Oxide Nanomaterials: Thermal

Dec 8, 2010 - Flame spray pyrolysis (FSP) was utilized to fabricate palladium in silica nanocomposites (core−shell structure) in a single step. The ...
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J. Phys. Chem. C 2011, 115, 1269–1276

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From Embedded to Supported Metal/Oxide Nanomaterials: Thermal Behavior and Structural Evolution at Elevated Temperatures† Stephanie B. Bubenhofer, Frank Krumeich, Roland Fuhrer, Evagelos K. Athanassiou, Wendelin J. Stark, and Robert N. Grass* Institute for Chemical and Bioengineering, ETH Zurich, CH-8093 Zurich, Switzerland ReceiVed: July 15, 2010; ReVised Manuscript ReceiVed: NoVember 19, 2010

Flame spray pyrolysis (FSP) was utilized to fabricate palladium in silica nanocomposites (core-shell structure) in a single step. The nanometer scale transformation of these materials to Pd dispersed on an amorphous silica matrix (oxide-supported noble metal) at elevated temperatures (500-900 °C) was investigated using transmission electron microscopy and CO chemisorption. A spatially resolved (1-D) population balance model was utilized to describe the transformation by diffusion and aggregation processes. The model describes the influence of temperature, matrix viscosity, particle sizes, and concentrations and enables a prediction of the morphology (core-shell vs supported) of metal/silica nanocomposites processed at elevated temperatures. The data are discussed in terms of aerosol formation mechanisms (consecutive vs simultaneous coagulation) and compared to literature data on the high-temperature formation of nanocomposites. To illustrate the validity of the physical model and mechanisms, a network modifier (CaO) was added to the glassy matrix of the composite (Pd/CaO/SiO2), decreasing the matrix viscosity and resulting in the predicted morphology. Introduction The relation between structure and function is of major importance at material interfaces which often excel in terms of chemical or physical effects.1-7 Metal/oxide interfaces account for a majority of heterogeneous catalysts, driving technically relevant industrial processes.8,9 As a direct consequence, scientific efforts are shifting from single-component nanoparticles (shape effects,10 degree of aggregation11) to multiphase nanocomposites.12 The increased interest in these materials arises from the possibility to contact two different materials at the nanometer scale and profit from a highly increased interaction area which can result in new physical effects.13-15 The thermal evolution (e.g., heat treatment during preparation or use of catalyst) of metal/oxide nanocomposites determines the performance of these materials. This study uses a hightemperature, dry aerosol investigation (flame synthesis) to follow the structural changes of oxide/metal nanocomposites. This method has been previously applied for the fabrication of core/ shell particles,14,16-22 janus-shaped particles,23 and noble metals supported on oxide carriers.24,25 The latter materials have gained great interest in the field of heterogeneous catalysis as reported by Strobel et al.,26-28 Hannemann et al.,29 and Somboonthanakij et al.30 Much is known regarding the structure-function (structureactivity) relationship of precious metal/metal oxide nanocomposites in heterogeneous catalysis (e.g., precious metal dispersion, state of reduction, pore size distribution).31,32 The formation of key structural elements during the catalyst preparation utilizing elevated temperatures, however, has still not been fully understood.14,28,33-35 More precisely, the formation of multicomponent (multiphase) nanoparticles from a single-phase (homogeneous) precursor solution is still under consideration. This systematic study on the model compound Pd/SiO2 demonstrates the morphological transition from metal embedded †

Part of the “Alfons Baiker Festschrift”. * Corresponding author. E-mail: [email protected].

in oxide to metal supported on oxide nanocomposites. This transition is followed in terms of metal crystallinity, surface area, and dispersion prior to and after additional heat treatments. The study shows how the nontraditional materials (metal inside an oxide) transform into the classical metal-on-oxide nanomaterial (i.e., supported catalyst).26-30 A physical diffusion/ aggregation model is used to discuss the structural evolution of metal/metal-oxide nanostructured particles in high-temperature processes and is used to further describe the nanocomposite formation mechanism at elevated temperatures. Experimental Methods Particle Synthesis. Materials. The Pd/SiO2 nanocomposite particles were prepared by conventional flame spray synthesis from a multicomponent precursor, obtained by mixing tetraethoxysilane (TEOS, ABCR, 98%) with a palladium precursor in different ratios corresponding to 2, 10, and 30 volume percent of palladium in the particles. For the Pd precursor (1.5 wt % Pd), tetraaminepalladium(II) hydrogen carbonate (Umicore, Hanau, D) was dissolved in 2-ethylhexanoic acid (Fluka Analytical, puriss. >99%) by stirring at 140 °C for 1 h. Prior to the flame spray synthesis, the precursor was diluted 2:1 (w/w) with toluene (technical) and filtered. For the formation of calcium comprising nanoparticles, calcium 2-ethylhexanoate36 (5.5 wt % Ca) was added to the precursor mixture in the appropriate proportions. The total metal concentration (Pd, Si, and Ca) in all precursors was kept constant at 1.7 wt %. Flame Spray Setup. The multicomponent precursor was delivered (3 mL min-1) through a capillary (diameter 0.4 mm) into a spray nozzle, where it was dispersed by oxygen (3 mL min-1) into an methane/oxygen flame (CH4, 1.13 L min-1; O2 2.4 L min-1). The nanoparticles were collected by a glass fiber filter (Whatman GF/A, 25.7 cm diameter) situated above the flame in the off-gas. A detailed experimental description of the flame spray setup can be found in Madler et al.37 The synthesis conditions are noted in the form (liquid precursor feed rate/

10.1021/jp106576k  2011 American Chemical Society Published on Web 12/08/2010

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TABLE 1: Prepared Materials with Physical Characteristics material

pretreatmenta

SSA BET (m2 g-1)

dBETb (nm)

Pd dispersionc (%)

dPdd (nm)

SiO2 2 vol % Pd/SiO2 “ “ “ “ 10 vol % Pd/SiO2 30 vol % Pd/SiO2 2 vol % Pd/SiO2/30 vol % CaO

no (260 °C) no (260 °C) 500 °C 700 °C 800 °C 900 °C no (260 °C) no (260 °C) no (260 °C)

272 284 280 232 146 232 193 128

8 7 7 9 15 7 6 15

0.2 1.1 7.0 12.4 4.1 -

16 9 27 -

a 260 °C ) max temperature prior to CO chemisorption. b From specific surface area (SSA) with dBET ) 6/(SSA × density). c Stoichiometry factor (Pd/CO) ) 2. d From Pd dispersion, only given for Pd disperson >1%.

Figure 1. Representative transmission electron microscopy images of 2, 10, and 30 vol % Pd in SiO2 showing an incorporation of the metal nanoparticles in the silica matrix and a clear increase in metal particle size for higher volume fractions.

dispersion gas feed rate) using milliliters/minute and liters/ minute as units. SiO2 nanoparticles (3/3 sprayed) were used as a reference. Particle Analysis. X-ray diffraction patterns were obtained with X’Pert PRO-MPD (Cu KR radiation, X’Celerator linear detector system, step size of 0.05°, 45 kV, 40 mA, ambient conditions). Transmission electron microscopy investigations were carried out with a FEI Tecnai F30 (FEG cathode, operated at 300 kV, point resolution ∼2 Å). Ten particles were considered on different images for particle size determination from TEM images. To get statistical information on the size distribution and particle size variability, 334 Pd clusters were measured for one sample (2 vol % Pd/SiO2 heat treated (700 °C)). A standard deviation of 0.5 nm was achieved. The specific surface area was determined by nitrogen adsorption (BET, Tristar Micromeritics Instruments), whereby the samples were kept at 140 °C for 3 h under vacuum prior to the measurement. To observe the palladium segregation in dependence of the temperature, the samples were treated at different temperatures (20 min of He flushing at 20 mL min-1, followed by heating at 10 °C min-1 in flowing hydrogen at 20 mL min-1 up to various final temperatures (1 h at the final temperature)). CO-pulse chemisorption was used to determine the Pd dispersion at the surface (at 35 °C, He flow 50 mL min-1, pulses 0.5 mL (10% CO in He), Micromeritics Autochem II2920). Before each chemisorption experiment, the sample was freshly reduced (1 h at 250 °C with a hydrogen flow rate of 20 mL min-1, followed by He flushing at 50 mL min-1 for 1.5 h at 260 °C). For the calculation of the surface average primary particle size, the density of Pd/SiO2 was calculated from the sum of the volume-weighted individual densities (dp,BET ) 6/(F · SSA)). For the adsorption of decanethiol, the samples (6-10 mg) were immersed in hexane (20 mL), and 0.2 mL of decanethiol was added. Following good mixing (3 min) and short ultrasonication (1 min), the particles were sedimented using a

benchtop centrifuge. The particles were washed with hexane, water, ethanol, and acetone, each time intensely agitating the powder with 10 mL of the liquid and subsequent separation by centrifugation. After the last washing step, the powder was allowed to dry in a vacuum centrifuge (Concentrator Plus, Eppendorf) at room temperature. IR spectra of the powders (after mixing in KBr, 5 wt % of sample) were recorded using a Bruker Tensor 27 equipped with a diffuse reflectance unit (resolution 4 cm-1, 200 scans). Results The conversion of silicon and palladium containing precursors via flame spray pyrolysis yielded brownish to black powders, depending on the Pd content (2, 10, and 30 volume percent of metal in the final powder). The powders exhibited high electrostatic charging evidencing their low electrical conductivity. X-ray diffraction patterns of the materials (Supporting Information, Figure S1) show an amorphous phase (silica) and a crystalline metallic palladium phase in all samples. The collected data do not give any evidence of oxidized palladium (PdO) as found in catalysts prepared by wet impregnation.38 While the BET specific surface area decreased from 284 m2 g-1 for 2 vol % Pd down to 193 m2 g-1 for 30 vol % Pd while keeping all synthesis conditions equal, the surface average primary particle size remained within 7 ( 1 nm (see Table 1). This result has been verified by the transmission electron micrographs (Figure 1), which show highly aggregated amorphous silica particles and individual crystalline palladium spheres. The average diameter of the palladium nanoparticles (dark spheres) can be estimated to 2-3 nm for the sample with the lowest palladium fraction (2 vol % Pd). It clearly increases with the rise in palladium volume fraction, to around 5-8 nm (30 vol % Pd). The specific particle size distribution of nanoparticles formed by flame spray synthesis is described by a geometric standard deviation of 1.3-1.5. For the materials

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Figure 2. Transmission electron micrographs of Pd/SiO2 (2 vol %) showing the as-prepared sample and samples treated for 1 h at 260, 500, 700, 800 and 900 °C. Diffusion of the metal clusters to the surface of the SiO2 matrix is observed for a treatment above 700 °C, and growth of the metal clusters can be observed after treating at 900 °C. Pd clusters clearly visible as on the silica surface are indicated with arrows.

with high volume fractions (10 and 30 vol % Pd), the metallic particles can be seen as embedded within the silica matrix. Due to the low contrast of silica in TEM imaging, the phase boundary of the matrix is often hard to identify. Especially in the case of the low palladium loading (2 vol % Pd), it is unclear from the images if the metallic clusters are situated on the surface of the silica matrix, or embedded within. To discriminate between palladium in silica and palladium on silica for the sample comprising 2 vol % Pd, the strong interaction between metallic palladium and carbon monoxide was utilized. CO pulsed chemisorption was carried out on the flame derived material. To guarantee a reduced Pd surface, the nanoparticles were first reduced in a hydrogen stream at 250 °C for 1 h as previously proposed by Strobel.26 Only minute amounts of CO adsorbed on the flame-derived material, indicating the absence of significant amounts of metallic palladium on the particle surface and corroborating a nanoparticle structure consisting of palladium in silica. The presence of palladium oxide species on silica can be excluded based on the argument that such species would have been reduced to metallic Pd during the highly reducing pretreatment (H2, 250 °C). The data stated in Table 1 are given as Pd dispersion, i.e., the fraction of Pd atoms present on the material’s surface (assuming a stoichiometry factor Pd/CO of 2).39 From Embedded to Supported Noble Metal Nanocomposites. The material comprising 2 vol % Pd was heated to final temperatures (10 °C min-1 in flowing hydrogen 20 mL min-1) in the range of 500-900 °C for 60 min. After each heating step, TEM images of the material were taken (Figure 2), and the palladium dispersion was measured by CO chemisorption. The adsorption data (Figure 3) shows that at temperatures below 500 °C the structure (Pd in SiO2) does not change. CO molecules are only adsorbed on the surface if the material has been treated at temperatures exceeding 700 °C. This observation reveals that the material has restructured at these temperatures, leaving Pd atoms on the surface. A look at the corresponding TEM images

Figure 3. Pd dispersion and calculated Pd particle size from CO-pulsed chemisorption as a function of heat treatment temperature.

(Figure 2, 700 °C, 800 °C) further supports these findings: with increasing temperatures, the Pd clusters move from the core of the silica matrix to the surface. Some clusters (indicated with arrows) can be clearly distinguished as present on the oxide surface. Further heating the material to 900 °C resulted in a sintering of the Pd clusters as evidenced by particle growth to 5-8 nm (Figure 2, 900 °C). Since the CO adsorption data are collective particle measurements data and can be used to calculate the accessible Pd metal surface area, we used the data to calculate the corresponding mean Pd cluster size (Table 1 and Figure 3 black squares).26 Such calculations are generally done under the assumption of spherical palladium clusters and full accessibility. If we consider the 700 °C sample, this assumption is obviously violated as the calculated particle size (dp,CO ) 16 nm) is much larger than the size visually determined from TEM images (dp,TEM ∼ 2 nm). Moreover, this value is also larger than the calculated Pd particle size after sintering to 800 °C (dp,CO ) 9 nm). Since splitting of metal Pd clusters during sintering makes little sense, this finding indicates that in the original, flame-derived material almost all Pd has been

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Figure 4. Mechanisms during high-temperature processing of single-phase, two-component precursors leading to metal-oxide (e.g., SiO2)/noble metal nanocomposites (e.g., Pd/SiO2). Following decomposition of the precursors, either a consecutive coagulation or a simultaneous coagulation of the two materials forms the particles. For simultaneous coagulation two diffusion processes have to be differentiated, metal cluster (particle) diffusion or ion/atom diffusion, leading (given enough time) to the same end products.

covered by silica and has therefore been inaccessible to CO. The maximal Pd dispersion (12.4%) is achieved after sintering to 800 °C and is in good agreement with the Pd dispersion of other flame-derived as well as commercial catalysts.26,40-43 Pd on glass nanoparticles could also be prepared directly in the flame (Figure 7a). This was done by incorporating a calcium source into the precursor and resulted in a material comprising Pd (2 vol %) dispersed on an amorphous calcium silicate glass (30 vol % CaO, Figure S3, Supporting Information). As the presence of CaO in the matrix impeded the use of CO adsorption (CO/CaO interferes with the CO titration of the Pd surface), adsorption of an aliphatic thiol (R-SH) was used to prove the presence of metallic palladium on the surface of the particles (Figure 7b). Analogous experiments on as-prepared and heated (700 °C) 2 vol % Pd/SiO2 also clearly show a functionalization on the heated particles (Pd on SiO2, Supporting Information, Figure S2). This procedure is in accordance with the formation of self-assembled monolayers on metallic Pd, Pt, and Au surfaces.44 Discussion Pd in Silica. The experimental data collected on Pd/SiO2 nanocomposite particles formed by flame spray pyrolysis clearly show that under standard flame operating conditions the metallic palladium is situated inside of an amorphous silicate matrix. While for high volume fractions of metal this is evident from transmission electron imaging, CO adsorption experiments are necessary to prove the absence of Pd on the surface for samples containing low metal contents. This is in contrast to earlier findings on similar flame processed materials, which all presented the metal on the surface of the ceramic carrier.26-30 On the basis of the previously published findings (metal on ceramic), a mechanism for the formation of this structure in flames was formulated by Johannessen and Koutsopoulos.45 The presented mechanism describes a two-step process, wherein the ceramic phase nucleates first and grows to its final size before the metal nucleates and deposits on the surface of the ceramic. The mechanism assumes that at a given high-temperature region of the flame the vapor pressure of the ceramic is much lower than the vapor pressure of the metal, resulting in a consecutive coagulation pattern (see Figure 4 and Figure 1 in Jensen et al.46). The new observations reported within this paper (Pd in silica) and previous findings for silver34 show that at least for the case

of Pd and Ag with silica the mechanism given by Johannessen and Koutsopoulos45 does not completely represent the structure formation of the composite particles. The presence of Pd within the silica implies that the metal has condensed prior to or simultaneously with the ceramic phase (see Figure 4). Further investigations by heat treatment (Figure 2, 700 and 800 °C) show that the Pd in silica nanomaterial can be easily transformed to a Pd on silica material without significantly changing the particle sizes of the metallic clusters. By comparing the TEM images (Figure 2) of the as-prepared Pd(2 vol %)/ SiO2 particles with the ones heated for 1 h at 700 °C, it is evident that Pd particles have shifted from the core of the silica to the surface, which is verified by the CO-chemisorption measurements (Figure 3). By additional heating, more Pd shifts to the surface of the silica matrix, which results in an increased Pd dispersion (Figure 3 and Table 1), with the maximal effect after a heat treatment of 800 °C (Pd dispersion ) 12.4%). The decreasing dispersion at 900 °C can be explained by the sintering and aggregation of the Pd at the surface of SiO2, which can also be observed as slightly bigger Pd clusters in the TEM image. The temperature-induced transformation of the material (Pd in SiO2 f Pd on SiO2) leads to the conclusion that the observation of a metal on a ceramic nanocomposite does not presuppose a consecutive coagulation mechanism as proposed by Johannessen and Koutsopoulos.45 Physical Description of Transformation. To describe the transformation of the Pd in SiO2 to Pd on SiO2 nanocomposites, a physical model based on Pd cluster diffusion and aggregation within silica spheres was derived. A schematic of the model is given in Figure 5. At elevated temperatures, metallic clusters are allowed to diffuse within the amorphous silica matrix (assumed as spherical). The direct consequences of this diffusion are the aggregation of the Pd clusters within the silica sphere and the dislocation of the Pd clusters to the surface of the matrix. It is assumed that the metallic clusters can not re-enter the silica matrix once on the surface, as thermodynamically the Pd on SiO2 conformation is more stable (minimizing the surface area of the interface Pd/SiO2). Under these assumptions, a spatially resolved (1-D, spherical) population balance of metal clusters diffusing within a spherical matrix can be derived47

From Embedded to Supported Metal/Oxide Nanomaterials

where ui is the normalized number of metal aggregates consisting out of i primary particles; z is the normalized radius of the matrix particles (r/rd); k stands for the Boltzmann constant; µ is the viscosity of the matrix; T is the temperature; and t stands for time. N0 is the initial number of primary metal particles per unit volume with diameter a in a matrix particle with diameter rd. D0i is the dimensionless diffusion coefficient of the ith aggregate and βij0 the dimensionless aggregation constant between the ith and jth aggregate. Assuming instantaneous coalescence (sintering) of the metallic clusters at the elevated temperatures, D0i ) i-1/3 and β0ij ) (i-1/3 + j-1/3) · (i1/3 + j1/3) are well-defined.48 The boundary conditions were chosen so that the metal clusters which diffuse out of the sphere are considered as fixed on the surface and no longer contribute to the diffusion and aggregation process within the sphere (ui(t, rd) ) 0). Details on the algorithms used for solving the equation and a validation of the model can be found in the Supporting Information. The first part in the equation represents the diffusion of the metallic clusters within the matrix, and the second part of the equation represents the aggregation (and instantaneous coalescence) of metallic clusters to give clusters of increased size. The equation shows that both processes are accelerated by

Figure 5. Schematic representation of diffusion and aggregation of metal clusters in an amorphous matrix. For higher volume fractions, aggregation is accelerated (see eq 1).

J. Phys. Chem. C, Vol. 115, No. 4, 2011 1273 increasing the temperature, both directly (factor of T) and indirectly via the influence of temperature on the matrix viscosity µ (the viscosity of silica decreases with increasing temperature). Whereas the aggregation term is accelerated by the initial particle number concentration N0 (equivalent to volume fraction φ0 for given initial cluster radius a; φ0 ) N0 · (4/3) · π · a3), diffusion is affected by geometric factors (radius of cluster a, radius of matrix rd). To describe the process during the heat treatment of the Pd(2 vol %)/SiO2 particles, the viscosity of the glass matrix has to be estimated. This was done by estimating the required characteristic diffusion length (ld ) 4 nm) of the Pd clusters (a ) 1 nm) required to dislocate from the center of the silica matrix to the surface during the heat treatment at 700 °C. Combining this characteristic diffusion length with the duration of the heat treatment (1 h) and the definition of the characteristic diffusion length ld ) (6Dt)1/2,49 the diffusivity of the Pd clusters, D, could be estimated. The Stokes-Einstein equation was further used to estimate the magnitude of the viscosity from the diffusivity (D ) (kT)/(6πµa)) resulting in a maximal viscosity of the silica matrix at 700 °C of ∼1 × 109 Pa s which is in line with literature data.50 The model was solved to physically describe the nanomaterial transformation during the heat treatment at 700 °C assuming an initial volume fraction of 2% of primary metal particles of a radius a ) 1 nm homogeneously dispersed within a silicate sphere of rd ) 5 nm (see Table 1, dBET). Figure 6a shows the concentration of the primary metal clusters (i ) 1) as a function of time (up to t ) 6.25 h) and space. Figure 6b depicts how the concentration of the doublets first increases with time at the center of the sphere (radius/rd ) 0) due to the aggregation of primary particles and then subsequently decreases due to diffusion and further aggregation to larger clusters. It has to be pointed out that in the presented case the role of aggregation is very small. The maximum number concentration the doublets ever reach within the sphere is ∼10% of the initial primary particle concentration (at radius/rd ) 0). The model solution for the primary particles within the matrix during the heat treatment is further shown in Figure 6c for various times, illustrating the diffusion profiles. These profiles were integrated to calculate the total amount of primary particles left within the sphere at a given time (Figure 6d, i ) 1). The same procedure for larger aggregates (i ) 2, ..., 6) revealed that within this heat treatment process the importance of larger aggregates is negligible. This is in line with a characteristic aggregation time of 3.1 h calculated from τ ) (3 µ)/(4kBTN0),48 which is longer than the actual heating process of 1 h. As expected, most of the primary particles diffused out of the sphere after the one hour heat treatment at 700 °C, and only a few larger aggregates could be observed. It has to be pointed out that in this case a continuous model was utilized to describe fairly discrete processes (only 2-10 metal clusters per silica sphere) and neglecting the effects of surface energy and matrix crystallinity. The concise conclusion of the model and the excellent agreement with the experimental findings (Figure 2) justify these simplifications for the case of 2 vol % Pd loading. For the case of higher Pd loadings (10 vol % and 30 vol %, see Figure 1), these simplifications can no longer be sustained due to the very discrete nature of the composite (one metal cluster per silica sphere) and the large silica/palladium interaction areas. Mechanism. Possible mechanisms for the formation of twophase composites in high-temperature, gas-phase processes have been previously reported for mixed oxide systems (SiO2/TiO2 and SiO2/FexOy)33,51 and discriminate between simultaneous

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Figure 6. Model results of eq 1 for a ) 1 nm metal nanoparticles in a rd ) 5 nm matrix particle (2 vol % metal). Evolution of the primary particles (a) and doublets (b) over time and radial position in the matrix. Radial distribution of primary particles at various times