Spontaneous Activation of CO2 and Possible Corrosion Pathways on

Feb 5, 2009 - Spontaneous Activation of CO2 and Possible Corrosion Pathways on the Low-Index Iron Surface Fe(100). Vassiliki-Alexandra Glezakou* and ...
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J. Phys. Chem. C 2009, 113, 3691–3696

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Spontaneous Activation of CO2 and Possible Corrosion Pathways on the Low-Index Iron Surface Fe(100) Vassiliki-Alexandra Glezakou* and Liem X. Dang Chemical and Materials Sciences DiVision, Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, MS K1-83, P.O. Box 999, Richland, Washington 99352

B. Peter McGrail EnVironmental Sustainability DiVision, Energy and EnVironment Directorate, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 ReceiVed: September 17, 2008; ReVised Manuscript ReceiVed: NoVember 19, 2008

Spin-polarized density functional theory (DFT) calculations and periodic slab models were used to study the reactive pathways leading to corrosion products on the low-index (100) surface of iron. We determined the binding energies of CO2 and the barrier to decomposition of adsorbed CO2 to O + CO, as well as to the formation of adsorbed CO32- and H2CO3 on the Fe(100) surface. The barriers of these pathways were determined with nudged elastic band (NEB) calculations. Short trajectories with DFT-based dynamics were employed to identify the most important species. These simulations (up to 0.5 ML coverage) show that CO2 is spontaneously activated and can bind with two or all three atoms assuming bent configurations strongly reminiscent of the radical CO2-. This spontaneous activation of CO2 is possible through charge rearrangement of the slab density. The CO2 decomposition to O + CO has a barrier of 5.0 kcal/mol. The subsequent formation of CO32- by reaction with an incoming CO2 is strongly favored thermodynamically. Interaction of H2O with the adsorbed CO2 forms a loosely bound complex that leads to the formation of surface-bound carbonic acid, with a barrier of approximately 35.0 kcal/mol. 1. Introduction The interaction of CO2 with iron has been the focus of intense interest for many years because of its importance in many fields, from corrosion,1,2 to catalysis,3,4 to CO2 recycling5 and green chemistry,6 as well as in efforts for its more effective use as C1 feedstock.7 Corrosion of low-alloy steels has a tremendous impact in the oil industry.8 In nature, CO2 sequestration in reactive reservoir rocks has also garnered much attention.9 In all cases, the leading assumption has been that these reactions are water-mediated, involving reactions such as10-13

CO2(g) + H2O(l) h H2CO3(aq)

(1)

+ HCO3-

(2a)

HCO3- h H+ + CO32-

(2b)

2H+ + 2e-fH2

(2c)

FefFe2+ + 2e-

(3)

+

H2CO3 h H

and a corrosion layer is formed on the steel surface:

Fe2+ + CO32-fFeCO3 2+

Fe

+ 2HCO3-fFe(HCO3)2

Fe(HCO3)2 f FeCO3 + CO2 + H2O

(4a) (4b) (4c)

However, there has been evidence that such reactions do in fact happen in the gas-solid interface, or condensed CO2 phases,14 with no or only traces of water present, in spite of the * To whom correspondence should be addressed. Tel.: 509-375-6961. E-mail: [email protected].

large thermodynamic stability of CO2. Solymosi,15 and Freund and Roberts16 have reviewed the surface chemistry of CO2 as has been studied by a variety of experimental surface science techniques. As all these studies point out, there is strong spectroscopic evidence that metal surfaces activate CO2 in a process that involves charge transfer from the surface to the CO2 moiety, which becomes bent, just as in the radical anion, CO2-. Charge transfer in the opposite direction from the molecule toward the surface would have left it linear. Depending on the nature of the metal surfaces and the presence or absence of adatoms, CO2 may further dissociate to O and CO, or transform to CO32- and CO. In contrast, physisorbed CO2 maintains its linear configuration. Interestingly, there are not many studies on iron, and the existing ones are, for the most part, experimental studies. The current state-of-the-art methodology and computing resources have made possible the sophisticated simulations of surfaces and reactions taking place on them, offering a reliable guide to subsequent experiments.17 Reactive pathways on surfaces are complicated and difficult to characterize; however, they can almost always be expressed in terms of fundamental processes which include adsorption, desorption and/or diffusion of species, charge transfer, structural rearrangements of the adsorbates and the surface atoms, as well as more radical transformations such as bond dissociation and bond formation. Obviously, the knowledge of how such processes interrelate is of paramount importance to elucidate these complicated, but interesting, phenomena. In this paper, we investigate the adsorption of CO2 alone or in the presence of another CO2 or H2O molecule on the Fe(100) surface. Theoretical calculations predict the following order of stability of iron surfaces: Fe(110) ∼ Fe(100) > Fe(111).18a

10.1021/jp808296c CCC: $40.75  2009 American Chemical Society Published on Web 02/05/2009

3692 J. Phys. Chem. C, Vol. 113, No. 9, 2009 Cleavage experiments, however, show that the (100) surface is the most likely cleavage plane;18b,c therefore, Fe(100) is a good reference for Fe surface reactivity. We are interested in knowing whether spontaneous activation of CO2 happens when it contacts a clean iron surface and what are the possible reactions following interaction with other CO2 molecules or H2O. In a recent paper, density functional theory (DFT) calculations by de la Pen˜a O’Shea et al. show that CO2 is spontaneously activated and adsorbed on pure cobalt surfaces.19 Is it possible that clean iron surfaces can activate CO2 in a similar fashion? Nassir and Dwyer report adsorption and dissociation of CO2 on Fe(100) using temperature-programmed desorption, X-ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy (UPS), highresolution electron energy-loss spectra (HREELS), and lowenergy electron diffraction.20 Can we use theoretical models to predict the possible reactive pathways following CO2 adsorption? How do the reactions in mainly CO2 phases differ from reactions 1-4 which happen in aqueous environments? We used a combination of DFT calculations and DFT-based molecular dynamics (MD) to identify the important configurations that lead in chemisorption of CO2, its reaction with other CO2 or H2O molecules, the binding energies of the adsorbed species, and further formation of corrosion products (adsorbed CO3 and H2CO3). 2. Methods and Computational Details Calculations were carried out by means of periodic spinpolarized DFT (unrestricted Kohn-Sham) with the appropriate pseudopotentials within the CP2K code.21 The generalized gradient approximation (GGA) with the PBE functional was used.22 The core electrons were modeled as norm-conserving pseudopotentials constructed for use with PBE, and the valence electron wave functions were expanded in a localized Gaussian double-ζ companion basis set derived from the pseudoatomic wave functions.23 For Fe, a 16-valence electron potential was used, whereas for C and O only the 1s electrons were described by the potential. The density was expanded in terms of an auxiliary plane wave basis with the standard cutoff of 280 Ry. Brillouin zone integration was performed using the Γ-point approximation for supercells of up to 11.0 × 11.0 × 11.0 Å3 of bcc Fe with careful checks performed on the lattice parameters and bulk modulus to verify convergence, see details in section 2.1. Optimizations were carried out until the maximum forces were below 0.02 eV/Å. Construction of surfaces was done with the commercially available package Materials Studio by Accelrys, visualization of structures was done with MacMolPlot,24 and the density plots were done with VMD.25 We also performed short ab initio MD trajectories (approximately 2-10 ps) to sample the configurational space more efficiently, also using CP2K. The simulations were run within the NVT ensemble and Nose´-Hoover thermostat chains and a 0.5-1.0 ps time step. 2.1. Cell Parameters and Surface Model. The interaction of CO2 with the iron surface was carried out within the periodic supercell approach. The lattice parameters used were determined using a 36-atom cell of bcc bulk iron and fitting the data to the Murnaghan equation of state.26 The fit gave a lattice parameter of 2.850 Å, in good agreement with the experimental value of 2.867 Å.27 From this fit we estimated the bulk modulus for iron at ∼150 GPa, in good agreement with the experimental value 170-190 GPa. From the expectation value of the total spin of the supercell (〈S〉 ) (S(S + 1))1/2), we obtained an estimate of the total spin per Fe atom. Using the spin-only formula for estimating the magnetic moment, µ ∼ (4S(S + 1))1/2, we

Glezakou et al.

Figure 1. Topology of binding sites of Fe(100), panel a. Molecular water is more likely to occupy, almost isoenergetically, the top or 2-fold sites (b and c), but when the H is directed toward the 4-fold site, it readily dissociates (d and e).

calculated a magnetic moment of approximately 1.6 µB per Fe atom, in fair agreement with the experimental value of ∼2.0 µB per Fe.27 Optimization of the lattice constant with a supercell containing a larger number of atoms did not change the results significantly, the variations being < ( 0.5% of the experimental value. These values indicate that the set of computational parameters used provide a realistic representation of the structural properties of bulk Fe with slightly larger discrepancies for the magnetic moment. A periodic slab of five atomic layers was carved along the (100) Miller plane using these optimized cell parameters with a 10 Å vacuum layer along the C-axis to minimize interactions between the surface slab and its periodic image. In all surface slab calculations, the top two layers were allowed to relax and the remaining three layers were fixed at their bulk lattice positions. The top layer contracted an average of 1%, whereas the second layer expanded less than 3%, in agreement with experimental and other theoretical works.18a In all subsequent simulations, only the top layer of Fe atoms was allowed to relax together with the adsorbates. For certain reactions of interest, after the corresponding equilibrium structures were determined, the minimum energy pathways connecting these minima of adsorbed species on the surface were optimized by means of the climbing image nudged elastic band (NEB) method of Jo´nsson and co-workers.28 In this approach, the reaction path is “discretized”, by way of a set of discrete images or “replicas” that are connected by elastic springs, to prevent the intermediate images from sliding to the closest minimum with additional constraints to force the highest energy point of the NEB toward a transition state. In these NEB searches we used 7-8 images to cover the path from one minimum to the other. 3. Results and Discussion The adsorption of CO2, H2O, and other possible reaction products on the low-index surface of iron Fe(100) was studied by means of periodic density functional methods. The topology of the possible binding sites on the Fe (100) surface is shown in Figure 1(a). The possible binding sites include a 1-fold (top),

Activation of CO2 and H2O on the Fe(100) Surface

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3693

TABLE 1: Structural Parameters for Adsorbates Used in This Studya species

R

% err

∠OCO, ∠OHO

% err

CO CO2 CO2-* H2O

1.150 (1.128) 1.182 (1.170) 1.267 (1.25) 0.978 (0.958)

+2.0 +1.0 +1.4 +2.0

180.0 (180.0) 134.3 (135.0) 103.0 (104.5)

0.0 -0.5 -1.4

a Distances in angstroms and angles in deg. Experimental parameters in parentheses (NIST database).

a 2-fold bridging, and a 4-fold hollow site. In the following sections we will discuss what are the preferred binding sites for CO, H2O, and CO2 and how this might influence their reactivity. Because CO/Fe(100)29 and H2O/Fe(100)30 have been studied before by others, they are included here as benchmark calculations. However, the interaction of H2O with adsorbed CO2 on Fe(100) has not been studied before and constitutes the core of this study. 3.1. Equilibrium Structures of CO, CO2, Radical CO2-, and H2O. To calibrate our methods, and provide a baseline for comparison between the ground-state equilibria of the adsorbates and the geometries of the adsorbed species, we calculated the equilibrium structures of CO, CO2, CO2-, and H2O. The resulting geometries are listed in Table 1. They show very good agreement with the accepted experimental values. The bond lengths are consistently longer by 1-2%, whereas angles are slightly smaller than the experimental ones by 0.5-1.4%. Unless otherwise noted, energy differences are taken with respect to the limit of noninteracting adsorbates and slab:

∆E ) E(adsorbate/slab) - E(adsorbate) - E(slab) The radical anion CO2- has attracted considerable attention in the past. In 1953, Walsh published a paper where he discusses the electronic orbitals and spectra of polyatomic molecules of the type AX2 in terms of his now renowned Walsh diagrams31 and how the shape of molecules is affected by a change in the symmetry of their electronic wave function. Whereas neutral CO2 is linear and thermodynamically very stable, its negative counterpart, the radical anion, is metastable, albeit kinetically stable. It is also bent, which is the result of stabilization of the anion brought about by the lowering of the symmetry of the electronic wave function. These two systems are separated by a barrier of 12.0-14.0 kcal/mol, which corresponds to the adiabatic electron affinity of CO2. Our calculations predict an electron affinity of about 16.1 kcal/mol, which we find in good agreement. 3.2. Calibration Calculations: Adsorption of CO and H2O on Fe(100). The adsorption of H2O and CO has been discussed before by others in detail, vide infra. In the next two subsections, we will briefly review these studies, and present our results, which will serve as a calibration of our methods with respect to the ones used in the literature. 3.2.1. CO Adsorption on Fe(100). Sorescu et al.29 have given a detailed account of the fundamental processes of adsorption, diffusion, and dissociation of a CO molecule on the Fe(100) surface. Their findings can be summarized as follows: (i) CO binds about equally at the on-top or 2-fold sites (∼31.0-35.0 kcal/mol) but much more strongly at the 4-fold site (∼47.0-48.0 kcal/mol). (ii) Experimental values of the on-top, 2-fold, and 4-fold sites are 12.8,32a 18.0,32a and 26.232a or 37.0.32b Although theory does overestimate the binding energies, the relative energies are in qualitative agreement.

As a calibration test, we calculated the adsorption of CO at the on-the top site, and we found it to be 33.6 kcal/mol, in very good agreement with the computed value by Sorescu et al. 3.2.2. H2O Adsorption on Fe(100). The interaction of water with Fe surfaces has also been the subject of extensive studies for a long time because of its importance in electrochemistry, corrosion, and catalysis. The investigation of the initial stages of interaction of water molecules with iron is therefore of great importance. We are particularly keen on understanding the nature of the species involved, their molecular orientation with respect to the surface, determination of reaction barriers, and how these may change in the presence of adsorbed CO2, which will be discussed in the following sections. Experimental studies by Dwyer et al.33 and Baro´ and Erley34 on H2O/Fe(110) have shown that H2O dissociates spontaneously between 130-160 K, for coverage less than 0.2 ML, whereas for higher coverage molecular water was detected. Others35 observed molecular water on the less packed Fe(100) for either low or high coverage at 100 K. Calculations by Eder et al.30 showed that H2O on Fe(100) or Fe(110) at low coverage spontaneously dissociates to surface-bound HO and H and further adsorption does not lead to spontaneous dissociation, which explains the experimentally observed molecular water. In our calculations of water molecules on Fe(100) we also observe both physisorption and spontaneous dissociation of water. Molecular H2O seems to prefer almost equally the ontop or 2-fold sites with binding energies of 9.1 and 11.3 kcal/ mol, respectively. These values are in good agreement with the ones obtained by Eder et al.30 of 8.1 kcal/mol. A combination of structural relaxations from different starting configurations and short MD trajectories revealed that the water moves in a rather flat potential and remains mostly physisorbed. However, when the oxygen finds itself at a 2-fold site with one hydrogen pointing toward the 4-fold hollow site, see Figure 1, panels (d) and (e), it readily dissociates to give chemisorbed OH and H. The energy difference between the 2-fold bridging position and the final dissociated state is found to be about 24.2 kcal/mol, also in excellent agreement with Eder et al. (∼25.4 kcal/mol). 3.3. CO2 Adsorption on Fe(100). When considering the interaction of CO2 with transition metals, it is reasonable to assume that CO2 can either bind through the O atoms, or form π-type interactions with CdO. For the bent anionic system, one would expect an additional binding site, through the central Carbon. Remarkably, structural characterizations either experimental36 or theoretical37-39 with one or more metals centers show that the binding interaction of CO2 with the metal centers involves mainly mixed carbon-oxygen bonding, and it is characterized by a bent configuration of CO2. Early studies of CO2 adsorption on metal surfaces were rather conflicting: as early as 195140 researchers reported irreversible CO2 adsorption on Ni single crystals. Later IR studies on NiO proposed the formation of CO32- as well as mixture of chemisorbed and physisorbed CO241 or the dissociative chemisorption of CO2 to CO and O at room temperature.42,43 Studies of CO2 adsorption on clean or alkali modified iron surfaces20,44 show that CO2 physisorbs and chemisorbs in almost all cases, and more often than not, it dissociates to CO and O. Our findings certainly agree with this picture: ab initio MD trajectories of several CO2 molecules (corresponding coverage up to 0.5 ML) on Fe(100) surface show that CO2 is spontaneously actiVated, assumes a bent structure, and binds to the surface. We also noticed that CO2 can bind in a number of different ways, all of which involve a bent configuration strongly suggesting charge transfer from the surface toward the CO2

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Glezakou et al.

Figure 4. Thermodynamically favorable products following CO2 adsorption on Fe(100), O-CO decomposition to produce adsorbed O and CO (P1) and further reaction of (P1) products with additional CO2 to give adsorbed CO32- (P2).

Figure 2. CO2 binding configuration on Fe(100).

TABLE 2: Structural Parameters and Binding Energies of CO2 on the Fe(100) Surfacea parameter

C1

C2

C3

C4

C5

RC-O1 RC-O2 RC-FEb RO1-FEb RO2-FEb ∠O1CO2 BE

1.347 1.349 2.109 2.082 2.054 117.6 -16.7

1.358 1.293 1.995 2.058 2.056 119.3 -14.5

1.313 1.304 1.985 2.005 2.047 120.4 -16.4

1.262 1.269 1.985 2.099 2.0667 132.9 -11.9

1.262 1.264 1.934 2.073 2.148 131.9 -12.9

C6 1.182 1.178 2.296 179.6 +9.1

a Binding energies in kcal/mol, distances in angstroms, and angles in deg. Configurations C1-C6 are shown in Figure 2. b To closest Fe neighbor.

Figure 5. NEB pathway connecting adsorbed CO2 (C1) and dissociated products O + CO on Fe(100). The two minima are connected with a low barrier of approximately 5 kcal/mol.

Figure 3. Electron density difference plot of adsorbed CO2 (configuration C2) (a), and spin-density plot of CO2- (b), showing radical character of adsorbed CO2.

moiety. Only physisorbed CO2 maintains its linear structure. The different binding configurations of adsorbed CO2 are shown in Figure 2. Table 2 summarizes the most important structural parameters and their binding energies taken with respect to the sum of energies of the surface and the ground state of the adsorbate (CO2). In all cases of chemisorbed CO2 (Figure 2, panels C1-C5), we see that it is stabilized on the surface with multiple bonds that, in most cases, involve all three centers of CO2. The most stable configuration, C1, binds by 16.7 kcal/ mol. We notice that this configuration has the longest C-O bonds, indicating that the longer, weaker C-O bonds facilitate multiple interactions with the surface and lead to stronger C-Fe and O-Fe bonds and overall to a more strongly bound system. Physisorbed CO2, Figure 2, panel C6, has a repulsive interaction with the surface, reflected in a slight shortening of the CO distance closest to the surface, and overall becomes destabilized compared to the noninteracting surface + CO2. The spontaneous activation of CO2 involves charge transfer from the surface toward the CO2 moiety, transforming it effectively to a bound CO2-. Figure 3 demonstrates exactly that: panel (a) shows the electron density difference plot of the adsorbed CO2 (here configuration C2) indicating a net increase

of electron density of the π-type orbitals of the molecule, shared almost equally by the terminal oxygens and the central carbon. In panel (b), we see the spin-density plot of the negatively charged radical species. Since the spin density arises from filling an empty state on bent neutral CO2 it should provide a direct comparison with the density difference obtained from the surface. The striking similarity between the two densities validates the intuitively simple proposition that CO2 adsorbs as CO2-. 3.4. CO2 Decomposition on Fe(100) to O + CO and Formation of CO3. Nassir and Dwyer have reported the adsorption and dissociation of CO2 on Fe(100) with XPS, UPS, and HREELS spectroscopies.20 Physisorbed, linear CO2 was also observed, but the chemisorbed species remained on the surface and eventually dissociated to CO and O. In this study, we also computed the dissociation process of adsorbed CO2/Fe(100) and found that the dissociated species OC + O/Fe(100) is more stable by 11.3 kcal/mol compared to the most stable C1 configuration, with both O and CO approximately situated at 4-fold sites. The C-O2 distance changes from ∼1.35 Å in the chemisorbed CO2 to almost 3.0 Å in the dissociated state, see Figure 4, panel P1. In order to obtain an estimate of the barrier for this dissociation process, we applied the NEB methodology29 on a series of configurations connecting the two points of interest, the adsorbed CO2 (here we chose the most stable C1 configuration) and adsorbed CO + O. Figure 5 shows the “replicas” that connect the two minima. The transition state is structurally very similar to the adsorbed species and mainly involves the

Activation of CO2 and H2O on the Fe(100) Surface

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3695

TABLE 3: Bader Analysis Charges on Replicas R1, R2, and R7 of NEB Following the Decomposition of Adsorbed CO2 to O + CO on Fe(100), Figure 6 replica

O2

C

O1

R1 (CO2) R2 (TS) R3 (O + CO)

-0.8 -1.7 -1.5

+1.1 +1.9 +0.9

-1.6 -1.5 -1.4

TABLE 4: Structural Parameters and Binding Energies of Other Corrosion Products on the Fe(100) Surfacea parameter RC-O1 RC-O2 RC-O3 RO3-H1 RO3-H2 RC-FE RO1-FE RO2-FE ∠O1CO2 ∆E

O + CO (P1)

CO3 + CO (P2)

1.278 2.981

1.257 1.332

1.979b

1.886b

2.006

2.162d

-11.3e

-29.9f

H2CO3 1.474 1.372 1.424 0.989 2.597c 2.169c 2.014c 107.5 +5.1,g -8.8h

CO2 · · · H2O

Figure 6. Interaction of adsorbed CO2 with H2O, adsorbed H2CO3 (P3) and van de Waals complex of adsorbed CO2 and molecular water (P4).

1.353 1.307 2.719 0.985 0.984 2.013 1.997 2.045 118.5 -10.1g

a Binding energies in kcal/mol, distances in angstroms, and angles in deg. Corresponding structures are shown in Figures 4 and 6. b C on CO to closest Fe neighbor. c To closest Fe neighbor. d O on CO3 to closest Fe neighbor. e Compared to configuration C1. f Compared to (P1) + CO2. g Compared to (C2) (Figure 2) + H2O, since CO2 resembles more (C2) configuration. h Compared to slab + H2CO3.

rearrangement of the CO moiety, disengaging the O double bridge from the surface so that CO can start moving away from O. The barrier for this rearrangement is fairly low, slightly over 5.0 kcal/mol, indicating that this reaction should be facile even at low temperatures. The final product, the adsorbed O and CO species at 4-fold sites, is considerably more stable, increasing the binding energy (compared to the noninteracting slab + CO2) to 28.0 kcal/mol. For replicas R1 (adsorbed CO2), R2 (transition state), and R7 (adsorbed O + CO), we performed a Bader analysis45 on their corresponding charge density grid. The results are summarized in Table 3. The net charge on the adsorbates is -1.3 (R1), -1.3 (R2), and -2.0 (R3), respectively. Clearly, the adsorbed CO2 is essentially CO2-. Although the geometry of the transition state is rather similar to the adsorbed CO2 (R1), the charge distribution is quite different, the charge separation becoming more pronounced as the adsorbed system prepares for dissociation. This charge reorganization, within the adsorbate only, happens quickly, assisted by the disengagement of O1 from the surface, and some of that density flows into O2. As the O2-C distance becomes bigger, the charge separation within the CO moiety balances out and O2 remains adsorbed essentially as O2-. We are interested in the interaction of metal surfaces with condensed CO2 phases, and since the dissociated state is significantly more stable, we also studied the interaction of a second CO2 molecule with the adsorbed O + CO. The result is adsorbed CO32-, as shown in Figure 4, panel P2. Select structural parameters are listed in Table 4. The C-O distances of the adsorbed CO3, are 1.295, 1.309, and 1.332 Å, consistent with the picture of resonant bonds in CO32-. The formation of CO32- lowers the energy of the adsorbed species by almost 30 kcal/mol compared to (C1) + CO2, indicating that this kind of transformation is thermodynamically favorable. 3.5. Reaction of Adsorbed CO2 with H2O and Formation of H2CO3. Ultimately, we are interested in studying the mechanisms of corrosion in condensed CO2 or water-saturated CO2 phases. CO2 in aqueous solutions readily becomes hydrated to produce the weak carbonic acid, H2CO3, which subsequently

Figure 7. NEB pathway connecting adsorbed H2CO2 (P3) and CO2-H2O bound complex on Fe(100). The two are connected through a significant barrier of slightly less than 20 kcal/mol, which is still much lower than the corresponding gas-phase barrier (48.3 kcal/mol, ref 46).

will equilibrate to HCO3- and CO32-. These anions play a very important role in electrochemical reactions and corrosion mechanisms. How do these mechanisms differ when water is not abundant? It is therefore important to understand the interaction of CO2 and H2O with Fe(100) in the presence of each other at the molecular level and trace the possible corrosion pathways when very little water is present. Nguyen et al. have conducted a systematic ab initio study of the gas-phase hydration mechanism of CO2 with a finite number of water molecules.46 Although carbon dioxide forms a reactant complex with a water molecule, it has to overcome a very large barrier (∼51.0 kcal/mol) before it can reach a transition state with a bent CO2 and finally form carbonic acid. Our study so far has proven that the formation of carbonate on the surface is a thermodynamically favorable process, once the adsorbed CO2 dissociates, a process with a small barrier. Obviously, CO2 reactions are facilitated by the activation of carbon dioxide by the metal surface. Is this also the case for the hydration of CO2? In Figure 6 we show the equilibrium structures of adsorbed H2CO3 (P3) and CO2 · · · H2O complex (P4). The last two columns in Table 4 contain select structural parameters of these systems. The complex (P4) is a stabilized complex compared to the energies of adsorbed CO2 + H2O, by 10.1 kcal/mol. Here we compare with the (C2) configuration, since CO2 in the equilibrium complex resembles that configuration more. The carbonic acid, however, is slightly destabilized with respect to the same reference, (C2) + H2O, by 5.1 kcal/mol, but stabilized by 8.8 kcal/mol compared to surface + H2CO3, and the relative energy difference between the complex (P4) and the carbonic

3696 J. Phys. Chem. C, Vol. 113, No. 9, 2009 acid (P3) becomes 15.2 kcal/mol. Quite interestingly, the gasphase difference between the corresponding systems is about 11 kcal/mol, very similar to their difference as adsorbed species. As in the case of C-O dissociation, we performed an NEB calculation to obtain an estimate of the barrier for the formation of the carbonic acid. The path as sampled by the NEB method and the corresponding replicas are shown in Figure 7. Although this transformation is still a thermodynamically unfavorable process with a barrier of ∼35.0 kcal/mol, this barrier is still quite smaller than the gas-phase barrier of 51.1 kcal/mol: adsorption stabilizes mainly the transition state. 4. Conclusions On the basis of a periodic slab model, we have shown that CO2 is spontaneously activated when in proximity with a clean Fe(100) surface. We have identified the possible binding configurations of CO2 on the surface and have determined their relevant binding energies, the most stable of which is bound by almost 17.0 kcal/mol. We determined the possible reactive pathways of adsorbed CO2 to dissociate and react with other CO2 or H2O molecules to produce corrosion products, CO32or H2CO3 in nonaqueous environments. Nudged elastic band calculations tracing the reactive pathways for CO2- decomposition to adsorbed O + CO and formation of carbonic acid show that the barriers for these two reactions are of the order of 5.0 and 35.0 kcal/mol, respectively. Although the adsorbed CO2 has a very small barrier to dissociation, and it can subsequently react with other free CO2 molecules to form CO32- on the surface, formation of H2CO3 is still a highly unlikely event at least under mild conditions, although the adsorbed CO2 · · · H2O complex is slightly bound. From this study, it is evident that corrosion of metal surfaces can happen in the absence of water and quite interestingly in the presence of one of the most benign and inert systems, CO2. Acknowledgment. The computer resources were provided by the Division of Chemical and Materials Sciences. V.-A.G. and B.P.M. gratefully acknowledge support received from the National Energy Technology Laboratory of DOE’s Office of Fossil Energy. L.X.D. acknowledges support from the Division of Chemical Sciences, Office of Basic Energy Sciences of the US DOE. The authors thank Dr. R. Rousseau for a critical review of the manuscript and useful discussions, as well as Dr. J. Jaffe, Dr. D. Mei and Dr. J. A. Franz for helpful comments. References and Notes (1) Nordsveen, M.; Nesˇiæc, S.; Nyborg, R.; Stageland, A. Corrosion 2003, 59, 443. (2) Nesˇiæc, S.; Lee, K.-L. J. Corrosion 2003, 59, 616. (3) Ford, P. C.; Rokicki, A. AdV. Organomet. Chem. 1988, 28, 139. (4) (a) Inoue, S.; Yamazaki, N. In Organic and Bioorganic Chemistry of Carbon Dioxide; Kodansha, L. T. D., Ed.; Wiley and Sons: New York, 1982. (b) Creutz, C. In Electrochemical and Electrocatalytic Reactions of Carbon Dioxide; Sullivan, B. P., Krist, K., Guard, H. E., Eds.; Elsevier: Amsterdam, The Netherlands, 1993. (5) Halmann, M. M. Chemical Fixation of Carbon Dioxide: Methods for Recycling CO2 into Useful Products; CRC Press: Boca Raton, FL, 1993. (6) (a) Behr, A. Angew. Chem., Int. Ed. Engl. 1988, 27, 661. (b) Miller, J. D. In Reactions of Coordinated Ligands; Braterman, P. S., Ed.; Plenum Press: New York, 1989; Vol. 1. (7) (a) Behr, A. In Catalysis in C1 Chemistry; Keim, W., Ed.; D. Reidel Publishing Co.: Dordrecht, The Netherlands, 1983; p 169. (b) Weissermel,

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