Transient Technique for Identification of True Reaction Intermediates

Tokyo, Japan, and Environmental Catalysis and Nanomaterials Laboratory, Department of Chemical Engineering (0211), Virginia Polytechnic Institute ...
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J. Phys. Chem. C 2008, 112, 1115-1123

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Transient Technique for Identification of True Reaction Intermediates: Hydroperoxide Species in Propylene Epoxidation on Gold/Titanosilicate Catalysts by X-ray Absorption Fine Structure Spectroscopy Juan J. Bravo-Sua´ rez,† Kyoko K. Bando,† Jiqing Lu,‡ Masatake Haruta,§ Tadahiro Fujitani,† and S. Ted Oyama*,†,# Research Institute for InnoVation in Sustainable Chemistry, National Institute of AdVanced Industrial Science and Technology, AIST Tsukuba West, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan, Zhejiang Key Laboratory for ReactiVe Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal UniVersity, Jinhua 321004, China, Department of Applied Chemistry, Graduate School of Urban EnVironmental Sciences, Tokyo Metropolitan UniVersity, 1-1 Minami-Osawa, Hachioji 192-0397, Tokyo, Japan, and EnVironmental Catalysis and Nanomaterials Laboratory, Department of Chemical Engineering (0211), Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24061 ReceiVed: September 17, 2007; In Final Form: NoVember 5, 2007

In situ ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy was used in combination with in situ Ti K-edge X-ray absorption near-edge structure (XANES) to study the formation of Ti-hydroperoxo species during the gas-phase epoxidation of propylene with H2 and O2 at reaction conditions over a Au-Ba/Ti-SiO2 (Ti-TUD) catalyst. The in situ UV-vis measurements showed growth of a signal due to Ti-hydroperoxo species when the catalyst was put in contact with H2/O2/Ar (1/1/8) and C3H6/H2/O2/Ar (1/1/1/7) gas mixtures at 423 K and 0.1 MPa. Changes in the area of the pre-edge peak centered at 4968.9 eV present in the Ti K-edge XANES spectra of the catalyst were used to estimate the Ti-hydroperoxo species coverages (θ) under operating conditions. Transient Ti K-edge XANES experiments with H2/O2/Ar (1/1/8) and C3H6/H2/O2/Ar (1/1/1/7) gas mixtures allowed the estimation of the net epoxidation rate by a novel method involving the determination of dθ/dt. It is shown that the Ti-hydroperoxo species are true intermediates because their initial rate of reaction measured from the in situ transient XANES data (3.4 × 10-4 s-1) has the same order of magnitude as the steady-state turnover frequency for propylene epoxidation based on the total Ti (2.5 × 10-4 s-1) measured in a catalytic flow reactor. This is the first use of XANES to measure the turnover rate of a catalyzed reaction.

Introduction Propylene oxide (PO) is a well-known chemical intermediate used in the synthesis of polyether polyols, propylene glycols, and propylene glycol ethers, which are subsequently employed in the preparation of many end products such as polyurethanes, cosmetics, hydraulic, antifreeze and brake fluids, coatings, inks, textile dyes, and solvents.1 Worldwide PO production has been estimated to be over 6.7 million tons in 2003, and its large size highlights PO’s importance in the chemical market.2 In the past decade, there has been an increasing interest in the development of new methods for the production of PO to replace inefficient conventional routes. The trend has been to move from multistage processes producing large amounts of low-value byproducts and coproducts, as in the case of the chlorohydrin and organic hydroperoxide processes, to simpler more direct processes.1,3,4 Examples of the organic hydroperoxide processes include the Sumitomo cumene-recycle process and the Shell styrene-monomer processes.1,3-5 Examples of direct processes are the hydrogen peroxide route, being commercialized by BASF-Dow6 and Degussa-Headwaters7 using titanosilicate * To whom inquiries should be addressed. E-mail: [email protected]. Tel: (540) 231-5309. Fax: (540) 231-5022. † National Institute of Advanced Industrial Science and Technology. ‡ Zhejiang Normal University. § Tokyo Metropolitan University. # Virginia Polytechnic Institute and State University.

catalysts,8,9 and the direct oxygen and hydrogen-oxygen routes being widely studied by many groups. From an environmental and atom efficiency point of view, these latter transformations reduce waste and are desirable,10 and a variety of catalysts have been reported for the direct oxygen route (e.g., Ag/CaCO 3,11-13 Ti/HSZ,14 Ti/SiO2,15 and MoO3/SiO216 catalysts) and hydrogenoxygen routes (e.g., Au/TiO2,17-19 Au/Ti-SiO2,20-26 Ag/TiSiO2,27 and Pd-Pt/Ti-SiO228,29 catalysts). Because of the high selectivity to PO (>90%) and the lower cost of the feedstocks, the hydrogen-oxygen route over Au/Ti-SiO2 catalysts has attracted great attention. In particular, two major catalysts have been developed for PO synthesis with propylene, H2, and O2 consisting of Au supported on microporous (TS-1, Ti/Si ) 1/100)23-26 and mesoporous (amorphous Ti-SiO2, Ti/Si ) 3/100)10,21,22 titanosilicates. Catalytic activity tests have resulted in space-time yields for the Au/TS-1 and the Au/(mesoporous)Ti-SiO2 of 116 and 92 gPO kgcat-1 h-1, respectively, at propylene conversions close to 8%, PO selectivities over 80%, and H2 efficiencies over 20%. These results are quite remarkable since it has been estimated that a commercially viable process would require values of C3H6 conversion >10%, PO selectivity >90%, and H2 efficiency >50%.10 In spite of the major advances in catalyst development for PO synthesis through the hydrogen-oxygen route, not as much work has been carried out to understand the reaction pathways.

10.1021/jp077501s CCC: $40.75 © 2008 American Chemical Society Published on Web 01/05/2008

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Figure 1. Possible sequence of steps for propylene epoxidation with H2 and O2 on Au-supported titanosilicates.

Some theoretical work has been carried out in this area,30-32 and the kinetics of PO synthesis over Au/TS-1 and Au-Ba/ Ti-TUD (mesoporous Ti-SiO2) have been recently investigated.33,34 From these reports, there seems to be agreement that the important steps during PO synthesis consist of (1) synthesis of hydrogen peroxide from hydrogen and oxygen on gold nanoparticles; (2) formation of Ti-hydroperoxo or peroxo species from hydrogen peroxide on tetrahedral Ti centers; (3) reaction of propylene with the Ti-hydroperoxide species to form PO; and (4) decomposition of hydrogen peroxide to water. A schematic of the reaction pathways is shown in Figure 1. The details will be discussed in the main body of the paper. The similarity in the power-rate law expressions for PO synthesis on the Au/(microporous) TS-133 (rPO ) k(H2)0.60(O2)0.31(C3H6)0.18) and the Au-Ba/(mesoporous) TiTUD34 (rPO ) k(H2)0.54(O2)0.24(C3H6)0.36) suggests that the sequence of steps occurring on both catalysts is similar. The reaction rate appears to be determined by two irreversible steps: the production of hydrogen peroxide on a gold site, and the epoxidation of propylene by a hydroperoxide species on a Ti site. Despite these studies, direct experimental evidence supporting this sequence of steps has been lacking. For example, Stangland et al. suggested the involvement of hydroperoxide species on the basis of a D2 kinetic isotope effect found for the PO reaction, but these species were not directly observed.35 Using inelastic neutron scattering (INS), Goodman and coworkers found the presence of hydroperoxide species on a Au/ TiO2 catalyst; however, the measurement conditions (20 K) were far from reaction conditions.36 More recently, Chowdhury et al. reported the presence of Ti-hydroperoxo species on a Au/ (mesoporous) Ti-SiO2 catalyst during in situ ultraviolet-visible (UV-vis) measurements at PO synthesis conditions, but did not confirm that it was a reactive species.37 Although these results support the formation of Ti-hydroperoxide species in the previously mentioned sequence, the sole detection of these species is not sufficient proof that they are true intermediates rather than spectators during the actual reaction.38-41 To demonstrate that these spectroscopically detected species are true intermediates, it is necessary to show that they are reacting at the same rate as the overall rate of reaction.40,42 This is because, in a catalytic cycle, the rate of each step is equal to the overall rate (divided by its stoichiometric number, the

Bravo-Sua´rez et al. number of times the step occurs for each cycle).43,44 Although numerous spectroscopic techniques are able to detect adsorbed species at reaction conditions,38 adequately designed experiments that prove that they are true intermediates are few. Some examples that use spectroscopic techniques to identify intermediates and kinetic methods to establish their mechanistic role include the Raman spectroscopy kinetic studies of the oxidation of acetone with ozone on manganese oxide-supported catalysts,40,42 the UV-vis spectroscopy kinetic studies of the reduction of chromium oxide catalysts with carbon monoxide,45 the reduction oxidation dynamics of VOx/γ-Al2O3 catalysts during propane oxidative dehydrogenation,46,47 and the Fourier transform infrared (FTIR) studies of methanol oxidation on MoO3/SiO2 catalysts.48 In the present study, we carried out in situ UV-vis and in situ X-ray absorption fine structure (XAFS) measurements during the direct propylene epoxidation with H2 and O2 mixtures at reaction conditions on a catalyst consisting of gold supported on a mesoporous titanosilicate (Au-Ba/Ti-TUD, Ti/Si ) 3/100). In situ UV-vis spectroscopy has proved to be useful for the study of PO synthesis at reaction conditions,10,12,13,37 and recent reviews provide useful descriptions of the technique.49,50 In this work, in situ transient UV-vis data are used to identify the formation of Ti-hydroperoxide species and to verify its characteristic behavior as a reaction intermediate. In situ XAFS spectroscopy is another powerful technique that can be used to obtain atom-specific structural and electronic information from solid inorganic catalysts under operating conditions.51-53 The Ti K-edge X-ray absorption near-edge structure (XANES) spectrum of titanosilicates with titanium in framework positions shows a pre-edge peak characteristic of tetrahedral Ti sites,51,52,54,55 and, in this work, changes in this spectral signature are used to monitor adsorption on the Ti sites. Coverage of this tetrahedral Ti is then followed during transient experiments at reaction conditions and analyzed to obtain a rate of reaction. To the best of our knowledge, this study presents the first report in which kinetic estimations are carried out by means of XANES spectroscopy to prove the identity of a reaction intermediate in a catalytic reaction. Experimental Section Materials. The following materials were used without any further purification: tetraethylammonium hydroxide solution (TEAOH, 20% aqueous solution, Merck), tetraethylorthosilicate (TEOS, Wako, 95.0%), tetrabutylorthotitanate (TBOT, TCI, 99.0%), chloroauric acid tetrahydrate (HAuCl4·4H2O, WAKO, 99.0%), Ba(NO3)2 (Chamelon Reagent, 99.0%), and Millipore water (Autopure WEX 3, Yamato). The Ti-TUD (Ti/Si ) 3/100) support was prepared by a modified sol-gel method.56,57 This method comprised the dropwise addition at room temperature of a TEAOH solution (30.2 g) to a clear mixture consisting of TBOT (2.20 g) and TEOS (42.6 g) under vigorous stirring followed by a dropwise addition of water (40 g). The resulting clear yellow solution was aged statically in a Teflon-lined autoclave at 373 K for 3 h, and then to this solution TEA (60 g) was added dropwise under vigorous stirring. The final homogeneous mixture was aged under stirring at room temperature for 24 h, dried at 373 K for 24 h, and then calcined in air at 973 K (1 K min-1) for 10 h. The gold-supported Ba-promoted Ti-TUD catalyst was prepared by a deposition-precipitation method (DP).57 In this method, the pH of HAuCl4‚4H2O (100 mg) in water (100 mL) at 343 K was brought to 8.8 using a 1 M Na2CO3 solution. Then, 1 g of the Ti-TUD support was added, and the suspension was stirred for 0.25 h, followed by the addition of Ba(NO3)2

True Intermediates in Propylene Epoxidation (50 mg) in water (5 mL), and stirring was continued for another 0.75 h at 343 K. Solids were filtered out, washed with water (100 mL), dried under vacuum at room temperature overnight, and calcined in air at 573 K (1.8 K min-1) for 4 h. The asprepared catalyst is designated as Au-Ba/Ti-TUD (9), where the (9) refers to the pH of preparation. Catalytic Reactivity Testing. Propylene epoxidation experiments were conducted at 423 K using C3H6 (Takachiho Chemical, g99.8%), H2 (from a hydrogen generator, OPGU2100S Shimadzu, g99.99%), and O2 (Tomoe Shokai, g99.5%) diluted with Ar (Suzuki Shokan, g99.9997%). In a typical run, 0.2 g of the Au-Ba/Ti-TUD (9) catalyst powder sample was loaded in a 0.8 mm OD quartz reactor, and the temperature was raised to 523 K (10 K min-1) under Ar flow (35 cm3 min-1) at 0.1 MPa, followed by a pretreatment with 10 vol % H2 in Ar for 0.5 h, and 10 vol % O2 in Ar for 0.5 h. After this pretreatment, the sample was cooled to the reaction temperature (423 K) under Ar flow. The flow rates of reactants (35 cm3 min-1) were in a ratio of C3H6/H2/O2/Ar ) 1/1/1/7, at a space velocity of 10 500 cm3 h-1 gcat-1. The absence of mass and heat transfer limitations was checked by means of the WeiszPrater and Mears criteria.58,59 A detailed description of the analytical system can be found elsewhere.57 Spectroscopic Measurements. In situ UV-vis spectra were collected using a large-compartment spectrometer (Varian Cary 5000). Samples in powder form were loaded in a reaction chamber (Harrick Scientific, model HVC-DRP) provided with a praying mantis diffuse reflectance attachment (DRP-XXX). Reaction conditions such as temperature, pressure, and gas flow rates were the same as those for the reactivity measurements. The temperature in the reaction chamber was followed by a thermocouple added to the Harrick cell just under the center of the sample. This thermocouple tracks the catalyst surface temperature and accounts for possible temperature differences with the control thermocouple mounted on the sample cup. Spectra were taken in the 2.0-6.2 eV (600-200 nm) range at a scan rate of 600 nm min-1, an averaging time of 0.1 s, and a data interval of 1 nm. For in situ UV-vis measurements at a given energy (wavelength), the Cary WinUV kinetics application was employed. Data was collected at 3.76 eV (330 nm), a spectral bandwidth (SBW) of 1 nm, a signal averaging time of 0.1 s, and a cycle time between 0.5 and 1 s. Three different sets of in situ experiments on the Au-Ba/Ti-TUD (9) catalyst at reaction conditions (423 K and 0.1 MPa) were carried out: (Set 1) Measurement of UV-vis spectra in the 2.0-6.2 eV (600-200 nm) range under H2 and O2 (H2/O2/Ar ) 1/1/8) for 8 h, and under C3H6, H2, and O2 (C3H6/H2/O2/Ar ) 1/1/1/7) for 8 h. (Set 2) Measurement of UV-vis intensity at constant energy (3.76 eV) under H2 and O2 (H2/O2/Ar ) 1/1/8) for 600 s, and under C3H6, H2, and O2 (C3H6/H2/O2/Ar ) 1/1/1/7) for 600 s. (Set 3) Measurement of UV-vis intensity at constant energy (3.76 eV) under consecutive H2/O2/Ar (1/1/8), Ar, and C3H6/ H2/O2/Ar (1/1/1/7) for 8 h each. Spectra were referenced to the same material under argon just before reaction started (difference spectra). Diffuse reflectance spectra were analyzed using the Kubelka-Munk function, F(RR), calculated from absorbance data.60 Curve fitting analyses were made with Gaussian-type peaks using the software PeakFit, version 4.12. In situ Ti K-edge XAFS measurements were carried out at beamline 9A of the Photon Factory in the Institute of Materials Structure Science, High-Energy Accelerator Research Organization (PF-IMSS-KEK) in Japan. All spectra were obtained in

J. Phys. Chem. C, Vol. 112, No. 4, 2008 1117 transmission mode using an in situ XAFS cell provided with a flow delivery system. Pretreatment and reaction conditions such as temperature, pressure, and gas flow rates were monitored and controlled from outside the radiation shield hutch. Two different experiments at reaction conditions were conducted: one in the presence of H2 and O2 (H2/O2/Ar ) 1/1/8) and another in the presence of C3H6, H2, and O2 (C3H6/H2/O2/Ar ) 1/1/1/ 7). XAFS spectra were obtained every 360 s in a step-scanning mode using the same pretreatment and reaction conditions as those used in the catalytic testing reactor with helium as the inert gas, at a total gas flow rate of 35 cm3 min-1. Analysis of XANES data was conducted with commercially available software (REX, Rigaku Co.). The titanium K-edge position of the sample was calibrated by setting the measured edge position of a titanium foil to a standard value (4964.8 eV). Ex situ Au LIII-edge XAFS measurements were also carried out at beamline 9A in transmission mode for the fresh and used catalyst at ambient conditions. In situ FTIR measurements were carried out with a JASCO FT/IR-610 spectrometer provided with an MCT detector. Samples were loaded in an in situ IR cell provided with KBr windows and equipped with KBr rods in the free space inside the cell to minimize gas contributions to the FTIR spectra. Sample wafers (diameter ) 1 cm) were prepared from 15 mg of material pressed at about 30 MPa. Pretreatment and reaction conditions (C3H6/H2/O2/Ar ) 1/1/1/7) were identical to those used in the catalytic testing reactor. Spectra were recorded at a 4 cm-1 resolution and averaged over 100 scans. High-resolution transmission electron microscopy (TEM) images of the Au-Ba/Ti-TUD (9) catalyst were obtained in a microscope (Philips CM200 UT) operated at 200 kV. Results Catalyst Preparation and Activity Measurements. The catalyst used in this study was identical to that studied by Lu et al.:57 a barium-promoted gold catalyst supported on a mesoporous titanosilicate (Ti/Si ) 3/100) prepared by DP at a pH of 9 (Au-Ba/Ti-TUD (9)). From ex situ Au LIII-edge XAFS measurements (Supporting Information) on the fresh catalyst and the catalyst after reaction, several observations can be made: (1) the catalyst presented a coordination number (CN) corresponding to a particle size (Dp) of 0.9 nm. Gold particles with a diameter of 0.9 nm contain about 28 gold atoms and have a dispersion close to 95%,61,62 and an exposed gold content of 5.3 µmol g-1; (2) the fresh catalyst contained cationic gold, which was partly reduced to metal under reaction conditions. The gold and barium content are estimated at 0.11 and 2.4 wt %, respectively, from inductively coupled plasma analysis.57 The total titanium and gold content for this catalyst are 476 and 5.3 µmol g-1, respectively. A high-resolution TEM image of the Au-Ba/Ti-TUD (9) catalyst shows the amorphous structure of this catalyst (Supporting Information). In this image, gold particles are hardly discernible, but this confirms the size of the small particles estimated from EXAFS measurements. The UV-vis spectra of the Ti-TUD (Ti/Si ) 3/100) support before and after contact with water vapor and aqueous hydrogen peroxide are presented in Figure 2. For the fresh support (Figure 2a), a broad band (200-300 nm) centered at 5.54 eV (224 nm) is observed that has been related to the presence of tri- and tetrapodal tetrahedral Ti sites.63 For the Ti-TUD support contacted with H2O in Ar at reaction temperature (Figure 2b), a broad feature (240-310 nm) around 4.58 eV (270 nm) develops, which has been ascribed to water adsorbed on Ti sites.64 For the support in contact with hydrogen peroxide

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Figure 2. UV-vis spectroscopy results for Ti-TUD (Ti/Si ) 3/100) support (a) fresh, referenced to Ba(SO)4; (b) contacted with H2O(3.5%)/ Ar at 423 K for 8 h, referenced to fresh sample under Ar at 423 K; and (c) contacted with H2O2 35%, referenced to the fresh sample.

Figure 3. Catalytic activity for Au-Ba/Ti-TUD (9) during propylene epoxidation with H2 and O2: (a) PO space-time yield (STY), (b) propylene conversion, and (c) PO selectivity. Reaction conditions: C3H6/H2/O2/Ar ) 1/1/1/7, space velocity 10500 cm3 h-1 gcat-1, 423 K, and 0.1 MPa.

(Figure 2c), a broad band (280-480 nm) centered at 3.64 eV (340 nm) is visible. Deconvolution of this feature shows the presence of three bands. Two major bands centered at 3.41 (364 nm) and 3.80 eV (326 nm) have been related to Ti-hydroperoxo species.63-67 An additional band at 4.21 eV (290 nm) is due to water coordinated to the Ti centers.63 There is no tetrahedral Ti signal (200-300 nm) in Figure 2b because the reference material was the Ti-TUD support. The catalytic performance over a period of 8 h for the AuBa/Ti-TUD (9) catalyst is presented in Figure 3. The conversion starts at about 2%, and drops during the first 3-4 h to reach a constant value of about 1%. The PO selectivity remains high (∼93%) and is relatively constant. Steady-state is reached after about 5 h of reaction, giving a space-time yield constant value of 25 gPO kgcat-1 h-1. The steady-state PO turnover rate based on total Ti is 2.5 × 10-4 s-1, and that based on surface gold atoms is 2.2 × 10-2 s-1. The water formation rate at steadystate conditions is about 2400 mmol kgcat-1 h-1, which corresponds to a turnover rate based on surface gold atoms of 1.3 × 10-1 s-1.

Bravo-Sua´rez et al.

Figure 4. In situ UV-vis spectroscopy results for Au-Ba/Ti-TUD (9) under H2/O2/Ar ) 1/1/8 gas mixture at 423 K and 0.1 MPa as a function of reaction time. Reference is sample at 423 K under Ar flow before reaction.

Figure 5. In situ UV-vis spectroscopy results for Au-Ba/Ti-TUD (9) under propylene epoxidation conditions (C3H6/H2/O2/Ar ) 1/1/1/ 7) at 423 K and 0.1 MPa as a function of reaction time. Reference is sample at 423 K under Ar flow before reaction.

In Situ UV-vis Spectroscopy Measurements. Figures 4 and 5 show the results for the Set 1 in situ UV-vis experiments. Figure 4 presents the in situ UV-vis results for the Au-Ba/ Ti-TUD (9) catalyst under reaction conditions in the presence of the H2/O2/Ar (1/1/8) gas mixture for 8 h. As observed in this figure, a broad feature arose in the 3.1-4.6 eV region (400270 nm), whose intensity increased with reaction time. Deconvolution of this feature (at 600 s) shows the presence of a major band centered at 3.82 eV (325 nm) assigned to Ti-hydroperoxo species, and two minor bands centered at 3.96 eV (313 nm) and 4.18 eV (297 nm) assigned to water coordinated to Ti sites. Figure 5 shows the in situ UV-vis results for the Au-Ba/TiTUD (9) catalyst under reaction conditions in the presence of the C3H6/H2/O2/Ar (1/1/1/7) gas mixture during 8 h. Also in this case, a wide peak in the 3.0-4.7 eV (400-264 nm) range was formed. This peak is similar to the one formed with the H2/O2/Ar (1/1/8) gas mixture, but is of lower intensity and broader. Deconvolution of the peak (at 600 s) yields a major band centered at 3.82 (325 nm) assigned to Ti-hydroperoxo

True Intermediates in Propylene Epoxidation

Figure 6. In situ UV-vis spectroscopy results at 3.76 eV (330 nm) for Au-Ba/Ti-TUD (9) under (a) C3H6/H2/O2/Ar ) 1/1/1/7 (bottom) and (b) H2/O2/Ar ) 1/1/8 (top) at 423 K and 0.1 MPa during the first 600 s of reaction. Reference is sample at 423 K under Ar flow before reaction.

Figure 7. In situ UV-vis spectroscopy results at 3.76 eV (330 nm) for Au-Ba/Ti-TUD (9) under consecutive gas mixtures of (a) H2/O2/ Ar ) 1/1/8 (left); (b) Ar (middle); and (c) C3H6/Ar ) 1/9 (right) at 423 K and 0.1 MPa for 8 h each. Reference is sample under Ar flow before reaction.

species, and two minor bands centered at 4.04 eV (307 nm) and 4.51 eV (275 nm) ascribed to water coordinated to Ti centers. Figure 6 presents the results for the Set 2 experiments: in situ UV-vis measurements at constant energy (3.76 eV) during the first 600 s of reaction under H2/O2/Ar (1/1/8) and C3H6/ H2/O2/Ar (1/1/1/7) gas mixtures. The intensity in both cases initially increased quickly, but later slowly. However, the observed intensity was appreciably larger under the H2/O2/Ar mixture than under the C3H6/H2/O2/Ar gas mixture. Figure 7 shows the results for the Set 3 experiments: in situ UV-vis measurements at constant energy (3.76 eV) at reaction conditions for 24 h under consecutive H2/O2/Ar (1/1/8), Ar, and C3H6/Ar (1/9) gas mixtures. During the first part of the run (Figure 7a), the H2/O2/Ar (1/1/8) gas mixture was introduced for 8 h. Initially, the intensity increased rapidly, but this increase was slower after about 2 h. In the second part of the run (Figure 7b), a treatment with Ar for 8 h at the same temperature, pressure, and space velocity was started. During the initial 2 h, the intensity dropped quickly, followed by a much slower reduction rate. After 8 h, the intensity was about 30% of the maximum obtained during the H2/O2/Ar flow. In the last part of the experiment (Figure 7c), the C3H6/Ar (1/9) gas mixture

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Figure 8. In situ Ti K-edge XANES spectroscopy results for AuBa/Ti-TUD (9) under propylene epoxidation conditions (C3H6/H2/O2/ Ar ) 1/1/1/7) at 423 K and 0.1 MPa as a function of reaction time.

Figure 9. Coverage on 4-fold coordinated Ti (θ, fractional decrease of pre-edge peak area in the Ti K-edge XANES spectra) for Au-Ba/ Ti-TUD (9) under (a) C3H6/H2/O2/Ar ) 1/1/1/7 (bottom) and (b) H2/ O2/Ar ) 1/1/8 (top) at 423 K and 0.1 MPa during the first 900 s of reaction.

was introduced for 8 h. The initial intensity first decreased rapidly, and then more slowly. In Situ XAFS Spectroscopy Measurements. Figure 8 presents the in situ Ti K-edge XANES spectra for the Au-Ba/ Ti-TUD (9) catalyst under reaction conditions with C3H6/H2/ O2/Ar (1/1/1/7) as a function of time. In situ Ti K-edge XANES results with the H2/O2/Ar (1/1/8) mixture are similar and not shown for the sake of brevity. The Ti pre-edge peak is centered at 4968.9 eV and decreases in intensity with reaction time. The position is characteristic of 4-fold coordinated Ti.54,55 The fractional decrease in the pre-edge peak area from its value before the introduction of the reactants is defined as the coverage, θ, on the Ti centers because the decrease in area is a result of an increase in the coordination of Ti. Figure 9 shows the increase in coverage during the first 900 s of reaction with H2/O2/Ar (1/1/8) and C3H6/H2/O2/Ar (1/1/1/7). The rate of increase of coverage is faster with H2/O2/Ar than with C3H6/ H2/O2/Ar. The slopes at time zero for the H2/O2/Ar and C3H6/ H2/O2/Ar transient curves are 7.1 × 10-4 and 3.7 × 10-4 s-1, respectively. In Situ FTIR Spectroscopy Measurements. In situ FTIR measurements at reaction conditions on the working Au-Ba/ Ti-TUD (9) catalyst are presented in Figure 10. Bands arising at 2980, 2939, and 2880 cm-1 are assigned to the C-H

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Figure 10. In situ FTIR spectroscopy results for Au-Ba/Ti-TUD (9) under propylene epoxidation conditions (C3H6/H2/O2/Ar ) 1/1/1/ 7) at 423 K and 0.1 MPa as a function of reaction time (in h): (a) 0, (b) 0.25, (c) 1, (d) 2, (e) 3, (f) 4, (g) 5, and (h) in Ar purge for 900 s.

stretching vibrations of bidentate propoxy species, which can result from PO decomposition on acidic Ti sites.68 The bands at 1572 and 1422 cm-1 are due to the CO stretching bands of formate and acetate species, the band at 1720 cm-1 can be assigned to the carbonyl stretch of adsorbed propanal, and the band at 1380 cm-1 comes from the symmetric bending of CH3 groups (Supporting Information).68,69 Figure 10 shows that, after 5 h of reaction, the bidentate species and the organic fragments detected by FTIR remained adsorbed after purging with argon for 900 s at 423 K. From in situ FTIR difference spectra (Supporting Information), it can be seen that bands at 3735 and 3375 cm-1 and a shoulder around 1643 cm-1 are formed under reaction conditions. The band at 3735 cm-1 can be assigned to the OH stretching vibration of isolated surface silanol groups, with some contribution from Brønsted OH groups close to Ti Lewis acid sites.70 The broad band at 3375 cm-1 has been related to H-bonded OH groups, which could possibly be associated with the presence of hydroperoxy species.71 An absorption band due to a peroxidic OO stretching mode (∼840 cm-1) related to the presence of hydroperoxy species was, however, not observed, probably because of the low surface concentration of the hydroperoxo species at the reaction conditions and because of a strong absorption at low wavenumbers (