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Correlation between microstructure and hydrogenolysis activity and selectivity of noble metal catalyst particles
C!atdysis Today, 12 (16923 269-262 B%evier Science FWii3heereB.V., Ametrstdam
CORREIATION
BETWEEN
ACITMTYANDSm-
I.D. -
269
MXROSTRUCSURE
AND
HYDROGENOLY’SIS
OF NOBLE METAL CATALYST PARTICLES’
and KR. KRAUSE?
Department of Chemical Engineering and Materials Science, University of Minnesota Minneapolis MN 55455
of~e~~p~,
~onel~on~~~~)givesdirectb~~~on
and locations of catalyst particles down to atomic dimensions. We have used TEM to correlate catalyst particle microstructure with reactivity and selectivity in alkane hydrogenolysis on & Pt, Ir, Ru, and Ni supported on SiO, by determi&q microreactor kinetics folltig heating stroq& in Hz at riDo”C(am~ealed) or oxidation in 0, ~~0~ reduction in H, to reform metal par@es
&her
by krw temperature
(oxidized). In these experiments we txBmine the
same catalyst repeatedly after these treatments, and thus effects due to loading or treatrnant differences are excluded. Ethane hydrogenolysisactivities are found to vary by up to 104 depending on oxidaticm or annealing pretreatment, and these changes are rever&e
in that atalysts can be cy&d
many times between these pretreatments. Selectivities and activation energies are also found ta vary significantly with these treatments, showing that treatment alters the structure of the surface, not just the surface area. TEM shows that high temperature oxidation followed by low temprature reduction in Hz transforms ~J.Iparticles from singe crystal metah (typica@
IWA diameter)
into tiny
clusters of metal particles (typically lak diameter) and that these microstructures are stable iudetitely
under reaction conditions, 18o-26ooC, Similar results are also reported far
Rh-Ce/SiC$ catalysts, but the iarge microstroctore change observed in TEM causes only a digfit enhancement in activity.
It has been known since the classic work of Sinfelt et al. [l-4] that hydrogenolysis of alkartes depends sensit.iveIyon dispersion of t&z meta& and more recently many investigaiom ‘936s research pwtially sponsored by NSF under Grant No. CBTgg22745.
210
have observed
a strong sensitivity to pretreatment
of the catalyst [5-q. Most of these
investigators observe different behaviour on different catalysts and infer structure sensitivity, while evidence of changes on a single catalyst sample gives much more direct information on structure sensitivity. For several years we have conducted systematic studies of the correlation particle microstructure
of catalyst
with reactivity of alkane bydrogenolysis [g-11] and CO hydrogenation
[lO,ll]. We find very large effects which agree very well with suggestions based on the effect of dispersion
and
microstructure
and reactivity. In this article we summarix
of C&,
pretreatment
but which explicitly
show the relationship
between
these results for the hydrogenolysis
C$&, and C,H,, on several metals supported on SiO, and also for the promoted
metal catalyst system Rh-CeBiO,. Hydrogenolysis is simply the cracking of an alkane in the presence of hydrogen to yield smaller alkanes and the mechanism has been the subject of several reviews [3,X2,13]. We examine the simplest of these systems: ethane, propane and n-butane,
C&+H2
= 2cH,
CJ-%+Hz
= C&-G+CH4
C,H,, + Hz = 2CJ-G C,H,, + H,
= CsHs + CHd
These are written as scission of one C-C! bond in the parent molecule, but of course multiple bonds may be broken. In all but ethane the selectivity of formation
of various
products is usually more important than overall activity, because one is frequently interested primarily
in selective hydrogenolysis
to make specific products
and methane
is usually
undesired.
Fixpedmental Metal particles on planar amorphous SiO, were used for TEM. The silica was prepared by vacuum depositing a -200
A filmof Si on a gold TEM grid which had been coated with
Formvar and then heating in 0, in a furnace to 800 T to oxidize the Si to SiO, and burn off the Formvar. Metals were then deposited by vacuum deposition of a 20 heating in H, at -600 ‘C to break up the fihu into 20-200
A filmfollowed by
A diameter particles. As shown in
Fig. 1, this produces isolated particles of fairly uniform size and spacing which have generally polyhedral outlines and are usually single crystals. Preparation
by decomposition
of salts [14]
has also been shown to produce the same general structures after heating in H, although
271
particle sizes and spacings are less uniform. Samples were examined using TEM after various heat treatments which were carried out by heating for several hours in flowing Hz or 0, in a tube furnace. A major advantage of these techniques
is that we are able to examine the same region of a sample repeatedly
following each heat treatment
so that we can follow the evolution
individual particles. This contrasts strongly with conventional
in microstructure
of
TEM of supported catalysts in
which one can only do “post mortem” analysis of samples after sectioning or other preparation techniques. This is possible because we use planar SiO, samples in which metal particles never overlap and which are stable enough to be transferred
repeatedly
between
furnace and
microscope to locate the same particles. Reactivity measurements These were all prepared
were in a microreactor
containing
by incipient wetness impregnation
(except Rh/Ce catalysts which were on Aerosil200)
- 0.1 grams of catalyst.
of metal salts on Cabosil SiO,
followed by calcining and heating in Hz
such that all metal loadings were 5% by weight. Metal surface areas were determined various stages of treatment
by Hz chemisorption
at
[U] and samples were heated such that
average particle sizes were - 100 A, the same approximate
size of particles used in TEM.
Reactants were typically 5% hydrocarbon, 35% H, and the balance He at atmospheric pressure with analysis by gas chromatography
[9]. Flow rates and total amounts of catalyst
were adjusted to maintain 2-10% total hydrocarbon conversion. This was important for higher alkanes to assure that continued hydrogenolysis of product fragments was minimized. The sequence of treatments
preparation
can be sketched as follows:
- H, 6OOT - reaction - 0,500T
with heat treatments
- H, 250°C - reaction
in flowing gases at temperatures
indicated for 4 hours and reaction at
180-26oT.
Fig. 1 shows TEZM micrographs of Rh particles on SiO,. Fig. la is after heating in H, while Fig. lb shows the same particles after heating in 0, at 5OOT. Upon conversion Rh,O,
to
the polyhedral metal particles have larger outlines and irregular shapes because the
oxide expands slightly and “wets” the SiO, XPS, electron diffraction, and dark field imaghtg all show that Rh metal is converted completely into microcrystalline
particles of RhzOs by this
treatment. However, upon heating in H, at 3OOT (Fig. lc), all particles shrink somewhat and
212
have tiny grams which electron diffraction and dark field imaging show to consist entirely of Rh metal. These clusters oniy slowly recrystallixe back into roughly their initial structures, approximately
as in Fig. la. The lower part of Fig. 1 indicates ideahxed cross sections of
particles shown in the micrographs treatment
to show the microstructures
indicated
in the
cyclic
process sketched above.
H2
600'1: Fig. 1. TEM micrographs of Rh particles on a planar amorphous SiO, film following treatments indicated for several hours at atmospheric pressure. Panel (a) is of single crystal Rh particles - 100 A indiameter, (b) is converted completely into crystalline Rh20s particles, and (c) is clusters of - 10 A diameter metal particles. The sketches show side views of particles inferred from the micrographs.
Thus, Fig. l(a) represents
the microstructure
of the “annealed” particles while Fig. l(c)
represents the “oxidixed” particles. Both are exclusively metal, and both have roughly the same location and size, but the annealed particles are polyhedral while the oxidized particles are clusters of tiny metal crystals. Hydrogen chemisorption
of supported catalysts shows that the
surface area of the Rh is higher by only a factor of two on the “oxidized” surface than on the “annealed” surface. Similar TEM experiments have been done for Ir [ 161, Ni [17], Pd [ 181, Pt [ 191, and Ru [20] and nonporous spheres have also been used as supports [21]. All metals except Pt readily form oxide and are readily reduced in H, to form the clusters similar to those shown in Fig. 1. Pt cannot be oxidixed by these treatments,
but we have shown [19] that Pt particles
form perfect cubes after heating in H, but spheres after heating in 0,. Fig. 2 shows ‘IEM micrographs of Ir particles on SiO, which reveal a side view of
273
Fig. 2. TEhf micrograph of Ir particles on SiO, showmg side views of particles after (a) annealing in H, at 6OOY!,(b) heating in 0s at SOOT, (c) heating in H, at 2XPC, and (d) heating in H, at 6OOT. Drastic changes occur upon forming IrO, (b) and reducing at low temperature (c). Structures are those sketched in the lower panel of Fig. 1
particles rather than the top view shown in Fig. 1. Here we can see that the particles are nearly as high as they are wide. The annealed particles are polyhedral, while the side view TEM shows that the Ir particles fhst become roughened near their surfaces and thus attain a totally different shape upon total oxidation, Fig. 2b. Reduction of IrOs, Fig. 2d, shows that all particles have formed small metal clusters, and the large particle at the right in the micrograph in fact appears to have cracked into two particles upon low temperature reduction to metallic Ir. These side view images confirm that particle shapes are approximately as indicated in the lower portion of Fig. 1.
Ethane J3ydrogenolysis We next see how these annealing and oxidizing treatments affect the reactivities of the catalysts, although the reactivity samples are necessarily porous rather than planar SiO, because it is necessary to have sufficient area to measure reactivity. Fig. 3 shows Arrhenius plots of the hydrogenolysis rate of CJ& (in moles CJ-& reacted per gram of catalyst per second) versus l!P for Rh, Ru, Ir, Ni, Pd, and Pt on SiO,. As noted above, all were for 5% loading with nearly identical preparation and treatment conditions. Surface areas are probably slightly different, but H, chemisorption measurements indicated that these varied by a much smaller amount than the 10’ to lo’ variations caused by oxidation-annealing treatments.
274
1o-4
I
I
I
I
I
I
I
I
1
2.2
2.3
1o-6 5I s f \M $
10-8
u” z -ii lo-lo
lo-l2 _ 1.8
1.9
2.0
2.1
1000/T (K-l)
Fig. 3. Plots of rates of Q& hydrogenolysis versus l/T on metals indicated at 5% loading on SiO, following high temperature heating in I$ (solid lines) and heating in 0, (dashed lines). Fig. 4 shows a plot of rate following oxidation and annealing sequences at 23OT for each metal Variations are largest for Rh and Ru, and smaller for Pt, Pd, and Ni. There is an irreversible sintering evident for all metals with repeated cycling, but the reversiile variations between oxidation and reduction cycles are much larger than this for all metals.
1o-4 Ru 0
1o-6 52 3 “h 1 \M zw 1o-a u” ‘;j s. $2 E lo-lo
annealed lo-l2
I
I
annealed I
oxidized
annealed I
oxidized
I
I
oxidized
Fig. 4. Plots of rates of CJ& hydrogenolysis at 23oOC on metals indicated on SiO, all at 5% loading. Oxidation produces catalysts which are much more active than after annealing in H, and these changes are reversible with continued oxidation-reduction cycling.
These dramatic and systematic variations with oxidation and reduction treatments have obvious correlation Treatment
with and interpretation
in Oz followed by low temperature
from the TEM micrographs
of Figs. 1 and 2.
reduction to metal produced very small
216
crystallites (- 10 A>which have a much higher activity than do the large (- 100 upon annealing
A>particles
Thus it is clear that there is a strong surface structure effect in C&
hydrogenolysis. This is much larger than could be accounted for by surface area changes, and the change in reaction activation energies evident in Fig. 3 shows that the mechanism
of
reaction must also be changing between the treatments. Several factors explain the differences between the metals. (1) Pt does not form a bulk oxide
under
these
transformation treatment
treatment
conditions,
and
therefore
camrot occur. All changes are therefore
the
metal-to-oxide-to-metal
due to shape changes caused by
in the gases. [19] (2) Ni and Pd sinter at so low a temperature
oxide is reduced temperature
and the metal sinters back to the annealed
(<2OOT) that the
state before the reaction
is reached. (3) Ir is quite refractory, and it may not be totally annealed to the
single crystal structure during H, treatment
at 600°C.
Butane and Propnue Eydrogenolysis Fig. 5 shows total rates of hydrogenolysis of C&Is and C,H,, on RhDiO, at 230°C. Rates are larger for the higher hydrocarbons
as expected
from the relative
rates of
chemisorption of primary and secondary carbons. While there is an overall decrease in activity with repeated treatment
in both annealed
and oxidized states due to irreversible
sintering,
there is a reversible variation in the total activity with C&Is and C,H,, of approximately
a
factor of 10 between annealed and oxidized treatments. This is somewhat smaller than the 104 variation noted for CJ& Fig. 6 shows Arrhenius plots of total activity and formation of products (C&Is, C$&, and Cl&) in C,,H,, hydrogenolysis. temperature
The curves show that activation
and that selectivities also are highly temperature
at low temperatures
energies vary with
dependent with very little CH,
and much more CH, at high temperatures.
The annealed surface makes much less CH_,than does the oxidized surface, and C-&V& is the dominant
product from C,H,, on the annealed
behaviour correlates well with microstructure
surface even at 26ooC. Again this
changes observed by TEh4, Figs. 1 and 2. The
oxidized surface with very small particles is much more reactive and tends to completely decompose the hydrocarbons into small fragments, while the less reactive annealed tends to break only one bond to form either two C& of parallel
and series reactions
have been
surface
molecules or CsHs and CH_,. The ideas
discussed in detail by Bond [22]. We also
investigated carefully the role of further hydrogenolysis and the extent to which the process can be regarded as a simple breaking of one C-C bond versus intermolecular carbon hydrogenation
[9].
reactions and
277
I
I
I
I
I
I
-5 -
6,
annealed
annealed
annealed
Fig. 5. Plots of rates of h~eno~~ of C$&, C&Z&,and C,H,, on 5% Rh on Si02 at 230% fobwing anrmabg in Hz and heating in 0, as dmrii previously.All rates vary by factors of > 10 by these treatments.
*O-91 1.8
I
1.9
I
I
I
2.0
2.1
2.2
1ooo/T UC-‘)
Fig. 6. Plots of hydrogenolysis rates of C.,H,, to form CI& C$&, and C;rg on 5% Rh on SiO, following treatments of catalyst in Hz (dashed lines) and in Oz (solid lines). Rates differ by - 100 between treatments, and selectivities to form various products are also different.
Ethne Ey~no~sis
on Rh-Ce/Si02 Cataiysts
The above results were on single metals on SiO, the most inert support in common
usage. We have observed similar microstructure
changes on A1,0, [23,24] and TiO, altbougb
we have not attempted similar reactivity characterization.
Ce is an important additive to noble
metals which alters rates and selectivities of several reactions ~clu~g We examined the microstructures
h~~no~~.
125,261
of Rh-Ce on SiO, using TEM 111,271 with methods
identical to those used for noble metals alone. We observed a dramatic alteration in the Ce between H, and 0, treatments as shown by the micrographs of Fig. 7 and the sketches below the micrographs. In H, the Ce forms noncrystalline
Ce+3 (both ce,O,
and Ce.&O,
phases)
which forms a rather uniform thin film which covers the SiO, while in & Ce forms crystalline CeO, crystals which associate with the oxidized Rh particles. This system is therefore interesting candidate to see how these changes affect hydrogenolysis selectivity.
an
279
a
b
Ce02 SiO2
After oxidation Fig. 7. TEM micrographs of Rh-Ce on SiO, (a) after heating in Hz at 6OOT and (b) after heating in 0, at 5OO“C.After heating in H, the Ce forms an amorphous -0, fihn on the SiO, while the Rh forms particles. After heating in 0, the Ce forms crystalline CeO, particles which associate with the Rh,O, particles. The presence of Ck increases the hydrogenolysis rate slightly, but the change is much smaller than the large changes in microstructure suggest.
Fig. 8 shows Arrhenius
plots of C&
hydrogenolysis activity of Rh with and without
addition of Ce after amrealing and oxidation treatments.
The presence of Ce increases rates
slightly (about a factor of 2), but this difference is much smaller than the differences produced by annealing and oxidizing Rh alone, as ilhrstrated in Fig. 3. We conclude, therefore, that the
Fig- 8. Plots of C.& bydsogenolysis ratesversus rmpature following treatmentunder conditions indicated for RM!e on SO2 for Ce loadings indkated. The presece of Ce increamt&e satessligbtly~ but t&edifference belween treatment eundltions is mu& larger.
281 microstmctural
changes in Rh which alter hydrogenolysisactivity are not strongly affected by
the presence of Ce even though TEM showsthat Ce associates stronglywith the Rh particles~ Evidently the reactive sites on the Rh are not stronglyaffected by the presence of Ce, and the only effect of adding Ce is to somehow slightly increase the activity of the Rh, perhaps through an increase in the Rh surface area.
These simple experiments demonstrate that there is a strong correlation between catalyst microstructure and activityand selectivityin alkane hydrogenolysis.The effects are (1) very large, typicallyat least a factor of 10, (2) nearly reversible, (3) comparable for several noble metals, and (4) quite reproducible between catalyst sampks. Since the variations are on single samples, it is clear that effects are caused by microstructure changes, not simply differencesbetween structures of samples with different loadings or preparation procedures. The interpretation of these observations also seems to be clear in that on the small clusters there are mostly low oration
sites whose activity is at least a factor of 10 h&her
than on the polyhedralannealed s&aces whichexpose predominantly(111) and (100) planes. These low coordination sites must be many orders of magnitude more reactive than those on (111) and (100) planes because there must be some of them even on annealed surfaces because of edges and comers. The absence of these highly reactive sites may in some situationsbe desirable if one wants a catalyst which is highlyselective in cracking a&anes to smaller aJkanesbut not to CH.,. These results show that variations with treatment can in many cases be even larger than variations between different metals.
References :. 3 4 5 6 7 8 9 10 11 12 13 14 15 16
J.H. Sinfelt and D.J.C. Yates, J. Catal., 8 (1%7) 82. D.J.C. Yates and J. H. Sinfelt, J. Catal., 8 (1%7) 348. J.H. Sinfelt, Catal., Rev., 3 (1%9) 175. J.L Carter, JA Cusnmano, and J.H. Sinfelt, J. Phys. Chem., 70 (1966) 2257. G.C. Bond and X Yide, J. Chem. Sot. Faraday Trans. l., 80 (1984) 3103. G.C. Bond, RR. Rajq and R. Bunch, J. Phys. Chem., 90 (1986) 4877. C. &al and S. Ferrer, Surf. Sci., 178 (1986) 850. C.P. Lee and LD. Schmidt, J. Catal., 101 (1986) 123. S. Gao and LD. Schmidt, J. Catal., 111 (1988) 210. C. Lee, S. Gao, and LD. Schmidt, “Reactivity and Structure of Metal Catalysts,” in Preparation of Catalysts IV, Elsevier Science pub., (1987) 421. T. Chojnacld, K. Krause, and LD. Schmidt, J. CataL, 128 (1991) 161. G. Maire and F. Garin, J. Molec. Catah, 48 (1988) 99. E.H. van Broekhoven and V. Ponec, Prog. Surf. Sci., 19 (1985) 351. T.P. Chojnacki and LD. Schmidt, J. Catal., 115 (1989) 473. S. Gao and LD. Schmidt, J. Catal., 115 (1989) 356. T. Wang and LD. Schmidt, J. CataL, 66 (1980) 301.
282
17 18 19 20 21 22 23 2
26 27
C. Lee and LD. Schmidt, J. Electrochem. Sot., 136 (1989) 2471. M. Chen and LD. Schmidt, J. CataL, 56 (1979) 198. T. Wang and LD. Schmidt, Surf. Sci., 163 (1985) 181. E.B. Prestridge, G.H. Via, and J.H. Sinfelt, J. Catal., 50 (1977) 115. AK. Datye and NJ. Long, Ultramicroscopy, 25 (1988) 203. G. Bond, J. Catal., 115 (1989) 286. J. Burkhardt and LD. Schmidt, J. CataL, 116 (1989) 240. T. Wang and LD. Schmidt, J. CataL, 70 (1981) 187. A. Kiennemann, R. Breault, J.-P. Hindermann, and M. L.aurin, J. Chem. Sot., Faraday Trans. I, 83 (1987) 2119. Y.-F. Yu Yao, J. Catal., 87 (1984) 152. K,R. Krause, P. Schabes-Retchkiman, and LD. Schmidt, J. Catal., to be published.
CORREIATION
BETWEEN
ACITMTYANDSm-
I.D. -
269
MXROSTRUCSURE
AND
HYDROGENOLY’SIS
OF NOBLE METAL CATALYST PARTICLES’
and KR. KRAUSE?
Department of Chemical Engineering and Materials Science, University of Minnesota Minneapolis MN 55455
of~e~~p~,
~onel~on~~~~)givesdirectb~~~on
and locations of catalyst particles down to atomic dimensions. We have used TEM to correlate catalyst particle microstructure with reactivity and selectivity in alkane hydrogenolysis on & Pt, Ir, Ru, and Ni supported on SiO, by determi&q microreactor kinetics folltig heating stroq& in Hz at riDo”C(am~ealed) or oxidation in 0, ~~0~ reduction in H, to reform metal par@es
&her
by krw temperature
(oxidized). In these experiments we txBmine the
same catalyst repeatedly after these treatments, and thus effects due to loading or treatrnant differences are excluded. Ethane hydrogenolysisactivities are found to vary by up to 104 depending on oxidaticm or annealing pretreatment, and these changes are rever&e
in that atalysts can be cy&d
many times between these pretreatments. Selectivities and activation energies are also found ta vary significantly with these treatments, showing that treatment alters the structure of the surface, not just the surface area. TEM shows that high temperature oxidation followed by low temprature reduction in Hz transforms ~J.Iparticles from singe crystal metah (typica@
IWA diameter)
into tiny
clusters of metal particles (typically lak diameter) and that these microstructures are stable iudetitely
under reaction conditions, 18o-26ooC, Similar results are also reported far
Rh-Ce/SiC$ catalysts, but the iarge microstroctore change observed in TEM causes only a digfit enhancement in activity.
It has been known since the classic work of Sinfelt et al. [l-4] that hydrogenolysis of alkartes depends sensit.iveIyon dispersion of t&z meta& and more recently many investigaiom ‘936s research pwtially sponsored by NSF under Grant No. CBTgg22745.
210
have observed
a strong sensitivity to pretreatment
of the catalyst [5-q. Most of these
investigators observe different behaviour on different catalysts and infer structure sensitivity, while evidence of changes on a single catalyst sample gives much more direct information on structure sensitivity. For several years we have conducted systematic studies of the correlation particle microstructure
of catalyst
with reactivity of alkane bydrogenolysis [g-11] and CO hydrogenation
[lO,ll]. We find very large effects which agree very well with suggestions based on the effect of dispersion
and
microstructure
and reactivity. In this article we summarix
of C&,
pretreatment
but which explicitly
show the relationship
between
these results for the hydrogenolysis
C$&, and C,H,, on several metals supported on SiO, and also for the promoted
metal catalyst system Rh-CeBiO,. Hydrogenolysis is simply the cracking of an alkane in the presence of hydrogen to yield smaller alkanes and the mechanism has been the subject of several reviews [3,X2,13]. We examine the simplest of these systems: ethane, propane and n-butane,
C&+H2
= 2cH,
CJ-%+Hz
= C&-G+CH4
C,H,, + Hz = 2CJ-G C,H,, + H,
= CsHs + CHd
These are written as scission of one C-C! bond in the parent molecule, but of course multiple bonds may be broken. In all but ethane the selectivity of formation
of various
products is usually more important than overall activity, because one is frequently interested primarily
in selective hydrogenolysis
to make specific products
and methane
is usually
undesired.
Fixpedmental Metal particles on planar amorphous SiO, were used for TEM. The silica was prepared by vacuum depositing a -200
A filmof Si on a gold TEM grid which had been coated with
Formvar and then heating in 0, in a furnace to 800 T to oxidize the Si to SiO, and burn off the Formvar. Metals were then deposited by vacuum deposition of a 20 heating in H, at -600 ‘C to break up the fihu into 20-200
A filmfollowed by
A diameter particles. As shown in
Fig. 1, this produces isolated particles of fairly uniform size and spacing which have generally polyhedral outlines and are usually single crystals. Preparation
by decomposition
of salts [14]
has also been shown to produce the same general structures after heating in H, although
271
particle sizes and spacings are less uniform. Samples were examined using TEM after various heat treatments which were carried out by heating for several hours in flowing Hz or 0, in a tube furnace. A major advantage of these techniques
is that we are able to examine the same region of a sample repeatedly
following each heat treatment
so that we can follow the evolution
individual particles. This contrasts strongly with conventional
in microstructure
of
TEM of supported catalysts in
which one can only do “post mortem” analysis of samples after sectioning or other preparation techniques. This is possible because we use planar SiO, samples in which metal particles never overlap and which are stable enough to be transferred
repeatedly
between
furnace and
microscope to locate the same particles. Reactivity measurements These were all prepared
were in a microreactor
containing
by incipient wetness impregnation
(except Rh/Ce catalysts which were on Aerosil200)
- 0.1 grams of catalyst.
of metal salts on Cabosil SiO,
followed by calcining and heating in Hz
such that all metal loadings were 5% by weight. Metal surface areas were determined various stages of treatment
by Hz chemisorption
at
[U] and samples were heated such that
average particle sizes were - 100 A, the same approximate
size of particles used in TEM.
Reactants were typically 5% hydrocarbon, 35% H, and the balance He at atmospheric pressure with analysis by gas chromatography
[9]. Flow rates and total amounts of catalyst
were adjusted to maintain 2-10% total hydrocarbon conversion. This was important for higher alkanes to assure that continued hydrogenolysis of product fragments was minimized. The sequence of treatments
preparation
can be sketched as follows:
- H, 6OOT - reaction - 0,500T
with heat treatments
- H, 250°C - reaction
in flowing gases at temperatures
indicated for 4 hours and reaction at
180-26oT.
Fig. 1 shows TEZM micrographs of Rh particles on SiO,. Fig. la is after heating in H, while Fig. lb shows the same particles after heating in 0, at 5OOT. Upon conversion Rh,O,
to
the polyhedral metal particles have larger outlines and irregular shapes because the
oxide expands slightly and “wets” the SiO, XPS, electron diffraction, and dark field imaghtg all show that Rh metal is converted completely into microcrystalline
particles of RhzOs by this
treatment. However, upon heating in H, at 3OOT (Fig. lc), all particles shrink somewhat and
212
have tiny grams which electron diffraction and dark field imaging show to consist entirely of Rh metal. These clusters oniy slowly recrystallixe back into roughly their initial structures, approximately
as in Fig. la. The lower part of Fig. 1 indicates ideahxed cross sections of
particles shown in the micrographs treatment
to show the microstructures
indicated
in the
cyclic
process sketched above.
H2
600'1: Fig. 1. TEM micrographs of Rh particles on a planar amorphous SiO, film following treatments indicated for several hours at atmospheric pressure. Panel (a) is of single crystal Rh particles - 100 A indiameter, (b) is converted completely into crystalline Rh20s particles, and (c) is clusters of - 10 A diameter metal particles. The sketches show side views of particles inferred from the micrographs.
Thus, Fig. l(a) represents
the microstructure
of the “annealed” particles while Fig. l(c)
represents the “oxidixed” particles. Both are exclusively metal, and both have roughly the same location and size, but the annealed particles are polyhedral while the oxidized particles are clusters of tiny metal crystals. Hydrogen chemisorption
of supported catalysts shows that the
surface area of the Rh is higher by only a factor of two on the “oxidized” surface than on the “annealed” surface. Similar TEM experiments have been done for Ir [ 161, Ni [17], Pd [ 181, Pt [ 191, and Ru [20] and nonporous spheres have also been used as supports [21]. All metals except Pt readily form oxide and are readily reduced in H, to form the clusters similar to those shown in Fig. 1. Pt cannot be oxidixed by these treatments,
but we have shown [19] that Pt particles
form perfect cubes after heating in H, but spheres after heating in 0,. Fig. 2 shows ‘IEM micrographs of Ir particles on SiO, which reveal a side view of
273
Fig. 2. TEhf micrograph of Ir particles on SiO, showmg side views of particles after (a) annealing in H, at 6OOY!,(b) heating in 0s at SOOT, (c) heating in H, at 2XPC, and (d) heating in H, at 6OOT. Drastic changes occur upon forming IrO, (b) and reducing at low temperature (c). Structures are those sketched in the lower panel of Fig. 1
particles rather than the top view shown in Fig. 1. Here we can see that the particles are nearly as high as they are wide. The annealed particles are polyhedral, while the side view TEM shows that the Ir particles fhst become roughened near their surfaces and thus attain a totally different shape upon total oxidation, Fig. 2b. Reduction of IrOs, Fig. 2d, shows that all particles have formed small metal clusters, and the large particle at the right in the micrograph in fact appears to have cracked into two particles upon low temperature reduction to metallic Ir. These side view images confirm that particle shapes are approximately as indicated in the lower portion of Fig. 1.
Ethane J3ydrogenolysis We next see how these annealing and oxidizing treatments affect the reactivities of the catalysts, although the reactivity samples are necessarily porous rather than planar SiO, because it is necessary to have sufficient area to measure reactivity. Fig. 3 shows Arrhenius plots of the hydrogenolysis rate of CJ& (in moles CJ-& reacted per gram of catalyst per second) versus l!P for Rh, Ru, Ir, Ni, Pd, and Pt on SiO,. As noted above, all were for 5% loading with nearly identical preparation and treatment conditions. Surface areas are probably slightly different, but H, chemisorption measurements indicated that these varied by a much smaller amount than the 10’ to lo’ variations caused by oxidation-annealing treatments.
274
1o-4
I
I
I
I
I
I
I
I
1
2.2
2.3
1o-6 5I s f \M $
10-8
u” z -ii lo-lo
lo-l2 _ 1.8
1.9
2.0
2.1
1000/T (K-l)
Fig. 3. Plots of rates of Q& hydrogenolysis versus l/T on metals indicated at 5% loading on SiO, following high temperature heating in I$ (solid lines) and heating in 0, (dashed lines). Fig. 4 shows a plot of rate following oxidation and annealing sequences at 23OT for each metal Variations are largest for Rh and Ru, and smaller for Pt, Pd, and Ni. There is an irreversible sintering evident for all metals with repeated cycling, but the reversiile variations between oxidation and reduction cycles are much larger than this for all metals.
1o-4 Ru 0
1o-6 52 3 “h 1 \M zw 1o-a u” ‘;j s. $2 E lo-lo
annealed lo-l2
I
I
annealed I
oxidized
annealed I
oxidized
I
I
oxidized
Fig. 4. Plots of rates of CJ& hydrogenolysis at 23oOC on metals indicated on SiO, all at 5% loading. Oxidation produces catalysts which are much more active than after annealing in H, and these changes are reversible with continued oxidation-reduction cycling.
These dramatic and systematic variations with oxidation and reduction treatments have obvious correlation Treatment
with and interpretation
in Oz followed by low temperature
from the TEM micrographs
of Figs. 1 and 2.
reduction to metal produced very small
216
crystallites (- 10 A>which have a much higher activity than do the large (- 100 upon annealing
A>particles
Thus it is clear that there is a strong surface structure effect in C&
hydrogenolysis. This is much larger than could be accounted for by surface area changes, and the change in reaction activation energies evident in Fig. 3 shows that the mechanism
of
reaction must also be changing between the treatments. Several factors explain the differences between the metals. (1) Pt does not form a bulk oxide
under
these
transformation treatment
treatment
conditions,
and
therefore
camrot occur. All changes are therefore
the
metal-to-oxide-to-metal
due to shape changes caused by
in the gases. [19] (2) Ni and Pd sinter at so low a temperature
oxide is reduced temperature
and the metal sinters back to the annealed
(<2OOT) that the
state before the reaction
is reached. (3) Ir is quite refractory, and it may not be totally annealed to the
single crystal structure during H, treatment
at 600°C.
Butane and Propnue Eydrogenolysis Fig. 5 shows total rates of hydrogenolysis of C&Is and C,H,, on RhDiO, at 230°C. Rates are larger for the higher hydrocarbons
as expected
from the relative
rates of
chemisorption of primary and secondary carbons. While there is an overall decrease in activity with repeated treatment
in both annealed
and oxidized states due to irreversible
sintering,
there is a reversible variation in the total activity with C&Is and C,H,, of approximately
a
factor of 10 between annealed and oxidized treatments. This is somewhat smaller than the 104 variation noted for CJ& Fig. 6 shows Arrhenius plots of total activity and formation of products (C&Is, C$&, and Cl&) in C,,H,, hydrogenolysis. temperature
The curves show that activation
and that selectivities also are highly temperature
at low temperatures
energies vary with
dependent with very little CH,
and much more CH, at high temperatures.
The annealed surface makes much less CH_,than does the oxidized surface, and C-&V& is the dominant
product from C,H,, on the annealed
behaviour correlates well with microstructure
surface even at 26ooC. Again this
changes observed by TEh4, Figs. 1 and 2. The
oxidized surface with very small particles is much more reactive and tends to completely decompose the hydrocarbons into small fragments, while the less reactive annealed tends to break only one bond to form either two C& of parallel
and series reactions
have been
surface
molecules or CsHs and CH_,. The ideas
discussed in detail by Bond [22]. We also
investigated carefully the role of further hydrogenolysis and the extent to which the process can be regarded as a simple breaking of one C-C bond versus intermolecular carbon hydrogenation
[9].
reactions and
277
I
I
I
I
I
I
-5 -
6,
annealed
annealed
annealed
Fig. 5. Plots of rates of h~eno~~ of C$&, C&Z&,and C,H,, on 5% Rh on Si02 at 230% fobwing anrmabg in Hz and heating in 0, as dmrii previously.All rates vary by factors of > 10 by these treatments.
*O-91 1.8
I
1.9
I
I
I
2.0
2.1
2.2
1ooo/T UC-‘)
Fig. 6. Plots of hydrogenolysis rates of C.,H,, to form CI& C$&, and C;rg on 5% Rh on SiO, following treatments of catalyst in Hz (dashed lines) and in Oz (solid lines). Rates differ by - 100 between treatments, and selectivities to form various products are also different.
Ethne Ey~no~sis
on Rh-Ce/Si02 Cataiysts
The above results were on single metals on SiO, the most inert support in common
usage. We have observed similar microstructure
changes on A1,0, [23,24] and TiO, altbougb
we have not attempted similar reactivity characterization.
Ce is an important additive to noble
metals which alters rates and selectivities of several reactions ~clu~g We examined the microstructures
h~~no~~.
125,261
of Rh-Ce on SiO, using TEM 111,271 with methods
identical to those used for noble metals alone. We observed a dramatic alteration in the Ce between H, and 0, treatments as shown by the micrographs of Fig. 7 and the sketches below the micrographs. In H, the Ce forms noncrystalline
Ce+3 (both ce,O,
and Ce.&O,
phases)
which forms a rather uniform thin film which covers the SiO, while in & Ce forms crystalline CeO, crystals which associate with the oxidized Rh particles. This system is therefore interesting candidate to see how these changes affect hydrogenolysis selectivity.
an
279
a
b
Ce02 SiO2
After oxidation Fig. 7. TEM micrographs of Rh-Ce on SiO, (a) after heating in Hz at 6OOT and (b) after heating in 0, at 5OO“C.After heating in H, the Ce forms an amorphous -0, fihn on the SiO, while the Rh forms particles. After heating in 0, the Ce forms crystalline CeO, particles which associate with the Rh,O, particles. The presence of Ck increases the hydrogenolysis rate slightly, but the change is much smaller than the large changes in microstructure suggest.
Fig. 8 shows Arrhenius
plots of C&
hydrogenolysis activity of Rh with and without
addition of Ce after amrealing and oxidation treatments.
The presence of Ce increases rates
slightly (about a factor of 2), but this difference is much smaller than the differences produced by annealing and oxidizing Rh alone, as ilhrstrated in Fig. 3. We conclude, therefore, that the
Fig- 8. Plots of C.& bydsogenolysis ratesversus rmpature following treatmentunder conditions indicated for RM!e on SO2 for Ce loadings indkated. The presece of Ce increamt&e satessligbtly~ but t&edifference belween treatment eundltions is mu& larger.
281 microstmctural
changes in Rh which alter hydrogenolysisactivity are not strongly affected by
the presence of Ce even though TEM showsthat Ce associates stronglywith the Rh particles~ Evidently the reactive sites on the Rh are not stronglyaffected by the presence of Ce, and the only effect of adding Ce is to somehow slightly increase the activity of the Rh, perhaps through an increase in the Rh surface area.
These simple experiments demonstrate that there is a strong correlation between catalyst microstructure and activityand selectivityin alkane hydrogenolysis.The effects are (1) very large, typicallyat least a factor of 10, (2) nearly reversible, (3) comparable for several noble metals, and (4) quite reproducible between catalyst sampks. Since the variations are on single samples, it is clear that effects are caused by microstructure changes, not simply differencesbetween structures of samples with different loadings or preparation procedures. The interpretation of these observations also seems to be clear in that on the small clusters there are mostly low oration
sites whose activity is at least a factor of 10 h&her
than on the polyhedralannealed s&aces whichexpose predominantly(111) and (100) planes. These low coordination sites must be many orders of magnitude more reactive than those on (111) and (100) planes because there must be some of them even on annealed surfaces because of edges and comers. The absence of these highly reactive sites may in some situationsbe desirable if one wants a catalyst which is highlyselective in cracking a&anes to smaller aJkanesbut not to CH.,. These results show that variations with treatment can in many cases be even larger than variations between different metals.
References :. 3 4 5 6 7 8 9 10 11 12 13 14 15 16
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