TiO2 Catalytic

Article pubs.acs.org/cm

Supported Mesoporous and Hierarchical Porous Pd/TiO2 Catalytic Coatings with Controlled Particle Size and Pore Structure Erik Ortel,† Sergey Sokolov,‡ Claudia Zielke,§ Iver Lauermann,§ Sören Selve,† Kornelia Weh,† Benjamin Paul,† Jörg Polte,† and Ralph Kraehnert*,† †

Department of Chemistry, Technical University of Berlin, Straße des 17. Juni 124, D-10623 Berlin, Germany Leibniz Institute for Catalysis, University of Rostock, Albert-Einstein-Str.29a D-18059 Rostock, Germany § Solar Energy Research, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany ‡

ABSTRACT: Control over the size of active metal particles and the structure of catalysts pore system is an essential requirement for the design of supported catalysts. Polymeric templates combined with a suitable metal-oxide precursor enable the synthesis of defined pore systems, whereas colloidal metal particles can provide access to the particle-size control. However, pore template, metal-oxide precursor, and colloidal metal particles combined in one synthesis solution are often not compatible with each other due to aggregation, precipitation, and dissolution processes. We present a new approach to the preparation of supported catalysts that permits the controlled coassembly of preformed colloidal metal nanoparticles, polymeric pore templates, and a metal-oxide precursor from a water-based solution. The synthesis is enabled by establishing under pH-neutral conditions the templating of defined pores using titanium(IV) bis(ammonium lactato) dihydroxide as an unconventional metaloxide precursor. The presented approach provides a modular strategy for the precise control of the catalysts nanostructure. This is illustrated for the synthesis of mesoporous as well as hierarchically porous Pd/TiO2 catalysts prepared from colloidal solutions of palladium nanoparticles. The catalysts show high activity and selectivity in the gas-phase hydrogenation of 1,3-butadiene. KEYWORDS: mesoporous materials, hierarchical porous, titanium oxide films, palladium nanoparticle, Pd-TiO2 catalyst, hydrogenation of 1,3-butadiene

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INTRODUCTION Supported catalysts, i.e. the combination of active metal particles dispersed on a porous metal-oxide support, represent one of the most important classes of catalyst in industrial hydrogenation and oxidation reactions as well as environmental applications. In particular automotive catalysis and microstructured reactors require the highest possible space-time yields to minimize the size and weight of catalytic converters.1,2 Efficient converters therefore necessitate the design and synthesis of catalysts that provide high catalytic activity and selectivity at a minimum of catalyst amount. The performance of reactions over supported catalysts is determined by two main factors: intrinsic catalytic properties and mass-transport phenomena. The intrinsic properties of catalytically active centers located on the surface of metal nanoparticles can be controlled by composition, size, and shape of the metal particles. Moreover, the transport of molecules to the active centers occurs via diffusion through the catalysts pore system, which can be controlled by the pore morphology of the support material.3 Here, small pores provide a large surface area where active particles can be dispersed and anchored. However, small pores decrease also effective diffusion rates of the reactants to the active sites due to an increased number of collisions © 2012 American Chemical Society

between gas molecules and pore walls. A balanced compromise of high surface area and fast pore diffusion can be realized in hierarchical pore systems, where large pores facilitate fast transport while the small pores provide the high surface for supporting active metal sites. In consequence, optimizing the performance of a supported catalyst requires control over the supports pore morphology as well as over the number and size of active metal nanoparticles (NPs) during the catalyst synthesis. Supported catalysts are commonly synthesized by impregnation of a catalyst support with a solution containing the dissolved metal precursor. Impregnation is typically followed by drying and thermal treatments (calcination, reduction), during which metal particles form on the supports surface. However, such syntheses often result in wide particle size distributions. Moreover, they do not provide rational and direct means of controlling particle size or the catalysts pore morphology. Several strategies for improved control over the nanostructure of supported catalysts have been reported. So-called nanocasting Received: April 6, 2012 Revised: August 29, 2012 Published: October 1, 2012 3828

dx.doi.org/10.1021/cm301081w | Chem. Mater. 2012, 24, 3828−3838

Chemistry of Materials

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to such a synthesis solution. TALH is water-soluble, forms pHneutral solutions, and hydrolyzes very slowly in the presence of water. Moreover, it is commercially available and nonhazardous. TALH was previously employed for the synthesis of titania nanostructures, e.g. via layer-by-layer technique as reported by Wang et al.21 and Azzaroni et al.22 Macroporous titania films have been prepared from TALH solutions by self-assembly of latex spheres.23,24 Chinthamanipeta et al.25 employed TALH to generate nanostructured titania rods via deposition of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer on surface, cross-linking them into cylinders and finally filling the polymer cylinders with TALH solutions. Pelentridou et al.26 synthesized TiO2 films of high surface area by deposition of TALH mixed with Triton X-100. However, micelle-templated three-dimensionally ordered pore systems based on TALH were not reported so far. We present for the first time the synthesis of titania-based supported catalysts with three-dimensionally ordered mesopore structure using TALH as titania precursor. Preformed colloidal Pd-NP’s are incorporated into the film synthesis to achieve homogeneous Pd-NP/TiO2 catalytic coatings with controlled pore size and particle size. Moreover, the first synthesis of TALHbased titania films with hierarchical meso-macro-porosity via dual-templating is demonstrated. Scheme 1 illustrates the

provides access to tunable pore morphologies in oxide materials by replicating the nanostructure of a template material into a locally ordered pore system.4 Employing micelles of amphiphilic block copolymers as pore template, oxide coatings with ordered and well-connected mesopores can be formed by a strategy called evaporation-induced self-assembly (EISA).5−7 Moreover, spheres of poly(methyl methacrylate) (PMMA) or polystyrene (PS) close-packed into 3D colloidal crystals have been used as templates for the synthesis of macroporous materials with threedimensionally ordered pore structure.8 The synthesis of hierarchical pore systems was accomplished by a combination of these meso- and macro-structure directing templates.9−13 For metal nanoparticles improved control over size and shape can be obtained when the particles are prepared in colloidal form prior to colloid deposition on the support.14−16 The absence of the support surface during particle formation and growth decreases heterogeneous nucleation in favor of homogeneous particle nucleation, which results in more narrow size distribution and often provides also means of controlling particle size and shape. However, subsequent immobilization of preformed colloidal particles on a catalyst support can result in preferred particle deposition on the outer surface of catalyst bodies, which leads to inhomogeneous distribution of the active sites. Moreover, aggregation of particles during deposition and drying facilitates particle sintering during subsequent thermal treatments, i.e. undesired broadening of the particle-size distribution. The goal of the current work was to design catalytic coatings where the active phase (palladium nanoparticles, Pd-NP) is distributed homogeneously in mesoporous and hierarchical meso-macro-porous metal-oxide films (titania), and where the synthesis strategy has the potential to control the size of Pd-NP, mesopores and macropores independently. Two different synthesis strategies were considered, i.e. one route that comprises a codeposition/coassembly of the ionic metal precursor with the oxide precursor and pore templates17,18 and the second route were the metal precursor is replaced by preformed colloidal metal particles.19 However, combining Pd-NP colloids with polymer templates and typical titania precursor solutions, such as anhydrous TiCl4 or acid-stabilized titanium alkoxides, proves to be extremely challenging since these precursor solutions are highly acidic and therefore dissolve Pd colloids. Moreover, the low pH of these precursor solutions decomposes various types of latex spheres employed as template. Furthermore, it also corrodes the surface of steel substrates,20 a material typically employed as the wall material of catalytic microreactors. In addition, low pH and high ion content of such synthesis solutions can destroy the electrostatic stabilization of the colloidal metal particles. Particle stabilization must therefore rely on additional stabilizing agents. However, these agents should not interfere with the self-assembly of the templating agents, which would obstruct pore formation. Moreover, they must be easily removable after film deposition; hence, stabilizers containing typical catalyst poisons such as sulfur cannot be used. Thus, coatings of supported catalysts with controlled properties can only be obtained if the required mixture of solvents, metal colloid, colloid stabilizer, oxide precursors, and pore template(s) form homogeneous and stable solutions. This requires compatible ingredients and solution conditions, preferably based on water as a solvent and conditions near neutral pH. We propose that the metal precursor titanium(IV) bis(ammonium lactato) dihydroxide (TALH) is a key component

Scheme 1. Illustration of the Synthesis Approach to Obtain (A) Mesoporous TiO2, (B) Hierarchical Meso-macro-porous TiO2, and (C) Hierarchical Meso-macro-porous Pd-NP/TiO2 Based on TALH as a Water-Soluble pH-Neutral TiO2 Source, Pore-Templating with Polymers and Preformed Colloidal PdNP

modular synthesis approach, which depending on the synthesis ingredients leads to (A) mesoporous titania, (B) hierarchical porous titania, or (C) Pd-NP/TiO2 catalytic coatings with controlled porosity. All catalysts proved to be highly active and selective in the gas-phase hydrogenation of 1,3-butadiene.

1. EXPERIMENTAL SECTION Colloidal Pd-NP’s were prepared by reduction of Pd precursors in aqueous solutions. Mesoporous titania films were synthesized by dipcoating of solutions containing TALH and micelles of either PEO-PPOPEO (Pluronic) or PEO-PB-PEO copolymers followed by calcination. Adding PMMA latex to this synthesis afforded films with hierarchical porosity. Catalytic coatings were prepared adding the Pd-NP to the respective oxide syntheses. Resulting catalysts were tested in gas-phase hydrogenation of butadiene. The corresponding synthesis and characterization procedures are described in detail below. 1.1. Chemicals and Materials. Aqueous (50 wt %) solution of Ti(IV) bis(ammonium latate) dihydroxide (TALH), Pluronic F127, and 3829



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Figure 1. TEM images of fresh Pd-NP colloidal solutions: (A) Pd-NP stabilized by PVP (PVP/Pd-NP), (B) Pd-NP stabilized by F127 (F127/Pd-NP). 1.4. Preparation of Pd-NP/TiO2 Catalyst. Mesoporous Pd-NP/ TiO2 catalytic coatings (template F127) and hierarchically porous PdNP/TiO2 (templates F127 and 632 nm sized PMMA) were prepared in one-step syntheses by adding the nanoparticles PVP/Pd-NP to the respective TALH-based dip-coating solutions. The amount of required Pd-NP solution was calculated to obtain 0.5 wt % Pd-loading in TiO2. The catalyst were calcined in flowing air with a ramp of 1 K/min to 400 °C and thereafter reduced in H2/Ar atmosphere (4 vol % H2) at 350 °C for 6 h. 1.5. Characterization. Scanning electron microscopy (SEM) images were collected on a JEOL 7401F scanning electron microscope. To determine the film thickness, coated silicon wafers were split into two pieces and imaged at the cross-section. Image J, version 1.39u (http://rsbweb.nih.gov/ij), was employed to determine the size of PdNP, the pore diameter, and film thickness and to derive fast Fourier transform (FFT) plots from the SEM images. Film morphology and crystallinity of film fragments removed from the substrates was studied by transmission electron microscopy (TEM; Zeiss EM Omega 812 X, 120 kV). Two-dimensional scattering angle X-ray spectroscopy (2DSAXS) patterns were collected on the μSpot beamline at the synchrotron BESSY II (Berlin, Germany). The patterns were recorded in transmission mode with the incoming X-ray beam positioned between 90 and 10° relative to the substrate surface. The SAXS data were processed employing the computer program FIT2D. The modulus of the scattering vector q is defined in terms of the scattering angle θ and the wavelength λ of the radiation used: thus q = 4π/λ sin(θ/2). Krypton adsorption was measured at 77 K on Quantachrome Autosorb-1-C. From this data, the coatings surface area was calculated via Brunauer− Emmett−Teller (BET) method. To determine the coating mass, the mass depth of each film was calculated by STRATAGem film analysis software (v 4.3) after a wavelength dispersive X-ray spectroscopy (WDX) analysis using a Cameca ″Camebax-microbeam″ electron microprobe at ZELMI (TU-Berlin). UV/vis absorbance spectra were recorded on AvaSpec-2048TEC-2 equipped with a Deuterium halogen light source (Avantes, Broomfield, USA). XRD data were collected on Bruker D8 Advance instrument (Cu Kα radiation). The average crystallite size was calculated applying the Scherrer equation. XPS spectra of deposited films were obtained at the U49-2 PGM-1 beamline at BESSY II using excitation energy of 950 eV. Electron kinetic energies were measured with a VG CLAM 4 electron analyzer equipped with 9 channeltrons. The respective photon energies were calibrated using the Au 4f5/2 photoemission peak (binding energy = 84.0 eV) obtained from a sputter-cleaned gold foil. The spectra were analyzed by determining binding energies of photoemission peaks and kinetic energies of Auger emission peaks, respectively, and additionally calculating the modified Auger parameter α′ = EB(XPS) + Ekin(Auger). The latter is independent

methyl methacrylate (99%) were purchased from Sigma-Aldrich. Polyvinylpyrrolidone (PVP) with a molar mass of 58 000 g mol−1, Na2PdCl4·3H2O, and NaBH4 (98%) were obtained from Alfa Aesar; L(+)-ascorbic acid (≥99%) was from Roth. All chemicals were used without further purification. The pore-template poly(ethylene oxide) 213 -block-poly(butadiene) 184 -block-poly(ethylene oxide) 213 (“10k-PB”, Mw = 28 740 g mol−1) was synthesized by PolymerService-Merseburg GmbH as described in our previous work.27 The macropore templates poly(methyl methacrylate) (PMMA) were synthesized by surfactant-free emulsion polymerization according to the procedure described elsewhere.28 Two batches of PMMA latex were prepared with sphere diameters of 403 ± 13 and 632 ± 19 nm, respectively. A 43 wt % suspension of PMMA spheres in water was used for the film syntheses. Si wafers and steel plates (grade 1.4301) were employed as substrates for film deposition. Before dip-coating, the Si wafers were rinsed with ethanol. The surface of steel plates was passivated prior to coating as described in ref 29 by grinding with 180grit sandpaper followed by calcination in air for 2 h at 600 °C. 1.2. Synthesis of Pd-NP Colloids. Pd-NP’s in aqueous solution were synthesized as reported in literature, either via reduction of dissolved Na2PdCl4 using Pluronic F12730 (further denoted as F127/ Pd-NP) or by reduction with ascorbic acid in presence of PVP31 (further denoted as PVP/Pd-NP). The F127/Pd-NP colloids were prepared by mixing an aqueous Pluronic F127 solution (2.98 mL, 10 wt %) with an aqueous solution of Na2PdCl4 (16 μL, 0.163 g/mL) at room temperature. For the synthesis of PVP/Pd-NP, Na2PdCl4 (70.5 mg) was dissolved in water (11.25 mL). Ascorbic acid (128.8 mg) and PVP (83.4 mg) were dissolved in a separate volume of water (18.75 mL). Then, these two solutions were mixed at 90 °C. 1.3. TiO2 Coatings. For the synthesis of mesoporous TiO2 films, the mesopore template (either Pluronic F127 or 10k-PB) was dissolved in a mixture of methanol and water. The clear template solution was then added dropwise under stirring to a 50 wt % aqueous TALH solution. The molar ratios of TALH:methanol:water:template were 1:47:16:0.0064 for F127 and 1:47:30:0.0029 for 10k-PB. Dip-coating was performed in a controlled atmosphere at 25 °C and 80% relative humidity with a withdrawal rate of 240 mm/min. The films were subsequently calcined in flowing air for 20 min at 400 °C (heating ramp 1 K/min). For the synthesis of hierarchical meso-macro-porous TiO2 films (further denoted as F127+PMMA), a 50 wt % TALH aqueous solution (2.44 g) was mixed with a PMMA suspension (4.70 g, 403 nm spheres) and water (3.4 mL). Then, F127 (0.40 g) was dissolved in this solution. Dip-coating was performed at 38 °C and 7% relative humidity with a withdrawal rate of 1 mm/min. The films were then calcined for 20 min at 400 °C in flowing air (heating ramp 1 K/min). 3830



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of peak shifts due to sample charging and was thus used to verify the determined binding energies. 1.6. Catalytic Reaction. The catalytic performance of mesoporous and of hierarchically porous catalyst films in the gas-phase hydrogenation of 1,3-butadiene was studied at temperatures between 40 and 150 °C. Pd-NP/TiO2 films were coated on both sides of steel plates (plate size 27 mm × 30 mm). For each catalytic run, five identical steel plates were stacked parallel into the reactor housing with 1.5 mm distance between the plates. The total mass of catalyst per run amounted to 2.0 mg. A test setup and procedure similar to the one described by Cukic et al.32 was used. A reaction mixture consisting of 5% butadiene (2.5 purity), 10% hydrogen (5.0 purity), and 85% nitrogen (5.0 purity) was passed through the reactor at a flow rate of 60 mL/min (STP) at 1.05 bar. The catalyst was then heated to 150 °C under reactive gas flow and equilibrated to reaction conditions for several hours. Thereafter, the temperature was decreased stepwise in 10 K increments to 40 °C with a dwell time of 60 min for each temperature set point. Analysis of the gas products was performed continuously every 7 min by online gas chromatograph (Agilent GC 7890 equipped with FID, TCD and columns HP Plot Al2O3, Molsieve 5A, HP Plot Q and DB FFAP.) The space-time yield (STY) was calculated as produced moles of butenes per second per kilogram of the catalyst [mol s−1 kg−1].

solution with no absorption band in the studied range (450−850 nm) (see TiCl4-spectrum in Figure 2A). In contrast, the mixture of TiCl4 with Pd-NP yielded a black colloid solution that absorbs light in the whole studied range but without a distinct maximum (Figure 2A, TiCl4+Pd-NP 0 h). After 24 h of stirring, the color of this solution changed from black to clear orange. Moreover, an absorption band with a maximum at 475 nm evolved (Figure 2A, TiCl4+Pd-NP 24 h), attributed to the dissolution of the Pd-NPs under these highly acidic conditions. Drelinkiewicz et al.33 observed a similar absorption band at 474 nm for PdCl2 solutions of very high acidity (HCl concentration 2 mol dm−3). The authors attributed this absorption band to an anionic Pd-complex of [PdCl4]2−. Indeed UV/vis spectra of solutions containing TiCl4 and Na2PdCl4 also feature a band with a maximum at 475 nm (Figure 2A, TiCl4+Na2PdCl4). Figure 2B shows for comparison the UV/vis spectra acquired for Pd-NP/TALH solutions. Pure TALH solution had a clear yellow color with no distinct absorption peak between 450 and 850 nm (Figure 2B, TALH). The solution turned black when Pd colloids were added (Figure 2B, TALH+Pd-NP 0 h). After 24 h of stirring, the spectrum remained almost unchanged (Figure 2B, TALH+Pd-NP 24 h). Moreover, the mixture did not show evidence of color changes or precipitation. Thus, Pd colloids are stable in TALH-based precursor solutions whereas acidic TiCl4based solutions either dissolved or precipitated Pd-NP. This makes TALH the precursor of choice for a one-step synthesis of porous catalytic coatings of titania-supported Pd-NP’s. 2.2. Mesoporous and Hierarchically Porous TiO2 Films Based on TALH. Besides being compatible with Pd colloids, TALH is also required to work as precursor for mesoporous and hierarchically porous titania films. Such TALH-based films templated by micelles of amphiphilic block copolymers have not been reported before. Figure 3 presents top-view SEM images at three magnifications together with 2D-SAXS patterns of TALHbased films templated with F127 (upper row), 10k-PB (middle row), and F127+PMMA (lower row), i.e. the combination of F127 micelles with the macropore template PMMA latex. SEM images recorded at high magnification (150 000× and 50 000×) clearly evidence ordered porosity imprinted by the polymer micelles. The films are highly porous with pores open to the outer film surface. The pore openings were 7 and 21 nm for F127 and 10k-PB respectively, hence pore size scaled with the molar mass of the respective pore template (MF127 < M10k‑PB). Moreover, codeposition of micelles of F127 with PMMA spheres leads to a hierarchical meso-macro-porous structure (SEM images in the lower row 150 000× and 50 000×). As evident from the high-magnification SEM image (150 000×), the mesopores cast by F127 micelles fully penetrate the walls of the macroporous PMMA-templated framework. Pore size determined from the micrographs amounted to 6 nm for the mesopores and 319 nm for the macropores. SEM images recorded at lower magnification (Figure 3, 1000×) attest a high coating quality of the mesoporous films. Both mesoporous films (F127 and 10k-PB) exhibit crack-free film surface on a millimeter scale. In contrast, a top-view SEM of the calcined hierarchical film (F127+PMMA) reveals that this film consisted of separated micrometer-sized domains. However, additional experiments show that the substrate coverage of these hierarchical films can be improved by repeating the coating (data not shown here). The FFT images corresponding to the mesoporous films templated by F127 and 10k-PB (Figure 3, insets in the 50 000× SEM images) show distinct rings, a clear indication of locally

2. RESULTS AND DISCUSSION 2.1. Compatibility of Pd-Colloids with TiCl4 versus TALH. To synthesize Pd-NP/TiO2 films in a one-step procedure, a stable solution containing TiO2-precursor and Pd-colloid is required. We therefore studied the stability of different mixtures of colloidal Pd particles in dip-coating solutions typically employed for the synthesis of porous titania. Hence, aqueous Pd-NP solutions were mixed with ethanolic TiCl4 or aqueous TALH solution at a ratio that would result in 0.5 wt % Pd content in the calcined catalyst. The stability of these colloids was followed by UV/vis spectroscopy. TEM images of the employed fresh colloids PVP/Pd-NP and F127/Pd-NP are shown in Figure 1A and B, respectively. The images indicate that PVP/Pd-NP and F127/Pd-NP are similar in size and show a narrow size distribution of 4.6 ± 1.2 nm (PVP/Pd-NP) and 3.8 ± 0.8 nm (F127/Pd-NP), respectively. Mixing the PVP/Pd-NP with ethanolic TiCl4 solution induced rapid precipitation, i.e. the solutions were not compatible with each other. In contrast, a mixture of PVP/Pd-NP with aqueous TALH precursor formed a brown transparent sol. Also the mixtures of F127/Pd-NP with either TiCl4 or TALH formed homogeneous solutions. UV/vis spectra of the F127/Pd-NP with TiCl4 and PVP/Pd-NP with TALH are shown in Figure 2 for different aging times. TiCl4 in ethanol is a clear yellow

Figure 2. UV/vis spectra of solutions based on (A) TiCl4 and (B) TALH with and without addition of Pd-NP and Na2PdCl4. Solutions containing Pd-NP were analyzed immediately after NP addition as well as after 24 h of aging to test for Pd-NP dissolution. 3831



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Figure 3. SEM top-view images at 150 000×, 50 000×, and 1000× magnifications and 2D-SAXS patterns (right column) of TALH-based TiO2 films templated with F127 (upper row), 10k-PB (middle row), and F127+PMMA (bottom row).

0.68 nm−1; dx = 9 nm). This difference in periodicity between F127 versus F127+PMMA templated films can be explained by a different shrinkage behavior of the coatings. During the synthesis solvent evaporation, conversions of TALH to TiO2 and removal of polymer templates result in reduction of the films volume, and consequently, the films shrink. For the thin mesoporous films, the shrinkage occurs exclusively perpendicular to the substrate without fracturing of the film as commonly reported in the literature for other oxide systems, which leads to the transformation of the pore geometry from spherical to ellipsoidal. In contrast, the F127+PMMA templated films show separated domains of hierarchical porosity after calcination (Figure 3, F127+PMMA, 1000×). It is assumed that during drying macrostructured domains are formed due to nonideal PMMAlatex packing. These shrink separately of each other due to a low structural rigidity between the domains. That leads to the formation of separated porous islands after calcination. Moreover, the hierarchical films are much thicker (approximately 4 μm) than the mesoporous films (about 200 nm) which typically results in increased local tension and in film fracturing. Hence, the macropore-containing films shrink not only perpendicular but also parallel to the substrate. In consequence, the mesopores in hierarchically porous films shrink uniformly in all three dimensions retaining the spherical mesopore shape, which as well

ordered mesopores. The periodic distances amount to 13.5 nm (F127) and 29.4 nm (10k-PB). In contrast, FFT-transformed images of the F127+PMMA sample show distinct spots. The spots reflect the high order of macropores cast by close-packed arrays of PMMA spheres. 2D-SAXS patterns recorded in transmission mode with an incoming X-ray beam perpendicular to the substrates surface (β = 90°) feature isotropic rings for all TiO2 samples templated with F127 and 10k-PB as well as F127+PMMA (not shown). Moreover, the patterns recorded in transmission at a smaller angle of β = 6° (F127), β = 10° (10k-PB), and β = 20° (F127+PMMA) relative to the substrate surface are shown in the right column of Figure 3. The scattering images of mesoporous TiO2 films templated with F127 and 10k-PB feature a pattern typical for a distorted cubic arrangement of mesopores. Similar patterns were previously observed for TiO2 films based on TiCl4 precursor templated with the same PEO-PB-PEO block copolymers.27 The ellipsoidal shape of the patterns indicates isotropic shrinkage of the mesostructure in the direction normal to the substrate.34−39 In contrast, the SAXS pattern of the F127+PMMA templated film (β = 20°) shows a homogeneous circular ring devoid of evidence for uniaxial mesopore contraction. Moreover, it is apparent that the d-spacing of films templated by only F127 is higher (qx = 0.45 nm−1; dx = 14 nm) than the d-spacing observed on the F127+PMMA films (qx = 3832



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results in a smaller pore diameter and lower d-spacing parallel to the substrate when compared to the mesoporous films. The pore morphology and crystallinity of the TALH-based TiO2-films were further studied by TEM and selected area electron diffraction (SAED). Figure 4 presents TEM images (A

crystallizes into anatase during the thermal treatment is in agreement with a recent report of Pelentridou et al.26 Moreover, Yu et al.40 synthesized mesoporous titania films from titanium tetraisopropoxide and Pluronic P123 template. After calcination at 400 °C they also observed anatase crystallites of 6.1 nm in size. Hence, templated TiO2 films based on TALH form crystallites of similar size as in mesoporous titania films based on commonly used alkoxide precursors. In summary, we demonstrate that titania films with templated mesopores can be synthesized from TALH precursor in a pHneutral solution via soft-templating with polymer micelles. Mesopores are locally ordered and their size can be conveniently adjusted by varying the molar mass of the block copolymer template.27 Moreover, the TALH-based synthesis can also produce hierarchical meso-macro-porous TiO2 thanks to pHneutral solutions rendering PMMA spheres stable. 2.3. Mesoporous and Hierarchical Porous Pd-NP/TiO2 Catalysts. In order to prepare catalytic coatings with controlled porosity, preformed colloidal PVP/Pd-NP’s were combined with solutions containing TALH and either F127 alone or F127 in combination with PMMA spheres. The dip-coated and dried films were calcined at 400 °C in air and subsequently reduced at 350 °C in H2/Ar. Figure 6 shows SEM images of the TALHbased mesoporous Pd-NP/TiO2 catalyst film templated by F127 (A) as well as TEM images (B−E) recorded at different magnifications. Moreover, Figure 6F presents for direct comparison a TEM image of a catalyst film synthesized in a similar way, but using highly acidic TiCl4 solution instead of TALH. The top-view SEM image of the TALH-based Pd-NP/ TiO2 coating (Figure 6A) appears to be very similar to that of the pure TiO2−F127 film (Figure 3), indicating that the presence of the Pd colloid is not detrimental to the self-assembly of template micelles. Figure 6B presents a Z-contrast TEM image of the films crosssection recorded in high-angle annular dark-field mode (HAADF). The image reveals that preformed Pd-NP (bright spots) are distributed throughout the entire films cross-section with the film being about 165 nm thick. The observed Pd-NP’s are 4.5 ± 1.4 nm in diameter, i.e. comparable in size to the diameter of 4.6 nm observed for the original colloid. Particle size remained almost unchanged after catalysis (4.9 ± 1.5 nm). In contrast, Pd-NP/TiO2 films prepared from the acidic TiCl4 precursor (Figure 6F) show Pd particles grown up to 15 nm in diameter during the catalyst synthesis. Hence, the size of Pd-NP in TALH-derived catalysts is clearly much narrower and closer to the original colloid size than in TiCl4-based films. The broad particle-size distribution in TiCl4-derived catalysts is probably related to partial dissolution of the colloidal Pd-NP before film deposition as well as disturbance of the colloids stability due to the low pH of TiCl4-based solutions. Additional high-resolution TEM (HRTEM) images of the TALH-based Pd-NP/TiO2 displayed in Figure 6C−E reveal in more detail the different crystallites that constitute the catalyst film. Crystallites consisting of anatase, Pd but also PdO were detected. In Figure 6D, the (101)-planes of anatase (d = 0.352 nm) can be clearly distinguished in the TiO2 matrix. Moreover, Figure 6C and D evidence typical Pd particles with lattice fringes corresponding to metallic Pd, i.e. (111)-planes with d-spacing of 0.225 nm and (200)-planes with d-spacing of 0.195 nm. The detected Pd-NP’s possessed nearly spherical shapes. However, in some cases also lattice fringes corresponding to (102) plane of tetragonal PdO (d = 0.202 nm) could be distinguished (Figure 6E). The corresponding particles possessed elliptical shapes

Figure 4. TEM-analysis (A and B) and SAED images (C and D) of TALH-based TiO2 films templated with F127 (A and C) and with F127+PMMA (B and D).

and B) and SAED patterns (C and D) recorded on the calcined films templated by either F127 (A and C) or the combination of F127 and PMMA latex (B and D). TEM images of TiO2 films templated by F127 indicate a fully mesoporous material. In contrast, titania films templated by the F127+PMMA combination reveal a network of mesopores completely penetrating the macropore walls. Moreover, the SAED patterns of both materials show isotropic diffraction rings with ring positions that match the reflections expected for anatase (powder diffraction files, PDF21-1272). The homogeneous diffraction rings indicate that the pore walls consist of randomly orientated anatase crystallites. This phase assignment is further corroborated by X-ray diffraction recorded on both films (Figure 5). The patterns show peaks at 2θ = 25.3°, which correspond to the (101) reflection of anatase. Crystallite sizes estimated from the peak width by Scherer equation were 6.0 and 4.7 nm for F127 and F127+PMMA, respectively. Our observation that TALH

Figure 5. XRD pattern in a 2-θ region of 20−30° of TALH-based TiO2 films templated with F127 and with F127+PMMA after calcination at 400 °C. 3833



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Figure 6. Electron microscopy images of F127-templated TiO2 film with incorporated colloidal Pd-NP. All images show a catalyst prepared from TALH (A−E) except for part F (from TiCl4, shown for direct comparison): (A) top-view SEM image, (B) cross-sectional high-angle annular dark-field TEM image, (C−E) high-resolution bright-field TEM image, (F) cross-sectional backscattered electron SEM image.

Imaging at high resolution revealed that the Pd-NP’s were 5.0 ± 1.4 nm in size before catalytic tests and 5.0 ± 1.2 nm after butadiene hydrogenation tests. Moreover, they are crystalline and their lattice spacings match the lattice parameters of (111) and (200) planes of metallic Pd (Figure 7D). To further evaluate the impact of the Pd-NP colloids presence on mesopore ordering, SAXS patterns were recorded in transmission mode (β = 90°) on films deposited with and without colloidal Pd-NP. Figure 8 shows the corresponding scattering curves as scattering intensity plotted vs scattering vector q. The patterns recorded on mesoporous F127-templated TiO2 show similar maxima at q = 0.47 nm−1 without Pd-NP (A, black curve) and q = 0.44 nm−1 with Pd-NP (B, black curve), respectively, which corresponds to d-spacings of approximately 14 nm. In contrast, hierarchical porous films templated by a combination of F127 and PMMA show without (A gray curve) and with Pd-NP (B gray curve) maxima at q = 0.68 nm−1 corresponding to a d-spacing of d = 9 nm. Hence, it is evident from SAXS analysis that in both systems, F127 and F127+PMMA, the ordering of mesopores is not significantly affected by addition of Pd-NP colloids to the dip-coating solution. The surface compositions of Pd-NP/TiO2 films templated by F127 and by F127/PMMA mixture were analyzed by XPS. Figure 9 shows XPS spectra of the films in the Ti 2p region (A and B) and the Pd 3d region (C and D). The binding energy (BE) for the peak in the Ti 2p3/2 amounted to 459.3 ± 0.1 and 459.4 ± 0.1 eV for F127 and F127+PMMA, respectively, which is

described by axis lengths of about 5 and 7 nm, respectively. On the basis of particle shape and dimensions, this observation is interpreted as the oxidative transformation of individual spherically shaped Pd metal particles into elongated PdO particles, which according to the unit cells of the cubic Pd lattice (a = b = c = 0.389 nm, PDF-C46-1043) and of tetragonal PdO (a = b = 0.304 nm, c = 0.533 nm, PDF-C75-584) should be accompanied by elongation of the oxidized single-crystals. Evidently, the applied heat treatment of the catalysts (20 min at 400 °C in air followed by 6 h at 350 °C in H2/Ar) yields metallic Pd-NP’s as well as a few PdO particles. Unfortunately, the limited number of observed particles in thinned crosssectional TEM samples did not allow for a statistical analysis of this effect. However, the data clearly suggest that independent of the formed Pd-NP phase (Pd or PdO) the initial colloidal particles did neither agglomerate nor sinter significantly. Catalysts with hierarchical porosity were prepared by a similar coating procedure. Figure 7 shows SEM (A and B) and TEM images (C and D) of a hierarchical Pd-NP/TiO2 film templated by a combination of F127 and PMMA latex. Ordered arrays of macropores with 470 nm in diameter are observed. Hence, PMMA spheres self-assemble during drying of the coating solution on the substrate. The film thickness measured on the SEM image of the cross-section amounted to ca. 4 μm (Figure 7B). TEM analysis of the hierarchical Pd-NP/TiO2 film revealed that the macropore walls are penetrated by templated mesopores of ca. 7 nm in diameter (Figure 7C). Moreover, Pd-NP’s distributed throughout the porous network are observed. 3834



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Figure 7. Electron microscopy images of F127+PMMA templated TiO2 film with incorporated Pd-NP: (A) top-view SEM image, (B) cross-sectional SEM image, (C) bright-field TEM image, (D) high-resolution bright-field TEM image.

indicate that both Pd-NP/TiO2 films contained two different Pd surface species. The binding energies for Pd 3d5/2 were 335.6 ± 0.1 and 336.5 ± 0.1 eV on the F127 film. For the F127+PMMA film, similar values of 335.7 ± 0.1 and 336.9 ± 0.1 eV were obtained. The peak positions and the observed BE difference of ca. 1 eV allow the assignment of the major peak to metallic Pd0 and a smaller peak of Pd2+,42 i.e. after heat treatment with 4% H2/ Ar a small amount of oxidized Pd is still present in both TiO2 films (see also the TEM results in Figure 6). The presence of small amounts of PdO in Pd/TiO2 also after hydrogen treatment has been explained by Wang et al. with the fact that palladium can be oxidized with the oxygen provided from the titania support.43 On the basis of XPS analysis, they identified both PdOx and metallic palladium species after reduction of Pd/TiO2 samples at 400 °C in 10% H2/He atmosphere. Peak deconvolution and quantitative analysis of the XPS data indicate that the surface-content ratio of Pd to TiO2 corresponds to approximately 1.3 ± 0.2 wt % in the F127-derived catalyst and 2.5 ± 0.2 wt % in the F127+PMMA catalyst. These values are higher than the 0.5 wt % of Pd in TiO2 expected from the bulk composition (i.e., the composition of the dip-coating solution). Hence, XPS indicates a high dispersion of Pd on the titania surface. In consequence, a significant portion of the provided PdNP’s is not enclosed in the titania wall through the codeposition of Pd-NP and TiO2 precursor and is therefore accessible for a surface-catalytic reaction. Moreover, the XPS data suggest that the surface ratio Pd:TiO2 is higher for the hierarchical Pd-NP/ TiO2 than for the mesoporous film. However, XPS records the local surface composition of the catalyst films, where it is not clear to which extent the internal pore surface of templated pores

Figure 8. SAXS patterns of TALH-based TiO2 films templated with F127 (black) and F127+PMMA (gray). Both materials are shown without (A) and with (B) incorporation of colloidal Pd-NP.

in good agreement with the BE values reported for TiO2 in literature.41 Moreover, the spectra recorded in the Pd 3d range 3835



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Figure 9. Measured and deconvoluted XPS spectra in the regions of Ti 2p and Pd 3d of F127 and F127+PMMA templated Pd-NP/TiO2 films (excitation energy 950 eV).

Figure 10. Catalytic performance of mesoporous (F127-templated, ■) and hierarchical porous (F127+PMMA-templated, Δ) Pd-NP/TiO2 catalyst in gas-phase hydrogenation of butadiene: (A) butadiene conversion vs temperature and (B) selectivity to 1-butene and total butene selectivity vs butadiene conversion.

400 °C, which is the same order of magnitude as observed here for TALH-derived films. In summary, the developed synthesis approach based on preformed colloidal Pd-NP’s, TALH titania precursor and different polymer templates (see Scheme 1) can produce PdNP/TiO 2 catalytic coatings with controlled meso- and hierarchical meso-macro-porosity as well as controlled particle size. The synthesis yields the high accessible surface area desired for catalytic applications. The presence of Pd-NP has no detrimental effect on film integrity, pore templating, and ordering. Moreover, the preformed colloidal Pd-NP endured

vs the outer surface of the catalyst film contributes to the photon signal (e.g., higher roughness factor in macroporous films). Hence, a precise quantification of the composition of the internal surface of the pore systems could not be obtained. The specific surface area of the catalyst films was analyzed by Kr adsorption. The BET surface areas of the Pd-NP/TiO2 films amounted to 197 m2/g for F127 and 166 m2/g for F127+PMMA. These values are similar to those reported in literature for titania with templated porosity prepared from other precursors. For example, Yu et al.40 reported 113 m2/g for TiO2 films based on titanium tetraisopropoxide/Pluronic P123 solution calcined at 3836



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regime space-time yields that are similar to values reported in literature, i.e. a comparable intrinsic catalytic activity and selectivity. It must be emphasized that both of the synthesized Pd-NP/ TiO2 catalysts, mesoporous as well as the hierarchically porous material, perform very similar although their pore structures differ significantly. This is the expected behavior in the kinetic regime, i.e. when mass transport in the catalyst by pore diffusion is rapid compared to the reactions kinetics. However, in cases where pore diffusion limits the catalytic performance, i.e. higher temperatures or faster reactions, the hierarchical system is expected to provide lower pore-diffusion resistance and thus deliver superior space-time yields. Moreover, the uniform and controlled structure of the developed Pd-NP/TiO2 catalysts makes them ideal model systems to study the impact of pore diffusion on the catalytic performance. In summary, the developed synthesis approach produces PdNP/TiO2 catalysts with intrinsic activity and selectivity comparable to catalysts reported in literature. Furthermore, the new TALH-synthesis route enables the synthesis of model-type catalysts with well-defined uniform pore structure, where the pore size and pore structure can be tuned easily independent of other catalyst parameters. 2.5. Conclusions. The commercially available water-soluble titanium precursor TALH was used to prepare mesoporous and hierarchically meso-macro porous TiO2 films by templateassisted dip-coating. Block copolymer templates (Pluronic F127 and 10k-PB) produced ordered porosity on a meso scale with narrow pore size distribution (F127: 7 nm, 10k-PB: 21 nm). Combining F127 with PMMA spheres casts hierarchically porous TiO2 films with accessible mesopores located inside of the macropore walls. Furthermore, mesoporous and hierarchically porous Pd-NP/TiO2 catalytic coatings were prepared in a one-step procedure by adding Pd colloids to the dip-coating solution. Due to the neutral pH and water tolerance of TALH a stable solution containing titanium precursor, preformed Pd colloids and two kinds of templates was formed. The solutions neutral pH enabled also the coating of pH-sensitive substrates such as stainless steel, which is the most common reactor material in catalysis applications. All synthesized Pd-NP/TiO2 films were nanocrystalline and exhibited high surface areas of about 160−200 m2 g−1. The mesoporous and hierarchically porous catalytic coatings proved to be highly active and selective in the gas-phase hydrogenation of 1,3-butadiene to butenes. The developed approach represents a modular concept for the synthesis of TiO2-based catalytic coatings. TALH is the key element of this versatile synthesis platform outlined in Scheme 1. The synthesis can produce mesoporous and hierarchical porous colloid-based catalysts with controlled pore sizes and particle diameters. With this new approach, intrinsic catalytic activity and pore diffusion can be tuned more precisely and independent of each other. Moreover, the provided flexibility can be extended also to metal colloids of other composition and particle sizes than the studied Pd-NP. Thus, the concept provides an unique platform not only for fundamental studies of catalytic reactions but also for the rational optimization of catalysts for different practical applications.

the synthesis and calcination and provides a high Pd dispersion in the final catalytic coatings. 2.4. Catalytic Performance in Butadiene Hydrogenation. Both Pd-NP/TiO2 catalysts templated with F127 as well as with F127+PMMA show high catalytic activity in selective hydrogenation of 1,3-butadiene. Figure 10A illustrates the effect of temperature on 1,3-butadiene conversion over the two studied catalysts. The butadiene conversion increased with temperature and was very similar for both catalysts over the studied temperature range. At 150 °C, the butadiene conversion amounted to 89% and 92% for Pd-NP/TiO2/F127 and PdNP/TiO2/F127+PMMA catalysts, respectively. Thus, the data indicate that the catalytic tests were performed in the kinetic regime, i.e. the absence of mass-transport limitations. Moreover, the Arrhenius plots constructed for both catalysts in the 40−80 °C interval were fitted by a straight line, yielding the same activation energy of 62 kJ/mol from the slope. This value compares well with activation energies reported in literature for Pd/Al2O3 catalyst, i.e. between 48−6644 and 75 kJ/mol.45 Due to strong metal−support interactions between Pd and TiO2 the presence of titania significantly increases the selectivity of Pd-based catalysts toward butenes in the butadiene hydrogenation reaction. Lee et al.46 reported e.g. that also the modification of Pd/SiO2 catalysts with TiO2 improved the 1butene selectivity. In our study the products observed for both tested Pd-NP/TiO2 catalysts were 1-butene, trans-2-butene, cis2-butene, and small amounts of n-butane. Figure 10B illustrates the selectivity to 1-butene and the total selectivity to butenes (sum of S1‑butene, Strans‑2‑butene, and Scis‑2‑butene) as a function of butadiene conversion. Both catalysts exhibit high selectivity to butenes and yield practically the same selectivity curve over the range of conversions. Moreover, both catalyst are highly stable under reaction conditions and feature identical activity and selectivity behavior also after several hours, in repeated runs under identical reaction conditions as well as after several months of storage in air. The selectivity to 1-butene of 55% observed in this work at butadiene conversions up to about 50% is in line with values reported in literature for other Pd-based catalysts.32 Moreover, the catalytic behavior observed here over Pd-NP/TiO2 is similar to that of Pd/Al2O3 recently reported by Pattamakomsan et al.47 They studied materials prepared via conventional impregnation of porous Al2O3 (surface area: 6 and 46 m2/g) with Pd (II) acetylacetonate. Their 0.5 wt % Pd catalyst with Pd particle size of 2−6 nm gave 50% conversion of 1,3-butadiene with a 1-butene selectivity of 50 to 60%. In addition, also the decrease in butene selectivity in favor of butane production as evident from Figure 10 for high butadiene conversion is consistent with the typical behavior of Pd based catalysts reported in literature.46 The catalytic performance of the developed porous Pd-NP/ TiO2 catalysts was compared to literature values computing the space-time yield (STY) of butene formation at a temperature of 50 °C. The experimentally derived STY over hierarchically porous Pd-NP/TiO2 at 50 °C amounted to 0.019 mol s−1 kg−1. This value is similar to the STY of 0.016 mol s−1 kg−1 obtained by Pattamakomsan for their Pd−Al2O3 catalyst loaded with 0.5 wt % Pd47 (calculation based on the diagrams in their publication). However, Lee et al.46 reported for a TiO2-modified Pd-SiO2 catalysts an STY of ca. 0.061 mol s−1 kg−1, i.e. about three times higher than over the hierarchical porous Pd-NP/TiO2. This higher STY observed by Lee et al. can in parts be explained by the higher Pd loading (1 wt % Pd) of their catalyst. In conclusion, the developed Pd-NP/TiO2 catalytic coatings show in the kinetic

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Corresponding Author

*E-mail: [email protected]. Author Contributions

C.Z. and I.L. contributed the XPS analysis of the materials. 3837



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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge experimental support by T. T. Ahner and D. Bernsmeier and recording of XRD data by B. Eckhardt. I.L. and C.Z. thank C.-H. Fischer, B. Höpfner, and A. Grimm for their contributions during the BESSY beamtime. E.O., K.W., B.P., and R.K. acknowledge generous funding from BMBF within the frame of the Nanofutur program (FKZ 03X5517A). R.K. thanks also the German Cluster of Excellence in Catalysis (UNICAT, EXC 314) funded by the German National Science Foundation (DFG) and managed by the Technical University Berlin for financial support.

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TiO2 Catalytic

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