on SiO2 Matrix at Elevated

Article pubs.acs.org/JPCC

Surface Diffusion of Pt Clusters in/on SiO2 Matrix at Elevated Temperatures and Their Improved Catalytic Activities in Benzene Oxidation Gang Liu,†,‡ Kun Yang,† Jiaqi Li,†,‡ Wenxiang Tang,†,‡ Junbo Xu,† Haidi Liu,† Renliang Yue,*,† and Yunfa Chen*,† †

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Pt in/on silica nanoparticles were synthesized by flame spray pyrolysis (FSP) followed by H2 reduction at different temperature and tested in benzene complete oxidation reaction. The Pt clusters diffusion from the interior to the exterior of the SiO2 matrix followed by aggregation in/ on the SiO2 matrix was observed with elevated temperature (300−1100 °C) and time (0−5 h). The aggregation of the Pt clusters on the surface of the SiO2 matrix was also evidenced by X-ray diffraction (XRD), transmission electron microscopy (TEM), and CO-pulse chemisorption. The effect of heat treatment temperature and time on the Pt/SiO2 structure was discussed. In combination with the experimental study, a further physical model describing the structural transformation was developed to complementarily depict the diffusion and aggregation process. The developed physical model correlated well with the experimental data. The catalytic activities increased with the elevated temperature until Pt3Si species was present at 1100 °C. The improved catalytic activities were attributed to the structural transformation induced by the elevated temperature. The rate of the Pt clusters diffusion to the surface and aggregation on the surface determined the Pt dispersion, which is the key mechanism in determining the catalytic activity. Al2O3 matrix in Pt−Sn/Al2O3 catalyst. van Vegten et al.25 proved that Pd clusters were stabilized in different supports (Al2O3, SiO2, ZrO2, TiO2, MgAl2O4, and CeO2) and thus inaccessible to catalytic reaction. It is also reported that Pd clusters were embedded in SiO2 matrix for flame-made Pd/SiO2 catalysts.20,26 Apparently, the embedded noble metal clusters cannot act as active sites during the catalytic reaction, which may directly decrease the catalytic activity. Therefore, effort has been focused on noble metal on SiO2 structure to fabricate exposure morphology and elevate the performance.27,28 Bubenhofer et al.26 investigated the thermal behavior and diffusion evolution of Pd/SiO2 and utilized a diffusion model to describe the process of noble metal clusters diffusion and aggregation in the SiO2 matrix. These references indicate that the noble metal clusters in the SiO2 matrix are mobile during heating. Catalytic behaviors of Pt-based catalysts, i.e., stability and activity, in benzene complete oxidation are complex and are closely related to Pt dispersion and Pt cluster size. Earlier, Papaefthimiou et al.29 reported the relationship between Pt dispersion and intrinsic activity of benzene oxidation over Pt/ SiO2. The intrinsic activity of benzene oxidation was found to

1. INTRODUCTION Supported noble metal catalysts have been applied in various catalytic reactions such as CO oxidation,1−3 stream reforming,4,5 NOx storage and reduction,6−8 and catalytic oxidation of volatile organic compounds (VOCs).9−11 Scientific efforts are focusing on the relationship between surface characteristics and catalytic performance. As a direct consequence, the structure of the catalyst is a key factor in determining the catalytic activity. In particular, complete benzene oxidation over supported noble metal catalysts which efficiently oxidize benzene to H2O and CO2 at lower temperature is a structure-sensitive reaction,10,12 so that proper exposure and dispersion of the noble metal are essential for the catalytic performance. With respect to fabricating exposure structure, flame spray pyrolysis is a facile method for supported noble metal catalysts,13,14 core−shell particles,15 and mixed oxide catalysts16 and has been widely investigated in NOx storage−reduction,6,17,18 hydrogenation reaction,19,20 and selective reduction of NO.21,22 However, problems still exist in controlling the structure of the catalyst when noble metal is loaded on the metal oxide matrix.20,23−25 The noble metal clusters are prone to be embedded in the metal oxide matrix, which inhibits the exposure of the noble metal clusters to the catalyst surface and thus decreases the catalytic activity. For example, Pisduangdaw et al.23 reported that some of the noble metal clusters were covered by © 2014 American Chemical Society

Received: February 10, 2014 Revised: August 18, 2014 Published: September 8, 2014 22719

dx.doi.org/10.1021/jp501434f | J. Phys. Chem. C 2014, 118, 22719−22729

The Journal of Physical Chemistry C

Article

calculated from the nitrogen adsorption−desorption isotherm using the BJH method and the SSA from a relative pressure of 0.03−0.2. Average particle size derived from SSA was calculated according to dBET = 6/(SSA × ρ), assuming the density of Pt/ SiO2 to 2.2 g/cm3. The reproducibility of the specific surface area was within 5%. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer (VG Scientific, UK) using 300 W Al Kα radiation. The base pressure during these measurements was approximately 3 × 10−9 mbar. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. A JEM 2100F electron microscope (JEOL) was used to characterize the morphology of the Pt/SiO2 catalysts. To get the statistical information on the Pt cluster size distribution, 200 Pt clusters were measured for each sample. CO-pulse chemisorption was performed to determine the Pt dispersion (35 °C, He flow 50 mL/min, pulse 0.5 mL, 10 vol % CO in He, Micromeritics Autochem II2920). Prior to chemisorption experiment, the samples were freshly reduced by H2 at 250 °C for 1 h with a flow rate of 20 mL/min, followed by 50 mL/min He flushing for 1 h at 260 °C. The Pt dispersion was calculated by assuming a CO to surface Pt atom ratio of 1:1. The reproducibility of the Pt dispersion was within 5%. H2-TPR was performed on a Micromeritics Chemisorb 2720 pulse chemisorption system (Micromeritics) equipped with a TPx system and a thermal conductivity detector (TCD). Samples of approximately 30 mg were heated from 100 to 750 °C at 10 °C/min and a mixture of H2 (10 vol %), and Ar was employed as a reducing atmosphere at a flow rate of 25 mL/min. 2.3. Catalytic Activity Test. Measurements of the catalytic oxidation of benzene were carried out in an atmospheric fixed bed reactor, heated from 110 to 250 °C. Catalyst samples of approximately 100 mg (40−60 mesh) were transferred into a quartz tube reactor (i.d. = 6 mm) and enclosed on both sides with two layers of silica wool. Temperature was monitored in the vicinity of the catalyst bed by means of a K-type thermocouple. To minimize the condensation of the products on the catalyst reactor, all gas lines were maintained at 110 °C. 100 ppm gaseous benzene balanced by synthetic air was feed into the atmospheric fixed bed reactor, and the total flow rate was 100 mL/min, corresponding to a weight hourly space velocity (WHSV) of 6000 mL/(g h). Exhaust gases from the catalytic reaction were analyzed by a gas chromatograph (GC, GC-2014, Shimadzu) equipped with a flame ionization detector (FID). The total conversion of benzene was calculated from the changes in benzene concentration between the inlet and the outlet gas. Volatile organic compounds mass spectrometer (SPIMS-1000, Hexin Mass Spectrometry) was used to further investigate the detail products in the final exhaust gases. Ionization energy was adjusted to 10.6 eV. Mass spectra data were analyzed by SPIMS_v1.0 software. Measurements of apparent activation energy (Ea) were performed in a separate experiment, where the conversion of benzene was kept to the values typically between 5% and 20%. The apparent activation energy was calculated from the slopes of the Arrhenius plots, where 1000 ppm gaseous benzene was used, corresponding to a WHSV of 60 000 mL/(g h).

increase with increasing Pt cluster size (decreasing Pt dispersion). Since then, it has been continuously argued that the Pt dispersion and the Pt cluster size may play a significant role in the catalytic activity. Ferreira et al.10 observed that the increase in Pd dispersion strongly reduced the intrinsic activity over Pd/ Al2O3 catalysts. Garetto et al.12 found that the light-off curves shifted to lower temperature with decreasing Pt dispersion, suggesting that benzene complete oxidation is a structure sensitive reaction over Pt-based catalysts. Apparently, keeping the noble metal clusters on the surface with a proper dispersion is one of the most important factors in determining the catalytic activity. In this paper, we focused on the effect of heat treatment temperature both on the structural transformation of Pt clusters in/on SiO2 matrix and on the catalytic activity of complete benzene oxidation. The influence of Pt clusters exposure, Pt dispersion, Pt cluster size, and Pt clusters diffusion/aggregation resulted from heat treatment on benzene complete oxidation were investigated. A complementary physical model concerning the effect of heat treatment temperature and time both on the structural transformation of the Pt/SiO2 catalyst and on the catalytic performance was further described.

2. EXPERIMENTAL SECTION 2.1. Catalyst Synthesis. Materials. Hexamethyldisiloxane (HMDSO, Sinopharm Chemical Reagent Co., Ltd., 99.0%) and platinum(II) acetylacetonate (Boren Co., Ltd., 99.9%) were dissolved in a mixture of ethanol and acetone (Sinopharm Chemical Reagent Co., Ltd., 3:1, vol %). The overall mole ratio of platinum(II) acetylacetonate to HMDSO was 1:200. According to the ratio, the theoretical Pt concentration was calculated to be 0.81 wt %. Flame Spray Pyrolysis. The method for preparing the Pt/SiO2 catalysts by FSP has been described earlier with some modifications.16 The precursor solution was dispersed to spray droplet by dispersion oxygen (4.0 L/min). The precursor feed rate (PFR) was adjusted to 5.535 mL/min. Support oxygen gas (4.0 L/min) and support methane gas (2.2 L/min) were ignited to form support flame, which subsequently initiated the combustion of the precursor solution. The gas flow rate was monitored by mass flow controller (D07-11C, Beijing Sevenstar Electronics Co., Ltd.). Particles were collected by a glass fiber filter with the aid of a vacuum pump. Heat Treatment. The flame-made catalysts were subjected to reduction in flowing H2 stream. The samples were located in a tubular furnace and pretreated by heating in flowing 5.0 vol % H2 balanced by N2 and kept at a certain temperature (300−1100 °C) for 1 h. To investigate the effect of heat treatment time on the flame-made catalysts, rapid thermal process (RTP-1000D4, MTI) was used in flowing 5.0 vol % H2 balanced by N2 at 500 °C for a certain time (0.5−5 h). The ramp temperature was 24 °C/s in order to minimize the effect of ramp time on the migration of the Pt clusters. 2.2. Catalyst Characterization. Elemental analysis was carried out using an inductively coupled plasma optical emission spectrometer (ICP-OES, Vista-MPX, Varian). The reproducibility of the Pt concentration was within 5%. Data concerning the phases of the catalysts were obtained by X-ray diffractometer (XRD, X’Pert Pro), applying scans from 10° to 70° (2θ) with Cu Kα (λ = 1.54 Å) radiation. Specific surface areas (SSA) were determined by employing the BET method, using data from an Autosorb-1-C-TCD (Quantachrome Instruments). Prior to measurements, powder samples (200 mg) were pretreated at 300 °C for 3 h to remove moisture. Pore size distribution was

3. RESULTS AND DISCUSSION 3.1. Physical and Chemical Properties. The X-ray diffraction patterns of the Pt/SiO2 catalysts treated under different temperature are shown in Figure 1. The broad peak with its centroid located at 2θ = 21.5° could be assigned to amorphous SiO2. The structure of the SiO2 is stable at 1100 °C. According to 22720



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dispersion of the Pt clusters in/on the SiO2 matrix. However, the measured Pt concentrations are much lower than the theoretical value (0.81 wt %). The decrease in the measured concentration of the Pt clusters compared with the theoretical value may result from the partial evaporation of the PtO2 species. It is reported that the metal loss was thought to occur at high temperature due to the volatilization of the PtO2 species.34−36 In case of 1 wt % Ru/Al2O3 only 0.6 wt % Ru was found.37 The volatile PtO2 species are prone to form in the oxidative flame at high temperature, which are subsequently subjected to rapid condensation on the wall of the collection setup. To get the statistical information on the cluster size distribution, 200 Pt clusters were measured for each sample. It is inevitable to neglect large clusters in the statistical process. The appearance of both the large clusters and the small clusters is prevalent in flame-made samples.38,39 The specific surface area of the Pt/SiO2 nanoparticles as a function of heat treatment temperature is shown in Figure 2.

Figure 1. X-ray diffraction patterns of the Pt/SiO2 nanoparticles treated at different temperature for 1 h. The broad peak with its summit located at 2θ = 21.5° suggests that the SiO2 is amorphous in the temperature range investigated.

JCPDS 65-2868, all the catalysts exhibit the characteristic peaks of metallic Pt (major peak at 2θ = 39.8°). The absence of PtOx diffraction peaks in the XRD patterns suggests that the Pt species exist in the form of metallic Pt.26 This is also evidenced by H2-TPR results in Figure S1 that there is no reduction peak corresponding to the PtOx species. The absence of the PtOx reduction peak suggests that the Pt species exist in the form of metallic Pt instead of PtOx. It should be noted that the metallic noble metal is obtained directly by FSP without postsynthesis reduction step.20,30,31 The peaks corresponding to the metallic Pt become broader and much intense with elevated heat treatment temperature (H2 atmosphere), suggesting the growth of the Pt clusters either in the form of Pt clusters aggregation in the SiO2 matrix or surface aggregation. It is of interest to note that the characteristic diffraction peaks of the Pt/SiO2 catalyst treated at 1100 °C exhibit a shift to higher 2θ (47.0°). It indicates that Pt−Si species (Pt3Si alloy)32,33 may form during high-temperature reduction process, which was prepared by thermal treatment of Pt and SiO2 matrix in hydrogen (JCPDS 43-1133).32 To determine the elemental composition of the materials obtained by FSP, selected samples were analyzed by ICP-OES. The ICP-OES results listed in Table 1 show that the Pt concentration of the Pt/SiO2 catalysts treated at different temperature is 0.24 ± 0.04 wt %, suggesting the homogeneous

Figure 2. Specific surface area of the Pt/SiO2 nanoparticles as a function of heat treatment temperature.

Apparently, the specific surface area decreases with the elevated temperature. The specific surface area is almost constant (130− 140 m2/g) below 700 °C and begins to drop dramatically with the elevated temperature from 700 °C (140.2 m2/g) to 1100 °C (64.8 m2/g). dBET was calculated according to specific surface area, as listed in Table 1. The increase in dBET and the loss in SSA (700−1100 °C) may result from the aggregation of the SiO2 matrix at high temperature. This is in accordance with the result in Breitscheidel’s study40 that partial sintering of the SiO2 matrix occurred and resulted in the decreased specific surface area during reduction. It is of interest to note that the specific surface area of the Pt/SiO2 powder is quite low compared with the counterpart prepared by similar FSP setup.26 One factor (dispersion gas rate) could be taken into consideration to explain the difference in specific surface area according to previous publication by Madler et al.41 It indicates that the specific surface area decreased with the increase of the oxygen flow rate. Flame height was reduced at higher oxygen flow rate, which led to a higher temperature during particle formation and therefore to pronounced sintering and to lower specific surface area. The oxygen flow rate is 4.0 L/min in this study, which results in the higher flame temperature and the lower specific surface area compared with the values in ref 26. The nitrogen adsorption−desorption isotherm and the pore size distribution of the Pt/SiO2 catalysts treated at different temperature are shown in Figure 3a,b. According to IUPAC classification, the nitrogen adsorption−desorption isotherm

Table 1. Physical Properties of the As-Prepared and the Treated Pt/SiO2 Nanoparticles at Different Heat Treatment Temperature for 1 h Pt/SiO2 catalysts

SSA (m2/g)

dBET (nm)

asprepared 300 °C 400 °C 500 °C 600 °C 700 °C 800 °C 900 °C 1100 °C

125.1

21.8

136.7 129.6 135.0 131.8 140.2 110.0 110.3 64.8

19.9 21.1 20.2 20.7 19.5 24.8 24.7 42.1

a b

Pt (wt %)

Pt dispersion (%)

0.25 0.20 0.25 0.26 0.26 0.28 0.21 0.25

3.06 6.81 9.90 4.23 3.37 3.64 0.47 0.33

dPt−COa (nm)

dTEMb (nm)

4.6 10.3 12.9 11.1

6.4 6.8 7.7 7.9 9.7 9.7 10.9 12.7

Average Pt cluster size derived from CO-pulse chemisorption. Average Pt cluster size derived from TEM. 22721



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Figure 3. N2 adsorption−desorption isotherm loop (a) and a histogram of the pore size distribution (b) for the Pt/SiO2 catalysts obtained by heat treatment at 300, 500, 800, and 1100 °C.

Figure 4. Representative TEM images and corresponding cluster size distribution of Pt clusters treated at different temperature for 1 h: (a) 300, (b) 400, (c) 500, (d) 600, (e) 700, (f) 800, (g) 900, and (h) 1100 °C. Note that the Pt clusters diffusion to the surface of the SiO2 matrix begin above 300 °C, and the growth of the Pt clusters is observed obviously with elevated temperature. The elevated temperature from 300 to 1100 °C is accompanied by the migration of the Pt clusters from the interior to the exterior of the SiO2 matrix and the aggregation on the surface.

flame-made Pd/SiO2 exhibited the typical patterns for mesopore and macropore. However, the reduction of the mesoporous structure happens after the Pt/SiO2 catalyst is treated at 1100 °C. It indicates that the porous structure collapsed and the specific surface area decreased drastically at high temperature (1100 °C).

corresponds to type IV isotherm. All the samples exhibit mesoporous structure with a small portion of macropores, even treated at high temperature. The structural property is in accordance with the work of Somboonthanakij et al.19 and Mekasuwandumrong et al.20 that the pore size distribution of the 22722



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This is in accordance with the publication of Madler et al.38 that the pore volume is attributed to larger pores (intraparticles pores between aggregates) and smaller pores (intraparticles pores between primary particles). Thus, porous structure may play an important role in maintaining high specific surface area. The Pt/SiO2 catalysts were subjected to heat treatment at 300−1100 °C for 1 h in 5 vol % H2−N2 flowing. TEM images and corresponding cluster size distribution of the Pt clusters are shown in Figure 4a−h. All samples exhibited crystalline Pt clusters dispersed in/on the amorphous SiO2 matrix. However, it is hard to identify the boundary of the SiO2 matrix due to the low contrast of the SiO2 in the TEM images. So it is impossible to identify where the Pt clusters locate only by TEM images. An additional source of uncertainty comes from the statistical Pt clusters size distribution, which results from the relatively low number of Pt clusters counted in TEM analysis (200 Pt clusters). 3.2. Pt Clusters Diffusion and Aggregation. The Pt/SiO2 catalysts were subjected to heat treatment in flowing 5.0 vol % H2−N2 at 300−1100 °C. It could be observed from the TEM images that the Pt clusters move from the interior of the SiO2 matrix to the boundary with the elevated temperature. Meanwhile, the growth of the Pt clusters is also observed when the Pt/SiO2 nanoparticles are subjected to heat treatment, which is supported by dTEM shown in Figure 4. The mean diameter of the Pt clusters begins to growth linearly from 6.4 to 12.7 nm once the Pt clusters are exposed to high temperature for 1 h. This phenomenon was also observed in Bubenhofer’s publication26 that a number of Pd clusters were embedded in the SiO2 matrix during flame synthesis. After heat treatment, the Pd clusters diffused to the surface of the SiO2 matrix and grew to larger clusters resulted from surface aggregation. These observations indicated that the Pt(Pd)/SiO2 nanoparticles have been subjected to restructure at high temperature, forming Pt(Pd) surface enrichment structure. XPS is a reasonable method to detect the surface composition on the SiO2 matrix, which could be a complementary evidence for the structural transformation. XPS measurements were carried out over the Pt/SiO2 catalysts treated at 300 and 900 °C for 1 h to determine the Pt concentration on the surface of the SiO2 matrix. The Pt 4f of Pt/SiO2-300 °C and Pt/SiO2-900 °C are shown in Figure S2. A distinct peak for Pt 4f was absent for Pt/SiO2-300 °C catalyst, indicating that the Pt concentration on the surface of the SiO2 matrix is low. The Pt clusters may be embedded in the SiO2 matrix, which could be hardly detected by X-ray. One possible explanation is that substantial numbers of the Pt clusters may be confined in the SiO2 matrix evidenced by TEM images and CO-pulse chemisorption results. The intensity of the specific peak corresponding to Pt 4f increases when the reduction temperature is elevated to 900 °C in Figure S2. Meanwhile, the Pt concentration on the SiO2 surface increases from 0.06 to 0.26 wt %, demonstrating the possible structural transformation during heat treatment. Additionally, previous publications concerning Pd/SiO2 and Ag/glass revealed that the noble metal may be confined in the SiO2 matrix during flame synthesis.26,42 The diffusion and aggregation of the Pt clusters could be further evidenced by CO-pulse chemisorption results. The COpulse chemisorption data are shown in Table 1 and Figure 5. The Pt dispersion increases from 3.06% to 9.90% in the temperature range of 300−500 °C, suggesting that Pt clusters begin to diffuse in this temperature range. In the temperature range of 500−1100 °C, the Pt dispersion decreases linearly from 9.90% to 0.33% with the elevated temperature, indicating the behavior of the Pt

Figure 5. Pt dispersion and dTEM as a function of heat treatment temperature.

clusters diffusion and aggregation on the surface. It is generally accepted that a high Pt dispersion corresponds to a small Pt cluster size, where the Pt clusters locate on the surface of the support.11,43 Actually, the higher Pt dispersion is related to the larger Pt cluster size in the temperature range of 300−500 °C in Figure 5. In the temperature range of 500−1100 °C, the higher Pt dispersion corresponds to the smaller Pt cluster size. TEM analysis suggests that the Pt clusters are located in the amorphous SiO2 matrix. The CO-pulse chemisorption experiment has also evidenced that the Pt clusters are absent on the surface of the SiO2 matrix. The rate of the Pt clusters diffusion from the core of the SiO2 matrix to the surface is larger than that of the Pt clusters aggregation on the surface with elevated temperature from 300 to 500 °C. On the other hand, the rate of the Pt clusters aggregation on the surface is faster than that of the Pt clusters diffusion to the surface in the temperature range of 500−1100 °C, which could be one reasonable explanation for the relationship between the Pt dispersion and the Pt cluster size indicated in Figure 5. As shown in Figure 5, the Pt dispersion is extremely low in the temperature range of 900−1100 °C. It is reported that Pt/SiO2 reduced at 650 °C showed diffraction rings of both metallic Pt and Pt3Si, and the decrease of CO chemisorption capacity could be attributed to the appearance of silicide (Pt3Si).32,44 However, we observe that the Pt3Si species are absent until 1100 °C in the XRD analysis. Therefore, it suggests that the dramatic decrease in the Pt dispersion at 1100 °C could be attributed to the formation of the Pt3Si species and the growth of the Pt clusters at high temperature and reduction atmosphere. The effect of heat treatment time on Pt dispersion is also studied. In order to minimize the influence of ramp time on the Pt clusters migration, a rapid ramp (24 °C/s) was used. The samples were kept in a flowing 5.0 vol % H2 balanced by N2 at 500 °C for 0.5−5 h. Figure 6 and Table S1 show Pt dispersion and dPt−CO as a function of heat treatment time. Apparently, the Pt dispersion decreases lineally from 6.78% to 0.11% with heat treatment time at 500 °C. As expected, dPt−CO increases with heat treatment time. 3.3. Physical Model of the Pt Clusters Diffusion and Aggregation. In combination with the experiment study, a physical model describing the structural transformation of the Pt/SiO2 catalysts with elevated temperature and time has been developed. The model consists of two processes: (1) the behavior of the Pt clusters in the SiO2 matrix including diffusion and aggregation; (2) the aggregation of the Pt clusters on the surface of the SiO2 matrix. Both process 1 and process 2 were solved with Matlab pdepe.44 Pt surface area and Pt cluster size 22723



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dimensionless diffusion coefficient of the ith aggregate, N0 is the initial number of Pt clusters per volume, and βij0 is the dimensionless aggregation coefficient between the ith and the jth aggregate. The first part and the second part of eq 1 represent the diffusion and aggregation of the Pt clusters within the SiO2 matrix, respectively. The viscosity of the SiO2 matrix is roughly estimated by Friuli Greig formula: ln μ = 10.8 + 9632.2/T. Figure 8a shows the concentration of the Pt clusters (i = 1) as a function

Figure 6. Pt dispersion and dPt−CO as a function of heat treatment time.

were used to evaluate the migration degree of the Pt clusters. A schematic model of the Pt clusters diffusion and aggregation in/ on the SiO2 matrix is depicted in Figure 7.

Figure 7. Schematic model of the Pt clusters diffusion and aggregation in/on the SiO2 matrix.

For the behavior of the Pt clusters in the SiO2 matrix including diffusion and aggregation (process 1 indicated in the schematic model), it is assumed that the primary Pt clusters homogeneously disperse in the SiO2 matrix. The metallic Pt clusters are assumed to migrate in the spherical SiO2 matrix, including diffusion to the surface of the SiO2 matrix and aggregation in the matrix. Once the Pt clusters have migrated to the surface, they could not enter the SiO2 matrix due to surface energy minimization. Based on these assumptions, a population balance of the Pt clusters diffusion and aggregation in the SiO2 matrix is developed and the basic values are similar to Bubenhofer’s publication.26 ⎡ ∂ui 4kT ⎢ 1 ∂ ⎛ 0 2 ∂ui ⎞ ⎜Di z ⎟ = 2 2 ⎢ 6μ ⎣ 4πard z ∂z ⎝ ∂t ∂z ⎠ ⎞⎤ ⎛ 1 + N0⎜⎜ ∑ βjk0 ujuk − ui ∑ βij0uj⎟⎟⎥ ⎥ j ⎠⎦ ⎝ 2 j+k=i t = 0: t > 0:

∂ui = 0, ∂z

u1 = 1, for z = 0;

Figure 8. Model results of the behavior of the Pt clusters in the SiO2 matrix. Evaluation of the Pt clusters, i = 1 (a), i = 2 (b). Pt cluster mass in the SiO2 matrix at different time and temperature (c).

ui ≠ 1 = 0 ui = 0,

of time and radius. Figure 8b shows the concentration of the Pt clusters (i = 2) increases with time at the center of the matrix resulted from the interior aggregation and then decreases due to the diffusion and aggregation to larger clusters. The model suggests that the process of the Pt clusters migration includes the Pt clusters diffusion to the surface and aggregation within the SiO2 matrix. Both diffusion and aggregation of the Pt clusters within the matrix are accelerated by elevating heat treatment temperature. Figure 8c describes the Pt cluster mass in the SiO2 matrix at different heat treatment temperature and time. The

for z = 1 (1)

where ui is the normalized number of metal aggregates consisting out of i Pt clusters, t stands for heat treatment time, k stands for Boltzmann constant, T stands for heat treatment temperature, μ is the viscosity of SiO2 matrix, a stands for the diameter of Pt clusters, a = 1 nm, rd stands for the diameter of SiO2 particles, rd = 5 nm, z is the normalized radius of the SiO2 particles, D0i is the 22724



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decrease in Pt cluster mass in the SiO2 matrix indicates the migration of Pt clusters to the SiO2 surface. It reveals that the Pt clusters are prone to migrate to the surface at high temperature. Meanwhile, the process of the Pt clusters migration to the matrix surface is dependent on heat treatment time. It has to be pointed out that the existing physical model neglects the effect of surface energy and matrix crystallinity,26 which is crucial for the catalytic activity. It is assumed that the number of the Pt clusters on the surface is kept to zero at any given time to drive the Pt clusters to the surface. Actually, the concentration of the Pt clusters on the surface is not zero during heat treatment. Therefore, this physical model is inadequate in determining the whole process of the Pt clusters migration. Furthermore, the present physical model does not refer to the behavior of the Pt clusters diffusion on the surface, which is important in determining the catalytic activity. Considering the relationship between the surface aggregation of the Pt clusters and their influence on catalytic activity, a more precise physical model concerning the Pt clusters aggregation on the surface was developed in the present study. The schematic model of the Pt clusters aggregation on the surface of SiO2 matrix is given in Figure 7. For the aggregation of Pt clusters on the surface of the SiO2 matrix (processes 1 and 2 indicated in the schematic model), the Pt clusters on the surface of the SiO2 matrix tend to further aggregation to minimize the surface energy45 with elevated heat treatment temperature and time. The population balance of the Pt clusters diffusion out of the SiO2 matrix and aggregation in/on the SiO2 matrix is delivered based on the results of Bubenhofer et al.26 ⎛ 4kTN0 ⎜ 1 6μ ⎜⎝ 2 ∂t ⎞ 0 − ui ,sur ∑ βij uj ,sur ⎟⎟ j ⎠

∂ui ,sur

= ui ,out +

t = 0:

∑

βjk0 uj ,suruk ,sur

Figure 9. Model results of the behavior of the Pt clusters in/on the SiO2 matrix (a), evolution of Pt surface area as a function of t at different T (b), Pt surface area as a function of T at different t (c), Pt cluster size as a function of t at different T (d), and Pt cluster size after 1 h heat treatment at different T (e).

j+k=i

u1,sur = 1, ui ≠ 1,sur = 0, ui ,out = 0

(2)

due to the Pt clusters diffusion to the surface of the SiO2 matrix and then decreases due to the Pt clusters aggregation on the surface of the SiO2 matrix above 700 °C. However, the Pt surface area increases linearly with heat treatment time below 500 °C. This is in accordance with the experimental study that the Pt dispersion decreases linearly with heat treatment time in Figure 6. Figure 9c shows the Pt surface area as a function of heat treatment temperature (300−1100 °C) at different heat treatment time (0−5 h). It is found that Pt surface area first increases with the increase of heat treatment temperature due to the Pt clusters diffusion to the surface of the SiO2 matrix and then decreases due to the Pt clusters aggregation on the surface of the SiO2 matrix. For example, the maximal Pt surface area for the Pt/ SiO2 nanoparticles treated for 1 h is achieved at 700 °C. Correspondingly, the Pt/SiO2 nanoparticles treated for 1 h at 500 °C exhibit the maximal Pt dispersion in the experimental study. Figure 9d shows the Pt cluster size as a function of heat treatment time at different heat treatment time. It is shown that the Pt cluster size increases linearly with heat treatment time. Figure 9e depicts the Pt cluster size after 1 h heat treatment at different heat treatment temperature. The Pt cluster size kept constant below 700 °C and increased dramatically above 800 °C. It suggests that the aggregation of the Pt clusters within the matrix is accelerated at temperature above 800 °C. The role of aggregation is negligible below 700 °C in this study, which is in accordance with ref 26.

where ui,sur is the normalized number of metal aggregates consisting out of i Pt clusters on the surface of SiO2 matrix, uj(k),sur is the normalized number of metal aggregates consisting out of j (k) Pt clusters on the surface of SiO2 matrix, and ui,out is the normalized number of metal aggregates consisting out of i Pt clusters diffusion to the surface at a given time t. The first part of eq 2 stands for the diffusion of the Pt clusters out of the SiO2 matrix, where the aggregation of the Pt clusters in the matrix is negligible.26 The second part of eq 2 represents the aggregation of the Pt clusters on the SiO2 matrix. The whole equation depicts the process of the Pt clusters migration from the interior to the exterior of the matrix and aggregation on the surface to minimize the surface energy at elevated temperature and time. The active sites accounting for the catalytic activity consist of two parts: (a) the Pt clusters diffusion from the interior to the exterior of the SiO2 matrix and (b) the Pt clusters aggregation on the surface of the SiO2 matrix. In the physical model, Pt surface area was used to evaluate the migration of the Pt clusters exposed to catalytic reactant on the SiO2 matrix. The result of the Pt clusters diffusion within the SiO2 matrix and aggregation on the surface is shown in Figure 9a. Figure 9b shows the Pt surface area as a function of heat treatment time (0−5 h) in the temperature range of 300−1100 °C. It is observed that the Pt surface area first increases with heat treatment time 22725



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during complete benzene oxidation over these catalysts could be proved to H2O and CO2 with its m/z locating at 18 and 44, respectively. As evidenced in Figures S3−S10, there is no other intermediate in the exhaust gases. The results indicate that the catalytic activity is highly related to the heat treatment temperature of the Pt/SiO2 catalysts. The Pt dispersion data are based on the CO-pulse chemisorption results. As discussed in sections 2.2 and 2.3, the structural transformation including the Pt clusters diffusion from the interior to the exterior of the SiO2 matrix and the Pt clusters aggregation on the surface of the SiO2 matrix is induced by elevated heat treatment temperature. The direct consequence of the structural transformation is the change in the Pt dispersion. The rate of the Pt clusters diffusion from the interior to the exterior of the SiO2 matrix is faster than that of the Pt clusters aggregation on the surface of the SiO2 matrix at lower temperature (300−500 °C), where the rate of the Pt clusters diffusion to the surface determines the Pt dispersion. The rate of the Pt clusters aggregation on the surface of the SiO2 matrix is faster than that of the Pt clusters diffusion from the interior to the exterior of the SiO2 matrix at higher temperature (500−1100 °C), where the rate of Pt clusters aggregation on the surface determines the Pt dispersion. Therefore, the Pt dispersion is given as the key mechanism in determining the catalytic activity, as indicated in Figure 5. Catalytic performance (T90) as a function of heat treatment temperature is depicted in Figure 11. In combination with the

Generally, Pt cluster size increases dramatically with heat treatment time at high temperature. However, there is lag between the result of the physical model and the experimental data. The lag is possibly attributed to the initial simplification of the model. It is the simplification (the nonsphericity of the SiO2 matrix, the heterogeneous distribution of the Pt clusters, and the roughly estimated viscosity, etc.) of the model that results in the semiquantitative validation of the simulation. On the other hand, the chosen diameter of the Pt cluster size and the SiO2 particle size in the simulation are not exactly related to the experimental data, where the initial parameters in the simulation are referred to previous publication.26 First, the diameter of the SiO2 matrix is hard to determine due to nonsphericity, overlap, and agglomeration of the SiO2 matrix. Second, the simulation process is aimed to complementarily evidence the process of the Pt clusters in/on the matrix coupled with the experimental results, where the trend in the simulation is highly related to the experiment. Therefore, the conclusion of the model is in agreement with the experimental study in principle, which suggests the validation of the model. This could be used to explain the effect of the structural transformation induced by heat treatment on benzene catalytic oxidation. 3.4. Effect of Heat Treatment Temperature on Catalytic Activity. Benzene catalytic oxidation was used as a model reaction to investigate the catalytic activity of these Pt/SiO2 catalysts. The catalytic performances (T10, T50, and T90, the temperature for 10%, 50%, and 90% benzene conversion) of the Pt/SiO2 catalysts treated at different temperature in benzene complete oxidation are shown in Table S2, and the light-off curves are depicted in Figure 10.

Figure 11. T90 as a function of heat treatment temperature. T90 decreases with heat treatment temperature elevated from 300 to 900 °C and increases with the temperature further elevated to 1100 °C.

analysis in Figure 5, the catalytic activity increases with the increased Pt dispersion in the temperature range of 300−500 °C. The Pt clusters located in the SiO2 matrix do not participate in the catalytic reaction as the active sites. The increased Pt dispersion is due to the Pt clusters diffusion to the surface of the SiO2 matrix, as depicted in the physical model in section 2.3. The rate of the Pt clusters diffusion to the SiO2 surface is larger than that of the Pt clusters aggregation on the SiO2 surface, which results in the Pt clusters diffusion to the surface of the SiO2 matrix and thus the increased number of the Pt clusters on the surface of the SiO2 matrix. On the other hand, the specific surface area and the pore size distribution do not change in this temperature range. It suggests that the number of the Pt clusters on the SiO2 matrix is the key factor in determining the catalytic activity. The improved catalytic activity is due to the increased number of the Pt clusters on the matrix resulted from the Pt clusters diffusion to the SiO2 surface. In the temperature range of 500−900 °C in Figures 10 and 11, the decrease of the Pt dispersion strongly improves the catalytic

Figure 10. Light-off curves of complete benzene oxidation over the Pt/ SiO2 catalysts treated at different temperature. Reaction conditions: benzene = 100 ppm, catalyst weight = 100 mg, gas flow rate = 100 mL/ min, WHSV = 6000 mL/(g h). Note the Pt/SiO2 catalyst treated under reduction atmosphere at 900 °C shows the lowest complete conversion temperature. Catalytic activity decreases dramatically with the further increase in heat treatment temperature to 1100 °C.

Generally, the catalytic performance is increasing in the order Pt/SiO2-300 °C < Pt/SiO2-400 °C < Pt/SiO2-500 °C < Pt/SiO2600 °C < Pt/SiO2-700 °C < Pt/SiO2-800 °C < Pt/SiO2-900 °C (T90 = 181.2, 175.5, 173.6, 167.6, 159.2, 155.7, and 148.6 °C, respectively) with elevated temperature from 300 to 900 °C. However, the catalytic activity decreases dramatically when the reduction temperature is elevated to 1100 °C with T90 = 209.0 °C. Hence, the Pt/SiO2 catalyst treated at 900 °C exhibited the best performance with T90 = 148.6 °C. Additionally, the products 22726



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at elevated temperature. The catalytic activity increases with the heat treatment temperature elevated from 300 to 900 °C and decreases when the heat treatment temperature is higher than 900 °C. The structural transformation is characterized by Pt clusters migration to the surface of SiO2 matrix and aggregation in the matrix and on the surface of the matrix. The changes in exposure structure, Pt cluster size, specific surface area, and Pt dispersion induced by elevated temperature and time have been evidenced. It indicates that the changes in Pt dispersion resulted from the structural transformation are responsible for the catalytic activity of benzene oxidation. Meanwhile, the model considering the influence of heat treatment temperature and time on the Pt/SiO2 nanoparticle structural transformation has been developed combined with experimental study. The diffusion and aggregation process of Pt/SiO2 induced by elevated heat treatment temperature may be a potential method in improving catalytic activity in heterogeneous catalysis.

activity, which suggests the dependence of Pt cluster size on benzene complete oxidation. The evolution of the Pt clusters aggregation on the surface of the SiO2 matrix with elevated heat treatment temperature may account for the improved catalytic activity. The decreased Pt dispersion is due to the aggregation of the Pt clusters on the SiO2 surface at higher temperature. The rate of the Pt clusters aggregation on the SiO2 matrix is larger than that of the Pt clusters diffusion to the SiO2 surface, which results in the growth of the Pt clusters and the decreased Pt dispersion. It suggests that the Pt dispersion is strongly related to the catalytic activity of benzene complete oxidation. Benzene complete oxidation over Pt or Pd catalysts has been reported to be a structure sensitive reaction that the light-off curves shift to lower temperature with the decreased Pt or Pd dispersion,10,12 which is in accordance with the catalytic performance in the temperature range of 500−900 °C in the present study. Therefore, the improved catalytic activity over Pt/SiO2 treated in the temperature range of 500−900 °C is attributed to the growth of the Pt clusters derived from the Pt clusters aggregation on the SiO2 surface. As shown in Figure 11, the Pt/SiO2 catalyst treated at 1100 °C under reduced atmosphere exhibits the decreased catalytic activity with its T90 higher than 200 °C. The possible reason for the dramatic decrease in catalytic activity of the Pt/SiO2 catalyst treated at 1100 °C may be attributed to the formation of the Pt3Si species and the decrease in specific surface area. The formation of the Pt3Si species has been evidenced by the peak shift in XRD and CO-pulse chemisorption analysis. Moreover, a further evidence for the decrease in the catalytic activity is depicted in Figure 12 and Table S3.

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ASSOCIATED CONTENT

* Supporting Information S

H2-TPR profile, XPS spectra, mass spectrum analysis, original catalytic data, and apparent activation energy data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel 86-10-82544895; Fax 86-1082544919 (R.Y.). *E-mail [email protected]; Tel 86-10-82627057; Fax 8610-82544919 (Y.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the National High Technology Research and Development Program of China (Grants 2012AA062702 and 2010AA064903), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB05050300), the National Natural Science Foundation of China (Grant 21306199), the Instrument Developing Project of the Chinese Academy of Sciences (Grant YZ200722), and the 12th Five-years National Key Technology R&D Program (Grants 2012BAJ02B03 and 2012BAJ02B07).

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Figure 12. Arrhenius plots of reaction rate ln(−r) vs 1000/T for benzene complete oxidation over the Pt/SiO2 catalysts treated at 300, 800, and 1100 °C. Straight lines correspond to the linear fitting, which are related to the apparent activation energies given in Table S3.

REFERENCES

(1) Ivanova, A. S.; Slavinskaya, E. M.; Gulyaev, R. V.; Zaikovskii, V. I.; Stonkus, O. A.; Danilova, I. G.; Plyasova, L. M.; Polukhina, I. A.; Boronin, A. I. Metal-Support Interactions in Pt/Al2O3 and Pd/Al2O3 Catalysts for CO Oxidation. Appl. Catal., B 2010, 97, 57−71. (2) Hinokuma, S.; Fujii, H.; Okamoto, M.; Ikeue, K.; Machida, M. Metallic Pd Nanoparticles Formed by Pd-O-Ce Interaction: A Reason for Sintering-Induced Activation for CO Oxidation. Chem. Mater. 2010, 22, 6183−6190. (3) Li, Y.; Yu, Y.; Wang, J.; Song, J.; Li, Q.; Dong, M.; Liu, C. CO Oxidation over Graphene Supported Palladium Catalyst. Appl. Catal., B 2012, 125, 189−196. (4) Chiarello, G. L.; Aguirre, M. H.; Selli, E. Hydrogen Production by Photocatalytic Steam Reforming of Methanol on Noble Metal-Modified TiO2. J. Catal. 2010, 273, 182−190. (5) Ferreira, A. P.; Araújo, J. C. S.; Liberatoria, J. W. C.; Damyanova, S.; Zanchetc, D.; Noronha, F. B.; Bueno, J. M. C. Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 2007.

Figure 12 shows the Arrhenius plots of reaction rate ln(−r) vs 1000/T for benzene complete oxidation over the Pt/SiO2 catalysts treated at 300, 800, and 1100 °C. The apparent activation energies of Pt/SiO2-300 °C and Pt/SiO2-800 °C show similar value toward complete benzene oxidation, as list in Table S3. However, the apparent activation energy of Pt/SiO2-1100 °C increases dramatically, which could be the reason for the decreased catalytic activity induced by the formation of the Pt3Si species.

4. CONCLUSIONS The complete oxidation of benzene has been tested over Pt/SiO2 nanoparticles by flame spray pyrolysis followed by heat treatment 22727



Article

sized by One-Step Flame Spray Pyrolysis in the Dehydrogenation of Propane. Appl. Catal., A 2009, 370, 1−6. (24) Ernst, F. O.; Buechel, R.; Strobel, R.; Pratsinis, S. E. One-Step Flame-Synthesis of Carbon-Embedded and -Supported Platinum Clusters. Chem. Mater. 2008, 20, 2117−2123. (25) van Vegten, N.; Maciejewski, M.; Krumeich, F.; Baiker, A. Structural Properties, Redox Behaviour and Methane Combustion Activity of Differently Supported Flame-Made Pd Catalysts. Appl. Catal., B 2009, 93, 38−49. (26) Bubenhofer, S. B.; Krumeich, F.; Fuhrer, R.; Athanassiou, E. K.; Stark, W. J.; Grass, R. N. From Embedded to Supported Metal/Oxide Nanomaterials: Thermal Behavior and Structural Evolution at Elevated Temperatures. J. Phys. Chem. C 2011, 115, 1269−1276. (27) Pal, S.; De, G. Formation of Au-Pt Bimetallic Nanoparticles in a Two-Layer SiO2 Films Doped with Au and Pt, Respectively, through Interlayer Diffusion. Phys. Chem. Chem. Phys. 2008, 10, 4062−4066. (28) Nakagawa, K.; Okayama, T.; Tanimoto, Y.; Sotowa, K. I.; Sugiyama, S.; Moriga, T.; Takenaka, S.; Kishida, M. Preparation of Carbon-Supported Pt Catalysts Covered with Microporous Silica Layers Using Organosilanes: Sintering Resistance and Superior Catalytic Performance for Cyclohexane Dehydrogenation. Appl. Catal., A 2012, 419, 13−21. (29) Papaefthimiou, P.; Ioannides, T.; Verykios, X. E. Performance of Doped Pt/TiO2 (W6+) Catalysts for Combustion of Volatile Organic Compounds (VOCs). Appl. Catal., B 1998, 15, 75−92. (30) Height, M. J.; Pratsinis, S. E.; Mekasuwandumrong, O.; Praserthdam, P. Ag-ZnO Catalysts for UV-Photodegradation of Methylene Blue. Appl. Catal., B 2006, 63, 305−312. (31) Schimmoeller, B.; Hoxha, F.; Mallat, T.; Krumeich, F.; Pratsinis, S. E.; Baiker, A. Fine Tuning the Surface Acid/Base Properties of Single Step Flame-Made Pt/Alumina. Appl. Catal., A 2010, 374, 48−57. (32) Lamber, R.; Jaeger, N. I. On the Reaction of Pt with SiO2 Substrates: Observation of the Pt3Si Phase with the Cu3Au Superstructure. J. Appl. Phys. 1991, 70, 457−461. (33) Wang, D.; Penner, S.; Su, D. S.; Rupprechter, G.; Hayek, K.; Schlogl, R. Silicide Formation on a Pt/SiO2 Model Catalyst Studied by TEM, EELS, and EDXS. J. Catal. 2003, 219, 434−441. (34) Monnet, F.; Schuurman, Y.; Aires, F. C. S.; Bertolini, J. C.; Mirodatos, C. Silicon Nitride Supported Platinum Catalysts for the Partial Oxidation of Methane at High Temperatures. Catal. Today 2001, 64, 51−58. (35) Jehn, H. High-Temperature Behavior of Platinum Group-Metals in Oxidizing Atmospheres. J. Less-Common Met. 1984, 100, 321−339. (36) Datye, A. K.; Xu, Q.; Kharas, K. C.; McCarty, J. M. Particle Size Distributions in Heterogeneous Catalysts: What do They Tell Us about the Sintering Mechanism? Catal. Today 2006, 111, 59−67. (37) Hannemann, S.; Grunwaldt, J. D.; Lienemann, P.; Gunther, D.; Krumeich, F.; Pratsinis, S. E.; Baiker, A. Combination of Flame Synthesis and High-Throughput Experimentation: The Preparation of AluminaSupported Noble Metal Particles and Their Application in the Partial Oxidation of Methane. Appl. Catal., A 2007, 316, 226−239. (38) Madler, L.; Stark, W. J.; Pratsinis, S. E. Flame-Made Ceria Nanoparticles. J. Mater. Res. 2002, 17, 1356−1362. (39) Strobel, R.; Pratsinis, S. E. Effect of Solvent Composition on Oxide Morphology during Flame Spray Pyrolysis of Metal Nitrates. Phys. Chem. Chem. Phys. 2011, 13, 9246−9252. (40) Breitscheidel, B.; Zieder, J.; Schubert, U. Metal-Complexes in Inorganic Matrices. 7. Nanometer-sized, Uniform Metal Particles in a SiO2 Matrix by Sol-Gel Processing of Metal-Complexes. Chem. Mater. 1991, 3, 559−566. (41) Madler, L.; Kammler, H. K.; Mueller, R.; Pratsinis, S. E. Controlled Synthesis of Nanostructured Particles by Flame Spray Pyrolysis. J. Aerosol Sci. 2002, 33, 369−389. (42) Rotzetter, A. C. C.; Luechinger, N. A.; Athanassiou, E. K.; Mohn, D.; Koehler, F. M.; Grass, R. N. Sintering of Core-shell Ag/Glass Nanoparticles: Metal Percolation at the Glass Transition Temperature Yields Metal/Glass/Ceramic Composites. J. Mater. Chem. 2010, 20, 7769−7775.

(6) Roy, S.; van Vegten, N.; Maeda, N.; Baiker, A. NOx Storage and Reduction over Flame-Made M/MgAl2O4 (M = Pt, Pd, and Rh): A Comparative Study. Appl. Catal., B 2012, 119−120, 279−286. (7) Buchel, R.; Strobel, R.; Baiker, A.; Pratsinis, S. E. Effect of the Proximity of Pt to Ce or Ba in Pt/Ba/CeO2 Catalysts on NOx StorageReduction Performance. Top. Catal. 2009, 52, 1709−1712. (8) Strobel, R.; Krumeich, F.; Pratsinis, S. E.; Baiker, A. Flame-Derived Pt/Ba/CexZr1‑xO2: Influence of Support on Thermal Deterioration and Behavior as NOx Storage-Reduction Catalysts. J. Catal. 2006, 243, 229− 238. (9) He, C.; Li, J.; Cheng, J.; Li, L.; Li, P.; Hao, Z.; Xu, Z. P. Comparative Studies on Porous Material-Supported Pd Catalysts for Catalytic Oxidation of Benzene, Toluene, and Ethyl Acetate. Ind. Eng. Chem. Res. 2009, 48, 6930−6936. (10) Ferreira, R. S. G.; de Oliveira, P. G. P.; Noronha, F. B. Characterization and Catalytic Activity of Pd/V2O5/Al2O3 Catalysts on Benzene Total Oxidation. Appl. Catal., B 2004, 50, 243−249. (11) Haneda, M.; Watanabe, T.; Kamiuchi, N.; Ozawa, M. Effect of Platinum Dispersion on the Catalytic Activity of Pt/Al2O3 for the Oxidation of Carbon Monoxide and Propene. Appl. Catal., B 2013, 142−143, 8−14. (12) Garetto, T. F.; Apesteguía, C. R. Structure Sensitivity and In Situ Activation of Benzene Combustion on Pt/Al2O3 Catalysts. Appl. Catal., B 2001, 32, 83−94. (13) Wang, Z. C.; Pokhrel, S.; Chen, M. M.; Hunger, M.; Madler, L.; Huang, J. Palladium-Doped Silica-Alumina Catalysts Obtained from Double-Flame FSP for Chemoselective Hydrogenation of the Model Aromatic Ketone Acetophenone. J. Catal. 2013, 302, 10−19. (14) Minnermann, M.; Pokhrel, S.; Thiel, K.; Henkel, R.; Birkenstock, J.; Laurus, T.; Zargham, A.; Flege, J. I.; Zielasek, V.; Piskorska-Hommel, E.; et al. Role of Palladium in Iron Based Fischer−Tropsch Catalysts Prepared by Flame Spray Pyrolysis. J. Phys. Chem. C 2011, 115, 1302− 1310. (15) Teleki, A.; Heine, M. C.; Krumeich, F.; Akhtar, M. K.; Pratsinis, S. E. In Situ Coating of Flame-Made TiO2 Particles with Nanothin SiO2 Films. Langmuir 2008, 24, 12553−12558. (16) Liu, G.; Yue, R. L.; Jia, Y.; Ni, Y.; Yang, J.; Liu, H. D.; Wang, Z.; Wu, X. F.; Chen, Y. F. Catalytic Oxidation of Benzene over Ce-Mn Oxides Synthesized by Flame Spray Pyrolysis. Particuology 2013, 11, 454−459. (17) Buchel, R.; Strobel, R.; Krumeich, F.; Baiker, A.; Pratsinis, S. E. Influence of Pt Location on BaCO3 or Al2O3 during NOx Storage Reduction. J. Catal. 2009, 261, 201−207. (18) Roy, S.; van Vegten, N.; Baiker, A. Single-Step Flame-Made Pt/ MgAl2O4: A NOx Storage-Reduction Catalyst with Unprecedented Dynamic Behavior and High Thermal Stability. J. Catal. 2010, 271, 125−131. (19) Mekasuwandumrong, O.; Somboonthanakij, S.; Praserthdam, P.; Panpranot, J. Preparation of Nano-Pd/SiO2 by One-Step Flame Spray Pyrolysis and Its Hydrogenation Activities: Comparison to the Conventional Impregnation Method. Ind. Eng. Chem. Res. 2009, 48, 2819−2825. (20) Somboonthanakij, S.; Mekasuwandumrong, O.; Panpranot, J.; Nimmanwudtipong, T.; Strobel, R.; Pratsinis, S. E.; Praserthdam, P. Characteristics and Catalytic Properties of Pd/SiO2 Synthesized by One-Step Flame Spray Pyrolysis in Liquid-Phase Hydrogenation of 1Heptyne. Catal. Lett. 2007, 119, 346−352. (21) Chiarello, G. L.; Ferri, D.; Grunwaldt, J. D.; Forni, L.; Baiker, A. Flame-Synthesized LaCoO3-Supported Pd. 2. Catalytic Behavior in the Reduction of NO by H2 under Lean Conditions. J. Catal. 2008, 253, 318−319. (22) Chiarello, G. L.; Grunwaldt, J. D.; Ferri, D.; Krumeich, R.; Oliva, C.; Forni, L.; Baiker, A. Flame-Synthesized LaCoO3-Supported Pd. 1. Structure, Thermal Stability and Reducibility. J. Catal. 2007, 252, 127− 136. (23) Pisduangdaw, S.; Panpranot, J.; Methastidsook, C.; Chaisuk, C.; Faungnawakij, K.; Praserthdam, P.; Mekasuwandumrong, O. Characteristics and Catalytic Properties of Pt-Sn/Al2O3 Nanoparticles Synthe22728



Article

(43) Christou, S. Y.; Efstathiou, A. M. Effects of Pd Particle Size on the Rates of Oxygen Back-Spillover and CO Oxidation under Dynamic Oxygen Storage and Release Measurements over Pd/CeO2 Catalysts. Top. Catal. 2007, 42−43, 351−355. (44) Sefcik, J.; Grass, R.; Sandkühler, P.; Morbidelli, M. Kinetics of Aggregation and Gelation in Colloidal Dispersions. Chem. Eng. Res. Des. 2005, 83, 926−932. (45) Winans, R. E.; Vajda, S.; Lee, B.; Riley, S. J.; Seifert, S.; Tikhonov, G. Y.; Tomczyk, N. A. Thermal Stability of Supported Platinum Clusters Studied by In Situ GISAXS. J. Phys. Chem. B 2004, 108, 18105−18107.

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