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High-resolution profile imaging studies of sulphur overlayers on platinum {100} surfaces
132
Surface
Science 23X (1990) 1322141 North-Holland
High-resolution profile imaging on platinum { lOO} surfaces
studies of sulphur
overlayers
J. Uppenbrink, A.I. Kirkland, D. Tang and D.A. Jefferson * Depurtment of Chemistry, Uniuers+ of Camhrrdge, Lensfield Roud, Camhrrdge CB2 IE W. UK Received
8 January
1990; accepted
for publication
23 April 1990
Previous investigations have shown that an ordered monolayer of sulphur on the (100) surfaces of small platinum particles can be observed directly by high-resolution electron microscopy. This study presents the results of extensive image simulations of different possible { 100) overlayer structures and adsorbtion sites for both (100) and (110) oriented particles, the results of this confirming the sulphur present in the experimental images as a c(2 X 2) overlayer with a four-fold adsorbate site symmetry.
1. Introduction The catalytic activity of metal catalysts can be strongly modified by the presence of surface adsorbates. In particular the strong interaction of sulphur with metal surfaces often leads to the poisoning of such catalysts [l-3]. To date, the structural consequences of sulphur adsorption on metal surfaces has been investigated mainly by means of model single crystal studies under UHV conditions [4-61, with important information on overlayer symmetries and on surface reconstructions having been obtained. However the properties of real catalysts differ substantially from the extended bulk samples used in single crystal studies, and it is therefore desirable to study also surface effects on small metal particles. Using high-resolution electron microscopy (HREM) in the profile imaging mode [7,8], it is possible to investigate the structural modification of small metal particles in specimens bearing a closer resemblance to commercial catalysts than has previously been possible. Previous work has shown that the treatment of platinum particles with low concentrations of H,S induces (100) facetting [l] and the concomitant formation of an ordered overlayer of sulphur atoms [2]. This overlayer has been identified as * To whom correspondence 0039-6028/90/$03.50
should
be addressed.
(i: 1990 - Elsevier Science Publishers
having a c(2 X 2)s structure from a consideration of the observed doubled superlattice spacing at the crystal edges and confirmed by means of image simulations based on a quasi-periodic unit cell. However the question of the site symmetry of individual adsorbate atoms has not been addressed. In this study high-resolution profile images of sulphur overlayers on small (100) facetted platinum particles in both (100) and (110) orientations have been obtained and both the overlayer geometry and the adsorption site symmetry are investigated using multislice simulations of model particles.
2. Experimental Specimens were prepared directly on a microscope grid. An aqueous alumina solution (2.5 g/8) was mixed with an aqueous solution of H,PtCl, (4.0 g/L’) to achieve a Pt : Al,O, ratio of 1 : 15 by weight. A drop of this mixture was placed on a grid, allowed to dry and then fired for l/2 h at 600” C in air. For the sulphur treatment the furnace was flushed with H,, heated to 490” C and held at this temperature in an atmosphere of 100 ppm H,S in H, for 20 h. The H,S gas flow was then removed, and the specimens were allowed to cool under H, flow. In a control experiment, the
B.V. (North-Holland)
J. Uppenbrink ef al. / HREM of suiphur overlayers on platinum
{IOO) surfaces
133
A x 50 A, supported on a computer generated background of amorphous A1,03 40 A in thickness. For the construction of the amorphous background, the atomic coordinates were generated on a purely random basis but were then tested to ascertain that they were in accordance with known limits of Al-Al and Al-O distances [11,12]: Those which did not satisfy these criteria were rejected. In this way a reasonably realistic amorphous support was generated, although no weight was given to the actual coordination symmetry. Both particle and background were sliced perpendicular to the incident electron beam with slice thicknesses and numbers of slices being given in table 1. The supercell was sampled on a 512 x 512 square array giving a sampling interval of 0.098 A per point, in order to eliminate artifacts due to an insufficiently small sampling interval [13]. For the actual image calculation the electron-optical parameters applicable to our microscope were used, and a Gaussian focal spread of 57 A (width at half height) was incorporated as an envelope function to limit the final image resolution to effectively - 1.75 A. Comparative simulations, in which the focal spread was incorporated as a numerical summation of 20 images, showed no differences when compared to those calculated using the envelope function, and therefore the envelope method was retained for ease of computation. Beam divergence was not included in the simulations, since the results of a similar trial calculation indicated that its effect
same thermal conditions were applied, with pure H, used throughout. The specimens were examined using a modified JEOL-200CX electron microscope (C, = 0.52 mm, C, = 1.05 mm) [9] operated at 200 kV, with an interpretable point-to-point resolution of 1.95 A as defined by the first zero in the phase contrast transfer function at a defocus slightly greater than the optimum and an absolute information limit of - 1.7 A. Micrographs were recorded at electron-optical magnifications of either - 330 000 or 475 000 X , For each group of particles a series of micrographs was recorded with focal increments of - 150 A between each. Care was taken to ensure the correct alignment of the illumination system [IO], to avoid the introduction of contrast artifacts in the image due to tilt of the incident beam. During electron beam irradiation neither thermally induced rearrangements of the particles nor noticeable carbon conta~nation were noted.
3. Image simulations Image simulations were carried out using a modified multislice routine available for the simulation of complete particles [ll]. The model particles were generated by truncation of a bulk fee supercell on six (100) surfaces, and were contained in a supercell with lateral dimensions of 50
Table 1 Slice parameters, overlayer geometries and site symmetries of sulphided platinum particles used in the simulations referred to in the text Overlayer
Adsorption site
None
c(2 x 2) P(2 X 2) c(2 x 2) PC2X 2) 42X2) PC2X 2) None c(2 X 2) PC2X 2)
Top TOP 2-fold 2-fold 4-fold /I-fold 4-fold 4-fold
Total number of atoms af
Total number of slices
(A)
970 970 1523 1320 1320 970 1523 970 1523 1523
13 14 14 14 13 14 27 27 27
1.8974 1.8974 2.0157 2.1057 2.1057 1.8974 2.1057 1.3870 1.3870 1.3870
Slice thickness
a) Refers to the number of platinum atoms in the particle, excluding the overlayer atoms.
Orientation
134
J. Uppenbrink et al. / HREM
ofsulphur
was merely to lower the image contrast slightly without removing any of the salient features. For each model one series of images was computed for
ouerlayers on platinum
(100) surfaces
defoci of 0 to -1400 A (underfocus) with increments of 200 A between each and another for defoci of 0 to -800 A with 100 A increments.
images of platinum particles supported on amorphous Fig. 1. Low-resolution and (100) facetting; (b) after treatment in 100 ppm H,S/H,
Al,O,: (a) after treatment in H, showing showing pronounced (100) facetting.
both
{ 111)
J. Uppenbrink et al. / HREM
ofsulphuroverlayers
on platinum (100) surfaces
135
4. Results and discussion Low-resolution TEM shows that the particles treated with H,S (size range 50-100 A) exhibit pronounced (100) facetting, when compared to those treated with pure H, (figs. la and lb). Subsequent profile high-resolution images show surface details in the form of a castellation with a repeat of - 3.9 A at the vertical edges of the particles. Images of cubic particles in a (100) orientation are shown in figs. 2a, 2b and 2c. This surface feature was visible over the entire range of
Fig. 3. Typical high-resolution images of unsulphided platinum particles (pure hydrogen treatment). No overlayers or strong {100) facetting can be observed.
Fig. 2. Typical high-resolution images of sulphided platinum particles: (a), (b), (c) (100) incidence with clear evidence for contrast due to an ordered overlayer at the particle edges; (d) (110) incidence.
defoci recorded experimentally and did not change in appearance during the observation period. Similarly an image of a cubic particle in a (110) orientation is shown in fig. 2d, and no distinctive surface features being observed. With pure H,, no overlayers were observed in high-resolution images (fig. 3) and the particles exhibited both (100) and {ill} facets, in accord with previous HREM studies on small platinum particles [14,15]. The multiply twinned forms frequently observed for other fee metals were almost entirely absent in the samples examined, again in agreement with results from other workers [16]. Previous single-crystal studies [4-61 and investigations of sulphided platinum particles [1,2] have shown that the two overlayers formed on Pt{lOO} are the c(2 x 2)s and the p(2 x 2)s overlayers (fig. 4). For each overlayer, three different individual adatom adsorption sites have to be considered, namely the top site (figs. 4b and 4e) where each sulphur atom is located immediately above a platinum atom on the surface, and the four-fold (figs. 4d and 4g) and two-fold sites (figs. 4c and 4f), where each sulphur atom is located above the spaces between four and two platinum surface atoms respectively.
136
.I. l/ppenhnnk
et 111./ HREM
of sulphurouerluyers on plarinum
(100)
.m+~e.~
392;i
1.96 A=
(a)
(e Fig. 4. Schematic diagrams of the model particles used in the image simulations for a (100) orientation with the relevant bulk and surface periodicities indicated: (a) no overlayer; (b. c, d) ~(2 x 2) overlayer; (e, f, g) p(2 x 2) overlayer; (b, e) top site; (c, f) two-fold site; (d, g) four-fold site.
Fig. 5. Simulated
images corresponding to the models of fig. 3 for a defocus of -400 A in all cases: (a) no overlayer: overlayer; (e, f, g) p(2 X 2) overlayer; (b, e) top site; (c, f) two-fold site; (d, g) four-fold site.
(b, c, d) ~$2 x 2)
J. Uppenbrink
et al. / HREM
of sulphur overlayers on platinum
To distinguish between the p(2 x 2) and the c(2 x 2) overlayer symmetries, both the relative contrast of the adatoms for (100) oriented particles and the overlayer periodicities for (110) oriented particles have been used. Simulations for the p(2 x 2)s overlayer on (100) oriented particles show fainter contrast due to the overlayer when compared to those for the c(2 x 2)s overlayer (fig. 5) and comparison with the experimental image of fig. 2a suggests that the latter geometry prevails in practice. In addition, the c(2 X 2) overlayers are clearly visible over the focal range from ca. - 200 to - 500 A, whereas the p(2 x 2) overlayers are only clearly visible at a defocus of ca. -400 A. Therefore both the magnitude of the contrast and its focal dependence may be used as diagnostic features to distinguish between the two geometries. However some care must be exercised in quantifying these results precisely, since the par-
(100) surfaces
137
ticle size in the simulations differs substantially from that of the experimental images. Differences in the simulated image contrast for different adsorption sites are also evident for (100) oriented particles (fig. 5). In the case of the top site, the simulated image shows a pronounced white fringe between the particle surface and the sulphur atoms, which is absent for both the twofold and the four-fold site. Furthermore for the two-fold site the sulphur atoms appear out of register with the positions of the platinum atoms in the particle. The simulated images show the same qualitative features for two different sulphur radii applied (fig. 6) one of them being the same as the platinum radius (1.3870 A) and the other the covalent sulphur radius (1.02 A). In this context it should be noted that the value of the Pt-S bond distance in the case of a monolayer of sulphur on platinum { lOO} is not precisely known,
Fig. 6. Simulated images for (100) incidence calculated for two different values of the Pt-S bond length for the c(2 x 2) overlayer and for a defocus of -400 A in all cases: (a, d) top site; (b, e) two-fold site; (c, f) four-fold site. (a, b, c) r(S) = 1.3870 A; (d. e. f) r(S) = 1.02 A.
138
J. Uppenbrmk et al. / HREM of suiphur ouerlayers on platinum (100) surfaces
although the bond is believed to be covalent [3,6]. The effect of the inclusion of either beam divergence up to a value of 0.75 mrad or an increased focal spread of 86 A does not remove the white fringe observed for the top site but only diminishes its contrast. Given these results it is clear from an examination of the experimental images of fig. 2a that the four-fold site symmetry with a c(2 x 2) geometry gives the best experimental/ calculated image match. To distinguish more clearly between the p(2 x 2) and the c(2 x 2) geometries, images of a particle in a (110) orientation were simulated for the case of four-fold adsorbate site symmetry only. The model particles used are shown in fig. 7 with the different periodicities for the two overlayers marked, and fig. 8 shows simulated images corresponding to fig. 7. From fig. 8 it is clear that the p(2 X 2) overlayer gives rise to a doubled repeat at the particle edges, whereas the c(2 x 2) overlayer has
the same spacing as the particle and can thus not be distinguished from the fringe contrast due to the particle; hence this projection may be used to distinguish the two different overlayer structures. The experimental image in fig. 2d does not show the doubled repeat of the p(2 x 2) overlayer and can thus be interpreted as having a c(2 x 2) overlayer symmetry in accordance with the results of the (100) orientation. Thus far only the surface detail in profile at the particle edges has been considered. However a (2 X 2) superlattice in the bulk of the particle is also often observed. Such a superlattice may arise either from the sulphur overlayer or alternatively may be due to formally forbidden fee reflections, which have been observed previously both in small particles [17,18] and in extended thin films [19,20]. This effect is illustrated in fig. 9, where the normalised intensity of the f (422) superlattice beam is plotted as a function of the number of (100) planes for (100) incidence with the normalised
2-26i .
I-96A
a
452A \\
3.92A-
Fig. 7. Schematic diagrams of the model particles used in the image simulations for a (110) orientation with the relevant bulk and surface periodicities indicated: (a) no overlayer; (b) c(2 X 2) overlayer, four-fold site; (c) p(2 X 2) overlayer. four-fold site.
J. Uppe~brink et al. / HREM
ofsulphurouerlayers on platinum
(100) surfaces
139
I-96W
a
b Fig. 8. Simulated
images
corresponding
c
to fig. 7 for a defocus of -400 ,k in all cases: (a) no overlayer; four-fold site; (c) p(2 X 2) overlayer, four-fold site.
intensity of the (000) and (200) beams plotted for reference. From these calculations it is apparent that the intensity of this formally forbidden beam oscillates with a period of two (100) planes with minima for even numbers of layers. However for the case of sulphided platinum particles a further complication arises in that an overlayer of sulphur atoms may also give rise to a bulk superlattice which will be indistinguishable from that due to the formally forbidden reflections. From a series of simulations of model particles calculated for both even and odd numbers of { lOO} planes in the
(b) ~(2 X 2) 0%rerlayer,
presence and absence of sulphur overlayers it becomes apparent that the effect of forbidden reflection contrast due to the odd numbers of {IOO} planes is far greater than that due to the sulphur overlayers, as might be expected from consideration of their scattering factors. Hence any superlattice observed in the bulk of the particle cannot be used reliably to give information about either the presence or the geometry of the overlayer, unless the particle dimension parallel to the incident beam is known precisely, a requirement that is almost impossible to meet in practice. In orien-
140
J. Uppenhrink et al. / HREM
ofsulphur
overlayers on plarinum
(100) surfaces
5. Conclusions
r
r z1
?.I,
/
F L 7
1.6
d
2 2
I.2
7
2 2
0.X
e
0.4
2 6 ‘W
2 r(
7
2 7
n
10
XI
2” humher
uf
10
slice
Fig. 9. Normalised diffracted intensities of the (a) (000). (b) (200) and (c) 4 (422) beams. plotted as a function of the total number of layers in the particle for [loo] incidence. The periodic variation of the amplitude of the $ (422) beam with every second layer should be noted.
tations other than (loo), where the exact dimensions of the particle may be observed, the effects of forbidden fee reflections may be allowed for.
The results of this study suggest the formation of the c(2 X 2)s overlayer under the conditions applied, in accordance with existing literature based on single-crystal { lOO} surface studies [4-61. The concomitant reorientation of the particles in the presence of sulphur is of importance for structure-sensitive reactions in catalysis, since in the absence of sulphur, particles with a far higher proportion of { 111) surfaces are preferentially formed [14-161. Furthermore, the likely overlayer site symmetry has been identified as the four-fold site from a separate consideration of the contrast at the edge of the particles for both (100) and (110) orientations, again in agreement with bulk surface studies [6]. However the bulk superlattice often observed cannot be used to give information about the overlayer due to complications in the image contrast arising from superlattice reflections. This work has shown that HREM, coupled with image simulations, can be used as a tool to investigate surface structures directly and is capable of identifying both overlayer geometries and probable adsorption sites on real catalyst particles, if the overlayers are sufficiently stable in the electron beam. This is an important development, since the identification of adsorption sites using full low energy electron diffraction (LEED) studies requires intensity measurements involving even greater computations than those used for the multislice calculations. Moreover, the behaviour of small metal particles may well differ from that of extended single-crystal surfaces and this method may hence lead to better models for the poisoning of actual catalysts containing a dispersed metal phase than has hitherto been possible. Modifications of adsorbates by other species, such as promoters, can also be observed in this manner, and preliminary studies using alumina supports containing small amounts of alkali have indicated that different particle morphologies are produced. For example, in the presence of trace amounts of alkali on a boehmite support no { lOO} facetting is observed and the nature of any sulphur overlay is still uncertain. Given the sensitivity of the image contrast to adsorbate symmetry, it is interesting to
J. Uppenbrink et al. / HREM of sulphur overlayers on platinum (100) surfaces
speculate whether could be elucidated
the exact role in this way.
of promoters
Acknowledgements We wish to acknowledge financial the SERC, the DAAD (J.U.) and (A.I.K.).
support from British Alcan
References VI P.J.F. Harris, Nature (London) 323 (1986) 792. PI D.A. Jefferson and P.J.F. Harris, Nature (London)
332 (1988) 617. Adv. Catal. 31 (1981) 135. [31 C.H. Bartholomev, (41 Y. Berthier, Surf. Sci. 36 (1972) 225. [51 W. Heegemann, K.H. Meister, E. Bechtold and K. Hayek, Surf. Sci. 49 (1975) 161. [61 T.E. Fischer and S.R. Kelemen, Surf. Sci. 69 (1977) 1. [71 D.J. Smith, Surf. Sci. 178 (1986) 462. PI D.J. Smith, R.W. Glaisher, P. Lu and M.R. McCartney, Ultramicroscopy 29 (1989) 123.
147
[9] D.A. Jefferson, J.M. Thomas, G.R. Millward, K. Tsuno, A. Harriman and R. Brydson, Nature (London) 323 (1986) 428. [lo] D.J. Smith, L.A. Bursill and G.J. Wood, Ultramicroscopy 16 (1985) 19. [ll] D.A. Jefferson and AI. Kirkland, Inst. Phys. Short Meetings Ser. 11 (1988) 71. [12] P.L. Gai, M.J. Goringe and J.C. Barry, J. Microsc. 142 (1986) 9. [13] L.D. Marks, Surf. Sci. 139 (1984) 281. [14] M.L. Sattler and P.N. Ross, Ultramicroscopy 20 (1986) 21. [15] L.R. Wallenberg, J.-O. Bovin, A.K. Petford-Long and D.J. Smith, Ultramicroscopy 20 (1986) 71. [16] D.G. Duff, P.P. Edwards, J. Evans, J.T. Gauntlett, D.A. Jefferson, B.F.G. Johnson, AI. Kirkland and D.J. Smith, Angew. Chem. (Int. Ed. Engl.) 28 (1989) 591. [17] V. Castano, A. Gomez and M.J. Yacaman, Surf. Sci. 146 (1984) L587. [18] N. Tanaka and J.M. Cowley, Mater. Res. Symp. Proc. 41 (1985) 155. [19] G. Nihoul, K. Abdelmoula and J.J. Metois, Ultramicroscopy 12 (1987) 353. [20] W. Krakow, Ultramicroscopy 4 (1975) 55.
Surface
Science 23X (1990) 1322141 North-Holland
High-resolution profile imaging on platinum { lOO} surfaces
studies of sulphur
overlayers
J. Uppenbrink, A.I. Kirkland, D. Tang and D.A. Jefferson * Depurtment of Chemistry, Uniuers+ of Camhrrdge, Lensfield Roud, Camhrrdge CB2 IE W. UK Received
8 January
1990; accepted
for publication
23 April 1990
Previous investigations have shown that an ordered monolayer of sulphur on the (100) surfaces of small platinum particles can be observed directly by high-resolution electron microscopy. This study presents the results of extensive image simulations of different possible { 100) overlayer structures and adsorbtion sites for both (100) and (110) oriented particles, the results of this confirming the sulphur present in the experimental images as a c(2 X 2) overlayer with a four-fold adsorbate site symmetry.
1. Introduction The catalytic activity of metal catalysts can be strongly modified by the presence of surface adsorbates. In particular the strong interaction of sulphur with metal surfaces often leads to the poisoning of such catalysts [l-3]. To date, the structural consequences of sulphur adsorption on metal surfaces has been investigated mainly by means of model single crystal studies under UHV conditions [4-61, with important information on overlayer symmetries and on surface reconstructions having been obtained. However the properties of real catalysts differ substantially from the extended bulk samples used in single crystal studies, and it is therefore desirable to study also surface effects on small metal particles. Using high-resolution electron microscopy (HREM) in the profile imaging mode [7,8], it is possible to investigate the structural modification of small metal particles in specimens bearing a closer resemblance to commercial catalysts than has previously been possible. Previous work has shown that the treatment of platinum particles with low concentrations of H,S induces (100) facetting [l] and the concomitant formation of an ordered overlayer of sulphur atoms [2]. This overlayer has been identified as * To whom correspondence 0039-6028/90/$03.50
should
be addressed.
(i: 1990 - Elsevier Science Publishers
having a c(2 X 2)s structure from a consideration of the observed doubled superlattice spacing at the crystal edges and confirmed by means of image simulations based on a quasi-periodic unit cell. However the question of the site symmetry of individual adsorbate atoms has not been addressed. In this study high-resolution profile images of sulphur overlayers on small (100) facetted platinum particles in both (100) and (110) orientations have been obtained and both the overlayer geometry and the adsorption site symmetry are investigated using multislice simulations of model particles.
2. Experimental Specimens were prepared directly on a microscope grid. An aqueous alumina solution (2.5 g/8) was mixed with an aqueous solution of H,PtCl, (4.0 g/L’) to achieve a Pt : Al,O, ratio of 1 : 15 by weight. A drop of this mixture was placed on a grid, allowed to dry and then fired for l/2 h at 600” C in air. For the sulphur treatment the furnace was flushed with H,, heated to 490” C and held at this temperature in an atmosphere of 100 ppm H,S in H, for 20 h. The H,S gas flow was then removed, and the specimens were allowed to cool under H, flow. In a control experiment, the
B.V. (North-Holland)
J. Uppenbrink ef al. / HREM of suiphur overlayers on platinum
{IOO) surfaces
133
A x 50 A, supported on a computer generated background of amorphous A1,03 40 A in thickness. For the construction of the amorphous background, the atomic coordinates were generated on a purely random basis but were then tested to ascertain that they were in accordance with known limits of Al-Al and Al-O distances [11,12]: Those which did not satisfy these criteria were rejected. In this way a reasonably realistic amorphous support was generated, although no weight was given to the actual coordination symmetry. Both particle and background were sliced perpendicular to the incident electron beam with slice thicknesses and numbers of slices being given in table 1. The supercell was sampled on a 512 x 512 square array giving a sampling interval of 0.098 A per point, in order to eliminate artifacts due to an insufficiently small sampling interval [13]. For the actual image calculation the electron-optical parameters applicable to our microscope were used, and a Gaussian focal spread of 57 A (width at half height) was incorporated as an envelope function to limit the final image resolution to effectively - 1.75 A. Comparative simulations, in which the focal spread was incorporated as a numerical summation of 20 images, showed no differences when compared to those calculated using the envelope function, and therefore the envelope method was retained for ease of computation. Beam divergence was not included in the simulations, since the results of a similar trial calculation indicated that its effect
same thermal conditions were applied, with pure H, used throughout. The specimens were examined using a modified JEOL-200CX electron microscope (C, = 0.52 mm, C, = 1.05 mm) [9] operated at 200 kV, with an interpretable point-to-point resolution of 1.95 A as defined by the first zero in the phase contrast transfer function at a defocus slightly greater than the optimum and an absolute information limit of - 1.7 A. Micrographs were recorded at electron-optical magnifications of either - 330 000 or 475 000 X , For each group of particles a series of micrographs was recorded with focal increments of - 150 A between each. Care was taken to ensure the correct alignment of the illumination system [IO], to avoid the introduction of contrast artifacts in the image due to tilt of the incident beam. During electron beam irradiation neither thermally induced rearrangements of the particles nor noticeable carbon conta~nation were noted.
3. Image simulations Image simulations were carried out using a modified multislice routine available for the simulation of complete particles [ll]. The model particles were generated by truncation of a bulk fee supercell on six (100) surfaces, and were contained in a supercell with lateral dimensions of 50
Table 1 Slice parameters, overlayer geometries and site symmetries of sulphided platinum particles used in the simulations referred to in the text Overlayer
Adsorption site
None
c(2 x 2) P(2 X 2) c(2 x 2) PC2X 2) 42X2) PC2X 2) None c(2 X 2) PC2X 2)
Top TOP 2-fold 2-fold 4-fold /I-fold 4-fold 4-fold
Total number of atoms af
Total number of slices
(A)
970 970 1523 1320 1320 970 1523 970 1523 1523
13 14 14 14 13 14 27 27 27
1.8974 1.8974 2.0157 2.1057 2.1057 1.8974 2.1057 1.3870 1.3870 1.3870
Slice thickness
a) Refers to the number of platinum atoms in the particle, excluding the overlayer atoms.
Orientation
134
J. Uppenbrink et al. / HREM
ofsulphur
was merely to lower the image contrast slightly without removing any of the salient features. For each model one series of images was computed for
ouerlayers on platinum
(100) surfaces
defoci of 0 to -1400 A (underfocus) with increments of 200 A between each and another for defoci of 0 to -800 A with 100 A increments.
images of platinum particles supported on amorphous Fig. 1. Low-resolution and (100) facetting; (b) after treatment in 100 ppm H,S/H,
Al,O,: (a) after treatment in H, showing showing pronounced (100) facetting.
both
{ 111)
J. Uppenbrink et al. / HREM
ofsulphuroverlayers
on platinum (100) surfaces
135
4. Results and discussion Low-resolution TEM shows that the particles treated with H,S (size range 50-100 A) exhibit pronounced (100) facetting, when compared to those treated with pure H, (figs. la and lb). Subsequent profile high-resolution images show surface details in the form of a castellation with a repeat of - 3.9 A at the vertical edges of the particles. Images of cubic particles in a (100) orientation are shown in figs. 2a, 2b and 2c. This surface feature was visible over the entire range of
Fig. 3. Typical high-resolution images of unsulphided platinum particles (pure hydrogen treatment). No overlayers or strong {100) facetting can be observed.
Fig. 2. Typical high-resolution images of sulphided platinum particles: (a), (b), (c) (100) incidence with clear evidence for contrast due to an ordered overlayer at the particle edges; (d) (110) incidence.
defoci recorded experimentally and did not change in appearance during the observation period. Similarly an image of a cubic particle in a (110) orientation is shown in fig. 2d, and no distinctive surface features being observed. With pure H,, no overlayers were observed in high-resolution images (fig. 3) and the particles exhibited both (100) and {ill} facets, in accord with previous HREM studies on small platinum particles [14,15]. The multiply twinned forms frequently observed for other fee metals were almost entirely absent in the samples examined, again in agreement with results from other workers [16]. Previous single-crystal studies [4-61 and investigations of sulphided platinum particles [1,2] have shown that the two overlayers formed on Pt{lOO} are the c(2 x 2)s and the p(2 x 2)s overlayers (fig. 4). For each overlayer, three different individual adatom adsorption sites have to be considered, namely the top site (figs. 4b and 4e) where each sulphur atom is located immediately above a platinum atom on the surface, and the four-fold (figs. 4d and 4g) and two-fold sites (figs. 4c and 4f), where each sulphur atom is located above the spaces between four and two platinum surface atoms respectively.
136
.I. l/ppenhnnk
et 111./ HREM
of sulphurouerluyers on plarinum
(100)
.m+~e.~
392;i
1.96 A=
(a)
(e Fig. 4. Schematic diagrams of the model particles used in the image simulations for a (100) orientation with the relevant bulk and surface periodicities indicated: (a) no overlayer; (b. c, d) ~(2 x 2) overlayer; (e, f, g) p(2 x 2) overlayer; (b, e) top site; (c, f) two-fold site; (d, g) four-fold site.
Fig. 5. Simulated
images corresponding to the models of fig. 3 for a defocus of -400 A in all cases: (a) no overlayer: overlayer; (e, f, g) p(2 X 2) overlayer; (b, e) top site; (c, f) two-fold site; (d, g) four-fold site.
(b, c, d) ~$2 x 2)
J. Uppenbrink
et al. / HREM
of sulphur overlayers on platinum
To distinguish between the p(2 x 2) and the c(2 x 2) overlayer symmetries, both the relative contrast of the adatoms for (100) oriented particles and the overlayer periodicities for (110) oriented particles have been used. Simulations for the p(2 x 2)s overlayer on (100) oriented particles show fainter contrast due to the overlayer when compared to those for the c(2 x 2)s overlayer (fig. 5) and comparison with the experimental image of fig. 2a suggests that the latter geometry prevails in practice. In addition, the c(2 X 2) overlayers are clearly visible over the focal range from ca. - 200 to - 500 A, whereas the p(2 x 2) overlayers are only clearly visible at a defocus of ca. -400 A. Therefore both the magnitude of the contrast and its focal dependence may be used as diagnostic features to distinguish between the two geometries. However some care must be exercised in quantifying these results precisely, since the par-
(100) surfaces
137
ticle size in the simulations differs substantially from that of the experimental images. Differences in the simulated image contrast for different adsorption sites are also evident for (100) oriented particles (fig. 5). In the case of the top site, the simulated image shows a pronounced white fringe between the particle surface and the sulphur atoms, which is absent for both the twofold and the four-fold site. Furthermore for the two-fold site the sulphur atoms appear out of register with the positions of the platinum atoms in the particle. The simulated images show the same qualitative features for two different sulphur radii applied (fig. 6) one of them being the same as the platinum radius (1.3870 A) and the other the covalent sulphur radius (1.02 A). In this context it should be noted that the value of the Pt-S bond distance in the case of a monolayer of sulphur on platinum { lOO} is not precisely known,
Fig. 6. Simulated images for (100) incidence calculated for two different values of the Pt-S bond length for the c(2 x 2) overlayer and for a defocus of -400 A in all cases: (a, d) top site; (b, e) two-fold site; (c, f) four-fold site. (a, b, c) r(S) = 1.3870 A; (d. e. f) r(S) = 1.02 A.
138
J. Uppenbrmk et al. / HREM of suiphur ouerlayers on platinum (100) surfaces
although the bond is believed to be covalent [3,6]. The effect of the inclusion of either beam divergence up to a value of 0.75 mrad or an increased focal spread of 86 A does not remove the white fringe observed for the top site but only diminishes its contrast. Given these results it is clear from an examination of the experimental images of fig. 2a that the four-fold site symmetry with a c(2 x 2) geometry gives the best experimental/ calculated image match. To distinguish more clearly between the p(2 x 2) and the c(2 x 2) geometries, images of a particle in a (110) orientation were simulated for the case of four-fold adsorbate site symmetry only. The model particles used are shown in fig. 7 with the different periodicities for the two overlayers marked, and fig. 8 shows simulated images corresponding to fig. 7. From fig. 8 it is clear that the p(2 X 2) overlayer gives rise to a doubled repeat at the particle edges, whereas the c(2 x 2) overlayer has
the same spacing as the particle and can thus not be distinguished from the fringe contrast due to the particle; hence this projection may be used to distinguish the two different overlayer structures. The experimental image in fig. 2d does not show the doubled repeat of the p(2 x 2) overlayer and can thus be interpreted as having a c(2 x 2) overlayer symmetry in accordance with the results of the (100) orientation. Thus far only the surface detail in profile at the particle edges has been considered. However a (2 X 2) superlattice in the bulk of the particle is also often observed. Such a superlattice may arise either from the sulphur overlayer or alternatively may be due to formally forbidden fee reflections, which have been observed previously both in small particles [17,18] and in extended thin films [19,20]. This effect is illustrated in fig. 9, where the normalised intensity of the f (422) superlattice beam is plotted as a function of the number of (100) planes for (100) incidence with the normalised
2-26i .
I-96A
a
452A \\
3.92A-
Fig. 7. Schematic diagrams of the model particles used in the image simulations for a (110) orientation with the relevant bulk and surface periodicities indicated: (a) no overlayer; (b) c(2 X 2) overlayer, four-fold site; (c) p(2 X 2) overlayer. four-fold site.
J. Uppe~brink et al. / HREM
ofsulphurouerlayers on platinum
(100) surfaces
139
I-96W
a
b Fig. 8. Simulated
images
corresponding
c
to fig. 7 for a defocus of -400 ,k in all cases: (a) no overlayer; four-fold site; (c) p(2 X 2) overlayer, four-fold site.
intensity of the (000) and (200) beams plotted for reference. From these calculations it is apparent that the intensity of this formally forbidden beam oscillates with a period of two (100) planes with minima for even numbers of layers. However for the case of sulphided platinum particles a further complication arises in that an overlayer of sulphur atoms may also give rise to a bulk superlattice which will be indistinguishable from that due to the formally forbidden reflections. From a series of simulations of model particles calculated for both even and odd numbers of { lOO} planes in the
(b) ~(2 X 2) 0%rerlayer,
presence and absence of sulphur overlayers it becomes apparent that the effect of forbidden reflection contrast due to the odd numbers of {IOO} planes is far greater than that due to the sulphur overlayers, as might be expected from consideration of their scattering factors. Hence any superlattice observed in the bulk of the particle cannot be used reliably to give information about either the presence or the geometry of the overlayer, unless the particle dimension parallel to the incident beam is known precisely, a requirement that is almost impossible to meet in practice. In orien-
140
J. Uppenhrink et al. / HREM
ofsulphur
overlayers on plarinum
(100) surfaces
5. Conclusions
r
r z1
?.I,
/
F L 7
1.6
d
2 2
I.2
7
2 2
0.X
e
0.4
2 6 ‘W
2 r(
7
2 7
n
10
XI
2” humher
uf
10
slice
Fig. 9. Normalised diffracted intensities of the (a) (000). (b) (200) and (c) 4 (422) beams. plotted as a function of the total number of layers in the particle for [loo] incidence. The periodic variation of the amplitude of the $ (422) beam with every second layer should be noted.
tations other than (loo), where the exact dimensions of the particle may be observed, the effects of forbidden fee reflections may be allowed for.
The results of this study suggest the formation of the c(2 X 2)s overlayer under the conditions applied, in accordance with existing literature based on single-crystal { lOO} surface studies [4-61. The concomitant reorientation of the particles in the presence of sulphur is of importance for structure-sensitive reactions in catalysis, since in the absence of sulphur, particles with a far higher proportion of { 111) surfaces are preferentially formed [14-161. Furthermore, the likely overlayer site symmetry has been identified as the four-fold site from a separate consideration of the contrast at the edge of the particles for both (100) and (110) orientations, again in agreement with bulk surface studies [6]. However the bulk superlattice often observed cannot be used to give information about the overlayer due to complications in the image contrast arising from superlattice reflections. This work has shown that HREM, coupled with image simulations, can be used as a tool to investigate surface structures directly and is capable of identifying both overlayer geometries and probable adsorption sites on real catalyst particles, if the overlayers are sufficiently stable in the electron beam. This is an important development, since the identification of adsorption sites using full low energy electron diffraction (LEED) studies requires intensity measurements involving even greater computations than those used for the multislice calculations. Moreover, the behaviour of small metal particles may well differ from that of extended single-crystal surfaces and this method may hence lead to better models for the poisoning of actual catalysts containing a dispersed metal phase than has hitherto been possible. Modifications of adsorbates by other species, such as promoters, can also be observed in this manner, and preliminary studies using alumina supports containing small amounts of alkali have indicated that different particle morphologies are produced. For example, in the presence of trace amounts of alkali on a boehmite support no { lOO} facetting is observed and the nature of any sulphur overlay is still uncertain. Given the sensitivity of the image contrast to adsorbate symmetry, it is interesting to
J. Uppenbrink et al. / HREM of sulphur overlayers on platinum (100) surfaces
speculate whether could be elucidated
the exact role in this way.
of promoters
Acknowledgements We wish to acknowledge financial the SERC, the DAAD (J.U.) and (A.I.K.).
support from British Alcan
References VI P.J.F. Harris, Nature (London) 323 (1986) 792. PI D.A. Jefferson and P.J.F. Harris, Nature (London)
332 (1988) 617. Adv. Catal. 31 (1981) 135. [31 C.H. Bartholomev, (41 Y. Berthier, Surf. Sci. 36 (1972) 225. [51 W. Heegemann, K.H. Meister, E. Bechtold and K. Hayek, Surf. Sci. 49 (1975) 161. [61 T.E. Fischer and S.R. Kelemen, Surf. Sci. 69 (1977) 1. [71 D.J. Smith, Surf. Sci. 178 (1986) 462. PI D.J. Smith, R.W. Glaisher, P. Lu and M.R. McCartney, Ultramicroscopy 29 (1989) 123.
147
[9] D.A. Jefferson, J.M. Thomas, G.R. Millward, K. Tsuno, A. Harriman and R. Brydson, Nature (London) 323 (1986) 428. [lo] D.J. Smith, L.A. Bursill and G.J. Wood, Ultramicroscopy 16 (1985) 19. [ll] D.A. Jefferson and AI. Kirkland, Inst. Phys. Short Meetings Ser. 11 (1988) 71. [12] P.L. Gai, M.J. Goringe and J.C. Barry, J. Microsc. 142 (1986) 9. [13] L.D. Marks, Surf. Sci. 139 (1984) 281. [14] M.L. Sattler and P.N. Ross, Ultramicroscopy 20 (1986) 21. [15] L.R. Wallenberg, J.-O. Bovin, A.K. Petford-Long and D.J. Smith, Ultramicroscopy 20 (1986) 71. [16] D.G. Duff, P.P. Edwards, J. Evans, J.T. Gauntlett, D.A. Jefferson, B.F.G. Johnson, AI. Kirkland and D.J. Smith, Angew. Chem. (Int. Ed. Engl.) 28 (1989) 591. [17] V. Castano, A. Gomez and M.J. Yacaman, Surf. Sci. 146 (1984) L587. [18] N. Tanaka and J.M. Cowley, Mater. Res. Symp. Proc. 41 (1985) 155. [19] G. Nihoul, K. Abdelmoula and J.J. Metois, Ultramicroscopy 12 (1987) 353. [20] W. Krakow, Ultramicroscopy 4 (1975) 55.