IntroductionToday, catalysts are mostly manufactured using

wet-chemical techniques. Incipient wetnessimpregnation, sol-gel, precipitation, grafting andsolid-state reactions, just to name a few, are batchprocesses requiring several aftertreatment steps,such as filtration, drying and calcination (1–3).

Flame technology is a scalable, continuous andwell-established method for production ofnanoparticles in large quantities. For decades it hasbeen used for large scale manufacture of simplecommodities, such as carbon black, pigmentarytitania, silica waveguide preforms, fumed SiO2 andalumina (4–6). Since the 1970s, flame-made mate-rials have been used as catalyst supports (i.e. Al2O3,SiO2, TiO2) (1, 2, 7) and as photocatalysts (mainlyTiO2) (8). Progress in flame technology during thelast decade, especially the development of the so-called flame-spray pyrolysis (FSP), contributeddecisively to the creation of new and sophisticatedmaterials for catalysis (9) as well as for sensors,biomaterials, microelectronics and other applica-tions (10). In fact, today it is possible to make upto 1 kg h–1 of nanostructured materials by flameaerosol technology even in an academic laboratory.Figure 1 shows such a pilot FSP unit in operationalong with a baghouse particle collection unit (10).

Among other materials, pgms supported onvarious ceramic carriers have been prepared by

flame technology during the last decade. Figure 2shows a schematic of wet and flame processes forsynthesis of supported pgm catalysts. In contrastto conventional multi-step wet chemistry tech-niques, such as precipitation of the support with itswashing, filtration, drying and calcination steps

Platinum Metals Rev., 2009, 53, (1), 11–20 11

Flame Synthesis of Supported PlatinumGroup Metals for Catalysis and SensorsNOVEL FLAME PROCESSES ALLOW SYNTHESIS OF SUPPORTED PGMs IN A SINGLE STEP

By Reto Strobel and Sotiris E. Pratsinis*Particle Technology Laboratory, Institute of Process Engineering, Department of Mechanical and Process Engineering,

ETH Zurich, Sonneggstrasse 3, CH-8092 Zurich, Switzerland; *E-mail: pratsinis@ptl.mavt.ethz.ch

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, whilerecently developed flame processes allow synthesis of supported pgms in a single step includingthe support material. Here, we describe flame processes and how finely dispersed supportedpgms are made in these flames. So Pt/Al2O3, Pd/ZnO, Rh/Al2O3, Pt/Ba/Al2O3 and others arehighlighted regarding their materials properties and performance as catalysts as well as ingas sensors.

DOI: 10.1595/147106709X392993

Fig. 1 Pilot plant for flame synthesis of nanostructuredmaterials with control unit (top) and baghouse filter forparticle collection (bottom) (10)

followed by impregnation with the pgm and thewashing to calcination sequence, flame synthesisallows preparation of the support and the pgm in asingle step. As-prepared particles can be separatedeasily from the gas phase by filtration. In general,flame-made supported pgms consist of finely dis-persed pgms on top of a nonporous support(Figure 3) exhibiting high external surface area andthus good thermal stability and mass transfer prop-erties (11–13).

Here, recent progress in flame synthesis of sup-ported pgm as well as gold and silver catalysts andsensors is presented along with the unique struc-tural and catalytic properties of these materials.Some examples which will be discussed hereinclude supported platinum and palladium cata-

lysts for enantioselective hydrogenation, bimetallicPd-Pt catalysts for catalytic combustion ofmethane, Pt/Ba/Al2O3 for NOx-storage reduc-tion, supported Au catalysts for selective oxidationof carbon monoxide, TiO2 and zinc oxide photo-catalysts containing Pt and Ag as well as Pt/tinoxide gas sensors.

Flame Synthesis of SupportedPGMs

Among other flame processes, FSP plays adominant role in the manufacture of supportedmetals due to its versatility in terms of applicableprecursors. Traditional flame synthesis as appliedfor the manufacture of fumed Al2O3, SiO2 or TiO2

relies on volatile precursors, which are evaporated

Platinum Metals Rev., 2009, 53, (1) 12

Fig. 2 Comparison of process steps involved in the preparation of Pt/Al2O3 catalysts either by flame synthesis (leftsequence) or conventional precipitation/impregnation techniques (right sequence)

prior to feeding them into the flame reactor (9). Asvolatile precursors at reasonable prices are rare andonly available for a limited selection of metals, thedevelopment of the FSP technique, which relies onprecursors dissolved in a combustible liquid(14–17), opened a new range of materials availablethrough flame processes, including supportedpgms. Possible precursors for the pgm and thesupport include metal salts (nitrates, carbonates,chlorides) or organometallic compounds, typicallyalkoxides, carboxylates or acetylacetonates. Thesemetal precursors are dissolved in an appropriatesolvent (such as alcohols, hydrocarbons or car-boxylic acids), fed through a spray nozzle anddispersed by a gas, resulting in fine droplets. Thisspray is ignited, forming a spray flame, where thesolvents and usually also the metal precursors areevaporated and combusted. Product particle for-mation then takes place by nucleation from the gasphase. Maximum flame temperatures typically liebetween 2000 and 2500ºC and are quenched with-in a few milliseconds down to 400ºC (18, 19).

Flame synthesis of supported pgms generallyresults in well dispersed pgm particles on top of aceramic carrier, which is the desired structure forcatalysts. This suggests that supported metals areformed by a sequential pathway in the flame (13,20). Figure 4 depicts the basic mechanism for formation of supported pgm particles. As thevolatility of the ceramic supports is generallylower than that of the pgm, the support formsfirst by nucleation at higher temperatures than thepgm. Then the support particles grow by coagula-tion and sintering in the hot flame environment.Later, as the temperature drops further away fromthe flame, the pgm also nucleates (21). One canimagine two possible pathways: homogeneousnucleation of the pgm and subsequent depositionon the support, or direct heterogeneous nucle-ation on the support. The latter is the more

Platinum Metals Rev., 2009, 53, (1) 13

Flame-made “nonporous” support

Conventional “porous” support

Fig. 3 Comparison of pgm distribution (dark) on flame-made nonporous and conventionally porous supports. Thenonporous nanoparticles of the flame-made supportresult in high accessibility of the pgms and high thermalstability of the support

Deposition of pgm

Homogeneous

nucleation

of pgm

Support growth and

agglomeration

Support nucleation

Heterogeneous

nucleation

of pgm

Fig. 4 Schematic for the formation of supported metals ina flame. In a first step the support is formed by nucleationfrom the gas phase, and then at lower temperatures pgmsnucleate heterogeneously on the support and/orhomogeneously from the gas with deposition on thesupport

probable pathway (9). Compared to flame synthe-sis of pgms in the absence of a support, theirdeposition and immobilisation on the supportslows down their sintering, resulting in particlestypically smaller than 5 nm. Table I gives anoverview of supported noble metals made inflames included in this review.

The FSP technique allows precise and repro-ducible control of the support surface area and thesize of the pgm particles. The most important fac-tor determining support and metal particle size isthe high-temperature particle residence time,which can be controlled by precursor and disper-sion gas flow rates during FSP (16, 22). Increasingprecursor flow rates and/or lowering the disper-sion gas flow rate results in longer flames andhigher temperatures, and thus larger support parti-cles as sintering after coagulation is enhanced. Thesize of the pgm particles is mostly determined bythe metal loading in terms of weight per surfacearea of the support. Figure 5 shows the Pd particlesize on Al2O3 for different Pd contents (1–7.5wt.%, controlled by the Pd concentration in theprecursor solution) and Al2O3 supports with differ-

ent specific surface areas (23). The Pd particle sizeshows a nearly linear dependence on the Pd load-ing, increasing with higher Pd loadings. This showsthat the control of Pd particle size is not indepen-dent of the support formation. In contrast, onlyfor Au the particle size was independent of thesupport surface area or composition, suggestinghomogeneous nucleation of Au particles and laterdeposition (24).

Support and metal formation can be decoupledby rapidly quenching the flame at a certain heighteither by expansion through a critical flow nozzle(9, 25) or by introduction of a cold gas stream (21).Depending on the point of quenching, particlegrowth can be stopped at any position in the parti-cle formation sequence outlined in Figure 4. Earlyquenching resulted in smaller support particles(TiO2) and polydisperse Pt particles (very large andvery fine) by incomplete evaporation of the Pt pre-cursor, while late quenching had no further effecton the support but froze the growth of the Pt par-ticles. This gave a minimum in Pt particle size byquenching exactly at the point of complete Pt pre-cursor evaporation (9, 21).

Platinum Metals Rev., 2009, 53, (1) 14

Table I

Overview of Flame-Made Supported Noble Metals with the Corresponding Support Material and TheirApplications

Metal Support Application References*

Platinum Al2O3 Enantioselective hydrogenation (13)BaCO3/Al2O3 NOx storage-reduction (28)CexZr1–xO2 Three-way catalyst (11) TiO2 Oxidation catalyst, photocatalysis (20, 26, 31)SnO2 Sensor (27)

Palladium Al2O3 Enantioselective hydrogenation (23)La-Al2O3 Catalytic combustion (12)La2O3 Catalytic combustion (36)SiO2 Hydrogenation (37)ZnO Sensor (38)

Rhodium Al2O3 Selective hydrogenation (40)CexZr1–xO2 Syngas production (41)

Gold Fe2O3 Selective CO oxidation (42)SiO2 Selective CO oxidation (24, 42)TiO2 Selective CO oxidation (24, 42, 43)

Silver TiO2 Photocatalysis, antimicrobial (44)ZnO Photocatalysis (45)

*See main text for Reference numbering

PlatinumPlatinum supported on TiO2 was the first sup-

ported pgm made in a flame by a vapour-fedaerosol flame process (20). It consisted of well dis-persed Pt clusters on top of the TiO2 support.However, the low volatility of the applied plat-inum(II) acetylacetonate (Pt(acac)2) precursorlimited the production rate to a few mg per hour.FSP quickly superseded that process, as it workswith liquid precursor solutions and easily allowssynthesis of large sample quantities within a fewminutes. All of the following examples involve cat-alysts that have been prepared by this method.

FSP of solutions containing Pt(acac)2 and alu-minium sec-butoxide resulted in the formation ofsmall Pt clusters (< 5 nm) well dispersed on top ofAl2O3 nanoparticles in the range 10 to 30 nm (13).Similar structural properties have been observedfor Pt on TiO2 (26), CexZr1–xO2 (11) or SnO2 (27).Flame-made Pt/Al2O3 catalysts were tested fortheir behaviour in enantioselective hydrogenation,where the open structure of the support stronglyincreased the catalytic activity due to improvedmass transfer properties, without losing selectivitycompared to commercial catalysts (13).

Pt on solid solutions of ceria-zirconia is a stan-dard automotive three-way catalyst, and whenprepared by FSP the catalyst did not lose its low-temperature oxygen exchange capacity afterexposure to high temperatures (1100ºC) (11). The

Pt clusters were oxidised in the as-prepared mate-rial and rather high temperatures were necessaryfor reduction into metallic Pt under a H2 atmos-phere.

More recently developed automotive catalystsare the so-called NOx storage-reduction (NSR)catalysts that are used for abatement of NOxunder lean conditions. Fully functionalPt/Ba/Al2O3 NSR catalysts could be prepared bytwo-nozzle FSP as shown in Figure 6 (28).Spraying and combusting the precursors of all

Platinum Metals Rev., 2009, 53, (1) 15

0 .0 0 .1 0 .2 0 .3 0.4 0.5 0.6 0.7

2 0

3 0

4 0

5 0

2

3

4

5

6

Pd

dis

pe

rsio

n (

%)

Pd loading (mg/m2

)

Pd

pa

rtic

le s

ize

(n

m)

Pd loading, mg m–2

Pd d

ispers

ion, %

Pd p

artic

le s

ize, n

m

Fig. 5 Pd dispersion andcorresponding Pd particle sizeof flame-made Pd/Al2O3 withdifferent Pd contents and Al2O3

surface area. The Pd dispersiononly depends on the Pd loadingper available Al2O3 surface (23)

Fig. 6 Two-nozzle flame synthesis as applied forpreparation of Pt/Ba/Al2O3 NOx storage-reductioncatalysts. The separation of Al2O3 (left flame) andPt/BaCO3 (right flame) particle formation affordscatalysts containing well-mixed but individual Al2O3 andBaCO3 particles (28)

three components together resulted in amorphousbarium species which were not active for NOxstorage. However, using two spray nozzles, one asan Al and the other as a Pt/Ba source, allowed syn-thesis of individual Al2O3 and BaCO3 particles withthe Pt clusters on top of them. These materialsexhibited similar behaviour to impregnated cata-lysts during NSR. At higher Ba loadings, however,the flame-made NSR catalysts showed faster andhigher NOx uptake (29). This is attributed to theunique formation of relatively unstable BaCO3 inflame-made materials, even at high Ba content.Similar observations could be made for flame-made Pt/Ba/CeO2-ZrO2. Replacing Al2O3 withCexZr1–xO2 allowed regeneration of these NSR cat-alysts after thermal deterioration (30).Well-dispersed Pt clusters were also obtained onTiO2 and increased the activity of TiO2 for photo-catalytic degradation of sucrose (26) and methylorange (31).

Beyond the widespread application of Pt andPd in catalytic processes, they can also be appliedto enhance the gas sensing properties of semicon-ducting metal oxide sensors (27). Flame synthesisof sensor materials is attractive as it allows synthe-sis of the active sensing material and depositionon a sensor substrate in one step (32). This way,very porous (up to 98% porosity) Pt/SnO2 sen-sors have been prepared, exhibiting a very lowdetection limit for CO (down to 1 ppm) with high

sensor response. Direct FSP deposition of aPd/Al2O3 layer acting as a water filter on top of asensing SnO2 layer decreased the influence ofhumidity on the sensor signal (33). Moreover,micropatterning of these directly deposited layersfollowed by flame annealing has made possible thesynthesis of well adherent Pt/SnO2 sensing layers(34).

PalladiumSimilar structural properties as for Pt were

observed for Pd on Al2O3 (23). Figure 7 shows ascanning transmission electron microscope(STEM) image of flame-made Pd/Al2O3 contain-ing 5 wt.% Pd and the corresponding Pd particlesize distribution. In the as-prepared materials, Pdwas in the form of PdO which was very stable andhardly reduced into metallic Pd compared toPd/Al2O3 prepared by other methods (35). Thiscan be traced to the generally strong metal–sup-port interactions of flame-made materials. ThesePd/Al2O3 catalysts exhibited good selectivity andactivity in enantioselective hydrogenation (23). Theeasy control of pgm particle size, as shown inFigure 5, was used to elucidate the structure sensi-tivity of the reaction.

Flame-made Pd supported on very stable lan-thanum-doped Al2O3 was used for catalyticcombustion of methane and investigated for itshigh-temperature stability (12). After annealing at

Platinum Metals Rev., 2009, 53, (1) 16

0 1 2 3 4 5

Pd diameter, nm

Fra

ction, %

40

30

20

10

50 nm

Fig. 7 STEMimage of flame-made Pd/Al2O3

containing 5 wt.%Pd. The number-based Pd particlesize distribution asderived from STEMimages shows atypical lognormaldistribution (dottedline) (23)

1100ºC, flame-made Al2O3 retained a surface areaof 90 m2 g–1 compared to 57 m2 g–1 for a commer-cial Al2O3. Adding a few weight per cent of Laincreased the thermal stability of flame-madeAl2O3 even further. The generally high thermal sta-bility of flame-made supports is traced to theabsence of small pores, which makes thempromising catalysts for high-temperature process-es. However, the deactivation of the catalyst incatalytic combustion up to 1000ºC was mainlycaused by sintering of Pd, with only marginaleffects from the stability of the support, prepara-tion method (impregnation, flame synthesis) or Lacontent (12). Similar observations have been madefor Pd on La2O3 particles which were made byspraying aqueous precursor solutions in a hydro-gen flame. These particles were then directlycollected in a water suspension that was directlyapplied to coat metallic honeycombs (36).

In a similar way Pd/SiO2 containing 0.5–10wt.% Pd has been prepared by FSP (37). Thesecatalysts have been tested for selective hydrogena-tion of 1-heptyne. Pd has also been added to ZnOparticles to improve their ethanol sensing behav-iour (38). Further, flame-made Pd/CeO2 particleshave been applied successfully in solid oxide fuelcells (39).

RhodiumA study comparing flame-made Rh/Al2O3 cat-

alysts with two commercial ones has beenpublished recently (40). The as-prepared flame-made material consisted of well dispersed Rh2O3

particles (< 5 nm) that needed relatively high tem-peratures (> 400ºC) to be reduced into metallicRh. In contrast to commercial catalysts, a largeamount of cationic Rh species were formed duringreduction, indicating a strong interaction betweenRh and the Al2O3 support. Similarly to the previ-ously described Pt/Al2O3 and Pd/Al2O3, theflame-made Rh/Al2O3 also showed much higheractivity in the chemoselective hydrogenation of3,5-di-(trifluoromethyl)-acetophenone than com-mercial catalysts, together with comparableselectivity. FSP was also applied for synthesis ofRh/CeZrO4 catalysts that were used for syngasproduction from butane (41).

Iridium, Osmium and RutheniumThese pgms are not as widely applied in cataly-

sis as Pt or Pd and thus no reports on flamesynthesis of them are available. So far they haveonly been prepared by FSP in combination withother metals as described later in the section onbimetallic systems.

Gold and SilverAlthough not belonging to the pgms,

supported Au will be discussed here as it hasattracted a lot of interest since the discovery of itscatalytic “non-inertness”. Therefore it is not sur-prising that flames have also been employed forthe manufacture of supported Au catalysts. Ausupported on TiO2 and SiO2 has been made byFSP (24). On both supports, the Au particle sizewas larger than the previously described pgms, andvaried between 3 and 15 nm depending on the Aucontent (1–4 wt.%). The catalytic behaviour ofFSP-made Au catalysts supported on SiO2, TiO2

and Fe2O3 in the selective oxidation of carbonmonoxide was investigated, revealing similar rela-tionships between Au particle size or supportmaterial and catalytic performance to those seen inconventionally prepared catalysts (42). Au/TiO2

was also directly deposited from the gas phaseonto surfaces in microsystems (43). Shadow mask-ing allowed deposition of these catalysts in smallmicrochannels, which were directly used for test-ing catalytic reactions in microreactors, here forinstance the oxidation of CO.

Silver is also not a pgm but exhibits similarstructural properties when made in flames. Theformation of small Ag clusters has already beenobserved on TiO2 (44), SiO2 (42) and ZnO (45).Besides having some interesting applications dueto its strong antimicrobial properties (44), Ag alsoenhances the photocatalytic activity of the sup-port. The addition of Ag increased thephotocatalytic activity of TiO2 for decompositionof stearic acid (44) and the activity of ZnO fordecomposition of methylene blue (45).

Bimetallic Systems Alloying pgms is often used as a method for

fine-tuning and optimising their properties. The

Platinum Metals Rev., 2009, 53, (1) 17

formation of bimetallic clusters can be difficultwhen working with conventional techniques.Flame synthesis facilitates formation of bimetallicclusters as both constituents are formed practicallysimultaneously in the flame. This has already beenexploited for synthesis of bimetallic Pd-Pt/Al2O3

where Pd and Pt were present in the same clustersand form a Pd-Pt alloy upon reduction (46). Figure8 shows an STEM image of this material with thecorresponding energy dispersive X-ray (EDX)analysis of a single metal particle, revealing thepresence of Pd and Pt in the same cluster. Here,the addition of 5% Pt to Pd increased its reducibil-ity and especially the noble metal cluster stabilityagainst sintering at high temperatures (up to800ºC), resulting in an improved resistance againstdeactivation during catalytic combustion ofmethane (46).

Similar observations were made for Au-Ag,used as a catalyst for selective CO oxidation. Thecatalyst was proved to consist of small alloyed Au-Ag bimetallic clusters (< 10 nm) dispersed onTiO2, Fe2O3 or SiO2 (42). Small alloyed Pt-Rh clus-

ters were also deposited on Al2O3 and evaluatedfor their behaviour in the partial oxidation ofmethane (47). Alloy formation was also observedfor unsupported Ag-Pd particles (48), whereas Pt-Ru was only partly alloyed either unsupported (49)or supported on Al2O3 (47). It can be assumed thatsimultaneous nucleation of noble metals in the gasphase or on the support, combined with similaroxidation states, leads to the formation of bimetallic clusters. This makes flame technology apromising alternative to wet-phase processes forpreparation of various bimetallic or even “multi-metallic” catalysts, as already shown for acombination of four pgms (Pt, Pd, Rh and Ru) onAl2O3 for partial oxidation of methane (47).

ConclusionsFlame aerosol technology is well suited for one-

step synthesis of supported pgms that can beapplied in catalysis and as gas sensors. In generalthe pgms form finely dispersed clusters on top ofnanostructured ceramic supports, which can con-sist of most metal oxides or their mixtures. A

Platinum Metals Rev., 2009, 53, (1) 18

AlC

O

Cu

Pt

PdPt

Cu

0 5 10

Energy, eV

60

40

20

Counts

20 nm

Pt

Fig. 8 STEM image of bimetallicPd-Pt/Al2O3, with EDX spot analysis of asingle Pd-Pt cluster (indicated by anarrow), revealing the presence of Pt and Pdin the same cluster on top of Al2O3

(adapted from (46)). Similar observationshave been made for Au-Ag (42) or Pt-Rh(47)

characteristic property of flame-made supportmaterials is their open structure, which gives highthermal stability and good mass transfer propertiesthat can result in catalysts with improved activityand longevity. Strong metal–support interactionsare often observed, resulting in stabilisation of thepgms as oxides or cationic species. Bimetallic andmultimetallic clusters are easily formed by thisflame aerosol technology. Rapid synthesis (below1 min for 100 mg) and reproducibility make flame

synthesis a promising method for high throughputscreening of catalysts with different pgm and support compositions.

AcknowledgementsFinancial support by the Swiss Commission

for Technology and Innovation (KTI grant No.8316-1) and the loan of platinum chemicals by Johnson Matthey PLC are gratefully acknowledged.

Platinum Metals Rev., 2009, 53, (1) 19

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The AuthorsReto Strobel received his diploma inChemistry from ETH Zurich, Switzerland,in 2002. He then joined the ParticleTechnology Laboratory at ETH andreceived his Ph.D. in 2006. Since thenhe has been a Lecturer at ETH andProject Leader of joint industry–ETHresearch programmes. His researchinterests lie in flame synthesis ofnanoparticles, focusing on control of

materials properties and their performance in catalysis and othersystems.

Sotiris E. Pratsinis obtained his Ph.D.from the University of California, LosAngeles (UCLA), U.S.A., in 1985. Hewas a professor at the University ofCincinnati, Ohio, U.S.A. (1985–2000)and since 1998 he has been Professorof Process Engineering and adjunctProfessor of Material Science at ETHZurich. His research focuses on thefundamentals of aerosol synthesis of

films and particles for catalysts, sensors and composites forelectroceramics and biomaterials. Together with his Ph.D.students he has published over 200 refereed articles, receivedseveral patents licensed to industry and contributed to thecreation of four spin-off companies.