Synthesis and Characterization of Cu-TiPILCs for SelectiveCatalytic Reduction of NO by Propylene in the Presence of Oxygenand H2O: Influence of the Calcination Temperature, the CopperContent, and the Cation Promoter (Ce/Ag)

Jose L. Valverde,* Fernando Dorado, Paula Sanchez, Isaac Asencio, andAmaya Romero

Facultad de Ciencias Quımicas, Departamento de Ingenierıa Quımica, Universidad de Castilla-La Mancha,13004 Ciudad Real, Spain

Cu ion-exchanged titanium pillared clays (Cu-TiPILCs) were studied as catalysts for selectivecatalytic reduction (SCR) of NO by propylene. The results showed that the catalysts were porousmaterials with copper species located in the interlayer. The catalytic activity of Cu-TiPILCswas found to depend on the calcination temperature of the TiPILC and the amount of copperloading. The optimum copper loading was about 6 wt %, which gave the higher activity at lowtemperatures. The effect of both ceria and silver as promoters on the activity and hydrothermalstability of Cu-TiPILCs for SCR of NO with C3H6 under oxidizing and wet conditions wasinvestigated. The addition of silver resulted in a small shift of the maximum of NO conversiontoward higher temperatures. However, the addition of ceria as a cocation to Cu-TiPILC catalystslightly decreased the reversible inhibition by water occurring under catalytic conditions in atemperature range between 200 and 275 °C. Ceria species have the ability to suppress theagglomeration of Cu2+ species and stabilize the catalyst when water is present in the feed stream.It was verified that a small amount of ceria (∼1 wt %) can provide the maximum promotioneffect.

1. Introduction

Much research related to the selective catalytic reduc-tion (SCR) of NOx by hydrocarbons was undertaken andreported in the literature because of its potential for theeffective control of NO emission under oxidant environ-ments.1-11 This reaction has been described as a poten-tial method to remove NOx from natural-gas-fueledengines, such as lean-burn gas engines in cogenerationsystems.12 SCR of NOx by hydrocarbons can also findimportant applications for learn-burn (i.e., O2-rich)gasoline and diesel engines where the noble-metal three-way catalysts are not effective in the presence of excessoxygen.13 Hydrocarbons would be the preferred agentsover NH3 because of the practical problems associatedwith its use, handling, and slippage through the reactor.A large number of catalysts, such as V2O5-WO3 (orMoO3)/TiO2, other transition-metal oxides (e.g., Fe, Cr,Co, Ni, Cu, Nb, etc.), and doped catalyst as well aszeolite-type catalysts (e.g., H-ZSM-5, Fe-Y, Cu-ZSM-5), have been found to be active in this reaction. Despitethe high activity of vanadium-based catalysts,14 majordisadvantages remain, such as their toxicity and highactivity for the oxidation of SO2 to SO3, which causescorrosion, and plugging of the reactor and heat exchang-ers. Hence, there are continuing efforts for developingnew catalysts.

An ion-exchange process involving the exchangeablecations of certain clays typically initiates the productionof pillared clays. Na-rich montmorillonite can be pillaredby different polyoxymetal cations. Upon calcination, the

resulting materials contain metal-oxide pillars, whichseparate the 2:1 layers. This modification allows inter-nal surfaces to be accessible for greater chemical activ-ity. These intercalated clays are commonly preparedfrom natural smectite clays because of the low chargedensity on the 2:1 layers and swelling ability.15,16

In recent years, the suitability of pillared clays ascatalyst supports has been explored, especially becauseof the textural and acidic properties of these solids.Hence, TiPILCs have been used as catalysts for the SCRof NOx.17-20 Potential applications of PILCs in catalyticprocesses of a redox nature would require the claystructure to accommodate transition-metal ions that areknown to easily change their oxidation state.

Cu-TiPILCs solids were active for the selectivereduction of NO by hydrocarbons, but the activitystrongly decreased with the presence of water in thefeed, thus limiting their use for treatment of emissionsfrom diesel and lean-burn engines. It is known that therate of NOx reduction into N2 is directly related to thenumber of isolated Cu2+/Cu+ ions located in the pillaredclay structure. When water is present in the feed, thenumber of isolated Cun+ ions decreases. At moderatetemperatures, the deactivation is mainly due to thepartial migration of copper to inaccessible sites, withthe degradation of the PILC itself being limited.

In the literature, attempts to stabilize different PILCsDeNOx catalysts have been performed by using variousmodifiers (La, Ce, etc.).16,17,21,22 We have recently re-ported the effect of cocations (Ce and Ag) in zeolitecatalysts on the migration and the subsequent ag-glomeration of exchanged ions.23 Ceria and silver havebeen chosen for several reasons. Ceria removes excessoxygen from the exhaust gas, and catalyzed by the

* To whom correspondence should be addressed. Tel.: +34-926295300. Fax: +34-926295318. E-mail: jlvalver@inqu-cr.uclm.es.

3871Ind. Eng. Chem. Res. 2003, 42, 3871-3880

10.1021/ie0209069 CCC: $25.00 © 2003 American Chemical SocietyPublished on Web 07/11/2003

metals, NOx is reduced to harmless nitrogen by CO andunburnt hydrocarbons. Furthermore, its presence hasother beneficial effects on the catalyst performance, suchas improving the dispersion of the active phase. Silveris, on the other hand, active by itself and possesses arelatively weak affinity for water. Furthermore, theoxide is relatively unstable and subsequent NO dis-sociation could be favored on the metallic part.23

The present paper deals with the activity, in thepresence and absence of water in the feed, of Cu ion-exchanged TiPILCs modified with Ce and Ag catalystsfor SCR of NO with propene. The influence of both theconcentration of copper and ceria and the calcinationtemperature of the starting TiPILC on the catalyticperformance have also been investigated.

2. Experimental Section

2.1. Catalyst Preparation. (a) Synthesis of TiP-ILCs. The starting clay was a purified montmorillonite(purified-grade bentonite powder from Fisher Co.),which had a particle size of 2 µm or less and a cation-exchange capacity of 97 mequiv/g of dry clay. Thepillaring solution of partially hydrolyzed Ti polycationswas prepared by first adding titanium metoxide to a 5M HCl solution to obtain a HCl/Ti molar ratio of 2.5.The resulting solution was aged for 3 h at roomtemperature. Then, 1 g of starting clay was dispersedin 0.75 L of deionized water for 3 h under stirring. Thepillaring solution was then slowly added with vigorousstirring into the suspension of clay until the amount ofpillaring solution reached that required to obtain a Ti/Clay ratio of 15 mmol of Ti/g of clay. The intercalationstep took about 16 h. The pH of the mixture was 1.9.Subsequently, the mixture was separated by vacuumfiltration or centrifugation and washed with deionizedwater until the liquid phase was chloride-free. Thesample was dried at 120 °C for 12 h and calcined for 2h.

(b) Ion Exchange of TiPILC with Cu2+ (Cu-TiPILC). A total of 1 g of TiPILC was added to 100 mLof a 0.05 M Cu2+ acetate solution. The mixture wasstirred for several hours at room temperature. The ion-exchanged product was collected by filtration or cen-trifugation, followed by washing five times with deion-ized water. The obtained solid sample was first driedat 120 °C in air for 12 h, and then the sample wascalcined at 400 °C in air for 2 h. After this pretreatment,the sample was ready for further experiments. Eachsample had different metal loadings, depending on howlong and how many steps were needed for the ion-exchange process. These samples were referred to as afunction of the metal loading. For instance, Cu4.0-TiPILC corresponds to a copper-exchanged TiPILC witha Cu content of 4.0 wt %. Table 1 summarizes theTiPILC-based samples prepared by ion exchange.

(c) Impregnation of Cu-TiPILC with Ceria (Ce-Cu-TiPILC). Several grams of Cu-TiPILC was placedin a glass vessel and kept under vacuum at roomtemperature for 2 h in order to remove water and othercompounds adsorbed on the clay. A known volume ofan aqueous Ce(NO3)3/AgNO3 solution (the minimumamount required to wet the solid) was then poured overthe TiPILC sample. After 2 h, the solvent was removedby evaporation under vacuum. The sample was thendried and calcined at 400 °C for 2 h. Again, thesesamples were referred to as a function of the metalloading. For instance, Ce1.0-Cu4.0-TiPILC corre-

sponds to a ceria-impregnated Cu-TiPILC sample witha Ce content of 1.0 wt %.

2.2. Catalyst Characterization. The X-ray diffrac-tion (XRD) patterns were obtained using a Philips modelPW 1710 diffractometer with Ni-filtered Cu KR radia-tion. To maximize the (001) reflection intensities, ori-ented clay-aggregate specimens were prepared by dry-ing clay suspensions on glass slides. The XRD patternof the parent clay exhibits a main X-ray peak at around9°. This peak is commonly assigned to the basal (001)reflection [d(001)]. In pillared clays, the d(001) peak wasfound to shift toward the lower 2θ region, which is aclear indication of the enlargement of the basal spacingof the clay. The d(001) peaks represent the distancebetween two clay layers, including the thickness of oneof the layers. The thermal stability of samples wasinvestigated by exposing the materials to temperaturesin the range of 200-600 °C for 2 h in air.

The surface area and pore size distribution weredetermined by using nitrogen as the sorbate at 77 K ina static volumetric apparatus (Micromeritics ASAP 2010sorptometer). Pillared clays were previously outgassedat 180 °C for 16 h under a vacuum of 6.6 × 10-9 bar.Specific total surface areas were calculated by using theBrunauer-Emmett-Teller (BET) equation, whereasspecific total pore volumes were evaluated from nitrogenuptake at a relative pressure of N2 (P/P0 ) 0.99). TheHorvath-Kawazoe method was used to determine themicroporous surface area and micropore volume.

To quantify the total amount of metals incorporatedinto the catalyst, atomic absorption measurements, withan error of (1%, were made by using a SPECTRAA220FS analyzer with simple beam and backgroundcorrection. The samples were previously dissolved inhydrofluoric acid and diluted to the interval of measure-ment. A mass analyzer (inductively coupled plasmaatomic emission spectrometry, Liberty-RL) was used todetermine the Ce content.

The total acid-site density and acid-strength distribu-tion of the catalysts were measured by temperature-programmed desorption of ammonia (TPDA), using aMicromeritics TPD/TPR 2900 analyzer. The sampleswere housed in a quartz tubular reactor and pretreatedin flowing helium (g99.9990% purity) while heating at15 °C/min up to the calcination temperature of thesample. After a period of 30 min at this temperature,the samples were cooled to 180 °C and saturated for 15min in an ammonia stream (g99.9990% purity). The

Table 1. Experimental Conditions Used for thePreparation of the Catalysts

samplecalcinationtemp (°C)

Cucontent(wt %)

Ce/Agcontent(wt %)

ion-exchangeconditions:

time and steps

TiPILC350 350TiPILC400 400TiPILC450 450TiPILC500 500Cu-TiPILC350 350 6.0 15 h and one stepCu-TiPILC400 400 6.0Cu-TiPILC450 450 6.0Cu-TiPILC500 500 6.0Cu4.0-TiPILC 400 4.0 6 h and one stepCu6.0-TiPILC 400 6.0 15 h and one stepCu7.4-TiPILC 400 7.4 15 h and two stepsCu9.0-TiPILC 400 8.9 15 h and three stepsAg2.0-Cu-TiPILC 400 6.0 1.0 15 h and one stepCe2.0-Cu-TiPILC 400 6.0 1.0Ce0.5-Cu-TiPILC 400 6.0 0.5Ce1.0-Cu-TiPILC 400 6.0 1.0Ce2.0-Cu-TiPILC 400 6.0 2.0

3872 Ind. Eng. Chem. Res., Vol. 42, No. 17, 2003

catalyst was then allowed to equilibrate in a helium flowat 180 °C for 1 h. Next, the ammonia was desorbed byusing a linear heating rate of 15 °C/min up to 500 °C.Temperature and detector signals were simultaneouslyrecorded. The area under the curve was integrated todetermine the relative total acidity of the catalyst. Twopeaks of ammonia desorption are observed at about 270°C (low desorption temperature) and 350 °C (highdesorption temperature), respectively. To obtain thestrength distribution, the desorption profiles were fittedusing the two peaks, with their maxima and widthsbeing held as constant as possible while fitting eachprofile. The average relative error in the acidity deter-mination was lower than 3%.

Temperature-programmed reduction (TPR) measure-ments were carried out with the same apparatus as thatdescribed above. After loading, the sample was out-gassed by heating at 15 °C/min in an argon flow up tothe calcination temperature of the sample and keptconstant at this temperature for 30 min. Next, it wascooled to room temperature and stabilized under anargon/hydrogen flow (g99.9990% purity, 83/17 volumet-ric ratio). The temperature and detector signals werethen continuously recorded while heating at 15 °C/minup to 500 °C. The liquids formed during the reductionprocess were retained by a cooling trap placed betweenthe sample and the detector. It was previously foundthat montmorillonite consumed some hydrogen at tem-peratures >500 °C because of the reduction of themetals in its structure. This consumption was negligibleat temperatures <500 °C and could be accounted forwithout significant error by subtraction in the TPRanalyses. TPR profiles were reproducible, with standarddeviations for the temperature of the peak maximabeing (2%.

All gas streams used for TPDA and TPR experimentswere further purified. The gas streams were passedthrough water/hydrocarbon and oxygen traps. Then, thegas streams, containing <1 ppm oxygen, water, orhydrocarbons, were directed to the apparatus for theexperiments.

2.3. Catalyst Activity Measurements (NO SCRTest). The experiments were carried out in a flow-typeapparatus designed for continuous operation at atmos-pheric pressure. This setup consisted of a gas feedsystem for each component with individual control bymass flowmeters, a fixed-bed downflow reactor, and anexit gas flowmeter. The reactor, a stainless steel tubewith an internal diameter of 4 mm, was filled with thecatalyst sample (0.25 g). A temperature programmerwas used with a K-type thermocouple installed incontact with the catalyst bed. The products wereanalyzed simultaneously by chemiluminiscence (NO-NO2-NOx Eco Physics analyzer) and by a Fouriertransform infrared analyzer (Perkin Elmer SpectrumGX) capable of measuring continuously and simulta-neously the following species: NO, NO2, N2O, CO2, andC3H8.

The catalysts were brought to identical reactionconditions for each test [1000 ppm C3H6, 1000 ppm NO,5% O2, 10% H2O (when used), and the balance He; spacevelocity (GHSV) of 15 000 h-1 and a flow rate of 125mL/min]. Before the analysis was started, the catalystswere preconditioned at each reaction temperature for30 min by flowing helium over the sample (125 mL/min)while it is was rapidly heated to the desired reductiontemperature. Thereafter, the temperature was kept

constant for the desired time. After this, the tempera-ture was reduced to 200 °C. The feeding gases weremixed and preheated prior to entering the reactor. Theexperiments were carried out at atmospheric pressure.

The reaction products were analyzed at 20, 50, 80,and 120 min from the beginning of the experiment. Theconversion shown in all figures is the arithmetic meanof these four analyses. The difference between the firstanalysis and the fourth one is always <2%. The resultsfrom a reproduced experiment showed that the mea-surement of NO conversion had an error of (5%.

3. Results and Discussion

3.1. Influence of the TiPILC Calcination Tem-perature. Figure 1 shows the XRD patterns of theparent TiPILC calcined at different temperatures andthat of the original clay. The XRD pattern of the parentclay exhibits a main peak at 2θ of about 9°, which iscommonly assigned to the basal (001) reflection [d(001)].The d(001) peaks of the pillared material were foundto shift toward the lower 2θ region, which it is a clearindication of an increase in the basal spacing of the clay.A wider peak can also be observed at 2θ = 7°, but it isless intense than the first peak. This peak has also beenobserved for TiPILCs reported by other authors andsuggests the presence of a proportion of the clayexchanged with some type of monomeric Ti species(smaller in size),24 a situation that leads to a smalleropening of the clay layers. On the other hand, basalspacing is not significantly affected by calcinationtemperatures lower than 500 °C, which suggests thatTi species are deposited principally between the silicatelayers.25 Nevertheless, it is quite reasonable that someof the Ti polymers remain at the morphological surfaceof the clay crystallites. Finally, the d(001) peak disap-pears at 600 °C as a result of the collapse of thestructure.

Figure 1. XRD patterns of TiPILC samples calcined at differenttemperatures.

Ind. Eng. Chem. Res., Vol. 42, No. 17, 2003 3873

Table 2 summarizes textural characteristics of tita-nium pillared and Cu ion-exchanged titanium pillaredclays calcined at different temperatures. The nitrogenadsorption/desorption isotherms of the samples over thewhole relative pressure range are shown in Figure 2.As expected, the intercalation with titanium polycationscaused a noticeable increase of both the specific surfacearea and the total and microporous specific pore vol-umes. All isotherms show that the pillared clays areessentially microporous with some mesoporosity.25 Theshape of the isotherms indicates the presence of twogroups of micropores differing in size: ultramicropores(filled with N2 at lower relative pressure) and super-micropores (filled at higher relative pressure).26 TiPILCscontain a noticeable amount of supermicropores. Theisotherm shape is a combination of type I (typical of

microporous solids) and type IV (characteristic of amesoporous solids). Upon calcination of the intercalatedsamples at different temperatures in order to obtain thepillared clays, some interesting changes took place,which can be summarized as follows: (i) The TiPILCsshowed a noticeable decrease of the specific surface areawhen the calcining process occurs. (ii) A small additionalincrease of the specific surface area occurs when thecalcination temperature increases. This effect is ac-companied by a simultaneous small increase in themicropore volume. However, the contribution of themesopores remains constant. (iii) The doping of thepillared samples with copper affects the textural prop-erties of the resulting material.

When the sample is calcined at 350 °C, a significantmicropore surface area loss took place compared withthe uncalcined sample. This fact can be explained interms of pillar restructuring and gallery height reduc-tion. Some authors described the process as a lateralredistribution and restructuring of the pillars, whichleads to a more uniform micropore structure.15 Theeffect of the calcination step on the texture of TiPILCswas systematically studied and found that the mi-cropore volume increased with the temperature forcalcination temperatures below to 500 °C. Calcinationtemperatures above this value resulted in a degradationof the pillared clays’ microstructure and, consequently,in a loss of the specific micropore volume. This trendhas been related to both the dehydroxylation of theinitial pillaring species and the progressive clay andpillar degradation. Taking into account that the titan-ium oligomers stem from the hydrolysis of an alkoxide,the surface area increase that takes place duringcalcination of the intercalated samples could be partiallyexplained by attending to the thermal decomposition ofthe alkoxide ligands, which further facilitates the accessto the porous network of the pillared clays.

On the other hand, when copper is introduced intothe clay structure, a noticeable decrease of the microporesurface area occurs. Two phenomena may be consideredas potential reasons for the observed changed: adecrease of the interlayer distance during doping orfilling and/or blocking of pores by copper species. X-ray

Figure 2. Nitrogen adsorption/desorption isotherms at very low pressures of the titanium pillared clays and copper-doped titaniumpillared clays.

Table 2. Textural Properties Derived from the NitrogenAdsorption/Desorption Isotherms at 77 K

sampledesignation

SBET(m2/g)a

Sint(m2/g)a

Sext(m2/g)a

Vp(cm3/g)b

Vµp(cm3/g)b

original clay 36 15 21 (58) 0.060 0.006TiPILC 355 327 28 (8) 0.263 0.204TiPILC350 305 269 36 (12) 0.251 0.189TiPILC400 314 282 32 (10) 0.260 0.204TiPILC450 319 287 32 (10) 0.270 0.213TiPILC500 338 306 32 (9) 0.286 0.219TiPILC600 280 226 54 (20) 0.239 0.105Cu6-TiPILC350 274 204 70 (25) 0.295 0.148Cu6-TiPILC400 294 230 64 (23) 0.319 0.171Cu6-TiPILC450 275 222 53 (21) 0.261 0.156Cu6-TiPILC500 256 193 63 (24) 0.267 0.148Cu4.0-TiPILC 295 232 63 (21) 0.318 0.172Cu6.0-TiPILC 294 230 64 (23) 0.319 0.171Cu7.5-TiPILC 271 185 88 (32) 0.315 0.140Cu9.0-TiPILC 256 164 92 (36) 0.306 0.126Ag1.0-Cu6-TiPILC 270 201 69 (25) 0.295 0.154Ce1.0-Cu6-TiPILC 283 212 71 (24) 0.307 0.162Ce0.5-Cu6-TiPILC 289 217 72 (25) 0.314 0.163Ce1.0-Cu6-TiPILC 283 212 71 (25) 0.307 0.162Ce2.0-Cu6-TiPILC 256 191 65 (25) 0.274 0.152

a Total surface area obtained from the BET equation (SBET),micropore area obtained from the t-plot method (Sint), and meso-pore area (Sext). The percent of the total surface area is given inparentheses. b Micropore volume obtained from the t-plot method(Vµp), and the total pore volume at P/P0 ) 0.99 (Vp).

3874 Ind. Eng. Chem. Res., Vol. 42, No. 17, 2003

investigations have shown that the interlayer distancesare not affected in a significant way by the dopingprocess.27 This fact would indicate that the mechanismresponsible for the reduction of both surface areas andpore volumes is the blocking of pores by copper species.As deduced in Table 2, a large contribution of mesoporesis also observed in pillared samples doped with copper.This fact suggests that an important relation could existbetween the textural properties after calcination andthe amount of copper present in the pillared clays.Therefore, it is likely that during the ion-exchangeprocess some of the copper species were depositedoutside the interlayer space. Upon calcination, thesespecies will give rise to external mesoporous copperparticles, which could contribute significantly to theincrease of the external surface area.28

In Figure 3, the catalytic behaviors of Cu-TiPILCscalcined at different temperatures for SCR of NO bypropylene in the presence of oxygen are shown. It isclear that the catalytic activity increased with thereaction temperature, reaching a maximum NO conver-sion and then decreasing at higher temperatures.Likewise, the C3H6 conversion (not shown) monotoni-cally increased with the reaction temperature, reaching100% at the temperature yielding the maximum NOconversion. The temperatures for maximum NO conver-sion were 240 °C for samples calcined at 350 and 400°C, 245 °C for the sample calcined at 450 °C, and 260°C for the sample calcined at 500 °C. The maximum NOconversion (always the same) shifted toward higherreaction temperatures whenever the calcination tem-perature of the samples increased. This can be due tothe fact that at higher calcination temperatures a strongdehydroxylation of the pillars could occur, thus decreas-ing the number of sites in which the Cu2+ ions could beanchored. In all cases, the formation of N2O is negligible.

According to these results, the sample calcined at 400°C was selected for further investigations.

3.2. Influence of the Copper Content. Table 2summarizes textural characteristics of Cu-TiPILCswith different copper contents. An increase in the Cucontent was accompanied by a decrease of the surfacearea and micropore volume. This indicated that Cuintroduced into the pillared matrix occupied the inter-layer area.29

The total acidity of the Cu-TiPILCs increased withthe Cu content. It can be noted that the strong acidity

increased with increasing Cu contents because of thefact that the number of isolated Cu2+ ions (which arestrong Lewis acid sites) increases. On the contrary, theweak acidity remains constant (Figure 4). It has beenreported that the majority of copper ions are associatedwith the titanium pillars as isolated Cu2+ ions, with therest forming clusters of an undefined location.27 Eachsubsequent doping increases the relative content ofcopper clusters. Because the preparation of the catalystsinvolved calcination in air at 400 °C, the clusteredspecies are believed to exist as patches of amorphousCuO, whose mesoporous structure could be responsiblefor the increase of the external surface found for thesesamples (Table 2).

The anchoring of the Cu(II) species can be due to thecoordination of these ions to the oxygens of the silicatelayers or to those of the pillar.27 Some studies havedemonstrated that the isolated Cu ions must be an-chored at the pillars rather than at the surface of thesilica layers,30 whereas CuO clusters should be coordi-nated with the oxygens of the silicate layers.

Figure 5a presents TPR spectra of the catalystsprepared with different Cu contents. The peak at thelowest temperature would be related to the presence ofCuO aggregates.17,31-34 The other two reduction peakssuggest a two-step reduction process of isolated Cu2+

species.33,35 The peak at the lower temperature (whichis overlapped with the peak at the lowest one) wouldindicate that the process of Cu2+ f Cu+ occurred. Theother peak at the highest temperature suggests that theproduced Cu+ was further reduced to Cu0. Because thestabilization of a monovalent copper does not take placewithin the CuO species, they are not able to carry outNO reduction, which demands monovalent copper. InSCR of NO, Cu oxide species can be expected tocontribute mainly to propene oxidation and NO oxida-tion to NO2 rather than to SCR of NO to nitrogen. Itcan be observed that the peak related to the reductionof CuO aggregates becomes increasingly pronouncedwith increasing Cu content. On the other hand, the peakrelated to the reduction of both CuO particles and Cu2+

was shifted to lower temperatures when the Cu contentincreases, showing that the metal support interactionis progressively weaker. This fact could be attributedto the formation, with increasing Cu loading, of largermetal particles. On these particles, dissociative H2chemisorption is easier. Because H atoms are betterreducing agents than H2 molecules, the large particles

Figure 3. SCR activities over Cu-TiPILCs calcined at differenttemperatures. Reaction conditions: NO ) C3H6 ) 1000 ppm, O2) 5%, catalyst ) 0.25 g, He ) balance, and total flow rate ) 125cm3/min.

Figure 4. Total acid-site density and acid-strength distributionof the catalysts prepared with different Cu contents.

Ind. Eng. Chem. Res., Vol. 42, No. 17, 2003 3875

formed at high metal content are reduced at lowertemperatures.36

Figure 6 shows the maximum NO conversion to N2of the catalysts prepared with different Cu contents. NOconversion reached for all of the samples a maximumas a function of temperature. As the Cu content in thesample increased, NO conversion increased (until acopper content in the range of 6-7.4 wt %), whereasthe maximum NO conversion is reached at a lowertemperature. On the other hand, the C3H6 conversion(not shown) monotonically increased with the temper-ature, reaching 100% at the temperature yielding themaximum NO conversion.

Figure 7 shows the H2 consumption in the processCu2+ f Cu+ for Cu-TiPILCs both calcined at differenttemperatures and with different Cu contents. It isinteresting to note that the maximum corresponds to acalcination temperature in the range of 400-450 °C anda Cu content of about 6%. This is indicative that a major

number of accessible and reducible copper ions arepresent in these samples. This number decreases whenboth the calcination temperature and the Cu contentincrease.

Because the best results were reached by usingsamples with a Cu content in the range of 6-7.5 wt %,sample Cu6.0-TiPILC was selected for further experi-ments.

3.3. Influence of the Cation Promoter (Ce, Ag)over the SCR of NO in the Presence of Water. Ceriaand silver are known to be active promoters37,38 forenhancing both the activity for SCR of NO and thehydrothermal stability. Particularly, ceria has been usedon PILC catalysts17,39 because of its oxygen storage andtransfer ability. On the other hand, the presence ofsilver provides a higher hydrothermal stability in zeolitematerials because of its relatively weak affinity for

Figure 5. TPR profiles: (a) influence of the copper content; (b) influence of the presence of ceria as a promoter (only peaks correspondingto Cu2+ f Cu+ and Cu2+ f Cu+ reduction processes have been considered).

Figure 6. Conversions of NO to N2 of the Cu-TiPILCs preparedwith different Cu contents. Reaction conditions: NO ) C3H6 )1000 ppm, O2 ) 5%, catalyst ) 0.25 g, He ) balance, and totalflow rate ) 125 cm3/min.

Figure 7. Hydrogen consumption (moles per gram of catalyst)for the Cu2+ f Cu+ reduction process as a function of Cu loadingand the calcination temperature.

3876 Ind. Eng. Chem. Res., Vol. 42, No. 17, 2003

water.38 However, this effect has not been checked inPILCs yet.

The influence of the presence of H2O in the feed onthe performance of the most promising catalyst wasstudied. In Figure 8 are shown the performances of thiscatalyst (Cu6.0-TiPILC) modified with ceria and silverwith and without the presence of water in the feed.Without water in the feed, the catalytic behavior of thesolid containing a small quantity of silver or ceria (1%)is slightly superior to that of Cu6.0-TiPILC. Theaddition of 10% H2O in the feed inhibited the SCR ofNO (the magnitude of the maximum NO conversion wasdecreased about 20-25%), causing a shift of the conver-sion vs temperature curve to higher temperatures. Thiseffect is fully reversible if the water is suppressed(Figure 8). Similar remarks can be deduced for propyl-ene conversion (Figure 9a).

H2O inhibits the reduction due to the competitiveadsorption of water with reactants for the same activesites.38 This observation supports the utilization of morepolar reducing agents such as alcohols and ethers, whichproved to be more effective than propylene when usedin wet streams over these catalysts.40 The reactiontemperature is an important parameter to check theeffect of water on the activity. Generally, the inhibitionby water is higher at small temperatures than at highones, at which some materials are tolerant to the waterpresence.41 With the present catalysts, the SCR reactiontakes place at low temperatures, so the water inhibitionwas significant.

On the other hand, ceria promoter allows one to keepa better activity in the presence of water in the feedthan that obtained with silver promoter. Furthermore,despite the reversible deactivation by water, the maxi-mum activity of the Ce-Cu-TiPILC under wet condi-tions appeared to be stable over extended periods of timeat Tmax (Figure 9b).

Because the best results were reached when ceriapromoter was used, this element was selected for furtherexperiments.

3.4. Influence of the Cerium Content. Noblemetal-ceria catalysts have been studied extensivelybecause of their application in automotive catalytic

converters. Clear evidences of the enhancement of theoxygen storage capacity of ceria by the noble metal andof improved redox activity of these catalysts have beenreported in the literature.42-46

Figure 10 shows the NO conversion to N2 of sampleswith different Ce contents in both the absence andpresence of water in the feed. In both cases, the SCRactivity increased with the Ce loading. This is probably

Figure 8. NO conversion to N2 in the presence/absence of waterover Ag/Ce-doped Cu-TiPILC. Reaction conditions: NO ) C3H6) 1000 ppm, O2 ) 5%, catalyst ) 0.25 g, H2O ) 10% (when used),He ) balance, and total flow rate ) 125 cm3/min.

Figure 9. NO conversion to N2 (and C3H6 conversion to CO2) inthe presence/absence of water over the catalyst Ce1.0-Cu6.0-TiPILC: (a) as a function of the reaction temperature; (b) as afunction of time at 240 °C. Reaction conditions: NO ) C3H6 )1000 ppm, O2 ) 5%, catalyst ) 0.25 g, H2O ) 10% (when used),He ) balance, and total flow rate ) 125 cm3/min.

Figure 10. NO conversion to N2 in the presence/absence of waterover Ce-doped Cu-TiPILC with different Ce contents. Reactionconditions: NO ) C3H6 ) 1000 ppm, O2 ) 5%, catalyst ) 0.25 g,H2O ) 10% (when used), He ) balance, and total flow rate ) 125cm3/min.

Ind. Eng. Chem. Res., Vol. 42, No. 17, 2003 3877

due to the fact that a synergistic effect exists betweenCe-Cu on pillared clays for the NO reduction bypropylene in the presence or absence of water. Morelikely, the structural function of cerium in Ce-Cu-TiPILCs samples could be to keep Cu2+ ions dispersedby inhibiting the migration of Cu2+ ions and preventingthe formation of large Cu clusters. Highly dispersedCu2+ ions are the preferred sites for the SCR reaction.47

However, when the Ce content was too big (2%),

significant surface area and pore volume losses tookplace because of the pore blockage by the cerium oxideparticles (Table 2). As a consequence of this poreblockage, the active sites accessibility decreased and,hence, NO conversion was slightly lower.

TPR results (Figure 5b) indicated after cerium ionswere introduced into Cu-TiPILCs, Cu2+ became moreeasily reducible by H2, and so the peaks appear at lower

Figure 11. Maximum NOx conversion of different catalysts reported in the literature by different authors as a function of the reactiontemperature.

Table 3. Catalysts, Maximum Conversion, and Reaction Conditions Obtained by Different Authors

ref catalyst

metal/metallic

oxide

metal/oxide

content(wt %) max conversion

reaction tempcorresponding

to the maxconversion (°C) feed

17 Ce-Cu-TiPILC Ce, Cu Ce, ND; Cu, 5.9 60% conversion of NO 300 [NO] ) 1000 ppm; [C3H6] ) 1000 ppm;[O2] ) 4%

57 CaO CaO pure 100% conversion of NO 850 [NO] ) 2000 ppm; [C3H6] ) 2.5%;[O2] ) 4%

56 CuO/Al2O3 Cu 2.7 97.3% yield of N2 500 [NO] ) 0.08%; [C3H6] ) 0.2%;[O2] ) 2%; [He] ) 97.72%

51 Cu-ZSM-5 Cu 7.7 75% conversion of NO 375 [NO] ) 2000 ppm; [C3H8] ) 1000 ppm;[O2] ) 2%; [CO] ) 800 ppm

52 Ag-ZSM-5, Ag-Al2O3 Ag, Ag 1.2 and3.0

26% and 80% conversionof NO

475 and 400 [NO] ) 1000 ppm; [C3H6] ) 1000 ppm;[O2] ) 5%

50 H-Cu-ZSM-5,H-Co-ZSM-5

Cu, Co 1.44 and 1.05 85% and 56% conversionof NO

350 and 450 [NO] ) 2000 ppm; [C3H6] ) 2000 ppm;[O2] ) 2%; balance He

55 Cu-MFI Cu 2.8 95% conversion of NO 450 [NO] ) 880 ppm; [C3H6] ) 800 ppm;[O2] ) 4%

49 Cu-MFI(L) Cu 3.5 65% conversion of NO 327 [NO] ) 1000 ppm; [n-octano] ) 750 ppm;[O2] ) 6.7%; [H2O] ) 2%

15, 20, 23,and 24

Cu-TiPILC Cu 6.5 50% yield of N2 240 [NO] ) 1000 ppm; [C3H8] ) 1000 ppm;[O2] ) 5%

53 FeOx/ZrO2 Fe 2.8 55% conversion of NO 400 [NO] ) 4000 ppm; [C3H6] ) 4000 ppm;[O2] ) 2%; balance He

54 Co-MOR Co 3 80% yield of N2 400 [NO] ) 1000 ppm; [C3H8] ) 1000 ppm;[O2] ) 5%

54 H-Ag130-Co130-MOR Ag, Co Ag, 1.4; Co, 2.6 72% yield of N2 425 [NO] ) 1000 ppm; [C3H8] ) 1000 ppm;[O2] ) 5%

3878 Ind. Eng. Chem. Res., Vol. 42, No. 17, 2003

temperatures. This may be related to the observedincrease of catalytic activity of NO reduction by propyl-ene.48

4. Conclusions

In this work, TiPILC-based catalysts ion-exchangedwith Cu were prepared and used for the SCR of NOusing propylene as the reducing agent. The calcinationtemperature of the TiPILC is an important factorbecause it affects both the textural characteristic andthe activity of the catalyst. When the catalyst is calcinedat high temperatures, a small shift toward highertemperatures of the maximum NO conversion occurs.This is probably due to the fact that a strong dehy-droxylation of the pillars occurs when the sample iscalcined at high temperatures, leading to a decrease ofthe number of sites where the Cu2+ ions could beanchored. The influence of the copper loading in theSCR activity was studied. The catalytic activity in-creased with the metal loading, reaching a maximumNO conversion, and then decreased at higher metalcontent. The maximum NO conversion was achieved fora copper loading up to 6-7.5 wt %, which gave thehigher activity at lower temperatures. These catalystsare porous materials with Cu species located in theinterlayer, present either as isolated Cu2+ ions anchoredat the pillars or as patches of amorphous CuO. Thelower activity of the sample containing more Cu wasattributed to the increased contribution from the clus-tered Cu species, which made part of the potentiallyactive centers inaccessible. Cu-TiPILCs showed asignificant loss of activity in the presence of 10% water,but this inhibition was fully reversible. To partlyprevent the inhibiting water effect, ceria was added asa cocation to Cu-TiPILC.

Ceria species have the ability to suppress the ag-glomeration of Cu2+ species and stabilize the catalystwhen water is present in the feed stream. Compositionoptimization studies showed that a small amount ofceria (∼1 wt %) can provide the maximum promotioneffect.

Certainly the catalytic activity of the catalysts re-ported on this work is far lower than that shown forother similar catalysts such as the zeolites. However,the temperatures corresponding to the highest conver-sions are 100-200 °C lower than those observed forother catalysts. Figure 11 shows the maximum conver-sion of different catalysts as a function of the reactiontemperature. Likewise, Table 3 shows the reactionconditions used by the authors referenced in this figure.

Acknowledgment

Financial support from European Commission (Con-tract ERK5-CT-1999-00001) and Consejerıa de Cienciay Tecnologıa de la Junta de Comunidades de Castilla-La Mancha (Project PC1-02-001) is gratefully acknowl-edged.

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Received for review November 13, 2002Revised manuscript received May 30, 2003

Accepted June 2, 2003

IE0209069

3880 Ind. Eng. Chem. Res., Vol. 42, No. 17, 2003