A Kotarba

A. Kotarba

B Azambre

B Ura

BAAL Setten van

D Fino

G Maniak

H An

JPA Neeft

K Krishna

M Yamawaki

M. Trębala

MS Gross

N Nejar

P. Legutko

P. Stelmachowski

S Bensaid

S Ito

W Bieniasz

Z. Sojka

Crossref

ORIGINAL PAPERRole of Electronic Factor in Soot Oxidation Process OverTunnelled and Layered Potassium Iron Oxide CatalystsP. Legutko • P. Stelmachowski • M. Tre˛bala •Z. Sojka • A. KotarbaPublished online: 26 February 2013 The Author(s) 2013. This article is published with open access at Springerlink.comAbstract This paper describes the investigations of thecatalytic activity in soot oxidation over well-defined ironoxide based materials. The nanostructuration of iron oxideby potassium into tunnelled (KFeO2) and layered(K2Fe22O34) ferrites and the surface promotion with CeO2results in the marked increase in the catalytic activity(decrease of the ignition temperature down to 210 C andT10 % to 310 C). The measurements of the catalysts workfunction showed that both nanostructuration and surfacepromotion with ceria of the best KFeO2 phase led toincrease of the electron availability (decrease of the workfunction). Strong correlation of the catalytic activity in sootcombustion of the Ce–K–Fe–O systems with the workfunction value was revealed for the first time in the modelstudies, and can be used as a guideline for optimisation ofthe real catalytic filters.Keywords Iron oxide  Potassium  Cerium oxide  Soot Diesel  Oxidation  Promotion  Ferrite  Work function1 IntroductionSoot particles emitted from diesel engines, due to theirsubmicron size and specific chemical composition, areamong the most hazardous urban air pollutants [1]. Nativesoot self-ignites at temperatures higher than 550 C, but itcould be burned in catalytic filters already at the temper-ature of the exhaust gases (200–400 C), provided anefficient catalyst is applied. The desired catalyst should notonly assure complete combustion at relatively low tem-peratures, but also be cheap and stable at broad range ofoperating conditions. Facile washcoating and the use ofecologically benign raw materials are another practicalrequirements. In addition to catalyst developments, severalapproaches have been adopted in the field of soot filtration[2] and filter regeneration [3], as they may affect thedesired morphology of the catalyst.A large number of catalytic formulations, includingsingle and mixed transition metal oxides, perovskite- andspinel-type oxides, and low melting point compounds,have been tested for the catalytic combustion of dieselsoot [4–7]. Despite of the progress made, the reactionmechanism for this process has been scarcely addressedand therefore a lot of work is still needed in order tounderstand and advance a rational catalyst design. Theinvestigated catalytic systems operate satisfactory onlyabove 500 C, but upon impregnation with alkali, amongwhich potassium [5, 8] is the most extensively investi-gated, their activity can be substantially enhanced [9]. Itspositive role on soot combustion has been discussed interms of: (i) an electron donor effect, increasing theoxygen reactivity; (ii) autogenic formation of low meltingpoint compounds, or eutectics with other components ofthe catalyst, wetting the soot surface to increase thecontact with the catalyst, (iii) the formation of a super-ficial carbonate intermediate and (iv) nanostructurizationof the parent oxide. The latter case is may be illustratedby the transformation of Fe2O3 into tunnelled and layeredferrites of KFeO2, and K2Fe22O34 structures. Such mate-rials exhibiting low work function and semiconductivityare promising generic materials for model redox catalyticDedicated to Professor Adam Bielan´ski on his 100th birthday.P. Legutko  P. Stelmachowski  M. Tre˛bala  Z. Sojka A. Kotarba (&)Faculty of Chemistry, Jagiellonian University, ul. Ingardena 3,30-060 Krako´w, Polande-mail: kotarba@chemia.uj.edu.pl123Top Catal (2013) 56:489–492DOI 10.1007/s11244-013-0003-8studies since electronic properties can be easily tuned byFermi level adjustment [10].On the other hand CeO2 alone or in combination withother metal oxides can be also highly active in oxidation ofsoot and it is suggested that the mechanism involved in such acase is based of the redox properties of ceria related to itsability to deliver the structural oxygen for the reaction [11].In the present work we described the nanostructurationof iron oxide into layered and tunneled K–Fe–O catalyst.The catalytic of the resultant ferrites can be furtherenhanced by doping with ceria nanoparticles. Both effectswere discussed in terms of electronic factor-structure–activity relationship.2 Experimental PartSeveral iron containing oxides were used in the study:a-Fe2O3, K2Fe22O34 as well as promoted and unpromotedKFeO2. a-Fe2O3 was obtained from Merck and used withoutfurther processing. The samples of K2Fe22O34 ferrite wereprepared by solid-state reaction of a stoichiometric mixtureof K2CO3 with a-Fe2O3 at 1200 C as described previously[12]. The KFeO2 sample was synthesized from K2CO3 witha-Fe2O3 according to the procedure described in [13]. Inbrief, the mixtures of finely ground powders were placed in aporcelain crucible and heated with the rate of 5 C/min instatic air up to the final temperatures of 800 C for KFeO2and 1200 C for K2Fe22O34. For 0.5–4 wt% Ce-dopedKFeO2 samples, the appropriate fraction of a-Fe2O3 powderwas replaced by CeO2. Four CeO2 types of precursors wereused for KFeO2 promotion: micrometric commercial cer-ium(IV) oxide (Merck), nanometric CeO2 obtained by sol–gel method, solid ammonium cerium(IV) nitrate (Merck)forming an oxide upon calcination and ammonium cer-ium(IV) nitrate water solution with subsequent impregna-tion of KFeO2, drying in 100 C for 1 h and calcination in400 C for 4 h. All such obtained samples were mixed withcarbon black, Printex 80 (Degussa AG) with catalyst to sootratios: 1/1, 4/1, 8/1. Both ‘‘loose’’ than a ‘‘tight’’ conditions[14] were tested in preliminary experiments, but since theresults were better reproducible in the latter conditions the‘‘tight contact’’ was applied in the comparative tests. Cata-lytic activity of the samples was determined by temperature-programmed oxidation (TPO) tests, carried out with thecatalyst/soot mixtures as described above. The amount ofsample loaded into the reactor varied from 50 to 100 mg.The soot combustion experiments were carried using aheating rate of 10 C/min, in the flow of 5 % O2 in He. Thegas phase composition (O2, CO, CO2, H2O) was monitoredby the QMS spectrometer (SRS RGA200). The ignitiontemperatures were determined with the use of the Arrhenius-like plot (lnX vs. 1/T), which gives rise to a sharp turningpoint defining the start of the oxidation process in a non-arbitrary way in contrast to more subjective yet commonlyuse method of T5 % or T10 %.The samples were characterized by X-ray diffraction(XRD) (X’pert Pro Philips) for phase composition, SEM(Philips XL 30 ESEM) for grain size morphology, N2-BET(Quantasorb Jr) and XPS (Prevac/VG SCIENTA R3000)for surface area and composition, and Kelvin Probe(McAllister KP6500) for evaluation of electronic proper-ties. The contact potential difference (VCPD) measurementswere carried out by the dynamic condenser method ofKelvin. To standardize the catalyst surface the measure-ments were carried out in vacuum of 10-7 mbar afterannealing the sample to 400 C. The work function valueswere obtained from a simple relation eVCPD = Ureference –Usample, using a standard stainless steel plate as a referenceelectrode (d = 3 mm, Ureference = 4.1 eV).3 Results and DiscussionTypical XRD patterns and the corresponding SEM imagesof the obtained ferrites are shown in Fig. 1. The diffractionFig. 1 The tunnelled (KFeO2) and layered (K2Fe22O34) structures ofthe generic potassium ferrites and the corresponding XRD patternsand SEM morphologies490 Top Catal (2013) 56:489–492123revealed that the obtained samples of KFeO2 (indexedwithin Pbca-JCPDS-ICDD-39-0892 databases as refer-ence) and K2Fe22O34 (P63/mmc-JCPDS-ICDD 31-1034)were virtually monophasic. The K2Fe22O34 has the hex-agonal layered structure and can be considered to be builtup from spinel blocks of c-Fe2O3 and separated by con-ducting potassium layers. The KFeO2 exhibits a tetragonaltunneled structure, that is not congruent with parentc-Fe2O3. Since this ferrite has no layer structure, K, Fe, andO constituents are spatially more uniformly distributed(Fig. 1).The introduction of ceria (at the level of 2 %) did not leadto any appreciable changes in the diffraction pattern. TheSEM images of the bare ferrites reveal different morpholo-gies of the KFeO2 and K2Fe22O34 grains (Fig. 1). Whereasfor the KFeO2 crystallites of 3–5 lm are interconnectedforming coarse blunt aggregates of about 10 lm, in the caseof K2Fe22O34 the hexagonal well-shaped crystallites of0.5–2 lm in diameter and thickness of *0.2 lm wereobserved, and resembled that of a single K?-b-ferrite withstoichiometric composition [12]. The EDX analysis showedthat the distributions of K and Fe are strongly correlated asexpected for the ferrite phases, whereas ceria segregates atthe surface in the form of spherical smaller nanocrystals withthe size in the range of 10–500 nm, depending on the appliedpreparation method. The XPS surface composition of theinvestigated samples indicated that within the surface layeronly the constituent elements were found with the diagnosticbinding energies for KFeO2 (BEFe2p3/2 = 709.7–710.0 eV,BEK2p3/2 = 292.2–292.4 eV, BEO1s = 530.4–530.8 eV)and K2Fe22O34 (BEFe2p3/2 = 710.2–710.4 eV, BEK2p3/2 =294.4–294.5 eV, BEO1s 529.4–529.5 eV). Small concen-tration of CeO2 combined with the overlapping Fe LMMAuger signal made it impossible to determine the Ce 3dbinding energies.The comparison of the soot combustion activity of theseveral investigated catalysts as a function of temperatureis presented in Fig. 2. The synergetic effect of nanostruc-turation of iron oxide by potassium into tunneled KFeO2and layered K2Fe22O34 and the surface doping with ceriacan be clearly noticed. The transformation of iron oxideinto ferrites by potassium addition lowers the light offtemperature by about 70 and 100 C for K2Fe22O34 andKFeO2, respectively. Yet, the reactivity of the most activephase KFeO2 can be further enhanced by *30 C upon the2 % CeO2 addition (Fig. 3, green points). In order to checkthe stability of the best material the 10 consecutive tests ofsoot oxidation were performed for the series of CeO2–KFeO2 catalysts (at each time a new portion of soot wasadded to the same catalyst at the same ratio of 8/1). Theresults revealed that the sustainable activity was main-tained within the experimental limit (the standard deviationof Tig \ 8.5 C).Fig. 2 Comparison of temperature-programmed oxidation of soot onseveral iron oxide based catalysts systems showing the synergeticeffect of iron oxide promotion by potassium and ceriaFig. 3 The correlation of catalytic activity (ignition and 10 %conversion temperatures) and electronic properties (work function)for the investigated iron oxide catalysts undoped and doped with 2 %of ceria (a). The effect of doping level of ceria on soot oxidationactivity of KFeO2 with the optimal content of ceria at 2 % (b)Top Catal (2013) 56:489–492 491123It is worth mentioning that for the whole series of theinvestigated samples a strong correlation of the catalyticactivity gauged by both the ignition (Tig) and 10 % con-version (T10 %) temperatures of soot combustion with thework function value exists (Fig. 3a). The observed corre-lations reveal that the electronic effect (availability ofelectrons for oxygen activation) governs the soot com-bustion process. It is valid with the same spectacular trendeither in the case of nanostructuration of iron oxide bypotassium (gray points) and for subsequent surface modi-fication of the best performing KFeO2 ferrite by 2 % ofCeO2 (green points). Such level of doping was found to beoptimal, as it can be inferred from the Fig. 3b. The char-acteristic Topping-like behavior confirms that the observedchanges of Tig can actually account for by the associatedwork function changes. As a result, those findings showthat the unique, concise parameter—the catalyst workfunction—plays the key role in the soot combustion pro-cess. The observations of a prime importance of catalystwork function modification for the activity was alsoobserved for other redox reactions [15, 16].As mentioned, the electronic promotion effect on sootcombustion can be explained in terms of enhanced ener-getic accessibility of the electrons for activation of thedioxygen molecule by the catalyst. Indeed, the facilitationof the electron transfer from the catalyst surface to O2molecules gives rise to the formation of strongly reactivesurface oxygen species, such as Ox- radicals (O- and O2-),which are known as efficient soot oxidation agents takeninto account in kinetic modeling of soot oxidation [17]. It iswell known that the work function controls the nature ofsurface oxygen speciation. Two main scenarios related todioxygen chemisorption include singly ionized molecular(O2-) and atomic (O-) species as revealed elsewhere [18].Nonetheless, at this stage the established quantitative workfunction—combustion temperature relationship can beused as a guidelining principle for the optimization of thecatalyst for soot combustion, and searching for newpromising washcoats of DPF filters.4 ConclusionsStrong correlation of the catalytic activity of the Ce–K–Fe–O systems (ignition and 10 % conversion temperatures ofsoot combustion) with the catalyst work function value wasrevealed for the first time. The nanostructuration of ironoxide by potassium into tunnelled (KFeO2) and layered(K2Fe22O34) ferrites, together with the surface promotionby ceria can be interpreted in the unique term of theelectronic promotion, gauged by the work function low-ering. It is proposed that the enhanced electron availabilityis beneficial for the generation of the surface reactiveoxygen species, initializing the soot combustion process.The established correlation was used as a guideline forsystematic optimization of the KFeO2 catalysts by ceriaresulting in soot ignition temperature of 210 C.Acknowledgments This study was sponsored by National ScienceCentre Polish Grant number 2011/01/B/ST4/00574.Open Access This article is distributed under the terms of theCreative Commons Attribution License which permits any use, dis-tribution, and reproduction in any medium, provided the originalauthor(s) and the source are credited.References1. van Setten BAAL, Makkee M, Moulijn JA (2001) Catal Rev43:4892. Bensaid S, Marchisio DL, Russo N, Fino D (2009) Catal Today147S:S2953. Bensaid S, Fino D (2011) Catal Today 176:4174. Fino D (2007) Sci Technol Adv Mater 8:935. 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Role of Electronic Factor in Soot Oxidation Process Over Tunnelled and Layered Potassium Iron Oxide Catalysts

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Role of Electronic Factor in Soot Oxidation Process Over Tunnelled and Layered Potassium Iron Oxide Catalysts

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