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and therefore they are not provided in this paper. Summary The trace element concentrations in the runoff from both watersheds are below the Interim Primary (Finished) Drinking Water Standards and Secondary (Finished) Drinking Water Regulations for copper, zinc, cadmium, lead, and chromium. There was no significant difference in the heavy metal concentration between the control watershed and the sludge application watershed runoff water. Since the application of sludge began, the mean value of conductivity, dissolved solids, TKN, NH3, NO2 + NO3,totalphosphorus, and TOC increased significantly in the runoff from the sludge application watershed. Due to changes in the runoff water quality in the control watershed, only the increases in HN3, NO2 + NO3, and TOC may actually be related to the sludge applications. Laboratory and sample collection problems have hampered the bacteriological analyses taken from the deep well and surface water runoff. The reliability of the total and fecal coliform counts was very questionable and was
Dev. 1981, 20, 574-593
therefore not included in this paper. Sampling and analysis procedures have been revised in an effort to obtain more reliable data.
Acknowledgment This demonstration is partially supported through appropriated funds from Congress. This paper was presented at the Second Chemical Congress of the North American Continent, San Francisco, Calif., Aug 25-28, 1980. Literature Cited Miller, F. A., I11 Tennessee Valley Authority Proposal for the Demonstration of the Application of Aerobically Digested Sewage Sludge to Farmland, Chattan-, Tenn., Sept 1977.Miller, F. A., I11 Tennessee Valley Authority Report on the Geologic Evaluation of the Proposed Slte for the Florence Sludge Demonstration Project, Chattanooga, Tenn., Nov 1978. Cannella, A. L.; Miller, F. A., 111 Tennessee Valley Authority Progress Report for the Florence Demonstration Project, Chattanooga, Tenn., Dec 1979.
Received for review October 8, 1980 Revised manuscript received August 3, 1981 Accepted September 1, 1981
REVIEW SECTION Advances in Catalysls by Alloys John K. A. Clarke” and Anne C. M. Creaner Department of Chemistry, University College, Belfield, Dublin 4, Ireland
Photoelectron spectroscopy has confirmed the view that each metallic constituent retains its chemical individuality in an alloy surface. The action of group 8-B-subgroup alloys in hydrocarbon skeletal reactions may be interpreted in terms of ensembles, electronic influences, and surface structure. Auger electron spectroscopy now permits reliable determination of surface metal composition in alloy catalysts. Significantly for catalyst selectivity control, the possibility of segregation of one element to step sites on alloy surfaces is recognized. The effect of a reactive gas atmosphere at even moderate temperatures in causing different kinds of spatial separation of the metallic elements in different alloys is now understood. More generally, the influence of preparative variables on the formation of supported catalysts has been clarified, in particular, by temperature-programmed reduction. EXAFS and Mossbauer spectroscopies are informative for highly dispersed bimetallic catalysts (“clusters”).Alloy work contributes substantially toward the view that size of surface ensemble controls chainpropagation in Fischer-Tropsch synthesis: control of selectivity through crystallite size distribution is accordingly now an important aim.
Introduction In a previous review published in 1975 (Clarke, 1975) the principal modes of action of alloy catalysts were described and then-current rationalizations of these were presented. The class of group 8-group 1B binary alloys was then attracting most interest, particularly for conversions of hydrocarbons, and could claim some measure 0196-432 1l81l1220-0574$01.25/0
of understanding. The “electronic factor” (in which the chemical influence of one metallic component on the other was examined) which had previously been much emphasised was now being less stressed, and interpretations of catalytic action in terms of surface “ensembles” of active site atoms were sought. A further influence, namely that of the surface-structure sensitivity (Boudart, 1969) was beginning to be specifically noted for its possible implications for 8-1B and related combinations (Burton et al., 1975; Hagen and Somorjai, 1976). Surface-structure sensitivity is now known to be a characteristic of the several types of hydrocarbon skeletal reactions even if some mechanistic details still remain unresolved (Dartigues et al., 1979); carbon monoxide hydrogenation may also be, for some metals, so that this factor now receives comparable attention to ensembles in rationalising alloy catalytic action. Even then, ensembles have an established position (Burton and Hyman 1975; Dalmon and Martin, 1980a; Dautzenberg et al. 1980). Electronic interaction between metal atoms remains possible, organometallic analogy being rich in parallels. However, no experimental techniques available can measure electron transfers with precision; XPS gives qualitative information. (See Spectroscopic Techniques in Structural Study of Supported Alloy Catalysts.) Surface enrichment of an alloy in one metallic component, from either Gibbs accumulation or caused by a reactive gaseous atmosphere, can now be measured reliably 0 1981 American Chemical Society
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and progress in the rationalization of data for the former since the mid-1970’s is good. The latter effect is at the stage of practical exploitation in one catalyst system designed for automobile emission control. (See Preparation of Alloys for Catalytic Studies.) The more extreme situation of phase separation is now open to study with high resolution electron microscopes. In this review we shall begin by describing progress in two fundamental areas which form a basis for understanding alloy catalyst action, namely the electronic structure of the class of d-metal/B-metal alloys, which can claim much prominence in industrial catalytic use at this time, and the mechanisms of hydrocarbon reactions on transition metal surfaces. (The terms B-metal and Bsubgroup element signify elements in the copper through germanium subgroups.) Surface compositions of alloys will be reviewed. An account is then presented of methods of preparation of alloy catalysts and control techniques which have been developed-for the most part during the period being reviewed. Some spectroscopic techniques of outstanding power for probing the structure, and to some extent the electronic characteristics, of supported alloy “clusters” are described with recent results. Researches on several alloy-type reforming catalysts developed in the early and mid-1970’s are included and recent work on Fischer-Tropsch catalysis by alloys is reviewed. We include a section on clusters of metal atoms, for some years recognized as an interfacial zone between surface catalysis and organometallic chemistry offering promise for elucidation of catalytic action. This is a more slowly evolving approach than catalyst chemists have perhaps expected, partly because of the synthetic complexity involved. Some helpful, some provocative, concepts are emerging here, however, on both the theoretical and practical fronts.
Electronic Structure of d-Metal/B-Metal Binary Alloys: a Msum6 A major factor which determines the activity and selectivity of a metal or alloy catalyst is the electronic structure of the metal atoms. The activity of transition metals for hydrogenations, for various skeletal reactions of hydrocarbons, and other processes, is generally ascribed to the presence of d-vacancies. Reliable measurement of the density of states N ( E ) curves of various metals and alloys has been possible in the last decade due to the development of ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Using these has revealed that for group &-1B binary alloys the densities of states of the alloy valence band is composed of two slightly deformed densities of states, one localized at each of the component metal atoms. The main features of the bulk and the surface (Yu et al., 1976; Seib and Spicer, 1970; Heimann et al. (1978) electronic structures of bimetallic alloys are thus shown to be represented by the individual metal features with intensities correlated to the alloy composition. Levels around the maxima of N(E) derive from wave functions having a high contribution of the d-orbitals and good activity in catalysis is associated with a high N ( E ) at the Fermi energy. High catalytic activity requires a not over-strong chemical bonding of reactant to the surface (Bond, 1962) and the metals of group 8 appear to combine the required attributes optimally to this end because the d-orbitals here do not contribute excessively to the strength of the surface bond. Catalytic activity is probably dependent as well on the role of vacant d-levels in lowering the energy of the activated complex. Having by now recalled that there is a relationship between catalytic action and d-band struc-
-
DENSITY OF STATES (62% C u - 3 8 % N i l
E
w
ENERGY BELOW E,
Figure 1. A comparison of the UPS spectrum of the annealed surface of a 10% Cu/90% Ni sample with a density of states calculated from CPA theory. (Reproduced with permission from Yu et al. (1976). Copyright 1976 Pergamon Press Ltd.)
ture of the metal, we shall survey some of the results from photoelectron spectroscopy of alloys and parallel theoretical advances. Seib and Spicer in the period 1968-70 (Seib and Spicer, 1970) showed that their UPS and optical data for the electronic structure of Ni-Cu alloys were incompatible with the older rigid d-band model of alloys and proposed rather that these data, particularly at high Ni concentrations, be interpreted by a minimum polarity model. This model, due to Lang and Ehrenreich (19681, proposed that each atomic site is electrically neutral and all the d-holes in Ni-Cu reside entirely at Ni sites. Stocks et al. (1971) later calculated realistic densities of states of Ni-Cu alloys using the coherent potential approximation due to Soven (1967) which postulates, in essence, that the elemental components preserve a good deal of their individuality in alloys so that the atoms of both components, including those in the surface, are in principle distinguishable. Enrenreich and Schwartz (1976) have reviewed the theory of the electronic structure of substitutional binary alloys including the methods used in their calculation. Results obtained by CPA and average T-matrix approximation fit experimental results so well as to predict detailed features of the Ni density of states in NiCu. A similar picture is found for the PdAg and CuZn alloy systems. The studies described in the preceding paragraph all concentrate on the electronic structure of the bulk alloy. Just how much surface compositions of equilibrated alloys can differ from the bulk had become very apparent toward the mid-1970’s (see Atomic Composition of Alloy Surfaces). Such deviations appear to be localized within the few outermost atomic layers. In a significant advance Spicer and his co-workers (Yu et al., 1976) showed that surface and bulk electronic structure could be studied essentially independently using UV photoemission in the photon energy range 6 > hv 121.2 eV, the latter end of the range being more surface-sensitive on account of the shorter escape depth. Figure 1 shows the experimental density of states of the (100) face of a nominal 10Cu/9ONi wafer compared with that calculated from the CPA theory for the measured surface composition. The surface electronic structure thus consists of Ni and Cu d-peaks, 2 eV apart. Later work on (110) NiCu from the same group (Ling et al., 1980), while it shows minor fine structure in corresponding spectra, confirms the conclusions drawn in this initial work. The magnitude of the peaks was correlated to surface composition but neither the shape nor energy position of the peaks was sensitive to changes in
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bulk composition, surface crystallinity, or local environment. These results, particularly considering the proximity of the surface and the rapid change in composition as one moves a few atomic layers into the bulk, imply that the d-states of the alloy components are strongly localized (each individual surface atom retaining much of its atomic character). This work strengthens the validity of an individual-atom viewpoint for the chemisorption properties of the alloys already being stressed by Sachtler and others (see Sachtler and van Santen, 1977). Measurement of density of states for alloy single crystals by angle-resolved photoemission has been developed in recent years (Neddermeyer, 1978) and these furnish confirmation of the picture given by the work of Spicer's group (Yu et al., 1976; Ling et al., 1980).
Researches on Reaction Mechanisms; Skeletal Reactions of Hydrocarbons, Application to Alloys It is in the area of hydrocarbon conversion that catalysis by alloys has been most profitable. These skeletal changes take place with greater selectivity (that is, with greater exclusion of destructive reaction) on a group 8-1B bimetallic combination than on the group 8 metal alone. Whether this fairly general result means that the reactive intermediate species in the different reactions have different site-type or specific electronic requirement (Clarke, 1975) must await better characterisation of these intermediates and, if possible, their surface structure requirements. A number of parallel routes for a given chemical transformation is a complicating possibility which must be accepted. Debate about mechanisms has continued during the intervening period. However, important clarifications have emerged and the bearing of these on a developing theory of alloy catalysis justifies an account at this point, of this work. (A further reaction catalyzed by metals and alloys, carbon monoxide hydrogenation, will be considered in a later section). Hydrogenolysis of hydrocarbons on transition metals will be treated first. The bond-shift route and the carbocyclic mechanism of skeletal rearrangement are then described, the latter obviously including dehydrocyclization as such. The process of ring enlargement (aromatization) follows. Conclusions reached about surface-structure dependence are presented. Finally, a comment is offered on possible consequent results of alloying platinum with gold for catalysis of alkane conversion. The work of Gault and his group is notable throughout. Alkane Hydrogenolysis. For certainly a decade it has been realized that hydrogenolysis of higher alkanes on platinum is unlikely to be by the same mechanism as that of ethane, i.e., through a 1,1,2,2-tetra-adsorbed or similar surface radical (Sinfelt, 1973). Anderson and Shimoyama (1973) pointed out that the latter reaction has a very high activation energy not found for the former. One possibility is through a 1,1,3-triadsorbed intermediate I (requiring
I
more than one metal atom) which can break either at the C2-C3 bond (Anderson and Avery, 1966) or more probably at the C1-C2 bond (Leclercq et al., 1977). Because Gault believed that there was an essential relationship between hydrogenolysis and alkane rearrangement by the bond-shift mechanism on platinum, (see next
Scheme Io
\
c-c
/
/ / M\
-
I
1
C C C
M
a Reproduced from Amir-Ebrahimi et al. (1979). Nouu. J. Chim.;Gauthier-Villars Editeur, Paris.
sub section), he favored for simple acyclic alkanes, such as n-pentane or isopentane, a metallocyclobutane intermediate for hydrogenolysis (Amir-Ebrahimi et al., 1979). Such an intermediate, which is a firmly established entity in organometallic chemistry, may be termed a 1,3-diadsorbed radical in the terminology of classical surface catalysis and needs one metal atom only. In view of the earlier work (Anderson and Avery, 1966; Leclercq et al., 1977), Gault agreed that in more complex hydrocarbons and where molecular structure permitted that hydrogenolysis of carbon-carbon bonds proceeds through a 1,1,3triadsorbed species. Leclercq et al. (1977) concluded from hydrogenolysis studies on a range of branched alkane structures that while 1,(1),&diadsorbed intermediates were most common, that 1 , 4 and 1,5- diadsorbed modes may be feasible as well. For example, the easy scission of the central C-C bond in 2,2,3,3-tetramethylbutane(the preponderant scission mode on Pt) must be via a 1,4-diadsorbed mode. The 1,Cdiadsorbed intermediate if attached to a single surface site is identical with a metallocyclopentane in organometallic chemistry. Fboney (see Clarke and Kane, 1980) notes a parallel with the observation of Grubbs and Miyashita (1978) that thermal decomposition of such entities leads to central scission of the C4 chain to two C2 units via alkene precursors. Clarke and Kane (1980) find this process to occur in neohexane hydrogenolysis on platinum films containing a trace of gold, other more indiscriminate hydrogenolysis occurring in small amount alongside. Hydrogenolysis of 3,3-dimethylpentane on Pt-Cu films can be similarly interpreted (Clarke et al., 1981). What these results emphasize especially is that hydrogenolysis of alkanes higher than ethane cant occur on a single Pt surface site. Foger and Anderson (1980a,b) have suggested that neopentane hydrogenolysis on very small Pt particles or on Ir takes place on a single metal surface atom, and choose on balance the decomposition of a metallocyclobutane. Clarke and Kane (1980) argue that there is a closely similar effect of minor gold incorporation into platinum in suppressing the hydrogenolysis reaction of neohexane and of neopentane. They suggest therefore that the latter also may take place on a single Pt atom site. Note that, in principle, any dehydrocyclization mechanism in reverse provides a hydrogenolysis mechanism. Single site intermediates are currently favored for these processes (see later sub-section). For different metals the dominant mechanism may be different because of distinctive chemical characteristics of the metal. On palladium, because scission of C-C bonds which are methyl-substituted is a prominent mode, a 1,Zdicarbene mechanism (Le., 1,1,2,2-tetradsorbed intermediate) is precluded. An alternative is a metallocyclobutane derived from a x-allyl adsorbed species by attack at C2by a hydrogen ad-atom, (Garin and Gault, 1975) (see later). One surface metal atom only was taken to be needed to bond the intermediates. Gault and Weisang (1979) believed that hydrogenolysis on iridium was by rupture of a 1,2-dicarbene species (Scheme I). Metallocarbene formation is slow enough to permit attachment of only two carbon atoms to the metal in the adsorbed species (compare nickel and cobalt below).
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 577 Scheme I1
Pt
I
L
Pt
Figure 2. Precursor (left) and bridged intermediate (right) from neopentane. (Reproduced with permission from Anderson and Avery (1967). Copyright 1967 Academic Press Inc.)
M
-Md
Figure 3. Mechanism for bond shift isomerization due to McKervey, Rooney and Samman, illustrated for neopentane rearrangement. (Reproduced with permission from Clarke and Rooney (1976). Copyright 1976 Academic Press Inc.)
On nickel and more particularly on cobalt, extensive cracking to methane indicates that several consecutive C-C bond ruptures occur before desorption (Gault and Rooney, 1979). Either multiple attachment to the metal is possible or surface product can become diadsorbed at an adjacent C atom and sustain further C-C scission, etc. (see Scheme 11). Bond Shift Rearrangement. The simpler (if only later recognized with metals) mode of skeletal isomerisation of alkanes is by means of a “bond shift”. Platinum appears to be the most active metallic catalyst. Three mechanisms were suggested in all during 1965-1980. The first, due to Anderson and Avery (1966) proposed a 1,1,3-triadsorbed radical (Figure 2) which changed through a transition state best described as a Dewar-type a-complex attached to two surface sites. The order of relative isomerisation rates neopentane > isobutane > n-butane was rationalized from simplified Huckel MO calculations on the basis of a hyperconjugative effect.
M
From the obsprved isomerisation of strained cage compounds for which a l,l,&triadsorbed intermediate is not possible McKervey, Rooney, and Samman (McKervey et al., 1973) proposed that even a-alkyl surface radicals can undergo skeletal rearrangement. This proposal was strengthened by the finding that “simple” exchange of deuterium with both the reactant (adamantene dimer) and isomerized product occurred when the reactions were carried out on palladium in the presence of deuterium, that is, only one hydrogen atom is removed a t a time during both the exchange and the isomerization process. More recently Karpiiiski and Guczi (1977) have made a similar observation in neopentane isomerization on platinum in the presence of a large excess of deuterium gas. The intermediate state proposed by Samman and Rooney involves three-center orbitals, has carbonium-ion character (Figure 31, and factors determining the MO’s and energy of the intermediate are considered by the same authors to be the same as those governing a-bonding in metal-aalkene complexes. Gault and his co-workers noted that palladium, unlike platinum, does not apparently catalyze skeletal isomerizations at a quaternary carbon atom (for a qualification of the finding, see Karpiiiski, 1980). They proposed (Muller and Gault, 1972)that an allylic species held at two metal atoms can undergo C-C bond scission by hydrogen atom attack to form a metallocarbene and a ?r-adsorbed olefin. Rotation of the a-adsorbed olefin, which is rapid, followed by reforming of the C-C bond gives isomerization. Alternatively, detachment of the two attached species, perhaps with addition of hydrogens to one or both, gives hydrogenolysis. Subsequently, Gault preferred a metallocyclobutane intermediate possibly formed by hydrogen atom addition to a a-allylic species bonded at one surface site, which undergoes decomposition to metallmbene and a-adsorbed olefin (Figure 4) (Gault et al., 1979). This
M’
\ Figure 4. Carbene/olefin mechanism of bond-shift rearrangement illustrated by the reaction of 2-methylb~tane-2-’~C.(Reproduced with permission from Gault et al. (1979). Copyright 1979 Societe Chimique de Belgique.)
578
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981
Scheme IV. Nonselective Mechanism for Ring Opening/ Closure with Dicarbene Intermediate'
Scheme 111'
0-Q-Q-m Reproduced from Amir-Ebrahimi et al. (1980). Nouu. J. Chim., Gauthier-Villars Editeur, Paris. a
i
Scheme V. Nonselective Mechanism for Ring Closure/ Scission with Cyclopentenyl Intermediate and Due to Rooneyu
Adapted from Clarke and Rooney (1976). Copyright 1976 Academic Press Inc. a
mechanism was thought to apply also (at the least) to platinum (see below). With platinum there is no need for intervention of the a-allyl species because of the known propensity of platinum to form a metallocyclobutane or l,&diadsorbed species (Gault et al., 1962). Gault has indeed argued (Gault et al., 1979) that this mechanism explains the rapid isomerization of 2-methylbutane-2-13Cto 2-methy1butane-3-l3Cbut not to n-pentane-2-13C,rather breaking up to ethane and propane because of the known rapid switch of ethylidene to 7r-ethylene in organometallic chemistry. Garin and Gault (1975) argued against the mechanism of Anderson and Avery on the grounds of what they considered to be the over-strong association of a hydrogenolysis route with the bond shift mechanism. Gault and co-workers more recently argue for two bond-shift mechanisms (carbene/olefin and Rooney-Samman), the kinetic characteristics of the two being different to an extent which appeared to accommodate the observed parallel reactions of 13C-labeledhexanes (Parayre et al., 1980). Specifically, the former reaction route took place with a lower activation energy than the latter and had a mechanistically associated hydrogenolysis reaction. Anderson (Foger and Anderson, 1978) retains a belief in the Anderson-Avery mechanism at least for one of the possible routes in alkane isomerisation and favors a single-site mechanism for a higher activation energy route on Pt (Foger and Anderson, 198Ob). Anderson and Foger (Foger and Anderson, 1980a) see a need also for supposing a single-sitebond-shift mechanism on iridium. Rooney argues that the Rooney-Samman mechanism (or a simple modification in which a-olefin bonding is replaced by a a-allylic bonding in the transition state and requiring similarly a one atom site (Clarke and Rooney, 1976)) rationalizes the various characteristics of alkane rearrangements observed on platinum. Vinyl shift is predicted to be much easier than methyl or ethyl shift from molecular orbital theory (Clarke and Rooney, 1976) so that large negative hydrogen pressure dependence indexes (Garin and Gault 1975) can be understood. Other preferred pathways (e.g., Parayre et al., 1980) may be rationalized on this mechanism by (a) inhibition of the formation of the transition state in the case of larger alkyl groups due to steric hindrance, (b) bond-shift reactions involving symmetrical intermediates being favored over the other reaction pathways, as expected theoretically, and (c) change in electronic characteristics of surface sites as in Pt-Cu alloys (e.g., de Jongste et al., 1980) resulting in altered activation energy. Of key interest for the central theme of this review, both Gault and Rooney have visualized bond-shift taking place wholly on a single platinum atom site. This position is not at variance with results for platinum alloys, and is on balance supported by them (van Schaik et al., 1975; Clarke et al., 1980). Carbocyclic Mechanism. A well-established route for
Reproduced with permission from Kane and Clark6
(1980). Copyright 1980 Royal Society of Chemistry. Scheme VIu
C
C
111
111
M
M
c - c' 0 M
\\
M
a Reproduced from Gault et al. (1979). Copyright 1979,Socidtk Chemique de Belgique.
isomerization of alkanes on platinum is by cyclization to a C5 ring intermediate, displacement of the point of attachment of the ring to the surface, and ring opening at a different C-C bond (see Gault et al., 1979). Three mechanisms have been recognized by Gault and co-workers and these can be distinguished by considering the hydrogenolysis of the Cs ring intermediate, exemplified by methylcyclopentane in what follows (Scheme 111). On highly dispersed supported platinum only a nonselective cyclic mechanism (NSCM) is found in which endocyclic C-C bonds are broken with approximately equal probability. A selective cyclic mechanism (SCM), in which di-secondary C-C bonds only are broken, becomes prominent on poorly dispersed platinum. A minor partly selective mechanism PSCM occurs in addition on poorly dispersed platinum above 250 OC: here, a substituted C-C bond is broken but with a lesser probability than the other C-C bonds. Gault proposed for NSCM that a a-olefinic surface intermediate (which permits the ring to be approximately parallel to the surface so having reduced steric blockage (see Bragin et al., 1972)) converts to a dicarbene species in the ring opening step (Scheme IV). The 2:2:1 ratio of ring C-C bond breaking probabilities observed experimentally could be rationalized on this mechanism (Amir-Ebrahimi and Gault, 1980). Rooney has proposed an alternative NSCM in which a cyclopentenyl surface radical converts to the open-chain pentadienyl form, which hydrogenates and desorbs (0'Donohoe et al., 1980) (Scheme V). The surface ringopening precursor is related to the adsorbed hexa-1,3,5triene precursor suggested for 1,6-cyclization ( P a d and TBtBnyi, 1973). Steps are reversed in ring-closure. For the SCM, Gault argued that the relevant intermediate is a dimetallocarbyne (Scheme VI). The steric requirement of each M=C-C structure being linear demands two contiguous metal atoms rather than one. Experimentally it is indeed found that on platinum, at least, very large crystallites are necessary for this mechanism to take place. However, on iridium catalysts irre-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 579
M'
M
Reproduced from Amir-Ebrahimi et al. (1980). Nouu. J, Chim., Gauthier-Villars Editeur, Paris.
spective of dispersion this is the sole reaction (Weisang and Gault, 1979). The type of bond-shift mechanism involving a metallocyclobutane + carbene + olefin step and favored by Gault (see earlier), provides also a means of ring scission. The Strasbourg group believed this to be the basis of the minor PSCM which takes place on larger platinum crystallites (Amir-Ebrahimi et al., 1980). This is illustrated in Scheme VI1 (upper route) in the case of 1,3-dimethylcyclopentane. Ring Enlargement. Aromatization of alkanes on platinum may come about by direct 1,6 ring closure or by 1,5 ring closure followed by ring enlargement (Clarke and Rooney, 1976; Parayre et al., 1980). The two routes compete for alkanes having six or more carbon atoms in a chain; for substituted pentanes only the second route in practice obtains. Gault believed that ring enlargement was by the carbene/olefm mechanism at least on platinum, and possibly on palladium (Scheme VII, lower route). This scheme is therefore identical mechanistically with that suggested by him for the minor PSCM of dehydrocyclization. Aromatization is not influenced by the dispersion of the platinum on the support so that, he argued, it may be assumed that the ring-enlargement reaction involves a single metal atom (Amir-Ebrahimi et al., 1980; AmirEbrahimi, 1978). From the foregoing account, then, the balance of opinion is that bond-shift, dehydrocyclization/carbocyclic isomerization (NSCM) and some of the defined modes of hydrogenolysis require a single atom site only, in line with organometallic analogues. Selective dehydrocyclization/ carbocyclic isomerisation and other (possibly majority) alkane hydrogenolysis modes require two contiguous metal atoms. Particle-Size Dependence. Earlier work (Barron et al., 1963) had reported that the carbocyclic mechanism became the predominant route for isomerization on highly dispersed supported platinum. Hydrogenolysis of a number of alkanes has subsequently been found to require special sites which are present more on small metal particles (Anderson and Shimoyama, 1973; Brunelle et al., 1976; see also Blakely and Somorjai (1976). More detailed findings on these processes from recent work (Dartigues et al., 1979) are as follows. (i) In the isomerization of 2-methylpentane to 3methylpentane, the percentage of cyclic mechanism remains constant over a very large range of metal dispersion. However, its nature changes from SCM to NSCM as the size of the metal particles decreases (SCM does not occur for d 400 "C)while for Pt-Pd distinct platinum and PdO particles are observed (Figure 6). The authors argue that distinct platinum and palladium particles will remain following reduction on hydrogen, whereas rhodium should remain within the original particle. R-Rh surfaces tend, therefore, to become rhodium-rich by oxidation-reduction treatment, while Pt-Pd alloys may have particles of both platinum and palladium, a difference arising basically from the different nucleation characteristics of palladium and rhodium oxides. Rhodium enrichment at the surface of Pt-Rh foils oxidized for a short period at 900 "C has been demonstrated using XPS (Schlatter and Taylor, 1977). This effect appears to underlie the improved action of Pt-Rh/alumina three-way catalysts following thermal ageing in decreasing ammonia formation from nitric oxide. While there are still chemical difficulties in the realization of supported Pt-Rh for practical use in automobile emission control (Yao and Shelef, 1980) the principle of producing a rhodium-enriched surface in this way is still being pursued on account of the lesser availability of rhodium in natural resources compared to platinum. The most severe conditions of oxidation are known to lead to gross segregation with Pt-Rh. Pt-Rh gauzes used in ammonia oxidation burners for extended periods have been shown to develop during the combustion two distinct crystalline systems (ca. 800-950 "C, pressures of ammonia/air (10%) of 1-10 atm), namely small rhodium-rich crystals and large well-developed platinum-rich crystals (Contour et al., 1977). Here volatilization of platinum oxide at high temperatures after the oxidation of both metals appears to be the key to movement of material leading to segregation of rhodium at the surface. Returning to the preparation of alloy catalysts for research use, de Jongste and Ponec (1980b) suggest that oxidation-reduction treatment of Pt-Au/alumina (e.g., 350 OC,2 h in oxygen; then 350 O C 2 h in hydrogen) leads to a separation of components and a production of very small platinum particles. This inference was based wholly on observed catalytic selectivity patterns but is clearly a provocative one for the now considerable body of fundamental work on supported group 8-1B alloy catalysts. A failure to prepare supported Pt-Pd supported clusters by the co-impregnation method (Ruiz-Vizcaya et al., 1978) has been ascribed to segregation of metals in an oxidizing atmosphere following the findings of Chen and Schmidt
(Grill and Gonzalez, 1980).
Spectroscopic Techniques in Structural Study of Supported Alloy Catalysts: Mossbauer, EXAFS, and X-ray Photoelectron Spectroscopies Structural study of supported bimetallic catalysts by X-ray diffraction ceases to be feasible at high metal dispersion. Mossbauer spectroscopy is applicable to systems either where one metallic component contains a Miiesbauer isotope or where a trace of such an isotope is incorporated. The recently developed EXAFS technique is proving to be an especially powerful one. Finally, X-ray photoelectron spectroscopy (XPS) can provide information on special aspects of supported metals and alloys. In what follows, application of these techniques is illustrated with special emphasis on catalysts of contemporary interest. Because of this choice of catalyst examples, the opportunity will be taken at the same time to indicate some developments in the interpretation of the action of these catalysts. Mossbauer Spectroscopy. This technique consists in the measurement of the intensity of recoil-less resonance absorption of y rays by nuclei bound in solids. The extremely high resolution of this resonance allows measurement of the separation of nuclear energy levels to one part in 1014. At this level of accuracy the weak interaction of the nucleus with its electronic environment can be measured, notably the oxidation state, details of chemical bonding, and symmetry of atomic environment of the absorber. The method is limited in that it applies best only to Fe, Sn, and Eu. However, experiments have been performed with other elements and an important tactic is that of doping systems with Mossbauer probe atoms. In principle, details of electron movement on alloying of two metals should be determinable from Mossbauer spectra. Interpretations are not straightforward, however (see, e.g., Delgass et al., 1979a),and Mossbauer information requires to be supplemented by that from a further technique such as charge transfer from XPS (Tricker, 1974) to allow orbital population to be inferred. In the examples which follow, uncontentious procedures are described which allow such basic questions as whether metals are well mixed and what is the surface composition to be answered unambiguously. Bartholomew and Boudart (1973) studied Pt-Fe/carbon samples having metal particle size of 15-30 A from chemisorption measurement. Separate distinguishable components of the spectrum are due to surface Fe atoms, bulk Fe atoms, and corresponding cationic Fe atoms. Surface iron was determined from the relative Mossbauer spectral area of surface components compared to the total spectral area. Reduced samples showed surface compositions substantially the same as the nominal bulk compositions. That a homogeneous metal mixture was attained in each sample was confirmed by the (deconvoluted) inner bulk doublet having narrow width, whereas it would have been broadened by different quadrupole splitting values corresponding to different Fe contents. Finally, exposure to oxygen at 300 "C (but not at room temperature) caused iron enrichment at the surface. This effect is rather beautifully demonstrated in Figure 7 due to Garten (1976a). Vannice and Garten (1975-1976) found that not all Pt/Fe preparation ratios give rise to homogeneous clusters of corresponding composition and that correspondence depends also on metal loading. They emphasize that catalytic results ascribed to a particular bimetallic cluster composition must be regarded as suspect until the nature of the clusters is independently confirmed. Bimetallic clusters of Fe-Pd on v-A1203were prepared
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 4, 1981 --
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