Reto Strobel

Flame aerosol synthesis of smart nanostructured materials

Recent advances in aerosol and combustion science and engineering now allow scalable flame synthesis of mixed oxides, metal salts and even pure metals in the form of nanoparticles and films with closely controlled characteristics. In this way, high purity materials with novel metastable phases are made that are not accessible by conventional wet-phase and solid state processes. Here, flame processes are classified into vapour-fed and liquid-fed ones depending on the employed state of the metal precursor. Liquid-fed flame processes are distinguished for their flexibility in producing materials of various compositions and morphologies that result in unique product functionalities. Parameters controlling the characteristics of flame-made particles and films are summarized and selected classes of materials are reviewed focusing on catalysts, sensors, biomaterials (orthopaedic, dental or nutritional), electroceramics (fuel cells, batteries) and phosphors exhibiting superior performance over conventionally made ones. Just a few years ago it seemed impossible to make these materials in the gas phase. Finally, health effects of such particles are discussed while future challenges and opportunities for flame-made materials are highlighted.

Aerosol flame synthesis of catalysts

Abstract A review of synthesis and performance of flame-made catalytic materials is presented. Emphasis is placed on flame technology for its dominance in aerosol manufacturing of materials of high purity with minimal liquid byproducts. Flame aerosol processes are characterized in terms of the precursor state supplied to the flame. During the last decade, a better understanding of aerosol and combustion synthesis of materials contributed to the development of one-step, dry synthesis of catalysts that are prepared conventionally by multi-step wet-phase processes. This includes TiO 2 -based photocatalysts, mixed oxides (e.g. V 2 O 5 /TiO 2 , TiO 2 /SiO 2 , perovskites, etc.) as well as supported metals (e.g. Pt/TiO 2 , Pd/Al 2 O 3 , Pt/CeO 2 /ZrO 2 , Pt/Ba/Al 2 O 3 , Ag/ZnO, Cu/ZnO/Al 2 O 3 bimetallic Pd/Pt/Al 2 O 3 and Au/TiO 2 ) made by single- or multi-nozzle flames. In general, highly crystalline and non-porous nanoparticles are formed during flame synthesis, resulting in materials with high thermal stability. Unique particle structures, only available through aerosol processes, lead to improved performance in various catalytic applications.

Flame-made Pd/La2O3/Al2O3 nanoparticles: thermal stability and catalytic behavior in methane combustion

Palladium nanoparticles supported on lanthanum-stabilized alumina were prepared by flame spray pyrolysis. The as-prepared materials were characterized by high-resolution transmission electron microscopy, CO chemisorption, nitrogen adsorption, X-ray diffraction and temperature programmed reduction. These materials were tested for the catalytic combustion of methane with a focus on the thermal stability of the support and the palladium particles. Flame spray pyrolysis afforded small palladium particles (<5 nm) attached to the surface of the supporting La2O3/Al2O3 ceramic nanoparticles with specific surface areas in the range of 50–180 m2 g−1. Compared to commercial reference materials the flame-made catalysts showed excellent thermal stability in terms of specific surface area up to 1200 °C and retarded γ- to α-alumina transformation. Catalysts were tested as-prepared (small Pd particles, <5 nm) and after sintering at 1000 °C (large Pd particles, 50–150 nm). By cycling the temperature several times from 200 to 1000 °C during catalytic combustion, it could be shown that all catalytic materials, regardless of specific surface area, lanthanum content, and preparation method (flame-synthesis or impregnated), exhibited similar catalytic performance after an initial conditioning cycle.

Luminescence and crystallinity of flame-made Y2O3:Eu3+ nanoparticles

Cubic and/or monoclinic Y2O3:Eu3+ nanoparticles (10–50 nm) were made continuously without post-processing by single-step, flame spray pyrolysis (FSP). These particles were characterized by X-ray diffraction, nitrogen adsorption and transmission electron microscopy. Photoluminescence (PL) emission and time-resolved PL intensity decay were measured from these powders. The influence of particle size on PL was examined by annealing (at 700–1300°C for 10 h) as-prepared, initially monoclinic Y2O3:Eu3+ nanoparticles resulting in larger 0.025–1 μm, cubic Y2O3:Eu3+. The influence of europium (Eu3+) content (1–10 wt%) on sintering dynamics as well as optical properties of the resulting powders was investigated. Longer high-temperature particle residence time during FSP resulted in cubic nanoparticles with lower maximum PL intensity than measured by commercial micron-sized bulk Y2O3:Eu3+ phosphor powder. After annealing as-prepared 5 wt% Eu-doped Y2O3 particles at 900, 1100 and 1300°C for 10 h, the PL intensity increased as particle size increased and finally (at 1300°C) showed similar PL intensity as that of commercially available, bulk Y2O3:Eu3+ (5 μm particle size). Eu doping stabilized the monoclinic Y2O3 and shifted the monoclinic to cubic transition towards higher temperatures.

Titania–silica doped with transition metals via flame synthesis: structural properties and catalytic behavior in epoxidation

The influence of trace amounts of transition metals (Cr, Mn, Fe, Co) on the structural and catalytic properties of silica and 1 wt% titania–silica nanoparticles prepared by flame synthesis has been investigated. The transition metal concentration was varied from 40 to about 2000 ppm by admixing the corresponding transition metal precursor to the silica and titania–silica precursor mixtures. These components were fed into a methane–oxygen diffusion flame, affording a single-step flame synthesis of the mixed oxide materials. The specific surface areas of all the powders thus prepared ranged from 60 to 240 m2 g−1, with oxygen flow being the most influential parameter. Diffuse reflectance UV-VIS and FT-IR spectroscopy showed that the Ti sites in the doped nanoparticles were unaffected by chromium, manganese, and cobalt, while iron led to the formation of a dual site with considerable acidity. Epoxidation of 2-cyclohexenol by tert-butylhydroperoxide (TBHP) was used to probe the effect of the transition metal dopants on the activity and selectivity of the titania–silica. A significant loss in selectivity to the epoxide was observed for all doped mixed oxides. The chromium-doped material was highly active, favoring radical processes. Even at a doping level of 40 ppm, considerable amounts of ketone by-products were formed. Manganese doping led to slow decomposition of the TBHP, but had no significant influence on the alkene conversion at Mn contents of up to 2000 ppm. Incorporation of iron afforded Lewis acid sites, leading to dehydration activity, as demonstrated by the occurence of water abstraction from the reactant. Cobalt-doped titania–silica showed only a weak tendency toward radical reactions, and no reactions due to Lewis acid sites were observed. Among the metal dopants, only chromium underwent significant leaching under reaction conditions.

Flame Synthesis of Supported Platinum Group Metals for Catalysis and Sensors

Platinum group metals (pgms) supported on a carrier material are widely applied as catalysts. These catalysts are conventionally prepared by wet-phase processes in several steps, while recently developed flame processes allow synthesis of supported pgms in a single step including the support material. Here, we describe flame processes and how finely dispersed supported pgms are made in these flames. So Pt/Al2O3, Pd/ZnO, Rh/Al2O3, Pt/Ba/Al2O3 and others are highlighted regarding their materials properties and performance as catalysts as well as in gas sensors.