2944
J . Phys. Chem. 1986, 90, 2944-2949
crown content was low, 2% in some In all cases where 2:l complexes could form, the reduced viscosity of the solution showed a steeply increasing dependence on polym-er concentration. Cross-linking in DC5lO must eventually inhibit those cooperative motions necessary for the production of phenyl pairs. At some point in a microscale thin-film metal vapor reaction some phenyl substituents must become unavailable for chemical reaction because they are unable to diffuse to other phenyl groups in time to undergo reaction with a metal atom. In this case when cross-linking reduces the number of motional degrees of freedom available to a polymer or polymer segment, or erects a barrier to diffusion of a metal atom, there must always be a residue of unreacted ligand. The consumption of arene should eventually cease with time without exhausting the supply of arene ligand. This is just what is observed in fhin-film metal ua~opDC51O experi&ents.~ This is not to say that'a free metal itom cannot react with an aromatic group; the product Of such a reaction, solvated by the polymer backbone or otherwise, must
be singularly unstable toward further reaction, as recently found for ( V ~ - C ~ H ~ ) V . ~ ~
Acknowledgment. The generous financial assistance of the Natural Sciences and Engineering Research Council of Canada's Operating and Strategic Grants Programmes and the Connaught Foundation of the University of Toronto is gratefully appreciated. The award of a 3M Corporation Grant to G.A.O. is also acknowledged with gratitude. (22) Ozin, G. A.; Andrews, M. P.; Mattar, S.; McIntosh, D. F.; Huber, H. x. J . Am. Chem. SOC. 1983,105,6170; Andrews, M. p.; Mattar, S. M.; Ozin, G. A. J. Phys. Chem. 1986, 90, 744. (23) In this paper the periodic group notation in parentheses is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the p-block elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., 111 3 and 13.)
-
Kinetic Modeling of Hydrocarbon and Oxygenate Formation Roger C. Baetzold* and John R. Monnier Research Laboratories, Eastman Kodak Company, Rochester, New York 14650 (Received: October 21, 1985) Kinetic modeling is used to describe quantitatively the hydrocarbon- and oxygenate-forming reactions from CO/H, over a model catalyst. In particular, the effects on product yield of reaction variables such as ease of CO dissociation or heats of adsorption of CO or H2are shown. A mechanism involving olefin formation and subsequent reaction is used to describe the unexpected low amounts of two-carbon hydrocarbons and three-carbon oxygenates observed in experiments with a model Ru catalyst. Several catalysts are described quantitatively by this mechanism. Smaller olefins are reincorporated into the chain growth sequence and can react with CO after they become enlarged by one carbon atom.
Introduction Considerable progress has been made in understanding the mechanism of linear oxygenate formation from syngas. One may consider hydrogenation, CO dissociation, chain growth, and C O insertion functions to occur on the catalyst surface. Experiments with a variety of conventional catalyst preparations' have provided support for the CO-insertion step as introducing the oxygen function into the oxygenate chain as the final chain enlargement step in which C is added to the hydrocarbon fragment. One aspect we have pursued involves kinetic modeling2 that attempts to make quantitative some of the ideas developed for various mechanisms. We summarize here some recent applications of the technique to syngas chemistry. Kinetic modeling is the mathematical expression of the rate of prodwt formation, given the reactant conditions. Obviously, this is a demanding exercise in which current theoretical methods must be applied beyond normal conservative limits. We cannot expect to project fine details from such an approach; rather we must look for the broad picture. Fortunately, the more general aspects of this approach can be used profitably in understanding and analyzing experimental results. A mechanism is taken as a starting point of this kinetic modeling. This is the CO-insertion model of oxygenate formation.' Given this mechanism, the fundamental energetics of chemisorption3 can be used to estimate rate constants. Experimental (1) (a) Pichler, H. Adu. Catal. 1952, 4, 271. (b) Pichler, H.; Schulz, H. Chem.-Ing.-Tech. 1970, 42, 1162. (c) Henrici-Olive, G.; Olive, S . Angew. Chem. 1976.88, 144. (d) Muetterties, E.L.; Stein, J. Chem. Reu. 1979, 79, 479. (e) Sachtler, W. M. H. Proceedings of the 8th International Congress on Catalysis, Berlin; Dechema: Frankfurt, 1984; Vol. 1, p 159. (fj Ichikawa, M.; Fukushima, T.; Shikaura, K.Proceedings of the 8th International Congress on Catnlysis, Berlin; Dechema: Frankfurt, 1984; Vol. 2, p 69. ( 2 ) Baetzold, R. C. J . Phys. Chem. 1984, 88, 5583. (3) (a) Shustorovich, E. Solid State Commun. 1982, 44, 567. (b) Shustorovich, E. J. Phys. Chem. 1984.88, 1927. (c) Baetzold, R. C . Solid State Commun. 1982, 44, 781. (d) Kerr, J. A. Chem. Rev. 1966, 66, 465.
0022-3654/86/2090-2944%0 1.5010
SCHEME I: Sequence of Reaction Steps," Methanation Reaction
.&CO(a) H2(g) & 2H(a) C+0 CO(a) co(g)
k(l,l)
k(2.l)
C+H-CH CH
" g = gas phase;
+H
43.1) k(3,2)
CH,
a = adsorbed; kH = k(5,l).
SCHEME II: Methanation Plus Chain Growth
chain growth
and theoretical information such as heats of adsorption of stable surface s p e ~ i e and s ~ gas-phase ~~ bond energies3dare incorporated into the model. With this overall expression of product formation we can examine the total system behavior rather than looking at separate parts in a piecemeal fashion. 0 1986 American Chemical Society
Hydrocarbon and Oxygenate Formation
The Journal of Physical Chemistry, Vol. 90, No. 13, 1986 2945
SCHEME III: Methanation, Chain Growth, and Oxygenate
surface concentrations of various species expressed ih conventional Fa r H = 1. With Langmuir-Hinshelwood terms, where these relations the rate of forming saturated hydrocarbon at steady state is given by
+ +
oxygenate CH3 + CO
2CH$O
CH3COH
C2H5+ CO -% C2HSC0 H
R(CA"ZN+Z) = rgN-lkHIHl[CH31
C2HSCOH
which holds for N = 1,2, 3, ..., m . The rate of formation of higher (two-carbon and larger) oxygenates is given by
+ co A HCO
H + HCO & H2C0 H + H2C0
R(CNH~N+~OH) = rgN-'kA [CO][CHj]
2H$O
R(CH,OH)= k'[H] [H3CO]
I;"
The objective of this paper is to quantify the performance of a model catalyst in terms of the kinetic model. A particular point that requires explanation is the low yield of C2 hydrocarbons often found4 in experimental studies of syngas reactions. To treat this situation, we expanded the kinetic model2to allow olefin formation and subsequent hydroformylation with CO. as a result of this analysis, a proposal is advanced for the role of alkali promoters in altering product yield. Some of the more general features of oxygenate formation will also be discussed.
Results We begin by considering a catalyst of homogeneous surface composition. Oxygenates can form by three sets of reactions. First there is CO dissociation and hydrogenation leading to CH, and product CH4. The second set of reactions involves chain growth through the interaction of various CH, species. Finally, higher oxygenate formation is permitted by the coupling of CH, species with CO. Of course, methanol formation through hydrogenation of CO is possible. These schemes are assembled in Schemes 1-111 showing the mechanism we have considered for oxygenate formation. The reactions leading to chain formation have been discussed in some detaiL2 We assume that CO insertion into alkyl is the rate-limiting step leading to oxygenate as compared to reductive elimination via hydrogenation of the acyl fragment from the metal surface M.
/"
II 0
-
M t H-C-R
II
0
Acyl has been reported in various IR spectroscopic studies of reaction mixtures. le*f Some straightforward and useful relations can be derived for the CO-insertion model. For simplicity, let us assume that all alkyl fragments of different chain lengths insert CO, hydrogenate to alkane, and grow by CH2 insertion; the rate constants are kA, kH, and k,, respectively. Equations 1-3 are defined by using
k,[CH21
rg= kg[CH21 + kH[Hl + kAICol
(6)
Although an alcohol species is written in eq 5, an aldehyde could just as well be considered if the rate of converting aldehyde to alcohol is slow compared to desorption. Note the familiar relationship of Schulz-Flory5 (eq 7). The slopes of both oxygenate
C2H4 + H
C3H7k" C3H6+ H
M-C-R
(5)
where N = 2, 3,4, ..., m. The rate of methanol formation is given by
H3C0 + H 2 CH30H olefin C2HS
(4)
(1)
(4) (a) Vannice, M. A. J . Cutul. 1979, 56, 236. (b) Anderson, R. B. In Cutulysis; Emmett, P.; Ed.;Reinhold: New York, 1956; Vbl. 4,p 109. (c) Jacobs, P.A.; VanWouwe, D. J. Mol. Cutul. 1982,17, 145. (d) Henrici-Olive, G.;Olive, S . Angew. Chem., In?. Ed. Engl. 1976, IS, 136.
- a log R(C"ZN+IOH) = log rs (7) aN dN and hydrocarbon activity vs. carbon number are equal. The total rate of formation of hydrocarbons is given by a log R(C"2N+2)
-
and the total rate of higher oxygenate formation is given by
m
cx'=-1 - x
O I X < l
j=o
in deriving eq 8 and 9. Thus the ratio of C2 and larger oxygenates to total hydrocarbons becomes
This yield of oxygenate to hydrocarbon is independent of the chain growth rate. It is clear that the ratio OX/HD may approach infinity, depending upon the rate constants and surface species concentrations in eq 10. Reducing [HI or increasing [CO] would increase the OX/HD value. Formation of high-selectivity C2 and larger oxygenates is favored if
since this will give a Schulz-Flory slope near zero. Clearly, this effect depends critically on chain growth, which involves CH2 concentration, which in turn is controlled in part by surface H and the rate of C formation from CO dissociation. A means of estimating the rate constants in hydrocarbon formation was given earliera2 Preexponential factors were estimated for rate constants or equilibrium constants by using transition-state theory. Activation energies were estimated from enthalpy changes computed for surface reaction steps along with the Polyani relation. This procedure worked well to explain the rate laws of methanation and the dependence of higher hydrocarbon formation on ease of CO dissociation. We now extend the mechanism to include oxygenate formation through CO insertion with metal-alkyl fragments. An empirical rate constant that gives an appropriate competition between hydrogenation and CO insertion is chosen for this purpose. The constants used before (5) Schulz, G. V. Z . Phys. Chem. 1935, B29.299. (b) Flory, P.J. J . Am. Chem. SOC.1936,58, 1877.
Baetzold and Monnier
The Journal of Physical Chemistry, Vol. 90, No. 13, 1986
2946
n
Hydrocarbons
Oxygenates
'&.
Hydrocarbon
.
.I
..+..... Oxygenate
IO-L
7
'u,
i
P
I
-1 8 c
'u,
10-4
z
P
I I
lo-'
I
IO-e
2
3
4
5
6
s:IIII 1
2
3
4
5
6
Carbon number Figure 1. Turnover number (number of product molecules per catalyst surface atom per second) for hydrocarbon and oxygenate formation for reaction from CO/H2 at 250 "C, 1 atm pressure, H 2 / C 0 = 3. The effective heat of CO adsorption AH (kcal/mol) is varied while all other parameters of Table 111 are constant.
and the empirical CO-insertion rate constant are shown in Table 111. We have treated methanol formation on the metal surface through a succession of equilibria involving hydrogenation of CO. The preexponential terms of the equilibrium constants were derived from transition-state theory. The enthalpy terms arise from computed enthalpy changes of surface reaction. Thermodynamic cycles were constructed as before,2 by using thermochemical bond energies3dand computed heats of a d s ~ r p t i o n . ~ These ~ " values are given in Table 111 along with an empirical rate constant for hydrogenation of methoxide. Now let us consider several variables important in the growth of oxygenate and hydrocarbon chains. The first is the heat of adsorption of C O to the catalyst under working conditions. This parameter controls the surface concentration of adsorbed C O and has a dramatic effect on the selectivity and slope (r,) of the Schulz-Flory type plot shown in Figure 1 . The highest rates of product formation and the lowest Schulz-Flory slope are associated with high CO concentration. Figure 2 shows the complex effect of surface H concentration on rates of product formation. At low H concentration, the rate of product formation is low. As the concentration of surface H is increased through increasing the heat of adsorption, activity increases until hydrogenation becomes dominant compared to chain growth and C O insertion, and CH4 becomes the dominant product. The energy of C O dissociation is important in determining the Schulz-Flory slope, as shown in Figure 3. The smallest slopes correlate with ease of C O dissociation. This reflects the facts that the concentrations of all CH, (x = 0-3) species depend upon C O dissociation and that CHI has a particularly significant effect on the slope, as suggested by eq 1 and 7. It is instructive to compare these calculations with previous catalytic data for oxygenate and hydrocarbon formation. And e r ~ o describes n~~ a cobalt catalyst that gives alcohols and hydrocarbons with a slope of -0.75 and a Synol catalyst that gives slopes of 0.61 for the oxygenates and 0.70 for the hydrocarbons. As shown earlier, these slopes define rs,and the range of kinetic parameters modeled in Figures 1-3 falls in the range of observed behavior for experimental catalysts. Another comparison with experiment can be made concerning the low methanol (relative to other oxygenates) yield in Figures 1-3. The Synol and other catalysts give low methanol, as noted by Anderson. Many of these
I
I
I
I
I
2
3 4
5
6
I
l
I
l
\
1 2 3 4 5
Carbon number Figure 2. Turnover number for hydrocarbon and oxygenate formation for reaction from CO/H2 at 250 OC, 1 atm pressure, H2/C0 = 3. The effective heat of H2 adsorption -AH (kcal/mol) is varied while all other parameters of Table 2 are constant.
lo-' F
'm
z
2 lo-'
lo-
:,15 2
I
I
I
I
I
I
Z
I
l
I
3
4
5
6
1
2
3
4
5
6
Carbon number Figure 3. Turnover number for hydrocarbon and oxygenate formation for reaction from CO/H2 at 250 OC, 1 atm pressure, H2/C0 = 3. The activation energy of CO decomposition (kcal/mol) is varied while all
other parameters of Table 111 are constant. catalyst features have been reported for a patented K/CuCoCr oxide catalyst.6 This catalyst gives a low methanol yield and a slope of -0.5. A recent study7 of modified Fe on silica catalysts gives alcohol and hydrocarbon slopes completely consistent with these computed model results. Now we address the problem of why some experimental data4 show significant deviations of Cz hydrocarbons from the linear relation. Examples of this behavior are found in the experimental data discussed later. One potential explanation of the deviation is that k, is larger for C2H5reaction with CH, than the corresponding k, for reaction of CH3, C3H,, C,H9, ... with CHI. Under ~
~~
~~~
( 6 ) Sugier, A.; Freund, E. I. F. P., US.Patent 4 122 110, 1978. (7) Razzaghi, A.; Hindermann, J.-P.; Kiennemann, A. Appl. Cata/. 1984, 13, 193.
The Journal of Physical Chemistry, Vol. 90, No. 13, 1986 2941 Hydrocarbons
Hydrocarbons
Oxygenates
Oxygenates
-----
I
All C2H4 Desorbs No C2H4 Desorbs
kg greater for C2H5 reaction than CH3, C3H7, C4Hg ... 10-3 -\
IO-^ -
r
'm
2
e
I
2
3 4
5
I
2
3
4
5
Carbon number Figure 4. Turnover number for hydrocarbon and oxygenate for parameters of Table 111 when k, is a constant for all N values (-) or k, is increased by a factor of 2 for N = 2 (- - -).
I
I
I
I
2
3 4
5
I
I
I
I
I
I
I
2
3
4
5
Carbon number Figure 6. Computed turnover number for hydrocarbon and oxygenate formation for the parameters of Table 111 when all of the ethylene formed desorbs (-) or reacts further (- - -).
El
Oxygenates 0,.
Experiment
-Theory
I
1
1 Figure 5. Schematic representation of the reaction pathway considered
for olefin generation and reaction. this circumstance, the computations in Figure 4 show that yields of C2 hydrocarbon and C3oxygenate drop significantly. Although the experimental data could be fit by this scheme, the mechanism is not chemically satisfying. There is no clear reason why C,H5 should react differently with CH2than the other members of the alkyl series. Thus, we will look for another cause of this effect. We now consider olefin formation from adsorbed alkyl species. The accepted mechanism* of this reaction involves 8-H elimination, as shown for C2HSin eq 12. The olefin that is formed may desorb R
R
CH2
CH
I
I
I
CH2
I ////
-
II + CH2
H
I
(12)
//// R . H . CH3. C 2 H 5 .
e..
into the gas phase and be detected as products or react on the surface with some of the species in the reaction mixture. This balance between the two possibilities will be a function of catalyst (8) See, for example: Bell, A. T. Catal. Reu. Sci. Eng. 1981, 23, 203.
I
I
I
I
I
I
1 2 3 4 5 6 7
I
I
T
I
I
1 2 3 4 5
Carbon number Figure 7. Experimental data showing turnover number for total hydro-
carbons or oxygenate on evaporated Ru clean or K promoted. Computed profiles for parameters discussed in the text are compared with experiment. composition and reaction conditions. The olefin/paraffin product ratio measured for iron-based hydrocarbon catalysts9 or K-promoted Fe hydrocarbon catalysts8 is much smaller for two-carbon molecules than for larger molecules. This suggests that ethylene is more effectively consumed than higher olefins by species in the reaction mixture. This modeling does not consider specific effects associated with flow conditions of the feed gas. Hence an effective value for the percentage desorption of olefin to the gas phase is used. A detailed microscopic model could be devised to consider desorption and readsorption of olefins over the time scale of the flowing gases in the reactor. W e will model this situation by allowing ethylene to either desorb or react with CH2 to add a carbon unit, whereas all other olefins will be considered to desorb. This is shown schematically (9)Dry, M. E.J . Mol. Catal. 1982, 17, 133.
2948
Baetzold and Monnier
The Journal of Physical Chemistry, Vol. 90, No. 13, 1986
TABLE I: Effect of K on Olefin/Paraffin Ratiosagb catalyst c2 c3 Ru/Si02 1.5 3.O (K) Ru/S O 2 2.8 6.4
TABLE II: Parameters Describing Ru Catalyst c 4
2.2 4.3
“Space velocities similar for both catalysts, so that changes in olefin/paraffin ratios are not due to differences in catalyst contact time. bHydrocarbons up to C, detected over Ru/SiO,. For (K)Ru/Si02; only hydrocarbons up to C4 were detected.
rg
catalyst
Ru
0.51 0.64
Ru/K
A. The potassium-promoted R u / S i 0 2 sample was prepared by adding the proper amount of a 0.027 M potassium ethoxide/ ethanol solution to an ethanolic slurry of an unused Ru/Si02 sample. After rotary evaporation and drying at 100 OC, the potassium loading was determined to be 3.3 wt %. All Fischer-Tropsch reactions were run in a single-pass, high-pressure flow reactor with on-line gas chromatography used for product analysis. Unless otherwise stated, reaction conditions were 250 OC and 825 psig overall pressure with a 1:l feed composition of H 2 / C 0 . Both catalysts were left on stream for a minimum of 4 h to ensure steady-state conditions. Finally, H 2 and C O conversions were kept below 2%,so that all reactions were kinetically, and not thermodynamically, controlled. The experimental data are reported in terms of activity (pmol/(g of catalyst-s)), which is proportional to the turnover number (s-I) computed here. Thus the experimental and theoretical curves are aligned by overlaying the computed and experimental data. Only one alignment is allowed per catalyst (i.e., the same for hydrocarbons, oxygenates, and promoted cases). Once the curves are approximately aligned, the k A constant is varied and chosen to give the correct alcohol yield and the amount of olefin formed. Then the amount of C2H4 desorbed is varied to get the best fit in the C2, C3 region. This characterizes the unpromoted catalyst. Figure 7 shows that the catalyst behavior can be described quite well. The promoted catalyst is described now only by varying the amount of surface hydrogen from the value on the unpromoted catalyst. This variable was considered first because it is known that H 2
H, atoms/cm2
0.24
1.3 X 10”
0.31
1.3 X loi4
C2H4
desorbed, % 1 .o 1.o
TABLE 111: Rate and Equilibrium Parametersa ( k = koe-E/RT) rate const or exponential
equilib const in Figure 5 . Note that direct reaction of ethylene with CO leading to C3 oxygenated product can be considered to be insignificant, since the C3 oxygenated product is formed in much lower amounts than expected. We test this model by computing two extreme cases in Figure 6. All of the ethylene formed either desorbs or reacts. Qualitatively, the behavior most closely resembling experiment (Figure 7 ) is found when all of the ethylene reacts with CH2. If all of the ethylene desorbs, the shapes of the plots are unlike experimental data. Thus, in fitting the behavior of actual catalysts, we will determine the percentage of ethylene that reacts or desorbs. The higher olefins are also formed by 0-H elimination from the appropriate alkyl fragments. This suggests that the higher olefins are less reactive than ethylene with the species in the syngas mixture. This is consistent with the generalization that ethylene has the greatest ability to coordinate to transition metals compared to higher olefins.4d We now consider a Ru/Si02 catalyst prepared by evaporation techniques. The model R u / S i 0 2 catalyst was prepared by evaporating Ru (at -10“ Torr) onto 100 A of a previously evaporated S i 0 2 film. Both evaporations were done sequentially in the same apparatus, so the catalysts were not exposed to atmosphere during preparation. Larger amounts of catalyst were accumulated by carefully solvent-stripping the catalyst film from a polymer support. The details of this preparation technique and subsequent catalyst characterization will be described in a forthcoming paper.1° After a 1-h H2 pretreatment at 300 OC, the final composition was 12 atom % Ru/Si02 with a total surface area of 28 m2/g of catalyst. Transmission electron micrographs and selective H2 chemisorption analyses confirmed that the Ru was present in discrete Ru crystallites, with diameters of 50-100
0.25 0.05
r A
Kl K2 k(1,l) k(2,l) k(2,2) k(3,l) k(3,2) k(4,1) k(42) 4 5 ~ )
k, kA K,
preexponential
term, kcal
9.7 x lo4 cm 1.6 X IO7 cm-’ 5 x 109 s-l 1 x lo4 cm2/atom s 5 x 1010 s-I 1x cm2/atom s 5 x 10’0 s-1 1x cm2/atom s 5 x 1010 s-I 1x cm2/atom s 5 x cm2/atom s cm2/atom s 4x
12 12 10.0 0.0 2.0 18.0 2.0 2.0 2.0 24.0 9.0 23.0 18.0 17.0 -23.0 4.0 23.0
1x 1x 1x
cm2/atom cm2/atom cm2/atom 0.1 cm2/atom s I x 1013 s-l
K2 K3 k’
k”
4Temperature = 250 ‘C, pressure = 1 atm, Hl/CO = 3.0. chemisorption is decreased by K on Fe catalysts” and that the promoter effectively decreases the hydrogenation rate, as shown by the increased olefin/paraffin ratio (see Table I). In our procedure no other constants are varied to fit the promoted-catalyst data. We see that the promoted catalyst can be fit quite well by this procedure. Certainly this procedure supports the idea that potassium functions as a promoter by reducing hydrogenation rates. The parameters characterizing the Ru catalyst are collected in Table 11. Note that the efficiency in recycling through ethylene is high. Also, although K promotion reduces H and thus rH,this effect leads to an increase in r0.Perhaps a final point should be made concerning catalyst characterization by using the parameters of Table 11. Basically, the r values (only two independent variables) and the percent C2H4 desorbed can describe a catalyst. The other variables, such as heats of adsorption (except to describe K promotion), C O dissociation rate, and the hydrogenation rates leading to CH,, are the same for each catalyst in this comparison. Thus, a simplified final description of the catalyst can be presented.
Discussion One conclusion of this paper concerns the role of alkali promoter in reducing the hydrogenation rate on the catalyst surface. This effect was simulated just by changing the amount of surface hydrogen. Other possible effects such as changes in the C O decomposition energy leading to changes in surface carbon and thus surface hydrogen are certainly affected by the alkali. Thus hydrogenation ability seems to be the result of a complex change induced by the alkali. The deviations from Schulz-Flory distributions for C2hydrocarbons and C3 oxygenates have been taken as evidence for a CO-insertion mechanism.’ This procedure is certainly supported by these calculations. There is another point worthy of mention. Ethylene does not react significantly with CO to give hydrocarbon over these catalysts. Otherwise the product rate of C3 oxygenate would not be low. Yet a significant amount of adsorbed ethylene must be getting back into the reaction pathway or else the Schulz-Flory plots would resemble the case of all C2H4desorption as computed in Figure 6. This plot shape is not found experimentally. Thus, we are forced to the conclusion that adsorbed ethylene reacts preferentially with a CH, species to give another ~
(10) Monnier, J. R ; Preuss, D R.; Olin, G. A,, manuscript in preparation.
(1 1) Arakawa, H.; Bell, A. T. Ind. Eng. Chem. Process Des. Deo. 1983, 22, 97
J. Phys. Chem. 1986, 90, 2949-2956 species that can insert CO and form oxygenate. Such a pathway might be
C2H4
-
+ CH2
CH2-CH2-CH2
co
H
C4 oxygenate
(13)
Such a reaction might also occur under hydroformylation conditions on heterogeneous surfaces. It is knownM that when ethylene is added to CO/H2 reaction mixtures, several pathways operate, including 2CH2
C3H4
+
I
Hydrogenation to ethane is particularly dominant on some catalysts, and breakage of C-C has led to increased amounts of CH4 from the reaction.4d Incorporation of olefin into the growing chain is observed;however, this reaction is stated to decrease as the olefin molecular weight increases.4d This conclusion is in line with our model assumptions.
2949
A number of important and unanswered questions are pointed out from this work. We know nothing about the relative reactivity of various CH, species with CO, olefin, or alkyl groups on a given surface. For kinetic analysis we have assumed CH, reactions with CH2 or CO as the dominant pathway to precursors of two-carbon products. Although this proposal may be reasonable, there are no separate data to support it.
Summary We have constructed a model of oxygenate and hydrocarbon formation that describes well the observed kinetics from CO/H2 reaction measured on a model Ru catalyst. Deviations of C2 hydrocarbon and C3 oxygenates from the Schulz-Flory relation are explained in terms of ethylene formation and reincorporation into larger hydrocarbon fragments. The role of K promoter in increasing oxygenate selectivity and decreasing rates of product formation is explained in terms of a lower surface H concentration. Some general effects of reactant variables have been investigated to show how these affect product formation. We believe this formulation is general and can be applied to a variety of catalyst formulations. Acknowledgment. We are grateful to D. R. Preuss for preparation of the catalyst samples. Registry No. CO, 630-08-0; Ru, 7440-18-8.
Thermal Decomposition of Benzene on the Rh( 111) Crystal Surface B. E. Koel, Cooperative Institute for Research in Environmental Sciences and Department of Chemistry, University of Colorado, Boulder, Colorado 80309
J. E. Crowell,+B. E. Bent, C. M. Mate, and G. A. Somorjai* Materials and Molecular Research Division, Lawrence Berkeley Laboratory and Department of Chemistry, University of California, Berkeley, Berkeley, California 94720 (Received: October 22, 1985)
Benzene decomposition on the Rh( 111) crystal surface has been studied over the temperature range of 300-800 K by high-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD),and low-energy electron diffraction (LEED). Our results show that benzene decomposition begins at 400 K, forming a mixture of CH and C2H species. The relative amounts of these two species vary with temperature; at 470 K, the concentration ratio of CH to C2H is 0.4. Above 500 K, these fragments dehydrogenate and condense to form C,H polymers. By 800 K, the adsorbed monolayer is completely dehydrogenated, forming a polymeric carbon monolayer with a vibrational spectrum similar to that of an ordered graphite monolayer. We propose that benzene decomposition on Rh(ll1) proceeds by decyclotrimerization to form three acetylenes which immediately decompose to CH and C2H species.
Introduction Surface science studies have revealed that, during catalyzed hydrocarbon conversion reactions over metal surfaces, the active catalysts are partially covered with hydrocarbon fragments with a characteristic H/C atomic ratio.' Catalyst deactivation occurs when this layer dehydrogenates completely to form a graphitic overlayer. Consequently, it is important to elucidate the nature of the stable hydrocarbon fragments that might be present on metal surfaces during catalytic reactions throughout the temperature range employed in catalytic reactions. Chemisorption studies of hydrocarbon molecules on rhodium single crystal surfaces are of fundamental importance to the development of a molecular level understanding of catalysis, since rhodium is a versatile catalyst for hydrocarbon reactions. Present address: Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260.
0022-3654/86/2090-2949$01 SO/O
In this study we have explored the decomposition of benzene adsorbed on the Rh( 111) crystal face as a function of temperature in the range of 300-800 K. The sequential hydrogen evolution that accompanies benzene decomposition was monitored by temperature-programmed desorption (TPD). The disordered carbonaceous fragments produced on the surface were studied as a function of decomposition temperature by vibrational spectroscopy using high-resolution electron energy loss spectroscopy (HREELS). We propose structural models for the benzene decomposition fragments and suggest a mechanism for benzene decomposition on Rh surfaces. In addition, we have explored the decomposition of C2, C3, and C4 alkenes and alkynes on Rh( 11 1) and have found that, above 500 K, the adsorbed decomposition products for these hydrocarbons are the same as for benzene on this surface.2 We know of no previous studies that identify the ( 1 ) Zaera, F.; Somorjai, G. A. J . Phys. Chem. 1982, 86,3070.
0 1986 American Chemical Society
