Flame Aerosol Synthesis Research Articles

ChemInform Abstract: Flame Aerosol Synthesis of Metal Oxide Catalysts with Unprecedented Structural and Catalytic Properties

AbstractReview: 178 refs.

Gas-phase photocatalytic activity of nanostructured titanium dioxide from flame aerosol synthesis

The experimental evaluation of gas-phase photocatalytic activity of a TiO2 nanopowder synthesized in a flame aerosol reactor was carried out in photocatalytic oxidation (PCO) of volatile organic compounds (VOCs). The nanopowder has an average particle size of 13nm, anatase content 97wt.% and the specific surface area of 102m2g−1. The performance was compared to the benchmark photocatalyst, the commercial pyrogenic titania P25, Evonik, with the average particle size of 21nm. The full-factorial experiments were carried out varying contact times, concentrations of pollutants and temperatures in continuous gas-flow mode degrading aliphatic acrylonitrile (AN) and aromatic toluene. Higher conversions at more stable performance were observed for the flame aerosol synthesized photocatalyst in degradation of both pollutants. While the primary particle size and specific surface area present the apparent reasons for improved PCO performance in adsorbable AN, these parameters cannot do the same in oxidation of poorly adsorbable toluene: the superior generation of hydroxyl radicals and, therefore, advanced oxidative activity are proposed as explanation. The intense dehydration of reduced size anatase crystallites at elevated temperature (130°C) presumably resulted in decreased OH-radicals production along with the improved desorption of HCN, the PCO by-product of AN. The safe performance is thus requiring lower operational temperatures. Slower deactivation and faster restoration of catalytic activity of flame aerosol synthesized catalyst under UV-A-radiation are discussed.

Simulation of nanoparticle synthesis in an aerosol flame reactor using a coupled flame dynamics–monodisperse population balance model

Flame aerosol synthesis is one of the commonly employed techniques for producing ultra fine particles of commodity chemicals such as titanium dioxide, silicon dioxide and carbon black. Large volumes of these materials are produced in industrial flame reactors. Particle size distribution of product powder is the most important variable and it depends strongly on flame dynamics inside the reactor, which in turn is a function of input process variables such as reactant flow rate and concentration, flow rates of air, fuel and the carrier gas and the burner geometry. A coupled flame dynamics–monodisperse population balance model for nanoparticle synthesis in an aerosol flame reactor is presented here. The flame dynamics was simulated using the commercial computational fluid dynamics software CFX and the particle population dynamics was represented using a monodisperse population balance model for continuous processes that predicts the evolution of particle number concentration, particle volume and surface area. The model was tested with published experimental data for synthesis of silica nanoparticles using different burner configurations and with different reactor operating conditions. The model predictions for radial flame temperature profiles and for the effects of process variables like precursor concentration and oxygen flow rate on particle specific surface area and mean diameter are in close agreement with published experimental data.

Cover Picture: Flame Aerosol Synthesis of Metal Oxide Catalysts with Unprecedented Structural and Catalytic Properties (ChemCatChem 8/2011)

Cover Picture: Flame Aerosol Synthesis of Metal Oxide Catalysts with Unprecedented Structural and Catalytic Properties (ChemCatChem 8/2011)

Characterization of La<SUB>0.8</SUB>Sr<SUB>0.2</SUB>MnO<SUB>3±<I>δ</I></SUB> Nanopowders Synthesized by Aerosol Flame Synthesis for SOFC Cathode

Lanthanum strontium manganite (La(0.8)Sr(0.2)MnO(3 +/- delta), LSM) powders with a high specific surface area (55.26 m2/g) were successfully synthesized by aerosol flame synthesis (AFS) technique. The crystallinity and morphology of the synthesized powders sintered at various temperatures were studied by XRD, TEM and BET. The synthesized powders exhibited spherical shape mostly in a few nanometer ranges with a relatively high crystallinity due to thermal plasma reactions in a high temperature of oxy-hydrogen flame. To analyze electrochemical performances of synthesized LSM powders, impedance spectroscopy (IS) was carried out with the symmetric cells prepared by slurry based electrostatic spray deposition (ESD) onto the YSZ electrolyte pellet. The interfacial polarization resistances were 3.04 ohms cm2 at 750 degrees C which is relatively lower than that of micro-porous film (7.24 ohms cm2) applying micro-sized powders deposited on same condition.

Continuous flame aerosol synthesis of carbon-coated nano-LiFePO 4 for Li-ion batteries

Core–shell, nano-sized LiFePO 4-carbon particles were made in one step by scalable flame aerosol technology at 7 g/h. Core LiFePO 4 particles were made in an enclosed flame spray pyrolysis (FSP) unit and were coated in-situ downstream by auto thermal carbonization (pyrolysis) of swirl-fed C 2H 2 in an O 2-controlled atmosphere. The formation of acetylene carbon black (ACB) shell was investigated as a function of the process fuel-oxidant equivalence ratio (EQR). The core–shell morphology was obtained at slightly fuel-rich conditions (1.0<EQR<1.07) whereas segregated ACB and LiFePO 4 particles were formed at fuel-lean conditions (0.8<EQR<1). Post-annealing of core–shell particles in reducing environment (5 vol% H 2 in argon) at 700 °C for up to 4 h established phase pure, monocrystalline LiFePO 4 with a crystal size of 65 nm and 30 wt% ACB content. Uncoated LiFePO 4 or segregated LiFePO 4–ACB grew to 250 nm at these conditions. Annealing at 800 °C induced carbothermal reduction of LiFePO 4 to Fe 2P by ACB shell consumption that resulted in cavities between carbon shell and core LiFePO 4 and even slight LiFePO 4 crystal growth but better electrochemical performance. The present carbon-coated LiFePO 4 showed superior cycle stability and higher rate capability than the benchmark, commercially available LiFePO 4.

Flame Aerosol Synthesis of Metal Oxide Catalysts with Unprecedented Structural and Catalytic Properties

AbstractIn the past two decades flame aerosol synthesis of novel materials has experienced significant growth in both industry and academia. Recent research is focused on the development of new materials in the nanosized range to be used in various applications, such as catalysts, gas sensors, pigments, and batteries. Several studies indicate that this scalable synthesis method can result in novel and metastable phases of mixed metal oxides of high purity, which may not be easy accessible by conventional wet‐ or solid‐state processes. Especially for catalytic applications this synthesis method is emerging as an attractive fast and single‐step production route for high surface area materials, often with unprecedented structural and catalytic properties. The large variety of possible organometallic precursors especially for the liquid‐fed aerosol flame synthesis makes this technique very versatile for catalyst synthesis.Using the example of the widely used vanadia‐based mixed oxide catalysts, we analyze the structural and catalytic properties of flame‐derived catalysts and compare them to corresponding catalysts prepared by classical wet‐chemistry methods. The often unique structural properties along with their control at proper synthesis conditions and their influence on catalyst performance in selected reactions are discussed. Subsequently, we give an overview of other recent flame‐made mixed metal oxide based catalysts and make an attempt to assess the potential and limitations of flame synthesis for the preparation of catalytic mixed metal oxide materials, and finally we identify future challenges in research.

Photocatalytic activity of nanostructured titanium dioxide from diffusion flame synthesis

Vapour fed flame aerosol synthesis has been employed to synthesize TiO 2 photocatalyst by oxidation of titanium tetrachloride (TiCl 4). The effect of precursor loading on particle growth and rutile phase formation was studied using laminar premixed flame with (Ar + TiCl 4)/CH 4/O 2/N 2 configuration and synthesis temperature of around 900 °C. The increase in precursor flow rate, as key synthesis parameter, from 2.5 to 29.5 mmol h −1 increased the average particle diameter, d BET , from 11 to 22 nm and decreased the specific surface area from 145 to 71 m 2 g −1 respectively. The samples were anatase-rich (up to 98 wt.%) and of increasing rutile fraction, when the primary TiO 2 particles exceeded the critical size of d c ≈ 11.6 nm. Degradation rate of aliphatic dichloroacetic acid (DCA) and aromatic 4-chlorophenol (4-CP), used as model pollutants for evaluation of photocatalytic activity, was higher for diffusion flame aerosol powders than for commercial pyrogenic TiO 2 products, P25 and P90, Evonik, of comparable sizes and anatase fractions. This effect was attributed to a lower degree of agglomeration resulting in a higher specific surface area. Photocatalytic activity of nanopowders is exclusively coupled with the anatase fraction. A beneficial effect due to accompanying rutile was not detected. The photocatalytic efficiency of nanopowders was found to decrease strongly with increase in average particle sizes larger than d c but was not positively affected by d lower than d c .

Dopant-free, polymorphic design of TiO 2 nanocrystals by flame aerosol synthesis

A dopant-free aerosol synthesis of highly crystalline TiO 2 nanoparticles (20–35 nm) with tunable polymorphic content is demonstrated by rapid flame spray pyrolysis. By controlling precisely the total ambient oxygen partial pressure of the combustion in a quartz tube enclosure, anatase content as high as 96 wt% (4 wt% rutile) was obtained at high oxic flame conditions, while rutile content as high as 94 wt% (6 wt% anatase) was obtained under anoxic flames. The polymorphic variability lies within a narrow range of combustion equivalence ratios, that is, 1.0< Φ<1.5. Unlike any other flame aerosol syntheses, the anatase and rutile crystallite sizes were similar within each sample. Under highly oxic flame conditions ( Φ<1.0), twinnings between anatase {0 1 1} planes could be observed, inferring oriented attachment taking place. Such mechanism could not, however, be seen under anoxic flame ( Φ>1.0) possibly due to physical hindrance by surface carbonaceous content (typically <2 wt%). The carbon content can be easily removed by short calcination without significantly affecting the surface areas and crystallite properties of the original TiO 2 nanocrystals, preserving hence its pristine state.

Modeling and simulation of carbon black synthesis in an aerosol flame reactor

Carbon black has diverse industrial applications such as a reinforcing agent in tires and as a black pigment in printing inks. Flame aerosol synthesis is the commonly employed route for large scale production of carbon black. A coupled flame dynamics – monodisperse population balance model for the synthesis of carbon black in an aerosol flame reactor is presented here. The population balance model was incorporated into the commercial computational fluid dynamics software CFX to simulate the effect of flame dynamics on particle synthesis. The model was tested with published experimental data for acetylene black synthesis through oxidative thermal decomposition of acetylene with oxygen in a premixed flame reactor. The predicted axial temperature profiles at different equivalence ratios (actual acetylene/oxidizer ratio divided by stoichiometric acetylene/oxidizer ratio) were reasonably close to experimental data. The effect of the equivalence ratio on process parameters like maximum temperature of the flame, specific surface area of particles and flame structure was investigated. It was observed that the maximum temperature of the flame and specific surface area of carbon black particles decrease upon increasing the equivalence ratio which is in agreement with published experimental studies. Simulation results are also presented for carbon black synthesis in a diffusion flame reactor with different burner designs.

Structural dependence of the efficiency of functionalization of silica-coated FeOx magnetic nanoparticles studied by ATR-IR

The efficiency of propylamino functionalization of magnetic silica-coated FeOx nanoparticles prepared by different methods, including coprecipitation and flame aerosol synthesis, has been evaluated by attenuated total reflection infrared spectroscopy (ATR-IR) combined with a specific surface reaction, thus revealing the availability of the grafted functional groups. Large differences in the population of reactive groups were observed for the investigated materials, underlining the tight relation between the structure of nanoparticles and their suitability for organic functionalization. The materials possessed different core structure, surface area, and porosity, as evidenced by transmission electron microscopy and nitrogen adsorption–desorption isotherms. Grafting of aminopropyl groups using a standard procedure based on reaction with (3-aminopropyl)trimethoxysilane as source of the propylamino groups was performed, followed by classical dry analysis methods to determine the specific concentration of the organic functional groups (in mmolg−1 of material). ATR-IR spectroscopy in a specially constructed reactor cell was applied as wet methodology to determine the chemically available amount of such functional groups, showing that the materials possess largely different loading capacity, with a variability of up to 70% in the chemical availability of the organic functional group. The amount of (3-aminopropyl)trimethoxysilane used for functionalization was optimized, thus reaching a saturation limit characteristic of the material.

Charge–discharge characteristics of nanocrystalline Co 3O 4 powders via aerosol flame synthesis

Nanocrystalline Co 3O 4 powders were synthesized by aerosol flame synthesis (AFS) method for the anode of lithium ion batteries and the basic electrochemical properties were investigated. The effects of synthesis conditions and heat-treatment temperature on the morphology, crystallite size and electrochemical properties were investigated. As-prepared soot contained Co 3O 4, CoO and Co(OH) 2, which were eventually converted into cubic spinel Co 3O 4 by post heat treatment. The as-prepared particle size was in the range of 10–30 nm and grew to 50–85 nm by the heat treatment. With growing particle size and improved crystallinity, charge–discharge capacity and cycle performance were improved and the discharge capacity of the powder heat-treated at 700 °C was 571 mAh/g after 30 cycles, which was better than Co 3O 4 powder reported in the previous literature.

Flame aerosol synthesis and characterization of pure and carbon coated titania nano powder

Carbon coated titanium dioxide (TiO 2) nano powder has been synthesized by combustion of TiCl 4 precursor in a laminar diffusion flame using inexpensive liquid petroleum gas (LPG) as a fuel, air as an oxidant, nitrogen gas as a carrier, and characterized with regard to phase(s), surface area, carbon content, morphology and optical absorption. The product is shown to contain both the anatase and rutile phases and exhibits (i) decrease in rutile content, (ii) increase in BET specific surface area, and (iii) increase in the amount of carbon (soot) present with increase in fuel flow rate. Further, the maximum attainable temperature depends on carbon content and determines the phase content and morphology of nano powder, e.g. spherical particles result and display reduced agglomeration when carbon content is more. The rutile phase essentially emerges by transformation of the anatase phase, formed initially with lattice parameters somewhat smaller than the bulk due to oxygen deficiency. On the other hand, use of oxygen (instead of air) leads to formation of spherical particles (average diameter ∼104 nm) of a pure anatase phase (as transformation to rutile phase is totally suppressed) with lattice parameters, a=3.776(5) Å, c=9.507(5) Å, close to bulk.

Flame synthesis of TiO 2 nanoparticles with high photocatalytic activity

Growth behaviour of TiO 2 particles, which has significant influence on their crystallinity and surface area, has been controlled by varying the process parameters of the flame aerosol synthesis. An existing method to determine the crystallinity of the powder has been modified which facilitates comparison of the degree of crystallinity of the nanoparticles synthesized by various methods. Resultant TiO 2 nanoparticles were characterized by XRD and nitrogen physisorption to determine the degree of crystallinity quantitatively and surface area, respectively, and the decomposition rate of an industrial dye, methylene blue, has been taken as the measure of the photocatalytic activity (PCA). TiO 2 nanoparticles showed improved photocatalytic activity compared to the commercial photocatalyst, Degussa P25. This is attributed to the increased degree of crystallinity and surface area of the TiO 2 nanoparticles compared to Degussa P25. The results of this study elucidate the importance of the gas-phase processes and strategy to synthesize highly active TiO 2 photocatalysts in a single step. Moreover, difference in the PCA of the nanoparticles synthesized by the similar gas-phase routes has been shown and the particles are tailor-made for improved PCA in the present study.

A dense oxygen separation membrane deriving from nanosized mixed conducting oxide

In this paper, nanosized SrCo 0.4Fe 0.5Zr 0.1O 3− δ (SCFZ) powders, which were synthesized via a flame aerosol synthesis (FAS) method, were successfully used to fabricate oxygen separation membranes with densified structure. XRD, TEM (HRTEM), O 2-TPD and TG were used to characterize the crystal structure, morphology, oxygen desorption property and oxygen non-stoichiometry of SCFZ-FAS powders. The densification process and the oxygen permeability of the SCFZ-FAS membranes were examined by SEM and the high temperature oxygen permeation measurements. The as-produced SCFZ-FAS powders were of the typical perovskite structure with high degree of crystallinity. Hard agglomerations, which were induced from the high temperature of the flame, were found among the nanosized rod-like SCFZ-FAS particles. Compared with SCFZ synthesized by the traditional solid-state reaction (SSR) method, the densification temperature of SCFZ membranes was reduced and the oxygen permeation flux was increased by 40% at the elevated temperatures (1073–1223 K) when SCFZ-FAS powders were used as the starting material. Long-term oxygen permeation measurement (1123 K, 180 h) showed that SCFZ-FAS possessed stable structure under low oxygen partial pressure (about 10 −3 atm) environment.

Synthesis of non-aggregated titania nanoparticles in atmospheric pressure diffusion flames

Flame aerosol synthesis has been employed to synthesize nanoscale titania (TiO 2) particles by oxidation of titanium tetraisopropoxide (TTIP) vapor. The influence of reactant mixing and flow rates on particle morphology, size and phase composition has been studied for two different diffusion flame configurations using transmission electron microscopy, X-ray diffraction and photon correlation spectroscopy. Spherical, loosely agglomerated powders with a minimum secondary particle size of 90 nm and a rutile content of up to 35 w % were obtained at low oxygen flow rates in the double diffusion flame, while large anatase-rich aggregates formed at high oxygen flow rates. It is shown that the degree of aggregation of the as-synthesized particles is represented better by the ratio of d PCS 3/ d TEM 3, than by the ratio of d BET 3/ d XRD 3 commonly used in literature. The differences observed in particle morphology and phase composition can be explained by considering their time–temperature history as a function of flame configuration and gas flow rates.

Titania-supported Pt and Pt–Pd nanoparticle catalysts for the oxidation of sulfur dioxide

Several types of titania (anatase) were used as supports for pure platinum and Pt–Pd bimetallic alloy catalysts. The preparation methods, normal wet impregnation technique and flame aerosol synthesis, obtained metal loadings of 2% by weight. The prepared catalysts were tested for SO 2 oxidation activity at atmospheric pressure in the temperature range 250–600 °C. The SO 2 to SO 3 conversion efficiency of the Pt–Pd alloy was significantly higher than that of the individual metals. The effects of the preparation method and the titania type used on the properties and activity of the resulting catalyst are discussed.

Mixed Phase Pt-Ru Catalyst for Direct Methanol Fuel Cell Anode by Flame Aerosol Synthesis

A spray-flame aerosol catalyzation technique was studied for producing Pt-Ru anode electrodes for the direct methanol fuel cell. Catalysts were produced as aerosol nanoparticles in a spray-flame reactor and deposited directly as a thin layer on the gas diffusion layer. The as-prepared catalyst was found to be a mixture of nanocrystalline, mostly unalloyed Pt and an amorphous phase mostly of Ru and to a lesser extent of Pt oxides on top of the crystalline phase. The flame-produced Pt1Ru1 demonstrated similar onset potential but higher activity compared to commercially available Pt1Ru1/Vulcan carbon. The kinetics of methanol oxidation on the mixed phase catalyst was also explored by electrochemical impedance spectroscopy.

Aerosol synthesis of titania nanoparticles: Effect of flame orientation and configuration

Flame aerosol synthesis was employed to make nanoscale titania particles with controlled size, morphology and crystallinity by oxidation of titanium-tetra-isopropoxide. The effect of upward and downward flame orientation on these particle characteristics was investigated for laminar premixed flame reactors. The burner orientation influenced the product powder crystallinity but had only a minor effect on particle size and morphology. Reactant mixing in four diffusion flame configurations was studied to make titania nanoparticles of 10 to 50 nm average primary particle diameter and 6 to 60 wt-% rutile and the balance anatase. This showed the flexibility of diffusion flame reactors to produce a range of nanoparticle qualities for applications in ceramics, catalysis or pigments.

Flame Aerosol Synthesis of Vanadia–Titania Nanoparticles: Structural and Catalytic Properties in the Selective Catalytic Reduction of NO by NH3

Flame aerosol synthesis has been used to prepare vanadia–titania nanoparticles with high activity for the selective catalytic reduction of NO by NH3. The mixed oxides were prepared from vanadium and titanium alkoxides which were evaporated into an argon stream and burned in a methane oxygen diffusion flame. Silica-containing samples were produced in a similar way by mixing hexamethyldisiloxane vapor into the precursor stream. Different flame structures were investigated for the effect of temperature and residence time on particle morphology, vanadia surface species, and overall catalytic activity. By changing the oxygen flow rate into the flame, particles with specific surface areas between 23 and 120 m2/g could be produced. High-resolution transmission electron microscopy (HRTEM) revealed that nanoparticles were spherical with diameters of 10 to 50 nm. X-ray photoelectron spectroscopy analysis indicated that vanadia was dispersed on the surface of the titania spheres. No indication for the presence of crystalline V2O5 could be found by X-ray diffraction or HRTEM. Catalysts with a vanadia surface loading of 10 μmol/m2 showed high activity with less than 1% N2O formation up to 350°C. Catalytic activity strongly depended on the vanadia loading; an increase from 2.5 to 7 μmol/m2 resulted in a 30 times higher activity per vanadium. Addition of silica lowered the overall activity but did not change the activation energy. Raman spectroscopy indicated the presence of vanadate clusters. Temperature-programmed reduction corroborated that no significant amount of vanadia entered the titania lattice to form an interstitial solution. The selective catalytic reduction activity of as-prepared vanadia–titania is comparable to the best catalysts obtained by wet chemical methods.