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The effect of shape and crystal structure of small particles on their catalytic activity
Surface Science 106(1981)472-477 North-Holland Publishing Company
THE EFFECT OF SHAPE AND CRYSTAL STRUCTURE THJZIR CATALYTIC ACTIVITY
OF SMALL PARTICLES
ON
M. Jose YACAMAN
and S. FUENTES and J.M. DOMINGUEZ ~~ti~tDMex~a~a de1 Pet&o, Ave. de los Cien Me@osNo. 151, M&co, DF Received 8 September 1980;accepted for publication 16 October 1980
It is shown by the use of weak beam electron microscopy techniques, that the crystallographicstructure of small particles on supported catalysts can be obtained. In the case of Rh, it is found that the particles are mainly icosahedral non-fee or fee cub-~tahedra1 depending on the strength of the interaction with the support. Pentane hydrogenolysis reaction using Rh particles of different structure is studied.
In the recent years, there has been a renewed interest on the effect of cluster size in catalytic activity for demanding reactions [l]. The main conclusion of some recent experiments [l-4] is the connection between catalytic activity and some specific arrays of atoms on the surface of small particles. The characterization of the crystal structure and shape of small particles becomes then a basic step to the understanding of the catalytic activity. This is however a rather difficult task because the most interesting particle sizes are between 5-25 A. In that range many of the standard characterization techniques fail. In the recent years electron microscopy of small clusters has been developed enormously. Especially important are the micro-diffraction techniques [5], the weak beam dark field methods [6] and the ultra high vacuum in-situ reaction studies [7]. Using high resolution dark field images, we have demonstrated [8] that shape ch~acterization of Pt particles supported on graphite can be carried out. In the present paper new possibilities of characterization of small metallic particles using transmission electron microscopy techniques will be presented. The cube-octahedral and icosahedral particles will be discussed. The correlation between the shape and the catalytic activity is studied for the case of Rh particles on different supports. 003!&6028/81/0000-0000/$02.50@ North-Holland Publishing Company
M.J. Yacamcin et al. I Effect of shape and
Fig. 1. ‘Thickness fringe profile for a Pt cube-octahedral
crystalstmhue
473
cluster supported on graphite.
2. The characterization of cube-octahedral particles
The particles which are the more often observed are the cube-octahedral particles. They can be characterized by their dark field thickness fringes obtained using the technique of Yacamin and Ocafia [l]. Fig. 1 shows the typical fringe pattern of a cube-octahedral particle. By comparison of the computed and experimental fringe profiles and by determining the fringe distance from many beam dynamical calculations [9], it is possible to know that the particle in the fig. 1 has a truncation at 70% of its height, along a (111) face. From informations of this kind, the complete number of sites available on the particle surface can be calculated with very good accuracy. Cuba-octahedral particles tend to appear when a strongly interacting support is used. Examples of this are Pt on MgO, Rh on TiOz, etc. In these cases the interfacial energy of the overgrowth and the substrate is very large, as it is well known from epitaxy studies [lo]. In some other cases such as Rh/C the interfacial energy is lower and no additional energy is supplied to the particle. In those conditions the same structure as the free particle (icosahedral) will be formed.
3. Characterization of bimetallic systems In the case of catalysts composed by two or more metals, in addition to the shape determination, it becomes also important to obtain information about the alloy state of the constituents. It is not clear in many cases, whether the system forms an alloy or there is a segregation of the metals out of the solid solution. We have studied the case of Pt-Ni
474
M.J. Yacamcin et al. / Effect of shape and crystal stmchne
bimetallic systems for a concentration of 15% Pt and 85% Ni. Dark field can be used also to form different kinds of images. For instance figs. 2a, 2b and 2c show a sequence of bright field and dark field images. In the last two figures, a (220) spot of Pt or Ni was used respectively. The bright field images show a hexagonal pattern inside of the larger hexagonal profile. Moire fringes of parallel type can be seen. The observed particle contrast is only consistent with the Pt being precipitated out of the solution and forming a “shell” surrounding the Ni. This effect might have a strong influence on the catalytic properties of the alloys.
Fig. 2. Sequence of pictures of Pt-Ni bimetallic system: (a) bright field; (b) (220) dark field using a Pt spot; (c) (220) dark field using a Ni spot.
M.J. Yacamdn et al. / Effect of shape and crystalstruchue
475
a Fig. 3. Computer simulated diffraction pattern for an icosahedral particle: (a) particle oriented with a five fold axis parallel to the electron beam; (b) a (111) face in contact with the support and perpendicular to the electron beam.
4. Contrast from very small particles
The most difficult case arises when the studied particles have sizes in the range of 5-25 A. For these particles there is an image blurring due to size effects, which does not allow the standard dark field image analysis to be applied. For instance, the method used by Yacamhn et al. [ll] and Yang et al. [12] to identify icosahedral structures which is based upon the observation of typical contrasts will not be applicable in this case. One therefore has to look for a different approach. A new method that can be applied is the reconstruction of the diffraction pattern of a single particle from a number of weak beam images. The procedure is time consuming but has a number of advantages over the microdiffraction methods (for instance less sample contamination). In a second step, the diffraction experimental pattern is compared with a computer calculated one. The structure can then be found by simple comparison. An example of calculated patterns for an icosahedral particle in two orientations is shown in figs. 3a and 3b. Using these patterns, the icosahedral particles were identified among Rh supported catalysts.
5. Catalytic activity and particle structure
In the present work, catalytic studies were carried out on Rh catalyst supported on a number of substrates. An important effect of the substrate was found to be the change of the crystal structure of the produced particles. For substrates with a not well defined
M.J. Yacamn’n et al. I Effect of shape and crystalstructure
4%
crystalline structure and with amorphous regions such as y-AlzOj, a mixture of icosahedral and cube-octahedral particles was found. The studied reaction was the hydrogenolysis of pentane which is a demanding reaction, i.e. its activity depends on the particle size. The turnover number was measured by gas chromatography and hydrogen chemisorption using the standard methods. Table 1 shows the results for a number of catalysts, The cracking products were: methane (C,), propane (CJ, ethane (C,), butane (C,). The turnover number value reported in the table is that corresponding to the total number of converted molecules. It was found experimentally that the concefftration of Ct was equal to the concentration of C, (and similarly for C3 and 0. Therefore multiple hy~ogenolysis was not present. The particle structure reported in table 1 was the result of examining about hundred particles per sample. A specific structure was quoted on table 1 when about 95% of the particles corresponded to that case. In the y-AlZ03 case the percentage of each type of particle is quoted on the table. From the results, several interesting points can be seen; the total activity of the particles is more depending on their size than on their crystal structure. The activity drops for particles smaller than 10 A and for particles larger than 25A. Catalysts with 80% dispersion have the highest selectivity for C2 and C,, with the Rh/SiO, system being the most selective. The most dispersed catalysts are the less selective ones, Therefore the general behavior of the selectivity is similar to that of the activity. However, the highest value for the selectivity is obtained for 12A particles with the icosahedral structure being the predomin~t. On the other hand the activity has its highest values for 2OA particles which are mainly cube-octahedral. When the particles are very small, they are composed by a few atoms. For instance the first complete icosahedron is obtained for 13 atoms and has a mean diameter of about 7 A. The next complete shell will
Table 1 Catalytic data for hydrogenolysis of pentane; D means the catalyst dispersion, Z is the mean particle diameter. .$ indicates the selectivity to Ci, Ton is the turnover number Catalyst
D (%)
Selectivity
Selectivity
s,+.G
sz+ s7
Activity Ton (h-l) at 150°C
Type of particle
Cubo-octahedral (20%) Icosahedral (80%) Cubo-octahedral (60%) Icosahedral (40%) Cubo-octahedral Cuba-octahedral
7s
l&25
12
S8
261
50
E-30
14
86
1260
35 14
20-40 50-80
24 34
76 66
266 7
80 50
lo-25 1530
7 18
93 82
515 1100
RhlC
100
610
70
30
1
RhiTiOa
100
10
32
62
190
Cuba-octahedral
Rh/MgO __~_----_
60
lR30
28
72
200
Cubo-octahedral
Rh/y-Al@,
Rh/SiOs
Icosahedral Cubo-octahedral Icosahedral
M.J. Yacama’n et al. I Effect of shape and crystalstructure
477
have 55 atoms (mean diameter of about 10 A) and the next one will have 147 atoms and so on. As the size of the particle increases, there is a higher probability for having clusters formed by incomplete shells. In those particles, special surface sites will be formed which will disappear when the shell is complete. Examples of those sites are the BS (in the Van Hardeveld and Hartog notation [13]) for the cube-octahedron and the B6 and B4 for the icosahedron. It is possible that those special sites might be responsible for the observed behavior for the activity and selectivity. Those sites will not be present in very small particles (which tend to be very homogeneous) and might explain the drop on the activity in those clusters. A more detailed surface site statistics is needed to compare with the experimental data. It is however, clear that the modern characterization techniques such as weak beam electron microscopy will allow in the future to correlate reaction kinetic data with particle crystal structure and surface site statistics in a more direct way.
References [l] J.M. Dartigue, A. Chambellan, S. Coroleur, P. Gault, A. Renouprez, Dalmai-Imelik, NOW. J. Chim. 3 (1979) 591. [2] S. Fuentes and F. Figueras, J. Catalysis 61 (1980) 443. [3] K. Foger and J.R. Anderson, J. Catalysis 54 (1978) 318. [4] J.M. Dominguez and M.J. Yacaman, J. Catalysis 64 (1980) 223. [5] R. Geiss, Appl. Phys. Letters 21 (1975) 4. [6] M.J. Yacaman and T. Ocatia, Phys. Status Solidi (a) 42 (1977) 571.
B. Moraweck, P. Bosch-Giral
and G.
[7] H. Poppa and D. Moorhead, Surface Sci. 106 (1961) 478. [8] M.J. Yacamln and J.M. Dominguez, J. Catalysis 64 (1980) 213. [9] M.J. YacamPn, K. Heinemann and H. Poppa, in: Physics and Chemistry of Solid Surfaces III, Ed. R. Vanselow (CRC Press, 1981). [lo] Epitaxial Growth A, Ed. J. Matthews (Academic Press, 1975). [ll] M.J. YacamPn, K. Heinemann, C.Y. Yang and H. Poppa, J. Crystal Growth 47 (1979) 187. [12] C.Y. Yang, M.J. Yacaman and K. Heinemann, J. Crystal Growth 47 (1979) 283. [13] R. van Hardeveld and F. Hartog, Surface Sci. 15 (1969) 189.
THE EFFECT OF SHAPE AND CRYSTAL STRUCTURE THJZIR CATALYTIC ACTIVITY
OF SMALL PARTICLES
ON
M. Jose YACAMAN
and S. FUENTES and J.M. DOMINGUEZ ~~ti~tDMex~a~a de1 Pet&o, Ave. de los Cien Me@osNo. 151, M&co, DF Received 8 September 1980;accepted for publication 16 October 1980
It is shown by the use of weak beam electron microscopy techniques, that the crystallographicstructure of small particles on supported catalysts can be obtained. In the case of Rh, it is found that the particles are mainly icosahedral non-fee or fee cub-~tahedra1 depending on the strength of the interaction with the support. Pentane hydrogenolysis reaction using Rh particles of different structure is studied.
In the recent years, there has been a renewed interest on the effect of cluster size in catalytic activity for demanding reactions [l]. The main conclusion of some recent experiments [l-4] is the connection between catalytic activity and some specific arrays of atoms on the surface of small particles. The characterization of the crystal structure and shape of small particles becomes then a basic step to the understanding of the catalytic activity. This is however a rather difficult task because the most interesting particle sizes are between 5-25 A. In that range many of the standard characterization techniques fail. In the recent years electron microscopy of small clusters has been developed enormously. Especially important are the micro-diffraction techniques [5], the weak beam dark field methods [6] and the ultra high vacuum in-situ reaction studies [7]. Using high resolution dark field images, we have demonstrated [8] that shape ch~acterization of Pt particles supported on graphite can be carried out. In the present paper new possibilities of characterization of small metallic particles using transmission electron microscopy techniques will be presented. The cube-octahedral and icosahedral particles will be discussed. The correlation between the shape and the catalytic activity is studied for the case of Rh particles on different supports. 003!&6028/81/0000-0000/$02.50@ North-Holland Publishing Company
M.J. Yacamcin et al. I Effect of shape and
Fig. 1. ‘Thickness fringe profile for a Pt cube-octahedral
crystalstmhue
473
cluster supported on graphite.
2. The characterization of cube-octahedral particles
The particles which are the more often observed are the cube-octahedral particles. They can be characterized by their dark field thickness fringes obtained using the technique of Yacamin and Ocafia [l]. Fig. 1 shows the typical fringe pattern of a cube-octahedral particle. By comparison of the computed and experimental fringe profiles and by determining the fringe distance from many beam dynamical calculations [9], it is possible to know that the particle in the fig. 1 has a truncation at 70% of its height, along a (111) face. From informations of this kind, the complete number of sites available on the particle surface can be calculated with very good accuracy. Cuba-octahedral particles tend to appear when a strongly interacting support is used. Examples of this are Pt on MgO, Rh on TiOz, etc. In these cases the interfacial energy of the overgrowth and the substrate is very large, as it is well known from epitaxy studies [lo]. In some other cases such as Rh/C the interfacial energy is lower and no additional energy is supplied to the particle. In those conditions the same structure as the free particle (icosahedral) will be formed.
3. Characterization of bimetallic systems In the case of catalysts composed by two or more metals, in addition to the shape determination, it becomes also important to obtain information about the alloy state of the constituents. It is not clear in many cases, whether the system forms an alloy or there is a segregation of the metals out of the solid solution. We have studied the case of Pt-Ni
474
M.J. Yacamcin et al. / Effect of shape and crystal stmchne
bimetallic systems for a concentration of 15% Pt and 85% Ni. Dark field can be used also to form different kinds of images. For instance figs. 2a, 2b and 2c show a sequence of bright field and dark field images. In the last two figures, a (220) spot of Pt or Ni was used respectively. The bright field images show a hexagonal pattern inside of the larger hexagonal profile. Moire fringes of parallel type can be seen. The observed particle contrast is only consistent with the Pt being precipitated out of the solution and forming a “shell” surrounding the Ni. This effect might have a strong influence on the catalytic properties of the alloys.
Fig. 2. Sequence of pictures of Pt-Ni bimetallic system: (a) bright field; (b) (220) dark field using a Pt spot; (c) (220) dark field using a Ni spot.
M.J. Yacamdn et al. / Effect of shape and crystalstruchue
475
a Fig. 3. Computer simulated diffraction pattern for an icosahedral particle: (a) particle oriented with a five fold axis parallel to the electron beam; (b) a (111) face in contact with the support and perpendicular to the electron beam.
4. Contrast from very small particles
The most difficult case arises when the studied particles have sizes in the range of 5-25 A. For these particles there is an image blurring due to size effects, which does not allow the standard dark field image analysis to be applied. For instance, the method used by Yacamhn et al. [ll] and Yang et al. [12] to identify icosahedral structures which is based upon the observation of typical contrasts will not be applicable in this case. One therefore has to look for a different approach. A new method that can be applied is the reconstruction of the diffraction pattern of a single particle from a number of weak beam images. The procedure is time consuming but has a number of advantages over the microdiffraction methods (for instance less sample contamination). In a second step, the diffraction experimental pattern is compared with a computer calculated one. The structure can then be found by simple comparison. An example of calculated patterns for an icosahedral particle in two orientations is shown in figs. 3a and 3b. Using these patterns, the icosahedral particles were identified among Rh supported catalysts.
5. Catalytic activity and particle structure
In the present work, catalytic studies were carried out on Rh catalyst supported on a number of substrates. An important effect of the substrate was found to be the change of the crystal structure of the produced particles. For substrates with a not well defined
M.J. Yacamn’n et al. I Effect of shape and crystalstructure
4%
crystalline structure and with amorphous regions such as y-AlzOj, a mixture of icosahedral and cube-octahedral particles was found. The studied reaction was the hydrogenolysis of pentane which is a demanding reaction, i.e. its activity depends on the particle size. The turnover number was measured by gas chromatography and hydrogen chemisorption using the standard methods. Table 1 shows the results for a number of catalysts, The cracking products were: methane (C,), propane (CJ, ethane (C,), butane (C,). The turnover number value reported in the table is that corresponding to the total number of converted molecules. It was found experimentally that the concefftration of Ct was equal to the concentration of C, (and similarly for C3 and 0. Therefore multiple hy~ogenolysis was not present. The particle structure reported in table 1 was the result of examining about hundred particles per sample. A specific structure was quoted on table 1 when about 95% of the particles corresponded to that case. In the y-AlZ03 case the percentage of each type of particle is quoted on the table. From the results, several interesting points can be seen; the total activity of the particles is more depending on their size than on their crystal structure. The activity drops for particles smaller than 10 A and for particles larger than 25A. Catalysts with 80% dispersion have the highest selectivity for C2 and C,, with the Rh/SiO, system being the most selective. The most dispersed catalysts are the less selective ones, Therefore the general behavior of the selectivity is similar to that of the activity. However, the highest value for the selectivity is obtained for 12A particles with the icosahedral structure being the predomin~t. On the other hand the activity has its highest values for 2OA particles which are mainly cube-octahedral. When the particles are very small, they are composed by a few atoms. For instance the first complete icosahedron is obtained for 13 atoms and has a mean diameter of about 7 A. The next complete shell will
Table 1 Catalytic data for hydrogenolysis of pentane; D means the catalyst dispersion, Z is the mean particle diameter. .$ indicates the selectivity to Ci, Ton is the turnover number Catalyst
D (%)
Selectivity
Selectivity
s,+.G
sz+ s7
Activity Ton (h-l) at 150°C
Type of particle
Cubo-octahedral (20%) Icosahedral (80%) Cubo-octahedral (60%) Icosahedral (40%) Cubo-octahedral Cuba-octahedral
7s
l&25
12
S8
261
50
E-30
14
86
1260
35 14
20-40 50-80
24 34
76 66
266 7
80 50
lo-25 1530
7 18
93 82
515 1100
RhlC
100
610
70
30
1
RhiTiOa
100
10
32
62
190
Cuba-octahedral
Rh/MgO __~_----_
60
lR30
28
72
200
Cubo-octahedral
Rh/y-Al@,
Rh/SiOs
Icosahedral Cubo-octahedral Icosahedral
M.J. Yacama’n et al. I Effect of shape and crystalstructure
477
have 55 atoms (mean diameter of about 10 A) and the next one will have 147 atoms and so on. As the size of the particle increases, there is a higher probability for having clusters formed by incomplete shells. In those particles, special surface sites will be formed which will disappear when the shell is complete. Examples of those sites are the BS (in the Van Hardeveld and Hartog notation [13]) for the cube-octahedron and the B6 and B4 for the icosahedron. It is possible that those special sites might be responsible for the observed behavior for the activity and selectivity. Those sites will not be present in very small particles (which tend to be very homogeneous) and might explain the drop on the activity in those clusters. A more detailed surface site statistics is needed to compare with the experimental data. It is however, clear that the modern characterization techniques such as weak beam electron microscopy will allow in the future to correlate reaction kinetic data with particle crystal structure and surface site statistics in a more direct way.
References [l] J.M. Dartigue, A. Chambellan, S. Coroleur, P. Gault, A. Renouprez, Dalmai-Imelik, NOW. J. Chim. 3 (1979) 591. [2] S. Fuentes and F. Figueras, J. Catalysis 61 (1980) 443. [3] K. Foger and J.R. Anderson, J. Catalysis 54 (1978) 318. [4] J.M. Dominguez and M.J. Yacaman, J. Catalysis 64 (1980) 223. [5] R. Geiss, Appl. Phys. Letters 21 (1975) 4. [6] M.J. Yacaman and T. Ocatia, Phys. Status Solidi (a) 42 (1977) 571.
B. Moraweck, P. Bosch-Giral
and G.
[7] H. Poppa and D. Moorhead, Surface Sci. 106 (1961) 478. [8] M.J. Yacamln and J.M. Dominguez, J. Catalysis 64 (1980) 213. [9] M.J. YacamPn, K. Heinemann and H. Poppa, in: Physics and Chemistry of Solid Surfaces III, Ed. R. Vanselow (CRC Press, 1981). [lo] Epitaxial Growth A, Ed. J. Matthews (Academic Press, 1975). [ll] M.J. YacamPn, K. Heinemann, C.Y. Yang and H. Poppa, J. Crystal Growth 47 (1979) 187. [12] C.Y. Yang, M.J. Yacaman and K. Heinemann, J. Crystal Growth 47 (1979) 283. [13] R. van Hardeveld and F. Hartog, Surface Sci. 15 (1969) 189.