SBA-15 Catalyst Prepared by Direct

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Highly active and stable Pt-Sn/SBA-15 catalyst prepared by direct reduction for ethylbenzene dehydrogenation: Effects of Sn addition Lidan DENG, Takuto Arakawa, Tomoyo Ohkubo, Hiroki Miura, Tetsuya Shishido, Saburo Hosokawa, Kentaro Teramura, and Tsunehiro Tanaka Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 06 Jun 2017 Downloaded from http://pubs.acs.org on June 7, 2017

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Industrial & Engineering Chemistry Research

Highly active and stable Pt-Sn/SBA-15 catalyst prepared by direct reduction for ethylbenzene dehydrogenation: Effects of Sn addition Lidan Deng,[a,b] Takuto Arakawa,[b] Tomoyo Ohkubo,[b] Hiroki Miura,[b,c,d] Tetsuya Shishido,[b,c,d]* Saburo Hosokawa,[a,c] Kentaro Teramura,[a,c] Tsunehiro Tanaka[a,c]* [a] Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan [b] Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan [c] Elements Strategy Initiative for Catalysts & Batteries Kyoto University, 1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan [d] Research Center for Hydrogen Energy-based Society, Minami-Osawa, Hachioji, Tokyo 192-0397, Japan

* Corresponding author: Tel: +81-42-677-2850 Fax: +81-42-677-2821 (T. Shishido) E-mail addresses: [email protected] (T. Shishido)

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Abstract A series of Pt-Sn/SBA-15 catalysts (Sn/Pt nominal ratios: 0-3) prepared by direct reduction were applied to ethylbenzene dehydrogenation to styrene. The characterization by X-ray diffraction, X-ray absorption fine structure, CO adsorption, transmission electron microscopy and X-ray photoelectron spectroscopy revealed the formation of highly dispersed and stable Pt-Sn alloy particles (PtxSny (x/y≧3) and PtSn alloys having Sn-rich surfaces) on SBA-15. Non-alloyed Sn existed as highly dispersed SnO2. 1Pt1Sn/SBA-15 (Sn/Pt nominal ratio=1) exhibited the highest activity, on which Pt3Sn alloy nanoparticles were mainly formed. In contrast, PtSn alloy was dominant on Pt-Sn/SBA-15 catalysts whose Sn/Pt nominal ratios were larger than 1, and the activity was decreased. Furthermore, 1Pt1Sn/SBA-15 exhibited a higher stability than Pt/SBA-15 and 1Pt1Sn/SiO2. The addition of Sn not only inhibits C-C bond cleavage, improves selectivity towards styrene, but also enhances the “drain-off” effect—allowing coke precursors migrate from the active metals to SBA-15 with large surface area. Keywords Dehydrogenation • Drain-off effect • Ethylbenzene • Pt-Sn alloys • SBA-15

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1. Introduction Styrene is one of the key chemicals for the production of polymers such as polystyrene, styrene-butadiene rubber (SBR), and acrylonitrile-butadiene-styrene resin (ABS resin). Fe-K-Cr oxide-based catalysts are commercially used for ethylbenzene dehydrogenation, which is operated in the presence of excess steam at high temperatures of 873–973 K.1, 2 These conditions are favorable to inhibiting C-C bond cleavage and guaranteeing the selectivity towards ethylbenzene dehydrogenation to styrene. The addition of potassium to iron oxide (hematite; Fe2O3) enhances the reactivity of iron oxide and reduces the formation of coke that causes catalyst deactivation.3 Small amounts of Cr2O3 are added to improve the stability of the catalyst. Although Fe-K-Cr catalysts are somewhat active and selective, these catalysts do possess disadvantages such as active Fe3+ site instability, potassium promoter loss and redistribution, and rapid deactivation as a result of coke deposition.4 Furthermore, a large excess of steam used in the process leads to huge energy consumption.5 Therefore, the development of new catalysts which achieves higher yields, selectivity towards styrene and stability under more environmentally-benign conditions is desired. Steam-free oxidative dehydrogenation (ODH) and dehydrogenation of ethylbenzene are two main alternative technologies. The former one is promising reaction because of its exothermicity, the absence of thermodynamic limitations and lower operation temperatures. The related catalyst systems such as V2O5/Al2O3 with NOx as the mild oxidant6, TiO2-ZrO2 with CO2 as the oxidant7 and carbon-based catalysts with air as the oxidant8, 9 have been reported. In these ODH reactions, H2 from ethylbenzene dehydrogenation were consumed. Since H2 is also the important clean energy, non-oxidative dehydrogenation of ethylbenzene seems more elegant and consistent with the concepts in green chemistry. During the catalysts reported such as nanodiamond10, TiO2-ZrO211, 12 and Pt-based catalysts, Pt-based catalysts have drawn much more attentions. Moran et al. reported effective ethylbenzene dehydrogenation over Pt-supported 3



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synthetic clays with styrene conversion at ca. 50% and high styrene selectivity at low reaction temperatures (673 K) in the absence of steam.13 In fact, many researchers have investigated the catalytic dehydrogenation reactions over platinum or modified platinum catalysts.14-16 Supported platinum-tin alloy catalysts are reported as the highly efficient catalysts for the dehydrogenation of ethane,17-20 propane,21-27 isobutane,28,

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and n-butane.30-33 The Pt-Sn

bimetallic catalysts often show superior catalytic activity and selectivity compared with the supported Pt monometallic catalysts. However, the mechanism explaining the modified catalytic behavior of Pt with Sn as the promoter remains a matter of debate. The accurate nature of the Pt-Sn systems is complicated and closely related to the methods of catalyst preparation. Colloidal synthesis has been generally applied to the preparation of Pt-Sn bimetallic nanoparticle and favors the control of structural properties. However, many factors during preparation make this method not facile.34 Kim et al. have prepared Pt-Sn/C catalysts using borohydride reduction method at room temperature and found both geometric and electronic effects with variation of Sn content.35 Furukawa, S. et al. once synthesized Pt-based intermetallic compounds such as PtmMn/SiO2: M = Co, Ge, Sn, Tl and Zn) by impregnation with the thermal reduction by H2. The solid solution such as Pt3Sn was observed.36 Our previous work also reveal that in the dehydrogenation of propane, direct thermal reduction by H2 after the catalyst impregnation induces much more active and selective catalyst structure (Pt3Sn alloy nanoparticles) than other reduction methods such as decomposition in inert gas and calcination-reduction.37, 38 Moreover, even though supported Pt-Sn catalyst is one of the most suitable dehydrogenation catalysts, deactivation due to coke deposition is still not avoidable and leads to decreased catalytic activity. Using the support with large specific surface area and uniform pore structure is probably one efficient way to increase the catalyst stability and lifetime.39-41 During mesoporous silica materials with well-ordered channels (pore size: 2-10 nm) and high 4


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surface areas (ca. 1000 m2 g-1) such as MCM-41, FSM-16, SBA-15, SBA-15 has been widely used as a catalyst support.42-44 Kumar et al. found that highly dispersed Pt nanoparticles on SBA-15 showed a high activity in propane dehydrogenation.45 Chen et al. observed that Pt/SBA-15 showed higher stability than Pt/SiO2 catalysts in the catalytic dehydrogenation of methylcyclohexane due to highly dispersed Pt nanoparticles located in the mesochannels of SBA-15.39 The ordered mesopores of SBA-15 favored the confinement of Pt nanoparticles and its further growth during the reaction. Thus, SBA-15 was used in the present work as the support to prepare Pt-Sn/SBA-15 catalysts by direct reduction for ethylbenzene dehydrogenation to styrene. We expected for the development of highly active and stable catalyst system for the dehydrogenation of ethylbenzene. X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), CO adsorption, transmission electron microscopy (TEM), High-angle annular dark field-scanning transmission electron microscopy (HADDF-STEM) and X-ray photoelectron spectroscopy (XPS) are combined to reveal both bulk and surface properties of Pt-Sn/SBA-15 catalysts. Raman spectroscopy and Temperature-programmed oxidation (TPO) were utilized to study the coke formation over the catalysts in ethylbenzene dehydrogenation. 2. Experimental 2.1. Catalyst preparation In a typical batch to synthesize SBA-15, tetraethoxysilane (TEOS, 22 g, >96.0%, TCI company) was dissolved in a mixture of Pluronic P123 (10 g, average Mn 5800, Sigma Aldrich) and H2O (240 ml). To this polymer solution, HCl (40 ml, 36 wt.%, Wako) was added under magnetic stirring over 30 min. The resulting mixture was stirred for a further 20 h at 308 K and subsequently hydrothermally treated at 368 K for 24 h. The precipitate was separated by filtration, washed with copious amounts of distilled water, dried at 353 K for 24 h and finally calcined at 550 °C for 6 h to give the SBA-15 porous silica material.46 5



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SBA-15 was used as a support to prepare Pt-Sn/SBA-15. SBA-15 was first impregnated with an aqueous solution of H2PtCl6･6H2O (99.9 %,Wako), stirred at 353 K for 3 h, and dried at 353 K for 20 h. The obtained solid was then added to an acetone solution of SnCl2･2H2O (97.0 %, Wako), stirred at 353 K for 3 h and dried at 353 K for 20 h to yield the Pt-Sn/SBA-15 precursor. The Pt content was 3 wt.%. The obtained precursors were denoted xPtySn/SBA-15, where x and y indicate the initial molar ratio of Pt to Sn. 2.2. Ethylbenzene dehydrogenation to styrene Ethylbenzene dehydrogenation to styrene was carried out using a quartz glass tube with an inner diameter of 8 mm as a fixed-bed flow reactor under atmospheric pressure. The dehydrogenation reaction typically employed 0.05 g of the catalyst precursor, which had been previously pelletized to particle sizes between 25-50 mesh, prior to reactor loading. Before starting the reaction, the catalyst precursor was reduced in situ with 20 vol.% H2 diluted with pure N2 at a total flow rate of 50 ml min-1 at 1073 K for 1 h. The reduced Pt-Sn/SBA-15 catalysts were then used for the catalytic reaction. After the reduced catalyst cooling to 773 K in N2 (100 ml min-1), the reaction started after the introduction of an ethylbenzene and N2 gas mixture to the reactor. Ethylbenzene (0.059 ml min-1; 0.48 mmol min-1) was fed by pumping (PU-980 intelligent HPLC pump) to a vaporizer (433 K) in the presence of the carry gas, N2, in the vaporizer. All the lines and valves between the cold trap and the reactor were heated to 393 K to prevent condensation of either ethylbenzene or the dehydrogenation products. The reaction line was maintained at 393 K. The liquid products (styrene, toluene, and benzene) were analyzed with an on-line FID gas chromatograph (Shimadzu GC-14B) equipped with a CBP-10 column (Φ25 µm, 30 m). The gaseous products (CO, H2 and CH4) were analyzed by an on-line TCD gas chromatograph (Shimadzu GC-8A) equipped with a Molecular Sieve-5A column. 2.3. Characterization 6


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The specific surface area was estimated from N2 isotherms obtained using a BELSORP-mini II (BEL Japan, Osaka, Japan) analyzer at 77 K and calculated with Brunauer-Emmett-Teller (BET) method. Prior to the measurement, the samples were pretreated under evacuation at 573 K for 3 h. The amount of CO adsorbed on the catalysts at room temperature was estimated by the CO pulse method with an Okura BP-2 instrument (Okura Riken, Japan) interfaced with a thermal conductivity detector (TCD). Prior to the measurements, the catalyst precursor was in situ reduced with 50 ml min-1 of 5 vol.% H2 diluted with N2 at 1073 K for 1 h. XRD patterns were obtained using a RINT-TTR III powder x-ray diffractometer (Rigaku, Tokyo, Japan) employing Cu Kα radiation (λ = 1.5405 Å). The samples were scanned from 2θ = 36-48° at a scanning resolution of 0.01°. High

angle

annular

dark

field-scanning

transmission

electron

microscope

(HAADF-STEM) images were obtained using a JEOL JEM-3200FS transmission electron microscope. During the sample preparation, methanol was used to disperse small amount of catalyst powder and the suspensions were deposited onto carbon-coated copper grids (JEOL. Ltd.) followed by the complete evaporation of the ethanol. X-ray absorption experiments were conducted at the BL01B1 SPring-8 synchrotron facility with 8 GeV ring energy and 99.5 mA stored current. Pt L3-edge X-ray absorption spectra were obtained using a Si (111) monochromator in transmission mode. Sn K-edge X-ray absorption spectra were recorded using a Si (311) monochromator in fluorescence mode. The data were analyzed with the REX 2000 Ver.2.5.9 (Rigaku) and FEFF 8.40 programs (Washington University). XPS spectra were obtained using a JEOL JPS-9010MX instrument with Mg Kα radiation (10 kV, 10 mA) in a chamber at a base pressure of ~10-7 Pa. Sputtering was performed using an Ar ion beam (6.4 mA, 400 V) for 5 s in one run. O 1s (532.9 eV) in SiO2 7



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was used to calibrate all spectra. Raman spectra were measured using a Raman microscope (NRS-3100; JASCO) equipped with a He-Ne laser as a light source (wavelength: 633 nm), and 90 s detector exposure time per spectrum. The spectra were processed using Spectra manager ver. 2. 08. 02 (JASCO). TPO analysis of coked catalysts after ethylbenzene dehydrogenation was conducted by combustion in air with an Okura BP-2 instrument (Okura Riken, Japan) coupled to a BELMass (BEL Japan, Osaka, Japan) mass spectrometer monitoring the signal corresponding to CO2 (m/e = 44). 3. Results and Discussion 3.1 Ethylbenzene dehydrogenation to styrene Figure 1 shows the conversion of ethylbenzene over Pt-Sn/SBA-15 catalysts with various Sn/Pt ratios as a function of time-on-stream (TOS) up to 240 mins. Figure 2 summarizes the effect of the Sn/Pt ratio on the initial activity (TOS=10 min) for the catalysts prepared by direct reduction. It can be found that the Sn/Pt ratio have an appreciable effect on the ethylbenzene conversion, which remarkably increases with increasing Sn/Pt ratio up to 1.0. Further increasing the Sn/Pt ratio caused notable decreases in activity. Overall, 1Pt1Sn/SBA-15 exhibited the highest activity among the catalysts tested, exhibiting an ethylbenzene conversion of 37.0% and styrene selectivity of 99.4%. As can be seen from Figs. 1 and S1, 1Pt1Sn/SBA-15 also exhibited the superior stability among the catalysts tested and maintained high activity at 240 mins. In contrast, ethylbenzene conversions over the SiO2 supported catalysts noteworthy decreased during the reaction. This difference indicates the promotional role of SBA-15 on both activity and stability. Furthermore, the stability was also improved by the addition of Sn into Pt/SBA-15. Ethylbenzene conversion over Pt/SBA-15 decreased significantly as a function of TOS, and 8


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Pt/SBA-15 showed relatively low activity after 240 mins. Thus, 1Pt1Sn/SBA-15 is the highly efficient catalyst system for ethylbenzene dehydrogenation to styrene. 3.2 Structural properties of Pt-Sn/SBA-15 catalysts 3.2.1 Physical properties BET specific surface area, amount of adsorbed CO which reflects the surface exposed Pt atoms, and average particle size estimated from the HADDF-STEM images for Pt-Sn/SBA-15 catalysts and 1Pt1Sn/SiO2 as reference were summarized in Table 1. Self-made SBA-15 (890 m2 g-1) shows much higher specific surface area compared to the SiO2 (311 m2 g-1). Among the SBA-15 supported catalysts, the BET surface area of Pt/SBA-15 (717 m2 g-1) was slightly larger than those of the supported Pt-Sn bimetallic catalysts (e.g., 647 m2 g-1 for 3Pt1Sn/SBA-15). The surface areas of the catalysts were found to gradually decrease with increasing Sn/Pt ratio. However, the Pt-Sn/SBA-15 catalysts still presented a type IV nitrogen adsorption isotherm, in addition to the SBA-15 and Pt/SBA-15 catalysts (Figure S2). Their Barrett-Joyner-Halenda (BJH) plots also show similar pore size distributions (Figure S3). These results indicate the presence of uniform and well-ordered channels within the two-dimensional (2D) hexagonal SBA-15 even after modification with Pt and Sn. The HAADF-STEM images together with particle size histograms of the Pt-Sn/SBA-15 catalysts are shown in Figure S4. Uniform and well-ordered hexagonal channels of SBA-15 with ~6.0 nm diameters and white spots assigned to Pt-containing nanoparticles (Pt or Pt-Sn alloys) are clearly observed. For Pt/SBA-15, small Pt nanoclusters having an average particle size of ~1.5±0.6 nm were observed, suggesting the highly dispersion of Pt when Pt/SBA-15 was prepared by direct reduction. With the addition of Sn and the Sn/Pt ratio ranging from 0 to 1.0, the average particle sizes estimated form HAADF-STEM images for the bimetallic catalysts were ca. 1.3-1.6 nm. Further increase of the nominal Sn/Pt ratio led to the slight 9



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increase of particle size. The particles were more heterogeneously present over 1Pt3Sn/SBA-15, and the average particle size is about 2.5 nm, larger than that on Pt/SBA-15 catalyst. 3.2.2 XRD patterns The XRD pattern of SBA-15 showed its characteristic low angle peaks assigned to 100, 110, 200 reflections in a 2D hexagonal lattice (Figure S5).47 Figure 3 shows the XRD patterns of Pt/SBA-15 and Pt-Sn/SBA-15 catalysts. First, there is no clear diffraction peak located at 2θ = 39.8° attributed to Pt metal (111) (ICSD 41525) on Pt/SBA-15, suggesting highly dispersed Pt metal particles on SBA-15 with high surface area. The result is agreeable with TEM observations. In contrast, a broad peak at 2θ = 39.0° was observed in 3Pt1Sn/SBA-15. A shift in the Pt metal characteristic peak to lower angles in the presence of Sn has indicated the formation of Pt-Sn alloys, as reported in the previous works for various PtxSny (x/y≧3) alloys such as Pt0.90Sn0.10 and Pt0.75Sn0.25 (Pt3Sn).48 The broad full-width at half maximum (FWHM) of the diffraction peak of 3Pt1Sn/SBA-15 suggests the coexistence of various PtxSny (x/y≧3) alloys and/or highly dispersed of these alloy particles. The peak attributed to the Pt3Sn alloy (111) (2θ = 39.0°) (ICSD 105796) was observed in 2Pt1Sn/SBA-15. Compared with 3Pt1Sn/SBA-15, the FWHM of the diffraction peak for 2Pt1Sn/SBA-15 was narrower, suggesting that the crystal size of the Pt3Sn alloys supported on 2Pt1Sn/SBA-15 is slightly larger than that on 3Pt1Sn/SBA-15. The diffraction intensity for 1Pt1Sn/SBA-15 is very weak. It is attributed to the very small particle size of the Pt-Sn alloy crystals or the amorphous structure of the species. XAFS analysis will help to clarify the nature of the system since this technique is not limited by the crystallinity of the catalysts. Further increasing Sn loading resulted in the appearance of two diffraction peaks at ~42° and ~44°, which are assigned to the PtSn phase (ICSD 42593) as observed on 1Pt2Sn/SBA-15. PtSn alloy was also detected on 1Pt3Sn/SiO2 in addition to the 10


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Pt0.96Sn0.4 alloy. Furthermore, the obvious diffraction peaks related to Sn compounds such as Sn metal and Sn oxides (SnO and SnO2) were not observed on all catalysts including 1Pt3Sn/SBA-15 catalyst with 5.2 wt.% Sn. XRD results clearly prove the formation of the Pt-Sn intermetallic solid solution when Pt-Sn/SBA-15 catalysts were prepared with the direct reduction. The narrow FWHM of the diffraction peak further suggested the small Pt-Sn nanoparticles (1.3-2.5 nm) on Pt-Sn/SBA-15 catalysts with various Sn/Pt ratios. XRD results of Pt-Sn/SBA-15 (Sn/Pt ratio = 1/3-1) indicated that the geometric structure of Pt changed with the addition of Sn from the face centered cubic Pt crystallites to the solid solution PtxSny alloy phase (fcc, x/y ≥3). PtxSny (x/y ≧3) alloy formation accompanied with a lattice constant expansion. It also should be noted that the Sn/Pt ratio in the Pt-Sn intermetallic phase is smaller than the nominal Sn/Pt when the catalyst was prepared with direct reduction. For example, Pt3Sn phase formed on Pt-Sn/SBA-15 with the nominal Sn/Pt being 1. It was indicated that Sn does not completely alloy with Pt even under the reduction at 1073 K. With increasing the nominal Sn/Pt ratio, Sn-rich intermetallic phase such as PtSn appeared. Different from PtxSny (x/y ≧3) alloys, PtSn alloy exhibited hexagonal closed-packed crystal structure. These differences in the geometric parameters would contribute to the different catalytic behaviors between the Pt-Sn/SBA-15 catalysts. 1Pt2Sn and 1Pt3Sn/SBA-15 catalysts on which PtSn alloy was the main species showed the low catalytic activities. Alcalá et al.49 once reported that surface transition species derived from ethanol via C–C and C–O bond cleavage on a Pt (111) plane are sensitive to geometric parameters of Pt atoms such as bond length and angles based on density functional theory (DFT) calculations. It was observed that the extended Pt lattice might facilitate C–C cleavage, thus improving catalytic activity for the electro-oxidation of alcohols. Herein, the addition of Sn results in improved ethylbenzene conversion without reducing selectivity, even though the lattice constant 11



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expanded as a result of alloy formation between Pt-Sn. We postulate that the geometry effect alone is not sufficient to explain the promotional effect of Sn. 3.2.3 XAFS analysis X-ray absorption near edge structure (XAFS) is sensitive to short range ordering and does not require crystallinity, thus it is more extremely useful tool to study the present Pt-Sn/SBA-15 catalysts on an atomic level. In order to elucidate the nature of the interaction between Pt and Sn in Pt-Sn/SBA-15 catalysts, XANES has been utilized to identify the electronic states of both Pt and Sn. Extended X-ray absorption fine structure (EXAFS) has been used to investigate geometric properties of supported Pt-Sn bimetallic nanocatalysts. Figure 4 shows Pt L3-edge XANES spectra of Pt-Sn/SBA-15 catalysts along with reference samples (Pt/SBA-15 reduced at 1073 K, Pt foil and PtO2). The Pt L3-edge X-ray absorption white lines correspond to electronic transitions from 2p3/2 core level states, the white line directly reflect the electronic properties of vacant d orbitals of platinum atoms. The white line intensities at the Pt L3-edge decreased with increasing Sn/Pt ratio from 0 to 1. The lower intensities over 3Pt1Sn/SBA-15, 2Pt1Sn/SBA-15 and 1Pt1Sn/SBA-15 catalysts than that of Pt/SBA-15 have suggested the electron donation from Sn to Pt with PtxSny (x/y ≧3) alloys’ formation. The absorption band at the Pt L3-edge of 1Pt3Sn/SBA-15 was slightly shifted to a higher energy and the white line intensity was slightly higher than that observed for 1Pt1Sn/SBA-15 and 1Pt2Sn/SBA-15, lower than that observed for 3Pt1Sn/SBA-15, indicating that the electronic state of platinum on 1Pt3Sn/SBA-15 is different from other supported bimetallic catalysts and the references Pt/SBA-15 and Pt foil. It could be originated from the appearance of more PtSn alloy on 1Pt3Sn/SBA-15. It has also been reported that Pt atoms in Pt-Sn alloys are more electron-rich owing to electron donation from Sn to Pt.50, 51 Based on XAFS measurements and density functional theory calculations, Xin et al.52 explained the chemisorption of simple adsorbates on metal 12


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surfaces (pure metals or alloys) in terms of the d-band model. Alloy formation triggers a change in d-band width localized on Pt. They demonstrated that the width of a local d-band is a function of the local geometry of the valence orbitals of the atoms and also related to d-band center, the catalytic reactivity. Zhou et al.53 also discussed the effect of Sn on the mechanism of propane dehydrogenation with DFT calculations of Pt-Sn model alloys. They found that the d-band of Pt broadened with increasing Sn content, which led to a downshift in the d-band center of the Pt-Sn alloy. The bonding strength of a propyl group and propylene on the Pt-Sn alloy decreased. Therefore, propylene desorption became easier on the surface of Pt3Sn and propylene dehydrogenation was more difficulty than Pt catalysts. The selectivity towards propane dehydrogenation was improved over Pt3Sn alloy. The electronic modification of the unfilled d band states of Pt atoms in the present work may also decrease the bond intensity between carbon atoms and surface Pt atoms, thereby reducing or preventing strong adsorption of intermediates and products in ethylbenzene dehydrogenation as mentioned above in the case of propane dehydrogenation. The ability of Pt atoms to adsorb other molecules such as ethylbenzene might simultaneously decrease. However, this negative effect would compromise the beneficial inhibiting of side reactions such as cracking, isomerization during the reaction and promotional stability. Therefore, Pt-Sn/SBA-15 (Sn/Pt nominal ratios: 0.3-1) showed higher activity, selectivity and stability than Pt/SBA-15. Figure 5(A) shows Pt L3-edge k3-weighted EXAFS spectra. The EXAFS oscillations of Pt/SBA-15 are similar to those of Pt foil, although the intensity of the oscillations is significantly weaker. The peak located at ~0.25 nm in the Fourier transforms (FTs) of Pt L3-edge k3-weighted EXAFS spectra of Pt/SBA-15 (Figure 5(B)) is attributed to Pt-Pt linkage. A smaller Pt-Pt contribution than those of Pt foil indicates the small Pt nanoparticles (ca. 1.7 nm) on 3Pt1Sn/SBA-15. With the addition of Sn, the EXAFS oscillations of Pt-Sn bimetallic 13



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catalysts were different from those of Pt foil and Pt/SBA-15. These results are attributed to a mixture of Pt-Sn and Pt-Pt bonds in the Pt-Sn alloys. Besides the different Pt-Sn alloy contents shown in these Pt-Sn bimetallic catalysts, differences in Pt-Sn alloy composition as well as their size as indicated from XRD results also might contribute to the slight spectral differences between these supported Pt-Sn bimetallic catalysts. Figure 6 shows Sn K-edge XANES spectra of Pt-Sn/SBA-15 catalysts with the different Sn/Pt ratio, together with the reference samples (Sn foil and SnO2). All Pt-Sn/SBA-15 catalysts exhibited similar white line positions. The intensities of these white lines are higher than that of the Sn foil but lower than that of SnO2 (Figure 6(B)), suggesting the presence of both Sn0 and Sn cations (SnII and SnIV) on the Pt-Sn/SBA-15 catalysts. The white line intensity was lowest on 1Pt1Sn/SBA-15, indicating that the fraction of alloyed Sn was highest on 1Pt1Sn/SBA-15 among all bimetallic catalysts. Figure 7(A) shows k3-weighted EXAFS spectra at the Sn K-edge for Pt-Sn/SBA-15 catalysts with various Pt/Sn ratios. Firstly, the peak assigned to the Sn-O linkage located at ~0.16 nm was found in FTs of Sn K-edge k3-weighted EXAFS oscillation for all Pt-Sn/SBA-15 catalysts (Figure 7(B)). Furthermore, peak intensities located at ~0.2-0.3 nm, which implied alloy formation between Pt and Sn, decreased with increasing Sn/Pt ratio. The EXAFS oscillations of the 3Pt1Sn/SBA-15 catalyst showed obvious differences from the 1Pt1Sn/SBA-15 and 1Pt3Sn/SBA-15 catalysts in this range. As mentioned above, the mutual interference of the scattered electrons from Pt and Sn atoms in the Pt-Sn catalysts can diminish EXAFS oscillations. Therefore, these results indicate that the Pt-Sn alloy particles coexisted with the highly dispersed SnO2 on the Pt-Sn/SBA-15 catalysts, and that the fraction of alloyed Sn decreased with increasing Sn/Pt ratio. Additionally, the different ratio of the PtSn/Pt3Sn on the Pt-Sn/SBA-15 catalysts indicated from the XRD data also resulted in the observed EXAFS oscillation changes. 14


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It was previously mentioned that Sn does not completely alloy with Pt when Pt-Sn/SBA-15 bimetallic catalysts were prepared with direct reduction. Thus the Sn/Pt ratio in the Pt-Sn intermetallic phase over Pt-Sn/SBA-15 catalysts is smaller than the nominal Sn/Pt. Our previous work showed that for Pt-Sn/SiO2 catalysts, Sn retained its oxidation state during the impregnation step, and part of the Sn alloyed with Pt to form Pt-Sn solid solution during high-temperature H2 gas reduction.37, 38 However, Sn does not completely alloy with Pt even under the reduction at 1273 K. The present work also proves that the non-alloyed Sn was present as Sn oxide (SnO2) suggested form the Sn K-edge XAFS spectra. Furthermore, even for 3Pt1Sn/SBA-15 on which the nominal Sn/Pt ratio was equal to the stoichiometric ratio of the Pt3Sn alloy, the non-alloyed Sn also existed, suggesting that non-alloyed Sn is inevitable when the Pt-Sn bimetallic catalysts were prepared by direct reduction. 3.2.4 Surface analysis The surface compositions of Pt-Sn/SBA-15 catalysts were investigated by CO adsorption and XPS measurements. The adsorption of CO on platinum surfaces has been widely investigated to probe the number and nature of surface Pt metal sites over the supported platinum materials.54 In the present work, the amount of CO adsorbed on Pt/SBA-15 is the largest (37.2 mmol g-1) among all catalysts tested, suggesting the largest number of accessible surface Pt sites on it. Interestingly, although the average particle size estimated form HAADF-STEM images was ca. 1.6 nm regardless of the Sn/Pt ratio up to 1.0, the amount of adsorbed CO on these catalysts remarkably decreased (e.g., 14.8 µmol g-1 for 3Pt1Sn/SBA-15). With increasing Sn/Pt ratio to 2 or 3, the amount of adsorbed CO decreased significantly and the particle size was slightly increased. It has suggested the surface structure was remarkably changed with the addition of Sn. Pt-Sn alloy formation is one reason for the special CO chemisorption behaviors of Pt-Sn/SBA-15 catalysts. Wang et al.34 once prepared Pt, and PtSn (random alloy) catalysts via 15



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a colloidal synthesis method, and found a notable decrease in the intensity of linearly adsorbed CO on Pt sites in the PtSn (random alloy) catalyst than the pure Pt analogue. They attributed this performance to both geometric and electronic modification of Pt with alloying between Pt and Sn. Geometrically, the dilution role of Sn during Pt-Sn alloy formation will block CO adsorption on Pt atoms. Electronically, the electronic states of Pt probably was modified by the alloying with Sn, thus CO adsorption was suppressed in the Pt-Sn/SBA-15 catalysts. On the other hand, there is only 1.7 µmol g-1 CO absorbed over 1Pt3Sn/SBA-15 on which PtSn and Pt0.96Sn0.4 alloys formed as suggested from XRD data. This dramatic suppression of CO on 1Pt3Sn/SBA-15 indicates the formation of Pt-Sn alloys is not the only reason. More surface information is needed to explain the CO adsorption on Pt-Sn bimetallic catalysts. Figure 8 shows the Pt 4f (A) and Sn 3d (B) XPS spectra of Pt-Sn/SBA-15 catalysts. The electronic properties of the metal and/or alloy particle surface were evaluated using the peak energies from the Pt 4f and Sn 3d XPS spectra. The Pt 4f7/2 and 4f5/2 peaks were located at 71.6 and 74.8 eV respectively. Based on these binding energies, the platinum valence in the Pt-Sn/SAB-15 catalysts was assigned to Pt0. The intensity of the Sn 3d5/2 peak shown in Sn 3d region increased with Sn/Pt ratio. The binding energies of the Sn 3d5/2 peaks for all the supported bimetallic catalysts were about 486.5 eV, higher than the values typically reported for metallic Sn (484.9 eV) and lower than those usually reported for Sn oxides (487.1 eV). Furthermore, it is evident that the binding energies of the Sn 3d5/2 peak shifted to higher energy values with increasing Sn/Pt ratio. Deconvolution of the Sn 3d XPS spectra of Pt-Sn/SBA-15 catalysts revealed that the surface Sn0/SnIV ratio decreased with increasing Sn/Pt ratio. These results indicate the coexistence of metallic Sn and Sn oxides on all Pt-Sn/SBA-15 catalysts, and that the fraction of SnO2 gradually increased with the increase of Sn/Pt ratio. This observation is agreeable with the Sn K-edge XAFS data shown above. 16


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Moreover, the surface Sn/Pt ratios of all the Pt-Sn/SBA-15 catalysts were significantly larger than their bulk Sn/Pt ones. This result confirmed Sn surface enrichment over the Pt-Sn/SBA-15 catalysts regardless of the nominal Sn/Pt ratio. This behavior may be interpreted by the nature of Sn, which has a lower surface energy than platinum. Kim et al. once prepared Pt-Sn/C and found the similar results35. In order to make clear Pt and Sn elements’ states at the different depth, XPS depth analysis were carried out. Figure 9 shows the change in Pt 4f and Sn 3d XPS spectra of 1Pt3Sn/SBA-15 by Ar ion sputtering. The peak intensity of the Pt 4f XPS spectrum increased after the sputtering. The Sn 3d peaks become smaller and broader to the lower-energy side as a function of sputtering time. This change is related to the change in Sn0/SnIV surface ratio at different surface depths. Normalizing the XPS peak areas by their respective atomic sensitivity factors allowed the composition between Sn and Pt (Sn/Pt ratio) at a given sputtering time to be evaluated and the results are shown in Figure 10. The variation in the (Sn0+SnIV)/Pt ratios for the Pt-Sn/SBA-15 catalysts were presented in Figure 10 (A). Figure 10 (B) and (C) show variation in the Sn0/Pt and SnIV/Pt ratios as a function of sputtering time, respectively. The decay curves for the SnIV/Pt ratio is significantly sharper than that for the Sn0/Pt ratio, indicating that part of the Pt and/or Pt-Sn alloy particle was covered with a Sn oxide (SnO2) layer. The surface Sn0/Pt ratio of 1Pt1Sn/SBA-15 decreased with increasing sputtering time, having a ratio approaching 1.3 after 20 s of sputtering, close to the nominal ratio. Depth analysis of 3Pt1Sn/SBA-15 and 1Pt3Sn/SBA-15 were also performed and revealed that the surface Sn0/Pt ratio of 3Pt1Sn/SBA-15 and 1Pt3Sn/SBA-15 decreased with increasing sputtering time as did the 1Pt1Sn/SBA-15 catalyst, approaching 0.6 and 3.0 after 25 s of sputtering, respectively. The surface Sn0/Pt ratio observed after sputtering for 25 s is smaller than the nominal Sn/Pt atomic ratio (Fig. 10 (B)). Thus, we postulate that Pt-Sn alloy particles (PtxSny (x/y≧3) and the PtSn core with a Sn-rich surface) were formed with the modified surface by the SnO2, and 17



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the excess of Sn was present as the highly dispersed SnO2 on SBA-15. The effects of the bulk Sn/Pt ratio on the structure of the Pt-Sn/SBA-15 catalysts prepared by direct reduction are shown in Scheme 1. 3.3 Characterization of the spent catalysts after ethylbenzene dehydrogenation Metal sinter during the reaction is often regarded as one important reason for the catalyst deactivation. In the present work, the obvious metal or alloy sintering was not observed over both Pt/SBA-15 and Pt-Sn/SBA-15 after dehydrogenation of ethylbenzene (Figure S6) based on the HAADF-STEM images and particle size histograms. These results clearly indicated that the stability of Pt or Pt-Sn alloys on SBA-15 resulted from the unique pore structures and high surface areas. De Jongh et al.41 once reported how the mesoporous support hinders sintering of active sites, and how the cage-like silica pores of SBA-16 notably enhanced catalyst stability by limiting the confined Cu particle growth during the reaction when compared with Cu on SiO2-gel host materials also possessing 3D and highly interconnected porosity. Takenaka et al.55 also observed that carbon nanotube-supported Pd catalysts confined with silica layers were highly durable in the oxygen reduction reaction. The silica layer having a porous structure prevented the diffusion of dissolved Pd species out of the layers. After 240 mins, Pt/SBA-15 showed relatively low activity while 1Pt1Sn/SBA-15 maintained high activity during the whole reaction time (240 mins). In order to make clear the reason for catalytic deactivation over Pt/SBA-15 and the improved role of Sn for the high stability over 1Pt1Sn/SBA-15, Raman spectra and TPO profiles were used to investigate the formation of coke on the Pt/SBA-15 and Pt-Sn/SBA-15 catalysts after ethylbenzene dehydrogenation for 4 h (Figure 11). As observed from Fig. S7, two peaks corresponding to carbon formed over the catalyst surface at 1320 cm-1 and 1590 cm-1 (D and G bands, respectively) were detected in Raman spectra, which indicated that carbon formation on the 18


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catalyst surface was the main cause for catalyst deactivation. D band at about 1320 cm-1 could be associated with defective and disordered structures. It could be concluded that they are carbon nanoparticles, amorphous carbon or defective filamentous carbon.56 Additionally, the sharper G bands at around 1590 cm-1 are attributed to the C=C stretching in aromatic hydrocarbon (graphitic-like carbon) in these catalysts.57 The relative higher intensity of G band indicated that the formation of ordered carbon deposits with aromatic rings were preferred during ethylbenzene dehydrogenation over Pt-Sn/SBA-15 catalysts. In fact, the presence of aromatic rings observed in the solid state

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C cross-polarization/magic angle

spinning NMR spectra on the spent 1Pt1Sn/SBA-15 catalyst (Figure S8) further demonstrated that coke formation mainly originated from the aromatic rings of ethylbenzene. Besides the nature of coke from Raman and

13

C CP/MAS NMR spectra, TPO profiles

further shown the amount of coke formed on the spent catalysts. In the case of Pt/SBA-15, a broad peak at ~662 K (α-peak) was observed together with a small peak at ~780 K (β-peak). With increasing Sn/Pt ratio in the Pt-Sn/SBA-15 catalysts up to a ratio of 1, the α-peak gradually decreased in intensity while the β-peak intensity increased. The amount of CO2 formed during the TPO process, calculated from the α and β-peaks, was maximized on 1Pt1Sn/SBA-15 (Table 2). Further increasing the Sn/Pt ratio beyond 1 caused both the TPO peak intensity and the amount of formed CO2 to decrease over 1Pt2Sn/SBA-15 and 1Pt3Sn/SBA-15 because of their poor catalytic activities. Taking ethylbenzene conversion into account, 1Pt1Sn/SBA-15 shows the highest surface coking yields, however, normalizing total coke yield to converted ethylbenzene on 1Pt1Sn/SBA-15 displayed significantly less coke when compared with Pt/SBA-15 (2.5% for Pt/SBA-15 and 0.6% for 1Pt1Sn/SBA-15, Table 2), indicating that the selectivity towards coke formation was inhibited by the addition of Sn (Scheme 2). α- and β-coke formation on Pt/SBA-15 and 1Pt1Sn/SBA-15 was studied as a function of 19



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reaction time (Figure 12). It is evident that the major coke species over Pt/SBA-15 is α-coke, which increased with reaction time before plateauing at 300 mins. At the same time, ethylbenzene conversion was negligible because of the loss in Pt/SBA-15 activity after 4 h of reaction (Fig. 12 (A)). However, ethylbenzene conversion over 1Pt1Sn/SBA-15 was significantly higher over the entire TOS (~700 mins). α- and β-peaks assigned to carbon formed on the active metal, and carbon located on the surface of the support, respectively were also observed on the spent 1Pt1Sn/SBA-15. It was found that α-coke was the main source of carbon, β-coke increased significantly as a function of reaction time on the spent 1Pt1Sn/SBA-15. This indicates that Sn can promote the “drain-off” effect to assist carbon precursor migration from the metal active sites to the support. This explains the reason why although a larger amount of coke was deposited on Pt-Sn/SBA-15 than Pt/SBA-15, the former catalyst exhibited higher activity and stability during the dehydrogenation of ethylbenzene. Despite coke formation being at its maximum on 1Pt1Sn/SBA-15, this catalyst showed significantly higher stability than Pt/SBA-15. Furthermore, the normalized coke yield with respect to converted ethylbenzene (selectivity to coke) remarkably reduced in the presence of Sn, indicating that the selectivity towards coke formation is reduced by the addition of Sn. The change in the TPO profiles between Pt and Sn-Pt catalysts suggests that the coke location changes in the presence of Sn, which plays the role of “drain-off”. Lieske et al.58 firstly proposed the drain-off effect. Recently, Shin et al.23, 27 also reported differences in coke mobility behaviors of Pt-Sn/Al2O3 and Pt-Sn/ZnAl2O4. Coke transportation from the metallic sites to the support was enhanced on the Pt-Sn/Al2O3 catalyst. In the present work, it appears that the “drain-off” effect maintains accessibility to the Pt-Sn active sites by assisting migration of the coke precursors from the vicinity of the active sites to the support, as shown in Scheme 2. The special pore structure and high specific surface area of SBA-15 also 20


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guarantee this “drain-off” effect. Conclusion Pt-Sn/SBA-15 catalysts prepared by direct reduction were used in ethylbenzene dehydrogenation to styrene. It was observed that the Sn/Pt ratio strongly influence the structural properties of the catalyst and the catalytic behavior. Sn surface enrichment was observed across all Pt-Sn/SBA-15 catalysts including the catalyst with low Sn/Pt nominal ratio. The 1Pt1Sn/SBA-15 catalyst, on which Pt was dominant as small Pt3Sn alloy nanoparticles (ca. 1.5 nm), exhibited the highest catalytic activity and stability among the catalysts tested. Characterization of the spent catalysts by TPO measurements revealed that the normalized coke yield, as a function of ethylbenzene conversion, remarkably reduced in the presence of Sn. Furthermore, Sn also enhances the “drain-off” effect—enabling the migration of coke precursors away from the active metals. Hence, enhanced coke amount formed on 1Pt1Sn/SBA-15 compared with Pt/SBA-15, while the former one showed much higher stability because of more coke preferentially location over SBA-15 with high specific surface area. Supporting Information Catalytic activities of Pt-Sn/SiO2 with various Sn/Pt ratio, nitrogen adsorption isotherm, pore size distributions and low angle XRD of SiO2, SBA-15 and Pt-Sn/SBA-15 catalysts, HADDF-STEM images of Pt-Sn/SBA-15 with various Sn/Pt ratio, Raman spectra and

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C

CP/MAS NMR of the spent Pt-Sn/SBA-15 with various Sn/Pt ratio. Acknowledgement The authors acknowledge financial supports from the Program for Elements Strategy Initiative for Catalysts & Batteries (ESICB) and Grant-in-Aid for Scientific Research (B) (Grant 26289305) commissioned by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. The XAFS experiments were conducted with the approval 21



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(proposal No. 2012A1297, and 2013B1478) of the Japan Synchrotron Radiation Research Institute (JASRI). The authors thank Mr. Eichi Watanabe for the helps during HADDF-TEM observations.

References and Notes (1) Cavani, F.; Trifiro, F. Alternative processes for the production of styrene. Appl. Catal., A 1995, 133, 219-239. (2) Vora, B. V. Development of dehydrogenation catalysts and processes. Top. Catal. 2012, 55, 1297-1308. (3) Herzog, B. D.; Rase, H. F. In situ catalyst reactivation: used ethylbenzene dehydrogenation catalyst with agglomerated potassium promoter. Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 187-196. (4) Meima, G. R.; Menon, P. G. Catalyst deactivation phenomena in styrene production. Appl. Catal., A 2001, 212, 239-245. (5) Mimura, N.; Takahara, I.; Saito, M.; Hattori, T.; Ohkuma, K.; Ando, M. Dehydrogenation of ethylbenzene over iron oxide-based catalyst in the presence of carbon dioxide. Catal. Today 1998, 45, 61-64. (6) Shiju, N. R.; Anilkumar, M.; Mirajkar, S. P.; Gopinath, C. S.; Rao, B. S.; Satyanarayana, C. V. Oxidative dehydrogenation of ethylbenzene over vanadia-alumina catalysts in the presence of nitrous oxide: structure-activity relationship. J. Catal. 2005, 230, 484-492. (7) Burri, D. R.; Choi, K. M.; Han, S. C.; Burri, A.; Park, S. E. Dehydrogenation of ethylbenzene to styrene with CO2 over TiO2-ZrO2 bifunctional catalyst. Bull. Korean Chem. Soc. 2007, 28, 53-58. (8) Su, D. S.; Delgado, J. J.; Liu, X.; Wang, D.; Schlogl, R.; Wang, L. F.; Zhang, Z.; Shan, Z. C.; Xiao, F. S. Highly ordered mesoporous carbon as catalyst for oxidative dehydrogenation of ethylbenzene to styrene. Chem. Asian J. 2009, 4, 1108-1113. (9) Su, D. S.; Maksimova, N. I.; Mestl, G.; Kuznetsov, V. L.; Keller, V.; Schlogl, R.; Keller, N. Oxidative dehydrogenation of ethylbenzene to styrene over ultra-dispersed diamond and onion-like carbon. Carbon 2007, 45, 2145-2151. (10) Zhang, J.; Su, D. S.; Blume, R.; Schlogl, R.; Wang, R.; Yang, X. G.; Gajovic, A. Surface chemistry and catalytic reactivity of a nanodiamond in the steam-free dehydrogenation of ethylbenzene. Angew. Chem. Int. Ed. 2010, 49, 8640-8644. (11) Wang, I.; Chang, W. F.; Shiau, R. J.; Wu, J. C.; Chung, C. S. Nonoxidative dehydrogenation of ethylbenzene over TiO2-ZrO2: effect of composition on surface properties and catalytic activities. J. Catal. 1983, 83, 428-436. (12) Wu, J. C.; Chuang, C. S.; Ay, C. L.; Wang, I. Nonoxidative dehydrogenation of ethylbenzene over TiO2-ZrO2 catalysts: the effect of pretreatment on surface properties and catalytic activities. J. Catal. 1984, 87, 98-107. (13) Morán, C.; González, E.; Sánchez, J.; Solano, R.; Carruyo, G.; Moronta, A. Dehydrogenation of ethylbenzene to styrene using Pt, Mo, and Pt-Mo catalysts supported on clay nanocomposites. J. Colloid

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(30) Bocanegra, S. A.; de Miguela, S. R.; Borbath, I.; Margitfalvi, J. L.; Scelza, O. A. Behavior of bimetallic PtSn/Al2O3 catalysts prepared by controlled surface reactions in the selective dehydrogenation of butane. J. Mol. Catal. A: Chem. 2009, 301, 52-60. (31) Kikuchi, I.; Haibara, Y.; Ohshima, M.; Kurokuwa, H.; Miura, H. Dehydrogenation of n-butane to butadiene over Pt-Sn/MgO-Al2O3. J. Jpn. Pet. Inst. 2012, 55, 33-39. (32) Kikuchi, I.; Ohshima, M.; Kurokawa, H.; Miura, H. Effect of Sn addition on n-butane dehydrogenation over alumina-supported Pt catalysts prepared by co-impregnation and sol-gel methods. J. Jpn. Pet. Inst. 2012, 55, 206-213. (33) Ballarini, A. D.; Zgolicz, P.; Vilella, I. M. J.; de Miguel, S. R.; Castro, A. A.; Scelza, O. A. n-Butane dehydrogenation on Pt, PtSn and PtGe supported on γ-Al2O3 deposited on spheres of α-Al2O3 by washcoating. Appl. Catal., A 2010, 381, 83-91. (34) Wang, X. D.; Altmann, L.; Stöver, J.; Zielasek, V.; Bäumer, M.; Al-Shamery, K.; Borchert, H.; Parisi, J.; Kolny-Olesiak, J. PtSn Intermetallic, coreshell and alloy nanoparticles colloidal synthesis and structural control. Chem. Mater. 2013, 25, 1400-1407. (35) Kim, J. H.; Choi, S. M.; Nam, S. H.; Seo, M. H.; Choi, S. H.; Kim, W. B. Influence of Sn content on PtSn/C catalysts for electrooxidation of C1–C3 alcohols: synthesis, characterization, and electrocatalytic activity. Appl. Catal., B 2008, 82, 89-102. (36) Furukawa, S.; Tamura, A.; Ozawa, K.; Komatsu, T. Catalytic properties of Pt-based intermetallic compoundsin dehydrogenation of cyclohexane and n-butane. Appl. Catal., A 2014, 469, 300-305. (37) Deng, L. D.; Shishido, T.; Teramura, K.; Tanaka, T. Effect of reduction method on the activity of Pt-Sn/SiO2 for dehydrogenation of propane. Catal. Today 2014, 232, 33-39. (38) Deng, L. D.; Miura, H.; Shishido, T.; Hosokawa, S.; Teramura, K.; Tanaka, T. Dehydrogenation of propane over silica-supported platinum–tin catalysts prepared by direct reduction: effects of tin/platinum ratio and reduction temperature. ChemCatChem 2014, 6, 2680-2691. (39) Chen, A. B.; Zhang, W. P.; Li, X. Y.; Tan, D. L.; Han, X. W.; Bao, X. H. One-pot encapsulation of Pt nanoparticles into the mesochannels of SBA-15 and their catalytic dehydrogenation of methylcyclohexane. Catal. Lett. 2007, 119, 159-164. (40) Liu, L.; Deng, Q. F.; Agula, B.; Zhao, X.; Ren, T. Z.; Yuan, Z. Y. Ordered mesoporous carbon catalyst for propane dehydrogenation. Chem. Commun. 2011, 47, 8334-8336. (41) Prieto, G.; Shakeri, M.; de Jong, K. P.; de Jongh, P. E. Quantitative relationship between support porosity and the stability of pore-confined metal nanoparticles studied on CuZnO/SiO2 methanol synthesis catalysts. ACS Nano 2014, 8, 2522-2531. (42) Huirache-Acuña, R.; Nava, R.; Peza-Ledesma, C. L.; Lara-Romero, J.; Alonso-Núñez, G.; Pawelec, B.; Rivera-Muñoz, E. M. SBA-15 mesoporous silica as catalytic support for hydrodesulfurization catalysts-review. Material 2013, 6, 4139-4167. (43) Taguchi, A.; Schuth, F. Ordered mesoporous materials in catalysis. Microporous Mesoporous Mater. 2005, 77, 1-45. (44) Stein, A. Advances in microporous and mesoporous solids—highlights of recent progress. Adv. Mater.

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2003, 15, 763-775 (45) Kumar, M. S.; Holmen, A.; Chen, D. The influence of pore geometry of Pt containing ZSM-5, Beta and SBA-15 catalysts on dehydrogenation of propane. Microporous Mesoporous Mater. 2009, 126, 152-158. (46) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores. Science 1998, 279, 548-552 (47) Bassil, J.; AlBarazi, A.; Costa, P. D.; Boutros, M. Catalytic combustion of methane over mesoporous silica supported palladium. Catal. Today 2011, 176, 36-40. (48) Durussel, P.; Massara, R.; Feschotte, P. The binary system Pt-Sn J. Alloys Compd. 1994, 215, 175-179. (49) Alcalá, R.; Mavrikakis, M.; Dumesic, J. A. DFT studies for cleavage of C-C and C-O bonds in surface species derived from ethanol on Pt(111). J. Catal. 2003, 218, 178-190. (50) Roman-Martinez, M. C.; Macia-Agullo, J. A.; Julieta Vilella, I. M.; Cazorla-Amoros, D.; Yamashita, H. State of Pt in dried and reduced PtIn and PtSn catalysts supported on carbon. J. Phys. Chem. C 2007, 111, 4710-4716. (51) Uemura, Y.; Inada, Y.; Bando, K. K.; Sasaki, T.; Kamiuchi, N.; Eguchi, K.; Yagishita, A.; Nomura, M.; Tada, M.; Iwasawa, Y. In situ time-resolved XAFS study on the structural transformation and phase separation of Pt3Sn and PtSn alloy nanoparticles on carbon in the oxidation process. Phys. Chem. Chem. Phys. 2011, 13, 15833-15844. (52) Xin, H. L.; Holewinski, A.; Schweitzer, N.; Nikolla, E.; Linic, S. Electronic structure engineering in heterogeneous catalysis: identifying novel alloy catalysts based on rapid screening for materials with desired electronic properties. Top. Catal. 2012, 55, 376-390. (53) Yang, M. L.; Zhu, Y. A.; Fan, C.; Sui, Z. J.; Chen, D.; Zhou, X. G. Density functional study of the chemisorption of C1, C2 and C3 intermediates in propane dissociation on Pt(111). J. Mol. Catal. A: Chem. 2010, 321, 42-49. (54) Tanabe, T.; Nagai, Y.; Hirabayashi, T.; Takagi, N.; Dohmae, K.; Takahashi, N.; Matsumoto, S.; Shinjoh, H.; Kondo, J. N.; Schouten, J. C.; Brongersma, H. H. Low temperature CO pulse adsorption for the determination of Pt particle size in a Ptcerium-based oxide catalyst. Appl. Catal., A 2009, 370, 108-113. (55) Takenaka, S.; Susuki, N.; Miyamoto, H.; Tanabe, E.; Matsune, H.; Kishida, M. Highly durable carbon nanotube-supported Pd catalysts covered with silica layers for the oxygen reduction reaction. J. Catal. 2011, 279, 381-388. (56) Guo, J. J.; Lou, H.; Zheng, X. M. The deposition of coke from methane on a Ni/MgAl2O4 catalyst. Carbon 2007, 45, 1314-1321. (57) Han, Z. P.; Li, S. R.; Jiang, F.; Wang, T.; Ma, X. B.; Gong, J. L. Propane dehydrogenation over Pt-Cu bimetallic catalysts: the nature of coke deposition and the role of copper. Nanoscale 2014, 6, 10000-10008. (58) Lieske, H.; Sarkany, A.; Volter, J. Hydrocarbon adsorption and coke formation on Pt/Al2O3 and Pt-Sn/Al2O3 catalysts. Appl. Catal. 1987, 30, 69-80.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure captions Figure 1

Catalytic performance of SBA-15 supported Pt and Pt-Sn catalysts during ethylbenzene dehydrogenation. ( ◯ : Pt/SBA-15; ⦿: 1Pt1Sn/SiO2; ▲: 3Pt1Sn/SBA-15; ◆: 2Pt1Sn/SBA-15; ●: 1Pt1Sn/SBA-15; ■: 1Pt2Sn/SBA-15; ▼: 1Pt3Sn/SBA-15).

Figure 2

Initial ethylbenzene conversion over Pt-Sn/SBA-15 catalysts as a function of Sn/Pt ratio.

Figure 3

X-ray diffraction patterns of SBA-15 supported Pt and Pt-Sn catalysts prepared by direct reduction: a) Pt/SBA-15, b) 3Pt1Sn/SBA-15, c) 2Pt1Sn/SBA-15, d) 1Pt1Sn/SBA-15, e) 1Pt2Sn/SBA-15, and f) 1Pt3Sn/SBA-15. (▲: Pt, ■: Pt0.96Sn0.4, ●: Pt3Sn, ◆: PtSn).

Figure 4

(A) Pt L3-edge X-ray absorption near edge spectroscopy (XANES) spectra of SBA-15 supported Pt and Pt-Sn catalysts and references: a) 1Pt3Sn/SBA-15, b) 1Pt1Sn/SBA-15, c) 3Pt1Sn/SBA-15, d) Pt/SBA-15, e) Pt foil, and f) PtO2. (B) White line intensities of Pt L3-edge XANES spectra over Pt-Sn/SBA-15 as a function of Sn/Pt ratio (--- relates to the Pt foil reference).

Figure 5

(A) k3-weighted extended X-ray absorption fine structure (EXAFS) oscillation at the Pt L3-edge and (B) their Fourier transforms of Pt/SBA-15 and Pt-Sn/SBA-15 catalysts reduced at 1073 K and references: a) 1Pt3Sn/SBA-15, b) 1Pt1Sn/SBA-15, c) 3Pt1Sn/SBA-15, d) Pt/SBA-15, e) Pt foil ((A): x 1/3, (B): x 1/4)), and f) PtO2 ((A and B): x 1/2).

Figure 6

(A) Sn K-edge XANES spectra of Pt-Sn/SBA-15 catalysts and references. a) 1Pt3Sn/SBA-15, b) 1Pt1Sn/SBA-15, c) 3Pt1Sn/SBA-15, d) Sn foil, and e) SnO2. (B) White line position of Sn K-edge XANES spectra over Pt-Sn/SBA-15 as a function of Sn/Pt ratio (--- relates to the Sn foil, SnO and SnO2 references). 26


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7

(A) k3-weighted EXAFS oscillation at the Sn K-edge and (B) their Fourier transforms of Pt-Sn/SBA-15 catalysts and references: a) 1Pt3Sn/SBA-15, b) 1Pt1Sn/SBA-15, c) 3Pt1Sn/SBA-15, d) Sn foil ((A): x 1/2, (B): x 1/4), and e) SnO2 ((A and B): x 1/2).

Figure 8

X-ray photoelectron spectroscopy spectra of: A) the Pt 4f level and B) the Sn 3d level of a) Pt/SBA-15, b) 3Pt1Sn/SBA-15, c) 1Pt1Sn/SBA-15, and d) 1Pt3Sn/SBA-15.

Figure 9

XPS spectra of: A) the Pt 4f level and B) the Sn 3d level over 1Pt3Sn/SBA-15 (etching time: 0 s (a); 5 s (b); 15 s (c); 25 s (d)).

Figure 10

The surface (Sn4++Sn0)/Pt ratio (A), Sn0/Pt ratio (B) and Sn4+/Pt ratio (C) over 3Pt1Sn/SBA-15 (▲), 1Pt1Sn/SBA-15 (◆) and 1Pt3Sn/SBA-15 (●) as a function of etching time.

Figure 11

Temperature-programmed oxidation profiles over ethylbenzene dehydrogenated spent catalysts (4 h at 773 K): a) Pt/SBA-15, b) 3Pt1Sn/SBA-15, c) 2Pt1Sn/SBA-15, d) 1Pt1Sn/SBA-15, e) 1Pt2Sn/SBA-15 and f) 1Pt3Sn/SBA-15).

Figure 12

The formation of α- and β-coke on Pt/SBA-15 (A) and 1Pt1Sn/SBA-15 (B) during ethylbenzene dehydrogenation as a function of reaction time.

Scheme 1.

The effect of Sn addition on the structure of Pt-Sn/SBA-15 catalysts.

Scheme 2.

The “drain-off” effect of Sn in the 1Pt1Sn/SBA-15 catalyst during the dehydrogenation of ethylbenzene.

27



40

Conversion of ethylbenzene / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 43

30

20

10

0 0

50

100

150

200

250

reaction time / mins Figure 1. Catalytic performance of SBA-15 supported Pt and Pt-Sn catalysts during ethylbenzene dehydrogenation. ( ◯ : Pt/SBA-15; ⦿: 1Pt1Sn/SiO2; ▲: 3Pt1Sn/SBA-15; ◆ : 2Pt1Sn/SBA-15; ●: 1Pt1Sn/SBA-15; ■: 1Pt2Sn/SBA-15; ▼: 1Pt3Sn/SBA-15).

28


Page 29 of 43

40 Conversion of ethylbenzene / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

20

10

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Sn/Pt ratio Figure 2. Initial ethylbenzene conversion over Pt-Sn/SBA-15 catalysts as a function of Sn/Pt ratio.

29



300 f) Intensity / cps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 43

e) d) c) b) a) 36

38

40

42

44

46

48

2θ / degree Figure 3. XRD patterns of SBA-15 supported Pt and Pt-Sn catalysts prepared by direct reduction: a) Pt/SBA-15, b) 3Pt1Sn/SBA-15, c) 2Pt1Sn/SBA-15, d) 1Pt1Sn/SBA-15, e) 1Pt2Sn/SBA-15, and f) 1Pt3Sn/SBA-15. (▲: Pt, ■: Pt0.96Sn0.4, ●: Pt3Sn, ◆: PtSn).

30


Page 31 of 43

1.35

(B)

(A)

0.5

f) e) d) c) b)

1.30 White line intensity (a. u.)

Normalized absorption

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.25

Pt foil 1.20

a)

1.15

11540

0.0

11560 11580 Photon energy /eV

0.5

1.0 1.5 2.0 Sn/Pt ratio

2.5

3.0

Figure 4. (A) Pt L3-edge XANES spectra of SBA-15 supported Pt and Pt-Sn catalysts and references: a) 1Pt3Sn/SBA-15, b) 1Pt1Sn/SBA-15, c) 3Pt1Sn/SBA-15, d) Pt/SBA-15, e) Pt foil, and f) PtO2. (B) White line intensities of Pt L3-edge XANES spectra over Pt-Sn/SBA-15 as a function of Sn/Pt ratio (--- relates to the Pt foil reference).

31



(A)

5

(B)

f)

SnO2

29200 Edge position /eV

e) FT of k χ(k)

d)

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 43

c)

29198 SnO

b) 29196

Sn foil

a) 0.5 0

2

4

6 8 10 -1 k /10 nm

12

1.0

14

1.5 2.0 Sn/Pt ratio

2.5

3.0

Figure 5. (A) k3-weighted EXAFS oscillation at the Pt L3-edge and (B) their Fourier transforms of Pt/SBA-15 and Pt-Sn/SBA-15 catalysts reduced at 1073 K and references: a) 1Pt3Sn/SBA-15, b) 1Pt1Sn/SBA-15, c) 3Pt1Sn/SBA-15, d) Pt/SBA-15, e) Pt foil ((A): x 1/3, (B): x 1/4)), and f) PtO2 ((A and B): x 1/2).

32


Page 33 of 43

(A)

(B)

8

0.5

f)

e)

e)

Normalized absorption

d) k χ(k)

c)

d)

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b)

c) a)

b) a) 29180

29200 29220 Photon energy /eV

29240

0

1

2

3

4

5

6

-1

r / 10 nm

Figure 6. (A) Sn K-edge XANES spectra of Pt-Sn/SBA-15 catalysts and references. a) 1Pt3Sn/SBA-15, b) 1Pt1Sn/SBA-15, c) 3Pt1Sn/SBA-15, d) Sn foil, and e) SnO2. (B) White line position of Sn K-edge XANES spectra over Pt-Sn/SBA-15 as a function of Sn/Pt ratio (--- relates to the Sn foil, SnO and SnO2 references).

33



(A)

10

(B)

8

e)

e)

d)

3

FT of k χ(k)

d) k χ(k)

c)

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 43

c)

b)

b)

a) a)

0

2

4

6

8

10

0

1

-1

k / 10 nm

2

3 4 r / 10 nm

5

6

-1

Figure 7. (A) k3-weighted EXAFS oscillation at the Sn K-edge and (B) their Fourier transforms of Pt-Sn/SBA-15 catalysts and references: a) 1Pt3Sn/SBA-15, b) 1Pt1Sn/SBA-15, c) 3Pt1Sn/SBA-15, d) Sn foil ((A): x 1/2, (B): x 1/4), and e) SnO2 ((A and B): x 1/2).

34


Page 35 of 43

(B)

0

550

0

Pt 4f7/2 74.5

Pt 4f5/2 71.3

(A)

1000 2+

Sn 487.3

d) 0

0

Pt 4f5/2

c) 0

Pt 4f7/2

0

Pt 4f5/2

Intensity /cps

Pt 4f7/2

Intensity /cps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

Sn 486.1

Sn

2+

0

Sn

b) 0

Pt 4f7/2

d)

c)

0

Pt 4f5/2 Sn

2+

Sn

0

b)

a) 80

78

76 74 72 70 68 Binding energy /eV

492

66

490

488 486 484 Binding energy /eV

482

Figure 8. X-ray photoelectron spectroscopy spectra of: A) the Pt 4f level and B) the Sn 3d level of a) Pt/SBA-15, b) 3Pt1Sn/SBA-15, c) 1Pt1Sn/SBA-15, and d) 1Pt3Sn/SBA-15.

35



Etching

(A)

300 0

Pt 4f5/2

0

Pt 4f7/2

15 s

Pt 4f5/2

Sn

0

Pt 4f7/2

c)

0

Pt 4f5/2

Sn

0s

80

0

d)

4+

Sn

0

c)

4+

Sn

Sn

0

b)

Pt 4f7/2

b) 0

Sn

0

5s

Pt 4f5/2

4+

d)

25 s

0

(B)

2.5

Intensity /kcps

time

Intensity /cps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 43

0

Pt 4f7/2

Sn

a)

4+

Sn

75 70 Binding energy /eV

490

0

485 Binding energy /eV

a) 480

Figure 9. XPS spectra of: A) the Pt 4f level and B) the Sn 3d level over 1Pt3Sn/SBA-15 (etching time: 0 s (a); 5 s (b); 15 s (c); 25 s (d)).

36


Page 37 of 43

10

2.0

(A)

8

(B)

Sn / Pt ratio

1.5 0

6

1.0

0

0

0

(Sn +Sn ) / Pt ratio

4

4+

0.5 2 0.0

0 0

0

5 10 15 20 25 Etching time / s

7


(C)

6

0

5 4

4+

Sn / Pt ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3 2 1 0 0


Figure 10. The surface (Sn4++Sn0)/Pt ratio (A), Sn0/Pt ratio (B) and Sn4+/Pt ratio (C) over 3Pt1Sn/SBA-15 (▲), 1Pt1Sn/SBA-15 (◆) and 1Pt3Sn/SBA-15 (●) as a function of etching time.

37



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 11. Temperature-programmed oxidation profiles over ethylbenzene dehydrogenated spent catalysts (4 h at 773 K): a) Pt/SBA-15, b) 3Pt1Sn/SBA-15, c) 2Pt1Sn/SBA-15, d) 1Pt1Sn/SBA-15, e) 1Pt2Sn/SBA-15 and f) 1Pt3Sn/SBA-15).

38


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Page 39 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 12. The formation of α- and β-coke on Pt/SBA-15 (A) and 1Pt1Sn/SBA-15 (B) during ethylbenzene dehydrogenation as a function of reaction time.

39



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 1. The effect of Sn addition on the structure of Pt-Sn/SBA-15 catalysts.

40


Page 40 of 43

Page 41 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 2. The “drain-off” effect of Sn in the 1Pt1Sn/SBA-15 catalyst during the dehydrogenation of ethylbenzene.

41



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 43

Table 1. Physical and CO chemisorption properties of Pt-Sn/SBA-15. Sample[a]

Sn/Pt ratio[b]

SBET 2

CO adsorption −1[c]

/m g

/µmol g

Average particle size

−1[d]

/nm[e]

SBA-15

-

890

-

-

Pt/SBA-15

-

717

37.2

1.5±0.6

Pt-Sn/SBA-15

1/3

647

14.8

1.6±0.9

Pt-Sn/SBA-15

1/2

639

15.5

1.3±0.8

Pt-Sn/SBA-15

1

598

13.6

1.5±0.6

Pt-Sn/SBA-15

2

561

8.8

2.4±1.7

Pt-Sn/SBA-15

3

534

1.7

2.5±2.4

SiO2

-

311

-

-

Pt-Sn/SiO2

1

300

6.5

2.4±1.0

[a] 3 wt.% Pt loading. [b] the initial reagent Sn/Pt ratio. [c] Surface area was determined by the Brunauer-Emmett-Teller model. [d] CO adsorption was measured using CO pulse experiments. [e] Average particle size was calculated from the histograms of ~200 bright dots which showed in high angle annular dark field-scanning transmission electron microscopy images.

Table 2. Selectivity of coke, amount of CO2 formed and CO2 converted ethylbenzene on Pt-Sn/SBA-15. Catalyst [a]

Sn/Pt

Amount of formed

CO2

amount

of Scoke

ratio

CO2/

converted ethylbenzene/

mmol g−1 cat−1[b]

mmol g−1 cat−1[c]

(%)[d]

Pt/SBA-15

0

3.9

150

2.5

Pt-Sn/SBA-15

1/3

2.8

599

0.5

Pt-Sn/SBA-15

1/2

4.7

777

0.6

Pt-Sn/SBA-15

1

4.8

754

0.6

Pt-Sn/SBA-15

2

1.3

372

0.3

Pt-Sn/SBA-15

3

1.0

61

1.5

[a] the spent catalysts after ethylbenzene dehydrogenation at 773 K for 4 h. [b] calculated with catalyst temperature-programmed oxidation profiles. [c] normalizing total CO2 amount to converted ethylbenzene. [d] Scoke(%)=100*amount of formed CO2/CO2 amount of converted ethylbenzene.

42


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Only

Ethylbenzene dehydrogenation to styrene over supported Pt-Sn catalysts could be enhanced by controlling the Pt-Sn alloy molar ratio. The addition of Sn not only improved ethylbenzene conversion and catalyst activity over Pt-Sn/SBA-15, but only enhanced the stability with “drain-off” effect—enabling the migration of coke precursors away from the active metals to SBA-15 silica having a high specific surface area.

43


SBA-15 Catalyst Prepared by Direct

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