Copper Promotion in CO Adsorption and Dissociation on the Fe(100

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Copper Promotion In CO Adsorption And Dissociation On The Fe(100) Surface Xinxin Tian, Tao Wang, Yong Yang, Yong-Wang Li, Jianguo Wang, and Haijun Jiao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp506794w • Publication Date (Web): 14 Aug 2014 Downloaded from http://pubs.acs.org on August 19, 2014

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The Journal of Physical Chemistry

Copper Promotion In CO Adsorption And Dissociation On The Fe(100) Surface

Xinxin Tian,

a,b,c

d

Tao Wang, Yong Yang,

a,b

Yong-Wang Li,

a,b

a

,a,d

Jianguo Wang, Haijun Jiao*

a) State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan,030001, China; b) National Energy Center for Coal to Liquids, Synfuels CHINA Co., Ltd, Huairou District, Beijing, 101400, China; c) University of Chinese Academy of Sciences, Beijing, 100049, China; d) Leibniz-Institut für Katalyse eV. an der Universität Rostock, Albert-EinsteinStrasse29a, 18059 Rostock, Germany. E-mail: [email protected]

Abstract Spin-polarized density functional theory computations have been carried out to study the adsorption and dissociation of CO on the clean as well as nCu-adsorbed and nCu-substituted Fe(100) surfaces (n = 1-3) at different coverage to explore Cu promotion effect in CO activation. Increasing Cu content not only lowers CO dissociation energies but also increases CO dissociation barriers as well as make CO dissociation thermodynamically less favorable, and the clean Fe(100) surface is most active in CO adsorption and dissociation. The nCu-substituted Fe(100) surface can suppress CO adsorption and dissociation more strongly than the nCu-adsorbed Fe(100) surface. CO stretching frequencies at different coverage have been computed for assisting experimental investigations.

Keywords: DFT, CO Activation; Copper Promotion, Iron Surface, High Coverage

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1. Introduction Fischer-Tropsch synthesis (FTS, CO + H2 → CxHy + H2O + CO2),

1-3

which converts synthesis gas (CO/H2) into liquid fuels and high 4

value-added chemicals, is an important technology in energy societies. Among various FTS active metals, Fe-based catalysts are widely used in industry because of their high activity and low price.

5-7

However, Fe-based FTS catalysts consist of not only pure iron

components (metallic iron, iron oxides and iron carbides) but also promoters such as copper, potassium and silica or zinc oxide. Particularly, the role of copper in Fe-based FTS has attracted great attention and has been extensively studied experimentally. 3+

9

Copper has been widely thought to facilitate the reduction of Fe species to metallic Fe, and to lower the reduction temperature.

8

10

11

Upon exposure to H2, de Smit et al., found that the reduction of Cu-promoted catalysts takes place easily at low temperature and is much faster than that of the un-promoted catalysts at high temperature, however no such significant difference could be observed upon exposure to synthesis gas. Recently, Cu promoter has been found to facilitate the consecutive conversion of magnetite to α-Fe under H2, while suppresses the formation of the χ-Fe5C2 phase under CO atmosphere.

12,13

The results verify the previous findings by

14

Wan et al., i.e.; Cu promoter facilitates the high dispersion of Fe2O3, significantly promotes the reduction and H2 adsorption, but severely suppresses CO adsorption and the carburization. In addition, copper can promote the carburization rate, activities of FTS and water-gas shift (WGS, CO + H2O → CO2 + H2) reaction, observations were reported on hydrocarbon selectivity.

18-21

15-17

increase the

22

as well as FTS reaction rate, while controversial

10,15,18,23

24

Experimentally, Gijzeman et al., studied CO adsorption and decomposition on the Fe(100), Fe(110) as well as Cu(111)-Fe, Cu(110)-Fe alloy surfaces using ellipsometry. They found that CO dissociates rapidly on Fe(100) and very slowly on Fe(110) at room temperature, and does not adsorb or dissociate on the pure Cu single crystals under their experimental conditions. On the Cu(111)Fe and Cu(110)-Fe alloy surfaces CO can adsorb and dissociates slowly at room temperature. Chonco et al.

12,13

studied the role of Cu

promoter using CuFe2O4 and CuFeO2 as model catalysts, and found that these copper-containing compounds show a higher CO2 selectivity, which should be due to the presence of a larger amount of small magnetite crystallites present during the FTS. 25

Theoretically, many models were proposed to reveal the Cu promotion effects. For example, Natter et al., applied cluster models to study the bonding strengths of water-gas shift intermediates on the Fe3O4(111) surface. They substituted both the surface and sub-surface iron ions by copper irons, and found that the bonding strength of the adsorbed intermediates on the surface is only slightly weakened for sub-surface substitution model, while much larger effect was found for surface substitution model. Pakiari et 26

al., studied ethylene adsorption over pure and small bimetallic clusters of FenCum (2 ≤ m+n ≤ 4), and found that replacing iron atoms with copper atoms results in a substantial improvement in ethylene adsorption on the bimetallic cluster and causes an ob27

vious increase in binding and interaction energies compared to the pure iron cluster. Zhao et al., studied CO adsorption and dissociation on the Cu-covered Fe(100) surface by applying periodic slab models and found that Cu monolayer covered Fe(100) surface has weaker CO adsorption energy and lower CO activation degree as well as much higher CO dissociation barrier (2.4 eV) compared 28

to the pure iron surface, while the co-adsorbed H largely facilitates CO dissociation via CHO formation. Elahifard et al., studied CO adsorption and dissociation on various Cu/Fe alloys over fcc-Cu(100) and bcc-Fe(100) surfaces with different CO coverage (25% and 50%) and found that Cu weakens CO adsorption and increases CO dissociation barriers considerably. Most recently, Zhao et al.,

29

studied CO dissociation on the Cu/Fe bimetallic catalyst and found that copper doping can reduce CO dissociation activity and their results partially validate the proposed dual site mechanism for higher alcohol synthesis on bimetallic catalysts. In this work we have carried out systematic theoretical studies to investigate the Cu promotion effect on the Fe(100) surface using both Cu adsorbed and substituted surface models under different Cu contents at different CO coverage. Our results provide some complementary understanding into Cu promotion effect in CO activation mechanisms in the initial stages of FTS. ~2~


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2 Computational methods and models 2.1 Methods: All calculations were performed at the DFT level within the Vienna Ab initio Simulation Package (VASP).

30,31

Periodic

32

slab models were used to model the catalyst surfaces. The projected augmented wave method (PAW) and the generalized gradient 33

approximation (GGA) were used. The exchange and correlation energies were calculated using the Perdew-Burke-Ernzerhof (PBE) 34

functional. The plane wave basis was set up to 400 eV. Spin-polarization was included for iron systems to correctly account for the 35

magnetic properties and this was found essential for an accurate description of adsorption energies. Geometry optimization was –4

done when the atomic force tolerance becomes smaller than 0.03 eV/Å and the energy difference was lower than 10 eV. A vacuum layer of 15 Å was set between the periodically repeated slabs to avoid strong interactions. The adsorption energy (Eads) of nCO molecule is defined as Eads = EnCO/slab – [Eslab + nECO], where ECO/slab is the total energy of the slab with nCO adsorption, Eslab is the total energy of the bare slab and ECO is the total energy of a free CO molecule in gas phase; and the more negative the Eads, the stronger the adsorption. For getting the saturation coverage, stepwise adsorption energy (ΔEads) was applied, i.e., ΔEads = E(CO)n+1/slab - [E(CO)n/slab + ECO], where a positive ΔEads for n+1 adsorbed CO molecules indicates the saturation adsorption with nCO molecules. Our stepwise adsorption energy defines the change of the adsorption energy by adding one more CO to the surface. All transition states were estimated using the climbing-image nudged elastic band (CI-NEB) method,

36,37

and the CO stretching fre-

quencies were analyzed to characterize a transition state with only one imaginary frequency. The CO dissociation barrier (Ea) is defined as Ea = ETS – EIS and the reaction energy (Er) is defined as Er = EFS – EIS, where EIS, EFS and ETS represent the total energy of the initially adsorbed CO molecule, final dissociated CO molecule (C+O atoms), and the CO dissociation transition state, respectively. Our previous study has proved that the zero-point energy (ZPE) corrections are rather small to either CO adsorption energies or CO dissociation barrier. Therefore, we used all energetics without ZPE corrections for our analysis and discussion.

38,39

2.2 Models: By using a 10×10×10 k-point grid, the computed equilibrium lattice constant and magnetic moment of bulk bcc-Fe are 2.83 Å and 2.22 µB, respectively, which agree well with the experimental values of 2.86 Å and 2.22 µB.

40,41

For studying Cu promotion

effect in CO adsorption and dissociation on the Fe(100) surface, we used a p(3×3) surface model with five atomic layers, where the top three layers along with adsorbates are relaxed and the bottom two layers are ﬁxed in their bulk positions. As shown in Figure 1, we used two doping models. The first one is the adsorption mode, where the Cu atoms are adsorbed on the surface (nCu-ads). The second one is the substitution mode, where the surface Fe atoms are substituted by Cu atoms with the formation of surface alloys (nCu-subs) as reported in previous study. wt%),

42-44

28,29

Since the copper loading is rather low in real FTS catalysts (0.1-2.0

the doping content of Cu in this study changes from one to three Cu atoms either on the surface or in the surface (1/9, 2/9,

3/9 ML), which is much lower than that in the previous study (1/4-1 ML).

27-29

On the basis of our detailed test (Figure S1) for the nCu-

ads model, the most stable adsorption site is the hollow site for one Cu atom, two nearest neighbored hollow sites for Cu2 and three nearest neighbored hollow sites for a linear Cu3. For the nCu-subs model, the most substitution sites are all the next-nearest neighbor substitution along the diagonal line for Cu2 and Cu3. (Figure 1) 3 Results and discussions 3.1 Adsorption sites: Figure 1 shows the schematic CO adsorption sites on the Fe(100), nCu-ads/Fe(100), and nCu-subs/Fe(100) surfaces (n = 1-3). The clean Fe(100) surface has top (T), bridge (B) and 4-fold hollow (H) sites. The 1Cu-ads/Fe(100) surface has ten adsorption sites, i.e.; four top (T1, T2, T3, T4), four bridge (B1, B2, B3, B4), and two 4-fold (H1, H2) sites, where the T4 site is atop of the adsorbed Cu atom. The 1Cu-subs/Fe(100) surface also has ten adsorption sites, i.e.; three top (T1, T2, T3), four bridge (B1, B2, ~3~



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B3, B4), and three 4-fold (H1, H2, H3) sites, where T3 site is atop of the substituted Cu atom. The 2Cu-ads/Fe(100) surface has seventeen adsorption sites, i.e.; five top (T1, T2, T3, T4, T5), nine bridge (B1, B2, B3, B4, B5, B6, B7, B8, B9), and three 4-fold (H1, H2, H3) sites, where T5 site is atop of the adsorbed Cu atom, and B9 site bridges the two adsorbed Cu atoms. The 2Cu-subs/Fe(100) surface has fourteen adsorption sites, i.e.; four top (T1, T2, T3, T4), six bridge (B1, B2, B3, B4, B5, B6), and four 4-fold (H1, H2, H3, H4) sites, where the T1 site is atop of the substituted Cu atom. Because of its high symmetry, the 3Cu-ads/Fe(100) surface has eight adsorption sites, i.e.; three top (T1, T2, T3) sites in which T3 site is atop of the adsorbed Cu atom, four bridge (B1, B2, B3, B4) sites in which B4 site bridges the two adsorbed Cu atoms, and one 4-fold (H1) site. There are only six adsorption sites on the 3Cu-subs/Fe(100) surface, i.e.; two top (T1, T2) sites in which T1 site is atop of the substituted Cu atom, two bridge (B1, B2) and two 4-fold (H1, H2) sites. 3.2 CO Adsorption at the lowest coverage: It should be noted that each adsorption site can have different CO orientations and not all possible adsorption sites can stably adsorb CO at the lowest coverage. For one CO molecule adsorption on the clean surface (Table 1), the most stable adsorption is at the hollow site with adsorption energy of -2.10 eV; and the bridge and top adsorptions are much less stable (-1.63 and -1.64 eV, respectively). The adsorbed CO molecules at the hollow sites have titled adsorption configurations, as indicated by the title angle between the surface normal and the CO molecular axis. These results agree very well with the results ob45

tained from periodic slab and cluster models as well as our previously reported data.

38,39

(Table 1) On the 1Cu-ads/Fe(100) surface, we found several CO adsorption configurations at the hollow sites as well as at the bridge and top sites, and the least stable adsorption configuration is atop of the adsorbed Cu atom (-1.11 eV). All adsorptions at the hollow sites have stronger adsorption energies than those at the bridge and top sites. Such energetic preference is also found for CO adsorption on the 2Cu-ads/Fe(100) and 3Cu-ads/Fe(100) surface. Therefore we will discuss only these most stable adsorptions at the hollow sites, and the results for the less stable adsorptions at the bridge and top sites are given in Supporting Information (Table S1). Taking the distance of the adsorbed CO molecule to the adsorbed Cu atom as reference, it is found that the remote CO adsorption is more preferred, i.e.; the longer this, the more stable the adsorption. On the 1Cu-ads/Fe(100) surface, for example, at the remote H1 site, the CO adsorption energy (-2.02 eV) is weaker than that at the H2 site (-2.11 eV). On the 2Cu-ads/Fe(100) surface, the adsorption at the remote H2 site (-2.07 eV) is more stable than that those at the H1 and H3 sites (-1.98 and -1.82 eV, respectively). On the 3Cu-ads/Fe(100) surface, all hollow sites are equivalent and the adsorption energy is -2.04 eV. Indeed, this remote dependent energy difference has electronic origin. For example, Bader charge analysis shows the charge transfer from surface Fe atoms to the Cu atoms by -0.28 e due to their difference in electronegativity and the surface Fe atoms become positively charged. The deformation charge density plot (Figure S2) shows the localization of the excess charge between the Cu adatom and the neighboring surface Fe atoms. Such electron transfer decreases with the increase of the Cu and Fe distance, and finally vanishes at the distance of Cu and Fe longer than 6Å (distance to the second diagonal Fe atom). Therefore, the Cu promotion effect is only short-ranged electronic interaction due charge transfer. On the 1Cu-subs/Fe(100) surface, several CO adsorption configurations at the hollow sites as well as at the bridge and top sites are located, and the least stable adsorption is atop of the substituted Cu atom (-0.90 eV). As expected from CO adsorption on the clean Fe(100) and nCu-ads/Fe(100) surfaces, the adsorptions at the hollow sites on the nCu-subs/Fe(100) surfaces are energetically more favorable than those at the bridge and hollow sites (Table S1). On the 1Cu-subs/Fe(100) surface, the CO adsorption energy depends on the distance between the adsorbed CO molecule and the substituted Cu atom; and the two remote H3 and H2 sites have stronger adsorption (-2.05 and -2.03 eV, respectively) than the H1 ~4~


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site (-1.74 eV). On the 2Cu-subs/Fe(100) surface, the stronger CO adsorption is found at the most remote H3 site (-1.93 eV), and those at the H2 and H4 sites are less stable (-1.76 and -1.72 eV, respectively), while the least stable adsorption is found at the H1 site (-1.55 eV). On the 3Cu-subs/Fe(100) surface, CO adsorption at the H2 site (-1.64 eV) is more stable than that at the H1 site (-1.54 eV). Our results show that the adsorption energy of one CO molecule decreases gradually with the increase of the substituted copper content, and this is in agreement with the previously reported results.

27-29

Detailed comparison shows that both nCu-ads/Fe(100) and nCu-subs/Fe(100) surfaces can suppress CO adsorption; however, the nCu-subs/Fe(100) surfaces can suppress CO adsorption more strongly than the nCu-ads/Fe(100) surfaces. For example, on the 3Cuads/Fe(100) surface, the CO adsorption energy of the most stable adsorption configuration is -2.04 eV, which is slightly lower than that on the clean Fe(100) surface (-2.10 eV), while that on the 3Cu-subs/Fe(100) surface is much lower (-1.64 eV). 3.3 CO dissociation at the lowest coverage: On the basis of these most stable adsorption configurations we have computed the CO dissociation barriers and dissociation energies. The adsorption configurations of the initial states, transition states and final states are shown in Figure 2. Corresponding dissociation pathway for the less stable adsorption configurations are given in Supporting Information (Figures S3-7). It is noted that for the final states we did not search the most stable adsorption states from surface diffusion of the adsorbed C and O atoms. For the dissociation of one CO molecule on the clean surface (Table 1), the computed 46

47

activation barrier (1.05 eV) is similar to that (1.06 eV, PW91) by Sorescu et al. and (1.11 eV, PW91) by Bromfield et al. based on a p(2x2) surface size, as well as (1.03 eV, PBE) in our previous report

38,39

based on a p(4x4) surface size. However, the barrier obtained

48

on a p(2x2) surface size with RPBE by Elahifard et al. is slightly higher(1.20 eV). (Figure 2) On the nCu-ads/Fe(100) surfaces (n=1-3), it is found that the computed CO dissociation barriers change hardly as compared to that on the clean Fe(100) surface; and they depend neither on the adsorption sites (or adsorption energies) nor on the number of the adsorbed Cu atoms at the lowest CO coverage. On the Cu-ads/Fe(100) surfaces, for example, the two most stable adsorption configurations have very close dissociation barriers (1.05 vs. 1.06 eV), but larger differences in adsorption energy (-2.11 vs. -2.02 eV). On the 2Cu-ads/Fe(100) surfaces, the computed CO dissociation barriers are 1.04, 1.07 and 1.00 eV, respectively, for the three most stable adsorption configurations, despite their significant differences in adsorption energies. On the 3Cu-ads/Fe(100) surface, the computed CO dissociation barrier is 1.08 eV, close to that on the clean surface (1.05 eV). On the nCu-subs/Fe(100) surfaces, it shows that the CO dissociation barriers depend on the adsorption sites or adsorption energies. On the 1Cu-subs/Fe(100) surface, for example, the adsorption energy and dissociation barrier (-2.05 and 1.05 eV) of the most stable H3 site are close to those on the clean Fe(100) surface (-2.10 and 1.05 eV), while the least stable H1 site has higher CO dissociation barrier (1.14 eV) and lower adsorption energy (-1.74 eV). The same trend is also found on the 2Cu-subs/Fe(100) surface, e.g.; the less stable H1, H2 and H4 sites (1.62, 1.76 and 1.23 eV) have higher CO dissociation barriers than the most stable H3 site (1.15 eV). On the 3Cu-subs/Fe(100) surface, the less stable H1 site has higher barrier than the more stable H2 site (1.64 vs. 1.22 eV). Our results show that the dissociation barrier of one adsorbed CO molecule increases gradually with the increase of the substituted copper content, while the dissociation energy becomes less exothermic, and this is in agreement with the reported results.

27-29

These results show clearly that at very low Cu content the surface adsorbed Cu atoms do not affect the CO dissociation significantly, while strong effect has been found for the Cu substituted surfaces, not only in lowering CO adsorption energies but also in increasing CO dissociation barriers. For a simple CO dissociation on the surface, the reaction mainly have three elementary steps; i.e., CO adsorption and desorption as well as dissociation. Based on a simple micro-kinetics models (Supporting Information), we computed the rate constants of CO ad~5~



Page 6 of 20

sorption, desorption and dissociation at 600K and 40atm on the clean and 3Cu-doped surfaces. Under this reaction condition, the Gibbs free energies of CO adsorption on the Cu doped surfaces (-1.06 and -0.66 eV for the 3Cu-ads and 3Cu-subs models, respectively) are lower than that on the clean surface (-1.12 eV). Accordingly, the CO adsorption rate decreases (desorption rate increases) 9 -1

from 2.42×10 s (4.12×10

-10 -1

8 -1

-9 -1

5 -1

-6 -1

s ) to 7.59×10 s (1.32×10 s ) for the 3Cu-ads model and 3.31×10 s (3.02×10 s ) for the 3Cu-subs 4

-1

4 -1

2

-1

model, while the CO dissociation rate also decreases from 1.88×10 s to 1.05×10 s for the 3Cu-ads model and 7.00×10 s for the 3Cu-subs model. The doped-Cu species also decreases the rates of not only CO adsorption but also dissociation. It is also found that CO dissociation is the rate-determining step on both clean as well as Cu-doped surfaces and the reaction order is zero. 3.4 CO adsorption at high coverage: Apart from the lowest coverage, we are also interested in the Cu promotion effect in CO adsorption and dissociation at higher coverage, since it is found experimentally that the surfaces of FTS catalysts are CO pre-covered because of the much stronger CO adsorption than H2.

49-51

In our previous work we found that CO adsorption and dissociation at high 38,39

coverage are quite different from those at the lowest coverage.

On the clean Fe(100) surface, the p(3x3) model surface can adsorb nine CO molecules on the basis of the stepwise CO adsorption energies (Table 2), and this corresponds a saturation coverage of 1 ML. The first five adsorbed CO molecules (nCO = 1-5) have 4-fold hollow adsorption configuration, and the value of ΔEads decrease due to the lateral repulsive interaction of the adsorbed CO molecules from nCO = 4 onwards. At nCO = 6, bridge adsorption configuration appears, and one bridge and five hollow adsorption configurations coexist. At nCO = 8, top adsorption configuration appears, and one top, two bridge and five hollow adsorption configurations coexist. At the saturation (nCO = 9), various adsorption configurations co-exist due to the distortion caused by repulsive lateral interaction (Figure 3). These results agree well with our previous results about high coverage CO adsorption on the p(4x4) model surface.

38,39

Although the most stable adsorption of one CO on the nCu-ads/Fe(100) surfaces was not obviously influenced by the adsorbed Cu species, the adsorption on the Cu-nearest sites is indeed hindered. At high coverage, it is to expect that CO adsorption should be influenced by the adsorbed Cu atoms. On the 3Cu-ads/Fe(100) surface (Figure S11), the first three CO molecules (nCO = 1-3) have 4-fold hollow adsorption configuration. At nCO = 4, bridge adsorption configuration appears, and at nCO = 7, top adsorption configuration appears. At nCO = 9, the adsorbed CO arrange orderliness with three hollow, three bridge, three top configurations coexisting. The saturation coverage can adsorb twelve CO molecules, three more than that on the clean surface. This is because of the adsorption of the Cu atoms at the hollow sites and the three extra CO molecules on the Cu top sites. At the saturation coverage (nCO = 12, Figure 3), the twelve CO molecules arrange regularly in four layers, the three CO on the hollow sites are the lowest layer, the three CO on the bridge sites are the second layer, the three CO on the Fe top sites are the third layer, and the three CO on the Cu top sites are the fourth layer. As shown in Table 2, the computed total CO adsorption energy on the 3Cu-ads/Fe(100) surface is lower than on the clean surface for n = 1-9, and the largest difference is found for n = 6 (1.34 eV). The same trend is also found for the stepwise CO adsorption energies. The total CO adsorption energies for n = 10-12 on the 3Cu-ads/Fe(100) surface also are very strong, however, the corresponding stepwise adsorption energies are rather low, they are lower than that of the last CO adsorption on the clean surface. This indicates the difference between the clean and the Cu-adsorbed surfaces in CO adsorption. On the 3Cu-subs/Fe(100) surface (Figure S12), the first three CO molecules (nCO = 1-3) have hollow adsorption configuration. At nCO = 4, bridge adsorption configuration appears. The saturation coverage has eight adsorbed CO molecules (nCO = 8, Figure 3), where the hollow, bridge and top sites are possible. All these adsorption configurations are strongly distorted and they cannot be clearly identified apart from those at the hollow sites. Compared to the clean surface, not only the total adsorption energies but also the ~6~


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stepwise adsorption energies are lower, indicating the stronger effect of Cu in suppressing CO adsorption. The total CO adsorption energies on the clean and 3Cu-doped surfaces show the decreasing order of Fe(100) > 3Cu-ads/Fe(100) > 3Cu-subs/Fe(100). Both Cu adsorbed and substituted surfaces can hinder the CO adsorption, and the Cu substituted surface can suppress CO adsorption more strongly than the Cu adsorbed surface. For comparison, the configurations and stepwise adsorption energies of CO on the 1Cu-doped surfaces at different coverage are given in Supporting Information for comparison (Figures S9-10). On the basis of these results, it is therefore to expect that higher Cu content can suppress CO adsorption even more strongly. (Figure 3) 3.5 CO dissociation at high coverage: On the basis of the most stable adsorption configuration at different coverage, we computed the first CO dissociation for estimating the Cu promotion effect at high coverage. The computed CO dissociation barriers and energies of the clean and 3Cu-doped surfaces are listed in Table 2. We used the negative adsorption energies as the desorption energies to estimate the preference of dissociation and desorption measure. On the clean Fe(100) surface (Figure S13), the first CO dissociation barrier for n = 1-6 is lower than the corresponding stepwise adsorption energy and all CO dissociations are exothermic, indicating that these first CO dissociations are favorable both kinetically and thermodynamically. For n = 7, the first CO dissociation energy becomes higher than its corresponding stepwise adsorption energy and the CO dissociation becomes endothermic by 0.13 eV, revealing that the first CO will prefer desorption instead of dissociation. Therefore, for n ≥ 7, CO desorption rather than CO dissociation is preferred. On the 3Cu-ads/Fe(100) surface (Figure S14), the first CO dissociation barrier for n = 1-4 is lower than the corresponding stepwise adsorption energy and all CO dissociations are exothermic, indicating that these first CO dissociations are favorable both kinetically and thermodynamically. For n = 5 and 6, the first CO dissociation energies become higher than its corresponding stepwise adsorption energies, although the CO dissociations are slightly exothermic, revealing that these first CO dissociations are kinetically not favored. It is therefore to expect that CO desorption rather than CO dissociation is preferred for n ≥ 5. On the 3Cu-subs/Fe(100) surface (Figure S15), for nCO = 1-3, the first CO dissociation barrier is lower than the corresponding stepwise adsorption energy and all CO dissociations are exothermic, and they are favorable both kinetically and thermodynamically. For n = 4, the first CO dissociation barrier becomes higher than its corresponding stepwise adsorption energy (1.37 vs. -1.29 eV) and the dissociation becomes endothermic by 0.08 eV. Therefore, for n ≥ 4, the first step should be CO desorption instead of CO dissociation. Detailed comparison among these three surfaces shows some general trends, i.e.; the clean surface can dissociate more CO molecules than the adsorbed and substituted surfaces (n = 6, 4 and 3, respectively), and the dissociation barrier is the lowest on the clean surface, followed by the adsorbed surface and the substituted surface, as well as CO dissociation on the clean surface is most exothermic, while least exothermic on the substituted surface. It demonstrates obviously that the Cu substituted surface can hinder CO dissociation more strongly than the Cu adsorbed surface. 3.6 Stretching frequencies of adsorbed CO molecules: In order to provide some references for additional experimental studies in identifying the Cu adsorbed and substituted surfaces as well as Cu sites, we have computed all CO stretching frequencies from the lowest to the saturation coverage on all surfaces. All these data are listed in Supporting Information (Table S1-S5). The ranges of the computed CO frequencies at saturation coverage are shown in Table 3. (Table 3) On the clean Fe(100) surface, early experimental studies

52-54

have revealed three molecular CO desorption states (α1, α2 and α3)

and one re-combinative desorption state of dissociated C and O atoms (β); and the α3 state represents the most stable 4-fold hollow site adsorption, which is also the precursor state for CO dissociation.

55-58

The three molecular adsorption states have characteristic

~7~



CO stretching frequencies based on the HREELS study,

59,60

Page 8 of 20

-1

-1

i.e., 1180-1245 cm for the α3 state and 1900-2070 cm for the α1 and α2

states. On the clean surface at saturation (nCO = 8), the calculated CO stretching frequencies mainly show three ranges, i.e.; 1174-1

-1

-1

1374 cm for CO at the hollow sites, 1865-1978 cm for CO at the bridge sites and 2048 cm for CO at the top sites. The computed CO frequency ranges agree well with the experimental values as well as our previous reported data by using a p(4x4) surface size.

38,39

At the saturation coverage (nCO = 12) on the 3Cu-ads/Fe(100) surface, the calculated CO stretching frequencies show four ranges; -1

-1

-1

i.e.; 1228-1248 cm for CO at the hollow sites, 1801-1856 cm for CO at the bridge sites, 1851-1908 cm for CO at the Fe top sites as -1

-1

well as 2089-2129 cm for CO at the Cu top sites. The computed frequencies of 2089-2129 cm is very similar to those of CO on the -1

-1

Cu(111) (2070 cm ) and Cu(100) (2088 cm ) surfaces.

61

On the 3Cu-subs/Fe(100) surface at the saturation coverage (nCO = 8), the calculated CO stretching frequencies mainly show two -1

-1

ranges; i.e.; 1221-1335 cm for the hollow sites, 1869-2001 cm for the bridge and top sites. It is interesting to mention that the tilted bridge and tilted top adsorption configurations are very similar and they have very close stretching frequencies. Therefore, it may be very difficult to distinguish their adsorption configurations using these CO frequencies. It can be seen that Cu species shifts the CO frequencies of hollow site on Fe(100) surface to higher wave numbers and narrows the band range. Since CO stretching frequencies are directly associated with the C-O distances at the adsorbed equilibrium states, the shift of the CO frequencies to higher wave numbers upon the increase of the coverage comes from the weakening of the Fe-CO interaction and therefore the shortening of the CO distance. For the stretching frequencies of all CO adsorption state except the one atop of the Cu species in ads-model, they are still rather small than the wave numbers on Cu(111) and Cu(100) surfaces.

61

This is

reasonable because the CO has more strong interaction with the iron surface than that with copper surface. Conclusion For understanding the Cu promotion effect in CO activation in the initial stage of iron-based Fischer-Tropsch synthesis, spinpolarized density functional theory computations have been carried out to study the adsorption and dissociation of CO on the clean as well as Cu-doped Fe(100) surfaces at different coverage. To model the nCu-doped Fe(100) surfaces, we have used both the nCuadsorbed and nCu-substituted surfaces (n = 1-3). At the lowest CO coverage, the CO adsorption energy depends on the distance between the adsorbed CO molecule and the doped Cu atoms; and the remote CO adsorption is more preferred. For the most stable adsorption configurations on the nCu-adsorbed Fe(100) surface (n = 1-3), the copper content does not affect the CO adsorption energies as well as CO dissociation barriers. On the nCu-substituted Fe(100) surface(n = 1-3), however, the CO dissociation barrier depends on the adsorption sites or adsorption energies, and the less stable adsorption configurations have higher dissociation barriers than those of the more stable adsorption configurations. At higher CO coverage, both 3Cu-adsorbed and 3Cu-substituted Fe(100) surfaces lower the CO adsorption energies and raise the first CO dissociation barriers, and the first CO dissociation becomes thermodynamically less favored as compared with the clean Fe(100) surface. The 3Cu-substituted Fe(100) surface can suppress CO adsorption and dissociation more strongly than the 3Cu-adsorbed Fe(100) surface. At CO saturation coverage, various adsorption configurations in equilibrium can co-exist. The clean (3x3) Fe(100) surface can adsorb nine CO molecules (1 ML), and the 3Cu-substituted Fe(100) surface can adsorb eight CO molecules (8/9 ML). However, the 3Cuadsorbed Fe(100) surface has twelve CO molecules (12/9 ML), nine coordinating with surface iron atoms and three coordinating with the adsorbed Cu atoms. To aid further experimental studies, CO stretching frequencies from the lowest to the saturation coverage have been computed. ~8~


Page 9 of 20 11 22 3 43 54 6 75 86 9 7 10 11 8 12 13 9 14 10 15 16 11 17 12 18 19 13 20 14 21 22 15 23 16 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Since copper suppresses CO adsorption, CO stretching frequencies are shifted to high wavenumbers. At saturation coverage, the 3Cuadsorbed Fe(100) surface has four characteristic CO frequency ranges, from CO adsorptions at the 4-fold-hollw sites to the bridge and top sites on surface iron atoms as well as the top sites on the adsorbed copper atoms; while the 3Cu-substituted Fe(100) surface has two characteristic CO frequency ranges, from the 4-fold-hollw sites as well as the undistinguishable bridge and top sites.

Acknowledgment: This work was supported by the National Basic Research Program of China (no. 2011CB201406), the National Natural Science Foundation of China (no. 21273262& 21273266), and the Chinese Academy of Science and Synfuels CHINA. Co., Ltd. We also acknowledge general financial support from the BMBF and the state of Mecklenburg-Vorpommern.

Supporting Information Available: Micro-kinetics models; Vibration frequencies of adsorbed CO at different coverage on the clean and Cu-doped Fe(100) surface (Table S1-5); Structures and relative energies of various nCu-doped/Fe(100) models (Figure S1); The deformation charge density plot of 1Cu-ads/Fe(100) surface (Figure S2); Structures of CO dissociation from various adsorption sites on the nCu-doped/Fe(100) surface (Figures S3-7); Structures and stepwise adsorption energies of CO on the clean and nCudoped/Fe(100) surfaces at different coverage (Figures S8-12); Dissociation structures of the first CO at different coverage on the clean and 3Cu-doped/Fe(100) surfaces (Figures S13-15). This material is available free of charge via the internet at http://pubs.acs.org.

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Page 10 of 20

-1

Table 1: CO distances (dC-O, Å) and stretching frequencies (υCO, cm ), height of C atom to the surface (hC, Å), angle between the surface normal and the CO molecular axis (∠Norm-C-O, °) and adsorption energies (Eads, eV) of the adsorbed initial states (IS), as well as CO dissociation barriers (Ea, eV), dissociation energies (Er, eV) and breaking C-O distances (dC-O, Å) in the transition states (TS)

CO (free)

dC-O (IS)

υC-O (IS)

1.14

2133

hC

∠Norm-C-O

Eads

Ea

Er

dC-O (TS)

1.05

-0.98

1.93

CO/Fe(100) H

1.32

1168

0.66

48.64

-2.10

B

1.19

1779

1.63

23.93

-1.63

T

1.18

1876

1.79

1.04

-1.64

CO/1Cu-ads/Fe(100) H1

1.32

1158

0.65

48.42

-2.02

1.06

-0.81

1.94

H2

1.32

1173

0.66

49.29

-2.11

1.05

-0.96

1.93


1.32

1175

0.63

48.67

-1.98

1.07

-0.78

1.93

H2

1.32

1153

0.63

49.73

-2.07

1.04

-0.97

1.93

H3

1.32

1149

0.66

50.29

-1.82

1.00

-1.13

1.91

1.32

1178

-2.04

1.08

-0.77

1.93


0.62

48.82

CO/1Cu-subs/Fe(100) H1

1.32

1187

0.64

47.14

-1.74

1.14

-0.87

1.95

H2

1.32

1153

0.63

48.70

-2.03

1.09

-0.65

1.94

H3

1.32

1140

0.63

49.06

-2.05

1.05

-0.90

1.93


1.28

1307

0.68

39.12

-1.55

1.62

-0.57

1.94

H2

1.29

1277

0.65

42.68

-1.76

1.51

-0.65

1.98

H3

1.32

1180

0.60

48.15

-1.93

1.15

-0.62

1.96

H4

1.29

1277

0.67

42.13

-1.72

1.23

-0.79

1.96


1.28

1306

0.64

39.54

-1.54

1.64

-0.50

1.98

H2

1.30

1252

0.61

43.44

-1.64

1.22

-0.51

1.96

~ 10 ~


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Table 2 CO total adsorption energy (Eads, eV), stepwise adsorption energy (ΔEads, eV), dissociation barriers (Ea, eV) and dissociation energies (Er, eV) of the first CO at different coverage on the clean Fe(100) and 3Cu-doped surfaces. Fe(100) 3Cu-ads/Fe(100) 3Cu-subs/Fe(100) nCO Eads ΔEads Ea Er Eads ΔEads Ea Er Eads ΔEads Ea Er 1 -2.10 -2.10 1.05 -0.98 -2.04 -2.04 1.08 -0.77 -1.64 -1.64 1.22 -0.50 2 -4.25 -2.15 1.08 -1.04 -3.97 -1.93 1.16 -0.71 -3.40 -1.76 1.27 -0.47 3 -6.29 -2.04 1.16 -0.85 -5.69 -1.72 1.32 -0.65 -5.04 -1.64 1.31 -0.19 4 -8.11 -1.82 1.36 -0.57 -7.25 -1.56 1.37 -0.56 -6.33 -1.29 1.37 0.08 5 -9.72 -1.61 1.28 -0.69 -8.72 -1.47 1.53 -0.32 -7.79 -1.46 1.46 0.43 6 -11.33 -1.61 1.48 -0.41 -9.99 -1.28 1.54 -0.18 -9.18 -1.39 1.87 0.48 7 -12.30 -0.97 1.46 0.13 -11.05 -1.06 -10.12 -0.94 8 -13.11 -0.81 -11.95 -0.90 -10.90 -0.78 9 -13.45 -0.34 -12.68 -0.74 -10.73 0.16 10 -13.01 0.44 -12.94 -0.26 11 -13.16 -0.21 12 -13.27 -0.12

~ 11 ~


The Journal of Physical Chemistry 11 22 33 4 5 6 7 8 9 10 11 12 13 14 15 16 4 17 5 18 6 19 7 20 8 21 9 22 10 23 11 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-1

Page 12 of 20

Table 3 Comparison of calculated CO vibrational frequencies (cm ) at saturated coverage on clean Fe(100) and 3Cu-doped/Fe(100) surfaces with that experimental results on Fe(100), Cu(100), and Cu(111) surfaces

a

Fe(100)

Fe(100)-p(3x3) b Fe(100)-p(4x4) 3Cu-ads/Fe(100) 3Cu-subs/Fe(100) Cu(100)

H (α3) 1180-1245 1174-1374 1179-1280 1228-1248 1221-1335

B (α2) 1900-2070 (α1 and α2) 1865-1978 1800-1850 1801-1856 1869-2001

Cu(111)

T/Fe (α1)

T/Cu

2048 2012 1851-1908

2089-2129 c

2088 d (2079-2088) c 2070 e (2078-2070)

(a) Experimental value from Ref. 59,60 (b) DFT calculated results from Ref. 39 (c) Experimental value from Ref. 61 (d) Experimental value from Ref. 62 (e) Experimental value from Ref. 63

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Figure 1: Possible CO adsorption sites on the Fe(100), nCu-ads/Fe(100), and nCu-subs/Fe(100) surfaces (n = 1-3)

~ 13 ~



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Page 14 of 20

Figure 2: Initial state (IS), transition state (TS) and final state (FS) of CO dissociation from most stable adsorption sites on the Fe(100) and doped nCu-ads/Fe(100) and nCu-subs/Fe(100) surfaces (black ball for C, red ball for O, blue ball for Fe and orange ball for Cu atoms)

~ 14 ~


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Figure 3: Structures of CO at saturation coverage on the clean Fe(100) and 3Cu-doped/Fe(100) surfaces (black ball for C, red ball for O, blue ball for Fe and orange ball for Cu atoms)

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Copper Promotion in CO Adsorption and Dissociation on the Fe(100

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