condensation of the oxide phase in the form of a precipitate, all the operations of this step occurring under inert gas atmosphere; d) filtration to obtain separation of the liquid phase, washing of said precipitate with said alcohol, drying in vacuum at the temperature 35-45° C and subsequent milling of said precipitate; e) calcination of said precipitate until the organic residues are completely removed. Brief description of the figures To the present invention are attached six Figures showing figure 1 the diffraction patterns of the precipitate obtained in the example immediately after precipitation (trace a) after ageing (trace b) and after calcination at 350 °C and 600 °C (trace c and d respectively) ; figure 2 the differential thermal analysis pattern of the precipitate obtained in the example before ageing and figure 3 the analysis pattern obtained for the precipitate after ageing: figure 4 EDS microanalysis carried out on samaria doped ceria obtained in the example after calcination at 600 °C; figure 5 a typical impedance spectrum measured in the range 10^"2 - 10⁶ Hz relative to a test in non- controlled atmosphere (air) ©-n a pellet obtained from precipitate of the example and sintered at 1200 °C and figure 6 a scanning electron micrograph (FE-SEM) on a pellet sintered from precipitate of the example. Description of the invention In the initial reaction of the process according to the present invention are preferentially used hexahydrate nitrates of the element, or' of the elements of the lanthanides group which oxide or mixed oxide are to be produced. As tertiary amines are preferentially used tertiary diamines, in a particularly preferred way aliphatic N'N' -tetralkylamines; an example of amine preferably used is tetramethyl-ethylenediamine . Both nitrates and amines are used as alcoholic solutions . It is preferentially used a linear or branched C₁-C5- aliphatic alcohol such as ethanol, 1-propanol and 1- butanol and 2-propanol; within the present invention 2- propanol is preferred. It is preferred to use 1-0.01 M nitrate solutions and 0.5-5 M amine solutions in alcohol. The relative amounts of nitrates of two different elements in the formation of mixed oxide depend on the relative stoichiometric ratio that will be present in the mixed oxide. For instance, in the cerium and samarium mixed oxide the stoichiometric ratio is 4:1. Peculiar characteristics of the present invention are that the reaction between nitrates and amine is carried out in an inert gas atmosphere, preferentially nitrogen, at room temperature and this is a noticeable difference with what described in the prior art. An essential characteristic of this invention procedure is the "ageing" stage of the reaction between nitrates and amine that is carried out leaving the reaction to proceed until complete condensation of the oxide phase is achieved.. Thanks to this ageing stage, usually lasting between 48 and 96 hours, it is possible to achieve nanocrystallinity at low temperatures, because the ageing stage causes large nucleation followed by limited growth of the crystallites . The completion of this stage, -_s_jse. complete condensation of the oxide phase, can be instrumentally monitored as shown, for instance, by the trace shown in figure 1, in particular in traces la, lb and lc. The precipitate obtained from the reaction is filtered to separate it from the liquid phase, then further washed with the same alcohol used to dissolve the reactants . It is then dried in vacuum at a temperature between 35 and 45 °C, then coarsely ground to undergo

^■ then the subsequent calcination step. It has been demonstrated to be particularly advantageous, according to the present invention, to carry out two successive calcination steps the first at 350 °C and the second at 600 °C. The precipitate, obtained according to the process of the present invention, is a nanocrystalline precipitate that maintains such property even after subsequent work up, such as sintering; what above mentioned is confirmed by the experimental data shown in figure 5. Among the oxides obtained according to the process of the present invention ceria (cerium oxide) as well as cerium and samarium ( (SmOi.s) ₀.₂- (Ce0₂) o.β) and cerium and gadolinium ( (Gdθι.5) 0.1 ^■ (Ce0₂) 0.9) mixed oxides can be mentioned. The precipitate obtained according to the process of the present invention can undergo further workings up to obtain ceramic products such as, for instance, pellets, films or sintered products that are particularly suitable to be used as sensors, catalysts, anodic elements for fuel cells, electrochemical permeation membranes and fillers for metallic alloys . Example Production of cerium and samarium mixed oxide Cerium nitrate hexahydrate (Ce (NO3) 3.6H₂0, Sigma

Aldrich, 99,99%) and samarium nitrate hexahydrate (Sm(N0₃)₃.6H₂0, Sigma Aldrich 99^;>99%) in stoichiometric cationic ratio 4:1 were used to prepare a 0.1 M solution in 2-propanol. The same alcohol was used to prepare a 1 M solution of tetramethyl-ethylenediamine (Sigma Aldrich 99,%). 150 ml of the solution containing the salts were added drop-wise to an equal volume of the amine solution and the reaction was kept under mild stirring. The suspension thus formed was left "ageing" maintaining stirring for 72 hours, then filtered. Three washing with the same alcohol were carried out to eliminate the residual amine. The precipitate was then dried in vacuum at a temperature of about 40 °C then mildly ground with an agate pestle in a mortar. The powder was then calcined in a tubular oven, in two different procedures, at the temperatures of 350 and 600 °C respectively. The presence of residual organic compounds as well as of residual nitrate groups caused a vigorous gas emission during the heating process. To avoid spilling of the powder obtained by the procedure from the crucible, the heating rate was kept at 2 K/min up to 250 °C then increased to 5 K/min up to the chosen calcinations temperature. After the drying process the powder was characterized by thermogravimetric and differential thermal analysis (TG-DTA, Netsch) . Analyses were carried out in air with a heating rate of 1 K/min up to 400 °C and 5 K/min up to 1200 °C. Phase identification was carried out by XRD (Philips X'pert 1900 using X'pert Plus software) using Cu_Koc radiation in the range 2Θ = 15-85°. Diameter and size of crystallites were determined by using Scherrer equation. Morphology and agglomeration of the powder were observed by FE-SEM (LEO 1535) . A small amount of powder was dispersed in ethanol and deposited on a graphite stub without metallization. Pellets were prepared to evaluate the electrochemical properties of the material. The powder was pressed uniaxially at 100 MPa in a cylindrical container, then presses isostatically at 200 MPa. Sintering of the pellets was carried out in air at 1200 °c for 2 hours with a heating rate of 5 K/min. Th©:., pellets external surfaces were covered with platinum paste electrically connected with gold wires . Electrochemical impedance spectroscopy was measured (Solartron FRA 1255 with electric interface 1296) in air at the selected temperatures of 150 and 600 °C in the frequency range 10⁷-10^~2 Hz. Figure 1 shows the diffraction PATTERN of the same ' precipitate after precipitation (a) , after ageing (b) and after calcination at 350 and 600 °C (c and d respectively) . By comparing the spectra obtained on the precipitate after ageing (b) and after calcination (c) it can be noted that they are substantially similar and this observation confirms the importance of the ageing step in the present invention procedure and the achievement of nanocrystalline species at room temperature. This is also confirmed by the comparison of figures 2 and 3 that show the patterns relative to differential thermal analysis carried out on the precipitate (figure 2) and on the aged precipitate (figure 3) . In the case of the aged precipitate (figure 3) the weight loss is markedly lower and this confirms that ageing (step c of the process of claim 1) effectively eliminates residual organic compounds and completes material oxidation leading to nanocrystalline structures containing oxide. Figure 4, showing the results of EDS microanalysis on a sample of SDC calcined at 600 °C, confirms the absence of contaminants. Figure 5, showing an impedance pattern (EIS) referring to a test carried out in non controlled atmosphere (air) on a pellet obtained by the example precipitate sintered at 1200 °C, confirms that the material shows a polycrystalline structure deriving from the nanocrystalline structure of the original crystallites even after sintering. Figure 6, a scanning electron micrograph (FE-SEM) , of a pellet sintered f om the same precipitate confirms data and deductions derived from figure 5. The present invention has been described with reference to its actuation preferred at the present time but it is understood that variations and modifications introduced by the competent technician are included in the present privative.

Claims

CLAIMS 1. A process for the production of nanocrystalline oxides and mixed oxides of elements of the lanthanides group having general formula I

(XOn)m (YOp)_r wherein X and Y are different elements of the lanthanides groups (Ce, Pr, Pm, S , Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb e La) and n, m, p and r are the stoichiometric indexes of the specific oxide or mixed oxide, one of the indexes m or r possibly being equal to zero, comprising the following steps : f) preparation of a solution either of the element hydrated nitrate or of each element of the hydrated nitrate of the oxide or respectively of the mixed oxide to be obtained in a C1-C5 aliphatic alcohol either linear or branched to obtain a 1 - 0.01 M solution; g) preparation, in inert gas atmosphere, of a solution of a tertiary amine in a C₁-C₅ aliphatic alcohol either linear or branched to obtain a 5- 0.5 M solution; h) addition of_. the solution obtained in step a) to the solution obtained in step b) under stirring and progress of the reaction until complete condensation of the oxide phase in the form of a precipitate, all the operations of this step occurring under inert gas atmosphere; i) filtration to obtain separation of the liquid phase, washing of said precipitate with said alcohol, drying in vacuum at the temperature 35-45° C and subsequent milling of said precipitate; j) calcination of said precipitate until the organic residues are completely removed.

2. The process according to claim 1, wherein said steps a) and b) are carried out using preferentially 2- propanol as solvent.

3. The process according to claims 1 and 2 wherein said step b) tetramethyl-ethylendiamine is preferentially utilized as amine 4. The process according to at least one of the previous claims wherein said step c) to the solution produced in step b) an identical amount of solution produced in step a) is added. 5. The process according to at least one of the previous claims, wherein in said step e) a second calcination is carried out so that a higher nanocrystallinity of the above mentioned precipitate is obtained. 6. The process according to at least one of claims from 1 to 5, wherein said precipitate of oxides or mixed oxides obtained in step e) is further worked up to ceramic products . 7. Oxides and mixed oxides of elements of the lanthanides group obtainable by the process as claimed in claims 1 to 6. 8. Mixed oxides according to claim 7 comprising (SrnOi.s) o.2 ^• (Ce0₂) ₀.₈ and (GdOi.s) o.i ^• (Ce0₂) ₀.g . :. 9. Use of oxides and mixed oxides as claimed in claims 7 or 8 as sensors, catalysts, anodic elements for fuel cells, electrochemical permeation membranes and filler for metallic alloys.