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Electron microscopy study of the interaction of Ni, Pd and Pt with carbon
Surface Science 227 (1990) 15-23 North-Holland
15
ELECTRON MICROSCOPY STUDY OF THE INTERACTION II. Interaction of palladium with amorphous carbon * Ryszard
LAMBER
Institute of Low Temperature
Nils JAEGER
and Structure Research, Polish Academy
and Giinter
Institut ftir Angewandte Received
of Sciences, P. 0. Box 937, 50-950 Wroclaw, Poland
SCHULZ-EKLOFF
und Physikalische
28 August 1989; accepted
A change incorporation top of the Pd graphitization
Chemie, Universitiit
for publication
Bremen, Postfach 330 440, D-2800 Bremen 33, Fed. Rep. of Germany
10 November
1989
in the lattice parameter of Pd following the evaporation of the metal onto a carbon support is interpreted by the of carbon atoms into the Pd lattice. The decomposition of this phase above 700 K leads to the deposition of carbon on crystallites or even to their encapsulation. At temperatures above 870 K larger Pd crystallites were found to catalyze the of amorphous carbon.
1. Introduction In a previous study it has been shown that highly dispersed nickel interacts strongly with an amorphous carbon substrate and catalyzes the conversion of amorphous to graphitic carbon at temperatures above 730 K [l]. A suggested mechanism involved the rupture of carbon-carbon bonds and the removal of carbon atoms from the bulk at temperatures as low as 730 K. Carbon atoms are dissolved in the nickel phase and subsequently precipitated as graphite. Extended studies on carbon-supported palladium particles have already been reported [2-61 with evidence for a strong interaction between palladium and the carbon support [2-51. Recently some interesting data on the penetration of carbon into palladium during the catalytic hydrogenation of acetylene [7,8] or during heating in an atmosphere of ethylene, acetylene or carbon monoxide [9,10] have been reported. It has been shown that an interstitial solid solution of carbon in palladium containing up to 15 at% of carbon can be formed [7-lo]. A neutron diffraction study showed
l
OF Ni, Pd AND Pt WITH CARBON
Work carried out at the University
0039~6028/90/$03.50 (North-Holland)
2. Experimental The experimental method and apparatus used were described in detail in a previous paper [l].
of Bremen.
0 Elsevier Science Publishers
that carbon atoms are incorporated in the octahedral sites of the Pd lattice [lo]. The lattice constant of the new phase is 0.39956 nm, which is 2.7% greater than the lattice constant of palladium [lo]. Ziemecki et al. [lo] reported that the Pd-C phase decomposes at 870 K in an inert atmosphere and at = 420 K in H, and 0,. Stachurski [8] observed the decomposition of the Pd-C phase at 610 K in an argon atmosphere or under vacuum and at 460 Kin H,. It was reported that at higher temperatures (1150 K) palladium is able to catalyze the graphitization of amorphous carbon [ll]. The present paper gives evidence (i) of a lattice parameter 2.8% larger compared to bulk Pd following the evaporation of Pd onto a carbon support, which is interpreted by the incorporation of carbon atoms into the Pd lattice, (ii) of the encapsulation of small Pd particles within the carbon support, and (iii) of the palladium-catalyzed graphitization of amorphous carbon.
B.V.
16
R. L.amber et al. / The interaction of Ni, Pd and Pt with carbon. II
Thin film carbon substrates were prepared by vacuum deposition from a carbon arc. Prior to the deposition of the metal EM grids covered with carbon substrates were placed in a resistanceheated tantalum sample holder and heat treated at 875 K for 1 h in vacuum. After the substrate had been cooled to 300 K, discontinuous films of Pd were deposited by vacuum evaporation from a resistance-heated tungsten boat. The temperature of the substrate increased to 320 K during the evaporation process. By controlling the amount of evaporated metal an average thickness of 0.4 nm of a nominal film could be reproducibly established. After the metal layers had been evaporated a thermal treatment of the Pd-C system in a vacuum lo-’ Pa was carried out. All specimens were examined in a Philips EM 420 T transmission electron microscope (TEM) and equipped with an energy dispersive X-ray analyzer (EDX) at 120 kV. The microscope was operated with the lowest possible illumination intensity to minimize an influence of the electron beam. In order to exclude the possibility of electron-beam-induced carbon deposition observations under high-flux conditions were performed. They did not show any evidence for deposition of carbon due to the electron beam. The absence of carbon deposition from residual hydrocarbons could also be confirmed in blank experiments with Pd on SiO,.
3. Results TEM pictures of the Pd-C specimen as obtained after metal evaporation and the following Table 1 Measured interplanar distances (d) parameter a0 = 0.38893 nm)
(hkl)
111 200 220 311 222
thermal treatment in vacuum in the temperature range 293-1100 K are shown in fig. 1. After evaporation small liquid-like particles of palladium are formed (fig. la). Electron diffraction analysis of the Pd-C specimen after evaporation showed the presence of diffraction rings characteristic for fee crystallites in random orientations (fig. 2a). From the diffraction patterns an average lattice constant of the particles a = 0.400 + 0.003 nm was obtained by calibration with Au reflections as an internal standard. (A small amount of Au was vacuum deposited onto part of the EM grid prior to the electron diffraction analysis. In some cases Au reflections from the grid material were used for calibration. During the calibration process the objective lens current was not changed). This result indicates a 2.8% increase of the lattice parameter of the Pd particles with respect to the bulk lattice parameter a, = 0.38893 nm (table 1). Heat treatment in vacuum at 500 K for 70 h resulted only in some sintering of the Pd particles (fig. lb). The lattice parameter Q = 0.397 f 0.003 nm obtained from the diffraction pattern shown in fig. 2b still clearly exceeds the Pd bulk lattice parameter. The lattice constant of the palladium crystallites approached the value reported for bulk palladium a = 0.389 nm (table 1) after heating at a temperature of 700 K for 72 h. A selected area diffraction pattern after this stage of thermal treatment is shown in fig. 2c. At the same time further sintering of the palladium particles was observed (fig. lc). Heating of the Pd-C specimen after the metal deposition to temperatures of 850 K or higher and then maintaining it at this temperature resulted in still greater changes in the
and corresponding lattice parameters (a) of carbon-supported
palladium (the Pd bulk lattice
Thermal treatment conditions After evaporation
500K
d (nm)
d (nm)
a (nm)
d (nm)
a (nm)
d (nm)
a (nm)
0.229 0.199 0.140 0.120 0.114
0.397 0.398 0.396 0.398 0.395
0.225 0.194 0.138 0.118 0.112
0.390 0.388 0.390 0.390 0.388
0.224 0.195 0.138 0.117 0.113
0.388 0.390 0.390 0.388 0.390
0.232 0.200 0.141 0.120 0.116
a (nm) 0.402 0.400 0.399 0.398 0.402
700K
1OOOK
R. timber
Fig. 1. Electron micrograph
et al. / The interuction of Ni, Pd and Pi with c&on.
II
of Pd on carbon substrate folfowing the evaporation of palladium (a) and subsequent at 500 K, 70 b; (c) 700 K, 72 h; (d) at 850 K, 48 h; (e) at 1000 K, 48 h.
morphology of the particles. In fig. Id an electron micrograph taken from a sample heated at 850 K for 48 h is shown. A number of large particles with sizes from 15 to 50 nm appeared. By means of dispersive X-ray analysis and the microdiffraction method these large particles have been identified as palladium_ Fig. 3 shows the EDX spectrum produced when the electron probe was focused on a large Pd particle.
17
beat treatment (b)
In order to recognize the location of the small and large Pd particles with respect to the carbon support, TEM experiments by systematic tilting of the specimens were carried out. Fig. 4 shows some of the electron micrographs taken from the same area of the Pd-C specimen tilted at different angles with respect to the electron beam. It is seen that in the case of the large Pd crystallites it is possible to tilt the specimen in such a way that a
18
Fig. 2. Selected area diffraction patterns after evaporation of palladium (a) and heat trsatment, (b) at 500 K, 70 h; (c) at 700 K, 72 b; (d) at 1000 K, 48 h.
%
z
Fig. 3. EDX spectrum obtained when the electron probe was focused on a large metal parMe. Au peaks are due to Au EM grid.
Pd particle is visible on the carbon surface in the picture plane (fig. 4b). The small Pd particles are covered by a substance with a weaker contrast, which is most probably carbon, or are visible just betow the carbon support surface. They seem to be encapsulated within the carbon support (figs. 4b, 4c, 4d). When the Pd-C specimens were heated at temperatures above 870 K a layering characteristic for graphitic carbon could be observed on the surface of some large Pd crystallites. When the temperature of the thermal treatment was raised to loo0 K the graphite skeletons around the large Pd particles increased in thickness and were clearly visible in low-magnification eiectron mi~ro~aphs (fig_ Ie). In the electron diffraction pattern shown in Fig. 2d two additional diffuse diffraction rings
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. II
19
single or twinned Pd crystals (fig. 6). The fact that microdiffraction patterns obtained from large Pd particles revealed in addition to the Pd reflections the presence of the fragments of the graphite diffraction rings (fig. 6) seems to support our interpretation concerning the 0.209 and 0.120 nm diffraction rings. Direct observations of the graphite layers on the Pd particles was possible by high-resolution electron microscopy. In fig. 7 three Pd particles covered by graphite layers are shown. In all cases the graphite layers outline the shape of the metal crystallites.
4. Discussion
Fig. 4. Palladium particles on the edge of the carbon support. (a) Specimen tilted 20 o clockwise - neither large nor small Pd particles are visible on the carbon surface in the picture plane. (b) Specimen tilted loo clockwise - a large Pd particle is visible on the carbon surface. (c) Specimen not tilted. (d) Specimen tilted 5O counter-clockwise. Arrows point towards small Pd particles embedded within the carbon support.
with d = 0.209 nm and d = 0.120 nm are observed due to graphitic carbon. To elucidate the origin of the additional diffraction rings, both large and small crystallites visible in fig. le were analyzed by means of micro- and nano-diffraction methods. Nanodiffraction analysis of small particles (4-5 nm) showed diffraction patterns typical for the fee Pd metal (fig. 5). Microdiffraction patterns produced by large crystallites can be also ascribed to
Changes of the lattice parameters of small metal particles with respect to the bulk values have already been reported [12-181. While in most cases a decrease of the lattice parameter has been observed [15-181, the 3% expansion of the lattice parameter of small Pd particles supported on MgO, Al,O,, mica and amorphous carbon has been reported [12-141. The results were explained by “a crystallographic effect, such as a change from icosahedral (or otherwise polyhedral or composite) structure to the fee crystal structure during particle growth” [14]. Recently Stachurski [7,8] and Ziemecki et al. [9,10] have reported that an interstitial solid solution of carbon in palladium can be formed when metallic palladium is exposed to a flow of carbon-containing gases at moderate temperatures. It is believed that the formation of the Pd-C phase is due to deposition of a carbonaceus overlayer, followed by an activated diffusion of carbon atoms through the metal lattice [lo]. Carbon atoms occupy the octahedral sites within the Pd lattice. As a result the lattice expands by about 2.7%. This change is visible in the X-ray diffraction pattern as a shift of the diffraction peaks towards lower values of 28. There are two possible explanations for the lattice expansion observed in our experiments. The first proposed by Heineman and Poppa [14] is a size effect due to a not fully developed fee structure of the small (l-2 nm) palladium particles. Another explanation could be the formation of a Pd-C phase like that
20
R. Lmnber et al. / The interaction of Ni, Pd and Pt with carbon. II
Fig. 5. Typical nanodiffraction patterns obtained from particles visible in fig. le. (a) A single Pd crystallite in the [OOl] orientation. (b) A single Pd crystallite in the [ill] orientation.
observed during heating of Pd in carbon-containing gases. There are some indications in support of the assumption that the lattice expansion observed in our experiments can be connected with the formation of a Pd-C phase. Firstly, the Pd particles in our experiments are in the 2-5 nm size
range (see figs. la and lb), while changes of the lattice parameter due to size effect are observed for pafladiurn particles in the 1-2 nm size range [14]. Palladium particles of 5 nm mean size are believed to have the bulk value of the lattice constant [14]. Secondly, we observed that the
Fig. 6. ~icr~iffrac~on patterns obtained from large pat-ticks visible in Eg. le. (a) The [Oll] Pd zone axis. {b) The ill2] Pd zone axis. Both rn~cr~ffract~o~ patterns reveal the presence of the fragments of the graphite diffraction rings.
R. L.amber et al. / The interaction of Ni, Pd and Pt with carbon. II
21
Fig. 7. High-magnification images of palladium particles after heating at 1000 K; (a) top view; (b, c) side views.
lattice parameter of palladium particles reached the bulk value after heating at = 700 K, which roughly corresponds to the decomposition temperature of the PdC& given as 610 K (vacuum and argon atmosphere) [8] and 870 K (inert atmosphere) [lo]. Thirdly, the extent of the lattice expansion observed corresponds to that reported for the Pd-C phase. The electron diffraction pattern shown in fig. 2a reveals a strong broadening of the diffraction rings. While some broadening of the diffraction lines is due to the small size of the Pd crystallites, the extent of it seems to indicate a higher degree
of disorder within the particle lattice. A similar very strong broadening of the diffraction lines of carbon-supported palladium was reported by Anton and Poppa [12]. It is known that when metal deposits on nonmetallic substrates are warmed up, structural defects present in the deposits after evaporation are removed, which leads to ordering of the metal lattice and sharper rings in the electron diffraction patterns [19]. Following the heat treatment of the Pd-C system in vacuum at a temperature of 500 K (fig. 2b) the diffraction lines are even broader compared to those obtained after Pd evaporation and a lattice parameter of a =
22
R. Lamber ei al. / The interaction of Ni, Pd and Pt with carbon. II
0.397 nm has been obtained. This observation is taken as further evidence for the formation of the Pd-C phase during Pd evaporation onto carbon support. Broad diffraction rings would indicate a high degree of disorder within the Pd-C lattice. Only after heating of the specimen at 700 K, which probably exceeds the temperature of the stability of the Pd-C phase, sharper diffraction rings were observed and the lattice parameter reached the value reported for bulk Pd. The decomposition of the Pd-C phase should lead to the precipitation of carbon onto the surface of the palladium particles. As a result the Pd crystallites are covered by carbon or even encapsulated in the carbon substrate (fig. 4), which prevents further sintering of the metal. Only a fraction of the metal crystallites is able to grow and form large globular palladium particles visible on the carbon surface (fig. 4). Growth of “abnormally large” metal particles in the supported metal catalysts has already been reported in the literature [20]. The mechanism by which they grow is not clear [20]. It was reported that chemical atmosphere in particular 0, or water vapor even at very low pressures strongly affects sintering of platinum crystallites supported on carbon and enabled the growth of large metal particles [21]. It is known that metal particles supported on carbon catalyze the gasification of the support by oxygen, hydrogen, water and carbon dioxide [22-241. Baker et al. [22] reported that the active particles in the process of catalytic oxidation of graphite were at a higher temperature than those which remained stationary on the surface 1221. They grew also much larger than inactive particles [22]. Taking into account the heterogeneity of the carbon surface and the fact that the vacuum in our experiments was not better than lop5 Pa, we cannot exclude the possibility of local, Pd-catalyzed gasification of the carbon support by rest gases like 9, H, or H,O present in the vacuum chamber. These processes could explain the growth of the large palladium particles observed in our experiments. It should be noted that the growth of such “abnormally large” Pd particles was not observed when more inert supports like SiO, were used [25]. Heat treatment of the Pd-C specimens at temperatures above 870 K led to the observation of a layering characteris-
tic for graphitic carbon around large Pd crystallites. The thickness of the graphite skeletons around the Pd particles was found to increase for thermal treatment at still higher temperatures (fig. le). That means that palladium like nickel is able to catalyze the graphitization of amorphous carbon. Nickel, however, seems to be a far more efficient catalyst for this process. It is believed that the graphitization process proceeds via a solution-precipitation mechanism [ll,l]. It should be noted that the small encapsulated Pd particles showed no evidence of activity in the process of the catalytic ~ap~t~ation of amorphous carbon. A reexa~nation of the results reported in ref. [l] for the Ni-C system revealed that the lattice parameter of the Ni crystallites following the evaporation exceeds the bulk value for Ni by about 3.4% and approaches the bulk value following the heat treatment at 670 K in vacuum. Since it has been suggested that in the case of Ni catalysts the formation of an interstitial solid solution Ni-C with the lattice constant a = 0.363 nm is possible [7], we tend to explain the increase of the lattice constant of the nickel supported on carbon also by the formation of a Ni-C phase.
5. Conclusions The study of highly dispersed palladium supported on carbon suggests that the metal strongly interacts with carbon leading to the incorporation of carbon atoms into the metal. During the evaporation of palladium onto the carbon substrate an interstitial solid solution Pd-C with a lattice parameter 2.8% larger than the bulk value is formed. Above a temperature of 700 K the Pd-C phase decomposes and the Pd crystallites are covered with or even encapsulated by carbon during heating at higher temperatures. Encapsulation of the Pd particles prevents further sintering but it was observed that a number of crystallites were able to sinter and form large globular Pd particles located on the carbon surface. The mechanism by which these large particles are formed is not quite clear but it seems to be connected with Pd catalyzed gasification of the carbon support by rest gases present in vacuum chamber during ther-
R. Lumber et al. / The interaction of Ni, Pd and Pi with carbon. II
ma1 treatment. Above 870 K these large palladium particles catalyze the conversion of amorphous to graphitic carbon. There was no evidence of the activity of the small encapsulated Pd particles in the graphitization process.
Acknowledgements We thank Drs. P. Tomaszewski and M. Wolcyrz for helpful discussions. R.L. gratefully acknowledges financial support by Volkswagenstiftung (AZ I/64 808).
References 111R. Lamber, N. Jaeger and G. Schulz-Ekloff, Surf. Sci. 197 (1988) 402.
121R.C. Baetzold, Surf. Sci. 36 (1972) 123.
131 J.F. Hamilton and P.C. Logel, Thin Solid Films 16 (1973) 49. [41 W.G. Egelhoff, Jr. and G.G. Tibbetts, Solid State Commun. 29 (1979) 53. PI Z. Bastl, 0. Pribyl and P. Mikusik, Czech. J. Phys. B 34 (1984) 981. 161 W.F. Maier, S.J. Chettle, R.S. Rai and G. Thomas, J. Am. Chem. Sot. 108 (1986) 2609. 171 J. Stachurski, Doctoral Thesis, Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw (1983).
23
181 J. Stachurski and A. Frackiewicz, J. Less-Common Met. 108 (1985) 249. 191 S.B. Ziemecki and G.A. Jones, J. Catal. 95 (1985) 621. WI S.B. Ziemecki, G.A. Jones, D.G. Swartzfager, R.L. Harlow and J. Faber, Jr., J. Am. Chem. Sot. 107 (1985) 4547. 1111 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. WI R. Anton and H. Poppa, in: 10th Int. Congr. on Electron Microscopy, Hamburg 1982, Vol. 2, pp. 509, 511. 1131 K. Heinemann, T. Osaka, H. Poppa and M. Avalos-Borja, J. Catal. 83 (1983) 61. P41 K. Heinemann and H. Poppa, Surf. Sci. 156 (1985) 265. WI F.W.C. Boswell, Proc. Phys. Sot. (London) A 64 (1951) 465. 1161 C.W. Mays, J.S. Vermaak and D. Kuhlmann-Wilsdorf, Surf. Sci. 12 (1968) 134. P71 H. Poppa, K. Heinemann and A.G. Elliot, J. Vat. Sci. Technol. 8 (1971) 471. WI C. Solliard and M. Flueli, Surf. Sci. 156 (1985) 487. I191 J.V. Sanders, in: Chemisorption and Reactions on Metallic Films, Vol. 1, Ed. J.R. Anderson (Academic Press, New York, 1971) ch. 1. PO1 P.J.F. Harris, E.D. Boyes and J.A. Cairns, J. Catal. 82 (1983) 127. WI Y.F. Chu and E. Ruckenstein, Surf. Sci. 67 (1977) 517. WI R.T.K. Baker, J.A. France, L. Rouse and R.J. Waite, J. Catal. 41 (1976) 22. v31 W.L. Holstein and M. Boudart, J. Catal. 72 (1981) 328. v41 W.L. Holstein and M. Boudart, J. Catal. 75 (1981) 337. WI R. Lamber, N. Jaeger and G. Schulz-Ekloff, J. Catal., in press.
15
ELECTRON MICROSCOPY STUDY OF THE INTERACTION II. Interaction of palladium with amorphous carbon * Ryszard
LAMBER
Institute of Low Temperature
Nils JAEGER
and Structure Research, Polish Academy
and Giinter
Institut ftir Angewandte Received
of Sciences, P. 0. Box 937, 50-950 Wroclaw, Poland
SCHULZ-EKLOFF
und Physikalische
28 August 1989; accepted
A change incorporation top of the Pd graphitization
Chemie, Universitiit
for publication
Bremen, Postfach 330 440, D-2800 Bremen 33, Fed. Rep. of Germany
10 November
1989
in the lattice parameter of Pd following the evaporation of the metal onto a carbon support is interpreted by the of carbon atoms into the Pd lattice. The decomposition of this phase above 700 K leads to the deposition of carbon on crystallites or even to their encapsulation. At temperatures above 870 K larger Pd crystallites were found to catalyze the of amorphous carbon.
1. Introduction In a previous study it has been shown that highly dispersed nickel interacts strongly with an amorphous carbon substrate and catalyzes the conversion of amorphous to graphitic carbon at temperatures above 730 K [l]. A suggested mechanism involved the rupture of carbon-carbon bonds and the removal of carbon atoms from the bulk at temperatures as low as 730 K. Carbon atoms are dissolved in the nickel phase and subsequently precipitated as graphite. Extended studies on carbon-supported palladium particles have already been reported [2-61 with evidence for a strong interaction between palladium and the carbon support [2-51. Recently some interesting data on the penetration of carbon into palladium during the catalytic hydrogenation of acetylene [7,8] or during heating in an atmosphere of ethylene, acetylene or carbon monoxide [9,10] have been reported. It has been shown that an interstitial solid solution of carbon in palladium containing up to 15 at% of carbon can be formed [7-lo]. A neutron diffraction study showed
l
OF Ni, Pd AND Pt WITH CARBON
Work carried out at the University
0039~6028/90/$03.50 (North-Holland)
2. Experimental The experimental method and apparatus used were described in detail in a previous paper [l].
of Bremen.
0 Elsevier Science Publishers
that carbon atoms are incorporated in the octahedral sites of the Pd lattice [lo]. The lattice constant of the new phase is 0.39956 nm, which is 2.7% greater than the lattice constant of palladium [lo]. Ziemecki et al. [lo] reported that the Pd-C phase decomposes at 870 K in an inert atmosphere and at = 420 K in H, and 0,. Stachurski [8] observed the decomposition of the Pd-C phase at 610 K in an argon atmosphere or under vacuum and at 460 Kin H,. It was reported that at higher temperatures (1150 K) palladium is able to catalyze the graphitization of amorphous carbon [ll]. The present paper gives evidence (i) of a lattice parameter 2.8% larger compared to bulk Pd following the evaporation of Pd onto a carbon support, which is interpreted by the incorporation of carbon atoms into the Pd lattice, (ii) of the encapsulation of small Pd particles within the carbon support, and (iii) of the palladium-catalyzed graphitization of amorphous carbon.
B.V.
16
R. L.amber et al. / The interaction of Ni, Pd and Pt with carbon. II
Thin film carbon substrates were prepared by vacuum deposition from a carbon arc. Prior to the deposition of the metal EM grids covered with carbon substrates were placed in a resistanceheated tantalum sample holder and heat treated at 875 K for 1 h in vacuum. After the substrate had been cooled to 300 K, discontinuous films of Pd were deposited by vacuum evaporation from a resistance-heated tungsten boat. The temperature of the substrate increased to 320 K during the evaporation process. By controlling the amount of evaporated metal an average thickness of 0.4 nm of a nominal film could be reproducibly established. After the metal layers had been evaporated a thermal treatment of the Pd-C system in a vacuum lo-’ Pa was carried out. All specimens were examined in a Philips EM 420 T transmission electron microscope (TEM) and equipped with an energy dispersive X-ray analyzer (EDX) at 120 kV. The microscope was operated with the lowest possible illumination intensity to minimize an influence of the electron beam. In order to exclude the possibility of electron-beam-induced carbon deposition observations under high-flux conditions were performed. They did not show any evidence for deposition of carbon due to the electron beam. The absence of carbon deposition from residual hydrocarbons could also be confirmed in blank experiments with Pd on SiO,.
3. Results TEM pictures of the Pd-C specimen as obtained after metal evaporation and the following Table 1 Measured interplanar distances (d) parameter a0 = 0.38893 nm)
(hkl)
111 200 220 311 222
thermal treatment in vacuum in the temperature range 293-1100 K are shown in fig. 1. After evaporation small liquid-like particles of palladium are formed (fig. la). Electron diffraction analysis of the Pd-C specimen after evaporation showed the presence of diffraction rings characteristic for fee crystallites in random orientations (fig. 2a). From the diffraction patterns an average lattice constant of the particles a = 0.400 + 0.003 nm was obtained by calibration with Au reflections as an internal standard. (A small amount of Au was vacuum deposited onto part of the EM grid prior to the electron diffraction analysis. In some cases Au reflections from the grid material were used for calibration. During the calibration process the objective lens current was not changed). This result indicates a 2.8% increase of the lattice parameter of the Pd particles with respect to the bulk lattice parameter a, = 0.38893 nm (table 1). Heat treatment in vacuum at 500 K for 70 h resulted only in some sintering of the Pd particles (fig. lb). The lattice parameter Q = 0.397 f 0.003 nm obtained from the diffraction pattern shown in fig. 2b still clearly exceeds the Pd bulk lattice parameter. The lattice constant of the palladium crystallites approached the value reported for bulk palladium a = 0.389 nm (table 1) after heating at a temperature of 700 K for 72 h. A selected area diffraction pattern after this stage of thermal treatment is shown in fig. 2c. At the same time further sintering of the palladium particles was observed (fig. lc). Heating of the Pd-C specimen after the metal deposition to temperatures of 850 K or higher and then maintaining it at this temperature resulted in still greater changes in the
and corresponding lattice parameters (a) of carbon-supported
palladium (the Pd bulk lattice
Thermal treatment conditions After evaporation
500K
d (nm)
d (nm)
a (nm)
d (nm)
a (nm)
d (nm)
a (nm)
0.229 0.199 0.140 0.120 0.114
0.397 0.398 0.396 0.398 0.395
0.225 0.194 0.138 0.118 0.112
0.390 0.388 0.390 0.390 0.388
0.224 0.195 0.138 0.117 0.113
0.388 0.390 0.390 0.388 0.390
0.232 0.200 0.141 0.120 0.116
a (nm) 0.402 0.400 0.399 0.398 0.402
700K
1OOOK
R. timber
Fig. 1. Electron micrograph
et al. / The interuction of Ni, Pd and Pi with c&on.
II
of Pd on carbon substrate folfowing the evaporation of palladium (a) and subsequent at 500 K, 70 b; (c) 700 K, 72 h; (d) at 850 K, 48 h; (e) at 1000 K, 48 h.
morphology of the particles. In fig. Id an electron micrograph taken from a sample heated at 850 K for 48 h is shown. A number of large particles with sizes from 15 to 50 nm appeared. By means of dispersive X-ray analysis and the microdiffraction method these large particles have been identified as palladium_ Fig. 3 shows the EDX spectrum produced when the electron probe was focused on a large Pd particle.
17
beat treatment (b)
In order to recognize the location of the small and large Pd particles with respect to the carbon support, TEM experiments by systematic tilting of the specimens were carried out. Fig. 4 shows some of the electron micrographs taken from the same area of the Pd-C specimen tilted at different angles with respect to the electron beam. It is seen that in the case of the large Pd crystallites it is possible to tilt the specimen in such a way that a
18
Fig. 2. Selected area diffraction patterns after evaporation of palladium (a) and heat trsatment, (b) at 500 K, 70 h; (c) at 700 K, 72 b; (d) at 1000 K, 48 h.
%
z
Fig. 3. EDX spectrum obtained when the electron probe was focused on a large metal parMe. Au peaks are due to Au EM grid.
Pd particle is visible on the carbon surface in the picture plane (fig. 4b). The small Pd particles are covered by a substance with a weaker contrast, which is most probably carbon, or are visible just betow the carbon support surface. They seem to be encapsulated within the carbon support (figs. 4b, 4c, 4d). When the Pd-C specimens were heated at temperatures above 870 K a layering characteristic for graphitic carbon could be observed on the surface of some large Pd crystallites. When the temperature of the thermal treatment was raised to loo0 K the graphite skeletons around the large Pd particles increased in thickness and were clearly visible in low-magnification eiectron mi~ro~aphs (fig_ Ie). In the electron diffraction pattern shown in Fig. 2d two additional diffuse diffraction rings
R. Lamber et al. / The interaction of Ni, Pd and Pt with carbon. II
19
single or twinned Pd crystals (fig. 6). The fact that microdiffraction patterns obtained from large Pd particles revealed in addition to the Pd reflections the presence of the fragments of the graphite diffraction rings (fig. 6) seems to support our interpretation concerning the 0.209 and 0.120 nm diffraction rings. Direct observations of the graphite layers on the Pd particles was possible by high-resolution electron microscopy. In fig. 7 three Pd particles covered by graphite layers are shown. In all cases the graphite layers outline the shape of the metal crystallites.
4. Discussion
Fig. 4. Palladium particles on the edge of the carbon support. (a) Specimen tilted 20 o clockwise - neither large nor small Pd particles are visible on the carbon surface in the picture plane. (b) Specimen tilted loo clockwise - a large Pd particle is visible on the carbon surface. (c) Specimen not tilted. (d) Specimen tilted 5O counter-clockwise. Arrows point towards small Pd particles embedded within the carbon support.
with d = 0.209 nm and d = 0.120 nm are observed due to graphitic carbon. To elucidate the origin of the additional diffraction rings, both large and small crystallites visible in fig. le were analyzed by means of micro- and nano-diffraction methods. Nanodiffraction analysis of small particles (4-5 nm) showed diffraction patterns typical for the fee Pd metal (fig. 5). Microdiffraction patterns produced by large crystallites can be also ascribed to
Changes of the lattice parameters of small metal particles with respect to the bulk values have already been reported [12-181. While in most cases a decrease of the lattice parameter has been observed [15-181, the 3% expansion of the lattice parameter of small Pd particles supported on MgO, Al,O,, mica and amorphous carbon has been reported [12-141. The results were explained by “a crystallographic effect, such as a change from icosahedral (or otherwise polyhedral or composite) structure to the fee crystal structure during particle growth” [14]. Recently Stachurski [7,8] and Ziemecki et al. [9,10] have reported that an interstitial solid solution of carbon in palladium can be formed when metallic palladium is exposed to a flow of carbon-containing gases at moderate temperatures. It is believed that the formation of the Pd-C phase is due to deposition of a carbonaceus overlayer, followed by an activated diffusion of carbon atoms through the metal lattice [lo]. Carbon atoms occupy the octahedral sites within the Pd lattice. As a result the lattice expands by about 2.7%. This change is visible in the X-ray diffraction pattern as a shift of the diffraction peaks towards lower values of 28. There are two possible explanations for the lattice expansion observed in our experiments. The first proposed by Heineman and Poppa [14] is a size effect due to a not fully developed fee structure of the small (l-2 nm) palladium particles. Another explanation could be the formation of a Pd-C phase like that
20
R. Lmnber et al. / The interaction of Ni, Pd and Pt with carbon. II
Fig. 5. Typical nanodiffraction patterns obtained from particles visible in fig. le. (a) A single Pd crystallite in the [OOl] orientation. (b) A single Pd crystallite in the [ill] orientation.
observed during heating of Pd in carbon-containing gases. There are some indications in support of the assumption that the lattice expansion observed in our experiments can be connected with the formation of a Pd-C phase. Firstly, the Pd particles in our experiments are in the 2-5 nm size
range (see figs. la and lb), while changes of the lattice parameter due to size effect are observed for pafladiurn particles in the 1-2 nm size range [14]. Palladium particles of 5 nm mean size are believed to have the bulk value of the lattice constant [14]. Secondly, we observed that the
Fig. 6. ~icr~iffrac~on patterns obtained from large pat-ticks visible in Eg. le. (a) The [Oll] Pd zone axis. {b) The ill2] Pd zone axis. Both rn~cr~ffract~o~ patterns reveal the presence of the fragments of the graphite diffraction rings.
R. L.amber et al. / The interaction of Ni, Pd and Pt with carbon. II
21
Fig. 7. High-magnification images of palladium particles after heating at 1000 K; (a) top view; (b, c) side views.
lattice parameter of palladium particles reached the bulk value after heating at = 700 K, which roughly corresponds to the decomposition temperature of the PdC& given as 610 K (vacuum and argon atmosphere) [8] and 870 K (inert atmosphere) [lo]. Thirdly, the extent of the lattice expansion observed corresponds to that reported for the Pd-C phase. The electron diffraction pattern shown in fig. 2a reveals a strong broadening of the diffraction rings. While some broadening of the diffraction lines is due to the small size of the Pd crystallites, the extent of it seems to indicate a higher degree
of disorder within the particle lattice. A similar very strong broadening of the diffraction lines of carbon-supported palladium was reported by Anton and Poppa [12]. It is known that when metal deposits on nonmetallic substrates are warmed up, structural defects present in the deposits after evaporation are removed, which leads to ordering of the metal lattice and sharper rings in the electron diffraction patterns [19]. Following the heat treatment of the Pd-C system in vacuum at a temperature of 500 K (fig. 2b) the diffraction lines are even broader compared to those obtained after Pd evaporation and a lattice parameter of a =
22
R. Lamber ei al. / The interaction of Ni, Pd and Pt with carbon. II
0.397 nm has been obtained. This observation is taken as further evidence for the formation of the Pd-C phase during Pd evaporation onto carbon support. Broad diffraction rings would indicate a high degree of disorder within the Pd-C lattice. Only after heating of the specimen at 700 K, which probably exceeds the temperature of the stability of the Pd-C phase, sharper diffraction rings were observed and the lattice parameter reached the value reported for bulk Pd. The decomposition of the Pd-C phase should lead to the precipitation of carbon onto the surface of the palladium particles. As a result the Pd crystallites are covered by carbon or even encapsulated in the carbon substrate (fig. 4), which prevents further sintering of the metal. Only a fraction of the metal crystallites is able to grow and form large globular palladium particles visible on the carbon surface (fig. 4). Growth of “abnormally large” metal particles in the supported metal catalysts has already been reported in the literature [20]. The mechanism by which they grow is not clear [20]. It was reported that chemical atmosphere in particular 0, or water vapor even at very low pressures strongly affects sintering of platinum crystallites supported on carbon and enabled the growth of large metal particles [21]. It is known that metal particles supported on carbon catalyze the gasification of the support by oxygen, hydrogen, water and carbon dioxide [22-241. Baker et al. [22] reported that the active particles in the process of catalytic oxidation of graphite were at a higher temperature than those which remained stationary on the surface 1221. They grew also much larger than inactive particles [22]. Taking into account the heterogeneity of the carbon surface and the fact that the vacuum in our experiments was not better than lop5 Pa, we cannot exclude the possibility of local, Pd-catalyzed gasification of the carbon support by rest gases like 9, H, or H,O present in the vacuum chamber. These processes could explain the growth of the large palladium particles observed in our experiments. It should be noted that the growth of such “abnormally large” Pd particles was not observed when more inert supports like SiO, were used [25]. Heat treatment of the Pd-C specimens at temperatures above 870 K led to the observation of a layering characteris-
tic for graphitic carbon around large Pd crystallites. The thickness of the graphite skeletons around the Pd particles was found to increase for thermal treatment at still higher temperatures (fig. le). That means that palladium like nickel is able to catalyze the graphitization of amorphous carbon. Nickel, however, seems to be a far more efficient catalyst for this process. It is believed that the graphitization process proceeds via a solution-precipitation mechanism [ll,l]. It should be noted that the small encapsulated Pd particles showed no evidence of activity in the process of the catalytic ~ap~t~ation of amorphous carbon. A reexa~nation of the results reported in ref. [l] for the Ni-C system revealed that the lattice parameter of the Ni crystallites following the evaporation exceeds the bulk value for Ni by about 3.4% and approaches the bulk value following the heat treatment at 670 K in vacuum. Since it has been suggested that in the case of Ni catalysts the formation of an interstitial solid solution Ni-C with the lattice constant a = 0.363 nm is possible [7], we tend to explain the increase of the lattice constant of the nickel supported on carbon also by the formation of a Ni-C phase.
5. Conclusions The study of highly dispersed palladium supported on carbon suggests that the metal strongly interacts with carbon leading to the incorporation of carbon atoms into the metal. During the evaporation of palladium onto the carbon substrate an interstitial solid solution Pd-C with a lattice parameter 2.8% larger than the bulk value is formed. Above a temperature of 700 K the Pd-C phase decomposes and the Pd crystallites are covered with or even encapsulated by carbon during heating at higher temperatures. Encapsulation of the Pd particles prevents further sintering but it was observed that a number of crystallites were able to sinter and form large globular Pd particles located on the carbon surface. The mechanism by which these large particles are formed is not quite clear but it seems to be connected with Pd catalyzed gasification of the carbon support by rest gases present in vacuum chamber during ther-
R. Lumber et al. / The interaction of Ni, Pd and Pi with carbon. II
ma1 treatment. Above 870 K these large palladium particles catalyze the conversion of amorphous to graphitic carbon. There was no evidence of the activity of the small encapsulated Pd particles in the graphitization process.
Acknowledgements We thank Drs. P. Tomaszewski and M. Wolcyrz for helpful discussions. R.L. gratefully acknowledges financial support by Volkswagenstiftung (AZ I/64 808).
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