Chapter III: Optimization of catalytic support synthesis [309804]

Chapter III: Optimization of catalytic support synthesis

III.1. Evaporation Self Assembly Method

III.1.1. Introduction

III.1.2. Experimental

III.1.2.1. Synthesis of the catalytic support

III.1.2.2. Characterization techniques

III.1.3. Results and discussion

III.1.3.1.Optimization of the solvent evaporation conditions

III.1.3.2.Optimization of the drying conditions

III.1.3.3.Optimization of the calcination step

III.1.4. Conclusions

III.2. Coprecipitation Method

III.2.1. Introduction

III.2.2.Synthesis

III.2.3.Characterization

III.1. Evaporation Self Assembly Method

III.1.1. Introduction

The mixed CexZr1-xO2 oxides, as a [anonimizat]. The distortion of the O2- subllatices in the mixed oxides permits a [anonimizat]. [1] . [anonimizat], given to their good stability and redox properties. Recent research has shown that a ceria-zirconia mixed oxide solution is a better catalyst than ceria. Zirconia improves the stability of ceria and prevents its sintering. The addition of zirconium to the structure of ceria greatly increases its oxygen mobility and enhances the reducibility of Ce4+ . Also the substitution of Ce4+ by Zr4+ [anonimizat] [2].

CeO2-ZrO2 [anonimizat], and surface area. Therefore structured CeO2-ZrO2 oxides as an ordered mesoporous structure with crystalline walls are expected to exhibit enhanced catalytic performances due to their large surface area and a certain degree of size and shape selectivity. Sanchez et al. [3] reported the first successuful fabrication of cubic mesostructured CeO2-ZrO2 (rich in Zr) [anonimizat] a preserved enhanced oxygen storage capacity (OSC), thermal stability and surface area.

The properties of CeO2-ZrO2 mixed oxides are strongly related to their structure and local order. [anonimizat]. The crystal structure of compositionally homogeneous CeO2-ZrO2 solid solutions has been studied by Yashima et al. [4]. [anonimizat] P42/nmc space group. [anonimizat] t form, as predicted by the equilibrium phase diagram is restricted by the solubility limit. Thus, the t’ form has a [anonimizat]. The stable phase in this compositional range is the mixture of the t form and a cubic phase. Finally, the t’’[anonimizat] (8c sites of the Fm3m space group) along the c axis. Other metastable phases have been as well reported and characterized for the 50 mol % CeO2-ZrO2. [5]

Several routes have been used for the synthesis of nanocrystalline CeO2-ZrO2 [anonimizat], [anonimizat], [anonimizat]mbustion. All of these produce compositionally polimer homogeneous materials [6].

Mesoporous Ce0.5Zr0.5O2 with a distorted cubic structure was obtained using a KLE polymer ( poly(ethylene-co-butylene) – block –poly (ethylene oxide)), as the template . Using this block copolymer template, allows the prediction of a mesoporous ceria film with a homogeneous pore size of about 10 nm and highly crystaline walls after the evaporation. This procedure is know as induced self assembly method [7]. Using it, Yuang et al. reported a highly ordered mesoporous structure with a broad ratios of cerium to zirconium. The deposition of Pd on this mixed oxide lead to active Pd/Ce1-xZrxO2 catalysts for the conversion of CO. [8]

The OSC of the CeO2-ZrO2 solid solution depends largely on the method of preparation. Most of the physical and chemical properties such as crystallinity, crystallite and particle sizes, and homogeneity change as a function of the preparation method, also influencing the OSC [9]. Such a behaviour has been confirmed by investigation of CeO2-ZrO2 prepared using various methods such as the solid-state reaction [10], high energy milling [11], coprecipitation [12], sol-gel [13], citrate complexation [14], complex polymerization [15], combustion synthesis [16], or hydrothermal method [17].

First synthesis of the mesoporous materials was reported in 1992 by Kresge and Beck [18,19]. Since that moment the intererest in this research has expanded all over the world due to wide applications in catalysis and other potential fields. Mesoporous materials are typically synthesized by self-assembly of the surfactant-type templates, among wich amphiphilic block copolymers are valuable directing agents to obtain relatively large mesopores. [20,21] . Particularly the commercially available PEO-PPO-PEO triblock copolymers often generate mesoporous materials with a pore size smaler than 20 nm [21].

However, the properties of this mesoporous materials largely depend on the preparation method, the nature of the various phases and their content. Almost other the synthesis of binary oxide systems via sol-gel is very often reported. Comparative studies in the preparation of CeO2-ZrO2 mixed oxides by two different chemical routs (ie citrate complexation and sol-gel) by Alifanti et al. indicated that for low ceria [22] contents , the sol-gel leads to a more homogeneous solid solution. However, an increase of the ceria content (to 20 >%) led to a segregation of phases. Under these conditions the citrate route led to lower surface areas (SSA) than the sol-gel the oxide was more homogenous a composition area a large Ce/Zr ratios. However, it is important to emphasize that the determination of the phase homogeneities in a nanostrucured Ce-Zr solid solution is very difficult using conventional characterization techniques. At the nanoscale level, the possibility of the existence of rich nanodomains of ceria – or zirconia –cannot be excluded.

Other possibilities to adjust the textural properties of the CeO2-ZrO2 mixed oxides for the preparation of the catalytic supports, are offered by a soft templating method such as an evaporation induced self assembly. This is a quit novel approach for the synthesis a higly ordered mesoporous Ce0.5Zr0.5O2 solid solutions with a 2D hexagonal mesostructure.

The “physical chemistry of organized matter” relies on the successful combination of sol-gel chemistry and self-assembly combination procedures. unique control of the texture of the materials even at nanometer scale. The synthesis strategy is based on a sol-gel process combined with an evaporation –induced self-assembly method in ethanol using block copolymer Pluronic P123 as the template and ceric nitrate and zirconium oxide chloride as the precursors. This method does not require an additional acid or base[8].

However, in spite of the many research and the reported achievements, the apparent simplicity of this system, both the phase diagram and structural properties of the Ce0.5Zr0.5O2 are still a matter of investigation. This is well supported by the fact that the phase transition and the structures occuring in the Ce0.5Zr0.5O2 are not easily detectable. As an example X-ray diffraction and Raman spectroscopy are impotent to detect microdomain heterogeneity of these solid solution.

Based on this state of the art, the purpose of the research discussed in this chapter is to optimize the synthetic conditions of the mesostructured materials Ce0.5Zr0.5O2 mixed oxides using the evaporation –induced self-assembly method. to make more clear this attempt they new considered three steps: (a) the first step considered the optimization of the solvent evaporation conditions,(b) the optimization of the drying process and (c) the thirth step the optimization of the calcination conditions. The materials were characterized in detail by using TGA, XRD in the region of low and wide angle, Raman Spectroscopy and N2 physisorption. The final scope was synthesis of ordered mesostructured materials with potential application in catalysis, such as water denitrification [23,24].

III1.2.1. Synthesis of the catalytic support

Evaporation induced self assembly method (EISA)

EISA is a method that …. a phase and, pore size control and a good crystallinity of materials. The experiment used a volatile with the capability to delay both hydrolysis and condensation rates of the metal species. Such a behaviour is highly recommended for the formation of ordered mesostructures. By the evaporation of the solvent, the surfactant becomes highly concentrated leading to a liquid-crystal incorporating the inorganic species and finnaly to ordered composite mesostructures. Controlling this important step allows to improve the properties of mixed oxides generating ordered mesostructures. The inorganic frameworks is then solidified via a post-treatement. The last step of the process was the removal of the polymer by calcinations. Mixed metal oxides with open pores were thus generated.

In fact, the general synthesis strategy is based on a sol-gel process combined with the evaporation –induced self-assembly method. Ethanol has been used as solvent and block copolymer Pluronic P123 as the template. Ceric nitrate and zirconium oxide chloride are the inorganic precursors. No additional acid or base has been used.

The synthesis started by dissolving 0.5 g of Pluronic P123 ( M av=5800, EO20PO70EO20, Aldrich) in 10 ml of ethanol. Then, quantitative CeNO3 6H2O (Sigma Aldrich) and ZrOCl2 8H2O (Sigma Aldrich) were added (the concentration of the Ce and Zr was 5mmol), in order to produce a mixed oxides with an atomic Ce:Zr ratio of 1:1. After stirring for 3h at room temperature, the homogeneous sol was transferred to an oven under desired temperature and humidity:

temperature: 40°C, relative humidity:

S1(40%rH), S2(50%rH), S3(70%rH)

The solvent evaporation have been carried out in a 25 ml Berzelius glass. After 48 h aging, the gel product was dried at 100°C for 24 h.

After 48 h aging, the gel product was dried at different temperatures. Accordingly the samples were denoted as:

S2. 1. – 60°C, 24h,

S2. 2.- 100°C, 24h,

S2. 3.- 60°C, 24h; 100°C, 24h

Calcination was carried out by increasing temperature from room temperature to 400°C with a small ramp 1°C min−1 ramping rate. After reaching 400°C the temperature was maintained for another 4h in air.

The resulted solids were labeled as Ce0.5Zr0.5O2 S1(40%rH), Ce0.5Zr0.5O2 S2(50%rH) and Ce0.5Zr0.5O2 S3(70%rH) , respectively.

The entire process is described in Table 1 and Scheme 1.

Table 1. Mixed oxides CeO2/ZrO2 support synthesis

Scheme 1. EISA synthesis steps

III.1.2.2. Catalyst characterization

Thermogravimetric analyses (TGA) were carried out with a TA Instrument DSC-TGA SDT 2960 thermal analyzer in the temperature range of 25–900 °C under air or N2. The heating rate was kept constant (10 °C/min).

The X-ray diffraction (XRD) patterns were recorded on a HUBER G-670 diffractometer fitted with an anti-cathode CuKα (λ = 1.54178 Å). XRD patterns were recorded over 2θ from 10 to 80°, with a scanning rate of 5 s/step and 0.05 step sizes.

Low-angle XRD patterns were recorded with a D8 advances X-ray diffractometer (Bruker AXS) fitted with a CuKα (λ= 1.54178 Å) radiation in the 2θ range 0.3–6° with a 0.02° steps.

Raman spectroscopy analysis was performed on a Labram Infinity Dylor spectrometer fitted with frequency-doubled Nd:YAG laser with an excitation radiation of 532 nm. Prior to measurements , the silicon line at 521 cm-1 was used for calibration.

The porosity of the samples was estimated from nitrogen physisorption measurements at −196 °C on a Micrometrics Tristar 3020 analyzer. Preliminary, the samples were cleaned after outgassing under secondary vacuum at 200 °C. The surface area was calculated from the BET equation and the pore volume (Vp) estimated using the adsorption branch of the nitrogen isotherm curve.

III.1.3. Results and discussion

Optimization of the solvent evaporation conditions

The thermogravimetric analysis was carried out in order to determine the most appropriate temperature for the calcinations of the supports.TGA-DTA analysis of the dried samples was carried out in air using a TA Intrument DSC-TGA SDT 2960 thermal analyser in the range of 25°C-900°C. The heating rate was maintained at 10°C/min. and the gas flow was 100ml/min..

Figure 1 shows the weight loss as a function of temperature for the samples gave been prepared.

Figure 1.Mass loss as a function of temperature for Ce0.5Zr0.5O2( S1, S2, S3) dried at 100°C

The TGA curve for Ce0.5Zr0.5O2 EISA shows three distinct weight loss regions (fig.1). The first loss, correspond to the thermal desorption of physically adsorbed water ant ethanol. It occurred in the range of 25°C-100°C. An weak exothermic peak at ca. 99°C in the DTA curves is associated to this process. The second loss (110-300°C) corresponds to the decomposition of the polymer P 123. This loss is accompanied to DTA peaks at ca. 245°C. The third steps corresponds to the formation of crystalline Ce0.5Zr0.5O2. Since thermal decomposition is finished by 400°C, the precursors were treated in air temperatures high than 400°C (Table 2).

Fig 2.Thermal decomposition of the Polymer Pluronic P123

Table 2. Weight loss for the prepared, prepared via EISA

The TG curves show a total weight loss of ca. 50%. The weight loss (25-200°C) corresponds to the evaporation of ethanol and adsorbed surface water and is associated to an endothermic peak at ca. 99°C in the DSC curves. The second (200-400°C) weight loss is associated with the loss of hydroxyl groups, combustion of organics (the elimination of organic surfactants through combustion) and iyts associated to an exothermic reaction (crystallization). A large peak in the DSC profile at 350°C corresponds to the loss of surfactant as indicated by the weight loss of Pluronic P123 presented in (Figure 5). No other peak in the DSC curve and no weight loss were observed in the TG curve for temperature higher than 400°C, such a behaviour confirm that the sample has been completely crystallized and all the organics have been removed.

XRD analysis

XRD patterns were collected following two different procedures, ie (i) in the range of 2θ from 0.5 to 5°, at a scan speed of 0.02°/step and counting times of 1 s/step and (ii) 2θ from 5 to 80°, at a scan speed of 0.02°/step and counting times of 0.5 s/step.

XRD patterns for the samples uncalcined and calcined at 400°C are presented in figure 3. The XRD patterns of Ce0.5Zr0.5O2 is indicative for well-crystallized inorganic frameworks with crystalline tetragonal phase.

Figure 3. Wide angle and small angle XRD patterns for the samples: S1, S2, S3

Raman analysis

Raman spectroscopic measurements can complement XRD data since this technique is also sensitive to amorphous structures.

The exposure time was 20 seconds. The Raman spectra are presented in figure 4.

Figure 4. Raman spectra of mesostructured Ce0.5Zr0.5O2 ( S1, S2, S3) calcined at 400 °C

Obviously, 6 Raman active modes (A1 g + 2B1 g + 3E2 g) are present in the spectra of a tetragonal structure characterized by a P42/nmc space group. A cubic fluorite structure has in its Raman spectrum of the F2 g mode an intense Raman line at 490 cm−1 ( Fig. 4).

Raman lines located at 155, 200, 300 and 620 cm−1 were discernible for all the three samples. All the three samples shows a broadening of the line at 490 cm−1 with shoulder at around 550 nm−1 which is attributed to the tetragonal (t) phase [25].The line near 300 and 180 cm−1 relate to the displacement of the oxygen atoms from their ideal fluorite lattice positions caused by the insertion of zirconium into the CeO2 lattice with the formation of the pseudo-cubic (t”) phase [26] The line at about 155 cm−1 accounts to the t-ZrO2 phase [25] According to the literature [25] the recorded Raman spectra confirm the stabilisation of the tetragonal structure for atomic Ce compositions lower than 0.5. For x≥0.52, the cubic structure coexists the tetragonal one. It becomes predominant for atomic Ce compositions richer than 0.75.

Nitrogen adsorption-desorption analysis

The pore size distribution was evaluated from the analysis of the adsorption branch of the isotherm. The BET surface area was measured by nitrogen adsorption at 78.3 K, after degassing the sample at 473 K for 4h.

The BET nitrogen adsorption-desorption isotherms are measured for all the mesoporous Ce1-xZrxO2 (x=0.5) samples after calcinations. The isotherms are depicted in Figure 5

.

Figure 5. Nitrogen adsorption-desorption isotherms of mesostructuredCe0.5Zr0.5O2 (SI, S2, S3) Corresponding pore size distribution curves deduced from the desorption branches

The nitrogen sorption isotherms of the sample exhibit a typical type-IV isotherm with a small step and a H4 type hysteresis loop at P/Po=0.5-0.9, which is characteristic for a mesoporous metal oxide prepared via a soft template route. A hysteresis of type H2 for CZ(EISA) may correspond to the presence of partially uniform mesopores. The surface area, pore volume and average pore size, calculated from the BET and BJH formations are comparable. For all the three samples it can be observed a narrow pore size distribution in the range og 2-6nm.

The BET surface area, pore volume and pore size distribution are detailed in Table 4.

Table 4. BET surface area, pore volume and pore size distribution

The analysis of results of structural and textural charcaterization let to the conclusion that the optimal synthesis conditions for a good mesostructured catalytic supports , correspond to 50%rH for a solvent evaporation step.

III.1.3.2. Optimization of the drying conditions

Synthesis of Ce1-xZrxO2 (x=0.5) using Pluronic P123 dried at different temperatures,

Synthesis of the catalytic supports followed the same (EISA route) as described for the optimization of the solvent evaporation. The optimization of drying conditions followed the optimization of the conditions for solvent evaporation step (50%rH in a climate chamber, 48h).

With this aim, after 48 h aging, at 50%rH, the gel product was dried at different temperatures:

ST 1 – 60°C, 24h,

ST 2- 100°C, 24h,

ST 3- 60°C, 24h; 100°C, 24h

The calcination was carried out by a slow increase of the temperature from room temperature to 400°C (1°C min−1 ramping rate) and heating the sample for another 4h at 400°C.

Structural and textural characterization

Thermogravimetric analysis

The TGA-DSC analysis was performed before calcination in order to receive information about the calcination process and to determine the complete conversion temperature of the nitrate precursor. Weight-loss curves are presented in the Figure 6.

Figure 6.Weight loss curves for the Ce0.5Zr0.5O2 (S2.1., S2.2., S2.3.)

The TG curves show a total weight loss of ca. 50%, in two distinct steps. The first weight loss (25-200°C) was ascribed to the evaporation of ethanol and surface water corresponding to the endothermic peak at ca. 99°C in the DSC curves. The second (200-400°C) weight loss is associated to the loss of the hydroxyl groups, the combustion of organics (the elimination of organic surfactants through the combustion leading to an exothermic effect) and crystallization. A large peak in the DSC profile at 350°C corresponds to the loss of surfactant that is in accordance to the DTA curve of the surfactant Pluronic P123 presented in figure 6. Again, since thermal decomposition was finished at 380°C, the sample has been treated in air at temperatures above 400°C to produce its complete decomposition. No peaks was found on the DSC curve and no weight loss have been observed on the TG curve after 400°C, thus confirming no additional weight loss ot phase transformation.

Tabel 5. Weight loss of different catalytic supports dried at 3 different temperatures

XRD analysis

The diffractograms were collected following the protocol described in the subchapter II.1.1.2.

XRD patterns for the samples (uncalcined and calcined at 400°C) are presented in figure 7. The XRD patterns of Ce0.5Zr0.5O2 are indicative for a well-crystallized inorganic framework corresponding to the crystalline tetragonal phase.

Figure 7 .Small angle XRD for dried and calcined Ce0.5Zr0.5O2 (S2.1. – 60°C, S2.2.- 100°C, S2.3.- 60°C, 100°C)

Pore ordering quality of the materials was assessed by low angles XRD (Fig. 7). As illustrated, a well-defined reflection centered at 0.55° is observed for the dried Ce0.5Zr0.5O2 (S2.3.- 60°C, 100°C) sample prepared following EISA protocol. This result evidences the genesis of an ordered mesophase (P6mm hexagonal symmetry) with respect to the conventional precipitation route. However, the calcination step was partially detrimental for this sample as observed by the broadening and shifting of the X-ray line toward higher 2θ angle (1.15°). Partial shrinkage of the mesostructure indubitably occurs during the decomposition of the surfactant in the presence of the inorganic species. Hence, the mesopores formation can be produced following the two steps of the drying process.

Figure 8. Wide angle XRD patterns for Ce0.5Zr0.5O2 (S2.1. – 60°C, S2.2.- 100°C, S2.3.- 60°C, 100°C) calcined at 400oC

Raman analysis

Figure 9 shows the Raman spectra collected from an exposure time of 20 seconds.

Figure 9. Raman spectra of Ce0.5Zr0.5O2 (S2.1. – 60°C, S2.2.- 100°C, S2.3.- 60°C, 100°C) calcined at 400oC

All the Raman spectra shown lines located at 155, 200, 300, 445 and 620 cm-1, that may correspond to the stable t phase nature. According to literature the Raman spectra recorded for ceria–zirconia mixed oxides, indicate the stabilisation of a tetragonal structure (for an atomic Ce composition small than 0.5. On the other hand, the cubic structure coexists for x≥0.52 and becomes predominant for atomic Ce composition higher than 0.75.

Nitrogen adsorption-desorption analysis

The pore size distribution curve was determined from the analysis of the adsorption branch of the isotherm. The BET surface area was measured by nitrogenadsorption at 78.3 K, which was performed after degassing the sample at 473 K for 4h.

The BET nitrogen adsorption-desorption isotherms were measured for all the mesoporous Ce1-xZrxO2 (x=0.5) samples after calcinations. The isotherms are depicted in Figure 10.

Figure 10. Nitrogen adsorption-desorption isotherms of mesostructured Ce0.5Zr0.5O2 (S2.1. – 60°C, S2.2.- 100°C, S2.3.- 60°C, 100°C) calcined at 400oC Corresponding pore size distribution curves deduced from the desorption branches

The nitrogen sorption isotherms of the sample exhibit a typical type-IV isotherm with a small step and a H4 type hysteresis loop at P/Po=0.5-0.9, wich is the characteristic for a mesoporous metal oxide prepared by a soft template route.

Table 6. Surface area and pore size distribution for the catalytic supports dried at different temperatures

The sample dried in 2 steps at 60°C, and 100 °C had a pretty high SSA, and a narrow pore size distribution in the range 2-6nm, that corresponded to the better values.

III.1.3.3. Optimization of the calcination step

Calicnation of Ce1-xZrxO2 (x=0.5) EISA supports

The final step of the optimization of the preparation process is the calcination. This step was investigated considering the results of the two previous steps. Accordingly the experiments considered the support prepared in the following conditions:

Calcination was carried out by increasing the temperature from room temperature with a small rate to 400°C (1°C min−1) and further calcination at 400°C for 1 h in N2 or air. The samples are denoted as S2.3. N2, air and S2.3. air

XRD analysis

XRD patterns for the uncalcined and calcined samples at 400°C are presented in figure 2. The XRD patterns of Ce0.5Zr0.5O2 is indicative of well-crystallized inorganic frameworks corresponding to the crystalline tetragonal phase.

Figure7 . Wide angle XRD for CeZrO2 (S2.3. air ans S2.3N2.air)

Figure8 .Small angle angle XRD for CeZrO2 (S2.3. air and S2.3N2,air) dried and calcined

Descriere

Raman analysis

The laser Raman spectra were recorded using a Jobin-YvonSPEX Raman spectrometer with a 30 mWAr ion laser (488.0 nm) and a 12 mW He-Ne laser (532.8 nm). The silicon line at 521cm-1 was used for calibration before measurements.

The exposition time is 20 seconds and accumulates two times. The Raman spectra are described in the figure 9, for all the synthesis.

Figure 9. Raman spectra of Ce0.5Zr0.5O2 (S2.3. air ans S2.3N2.air) calcined at 400 °C in air , respectively 400 in N2, 400 in air

In all the Raman spectra we can observed the appearance of 5 Raman bands located at 155, 200, 300, 445 and 620 cm-1, that reveals its stable t phase nature. According literature Raman spectra recorded on ceria–zirconia mixed oxides are agree with the stabilisation of a tetragonal structure for atomic Ce composition lower than 0.5. On the other hand, the cubic structure coexists for x≥0.52 and becomes predominant for atomic Ce composition higher than 0.75.

2.4. Nitrogen adsorption-desorption analysis

SSA was registred using a Micrometrics Flow Sorb III, surface Area Analyzer and the nitrogen adsorption and desorption isotherms at 77.35 K were measured using an adsorption analyzer Micrometrics, Tri Star II 3020. The pore size distribution curve was taken from the analysis of the adsorptionbranch of the isotherm. The BET specific surface area was measured by nitrogenadsorption at 78.3 K, which was performed after degassing the sample at 473 K for 4h.

Figure 10. Nitrogen adsorption-desorption isotherms of mesostructuredCe0.5Zr0.5O2(S2.3. air ans S2.3N2.air )

Corresponding pore size distribution curves deduced from the desorption branches

The BET nitrogen adsorption-desorption isotherms are measured for all the mesoporous Ce1-xZrxO2 (x=0.5) samples after calcination and the isotherms are described in figure 10.

The nitrogen sorption isotherms of the sample exhibit a typical type-IV isotherm with a small step and a H4 type hysteresis loop at P/Po=0.5-0.9, wich is the characteristic of mesoporous metal oxide prepared by soft template route.

Table 7. BET surface area, pore volume and pore size distribution for supports dried in air or N2

III.1.4. Conclusions

According with the processes describe above, we selected as optimal conditions for the synthese of catalytic support, the next conditions:

For the solvent evaporation step 50%rH, for 48 h, in a climate chamber;

For drying step 60 o C ,24 and 100 o C, 24h;

For calcinations step, 4000C, in air, for 4h.

III.2. Coprecipitation Method la partea de bibliografie

III.2.1. Introduction

In this process the desired component is precipitated from the solution. Co-precipitation is used for simultaneous precipitation of more than one component. Catalysts based on more than one component can be prepared easily by co-precipitation. The precipitation process is used for preparation of bulk catalysts and support material such as Al2O3, SiO2, TiO2, ZrO2 , CeO2, etc.

Coprecipitation reactions involve the simultaneous occurrence of nucleation, growth, coarsening , and/or agglomeration processes.

Coprecipitation reactions exhibit the following characteristics:

The products are generally insolubles species formed under conditions of high supersaturation.

Nucleation is a key step, and a large number of small particles will be formed.

Secondarry processes , such as Ostwald ripening and aggregation , dramatically affect the size, morphology, and properties of the products.

The supersaturation conditions necessary to induce precipitation are usually the results of a chemical reaction.

XAy(aq)+ +yBx(aq)- AxBy (s) (1)

Typical coprecipitation synthetic methods are:

Metals formed from aqueous solutions, by reduction from nonaqueous solutions , electrochemical reduction, and decompositions of metallorganic precursors.

Oxides formed from aqueous and nonaqueous solutions.

Metal chalconides formed by reactions of molecular precursors.

Microwave /sonication assited coprecipitation

Advantages and disadvantages of coprecipitation method :

Coprecipitation method offers some advantages:

Simple and rapid preparation

Easy control of particle size and composition

Various possibilities to modify the particle state and overall homogeneity

Low temperature

Energy efficient

Does not involve use of organic solvent.

Disadvantages include:

Not applicable to uncharged species

Trace impurities may also get precipitated with the product

Time consuming

Batch to batch reproducibility problems

This method does not work well if the reactants have very different precipitations rates. [27]

During precipitation, several processes occurs and the major steps are :

liquid mixing/supersatuartion

nucleation

crystal growth to form primary products

aggregation of the primary particles

Initial mixing or interdispersing of components in the solution has a significant effect on the precipitation. Good mixing result in a more homogeneous product particularly in case of co- precipitation. Rate of stirring primarily affects the nucleation whereas growth rate is much less influenced by this factor. Stirring rate also affect the aggregation. Aggregate size can be influenced by changing the stirring rate and the manner of mixing.

Fig 11. Parameters affecting supersaturation

Parameters affecting supersaturation is shown in Fig. 11. In supersaturated region the system is unstable and precipitation occurs with any small disturbance. The supersaturaton region is approached either by increasing the concentration through evaporation, lowering the temperature or by increasing pH. The solubility of a component increases with temperature as shown in Fig. 11. The solubility curve is also function of pH. As pH increases solubility decrease and curve shift from 1 to position 2. Then the point which was initially in solution region becomes in supersatured region. The increase in pH is the most convenient method for precipitation. The reaction during precipitation, , is controlled by increasing the pH through addition of a basic solution. Hence by raising the pH value of a solution by addition of alkaline or ammonium hydroxide the corresponding metallic hydroxide compounds can be made insoluble and precipitated from solution. Commonly used reagents are NaOH, KOH, NH4OH, carbonates and bicarbonates. Particles within supersaturated region develop in two steps : nucleation and growth.

Nucleation may proceed spontaneously through the formation of M(OH)n entities or be initiated with seed materials such as dust, particle fragments, roughness of vessels surface. Addition of seed material enhances rate of nucleation. The nucleus is defined as the smallest solid phase aggregate of atoms, molecule or ions which is formed during a precipitation and which is capable of spontaneous growth. For nucleation to start the solution need to be super saturated. Only when the concentration exceeds a critical threshold value a nucleus will form and the precipitation will begin. As long as the concentration of the species stays above the nucleation threshold, new particles are formed. Nucleation starts with the formation of clusters which are capable of spontaneous growth by the subsequent addition of monomers until a critical size is reached.  Cluster smaller than this size tends to re-dissolve, while larger cluster continues to grow. As soon as the concentration falls below the critical concentration due to consumption of the precursors by nucleation or by the growth process, only particle growth of existing particles continues. Growth proceed through adsorption of ions on surface of seeded particle. This growth is a function of concentration, temperature and pH. Rates of nucleation and growth can be independently controlled. If nucleation is faster than growth, the system produces a narrow distribution of small particles. Fast growth results in narrow distribution of large particles.

Several equations are proposed for nucleation rate and the most used is :

where β is the pre-exponential term, σ is solid –fluid interfacial energy, is solid molecular volume and T is the temperature. The super saturation ‘s’ is defined as the ratio of actual concentration to solubility; s

The equation can be simplified as                   
Thus nucleation depends strongly both on concentration and temperature. There is a critical super saturation concentration below which nucleation is very slow and above which nucleation is very fast.

There are several mechanisms of crystal growth and most of these lead to the simple equation of growth rate, .  The ‘k’ is the kinetic coefficient, ‘c’ is the actual concentration and   ‘ceq’ is the equilibrium concentration. The exponent ‘n’ is usually only 1-2 and often close to 1.
Hence the dependency of the crystallite growth rate on concentration is closer to a linear function while nucleation rate increases exponentially with concentration. Therefore high super-saturation level promote nucleation rather than crystal growth and favor the precipitation of highly dispersed materials. In contrast precipitation from the more dilute solution tends to produce fewer but larger crystals.

Apart from nucleation and crystal growth, aggregation is also an important step. Aggregation leads to fewer and larger but yet porous particles. It is the formation of clusters of nano-scale primary particles into micrometer scale secondary particles. Physical and chemical forces can hold these particles together. Porosity is then determined by how the particles are stacked and the pores are considered as void spaces between the primary particles. Because of very high super-saturation during the precipitation of most base metal hydroxides or carbonates, nucleation is spontaneous.

Process variation

Precipitation process can be carried out in different ways. The process can be carried out either in batch mode or in continuous mode. The other process variation that affects the precipitate properties is the sequence of addition of the starting materials.

In a batch process, the salt solution from which the metal hydroxide is to be precipitated is taken in a vessel and the precipitating agent is added. The advantage of this method is its simplicity. However, variation of batch composition during precipitation process is a major limitation. This can lead to differences in the properties of the precipitate formed in the initial and final stages. The continuous process involves continual simultaneous addition of salt solution and precipitating agent to a vessel with simultaneous withdrawal of precipitate. This process has a higher demand on process control. All the parameters (pH, temperature, concentration, residence time) can be controlled as desired.

The order of addition of starting materials also affects the final properties of the precipitated catalysts. Different schemes of addition of starting materials in precipitation process is shown in Fig. 12.When metal solution is added to the precipitating agent, the product tend s to be homogeneous since the precipitating agent is present in large excess. This process is particularly important in co-precipitation as it give more homogeneous product than the process where the precipitating agent is added to a mixed metal solution. In the latter case, the hydroxide with lower solubility tends to precipitate first, resulting in formation of non-homogeneous product. Simultaneous addition of both reagents to a buffer solution of constant pH results in better homogeneity and process control. In this process, ratio of metal salt and precipitating agent can be controlled. However, product at the start and at the end may vary due to change in concentration of other ions that are not precipitated. These counter ions tend to occlude in larger extent in final products. Aging is also longer for final products. Aging represent time of formation of coprecipitated and its separation from solution. Aging results in change in structure and properties of hydroxide network. Aging leads to more crosslinked network.

Fig.  12. Different schemes of addition of starting materials in precipitation process

Advantages and disadvantages: The main advantages of the precipitation process is the possibility of creating pure and homogenous material. However the major disadvantages include necessity of the product separation after precipitation and generation of the large volume of salt containing solutions. There is also difficulty in maintaining a constant product quality throughout the whole precipitation process if the precipitation is carried out discontinuously.

Process parameters

In addition to the process variations discussed above there are many other parameters that affect the final product properties as shown in Fig.12. The properties of the final product that are affected include phase formation, chemical composition, purity, particle size, surface area, pore size and pore volumes. It is necessary to optimize the parameters to produce desired products.

Fig. 13. Parameters affecting the properties of the precipitate

Effect of raw materials: The precursors are usually chosen with counter ions that can easily be decomposed to volatile products during heat treatment steps. Nitrates and carbonate salts are preferably used as metal precursors whereas ammonia or sodium carbonate as the precipitating agent. Chloride and sulphates ions act as poisons in many catalytic processes. Such ions should be avoided in the precipitation process. However if the precipitation is needed to be carried out in the presence of these ions then repeated washing steps are necessary to remove these ions from the precipitate.

The nature of the counter ions present in the solution can also influence particle morphology, particle sizes and phase distribution.  It has been observed that preparation of MoO3 from  Na2MoO4 precursor salt results in small particles with relatively high surface area whereas use of (NH4)6Mo7O24 as precursor salt results in larger particles.

Effect of pH: pH directly control the degree of super saturation and hence is expected to affect the final properties. But the influence of pH is not simple and has to be investigated experimentally for the specific system. In aluminium oxide system, the precipitation pH is one of the parameters that determines the phase formation. In general it has been found that precipitation in alkaline medium (pH > 8) leads to the formation of bayerite (β-Al(OH)3), while precipitation in more acidic conditions favors formation of boehmite (γ-AlO(OH)).

Effect of concentration and composition: It is desirable to precipitate at high concentration levels of metal ions. This increases the space time yields by decreasing the vessel volume for the same mass of precipitate. Moreover higher degree of super-saturation leads to faster precipitation. Higher concentration levels also results in smaller particle size and higher surface areas due to increased nucleation rates.

If the catalysts are prepared by co-precipitation, the composition of solution determines the composition of the final product. Deviation from solution composition generally occurs if the solubilities of the different components differ significantly and the precipitation is not complete. The precipitation can be carried out simultaneously or sequentially. When the solubility of the components is not too different, then the precipitation will occur almost simultaneously. However, if the solubility of the components differs significantly then the component with lower solubility will preferentially precipitate resulting in sequential precipitation.

Effect of solvent:For preparation of bulk catalysts and supports, water is almost exclusively used as the solvent for economic reason. Organic solvents are much more expensive to use. Further more, solubilities of most metal salts used as the precursors are lower in organic solvents. Organic solvents are also environmentally hazardous. So use of organic solvents is very limited. These are used only in specific cases where product quality obtained is better by using organic solvent.

Effect  of temperature: The precipitation temperature is a decisive factor in controlling precipitate properties such as primary crystallite size, surface area and the phase formed. Till date it is very difficult to predict the exact nature and extent of effect of the precipitation temperature on the properties and is generally determined experimentally. Nucleation rates are extremely sensitive to temperature. In general, most precipitation process es are carried out above room temperature, often close to 373 K for obvious reason that the precipitation is more rapid. A higher temperature may result in an increase in crystallite size, though this depends on the kinetics of different elementary processes. Sometimes, no effect of temperature or even lowering of size of the crystallites is observed as in the case of ZnO system. Temperature also affects the phase formation. During preparation of Ni/SiO2 catalysts, at high temperatures nickel hydro-silicate is obtained while at lower temperatures, the main precipitate is nickel hydroxide.

When use of high temperature is detrimental, the rotary evaporator is often used to remove the solvent from slurry solution. Rotary evaporator is a vacuum evaporator in which pressure is lowered above the slurry so that boiling point of the solvent is reduced and it can be removed without using excessive heating. In the evaporator a rotating evaporating flask is connected to vapor duct to draw off the vapor and thereby, reduce the pressure within evaporator system. Sample solution is gently heated in bath, usually water bath, to enhance the solvent removal. The separated solvent vapor can be condensed back using a condenser and collected in a separate flask.

Effect of Additives: Additives are substance which are not a necessary ingredients of a precipitation reactions. The properties of the precipitates can strongly be influenced by additives.  The most widely used additives are organic molecules which are added to the precipitate in order to control the pore structure. Such organic molecules can later be removed from the precipitate in the calcination step.

A very promising route for the preparation of the high surface area oxides is the use of surfactants as additives. Removal of the surfactant by calcinations steps leaves a well defined pore network. The pore diameter can be adjusted in the range of 2-10 nm. These all are treated as trade secrets and details are not available in the public domain.[28]

III.2.2 Synthesis

Mixed oxide support and catalyst can be prepared by coprecipitation method. As discussed earlier, for coprecipitation, the solubility of the two components should be in similar range for simultaneous precipitation resulting in homogeneous product. Otherwise the precipitation will be sequential resulting in non-homogeneous product.

For the co-precipitation method (COP), Ce(NO3)3·6H2O and ZrOCl2·8H2O were dissolved in distilled water under magnetic stirring and then heated at 60 °C. Cerium and zirconium hydroxides were precipitated by adding dropwise ammonia solution up to pH= 10 and the suspension was stirred for 4 h. The solid precursor was washed with distilled water several times before aging at room temperature for 48 h, and drying at 120 °C for 24 h [29].

III.2.3. Characterization

The structural and the textural chacterization of support synthesized by Copp method were made in comparation with the support synthesized by EISA method.

Descriere CeO2-ZrO2 comparatie cu EISA – articol publicat, cap V

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