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The morphology of platinum catalyst particles studied by transmission electron microscopy
Surface Science 185 (1987) IA59-L466 North-Holland, Amsterdam
L459
SURFACE SCIENCE LETTERS THE M O R P H O L O G Y OF PLATINUM CATALYST PARTICLES STUDIED BY TRANSMISSION ELECTRON MICROSCOPY P.J.F. HARRIS * Department of Physical Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EP, UK Received 18 February 1987; accepted for publication 12 March 1987
The effect of various heat treatments on the surface structure of supported platinum particles was studied using transmission electron microscopy. Heating in a vacuum furnace resulted in the formation of sharp (111) facets, probably as a consequence of carbon deposition, while heating in the presence of hydrogen sulphide induced (100) faceting. In contrast, heat treatments in hydrogen tended to reduce the degree of faceting, resulting in particles with almost circular profiles.
A more complete knowledge of the factors which control the structure of small metal particles could lead to a fuller understanding of the activity and selectivity of supported metal catalysts. Transmission electron microscopy (TEM) offers perhaps the best hope of achieving progress in this field since it is the only technique which can provide direct images of individual metal particles. Recent work has tended to concentrate on the application of high resolution lattice imaging to the study of metal particles (e.g. refs. [1-3]), but equally valuable results can be obtained using conventional diffraction contrast microscopy. In this study bright field TEM was used to investigate the effect of various heat treatments on the morphology of alumina-supported platinum particles. Lattice imaging of the metal particles was not attempted, but in one case it was possible to obtain a direct image of an ordered layer of adsorbate on a particle surface. The specimens used for this work were prepared by a recently developed technique based on the sol-gel process [4,5]. This technique enables thin films of platinum/alumina to be prepared using "wet chemistry" rather than vacuum deposition, thus providing a reliable model of a real catalyst. The freshly prepared specimens consisted of sheets of microporous ~/-alumina on which platinum particles with a mean diameter of 50 ,~ were supported. Most of the particles in the fresh catalyst were single crystals, and some of them appeared to be approximately octahedral in shape, indicating a degree of (111) * Present address: TI Research, Hinxton, Saffron Walden, Essex CB10 1RH, UK.
0039-6028/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L460
P.J.F. Harris / TEM of platinurn catalyst particles
200A i.ll
ii ii
Fig. 1. Single crystal platinum particles in specimens subjected to the following heat treatments: (a) 7 0 0 ° C i n a i r for 1 h; (b) 7 0 0 ° C i n air for ] h, t h e n g 0 0 ° C i n v a c u u m ( - 1 0 6 Torr) for 30 min; (c) 700 ° C in air for 1 h, then 500 o C in H 2S / H 2 for 16 h.
faceting. Previous work [4,5] has shown that heat treatments in one atmosphere of air at 600 ° C and above cause growth of the platinum particles, with some of the particles growing at greatly accelerated rates compared to the majority of the population. These fast-growing particles often exhibited very characteristic shapes, the most common of which appeared to be the triangular plate, and usually contained twin boundaries, while the smaller particles were mainly single crystals. As in the fresh catalyst, many of these single crystal particles could be described as rounded octahedra; an example is shown in fig. la. In the present work, specimens heated at 7 0 0 ° C in air for 1 h were subjected to various further heat treatments in order to investigate any changes in the morphology of the platinum particles which might be produced. For simplicity, attention was generally focused on the shapes of the single crystal particles. The microscope employed was a J E O L 200CX instrument, operated with an accelerating voltage of 200 kV. The first set of experiments, some of which have been described previously [6], involved heat treatments under a moderate vacuum. The specimens were placed in a vacuum furnace, which was pumped down to approximately 10 6 Torr and slowly heated to 900°C. After a period 10-30 minutes at this temperature the specimens were allowed to cool. The result of such heat treatments was to transform the morphology of many of the platinum particles, inducing the formation of sharp facets, as can be seen in fig. l b which shows a typical single crystal following vacuum heating. In the earlier study [6] it was suggested that the shapes of the faceted particles indicated the presence of both (111) and (100) surface planes, but a more detailed investigation has shown that most of the profiles observed can be explained in terms of exclusively (111) faceting. For example, the particle in fig. l b probably has a shape of the type sketched in fig. 2, in which all the faces are at 70.5 ° to each other. The most likely explanation for this strong particle faceting is that
P.J.F. Harris / TEM of platinum catalyst particles
L461
Fig. 2. Approximateshape of particle shown in fig. lb. carbon is deposited onto the platinum particles during heat treatment in t h e vacuum furnace, as a result of cracking of residual hydrocarbons, and that this stabilises the (111) planes of platinum. This would broadly agree with studies of single crystal platinum specimens [7,8] which have shown that most faces other than (111) tend to restructure in the presence of adsorbed carbon. Micrographs of the vacuum heated specimens often clearly showed the presence of a contaminating layer on the surfaces of the particles, as well as on parts of the support. In some cases it was possible to obtain lattice images of the adsorbed layer, indicating that carbon had deposited onto the platinum surface in an ordered manner. Fig. 3 shows a carbon-covered platinum particle in which lattice fringes with a spacing of 3.9 .~ can be seen. In general it appeared that the carbon fringes ran parallel with the platinum (100) planes, but the exact structure of the overlayer has not yet been determined. It is possible that the phenomenon of carbon-induced faceting could provide an explanation for the changes in catalytic selectivity which sometimes occur following carbon deposition [9]. Although the experiments described above involved the deposition of a thick layer of carbon onto the platinum surface, which would destroy all catalytic activity, it is known that faceting can be induced by relatively small amounts of adsorbate [10]. The result might be a change in the activity of the catalyst towards structure-sensitive reactions rather than complete deactivation. The second set of experiments was aimed at investigating the effect of adsorbed sulphur on particle morphology. Heat treatments were carried out in a mixture containing 100 vpm hydrogen sulphide in hydrogen, which was passed over the specimens in a controlled atmosphere furnace at a temperature of 500 ° C for 16 h. Again, the result was to induce strong faceting of many of the particles but in this case the particle shapes observed were indicative of (100) facets. Thus, single crystal particles in the specimens exposed to hydrogen sulphide frequently exhibited square or rectangular profiles; fig. lc shows
L462
P.J.F Harris / TEM of platinum catalyst particles
Fig. 3. Faceted platinum particle in vacuum-heated specimen showing presence of lattice fringes from adsorbed layer.
a typical example. It might be argued that profiles of this kind could arise from shapes such as pyramids or octahedra, and do not necessarily imply (100) faceting. However images of such particle shapes would be expected, at least in some cases, to display evidence of thickness fringes, and this type of contrast was not observed here. Moreover, studies of macroscopic metal surfaces have also indicated that sulphur adsorption stabilises (100) surfaces. Low energy electron diffraction studies by McCarroll et al. [11,12] showed that the (111) face of nickel reorients to (100) when exposed to hydrogen sulphide, while Schmidt and Luss using scanning electron microscopy [13] observed the formation of rectangular facets on the surface of platinum-rhodium gauzes treated with hydrogen sulphide. It is well known that sulphur poisoning, like carbon poisoning, can sometimes produce changes in selectivity without deactivating the catalyst completely [14]. Again, these changes might be explained in terms of adsorptioninduced faceting of the type observed here, a suggestion first made by Somorjai [15]. A fuller description of the sulphur poisoning experiments has been given elsewhere [16]. The final experiments involved heat treating the air-sintered specimens in flowing hydrogen at 400 °C for 70 h. In this case the effect of heat treatment was the opposite of that observed in the experiments described above: the particles tended to become less faceted, often developing almost circular profiles. This could be most clearly observed in the "abnormally large" particles, whose shapes were frequently highly anisotropic in the air-heated
P.J.F. Harris / TEM of platinum catalystparticles
L463
Fig. 4. Micrographs showing change in platinum particle shape induced by heat treatment in hydrogen: (a) specimen heated at 700 o C in air for 1 h; (b) heated at 400 o C in hydrogen for 70 h following a similar air heat treatment.
L464
P.J.F. Harris / T E M of platinum catalystparticles
catalysts. Fig. 4a shows a region of an air-heated specimen in which a number of the large particles can be seen, while fig. 4b shows the change to approximately spherical shapes that occurred following heat treatment in hydrogen. This rounding effect was also observed in some of the small single crystal particles but was less obvious. Wang et al. [17] have recently reported results which appear to conflict with the findings of the present study: they found that heating supported platinum particles in hydrogen produced a change from almost spherical shapes to morphologies indicative of (100) faceting. The reasons for the differences between the observations of these workers and the results reported here are not clear, but may be related to the different form of specimen used. When considering the structures of small metal particles it is essential that both kinetic and thermodynamic effects are taken into account. If the particles are growing rapidly their shapes are likely to be dependent on the mechanism of growth, and may be very different from the thermodynamically most stable forms. This is certainly the case when platinum/alumina specimens are heated in air, where twin-accelerated anisotropic growth leads to plate-like or needlelike shapes with very high surface area-to-volume ratios, as noted above. However, in the experiments reported here particle growth was relatively slow, so it is reasonable to assume that the shapes observed were determined largely by thermodynamic factors. The theory governing the equilibrium shapes of crystals was first developed by Wulff and is based on the assumption that such shapes must obey the relation (neglecting any metal-substrate interactions): E
SlY i = minimum,
all faces
where S, is the area and Vi the surface energy of face i. Wulff showed that a polyhedron satisfying this condition could be constructed by plotting the surface energy in polar coordinates, drawing planes tangential to the curve in directions normal to the major crystallographic directions and taking the inner envelope of these planes [10,18]. On the basis of the results reported here it is possible to infer the approximate form of the Wulff construction for platinum particles covered with carbon, sulphur and hydrogen. The vacuum heating experiments indicated that carbon deposition reduces the surface energy of the (111) planes more than that of any of the other planes, and this would suggest a Wulff plot of the form shown in fig. 5a. In three dimensions the plot would have the shape of a regular octahedron. Sulphur adsorption, on the other hand, apparently stabilises the (100) faces, so that the same section through the Wulff plot would have the form shown in fig. 5b. A (100) section would in this case be a square, and the overall shape cubic. The effect of exposure to hydrogen appears to be to reduce the anisotropy of surface energy, implying that the Wulff plot would approximate to a sphere, with many cusped minima at roughly the same distance from the origin.
P.J.F Harris / TEM of platinum catalyst particles
L465
<001> a.
i
b.
>
\
<1i0>
<110>
urface energy
Fig. 5. Approximate form of the Wulff plot [(110) section] for (a) carbon-coveredplatinum particle, (b) sulphur-coveredplatinum particle. An obvious problem with TEM studies of the kind described here is the possibility of specimen contamination in the microscope. Since TEM vacua are rarely better than 10 -7 Torr and almost invariably contain organic species originating from the pumping system, it is virtually inevitable that some carbonaceous material will build up on the specimen surface while in the microscope. The presence of this contamination, coupled with heating due to the incident electron beam, could lead to adsorbate-induced changes in surface structure; indeed "strong faceting" effects in small particles have occasionally been observed by the present author when catalyst specimens have been illuminated for long periods in microscopes with relatively dirty vacua. However, in the present study great care was taken to minimise contamination and beam heating, and there is no reason to suppose that any of the effects reported here were microscope-induced. Complete elimination of the contamination problem would require the use of an ultra-high vacuum TEM. The author thanks Professor J.M. Thomas for many discussions and Dr. M.J. Stowell (TI Research) for critically reading the manuscript. The work was supported by an SERC Postdoctoral Fellowship, and materials for specimen preparation were supplied by the Harwell Laboratory.
References
[1] L.D. Marks and D.J. Smith, Nature 303 (1983) 316. [2] J.-O. Bovin, R. Wallenbergand D.J. Smith, Nature 317 (1985) 47. [3] D.A. Jefferson, J.M. Thomas, G.R. Millward, K. Tsuno, A. Harriman and R.D. Brydson, Nature 323 (1986) 428.
L466 [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
P.J.F Harris / T E M of platinum catalyst particles
P.J.F. Harris, E.D. Boyes and J.A. Cairns, J. Catalysis 82 (1983) 127. P.J.F. Harris, J. Catalysis 97 (1986) 527. P.J.F. Harris, Appl. Catalysis 16 (1985) 439. B. Lang, Surface Sci. 53 (1975) 317. D.W. Blakely and G.A. Somorjai, Surface Sci. 65 (1977) 419. P.P. Lankhorst, H.C. De Jongste and V. Ponec, in: Catalyst Deactivation, Eds. B. Delmon and G.F. Froment (Elsevier, Amsterdam, 1980) p.43. M. Drechsler, in: NATO Advan. Sci. Inst. Ser., Ser. B, Vol. 86, Surface Mobilities on Solid Materials, Ed. Vu Thien Binh (Plenum, New York, 1983) p. 405. J.J. McCarroll, T. Edmonds and R.C. Pitkethly, Nature 223 (1969) 1260. J.J. McCarroll, Surface Sci. 53 (1975) 297. L.D. Schmidt and D. Luss, J. Catalysis 22 (1971) 269. C.H. Bartholomew, P.K. Agrawal and J.R. Katzer, Advan. Catalysis 31 (1982) 135. G.A. Somorjal, J. Catalysis 27 (1972) 453. P.J.F. Harris, Nature 323 (1986) 792. T. Wang, C. Lee and L.D. Schmidt, Surface Sci. 163 (1985) 181. G. Wulff, Z. Krist. 34 (1901) 449.
L459
SURFACE SCIENCE LETTERS THE M O R P H O L O G Y OF PLATINUM CATALYST PARTICLES STUDIED BY TRANSMISSION ELECTRON MICROSCOPY P.J.F. HARRIS * Department of Physical Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EP, UK Received 18 February 1987; accepted for publication 12 March 1987
The effect of various heat treatments on the surface structure of supported platinum particles was studied using transmission electron microscopy. Heating in a vacuum furnace resulted in the formation of sharp (111) facets, probably as a consequence of carbon deposition, while heating in the presence of hydrogen sulphide induced (100) faceting. In contrast, heat treatments in hydrogen tended to reduce the degree of faceting, resulting in particles with almost circular profiles.
A more complete knowledge of the factors which control the structure of small metal particles could lead to a fuller understanding of the activity and selectivity of supported metal catalysts. Transmission electron microscopy (TEM) offers perhaps the best hope of achieving progress in this field since it is the only technique which can provide direct images of individual metal particles. Recent work has tended to concentrate on the application of high resolution lattice imaging to the study of metal particles (e.g. refs. [1-3]), but equally valuable results can be obtained using conventional diffraction contrast microscopy. In this study bright field TEM was used to investigate the effect of various heat treatments on the morphology of alumina-supported platinum particles. Lattice imaging of the metal particles was not attempted, but in one case it was possible to obtain a direct image of an ordered layer of adsorbate on a particle surface. The specimens used for this work were prepared by a recently developed technique based on the sol-gel process [4,5]. This technique enables thin films of platinum/alumina to be prepared using "wet chemistry" rather than vacuum deposition, thus providing a reliable model of a real catalyst. The freshly prepared specimens consisted of sheets of microporous ~/-alumina on which platinum particles with a mean diameter of 50 ,~ were supported. Most of the particles in the fresh catalyst were single crystals, and some of them appeared to be approximately octahedral in shape, indicating a degree of (111) * Present address: TI Research, Hinxton, Saffron Walden, Essex CB10 1RH, UK.
0039-6028/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
L460
P.J.F. Harris / TEM of platinurn catalyst particles
200A i.ll
ii ii
Fig. 1. Single crystal platinum particles in specimens subjected to the following heat treatments: (a) 7 0 0 ° C i n a i r for 1 h; (b) 7 0 0 ° C i n air for ] h, t h e n g 0 0 ° C i n v a c u u m ( - 1 0 6 Torr) for 30 min; (c) 700 ° C in air for 1 h, then 500 o C in H 2S / H 2 for 16 h.
faceting. Previous work [4,5] has shown that heat treatments in one atmosphere of air at 600 ° C and above cause growth of the platinum particles, with some of the particles growing at greatly accelerated rates compared to the majority of the population. These fast-growing particles often exhibited very characteristic shapes, the most common of which appeared to be the triangular plate, and usually contained twin boundaries, while the smaller particles were mainly single crystals. As in the fresh catalyst, many of these single crystal particles could be described as rounded octahedra; an example is shown in fig. la. In the present work, specimens heated at 7 0 0 ° C in air for 1 h were subjected to various further heat treatments in order to investigate any changes in the morphology of the platinum particles which might be produced. For simplicity, attention was generally focused on the shapes of the single crystal particles. The microscope employed was a J E O L 200CX instrument, operated with an accelerating voltage of 200 kV. The first set of experiments, some of which have been described previously [6], involved heat treatments under a moderate vacuum. The specimens were placed in a vacuum furnace, which was pumped down to approximately 10 6 Torr and slowly heated to 900°C. After a period 10-30 minutes at this temperature the specimens were allowed to cool. The result of such heat treatments was to transform the morphology of many of the platinum particles, inducing the formation of sharp facets, as can be seen in fig. l b which shows a typical single crystal following vacuum heating. In the earlier study [6] it was suggested that the shapes of the faceted particles indicated the presence of both (111) and (100) surface planes, but a more detailed investigation has shown that most of the profiles observed can be explained in terms of exclusively (111) faceting. For example, the particle in fig. l b probably has a shape of the type sketched in fig. 2, in which all the faces are at 70.5 ° to each other. The most likely explanation for this strong particle faceting is that
P.J.F. Harris / TEM of platinum catalyst particles
L461
Fig. 2. Approximateshape of particle shown in fig. lb. carbon is deposited onto the platinum particles during heat treatment in t h e vacuum furnace, as a result of cracking of residual hydrocarbons, and that this stabilises the (111) planes of platinum. This would broadly agree with studies of single crystal platinum specimens [7,8] which have shown that most faces other than (111) tend to restructure in the presence of adsorbed carbon. Micrographs of the vacuum heated specimens often clearly showed the presence of a contaminating layer on the surfaces of the particles, as well as on parts of the support. In some cases it was possible to obtain lattice images of the adsorbed layer, indicating that carbon had deposited onto the platinum surface in an ordered manner. Fig. 3 shows a carbon-covered platinum particle in which lattice fringes with a spacing of 3.9 .~ can be seen. In general it appeared that the carbon fringes ran parallel with the platinum (100) planes, but the exact structure of the overlayer has not yet been determined. It is possible that the phenomenon of carbon-induced faceting could provide an explanation for the changes in catalytic selectivity which sometimes occur following carbon deposition [9]. Although the experiments described above involved the deposition of a thick layer of carbon onto the platinum surface, which would destroy all catalytic activity, it is known that faceting can be induced by relatively small amounts of adsorbate [10]. The result might be a change in the activity of the catalyst towards structure-sensitive reactions rather than complete deactivation. The second set of experiments was aimed at investigating the effect of adsorbed sulphur on particle morphology. Heat treatments were carried out in a mixture containing 100 vpm hydrogen sulphide in hydrogen, which was passed over the specimens in a controlled atmosphere furnace at a temperature of 500 ° C for 16 h. Again, the result was to induce strong faceting of many of the particles but in this case the particle shapes observed were indicative of (100) facets. Thus, single crystal particles in the specimens exposed to hydrogen sulphide frequently exhibited square or rectangular profiles; fig. lc shows
L462
P.J.F Harris / TEM of platinum catalyst particles
Fig. 3. Faceted platinum particle in vacuum-heated specimen showing presence of lattice fringes from adsorbed layer.
a typical example. It might be argued that profiles of this kind could arise from shapes such as pyramids or octahedra, and do not necessarily imply (100) faceting. However images of such particle shapes would be expected, at least in some cases, to display evidence of thickness fringes, and this type of contrast was not observed here. Moreover, studies of macroscopic metal surfaces have also indicated that sulphur adsorption stabilises (100) surfaces. Low energy electron diffraction studies by McCarroll et al. [11,12] showed that the (111) face of nickel reorients to (100) when exposed to hydrogen sulphide, while Schmidt and Luss using scanning electron microscopy [13] observed the formation of rectangular facets on the surface of platinum-rhodium gauzes treated with hydrogen sulphide. It is well known that sulphur poisoning, like carbon poisoning, can sometimes produce changes in selectivity without deactivating the catalyst completely [14]. Again, these changes might be explained in terms of adsorptioninduced faceting of the type observed here, a suggestion first made by Somorjai [15]. A fuller description of the sulphur poisoning experiments has been given elsewhere [16]. The final experiments involved heat treating the air-sintered specimens in flowing hydrogen at 400 °C for 70 h. In this case the effect of heat treatment was the opposite of that observed in the experiments described above: the particles tended to become less faceted, often developing almost circular profiles. This could be most clearly observed in the "abnormally large" particles, whose shapes were frequently highly anisotropic in the air-heated
P.J.F. Harris / TEM of platinum catalystparticles
L463
Fig. 4. Micrographs showing change in platinum particle shape induced by heat treatment in hydrogen: (a) specimen heated at 700 o C in air for 1 h; (b) heated at 400 o C in hydrogen for 70 h following a similar air heat treatment.
L464
P.J.F. Harris / T E M of platinum catalystparticles
catalysts. Fig. 4a shows a region of an air-heated specimen in which a number of the large particles can be seen, while fig. 4b shows the change to approximately spherical shapes that occurred following heat treatment in hydrogen. This rounding effect was also observed in some of the small single crystal particles but was less obvious. Wang et al. [17] have recently reported results which appear to conflict with the findings of the present study: they found that heating supported platinum particles in hydrogen produced a change from almost spherical shapes to morphologies indicative of (100) faceting. The reasons for the differences between the observations of these workers and the results reported here are not clear, but may be related to the different form of specimen used. When considering the structures of small metal particles it is essential that both kinetic and thermodynamic effects are taken into account. If the particles are growing rapidly their shapes are likely to be dependent on the mechanism of growth, and may be very different from the thermodynamically most stable forms. This is certainly the case when platinum/alumina specimens are heated in air, where twin-accelerated anisotropic growth leads to plate-like or needlelike shapes with very high surface area-to-volume ratios, as noted above. However, in the experiments reported here particle growth was relatively slow, so it is reasonable to assume that the shapes observed were determined largely by thermodynamic factors. The theory governing the equilibrium shapes of crystals was first developed by Wulff and is based on the assumption that such shapes must obey the relation (neglecting any metal-substrate interactions): E
SlY i = minimum,
all faces
where S, is the area and Vi the surface energy of face i. Wulff showed that a polyhedron satisfying this condition could be constructed by plotting the surface energy in polar coordinates, drawing planes tangential to the curve in directions normal to the major crystallographic directions and taking the inner envelope of these planes [10,18]. On the basis of the results reported here it is possible to infer the approximate form of the Wulff construction for platinum particles covered with carbon, sulphur and hydrogen. The vacuum heating experiments indicated that carbon deposition reduces the surface energy of the (111) planes more than that of any of the other planes, and this would suggest a Wulff plot of the form shown in fig. 5a. In three dimensions the plot would have the shape of a regular octahedron. Sulphur adsorption, on the other hand, apparently stabilises the (100) faces, so that the same section through the Wulff plot would have the form shown in fig. 5b. A (100) section would in this case be a square, and the overall shape cubic. The effect of exposure to hydrogen appears to be to reduce the anisotropy of surface energy, implying that the Wulff plot would approximate to a sphere, with many cusped minima at roughly the same distance from the origin.
P.J.F Harris / TEM of platinum catalyst particles
L465
<001> a.
i
b.
\
<1i0>
<110>
urface energy
Fig. 5. Approximate form of the Wulff plot [(110) section] for (a) carbon-coveredplatinum particle, (b) sulphur-coveredplatinum particle. An obvious problem with TEM studies of the kind described here is the possibility of specimen contamination in the microscope. Since TEM vacua are rarely better than 10 -7 Torr and almost invariably contain organic species originating from the pumping system, it is virtually inevitable that some carbonaceous material will build up on the specimen surface while in the microscope. The presence of this contamination, coupled with heating due to the incident electron beam, could lead to adsorbate-induced changes in surface structure; indeed "strong faceting" effects in small particles have occasionally been observed by the present author when catalyst specimens have been illuminated for long periods in microscopes with relatively dirty vacua. However, in the present study great care was taken to minimise contamination and beam heating, and there is no reason to suppose that any of the effects reported here were microscope-induced. Complete elimination of the contamination problem would require the use of an ultra-high vacuum TEM. The author thanks Professor J.M. Thomas for many discussions and Dr. M.J. Stowell (TI Research) for critically reading the manuscript. The work was supported by an SERC Postdoctoral Fellowship, and materials for specimen preparation were supplied by the Harwell Laboratory.
References
[1] L.D. Marks and D.J. Smith, Nature 303 (1983) 316. [2] J.-O. Bovin, R. Wallenbergand D.J. Smith, Nature 317 (1985) 47. [3] D.A. Jefferson, J.M. Thomas, G.R. Millward, K. Tsuno, A. Harriman and R.D. Brydson, Nature 323 (1986) 428.
L466 [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
P.J.F Harris / T E M of platinum catalyst particles
P.J.F. Harris, E.D. Boyes and J.A. Cairns, J. Catalysis 82 (1983) 127. P.J.F. Harris, J. Catalysis 97 (1986) 527. P.J.F. Harris, Appl. Catalysis 16 (1985) 439. B. Lang, Surface Sci. 53 (1975) 317. D.W. Blakely and G.A. Somorjai, Surface Sci. 65 (1977) 419. P.P. Lankhorst, H.C. De Jongste and V. Ponec, in: Catalyst Deactivation, Eds. B. Delmon and G.F. Froment (Elsevier, Amsterdam, 1980) p.43. M. Drechsler, in: NATO Advan. Sci. Inst. Ser., Ser. B, Vol. 86, Surface Mobilities on Solid Materials, Ed. Vu Thien Binh (Plenum, New York, 1983) p. 405. J.J. McCarroll, T. Edmonds and R.C. Pitkethly, Nature 223 (1969) 1260. J.J. McCarroll, Surface Sci. 53 (1975) 297. L.D. Schmidt and D. Luss, J. Catalysis 22 (1971) 269. C.H. Bartholomew, P.K. Agrawal and J.R. Katzer, Advan. Catalysis 31 (1982) 135. G.A. Somorjal, J. Catalysis 27 (1972) 453. P.J.F. Harris, Nature 323 (1986) 792. T. Wang, C. Lee and L.D. Schmidt, Surface Sci. 163 (1985) 181. G. Wulff, Z. Krist. 34 (1901) 449.