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J. Phys. Chem. C 2010, 114, 14224–14232
Colloidal Nanoparticles Embedded in Ceramers: Toward Structurally Designed Catalysts P. Sonstro¨m,† M. Adam,‡ X. Wang,† M. Wilhelm,*,‡ G. Grathwohl,‡ and M. Ba¨umer*,† Institute of Applied and Physical Chemistry, UniVersity of Bremen, Leobener Strasse NW2, 28359 Bremen, Germany, and Ceramic Materials and Components, UniVersity of Bremen, Am Biologischen Garten 2/IW3, 28359 Bremen, Germany ReceiVed: June 25, 2010
Ceramers constitute a new type of porous hybrid ceramic which is obtained by pyrolysis of polysiloxane precursors with organic side groups at temperatures low enough that part of the polymer has not yet been decomposed (≈500 °C). By using different precursors, interesting possibilities arise to change the structural and mechanical properties of this monolithic material over a wide range. Moreover, the surface chemical properties (such as basicity or hydrophilicity) can be modified by varying the organic side groups. In order to add a catalytic function to such a ceramer, different preparation techniques were employed in this study with the objective of incorporating nanoparticles into the ceramers (addition of ionic precursors vs preformed colloidally synthesized nanoparticles during the synthesis). The resulting materials were structurally characterized and catalytically studied using the oxidation of CO as a test reaction. Our results reveal that especially colloidally prepared nanoparticles provide attractive options to manufacture tailored catalysts since the particle sizes, as controlled by the colloidal synthesis, can be well-preserved. It could be shown that aminopropyltriethoxysilane (APTE) leads to a homogeneous distribution of nanoparticles in the ceramer matrix and an enhanced CO oxidation activity after sample activation, an effect that can be attributed to a more effective binding of the Pt nanoparticles to the precursors of the material hindering sintering and agglomeration. The mass transport limitation due to pore diffusion was characterized for samples in the form of larger grains and finely ground powders by determining the macro kinetics (reaction orders and activation energies) and calculating the Thiele modulus. The comparison reveals mass transport limitation due to microporosity. We discuss strategies to avoid such restrictions and optimize mass transport of gaseous reactants paving the way to ceramer-based monolithic catalyst with high structural stability and an optimized distribution of monodisperse nanoparticles. 1. Introduction For industrial applications, monolithic catalysts offer several advantages1 (e.g., lower energy input, high safety, controllable mass transport, easy catalyst separation) and are in many cases preferable compared to classical packed bed reactors. Patcas et al.,2 for example, reported that monolithic ceramic foams are superior as compared to traditional pellet beds as far as mass transfer and pressure drops are concerned. Today, monolithic catalysts are intensively used in automobile exhaust treatment1 and other combustion reactions (e.g., of fuel gas3). Such applications are mainly based on low-surface-area cordierite honeycombs3,4 with high thermal and mechanical stability which, in order to increase the surface area,5 are often wash-coated with oxides, such as alumina. To additionally benefit from high thermal conductivity, also metallic monoliths6 are sometimes used (e.g., alumina-coated steel7). Another area where monolithic catalysts attract more and more attention are low- or medium-temperature applications (e.g., methanol steam reforming,5 oxidative dehydrogenation of propane,8 and liquid-phase hydrogenations9). Here, catalysts with a large specific surface area10 are in the focus of research. Specifically, porous materials such as foams11 or aerogels12,13 * To whom correspondence should be addressed. Tel.: +(49) 42121863170. Fax: +(49) 421-21863188. E-mail:
[email protected] (M.B.);
[email protected] (M.W.). † Institute of Applied and Physical Chemistry. ‡ Ceramic Materials and Components.
are investigated as monolithic catalytic supports, sometimes with the drawback of low mechanical stability.6 A versatile use of monolithic catalysts is confronted with the need to synthesize tailored, well-defined porous structures with optimized mass and heat transfer properties and low pressure drops.2,14 Structural imperfections shown to have a significant influence on these characteristics should be avoided.15 If the monolith itself is not catalytically active, an active component has to be added to the monolithic structure. To this end, many different procedures have been developed, ranging from incorporation of the active material into the monolith (e.g., by extrusion16) or into the wash-coat slurry (e.g., sol-gel routes17) before its deposition onto the monolith to direct deposition of the catalyst1 on the surface of the monolith (e.g., by dip-coating3 or impregnation18). However, a homogeneous distribution of the active component in or on the monolith is often difficult to achieve leading to concentration gradients.19,20 Additionally, good adhesion of the active material to the monoliths can be a problem so that leaching occurs during catalytic reactions in solvents.21 Furthermore, in the case of nanoparticles sintering22 or aggregation23 can result in severe activity losses if the particles are not sufficiently anchored to the surface of the monolith. Thus, a wider use of monolithic catalysts in industrial applications poses the challenge to create a tailored, well-defined monolithic catalyst with a structure that offers a wide range of possibilities to control and optimize pressure drops and mass and heat transfer. Furthermore, the structure of the monolith
10.1021/jp1058897 2010 American Chemical Society Published on Web 08/03/2010
Colloidal Nanoparticles Embedded in Ceramers should enable not only physical but also chemical bonding of nanoparticles to allow for a homogeneous distribution and good adhesion of the active material, ideally maintaining its catalytic properties even under harsher reaction conditions (e.g., high temperatures). In the past few years, porous silicon based materials have attracted much attention for the synthesis of tailored, hierarchical structured monoliths.24-28 Especially promising results for the preparation of such materials have been obtained in terms of “integrative chemistry”,29-33 an approach that allows one to create complex multiscale architectures containing hierarchical trimodal pore structures with tunable surface functionality. Such functionalized surfaces were shown33,34 to facilitate nanoparticle nucleation and anchoring to create highly active catalysts. Although materials such as these have already been widely used in the field of catalysis, ranging from photo-35 and base36 catalysis as well as Heck reactions34 in the liquid phase to heterogeneous gas-phase reactions,37,38 the use of so-called ceramers, microporous hybrid ceramics with large surface areas that can be obtained24 from polysiloxane precursors by pyrolysis in inert gas at temperatures between 400 and 700 °C, as monolithic supports in catalysis is still scarce.39 In this study, we have chosen ceramers as a versatile material for the synthesis of monolithic supports because they offer distinct advantages as compared to many silicon-based materials or other monoliths, such as the widely used cordierite: first, in analogy to emulsion-based functionalized silicon monoliths (organo-Si(HIPE)),33 a large number of different (poly)siloxanes40 can be used as precursors, but in contrast to these materials the organosilane amount is not limited to a certain range in order to create stable monolithic structures. This provides even more flexibility to obtain various hybrid ceramic materials with different structural and mechanical properties (e.g., porosity,41 thermal stability,42 and tensile strength43). Second, after pyrolysis part of the polysiloxane still has not been decomposed.41 These organic residues possess, of course, a much higher thermal stability than the unpyrolyzed organosilanes and also offer various options to tune the surface chemistry (e.g., by changing acid/base properties44 and hydrophobicity,39 selective binding33,45 of the catalyst to the support). This renders ceramers highly flexible with respect to adapting structural and chemical properties specifically to the desired application. While the general applicability of this monolithic approach for catalysis has already been briefly discussed in a previous work,39 several new issues are addressed in this paper. In particular, we compare here the previously reported approach to generate nanoparticles in situ during the synthesis of the ceramer by taking advantage of the complexation properties of the polysiloxane precursor for metal ions with the use of ex situ, i.e., colloidally prepared, nanoparticles. Although the first strategy allows convenient one-pot syntheses, the second one offers the distinct advantage to tune particle size and morphology independently of the monolithic structure. In both cases, the metal is added prior to cross-linking and pyrolysis, which allows for optimal inclusion of the nanoparticles into the ceramer. In contrast to systems34 where the particles are created in a subsequent step and located mainly on the outer pore walls, this approach maximizes, e.g., sintering protection. To shed more light onto the anchoring of Pt to the ceramer, we will furthermore discuss experiments with a bifunctional surfactant, i.e., aminopropyltriethoxysilane (APTE), which was additionally added in some of the preparations to enhance the binding of the nanoparticles to the preceramic matrix.
J. Phys. Chem. C, Vol. 114, No. 33, 2010 14225 Finally, we will deal with the question whether the microporosity of the ceramer is sufficient to enable effective transport of gaseous reactants (e.g., CO) through the monolithic system. To this end, kinetic measurements were performed with small grains of ceramer samples and compared with samples ground to fine powder. Reaction orders and activation energies were determined for CO oxidation. Aiming at the quantification of mass transport limitation, the Thiele modulus and concentration profiles are calculated. On the basis of these results, possible ways to circumvent or minimize mass transport limitation are discussed. 2. Experimental Section Ex situ Pt nanoparticles were synthesized by the ethylene glycol method according to Wang et al.46 Briefly, H2PtCl6 from Chempur was dissolved in ethylene glycol (EG, Fluka), mixed with a solution of sodium hydroxide (≈0.4 M) in EG at room temperature and then stirred at 160 °C for 3 hours. Addition of 1 M hydrochloric acid in water to the black solution resulted in precipitation of Pt nanoparticles. For further use the nanoparticles were redispersed in tetrahydrofuran (THF, Riedel-de Hae¨n) or acetone (Sigma-Aldrich). For the preparation of platinum-containing polysiloxane materials 3-aminopropyltriethoxysilane (APTE, ABCR) was dissolved in THF and a solution of H2PtCl6 or preformed Pt nanoparticles in THF were added under stirring. To get a complete complexation of the platinum ions/particles by amino groups, a molar ratio of 8:1 between APTE and platinum ions/ atoms was chosen. A methylphenyl polysiloxane (H44, Wacker Chemie) was added to this solution. After partially evaporating the solvent, the sample was cross-linked at temperatures up to 170 °C. Samples with 0.5, 1, and 2 mol % Pt (compare with Table 1) were prepared. Some samples were also synthesized without APTE as surfactant. For platinum-free polysiloxane samples, which were used as reference and support material, a solution of APTE in THF was mixed with H44. After evaporation of the solvent the material was also cross-linked as described above. For comparison, in one case Pt nanoparticles were subsequently supported on a ceramer. This sample (denoted Pt/ ceramer) was synthesized by a colloidal deposition method: grains (diameter 0.315 < x < 0.45 mm) of the platinum-free support (ceramer pyrolyzed at 500 °C) were added to an acetone solution containing colloidal Pt nanoparticles synthesized as described above. The solution was manually stirred until the solvent evaporated to obtain a supported Pt catalyst (metal loading 2.03 wt % as determined by atomic absorption spectroscopy, AAS) as reference material. The conversion of the polysiloxanes to a hybrid ceramic material was performed by an inert gas pyrolysis (under flowing nitrogen) at temperatures between 500 and 1000 °C. The specific surface areas were determined by BET nitrogen adsorption/ desorption isotherms recorded at 77 K (Gemini 2375, Micromeritics) after the samples were outgassed for 20 h at 100 °C under flowing argon. To determine the pore size distribution, nitrogen adsorption/desorption isotherms were recorded up to p/p0 ) 1 (ASAP 2010, Micromeritics) using DFT for the calculation. Before measurements the samples were outgassed for 24 h at 100 °C in vacuum. Bright-field TEM images were recorded with a Tecnai F20 S-TWIN electron microscope (FEI) operated at 200 kV, whereas a voltage of 120 kV was used for scanning transmission electron microscopy/energy-dispersive X-ray spectrometry (STEM/EDX) measurements. Table 1 summarizes the Pt content of the samples used for catalytic measurements. To render a comparison between
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TABLE 1: Calculated Pt Content of the Different Samples Pyrolyzed at 500 °C sample ex situ, APTE ex situ in situ, APTE in situ ceramer
Pt content (wt %)a c
2.17 (2.00 ) 2.37 2.32 (2.26c) 2.33 2.30 (2.03d)
sampleb
Pt content (wt %)a
∼0.5 mol % Pt ∼1.0 mol % Pt ∼2.0 mol % Pt
1.16 2.32 (2.26c) 5.13
a Values calculated on the basis of the used precursors. b All samples in this column are the APTE-containing in situ samples depicted in Figure 6. c Determined by elemental analysis. d Determined by atomic absorption spectroscopy.
samples synthesized on different routes possible, the metal loading for these samples was kept in the same range as was controlled by elemental analysis (performed by Mikroanalytisches Labor Pascher, Remagen, Germany) and AAS with a 5FL flame AAS (Zeiss). The catalytic experiments were carried out in a fixed bed reactor using 3 vol % CO in synthetic air (total flow: 50 mL/ min). In general, the original samples were broken into grains of a defined size (diameter 0.315 < x < 0.45 mm) of which 10 mg were mixed with 800 mg of quartz (Roth, diameter 0.4 < x < 0.8 mm, calcined for 6 h at 1000 °C). Some measurements were also carried out by using powder samples (obtained by crushing the original samples) to elucidate the influence of mass transport limitation. The samples mixed with quartz were placed as a thin layer (≈5 mm) on quartz wool in a quartz tube (diameter ) 15 mm). Another piece of quartz wool was put on top of the sample to prevent particles from being carried away by the gas flow. For analysis of products (and reactants) a photometric detector for CO/CO2 (Hartmann & Braun URAS 3G) was used. For experiments in which the temperature was increased/decreased stepwise, the temperature was kept constant for at least 15 min. The kinetic measurements were performed at different temperatures in order to find suitable reaction conditions for each sample. In detail, the reaction order for CO was determined at 140 °C (powder samples) and 180 °C (grains), whereas the reaction order for O2 was measured at 180 °C (powder samples) and 220 °C (grains). Turnover frequencies (TOFs) were calculated by assuming spherical particle shape and accessibility of all Pt surface atoms. Because all Pt particles are in contact with the ceramer and a part of their surface is not available for CO oxidation, the calculated TOF values represent lower limits. Due to mass transport limitation, in the case of the grains, TOFs are only given in the case of the powdered sample. In the former case, CO2 conversion is given in mmol/s/gPt. The metal loadings and average particle sizes were taken from Tables 1 and 2, respectively. 3. Results and Discussion 3.1. Structural Properties. The transmission electron micrographs (TEM) depicted in Figure 1 show that the incorporation of Pt nanoparticles into the ceramers can be obtained in two synthetic ways: the direct in situ synthesis (Figure 1A) where H2PtCl6 is added to the polysiloxane precursor,39 and the ex situ synthesis (Figure 1C), where the Pt nanoparticles are synthesized by a colloidal method46 in a preceding step and added to the polysiloxane precursor subsequently. (The latter approach makes use of a versatile and simple route (cf. the Experimental Section) to obtain mono- and also bimetallic nanoparticles47 with a narrow size distribution, which cansif
TABLE 2: Particle Size Distribution, Specific Surface Area, and Cumulative Pore Volume in the Microporous Range of Selected Samples
samplea ex situ, no APTE ex situ in situ, no APTE in situ in situ, 500 °Cf in situ, 600 °Cf in situ, 800 °Cf in situ, 1000 °Cf
mean surface cumulative particle deviation area pore volume size (nm)b (nm)b (m2/g)c e2 nm (cm3/g)d ≈4.4e 1.98 ≈4.2e 2.82 3.34 2.60 3.05 3.75
e 0.30 e 0.40 0.43 0.55 0.66 0.94
563 362 593 443 418 186
