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Comparative investigation of the morphology of nickel- and copper-graphite
Carbon Printed
Vol 29. No. 7, pp. 915-919. in Great Britain.
1991 Copyright
0
0008.6223191 $3.00+ .OO 1991 Pergamon Press plc
COMPARATIVE INVESTIGATION OF THE MORPHOLOGY OF NICKEL- AND COPPER-GRAPHITE ALOIS FCJRSTNER,* FERDINAND HOFER,~ and HANS WEIDMANN* *Institute of Organic Chemistry and fResearch Institute of Electron Microscopy and Fine Structure Research, Technical University, A-8010 Graz, Austria (Received
15 June 1990; accepted in revised form 26 November 1990)
Abstract-In
view of the considerable activation effects observed with various metals and metal combinations on graphite, recent applications in interesting chemical transformations called for an investigation of the kind of distribution and the degree of dispersion of both nickel and copper. As a result, in both cases evenly distributed finely dispersed layers of these metals were found which, in contrast to the general assumption, showed no sign of intercalation between the graphite layers. Various samples obtained by different procedures, with or without consecutive treatment, were studied. Electron microscopic methods in combination with microanalytical means, such as EDX- and EELS-spectroscopy, revealed intimate morphology and structure details. Key
Words-Nickel-graphite,
copper-graphite,
electron microscopy.
1. INTRODUCTION
Ni(O)-complexes, particularly Ni(CO), (for review, see [15,28]. 3. Doubts in the laminar structure of the commercially available Ni-Graphimet[l6-18,7] since the corresponding Pt-Graphimet was found to consist of graphite-supported platinum[5,19]. 4. The Ni-Graphimet[7,17,18] and the Ni-graphite[l9] obtained by reduction of nickel salts by C,K may be seen to be morphologically identical. 5. With the exception of zinc/silver-graphite[8], the reported activation of nickel-graphite by copper[9] seems to be the only example of enhanced activity by metal dotation.
Three
closely related procedures are used for the reduction of metal salts with formation of highly reactive metals comprising molten potassium (for review, see[l]), lithium naphthalenide[Z], or potassium-graphite laminate (C,K) (for review, see[3]). While all of these reducing agents form metals with high degrees of reactivity and are most useful in various kinds of metal-induced reactions, the graphite-supported metals most favourably combine high efficiency and ready as well as general applicability[3]. In view of these interesting properties, detailed studies of the morphology of metal-graphite combinations were called for; the first results of which invariably showed finely dispersed graphite-supported[4,5] instead of the previously assumed intercalated metals[6,7]. In addition, the long-known activation effect by metal/metal coupling was found to be distinctly enhanced by employing graphite as the most favourable support (for ZnIAg-Graphite, see [8]; for NiiCu-Graphite, see [9]). Considering the interesting results with highly reactive zinc[3], zinc/silver[8] (for recent advances see [10,29]), magnesium[ll], and titanium[ll], each supported by graphite, various applications of active nickel and copper reagents favoured a study of these metals in combination with graphite for the following reasons:
As in previous investigations[4,5], a combination of X-ray diffraction, analytical electron microscopy, and microanalytical methods such as EDX- and EELSspectroscopy were used in this study and the results follow.
2. EXPERIMENTAL
2.1 General Diglyme was distilled over LiAlH, under reduced pressure prior to use; potassium, NiBr, * HzO, CuCI,, and CuBr were purchased from Fluka AG, Switzerland, Ni-Graphimet (nickel, 1% “in graphite”) from Ventron-Alfa Products, FRG. The commercially available NiBr, . H,O was dried by heating at 160°C under reduced pressure (18 Torr) for three hours. Graphite (KS44, Lonza AG, Switzerland) was used in all preparations.
1. The applicability of activated nickel as a selective hydrogenation catalyst[l2] as well as its use in organo-metallic reactions[9] (for synthetic applications of Riecke-Ni see [13,25-271). 2. The direct formation of rr-allylnickel complexes [13,14, 25-271 avoiding the generally hazardous
2.2 Metal-graphite preparations All samples were obtained by introducing either NiBr, (1.68 g, 7.68 mmol) or CuCl, (1.03 g, 7.65 915
A. F~JRSTNER et al.
916
mmol) into freshly prepared suspensions of C,K (15.34 mmol) in diglyme (25 ml) under argon at room temperature, followed by keeping the reaction mixtures at lOO-110°C for 17 hours. For the preparation of nickel/copper-graphite, a mixture of NiBr, (1.50 g, 6.86 mmol) and CuCl, (0.11 g, 0.82 mmol) was used instead. Oxidized nickel-graphite was obtained by filtering Ni-graphite obtained as described above. After washing it with water followed by THF, the sample was dried in vacua. 2.3 Electron microscopy Depending on the electron microscopic technique employed in each case, part of the suspension was quickly dispersed on either (SiO), or amorphous carbon thin films, each mounted on aluminium grids. The investigations were performed by means of a Philips EM 420 transmission electron microscope (TEM), operated at 120kV. This microscope was equipped with an energy-dispersive X-ray detecting system (EDX) as well as an electron energy-loss spectrometer (EELS). EELS analyses were performed in the TEM-mode with diffraction-coupling. In order to obtain representative information in the TEM investigations, 20 different areas were analyzed in each sample. X-ray diffraction patterns of the powder samples employing Cu-K radiation were recorded with a Siemens one-circle diffractometer.
3. RESULTS AND DISCUSSION
There are essentially two major problems with the results of the morphological investigations of nickeland copper-graphite preparations. One concerns the yet undecided situation of graphite-supported or intercalated metal, the other has to do with the method used for the formation and preparation of the samples for analyses. In this respect, the solubility of the metal salts that influences their rates of reduction, as well as the sample deterioration caused by insufficient precautions and/or inappropriate work-up conditions, received only little attention[6,7]. This situation demanded reinvestigation. As a first result, nickel as well as copper salts, because of their comparatively low solubilities in Lewis-base solvents, showed unexpectedly low rates of reductions. This is particularly noteworthy in view of the high activity of C,K as a reducing agent, found to be generally superior to other methods for metalsalt reductions[3]. Thus, in order to ensure complete reduction, a necessary prerequisite for unambigous discrimination between intercalated or graphite-supported metals, both nickel and copper salts needed to be treated with C,K for up to 17 hours in diglyme, as the preferred solvent allowing higher temperatures. The samples obtained by this comparatively forceful procedure showed fine dispersions of metallic partitles on graphite[20].
However, since X-ray diffraction lines (XRD) are broadened in case of particle sizes below about 0.1 pm, these particles are difficult to detect. With the exception of Cu-graphite, all investigated samples showed only very weak and broad lines for the metallic species in the XRD-pattern. Combined use of TEM and electron diffraction was necessary for a complete characterization of these samples. 3.1 Ni-graphite The TEM bright field image of Ni-graphite (Fig. 1 a) shows uniformly and finely distributed nickel on graphite, the average particle size of which was 5 nm (the actual size varied in the range 2-7 nm). In the EELS-spectra (Fig. 1 c) taken from regions of uncovered and covered graphite, nickel is only observed in the latter, a first indication against intercalation. This result is supported by electron diffractions of various graphite flakes exhibiting no dectectable alterations of the graphite lattice. Figure 1 b shows a typical electron-diffraction pattern of nickel particles on a single graphite flake. Besides the (hkO)-reflections of the graphite crystal lying on the basal plane, diffuse rings caused by KBr and nickel can be observed. Following from the results of Evans and Thomas[21], in case of intercalation additional reflections should be observed in an (hkO)-orientation. However, these were absent in the nickel-graphite samples investigated, and X-ray diffraction showing only reflections of graphite and KBr also made intercalation unlikely. 3.2 Cu-graphite In case of Cu-graphite, distinctly larger copper particles (5-80 nm) were found on the graphite flakes (Fig. 2) less evenly distributed than nickel on the graphite surface. Electron and X-ray diffractions showed no indication for intercalation of copper in graphite. Copper lines are clearly detectable by XRD due to the larger particle size. 3.3 Ni-graphite (Hz0 treated) and Ni-Graphimet Contrary to the results invariably obtained with samples protected from even traces of moisture and oxygen, water-treated nickel-graphite samples[l2] exhibited the significant structural changes described below. Their TEM-image (Fig. 3 a) shows two kinds of particles that could be identified by EELS-analyses (Fig. 3 b). The very fine particles consisted exclusively of nickel-oxide, which was identified by the determination of the ratio of nickel and oxygen from the EELS-spectrum according to the analytical method described in [22]. While detailed surface investigations by photoelectron spectroscopy (XPS) showed the presence of Ni(OH), as previously assumed[20], under the conditions of electron microscopy this compound suffers dehvdration with formation of NiO. Therefore, only J
917
The morphology of nickel- and copper-graphite
(4
(b)
I
500
I
600
I
I
700
800 -
I
900 nE LeVI
(cl Fig. 1. Ni-graphite: (a) TEM bright field image of Ni-graphite; (b) Electron diffraction pattern of Nigraphite; bright reflections are due to graphite; (c) EELS-spectra of specimen areas marked in Fig. 1 (a) (1) Ni-particles covering the graphite support (2) graphite support.
NiO is found by electron microscopic techniques. According to this, the coating of the larger nickel particles (Fig. 3 a) amounts to approximately 2-3 nm in diameter and most likely consists of Ni(OH):, which explains the reduced catalytic activity in hydrogenation reactions and the increased stability in comparison to the nickel-graphite samples obtained under anhydrous conditions[l2]. The electron microscopic investigation of Ni-Graphimet (Fig. 4) showed the presence of nickel, nickel oxide, and nickel chloride on graphite. As with watertreated Ni-graphite the Ni-particles were found to be covered by thin layers of nickel oxide. Both X-ray and electron diffraction revealed no sign of intercalated nickel, although there are indications of
very low (- 0.1 at %) amounts of nickel in the graphite detectable by EDX-microanalyses of uncovered graphite areas. The water-treated nickel-graphite was found to be essentially identical to the commercially available Ni_Graphimet[l7,18]. This proves that, independent of the mode of preparation, all samples consist of nickel supported by graphite. In view of this result the C,K-based procedure is superior. 3.4 Nil&-gruphite In addition to the pronounced activation of zinc by the ZniAg-graphite combination[8], the NiiCugraphite reagent recently described[9] was of interest in connection with the present study.
A.
918
et al.
FihSTNER
salts. In view of the preceding results, the reduction of mixtures of nickel and copper salts, unlike that of salts of zinc and silver[23], does not lead to a nickel-copper couple. The only plausible explanation to be offered for the coupling reaction described[9] consists in a Cu(I)-salt-assisted, nickelinduced Wurtz-type reaction (for Cu(1) assisted reactions of halocompounds see [24]).
4. CONCLUSIONS
In summary, various samples, such as Ni-, Cu-, Ni/Cu-graphite, and Ni-graphite washed with water, were analyzed by X-ray diffraction and electron microscopy with the following results: 1. All samples consisted of dispersed metal or metal oxide supported on graphite, with no sign of intercalation. 2. While Ni was found to be finely and evenly distributed, the Cu on graphite showed a less even distribution with some areas of fine and others of rather coarse particles. 3. Washing of Ni-graphite resulted in the formation of nickel oxide and nickel on graphite found to be essentially identical with the commercial Ni-Graphimet. 4. In Ni/Cu-graphite the metals are separately deposited and no alloy formation was detectable.
Fig. 2. TEM bright field image of t&graphite.
In a reinvestigation, a mixture of NiBr, and CuCl, (molar ratio 8: 1) was reduced forming a complex system which essentially consisted of metallic nickel, a-Cu(I)Br, potassium halide, and unreacted C,K. While the TEM-image (Fig. 5) shows nickel particles distributed on graphite, only a detailed EDXspectroscopic investigation of a variety of such particles allowed the identification of small amounts of discrete Cu particles. Intermetallic Cu-Ni alloys could not be detected. The formation of the cuprous salt can only be explained by halide interchange prior to the reduction, causing a severe change of solubilities that, in turn, influenced the result of the reduction of these
Acknowledgements-We
gratefully acknowledge financial support by the Fonds zur Forderung der Wissenschaftlichen Forschung, Vienna (grant 6652CHE). We also thank Dr. M. Ebel (Technical University of Vienna) for the XPS measurements.
1
Ni L23 kf---
I
500
1
600
1
I
700
800 -
(4
I
900 AE LeVl
(4
Fig. 3. Ni-graphite (water-treated) (a) TEM bright field image of water-treated Ni-graphite showing Ni and NiO-particles (b) EELS-spectra of specimen areas marked in Fig. 2 (a) (1) Ni-particle on the graphite support (2) NiO-particles on the graphite support.
YlY
The morphology of nickel- and copper-graphite REFERENCES
R. D. Rieke, Top. Cur. Chem. 59, 1 (1975). 2. R. D. Rieke, P. T. J. Li, T. P. Burns, and S. T. Uhm, J. Org. Chem. 46, 4324 (1981). 3. R. Csuk, B. I. Glanzer, and A. Fiirstner, Adv. Organomet. Chem. 28, 85 (1988). 4. A. Fiirstner, H. Weidmann, and F. Hofer. /. Chem. Sot., Dalton Trans. 2023 (1988). 5. A Fiirstner, F. Hofer, and H. Weidmann. J. Catal. 118, 502 (1989). 6. D. Braga, A. Ripamonti, D. Savoia, C. Trombini, and A. Umani-Ronchi. J. Chem. Sot.. Dalton Trans. 2026 (1979). 7. M. E. Volpin, Y. N. Novikov, N. D. Lapkina, V. I. Kasatochkin, Y. T. Struchkov, M. E. Kazakov, R. A. Stukan, V. A. Povitskij, Y. S. Karimov, and A. V. Zvarikina, J. Am. Chem. Sot. 97, 3366 (1975). 8. R. Csuk, A. Fiirstner, and H. Weidmann. J. Chem. Sot., Chem. Commun. 775 (1986). 9. P. Marceau. F. Beauin. and G. Guillaumet. J. OreanII
omet. Chem. 342,137
(1988).
10. A. Fiirstner and H. Weidmann, J. Org. Chem. 54,2037 Fig. 4. TEM bright fteld image ot Ni-Graphimet, arrow marks a Ni-particle covered by a thin oxide layer.
(1989).
11. A. Fiirstner, R. Csuk, C. Rohrer, and H. Weidmann, J. Chem. Sot., Perkin Trans. I 1729 (1988) and lit. cit. 12. D. Savoia, E. Tagliavini, C. Trombini, and A. UmaniRonchi. J. Org. Chem., 46, 5340. 5344 (1981). 13. H. Matsumoto. S. I. Inaba. and R. D. Rieke. J. Org. Y Chem. 48, 840 (1983).
14. R. D. Rieke, A. V. Kavaliunas, L. D. Rhyne, and D. J. J. Fraser, J. Am. Chem. Sot. 101, 246 (1979). 15. M. F. Semmelhack, Org. React. 19, 115 (1972). 16. Alfa Division, Ventron Corp., Sales Catalogue p. 208 (1986). 17. D. J. Smith, R. M. Fischer, and L. A. Freeman, J. Caral. 72, 51 (1981). 18. G. Sirokman, A. Mastalir, A. Molnar, M. Bartok, 2. Sachay, and L. Guczi, J. Catal. 117, 558 (1989). 19. E Notheisz, A. Mastalir, and M. Bartok. in preparation. 20. R. Erre, A. Messaoudi, and F. Beguin, Synch. Met. 23. 493 (198X). 21. E. L. Evans and J. M. Thomas, J. Solid State Chem. 14, YY (1975). 22. F. Hofer and P. Golob, Micron and Microscopica Acta 19, 73 (1988). 23. A. Fiirstner, E Hofer, A. Zechmann, and H. Weidmann, in preparation. 24. G. H. Posner, Org. React. 19, 1 (1972). 25. S. I. Inaba and R. D. Rieke, Synthesis 842 (1984). 26. S. I. Inaba, H. Matsumoto, and R. D. Rieke, J. Org. Chem. 49, 2093 (1984).
Fig. 5. TEM bright field image of NiiCu-graphite
27. S. I. Inaba, and R. D. Rieke, J. Org. Chem. 50, 1373 (1985). 28. D. C. Billington, Chem Sot. Rev. 93 (1985). 29. A. Ftirstner and H. Weidmann, J. Org. Chem. 55, 1363 (1990).
Vol 29. No. 7, pp. 915-919. in Great Britain.
1991 Copyright
0
0008.6223191 $3.00+ .OO 1991 Pergamon Press plc
COMPARATIVE INVESTIGATION OF THE MORPHOLOGY OF NICKEL- AND COPPER-GRAPHITE ALOIS FCJRSTNER,* FERDINAND HOFER,~ and HANS WEIDMANN* *Institute of Organic Chemistry and fResearch Institute of Electron Microscopy and Fine Structure Research, Technical University, A-8010 Graz, Austria (Received
15 June 1990; accepted in revised form 26 November 1990)
Abstract-In
view of the considerable activation effects observed with various metals and metal combinations on graphite, recent applications in interesting chemical transformations called for an investigation of the kind of distribution and the degree of dispersion of both nickel and copper. As a result, in both cases evenly distributed finely dispersed layers of these metals were found which, in contrast to the general assumption, showed no sign of intercalation between the graphite layers. Various samples obtained by different procedures, with or without consecutive treatment, were studied. Electron microscopic methods in combination with microanalytical means, such as EDX- and EELS-spectroscopy, revealed intimate morphology and structure details. Key
Words-Nickel-graphite,
copper-graphite,
electron microscopy.
1. INTRODUCTION
Ni(O)-complexes, particularly Ni(CO), (for review, see [15,28]. 3. Doubts in the laminar structure of the commercially available Ni-Graphimet[l6-18,7] since the corresponding Pt-Graphimet was found to consist of graphite-supported platinum[5,19]. 4. The Ni-Graphimet[7,17,18] and the Ni-graphite[l9] obtained by reduction of nickel salts by C,K may be seen to be morphologically identical. 5. With the exception of zinc/silver-graphite[8], the reported activation of nickel-graphite by copper[9] seems to be the only example of enhanced activity by metal dotation.
Three
closely related procedures are used for the reduction of metal salts with formation of highly reactive metals comprising molten potassium (for review, see[l]), lithium naphthalenide[Z], or potassium-graphite laminate (C,K) (for review, see[3]). While all of these reducing agents form metals with high degrees of reactivity and are most useful in various kinds of metal-induced reactions, the graphite-supported metals most favourably combine high efficiency and ready as well as general applicability[3]. In view of these interesting properties, detailed studies of the morphology of metal-graphite combinations were called for; the first results of which invariably showed finely dispersed graphite-supported[4,5] instead of the previously assumed intercalated metals[6,7]. In addition, the long-known activation effect by metal/metal coupling was found to be distinctly enhanced by employing graphite as the most favourable support (for ZnIAg-Graphite, see [8]; for NiiCu-Graphite, see [9]). Considering the interesting results with highly reactive zinc[3], zinc/silver[8] (for recent advances see [10,29]), magnesium[ll], and titanium[ll], each supported by graphite, various applications of active nickel and copper reagents favoured a study of these metals in combination with graphite for the following reasons:
As in previous investigations[4,5], a combination of X-ray diffraction, analytical electron microscopy, and microanalytical methods such as EDX- and EELSspectroscopy were used in this study and the results follow.
2. EXPERIMENTAL
2.1 General Diglyme was distilled over LiAlH, under reduced pressure prior to use; potassium, NiBr, * HzO, CuCI,, and CuBr were purchased from Fluka AG, Switzerland, Ni-Graphimet (nickel, 1% “in graphite”) from Ventron-Alfa Products, FRG. The commercially available NiBr, . H,O was dried by heating at 160°C under reduced pressure (18 Torr) for three hours. Graphite (KS44, Lonza AG, Switzerland) was used in all preparations.
1. The applicability of activated nickel as a selective hydrogenation catalyst[l2] as well as its use in organo-metallic reactions[9] (for synthetic applications of Riecke-Ni see [13,25-271). 2. The direct formation of rr-allylnickel complexes [13,14, 25-271 avoiding the generally hazardous
2.2 Metal-graphite preparations All samples were obtained by introducing either NiBr, (1.68 g, 7.68 mmol) or CuCl, (1.03 g, 7.65 915
A. F~JRSTNER et al.
916
mmol) into freshly prepared suspensions of C,K (15.34 mmol) in diglyme (25 ml) under argon at room temperature, followed by keeping the reaction mixtures at lOO-110°C for 17 hours. For the preparation of nickel/copper-graphite, a mixture of NiBr, (1.50 g, 6.86 mmol) and CuCl, (0.11 g, 0.82 mmol) was used instead. Oxidized nickel-graphite was obtained by filtering Ni-graphite obtained as described above. After washing it with water followed by THF, the sample was dried in vacua. 2.3 Electron microscopy Depending on the electron microscopic technique employed in each case, part of the suspension was quickly dispersed on either (SiO), or amorphous carbon thin films, each mounted on aluminium grids. The investigations were performed by means of a Philips EM 420 transmission electron microscope (TEM), operated at 120kV. This microscope was equipped with an energy-dispersive X-ray detecting system (EDX) as well as an electron energy-loss spectrometer (EELS). EELS analyses were performed in the TEM-mode with diffraction-coupling. In order to obtain representative information in the TEM investigations, 20 different areas were analyzed in each sample. X-ray diffraction patterns of the powder samples employing Cu-K radiation were recorded with a Siemens one-circle diffractometer.
3. RESULTS AND DISCUSSION
There are essentially two major problems with the results of the morphological investigations of nickeland copper-graphite preparations. One concerns the yet undecided situation of graphite-supported or intercalated metal, the other has to do with the method used for the formation and preparation of the samples for analyses. In this respect, the solubility of the metal salts that influences their rates of reduction, as well as the sample deterioration caused by insufficient precautions and/or inappropriate work-up conditions, received only little attention[6,7]. This situation demanded reinvestigation. As a first result, nickel as well as copper salts, because of their comparatively low solubilities in Lewis-base solvents, showed unexpectedly low rates of reductions. This is particularly noteworthy in view of the high activity of C,K as a reducing agent, found to be generally superior to other methods for metalsalt reductions[3]. Thus, in order to ensure complete reduction, a necessary prerequisite for unambigous discrimination between intercalated or graphite-supported metals, both nickel and copper salts needed to be treated with C,K for up to 17 hours in diglyme, as the preferred solvent allowing higher temperatures. The samples obtained by this comparatively forceful procedure showed fine dispersions of metallic partitles on graphite[20].
However, since X-ray diffraction lines (XRD) are broadened in case of particle sizes below about 0.1 pm, these particles are difficult to detect. With the exception of Cu-graphite, all investigated samples showed only very weak and broad lines for the metallic species in the XRD-pattern. Combined use of TEM and electron diffraction was necessary for a complete characterization of these samples. 3.1 Ni-graphite The TEM bright field image of Ni-graphite (Fig. 1 a) shows uniformly and finely distributed nickel on graphite, the average particle size of which was 5 nm (the actual size varied in the range 2-7 nm). In the EELS-spectra (Fig. 1 c) taken from regions of uncovered and covered graphite, nickel is only observed in the latter, a first indication against intercalation. This result is supported by electron diffractions of various graphite flakes exhibiting no dectectable alterations of the graphite lattice. Figure 1 b shows a typical electron-diffraction pattern of nickel particles on a single graphite flake. Besides the (hkO)-reflections of the graphite crystal lying on the basal plane, diffuse rings caused by KBr and nickel can be observed. Following from the results of Evans and Thomas[21], in case of intercalation additional reflections should be observed in an (hkO)-orientation. However, these were absent in the nickel-graphite samples investigated, and X-ray diffraction showing only reflections of graphite and KBr also made intercalation unlikely. 3.2 Cu-graphite In case of Cu-graphite, distinctly larger copper particles (5-80 nm) were found on the graphite flakes (Fig. 2) less evenly distributed than nickel on the graphite surface. Electron and X-ray diffractions showed no indication for intercalation of copper in graphite. Copper lines are clearly detectable by XRD due to the larger particle size. 3.3 Ni-graphite (Hz0 treated) and Ni-Graphimet Contrary to the results invariably obtained with samples protected from even traces of moisture and oxygen, water-treated nickel-graphite samples[l2] exhibited the significant structural changes described below. Their TEM-image (Fig. 3 a) shows two kinds of particles that could be identified by EELS-analyses (Fig. 3 b). The very fine particles consisted exclusively of nickel-oxide, which was identified by the determination of the ratio of nickel and oxygen from the EELS-spectrum according to the analytical method described in [22]. While detailed surface investigations by photoelectron spectroscopy (XPS) showed the presence of Ni(OH), as previously assumed[20], under the conditions of electron microscopy this compound suffers dehvdration with formation of NiO. Therefore, only J
917
The morphology of nickel- and copper-graphite
(4
(b)
I
500
I
600
I
I
700
800 -
I
900 nE LeVI
(cl Fig. 1. Ni-graphite: (a) TEM bright field image of Ni-graphite; (b) Electron diffraction pattern of Nigraphite; bright reflections are due to graphite; (c) EELS-spectra of specimen areas marked in Fig. 1 (a) (1) Ni-particles covering the graphite support (2) graphite support.
NiO is found by electron microscopic techniques. According to this, the coating of the larger nickel particles (Fig. 3 a) amounts to approximately 2-3 nm in diameter and most likely consists of Ni(OH):, which explains the reduced catalytic activity in hydrogenation reactions and the increased stability in comparison to the nickel-graphite samples obtained under anhydrous conditions[l2]. The electron microscopic investigation of Ni-Graphimet (Fig. 4) showed the presence of nickel, nickel oxide, and nickel chloride on graphite. As with watertreated Ni-graphite the Ni-particles were found to be covered by thin layers of nickel oxide. Both X-ray and electron diffraction revealed no sign of intercalated nickel, although there are indications of
very low (- 0.1 at %) amounts of nickel in the graphite detectable by EDX-microanalyses of uncovered graphite areas. The water-treated nickel-graphite was found to be essentially identical to the commercially available Ni_Graphimet[l7,18]. This proves that, independent of the mode of preparation, all samples consist of nickel supported by graphite. In view of this result the C,K-based procedure is superior. 3.4 Nil&-gruphite In addition to the pronounced activation of zinc by the ZniAg-graphite combination[8], the NiiCugraphite reagent recently described[9] was of interest in connection with the present study.
A.
918
et al.
FihSTNER
salts. In view of the preceding results, the reduction of mixtures of nickel and copper salts, unlike that of salts of zinc and silver[23], does not lead to a nickel-copper couple. The only plausible explanation to be offered for the coupling reaction described[9] consists in a Cu(I)-salt-assisted, nickelinduced Wurtz-type reaction (for Cu(1) assisted reactions of halocompounds see [24]).
4. CONCLUSIONS
In summary, various samples, such as Ni-, Cu-, Ni/Cu-graphite, and Ni-graphite washed with water, were analyzed by X-ray diffraction and electron microscopy with the following results: 1. All samples consisted of dispersed metal or metal oxide supported on graphite, with no sign of intercalation. 2. While Ni was found to be finely and evenly distributed, the Cu on graphite showed a less even distribution with some areas of fine and others of rather coarse particles. 3. Washing of Ni-graphite resulted in the formation of nickel oxide and nickel on graphite found to be essentially identical with the commercial Ni-Graphimet. 4. In Ni/Cu-graphite the metals are separately deposited and no alloy formation was detectable.
Fig. 2. TEM bright field image of t&graphite.
In a reinvestigation, a mixture of NiBr, and CuCl, (molar ratio 8: 1) was reduced forming a complex system which essentially consisted of metallic nickel, a-Cu(I)Br, potassium halide, and unreacted C,K. While the TEM-image (Fig. 5) shows nickel particles distributed on graphite, only a detailed EDXspectroscopic investigation of a variety of such particles allowed the identification of small amounts of discrete Cu particles. Intermetallic Cu-Ni alloys could not be detected. The formation of the cuprous salt can only be explained by halide interchange prior to the reduction, causing a severe change of solubilities that, in turn, influenced the result of the reduction of these
Acknowledgements-We
gratefully acknowledge financial support by the Fonds zur Forderung der Wissenschaftlichen Forschung, Vienna (grant 6652CHE). We also thank Dr. M. Ebel (Technical University of Vienna) for the XPS measurements.
1
Ni L23 kf---
I
500
1
600
1
I
700
800 -
(4
I
900 AE LeVl
(4
Fig. 3. Ni-graphite (water-treated) (a) TEM bright field image of water-treated Ni-graphite showing Ni and NiO-particles (b) EELS-spectra of specimen areas marked in Fig. 2 (a) (1) Ni-particle on the graphite support (2) NiO-particles on the graphite support.
YlY
The morphology of nickel- and copper-graphite REFERENCES
R. D. Rieke, Top. Cur. Chem. 59, 1 (1975). 2. R. D. Rieke, P. T. J. Li, T. P. Burns, and S. T. Uhm, J. Org. Chem. 46, 4324 (1981). 3. R. Csuk, B. I. Glanzer, and A. Fiirstner, Adv. Organomet. Chem. 28, 85 (1988). 4. A. Fiirstner, H. Weidmann, and F. Hofer. /. Chem. Sot., Dalton Trans. 2023 (1988). 5. A Fiirstner, F. Hofer, and H. Weidmann. J. Catal. 118, 502 (1989). 6. D. Braga, A. Ripamonti, D. Savoia, C. Trombini, and A. Umani-Ronchi. J. Chem. Sot.. Dalton Trans. 2026 (1979). 7. M. E. Volpin, Y. N. Novikov, N. D. Lapkina, V. I. Kasatochkin, Y. T. Struchkov, M. E. Kazakov, R. A. Stukan, V. A. Povitskij, Y. S. Karimov, and A. V. Zvarikina, J. Am. Chem. Sot. 97, 3366 (1975). 8. R. Csuk, A. Fiirstner, and H. Weidmann. J. Chem. Sot., Chem. Commun. 775 (1986). 9. P. Marceau. F. Beauin. and G. Guillaumet. J. OreanII
omet. Chem. 342,137
(1988).
10. A. Fiirstner and H. Weidmann, J. Org. Chem. 54,2037 Fig. 4. TEM bright fteld image ot Ni-Graphimet, arrow marks a Ni-particle covered by a thin oxide layer.
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