Aerosol flame synthesis of catalysts

Advanced Powder Technol., Vol. 17, No. 5, pp. 457– 480 (2006) © VSP and Society of Powder Technology, Japan 2006. Also available online - www.brill.nl/apt

Invited review paper Aerosol flame synthesis of catalysts RETO STROBEL 1,2 , ALFONS BAIKER 2 and SOTIRIS E. PRATSINIS 1,∗ 1 Particle

Technology Laboratory, Department of Mechanical and Process Engineering, ETH Zurich, Sonneggstrasse 3, 8092 Zurich, Switzerland 2 Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Hönggerberg, HCI, 8093 Zurich, Switzerland Received 30 March 2006; accepted 21 April 2006 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 TiO2 based photocatalysts, mixed oxides (e.g. V2 O5 /TiO2 , TiO2 /SiO2 , perovskites, etc.) as well as supported metals (e.g. Pt/TiO2 , Pd/Al2 O3 , Pt/CeO2 /ZrO2 , Pt/Ba/Al2 O3 , Ag/ZnO, Cu/ZnO/Al2 O3 bimetallic Pd/Pt/ Al2 O3 and Au/TiO2 ) 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. Keywords: Flame synthesis; spray pyrolysis; aerosol synthesis; flame spray pyrolysis; catalysts; nanoparticles; catalyst preparation; supported metals; perovskites; mixed oxides; photocatalysis.

1. INTRODUCTION

By definition heterogeneous catalysis occurs on the nanoscale. The control and design of the nanostructure is crucial for catalytic materials with good performance [1]. This includes chemical and physical properties not only of the active site, where the reaction takes place, but also of the surrounding material, i.e. of the support for dispersed metal nanoparticles [2, 3]. Mechanical strength, thermal stability and porosity of the material are very important for catalytic applications. Flame synthesis of catalytically active nanoparticles can give access to novel structures and materials that are not available through conventional techniques [4, 5]. Today, ∗ To

whom correspondence should be addressed. E-mail: [email protected]

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Figure 1. Comparison of conventional wet-phase and flame methods for the synthesis of catalysts as illustrated for Pt/Al2 O3 . Flame synthesis allows preparation of the catalyst in one step [48], whereas the conventional route by precipitation and impregnation is a multi-step process [1].

mostly wet-phase techniques are used for the industrial manufacturing of catalysts: incipient wetness impregnation, sol-gel, precipitation, grafting and solid-state reactions, just to name a few, are batch processes requiring several after-treatment steps, such as filtration, drying and calcination [1, 6, 7]. Figure 1 shows a schematic of wet and flame processes for the synthesis of catalysts. Flame technology is a scalable, continuous and well-established method for the production of nanoparticles in large quantities. For decades it has been used for the large-scale manufacture of simple commodities, such as carbon black, pigmentary titania, waveguide preforms, fumed silica and alumina [8 –10]. Production rates of these materials can be of the order of 25 t/h and the corresponding reactors resemble best the rockets of a departing space shuttle [10]. Flame-made materials have been used as catalyst supports (i.e. Al2 O3 , SiO2 , TiO2 ) [1, 6, 11] and as photocatalysts (mainly TiO2 ) since the 1970s [12]. In fact, the most often applied TiO2 photocatalyst is manufactured by aerosol flame synthesis and marketed by Degussa as P25. A decade later, Ulrich proposed to use flames for the production of more complex catalytic materials [8]. However, it took more than another decade until the first works on flame-made materials and their catalytic properties were published [13 –16]. Since then, flame synthesis of catalytic materials has attracted

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many researchers, resulting in rapid progress towards more sophisticated catalysts. In general, flame-made materials are non-porous with a high external surface area that went through high temperatures and cooling rates, resulting in some unique phases and good thermal stability and mass transfer during catalysis [17, 18]. Here, recent progress in the flame synthesis of solid catalysts is presented along with the unique structural and catalytic properties of these materials. Other gasphase processes for catalyst manufacture such as chemical vapor deposition [19], plasma processing [20] as well as spray pyrolysis [21, 22], which was applied for synthesis of perovskite [23, 24], spinel [25] and noble metal [26 –29] catalysts, are beyond the scope of this paper and have been summarized elsewhere.

2. FLAME PROCESSES FOR CATALYST PREPARATION

Various aerosol reactors and methods have been developed for the synthesis of a wide variety of metal and metal oxide particles. Compared to wet-chemical routes with various post-treatment steps, such as filtration, washing, drying and calcination, gas-phase processes allow the preparation of the desired material without any further post-processing. Flame processes for catalyst preparation also have the advantage that multicomponent forms of catalysts can be readily prepared and their mixing can be controlled to obtain different distributions to prepare tailormade nanocomposites. In this review, processes for aerosol flame synthesis (AFS) are classified first depending on the precursor state fed to the flame in vapor-fed AFS (VAFS) and liquid-fed AFS (LAFS). In the latter, if the liquid precursor solution drives the flame process (contributes more than 50% of the energy) it is called flame spray pyrolysis (FSP). If a non-combustible solution is fed into the flame then LAFS is called flame-assisted spray pyrolysis (FASP). 2.1. VAFS This is the most common industrial flame process for synthesis of various ceramic commodities, such as fumed silica, waveguide preforms, alumina and pigmentary titania [8 – 10]. In this process volatile metal precursors (e.g. chlorides) are evaporated and fed into a flame that either supports the process (e.g. H2 /O2 flame in fumed silica manufacture) or ignites the process as in pigmentary TiO2 production (Fig. 2a). The metal precursor is converted into the metal oxide and starts to form particles by nucleation from the gas phase (Fig. 3). This is a simple and scalable process, as industrial practice and recent studies [30, 31] show. However, a drawback is the need for volatile precursors, which limits the application of VAFS to a few products where volatile precursors are available for a reasonable price, such as for SiO2 , TiO2 , Al2 O3 , V2 O5 , SnO2 , etc. [9, 10]. In the field of heterogeneous catalysts, VAFS has been used for the synthesis of TiO2 /SiO2 [32], V2 O5 /TiO2 [16, 33], Pt/TiO2 [34] and Cu/ZnO/Al2 O3 [35] (cf. Table 1).

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Figure 2. Schematic of selected flame configurations used for the synthesis of heterogeneous catalysts. (A) VAFS, (B) FASP and (C) FSP.

2.2. FASP In FASP a non-combustible liquid precursor is dispersed into fine droplets that are evaporated and pyrolyzed by an external flame, as was first developed for the synthesis of ZnO [36] and superconducting materials [37]. Instead of an electrically heated tube as in conventional spray pyrolysis [21, 22], an external hydrogen or hydrocarbon flame is used as the energy source during FASP (Fig. 2b) [38]. In general, aqueous solutions of metal salts (i.e. nitrates) are sprayed in the external flame where the solvent evaporates from the droplets and metal precursors are converted in products. Supplying the energy by an external vapor flame allows much higher process temperatures and cooling rates compared to electrically heated hot-wall reactors where temperatures are usually relatively low (below 2000◦ C) and residence times longer (up to a few seconds). Depending on process conditions, hollow, micron- or nano-sized particles are formed when the precursor reacts in the droplet or gas phase, respectively (Fig. 3) [21, 22, 38]. Apart from photocatalytic materials [39], mainly perovskite and spinel structures [13, 40] have been made by FASP (Table 1). 2.3. FSP In FSP the metal precursor is a combustible liquid that is sprayed and ignited, resulting in product nanoparticles. Although this method was developed as early as 1977 by Sokolowski et al. for the synthesis of Al2 O3 [41], it took nearly two decades until other researchers used it for the synthesis of nanoparticles [42 –45]. The organic precursor solution is dispersed either ultrasonically [41, 44] or by gas convection through a nozzle [42, 43] forming a fine spray which is ignited (Fig. 2).

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Figure 3. Schematic of the particle formation processes occurring during spray pyrolysis, FSP and VAFS. Two basic particle formation processes can be observed: particle formation in a droplet leading to large or hollow particles (spray pyrolysis and partly in FSP) and particle formation from the gas phase giving small nano-sized particles (VAFS, FSP and partly spray pyrolysis). More detailed particle formation mechanisms can be found elsewhere [38, 112]. This figure is published in colour on http://www.ingenta.com

The metal precursors evaporate in this spray flame and are combusted. Particles are then formed by nucleation from the gas phase (Fig. 3). The process features short residence times (a few milliseconds) and high maximum process temperatures (up to 3000 K) [46]. The main advantage of FSP over FASP is the formation of generally homogeneous, nano-sized particles through control of the precursor– solvent composition [47]. FSP has been used for the synthesis of many different solid catalysts, such as Pt/Al2 O3 [48], Pt/CeZrO2 [18], various perovskites [14, 49], etc. (Table 1).

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Table 1. Overview of catalytic materials made in flames, and related processes, and their evaluated catalytic application Material

Catalytic application

Reference

VAFS TiO2 TiO2 /SiO2 V2 O5 /TiO2 Pt/TiO2 Cu/ZnO/Al2 O3

photocatalysis epoxidation of cyclohexenol SCR of NO by NH3 SO2 oxidation methanol synthesis

[12, 61, 64, 65] [32] [16] [34] [35]

FASP TiO2 ZnO MoO3 /TiO2 SrMnO3 , NiMn2 O4 La1 – x Cex CoO3 LaBO3 (B Co, Mn, Fe) SrTiO3

photocatalysis photocatalysis photocatalysis methane combustion methane combustion methane combustion methane combustion

[39] [72] [70] [13] [15] [91, 92] [95]

FSP LaMnO3 Sr1 – x Agx TiO3 LaCoO3 V2 O5 /TiO2 V2 O5 /TiO2 Cex Zr1 – x O2 (Si,Al) Pt, Pd/Al2 O3 Pd/La2 O3 /Al2 O3 Au/TiO2 Pt/TiO2 Pt/Cex Zr1 – x O2 Pd–Pt/Al2 O3 Au–Ag/TiO2 , Fe2 O3 , SiO2 Ag/ZnO

methane combustion methane combustion methane combustion synthesis of phthalic anhydride SCR of NO by NH3 dynamic oxygen exchange (TWCs) enantioselective hydrogenation methane combustion CO oxidation photocatalysis dynamic oxygen exchange (TWCs) methane combustion selective CO oxidation photocatalysis

[14] [49] [94] [53] [76] [83, 90] [48, 100] [17] [56] [68] [18] [103] [104] [73]

2.4. Direct deposition Instead of collecting the as-prepared powder from the gas phase by filtration, electrostatic precipitation or in a cyclone, gas-phase preparation methods offer the possibility to directly deposit catalytically active materials on a substrate. Hunt et al. prepared thin films by combusting organic precursor solutions [45]. Direct flame deposition of the particles results in nanoporous films. For application in catalysis, substrates of various geometries have been used, such as flat plates [50, 51], tubes [52] or foams [53]. Cooling of the substrate enhances particle deposition by thermophoresis as observed for directly deposited Pt/SnO2 particles, which were applied as porous gas-sensing films [54]. Choy and Seh [55] used

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Figure 4. Schematic of the direct deposition technique as used to coat flame-generated particles onto a mullite foam. The deposited V2 O5 /TiO2 layer was extremely porous. These ready-to-use catalysts were successfully tested in the synthesis of phthalic anhydride (adapted from Ref. [53]).

FASP for the synthesis of Ni/Al2 O3 directly deposited on a flat substrate resulting in homogeneous, porous coatings. Larger particles and lower porosity were observed for higher substrate temperatures. Unfortunately no catalytic tests were reported. Figure 4 depicts V2 O5 /TiO2 nanoparticles that were deposited directly after FSP synthesis on a mullite foam resulting in highly porous, patchy catalyst layers of controlled structure [53]. This ready-to-use catalyst was successfully tested for the synthesis of phthalic anhydride from o-xylene exhibiting better performance than flame- or wet-made catalyst pellets. Thybo et al. [56] deposited FSP-made Au/TiO2 directly on a microreactor. The use of shadow masks allowed catalyst deposition in well-defined patterns with good adhesion on various substrates. These catalysts were tested for CO oxidation after deposition into a microreactor channel.

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3. CATALYSTS

Heterogeneous catalysis relies on different materials. Virtually all elements of the periodic table have been used either in the function of active species, supporting material, promoter, stabilizer or additive. Various catalytic materials have been prepared by flame aerosol processes, such as metal oxides, mixed metal oxides and supported noble metals. 3.1. Photocatalysts Apart from ceramic supports, photocatalysts are so far the only catalytic materials which are industrially produced by flame processes. The standard TiO2 photocatalyst is sold commercially by Degussa as P25 [10, 57]. TiO2 is one of the most often applied semiconductor photocatalysts as its redox potential lies within its band-gap energy [57 –59]. Environmental cleanup of organic and inorganic pollutants in air and wastewater is the main application of photocatalysts [57 –60]. In 1971 Formenti et al. [12] prepared various metal oxides by hydrolysis/combustion of the corresponding metal chlorides by VAFS in an oxy/hydrogen flame. Compared to other metal oxides (ZrO2 , GeO2 or SnO2 ), titania particles showed by far the highest activity for photooxidation of isobutene into acetone. The flame-made TiO2 was also much more active than corresponding aero- or xerogels [12]. The influence of conventional spray pyrolysis and FASP for the synthesis of TiO2 particles on their photocatalytic activity was investigated, showing that FASP-made TiO2 resulted in the highest performance [39]. VAFS-made titania particles were studied for photodestruction of phenol [61, 62] and salicylic acid [61], confirming the superior performance of flame-made TiO2 particles. Compared to pure anatase powders, the presence of small amounts of rutile resulted in more active catalysts [63]. The influence of particle size on photocatalytic properties was elucidated for flamemade TiO2 particles in the range of 6–50 nm [61, 62, 64]. Varying the residence time of the particles in the hot flame zone during preparation allowed precise control of TiO2 particle size and crystallinity. For very small particles (under 14 nm) quantum size effects resulted in an increased band-gap energy [64]. Concerning the photocatalytic oxidation of phenol in aerated suspensions, an optimum particle size between 20 and 40 nm was found [61, 64], whereas in non-aerated suspensions smaller particles exhibited the best performance [62]. Also, for the decomposition of methylene blue on TiO2 films, smaller particles (15 nm) were more active [65]. These effects were attributed to decreased quantum efficiencies for smaller particles and less available surface area for oxidation reactions on larger particles [64]. Other factors affecting the photocatalytic activity are charge transfer efficiency, OH. generation or Ti3+ defect sites as observed for different flame-made TiO2 particles [66]. TiO2 films directly deposited during VAFS have been used for the partial oxidation of cyclohexane under UV irradiation and compared to other deposition techniques [50]. The good morphological control of the flame-made deposits resulted in high selectivity and yields for the selective oxidation processes. Due to the high

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surface area and non-porous surfaces of the flame-deposited catalyst, coking and deactivation was at a minimum for the flame-deposited materials. Various dopants have been added to flame-made TiO2 particles to enhance the photocatalytic activity or shift its photocatalytic response into the visible light regime. This can be achieved by surface doping (supported metals or substitution of Ti in the crystal lattice). Si [61, 62], Fe and Zn [67] by VAFS as well as Pt [68] or Fe [69] by FSP and Mo by FASP [70] have been used as dopants for flamemade TiO2 . Titania particles doped with Cr, Fe, V or Zn were prepared by VAFS resulting in powders with a rather low surface area (around 20 m2 /g) [67]. The addition of Zn together with Fe resulted in the highest activity in the photocatalytic oxidation of 2-propanol, which was even higher than for P25. For Pt/TiO2 and MoO3 /TiO2 , adding dopant nearly doubled the photocatalytic activity compared to that of Degussa P25. For FSP-made Pt/TiO2 , a Pt content of 0.5 at% was optimal for the mineralization of sucrose [68], whereas for MoO3 /TiO2 , a MoO3 content of 2.5 at% showed the highest activity for the decomposition of dichlorobenzene [70]. The substitutional replacement of Ti4+ by iron reduced the band-gap effectively [69, 71], thus strongly enhancing the photocatalytic activity under visible light for mineralization of oxalic acid [69]. Apart from TiO2 , ZnO-based catalysts are often applied in photocatalysis since ZnO exhibits a similar band-gap as TiO2 . Jang et al. prepared ZnO particles by conventional spray pyrolysis and FASP of aqueous Zn(NO3 )2 solutions [72]. Similar to TiO2 [39] FASP-made ZnO particles exhibited higher photocatalytic activity that was attributed to their morphologies. Nano-sized ZnO particles (around 20 nm) with a similar photocatalytic behavior as P25 were formed by FASP, whereas larger particles (around 1 μm) with low activity were formed by spray pyrolysis (SP) [72]. Height et al. reported an increased photodegradation of methylene blue for larger ZnO particles (32 nm) [73], whereas Jang et al. observed the best performance for small particles (18 nm) [72]. The photocatalytic activity of ZnO nanoparticles could be further improved by adding Ag clusters on the ZnO surface during FSP synthesis [73]. 3.2. Vanadia-based catalysts Vanadia-coated titania for selective catalytic reduction (SCR) of NO with NH3 exhibits a V2 O5 surface layer structure on TiO2 as prepared by VAFS in counter[33] or co-flow [16] diffusion flames. As V2 O5 is more volatile than TiO2 , the latter particles are formed first, and vanadia condenses on them further downstream and forms a surface layer [16, 74]. Different flame parameters altered the specific surface area (SSA) from 20 to 120 m2 /g [16]. Using a counterflow diffusion flame resulted in a SSA of 40–50 m2 /g for V2 O5 /TiO2 and 80–90 m2 /g for V2 O5 /Al2 O3 [33]. Higher SSA as well as higher vanadia loading resulted in higher activities for the selective catalytic reduction of NO by NH3 [16]. The process for the synthesis of V2 O5 /TiO2 was scalable up to a production rate of 200 g/h without any loss in catalytic activity [75]. V2 O5 /TiO2 -based SCR catalysts were also prepared

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by FSP [76]. The addition of tungsten oxide and especially silica increased dramatically the thermal stability of the material concerning loss of specific surface area and prevented the transformation of anatase into rutile. The addition of a few wt% SiO2 to the V2 O5 /W2 O3 /TiO2 catalysts improved the activity in the SCR of NO by NH3 . V2 O5 /TiO2 as-prepared by FSP and directly deposited on mullite foams were tested for the oxidation of o-xylene to phthalic anhydride [53]. Well-dispersed vanadia species on the TiO2 particles and the absence of crystalline V2 O5 resulted in a very high activity of these catalysts. Smaller particles with higher accessible surface area further increased the catalytic activity. Vanadium phosphorous oxide (VPO) catalysts were prepared by spray pyrolysis [77] and VAFS in opposed jet diffusion flames [78]. For both processes, the asprepared material consisted mainly of VOPO4 · nH2 O, which could be converted into water-free, crystalline VPO structures upon heating. However, small VPO nanoparticles were formed by VAFS [78], whereas larger and hollow particles were observed for VPO made by spray pyrolysis [77]. The spray pyrolysis-made catalysts were tested for the oxidation of butane to maleic anhydride, revealing a strong influence of the reactor temperature during spray pyrolysis on the catalytic performance [77]. However, no catalytic results were reported for the VAFS-made catalysts [78]. 3.3. TiO2 /SiO2 epoxidation catalysts These catalysts were made by VAFS of titanium tetraisopropoxide and hexamethyldisiloxane with a SSA in the range of 50–300 m2 /g [32]. With up to 3 wt% TiO2 , the titania was atomically dispersed over the surface of the SiO2 particles. The absence of Ti hydroxides and the formation of individual, tetrahedral TiO2 sites [79] improved the activity and selectivity of these catalysts for epoxidation of 2-cyclohexenol by tert-butylhydroperoxide. The properties of materials could be conserved during VAFS scale-up from 6 up to 500 g/h [80]. Impurities in the form of transition metals, which commonly occur during wet chemical processes, can result in a dramatic loss in catalytic performance. The selectivity of these TiO2 /SiO2 epoxidation catalysts was lowered by very small amounts (less than 50 p.p.m.) of Fe, Cr, Co or Mn [81]. This demonstrates the advantage of flame processes with regard to the preparation of high-purity materials. In particular, the VAFS process gives access to very pure materials since non-volatile impurities are separated during evaporation of the metal precursors. This has been fully exploited in the manufacture of optical fiber preforms [10]. 3.4. Ceria-based catalysts The unique ability of ceria to store and release O2 makes automotive three-way catalysts (TWCs) the most important application of ceria [82]. As ceria has to sustain high temperatures, its thermal stability is critical for TWCs. The addition

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of zirconia as a stabilizer strongly enhances the resistance of ceria against thermal deactivation [83]. However, different preparation techniques also affect the thermal stability of ceria. As flame processes generally form highly crystalline and nonporous nanoparticles, a high thermal stability of such materials can be expected, making flame processes particularly attractive for the synthesis of ceria-based catalysts. Hence, various (flame) spray pyrolysis methods have been applied for the synthesis of ceria nanoparticles. Depending on process conditions, especially temperature and precursor formulation, either hollow, micron- or nano-sized CeO2 particles have been formed [84]. Emulsion combustion of aqueous emulsions in kerosene resulted in porous, micronsized ceria particles [85]. Spray pyrolysis of aqueous Ce nitrate solutions at low temperatures in a hot-wall reactor (400◦ C) resulted in precipitation and conversion of the precursor at the droplet surface, giving large, hollow particles (Fig. 3) [86]. In contrast, higher temperatures occurring during FSP of appropriate Ce precursors led to complete evaporation of the precursor and formation of nano-sized particles with a SSA up to 250 m2 /g [84, 87]. It should be noted, however, that appropriate precursors during FSP synthesis of CeO2 resulted in the desired nanostructure [84]. These nano-sized ceria particles exhibited good thermal stability compared to other preparation methods [84] and a similar dynamic oxygen exchange capacity compared to precipitated CeO2 [83]. Scale-up of FSP for synthesis of ceria particles up to 500 g/h was achieved without loosing its high SSA [87]. Further stabilization of flame-made ceria is achieved by addition of Zr. Stark et al. demonstrated that solvent composition strongly affects mixing of FSP-made CeO2 and ZrO2 in the solid phase [47]. Low-boiling solvents resulted in segregated ceria and zirconia phases [47, 88]. In contrast, high-boiling precursor solvents led to homogeneous solid solutions of Cex Zr1 – x O2 over the range of Ce:Zr ratios with surface areas around 150 m2 /g and very high thermal stability, which can be attributed to the non-porous nature and the high crystallinity of flame-made materials [83, 89]. However, these solid solutions exhibited rather low oxygen exchange capacities compared to precipitated catalysts, which was attributed to their high crystallinity and low defect concentration [83]. Introducing artificial defects by doping ceria/zirconia with low amounts of Si increased the oxygen storage capacity [90]. 3.5. Perovskites Perovskites are mixed oxides with the general formula ABO3 , where for catalytic applications A usually is a lanthanide ion and B a transition metal ion, both of which can be partially substituted. Perovskites exhibiting high thermal stability materials are attractive for catalytic combustion of natural gas in turbines for electricity generation — a high-temperature process. Two kinds of such catalysts are generally used and have been prepared by flame processes: perovskites [40] and supported noble metals (as will be described later [17]). The cheaper perovskites generally

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show lower activity, but better thermal stability compared to noble metal-based catalysts, where severe sintering of noble metals results in their deactivation. Traditional preparation methods such as calcination milling or the sol-gel citrate (SGC) method are not suitable for synthesis of perovskites exhibiting both high surface area (as needed for high activity) and high thermal stability. In recent years, flame technology turned out to be promising for the synthesis of perovskites of high surface area and thermal stability. The use of spray pyrolysis for the synthesis of various hollow, micron-sized perovskites has been summarized earlier [23, 24]. Here, we focus on flame-based processes for synthesis of perovskite catalysts. In an earlier work, Kriegel et al. [13] made perovskites by FASP of organic precursor solutions in an oxy/hydrogen flame. The as-prepared SrMnO3 was more active for methane combustion than the corresponding material prepared by solidstate reaction and showed a high surface area of 45 m2 /g. Synthesis of LaMnO3 - and LaCoO3 -based catalysts has been carried out by both FASP [15, 91 – 93] and FSP [14, 94]. The former generally resulted in non-porous particles with a surface area between 20 and 30 m2 /g [15, 91], whereas 70 m2 /g was achieved by the latter [94]. The phase purity of the perovskite crystal structure was exceptionally high for all flame-derived materials. Flame-made perovskites showed a similar or even higher activity for the catalytic combustion of methane than corresponding catalysts made by conventional SGC and other methods [15, 92]. However, the most notable improvement concerns the deactivation of the catalysts by sintering at high temperatures. Although the surface area was similar, the non-porous structure of flame-derived perovskites prevented their sintering and thus deactivation compared to SGC materials [40, 93]. The shorter residence times at high temperatures during FSP synthesis of LaCoO3 resulted in the highest SSA (up to 70 m2 /g) and catalytic activity. However, materials with a very high surface area sintered faster during reaction and thus a faster deactivation was observed [94]. The partial substitution of La by Ce improved the activity of LaCoO3 catalysts [15, 93]. This was attributed to higher oxygen mobility in the crystal lattice with small amounts of Ce being present. SrTiO3 -based perovskites were prepared by flame processes and tested for catalytic combustion of methane [49, 95]. Similar to the previously described LaCoO3 , the highest surface area for SrTiO3 was achieved by FSP (110 m2 /g) [49] rather than FASP (12 m2 /g) [95]. Both preparation methods afforded highly crystalline materials with catalytic activity similar to SGC-derived materials. A partial substitution of Sr by Ag strongly increased the activity and thermal stability for FSP-made SrTiO3 [49]. Concerning deactivation at high temperatures, however, the highest stability was observed for FASP- and SGC-derived materials, whereas FSP-derived SrTiO3 was less resistant. It can be summarized that various highly crystalline perovskites can be prepared by flame techniques in one step. In general, these materials exhibit a high surface area and good activity for methane combustion. The absence of internal micro- or

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Figure 5. Two-nozzle flame synthesis of Pt/Ba/Al2 O3 catalysts. Al2 O3 and BaCO3 particles are formed separately resulting in individual BaCO3 and Al2 O3 particles (see transmission electron micrograph on the right, Ba particles are colored in red) exhibiting good behavior as NSR catalysts (adapted from Ref. [96]). This figure is published in colour on http://www.ingenta.com

mesopores often leads to a high resistance against sintering at high temperatures and consequently lower deactivation of the catalysts. 3.6. NOx storage reduction catalysts NOx storage reduction (NSR) catalysts are applied for exhaust gas cleanup from engines operating under fuel-lean conditions. Typical NSR catalysts consisting of Pt and Ba supported on Al2 O3 were prepared by FSP in one step [96]. Spraying all three components together resulted in amorphous barium species, which were not active for NOx storage. However, using a two-nozzle process, one as aluminium and the other as a barium/platinum source, allowed synthesis of individual Al2 O3 and BaCO3 particles as shown in Fig. 5. These materials exhibited a similar behavior during NSR as impregnated catalysts. At higher Ba loadings, however, the flamemade catalysts showed faster and higher NOx uptake. This could be attributed to the unique formation of relatively unstable BaCO3 in flame-made materials even at high Ba content. 3.7. Supported metal catalysts Supported noble metals are widely used as catalysts in processes ranging from fine chemical synthesis to exhaust gas treatment and even oil refining. Small active metal clusters are generally dispersed on high surface area supports such as TiO2 , Al2 O3 and SiO2 . Various flame techniques have been applied for preparation of supported noble metal catalysts in one step, although FSP is the dominant technique as it allows high throughput of the noble metal. The relatively high vapor pressure

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Figure 6. Formation of nanoparticles in flames. After evaporation and combustion of the precursors, particle formation starts by nucleation from the gas phase. These particles coagulate and, depending on the temperature, sinter into larger particles or agglomerate. In the case of supported noble metal catalysts, noble metal particle formation starts after support formation by homogeneous nucleation from the gas phase and/or heterogeneous nucleation on the support.

of noble metals compared to that of the supporting metal oxides allows the cosynthesis of support with the noble metal. In a first step, the support is formed and then downstream noble metal particles nucleate at lower temperatures either heterogeneously on the support or homogeneously in the gas phase with later deposition on the support (Fig. 6). 3.7.1. Platinum. Vapor-fed aerosol flame synthesis was used for the preparation of Pt/TiO2 [34]. Ti isopropoxide and Pt acetylacetonate were evaporated, and burned in a premixed methane flame. The as-prepared material consisted of small Pt particles (below 5 nm) supported on TiO2 nanoparticles exhibiting similar performance in the oxidation of SO2 as impregnated catalysts. In contrast to the VAFS process, FSP allows much higher production rates for supported noble metal catalysts as liquid feeds can provide much higher throughputs. Pt/Al2 O3 (1–7.5 wt% Pt) or Pt/TiO2 (0.1–4 at% Pt) made by FSP consisted of well-dispersed Pt particles (below 5 nm) on top of γ -Al2 O3 or TiO2 particles (Fig. 7b) [48, 68]. By adjusting the precursor concentration and flame temperature profile, the specific surface area of both catalysts could be controlled between 70 and 150 m2 /g. The platinum dispersion on both supports depended strongly on Pt content and support surface area, resulting in an almost linear increase of Pt particle size with the Pt loading on Al2 O3 or TiO2 . The catalytic properties of flame-made Pt/Al2 O3 were evaluated for enantioselective hydrogenation of ethyl pyruvate. As shown in Fig. 8, the non-porous structure of the flame-made material resulted in much higher activity compared to commercial porous catalysts due to the abscence of intraparticle mass transfer limitations [48].

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Figure 7. Electron microscopy images of supported noble metals produced by FSP. (A) Pd/Al2 O3 [100], (B) Pt/TiO2 [68] and (C) Au/SiO2 [102].

Figure 8. Behavior of flame-made and commercial Pt/Al2 O3 catalysts containing 5 wt% Pt in the enantioselective hydrogenation of ethyl pyruvate. The non-porous structure of the flame-made catalyst resulted in a 5 times higher activity (adapted from Ref. [48]).

High thermal stability and very small Pt clusters were observed for Pt supported on ceria/zirconia at low Pt loadings (below 2 wt%) [18]. As the Pt clusters were very small, Pt was mainly present in its oxidized form and hardly reduced up to 300◦ C in 5% H2 . Compared to catalysts prepared by precipitation and impregnation, the thermally more stable flame-made Pt/ceria-zirconia did not lose its low-temperature oxygen exchange capacity after high-temperature treatment. Rapid quenching of the flame was applied for Pt/TiO2 to control the size of noble metal clusters independently of that of the TiO2 support [97]. Two different quenching devices have been applied for FSP-made Pt/TiO2 : cooling by a gas stream [97, 98] and by expansion of the flame gases into a supercritical nozzle similar to that used for VAFS [99]. The extra quenching of the flame resulted in a steep drop of flame temperature [97]. Figure 9 shows SSA and corresponding Pt dispersion for Pt/TiO2 prepared by FSP and a supercritical nozzle as a function of the burner–nozzle distance (BND). Quenching by this supercritical nozzle was more

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Figure 9. Pt dispersion and SSA of 5 wt% Pt on TiO2 made by quenching the flame in a supercritical nozzle as a function of BND.

efficient as by a gas stream resulting in very high surface area at low BND. However, incomplete evaporation of Pt at early stages in the flame resulted in the formation of large residual particles beneath smaller particles formed through nucleation from the gas phase [97]. If the flame was quenched after complete evaporation of the Pt precursor, smaller Pt clusters were formed compared to an unquenched flame. In this operation window the Pt particles size could be controlled without altering the surface area of the TiO2 support. 3.7.2. Palladium. Figure 7a depicts similar structural properties for Pd/Al2 O3 [100] as well as Pd supported on La-stabilized Al2 O3 [17] as for Pt/Al2 O3 . The Pd/Al2 O3 catalysts were also evaluated for enantioselective hydrogenation, and exhibited good selectivity and activity. The easy control of noble metal particle size was used to elucidate the structure sensitivity of the reaction [100]. Similar to Pt/ceria/zirconia [18], alumina-based supports showed good thermal stability [17]. After annealing at 1100◦ C, flame-made Al2 O3 retained 90 m2 /g compared to 57 m2 /g for a commercial alumina. Adding a few wt% of La increased even further the thermal stability of flame-made alumina. High thermal stability of γ -Al2 O3 was also observed for hollow, La-doped alumina particles made by emulsion combustion [101]. In general, the high thermal stability of these materials concerning loss in specific surface area can be attributed to the absence of small pores, which makes them promising catalysts for high temperature processes. Consequently, flame-made Pd/La-Al2 O3 was tested for catalytic combustion of methane, where the catalysts have to sustain temperatures up to 1000◦ C. However, the deactivation of the catalyst was mainly caused by sintering of palladium, with only marginal effects from the stability of the support, preparation method (impregnation, flame synthesis) or La content.

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3.7.3. Copper and gold. Cu/ZnO/Al2 O3 , a typical catalyst for methanol synthesis, was prepared by VAFS [35]. The as-prepared material consisted of ZnAl2 O4 with small ZnO and CuO crystallites on its surface. Upon reduction in hydrogen at 220◦ C, small Cu particles (10 nm) were formed. The use of hydrogen instead of methane as a fuel during VAFS nearly doubled the catalytic activity in methanol synthesis from syngas. The FSP was applied for synthesis of Au supported on SiO2 or TiO2 [56, 102]. Here, the gold clusters were larger (around 16 nm for 4 wt% Au on TiO2 ) compared to the previously described systems and Au particle size was independent of the support composition on its surface area (Fig. 7c). This indicates that Au particles probably form independently of the support by homogeneous nucleation or exhibit a high mobility on the support surface. 3.7.4. Bimetallic catalysts. Bimetallic Pd–Pt on Al2 O3 and Au–Ag on SiO2 , TiO2 or Fe2 O3 were also prepared by FSP [103, 104]. For the Pd–Pt/Al2 O3 , scanning/tuneling electron microscopy (STEM)/energy dispersive X-ray analysis (EDX) and extended X-ray absorption fine structure (EXAFS) analysis detected Pd and Pt in the same clusters, and formation of Pd–Pt alloys upon reduction. Figure 10 shows an STEM image of this material with the corresponding EDX analysis of a single metal particle. The addition of Pt to Pd and vice versa increased the stability of the noble metal clusters against sintering at high temperatures, resulting in an improved resistance against deactivation during catalytic combustion of methane [103]. Concerning the formation of small bimetallic clusters, similar observations were made for Au–Ag as used for selective CO oxidation. The catalyst consisted of small alloyed Au–Ag clusters (below 10 nm) dispersed on titania, iron oxide or silica [104]. Alloy formation was also observed for unsupported Ag–Pd particles [105], whereas Pt–Ru was only partly alloyed [106]. It can be assumed that simultaneous nucleation of noble metals in the gas phase or on the support and similar oxidation states lead to formation of bimetallic clusters. This makes flame technology a promising alternative to wet-phase processes for the preparation of various bimetallic catalysts.

4. CONCLUDING REMARKS

With regard to supported metal catalysts, it can be summarized that FSP of supported noble metals generally results in well-dispersed noble metal particles. It can be assumed that noble metals form by heterogeneous nucleation on the support (Fig. 6). Their particle size strongly depends on the surface area of the support and the strength of the metal–support interaction. Stronger metal–support interactions result in less sintering of the noble metals on the surface of the support at the high temperatures during synthesis. For Pt, the highest dispersions were observed on ceria/zirconia [18] followed by titania [68] and alumina [48]. Apart

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Figure 10. Electron microscopy image of bimetallic Pd–Pt on alumina. The inset shows EDX analysis of a single metal particle revealing the presence of Pt and Pd in the same particles (adapted from Ref. [103]).

from the properties of the support, the ‘nobleness’ of the supported metal has a strong influence on the metal–support interactions. It can be assumed that partly oxidized noble metals, as observed for Pd, Pt and Ag [18, 103, 104], exhibit strong metal–support interactions, preventing extensive sintering during the high temperature flame processes. As a result, small particles (below 5 nm) are formed. In contrast, more noble metals like Au form larger particles at similar loadings. This can be attributed to lower metal–support interactions as no oxides are formed [104] and to the relatively low melting point of this metal, which enhances sintering on the support during flame synthesis. Flame-based aerosol techniques are suitable processes for the synthesis of a wide variety of catalytic materials. Well-known catalysts such as supported noble metals

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as well as novel materials that are not accessible through conventional methods can readily be made in flames. In general, flame-made catalysts are non-porous and of controlled crystallinity, often possessing high thermal stability. For some specific catalytic applications flame-made materials exhibit superior performance compared to conventionally prepared ones. Proven scalability and continuous production make flame processes attractive for the industrial production of catalysts. Optimization of existing catalytic systems which are prepared by conventional techniques is only one goal for flame aerosol techniques. As flame techniques can result in the formation of unique materials, it seems more promising to use them for the development of novel catalytic materials which are not commonly used. This includes especially mixed oxides, where novel phases have already been reported [107], but also metal salts which have recently been prepared by flame synthesis [108 – 111]. These materials may exhibit improved and unexpected catalytic properties. Acknowledgements We thank ETH Zurich for support (TH Gesuch) over the last 6 years. For many helpful suggestions and comments, we thank Professor L. Forni (University of Milano, Italy), Professor P. Biswas (Washington University, USA), Professor T. Johannessen (TU Lyngby, Denmark), Professor W. J. Stark (ETH Zurich, Switzerland), Dr. T. Tani (Toyota Central R&D, Japan), W. Y. Teoh (University of New South Wales, Australia) and Dr. Neppolian (Osaka Prefecture University, Japan), as well as Mr. G. Varga, Dr. H. Mühlenweg and Dr. R. Vormberg from Degussa AG, Germany.

REFERENCES 1. G. Ertl, H. Knözinger and J. Weitkamp (Eds), Handbook of Heterogeneous Catalysis, Vol. 1. Wiley-VCH, Weinheim (1997). 2. A. T. Bell, The impact of nanoscience on heterogeneous catalysis, Science 299, 1688–1691 (2003). 3. D. R. Rolison, Catalytic nanoarchitectures — the importance of nothing and the unimportance of periodicity, Science 299, 1698–1701 (2003). 4. W. J. Stark, S. E. Pratsinis and A. Baiker, Heterogeneous catalysis by flame-made nanoparticles, Chimia 56, 485–489 (2002). 5. T. Johannessen, J. R. Jenson, M. Mosleh, J. Johansen, U. Quaade and H. Livbjerg, Flame synthesis of nanoparticles — applications in catalysis and product/process engineering, Chem. Eng. Res. Des. 82, 1444–1452 (2004). 6. G. Ertl, H. Knözinger and J. Weitkamp (Eds), Preparation of Solid Catalysts. Wiley-VCH, Weinheim (1999). 7. J. A. Schwarz, Methods for preparation of catalytic materials, Chem. Rev. 95, 477–510 (1995). 8. G. D. Ulrich, Flame synthesis of fine particles, Chem. Eng. News 62, 22–29 (1984). 9. M. S. Wooldridge, Gas-phase combustion synthesis of particles, Prog. Energ. Combust. 24, 63–87 (1998).

476

R. Strobel et al.

10. S. E. Pratsinis, Flame aerosol synthesis of ceramic powders, Prog. Energ. Combust. 24, 197– 219 (1998). 11. D. Koth and H. Ferch, Eigenschaften hochtemperaturhydrolytisch hergestellter Kieselsäuren und Metalloxide für Katalysatoren, Chem. Ing. Tech. 52, 628–634 (1980). 12. M. Formenti, F. Juillet, P. Meriaudeau, S. J. Teichner and P. Vergnon, Preparation in a hydrogen–oxygen flame of ultrafine metal oxide particles, J. Colloid Interf. Sci. 39, 79–89 (1972). 13. R. Kriegel, J. Töpfer, N. Preuss, S. Grimm and J. Böer, Flame pyrolysis: a preparation route for ultrafine powders of metastable β-SrMnO3 and NiMn2 O4 , J. Mater. Sci. Lett. 13, 1111–1113 (1994). 14. G. Saracco, F. Geobaldo, D. Mazza and G. Baldi, New method for catalyst powder manufacturing based on solvent combustion, J. Therm. Anal. Calorim. 56, 1435–1442 (1999). 15. R. A. M. Giacomuzzi, M. Portinari, I. Rossetti and L. Forni, A new method for preparing nanometer-size perovskitic catalysts for CH4 flameless combustion, Stud. Surf. Sci. Catal. 130, 197–202 (2000). 16. W. J. Stark, K. Wegner, S. E. Pratsinis and A. Baiker, Flame aerosol synthesis of vanadia-titania nanoparticles: structural and catalytic properties in the selective catalytic reduction of NO by NH3 , J. Catal. 197, 182–191 (2001). 17. R. Strobel, S. E. Pratsinis and A. Baiker, Flame-made Pd/La2 O3 /Al2 O3 nanoparticles: thermal stability and catalytic behavior in methane combustion, J. Mater. Chem. 15, 605–610 (2005). 18. W. J. Stark, J. D. Grunwaldt, M. Maciejewski, S. E. Pratsinis and A. Baiker, Flame-made Pt/ceria/zirconia for low temperature oxygen exchange, Chem. Mater. 17, 3352–3358 (2005). 19. P. Serp, P. Kalck and R. Feurer, Chemical vapor deposition methods for the controlled preparation of supported catalytic materials, Chem. Rev. 102, 3085–3128 (2002). 20. C. J. Liu, G. P. Vissokov and B. W. L. Jang, Catalyst preparation using plasma technologies, Catal. Today 72, 173–184 (2002). 21. G. L. Messing, S. C. Zhang and G. V. Jayanthi, Ceramic powder synthesis by spray-pyrolysis, J. Am. Ceram. Soc. 76, 2707–2726 (1993). 22. K. Okuyama and I. W. Lenggoro, Preparation of nanoparticles via spray route, Chem. Eng. Sci. 58, 537–547 (2003). 23. W. R. Moser, Hydrocarbon partial oxidation catalysts prepared by the high-temperature aerosol decomposition process, ACS Symp. Ser. 523, 244–261 (1992). 24. W. R. Moser and J. D. Lennhoff, A new high-temperature aerosol decomposition process for the synthesis of mixed metal-oxides for ceramics and catalysts and their characterization, Chem. Eng. Commun. 83, 241–259 (1989). 25. P. Fortunato, A. Reller and H. R. Oswald, Generation of mixed metal oxides by use of an ultrasonic aerosol thermal decomposition process, Solid State Ionics 101, 85–89 (1997). 26. D. Li, N. Ichikuni, S. Shimazu and T. Uematsu, Catalytic properties of sprayed Ru/Al2 O3 and promoter effects of alkali metals in CO2 hydrogenation, Appl. Catal. A 172, 351–358 (1998). 27. T. Uematsu, S. Shimazu, T. Kameyama and K. Fukuda, New preparation methods for active superfine catalysts by spray reaction, Stud. Surf. Sci. Catal. 75, 1809–1812 (1993). 28. T. Uematsu, L. Fan, T. Maruyama, N. Ichikuni and S. Shimazu, New application of spray reaction technique to the preparation of supported gold catalysts for environmental catalysis, J. Mol. Catal. A 182, 209–214 (2002). 29. W. R. Moser, J. A. Knapton, C. C. Koslowski, J. R. Rozak and R. H. Vezis, Noble-metal catalysts prepared by the high-temperature aerosol decomposition (HTAD) process, Catal. Today 21, 157–169 (1994). 30. K. Wegner and S. E. Pratsinis, Scale-up of nanoparticle synthesis in diffusion flame reactors, Chem. Eng. Sci. 58, 4581–4589 (2003). 31. K. Wegner and S. E. Pratsinis, Gas-phase synthesis of nanoparticles: scale-up and design of flame reactors, Powder Technol. 150, 117–122 (2005).

Aerosol flame synthesis of catalysts

477

32. W. J. Stark, S. E. Pratsinis and A. Baiker, Flame made titania/silica epoxidation catalysts, J. Catal. 203, 516–524 (2001). 33. P. F. Miquel, C. H. Hung and J. L. Katz, Formation of V2 O5 -based mixed oxides in flames, J. Mater. Res. 8, 2404–2413 (1993). 34. T. Johannessen and S. Koutsopoulos, One-step flame synthesis of an active Pt/TiO2 catalyst for SO2 oxidation — a possible alternative to traditional methods for parallel screening, J. Catal. 205, 404–408 (2002). 35. J. R. Jensen, T. Johannessen, S. Wedel and H. Livbjerg, A study of Cu/ZnO/Al2 O3 methanol catalysts prepared by flame combustion synthesis, J. Catal. 218, 67–77 (2003). 36. B. S. Marshall, I. Telford and R. Wood, Field method for determination of zinc oxide fume in air, Analyst 96, 569–578 (1971). 37. M. R. Zachariah and S. Huzarewicz, Aerosol processing of YBaCuO superconductors in a flame reactor, J. Mater. Res. 6, 264–269 (1991). 38. L. Mädler, Liquid-fed aerosol reactors for one-step synthesis of nano-structured particles, KONA 22, 107–120 (2004). 39. W. Lee, Y. M. Gao, K. Dwight and A. Wold, Preparation and characterization of titanium(IV) oxide photocatalysts, Mater. Res. Bull. 27, 685–692 (1992). 40. L. Forni and I. Rossetti, Catalytic combustion of hydrocarbons over perovskites, Appl. Catal. B 38, 29–37 (2002). 41. M. Sokolowksi, A. Sokolowska, A. Michalski and B. Gokieli, The ‘in-flame-reaction’ method for Al2 O3 aerosol formation, J. Aerosol. Sci. 8, 219–229 (1977). 42. L. Mädler, H. K. Kammler, R. Mueller and S. E. Pratsinis, Controlled synthesis of nanostructured particles by flame spray pyrolysis, J. Aerosol. Sci. 33, 369–389 (2002). 43. C. R. Bickmore, K. E. Waldner, D. R. Treadwell and R. M. Laine, Ultrafine spinel powders by flame spray pyrolysis of a magnesium aluminum double alkoxide, J. Am. Ceram. Soc. 79, 1419–1423 (1996). 44. A. Kilian and T. F. Morse, A novel aerosol combustion process for the high rate formation of nanoscale oxide particles, Aerosol Sci. Technol. 34, 227–235 (2001). 45. A. T. Hunt, W. B. Carter and J. K. Cochran Jr, Combustion chemical vapor deposition: a novel thin film deposition technique, Appl. Phys. Lett. 63, 266–268 (1993). 46. R. Jossen, Controlled synthesis of mixed oxide particles in flame spray pyrolysis, Dissertation 16401, ETH, Zurich (2006). 47. W. J. Stark, L. Mädler, M. Maciejewski, S. E. Pratsinis and A. Baiker, Flame synthesis of nanocrystalline ceria-zirconia: effect of carrier liquid, Chem. Commun., 588–589 (2003). 48. R. Strobel, W. J. Stark, L. Mädler, S. E. Pratsinis and A. Baiker, Flame-made platinum/alumina: structural properties and catalytic behaviour in enantioselective hydrogenation, J. Catal. 213, 296–304 (2003). 49. L. Fabbrini, A. Kryukov, S. Cappelli, G. L. Chiarello, I. Rossetti, C. Oliva and L. Forni, Sr1 – x Agx TiO3±δ (x = 0, 0.1) perovskite-structured catalysts for the flameless combustion of methane, J. Catal. 232, 247–256 (2005). 50. E. Sahle-Demessie, M. Gonzalez, Z. M. Wang and P. Biswas, Synthesizing alcohols and ketones by photoinduced catalytic partial oxidation of hydrocarbons in TiO2 film reactors prepared by three different methods, Ind. Eng. Chem. Res. 38, 3276–3284 (1999). 51. J. Karthikeyan, C. C. Berndt, J. Tikkanen, J. Y. Wang, A. H. King and H. Herman, Nanomaterial powders and deposits prepared by flame spray processing of liquid precursors, Nanostruct. Mater. 8, 61–74 (1997). 52. M. Miki-Yoshida, V. Collins-Martínez, P. Amézega-Madrid and A. Aguilar-Elguézabal, Thin films of photocatalytic TiO2 and ZnO deposited inside a tubing by spray pyrolysis, Thin Solid Films 419, 60–64 (2002).

478

R. Strobel et al.

53. B. Schimmoeller, H. Schulz, S. E. Pratsinis, A. Bareiss, A. Reitzmann and B. KraushaarCzarnetzki, Ceramic foams directly-coated with flame-made V2 O5 /TiO2 for synthesis of phthalic anhydride, J. Catal. (2006) (in press). 54. L. Mädler, A. Roessler, S. E. Pratsinis, T. Sahm, A. Gurlo, N. Barsan and U. Weimar, Direct formation of highly porous gas-sensing films by in situ thermophoretic deposition of flamemade Pt/SnO2 nanoparticles, Sens. Actuators B 114, 283–295 (2006). 55. K. L. Choy and H. K. Seh, Fabrication of Ni-Al2 O3 -based reforming catalyst using flameassisted vapour deposition, Mat. Sci. Eng. A 281, 253–258 (2000). 56. S. Thybo, S. Jensen, J. Johansen, T. Johannessen, O. Hansen and U. J. Quaade, Flame spray deposition of porous catalysts on surfaces and in microsystems, J. Catal. 223, 271–277 (2004). 57. M. A. Fox and M. T. Dulay, Heterogeneous photocatalysis, Chem. Rev. 93, 341–357 (1993). 58. M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95, 69–96 (1995). 59. A. L. Linsebigler, G. Lu and J. T. Yates, Photocatalysis on TiO2 surfaces: Principles, mechanisms, and selected results, Chem. Rev. 95, 735–758 (1995). 60. D. F. Ollis and H. Al-Ekabi, Photocatalytic Purification and Treatment of Water and Air. Elsevier, Amsterdam (1993). 61. G. P. Fotou and S. E. Pratsinis, Photocatalytic destruction of phenol and salicylic acid with aerosol-made and commercial titania powders, Chem. Eng. Commun. 151, 251–269 (1996). 62. G. P. Fotou, S. Vemury and S. E. Pratsinis, Synthesis and evaluation of titania powders for photodestruction of phenol, Chem. Eng. Sci. 49, 4939–4948 (1994). 63. D. C. Hurum, A. G. Agrios, K. A. Gray, T. Rajh and M. C. Thurnauer, Explaining the enhanced photocatalytic activity of Degussa P25 mixed-phase TiO2 using EPR, J. Phys. Chem. B 107, 4545–4549 (2003). 64. C. B. Almquist and P. Biswas, Role of synthesis method and particle size of nanostructured TiO2 on its photoactivity, J. Catal. 212, 145–156 (2002). 65. H. D. Jang, S. K. Kim and S. J. Kim, Effect of particle size and phase composition of titanium dioxide nanoparticles on the photocatalytic properties, J. Nanopart. Res. 3, 141–147 (2001). 66. W. Y. Teoh, F. Denny, D. Friedmann, L. Mädler and S. E. Pratsinis, Photocatalytic mineralisation of organic compounds: a comparison of flame-made TiO2 catalysts, Top. Catal. (2006) (in press). 67. B. Neppolian, H. S. Jie, J. P. Ahn, J. K. Park and M. Anpo, The preparation of TiO2 nanoparticle photocatalysts by a flame method and their photocatalytic reactivity for the degradation of 2-propanol, Chem. Lett. 33, 1562–1563 (2004). 68. W. Y. Teoh, L. Mädler, D. Beydoun, S. E. Pratsinis and R. Amal, Direct (one-step) synthesis of TiO2 and Pt/TiO2 nanoparticles for photocatalytic mineralisation of sucrose, Chem. Eng. Sci. 60, 5852–5861 (2005). 69. W. Y. Teoh, R. Amal, L. Mädler and S. E. Pratsinis, Flame sprayed visible light-active Fe-TiO2 for photomineralisation of oxalic acid, Catal. Today (2006) (in press). 70. W. Lee, Y. R. Do, K. Dwight and A. Wold, Enhancement of photocatalytic activity of titanium(IV) oxide with molybdenum(VI) oxide, Mater. Res. Bull. 28, 1127–1134 (1993). 71. Z. M. Wang, G. X. Yang, P. Biswas, W. Bresser and P. Boolchand, Processing of iron-doped titania powders in flame aerosol reactors, Powder Technol. 114, 197–204 (2001). 72. Y. J. Jang, C. Simer and T. Ohm, Comparison of zinc oxide nanoparticles and its nanocrystalline particles on the photocatalytic degradation of methylene blue, Mater. Res. Bull. 41, 67–77 (2006). 73. M. J. Height, S. E. Pratsinis, O. Mekasuwandumrong and P. Praserthdam, Ag–ZnO catalysts for UV-photodegradation of methylene blue, Appl. Catal. B 63, 305–312 (2005). 74. P. F. Miquel and J. L. Katz, Flame synthesis of nanostructured vanadium oxide based catalysts, Stud. Surf. Sci. Catal. 91, 207–216 (1995).

Aerosol flame synthesis of catalysts

479

75. W. J. Stark, A. Baiker and S. E. Pratsinis, Nanoparticle opportunities: Pilot-scale flame synthesis of vanadia/titania catalysts, Part. Part. Syst. Char. 19, 306–311 (2002). 76. R. Jossen, M. C. Heine, S. Augustine, M. K. Akhtar and S. E. Pratsinis, Thermal stability and catalytic activity of flame-made silica–vanadia–tungsten oxide–titania, Appl. Catal. B (2006) (in press). 77. P. M. Michalakos, H. E. Bellis, P. Brusky, H. H. Kung, H. Q. Li, W. R. Moser, W. Partenheimer and L. C. Satek, Synthesis of vanadium phosphorus oxide catalysts by aerosol processing, Ind. Eng. Chem. Res. 34, 1994–2000 (1995). 78. P. F. Miquel and J. L. Katz, Formation and characterization of nanostructured V–P–O particles in flames — a new route for the formation of catalysts, J. Mater. Res. 9, 746–754 (1994). 79. J. D. Grunwaldt, C. Beck, W. Stark, A. Hagen and A. Baiker, In situ XANES study on TiO2 – SiO2 aerogels and flame made materials, Phys. Chem. Chem. Phys. 4, 3514–3521 (2002). 80. W. J. Stark, H. K. Kammler, R. Strobel, D. Gunther, A. Baiker and S. E. Pratsinis, Flame-made titania/silica epoxidation catalysts: toward large-scale production, Ind. Eng. Chem. Res. 41, 4921–4927 (2002). 81. W. J. Stark, R. Strobel, D. Gunther, S. E. Pratsinis and A. Baiker, Titania–silica doped with transition metals via flame synthesis: structural properties and catalytic behavior in epoxidation, J. Mater. Chem. 12, 3620–3625 (2002). 82. J. Kaspar, P. Fornasiero and M. Graziani, Use of CeO2 -based oxides in the three-way catalysis, Catal. Today 50, 285–298 (1999). 83. W. J. Stark, M. Maciejewski, L. Madler, S. E. Pratsinis and A. Baiker, Flame-made nanocrystalline ceria/zirconia: structural properties and dynamic oxygen exchange capacity, J. Catal. 220, 35–43 (2003). 84. L. Mädler, W. J. Stark and S. E. Pratsinis, Flame-made ceria nanoparticles, J. Mater. Res. 17, 1356–1362 (2002). 85. T. Tani, N. Watanabe, K. Takatori and S. E. Pratsinis, Morphology of oxide particles made by emulsion combustion method, J. Am. Ceram. Soc. 86, 898–904 (2003). 86. M. ValletRegi, F. Conde, S. Nicolopoulos, C. V. Ragel and J. M. GonzalezCalbet, Synthesis and characterization of CeO2 obtained by spray pyrolysis method, Mater. Sci. Forum 235, 291–296 (1997). 87. R. Maric, M. Oljaca, B. Vukasinovic and T. Hunt, Synthesis of oxide nanopowders in NanoSpraySM diffusion flames, Mater. Manuf. Process. 19, 1143–1156 (2004). 88. A. C. Sutorik and M. S. Baliat, Solid solution behavior of Cex Zr1 – x O2 nanopowders prepared by flame spray pyrolysis of solvent-borne precursors, Mater. Sci. Forum 403, 371–376 (2002). 89. R. M. Laine, T. Hinklin, G. Williams and S. C. Rand, Low-cost nanopowders for phosphor and laser applications by flame spray pyrolysis, Metastable Nanocryst. Mater. 8, 500 (2000). 90. H. Schulz, W. J. Stark, M. Maciejewski, S. E. Pratsinis and A. Baiker, Flame-made nanocrystalline ceria/zirconia doped with alumina or silica: structural properties and enhanced oxygen exchange capacity, J. Mater. Chem. 13, 2979–2984 (2003). 91. I. Rossetti and L. Forni, Catalytic flameless combustion of methane over perovskites prepared by flame-hydrolysis, Appl. Catal. B 33, 345–352 (2001). 92. E. Campagnoli, A. Tavares, L. Fabbrini, I. Rossetti, Y. A. Dubitsky, A. Zaopo and L. Forni, Effect of preparation method on activity and stability of LaMnO3 and LaCoO3 catalysts for the flameless combustion of methane, Appl. Catal. B 55, 133–139 (2005). 93. R. Leanza, I. Rossetti, L. Fabbrini, C. Oliva and L. Forni, Perovskite catalysts for the catalytic flameless combustion of methane — preparation by flame-hydrolysis and characterisation by TPD-TPR-MS and EPR, Appl. Catal. B 28, 55–64 (2000). 94. G. L. Chiarello, I. Rossetti and L. Forni, Flame-spray pyrolysis preparation of perovskites for methane catalytic combustion, J. Catal. 236, 251–261 (2005).

480

R. Strobel et al.

95. C. Oliva, L. Bonoldi, S. Cappelli, L. Fabbrini, I. Rossetti and L. Forni, Effect of preparation parameters on SrTiO3±δ catalyst for the flameless combustion of methane, J. Mol. Catal. A 226, 33–40 (2005). 96. R. Strobel, M. Piacentini, L. Mädler, M. Maciejewski, A. Baiker and S. E. Pratsinis, Two-nozzle flame synthesis of Pt/Ba/Al2 O3 for NOx storage, Chem. Mater. 18, 2532–2537 (2006). 97. H. Schulz, L. Mädler, R. Strobel, R. Jossen, S. E. Pratsinis and T. Johannessen, Independent control of metal cluster and ceramic particle characteristics during one-step synthesis of Pt/TiO2 , Mater. Res. 20, 2568–2577 (2005). 98. J. P. Hansen, J. R. Jensen, H. Livbjerg and T. Johannessen, Synthesis of ZnO particles in a quench-cooled flame reactor, AIChE J. 47, 2413–2418 (2001). 99. K. Wegner, W. J. Stark and S. E. Pratsinis, Flame-nozzle synthesis of nanoparticles with closely controlled size, morphology and crystallinity, Mater. Lett. 55, 318–321 (2002). 100. R. Strobel, F. Krumeich, W. J. Stark, S. E. Pratsinis and A. Baiker, Flame spray synthesis of Pd/Al2 O3 catalysts and their behavior in enantioselective hydrogenation, J. Catal. 222, 307– 314 (2004). 101. T. Tani, A. Morikawa, H. Sobukawa, M. Kimura, K. Takatori and A. Suda, Powder characteristics and three-way catalytic activity of hollow alumina made by the emulsion combustion method, J. Ceram. Soc. Jpn. 113, 473–477 (2005). 102. L. Mädler, W. J. Stark and S. E. Pratsinis, Simultaneous deposition of Au nanoparticles during flame synthesis of TiO2 and SiO2 , J. Mater. Res. 18, 115–120 (2003). 103. R. Strobel, J. D. Grunwaldt, A. Camenzind, A. Baiker and S. E. Pratsinis, Flame-made alumina supported Pd-Pt nanoparticles: structural properties and catalytic behavior in methane combustion, Catal. Lett. 104, 9–16 (2005). 104. S. Hannemann, J.-D. Grunwaldt, F. Krumeich, P. Kappen and A. Baiker, Electron microscopy and EXAFS studies on oxide-supported gold-silver nanoparticles prepared by flame spray pyrolysis, Appl. Surf. Sci. (2006) (in press). 105. H. Keskinen, J. M. Makela, M. Vippola, M. Nurminen, J. Liimatainen, T. Lepisto and J. Keskinen, Generation of silver/palladium nanoparticles by liquid flame spray, J. Mater. Res. 19, 1544–1550 (2004). 106. D. Chakraborty, H. Bischoff, I. Chorkendorff and T. Johannessen, Mixed phase Pt-Ru catalysts for direct methanol fuel cell anode by flame aerosol synthesis, J. Electrochem. Soc. 152, A2357A2363 (2005). 107. J. A. Azurdia, J. Marchal, P. Shea, H. P. Sun, X. Q. Pan and R. M. Laine, Liquidfeed flame spray pyrolysis as a method of producing mixed-metal oxide nanopowders of potential interest as catalytic materials. Nanopowders along the NiO–Al2 O3 tie line including (NiO)0.22 (Al2 O3 )0.78 , a new inverse spinel composition, Chem. Mater. 18, 731–739 (2006). 108. S. Loher, W. J. Stark, M. Maciejewski, A. Baiker, S. E. Pratsinis, D. Reichardt, F. Maspero, F. Krumeich and D. Günther, Fluoro-apatite and calcium phosphate nanoparticles by flame synthesis, Chem. Mater. 17, 36–42 (2005). 109. M. Huber, W. J. Stark, S. Loher, M. Maciejewski, F. Krumeich and A. Baiker, Flame synthesis of calcium carbonate nanoparticles, Chem. Commun., 648–650 (2005). 110. R. N. Grass and W. J. Stark, Flame synthesis of calcium-, strontium-, barium-fluoride nanoparticles and sodium chloride, Chem. Commun., 1767–1769 (2005). 111. R. Strobel, M. Maciejewski, S. E. Pratsinis and A. Baiker, Unprecedented formation of metastable monoclinic BaCO3 nanoparticles, Thermochim. Acta 445, 23–26 (2006). 112. T. T. Kodas and M. J. Hampden-Smith, Aerosol Processing of Materials. Wiley-VCH, Weinheim (1999).