Flame Spray Pyrolysis Process Research Articles

Photocatalytic mineralisation of organic compounds: a comparison of flame-made TiO2 catalysts

A comparative photocatalytic analysis was carried out on TiO2 made in a Flame Spray Pyrolysis (FSP) process and flame-made Degussa P25. Both have similar crystallinity, phase composition, phase segregation and a non-porous surface. Hence comparison was made based on their difference in specific surface area, organic adsorption and the amount of OH• generated upon illumination. The photocatalytic activity tests were carried out using the following series of organic compounds: sucrose, glucose, fructose, maleic acid, glyoxylic acid, oxalic acid, isobutyric acid, phenol and methanol. FSP-made TiO2 outperformed P25 for saccharides mineralisation, while for phenol and methanol mineralisation P25 was better than FSP-made TiO2. Similar mineralisation rates were observed for both FSP-made and P25 TiO2 for the mineralisation of carboxylic acids. This shows that the relative performance of the photocatalysts depends on the type of organic compounds to be degraded. The high surface area and possibly a more efficient interfacial charge transfer of FSP-made TiO2 provided an efficient pathway for saccharides mineralisation. As for phenol and methanol, the mineralisation rates were higher when using P25 due to the greater amount of OH• radicals generated by this photocatalyst. The fast mineralisation rates of carboxylic acids made degradation of these organic compounds to be less affected by the TiO2 photocatalyst properties and conditions tested in this work.

Two-Nozzle Flame Synthesis of Pt/Ba/Al2O3 for NOx Storage

A novel two-nozzle flame spray pyrolysis (FSP) process has been developed for one-step preparation of Pt/Ba/Al2O3 particles as used for NOx storage-reduction (NSR) catalysts. The materials were characterized by transmission electron microscopy, CO chemisorption, nitrogen adsorption, X-ray diffraction, and temperature programmed decomposition and tested for their NOx storage behavior. The use of two separate nozzles, one as aluminum and the other as a barium/platinum source, resulted in individual Al2O3 and monoclinic BaCO3 nanoparticles, exhibiting good NOx storage activity. In contrast, using a single-nozzle process resulted in Al2O3 particles with amorphous Ba species with negligible NOx storage capacity. Increasing the internozzle distance resulted in late mixing of the two flame products and increased the amount of crystalline BaCO3. At ambient conditions, as-prepared monoclinic BaCO3 transformed into orthorhombic BaCO3. Independent of the Ba loading, nanocrystalline BaCO3 showed a low thermal stability (decomposition below 900 °C, LT-BaCO3) that was distinctly different from its “bulk” behavior (decomposition above 900 °C).

Cubic or monoclinic Y 2O 3:Eu 3+ nanoparticles by one step flame spray pyrolysis

Continuous, single-step synthesis of monocrystalline Y 2O 3:Eu 3+ nanophosphor particles (10–25 nm in diameter and 5 wt% Eu) was achieved by flame spray pyrolysis (FSP). The effect of FSP process parameters on materials properties was investigated by X-ray diffraction (XRD), nitrogen adsorption (BET) and transmission electron microscopy (TEM). Photoluminescence (PL) emission were measured as well as the time-resolved PL-intensity decay. Controlled synthesis of monoclinic or cubic Y 2O 3:Eu 3+ nanoparticles was achieved without post-treatment by controlling the high temperature residence time of these particles. The cubic nanoparticles exhibited longer decay times but lower maximum PL intensity than commercial micron-sized bulk Y 2O 3:Eu 3+ phosphor powder.

Direct (one-step) synthesis of [formula omitted] and [formula omitted] nanoparticles for photocatalytic mineralisation of sucrose

TiO 2 and Pt / TiO 2 nanoparticles were made by a one-step flame spray pyrolysis (FSP) process that resulted in mostly anatase (69–85 wt%) powders with controlled specific surface area and crystallite size. These particles resulted in shorter half-lives for sucrose photomineralisation, up to 50% lower than Degussa P25. Co-precipitation of Pt on TiO 2 during FSP increased the rutile content and slightly increased the specific surface area. Close control over Pt deposit size during this process was possible by varying the Pt concentration in the feed precursor. The dispersion of the Pt was high, 45–77% (at 4.0–0.1 atom% Pt) and corresponded to metal deposit size of 2.5–1.4 nm, respectively. An optimum photocatalytic activity was observed at 0.5 atom% Pt loading. At low Pt loading (0.1 atom% Pt), the activity was lower than that of FSP-made TiO 2 since the high photocurrent density of the Pt deposits increased the electron-hole recombination. The deposit size was also too small to establish sufficient electrical contact for efficient interfacial charge transfer between the photocatalysts and sucrose. Additional studies on the photocatalytic mineralisation of sucrose under oxygen enriched conditions reaffirmed the postulation that both FSP-made TiO 2 and improved Pt / TiO 2 photocatalysts favoured a reductive pathway which was different and faster than the pathway followed when using Degussa P25 TiO 2 . The intricate relationship between photocatalyst characteristics and its performance is highlighted.

Noble Metal/Ceramic Composites in Flame Processes

Noble metals on metal oxides play a major role in the performance of electrodes, catalysts and many other applications. Today, impregnation of noble metals on metal oxides is done in the wet phase involving multiple steps. The physicochemical nature of the support, the precursor and the reaction conditions influence the resulting noble metal particles size in those systems [1]. For every specific application the particle size and the metal/metal oxide interaction affect the performance of these nano-composite materials [2]. Recently, aerosol processes have been successfully used to produce platinum [3,4], palladium [5], silver [6] and gold [7] crystallites on Al2O3 [3,5], SiO2 [7] and TiO2 [4,6,7] in a single step.. The as-prepared materials exhibited a high external specific surface area (40 – 320 m g) [3-7] with a high degree of crystallinity and an excellent noble metal distribution [3-7] with metal particles smaller than 7 nm on the metal oxide [3-7]. This makes them attractive for reactions with mass transfer limitation and for high temperature applications. An intimate contact of the two particles was observed in HRTEM images [3-5,7]. For a specific metal oxide support the metal particle size is mainly dependent on its loading [3,7]. In this study, the role of the supporting metal oxide on the noble metal particle size was systematically investigated for the flame spray pyrolysis process. The materials were produced at fixed process conditions such as resident time of the particles in the flame, energy input, maximum temperature and cooling rate. Having the same surface area of the support and metal loading, the materials exhibited different noble metal particle sizes. A fundamental understanding of the mechanisms of metal particle formation in the flame and the effect of the metal oxide support is presented for the controlled production of a desired metal particle size. With these concepts the properties of the noble metal on the metal oxide support and the interaction with the support can be tailored for specific applications.

Nanoparticle synthesis at high production rates by flame spray pyrolysis

Scaling-up of nanoparticle synthesis by the versatile flame spray pyrolysis process at production rates up to 1.1 kg/ h is investigated. Product silica powder is collected continuously in a baghouse filter unit which is cleaned periodically by air-pressure shocks. The effect of powder production rate, dispersion gas flow rate and precursor (hexamethyldisiloxane, HMDSO) concentration on product particle size, morphology and carbon content is investigated. Droplet size distributions of the cold spray are measured by laser diffraction, while N 2 adsorption (BET), transmission electron microscopy and thermogravimetric analysis coupled with a mass spectrometer are employed to characterize the product powder. The product primary particle size was precisely controlled from 10 to 75 nm and compared to a well-established vapor-fed flame aerosol reactor.

Controlled synthesis of nanostructured particles by flame spray pyrolysis

The flame spray pyrolysis (FSP) process was systematically investigated using an external-mixing gas-assisted atomizer supported by six premixed methane–oxygen flameletes. The effect of oxidant and precursor fuel composition on the size of FSP-made silica primary particles (8– 40 nm ) was studied using as precursor hexamethyldisiloxane (HMDSO) dissolved in ethanol, iso-octane or methanol. As oxidant air and pure oxygen were used, that served also as droplet dispersion gases. Droplet size distributions were measured by laser diffraction, while droplet lifetimes were calculated using a spray combustion model to explain for the first time the difference in flame structure and especially product powder characteristics when air or oxygen was used as oxidant/dispersion gas. The spray flame temperature was measured by Fourier transform infrared (FTIR) emission/transmission (E/T) spectroscopy. The liquid solvent (fuel), especially its enthalpy content, was an important parameter in FSP as it affected the total net heating value of the spray flame. It is shown for the first time also how the specific surface area of the FSP-made particles can be systematically controlled through the oxidant flow rate and precursor/fuel composition.

FLAME SPRAY PYROLYSIS FOR SYNTHESIS OF NANOPARTICLES

Bismuth oxide is used as additive in paints and in cataphoresis, and as substitute for lead oxide in glass or porcelain. The presence of a small amount of Bi{sub 2}O{sub 3} in calcined ZnO ceramics promotes non-linear current voltage characteristics that are used in varistor production. Also tin oxide based gas sensors for carbon monoxide, exhibit improved sensitivity and selectivity when doped with Bi{sub 2}O{sub 3}. Furthermore, the total oxidation of isobutene is enhanced on bismuth containing tin oxide catalysts. Bismuth oxide itself serves also as catalyst for conversion of propylene to 1,5 hexadiene and can be regenerated when supported on {alpha}-Al{sub 2}O{sub 3}. In electrolyte, varistor, sensor and catalyst applications high purity and small Bi{sub 2}O{sub 3} particles with controlled morphology are required. Especially in catalysts the available surface area (below 17 m{sup 2}/g) of Bi{sub 2}O{sub 3} limits applications at the moment. Typically, Bi{sub 2}O{sub 3} is prepared by oxidation of bismuth metal at 750-800 deg C or by thermal decomposition of carbonates or by addition of alkali-metal hydroxides to a bismuth salt solution. High temperature synthesis of Bi{sub 2}O{sub 3} from precursor gases was proposed by Liu and Kleinschmit using the established Aerosil route of reacting chloride precursors in a hydrogen / air (or oxygen) flame, but the volatility of the starting materials poses the most severe limitation. Suzuki et al. prepared spherical bismuth oxide particles of about 26 nm in diameter by the ICP-method. Spraying an aqueous bismuth nitrate solution into a hot wall reactor resulted in Bi{sub 2}O{sub 3} particles of about 100 nm in diameter. Here, synthesis of bismuth oxide nanoparticles is investigated by flame spray pyrolysis (FSP) as flame technology is used for manufacture of oxide commodities and as such FSP has high potential for manufacture of oxide nanoparticles. The FSP process, in particular, has the ability to synthesize metal oxides and mixed metal oxides of high purity at high production rates and under controlled conditions. Here bismuth nitrate is used as raw material since it is widely used in wet chemical processing of bismuth oxide and it is one of the cheapest precursors available. However, in conventional spray pyrolysis this precursor has resulted in hollow particles. Therefore special emphasis is placed on the control of morphology and primary particle size of the product bismuth oxide nanoparticles.