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Chemical Engineering Journal 176– 177 (2011) 253– 259

Contents lists available at ScienceDirect

Chemical Engineering Journal

j ourna l ho mepage: www.elsev ier .com/ locate /ce j

new concept for a self-cleaning household oven

ietro Palmisanoa, Simelys P. Hernandezb, Murid Hussaina,c, Debora Finoa, Nunzio Russoa,∗

Department of Materials Science and Chemical Engineering, Politecnico di Torino, 10129 Torino, ItalyCenter for Space Human Robotics, Italian Institute of Technology, Corso Trento 21, 10129, Torino, ItalyDepartment of Chemical Engineering, COMSATS Institute of Information Technology Lahore Campus, M.A. Jinnah Building, Defence Road, Off Raiwind Road, Lahore 54000, Pakistan

r t i c l e i n f o

rticle history:eceived 6 December 2010eceived in revised form 28 February 2011ccepted 21 March 2011

eywords:

a b s t r a c t

The present work has the aim of developing an oven wall, which offers self-cleaning properties, viacatalytically enhanced thermal oxidation of soiling materials at temperatures within the available rangeof standard household ovens. The catalytic combustion of solid food soils, using CeO2 in powder form ina pilot plant, has confirmed the possibility of reducing the operation temperature by about 100–150 ◦Ccompared to non-catalytic oxidation and, at the same time, of diminishing CO emissions. After having

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studied different CeO2 deposition techniques over a standard enamel, the best catalytic activity wasobtained using the urea method, which yields both high dispersion and good adhesion of the catalystover the support. It has been confirmed, by means of gravimetric tests that, if such a catalyzed enamelis used, it is possible to achieve a temperature reduction from 550 ◦C (typical of a commercial pyrolyticoven) to a lower temperature of around 400–450 ◦C in the self-cleaning step, through a new catalyticoven concept.

. Introduction

Nowadays, due to globalization, manufacturers who are will-ng to maintain the top position on the market share ranking areocusing their attention on the conception of innovative solutions tomprove the usability of their products [1,2]. The self-cleaning ovenechnology has been developed in order to facilitate oven cleaning,ormally through the use of solvents, which dissolve only certaineposits, or of mechanical agents (i.e. powders, metallic or artifi-ial fibers), which result in an operation that is long and difficult [3].n general, self-cleaning ovens can be divided into two categories:yrolytic and catalytic.

The pyrolytic self-cleaning oven is the main commercial solu-ion currently on the market; it implements high temperature>500 ◦C), long-lasting (1–3 h) cycles, in order to incinerate therganic residues that are then easily removed as dust [4,5]. How-ver, such high temperatures in the cleaning step entail the usef special materials, particular cooling designs and high energyonsumption, which result in important safety and cost issues [3,6].

The modern version of self-cleaning domestic ovens has rede-ned the concept of the pyrolytic oven, resulting in a catalytic one,

n which the cleaning step occurs over a lower temperature rangehan 350 ◦C [2]. Cleaning can therefore be performed both duringnd after cooking, in this way avoiding a specific heat-proofing

∗ Corresponding author. Tel.: +39 011 5644710; fax: +39 011 5644699.E-mail address: nunzio.russo@polito.it (N. Russo).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.03.083

© 2011 Elsevier B.V. All rights reserved.

design. This represents a goal in terms of energy savings for theenvironment and costumers, and is coupled to a modern conceptof more economic core and trimming materials for the producers.

Different materials were used in the past for this purpose,such as, copper, vanadium, bismuth, molybdenum, manganese,iron, nickel, tin, niobium, chromium, tungsten, rhenium, platinum,cobalt, and their oxides, either alone or in mixtures. However,although the catalytic self-cleaning oven has been the subject ofresearch for many years, there is still room for improvement inthe properties of the oven wall coating, to create layers with ahigh resistance to heat and abrasion, and at the same time a goodcatalytic activity for oven soil produced by different types of foodresiduals [7].

The ability of cerium (IV) oxide, CeO2, as a catalyst to pro-mote the combustion of carbonaceous materials, in particular solidspecies, is well known in the literature [7–10]. Many applicationsusing lanthanide oxides were improved in the past by our group,and good performances have been obtained for the heterogeneouscatalytic combustion of carbonaceous materials [8–10]. Therefore,the field of household oven appliances represents a domain of uti-lization and optimization of the CeO2 catalyst.

The present work is aimed at developing oven walls with self-cleaning properties via catalytically enhanced thermal oxidation ofthe soiling material, within the standard range of household oven

temperatures. Thus, the target of this work was to verify the cat-alytic activity of CeO2 catalysts, in the form of powder and as acoating, towards the most representative food soils. Four differentsynthesis techniques have been studied for the CeO2 deposition

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54 P. Palmisano et al. / Chemical Engin

ver a enameled oven-tray furnished by the Indesit Company, inrder to individuate the most efficient coating in terms of adher-nce, brightness and self-cleaning performance, with respect to theeference enamel of a commercial pyrolytic oven tray furnished byhe Indesit Company.

. Materials and methods

.1. Catalyst preparation and characterization

The first part of this work was focused on assessing the capa-ility of the CeO2 to promote the oxidation of cooking soils. Theatalyst was synthesized by means of the “Solution Combustionynthesis” (SCS) method [11]. A concentrated aqueous solution ofhe metal nitrate precursor and urea, used as sacrificial fuel, waslaced into a crucible in an oven preheated at 600 ◦C for a few min-tes in order to ignite a very fast synthesis reaction. The adoptionf the SCS method entails the formation of pure oxide catalystsith rather high specific surface areas in the absence of any car-

ier. The obtained catalyst was characterized by means of X-Rayiffraction using a PW1710 Philips diffractometer equipped with aonochromator for the Cu-K� radiation. BET specific surface areasere measured using a Micromeritics ASAP 2010 analyzer. The two

echniques allowed the particle size and the specific surface areao be determined.

The second step regarded the comparison of various CeO2 depo-ition methods over pieces (5 cm × 5 cm) of oven walls, based on

commercial ceramic enamel provided by the Indesit Company.his enamel will be called the “standard” sample for the followingomparisons. Moreover, a second pirò enamel, which allows a self-leaning procedure at about 550 ◦C in a pyrolytic oven producedy the Indesit Company, was used as a “reference” for comparisonith the new catalyzed enamels. The two “fresh” enamels were

lso provided in powder form by the Indesit Company in order toe characterized by XRF analysis for subsequent comparisons.

The deposition techniques considered in this part of the workre: powder coating (PC), sol–gel precipitation (SG), gel combus-ion (GC) and urea decomposition (WU). The PC deposition wasbtained by keeping the steel support recovered with a commercialnamel in a muffle oven at 850 ◦C for 10 min. At this temperature,he enamel begins its transition towards the liquid phase. Afterhis time, the support was extracted from the oven and a layerf powder CeO2 was uniformly deposited by gravity. The opera-ion was repeated until an increment weight of almost 10% waschieved.

The SG method consisted of the preparation of a water solu-ion in which the CeO2 powder, prepared by SCS (10 g/l), citric acid20 g/l), hydrochloric acid 5% w/w (25 ml/l) had previously been dis-olved. The solution was kept at 80 ◦C while it was stir agitated for

h. The obtained gel was brushed over the surface and the wholeample was then introduced into a muffle oven set at 650 ◦C for

h. The operation was repeated until a 10% weight increment waseached.

The GC deposition foresaw a initial water solution of Ce(NO3)320% w/w respect water), citric acid and urea (molar ratio 1:1:2).he homogenous solution was kept at 100 ◦C while it was stir agi-ated in order to achieve a gel; the obtained gel was then brushednto the enamels. The enamels were then introduced into a muffleven and kept at 600 ◦C. The reaction of Ce(NO3)3 towards CeO2ccurred in a short time (about 10 min). The whole operation wasepeated until an increment in the enamel weight corresponding

o 10% was reached.

WU is a variant of the GC method, in which cerium nitrate isirectly substituted by CeO2 produced by SCS. The used molar ratioas the same as that of the GC deposition, as was the weight ratio

g Journal 176– 177 (2011) 253– 259

between the CeO2 and water (20%). This method was also repeateduntil a 10% increment of the enamel weight was achieved.

The obtained catalityc layers were evaluated in terms of adhe-sion, penetration and exposed surface, with respect to the standardenamel layer used as a support, using a Scanning Electron Micro-scope (Philips, Model 515) and EDS analyses.

2.2. Catalytic activity evaluation

Since oven soiling is mainly generated by food residuals; differ-ent typologies of commercial alimentary products were selectedfor the catalytic activity tests. Hence, a milk concentrate in pow-der form by Nestlè, tomato paste by Mutti, a sauté cube of friedingredients by Knorr and apricot jam by Santarosa were selectedas representative soils, since these products are recognized to bedifficult to remove from pyrolytic ovens.

The catalytic activity of the CeO2 powder, synthesized by SCS,towards the combustion of the selected soils was tested in a Tem-perature Programmed Combustion apparatus (TPC), previouslydescribed in Ref. [8]. TPC runs were subsequently performed foreach catalyst/soil combination, and the same procedure was usedon the reference runs on bare soil. A TPC sample was prepared using100 mg of a mixture of soil and powdered catalyst in a 1:9 w/w ratio,mixed in an agate mortar. All the soils were oven dried for 2 h at120 ◦C to obtain powders that could easily be mixed with the cat-alytic one. This preparation leads to contact conditions between thecatalyst and reagent that can be described as “tight”; such a situa-tion is not completely comparable to what is actually achievable onan oven muffle surface but enables a much higher degree of repro-ducibility, which is essential for activity screening studies. Silicapellets with diameters ranging from 250 to 425 �m were used asthermal dispersers in a quantity of 250 mg per run. The controlledcombustion was carried out in a continuous air flow of 100 Nml/minwith a gradual increase in temperature of 5 ◦C/min, and was mon-itored via an NDIR analyzer (ABB instrument), by measuring theCO2 and CO concentrations in the reactor outlet gases. The CO/CO2outlet concentrations increased, starting from the carbonaceoussubstance ignition temperature, then reached a maximum, andfinally decreased as a consequence of carbon consumption. Thus,the temperature corresponding to the CO2 peak (Tp) was taken as anindex of the catalytic activity, while the CO release was qualitativelyconsidered as an indicator of catalyst selectivity.

Gravimetric experiments were carried out in order to test theperformance of the home-made CeO2 catalyzed enameled oventrays towards the abatement of cooking soils (listed above, withthe addition of extra virgin olive oil). For this purpose, sampleswere placed in a furnace at 300 ◦C and 400 ◦C (for 1 h), and theirweights were measured before and after the thermal treatment,which simulated a cooking cycle. One hour of holding time was setas a strategic target for this new concept of self-cleaning oven, inwhich the main aim was to obtain the maximum efficiency, witha lower power consumption. The soils had previously been moltenand dissolved in an aqueous solution to a concentration of 100 g/land deposited, by pipetting, over 5 cm × 5 cm samples. Before thegravimetric experiments, the soiled samples had been introducedinto an oven at 120 ◦C to dehydrate the soils. Hence, the activity ofthe catalytic coatings was evaluated as the percentage value of theresidual soil after each cleaning cycle, as defined previously.

3. Results and discussion

3.1. Characterization and catalytic performance evaluation of the

powder catalysts

As far as the catalyst powder characterization is concerned, theprepared catalysts were found to be well crystallized in the XRD

P. Palmisano et al. / Chemical Engineering Journal 176– 177 (2011) 253– 259 255

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of all the studied food residuals. It is evident that a drastic reduc-tion in CO formation occurs compared to the un-catalyzed reaction.In fact, the catalyzed combustion never rises above 250 ppm ofthe CO concentration in the gas outlet, whereas the un-catalyzed

Table 1Temperatures at 10%, 50% and 90% of conversion of different soils calculated fromtemperature programmed combustion (TPC) curves.

Un-catalized combustion

Soil T10%a T50%b T90%c

Tomato 350 ◦C 500 ◦C 600 ◦CFried ingredients 300 ◦C 400 ◦C 500 ◦CApricot jam 330 ◦C 460 ◦C 510 ◦CConcentrated milk 330 ◦C 515 ◦C 520 ◦C

CeO2 catalyzed combustion

Soil T10%a T50%b T90%c

Tomato 225 ◦C 450 ◦C 500 ◦CFried ingredients 230 ◦C 335 ◦C 435 ◦CApricot jam 220 ◦C 235 ◦C 260 ◦C

Fig. 1. Temperature programmed combustion (TPC) CO

nalysis (not reported). All the main diffraction peaks listed in theeference JPCDS card of CeO2: PDF 65-2975 were observed. TheESEM analysis results (not reported) highlighted a very foamytructure, which is a typical feature of catalysts synthesized by SCS,s reported in our previous work [7]. In addition, the crystal sizef the considered catalysts ranged from 60 to 100 nm, which is inubstantial agreement with the measured specific surface area of0 m2/g.

The curves of the TPC experiment related to the CO2 outlet con-entration, during the non-catalyzed and catalyzed combustion ofoils, are reported in Fig. 1. It is evident that CO2 emission occurredn all cases, and it involved a multiple-step process: this suggestshat the first step of combustion had much faster kinetics. Oneypothesis that could explain such behaviour is that during the firstombustion stage, the direct oxidation of the organic compoundsas accompanied by the formation of difficult to oxidise interme-iates, which were consequently completely oxidised at a higheremperature in the second combustion stage and so on in the subse-uent steps. In some cases, this second stage in fact occurred at theypical temperatures at which carbonaceous compounds usuallyurn [12]. In addition, temperatures related to 3 different conver-ion ratios: 10, 50 and 90%, were calculated in function of the CO2rea in the TPC plots, and are reported in Table 1.

Comparing the TPC results regarding CO2 production (Fig. 1 andable 1), it is interesting to note that all the catalyzed oxidationseached 10% soil conversion at about 200 ◦C, which is 100 ◦C lowerhan the homologues without the catalyst. However, as the reactionroceeded, distinct behaviour was observed for each food residualue to the complex and different nature of each product. Amonghe different soils, apricot jam showed the best performance in theresence of CeO2, with a 50% abatement at 235 ◦C and a combus-ion peak at 260 ◦C, in correspondence to 90% of removal, 250 ◦Cower than the first peak of the non-catalytic reaction (registeredt 450 ◦C). Also in the case of the tomato paste, the catalytic combus-ion was almost completed at a 100 ◦C lower temperature than then-catalyzed one. Such results could be attributed to the fact thathe carbohydrates presents in the apricot jam, and in the tomatoaste, burn more easily than the more complex compositions of the

ther two considered soils.

The concentrated milk showed a first CO2 peak at 220 ◦C,lthough such a temperature is equal to only 10% of abatementf this soil, subsequently, 50% and 90% of the conversion were

s of catalytic and non-catalytic combustion of the soils.

obtained with 155 ◦C and 40 ◦C of temperature reduction, respec-tively, compared to the non-catalyzed reaction. Finally, the friedingredients showed the lowest performance of the catalyzed com-bustions, with a global reduction in the combustion temperatureof the different reaction steps of about 70 ◦C. These results couldbe explained by the very complex reaction mechanism that occursduring the combustion of these foods, which probably involvesthe generation of more than one reaction intermediate, as sug-gested by the multiple CO2 peaks observed in the TPC plots. Infact, the concentrated milk has a complex matrix composed ofproteins, carbohydrates and minerals (calcium, magnesium, etc.),among others; while the fried ingredients contain 41 wt% of car-bohydrates, 29 wt% of fats and around 7 wt% of proteins, whichare derived from different vegetables and cooking oil (and the oildegradation products produced during its elaboration).

On the other hand, the TPC curves of the CO generated during thecombustion of different soils, reported on Fig. 2, demonstrate thatthe presence of the CeO2 catalyst improves the oxidation efficiency

Concentrated milk 220 ◦C 360 ◦C 480 ◦C

a Temperature corresponding to 10% of soils conversion.b Temperature corresponding to 50% of soils conversion.c Temperature corresponding to 90% of soils conversion.

256 P. Palmisano et al. / Chemical Engineering Journal 176– 177 (2011) 253– 259

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pyrolytic enamel surface. Such results also reveal that some poten-tial catalytic oxides are already included in the pyrolytic enamel;cerium, barium and zirconium oxides are well known in the

Table 2X-ray fluorescence (XRF) analysis of the commercial enamels furnished by the Inde-sit Company.

Standard enamel Pyrolytic enamel

Compound %Weight Compound %Weight

MgO 3.33 MgO –Al2O3 37.14 Al2O3 1.41S 16.22 S –Cl 3.15 Cl –K2O 0.37 K2O 6.23CaO 7.28 CaO 7.11TiO2 3.36 TiO2 2.66Cr2O3 2.24 Cr2O3 0.5Fe2O3 13.29 Fe2O3 1.22NiO 2.59 NiO 4.15CuO 0.49 CuO 0.6ZnO 3.52 ZnO 0.83PdO 2.21 Na2O 0.23Yb2O3 1.96 SiO2 28.91PtO2 2.09 P2O5 0.45PbO 0.71 MnO 1.45

Co2O3 1.84As2O3 0.023Rb2O 0.014SrO 0.075ZrO2 17.94Nb O 0.016

Fig. 2. Temperature programmed combustion (TPC) CO

nes surpassed 1000 ppm in some cases. Such behaviour could bexplained by the fact that, during the first step of the un-catalyzedeaction, some oxygen shortage is created due to the quickness ofhis reaction step, with a consequent higher production of CO [7]. Atresent, pyrolytic ovens use a gas exhaust post-treatment reactoror the abatement of CO, which generates high costs for such sys-ems. Instead, the CeO2 catalyst has been seen to work as an oxygenump, promoting the complete oxidation of the soils [13,14]. Thisotable result in reduction of CO emissions improves the safety ofhe system, since CO is considered to be an asphyxiating gas, andhis implies a further cost reduction in the concept of the catalyticelf-cleaning oven.

The overall results obtained for the CeO2 catalyst in powder formre remarkable and could therefore be promising for a structuredystem, with the aim of reducing the temperature of the self-leaning step from the 550 ◦C currently employed in the pyroliticvens, to a new concept of catalytic oven in which the target fore-ees a dwelling of 1 h in a range of 400–450 ◦C.

.2. Structured system

.2.1. Characterization of the commercial and catalyzed enamelsThe XRF characterization results of the powders used for the

roduction of commercial enamels, the standard and the pyrolyticne, are reported in Table 2.

It is evident that the precursor powder used for the standardnamel is mainly constituted by a ceramic matrix of alumina37.1 wt%) with considerable quantities of iron(III) oxide (13.3 wt%)nd calcium oxide (7.3 wt%). It is known in the literature that CaO isommonly used in enamels, since it increases the melting point andiscosity of the material, thus improving its mechanical strengthnd thermal stability [15]. In addition, the noticeable quantity of Sompounds (16.2 wt%) could play a role as surface-active groups,hich could broaden the firing range of the material and reduce itsorosity [16]. The characteristic black colour of this type of ovenall is created by the presence of both Fe2O3 and a small quantity

f NiO (2.6 wt%). Some titanium oxide (3.4 wt%) is also included to

upply a high degree of brightness while the presence of zinc oxides justified because of its antibacterial behaviour [17]. This precur-or powder for the standard enamel also contained other importantxides such as PtO2 and PdO, which are already used in many appli-

of catalytic and non-catalytic combustion of the soils.

cations as catalyst. The small amounts of other metal oxides couldact as additives to promote the bonding processes.

On the other hand, the precursor powder for the pyrolyticenamel principally contains a ceramic silica matrix (29 wt%) with arelatively high quantity of zirconia (around 17 wt%), which in gen-eral is used to obtain an outstanding alkali resistance. In the sameway, other white metal oxides (barium, calcium and cerium oxide)are present in high percentages (around 10 wt%), which in combi-nation with some NiO (4 wt%), probably gives the grey colour of the

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Sb2O3 1.35BaO 11.36CeO2 11.47Ta2O5 0.11

P. Palmisano et al. / Chemical Engineering Journal 176– 177 (2011) 253– 259 257

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ig. 3. Scanning electron microscope (SEM) pictures of the self-cleaning tested enand pyrolytic), (C, D, E and F) enamels catalyzed by powder coating, wet-urea, gel-c

iterature as promoters of solid carbonaceous substances oxidation18].

The SEM characterization results obtained for the commer-ial enameled pieces and home-made CeO2 layers coated onto atandard enameled sample are reported in Fig. 3. The commer-ial standard enamel (Fig. 3A) presents a homogenous surface thats slightly covered by clusters of micrometric dimension, essen-ially due to the presence of TiO2. Instead, the commercial pyrolyticnamel (Fig. 3B) showed a surface that was covered completelyy ceria and zirconium clusters of about 1 micron in diameter,hich were partially drowned inside the matrix. The CeO2 coated

namels, obtained through the powder coating deposition methodFig. 3C), highlighted an appreciable covering of the substrate by

icrometric clusters of ceria, which were well drowned in theatrix. On the contrary, the urea method produced a good adhe-

ion with a less relevant degree of ceria crystal drowning (Fig. 3D),ut the catalyst dispersion was homogenous in all directions.

nstead, it is possible to notice, in Fig. 3E, that the gel combus-ion method was characterized by the formation of higher ceriarystals with an average diameter of 10 �m. In this case, the adhe-ion of the catalyst over the support was remarkably good, inpite of the fact that the catalyst was almost not drowned in theatrix. In addition, the distribution of the catalyst is evidently

ot homogenous, due to the poor dispersion and relevant crys-al agglomeration. The last method, represented by the sol–geleposition (Fig. 3F), showed the worst results in terms of cata-

yst adhesion, since the ceria film was almost not anchored tohe support. Such a phenomenon could be explained by the lowerxothermic reaction during deposition, compared to the other syn-hesis methods, which causes ceria to penetrate less inside theeramic matrix.

.2.2. Catalytic activity assessment of the coated enamelsThe residual soil amounts, obtained after treatment of the refer-

nce and the catalyzed enamels at 300 ◦C and 400 ◦C, respectivelyor 1 h, are reported in Fig. 4. Such values provide direct evidencef the catalytic efficiency of CeO2 coatings, obtained through dif-erent deposition techniques, for the abatement of different typesf food residuals, compared to the performance of the standard

on-catalyzed enamel. The black bar in the histogram shows theverage removal of all the residual soils at 300 ◦C and 400 ◦C, forach coating. Hence, it is easily seen that the catalytic enamels pro-uce a better result in every case compared to the non-catalytic

Magnification 1000× for all micrographs. (A and B) commercial enamels (standardstion and sol–gel deposition methods, respectively.

one. Although the catalytic performances at 300 ◦C are interesting,in terms of soil reduction, the remaining quantity of un-burnt soilsis too high to allow the development of a new self-cleaning ovenoperating 300 ◦C as the maximum temperature. At the same time,it is important to notice that the urea deposition method has fur-nished important results compared with the pirò enamel (19% ofresidual soil against 36%) already at 300 ◦C. Thus, a self-cleaningsolution which includes a first dwelling period at 300 ◦C and a sec-ond one at 400 ◦C for a shorter time could be hypothesised. Sucha solution could be valuable, considering the reduction in powerconsumption.

The results obtained at 400 ◦C appear to reflect the behaviouralready observed at 300 ◦C, confirming the best self-cleaning per-formance with the urea deposition method which furnished aremarkable residual soil average value of about 3%. All the otherceria deposition methods were similar, with residual average soilvalues of between 7 and 10%, while the standard and the pyrolyticenamels showed values of 25% and 13% respectively.

As shown in Fig. 3, the morphology and Crystallinity of the CeO2layer on the enamels was influenced to a great extent by the deposi-tion method. Consequently, it may be possible to find a correlationbetween the catalytic activity of the catalyzed enamels and theCeO2 coating characteristics. A low drowning degree, coupled to agood dispersion of small micrometric particles, seems to be essen-tial to increase the catalytic activity of the CeO2 layer obtained bymeans of the urea method. This is most probably due to the highercatalytic surface area available in this case, which increased thecontact between the catalyst particles and the soils, where the oxy-gen pump capacity of the ceria catalyst was better exploited for theoxidation of the food residuals.

This consideration is also supported by the SEM pictures (madein cross-section form) reported in Fig. 5, in which the CeO2 pene-tration inside the enamel thickness and its interface layer betweenthe ceramic matrix and the outside are evaluated. It is interesting tonotice that the thickness of the ceria layer can be directly correlatedto the capability of removing soils with catalytic combustion. Fig. 5Aand B, corresponding to the urea and gel combustion method,showed a thickness of the catalytic layer of 13 and 30 �m, respec-tively, the first one (urea deposition) having a perfect interface

between the enamel and the outer side; therefore, the lower thethickness, the better the catalytic activity.

Moreover, the drowning degree also seems to have an influ-ence on the properties of the materials in the combustion of the

258 P. Palmisano et al. / Chemical Engineering Journal 176– 177 (2011) 253– 259

Fig. 4. Thermo gravimetric results used to evaluate the consumption and residual soil after isothermal steps at 300 ◦C and 400 ◦C.

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ig. 5. Scanning electron microscope (SEM) cross-section pictures of the catalyzedoating method.

oils. The high drowning degrees were actually detrimental to thectivity, as shown in Fig. 5D, which was obtained in backscatteringode on the sample prepared by the powder coating method; it can

e observed that CeO2 penetrated all the standard enamel. Finally,ig. 5C, which corresponds to the sol–gel method deposition, con-rms that catalyst adhesion is not optimal when this system is usedince the catalytic phase is almost unnoticeable.

. Conclusion

The catalytic combustion of four different solid food soils in pilot plant, using CeO2 in powder form, has allowed the tem-

els. (A) urea method, (B) gel combustion method, (C) sol–gel method, (D) powder

perature operation window to be reduced by about 100–150 ◦Ccompared to non-catalytic oxidation.

The best CeO2 deposition method, over a standard enamel sup-plied by the Indesit Company, has been obtained using the ureamethod, which produces a high dispersion of the catalyst crystalsover the enamel surface coupled with a good adhesion on the sup-port. Moreover, the thinnest and less drowned film obtained withsuch a technique also shows the best catalytic results, which are

comparable with the actual performance of pyrolitic ovens thatwork at more severe operative conditions (550 ◦C for 1 h).

The gravimetric tests with solid soils and extra virgin olive oilhave confirmed that a self-cleaning cycle at 400 ◦C for 1 h could be a

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alid alternative to the commercial pyrolitic solution actually usedy the Indesit Company which works at 550 ◦C.

cknowledgments

The authors would like to thank the Indesit Company S.p.A. forhe financial support.

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