Supported metal catalysts; preparation, characterisation, and function: Part VI. Hydrogenolysis of ethane, propane, n-butane and iso-butane over supported platinum catalysts

Article abstract of DOI:10.1006/jcat.1998.2021

The catalytic properties of three supported platinum catalysts, 0.75 wt% Pt/SiO₂, 0.89 wt% Pt/Al₂O₃, and 0.48 wt% Pt/MoO₃, are reported for the hydrogenolysis of ethane, propane, n-butane, and iso-butane. The reactions were carried out in a flow reactor at atmospheric pressure. The apparent activation energies, E_app, for the hydrogenolysis reactions and the isomerisation reactions of n-butane and iso-butane, all show a compensation effect with the corresponding values of ln A_app. This compensation effect can be accounted for by the dehydrogenative chemisorption of the alkanes as the first step in the reaction mechanisms to both hydrogenolysis and isomerisation. No apparent structure sensitivity was demonstrated by the three catalysts for the hydrogenolysis of ethane. The Pt/Al₂O₃ catalyst showed the highest activity of the three catalysts for the hydrogenolysis of propane. This high activity can be related to the strong adsorption of alkane fragments on platinum sites on the Pt/Al₂O₃ catalyst which exhibit δ+ polarisation. Over the Pt/SiO₂ and Pt/Al₂O₃ catalysts the isomerisation of n-butane and iso-butane can be explained by a metal-only catalysed isomerisation route. Very high levels of isomerisation over the Pt/TMoO₃ catalyst agree with a bifunctional mechanism involving acid sites on the MoO₃ support.

Full text of DOI:10.1006/jcat.1998.2021

JOURNAL OF CATALYSIS 176, 225–234 (1998)
ARTICLE NO. CA982021
Supported Metal Catalysts; Preparation, Characterisation, and Function
Part VI. Hydrogenolysis of Ethane, Propane, n-Butane and iso-Butane
over Supported Platinum Catalysts
�
,1
S. D. Jackson, G. J. Kelly,† and G. Webb†
�
ICI Katalco, Research and Technology Group, P.O. Box 1, Billingham, Cleveland TS 23 1LB, United Kingdom;
and †Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, Scotland, United Kingdom
Received October 28, 1997; revised January 15, 1998; accepted January 16, 1998
of cyclopropane over three of these catalysts, a 0.75 wt%
The catalytic properties of three supported platinum catalysts, Pt/SiO2, a 0.89 wt% Pt/Al2O3, and a 0.48 wt% Pt/MoO3
.75 wt% Pt/SiO2, 0.89 wt% Pt/Al2O3, and 0.48 wt% Pt/MoO3, are all made by impregnation. This paper investigates the hy-
0
reported for the hydrogenolysis of ethane, propane, n-butane, and
iso-butane. The reactions were carried out in a flow reactor at atmo-
spheric pressure. The apparent activation energies, Eapp, for the hy-
drogenolysis reactions and the isomerisation reactions of n-butane
and iso-butane, all show a compensation effect with the correspond-
ing values of ln Aapp. This compensation effect can be accounted
for by the dehydrogenative chemisorption of the alkanes as the first
step in the reaction mechanisms to both hydrogenolysis and iso-
merisation. No apparent structure sensitivity was demonstrated by
drogenolysis reaction of ethane, propane, n and iso-butane
over the same three platinum catalysts and attempts to
2
relate the catalytic properties to the known physical prop-
erties of the catalysts and to the previous reaction studies.
The hydrogenolysis reaction of simple alkanes over metal
catalysts has been known for many years and has been in-
corporated into many reviews of hydrocarbon reactions
(
6–10). Hydrogenolysis can be defined as, “the breakage
the three catalysts for the hydrogenolysis of ethane. The Pt/Al2O3 of carbon–carbon bonds with the uptake of hydrogen.”
catalyst showed the highest activity of the three catalysts for the As each hydrogenolysis reaction involves the rupture of
hydrogenolysis of propane. This high activity can be related to one carbon–carbon bond and the formation of two carbon-
the strong adsorption of alkane fragments on platinum sites on the
Pt/Al2O3 catalyst which exhibit �+ polarisation. Over the Pt/SiO2
and Pt/Al2O3 catalysts the isomerisation of n-butane and iso-butane
canbeexplainedbyametal-onlycatalysedisomerisationroute. Very
high levels of isomerisation over the Pt/MoO3 catalyst agree with a
and also because of its industrial applications. Defined by
bifunctional mechanism involving acid sites on the MoO3 support.
�
c 1998 Academic Press
hydrogen bonds they are always exothermic.
The hydrogenolysis of alkanes over supported metal
catalysts maintains its interest both for the theoretical
aspects of this most fundamental hydrocarbon reaction
Boudartasa“structuresensitivereaction,”thespecificrates
of alkane hydrogenolysis reactions are often highly depen-
dent on the metal particle size of the active phase (11). In
the reforming of light hydrocarbons (in the range C5–C11)
INTRODUCTION
hydrogenolysis has to be minimised to achieve a better se-
This work is a continuation of a series of papers in lectivity of production of aromatics. Therefore a greater
which catalytic activity and selectivity have been related understanding of the hydrogenolysis mechanism, and the
back to an extensive study of catalyst preparation and identification and removal of the reactive sites responsi-
characterisation (1–5). A series of platinum and nickel ble for the reaction should lead to more highly selective
catalysts were prepared using a variety of methods (impreg- reforming catalysts (12).
nation, coprecipitation and metal vapour deposition) and
The metal function of supported catalysts has a large in-
were characterised using a wide range of physical and chem- fluence on the hydrogenolysis characteristics. The group
ical techniques (electronic spectroscopy, TGA, TPR, XPS, VIII metals show a clear tendency for hydrogenolysis
HRTEM, EXAFS, CO/O2/hydrocarbon chemisorption).
Paper V in this series investigated the hydrogenation of
2
These catalysts were denoted as Pt/silica(I), Pt/alumina(I), and Pt/
ethene, 1,3-butadiene, and 2-butyne and the hydrogenolysis
molybdena(I) in Parts I–III, where (I) denotes preparation by impregna-
tion; this distinguished them from catalysts prepared by coprecipitation
Now at ICI Katalco, Research and Technology Group, P.O. Box 1, which were dented (C). This annotation is not used in this paper because
1
Billingham, Cleveland TS 23 1LB, UK.
all catalysts used were prepared by the impregnation method.
225
0021-9517/98 $25.00
Copyright �c 1998 by Academic Press
All rights of reproduction in any form reserved.

2
26
JACKSON, KELLY, AND WEBB
activity to decrease when proceeding from right to left the Pt/silica, and 0.48 wt% Pt for the Pt/molybdenum triox-
across a period (6). Within the 4d series (Ru, Rh, Pd) maxi- ide catalyst. Before use the finely divided catalyst powders
mum activity is found at Ru for the 5d series (Os, Ir, Pt) were pressed into discs, crushed, and sieved between 180
at Os. In the first transition series Co and Ni both ex- and 500 �m meshes.
hibit comparably high activity (10). The metals also vary in
A summary of the characterisation work carried out
their depth of hydrogenolysis, that is, in how many carbon– on these catalysts is as follows (1–4). After reduction the
carbon bonds broken per visit of a reactant hydrocarbon Pt/SiO2 catalyst was shown to be chloride free with the
to the catalyst surface. Single hydrogenolysis is defined as platinum metal present in the zero oxidation state (by
the breakage of a single carbon–carbon bond per visit to XPS) with a large fraction of very small metal particles
the catalyst surface. Multiple hydrogenolysis involves the (� 0.7 nm) and minimal metal–support bonding (as judged
breakage of more than one bond per visit. The metals Pt by Pt L3-edge EXAFS spectroscopy). After reduction the
and Pd have a high selectivity for single hydrogenolysis. On Pt/Al2O3 catalyst retained a significant amount of chlorine
the other hand, W, Ta, Fe, and Co all have a high tendency (1.01 wt%) which was associated with the support, the plat-
�+
for total degradation to methane. The metals Ir, Rh, Os, Re, inum was shown to be present as Pt (by XPS), the metal
Ru, and Ni all lie somewhere between these two extremes. was again highly dispersed and there was substantial metal–
This depth of hydrogenolysis, however, has no relation support bonding evident from the EXAFS spectrum (co-
to the selectivity of the reaction, that is, the tendency for ordination number of O about Pt was 0.8). The Pt/MoO3
particular bonds to be broken. Ni, Pd, and Rh show high se- catalyst showed minimal chlorine content after reduction.
lectivity for terminal bond splitting (demethylation). Other Under the reduction conditions the support formed the
4
d and 5d group VIII metals (particularly Pt and Ir) show molybdenum bronze HxMoO3, where x was ca 1.0 (by TGA,
less selective cracking, leading to a largely statistical spec- TPR, and XPS). The platinum retained a �+ character in
trum of primary products. Under hydrogenolysis conditions the reduced state (by XPS). This is consistent with hydro-
isomerisation of alkanes can also take place (10). Most of gen bronze formation or the formation of MoOx species
the group VIII metals exhibit activity, in at least one alkane (where x < 3). This material had a wide distribution of Pt
isomerisation reaction. The order of activities usually fol- particles ranging from 0.7 to 20 nm. From the chemisorption
lows the sequence Rh > Ir > Pt > Pd > Ni.
of carbon monoxide and dioxygen the following platinum
Simple alkanes such as ethane, propane, n-, and iso- dispersions were calculated for the three catalysts:
butane can be used as “archetype” molecules for studying
hydrogenolysis reactions as they allow the comparison be-
PtAl2O3 � Pt/SiO2 � Pt/MoO3.
tween possible 1,2; 1,3; and 1,4 adsorptions of hydrocarbons
on the catalyst surface (5). Iso-butane and n-butane are also
useful molecules for studying bond shift isomerisation re-
actions.
This paper reports on the hydrogenolysis of these alka-
nes over three well characterised supported platinum cata-
lysts, using two traditional supports alumina and silica and
one less conventional material molybdenum trioxide. The
relative activities and selectivities of these catalysts illus-
trate important mechanistic aspects of the hydrogenolysis
reaction over platinum catalysts and highlights the major
influence of the support on the reaction characteristics.
(ca 90%)
(ca 15%)
The hydrogenolysis reactions of ethane, propane, n-, and
iso-butane were carried out in a stainless steel microre-
actor, designed to produce a controlled flow of helium,
dihydrogen, and hydrocarbon over a small charge (approx.
0
.15 g) of catalyst. The He (Grade A, B.O.C.), H2 (B.O.C.),
and hydrocarbon (all B.O.C., Grade N.2.0) feeds were
purified before use by passing through dioxygen filters and
molecular sieves (Prolab 4A). The reactor was operated
at atmospheric pressure. The catalysts were reduced by
heating to 693 K at a heating rate of 10 K per minute in a
�
1
flow of dihydrogen of 40 ml min . The samples were then
held in flowing dihydrogen at 693 K for 3 h to complete
the reduction. After reduction the samples were cooled
EXPERIMENTAL
The preparation and characterisation of these catalyst in flowing dihydrogen to the reaction temperature before
has been reported in detail in previous papers (1–3). The introducing the He/H /hydrocarbon (approx. 4 : 4 : 1 ratio)
2
catalysts were prepared by the impregnation of hexachloro- mixture. The flows of He, H , and hydrocarbon were
2
platinicacid, H2PtCl6 (JohnsonMatthey ChemicalsLtd.) on controlled by mass flow controllers.
to three support materials namely alumina (Degussa Alu-
In order to reach steady state conditions in the reac-
minium Oxid C, surface area 102 m g ), silica (Cab-O-Sil, tor, the reactant gases were flowed through the catalyst
2
� 1
2
� 1
surface area 203 m g ), and molybdenum trioxide (BDH for 20 min before analysing the reaction products, after
2
� 1
Analar grade 99.5%, surface area 2.1 m g ). The platinum the analysis He/H was flowed through the catalysts for
2
contents of these catalysts were determined by ICP-MS as 15 min; after this process the He/H /hydrocarbon mixture
2
follows, 0.89 wt% Pt for the Pt/alumina 0.75 wt% Pt for was again introduced into the reactor in order to study the

HYDROGENOLYSIS OVER PLATINUM CATALYSTS
227
to
�
1
� 1
reactionatdifferenttemperatures. Usingthisprocessrepro-
1.451 = conversion factor (converts ml min
g
cat
�
1
� 1
ducibility of reaction rates was observed even after temper-
ature cycling. The reaction products were analysed by gas
chromatography (Perkin Elmer 8500) using a Poropak QS
packed column and a flame ionisation detector.
molecules s surface platinum atom ) (13).
A similar set of equations was used to calculate the isomeri-
sation turnovers by replacing XHYD in the equation above
with XISOM, where for n-butane
Treatment of Results
iso-C4
XISOM =
The following treatment of the raw data was used to de-
rive the product selectivities and activities.
A + iso-C4 + n-C4
(a) Fractional conversion. The following equations were and for iso-butane
used to calculate the moles of reactant hydrocarbon (de-
noted as A) which had undergone hydrogenolysis: for
ethane A = C1/2; for propane A = (C1 + 2C2)/3; for n- and
iso-butane A = (C1 + 2C2 + 3C3)/4, where Ci is the mole
fraction of the products formed containing i carbon atoms
n-C4
XISOM =
.
A + iso-C4 + n-C4
.
(
i < 3).
RESULTS
For ethane and propane the fractional hydrogenolysis
conversions (XHYD) were then calculated from the equa-
tions: for ethane XHYD = A/(A + C2); for propane XHYD =
A/(A + C3).
(
i) Nonsteady State Condition
In order to reach steady state conditions the reactant
gases were flowed through the catalyst samples for 20 min
before analysing the reaction products. Analyses taken dur-
ing this initial 20-min period demonstrated a hydrogenoly-
sis activity rising to a maximum after 5 min before falling
to a steady state value. This initial activation of the cata-
lyst (found for all test alkanes over all the catalysts) only
occurred during the first contact of the catalyst with the re-
actant alkane. After the analysis a He/H2 stream was passed
over the catalyst for 15 min. During this time no products
were detected and, on resumption of the reaction at the
next temperature, no comparable rise and fall in activity
before reaching steady state conditions was shown.
For n-butane and iso-butane, as isomerisation can also
take place, the fractional conversions were calculated from
the equation
A
XHYD =
,
A + iso-C4 + n-C4
where iso-C4 = mole fraction of iso-butane and n-C4 = mole
fraction of n-butane.
(b) Selectivity. For propane and the butanes, where there
is a choice of hydrogenolysis products, the selectivity to
each product is denoted by Si, where Si = Ci/A.
For propane hydrogenolysis it can be shown that S1 =
3
4
� 2S2 and for the hydrogenolysis of butanes that S1 =
� 3S3 � 2S2.
(
ii) Ethane Hydrogenolysis
The fractional conversions for the hydrogenolysis of
The selectivity of isomerisation over hydrogenolysis
ethane(XHYD)overthethreecatalystsareshowninTable 1.
The apparent activation energies for the reaction are in the
for the butanes can be calculated from the equations: for
n-butane ISOM/HYD = iso-C4/A; for iso-butane ISOM/
HYD = n-C4/A.
�
1
range 201 to 253 kJ mol . This is consistent with literature
values for this reaction (6, 14). The Pt/Al2O3 and Pt/SiO2
catalysts showed very similar behaviours for this reaction
in terms of turnover and activation energies. The Pt/MoO3
catalyst, however, showed a steeper activation energy plot.
Both the Pt/MoO3 and Pt/SiO2 catalysts showed a loss of
linearity in their activation energy plots at higher tempera-
tures.
(c)Turnover. Thereactionratesandturnoverfrequencies
for the hydrogenolysis reactions were calculated as
F � XHYD
F � XHYD
W � L � d
rHYD =
, tHYD = 1.451 �
,
W
where
rHYD = reaction rate (ml min
�
1
� 1
g
)
cat
�
1
(iii) Propane Hydrogenolysis
tHYD = hydrogenolysis turnover (molecules s surface
�
1
platinum atom
)
The most marked difference in the reaction of propane
over the three catalysts, as compared to ethane, was that
the catalysts no longer reacted with similar turnovers in the
same temperature range (Fig. 1). For comparison purposes
Table 2 shows the temperature at which the hydrogenoly-
�
1
F = flow rate of hydrocarbon (ml min
XHYD = fractional conversion
W = weight of catalyst (g)
)
L = % Pt loading of catalyst
d = % dispersion of Pt
sis turnover = 0.02 molecules s� surface platinum atom .
1
� 1

2
28
JACKSON, KELLY, AND WEBB
TABLE 1
Hydrogenolysis of Ethane, Propane, n-, and Iso-Butane over Pt/Al2O3, Pt/SiO2, and Pt/MoO3 Catalysts
Hydrogenolysis
Isomerisation
� 1
Ea (kJ mol )
�
Catalyst
Pt/Al2O3
Alkane
Temp. (K)
XHYD
Ea (kJ mol 1)
ln A
XISOM
ln A
2.8 � 10� � 0.125
3
219
150
135
105
32.9
25.1
19.3
15.7
C2H6
C3H8
n-C4H10
iso-C4H10
683–763
603–673
693–753
643–713
�
3
6.3 � 10 � 0.15
0.017� 0.10
�
�
3
� 3
1.4 � 10 � 9.3 � 10
161
116
24.8
19.4
3
0.016� 0.12
2.1 � 10 � 0.019
�
�
3
Pt/SiO2
C2H6
C3H8
n-C4H10
iso-C4H10
683–763
653–733
593–673
653–733
2.3 � 10 � 0.058
201
142
94
30.2
21.8
13.8
17.2
3
7.1 � 10 � 0.11
� 3
� 3
7.1 � 10 � 0.033
5.1 � 10� � 0.051
4.6 � 10 � 0.033
102
148
15.1
26.3
3
� 3
123
7.3 � 10 � 0.085
�
3
Pt/MoO3
C2H6
C3H8
n-C4H10
iso-C4H10
683–763
683–763
693–753
633–733
1.2 � 10 � 0.068
253
297
66
41.4
46.7
6.5
�
4
7.9 � 10 � 0.085
� 3
� 3
8.4 � 10 � 0.020
5.6 � 10� � 0.030
8.5 � 10 � 0.13
274
179
45.4
28.9
3
� 3
85
10.5
6.7 � 10 � 0.20
Note. Units of A-molecules s� 1 surface platinum atom� 1
.
From this table the three catalysts had the following reac- propane to form methane, is increasingly taking place,
tion temperatures,
C3H8(g) → 3CH4(g).
Pt/Al2O3 622 K < Pt/SiO2 665 K < Pt/MoO3 707 K.
The Pt/Al2O3 and Pt/SiO2 catalysts showed similar activa-
tion energies for the hydrogenolysis of propane (150 and S1 values slightly less than 1. At higher temperatures the
Between 603 and 633 K the Pt/Al2O3 catalyst showed
�
1
1
42 kJ mol , respectively). The Pt/MoO3 catalyst demon- Pt/Al2O3 catalyst showed S1 values slightly higher than 1,
�
1
strated a steeper activation energy plot (Ea = 297 kJ mol ). suggesting that a small amount of multiple hydrogenolysis
None of the activation energy plots showed a loss of linear- was taking place. In the temperature range of overlap be-
ity at higher temperatures.
tween the Pt/Al2O3 and Pt/SiO2 catalysts (653–673 K) the
Figure 2 shows the selectivity for methane (S1) during selectivities shown by the two catalysts were very compa-
the hydrogenolysis of propane over the three catalysts. If rable. The Pt/SiO2 catalyst showed S1 values slightly higher
S1 = 1 then the single hydrogenolysis of 1 mole of propane than 1 at all temperatures.
to 1 mole of ethane and 1 mole of methane is taking place,
The Pt/MoO3 catalyst showed S1 values which rose with
temperature from 0.96 at 683 K to 1.27 at 673 K.
C3H8(g) → C2H6(g) + CH4(g).
At values of S1 > 1 the multiple hydrogenolysis reaction of
TABLE 2
Temperature at Which Hydrogenolysis Turnover = 0.02
�
1
� 1
Molecules s Surface Platinum Atom
Catalyst
Pt/Al2O3
Alkane
Temp. (K)
C2H6
C3H8
n-C4H10
iso-C4H10
715
622
698
644
Pt/SiO2
C2H6
C3H8
n-C4H10
iso-C4H10
707
665
636
698
Pt/MoO3
C2H6
C3H8
n-C4H10
iso-C4H10
700
707
753
703
FIG. 1. Hydrogenolysis of propane over Pt/Al2O3 (᭿), Pt/SiO2 (♦),
and Pt/MoO3 (᭹).

HYDROGENOLYSIS OVER PLATINUM CATALYSTS
229
FIG. 2. Selectivity of propane hydrogenolysis over Pt/Al2O3 (᭿),
Pt/SiO2 (♦), and Pt/MoO3 (᭹).
FIG. 4. Selectivity of n and iso-butane hydrogenolysis over Pt/Al2O3
S1 (᭿), S2 (᭹), S3 (ᮀ), and ISO/HYD (♦)).
(
(iv) n-Butane and Iso-Butane Hydrogenolysis
With the reactions of n and iso-butane over the catalysts,
isomerisation and hydrogenolysis reactions have very dif-
ferent gradients for both n and iso-butane. For both the
butanes the isomerisation reaction has a higher activation
energy than the hydrogenolysis reaction (Table 1). The ac-
tivation energy plots for both of the isomerisation reactions
show a loss of linearity at higher temperatures.
In terms of comparing hydrogenolysis activity (Table 2)
for the reaction of n-butane the order for the catalysts were
as follows:
in addition to the hydrogenolysis reaction, bond shift iso-
merisation reactions also take place. Figure 3 shows the
activation energy plots for the hydrogenolysis and isomeri-
sation of the butanes over the Pt/Al2O3 catalyst.
With the Pt/Al2O3 catalyst (Fig. 3) it is clear that there
is a close relationship between the hydrogenolysis and iso-
merisation reactions of both the butanes. Although there is
considerably more hydrogenolysis than isomerisation tak-
ing place the activation energies for the reactions are obvi-
ously very similar.
Pt/SiO2 636 K < Pt/Al2O3 698 K < Pt/MoO3 753 K.
With the Pt/SiO2 catalyst there is also a good correlation
between the activation energies of the hydrogenolysis and With the reaction of iso-butane the order changes:
isomerisation reactions (Table 1). With the Pt/SiO2 cata-
Pt/Al2O3 644 K < Pt/SiO2 698 K < Pt/MoO3 703 K.
lyst there is much more isomerisation taking place than over
the Pt/Al2O3 catalyst. For the two isomerisation reactions
there is a loss of linearity shown in the activation energy
plots at higher temperatures.
The Pt/MoO3 catalyst shows the largest amount of iso-
merisation activity. The activation energy plots for the
From a comparison of Figs. 4, 5, and 6 it is clear that the
three Pt catalysts have very different selectivities towards
isomerisation over hydrogenolysis. For the Pt/Al2O3 cata-
lyst the ISO/HYD ratio is around 0.08 for n-butane and
0
.13 for iso-butane (Fig. 4). Over the Pt/SiO2 catalyst the
ISO/HYD ratio is considerably higher at around 0.75 for
n-butane and up to values over 2 for iso-butane (Fig. 5). The
FIG. 3. Hydrogenolysis and isomerisation of n and iso-butane over
Pt/Al2O3 (n-butane hydrogenolysis (᭿), n-butane isomerisation (᭹), iso-
butane hydrogenolysis (ᮀ), and iso-butane isomerisation (♦)).
FIG. 5. Selectivity of n and iso-butane hydrogenolysis over Pt/SiO2
(S1 (᭿), S2 (᭹), S3 (ᮀ), and ISO/HYD (♦)).

2
30
JACKSON, KELLY, AND WEBB
FIG. 6. Selectivity of n and iso-butane hydrogenolysis over Pt/MoO3
S1 (᭿), S2 (᭹), S3 (ᮀ), and ISO/HYD (♦)).
(
FIG. 7. S1/S3 ratio during hydrogenolysis of n and iso-butane. (Pt/
Al2O3 n-butane (᭹), Pt/Al2O3 iso-butane (᭺), Pt/SiO2 n-butane (᭜),
Pt/SiO2 iso-butane (♦), Pt/MoO3 n-butane (᭿), and Pt/MoO3 iso-butane
Pt/MoO3 catalyst shows considerable isomerisation under
reaction conditions, with ISO/HYD values as high as 7.5
and 9.1 for n and iso-butane, respectively (Fig. 6).
(
ᮀ)).
The selectivities of the reactions of n and iso-butane over
3
The reaction of n-butane over the Pt/MoO catalyst
the three catalysts are shown in Figs. 4, 5, and 6. The single (Fig. 7) shows the highest values of S /S of the three cata-
1
3
hydrogenolysis reaction of n-butane can be expressed by lysts, with S /S values as high as 2.57. The Pt/MoO cata-
1
3
3
the following two reactions:
lyst also shows a high selectivity towards ethane during iso-
butane hydrogenolysis, particularly at lower temperatures.
n-C4H10(g) + H2(g) → C3H8(g) + CH4(g)
n-C4H10(g) + H2(g) → 2C2H6(g).
(
v) Compensation Effect
The Arrenhius parameters and turnover frequencies for
The selectivity towards single hydrogenolysis can be fol-
lowed from the S1/S3 ratio: if S1/S3 = 1 then only single hy-
drogenolysis is taking place; if S1/S3 > 1 then multiple hy-
drogenolysis is taking place. For iso-butane the only single
hydrogenolysis reaction that can take place is
all the hydrogenolysis and isomerisation reactions are sum-
marised in Table 1. Plotting the ln A values against the
apparent activation energies gives a convincing compensa-
tion effect plot (Fig. 8). The equation for the line obtained
is
ln A = 0.1711Ea � 2.587.
iso-C4H10(g) + H2(g) → C3H8(g) + CH4(g).
Any ethane that is formed from iso-butane must be pro-
duced by the breakage of two carbon–carbon bonds. The
value of S2 can therefore be used to follow the multiple hy-
DISCUSSION
A nonsteady state behaviour was noted for the first con-
drogenolysis of iso-butane as well as the S1/S3 ratio above. tact of each of the reduced catalysts with the reactant gases.
The values of S1/S3 for the reactions of the butanes over the
three catalysts are shown in Fig. 7.
The reactions of n- and iso-butane over the Pt/Al2O3
catalyst (Fig. 7) show only small amounts of multiple hy-
drogenolysis as shown by the S1/S3 ratio being slightly >1
in both cases and the small amount of ethane formed at low
temperature with iso-butane.
With the reaction of n-butane over the Pt/SiO2 catalyst
(Fig. 7) there is a crossover between S1/S3 < 1 in the tem-
perature range 593–613 K and S1/S3 > 1 from 613 to 673 K.
The highest value of S1/S3 over the Pt/SiO2 catalyst (S1/S3 =
1
.61) is higher than the highest value over the Pt/Al2O3
catalyst (S1/S3 = 1.29). With the reaction of iso-butane over
the Pt/SiO2 catalyst S1/S3 values are slightly higher than
1
and more ethane is formed than over the Pt/Al2O3
FIG. 8. Compensation effect of alkane hydrogenolysis and isomeri-
sation (hydrogenolysis (᭜) and isomerisation (᭺)).
catalyst.

HYDROGENOLYSIS OVER PLATINUM CATALYSTS
231
This took the form of an initial activation process followed totheapparentactivationenergyfromthedehydrogenative
by a deactivation. A similar nonsteady initial state has been chemisorption of the alkane (step (ii) in the hydrogenolysis
demonstrated for the hydrogenolysis of ethane over Os and mechanism). In other words, at higher temperatures there
Ru clusters (15, 16). The initial activation/deactivation of is an increase in the surface concentration of the reactive
�
the catalyst before reaching steady state is probably due to form of the alkane (CmH ) which leads to higher rates of re-
x
the formation of a reactive carbonaceous overlayer on the action. One implication of the existence of a compensation
platinum surface. Subsequent treatment of the catalyst with effect is that it is doubtful if any useful mechanistic infor-
the He/H2 stream does not appear to remove this overlayer mation about the rate determining step can be gained by
or “clean” the surface back to its original state. The reactive comparing overall activation energies. On the other hand,
catalyst surface that is carrying out the hydrogenolysis reac- several important observations can be made from inspec-
tions is therefore generated in situ and contains a carbona- tion of the compensation plot itself. For instance, ethane
ceous component. The involvement of this carbonaceous is often found to have a lower hydrogenolysis reactivity
component is therefore significant in any consideration of than the longer alkanes and a higher apparent activation
the active site for the hydrogenolysis reaction.
energy (14, 22, 27). This has been explained (22) as being
Several kinetic models for hydrogenolysis reactions over due to the difficulty of dehydrogenating ethane, as com-
metal catalysts have been suggested previously (17–22). pared to higher alkanes (i.e., the kinetic parameters are a
Many of the models agree with the following general re- consequence of the thermochemical stability of the reac-
action mechanism (22). Step (iii), the C–C bond breakage tant alkane). This holds for the Pt/Al2O3 catalyst and the
step, is generally agreed to be the rate determining step:
Pt/SiO2 catalyst where ethane has a lower reactivity than
the other alkanes and a higher activation energy. Over the
Pt/MoO3 catalyst however the highest activation energy is
for the hydrogenolysis of propane, not ethane.
Another interesting observation is that the isomerisation
reactions lie on the same compensation effect plot as the
hydrogenolysis reactions. For the bond-shift isomerisation
reactions of n- and iso-butane three possible mechanism
can be envisaged:
(i) dissociative chemisorption of dihydrogen,
�
�
KH, H2 + 2 ⇔ 2H ;
ii) dehydrogenative chemisorption of the alkane,
(
�
�
�
KC, CmHn + (n + 1 � x) ⇔ CmH + (n � x)H ;
x
(iii) fragmentation of this dehydrogenated species,
�
�
�
�
(i) a bifunctional mechanism involving acid sites on the
support,
k1, CmH + H → CpH + Cm� pH
;
x
y
x� y+1
(
ii) a metal-only catalysed rearrangement,
(
iv) rehydrogenation and desorption of the products,
(
iii) a support-only catalysed rearrangement.
�
�
�
kj, CpH + (2p + 2 � y)H → CpH2p+2 + (2p + 3 � y) .
y
These three mechanisms are shown in Fig. 9. Mechanisms
The compensation behaviour noted (Fig. 8) for this set (i) and (ii) both involve a dehydrogenation step on the
of reactions has previously been shown for alkane hy- metal. The dehydrogenation gives these two mechanisms
drogenolysis over Pt catalysts (23) and other metals (24). something in common with the general mechanism of hy-
Compensation behaviour has also been shown by many drogenolysis discussed earlier, that is, the formation of a de-
other heterogeneous and homogeneous catalytic processes hydrogenatedorganicintermediateonthemetalfunctionof
(25, 26). Various theories have been proposed to explain the the catalyst. The formation of a common intermediate (i.e.,
compensation effect, including enthalpy–entropy relation- a dehydrogenated alkane) that leads to both hydrogenol-
ships, energetically heterogeneous catalyst surfaces, and ysis and isomerisation may explain why both reactions lie
variations in the availability of surface reactants. These the- on the same compensation energy plot. Over the Pt/Al2O3
ories and others are discussed at considerable length in a re- and Pt/MoO3 catalysts all three mechanisms of isomerisa-
view by Galwey (24). The compensation effect plot (Fig. 8) tion are possible as both of these supports have acid sites
is in very good agreement with the compensation effect available for isomerisation. On the Pt/SiO2 catalyst only a
plot derived by G. C. Bond et al. from the hydrogenolysis metal-only catalysed rearrangement (ii) is possible as there
and isomerisation reactions of ethane, propane, n-butane, are no acid sites on the silica support strong enough to carry
n-pentane, n-hexane, and n-heptane over EUROPT-1 (14). out isomerisation reactions. Various reaction mechanisms
From a study of alkane hydrogenolysis (22) G. C. Bond pro- have been suggested for metal-only bond shift isomerisa-
posedthatthecompensationeffectwasaresultoftheappar- tion reactions (28–31). A common feature of all these mech-
ent activation energies (Eapp) as measured from rate data anisms is that they all invoke 1,3 diadsorbed alkane inter-
being much higher than the true activation energies (Et) for mediates. The mechanism shown in Fig. 9 for the metal-only
the reactions. This discrepancy between apparent and ac- isomerisation follows the work by Garin et al. and differs
tual activation energies arises from a positive contribution from the other suggested mechanisms in that the alkane

2
32
JACKSON, KELLY, AND WEBB
lysts the activation energies for the isomerisation reactions
are considerably higher than for the hydrogenolysis reac-
tions. The Arrenhius parameters of the isomerisation re-
actions still fit, however, on the compensation effect plot
(
Fig. 8). These two observations put together point toward
an isomerisation mechanism which, although it still involves
the metal function of the catalyst, does not have the same
rate-determining step as the hydrogenolysis reaction. This
would fit more closely to an isomerisation taking place by
a bifunctional mechanism involving the support (Fig. 9(i))
rather than by a metal-only reaction on the platinum. The
MoO3 support itself will have acidic sites present (34). Un-
der the standard reduction conditions the support is known
to form the molybdenum bronze HxMoO3, where x is ca
1
.0 (1–3). The formation of hydrogen molybdenum bronzes
from spillover hydrogen can lead to structural modifica-
tions of the MoO3 (32). When the hydrogen atoms enter
the MoO3 lattice there is a tendency for the oxide to cleave
in planes parallel to its c crystallographic axis. This may lead
to the formation of defect sites on the support and increase
the number of acidic sites present. The high multiple hy-
drogenolysis activity of the Pt/MoO3 catalyst with propane,
n, and iso-butane and also the high isomerisation activity of
this catalyst with the butanes may all be related to reactions
taking place on acid sites on the support. These reactions
may all follow a bifunctional mechanism with the transfer
of adsorbed organic intermediates from the platinum to the
support. The activation of propane over the Pt/MoO3 cata-
lyst is more difficult than the activation of ethane, which is
contrary to the results derived from the Pt/Al2O3 and the
Pt/SiO2 catalysts and literature results (14). This, coupled
with the multiple hydrogenolysis activity, suggests that the
transfer of organic intermediates from the metal to react
on the support is a more activated process than a reaction
solely on the metal.
FIG. 9. Isomerisation mechanisms of n and iso-butane.
intermediate is diadsorbed on a single platinum atom. The
Hydrogenolysis of small alkanes over supported metal
correlation that is shown between the activation energies catalysts are often found to be structure sensitive in terms of
for the hydrogenolysis and isomerisation of the butanes activity (35–38), or in some cases in terms of selectivity (39).
over the Pt/SiO2 catalyst supports the theory that the iso- For the hydrogenolysis of ethane the three catalysts reacted
merisation, as well as the hydrogenolysis reaction, is taking with similar turnover numbers in the temperature range
place on the metal function of the catalyst. Compared to the tested. There was no obvious structure sensitivity shown
Pt/SiO2 catalyst very little isomerisation activity is shown by for these catalysts, or therefore, for the hydrogenolysis of
the Pt/Al2O3 catalyst. As discussed previously, the isomeri- ethane. Any discussion of surface sensitivity when compar-
sation that does take place over the Pt/Al2O3 catalyst can ing these three platinum catalysts is complicated, however,
either takes place via acid sites on the support (Figs. 9(i) by factors such as the existence of the �+ polarised sites
and (iii)) or by the metal-only route. The close correlation on the Pt/Al2O3 and Pt/MoO3 catalysts and in the case of
between the activation energies for the hydrogenolysis and the Pt/MoO3 catalyst the existence of a wide distribution of
isomerisation reactions of the butanes over the Pt/Al2O3 platinum particle sizes (from 0.7 to 20 nm). The wide dis-
catalyst does suggest that both reactions are taking place tribution of the platinum particles on the Pt/MoO3 catalyst
on the metal function of the catalyst; i.e., the isomerisation make the detection of structure sensitivity particularly diffi-
is also taking place by a metal-only route on the Pt/Al2O3 cult as the smallest particles may be responsible for the ma-
catalyst. The Pt/MoO3 catalyst shows the highest selectiv- jority of the reaction (5). A turnover number as calculated
ity towards isomerisation with the reactions of the butanes. from an averaged platinum particle size may not therefore
Unlike the reactions over the Pt/SiO2 and Pt/Al2O3 cata- be applicable.

HYDROGENOLYSIS OVER PLATINUM CATALYSTS
233
For the hydrogenolysis of propane the three catalysts The metal-only isomerisation of butanes involves the fol-
showed a range of activities; i.e., lowing process (Fig. 9):
i) the breakage of a carbon–carbon bond
(
Pt/Al2O3 > Pt/SiO2 > Pt/MoO3,
ii) skeletal rearrangement, followed by
iii) reformation of a carbon–carbon bond.
(
(
comparing in the first instance the activities of the Pt/Al2O3
catalyst and the Pt/SiO2 catalysts. Although the Pt/Al2O3
catalyst has a marginally higher dispersion, as shown by
EXAFS, than the Pt/SiO2 catalyst, such a large difference in
activity appears to be too large to be accounted for solely in
terms of platinum particle size differences between the two
catalysts. In a previous paper the hydrogenolysis reaction of
cyclopropane was carried out over all three platinum cata-
lysts (5). The hydrogenolysis of cyclopropane was shown
to follow the Bond–Newham rate equation (33). The rate
coefficient for the reaction was considerably higher for the
Pt/Al2O3 catalyst (k = 43.9) than for the Pt/SiO2 catalyst
The �+ polarisation of the platinum sites on the Pt/Al2O3
catalyst would appear to have the effect of suppressing
either step (ii) or step (iii) in this process relative to the
Pt/SiO2 catalyst. The stronger adsorption of the dehydro-
genated C3 fragments that would be involved in steps (ii)
and (iii) on the �+ sites on the Pt/Al2O3 catalyst may inhibit
isomerisation and favour, instead, the hydrogenolysis reac-
tion pathway. Although the polarisation of the platinum on
the Pt/Al2O3 catalyst is strong enough to have an effect on
the isomerisation selectivity it does not appear to be strong
enough to produce other effects such as increased multiple
hydrogenolysis. This would manifest itself as high S1/S3 ra-
tios for the reactions of both n and iso-butane and high S2
figures for the reaction of iso-butane. This is not the case,
however, on the Pt/Al2O3 catalyst relative to the Pt/SiO2
catalyst.
(
k = 10.6). Other results from this study showed that cy-
clopropane adsorbed much more strongly on the Pt/Al2O3
catalyst than on the Pt/SiO2 catalyst. This stronger adsorp-
tion was related to the existence of platinum sites which ex-
hibited �+ polarisation on the Pt/Al2O3 catalyst, sites which
then interacted strongly with the incipient �-system of the
strained hydrocarbon (5). The Pt/SiO2 catalyst, on the other
CONCLUSIONS
�+
hand, did not show any evidence for the existence of Pt
sites. In the case of the Pt/Al2O3 catalyst the enhanced activ-
A clear compensation effect is shown between the ap-
ity it demonstrated, as compared to the Pt/SiO2 catalyst, in parent activation energies and preexponential factors for
the hydrogenolysis of cyclopropane appears to be as a con- the hydrogenolysis reactions of ethane, propane, n, and iso-
sequence of the �+ polarisation of the platinum sites, rather butane over the three platinum catalysts. This effect may be
than the presence of marginally smaller platinum parti- due to the dehydrogenative chemisorption of the alkanes
cles. The strong adsorption properties of the Pt/Al2O3 cata- prior to the carbon bond breakage step. There is a close re-
lyst for dehydrogenated C3 fragments may also explain its lationship between the mechanisms of hydrogenolysis and
higher activity for propane hydrogenolysis as compared to isomerisation of the butanes over the Pt/Al2O3 and Pt/SiO2
the Pt/SiO2 catalyst. Several crystallographic models have catalysts. Both the isomerisation and hydrogenolysis reac-
been proposed that relate the structure sensitivity to the tions appear to take place on the platinum functions of
existence of particular sites on very small metal crystal- the catalysts. Many of the characteristics of the Pt/MoO3
lites (40–43). The structural sensitivity shown in this case catalyst are not typical of a platinum catalyst, i.e. multiple
is not due to the existence of special sites on crystallites hydrogenolysis and large amounts of isomerisation. These
but rather on the electronic perturbation of the platinum characteristics are consistent with reactions taking place
crystallites that takes place with the Pt/Al2O3 catalyst. This over acid sites on the support and bifunctional reaction
electronic root of structure sensitivity may explain why cer- mechanisms. The higher activity of the Pt/Al2O3 catalyst
tain reactions are either structure sensitive or insensitive. as compared to the Pt/SiO2 catalyst for the hydrogenolysis
Rate enhancement will only occur if the electronic pertur- of propane appears to be linked with the �+ polarisation of
bation of the site in some way promotes the reaction (i.e. the platinum sites on the Pt/Al2O3 catalyst.
by interacting with the incipient �-system of the strained
cyclopropane). It is difficult to compare the relative activi-
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