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Effect of Preadsorbed S on the Adsorption of CO on Co(0001) S. H. Ma,*,†,‡ Z. Y. Jiao,† T. X. Wang,† and X. T. Zu*,‡ College of Physics and Information Engineering, Henan Normal UniVersity, Xinxiang, Henan 453007, China, and Department of Applied Physics, UniVersity of Electronic Science and Technology of China, Chengdu 610054, China ReceiVed: December 10, 2008; ReVised Manuscript ReceiVed: July 16, 2009
Coadsorption of CO and S on Co(0001) was studied using periodic density functional theory calculations, and the calculated adsorption energies, optimized geometries, and electronic structures are compared in more detail with those of single species adsorption systems. The theoretical results agree with the general trend of experimental studies and also indicate that the presence of S has a different effect on the adsorption of CO as the S coverage decreases from 0.25 to 0.11 ML, indicative of the different interaction nature between CO and S coadsorbed on Co(0001). I. Introduction In heterogeneous catalytic processes, residual sulfur as one common poison, has a negative effect on CO-related reactions.1 An understanding of how sulfur interacts with CO is of importance, since the lateral interaction between adsorbates is one of the key mechanisms affecting chemical reactions on transition metal surfaces. Experimental studies reported that S and CO coadsorbed on Ni(111), Pt(111), Rh(111), and Ru(0001) supports a type of long-range interaction between S and CO, whereas on Ni(100) and Ni(110) surfaces an opposite shortrange interaction was drawn, based on the literature of Zhang et al.2 and Lahtinen et al.3 In contrast, the Co(0001) experimental studies of Lahtinen et al. indicate a combination of local and long-range interactions between S and CO.3 On the theoretical hand, Feibelman and Hamann4 concluded that on Rh(001) the interaction between S and molecules is longrange in nature. However, on Rh(111), it was shown from the density functional theory (DFT) calculations of Zhang et al.,2 where the substrates were fixed at their bulk positions, that the presence of S does not significantly influence the adsorption properties of CO, suggesting a short range in nature between CO and S interaction. Wimmer et al.5 concluded that the poisoning effect of S has a complex nature involving direct interaction between S and the adjacent CO molecule for the Ni(100)-c(2 × 2)-(S + CO) system. Additionally, it was found that S introduces a long-range modification (Friedel oscillations) to the local density of states when CO and S coadsorbed on Cu(100)6 and that under O preadsorbed on Ru(0001) the adsorption energy of CO can be changed depending on the adsorption sites and coverages.7 Han et al.8 obtained that the effect of surface relaxation is important in mediating the interaction between coadsorbates O and CO on Pt(111). On the basis of the above debates, further studies are required to examine the interaction between coadsorbates on surfaces. To study the effect of S on the adsorption of CO on Co(0001) as the S coverage changes, we have carried out a series of ab initio DFT total energy calculations for p(2 × 2)-CO, p(2 × 2)-S, and the experimentally observed coadsorption p(2 × * Corresponding author. E-mail:
[email protected]; (S. H. M.);
[email protected]; (X. T. Z.). † Henan Normal University. ‡ University of Electronic Science and Technology of China.
2)-(S + CO) overlayers,3 as well as those of p(3 × 3)-S, p(3 × 3)-CO, and p(3 × 3)-(S + CO). The calculated results such as adsorption energies and site preference, optimized geometries, and electronic structures (work function and density of states) are compared in more detail with those of single species adsorption systems. Last, the interaction nature between S and CO is discussed and main conclusions are summarized. II. Computational Method Ab initio total energy density functional theory (DFT) calculations were carried out as implemented in the Vienna ab initio simulation package (VASP).9,10 We use the revised version of the Perdew-Burke-Ernzerhof (RPBE) functional11,12 within the generalized gradient approximation (GGA) to calculate the exchange-correlation energy. The electron-ion interaction was described by projector-augmented wave (PAW) potentials.13,14 The Kohn-Sham one-electron valence eigenstates were expanded in terms of plane-wave basis sets with a cutoff energy of 400 eV. The Co(0001) surface was modeled by a five-layer and a fourlayer slab for p(2 × 2) and p(3 × 3) overlayers, respectively. Adsorbates S and CO were placed on one side of the slab, which was separated by a 14 Å thick vacuum region. During structure optimization, the bottom two layers were fixed at their calculated bulk positions, while the topmost three metal layers (only the uppermost two layers along the surface normal direction for p(3 × 3)) and adsorbates were allowed to relax until the Hellmann-Feynman forces converged to less than 0.03 eV/Å. The k-point sampling of the two-dimensional Brillouin zone was performed using the Monkhorst-Pack scheme15 with a Γ-centered mesh of 8 × 8 × 1 and 6 × 6 × 1, respectively. Methfessel-Paxton smearing16 of σ ) 0.1 eV and the corrected energy for σ f 0 was employed. Test calculations for increasing k-points, energy cutoffs, and thicker layers show that the adsorption energies differed by 40 meV. Work function results were calculated by taking the difference of the average vacuum potential and the Fermi energy. The lattice constants for bulk cobalt, a ) 2.52 Å and c/a ) 1.62 calculated previously,17 are applied in this study, which differ by 0.5% from the experimental values of a ) 2.507 Å and c/a ) 1.62.18
10.1021/jp810950a CCC: $40.75 2009 American Chemical Society Published on Web 08/17/2009
Coadsorption of CO and S on Co(0001)
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TABLE 1: Calculated Results for the p(2 × 2) Adsorption Overlayersa CO Ead/∆Ead (eV)
top hcp-hollow fcc-hollow bridge
1.43 1.37 1.38 1.25
fcc-hollow hcp-hollow
5.03 4.99
dC-O (Å)
dC-Co (Å)
p(2 × 2)-CO, θCO ) 0.25 ML 1.168 1.196 1.194 1.189
dS-Co (Å)
1.750 1.991 1.992 1.896
ν (cm-1) 1969 1747 1763 1812
p(2 × 2)-S, θS ) 0.25 ML 2.204 2.198 p(2 × 2)-S + CO, θtotal ) 0.5 ML most stable (a) S(fcc) + CO(hcp)-NN (b) S(hcp) + CO(fcc)-NN least stable (c) S (fcc) + CO(top)-NN (d) S(hcp) + CO(top)-NN
0.83/0.54 0.83/0.53
1.197
1.974
2.117
1742
0.45/0.98
1.164
1.766
2.177
1971
a Adsorption energy Ead was calculated by Ead ) Eadsorbate + Eclean - E(adsorbed/surface) for pure CO or S adsorption system, and a positive Ead corresponds to a stable adsorption; dC-O, dC-Co, dS-Co: C-O bond length and the distances of C and S atoms from the nearest surface Co atom, respectively. ν: C-O stretch frequency.
III. Results III.A. Coadsorption of CO at a S Coverage of 0.25 ML. For the p(2 × 2)-CO and p(2 × 2)-S adsorption systems on Co(0001), the calculated results are given in Table 1, corresponding to coverages of θCO ) 0.25 ML and θS ) 0.25 ML, respectively. It is shown that the CO on-top site is the most favorable with an adsorption energy of 1.43 eV, slightly leading the fcc- and hcp-hollow sites by about 60 meV. Bridge is the least stable site with a binding energy of 1.25 eV. Compared to free CO, in higher coordination sites, the increasing C-O bond length is in agreement with the decreasing C-O stretching frequency and on-top site; these changes are around 2.1 and -7.6% in amplitude, respectively. The site preference of CO, optimized adsorption geometries, and C-O stretching frequency are in agreement with the results calculated by Gajdosˇ et al. using the PW91 functional.19 At θS ) 0.25 ML, adatom S favors hollow sites with a binding energy of 5.0 eV and a S-Co bond length of 2.20 Å, which are consistent with previously calculated results.20,21 A. Coadsorption Energy. In the coadsorption system, the adsorption energy of CO is calculated by the formula7,8 S/Co CO (CO+S)/Co ECO ad ) Etotal + Efree - Etotal
(1)
(CO+S)/Co S/Co CO where Etotal , Etotal , and Efree represent the total energy of (CO + S)/Co(0001) and S/Co(0001) and the free energy of the CO molecule, respectively. A positive adsorption energy corresponds to a stable structure. The effect of S on CO adsorption CO , is the change in CO adsorption energy with and energy, ∆Ead without S coadsorbed. For the coadsorption system, we mainly consider such site configurations as CO adsorption on top or hollow sites when S is adsorbed in hollow sites; the calculated energies and optimized coadsorption structures are shown in Table 1 and Figure 1, respectively. It can be seen from Table 1 that CO adsorption energy is substantially reduced upon S being preadsorbed on Co(0001), in agreement with the experimental result that the presence of S weakens the bonding of CO to the Co(0001) surface.3 The most stable structures are both CO and S in 3-fold hollow sites shown in Figure 1a,b with a binding energy of 0.80 eV, leading to the least stable ones displayed in Figure 1c,d, i.e., CO ontop Co next nearest from S in a hollow site about 0.4 eV. With
Figure 1. Top view for the most and least stable coadsorption structures in a p(2 × 2)-(S + CO) overlayer (solid lines) (N, nearest; NN, next nearest).
respect to that of the corresponding site in CO/Co(0001), the decreases in CO adsorption energy are close to 0.6 and 0.9 eV for the most stable and least stable structures, respectively, and vice versa. These results also show that CO favors hollow sites when S is coadsorbed, different from the on-top site in CO/ Co(0001), while CO does not affect S adsorption in hollow sites. This is consistent with experimental observations of CO being pushed to hollow sites from the preferential on-top site with S still in hollow sites.3 Also, it is found that the difference of CO adsorption energy between the most and least stable structures is about 0.38 eV, much higher than that of 60 meV for the tophcp-site energy difference in the CO/Co(0001) system. This suggests that the decreasing CO adsorption energy is not due to the change of CO site preference but the influence of preadsorbed S. Moreover, we find that the most stable structures, Figure 1a,b, occur when both S and CO occupy hollow sites and CO shares a bond with the same Co atoms to which S is bonded during coadsorption; namely, the coadsorbed species have to compete with each other for bonding from the surface atoms. For the least stable structures, Figure 1c,d, however, S bonds with three surface Co atoms, while CO bonds with another one, and there is no direct bonding competition over the same metal atom from the adsorbates. This site preference for S and CO coadsorption on Co(0001) is not consistent with the general “bonding competition” model.22 Additionally, unstable structures e and f in Figure 1 finally optimized away from the initial arranged sites to others.
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Figure 2. Calculated changes in work function (WF) (a) for the p(2 × 2)-S and p(2 × 2)-CO systems from that of clean Co(0001) and (b) for the p(2 × 2)-(CO + S) coadsorption system.
B. Geometries and C-O Stretch Frequency. From Table 1, we found that the adsorption geometries of CO such as C-O and C-Co bond length and C-O stretch frequency ν are nearly not affected by the presence of S on Co(0001). As for the most stable structures, the distances of C-Co and S-Co, dC-Co and dS-Co, slightly decrease by 0.9 and 3.6% compared to that of the individual adsorption systems. These suggest that S has a minor effect on the strength of the Co-C and C-O bonds, and vice versa, implying that the bonding of Co-C and S-Co is local, similar to that of p(2 × 2)-(S + CO)/Rh(111).2 Concerning the significant decrease in adsorption energy and negligible changes in adsorption geometries, we further examined the surface relaxation. Compared to the buckling values of 0.09 and 0.16 Å for CO individual adsorption on-top and in hollow sites, calculated results show that, when S and CO coadsorbed on Co(0001), the most stable structures induce a more pronounced surface corrugation with an increasing buckling of 0.2 Å, while the surfaces of the least stable structures tend to be flatter with a decreasing buckling of 0.1 Å. This is similar to that of CO and O coadsorption on Pt(111), and as suggested by Han et al.,8 it is possible that the surface relaxations are in mediating the interaction between S and CO with such a short separation of 2.9 Å (the distance of S from C) in p(2 × 2)-S + CO/Co(0001) structure. C. Work Function Analysis. Calculated changes in work function (WF) (Figure 2a) indicate that single S adsorption at 0.25 ML increases the WF by 0.55 eV compared to a clean Co(0001) surface, slightly larger than the experimental value of 0.2 eV.3 Correspondingly, the dipole moment of the sulfur layer is calculated to be -0.17 D using the Helmholz equation, close to the experimental value of -0.12 D.3 In contrast, CO adsorption alone induces a more striking increase in WF about 1.0 eV on-top site and 1.4 eV in hollow sites, respectively, yielding the CO overlayer dipole moments of -0.32 and -0.44 D. These results agree well with the experimental values of 1.2 eV in work function change and -0.34 D in dipole moment for the CO on-top site at the coverage of 0.33 ML.23 For the most and least stable coadsorption structures in the coadsorption system, the calculated changes in WF compared
Ma et al. to the clean Co(0001) surface (O in Figure 2b) are close to 1.3 and 1.0 eV, respectively, which are almost the same as that of pure CO adsorption (1 in Figure 2b). This suggests that there exists electrostatic (dipole-dipole) repulsion to some extent between adsorbates S and CO, yielding a mutual depolarization7 and resulting in the changes in WF being less than that of the sum of CO and S individual adsorption on Co(0001), which is slightly different from the experimental analysis of a rather small mutual depolarization between S and CO.3 D. Density of States. To further illustrate the effect of S adsorption on CO electronic structures, Figure 3 presents the comparison of the local density of states (DOS) around adsorbates CO and S between single species adsorption and coadsorption systems. It is shown by Figure 3a that, for the most stable structures, i.e., both S and CO in hollow sites, the CO 4σ, 1π, and 5σ peak positions apparently shift downward in energy compared to that of the CO on-top site for CO/ Co(0001), and the 1π and 5σ orbitals are almost degenerate at 7.0 eV below the Fermi energy EF. Moreover, the 2π orbital is almost at the same position as that of CO in the hcp-hollow site in the CO/Co(0001) system, but with respect to that of CO on-top alone, it slightly extends to further lower energies below EF, implying negligible changes in C-O bond length and bonding strength with and without S preadsorbed on Co(0001). Compared to that of S/Co(0001), two peaks of adatom S appear at -10.0 and -6.8 eV relative to the Fermi energy, which is due to an overlap of the S states with the molecular 4σ and 5σ states of CO. These peaks could also be regarded as an indication of a direct interaction between CO and S in the p(2 × 2)-S + CO overlayer. As for the least stable structures, the DOS displayed in Figure 3b also exhibits similar changes for CO and S but to a lesser extent. This can be explained by the fact that in the most stable structures adsorbates CO and S bond directly with the same metal atom (see Figure 1a,b), resulting in a direct strong repulsion due to bonding competition, which affects the density of states more than that of the least stable ones. Figure 4 clearly compares the DOS of the most with that of the least stable structures along with that of the nearest-neighbor surface Co atoms. We can see that the 4σ, 1π, 5σ, and 2π orbitals of CO and p states of S are involved in hybridization with substrate d bands in the coadsorption system, main contribution from the mixing of S p states, 5σ and 2π of CO with substrate d bands in the energy region of [-7.4 eV, +4.0 eV]. Compared to that of the most stable, 4σ-like state of S in the least stable structure slightly shifts upward in energy, whereas the d band center of the nearest-neighbor Co away from S shifts downward in the energy range from the Fermi energy EF to -5.0 eV. A possible explanation might be that the presence of S, being electronegative and withdrawing some electrons from the surface, decreases the backbonding capabilities of the substrate and induces the metal d band shifting downward to form a strong S-Co covalent bond, strongly weakening the C-Co bond of the least stable structures. Above all, the distribution of DOS in the p(2 × 2)-(S + CO)/Co(0001) system indicates that adatom S does affect the CO molecular states, similar to that of previous studies of c(2 × 2)-(S + CO)/Ni(001).5 III.B. Coadsorption of CO at a S Coverage of 0.11 ML. At a lower coverage of 0.11 ML, CO adsorption in hollow sites is predicted to be the most stable site with an adsorption energy of 1.30 eV, slightly leading the on-top site by about 60 meV in energy, according to our calculations in Table 2. The distances of C-O, nearest C-Co and C-O stretch frequency are nearly
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Figure 3. Density of states around adsorbates CO and S for the p(2 × 2)-S + CO overlayer, compared with that of p(2 × 2)-CO and p(2 × 2)-S: (a) the most stable; (b) the least stable (the vertical dotted lines denote the Fermi energy EF).
Figure 4. Density of states for the most and least stable structures in the p(2 × 2)-S + CO overlayer.
the same as that at 0.25 ML. For S/Co(0001), the adsorption energy and S-Co bond length in hollow sites are close to the values of 5.0 eV and 2.21 Å, respectively, almost identical to those at θS ) 0.25 ML. For the p(3 × 3)-(S + CO)/Co(0001) coadsorption system, Table 2 shows that there is very little change in CO adsorption energy compared to that of p(3 × 3)-CO/Co(0001), with a maximum reduction of 100 meV, and vice versa. Both CO and
Figure 5. Top view for CO upward vertically (a,b) and slightly tilted (c,d,e) structures in a p(3 × 3)-(S + CO) overlayer (solid lines). The dashed lines are drawn as a guide to the eye for CO site preference away from S.
S in hollow sites (see Figure 5a) are still the most favorable sites with an adsorption energy of 1.30 eV. For other coadsorption structures, Figure 5b-d, the adsorption energies of CO
TABLE 2: Calculated Results for the p(3 × 3) Adsorption Overlayersa CO Ead/∆Ead (eV)
top hcp-hollow fcc-hollow
1.24 1.30 1.29
fcc-hollow hcp-hollow
5.02 5.03
dC-O (Å) (R)
dC-Co (Å)
dS-Co (Å)
ν (cm-1)
∆Φ (eV)
1947 1728 1748
0.4
p(3 × 3)-CO, θCO ) 0.11 ML 1.169 1.755 1.198 1.965 1.195 1.985
0.6
p(3 × 3)-S, θS ) 0.11 ML 2.206 2.207
0.2
p(3 × 3)-S + CO, θtotal ) 0.22 ML CO upward vertically (a) S(fcc) + CO(fcc/hcp) (b) S(hcp) + CO(fcc/hcp) CO tilted from surface normal (c) S(fcc/hcp) + CO(top)
a
1.30/0.0 1.19/0.1
1.195 1.193
2.000 1.984
2.201 2.198
1742 1754
0.8
1.20/0.04
1.167 (0.5°) 1.169 (3.9°) 1.169 (3.9°)
1.763
2.184
1949
0.6
1.763
2.171
1890
0.6
1.766
2.156
1893
0.6
(d) S(fcc) + CO(top)-NN
1.16/0.08
(e) S(hcp) + CO(top)-NN
1.14/0.1
R: the angle of the C-O bond axis tilted away from the surface normal. ∆Φ: the change in work function (WF).
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Ma et al.
Figure 6. Density of states for the coadsorption structure in Figure 5d along with that of the CO on-top site in the p(3 × 3)-CO overlayer.
are nearly degenerate at ∼1.20 eV, whether CO is in hollow or on-top sites. Such small decreases in adsorption energy suggest that S-Co and C-Co bonding is rather localized in such a sparse p(3 × 3)-(S + CO) overlayer, in spite of CO on-top or hollow sites. This is similar to that of p(3 × 3)-(S + CO)/ Rh(111),2 where CO occupies an on-top site and S a hollow site, but significantly different from that of the denser p(2 × 2)-(S + CO)/Co(0001) overlayer. From the geometries and C-O stretch frequency in Table 2, we find that the distances of C-O, C-Co, and S-Co for coadsorption structures of Figure 5a-c are almost the same as that of corresponding sites in the p(3 × 3)-CO/Co(0001) system, as well as the C-O stretch strength. Other optimized structures of Figure 5d, i.e., S in hollow and CO on-top sites, show that the CO molecule is slightly tilted up to 3.9° away from the surface normal, decreasing the bond length of S-Co to 2.17 Å and weakening the C-O stretch strength by ∼60 cm-1. Furthermore, the distances of adatom S away from atom C of molecular CO are 4.3, 3.8, and 3.4 Å for the structures in Figure 5a, b,c, and d, respectively, larger than that of 2.9 Å in the denser p(2 × 2)-(S + CO) structure. The surface relaxation effect, taking Figure 5a and d as examples, is insignificant with the buckling of 0.13 and 0.04 Å in surface and subsurface layers, respectively, with respect to that of 0.14 and 0.01 Å for the CO on-top site, as well as that of 0.09 and 0.04 Å for CO in hollow sites for p(3 × 3)-CO/Co(0001). Compared to clean Co(0001), from Table 2, the increases in WF (∆Φ) are 0.8 and 0.6 eV for the coadsorption structures in Figure 5, which just correspond to that of the sum of 0.2 eV for S in hollow sites in the p(3 × 3)-S and 0.6 or 0.4 eV for CO in hollow sites or on-top in the p(3 × 3)-CO overlayer, respectively. This indicates a rather small mutual depolarization between adsorbates S and CO in the sparse p(3 × 3)-(S + CO) overlayer, slightly different from that in the dense p(2 × 2)-(S + CO) one. Moreover, Figure 6 presents the comparison of density of states (DOS) between the single CO on-top site and the coadsorption structure, Figure 5d, CO tilted on-top Co with S in hcp-hollow site. It is obvious that the number of peaks, the peak shape, and the peak positions of the DOS around CO and
S as well as nearest-neighbor Co atoms are almost identical to each other in these systems. IV. Discussion and Conclusions We have studied the coadsorption of CO and S on Co(0001) on the basis of ab initio density functional theory calculations as the S coverage decreases from 0.25 to 0.11 ML. It is indicated that the effect of preadsorbed S on CO adsorption properties depends on the coverage of S. At a higher S coverage of 0.25 ML, the calculated results indicate that the presence of S significantly decreases the CO adsorption energies and induces CO favorable in hollow sites, different from the CO on-top site in the p(2 × 2)-CO/Co(0001) system. One might therefore expect that the C-Co bond length would be elongated and the C-O stretch strength weaker; however, the main structures such as C-O, C-Co, and S-Co bond distances as well as C-O stretching strength are almost the same as that of CO/Co(0001). This is similar to what Hoeft et al.24 have studied for the c(2 × 2)-(CO + H)/Ni(001) coadsorption system using DFT calculations, where the adsorption energy of CO is strongly reduced to 0.73 eV from that of 1.43 eV for the single CO on-top site on Ni(001), while the C-Ni bond length is the same as that in the CO/Ni(001) system. The possible explanations are (i) the adsorption energy of CO may not be a fair measure of the local Co-C bond strength,24 and the striking decrease in adsorption energy must be associated with some form of energy cost, such as arising from a large surface buckling (structural modification)24 when S and Co coadsorbed on Co(0001), and (ii) the Co-S and Co-C bondings are local both with and without S coadsorbed, resulting in insignificant changes in these bond distances and strength, which shows that the interaction between S and CO is mainly shortrange in nature and local bonding.2 Moreover, there exists a somewhat direct interaction between CO and S, clearly shown by the changes of the density of states around CO and S as well as the insignificant work function increase compared to the pure CO adsorption system. Therefore, we can conclude that, at a higher S coverage of 0.25 ML, the interaction between S and CO on Co(0001) is complex, i.e., a combination of longrange in nature to some extent and somewhat local bonding.
Coadsorption of CO and S on Co(0001) At a lower S coverage of 0.11 ML, it is shown that the presence of S almost does not affect the adsorption properties of CO, similar to that of p(3 × 3)-(S + CO)/Rh(111). This indicates that the interaction between CO and S is mainly shortrange in nature. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 10774039) and the Basic Research Program of Education Bureau of Henan Province in China (No. 2009B140005). References and Notes (1) (2) (3) (4) (5) 2618. (6) (7) (8)
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