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2006, 110, 21387-21393 Published on Web 10/11/2006
Nanocatalysis on Tailored Shape Supports: Au and Pd Nanoparticles Supported on MgO Nanocubes and ZnO Nanobelts Garry Glaspell, Hassan M. A. Hassan,† Ahmed Elzatahry,‡ Lindsay Fuoco, Nagi R. E. Radwan,† and M. Samy El-Shall* Department of Chemistry, Virginia Commonwealth UniVersity, Richmond, Virginia 23284-2006 ReceiVed: August 8, 2006; In Final Form: September 15, 2006
Active gold and palladium nanoparticles supported on MgO nanocubes and ZnO nanobelts and transitionmetal-containing MgO nanobelts were synthesized by combining evaporation and deposition-precipitation techniques. The high activity and stability of the Au/CeO2 and Pd/CeO2 nanoparticle catalysts deposited on the MgO cubes are remarkable and imply that a variety of efficient catalysts can be designed and tested using this approach. The significant increase in the concentration of corner and edge sites in MgO nanocubes make them well-defined supports to study the detailed mechanism of the catalytic activity enhancement.
Nanocatalysis is a phenomenon of significant fundamental research and important practical applications in a variety of fields such as chemistry, physics, materials, and environmental and atmospheric sciences in addition to its traditional significance in advancing the petroleum field.1-3 It encompasses supported or unsupported nanometer-scale metal catalytic structures (spherical nanoparticles, nanorods, nanoplates, nanocubes, etc.) where the catalytic phenomena are specific to that length scale and, in general, are related to the high surface area and the density of the unsaturated surface coordination sites of the nanocatalysts.1-6 The origins of enhancements in activity and selectivity can be explained by dispersion factors or by quantum factors unique to this special length scale. Various factors identified include the emergence of electronic and/or atom-packing shell structures, along with fundamentally altered interactions with the support.1-6 Most of the studies in homogeneous catalysis involve spherical nanoparticles or nanoparticles of undetermined shapes. However, recent studies have examined the effects of the nanoparticles’ shape on the activation energy and the rates of the catalytic reaction.7-11 The influence of particle shape on the catalytic activity depends on the increase in the density of the active corner/edge sites which correlates with strong surface activity.7-11 This correlation is attributed to the fact that nanocrystals of different shapes have different facets with different fractions of atoms located at different corners, edges, and different defect sites.12 Thus, nanoparticle catalysts such as Au or Pd with identical sizes and shapes may exhibit different activities when supported on metal oxides having different shapes. Therefore, it is important to develop ways of tailoring the design and formation of new nanocatalyst systems with controlled sizes and shapes that result in high catalytic activity. Recent approaches have utilized carbon nanotubes as supporting materials for the dispersion and assembly of metal nanoparticle † Permanent address: Department of Chemistry, Suez Canal University, Suez, Egypt. ‡ Permanent address: Mubarak City for Scientific Research and Technology Applications (MuCSAT), New Bourg El-Arab, Alexandria, Egypt.
10.1021/jp0651034 CCC: $33.50
catalysts such Au and Pt.13-15 The increase in the catalytic efficiencies has been attributed to the increased surface areas and the well-dispersed nature of the carbon support and catalyst.15 Herein, we report the synthesis and characterization of Au and Pd nanoparticle catalysts supported on metal oxides of different shapes and demonstrate their excellent catalytic activities for the oxidation of carbon monoxide. The low temperature oxidation of carbon monoxide is one of the current important environmental issues, since small exposure (ppm) to this odorless invisible gas can be lethal.16 Therefore, the discovery that Au nanoparticles between 2 and 5 nm are exceptionally active for low temperature CO oxidation has stimulated extensive research to develop highly active catalysts to remove even a small amount of CO from the local environment.4-6,17-25 There is also a strong incentive to develop active supported catalysts that utilize small amounts of the noble metals such as gold and palladium. The catalytic activity of the Au-based catalysts depends on various factors such as the type of the support, the Au precursor, the preparation conditions, the pretreatment conditions, and the catalytic reaction conditions.17-25 Among these factors, the nature and shape of the support are expected to play a major role given the strong tendency of Au nanoparticles to efficiently adsorb CO molecules but the inability to activate oxygen molecules.17-19 Highly ionic metal oxide supports such as MgO and ZnO exhibit both Lewis base and Lewis acid characteristics as usually reflected in their high surface reactivity which depends on both particle size and shape. MgO is considered a model system for solid state and surface studies because of its simple rock-salt crystal structure and highly ionic and nonconducting nature. It also exhibits catalytic activity for a wide variety of reactions.26,27 This activity has been attributed to the presence of defect sites such as corners, edges, steps, and kinks.28,29 Klabunde and coworkers have prepared high surface area MgO with average crystalline sizes of about 4 nm and demonstrated that these materials are especially effective as “destructive adsorbents” for the destruction of toxic chemicals.28-31 Recently, nanocubes of MgO have been synthesized by chemical vapor deposition © 2006 American Chemical Society
21388 J. Phys. Chem. B, Vol. 110, No. 43, 2006
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Figure 1. TEM micrographs of (A) MgO nanocubes formed by thermal evaporation at 1200 °C and 760 Torr O2, (B) same as part A but using 100 Torr O2, (C) MgO wires formed by thermal evaporation at 10 Torr O2, (D) MgO nanoparticles formed by the LVCC method at 900 Torr O2, and (E and F) ZnO belts formed by thermal evaporation at 860 °C and 300 Torr O2.
followed by annealing at 1170 K under high vacuum conditions.32 The significant increase in the concentration of corner and edge sites in MgO nanocubes make them well-defined supports to study the detailed mechanism of the catalytic activity enhancement. In this letter, we report the synthesis of Au and Pd nanoparticle catalysts supported on MgO nanocubes and ZnO nanobelts and compare their catalytic activities for CO oxidation with the catalysts supported on ceria nanoparticles. The results clearly demonstrate that the combination of active and inactive supports of controlled shapes can lead to remarkable enhancement of the catalytic activity of the Au- and Pd-based nanoparticle catalysts. The MgO nanocubes and the ZnO nanobelts were generated by thermal evaporation of the metal powder (1190 °C for Mg and 910 °C for Zn) in the presence of oxygen in a tube furnace under controlled pressure and flow rate. MgO belts terminated by transition metal (Fe, Co, Ni) tips were also synthesized by combining laser ablation of the transition metal targets with the thermal evaporation of Mg in the presence of oxygen (Supporting Information). Dispersed Au and Pd nanoparticles supported on the MgO nanocubes and ZnO nanobelts were prepared using the deposition-precipitation (DP) method.33,34 Specifically, the appropriate amount of the HAuCl4 or Pd(NO3)2 aqueous solution was added to a suspension of the MgO or ZnO support. The pH of the solution was adjusted to 9 by the
dropwise addition of NaOH while stirring. After stirring the solution for an hour, the nanoparticles formed on the support were filtered and dried under vacuum. The Au/CeO2 and Pd/CeO2 supported on the MgO cubes and ZnO belts were also prepared using microwave irradiation (MWI) synthesis.35 In this case, an appropriate amount of Ce(NO3)4 solution in ethanol and the metal precursor (HAuCl4 or Pd(NO3)2) were added to the suspension of the MgO cubes or the ZnO belts. While stirring, 10 N NaOH (Alfa Aesar) was added dropwise until the pH of the resulting solution was ∼10. The resulting solution was then placed in a conventional microwave. The microwave power was set to 33% of 650 W and operated in 30-s cycles (on for 10 s, off for 20 s) for 10 min. The resulting powder was washed with distilled water and ethanol and left to dry. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to characterize the nanocrystalline materials. TEM images were obtained using a Joel JEM-1230 electron microscope operated at 120 kV. XRD spectra were obtained using an X’Pert Philips Materials Research Diffractometer, with Cu KR radiation. For the CO catalytic oxidation, the sample was placed inside a Thermolyne 2100 programmable tube furnace reactor.35 The sample temperature was measured by a thermocouple placed near the sample. In a typical experiment, 4 wt % CO and 20 wt % O2 in He were passed over the sample while the temperature
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Figure 2. XRD patterns of (a) MgO nanocubes, (b) ZnO nanobelts, and (c and d) MgO and ZnO nanoparticles, respectively, prepared by the LVCC method.
Figure 3. TEM micrographs of (A) 1% Au deposited on MgO nanocubes, (B) 2% Pd loaded ZnO nanobelts, (C and D) 1% Au/24% CeO2 deposited on ZnO nanobelts, (E) 5% Au deposited on ZnO nanobelts, (F) 1% Au/24% CeO2 deposited on MgO nanocubes, (G) 1% Pd/24% CeO2 deposited on ZnO nanobelts.
was ramped. The gas mixture was set to flow over the sample at a rate of 100 cm3/min controlled via MKS digital flow meters. The conversion of CO to CO2 was monitored using an infrared gas analyzer (ACS, Automated Custom Systems Inc.). All of the catalytic activities were measured (using a 20 mg sample) after a heat treatment of the catalyst at 300 °C in the reactant
gas mixture for 15 min in order to remove moisture and adsorbed impurities. Figure 1A displays a typical TEM image of the MgO nanocubes formed via thermal evaporation in oxygen at 760 Torr. The as-synthesized nanocubes are quite uniform with an average length of ∼130 nm. However, reducing the pressure
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Figure 4. CO oxidation on Au (A, C, and E) and Pd (B, D, and F) deposited on MgO nanocubes and ZnO nanobelts.
within the furnace to 100 Torr shrinks the length of the cubes to ∼45 nm, as shown in Figure 1B. Thus, by carefully controlling the pressure within the furnace, MgO nanocubes of different sizes can be prepared. It should be noted, however, that when the oxygen pressure is below 10 Torr, long wires rather than cubes are formed, as shown in Figure 1C. Interestingly, laser evaporation of a Mg target using the laser vaporization controlled condensation (LVCC) method36 in the presence of an oxygen atmosphere produces a mixture of very small cubes (less than 10 nm) and spherical particles, as shown in Figure 1D. For ZnO, thermal evaporation of Zn in the presence of oxygen at 300 Torr produces the ZnO nanobelts shown in Figure 1E which appear to grow from the original tetrahedral cores shown in Figure 1F. The average width of the ZnO nanobelts is ∼30 nm. The XRD patterns of the MgO nancubes and the ZnO nanobelts, shown in Figure 2, match well with the ICCD reference patterns 01-075-1525 and 01-079-0205, corresponding to the crystal structures of the bulk MgO and ZnO, respectively.
Also, the XRD pattern of the MgO nanocubes prepared by thermal evaporation is similar to that of the MgO nanoparticles prepared by the LVCC method, as shown in Figure 2a and c. Since the growth of the cubes occurs in three dimensions, the XRD patterns for the MgO nanoparticles and nanocubes show similar peak intensities. However, the XRD pattern of the ZnO belts (Figure 2b) shows a significant enhancement in the intensity of the diffraction (1 1 0) peak, indicating that the belts are grown along the [1 1 0] direction.37 Figure 3 displays several representative TEM images of the Au and Pd nanoparticle catalysts deposited on the MgO cubes and the ZnO belts using the DP method. It is clear that the Au and Pd nanoparticles are well dispersed on the MgO cubes and the ZnO belts with no evidence of large aggregates of Au or the Pd nanoparticles existing independently. The average size of the deposited Au and Pd nanoparticles are ∼9 and ∼12 nm, as determined from the analysis of several TEM images. Figure 4 compares the catalytic activities of the bare MgO nanocubes and ZnO nanobelts with the deposited nanoparticles
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Figure 5. TEM micrographs of MgO wires and belts initiated by (A) Co, (B) Fe, and (C) Ni during the thermal evaporation of Mg at 1200 °C in O2. (D) CO oxidation on the MgO belts incorprating transition metal oxides.
of Au, Pd, Au/CeO2, and Pd/CeO2 on the same supports. Clearly, both the bare MgO cubes (130 nm) and ZnO belts (30 nm) exhibit low activity for the CO oxidation, as indicated by the 50% conversions at 350 and 370 °C, respectively. However, the ZnO belts exhibit significantly higher activity for the 100% CO oxidation (381 °C) as compared to the MgO cubes (82% at 435 °C). Figure 4A and B compare the concentration effects (wt %) of the Au and Pd nanoparticles supported on the MgO cubes. In the case of Au nanoparticles, the activity increases significantly with increasing concentration of the Au nanoparticles within the 1-5% (wt) range. However, for Pd nanoparticles, only a small enhancement in the activity is observed by increasing the Pd loading from 2 to 5% (wt). The observed concentration effect can be explained by increasing the number of dispersed active sites before reaching the aggregation limit where the catalytic activity is expected to decrease. In both systems, higher concentrations of the metal nanoparticles (7.5 wt %) supported on the MgO cubes resulted in decreasing catalytic activity. This suggests that the 5% (wt) seems to be near the optimum concentration allowed using the DP method
to achieve good dispersion of the Au or Pd nanoparticles on the MgO cubes. Interestingly, the 5% (wt) Au nanoparticles supported on the MgO cubes show a much higher activity (50% conversion at 128 °C) than the same concentration of the Au nanoparticles supported on the ZnO belts (50% conversion at 280 °C), as shown in parts C and E of Figure 4, respectively. This indicates that the activity of the Au catalyst is strongly dependent on the type and shape of the metal oxide support. This is not the case for the Pd catalyst where the performance of the 5% Pd nanoparticles supported on the MgO cubes (100% conversion at 154 °C) is nearly similar to that supported on the ZnO belts (100% conversion at 162 °C). As compared to Au, the Pd catalysts show very sharp conversion curves immediately past the light-off temperatures. Figure 4 shows also a comparison of the activities of the Au and Pd nanoparticle catalysts deposited on the MgO cubes and ZnO belts with the activities of the same catalysts supported on CeO2 nanoparticles. For the Au catalyst, the activity is higher on the CeO2 support than on the MgO cubes or the ZnO belts. However, for the Pd catalyst, the difference between the
21392 J. Phys. Chem. B, Vol. 110, No. 43, 2006 activities on the different supports (CeO2 spherical particles, MgO cubes, and ZnO belts) is considerably small. This suggests that the influence of the shape of the support on the catalytic activity varies depending on the nature of the metal-support interaction. The most interesting result shown in Figure 4 is the enhancement of the activity of the Au supported CeO2 nanoparticles upon the deposition on the MgO cubes. In this case, the MWI synthesis of the Au/CeO2 catalyst in the presence of the MgO cubes is expected to produce Au nanoparticles supported on spherical CeO2 particles, as demonstrated in previous work.35 These Au/CeO2 nanoparticles are then deposited on the MgO cubes, although the possibility of depositing Au nanoparticles directly on the MgO cubes cannot be eliminated. However, the very different activity of the Au/CeO2/ MgO catalyst system as compared to the Au/MgO catalyst indicates the presence of strong Au-CeO2 interaction in the Au/CeO2/MgO catalyst system. By using only 1 (wt) % Au supported on 24 (wt) % CeO2 nanoparticles supported on 75 (wt) % MgO cubes, a remarkable improvement in the activity of the Au/CeO2 catalyst is observed with a 70% CO conversion and a full conversion (100%) achieved at 25 and 91 °C, respectively. Furthermore, the long-term stability of this catalyst that utilizes such a small concentration of the Au nanoparticles is excellent, as indicated by no decrease in activity even after 10 h of the reaction time. It should be noted that the same catalytic activity is observed after storing the particles in air for several days following a heat treatment of the catalyst at 300 °C in the reactant gas mixture for 15 min in order to remove moisture and adsorbed impurities. Although the catalytic enhancement is also observed for the Pd/CeO2 catalyst supported on the MgO cubes relative to the Pd/CeO2 (spherical particles), the activity of the Pd/CeO2/MgO catalyst is less than that of the Au/CeO2/MgO catalyst. This is probably due to the differences in the metal-support interactions of the Pd/MgO and Au/MgO systems. Since the remarkable activity of the 1% Au/24% CeO2/75% MgO cubes is not observed when 1% Au/24% CeO2 is deposited on 75% ZnO belts, this may suggest that the enhancement is due to the interaction with the cube-shaped support. This result demonstrates that MgO nanocubes with active corners and edges can lead to significantly enhanced catalytic activity. It is important to compare the catalytic activity of the 1% Au/24% CeO2 supported on the MgO cubes with other catalyst systems used for the CO oxidation. The observed activities of 70 and 100% CO conversions achieved at 25 and 91 °C, respectively, for the Au/CeO2/MgO (cubes) catalyst are much better than those reported for the Au nanoparticles supported on the surface of the CeO2 nanorods where 81% CO conversion was achieved at 220 °C.9 The performance of the Au/CeO2/ MgO (cubes) catalyst is also better than that of the β-MnO2 nanorods where with 90% CO conversions were achieved at 126 °C for the pure β-MnO2 nanorods and at 80-90 °C for the Ag-doped MnO2 nanorods.10 The current Au/CeO2/MgO (cubes) catalyst is also significantly superior to the quasicubic R-Fe2O3 nanoparticles which showed a 100% CO oxidation at 230 °C.38 Furthermore, the activity of the Au/CeO2/MgO (cubes) catalyst is much higher than that of the bimetallic PdAu nanoparticles supported on TiO2 where the 1 and 100% CO conversions were reported to occur at 150 and 250 °C, respectively.39 However, the low activity of the bimetallic PdAu nanoparticles39 supported on TiO2 appears to be unusual given the typical high activity of the Au nanocrystals supported on nanosized TiO2 which
Letters exhibit 50% CO conversion in the temperature range -25 to 13 °C depending on the pretreatment conditions.20 We have also explored the catalytic activity of the MgO nanobelts containing transition metal oxide tips. These nanobelts were synthesized by coupling the laser ablation of selected transition metal targets with the thermal evaporation of the Mg powder in the presence of an oxygen atmosphere within a tube furnace (Supporting Information). As shown in Figure 5, the TEM images suggest that the MgO nanobelts are grown from attached transition metal nanoparticle tips through the vaporliquid-solid (VLS) mechanism.40,41 The diameters of the belts were determined to be 10, 100, and 60 nm for the ablated Co, Fe, and Ni samples, respectively. It is interesting that the morphology of the tips depends on the nature of the metal catalyst. The spherical clusters observed for the Co/MgO belts (Figure 5A) are similar to those observed for the Co-initiated MgO fibers.42 However, in the case of Fe and Ni, cubic and cylindrical tips were observed, as shown in parts B and C of Figure 5, respectively. These transition metal oxide tips attached to the MgO belts can provide suitable catalytic centers with variable activities for CO oxidation. The CO conversion curves over the MgO belts incorporating the transition metal oxides are shown in Figure 5D. It is clear that the highest catalytic conversion is observed for the Fe-initiated MgO nanobelts with 100% CO conversion occurring at 293 °C. On the other hand, Co/MgO belts show 50% CO oxidation at temperatures below 200 °C. Optimization of the parameters that control the dimensions of the belts and the shape of the attached transition metal catalysts is expected to improve the catalytic properties of these systems. The MgO belts containing terminal transition metal oxide tips should prove to be interesting model systems for studying size- and shape-dependent catalytic and magnetic properties and also for a variety of applications in devices and sensors and possibly biomedical applications. In conclusion, active Au and Pd nanoparticle catalysts deposited on MgO nanocubes and ZnO nanobelts were synthesized by combining the thermal evaporation and depositionprecipitation techniques. The high activity and stability of the Au/CeO2 and Pd/CeO2 nanoparticle catalysts deposited on the MgO cubes are remarkable and imply that a variety of efficient catalysts can be designed and tested using this approach. The higher crystallinity and larger size of the nanocubes and nanobelts can provide higher thermal stability which ultimately enhances the durability of the catalyst. The transition-metalcontaining MgO nanobelts provide good model systems for studying the size- and shape-dependent catalytic and magnetic properties in high temperature materials. Acknowledgment. The research described herein was supported by the National Science Foundation through the USEgypt Cooperative Research Program (OISE-0413971). Acknowledgment is also made to the Donors of the American Chemical Society Petroleum Research Fund (PRF#41602-AEF) and to Philip Morris USA for the partial support of this research. Supporting Information Available: Experimental setup for the synthesis of MgO belts using transition metal catalysts. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Somorjai. G. A. Introduction to Surface Chemistry and Catalysis; Wiley Publishers: New York, 1994. (2) Moser, W. R., Ed. AdVanced Catalysts and Nanostructured Materials; Academic Press: San Diego, CA, 1996.
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