189
J. Phys. Chem. 1993,97, 189-192
Dehydrogenation of Adsorbed Methoxy on Clean and Oxidized Metals. An Electronic Effect and Its Implications Paul Shilled and Alfred B. Anderson* Chemistry Department, Case Western Reserve University, Cleveland, Ohio 441 06 Received: August 18, 1992; In Final Form: October 20, 1992
-
By means of an atom superposition and electron delocalization molecular orbital (ASED-MO) study it is shown that the reaction on P t ( l 1 l), CH3O(ads) CHsO(ads) H(ads) (i), may be viewed as a hydride transfer and has a relatively low activation barrier. By contrast, the reaction CH3O(ads) O(ads) CHtO(ads) OH(ads) (ii) has a higher barrier because O(ads) is formally 02-, having formed two electron holes at the top of the filled Pt valence band, and in reaction ii there is a net promotion of two electrons from lower-lying 0 lone-pair orbitals to the hole orbitals. On the basis of the electron promotion concept, pathways for dehydrogenation of other hydrocarbon and oxygenate intermediates on oxidized metal surfaces are suggested.
Introduction The dehydrogenation of methoxy, CH30(ads), on transition metals has been studied in recent years. On more active metals, H(ads) and CO(ads) form and they eventually desorb as the temperature is raised.l On less active metals, formaldehyde desorption has been observed.2~3Coadsorbed oxygen, O(ads), can lead to formaldehydeformation on some of the active metals.' Methoxy is formed by methanol dehydrogenation. The reaction is CH30H(ads)
-
CH,O-(ads)
+ H-(ads) + 2h+
(1)
where h+ stands for a hole at the top of the metal valence band due to oxidation of the surface. This is a convenient notation that is an alternative to assigning oxidation states to surface metal atoms. Metal-oxygen single bond strengths are typically 2'/2 eV, as are metal-hydrogen bond strengths, and the sum of these is about equal to the 0-H bond strengths in methanol, which explainsthis activity. When oxygen is present on the surface, the reaction CH,OH(ads)
+ 02-(ads) + 2h+
-
+ h+
-
CH,O(ads)
+ H-(ads) + h+
(3)
The purpose of this study is to explain why hydrogen does not transfer to the adsorbed oxygen or to an adsorbed hydroxyl instead. This would involve changes in oxidation state: CH,O-(ads)
+ 02-(ads) + 3h'
-
CH,O(ads)
+
-
+
metal, as in eq 3, the proton takes two electrons, forming H-(ads), and there is no electron promotion. Considerations of electron promotion energies attendant to bond formationon metal surfaceshave been found to be important in the past. The bond between CH3' and 02-in CH3O- adsorbed on Mo( 110) was calculated to be 40% of the strength of the 0-C bond in CH30H.S This was shown to be because when the u bond in CH30-(ads) is formed between CH3' and 02-(ads) + 2h+ an electron is promoted in the antibonding us counterpart orbital to a hole at the top of the occupied Mo valence band. This explained why some of the methoxy, when prepared on a partially oxidized Mo( 110) surface, released CH3' (g) when heated.6 In this paper we present the results of a theoretical study which support the above conjecture concerning the higher activation energy for reaction 4 compared to that for reaction 2 despite the fact that OH bonds in alcohols are about twice as strong as MH bonds. Forour study we havechosen thePt( 111) surface, modeled by a cluster, and use the atom superposition and electron delocalization molecular orbital (ASED-MO) theory.'
Model
+ OH-(ads) + 2h'
CH,O-(ads)
(2) is believed to occur with a rather low activation energy, 0.24 f 0.03 eV in the case of Pt(ll1) with a p(2 X 2) 0 ~ v e r l a y e rIn .~ this case the OH bond in methanol is replaced by the OH bond in the adsorbed hydroxyl in a proton transfer. Proton-transfer reactions are believed to be generally facile, and this surface reaction is no exception. On clean and oxidized metal surfaces, the rate-limiting step in methoxy decomposition is believed, based on thermal desorption profiles,ls4 to be the transfer of H to a metal site: CH30-(ads)
+
+ +
OH-(ads) h+ (4) Thesurface reduction by theeliminationof 2h+makes this reaction different from the hydride transfer in eq 3. In eq 4 two electrons are promoted from lower-lying 0 2p lone-pair orbitals to the higher-lying metal Fermi level. When the hydrogen goes to the 'Current address: Packard Electric, P.O.Box 431, Warren, OH 44486.
0022-36541581 2097-0 189$04.00/0
The bulk-superimpsable two-layer 20-atom cluster in Figure 1 was used to model the Pt( 111) surface. The ASED-MO theory is a semiempirical approach for determining approximate molecular structures, force constants, and reaction energy surfaces and explainingthem by means of perturbation theoretic concepts applied to the calculated orbitals and their energies. In this work the total energy is a sum of repulsive pairwise atom superposition energies and electron delocalization energies as defined by integrating electrostatic forces on the nuclei as isolated atom superimposed into a molecular configuration. The atom superposition components are calculated using isolated atom charge density distributionfunctionsand the total electrondelocalization energy is approximated by the valence molecular orbital binding energy obtained using a modified extended Hiickel Hamiltonian. The C, 0,and Pt parameters, in Table I, are from a CO adsorption study.* The transition states were found by tilting the methyl group in 5-deg steps and stretching the CH bond in 0.01-A steps toward the accepting atom, either Pt or 0. All independent adsorbate coordinates were completely optimized at each step, and the transition states were found graphically as saddle points on the energy surfaces. As expected for theoretical calculations on large systems, our structures and energies have systematic errors; for example, the CH bond length in methoxy is calculated to be 0.1 1 A too long. The changes in calculated properties are the focus here and are used to generate chemical understanding with the help of perturbation-molecular orbital theory analysis. 0 1993 American Chemical Society
Shiller and Anderson
190 The Journal of Physical Chemistry, Vol. 97, No. 1 , 1993
CH3O
0
oxidation to C O by surface 0 2 - (or OH-) will have relatively high activation energies for those steps where the dehydrogenation proceeds by hydride transfer. Thus, for the first step in methanol reaction on an oxidized metal or on a metal oxide CH30-(ads)+ OH-(ads) + 2h'
f \
CH30H(ads)+ O*-(ads) + 2h*
CHzOH-(ads)+ OH-(ads) + 2h*
Figure 1. Looking down onto the two-layer 20-atom cluster model of the Pt(l11) surface, showing the CH30 binding site and the 0 binding site used in the dehydrogenation calculations.
Methoxy was calculated to be most stable when bound to the Pt( 111) surface cluster model in the 1-fold atop site, based on calculated binding energies of 4.04,3.37, and 3.34 eV for this and the respective 2- and 3-fold sites. The energy for dissociating CH3' (g) is 2.0 eV for the most stable site and decreased to 1.8 eV for the other two sites. These are higher than the methyl radical dissociation energies calculated over Mo( 110)j and, as will be seen below, are significantly higher than the calculated activation energies for dehydrogenation on the surface. Over the surface model in the absence of adsorbed 0, a transition state was reached when the methyl group was tilted 45O from the vertical and the CH bond was stretched 0.29 A, as shown in Figure 2. The calculated activation energy is 0.68 eV. When H was transferred to 0 in a 3-fold site a very high activation energy of 2.70 eV was obtained and with the 0 moved to the adjacent 1-fold site the activation energy decreased to 1.42 eV. Though 0 is usually found in high-coordinate sites on transition metal surfaces, a shift to 1-fold yields a much lower energy pathway so this is the one analyzed. Structure information for this pathway is also shown in Figure 2. As is evident there, the OH bond must have become quite well formed in the transition state because the OH internuclear distance is short. According to the electronic structure diagram for H transfer from the methoxy to surface Pt in Figure 3, on going from CH3O(ads) to OCH2(ads)-H..Pt to OCH2(ads) + H(ads) the C-H u bonding orbital evolves into a three-center C--H.-Pt donation bonding orbital and finally to a Pt-H u orbital. The antibonding counterpart C-H-O u* orbital is rendered nearly nonbonding at the transition state by the mixing in of the higher-lying C-H u* orbital, with the result that the 1s contribution centered on the proton is nearly canceled. This orbital evolves into the CO A bond by mixing with what was a methoxy lone-pair orbital as the remaining CH2O becomes planar formaldehyde. The overall process is H- transfer from methoxy to the Pt surface. The activation energy is low because there is no net reduction of the metal surface but only some rearrangement of the electronic structure, as indicated by the bold arrow in Figure 3. This shows that a valence d,z-type orbital beneath the Fermi level with which the C H u orbital has an antibonding interaction is only slightly destabilized in the transition state. When the methoxy H transfers to the oxygen atom on the surface it is again as H- so that 02-(ads) becomes OH-(ads) and two electrons are promoted from the 0 2p levels up to holes at the Fermi level as shown by the bold arrow in Figure 4. The large 0 2 p H 1s overlap in the transition state shows that the OH bond is near to being completely formed. This stability counteracts the promotion of two 0 lone-pair electrons -3 eV to the Fermi level, resulting in an activation energy of only 1.42 eV.
Conclusions From the above it is concluded that while proton exchange reactions will be facile on oxidized metal surfaces, methanol
(5b)
either reaction has a low barrier. Focusing on eq Sa, the next step in methoxy oxidation is hydride transfer, eq 4, and has a high barrier. Formyl formation
-
+ 02-(ads) + 2h'
CH,O(ads)
Results and Discussion
(5a)
+
+
CHO-(ads) OH-(ads) 2h' (6) is by proton transfer and will proceed with a low barrier, which probably accounts for the rapid oxidation of formaldehyde over selective oxidation catalysts. The adsorbed CHO- will be relatively stable on the oxidized surface because its oxidation is by hydride transfer:
-
+ 02-(ads) + 3h'
CHO-(ads)
+
+
CO(ads) OH-(ads) h' (7) There are two decomposition pathways for the product of reaction (5b): CHzO(ads)+ OH-(ads) + h' CHsOH-(ads) + 02-(ads) + 3h*
(Ea)
f
\
CHOH*-(ads) + OH-(ads) + 3h* (ab)
The product 8a forms with a higher barrier and decomposes according to eqs 6 and 7. Product 8b can decompose by two pathways
+ OH-(ads)
COH'(ads)
(sa)
f CHOH*-(ads)+ OZ-(ads)+ 4h* \ CHO-(ads) + OH-(ads) + 2h'
(9b)
Pathway 9a eliminates four holes and should have a higher barrier than formyl formation (9b) where only two holes are eliminated. Formyl can be oxidized by adsorbed oxygen as in eq 7. Should COH+(ads) form it will easily deprotonate: COH+(ads)
+ 02-(ads) + h+
-
+
+
CO(ads) OH-(ads) h' (10) It is interesting that methane oxidation by 02on the oxidized surface can proceed by a series of low barrier proton transfers.
CH,
+ 02-(ads) + 2h'
-
CH;(ads) CH,-(ads)
+ 02-(ads) + 3h' + 02-(ads) + 4h'
CH3-(ads) CH3-(ads)
+ 02-(ads) + 5h'
(1 1)
OH-(ads)
+ 3h'
(12)
OH-(ads)
+ 4h'
(1 3)
OH-(ads)
+ 5h'
(14)
-+ -+ -+
CH,'-(ads) CH,'-(ads)
+ OH-(ads) + 2h'
C4-(ads)
The Journal of Physical Chemistry, Vol. 97, No. 1 , 1993 191
Dehydrogenation of Methoxy on Metals
TABLE I: Parameters Used in the Calculation: Principal Quantum Number, n; Valence State Ionization Potential, 1P (ev); SIater Orbital Exponents, 1 (au); and Coefficients for Double 5 d Functions, c' P
S
atom
n
H
1
C
2 2 6
0 Pt
IP 12.10 15.09 26.98 10.50
d
r
n
IP
f
n
IP
1.200 1.658 2.146 2.55
2 2 6
9.76 12.12 6.46
1.618 2.127 2.250
5
11.10
CI
0.6558
fl
c2
f2
6.0130
0.5715
2.3960
Values for C, 0, and Pt are from ref 8 and for H the standard exponent is used and its IP is decreased 1.5 eV from experimental as was done for C and 0. 0
#*A 1.43-1
0
0
1.60-1
k1.60 pt-pt
'r
plj
i
-
-15
\'--!
-1
-14
C
w 1.25-1
-1 8
c
Figure 2. An energy versus reaction coordinate plot for the reactions studied. AE is the change for methoxy passing through a transition state, forming H2CO and OH products over the Ptzo cluster. The CH3O and H2CO are bound to the central atom in Figure 1.
-28 -29
Pt-Pt
Pt-Pt
Figure 3. Orbital correlation diagram for the dehydrogenation of C H 3 0 on the Pt(ll1) cluster model of Figure 1. The bold arrow indicates electronic rearrangement.
TABLE II: Calculated CH3O(ads) Properties at Three Symmetric Sites, 1-fold as Shown in Figure 1, 2-fold Bridging Adjacent to This Site, and 3-fold Hollow above a Second-Layer Atom, Also Adjacent to This 1-fold Site: CH30(ads) Binding Energy to This Cluster, BE (eV); Equilibrium H-O and C-0 Distances, Rpn, and RCO(A); and C-0 Bond Strength for Generation of Free CHC, &O (eV) BE 4.04 3.37 3.34
site 1 -fold 2-fold 3-fold
RPto
Rco
Dco
1.60 1.78 1.87
1.43 1.46 1.46
1.99 1.81 1.75
TABLE III: Energy Change, AE (ev), for the Transition State and Product Structures Shown in Figure 2 AE mechanism H to Pt H to 0 (ads)
activation energy 0.68 1.42
reaction energy'
-0.65 0.28
In both cases the formaldehyde product is perpendicular to the surface mode1,coordinated throughoat a 1-foldsitewithpt-Odistanceoptimized to be 1.66 A.
Ethane and higher alkanes should lose H on this surface to form ethyl or its analogs:
C,H,
+ 02-(ads) + 2h+
-
CH,CH,-(ads)
+ OH-(ads) + 2h+ (15 )
Subsequent 6 H abstraction presents two possibilities: CH2CHt-(ads) CH2CH,(ads)-
+
02-(ads)
+ 3hC
I \
CH&HZ(g)
+ OH-(ads) + 3h'
+ OH-(ads) + h*
(16a)
(la)
The formation of adsorbed ethylene (16a) is expected to be
1
-29
Figure 4. As in Figure 3 but with H transfer to 0 on the surface.
preferred since the number of holes is unchanged while reductive elimination of ethylene (16b) destroys two holes. Dissociation of adsorbed ethylene to two adsorbed methylene fragments is likely to be an easy process because two more holes form. Rearrangement to adsorbed ethylidyne is also possible by these arguments and is observed on clean Pt(l1 l).9 However, on oxygen-saturated Pt( 111) ethylene adsorption is weak enough that it desorbs on heating instead of rearranging to ethylidyne.9 The conclusions given above for eqs 8-16 are guidelines. As adsorbed oxygen is eliminated as water, either by hydrogen transfer from adsorbed hydrocarbon fragments to adsorbed hydroxyl or by proton transfer between adsorbed hydroxyls, the surface metal sites will become available for accepting the hydride
192 The Journal of Physical Chemistry, Vol. 97, No. 1, 1993
anion from the intermediates and so alcohol decomposition to carbon monoxide will proceed easily.
Acknowledgment. Support for this work has been provided by the Defense Advanced Research Projects Agency. References and Notes (1) Lu,J.-P.; Albert, M.; Bernasek, S.L.; Dwyer, D.S.Surf. Sci. 1989, 218, 1.
Shiller and Anderson (2) Fu, S.S.;Somorjai, G. A. J. Phys. Chem. 1992, 96, 4542. (3) Wachs, 1.; Madix, R. J . Cuul. 1978, 53, 208. (4) Akhter, S.;White, J. M. Surf. Sci. 1986, 167, 101. ( 5 ) Shiller, P.; Anderson, A. B. J. Phys. Chem. 1991, 95, 1396. (6) Serafin, J. G.; Friend, C. M. J . Am. Chem. Soc. 1989, 111, 8967. (7) Anderson, A. B. J . Chem. Phys. 1975,62, 1187. Anderson, A, B.; Grimes, R. W.; Hong, S.Y.J . Phys. Chem. 1987, 91, 4245. (8) Shiller, P.;Anderson, A. B. Surf.Sci. 1990, 236, 225. (9) Cassuto, A.; Mane, M.; Jupille, J.; Tourillon, G.; Parent, Ph. J . Phys. Chem. 1992, 96, 5987.
