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Influence of the Microstructure of Gold-Zirconia Yolk-Shell Catalysts on the CO Oxidation Activity Arti Dangwal Pandey,† Robert Gu¨ttel,† Matteo Leoni,‡ Ferdi Schu¨th,† and Claudia Weidenthaler*,† Max-Planck-Institut fu¨r Kohlenforschung, Mu¨lheim (Ruhr), Germany, and Department of Materials Engineering and Industrial Technologies, UniVersity of Trento, Via Mesiano, 77-38123 Trento, Italy ReceiVed: July 12, 2010; ReVised Manuscript ReceiVed: September 16, 2010
The gold-zirconia yolk-shell system is an interesting catalyst for CO oxidation. The size distribution of the gold nanoparticles is very narrow, and they are well separated from each other also after treatment at high temperature, which is due to their encapsulation in crystalline zirconia hollow spheres. Because this allows thermal and chemical treatment without affecting the size distribution, different defect structures of the gold nanoparticles can be induced, and the effect on catalytic activity can be investigated. Line profile analysis of the powder diffraction data based on the whole powder pattern modeling approach was used to determine the domain size distribution and lattice defects present in this two-phase system. The influence of different diffractometer setups on the results of the line profile analysis was also investigated. Variation of the chemical and thermal treatment procedures allowed altering the microstructure of the system. The resulting catalysts showed substantial variation in the activity for CO oxidation. Lower dislocation densities and less stacking faults result in decreased catalytic activity. These contributions to activity could be studied without any superimposed size effect due to the constant gold particle sizes. Introduction Nanoparticles are attracting great interest due to their properties that are different from those of bulk materials.1 The properties of nanostructures are, among other factors, controlled by their microstructure, which can be manipulated to exploit their unique physical and chemical characteristics for various applications.2-4 Compared to bulk gold and other transition metals, gold nanoparticles supported on metal oxides have increased catalytic activity in many reactions, such as the catalytic hydrogenation of unsaturated alcohols and aldehydes,5O2 reduction,6 and CO oxidation.7 Among these reactions, CO oxidation is most intensively studied because it is, on the one hand, very simple, allowing fundamental studies, but is also interesting for several applications, including the highly demanding treatment of automotive exhaust gas and in CO2 lasers.8 There have been many attempts to understand the factors controlling the activity of gold nanoparticles. The particle size effect has already been investigated in detail: a strong increase in the activity for gold nanoparticles smaller than 5 nm has been observed.9 Moreover, the availability of defect sites at the interface between the small gold particles and the support is supposed to be an important feature of the active sites.10-12 High catalytic activity is generally assumed to occur at active sites, such as defects on the gold surface. Pioneering work by Mohr et al. on the influence of the geometrical structure of the supported gold catalysts in the partial hydrogenation of acrolein was based on detailed TEM investigations.13 Their study showed that a higher degree of rounded particles and a lower number of multiply twinned particles (MTPs) resulted in a higher * To whom correspondence should be addressed. E-mail: weidenthaler@ mpi-muelheim.mpg.de. † Max-Planck-Institut fu¨r Kohlenforschung. ‡ University of Trento.
turnover frequency. To obtain deeper insight into the effect of the gold microstructure, a study of the dependence of the catalytic behavior of gold nanostructures on the number of defects, such as stacking faults, twins, or dislocations, would be very interesting. For bulk materials, it is well known that defects or dislocations might act as active sites in heterogeneous catalysis.14,15 However, measuring the concentration of defects becomes rather complex for nanostructures as compared with bulk materials. For example, the dislocation etch pit technique (producing micrometer-sized etch pits on the crystal surface) is an effective way to study dislocation densities and their distribution in both nonmetallic16 and metallic crystals.17,18 However, this technique cannot be applied to nanostructures due to their very small sizes. High-resolution TEM (HR-TEM) is a powerful technique for the direct visualization of local structures, but it is rather complicated to obtain information representative of the whole sample. For a statistically meaningful analysis, hundreds of particles need to be analyzed. Additionally, larger particles which cannot be transmitted by the electron beam are not considered in the analysis. Moreover, for the presented gold-zirconia (Au@ZrO2) nanostructures and also for most of the applied materials that are, in general, multi-phase compounds, the observation of detailed lattice information is additionally hampered by the interfering projection of lattice patterns of different phases (zirconia and gold). Thus, the systematic and quantitative study of lattice imperfections of catalytic materials, such as twinning, stacking faults, or dislocations, by more appropriate techniques is required. X-ray powder diffraction (XRD) line profile analysis is a powerful technique not only for characterization of size and morphology of domains but also for quantification of lattice defects present in nanomaterials.19 Any physical or chemical treatment changing the nature of imperfections or the amount of defects in a material induces appreciable variations in the intensity distribution in XRD patterns and, therefore, changes the line profile. The shift, the
10.1021/jp106436h 2010 American Chemical Society Published on Web 10/21/2010
Au-ZrO2 Yolk-Shell Catalysts for CO Oxidation TABLE 1: Sample Treatment Conditions sample series numbers I
1, 2, 3 4, 5 6, 7
II
8, 9 10
III
11 12, 13 14 15
samples
additional treatments
as-made no gold-leached NaCN treatment below 50 °C quenched sample heated to 900 °C with 2 K min-1 keeping sample for 2 min at 900 °C quenching in ice water for 5 min as-made no quenched sample heated to 900 °C with 2 K min-1 keeping sample 5 min at 900 °C quenching in ice water for 30 min as-made no heat-treated sample heated to 800/900 °C with 2 K min-1 cooling with 2 K min-1 to room temperature quenched I quenching from 900 °C in ice water for 5 min quenched II quenching from 900 °C in ice water for 30 min
broadening, and the asymmetry of reflections of a measured profile are related to the fault densities and significantly depend on the (hkl) indices of the reflections.20 During recent developments in the field of line profile analysis, whole powder pattern modeling (WPPM) has emerged as a powerful technique for microstructural analysis of nanomaterials.19 The method is based on a direct physical modeling of the microstructure from XRD data in terms of the density of specific lattice defects, shape, and size distribution of coherently diffracting domains (aka crystallites). Furthermore, powder patterns of multiphase samples with strongly overlapping peaks can be analyzed to retrieve detailed microstructural information. Measuring XRD patterns of a material with different experimental setups might result in diffraction patterns with different resolutions or backgrounds, but the data should contain the same information on the microstructure. Thus, the comparison of retrieved microstructural results for the same material, investigated with different diffractometer configurations, will be helpful to specifying more suitable measurement configurations and confirm the reliability of the analysis. In this paper, we report on the influence of the microstructure on the catalytic activity of gold-zirconia (Au@ZrO2) yolk-shell catalysts.21-23 A series of systematic diffraction experiments and catalytic measurements were performed. Not only as-made samples were analyzed but also samples that had been calcined at different temperatures or quenched in order to appreciablychange the microstructure. The domain size distribution, dislocation density, dislocation character, stacking faults, and twin fault probability were retrieved using the PM2K software24 based on the WPPM approach. The results of line profile analysis for the same material measured with different XRD configurations are discussed in order to appraise the influence of the experimental setup. Experimental Section Material Synthesis and Treatments. The standard goldzirconia Au@ZrO2 yolk-shell material was synthesized according to the literature21 in three different batches (series I-III, Table 1). To change the microstructural properties of the surface, the gold cores of one portion of the as-prepared Au@ZrO2 sample were partially leached (series I, sample 4, Table 2), using a modification of a previously described method.25 The original procedure describes the gold leaching of the intermediate Au@SiO2material, which requires a temperature treatment at 800 °C in a subsequent preparation step. To avoid high temperatures, which could distort the effect of leaching on the surface microstructure, instead of the intermediate, the final material Au@ZrO2 was treated with NaCN below 50 °C. The amount of the leaching agent NaCN was adjusted to remove
J. Phys. Chem. C, Vol. 114, No. 45, 2010 19387 about 10% of the gold mass, which reduces the size of the gold particles only slightly (less than 5%). Thus, the influence of the modified gold microstructure on the catalytic behavior can be studied without an overlapping effect that could be caused by a change in the particle size. For all three batches, a portion of the standard sample was heated to 900 °C in air and then quenched in ice water. The hot samples were manually transferred into ice water (0 °C) within less than 30 s under an ambient atmosphere and kept there for 30 min. Also, some of the samples (series III) were heat-treated at 800 and 900 °C (HT 800/HT 900), with a heating (and subsequent cooling) rate of 2 K/min under air. Finally, the samples were stored at room temperature. The samples were treated before and after quenching in different ways (described in Table 1) in order to vary the final defect contents in the material, following the observation by Mori and Meshii for quenching of gold.26 The nomenclature of the samples and the experimental results of the microstructural analysis are given in Table 2. The catalytic measurements were performed in a fixed-bed reactor using 50 mg of material and a gas flow of 67 mL/min (1 vol % CO in air), following the protocol described elsewhere.27 The conversion of CO to CO2 was measured in the temperature range between 0 and 300 °C with a heating rate of 2 K/min. In the first measurement run, the sample is activated, whereas the following runs show the actual catalytic behavior. The conversion curves are reproducible within 5 K. XRD Measurements and WPPM. Data were collected on different diffractometers and with different diffraction geometries in order to differentiate the influence of the measuring conditions on the microstructural data. X-ray powder diffraction data were collected in reflection geometry on a Bragg-Brentano diffractometer (X’Pert, PANalytical) using Cu KR1,2 radiation, a secondary monochromator, and a position-sensitive real-time multistrip detector (X’Celerator, 2.12° 2θ active length). The primary slits were sufficiently narrow (divergence slit, 0.5°; Soller slit, 0.01 rad; antiscatter slit, 1°) to provide a symmetrical instrument profile. Data were collected in the range between 20 and 100° 2θ. Some of the samples were also investigated in transmission mode in Debye-Scherrer geometry with Cu KR1 radiation. The samples were filled into glass capillaries (0.5 mm diameter) and mounted on an X’Pert diffractometer (PANalytical) using a primary hybrid monochromator. The monochromator consists of a parabolic X-ray mirror and an asymmetric Ge monochromator and generates a monochromatic beam that is parallel in one direction. The following slit configuration was used: primary soller slits, 0.04 rad; divergence slit, 0.25°; antiscatter slit, 0.5°; secondary soller slits, 0.04 rad; antiscatter slit, 0.5°. As a third instrumental setup, another Bragg-Brentano diffractometer (STOE THETA/THETA) was used. This instrument is equipped with a secondary graphite monochromator (Cu KR1,2 radiation) and a proportional gas detector. Divergence and receiving slits were set to 0.8 mm, and the width of the horizontal mask was 4 mm. The diffractometer configurations and the results of the microstructural analysis are summarized in Table 3. In each case, the instrumental profile (IP) was experimentally determined by the analysis of NIST SRM 640c silicon as a reference material. Even if the 640c standard is not certified for line profile broadening, it shows peak profiles comparable in width with those of the proper SRM 660a standard. The Caglioti et al. relationships were used to parametrize the instrumental effects over the whole investigated range.28 XRD patterns of as-prepared, leached, and quenched Au@ZrO2
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TABLE 2: Microstructural Parameters Retrieved from Line Profile Analysis and Corresponding Catalytic Behaviora samples
sample number
D_Au (nm)
series I as-made_bc as-made_ac as-made_ac leached_bc leached_ac quenched_bc quenched_ac series II as-made_bc as-made_ac quenched _ac series III as-made_ac HT 900C_ac HT 800C_ac quenched_I_ac quenched_II_ac
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
F (1E15/m2)
R (%)
m
β (%)
T60 (°C)
sd
D_zir (nm)
sd
14.8 15.0 15.2 14.7 14.6 15.1 15.1
2.3 2.5 2.9 2.5 2.9 3.4 3.1
2.1 2.2 2.1 2.0 1.9 4.8 4.8
0.5 0.5 0.5 0.5 0.5 1.5 1.5
20.33 21.63 20.80 18.35 18.27 10.40 12.48
1.34 1.61 1.67 1.72 1.90 0.73 0.85
0.32 0.35 0.47 0.71 0.72 0.47 0.20
0.20 0.23 0.25 0.30 0.34 0.23 0.20
1.90 2.28 1.84 1.79 1.81 0.91 1.07
0.11 0.14 0.13 0.15 0.15 0.07 0.08
5.43 4.89 5.59 5.06 5.17 3.39 3.06
0.33 0.38 0.40 0.44 0.48 0.24 0.26
14.0 14.0 14.3
2.5 2.5 4.3
1.3 1.3 4.0
0.3 0.3 0.8
17.26 16.35 11.07
1.52 1.68 1.44
0.47 0.46 0.51
0.29 0.34 0.47
1.76 1.97 1.12
0.10 0.15 0.13
5.31 5.49 4.76
0.37 0.42 0.43
140 250
15.2 16.7 15.3 16.4 16.4
5.1 6.9 5.1 7.1 6.9
2.3 4.6 3.7 5.6 4.9
0.6 1.5 0.8 2.2 1.6
16.54 9.46 12.46 8.52 9.49
0.90 0.68 0.84 0.54 0.66
0.51 0.87 0.65 0.55 0.80
0.21 0.29 0.25 0.25 0.28
1.57 0.61 1.15 0.46 0.64
0.10 0.06 0.08 0.05 0.06
5.76 5.27 5.19 4.38 4.93
0.30 0.22 0.27 0.19 0.22
100 235 165 270 280
error
error
error
error
85 95 135 200
a Abbreviations: bc, before catalysis; ac, after catalysis; D, mean domain size; sd, standard deviation of the domain size distribution; F, dislocation density; m, dislocations edge character; R, stacking fault probability; β, twin fault probability; T60, temperature for 60% CO conversion.
TABLE 3: Microstructural Parameters for Samples 9 (As-Made, Series II) and 10 (Quenched, Series II) Retrieved from Line Profile Analysis of XRD Data Measured under Different Diffractometer Configurations for All Given Samples after Catalysisa X-Pert-PRO diffractometer, reflection geometry (Cu KR1,2 radiation), secondary monochromator, position-sensitive detector sample 9 sample 10
D_Au (nm)
sd
D_zir (nm)
sd
F (1E15/m2)
Re (nm)
m
R%
β%
14.0 14.3
2.5 4.3
1.3 4.0
0.3 0.8
17.26 11.07
4.00 4.00
0.47 0.51
1.76 1.12
5.31 4.76
X-Pert-PRO transmission geometry (Cu KR1 radiation), primary monochromator, position-sensitive detector sample 9 sample 10
D_Au (nm)
sd
D_zir (nm)
sd
F (1E15/m2)
Re (nm)
m
R%
β%
14.9 15.1
1.9 4.8
1.5 4.0
0.4 0.8
31.28 27.03
2.65 2.63
0.00 0.14
1.27 0.74
5.89 5.21
STOE THETA/THETA reflection geometry (Cu KR1,2 radiation), secondary monochromator, proportional gas detector sample 9 sample 10
D_Au (nm)
sd
D_zir (nm)
sd
F (1E15/m2)
Re (nm)
m
R%
β%
14.1 14.2
2.5 3.7
1.2 3.6
0.3 0.8
17.85 9.85
4.00 4.00
0.85 0.73
1.87 1.44
5.50 4.06
Abbreviations: D, mean domain size; sd, standard deviation of the domain size distribution; Re, effective outer cutoff radius; F, dislocation density; m, dislocations edge character; R, stacking fault probability; β, twin fault probability. a
powders were modeled with the PM2K software based on the WPPM approach for line profile analysis. WPPM is based on physically sound models for the microstructure of the material under study.19,29 The line profiles produced by a domain size distribution and by various kinds of defects (such as dislocations, planar faults, grain surface relaxation, antiphase boundaries, and concentration gradients) are folded together with the instrumental component, and the pattern is then directly synthesized through (fast) Fourier transformation. Experimental data were modeled by refining the following parameters: µ and σ defining the log-normal distribution of (spherical) domain diameters, dislocation density F, dislocation character (edge fraction m), stacking fault probability R, and twin fault probability β. The same effective outer cutoff radius Re was chosen for all specimens: the value was evaluated from a first refinement run. Because the specimens are uniform in nature, the approximation is not severe and helps in stabilizing the dislocation density results. Dislocations were considered by means of Wilkens’ model. The average contrast factors, accounting for the elastic anisotropy of the system and necessary to reproduce the observed broadening anisotropy, were calculated according to the recently presented procedure30 using the literature single-crystal elastic constants for gold (c11 ) 192.9
GPa, c12 ) 163.8 GPa, and c44 ) 41.5 GPa).31 Calculations were done for edge and screw dislocations, for the 1/2{1-11}〈110〉 slip system, the primary for fcc structures. For a cubic system, the average contrast factor can be parametrized as j hkl ) A + BH2 H2 ) (h2k2 + h2l2 + k2l2)/(h2 + k2 + l2)2 C
where the coefficients A and B are equal to Aedge ) 0.330506, Bedge ) -0.512181, Ascrew ) 0.281476, and Bscrew ) -0.647056 for edge and screw dislocations in gold, respectively. Warren’s treatment for stacking faults was employed, with the formulas properly corrected according to the latest findings of EstevezRams et al.32 The unit cell parameter, all peak intensities, a specimen displacement error, and the coefficients of a Chebyshev polynomial background were also simultaneously refined. Transmission Electron Microscopy and Gas Adsorption Measurements. Transmission electron microscopy (TEM) images of samples were obtained with an HF2000 microscope (Hitachi) equipped with a cold field emission gun. The acceleration voltage was 200 kV. Samples were prepared on a lacey carbon grid.
Au-ZrO2 Yolk-Shell Catalysts for CO Oxidation
Figure 1. XRD patterns of sample 8 measured in (a) Debye-Scherrer (transmission) geometry and (b) Bragg-Brentano (reflection) geometry.
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Figure 2. TEM image of the Au@ZrO2 yolk-shell material.
Nitrogen physisorption was measured on an ASAP 2010 instrument (Micromeritics) after degassing the samples at 200 °C in vacuum for 3 h. All physisorption data were processed with the nonlocal density functional theory (NLDFT) algorithm of the Autosorb software package (Quantachrome) using the respective kernels based on equilibrium adsorption (desorption) of nitrogen in cylindrical pores in oxidic materials (silica). The specific surface area was determined from data points between 0.05 and 0.2 p/p0 using the BET method. Results and Discussion Effect of Data Collection Configurations. To check the influence of the powder diffraction measurement conditions on the microstructural results and to find the most suitable configuration for data collection, some samples were measured under different configurations. The measured patterns are given in Figure 1, and the refinement results are summarized in Table 3. The as-made sample 8 was filled into a glass capillary (0.5 mm Ø) and measured in transmission geometry (Figure 1, pattern a). The high background in the lower 2θ range is due to the scattering of the glass capillary. Additionally, the same sample was prepared on a flat sample holder and measured on the same diffractometer, but this time in Bragg-Brentano geometry (Figure 1, pattern b). In this case, the background is flatter and can be modeled more precisely. For both geometries, the calculated mean domain sizes are in the same range within the standard deviation. However, the values obtained for the dislocation densities are significantly different for transmission and reflection XRD setups. The values for the dislocation densities obtained for the capillary measurement in transmission geometry are more than a factor of 2 higher than the values obtained from the Bragg-Brentano setup in reflection geometry. This shows the strong influence of the background on the calculated microstructural data. The high background obtained for the capillary data could not be modeled properly by mathematical functions and was strong enough to bias the results. The microstructural data for both domain size and defect concentration of samples measured on different Bragg-Brentano diffractometers are almost identical (Table 3). In contrast, the microstructural data obtained from the transmission data of the sample measured in a glass capillary deviate significantly from these results (Table 3). In general, the absolute values depend significantly on the geometry and optics of the diffraction setup, but the observed trends are similar.
Figure 3. Measured X-ray powder diffraction pattern of sample 8 (black points) of the investigated Au@ZrO2 system and fitted profile (red line). The upper and lower markers below the difference curve represent the peak positions for zirconia and gold.
From these results, the Bragg-Brentano geometry with a lower background can be recommended for such types of measurements. If possible, capillaries should not be used because the high scattering background leads to problems during the refinement of the microstructural parameters. Microstructural Analysis of Diffraction Data. The investigated material contains homogeneously distributed single gold nanoparticles, encaged inside ZrO2 hollow spheres. The diameter of the gold nanoparticles is 15 nm, and the diameter of the ZrO2 spheres is about 100 nm, as determined from TEM images (Figure 2). The XRD pattern in Figure 3 shows the measured data, the modeled line profile, and the difference curve. The diffraction pattern exhibits highly overlapping peaks originating from the ZrO2 and Au phases. Without a proper account for the background and for the correct shape of the peak profile tails, a separation of the various microstructure-related effects on the line profile is doubtful and traditional methods, such as the Scherrer formula33 or the Williamson-Hall plot,34 are prone to fail here. The line profile broadening due to the finite size of the scattering domains is assumed to be caused by a log-normal distribution of spherical domains, found appropriate for several real cases.35 The distribution parameters, that is, µ ) 3 and σ ) 0.3 (corresponding to a mean diameter ) 20 nm), were chosen as starting parameters for the refinements. These starting
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Figure 4. Size distribution of gold nanoparticles for sample 8: evaluation of TEM images (histogram, 200 particles) and log-normal distribution refined by WPPM (line).
Pandey et al. on the thermal behavior of zirconia hollow spheres up to 1100 °C.22 The high-resolution TEM images of heated and quenched samples also confirm that the zirconia particles grow substantially during heating and quenching, whereas the porosity is maintained (Figure S1, Supporting Information), albeit shifted to larger pore sizes, in agreement with the coarsening of the zirconia particles. Correspondingly, nitrogen adsorption measurements reveal a significant decrease of the specific surface area from 274 m2 g-1 for the untreated sample 11 to 93 m2 g-1 for the thermally treated sample 12 due to the growth of zirconia particles (Supporting Information, Figure S2). The analysis of the pore size distribution reveals two different pore sizes for sample 11: smaller pores of 3.5 nm and large pores with a broad distribution and a maximum at 17 nm (Supporting Information, Figure S3). The smaller 3.5 nm pores disappear during subsequent temperature treatment, whereas the large pores are preserved (Supporting Information, Figures S2 and S3).Thus, TEM and sorption data agree well. The corresponding refined X-ray powder patterns are shown in Figure S4 (Supporting Information). The difference curve is regular and flat in all three cases. The goodnessof-fit values lie in the range of 1.1-1.2, demonstrating the good performance of the numerical procedure, even without considering any dislocations or faults in the zirconia phase. Thus, we assume negligible amounts of defects present in the zirconia phase, and therefore, this should not influence the catalytic activity.
Figure 5. XRD patterns of (a) as-prepared, (b) leached, and (c) quenched Au@ZrO2 samples of series I.
values correspond to a much broader distribution than that observed from the TEM observations. For small crystallites of materials with a cubic crystal structure, the spherical shape is the most general one and fully consistent with the TEM observations. The flat difference curve shown in Figure 3 together with the final refinement values of weighted sum of squares, χ2 (6286.37), Rwp (2.36%), Rexp (2.05%), and goodness of fit (GoF ) 1.15), demonstrates the good performance of the algorithm. For a comparison of the domain size distribution obtained from powder diffraction data with those from TEM studies, sizes of about 200 gold particles in representative TEM images were analyzed manually for a statistical overview of the particle size distribution. A refined value for the gold domain size of 14 nm with a standard deviation of 2.4 nm is obtained from WPPM modeling, which is in very good agreement with TEM observations, as seen in Figure 4. The measured XRD patterns of as-prepared, leached, and quenched materials of series I are shown in Figure 5. The differences in the profiles of as-made and leached samples are not significant. In contrast to this, for the quenched sample, the diffraction pattern shows narrower peaks for zirconia, whereas the gold reflections seem to be unaffected. The narrowing of the zirconia reflections results from grain sintering, leading to the formation of larger domains. The TEM images (Supporting Information, Figure S1) clearly show no destruction of the system during quenching, which is in line with earlier reports
The structural and microstructural parameters retrieved from WPPM modeling for as-made and modified samples are summarized in Figures 6 and 7 and in Table 2. From the evolution of the domain size and the size distribution for both phases, gold and zirconia, as shown in Figure 6, the following are evident:
Figure 6. Domain size distributions for gold nanoparticles (a) and zirconia shells (b) for as-prepared (black square), leached (red circles), and quenched (blue triangles) samples.
Au-ZrO2 Yolk-Shell Catalysts for CO Oxidation
Figure 7. Results of line profile analysis: (a) Change of dislocation densities with treatment conditions is shown for the samples of series I before and after catalysis. (b) Stacking and twin fault probability for as-made and quenched samples of series I, II, and III.
(a) The mean domain size of gold is unaffected by quenching, showing the stabilizing effect of the zirconia shell. Leaching decreases the size of gold by less than 5% (Figure 6a). (b) The mean domain size of zirconia increases due to sintering, and the size distribution is broadened. As expected, treatment with NaCN does not change the size of the zirconia particles (Figure 6b). The main results for defects present in the gold particles can be summarized as follows: (a) Treatment under reaction conditions does not influence the microstructure of the catalyst (Figure 7a). (b) Dislocation densities do not change during catalysis.
J. Phys. Chem. C, Vol. 114, No. 45, 2010 19391 (c) After leaching, the dislocation density has decreased slightly, whereas the quenched samples have a significantly reduced dislocation density (Figure 7a). (d) Both, stacking fault and twin fault probabilities, decrease slightly after quenching (Figure 7b). The observed dislocation density for gold in the as-made sample is about 2 × 1016 m-2, which is 1-2 orders of magnitude higher than previously reported values based on TEM investigations on colloidal gold.36,37 The use of different synthesis methods or different investigation techniques could account for this difference. At this point, it has to be emphasized that TEM is a local probe, whereas XRD analyzes the whole sample (dislocations not correctly oriented under the TEM beam may be missed). The gold leaching from the surface results in an almost unchanged dislocation density, suggesting that dislocations (or effects mimicking the strain field of this type of defect) are distributed over the whole sample volume and not only on the surface. On the other hand, small changes in fault probabilities suggest that twins and stacking faults also extend through the whole crystal (see the HR-TEM, Figure 8). The refined values of m, representing the fraction of edge character of dislocations (Table 2), show random changes after the different treatments. For the quenched material, the significant decrease in dislocation and fault densities observed here is contrary to the behavior reported for gold foils.38-40 This can be understood by the fact that, in bulk material, most of the defects remain trapped inside the material due to the large dimensions, which is certainly not the case for nanoparticles. Various possible dislocation-stacking fault interactions for gold are reported to result in annihilation of the defects,41 which could also account for the changes observed here. It is also known from quenching experiments on gold that immediate postquenching treatment strongly influences the final fault densities,26 which is considered to be the reason for different fault probabilities for the quenched samples in series I, II, and III. High-Resolution TEM Observations. The high-resolution TEM observations on all samples showed the presence of stacking faults, as exemplarily shown in Figure 8. For these two images, the shells were partly destroyed, and HR-TEM images in Figure 8 were taken from such gold particles that were outside the shells. Otherwise, the superposition of the zirconia shell would not allow the detection of the microstructural details. Some decahedral MTPs were also observed, in agreement with publications on gold nanoparticles.42,43 The typical stacking faults extend through the whole particle.44 The TEM measurements were performed only to investigate the nature of defects present and not with the aim of any statistical study. For this, the analysis of hundreds of individual nanoparticles would be required. Thus, twinning and stacking faults
Figure 8. HR-TEM images of investigated gold nanoparticles encapsulated by crystalline zirconia hollow spheres showing an MTP particle (a) and stacking faults extending to the surface (b).
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Figure 9. CO conversion as a function of temperature for representative materials (sample number 5, leached; 7, heated to 900 °C and quenched; 11, as-made; 12, heated to 900 °C and slow cooling; 13, heated to 800 °C and slow cooling; 15, heated to 900 °C and quenched).
observed by TEM investigations show the presence of such defects and their nature but are considered to be local information. Catalytic Results. For evaluation and comparison of the catalytic activity of the different samples, the temperature for 60% CO conversion, T60, was chosen. Although the temperature for 50% conversion is common in the literature, T60 is more suitable for our data because, for one sample, 50% conversion was reached in the transient measurements at two different temperatures (Figure 9, sample 6). The low-temperature local maximum of the conversion curves could qualitatively be reproduced for all materials treated at 900 °C. However, it appeared more appropriate to compare primarily the activities of the catalyst for the branch that extends to full conversion, and thus, T60 was chosen as a unique reference point, while the full catalytic behavior is given in the conversion curves. In addition, constant temperature measurements at the low-T maximum revealed rapid deactivation, suggesting that this is a transient phenomenon. This also suggests that proper comparison should involve the high-temperature branch. Unusual “U”shaped conversion curves have also been observed for Au on Mg(OH)2 as a support.45 An interaction between catalysts and the support, which is supposed to supply reactive oxygen, is discussed as an origin for this behavior. However, this is a hypothesis at present and has not been proven conclusively. In this paper, the measured catalytic activity is discussed in relation to the microstructure of the gold core. Effects of structural and microstructural changes of the zirconia crystallites on the catalytic activity have been neglected. This assumption is supported by previous reports, where no effect of heat treatment on the catalytic activity was observed.21 These results show comparable activity, although zirconia crystallites grow during heat treatment, whereas the pore size distribution of the zirconia shell is only slightly affected. The highest treatment temperature used here, 900 °C, is higher than in previous studies, and thus, coarsening of the zirconia particles is more pronounced, leading also to the shift of the pore size distribution to higher values. If there were mass transfer limitations, these should be alleviated by bigger pores, and the catalysts should show higher apparent activities. This, however, is obviously not the case, and thus, an effect on mass transfer can be excluded. Figure 9 shows conversion-temperature curves for representative materials. The curves can be classified into materials treated below 800 °C (class I) and materials treated at 900 °C (class II).The materials of class I (sample numbers 1-5, 8, 9,
11, 13) show no local conversion maximum at temperatures below 100 °C, whereas class II materials exhibit a pronounced maximum between 60 and 90 °C. Furthermore, class II materials are less active and thus require a higher T60. When materials 12 and 13 are compared, the effect of treatment temperature becomes obvious. Both samples are prepared from the same batch by heating to 800 or 900 °C, followed by slow cooling. This observation suggests the conclusion that the thermal treatment is the key parameter affecting the catalytic activity. A higher treatment temperature can cause healing of defects, such as dislocations (Figure 10), which may affect the activity, as will be discussed in the following sections. The changes in dislocation density and faults, which are due to additional treatment steps of the as-made materials, occur simultaneously and may, therefore, influence the activity in a combined manner. By comparing the variation in activity with respect to dislocations and faulting separately, it is, however, possible to separate their effects to some extent. In the discussion, particle size effects of the gold core can be neglected because the average domain size of gold does not change significantly (Table 2). Influence of Dislocation Density. The obtained relation between dislocation density and T60 (Figure 10) clearly demonstrates the continuous drop in the activity of the materials with decreasing dislocation density. It is also interesting to see that the activity decreases more sharply for quenched samples with respect to the very small difference in F values (dislocation density). Moreover, because heterogeneous catalysis is a surface phenomenon, the results further suggest that most of the dislocations extend to the surface, and one may even speculate that the surface terminations of the dislocations act as the active sites in catalysis. Samples calcined at 800 and 900 °C and subsequently cooled slowly (samples 13 and 12) fit very well into the group of samples with reduced dislocation density and correspondingly lower catalytic activity. At this point, the reproducibility of the microstructural data should be discussed. Sample 15 was prepared in order to reproduce sample 10, and sample 14 was produced identically to sample 7. Even though we tried to exactly reproduce the treatment procedure, the microstructural data of the samples differ. On the other hand, the catalytic performance of the samples perfectly fits with their microstructural properties. This underlines that the microstructure of this system is extremely sensitive to the preparation procedure. To obtain exactly the same material with the same microstructural properties, and thus
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Figure 10. Correlation between dislocation density and catalytic activity. The samples in the left cloud represent as-made and leached samples (temperatures < 800 °C, class I). The samples in the right cloud are calcined and quenched samples (temperatures > 800 °C, class II).
Figure 11. Stacking fault and twin fault probabilities plotted against temperature T60 (left, class I; right, class II).
the same catalytic properties, requires reproducible heat treatment conditions of the catalysts. Influence of Stacking and Twin Fault Probability. It was generally observed for this system that the twin fault probability exceeds the stacking fault probability by a factor of 2. However, the twin fault probability is almost identical for all samples and does not seem to have much influence on their catalytic properties (Figure 11). In contrast, for calcined and quenched samples (Figure 11, right), the stacking fault probability slightly decreases as compared with the as-made samples (Figure 11, left). Even though the effect is small, higher temperatures are required for 60% CO conversion with decreasing numbers of stacking faults present in the material. The dotted line indicates a separation of the samples into two groups: the samples in the left field of the graph represent asmade and leached samples (class I), whereas the samples in the right field represent samples that were additionally heattreated at 900 °C (class II). One exception is sample 13, which was prepared by additional calcination at 800 °C. The temperature for 60% conversion is much lower compared with the sample calcined at 900 °C (sample 12). However, a closer look
on dislocation density and stacking fault probability shows that sample 13 has both a higher dislocation density and a higher stacking fault probability than sample 12 (Table 2). Higher calcination temperatures seem to heal dislocations and stacking faults, and accordingly, the catalysts become less active. The HR-TEM images shown in Figure 8 reveal that stacking faults extend to the surface and, hence, cause the formation of steps on the surface of gold particles. As steps on the surface are known active sites for catalysis, this further supports our conclusion that the presence of stacking faults increases the activity of gold nanoparticles. Very low catalytic activity for quenched gold wire has also been reported for the decomposition of hydrogen peroxide.46 In that case, the author suggested that dislocations do not seem to be generated during quenching, and the surface emergent point defects of quenched gold have no effect on the catalytic activity. In our case, we have also found a strongly reduced number of dislocations in quenched samples, but no information on point defects were obtained either by HR-TEM or by line profile analysis. In the discussion above, the altered catalytic activity has only been correlated with the gold microstructure; no other possible effects, such as the
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zirconia microstructure, were considered. Moreover, it is not clear how the interface between gold and zirconia has changed during quenching. The contact between gold and the support might influence the activity as well.47 Li et al.48 had analyzed how the contact area between gold and the zirconia support changes, if the zirconia particles become larger. Also in our study, the zirconia particle size increases during heating at high temperature. However, in the case of Li et al., the zirconia particles were substantially larger than the gold particles, whereas this was the opposite in our study (see Table 2). Thus, coarsening of the zirconia would not be expected to have a pronounced effect on contact area. In addition, the changes in defect concentration of the gold, as discussed above, can fully explain the changes in activity, and thus, we consider these to be the dominating factor, although a contribution from a change in the interface cannot be excluded. The influence of dislocations and other faults on the catalytic activity of gold nanoparticles, at least of particles in the size range investigated here, is thus highly probable. Conclusion The influence of the microstructure of metal nanoparticles on their catalytic activity in CO oxidation has been studied for the first time in detail by WPPM of laboratory X-ray diffraction data. The combination of catalytic measurements and microstructural analysis allows a comprehensive discussion of the structure-properties relationship of the gold-zirconia yolk-shell system used for CO oxidation. On the one hand, by evaluation of powder diffraction data, the variation of the size distribution depending on the treatment conditions could be analyzed. On the other hand, the detailed microstructural analysis showed quantitatively a variation of the defect concentration, induced by postsynthesis treatment of the material. Quenching from 900 °C or calcination and subsequent slow cooling leads to a reduction of the dislocation density and, with this, a much lower catalytic activity. TEM investigations prove the presence of twins and stacking faults in the gold nanoparticles. The analysis of the powder diffraction data reveals that the density of twin faults is not affected by thermal treatment, whereas the stacking fault probability is decreased with additional heat treatment. HRTEM observations show that stacking faults extend to the surface and cause the formation of steps. Thus, the present study suggests that the surface termination of dislocations and stacking faults in gold nanoparticles act as active centers in CO oxidation. Because size effects on the catalytic performance can be excluded, defects have probably the most significant influence on the catalytic properties. Additionally, the influence of dislocations and faults could be separated. High activity correlates with high dislocation density and stacking fault probabilities. The temperatures for 60% CO conversion of the materials treated at 900 °C are significantly higher than for the as-made catalysts. Whether these results can be extrapolated to the smaller gold particles with sizes below 5 nm, which have the highest normalized activity for CO oxidation, remains to be seen. Acknowledgment. The authors are thankful to Mr. Axel Dreier and Mr. Spliethoff for TEM investigations and to Mr. Ulrich Holle for supporting diffraction measurements. Financial support from DFG (SFB 558) is also acknowledged. Supporting Information Available: TEM images of asmade and heat-treated samples, nitrogen adsorption isotherms,
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