Physicochemical Characterization of Highly Dispersed Platinum and

Article pubs.acs.org/JPCC

Physicochemical Characterization of Highly Dispersed Platinum and Chromium on Zeolite Beta Yoshiyuki Izutsu,† Yuki Oku,† Yusuke Hidaka,† Naoki Kanaya,† Yoshiki Nakajima,† Jun Fukuroi,† Kaname Yoshida,‡ Yukichi Sasaki,‡ Yasushi Sekine,† and Masahiko Matsukata*,† †

Faculty of Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan Japan Fine Ceramics Center, 2-4-1, Mutsuno, Atsuta, Nagoya, Aichi 456-8587, Japan

‡

S Supporting Information *

ABSTRACT: Structures of platinum and chromium supported on zeolite beta were investigated by XAFS, XPS, UV−vis, NH3-TPD, XRD, CO chemisorption, and molecular dynamics simulation. Both platinum and chromium were uniformly dispersed in the micropore of zeolite beta. Loading of chromium helped platinum to disperse highly and stabilized in the micropore of beta. Major species of platinum on PtCr/beta after calcination at 773 K was Pt2+ forming a Pt−O bond. The Pt−O bond disappeared, and a Pt−Pt bond did not appear by reducing PtCr/beta in hydrogen, accompanying formation of Pt0. Chromium was loaded as chromate anion in the micropore of zeolite. Results of molecular dynamics simulation showed that Pt2+ associated with CrO42− in the micropore of zeolite beta was more stable than those in the absence of chromium species. We concluded that CrO42− electrostatically stabilizes Pt2+ and inhibits migration and aggregation of platinum.

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INTRODUCTION Zeolite catalysts modified with noble metals are active for skeletal isomerization of alkanes. Alkanes having a carbon number of seven or more are readily cracked via the beta scission mechanism.1,2 Therefore, development of catalysts for alkane isomerization has been focused on products selectivities.3−9 We recently reported that PtCr/beta catalyst gave branched alkanes at higher selectivities in n-heptane isomerization at 513 K, while cracking is significantly suppressed in comparison with Pt/beta catalyst.10 Platinum supported on zeolite beta was highly dispersed by the addition of chromium and would contribute to the suppression of cracking. Gallezot et al.11−14 reviewed literatures on platinum dispersion of zeolite. They discussed three kinds of methods to determine the physicochemical states of platinum and palladium supported on faujasite-type zeolite. Small-angle X-ray scattering (SAXS) is a useful technique to determine the size of platinum particles supported on Y zeolite. Platinum particles loaded on Y zeolite by ion exchange and reduced at 573 K were estimated to be 6−13 Å.12 Another method is direct observation of platinum particles with a transmission electron microscope (TEM). Location and size of metal particles can be drawn from the TEM images. Platinum particles supported on faujasite were observed with TEM, and their sizes depend on treatments such as calcinations, reduction, and evacuation.13 From the results of X-ray diffraction (XRD), palladium particles were located on the extra surface of NaNH4Y zeolite, and their size was calculated to be 20 Å.14 Chemisorption of gas, commonly hydrogen or carbon monoxide (CO), has often been used to determine particle size of metals on zeolite. In the case of hydrogen adsorption, the ratio of H/Pt must be known © 2014 American Chemical Society

to determine the dispersion of platinum. Its ratio depends on adsorption conditions such as temperature and equilibrium pressure. When platinum is supported on zeolite Y, H/Pt was reported to be unity12,15,16 or two.17−19 CO could be chosen as probe gas because the ratio of CO/Pt is always unity. As for structures of chromium loaded on zeolite, there have been some literatures. Tzou et al.20,21 and Jiang et al.22 reported that noble metal including bimetal system PtFe or RhCr were highly dispersed on NaY though information on the locations of two kinds of elements on zeolite were not given. Dzwigaj et al.23 and Tielens et al.24 analyzed the structure of CrSiBEA prepared by an evaporation to dryness method with an aqueous solution of chromium nitrate. They claimed that chromium was introduced to open T atom site of beta zeolite as trivalent cation and was hexavalent after calcination. Zhang et al.25 showed that the ratio of Cr6+/Cr3+ in Cr/ZSM-5 prepared by incipient wetness followed by calcination was large when the zeolite crystallite was small and its external surface area was large. The activity for propane dehydrogenation was higher over hexavalent-Cr rich catalyst. Joyner et al.26−28 characterized alloy particles containing Pt and an additional kinds of metal (Cr, Ga, Ge, Ir, Re, or Sn) supported on ZSM-5 by an impregnation method on the basis of the results of extended Xray absorption fine structure (EXAFS) analysis. Particles of Cr−Pt alloy of 12−20 nm in size were observed by TEM and found to be located on the external surface of ZSM-5. The catalytic activity for ethane aromatization was decreased by Received: January 9, 2014 Revised: April 13, 2014 Published: April 14, 2014 10746

dx.doi.org/10.1021/jp500232s | J. Phys. Chem. C 2014, 118, 10746−10753

The Journal of Physical Chemistry C

Article

Figure 1. Typical TEM images: (a, b) 0.33 wt % Pt/beta; (c, d) 0.36 wt % Pt 1.1 wt % Cr/beta.

aspirator (AS ONE, GAS-1) for 30 min. After that, the suspension was heated to 353 K and kept for 6 h. The amounts of platinum and chromium loaded were controlled by using the aqueous solution having an appropriate concentration. Cationexchanged zeolite was dried in an oven (393 K) and calcined at 773 K unless otherwise mentioned. When we exchanged chromium ions in Cr/beta before calcination to sodium ions, the sample was suspended in the aqueous solution of sodium nitrate and heated and dried under the same conditions as those used in the catalyst preparation. Elemental analyses were carried out to determine the amounts of platinum and chromium loaded on zeolite beta. Chemical compositions of samples solved in aqua regalis and/ or hydrofluoric acid were measured by using inductively coupled plasma atomic emission spectroscopy (ICP-AES, SPECTRO CIROS CCD). The amounts of metals loaded were determined to be 0.33 wt % Pt for Pt/beta, 1.1 wt % Cr for Cr/beta, 0.36 wt % Pt, and 1.1 wt % Cr for PtCr/beta unless otherwise mentioned. Extraction of platinum and/or chromium in PtCr/beta into water was attempted to examine the stability of these elements occluded in the micropores of zeolite beta. 500 mg of catalyst was soaked in 25 mL of water. The slurry was stirred and evacuated by an aspirator. Catalyst was filtered and washed with 50 mL of water. This process was repeated five times and after them dried at 393 K in an oven overnight. Element composition of catalyst after extraction was determined by using ICP-AES.

chromium addition. The authors supposed the influence of electrostatic interaction between alloy and zeolite. We recently reported that platinum and chromium loaded zeolites were useful as catalysts for the skeletal isomerization of n-heptane, and these elements were highly and uniformly dispersed in the micropores of zeolite beta.10 While Liu et al.29 similarly reported the positive effect of Cr addition on Pt/ dealuminated Hβ for n-heptane isomerization, the mechanism of change of platinum dispersion and the effect of Pt dispersion on products selectivity are open questions. In this study, the physicochemical structure and states of platinum and chromium in PtCr/beta were investigated with a variety of analytical techniques and molecular dynamics (MD) simulation.

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EXPERIMENTAL SECTION Zeolite beta with Si/Al ratio = 15 was synthesized by a dry-gel conversion method. The compositions of aluminosilicate hydrogel to obtain zeolite beta were SiO2/Al2O3/Na2O/ TEAOH/H2O = 1/0.033/0.1/0.37/0.030. The precursor mixture was dried and kept at 448 K for 16 h in an autoclave. The preparation method of zeolite beta was described elsewhere.10 Cation in zeolite beta was exchanged to ammonium ion in an aqueous solution of ammonium nitrate. H-beta was obtained by calcination of NH4-beta at 773 K for 3 h in air. Proton in zeolite beta was partially exchanged to platinum and/or chromium. Hbeta was suspended in an aqueous solution of tetraammine platinum nitrate and/or chromium nitrate, evacuated with an 10747



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find platinum particles by FE-SEM, they were hardly observed since they are probably hidden in or behind the aggregated zeolite crystals. Figure 2 shows the Fourier transformed XAFS results for Pt/ beta, PtCr/beta, and references (Pt foil and PtO2). Pt foil gave

Platinum particles in the catalysts were observed via a scanning transmission electron microscope (STEM), and elements of observed clusters were identified with energy dispersive X-ray spectroscopy (EDX). Valence platinum was examined using X-ray photoelectron spectroscopy (XPS, ALVAC-PHI ESCA1800). Pt 4f5/2 was conducted using nonmonochromatic Mg Kα radiation. Binding energies were calibrated with C 1s at 284.8 eV. When being reduced catalyst, it was heated to 673 K in flowing hydrogen at 10 K min−1 and kept for 60 min. X-ray absorption fine structures (XAFS) for Pt LIII-edge were observed at the BL14B2 station of Spring-8 in Japan. Crystalline Si(111) was used to produce a monochromatic Xray beam. Measurements were performed at room temperature using a fluorescence method. Fourier transformation of k3weighted EXAFS spectra was obtained in the Δk range of 3−12 Å−1. The spectra after Fourier transformation were fitted with Pt foil for Pt−Pt bond and PtO2 for Pt−O bond. The valence state of chromium was investigated by UV−vis (MC-2530 for light source and MCPD-3000 for detector, Otsuka Electronics). Light reflected from samples spread on a filter paper was detected. The acid amount of catalyst was examined by ammonia temperature-programmed desorption (NH3-TPD, BELCAT-B, Nippon Bell). The sample was heated in flowing air for 673 K, purged by helium, and reduced in flow of hydrogen for 30 min. Ammonia was introduced at 373 K to adsorb on catalyst and was desorbed by heating at 10 K min−1 in flowing helium. Desorbed ammonia was detected with a thermal conductivity detector (TCD). Chemisorption of CO was measured by the constant volume adsorption method with a glass vacuum apparatus (Makuhari Rika Glass). A catalyst cell was evacuated and heated for 523 K. Introduction of 20−30 kPa of hydrogen into the cell and evacuation were repeated three times to reduce catalyst. The cell evacuated was cooled to 323 K. First, the isotherm as the sum of physisorption and chemisorption of CO was taken. Physically adsorbed CO was then removed by evacuation, and the isotherm of the physisorption of carbon monoxide was taken. The isotherm of CO chemisorption was evaluated as difference of these two isotherms. The amount of platinum exposed to CO was estimated as the saturated value of CO uptake. The “Universal” module in the Materials Studio software (Ryoka System) was used to conduct force field computations. The details of the module for both electrostatic terms and van der Waals terms were the following; summation: atom based; truncation: cubic spline; cutoff distance: 12.5 Å; spline width: 1 Å; and buffer width: 0.5 Å.

Figure 2. Fourier transform of Pt LIII-edge EXAFS: (a) Pt foil, (b) PtO2, (c) 0.33 wt % Pt/beta, (d) 0.36 wt % Pt 1.1 wt % Cr/beta, and (e) reduced 0.36 wt % Pt 1.1 wt % Cr/beta.

one distinct peak at R = 2.6 Å, indicating Pt−Pt bond, and a Pt−O bond appeared at R = 1.6 Å for PtO2. Both peaks corresponding to Pt−Pt and Pt−O bonds are seen in the spectrum for Pt/beta (Figure 2c). The spectrum for Pt/beta was taken after the calcination of Pt/beta. Since platinum in Pt/ beta formed small particles as mentioned above, the appearance of the peak for Pt−Pt bond is in agreement with the formation of metallic platinum particles in Pt/beta and in addition the presence of Pt−O bond indicates that the surface of metallic platinum particles was oxidized, or together with metallic particles on the external surface oxidized platinum nanoparticles that are hardly seen in the TEM images dispersed in zeolite beta. On the other hand, PtCr/beta after calcination showed only one peak corresponding to Pt−O bond, implying that in PtCr/ beta platinum formed tiny PtO2 clusters or monatomic oxidized platinum were dispersed probably in the micropores of zeolite beta. The XAFS spectrum of PtCr/beta reduced by hydrogen at 673 K gave deep insight into the dispersion state of platinum on PtCr/beta. Interestingly, no peak appeared for reduced PtCr/beta, strongly suggesting that platinum in PtCr/beta was close to monatomic. The TEM observations shown in Figure 1 are consistent with such discussion. Table 1 lists the fitting results of XAFS analysis. The coordination number (CN) of Pt−Pt in Pt/beta was 5.26, which was smaller than 12 of the typical cluster of platinum. Such a smaller CN means that platinum formed nanometersized particles since about half of the atoms of platinum are suggested to be exposed to the particle surface. The XAFS results of Pt/beta also showed the presence of Pt−O bonds and its CN was 1.90, being less than 4 of typical PtO2. We suppose that the surface platinum atoms on the metallic platinum particles in Pt/beta would form Pt−O bonds. PtCr/beta showed solely Pt−O bonds, and the CN was 7.29, being larger than the CN in Pt/beta. In comparison with Pt/

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RESULTS AND DISCUSSION State of Platinum in PtCr/beta. We previously reported that when being supported on zeolite beta with chromium, platinum was more highly dispersed than that on beta in the absence of chromium.10 In this study, we attempted to determine the size of platinum particles formed in zeolite beta in the presence of chromium. Figure 1 shows typical TEM images of Pt/beta and PtCr/ beta. Black dots observed in Figures 1a,b of Pt/beta correspond to cubic platinum particles with 20−30 nm in size. However, as typically seen in Figures 1c,d, particles were hardly observed in PtCr/beta. These images strongly suggest platinum in PtCr/ beta is highly dispersed. In addition, though we attempted to 10748



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Al3+ peak appearing at 77.6 eV, and no signal appeared at 71.3 eV. Thus, we consider that major platinum species in PtCr/beta was Pt2+ because the peak attributed to Pt4+ was very small. After the reduction of PtCr/beta with hydrogen, a new peak emerged at 71.3 eV, which is attributed to Pt0. These XPS results fully coincide with the results obtained by the XAFS measurements: namely, platinum in PtCr/beta was Pt2+ after calcination and is able to be reduced to metal. Bulk properties of platinum were further investigated by using the XANES spectra along with the results obtained from the XPS measurements. On the basis of the XANES results, we discuss the valence of platinum in zeolite beta. Figure 4 shows

Table 1. Curve Fitting Results of Fourier Transform of Pt LIII-Edge EXAFSa sample Pt foil PtO2 Pt/beta PtCr/beta PtCr/beta (reduced)

shell

CN

Pt−Pt Pt−O Pt−Pt Pt−O Pt−O

12 4 5.26 1.90 7.29

R/Å

dE

2.78 2.15 2.78 −2.68 2.15 1.32 1.97 −0.08 fitting impossible

DW/Å

MF

0.77 0.06 0.06

7.00 7.00 7.00

a CN: coordination number, R: distance of the pair of elements; dE: fitting difference; DW: Debye−Waller factor; MF: photoelectron mean free path. Element loading: 0.33 wt % Pt/beta, 0.36 wt % Pt 1.1 wt % Cr/beta.

beta, a larger portion of platinum atoms was oxidized. In addition, no distinct peak was observed in the Fourier transformed XAFS spectra for PtCr/beta reduced by hydrogen, indicating that neighboring elements were hardly found on PtCr/beta. These results suggest that platinum particles in PtCr/beta should be much smaller than those in Pt/beta, possibly being close to monatomic. Let us discuss the valence states of platinum supported on zeolite beta using the results of XPS and XANES measurements. Figure 3 illustrates the XPS spectra of catalysts and reference samples. The peaks of Pt foil corresponding to Pt0 appeared at

Figure 4. Pt LIII-edge XANES spectra of (a) Pt foil, (b) PtO2, (c) 0.33 wt % Pt/beta after calcination, (d) 0.36 wt % Pt 1.1 wt % Cr/beta after calcination, and (e) 0.36 wt % Pt 1.1 wt % Cr/beta after reduction.

the absorption spectra of XANES measurements. The adsorption was obtained for Pt/beta in the range of 11 590− 11 600 eV and was similar to that of Pt foil. In the case of PtCr/ beta, The XANES spectrum obtained with the sample after calcination was similar to that with PtO2, while the sample after reduction gave a similar absorption spectrum to Pt foil. Thus, it was again suggested that while platinum in PtCr/beta was oxidized, the valence state of platinum was changed to Pt0 by reduction with hydrogen. Consequently, platinum in PtCr/beta was highly dispersed, possibly in close to monatomic state, in the micropores of zeolite beta. Platinum species in PtCr/beta was oxidized after calcination and readily reduced to metallic state, Pt0, with hydrogen. Even through atomically dispersed platinum species became metallic after reduction, no indication of sintering to form metal particles was observed. In other words, atomically dispersed platinum species is fairly stable in the presence of chromium in the micropores of zeolite beta. State of Chromium of PtCr/beta. Next, we discuss the state of chromium in PtCr/beta. Commonly chromium forms chromium oxide (Cr2O3) as trivalent and chromic acid (2H+ + CrO42−) and its salt as hexavalent. Chromium oxide is green solid and insoluble in water. Chromic acid and its salt of chromate are yellow, red, or bistered, and many of them are soluble in water. Chromium in PtCr/beta and Cr/beta were originally trivalent since chromium nitrate was used as precursor, and

Figure 3. XPS spectra of Pt 4f5/2: (a) Pt foil, (b) PtO2, (c) 0.33 wt % Pt/beta after calcination, (d) 0.36 wt % Pt 1.1 wt % Cr/beta after calcination, and (e) 0.36 wt % Pt 1.1 wt % Cr/beta after reduction.

71.3 and 74.6 eV of the binding energy. PtO2 containing Pt4+ shows peaks appearing at 74.6 and 77.6 eV. PtO2 is readily reduced partly to Pt2+ and gives additional peaks appearing at ∼73 and ∼76 eV.30,31 Since a peak attributed to Al3+ in zeolite beta appeared at 74.9 eV and partially overlapped with the signals for Pt0, Pt2+, and Pt4+, we will make qualitative discussion using the peaks at 71.3 eV for Pt0 and 77.6 eV for Pt4+. Pt/beta after calcination gave one intense peak at 71.3 eV, which is attributed to Pt0, indicating a large portion of platinum in Pt/beta formed metallic clusters. On the other hand, PtCr/ beta after calcination showed only a small shoulder beside the 10749



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the color of these samples was green before calcination. On the other hand, the color of PtCr/beta and Cr/beta became yellow after calcination. Formation of chromium salt can be ruled out in these samples, and we observed that yellow species was readily eluted off when these samples were immersed into water. From these observations, we concluded that trivalent chromium ion supported in the micropores of zeolite beta by ion-exchange was oxidized to hexavalent in the course of calcination to form chromate anion. The UV−vis spectra were taken with Cr-containing samples. Figure 5 compares the UV−vis spectra for 1.1 wt % Cr/beta Figure 6. Acid amounts determined by ammonia-TPD for PtCr/beta with different chromium loadings (Pt loading: 3.3−3.6 wt %; Cr loading: 0−1.2 wt %).

procedure one trivalent chromium was changed to chromic acid (2H+ + CrO42−) interacting with one proton of zeolite. The hydrogen source for proton should be hydroxide of Cr(OH)2− or water absorbed in zeolite. In the case that one chromic acid occupies one Brönsted acid site of zeolite, chromic acid kills the original Brönsted acid and produces a couple of Brönsted acids because chromic acid has two protons. It was also suggested that chromium was highly dispersed in the micropore of zeolite beta and hardly sintered to form chromia (Cr2O3) particles during calcination. In other words, chromic acid was stabilized in the micropore of zeolite beta. We conclude that these acid sites additionally formed by the introduction of chromic acid in zeolite beta did not contribute to the skeletal isomerization of n-heptane, since we have previously reported that the catalytic activity was not increased but decreased by the chromium addition.10 Though the properties of acid sites newly generated acid sites by the introduction of chromic acid are still an open question, the positions of two kinds of protons of zeolitic and chromic acids, or the space around such protons in the micropore of zeolite, might be different, leading to different catalytic properties. Interaction of Platinum and Chromium in PtCr/beta Catalyst. How can one explain the reason for the observation that the aggregation of platinum was suppressed in the presence of chromium acid in the micropores of zeolite beta? One plausible explanation is electrostatic stabilization by forming a pair of Pt2+ + CrO42−. We soaked 0.36 wt % Pt1.1 wt % Cr/ beta in water and compared the platinum and chromium contents before and after extraction, as listed in Table 2. The

Figure 5. UV−vis spectra of 1.1 wt % Cr/beta (a) before and (b) after calcination.

before and after calcination. Green Cr/beta before calcination gave two bands at 421 and 594 nm, being attributed to Cr3+.32−34 On the other hand, yellow Cr/beta after calcination showed three bands at 271, 379, and 450 nm, being attributed to Cr6+,32,33,35 and no band being attributed to Cr3+ vanished. These UV−vis results agreed with the observations described above. The ammonia-TPD spectrum was recorded for PtCr/beta having a wide range of chromium loading (0−1.2 wt %) since ammonia would adsorb on chromic acid (2H+ + CrO42−) formed in the micropore of zeolite beta. Deconvolution of ammonia desorption spectra was carried out to determine the acid amount from the desorption peak appearing at higher temperature. Original TPD spectra are shown in the Supporting Information (Figure S1). Figure 6 shows the relationship between chromium loading and the acid amount of PtCr/beta. The acid amount of Pt/beta without chromium (loading = 0) was 765 mmol g−1. The acid amount linearly increased with increasing chromium loading. The slope of this linear relation was 0.943 (∼1), meaning that one chromium supported on zeolite beta led to the formation of one acid site to adsorb ammonia in addition to the acid sites that zeolite beta originally had. In order to understand the effect of support on the valence state of chromium, we prepared Cr/Al2O3 and Cr/SiO2 by an impregnation method. The color of Cr/Al2O3 was yellow, indicating that chromium was mainly hexavalent on Al2O3. On the other hand, that of Cr/SiO2 was green, strongly suggesting that trivalent chromium was stable on SiO2. From these observations, chromium loaded on solid acid would form Cr6+ during calcination. We suppose that in the course of loading

Table 2. Platinum and Chromium Content of PtCr/beta before and after Extraction before after

Pt content (μmol g−1)

Cr content (μmol g−1)

18.6 17.2

221 39.0

amounts of platinum and chromium were determined by chemical analysis using ICP-AES. While the content of platinum slightly decreased from 18.6 to 17.2 μmol g−1, 82.4% of chromium was extracted in water, and the content of chromium markedly decreased from 221 to 39.0 μmol g−1. Chromium still remained in zeolite beta after the extraction, and its amount was 2.27 times larger than that of platinum. One would thus be interested in if remaining chromium after the extraction treatment had interaction with platinum. We 10750



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trends since the ability of PtCr/beta to inhibit cracking disappeared by the chromium extraction. The selectivity to cracking significantly decreased to 0.462% from 4.98% by the chromium addition but rose by the chromium extraction. The calcination after extracting chromium resulted in a higher conversion of 58.3% and a higher cracking selectivity of 9.79%. Though in this reaction the product selectivity substantially depends on the level of conversion,10 the inhibition of cracking on PtCr/beta was evident because the cracking selectivities at ∼30 and ∼50% of n-heptane conversion on PtCr/beta were much lower than that on Pt/beta, as shown in Table 3. As a result, we conclude that the presence of chromium was essential to highly disperse and stabilize platinum in the micropore of zeolite beta. Judging from the results of XRD and catalytic performance for the isomerization of n-heptane, chromium species that eluted in water was effective for dispersing platinum in the micropore of zeolite beta and inhibiting cracking. A plausible interpretation was stabilization by the electrostatic interaction between Pt2+ and CrO42−. Energies of various configurations of platinum and/or chromium species zeolite beta were calculated by using the MD simulation. Their geometry was optimized and the energies were calculated. We prepared the models of zeolite beta including Al3+, proton (H+), chromate (CrO42−), and platinum cation (Pt2+) and gave appropriate charges to the models. Cations and/or anions were introduced in micropores of zeolite framework and optimized by calculation. Table 4 lists the simulation results. The optimized energies are shown as the values per eight unit cell of zeolite beta

attempted to examine the structure of platinum in PtCr/beta by XRD. Figure 7 shows the results of XRD measurements. If

Figure 7. XRD patterns of (a) 0.33 wt % Pt/beta, (b) 0.36 wt % Pt 1.1 wt % Cr/beta, (c) PtCr/beta after extraction, and (d) PtCr/beta after extraction and calcination.

platinum forms crystallite such as that in Pt/beta, the reflection peak corresponding to Pt(111) should appear at 2θ = 39.8°. However, PtCr/beta both before and after the extraction of chromium did not give such a reflection peak, suggesting that platinum was still highly dispersed in PtCr/beta after the extraction. We observed the reflection peak for Pt(111) after the calcination at 773 K following the extraction, indicating that platinum was aggregated in the course of calcination. The highly dispersed platinum species was no longer stable upon heating in the absence of a sufficient amount of chromium. We compare that the catalytic activities and product selectivities of two kinds of PtCr/beta, that is, PtCr/beta after the extraction and after the calcination following the extraction, for the skeletal isomerization of n-heptane, as listed in Table 3. The reaction was carried out at 513 K in a flow reactor with a fixed bed of catalyst. The reaction procedure was described in detail elsewhere.10 PtCr/beta gave 32.8% of n-heptane conversion while Pt/beta showed 53.1%. Chromium was eluted out from PtCr/beta in the course of extraction. After the extraction of chromium, the level of conversion increased to 51.1% that was almost the same as that on Pt/beta. The product selectivities also show similar

Table 4. Energies Calculated by the MD Simulation no. of ions/(8 unit cells)−1 8 × H2CrO4 beta Cr/beta Pt/beta PtCr/beta

Al3+

H+

Pt2+

CrO42−

energy (kcal (8 unit cells)−1)

0 32 32 32 32

16 32 48 30 46

0 0 0 1 1

8 0 8 0 8

−3462 −2045 −6158 −2079 −6287

containing 32 Al3+. The amount of chromium was fixed to be 8 per eight unit cell. The optimized energies of eight chromic acids (2H+ + CrO42−) and zeolite beta with protons consisting of eight unit cells are −3462 and −2045 kcal, respectively, and the sum of them is −5507 kcal. The energy of zeolite beta containing 2H+ + CrO42− in the micropores was −6158 kcal, which is less than the sum of the energies of isolated zeolite

Table 3. Reaction Results of n-Heptane Skeletal Isomerizationa yield (%) catalyst

W/F (g h mol−1)

monobranched alkanes

multibranched alkanes

cracking products

n-heptane conv (%)

Pt/beta Pt/beta PtCr/beta PtCr/beta PtCr/beta (extracted) PtCr/beta (extracted and calcined

5.09 10.2 10.2 20.4 10.2 10.2

23.0 39.2 26.2 38.9 35.3 38.0

4.14 8.86 4.22 6.64 9.30 10.5

2.26 4.98 0.462 1.34 6.46 9.79

29.4 53.1 32.8 46.8 51.1 58.3

Flow rates, n-heptane/N2/H2 = 2/2/40 mL min−1 (STP). Reaction temperature, 513 K. Element loading: 0.33 wt % Pt/beta, 0.36 wt % Pt 1.1 wt % Cr/beta. a

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framework and chromate anions. Thus, chromates in the micropore of zeolite beta are stabilized. Further, the energy was calculated when Pt2+ was replaced with a pair of H+ in the presence of chromate. The structure containing chromate is much more stable than that in the absence of chromate and further introduction of Pt2+ slightly stabilized the structure by 129 kcal, from −6158 to −6287 kcal. Here, we discuss the ratio of platinum and chromium to increase platinum dispersion. Figure 8 shows the CO uptakes

Figure 8. CO uptakes on PtCr/beta containing different amounts of platinum and chromium. Chromium nitrate concentration/mol L−1: (●) 0, (⧫) 0.0005, (▲) 0.001, and (■) 0.002−0.02. Pt loading: 0− 0.83 wt %; Cr loading: 1.1 wt %.

for PtCr/beta having different amounts of platinum and chromium as a function of the amount of platinum loaded. For instance, when 0.36 wt % of Pt and 1.1 wt % of Cr were loaded on zeolite beta, the CO uptake was 9.2 mmol g−1. The CO uptake increased with increasing amount of chromium loaded. It is noteworthy that platinum was highly dispersed when a sufficient amount of chromium was loaded. All the CO uptake results of PtCr/beta prepared using solutions containing chromium nitrate in the concentration range of 0.002−0.2 mol L−1 are plotted on the same straight line (plotted as solid squares). On the other hand, the CO uptake of chromium-free Pt/beta reached a plateau above 0.06 wt % of Pt loading. With a sufficient amount of chromium, platinum was dispersed in close to monatomic state. Judging from the CO uptake results the minimum Cr/Pt ratio necessary for highly dispersing platinum is 1.2, of which this ratio means the lower limit that chromate to interact with platinum to inhibit migration and aggregation.

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Figure 9. Schematic model for the interaction of platinum, chromium, and zeolite beta.

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ASSOCIATED CONTENT

S Supporting Information *

Ammonia TPD spectra of PtCr/beta with different amounts of chromium loading (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

CONCLUSIONS

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Platinum and chromium species supported on zeolite beta were characterized. The EXAFS and XPS results strongly suggested that platinum was dispersed as close to monatomic state. The UV−vis spectra indicated that chromium formed chromate in zeolite. The MD simulation suggested that Pt2+ was more stabilized in the presence of chromium in the micropores of zeolite beta. The electrostatic interaction of Pt2+ and CrO42− would inhibit migration and aggregation of platinum during calcination. We propose the interactions of platinum, chromium and zeolite beta, as shown in Figure 9. Chromate is stabilized on acid site of zeolite. Platinum cation is anchored with chromate by electrostatic interaction, resulting in that platinum would hardly migrate and aggregate.

AUTHOR INFORMATION

Corresponding Author

*Ph +81-3-5286-3850, e-mail [email protected] (M.M.). Notes

The authors declare no competing financial interest.

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REFERENCES

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Physicochemical Characterization of Highly Dispersed Platinum and

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