CO adsorption on ultrathin MgO films grown on a Mo(100) surface: an IRAS study

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CO adsorption on ultrathin MgO films grown on a Mo( 100) surface: an IRAS study Jian-Wei He, Cesar A. Estrada, Jason S. Corneille, Ming-Cheng Wu and D. Wayne Goodman

*

Department of Chemistry, Texas A&M Unicersity, College Station, TX 77843-3255, USA

Received 10 June 1991; accepted for publication 23 August 1991

CO adsorption on MgO ultrathin films grown on a Mo(100) surface is studied using infrared reflection absorption spectroscopy GRAS). CO adsorbed on 7 ML of MgO shows a stretch frequency at 2178 cm-’ which is blue-shifted relative to that of gas-phase CO (2143 cm-‘). This blue-shift is believed to arise from electron charge donation from the CO% orbitals to the MgO surface. The heat of adsorption of CO on 7 ML of MgO is estimated to be 9.9 kcal/mol using an isothermal adsorption method. CO adsorbed on the MgO thin films desorbs between 100 and 180 K, as indicated by temperature-programmed desorption.

1. Introduction Due to the importance of magnesium oxide (MgO) in heterogeneous catalysis [1,2], particularly in the oxidative coupling of methane [3], the interaction of CO with MgO surfaces has received considerable attention both experimentally and theoretically [4-61. In addition, since chemisorption and reaction of CO on oxide surfaces occur with the participation of oxygen, surface hydroxyl groups and coordinatively unsaturated surface cations, CO has been extensively used as a probe molecule to elucidate the nature of active sites on oxide surfaces (ref. [21 and references therein). In an early study using infrared spectros~py, Guglielminotti et al. [4] reported that CO, adsorbed on MgO powder at room temperature, yielded several absorption bands in the 1000-2200 cm-’ range. These bands have been assigned to correspond to CO molecules weakly bound to cationic sites and to negatively charged polymeric CO clusters. Plater0 et al. [5] reported that CO adsorbed onto polycrystalline MgO at 77 K ex-

* To whom correspondence ~39-6028/92/$05.00

should be addressed.

hibits IR peaks around 2150 cm-i. In addition, these CO adsorption states show a red-shift in the CO stretch frequency as the CO coverage is increased. Using an ab initio theoretical method, Colbourn and Mackrodt f6] found that CO binds to a defect-free Mg~lOO) surface only at or near a Mg*+ ion with a binding energy of _ 9 kcal/mol. The preferable configuration is with the CO perpendicular to the surface with the C atom attached to a Mg2+ ion. This calculation further predicted that the charge transfer between CO and the surface is small (O.O07e-I, and involves transfer of electrons from the CO.% orbitals to the oxide surface. An improved calculation by Pope et al. [61 computes the binding energy of CO to Mg2+ to be 8.6 kcal/mol for binding through C (Mg2+-CO) and IO.6 kcal/mol for binding through oxygen (Mg’+-00. Recently, we have studied CO adsorption onto MgO ultrathin films grown on a Mo(100) substrate using infrared reflection absorption spectroscopy (IRAS). The results are presented and discussed in this paper. Compared with a bulk single-crystal oxide, the study of thin oxide films grown on a metal substrate offers several advantages. For example, (a) it enables one to circumvent surface charging in the use of electron spec-

0 1992 - Elsevier Science Publishers B.V. Ail rights reserved

troscopies to characterize the thin film oxides, and (b) the properties of the thin film oxides, such as the density of surface defects and the size of clusters, can be tailored by varying the film thickness, the oxidation conditions, etc.

2. Experimental The experiments were performed in an ultrahigh vacuum chamber, described previously 171, equipped with infrared spectroscopy, Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). The sample was spot-welded to two Ta wires on the backface of the crystal which allowed resistive heating of the sample to 1500 K and cooling to 80 K. In addition, the sample could be heated to 2300 K via e--beam heating. The Mo(100) sample was cleaned using cycles of oxidation in oxygen (2 X 10s7 Torr, and a sample temperature, T,, of 1600 K) and annealing in vacuum CT, = 18~-20~ Kj. After this procedure, AES indicated a clean surface, with C, 0 and S impurities less than 1 at%, and

7 MLMgO/MO(iOO)

I

VA

1

/

MO

Ms 10

130

KINETIC

250

370

ENERGY

490

610

(elf)

Fig. 1. Auger electron spectra of clean, covered with 0.9 ML

of Mg, and covered with 7 ML of MgO Mo(100) surfaces.

LEED exhibited a sharp (1 x 1) substrate pattern. The infrared spectra were obtained in the single-reflection mode at an 85” incident angle with 4 cm-’ resolution. The spectra shown are raw data, corrected only for the baseline. High-purity Mg ribbon wrapped around a tungsten filament was used for the Mg deposition. Before each dose, the Mg source was extensively degassed. As illustrated in the AES spectra in fig. 1, no impurities accumulated on the surface during the Mg deposition and oxidation.

3. Results and discussion 3.1. Preparation of MgO thin films The MgO thin films were prepared using two methods: In the first method, Mg was deposited onto the Mo(100) surface in vacuum at a sample temperature of 90 K. The Mg coverage was then determined using the established relationship of the Mg(44 eV)/M~l86 eV) AES ratio versus Mg coverage [81. Next, the Mg/Mo(l~) surface was oxidized at 400 K in 1 X lo-’ Torr of 0, for 15 min. In the second method, Mg was deposited in an 0, background of 1 x lo-’ Torr at 400 K. The deposition rate was approximately 1 ML per minute (1 ML = 1.0 X 1015atoms/cm2, the atomic density of the Mo(100) surface). The MgO films synthesized with these two procedures displayed identical properties regarding CO adsorption. Fig. 1 shows the AES spectra of a clean Mo~lOO) surface, 0.9 ML of Mg and 7 ML of oxidized Mg on a Mo(l~) surface. These spectra indicate that no detectable carbon is present on any of these surfaces. Metallic Mg and Mg2” in magnesium oxide are characterized by AES peaks at 44 eV (MgL,,VV) and 32 eV (MgL,,,OL,,, OL,,), respectively [9,10]. The films prepared using these procedures have also been characterized by low-energy electron diffraction (LEED), electron energy loss spectroscopy (ELS), and high-resolution electron energy loss spectroscopy (HREELS) IS]. The detailed results regarding synthesis and characterization of ultrathin MgO films on MO (100) are presented elsewhere [8]. It has been shown that the HREELS spectra of the

J.-W He et ccl. / CO adsorption on

166

MgOfilms an Mo(lOO)

magnesium oxide thin films are nearly identical to those of single-c~stal Mg~lOO). The MgO films exhibit a one-to-one stoichiomet~ and exhibit the characteristic electronic properties of bulk MgO. LEED studies also have shown that the MgO films grow epitaxially with the (100) face parallel to the Mo(100) surface. The growth mode (Frank-van de Merwe or StranskiKrastanov) and the morphology of the MgO films are not well understood at present. However, studies regarding the morphology and microstructure of the MgO/Mo(lOO) system are currently underway in this laboratory.

.

.

-.

3.2. CO IR spectra at w6m.s CO insures Fig. 2 presents the IR spectra of CO adsorbed onto 7 ML of MgO supported on a MO (100) surface as a function of CO exposure. CO adsorption and IR spectral collection were carried out at a sample temperature of 90 K. Fig. 3 shows the peak frequency and integrated intensity versus the logarithm of the CO exposure. Two points in figs. 2 and 3 are noteworthy:

ho.02% I h

CO/MgO/Mo(lOO) 6~~0

=7ML

1

2170

CO EXPOSURE

6L

2200

2100

FREQUENCY

2000

(cm-‘)

Fig. 2. IR spectra of CO on 7 ML of MgO supported on a Mo(100) surface at the indicated CO exposures. The sample temperature during the CO exposure and IR spectral coliection was 90 K.

L_

0

i 1

LOG [EXF&“RE

(L;]

Fig. 3. The frequency and integrated intensity of the CO IR peak in fig. 2 as a function of log(C0 exposure).

(1) The CO adsorbed onto the MgO films shows a stretch frequency at 2178 cm-’ which is 35 cm-’ higher than that of gas-phase CO (2143 cm-‘). This blue-shift in the CO stretch frequency implies that the bonding between CO and the MgO surface arises mainly from electron donation from the CO5u orbitals to the MgO. Upon CO adsorption onto a solid surface, electrons in the C05a orbitals are donated to the substrate, and electrons from the substrate backdonated into the CO 2~* orbitals. On a metal surface, the electron charge transfer primarily consists of the backdonation. Because the 2~r* orbitals of CO are antibonding, this backdonation causes a weakening of the C-O bond, and, in turn, leads to a red-shift in the CO stretch frequency. Upon CO adsorption onto a Mg2+ cation site on a MgO surface, the charge transfer has been predicted to be from the CO5a orbitals to the Mg2+, rather than from the MgO surface to CO [6]. This 5a donation consequently strengthens the C-O bond, due to a stabilization of the

.I.- W. He et al. / CO ahorption

CO molecular orbitals that arises from the formation of a slightly positively charged CO molecule (CO”). Theoretical calculations have shown that the CO 5a orbital consists primarily of a lone pair of electrons on the carbon atom and essentially is nonbonding in character ill]. The removal of electron charge from the 5a orbital increases the electron affinity of the C atom, consequently stabilizing the CO molecular orbitals, and thereby strengthening the C-O bond [ill. It is, therefore, concluded that the blue-shift in the stretch frequency of CO on MgO arises primarily from the C05a donation, the essential component of the CO-MgO bond. A blue-shifted CO stretch frequency has been also reported for CO adsorbed onto other oxides such as TiO,, ZnO, and NiO

La. (2) In fig. 2, as the CO coverage increases, the peak frequency remains unchanged. CO adsorbed onto a metal surface usually displays a blue-shift in the stretch frequency as the CO coverage increases. The blue-shift is due to CO-CO interactions (vibrational coupling) and competition among CO molecules for backdonation from the surface (electrostatic effect) [12,131. On a transition metal surface, both effects lead to a blue-shift of the CO stretch frequency. On an oxide surface (NiO), however, it has been demonstrated that the dipole-dipole coupling blue-shifts the CO frequency (27 cm-‘), whereas the electrostatic effect red-shifts the CO frequency (43 cm-‘) [5]. Therefore, the invariant frequency in fig. 2 (for low CO surface coverages) may arise because of the compensation between a blue-shift CO due to dipole-dipole coupling and a red-shift CO due to electrostatic effects. For CO on a transition metal surface, as discussed above, electron charge transfers from the metal to the C02~* orbitals. As the CO coverage increases, more CO molecules compete for the electron charge. Consequently, less charge is transferred to each CO molecule, and the CO stretch frequency blueshifts accordingly. However, for the CO-MgO system, an increase in the CO surface coverage increases the number of electron donors, and thus decreases the overall electron charge transferred from each CO molecule to the substrate. As a result, the CO stretch frequency red-shifts

on MgOfilms

167

on Mo(100)

t&,o

=7ML

2170

CO PRESSURE

FREQUENCY

Fig. 4. IR spectra of CO on 7 ML of Mo(100) surface as a function of the sure. The sample temperature during spectral collection was

(cm-‘) MgO supported on a background CO presCO exposure and IR 90 K.

and approaches that of gas-phase CO. This redshift is clearly evident in the isothermal adsorption experiment, as shown in the following section. 3.3. Isothermal adsorption of CO In order to estimate the adsorption energy of CO on the MgO thin films, isothermal adsorption measurements were carried out. Fig. 4 shows the IR spectra of CO on 7 ML of MgO in background CO at the indicated pressure. The sample temperature during the spectral collection was 90 K. In fig. 4, as the CO pressure is increased, the CO IR intensity increases continuously, and the peak frequency red-shifts slightly. This experiment clearly indicates that only a portion of the MgO surface sites are populated with CO by a direct CO exposure at 90 K. The remaining adsorption sites apparently have a smaller activation energy for desorption and can only be populated either in background CO or at substrate temperatures < 90 K. The red-shift of the CO frequency (6 cm-‘) in fig. 4 is d ue to an electrostatic effect, i.e., an increase in the CO surface coverage leads

J.-W.. He et al. / CO adsorption on MgO films on Mo(lO0)

168

0.0 -

’ -5.0

This value agrees well with the theoretical one of 9.0 kcal/mol for CO on a MgO(100) surface 161, and the desorption activation energy of 10.6 kcal/mol estimated for CO on MgO thin films from temperature-programmed desorption measurements (see next section).

0

94K

c

98K

”

102K

3.4. CO temperature-programmed

I -6.0

-7.0

-8.0

LOG [CO PRESSURE

(TORR)]

Fig. 5. Frequency and integrated intensities of the IR peak of CO on 7 ML of MgO supported on a MoflOO) surface as a function of Iog(C0 pressure).

to a decrease in the donated charge from each CO molecule to the MgO substrate. Isothermal adsorptions were conducted at several temperatures, and the integrated IR intensities acquired as a function of CO exposure (fig. 5). Clausius-Clapyron plots of the CO coverages as indicated by the integrated IR intensities versus l/T are shown in fig. 6. The slopes correspond to a CO heat of adsorption of 9.9 kcal/mol.

r z

INTEGRATED

-3 1

5

g 2

-5 1

.

w g

-6.

.

B o

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desorption

Fig. 7 shows the temperature programmed desorption spectra of CO from N 10 ML of MgO on a Mo(100) surface following CO exposure at the indicated levels at 100 K; the heating ramp rate was 7 K/s. The TPD spectra exhibit a desorption feature with a peak temperature of 180 K. This peak temperature remains constant with an increase in CO coverage, indicating first-order desorption kinetics. This peak clearly does not saturate at CO exposures as high as 50 L, consistent with the IR results that showed a continuous increase in the CO IR intensity in the 2-1000 L CO exposure range (fig. 3). Assuming a frequency

/ I

CO/MgO/Mo(lOO)

A

g,,,go= -10

ML

INT.

.

0.005 1

.

0.004

.

0.003

:

s -a

t

-4 -9 tLYId 0.9

100

1.0

1.1

1 /T (K-’ )x1 O-2 Fig. 6. Clausius-Clapyron plots for CO adsorption on 7 ML of MgO supported on a MO (1001 surface. The data were taken from fig. 5.

260

420

560

TEMPERATURE

740

900

(K)

Fig. 7. Temperature-programmed desorption spectra of CO on 10 ML of MgO supported on a MotlOO) surface. The MgO thin film was dosed with CO at 100 K: (al 0.2 L. (bl 0.4 L, (c) 1.2 L, (d) 5 L, (e) 12 L, (fl 25 L and (g) 50 L. The heating ramp rate was 7 K/s.

169

J.-W He et al. / CO adsorption on MgO films on Mo(lO0)

factor of 1013, the desorption activation energy is estimated to be 10.6 kcal/mol using the Redhead approximation [14]. This value is in good agreement with the adsorption energy of 9.9 kcal/mol obtained in the isothermal adsorption experiments described in the previous section. The integrated area of spectrum (g) in fig. 7 is equal to that of the “(Y” peak in the TPD spectrum of CO from a Mo(100) surface. Previous work suggests that this “(Y” peak at saturation corresponds to - 0.5 ML of CO [El. The spectrum (g) in fig. 7, therefore, corresponds to approximately 0.5 ML of CO on the MgO thin films.

I 0

1

1

2

3

4

5

eMgO

(ML)

6

7

Fig. 9. The integrated intensity of the CO IR peak in fig. 8 as a function of MgO coverage.

3.5. CO IR spectra at various A4gO coverages Fig. 8 shows the IR spectra of CO adsorbed onto MgO thin films supported on a Mo(100) surface at several MgO coverages. The CO exposure (- 1000 L) and IR spectral collection were carried out with the sample at 90 K. For the IR spectrum acquired at zero coverage of MgO, the Mo(100) surface was exposed at 400 K to lo-’ Torr of oxygen for 15 min (the same temperature

0 !,L._,. ,.i,,::,+&+&j 2200

2100

FREQUENCY

2000

(cm-‘)

Fig. 8. IR spectra of CO adsorbed on various coverages of MgO on Mo(100). CO exposure ( - 1000 L) and IR spectral collection were carried out at a sample temperature of 90 K.

and oxygen background pressure used for the preparation of the MgO thin films). For this oxidized Mg(100) surface, a peak at 2201 cm-i following CO adsorption is evident. As the MgO coverage increases above 0.9 ML, a CO stretch frequency at 2178 cm-’ is evident and remains essentially unchanged up to 7 ML. A plot of the CO IR intensity as a function of MgO coverage is presented in fig. 9. Initially, the integrated CO intensity increases from 0 to - 1 ML. A further increase in the MgO coverage from l-7 ML leads to a decrease in the integrated intensity. In the absence of significant changes in the IR crosssection of CO between a MgO coverage of 1 and 7 ML, integrated intensity should merely reflect the overall capacity of the surface to adsorb CO. These results then suggest that the MgO films formed at the l-2 ML level are relatively rough and apparently become considerably smoother as the coverage increases above 2 ML. It is also possible that an increase in the MgO film thickness results in an increase in the number of steps (enhanced film roughness). CO could then adsorb on the steps with the C-O axis parallel to the substrate surface and thus will not have a component of the vibrational dipole in the direction perpendicular to the surface. Consequently, the population of CO assuming the perpendicular bonding geometry on the MgO films is decreased leading to the decrease in the CO IR intensity, as observed in fig. 9.

170

J.-W. He et al. / CO adsorption on MgO films on Mo(100)

4. Summary and conclusions (1) CO adsorbed onto 7 ML of MgO ultrathin films grown on a Mo(100) surface shows a stretch frequency at 2178 cm-’ which is 35 cm-’ higher than the stretch frequency of gas-phase CO (2143 cm-i). This blue-shift in CO stretch frequency is explained as due to the formation of the COMgO bond that predominantly consists of electron donation from the Co50 orbital to the MgO substrate. (2) The heat of CO adsorption on 7 ML of MgO thin films is deduced to be 9.9 kcal/mol utilizing an isothermal adsorption method. (3) CO adsorbed onto 10 ML of MgO thin films exhibits a peak at N 180 K in the temperature-programmed desorption spectra with a corresponding desorption activation energy of 10.6 kcal/mol. (4) As the CO coverage increases, the CO stretch frequency red-shifts and is interpreted to arise from an electrostatic effect. An increase in the CO coverage increases the number of electron donors, and thus decreases the charge donated from each CO molecule to the substrate. The CO stretch frequency thereby red-shifts toward that of gas-phase CO. (5) The IR integrated intensity of CO on the MgO thin films decreases in the 1-7 ML MgO coverage range, suggesting that the MgO films become significantly smoother as the coverage is increased above the first monolayer.

Acknowledgement We acknowledge with pleasure the support of this work by the Department of Energy, Office of

Basic Science, Division of Chemical Sciences and the Gas Research Institute.

References 111B.C.

Gates, J.R. Katzer and G.C.A. Schuit, Chemistry of Catalytic Process (McGraw-Hill, New York, 1979). of Adsorbed 121A.A. Davydov, Infrared Spectroscopy Species on the Surfaces of Transition Metal Oxides, Ed. C.H. Rochester (Wiley, New York, 1990). [31 J.H. Lunsford, Langmuir 5 (19891 12. S. Coluccia, E. Garrone, S. Cerruti [41 E. Guglielminotti, and A. Zecchina, J. Chem. Sot. Faraday Trans. I, 75 (19791 96. [51 E.E. Platero, D. Scarano, G. Spoto and A. Zecchina, J. Chem. Sot. Faraday Trans. I, 80 (1985) 183. (61 E.A. Colbourn and W.C. Mackrodt, Surf. Sci. 143 (1984) 391; S.A. Pope, I.H. Hillier, M.F. Guest, E.A. Colburn and J. Kendrick, Surf. Sci. 139 (19841 299. 171J.-W. He, W.K. Kuhn, L.-H. Leung and D.W. Goodman, J. Chem. Phys. 93 (1990) 7463. RI M.-C. Wu, J.S. Corneille, C.A. Estrada, J.-W. He and D.W. Goodman, Chem. Phys. Lett. 182 (1991) 472. [91 V.M. Bermudez and V.H. Ritz, Surf. Sci. 82 (1979) L601. [lOI H. Namba, J. Darville and J.M. Gilles, Surf. Sci. 108 (19811 446. 10 (1960) 212. [ill H.H. Jaffe and M. Orchin, Tetrahedron [la F.M. Hoffmann, Surf. Sci. Rep. 3 (1983) 107. [131 R. Ryberg, in: Advances in Chemical Physics Ed. R.P. Lawley, (Wiley, New York, 1989) p. 1. [I41 P.A. Redhead, Vacuum 12 (19621203. 1151 E.I. Ko and R.J. Madix, Surf. Sci. 109 (1981) 221.