Effect of the nature of the support on molybdenumcatalytic behavior in diesel particulate combustion

Silvana Brauna,*, Lucia G. Appelh, Martin Schmalc"Departamento de Qufmica da Pantiflcia Universidade Cat6lica da Ria de Janeiro, PUC-Ria,

R. Marques de Saa Vicente, 225 Cavea, 22453-900 Ria de Janeiro, Brazilhlnstituto Nacianal de Tecnalagia, Ministeria da Ciencia e Tecnalagia, Av. Venezuela 82 s. 518, 20081-310 Ria de Janeiro, Brazil

'NUCATIPEQICOPPE, Universidade Federal da Ria de Janeiro-UFRJ, Centro de Tecnalagia bl.C, 68502-970 Ria de Janeiro, Brazil

Mo/Si02 and Mo/Ti02 catalysts with three different molybdenum contents were prepared using non-porous supports and thethermal spreading method for the combustion of a particulate material (PM). The results of scanning electron microscopy (SEM)and N2 adsorption/desorption techniques showed that the thermal spreading preparation method does not induce relevanttextural changes on the supports. X-ray diffraction (XRD) results showed the occurrence of thermal spreading of Mo03 ontosilica and titania supports. Diffuse reflection spectroscopy (DRS) results provided clear evidence of different Mo species onthese systems: highly dispersed species on the silica catalysts and polymolybdates on the titania catalysts. It may be inferred thatwhen prepared by the thermal spreading method the nature of the support determincs the kind of molybdenum species formed inthese catalysts, irrespective of the Mo content. The reactive data were evaluated by differential scanning calorimetry (DSe),using a physical mixture of PM and the catalysts. The silica-supported catalysts showed higher reactivity for PM combustionthan the titania-supported ones, being the most active the systems with the Mo monolayer. The results suggested that thedispersed species are far more active than the polvmolybdates or Mo03 itself.

and also by HC, SO) or H2S04, and water adsorbed atthe surface [2,3],

There are a few studies published on the develop-ment of catalytic converters for particulates fromdiesel. Such converters consist of filters with a cata-lytic coating, allowing the retention of PM and itscombustion [4]. Many catalysts have been evaluatedfor this reaction, and metal transition oxides [5-10],precious metal supported [11] and also more complexsystems like Mo/K/Cu [9,12] and Co/Mg/K [13,14]could be mentioned. However, nowadays the most

The presence of particulate material (PM) in theatmosphere is one of the main environmental problemsof urban areas concerning the exhaust diesel engines[I]. These particulate materials are agglomerates con-stituted by carbon nuclei with some inorganic material,

Corresponding author. Tel.' ~55-21-3114-1813;fax: +55-21-2274-1323.E-mail address:braun@rdc.puc-rio.br (S. Braun).

promising catalysts are based on eutectic mixtures ofvanadium, molybdenum and alkaline-metal oxides[] 5-19], which are mobile liquids. However, accord-ing to van Setten [20] they could not be applied topassenger cars, trucks and busses due to their lowactivity, i.e. these coating systems are not efficientbecause the particulate burn off temperature is higherthan the ordinary temperature in the diesel exhaust[1,2].

Previous studies about this subject have shown thatMoO} seems to be active for this reaction [9]. On theother hand, Mo-supported titania catalyst are veryactive in oxidation reactions [21,22]. It is also wellknown that the nature of the support as well as Mocontent determines the formation of Mo surface spe-cies [23-25], which show different catalytic perfor-mances. However, there is no information on how Mo-supported catalysts and their different species affectthe particulate material combustion.

Thus, the aim of this work is to study the effect ofthe nature of the support and the molybdenum contentfor Mo/Ti02 and Mo/Si02 systems on the perfor-mance of the combustion of a model particulatematerial.

Two commercial supports (Degussa AG) with simi-lar morphological and textural properties were used:aerosil OX50 silica (further designated S) with anaverage particle size of 40 nm, and Ti02 P25 (furtherdesignated T) 80% as anatase, with an average particlesize of 21 nm, both with apparent density of130 g dm -} [26]. These supports were chosen in orderto obtain non-porous materials, since only the externalsurface area seems to be effective in the combustionreaction of particulates. These materials were pre-heated in a muffle furnace for 4 h. Molybdenumwas added to the supports by thermal spreading, asdescribed previously [24,27]. This preparation methodwas an important choice to avoid possible texturalalterations of the supports [28]. Thus, MoO} and thesupport were ground in a mortar for 10 min, and themixture calcined in a muffle furnace at 773 K for 24 h,at a heating rate of 5 K min -I. The molybdenum

contents were chosen based on previous data thatallows to a good dispersion lower than or equal tothe monolayer, which is reached at 4 llmol Mo per m2

for silica [29] and 8 llmol m - 2 for Ti02 [30]. Thesevalues were calculated from OH groups of the sup-ports reacting with MoO}. These catalysts were clas-sified according to the Mo content, in llmol m-2 ofsupport, followed by a letter that identifies the support:8T, 4T, 2T, 8S, 4S, 2S and ] S.

Catalyst 8S contains Mo content above the mono-layer [27,29] and was used for comparison with MolTi02 system. Physical mixtures of MoO} + Si02 andMoO} + Ti02 were used for comparison with therespective catalysts, here classified by the previousidentification followed by subscript mf.

A model particulate material (Printex-V, Degussa)was used for the reaction. The textural properties ofthis particulate (PM) are average particle size of25 nm, specific area of 72 m2 g -1 and apparent densityof ]60 g dm-3 [26].

The following characterization techniques wereemployed:

• X-Ray diffraction (XRD): a Philips PW 1410 pow-der diffractometer with Cu Kcx radiation (40 kV,30 mA) plus a Ni filter was used with an angularrange between ]9 and 800 in a 0.02° per step and0.8 s counting per step.

• Diffuse reflection spectroscopy (DRS): these mea-surements were carried out on a Varian Cary 5spectrophotometer (Harrick Scientific) with a dif-fuse reflectance accessory of Praying Mantis geo-metry. The samples (2 mm thickness) were placedin the sample holder and the supports Si02 and Ti02

were used as references. The spectra were recordedin the range between 190 and 800 nm with a scan-ning speed of ] 800 mm min - I.

• Surface area and pore size distribution: these ana-lyses were performed on Micromeritics ASAP equip-ment, after pretreatment at 393 K under vacuum.

• Scanning electron microscopy (SEM): the sampleswere examined in a LEICA S440 scanning electronmicroscope, operating at 20 kV, 100-50 pA andworking distance of ] 0-13 mm. The samples werecoated with a thin layer of Au (10 nm).

The reactivity experiments related to PM combus-tion reaction were carried out by experiments ofdifferential scanning calorimetry (DSC) in a RigakuTG 8110 balance. The catalyst was mixed with PM ata 2: 1 catalystPM (w/w) ratio by gentle shaking in abottle. The reaction was carried out on 2 mg of mixedsample under flowing synthetic air at 48 ml min - t andat a heating rate of 10K min - I. The performance ofthe catalysts was evaluated taking the maximum tem-perature of the exothermic peak. The differencebetween the maximum temperature of the DSC curvesof the PM/catalyst mixture and the PM/support mix-ture indicated the catalytic effect.

Table I displays the specific surface area of thesupports and catalysts. Comparing these values, itseems that the thermal spreading preparation methodhardly influences the specific area of the supports. Forthe catalysts, these values decrease as the molybde-num content increases, but this effect is more pro-nounced for the titania-supported catalysts.

Fig. I shows the pore size distribution curves forsilica and for catalysts IS, 2S and 4S, whereas Fig. 2,for titania and catalysts 2T, 4T and 8T. All samplesexhibit a large range of pore diameter, and dV/dlog Dvaries only slightly between 20 and 100 A, while itincreases from 100 to 1000 A. This feature is morepronounced for titania-supported catalysts. It seemsthat the contribution of macropores is significant forthe total porosity. However, these supports do notpresent pores but only interstices resulting from thepacking spheres. Such interstices look like pores in theN2 adsorption/desorption measurements and their dif-ferent sizes may explain the random packing of thespherical particles [281. As the pore size distribution

Table ISpecific area of the supports and the catalysts prepared

0,40

0,35

0,30

6' 0,25Ol.2 0,20'0

> 0,15'0

0,10

0,05

0,0°10 100pore diameter I A

Fig. I. Pore size distribution curves in a d V/dlog f) vs. porediameter plot for: (a) the silica. (b) IS. (e) 2S and (d) 4S catalysts.

curves of the supports and the catalysts are similar itmay be inferred that the thermal spreading preparationmethod does not cause relevant textural alterations ofthe supports, and hence non-porous systems with highspecific area can be achieved.

The scanning microscope images of silica andtitania are displayed in Figs. 3 and 4, respectively.The images of both the supports are very similar andthey indicate that the microspheres of such supportsare not well dispersed, but are agglomerations with anaverage diameter of the order of micrometers. Theimages of catalysts 4S and 8T (Figs. 5 and 6, respec-tively) present similar features to those of the supports

0,40

0,35

0,30

6' 0,25Ol0 0,20=0

> 0,15'0

0,10

0,05

0,0°10 100pore diameter / A

Fig. 2. Pore size distribution curves in a dV Idlog 0 vs. porediameter plot for: (al Ti02• (bl 4T and (e) ~ncatalysts.

and agree very well with the results from the nitrogenadsorption/desorption technique. Thus, the agglom-eration of the catalysts particles was neither due to theactive phase nor of the preparation method. Fig. 7shows that agglomeration may even occur on theparticulates.

Fig. 8 shows the diffractogram of silica (pattern a)and the corresponding Mo/Si02 systems: 2S (b), 4S(c), 8S (d) and 4Smf (e), and Fig. 9 displays thediffractograms of Ti02 (a) and those of catalysts 4T(b), 8T (c), and 8Tmf (d). Both figures show markedpeaks concerning the main reflection planes of Mo03.

Comparing the diffraction patterns of TiOTsupported

Fig. 7. Scanning electron microscopy images of the particulatematerial.

catalysts with the support itself, it seems that they arealmost alike. Similar results were observed for silica-supported catalysts, although, some reflections forMo03 show very low intensity on sample 4S. How-ever, sample 8S exhibits Mo03 peaks as expected forthe higher contents of Mo03, exceeding the mono-layer. Comparison of 4Smf and 8Tmf patterns with thecorresponding catalysts show that the initial Mo03

crystalline phase was transformed into amorphous Mospecies, although, Raman spectroscopic analyses

40 50 602 e /0

Fig. 8. XDR patterns of: (a) silica, (b) 2S, (c) 4S (d) 8S and (e)4Smf. The marked peaks assigned to the main MoO] planesreflection.

10 20 30 40 50 60 702 e / 0

Fig. 9. XDR patterns of: (a) titanium oxide, (b) 4T, (c) 8T and (d)8Tmf. The marked peaks assigned to the main MoO] planesreflection.

already published showed that some small crystallitesof Mo03 remained in the silica-supported catalysts[27]. Such results combine with the following DRSspectra results provide evidence of the spreading ofMo03 over those supports [27,30].

Figs. 10 and I I show the DRS spectra of thesamples. Fig. 10 displays the curves of the silica

0,8

0,6

i? 0,4LL

0,2

0,0

200

~e(x 10)'~fY 10)

300 400'A / nm

Fig. 10. Diffuse reflectance spectra of: (a) 4S, (b) 8S, (c) 2S, (d)IS. (e) 8Smf and (f) 4Smf. Spectra of 8Smf and 4Smf samples wereamplified for better visualization.

0,08

0,06

iY0,04~

l.L

0,02

0,00

200 300 400 500j, / nm

Fig, I L Diffuse reflectance spectra of: (a) 8T,nf' (b) 8T (c) 4T and(d) 2T

systems: 4S (curve a), 8S (b), 2S (c), IS (d), 8SIll1

(e)and 4SIllf (f), while Fig, II shows the curves of Ti02samples: 8Tm! (a), 8T (b), 4T (c) and 2T (d), Thesespectra show absorption bands in the UV- Vis regionbetween 200 and 400 nm, which are assigned toMo(VI) ion with dOconfiguration due to ligand-metalcharge transfer (LMTC): 02

-7 M06 ,

Silica-supported catalysts present absorption max-ima between 230 and 240 nm (curves a, b, c and d), Asit is seen, these bands are broad and with a shoulder inthe region at higher wavelength. However, the physi-cal mixtures 8SIllf and 4SIll1 show the absorptionmaximum around 360 nm, Qualitatively, it is possibleto notice some differences between the absorptionintensities of the samples (Kubelka-Munk function),The catalysts show higher intensities than the physicalmixtures, increasing as follows: IS < 2S < 4S. whichmeans that it increases as the amount of Mo does. Thiscan be also noticed on the physical mixtures, wherethe absorption intensity of 8Srnf is higher than that of4SIll!, However, it is important to point out that theintensity of the catalyst 8S is lower than the 4S,Similar behavior is observed on TiOrsupported cat-alysts, whose intensities also increase as the amount ofMo does (2T < 4T < 8T), On the other hand, differ-ently than the silica system, the absorption intensity ofmixture 8TIll1 is higher than that of the catalyst Whenthe band of the spectra of the catalysts is deconvoluted,they present two others with maxima around 360 and

410 nm, Mixture 8TIllf presents only one band with theabsorption maximum at 360 nm,

The band position of DRS spectrum dependsstrongly on structural features, such as cluster sizeand polyanion symmetry containing Mo ion. Fournieret aL [31] have shown that the condensation degree,i,e, the polyanion size, as well as its dispersion over thesupport influences the band position of the DRSspectra far more than the local symmetry of Mo(VI)ion (inner sphere of coordination). Therefore, thehigher the wavelength of the absorption maximum,the higher the molybdenum amount ions surroundingthe molybdenum center, which may be related to thedifferent molybdenum species and their dispersionover the support Thus, it is possible to infer thatthe Mo species formed on the Mo/Si02 catalystsare far more dispersed than those on the Mo/Ti02

systems, However, Mo on Mo/Si02 catalysts cannotbe considered more dispersed than on Mo/Ti02 sam-ples due to the Mo03 presence,

By means of the empirical relation proposed byWeber [32l, the absorption edges energy of the band ofDRS spectra can be associated to the relative distancebetween Mo ions. The positions of absorption edgeswere determined by finding out the energy intercept ofa straight line fitted through the low energy rise in theplot of [F(R) x hv]e versus hv. where F(R) is theKubelka-Munk function for an infinitely thick sampleand hv is the energy of an incident photon, Accordingto that work, the energy values around 2,7 eV areassigned to polyanion [M060]lJf-, while for otherMo species. such as MoO" and polyanions[M0702416

, [Moe07f- and [Mo0412-, the energy

values are around 3,0, 3,3. 3,9 and 4.3 eV, respectively,The plots of [F(R) x hvf versus hv for the Mo/Si02

and Mo/TiOe catalysts are shown in Figs. 12 and 13,respectively. The calculated energy values for MolSiOe catalysts are 3.8 ± 0.1 eV, and therefore, thecondensation degree of Mo species formed on silicasurface is low, regardless of the Mo content On theother hand, the curves of MoITi02 catalysts can bedeconvoluted in two other curves, and application ofthe Weber transformation exhibits two energy values:2.7::r:0.1 and 3.2±0.1 eY. They are attributed to[M06019f and [M070e416 polyanions. Therefore,it is strong evidence that the main molybdenum spe-cies present in TiOe catalysts are highly condensedbidimensional polymolybdates,

201816

'>' 14.c 12><~ 100

0a:- 8~6420

1 2 3 4 5

hv / eV

Fig. 12. Positions of absorption edges energy determined by theplot of F(R) x hl'f vs. hI' for: (a) 4S. (b) 2S and (c) IS samples.

Machej et al. [33] showed by Raman spectroscopythe presence ofbidimensional polymeric molybdenumas the main species of Mo/Ti02 samples. which wereformed by highly distorted octahedral MoOn' due to astrong interaction with the support. When hydrated. itforms rM0702~]6 - and rMox02n]~ clusters. Thus.DRS results indicate the presence of bidimensionalpolymolybdates on TiOrsupported catalysts, irre-spective of the Mo03 content used for the preparationby thermal spreading.

Comparing the spectra of silica-supported catalystswith those of Ti02-supported catalysts. it can beobserved that the dispersed species display absorption

N 0,04

>..c: 0,03><-8 0,020:::'-'~

0,01

0.001 2 3

hv I eVFig. 13. Positions of absorption edges energy determined by theplot of [F(R) x. hl',2 YS. hI' for: (a) 8T. (bl 4T and (c) 2T samples.

~

~ 860~Q)

g 8402

Fig. 14. Combustion temperature of PM with the silica and titaniasupports and the catalysts prepared.

bands intensities much higher than those more aggre-gated. such as the polymolybdates, so. the intensityincrease as the condensation degree decrease. Prob-ably. this fact does not allow us to see the band relativeto MoO, for catalyst 8S, even though this compoundmay be verified by XRD results. Considering that DRSis a bulk technique. it is possible that the absorptionband intensity present on catalyst 8S is lower than thaton catalyst 4S probably due to the remaining MoO"since the weight of the sample is practically constantand, therefore. sample 8S has a lower amount ofdisperse phase when compared to 4S sample.

Fig. 14 shows the DSC results of the PM combus-tion in the presence of the supports and the catalysts. Itshows that the performances of the silica-supportedcatalysts are rather distinct from that of the Ti02-supported catalysts. The Mo/Si02 catalysts are, irre-spective of the molybdenum contents, more efficient,burning PM at a lower combustion temperature. ThePM burns off at 898 K in the presence of silica, but thecombustion temperature decreases to 854, 840 and819 K in the presence of catalysts IS, 2S and 4S,respectively. Thus. the difference of the PM combus-tion temperature between catalysts IS, 2S and 4S andsilica is 44, 58 and 79 K, respectively.

The scanning microscope images show that thecontact between catalyst and PM means the contactbetween the agglomerates of PM and agglomerates ofcatalyst particles. Hence. it could be suggested that theactive surface of the catalyst is not the surface of theparticles but the surface of the agglomerates instead.

On the other hand, pore size distribution curves ofthese materials show that the contribution of inter-stices is significant for the total porosity. Therefore,these interstices may also allow the contact betweencatalyst and small sized or partial burning PM beyondthe surface ofthe agglomerated. In the case of oxygen,it could be proposed that it came from Mo species, asdescribed the catalytic oxidation mechanisms. Therole of the air would be the catalysts reoxidation.

Taking into account the scanning microscopeimages and the pore size distribution curves resultsit could be proposed that the contact between catalystand PM is almost the same for the different samples.Therefore, the DSC results are mainly related to thenature of these materials.

Comparing the combustion temperature of PM with4S and 4Smf, it turns out that the molybdenum specieson the catalyst are far more active than Mo03 itself.

The combustion temperature of PM with catalysts8S and 4S are very similar, although, the first samplehas twice the amount of Mo present in the latter. DRSand XRD results of catalysts 8S and 4S showed thepresence of Mo dispersed species and Mo03. So thecatalytic behavior of the silica-supported catalysts isassociated with these species. Taking into account thatMo03 content on 8S is much higher than that on 4S, itturns out that the Mo dispersed species are more activethan the Mo03 remaining from the spreading process.

Considering the temperature range of DSC experi-ments one can suppose that Mo03 spreading can occurduring the reaction, which means that Mo03 couldmigrate to the particulate. However, even regardingthese phenomena as possible, it is worth pointing outthat, based on the results obtained, what is reallyrelevant is the catalytic behavior of the Mo dispersedspecies. In addition, although, it is well known thatMo03-supported catalysts are not stable at high tem-perature in the presence of water vapor, recent resultsobtained in our laboratory have shown that the cata-lytic behavior is the same even after three consecutivecombustion reactions of the particulate material (DSCexperiments). So the Mo dispersed species seem to bestable in the reaction condition, and these results are inagreement with the conclusions mentioned earlier.

The temperature decrease of the PM combustion onthe TiOrsupported catalysts is much lower than thaton the silica-supported ones, being significant only forcatalyst 8T. The combustion temperature of PM is

901 K with Ti02, and 901, 888 and 844 K, with cat-alysts 2T, 4T and 8T, respectively. Again, comparingthe combustion temperature of 8T and 8Tmf, the surfacepolymolybdates of the TiOrsupported catalysts appearto have catalytic performance similar to that of Mo03.

So, the nature of the support determines whichmolybdenum species are formed on those catalysts,irrespective of the Mo content used by the thermalspreading. In addition, as textural and morphologicalproperties of both TiOr and Si02-based catalysts arevery similar, it may be suggested that the contactbetween both the catalytic systems with PM is verysimilar too. Thus, it may be inferred that Mo speciesexhibit distinct performance in the PM combustionreaction, being the dispersed species far more activethan the polymolybdates or Mo03 itself. Although,these systems still cannot be employed in practicalconditions, the studies concerning the behavior of theMo dispersed species on silica-supported catalystsmay suggest a way to develop real catalytic systems.

The results suggested that the Mo dispersed specieson silica catalysts are far more active in diesel parti-culate combustion than the polymolybdates or Mo03

itself.

The authors thank Dr. Arnaldo Alcover Neto(CETEM/CNPq) for the SEM micrographs andDegussa for the supply of supports and particulatematerial samples. This work was supported by theConselho Nacional de Desenvolvimento Cientffico eTecnologico, CNPq, Brazil.

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