Elucidation of Pt Clusters in the Micropores of Zeolite Nanoparticles

Nov 16, 2010 - (38) The radiolitic products are reported in eq 1, and numbers in ..... of the films along the support are clearly seen from the SEM pi...
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J. Phys. Chem. C 2010, 114, 20974–20982

Elucidation of Pt Clusters in the Micropores of Zeolite Nanoparticles Assembled in Thin Films Ivan Yordanov,† Raphael Knoerr,‡ Vincent De Waele,‡ Mehran Mostafavi,‡ Philippe Bazin,† Se´bastien Thomas,† Micke¨l Rivallan,† Louwanda Lakiss,† Till H. Metzger,§ and Svetlana Mintova*,† Laboratoire Catalyse & Spectrochimie, ENSICAEN - UniVersite´ de Caen - CNRS, 6, BouleVard du Mare´chal Juin, 14050 Caen, France, Laboratoire de Chimie Physique, UMR-8000, CNRS - UPS Bat. 349, 91405 Orsay, France, and European Synchrotron Radiation Facility, ESRF, BP 220, 38043 Grenoble, France ReceiVed: June 15, 2010; ReVised Manuscript ReceiVed: October 22, 2010

Platinum clusters are prepared by γ radiolysis of beta zeolite suspensions (Pt-Beta) in the presence of a templating agent (tetraethylammonium hydroxide, TEAOH). Further, the Pt-Beta suspensions were stabilized with methyl cellulose and deposited in thin films with a thickness of 220 nm on silicon wafers. Elucidation of the size, distribution, and stability of the Pt clusters in the colloidal suspensions and thin films is provided by high-resolution transmission electron microscopy (HRTEM) combined with grazing incident X-ray diffraction (GI-XRD) measurements. The lateral length of the Pt clusters immobilized in the channels of the beta nanocrystals is between 1 and 2 nm. The presence of crystalline fringes with spacings of 0.23 and 1.26 nm corresponding to cubic Pt and zeolite beta are clearly seen in the HRTEM. The homogeneous distribution of the clusters along the film thickness is confirmed by GI-XRD measurements at two penetration depths. Besides, the location of the Pt clusters in the channels of beta nanoparticles is studied by FTIR spectroscopy. The Pt clusters confined in the channels of beta crystals decrease the total micropore volume and also lower the water sorption capacity in comparison with pure beta zeolite. Introduction The morphology- and size-controlled preparation of nanostructures is a great challenge in materials chemistry because their dimensionally, shape, and stability are effectively tuned by their intrinsic chemical and physical properties.1-3 In this respect, the preparation of nanosized porous materials with small metal clusters stabilized within their channels combined with characterization of the size-dependent electronic, adsorptive, and catalytic properties is still of continuous interest and a great challenge. The standard chemical reduction of metal cations (M+) to metal (Me0) clusters often results in a rather broad size distribution even in the case of using micro- and mesoporous hosting materials for preparation of confined metal clusters.4-7 However, metal clusters with a size close to 1 nm in diameter consisting of only 10 atoms or fewer have been designed by applying the confinement synthesis approach.8-10 The porous materials are confining the extremely small and uniform metal clusters, thus providing the possibility for high dispersion and restriction from agglomeration and leaching.11,12 Once the small metal precursors are captured in the channel system of molecular sieves, the subsequent treatment leads to the formation of larger metal clusters if the conditions for preparation are not optimized or even to demolition of the crystalline porous materials. Complications may arise as smaller species are transported out of the pores onto the exterior surfaces or as the encaged structures grow too large to fit and cause the porous crystallites to burst. * To whom correspondence should be addressed. Fax: + 33 2 31 45 28 22. E-mail: [email protected]. † ENSICAEN - Universite´ de Caen - CNRS. ‡ Laboratoire de Chimie Physique, UMR-8000, CNRS - UPS Bat. 349. § European Synchrotron Radiation Facility, ESRF.

The preparation of ligand-stabilized metal clusters has been reported, and very often they are agglomerating and sintering during removal of the ligands by subsequent treatment.13,14 The mean diameters of single or mixed clusters stabilized by various ligands are ranging from 1.3 to 4 nm depending on the preparation techniques, whereas the clusters stabilized by polymers have larger sizes, suggesting the formation of aggregates.15 Among various techniques, the radiolytic reduction of aqueous metal ions is one of the simple, clean, and well-controlled methods for production of homogeneous small metal particles in either colloidal dispersions or powders.16-23 A γ radiolysis process was applied to reduce metal ions in hydrated and dehydrated micro- and mesoporous materials stabilized as powders and suspensions.24,25 In this case, the microporous materials (zeolites) are considered as nanoreactors for trapping clusters under dynamic radiolytic treatment conditions. The interest in fabrication of molecular sieves with metal clusters is due to their applications not only as selective catalysts but also in the production of H2 from CH4, reduction of exhaust gases from automobiles, production of methanol fuel cells,26,27 solar-water devices, 28,29 photochemistry, optoelectronics, and chemical sensors.30,31 The high cost and limited supply of the noble metals in these applications remain a challenge that demands its efficient commercial usage. The manipulation of the size and shape of the noble materials at the nanoscale can contribute to low metal usage and achieve the necessary performance and cost reduction, eventually. In addition, the continuous interests in the characterization of the physical and physicochemical properties of nanometer-sized metal clusters with regards to particle size effects in catalysis and electronic devices have stimulated the efforts to prepare them with a uniform size and homogeneous dispersion. The most applied

10.1021/jp105490g  2010 American Chemical Society Published on Web 11/16/2010

Pt Clusters in Zeolite NPs in Thin Films

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TABLE 1: Chemical Composition and Solid Concentration of Pt-Beta Crystals in the Colloidal Suspension

sample

Si

Al

Pt

Si/Al

Pt/Al

zeolite concentration in suspension (g/L)

Pt-Beta

29.31

2.43

2.32

11.59

0.13

6.5

zeolite (wt %)

elemental molar ratios

methods for characterization of metal clusters are UV-vis and FTIR spectroscopies.32,33 CO adsorption measurements on Mecontaining porous material powders revealed the formation of various carbonyls involving metal clusters in different oxidation states. On the other hand, obtaining direct and clear evidence for the presence of subnanometer clusters in a microporous matrix is a very challenging task, and one faces additional difficulties in the manipulation of these materials assembled in thin films. In the present work, direct indications for the preparation of Pt clusters within the micropores of zeolite beta nanocrystals stabilized in colloidal suspensions and thin films are presented. The Pt clusters in beta suspensions and films are studied by GI-XRD, UV-vis, and HRTEM combined with H2O, N2, and CO adsorption measurements. Experimental Section Preparation of Zeolite Beta Nanoparticles Loaded with Platinum (Pt-Beta). Nanosized beta crystals were synthesized from a clear precursor suspension having the following chemical composition: 15 TEAOH:1 Al2O3:25 SiO2:375 H2O. The initial compounds, Cab-o-sil M5 (Riedel-DeHa¨en) as a silica source, aluminum sec-butoxide (Aldrich) as an aluminum source, tetraethylammonium hydroxide (TEAOH, 35 wt %, Aldrich) as an organic template, and deionized water were mixed in one pot (total volume of 500 mL) and agitated until the clear suspension was obtained. The as-prepared precursor suspension was aged at room temperature for 3 days prior to further hydrothermal treatment at 100 °C for 3 days. The crystalline zeolite beta nanoparticles were purified by two-step centrifugation (24 500 rpm, 4 h), and after each step, the nanoparticles were redispersed in distilled water. The crystalline nanosized beta sample with Si/Al ) 14 was loaded with Pt by an ion-exchange treatment using [Pt (NH3)4](NO3)2 as a platinum source. The ion exchange was carried out in one step (60 °C, 24 h), and then the Pt-Beta nanoparticles were washed three times in order to remove the excess Pt. The chemical composition of the beta sample determined by X-ray fluorescence analysis is given in Table 1. γ Irradiation of the Pt-Beta Suspension. The radiolysis of the colloidal suspension containing Pt ion-exchanged beta nanoparticles was performed using the γ-ray emitted from a 60 Co panoramic source. The dose rate was 2000 Gy/h as determined by Fricke dosimeter. Isopropanol (0.2 M) was added to the suspensions in order to convert the oxidizing HO• radicals and the H• atoms generated during the radiolysis of water into reducing alcohol radicals. The suspensions were saturated with N2 and kept under this controlled atmosphere during the irradiation and optical UV-vis measurements. The sample (PtBeta) was irradiated stepwise up to 10 kGy, and the UV-vis spectra were recorded for each irradiation dose (Table 1). Two samples denoted as Pt-Beta-1 and Pt-Beta-2 were irradiated up to partial (30%) and complete (100%) reduction of the Pt2+ cations, respectively. These samples (suspensions) were assembled in thin films by a spin-on process. Assembling of Pt-Beta Nanoparticles in Thin Films. Silicon (100) wafers (2 × 2 cm) were precleaned with ethanol and

concentration of Pt2+ in suspension (mol/L) 7.6 × 10-4

acetone for 30 s under spinning at 3000 rpm using a Laurell WS-400B-6NPP-Lite-AS spin-coater.34,35 The Pt-Beta suspensions with a solid concentration of 1 wt % were either used directly or mixed with binder prior to film deposition. As a binder, methyl cellulose was mixed with the zeolite suspension in the ratio of 1:10. To ensure the preparation of smooth and homogeneous films, the coating suspensions were filtrated through syringe filters (450 nm) prior to spinning. To facilitate the preparation of thick beta films, the spin-coating procedure (acc, 5000 rpm s-1; 2000-5000 rpm for 60 s) was repeated up to 10 times. After each coating step, the films were dried at 60 °C in air for 60 min. From the numerous films with variable thicknesses, only the films prepared from suspensions Pt-Beta-1 and Pt-Beta-2 at a spinning rate of 4000 rpm for 60 s (1 layer), 1600 rpm for 30 s (3 layers), and at 3600 rpm for 60 s (2 layers) will be discussed in detail. Both samples Pt-Beta-1 and Pt-Beta-2 have a thickness of about 220 nm. A scheme representing the entire experimental procedure for preparation of nanosized Pt-Beta crystals and thin films is given in the Supporting Information (see Figure S1). Characterization of Pt-Beta Suspensions, Powders, and Films. X-ray diffraction (XRD) patterns of powder samples were recorded using a STOE STADI-P diffractometer in DebyeScherrer geometry (Cu KR, λ ) 0.15406 nm), with a 0.028 step size and a 1 s step time. The chemical composition of PtBeta was determined by X-ray fluorescence (XRF) analysis with a MagiX PHILIPS PW2540. The size of the crystals and the thickness of the films were determined by scanning electron microscopy (SEM) recorded on a Philips XL30 FEG. In addition, the size of the particles was measured with dynamic light scattering (DLS) in colloidal suspensions without dilution using a Malvern Zetasizer-Nano instrument equipped with a 4 mW He-Ne laser (633 nm) providing scattering information at 173°. High-resolution transmission electron micrographs (HRTEM) were taken from the suspensions deposited on Plano holey carbon-coated copper grids using an FEI 300 TEM microscope operated at 300 kV. The suspensions were deposited on the grids in a glovebox under a nitrogen atmosphere, followed by drying in vacuum for 2 h prior to the TEM study. The reduction of Pt2+ to Pt0 was followed by a UV-visible spectrophotometer using a Hewlett-Packard 8453A. The absorption spectra of the Pt-Beta suspension were recorded as a function of the irradiation dose. The structural features of the films deposited from Pt-Beta on silicon wafers were studied with grazing incidence X-ray diffraction (GI-XRD) using synchrotron X-ray radiation (8 keV, λ ≈ 0.155 nm) at beamline ID01 at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The GI-XRD measurements were carried out at two incident angles, Ri ) 0.05 and 0.1°. The scattering geometry for the grazing incidence diffraction is shown elsewhere.36,37 The size of Pt clusters and beta crystals was estimated using the Scherrer equation. The diffraction peaks at 40, 46.5, and 67.85° 2θ corresponding to cubic Pt and the peak at 7.1° 2θ corresponding to zeolite beta

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were used for size determination of Pt and zeolite crystals, respectively. The thickness and the refractive index (n) of the films were determined by a Rudolph SE800 ellipsometer (all measurements were performed at 65, 70, and 75° in the spectral range of 200-1000 nm). The values of the refractive index of the films were taken at a wavelength of 500 nm. Sorption of N2, H2O, and CO of Pt-Beta Samples. To elucidate the degree of filling of beta micropores with Pt clusters and to determine the change in their porosity under postsynthesis treatments, nitrogen (N2) sorption measurements were performed on the samples with a Micromeritics ASAP 2420 analyzer (the samples were heated at 500 °C and outgassed at 350 °C). Additionally, water sorption isotherms of the same samples were measured at 22 °C using a CiSorp water sorption analyzer (CI Electronics, U.K.). An equilibrium weight of the samples, at each relative humidity (RH) in the range of 5-85%, of (0.001% per min was used. The water sorption isotherm of the Pt-Beta film was recorded as well. CO adsorption on Pt-Beta powder samples was followed with in situ FTIR reordered on a Nicolet Magna IR 560 spectrometer (optical resolution of 4 cm-1 and accumulating 64 scans). The freeze-dried Pt-Beta were calcined at 550 °C for 6 h prior CO adsorption. Self-supported pellets (ca. 10 mg cm-2) were prepared from the samples and treated directly in the IR cell. To remove the adsorbed water, the samples were heated under vacuum at 100 °C for 3 h. The adsorption of CO (99.997%, Air Liquide) was performed at ambient temperature. Doses of CO (2.3 × 10-4 mmol · g-1) were sent on the activated samples, and the IR spectra were collected. The change of the νCO IR band corresponding to the CO chemisorbed on platinum was followed. It is important to note that no residual gas phase in the IR cell before sample saturation and no bridged CO or carbonates were detected. Results and Discussion γ Irradiation of Pt-Beta Suspensions. The chemical effects of an ionizing radiation are fully quantified in terms of dose and radiolytic yield. The former refers to the amount of energy deposited in the sample by the radiation (in Gy, or J kg-1), whereas the later quantifies the concentration of species formed per Gy deposited in the sample. The radiolytic yield is expressed in terms of g-values, given in mol Gy-1. The irradiation of a sample placed in the field of a panoramic γ source results in a highly uniform dose deposition, which is mainly proportional to the electronic density of the irradiated media.38 In principle, the quantitative description of the dose deposition in a solid-liquid system is very complex. However, in our case, the Pt-Beta suspension contains 99.9 wt % water and zeolite particles with a size below 100 nm. Under γ-rays, the zeolite suspension behaves as a highly diluted aqueous suspension, and therefore, no effect from the direct interactions between the γ-ray and the zeolite beta particles is expected. As a consequence, the radiolytic species involved in the experiments resulted only from the radiolysis of the water molecules. The γ-rays decompose the water molecules into radicals and molecular species. The radiolytic products delivered from the decomposition of water and their respective g-value have been established by numerous scavenging and pulse-radiolysis studies.38 The radiolitic products are reported in eq 1, and numbers in brackets refer to the radiolytic yield in µmol Gy-1. Additionally, the 0.2 M (CH3)2CHOH added in the Pt-Beta sample scavenged the oxidizing HO• radicals and the H• species to form the reducing R-methyl-hydroethyl radicals (R-H2C-HCOH-CH3):

Yordanov et al.

water f γ ray esolv (0.28), H• (0.06), HO• (0.28), H2O2 (0.07), H3O+ (0.28) (1) H2C-HCOH-CH3 + HO• (H•) f H2C-•COH-CH3 + H2O (2) According to eqs 1 and 2, the radiolysis of the Pt-Beta suspension leads to the formation of strong reducing agents, that is, the solvated electron [E°(H2O/esol ) ) -2.87 VNHE ] and the R-methyl-hydroethyl radical [E°((CH3)2CO/(CH3)2•COH) ) -1.8 VNHE] with well-determined radiolytic yields.38 From the known g-values reported in eq 1, the total yield of formation of reducing species in the Pt-Beta suspension is given by g(reducing species) ) g(esolv ) + g((CH3)2•COH) ) 0.62 µmol/Gy

Hereafter, the R-methyl-hydroethyl radical will be denoted as •ROH. Further, the platinum clusters will be formed by reduction of the Pt2+ cations into zero valence platinum Pt0 and further coalescence in the zeolite nanocrystals. This mechanism has been investigated in solution by pulse radiolysis for different platinum complexes.39 The formation of platinum metals is initiated by reduction of divalent cations (Pt2+) according to eq 3: (•ROH) f Pt+ Pt2+ + esolv

(3)

In solutions, the PtI is observed as a transient species with an absorption band at 310 nm.39 The PtI species disappeared mainly by disproportionation and yields equal amounts of Pt0 and PtII (reaction 4). Besides, the direct formation of Pt0 according to eq 5 is also possible, notably in the zeolite due to the hindered mobility of the cations owing to the vicinity of the zeolite surface and metal particles:

Pt+ + Pt+ f Pt0 + Pt2+

(4)

Pt+ + esolv (•ROH) f Pt0

(5)

According to the reaction scheme described with eqs 3-5, the radiolytic yield of formation of metal platinum is

g(Pt0) ) 0.5/g(reducing species) ) 0.31 µmol Gy-1 In fact, specific reactions may increase or diminish this theoretical value due to the state of complexation of the Pt cations or to the strong oxidant character of PtI.39 These particular aspects of the chemistry of platinum complexes have not been discussed for systems containing zeolites up to now. However, the formation of metal platinum in the zeolite colloidal suspension can be quantitatively followed by the change of the UV-vis spectra of samples in the course of γ irradiation and thus could provide information about the reaction scheme and efficiency of radiolysis. In Figure 1, the absorbance of the Pt-Beta suspension at different irradiation doses ranging from 0 to 10 000 Gy is shown. Before irradiation (0 Gy), the spectrum is dominated by the

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Figure 1. Absorption spectra of the Pt-Beta suspension irradiated with different doses from 0 to 10 000 Gy. Inset, left: spectrum before irradiation is subtracted from all spectra. Inset, right: change of absorbance versus irradiation dose.

Figure 2. Particle size distribution of pure beta, ion-exchanged beta, Pt-Beta-1, and Pt-Beta-2 suspensions determined by a back-scattered DLS technique.

light-scattering signature from the colloidal zeolite particles and a small contribution due to the absorption of the (Pt(NH3)42+) complex.40 To visualize better the changes of the absorbance upon irradiation of the Pt-Beta suspension, the spectrum before irradiation was subtracted from each spectrum and shown as the inset in Figure 1, left. Considering that the zeolite lightscattering is constant during the irradiation, and the absorption coefficient from the platinum complex (ε) is about 100 L mole-1 cm-1, thus, above 250 nm, it is at least 10 times less than the extinction coefficient (ε) from platinum atoms in clusters is in the range of 2000-5000 L mole-1 cm-1.40,41 Therefore, the optical changes observed in Figure 1 are assigned to the formation of platinum clusters. Upon irradiation, a large band with a maximum at 249 nm appears (1 kGy), and this band increases strongly in intensity to reach a maximum at 6 kGy. For irradiation doses higher than 6 kGy, the absorbance of the samples does not change significantly; only a broadening of the absorption band in the red wing of the spectrum is observed. For example, at 10 kGy, the spectrum is not evolved anymore (see the inset in Figure 1, left). The change in the intensity of absorbance versus irradiation dose is plotted for λ ) 300 nm (see the inset in Figure 1, right). The absorbance increases linearly with the dose until it reaches a plateau. From the extrapolation of the linear part of the curve, the plateau is reached for a dose around 3 kGy (see Figure 1, right). The presence of the plateau indicates either the full reduction of all accessible Pt2+ cations or a stage of coalescence from which no significant modification of the size and shape of the metal particles with further irradiation is observed. The total amount of reducing species formed at an irradiation dose of 3 kGy, cred ) 1.8 mM, is comparable with the theoretical amount necessary to reduce the incorporated Pt2+, that is, ctheo ) 2 × 0.76 ) 1.52 mM. However, on the basis of the extinction coefficient at 300 nm equal to 3000 L mol-1 cm-1,41 the amount of reduced platinum atoms is estimated to be 0.26 × 10-4 mole L-1, OD300 ) 0.8. This estimation leads to the conclusion that about 1/3 of the initial Pt2+ cations have been reduce to Pt0. It is worth noting that the amount of reduced Pt2+ cations is fully controlled by the irradiation dose, as can be seen from the data presented in Figure 1. It is also important to outline that no change of the Pt-Beta sample during aging after each irradiation step is observed as was reported earlier by A. Henglein et al.41 In this paper, an autocatalytic reaction yielding the formation of a

higher Pt0 amount than that expected from stoichiometry is reported. Such an autocatalytic process is not observed in the case of the Pt-Beta sample, and absorbance is increased linearly with the dose. All these profound differences with the reactivity observed during the radiolytic reduction of platinum in aqueous solutions confirmed that the clusters are firmly confined into the zeolite pores. Additionally, it is noted that the position of the absorption band of the platinum cluster in the Pt-Beta suspension is strongly red shifted compared with the value reported for small nanometric-sized Pt clusters; the band is usually observed around 215 nm. This could be assigned to the surrounding effect of zeolite particles in which the Pt clusters are stabilized or to the small size and nonspherical shape of the formed Pt particles. Characterization of Irradiated Pt-Beta Suspensions. The stability of zeolite nanoparticles in the suspensions before and after ion exchange and γ irradiation was followed with DLS. The particle size distribution in all suspensions did not change significantly for 3 months at ambient conditions. The DLS curves for suspensions Pt-Beta-1 (30% Pt reduced) and Pt-Beta-2 (100% Pt reduced) prior film deposition in comparison to asprepared and after ion-exchanged beta suspensions are shown in Figure 2. The DLS curves reveal a monomodal particle size distribution in the suspensions before and after γ radiolysis. On the basis of the cumulant analysis, the average hydrodynamic diameter of crystalline particles in the beta suspension is 30 nm. The polydispersity index of about 0.01 shows that the samples display a very narrow particle size distribution. The DLS curves of the ion-exchanged β- and γ-irradiated samples (Pt-Beta-1 and Pt-Beta-2) are similar. The maximum of the DLS peak is shifted to the higher number, but the width of the curves for the three suspensions is the same. The difference in the particle distribution curves is explained with a partial agglomeration of the beta nanoparticles due to the several steps of postsynthesis treatment of the suspensions, including ion exchange, purification, and γ irradiation. Besides, the mean size of the crystalline beta particles assembled in the films is determined by GI-XRD based on the Scherrer equation (see the section below). The high crystallinity, morphological appearance, and size of the individual zeolite particles in samples Pt-Beta-1 and PtBeta-2 are confirmed by HRTEM. The TEM pictures from both samples Pt-Beta-1 and Pt-Beta-2 are depicted in Figure 3. As can be seen, the beta particles have a size of about 10-15 nm

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Figure 3. HRTEM pictures of (a) Pt-Beta-1 and (b) Pt-Beta-2 nanocrystals. M ) 10 nm. Samples (c) Pt-Beta-1 and (d) Pt-Beta-2 at high magnification: two types of fringes with a d-spacing of 1.26 and 0.23 nm corresponding to beta zeolite and Pt clusters, respectively, are seen. M ) 2 nm.

with sharp crystalline edges and all contain crystalline fringes. The nanosized crystals tend to agglomerate during the drying process, but no intergrown crystals or big aggregates are identified in the TEM pictures taken from both samples. The entire preservation of zeolite beta nanocrystals and lack of any other side phase different from zeolite beta is revealed as well (Figure 3a,b). The absence of individual Pt clusters at the periphery of the beta zeolite crystallites is confirmed by the numerous TEM pictures taken from the Pt-Beta-1 and Pt-Beta-2 samples. It is important to note that the beta zeolite nanoparticles were stable under the beam and did not amorphize immediately, as it is expected for nanosized zeolites. Both zeolite nanoparticles (size of 10-15 nm) and Pt clusters (1-2 nm) have been preserved completely during the TEM study. The most intriguing information for the presence of Pt clusters within the pores of the beta particles is provided from the images taken at high magnification (Figure 3c,d). The presence of two types of crystalline fringes (0.23 nm) and (1.26 nm) that correspond to cubic Pt clusters and zeolite beta are clearly distinguished in the two samples. The Pt clusters with a (220) plane are aligned in the channels of beta zeolite running along (100), which corresponds to 12-membered rings with a size of 6.6 × 7.5 Å. The presence of Pt clusters with a size (length) from 1 to 2 nm located in the channels of beta nanocrystals is noticeable. Such an observation is in a good agreement with the GI-XRD results and optical absorption data recorded from the same material (see the sections below). Besides, the energydispersive X-ray microanalysis (EDX) was used to confirm the presence of Pt in zeolite crystals. The expected three Pt peaks at 2.1 keV (MR, Mβ), 9.4 keV (LR), and 11.2 keV (Lβ) are present in the Pt-Beta-1 and Pt-Beta-2 samples (Figure 4). The existence of Pt is also confirmed by X-ray fluorescence (XRF) analyses carried out on Pt-Beta-1 and Pt-Beta-2 samples. The concentration of Pt in both samples is about 0.75%. Having in mind that the pore opening of zeolite beta is 0.77 × 0.67 nm, a partial replacement of the template (TEA+ with a size of 0.54 × 0.53 nm) with the platinum (tetraammineplatinum with a size of 0.44 × 0.45 nm) is achieved via a one-step ion-exchange procedure. The size of the beta nanoparticles measured by DLS was slightly bigger than that determined with HRTEM. This could

Figure 4. EDX-TEM data of Pt-Beta-1 and Pt-Beta-2 samples.

be explained by the surface structure, concentration, and ions present in the dispersion medium (either water or ethanol was used as the solvent). It is important to note that back-scattered DLS measurements were carried out in the zeolite suspensions without dilution. In conclusion, the combination of these two characterization methods (HRTEM and DLS) firmly confirms that the diameter

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Figure 6. Water vapor adsorption isotherms of pure beta and Pt-Beta-2 at 22 °C. Inset: water vapor adsorption isotherm of the Pt-Beta-2 film.

Figure 5. N2 sorption isotherms for (a) pure beta, (b) Pt-Beta-1, and (c) Pt-Beta-2 samples: (A) entire (0-1 p/p0) and (B) micropore (0-0.5 p/p0) regions.

of beta crystals is between 10 and 15 nm. It is important to note that a second subnanometer generation of particles has not been distinguished in both suspensions, thus suggesting the absence of isolated Pt clusters outside of the zeolite matrix. Sorption of N2, H2O, and CO on Irradiated Pt-Beta Powders. The degree of loading of beta with Pt clusters is evaluated from the N2 sorption isotherms recorded on samples Pt-Beta-1, Pt-Beta-2, and pure Beta. All samples exhibit type-I isotherms (Figure 5). The most important observation is the decrease of the micropore volume from 0.20 to 0.11 and 0.10 cm3/g for pure beta, Pt-Beta-1, and Pt-Beta-2 samples, respectively. This can be explained by a partial filling of the zeolite micropores with the Pt clusters. However, this is a speculative statement, and further characterization of the samples via sorption of probe molecules will be carried out to analyze the degree of blocking of the zeolite micropores with the platinum clusters. The presence of Pt within the pores of the zeolite beta nanoparticles is also confirmed by recording the water adsorption isotherms on pure beta and Pt-Beta-2 powder samples (Figure 6). As can be seen, no significant adsorption at low relative humidity characteristic of hydrophobic materials is measured. The pure beta sample adsorbed a considerably higher amount of water vapor at all relative humidities, whereas the decreased amount of water vapor in Pt-Beta-2 can be explained with the partial filling of the zeolite channels with Pt clusters. No change in the surface selectivity for both samples is observed, which proves that Pt clusters are located in the channels, and not at the surface, of the zeolite particles. Besides, the aperture of the channels of zeolite beta could be blocked by the Pt clusters, which could cause the lower water sorption of the Pt-Beta

Figure 7. Evolution of IR spectra in the CO stretching region for (A) Pt-Beta-1 and (B) Pt-Beta-2 samples (outgassed at 100 °C in a vacuum down to 10-3 Pa). The amount of the introduced dose of CO is 2.3 × 10-4 mmol · g-1. (a) Spectrum recorded after the first CO dose. Spectrum (h) corresponds to 16.1 × 10-4 mmol · g-1 CO. All spectra are presented after subtraction of the corresponding spectrum before CO adsorption.

samples. The water adsorption isotherm of thin film prepared from Pt-Beta-2 suspension is presented as an inset in Figure 6. The very low mass change for the film is due to the small amount of Pt-Beta-2 material deposited in the film with a thickness of 217 nm. In-situ CO adsorption spectra on self-supported pellets of PtBeta-1 and Pt-Beta-2 were carried out, and the spectra are shown in Figure 7. In the case of CO adsorbed to metallic Pt species in linear and bridged arrangements, the bands in the regions of 2110-2104 and 1770 -1860 cm-1 are expected, respectively. In the Pt-Beta-1, the CO band centered at 2079 cm-1 (Figure 7A) does not shift significantly when increasing the amount of CO adsorbed on the sample. This band is attributed to the ν(CO)

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Figure 8. SEM pictures with a side view of (a) Pt-Beta-1 and (b) Pt-Beta-2 films and top view of (c) the Pt-Beta-2 film.

vibration of carbon monoxide linearly bonded with Ptn clusters.42-44 The absence of any bands at a wavenumber higher than 2100 cm-1 indicates that the Pt is not in the cationic form, Ptδ+, and is completely reduced to Pt0. The IR spectrum of sample Pt-Beta-2 at a low CO dose contains a weak band at 2074 cm-1 and a shoulder at 2052 cm-1 (Figure 7B), which also do not shift with the amount of adsorbed CO. The increase of the irradiation dose shifts the maximum of the ν(CO) band about +6 cm-1. This can be related to a more intense back-donation, which is due to the presence of larger clusters. The broad shoulder observed at 2052 cm-1 for the PtBeta-2 sample is referred to the presence of less coordinated Pt atoms at the extremities of some clusters. Characterization of Pt-Beta Films. The top and side views of the Pt-Beta-1 and Pt-Beta-2 films deposited on silicon wafers are shown in Figure 8. As can be seen, the silicon wafers are entirely covered with homogeneous films with a thickness of about 220 nm (see Figure 8a,b) deposited by several deposition

Yordanov et al. steps. The size of the individual crystals is very difficult to estimate from the top and side views of the SEM pictures. However, the high density and the homogeneity of the films along the support are clearly seen from the SEM pictures. To evaluate the degree of filling of the micropores of zeolite beta with Pt clusters, ellipsometric measurements were carried out on pure Beta and Pt-Beta-1 and Pt-Beta-2 films. For the ellipsometric measurements, a theoretical model was developed based on the assumption that the film consists of nanoparticles and a certain fraction of voids are present as well. The refractive indexes (n) of the three films are measured in the spectral range of 200-1000 nm, while the n-values at a wavelength of 500 nm are used for comparison. The refractive index of the films is increased from 1.14 to 1.20, which is expected for porous beta-type material. In all cases, methyl cellulose was used as a binder and burned prior to the ellipsometric measurements. This result reveals that the porosity of the materials is decreased by incorporation of Pt in the channels; and therefore, the refractive index is increased. The structure and the presence of both crystalline zeolite particles and Pt clusters in the films are studied by GI-XRD. The GI-XRD setup used for the measurements of the zeolite films is schematically presented in Figure 9a. To prove the presence of Pt clusters in the films, the radial 2Θ scans of the Pt-Beta-1 and Pt-Beta-2 samples at higher angle are recorded (Figure 9b,c). The main Bragg peaks corresponding to zeolite beta are measured at two incident angles (0.05° and 0.1°), which correspond to penetration depths of 8 and 45 nm, respectively. The most intense peak of zeolite beta at 7.3° 2Θ with an (hkl) value of (101) is shown as an inset in Figure 9c. The incident angles were kept constant (either 0.05° or 0.1°), and the scattered intensity was integrated over all channels of the positionsensitive detector (PSD). As can be seen, most of the Bragg peaks belong to zeolite beta, which is clear proof that the crystalline structure is completely preserved after the γ irradiation of the Pt-Beta suspensions. The diameter (d) of the beta crystallites is determined by the Scherrer equation using the (101) peak. The size of beta crystallites is about 10 nm, which is in a good agreement with the DLS data (Figure 2) and the TEM pictures (Figure 3). In addition to the peaks of the beta crystalline phase, three main peaks corresponding to cubic platinum with (hkl) values of (111), (200), and (220) are observed in the films at both penetration depths (Figure 9). The size of the Pt clusters and the distribution along the film thickness are evaluated using the three reflections for Pt0 (Table 2). The size of the Pt clusters deep inside the films is equal for both samples, that is, about 1 nm. However, a negligible difference in the size of the clusters located very close to the film surface is measured. The Pt clusters in the Pt-Beta-1 and Pt-Beta-2 films are 1.7 and 1.2 nm, respectively, which agrees well with the observation made by FTIR spectroscopy (Table 2). These data permit assuming that, with increasing the irradiation dose, the size of the platinum clusters is slightly increased especially at the surface of the films. This can be explained with an agglomeration of clusters that can be leached from the pores of the crystalline beta particles. It is important to note that no metal oxides are detected in the samples with GI-XRD. One of the interesting features of zeolite films is the crystal orientation throughout their thickness. The depth-depend scans on the specific setting of the incident and exit angles are recorded, but no differences in the orientation of the particles along the films is detected. This result is expected for films deposited by a spin-coating technique using suspensions with

Pt Clusters in Zeolite NPs in Thin Films

J. Phys. Chem. C, Vol. 114, No. 49, 2010 20981 zeolite nanoparticles and Pt clusters are completely preserved. The use of nanosized zeolite crystals stabilized in water suspensions led to a fairly homogeneous distribution of reduced platinum species over zeolite particles. The pure beta and Ptbeta suspensions were combined with methyl cellulose binder in order to prepare stable thin films by a spin-on process. The pure beta films display a constant thickness and smooth surfaces and show a low refractive index, which is typical for highly porous silica-based materials. The Pt-Beta films have, on the contrary, a higher refractive index, which proves the location of the clusters within the channels of zeolite particles. These data are supported by the decrease of the micropore volume of the Pt-Beta samples in comparison with pure beta measured by nitrogen and water sorption. Besides, the IR of Pt-Beta samples reveal the vibrations of carbon monoxide linearly bonded with platinum metallic clusters. Direct evidence for the presence of Pt clusters in the channels of zeolite beta assembled in films is provided by GI-XRD experiments. It is demonstrated that the Pt clusters have the same size and are homogeneously distributed along the film thickness. The data are consistent with the HRTEM study, confirming the presence of two types of crystalline fringes (0.23 nm) and (1.26 nm) that correspond to cubic Pt clusters with a (220) plane aligned in the channels of beta zeolite crystals (101). This method is promising for the preparation of metalcontaining zeolites with a homogeneous distribution of the clusters within zeolite nanoparticles and thin films. Acknowledgment. The authors gratefully acknowledge funding from the SRIF PNANO ANR Project. Supporting Information Available: A schematic representation of the entire experimental procedure. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 9. (a) Principal scheme of the GI-XRD used for characterization of zeolite films (ESRF-Grenoble, beamline 01) and GI-XRD data of (b) Pt-Beta-1 and (c) Pt-Beta-2 films in the range of 27-77° 2Θ at incident angles of 0.05° and 0.1°. Inset of (c): XRD pattern in the range of 5-10° 2Θ, containing the most intense peak of zeolite beta (7.3° 2Θ).

TABLE 2: Characteristics of Pt-Beta Films particle size (nm)b sample

thickness (nm)

Pt-Beta-1 Pt-Beta-2

223 217

a

a

Ri ) 0.05°

Ri ) 0.3°

1.7 1.2

1.0 1.0

Based on ellipsometry. b Based on GI-XRD measurements.

essentially spheroidal particles, resulting in the formation of disoriented films. The binder used to improve the adhesivity of the films introduced additional disordering, and therefore, no orientation has been recorded for the Pt-Beta films. Conclusions Stable colloidal suspensions of small zeolite beta nanocrystals (10-15 nm) were prepared and loaded with Pt ions by ion exchange. γ radiolytic reduction of Pt+ to Pt0 in zeolite suspensions has been performed in order to prepare stable platinum clusters under mild conditions where both crystalline

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