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Morphology Changes of Supported Rh Particles During Thermal Treatments as Probed by Catalytic Measurements
C.H. Bartholomew and J.B. Butt (Editors), Catalyst Deactiuation 1991 0 1991 Elsevier Science PublishersB.V., Amsterdam
613
MORPHOLOGY CHANGES OF SUPPORTED Rh PARTICLES DURING THERMAL TREATMENTS AS PROBED BY CATALYTIC MEASUREMENTS 1
1
1
2
W. E. ALVAREZ2, R. J. FENOGLIO , G. M. NUfiEZ , A. J. ROUCO and D. E. RESASCO ’INTEMA, Institute of Materials Science and Technology, University of Mar del Plata, Juan B.Justo 4302, Mar del Plata (Argentina) 2Monomeros Vinilicos, Polo Petroquimico, Bahia Blanca (Argentina)
SUMMARY The effects of oxidation - reduction cycles on the activity and selectivity of supported Rh catalysts were investigated using the hydrogenolysis of methylcyclopentane (MCP) as a test reaction. From the analysis of catalytic properties and reduction profiles it is concluded that, on silica-supported catalysts, following the initial oxidation at 400 C, successive reduction treatments at increasing temperatures cause a progressive reconstruction of the Rh particles. On g-alumina-supported catalysts the situation is more complex. The inter-actionof Rh with the support during the initial oxidation makes a fraction of the Rh inaccessible to the gas phase. Only after subsequent oxidation - reduction cycles do they behave like the silica-supported catalysts INTRODUCTION
In the past few years the study of morphology changes undergone by
supported Rh particles during oxidation / reduction thermal treatments has
attracted considerable attention (refs 1-4). These phenomena are of
particular importance in the deactivation of automotive exhaust catalysts for which rhodium is the main active ingredient for the reduction of NO. In the present work, we investigate the effects of such morphology
changes on catalytic properties by using the hydrogenolysis of MCP as a
test reaction. Because of its high sensitivity to surface structure (ref.
51, this reaction results particularly useful for probing changes in the
morphology of the metal particles. It is well know that on smooth surfaces with a large fraction of highly coordinated etoms, the MCP ring opening mainly yields 2MF (2-methylpentane) and 3MP (3-methylpentane).
On
the other hand, on irregular surfaces with a high number of uncoordinated atoms, the percentage of n-hexane increases and the
product distribution tends to the statistical (2MP:3MP:nHex:40:20:40) (refs. 6-7).
614 EXPERIMENTAL The catalyst samples were prepared by supports:
-
impregnating the two
Degussa C y-A1203, 100 m2/g and Aerosil 380 SiOz, 380 m2/g
-
with a rhodium chloride (RhC13.3H20) solution to produce a Rh concentration of 2 wt% in the catalysts. The catalysts were characterized by temperature
programmed reduction (TPR), transmission electron microscopy (TEM) and
catalytic activity measurements. The TPR apparatus used in this work has
been described elsewhere (ref. 8). For each TPR run, the sample (40 mg) was previously oxidized in a 5% 0 /He mixture at either 200 or 400’C for 30 2
min, following a linear increasing temperature at 7’C/min. Then, the TPR was started from O’C using the same heating rate of 7’C/min
3
in 30 cm /min
flow of 5% H /Ar mixture. The activity measurements for the hydrogenolysis 2 of MCP were conducted in a pulse reactor. The previously oxidized samples
were reduced in situ at various temperatures in pure H2. For each run, a gaseous 400 pl pulse of 6.8% MCP in hydrogen was injected into the 22
czn"/min H stream, which acted as a carrier. The products were directly 2
sent to a gas chromatograph and separated by a column packed with 30%
DC200/500 on Chromosorb P
A/W 60-80.
JEOL TEM 100 CX.
The electron microscope used was a
RESULTS
Thermal Programmed Reduction (TPR)
Reduction profiles obtained on the Rh/Si02 catalyst after different
pretreatments are shown in Fig. la. When the fresh sample was pre-oxidized at 400’C, the TPR profile (denoted as 1-ox-400 in the figure) presented a main H2 consumption peak at about 82’C and much smaller peaks at 30-5O’C.
It was observed that after the first TPR, the subsequent oxidation -
reduction cycles showed a reproducible behaviour. In every case, the size
of the reduction peak decreased as the pre-oxidation temperature was lowered to 2OO’C and increased again as it was increased to 400’C.
It can be noted that most of the Rh was reduced below 150’C for all the samples investigated. When the sample was left at room temperature in air for
several days, the following TPR showed a small H consumption at low 2 reduction temperatures (about 5OOC). The sequence of TPR profiles for the RNAl 0 catalyst after different 23 pretreatments are shown in Fig. lb. A clear difference is observed in the first oxidation (400’C) - reduction cycle (denoted as 1-ox-400). relative to the subsequent cycles. In the first cycle, the H consumption was 2
significantly lower and extended to higher reduction temperatures.
615
Rh
I
' -> ,
0
\
50
A :
'L5-OX-200
100
150
/
alumina
6-OX-400
'\
/'
7-OX-200
4-OX-25
200
250
300
360
400
0
50
100
150
200
250
TEMPERATURE
TEMPERATURE (C)
300
360
400
300
350
400
(C)
3-OX-400 2-OX-200
,/ 0
50
100
150
200
250
TEMPERATURE
300
(C)
350
-400
/
1-OX-400 400
0
50
100
150
200
250
TEMPERATIJRE (C)
Fig.1. TPR profiles of succesive oxidation - reduction cycles on Rh catalysts. For each TPR the order in the sequence and the oxidation temperatures are indicated. (a) left panel: silica-supported catalysts; (b) right panel: alumina-supported catalysts. Under the assumption that the initial oxidation at 400’C was responsible
for the low H consumption on the alumina - supported catalyst, we carried 2
out two additional thermal programmed reductions, drastically varying the
initial oxidation conditions. In one case, the fresh sample was heated in
the 0 /He mixture at a very low rate, l’C/min, up to 400’C and left for 1 h 2
at that temperature (sample 1-ox-400(L)). In the second case, the fresh sample was suddenly transferred, in the same 0 /He mixture, from room 2
616
temperature to 400-C and left at that temperature for 1 h (sample l-ox-4OO(S)).
The TPR profiles following these two pre-treatments are
compared in Fig. 2. A clear difference is observed. The sample oxidized at
a lower rate exhibits a lower H2 consumption, indicating that, indeed, the initial oxidation plays an important role in the loss of accessibility of
Rh f o r the subsequent reduction. However, as also shown in Fig. 2, the
difference observed in the f i r s t cycle disappears when subsequent cycles are compared. The H2 consumption and position of the peaks
are very similar when the two used samples are subsequently o x i d i z e d under the same standard conditions (heating rate, 7OC/min).
2-OX-400 (L)
Fig. 2. Reduction profiles of Rh / alumina catalysts after different oxidation pretreatments. (L)low heating rate: CS! heated capidly from 25 C to 400 C. Below: first cycles; Above: second cycles
2-OX-400 ( S )
0
50
100
160
200
250
TEMPERATURE
300
(C)
350
400
Electron microscopy (TEM)
The catalysts were examined after an initial oxidation at 4OO0C followed
by reduction at two different temperatures (150’C and 500’C).
Both samples,
Rh/Si02 and Rh/A1203, when reduced at 15O’C, exhibited very small crystallites, i.e. smaller than 10 A.
The sample reduced at 500-C suffered
a moderate sintering, exhibiting particles in the range 10-20 A.
Hydrogenolysis of MCP
We have studied the variation of overall MCP conversion after an initial
oxidation at 400’C followed by reduction at subsequently higher
temperatures. The results for the Rh/Si02 catalyst are plotted in Fig. 3a.
On this catalyst, the overall conversion initially decreases with
increasing reduction temperature, then it starts increasing, reaching a
617 maximum after a reduction temperature of about 35O0C and then markedly
decreases.
It can be noted that the same pattern is reproduced after
several oxidation - reduction cycles. The maximum conversion in each cycle
was found to increase after each consecutive cycle.
On the other hand, the variation of overall conversion with reduction
temperature for the alumina-supported catalyst is illustrated in Fig. 3b.
The observed trend is significantly different from that reported above or
the silica-supported catalyst. Over the entire initial cycle, the activity
is relatively low and changes little with temperature of reduction. But, after the second cycle, it is found to vary in the same way as Rh/SiOg,
exhibiting a maximum after reduction at 350'C. CONVERSION (%)/mQ Rh I
150
250
350
I
450
REDUCTION TEMPERATLIRE (C)
CONVERSION (%)/mQ Fin I
150
250
350
450
REDUCTION TEMPERATilRE (C)
Fig. 3. Variation of conversion as a functi9n of reduction temperature after an initial oxidation treatment at 400 C. First ( a1; third ( A 1 and fifth cycle ( 0 ) . (a) left: RWsilica catalyst; (b) right: RWalumina catalyst. In parallel to the changes in overall conversion, significant
changes in product distribution are observed for the two catalysts
during the oxidation - reduction cycles. Fig. 4a shows the variation of the percentage of n-hexane in products over different cycles for the case of Rh/Si02. Except for a slight increase at low reduction
temperatures observed for the first cycle, the percentage of n-hexane decreases with temperature of reduction over the whole range. In the case of Rh/A1203, the variation of product distribution
with temperature of reduction shows a contrasting trend for the first cycle compared to the subsequent cycles. A s depicted in Fig. 4b,
during the first cycle the percentage of n-hexane increases with
reduction temperature over the entire cycle. By contrast, it exhibits
a maximum at intermediate temperatures during the third cycle.
,
618
35;
20
n-Hexane in products
% n-Hexane in products
35,
i 15t
10’ 150
1
250
350
10 150
450
260
350
450
REDUCTION TEMPERATURE (C)
REDUCTION TEMPERATURE (C)
Fig. 4. Variation of percentage of n-hexane in products as a funcLion of reduction temperature after an initial oxidation treatment at 400 C. First ( 0 ) ;third ( A ) and fifth cycle ( 0 ) . (a) left: Wsilica catalyst; (b1 right: RWaluminacatalyst. DISCUSSION
Wong and McCabe ( r e f s . 3-4) have studied the oxidation reduction cycles of Rh/Si02 and Rh/A1203 catalysts in great detail.
Their studies mainly focused on oxidation temperatures significantly
higher than those used in this work.
Their main conclusions were
that on silica the only effect of the oxidation
-
reduction cycles is
Rh crystallite sintering. On alumina the situation is more complex.
An initial oxidation treatment at
8OOOC
causes the diffusion of some
Rh into the bulk of the support. This Rh, buried into the alumina,
can be restored back to the surface by a high temperature reduction.
After this reduction treatment no diffusion into the alumina is
observed during a subsequent oxidation treatment at 500 C. In accord with these authors, we propose that on Si02, the observed
changes in catalytic activity and selectivity can be interpreted in terms
of changes in the Rh crystallite morphology. As described in a previous
paper (ref. 91, we postulate that following oxidation at 400’C,
the
subsequent reduction at low temperatures leaves the metal particle with a
very open structure. An increase in the reduction temperature causes an
annealing of the metal particle. During this process, the surface of the
particle undergoes profound changes, i.e. conversion of open planes into
closely packed planes. These changes may significantly affect the catalytic
properties (ref. 101, causing the observed variations in activity and
619
selectivity.
Indeed, as shown in Fig. 3, the overall activity significantly changes
as the reduction temperature is increased. It is generally accepted that
hydrogenolysis reactions require a relatively large ensemble of atoms to constitute the active site. The maximum in activity after reduction at 350’C
would indicate that, at that temperature, an optimal configuration of
the surface atoms is reached. Also, as illustrated in Fig. 4, as the
surface of the metal particle becomes more closely packed, the percentage
of n-hexane in products decreases, which would be consistent with an increase in the coordination numbei of the Hh atoms.
It is interesting to compare activity and selectivity data obtained on a
3% R W S i O
catalyst whose particle size was in the range 40-50 A (ref. 91,
2
with the trends reported in this work for a highly dispersed 2% RWSiO2 (about 10
A).
In the previous work, it was observed that the activity
maximum was obtained after reduction at about 250’C while the maximum
activity was found to decrease after each cycle. Here, the maximum appears after reduct,ionat 350 C and it increases a.fter each succesive oxidation-
reduction cycle. These opposing trends indicate that the effects of the proposed reconstruction on the surface morphclogy strongly depends on the
original size of the metal crystallite. After all, it is reasonable to expect that smaller particles, when oxidized at
4OO0C, be
spread
more readily over the support than large particles (ref. 11, and
consequently, a higher temperature would be required to reach the high
activity configuration.
The patterns of the selectivity towards n-hexane are also different.
Whi1.e. j n the case of relatively large particles the percentage of n-hexane
is very low and changes little during oxidation/reduction cycles, here it drastically changes over Lhe cycle as a consequence of the surface
smoothing process. Again, this effect would be
particles.
more marked on the smaller
In the case of Rh/A1203, our results make evident a strong influence of
the support during the oxidation - reduction cycles. Both, the relatively
low activity (Fig. 3b) and H
2
consumption (Fig. la and 2 ) observed during
the first cycle suggest that after the initial oxidation at 400'C an
important fraction of Rh is not accessible to the gas phase, and it is only
restored af+.er reduction at. high temperatures. A possible explanation for this phenomenon could be the diffusion of Rh into the alumina support described above. However, according to Wong and McCabe (ref. 4). this
process would require significantly higher temperatures than those used in this work.
620
The fact that the low accessibility of Rh is only evident during the
first cycle (fresh sample) would indicate that the interaction responsible f o r this phenomenon might rather be between the support and rhodium precursor. A second possibility, perhaps related to this, might be that a fraction of Rh may become partially covered by support species
during the initial stages of the preparation, i.e. during the impregnation
with the acidic solution due to dissolution and re-deposition of the alumina (refs. 11-12). If that were the case, those species would be removed from the metal surface by a high temperature oxidation cycle.
-
reduction
After the second cycle the effect of the support is less pronounced and
both, activity and selectivity follow about the same pattern as the RWSiO 2 catalyst. ACKNOWLEDGEMENTS
We acknowledge Fundacih Antorchas and CONICET of Argentina for
financial support of this work. REFERENCES 1 2
C. Lee and L. D. Schmidt, J. Catal. 101 (1986) 123. E. J. Braunschweig, A. D. Logan, A. K. Datye and D. J. Smith, J. Catal.
3
C. Wong and R. McCabe, J. Catal. 107 (1987) 535. C. Wong and R. McCabe, J.Cata1. 119 (1989) 47. M. Che and C. 0. Bennett, in D.D. Eley et al. (Eds.1, Advances Catalysis, Academic Press, San Diego, 1989, Vol. 36, p. 68. F. Gault, in D.D. Eley et al. (Eds.1, Advances in Catalysis, Acade Press, New York, 1983, Vol 30, p. 1. J. G. van Senden, E. H. van Broekhoven, C. T. J. Wreesman, V. Ponec, J. Catal. 87 (19851 468. D. E. Damiani, E. D. Perez Millman and A. J. Rouco, J. Catal. 101
4 5 6 7 8
118 (1989) 227.
(1956) 162.
9
10
R. J. Fenoglio, G. M. Nufiez and D. E. Resasco, J. Catal. 121 (1990) 7 G.M. Nuiiez, A.R. Patrignani and A . J . Rouco, J. Catal. 101 (1986)
11
E. Santacesaria, S. Carra and I. Adami, Ind. Eng. Chem. Prod. Res. De
12
A. A. Castro, 0. A. Scelza, E. R. Benvenuto, F. T. Baronetti, S.R. Miguel and J. M. Parera, in Poncelet et al. (eds.1, Stud. Surf. Sci. Catal., Elsevier, Amsterdam, 1983, Vol. 16, p.47.
162.
16 (19771 41.
613
MORPHOLOGY CHANGES OF SUPPORTED Rh PARTICLES DURING THERMAL TREATMENTS AS PROBED BY CATALYTIC MEASUREMENTS 1
1
1
2
W. E. ALVAREZ2, R. J. FENOGLIO , G. M. NUfiEZ , A. J. ROUCO and D. E. RESASCO ’INTEMA, Institute of Materials Science and Technology, University of Mar del Plata, Juan B.Justo 4302, Mar del Plata (Argentina) 2Monomeros Vinilicos, Polo Petroquimico, Bahia Blanca (Argentina)
SUMMARY The effects of oxidation - reduction cycles on the activity and selectivity of supported Rh catalysts were investigated using the hydrogenolysis of methylcyclopentane (MCP) as a test reaction. From the analysis of catalytic properties and reduction profiles it is concluded that, on silica-supported catalysts, following the initial oxidation at 400 C, successive reduction treatments at increasing temperatures cause a progressive reconstruction of the Rh particles. On g-alumina-supported catalysts the situation is more complex. The inter-actionof Rh with the support during the initial oxidation makes a fraction of the Rh inaccessible to the gas phase. Only after subsequent oxidation - reduction cycles do they behave like the silica-supported catalysts INTRODUCTION
In the past few years the study of morphology changes undergone by
supported Rh particles during oxidation / reduction thermal treatments has
attracted considerable attention (refs 1-4). These phenomena are of
particular importance in the deactivation of automotive exhaust catalysts for which rhodium is the main active ingredient for the reduction of NO. In the present work, we investigate the effects of such morphology
changes on catalytic properties by using the hydrogenolysis of MCP as a
test reaction. Because of its high sensitivity to surface structure (ref.
51, this reaction results particularly useful for probing changes in the
morphology of the metal particles. It is well know that on smooth surfaces with a large fraction of highly coordinated etoms, the MCP ring opening mainly yields 2MF (2-methylpentane) and 3MP (3-methylpentane).
On
the other hand, on irregular surfaces with a high number of uncoordinated atoms, the percentage of n-hexane increases and the
product distribution tends to the statistical (2MP:3MP:nHex:40:20:40) (refs. 6-7).
614 EXPERIMENTAL The catalyst samples were prepared by supports:
-
impregnating the two
Degussa C y-A1203, 100 m2/g and Aerosil 380 SiOz, 380 m2/g
-
with a rhodium chloride (RhC13.3H20) solution to produce a Rh concentration of 2 wt% in the catalysts. The catalysts were characterized by temperature
programmed reduction (TPR), transmission electron microscopy (TEM) and
catalytic activity measurements. The TPR apparatus used in this work has
been described elsewhere (ref. 8). For each TPR run, the sample (40 mg) was previously oxidized in a 5% 0 /He mixture at either 200 or 400’C for 30 2
min, following a linear increasing temperature at 7’C/min. Then, the TPR was started from O’C using the same heating rate of 7’C/min
3
in 30 cm /min
flow of 5% H /Ar mixture. The activity measurements for the hydrogenolysis 2 of MCP were conducted in a pulse reactor. The previously oxidized samples
were reduced in situ at various temperatures in pure H2. For each run, a gaseous 400 pl pulse of 6.8% MCP in hydrogen was injected into the 22
czn"/min H stream, which acted as a carrier. The products were directly 2
sent to a gas chromatograph and separated by a column packed with 30%
DC200/500 on Chromosorb P
A/W 60-80.
JEOL TEM 100 CX.
The electron microscope used was a
RESULTS
Thermal Programmed Reduction (TPR)
Reduction profiles obtained on the Rh/Si02 catalyst after different
pretreatments are shown in Fig. la. When the fresh sample was pre-oxidized at 400’C, the TPR profile (denoted as 1-ox-400 in the figure) presented a main H2 consumption peak at about 82’C and much smaller peaks at 30-5O’C.
It was observed that after the first TPR, the subsequent oxidation -
reduction cycles showed a reproducible behaviour. In every case, the size
of the reduction peak decreased as the pre-oxidation temperature was lowered to 2OO’C and increased again as it was increased to 400’C.
It can be noted that most of the Rh was reduced below 150’C for all the samples investigated. When the sample was left at room temperature in air for
several days, the following TPR showed a small H consumption at low 2 reduction temperatures (about 5OOC). The sequence of TPR profiles for the RNAl 0 catalyst after different 23 pretreatments are shown in Fig. lb. A clear difference is observed in the first oxidation (400’C) - reduction cycle (denoted as 1-ox-400). relative to the subsequent cycles. In the first cycle, the H consumption was 2
significantly lower and extended to higher reduction temperatures.
615
Rh
I
' -> ,
0
\
50
A :
'L5-OX-200
100
150
/
alumina
6-OX-400
'\
/'
7-OX-200
4-OX-25
200
250
300
360
400
0
50
100
150
200
250
TEMPERATURE
TEMPERATURE (C)
300
360
400
300
350
400
(C)
3-OX-400 2-OX-200
,/ 0
50
100
150
200
250
TEMPERATURE
300
(C)
350
-400
/
1-OX-400 400
0
50
100
150
200
250
TEMPERATIJRE (C)
Fig.1. TPR profiles of succesive oxidation - reduction cycles on Rh catalysts. For each TPR the order in the sequence and the oxidation temperatures are indicated. (a) left panel: silica-supported catalysts; (b) right panel: alumina-supported catalysts. Under the assumption that the initial oxidation at 400’C was responsible
for the low H consumption on the alumina - supported catalyst, we carried 2
out two additional thermal programmed reductions, drastically varying the
initial oxidation conditions. In one case, the fresh sample was heated in
the 0 /He mixture at a very low rate, l’C/min, up to 400’C and left for 1 h 2
at that temperature (sample 1-ox-400(L)). In the second case, the fresh sample was suddenly transferred, in the same 0 /He mixture, from room 2
616
temperature to 400-C and left at that temperature for 1 h (sample l-ox-4OO(S)).
The TPR profiles following these two pre-treatments are
compared in Fig. 2. A clear difference is observed. The sample oxidized at
a lower rate exhibits a lower H2 consumption, indicating that, indeed, the initial oxidation plays an important role in the loss of accessibility of
Rh f o r the subsequent reduction. However, as also shown in Fig. 2, the
difference observed in the f i r s t cycle disappears when subsequent cycles are compared. The H2 consumption and position of the peaks
are very similar when the two used samples are subsequently o x i d i z e d under the same standard conditions (heating rate, 7OC/min).
2-OX-400 (L)
Fig. 2. Reduction profiles of Rh / alumina catalysts after different oxidation pretreatments. (L)low heating rate: CS! heated capidly from 25 C to 400 C. Below: first cycles; Above: second cycles
2-OX-400 ( S )
0
50
100
160
200
250
TEMPERATURE
300
(C)
350
400
Electron microscopy (TEM)
The catalysts were examined after an initial oxidation at 4OO0C followed
by reduction at two different temperatures (150’C and 500’C).
Both samples,
Rh/Si02 and Rh/A1203, when reduced at 15O’C, exhibited very small crystallites, i.e. smaller than 10 A.
The sample reduced at 500-C suffered
a moderate sintering, exhibiting particles in the range 10-20 A.
Hydrogenolysis of MCP
We have studied the variation of overall MCP conversion after an initial
oxidation at 400’C followed by reduction at subsequently higher
temperatures. The results for the Rh/Si02 catalyst are plotted in Fig. 3a.
On this catalyst, the overall conversion initially decreases with
increasing reduction temperature, then it starts increasing, reaching a
617 maximum after a reduction temperature of about 35O0C and then markedly
decreases.
It can be noted that the same pattern is reproduced after
several oxidation - reduction cycles. The maximum conversion in each cycle
was found to increase after each consecutive cycle.
On the other hand, the variation of overall conversion with reduction
temperature for the alumina-supported catalyst is illustrated in Fig. 3b.
The observed trend is significantly different from that reported above or
the silica-supported catalyst. Over the entire initial cycle, the activity
is relatively low and changes little with temperature of reduction. But, after the second cycle, it is found to vary in the same way as Rh/SiOg,
exhibiting a maximum after reduction at 350'C. CONVERSION (%)/mQ Rh I
150
250
350
I
450
REDUCTION TEMPERATLIRE (C)
CONVERSION (%)/mQ Fin I
150
250
350
450
REDUCTION TEMPERATilRE (C)
Fig. 3. Variation of conversion as a functi9n of reduction temperature after an initial oxidation treatment at 400 C. First ( a1; third ( A 1 and fifth cycle ( 0 ) . (a) left: RWsilica catalyst; (b) right: RWalumina catalyst. In parallel to the changes in overall conversion, significant
changes in product distribution are observed for the two catalysts
during the oxidation - reduction cycles. Fig. 4a shows the variation of the percentage of n-hexane in products over different cycles for the case of Rh/Si02. Except for a slight increase at low reduction
temperatures observed for the first cycle, the percentage of n-hexane decreases with temperature of reduction over the whole range. In the case of Rh/A1203, the variation of product distribution
with temperature of reduction shows a contrasting trend for the first cycle compared to the subsequent cycles. A s depicted in Fig. 4b,
during the first cycle the percentage of n-hexane increases with
reduction temperature over the entire cycle. By contrast, it exhibits
a maximum at intermediate temperatures during the third cycle.
,
618
35;
20
n-Hexane in products
% n-Hexane in products
35,
i 15t
10’ 150
1
250
350
10 150
450
260
350
450
REDUCTION TEMPERATURE (C)
REDUCTION TEMPERATURE (C)
Fig. 4. Variation of percentage of n-hexane in products as a funcLion of reduction temperature after an initial oxidation treatment at 400 C. First ( 0 ) ;third ( A ) and fifth cycle ( 0 ) . (a) left: Wsilica catalyst; (b1 right: RWaluminacatalyst. DISCUSSION
Wong and McCabe ( r e f s . 3-4) have studied the oxidation reduction cycles of Rh/Si02 and Rh/A1203 catalysts in great detail.
Their studies mainly focused on oxidation temperatures significantly
higher than those used in this work.
Their main conclusions were
that on silica the only effect of the oxidation
-
reduction cycles is
Rh crystallite sintering. On alumina the situation is more complex.
An initial oxidation treatment at
8OOOC
causes the diffusion of some
Rh into the bulk of the support. This Rh, buried into the alumina,
can be restored back to the surface by a high temperature reduction.
After this reduction treatment no diffusion into the alumina is
observed during a subsequent oxidation treatment at 500 C. In accord with these authors, we propose that on Si02, the observed
changes in catalytic activity and selectivity can be interpreted in terms
of changes in the Rh crystallite morphology. As described in a previous
paper (ref. 91, we postulate that following oxidation at 400’C,
the
subsequent reduction at low temperatures leaves the metal particle with a
very open structure. An increase in the reduction temperature causes an
annealing of the metal particle. During this process, the surface of the
particle undergoes profound changes, i.e. conversion of open planes into
closely packed planes. These changes may significantly affect the catalytic
properties (ref. 101, causing the observed variations in activity and
619
selectivity.
Indeed, as shown in Fig. 3, the overall activity significantly changes
as the reduction temperature is increased. It is generally accepted that
hydrogenolysis reactions require a relatively large ensemble of atoms to constitute the active site. The maximum in activity after reduction at 350’C
would indicate that, at that temperature, an optimal configuration of
the surface atoms is reached. Also, as illustrated in Fig. 4, as the
surface of the metal particle becomes more closely packed, the percentage
of n-hexane in products decreases, which would be consistent with an increase in the coordination numbei of the Hh atoms.
It is interesting to compare activity and selectivity data obtained on a
3% R W S i O
catalyst whose particle size was in the range 40-50 A (ref. 91,
2
with the trends reported in this work for a highly dispersed 2% RWSiO2 (about 10
A).
In the previous work, it was observed that the activity
maximum was obtained after reduction at about 250’C while the maximum
activity was found to decrease after each cycle. Here, the maximum appears after reduct,ionat 350 C and it increases a.fter each succesive oxidation-
reduction cycle. These opposing trends indicate that the effects of the proposed reconstruction on the surface morphclogy strongly depends on the
original size of the metal crystallite. After all, it is reasonable to expect that smaller particles, when oxidized at
4OO0C, be
spread
more readily over the support than large particles (ref. 11, and
consequently, a higher temperature would be required to reach the high
activity configuration.
The patterns of the selectivity towards n-hexane are also different.
Whi1.e. j n the case of relatively large particles the percentage of n-hexane
is very low and changes little during oxidation/reduction cycles, here it drastically changes over Lhe cycle as a consequence of the surface
smoothing process. Again, this effect would be
particles.
more marked on the smaller
In the case of Rh/A1203, our results make evident a strong influence of
the support during the oxidation - reduction cycles. Both, the relatively
low activity (Fig. 3b) and H
2
consumption (Fig. la and 2 ) observed during
the first cycle suggest that after the initial oxidation at 400'C an
important fraction of Rh is not accessible to the gas phase, and it is only
restored af+.er reduction at. high temperatures. A possible explanation for this phenomenon could be the diffusion of Rh into the alumina support described above. However, according to Wong and McCabe (ref. 4). this
process would require significantly higher temperatures than those used in this work.
620
The fact that the low accessibility of Rh is only evident during the
first cycle (fresh sample) would indicate that the interaction responsible f o r this phenomenon might rather be between the support and rhodium precursor. A second possibility, perhaps related to this, might be that a fraction of Rh may become partially covered by support species
during the initial stages of the preparation, i.e. during the impregnation
with the acidic solution due to dissolution and re-deposition of the alumina (refs. 11-12). If that were the case, those species would be removed from the metal surface by a high temperature oxidation cycle.
-
reduction
After the second cycle the effect of the support is less pronounced and
both, activity and selectivity follow about the same pattern as the RWSiO 2 catalyst. ACKNOWLEDGEMENTS
We acknowledge Fundacih Antorchas and CONICET of Argentina for
financial support of this work. REFERENCES 1 2
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