Langmuir 1985, 1, 262-264
262
Table 111. Values of Henry's Constant H4PhClz 9.01 x 10-2 L/g H3PhC~o 1.24 X L/g H3PhCg 2.27 x 10-3L/g
dep. ndence on tail length. N ~ n found n ~ ~ Henry's law in the presence of added electrolyte, but when no additional electrolyte was added, the isotherms exhibited a decreasing slope throughout region I. Since Wakamatsu and Fuerstenau performed their experiments at pH 7 compared to pH 8, here, in their study larger values of the surface charge are possible. Hence as + uI maI
>>
c3
-=
In this case, increasing the tail length will not change the Henry constant-a trend they reported. Finally, in the presence of little or no added salt, eq 57 takes the form
=(
r1
Ps-
)
As- (as + d2
PS- + P N ~ + 2a2
naI
4 k T e ~ exp(
-E +
-)+ Q,
uI
CO
(63)
In this limit, rIis not linearly related to p s and a decreasing slope with increasing ps- will be observed. This corresponds to the findings reported by N ~ n n . ~ ~ Acknowledgment. The research was supported by the U.S Department of Energy, the Robert A. Welch Foundation, Amoco Production Co., Arco Oil and Gas Co., British Petroleum, Chevron Oil Field Research, Conoco, Inc., Dowel1 Division of Dow Chemical Elf-Aquitaine, Exxon Production Research Co., GAF Corp., Getty Oil Co., Gulf Research and Development Co., Mobile Research and Development Co., Shell Development Co., Sun Production Co., Tenneco Oil Co., Texaco U.S.A., Union Oil Co., and Witco Chemical Corp.
Letters Site Specificity in the Room Temperature Deuterium Exchange of Cyclopentane over Pt/CPG Catalysts Robert L. Augustine* and Russell Wesdyk Department of Chemistry, Seton Hall University, South Orange, New Jersey 07079 Received July 20, 1984. In Final Form: December 13, 1984 Several single-turnover (STO) characterized Pt/CPG catalysts were used for the H/D exchange on cyclopentane. The data obtained indicated that polydeuterium exchange, which occurred by way of a multiple a,b-exchangeprocess, took place on those sites responsible for direct alkene hydrogenation in the STO procedure. Monodeuterated products, which are formed by an a-substitution process, are produced by reactions on those surface sites that are not involved in alkene hydrogenation or isomerization. Mechanistic considerations indicate that the a,P-exchangesites are more coordinately unsaturated than are those on which a-exchange occurs. The catalytic H/D exchange on alkanes has been rather extensively studied as a means of determining the nature of the interaction between hydrocarbons and catalyst surfaces.' While the results obtained have provided a considerable amount of information concerning this process, they were not always readily interpretable on the basis of a single type of metal/hydrocarbon interaction. Instead, it has been proposed that at least two different types of sites are present on the catalyst surface.2 It has been suggested that on one of these simple monodeuterium exchange takes place through an a-exchange process while another type of site produces polydeuterated products through a series of a,p exchanges taking place by way of a sequence of 1,Zdiadsorbed intermediates. This difference can most easily be seen by reference to Scheme I in (1) Burwell, R. L., Jr. Acc. Chem. Res. 1969,2,289; Catal. Rev. 1972, 7, 25. Kemball, C. Catal. Reu. 1971,5, 33. (2) Anderson, J. R.; Kemball, C. Proc. R. SOC.London, Ser. A 1954,
223,361. Rowlinson, H. C.; Burwell, R. L., Jr.; Tuxworth, R. H. J.Phys. Chem. 1955,59,226. Inoue, Y.; Herrmann, J. M.; Schmidt, H.; Burwell, R. L., Jr.; Butt, J. B.; Cohen, J. B. J. Catal. 1978, 53, 401.
0743-7463/85/2401-0262$01.50/0
Table I. Pt/CPG Catalyst Characterization Data" % surface sitesb
catalyst type 1 type I1 type I11 types I1 and I11 dispersion
1' 72 7 21 28
2' 64 11 25 36
3c 60 12 28 40
4d 32 22 46 68
5c 28 23 49 72
0.88
0.66
0.49
0.44
0.61
Obtained by using the single-turnover procedure described in detail in ref 5. *Type I11 sites promote direct alkene hydrogenation, type I1 are alkene isomerization sites, and type I sites only absorb hydrogen. These types of sites previously have been tentatively labeled5 3M, 2M,and M6 sites, respectively. '6.9% Pt/ CPG. d6.1%Pt/CPG.
which the generally accepted general mechanism for deuterium exchange' is depicted. Surface sites that have a high degree of coordinative unsaturation such as corners or kinks should more readily accommodate the diadsorbed species formed in step 3. Alternate reversal of this step with deuterium incorporation (step 3) and reformation of the 1,2-diadsorbedspecies with the breaking of other C-H 0 1985 American Chemical Society
Letters
Langmuir, Vol. 1, No. 2, 1985 263 Scheme I D2
e
Table 11. Cyclopentane/D, Exchange Data
20
% exchange
4
catalyst
1 77
2 71
9 4 3 1 1
ds
5 3 4 3 0 3 3
d9 dl0
1 1
3 1
7 5 1
% conversion
2.2
2.4
3.0
dl d2 d3
d4 dS d6
dl
U
bonds accounts for the formation of polyexchanged products in these reactions. Single-atom surface sites such as terrace or face atoms do not have a high degree of coordinative unsaturation and, thus, would be expected to accommodate only the monoadsorbed species formed in step 2. Reversal of this step using an adsorbed deuterium would give the monodeuterated material (step 2). Recent data, however, have shown that there is no difference in the exchange patterns observed on exposure of deuterium and a variety of hydrocarbons to either the flat(ll1) or the kinked (10,8,7) faces of platinum singlecrystal ~atalysts.~ These results indicate that there is no site specificity for CY or CY,^ exchange in these reactions. It must be recognized, though, that these single-crystal studies were run at temperatures of 225-375 OC and, thus, may not be correlatable with previously reported exchange reactions which were run at considerably lower temperatures, usually ambient. We have recently developed a single turnover procedure4 by which it is possible to determine on a dispersed Pt catalyst the number of surface sites that can directly promote alkene hydrogenation (type 1111, as well as the number of those sites on which double-bond isomerization takes place (type 11) and those surface sites that adsorb H2but do not promote any olefin reaction (type I)? While a precise description of the nature of these different types of sites is not possible at this time as a working hypothesis the hydrogenation sites have been equated with the 3M (comer or kink) site designation proposed by Siege1.G The isomerization sites have been labeled as 2M (step or edge) sites and those on which only hydrogen adsorption takes place as 'M (face or terrace) sites. It was further found that changing the reduction temperature and the quantity of metal present gave a number of Pt/CPG (controlled pore glass) catalysts having a wide variation in site densities as determined by the singleturnover procedure.' With these catalysts available, it was felt that a determination of whether there is any site specificity in the room temperature alkane/deuterium
4 2
3 57 5 4 4 4
6 8
4 55
8 1 1 1 5 8 9
5 45 5
3 5 5 3 5
8
9 12
3
7
1.8
3.4
exchange process should be possible. To accomplish this a series of Pt/CPG catalysts having the site densities shown in Table I' were used to promote the room temperature deuterium exchange on cyclopentane using the single-turnover technique! The catalyst (15 mg) was placed in the reactor tube of our singleturnover apparatus415with the reactor modified by the insertion of a tee with a septum between the catalyst and the reactor inlet. A microcomputer-controlled VG Instruments SX200 quadrupole mass spectrometer was interfaced with the gas chromatograph used for on-line product analysis. The system was swept with deoxygenated zero grade He, and a 50-pL pulse of D, (purified by passage through an Englehard Deoxo Unit, a drying column, a purifier, and a filter) was introduced into the reactor to saturate the catalyst surface and the excess swept out of the reactor by the carrier gas stream. This was followed by the injection of 1pL of distilled and degassed cyclopentane into the reactor through the septum. The hydrocarbon was swept over the catalyst, through the gas chromatograph, and into the mass spectrometer where it was analyzed using a 16-chwel specific mass detector set to provide the digital readout and continuous recording of the partial pressures of species with m / e values of 70-80. The background was subtracted from these values and they were then corrected for natural isotopic abundance. The exchange results obtained for reactions run to approximately the same degree of conversion over this series of catalysts are listed in Table II.8 Comparison of the site densities listed in Table I with the exchange data presented in Table I1 shows that there is a direct relationship between the amount of monodeuterated species formed and the relative number of type I sites present. On the other hand, as the amount of the type I1 and/or I11 sites increases the extent of polydeuterium exchange also increases. The present data do not permit a distinction to be made as to whether either or both of these sites is responsible for the observed polyexchange. There is no correlation between any of the exchange patterns and the catalyst dispersions. It is clear from these results, that the previous assumptions2 concerning the presence of different types of sites having different exchange capabilities were valid. Further, if the mechanism shown in Scheme I is correct, these data can also be used to support the premise that the type I1 and I11 sites are more coordinatively unsaturated than are the type I sites. The lack of site specificity noted in the single-crystal-catalyzedexchange reaction3was probably a result of the high temperature used in that
~
(3) Davis, S. M.;Somorjai, G. A. J. Phys. Chem. 1983,87, 1545. (4) Augustine, R. L.;Warner, R. W. J. Org. Chem. 1981, 46, 2614. (5) Augustine, R. L.;Warner, R. W. J. Catal. 1983,80, 358. (6) Siege], S.;Outlaw, J., Jr.; Garti, N. J. Catal. 1978,52, 102. (7) Kelly, K. P. Ph.D. Dissertation, Seton Hall University, South Orange, NJ, 1983.
(8) Various corrections based on the fragmentation pattern of cyclopentane were applied to these data to evaluate the effect that the fragmentation of more highly deuterated species would exert on these values. Such corrections had, at most, only a minimal impact on these data and had no effect at all on the observed trends.
264
Langmuir 1985, 1, 264-266
study as well as the fact that the exchanges were run in an excess of deuterium while in the present work only a limited amount of deuterium was available on the catalyst.
Acknowledgment. The SX200 Mass Spectrometer was
purchased with funds provided by Grant PRM-8112908 from the National Science Foundation. This research was supported by a grant from the Procter and Gamble Co. Registry No. Pt, 7440-06-4;cyclopentane,287-92-3.
Chemisorption of Molecular Nitrogen on Palladium Surfaces at and above Room Temperature Eizo Miyazaki,* Isao Kojima, and Sumio Kojima Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152, Japan Received August 6, 1984. In Final Form: November 27, 1984 The adsorption of nitrogen on polycrystalline Pd surfaces under ultrahigh vacuum (uhv) conditions has been studied at and above room temperature by means of TPTD-MS (temperature programmed thermal desorption mass spectrometry), isotopic exchange reaction, and theoretical calculations of cluster models. It is found that a nitrogen molecule in the ground state easily adsorbs at and above room temperature on the Pd surface if the surface was in advance exposed to the atomic nitrogen produced by a hot tungsten filament, and it is suggested that an atomic nitrogen, which is strongly Pound to the surface layer and negatively charged, may create an active site required for chemisorption of a molecular nitrogen. The interaction of a nitrogen molecule with metal surfaces is of considerable interest for both science and technology, in particular from nitrogen fixation It is well-known that this molecule chemisorbs dissociatively at and above room temperature on the transition d metals such as Ti, Zr,Nb, Ta, Cr, Mo, and W, which are situated in the left region of the periodic table, and that on these metals the chemisorbed nitrogen atoms diffuse into bulk a t higher temperature to form the metal nitrides."" However, the adsorption of a nitrogen molecule does not occur at and above room temperature on noble metals such as Pd and Pt either dissociatively or molecularly and the corresponding metal nitride is not formed even at high temperature.4v6 These general trends for the chemisorption of a nitrogen molecule on various transition d metals are understandable from the potential energy curves calculated by one of the present authorss in which the activation energy for dissociation from a nitrogen molecule is considerably higher on the noble metals. The weak molecularly adsorbed states with desorption energy of 10-60 kJ/mol have been observed at lower temperature on various metals, but when the surfaces were allowed to warm up toward room temperature, the adsorbates simply desorb.49'271321
On the other hand, nitrogen is known to adsorb on Pt, Ir, Rh, Pd after the gas was atomized by a high-frequency di~charge,'~J~ hot W filament,16J7or Pd surface by electron bombardment for more than 1h.l* However, the details of the adsorbate characteristics have not been elucidated. We first report here that the chemisorption of a molecular nitrogen occurs with a desorption energy of 123 kJ/mol at and above room temperature on the Pd surface if the surface was exposed in advanced to the atomic nitrogen produced by a hot W filament and that this chemisorptive site for molecular nitrogen is induced by the presence of the most strongly adsorbed or absorbed nitrogen. A polycrystalline Pd plate (18 X 15 X 0.1 mm, 99.99%) obtained from Johnson-Matthey Ltd.,a tungsten filament for producing the nitrogen atoms in the gas phase, and a mass spectrometer were set in a uhv chamber. The base torr (1torr = 133.3 Pa). The appressure was 3 X paratus used is similar to the one reported by Wilf and Dawson.17 In the TPDS-MS measurements, the temperature of the Pd plate was raised at constant rate of 15 K/s by a IR lamp and monitored by a Pt-Pt/Rh thermocouple which was spot welded to the Pd sample. The sample plate was heated at 1073 K in vacuo and then cleaned by alternate treatment of exposure to oxygen and hydrogen.lg
(1)Leigh, G. J. "The Chemistry and Biochemistry of Nitrogen Fixation"; Postgate, J. R., Ed.; Plenum: London, 1971. (2)Hardy, R. W. A ' Treatise on Dinitrogen Fixation"; Wiley: New York. 1979. - _._ , -(3)Ozaki, A.; Aika, K. Catal.: Sci. Technol. 1981,1, 87. Ogata, Y.; Aika, K.; Onishi, T. Surf. Sci. 1984,140,L285. (4)Broden, G.; Rhodin, T. N.; Brucker, C.; Benbow, R.; Hurrych, Z. Surf. Sci. 1976, 59,593. (5)Miyazaki, E. J. Catal. 1980,65,84 and references therein. (6) Brearley, W.; Surplice, N. A. Surf. Sci. 1977,62,93. (7)Yates, J. T.,Jr.; Madey, T. E. J. Chem. Phys. 1966,43, 1055. (8)Bozso, F.; Ertl, G.; Weies, M. J. Catal. 1977,50,519. (9)Housley, H.; King, D. A. Surf. Sci. 1977,62,93. (IO) Foord, J. S.; Goddard, P. J.; Lambert, k.M. Surf. Sci. 1980,94, 339. (11)Kishi, K.;Roberta, M. W. Surf. Sci. 1977,62,252.
(12)Shigeishi, P. A.; King, D. A. Surf. Sci. 1977,62,379. (13)Hendrickx, H. A. C. M.; Hoek, A.; Nieuwenhuys, B. E. Surf. Sci. 1983,81,135 and references therein. (14)Schwaha, K.;Bechtold, E. Surf. Sci. 1977,66,343. (15)Kiss, J.;Berko, A,; Solymosi, F. Roc. Znt. Conf. Solid Surf. 4th 1980,1, 521. (16)Mimeat, V. J.; Hansen, R. S., J. Phys. Chem. 1966, 70, 3001. (17)Wilf, M.; Dawson, P. T. Surf. Sci. 1976,60,561. (18)Kunimori, K.; Kawai, T.; Kondow, T.; Onishi, T.; Tamaru, K. Surf. Sci. 1976,54, 525. (19)Kojima, I.; Miyazaki, E.; Yasumori, I. J. Chem. Soc., Faraday Trans. 1 1982,78,1423. (20)Obuchi, A.;Naito, S.; Onishi, T.; Tamaru, K. Surf. Sci. 1982,122, 235. (21)Ibbotaon, D. E.; Wittrig, T. S.; Weinberg, W. H. Surf. Sci. 1981, 110. 313.
0743-746318512401-0264$01.50/0 0 1985 American Chemical Societv
