Tuning of Texture and Structure of Copper-Containing Nanocomposite

Jan 16, 2008 - For studies of textural tuning, structural tuning, or materials sintering, copper/aluminum and copper/zinc nanocomposite materials were...
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J. Phys. Chem. C 2008, 112, 1446-1454

Tuning of Texture and Structure of Copper-Containing Nanocomposite Oxide Materials Yu Xing,† Zhenxin Liu,† Sinue Gomez,‡,§ and Steven L. Suib*,†,‡ Department of Chemistry, and Department of Chemical, Materials & Biomolecular Engineering, UniVersity of Connecticut, Storrs, Connecticut 06269-3060 ReceiVed: September 15, 2007; In Final Form: NoVember 12, 2007

For studies of textural tuning, structural tuning, or materials sintering, copper/aluminum and copper/zinc nanocomposite materials were prepared. Resistance to sintering of different phases was investigated. Thermal analysis methods were used to design feasible thermal treatment methods that can avoid destructive damages to gels. X-ray diffraction and nitrogen sorption measurements were used for structural and textural analysis. Compared with the wide distributions of pore sizes and low surface areas of the products prepared via conventional coprecipitation methods, a novel urea-gelation/thermal-modification method was developed to produce CuO/Al2O3 nanocomposites with narrow distributions of pore sizes and high surface areas. In comparison with the products of conventional coprecipitation methods, this novel urea-gelation/thermalmodification method produces copper/aluminum nanocomposites with significantly higher specific copper loading, which should be valuable in apparatus that have space limitations, such as vehicle fuel cell systems. Stepwise reduction and reoxidation were studied for the structural tuning and purification of Cu-Al-O spinel phases with isotropic and gradual unit-cell contractions. The textural and structural features of some copper/ aluminum nanocomposite materials were observed by electron microscopy methods, that is, field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and convergent beam electron diffraction (CBED).

1. Introduction Porous inorganic and organic materials have extensive uses.1-6 Considerable endeavors were made in the preparation of porous materials, or the utilization of porous materials as the templates to synthesize nanometer-sized matter.7-9 Great progress in the preparation of porous inorganic materials has been seen. New morphologies and chemical compositions are now achievable on a wide range of length scales.10 Porous solids are of scientific and technological interest because of their ability to interact with atoms, ions, and molecules not only at their surfaces but also throughout the bulk of the material.11 The texture of a solid material or a heterogeneous catalyst is the detailed geometry of the void space in the particles, from the intergranular voids in the agglomerates down to the pore size distribution at the finest level of resolution, from the shape of the particles and their external surface area down to the pore shapes and the extent of the accessible internal surface. The parameters describing the material/catalyst texture may include the specific surface area, porosity, pore shape, pore size distribution, average pore size, particle size, and the shapes and the sizes of agglomerates of particles.12,13 The number, the shape, and the size of the pores greatly affect the extent of the accessible internal surface area of the catalyst and also govern the heat and mass transfer phenomena that generally occur in practical conditions and sometimes can modify the selectivity of the reaction and markedly influence the thermal stability of * Corresponding author. E-mail: [email protected]. † Department of Chemistry, University of Connecticut. ‡ Department of Chemical, Materials & Biomolecular Engineering, University of Connecticut. § Present address: Corning Incorporated, Science and Technology, SPFR-5, Corning, NY, 14831.

the catalyst. The structure of a solid material or a heterogeneous catalyst is the distribution of its atoms or ions.12 The properties of materials or catalysts will undoubtedly be affected by their structures. Copper-containing mixed metal oxide (MMO) materials such as CuO/Al2O3 and CuO/ZnO/Al2O3, and their reduced materials such as Cu0/Al2O3 and Cu0/ZnO/Al2O3, may be used as catalysts for oxidation, hydrogenation, or dehydrogenation reactions.14 Supported Cu0 may be employed as electronically conducting materials in solid oxide fuel cells (SOFCs).15 AB2O4 spinels may be used as catalysts in areas like abatement of NOx, or they can be useful in microwave electronics.16 Therefore, the structural transformation of spinels has been of interest.17 Herein, we report detailed studies on the textural tuning of copper/aluminum nanocomposite materials via a novel ureagelation/thermal-modification method in comparison with conventional coprecipitation methods (by which commercial copperbased materials are manufactured18), the structural tuning of Cu-Al-O spinel phases via stepwise reduction under methanolsteam reforming conditions, and the resistance to sintering of different phases. A traditional homogeneous deposition-precipitation (HDP) method with the use of urea at 90 °C, which does not produce gels but produces precipitates, has been developed since 1970s by Geus et al. as an important method for the preparation of highly dispersed and highly loaded oxide-supported metal catalysts.19 In a typical HDP operation, a porous support such as silica is pre-added to a solution containing urea and a soluble metal salt, and then the pH of the suspension at room temperature is adjusted to 3.0 by adding HNO3, and then the precipitated metal-containing particles are deposited on the surface of the support at 90 °C, and then the solid product is filtered, washed, dried, and calcined.19

10.1021/jp077440y CCC: $40.75 © 2008 American Chemical Society Published on Web 01/16/2008

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TABLE 1: Denomination of Uncalcined Xerogel/Precipitates ratio of CuO/ZnO/Al2O3

a

denomination of xerogel/precipitates

nominal ratio (wt %)

actual ratio (wt %)

C100-NaHCO3 Z100-NaHCO3 b A100-NaHCO3 CA50-NaHCO3 CA50-Na2CO3 CA50-(NH4)2CO3 CA50-Urea-G c CA50-Urea-P d CA30-(NH4)2CO3 CZ50-NaHCO3 CZ50-Na2CO3

100/0/0 0/100/0 0/0/100 50/0/50 50/0/50 50/0/50 50/0/50 50/0/50 30/0/70 50/50/0 50/50/0

100.0/0/0 0/100.0/0 0/0/100.0 49.8/0/50.2 51.4/0/48.6 48.9/0/51.1 53.2/0/46.8 91.4/0/8.6 27.2/0/72.8 50.0/50.0/0 49.5/50.5/0

a Elemental composition. b “Z”, zinc. c Dried at 180 °C. “C”, copper; “A”, aluminum; “50”, nominal ratio for prior element; “Urea”, precipitant; “G”, gelation. Others are defined similarly. d “P”, precipitation.

In this report, a novel method was carefully designed and successfully carried out where no support was pre-added to the solution and no preadjustment of pH was made during the synthesis. This novel urea-gelation/thermal-modification method used excess urea (e.g., 3.0 times the stoichiometric requirement) at 100 °C to produce copper/aluminum gels (gelation does not occur at lower temperatures such as 90 °C), was not filtered, and a stepwise thermal modification was applied to the gels. During the formation of copper/aluminum gels, excess urea (gelation does not occur at 1.0 equiv) was used as a gelation agent, whereas in the well-designed stepwise thermal modification the excess urea that was not removed but was buried in the gels during gelation was used as a pore-creating reagent. In summary, this paper focuses on the materials chemistry (e.g., synthesis, thermal modification) of copper-containing nanocomposites and their textural/structural features. The catalytic properties of these materials will be covered elsewhere. 2. Experimental Section 2.1. Preparation of Materials. The sources of Cu, Al, and Zn were Cu(NO3)2‚3H2O, Al(NO3)3‚9H2O, and Zn(NO3)2‚6H2O, respectively, (all from Strem Chemicals, Inc.), while the dosages were all based on the nominal ratios in Table 1. Other chemicals were purchased from Aldrich. Gel CA50-Urea-G was produced by precipitation and subsequent gelation at 100 °C under vigorous stirring, by reacting 3.0 times the stoichiometric requirement of the precipitant urea with the solutions of Al and Cu sources. Gel CA50-Urea-G was dried at 100, 150, or 180 °C to produce different xerogels. The difference in synthesis between precipitate CA50-Urea-P and gel CA50-Urea-G is that the former is the produced precipitate before gelation, which was filtered, washed, and then dried at 100 °C. Other materials in Table 1 are all precipitates, prepared with 1.05 times the stoichiometric requirements of precipitants at 50 °C under vigorous stirring by coprecipitation methods, filtered after 1 h digestion, washed with deionized water, and then dried at 100 °C. Air is the atmosphere for drying and calcination. 2.2. Reduction of Materials. The 250-600 °C and 250800 °C reduction experiments were carried out in helium by loading the sample powder in quartz boats in a tubular furnace, fueled with CH3OH/H2O mixture (molar ratio 1/1) at a methanol-based weight hourly space velocity of 0.36 h-1. Hydrogen-rich gases can be produced over copper-containing solids under methanol-steam reforming conditions and can reduce copper species in situ.20 At each temperature stage, the

sample powder was quickly cooled down to room temperature, after 2 and 50 h during the 250-600 °C experiment, or after 5 and 50 h during the 250-800 °C experiment, for XRD scans. 2.3. Characterization of Materials. The elemental compositions of the products were determined by using an IRIS ICPOES. The crystalline phases were identified by powder X-ray diffraction using a Scintag XDS-2000 diffractometer with Cu KR radiation. Volume-weighted average crystallite sizes were determined by X-ray diffraction line broadening analysis (LBA), where the data of integral line widths were employed with a Scherrer constant of 1.0. Instrumental broadening was corrected via the Warren correction.21 A Micrometrics ASAP 2010 surface area system was used for nitrogen sorption measurements. Desorption branches of isotherm plots were used for BJH pore distribution analysis. Sufficient data points at low partial pressure were taken to allow the Horvath-Kawazoe calculation. Thermal analyses were carried out using a DSC 2920 Differential Scanning Calorimeter and a Perkin-Elmer TGA-7 with ramps of 10 °C/min. Air was used in the TGA research to be consistent with the calcination of samples that was also conducted in air. Nitrogen was used in the DSC studies because this DSC instrument has to be operated under an inert (argon, helium, or nitrogen) atmosphere. Temperature-programmed reduction (TPR) was monitored continuously with a portable MKS-UTI MSS quadrupole residual gas analyzer mass spectrometer (MS). TPR analysis consisted of placing 100 mg of the sample in a quartz tubular reactor, and heating it in situ in a 30 mL/min gas stream comprising 8% (by mol) H2 and 92% He at a linearly programmed rate of 5 °C/min from 25 to 750 °C. Field-emission scanning electron microscopy (FESEM) studies were carried out using a Zeiss DSM 982 Germini FESEM with a Schottky emitter operated at 10 kV and a beam current of about 1 µA. High-resolution transmission electron microscopy (HRTEM) combined with convergent beam electron diffraction (CBED) was employed for structure analysis using a JEOL 2010 FasTEM with an accelerating voltage of 200 kV. 3. Results and Discussion 3.1. Resistance to Sintering of Different Phases. A group of uncalcined nanoparticles and nanocomposites were prepared by coprecipitation or gelation methods using different precipitants. A gel was formed only when urea was used as the precipitant, while precipitates were formed when other precipitants (i.e., NaHCO3, Na2CO3, (NH4)2CO3) were used. As listed in Table 1, except CA50-Urea-P, the actual ratios of CuO to Al2O3 of copper/aluminum precipitates/xerogels are all close to their nominal ratios. Powder X-ray diffraction (XRD) studies and thermogravimetric analysis (TGA) show that the precipitates C100NaHCO3, Z100-NaHCO3, and A100-NaHCO3, are crystalline malachite (Cu2CO3(OH)2) phase, crystalline Zn3(OH)4CO3 phase, and X-ray amorphous Al(OH)3 phase, respectively. After calcination, these precipitates decomposed into crystalline CuO phase, crystalline ZnO phase, and X-ray amorphous Al2O3 phase, respectively. Figure 1 shows that the Al2O3 phase has a much higher BET specific surface area (SA) than the CuO or ZnO phase, which suggests that the Al2O3 phase will have a dominant effect on the surface area of a copper/aluminum nanocomposite and might have a much smaller average particle size than CuO or ZnO crystallites. Even after heat-treatment at 800 °C, Al2O3 particles have a size range of only 8-10 nm, which will be discussed at the end of the Results and Discussion section. Sintering leads to a gradual increase in the average size of crystallites or growth of the primary particles in a material or

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Xing et al. TABLE 2: Average Crystallite Sizes and Surface Areas of Calcined Nanocomposites materials before calcination CA50-NaHCO3 CA50-Na2CO3 CZ50-NaHCO3 CZ50-Na2CO3

Figure 1. Average crystallite sizes (L) and BET specific surface areas (SA) of pure oxides after calcination of C100-NaHCO3, Z100NaHCO3, and A100-NaHCO3. “LCuO” and “LZnO” denote the average sizes of CuO and ZnO crystallites, in the directions perpendicular to the CuO(-111) and ZnO(101) diffracting planes, respectively. (1/2)SA denotes half of the value of SA.

a catalyst at temperatures below the melting point of the crystallites or particles. Sintering leads to a decrease in surface area.22 Textural changes such as growth of particle size, morphology change, pore size change, or closure of open pores can be caused or affected by sintering of the components of materials. Resistance to sintering of materials is therefore important in the stability and lifetime of materials under practical conditions. Figure 1 shows that temperature has greater effects on sintering of materials than time. The Al2O3 phase shows better resistance to sintering than the CuO or ZnO phase. The average crystallite size (23.4 nm) of pure phase CuO from the calcination (400 °C for 3 h) of C100-NaHCO3 is greater than that (16.0 nm) of pure phase ZnO from the calcination (400 °C for 3 h) of Z100-NaHCO3. This is likely not due to sintering but due to the effects of the crystallite sizes of their uncalcined precursors: the average crystallite size (25.1 nm, derived from the peak at 2 theta of 14.8°) of the uncalcined precursor (Cu2CO3(OH)2) of pure phase CuO is greater than that (14.0 nm, derived from the peak at 2θ of 13.1°) of the uncalcined precursor (Zn3(OH)4CO3) of pure phase ZnO. The XRD patterns of uncalcined precursors of these pure phase oxides are shown in the Supporting Information (Figure 1). The average crystallite size of pure phase CuO from the calcination (700 °C for 3 h) of C100-NaHCO3 is 1.3 times larger than that calcined at 400 °C for 3 h, whereas the average crystallite size of pure phase ZnO from the calcination (700 °C for 3 h) of Z100NaHCO3 is 2.2 times larger than that calcined at 400 °C for 3 h. These data indicate that pure phase CuO has better resistance to sintering than pure phase ZnO. X-ray diffraction line broadening analysis (LBA) is widely used for characterizing the average size of crystallites such as supported metal crystallites, and is well suited for the study of sintering.21 Table 2 shows the average crystallite sizes and BET specific surface areas of calcined copper/aluminum and copper/ zinc nanocomposites. The XRD patterns are shown in the Supporting Information (Figures 2and 3). The precipitant NaHCO3 was used to manufacture commercial CuO/ZnO/Al2O3 catalysts.18 Sample M1 was prepared similarly and used here, representing the ZnO-free “commercial” product. The average sizes of CuO crystallites in calcined CA50-NaHCO3 and CA50-Na2CO3 were obviously smaller than those in calcined C100-NaHCO3. These data suggest that resistance to sintering of CuO crystallites can be improved significantly by an Al2O3 carrier. The role of ZnO in commercial CuO/ZnO/Al2O3

CA50-(NH4)2CO3 CA30-(NH4)2CO3 CA50-Urea-P

calcination process

LCuO/ nm a

LZnO/ nm b

SAc/ (m2/g)

coded

400 °C/3 h 400 °C/212 h 700 °C/3 h 400 °C/3 h 400 °C/212 h 700 °C/3 h 400 °C/3 h 400 °C/212 h 700 °C/3 h 400 °C/3 h 400 °C/212 h 700 °C/3 h 700 °C/3 h 750 °C/3 h 400 °C/3 h

15.1 15.7 15.2 9.0 13.7 15.9 7.8 15.2 42.8 5.1 10.8 39.9 9.8 N/Ae 26.3

N/A N/A N/A N/A N/A N/A 10.9 23.4 81.1 7.5 18.1 76.9 N/A N/A N/A

70 66 63 122 129 81 35 17 5 58 24 4 47 105 39

M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15

a LCuO(200), average size of CuO crystallites in the direction perpendicular to the (200) planes. Others are defined similarly. b LZnO(100).c BET specific surface area. d Codes of calcined materials. e No CuO phase. LCuAl2O4(311) ) 4.7 nm.

Figure 2. Differential scanning calorimetry (DSC) and thermogravimetry (TGA) curves of xerogel CA50-Urea-G (dried at 100 °C). (a) DSC in nitrogen. (b) TGA in air.

catalysts is not as an active species but as a poison absorber and a support, while the role of Al2O3 is the major support.23 The differences of average sizes of CuO crystallites and the differences of BET specific surface areas in the copper/zinc nanocomposites, which were calcined under different conditions, were much more significant than those in the calcined copper/ alumina nanocomposites. These data suggest that the Al2O3 phase is a much better support for the stabilization of CuO nanoparticles than the ZnO phase. Among the calcined copper/ zinc nanocomposites, especially after calcination at high temperature of 700 °C, the average sizes of CuO crystallites were significantly smaller than those of ZnO crystallites. These data further suggest that the CuO phase has better resistance to sintering than the ZnO phase. Furthermore, the Cu0 crystallites obtained from the calcination (400 °C for 3 h) and then the reduction (250 °C for 50 h) of C100-NaHCO3 had an average size, LCu(111), of 39.9 nm, which was significantly larger than the average size (22.9 nm) of ZnO crystallites after the calcination (400 °C for 212 h) of Z100-NaHCO3. These data suggest that the ZnO phase has better resistance to sintering than the Cu0 phase. From the above analysis, resistance to sintering of different phases decreases in the following order: Al2O3 > CuO > ZnO > Cu0. Sintering, of which the mechanism includes atomic migration and particle migration, is strongly temperature-dependent. The melting point usually plays an important role. During the

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Figure 3. Isotherm plots of physical sorption of calcined copper/ aluminum nanocomposite materials. (a) Nanocomposites prepared with the use of urea. (b) Nanocomposites prepared without the use of urea.

increase of temperature, first, when the Hu¨ttig temperature ()0.3 Tmelting,K) is reached, atoms at defects will become mobile. Later, when the Tamman temperature ()0.5 Tmelting,K) is reached, atoms from the bulk will exhibit mobility, and at the melting temperature the mobility will be so high that liquid-phase behavior will be observed.24 The melting points of different phases decrease in the following order: Al2O3 (2053 °C) > ZnO (1974 °C) > CuO (1446 °C) > Cu (1084.6 °C).25 Except for the order of CuO and ZnO, the order of resistance to sintering is consistent with the order of melting points. These data are in line with the use of ceramic supports like alumina and silica for supported metal catalysts. The small metal or metal oxide crystallites often are anchored to these supports by van der Waals forces or chemical bonds, thus avoiding sintering.24,26 On the basis of considerations of thermal sintering, CuO or Cu0 nanoparticles are supposed to be supported by a carrier with better resistance to sintering than ZnO. However, Cu/ZnO, CuO/ ZnO, and other formulations where ZnO acts as a major support are still shown in some literature.27 Unless the operation temperature is very low, the ZnO phase itself needs a refractory carrier to improve its own poor resistance to sintering. There is only a small difference in the melting point (less than 100 °C) between Al2O3 and ZnO, whereas the difference between ZnO (or Al2O3) and CuO/Cu0 melting points is hundreds of degrees. From this, one would expect ZnO to be almost as good as Al2O3 with respect to resisting sintering, but clearly this is not the case. The abnormally low resistance to sintering of the ZnO phase indicates that, in this case, the melting point alone does not reflect the observed differences. The melting point seems to not be the only reason for the resistance to sintering. 3.2. Tuning the Texture of Copper/Aluminum Nanocomposites. Differential scanning calorimetry (DSC) can be applied for both physical changes like melting and chemical reactions

Figure 4. Pore size distribution of calcined copper/aluminum nanocomposite materials. (a) BJH pore size distribution of the nanocomposites prepared with the use of urea. (b) BJH pore size distribution of the nanocomposites prepared without the use of urea. (c) HorvathKawazoe micropore size distribution.

like decompositions.28 The thermal decompositions of the copper/aluminum precipitates prepared via conventional coprecipitation methods were all endothermic [Supporting Information (Figure 4)]. As shown in Figure 2, the thermal decomposition of the xerogel CA50-Urea-G (dried at 100 °C) prepared with the use of urea has not only endothermic but also exothermic peaks in the DSC curve. The endothermic peaks were possibly due to the decomposition of copper/aluminum-containing species, whereas the exothermic peaks were possibly due to the decomposition of excess urea or urea-related organic species. Thermal treatment of the urea-related precipitate/xerogels is thus of great importance. Table 3 shows the comparison of thermal modification methods on the gel CA50-Urea-G. DSC and TGA curves in Figure 2 were used to design the thermal

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TABLE 3: Results of Stepwise Thermal Modification on the Gel CA50-Urea-G code

drying temp

calcination process

LCuO/nm a

SA/(m2/g)

M16 M17 M18 M19 M20 M21

100 °C 150 °C 180 °C 180 °C 180 °C 180 °C

400 °C/3 h 400 °C/3 h 400 °C/3 h 400 °C/212 h 250 °C/2 h, 400 °C/3 h 250 °C/2 h, 400 °C/212 h

29.5 29.5 9.4 9.4 7.6 7.4

8 8 160 155 217 245

a

LCuO(-111).

modification methods. The DSC curve shows that endothermic decomposion occurs below 257 °C, whereas exothermic decomposition occurs above 257 °C. Endothermic decomposion is usually harmless, whereas exothermic decomposion might damage or destroy the texture and structure of a sample. This explains why sample M16, which was formed by drying the gel CA50-Urea-G at 100 °C and subsequent calcination at 400 °C (furnace temperature), has a SA of only 8 m2/g, which means that both the CuO phase and even the Al2O3 phase were significantly sintered. The XRD patterns of some calcined xerogels are shown in the Supporting Information (Figure 5). High-temperature products, that is, R-Al2O3, CuAl2O4, and CuAlO2, were found in sample M16. The CuAl2O4 spinel phase is usually formed by the solid-solid reaction between CuO and Al2O3 phases at 600 °C or a higher temperature,29 whereas CuAlO2, a delafossite ABO2 phase, is usually formed at a high temperature like 1160-1200 °C.30 These data suggest that although the temperature of the muffle furnace during calcination was only 400 °C the temperature in sample M16 was much higher than 400 °C. The Cu0 phase found in sample M16 was formed possibly via the reduction by urea. The TGA curve in Figure 2 shows that weight loss became significant at 165 °C and became almost constant above 278 °C. Combining the TGA and DSC curves, the thermal treatments between 165 and 257 °C may relieve the destructive exothermic effects by decomposing and releasing most exothermic components. Without thermal treatments in this “safety zone” (165-257 °C), calcination of CA50-Urea-G may be destructive to the texture and structure of the sample. Sample M17, which was formed by drying the gel CA50-Urea-G at 150 °C, a temperature not in the “safety zone”, and subsequent calcination at 400 °C, was significantly sintered and was very similar to sample M16. Sample M18, which was formed by drying the gel CA50-Urea-G at 180 °C, a temperature in the “safety zone”, and subsequent calcination at 400 °C, has a much higher SA (160 m2/g) and a much smaller average size (9.4 nm) of CuO crystallites than samples M16 and M17. Besides the drying at 180 °C, samples M20 and M21 were prepared with an extra thermal treatment at 250 °C before the final calcination at 400 °C and have improved higher SA values (217 and 245 m2/g, respectively) than sample M18. None of the CuAl2O4, CuAlO2, R-Al2O3, and Cu0 phases were found in samples M18-M21, which consist of only crystalline CuO and X-ray amorphous phases. These data show that, when the destructive exothermic decomposition is controlled via welldesigned stepwise thermal modification, exothermic processes can be employed to produce CuO/Al2O3 nanocomposites with high specific surface areas. If the thermal decomposition of a sample (e.g., the precipitates formed by using precipitants like NaHCO3, Na2CO3, or (NH4)2CO3) is endothermic, then its heat-treatment might be as simple as a single-step calcination. No significant improve-

Figure 5. Stepwise reduction of calcined CA50-(NH4)2CO3 (M13) under 250-600 °C methanol-steam reforming (MSR) conditions as well as reoxidation of the reduced material. (a) XRD patterns: a, CuAl2O4 phase; b, CuO phase; c, Cu0 phase; d, Al2O3 phase. 1, before reduction; 2, after reduction at 250 °C; 3, after reduction at 250/350 °C; 4, after reduction at 250-450 °C; 5, after reduction at 250-600 °C; 6, after reoxidation at 200 °C. (b) gradual, isotropic decreases of interplanar distances of Cu-Al-O spinel phase during reduction. The samples were step-scanned at 2θ of 41.3-45.3° (Cu0(111) peak) with a preset time of 6 s and were scanned continuously at other angles with a rate of 2°/min.

ments or benefits were observed in the stepwise thermal modification of these conventional coprecipitation products. If the thermal decomposition of a sample (e.g., the gel formed by using urea) contains exothermic peaks in its DSC curve, then the heat-treatment might have to be stepwise in order to avoid a temperature runaway and consequent sintering/solid-state reactions. The calcination of a small quantity of urea-containing Cu/Al composite materials in a muffle furnace was safe, but special precautions regarding hazards (e.g., temperature runaway) might have to be taken during the heat-treatment of a large quantity of urea-containing Cu/Al composite materials. Table 4 shows the textural features of calcined copper/ aluminum nanocomposite materials, of which the isotherm plots of physical sorption are shown in Figure 3. Each isotherm plot in Figure 3 has a characteristic hysteresis loop and exhibits a Point B, at which monolayer coverage is complete and multilayers start to occur. Therefore, all isotherm plots in Figure 3 are of type IV. A classification of the hysteresis loops into five main groups has been proposed by de Boer.12,31 The hysteresis loops of samples M18-21, which were prepared with the use of urea and have almost equivalent amounts of CuO and Al2O3 phase, are much closer to type A than to type E. Unlike the non-parallel branches in the type E hysteresis loop, the two branches in their hysteresis loops are nearly parallel over an appreciable range

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TABLE 4: Textural Features of Calcined Copper/Aluminum Nanocomposite Materials

code

nanocomposites before calcination

calcination process

surface areaa/ (m2/g)

M1 M2 M4 M5 M13 M15 M18 M19 M20 M21

CA50-NaHCO3 CA50-NaHCO3 CA50-Na2CO3 CA50-Na2CO3 CA50-(NH4)2CO3 CA50-Urea-P CA50-Urea-G CA50-Urea-G CA50-Urea-G CA50-Urea-G

400 °C/3 h 400 °C/212 h 400 °C/3 h 400 °C/212 h 700 °C/3 h 400 °C/3 h 400 °C/3 h 400 °C/212 h 250 °C/2 h,400 °C/3 h 250 °C/2 h,400 °C/212 h

70 66 122 129 47 39 160 155 217 245

av pore diameterb/ nm

pore volume Vpore c/ (cm3/g)

particle porosity (p)d

solid density Fsolid e/ (g/cm3)

particle density Fpf/ (g/cm3)

specific copper loadingg/ (g/cm3)

21.6 22.9 10.3 13.7 22.4 15.4 4.3 4.6 4.2 4.1

0.38 0.38 0.32 0.44 0.26 0.15 0.17 0.18 0.23 0.25

65.9% 65.8% 61.8% 69.4% 55.4% 48.0% 47.0% 48.0% 54.2% 56.2%

5.10 5.10 5.14 5.14 4.73 6.10 5.18 5.18 5.18 5.18

1.74 1.75 1.96 1.57 2.11 3.17 2.75 2.69 2.37 2.27

0.70 0.70 0.78 0.63 0.82 2.32 1.12 1.09 0.96 0.92

a BET specific surface area. b Average pore diameter (4V/A by BET). c Single point total pore volume of pores less than 2823.0542 Å diameter at P/P0 0.99311705. d p ) Vpore/Vparticle ) Vpore/(Vpore + Vsolid) ) Vpore/(Vpore + 1/Fsolid). e Also called skeletal density: mass per unit volume of solid, calculated by Σ(Xi*Fi), where Xi and Fi are, respectively, the concentration and solid density of a component. f Also called apparent density: mass per unit volume of pellet. Fp ) 1/(Vpore + Vsolid) ) 1/(Vpore + 1/Fsolid). g Loaded amount of copper per unit volume of pellet.

of gas uptake. Therefore, the hysteresis loops of samples M1821 belong to type A, indicating the presence of a mesoporosity formed by “prism-shaped” or “cylinder-shaped” pores in these samples. The hysteresis loops of samples M1, M2, M4, M5, and M13, which were prepared without the use of urea, are clearly of type D, indicating the presence of a mesoporosity created by slitshaped pores that are formed by non-parallel plates. The hysteresis loop of sample M15, which was prepared with the use of urea and contains 91.4% (by weight) of CuO phase and 8.6% of Al2O3 phase, is in between types B and D and is a little closer to the former, indicating the presence of a mesoporosity created by slit-shaped pores, which are formed by both parallel plates and non-parallel plates. During the synthesis of CA50-Urea-G at 100 °C, precipitate particles were first produced from the clear solution and later a uniform, blue-color gel was quickly formed while vigorous stirring was continued for about 20 min after the formation of the gel. The precipitate particles were filtered before the formation of the gel, washed with deionized water, and then desiccated at 100 °C. Precipitate CA50-Urea-P, the uncalcined precursor of sample M15, was obtained in this way for the study of the preparation process. As listed in Table 1, precipitate CA50-Urea-P contains much more CuO than Al2O3 (elemental compositions), indicating that the copper-containing precipitate was formed prior to the aluminum-containing species. The complete formation of aluminum-containing species during the synthesis of CA50-Urea-G caused the formation of a gel product, possibly via the crosslinking of aluminum-containing species in urea solution. The gel transformed to a dense fluid during drying at 180 °C and finally became a xerogel, with visible macropores and significant volume expansion. Calcination of this xerogel produces samples M18-21. Figure 4a and b shows the pore size distributions calculated from the Barrett-Joyner-Halenda (BJH) method. Samples M18-21, which were prepared with the use of urea and have hysteresis loops of type A, have narrow distributions of pore sizes and small average pore diameters of 4.1-4.6 nm. The major pore diameters of samples M18-21, are 3.78, 3.40, 3.35, and 3.38 nm, respectively. As shown in Figure 4a and Table 4, sample M15 has a significantly wider distribution of pore sizes and a larger average pore diameter (15.4 nm) than samples M18-21, indicating that once the content of Al2O3 in a sample is very low and exothermic urea-related components are completely washed away the produced CuO/Al2O3 nanocom-

posites may no longer have narrow distributions of pore sizes or small average pore diameters. As shown in Figure 4b and Table 4, samples M1, M2, M4, M5, and M13, which were prepared without the use of urea and have hysteresis loops of type D, have wide distributions of pore sizes and large average pore diameters of 10.3-22.9 nm. Except for samples M15 and M13, all samples in Figure 4a and b are similar in both structure and chemical composition, but they belong to two distinct groups of pore size distributions. These data clearly show that the narrow distributions of pore sizes are caused by suitable employment of urea. These data show that the texture of copper/aluminum nanocomposite materials can be finely tuned via well-designed synthesis and thermal modification methods. As shown in Table 4, samples M18-21 have BET specific surface areas of 155-245 m2/g. Samples M1, M2, M4, and M5 have BET specific surface areas of 66-129 m2/g. Comparing the calcinations at 400 °C for 3 h and 212 h, the decrease of BET specific surface area in these eight samples is not significant, indicating that at 400 °C the sintering of Al2O3 phases in these samples are negligible. As shown in Tables 2 and 3, the average sizes of CuO crystallites of these samples at 400 °C are stable, except for samples M4 and M5. As shown in Table 4, the pore volumes and particle porosities in samples M1, M2, M4, and M5 are greater than those in samples M18-21. The particle densities of samples M1, M2, M4, and M5 are significantly smaller than those of samples M18-21. As a result, the specific copper loadings in samples M18-21 are significantly greater than those in samples M1, M2, M4, and M5. For example, the specific copper loading in sample M21 is 32% greater than that in sample M2, and is 47% greater than that in sample M5. Meanwhile, the BET specific surface area in sample M21 is 273% greater than that in sample M2, and is 89% greater than that in sample M5. A relatively high specific metal loading is valuable in an apparatus that has space limitations, such as a vehicle fuel cell system. Catalysts with relatively high specific metal loadings may be used to decrease the space occupancy of fuel processing units of fuel cell systems and may be used to increase the fuel processing capability and output of an apparatus with a certain space occupancy. In comparison with the products of conventional coprecipitation methods, samples M20 and M21 have large and stable specific surface areas, small and stable average sizes of CuO crystallites, narrow distributions of pore sizes that are small but

1452 J. Phys. Chem. C, Vol. 112, No. 5, 2008 large enough for catalytic reactions, acceptable particle porosities and pore volumes, and significantly higher specific copper loadings. Tables 3 and 4 show that proper use of urea may create nanopores in the products, whereas improper use of urea may destroy the porous structure, cause severe sintering, and produce unfavorable phases. In our previous report, the samples (e.g., sample M20) having narrow distributions of pore sizes with small major pore diameters (e.g., around 3.5 nm) stabilized the supported Cu0 nanoparticles in wide temperature ranges by confining the growth of copper nanoparticles.32 3.3. Tuning the Structures of Cu-Al-O Spinel Phases. To study the tuning of the spinel structure, we chose two typical samples M13(CuAl2O4/CuO) and M14(CuAl2O4/Al2O3) that contain considerable CuAl2O4 spinel phase for reduction studies. The XRD pattern for sample M13, shown as curve 1 in Figure 5a, suggests that sample M13 is composed of crystalline CuAl2O4 and CuO phases. The significant intensity of the peaks of CuAl2O4 phase and the absence of the only sharp peak of Al2O3 phase at 2θ of 38.4°, indicates that the Al2O3 phase has been consumed to form a CuAl2O4 phase via the solid-solid reaction with the CuO phase at 700 °C. Sample M13 has a large average pore diameter of 22.4 nm and a wide distribution of pore sizes in the mesopore and macropore regions, as shown in Table 4 and Figure 4b. Futhermore, the micropore size distribution calculated from the Horvath-Kawazoe (H-K) method, as shown in Figure 4c, suggests that sample M13 has micropores distributed in the range of 0.47-1.51 nm, with a major micropore size of 0.64 nm. The micropore volume of sample M13 is 0.015 cm3/g, which is 5.7% of the total pore volume. The micropore area of sample M13 is 30.4 m2/g, which is 64.8% of the BET specific surface area. Reduction of the CuAl2O4-containing sample (M13), was carried out in a temperature range of 250-600 °C under methanol-steam reforming (MSR) conditions, from which reducing gases like H2 will be produced for reduction. The temperature of 250 °C is the temperature we used in the reduction, but the lowest temperature for reduction was not determined. Figure 5a shows that the CuO phase in sample M13 was completely reduced to a Cu0 phase at 250 °C. During the reduction at different temperature stages from 25 to 600 °C, the peaks of the CuAl2O4 phase became less and less intense and gradually shifted to larger scanning angles, while at the same time the peaks of the Cu0 phase became more and more intense but did not shift, and therefore could be used as a reference material. These XRD data show that the CuAl2O4 phase was gradually reduced to Cu0, Al2O3, and unit-cell contracted Cu-Al-O spinel phases at different temperature stages. The final average size of Cu0 crystallites is 14.6 nm after the 250-600 °C reduction experiment. Figure 5b shows that the interplanar distances in different orientations of the Cu-Al-O spinel phases decreased in almost the same trend and extent, indicating that the unit-cell contraction of Cu-Al-O spinel phases is isotropic and gradual and depends mainly on reducing temperature. The gradual decreases of interplanar distances are clearly signs of the formation of a series of Cu-Al-O solid solutions, in which some relatively large Cu2+ cations33 were gradually replaced by some relatively small Al3+ cations.33 As a result, the unit-cell contracted CuAl-O spinel phases will become rich in aluminum and insufficient in copper. The BET specific surface area of sample M13 before reduction is 47 m2/g, whereas that of sample M13 after the 250-600 °C reduction experiment is 74 m2/g. This increase is mainly due to the releasing of Al2O3 nanoparticles during the reduction of the CuAl2O4 phase. Curve 6 in Figure

Xing et al.

Figure 6. XRD patterns during stepwise reduction of calcined CA30(NH4)2CO3 (M14) under 250-800 °C methanol-steam reforming (MSR) conditions. a, CuAl2O4 phase; b, Cu0 phase; c, Al2O3 phase. 1, before reduction; 2, after reduction at 250 °C; 3, after reduction at 250/ 500 °C; 4, after reduction at 250-600 °C; 5, after reduction at 250700 °C; 6, after reduction at 250-800 °C. The samples were stepscanned at 2θ of 41.3-45.3° (Cu0(111) peak) with a preset time of 6 s and were scanned continuously at other angles with a rate of 4°/min.

5 is the XRD pattern of sample M13 after the reduction experiment and subsequent reoxidation, which shows that Cu0 phase was reoxidized into CuO phase while the peaks of the unit-cell contracted Cu-Al-O spinel phase did not change. This reoxidation experiment provides a method to purify the produced unit-cell contracted Cu-Al-O spinel materials: Cu0 phase can be oxidized to CuO phase, which may be removed together with Al2O3 phase by dilute acid leaching. Reduction of another CuAl2O4-containing sample (M14), which consists of CuAl2O4 and Al2O3 phases and does not contain CuO phase, was carried out in the temperature range of 250-800 °C under methanol-steam reforming (MSR) conditions, as shown in Figure 6. These XRD data show that the CuAl2O4 phase was gradually reduced to Cu0, Al2O3, and unitcell contracted Cu-Al-O spinel phases. The unit-cell contracted Cu-Al-O spinel phases existed even after reduction at 700 °C and 800 °C, although with much lower concentrations than other temperature stages. The XRD pattern (2) in Figure 6 confirms that Cu0 can be formed by reducing CuAl2O4 phase at 250 °C. The average sizes of Cu0 crystallites at the end of 500, 600, 700, and 800 °C stages are 8.5, 10.6, 15.6, and 26.6 nm, respectively, indicating that the sintering of Cu0 crystallites at 700 and 800 °C is more significant than at 500 and 600 °C. The average size (10.6 nm) of Cu0 crystallites in sample M14 at the end of the 600 °C stage is significantly smaller than that (14.6 nm) of sample M13, indicating that the resistance to sintering of supported metal nanoparticles can be improved by increasing the content of the support. Other than the R-Al2O3 phase in samples M16 and M17, the Al2O3 phases in all samples in this paper are X-ray amorphous. Other than a sharp peak at 2θ of 38.4°, the lines in the XRD patterns of these Al2O3 phases were mostly diffuse. Figure 7 shows the 25-750 °C temperature-programmed reduction (TPR) plot of calcined CA50-(NH4)2CO3 (sample M13). Peaks 1-5 are the decreases of hydrogen partial pressures and represent the consumption of hydrogen. Hydrogen-containing gases are the reducing reagents for both the reduction experiments and TPR studies. Therefore, the data of the reduction experiments may provide information for potential use in TPR studies. The data in Figures 5 and 6 suggest that

Tuning of Texture and Structure

J. Phys. Chem. C, Vol. 112, No. 5, 2008 1453

Figure 7. Temperature-programmed reduction (TPR) study of calcined CA50-(NH4)2CO3 (M13). 1)-5): H2 consumption peaks.

the first TPR peak at about 210 °C should be attributed to the reduction of CuO to Cu0, the second TPR peak at about 250 °C should be attributed to the reduction of the fresh CuAl2O4 phase, while other three TPR peaks represent the gradual, stepwise reduction of the unit-cell contracted Cu-Al-O spinel phases. The third TPR peak, occurring from 320 °C and reaching its maximum at 390 °C, is most intense, indicating that the reduction of the Cu-Al-O spinel phase at 320-390 °C is the most significant part. Other than the second TPR peak, which has the contribution from the reduction of the fresh CuAl2O4 phase and has been confirmed by the XRD pattern (2) in Figure 6, all attributions of TPR peaks are consistent with the literature.34 Reduction of other samples to Cu0/Al2O3, homogeneity (dispersion) of metals within the solids, effects of mixing homogeneity and pore size distribution on the stabilization of supported metal nanoparticles, and other properties of the samples were addressed in our previous report.32 3.4. Electron Microscopy Studies of Materials. Electron microscopy studies can be used for the microstructure characterization of metal/metal oxide nanocomposites.35 The typical samples produced by different precipitants were observed by FESEM or HRTEM. Figure 8 shows the results of the electron microscopy studies of copper/aluminum nanocomposite materials. Although the resolution of FESEM may be up to 1 nm, individual Al2O3 nanoparticles are still invisible because of their poor electrical conductivity and contrast. FESEM was thus used to observe the morphology of Al2O3 aggregates or individual CuO/Cu0 nanoparticles. HRTEM can be used to observe any of these individual nanoparticles. As shown in Figure 8a and b, the morphology of the Al2O3 phase in sample M20 consists of an assemblage of layered crescent-shaped sheets that rigidly joined together, on which CuO crystallites were anchored as dispersed arrays with crystallite sizes of 7-8 nm. Figure 8c and d shows the mesopores in samples M4 and M1, respectively. The mesopores in samples M4 seem smaller than those in sample M1. The magnifications of Figure 8c and d are so large for FESEM (scale bars: 50 nm) that these two images seem somewhat indistinct. Figure 8e and f shows that the reduced sample M13 is composed of randomly joined particles, which created pores widely distributed in both the mesopore and macropore ranges. The dark-color substances in the HRTEM image of the reduced sample M14 (Figure 8g) are mostly Cu0 single crystals, which contribute to the diffraction spots of the electron diffraction pattern (Figure 8h), and some randomly oriented, unit-cell contracted Cu-Al-O spinel polycrystals that give rise to the diffraction rings. The light-color substances in Figure 8g are the Al2O3 nanoparticles, which seem cohesive but act as a carrier for dispersed Cu0 particles. Individual Al2O3 particles are round flakes with a diameter of about 8-10 nm

Figure 8. Electron microscopy studies of copper/aluminum nanocomposite materials. (a and b) FESEM images of M20 (scale bars: 200 nm for a and 100 nm for b). (c) FESEM image of M4 (scale bar: 50 nm). (d) FESEM image of M1 (scale bar: 50 nm). (e and f) FESEM images reduced M13 after the 250-600 °C reduction (scale bars: 500 nm for e and 200 nm for f). (g and h) Reduced M14 after the 250800 °C reduction: (g) HRTEM image (scale bar: 100 nm); (h) CBED pattern of a dark-color substance.

and are randomly aggregated, while some are sintered as fingerlike bars with a width of a single particle and a length of 4075 nm. 4. Conclusions In summary, copper/aluminum and copper/zinc nanocomposite materials were prepared for studies of textural tuning, structural tuning, or material sintering. Resistance to sintering of different phases was found to decrease in the following order: Al2O3 > CuO > ZnO > Cu0. Thermal analyses including TGA and DSC were used to design feasible thermal modification methods that avoid destructive damages and create special pore size distributions. The porous texture of copper/aluminum nanocomposite materials can be finely tuned via well-designed synthesis and thermal modification methods. Conventional coprecipitation methods produce the CuO/Al2O3 nanocomposites with wide distributions of pore size, large average pore sizes of 10.3-22.9 nm, and low surface areas of 66-129 m2/g. The novel urea-gelation method, together with stepwise thermal modification, which makes the exothermic effects of urea

1454 J. Phys. Chem. C, Vol. 112, No. 5, 2008 controllable, produces the CuO/Al2O3 nanocomposites with narrow distributions of pore sizes, small average pore sizes of 4.1-4.6 nm, and high surface areas of 155-245 m2/g. In comparison with the products of conventional coprecipitation methods, this novel urea-gelation/thermal-modification method produces copper/aluminum nanocomposites with significantly higher specific copper loading, which should be valuable in apparatus which have space limitations, such as vehicle fuel cell systems. The isotropic and gradual unit-cell contractions were observed during the structural tuning of the CuAl2O4 phase via stepwise reduction under methanol-steam reforming conditions. Acknowledgment. We thank Dr. Francis Galasso for helpful suggestions, Dr. Jim Romanow for helping to collect FESEM images, and the Institute of Materials Science at University of Connecticut for use of the microscopy facilities. This work was supported by the Geosciences and Biosciences Division of the Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy. Supporting Information Available: Powder X-ray diffraction patterns and DSC curves are included. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Cansell, F.; Aymonier, C.; Loppinet-Serani, A. Curr. Opin. Solid State Mater. Sci. 2003, 7, 331-340. (2) Xu, Z.; Zhang, Q.; Fang, H. H. P. Crit. ReV. EnViron. Sci. Technol. 2003, 33, 363-389. (3) Witula, T.; Holmberg, K. Langmuir 2005, 21, 3782-3785. (4) McLeary, E. E.; Jansen, J. C.; Kapteijn, F. Microporous Mesoporous Mater. 2006, 90, 198-220. (5) de Angelis, A.; Ingallina, P.; Perego, C. Ind. Eng. Chem. Res. 2004, 43, 1169-1178. (6) Thomas, K. M. Catal. Today 2007, 120, 389-398. (7) Velev, O. D.; Kaler, E. W. AdV. Mater. 2000, 12, 531-534. (8) Huczko, A. Appl. Phys. A 2000, 70, 365-376. (9) Lee, J.; Kim, J.; Hyeon, T. AdV. Mater. 2006, 18, 2073-2094. (10) Toberer, E. S.; Seshadri, R. Chem. Commun. 2006, 3159-3165. (11) Davis, M. E. Nature 2002, 417, 813-821. (12) Lecloux, A. J. In Catalysis: Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1981; Vol. 2, pp 173, 178-179. (13) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Simmieniewska, T. Pure Appl. Chem. 1985, 57, 603619. (14) (a) Sagar, G. V.; Rao, P. V. R.; Srikanth, C. S.; Chary, K. V. R. J. Phys. Chem. B 2006, 110, 13881-13888. (b) Twigg, M. V. Catalyst Handbook; Manson Publishing: London, 1996; pp 312, 456. (c) Fridman,

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