Langmuir 1991, 7, 2508-2513
2508
Chemisorption and Surface Reactions from the Lewis Acid-Base Point of View Peter C. Stair Ipatieff Laboratory and Department of Chemistry, Northwestern University, Evanston, Illinois 60208 Received April 10, 1991. In Final Form: June 14,1991 The concept of Lewis acids and bases is one of the most useful in chemistry. In this paper, modern thinking on this topic is reviewed and the concept is applied to understanding chemisorptionand catalytic reactions on chemically modified molybdenum surfaces. ence in the separated molecules. Absolute hardness, qs, is defined by
T h e Lewis Acid-Base Concept The Lewis acid-base concept is one of the most generally useful classification schemes in chemistry. It is a tool for systematizing reactive molecules and reactive sites on molecules that provides insight into the nature of their reactivity. For an extensive review of the subject the reader is referred to ref 1. Briefly, a Lewis acid is an atom, molecule, or ion that tends to act as an electron acceptor while a Lewis base is an atom, molecule, or ion that tends to act as an electron donor. An acid and base interact by electron donation from the base to the acid. The result of this interaction is the formation of a chemical bond between the acid and base creating an “acid-base adduct”.
+
where (IS - As) is a finite difference approximation to (a2E/aW)z.Softness, Q, is the inverse of hardness, u = l / q . According to Koopman’s theorem -I is the highest occupied molecular orbital (HOMO) energy and -A is the lowest unoccupied molecular orbital (LUMO)energy. Thus “hard molecules have a large HOMO-LUMO gap, and soft molecules have a small HOMO-LUMO gap”.3 Soft species also tend to have high polarizability since this is favored by a small HOMO-LUMO gap. A corresponding property of surfaces, namely “stiffness”, with the same physical basis has been discussed by Feibelman.‘ The electron transfer, AN, associated with acid-base bonding is given to first order by2
-
A :B A:B (1) In its most general form, an acid-base interaction includes any degree of electron transfer (even simple polarization) of nonbonding, bonding, or antibonding electrons from the base to an unfilled nonbonding, bonding, or antibonding electronic state on the acid. The electronic states involved may be localized on a single atom or delocalized over several atoms in a molecule. Usually only closedshell interactions are considered to be of an acid-base type, and usually a pair of electrons is involved. Strictly speaking, the acid-base concept applies only to the incipient reaction, Le., to the initial interaction of reactant molecules as they come together. This follows from the fact that bonding is described in terms of properties of the separated molecules before any appreciable distortion has occurred. Recently Parr and Pearson, using density functional theory, developed the concepts of absolute electronegativity and absolute hardness to quantify the propensity for electron transfer in acidbase interactions in terms of properties of the separated molecules.2 Acid or base strength is expressed in terms of the chemical potential, p, of density functional theory, which can be written
AN = (XAO - x,e)/2(9A
+
(4)
where and XBO are the electronegativities of the separated (undistorted) acid and base. Equations 3 and 4 show that “hardness” is “resistance of the chemical potential to change in the number of electrons”.2 Soft acid-base pairs favor electron transfer. Hard acid-base pairs disfavor electron transfer. Extensive tabulations of absolute electronegativity and hardness for atoms, monatomic cations, and neutral molecules have been compiled by Pearson.6 The data are shown in Figure 1 as a plot of absolute electronegativity vs absolute hardness. I t is evident from Figure 1that x generally increases with charge whereas q remains within a relatively narrow region below 10 eV except for a few cations. As shown by the alkali metals, both x and q tend to increase with decreasing cation ion size. Neutral atoms and molecules are clustered in the region of low electronegativity and hardness; i.e., they are relatively soft and basic. Metal surfaces, while not shown in Figure 1must certainly be located in the lower left corner of the plot. Their HOMO-LUMO gap is zero, giving them a hardness of zero, while their electronegativity is equal to their work function, typically in the range 4-5 eV. The concept of hard and soft was originally introduced by Pearson as a descriptive classification scheme.6 “A soft base [is] one in which the valence electrons are easily XAO
= -(aE/aN), = xS = 1 / 2 ( ~ s+ A,) (2) where E is the total energy, N is the total number of electrons, 2 is the total nuclear charge, and xs, Is, and As are the absolute electronegativity, the ionization potential, and the electron affinity, respectively, of species S. From this point of view, strong acids have large electronegativities while strong bases have small electronegativities. Acid-base bond formation is due to electron transfer driven by equalization of the electron chemical potential differ-pS
(3) Pearson, R. G. R o c . Natl. Acad. Sci. U.S.A. 1986,83, 8440. (4) Feibelman, P. J. Annu. Reu. Phys. Chem. 1989,40, 261. ( 5 ) Pearson, R. G. Inorg. Chem. 1988,27,734; J. Org. Chem. 1989,54,
~~_______
(1) Jensen, W. B. The Lewis Acid-Base Concepts-An Overview; Wiley: New York, 1980. (2) Parr, R. G.; Pearson, R. G. J. Am. Chem. SOC.1983, 105, 7512.
1423. (6) Pearson, R. G. J. Am. Chem. Soc. 1963,85, 3533.
0
1991 American Chemical Society
Langmuir, Vol. 7, No. 11, 1991 2509
Chemisorption and Surface Reactions
-
50
I
I
>
o Mg2+
O'Sc3+
0 0 0
Lit
Y
n
1
lecular orbital m. firs = J4r(i)H14a(i)d7i is the resonance integral of the perturbation Hamiltonian, HI, between atomic orbitals r and s. E m is the energy of molecular orbital m. The third term
I
l
rs
. . A
v)
J3
0
Q
0
0 ,
0
20
10
30
1 + CATIONS 2 C CATIONS. 3 C CATIONS
40
Absolute Hardness (eV)
Figure 1. Absolute electronegativityvs absolute hardness.
distorted-polarized-or-removed. A hard base has the opposite properties, holding on to its valence electrons much more tightly. We also define a hard acid as one of small size, high positive charge and with no valence electrons that are easily distorted or removed .... A soft acid is one in which the acceptor atom is of large size, small or zero positive charge, or has several valence electrons which are easily distorted or removed".6 This qualitative description is clearly consistent with absolute hardness-softness as defined by eq 3 and the properties of the species represented in Figure 1. The utility of the hard-soft classification is derived from a qualitative empirical observation sometimes called "Pearson's principle": hard acids prefer to bind to hard bases and soft acids prefer to bind to soft bases.' A hard-soft pairing is unfavorable. The fundamental origin of Pearson's principle was elucidated by Klopman8and extended by Jensen.' They considered the bond energy of an acidbase pair as a perturbation on the energy of the separated molecules in order to examine the nature of the interactions involved. Note this approach is in harmony with the fundamental principle of the concept of acids and bases that bonding can be understood in terms of properties of the separated molecules. The perturbation energy is written in three terms: a w r t = @h + @rb + m e p l (5) an electrostatic term, M c h , an electron-transfer (covalent bond) term, hEorb,and a repulsive term, AErepl, due to electron-electron repulsion. The electrostatic term is given by r.8
where the sum is over all pairs of atoms, rs, of charge Q in the acid and base. The electron-transfer term is given by
where the summation is over all interacting pairs of occupied, m, and unoccupied, n,orbitals and over all pairs of atoms rand s on the acid and base. C r is the coefficient of atomic orbital r in the LCAO representation of mo(7) See, for example: Pearson, R.G. Hard and Soft Acids and Bases; Dowden, Hutchinson and Roes: Stroudenburg, PA, 1973. Pearson, R.G. In Bonding Energetics in Organometallic Compounds;Marks, T. J., Ed.; ACS Symposium Series 428;American Chemical Society: Washington, DC, 1990,p 251. (8)Klopman, G. J. Am. Chem. SOC.1968,90, 223.
m
n
is due to Jensenl and represents the repulsive interactions between occupied orbitals. Sr8 = J&(i)&(i) d7i is the overlap integral. It is clear from eqs 6 and 7 and the discussion above that the hard-hard interaction is electrostatic bonding and the soft-soft interaction is covalent bonding. Therefore, absolute hardness and softness are simply measures of the propensity for electrostatic and covalent bonding between acids and bases. Equation 7 also serves to guide our thinking in an assessment of adsorbate-surface interactions that determine the energy of covalent chemisorption bonds. Since unoccupied orbitals are generally higher in energy than occupied orbitals (E, > E m ) , bonding is favored by low values of E, and high values of E m . Often, only the frontier orbitals are considered. Then E m is identified with the highest occupied molecular orbital (HOMO) of the base and E n is the energy of the lowest unoccupied molecular orbital (LUMO) of the acid. The interacting orbitals must also overlap in order that & be appreciably greater than zero, and two aspects of orbital overlap should be noted. First, the interacting orbitals must be directed properly and be of the proper sign for positive overlap. Second, the magnitude of the overlap integral will be maximal for orbitals of approximately the same size.s This latter property may also figure into the soft-soft and hard-hard preference. In a soft-soft interaction both donor and acceptor orbitals will be relatively delocalized. In a hardhard interaction the orbitals will both be more localized. Thus, for soft-soft and hard-hard interactions, the size of donor and acceptor orbitals is similar. There is an orbital size mismatch associated with a hard-soft interaction that lowers the covalent bond energy.
Application to Surfaces The concept of Lewis acids and bases applied to surfaces has been codified into a set of qualitative rules to describe the physical and chemical properties pertaining to metals chemically modified by incorporation of electronegative elements (0,C, N) into the surface region.1° (1)Electronegative modifiers produce electron-deficient metal atoms (cations) and electron-rich adatoms (anions). (2) The degree of electron transfer away from surface metal atoms increases with modifier coverage and/or modifier electronegativity. (3) The surface polarizability decreases with increasing modifier coverage as the surface region transforms from metal to metal compound. (4) The metal cations are Lewis acid sites, and the adatom anions are Lewis base sites. (5) The strength of acid sites increases with modifier coverage and/or modifier electronegativity. The strength of base sites decreases with modifier coverage. ( 6 ) The acid sites become harder in character with increasing modifier coverage. The first three rules describe the changes in surface electronic structure that are responsible for the appearance (9)Stair, P. C. J. Am. Chem. SOC.1982,104, 4044. (10)Stair, P. C. In Bonding Energetics in Organometallic Compounde, Marks,T. J.,Ed.;ACSSympoeium Series428;AmericalChemicalSociety Washington, DC, 1990; p 239.
Stair
2510 Langmuir, Vol. 7, No.11, 1991 Table I. M0(3d~/~) Surface Binding Enerdes (BE) and Oxidation States modifier coverage, 1016 atoms/cmz BE, eV oxidation state 227.5 0.0 clean B 1.1 227.6 0.4 1.0 227.7 0.9 C
8x0 0 0 0 0 0
1.0 0.3 0.8 1.0 1.2 1.4
227.8 227.8 227.8 227.9 228.0 228.4
Molybdenum Oxidation States (Rules 1 and 2) XPS measurements of chemically modified metal surfaces contain contributions from both surface and bulk metal atoms. The component due to surface molybdenum atoms was determined with a procedure established by Citrin et al.,'Qwhich is based on the changing contribution of surface and bulk components to the measured signal as a function of photoemission takeoff angle. The surface molybdenum atom core levels shift to higher binding energy with respect to the clean surface for all adlayers studied as shown by the data in Table I. The surface designated 0-CO was a 50:50 mixture of atomic carbon and oxygen produced by the dissociative adsorption of CO. Three aspects of the results are of significance for understanding the chemistry of the chemically modified molybdenum surfaces. First, the surface Mo core level binding energy increases monotonically with increasing oxygen coverage, indicative of progressive molybdenum oxidation. Second, it is evident that the most rapid increase in molybdenum binding energy occurs between 1.0 X 1015and 1.4 X l O I 5 atoms/cm2 oxygen coverage as indicated by the plot in Figure 2. This is due to formation of a thin oxide layer on the surface.15 The final point of interest concerns the effective oxidation states in Table I assigned to molybdenum atoms (11) Overbury, S. H.; Stair, P. C. J. Vac. Sci. Techno[.1983, AI, 1055. (12)DeKoven, B. M.; Overbury, S. H.; Stair, P. C. Phys. Reu. Lett. 1984, 53, 481. (13) Grant, J. L.; Fryberger, T. B.; Stair, P. C. Surf. Sci. 1985,159,333. (14) Grant, J. L.; Fryberger, T. B.; Stair, P. C. A p p f .Surf. Sei. 1986,
-26. - , 472. - -
(15) Fryberger, T. B.; Stair, P. C. Chem. Phys. Lett. 1982,93, 151. (16) Stair, P. C. Isr. J . Chem. 1982, 22, 380.
(17) Deffeyes, J. E.; Smith, A. H.; Stair, P. C. Surf. Sci. 1985,163,79. (18) Deffeyes, J. E.; Smith, A. H.; Stair, P. C. Appl. Surf. Sei. 1986,
(19) Citrin, P. H.; Wertheim, C. 3160.
K.; Baer, Y. Phys. Reu.
1 " " " ' " 1 ' 1 ' 1 ' 1
1983,828,
?
I
220.3
I
I
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1.3 1.3 1.3 1.8 2.2 4.0
of acidic and basic sites. The identity of these sites, their strength as a function of modifier coverage, and their hardsoft character are governed by the last three rules. Over the past several years we have amassed a body of physical and chemical evidence from chemisorption experiments on Mo(100) surfaces chemically modified by monolayer quantities of electronegative atomic absorbates (B, C, 0, S) to test the acid-base rules. These surfaces were prepared by dosing the clean Mo(100) surface with appropriate precursors (BzHs, C2H4,02, H2S) followed by heating to drive off hydrogen where necessary. Carbon and oxygen coverageswere determined by XPS and Auger measurements calibrated against the saturated 8-CO surface, which consists of 5 X 1014/cm2each of atomic carbon and oxygen formed by dissociative adsorption of CO. Boron and sulfur coverages were obtained by calibration against carbon from measurements of adsorbed B(OCH& and CS2. The results of these studies are summarized in the remainder of this paper. The details of this research can be found in refs 11-18.
26. - - , 517 - - ..
. ,
h
1 14
0.0
0.2
0
Oxygen Coverage (otoms/cm2
16
x 10'5)
Figure 2. Mo(3dsp) binding energy vs oxygen coverage. 2.0,
1
I
1
1.0 MC 0
1.0 ML
1.0 ML
1.1 ML B
0.0
I
1 .o
8-CO
C I
/ I
I
2.0
3.0
4.0
MODIFIER ELECTRONEGATIVITY
Figure 3. Mo oxidation state vs modified electronegativity.
on the chemically modified surfaces. The adlayer a t 1.4monolayer (ML) oxygen coverage is an oxide with a polarizability nearly equal to that of Moo2 (see below), therefore, the molybdenum atoms in the layer are assigned an oxidation state corresponding to this oxide, namely, +4. Effective oxidation states are assigned on the rest of the surfaces by assuming a linear relationship between surface binding energy shifts and oxidation state. An important observation in connection with the assigned oxidation states for different adlayers at constant 1-ML coverage, is the linear increase in surface oxidation state with increasing modifier electronegativity (see Figure 3). Linearity implies a direct relation between the degree of electron transfer frommetal to adsorbate and the adsorbate electronegativity. This relationship can be understood as a consequence of the electronegativity equalization postulate of Sanderson,20which states that when two or more atoms bond to each other electron transfer adjusts their electronegativities to some common intermediate value. This postulate is also consistent with eq 2 and the usual thermodynamic meaning of a chemical potential. The zero oxidation state intercept at an electronegativity of 1.6 instead of the value of 1.8 assigned to pure molybdenum implies that the assigned oxidation states are slightly high. This could arise because the oxide film has not fully developed at an oxygen coverage of 1.4 X 1015atoms/cm2. This interpretation is supported by comparison of the surface relaxation energies discussed below. Surface Polarizability (Rule 3) The shift in measured core level binding energies between a rare gas such as xenon in the gas phase and (20) Sanderson, R. T. J. Am. Chem. SOC.1952, 74, 272.
Langmuir, Vol. 7, No.11,1991 2511
Chemisorption and Surface Reactions
Table 11. Mo( 100) Surfaces Used for Chemisorption
Experiments
surface adatom positions 1.0 ML 0 4-fold hollow 0.3 A 1.2 ML 0 above Mo 1.5 ML 0 disordered 1.0 ML C 4-fold hollow 0.3 A above Mo 0.8 ML S I-fold hollow + bridged 1 A above Mo
Mo oxdn state 1.8 2.2 4.0 0.9
commenta Chemisorbed phase mix of chemisorbed phase and oxide oxide chemisorbed phase
chemisorbed phase
1 8 '
0'0
'
0'2
'
0'4
'
0'6 '
0'8 '
1'0
'
Oxygen Coverage ( a t o m s / c m *
1'2 '
1'4
'
1'6
x 1015)
Figure 4. Surface relaxation energy vs oxygen coverage. adsorbed on a surface results from a combination of chemical shift, local potential at the site of the adsorbate, and stabilization of the photoionization core hole by polarization of the substrate electron density. As discussed in ref 16, the contribution due to substrate polarization is related to the surface electronic polarizability and can be isolated from the other contributions to a good approximation by measurements of the xenon gas-phase and adsorbed-phase "Auger parameter", a. a is defined as the difference between the u k l ) core level Auger electron kinetic energy, KAMerGkl),and the 0') core level photoelectron kinetic energy, KPEG). a I p w e r Gkl) - KPEG) (9) The difference in measured values of a for xenon in the gas phase and on the surface is equal to twice the polarization relaxation energy of the substrate, ELE. A a = a(ads) - a(gas) = 2ELE
(10) The details of this argument can be found in ref 16. Measured values of EiEfor xenon adsorbed on the Mo(100) surface with varying coverages of atomic oxygen are plotted in Figure 4. The relaxation energy and hence the surface polarizability decreases monotonically with increasing oxygen coverage with a pronounced break in the slope at a coverage corresponding to one monolayer. At (1.4-1.5) X 1015atoms/cm2, the surface relaxation energy has nearly reached the value measured for xenon adsorbed on bulk MoO2. The data are interpreted as evidence for (1) the presence of a chemisorbed oxygen phase below 1015atoms/cm2with a surface polarizability characteristic of a metal and (2) the formation of oxide above 1015atoms/ cm2, which is completed at an oxygen coverage of 1.5 X 1015atoms/cm2. Moreover the decrease in surface electronic polarizability must certainly be associated with a reduction in the Fermi level density of states at the surface and perhaps formation of a gap between filled and unfilled states. According to the concept of absolute hardness, such a gap would increase the surface hardness from a value of zero, characteristic of a metal, to a finite value. Associated with this would be a transformation from the very delocalized electronic structure of a metal to a more localized electronic structure characteristic of an oxide.
Acid Properties of Chemically Modified Mo( 100) (Rules 4-6) Temperature-programmed desorption measurements of desorption activation energies were performed for ammonia, dimethyl ether, phosphine, propene, 3,3,3-trifluoropropene, ethene, and carbon monoxide adsorbed on
1 -
1004
0-4
co-
' 8 20
'
i
H
S(O8)
C(1O)
O(10)
O(12)
O(15)
SURFACE MODIFIER ( a t o m s / d x i 015)
Figure 5. Desorption energies for various Lewis bases. the five surfaces listed in Table 11. The desorption spectra were measured a t four heating rates ranging from 3 to 150 K/s. They were analyzed by finding desorption rates and surface temperatures at several common coverageson each desorption curve. These data were used to construct an Arrhenius plot of log (desorption rate) vs 1/T a t each coverage.21 Desorption activation energies and prefactors were calculated from the slopes and intercepts of these plots. No significant coverage dependence of the desorption kinetic parameters was observed. Therefore, a single value of the activation energy is sufficient for purposes of comparing the various molecule-surface combinations. The measured desorption activation energies (surface bond dissociation energies), D(B-Mo*+), are summarized in Figure 5. The Lewis bases, NH3, CH30CH3, PH3, CH3CHCH2, and CH2CH2, exhibit increasingD(B-Mob+) with increasing oxygen coverage. This trend reflects an increase in the Lewis acidity of the surface molybdenum atoms with increasing oxygen coverage and is consistent with the oxidation-state changes measured by XPS. The opposite trend is observed for CF3CHCH2 and CO, suggesting that electron transfer from surface to adsorbate is an important contribution to the bonding. D(B-Mo*+) measured for all the molecules adsorbed on the sulfurmodified surface is much lower than that observed for any of the other surfaces. This effect cannot be explained by electron transfer from molybdenum to sulfur else a similar effect would be observed with the oxygen-modified surfaces. Rather, the weak bonding to sulfur-modified Mo(100) must be due to steric blocking of the molybdenum atoms by the sulfur overlayer. The greater influence of steric blocking with sulfur compared to the carbon and oxygen modifiers can be attributed to its increased size and the larger sulfur to molybdenum layer spacing (see (21)Taylor, J. L.; Weinberg,
W. H. Surf. Sci. 1978, 78, 259.
Stair 7
B
0
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6
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-
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3-
+
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CH3CH=CH2
-
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1
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0
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95 0
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55 0
D(B-Mo+),
D(B-K+),
D(B-Li+),
kJ/mol
kJ/mol
kJ/mol
138.4 118.4
75 87
164 165
base NH3 (CH3)zO
1 0
Table 11). Furthermore, this result suggests that bonding of all the molecules to the modified surfaces involves coordination to molybdenum rather than to the modifier adatoms. Otherwise, bonding to the sulfur-modified surface would be enhanced by the greater accessibility to the sulfur layer. Figure 6 depicts the acid-base properties of the probe molecules employed in our experiments (CF3CHCH2 excepted) as a plot of electronegativity vs hardness. CO is clearly the most acidic of the probe molecules, andD(C0Mob+)exhibits the expected trend for acid-base bonding to the surfaces studied. While there is a rough correlation between base strength as measured by electronegativity and D(B-Mo*+) on 1.5-ML O-M0(100), the correlation is certainly not quantitative or even monotonic since D(NH3Mod+)> D((CH3)20-Mo6++) andD(PH3-Mo6+) > D(C3H6Mod+)are in the wrong order. Electrostatic interactions (hardness) might be invoked to explain the order of desorption energies. However, all of these molecules are relatively soft on the scale of Figure 1, and the value of D(PH3-Mod+)is clearly in disagreement with predictions based on Figure 6. Moreover, the magnitude and order of desorption energies is inconsistent with purely electrostatic bonding. Table I11compares the dissociation energies for ammonia and dimethyl ether bonded to K+ and Li+ (reported in refs 22 and 23) to their desorption activation energies on the 1.5-ML 0-Mo(100) surface. Since the bonding to K+ and Li+ is purely electrostatic, deviations in either the magnitude or order of the desorption activation energies from D(B-K+) or D(B-Li+) are attributable to covalent contributions to the chemisorption bond. Differences in D(B-K+) and D(BLi+) arise primarily because of stronger electron-electron repulsion due to the larger number of electrons in K+. Surface Mo cations should resemble K+ more than Li+ in terms of their size and electron number. However, the bonding of ammonia and dimethyl ether to surface Mo is much stronger than to K+,and the order of bond strengths is opposite that of D(B-K+). These differences are
indicative of a predominant contribution by covalent bonding to chemisorption on the modified Mo(100) surfaces. The desorption activation energies do show a linear correlation with the measured proton affinitiesF4as shown in Figure 7. This correlation confirms the validity of the acid-base model of bonding for these adsorbates. It also suggests that orbital overlap effects are quite important. The Lewis acid sites on all of the oxygen-modified surfaces must be spatially localized and the low D(B-Mo6+) values measured for ethene and propene relative to the long-pair electron donors NH3, (CH3)20, and PH3 reflect the poor overlap between donor *-orbitals and acceptor orbitals in the surface. The slope of the correlation line in Figure 7 decreases with decreasing oxygen coverage, indicating that the surface electronic structure is more delocalizeda t lower oxygen coverages. This chemical trend agrees with the interpretation of surface polarizability measurements discussed above. Figure 7 also shows the correlation between measured affinities for the gas-phase Al+ (filled circles) obtained from r e f e r e n ~ e .As ~ ~is evident from the figure, the correlation curve through the Al+ data parallels the curve through the desorption data. This suggests that the bonding to the surface and to Al+ is similar, and therefore, the Lewis acid-base model correctly describes the bonding in these systems. Comparison of D(B-Mo*+) for the molecules adsorbed on 1.0 X 1015 atoms/cm2 carbon and oxygen-modified surfaces provides a test for the effect of surface electronic structure changes with little or no change in the surface geometry. The activation energies are lower for desorption from the carbon-modified surface than from the oxygen-modified surface. This trend supports the model for surface Lewis acidity and is in agreement with the measured molybdenum oxidation states. The corresponding desorption energy increase observed for PH3 between 0-Mo(100) and C-Mo(100) is not consistent with the acidbase bonding model but may be an artifact of the partial decomposition that accompanies PH3 adsorption on these
R.; Kebarle, P. J. Am. Chem. SOC.1976,98,6133. (23) Woodin, R.L.;Beauchamp, J. L. J. Am. Chem. SOC.1978, 100, 501.
(24) Aue, D. H.; Bowers, M. T. In Gas Phase Ion Chemistry; Bowers, M. T., Ed.; Academic Press: New York, 1979; Vol. 2, p 1. (25) Uppal, J. S.;Staley, R. H. J . Am. Chem. Soc. 1982,104,1235.
(22) Davidson, W.
Langmuir, Vol. 7, No. 11, 1991 2513
Chemisorption and Surface Reactions surfaces; i.e., the surface from which desorption occurs has an altered composition. A similar argument can be put forth to explain why the desorption energy of CF3CHCH2 does not follow the same trend as CO between the carbon- and oxygen-modified surfaces. The role of surface structure has not been considered in the above discussion. No evidence for metal atom restructuring was observed for carbon- and sulfur-modified surfaces or for oxygen coverages below 1 X 1015 atoms/ cm2.11J2 However, for oxygen coverages above this value, restructuring must take place to accommodate the formation of the oxide phase a t 1.4 ML. The influence of such restructuring on the chemical trends with oxygen coverage depicted in Figure 5 is difficult to assess. There are no obvious steric effects that might be reflected by, for example a reversal in the relative desorption energies of CHsOCH3 and PH3. Moreover, the desorption energies for all the probe molecules change in a continuous and monotonic fashion with increasing oxygen coverage. If these progressive changes were dominated by corresponding changesin surface structure, the latter would also have to evolve continuously, which seems unlikely. Methylcyclopropane Hydrogenolysis The physical and chemical properties of the oxygenmodified molybdenum surfaces described above indicate the formation of acidic sites with variable strength and hard-soft character as a function of oxygen coverage. The hydrogenolysis of methylcyclopropane (MCP) was investigated to probe the catalytic properties of these surfaces. A full account of this study appears elsewhere.% MCP may undergo single hydrogenolysis (ring opening) to form n-butane or isobutane or double hydrogenolysis to form either equal amounts of methane and propane or two molecules of ethane. The formation of isobutane is taken as evidence for reaction catalyzed by metal sites whereas n-butane formation is characteristic of acidic sites.n nButane product is not favored by metal-catalyzed hydrogenolysis due to the electron-donating methyl substituent, which strengthens the adjacent bonds and weakens the bond opposite the substituent.2s Production of isobutane by an acid-catalyzed reaction is unfavorable due to the required formation of a primary carbenium ion intermediate.29 MCP hydrogenolysis was carried out over oxygenmodified Mo(ll1) surfaces with varying oxygen coverages. The reaction conditions (5 Torr of MCP; 750 torr of H2; 100 "C) were similar to those reported for MCP hydrogenolysis catalyzed by low-valent Mo supported on alumina.27 The turnover frequencies per surface Mo atom as (26) Touvell, M. 5.;Stair, P. C. J . Catal. 1991, 130,556. (27) Chung, J . 4 . ; Burwell, R. L., Jr. J . Catal. 1989, 116, 519. (28) Ghther, H.Tetrahedron Lett. 1970, 5173. (29) Brouwer, D.M. In NATO AduancedStudy Institue on Chemistry and Chemical Engineering of Catalytic Processes; Prins, R., Schuit, G. C. A., Ede.; Sijthoff & Noordhoff: Alphen aan den Rijn,The Netherlands, 1980; p 137.
0.60
1
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lsobutone Butane
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5 Torr MCP 7 5 0 Torr HZ 373 K
A
Mo(1 1 1 ) A
A
A
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0
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,
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Moo?
I
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1
2.00
0 Concentration ( a t o m s / c m * XI 0") Figure 8. Methylcyclopropane hydrogenolysis turnover frequencies.
a function of oxygen coverage are plotted in Figure 8. Note that for oxygen coverages below 1015 atoms/cm2 isobutane is the major product while above 1015atoms/cm2 the major product is n-butane. These results are indicative of catalytic function that is predominantly metallic at low oxygen coverage and acidic at coverages greater than 1015 atoms/cm2. The variation in catalytic function with increasing oxygen coverage is in excellent agreement with surface characterization by molybdenum core level shifts and surface polarizability measurements. Clearly the physical measurements were performed on the actual catalytically active sites, and the nature of the active sites has been established. The catalytic results combined with the spectroscopic data also serve to calibrate the minimum molybdenum oxidation state required for acid-catalyzed C-C bond breaking and formation of a secondary carbenium ion, namely, Mo(1V). Finally, it should be noted that the present work calls into question a previous report by Holl et. aL30 that zerovalent molybdenum catalysts possess acidic function. In light of the present results and the fact that molybdenum has a very high affinity for oxygen, it is likely that the acidic function was imparted by a thin oxide layer, which covered the molybdenum particles in their catalyst. Similar observations of acid function on other zero-valent metal catalysts may also be spurious due to the presence of oxide overcoatings. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grants CHE-8206104,CHE-8505723, and CHE-8821781. Registry No. Mo, 7439-98-7; B, 7440-42-8; 7782-44-7; S, 7704-34-9. ~~~
C,7440-44-0; 0,
~
(30) Holl,Y.;Garin, F.; Maire, G. J. Catal. 1988, 113, 569.
