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CO adsorption on oxygen-modified molybdenum surfaces
Journal of Physics and Chemistry of Solids 72 (2011) 744–748
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CO adsorption on oxygen-modified molybdenum surfaces N.V. Petrova Institute of Physics of National Academy of Sciences of Ukraine, Prospect Nauki 46, Kiev 03028, Ukraine
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2010 Received in revised form 3 January 2011 Accepted 14 March 2011 Available online 21 March 2011
Structures of carbon monoxide layers on the oxygen-modified Mo(1 1 0) and Mo(1 1 2) surfaces have been investigated by means of density-functional (DFT) calculations. It is found that CO molecules adsorb at hollow sites on the O/Mo(1 1 0) surface and nearly atop Mo atoms on the O/Mo(1 1 2) surface. The favorable positions for adsorption are shown to be near protrusions of electron density above the Mo surface atoms. The presence of oxygen on the molybdenum surface significantly reduces the binding energy of the CO molecule with the substrate; on the oxygen-saturated Mo(1 1 0) surface, the adsorption of CO is completely blocked. The calculated local densities of states (LDOS) demonstrate that the O 2s peak for O adsorbed on Mo(1 1 0) surface is at 19 eV (with respect to the Fermi level), while for the oxygen atom of an adsorbed CO molecule the related 3s molecular orbital gives rise to a peak at 23 eV. This difference stems from the bonding of the O atom either with Mo surface for adsorbed O or with C atom in adsorbed CO, and therefore the position of the O 2s peak in photoemission spectra can serve as a convincing argument in favor of either the presence or absence of the CO dissociation on Mo surfaces. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Surfaces C. Ab initio calculations D. Surface properties D. Electronic structure
1. Introduction Molybdenum oxide is one of the most widely used catalysts for various chemical reactions. This is partly due to the variety of oxidation states of Mo, so that the reactivity of the surface can be easily controlled by changing the concentration of oxygen and other modifiers [1]. In turn the adsorption of CO on transition metals and oxides is one of the elementary steps of many catalytic reactions, such as the car exhaust catalysis and the Fischer– Tropsch synthesis [2]. The CO adsorption on oxygen-modified Mo(1 1 0) and Mo(1 1 2) surfaces has been studied experimentally by temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS) and infrared absorption spectroscopy (IRAS) [3–5]. It has been found that TPD spectra for CO on Mo(1 1 2) strongly depend on the oxygen coverage. With increasing oxygen coverage, the low-temperature spectral peak shifts from 305 to 280 K, and a new peak arises at 220 K. On the p(1 2)– O/Mo(1 1 2) surface (with oxygen coverage yO ¼1) only the high temperature peak (280 K) was observed and the binding energy for CO in this adsorption state was estimated as 0.75 eV [3]. The calculated binding energies for CO with clean Mo(1 1 2) and Mo(1 1 0) surfaces are about 1.76–2.1 eV depending on the CO coverage [6,7]. For the coadsorbed CO and oxygen on W(1 1 0) and W(1 1 3) surfaces, a similar decrease of the heat of adsorption of CO with increasing oxygen concentration was reported [8].
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The coadsorption of oxygen and CO on Ru(0 0 0 1) surface has been studied by thermal desorption spectroscopy, LEED and HREELS [9,10]. It has been shown that for the lower-coverage phases of oxygen (yO ¼0.5), CO adsorbs on the surface and the bond between CO and Ru is weakened by the coadsorption with oxygen, as indicated by the significant decrease of the CO desorption temperature [9,10]. For the oxygen coverage close to 1.0, for which the CO oxidation rate was assumed to be optimal, the adsorption of CO is nearly prohibited [11]. In contrast, CO molecules prefer to desorb as CO2 from the oxygen-modified Pt(1 1 1) (with the activation energy of the oxidation reaction being about 0.5–1.1 eV [12–15]). The activation energy for the CO desorption from clean Pt(1 1 1) surface is about 1.2 eV [16–23]. In present paper, the adsorption of CO on the oxygen-modified Mo(1 1 0) and Mo(1 1 2) surfaces is studied by means of densityfunctional (DFT) calculations of binding (adsorption) energies and local densities of states (LDOS). It is found that the presence of one monolayer (ML) of oxygen on the Mo(1 1 2) surface substantially reduces the binding energy of the CO molecule, while on Mo(1 1 0) it completely blocks the adsorption of CO.
2. Method of calculations The DFT semirelativistic calculations were carried out using CASTEP code [24], ultrasoft pseudopotentials [25] and generalized gradient approximation (GGA) in the Perdew–Burke–Ernzerhof form [26]. The surfaces were simulated by using the repeat-slab
N.V. Petrova / Journal of Physics and Chemistry of Solids 72 (2011) 744–748
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Fig. 1. (a)–(c): Optimized structures of oxygen-modified Mo surfaces: (a) (1 1) O/Mo(1 1 2) (yO ¼1.0 ML), (b) (1 1) O/Mo(1 1 0) (yO ¼1.0 ML) and (c) (2 1) O/Mo(1 1 0) (yO ¼ 0.5 ML); (d)–(f) corresponding distributions of the density of unoccupied electron states near the Fermi level along the surface. Arrows indicate the protrusions in density near Mo atoms as possibly favorable places for CO adsorption.
model, with CO and O adsorbed on one side of the slab. The surface unit cells were chosen to be (1 1) for Mo(1 1 2) and (2 1) for Mo(1 1 0) and the thickness of the slabs was of 7 layers ˚ both for Mo(1 1 2) and Mo(1 1 0), with the vacuum gap 10 A. The positions of the oxygen atoms, CO molecules and Mo atoms of three surface layers were optimized (using BFGS [27] optimization procedure) until the forces on atoms converged to less than ˚ The efficiency of the Brillouin zone sampling, using 0.03 eV/A. various k-point lattices, was carefully verified by increasing the number of k-points until the required 0.01 eV convergence of total energies and about 0.005 A˚ accuracy of atomic positions were achieved (6 9 1 and 6 6 1 Monkhorst–Pack [28] sets of special k-points were found sufficient for Mo(1 1 2) and Mo(1 1 0), respectively). All calculations were performed with cut-off energy of 340 eV. The local densities of states (LDOS) were calculated using the linear interpolation scheme. Estimates of partial LDOS were obtained by expansion of electron densities into spherical harmonics around specified atoms. The atomic radii were about half of the nearest-neighbor interatomic distance. Before depositing CO and oxygen atoms, the slabs were relaxed, i.e. all the atoms were allowed to adjust their positions to minimize total energy of the system. The optimization of atomic positions for clean Mo(1 1 0) and Mo(1 1 2) surfaces, in agreement with results of other calculations [7,29], resulted in 5% contraction of the topmost Mo surface layer with respect to the related interplane distance in bulk Mo (the estimated lattice ˚ Only minor relaxation shifts constant of the bulk Mo was 3.15 A). were found for the second and the third layers. Optimization of atomic positions of adsorbed oxygen atoms along with Mo atoms of the three surface layers led on average to a ‘‘backward’’ relaxation shift of the Mo(1 1 0) and Mo(1 1 2) surfaces. The binding energies (positive) of CO were defined as –Eb ¼E– EO þ Mo –ECO, where E, EO þ Mo and ECO are the total energies of the COþO/Mo adsorption system, the oxygen-precovered Mo surface and a CO molecule, respectively. The energies were determined taking into account the relaxation of the oxygen-covered molybdenum surface as well as the CO-induced surface relaxation. The coverage y for all species was defined with respect to the number of substrate atoms in the surface unit cell.
3. Results and discussion 3.1. Structures and binding energies In the present paper, the adsorption of CO was studied on three different oxygen-modified molybdenum surfaces: (1 1)
O/Mo(1 1 2) (yO ¼1.0 ML), (1 1) O/Mo(1 1 0) (yO ¼1.0 ML) and (2 1) O/Mo(1 1 0) (yO ¼ 0.5 ML). On the Mo(1 1 2) surface, favorable adsorption positions for O atoms at yO ¼1.0 ML are quasithreefold sites in furrows [29]. In LEED experiments for O/Mo(1 1 2) at yO ¼1.0 ML one originally observes a (1 1) pattern, while an annealing to the 500–600 K leads to the formation of (1 2) pattern [3,30]. The results of DFT calculations [29] show that total energies for these structures are almost the same, so they might be considered equally favorable. The LEED studies of structures of oxygen layers on the Mo(1 1 0) surface [31–33] have shown that oxygen forms various structures, starting from c(2 2) O at y ¼ 0.25 and finishing with (1 1) O at yO ¼1.0. Previous DFT calculations [34] suggested that triply coordinated hollow sites of the Mo(1 1 0) surface are strongly favorable for oxygen atoms for all coverages and the p(2 1)O structure forming at yO ¼ 0.5 is found to be favorable with regard to other possible structures. The structures of oxygen-modified surfaces used in the present paper as the substrate for CO adsorption are shown in Fig. 1(a)–(c). The distributions of densities of unoccupied electron states on the oxygen-modified surfaces in the energy range from EF to 1 eV (which can be expected to be responsible for the formation of the CO–surface adsorption bonds) are presented in Fig. 1(d)–(f). It can be seen that the density distributions for the (2 1) O structure on the Mo(1 1 2) surface and for the (1 2) O structure on the Mo(1 1 0) surface reveal significant protrusions above the Mo atoms, while for the (1 1) O structure on the Mo(1 1 0) surface the protrusions are small and are located above the oxygen atoms. In contrast, distributions of electron densities of occupied states near EF have protrusions only above O atoms and, as such reproduce just the ballmodels of the surfaces without any significant differences between distributions of densities for these systems. To obtain the optimal CO structure on the oxygen-modified molybdenum surface, the CO molecule was placed at various possible positions in the unit cell. On O/Mo(1 1 2) surface, such sites are quasi-threefold in furrow, and twofold in furrow and atop Mo atoms. In the course of optimization (along with two molybdenum and oxygen layers), CO molecules initially deposited at arbitrary positions on the surface, move towards on-top sites on Mo rows. In the optimized structure, CO molecules at the ontop sites are tilted by 22.41 with respect to the normal to the surface (Fig. 2(a)). The other sites appear unstable and the optimization procedure leads either to desorption or displacement of CO molecules to the on-top positions. The strong preference of the on-top sites with respect to sites in furrows results in the formation of a p(1 1) CO structure. The adsorption of CO does not significantly affect the location of the adsorbed oxygen atoms, so that the bond length of O atoms
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Fig. 2. Optimized CO structures on: (a) (1 1) O/Mo(1 1 2) (yO ¼1.0 ML) surface and (b) (2 1) O/Mo(1 1 0) (yO ¼ 0.5 ML) surface.
3.2. Density of states To reveal correlations of binding energies and adsorption properties with related changes of the electronic structure of the surface, the local densities of states have been calculated for several characteristic structures. The estimated DOS for the (COþO)/Mo(1 1 0) and (COþO)/Mo(1 1 2) surfaces, presented in Fig. 3, were constructed for the surface layer consisting of a CO molecule, adsorbed oxygen atom and two Mo atoms of the upper layer. The peaked structure in the range from EF to 5 eV in both spectra is originated by Mo d states, while the other peaks in the spectra are originated from CO molecular orbitals and atomic states of adsorbed oxygen. Specifically most of the peaks can be attributed to molecular orbitals of CO, as indicated in Fig. 3, while
DOS (states/eV)
6
1π
5 4
5σ 3σ
3 4σ
2 1 0 -25
-20
-15
-10
-5
0
E (eV)
6 DOS (states/eV)
˚ The with substrate atoms remains constant within 0.02 A. obtained binding energy of CO with the O/Mo(1 1 2) surface, Eb ¼0.87 eV, is in reasonable agreement (taking into account known overestimates of GGA binding energies) with the experimental value of 0.75 eV [3]. It should be noted that the binding energy on the oxygen-modified Mo(1 1 2) surface is 1 eV less than the binding energy of CO with a clean Mo(1 1 2) surface (1.76–2.1 eV at various CO coverages [6,7]), while the bond length ˚ adsorbed on O/Mo(1 1 2) is almost in the CO molecule (1.16 A) equal to that for CO on a clean Mo(1 1 2) surface. The CO adsorption on the oxygen-modified Mo(1 1 0) surface has been simulated in the same way. Nonetheless, in contrast to O/Mo(1 1 2) surface, on which CO adsorbs with a substantial binding energy, the CO molecule absolutely does not stick to the oxygen-saturated Mo(1 1 0) surface. Evidently, this is due to the fact that adsorption of the oxygen monolayer on the Mo(1 1 0) surface causes a complete poisoning of the molybdenum surface that leads to the blockage of the CO adsorption. Note that this feature apparently correlates with the absence of protrusions in the density of electron states above Mo atoms for the oxygensaturated Mo(1 1 0) surface (see Fig. 1(e)). On Mo(1 1 0) surface partly covered by oxygen, the adsorption of CO is possible. The optimized CO structure on the p(2 1) O/Mo(1 1 0) surface (yO ¼0.5) is presented in Fig. 2(b). The CO molecules occupy quasi-threefold positions near topmost Mo atoms and form a p(2 1) structure. The adsorbed CO molecules in this configuration are tilted by 15.721 with respect to the normal to the surface and do not noticeably affect the position of the adsorbed oxygen atoms. The obtained binding energy for CO molecules is found to be 1.05 eV, which is 1.0 eV less than the binding energy of CO with a clean Mo(1 1 0) surface (2.1 eV [7]). Like in the case of adsorption on O/Mo(1 1 2) surface, the positions of the adsorbed CO molecules correlate well with the location of protrusions in the density of the electron states (see Fig. 1(d) and (f)).
5σ
5 4
1π
3σ
3
4σ
2 1 0 -25
-20
-15
-10 E (eV)
-5
0
Fig. 3. Calculated local density of states for (a) (1 1) CO/O/Mo(1 1 2) and (b) for (2 1) CO/O/Mo(1 1 0) structures. Contributions from CO molecular orbitals to DOS are indicated.
the peak at 19 eV, which has no counterpart in the spectrum of CO molecule, is originated by the adsorbed O. The DOS for (COþO)/Mo(1 1 0) and (COþO)/Mo(1 1 2) are found to be very similar. A small difference between positions of the peaks originating from 3s CO orbital, i.e. 23 eV for (COþO)/Mo(1 1 0) and 24 eV for CO þO/Mo(1 1 2), can be attributed to different coordination numbers for CO molecules on Mo(1 1 0) and Mo(1 1 2) surfaces. As on the O/Mo(1 1 2) surface CO adsorbs nearly atop Mo it is bonded only with the Mo atom of the topmost row while on O/Mo(1 1 0), CO adsorbs in hollow sites; so there are three Mo atoms that contribute to CO–Mo bonding. As a result, the binding energy of CO molecule with the O/Mo(1 1 0) surface exceeds that of CO with the O/ Mo(1 1 2) surface by 0.18 eV, which may also affect the position of the CO 3s peak. The contribution of particular atomic states to the net spectrum can be determined from partial local densities of states.
N.V. Petrova / Journal of Physics and Chemistry of Solids 72 (2011) 744–748
DOS (states/eV)
3
O on Mo(110) O 2p
2 O 2s 1
0 -25
-20
DOS (states/eV)
3
2
-15
-10 E (eV)
-5
0
O in CO on O/Mo(110)
3σ
5σ 1π
1
4σ
0 -25
-20
DOS (states/eV)
3
-15
-10 E (eV)
-5
0
C in CO on O/Mo(110)
2
5σ 1
4σ
3σ
1π
0 -25
-20
-15
-10 E (eV)
-5
0
Fig. 4. (a) Local densities of states for CO/O/Mo(1 1 0) on adsorbed oxygen atom, (b) oxygen atom of adsorbed CO molecule and (c) carbon atom in adsorbed CO molecule . The s and p states are shown by black and red lines, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The calculated LDOS on the adsorbed O as well as on O and C atoms in CO molecule adsorbed on the Mo(1 1 0) surface are shown in Fig. 4(a)–(c). For an adsorbed oxygen atom (Fig. 4(a)), the peak at 19 eV originated by O 2s state, while the characteristic peaks at 6.5 and 5 eV originated predominantly by the interaction of O 2p electrons with Mo surface. In contrast, for the oxygen atom in adsorbed CO molecule, related CO 3s peak in LDOS (which stems from the O 2s state) is at –23 eV (Fig. 4(b)). The peaks 23, –9, 7 and 5 eV in the LDOS on the carbon atom (Fig. 4(c)) the and can be attributed to 3s, 4s, 5s and 1p molecular orbitals of adsorbed CO, respectively. The significant difference in the position of the O 2s peak for adsorbed oxygen atom and 3s peak of a CO molecule is due to the bonding of the O either with Mo or C. This feature can serve as a decisive argument in the vital discussion about the form of CO adsorption on Mo surfaces. The angle-resolved ultraviolet
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photoemission spectroscopy (UPS) as well as the near-edge X-ray absorption fine structure studies [35–37] indicates that at low coverages CO molecules on W and Mo surfaces are tilted (the b state), while at high coverages they are oriented normal to the surface with the C atom down (the a state). Using high-resolution core-level spectroscopy and UPS for CO adsorption on Mo(1 1 0), Jaworowski et al. [37] suggested thermally activated dissociation of CO molecules in the b state (i.e. at low CO coverages). In contrast the TDS and UPS studies of CO adsorption on W(1 1 0) and Mo(1 1 0) surfaces [38–40] suggested that the b state corresponds to a tilted orientation of CO on W(1 1 0) and Mo(1 1 0) but not to dissociated CO molecules, thus indicating an absence of CO dissociation. It was concluded therefore that the b peak in thermodesorption spectra in fact corresponds to the desorption of virgin CO rather than the recombinative desorption of previously dissociated CO [38–40]. As the order of desorption of b-CO might be considered unconvincing, it is interesting to compare the evidences proand contra-CO dissociation on Mo based on XPS and UPS studies in Refs. [37,,38], respectively. Thus, in Ref. [37] the dissociation of CO on Mo(1 1 0) was suggested predominantly from; (i) the observation of seemingly related chemical shift of the C 1s line (it was claimed that the O 1s line is diffuse and hence difficult to observe) and (ii) a picturesque transformation of the UPS spectrum with heating from 180 to 430 K, which was believed to provide the evidence of CO dissociation. It should be noted, however, that the carbon atom is bound to the surface, being adsorbed either separately or as a part of a CO molecule. Hence the shift of the C 1s line does not necessarily indicate the dissociation of CO, but could be caused by the changing distribution of electrons when the CO coverage decreases. The changes in the valence band spectrum, in turn, could hardly prove the dissociation because of the overlap of the wide O 2p peak (about 5–7 eV binding energy, see Fig. 4(a) and (c)) and the combined 1p þ 5s peak of adsorbed CO molecule, observed in the photoemission spectra in the same energy range [37]. Hence the changes observed in the valence band on heating can be explained by a partial desorption of CO, which at 180 K can form a multilayer film on Mo(1 1 0), while after heating to 430 K only one monolayer remains on the surface [38]. The observed changes in the valence band spectra should be attributed therefore to different binding energies with the surface for CO molecules in the submonolayer and multilayer films and do not necessarily indicate the dissociation of CO. Kim et al. [38] arrived at similar conclusions in their UPS study of the valence band of CO/Mo(1 1 0) and CO/W(1 1 0) systems. In particular they found that the peak at a binding energy of 7.0 eV can be assigned to the overlap of the 5s and 1p orbitals of b-CO, whereas the feature at about 11 eV is ascribed to the 4s orbital. In addition, while the chemisorbed CO species were detected by the presence of characteristic peaks at –8 and –11 eV binding energies, the valence band spectra did not reveal any adsorbed O atoms on the surface. It is the difference of binding energies of the O 2s and peaks in photoemission spectra that can prove or discard the dissociation of CO on the Mo surface. Specifically the 19 eV peak is characteristic of an adsorbed O atom (see Fig. 4(a)), so its appearance in the spectra would prove the dissociation, while the presence of the 23 eV peak corresponds to the CO 3s molecular state. It should be noted also that DOS for CO/Ni(1 1 1) [41], where the adsorption of CO is definitely nondissociative, demonstrate the 3s peak at 24 eV. Unfortunately, in Refs. [38–40], the spectra of the valence band for CO on Mo(1 1 0) and W(1 1 0) are given only up to 17 eV binding energy, so that the calculated positions of the O 2s and CO 3s peaks can be compared only with the UPS results presented in
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Ref. [37]. In the photoemission spectra for CO on Mo(1 1 0), obtained after the annealing at T¼430 K (presented in Fig. 4 of [37]), the peak at 22 eV is well discernable while there is no sign of the 19 eV peak, which, as followed from present calculations, is characteristic of adsorbed O. This means that the spectrum for CO on Mo(1 1 0), despite the significant transformation of the valence band after heating with respect to the spectrum obtained at T¼180 K, still corresponds to adsorbed non-dissociated CO rather than separately adsorbed C and O.
[3] [4] [5] [6] [7] [8] [9] [10] [11]
4. Conclusion
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
It is found that the CO molecules will adsorb at hollow sites on the O/Mo(1 1 0) surface and nearly atop Mo atoms on the O/Mo(1 1 2) surface. Adsorbed CO molecules are tilted with respect to the normal to the surface. The favorable positions for adsorption correlate with protrusions of the spatial density of vacant electronic states above the Mo surface atoms. The presence of oxygen on the molybdenum surface significantly reduces the binding energy of the CO molecule with the substrate, and on the oxygen-saturated Mo(1 1 0) surface the adsorption of CO is completely blocked. The calculated local densities of states (LDOS) demonstrate that the O 2s peak for O adsorbed on Mo(1 1 0) surface is at 19 eV (with respect to the Fermi level) while for the oxygen atom of an adsorbed CO molecule the related 3s molecular orbital gives rise to the peak at –23 eV. This difference stems from the bonding of the O atom either with Mo surface for adsorbed O or with C atom in adsorbed CO and therefore the position of the O 2s (or related CO 3s) peak in photoemission spectra can serve as a convincing argument in favor of either the presence or absence of CO dissociation on Mo surfaces. References [1] G. Wu, B. Bartlett, W.T. Tysoe, J. Mol. Catal. A 131 (1998) 91. [2] F. Morales, B.M. Weckhuysen, Catalysis 19 (2006) 1; M. Bowker, A.F. Carley, M. House, Catal. Lett. 120 (2008) 34; O.G.M. Flores, S. Ha, Appl. Catal. A: Gen. 352 (2009) 124.
[12] [13]
[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]
[36] [37] [38] [39] [40] [41]
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CO adsorption on oxygen-modified molybdenum surfaces N.V. Petrova Institute of Physics of National Academy of Sciences of Ukraine, Prospect Nauki 46, Kiev 03028, Ukraine
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2010 Received in revised form 3 January 2011 Accepted 14 March 2011 Available online 21 March 2011
Structures of carbon monoxide layers on the oxygen-modified Mo(1 1 0) and Mo(1 1 2) surfaces have been investigated by means of density-functional (DFT) calculations. It is found that CO molecules adsorb at hollow sites on the O/Mo(1 1 0) surface and nearly atop Mo atoms on the O/Mo(1 1 2) surface. The favorable positions for adsorption are shown to be near protrusions of electron density above the Mo surface atoms. The presence of oxygen on the molybdenum surface significantly reduces the binding energy of the CO molecule with the substrate; on the oxygen-saturated Mo(1 1 0) surface, the adsorption of CO is completely blocked. The calculated local densities of states (LDOS) demonstrate that the O 2s peak for O adsorbed on Mo(1 1 0) surface is at 19 eV (with respect to the Fermi level), while for the oxygen atom of an adsorbed CO molecule the related 3s molecular orbital gives rise to a peak at 23 eV. This difference stems from the bonding of the O atom either with Mo surface for adsorbed O or with C atom in adsorbed CO, and therefore the position of the O 2s peak in photoemission spectra can serve as a convincing argument in favor of either the presence or absence of the CO dissociation on Mo surfaces. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Surfaces C. Ab initio calculations D. Surface properties D. Electronic structure
1. Introduction Molybdenum oxide is one of the most widely used catalysts for various chemical reactions. This is partly due to the variety of oxidation states of Mo, so that the reactivity of the surface can be easily controlled by changing the concentration of oxygen and other modifiers [1]. In turn the adsorption of CO on transition metals and oxides is one of the elementary steps of many catalytic reactions, such as the car exhaust catalysis and the Fischer– Tropsch synthesis [2]. The CO adsorption on oxygen-modified Mo(1 1 0) and Mo(1 1 2) surfaces has been studied experimentally by temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS) and infrared absorption spectroscopy (IRAS) [3–5]. It has been found that TPD spectra for CO on Mo(1 1 2) strongly depend on the oxygen coverage. With increasing oxygen coverage, the low-temperature spectral peak shifts from 305 to 280 K, and a new peak arises at 220 K. On the p(1 2)– O/Mo(1 1 2) surface (with oxygen coverage yO ¼1) only the high temperature peak (280 K) was observed and the binding energy for CO in this adsorption state was estimated as 0.75 eV [3]. The calculated binding energies for CO with clean Mo(1 1 2) and Mo(1 1 0) surfaces are about 1.76–2.1 eV depending on the CO coverage [6,7]. For the coadsorbed CO and oxygen on W(1 1 0) and W(1 1 3) surfaces, a similar decrease of the heat of adsorption of CO with increasing oxygen concentration was reported [8].
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The coadsorption of oxygen and CO on Ru(0 0 0 1) surface has been studied by thermal desorption spectroscopy, LEED and HREELS [9,10]. It has been shown that for the lower-coverage phases of oxygen (yO ¼0.5), CO adsorbs on the surface and the bond between CO and Ru is weakened by the coadsorption with oxygen, as indicated by the significant decrease of the CO desorption temperature [9,10]. For the oxygen coverage close to 1.0, for which the CO oxidation rate was assumed to be optimal, the adsorption of CO is nearly prohibited [11]. In contrast, CO molecules prefer to desorb as CO2 from the oxygen-modified Pt(1 1 1) (with the activation energy of the oxidation reaction being about 0.5–1.1 eV [12–15]). The activation energy for the CO desorption from clean Pt(1 1 1) surface is about 1.2 eV [16–23]. In present paper, the adsorption of CO on the oxygen-modified Mo(1 1 0) and Mo(1 1 2) surfaces is studied by means of densityfunctional (DFT) calculations of binding (adsorption) energies and local densities of states (LDOS). It is found that the presence of one monolayer (ML) of oxygen on the Mo(1 1 2) surface substantially reduces the binding energy of the CO molecule, while on Mo(1 1 0) it completely blocks the adsorption of CO.
2. Method of calculations The DFT semirelativistic calculations were carried out using CASTEP code [24], ultrasoft pseudopotentials [25] and generalized gradient approximation (GGA) in the Perdew–Burke–Ernzerhof form [26]. The surfaces were simulated by using the repeat-slab
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Fig. 1. (a)–(c): Optimized structures of oxygen-modified Mo surfaces: (a) (1 1) O/Mo(1 1 2) (yO ¼1.0 ML), (b) (1 1) O/Mo(1 1 0) (yO ¼1.0 ML) and (c) (2 1) O/Mo(1 1 0) (yO ¼ 0.5 ML); (d)–(f) corresponding distributions of the density of unoccupied electron states near the Fermi level along the surface. Arrows indicate the protrusions in density near Mo atoms as possibly favorable places for CO adsorption.
model, with CO and O adsorbed on one side of the slab. The surface unit cells were chosen to be (1 1) for Mo(1 1 2) and (2 1) for Mo(1 1 0) and the thickness of the slabs was of 7 layers ˚ both for Mo(1 1 2) and Mo(1 1 0), with the vacuum gap 10 A. The positions of the oxygen atoms, CO molecules and Mo atoms of three surface layers were optimized (using BFGS [27] optimization procedure) until the forces on atoms converged to less than ˚ The efficiency of the Brillouin zone sampling, using 0.03 eV/A. various k-point lattices, was carefully verified by increasing the number of k-points until the required 0.01 eV convergence of total energies and about 0.005 A˚ accuracy of atomic positions were achieved (6 9 1 and 6 6 1 Monkhorst–Pack [28] sets of special k-points were found sufficient for Mo(1 1 2) and Mo(1 1 0), respectively). All calculations were performed with cut-off energy of 340 eV. The local densities of states (LDOS) were calculated using the linear interpolation scheme. Estimates of partial LDOS were obtained by expansion of electron densities into spherical harmonics around specified atoms. The atomic radii were about half of the nearest-neighbor interatomic distance. Before depositing CO and oxygen atoms, the slabs were relaxed, i.e. all the atoms were allowed to adjust their positions to minimize total energy of the system. The optimization of atomic positions for clean Mo(1 1 0) and Mo(1 1 2) surfaces, in agreement with results of other calculations [7,29], resulted in 5% contraction of the topmost Mo surface layer with respect to the related interplane distance in bulk Mo (the estimated lattice ˚ Only minor relaxation shifts constant of the bulk Mo was 3.15 A). were found for the second and the third layers. Optimization of atomic positions of adsorbed oxygen atoms along with Mo atoms of the three surface layers led on average to a ‘‘backward’’ relaxation shift of the Mo(1 1 0) and Mo(1 1 2) surfaces. The binding energies (positive) of CO were defined as –Eb ¼E– EO þ Mo –ECO, where E, EO þ Mo and ECO are the total energies of the COþO/Mo adsorption system, the oxygen-precovered Mo surface and a CO molecule, respectively. The energies were determined taking into account the relaxation of the oxygen-covered molybdenum surface as well as the CO-induced surface relaxation. The coverage y for all species was defined with respect to the number of substrate atoms in the surface unit cell.
3. Results and discussion 3.1. Structures and binding energies In the present paper, the adsorption of CO was studied on three different oxygen-modified molybdenum surfaces: (1 1)
O/Mo(1 1 2) (yO ¼1.0 ML), (1 1) O/Mo(1 1 0) (yO ¼1.0 ML) and (2 1) O/Mo(1 1 0) (yO ¼ 0.5 ML). On the Mo(1 1 2) surface, favorable adsorption positions for O atoms at yO ¼1.0 ML are quasithreefold sites in furrows [29]. In LEED experiments for O/Mo(1 1 2) at yO ¼1.0 ML one originally observes a (1 1) pattern, while an annealing to the 500–600 K leads to the formation of (1 2) pattern [3,30]. The results of DFT calculations [29] show that total energies for these structures are almost the same, so they might be considered equally favorable. The LEED studies of structures of oxygen layers on the Mo(1 1 0) surface [31–33] have shown that oxygen forms various structures, starting from c(2 2) O at y ¼ 0.25 and finishing with (1 1) O at yO ¼1.0. Previous DFT calculations [34] suggested that triply coordinated hollow sites of the Mo(1 1 0) surface are strongly favorable for oxygen atoms for all coverages and the p(2 1)O structure forming at yO ¼ 0.5 is found to be favorable with regard to other possible structures. The structures of oxygen-modified surfaces used in the present paper as the substrate for CO adsorption are shown in Fig. 1(a)–(c). The distributions of densities of unoccupied electron states on the oxygen-modified surfaces in the energy range from EF to 1 eV (which can be expected to be responsible for the formation of the CO–surface adsorption bonds) are presented in Fig. 1(d)–(f). It can be seen that the density distributions for the (2 1) O structure on the Mo(1 1 2) surface and for the (1 2) O structure on the Mo(1 1 0) surface reveal significant protrusions above the Mo atoms, while for the (1 1) O structure on the Mo(1 1 0) surface the protrusions are small and are located above the oxygen atoms. In contrast, distributions of electron densities of occupied states near EF have protrusions only above O atoms and, as such reproduce just the ballmodels of the surfaces without any significant differences between distributions of densities for these systems. To obtain the optimal CO structure on the oxygen-modified molybdenum surface, the CO molecule was placed at various possible positions in the unit cell. On O/Mo(1 1 2) surface, such sites are quasi-threefold in furrow, and twofold in furrow and atop Mo atoms. In the course of optimization (along with two molybdenum and oxygen layers), CO molecules initially deposited at arbitrary positions on the surface, move towards on-top sites on Mo rows. In the optimized structure, CO molecules at the ontop sites are tilted by 22.41 with respect to the normal to the surface (Fig. 2(a)). The other sites appear unstable and the optimization procedure leads either to desorption or displacement of CO molecules to the on-top positions. The strong preference of the on-top sites with respect to sites in furrows results in the formation of a p(1 1) CO structure. The adsorption of CO does not significantly affect the location of the adsorbed oxygen atoms, so that the bond length of O atoms
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Fig. 2. Optimized CO structures on: (a) (1 1) O/Mo(1 1 2) (yO ¼1.0 ML) surface and (b) (2 1) O/Mo(1 1 0) (yO ¼ 0.5 ML) surface.
3.2. Density of states To reveal correlations of binding energies and adsorption properties with related changes of the electronic structure of the surface, the local densities of states have been calculated for several characteristic structures. The estimated DOS for the (COþO)/Mo(1 1 0) and (COþO)/Mo(1 1 2) surfaces, presented in Fig. 3, were constructed for the surface layer consisting of a CO molecule, adsorbed oxygen atom and two Mo atoms of the upper layer. The peaked structure in the range from EF to 5 eV in both spectra is originated by Mo d states, while the other peaks in the spectra are originated from CO molecular orbitals and atomic states of adsorbed oxygen. Specifically most of the peaks can be attributed to molecular orbitals of CO, as indicated in Fig. 3, while
DOS (states/eV)
6
1π
5 4
5σ 3σ
3 4σ
2 1 0 -25
-20
-15
-10
-5
0
E (eV)
6 DOS (states/eV)
˚ The with substrate atoms remains constant within 0.02 A. obtained binding energy of CO with the O/Mo(1 1 2) surface, Eb ¼0.87 eV, is in reasonable agreement (taking into account known overestimates of GGA binding energies) with the experimental value of 0.75 eV [3]. It should be noted that the binding energy on the oxygen-modified Mo(1 1 2) surface is 1 eV less than the binding energy of CO with a clean Mo(1 1 2) surface (1.76–2.1 eV at various CO coverages [6,7]), while the bond length ˚ adsorbed on O/Mo(1 1 2) is almost in the CO molecule (1.16 A) equal to that for CO on a clean Mo(1 1 2) surface. The CO adsorption on the oxygen-modified Mo(1 1 0) surface has been simulated in the same way. Nonetheless, in contrast to O/Mo(1 1 2) surface, on which CO adsorbs with a substantial binding energy, the CO molecule absolutely does not stick to the oxygen-saturated Mo(1 1 0) surface. Evidently, this is due to the fact that adsorption of the oxygen monolayer on the Mo(1 1 0) surface causes a complete poisoning of the molybdenum surface that leads to the blockage of the CO adsorption. Note that this feature apparently correlates with the absence of protrusions in the density of electron states above Mo atoms for the oxygensaturated Mo(1 1 0) surface (see Fig. 1(e)). On Mo(1 1 0) surface partly covered by oxygen, the adsorption of CO is possible. The optimized CO structure on the p(2 1) O/Mo(1 1 0) surface (yO ¼0.5) is presented in Fig. 2(b). The CO molecules occupy quasi-threefold positions near topmost Mo atoms and form a p(2 1) structure. The adsorbed CO molecules in this configuration are tilted by 15.721 with respect to the normal to the surface and do not noticeably affect the position of the adsorbed oxygen atoms. The obtained binding energy for CO molecules is found to be 1.05 eV, which is 1.0 eV less than the binding energy of CO with a clean Mo(1 1 0) surface (2.1 eV [7]). Like in the case of adsorption on O/Mo(1 1 2) surface, the positions of the adsorbed CO molecules correlate well with the location of protrusions in the density of the electron states (see Fig. 1(d) and (f)).
5σ
5 4
1π
3σ
3
4σ
2 1 0 -25
-20
-15
-10 E (eV)
-5
0
Fig. 3. Calculated local density of states for (a) (1 1) CO/O/Mo(1 1 2) and (b) for (2 1) CO/O/Mo(1 1 0) structures. Contributions from CO molecular orbitals to DOS are indicated.
the peak at 19 eV, which has no counterpart in the spectrum of CO molecule, is originated by the adsorbed O. The DOS for (COþO)/Mo(1 1 0) and (COþO)/Mo(1 1 2) are found to be very similar. A small difference between positions of the peaks originating from 3s CO orbital, i.e. 23 eV for (COþO)/Mo(1 1 0) and 24 eV for CO þO/Mo(1 1 2), can be attributed to different coordination numbers for CO molecules on Mo(1 1 0) and Mo(1 1 2) surfaces. As on the O/Mo(1 1 2) surface CO adsorbs nearly atop Mo it is bonded only with the Mo atom of the topmost row while on O/Mo(1 1 0), CO adsorbs in hollow sites; so there are three Mo atoms that contribute to CO–Mo bonding. As a result, the binding energy of CO molecule with the O/Mo(1 1 0) surface exceeds that of CO with the O/ Mo(1 1 2) surface by 0.18 eV, which may also affect the position of the CO 3s peak. The contribution of particular atomic states to the net spectrum can be determined from partial local densities of states.
N.V. Petrova / Journal of Physics and Chemistry of Solids 72 (2011) 744–748
DOS (states/eV)
3
O on Mo(110) O 2p
2 O 2s 1
0 -25
-20
DOS (states/eV)
3
2
-15
-10 E (eV)
-5
0
O in CO on O/Mo(110)
3σ
5σ 1π
1
4σ
0 -25
-20
DOS (states/eV)
3
-15
-10 E (eV)
-5
0
C in CO on O/Mo(110)
2
5σ 1
4σ
3σ
1π
0 -25
-20
-15
-10 E (eV)
-5
0
Fig. 4. (a) Local densities of states for CO/O/Mo(1 1 0) on adsorbed oxygen atom, (b) oxygen atom of adsorbed CO molecule and (c) carbon atom in adsorbed CO molecule . The s and p states are shown by black and red lines, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The calculated LDOS on the adsorbed O as well as on O and C atoms in CO molecule adsorbed on the Mo(1 1 0) surface are shown in Fig. 4(a)–(c). For an adsorbed oxygen atom (Fig. 4(a)), the peak at 19 eV originated by O 2s state, while the characteristic peaks at 6.5 and 5 eV originated predominantly by the interaction of O 2p electrons with Mo surface. In contrast, for the oxygen atom in adsorbed CO molecule, related CO 3s peak in LDOS (which stems from the O 2s state) is at –23 eV (Fig. 4(b)). The peaks 23, –9, 7 and 5 eV in the LDOS on the carbon atom (Fig. 4(c)) the and can be attributed to 3s, 4s, 5s and 1p molecular orbitals of adsorbed CO, respectively. The significant difference in the position of the O 2s peak for adsorbed oxygen atom and 3s peak of a CO molecule is due to the bonding of the O either with Mo or C. This feature can serve as a decisive argument in the vital discussion about the form of CO adsorption on Mo surfaces. The angle-resolved ultraviolet
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photoemission spectroscopy (UPS) as well as the near-edge X-ray absorption fine structure studies [35–37] indicates that at low coverages CO molecules on W and Mo surfaces are tilted (the b state), while at high coverages they are oriented normal to the surface with the C atom down (the a state). Using high-resolution core-level spectroscopy and UPS for CO adsorption on Mo(1 1 0), Jaworowski et al. [37] suggested thermally activated dissociation of CO molecules in the b state (i.e. at low CO coverages). In contrast the TDS and UPS studies of CO adsorption on W(1 1 0) and Mo(1 1 0) surfaces [38–40] suggested that the b state corresponds to a tilted orientation of CO on W(1 1 0) and Mo(1 1 0) but not to dissociated CO molecules, thus indicating an absence of CO dissociation. It was concluded therefore that the b peak in thermodesorption spectra in fact corresponds to the desorption of virgin CO rather than the recombinative desorption of previously dissociated CO [38–40]. As the order of desorption of b-CO might be considered unconvincing, it is interesting to compare the evidences proand contra-CO dissociation on Mo based on XPS and UPS studies in Refs. [37,,38], respectively. Thus, in Ref. [37] the dissociation of CO on Mo(1 1 0) was suggested predominantly from; (i) the observation of seemingly related chemical shift of the C 1s line (it was claimed that the O 1s line is diffuse and hence difficult to observe) and (ii) a picturesque transformation of the UPS spectrum with heating from 180 to 430 K, which was believed to provide the evidence of CO dissociation. It should be noted, however, that the carbon atom is bound to the surface, being adsorbed either separately or as a part of a CO molecule. Hence the shift of the C 1s line does not necessarily indicate the dissociation of CO, but could be caused by the changing distribution of electrons when the CO coverage decreases. The changes in the valence band spectrum, in turn, could hardly prove the dissociation because of the overlap of the wide O 2p peak (about 5–7 eV binding energy, see Fig. 4(a) and (c)) and the combined 1p þ 5s peak of adsorbed CO molecule, observed in the photoemission spectra in the same energy range [37]. Hence the changes observed in the valence band on heating can be explained by a partial desorption of CO, which at 180 K can form a multilayer film on Mo(1 1 0), while after heating to 430 K only one monolayer remains on the surface [38]. The observed changes in the valence band spectra should be attributed therefore to different binding energies with the surface for CO molecules in the submonolayer and multilayer films and do not necessarily indicate the dissociation of CO. Kim et al. [38] arrived at similar conclusions in their UPS study of the valence band of CO/Mo(1 1 0) and CO/W(1 1 0) systems. In particular they found that the peak at a binding energy of 7.0 eV can be assigned to the overlap of the 5s and 1p orbitals of b-CO, whereas the feature at about 11 eV is ascribed to the 4s orbital. In addition, while the chemisorbed CO species were detected by the presence of characteristic peaks at –8 and –11 eV binding energies, the valence band spectra did not reveal any adsorbed O atoms on the surface. It is the difference of binding energies of the O 2s and peaks in photoemission spectra that can prove or discard the dissociation of CO on the Mo surface. Specifically the 19 eV peak is characteristic of an adsorbed O atom (see Fig. 4(a)), so its appearance in the spectra would prove the dissociation, while the presence of the 23 eV peak corresponds to the CO 3s molecular state. It should be noted also that DOS for CO/Ni(1 1 1) [41], where the adsorption of CO is definitely nondissociative, demonstrate the 3s peak at 24 eV. Unfortunately, in Refs. [38–40], the spectra of the valence band for CO on Mo(1 1 0) and W(1 1 0) are given only up to 17 eV binding energy, so that the calculated positions of the O 2s and CO 3s peaks can be compared only with the UPS results presented in
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Ref. [37]. In the photoemission spectra for CO on Mo(1 1 0), obtained after the annealing at T¼430 K (presented in Fig. 4 of [37]), the peak at 22 eV is well discernable while there is no sign of the 19 eV peak, which, as followed from present calculations, is characteristic of adsorbed O. This means that the spectrum for CO on Mo(1 1 0), despite the significant transformation of the valence band after heating with respect to the spectrum obtained at T¼180 K, still corresponds to adsorbed non-dissociated CO rather than separately adsorbed C and O.
[3] [4] [5] [6] [7] [8] [9] [10] [11]
4. Conclusion
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
It is found that the CO molecules will adsorb at hollow sites on the O/Mo(1 1 0) surface and nearly atop Mo atoms on the O/Mo(1 1 2) surface. Adsorbed CO molecules are tilted with respect to the normal to the surface. The favorable positions for adsorption correlate with protrusions of the spatial density of vacant electronic states above the Mo surface atoms. The presence of oxygen on the molybdenum surface significantly reduces the binding energy of the CO molecule with the substrate, and on the oxygen-saturated Mo(1 1 0) surface the adsorption of CO is completely blocked. The calculated local densities of states (LDOS) demonstrate that the O 2s peak for O adsorbed on Mo(1 1 0) surface is at 19 eV (with respect to the Fermi level) while for the oxygen atom of an adsorbed CO molecule the related 3s molecular orbital gives rise to the peak at –23 eV. This difference stems from the bonding of the O atom either with Mo surface for adsorbed O or with C atom in adsorbed CO and therefore the position of the O 2s (or related CO 3s) peak in photoemission spectra can serve as a convincing argument in favor of either the presence or absence of CO dissociation on Mo surfaces. References [1] G. Wu, B. Bartlett, W.T. Tysoe, J. Mol. Catal. A 131 (1998) 91. [2] F. Morales, B.M. Weckhuysen, Catalysis 19 (2006) 1; M. Bowker, A.F. Carley, M. House, Catal. Lett. 120 (2008) 34; O.G.M. Flores, S. Ha, Appl. Catal. A: Gen. 352 (2009) 124.
[12] [13]
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[36] [37] [38] [39] [40] [41]
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