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Single atom sites and hydrocarbon reaction mechanisms
113
catalysis Today 12 (1992) 113-129 Elsevier Science publishersB.V., Amsterdam
W.R. PATTERSON and J.J. RGGNEY
School of Chemistry, The Queen’s University of Belfast, David Keir Building, Stramnilhs Road, BELFAST BT9 SAG, N. Ireland
The following is the summary of a review article in Nature, 1984, entitled, “The active sites of acidic ahuninosilicate catalysts”, by Weiss and coworkers [l] of Mobil, which aptly bastes
a Freud
dilemma for all of heterogen~us cata?@s.
‘The availability of solid acidic catalysts based on zeolite ZSM-5 makes possible observations on the nature and the absolute rate bebaviour of the individualprotonic sites. Tetrahedral ahuninium atoms are the highly reactive ingredients even at a level of parts per million, or less. Turnover numbers for several hydrocarbon reactions equal or exceed familiar enzymatic turnover values. Intensive catalytic activities can result from aluminium levels likely to be iguored by the experimenter.” This statement is completely justified since the nmbers and strength distributions of the protonic centres have been estimated by a variety of titration techniques using bases [2J. A similar methodology has also been employed to examine the redox sites in refractory oxides such as y-AlzOs and GazO, which promote a variety of catalytic reactions ~~lu~g
exchange
with deuterium of alkanes and alkenes, and double bond isomerism in alkenes. Here elegant studies [3,475] have revealed that gases such as H& SO, CO* etc., may be used as in siti selective, quantitative, and reversible poisons for certain features of these reactions. Again the major and ~eq~~
fiudiug is that the site densities are very low. The bang
general question is therefore valid and very importaut. Are site densities invariably low for aJl heterogeneous catalysts, and especially for reactions of paraffms?
This question is
particularly apposite for metal catalysts where there is no consensus on what constitutes an active site, one surface atom, or a contiguous pair, or a multinuclear ensemble. Turnover nmnbers are rarely if ever expressed in rates per c&c$&
site because the densities of such
sites are not known; rather they are expressed as rates per exposed metal atom, or unit metal surfam area, as measured by Hu 0,
or Co adsorption. Methods of iden~
sites and
estimating their densities are very limited and the mechanisms of reactions much less clear-cut
0920~5861/92/$17.~ 0 1992 Elsevier Science PublishersB.V. All rights reserved.
114
than for the acidic and redox surfaces.
Surface physics techniques
have been increasingly
employed especially during the past two decades in attempts to reveal surface intermediates. Frequently,
however, the adsorbed hydrocarbon entities which are most abundant
and most
readily identified are not catalytically competent, or are simply “spectator” species, as found for the celebrated ethylidyne entity [6] which bridges three metal atoms and is formed under certain conditions from chemisorbed constitutes
a reactive
compounded
ethane, ethylene or acetylene.
site and the true estimation
of the number
by the observation that many hydrocarbon
are multi-step in nature.
The dilemma of what of these is further
reactions at elevated temperatures
Some of these steps are regarded as essentially surface structure
insensitive [I, whereas a few, notably ethane and neopentane
hydrocracking, are classifted as
structure sensitive. The word “insensitive” is used where rates of reaction per unit metal area are, by and large, independent
of the method of preparation
and pretreatment
and are a function only of the identity of the metal involved. in these cases that all the exposed metal atoms contriiute
of the catalyst
Generally it has been assumed signi&antly
to the reactivity, but
the alternative proposition that the site densities are low while remaining reasonably constant cannot be excluded. In contrast, the rates of structure sensitive reactions depend strongly on the history of the catalyst and the reaction conditions. insensitive reactions are largely independent reactions are dependent
Thus, it follows that the rates of
of particle size, whereas the rates of sensitive
on this variable.
The idea that certain individual metal atoms may constitute the reactive sites has been consistently advocated to explain many hydrocarbon reactions since it was first communicated [g,9] as “the x-bond theory of catalysis”. The essence of this idea is that various e- and Abonded intermediates
interconvert
as transient
ligands of the same surface atom or ion,
preferentially coordinatively highly unsaturated and presumably only present in relatively small numbers [lo]. The alternative view that a site consists of ensembles of metal atoms is widely accepted [ll], but with little or no experimental homogeneous
justification,
and without cotmterparts
in
catalysis.
Investigations
of chemisorbed species using surface physics techniques may therefore
be merely complementary
to the chemical probe approach
in the field of heterogeneous
catalysis. The claim [6] that surface science and solid state science are the twin pillars of an understanding
of this difficult subject must be critically assessed since it is possible that only
relatively few sites are really effective under steady-state conditions, with the vast bulk of chemisorbed
material on the surface not directly involved in each catalytic cycle.
juncture in the development
At this
of the theoty of metal catalysis it is essential that such questions
be put clearly without bias since there is much confusion arising from the sheer plethora of
115
data and mechanistic suggestions, many of which are conflicting and chemically unrealistic. The purpose of this review therefore is to reappraise
some of the more significant features
in a fashion which will illuminate the true nature of the active sites on the surface of a metal catalyst. Mechanisms of Structure Insensitive Reactions Structure insensitive reactions are basically those involved only in the making and breaking
of
C-H
deuterium-exchange [12].
Some
bonds.
examples
are
the
hydrogenation
reactions of paraffins and the exchange and hydrogenation
The 1,2-bond shift isomerixation
reactions also fall into this category.
reaction
of parafiks
to hydrocarbon
carbyne intermediates
are an essential part of the mechanism.
of benzene
[13] and certain cyclixation
reaction depends on whether or not carbene and/or
theory one surface metal atom is enough
hydrocarbon transients for insensitive reactions. hydrogenation
alkenes,
The real distinction between the terms sensitive and
insensitive as applied
In terms of the x-bond
of
to hold the
For example, ethane exchange and ethylene
can be envisaged as the interconversion
of u-bonded
ethyl and x-bonded
ethylene (x = H or D) as shown in reaction (1).
-X
‘2’5 + M
cx2
* X
cx2 f M
There is ample evidence for this mechanism from direct spectroscopic investigations from exchange reactions of suitable model pmbes such as heptacyclotetradecane can potentially form an eclipsed 1,2diadsorbed
intermediate,
[14] and [U] which
as advocated by Burwell[16],
but not a x-bonded analogue; only simple exchange is observed [15] as expected if n-bonding is the correct description
of the 1,2diadsorbed
species required by the multiple exchange
process. Multiple interconversion
exchange and hydrogenation of x-bonded
There is considerable
of benzene
and a-bonded intermediates
also can be accounted
for by
as shown in reaction (2)
confusion about the mechanism of 1,2bond
shift isomerism in
paraffins [17], even though there is now much evidence [18] that the only viable general pathway is rearrangement
of surface alkyls in a manner akin to that of carbonium ions. The
lowering of the energy barrier via x-bonding of the half-reaction-state
complex to a surface
116
*@:
x@x + xj$x
%
M
X
atom is regarded as being very important but the degree of o-bonding between the final and initial radicals and the metal is a moot question. the classical example of neopentane
Reaction (3) exemplifies this mechanism for
conversion into isopentane.
CH3 I {HfC-Cn, I cH3 M
CH l3 %-
z+
./‘“3 c\ : CH3 IA
(3)
It is worth noting that in this scheme a C-C bond is never really cleaved as it is in the alternative suggestion of an olefin-metathesis
type mechanism involving metallocyclobutanes
[19] (reaction (4)). There is convincing evidence [20] that the metathesis
mechanism
for bond shift is
confined to methyl shift in simple alkanes and is only significant on metals such as Ni and Pd at elevated temperatures
and low hydrogen pressures, i.e. conditions under which there is also
substantial concomitant homologation when there
is simultaneous
and methanation
formation
reactions [21]. Homologation
from paraffins
of transient
occurs
olefins and surface
methylenes arising from extensive C-C fission. Migration of olefin, as formed by reaction (l), to a site with an adsorbed methylene
then results in cyclobutanation
and ultimately chain
lengthening, as shown in reaction (4). On W catalysts linear homologation
of paraffins is an
117
H3c\
fH3
/c\
cH\3 lCH3 -
cH2\chC$
Cq/2
JP
JP
/“P
,CH3
CH2 \/‘CH3
important specific reaction accompanying substantial methanation
(4
[ZO]. Here the metathesis
reaction is clearly operating but does not give bond shift; for example, n-pentane isopentane, but only n-hexane and n-butane. electrophilic
does not give
Clearly the methylene intermediates
and add specifically to the terminal C-atom of intermediate
selectivity is reflected by the high ratios of degenerate
are highly
alk-1-ene.
This
to productive methathesis found for
terminal alkenes using W-based catalysts [22]. The preferred pathway is as shown in reaction (5) and not that in reaction (6).
=%=
RdC’CHR’
(5)
‘w’
tiCH ,,kw
AH2
FkH
-
(6)
118
Both (5) and (6) would have to be significant contribute to 1,2-bond shift. Furthermore,
for the metathesis
the intermediate
itself in order to make an isomeric metallacycle.
mechanism
to
olefin has to rotate to reorientate
Mobility of olefin is indeed required if
migration of alkene across the surface leads to substantial homologation but this mobility does not simultaneously surface
methylenes
afford 12-bond
shift on W [20]. If the hydrogen pressure is high the
are preferentially
hydrogenated
to methane.
The
homologation
mechanism with migration of transient olefin also provides a novel way of considering the mechanism paraffins.
of Fischer-Tropsch
synthesis [23] and Schultz-Flory
The initial step is the combination
homologation participation
distributions
of product
of two methylenes to give ethylene. Statistical
is then possible via migration in reversible metallacyclobutane
of ethylene
and all product
alkenes with
formation as shown in reaction (4).
We wish to stress therefore that while the metathesis-type
reactions are important on
metal surfaces they are not responsible in almost all cases for bond shift isomerixation. Pt is the metal par cmdence for bond shift [l&18,24], but it is poor at methanation high hydrogen pressures.
Furthermore,
the metathesis
mechanism
confined to methyl shift and simply camiot explain rearrangement the required isomeric metallacyclobutanes
Thus even at
for isomerixation
is
of cycloalkanes [WI, since
cannot form in these cases.
Any intermediate
alkylidene higher than methylene rearranges too readily via a rapid 1,2 H-shift at elevated temperatures
to the corresponding
alkene as shown in reaction (7).
RCH
CH2
(71
=t-
The only viable 12-bond shift mechanism for Pt is therefore the alkyl radical one. It is even possible that just as transient olefins migrate across the surface, free radicals formed at certain sites also migrate and then rearrange on other sites. This aspect of the mechanism could be quite important at elevated temperatures The recent elegant synthesis of dodecahedrane,
where 1,kyclixation
reactions also occur.
CL&&,,,from its isomeric precursor pagodane
using Pd and Pt catalysts in excess hydrogen at cu. 23X! is unquestionably
a nice example of
l,S-ring closure by a free radical mechanism [26]. Inter-site transfer of transients in metal catalysis may be much more widespread than hitherto considered.
It is a complicating factor in discussing kinetics and reaction insensitivity
and may even extend, as will be discussed in more detail later, to exchange of paraffins with D, at ambient temperatures.
119 Site Jhnsities
and Insensitivity
The r-bond theory with its emphasis on mononuclear sites is vindicated by the mechanistic evidence apart from the understanding it gives to heterogeneous catalysis in the light of parallel developments in organometallic chemistry and homogeneous metal catalysis. Indeed in the latter field essentially every known reaction is mononuclear which emphasizes the importance of the cis-ligand insertion step both on surfaces and in solution. It might appear therefore that every exposed metal atom in a surface is a catalytically active centre for all of the above reactions. As yet there is no direct spectroscopic evidence concerning this important issue but a considerable amount of good indirect evidence in recent years points to the conclusion that the really reactive sites are relatively few in number even for simple hydrogenation reactions. The surface atoms in an ideal metal crystal can be considered in terms of their coordinative unsaturation, which is least for atoms in the (111) plane and is greatest for apical atom sites. Atoms in other planes, terraces and edges have intermediate degrees of saturation. If all exposed surface atoms were catalytically active, irrespective of coordination, a series of Arrhenius equations could be considered with the summation overall givingthe actual rate of reaction per unit metal area provided that the A factors, which reflect the densities of the different types of sites are expressed in this way. In the simplest case the kinetics may be assumed to be the same with only the A and E factors changing: b
= Zk
= BAexp(-EJRT)
An internal compensation effect will then arise where the sites of lowest unsaturation will be the most numerous (e.g. (111) plane) but with the highest activation energy, whereas the lowest activation energy sites, e.g. comers, should be scarce. Various distributions of the fractions of the total rate as a function of site type may be constructed, and it is evident thal the low index metal planes may be essentially inert because the E values are too high, even though the corresponding A factors are quite favourable. This would leave the coordinatively highly unsaturated metal atoms which may be regarded as defects, as the only sites really effective.
In the extreme the distriiution may be so narrow that only one type of site
contributes overwhelminglyto the total reaction. In the light of these ideas the descriptior “structure insensitive” is never more than an approximation. Nevertheless it may be quite a good approximation when the rate per unit metal area is largely independent of the methot of preparation and pre-conditioning of the catalyst and is a function only of the identity of thf metal. However, the expression of activities in apparent rather than true turnover numben conceals the possibility that catalysis may be due to defect sites, possibly of more than one type, the densities of which stay fairly constant. Recent work on Pt/riO, catalysts in the SMSI and nonSMS1 states provides mucl
120
evidence that this interpretation
of structure insensitivity is correct. The catalysts preheated
to give the SMSI state have greatly reduced capacities to chemisorb hydrogen but retain much of their activities for bond-shift isomerixation
[24], olefin hydrogenation
exchange
The characteristic
[271, and H&
exchange
[28].
double
[25], cyclopentane U-shaped
initial
distribution for cyclopentane exchange on Pt is maintained (did, isomers, and ds-d,, isomers). This suggests that the active sites are still the same as those on non-SMSI pt/riOs, on films [29], and on metal dispersed on conventional
supports [30]. Since the H-H bond is more
readily broken on metals than C-H bonds this is a truly surprising result unless the site densities are low and the planes essentially inert for paraffin activation as claimed. Why then are the multiplicities in initial exchange reactions the same for SMSI and non-SMSI metal if as hitherto assumed a surface pool of numerous rapidly scrambling highly mobile H and D adatoms is required? sine qua non for any
The availability of a large excess of highly reactive D atoms is a kinetic meaningful
interpretation
of exchange reactions [29], yet the SMSI
catalysts quite evidently do not have such a pool. real&ration that molecular Horiuti-Polyani
The answer to this dilemma lies in the
hydrogen may be the active species.
Thus, while the original
mechanism with respect to D adatoms and alkyl reversal may be operative it
is not essential.
Recent work using model probes has provided evidence [25] that alkyl/olefin
interconversion
can involve molecular hydrogen as a weakly held transient ligand of the same
metal atom as shown in reaction (8).
c2Y ?/”
=
cX2
cx2
L-
‘f
(81 cx2Tcx2 M
4
+
x2
X Such a scheme is analogous counterpart
to the Eley-Rideal
mechanism
and has a homogeneous
in a recently discovered reaction mechanism [31]. Indeed the latter explains why
HJDs scrambling takes place at almost the same rates on SMSI and norAMS
PVIYO, [24]
provided that only “defect” sites are involved and these are little altered both in densities and reactivities by the SMSI pretreatment.
The molecular orbital explanation
exactly analogous to that of reversible metallocyclopentane metaldiolefin
complex (reaction (9)). Ethanation
occurs by this mechanism [33].
of reaction (8) iz
formation from the corresponding
of paratfins as on Ir catalysts [32] probabh
121
The behaviour of SMSI Pt with respect to Hs uptake can therefore be attriiuted to an electronic factor, i.e. donation of electrons from partly reduced TiO, in contact with small metal crystallites [24]. The surface atoms in low index planes are most sensitive to this factor and lose their capacity to retain weakly adsorbed H adatoms, but the defect atoms are left largely unaffected. This novel idea may explain why C-H and H-H bond-fission reactions, and some C-C bond-forming reactions [34] are relatively insensitive to the SMSI effect. SMSI Pt in some respects therefore can be regarded as intermediate between normal Pt and the coinage metals, Au and Cu. The latter do not readily dissociatively chemisorb Hz but are effective catalysts for many hydrogenation reactions [25]. SMSI Pt is therefore catalytically aldn to Pt alloyed with Cu or Au [35] where extensive incorporation of the coinage metals does not eliminate many of the C-H reactions includingbond shift, isomerixation of n-pentane persists to quite high levels of Au [35].
Rearrangement of model probes such as
1,ldimethylcyclopentane and methylcyclopentane is still found on alloys of high gold content in the total absence of methane formation and thus C-C fission [32]. Progressive carbiding of Pt also results in a similar shift from extensive hydrogenolysisto a high selectivity for C-H reactions including bond shift as well as some cyclixation reactions [12]. Structure Sensitive Reactions The really structurally sensitive factor is the ability of the surface to chemisorb H, as a function of SMSI pretreatment, alloying, carbiding, sulphiding and other chemical modifications. Ethane and neopentane hydrogenolysisare the classical examples of structure sensitivitywith rates falling by several orders of magnitude on alloying Croup VIII metals with small amounts of Au or Cu [35]. The activation energy for ethane cracking on PtAXO, remains the same irrespective of pretreatment, even though the rate tXls dramatically after SMSI preconditioning [24]. The decreases in rate parallel the decreases in Hz uptake. Why then, if the same “defect” sites are responsible, are these C-C fission reactions so different tc C-H reactions? The answer lies in the mechanisms. All the intermediates involved in the C-H reactions, bond shift isomerixations, and certain cyclixations, involve only (I and/orr-bonded species, alkyb olefm, allyl, etc.
The surface mononuclear organometallic complexes arc
capable of activating and reacting with molecular H, from the gas phase as shown in reactior (8). A key point here is that for such x-complexes dr-pn* back-bonding is an importan component which helps to create the necessary d-orbital vacancies for H, activation. FOI example, pure Cu and Au do not dissociatively adsorb Hz readily but quite rapia hydrogenate a strongly complexing strained olefin such as norbomene [WI, which indicates that it is the x-complexes which activate H, (reaction (8)). Hydrogen is also used as a chair
122
transfer agent in Ziegler-Natta
polymerixation
of n-alk-l-enes
so even metal-alkyls
totally
deficient in d-electrons may react directly in this fashion (reaction (10)).
R I Ti
+
H2
Ti-H
!i
w
.
RH
(10)
H
By way of contrast, hydrocracking of ethane and neopentane and metallacarbynes
involve metallacarbenes
(in the above examples, methylene, methine and even single C atoms).
Our contention is that surface organometallic species of this type are not hydrogenated rapidly off the surface. For example, ethylidyne once formed is remarkably stable to hydrogenation [6]. If there is a deficiency of surface H atoms the initial highly reactive CHz = M species either homologates by adding to alkene or reacts further to produce more stable bridging species and even surface carbide. The bridging species such as ethylidyne and surface carbide are effectively partial poisons. The hydrogen adatom factor is therefore crucial for high rates of reaction as can be seen from a steady state treatment
of the following mechanism
for
ethane cracking (reaction (11)).
CH2- CH2 / M
\L= M
M
/R M
M
etc.
MMM
(11)
Cracking at one metal atom is even a distinct possibility as occurs for ethylene homologation on some heterogeneous x-complex
can
metallacyclobutane
be
regarded
as
olefin metathesis MO catalysts [36]. Here the ethylene a
metallacyclopropane
reacting
analogously
ta
(reaction (12)).
CH2 CH2 T M
cc
Ctq”’
s
‘3
JH2 M
(12
123
It is of considerable interest that the ag cracking of neohexane to neopentane and methane declines much less rapidly than ethane cracking with an increasing SMSI state of Pt/riO, [24], or on alloying with Cu [37]. A possible explanation is that it is easier to form the disparate metallacarbenes, namely, neopentylidene and methylene, than two methylenes, but it may be more correct to say that the bulky tert-butylgroup protects adjacent atom sites and keeps them free for hydrogen chemisorption such that there is a reasonable steady-state density of neighbouring H adatoms. Such steric effects of alkyl groups in adsorbed alkenes have been shown to influence greatly rates of ortholpara hydrogen conversion [38]. The bonding requirements in the carbene/carbyne mechanism of hydrogeno@sis not only render rates very sensitive to the density of hydrogen adatoms but also clearly indicate that only the most coordinatively unsaturated electron deficient atoms will be significantly active. This view is justified by the consideration that the atom in question may have to hold simultaneously a methylene ligand, an isobutene ligand, and two hydrogen atoms in order to be a really effective centre for hydrogenolysis of neopentane. It is not surprising therefore that highly dispersed metal atoms are extremely active for this reaction [24]. The ultimate in dispersion with very high activities is to have isolated individual metal atoms in zeolites perhaps present as singly charged ions as suggested by Karpifnki based on esr evidence [13]. There is a homogeneous counterpart where Pt and Pd ions in the presence of hard acids and anions such as [CP$OJ
are quite effective for activation of small paraffins including methane
[39]. The metal ions are believed to be strong nucleophiles attacking the C-H bond in the following fashion (reaction 13).
M
n’
+
R-H
-
“(!?!.+~
+
H’
The heterogeneous mechanism is likely to be the same, affording preferential attack at terminal C-H bonds in paraf6ns. This is exactly what occurs with a very highly dispersec PfliO,
catalyst in the nonSMS1 state [49] which is found to be extremely active for
hydrogenolysisof neohexane to isopentane and where n-pentane is attacked preferentially al the terminal C atoms in contrast to the behaviour of the standard Pt/SiO, (EuroPt) catalyst! [17].
This unusual N&like behaviour of Pt with respect to hydrogenolysis and selectiw
demethylation such as methylcyclopentane conversion into cyclopentane has also been fount with very highly dispersed Pt/SiO, catalysts [41] including those prepared using Chini’s cluster compounds [42]. Other workers [43] have recently confirmed that minute particles of Pt madt from [ptls(CO),]z
on r-AlsOs have a very high selectivity for hydrogenolysis of neopentane
124
When the defect crystal structures were annealed by reduction in Hs at 1070 K without change in crystallite size the reaction shifted in the direction of isomerixation.
Karpinski [13] has also
shown that, whereas dispersed Pd catalysts only give hydrogenolysis
of neopentane,
(111)
oriented fihns are highly active and selective for isomerixation and has confirmed that the key species in the bond-shift reaction is the neopentyl radical. Detailed analyses of the reactions of the model probe, 2,2,4,4_tetramethylpentane
with D, on films of Rh, Ir, Pd and Pt also
provided conclusive evidence [l&t] that isomerlxation mechanism.
All
metallacyclobutanes
the
evidence
therefore
occurs via the surface alkyl radical
points
to
the
conclusion
are formed readily, the corresponding olefln-metahacarbenes
that,
when
react rapidly
in excess H2 to give methane and lower paraffins, but never under these conditions afford isomerixation.
Homogeneous
metallacyclobutane
complexes of Pt have been extensively
studied by Puddephatt and coworkers [44] and are well-known for the whole transition series. Although very highly dispersed metal catalysts have not been studied using paraffin/D, exchange reactions there is a report [30] that when Pt/xeolite catalysts are subjected to cycles of 0s and then H, pretreatment
at elevated temperatures
(which might be expected to result
in highly dispersed Pt atoms and/or ions) the multiple exchange distributions characteristic of standard
Pt catalysts
methylcyclopentane
shift dramatically
[30].
towards
Clearly, hydrogenation
the d, isomer
for cyclopentane
become a much more likely event than conversion into adsorbed alkene.
As a consequence,
the ap exchange process becomes dominated by the simple exchange reaction. direction expected [45] because electron-deficient
and
of the initially formed alkyl species has
coordinatively unsaturated
This is the
metal, especially
ions, should be very reactive towards hydrogen but have a poorer dx-px* bonding propensity, thus disfavouring reversal of e-bonded
alkyl to x-bonded
alkene.
A clear example of this
trend is the formation of high molecular weight polymer from ethylene using catalysts based on Ti and other early transition metal ions with few or no d-electrons whereas Ni, Rh and other Group VIII metal ions tend to terminate the propagation
reaction at the dimer stage
(reaction 14) [46]. CH-
CH2--CH2-CH2-CH3 rc”
Hydrogenolysis
I M
CH2-CH3 (14
s= M-H
of simple paraffins from ethane to n-pentane
has been extensively
studied [471 using single crystals of Ir and Rh ((100) and (111) faces) and SiO,-supported metals. As might be expected for reactions involving carbene and carbyne species there is a
125 great deal
of sensitivity towards
roughening pretreatment
the
type of crystal face expo~A, and to the annealing or
given; this is true for terminal vs internal C-C scission, single versus
multiple C-C scission, and changes in the kinetic parameters.
Depending
on chain length
there are indications here of all possible modes of C-C fission via metallacyclopropanes dimetallacyclobutanes
(a@ mode), metallacyclobutanes
At higher ternary
(9,ll)).
multiple hy~~no~~
and metallacyclopemanes of n-pentane
or
(reactious
to methane takes over
from single C-C fission, and significantly the activation energy changes to a different value at this juncture because the rate determining desorption.
step changes from C-C bond s&ion
to methane
Reducing the hydrogen pressure during reaction causes the same switch from
single to multiple GC fission. These results confhm the argument that the H adatom factor is very important in detenninin g the behaviour of structure sensitive reactions but they also support the notion in line with observations
in organometallic
activity of individual metal atoms is very dependent
chemistry that the catalytic
on the degree of coordination
by and
spatial disposition of nearest neighbours. C-E Reactions and DePeets There are many pnzzling features about reactions involving C-H bond fission, especially exchange reactions with Du if the term insensitive is to imply that all exposed metal atoms are active. Thus the U-shape of the initial ~~~utio~
for ethanelf),
exchange found using Pt
fihns [29] and many other forms of this metal, inchuiing a (111) single crystal [48], is a general characteristic which also extends to the behaviour of high alkanes and cycloalkanes on various Pt catalysts [45]. This type of distribution requires two or more types of site whose numbers and relative proportions or prevalent
stay fairly constant and independent
of the active surface.
of the method of preparation
They may act ~dependent~
[29] or ~terna~e~,
paraffin activation may occur mainly on one type of site, and is then followed by inter-site migration
of reactive
cyclopentane/Dz own douche
transients,
e.g. alkenes
and possibly
alkyl radicals
[45].
The
reaction [29], which is now a classical probe with each metal exhibiting its behaviour,
mobility of intermediate
shows that even at ambient temperatures
cyclopentenes,
there is considerable
including roll-over and inter-site transfer 1451. This
topic will be discussed in greater detail elsewhere [49], but it is worth noting here that while the densities of the active defect metal atoms may be low they seem to exist in pairs since the presence of sites of one type requires the presence of sites of a second type. For example, a comer atom-site is always accompanied by an edge atom-site. transients
is not so snrprising
since it is the foundation
Inter-site transfer of olefinic
of classical dual functional
reforming catalysis where metals on acidic supports are used.
oil
126
Two additional examples illustrate that defect sites are responsible for C-H reactions. The first is provided by Burwell’s work [16] using the model probe di-t-butylacetylene hydrogenation
studies.
not be expected to react readily on surface planes yet it hydrogenates cyclopentene.
in
Because of the sheer bulk of the t-butyl groups this substrate would
In order to accommodate
the idea that hydrogenation
almost as readily as reactions are basically
structure insensitive with the ensuing assumption that all exposed metal atoms are catalytically active, Burwell speculated that the strongly complexing acetylene IiteralJy lifts the metal atom up out of the plane.
An alternative
explanation
which now seems evident is that C-H
reactions only occur on relatively few centres, ie the defect atoms at comers and edges etc. Hydrogenation
of ethylene on silica-supported
Ru-Cu catalysts has been studied [50]
using solid state 13C mm and this work provides the second example that special sites are essential for the simplest hydrogenations. the monometallic unsaturated
The bimetallic catalysts are much less active than
Ru catalyst and, since Cu is known to populate the coordinativeiy
highly
sites such as edges and comers, it is postulated that these play a crucial role in
hydrogenation
reactions.
The real mystery at the heart of this dilemma of so-called insensitivity
is that the
catalysts may have to be produced at great extremes of the roughness associated with high dispersion or the smoothness of extended flat arrays of atoms before the rates per unit metal area change significantly for this category of reactions. Thus when supported Ru catalysts are prepared
by treating
SiO, with (q4-cycloocta-l,S-diene)
(n’-cycloocta-1,3,Qriene)Ru(O)
followed by reduction with H, particles of highly dispersed metal are obtained [Sl]. The rates of hydrogenation
of cyclohexene and benzene
at ambient temperatures
show pronounced
maxima at particle diameters of cu. 2.5 mn. The variation of rate with particle sixe is very reminiscent
of the theoretical distribution
effective in Ns chemisorption
on metals as a function of particle size. A similar dependence
of rates of benxene hydrogenation Rh/siO,
curves calculated [52] for defect sites found to be
on particle size has also been noted for Ni/SiO, [52] and
[54]. An examination
of the literature
on deuterium
exchange reactions
on Pt reveals
examples which illustrate the effect of severe annealing on activity. The rates of exchange of methane per unit area are broadly the same for polycrystalline films [55] and powders [56] with similar values for activation energies and pre-exponential
factors. However, in the case
of fihns prepared and annealed at 5OOT [571 the pre-exponential
factor is reduced by cu. 10s
although the activation energy is in agreement with the other studies. Deuterium
distriiution
patterns show that the same exchange process is observed irrespective of catalyst origin. A similar result is found with the exchange of ethane on polycrystahine
fihns [58], (Ill)-single
crystals [48] and powders [59,60] compared with severely annealed fihns [Sq.
Rates of
exchange on the latter are Id-10’ times lower than on the other catalysts, but again it should be noted that the W-shaped distriiution in the deuteroisomers is preserved throughout. Thus, despite siguificant variations in preparative technique, rates per unit metal area are only influenced in a major fashion by treatment likely to eliminate most defect sites. Remarkably, the initial
~m~utions
of deute~~rne~
remain the same
showhrg that the detailed
mechanism and nature of the sites responsrble do not change. Conchsions In 1925 Taylor 1611 suggested that crystal edges and comers, gram boundaries, and other physical irregularities of the surface may provide active centres of unusually high catalytic activity. Some forty years later this idea was greatly developed through the r-bond theory of catalysis [8] with its emphasis on single atom sites, coordinative unsaturation, and the relationship of the surface reactions to organometallic chemistry and to homogeneous catalysis. More recently surface physics techniques assumed great importance in the area of gas solid reactions; the relevance of this research to the field of heterogeneous catalysis, especially hydrocarbon reactions on metals, is apparently justified by the observation that many of these reactions are largely structure insensitive. However, in the present review we have
summarized
recent work which provides a convhxing case that the term insensitive is
something of a misnomer and that catalysis by metals for all hydrocarbon reactions is indeed due to relatively low numbers of defects which are basically single atom sites. More than one type of defect site may operate distinctively but in concert in a given reaction, as apparently happens with comer and nearest-neighbour edge atoms on Pt in cataly&rg paraffin exchange with D,. As in the ~~o~~te
field of catalysis fl] site densities are therefore much lm
and real turnover numbers much higher than those calculated on the basis of surface areas. An additional major difficulty emerging for kinetic analysis and the estimation of true turnover numbers is that even under mild conditions there may be considerable transfer of transient ~te~e~at~,
e.g. alkenes, but also free radicals, from one type of defect site to
another. Mobility of intermediates does not necessarily imply total detachment from the metal surface in the act of transfer, as may happen in certain spillover phenomena and classical dual-functional catalysis in oil reforming.
The more general idea of dual functionality
described in this review could also be of special value in the third broad class of heterogeneous catalysts namely, semiconductor oxides.
For example, formation of free
radicals at Bi3’ ion sites followed by migration to and reaction on MO’+ ion sites may well occur in selective oxidation of allrenes on bismuth molybdate.
128 In conclusion these two concepts of low site densities and possible mobility of transients may be important in ah areas of heterogeneous
catalysis. Such concepts emphasize the value
of the chemical probe approach for synthetic innovation and mechanistic understanding
as well
as serving as a basis for a unifying philosophy of ah cataIysis including the homogeneous
and
enzymatic areas. Acknowledgements We thank the EC Directorate for Science for financial assistance and Professor J.K.A. Clarke for helpful discussion. References
5 6 7 8 9 10 11 12
13 14 15 16 17 18a 18b 19 20 21 22 23 24 25 26 27 28
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29
30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
61
C. KembaII, Adv. Catal., 11 (1959) 223. N. Poole and D.k Whan, Proc. 8th Int. Congr. CataI. (Berlin 1984), Verlag Chemie, Weinhehq 1989, vol 4 p. 345. P.M. Hodges, S.k Jackson, J. Jacke, M. Poliakoff, J.J. Turner and F.-W. GreveIs, J. Am. Chem. Sot., 112 (1990) 1234; W.D. Harman and H. Taube, J. Am. Chem. Sot., 112 (1990) 2261. A.F. Kane and JKA. Clarke, J. Chem. Sot., Faraday Trans. I, 76 (1980) 1640; J.R. Engstrom, D.W. Goodman and W.H. Weinberg, J. Am. Chem. Sot., 108 (1986) 4653. S.J. McLain, J. Sancho and R.R. S&rock, J. Am. Chem. Sot., 101 (1979) 5451; R.H. Grubbs and A. Miyaskita, J. Am. Chem. Sot., 100 (1978) 1300. J.B.F. Anderson, R. Burch and J.k Cairns, J. Catal., 107 (1987) 364. J.K.A. Clarke, Ind. Eng. Chem. Prod. Res. Dev., 20 (1981) 574. P.P. O’NeiII and J.J. Rooney, J. Am. Chem. Sot., 94 (1972) 4383. J.KA_ Clarke, KM.G. Rooney and T. Baird, J. CataI., 113 (1988) 374. G.H. Twigg and E.K. RideaI, Trans. Faraday Sot., 36 (1940) 533. A Sen, Platinum Metals Rev., 35 (1991) 126. J.KA Clarke, R.J. Dempsey and T. Baird, J. Chem. Sot. Faraday Trans. I, 86 (1990) 2789. J.KA Clarke, kC.M. Creaner and T. Baird, Applied CataL, 9 (1984) 85. 0. Zahra, F. Garin and G. Maire, Faraday Disc. Chem. Sot., 72 (1981) 33. B.E. Hardy, J.A. Dumesic and S.H. Langer, J. Catal., 126 (1990) 73. R.J. Puddephatt, Coord. Chem. Rev., 39 (1980) 149. B.F. Hegarty and J.J. Rooney, J. Chem. Sot., Faraday Trans. I, 85 (1989) 1861. R. Cramer, Act. Chem. Res., 1 (1968) 186. AD. Logan, K Sharoudi and AK Datye, J. Phys. Chem., 95 (1991) 5568. F. Zaera and GA. Somorjai, J. Phys. Chem., 84 (1985) 3211. W.R. Patterson and J.J. Rooney, to be published. M. Sprock, M. Pruski, B.G. Gerstein and T.S. King, CataI. Letts., 5 (1990) 395. N. Kitajima, A. Kono, W. Ueda, Y. Moro-oka and T. Ikawa, J. Chem. Sot. Chem. Commun., (1986) 674. R. van Hardeveld and J. Hartog, Surf. Sci., 15 (1%9) 189. J.W.E. Coenen, R.Z.C. van Meerten and H.Th. Rijnten, Proc. 5th Int. Congr. CataL (Palm Beach 1972), North Holland Publ. Co., Amsterdam, 1973, vol 1 p. 671. W.F. Graydon and MD. Lang.an, J. Catal., 69 (1981) 180. C. Kernball, Proc. Roy. Sot., A217 (1953) 376. D.W. McKee and F.J. Norton, J. Phys. C&em., 68 (1964) 481. L. Guczi and Z. KarpifAi, J. Catal., 68 (1981) 190. J.R. Anderson and C. Kembah, Proc. Roy. Sot., A223 (1954) 361. L. Babemico, L. Guczi, K Matusek, A Sarkany and P. Tetenyi, Proc. 6th Int. Congr. CataL (London 1976), Chem. Sot., London, 1977, vol 1 p. 456. L Guczi, A. Sarkany and P. Tetenyi, J. Chem. Sot. Faraday Trans. I, 70 (1974) 1971. H.S. Taylor, Proc. Roy. Sot., A108 (1925) 105.
catalysis Today 12 (1992) 113-129 Elsevier Science publishersB.V., Amsterdam
W.R. PATTERSON and J.J. RGGNEY
School of Chemistry, The Queen’s University of Belfast, David Keir Building, Stramnilhs Road, BELFAST BT9 SAG, N. Ireland
The following is the summary of a review article in Nature, 1984, entitled, “The active sites of acidic ahuninosilicate catalysts”, by Weiss and coworkers [l] of Mobil, which aptly bastes
a Freud
dilemma for all of heterogen~us cata?@s.
‘The availability of solid acidic catalysts based on zeolite ZSM-5 makes possible observations on the nature and the absolute rate bebaviour of the individualprotonic sites. Tetrahedral ahuninium atoms are the highly reactive ingredients even at a level of parts per million, or less. Turnover numbers for several hydrocarbon reactions equal or exceed familiar enzymatic turnover values. Intensive catalytic activities can result from aluminium levels likely to be iguored by the experimenter.” This statement is completely justified since the nmbers and strength distributions of the protonic centres have been estimated by a variety of titration techniques using bases [2J. A similar methodology has also been employed to examine the redox sites in refractory oxides such as y-AlzOs and GazO, which promote a variety of catalytic reactions ~~lu~g
exchange
with deuterium of alkanes and alkenes, and double bond isomerism in alkenes. Here elegant studies [3,475] have revealed that gases such as H& SO, CO* etc., may be used as in siti selective, quantitative, and reversible poisons for certain features of these reactions. Again the major and ~eq~~
fiudiug is that the site densities are very low. The bang
general question is therefore valid and very importaut. Are site densities invariably low for aJl heterogeneous catalysts, and especially for reactions of paraffms?
This question is
particularly apposite for metal catalysts where there is no consensus on what constitutes an active site, one surface atom, or a contiguous pair, or a multinuclear ensemble. Turnover nmnbers are rarely if ever expressed in rates per c&c$&
site because the densities of such
sites are not known; rather they are expressed as rates per exposed metal atom, or unit metal surfam area, as measured by Hu 0,
or Co adsorption. Methods of iden~
sites and
estimating their densities are very limited and the mechanisms of reactions much less clear-cut
0920~5861/92/$17.~ 0 1992 Elsevier Science PublishersB.V. All rights reserved.
114
than for the acidic and redox surfaces.
Surface physics techniques
have been increasingly
employed especially during the past two decades in attempts to reveal surface intermediates. Frequently,
however, the adsorbed hydrocarbon entities which are most abundant
and most
readily identified are not catalytically competent, or are simply “spectator” species, as found for the celebrated ethylidyne entity [6] which bridges three metal atoms and is formed under certain conditions from chemisorbed constitutes
a reactive
compounded
ethane, ethylene or acetylene.
site and the true estimation
of the number
by the observation that many hydrocarbon
are multi-step in nature.
The dilemma of what of these is further
reactions at elevated temperatures
Some of these steps are regarded as essentially surface structure
insensitive [I, whereas a few, notably ethane and neopentane
hydrocracking, are classifted as
structure sensitive. The word “insensitive” is used where rates of reaction per unit metal area are, by and large, independent
of the method of preparation
and pretreatment
and are a function only of the identity of the metal involved. in these cases that all the exposed metal atoms contriiute
of the catalyst
Generally it has been assumed signi&antly
to the reactivity, but
the alternative proposition that the site densities are low while remaining reasonably constant cannot be excluded. In contrast, the rates of structure sensitive reactions depend strongly on the history of the catalyst and the reaction conditions. insensitive reactions are largely independent reactions are dependent
Thus, it follows that the rates of
of particle size, whereas the rates of sensitive
on this variable.
The idea that certain individual metal atoms may constitute the reactive sites has been consistently advocated to explain many hydrocarbon reactions since it was first communicated [g,9] as “the x-bond theory of catalysis”. The essence of this idea is that various e- and Abonded intermediates
interconvert
as transient
ligands of the same surface atom or ion,
preferentially coordinatively highly unsaturated and presumably only present in relatively small numbers [lo]. The alternative view that a site consists of ensembles of metal atoms is widely accepted [ll], but with little or no experimental homogeneous
justification,
and without cotmterparts
in
catalysis.
Investigations
of chemisorbed species using surface physics techniques may therefore
be merely complementary
to the chemical probe approach
in the field of heterogeneous
catalysis. The claim [6] that surface science and solid state science are the twin pillars of an understanding
of this difficult subject must be critically assessed since it is possible that only
relatively few sites are really effective under steady-state conditions, with the vast bulk of chemisorbed
material on the surface not directly involved in each catalytic cycle.
juncture in the development
At this
of the theoty of metal catalysis it is essential that such questions
be put clearly without bias since there is much confusion arising from the sheer plethora of
115
data and mechanistic suggestions, many of which are conflicting and chemically unrealistic. The purpose of this review therefore is to reappraise
some of the more significant features
in a fashion which will illuminate the true nature of the active sites on the surface of a metal catalyst. Mechanisms of Structure Insensitive Reactions Structure insensitive reactions are basically those involved only in the making and breaking
of
C-H
deuterium-exchange [12].
Some
bonds.
examples
are
the
hydrogenation
reactions of paraffins and the exchange and hydrogenation
The 1,2-bond shift isomerixation
reactions also fall into this category.
reaction
of parafiks
to hydrocarbon
carbyne intermediates
are an essential part of the mechanism.
of benzene
[13] and certain cyclixation
reaction depends on whether or not carbene and/or
theory one surface metal atom is enough
hydrocarbon transients for insensitive reactions. hydrogenation
alkenes,
The real distinction between the terms sensitive and
insensitive as applied
In terms of the x-bond
of
to hold the
For example, ethane exchange and ethylene
can be envisaged as the interconversion
of u-bonded
ethyl and x-bonded
ethylene (x = H or D) as shown in reaction (1).
-X
‘2’5 + M
cx2
* X
cx2 f M
There is ample evidence for this mechanism from direct spectroscopic investigations from exchange reactions of suitable model pmbes such as heptacyclotetradecane can potentially form an eclipsed 1,2diadsorbed
intermediate,
[14] and [U] which
as advocated by Burwell[16],
but not a x-bonded analogue; only simple exchange is observed [15] as expected if n-bonding is the correct description
of the 1,2diadsorbed
species required by the multiple exchange
process. Multiple interconversion
exchange and hydrogenation of x-bonded
There is considerable
of benzene
and a-bonded intermediates
also can be accounted
for by
as shown in reaction (2)
confusion about the mechanism of 1,2bond
shift isomerism in
paraffins [17], even though there is now much evidence [18] that the only viable general pathway is rearrangement
of surface alkyls in a manner akin to that of carbonium ions. The
lowering of the energy barrier via x-bonding of the half-reaction-state
complex to a surface
116
*@:
x@x + xj$x
%
M
X
atom is regarded as being very important but the degree of o-bonding between the final and initial radicals and the metal is a moot question. the classical example of neopentane
Reaction (3) exemplifies this mechanism for
conversion into isopentane.
CH3 I {HfC-Cn, I cH3 M
CH l3 %-
z+
./‘“3 c\ : CH3 IA
(3)
It is worth noting that in this scheme a C-C bond is never really cleaved as it is in the alternative suggestion of an olefin-metathesis
type mechanism involving metallocyclobutanes
[19] (reaction (4)). There is convincing evidence [20] that the metathesis
mechanism
for bond shift is
confined to methyl shift in simple alkanes and is only significant on metals such as Ni and Pd at elevated temperatures
and low hydrogen pressures, i.e. conditions under which there is also
substantial concomitant homologation when there
is simultaneous
and methanation
formation
reactions [21]. Homologation
from paraffins
of transient
occurs
olefins and surface
methylenes arising from extensive C-C fission. Migration of olefin, as formed by reaction (l), to a site with an adsorbed methylene
then results in cyclobutanation
and ultimately chain
lengthening, as shown in reaction (4). On W catalysts linear homologation
of paraffins is an
117
H3c\
fH3
/c\
cH\3 lCH3 -
cH2\chC$
Cq/2
JP
JP
/“P
,CH3
CH2 \/‘CH3
important specific reaction accompanying substantial methanation
(4
[ZO]. Here the metathesis
reaction is clearly operating but does not give bond shift; for example, n-pentane isopentane, but only n-hexane and n-butane. electrophilic
does not give
Clearly the methylene intermediates
and add specifically to the terminal C-atom of intermediate
selectivity is reflected by the high ratios of degenerate
are highly
alk-1-ene.
This
to productive methathesis found for
terminal alkenes using W-based catalysts [22]. The preferred pathway is as shown in reaction (5) and not that in reaction (6).
=%=
RdC’CHR’
(5)
‘w’
tiCH ,,kw
AH2
FkH
-
(6)
118
Both (5) and (6) would have to be significant contribute to 1,2-bond shift. Furthermore,
for the metathesis
the intermediate
itself in order to make an isomeric metallacycle.
mechanism
to
olefin has to rotate to reorientate
Mobility of olefin is indeed required if
migration of alkene across the surface leads to substantial homologation but this mobility does not simultaneously surface
methylenes
afford 12-bond
shift on W [20]. If the hydrogen pressure is high the
are preferentially
hydrogenated
to methane.
The
homologation
mechanism with migration of transient olefin also provides a novel way of considering the mechanism paraffins.
of Fischer-Tropsch
synthesis [23] and Schultz-Flory
The initial step is the combination
homologation participation
distributions
of product
of two methylenes to give ethylene. Statistical
is then possible via migration in reversible metallacyclobutane
of ethylene
and all product
alkenes with
formation as shown in reaction (4).
We wish to stress therefore that while the metathesis-type
reactions are important on
metal surfaces they are not responsible in almost all cases for bond shift isomerixation. Pt is the metal par cmdence for bond shift [l&18,24], but it is poor at methanation high hydrogen pressures.
Furthermore,
the metathesis
mechanism
confined to methyl shift and simply camiot explain rearrangement the required isomeric metallacyclobutanes
Thus even at
for isomerixation
is
of cycloalkanes [WI, since
cannot form in these cases.
Any intermediate
alkylidene higher than methylene rearranges too readily via a rapid 1,2 H-shift at elevated temperatures
to the corresponding
alkene as shown in reaction (7).
RCH
CH2
(71
=t-
The only viable 12-bond shift mechanism for Pt is therefore the alkyl radical one. It is even possible that just as transient olefins migrate across the surface, free radicals formed at certain sites also migrate and then rearrange on other sites. This aspect of the mechanism could be quite important at elevated temperatures The recent elegant synthesis of dodecahedrane,
where 1,kyclixation
reactions also occur.
CL&&,,,from its isomeric precursor pagodane
using Pd and Pt catalysts in excess hydrogen at cu. 23X! is unquestionably
a nice example of
l,S-ring closure by a free radical mechanism [26]. Inter-site transfer of transients in metal catalysis may be much more widespread than hitherto considered.
It is a complicating factor in discussing kinetics and reaction insensitivity
and may even extend, as will be discussed in more detail later, to exchange of paraffins with D, at ambient temperatures.
119 Site Jhnsities
and Insensitivity
The r-bond theory with its emphasis on mononuclear sites is vindicated by the mechanistic evidence apart from the understanding it gives to heterogeneous catalysis in the light of parallel developments in organometallic chemistry and homogeneous metal catalysis. Indeed in the latter field essentially every known reaction is mononuclear which emphasizes the importance of the cis-ligand insertion step both on surfaces and in solution. It might appear therefore that every exposed metal atom in a surface is a catalytically active centre for all of the above reactions. As yet there is no direct spectroscopic evidence concerning this important issue but a considerable amount of good indirect evidence in recent years points to the conclusion that the really reactive sites are relatively few in number even for simple hydrogenation reactions. The surface atoms in an ideal metal crystal can be considered in terms of their coordinative unsaturation, which is least for atoms in the (111) plane and is greatest for apical atom sites. Atoms in other planes, terraces and edges have intermediate degrees of saturation. If all exposed surface atoms were catalytically active, irrespective of coordination, a series of Arrhenius equations could be considered with the summation overall givingthe actual rate of reaction per unit metal area provided that the A factors, which reflect the densities of the different types of sites are expressed in this way. In the simplest case the kinetics may be assumed to be the same with only the A and E factors changing: b
= Zk
= BAexp(-EJRT)
An internal compensation effect will then arise where the sites of lowest unsaturation will be the most numerous (e.g. (111) plane) but with the highest activation energy, whereas the lowest activation energy sites, e.g. comers, should be scarce. Various distributions of the fractions of the total rate as a function of site type may be constructed, and it is evident thal the low index metal planes may be essentially inert because the E values are too high, even though the corresponding A factors are quite favourable. This would leave the coordinatively highly unsaturated metal atoms which may be regarded as defects, as the only sites really effective.
In the extreme the distriiution may be so narrow that only one type of site
contributes overwhelminglyto the total reaction. In the light of these ideas the descriptior “structure insensitive” is never more than an approximation. Nevertheless it may be quite a good approximation when the rate per unit metal area is largely independent of the methot of preparation and pre-conditioning of the catalyst and is a function only of the identity of thf metal. However, the expression of activities in apparent rather than true turnover numben conceals the possibility that catalysis may be due to defect sites, possibly of more than one type, the densities of which stay fairly constant. Recent work on Pt/riO, catalysts in the SMSI and nonSMS1 states provides mucl
120
evidence that this interpretation
of structure insensitivity is correct. The catalysts preheated
to give the SMSI state have greatly reduced capacities to chemisorb hydrogen but retain much of their activities for bond-shift isomerixation
[24], olefin hydrogenation
exchange
The characteristic
[271, and H&
exchange
[28].
double
[25], cyclopentane U-shaped
initial
distribution for cyclopentane exchange on Pt is maintained (did, isomers, and ds-d,, isomers). This suggests that the active sites are still the same as those on non-SMSI pt/riOs, on films [29], and on metal dispersed on conventional
supports [30]. Since the H-H bond is more
readily broken on metals than C-H bonds this is a truly surprising result unless the site densities are low and the planes essentially inert for paraffin activation as claimed. Why then are the multiplicities in initial exchange reactions the same for SMSI and non-SMSI metal if as hitherto assumed a surface pool of numerous rapidly scrambling highly mobile H and D adatoms is required? sine qua non for any
The availability of a large excess of highly reactive D atoms is a kinetic meaningful
interpretation
of exchange reactions [29], yet the SMSI
catalysts quite evidently do not have such a pool. real&ration that molecular Horiuti-Polyani
The answer to this dilemma lies in the
hydrogen may be the active species.
Thus, while the original
mechanism with respect to D adatoms and alkyl reversal may be operative it
is not essential.
Recent work using model probes has provided evidence [25] that alkyl/olefin
interconversion
can involve molecular hydrogen as a weakly held transient ligand of the same
metal atom as shown in reaction (8).
c2Y ?/”
=
cX2
cx2
L-
‘f
(81 cx2Tcx2 M
4
+
x2
X Such a scheme is analogous counterpart
to the Eley-Rideal
mechanism
and has a homogeneous
in a recently discovered reaction mechanism [31]. Indeed the latter explains why
HJDs scrambling takes place at almost the same rates on SMSI and norAMS
PVIYO, [24]
provided that only “defect” sites are involved and these are little altered both in densities and reactivities by the SMSI pretreatment.
The molecular orbital explanation
exactly analogous to that of reversible metallocyclopentane metaldiolefin
complex (reaction (9)). Ethanation
occurs by this mechanism [33].
of reaction (8) iz
formation from the corresponding
of paratfins as on Ir catalysts [32] probabh
121
The behaviour of SMSI Pt with respect to Hs uptake can therefore be attriiuted to an electronic factor, i.e. donation of electrons from partly reduced TiO, in contact with small metal crystallites [24]. The surface atoms in low index planes are most sensitive to this factor and lose their capacity to retain weakly adsorbed H adatoms, but the defect atoms are left largely unaffected. This novel idea may explain why C-H and H-H bond-fission reactions, and some C-C bond-forming reactions [34] are relatively insensitive to the SMSI effect. SMSI Pt in some respects therefore can be regarded as intermediate between normal Pt and the coinage metals, Au and Cu. The latter do not readily dissociatively chemisorb Hz but are effective catalysts for many hydrogenation reactions [25]. SMSI Pt is therefore catalytically aldn to Pt alloyed with Cu or Au [35] where extensive incorporation of the coinage metals does not eliminate many of the C-H reactions includingbond shift, isomerixation of n-pentane persists to quite high levels of Au [35].
Rearrangement of model probes such as
1,ldimethylcyclopentane and methylcyclopentane is still found on alloys of high gold content in the total absence of methane formation and thus C-C fission [32]. Progressive carbiding of Pt also results in a similar shift from extensive hydrogenolysisto a high selectivity for C-H reactions including bond shift as well as some cyclixation reactions [12]. Structure Sensitive Reactions The really structurally sensitive factor is the ability of the surface to chemisorb H, as a function of SMSI pretreatment, alloying, carbiding, sulphiding and other chemical modifications. Ethane and neopentane hydrogenolysisare the classical examples of structure sensitivitywith rates falling by several orders of magnitude on alloying Croup VIII metals with small amounts of Au or Cu [35]. The activation energy for ethane cracking on PtAXO, remains the same irrespective of pretreatment, even though the rate tXls dramatically after SMSI preconditioning [24]. The decreases in rate parallel the decreases in Hz uptake. Why then, if the same “defect” sites are responsible, are these C-C fission reactions so different tc C-H reactions? The answer lies in the mechanisms. All the intermediates involved in the C-H reactions, bond shift isomerixations, and certain cyclixations, involve only (I and/orr-bonded species, alkyb olefm, allyl, etc.
The surface mononuclear organometallic complexes arc
capable of activating and reacting with molecular H, from the gas phase as shown in reactior (8). A key point here is that for such x-complexes dr-pn* back-bonding is an importan component which helps to create the necessary d-orbital vacancies for H, activation. FOI example, pure Cu and Au do not dissociatively adsorb Hz readily but quite rapia hydrogenate a strongly complexing strained olefin such as norbomene [WI, which indicates that it is the x-complexes which activate H, (reaction (8)). Hydrogen is also used as a chair
122
transfer agent in Ziegler-Natta
polymerixation
of n-alk-l-enes
so even metal-alkyls
totally
deficient in d-electrons may react directly in this fashion (reaction (10)).
R I Ti
+
H2
Ti-H
!i
w
.
RH
(10)
H
By way of contrast, hydrocracking of ethane and neopentane and metallacarbynes
involve metallacarbenes
(in the above examples, methylene, methine and even single C atoms).
Our contention is that surface organometallic species of this type are not hydrogenated rapidly off the surface. For example, ethylidyne once formed is remarkably stable to hydrogenation [6]. If there is a deficiency of surface H atoms the initial highly reactive CHz = M species either homologates by adding to alkene or reacts further to produce more stable bridging species and even surface carbide. The bridging species such as ethylidyne and surface carbide are effectively partial poisons. The hydrogen adatom factor is therefore crucial for high rates of reaction as can be seen from a steady state treatment
of the following mechanism
for
ethane cracking (reaction (11)).
CH2- CH2 / M
\L= M
M
/R M
M
etc.
MMM
(11)
Cracking at one metal atom is even a distinct possibility as occurs for ethylene homologation on some heterogeneous x-complex
can
metallacyclobutane
be
regarded
as
olefin metathesis MO catalysts [36]. Here the ethylene a
metallacyclopropane
reacting
analogously
ta
(reaction (12)).
CH2 CH2 T M
cc
Ctq”’
s
‘3
JH2 M
(12
123
It is of considerable interest that the ag cracking of neohexane to neopentane and methane declines much less rapidly than ethane cracking with an increasing SMSI state of Pt/riO, [24], or on alloying with Cu [37]. A possible explanation is that it is easier to form the disparate metallacarbenes, namely, neopentylidene and methylene, than two methylenes, but it may be more correct to say that the bulky tert-butylgroup protects adjacent atom sites and keeps them free for hydrogen chemisorption such that there is a reasonable steady-state density of neighbouring H adatoms. Such steric effects of alkyl groups in adsorbed alkenes have been shown to influence greatly rates of ortholpara hydrogen conversion [38]. The bonding requirements in the carbene/carbyne mechanism of hydrogeno@sis not only render rates very sensitive to the density of hydrogen adatoms but also clearly indicate that only the most coordinatively unsaturated electron deficient atoms will be significantly active. This view is justified by the consideration that the atom in question may have to hold simultaneously a methylene ligand, an isobutene ligand, and two hydrogen atoms in order to be a really effective centre for hydrogenolysis of neopentane. It is not surprising therefore that highly dispersed metal atoms are extremely active for this reaction [24]. The ultimate in dispersion with very high activities is to have isolated individual metal atoms in zeolites perhaps present as singly charged ions as suggested by Karpifnki based on esr evidence [13]. There is a homogeneous counterpart where Pt and Pd ions in the presence of hard acids and anions such as [CP$OJ
are quite effective for activation of small paraffins including methane
[39]. The metal ions are believed to be strong nucleophiles attacking the C-H bond in the following fashion (reaction 13).
M
n’
+
R-H
-
“(!?!.+~
+
H’
The heterogeneous mechanism is likely to be the same, affording preferential attack at terminal C-H bonds in paraf6ns. This is exactly what occurs with a very highly dispersec PfliO,
catalyst in the nonSMS1 state [49] which is found to be extremely active for
hydrogenolysisof neohexane to isopentane and where n-pentane is attacked preferentially al the terminal C atoms in contrast to the behaviour of the standard Pt/SiO, (EuroPt) catalyst! [17].
This unusual N&like behaviour of Pt with respect to hydrogenolysis and selectiw
demethylation such as methylcyclopentane conversion into cyclopentane has also been fount with very highly dispersed Pt/SiO, catalysts [41] including those prepared using Chini’s cluster compounds [42]. Other workers [43] have recently confirmed that minute particles of Pt madt from [ptls(CO),]z
on r-AlsOs have a very high selectivity for hydrogenolysis of neopentane
124
When the defect crystal structures were annealed by reduction in Hs at 1070 K without change in crystallite size the reaction shifted in the direction of isomerixation.
Karpinski [13] has also
shown that, whereas dispersed Pd catalysts only give hydrogenolysis
of neopentane,
(111)
oriented fihns are highly active and selective for isomerixation and has confirmed that the key species in the bond-shift reaction is the neopentyl radical. Detailed analyses of the reactions of the model probe, 2,2,4,4_tetramethylpentane
with D, on films of Rh, Ir, Pd and Pt also
provided conclusive evidence [l&t] that isomerlxation mechanism.
All
metallacyclobutanes
the
evidence
therefore
occurs via the surface alkyl radical
points
to
the
conclusion
are formed readily, the corresponding olefln-metahacarbenes
that,
when
react rapidly
in excess H2 to give methane and lower paraffins, but never under these conditions afford isomerixation.
Homogeneous
metallacyclobutane
complexes of Pt have been extensively
studied by Puddephatt and coworkers [44] and are well-known for the whole transition series. Although very highly dispersed metal catalysts have not been studied using paraffin/D, exchange reactions there is a report [30] that when Pt/xeolite catalysts are subjected to cycles of 0s and then H, pretreatment
at elevated temperatures
(which might be expected to result
in highly dispersed Pt atoms and/or ions) the multiple exchange distributions characteristic of standard
Pt catalysts
methylcyclopentane
shift dramatically
[30].
towards
Clearly, hydrogenation
the d, isomer
for cyclopentane
become a much more likely event than conversion into adsorbed alkene.
As a consequence,
the ap exchange process becomes dominated by the simple exchange reaction. direction expected [45] because electron-deficient
and
of the initially formed alkyl species has
coordinatively unsaturated
This is the
metal, especially
ions, should be very reactive towards hydrogen but have a poorer dx-px* bonding propensity, thus disfavouring reversal of e-bonded
alkyl to x-bonded
alkene.
A clear example of this
trend is the formation of high molecular weight polymer from ethylene using catalysts based on Ti and other early transition metal ions with few or no d-electrons whereas Ni, Rh and other Group VIII metal ions tend to terminate the propagation
reaction at the dimer stage
(reaction 14) [46]. CH-
CH2--CH2-CH2-CH3 rc”
Hydrogenolysis
I M
CH2-CH3 (14
s= M-H
of simple paraffins from ethane to n-pentane
has been extensively
studied [471 using single crystals of Ir and Rh ((100) and (111) faces) and SiO,-supported metals. As might be expected for reactions involving carbene and carbyne species there is a
125 great deal
of sensitivity towards
roughening pretreatment
the
type of crystal face expo~A, and to the annealing or
given; this is true for terminal vs internal C-C scission, single versus
multiple C-C scission, and changes in the kinetic parameters.
Depending
on chain length
there are indications here of all possible modes of C-C fission via metallacyclopropanes dimetallacyclobutanes
(a@ mode), metallacyclobutanes
At higher ternary
(9,ll)).
multiple hy~~no~~
and metallacyclopemanes of n-pentane
or
(reactious
to methane takes over
from single C-C fission, and significantly the activation energy changes to a different value at this juncture because the rate determining desorption.
step changes from C-C bond s&ion
to methane
Reducing the hydrogen pressure during reaction causes the same switch from
single to multiple GC fission. These results confhm the argument that the H adatom factor is very important in detenninin g the behaviour of structure sensitive reactions but they also support the notion in line with observations
in organometallic
activity of individual metal atoms is very dependent
chemistry that the catalytic
on the degree of coordination
by and
spatial disposition of nearest neighbours. C-E Reactions and DePeets There are many pnzzling features about reactions involving C-H bond fission, especially exchange reactions with Du if the term insensitive is to imply that all exposed metal atoms are active. Thus the U-shape of the initial ~~~utio~
for ethanelf),
exchange found using Pt
fihns [29] and many other forms of this metal, inchuiing a (111) single crystal [48], is a general characteristic which also extends to the behaviour of high alkanes and cycloalkanes on various Pt catalysts [45]. This type of distribution requires two or more types of site whose numbers and relative proportions or prevalent
stay fairly constant and independent
of the active surface.
of the method of preparation
They may act ~dependent~
[29] or ~terna~e~,
paraffin activation may occur mainly on one type of site, and is then followed by inter-site migration
of reactive
cyclopentane/Dz own douche
transients,
e.g. alkenes
and possibly
alkyl radicals
[45].
The
reaction [29], which is now a classical probe with each metal exhibiting its behaviour,
mobility of intermediate
shows that even at ambient temperatures
cyclopentenes,
there is considerable
including roll-over and inter-site transfer 1451. This
topic will be discussed in greater detail elsewhere [49], but it is worth noting here that while the densities of the active defect metal atoms may be low they seem to exist in pairs since the presence of sites of one type requires the presence of sites of a second type. For example, a comer atom-site is always accompanied by an edge atom-site. transients
is not so snrprising
since it is the foundation
Inter-site transfer of olefinic
of classical dual functional
reforming catalysis where metals on acidic supports are used.
oil
126
Two additional examples illustrate that defect sites are responsible for C-H reactions. The first is provided by Burwell’s work [16] using the model probe di-t-butylacetylene hydrogenation
studies.
not be expected to react readily on surface planes yet it hydrogenates cyclopentene.
in
Because of the sheer bulk of the t-butyl groups this substrate would
In order to accommodate
the idea that hydrogenation
almost as readily as reactions are basically
structure insensitive with the ensuing assumption that all exposed metal atoms are catalytically active, Burwell speculated that the strongly complexing acetylene IiteralJy lifts the metal atom up out of the plane.
An alternative
explanation
which now seems evident is that C-H
reactions only occur on relatively few centres, ie the defect atoms at comers and edges etc. Hydrogenation
of ethylene on silica-supported
Ru-Cu catalysts has been studied [50]
using solid state 13C mm and this work provides the second example that special sites are essential for the simplest hydrogenations. the monometallic unsaturated
The bimetallic catalysts are much less active than
Ru catalyst and, since Cu is known to populate the coordinativeiy
highly
sites such as edges and comers, it is postulated that these play a crucial role in
hydrogenation
reactions.
The real mystery at the heart of this dilemma of so-called insensitivity
is that the
catalysts may have to be produced at great extremes of the roughness associated with high dispersion or the smoothness of extended flat arrays of atoms before the rates per unit metal area change significantly for this category of reactions. Thus when supported Ru catalysts are prepared
by treating
SiO, with (q4-cycloocta-l,S-diene)
(n’-cycloocta-1,3,Qriene)Ru(O)
followed by reduction with H, particles of highly dispersed metal are obtained [Sl]. The rates of hydrogenation
of cyclohexene and benzene
at ambient temperatures
show pronounced
maxima at particle diameters of cu. 2.5 mn. The variation of rate with particle sixe is very reminiscent
of the theoretical distribution
effective in Ns chemisorption
on metals as a function of particle size. A similar dependence
of rates of benxene hydrogenation Rh/siO,
curves calculated [52] for defect sites found to be
on particle size has also been noted for Ni/SiO, [52] and
[54]. An examination
of the literature
on deuterium
exchange reactions
on Pt reveals
examples which illustrate the effect of severe annealing on activity. The rates of exchange of methane per unit area are broadly the same for polycrystalline films [55] and powders [56] with similar values for activation energies and pre-exponential
factors. However, in the case
of fihns prepared and annealed at 5OOT [571 the pre-exponential
factor is reduced by cu. 10s
although the activation energy is in agreement with the other studies. Deuterium
distriiution
patterns show that the same exchange process is observed irrespective of catalyst origin. A similar result is found with the exchange of ethane on polycrystahine
fihns [58], (Ill)-single
crystals [48] and powders [59,60] compared with severely annealed fihns [Sq.
Rates of
exchange on the latter are Id-10’ times lower than on the other catalysts, but again it should be noted that the W-shaped distriiution in the deuteroisomers is preserved throughout. Thus, despite siguificant variations in preparative technique, rates per unit metal area are only influenced in a major fashion by treatment likely to eliminate most defect sites. Remarkably, the initial
~m~utions
of deute~~rne~
remain the same
showhrg that the detailed
mechanism and nature of the sites responsrble do not change. Conchsions In 1925 Taylor 1611 suggested that crystal edges and comers, gram boundaries, and other physical irregularities of the surface may provide active centres of unusually high catalytic activity. Some forty years later this idea was greatly developed through the r-bond theory of catalysis [8] with its emphasis on single atom sites, coordinative unsaturation, and the relationship of the surface reactions to organometallic chemistry and to homogeneous catalysis. More recently surface physics techniques assumed great importance in the area of gas solid reactions; the relevance of this research to the field of heterogeneous catalysis, especially hydrocarbon reactions on metals, is apparently justified by the observation that many of these reactions are largely structure insensitive. However, in the present review we have
summarized
recent work which provides a convhxing case that the term insensitive is
something of a misnomer and that catalysis by metals for all hydrocarbon reactions is indeed due to relatively low numbers of defects which are basically single atom sites. More than one type of defect site may operate distinctively but in concert in a given reaction, as apparently happens with comer and nearest-neighbour edge atoms on Pt in cataly&rg paraffin exchange with D,. As in the ~~o~~te
field of catalysis fl] site densities are therefore much lm
and real turnover numbers much higher than those calculated on the basis of surface areas. An additional major difficulty emerging for kinetic analysis and the estimation of true turnover numbers is that even under mild conditions there may be considerable transfer of transient ~te~e~at~,
e.g. alkenes, but also free radicals, from one type of defect site to
another. Mobility of intermediates does not necessarily imply total detachment from the metal surface in the act of transfer, as may happen in certain spillover phenomena and classical dual-functional catalysis in oil reforming.
The more general idea of dual functionality
described in this review could also be of special value in the third broad class of heterogeneous catalysts namely, semiconductor oxides.
For example, formation of free
radicals at Bi3’ ion sites followed by migration to and reaction on MO’+ ion sites may well occur in selective oxidation of allrenes on bismuth molybdate.
128 In conclusion these two concepts of low site densities and possible mobility of transients may be important in ah areas of heterogeneous
catalysis. Such concepts emphasize the value
of the chemical probe approach for synthetic innovation and mechanistic understanding
as well
as serving as a basis for a unifying philosophy of ah cataIysis including the homogeneous
and
enzymatic areas. Acknowledgements We thank the EC Directorate for Science for financial assistance and Professor J.K.A. Clarke for helpful discussion. References
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