11994
J. Phys. Chem. B 2001, 105, 11994-12002
Characteristics of Copper Particles Supported on Various Types of Graphite Nanofibers J. Ma, C. Park, N. M. Rodriguez,† and R. T. K. Baker*,† Chemistry Department, Northeastern UniVersity, Boston, Massachusetts 02115 ReceiVed: July 5, 2001; In Final Form: October 3, 2001
We have used a variety of experimental techniques, including high-resolution transmission electron microscopy, X-ray diffraction, and adsorptive decomposition of N2O to examine the characteristics of copper particles dispersed on different types of graphite nanofiber supports. When copper was dispersed on the edge sites of graphite nanofibers the particles adopted a relatively thin faceted morphology; characteristics that are associated with the establishment of a strong metal-support interaction. This behavior is to be contrasted with that observed when the metal was supported on active carbon or the basal planes of graphite. In these cases, the particles tended to acquire a globular geometry typically encountered in systems were there are relatively weak interfacial forces between the metal and the support. Structural modeling of the arrangement of copper atoms on the prismatic and basal plane surfaces of graphite indicated that formation of preferred crystallite orientations occurred at certain support locations. It was found that the atomic arrangement of Cu(110) exhibited the closest match with the graphite “zigzag” face, whereas that of Cu(100) was in register with a section of the “armchair” face configuration. On the other hand, the atomic arrangement of Cu(111) provided a good fit with the graphite basal plane. The ramifications of these features on the impact of different types of graphite nanofiber supports on the catalytic performance of copper are discussed.
Introduction High-resolution transmission electron microscopy (HRTEM) has traditionally been used to determine the size of very small particles. It has recently been recognized that this technique can provide a great deal of fundamental information regarding the particle characteristics, such as morphology, crystalline orientation, and lattice parameters. In addition, by using a combination of selected area electron diffraction and dark field imaging techniques it may be possible to establish the existence of preferred crystallite orientations and how such features change on the various support materials. The morphological characteristics of metal particles are governed to a large degree by two factors; the chemical nature of the surrounding gas phase and the strength of the interaction with a support medium.1 It has been found that metal particles that exhibit a strong interaction with the substrate tend to adopt a “pill-box” morphology.2 Over the years, considerable research effort has been devoted to the study of the interactions between metal particles and various support media.3 It is now generally accepted that the strength of a metal-support interaction can exert a significant impact not only on the activity, but also on the selectivity pattern displayed by a catalyst system. The events occurring at the metal-support-gas interface can be described from a thermodynamic standpoint by reference to Young’s equation
γgs ) γms + γmgcosθ
(1)
where θ is the contact angle between the metal particle and the support, γ is the surface energy and the subscripts s, m, and g refer to support, metal and gas, respectively. The ability of the * To whom all correspondence should be addressed. † Current address: Catalytic Materials Ltd., West Holliston Professional Park, Holliston, MA 01746.
metal particles to undergo a transformation from a nonwetting (θ > 90) to a wetting state (θ < 90°) indicates that a significant degree of atomic mobility exists, particularly in the surface layers. It follows, therefore, that there will be a minimum temperature, related to the melting point of the metal or metal oxide, where such a phenomenon can occur. Spreading of the metal, in form of a thin film, along the support surface will take place when the work of adhesion is greater than the work of cohesion within the particle; conditions where a strong metal-support interaction exists can be expressed as
cosθ )
γgs - γms γmg
(2)
A survey of the literature reveals that there are relatively few papers dealing with morphological aspects of supported copper particles. In an investigation of the copper/TiO2 system treated in hydrogen at 400 °C conducted by Boccuzzi and co-workers,4 the metal particles observed in a cross-sectional view exhibited a large contact area with the support. These features were rationalized according to the notion that a strong interaction was established between the metal and support. In the present study, the morphologies of graphite nanofiber (GNF) supported copper particles were investigated as a function of the particular type of nanofiber and sample pretreatment conditions. From these observations, we have attempted to establish the relationship between the strength of the metal-support interaction and the GNF supports. The particle size distribution of supported catalysts can also be used to obtain a measure of the strength of metal-support interactions between components. A decrease in the average particle size of Pt on a TiO2 support was found when the system was heated in H2 at 500 °C, and this finding led to the belief that this was a criterion for the establishment of a strong metalsupport interaction.5,6 Later, this feature was found not to be a
10.1021/jp012551x CCC: $20.00 © 2001 American Chemical Society Published on Web 11/07/2001
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critical factor because in other comparable systems, the metal particles were observed to have rather large sizes. This phenomenon was shown to be due to the formation of a “pillbox” structure that was created by a spreading action of metal on the support surface.2 In another investigation, the surface atoms of a metal particle were found to exhibit significant mobility upon heating. Baker7 demonstrated that a direct relationship existed between the observed mobility temperature of small particles (∼10 nm diameter) on the basal plane of graphite and the respective bulk melting point of the metal. These temperatures were close to half the melting point of the corresponding metal when expressed on the absolute scale, a temperature known as the Tammann Temperature in solid-state physics.8 The induced mobility could result in changes in the particle morphology and particle size by the subsequent collision with other particles to cause sintering under certain heating conditions. Therefore, the change in the supported metal particle size under different reactions conditions could be used to determine the sintering characteristics of supported catalysts. The measurement of particle size is bound to be somewhat difficult when the supported particles are nonuniform in both size and shape. A variety of experimental techniques used for size characterization have limitations that are dictated by the particle dimensions, both width and thickness, and furthermore, such techniques usually give dramatically different values for the “average” size of the same assembly of particles. The particle size characterization is normally performed using a combination of wide-angle X-ray diffraction, transmission electron microscopy, small-angle X-ray scattering, and chemisorption methods.9 Metal dispersion, which represents the ratio of exposed surface atoms to total atoms in a particle, has a direct relationship with the catalytic activity and as a consequence, the determination of this parameter is essential. Traditionally, the metal dispersion is measured using CO or H2 chemisorption, however, neither of these adsorbates can provide an accurate means of determining the dispersion of supported copper catalysts.10 Furthermore, attempts to use O2 chemisorption have also been fraught with problems because oxidation of the surface copper atoms ensues and continues into the subsurface region.11 The adsorptive decomposition of N2O has been found to provide a satisfactory method of measuring the metal surface area of both pure and supported copper catalysts.12-15 Under mild conditions, the adsorptive decomposition of N2O can take place on the metallic surface of copper to produce gas-phase nitrogen according to the following reaction scheme
N2O (g) + 2Cu (s) f N2 (g) + Cu-O-Cu (s)
(3)
The oxidation of copper by N2O was found to proceed only on the surface and lead to the Cu+ oxidation state by Narita and co-workers using the UPS technique.16 The moles of N2 evolved is also generally agreed to be stoichiometrically equivalent to twice that of the surface copper atoms corresponding to the full surface coverage (θ)1). There is some conjecture regarding the optimum temperature for the N2O decomposition reaction and the effect of the temperature on the extent of subsurface oxidation. Dell and co-workers used 20 °C as the chemisorption temperature in their measurement,12 whereas Scholten and Konvalinka14 suggested that performing the experiments at 90° to 100 °C could provide a more reliable result as the bulk oxidation would start above 120 °C. Support for this statement was presented by Evans and co-workers,15 who carried out the reaction at 90 °C using a single pulse of excess amounts of N2O. Burch and co-workers investigated this system using a frontal chromatography technique and claimed that the resulting
surface Cu2O monolayer was impervious to oxygen at 60 °C, and only slight penetration of the gas into subsurface layer occurred at higher temperatures.17 Some investigators have even proposed using temperatures up to 120 °C as they considered that the bulk oxidation would not occur until 150 °C.13 Various techniques have been developed to perform the measurement of the copper surface area using the adsorptive decomposition of N2O.13,17-21 The application of this characterization method has been focused on the methanol synthesis catalysts, i.e., copper supported on traditional catalyst support media such as γ-Al2O3, SiO2, and ZnO. Very few studies have involved the discussion of the effects of the different support types or the sample pretreatment procedure, an exception being an investigation by Birch and co-workers 17 who claimed that the metal dispersion was related to the sample history. In the current study, a pulse reactor system connected to a gas chromatography unit was used to carry out the determination of the metal dispersion of the carbon nanofiber supported copper catalysts. The influence of different copper precursors on these materials and the sample pretreatment step were investigated and correlated with the results obtained from other characterization methods. Experimental Section Materials. The graphite nanofibers used in this work were synthesized according to the following procedures. A nickelbased catalyst was used to produce graphite nanofibers possessing a “herring-bone” like structure, referred to as GNF-H, from the catalytic decomposition of ethylene at 600 °C. Various iron-based catalysts were used to prepare the other three types of graphite nanofibers with diverse structural arrangements from the decomposition of CO/H2 at 600 °C. These latter three types of nanofibers are identified as “platelet”, “ribbon”, and “tubular” forms, designated as GNF-P, GNF-R, and GNF-T, respectively. The detailed steps involved in the preparation procedures of certain graphite nanofibers were identical to those reported in previous literature studies.22-24 Following the growth of graphite nanofibers, the metal particles present within the structures were removed by dissolution in 1.0 M HCl over a period of 1 week prior to impregnation of the copper precursor salts. Finally, TEM and XRD techniques were used to examine these materials to ensure the complete removal of the original metal catalyst particles. No diffraction signals of residual metal particles were found to be present in the XRD spectra after the demineralization process. This observation was also supported by the random survey of a number of samples under the electron microscope, which showed a complete absence of metal particles. The GNF supported copper catalysts were prepared according to a standard incipient wetness impregnation method. Cu(NO3)2 (99.9%) obtained from Fisher Scientific Co. and Cu(Ac)2 (99.9%) purchased from Alpha Chemical Co. were used as the copper precursor salts and were dissolved in ethanol and added dropwise to the graphite nanofiber support media. After the solvent was evaporated, the impregnated samples were dried at 110 °C overnight and calcined in air at 180 °C for 20 h to decompose the copper precursors. This step was followed by reduction in 10% H2/He at 300 °C for 24 h. Finally, the reduced samples were passivated in 2% O2/He before being taken out of the reactor. This process creates a protective oxide skin on the surface of metallic copper, which prevents the metal particles from sintering and attacking the carbon as a result of the extreme exothermic reaction between chemisorbed hydrogen on the metal surface and the oxygen in air.
11996 J. Phys. Chem. B, Vol. 105, No. 48, 2001
Figure 1. Schematic diagram of the pulse reaction system used for the measurement of copper dispersion.
Unless specifically mentioned as being derived from Cu(Ac)2, all the catalysts used in this work were prepared from Cu(NO3)2. To simplify the notation, Cu/GNF and Cu/GNFA are used as the representation of catalysts prepared from Cu(NO3)2 and Cu(Ac)2, respectively when the behavior of such systems are compared. The gases used in this investigation oxygen (99.999%), hydrogen (99.999%), CO (99.99%) helium (99.999%) and N2O (99.995%) were purchased from Med Tech and used without further purification. Techniques and Procedures. A JEOL 100CX electron microscope was used in this study to obtain suitable micrographs for the measurement of the metal particle size distribution. The image was transferred to a close-circuit TV monitor connected to a built-in camera with an increased magnification between 300,000 and 1,200,000 times. Normally, about 400 to 600 metal particles were randomly selected and measured from different areas on a given sample in order to obtain the typical sizes of a diverse specimen area. High-resolution studies were performed with a JEOL 2000 EX II electron microscope, an instrument that has a lattice resolution of 0.18 nm. Suitable transmission specimens were prepared by ultrasonic dispersion of a selected catalyst sample in 2-butanol and application of a drop of the supernate onto a holey carbon support film. XRD patterns of the GNF supported copper catalysts were obtained with a Rigaku 300 X-ray diffractometer using nickel filtered Cu Ka radiation. Diffraction patterns were recorded over a range of 2θ angles from 10 to 90°. Spectra analysis was performed using the “Jade” software combined with the “JCPDS” database. Temperature programmed reduction (TPR) studies of copper oxide on the different types of GNF were performed with a custom-designed unit equipped with a thermal conductivity detector. In these experiments a 10% H2/N2 gas mixture was used as both the reduction and carrier gas for the detector. The sample was heated at 10 °C/min in the gas mixture at a total flow rate of 50 mL/min. A cold trap was inserted in the line to remove the water generated during the reaction. Chemisorption measurements were performed in an apparatus that is shown in the schematic diagram, Figure 1. Helium was used as the carrier gas for both reactor and GC procedures. Two 4-port valves and one 6-port valve were employed to control the reactor and injection systems. When the 4-port valves were set as indicated by the solid lines, the system was in the bypass
Ma et al. mode. The catalyst in the reactor was then isolated from the chromatography system and subjected to various treatments including reduction with hydrogen or CO. In the pulse reaction configuration, the 4-port valves were switched to the position shown by the dashed lines, where the gas stream containing N2O and He flowed through the six-port valve and filled the sample loop with a volume of 0.5 mL. By switching the 6-port valve, a pulse of diluted N2O was injected into the reactor, and the evolved gases including N2 and N2O were then passed through a 5A molecular sieve GC column and analyzed by the thermal conductivity detector. A 50 mg sample was first treated at its original reduction temperature, i.e., 200 °C, 300 °C, or 400 °C, in 10% H2/He for 30 min. Following this step the sample was cooled to room temperature in flowing helium for a sufficient time to ensure the complete removal of the possible presence of surface adsorbed hydrogen species. Failure to perform this step in an efficient manner would have resulted in an over estimation of surface metal atoms. The potential such a problem was emphasized by Berndt and co-workers 21 from their studies of the desorption of H2 from a copper surface. The pulse reaction was carried out at 60 °C, 80 °C, 120 °C and 150 °C by injecting 0.5 mL of 4% N2O/He mixture into the reactor. The evolved gases were analyzed by the GC. In a typical experiment, the pulse of N2O into the reactor would be continued until the GC detected no evolved N2. The total amount of N2 produced was used to calculate the number of surface copper atoms and the metal dispersion, represented as D. Results (a) Temperature Programmed Reduction (TPR). When Cu(NO3)2 was used as the precursor salt, the GNF supported copper oxide particles formed during calcination were easily reduced to the metallic state in the presence of 10%H2/N2. The reduction profiles of all the GNF supported metal oxide particles were similar showing that the process commenced at 120 °C and was completed in a single step when the temperature was progressively raised to 220 °C. This behavior is to be contrasted with that found when copper oxide was dispersed on other types of carbonaceous solids where it was necessary to increase the temperature to 400 °C in order to achieve reduction to the metallic state and the process occurred in a stepwise fashion.25 The reduction profiles of samples prepared from CuAc2 showed a completely different pattern of behavior to that derived from the metal nitrate precursor salt. This comparison is shown for the two copper sources dispersed on the GNF-H material in Figure 2. Analogous sets of profiles were observed for CuO supported on the other three types of GNF structures. It is evident therefore that the reduction of the organic salt is a much more complex process. (b) Metal Particle Size Measurements. Transmission electron microscopy examination can provide not only direct information of particle size, but also the size distribution over a certain range. The average particle size can then be obtained by calculation from the distribution pattern. Because only twodimensional images can be obtained from TEM, specific morphological characteristics of some particles such as the “pillbox” structure can result in some distorted images depending upon their orientations. Thus, the measurement technique with nonuniform particle shapes can affect the final result. In the current study, the diameter, which is defined as any straight line crossing the center of mass of the particle and terminating at the particle boundary, was used when the particle adopted a uniform globular or hexagonal shape. Other particles with
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J. Phys. Chem. B, Vol. 105, No. 48, 2001 11997
Figure 2. Comparison of the reduction profiles of CuO/GNF-H using different metal precursor salts.
Figure 3. Definition of the particle size dimensions used in this study.
nonuniform shapes were measured according to Cadle’s definition,26 where equivalent-circle diameter, average distance from different directions, or the distance between opposite sides of a particle and on a line bisecting the projected area were employed to represent the particle size as shown schematically in Figure 3. It should be stressed that particles with sizes smaller than 2 nm were very difficult to detect due to the resolution limit and therefore were not accounted for in the distribution profiles. On GNF-H-type graphite nanofibers, a broad distribution pattern was evident following reduction of the sample at 200 °C, possibly due to the initial stage of rupture of the oxide film and subsequent nucleation of copper particles. After reduction at 300 °C, copper particles were found to exhibit good stability with quite a uniform size range. Very similar distribution patterns were observed from the samples reduced either at higher temperatures or for various periods of time at 300 °C, as shown in Figures 4 and 5, respectively. This same pattern of behavior was observed on the GNF-P and GNF-R types of graphite nanofibers indicating the excellent stability of supported copper particles under these conditions. In contrast, the GNF-Tsupported copper samples exhibited a much broader particle size distribution as shown in Figure 6, which was similar to that observed on a copper/graphite system.25 It was found that the use of different copper precursor salts had a significant impact on metal particle size distribution patterns due to different interactions existing between such salts and the graphite nanofiber surfaces during the impregnation and calcination steps. The edge regions of these support materials possess various oxygenated functionalities that exhibit strong interactions with polar groups. In this regard, when the covalent molecule, CuAc2, was used as the precursor salt, a broadening in the distribution patterns of copper particle sizes was observed on GNF-H, P, and R type nanofibers (Figure 7). On the other hand, no major changes in the distribution pattern could be detected on GNF-T, when compared with that obtained with the catalyst prepared from Cu(NO3)2.
Figure 4. Particle size distribution of a 5 wt % Cu/GNF-H catalyst reduced at various temperatures for 24 h.
Figure 5. Particle size distribution of a 5 wt % Cu/GNF-H catalyst reduced at 300 °C for various periods of time.
11998 J. Phys. Chem. B, Vol. 105, No. 48, 2001
Ma et al. TABLE 1: Metal Particle Sizes of 5 wt % Cu/GNF-H from XRD and TEM reduction temp (°C)
T (hours)
XRD (nm)
Σnd/Σn (nm)
TEM Σnd3/Σn2 (nm)
Σnd4/Σn3 (nm)
200 300 400 300 300a
24 24 24 72 24
22.0 22.9 23.3 24.5 26.8
21.7 12.2 13.4 12.2 22.8
35.8 25.9 23.5 26.2 31.1
41.0 34.1 34.4 32.3 35.4
a
Cu/GNF-HA.
TABLE 2: Metal Particle Sizes of 5 wt % Cu/GNF-P from XRD and TEM reduction temp (°C)
T (hours)
XRD (nm)
Σnd/Σn (nm)
TEM Σnd3/Σn2 (nm)
Σnd4/Σn3 (nm)
200 300 400 300 300a
24 24 24 72 24
23.3 24.9 23.8 21.1 33.2
15.8 15.6 14.2 15.0 30.9
23.7 26.8 23.2 24.6 39.0
28.4 34.6 29.4 29.8 42.4
a
Cu/GNF-PA.
TABLE 3: Metal Particle Sizes of 5 wt % Cu/GNF-R from XRD and TEM reduction temp (°C)
T (hours)
XRD (nm)
Σnd/Σn (nm)
TEM Σnd3/Σn2 (nm)
Σnd4/Σn3 (nm)
200 300 400 300 300a
24 24 24 72 24
13.6 21.2 27.8 28.9 38.3
14.2 15.7 18.6 20.1 22.7
22.2 29.1 28.8 31.9 33.0
29.5 35.2 34.3 37.3 38.7
Figure 6. Particle size distribution of a 5 wt % Cu/GNF-T catalyst reduced at various temperatures for 24 h. a
Cu/GNF-RA.
where dv is the mean of the distribution of the volume of the active fraction as a function of the size. Besides the volume weighted average, other average sizes, e.g., dn and ds, can be derived from different calculation methods
dnn )
dns )
Figure 7. Particle size distribution of a 5 wt % Cu/GNF-P catalyst prepared from different copper precursor salts (reduction conditions: 300 °C, 24 h).
X-ray line broadening is sensitive to the bulk properties of the particles, thus it must be compared to the volume-weighted average size obtained from TEM
dv )
Σnid4i Σnid3i
volume weighted
(4)
Σnid Σni Σnid3i Σnid2i
number weighted
(5)
surface-area weighted
(6)
The surface-area weighted size ds is proportional to the volumeto-surface ratio of the active phase, the reciprocal of which is the dispersion D. If a sample exhibits a mono-dispersion distribution model, the size calculated from all the different methods will be identical. Otherwise significant differences may appear depending upon the poly-dispersion characteristics of the sample. The particle sizes of the copper particles dispersed on the four types of GNF materials as calculated from XRD using the Scherrer equation and measured from TEM are listed in Tables 1 to 4, where k is assigned to be 0.9, and b1/2 was obtained by measuring the breadth of the diffraction peak at half of intensity. The diffraction peaks, d Cu(111) ) 2.088 Å and d Cu2O(111) ) 2.465 Å, were chosen for the measurement of the crystallite sizes of metallic and cuprous phases, respectively. Instrument line broadening was automatically corrected by Warren’s method incorporated with the “Jade” program. From the calculated results, the average particle sizes were found to be in reasonable agreement to the trend presented by
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J. Phys. Chem. B, Vol. 105, No. 48, 2001 11999
TABLE 4: Metal Particle Sizes of 5 wt % Cu/GNF-T from XRD and TEM reduction temp (°C)
T (hours)
XRD (nm)
Σnd/Σn (nm)
TEM Σnd3/Σn2 (nm)
Σnd4/Σn3 (nm)
200 300 400 300 300a
24 24 24 72 24
19.2 20.8 22.1 20.5 36.7
23.6 24.9 23.4 25.6 23.2
38.0 39.6 36.6 38.0 36.1
42.6 43.7 41.8 42.4 41.7
a
Cu/GNF-TA.
Figure 10. High-resolution TEM micrograph showing the appaerance of a copper crystallite located on the edge of a GNF-P support.
Figure 8. Temperature dependence of N2O adsorptive decomposition over Cu/GNF-H catalysts that were reduced at 200 °C (); at 300 °C (); and at 400 °C () for 24 h, respectively.
Figure 9. Comparison of metal dispersion values of Cu/GNF-H catalysts reduced at different temperatures from N2O adsorptive decomposition and TEM measurements.
the particle size distribution measurement. Nevertheless, some significant differences do exist between the XRD and TEM results in several systems. The results of Cu on GNF-H, GNFP, and GNF-R were quite close to the volume-area ds values. Obvious differences did appear between the sizes derived from different calculation methods due to the various distribution patterns. (c) Copper Dispersion from Adsorptive Decomposition of Nitrous Oxide. The temperature dependence of pulsed N2O adsorptive decomposition on GNF-H supported copper is shown in Figure 8. A very strong signal of liberated nitrogen was detected from the Cu/GNF-H samples, indicative of the existence of a relatively high metal dispersion. It can be seen that the amount of nitrogen released from the adsorptive decomposition of N2O increased almost linearly as the pulse temperature was raised from 55 °C to 150 °C. The calculated metal dispersion was found to be highest for the catalyst reduced at 300 °C as presented in Figure 9. No significant changes in the metal dispersion were observed between the catalyst reduced at 300 °C for 24 and 72 h, which is consistent with the particle
size distribution measurement. Dramatic differences were apparent, however, with the catalyst prepared from CuAc2, which produced a poor metal dispersion. Similar trends to that described above were found for copper dispersed on GNF-P and GNF-R type of materials, whereas the 5wt % Cu/GNF-T sample exhibited a much lower metal dispersion based on the N2O adsorptive decomposition measurements. (d) Morphological Characteristics of Supported Copper Particles. High-resolution TEM examinations of the GNF supported metal catalysts derived from a Cu(NO3)2 precursor salt showed that the copper particles acquired a very thin, flat and faceted morphology. These features can be seen from the electron micrograph of a Cu/GNF-P sample presented in Figure 10. Close inspection of the metal crystallites on the graphite nanofiber supports revealed the existence of some fascinating features regarding the structural details of the catalyst samples. It was possible to discern the appearance and direction of the lattice fringe images on both the metal particles and graphite nanofibers. In some cases, the fringes of the metal crystallite were aligned in the same direction as those of the nanofibers, whereas there was also examples where the two sets of fringes were at an angle of about 74° with respect to one another. Measurement of the lattice spacing of the metal crystallites gave a value of 0.54 nm, which could not be matched to index planes that are observable by X-ray diffraction. Clearly, the structural arrangement of the GNF is exerting a significant impact on the configuration of copper atoms comprising these crystallites. Furthermore, the fact that one can actually observe the presence of the lattice fringe images in these particles confirms the claim that they are very thin, relatively flat and highly crystalline in nature. Catalysts prepared from CuAc2 appeared to generate metal particles that exhibited significantly different morphological characteristics to those derived from Cu(NO3)2. Although the crystallites maintained a flat and rather thin morphology, they were in general, much larger and adopted a “willow leaf-like” outline when the former precursor salt was used as the copper source. These features can be seen in Figure 11, an electron micrograph of a sample produced from the dispersion of CuAc2 on the GNF-H type material. Discussion It is well-known that very small particles,
