Density Functional Theory Study of CO and Hydrogen Co-adsorption

J. Phys. Chem. C 2007, 111, 4305-4314

4305

Density Functional Theory Study of CO and Hydrogen Co-adsorption on the Fe(111) Surface Zhong-Yun Ma,† Chun-Fang Huo,† Xiao-Yuan Liao,† Yong-Wang Li,† Jianguo Wang,† and Haijun Jiao*,†,‡ State Key Laboratory of Coal ConVersion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, P R China, and Leibniz-Institut fu¨r Katalyse e.V. an der UniVersita¨t Rostock, Albert-Einstein-Strasse 29a, 18059 Rostock, Germany ReceiVed: October 6, 2006; In Final Form: December 21, 2006

Spin-polarized density functional theory calculations have been carried out to characterize CO and H2 coadsorption on the Fe(111) surface. At 0.5 monolayer (ML) CO coverage, the most-stable adsorption configurations are close to those of individually adsorbed CO and H, and the interaction of adsorbed CO and H does not lead to any hydrogenated product. As the ratio of H2/CO increases, the most-table co-adsorption site of CO changes from the shallow-hollow site (sh) to the top site (t), while H maintains approximately at the top-shallow bridge site (tsb) or the quasi-fourfold site (qff). At 1 ML CO coverage, a similar trend is found where CO moves up to the first-layer Fe surface, while H prefers to diffuse into the subsurface when the H2/CO ratio increases. At very high H2/CO ratios, the most-stable configuration of CO has adsorption at the top site (t). In addition, formyl (CHO) is formed at H2/CO ratios of 1/2 and 1/1, deriving from hydrogenation of adsorbed CO in the bridge-like site (brl). The interaction nature of CO-H is also analyzed by the DOS analysis.

1. Introduction Iron-based catalysts are used widely in Fischer-Tropsch synthesis (FTS) for converting CO and H2 to high-molecularweight hydrocarbons,1 and R-Fe has been proposed to be one of the active phases.2 However, the corresponding reaction mechanism remains uncertain despite tremendous efforts over the past few decades. Particularly, it is experimentally very difficult to determine the initial states (surface species) of the reactions over iron-based catalysts under FTS conditions, which are essential to gain insight into the mechanism of FTS. Valuable information about the initial steps of the reactions may be obtained from the study of co-adsorbed CO and H2 on the surface. The co-adsorption of CO and H2 on low-index surfaces of transition metals has attracted particular attention, such as Fe(100),3 Ru(0001),4 Ni(100),5 Ni(111),6,7 and Gd(0001).8 Experimental investigations into the interaction between CO and H2 on Fe(100)3 showed that CO adsorption blocks the subsequent dissociative adsorption of H2, and in turn preadsorption of H2 reduces the binding energy of the subsequent CO adsorption and then inhibits the dissociation of CO. Density functional theory (DFT) calculations on Ru(0001)4 revealed that the lateral interaction between H and CO is repulsive, and Hads and COads prefer to form islands rather than mixed structures. In addition, the formation of formyl (CHO) and formate (HCOO) was detected by vibrational frequency analysis on Ni(111).6 Theoretical calculations7 showed that the best-fit geometry for adsorbed CO is the hexagonal-close-packed (hcp) hollow site on Ni(111) and that for adsorbed H atom is the facecentered-cubic (fcc) hollow site in the co-adsorbed phase. For the very open Fe(111) surface, rather little attention has been paid despite the proposed high catalytic activity.9 Using * Corresponding author. E-mail: [email protected]. † State Key Laboratory of Coal Conversion. ‡ Leibniz-Institut fu ¨ r Katalyse e.V. an der Universita¨t Rostock.

Figure 1. Top and side view of Fe(111). (a) Top surface cells used for calculations: centered rectangular for c(x3 × 1) with 0.5 ML CO and rhombus for p(1 × 1) with 1 ML CO; (b) side view of the p(1 × 1) and c(x3 × 1) slabs.

thermal energy atom scattering (TEAS), Bernasek et al.10 investigated the co-adsorption of CO and H2 on Fe(111) under ultrahigh vacuum (UHV) conditions. It was found that if the surface is first saturated with CO then H2 will not adsorb, but when H2 is adsorbed first and even at saturation coverage CO will adsorb and displace H2 from the surface. However, this work is only a macroscopic characterization of the interaction between CO and H2 and does not involve the identification of the concrete co-adsorption sites. In our previous studies, individual adsorption of CO and H2 at different coverage on Fe(111) was investigated.11,12 We found that the shallow-hollow (sh, bonded to the exposed second-layer Fe atoms) adsorption of CO is the most-stable site at 1/3 and 1/2

10.1021/jp066575l CCC: $37.00 © 2007 American Chemical Society Published on Web 02/27/2007

4306 J. Phys. Chem. C, Vol. 111, No. 11, 2007

Ma et al.

Figure 2. Configurations of CO and H2 co-adsorption on Fe(111) at 0.5 ML CO and H2/CO ) 1/2 (Fe atom in purple, C in gray, O in red and H in white); to distinguish the different bonds, Fe atoms in the c(x3 × 1) surface cell are labeled as a-k as shown in 1, Fe(b, d, g, h, k) for Fe1, Fe(a, e, i) for Fe2, and Fe(c, f, j) for Fe3.

monolayer (ML) coverage, while both shallow-hollow (sh) and bridge (br, bonded to both the first- and second-layer Fe atoms) adsorption can coexist at 1 ML. For H2 adsorption, the most favored adsorption of atomic H is the top-shallow bridge site (tsb, bridged over the first- and second-layer Fe atoms), followed by the quasi-fourfold site (qff, not only bonded to the secondand third-layer Fe atoms but also interacted with two nearby first-layer Fe atoms), while the top site (t, bonded to the exposed first-layer Fe atom) is not competitive. Furthermore, the adsorbed atomic H has a high mobility as indicated by the small diffusion barriers. These results are in very good agreement with the experiments. To further elucidate the interactions between CO and H2, and the formation of initial species on the Fe(111) surface, we perform a systematic DFT study on the coadsorption of CO and hydrogen at 0.5 and 1 ML CO coverage with different H2/CO ratios. 2. Methods and Models All calculations were performed with the CASTEP module13 in the Materials Studio 2.2 program of Accelrys Inc. The exchange and correlation energies were calculated using the Perdew, Burke, and Ernzerhof (PBE) functional14 with the generalized gradient approximation (GGA).15 The electronion interactions were described by ultrasoft pseudopotentials

(USPP)16 with the plane-wave basis set cutoff at 340 eV. A Fermi smearing of 0.1 eV was utilized. Spin polarization having major effects on the adsorption energies for magnetic systems17 was included to correctly account for its magnetic properties. The convergence criteria for structure optimization and energy calculation were set to MEDIUM quality with the tolerance for SCF, energy, maximum force, and maximum displacement of 2.0 × 10-6 eV/atom, 2.0 × 10-5 eV/atom, 0.05 eV/Å, and 2.0 × 10-3 Å, respectively. As pointed out by Hafner et al.,18 the present GGA functional underestimates the virtual levels of CO and overestimates the adsorption energies of CO on close-packed metal surfaces and even gives the wrong adsorption sites for some. It is reported that PBE is reliable for geometry but overestimates the adsorption energies;19 in contrast, the revised PBE (RPBE) can give not only reliable geometries but also reasonable adsorption energies. This behavior has been further tested and validated for hydrogen adsorption on Fe(111)12 and oxygen adsorption on R-Mo2C(0001).20 Compared to PBE, RPBE lowers the hydrogen and oxygen adsorption energies by about 0.3 and 0.5 eV, respectively. For hydrogen adsorption on Fe(111), the RPBE adsorption energy agrees very well with the available experimental data, and this excellent agreement give us the confidence for using PBE for geometry optimization and RPBE for

Density Functional Theory Study

J. Phys. Chem. C, Vol. 111, No. 11, 2007 4307

TABLE 1: Computed Bond Lengths (d, Å) and Adsorption Energies (Eads, eV) for CO and H2 Co-adsorption on Fe(111) with 0.5 ML CO and H2/CO ) 1/2 no.

CO + H

Eads a

dC-O

1

sh + tsb

1.188

1.752

2

sh + qff

-2.63 (-3.12) -2.58 (-3.11)

1.190

1.755

3

sh + qff

-2.57 (-3.09)

1.189

1.750

4

sh + sh

1.189

1.753

5

br + tsb

1.190

6

br + qff

-2.58 (-3.06) -2.61 (-3.11) -2.20 (-2.76)

1.755(a) 2.299(b) 1.773(a) 2.090(b)

1.190

7

brl + tsb

-2.42 (-3.01)

1.234

8

brl + qff

-2.33 (-2.98)

1.233

9

brl + tf

-2.12 (-2.80)

1.246

10

brl + tf

-1.89 (-2.52)

1.227

dC-Fe b

1.829(a) 2.137(b) 2.039(k) 1.829(a) 2.111(b) 2.039(k) 1.913(a) 2.132(b) 1.913(k) 1.859(a) 2.299(b) 1.955(k)

dO-Fe

dH-Feb 1.820(b) 1.612(e) 1.893(b) 1.978(c) 2.103(d) 1.657(e) 2.012[b] 2.027(b) 1.624(e) 2.130(f) 1.583(e)

2.149 2.131

2.045 2.179

1.846(d) 1.614(e) 1.667[a] 1.741(c) 1.987(d) 2.058(h) 1.790(d) 1.630(e) 1.950(b) 1.849(c) 2.006(h) 1.688(i) 1.730[a] 1.770(c) 1.790(d) 1.637(a) 1.852(b) 1.727(f)

PBE values in parentheses. b Letters a-k for the labeled Fe atoms in 1 (Figure 2). The symbol in parentheses means the bonded Fe atom in a c(x3 × 1) cell, and that in square brackets hints the bonded Fe atom in the adjacent c(x3 × 1) cell.


mode

dC-O dC-Fe b dO-Fe

(CO + H2) 11 sh + tsb/qff -2.97 1.189 1.756(a) (-3.65) 12 sh + tsb/qff -2.97 1.189 1.758(a) (-3.64)

dH-Fe b

dH-H

A

B

1.975(b) 1.828(c) 2.067(d) 1.637(e) 1.764(d) 1.606(e)

1.769(b) 1.607(i)

15 br + t

2.004[b] 2.022(b) 1.628(e) 1.860(f) 1.670(a) 1.789(b) 1.748(j) 1.644[a] 1.755(c) 2.068(d) 1.961(h) 1.720 0.891

19 brl + t

1.894(c) 1.839(h) 1.634(i) 1.656[a] 1.840(c) 1.960(d) 2.074(h) 1.606(a) 2.163(b) 1.794(f) 1.875(g) 3.074 0.759

13 br + tsb/tf

-2.71 1.188 1.767(a) (-3.39) 2.173(g)

1.849(b) 1.620(e)

14 br + tsb/qff

-2.68 1.189 1.773(a) (-3.37) 2.110(b)

1.837(b) 1.620(e)

-2.15 1.191 1.765(a) 1.691 (-2.76) 2.272(b) 16 brl + qff/qff -2.64 1.242 1.877(a) 2.062 1.903(c) (-3.47) 2.136(b) 1.787(d) 1.938(k) 1.645(e) 17 brl + tsb/qff -2.50 1.248 1.927(a) 2.115 1.806(b) (-3.32) 2.095(b) 1.620(e) 2.128(j) 2.001(k) 18 brl + qff/qff -2.42 1.231 1.858(a) 2.140 1.959(c) (-3.22) 2.280(b) 1.868(d) 1.958(k) 1.663(e)

20 t + tf/qff

a

calculating adsorption energy. Therefore, we use the RPBE energies for discussion, and the PBE values are provided for comparison. As shown in Figure 1, Fe(111) is a step-like or defect surface with atoms of both the second and the third layers exposed. The iron atoms of the first, second, and third layers are labeled as Fe1, Fe2, and Fe3, respectively. To model this surface, we used a slab with seven layers (Figure 1b); the top three layers were allowed to relax, and the bottom four layers were kept in their bulk positions (3Fe/4Fe). This model has been proven to be reasonable in our previous studies.11,12 In addition, the vacuum thickness of the slab was set to 10 Å to separate neighbor slabs. The calculated lattice constant and magnetic moment for R-Fe (bcc, ferromagnetic) of 2.826 Å and 2.24 µB are in nice agreement with the available experimental values of 2.866 Å and 2.22 µB.21 At the Fe(111) surface, magnetism is enhanced and the magnetic moment of the first (Fe1), second (Fe2), and third (Fe3) layers are 3.14, 2.42, and 2.52 µB, respectively. The same changes are found for the Fe(110) and Fe(100) surfaces (2.74, 2.53, and 2.54 µB vs 3.05, 2.52, and 2.65 µB).22 For studying the co-adsorption of CO and H2 on Fe(111), it is necessary to consider the adsorption order. Because CO adsorption on Fe(111) is stronger than H2, as found in both experimental10 and our previous theoretical studies,11,12 it is reasonable to consider that CO will adsorb prior to H2 under syngas environment. Therefore, H2 was placed on the preadsorbed CO systems before implementing the optimization steps.

Eads a

21 t + tsb/qff

22 t + tsb/tf 23 t + tf/tf 24 t + tsb/tsb

-1.80 1.243 1.844(a) 2.056 2.332 (-2.35) 2.137(b) 1.981(k) -2.44 1.173 1.783 1.910(c) 2.094[b] (-3.11) 1.861(d) 1.945(b) 1.669(e) 1.638(i) 1.908(j) -2.43 1.172 1.769 1.673(d) 2.026[b] (-3.10) 1.639(e) 2.020(b) 1.643(e) 1.814(f) -2.38 1.168 1.772 1.670(h) 1.731[a] (-3.03) 1.684(i) 1.833(c) 1.711(d) -2.30 1.173 1.167 1.990(c) 1.993(c) (-2.99) 1.768(d) 1.760(h) 1.655(e) 1.656(i) -2.28 1.181 1.760(d) 1.707(a) 1.685(a) (-2.99) 2.285(e) 1.595(g) 1.626(k)

a

PBE values in parentheses. b Letters a-k for the labeled Fe atoms in 1 (Figure 2). The symbol in parentheses means the bonded Fe atom in a c(x3 × 1) cell, and that in square brackets hints the bonded Fe atom in the adjacent c(x3 × 1) cell.

Two different surface cells were used to produce two different CO coverages by putting one CO molecular on one side of the slabs, c(x3 × 1) for 0.5 ML and p(1 × 1) for 1 ML, corresponding to the centered rectangular and the rhombus cells in Figure 1a, respectively. Brillouin-zone sampling is performed on Monkhorst-Pack special points.23 The K-point mesh is chosen to be (3 × 5 × 1) for c(x3 × 1) and (6 × 6 × 1) for p(1 × 1). Because the ratio of H2/CO lies in the range of 0.5 to 1.7 in a working iron catalyst,24 the ratios of H2/CO as 1/2, 1/1, and 2/1 are used, corresponding to 1/2H2, 1H2, and 2H2 on the surface, respectively. The adsorption energy for mCO and nH2 is defined as Eads ) E(mCO/nH2/slab) - [E(slab) + mE(CO) + nE(H2)], where m and n are the numbers of the adsorbed CO and H2, respectively, E(mCO/nH2/slab) is the total energy for the slab with absorbed CO and H2 on the surface, E(slab) is the total energy of the corresponding bare Fe slab, and E(CO) and E(H2) are the total energies of free CO and H2, respectively. Thus, the more negative the energy, the stronger the adsorption.


Ma et al.

Figure 3. Configurations of CO and H2 co-adsorption on Fe(111) with 0.5 ML CO and H2/CO ) 1/1 (Fe atom in purple, C in gray, O in red and H in white); to distinguish the different bonds, Fe atoms in the c(x3 × 1) surface cell are labeled as a-k as shown in 1, Fe(b, d, g, h, k) for Fe1, Fe(a, e, i) for Fe2, and Fe(c, f, j) for Fe3.

In this paper, the definition of CO or H coverage is the number of the adsorbed CO molecule or H atom over the number of the first layer Fe atom. 3. Results 3.1. Structure and Energy. 3.1.1. nH2/CO Co-adsorption (0.5 ML CO; n ) 1/2, 1, 2). For CO and H2 co-adsorption with H2/CO ratios of 1/2, 1/1, and 2/ , 1 CO molecule, and 1/ , 1, as well as 2 H molecules were 1 2 2 put in the c(x3 × 1) cell, and this corresponds to 0.5 ML CO coverage, and the H coverage of 0.5, 1, and 2 ML, respectively. At 0.5 ML CO and H2/CO ) 1/2 (0.5 ML H), ten co-adsorption configurations (1-10) are obtained (Figure 2), and the calculated adsorption energies and structural parameters are given in Table 1. In 1-4, CO adsorbs at the shallow-hollow site (sh/Fe2), while

H locates in turn at the top-shallow bridge site (tsb), the quasifourfold site (qff), and the shallow-hollow site (sh). They have very close bond parameters and adsorption energies, and their C-O and C-Fe2 bond lengths are very close to those of individual CO adsorption (1.190 and 1.752 Å, respectively),11 indicating that there is no obvious interaction between the coadsorbed CO and H. In 5 and 6, CO adsorbs at the bridge site (br), while H occupies the top-shallow bridge site (tsb) and the quasi-fourfold site (qff), respectively. 5 is almost the same as 1-4 in structural parameters and adsorption energies, but 6 is less stable than 5 (-2.20 vs -2.61 eV). In 7-10, CO adsorbs at the bridge-like site (brl), and they are much less stable than 1-5. This is due to the crowded co-adsorption, which results in CO and H repulsive interactions. Furthermore, co-adsorption with CO or H at the top site (t) cannot be found.



Figure 4. Configurations of CO and H2 co-adsorption on Fe(111) 0.5 ML CO and H2/CO ) 2/1 (Fe atom in purple, C in gray, O in red, and H in white); to distinguish the different bonds, Fe atoms in the c(x3 × 1) surface cell are labeled as a-k as shown in 1, Fe(b, d, g, h, k) for Fe1, Fe(a, e, i) for Fe2, and Fe(c, f, j) for Fe3.

For H2/CO ) 1/1 (1 ML H), there are fourteen configurations (11-24), and the calculated adsorption energies and structural parameters are listed in Table 2. As shown in Figure 3, it is noteworthy that the H adsorbed sites can be considered as the combinations of the occupied sites in 1-10. For instance, 11 has the same adsorbed H sites as 1 (tsb) and 2 (qff), and 12 has the same adsorbed H sites as 3 (qff) and 5 (tsb). As given in Table 2, 11 and 12 are the most-stable configurations, while other combinations (13, 14, 16-18) are less stable, and those with H or CO at the top sites (15, 19, or 20-24) are the least stable. With further increase of the H2/CO ratio, the situation changes considerably. For 0.5 ML CO and H2/CO ) 2/1 (2 ML H), the adsorption structures (25-39) in Figure 4 become more complicated, and the same is also true for the adsorption energies

(Table 3). As shown in Figure 4, the most-stable configuration has adsorbed CO at the top site (37 and 38), closely followed by those at the bridge (br) or bridge-like (brl) sites (26 and 29 or 34). This indicates that the most-stable CO adsorption site changes considerably with the increased hydrogen coverage, and the resulted stability order differs from those of individual CO adsorption, for example, the shallow-hollow site (sh), the bridge site (br), the brige-like site (brl), and the top site (t) at 0.5 ML.11 It is noteworthy that the more stable configuration (29) has adsorbed molecular hydrogen at the top site, indicating the possibility of the coexistence or equilibrium of CO and hydrogen at different adsorption sites at this coverage. 3.1.2. nH2/CO Co-adsorption (1 ML CO; n ) 1/2, 1, 2). For testing the influence of CO coverage on the co-adsorption


Ma et al.


mode

Eads a

dC-O

dC-Fe b

dH-Fe b

dO-Fe

(CO + 2H2)

B

C

D

1.949(c) 1.889(d) 1.602(e) 1.909(c) 1.789(d) 1.619(e)

1.749(c) 1.695(h) 1.633(i) 1.998(c) 1.823(h) 1.603(i)

1.807(b) 2.379(c) 1.593(e)

1.793(b) 2.393(c) 1.594(i)

1.756(b) 1.642(e) 1.746(f) 2.076[b] 1.964(b) 1.593(i) 2.146(j) 1.896(b) 1.611(e) 1.824(f)

1.560(b) 1.826(e)

1.646(b) 1.752(c) 1.746(i) 1.942(b) 1.871(c) 2.030(h) 1.653(i) 2.082

2.451

0.766

2.043

0.775

1.793(b) 1.616(e)

2.294

3.039

0.758

2.075(b) 1.911(c) 2.023(d) 1.664(e) 1.953(c) 1.828(d) 1.632(e) 1.861(b) 1.577(e) 2.337(f) 1.736(b) 1.610(e)

2.881

3.271

0.755

3.074

0.759

1.944(c) 1.754(d) 1.662(e)

1.896(c) 1.850(h) 1.637(i) 1.962(c) 1.828(h) 1.620(i) 1.878(b) 2.015(c) 2.063(h) 1.609(i) 2.038(c) 1.696(h) 1.645(i) 1.828(c) 1.722(h) 1.710(i) 1.930(c) 1.688(h) 1.663(i)

1.847(c) 1.677(d) 1.731(e)

1.839(c) 1.676(h) 1.729(i)

25

sh + tsb/tf/tf/tf

-2.66 (-3.72)

1.191

1.774

1.762(b) 1.612(i)

26

br + tf/tf/tf/qff

-2.94 (-4.02)

1.193

1.891(a) 1.934(k)

1.602(a) 1.846(f) 1.763(g)

27

br + qff/tf/tf/tf

-2.82 (-3.89)

1.192

1.792(a) 2.053(b)

28

br + tf/tsb/tf/tsb

-2.51 (-3.59)

1.184

2.133(a) 1.788(b)

29

br + t/qff/qff

-2.90 (-3.73)

1.192

1.756(a) 2.299(b)

1.581[a] 1.808(c) 1.946(d) 2.075(h) 1.623[a] 1.847(c) 1.852(d) 2.244

30

br + tf/tsb/t

-2.59 (-3.47)

1.197

1.773(a) 2.070(b)

31

brl + qff/tsb/t

-2.40 (-3.31)

1.250

32

brl + qff/qff/t

-2.40 (-3.23)

1.230

1.932(a) 2.090(b) 2.128(j) 1.996(k) 1.850(a) 2.282(b) 1.972(k)

33

brl + t/tf/tf

-2.55 (-3.46)

1.241

34

brl + tf/tf/tf/tf

-2.93 (-4.08)

1.224

35

brl + tf/tsb/qff/tf

-2.69 (-3.84)

1.222

36

t + tf/tsb/tf/tf

-2.83 (-3.83)

1.166

1.759

37

t + tf/tsb/tf/tsb

-2.97 (-3.98)

1.165

1.757

38

t + tsb/tf/tf/tqff

-3.01 (-4.04)

1.168

1.758

39

t + tsb/tf/tf/tsb

-2.70 (-3.71)

1.166

1.753

1.868(a) 2.136(b) 1.950(k) 1.882(a) 2.322(b) 1.903(k) 1.920(a) 2.282(b) 1.914(k)

2.087

2.153

2.070 2.185 2.166

dH-H

A

1.652[a] 1.751(c) 1.895(d) 1.653[a] 1.956(b) 1.846(c) 2.078(h) 1.603(a) 2.149(b) 1.790(f) 1.896(g) 2.337 1.606(a) 1.927(f) 1.703(g) 1.640(a) 1.757(b) 1.745(f) 1.766(a) 1.830(f) 1.661(g) 1.703(a) 1.885(f) 1.663(g) 1.695(a) 1.603(g) 1.645(a) 1.689(g)

2.144[b] 1.882(b) 1.621(e) 2.028(f) 1.772(b) 1.621(e)

1.799(b) 1.601(e) 1.854(b) 1.608(e)

1.524(b) 2.006(i)

1.859(c) 1.759(d) 1.628(e) 1.657[a] 1.810(c) 1.707(d) 1.764[a] 1.929(c) 1.621(d) 1.752(a) 1.575(k) 2.115[b] 1.914(b) 1.634(i) 1.936(j) 1.644(a) 1.696(k)

a PBE values in parentheses. b Letters a-k for the labeled Fe atoms in 1 (Figure 2). The symbol in parentheses means the bonded Fe atom in a c(x3 × 1) cell, and that in square brackets hints the bonded Fe atom in the adjacent c(x3 × 1) cell.

configurations, we have calculated the co-adsorption with 1 ML CO coverage and H2/CO ratios of 1/2, 1/1, and 2/1 ML. For 1 ML CO coverage and H2/CO ) 1/2 (1 ML H), the coadsorption of one CO and one H atom yields seven configurations (40-46). The calculated bond parameters and adsorption energies are given in Table 4. As shown in Figure 5, the moststable configurations are 41 (-1.95 eV) and 42 (-1.91 eV), while the other configurations are less stable. In 41 and 42, CO adsorbs at the bridge and bridge-like sites, respectively, and H adsorbs at the quasi-fourfold site (qff). It is also interesting to note the formation of adsorbed formyl (CHO) on the surface (46), but the calculated adsorption energy of -1.58 eV is lower than that of 41 (-1.95 eV). The least-stable configuration has adsorbed molecular H2 at the top site (40).

For 1 ML CO and H2/CO ) 1 (2 ML H) there are eight configurations (47-54) in Figure 6 of CO and H2 co-adsorption, and the calculated bond parameters and adsorption energies are given in Table 5. Because of the high coverage of hydrogen, the most-stable configuration has adsorbed CO at the top site (52 and 53), while the other configurations are less stable. Particularly configurations with adsorbed CO at the shallowhollow (sh) site or bridge (br) site and adsorbed molecular H2 at the top site (47 and 48) are the least stable. Interesting structural parameters are found in 51, in which one of the dissociated H2 embeds into the Fe bulk and forms bonds with the Fe1, Fe2, Fe3 and even the forth-layer Fe atoms (Fe4), and stays in a tetrahedron cavity (fourfold (ff)), and the other H still stay in the threefold site (tf) on the surface. For



TABLE 4: Computed Bond Lengths (d, Å) and Adsorption Energies (Eads, eV) for CO and H2 Co-adsorption on Fe(111) with 1 ML CO and H2/CO ) 1/2 no.

CO + H

40

sh + t

41

br + qff

42

brl + qff

43

Eads a

dC-O

-0.98 (-1.52) -1.95 (-2.50)

1.177

1.782

1.517

1.188

1.781(a) 2.128(b)

-1.91 (-2.54)

1.222

1.873(a) 1.960(b) 2.308(d)

2.176

brl + tf

-1.80 (-2.43)

1.224

2.151

44

t + tsb

1.161

45

t + tf

-1.62 (-2.12) -1.80 (-2.28)

1.882(a) 2.017(b) 2.214(d) 1.780

1.164

1.808

46

CHO

1.288

1.859

1.608[a] 2.027(b) 2.041[c] 1.749(e) 1.618[a] 2.100(b) 1.929[c] 1.756(e) 1.663[a] 1.728[b] 1.686(e) 1.722(a) 1.572(c) 1.701(a) 1.806(d) 1.801(e) 1.104 (dC-H)

-1.58 (-2.14)

dC-Fe b

dO-Fe

1.962

dH-Fe b

a PBE values in parentheses. b Letters a-e for the labeled Fe atoms in 40 (Figure 5). The symbol in parentheses means the bonded Fe atom in a p(1 × 1) cell, and that in square brackets hints the bonded Fe atom in the adjacent p(1 × 1) cell.

testing the possibility of H in Fe bulk, additional calculations with CO removed from 51 were carried out. The embedded H goes back to the surface without any barrier, and it is obvious that the presence of bulk H in this case is due to the repulsive interaction between H and CO in 51. Indeed, embedded hydrogen into transition metals has been found in both experimental25,26 and theoretical27-29 studies. It is noteworthy that hammering a H atom into bulk is a nice way to store potential energy, and the subsequent release can increase the reactivity of the reactant. Therefore, this is a widely useful mean of producing more active species in heterogeneous catalysis. For 1 ML CO and H2/CO ) 2/1 (4 ML H) there are only two adsorption configurations (55 and 56) in Figure 7 under such hydrogen-rich conditions, and the calculated structural parameters are listed in Table 6. In 55, one H2 dissociates at the threefold site (tf) and another adsorbs molecularly at top site (t), while CO sits in the shallow-hollow site (sh). In 56, both H2 are dissociated and CO sits in the top site (t). As expected, 56 with CO at the top and dissociated H is more stable than 55 with CO at the shallow-hollow site (sh) and H2 at the top (-1.53 vs -0.80 eV). It is also noteworthy that all other initial structures

Figure 5. Configurations of CO and H2 co-adsorption on Fe(111) with 1 ML CO and H2/CO ) 1/2 (Fe atom in purple, C in gray, O in red, and H in white); to distinguish the different bonds, Fe atoms in the p(1 × 1) surface cell are labeled as a-e as shown in 40, Fe(b, c, d) for Fe1, Fe(a) for Fe2, and Fe(e) for Fe3.


Ma et al.

Figure 6. Configurations of CO and H2 co-adsorption on Fe(111) with 1 ML CO and H2/CO ) 1/1 (Fe atom in purple, C in gray, O in red, and H in white); to distinguish the different bonds, Fe atoms in the p(1 × 1) surface cell are labeled as a-e as shown in 40, Fe(b, c, d) for Fe1, Fe(a) for Fe2 and, Fe(e) for Fe3.

with CO at other sites have been optimized into 56; this indicates the strong repulsive interaction between CO and H2, and this interaction can be reduced or minimized by moving CO to the top site. 3.2. DOS Analysis. The binding mechanism of CO on transition-metal surfaces shows the primary interactions of the expected forward- and back-donation, the mixing of CO 5σ with surface dz2 and s states, and the mixture of CO 2π* with metal dxz,yz.30 It was also thought that CO 1π to metal interaction plays an important role in weakening the CO bond in addition to 2π* back-donation when CO was inclined to the surface.31 On the basis of molecular orbital theory (four-electron repulsive interactions),32 the interaction between H-1s and CO-4σ, 1π, or 5σ will weaken the C-O bond,33 while the interaction between H-1s and CO-2π* will strength the C-O bond. Therefore, the competition of these two reverse effects on C-O bond strength may induce the C-O elongated, unchanged, or shortened with respect to the corresponding cases in CO individual chemisorption system.11 In our previous DOS analysis of CO adsorption on Fe(111), 5σ band shifts downward with respect to free CO and overlaps with 1π band (Figure 8). The main difference of DOS of adsorbed CO at the bridge (br) and bridge-like (brl) site is that there is a new peak at around -7.5 eV in the latter, which should

be assigned to 1π-derived state due to the O-Fe direct interaction, resulting in the elongated CO bond (1.221 Å). For co-adsorption of CO and H, there are two limited cases. At low coverage, the configurations and energies of the coadsorbed CO and hydrogen mimic those of the most-stable states of their individually adsorbed counterparts. At high coverage, however, the configurations and energies of the co-adsorbed CO and hydrogen mimic those of the least-stable states of their individual adsorbed counterparts. Because both the most-stable and least-stable states of individual adsorption have been analyzed,11,12 we have calculated the most-stable intermediate states (41 and 42) to examine the interaction of CO and H under co-adsorption with 1 ML CO and H2/CO ) 1/2 (1 ML H). As shown in Figure 8, 41 with CO at the bridge site (br) displays two peaks among -5 and -8.5 eV, and they are very similar with the DOS of individually adsorbed CO at the bridgelike (brl) site, indicating that the H-1s orbital interacts with CO1π, causing the splitting of this state. This conclusion is in agreement with the prediction of Xu et al.7 Like 41, 42 presents the 1π-derived state splitting to a larger extent in the energy range of -5 and -8.5 eV. The computed DOS for the adsorbed H in 41 and 42 (Figure 8) shows that the H-1s may interact with CO-4σ, 1π, 5σ, and 2π* because of the presence of peaks in their corresponding energy positions.




Eads a

mode

dC-O dC-Fe b dO-Fe

(CO + H2) 47 sh + t 48 br + t 49 sh + tf/tf

-1.00 1.202 1.823(a) (-1.75) 2.182(b) 2.211(d) -1.00 1.201 1.895(a) (-1.76) 1.916(b) -1.65 1.188 1.819(a) (-2.41) 2.242(b) 2.259(d) -1.67 1.198 1.915(a) (-2.44) 1.950(b)

dH-Fe b B

1.656

1.648

0.849

1.599

1.605

0.880

1.590[a] 1.777(b) 1.791(e) 50 br + tf/tf 1.588[a] 1.852(b) 1.858(e) 51 brl + tf/ff -1.47 1.228 1.885(a) 2.109 1.606[a] (-2.26) 1.960(b) 1.758(b) 2.312(d) 1.893(e) 52 t + tsb/tf

-2.04 1.163 1.760 (-2.73)

53 t + tf/tf

-1.98 1.162 1.771 (-2.65)

54 CHO,H

dH-H

A

1.759(a) 1.550(c)

1.736[a] 1.644(b) 1.912(e) -1.59 1.290 1.931(a) 1.957 1.106 (-2.37) 2.071(b) (dC-H)

1.589[a] 1.776(d) 1.797(e) 1.608[a] 1.717(d) 1.739(e) 1.650[a] 1.659[b] 1.656(e) 1.660 1.739(a) 1.682(b) 1.836(e) 1.737[a] 1.643(d) 1.908(e) 1.653[a] 1.816(b) 1.727(e)


However, the relative peak intensities indicate that the adsorbed CO and H interactions mainly come from the 1π orbital at 1 ML CO and H2/CO ) 1/2. 4. Summaries and Conclusions CO and hydrogen co-adsorption on Fe(111) at 0.5 and 1ML CO coverages and three H2/CO ratios (1/2, 1/1, and 2/1) has been investigated at the level of density functional theory. At defined CO coverage, as expected, the most-stable co-adsorption patterns change with the increased H2/CO ratio. At 0.5 ML CO and H2/ CO ) 1/2, the configurations with CO adsorbed at the shallowhollow site (sh) are the most stable (1-5), and no top site (t) adsorptions of both CO and H can be found. At H2/CO ) 1/1, CO adsorbs at the shallow-hollow site (sh) is still the most stable (11 and 12), while the configurations with CO adsorbed at the top site (t) becomes possible. At H2/CO ) 2/1, configurations with CO adsorbed at the shallow-hollow site (sh) become less stable because of the crowded distribution of surface species, while CO adsorbed at the top site (t) becomes more favored (37 and 38). Apart from CO, H adsorption sites of the topshallow bridge (tsb), the quasi-fourfold (qff), the shallow-hollow

Figure 7. Configurations of CO and H2 co-adsorption on Fe(111) with 1 ML CO and H2/CO ) 2/1 (Fe atom in purple, C in gray, O in red, and H in white); to distinguish the different bonds, Fe atoms in the p(1 × 1) surface cell are labeled as a-e as shown in 40, Fe(b, c, d) for Fe1, Fe(a) for Fe2, and Fe(e) for Fe3.

(sh), and the threefold (tf) sites have little influence on the stability of co-adsorbed configurations. It is also interesting to note the embedded hydrogen into Fe bulk due to the lateral repulsive interaction at high coverage. At 0.5 ML CO coverage, no hydrogenation products are formed; even the H atom is initially attached to the C or O atom of CO, and optimization led to the separated states. At 1 ML CO, surface-adsorbed formyl (CHO) is formed (46 and 54). However, adsorbed formyl species can only be formed with CO initially at the bridge-like site (brl), and CO initially at the shallow-hollow (sh), bridge (br), and top (t) sites do not lead to the formation of the C-H bond. Nevertheless, surface formyl is less stable compared to co-adsorbed CO and H. This is in good agreement with the theoretical investigations by Morgan et al.34 that formyl species adsorbed on Ru(001) are unstable and rapidly decompose to co-adsorbed CO and H. In addition, the DOS analysis shows that the H-1s may interact with CO-4σ, 1π, 5σ, and 2π*, leading to the C-O bond be weakened or enhanced. The relative interaction orientation of CO and H is tightly correlated with the change of the C-O bond length. Acknowledgment. This work has been supported by the National Nature Science Foundation of China (20590361 and


mode

Eads a

dC-O

dC-Fe b

(CO + 2H2)

dH-Fe b A

55

sh + t/tf/tf

-0.80 (-1.82)

1.197

56

t + t/tsb/tf/tf

-1.53 (-2.60)

1.170

1.816(a) 2.216(b) 2.251(d) 1.727

B

1.997

2.026

1.494

1.749(a) 1.531(d)

dH-H C

D

1.604[a] 1.774(b) 1.771(e) 1.750[a] 1.630(b) 1.748(e)

1.604[a] 1.775(d) 1.764(e) 1.733[a] 1.628(d) 1.755(e)

0.760



Figure 8. Total density of states of co-adsorbed CO and H in configurations 41 and 42 with 1 ML CO and H2/CO ) 1/2.

20573127) and the National Outstanding Young Scientists Foundation of China (No. 20625620). References and Notes (1) Anderson, R. B. The Fischer-Tropsch Reaction; Academic Press: London, 1984. (2) Dwyer, D. J.; Somorjai, G. A. J. Catal. 1978, 52, 291. (3) Benziger, J. B.; Madix, R. J. Surf. Sci. 1982, 115, 279. (4) Ciobica, I. M.; Kleyn, A. W.; Van Santen, R. A. J. Phys. Chem. B 2003, 107, 164.

Ma et al. (5) Li, J.; Schiøtt, B.; Hoffmann, R.; Proserpio, D. M. J. Phys. Chem. 1990, 94, 1554. (6) Bertolini, J. C.; Imelik, B. Surf. Sci. 1979, 80, 586. (7) Xu, L.; Xiao, H. Y.; Zu, X. T. Chem. Phys. 2006, 323, 334. (8) Getzlaff, M.; Bode, M.; Wiesendanger, R. Appl. Surf. Sci. 1999, 142, 428. (9) Spencer, N. D.; Schoonmaker, R. C.; Somorjai, G. A. J. Catal. 1982, 74, 129. (10) Bernasek, S. L.; Zappone, M.; Jiang, P. Surf. Sci. 1992, 272, 53. (11) Chen, Y.-H.; Cao, D.-B.; Yang, J.; Li, Y.-W.; Wang, J.; Jiao, H. Chem. Phys. Lett. 2004, 400, 35. (12) Huo, C.-F.; Li, Y.-W.; Wang, J.; Jiao, H. J. Phys. Chem. B 2005, 109, 14160. (13) (a) Payne, M. C.; Allan, D. C.; Arias, T. A.; Joannopoulos, J. D. ReV. Mod. Phys. 1992, 64, 1045. (b) Milman, V.; Winkler, B.; White, J. A.; Pickard, C. J.; Payne, M. C.; Akhmataskaya, E. V.; Nobes, R. H. Int. J. Quantum Chem. 2000, 77, 895. (14) Perdew, J. P.; Burke, S.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (15) White, J. A.; Bird, D. M. Phys. ReV. B 1994, 50, 4954. (16) Vanderbilt, D. Phys. ReV. B 1990, 41, 7892. (17) Ge, Q.; Jenkins, S. J.; King, D. A. Chem. Phys. Lett. 2000, 327, 125. (18) Gajdosˇ, M.; Eichler, A.; Hafner, J. J. Phys.: Condens. Matter. 2004, 16, 1141. (19) (a) Hammer, B.; Hansen, L. B; Nørskov, J. K. Phys. ReV. B 1999, 59, 7413. (b) Zhang, Y.; Yang, W. Phys. ReV. Lett. 1998, 80, 890. (20) Ren, J.; Huo, C.-F.; Wang, J.; Li, Y.-W.; Jiao, H. Surf. Sci. 2005, 596, 212. (21) Kittel, C. Introduction to Solid State Physics; Wiley: New York, 1996. (22) Błon´ski, P.; Kiejna, A.; Hafner, J. Surf. Sci. 2005, 590, 88. (23) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (24) (a) Davis, B. H. Catal. Today 2003, 84, 83. (b) Dry, M. E. Catal. Today 2002, 71, 227. (25) Johnson, A. D.; Daley, S. P.; Utz, A. L.; Ceyer, S. T. Science 1992, 257, 223. (26) Daley, S. P.; Utz, A. L.; Trautman, T. R.; Ceyer, S. T. J. Am. Chem. Soc. 1994, 116, 6001. (27) Ledentu, V.; Dong, W.; Sautet, P. J. Am. Chem. Soc. 2000, 122, 1796. (28) Jiang, D. E.; Carter, E. A. Phys. ReV. B 2004, 70, 064102. (29) Sorescu, D. C. Catal. Today 2005, 105, 44. (30) Sung, S.-S.; Hoffmann, R. J. Am. Chem. Soc. 1985, 107, 578. (31) (a) dePaola, R. A.; Hrbek, J.; Hoffmann, F. M. J. Chem. Phys. 1985, 82, 2484. (b) Eberhardt, W.; Hoffmann, F. M.; dePaola, R. A.; Heskett, D.; Strathy, I.; Plummer, E. W. Phys. ReV. Lett. 1985, 54, 1856. (32) Albright, T. A.; Burdett, J. K.; Whangbo, M. H. Orbital Interaction in Chemistry; Wiley: New York, 1985; pp 12-38. (33) Ziegler, T.; Versluis, L.; Tschinke, V. J. Am. Chem. Soc. 1986, 108, 612. (34) Morgan, G. A., Jr.; Sorescu, D. C.; Zubkov, T.; Yates, J. T., Jr. J. Phys. Chem. B 2004, 108, 3614.

Density Functional Theory Study of CO and Hydrogen Co-adsorption

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