J . Phys. Chem. 1985, 89. 4150-4155
4150
intensities. The line widths were analyzed by’,’ AH(M)=a+PM+y@+6M3
(5)
Our results for VOAA-toluene (no added naphthalene) in the temperature range 3 13 > T > 282 K were compared with those of Wilson and Kivelson* and Hwang et a].;’ these authors used the M = -3/2 line as their standard and calibrated the magnetic field using a gauss meter and frequency counter. Our values of the line width parameters were found to be in good agreement with those from ref 1 and 7, justifying the use of the TCNE anion as the line width standard in the studies reported here. Figure 2 shows our y values for VOAA in toluene as well as several values from ref 1. Our value of y = 0.308 G for VOAA in benzene at 25 O C is also consistent with that of Wilson and Kivelson (y = 0.305 G at 24 oC).2 The reorientational correlation times, io, for all of our samples were obtained from y by using eq 7-9 of ref 6. The combination of anisotropic hyperfine parameters, Pa = a, - (1/2)(a, + a,), is needed to calculate io from y,and the value determined for VOAA in toluene was used. This is not a source of uncertainty in the analysis of the line width data for VOAA in the ethers; measurements of the isotropic hyperfine splitting constant, a. = (l/3)(ax a, + aJ, in M T H F fluid solution combined with the value of a, found in an M T H F glass showed that Pa = (3/2)(a, - ao) for VOAA in M T H F was within 1% of that for VOAA in toluene. The line width parameter y (rather than @)was used to determine r@because fl is subject to larger experimental uncertainties than y5J7and because P is a strong function of the nonsecular spectral densities.’V7 y , which depends weakly on the nonsecular spectral densities, is almost directly proportional to r0. There are still questions concerning the functional form of the nonsecular spectral densities; recent studies of VOAAI3 used i g determined from y to check models for the nonsecular spectral densities appearing in the theoretical expression for fl. In a related study of a series of vanadyl P-diketonate complexes, Eagles and M ~ C l u n gfound ~ ~ that the value of ~r~obtained from /3 and y differed slightly but that the value of ~r~derived from the total M-dependent line width, W(M)= PM + r@ + 6M3,was in close agreement with the value found from y. Our interest in this paper
+
(34) Eagles, T. E.; McClung, R. E. D. Can.J . Phys. 1975, 53, 1492.
is the dependence of ig i.e. K, , on solvent. Consequently, y has been used to calculate io. K was determined from eq 1 . We have used r3 = 5 5 A3 for all of our samples since the molecular radius, r, for VOAA in benzene,’ toluene,]’ and THF” is the same within experimental error. In the temperature range 9-40 ‘C we determined K = 0.64 f 0.04 for VOAA in toluene; both the value of K (0.64) and its characteristic uncertainty (f0.04) are the same as in ref 7. The uncertainty for the K values of the four ethers is also f0.04. As noted in section 11, K for VOAA in benzene at 25 OC also agrees with a previous determination.*S7 We studied VOAA in liquid naphthalene at 100, 120, and 140 OC while Wilson and Kivelson2 performed measurements at 97 and 147 C . Using all five sets of data, we find K = 0.59 f 0.06. The viscosity data of ref 35 have been used for liquid naphthalene; this data is consistent with other Although examination of all data for liquid naphthalene does indicate some ~ncertainty,~’ several determinations are in close agreement at 80 and 100 “C (see data from ref 830 and 930 in LandoltB e r n ~ t e i nin~ addition ~ to our ref 24-26, 35, 36, and 38). If only the VOAA line width data in liquid naphthalene at 97 and 100 “ C are considered, we still find K = 0.59. Acknowledgment. The variable-temperature unit used in this work was purchased with funds provided by the St. Louis University Beaumont Faculty Development Fund and the Department of Chemistry, St. Louis University. B.A.K. also received a summer stipend from the Beaumont Faculty Development Fund and 0.-W.Y. was partially supported by a grant (to B.A.K.) from the Research Corporation. The final draft of this paper was written while B.A.K. was on sabbatical leave at the Institute of Materials Science (IMS) of the University of Connecticut; the assistance of the IMS staff with its preparation is gratefully acknowledged. Registry No. Dimethoxyethane, 110-71-4; tetrahydrafuran, 109-99-9; 2-methyltetrahydrofuran,96-47-9;tetrahydropyran, 142-68-7;benzene, 7 1-43-2; toluene, 108-88-3;naphthalene, 91 -20-3. (35) Bingham, E. C.; Hatfield, J. E. J . Appl. Phys. 1935, 6, 64. (36) Weast, R. C., Ed. “Handbook of Chemistry and Physics”, 56th ed.; Chemical Rubber Publishing Co.: Cleveland, OH, 1975. (37) Andrussow, L.; Schramm, B. “Landolt-Bernstein Zahlenwerte and Funktionen”, 6th ed.; Springer-Verlag: New York, 1969. (38) Saji, K. Busseiron Kenkyu 1956, No. 96, 8 3 . See: Chem. Absfr. 1957, 51, 5489b.
A Theoretical Model for Methanol Formation from CO and H, on Zinc Oxide Surfaces R. C . Baetzold Research Laboratories, Eastman Kodak Company, Rochester, New York 14650 (Received: October 29, 1984; I n Final Form: May I O . 1985)
Models are developed for the polar (0001) and nonpolar (1010) surfaces of ZnO in order to consider methanol formation from adsorbed carbon monoxide and hydrogen atoms. The heats of adsorption of H,CO and OH,CO ( x = 0-3) species involved in methanol formation are computed to determine the enthalpy changes of reaction. Reaction sequences involving formyl or formate intermediates are considered. We propose that the reaction mechanism is catalyzed by the Cu+to proceed through a methoxy intermediate on Cu+/ZnO with a lowering of the energy pathway. The ZnO surfaces are poor donors and function primarily as acceptors of electron density from CO. The donor role of Cu+ is demonstrated on the polar surface by increasing the heat of adsorption of acceptor adspecies and decreasing the heat of adsorption of donor adspecies.
Introduction Zinc oxide is a material of significant catalytic importance. It is best known as one of the components of a commercial catalyst that is nearly fully selective to methanol in CO/H2 gas feeds.’ ( 1 ) Kung, H. H. Carol. Rev. Sci. Eng. 1980, 22, 235.
0022-3654/85/2089-4150$01.50/0
In addition, this rather basic oxide has been used as a support for a number of catalysts. As is typical for many of the oxide surfaces, OH groups are present unless the surface has h e n treated to specifically dehydroxylate it. Likewise, oxygen vacancies on the surface of this oxide can cause an adjacent metal ion to have reduced coordination number. Thus the surface composition of 0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985 4151
Methanol Formation from C O and H2 on ZnO 0
O
ti
\c'
I
H
"
zn*+
H H
\c/ & I zn"
I
z nu
HO
Zn"
H
H H W Y
Z ~ U _ CH,OH
OH
\ I
r - c
4n++
H_
\y
H H W
(1)
0
J"
HOHH
H
- - t L
CH30H I
0
Zn-
Figure 2. Mechanism of methanol formation considered by Muetterties and Stein."
v
v = oxygen voconcy
Figure 1. Mechanisms of methanol formation considered by Kung.'
@-@
ZnO is sensitive to treatment conditions, and therefore ZnO has variable catalytic properties. Both hydrogen and carbon monoxide are adsorbed to the surface of ZnO. Heterolytic cleavage of H2 according to eq 1 is known
H2
t -2n-0-
I
1
- -"
zn-0-
I
7I
(1)
on nonpolar surface^.^^^ The Zn-H and 0-H bonds are readily detected3 with infrared spectroscopy. The heat of adsorption at low coverage has been measured to be 12 kcal/m01~*~ on catalyst powders. Carbon monoxide adsorbs at the Zn cation with a heat of adsorption of 12 kcal/mol$' as measured on powders and single-crystal surfaces. In CO/H2 coadsorption studies, the adsorption of CO did not block sites for H2 adsorption as written in eq 1, but under some conditions there was mutual enhancement of adsorption.'q4 There have been several recent spectroscopic studies of the interaction of C O with ZnO single-crystal surfaces. By the use of high-resolution electron energy-loss spectroscopy (EELS) the C O vibrational mode at 2202 cm-l and the Zn-C vibrational mode at 250 cm-I have been detected.6 The increase in CO vibration frequency relative to the gas-phase value, also detected by IR on powder samples, is interpreted to indicate net charge transfer from CO to the ZnO surface. In contrast, for metal surfaces the reverse situation holds. Thus, C O is suggested to be a net donor on ZnO surfaces. Angle-resolved UPS studies on various ZnO surfaces have shown that C O bonds to the Zn ion through C, forming "approximately linear Zn-C-O c o m p l e x e ~ " . ~ - ~ The active methanol catalyst contains Cu+ mixed with the ZnO lattice. It has been p r o p o ~ e d ~that . ' ~ Cu+ is dissolved in the lattice in this catalyst. It is thought that Cu' enhances adsorption of C O by forming a Cu+-CO bond stronger than the Zn2+-C0 bond.' In addition, the presence of Cu+ increases the density of 0 vacancies, as required by charge balance. In this view the H atoms bound to ZnO act to reduce the C O to methanol. Several proposals have been advanced to describe methanol formation mechanism on ZnO; we will cite only a few. Kung' has described three that seem fairly basic and are shown in Figure (2) Kokes, R. J.; Dent, A. L.; Chang, C. C.; Dixon, L. T. J . Am. Chem. SOC.1972, 94,4429. (3) Griffin, G. L.; Yates, J. T. J . Chem. Phys. 1982, 77, 3744. (4) Griffin, G. L.; Yates, J. T. J . Chem. Phys. 1982, 77, 3751. (5) Gay, R. R.; Nadine, M. H.; Solomon, E. I.; Henrich, V. E.; Zeiger, H.J. J . Am. Chem. SOC.1980, IO.?, 6752. (6) D'Amico, K. L.; McFeely, F. R.; Solomon, E. I. J . Am. Chem. SOC. 1983, 105, 6380. (7) McClellan, M. R.; Trenary, M.; Shinn, N. D.; Sayers, M. J.; DAmico, K. L.; Solomon, E. I.; McFeely, F. R. J . Chem. Phys. 1981, 74, 4726.
(8) DAmico, K. L.; Trenary, M.; Shinn, N. D.; Solomon, E. I.; McFeely, F. R. J. Am. Chem. SOC.1982, 104, 5102. (9).Herman, R. G.; Klier, K.; Simmons, G. W.; Finn, B. P.; Bulko, J. B.; Kobylinski, T. P. J. Catal. 1979, 56, 407. (10) Visser-Lulrink,G.; Matulewicz, E. R. A,; Hart, J.; Mol, J. C. J . Phys. Chem. 1983,87, 1470.
Figure 3. Top view of the (1010) nonpolar cleavage surface of ZnO. Circled ions are in the top plane; other ions are in the next deeper plane.
1. In the first, the formyl species (CHO) is formed and successively hydrogenated to hydroxymethylene (CHOH), then to hydroxymethyl (CH20H), before final hydrogenation to methanol. In the second mechanism, formate ( C 0 2 H ) is formed by interaction of CO with surface hydroxyl groups. A sequence of hydrogenation and water elimination steps lead to methoxide (OCH3), which is subsequently hydrogenated to methanol. Kung has also described a mechanism whereby C O is bound to the ZnO surface and the 0 end of the molecule tips toward an oxygen vacancy. Upon hydrogenation of carbon, a methoxide is formed. Mechanism 111 was stated to be more plausible than I or 11. Muetterties and Stein" also considered various general schemes for methanol formation. Figure 2 shows a proposal of theirs that involves similar intermediates, though different reaction pathways, to those discussed by Kung. In addition to this general scheme, Muetterties and Stein considered that either formyl (CHO) or surface hydroxide groups (OH) could provide a source of the hydrogen for hydrogenation steps. The purpose of this work is to computationally examine the interaction of alcohol precursor species with ZnO and Cu+/ZnO surfaces. Previously,I2the computational procedures were applied to treat the metallic bonding in transition-metal syrfaces that interacts with various molecular and radical species. It is not clear how the methods will work for a covalent-ionic material such as ZnO. However, to the extent that the methods are successful, key questions within the methanol-forming mechanism from CO/H2 will be examined. Model Calculations. Zinc oxide is a typical compound semiconductor of the 11-VI class. It has the zinc blende structure with tetrahedral bonding at oxygen and zinc ions. Both polar and nonpolar low-index surfaces of ZnO can be prepared. The polar surfaces have shown a greater reactivity for catalytic methanol formation from C O and H2than the nonpolar [ lOTO] surface^.^,'^ They are much more demanding to model theoretically with the tight-bonding procedures. This is because of the obvious electrostatic effects associated with a complete surface plane of similarly charged ions. The nonpolar surface is sketched from above in Figure 3. Note that because of the 3.25-A spacing between (11) Muetterties, E. L.; Stein, J. Chem. Reu. 1979, 79, 479. (12) Baetzold, R. C. J . Am. Chem. SOC.1983, 105, 4271. (13) Bowker, M.; Houghton, H.; Waugh, K. C.; Giddings, T.; Green, M. J . Catal. 1983, 84, 252.
4152 The Journal of Physical Chemistry, Vol. 89, No. 19, 1985
Baet zold
TABLE I: Parameters of the Calculation atom Zn
0
C H
cut
bond Zn2+-C
c-0
orbital 3d
energy, eV -20.00
4s 4P 2s 2P 2s 2P 1s 3d
-9.00 -9.00 -32.30 -14.80 -2 1.40 -1.40 -13.60 -13.00
4s 4P length. 8, 2.15 1.2
-7.00 -7.00 bond C-H 02--c
exponent 5.95 (0.593)" 2.30 (0.574) 2.75 2.75 2.275 2.275 1.625 1.625 1.300 5.95 (0.593) 2.30 (0.574) 2.15 2.15 length, 8, 1.1 2.15
Double { exponents with coefficients in parentheses.
like ions in a single plane, lateral interactions take place dominantly through an interplanar mechanism. A surface ion is directly bonded to one ion in the same plane and two ions in the plane below. There is no good source of data to use in assigning the parameters for Zn and 0 ions in the ZnO surface. Therefore, we have fixed the orbital-energy parameters through the use of photoemission spectra. Oxygen values are taken as standard for the 0 atom as used in extended Hiickel calculations. Zinc parameters for the d orbitals are taken so as to properly position the Zn-3d component of the density of states relative to the 0-2p component. The Zn-4s orbitals are estimated to be 7 eV less bound than the 0 - 2 p orbitals.I4J5 This determines the orbital energies used in Table I. The Slater orbital exponents are double {for the 3d and standard single { fbr the s and p orbitals. The density of states for the (1010) surface calculated with our parameters is shown in Figure 4. The first occupied band, centered at -14 eV, is due to 0 . 2 ~levels with a small admixture of Zn-4s and Zn-4p levels. The second occupied band is centered at -20 eV and is composed primarily of Zn-3d levels. There is an unoccupied band starting at -9 eV, which is composed primarily of Zn-4s and Zn-4p levels. Comparison with the angle-integrated He I1 spectrum from the (1010) ZnO surface5 is also shown in Figure 4. It is clear that the spectra can be aligned to reproduce the major peaks. Of course, intensities would not be expected to agree between the two curves because of the different photoemission cross sections for s, p, and d orbitals. In addition, the unoccupied levels would not be measured in the experiment. The gap between occupied and unoccupied bands computed here to be 3.0 eV agrees well with the value 3.3 eV computed earlierI4 for ZnO ( i o i o ) . The method of calculation is tight binding12 in which the covalent type of interactions are treated. Possible ionic interactions between adsorbate and the surface are not treated explicitly but incorporated through the parameters. Intralayer matrix elements are computed for nine ZnO units within a rectangular array, as can be Seen from Figure 3. This array includes all of the significant interactions of the ZnO basis. The interlayer interactions are computed between a ZnO unit and the four nearest ZnO units in each adjacent layer. The procedure is used for four ZnO layers interacting with a single adsorbate species per ZnO unit. Only one ZnO surface is covered. We do not treat adsorbateadsorbate interactions, which would be small on the open (1070) surface. More details of the calculation are in Appendix A. We have simulated a Cu+ substituted for ZnZ+in the top surface layer. Here again we lack good parameters for Cu+ in the ZnO lattice. We expect the d levels of Cu+ to be shifted toward vacuum relative to those of Zn2+ because of its smaller charge. Compu(14) Ivanov. I.; Pollmann, J. Solid State Commun. 1980, 36, 3 5 1 . (15) Gopel, W.; Pollmann, J.; Ivanov, I.; Reihl, B. Phys. Rev. B 1982, 26,
3144.
- 20
-15
-10
Energy, eV Figure 4. Computed total density of states for ZnO (1010) surface film of four layers. The components due to Cut added in the band-gap region are also noted. The He I1 angleintegrated UPS spectrum of ZnO (1010) is also shown.s
tationally the Cu+ has basically a d'O configuration so that its net charge is properly described. Lacking good experimental information to pin these levels, we place them at -13 eV, which is reasonable compared to the atomic Cuo levels. This parameter is taken as empirical. Thus, it must be remembered that although we will refer to Cu+ impurities in our calculation we should not be this specific. We are really computing for a univalent ion having dl0 occupancy and d levels destabilized relative to Zn2+ as Cu+ must be. Unfortunately, there is no simple way to introduce into the periodic lattice isolated oxygen vacancies that might be expected to accompany the substitutional Cu+ impurity. An important effect of a univalent impurity, such as the Cu+ which we have simulated, is to introduce occupied levels into the band-gap region of ZnO. The additional levels introduced by Cu+ a t the top of the valence band are sketched in Figure 4. These levels are significant in promoting electron transfer to an adsorbate because their ionization potential is almost 2 eV less than the ionization potential for ZnO. We will find these levels to be key for the stabilization of 4CH3on Cu+ vs. Zn2+,as will be discussed later. We have begun the calculations with an eye toward checking the C parameters of CO to determine that proper scaling is maintained with respect to the ZnO lattice. The key parameter here is the C,, level, which, as we have learned from calculations on metal surfaces,I2 is important in determining the relative positions of the ll and ll* levels vs. metal Fermi level and controls the direction of charge transfer. In the case of ZnO we varied C2, from -1 1.4 to -7.4 eV. We find that, for the preferred bonding site next to Zn2+, the direction of electron transfer from CO to the ZnO lattice is independent of this parameter. The Mulliken analysis gives charges of 0.3+ to 0.4+ on CO as CZpis varied from -1 1.4 to -7.4 eV. The heat of adsorption (Q) is dependent on this parameter, however. Q varies from -0.40 to +0.63 eV as the 2p level is varied from -1 1.4 to -7.4 eV. The experimental value ~ * ~ eV). for the heat of CO adsorption on ZnO is 12 k c a l / m ~ l (0.52 Thus, a C2, ionization level of -7.4 eV was used throughout these calculations. Results Energetics of Adsorption. CO. We consider bonding of C O normal to the (1010) surface on Zn2+ and 02-ions. The C end of the molecule is placed next to the surface, as known from e ~ p e r i m e n t . ~Under . ~ this circumstance we compute a repulsive interaction for bonding next to 02-with an attractive interaction for bonding to Zn2+. The Zn2+ site is preferred, in accord with experimental e v i d e n ~ e . ~The . ~ repulsive 0 2 - - C 0 interaction is a classic case of one between two closed-shell species, which is always repulsive. For Zn2+ the 3d levels are not completely full, owing to s,p hybridization, and the 4s and 4p levels can interact
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985 4153
Methanol Formation from CO and H2 on ZnO
0
9 . H
ti
/ / C
9-. .CH2
I
/"\
.*, i
Zn-0
i n -0
m
n
I
0-CH,
Zn-0
m
0
Zn-0
I
Zn-0
P Figure 5. Orientations of chemisorbed H 2 C 0 on ZnO considered.
with the closed-shell CO. The 4s and 4p levels of Zn2+ accept electrons from CO, as in the Blyholder model generally accepted for metal C O binding,16 leading to the positive charge on CO. Because the 3d levels are so deep in energy on Zn2+ compared to zerovalent metal surfaces, there is no significant back-bonding with the ll*levels of CO. Thus, the acceptor role for CO, which is known on metals, is significantly diminished on ZnO. The computed heat of CO adsorption is 0.63 eV, of which only 0.06 eV is found to be due to the acceptor function when we use an orbital energy decomposition scheme" described before. For CO bonding next to Cu+,a heat of adsorption of 0.97 eV is computed. The charge on C O is 0.41+ vs. 0.58+ for adsorption on ZnO. Thus C O is bound more strongly to Cu+ than to Zn2+, and there is a stronger acceptor function for C O next to Cu'. The d levels of Cu+ are higher in energy than the d levels of Zn2+ (Table I), and thus they are better able to donate electrons to the II* levels of CO. This effect accounts for the additional bond strength on Cu+ vs. Zn2+, but CO remains a net donor. CHO. The species C H O bonds preferentially at Zn2+ ions through carbon. The heat of adsorption is Q = 1.59 eV on ZnO and 2.09 eV on Cu+/ZnO. The preferred geometry has both 0 and H bent 30° away from the surface. This species is a donor having a charge of 0.21+ on the ZnO surface. C H 2 0 . This species C H 2 0 is weakly stabilized, if at all, on the ZnO surfaces. Several geometries have been investigated in an attempt to find stabilization, as shown in Figure 5. The most stable geometry is V with the C-0 axis perpendicular to the surface. On ZnO this interaction is repulsive (Q = -0.26 eV), but on Cu+/ZnO, Q = 0.26 eV. This species is nearly neutral on all surfaces. Other configurations are very close in energy to V. For example, structure I1 involving bonding through oxygen is only 0.21 eV less stable than V on the (1010) surface. Thus, the possibility of having an upright CH20 species on ZnO seems real. CH,O. The methoxide species has been examined with 0 bound to Zn2+along the surface normal. It is an electron acceptor with charges of 0.95- on ZnO and Cu+/ZnO. The heat of adsorption is Q = -0.24 eV on ZnO and Q = 2.01 eV on Cu+/ZnO. The Cu+ ion greatly stabilizes this species because of the impurity energy levels it creates in the band gap. Electron transfer from these levels to OCH3 can occur readily because of the lower ionization potential. CH20H. The hydroxymethyl species, unlike methoxide, is an electron donor on ZnO and Cu+/ZnO surfaces. The charge is 0.21+ on ZnO and 0.51+ on Cu+/ZnO. The pseudo-tetrahedral geometry of C H 2 0 H leads to a heat of adsorption of 1.65 eV on ZnO and 2.25 eV on Cu+/ZnO. OCH,O. We have considered various of the formate-type species OCH,O (x = 1, 2, 3) that may be formed on the ZnO surface. Three adsorption geometries are considered for OCHO, where rigid 120' bonding angles between the C-X ligands are assumed. (16) Blyholder, G. J . Phys. Chem. 1974, 68, 2772. (17) Baetzold, R. C. Phys. Rev. B 1984, 29, 4211.
0
Zn
0 Zn
i n
B
A
C
Each species is computed to be an anion with charges of 0.89-, 0.95-, and 0.94- for A, B, and C, respectively. The heats of adsorption are computed to be 0.80,0.46, and 0.60 eV, respectively, for structures A, B, and C. The energetic comparisons based upon our approximate calculations suggest that each species is close in stability and either may be present on this ZnO surface. Using A as the preferred structure, we may consider the reaction OH + C O OCOH (2) written in Figure 1 and invoked in some reaction schemes.' This is an exothermic reaction for which we compute AH = -67 kcal on the (1010) surface and AH = -57 kcal on the Cu+ (1010) surface. Despite the simplicity of these calculations, these large exothermic values certainly suggest that this reaction will proceed and that surface hydroxide is readily converted to formate by CO. This would support the reaction mechanism of Figure 1 involving formate. We note, however, that spectroscopic evidence for formate produced by reaction 1 is lacking, and some authors18J9 ascribe formate formation from C 0 2 and H. Computations have been performed for other intermediates that have been assumed in the formate-type mechanism.
-
H
H,~,o-H
"\C
rH C
p
0
D
E
Plausible structures having trigonal (D) or tetrahedral (E) arrangements of ligands around the central carbon atom were considered. Structure D is computed to have a charge of 0.04+, making it basically neutral, whereas structure E has an anionic charge of 0.95-. These species are weakly bound to the (1010) ZnO surface, having repulsive interactions of 7 and 4 kcal/mol, respectively, for D and E. Despite this fact, our estimates suggest that reaction pathways leading to species E by successive additions of H (as in Figure 1) are exothermic and thus possible reaction pathways leading to methanol. It is not clear, however, from a mechanistic viewpoint how water is eliminated and additional H atoms are incorporated in the later steps shown in mechanism I1 of Figure 1. Thus, we have not investigated this mechanism further. (0001) ZnO Surface. We have performed preliminary computations on the (0001) ZnO film structure in order to make comparisons with the nonpolar surfaces considered in this work. Within a given plane only like ions with a hexagonal arrangement consistent with tetrahedral bonding are present. The next plane contains oppositely charged ions having three nearest neighbors to each ion in the plane above. This film structure was modeled by a Zn-0 basis with the appropriate translation vectors. Parameters identical with the ones used for the nonpolar surface were adopted. We find a band structure for the four-layer arrangement sjmilar to that computed in Figure 4. A band gap of 3.0 eV at k = 0 is computed. At least one reservation about this idealized model of the (0001) ZnO surface must be pointed out. Experimental LEED studies of the polar surfaces of ZnO give evidence for reconstruction.20 (18) Bowker, M.; Houghton, H.; Waugh, K. J . Chem. Soc., Faraday Trans. 1 1981, 77, 3023. (19) Ueno, A.; Onishi, T.; Tamaru, K. Trans. Faraday Soc. 1970,66,756. (20) Henrich, V. E.; Zeiger, H. J.; Solomon, E. I.; Gay, R. R. Surf. Sci. 1974, 46, 293.
4154
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985
Baetzold
sDecies
co OCH, CHZOH HCO HZCO OCHO OH species
charge. au +0.58 -0.95 +0.21 +0.21 +0.05 -0.94 -0.95 charge, au
co OCH,
CHZOH HCO H2CO OCHO OH
f0.51 -0.94 +0.68 +0.61 +0.06 -0.89 -0.95
ZnO (1010) 0.63 -0.24 1.65 1.59 -0.26 0.80 1.64 ZnO (0001) 0.68 0.56 1.83 1.87 -0.28 1.10 1.81
Cu+/ZnO (1010) 0.97 2.01 2.25 2.09 0.26 3.14 3.90
Cu+/ZnO (0001)
“Reference 21. bComputed from the sum of bond energies from known data.
One mode13q4of this (2 X 2) reconstructed surface has 1/4 of the Zn cations missing from the top plane. This is an attractive model, since it would reduce charge imbalance associated with a complete layer of like-charged ions. Our film computations do not explicitly contain missing Zn ions, although they do not contain explicit terms representing the (0001) surface charge imbalance. Thus, the polar model of the (0001) surface treats charge imbalance on an equal footing with that of the (1070) surface model with the same parameters. Table I1 compares the heats of adsorption of the various surface species computed for the (0001) surface of ZnO and the (1070) surface. The same species geometries were used for each surface calculation. The heat of adsorption is generally larger on the (0001) surface than the (lOT0) surface. This is accomplished despite the fact that some species such as C O and H C O are electron donors and OCH3and OH are electron acceptors. This effect of greater heat of adsorption on the (0001) Zn surface is attributed to the greater separation of adsorbed species and 0 ions relative to the separation on the (1070) surface. There is a repulsive interaction between species such as CO or OCH3with the polar 0 ZnO surface. This is a case of repulsions between closed shells described earlier. This same effect explains the greater heat of adsorption on (OOO1) Zn surfaces than (1010) ZnO surfaces, although the effect is not the only factor to be considered. The role of Cu+ as a donor on the (0001) surface may be seen in the computed heats of adsorption in Table 11. The Mulliken charges of species on the (OOO1) surface are shown, and in all cases the presence of Cu+ decreases the heat of adsorption of positively charged adsorbates and increases the heat of adsorption of negatively charged adsorbates. Methanol Formation. We may consider the enthalpy changes involved in methanol formation at room temperature. The experimental enthalpies of foimation of H2,CO, H2C0, and CH,OH as stable gas-phase molecules are considered. H2 + CO H,CO AH = -1.3 kcal/mol H,
+ H2C0
-
-
H,COH
AH = -20.4 kcal/mol
Thus in the gas phase there is a considerable enthalpy change
HCO (9)
____)
AH
0 . 7 8 e V = 18 kcal
Figure 6. Thermodynamic cycle that allows computation of the enthalpy change of surface reaction from corresponding values in the gas phase and computed or experimental heats of adsorption.
0.45 2.86 1.70 1.78 -0.26 3.39 3.87
TABLE 111: Thermochemical Bond Energies bond gas-phase bond energy,ReV 0.78 H-CO 3.83 H-CHO 0.96 H-CH20 4.43 H-OCH, 1 .39b H-OCH2 4.00 H-CHIOH 4.45 0-H 11.13 c-0
0.78eV
+
TABLE 11: Computed Charge and Heats of Adsorption of Surface SoeCies heat of adsorption, eV
Methoxy Mechanism
- zno
.._.._ Cu+/ZnO
.......
.._....
I -
.......
-
fll 4H+CO
3H+HCO
2H+HzCO
H+H$O
H,COH
Figure 7. Schematic of the enthalpy change associated with gaseous methanol formation on ZnO and Cu+/ZnO starting from adsorbed CO and H and passing through methoxy intermediate. TABLE IV: Enthalpy of Surface Reaction AH, kcal/mol
cu+/ cut/ ZnO surface reaction l.H+CO-.HCO 2. H + HCO H2CO 3. H + H,CO H,CO 4. H + H3CO H3COH 5 . H + HZCO HZCOH 6. H + H,COH H3COH 7. OH + CO OCOH
--
-
-+
-+
(ioio)
18 12 36 -49 -24 4 -61
ZnO (0001) 12 19 17 -31 -27 9 -53
ZnO
(ioio) 14 11 -4
2
-28 22 -57
ZnO (0001) 9 16 -36 22 -7
5 -87
favoring the forward reaction. Kinetic barriers, of course, must be overcome for this reaction to proceed. Some proposed reactions in methanol formation mechanisms can be evaluated from the calculated heats of adsorption (Table 11) and experimental thermochemical bond energies determined in the gas phase (Table III).21 These can be evaluated by a thermodynamic cycle along with the experimental heat of H 2 adsorption (12 kcal/mol). Figure 6 is a representative cycle for HCO. A similar procedure was used for each entry in Table IV, where the properties on ZnO and Cu+/ZnO are determined. The entries in this table refer to enthalpy changes of adsorbed initialand final-state species at 298 K. Specific structures of the adsorbed species have been defined in earlier parts of this paper. Error limits cannot be accurately placed on these values because of the complexities of the surface species we are modeling. Interpretations based upon trends represent the best policy. The complexities of the mechanistic proposals for methanol formation precluded investigation of each mechanism in detail at this point. Instead, we studied the intermediate species common to the mechanisms, where we might be able to learn the role of ZnO. We consider that in any given sequence of steps leading to methanol formation there should be no big energy barriers. If there is a large barrier computed in the enthalpy term, this must also be true for the activation energy. Of course, a small or (21) Kerr, J. A. Chem. Rev. 1966, 66, 465.
The Journal of Physical Chemistry, Vol. 89, No. 19, 1985 4155
Methanol Formation from C O and H2 on ZnO Hydroxymethyl Mechanism
-
ZnO
-.-... Cu+/ZnO
....._
......
.._
-
-
- I
I
_..._.
4H+CX)
3H+HCO
2H+H2CO
H+H,CO
H3COH
Figure 8. Schematic of the enthalpy change associated with gaseous methanol formation on ZnO and Cu+/ZnO starting from adsorbed CO and H and passing through the hydroxymethyl intermediate.
negative enthalpy change does not guarantee a small activation barrier. Let us consider C H 3 0 H formation on ZnO using the data in Table IV to prepare Figures 7 and 8 showing the enthalpy changes associated with methanol formation on the (lOT0) film. Formation of CH30- from C O and H surface species bound to a Zn2+ ion is energetically very demanding on the ZnO surface, whereas -CH20H could be formed much more easily. This situation can be contrasted with Cu+/ZnO, where the -OCH3 group is more stable and could be formed more readily. Also, on Cu+/ZnO the CHzOH group would be formed readily, but there is a large bamer to its subsequent hydrogenation to methanol. Thus, we would consider the hydroxymethyl mechanism as most likely on ZnO and the methoxide mechanism as most likely on Cu+/ZnO. Certainly the smaller energy barriers encountered in methanol formation from CO/H2 on the Cu+/ZnO surface compared to the ZnO surface could account for the role of Cu+ catalysis in this reaction. The data in Table IV show that the (0001) ZnO surface would behave qualitatively like the nonpolar (1070) surface just described. The presence of Cu+ gives a more stable adsorbed methoxide species, which favors formation of this species from C O and H. However, we note that the enthalpic barriers to methanol formation proceeding through hydroxymethyl on the polar (000 1) surface are not so large as to preclude this possibility. We may consider the stability of various species on these surfaces. Decomposition by loss of successive H atoms is favored by enthalpy for H 2 C 0 or HCO on each of the surfaces considered. Meanwhile, only for the copper-doped surfaces do we compute favorable enthalpy change for methanol to decompose to methoxide and hydrogen. On the polar and nonpolar surfaces, formation of methoxide by the back-reaction 4 in Table IV is unfavorable. For methanol to decompose on these surfaces, we would need to invoke a mechanismI8 such as 0 + CH30H OH CH30 OH
--
+ CH30H
+ H 2 0+ C H 3 0
(3)
to explain the instability on these intrinsic surfaces. Experimentally, formation of methoxide from methanol is observed22 as well as decomposition of H 2 C 0 to H 2 and C O on ZnO surfaces.
'*
(22) Cheng, W. H.; Akhter, S.; Kung, H. H. J . Catal. 1983, 82, 341.
The prediction that Cu+ catalyzes a different route for methanol formation than in its absence on ZnO can be understood in terms of the occupied states created within the ZnO band gap. Methoxide is stabilized as an anion, which requires electron donation from the ZnO surface. The impurity states induced by Cu+ have a smaller ionization potential than the intrinsic ZnO surface and thus can promote methoxide stabilization. In addition, the copper ion strengthens the C O bond to the surface, but this bond remains of moderate strength so as not to interfere with subsequent hydrogenation. In addition, calculations for Cuo bound on the surface of the ZnO film have given results analogous to those for the Cu+ lattice ion. A strong stabilization of methoxide is observed in each case. Conclusions
Models of the (IOTO) nonpolar and (0001) polar faces of ZnO have been presented. These parameterized calculations represent the interaction of absorbates with the ZnO surface well. We predict C O to be a net donor, in agreement with experimental interpretations of vibrational data. In the presence of Cu+, the CO-surface bond energy is increased relative to the ZnO surface, with C O being a weaker donor. The computed site of C O adsorption next to the metal ion agrees with experiment. The orientation of CO to the ZnO surface is perpendicular for (1010) and (0001) surfaces. The enthalpy changes for various surface reactions have been estimated. These values are based upon computed heats of adsorption and experimental gas-phase bond energies. These enthalpy changes are correlated in an unknown way with activation energies, so they are not a complete guide for selecting reaction sequences. They do indicate that formation of H 2 C 0 from H and C O reactions requires energy input on all ZnO surfaces. An important role for Cu+ in the methanol-forming mechanism has been predicted. This involves stabilization of methoxy (OCH3) groups by interaction with the surface states induced within the band gap by Cu'. These states can donate charge to the strongly electron-withdrawing methoxy group. In the absence of Cu+, a hydroxymethyl mechanism appears to be a possible reaction course. This mechanistic proposal should be amenable to experimental tests.
Appendix A The procedure used to compute the band structure of ZnO films is similar to that used earlier for transition-metal films.I2 The determinant det(H - E S ) = 0
('41)
is solved in complex number space. In this regard, the Hamiltonian ( H ) or overlap (S) matrices are computed as a sum over several unit cells
where R, is the position vector of a unit cell and k is the wavevector or point in the two-dimensional Brillouin zone. The integral in eq A2 is evaluated by using standard Huckel procedures to compute diagonal and off-diagonal terms.I2 The complex-number secular determinant is solved at 9 points in E space uniformly distributed in the area enclosed by = 0 to k = ( l , O ) r / a to k = ( l , l ) r / u . The results are averaged over the points to compute energy or charge distribution by the Mulliken procedure. The basis atoms consist of a single Zn-0 unit, as described earlier in the text. Registry No. ZnO, 1314-13-2; CO, 630-08-0: CH,OH, 67-56-1; Cu, 7440-50-8.
