- Email: [email protected]
Electron microscopy study of the interaction of Ni, Pd and Pt with carbon
Surface Science 197 (1985) 402-414 North-Holland, Amsterdam
Ryszard LAMBER Institute for Low Temperature
end P-0. Box 937, SO-950 WrocCaw, Poland
Structure
Research,
Nils JAEGER and Giinter SCHULZ-EKLOFF Institutftir ~ngewandte und Physikalisci., Chemie, Universitiir
Polish
Academy
of
Sciences,
Bremen, Postjach 33n 440,
D-2800 Bremen 33, Fed. Rep. oj Germany Received
13 July 1987; accepted
for publication
20 November
1987
The aggregation of nickel supported on amorphous carbon during heating in vacuum and at .-. : . . IUW hydr~ogeu p~rssuics kit Put temperaturesrange ate 1000 5x W’~Yr~esi~galru ny I~C~~E!J VL &a:~~~ -l..*;,e It was established that iL>V .I... . ,.. electron microsco?~, electron diffraction and microdiffraction. the interaction or i+!y dispersed nickel with amorphous carbon substrates is strong enough to break away carbon atoms from the bulk at relatively low temperatures ( - 700 K) Cxbnn atoms dissolved in mckel precipitate as gra$titc. .2,= ., _ res~!t the aggregation of nickel is accompanied by the conversion of the amorphous carbon substrate to graphitic carbon.
Supported metal catalysis consist of highly dispersed metal phases on suitable carriers like ceramic oxides, zeolites or a orphous carbon. The activity of the metal in catalytic reactions depends on the preparation conditions among which te perature is one of the decisive parameters. The activity catalysts tends decrease with - e due to a num f chemical and al phenomena ical interaction lvith the support, ~;~nte~~g or even encapsulation of the metal particles Or?: uring the pas\ years consi eta1 support iiatcracaion on t he properties can rag between t * Work carried out at the Universitlt
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. I
403
strong metal-support interaction (SMSi) was introduced by Tauster et ai. [1,2] following their discovery of the suppression of hydrogen and CO chemisorption on titania supported platinum group metals af~:er the catalysts had been treated in hydrogen at temperatures above 700 K. Although it was claimed that only transition metal oxides such as TiO 2 give rise to strong interactions with metals [2] there is now increasing evidence for similar phenomena involving non-transition metal oxides such as the widely used silica [3-9], alumina [10-15] and magnesia [16]. Strong chemical interaction between the metal and the substrate should also be considered in the case of Group VIII 3 metals supported on carbon. It is known that Ni can form a metastable Ni3C compound with carbon [17]. There is evidence for strong interaction between Pd and carbon [18]. However, the formation of a palladium carbide has not yet been established. Recently it has been shown that the formation of a metastable platinum carbide (PtEC) is possible in the Pt-C system [19]. It is known that the conversion of disordered carbons to graphite can be accelerated by metals [20,21]. Extended experimental studies on the sintering mechanism of various metals supported on carbon substrates have already been reported [22-30]. The present investigation was carried out in order to recognize in more detail the metal support interaction in Group VIII 3 metal/carbon systems and to claruy tn~ . . . . . ,)f t;,,~ metal on structure changes L,, ,.,~,,,,.,.. In ,h.. present study the method of transmission electron microscopy, electron diffraction and microdiffraction have been used. The utility of these methods in the study of supported metal catalysts is well documented [31,32]. Evidence for the nickel catalyzed conversion of amorphous carbon to graphite at low temperatures will be presented. Results concerning the P d - C and Pt-C systems will be reported in a subsequent publication [33]. ltlllltUK;i]t~,l~,
"
- ..t
-
~
,
2. Experimental Thin film carbon substrates were prepared by vacuum deposition from a carbon arc onto a fresh air-cleaved face of a NaCl single crystal. After deposition the NaCI substrate was immersed into a large amount of distilled water and the floating carbon films were picked up onto gold electron microscope grids. Prior to the deposition of r,~ckel the EM grids were placed in a resistanceh o t r o d ~ a n t a l n m ¢~rnn|P h n | d e . r n n d h e a t t r e a t e d at the upper temperature limit of the experiments (875 K) for 1 h in vacuum (10 -5 Pa). The temperature was measured using a P t - ( P t - R h ) thermocouple spot-welded directly to the sample holder. Electron diffraction analysis of the C substrates after this thermal treatmc~tt showed only the presence of two diffuse halos indicating that the substrates were amorphous, in accordance with observations of Mc Lintock and Orr [34].
404
R. Lamber et ai. / The interaction of Ni, P d and Pt with carbon• 1
:v~.~=~ ¸lr~
~
~,~~ ~ : ~ ~ , ~
~ i ~!~'~:~ ii~i~i~,~:% ~, .
.... ~
~ ~ ~.~
•
Fig. 1 Electron micrographs of Ni on carbon substrates (a) following the evaporation of nickel (specimen 1); and subsequent treatment (b) at 670 K, 60 h, vacuum (specimen 2); (c) at 730 K, 80 h, vacuum (specimen 3); (d) at 820 K, 10 h, H 2 (specimen 4); (e) at 850 K, 80 h, H 2 (specimen 5); ~f) at 870 K, 60 b, vacuum (specimen 6): fg) additional treatment of specimen 3 at ]0~0 K, 24 h, vacuum. Note different magnifications.
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. 1
405
Table 1 Thermal treatment conditions Specimen
Atmosphere
1 2 3
Vacuum Vacuum Vacuum
4 5 6
H2 H2 Vacuum
Temperature
Time
(K)
(h)
293 670 730 1000 820 850 870
60 80 24 10 80 60
Fig. la lb, 2a lc, 2b lg, 2d ld le, 2c lf, 3a, 3b, 3c, 3d
Discontinuous films of Ni were deposited by vacuum evaporation from a resistance heated tungsten boat (fig. la). By controlling the amount of the evaporated metal an average film thickness of 0.4 nm could be reproducibly established. During evaporation the pressure was kept below 2 x 10 -4 Pa. After the Ni layer had been evaporated, a thermal treatment of the N i - C system in a vacuum (10 -5 Pa) or hydrogen atmosphere (6 × 10 -z Pa) was carried out. The details of this thermal treatment are shown in table I for six representative samples, which were chosen from a larger number of systematic experiments. All species were examined in a Phillips EM 420 transmission electron microscope operated at 120 kV and equipped with an energy dispersive X-ray analyzer (EDX). With EDX no contaminants were detectable in the investigated specimens.
3. Results A TEM picture of the N i - C specimen 1 as obtained after the metal evaporation is shown in fig. la. Nickel is present as small particles of sizes in the range 1.5-3 nm. The thermal treatment in vacuum at 670 K (60 h) (specimen 2) resulted in some sintering of the metal, i.e. larger liquid-like particles of nickel are formed (fig. lb). Only the nickel diffraction rings can be observed in the electron diffraction pattern (fig. 2a). Following the deposition ,~f ~i ~n~,-im~n ~ , ~ h~t~d in vacuum at 730 K for 80 h. This heat treatment caused distinct changes in the N i - C system (fig. lc). Hn the vicinity of most nickel particles regions with a stronger contrast are visible. The difference in intensity is caused by a local stronger scattering of the electron beam by the support and is connected with structural changes of the carbon substrate. This is confirmed by the diffraction pattern obtained from this specimen (fig. 2b). In addition to the spotted nickei diffraction rings two strong, diffuse rings
406
1~ Lamber et al. / The interaction of Ni, Pd and Pt with carbon. I
Fig. 2. Selected area diffraction patterns for: (a) specimen 2 (67(I K, 60 h); (b) specimen 3 (730 K, 80 h)~ (c) specimen 5 (850 K, 80 h); (d) specimen 3 following additional heat ~reatment at 1000 K, 24 h.
which correspond to interplanar spacings d = 0.336 and 0.209 nm, and a weak ring with d = 0.122 nm can be observed (fig. 2b). The presence of these diffraction rings can be explained by the formation of graphitic carbon, which consists of stacked " b a s a l " planes at 0.335 nm intervals. The interplanar spacings corresponding to the observed rings are listed in table 2. Two specimens were heated in a hydrogen atmosphere at a pressure of 6 × 10 .2 Pa° Specimen 4 was heated at 820 K (10 h) and fig. l d shows that graphitization of carbon takes also place under conditions where an efficient ~T~ ~ . ~ .... ~ ~ ' ~ ' ~ ' ~ ' ~ of ~". . . . ~,~,., ~ , ~ , ~ t ~ ~.~n ~ expected [35! Th~ presence of large n i c k d particles (fig. l d ) indicates a faster sintering of ~he metai, which may, be c©aaected with nickel catalyzed hydrogena~ioe of ~he carbon subs~rate. The thermal treatment of specimen 5 was carried om at 850 K (80 h). An electron micrograph and an electron diffraction pattern of this sample are shown ia figs. le and 2c. Heating at higher temperature for 80 h ~:aused an increase o~ the degree of the conversion of carbon substrate to
R. Lamber etal. / The interaction of Ni, Pd and Pt with carbon. I
407
Table 2 Measured interplanar distances d (am) in comparison with inte:'p~anar distances reported for nickel and graphite d measured
d reported for
for
Specimen 3 (730 K)
Specimen 6 (870 K)
0.336 vs 0.209 m 0.203 s
0.337 vs 0.211 s 0.204 s
0.203
(111)
0.176 s
0.177 s
0.176
(200)
-
0.168
0.125 m 0.122 w
0.125 m 0.122 s
0.125
0.106 m 0.102 w
0.106 m 0.102 w
0.106 0.102
vs:
Nickel
d
Graphite
(hkl)
w
d
(hkl)
0.336 0.213 0.203 0.180
(002) (100) (101) (102)
0.168 0.i54
(004) (103)
0,123 0.116 0.114 0.112 0.105
(110) (112) (105) (006) (201)
(220)
(311) (222)
very strong, s: strong, m: medium, w: weak.
gr~_phitlc c~rbon. In addition, trails of graphitic carbon are indicative for the migration of Ni crystals. Comparison of fig. le and fig. lf, which was taken from specimen 6 after heating in vacuum at 870 K, shows that the presence of a hydrogen atmosphere apparently facifitates particle migration. The area left behind the migrating particles appears to be graphitic carbon° D~ring heating in a vacuum the large nickel particles are rather immobile and surrounded by graphitic carbon. This can be seen in fig. 3a where a high resolution micrograph of a nickel particle surrounded by graphite is shown. The fact that some nickel particles are really encapsulated is supported by observations of .nickel crystallites on the edges of folded parts of the carbon films. In fig. 3b the side view of such a Ni particle is shown. The graphite layers outlining the shape of the metal crystallite are visible also below the surface of the carbon substrate. In some cases the nickel crystaUites were only partially covered by graphite layers. This is shown in fig. 3c for a particle wi~h a well defined shape, the (110) plane parallel to the substrate and (100) planes predominantly exposed. !n these cases the nickel particles are still able to move across the carbon subs~rate. The migration of Ni particles is illustrated by fig. 3d. This picture shows tha~ L~ic ,~: .~de has ~.~oved across the carbon substrates leaving behind a trail of graplLdc carbon. This particle, is also surrounded by graphite layers. GrapNte layers w~th different orientation overlap and prouu,.c .... mo~re " va[terns (marked as "M') in fig. 3d. ~t should be noted that most of the support area shown in fig. 3d e~d~ibits the layering characteristic for grap~itic carboe.
408
R. Lamber et al. / The interaaion of Ni, Pd and Pt with carbon. 1
b J
"..-.,
"
-
.
Fig. 3. Hi~J~ magnification images of nickel parfictes afr..-r graphit~zafion at 810 K; (a) an encapsulated particle; Cb) side ~sew of encapsulated particle; (c) side view of partially encapsulated Ni particle; (d) a '° mo~Sng panicle" with trai~s o{ graphite, overlaps:rig basal graphite planes with different orientations produce m¢ !~ patterns (" °A').
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. I
409
Fig. 3. Continued.
Specimen 3 heated in vacuum for 730 K (fig. lc) was in addition annealed at 1000 K for 24 h. This heat treatment caused some further sintering of the metal particles (fig. lg). An eiectron diffraction pattern taken from this specimen is shown in fig. 2d. In comparison with the diffraction pattern obtained for the same specimen after the heat treatment at 730 K (fig. 2b) one notes the higher intensity of the graplfite diffraction rings. Rings corresponding to the (002), (004), (106) and (110) planes of graphite are observed. The fact that the (110) ring is narrow and the (002) and (004) rings are diffuse indicates that the order within the basal planes is more extensively developed than the order between planes. This seems to be supported by the fact that the (hOl) and (kkl) rings couid not be detected (table 2). In order to study a possible correlation of activity of nickel particles in the with respect to the carbon substrate (e~ectrc~n be~,..m~ ~ ~ ~ e ~ d of ~-~crodifo fraction has bee~ us,:do A few typical convergent beam diffraction patterns obtained from Large nickel particles are shown in fig. 4. Nickel particles with different oriev.iations have been obsen,ed, however the mai~ orientation is (!12). In fig. 4d one sees a microdiffraction pattern obtained f~om grapNtfic
410
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. I
d
Fig. 3. Continued.
carbon located in the vicinity of a nicke~ partic!e. Sharp rings due to graphite are visible.
4. Discussion Normally ~',~e conversior~ of disordered carbon into graphite without a catalyst takes ptace at temperatures between 2300-3500 K depending on ~he source of carbon [36]. The presented results show that nicke~ may catalyze the
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. I
411
Fig. 4. Microdiffraction patterns from nickel particles: (a) in the (11i2) oriemation; (b) in ~he (100) orientalion; (c) in the (123) orientation; (d) microdiffracfion pa~ern from a graphi6zed area in the vicinity of a nickel particle. In fig. 4a-~c ~he fragments of ~he graphite diffrac6on rings are also visible.
conversion of amorphous to graphitic carbon at temperatures as !,ew as 730 K. At this temperature the degree of order of the obtained graphitic carbon was not very high, as evidenced by the diffuse diffraction rings in fig. 2b. Higher degrees of order were obtained at higher temperatures. It has been found that the graphitization of amorphous carbon proceeds in a vacuum as well as in a hydrogen atmosphere. ~v~ecnamsm qf mce(e~-cmatyzea gray~hi~izatiorg
~t is known that trar~sit~on, alkali and a~katine earth metals accelerate the graphitization of carbon [20]. tt is believed ~hat the catalyzed coavers~or~ ef disordered carbon to graphite proceeds by a solution-precipitation mechanism. This is supported by observations tha~ graphite is formed from carbon dissolved in solid nickel and cobalt [37,38] or in ~iquid metals [391o The first step has to be the rupture of carbon-carbon bor~ds by :he catalyst particle at
412
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. I
the interface between the disordered carbon and the metal. This is foUowed by the dissolution of carbon in the metal, diffusion and precipitation as graphitic carbon. The driving force is the free energy difference between the initial and final forms of carbon [40]. A detailed in situ transmission electron microscopy study of the palladiumcatalyzed conversion of amorphous to graphitic carbon has been reported by Holstein et al. [21]. They observed the behavior of palladium particles supported on the amorphous carbon substrate during heating in vacuum at 1150 K. It has been shown that after an induction time palladium particles begin to react with the amorphous carbon support. The result is the formation of an "intermediate" which was still identified as crystalline palladium and was assumed to catalyze the graphitization of carbon. The next stage of graphitization involves the break-up of the intermediate and the exposure of a more crystalline form of carbon. We believe that the same mechanism is operating in the nickel catalyzed conversion of amorphous to graphitic carbon. However, nickel may accelerate this conversion at temperature as low as 730 K, i.e. that nickel is able to break away carbon atoms from the bulk at relatively low temperatures. Comparison of figs. l b and lc shows that heating ~t 730 K results in distinct changes in the morphology of the nickel particles. This could be due to strong chemical interaction between nickel particles and the carbon substrate, which leads to wetting of the substrate by the metal. The wetting is probably accompanied by the diffusion of carbon into the nickel leading to the formation of a solid solution or a carbide at the metal-carbon interface. However, we were not able to identify a nickel-carbon compound, it is assumed that the nickel in this state catalyzes the graphitization of the disordered carbon. The interaction between nickel and graphite is probably weaker and when nickel is in contact with a graphitic support a contraction of the particles can be expected. As a result larger, globular metal particles are formed and can be observed in figs. le and If after heating to temperatures above 850 K. Whe;e these larger nickel particles are still in contact with disordered carbon, the catalyzed conversion to graphite can continue. The changing contact angles at the metal-carbon interface may result in a movement of the nickel particles across the carbon substrate leaving behind a graphitized area (fig. 3d). When the graphite layers surround the whole particle symmetrically (figs. 3a and 3b) the particle is encapsulated in the graphite skeleton and is not able to move across the substrate. Fig. 3c shows a partially encapsulated particle, which might be in a stage where it is still able to move. Careful inspection of figs. 3a and 3d show that the outer layers of graphite in general is not disturbed by the carbon substrate, which indicates that they are present above the support. However, occasionally the buckled edge of attack of graphite layers could be observed (marked as "B" in fig. 3d), which is evidence for the growth of graphite layers below the surface of the carbon substrate.
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. I
413
During the study of' segregation of dilute solid solutions of C i~a Ni, Blakely et al. [41,42] have established that in the case of N i ( l l l ) surface, carbon atoms formed an epitaxial graphite monolayer. One might expect that the activity of nickel Farticles in the amorphous-graphitic carbon conversion may be correlated with the orientation with respect to the substrate. The microdiffraction study showed that the active nickel particles are in different orientations with respect to the substrate, however most often the (112) orientation was observed. It may be rationalized with the other observation by Blakely et al. [43] that with surfaces of Ni other than (111), the appearance of the carbon overlayer is accompanied by a surface reconstruction in which faceting occurs to expose only sections of (111) planes.
5. Conclusions The study of the interaction of highly dispersed nickel with disordered carbon substrates by means of transmission electron microscopy revealed that nickel catalyzes effectively the conversion of amorphous to graphite carbon at temperatures above 730 K. A suggested mechanism involves the rupture of carbon-carbon bends and the removal of carbon atoms from the bulk at temperatures as low as 700 K. Carbon atoms are dissolved in the nickel phase and subsequently precipitated as graphite. The conversion of amorphous to graphitic carbon proceeds s!ewly at 730 K and the degree of order of the graphJtic carbon is not high. At temperatures above 870 K carbon with a higher degree of order can be obtained but the faster sintering of the nickel particles causes a reduction of the metal-substrate contact area. Trails of graphidzed areas are interpreted by moving metal crystallites. A detailed mechanism for this movement cannot be given at the present time. The microdiffraction study showed that the particles active for the graphitization process are in different orientations with respect to the carbon substrate. Acknowledgements We thank Mr. G. Ernst for his skillful technical assistance. R.L. gratefully acknowledges financial support by Deutscher Akademischer Aus~auschdienst. References [1] [2] [3] [4]
S.J. Tausler, S.C. Fung and R.L. Garten, J. Am. Chem. Soc. 100 (1978) 170. S.J. Tauster and S.C. Fung, J. Catalysis 55 (1978) 29. G.R. Wilson and W.K. Hall, J. Catalysis 24 (1972) 306. R.L. Moss, D. Pope, B.J. Davis and D.H. Edwards, 3. Catalysis 58 (1979) 206.
414
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. I
[5] G.A. Martin and J.A. Dalmon, Reaction Kinet. Catalysis Letters 16 (1981) 325. [6] [7] [8] [9]
G.A. Martin, R. Dutartre and J.A. Dalmon, Reaction Kinet. Catalysis Letters 16 (1981) 329. G.A. Martin and J.A. Dalmon, J. Catalysis 75 (1982) 233. H. Praliaud and G.A. Martin, J. Catalysis 72 (1981) 394. IL Lamber, Thin Solid Films 128 (1985) L29; R. Lamber and W. Romanowski, J. Catalysis 105 (1987) 213. [10] F.M. Dautzenberg and H.B.M. Wolters, J. Catalysis 51 (1978) 26. [11] G3. Den Otter and F.M. Dautzenberg, J. Catalysis 53 (1978) 116. [12] IC Kunimori, T. Okouchi and T. Uchijima, Chem. Letters (1980) 1513. [131 IC Kunimori, Y. Ikeda, M. Soma and T. Uchijima, J. Catalysis 79 (1983) 185. [14] J.E.E. Baglin, G3. Clark, J.F. Ziegler and J.A. Cairns, J. Mol. Catalysis 20 (1983) 299. [15] J.A. Cairns, J.E.E. Baglin, G.J. Clark and J.F. Ziegler, J. Catalysis 83 (1983) 301. [16] J. Adamiec, S.C. Wanke, B. Tesche and U. Klengler, in: Meta~,-Support and Metal-Additive Effects in Catalysis, Eds. B. Imelik et al. (Elsevier, Amsterd~l, 1982) p. 77. [17] E. Fromm and E. GebhardL Gase und Kohlenstoff in Metallen (Springer, Berlin, 1976). [18] R.C. Baetzold, Surface Sci. 36 (1972) 123. [19] M.J. Witcomb, U. Dahmen and K.H. Westmacott, Acta Met. 31 (1983) 743. [20] H. Marsch and A.P. Warburton, J. Appl. Chem. 20 (1970) 113. [21] W.L. Holstein, R.D. Moorhead, H. Poppa and M. Boudart, in: Chemistry and Physics of Carbon, Vol. 18, Ed. P.L. Walker (Dekker, New York, 1982) p. 139. [22] J.A. Bett, K. Kinoshita and P. Stoneheart, J. Catalysis 35 (1974) 307. [23] R.T.K. Baker, R.S. Harris and R.B. Thomas, Surface Sci. 46 (1974) 311. [24] P. Ehrburger, P.O. Mahajan and P.L. Walker, Jr., J. Catalysis 43 (1976) 61. [25] P. Ehrburger and P.L. Walker, Jr., J. Catalysis 55 (1978) 63. [26] Y.7. Chu and E. Ruckenstein, Surface Sci. 67 (1977) 517. [27] R.T.K. Baker, E.B. Prestridge and R.L. Garten, J. Catalysis 56 (1979) 390. [28] M. Arai, T. Ishikawa and Y. Nishiyama, J. Phys. Chem. 86 (1982) 577. [29] M. Arai, T. Ishikawa, T. Nakayama and Y. Nishiyama, J. Colloid Interface Sci. 97 (1984) 254. [30] M. Arai and Y. Nishiyama, J. Colloid Interface Sci. 104 (1985) 175. [31] T. Baird, in: Catalysis (A Specialist Periodic Report), Vol. 5 (The Royal Society of Chemistry, London, 1982) p. !72. [32] J.V. Sanders, in: Catalysis, Science and Technology, Voi. 7 (Springer, Berlin, 1985) ch. 2. [33] R. Lamber, N. Jaeger and G. Schulz-Eldoff, in preparation. [34] I.S. McLintock and J.C. Orr, in: Chemistry and Physics of carbon, Vol. 1I, Ed. P.L. Walker, Jr. (Dekker, New York, 1973) p. 243. [35] A. Tomita and Y. Tamai, J. Catalysis 27 (1972) 293. [36] D.B. Fischbach, in: Chemistry and Physics of Carbon, Vol. 7, Ed. P.L. Walker, Jr. (Dekker, New York, 1971) ch. 1. [37] F.J. Derbyshire, A.E.B. Presland and D.L. Tnmm, Carbon 10 (1972) 114. [38] F.J. Derbyshire, A.E.B. Presland and D.L. Trimm, Carbon 13 (1975) 111. [39] S.B. Austermann, in: Chemistry and Physics of Carbon, Vol. 4, Ed. P.L. Walker, Jr. (Dekker, New York, 1968) p. 137. [40] E. Fitzer and B. Kegel, Carbon 6 (1968) 433. [41] J.C. Shelton. H.R. Patil and J.M. Blakely, Surface Sci. 43 (1974) 493. [42] M. Eizenberg and J.M. Blakely, Surface Sci. 82 (1979) 228. [43] M. Eizenberg and J.M. Blakely, J. Phys. Chem. 71 (1979) 3467.
Ryszard LAMBER Institute for Low Temperature
end P-0. Box 937, SO-950 WrocCaw, Poland
Structure
Research,
Nils JAEGER and Giinter SCHULZ-EKLOFF Institutftir ~ngewandte und Physikalisci., Chemie, Universitiir
Polish
Academy
of
Sciences,
Bremen, Postjach 33n 440,
D-2800 Bremen 33, Fed. Rep. oj Germany Received
13 July 1987; accepted
for publication
20 November
1987
The aggregation of nickel supported on amorphous carbon during heating in vacuum and at .-. : . . IUW hydr~ogeu p~rssuics kit Put temperaturesrange ate 1000 5x W’~Yr~esi~galru ny I~C~~E!J VL &a:~~~ -l..*;,e It was established that iL>V .I... . ,.. electron microsco?~, electron diffraction and microdiffraction. the interaction or i+!y dispersed nickel with amorphous carbon substrates is strong enough to break away carbon atoms from the bulk at relatively low temperatures ( - 700 K) Cxbnn atoms dissolved in mckel precipitate as gra$titc. .2,= ., _ res~!t the aggregation of nickel is accompanied by the conversion of the amorphous carbon substrate to graphitic carbon.
Supported metal catalysis consist of highly dispersed metal phases on suitable carriers like ceramic oxides, zeolites or a orphous carbon. The activity of the metal in catalytic reactions depends on the preparation conditions among which te perature is one of the decisive parameters. The activity catalysts tends decrease with - e due to a num f chemical and al phenomena ical interaction lvith the support, ~;~nte~~g or even encapsulation of the metal particles Or?: uring the pas\ years consi eta1 support iiatcracaion on t he properties can rag between t * Work carried out at the Universitlt
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. I
403
strong metal-support interaction (SMSi) was introduced by Tauster et ai. [1,2] following their discovery of the suppression of hydrogen and CO chemisorption on titania supported platinum group metals af~:er the catalysts had been treated in hydrogen at temperatures above 700 K. Although it was claimed that only transition metal oxides such as TiO 2 give rise to strong interactions with metals [2] there is now increasing evidence for similar phenomena involving non-transition metal oxides such as the widely used silica [3-9], alumina [10-15] and magnesia [16]. Strong chemical interaction between the metal and the substrate should also be considered in the case of Group VIII 3 metals supported on carbon. It is known that Ni can form a metastable Ni3C compound with carbon [17]. There is evidence for strong interaction between Pd and carbon [18]. However, the formation of a palladium carbide has not yet been established. Recently it has been shown that the formation of a metastable platinum carbide (PtEC) is possible in the Pt-C system [19]. It is known that the conversion of disordered carbons to graphite can be accelerated by metals [20,21]. Extended experimental studies on the sintering mechanism of various metals supported on carbon substrates have already been reported [22-30]. The present investigation was carried out in order to recognize in more detail the metal support interaction in Group VIII 3 metal/carbon systems and to claruy tn~ . . . . . ,)f t;,,~ metal on structure changes L,, ,.,~,,,,.,.. In ,h.. present study the method of transmission electron microscopy, electron diffraction and microdiffraction have been used. The utility of these methods in the study of supported metal catalysts is well documented [31,32]. Evidence for the nickel catalyzed conversion of amorphous carbon to graphite at low temperatures will be presented. Results concerning the P d - C and Pt-C systems will be reported in a subsequent publication [33]. ltlllltUK;i]t~,l~,
"
- ..t
-
~
,
2. Experimental Thin film carbon substrates were prepared by vacuum deposition from a carbon arc onto a fresh air-cleaved face of a NaCl single crystal. After deposition the NaCI substrate was immersed into a large amount of distilled water and the floating carbon films were picked up onto gold electron microscope grids. Prior to the deposition of r,~ckel the EM grids were placed in a resistanceh o t r o d ~ a n t a l n m ¢~rnn|P h n | d e . r n n d h e a t t r e a t e d at the upper temperature limit of the experiments (875 K) for 1 h in vacuum (10 -5 Pa). The temperature was measured using a P t - ( P t - R h ) thermocouple spot-welded directly to the sample holder. Electron diffraction analysis of the C substrates after this thermal treatmc~tt showed only the presence of two diffuse halos indicating that the substrates were amorphous, in accordance with observations of Mc Lintock and Orr [34].
404
R. Lamber et ai. / The interaction of Ni, P d and Pt with carbon• 1
:v~.~=~ ¸lr~
~
~,~~ ~ : ~ ~ , ~
~ i ~!~'~:~ ii~i~i~,~:% ~, .
.... ~
~ ~ ~.~
•
Fig. 1 Electron micrographs of Ni on carbon substrates (a) following the evaporation of nickel (specimen 1); and subsequent treatment (b) at 670 K, 60 h, vacuum (specimen 2); (c) at 730 K, 80 h, vacuum (specimen 3); (d) at 820 K, 10 h, H 2 (specimen 4); (e) at 850 K, 80 h, H 2 (specimen 5); ~f) at 870 K, 60 b, vacuum (specimen 6): fg) additional treatment of specimen 3 at ]0~0 K, 24 h, vacuum. Note different magnifications.
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. 1
405
Table 1 Thermal treatment conditions Specimen
Atmosphere
1 2 3
Vacuum Vacuum Vacuum
4 5 6
H2 H2 Vacuum
Temperature
Time
(K)
(h)
293 670 730 1000 820 850 870
60 80 24 10 80 60
Fig. la lb, 2a lc, 2b lg, 2d ld le, 2c lf, 3a, 3b, 3c, 3d
Discontinuous films of Ni were deposited by vacuum evaporation from a resistance heated tungsten boat (fig. la). By controlling the amount of the evaporated metal an average film thickness of 0.4 nm could be reproducibly established. During evaporation the pressure was kept below 2 x 10 -4 Pa. After the Ni layer had been evaporated, a thermal treatment of the N i - C system in a vacuum (10 -5 Pa) or hydrogen atmosphere (6 × 10 -z Pa) was carried out. The details of this thermal treatment are shown in table I for six representative samples, which were chosen from a larger number of systematic experiments. All species were examined in a Phillips EM 420 transmission electron microscope operated at 120 kV and equipped with an energy dispersive X-ray analyzer (EDX). With EDX no contaminants were detectable in the investigated specimens.
3. Results A TEM picture of the N i - C specimen 1 as obtained after the metal evaporation is shown in fig. la. Nickel is present as small particles of sizes in the range 1.5-3 nm. The thermal treatment in vacuum at 670 K (60 h) (specimen 2) resulted in some sintering of the metal, i.e. larger liquid-like particles of nickel are formed (fig. lb). Only the nickel diffraction rings can be observed in the electron diffraction pattern (fig. 2a). Following the deposition ,~f ~i ~n~,-im~n ~ , ~ h~t~d in vacuum at 730 K for 80 h. This heat treatment caused distinct changes in the N i - C system (fig. lc). Hn the vicinity of most nickel particles regions with a stronger contrast are visible. The difference in intensity is caused by a local stronger scattering of the electron beam by the support and is connected with structural changes of the carbon substrate. This is confirmed by the diffraction pattern obtained from this specimen (fig. 2b). In addition to the spotted nickei diffraction rings two strong, diffuse rings
406
1~ Lamber et al. / The interaction of Ni, Pd and Pt with carbon. I
Fig. 2. Selected area diffraction patterns for: (a) specimen 2 (67(I K, 60 h); (b) specimen 3 (730 K, 80 h)~ (c) specimen 5 (850 K, 80 h); (d) specimen 3 following additional heat ~reatment at 1000 K, 24 h.
which correspond to interplanar spacings d = 0.336 and 0.209 nm, and a weak ring with d = 0.122 nm can be observed (fig. 2b). The presence of these diffraction rings can be explained by the formation of graphitic carbon, which consists of stacked " b a s a l " planes at 0.335 nm intervals. The interplanar spacings corresponding to the observed rings are listed in table 2. Two specimens were heated in a hydrogen atmosphere at a pressure of 6 × 10 .2 Pa° Specimen 4 was heated at 820 K (10 h) and fig. l d shows that graphitization of carbon takes also place under conditions where an efficient ~T~ ~ . ~ .... ~ ~ ' ~ ' ~ ' ~ ' ~ of ~". . . . ~,~,., ~ , ~ , ~ t ~ ~.~n ~ expected [35! Th~ presence of large n i c k d particles (fig. l d ) indicates a faster sintering of ~he metai, which may, be c©aaected with nickel catalyzed hydrogena~ioe of ~he carbon subs~rate. The thermal treatment of specimen 5 was carried om at 850 K (80 h). An electron micrograph and an electron diffraction pattern of this sample are shown ia figs. le and 2c. Heating at higher temperature for 80 h ~:aused an increase o~ the degree of the conversion of carbon substrate to
R. Lamber etal. / The interaction of Ni, Pd and Pt with carbon. I
407
Table 2 Measured interplanar distances d (am) in comparison with inte:'p~anar distances reported for nickel and graphite d measured
d reported for
for
Specimen 3 (730 K)
Specimen 6 (870 K)
0.336 vs 0.209 m 0.203 s
0.337 vs 0.211 s 0.204 s
0.203
(111)
0.176 s
0.177 s
0.176
(200)
-
0.168
0.125 m 0.122 w
0.125 m 0.122 s
0.125
0.106 m 0.102 w
0.106 m 0.102 w
0.106 0.102
vs:
Nickel
d
Graphite
(hkl)
w
d
(hkl)
0.336 0.213 0.203 0.180
(002) (100) (101) (102)
0.168 0.i54
(004) (103)
0,123 0.116 0.114 0.112 0.105
(110) (112) (105) (006) (201)
(220)
(311) (222)
very strong, s: strong, m: medium, w: weak.
gr~_phitlc c~rbon. In addition, trails of graphitic carbon are indicative for the migration of Ni crystals. Comparison of fig. le and fig. lf, which was taken from specimen 6 after heating in vacuum at 870 K, shows that the presence of a hydrogen atmosphere apparently facifitates particle migration. The area left behind the migrating particles appears to be graphitic carbon° D~ring heating in a vacuum the large nickel particles are rather immobile and surrounded by graphitic carbon. This can be seen in fig. 3a where a high resolution micrograph of a nickel particle surrounded by graphite is shown. The fact that some nickel particles are really encapsulated is supported by observations of .nickel crystallites on the edges of folded parts of the carbon films. In fig. 3b the side view of such a Ni particle is shown. The graphite layers outlining the shape of the metal crystallite are visible also below the surface of the carbon substrate. In some cases the nickel crystaUites were only partially covered by graphite layers. This is shown in fig. 3c for a particle wi~h a well defined shape, the (110) plane parallel to the substrate and (100) planes predominantly exposed. !n these cases the nickel particles are still able to move across the carbon subs~rate. The migration of Ni particles is illustrated by fig. 3d. This picture shows tha~ L~ic ,~: .~de has ~.~oved across the carbon substrates leaving behind a trail of graplLdc carbon. This particle, is also surrounded by graphite layers. GrapNte layers w~th different orientation overlap and prouu,.c .... mo~re " va[terns (marked as "M') in fig. 3d. ~t should be noted that most of the support area shown in fig. 3d e~d~ibits the layering characteristic for grap~itic carboe.
408
R. Lamber et al. / The interaaion of Ni, Pd and Pt with carbon. 1
b J
"..-.,
"
-
.
Fig. 3. Hi~J~ magnification images of nickel parfictes afr..-r graphit~zafion at 810 K; (a) an encapsulated particle; Cb) side ~sew of encapsulated particle; (c) side view of partially encapsulated Ni particle; (d) a '° mo~Sng panicle" with trai~s o{ graphite, overlaps:rig basal graphite planes with different orientations produce m¢ !~ patterns (" °A').
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. I
409
Fig. 3. Continued.
Specimen 3 heated in vacuum for 730 K (fig. lc) was in addition annealed at 1000 K for 24 h. This heat treatment caused some further sintering of the metal particles (fig. lg). An eiectron diffraction pattern taken from this specimen is shown in fig. 2d. In comparison with the diffraction pattern obtained for the same specimen after the heat treatment at 730 K (fig. 2b) one notes the higher intensity of the graplfite diffraction rings. Rings corresponding to the (002), (004), (106) and (110) planes of graphite are observed. The fact that the (110) ring is narrow and the (002) and (004) rings are diffuse indicates that the order within the basal planes is more extensively developed than the order between planes. This seems to be supported by the fact that the (hOl) and (kkl) rings couid not be detected (table 2). In order to study a possible correlation of activity of nickel particles in the with respect to the carbon substrate (e~ectrc~n be~,..m~ ~ ~ ~ e ~ d of ~-~crodifo fraction has bee~ us,:do A few typical convergent beam diffraction patterns obtained from Large nickel particles are shown in fig. 4. Nickel particles with different oriev.iations have been obsen,ed, however the mai~ orientation is (!12). In fig. 4d one sees a microdiffraction pattern obtained f~om grapNtfic
410
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. I
d
Fig. 3. Continued.
carbon located in the vicinity of a nicke~ partic!e. Sharp rings due to graphite are visible.
4. Discussion Normally ~',~e conversior~ of disordered carbon into graphite without a catalyst takes ptace at temperatures between 2300-3500 K depending on ~he source of carbon [36]. The presented results show that nicke~ may catalyze the
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. I
411
Fig. 4. Microdiffraction patterns from nickel particles: (a) in the (11i2) oriemation; (b) in ~he (100) orientalion; (c) in the (123) orientation; (d) microdiffracfion pa~ern from a graphi6zed area in the vicinity of a nickel particle. In fig. 4a-~c ~he fragments of ~he graphite diffrac6on rings are also visible.
conversion of amorphous to graphitic carbon at temperatures as !,ew as 730 K. At this temperature the degree of order of the obtained graphitic carbon was not very high, as evidenced by the diffuse diffraction rings in fig. 2b. Higher degrees of order were obtained at higher temperatures. It has been found that the graphitization of amorphous carbon proceeds in a vacuum as well as in a hydrogen atmosphere. ~v~ecnamsm qf mce(e~-cmatyzea gray~hi~izatiorg
~t is known that trar~sit~on, alkali and a~katine earth metals accelerate the graphitization of carbon [20]. tt is believed ~hat the catalyzed coavers~or~ ef disordered carbon to graphite proceeds by a solution-precipitation mechanism. This is supported by observations tha~ graphite is formed from carbon dissolved in solid nickel and cobalt [37,38] or in ~iquid metals [391o The first step has to be the rupture of carbon-carbon bor~ds by :he catalyst particle at
412
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. I
the interface between the disordered carbon and the metal. This is foUowed by the dissolution of carbon in the metal, diffusion and precipitation as graphitic carbon. The driving force is the free energy difference between the initial and final forms of carbon [40]. A detailed in situ transmission electron microscopy study of the palladiumcatalyzed conversion of amorphous to graphitic carbon has been reported by Holstein et al. [21]. They observed the behavior of palladium particles supported on the amorphous carbon substrate during heating in vacuum at 1150 K. It has been shown that after an induction time palladium particles begin to react with the amorphous carbon support. The result is the formation of an "intermediate" which was still identified as crystalline palladium and was assumed to catalyze the graphitization of carbon. The next stage of graphitization involves the break-up of the intermediate and the exposure of a more crystalline form of carbon. We believe that the same mechanism is operating in the nickel catalyzed conversion of amorphous to graphitic carbon. However, nickel may accelerate this conversion at temperature as low as 730 K, i.e. that nickel is able to break away carbon atoms from the bulk at relatively low temperatures. Comparison of figs. l b and lc shows that heating ~t 730 K results in distinct changes in the morphology of the nickel particles. This could be due to strong chemical interaction between nickel particles and the carbon substrate, which leads to wetting of the substrate by the metal. The wetting is probably accompanied by the diffusion of carbon into the nickel leading to the formation of a solid solution or a carbide at the metal-carbon interface. However, we were not able to identify a nickel-carbon compound, it is assumed that the nickel in this state catalyzes the graphitization of the disordered carbon. The interaction between nickel and graphite is probably weaker and when nickel is in contact with a graphitic support a contraction of the particles can be expected. As a result larger, globular metal particles are formed and can be observed in figs. le and If after heating to temperatures above 850 K. Whe;e these larger nickel particles are still in contact with disordered carbon, the catalyzed conversion to graphite can continue. The changing contact angles at the metal-carbon interface may result in a movement of the nickel particles across the carbon substrate leaving behind a graphitized area (fig. 3d). When the graphite layers surround the whole particle symmetrically (figs. 3a and 3b) the particle is encapsulated in the graphite skeleton and is not able to move across the substrate. Fig. 3c shows a partially encapsulated particle, which might be in a stage where it is still able to move. Careful inspection of figs. 3a and 3d show that the outer layers of graphite in general is not disturbed by the carbon substrate, which indicates that they are present above the support. However, occasionally the buckled edge of attack of graphite layers could be observed (marked as "B" in fig. 3d), which is evidence for the growth of graphite layers below the surface of the carbon substrate.
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. I
413
During the study of' segregation of dilute solid solutions of C i~a Ni, Blakely et al. [41,42] have established that in the case of N i ( l l l ) surface, carbon atoms formed an epitaxial graphite monolayer. One might expect that the activity of nickel Farticles in the amorphous-graphitic carbon conversion may be correlated with the orientation with respect to the substrate. The microdiffraction study showed that the active nickel particles are in different orientations with respect to the substrate, however most often the (112) orientation was observed. It may be rationalized with the other observation by Blakely et al. [43] that with surfaces of Ni other than (111), the appearance of the carbon overlayer is accompanied by a surface reconstruction in which faceting occurs to expose only sections of (111) planes.
5. Conclusions The study of the interaction of highly dispersed nickel with disordered carbon substrates by means of transmission electron microscopy revealed that nickel catalyzes effectively the conversion of amorphous to graphite carbon at temperatures above 730 K. A suggested mechanism involves the rupture of carbon-carbon bends and the removal of carbon atoms from the bulk at temperatures as low as 700 K. Carbon atoms are dissolved in the nickel phase and subsequently precipitated as graphite. The conversion of amorphous to graphitic carbon proceeds s!ewly at 730 K and the degree of order of the graphJtic carbon is not high. At temperatures above 870 K carbon with a higher degree of order can be obtained but the faster sintering of the nickel particles causes a reduction of the metal-substrate contact area. Trails of graphidzed areas are interpreted by moving metal crystallites. A detailed mechanism for this movement cannot be given at the present time. The microdiffraction study showed that the particles active for the graphitization process are in different orientations with respect to the carbon substrate. Acknowledgements We thank Mr. G. Ernst for his skillful technical assistance. R.L. gratefully acknowledges financial support by Deutscher Akademischer Aus~auschdienst. References [1] [2] [3] [4]
S.J. Tausler, S.C. Fung and R.L. Garten, J. Am. Chem. Soc. 100 (1978) 170. S.J. Tauster and S.C. Fung, J. Catalysis 55 (1978) 29. G.R. Wilson and W.K. Hall, J. Catalysis 24 (1972) 306. R.L. Moss, D. Pope, B.J. Davis and D.H. Edwards, 3. Catalysis 58 (1979) 206.
414
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. I
[5] G.A. Martin and J.A. Dalmon, Reaction Kinet. Catalysis Letters 16 (1981) 325. [6] [7] [8] [9]
G.A. Martin, R. Dutartre and J.A. Dalmon, Reaction Kinet. Catalysis Letters 16 (1981) 329. G.A. Martin and J.A. Dalmon, J. Catalysis 75 (1982) 233. H. Praliaud and G.A. Martin, J. Catalysis 72 (1981) 394. IL Lamber, Thin Solid Films 128 (1985) L29; R. Lamber and W. Romanowski, J. Catalysis 105 (1987) 213. [10] F.M. Dautzenberg and H.B.M. Wolters, J. Catalysis 51 (1978) 26. [11] G3. Den Otter and F.M. Dautzenberg, J. Catalysis 53 (1978) 116. [12] IC Kunimori, T. Okouchi and T. Uchijima, Chem. Letters (1980) 1513. [131 IC Kunimori, Y. Ikeda, M. Soma and T. Uchijima, J. Catalysis 79 (1983) 185. [14] J.E.E. Baglin, G3. Clark, J.F. Ziegler and J.A. Cairns, J. Mol. Catalysis 20 (1983) 299. [15] J.A. Cairns, J.E.E. Baglin, G.J. Clark and J.F. Ziegler, J. Catalysis 83 (1983) 301. [16] J. Adamiec, S.C. Wanke, B. Tesche and U. Klengler, in: Meta~,-Support and Metal-Additive Effects in Catalysis, Eds. B. Imelik et al. (Elsevier, Amsterd~l, 1982) p. 77. [17] E. Fromm and E. GebhardL Gase und Kohlenstoff in Metallen (Springer, Berlin, 1976). [18] R.C. Baetzold, Surface Sci. 36 (1972) 123. [19] M.J. Witcomb, U. Dahmen and K.H. Westmacott, Acta Met. 31 (1983) 743. [20] H. Marsch and A.P. Warburton, J. Appl. Chem. 20 (1970) 113. [21] W.L. Holstein, R.D. Moorhead, H. Poppa and M. Boudart, in: Chemistry and Physics of Carbon, Vol. 18, Ed. P.L. Walker (Dekker, New York, 1982) p. 139. [22] J.A. Bett, K. Kinoshita and P. Stoneheart, J. Catalysis 35 (1974) 307. [23] R.T.K. Baker, R.S. Harris and R.B. Thomas, Surface Sci. 46 (1974) 311. [24] P. Ehrburger, P.O. Mahajan and P.L. Walker, Jr., J. Catalysis 43 (1976) 61. [25] P. Ehrburger and P.L. Walker, Jr., J. Catalysis 55 (1978) 63. [26] Y.7. Chu and E. Ruckenstein, Surface Sci. 67 (1977) 517. [27] R.T.K. Baker, E.B. Prestridge and R.L. Garten, J. Catalysis 56 (1979) 390. [28] M. Arai, T. Ishikawa and Y. Nishiyama, J. Phys. Chem. 86 (1982) 577. [29] M. Arai, T. Ishikawa, T. Nakayama and Y. Nishiyama, J. Colloid Interface Sci. 97 (1984) 254. [30] M. Arai and Y. Nishiyama, J. Colloid Interface Sci. 104 (1985) 175. [31] T. Baird, in: Catalysis (A Specialist Periodic Report), Vol. 5 (The Royal Society of Chemistry, London, 1982) p. !72. [32] J.V. Sanders, in: Catalysis, Science and Technology, Voi. 7 (Springer, Berlin, 1985) ch. 2. [33] R. Lamber, N. Jaeger and G. Schulz-Eldoff, in preparation. [34] I.S. McLintock and J.C. Orr, in: Chemistry and Physics of carbon, Vol. 1I, Ed. P.L. Walker, Jr. (Dekker, New York, 1973) p. 243. [35] A. Tomita and Y. Tamai, J. Catalysis 27 (1972) 293. [36] D.B. Fischbach, in: Chemistry and Physics of Carbon, Vol. 7, Ed. P.L. Walker, Jr. (Dekker, New York, 1971) ch. 1. [37] F.J. Derbyshire, A.E.B. Presland and D.L. Tnmm, Carbon 10 (1972) 114. [38] F.J. Derbyshire, A.E.B. Presland and D.L. Trimm, Carbon 13 (1975) 111. [39] S.B. Austermann, in: Chemistry and Physics of Carbon, Vol. 4, Ed. P.L. Walker, Jr. (Dekker, New York, 1968) p. 137. [40] E. Fitzer and B. Kegel, Carbon 6 (1968) 433. [41] J.C. Shelton. H.R. Patil and J.M. Blakely, Surface Sci. 43 (1974) 493. [42] M. Eizenberg and J.M. Blakely, Surface Sci. 82 (1979) 228. [43] M. Eizenberg and J.M. Blakely, J. Phys. Chem. 71 (1979) 3467.