0263–8762/03/$23.50+0.00# Institution of Chemical Engineers

www.ingentaselect.com=titles=02638762.htm Trans IChemE, Vol 81, Part A, August 2003

DETAILED CHARACTERIZATION OF VARIOUS POROUSALUMINA-BASED CATALYST COATINGS WITHIN

MICROCHANNELS AND THEIR TESTINGFOR METHANOL STEAM REFORMING

R. ZAPF1, C. BECKER-WILLINGER3, K. BERRESHEIM2, H. BOLZ3, H. GNASER2,V. HESSEL1, G. KOLB1, P. LOB1, A.-K. PANNWITT3 and A. ZIOGAS1

1Institut fur Mikrotechnik Mainz GmbH (IMM), Mainz, Germany2Institut fur Ober�achen- und Schichtanalytik GmbH (IFOS) und Fachbereich Physik,

Universitat Kaiserslautern, Kaiserslautern, Germany3Institut fur Neue Materialien gem. GmbH (INM), Saarbrucken, Germany

A fundamental study concerning the preparation of porous alumina washcoats inmicrochannels for the application in heterogeneous gas phase catalysis wasperformed, focusing on the pre-treatment of the microstructures, properties, and

adhesion of the washcoats as well as the testing of the prepared catalysts. Steel microstruc-tures, which are manufactured by wet-chemical etching with chloride solutions, showsigni�cant chlorine content at the surface due to the etchant. Anodic oxidation and thermaltreatment of the microstructures signi�cantly reduce the undesirable chlorine content, which isassumed to have deleterious effects on the catalyst activity. Good adhesion of the porouscatalysts, deposited by a two-step process, washcoating and wet impregnation, was demon-strated by a mechanical test. Cross-sectional pro�le accuracy was reasonable and reliable. Atthe example of a CuO=Cr2O3=Al2O3 system, the distribution of the impregnated componentswithin the washcoat, in lateral (depth of the coating) and horizontal directions (at the coating’ssurface), was studied by secondary ion mass spectrometry (SIMS). It turned out that Cr2O3

was homogeneously distributed both in horizontal and lateral direction, whereas the content ofCuO decreased with the washcoat depth and islands of accumulated material on the surfacewere formed. The activity of the CuO=Cr2O3=Al2O3 system was investigated using differentalumina carriers for methanol steam reforming. The activity found was correlated with thetotal catalyst surface area offered to the reaction system.

Keywords: catalysis; microchannels; washcoat; alumina; methanol steam reforming.

INTRODUCTION

The development in microreaction technology initiated aquest for applications where the new tool has signi�cantadvantages over conventional reactors (Ehrfeld et al., 1999,2000; Gavriilidis et al., 2002; Fletcher et al., 2002; Wegenget al., 2000; Jensen, 2001; De Mello and Wootton, 2002).One �eld of application is heterogeneously catalysed gasphase reactions (Besser et al., 2002; Kestenbaum et al.,2000a; Wießmeier and Honicke, 1996; Srinivasan et al.,1997;Fichtneret al., 2001;Rebrov et al., 2001a; Rouge et al.,2001; Worz et al., 2001; Wan et al., 2002; Cominos et al.,2002; Hessel et al., 2000). In this way the catalysts and theircarriers have become the focus of scienti�c investigationsontheir preparation, morphology, porosity, composition etc.

Catalysts and their carriers are provided in microchannelsby various means and in various geometric forms, such asbulk materials or granular beds (Fichtner et al., 2001; Hesselet al., 2000; Kestenbaum et al., 2000; Ajmera et al., 2001).

However most often catalyst=carrier coatings in microchan-nels are applied as documented by many reports on theirapplication for various gas phase processes (Ehrfeld et al.,2000). These coatings are made most prominently via thewashcoat route followed by wet impregnation (see e.g.Rouge et al., 2001; Liauw et al., 2000; Pfeifer et al.,2002; Dupin, 1984). Anodic oxidation is also widely used(see e.g. Liauw et al., 2000; Rebrov et al., 2001b) togenerate porous oxide carrier layers, as far as aluminiumreactors can be employed. Besides this, thin �lm techniquessuch as CVD (see e.g. Liauw et al., 2000) and PVD, namelysputtering (Jensen et al., 1997), serve to generate thincatalyst �lms. To complete this list of approaches towardscatalyst=carrier coatings, various other techniques weretested such as aerosol techniques (Franz et al., 2000), sol–gel techniques (Zhao and Besser, 2002), an advancedplasma electrochemical process belonging to anodic sparkdeposition (Gorges et al., 2002), and �nally electrolysis(Kusakabe et al., 2001).

721

Detailed description of the procedure for preparation ofcatalysts=carriers, for instance, is given explicitly for wash-coats (see e.g. Rouge et al., 2001; Pfeifer et al., 2002). Atypical sequence of catalyst preparation steps is given thereas follows. The inlet and outlet chambers, encompassing themicrochannel arrays on microreactor plates, are protectedwith a thin polymer �lm. The suspension is deposited on themicrochannel plate and the excess suspension is wiped off.The coating derived in such a manner is dried at roomtemperature accompanied by shrinkage of the washcoat.Having cleaned the top parts of the microchannel �ns, thedried washcoats are calcined typically at 500–600¯C to burnout the binder and to remove the protecting polymer�lm (Rouge et al., 2001; Liauw et al., 2000). Thereafter,the catalyst is brought into the washcoat by means ofimpregnation.

At present, only very few comprehensive reports are solelyattributed to catalyst=carrier coatings in microchannels andtheir basic characterization in terms of preparation, morphol-ogy, porosity, composition etc. One of theses exceptions isPfeifer et al. (2002), which is concerned about a characteriza-tion of ZnO washcoats made by dispersion of nanoparticlesand electrophoretic deposition of Al2O3 nanoparticles.

The majority of other, less-detailed reports on catalyst=carrier coatings in microchannels focuses mainly on directsurface imaging, determining porosity and sometimes ongiving cross-sectional pro�les. In numerous publications,SEM or other types of images of such catalyst surfaces arefound, most often before and seldom after use (Kestenbaumet al., 2000b,c). Cross-sectional pro�les of coatings arerarely given; the few images shown so far indicate non-uniform pro�les both in semicircular and rectangular chan-nels. Correspondingly the coating depth varies according toits position in the channel (Franz et al., 2000; Zhao andBesser, 2002; Walter et al., 2001). Other detailed informa-tion on microreactor catalyst characterization is rarely given,an exception being the simulation of a temperature pro�leinside an alumina layer (Rebrov et al., 2001a).

METHODS AND MATERIALS

Materials and Coating Procedure

For the preparation of g-alumina washcoats a suspensionwith a standard composition of 20 g g-alumina (Alfa Aesar,3 micron-APS powder), 75 g deionized water, 5 g binderpolyvinyl alcohol (Fluka, polyvinyl alcohol 100,000) and1 g acetic acid was generated. In the �rst step, the binder wasdissolved in water in a beaker by stirring smoothly with alaboratory magnetic stir bar at 60¯C for 2 h and left withoutstirring overnight. The alumina powder and acetic acid wereadded successively, again without stirring. This mixture wasthen stirred overnight. In order to remove air bubblesentrapped in the viscous mixture, a suf�cient period forthe self-release of the bubbles was necessary, typicallyranging from 3 days to 2 weeks.

For the preparation of the washcoats with a-alumina(particle size 1 mm) or boehmite (particle size 10 nm)precursor, which is transformed later in the calcinationstep to g-alumina, a similar procedure was applied.

The alumina washcoats were applied onto the etched chan-nels of the stainless steel platelets using conventional wash-coating methods such as described by Liauw et al. (2000).

Different from this reported procedure, removal of excessmaterial that remains on the �ns is done here before drying(see below), whereas Liauw et al. (2000) carried this out afterdrying or calcination. The washcoated plates were thencalcined in an oven for 1 h at a temperature of 600¯C. Beforeimpregnation the washcoats were evacuated in an exsiccatorconnected to a vacuum pump to remove air from the pores.Pure carbon dioxide was then passed to �ll the pores. For theimpregnation, 10 wt% aqueous solutions of the targetcompound [metal nitrates, Cu(NO3)2, Cr(NO3)3] were used.After impregnation and drying, the impregnated washcoatswere calcined again. The temperature was set 20–50¯C abovethe respective operation temperature of the reaction applied.This was done to keep the porosity as high as possible.Calcination at higher temperatures usually leads to a decreaseof the speci�c surface area of the active component. Beforetesting, the catalysts were reduced (5% H2, 95% N2) at atemperature of 250¯C for 2 h.

Microstructuring

Plates of stainless steel (X2CrNiMo17 12 2 and X6CrNi-MoTi17 12 2) were etched by means of a photoetchingtechnique (Striedieck, 1991) by a commercial provider(Atztechnik Herz, Epfendorf, Germany), based on wet-chemical etching employing aqueous iron trichloride solu-tions. Three types of plates with different dimensionsof the semi-circular microchannels were manufactured:(a) 500 mm £ 300 mm; (b) 750mm £ 300mm; (c) 500 mm £70 mm (width £ depth).

Catalyst Testing Reactor

A reactor specially designed for the determination ofcatalyst activity in parallel (primary screening) or serialoperation (catalyst optimization) was used for methanolsteam reforming. The reactor housing was made of stainlesssteel (X10CrAlSi18) and graphite served as material for thegaskets. The reactor is part of a specially constructed set-upfor methanol steam reforming, see Kolb et al. (2002)for more details on a further version of this reactor.The reactor (outer dimensions: approx. 70 £ 55 £ 64 mmwithout tubing) comprised a housing (X10CrAlSi18)with ten cartridges on which ten microstructured plates(X10CrAlSi18, length: 50 mm, geometry a) were positionedand graphite served as material for the gaskets. By simpleexchange of the end caps a decision could be made whetherto operate the micro reactor serially or in parallel. Forparallel operation the ten microstructured plates were fedsimultaneously by ten substreams which emerged from onecommon stream. The ten substreams left ten separate outletsthat were analyzed serially. The reactor is part of a speciallyconstructed set-up (Cominos et al., 2003) for methanolsteam reforming, that comprises liquid feed tanks, acommercial evaporator (Bronkhorst Company) and severalgas feed lines with mass �ow controllers (BronkhorstCompany).

Methods

X-ray photoelectron spectroscopy (XPS)The measurements were performed with the XPS system

PHI 5500 of Physical Electronics, equipped with asmall spot supplement. The excitation was induced by Al

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722 ZAPF et al.

K-radiation (1486.6 eV). The analysed area was 1 mm £3 mm and for the angle between the analyser and the sample70¯ was chosen.

Nitrogen sorptionThe speci�c surface area and pore size distributions were

determined by nitrogen sorption using a Sorptomatic 1990(Carlo Erba Instruments) automatic apparatus and calculatedby the BET method (surface area) respective the BJHmethod (pore size distribution). The measurements wereperformed with coatings deposited on polymer foils andtreated in the same manner as the washcoats.

X-ray diffraction (XRD)The XRD investigation of the alumina washcoats was

performed using a Siemens D5000 diffractometer at grazingincidence (1¯). The diffracted radiation was observed in theangle range (2y) of 20–70¯ (step width 0.03¯) at roomtemperature.

Scanning electron microscopy (SEM)SEM characterization of the samples was conducted with

Model JSF 6400 F from JEOL.

Optical microscopyThe pro�les of the coated and uncoated platelets were

recorded by means of a UBM Microfocus with a resolutionof 200 points per mm.

Secondary ion mass spectroscopy (SIMS)These measurements were performed on two different

instruments. The depth-pro�le analyses were carried out ona double-focusing magnetic sector instrument (CamecaIMS-4f; Migeon et al., 1986). A 5.5 keV Cs‡ primary-ionbeam was used for sputtering (incidence angle ¹42¯ offnormal) and positive molecular secondary ions of thegeneral type MCs‡ (where M constitutes the element ofinterest) were detected; monitoring these species typicallyfacilitates the quantitativeevaluation of the measured signals(Gnaser, 1994). The primary-ion current density amountedto about 0.7 mA cm¡2, which translates into an erosion rateof approximately 1 nm s¡1 (this estimate is based on the

density of crystalline Al2O3); the circular analysis area(¹30 mm in diameter) was centred within the etched chan-nels of the support material. The lateral element distribu-tions were recorded in a time-of-�ight mass spectrometer(Charles Evans & Associates TRIFT 3; Schuler et al., 1990);for these analyses a 25 keVGa‡ ion beam with a beamdiameter of ¹1 mm was employed for sputtering. Becausethe total ion �uence required for taking the images in thoseexperiments corresponded only to a fraction of an atomiclayer, the analytical information relates to the topmostsurface of the specimens.

RESULTS AND DISCUSSION

Effect of Etching Agents

Since the microstructures were manufactured by wet-chemical etching using aqueous iron trichloride solutions,chlorine could migrate in the stainless steel of the micro-structured plates. Former studies by other authors showedthat the presence of chlorine can lead to catalyst deactivation(Twigg, 1996).

Accordingly, XPS measurements, which allow for aquanti�cation of the composition of the stainless steelsurface, were performed selectively in the microchannelregion of the respective plate in order to study the effectsof etching on the surface of the microstructures. A non-microstructured, i.e. non-etched, steel plate was taken asreference. A 550% increase of chlorine was found afterthe etching procedure. Therefore efforts were made toreduce the amount of chlorine by electrochemical andthermal methods, which are frequently applied pre-treat-ment methods for similar purposes (see Figure 1). Besidesusing these two techniques, combinations thereof wereapplied. This resulted in seven different types of pre-treatment.

All these experiments show that it is possible to reducethe chlorine amount signi�cantly, in the order of 75–95%.The chlorine content is reduced to values close to thechlorine content before etching (close to the detectionlimit of XPS). The best results in terms of reducing thechlorine content were obtained by calcination of the plateletsat 800¯C. In this case, the chlorine concentration was evenlower than the reference.

Characterization of the Washcoats

As a �rst step the temperature stability of the g-aluminawashcoats was evaluated, as an extension of the operationtemperature of the catalysts up to 800¯C is crucial for awidespread applicability (e.g. for hydrocarbon reforming).The speci�c surface area, the pore size distribution and thethermal stability of the phases were applied to judge carrierstability.

Speci�c surface=pore size distributionBy varying the calcination temperature of the g-alumina

washcoat, its properties are affected signi�cantly. Inparticular, the speci�c surface area is decreased at highercalcination temperatures, while the pore diameter does notchange signi�cantly (Table 1).

Figure 1. Atomic concentration of chlorine in dependence of the pre-treatment.

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CHARACTERIZATION OF ALUMINA-BASED CATALYST 723

XRD measurementsThe obtained diagrams (see Figure 2) show good accor-

dance with the data from the literature reference JCPDS(Huang et al., 1990; Shirasuka et al., 1976), with a smallshift to higher 2y values. The (*)-marked re�ex belongs tothe stainless steel substrate. Hence the �ndings prove thatcoatings with single phases were achieved.

At temperatures exceeding 700¯C a phase transition ofg-alumina to phases intermediate (e.g. d) to a-alumina maywell occur. Accordingly, washcoats with different calcina-tion temperatures were prepared in the temperature range of600–800¯C. Their XRD diagrams (see Figure 3) showed the

higher crystallinity of g-alumina (cubic, JCPDS 74-2206Shirasuka et al., 1976) with increasing temperature, but noindication of other alumina phases is found. Thereforemeasurements of the corresponding powders wereperformed revealing the formation of minor amounts of amonoclinic alumina phase (JCPDS 79-1559; Zhou andSnyder, 1991). At 800¯C the sample is still mostlycomposed of g-alumina.

Pro�lesBy applying the washcoat method to different channel

geometries, signi�cant differences concerning the g-aluminawashcoat pro�les were observed, especially for microstruc-tured platelets with different channel depths, e.g. 70 and300 mm (see Figure 5). Owing to the strong surface tensionof the aqueous suspension, thicker washcoat layers wereobtained near the channel walls than in the centre of thechannel. By using a suspension with a g-alumina content of30%, a thickness of between 20 (centre of the channel) and70 mm (near the channel walls) was found for microchannels500 mm (see Figure 5a) and 750 mm (see Figure 5b) wide.

For shallow channels with a depth of 70 mm, the washcoatlayer shows a more uniform thickness. While the deeperchannels have a maximum deviation in coating thickness by

Figure 2. Comparison of the XRD diagrams of the washcoats with referencedata: g-alumina, calcined at 600 C; a-alumina, calcined at 800 C.

Figure 3. XRD diagrams of g-alumina washcoats calcined at differenttemperatures.

Figure 4. Comparison of XRD diagrams of g-alumina powders calcined atdifferent temperatures with reference data.

Table 1. Properties of alumina washcoats as determined by nitrogensorption.

SystemCalcination

temperature (¯C)Speci�c surfacearea (m2g¡1)

Median poreradius (nm)

g-Al2O3 600 69 17g-Al2O3 700 63 16g-Al2O3 800 56 19a-Al2O3 800 7 17

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724 ZAPF et al.

a factor of 3.5, the shallow 70 mm channels have a coatingthickness of 10–15 mm in the centre of the channeland 15–20 mm at the channel wall, i.e. a deviation by afactor of 2 in the worst case (see Figure 5c).

AdhesionThe good adhesion of the g-alumina washcoat layers

reported here is evidenced by two �ndings: �rstly, SEMimaging (see Figure 5) reveals that there are no uncoatedareas or voids and that the coatings are crack-free. Secondly,a test for the mechanical stability of the coatings wasapplied. The coated catalyst platelets were placed on aguided metal block (weight 664.4 g) and fell down adistance of 0.5 m, reaching a velocity of 3 m s¡1 at theimpingement to a stainless steel plate. The determinedweight loss of the coated platelet was used as a referencevalue for judging adhesion quality. The washcoats that wereprepared according to the method described above show novisual defects after this test and the weight loss wasdetermined to be less than 1%.

Uniformity of coating in various microchannelsIn order to study the uniformity of the coating technique

applied, the mass of coating in each microchannel on amicro-structured plate was determined. Optical pro�lingtechniques using autofocusing instruments such as theUBM Microfocus allow an accurate and non-destructiveway of measuring surface pro�les. By determining cross-sectional areas of uncoated and coated microchannels, thecross-sectional area of the coating results as the difference ofboth. It is proportional to the respective coating massprovided that the density is constant over the whole coatingand that there are no signi�cant changes of the coatingpro�le along the channel.

In Figure 6, the catalyst mass, derived from the cross-sectional area, is given as normalized values (catalyst mass inone channel=average mass of all channels). The values weredetermined for a g-alumina coating on microstructuredplatelets with a channel depth of 300 and channel width of500 mm.

These measurements show only small maximal deviations( § 5%) with respect to the mass of the coating within thechannels. Even though the deviations are expected to levelout over the length of the channels, an exact quanti�cation

of the quality of the �ow distribution needs to be performedas a future task.

Characterization of the Active Components

In-depth monitoring of element concentrations—depthpro�ling

The distribution of Cu and Cr within the g-Al2O3 coatingwas studied by monitoring concentration–depth pro�les ofall relevant elements by means of secondary ion massspectrometry (SIMS). This is done in an erosive mannerdue to the removal of atoms by sputtering, starting from thecoating’s top, yielding plots of the atomic concentrations ofthe elements vs the erosion time (in seconds; typically a rateof approximately 1 nm s¡1 is applied). Besides Cr, Cu, Aland O, the main constituent of the stainless steel plate,namely Fe, was analysed.

Figure 7 is typical for many other spectra, which, althoughbeing different in detail, show the same general features as aresult of the above mentioned coating procedure:

concentrations up to 13 wt% of active species;formation of four major zones of distinct concentrationlevels—surface region, alumina carrier (alumina wash-coat), intermediate region and steel plate (see Figure 7);

Figure 5. Washcoat cross-sectional pro�les for microchannels of various dimensions (maximum width and depth of the semi-circular microchannels:(a) 500mm 300mm; (b) 750mm 300mm; (c) 500mm 70 mm).

Figure 6. Mass of g-alumina coating in each microchannel on one micro-structured plate (300mm 500 mm).

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CHARACTERIZATION OF ALUMINA-BASED CATALYST 725

enrichment or depletion of components at the surface;constant or decreasing pro�les of components within thealumina washcoat.

The concentration of the components at the surface and asurrounding small portion of the coating below that has athickness of about 0.5 mm, the so-called surface region isde�nitely different from the residual major fraction of thecoating. This is clearly visible for the distribution of Fe andCu that have contents about 500% and 200%, respectively,higher at the surface as compared within the washcoat. Thisenrichment is obviously so pronounced that it displaces to aminor part even the content of the carrier material alumina.This indicates either a �lling of the pores in the carrier withCu far above average or, more likely, formation of Cu-richlayers or islands thereof on top of the carrier (see Figure 8for more information). The Cr distribution remains nearlyconstant over the whole depth.

Figure 7. Analysis of the distribution of the components within the catalystCuO=Cr2O3=g-Al2O3 by SIMS.

Table 2. Comparison of the values for the content of the detectedelements in the different regions.

ElementsContent(atom%)

Content(wt%)

c (Cr)surface 5.5 12.8c (Cr)surface-alumina carrier 4.7 10.9c (Cr)alumina carrier-intermediate region 5.1 11.9

c (Cu)surface 3.2 9.1c (Cu)surface-alumina carrier 1.7 4.8c (Cu)alumina carrier-intermediate region 0.7 2.0

c (Fe)surface 0.7 1.8c (Fe)surface-alumina carrier 0.1 0.3c (Fe)alumina carrier-intermediate region 0.1 0.3

c (Al)surface 31.8 38.4c (Al)surface-alumina carrier 34.6 41.7c (Al)alumina carrier-intermediate region 35.1 42.3

c (O)surface 59.1 42.2c (O)surface-alumina carrier 57.5 41.1c (O)alumina carrier-intermediate region 58.6 41.9

Figure 8. Lateral distribution of aluminium (a), copper (b) and chromium(c) within the catalyst CuO=Cr2O3=g-Al2O3; brighter areas show zones withhigher metal content.

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726 ZAPF et al.

The fact that the impregnated element Cr is present incontents as high at the bottom as at the surface is anotherhint for a continuous pore network throughout the wholedepth of the coating. It, moreover, demonstrates that theimpregnating solution penetrates the carrier to the steelinterface, i.e. that the evacuation method is effective. Inconsequence, this points to another reason than incompletewetting to explain the Cu enrichment at the surface, sinceboth corresponding salts Cu(NO3)2 and Cr(NO3)3 wereoffered in aqueous solutions of similar concentration andviscosity. The catalyst had a signi�cantly higher loading ofchromium than copper because chromium was the �rstimpregnant, followed by copper. At present, one can onlyspeculate on the detailed reason. Principally, this may be dueto existing coverage of alumina sites by the �rst impregnant,making a second coating more unlikely, up to even partlyclosure of pores by this.

In the ‘steel plate’ region (long erosion times), largechanges in concentration are observed. Here, in an inter-mediate region, both the steel plate and the alumina carrierare measured. This stems �rst from the curved pro�le of themicrochannel bottom giving varying concentration distribu-tion in horizontal position. To a minor extent, the surfaceroughness of the etched surface adds to the horizontalchanges in concentration. At very long erosion times, onlyelements of the steel plate are found. Since Cr is also amajor content of the steel, the concentration of Cr reaches astationary value.

Lateral monitoring of element concentration—surfaceimaging

By using SIMS as well, the lateral distribution of Cu, Crand Al on the surface of the g-alumina washcoat was studiedvia monitoring their distribution (see Figures 8 and 9). Theircontent within the analysed area of 200 mm £ 200 mm isrepresented by different brightnesses. Brighter spots repre-sent higher metal content.

‘O’-shaped zones enriched mainly in Cu and to a lowerextent in Cr of a size of about 30 mm £ 25 mm are found (seeFigure 8b and c). These enriched zones covercompletely theirrespective underlying alumina surface; i.e. no Al is found inthe zones, thereby revealing an ‘inverse-zoned’ image inFigure 8(a). Cr is, apart from being accumulated in thesezones, also �nely distributed in the interstices between thezones, whereas Cu seems to be more or less solely concen-trated in the zones. In view of the zoned structure, the strongincrease in Cu concentration at the coating surface detectedalso by SIMS in Figure 9 shows that the corresponding depthpro�le was performed within such a zone.

Besides zoned surface structures, representing inhomo-geneously coated areas, there are also plates with areas ofhomogeneous distribution of the active components (seeFigure 9). These plates were dabbed off after removing theimpregnated plates from the impregnant. Compared withFigure 8(b) and in particular Figure 8(c), the respectiveaverage contents of Cr, and more pronounced for Cu, seemto be lower, due to the absence of species accumulation inzones.

Catalyst Performance

Catalysts of the system CuO=Cr2O3=Al2O3 with the sameamount of catalyst (1.50mg, weight ratio Cu=Cr ˆ 5) but

Figure 9. Lateral distribution of aluminium (a), copper (b) and chromium (c)within the catalyst CuO=Cr2O3=g-Al2O3; brighter areas show zones withhigher metal content.

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CHARACTERIZATION OF ALUMINA-BASED CATALYST 727

different alumina carriers (g-Al2O3, a-Al2O3, boehmite; seeTable 3) prepared by washcoating=co-impregnation, wereapplied for steam reforming of methanol (CH3OH ‡ H2O „CO2 ‡ 3H2) using a methanol–water feed molar compositionof 1:2 and a �ow rate of 1.5 g h¡1. Nitrogen was used as carriergas with a �ow rate of 50 mlmin¡1 for each plate. The reactionwas performed at 250¯C with a total �ow rate of 11.7mlmin¡1

for each catalyst plate and an absolute pressure of 1100mbar.The total �ow rate led to a residence time (t) of 0.46 s and alinear velocity (w) of 0.11m s¡1.

The measurements (see Figure 10) were performed withcatalysts, which were deactivated for 15 h prior to theactivity determination to achieve a stable operation. Theyshow, that the available speci�c surface of the washcoatsdetermine the activity of the catalyst, if the amount ofcatalyst is equal. According to Table 3, the largest surfacearea was available for the catalyst with the g-alumina carrier,thus leading to the highest activity of the three materialsapplied, followed by the boehmite and the a-alumina.

CONCLUSIONS

By applying a washcoating=co-impregnation technique itwas feasible to prepare alumina-based catalyst coatings ofreasonable pro�le accuracy, very good adhesion, highporosity, very good mass distribution between the variousmicrochannels on one plate, and reasonably good catalystdistribution. Beyond solving some of the main issues ofestablishing proper coating processes for microchannels,some further main issues to be solved could be clearlyoutlined and these will serve as a guideline for futureexperimental investigations. Among these will be the

achievement of a better distribution of the catalyst inthe washcoat in both lateral and horizontal direction. Theactivity of CuO=Cr2O3=Al2O3 catalysts with differentalumina precursors was demonstrated for steam reformingof methanol.

Detailed information on the carrier=catalyst propertiessuch as pro�le accuracy, adhesion, porosity, carrier andcatalyst distribution and control thereof are seen as theimportant step towards establishing a knowledge-basedmicro-chemical processing. It is the interplay betweensimulation=modelling and the advantageous properties ofmicro�uidic �ow, demonstrated successfully for otherpurposes (Ehrfeld et al., 2000), which may also promotecatalyst testing with microdevices. To enable this, as highdemands on the quality of the catalyst coatings have to beposed as on the microstructure itself.

REFERENCES

Ajmera, S.K., Delattre, C., Schmidt, M.A. and Jensen, K.F., 2001, Anovel cross-�ow microreactor for kinetic studies of catalyticprocesses, in Microreaction Technology, IMRET 5: Proceedings ofthe 5th International Conference on Microreaction Technology,Matlosz, M., Ehrfeld, W. and Baselt, J.P. (eds) (Springer, Berlin,Germany), pp 414–423.

Besser, R.S., Ouyang, X. and Surangalikar, H., 2002, Fundamental char-acterization studies of a microreactor for gas-solid heterogeneous cata-lysis, in Proceedings of the 6th International Conference onMicroreaction Technology, IMRET 6, pp 254–261.

Cominos, V., Hardt, S., Hessel, V., Kolb, G., Lowe, H., Wichert, M. andZapf, R., 2003, A methanol steam micro-reformer for low power fuel cellapplications (submitted to Chemical Engineering Communication).

De Mello, A. and Wootton,R., 2002,But what is it good for? Applicationsofmicroreactor technology for the �ne chemical industry, Lab on a Chip, 2:7N–13N.

Dupin, T., 1984, Alumina coating compositions for catalyst supports andprocess for their formulation, Rhone-Poulenc Specialites Chimiques,Courbevoie, France, US Patent 4529718.

Ehrfeld, W., Hessel, V. and Haverkamp, V., 1999,Microreactors, in Ullmann’sEncyclopedia of Industrial Chemistry (Wiley-VCH, Weinheim,Germany).

Ehrfeld, W., Hessel, V. and Lowe, H., 2000, Microreactors (Wiley-VCH,Weinheim, Germany).

Fichtner, M., Mayer, J., Wolf, D. and Schubert, K., 2001, Microstructuredrhodium catalysts for the partial oxidation of methane to syngas underpressure, Ind Eng Chem Res, 40(16): 3475–3483.

Fletcher, P.D.I., Pombo-Villar, E., Warrington, B.H., Watts, P., Wong, S.Y.F.and Zhang, X., 2002, Microreactors: principle and applications in organicsynthesis, Tetrahedron, 58(24): 4735–4757.

Franz, A.J., Ajmera, S.K., Firebaugh, S.L., Jensen, K.F. and Schmidt, M.A.,2000, Expansion of microreactor capabilities through improved thermalmanagement and catalyst deposition, in Microreaction Technology:3rd International Conference on Microreaction Technology, Proceedingsof IMRET 3, Ehrfeld, W. (ed.) (Springer, Berlin, Germany), pp 197–206.

Gavriilidis, A., Angeli, P., Cao, E., Yeong, K.K. and Wan, Y.S.S., 2002,Technology and application of microengineered reactors, Trans IChemE,80(A1): 3–30.

Gnaser, H., 1994, J Vac Sci Technol A, 12: 452–456.

Table 3. Catalyst data for the systems CuO=Cr2O3=Al2O3.

CarrierAmount of

washcoat (mg)Speci�c surfacearea (m2g¡1)

Available speci�csurface (m2)

Amount of catalyst(CuO=Cr2O3) (mg)

a-Al2O3 17.50 7 0.12 1.50g-Al2O3 13.20 69 0.91 1.50Boehmite 4.70 160 0.75 1.50

Figure 10. Methanol conversion of the catalysts CuO=Cr2O3=Al2O3

prepared with different alumina carriers.

Trans IChemE, Vol 81, Part A, August 2003

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ACKNOWLEDGEMENT

The authors thank the Universite Paul Sabatier (Laboratoire de Chimiedes Materiaux Inorganiques et Energetiques, Toulouse) and the Albert-Ludwigs-University (Institut fur Anorganische und Analytische Chemie,Freiburg) for the XRD measurements. Furthermore, the authors thank theGerman Ministry of Education and Research (BMBF) which supported theinvestigations reported here by a project under contract number 03C0312B.

ADDRESS

Correspondence concerning this paper should be addressed to Dr R. Zapf,Institut fur Mikrotechnik Mainz GmbH, Carl-Zeiss-Str. 18-20, 55129Mainz, Germany.E-mail: zapf@imm-mainz.de

Trans IChemE, Vol 81, Part A, August 2003

CHARACTERIZATION OF ALUMINA-BASED CATALYST 729

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