J . Phys. Chem. 1985, 89, 4979-4983
Ti02and to produce a deep reduction of the oxide by the hydrogen adsorbed on the metaLg Conclusions The hydrogen adsorption on Rh/Ti02 catalyst samples (2.5 wt % of rhodium), heated under hydrogen above 573 K, gives for Hz pressures 1 5 torr 'H N M R spectra composed of two lines centered a t -0 and -130 ppm. On the basis of temperature independence of the position of the shifted line and on the dependence of T I values on the H2 pressure, these two lines have been assigned respectively to OH groups on the support and hydrogen adsorbed on the rhodium particles. The present results indicate therefore that the existing c o n t r o ~ e r s yabout ~ , ~ the origin of the shifted line on Rh/Ti02 system should be decided in favor of its assignment to metal-bonded hydrogen. The analysis of the changes in the shifted line position with the amount of adsorbed hydrogen indicates not only the presence of two types of adsorbed hydrogen but also a change of the characteristics of the stronger metal-hydrogen bond with the amount of hydrogen weakly adsorbed on the metal. Outgassing the sample a t 473 K eliminates all the hydrogen retained on the surface of the metal particles.
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Heating the Rh/Ti02 sample under hydrogen at temperatures close to 773 K diminishes the metal ability to adsorb hydrogen by favoring strong interactions between metal and support (SMSI). This capability is recovered by exposing the sample to air. However, the detection of the shifted line in the N M R spectra needs the previous elimination of the strongly adsorbed H 2 0 or O2 molecules. Finally the interaction of the air-exposed sample M H R outgassed at 523 K with H2 at increasing pressures produces the transfer of hydrogen to the support (spillover) once the most energetic metal sites for adsorption are saturated. This spillover is favored at high H2 pressures or by increasing the sample temperature. Outgassing of the sample at room temperature does not eliminate the hydrogen transferred to the support during these treatments. Acknowledgment. We thank the Spain-US. Cooperation Treaty (Project IE 79170) and the FundaciBn Rambn Areces for financial support. We acknowledge Drs. G. Munuera, J. C. Conesa, and J. Soria for stimulating discussions and critical revision of the manuscript. Registry No. H2, 1333-74-0; Rh, 7440-16-6; TiOz. 13463-67-7.
Transferability of the cis- and trans-Dlfluoroethylene Polar Tensors Mozart N. Ramos, B. B. Neto, Departamento de Quimica Fundamental, Universidade Federal de Pernambuco, 50.000 Recife, PE, Brazil
and Roy E. Bruns* Instituo de Quimica, Universidade Estadual de Campinas, 13100, Campinas. SP, Brazil (Received: February 20, 1985)
Ab initio and semiempirical molecular orbital methods are used to determine the directions of the aj/aQ;s of trans-C2H2F2. These directions and the experimental vibrational intensities are used to calculate the polar tensor of this molecule. The experimental polar tensors of cis- and trans-C2H,F2 are found to be almost identical, indicating that the electronic structures of these molecules in the ground state are practically equivalent, taking symmetry differences into account. Both ab initio 9s5p/3s2p and semiempirical MNDO calculations predict similar atomic polar tensors for these molecules.
Introduction Transference of atomic polar tensors' (APT) from reference molecules with the aim of evaluating the infrared gas-phase integrated intensities of other molecules has been the subject of various recent articles.2 Due to the limited amount of experimental intensity data available, statistical methods are not yet feasible for determining reference APT'S which are most suitable for transference to the molecule whose intensities are to be determined. Person and co-workers3have suggested that the fluorine and hydrogen APT's of methyl fluoride are reasonable choices in attempts to calculate the intensities of other fluoromethane molecules. Their calculated intensities for CH2F2,CHF,, and CF4 using these reference APT's are always within a factor of two of the experimental values and for strong bands the agreement is much better. Although the methyl fluoride APT's appear to (1) (a) W. B. Person and J. H. Newton, J . Chem. Phys., 61, 1040 (1974); (b) J. F. Biarge, J. Herranz, and J. Morcillo, An. R. SOC.Esp. Fis. Quim., Ser. A , 57, 81 (1961). (2) (t)W.B. Person in "Vibrational Intensities in Infrared and Raman Spectra , W. B. Person and G. Zerbi, Eds., Elsevier, Amsterdam, 1982, Chapter 14; (b) R. E. Bruns, Y.Hase, and I. M. Brinn, J. Phys. Chem., 84, 3593 (1980); (c) 0. M. Herrera, M. N. Ramos, and R. E. Bruns, Spectrochim. Acta, P a r t A , 39, 1111 (1983). (3) J. H. Newton, R. A. Levine, and W. B. Person, J . Chem. Phys., 67, 3282 (1977).
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permit the semiquantitative determination of the intensities of most molecules containing hydrogen and fluorine atoms, this choice is by no means optimal in the sense of minimizing the deviations between calculated and experimental values for molecules in general. Certainly a theoretical study aimed at establishing criteria for the selection of reference APT's for intensity calculations is welcome. Such a study would increase our understanding of the extent of the validity of transferring APT's and possibly allow the formulation of systematic methods for modifying APT'S to increase the accuracy of theoretical infrared intensity determinations. The most appropriate starting point of this study might well be an analysis of the APT's of cis- and trans-difluoroethylene. The polar tensors of these molecules appear to be equivalent since the APT's of the cis compound are capable of reproducing the experimental vibrational intensities of the trans isomer within propagated experimental error! However, the experimental polar tensor of trans-difluoroethylene has not yet been determined due to lack of experimental information about the directions of the dipole moment derivatives with the respect to the normal coordinates, the @/aQi's. In spite of the high molecular symmetry ~
(4) (a) R. 0. Kagel, D. L. Powell, M. J. Hopper, J. Overend, M. N. Ramos, A. B. M. S. Bassi, and R. E. Bruns, J . Phys. Chem., 88, 521 (1984); (b) R. 0. Kagel, D. L. Powell, J. Overend, M. N. Ramos, A. B. M. S. Bassi, and R. E. Bruns, J. Chem. Phys., 78, 7029 (1983).
0 1985 American Chemical Society
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Ramos et al.
TABLE 1: Fundamental Intensities (km mol-') and Directions of the a$/aQ,'s (Given in Parentheses) of trans-C2H2F2 sym species
descriptionD
BU
C H stretch C H bend C F stretch CF bend sym CH wag torsion
Au
Y,
cm-'
3116 1274 1160 338 873 325
exptb 9.5 f 14.7 f 217.7 f 1.5 f 56.7 f 12.7 f
ris-C2H2F2C 0.2 0.2 4.4 1.5 0.7 1.4
9.9 6.0 205.1 9.1 40.0 9.1
(128') (198') (221') (312')
MNDO
CNDO/2
4-31G
9s5p/3s2p
31.9 (340') 20.5 (190') 214.9 (222') 21.5 (312') 24.9 14.0
35.9 (99") 33.4 (193') 186.2 (218") 9.3 (314') 19.1 16.4
5.1 (63') 10.7 (193') 275.2 (221') 23.8 (310') 81.9 21.2
14.9 (67') 14.2 (186') 337.7 (221') 22.1 (309') 93.7 16.5
c.
a Band assignments given in N.C. Craig and E.A. Entemann, J . Chem. Phys., 36, 243 (1962); N . Craig and J. Overend, J . Chem. Phys., 51, 1127 (1969). bR.O.Kagel, Ph.D. Dissertation. University of Minnesota, 1964. 'Values obtained by transference of P, (cis-C2H2F2)to the trans isomer.
(C,,J of trans-difluoroethylene, the directions of the djj/aQ, are not restricted to orientations parallel to the principal symmetry axes. Although band contour analyses could be used to determine the derivative directions5 a more efficient method might be the calculation of these quantities using quantum chemical approaches.6 Schmeidekamp et al.' have shown that the theoretical directions of the permanent dipole moments of NH,F, NHF,, PH,F, and PHF, determined using (7s3pld/7s3p/3s) and (lOs6pld/7s3p/3s) basis set ab initio wave functions are in excellent agreement with the experimental values even though the corresponding predictions of the magnitudes of these moments were not exceptionally good. One might expect that accurate directions of the @/aQ, for trans-difluoroethylene can also be obtained from wave functions of similar quality. These theoretical directions combined with the experimental magnitudes of the absolute intensities would allow an accurate determination of the polar tensor of trans-difluoroethylene. In summary, a comparison of the experimentally determined polar tensors of cis- and trans-difluoroethylene is presented. A parallel comparison of quantum chemically calculated polar tensors of these molecules is also carried out with the intent of determining whether the theory reflects the similarities (or differences) in the polar tensor elements for the two isomers. Finally, an examination of the values of the quantum chemical contributions, equilibrium charge movement, charge flux, and interference terms,*s9to the dipole moment derivatives is made to increase our understanding of the similarities found for the changes in electronic structure accompanying small vibrations for these two molecules. Calculations Theoretical values of the polar tensors were obtained from ab initio wave functions calculated by using 4-3 1G and 9s5p/3s3p basis sets.I0 The calculations were performed with the HONDO program" on a PDP-10 computer. Semiempirical quantum chemical values were also obtained by the CND0I2and MND0I3 methods. Experimental equilibrium g e o m e t r i e ~ ,given '~ by the bond lengths and angles presented in Figure 1, were used for both cis- and trans-difluoroethylene. All of the polar tensor elements were evaluated with the finite difference approximation ap,/aa = A p , , / h
(0
= x and y )
with atomic Cartesian displacement of f 0 . 0 1
(1)
A from the equi-
(5) M. Diem, L. A. Nafie, and D. F. Burow, J . Mol. Specfrosc.,71, 446 (1978). ( 6 ) M. N. Ramos, J . Mol. Srrucr., 108, 241 (1984). (7) A. Schmeidekamp, S. Skaarup, P. Pulay. and J. E. Boggs. J . Chem. Phys., 66. 5769 (1977). (8) W. T. King and G. B. Mast, J . Phys. Chem., 80, 2521 (1976); W . T. King in "Vibrational Intensities in Infrared and Raman Spectra", W . B. Person and G. Zerbi, Eds., Elsevier, Amsterdam, 1982; Chapter 6. (9) W. B. Person, B. Zilles. J. D. Rogers, and R. G. A. Maia. J . Mol. Stsurf., 80, 297 (1982). (10) T. H. Dunning, Jr., and P. J. Hay in "Modern Theoretical Chemistry'. Vol. 3. H. F. Schaefer 111, Ed., Plenum, New York, 1977, Chapter 1. ( 1 1 ) M. Dupuis, J. Rys, and H. F. King, QCPE. No. 403 (1978). (12) J. A. Pople and D. L. Beveridge, 'Approximate Molecular Orbital Theory", McGraw-Hill, New York, 1970; R. E. Bruns. QCPE. No. 240 (1974). (13) M. J. S. Dewar and W. Thiel, J . A m . Chem. Soc.. 99. 4899, 4907 (1977).
Y
t
/
H4
F5
Figure 1. The Cartesian coordinate system for trans-difluoroethylene. The direction of the ap'/aQ, vector is measured by the angle 8.
librium geometries. Theoretical values of 2.77, 3.01, 3.44, and 3.61 D for the equilibrium dipole moment of cis-difluoroethylene were obtained with C N D 0 / 2 , MNDO, 4-31G, and 9s5p/3s2p basis set wave functions. These values are to be compared with the experimental values of 2.42 D.13 The theoretical intensity values were calculated with the equation
where i represents the ith fundamental vibration and x and y are the Cartesian coordinates defined in Figure 1. The components of the ajj/dQ, vectors in this equation were obtained by multiplying the polar tensor by the appropriate transformation matrices containing information about the normal coordinates, molecular geometry, atomic masses, and permanent dipole moment.I4 Theoretical values of the directions of the ajj/aQ, vectors were obtained from tan 8, =
(a~y/dQi)/(a~Y/dQi)
(3)
Theoretical contributions to the polar tensor elements are calculated by the charge-charge flux-overlap (CCFO) The APT of the ath atom is considered to be a resultant of three electronic contributions p,c4 = P X ( n )(charge) + P x ( a )(charge flux) + PXcD) (overlap) (4) The first term is simply the contribution due to unit displacements of the equilibrium net charge of the a t h atom. The charge flux contribution results from changes in the atomic net charges of all the atoms upon unitary displacements from geometrical equilibrium. The final term represents the quantum mechanical interference terms to the APT and includes commonly known homopolar and hybridization effects." Results Theoretical values of the fundamental intensities and ajj/aQ, directions obtained from the semiempirical and ab initio wave functions are presented in Table I along with the experimental intensity values of trans-difluoroethylene. Included in this table (14) V. W . Laurie and D. T. Pence, J . Chem. Phys., 38, 2693 (1963). (15) G . Riley, S. Suzuki, and W. J. Orville-Thomas in "Vibrational Intensities in Infrared and Raman Spectra", W. B. Person and G. Zerbi, Eds., Elsevier. Amsterdam, 1982, Chapter 8.
The Journal of Physical Chemistry, Vol. 89, No. 23, 1985 4981
Polar Tensor Transferability TABLE 11: Experimental Atomic Polar Tensors for t r a n ~ - C , H , F , ~
pX (C),
[:r.
0.46 0.57 0 (0.50) (0.46) (0)
-0.47 (-0.50)
-0.55 (-0.50)
(0.17) (0.58) y 5 (0)
p X ( F ) =8:-[ (-0.21)
4 :(-0.56) -
-:,'I 0 (0)
(0)
[
0.oi
(0.01,
pXW=
-0.02
0
(0.06) ( 0 )
(0.04) 1-0.02) - y 2
(:,18] 0)
(0) (0) (-0.06) (0) (0) (-0.10) (0) (0) (0.16) XH = 0.063 (0.060,1 xc = 0.51 (0.30)b XF = 0.32 (0.31) a Corresponding values for the cis isomer are shown in parentheses. Units of electrons, e. Calculated from the experimental intensities directions obtained from ab initio (9s5p/3s2p) calculations. 1 e = 1.602 X l O - " C = 4.803 D K ' . & =
are values of the intensities and directions of the a$/aQi obtained by transferring the cis-difluoroethylene hydrogen and fluorine APT's to the trans isomer. Since the cis APT's are capable of reproducing the trans intensities very accurately, reasonable confidence in the directional values calculated with these APT's is justified. CH Stretching Mode. Both intensity and directional values vary considerably with the type of calculation used. Considering the small experimental intensity value, the intensities calculated by using the ab initio wave functions are in reasonable agreement with the experimental values whereas the semiempirical values are too high by a factor of three or more. The directions of d$/aQj calculated with the a b initio wave functions (63' and 67') are almost the same; however, they are much different than the value obtained by transferring the cis APT's (128') which resulted in an intensity value almost identical with the experimental one. The latter value corresponds to an orientation of the d$/aQi which is almost parallel to the C H bonds. On the other hand, the ab initio directions are almost parallel to the C F bonds. These two alternative values correspond to two different interpretations of the significance of the dipole moment derivative contributions involved. The value close to 120' could result from the movement of the equilibrium hydrogen charges and from charge flux terms restricted to the two CH bonds. The 60' values would be expected if charge flux contributions within the C F bonds are predominant. Although, it is not possible to choose between these alternative values based on the calculated results for this mode this uncertainty may not cause serious errors in the evaluation of the APT's of the trans molecule for two reasons. First, since the C H stretching band is weak, errors in the direction of the corresponding a$/aQj value will have a relatively small effect on the polar tensor values. Second, both alternative angles yield similar values for the y component of the 8$/aQi and the errors propagated into the polar tensor values will be localized in polar tensor elements involving the x component of the dipole moment. C-H Deformation Mode. With the exception of the CNDO calculated intensity value, all the theoretical values are in reasonable agreement with the small experimental value of 14.7 km mol-'. Much more striking, however, is the excellent agreement in the calculated directions of a$/dQi, all around 190'. This direction is approximately perpendicular to the C H bonds. CF Stretching Mode. The intensities calculated with MNDO theory and transferred APT'S from the cis isomer are in excellent agreement with the experimental value of 217.7 km mol-'. The ab initio values, overestimate the experimental value by deviations which are even larger than the discrepancy of the C N D O calculated intensity. Possibly the inclusion of d orbitals is necessary to correctly describe the polarization effects in the C F bond. Here we again see the acute dependence of the stretching mode intensity on the basis set used. A 4-31G basis results in a value of 275.2 krn mol-' whereas the 9s5p/3s2p basis predicts an even higher value, 337.7 km mol-l. On the other hand, all the theoretical methods result in calculated directions (-220') which are in close agreement, suggesting that these directional properties are much less sensitive to basis set variations than are the intensity values. Recent MNDO and 9s5p/3s2p basis set results16 for nitrosyl cyanide also (16) M. N. Ramos and B. B. Neto, J . Mol. Strucr., submitted for publi-
cation.
show excellent agreement for the directions of the a$/aQi values whereas the intensity values are very different. It is worth noting that this a$/aQi direction is almost parallel to the C F bonds, suggesting that equilibrium charge movements of the fluorine atoms and charge flux effects restricted to the C F bonds are predominant in determining the dipole moment derivative value. CF Bending Mode. Again all theoretical methods predict essentially the same orientation of the dipole moment derivative for this mode, -310'. This value indicates that the change in dipole moment direction is almost perpendicular to the directions of the C F bonds. With the CCFO model this direction can be explained by large contributions from the movement of the equilibrium fluorine charges. The theoretical intensity values vary considerably depending on the theoretical method used. All values are much larger than the very small experimental intensity value (Table I). Part of the discrepancy may be attributed to errors in the graphical ~ e p a r a t i o nsince ~ ~ this mode of B, symmetry at 338 cm-I is overlapped by an A, symmetry torsional mode at 325 cm-I. However, the total intensity of the overlapped band system is 14.2 km mol-' which is much less than the intensity values predicted by the ab initio and MNDO wave functions leading to the conclusion that these theoretical values are in serious disagreement with the absolute experimental intensity values. At this point it seems appropriate to explain why the calculation of the directions of the a$/aQls appears to be much more precise than the absolute magnitudes of these vectors. The directions are determined by the relative values of changes in the x and y components of the dipole moment for unit displacements in the normal coordinates (see eq 3). If the quantum mechanical interference contributions are small, as suggested later in Tables I11 and IV, and the charge flux effects are predominantly restricted to the C F (or CH) bonds being stretched, the ratio in eq 3 is independent of the equilibrium charge calculated for the displaced F (or H ) atoms and the changes in the charges suffered by the C and F (or C and H ) atoms for the unit displacement. An analogous argument can be made for the bending motions. The absolute values, on the other hand, depend directly on these quantities. This explanation is consistent with the fact that for the three a$/aQi where the different quantum chemical methods predict essentially the same direction, the dipole moment changes have directions which are either almost parallel (for the C F stretch) or perpendicular (for the C H and C F deformations) to the bonds involved in the displacement. Finally, these results do not allow a definite conclusion to be drawn as to the relative merits of the 4-31G and 9s5p/3s2p basis sets in describing changes in the electronic structures accompanying the vibrational distortions of the difluoroethylenes. Both basis sets overestimate the C F stretching and bending intensities as well as the C H wagging intensity. Agreement for the intensities of the other normal coordinates is much better, at least if absolute differences are used as a criterion. However, relative errors in the theoretical values of these intensities are large as is usually the case for bands with small experimental intensities. In the following discussion, charge-charge flux-overlap contributions to the dipole moment derivatives are calculated from the 9s5p/3s2p results, in view of the increased sophistication of this basis set relative to the 4-31G set. A P T s ofthe Cis and Trans Isomers. In Table I1 experimental values of the cis-difluoroethylene APT's are compared with the values obtained for the trans isomer by using the experimental
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The Journal of Physical Chemistry, Vol. 89, No. 23, 1985
---/I
-
.-5 3
J . Phys. Chem. 1985, 89, 4983-4988 intensities for this molecule and the ap’/dQi directions obtained theoretically. For the derivative corresponding to the C H stretch two sets of A m ’ s were calculated, one corresponding to an angle of 128O determined by transference of the cis APT‘Sand the other by using 67O as indicated by the a b initio calculation using the 9sSp/3s2p basis set. Since the two sets have very similar values only one set (67’ set) is presented in Table 11. The largest discrepancy between cis and trans polar tensor elements occurs for dp,/ay, where 0.57 and 0.46 e values are encountered. In general, the differences between corresponding polar tensor elements for the cis- and trans-difluoroethylenes is of the same magnitude as the expected experimental errors in these values. Very similar polar tensors for these molecules are certainly not surprising in view of the fact that the only structural differences for these isomers occurs in the cis-atom interactions; two Ha-F cis interactions in the cis isomer are replaced by the Ha-H and F-F cis interactions in the trans isomer. Now that the experimental APT’s of cis- and trans-difluoroethylene have been shown to be almost identical it is natural to investigate whether quantum chemical calculations also predict practically equivalent APT’s. Tables I11 and IV contain values of the APT’s for both molecules calculated with the 9s5p/3s2p basis set and MNDO wave functions. In addition, the values for the three principle contributions to the APT’s as defined by the CCFO model are presented. Independent of the level of calcu-
4983
lation the APT elements for the trans molecule are almost equivalent to their corresponding cis isomer values. Furthermore this equivalence between polar tensor elements is preserved even if the APT’s are decomposed into their different CCFO contributions. The largest difference observable in these tables, 0.05e, occurs for the ab initio estimates of the overlap contribution to the dp,/dxH value and the total apZ/azFvalue. The above theoretical and experimental results show that the electronic structures and their variations for small vibrations are essentially equivalent for cis- and trans-difluoroethylene if differences in symmetry are taken into account. This result may hold in general for cis-trans isomers. For example, the infrared fundamental intensities of trans-CzHzClz have also been successfully calculated by transference of the AFT’S of its cis isomer.17 These results also suggest the intriguing possibility that quantum chemical results, even at the semiempirical level, may serve as a criterion for choosing experimental reference APT’s to be used in the polar tensor transference procedure. Studies are currently being carried out in our laboratories to determine the usefulness of this criterion for other molecules. Registry No. cis-C2H2F2,1630-77-9; trans-C2H,F2, 1630-78-0. (17) M. J..Hopper, J. Overend, M. N . Ramos, A. B. M. S. Bassi, and R. E. Bruns, J. Chem. Phys., 79, 19 (1983).
An ESCA Study of Alkali Promoter Effects on Slllca-Supported Ruthenium Catalysts Joseph Z. Shyu,+ James G. Goodwin, Jr.,** and David M. Hercules5 Departments of Chemistry and Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261 (Received: March 25, 1985)
An ESCA study has been carried out to characterize the effect of alkali promotion on ruthenium-silica catalysts. Alkali promotion of Ru was found to result in a significant increase in the atomic relaxation energy in addition to a presumed reduction in the work function. While it was not possible to determine the amount, if any, of electron charge transfer from the promoter to the Ru, it was also not possible to rule out such a transfer. The anions associated with the alkali ions, however, appeared to have little effect on the electronic structure of the Ru. The study reported in this paper is one of the most complete investigations to date of the modifcation by alkali promotion of the electronic structure of a supported transition-metal catalyst.
Introduction Alkali metals are known to be promoters in many catalyst systems. For example, potassium is used as a promoter for ammonia synthesis on Fe catalysts,’ for the synthesis of higher alcohols on Cu/ZnO-based catalysts,z and in the Fischer-Tropsch synthesis on Fe3 and Ru4 catalysts. It has been reported that, in the case of potassium ion promoted Fe catalysts, K+ became more electropositive when interacting with the Fe.536 Also, significant reduction of the work function (4) of Fe was observed by doping However, correlation between promoter effects on NH3 with synthesis activity and the decreased work function of Fe catalysts as a result of K+ promotion indicated that the observed promoter effect was not simply related to modifications in the work function of Fe.6 This suggests that other factors such as the electron density in the valence band and the chemical environment of Fe may also be important in alkali promotion. Dry et al.’ proposed charge transfer from alkali promoters to Fe to explain the promotion effect, based on a strengthened carbon-iron bond during the Fischer-Tropsch synthesis. It has been suggested by Broden et aL5 that the major function of alkali promoters is to modify the electronic structure of iron by electron donation. Ozaki proposed K+.536
Current address: Ford Motor Company, Dearborn, MI. *Department of Chemical and Petroleum Engineering. Department of Chemistry.
a similar mechanism for alkali promotion of Ru catalysts.* Recently, Somorjai et aL9 have observed a strong enhancement of electron back-donation of Pt to chemisorbed carbon monoxide caused by potassium promotion. They attributed this phenomenon to increased Pt electron density as a result of potassium promotion. Although the evidence derived from chemisorption and catalytic reaction studies favors the idea of electron donation by alkali promoters, little direct measurement of changes in the electron density of a catalyst induced by alkali promoters has yet been reported . Charge transfer between alkali-metal atoms and transitionmetal catalysts requires that the first ionization potential (IP) of (1) K. Aika. H. Hori. and A. Ozaki. J. Catal.. 27. 424 (1972). (2) G. Natta, U. Colombo, and I. Pasquon in k a ~ a l y s i k Voi. , 5, P. H. Emmett, Ed., Rheinhold, New York, 1957, p 141. (3) R.B. Anderson in “Catalysis”, Vol. 4, P. H. Emmett, Ed., Rheinhold, New York, 1956, p 331. (4) C. H. Yang, J. G. Goodwin, Jr., and G. Marcelin, “Proceedings of the 8th International Congress on Catalysis”, Vol. 4, Verlag Chemie, Wertheim, 1984, pp 263-273. (5) G. Broden, G. Gafner, and H. P. Bonzel, Surf. Sci., 84. 295 (1979). (6) G. Ertl, M. Weiss, and S. B. Lee, Chem. Phyg. Lett., 60, 391 (1979). (7) M. E. Dry, T. Shingles, L. J. Boshoff, and G. J. Oosthuizen, J. Catal., 15, 190 (1969). (8) A. Ozaki, Acc. Chem. Res., 14, 16 (1981). (9) E. L. Garfunkel, J. E. Crowell, and G. A. Somorjai, J . Phys. Chem., 86, 310 (1982).
0022-3654185 12089-4983%01.50/0 , 0 1985 American Chemical Society I
,
