Catalysis Today, 7 (1990 ) 139-155 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

139

THE FORMATION AND ROLE OF CARBONACEOUS RESIDUES

IN METAL-CATALYSED REACTIONS OF HYDROCARBONS.

Geoffrey WEBB

Department of Chemistry, The University, GLASGOW G12 8QQ, Scotland.

SUMMARY

In the course of this review some of the evidence for the formation of surface carbonaceous residues resulting from the interaction of hydrocarbons with metal surfaces, the mechanism of formation of such residues and, finally, the effects of the presence of the carbonaceous deposits on the catalytic behaviour of the surface is examined. Particular attention is paid to the adsorption and hydrogenation of unsaturated hydrocarbons and to the reforming of hydrocarbons in the context of the refining of petroleum naphthas. A brief mention is also made of the formation and role of carbonaceous residues in reactions of carbon monoxide in the presence of hydrogen from the standpoint of hydrocarbon synthesis and the production of oxygenated products.

INTRODUCTION

The formation of carbonaceous deposits’ in the reactions of hydrocarbons on metal

catalyst surfaces is, of course, not new. If one looks at the pioneering work of Sabatier et al.

(ref.1) there are many references to the formation of surface carbonaceous deposits and to

the detrimental effects of these on the catalytic activity. Again in, for example, the studies

of Beeck (ref.2) on the interaction of hydrocarbons with evaporated metal films he refers to

the self-hydrogenation of ethylene and to the consequent formation of surface carbona-

ceous deposit (C,Ho.&, which poisoned the catalyst surface. However, it is in the last two

decades that much more attention has been paid to the formation of such deposits and to

their role in determining the catalytic behaviour of the surface, rather than simply acting as

a catalyst poison, which was the commonly held view certainly up to the mid-1960’s. In-

deed, in 1982, Somorjai and Zaera (ref.3) stated “IKf!rin seconds after the start of a hydra-

carbon reaction at atmospheric pressures, an equivalent of a carbonaceous layer is deposited

on a catalyst surface and the reaction then occurs in the presence of this deposit. Indeed, a

clean transition-metal surface cannot readily catalyse hydrocarbon conversion reactions.”

The subject has, and continues to, attract much attention and this is reflected in the

extensive literature. In consequence, this review does not set out to be exhaustive or

1. Throughout this review the term carbonaceous residue will be used to indicate both surface carbon, in any form, and h drocarbonaceous species, of varying C/H ratio, which can be eonsidered to be ‘permanent1 J ’ retained on the surface during a normal catalytic reaction.

0920-5861/90/$05.95 0 1990 Elsevier Science Publishers B.V.

140

comprehensive, but rather has the objective of focussing on the some of the main features

regarding the formation of the deposits; their mechanism of formation and chemical identi-

ty, and of their role in determining the catalytic behaviour of the surface.

HYJlROCARRON HYDROGENATION REACTIONS

As noted above, the formation of carbonaceous surface residues resulting from the

adsorption of unsaturated hydrocarbons on metal catalysts was established in the early days

of catalysis. However, over the past 20 or so years, there has been an increased interest in

hydrocarbon adsorption, particularly ethene and ethyne, although more recently higher

hydrocarbons have also been examined, from the standpoint of defining the working cata-

lyst under reaction conditions. Much of this work has concentrated on examining the nature

of the carbonaceous overlayer and on its role in determining the catalysis. The results of

these studies has led to the interesting conclusion that the hydrocarbon species which

actually participates in the catalytic reaction is not adsorbed on an extended metal surface

and may even be adsorbed on a (hydro)carbonaceous overlayer, which is effectively cover-

ing the metal.

One of the earliest studies in which this idea was advanced was that reported by

Gamer and Hansen (ref.4), who used F.E.M. to investigate the adsorption and hydrogena-

tion of ethene on stepped tungsten surfaces. They concluded that the metal suface was

effectively covered with a carbonaceous overlayer and that the reaction actually occurred in

a second hydrocarbon layer adsorbed on the first. These workers also suggested the possi-

bility that hydrogen transfer between the carbonaceous overlayer and associatively ad-

sorbed ethene in the second layer was responsible for hydrogenation of the latter. Subse-

quently, Weinberg t3 uZ.(ref.S), reported an extensive study of the adsorption of ethene on a

Pt(ll1) surface using LEED, flash desorption, atomic and molecular beam scattering and

vacuum ultra-gravimetry. They concluded that the ethene was adsorbed irreversibly and

dissociatively in the first monolayer to form an ordered (2 x 2) array of acetylenic residues.

Reversible adsorption and subsequent hydrogenation of ethene was claimed to occur in a

second monolayer which was adsorbed in register with the acetylenic primary layer. These

workers also stated that the extent of carbonaceous overlayer was greater with ethyne than

with ethylene and that hydrogen released in the dissociative adsorption acted as a poison

for the further adsorption of ethene on the metal. A similar conclusion regarding the

formation of a carbonaceous overlayer and hydrogenation from a second layer was drawn

by Kesmodel et al. (ref.6) from their LEED studies of the interaction of ethyne with

Pt(ll1) surfaces. DUS (ref.7) from capacitor studies of ethene hydrogenation on evaporat-

ed palladium films also claimed the existence of two adsorbed layers; a first layer resulting

from dissociative adsorption of ethene and a second associatively adsorbed ethene layer

from which hydrogenation occurred.

Evidence for carbonaceous overlayer formation on supported metal catalysts has

been obtained from the extensive 14C-radiotracer studies of Thomson, Webb and co-

141

workers (refs.8-13). Using both silica- and alumina-supported metals it has been shown

that, at 298K, the adsorption of 14C-ethene and I4 C-ethyne occurs in two distinct stages.

As shown in Fig. 1, the adsorption isotherms consist of a non-linear irreversible primary

region, in which the species are predo~nantly dissociatively adsorbed, and at higher pres-

sures, a linear secondary region. Examination of the gas phase in contact with the surface

during the primary adsorption process, with ethene or ethyne, revealed only ethane, imply-

ing the occurence of dissociative adsorption and “self-hydrogenation”.

0

0 2 I Total molecules in gas phose x~O-‘~) 6 t

8

Fig. 1. Isotherm for the adsorption of 14C2H4 on a Pd/Si02 catalyst sample at

ambient temperature (0) and composition of the gas phase during the build-

up of the isotherm (0, ethane; l , ethene).

This behaviour has been found to be typical for a whole range of different supported

metal catalysts and to be independent of the nature of the support. The species which

undergoes hydrogenation is located in the secondary region and determination of the

corresponding I4 C-carbon monoxide adsorption isotherms, which show the expected

Langmuir-type behaviour, lead to the conclusion that the primary region corresponds to

effective monolayer coverage of the metal by the permanently retained hy~o~rbona~ous

species. From the yields of hydrogenated product formed in the primary adsorption region,

the average composition of the primary adsorbed layer has been calculated to be as shown

in Table 1.

Brandreth et A(ref.14) using a pulsed flow technique examined the adsorption and

retention of ethene and propyne on a variety of supported rhodium catalysts. This study is

of particular interest in that it shows a distinct effect of metal precursor on the formation of

the carbonaceous species which, indirectly, could be attributed to the different metal dis-

142

persions, in terms of the greater amounts of hydrogen retained by the catalysts derived

from the nitrate and oxide than from the chloride. The amounts of each hydrocarbon re-

tained by the catalyst was greater for the catalysts derived from Rh203 or Rh(NO& than

for those from RhC13 , even though the former retained more catalyst hydrogen from acti-

vation and, initially, gave more hydrogenation of the adsorbate ethene. Some typical re-

sults obtained from this study are shown in Table 2.

TABLE 1.

Composition of the primary adsorbed layer resulting from the adsorption of various unsatu-

rated hydrocarbons on a variety of supported metal catalysts.(refs.8-13)

Catalyst Adsorbate Compositon of

Hydrocarbon Primary Layer

5%Pd-SiO2

5%Pd-A1203

5%Rb-si02

5%Ir-SiO2

6.3%Pt-Si02

=2*4 C2H2.7

C2H4 ‘ZH2. 65

‘2*4 C2H3. 1

=2*4 C2H2.9

‘2*4 ‘ZH3. 1

S%Pd-Si02

5%l?b-A1203

5%Rb-Si02

5%Ir-Si02

6.3%Pt-Si02

25.4%Ni-Si02

=2*2 (32Hl.4

c2H2 CZHl.75

c2H2 ‘lHl. 8

c2*2 ‘2*1.6

c2H2 CZHl.75

=2*2 ‘2*1.85

5%Pd-Si02

5%Pd-A1203

5%Rb-A1203

C3H8 C3H4.7

‘3*8 ‘3*4.6

‘3*8 C3H5.2

HYDROCARBON REFORMING REACTIONS

Not surprisingly in view of their commercial importance, the formation of carbona-

ceous deposits, commonly referred to as catalyst coke, resulting from the reactions of

143

hydrocarbons on metal surfaces, particularly platinum based-catalysts, under reforming

conditions of moderate pressures and relatively high temperatures has attracted considera-

ble attention, both from the standpoint of catalyst lifetime and, more recently, with respect

to the changes in catalyst activity and selectivity induced by their presence on the surface

during reaction.

TABLE 2.

Adsorption of Ethene and Propyne on Supported Rhodium Catalysts. (ref.14).

Catalyst

WNo3)3/Sio2 (0.9%w/w Rh)

Rb203/Si02

(0.9%w/w Rh)

RbC13/Si02

(1.6%w/w Rb)

Adsorbate Amount Adsorbed

maol(g.catalyst)-l

‘2*4 87.28

C3R4 96.15

‘2*4 72.56

C3H4 96.35

‘2*4 43.70

C3H4 56.08

Composition of

Retained Species

lst.pulse Steady state

CZHO C2H2.5

‘3*2.3 C3H4

CZHO '2*1.8

C3*1,3 c3*4

c2*0 '2*3-l

C3H3.7 c3*4

Extensive studies have been carried using both commercial catalysts which contain

both a metallic function and an acidic function, commonly obtained by the incorporation of

chloride into the support, and on platinum single crystals. Commonly these studies have

concentrated on the origins and mechanism of formation of the surface “coke” and on its

characterisation. It is generally considered that the formation of the coke is initiated by the

retention, on the surface, of strongly adsorbed hydrocarbonaceous species, which subse-

quently undergo condensation and polymerisation reactions with loss of hydrogen to give

highly hydrogen-deficient residues (refs.1521). The complicating feature of such studies is

that the precursor molecules are not generally the reactant molecules themselves, but

reaction intermediates. This latter point was elegantly demonstrated by Bertolacini and

Pellet (ref.lS), who showed that an intimate mixture of Pt/g-Al203 and Re/l -Al203 had a

higher activity for naphtha reforming than either Pt/l-A1203 itself or Pt/l -Al203 fol-

lowed, further upstream, by Re/t-A1203, due to the ability of the Re in the mixture to

remove the highly reactive coke precursor molecules. Small unsaturated molecules have

often been found to act as coke-precursors. Thus, for example, Cooper and Trimm (ref.22)

showed that, under similar reaction conditions on Pt/a-Al203/Cl-, the rate of coke forma-

144

tion from a-hexene was 3 times greater than that from n-hexane. These workers also found

that the amount of coke deposited from a methylcyclopentane feedstock greatly exceeded

that produced from a variety of other C6-hydrocarbons including benzene and cyclohexane,

supporting the view (ref.15,20) that the C5 naphthenic structure was particularly potent as a

coke precursor. Bertolacini and Pellet (ref.l5), studying reforming of a cyclopentane-rich

feedstock, attributed coke formation to the reaction sequence:-

cyclopentane _I___t cyclopentadiene

polyaromatics - naphthalene

and Levinter et al. (ref.l9), using petroleum naphthas, suggested the following scheme:

naphtha 4 dienes + resins + asphaltenes 4 carboids

Beltramini et al. (ref.20) have also shown that naphthenic and aromatic feedstocks are

relatively better coking agents than paraffinic species. These workers also found a general

trend for the extent of coking to increase with increasing molecular weight for both aromat-

ic and paraffinic feedstocks.

The possible role of hydrogen acceptor molecules in the formation of surface carbo-

naceous deposits has also been the subject of debate. Thus, the high coking tendency of

naphthenes (refs.15,18,22) has been ascribed to their ability to donate hydrogen to unsatu-

rated acceptor molecules.

The rate of formation of carbonaceous residues as a function of the experimental

conditions has been the subject of extensive study. The collective results may be summa-

rised as showing that the rate of catalyst coking increases with (a) increasing temperature

(ref.24); (b) decreasing total pressure (25); (c) increasing hydrocarbon/H2 reactant ratio

(refs.26,27) and (d) decreasing space velocity (ref.16).

Other factors which have been established as having an effect on the extent of

carbonaceous deposition include metal loading and metal dispersion of the catalyst,

presence of surface chlorine or sulphur and the presence of a second metal, commonly

rhenium or iridium, as a promoter for the platinum. Barbier et aL(ref.28) have shown that

the rate of coking from cyclohexane increases with increasing metal loading, whilst Lank-

horst et al. (ref.29), using n-hexane and a series of Pt/Si02 catalysts have shown that resist-

ance to coking increases as the metal dispersion increases. This latter effect is attributed to

the greater proportion of edge and comer sites on the smaller crystallites, which according

to Somorjai and Blakely (ref. 30) are less vulnerable to coke deposition than more coordi-

natively saturated metal atoms in extended crystal planes. Sulphur, when present in small

amounts are known to promote improved selectivity and activity due to their ability to

145

selectively suppress the intrinsically high dehydrogenation activity of the metal (ref.31) and

hence the rate of production of unsaturated coke precursors, although excess amounts of

sulphur lead to catalyst poisoning (ref.32). The presence of excess chlorine has been shown

to lead to an enhancement in catalyst coking due it is suggested (refs.27,32) to the chlorine

decrease in the rate of rehydrogenation of unsaturated coke precursors due to the chlorine

blocking the spillover of hydrogen from the metal to the support.

Whilst it is well established that the incorporation of a second promoting metal in

platinum reforming catalysts leads to enhanced lifetime and improved selectivity consider-

able debate still ensues as to the precise mode of action of the promoter. Although a full

discussion of the role of metal promoters is outside the scope of this review, it is worth

noting that a commonly held view is that the promoter inhibits the formation of carbona-

ceous residues. Thus, for example, Magitfalvi et al. (ref.33) and Bertolacini and Pellet

(ref.15) have suggested that the presence of Re provides a route for the interception.of Cg-

naphthenic coke precursors, formed on the Pt, and their reconversion to paraffinic species

by hydrogenolysis. Similarly, Carter et al. (ref.34) have concluded that the high hydroge-

nolysis activity of iridium was responsible for the greatly reduced coking rate of Pt-Ir&

-Al2O3/Cl- catalysts, relative to Pt/J-Al203/Cl- catalysts.

STRUCTURE a CARBONACEOUS LAYER

What is the chemical nature of the carbonaceous overlayer? Here one has to turn

to spectroscopic studies, both supported metal catalysts (infra red) and, on single crystal

metal surfaces (EELS and RAIRS) , as recently reviewed by Sheppard (ref.35) and to other

surface science techniques applied to model single crystal surfaces (see e.g. ref.3).

1. Unsaturated Hvdrocarbons.

Whilst many of the early infra red studies of the adsorption on supported metals have been

severely hampered due to the relatively low intensities of relevant bands (ref.36), although

this problem is now less severe with the advent of FIX&such studies nevertheless revealed

that, on silica-supported metals, at least some of the adsorbed ethene or ethyne was disso-

ciatively adsorbed. Thus, for example, adsorption of ethene at ambient temperature on

“bare” Pt-Si02 gives spectra consistent with species of types (1) and (2), in Fig. 2.

H H CH I 3 C

/I’*

Fig. 2. Adsorbed species retained on the surface following the adsorption of

C2H4 on Pt/Si02 as identified by infra red spectroscopy.

Admission of hydrogen to this preadsorbed ethene results in production of ethane

and an increase in the band intensities, implying the presence of surface carbidic species

146

from the initial adsorption, whilst at slightly higher temperatures (95OC) bands correspond-

ing to surface n_butyl groups have been observed (refs.37-39). A recent study of ethene on

Pt/SiO2 (ref.35) has also shown the presence of bands ascribable to the ethylidyne species

(3). Use of 13C-ethene showed that the adsorbed species retained their C2-integrity, no

redistribution of the I3 C-label being observed on hydrogenating the retained species from

the surface (ref.40). Similar results have been obtained for Ni/Si02, the only difference

being that the species arising from adsorption at ambient temperature are C4-units rather

than C2-species (ref.39,41). Surface alkyl species of the type CH3(CH2)n, where n 2 4

(Pt) or n = 3 (Ni), have also been observed following the adsorption of ethyne on silica-

supported platinum, palladium and nickel catalysts (ref.42) and on alumina-supported

palladium (ref.43) and platinum (44). Temperature programmed desorption studies on

ethene or ethyne precovered silica- or alumina-supported platinum (ref.45) and rhodium

(ref.46) have also show the presence of highly unsaturated linear polymeric species.

Extensive studies of the adsorption of various unsaturated hydrocarbons on single

crystal platinum surfaces by Somorjai and co-workers has revealed that, on “bare” metal

surfaces a whole range of species exist depending upon the temperature. Hydrogen ther-

mal desorption spectra (ref.f), as shown in Fig. 3, show that there is a progressive dissocia-

tion of the adsorbed species, which ultimately forms a graphitic layer (ref.47). 14C-radio-

tracer studies (ref.48) have shown that there is a direct correlation between the extent of

14C-carbon retention on the surface and the hydrogen content of the carbonaceous over-

layer as shown in Fig. 4.

I I I I 600 600

1 (K)

Fig. 3. Hydrogen thermal desorption spectra illustrating the sequential dehydrogenation of

ethene, propene and cis-but-Zene chemisorbed on Pt( 111) at about 120K (ref.3)

(Reproduced wifh permission from J. Phys. Chem.)

147

2.0 -1.0

S ._

1.5- y ‘s E LL

0 1

0)

l.O- -0.5 % v)

B *

0.5- t .- tt z 4,

O- 273 473 673 ,o E

Adsorption Temperature (K 1

Fig. 4. Composition and reactivity of 14C2H4 chemisorbed on Pt(ll1) at 320-370K showing

the correlation between the irreversibly adsorbed fraction and the (H/C)

content of the strongly bound surface species. (Reproduced by permission from

3. GUaI.)

2. Saturated Hvdrocarbons.

Somorjai et al. (ref.26) used AES together with hydrogen thermal desorption stud-

ies to investigate the chemical composition of coke deposits on a variety of platinum single

crystal faces, which had been used for g-hexane conversion in the temperature range 300-

450°C. They showed that the stoichiometry of the residues ranged between H/C values of

1.6 at 300°C and 1.0 at 405OC, compositions which might be considered as being interme-

diate between those of polyacetylene (1.0) and polyethylacetylene (l.S), or related to sur-

face structures of the type:

II II II S S S

(H/04/3)

(I)

H3C\ /CH, H3C\ 73

\c/c\c/c\c

II II II S S S

(H/C-1.5)

(II)

148

Differential thermal analysis during temperature programmed oxidation studies of

cokes deposited on Pt/Al203/Cl-, Pt/Si02, and acidic Al203 led Figoli ef al. (ref.25) to

conclude that the coke deposited on the metallic function had a higher H/C ratio and was

therefore less highly polymerised, than that on the support phase. A similar conclusion was

reached by Parera et al. (ref.50).

Raman spectroscopy has proved to be especially useful for coke characterisation,

particularly for determining the relative amounts of pregraphitic condensed polyaromatic

rings (giving a band at 1355 cm-l) and graphitic coke band at 1600 cm-l) on both Pt/acid

Al203 and rhenium or iridium promoted platinum catalysts (ref.51), as shown in Fig. 5.

CATALYl’iC EFFECTS m CARBONACEOUS OVERLAYERS

Turning to a consideration of the effects of the presence of catalyst coke on the

catalytic behaviour of metal, it is well established that by far the greatest effect is to poison

the catalyst surface thereby causing deactivation; this is irreversible when the carbonaceous

overlayer becomes graphitic (ref.47). In spite of this poisoning effect, however, it is also

becoming increasingly clear that the presence of the carbonaceous deposits have some

beneficial effects in terms of influencing the catalytic behaviour of the surface.

Examining first hydrocarbon hydrogenation reactions, Somorjai et al. (ref.47,52,53)

claimed that, on platinum single crystal surfaces below 120°C, the carbonaceous adlayer,

which has a stoichiometry C2H3, is removed as rapidly as ethene molecules are hydroge-

nated to ethane. In consequence, it was been concluded that the surface effectively remains

clean throughout the normal hydrogenation reaction. However, this conclusion is contrary

to the observations with l4 C-radiotracer studies on highly dispersed supported metal cata-

lysts (refs.8-12), which show that, during the hydrogenation reaction, there is a progressive

build up of strongly held carbonaceous species, as shown, for example in Fig. 6 for ethyne

hydrogenation on Ni/SiO2, until after a number of reactions a steady state catalytic activity

is achieved.

A number of effects of the carbonaceous overlayer in catalytic hydrogenation have

been established. In the hydrogenation of ethyne over 6.3%Pt/Si02 (EURO-Pt-l), the

selectivity for ethene formation increases from ca.85% on a freshly reduced sample to

>95% as the catalyst reaches a steady state activity when it is effectively covered by a

carbonaceous layer (ref.54). Yasamouri et al. (ref.55) have also reported that the irreversi-

ble adsorption of ethyne on to palladium surfaces results in a stabilisation of the activity for

the hydrogenation of ethene and ethyne, and also acts as a surface template for catalytic

conversion of ethyne to benzene; an effect which they claim is similar to the tartaric acid

modification of nickel surfaces for enantioselective hydrogenation of p-ketoesters (ref.56).

149

Fig. 5. Raman spectra of coked monometallic Pt/Al203 catalysts showing bands ascribable

to graphitic and pre-graphitic condensed polyaromatic rings (ref.51).

(Reproduced by permission from Applied Catalysis.)

The ability of the carbonaceous overlayer to reversibly store hydrogen is clear from

the infra red studies of Sheppard et al. (ref.3739), who found that admission of hydrogen to

hydrocarbon precovered surfaces resulted in an increase in the band intensities

in the C-H region, whilst subsequent evacuation of the surface resulted in a decrease in the

intensity of these bands. It has also been have proposed (refs.4,13) that the hydrogenation

of associatively adsorbed alkenes/alkynes involves the carbonaceous overlayer as hydrogen

transfer medium. Using platinum single rystal surfaces, Somorajai and Zaera (ref.3) have

concluded that the presence of active carbonaceous residues containing CH or CH2 frag-

ments are capable of storing about 10 times as much hydrogen as a clean metal surface,

150

whilst Davis et al (ref.26) have stated that in the presence of catalytically active CxHy

carbonaceous species hydrogenation, dehydrogenation and deuterium exchange reactions

are typically lo-1C3 times faster than skeletal rearrangements. The use of the carbona-

ceous overlayer as a hydrogen transfer medium has also proposed for the hydrogenation of

carbon monoxide on carbon-covered surfaces (ref.57). It is also interesting to note that in a

study of the hydrogenation of (2,2),(5,5)-tetramethylhex-3-yne over various platinum catalysts,

Burwell and co-workers (ref.58,59) reported that the activity of the catalyst was increased

when a series of cyclopentene reactions, which resulted in the formation of carbonaceous

deposits, had been carried out to bring the catalyst to a steady state activity. This suggests that

the effect of the carbonaceous overlayer was to effectively create active sites for the hydroge-

nation of this very sterically hindered molecule.

-7 <

6

I I # 1 I I 5 10 15 20

time/min

Fig. 6. Variation in surface (0) and gas phase (0) count rates and the corresponding

pressure fall-time curve observed in the hydrogenation of 14C2H2 on

a freshly reduced Ni/Si02 catalyst (ref.12).

Similarly, in hydrocarbon conversion reactions it has become clear that the presence

of carbonaceous deposits (“coke”) has beneficial effects in terms of promoting catalyst

selectivity (refs.26,29,60,61). Using Pt single crystal surfaces for n-hexane conversion, Davis

et al. (ref.26) have shown that residue formation is accompanied by a significant increase in

the selectivity for aromatisation and hydrogenolysis relative to isomerisation and Cg-cycli-

151

sation. Furthermore, within the isomerisation reactions, the (2-methylpentane/3-methyl-

pentane) ratio was lower on carbon covered surfaces. Similar results have been reported by

Kapinski and Kosecielski (ref.60) using silica-supported Pt-Pd alloy catalysts, whilst Lank-

horst et al. (ref.29), using Pt/SiQ2 catalysts for n-hexane conversion have also observed

that the major effect of the carbonaceous residue was to increase the selectivity for dehy-

drocyclisation relative to the isomerisation. Christoffel and Pad1 (ref.61) have claimed

that, in the reaction of metbylcyclopentane over Pt/y-Al203/Cl-, the formation of surface

carbonaceous residues results in a partial deactivation of the Pt function in that it loses its

isomerisation and hydrogenolysis activity, but retains its dehydrogenation activity. Howev-

er, Figoli et al. (ref. 25) have provided evidence for selective loss of dehydrocyclisation

activity with increasing coking during naphtha reforming over a bifunctional Pt/A1203

catalyst, whilst Beltramini et al. (ref.21) have observed that in the reaction of g-hexane over

Pt/y -Al2O3/Cl-, the formation of carbonaceous residues produces a significant increase

in the isomerisation activity and simultaneous decrease in the dehydrocyclisation activity.

In a recent study of the reaction of n-heptane over Pt/@203/Cl- (ref.62), we have found

that the lay down of carbonaceous residues promoted the isomerisation selectivity, whilst

suppressing the aromatisation and hydrocracking selectivities; the hydrogenolysis selectivity

remained almost constant. Use of 14C-labelled a-heptane as tracer has also shown that,

during a-heptane reforming, exchange of carbon between the strongly adsorbed and re-

tained carbonaceous residue and gas phase hydrocarbon occurs; the 14C-label being

incorporated in almost all of the products, as shown in Table 3, although the isotopic distri-

butions cannot simply be described in terms of molecular exchange between labelled

adsorbed n_heptane and unlabelled gas phase n-heptane reactant molecules. These obser-

vations suggest that at least part of the carbonaceous residue, although strongly adsorbed

and, therefore, effectively retained, by the catalyst surface is present as a non-polymerised,

non-condensed species.

A very recent study by Querini et al. (ref.63) has sugested that on commercial

Pt-Re-S/A1203/Cl catalysts the carbonaceous overlayer on the metal reaches a steady

state, resulting in a steady state activity for cyclohexane dehydrogenation and cyclopentane

hydrogenolysis. This study also suggests that n-heptane dehydrocyclisation is purely con-

trolled by the acidic function on the catalyst.

Somorjai et al., from platinum single crystal work, have also deduced that certain reactions

are also sensitive to the structure of the carbonaceous overlayer. Thus, conversion of

cyclohexene to benzene is poisoned unless the adlayer has an ordered structure (ref.64),

whilst the aromatisation of a-heptane to toluene only occurs in the presence of an ordered

adlayer (ref.65), although cyclohexane hydrogenolysis is independent of the extent of order-

ing of the carbonaceous overlayer.

CATALYTIC EFFECTS e CARBONACEOUS RESIDUES m CARBON MONOXIDE

HYDROGENATION

152

Finally a brief mention should be made of the effects of the presence of hydrocar-

bonaceous residues, which have been observed in non-hydrocarbon reactions on metal

surfaces. In particular, there is a wealth of data concerning the formation and role of

carbonaceous residues in the hydrogenation of carbon monoxide, both in the context of

hydrocarbon synthesis (refs.66-68) and in the formation of oxygenated products (refs.58,69-

71). Concentrating on the latter by way of an example, Ponec and co-workers (ref.71) have

suggested that, with rhodium catalysts, partial blocking of the surface by hydrocarbona-

ceous residues enhances the formation of methanol, relative to methane. Subsequently,

Jackson and co-workers have shown that the carbon monoxide is not hydrogenated directly

to methane, but goes through the hydrocarbonaceous residue present on the surface

(ref.57). They have also suggested that the carbonaceous residue plays a central role in

the mechanism in that it effectively supplies both hydrogen and CHZ-units to other reactive

surface species. These workers have also used 13C-labelling and transient tracer tech-

niques (ref.70) to show that, over supported rhodium catalysts, during non-steady state

operation there is a laydown of hydrocarbonaceous material, which (i) acts as a hydrogen

transfer medium to supply hydrogen to surface intermediates, and (ii) acts as a reservoir of

carbon atoms, which may desorb as Cl-species through hydrogenolysis of the carbonaceous

overlayer.

TABLE 3.

Distribution of 14C-label in products from reaction of 12Gg-heptane at 500°C on

a Pt/y -Al203/Cl- catalyst precovered with l-14C-E-heptane. (ref.62).

Hydrocarbon %age Yield Total Counts

cl - c3 22.07 4407

11- +i-c, 3.25 1379

n- + i-c5 7.16 1241

n- + i-c, 0.23 305

rJ_- + i-c, 54.96 44188

Toluene 14.88 17792

GENERAL CONCLUSIONS

In conclusion, the examples presented above clearly demonstrate that the presence

of carbonaceous residues on the surface of a metal catalyst, which result from the interac-

tion of hydrogen with the hydrocarbon during the catalytic transformation play a significant

role in determining both catalyst activity and selectivity. Their work on single crystal plati-

num surfaces has led Somorjai and his co-workers to propose a working model of the cata-

153

lyst surface to be as shown in Fig. 6.

Fig. 6. Model of the working surface composition of platinum reforming catalysts (ref.26).

{Repduced by permission from J. CataL)

From the work carried out to date, it is also important to recognise that the pread-

sorption of hydrocarbon and investigations of the carbonaceous species resulting from this,

may not give a true picture of the surface under reaction conditions. Clearly, in the future,

in sihr studies are required in order to establish the chemical nature of the surface carbona-

ceous deposits which are present during, for example, the hydrogenation of unsaturated

hydrocarbons. In such studies it is essential to establish the role of the support in determin-

ing the extent of formation and retention of carbonaceous residues and to determine the

contribution, if any, of the species retained on the support to the reactions on the metal.

Further, although it has been well established that in hydrocarbon conversion reactions, the

specific rates measured on extended single crystal surfaces and on supported catalysts are

similar (ref. 3), nevertheless, it still remains necessary to establish unequivocally that the

carbonaceous deposits formed on the extended crystallites are also formed on those very

small metai particles often associated with highly dispersed working supported metal cata-

lysts.

However, in spite of these reservations, it is clear that the observed effects of the

carbonaceous residues cannot be simply interpreted in terms of a poisoning of the surface

by the presence of the strongly adsorbed carbonaceous residues. Indeed, in many cases the

presence of the carbonaceous overlayer may be considered to be beneficial and, in some

154

cases, even essential, to the catalysis.

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