Adsorption and Dissociation of CO on a FeâCo Alloy (110) Surface

J. Phys. Chem. C 2008, 112, 3679-3691

3679

Adsorption and Dissociation of CO on a Fe-Co Alloy (110) Surface: A Theoretical Study John M. H. Lo and Tom Ziegler* Department of Chemistry, UniVersity of Calgary, Calgary, Alberta, T2N 1N4 Canada ReceiVed: October 24, 2007; In Final Form: December 19, 2007

The results from a density functional theory study on the structure and reactivity of CO adsorbed on the face-centered FeCo(110) surface were reported. It is found that CO adsorbs preferentially on the on-top (OT)Co and long-bridge (LB)-Co sites with the computed binding energies ranging from 43 to 33 kcal/mol for 0.125 and 0.500 ML, respectively. The strong Co-CO bond is attributed to the alloy formation between Fe and Co that alters the local electronic structure of surface Co atoms. Several CO decomposition paths have been explored, and all paths are found to be endothermic, as in the cases of Fe(110) and Co(0001). The path that leads to scission of CO at the LB-Co site with formation of C and O adatoms coadsorbed at LB-Co and transcription factor (TF)-Co sites, respectively, is kinetically the most feasible (Ef ) 45.4 kcal/mol) and least endothermic (∆E ) 10.6 kcal/mol). High reaction temperatures are thus necessary to facilitate the CO dissociation on FeCo(110).

1. Introduction The chemisorption and dissociation of CO is of great importance to industry because of its relevance to catalytic processes such as methanol reforming,1 heterogeneous hydroformylation,2 and the Fischer-Tropsch synthesis,3 which are all widely employed in the production of valuable fuels and hydrocarbons. Therefore, the understanding of the details concerning these processes is of great importance. To this end, it is therefore not surprising that extensive spectroscopic and kinetic measurements, accompanied by theoretical calculations, have been performed to characterize the structure and bonding of CO on metal surfaces. Currently, the chemisorption of CO has been explored for almost all transition metals4-11,14 and some p-block elements.12,13 Many theoretical investigations previously reported in the literature have focused on merely the CO adsorption on pure metal surfaces.4-11,14-18 Similar studies concerning the systems of supported and bimetallic alloys on the other hand have recently become popular because of the unique catalytic properties these alloys exhibit compared to the pure components.9,19-21 For instance, the doping of Pt on Ni can significantly enhance the aromatization of linear hexane, the activity which is absent in the case of pure Ni,22 while replacing Pt with Au suppresses the surface coating of Ni by graphite during the steam-reforming of n-butane.23 The existence of such distinctive properties are generally attributed to the intermetallic interaction of the constituents. In this work, the theoretical investigation of the interactions of CO with the Fe-Co alloy has been conducted. The binary Fe-Co alloys are unusually soft ferromagnetic materials that show large saturation inductions and a modest anisotropy, the properties desirable for the manufacture of high-tempearture magnets and electric aircraft engines.24,25 In addition to their potential applications in electrical engineering, Fe-Co alloys have been found to be highly effective in catalyzing the Fischer-Tropsch synthesis as Fe and Co do, but they offer several advantages over the traditional Fe and Co catalysts: * Corresponding author. E-mail: [email protected].

lower reaction temperatures, higher CO conversion, no carburization-induced poisoning of catalysts, and higher water-gas shift activity.26-29 It has been proposed that the presence of Co atoms destabilizes the formation of carbide in Fe from the dissociation of CO, thus slowing the rate of degradation of the catalyst.26 Nevertheless, no persuasive proof of this conjecture has yet been put forward based on experimental or theoretical investigations, and no information concerning the influences of Co on the activation barriers and pathways of CO on Fe-Co alloy surfaces has been provided. We shall therefore in the present work foster insights from first-principle calculations into the electronic factors determining the synergetic effects imposed by alloying Co and Fe. In particular, the site preference for CO adsorption and the favored dissociation channels of CO on the FeCo(110) surface will be compared to the corresponding properties on the structurally similar Fe(110) surface. 2. Computational Details All the results were obtained from the first-principle periodic density functional theory (DFT) as implemented in the program suite of VASP.30-32 Because of the magnetic nature of the FeCo alloys, spin-polarization was assumed in all calculations. The exchange-correlation effects were described by the Perdew-Burke-Ernzerhof (PBE) functional33,34 within the generalized gradient approximation (GGA). The ionic cores were represented by the projector augmented wave (PAW) method,35,36 while the electronic wave functions were expressed in terms of the plane-wave basis sets with the kinetic energy cutoff of 400 eV. It has been known that the Fe-Co alloys exist in a wide spectrum of atomic composition of Co, ranging from iron rich Fe15Co to the FeCo15 counterpart. At low temperatures ( TF-Fe ∼ TF-Co > SB ∼ OT-Fe > LB-Fe. In general, it is the most favorable if the adsorption involves only the coordination to Co atoms, and a stronger binding is achieved when more Co-C bonds are formed. Vibrational frequency analysis also yields the consistent results; only the OT-Co and LB-Co modes, which contain no Fe-C bonds, are minima, having the corresponding CdO stretching frequency of 1933 and 1715 cm-1 respectively. The LB-Fe mode is a second-order saddle point while the other modes are transition states. These are interesting outcomes in contrast to the situations of CO adsorption on Co(0001) and Fe(110) where the CO adsorption at the OT-sites are the most energetically stable and are true minima.48,54 Obviously, the disfavored coordination of CO to Fe atoms biases the site preference for CO adsorption toward Co and forces the OT-Fe mode to become a transition state.

3682 J. Phys. Chem. C, Vol. 112, No. 10, 2008

Lo and Ziegler

Figure 2. The site-projected orbital-resolved LDOS plots for adsorbed CO at the OT-Co sites of the FeCo(110) surface for the 0.125 ML coverage. The plots include the contributions from the 4s-, 4p-, and 3d-sub-bands of Co and the σ- and π-bands of CO molecule. The red and blue curves represent the spin-up and spin-down electrons, respectively. The Fermi energy level (F) is indicated by the black line in each diagram. Cartesian coordinates are defined according to Figure 1.

3.2. CO Adsorption at Medium Coverage: 0.250 ML. Also included in Table 1 are the structural parameters and the corresponding adsorption energies of CO molecules on FeCo(110) at 0.250 ML coverage. As expected, the adsorption of CO at this coverage is generally weaker than that at 0.125 ML coverage; for instance, the computed binding energy of CO at the OT-Co site is reduced from 42.6 to 35.1 kcal/mol when the surface coverage is doubled. A similar trend is also observed for other modes of adsorption; the reduction ranges from 5.0 kcal/mol for the LB-Fe mode to 11.4 kcal/mol for the TF-Fe mode. Simultaneously, the CdO bonds are found to be compressed by approximately 0.01 Å, indicating a lesser extent of back-donation from the metal surface to the antibonding 2πMO of CO. These influences, although not substantial, may originate from the electrostatic dipole interactions and steric repulsion between coadsorbed CO molecules on FeCo(110) that weaken the surface-CO adsorption. A different trend of stability from that for the lower surface coverage, OT-Co ∼ LB-Co > LB-Fe > OT-Fe ∼ SB ∼ TF-Fe ∼ TF-Co, is observed. A remarkable change is noticed for the OT-Fe and LB-Fe states, which become more stable with respect to TF-Fe and TF-Co states. The OT-Co adsorption mode remains the most stable configuration, but the associated adsorption energy is slightly decreased. This state is found from the normalmode analysis to be a true minimum; its higher CdO stretching frequency of 1983 cm-1 compared to the corresponding value (1933 cm-1) at lower coverage (0.125 ML) is consistent with the weaker occupation of the 2π-MO.

The vibrational frequency calculations also reveals another stable configuration corresponding to the LB-Co mode, which lies less than 0.1 kcal/mol above the OT-Co conformation. This state, as the OT-Co mode, illustrates the positive correlation of the CdO bond strength (1.194 cf. 1.202 Å), and thus the vibrational mode (1776 cf. 1715 cm-1), to the surface coverage. The theoretical calculations of Jiang and Carter16 also predicted that both OT and LB modes are stable for CO adsorption on Fe(110), although the HREELS study of Erley48 did not observe the expected signal in the region of 1650-1800 cm-1. Adsorption of CO at either TF-Co or TF-Fe site is not preferred; however, these states are nearly degenerate to the SB and OT-Fe modes, in which the difference in their adsorption energies is smaller than 1 kcal/mol. Accordingly, it is possible that CO may rapidly scramble between these sites and relax to the neighboring stable OT-Co and LB-Co sites via the TF-Co f OT-Co and TF-Fe f LB-Co paths. There has been a controversy about the exact surface topology of adsorbed CO molecules at the OT-sites on Fe(110) at 0.250 ML. While Erley proposed a c(2 × 4) pattern in the [11h0] direction (type 1) based on the HREELS and LEED observations,48 the later DFT studies by Jiang and Carter did not support this and suggested that a p(2 × 2) surface structure (type 2) is instead more favorable.16 In this work, both possible arrangements for CO at the OT-Co sites of FeCo(110) were explored. Furthermore, an additional structure that contains

CO on a Fe-Co Alloy (110) Surface

J. Phys. Chem. C, Vol. 112, No. 10, 2008 3683

Figure 3. The site-projected orbital-resolved LDOS plots for adsorbed CO at the OT-Fe sites of the FeCo(110) surface for the 0.125 ML coverage. The plots include the contributions from the 4s-, 4p-, and 3d-sub-bands of Fe and the σ- and π-bands of the CO molecule. The red and blue curves represent the spin-up and spin-down electrons, respectively. The Fermi energy level (F) is indicated by the black line in each diagram. Cartesian coordinates are defined according to Figure 1.

the adsorbed CO molecules along the [001] direction (type 3) was investigated. These three structures are illustrated in Figure 5. The type 3 structure is the least stable with a binding energy of only 30.5 kcal/mol; the destabilization is mostly attributed to the short Co-Co bond length (2.8418 Å), which is significantly smaller than the van der Waals diameter of a CO molecule (3.15-3.17 Å as derived from the saturated CO layers on the Pd(111) and Pt(111) surfaces58) that leads to a great repulsion between the adsorbed CO molecules at neighboring OT-Co sites. On the other hand, type 1 and 2 structures are degenerate in stability, a consequence that may result from the similar intermolecular separation of CO molecules in the two phases (4.0189/5.6836 cf. 4.9221 Å). Therefore, both phases may coexist in the CO monolayer on FeCo(110) for the surface coverage below 0.500 ML. The interchange of these structures would possibly be facile at room temperatures because a barrier of ∼2 kcal/mol associated with the diffusion OT-Co f TS f LB-Co f TS f OT-Co′, through which c(2 × 4) f p(2 × 2) phase transition proceeds, may be estimated based on the small energy difference between the OT-Co and LB-Co modes (∆E < 0.1 kcal/mol). Not only for the most stable OT-Co mode, but also the other favorable LB-Co mode have the effects of various packing patterns on the binding energies of CO molecules been explored for the 0.250 ML coverage. Different from the OT-Co mode where three possible surface structures could be constructed, only two surface configurations were considered for the LBCo mode, corresponding to the line (type 1) and alternate (type

2) arrangements depicted in Figure 6. Both structures possess CO molecules adsorbed in rows along the [001] direction of FeCo(110); however, they differ by the relative positions of two neighboring CO. For the type 1 configuration, two adjacent CO molecules share a common Co atom, and a continuous CO “chain” on alternate rows of Co atoms is thus formed. As expected from the fact that the competition exists between two CO molecules for the Co electron density donated to the CO antibonding 2π-MO, the binding interaction for both CO molecules is weakened compared to that at 0.125 ML coverage. In addition, the short CO separation (2.8418 Å) contributes to the strong van der Waals repulsion that further destabilizes the type 1 phase of CO adsorption at the LB-Co sites. The type 2 configuration on the other hand has both the problems of competing back-donating electrons from Co atoms and van der Waals interaction between coadsorbed CO molecules alleviated by displacing alternate CO molecules on a row to the next Co row; in other words, the surface topology is changed from the compact p(1 × 4) pattern to the more open p(2 × 2) pattern. This shift leads to the increase of 3.6 kcal/mol in the computed adsorption energy. 3.3. CO Adsorption at High Coverage: 0.500 ML. The LEED studies by Erley48 and Papp50 have revealed that the saturated surface coverage of CO on Fe(110) and Co(0001) is around 0.6-0.7 ML. Further increase in CO exposure leads to the formation of overlayers and distortion of the surface topology. Therefore, it is believed that 0.500 ML would be near the saturation limit for the CO coverage on the FeCo(110) surface.


Lo and Ziegler

Figure 4. The site-projected orbital-resolved LDOS plots for adsorbed CO at the LB-Co sites of the FeCo(110) surface for the 0.125 ML coverage. The plots include the contributions from the 4s-, 4p-, and 3d-sub-bands of Co and the σ- and π-bands of the CO molecule. The red and blue curves represent the spin-up and spin-down electrons, respectively. The Fermi energy level (F) is indicated by the black line in each diagram. Cartesian coordinates are defined according to Figure 1.

Figure 5. The three possible surface configurations of CO adsorption at the OT-Co sites of FeCo(110) at 0.250 ML coverage. The unit cell for each pattern is represented by a blue box.

Table 2 summarizes the important data regarding the adsorption of CO on FeCo(110) at 0.500 ML coverage. As in the case of 0.250 ML, the computed binding energies are reduced compared to 0.125 ML because of the strong repulsive interaction between coadsorbed CO molecules and their competition for surface valence band electrons. For instance, it is observed that the most stable configuration, OT-Co mode, possesses the

adsorption energy of 32.6 kcal/mol, which is 23% smaller than the corresponding value for the 0.125 ML coverage. The most dramatic changes in binding energy occur for the TF-Fe and TF-Co modes where the reductions are as large as 33%. CdO bonds are all shortened and their corresponding CdO stretching frequencies are increased (e.g., 1990 versus 1933 cm-1 for OTCo mode at 0.500 and 0.125 ML, respectively), which are in



Figure 6. The two possible configurations of CO adsorption at the LB-Co sites of FeCo(110) at 0.250 ML coverage. The unit cell for each pattern is represented by a blue box.

TABLE 2: Adsorption Energies of CO on FeCo(110) and the Associated Structural Parameters for 0.500 ML Surface Coveragea adsorption mode

r(C-O)/Å

OT-Co OT-Fe SB TF-Fe TF-Co LB-Co LB-Fe OT-Co (dis.) OT-Fe (dis.) OT-Co/OT-Fe LB-Co/LB-Fe OT-Fe/LB-Fe OT-Co/LB-Co

1.167 1.164 1.181 1.181 1.185 1.193 1.190 1.171 1.168 1.167/1.164 1.188/1.189 1.163/1.191 1.166/1.194

a

r(Fe-C)/Å

r(Co-C)/Å BE/kcal mol-1 1.768

1.819 1.907 1.859 2.189

1.976 2.261 1.840 1.967

1.986 1.775 1.811 1.836 1.775 2.006 1.992 1.819/1.989 1.768/1.967

37.3 (32.6) 33.2 (28.7) 33.1 (28.6) 32.1 (27.6) 31.9 (27.4) 35.9 (31.7) 34.4 (30.2) 36.6 (31.9) 33.6 (29.1) 31.9 (27.3) 33.5 (29.3) 34.0 (29.7) 36.7 (32.0)

ZPE-corrected binding energies are shown in parentheses.

line with the weaker back-donation to the antibonding CO 2πMO, although the amounts are not substantial. The same order of site preference for the CO adsorption as in the case of 0.250 ML: OT-Co > LB-Co > LB-Fe > OT-Fe ∼ SB > TF-Fe ∼ TF-Co, is predicted for the 0.500 ML coverage. The trend that CO prefers the coordination to Co is still observed, as illustrated by the highest stability of OT-Co and LB-Co modes in which only Co-C bonds are involved. Unlike the case of 0.250 ML coverage, the OT-Co, LB-Co, LBFe, and OT-Fe modes are true minima according to the vibrational frequency analysis. An increase in the energy separation between the OT-Co and LB-Co state when the surface coverage increases from 0.250 to 0.500 ML is attributed to the fact that all LB-Co sites are occupied by CO at this coverage, and two adjacent CO molecules are bound to the same Co atom via the 2π-3dxz interaction. The competition of electron density from Co therefore reduces the strength of both Co-C bonds. On the other hand, no such destabilization occurs for the OTCo mode since the 5σ-3dz2 interaction still dominates the Co-C interaction at the OT-Co sites. The same effect also influences the LB-Fe mode, leading to the diminished energy separation between the CO adsorption at the OT-Fe and LB-Fe sites. The observed order of stability LB-Fe > OT-Fe is identical to that predicted by the GGA-PAW/PBE calculations for the CO adsorption on Fe(110) at the same surface coverage,14,16,18 even though only by using the PKZB meta-GGA functional can the

Figure 7. Schematic representation of the distorted p(2 × 1) surface arrangement of CO molecules adsorbed on FeCo(110) at 0.500 ML surface coverage (hatched, Co; striped, Fe; red, O). (a) OT-Co mode; (b) OT-Fe mode.

experimental stability trend OT-Fe > LB-Fe be obtained.16 Therefore, the current trend may not be fully accurate because of the use of the PBE functional. Further studies are thus necessary to determine the relative stability of the OT-Fe and LB-Fe configurations. It has been proposed from the LEED results that the CO monolayer on Fe(110) adopts the distorted p(2 × 1) surface structure for CO exposure higher than 0.7 L to eliminate the repulsive van der Waals interaction between CO molecules along a close-packed row.48 This proposition was subsequently confirmed by the theoretical investigations conducted individually by Stibor et al.,14 Jiang et al.,16 and Sun et al.18 using the GGA-PBE/PAW method; in these studies, the distorted configuration is more favored than the symmetric configuration by 0.01 to 0.04 eV. This distortion phenomenon was also considered in this work for the OT-Co and OT-Fe modes. Figure 7 depicts the resulting p(2 × 1) structures for these adsorption modes, and their associated adsorption energies are listed in Table 2. In both cases, the CO molecules are tilted by ∼14° from the surface normal and are displaced from the ideal OTsites (0.5 Å for the OT-Co mode and 0.6 Å for the OT-Fe mode toward the neighboring TF-site). These observations agree well with the data obtained by Jiang and Carter.16 Interestingly, the distorted OT-Co configuration is found to be less stable than the upright counterpart with a marginal energy difference (∼0.7 kcal/mol), while the tilted arrangement slightly stabilizes the OT-Fe mode by about 0.4 kcal/mol. The reasons for these contrasting effects are uncertain but are possibly related to the enhanced 3dxz f 2π back-donation, which is more pronounced for the OT-Fe mode than the OT-Co mode (Figures 2 and 3), when the CO molecule adsorbed at the OT-Fe site is bent toward the adjacent TF-Fe site. For clean single-crystal Co(0001) and Fe(110) surfaces, the packing of CO molecules at high coverage is largely governed by the compromise between the attractive interaction of CO with the surface and the van der Waals repulsion between coadsorbed CO molecules, and this in consequence leads to the


Lo and Ziegler

Figure 8. Unusual structures of CO overlayer on FeCo(110) at 0.500 ML coverage. (a) OT-Co/OT-Fe; (b) LB-Co/LB-Fe; (c) OT-Fe/LB-Fe; (d) OT-Co/LB-Co. Blue and red circles represent the possible repeating units of CO adsorbates. (Labels: Fe (striped); Co (hatched); O (equatorial)).

TABLE 3: Adsorption Energies of C and O on FeCo(110) at 0.125 ML Coveragea adsorption mode OT-Co OT-Fe SB TF-Co TF-Fe LB-Co LB-Fe a

BE (C)/kcal mol-1

BE (O)/kcal mol-1

114.3 (113.5) 115.2 (114.4) 137.1 (135.5) 156.8 (154.7)

104.1 (103.4) 107.8 (107.1) 125.9 (124.6) 141.6 (139.9) 133.1 (131.4) 132.2 (130.5) 136.1 (134.4)

155.9 (153.8) 155.6 (153.5)

ZPE-corrected binding energies are shown in parentheses.

most favorable orthorhombic or distorted p(2 × 1) close packing.14 More varieties of surface structures are nevertheless noticed for FeCo(110), which is constructed from having alternate rows of Fe atoms on Fe(110) along the [001] direction replaced by Co atoms, because CO molecules can simultaneously be bound to Fe and Co atoms. Considering the fact that the OT-sites and LB-sites are favorable, four additional surface structures were examined: (i) OT-Co/OT-Fe, (ii) LBCo/LB-Fe, (iii) OT-Fe/LB-Fe, and (iv) OT-Co/LB-Co. Their structural parameters and associated adsorption energies are included in Table 2. Type (i) structure originates from the OT-Co configuration and is formed from displacing alternate rows of CO molecules to their neighboring rows of Fe atoms. The resulting surface structure can be generally described by either c(1 × 4) or p(1 × 4) units (shown by the blue and red boxes in Figure 8a, respectively); however, the central CO molecule in an c(1 × 4) unit is shifted from the LB-Fe site to a neighboring OT-Co site, while those in a p(1 × 4) unit are moved from the OT-Co sites to the OT-Fe sites. It is clearly illustrated that the CO molecules in this configuration are more closely packed than both the pure OT-Co and OT-Fe modes; the lateral interaction between them is thus significantly enhanced, which destabilizes this config-

uration. The computed average binding energy for this state is only 31.9 kcal/mol, which is even lower than that for the OTFe mode. Similarly to the type (i) configuration, the type (ii) structure (Figure 8b) cannot be properly described by the perfect c(1 × 4) or p(1 × 4) units because the central CO molecules are displaced from the ideal positions. This adsorption pattern may be viewed as an image of type (i) configuration shifted along the [001] direction by half of the Co-Co bond distance. The strong repulsion between coadsorbed CO in close proximity dramatically reduces the adsorption energy of CO molecules, as in the type (i) case. This state with the CO binding energy of 29.3 kcal/mol is more favored than the related type (i) configuration but is less preferred compared to the LB-Fe and LB-Co modes. Type (iii) and (iv) states possess surface structures closely related to one another; their configurations could be interchanged by shifting the entire CO overlayer along the [11h0] direction. Both adsorption patterns can be described by c(1 × 4) or p(1 × 4) supercells, and no displacement of CO molecules that breaks the unit cell symmetry is present. The type (iii) configuration is obtained by shifting alternate rows of CO molecules in the OT-Fe mode to their neighboring LB-Fe sites. It is expected that type (iii) structures would have a stability comparable to the LB-Fe configuration because the OT-Fe mode is less favored than the LB-Fe mode at high surface coverage. The computed stability trend OT-Fe/OT-Fe < OT-Fe/LB-Fe ∼ LB-Fe/LB-Fe is found to be in agreement with this prediction. On the other hand, the opposite trend OT-Co/OT-Co > OTCo/LB-Co > LB-Co/LB-Co is observed for the type (iv) state, which is attributed to the fact that shifting the CO molecules from the OT-Co sites to the LB-Co sites always reduces their adsorption energies, as indicated in Tables 1 and 2. 3.4. CO Dissociation on FeCo(110). The dissociation process of CO on the FeCo(110) surface was investigated in the present



Figure 9. Preferred structures and their associated relative energies for C and O coadsorption on FeCo(110).

Figure 10. Four minimum energy profiles (category 1) for the dissociation of CO at OT-Co site on FeCo(110). Only the transition and product states are included. For all channels, the initial state corresponds to the CO molecule adsorbed at the central OT-Co site of the (2 × 2) unit cell.

study because it corresponds to a major channel of providing surface carbide that may undergo hydrogenation reactions with surface hydrogen that lead to the production of various types of hydrocarbons. Currently, two starting CO configurations were considered: OT-Co and LB-Co since they appear to be true minima for the whole range of surface coverage below the experimental limit (∼0.7 ML). In particular, the surface concentration of CO was assumed to be 0.125 ML; in this situation, CO molecules adopt a loose p(2 × 4) packing configuration, which provides sufficient space for CO dissociation. It is believed that increasing the concentration to 0.250 ML may only change the activation barriers slightly as there exists no competition of adsorption sites for neighboring CO molecules in the most favorable p(2 × 2) configuration. Calculations have been performed to identify the most favorable adsorption sites for C and O on FeCo(110). Both species are bound strongly to the FeCo(110) surface, as in the cases of the single-crystal Fe and Co counterparts.16,59,60 A site selectivity can be clearly seen from Table 3 where C and O favor the adsorption at high-coordination sites and in particular the TF-Co sites. The calculated adsorption energies for C and O at TF-Co sites are 154.7 and 139.9 kcal/mol, respectively. Interestingly, the C atom residing at the TF-Fe site is unstable and relaxes to an adjoining LB-Co site. The site preference for C and O becomes slightly complicated when the lateral

interaction arising from co-adsorption of C and O is taken into account. In Figure 9 the three most favored structures are illustrated; it is found that C moves to a LB-site (either LB-Co or LB-Fe) when O is present at an adjacent TF-site to minimize the destabilization from shared atoms. Two categories of dissociation paths exist for CO on FeCo(110) because two stable adsorption configurations are available, namely, the OT-Co and LB-Co modes. The first group comprises the dissociation channels of CO at the OT-Co site, while the other group concerns the scission of CO at the LBCo site. Figure 10 depicts four dissociation paths that originate from the OT-Co mode (category 1). A common feature observed for the four paths is that the dissociation proceeds through the initial displacement of CO from the OT-Co site, followed by the CdO bond breaking over either a Co atom (paths 1 and 4) or an SB-site (paths 2 and 3). Similarly to the CO dissociation on the close packed Co(0001) surface, these processes are highly endothermic and possess exceptionally high activation barriers. For instance, the computed activation energy and endothermicity for the CdO bond rupture to C and O at opposite TF-sites (path 1) are, respectively, 61.6 and 15.5 kcal/mol. These values are comparable to the corresponding quantities (∼56.0 and 29.8 kcal/mol) determined for the Co(0001) surface.60 Dissociation of CO to C and O at the less stable coadsorption configurations, such as those shown in Figure 9, increases considerably both


Lo and Ziegler

Figure 11. Four minimum energy profiles (category 2) for the dissociation of CO at LB-Co site on FeCo(110). Only the transition and product states are included. For all channels, the initial state corresponds to the CO molecule adsorbed at the central LB-Co site of the (2 × 2) unit cell.

the reaction barriers and reaction energies due to the strong repulsive interaction between C and O, especially when metal atoms (either Fe or Co) are shared in the final states.60 The only exception occurs for the process that gives rise to the C atom at the TF-Fe site and O atom at the LB-Fe site (path 3); for this path the overall activation barrier is lowered to 48.4 kcal/mol, while the product state is 25.2 kcal/mol endothermic with respect to the CO molecule in the OT-Co mode. The reduced barrier may be attributed to the initial shift of CO from the OT-Co site to the LB-Co site, which weakens the CdO bond strength and facilitates its scission. It is worth noting that two transition states are found for this path where the first one is 18.3 kcal/mol lower in energy than the second one. The two transition states are connected by an intermediate, which is stabilized by 2.2 kcal/mol with respect to the first transition state. The global transition state is composed of the stretched CO molecule (1.795 Å) that is 1.257 Å over a TF-Fe site (path 3) and is parallel to the substrate surface. Four dissociation paths that originate from the CO molecule at the LB-Co site (category 2) were also considered. In general, these paths possess lower reaction barriers compared to those in category 1 because of the back-donation from the bridge Co atoms to the CO 2π-MO that weakens the CdO bond and makes its dissociation more facile. This trend is clearly reflected by the data summarized in Table 4. For instance, both path 4 and path 8 involve the dissociation of CO to two adjacent LB-CO sites over a Co atom; shifting CO from the OT-Co site to LBCo site, which costs less than 1 kcal/mol, lowers the forward reaction barrier by 10 kcal/mol. The same effect is also observed for paths 3 and 5, which deal with the CdO bond scission over an SB-site. The activation energy is reduced from 48.4 kcal/ mol for path 3 to 45.3 kcal/mol for path 5 when CO is displaced from the OT-Co site to the LB-Co site. It is intriguing to note the similarity between path 5 and path 7; their transition states are essentially identical, with the differences in Ef and CO bond distances being 0.1 kcal/mol and 0.005 Å respectively, and their final states resemble each other closely. The decrease in

TABLE 4: Dissociation Channels of CO on FeCo(110) at 0.125 ML Coveragea

a

channel

Ef

Eb

Path 1 Path 2 Path 3 Path 4 Path 5 Path 6 Path 7 Path 8

61.6 62.2 48.4 77.2 45.3 60.7 45.4 68.1

46.1 36.4 23.2 49.2 20.3 49.8 34.8 40.1

All energies are ZPE-corrected and given in kcal/mol.

endothermicity of path 7 is caused by the migration of the O adatom from the LB-Fe site to the nearest TF-Co site, which enhances its adsorption via the additional Co-O bond (at the TF-Co site), thereby stabilizing the resulting configuration by 14.5 kcal/mol. According to the data presented in Table 4, paths 5 and 7 are the most probable minimum energy paths for the CO dissociation on FeCo(110). Nevertheless, their activation energies are significantly higher than the value deduced for the CO dissociation on Fe(110)16 while smaller than that for the corresponding process on Co(0001).60 Note also that all these paths are endothermic by 11∼28 kcal/mol. Therefore, the CO dissociation on FeCo(110) is thermodynamically hindered and is seemingly not possible at ambient temperatures; rather CO may desorb molecularly from the surface. Under the FischerTropsch reaction conditions, however, these processes could be kinetically assisted by the high reaction temperatures (>200 °C), and the CO decomposition may occur competitively with CO desorption, as in the case of Fe(110).16 The activation barriers for CO decomposition on FeCo(110) lie in between the corresponding values for CO dissociation on Fe(110) and Co(0001). It has been observed experimentally that alloying Fe with a small amount of Co significantly slows down



TABLE 5: Comparison of PBE and RPBE CO Adsorption Energiesa coverage

site

PBE

RPBE

∆EPBE-RPBE

0.125 ML

OT-Co OT-Fe SB TF-Fe TF-Co LB-Co LB-Fe OT-Co OT-Fe SB TF-Fe TF-Co LB-Co LB-Fe OT-Co OT-Fe SB TF-Fe TF-Co LB-Co LB-Fe

42.6 37.8 38.6 41.3 40.7 42.5 36.6 35.1 29.9 29.9 29.8 29.7 35.1 31.6 32.6 28.7 28.6 27.6 27.4 31.7 30.2

35.9 30.9 30.8 34.3 33.7 34.2 28.6 27.1 21.6 20.4 20.0 19.7 24.8 21.7 24.6 20.7 19.4 18.2 17.6 21.9 20.4

6.7 6.9 7.8 7.0 7.0 8.3 8.0 8.0 8.3 9.5 9.8 10.0 10.3 9.9 8.0 8.0 9.2 9.4 9.8 9.8 9.8

0.250 ML

0.500 ML

a


the rate of CO conversion and enhances the selectivity toward the production of C6+ hydrocarbons.52 The present results are in line with this conclusion; the higher CO activation energy on FeCo(110) compared to Fe(110) infers smaller surface concentrations of C and O, which favor the subsequent C-C coupling reactions leading to the formation of long-chain compounds. Therefore, it is suggested that bimetallic Fe-Co catalysts should be selective for high-weight hydrocarbon products and demonstrate low methanation activity. 3.5. Influences of Functionals on Energetics and Site Preference. It has been shown from the work of Hammer, Hansen, and Nørskov that the GGA-PBE functional generally overestimates the chemisorption energies of atomic and small molecular adsorbates by about 0.6 eV, although it is able to yield geometries with very good agreement with experiments.61 To improve the adsorption energies, Nørskov et al. proposed a revised version of the PBE functional (RPBE) that reduces the discrepancy between the theoretical and experimental adsorption energies to be within 0.3 eV.61 In a subsequent study of Jiang and Carter, they confirmed the effectiveness of using RPBE functional in the determination of the adsorption energies for CO adsorption on Fe(110);16 in these calculations, both PBE and RPBE functionals predict the right site preference at OTsites while RPBE gives the adsorption energies only differing from the experimental value by 0.3 eV. To explore the influences of RPBE functional on the energetics regarding the CO adsorption on FeCo(110), RPBE single-point calculations were performed for the PBE-optimized geometries. Full RPBE geometry optimizations have also been carried out, but the resulting structures are essentially identical to the PBE geometries with the negligible difference in bond distances being 0.001 Å. The computed RPBE binding energies for CO adsorption at 0.125, 0.250, and 0.500 ML are listed in Table 5. All RPBE energies are lower than the corresponding PBE values. At 0.125 ML, the average change in binding energies due to the use of RPBE functional is 7.4 kcal/mol, which agrees with the observations of Nørskov et al.61 For higher coverage, however, the RPBE corrections of binding energies are slightly increased; the mean ∆EPBE-RPBE ) EPBE - ERPBE becomes 9.4 and 9.1 kcal/mol for 0.250 and 0.500 ML respectively.

TABLE 6: Comparison of PBE and RPBE CO Activation Energiesa Ef

Eb

path

PBE

RPBE

∆E

PBE

RPBE

∆E

1 2 3 4 5 6 7 8

61.6 62.2 48.4 77.2 45.3 60.7 45.4 68.1

66.0 66.5 52.8 80.4 49.6 58.4 49.6 66.1

4.4 4.3 4.4 3.2 4.3 -2.3 4.2 -2.0

46.1 36.4 23.2 49.2 20.3 49.8 34.8 40.1

43.0 33.6 21.3 44.3 14.6 36.0 28.7 36.4

-3.1 -2.8 -1.9 -4.9 -5.7 -13.0 -6.1 -3.7

a


It is noticed that RPBE favors the OT-Fe configuration over the LB-Fe configuration at all surface coverage. Except for 0.125 ML where both the PBE and RPBE functionals yield the same order of stability OT-Fe > LB-Fe, the PBE and RPBE predictions for 0.250 and 0.500 ML are opposite. Contrary to the PBE results that LB-Fe configuration is more stable than the OT-Fe configuration by 1.5∼1.7 kcal/mol, RPBE predicts that OT-Fe is more energetically favorable instead, but the two configurations are nearly degenerate (∆E ) 0.1∼0.3 kcal/mol). Similar contradictions have also been observed in the case of CO adsorption on Fe(110)16 and have been attributed to the differential accuracy of these functionals on treating chemical bonds with varying bond orders.62 Therefore, only by using functionals beyond the GGA, which can treat systems of various sizes with the same accuracy, may the site preference for CO adsorption be correctly predicted. One such functional is PKZB.63 Unfortunately, self-consistent calculations using this functional are not yet available in the VASP program. Apart from the binding energies associated with the CO adsorption on FeCo(110), the effects of RPBE on the activation energies of adsorbed CO have also been investigated. Singlepoint calculations with the RPBE functionals were conducted for the PBE-optimized geometries of adsorbed CO, transition states, and coadsorbed C and O. The resulting activation energies for the eight decomposition pathways are illustrated in Table 6. Several trends can be noticed from the data. First, both the PBE and RPBE functionals yield the same prediction that the pathways 5 and 7 are the most possible channels of CO decomposition. Second, except for the pathways 6 and 8, replacing PBE with RPBE systematically increases the CO activation energies by about 4 kcal/mol. The situation of the backward reaction barriers is slightly different; all Eb values are reduced, but the magnitude varies from 1.9 kcal/mol for the pathway 3-13.0 kcal/mol for the pathway 6. These observations suggest that RPBE corrections for the atomic systems, such as C and O, are larger relative to the molecular systems like CO molecules. According to the proposition of Zupan et al.,64 the greater destabilization effects on the coadsorbed C and O atoms compared to CO are due to the larger reduction of density gradients and free surface area for the former system upon chemisorption, which is favored by GGA functionals. Consequently, the backward reaction barriers for all eight pathways are lowered thus causing these pathways to be more endothermic. 4. Conclusions The processes of CO adsorption and dissociation on the compact FeCo(110) surface below the saturated surface coverage of 0.500 ML has been investigated using spin-polarized DFT. It is found that CO favors the OT-Co and LB-Co sites regardless of the surface coverage because the Co-CO bond is stronger

3690 J. Phys. Chem. C, Vol. 112, No. 10, 2008 than the Fe-CO bond for the 1:1 Fe-Co alloy. The computed binding energies for CO adsorption at the OT-Co site and LBCo sites are larger by 7 kcal/mol than the corresponding values for CO adsorption on the structurally related Co(0001) surface. On the other hand, the Fe-CO bonds are all weakened in the presence of Co atoms; the adsorption energies associated with CO at OT-Fe and LB-Fe sites are 1∼5 kcal/mol smaller than those for the same coordination modes on pure Fe(110). In addition, vibrational frequency analysis reveals that only the OT-Co and LB-Co modes are true minima up to the saturation surface coverage, while the other modes are either transition states or second-order saddle points. These observations are in contrast to the situation of Fe(110) where CO adopts the tilted configurations at the OT-Fe and LB-Fe sites at 0.500 ML so as to minimize the van der Waals repulsion between the coadsorbed CO molecules. These configurations have been studied in this work, but they are found to be less stable than the upright configurations. The decomposition of CO on FeCo(110) has been explored at 0.125 ML where sufficient space is available for C and O co-adsorption. As on the Fe(110) and Co(0001) surfaces, this process is highly endothermic and kinetically demanding. Because both OT-Co and LB-Co configurations are stable, two groups of dissociation channels were studied: one originating from the OT-Co mode and the other from the LB-Co mode. For the channels starting from CO at the OT-Co site, the CO scission always proceeds initially via the displacement of CO from the on-top site to the neighboring LB-Co site, followed by the bending and stretching of the CdO bond. It is noticed that higher reaction barriers result when the transition states involve the migration of O over an on-top site (either OT-Co or OT-Fe site), whereas the paths involving the CO dissociation over an SB-site (i.e., Fe-Co bridge) are more energetically accessible. Among the eight minimum energy paths considered in this work, path 7, which concerns the CO decomposition into C and O adsorbed at the distant LB-Co and TF-Co sites, is the most probable channel. Neverthelsss, high reaction temperatures (∼200 °C) are apparently required to trigger the process, and this process would be competitive to the molecular desorption at lower temperatures, which is the situation that has been experimentally observed for CO adsorption on Fe(110). To investigate the influences of different functionals on the CO adsorption energies and dissociation barriers, calculations utilizing the RPBE functionals and PBE-optimized geometries were performed. It is observed that the resulting binding energies are systematically lowered by 7 to 10 kcal/mol while the site preference remains the same. The effects on the activation barriers of CO decomposition are more pronounced. Except for the pathways 6 and 8, the forward reaction barriers for the other pathways are increased by approximately 4 kcal/mol. On the other hand, all the backward reaction barriers are reduced, which is attributed to the greater destabilization of coadsorbed C and O than CO caused by all GGA-type functionals. Despite the influences on energetics, both PBE and RPBE functionals predict that pathways 5 and 7 are likely responsible for the CO decomposition on FeCo(110). Acknowledgment. The authors would like to acknowledge the financial supports by the Alberta Ingenuity Funds in the form of a postdoctoral fellowship (to J.M.H.L.) and the computer resources provided by the Western Canada Research Grid and the Department of Chemistry of the University of Calgary. T.Z. also wants to thank the Canadian Government for a Canada Research Chair in Theoretical Inorganic Chemistry.

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Adsorption and Dissociation of CO on a FeâCo Alloy (110) Surface

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Adsorption and Dissociation of CO on a FeâCo Alloy (110) Surface