Kinetics of Oxygen Titration by
CO on Rh
dicates that HNO species are less numerous than NH* species. A greater reactivity of these last species may also account for the N2/N20 ratio. From eq 1-3, which are equivalent t o those considered for isobutane oxidation under the same conditions,l, it may also be seen that NH, consumption is proportional to the generation rate of holes created by the UV photons. At least one photohole is required to transform one NH, molecule, which means that the quantum yield cannot exceed 1. From the mere photonic point of view, this upper limit can be reached only if all photoholes are involved in neutralization of 0- species. 0; species, which also exist at the surface of UV-irradiated Ti02,7J2,19exhibit a reactivity much smaller than that of 0- species,20 so that their interaction with holes is less probable.21 Because of the coverage in negative oxygen species the surface is depleted of electrons and the recombination of electron-hole pairs is negligible. In conclusion, the mechanism which is tentatively proposed involves two types of oxygen adsorbed species: a first one, dissociated, activated by the photoholes, and responsible for the attack of the adsorbed NH, molecules; a second one, molecular, which oxidizes the resulting NH radicals into HNO intermediates whose concentration is supposed to be independent of NH3 pressure at the stationary state.
Acknowledgment. The authors thank Dr. F. Juillet who initiated this study while being adviser for the thesis of one of us (H.M.).
The Journal of Physical Chemistry, Vol. 83, No. 17, 1979
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References and Notes (1) S. Z. Levine and J. G. Calvert, Chem. Phys. Lett., 48, 81 (1977). (2) G. Gopalarao, Z . Pbys. Chem. A , 184, 337 (1939); G. Gopalarao and Ch. 1. Varadanam. J . Indian Chem. Soc., 18, 361 (1941). W. R. Mc Lean and M. Richtie, J . Appl. Cbem., 15, 452 (1965). I. E. Den Besten and M. Qasim, J . Catal., 3 , 387 (1964). A. V. Alekseev, D. V. Pozdnyakov, A. A. Tsyganenko, and V. N. Filimonov, React. Kinet. Catal. Lett., 5, 9 (1978). F. Juillet, F. Lecomte, H. Mozzanega, S.J. Teichner, A. Thevenet, and P. Vergnon, Faraday Symp., 7, 57 (1973). M. Formenti, F. Juillet, P. Meriaudeau, and S.J. Teichner, Cafal. h e . Int. Congr., 5tb, 7972, 2, 1011 (1973). N. Djeghri, M. Formenti, F. Juillet, and S.J. Telchner, Faraday L)iscuss., 58, 185 (1974). M.-N. Mozzanega, J.-M. Herrmann, and P. Pichat, Tetrahedron Lett ., 2965 (1977). H. Courbon, M. Formenti, and P. Pichat, J . Phys. Cbem., 81, 550 (1977). H. Mozzanega, These CNAM, Lyon, France, 1975. J.-M. Herrmann, J. Disdier, and P. Pichat, “Proceedings of the 7th International Vacuum Congress and 3rd International Conference on Solid Surfaces”, Vol. 11, R. Dobrozemsky et ai., 1977, p 951. J.-M. Herrmann, J. Dlsdler, M.-N. Mozzanega, and P. Pichat, presented in part at the 6th North American Meeting of the Catalysis Society, Chicago, Ill., March, 1979, J . Catal., in press. N . I. Il’chenko and G. I. Golodets, J . Catal., 38, 57 (1975). M. Primet, P. Pichat, and M.4. Mathleu, J . Pbys. Cbem., 75, 1221 (1971); G. D. Parfitt, J. Ramsbotham, and C. H. Rochester, Trans. Faraday SOC., 87, 841 (1971); N. D. Parkyns, “Symposium on Chemisorption Catalysts”, Institute of Petroleum, London, 1971, p 150. J.-E. Germain and R. Perez, Bull. SOC. Chim. Fr., 2042 (1972). N. Giordano, E. Cavaterra, and D. Zema, J . Catal., 5, 1325 (1966), and references cited in ref 16 and 18. J. Zawadski, Faraday Discuss., 8 , 140 (1950). S. Fukuzawa, K. M. Sancier, and T. Kwan, J. Cafal., 11, 384 (1968). J. H. Lunsford, Catal. Rev., 8, 135 (1973). J.-R. Martin, ThBse, Lyon, France, 1978.
Kinetics of Oxygen Titration by Carbon Monoxide on Rhodium Charles T. Campbell,+ Shei-Kung Shl, and J. M. White” Department of Chemistry, University of Texas, Austin, Texas 78712 (Received February 2, 1979) Publication costs assisted by the Robert A. Welch Foundation
Data for the low pressure transient kinetics of oxygen titration by carbon monoxide on polycrystalline Rh are reported for temperatures between 360 and 779 K. The time dependence of CO pressure and COz evolution are followed when a constant flux of CO is introduced onto a Rh wire predosed in oxygen. The results of a variety of simulations are compared with these experimental transients and it is shown that all the data are satisfactorily accounted for by a complex Langmuir-Hinshelwood reaction path. The model involves three features: (1) an activation energy which depends on oxygen coverage below 529 K, (2) the inhibition of CO adsorption by oxygen coverage above 529 K, and (3) a drastic increase in the activation energy around 529 K.
I. Introduction As one of the simplest examples of a heterogeneous catalytic reaction, the oxidation of carbon monoxide over transition metals has enjoyed a great deal+of careful investigation aimed a t achieving a fundamental understanding of the kinetic and mechanistic details of the reaction pathway. As yet, however, these studies have failed to produce a fully coherent picture of the process. A case in point is the confusion over whether the production of COz occurs via an Eley-Rideal (ER) or a Langmuir-Hinshelwood (LH) combination of CO and oxygen.l-19 The majority of these studies have been of two types: (1)those providing qualitative features over a large National Science Foundation Trainee.
range of experimental variables (temperature, pressure, coverage);l-1° and (2) those providing quantitative determinations of the kinetic parameters over a narrower range of ~ariab1es.ll-l~ In this paper we attempt, by means of simulation, to combine these qualitative and quantitative results into a coherent model which is suitable over a wide range of temperatures and pressures. In this approach various kinetic models are numerically solved in an effort to accurately simulate the results of experimental measurements and furnish predictions at conditions far-removed from those used to determine the parameters. In this report we present a reaction model which adequately simulates the transient COz production rates during the titration of preadsorbed oxygen on Rh over a range of
0022-3654/79/2083-2255$01 .00/0 0 1979 American Chemical Society
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The Journal of Physical Chemistty, Vol. 83, No. 17, 1979
conditions in which the CO coverage and the rate constant vary by seven orders of magnitude. In a previous study on Rh it was shown that the LH reaction between coadsorbed oxygen and CO proceeds, for 360 K IT I 500 K when the coverages (0, and 0,) exceed about 0.1 (coverages (0) are defined relative to their maximum values, N-), via a path whose activation energy decreases with oxygen coverage." It was also shown that some very interesting features, previously ascribed to the ER mechanism, were qualitatively accounted for by this LH model. A major driving force for the present investigation has been to determine if this same LH reaction path alone can describe the features of the oxidation reaction at higher temperatures where CO coverages are negligible and the ER mechanism was previously though to predominate.' These results bring to light characteristics of the reaction which have left previous investigators with conflicting opinions on the reaction mechanism.
11. Experimental Section The apparatus, sample, and experimental techniques were the same as used in a previous study.'l The ultra-high vacuum chamber, typical of those used for flash desorption spectroscopy and kinetic studies, was equipped with a mass spectrometer and ion gauge. The chamber pumping was conductance limited with S/V = 1.9 s-' for 02. The polycrystalline Rh wire was resistively heated, and temperatures were measured by an attached thermocouple. In a typical experiment, clean Rh at the desired temperature was exposed to 25 L (Langmuir = lo4 torr s) of oxygen, giving a coverage of 0.83.11 Then CO gas was immediately introduced a t a constant leak rate, which was determined from the final steady-state CO pressure (Pco"). The transient responses of m / e 28 and 44 were followed in identical experiments with the mass spectrometer, and, from these, the pressures of CO (PCo(t)) and C02 (PCoz(t)) were determined a t time (t). The COz production rate, Rco,(t), was determined throughout the titration from the increase in COz pressure above background. 111. Simulation Techniques In order to test the applicability of a number of kinetic models to the present data, the rate equations defining the models were solved numerically by standard techniques to provide Rco,(t) and Pco(t) for comparison with the experimental resultsa20 The numerical solution requires initial values for Oo and OcO, and the input leak rate of CO(g), Ri,(t). It takes into consideration the system response time by calculating Ri,(t) from P,(t) multiplied by the pumping speed, where P,(t) is determined from a background experiment in which CO is introduced in the absence of an absorbing Rh surface. A difference between P,(t) and Pco", only observed for times less than 1 min, arises from a combination of effects which we will refer to as system response time and which include the time to open the leak valve, the apparatus response time, and wall effects. The kinetic models were all variations of the general form:
RaCo= kaPco(l - Oco - ~ 0 0 )
(111.1)
RdCo = k&Co
(111.2)
Rcoz = kLHOoOCo = -dOo/dt
(111.3)
Here, RaCoand RdCoare the adsorption and desorption rates for chemisorbed CO (CO(a)). Oxygen desorption is not included in the model because it is negligible at the temperatures and oxygen coverages under considera-
C.T.
Campbell, S.-K. Shi, and J. M. White
It was verified experimentally that JomRCOz d t was constant over the range of temperatures here (360-779 K) so that penetration of oxygen into the bulk is unimportant, just as on Pd.18 The adsorption rate constant, k,, was calculated by assuming an initial sticking probability for CO of unity, determined from analysis of an adsorption transient at 360 K. This value is in reasonable agreement with previous r e s ~ l t s ' , ~and ~ - ~is~ confirmed by the simulation results below. The variable parameter, x, is a measure of the extent to which adsorbed oxygen inhibits CO adsorption. The CO desorption rate constant is assumed to be of the form:
Values of Ad = 3.15 X s-' and Ed = 31 kcal mol-' were used because they agree with flash desorption r e s ~ l t s ~ ~ - ~ ~ and predict an equilibrium CO coverage (Ocoeq) of 0.72 at 442 K with Pco = 5.25 X torr which agrees with a value measured by adsorption transient and subsequent flash desorption in the present experiments. In one model, however, Edis allowed to decrease with oxygen coverage via E d = E d 0 - PO60 (111.5) where E d 0 = 31 kcal mol-'. The Langmuir-Hinshelwood rate constant, kLH,was assumed to be of the form:
where the activation energy was allowed to vary as E L H = Eo - CYOOO (111.7) In this situation kLH can be written in terms of a coverage-independent factor (111.8) and a coverage-dependent factor, exp[aoOo/(RT)J. Thus (111.9) For a given temperature, three parameters (ko(T),a0,and x , or Po) were varied to provide the best fit to the data as judged by visual comparison of the Rco,(t) and Pco(t) curves. Experimental error limits were considered in this fitting process, and special attention was paid to key kinetic features such as induction time and completion time (when Oo I0.05). After determining values for ko(T) at every temperature, they were fit to eq 111.8 in order to determine ALH and E,. As described below, it was found that eq 111.8 only holds for limited temperature (or CO coverage) ranges. Monolayer coverages (Nmax) of CO(a) and O(a) were assumed to be 5 x 1014particles cm-2 in agreement with the present results and only slightly larger than previous values4p5near 4 x 1014particles cm-2. Mass balance was also required in the calculations:
Reo,
(111.10)
where RpumpCoa(S/ V)Pco is the rate of CO pumping, Tg
Kinetics of Oxygen Titration by CO
The Journal of Physical Chemistry, Vol. 83, No. 17, 1979
on Rh
2257
TABLE I: Kinetic Models -
Po, kcal model
mol-L
X
a,, kcal
mol-'
E,, kcal
A L H , moles-'
cules
mol-'
problems
A
0
0
4.4
5.71 X
lozo
14.3
B
1
0
0
1.38 x
loz6
25.0
c
0
5.0
0
2.40
D
1 above 529 K 0 below 529 K 1 above 529 K 0 below 5 2 9 K
0
0
1.38 X l o z 6
25.0
0 0
0 4.4
1.38 X 5.71 X
loz6 lo2'
25.0 14.3
E
not enough inhibitionabove 529 K, rate t o o small above 529 K rate far too low a t 360 K, too much inhibition below 529 K too much induction period a t 529-624 K, rate too small at 360 K rate far too low a t 360 K
24.5
X
none time/min
5
-
~
L
,*_,._..-..
529K
- ..-,.
4
b +
9
-. Q
3
8
2
v
a
I
0 5
-
4
L
& -
c:
3
9 \
2
V
a 1 0
c ,
0
I
2
1
,
I
4
I
6
,
,
8
I
, '
10
tirne/min.
Figure 2. (a) Experimental CO pressure transients which accompany the results of Figure l a . I n these experiments, P,," = 3.8 X lo-*, 4.6 X lo-', 5.0 X lo-', 3.6 X IO-', 3.6 X and 3.6 X lO-'torr, respectively, in order of increasing temperature. (b) Pco( t ) curves simulated by model E. TIMf-lmin.
Figure 1. (a) Experimental COP production curves for 360-779 K. Monolayer = 5 X I O l 4 cm-2. (b) Theoretical simulations by model E of Table I. Note that models A and E are identical below 529 K, and models B, D, and E are identical above 529 K. At 529 K, model E uses the high temperature parameters only with an intermediate value of x = 0.5.
is the gas temperature (300 K), and k B is Boltzmann's constant.
IV. Results Figures l a and 2a show the experimental Rco ( t ) and ~ c o ( tcurves, ) respectively. Several qualitative {eatures which guided our choice of trial kinetic models can be noted. These features have been discussed in detail,20and we present here only the conclusions. First, the sticking probability for CO, even in the presence of large amounts of O(a), remains near unity at 360 and 442 K until CO(a) begins to accumulate. This implies that x is near zero at these temperatures. Second, the induction times (time at which RcOzmaximizes) for the 624-779 K experiments greatly exceed the system response time (-1 min) and accepted forms of the ER mechanism are ruled out. According to the LH model, there is some induction time because CO(a) must accumulate for the reaction rate to maximize. This time, however, was shown by simulations in which the instrument response time was artificially set
t o zero (P,(t) = Pcomfor all t ) to be less than 1s above 529 K, far too low to explain the measured induction times of up to 5.5 min. We are therefore forced to make some additional assumption to explain the observed induction times here. The simplest assumption, which preserves the form of a LH model, is that adsorbed oxygen inhibits CO adsorption, either directly (x > 0), or indirectly by affecting the desorption rate (Po > 0). The simulation analysis is summarized in Table I, in which a number of kinetic models are presented along with a brief description of their associated problems. Although numerous trials were made, Table I shows only selected attempts for the given kinetic assumptions (x, Po, or cyo nonzero). Only model E was satisfactory throughout the entire temperature range investigated. The RcoJt) and Pco(t) curves generated with model E are shown in Figures l b and 2b for comparison with experiment. The extent of agreement is very good for the Rcoz curves, perhaps somewhat poorer in Pco(t). Comparison for the PcO(t) curves is complicated by the experimental difficulties of (1)reliable determination of the m l e 28 cracking fraction for C02, (2) determination of the proper mass spectrometer sensitivity for CO vs. COz, and (3) errors in determining the relative maximum coverages of CO(a) and O(a). Nevertheless, the agreement is quite satisfactory. In model E, values for Eo (14.3 kcal mol-'), A L H (5.7 X lozomolecules cm-* s-l), and a. (4.4 kcal mol-')
2258
The Journal of Physical Chemistry, Vol. 83, No. 17, 1979
EXPERIMENT
0
2
4
6
a
C. T. Campbell, S.-K. Shi, and J. M. White
-
IO
t h e / mln.
Figure 3. Comparison of models with experiment at 360 K. Curve D' uses model D, except with zero system response time and k,, X 100. The low rate demonstrated in model D at this temperature indicates the need for a change in E,, as the temperature decreases. The induction time experimentally observed is smaller than predicted by model D'. Model E shows that this could be due to coverage dependence in EM rather than to a contribution from an Eley-Rideal reaction pathway.
t i m e / rnin.
Figure 4. Comparison of models with experiment at 442 K. The failure of model B indicates the absence of oxygen inhibaion at this temperature.
are used below 529 K and these are in excellent agreement with our previous results (14.3 kcal mol-l, 4.7 x 1020 molecules cm-2 s-l, and 4.4 kcal mol-', respectively).'l Above 529 K, however, a dramatic change in the kinetic parameters is required to match the data. Inhibition by oxygen becomes important (x = 1) and the activation energy increases to 25 kcal while its variation with 8, disappears (ao= 0). The need for such a complex model is not readily apparent, but as described below models which used a single activation energy invariably failed. The necessity for such a change appeared at first to confirm our earlier assertion that the ER mechanism is also important, especially at higher temperatures.' However, this conclusion must be reexamined in view of the recent results which provide direct evidence (by molecular beam) that the CO oxidation reaction on Pd(ll1) occurs via the LH path alone.lsJg In addition, a kinetic transition very similar to that indicated by model E was noted!18 In view of these results, we will retain the Langmuir-Hinshelwood kinetics displayed by model E, in spite of its complexity. We should point out that previous results have also indicated a vast similarity between Rh and P d in the kinetic features of this reaction.lA10 In the following paragraphs we show how simpler models failed, and in doing so demonstrate three important points. First, a change in the extent to which oxygen inhibits CO adsorption is necessary. Second, a coverage-dependent effect (such as a. = 4.4 kcal moll1) is needed in the LH description at low temperatures. Finally, a marked increase in ELHoccurs above 529 K. The problems of these simpler models are summarized in Figures 3 and 4 for 7' = 360 and 442 E(, respectively. As noted by comparing the experimental COzproduction rate to the CO collision frequency, there is no significant inhibition to CO adsorption by preadsorbed oxygen at 442 K.20 Thus model B of Table I fails as shown in Figure 4. However, the necessity for inhibitory effects above 529 K has already been shown. Simulations showed that x = 1 was necessary to provide the observed induction times in the high temperature range. In our models it was essential to incorporate a change with temperature in the extent to which oxygen inhibits CO adsorption. Model C was an unsuccessful attempt to indirectly model this effect by allowing the CO desorption rate constant to increase with Bo (Po = 5 kcal mol-l in eq 111.5). This type of formalism fails because the transition (from inhibition to none) occurs at too low a temperature. Returning to curve D' of Figure 3, we can understand the need for the use of a. = 4.4 kcal mol-l at low tem-
peratures. In this simulation a t 360 K, x = 0 so there is no inhibition to the reaction onset due to adsorbed oxygen. Note that the rate constant, hLH, is large enough as evidenced by the proper time scale for the reaction. However, the induction period (2 min) in this model is too large when compared to the experimental result. Doubling hLH only reduces this induction time to 1.5 min, and the maximum rate becomes too large. Clearly, the classical LH reaction simply does not turn on fast enough at 360 K, even in the absence of any oxygen inhibition. This problem was faced previouslylJl and, at first, it was concluded that the ER mechanism (which shows zero induction time past system response) was imp0rtant.l A later study,ll however, pointed out that a LH mechanism in which a. = 4.4 kcal mol-l provides for sufficiently rapid turn-on. This conclusion is confirmed here as shown in curve E of Figure 3. A major effort was not made to optimize the value of a. because of the agreement with our previous results which relied on more extensive low temperature data.ll The trend, however, is clear. We also noted in simulations that maintaining large a. above 529 K is impossible, for it decreases the induction time drastically. The final point which our simulations demonstrated is that a marked change in the activation energy, ELH, occurs near 529 K. This fact is most saliently observed in the almost-successful model D, which is shown in Figures 3 and 4. This model uses a single activation energy throughout, while allowing a change in x from 0 to 1above 526 K. At 529 K, x = 0.5. This model provides very good agreement with experiments at 529 K and above (note that models D and E are identical in this range) and the activation energy here is clearly satisfactory. As shown in Figure 4, kLH at 442 K in model D is still reasonably satisfactory. Yet at 360 K (Figure 3), model D provides a rate constant which is a factor of 100 too low! Curve D' of Figure 3 is model D, except kLHhas been multiplied by 100 (also system response time was forced to zero). The necessity for a slower decrease in hLH from 442 to 360 K is obvious, and the activation energy change from 25 kcal mol-l to 14.3 kcal mol-' below 529 K accounts for the success of model E shown in Figures 1 and 2. V. Discussion There are several comments that should be made concerning the results which are summarized as model E of Table I. The magnitude of ELH,especially at high temperatures, conflicts with previous assertions11i26that this energy corresponds to the barrier to surface diffusion of CO(a). Field emission results on Ir2' which are in
The Journal of Physical Chemistry, Vol. 83, No. 17, 1979
Reactions of Ethylene and 1-Butene over LaCo,N,
agreement with recent calculationszs suggest a much smaller barrier for this process. We suggest that E L H is a measure of the energy required to form some reactive intermediate, perhaps C02*(a).29 The fact that E L H appears to decrease with oxygen coverage a t low temperatures could be the result of considerable inhomogeneity in the oxygen adlayer, perhaps even induced by the titration reaction itself. This possibility has been discussed p r e v i o ~ s l y . ~ ~The J ~ Jobserved ~ existence of several phases of adsorbed oxygen on Rh30331 and various configurations of coadsorbed layers of CO and oxygen on Pd(lll)13J8J9make this a reasonable hypothesis. The change in activation energy could be the result of a phase transformation in the oxygen adlayer. Tucker, in fact, observed a very high-coverage phase of O(a) on Rh(llO), grown only above 570 K, which was inert to reaction with CO(g)30J1below 320 K. A similar change in ELHon P d ( l l 1 ) is though to be brought about by the presence of CO(a), which compresses the O(a) adlayer.l8J9 We should note that significant inhibition of CO adsorption and reaction by O(a) has not to our knowledge been observed on P d or Pt, even at high temperatures. Following the lines of the explanation of this effect offered by Conrad et al.,19 we believe that this could be a result of a slightly smaller spacing between Rh atoms on the surface. In an oxygen adlayer of equivalent (2 X 2) symmetry on Rh there would be less room for the impinging CO molecule to "squeeze" in between oxygens. It is unknown whether the small size decrease of about 0.14 A in 0-0 distance could cause this result. We are presently investigating whether oxygen inhibition also occurs at steady state. Preliminary results are a f f i r m a t i ~ e The .~~ model we support here is derived under the assumption of homogeneous distributions of adsorbates on the surface. The overall complexity of this model is perhaps an indication of significant inhomogeneity in the reactivity of the adspecies. Future experiments with LEED, surface electron microscopies, and/or molecular beam surface scattering could clarify this point.
2259
Acknowledgment. This research is supported in part by the Robert A. Welch Foundation. References and Notes C. T. Campbell and J. M. White, J. Catal., 54, 289 (1978). G. Ertl and P. Rau, Surf. Sci., 15, 443 (1969). T.Matsushima and J. M. White, Surf. Sci., 67, 122 (1977). (a) T. Matsushima and J. M. White, J. Catal., 39, 265 (1975); (b) T. Matsushima, B. A. Almy, D. C. Fop, J. S. Close, and J. M. White, bid., 39, 277 (1975). (5) T.Matsushima, C. J. Mussett, and J. M. White, J. Catal., 41, 397 (1976). (6) R. A. Shigeishi and D. A. King, Surf. Sci., 75, L397 (1978). (7) C. A. Bscker, J. P. Cowin, L. Wharton, and D. J. Auerback, J. Chem. Phys., 67, 3394 (1977). (8) T. Matsushima, Surf. Scl., submitted for publication. (9) H. P. Bonzel and R. Ku, Surf. Sci., 33, 91 (1972). (10) N. W. Cant, P. C. Hicks, and B. S. Lennon, J . Catal., 54, 372 (1978). (11) C. T. Campbell, S-K. Shl, and J. M. White, Appl. Surf. Sci., 2, 382 (1979). (12) T. Engei and G. Ertl, Chem. Phys. Lett., 54, 95 (1978). (13) G. Ertl and J. Koch in "Adsorption-Desorption Phenomena", F. Ricca Ed., Academic Press, New York, 1972, p 345. (14) V. P. Ivanov, 0. K. Boreskov, V. I. Savchenko, W. F. Egelhoff, and W. H. Weinberg, J. Catal., 46, 269 (1977). (15) H. P. Bonzel and J. J. Burton, Surf. Sci., 52, 223 (1975). (16) P. A. Zhdan, G. K. Boreskov, W. F. Egelhoff, and W. H. Weinberg, Surf. Sci., 61, 377 (1976). (17) J. M. White and A. Golchet, J. Chem. Phys., 66, 5744 (1977). (18) T. Engel and G. Ertl, J. Chem. Phys., 69, 1267 (1978). (19) H. Conrad, G. Ertl, and J. Kuppers, Surf. Sci., 76, 323 (1978). (20) C. T. Campbell, S-K. Shi, and J. M. White, J . Vac. Scl. Techno/., in press. (21) P. A. Thiil, J. T. Yates, and W. H. Weinberg, Surf. Sci., 82, 22 (1979). (22) J. T. Yates, P. A. Thiel, and W. H. Weinberg, Surf. Sci., 82, 45 (1979). (23) B. A. Sexton and G. A. Somorjai, J. Catal., 46, 167 (1977). (24) D. G. Castner, B. A. Sexton, and G. A. Somorjai, Surf. Scl., 71, 519 (1978). (25) R. A. Marbrow and R. M. Lambert, Surf. Sci., 67, 489 (1977). (26) T. Matsushima, D. B. Almy, and J. M. White, Surf. Sci., 67, 89 (1977). (27) V. D. Beiov, Y. K. Ustinov, and A. P. Komar, Kinet. Katal., 18, 1448 (1977). (28) G. Doyen and G. Ertl, Surf. Sci., 69, 157 (1977). (29) I. Kobai, M. Senegacnik, and B. Barlic, J. Chem. Phys., 69, 174 (1978). (30) C. W. Tucker, J. Appl. phys., 37,4147 (1968); corrections, 36, 2696 (1967). (31) C. W. Tucker, J . Appl. Phys., 37, 3013 (1986). (32) Y. Kim, S-K. Shi, and J. M. White, J. Catal., submitted for publication. (1) (2) (3) (4)
Hydrogenation of Ethylene and Isomerization of 1-Butene over LaCo,N, K. Soga," T. Sano, M. Sato, and 5. Ikeda Research Laboratory of Resources Utilization, Tokyo Instltute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 227, Japan (Received January 4, 1979) fublication costs assisted by the Tokyo Institute of Technology
It was found that LaCo5 absorbed a considerable amount of nitrogen in atomic form to give LaCo5N, under low pressures above about 250 "C. The rate of desorption of nitrogen below 350 "C was less than 0.016 mmol of N2 (g of L ~ C O ~ N h-' ~ . at ~ ~350 ) -"C. ~ The hydrogenation of ethylene was studied by passing a mixture of ethylene and hydrogen over the Lac05 and LaCo6N, ( n = 0.02,0.05,0.08,0.10,0.13,0.16,0.22,0.28, and 0.33) in the temperature range from -57 to -78 "C. The reaction proceeded at much higher rates over the LaCo5N, than over the LaCo5. The hydrogen-deuterium equilibration reaction was examined during the hydrogenation, but no hydrogen deuteride was detectable. The isomerization of 1-butene was also studied over a similar temperature range over the same catalysts. The reaction was hardly dependent upon the catalyst used.
Introduction Some activated intermetallic compounds of transition metals, such as, LaNi,, LaCo5, PrCo5, CeCo,, etc., are remarkable absorbers of hydrogen both from the standpoints of the amounts of hydrogen which can be absorbed and the rapidity of absorption and des0rption.l The 0022-3654/79/2083-2259$01 .OO/O
physical properties of these compounds and hydrogen systems have been extensively investigated mainly by N e u m a w 2 van Vucht et aL3 and van On the other hand, Wallace et al.576have recently reported that these intermetallic compounds catalyze the reaction of carbon monoxide and hydrogen to form 0 1979 American Chemical Society
