TiO2, TiO2/Ag and TiO2/Au photocatalysts prepared by spray pyrolysis

TiO₂, TiO₂/Ag and TiO₂/Au photocatalysts exhibiting a hollow spherical morphology were prepared by spray pyrolysis of aqueous solutions of titanium citrate complex and titanium oxalate precursors in one-step. Effects of precursor concentration and spray pyrolysis temperature were investigated. By subsequent heat treatment, photocatalysts with phase compositions from 10 to 100% rutile and crystallite sizes from 12 to 120 nm were obtained. A correlation between precursor concentration and size of the hollow spherical agglomerates obtained during spray pyrolysis was established. The anatase to rutile transformation was enhanced with metal incorporations and increased precursor concentration. The photocatalytic activity was evaluated by oxidation of methylene blue under UV-irradiation. As-prepared TiO₂ particles with large amounts of amorphous phase and organic residuals showed similar photocatalytic activity as the commercial Degussa P25. The metal incorporated samples showed comparable photocatalytic activity to the pure TiO₂ photocatalysts.     

LASER RAMAN SPECTROSCOPY OF PRECIPITATES: APPLICATIONS IN CATALYSIS

Precipitates are important precursors in the preparation of catalysts, pigments, fillers, and other metal oxides and sulfides. Despite the extensive investigations of these colloidal systems, the mechanism of precipitate formation and particle growth remains unclear. The processes involved during precipitation are complex and are sensitive to many environmental factors. The solutions of the metal salts used in the precipitation also may be complex, particularly for mutli-valent ions. Precipitate solutions are extremely difficult to characterize in situ with most spectroscopic techniques. The use of Raman spectroscopy is highly advantageous since precipitates may be examined in aqueous solution without interference from the presence of water.

A series of bismuth molybdate selective oxidation catalysts were characterized during preparation using in situ Raman spectroscopy. Spectra were recorded during the precipitation and aging of stoichiometric γ-Bi₂MoO₆ and Bi₂Mo₃O₁₂ catalysts. The formation of these phases was related to the slate of aggregation of molybdenum and bismuth species. Various techniques were used to obtain the Raman spectra of the precipitates, which are briefly described.     

Microstructure and synthesis mechanism of dysprosia-stabilized zirconia nanocrystals via chemical coprecipitation

In this study, zirconia (ZrO₂) and dysprosia-stabilized zirconia (DySZ) nanocrystals were synthesized using a chemical coprecipitation method. The crystal structure and micromorphology of the as-synthesized powders, as well as the structural evolution from precursors to oxides were investigated, and the synthesis mechanism was also examined. Results show that pure ZrO₂ powders mainly comprise the monoclinic ZrO₂ phase with trace tetragonal ZrO₂, while the DySZ powders exhibit a tetragonal ZrO₂ structure. In addition, the crystal growth rate of DySZ is far slower than that of the pure ZrO₂ under elevated calcination temperature. The addition of Dy could significantly improve the phase stability of DySZ powder and effectively inhibit the crystal growth of DySZ. In the DySZ precursor, the binding energy of chemical bonds is significantly difference than in the ZrO₂ precursor. A composite hydroxide can be formed with -Zr-OH-Dy- and -Zr-OH-Zr- units in the tetramer structure because of the in-situ substitution of Zr by Dy atoms. Both the ZrO₂ and DySZ precursors exhibit analogous dehydration and crystallization behaviours in calcination process. Dy-doping plays a significant role in stabilizing both the intermediate product and the DySZ crystal.     

Methanol decomposition to synthesis gas over supported Pd catalysts prepared from synthetic anionic clays

Supported Pd or Rh catalysts were prepared by the solid-phase crystallization method starting from hydrotalcite anionic clay minerals based on [Mg₆Al₂(OH)₁₆CO ₂ ²⁻ ]·4H₂O as the precursors. The precursors were prepared by a coprecipitation method from the raw materials containing Pd²⁺ and various trivalent metal ions which can replace each site of Mg²⁺ and Al³⁺ in the hydrotalcite. Rh³⁺ was also used for preparing the catalyst as comparison. The precursors were then thermally decomposed and reduced to form supported Pd or Rh catalysts and used for the methanol decomposition to synthesis gas. Among the precursors tested, use of Mg–Cr hydrotalcite containing Pd²⁺ resulted in the formation of efficient Pd supported catalysts for the production of synthesis gas by selective decomposition of methanol at low temperature. Although Pd²⁺ cannot well replace the Mg²⁺ site in the hydrotalcite, the Pd supported catalyst (Pd/Mg–Cr) prepared by the solid-phase crystallization method formed highly dispersed Pd metal particles and showed much higher activity than that prepared by the conventional impregnation method. When the precursor was prepared under mild conditions, more fine particles of Pd metal were formed over the catalyst, resulting in high activity. It is likely that the high activity may be due to the highly dispersed and stable Pd metal particles assisted by the role of Cr as the co-catalyst. This revised version was published online in November 2006 with corrections to the Cover Date.     

Methanol Decomposition to Synthesis Gas Over Supported Pd Catalysts Prepared from Hydrotalcite Precursors

Supported Pd catalysts were prepared by solid-phase crystallization (spc) starting from MgAl hydrotalcite anionic clay minerals as the precursors, and were tested for the methanol decomposition to synthesis gas. The precursors based on [Mg₆Al₂(OH)₁₆CO₃ ^2-] · 4H₂O were prepared by coprecipitation from raw materials containing Pd²⁺, Mg²⁺ and Al³⁺ ions as the components of hydrotalcite. The precursors were thermally decomposed and reduced to afford supported Pd catalysts on MgAl mixed oxide. Pd-supported catalysts as a reference were also prepared by the impregnation (imp) method. The spc-Pd catalysts thus prepared afforded highly dispersed Pd metal particles and showed higher activity as well as lower activation energy than the imp-Pd catalysts. When the precursor was prepared under mild conditions, finer particles of Pd metal were formed over the catalyst, resulting in a high activity. In the case of spc-Pd catalysts, carbon monoxide adsorbed on Pd easily desorbed, compared with imp-Pd catalysts. It is likely that the high activity is due to the highly dispersed and stable Pd metal particles and the easy desorption of carbon monoxide.     

Synthesis of Complex Oxides of Rare Elements of Groups IV and V (A Review)

The prospects of using various methods of synthesis for producing extra-pure complex oxides of alkali and bivalent elements with rare elements of groups IV and V of precisely prescribed chemical and granulometric compositions are analyzed. It is demonstrated that only the sol-gel method with inorganic precursors yields nanosize powders of narrow granulometric classes. Epitaxial coatings based on complex oxides can be synthesized by the sol-gel method using organic precursors or true solutions of reactant components. __________ Translated from Steklo i Keramika, No. 8, pp. 14 – 18, August, 2005.     

Electrochemical performance and local cationic distribution in layered LiNi1/2Mn1/2O2 electrodes for lithium ion batteries

LiNi_1/2Mn_1/2O₂ electrodes with layered structure were synthesized by solid-state reaction between lithium hydroxide and mixed Ni,Mn oxides obtained from co-precipitated Ni,Mn carbonates and hydroxides and freeze-dried Ni,Mn citrates. The temperature of the solid-state reaction was varied between 800 and 950 °C. This method of synthesis allows obtaining oxides characterized with well-crystallized nanometric primary particles bounded in micrometric aggregates. The extent of particle agglomeration is lower for oxides obtained from freeze-dried Ni,Mn citrates. The local Mn⁴⁺ surrounding in the transition metal layers was determined by X-band electron paramagnetic resonance (EPR) spectroscopy. It has been found that local cationic distribution is consistent with α,β-type cationic order with some extent of disordering that depends mainly on the precursors used. The electrochemical extraction and insertion of lithium was tested in lithium cells using Step Potential Electrochemical Spectroscopy. The electrochemical performance of LiNi_1/2Mn_1/2O₂ oxides depends on the precursors used, the synthesis temperature and the potential range. The best electrochemical response was established for LiNi_1/2Mn_1/2O₂ prepared from the carbonate precursor at 900 °C. The changes in local environment of Mn⁴⁺ ions during electrochemical reaction in both limited and extended potential ranges were discussed on the basis of ex situ EPR experiments.     

A green and economical route to the precursor for the synthesis of single crystal LiNi0.5Co0.2Mn0.3O2

Single crystal LiNi_1-x-yCo_xMn_yO₂(NCM) cathode materials are typically synthesized using spherical polycrystalline hydroxides which are often prepared via coprecipitation reactions. However, the spherical morphology of polycrystalline hydroxides is not essential for the precursor in the synthesis of single crystal NCM, and also the coprecipitation process is not environmentally friendly and cost-effective enough. Herein a new process based on room-temperature solid-state metathesis reactions is developed to prepare the precursor for the synthesis of single crystal LiNi_0.5Co_0.2Mn_0.3O₂(NCM523). The whole process is free of any undesirable chemicals, and the resulting nanosize precursor can facilitate the synthesis of micron-level single crystal NCM523 at relatively lower sintering temperatures with less lithium excess. Moreover, the obtained single crystal NCM523 can exhibit comparable reversible capacities as compared with that synthesized from the coprecipitated spherical polycrystalline hydroxides. This work demonstrates a green and economical route to the precursor for the synthesis of single crystal NCM.     

Novel nanocasting method for synthesis of ordered mesoporous metal oxides

Novel one-step high vacuum nanocasting method is developed to prepare ordered mesoporous rare earth and transition metal oxides with high-surface area using mesoporous KIT-6 as hard templates. Characterization of these mesostructured materials was performed by the X-ray powder diffraction, high resolution transmission electron microscopy and N₂ adsorption–desorption isotherms. In this synthesis method, metal oxide precursors are effectively filled into the channels of hard templates by a one-step high vacuum impregnation, and the excess precursors can be removed through vacuum filtration. It is a very effective nanocasting method and has the wider application in the preparation of mesoporous metal oxides with high surface area and high purity.     

Electrochemical synthesis of perovskite oxides

Hydroxide precursors to perovskite oxides of the formula, LnMO₃ (Ln = La, Pr, Nd; M = Al, Mn, Fe) and LaMO₃ (M = Co and Ni) were synthesized by electrogeneration of base by cathodic reduction of the appropriate mixed-metal nitrate solution. The precursors were heat treated at different temperatures to obtain the perovskite oxides. The bath composition for various systems was optimized to get a single-phase oxide product. This method can be adapted as a simple route to the synthesis of perovskite oxide coatings on conducting substrates.     

Biotechnological fabrication of LaMnO3-carbon catalyst for n-butanol conversion to ketones

An attempt to synthesize a hybrid carbon-based material for catalytic purposes was made using a biotechnological approach. The material consists of a carbon matrix in which LaMnO₃ nanocrystallites are dispersed. The perovskite-type oxide crystallites were synthesized in the carbon matrix by the transformation of lanthanum and manganese salts, which penetrated the living tissues of a plant due to the natural transport of metal ions. Cellulose/lignin rich plant stems were carbonized at 600-800 °C for 1-22 h, yielding a microporous carbon matrix in which nanocrystallites of the complex oxide were uniformly distributed. The plant (Salix viminalis) was selected because of its high tolerance to heavy metal ions. The formation of carbon matrix from organic precursor is a key factor in the synthesis of the complex inorganic nanocrystallites of high dispersion. The hybrid carbon-based materials were tested as catalysts for unconventional conversion of n-butanol to heptanone-4. The catalysts exhibited high selectivity and product yield, despite the extremely low content of LaMnO₃ (0.3% atomic). The paper proves that catalysis-oriented functionalization of carbons may begin at a very early stage through chemical modification.     

10-mol%-Gd2O3-Doped CeO2 Solid Solutions via Carbonate Coprecipitation: A Comparative Study

10mol% Gd₂O₃-doped CeO₂ solid solutions (20GDC) have been synthesized via carbonate coprecipitation using ammonium bicarbonate (AHC) and urea as the precipitants. The precursors and the resultant oxide powders were characterized via chemical analysis, X-ray diffractometry (XRD), Brunauer–Emmett–Teller (BET) analysis, and high-resolution scanning electron microscopy (HRSEM). Sinterabilities of the 20GDC oxides in air were studied by constant-rate-of-heating (CRH) sintering and the conventional ramp-and-holding sintering methods. The precursor processed by both methods is hydroxyl carbonate but shows quite different particle morphologies in the two cases. Highly sinterable 20GDC oxides that can be densified to >99% of the theoretical at 1050°C within 4 h have been obtained via the AHC method.     

Catalytic reduction of N2O by ethylbenzene over novel hydrotalcite-derived Mg–Cr–Fe–O as an alternative route for simultaneous N2O abatement and styrene production

New hydrotalcite-like materials containing magnesium, chromium, and/or iron were synthesized by the coprecipitation method and then thermally transformed into mixed metal oxides. The obtained catalysts were characterized with respect to chemical composition (XRF), structural (XRD, Mössbauer spectroscopy) and textural (BET) properties. The catalytic performance of the hydrotalcite-derived oxides was tested in the N₂O decomposition and the N₂O reduction by ethylbenzene. An influence of N₂O/ethylbenzene molar ratio on the process selectivity was studied. The relationship between catalytic performance and structure of catalysts was discussed.     

A novel 2D polymeric structure of Cu–Na complex, in which phthalate gives seven coordination sites

A novel copper and sodium hetero-nuclear complex {[(H₂O)Na]₂Cu(phth)₂}_n (phth=phthalate) was synthesized and its crystal structure has been determined by X-ray diffraction methods. The complex structure consists of two-dimensional (2D) infinite layers, in which it is first observed that one phthalate ligand coordinates to five metal ions and gives seven coordination sites.     

Metal-Controlled Assembly of Two Coordination Polymers Built from 4,4′-Methylenedibenzoic Acid with or Without Methyl-Functionalized N-Donor Ligand

By changing the metal salts, two new transition metal-based coordination polymers (CPs), {[Co₃(DBA)₃(dmbpy)]·(DMF)·(H₂O)₄}_n (1), and {[(Me₂NH₂)₂Cd(DBA)₂]·(DMF)₃}_n (2) ((H₂DBA = 4,4′-methylenedibenzoic acid, dmbpy = 3,3′-dimethyl-4,4′-bipyridine) has been synthesized under the same reaction conditions (DMF and 130 °C) and were structurally characterized. Both compounds are synthesized using dual linkers (H₂DBA and dmbpy). 1 performs a 2-fold interpenetration 3D framework with pcu topology and is stable in aqueous solutions at pH range from 5 to 9, whereas 2 shows a 2D (4,4) network and is unstable in air. Furthermore, the magnetic and 3D luminescent properties of 1 and 2 are also investigated, respectively. The results suggest that metal salts influence on the final resulting CPs and the introduction of water repellent functional groups within the structure can largely enhance the hydrophobic properties of CPs.     

Study by high-throughput experimentation of the effect of the pretreatment and precursors on the catalytic activity of tungstated zirconia catalysts

High-throughput experimentation (HTE) was used for the synthesis, characterization and catalytic evaluation of tungstated zirconia (WZ) materials doped with manganese. A library with three series of MnO_y–WO_x–ZrO₂ catalysts was prepared by different methods (surfactant-assisted coprecipitation and impregnation), using different precursors and pretreatments (washing and/or aging). These materials were characterized by XRD, Raman and MET (HRTEM and EDS). Different nanostructures were observed depending on the synthesis method. Highly dispersed WO₃ catalysts calcined at 800 °C were obtained by coprecipitation method, while the impregnation method brings the segregation of both, the monoclinic WO₃ and ZrO₂ phases. The catalytic activity of the Pt-promoted MnO_y–WO_x–ZrO₂ catalysts (0.3 wt.% Pt) was evaluated in the n-hexane hydroisomerization reaction. Although, in the coprecipitated catalysts similar WO_x nanostructures were observed, the catalytic activity mainly depends on the zirconia precursor, as well as aging and washing pretreatments, following the order: ZrOCl₂ (coprecipitation) > ZrO(NO₃)₂ (coprecipitation)  Zr(OH)₄ (impregnation). The catalysts with highest activity were pretreated by washing and aging.     

Review of SO2-4/MxOy solid superacid catalysts

Some metal oxides modified with sulfate ions form highly acidic or superacidic catalysts. SO₂₋⁴/M _x O _y solid superacid catalysts, play a vital role in more and more fields such as organic synthesis, fine chemicals, pharmaceuticals, and means for strengthening environmental safeguards. This review highlights the recent development of solid superacid catalysts based on SO₂₋⁴/M _x O _y , including synthesis method, characterization of acid sites and acid strength, and applications.     

Synthesis and Investigation of Solid Solutions Based on Indium Oxide in In<Subscript>2</Subscript>O<Subscript>3</Subscript>–MeO<Subscript>2</Subscript> (Me = Zr,Sn, Ti) Systems

Solid solutions in In₂O₃–MeO₂ (Me = Zr, Sn, Ti) systems based on indium oxide are synthesized by the coprecipitation method. It is found that the ultrasound treatment of the coprecipitation products reduces the degree of agglomeration of the initial particles by a factor of 3 and initiates the crystallization process of the precipitates. Nanocrystal (5–8 nm) precursor powders are obtained at 400°C. The optimal regime for sintering powders based on In₂O₃ to form a ceramic with a dense microstructure is chosen. The influence of the temperature, alloying additives, and the partial pressure of oxygen on the specific conductivity of indium oxide solid solutions is studied.     

Synthesis of nanosized (Pb,Sr)TiO3 perovskite powders by coprecipitation processing

Nanosized perovskite lead strontium titanate (Pb_xSr_1−x)TiO₃ (PST) powders were successfully prepared by a simple coprecipitation method. Lead-strontium titanyl oxalate (PSTO) precursor was first synthesized at room temperature, and the precursor was then calcined at 600 °C for 1 h to produce the single phase perovskite PST powders. Characterization studies were carried out on the as-dried precursor and the calcined PST powders by various techniques. The results showed a strong dependence of the chemical composition of final PST powders on pH value in the coprecipitation reaction. PST powders with desired composition could be synthesized by adding 25 mol% excess Sr. PST particles were found to be spherical in nature with an average size of 10 nm.