Hydrogenolysis and isomerization of n-butane on low-index Pt single crystals and polycrystalline Pt foil

Article abstract of DOI:10.1006/jcat.1996.0060

The hydrogenolysis and isomerization of n-butane over Pt(100) and Pt(111) single crystals and a poly crystalline Pt foil have been studied over a wide range of reaction temperatures (510-610 K) and hydrogen partial pressures (7.5-250 Torr). The activity and selectivity have both been determined as a function of temperature and hydrogen partial pressure. The more open Pt(100) surface was an order of magnitude more active than the close-packed Pt(111) surface. Hydrogenolysis selectivities were invariant with reaction temperature over the range studied. Hydrogen partial pressure affected both the activity and product distribution. Both single-crystal surfaces exhibit a decrease in the activity as the H₂ pressure is decreased below a critical amount and this decrease is directly attributable to the formation of a carbonaceous surface residue. The amount of isomerization was always less than the amount of hydrogenolysis regardless of the n-butane to hydrogen ratio or the reaction temperature.

Full text of DOI:10.1006/jcat.1996.0060

JOURNAL OF CATALYSIS 159, 23–30 (1996)
ARTICLE NO. 0060
Hydrogenolysis and Isomerization of n-Butane on Low-Index Pt
Single Crystals and Polycrystalline Pt Foil
Shane L. Anderson,ꢀ Janos Szanyi,† Mark T. Paffett,† and Abhaya K. Datyeꢀ
ꢀ
†
Department of Chemical & Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131; and Chemical Science & Technology
Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Received January 12, 1995; revised October 20, 1995; accepted November 27, 1995
catalyst system, one in which three-dimensional particle
shapes and the catalytic reactivity of the metal surfaces
can both be determined. This is accomplished by using ox-
ide particles with simple geometric shapes as model sup-
ports. Nonporous oxide supports have the advantage that
the metal particles are located exclusively on the convex
surfaces of the oxide particles and therefore can be exa-
mined readily by high-resolution transmission electron mi-
croscopy. Using these model supports, we have found that
large particles of Pt supported on silica exhibited a morpho-
logical dependence on the atmosphere in which they had
been treated (9). When heated at 923 K in 200 Torr of H2,
large Pt particles exhibited well-defined cubo-octahedral
shapes with prominent (111) surface facets. When the
particles were subsequently heated in 70 Torr of O2, the
particles become much more rounded with the (111) facets
being less well defined. This thermally induced restructur-
ing was completely reversible. Studies of the reactivity be-
havior of these model catalysts of controlled morphology
are presently under way in our laboratory. To establish a
connection between morphology and reactivity, we have
chosen to examine hydrocarbon conversion reactions on
these Pt catalysts.
In view of its commercial importance as a catalyst for
hydrocarbon conversion reactions, the behavior of Pt cata-
lysts has been intensively studied. Bond (3) has elegantly
summarized our current understanding of the variables that
control the catalytic behavior of supported Pt (1–3). The
study of well-defined single crystals of Pt by Somorjai and
co-workers (10–12) provides direct information about the
role of surface structure on reactivity of hydrocarbons. One
problem encountered when comparing literature data from
single-crystal investigations with that from supported cata-
lysts is the large discrepancy in the reaction conditions used.
The majority of studies on supported Pt are performed un-
der conditions of flowing gases at atmospheric or higher
pressure while single-crystal studies have been carried out
in batch reactors, usually at subatmospheric pressures. The
reactant pressure, as will be shown, can lead to quite signi-
ficant differences in catalytic behavior.
The hydrogenolysis and isomerization of n-butane over Pt(100)
and Pt(111) single crystals and a polycrystalline Pt foil have been
studied over a wide range of reaction temperatures (510–610 K) and
hydrogen partial pressures (7.5–250 Torr). The activity and selec-
tivity have both been determined as a function of temperature and
hydrogenpartialpressure. ThemoreopenPt(100)surfacewasanor-
der of magnitude more active than the close-packed Pt(111) surface.
Hydrogenolysis selectivities were invariant with reaction tempera-
ture over the range studied. Hydrogen partial pressure affected both
the activity and product distribution. Both single-crystal surfaces
exhibit a decrease in the activity as the H2 pressure is decreased
below a critical amount and this decrease is directly attributable
to the formation of a carbonaceous surface residue. The amount
of isomerization was always less than the amount of hydrogenol-
ysis regardless of the n-butane to hydrogen ratio or the reaction
c
temperature. ꢀ 1996 Academic Press, Inc.
INTRODUCTION
The effect of supported metal morphology on catalytic
reactivity has been the topic of a large number of inves-
tigations over the past twenty-five years (1–5). Generally,
morphology is conventionally varied by changing the size
of the metal particles (which in turn affects the coordina-
tion of surface atoms and their environment). Metal parti-
cle morphology is also thought to be affected by the oxide
support, but conclusive evidence is often lacking since par-
ticle shapes in supported catalysts are not easy to define
unequivocally. More recently, there have been a number of
studies where metal particle shapes were varied by anneal-
ing particles in different gaseous atmospheres at elevated
temperatures (6–8). These experiments were performed on
model systems where the metal was evaporated on planar
thin films of oxide support such that particle shapes could
be unambiguously determined. These model systems, how-
ever, are not convenient for reactivity measurement. Hence
the precise effect of metal particle shape and morphology
on catalytic reactivity continues to be difficult to establish.
We have chosen to work with a different kind of model
23
0021-9517/96 $18.00
c
Copyright ꢁ 1995 by Academic Press, Inc.
All rights of reproduction in any form reserved.

24
ANDERSON ET AL.
The catalytic properties of supported Pt model catalysts of cleaning the crystal with an oxidation/anneal cycle, an-
are currently being investigated in our laboratory, however, alyzing the surface composition with AES, retracting the
as mentioned above, there is only limited kinetic data avail- crystal into the reactor and performing the analytic reac-
able on single-crystal Pt model catalysts with which these tion, evacuating the reactor, and finally performing post
results on supported catalyst can be compared. Therefore, reaction surface analysis.
as a first step, we have examined the catalytic behavior
Reactant gases were research grade n-butane (99.9%),
of Pt(111) and (100) single crystals and polycrystalline Pt UHP H2 (99.9995%), UHP O2 (99.9985) from Matheson,
foil. The structure-sensitive reaction of isomerization and and UHP Ar (99.9998%) from Trigas. The n-butane was
hydrogenolysis of n-butane have been chosen as probe re- further purified by a set of five freeze/thaw cycles. The H2
actions. We have employed awiderangeof reactiontemper- was stored in a glass bulb under liquid nitrogen during use.
atures and hydrogen partial pressures so that comparisons The main impurities in the n-butane feed were ꢄ0.04%
can be made with the silica-supported Pt catalysts. In a sub- propane and ꢄ0.06% iso-butane as measured by gas chro-
sequent publication, we will report the reactivity behavior matography. All gas analysis was done with either an HP
of supported metal catalysts whose morphology has been 5400 or an HP 5890 gas chromatograph equipped with a
varied by annealing treatments to gain insight into the role flame ionization detector. Separations were accomplished
of Pt surface morphology on catalytic behavior.
with either 6-ft Poropak N or 6-ft 0.19% picric acid on Car-
bowax packed columns.
EXPERIMENTAL
RESULTS
All reaction experiments and surface analysis were per-
formed in one of two systems. Each was a UHV chamber
coupled to a high-pressure reactor via a gate valve. The base
The number of Pt atoms was assumed to be
1.505 ꢂ 1015/cm2 for the (111) crystal and 1.470 ꢂ 1015/cm2
for the (100) crystal. The atom density for the polycrys-
talline foil was assumed to be 1.5 ꢂ 1015 Pt atoms per cm2.
Activity measurements are reported as a turnover fre-
quency (TOF), i.e., the number of n-butane molecules that
react per surface Pt atom per second. The TOF was calcu-
lated using the following equation:
ꢃ10
pressure for each system was 2.0 ꢂ 10
Torr. Typical op-
erating pressures were between 7.0 ꢂ 10ꢃ10 and 3.0 ꢂ 10ꢃ9
Torr. The first system had the capability for a wide range of
surface analysis experiments, including LEED, XPS, AES,
inert gas LEISS, and thermal desorption experiments. The
second system utilized AES and TPD to monitor surface
composition, in addition a variable energy ion gun was
available for sample cleaning. Both systems have been de-
scribed in detail elsewhere (13, 14).
[(PnC )(Vrctr)/RT ]XnC NA
4
4
TOF =
,
2strxn
where 2s is the number of surface Pt atoms, XnC is the
fractional conversion, NA is Avagadro’s number, trxn is the
reaction time in seconds, Vrctr is the reactor volume (liters),
The Pt(100) and Pt(111) single crystals and the polycrys-
talline foil had an exposed surface area of approximately
1 cm2. The crystals were spot welded between two tungsten
heating leads that were in turn spot welded to two stainless
steel supports. The supports were mounted on the copper
feed throughs of a standard UHV sample manipulator. The
samples were resistively heated by a DC power supply that
was current controlled by a feedback PID controller. Sam-
ples could be heated to 1300 K. The crystals and foil were
initially cleaned by a series of sputter/oxidize/anneal cycles.
The samples were sputtered with 2.5 keV Ar ions at approx-
imately 10–15 ꢀA for 20 min with TPt = 873 K. The samples
were then oxidized at 923 K in 5.0 ꢂ 10ꢃ7 Torr of O2. Fol-
lowing the oxidation, the crystals were then annealed to
1250 K.
The long-range ordering of both crystals has been char-
acterized by LEED and each exhibited a sharp pattern
corresponding to its orientation. Carbon monoxide ther-
mal desorption was performed on each of the crystals and
the results closely matched literature observations (15, 16).
The surface composition was routinely monitored for car-
bon, oxygen, and impurities such as Si, S, and B diffusing
out from the bulk. A typical reaction sequence consisted
4
PnC is the initial pressure of the n-butane in the reactor
4
(Torr), T is the temperature (K), and R is the ideal gas
constant (liter Torr)/(mole K).
In Fig. 1 the activity for the hydrogenolysis and isomer-
ization of n-butane over each of the Pt catalysts is shown
in Arrhenius form. The reaction conditions were 50.0 Torr
H2 and 0.5 Torr n-butane. The activation energies, deter-
mined from the least-squares fit of the Arrhenius plots, are
25.0 ꢅ 2.9 and 27.3 ꢅ 3.4 kcal/mole for the (100) and (111)
facets, respectively. The activation energies were computed
by a least-squares fit to the data. The Pt(100) crystal face is
nearly an order of magnitude more active than the Pt(111)
face for the hydrogenolysis of n-butane, while the polycrys-
talline Pt was approximately a factor of two more active
than the Pt(100) crystal. The TOFs reported here for both
crystals differ from those reported by Davis et al. (10), due
in part to the different reaction conditions used. Davis et al.
used a H2 : nC4 ratio of 10 : 1 and a total pressure of 220
Torr while the total pressure used in our study was 50.5 Torr
with a H2 : nC4 ratio of 100. As will be shown below, the H2

n-BUTANE OVER Pt(100) AND Pt(111)
25
FIG. 3. Selectivity of n-butane hydrogenolysis and isomerization as a
function of hydrogen partial pressure over Pt(100).
FIG. 1. Arrhenius plot for the hydrogenolysis and isomerization of
n-butane over Pt(100), Pt(111), and polycrystalline Pt foil.
remains constant over the temperature range investigated
and is approximately equal for the (100) and (111) surfaces.
The higher methane production seen on the (100) plane
comes at the expense of the propane formation, which is
significantly lower over the (100) surface than on the (111)
surface. For Pt(111), the average methane to propane ratio
is 2.1, while for Pt(100) the ratio is 13.6. Over the tem-
perature range 533 to 575 K, the selectivity of the polycrys-
talline foil, not shown here, was identical to that of the (100)
surface.
partial pressure greatly influences the catalytic activity for
hydrogenolysis.
The product distributions, as a function of reaction tem-
perature, for both surfaces are shown in Fig. 2. The first
salient feature to point out is that over the temperature
range studied, neither surface is capable of isomerizing
n-butane to iso-butane. The hydrogenolysis products do not
vary significantly with temperature for either surface. Both
surfaces exhibit a high degree of multiple hydrogenolysis,
–
i.e., cleavage of more than one C C bond in the molecule.
The effect of the hydrogen partial pressure on the selec-
tivity for the (100) surface is depicted in Fig. 3. It should be
noted here that the total pressure for this set of experiments
was held constant at 250.5 Torr and the pressure of the
n-butanewasheldat0.5Torr. AstheH2 pressurewasvaried,
the balance was made up with argon. Clearly, the extent of
isomerization is dependent on the ratio of hydrogen to hy-
drocarbon. The amount of iso-butane produced gradually
increases with decreasing hydrogen partial pressure. Also,
as the H2 to hydrocarbon ratio drops below 25, the rate of
ethane formation begins to increase, and at an H2 pressure
of 7.5 Torr, over 30% of the product formed is ethane, while
the amount of propane produced falls to zero.
The multiple hydrogenolysis is manifested in methane be-
ing the dominant product over both surfaces. The amount
of ethane formed, from cleavage of the central C C bond,
–
The PH dependence of the product selectivities for
2
Pt(111) is displayed in Fig. 4. Unlike the Pt(100) surface, the
extent of n-butane isomerization is negligible on Pt(111)
and iso-butane initially present as contamination in the
n-butane feed in fact begins to crack at low hydrogen par-
tial pressures. The negative amount of iso-butane shown
at the lowest H2 pressure is caused by the procedure used
to correct for the iso-butane present in the feed, a proce-
dure that is not perfect when dealing with small amounts
of iso-butane produced. Multiple hydrogenolysis seems to
be subsiding at the low hydrogen pressures as evidenced by
FIG. 2. Selectivity of n-butane hydrogenolysis and isomerization as
a function of reaction temperature over Pt(100) (left panel) and Pt(111)
(right panel).

26
ANDERSON ET AL.
FIG. 6. Activity for n-butane hydrogenolysis and isomerization and
corresponding carbon to Pt Auger ratio as a function of hydrogen partial
pressure for Pt(111).
FIG. 4. Selectivity of n-butane hydrogenolysis and isomerization as a
function of hydrogen partial pressure over Pt(111).
the increased amount of ethane and propane and the lower
methane selectivity.
accompanied by the buildup of a carbonaceous residue on
the surface of the crystal as was determined by postreac-
tion AES analysis and is shown in Fig. 5. Davis et al. (11)
have reported a similar poisoning of Pt surfaces by depo-
sition of a nongraphitic form of carbon. A similar activity
pattern is observed for the Pt(111) surface as displayed in
Fig. 6. As the hydrogen partial pressure is lowered, a rapid
decrease in the activity is seen. Again, the accumulation of
carbonaceous deposits on the catalyst surface is evident at
low H2 pressures. The direct comparison of the activity pat-
terns of the two surfaces is not straightforward because of
the different total pressure applied on the two surfaces.
Figure 7 demonstrates the deactivation of the Pt(111)
surface as a function of reaction time at a given reaction
temperature and reactant ratio. The procedure for this ex-
periment was to run the reaction for a given time, analyze
the reaction gas mixture with gas chromatography, transfer
the catalyst into UHV to measure the C/Pt ratio, and then
perform another reaction on the carbon-covered surface
without cleaning the crystal. The activity drops by a factor
of 10 after a reaction time of 25 min whereas the amount of
carbon deposited increases. This deactivation is also seen
for both the (100) and polycrystalline Pt samples. For the
Pt(100) surface, the deactivated surface is approximately
10% as active as the initially clean surface. It is interest-
ing to analyze the product distribution for each surface as
a function of reaction time. Figure 8 shows the change in
product selectivity as a function of reaction temperature
over the Pt(111) surface. The amount of methane formed
on the carbon-covered surface is significantly higher than
that on the clean surface, i.e., 60% as opposed to 38%. This
increase in methane selectivity comes at the expense of both
ethane and propane which are seen to decrease. For the
The effect of hydrogen partial pressure on the activity of
the Pt(100) is depicted in Fig. 5. Clearly, the activity goes
through a maximum over a H2 pressure range of approx-
imately 50 to 100 Torr which corresponds to hydrogen to
n-butane ratio between 100 and 200. As the H2 pressure
is increased further the activity drops. Such behavior has
been reported in the literature for other Group VIII met-
als (17). The decrease in the activity at low H2 pressures is
FIG. 5. Activity for n-butane hydrogenolysis and isomerization and
corresponding carbon to Pt Auger ratio as a function of hydrogen partial
pressure for Pt(100).

n-BUTANE OVER Pt(100) AND Pt(111)
27
after about 10 min. No attempt was made to extrapolate
back to zero reaction time due to the rapid deactivation
at short times and the consequent difficulty of getting a
precise estimate of the initial rate. The reaction times varied
with temperature, with shorter times being used at higher
temperatures so that conversions did not exceed 5%.
The higher reactivity of the (100) surface compared to
the (111) surface is similar to that seen for n-butane hy-
drogenolysis on Rh (18) and ethane hydrogenolysis on Ni
(19). The higher activity of the (100) surface has been at-
tributed to the more open and atomically rougher surface
structure of this crystal face (19). This interpretation is con-
sistent with the observed activity of the polycrystalline foil
being even greater than that of Pt(100), since the poly-
crystalline foil has a larger fraction of surface defects and
grain boundaries and therefore a larger number of coordi-
natively unsaturated surface atoms. One of the most impor-
tant structural differences between Pt(111) and Pt(100) is
the presence of threefold hollow sites on the former sur-
face. It has been shown (20–22) that these threefold hollow
FIG. 7. Activity and carbon to Pt Auger ratio for n-butane hy-
drogenolysis and isomerization as a function of reaction time for Pt(111).
Pt(100) and polycrystalline surfaces, the selectivities are in- sites are capable of forming stable alkylidyne species which
variant with the reaction time, and the methane selectivity are unique to surfaces that have threefold symmetry. If the
remains between 60 and 75% for both surfaces.
surface intermediate formed on the (111) surface is much
more stable than that formed on the (100) surface a lower
activity should be expected for the (111) surface.
DISCUSSION
Another significant difference between the (111) and
(100) surfaces is their ability to bind hydrogen. Any differ-
ence in surface hydrogen concentration is important during
the hydrogenation of the adsorbed hydrocarbon species
The results of this study show that the Pt(100) surface is
over an order of magnitude more reactive than the Pt(111)
surface for the hydrogenolysis of n-butane under identical
reaction conditions. The polycrystalline Pt foil is about a
factor of 2 greater in activity than the Pt(100) surface. The
activitiesreportedintheArrheniusplotsarefordeactivated
surfaces and are not the initial rates. Deactivation was quite
rapid, and for example at 575 K, the activity leveled off
–
after C C bond cleavage. It has been demonstrated (23,
24) that hydrogen bonds to Pt(100) more strongly than to
Pt(111), therefore the (100) surface should have a higher
relativecoverageofHatoms. Consequently, thehydrogena-
tion of the adsorbed hydrocarbon species is often enhanced
on the (100) surface thus resulting in higher hydrogenolysis
activities. This is consistent with the data shown on Figs. 5
and 6 where the rollover in reaction order from positive to
negative occurs at a lower pressure for the (100) surface.
Under our experimental conditions, the rate of reaction on
Pt(111)remainedpositiveordereven atthehighestH2 pres-
sures investigated. These results for Pt(111) and Pt(100) dif-
fer significantly from those of Davis et al. (10), who found
that the activity of Pt(100) was nearly the same as that for
Pt(111) at 573 K. The discrepancies could be caused, in part,
by the differences in reaction conditions applied. The par-
tial pressures of the reactants used in the study of Davis et al.
(10) were 10 Torr of n-butane and 200 Torr in H2, whereas
the activities in this study were determined in a gas mixture
of 0.5 Torr n-butane and 50 Torr H2.
Also associated with the different surfaces are changes
intheirrespectiveproductdistributions. Thepolycrystalline
and (100) surfaces of Pt exhibit a higher degree of multiple
hydrogenolysis, manifested in the high methane selectivity,
than does the (111) plane of Pt. The general trends in pro-
duct selectivities are in accord with product distributions
FIG. 8. Selectivity for n-butane hydrogenolysis and isomerization as
a function of reaction time for Pt(111).

28
ANDERSON ET AL.
observed for the Rh(111) and Rh(100) (18) surfaces and for isomerization. The reason for the high isomerization selec-
the Ir(111) and Ir(110) (25) surfaces. Clearly, the selectivity tivity reported by Davis et al. (10) is not clear from analysis
for alkane hydrogenolysis is sensitive to the morphology of results reported here or in the literature.
of the catalyst surface. The selectivities are invariant with
Hydrogenolysis and isomerization of n-butane were per-
temperature over the range studied, as might be expected formed on the Pt(100) surface at 575 K and 20 Torr on
for catalysts that exhibit significant multiple hydrogenolysis n-butane and 200 Torr of hydrogen. Again, the selecti-
reactions.
vity towards hydrogenolysis far exceeded that of isomeriza-
The selectivities observed in our study are in marked con- tion. Under these conditions 18% iso-butane was formed,
trast to those reported by Davis et al. (10), who found that withthehydrogenolysisselectivityformethane, ethane, and
Pt(111) exhibited a statistical distribution (i.e., one third propane equal to 52, 21, and 9%, respectively.
each of methane, ethane, and propane) of hydrogenoly-
A comparison of the product distributions for the (100)
sis products while Pt(100) yielded 60% ethane and equal and (111) surfaces reveal both differences and similarities
amountsofmethaneandpropane. Inadditiontodifferences between the two surfaces. Both surfaces exhibit a prefer-
in hydrogenolysis selectivity, Davis (10) reported that both ence for methane production and both produce approxi-
the (100) and (111) surfaces yielded more isomerization mately equal amounts of ethane. Besides the striking sim-
products than hydrogenolysis products. For all conditions ilarity of producing about the same amount of ethane
used in our study, the selectivity toward hydrogenolysis far (ꢄ25%) its selectivity is independent of the temperature,
exceeded that of isomerization. In fact, even at the low- over the range studied. This could be explained by the pos-
est reaction temperature studied, the amount of methane sibility that the ethane is produced on defect sites such as
formed was greater than what would be expected from a edge or corner atoms. Both crystals were approximately
statistical distribution. The relative temperature invariance 1 cm2 per side and 1 mm thick. This would suggest that the
of the product distribution indicates that the mechanistic number of defect atoms would be nearly the same, which
pathwayisnotchangingoverthetemperaturerangeinvesti- then would imply that the Pt atoms on both (100) and
–
gated. It has been suggested (30) that the n-butane adsorbs (111) surfaces cleave terminal C C bonds preferentially.
through the 1,2-carbon atoms and this leads to the pre- Differences between the surfaces manifest themselves in
–
ferential cleavage of the terminal C C bonds. Breaking of the amount of C3Hy that is hydrogenated off the surface
the terminal bond will yield CHx and C3Hy surface species. to form propane. On the (100) surface practically all of
–
The C3Hy can undergo further cleavage to yield C2Hz and the C3Hy species undergo further C C bond cleavage to
CHa or it can be hydrogenated off from the surface to form form exclusively methane. On the (111) surface a portion
propane. Our product distribution results suggest that the of the C3Hy is hydrogenated off to form propane and the
–
predominantpathwayforethaneformationistheC Cbond remainder undergoes hydrogenolysis to produce methane
cleavage of the adsorbed C3Hy fragments. and ethane. The implication here is that the C3Hy species is
The very high isomerization selectivities reported by more strongly adsorbed to the Pt(100) surface than to the
Davis et al. (10) are somewhat surprising since the great (111) surface.
majority of studies of n-butane and hydrogen on Pt report
Low-energy electron diffraction was used to determine
significantly less isomerization than hydrogenolysis. Seve- that the crystals were highly ordered, therefore one cannot
ral other research groups that have studied the reaction of attribute the different product distributions to imperfec-
n-butane and hydrogen (on supported Pt and oriented Pt tions in these crystals. We therefore conclude that the differ-
films) report significantly less isomerization activity, where ent selectivities in the earlier study by Davis et al. (10) must
in fact the hydrogenolysis activity is always greater than the be attributed in part to the different partial pressures or
isomerization activity (26–29). For n-butane plus H2 on an differences in methods used to correct for the background
oriented Pt(100) film, Anderson and Avery (26) report an concentration of iso-butane in the n-butane feed.
isomerization selectivity of 16% at 573 K and H2 : nC4 of
The experimental determination of the hydrogen partial
12. The same group reports an isomerization selectivity of pressure dependence is not straightforward since the rapid
12% on a Pt(111) oriented film at a reaction temperature deactivation at low H2 partial pressures makes it difficult to
of 593 K. Their investigation of n-butane isomerization on obtain reliable reaction rates. However, it is clear that the
an unoriented Pt film yield 11% selectivity at 553 K.
low activity at low partial pressures of H2 is caused by large
Dowie et al. (27) also report isomerization selectivities amounts of carbon that are deposited on the surfaces at low
which are less than hydrogenolysis selectivities for unori- H2/n-butane ratios. This is different from what is seen for
ented Pt films. Their isomerization selectivity values range either Ir (25) or Rh (18). For Ir it has been shown that as
from 8 to 44% depending on the reaction conditions. Two the H2 pressure decreases, the activity levels off but does
other groups (28, 29) studying the reaction of n-butane and not drop (25). This has been ascribed to a change in the
–
hydrogen on supported Pt also report that hydrogenoly- rate-limiting step from C C bond cleavage to desorption of
sis is the dominant reaction pathway when compared to the hydrogenolysis products. At H2 : nC4 ratios greater than

n-BUTANE OVER Pt(100) AND Pt(111)
29
150, the rate can be expressed as the competitive adsorption
of H2 and n-butane with the cleavage of C C bonds being
CONCLUSIONS
–
The activities and selectivities for the hydrogenolysis and
isomerization of n-butane have been measured for poly-
crystalline Pt and Pt(100) and Pt(111) single crystals. The
experiments were performed over wide ranges of reaction
temperatures and H2 partial pressures. The open (100) sur-
face was approximately an order of magnitude more ac-
tive than the close-packed (111) surface, in accordance with
what has been reported for Rh and Ni single crystals. Pro-
duct distributions for the single crystals were different in
that the (100) face had a higher methane selectivity, indi-
cating more multiple hydrogenolysis. Neither surface pro-
duced substantial amounts of iso-butane, as the quantity
of hydrogenolysis products was always much higher than
thatofisomerization. Bothsingle-crystalsurfacesshowcon-
siderable deactivation at low H2 pressures due to carbon de-
position, withslightchangesintheselectivityatlowH2 pres-
sures. Also, both surfaces demonstrate a loss in activity as a
function of reaction time and concomitantly there is an in-
crease in the amount of carbon on the surface. These results
will be used to provide a baseline for determining the effect
of supported particle microstructure for a silica-supported
Pt catalyst where the morphology can be controlled by ther-
mal treatment in different gaseous atmospheres.
the rate-limiting step. This has also been reported to be
the case for n-butane hydrogenolysis over Ir single crystals,
but for H2 : nC4 ratios greater than 20. The decrease in the
concentration of surface hydrogen is regarded to be respon-
sible for this change in rate limiting step. On Pt surfaces, it
appears that the carbon deposited at low H2/n-butane ra-
tios acts to block or modify the available reaction sites. For
Pt(111), not only is the amount of surface carbon at low H2
pressures causing a decrease in activity but it changes the
product distribution as well. One possible explanation for
this observation is that the carbon preferentially blocks the
–
sites where internal C C bond cleavage takes place, thus
yielding the higher methane concentration. A second pos-
sibility is that the surface carbon is being hydrogenated and
desorbing as methane. This change in selectivity is not seen
on either the Pt(100) or polycrystalline Pt surface, where
the amount of methane is already high.
As seen from Fig. 5, there is a maximum in activity with
increasing H2 pressure, and such a maximum has also been
reported to occur over supported Pt/silica catalysts (3, 29).
Competition between the parent hydrocarbon and H2 for
surface sites leads to a drop in activity at high H2 pressures.
Comparison of Figs. 5 and 6 shows that the TOF begins
to roll over at a higher hydrogen pressure for the Pt(111)
surface than for the Pt(100) surface. This clearly supports
the evidence that the (100) surface has a higher concen-
tration of hydrogen than the close-packed (111) surface, at
an equivalent temperature. Both surfaces exhibit a loss in
activity as a function of reaction time, for a given temper-
ature. This loss in activity correlates well with a increase
in the amount of carbon on the surface. In addition to the
change in activity, in Pt(111) surface also shows a change
in selectivity with the more deactivated surface producing
an increased amount of methane. What is not clear yet is
whether the increased rate of methane formation is a result
of the hydrogenation of the accumulated surface carbon
or an enhancement in the terminal bond scission pathway
relative to the internal bond scission pathway.
Two important aspects have emerged from this study, the
first is that neither Pt(100) or (111) are capable of the iso-
merization of n-butane to iso-butane under the experimen-
tal conditions of this study. The second aspect is that dif-
ferences in hydrogenolysis activities and selectivities exist
between the two crystal faces and these differences are sig-
nificant enough to help in the study of supported Pt catalysts
with known particle morphology. The aim of this study has
been to establish the effect of temperature and H2 partial
pressure on the catalytic behavior of unsupported, well-
defined Pt surfaces for the reaction of n-butane and hydro-
gen. These results will be used to elucidate the effect of
particle microstructure on the reaction of n-butane and H2
for silica supported Pt catalysts.
ACKNOWLEDGMENTS
This research has been supported by the American Chemical Society,
Petroleum Research Fund Grant 28656-AC5. S. L. Anderson would like
to thank the Los Alamos Graduate Research Program.
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