Au/TiO2 晶相与形貌的影响

r n Applied Catalysis B: Environmental 91 (2009) 470–480 cr ru n in nd xi d io Contents lists available at ScienceDirect Applied Catalysis B l 1. Introduction Oxide supported Au catalysts have been demonstrated to be highly active already at low temperatures for various oxidation and reduction reactions,mostprominently theCOoxidation reaction [1– 3], which in turn is of interest in environmental catalysis applications such as the preferential oxidation of CO in H2 feed gases for fuel cells. It turned out, that in addition to other properties, the nature of the supporting oxide has a significant effect on the activity of the catalyst [4–7]. Recently, mesoporous oxides were introduced as alternative support materials, which may be attractive, e.g., because of their high surface area or the stabilization of the Au nanoparticles in the mesopores of the support [7–15]. For example, Au catalysts supported on mesoporous aluminosilicates showed a higher COoxidation activity [9–11] thanAu/SiO2 catalysts [6,16]. Similar observations of a higher activity were reported also for the much more active mesoporous Au/TiO2 catalysts [8,12– 15,17], when comparing with highly active Au/TiO2 catalysts based on non-porous titania [2,18–24]. Comparing different mesoporous Au catalysts, Overbury et al. found a much higher CO oxidation activity on Au/TiO2 than on Au/SiO2 in temperature screening measurements [7]. Furthermore, it was reported that also the crystalline phase of the (non-porous or mesoporous) TiO2 support influences the COoxidation activity [13,25–27]. Schwarz et al. found that the activity of non-porous Au/TiO2 catalysts decreased from brookite via anatase to rutile for catalysts activated by calcination at 300 8C, while after reduction at 150 8C the activities were compar- able [25], and similar observations were reported by Yan et al. for mesoporous TiO2 supported catalysts [26]. These authors explained this by an increasing tendency for Au particle sintering during calcinationat300 8C intheorderbrookite < anatase< P25(anatase/ rutile)� rutile Au/P25 (non-porous rutile/ anatase) > Au/rutile (mesoporous) [13]. In a comparative study on the activity of porous and non-porous Au/TiO2 catalysts prepared from different crystalline phases of TiO2 (anatase, rutile, P25), Comotti et al. concluded that the crystalline phase has no significant influence on the catalytic properties of the unconditioned catalysts [27]. This was different when the catalysts were calcined before reaction,where the P25 based catalyst showeda significantlyhigher thermostability than catalysts supported on anatase or rutile.While after calcination at 250 8C, the activities are comparablewith that of the unconditioned catalysts and similar for all catalysts, calcination at 350 8C leads to lower activities for the (mesoporous) rutile or (mesoporous) anatase supported catalysts, while the P25 supported catalyst retains its activity. The decay in activity, however, is not caused by Au nanoparticle sintering, since the mean particle size of �3.0 nm was identical for 250 and 350 8C calcination and all catalysts.On theotherhand,Wangetal. reporteda significanthigher Deactivation Au/TiO2 CO oxidation � 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +49 731 50 254 50; fax: +49 731 50 254 52. E-mail address: juergen.behm@uni-ulm.de (R.J. Behm). 0926-3373/$ – see front matter � 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2009.06.016 Influence of the crystalline phase and su the CO oxidation activity of mesoporous Y. Denkwitz a, M. Makosch a, J. Geserick b, U. Ho¨rma a Institute of Surface Chemistry and Catalysis, Ulm University, D-89069 Ulm, Germany b Institute of Inorganic Chemistry I, Ulm University, D-89069 Ulm, Germany cCentral Facility of Electron Microscopy, Ulm University, D-89069 Ulm, Germany A R T I C L E I N F O Article history: Received 13 March 2009 Received in revised form 15 June 2009 Accepted 18 June 2009 Available online 24 June 2009 Keywords: Mesoporous catalysts Activity A B S T R A C T The influence of the TiO2 deactivation behavior of st loading in the CO oxidatio reaction conditions and by were controlled by the pH a loading of themesoporous o catalysts. The resulting tren stability/deactivation behav journa l homepage: www.e face area of the TiO2 support on Au/TiO2 catalysts n c, S. Selve c, U. Kaiser c, N. Hu¨sing b, R.J. Behma,* ystalline phase and of the surface area on the activity and stability/ cturally well-defined mesoporous Au/TiO2 catalysts with comparable Au reaction was investigated by kinetic measurements under differential situ DRIFTS. The crystalline phase and surface area of the TiO2 substrate the type of the structure-directing surfactants applied in the synthesis. Au des was performed by the same deposition–precipitation procedure for all s in the CO oxidation behavior, including the TOF based activities and the r are discussed. : Environmental sev ier .com/ locate /apcatb Table 1 TiO2 crystallite size, BET surface area, pore diameter, pore volume, Au loading and weight loss (TGA) of Au/TiO2 catalysts based on different (mesoporous) support materials. Catalyst TiO2 support synthesized with pH Crystalline phase TiO2 crystallite size (nm) BET surface area (m2 g�1) Pore diameter (nm) Pore volume (cm3 g�1) Au loading (wt.%) Weight loss <500 8C (wt.%) Weight loss <990 8C (wt.%) Au/TiO2(P25) – – Anatase/rutile 21/30 58 – – 3.1 2.7 2.8 Au/TiO2(1) Brij56, EGMT 2 Anatase 9.9 106 7.5–8.2 0.18 3.0 4.6 4.8 Au/TiO2(2) Brij56, EGMT 2 Anatase 9.9 106 7.5–8.2 0.18 2.6 3.9 4.1 Au/TiO2(3) SDS, EGMT 2 Anatase 6.6 175 4.4–6.3 0.28 2.6 8.5 9.2 Au/TiO2(4) SDS, EGMT �1.2 Anatase/rutile 5.6 240 3.5 0.32 4.2 11.0 11.8 Au/TiO2(5) SDS, EGMT 0 Rutile 6.7 160 a 0.22 4.8 6.8 9.3 a Not well defined. Y. Denkwitz et al. / Applied Catalysis B: Environmental 91 (2009) 470–480 471 activity of Au catalysts based on mesopous rutile (5% anatase) after calcination at 500 8C compared to Au/P25 [14]. Moreau et al. reported a higher activity of a non-conditioned Au/rutile catalyst (100 m2 g�1) compared to a similarly treated Au/anatase catalyst (90 m2 g�1) [24]. These authors also studied the influence of the surface area of the anatase support on the CO oxidation activity, and found that the activity decreases with increasing surface area in the order 37 m2 g�1 > 45 m2 g�1 > 90 m2 g�1 > 240 m2 g�1 > 305 m2 g�1. Comparable observations of a decreasing activity with increasing BET surface area were reported also by Zhu et al. for mesoporousAu/TiO2 catalysts, comparing a55m 2 g�1 catalyst anda 141 m2 g�1 catalyst [15]. Summarizing these results, theactivityand stability of mesoporous Au/TiO2 catalysts seem to be affected by various properties, including (i) the size of the oxide crystallites and Au nanoparticles and the respective surface areas, (ii) the structure and size of the oxide pores, and (iii) the crystalline phase of the TiO2 support (anatase/rutile). Clear trends, however, can hardly be extracted from these data because of the wide variations in parameters. For a more detailed understanding, we started a systematic study on the CO oxidation behavior of Au catalysts based on mesoporous TiO2 whose structural properties were systematically varied. Recently we reported that mesoporous Au/TiO2 catalysts prepared by sol–gel processing and subsequent Au deposition on the mesoporous TiO2 via a deposition–precipitation procedure show a higher CO oxidation activity than catalysts prepared in a similar way from a commercially available (P25, Degussa) non- porous support, with a comparable Au particle size and Au metal loading [12]. In the present paper, we report on the influence of the TiO2 crystalline phase of mesoporous Au/TiO2 catalysts on their CO oxidation activity and deactivation behavior. Mesoporous TiO2 materials with high surface areas and of different crystalline phases were synthesized via sol–gel processing as described in Ref. [28]. For comparison, we also include a standard Au/TiO2 catalyst based on a P25 support. The structure and surface composition of the catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) (Section 3.1). Stimulated by our recent finding of a pronounced effect of a catalyst drying step prior to catalyst Table 2 Mean Au particle size, Au(4f):Ti(2p) intensity ratio, absolute and relative activity for C calcination in 10% O2/N2 at 400 8C (30 min) (N15-O400) and after re-calcination in 10% Catalyst Au loading (wt.%) Au-particle size (nm) Au(4f):Ti(2p) ratio Initial/fina (�10�4 mo Au/TiO2(P25) 3.1 2.9 � 0.8 0.13 34.3/9.4 Au/TiO2(1) 3.0 4.1 � 0.8 0.05 84.1/27.9 Au/TiO2(2) 2.6 3.7 � 0.9 0.04 87.1/25.7 Au/TiO2(3) 2.6 3.0 � 0.7 0.04 37.2/12.4 Au/TiO2(4) 4.2 6.1 � 1.3 0.04 33.9/6.0 Au/TiO2(5) 4.8 6.1 � 1.5 0.03 34.8/9.2 conditioning on the CO oxidation behavior of P25 supported Au/TiO2 catalysts [29], we also evaluated the influence of a similar catalyst pre-treatment on the reaction behavior (Section 3.2). The activities as well as the deactivation and re-activation behavior during CO oxidation were evaluated by kinetic measurements under differential reaction conditions (Section 3.3). Mechanistic informationwas obtained from in situ IRmeasurements performed under similar reaction conditions, employing diffuse reflectance IR spectroscopy (DRIFTS). 2. Experimental set-up and procedures 2.1. Preparation of the mesoporous TiO2 support and the Au catalysts The mesoporous TiO2 support material was prepared via an ethylene glycol-modified titanium precursor (bis(2-hydro- xyethyl)titanate, EGMT) [28]. EGMTwas synthesized bymodifying a procedure described by Xia et al. [30]. For the catalysts (1) and (2), the mesoporous TiO2 support was synthesized by dissolving polyethylene oxide hexadecylether (Brij56) in dilute hydrochloric acid (pH 2). Subsequently, EGMT was added, and the solution was ultrasonicated for 5 h at 60 8C. The resulting suspension was aged at 60 8C for 24 h, and then centrifuged. The precipitate was washed withwater three times and subsequently calcined at 400 8C for 4 h. The mesoporous support for the catalysts (3)–(5) was synthesized with sodium dodecyl sulfate (SDS) and EGMT to obtain higher surface areas. The different polymorphs of titania (anatase and rutile) were accessible by variation of the pH-value (see Table 1). The Au/TiO2 catalysts were prepared via a deposition– precipitation procedure described in [23,31]. In short, TiO2 was suspended in water at 60 8C at a pH of 5–5.5, followed by addition of tetrachloroauric acid (HAuCl4�4H2O). A constant pH value was maintained by addition of Na2CO3 solution. After additional 30 min of stirring, the precipitate was cooled to room temperature, filtered, washed and dried overnight at room temperature under vacuum. The catalysts were stored in the dark at �10 8C. The Au metal loading (Table 2) was determined by inductively coupled plasma atom emission spectroscopy (ICP-AES). If not mentioned otherwise, the raw catalysts were dried in 20 N ml min�1 N2 at 100 8C for 15 h, then calcined (10% O2 in N2) at 400 8C for 30 min, O oxidation of various Au/TiO2 catalysts after drying in N2 at 100 8C (15 h) plus O2/N2 at 400 8C (30 min). l activity l s�1 gAu �1) Initial/final TOF (s�1) Activity after 1000 min reaction (%) Initial activity after re-calcination (%) 2.0/0.5 28 n.m. 6.4/2.1 33 n.m 6.3/1.9 30 n.m. 2.2/0.7 33 94 4.0/0.7 18 82 4.2/1.1 26 84 Administrator 打字机 Administrator 打字机 Administrator 打字机 乙二醇钛 Administrator 打字机 Administrator 打字机 Administrator 打字机 Administrator 打字机 Administrator 打字机 Administrator 打字机 Administrator 打字机 Administrator 打字机 Administrator 打字机 adsorbate concentration [36]. For possible deviations from this linear behavior see [37]. Background subtraction and normal- ization of the spectra was performed by subtracting spectra recorded in a flow of N2 at the reaction temperature directly before starting CO oxidation. To remove changes in the reflectivity of the respective catalysts, the spectra were scaled to similar background intensity at 2430 cm�1, which did not interfere with any other signals and where the shape of the raw spectrum changed little during the measurements. The gas phase CO signal was removed by subtracting the spectral region of gas phase CO (2040– 2240 cm�1) from a spectrum recorded on pure a-Al2O3 in CO containing atmosphere, which was verified in test experiments to not adsorb CO under present reaction conditions. 3. Results and discussion 3.1. Catalyst surface properties In the following, we present and discuss results on the structure and surface composition of the different Au/TiO2 catalysts, which were obtained by different techniques including X-ray diffraction, Y. Denkwitz et al. / Applied Catalysis B: Environmental 91 (2009) 470–480472 and finally cooled down to the reaction temperature in N2 before the reaction measurement. The concentration of Cl was below the detection limit of the ICP-AES and XPS measurements. 2.2. Catalyst characterization The surface area and the pore diameter of the mesoporous support material and of the different catalysts were determined by N2 sorption measurements (Autosorb MP1, Quantachrome). The resulting surface areas were calculated using the Brunauer– Emmett–Teller (BET) relation in the p/p0 range of 0.05–0.30. The pore size distributionwas evaluated from the desorption branch of the isotherms, using the procedure developed by Barrett, Joyner and Halenda (BJH) [32]. XRD measurements were performed on a PANalytical MPD PRO instrument, using Cu-Ka radiation (l = 0.154 nm) to evaluate the titania structure and the titania particle sizes. The size and particle distribution of the Au nanoparticles were derived from TEM images obtained in a Philips CM 20 instrument (200 kV). Typically, several hundred Au nanoparticles (>400) were evaluated per sample for determining the size distribution of the Au nanoparticles. The relative surface concentrations and the oxidation states of Au, Ti, and O were determined by X-ray photoelectron spectroscopy, using mono- chromatized Al-Ka radiation for excitation (PHI 5800 ESCA system, Physical Electronic). The binding energies (BEs) were calibrated with respect to the bulk Ti(2p3/2) signal at 459.2 eV for Ti 4+ [33]. Subtraction of a Shirley background and peak fitting were performed using a public domain XPS peak fit program (XPSPEAK4.1 by R. Kwok). Thermogravimetry (TGA) measure- ments were performed on a STA 449C Jupiter instrument (Netsch Gera¨tebau GmbH). Finally, temperature programmed desorption measurements were performed in a quarz tube micro reactor, which was connected to a mass spectrometer (Atomica IMR-MS 1500). TPD spectra were recorded in a N2 stream with a temperature ramp of 5 8C min�1, starting at 30 8C. 2.3. Activity measurements The catalytic activity of the catalysts was determined in reaction measurements performed at atmospheric pressure in a quartz tube micro-reactor under differential reaction conditions, using typically 65–70 mg diluted catalyst powder. In order to limit the conversion to values between 5 and 20%, the catalyst was diluted with a-Al2O3, which is not active for CO oxidation under present reaction conditions. The measurements were carried out with a gas flow of 60 N ml min�1 in 1 kPa CO, 1 kPa O2 and balance N2 at 80 8C. Incoming and effluent gases were analyzed by on-line gas chromatography (Dani GC 86.10HT), using H2 as carrier gas. High-purity reaction gases (CO: 4.7, O2: 5.0 and N2: 6.0; Westphalen) were passed through a moisture filter (Varian GC- MS filter CP 17973) to ensure water concentrations of below 0.1 ppm. The reaction rates were determined from the CO2 partial pressure; for further information on experimental details see ref. [34].With rates below 10�5 mol s�1 cm�3, mass and heat transport problems were negligible [35]. 2.4. Infrared measurements In situ IR measurements were performed by diffuse reflectance infrared fourier transform spectroscopy using a commercial in situ reaction cell (Harricks, HV-DR2). The spectra were recorded in a Magna 560 spectrometer (Nicolet) equipped with a MCT narrow band detector. About 20 mg of a-Al2O3 diluted catalyst (dilution 1:2) were used as catalyst bed. Typically, 400 scans (acquisition time 3 min) were co-added per spectrum. The intensities were evaluated in Kubelka-Munk units, which are linearly related to the N2 sorption, thermogravimetric analysis, infrared spectroscopy, transmission electron microscopy as well as X-ray photoelectron spectroscopy. Based on the characteristic reflections in the diffractograms (a) and (b) of the as-synthesized catalysts in Fig. 1, the synthesis of mesoporous TiO2 with EGMT and Brij56 or SDS at pH 2 results in anatase. In contrast, for TiO2 synthesized at pH < 0, only the characteristic rutile reflections are observed (Fig. 1, diffractogram (d)), while for synthesis at pH � 1.2 reflections of anatase and rutile coexist (Fig. 1, diffractogram (c)). Obviously, the resulting crystal- line phase of TiO2 depends sensitively on the pH during the synthesis (see also Table 1) [28], in addition to the choice of the titania precursor and the temperature [38]. Additionally, a catalyst supported on commercial, non-porous TiO2 (P25, Degussa) was characterized (Fig. 1e), which shows reflections of anatase and rutile [26]. Based on the width of the reflections, the crystallite sizes of the mesoporous supports are rather small, about 10 nm for the TiO2 obtained when using Brij56 as surfactant and about 6– 7 nmwhen using SDS, independent of the crystalline phase of TiO2. Since no Au reflections were observed in the raw catalysts by XRD, the Au particles are too small for detection by XRD. For the Au/ TiO2(1) and Au/TiO2(2) catalysts, which are based on the same support material, we obtained BET surface areas of 106 m2 g�1 and Fig. 1. XRD pattern of the different as-synthesized Au/TiO2 catalysts: (a) Au/TiO2(2), which is supported on the same material as catalyst (1), (b) Au/TiO2(3), (c) Au/ TiO2(4), (d) Au/TiO2(5), (e) Au/TiO2(P25). pore diameters of 8 nm. The interaction of SDS as the structure- directing agent in the synthesis protocol with the growing titania species is due to its ionic character completely different from that of the previously applied Brij56 as a non-ionic surfactant (samples (1) and (2)). In our case, this interaction results in larger specific surface areas of 175, 240 and 160 m2 g�1 for samples (3)–(5), respectively. The thermal stability of the different unconditioned catalysts was characterized by TGA. With increasing BET surface area, the weight loss of the mesoporous catalysts increased (see Table 1). This weight loss is mainly attributed to the desorption of molecularly adsorbed water and recombination of OH groups [39], whose amount increases with increasing BET surface area. At higher temperatures (>500 8C), the catalyst weight loss (0.7–2.6%, see Table 1) is caused by reactive desorption of carbon containing species, which are residuals from the TiO2 synthesis (see also Section 3.2). These species are also visible in IR spectra recorded on the different catalysts (Fig. 2). Prior to the IR measurements, the catalysts were dried at 100 8C for 15 h and calcined at 400 8C. Signals at 2352 and 1358 cm�1 belong to the carbon containing species, which are not observed on the P25 based catalyst (Fig. 2, spectrum (e)). The first signal is usually related to gas phase CO2 resulting from CO oxidation (CO2 was