Amorphous citrate methods

In terms of application, the high specific surface area and the nano-size obtained from FH provide catalytic materials with improved resistance to high temperature treatment As an example, the methane combustion was used as a probe reaction. Although LaMnOs and LaCoOs+g prepared by the amorphous citrate method had lower temperature of methane oxidation, the corresponding catalysts prepared by FH demonstrated higher resistance to sintering after forced exposiu-e to reaction conditions at 800 °C for 24 h [76]. The catalysts prepared by the citrate method possess small particles obtained at low calcination temperature, for example, 600-700 °C. Thermal resistance is obtained only after calcination at 850-900 °C but at the expense of the specific surface area, whereas an ideal compromise is obtained by FH. 

Pd-doped catalysts have been produced by USS [82]. The fingerprint of Pd adopting the octahedral coordination of Fe in LaFeo,95Pdo,o503 has been observed in the XANES spectra of the material prepared by spray synthesis (27m /g) similarly to the preparation by the amorphous citrate method (14m /g) [17,82]. In contrast, the flame-made material of the same composition (22m /g) exposed metallic Pd particles on LaFeOs similarly to preparation by solution combustion. The different nature of the Pd species obtained by changing the synthesis method dramatically influences their catalytic performance, since PdO nanoparticles exposed at the surface of the mixed oxide exhibit catalytic activity, whereas Pd—O species in the bulk of the mixed oxide are inactive, at least in the case of methane oxidation [27]. In contrast to LaFeOs, LaMnOs did not allow Pd to adopt the octahedral coordination irrespective of synthesis method. Therefore, the coordination of Pd strongly depends on both the composition of the perovskite-type oxide and the synthesis method. 

Popa et al. [158] compared the synthesis of the perovskite LaCoOs by PCR with the non-polymerizable route, using citric acid as a complexing agent, without the use of ethylene glycol (also known as the amorphous citrate method). According to the X-ray diffraction results, the materials prepared by the amorphous citrate method showed some minor unidentified diffraction peaks, in addition to LaCoOs... 

To prepare a high surface area amorphous phosphate precursor AIPO4, the citrate method was used [6]. To reach an Al/P ratio fixed at 1, 0.667 mole of A1(N03)3-9H20 (Merck) and 0.667 mole of (NH4)H2P04 (Merck) were dissolved... 

Teraoka et al. [41, 42] have applied the amorphous citrate process to prepare unsupported (or neat) and supported perovskites of the type LaMn, LaCo, LaMnCu, LaCoFe, LaCaCo, LaCaMn, LaSrMn, LaSrCo, LaSrCoCu, and LaSrCoFe. The use of the citrate method for preparing atomically homogeneous oxides on supports has been a very important extension of the citrate method over the years. Nitrates of constituent metals of the required perovskite were dissolved in water and mixed with an aqueous solution of citric acid (molar ratio of citric acid to total metals 1 1). Water was evaporated from the mixed solution... 

Le et al. (2006) employed "amorphous citrate" gel method to prepare single phase ceramic powders of LaNiOs with controlled grain size by thermal annealing. The target product was obtained at 650 °C with particle sizes from 30 to 65 ran at annealing temperatures from 650 to 750 °C, while the crystalline sized varied from 10 to 15 ran. Accordingly, a wide... 

This study focused on the deactivation of the Mn/Ce catalysts during reaction. The catalytic oxidation of phenol in aqueous solution to carbon dioxide, water and other side-products was selected as the test reaction. Catalysts were prepared from amorphous precursors using the citrate method and controlling the calcination temperature. Activity performance as a function of the time on stream was studied by simultaneously analyzing the conversion of phenol, the total organic carbon content of the catiyst, the cations eluted and the elemental composition of both cerium and manganese. Experimental conditions were widely varied. Fresh and used catalysts were also analyzed by BET surface area, X-Ray Diffraction and X-Ray Photoelectron Spectroscopy. 

Citrate method The preparation procedure consisted in the dissolution of the metal salts, namely Mn(N03)2 4H20 and Ce(N03)3 6H20, in distilled water, the complexation of the metallic cations with citric acid and the rapid concentration of the liquid by evaporation under vacuum. The viscous liquid was dried at 80°C and the amorphous precursor obtained was decomposed in air prior to calcination. Owing to the complexing property of the citrate anion, this procedure leads to the formation of finely dispersed two-phase systems or favours the formation of mixed oxide phases upon calcination, when these exist. Samples were calcined for 5 hours at 200, 300,400 or 500°C. 

All the perovskites were prepared by the method of the amorphous citrate percursor [12] and were present as pure phases with no contaminants of Ln,0, or cobalt oxides according to the powder XRD analyses. 

Citrate method The amorphous citrate precursors were prepared using a similar procedure as given by Courty et al. [5]. Adequate amounts of La(N03)3-6H20 and A1(N03)3-9H20, from FLUKA (puriss p.a.), were dissolved in deionized water to obtain a 1.25 M solution. Citric acid monohydrate (FLUKA, puriss p.a.) was added to the solution, with a molar ratio citric acid / total cations = 1. The solution was concentrated in a rotary evaporator at 330 K and 20 mbar for 3 h. The viscous and vitreous product was finally dried in a vacuum stove at 370 K and 90 mbar for 20 h. After drying, a foaming precursor was obtained, which was calcined in air at 1070 K for 8 h, at 1170 for 12 h, and finally at 1370 K for 8 h. The support materials were crushed and sieved. The size fracrion between 100 and 300 pm was used for catalyst preparation. 

Citrate method is probably the most widespread and the most effective route to create high-surface-area perovskites (Figure 18.4) [23]. Additionally, the decomposition of the amorphous citrate precursors leads to mixed oxides or solid solutions of high homogeneity. Although often considered as a sol-gel method, citrate is a specific route. The comparison of the two mild chemical... 

Baythoun M.S.G., Sale F.R. Production of strontium-substituted lanthanum manganite perovskite powder by the amorphous citrate process. J. Mater. Sci. 1982 17 2757-2769 Bernard M.I.B., Soledade L.E., Santos I.A., Leite E.R., Longo E., Varela J.A. Influence of the concentration of 86203 and the viscosity of the precursor solution on the electrical and optical properties of Sn02 thin films produced by the Pechini method. Thin Solid Films 2002 405 228-233... 

One other notable method has been used in the preparation of mixed transition metal molybdates, amongst many other oxide systems. This novel method(TT) involves preparation of the mixed metal oxides via an amorphous precursor such as a citrate salt of the appropriate metals, and then thermal decomposition of the complex to yield the resulting mixed oxides. The experimental procedures are described in four French patents(78-81), giving details of many different preparations including a proposed M0O3 rich, chromium doped iron molybdate, prepared as a possible selective oxidation catalyst. 

The citrate gel method was developed by Marcilly et al. (99) and can be illustrated by the synthesis of the ceramic superconductor YBa2Cu307 (100). Nitrate solutions of Y, Ba, and Cu were added to citric acid solution, and the pH was kept at 6 to prevent precipitation of barium nitrate. Heating the solution at 75°C in air produced a viscous liquid containing polybasic chelates. Further heating at 85°C in a vacuum produced an amorphous solid that was pyrolyzed in air at 900°C to produce a crystalline powder. 

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