Promotional Effect of Reduction Treatments of PtIn(ferrierite) on Its

L. B. Gutierrez,† J. M. Ramallo-Lo´pez,‡ S. Irusta,† E. E. Miro´,† and F. G. Requejo*,‡. Fac. Ingenierı´a Quı´mica, INCAPE (CONICET), ...
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J. Phys. Chem. B 2001, 105, 9514-9523

Promotional Effect of Reduction Treatments of PtIn(ferrierite) on Its Activity in the SCR of NO with Methane. Kinetics and Novel Characterization Studies L. B. Gutierrez,† J. M. Ramallo-Lo´ pez,‡ S. Irusta,† E. E. Miro´ ,† and F. G. Requejo*,‡ Fac. Ingenierı´a Quı´mica, INCAPE (CONICET), UNL, Santiago del Estero 2829, 3000 Santa Fe, Argentina, and Instituto de Fı´sica de La Plata (CONICET), Dep. de Fı´sica, Fac. Ciencias Exactas, UNLP CC/67 1900 La Plata, Argentina ReceiVed: February 27, 2001; In Final Form: July 5, 2001

K-ferrierite co-exchanged with Pt and In (PtIn(ferrierite)), with different pretreatments, is characterized by EXAFS (extended X-ray absorption fine structure), TDPAC (time differential perturbed angular correlation), TPR (temperature-programmed reduction), and hydrogen chemisorption. The presence of different In and Pt species is correlated with activity and selectivity during the NO selective reduction (SCR) with CH4 in the presence of excess oxygen. The reduction of PtIn(ferrierite) catalyst under controlled conditions (hydrogen at 350 or 500 °C) strongly promotes the activity of the said catalyst. The reduction at 350 °C results in the formation of small Pt° crystals, stable under wet reaction conditions, which probably favor both the oxidation of NO to NO2 and the adsorption of the said species, thus increasing the overall reaction rate. The reduction at 500 °C further increases the reaction rate because it promotes the formation of new (InO)+ active species from the reduction of In2O3 at the catalyst external surface. Small crystals of Pt° are also formed when the calcined catalyst is treated under wet reaction conditions, thus increasing the reaction rate to an extent similar to the one observed for the catalyst reduced at 350 °C.

1. Introduction The increasing interest in the study of the nitric oxide reduction with hydrocarbons in the presence of oxygen excess is supported in both practical and academic aspects. The development of a suitable NOx reduction catalyst would be highly desirable for its use in both mobile and stationary sources. On the other hand, academic interest arises from the complex reaction mechanism and from the nature of the active sites involved. In this vein, the use of CH4 as a substitute for NH3 in stationary sources has become of particular interest. Since Li and Armor1,2 reported that Co-ZSM5 is an effective catalyst for this reaction, a great variety of materials have been studied3 with the same purpose. To develop active and stable catalysts, the cooperation effect of catalytic species has recently been studied.4 In this vein, in pioneering works, Ogura et al.5,6 performed several studies on the effect of the addition of precious metals (Pt, Rh, and Ir) to In-HZSM5. They reported that such solids are highly selective for the reduction of NO with methane in feed streams containing up to 10% water vapor. The authors suggested that the bifunctional catalysis of such solids is remarkably facilitated by the coexistence of the active sites in the pore of the zeolite, what they call “intrapore catalysis”. They also proposed the formation of (InO)+ species as active centers for the dehydrogenation of methane with NO2, which has been proposed as the limiting step of the reaction mechanism.7,8 As regards the role of noble metals, these authors also suggested that it is not only to promote NO oxidation but also to enhance the ability of the catalyst to adsorb NO, and to increase the amount of * Corresponding author. E-mail: [email protected]. Present address: Materials Science Division, LBNL, One Cyclotron Road, Mailstop 66-200, Berkeley, CA 94720. † INCAPE (CONICET). ‡ Instituto de Fı´sica de La Plata (CONICET).

NO2 adsorbed species in the In active centers,5 thus improving the catalysts resistance of the to water poisoning.9 In previous studies10-12 we characterized In species on In/ H-ZSM5 catalysis. The main species identified were In2O3 (indium sesquioxide crystals), In+Z- and (InO)+Z- (different indium species exchanged in the zeolitic matrix), and highly dispersed noncrystalline In oxide species not bonded to the zeolite matrix. Impregnated catalysts, followed by calcination at 500 °C, had low activity for the reduction of NO with CH4 in the presence of excess oxygen, showing only In2O3 and noncrystalline In oxide species. Treatments at 750 °C in O2, or at 500 °C in H2 followed by reoxidation at the same temperature resulted in active catalysts showing an appreciable concentration of (InO)+Z- active species. The same active species were formed after the indium ionic exchange of NH4-ZSM5 was followed by calcination at 500 °C. We have also shown that the perturbed angular correlation (TDPAC) technique is a powerful tool for the identification and quantification of the indium species present on the surface of zeolitic supports. In recent works13,14 we reported that In and In,Pt coexchanged in the ferrierite structure [(Na,K)3.7(AlO2)3.7(SiO2)32.3] resulted in active and selective catalysts for the NO selective reduction with CH4. When a dry feed was used, both monometallic and bimetallic catalysts behaved similarly. However, when water was present in the feed, while In(ferrierite) lost activity in the whole temperature range, PtIn(ferrierite) increased its activity at temperatures above 450 °C. When water was removed, while In(ferrierite) almost resumed its original activity, PtIn(ferrierite) resulted in a catalyst with higher activity. The EXAFS characterization indicated that there is a relationship between the activation of the bimetallic catalyst and the formation of small metal clusters on the outer surface of the zeolite due to the in situ reduction of PtO2 crystals under wet reaction conditions. These small metal clusters would increase

10.1021/jp010748j CCC: $20.00 © 2001 American Chemical Society Published on Web 09/11/2001

Effect of PtIn(ferrierite) Reduction the velocity of the NO oxidation reaction step, thus increasing the overall reaction rate. The combination of EXAFS (extended X-ray absorption fine structure), and TDPAC (time differential perturbed angular correlation) techniques, combined with TPR (temperatureprogrammed reduction), H2 chemisorption, and activity measurements, resulted in a powerful tool for the characterization of these bimetallic catalysts.14 The importance of the extended X-ray absorption fine structure (EXAFS) technique in catalysis studies has long been acknowledged.15 This powerful technique refers to the oscillatory structure in the X-ray absorption coefficient beyond 50 eV of threshold, where the photoelectron backscattering responsible for this phenomenon is relatively weak. EXAFS analysis is no longer limited to first neighbors, and distance determinations are now often comparable in accuracy to those from X-ray diffraction measurements.16 On the other hand, the time differential perturbed angular correlation (TDPAC) technique is a useful characterization tool for catalytic systems, as it allows in situ studies of dispersed and diluted (as low as ppm) phases on catalysts.17,18 This technique, through the measurement of the local electric field gradient (EFG) at the radioactive probe site, can give information about the characteristics (coordination, symmetry, distortions, etc.) of the different environments of the radioactive probes, their concentrations, and modifications related to in situ conditions. This is possible by means of the hyperfine interaction between the nucleus of the probe and the EFG produced by the extranuclear (ion and electronic) charges.19 A brief and clear description of this technique and of the typical equipment set up was published by Vogdt and co-workers.20 Keeping in mind our recent work, where we present a strong activation effect of PtIn(ferrierite) under wet reaction conditions, which maybe originated in the in situ reduction of the PtO2 crystals, the aim of the present contribution is to gain insight about such interesting phenomenon. We report here an investigation of the effect of the PtIn(ferrierite) reduction (in a controlled hydrogen atmosphere) on the activity for the SCR of NOx with methane. To this end, the nature of the species present after different calcination and reduction treatments was studied using EXAFS, TDPAC, TPR, hydrogen chemisorption, and catalytic reaction tests including water in the feed. We also discuss the origin of the dynamical processes at In sites when samples are under reductive conditions. 2. Methods 2.1. Catalyst Preparation and Pretreatments. A commercial K-ferrierite [(K)3.7(AlO2)3.7(SiO2)32.3] Tosoh HSZ720KOA, Lot No. 5001, with a Si/Al ratio of 8.8 determined by chemical analysis was the starting material. The indium-exchanged sample was prepared by the standard ionic exchange method, stirring an aqueous solution of In(NO3)3 (0.003 M) and K-ferrierite at room temperature for 24 h, followed by filtering and distilled water washing. After that, the sample was dried in a stove at 120 °C and calcined in a dry oxygen atmosphere by heating to 500 °C at 5 °C/min and holding the final temperature for 12 h (standard calcinations pretreatment). In this way, a sample with 0.52 wt % Inexchanged was obtained. Indium-impregnated samples were also prepared by the conventional wet impregnation method, stirring an aqueous solution of In(NO3)3 and either K-ferrierite or NH4-ferrierite at 80 °C until all water was evaporated, followed by drying in a stove at 120 °C for 12 h and, finally, the standard calcination procedure. Samples with 2.0 wt % In-impregnated were prepared in this way.

J. Phys. Chem. B, Vol. 105, No. 39, 2001 9515 A Pt-exchanged sample was prepared using a Pt(NH3)4(NO3)2 aqueous solution with the desired concentration in order to obtain a sample with ca. 0.5 wt % Pt. The bimetallic (Pt,In) catalyst was obtained from a monometallic (Pt) sample with a second exchange with an In3+ solution following the technique described for In(ferrierite). After calcination, a portion of all the prepared samples was pretreated under hydrogen flow either at 350 or 500 °C. 2.2. Reaction Experiments. Steady-state reaction experiments were performed using a single-pass flow reactor made of fused silica with an inside diameter of 5 and 300 mm long, operating at atmospheric pressure. The reacting mixture was obtained by mixing four gas lines independently controlled with mass flow controllers; the details of this apparatus are given elsewhere.21 The conversions for the selective reduction reaction were calculated in terms of N2 production as CNO ) 2[N2]/[NO], and for CH4 as CCH4 ) [COx]/[CH4], where [N2], [COx] are gasphase concentrations after reaction and [NO] and [CH4] are feed concentrations. The carbon balance was always better than 95% and the conversions reported were determined after the steady state was reached (usually after 1 h on stream). The gas blends were analyzed before and after reaction using a SRI 9300 B GC instrument. Zeolite 5A was used to separate N2, O2, NO, CO, and CH4 and Chromosorb 102 to analyze CO2 and N2O. 2.3. Catalysts Characterization. 2.3.1 Temperature-Programmed Reduction (TPR). Temperature-programmed reduction experiments (TPR) were carried out in an Okhura TS-2002 instrument. Typically, 50 mg of the solid was pretreated in an oxygen atmosphere by heating to 500 °C at 5 °C/min and holding the final temperature for 4 h. Afterward, the TPR was performed using 2% hydrogen in argon, 30 cm3/min, with a heating rate of 10 °C/min, from 25 to 750 °C. 2.3.2. Hydrogen Chemisorption. The hydrogen adsorption was determined by the constant volume method in a conventional vacuum equipment. As a standard procedure, prior to adsorption, the catalysts (100 mg) were pretreated in a O2 flow (350 or 500 °C) and reduced in a H2 flow (350 or 500 °C). Afterward, they were evacuated (10-5 Torr) at the reduction temperature for 2 h. After cooling at room temperature, the first adsorption isotherm was performed, followed by evacuation at room temperature for 2 h. Then, the second isotherm was carried out. The amount of chemisorbed hydrogen was determined by the difference between the zero pressure extrapolation of the two isotherms. 2.3.3. X-ray Absorption Spectroscopy. The X-ray absorption spectra were measured at the XAS beamline at the LNLSNational Synchrotron Light Laboratory, Campinas, Brazil. The EXAFS spectra of the Pt LIII edge (11.6 keV) were recorded at room temperature in air using two Si(220) channel-cut crystal monochromators. The EXAFS data were extracted from the measured absorption spectra by standard methods.22,23 Raw data files were averaged and the preedge region was approximated by a modified Victoreen curve. Normalization was completed by dividing by the height of the absorption edge, and the background was subtracted using cubic spline routines. The main contributions to the spectra were isolated by inverse Fourier transformation of the final EXAFS functions. The analysis was performed on these Fourier-filtered data. Three samples were measured by EXAFS in this work: fresh PtInFER calcined, reduced at 350 °C, and the same reduced catalyst used either in dry or wet reaction conditions. Results

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were compared with those for nonreduced samples reported elsewhere.14 To obtain quantitative information, EXAFS measurements of two reference compounds were used: β-PtO2 and Pt foil.14 2.3.4. TDPAC Technique. Since this interesting technique has been scarcely applied in the catalysis field, a brief description of its principles will be addressed. The TDPAC technique rests on the fact that, due to the conservation of angular momentum, the direction of emission of a photon in a nuclear decay is strictly correlated with the orientation of the nuclear spin.19 Then, in a radioactive source with the nuclei randomly oriented, it is possible to select a set of nuclei with a particular spin orientation detecting the first radiation γ1 of a γ-γ nuclear cascade in a fixed direction B k1. The detection of the second γ-ray, γ2, will therefore show a certain angular distribution pattern with respect to the B k1 direction. The density of probability W(θ,t) of the second emission in the B k2 direction, detected at an angle θ with respect to B k1, will be perturbed if interaction with extra nuclear fields occurs during the t time the nuclei remain in the intermediate level. Thus, W(θ,t) is written as19

W(θ,t) ) 1 + A22G22(t)P2(cos θ)

(1)

A22 being the anisotropy of the cascade that depends only on nuclear properties, and P2(cos θ) is the second-order Legendre polynomial, which reflects the angular dependence of W(θ,t). Higher terms in eq 1 are negligible in the case of the 111Cd cascade due to the small values of the A22 coefficients. In the absence of magnetic fields, the G22(t) perturbation function contains all the information on the hyperfine interactions between nuclear electric quadrupole moment eQ with an extra nuclear EFG. These EFG result mainly from the local charge density distribution, due to the r-3 dependence of the EFG with the distance between the probe and the electronic charge, which, in turn, depends on the electric characteristics of the atomic near surroundings and on the nature of chemical bonds. For polycrystalline samples and a nuclear spin I ) 5/2 of the intermediate nuclear level of the cascade, the G22(t) perturbation factor for static electric quadrupole interactions has the form19 3

G22(t) )

∑i fi(S20,i + n)1 ∑S2n,i cos(ωnit)e-δ ω t) i ni

(2)

where fi is the relative fraction of nuclei that experiments a given perturbation. The ωn frequencies are related to the quadrupole frequency ωQ ) eQVZZ/40p by ωn ) gn(η)ωQ. The gn and Sn coefficients are known functions24 of the asymmetry parameter η ) (VYY - VXX)/VZZ, where Vii are the principal components of the EFG tensor and |VXX| < |VYY| < |VZZ|. The exponential functions account for a Lorentzian frequency distribution of relative width δ around ωn. In this work, a four CsF detector fast-fast coincidence system in a coplanar arrangement, with a time resolution of 1 ns was used to perform the TDPAC experiments. All experiments were performed at 500 °C in order to avoid electronic relaxation phenomena also known as “aftereffect” produced by the electron capture decay of the 111In f 111Cd.25,26 Eight coincidence spectra (four at 90° and four at 180°) of all possible start-stop combinations of the four detectors were recorded simultaneously in a multichannel analyzer. A coincidence Cij(t) is the result of the detection of the two γ-rays from a double cascade decay coming from the same nucleus in detectors i and j separated an angle of θ with an interval of t (seconds) between them. From

the coincidences at two angles (90° and 180°) these two relations can be constructed:

W(180°,t) ) [C13(t) + C31(t)]1/2[C24(t) + C42(t)]1/2 (3) and

W(90°,t) ) [C12(t) + C21(t)]1/2[C34(t) + C43(t)]1/2

(4)

Finally, the asymmetry ratio R(t) becomes

R(t) ) 2

[W(180°,t) - W(90°,t)] ≈ Aexp 22 G22(t) W(180°,t) + 2W(90°,t)

(5)

Aexp 22 being the effective anisotropy of the cascade for a certain experimental condition. Performing a nonsquare fit of the theoretical perturbation factor Aexp 22 G22(t) to this R(t) spectra, one can obtain the hyperfine parameters (relative concentration f, quadrupole frequency ωQ, asymmetry parameter η, and distribution percentage δ) for each probe’s inequivalent site. Under certain circumstances, perturbed angular correlation of γ-rays emitted from a nuclide after an electron capture (EC) decay is strongly influenced by excited electronic states produced by the EC decay and the subsequent Auger process. This phenomenon, the so-called aftereffect of EC decay, is generally thought to be undesirable in applying TDPAC to solid state physics or chemistry, because the hyperfine frequencies may be widely spread due to such effects, sometimes to the extent that the hyperfine interactions inherent in host materials are completely smeared out. However, these aftereffects themselves are very interesting and important phenomena because they include information on the electronic excited states and their relaxation to the ground state of the solids. Depending on how fast it can be removed, it is possible to infer the electronic availability at probe’s surrounding. 3. Results 3.1. Reaction Results for the SCR of NOx with Methane. Effect of the Reduction on Catalytic Activity. 3.1.1. Monometallic Catalysts. Data reported in a previous work show that K-ferrierite exchanged with indium (0.52 wt %) and calcined in oxygen at 500 °C is an active solid and highly selective, showing 60% of NO conversion at 450 °C (GHSV ) 6500 h-1).14 This conversion decreases about 44% when water (2%) is added to the reacting stream. However, when the addition of water is stopped, the NO conversion is almost totally recovered.14 Now, we found that when this catalyst is reduced for 1 h at 350 °C, it is much less active than the calcined one. The maximum NO conversion (40%) is reached at 450 °C but with a very low selectivity. Moreover, the subsequent reduction of the solid at 500 °C for 1 h results in an almost inactive catalyst. K-ferrierite impregnated with In(NO3)3 (see methods section) resulted in a totally inactive catalyst, in both the calcined and reduced form. However, when NH4-ferrierite was impregnated, it resulted in an active catalyst when reduced at 500 °C. These results are not surprising, since they follow the same pattern observed by Kikuchi et al.7 and Miro´ et al.11 for In-ZSM5 impregnated catalysts. It has been readily shown7,12 that acid sites are necessary to form (InO)+ active species after calcination at 750 °C or reduction at 500 °C. The (InO)+ species are also formed in the zeolite framework when the preparation procedure is the ionic exchange rather than impregnation, thus explaining

Effect of PtIn(ferrierite) Reduction

Figure 1. Effect of pretreatment on catalytic behavior of monometallic solids. Solid symbols: InFer prepared by ionic exchange. Open symbols: InFer prepared by wetness impregnation (on NH4-ferrierite). Key: (9) calcined 500 °C, (b) reduced 350 °C, (4) reduced 500 °C, (]) Calcined 750 °C. Reaction conditions: GHSV 6500 h-1, NO ) 1000 ppm, CH4 ) 1000 ppm, O2 ) 2%.

the results described above. Figure 1 displays a summary of the catalytic results obtained with monometallic catalysts. 3.1.2. Bimetallic Catalyst: PtIn(ferrierite). In our previous work14 we studied the catalytic behavior of the nonreduced bimetallic catalyst under both dry and wet conditions. Under dry conditions the behavior of PtIn(ferrierite) is comparable to that observed for the monometallic catalyst. However, when water was included in the feed, instead of the expected activity decrease, higher NO conversions were obtained at temperatures above 450 °C. When water was removed, the NO conversion was higher in the whole temperature range if compared with the behavior of the fresh catalyst. It was suggested that this interesting result is ascribed to the in situ reduction of small PtO2 crystals. Thus, to gain insight into this phenomenon, the present work is mainly focused on the study of the catalyst reduction under controlled conditions. The maximun NO conversion for PtIn(ferrierite) nonreduced sample, under dry conditions, is 50% at 450 °C.14 In Figure 2 it can be observed that the reduction of the PtIn(ferrierite) at 350 °C provokes an increase of this value, reaching 75% at 450 °C. Both the NO to N2 conversion on the sample reduced at 350 °C and the selectivity are lower when water vapor is incorporated. When the flow of water is suspended, the activity is recovered. The activation effect becomes more important when the reduction temperature is 500 °C (Figure 3). The activity yielded by the catalyst reduced at 500 °C is close to 100% at 450 °C, decreasing to 80% when water is added. This effect is totally reversible. By comparison of these results with those of the previous work14 it can be seen that the promoting effect of water on calcined PtIn(ferrierite) is very similar to that observed with the sample reduced at 350 °C, which appears to be in agreement with the activation mechanism proposed in ref 14. 3.2. TPR Characterization Results. Previous results of TPR characterization of the nonreduced, fresh PtIn(ferrierite) show that all the peaks corresponding to the monometallic samples (Pt(ferrierite) and In(ferrierite)) were present.14 The only difference is a shift to a lower temperature of the wide signal

J. Phys. Chem. B, Vol. 105, No. 39, 2001 9517

Figure 2. Catalytic behavior of PtInFer reduced at 350 °C: (b) 0% H2O, (O) 2% H2O, (X) 0% H2O after treatment with water. Reaction conditions: see Figure 1.

Figure 3. Catalytic behavior of PtInFer reduced at 500 °C: (9) 0% H2O, (0) 2% H2O, (%) 0% H2O after treatment with water. Reaction conditions: see Figure 1.

ascribed to the reduction of (InO)+Z- species and highly dispersed noncrystalline In oxide species not bonded to the zeolite matrix, and a slight decrease of the high-temperature peak ascribed to In2O3. This result suggested that only a weak interaction between the In and Pt species supported in the ferrierite structure would exist.14 The profiles of on-stream used bimetallic samples are depicted in Figure 4. (a) used nonreduced catalyst, (b) used catalyst reduced at 350 °C, and they are compared with that of the fresh, nonreduced sample (c). Two reduction zones can be observed in both used samples. The low-temperature zone, already assigned to exchanged (InO)+ species and/or highly dispersed noncrystalline In oxide species not bonded to the zeolite matrix, is predominant in both cases. In the reduced solid (b), it can be observed that the fraction of Pt which has been reduced at 350 °C during the pretreatment is not reoxidized under reaction conditions, and the remaining has migrated to more easily reducible positions. The lowtemperature peak ascribed to well-dispersed In species shifts to even lower temperatures. Besides, the intermediate reduction

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Figure 4. TPR pofiles of PtInFer catalyst. (A) Calcined used under reaction conditions. (B) Reduced used under reaction conditions. (C) Calcined fresh.

TABLE 1: Hydrogen Chemisorption (mol of H/mol of Pt0) on Pt and Pt,In Exchanged K-Ferrierite reduction temp catalyst and pretreatment a

PtFerr PtFerrb PtInFerra PtInFerrb PtInFerrc

T ) 350 °C

T ) 500 °C

1.2 0.9 0.8 0.9 0.2d

0.9 0.3 1.0 0.9

a Calcination at 350 °C. b Calcination at 500 °C. c Calcination at 500 °C, followed by reaction under wet conditions. d Chemisorption value measured without reduction.

peak corresponding to Pt at some exchange position14 is not detected. On the contrary, the catalyst which has not been reduced prior to the reaction seems to preserve the species present in the fresh catalyst, but now with a different distribution (the broad low-temperature peak seems to contain all the peaks of the fresh bimetallic sample). 3.3. Hydrogen Chemisorption Results. Table 1 shows the dispersion values obtained for the Pt and PtIn catalysts with different pretreatments (calcination at 350 °C or 500 °C followed by reduction at 350 °C or 500 °C). In all cases the amount of chemisorbed hydrogen is referred to the amount of Pt° present in the reduced catalyst. An exception is the datum for used sample (a), which was not reduced, and the amount of hydrogen chemisorption is referred to the total amount of Pt. It should be pointed out here that hydrogen adsorption was observed neither in the support nor in the In-ferrierite catalyst. Pt(ferrierite). For this solid, only a partial reduction of Pt is achieved (ca. 53%) when reduced at 350 °C. At 500 °C, the reduction is complete. This result is in agreement with previously reported TPR data.14 The catalyst containing only Pt calcined either at 350 °C or 500 °C and reduced at 350 °C, shows a high degree of dispersion. However, when it was calcined at 500 °C and reduced at the same temperature, a

decrease in dispersion was observed. This decrease in dispersion when increasing the reduction temperature has already been noticed by Sachtler and co-workers27 for a PtYZeolite catalysts and may be attributed to the growth of Pt particles at the high reduction temperature used. PtIn(ferrierite). The co-exchange of In appears to stabilize the small Pt particles inside zeolite channels when reduced at 500 °C. These results are different from those found by Meriandeau et al.28 who reported that the addition of In decreases dispersion due to the formation of PtIn particles. The difference with our results may be due to the fact that the amounts of Pt and In they used (5% Pt, 0-5% In) are quite higher than those used in our catalysts. It should be remarked that in no case was the formation of bimetallic PtIn particles observed in our samples.14 Our results are in agreement with those reported in ref 29 for PtFeYZeolite, in which the presence of Fe increased the Pt dispersion because the Pt migration and particle growth is hindered by the presence of Fe blocking the zeolite channels. An interesting hydrogen chemisorption result was obtained for the PtIn(ferrierite) catalyst, which has been under wet reaction conditions (last result in Table 1). The hydrogen adsorption has been carried out in this sample without previous reduction treatment. Despite that, the sample adsorbs hydrogen (0.2 mol of H per mole of total Pt), indicating the formation of Pt° during the wet reaction, which is in agreement with our previous work,14 in which we detected Pt° particles by EXAFS characterization of this sample. 3.4. EXAFS Measurements. EXAFS experiments were performed in order to determine the local ordering characteristics at Pt sites. The effect of the reduction treatment (H2, 350 °C) on the Pt species structure was studied with fresh and used catalysts using the EXAFS technique. Pt L3-edge k3-weighted. EXAFS (k vs k3χ(k)) spectra of PtIn(ferrierite) (after standard calcinations pretreatment) (a), PtIn(ferrierite) reduced at 350 °C (b), and PtIn(ferrierite) reduced at 350 °C and used in dry reaction (c) and wet reaction conditions (d) are shown in Figure 5 (left). The corresponding k3-weighted Fourier transforms of the EXAFS spectra are shown in Figure 5 (right). The experimental Pt L3-edge EXAFS function (χ(k)) is plotted against the photoelectron wave vector (k) after the smooth background decay has been subtracted from the data above the absorption edge using standard procedures.29 The EXAFS oscillations of the fresh samples (Figure 5a,b) are clearly different. This fact is shown in their Fourier transforms, which indicate that there is a change in the local Pt environment due to reduction treatment. The EXAFS spectra of reference samples (PtO2 and Pt foil) are shown in Figure 6. Before any phase correction is made, the Fourier transform of Pt L3-edge k3-weighted EXAFS for PtO2 (Figure 6a, left) exhibits peaks at 1.7 and 3.0 Å due to the Pt-O shell and Pt-O-Pt shell, respectively. The Fourier transform of the Pt L3-edge k3-weighted EXAFS for Pt foil (Figure 6b, left) exhibits peaks at 2.1 and 2.6 Å due to the scattering of neighboring Pt atoms. The peak at 2.1 Å is caused by nonlinear parameters (phase shift, backscattering amplitude) of platinum. The Fourier spectra of the calcined sample and the reduced samples after being used (Figure 5a,c,d) exhibit a peak at 1.7 Å. The peak position is the same as the first one observed for PtO2. Unlike PtO2, no peak appeared at 3.0 Å. The order of intensity of the peak at 1.7 Å is PtO2 > PtIn(ferrierite) calcined fresh > PtIn(ferrierite) reduced used (dry) > PtIn(ferrierite)

Effect of PtIn(ferrierite) Reduction

k3-weighted

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k3χ(k))

Figure 5. Pt L3-edge EXAFS (k vs spectra of PtIn/ FER calcined (a), PtIn/FER reduced (b), and PtIn/FER reduced used in dry (c) and wet conditions (d) catalysts (right) and the k3-weighted Fourier transforms of the EXAFS corresponding spectra (left).

Figure 6. Pt L3-edge k3-weighted EXAFS (k vs k3χ(k)) spectra of PtO2 (a) and k-weighted EXAFS (k vs kχ(k)) of Pt foil (b) (right) and the corresponding k3-weighted Fourier transforms of the EXAFS spectra (left).

reduced used (wet). This peak indicates that Pt-O shells are present in all these samples and their intensities are inversely related with the dispersion degree of the Pt-oxidized species. The sample that exhibits peaks at 2.1 and 2.6 Å as in Pt foil is PtIn(ferrierite) reduced fresh and used under wet reaction conditions. This suggests that Pt is present in the metallic state in these samples. The peak heights are less than half that for the Pt foil, indicating that the size of the metallic Pt particles is small. To quantitatively analyze these data, the main peaks of each transform were isolated by Fourier filtering (below 1.0 Å and above 2.1 Å for the Pt-O shell and below 2.1 Å and above 3.4 Å for the Pt-Pt shell) and were back-transformed to k space. To obtain the first shell distance, R, and the coordination number, N, for Pt bonds, the resulting inverse transforms were analyzed by a standard fitting procedure30 using the bulk metal first shell Pt-Pt and the PtO2 first shell Pt-O phase and amplitude as reference (Figures 7 and 8 and Table 2). Reference values for the first Pt-Pt and Pt-O shells are from crystallographic data.31

Figure 7. Inverse Fourier transform of the major peak (∆R ) 1-2.1 Å) of the Fourier transform of the EXAFS spectra for PtIn/FER calcined (a) and PtIn/FER reduced used in dry (b) and wet conditions (c) (circles) and the calculated EXAFS function for Pt-O distances (solid curve).

Figure 8. (a) Inverse Fourier transform of the second peak (∆R ) 2.1-3.4 Å) of the Fourier transform of the EXAFS spectra for PtIn/ FER fresh (a) and used under wet conditions (b) (circles) and the calculated EXAFS function for Pt-Pt distances (solid curve).

TABLE 2: Pt L3 EXAFS Results (Figures 7 and 8) (Error in Last Digit in Parentheses) catalyst

pair

Na

Db (Å)

102σ2 (Å)

PtInFerr calcined PtInFerr reduced PtInFerr reduced used (dry) PtInFerr reduced used (wet)

Pt-O Pt-Pt Pt-O

4.8(2) 8(1) 4.4(2)

2.00(1) 6.4(7) × 10-2 0.6(1) 2.66(1) 4(1) × 10-2 -5(1) 1.99(1) 5.6(5) × 10-2 -1.1(1)

Pt-O

3.6(2)

1.99(1) 9.2(2) × 10-1 -0.5(1)

Pt-Ptc 6.4 (5) 2.72(1) 1.0(1) a

E0 shift

0.4(1) b

Average coordination number for the corresponding pair. Firstneighbor distance for the corresponding pair. c Third and fourth cumulants were used in these fittings.

For PtIn(ferrierite) calcined fresh and PtIn(ferrierite) reduced used under dry conditions, only a Pt-O shell was fitted, indicating that platinum on the zeolite is in the form of PtOx. For PtIn(ferrierite) reduced fresh only a Pt-Pt shell was fitted, indicating that the Pt on this sample is in metallic form. For

9520 J. Phys. Chem. B, Vol. 105, No. 39, 2001

Gutierrez et al. TABLE 3: Fitted Values of the Parameters Characterizing the Observed Interactions in the PtIn/FER TDPAC Spectra of Figure 9 condition at air at H2a at air bulk In2O3b

b

Figure 9. Left: points: PAC spectra taken at 500 °C of PtIn/FER at air (a), in H2 atmosphere (b), and at air after the reduction process (c); curve line, fitted function A22G22(t). Right: their corresponding Fourier transforms.

PtIn(ferrierite) used under wet conditions, both Pt-O and PtPt shells were fitted. However, peaks arising from the second and third shells of the face-centered cubic (fcc) Pt-metal structure are absent. The average coordination number of the Pt-Pt shell for these two samples is smaller than the value of 12, which is the average coordination number for the Pt foil, indicating the small size of the crystals. The fact that no metallic platinum was found in the PtIn(ferrierite) reduced after the reaction in dry conditions indicate that the Pt particles are oxidized during the reaction in dry condition but not during the reaction in wet condition. Moreover, it should be noted that no Pt-O shells are found in the reduced sample after the wet reaction, indicating that there is a reduction effect on Pt during this treatment. Evidence for higher coordination shells is slight since the peak intensities at appropriate distances are on the order of the noise. 3.5. TDPAC Measurements. To determine the nature of In sites in the samples, TDPAC measurements were performed at 500 °C. Figure 9 shows the obtained TDPAC spectra (left) and their corresponding Fourier transforms (right) for PtIn(ferrierite) at air (only these data were already published in ref 14, depicted here for comparison purposes) (a), in H2 flux (b), and at air after H2 flux (c). It is clear from the spectra that the species present in every step of the treatment are very different and that reduction at 500 °C has an important effect on the final configuration of indium on the catalyst surface. The spectra in Figure 9a indicate the existence of, at least, three hyperfine interactions (Table 3). The interactions labeled as I1 and I2 have the hyperfine parameters of 111In in In2O3.26 This indicates that 75% of indium is forming In2O3. Slight differences with the pure In2O3 case should be associated with the small dimensions of the crystallites present in the catalysts.12 The third interaction I3 has the same hyperfine parameters as the ones observed in the monometallic sample.14 When a fraction of In probes has very similar nearest neighborhoods (but not identical), a finite frequency distribution around a mean “precession frequency” due to inhomogeneities can be obtained. The “inhomogeneities” fitted for I3 can be attributed to 111In-probes located in slight-different sites of exchange in the zeolite as in In-H-ZSM5 catalysts.12

population frequency asymmetry distribution interf (%) ωQ (MHz) parameter η δ (%) action 45(6) 28(6) 27(6) 100 36(5) 8(1) 56(3) 77(2) 23(2)

124(2) 151(2) 177(2) 134(15) 122(1) 156(2) 188(9) 119.1(5) 155.2(5)

0.66(3) 0.24(3) 0.35(2) 0.5(1) 0.71(2) 0.15(5) 0.6(2) 0.71(1) 0

10(2) 5(1) 10(3) 100 4(1) 1(5) 35(8) 1.4(4) 1.0(4)

I1 I2 I3 Ih I1 I2 I3 I1 I2

a Relaxation parameter of 0.010(1) 1/ns was fitted for this interaction. From reference 26.

During the reduction treatment at 500 °C, the TDPAC spectrum (Figure 9b) is completely different. The spectrum obtained is clearly dominated by the exponential attenuation characteristic of a time-dependent interaction. This interaction may be originated in the atomic relaxation process, which follows the nuclear electron capture of 111In (usually called aftereffects). It has been shown in the literature that this anomaly in the TDPAC spectra can be eliminated with the addition of donor dopants that provide the required electrons at the probe site25,26 or by raising temperature in order to give the valence electron the necessary mobility to accomplish the electronhole recombination.32 As no aftereffects were observed in all other measures of our samples at 500 °C, the reason for the observed relaxation could be associated with the presence of H2. This adsorption could be modifying the near In surrounding (TDPAC probe) during the detection between γ1 and γ2 in the TDPAC experiments, giving a mean configuration in the near neighborhood and the respective loss of information (evidenced through the exponential dumping in the TDPAC spectrum). A time dependent type interaction can also occur when the environment of the probe changes rapidly. This situation can occur when rapidly moving defects such as vacancies contribute significantly to the hyperfine fields experienced by the probe nucleus.33 When the sample is then calcined at 500 °C in air, the original hyperfine interactions are recovered (Figure 9c) with some differences in their populations. Both In2O3 sites are present, indicating that 44% of indium atoms are forming the oxide. The hyperfine parameters of the third site correspond again to (InO)+ species at exchange sites and represents 56% of the total atoms, indicating an increase of In atoms forming the active phase (Table 3). 4. Discussion The activation phenomenon described in the Results, in PtIn(ferrierite) used under wet reaction conditions and reduced in hydrogen at 350 or 500 °C will be discussed in this section by taking into account the systematic characterization carried out. Since the indium-exchanged K-ferrierite sample does not show any activation behavior when either reduced or treated under a wet reaction atmosphere, it can be concluded that the presence of Pt is necessary for the observed activation phenomenon. In the Results it has been shown that the L3-Pt EXAFS oscillations and their corresponding Fourier transforms of PtIn(ferrierite) before and after the reduction treatments are different. Table 2 shows the fitted parameters for these samples. The oxygen average coordination number for the PtIn(ferrierite) sample before reduction is 5, which we have previously interpreted as an average of two coordination numbers corre-

Effect of PtIn(ferrierite) Reduction sponding to two different Pt species.14 The first species would be Pt at exchange sites, and the other would be an oxide species with a coordination number for the pair Pt-O of 6.31 In effect, taking the Pt(ferrierite) spectrum as the corresponding one for Pt in exchange sites, one can obtain a difference spectrum corresponding to the other species present by subtracting it (weighted taking into account the average coordination number) from the spectrum for bimetallic catalyst. The obtained EXAFS spectrum is very similar to that of PtO2 and the coordination number fitted for the Pt-O pair is 6. The TPR profiles showed the presence of similar species (Figure 4c). Despite the appearance of the peak at 1.7 Å, indicating that a Pt oxide phase is present, the Pt-Pt scattering peak at 3.0 Å for the configuration of Pt-O-Pt is not observed (see Figure 5.d). The absence of the Pt-O-Pt shell suggests that the Pt oxide phase is not present as bulk Pt oxide but as a very dispersed species. Some authors assigned this Pt configuration to a thin layer of Pt oxide.34 This shows that Pt is forming two different species in the bimetallic samples. One corresponds to Pt at exchange sites (ca. 50%) and the other to an oxide species, probably segregated on the external surface. After the reduction treatment, no oxide phase is present and only Pt-Pt bonds are observed (Figure 5b and Table 2). The fitted average coordination number of 8 is lower than that of bulk Pt, indicating the small size of Pt particles. When small metal clusters are examined by EXAFS, the average coordination number is smaller than the one observed in the bulk because of the high proportion of surface atoms. This effect is dependent on the size and shape of the metal cluster. The platinum particle size in PtIn(ferrierite) after reduction may be estimated from coordination number N obtained for the Pt-Pt shell according to the method proposed by Greegor and Lytle,35 assuming the spherical particle of the fcc package. From our results, the diameter of the metallic particles is estimated to be between 12 and 14 Å. The Pt-Pt distance fitted for these clusters is 2.65 Å instead of the bulk Pt bond length of 2.77 Å. The Pt-Pt bond contraction found is consistent with previous results.36 In effect, it has been reported that near-neighbor distances determined from EXAFS measurements of “bare” (i.e., without adsorbed gases) Pt clusters show contractions of approximately 0.07 Å (2.5%) relative to the bond length in bulk Pt. The origin of these bond contractions in small Pt clusters is uncertain and has been deemed to be intrinsic to the cluster.36 These results are very similar to the ones we have found considering the accuracy of the EXAFS technique for the determination of bond distances. The fact that there is no evidence of Pt-In bonds in the bimetallic sample before and after the reduction indicates that there is no direct interaction between In and Pt, in agreement with TPR results. Thus, no bimetallic In-Pt entity is present in the PtIn(ferrierite) catalyst. After the reduced catalyst is used, a change in the Ptsourroundings is observed. A Pt-O shell is observed after the reaction in both dry and wet conditions, and no Pt-Pt bonds are present after the dry reaction. The average coordination number of the Pt-O shell obtained for the calcined PtIn(ferrierite) sample used in dry reaction conditions is 4.4 (see Table 2), which can be again interpreted as an average from two different coordination numbers corresponding to the two species found before: Pt in exchange sites and a platinum oxide phase. Under this supposition, and assuming that the spectra for Pt(ferrierite) correspond to Pt2+ ions at cationic exchange sites,14 one can obtain the spectra for each species in an 8:2 ratio (exchanged:oxide). Thus, Pt is completely oxidized after the reaction under dry conditions and no metallic particle is

J. Phys. Chem. B, Vol. 105, No. 39, 2001 9521 found. We could arrive at similar conclusions by observing the corresponding TPR profile (Figure 4.a). In contrast, after the reaction in wet conditions, the average coordination number of Pt-O shell corresponds to that of Pt at exchange sites. No oxide phase is present after the catalyst was used in the presence of water. A metallic phase is still present, the coordination number for the Pt-Pt bond being slightly smaller than that of the fresh sample (Table 2) resulting in smaller Pt particles (between 10 and 12 Å in diameter according to the cited method in ref 35). This result agrees very well with TPR data (Figure 4), in which it is clearly seen that a fraction of the reduced Pt is still present after reaction under wet conditions. The presence of this metallic phase would be responsible for the higher activity of the bimetallic catalyst under wet reaction conditions. The presence of small metal clusters may accelerate the velocity of the NO + 1/2O2 f NO2 reaction step and also the NO2 adsorption capacity, thus increasing the overall reaction rate. The methane oxidation reaction with adsorbed NO2 has been proposed to be the limiting rate step7,8 and to occur in In active species.5 On the other hand, it has been reported that in metal zeolite based catalysts sodium inhibits the adsorption of methane by enhancing the adsorption of NO over Pd-based catalysts.37 We accordingly think that the residual content of potassium in our solids (c.a 0.08 wt %) could play a similar role. Probably some effect on the dispersion and redispersion of metal species also may take place. Other authors have shown that the addition of potassium to Pt/L-zeolite reduces the size of the platinum surface ensembles.38,39 However, due to the low potassium content, such effects should be rather small. The zeolitic microstructure could also affect the performance of the catalysts studied in this work. As a matter of fact, Ogura and co-workers have shown that microporosity plays an essential role not only on the position of the cationic metal, but also on diffusivity effects of gaseous molecules40 (reactives, products, and intermediates). Considering the previous analysis for the Pt clusters found in the catalysts used under wet conditions, we can draw some conclusions regarding the place where these particles are located. As the size of the biggest ferrierite channels is 4.2 × 5.4 Å in diameter, and the diameter of the Pt clusters is between 10 and 14 Å, these metallic particles cannot be found inside the zeolite channels and should be located on the external surface. This fact is further supported by the low hydrogen chemisorption observed in this sample (Table 1). TDPAC measurements and TPR experiments show the presence of exchanged (InO)+ “intrapore” species in both the calcined and the reduced catalyst. Indium sesquioxide is also present in both samples, but it has been shown that this oxide is not active for the SCR of NO.12 Considering that their hyperfine parameters are very close to those reported in the literature for bulk In2O3 26 (although their distributions are a bit high and the population relation is not 3:1), one can infer that their size must be bigger than 30 Å in diameter so that they must be segregated on the external surface of the zeolite. In effect, even though the In2O3 phase was not observed in our samples by XRD, the low distribution parameter δ fitted for In sites in In2O3 (see Table 3) indicates that In2O3 crystallites are at least 30 Å in diameter. In other words, when a set of probes (111In at In sites) has very highly distorted near neighborhoods, like in a low dimensional In2O3 cluster, a finite frequency distribution δ (no less than 20%) around a mean “precession frequency” due to inhomogeneities should be observed. Then, In2O3 present in our catalysts should

9522 J. Phys. Chem. B, Vol. 105, No. 39, 2001 be located in the zeolite external surface because crystallites bigger than 30 Å in diameter cannot enter in zeolite channels. The reduction of this oxide proceeds in the way proposed by Beyer et al.41:

In2O3 + 2H2 f In2O + H2O In2O + 2H+Z- f 2In+Z- + H2O The observed hyperfine interaction for the sample in H2 should corresponds to this reduced site. An interesting point is the exponential damping observed in the spectra. This attenuation is clearly indicating a time dependent interaction. This kind of process can be associated with different causes. One possibility is the aftereffects following the nuclear electron capture of the probe (111In). This perturbation is ultimately produced by a hole trapped in the impurity center introduced by the 111Cd decay product in the band gap of the semiconductor.25 Two different electron-hole recombination mechanisms were identified: electrons coming from the conduction band mostly dominate the recombination process at low temperature (from -259 to -73 °C), while the thermal excitation of electrons from the valence band is the most important source of electron supply at higher temperatures.42 For the case of In2O3, no attenuation is observed at 500 °C.32 As this relaxation effect is not observed in the sample measured in air at 500 °C, there must be a modification of the electronic structure of the probe due to the presence of H2. It has been reported that the chemisorption of H2 on Pt atoms induces an antibonding state above the Fermi level, which enlarges the number of d holes.43 A similar effect could be produced on In atoms, reducing the concentration of electrons capable of recombining at that temperature. As we have already said, a second cause for a time dependent type interaction could be a configuration where the environment of the probe changes rapidly. This situation can occur when rapidly moving defects such as vacancies contribute significantly to the hyperfine fields experienced by the probe nucleus.33 In our particular case, an oxygen vacancy (created by reduction with hydrogen), which may be free to jump between equivalent near-neighbors positions, seems to be the reason for the relaxation process. Such effects have been already reported when using 111In PAC spectroscopy to study trapping and hopping of oxygen vacancies near indium and its daughter isotope, cadmium, in a variety of zirconia-based ceramics44,45 and ceria oxide.46 However, further analysis is required to decide the nature of this behavior and will be discussed in a future work. When the sample is reoxidized in air at 500 °C, a part of In2O3 is recovered but in a smaller quantity that in the calcined sample. The distribution of the two sites is smaller than those of the calcined sample, meaning that the crystals are bigger after the reduction treatment. The rest of In atoms are present in the form of (InO)+ species at cationic exchange sites, which are the active sites for the SCR of NO. This represents an increase of 100% in the number of indium active sites. However, some differences in the hyperfine parameters before and after reduction are found, which may indicate that after reduction the fitted interaction corresponds to more than one site and the increase in the population would be smaller. Because of the high distribution there is no possibility of fitting two different sets of parameters for this interaction to clarify this point. This apparent increase in the population of the active site can be another reason for the increase in the conversion percentage observed. There is also an important increase in the distribution

Gutierrez et al. percentage of these sites, which may also be beneficial for the SCR conversion. 5. Conclusions Both the reduction at 350 °C and reaction under wet conditions lead to the formation of small Pt° crystals, which increases the reaction rate of SCR of NO with methane. The Pt° crystals formed during reduction at 350 °C are stable under wet reaction conditions, but not under dry reaction conditions, as seen by TPR and EXAFS. The reduction at 500 °C further increases the reaction rate, due to the formation of new (InO)+ active sites, as seen in TDPAC results. During the reduction treatment a time dependent interaction is observed by TDPAC related with the existence of a relaxation process at the probe site. Neither EXAFS results, nor TDPAC characterization, show the presence of intermetallic Pt-In species both in calcined and reduced catalysts. The presence of exchanged In favors the Pt dispersion, as seen in chemisorption results. Acknowledgment. This research was partially supported by LNLS-National Synchrotron Light Laboratory, Campinas, Brazil, under Project XAFS 350/98. We are also grateful to Agencia de Promocio´n a la Investigacio´n (Proj. 14-6971) and UNL (CAI+D ‘96 Program) and CONICET, Argentina, through grant PEI-0132/98. Thanks are finally given to Helio Tolentino and Maria do Carmo Martins Alves (LNLS) for valuable discussion, to Ing. Jorge Runco for his technical assistance in the experimental TDPAC setup, and to Elsa Grimaldi for the edition of the English manuscript. References and Notes (1) Li, Y.; Armor, J. N. Appl. Catal. B 1992, 1, 131. (2) Armor, J. N. Catal. Today 1995, 26, 147. (3) Li, Y.; Armor, J. N. J. Catal. 1994, 150, 376. (4) Hamada, H. Catal. SurVeys Jpn. 1997, 1, 53. (5) Kikuchi, E.; Ogura, M. Catal. SurVeys Jpn. 1997, 227. (6) Ogura, M.; Hayashi, M.; Kikuchi, E. Catal. Today 1998, 42, 159. (7) Kikuchi, E.; Ogura, M.; Terasaki, I.; Goto, Y. J. Catal. 1996, 161, 465. (8) Cowan, A.; Du¨mpelman, R.; Cant, N. Stud. In Surf. Sci. Catal. 1996, 101, 671. (9) Ogura, M.; Kikuchi, E. Stud. Surf. Sci. Catal. 1996, 101, 671. (10) Ramallo-Lo´pez, J. M.; Requejo, F. G.; Renterı´a, M.; Bibiloni, A. G.; Miro´, E. E. Hyp. Int. 1999, 120/121, 529. (11) Requejo, F. G.; Ramallo-Lo´pez, J. M.; Lede, E.; Miro´, E. E.; Pierella, L.; Anunziata, O. Catal. Today 1999, 54, 553. (12) Miro´, E. E.; Gutierrez, L.; Ramallo Lo´pez, J. M.; Requejo, F. G. J. Catal. 1999, 188 (2), 375. (13) Gutierrez, L.; Miro´, E. E.; Petunchi, J. O. Stud. Surf. Sci. Catal. 2000, 130D, 1535. (14) Ramallo Lo´pez, J. M.; Requejo, F. G.; Gutierrez, L.; Miro´, E. E. Appl. Catal. B 2001, 29, 35. (15) Lytle, F. W.; Wei, P. S. P.; Greegor, R. B.; Via, G. H.; Sinfelt, J. H. J. Chem. Phys. 1979, 70, 4849. (16) Rehr, J. J.; Ankudinov, A.; Zabinsky, S. I. Catal. Today 1998, 39, 263. (17) Requejo, F. G.; Bibiloni, A. G. Phys Stat. Sol. (a) 1995, 148, 497. (18) Requejo, F. G.; Bibiloni, A. G.; Langmuir 1996, 12, 2 (1), 51. (19) Frauenfelder, H.; Steffen, R. M. In Siegbahn, K., Ed. Alpha-Beta and Gamma-Ray Spectroscopy; North-Holland: Amsterdam, 1968; Vol. 2, p 997. (20) Vogdt, C.; Butz, T.; Lerf, A.; Kno¨zinger, H. J. Catal. 1989, 116, 31. (21) Vasallo, J.; Miro´, E. E.; Petunchi, J. O. Appl. Catal. B 1995, 7, 65. (22) Cook, Jr, J. W.; Sayers, D. E. J. Appl. Phys 1991, 52, 5024. (23) van Zon, J. B. A. D.; Koningsberger, D. C.; van’t Blik, H. F. J.; Sayers, D. E. J. Chem Phys. 1985, 12, 5742.

Effect of PtIn(ferrierite) Reduction (24) Mendoza-Ze´lis, L. A.; Bibiloni, A. G.; Caracoche, M. C.; Lo´pezGarcı´a, A.; Martı´nez, J. A.; Mercader, R. C.; Pasquevich, A. F. Hyp. Int. 1977, 3, 315. (25) Bibiloni, A. G.; Massolo, C. P.; Desimoni, J.; Mendoza Zelis, L. A.; Sa´nchez; F. H.; Pasquevich; A. F.; Damonte, L.; Lo´pez Garcı´a, A. R. Phys. ReV. B 1985, 32, 2393. (26) Habenicht, S.; Lupascu, D.; Uhrmacher, M.; Ziegeler, L.; Lieb, K. P.; SOLDE Collaboration. Z. Phys. B 1996, 101, 187. (27) Tzou, M. S.; Teo, B. K.; Sachtler, W. M. H. J. Catal. 1989, 116, 350. (28) Me´riaudeau, P.; Naccache, C.; Thangaraj, A.; Bianchi, C.; Carliand, R.; Narayanan, S. J. Catal. 1995, 152, 313. (29) Tzou, M. S.; Teo, B. K.; Sachtler, W. M. H. React. Kinet. Catal. Lett. 1987, 35, (1-2), 207. (30) Koningsberger, D. C., Prins, R., Eds. X-ray Absorption Techniques of EXAFS, SEXAFS and XANES; Wiley: New York, 1988. (31) Wyckoff, R. Crystals Structures, 2nd ed.; Interscience: New York, 1965; Vols. 1 and 2. (32) Bibiloni, A. G.; Desimoni, J.; Massolo, C. P.; Mendoza-Ze´lis, A. F.; Pasaquevich, F. H.; Sa´nchez, L.; Lo´pez-Garcı´a, A. Phys. ReV. B 1984, 29, 1109. (33) Evenson, W. E.; McKale, A. G.; Su, H. T.; Gardner, J. A. Hyp. Int. 1990, 61, 1379.

J. Phys. Chem. B, Vol. 105, No. 39, 2001 9523 (34) Shishido, T.; Tanaka, T.; Hattoru, H. J. Catal. 1997, 172, 24. (35) Greegor, R. B.; Lytle, F. W. J. Catal. 1980, 63, 476. (36) Boyanov, B. I.; Morrison, T. I. J. Phys. Chem. 1996, 100, 16310. (37) Yentekakis, I. V.; Lambert, R. M.; Konsolakis, M.; Kiousis, V. M. Appl. Catal. B. 1998, 18, 293. (38) Hill, J. M.; Cortright, R. D.; Dumesic, J. A. Appl. Catal. A 1998, 168, 9. (39) Cortright, R. D.; Hill, J. M.; Dumesic, J. A. Catal. Today 2000, 55, 213. (40) Ogura, M.; Ohsaki; Kikuchi, E. Micropor. Mater. 1998, 21, 533. (41) Beyer, H. K.; Miha´lyi, R. M.; Minchev, Ch.; Neinska, Y.; Kanazirev, V. Micropor. Mater. 1996, 7, 333. (42) Massolo, C. P.; Desimoni, J.; Bibiloni, A. G.; Mendoza-Ze´lis, L. A.; Sa´nchez, F. H.; Pasquevich, A. F.; Lo´pez-Garcı´a, A. R. Hyp. Int. 1986, 30, 1. (43) Hammer, B.; Nørskov, K. K. Nature 1995, 376, 238. (44) Gardner, J. A.; Jaeger, H.; Su, H. T.; Haygarth, J. C. Physica B 1988, 150, 223. (45) Su, H. T.; Wang, R.; Fuchs, H.; Gardner, J. A.; Evenson, W. E.; Sommers, J. A. J. Am. Ceram. Soc. 1990, 73, 3215. (46) Wang, R.; Gardner, J. A.; Evenson, W. E.; Sommers, J. A. Phys. ReV. B 1993, 47, 638.