A First-Principles Study of Oxygenates on Co Surfaces in Fischer

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J. Phys. Chem. C 2008, 112, 9464–9473

A First-Principles Study of Oxygenates on Co Surfaces in Fischer-Tropsch Synthesis Jun Cheng,† P. Hu,*,† Peter Ellis,‡ Sam French,‡ Gordon Kelly,§ and C. Martin Lok§ School of Chemistry, The Queen’s UniVersity of Belfast, Belfast BT9 5AG, U.K., Johnson Matthey Technology Centre, Reading RG4 9NH, U.K., and Johnson Matthey Technology Centre, Billingham CleVeland, TS23 1LB, U.K. ReceiVed: March 14, 2008; ReVised Manuscript ReceiVed: April 14, 2008

Extensive density function theory calculations are performed to study the mechanism of the formation of aldehyde and alcohol on Co surfaces in Fischer-Tropsch synthesis, a challenging issue in heterogeneous catalysis. Three possible pathways for the production of formaldehyde and methanol on flat and stepped Co(0001) surfaces are investigated: (i) CO + 4H f CHO + 3H f CH2O + 2H f CH3O + H f CH3OH; (ii) CO + 4H f COH + 3H f CHOH + 2H f CH2OH + H f CH3OH; and (iii) the coupling reactions of CH2 + O f CH2O and CH3 + OH f CH3OH. It is found that these pathways are generally favored at step sites, and the preferred mechanism is pathway (i) via CHO. Furthermore, the three traditional chain growth mechanisms in Fischer-Tropsch synthesis are semiquantitatively compared and discussed. Our results suggest that the two mechanisms involving oxygenate intermediates (the CO-insertion and hydroxycarbene mechanisms) are less important than the carbene mechanism in the production of long chain hydrocarbons. However, the CO-insertion mechanism may be responsible for the production of long-chain oxygenates. 1. Introduction Fischer-Tropsch (FT) synthesis has been the subject of intense research work for many decades because it gives access to industrially important chemicals from the starting point of simple inorganic molecules (CO and H2).1–8 Especially in the past 20 years, tremendous efforts have been dedicated to this process because of the unremitting rising price of crude oil. The diversity of FT products is a very interesting feature of the process, which consists of mainly paraffins and olefins with a carbon number of up to 100, and low levels of oxygenates including alcohols, aldehydes, ketones and acids. The production of hydrocarbons has been extensively studied in the literature, and great progress has been achieved.9,10 However, the formation of oxygenates has been inadequately addressed, and how they are produced in FT synthesis remains unclear. Aiming to provide insight into the mechanism of oxygenate formation, we have studied the synthesis of alcohol and aldehyde on Co surfaces using density functional theory (DFT). The amount of oxygenates produced under typical FT reaction conditions is very small, and they are therefore often neglected. Yates and Satterfield11 found that 1-alcohols from C1 to C5 along with acetaldehyde and propionaldehyde were produced using a Co-Mg catalyst supported on diatomaceous earth in a slurry reactor. At C5, the yield of alcohol was approximately 1/100th that of alkane and alkene. The yields of alcohols were found to follow a “double-R” Schulz-Flory distribution, where the amount of alcohol drops off rapidly with carbon number at low carbon numbers, and decreases less quickly at higher carbon numbers. Addition of a promoter can be used to modify the product distribution. For instance, Co-Mn catalysts have been shown to produce oxygenated materials.12,13 Morales et al. suggest that MnO sites are responsible for the formation of CxHyOz inter* Corresponding author. † The Queen’s University of Belfast. ‡ Johnson Matthey Technology Centre, Reading. § Johnson Matthey Technology Centre, Billingham Cleveland.

mediates,14 which lead to the formation of oxygenated compounds. Co-Cu catalysts are well-known15,16 for C1 to C6 alcohol synthesis. Fe catalysts, in contrast, can give significantly higher yields of oxygenated compounds. Yang et al.17 found selectivities to oxygenates (mainly alcohols and esters) in excess of 30% for an Fe-Mn catalyst in a fixed bed reactor. Addition of promoters such as K17 reduces the oxygenate selectivity. Traditionally, three types of chain growth mechanisms have been proposed, namely the carbene mechanism,18 the hydoxycarbene mechanism,19 and the CO-insertion mechanism.20 With the support of a great deal of experimental21–26 and theoretical work,27–34 the carbene mechanism is often referred to as the most probable FT mechanism. Our recent work provides a quantitative understanding of the chain growth of the carbene mechanism by comparing the reaction rates of all possible C-C coupling pathways.35,36 Nevertheless, the latter two mechanisms, both of which involve oxygenate intermediates, have not been adequately investigated. In particular, the CO-insertion mechanism could be an alternative possibility responsible for the formation of alcohols, aldehydes, and other oxygenates. In this study, we will further contrast these two mechanisms with the carbene mechanism in terms of the reaction rate. To study the formation of alcohol and aldehyde, we started with formaldehyde and methanol, and then further extended to C2+ alcohols and aldehydes. In the literature, methanol chemistry has been extensively studied because of its relevance to two important catalytic processes, namely, methanol synthesis37,38 and methanol decomposition.39–41 Neurock studied CO hydrogenation to methanol over Pd(111) clusters using DFT calculations.42 The author calculated the adsorption energetics of surface intermediates, formyl (CHO), formaldehyde (CH2O), methoxy (CH3O), and methanol (CH3OH) on different sizes of Pd cluster, and the reaction barrier of the first hydrogenation step (CO + H f CHO) was computed to be 78 kJ/mol (0.81 eV). In another theoretical work, Remediakis et al. considered an alternative pathway starting with the formyl isomer (COH) on Ni(111).43 Although COH was found to be more stable than

10.1021/jp802242t CCC: $40.75  2008 American Chemical Society Published on Web 06/04/2008

Oxygenates on Co Surfaces in FT Synthesis

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9465

Figure 1. Top view and side view (insets) of oxygenate intermediates adsorbed on flat Co(0001): (a) CHO; (b) CH2O; (c) CH3O; (d) CH3OH; (e) COH; (f) CHOH; (g) CH2OH. The big blue balls are Co atoms, the gray ones are C atoms, the red ones are O atoms, and the white ones are H atoms. This notation is used throughout this paper.

CHO on Ni(111), the authors concluded that the reaction pathway via CHO is still slightly favored. The reverse reaction, methanol decomposition, is the principle process occurring on the anodes of direct methanol fuel cells (DMFCs),44–46 and has attracted great interest in recent years. Greeley and Mavrikakis studied two competitive paths for methanol decomposition on Pt(111):47,48 O-H scission to CH3O, followed by sequential dehydrogenation to CH2O, CHO, and CO, and C-H scission to CH2OH, then to CHOH, COH, and CO. Their microkinetic analysis showed that C-H scission in methanol is the initial decomposition step with the highest net rate under realistic reaction conditions. This result indicates that the reaction pathway of methanol synthesis via the intermediate COH may be faster than that involving the intermediate CHO on Pt(111), which is opposite to the previous findings42,43 on Pd(111) clusters and Ni(111). Another first-principles study of methanol decomposition on Pd(111) by Zhang and Hu investigated an additional possibility through initial C-O cleavage.49 Therefore, at least three pathways for methanol synthesis can be found in the literature. Which one is responsible for the synthesis of methanol on the Co surface in FT synthesis? This key question will be addressed in the present work. In addition, experimental work showed that methanol synthesis is structure-sensitive on different Cu surfaces.50,51 Also, some other studies on methanol decomposition concluded that defects can significantly modify methanol chemistry on Pt(111) surfaces.52–57 To the best of our knowledge, however, the structure sensitivity of methanol chemistry has not been tackled using DFT. Therefore, we have investigated the reaction pathways on the flat Co(0001) and monatomic steps in order to determine the active sites for oxygenate formation. To address the above issues, we have performed extensive DFT calculations on the thermochemistry and activation energies for elementary steps in oxygenate synthesis on the flat and stepped Co(0001) surfaces. The paper is arranged as follows. In the next section, some calculation details will be given. Then, we will present our calculation results of oxygenate formation. In the discussion section, we will study the mechanism of the formation of aldehyde and alcohol and determine the active site. Furthermore, the hydoxycarbene mechanism and the COinsertion mechanism will be discussed in contrast to the carbene mechanism. Finally, some conclusions will be drawn. 2. Method Inthiswork,theSIESTAcode58 wasusedwithTroullier-Martins norm-conserving scalar relativistic pseudopotentials.59 A dou-

ble-ζ plus polarization (DZP) basis set was utilized. The localization radii of the basis functions were determined from an energy shift of 0.01 eV. A standard DFT supercell approach with the Perdew-Burke-Ernzerhof form of the generalized gradient approximation (GGA) functional60 was implemented, and the Kohn-Sham orbitals were expanded in a localized (double-ζ) basis set with a mesh cutoff of 200 Ry. Spin polarization was included in the calculations. The accuracy of the setting was tested in our previous work.35 In studying the reactions on terraces, the p(2 × 2) unit cell was utilized, and surface Monkhorst Pack meshes of 5 × 5 × 1 k-point sampling in the surface Brillouin zone was utilized. In studying the reactions at steps, a p(4 × 2) unit cell was utilized, and the stepped Co(0001) was modeled by removing two neighboring rows of cobalt atoms on the top layer. The surface Monkhorst Pack meshes of 3 × 5 × 1 k-point sampling in the surface Brillouin zone was used on the stepped surface. In the calculations, the surface was modeled by four layers of metals, in which the bottom two layers of metal atoms were fixed and the top two layers and the adsorbates were relaxed. The transition states (TSs) were searched using a constrained optimization scheme.61–63 The distance between the reactants is constrained at an estimated value, and the total energy of the system is minimized with respect to all the other degrees of freedom. The TSs can be located via changing the fixed distance, and must be confirmed by the following two rules: (i) all forces on atoms vanish; (ii) the total energy is a maximum along the reaction coordinate but a minimum with respect to the rest of the degrees of freedom. 3. Results We first calculated the chemisorption of all the relevant oxygenate intermediates on both the flat and stepped Co(0001) surfaces reported in section 3.1. Considering these structures as initial states (ISs) or final states (FSs), we further located the TSs of the relevant elementary steps at both sites and determined the barriers (section 3.2). In section 3.3, the coupling of CH3 + CO is reported, and the sequential hydrogenation reactions to ethanol are investigated. 3.1. Intermediates on Flat and Stepped Co(0001). The most stable configurations of the intermediates on flat Co(0001) obtained from our DFT calculations are shown in Figure 1. All the intermediates prefer the 3-fold hexagonal close-packed (hcp) hollow site except methanol, which is on the top site. For adsorbed CHO and CH2O, the C atom binds with two Co atoms, and the O atom binds with the other Co atom. CH3O adsorbs

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TABLE 1: Selected Geometric and Energetic Parameters of Oxygenate Intermediates on the Flat Co(0001) species

BEa (eV)

configuration

dC-Ob (Å)

CHO COH CH2O CHOH CH3O CH2OH CH3OH

-2.20 -4.38 -0.86 -3.82 -2.63 -1.72 -0.29

hcp: bound through C and O hcp: bound through C hcp: bound through C and O hcp: bound through C hcp: bound through O hcp: bound through C top: bound through O

1.308 1.366 1.350 1.394 1.433 1.463 1.434

dO-Coc (Å)

dC-Cod (Å)

2.012

2.006, 1.887, 2.165, 1.972,

1.951

2.008 1.924, 1.927 2.188 1.972, 2.229

2.054,2.057, 2.058 2.123, 2.125, 2.664 2.232

a BE is the binding energy with respect to the gaseous radical or molecule. b dC-O is the distance between C and O. c dO-Co is the distance between O and the nearest Co atoms. d dC-Co is the distance between C and the nearest Co atoms.

Figure 2. Top view and side view (insets) of oxygenate intermediates adsorbed on stepped Co(0001): (a) CHO; (b) CH2O; (c) CH3O; (d) CH3OH; (e) COH; (f) CHOH; (g) CH2OH.

TABLE 2: Selected Geometric and Energetic Parameters of Oxygenate Intermediates on the Stepped Co(0001)a species

BE (eV)

configuration

dC-O (Å)

dO-Co (Å)

dC-Co (Å)

CHO COH CH2O CHOH CH3O CH2OH CH3OH

-2.82 -4.32 -1.23 -4.20 -3.03 -2.22 -0.44

corner: bound through C and O corner: bound through C edge bridge: bound through C and O edge bridge: bound through C edge bridge: bound through O edge bridge: bound through C and O edge top: bound through O

1.387 1.396 1.365 1.383 1.425 1.470 1.440

2.031, 2.034

1.983, 1.986, 2.149 2.014, 2.019, 2.030, 2.030 1.990 1.959, 1.962

a

1.881 1.956, 1.967 2.041 2.151

1.977

See the caption of Table 1 for parameter definitions.

on the hcp site via the O atom binds to the surface with the saturated CH3 group away from the surface. Regarding the adsorbed COH, CHOH, and CH2OH, they adsorb on the surface via the unsaturated C atom with the saturated OH group away from the surface. Since there is no unsaturated atom in CH3OH, it binds weakly on the surface through the O atom. Some geometric and energetic information is given in Table 1, and the binding energies (BEs) are referenced to the corresponding free radical or molecule in the gas phase. In comparison with other DFT work, our calculated results on Co(0001) are generally consistent with those on Pd, Ni, and Pt surfaces. For example, CH3OH was found to adsorb on the top site on Pd(111),49 Ni(111),43 and Pt(111)47,48 with a very weak binding energy (BE) of around -0.3 eV. It was reported that CH3O adsorbs on the hollow site on Pd19 clusters with a BE of -195 kJ/mol (-2.02 eV),42 which is in agreement with our work. However, there are some differences. For example, CH3O prefers the top site on Ni(111)43 and Pt(111)47,48 with BEs of -1.86 eV and -1.54 eV, respectively. For the adsorption of CH2O, many authors found that on Pd(111),42 Ni(111),43 and Pt(111)47,48 the most favorable mode was the di-σ structure, in which CH2O adsorbs on the 2-fold bridge site with the C and O atom binding to one metal atom each. However, this is not the case on Co(0001). From our calculations, the di-σ structure

is about 0.24 eV less stable than the structure shown in Figure 1(b). On Pt(111),47,48 CHO was reported to prefer the top site, whereas it is favored on the hollow site on Pd(111),42 Ni(111),43 which is the same as our result on Co(0001). For COH, CHOH, and CH2OH, the geometric and energetic results from our calculations also agree well with those on Pt(111),47,48 except for the adsorption site of CH2OH: it is on the top site via the C atom on Pt(111), while it prefers the hollow site on Co(0001). In order to investigate the influence of surface defects on oxygenate formation, we have also studied the chemisorption of the oxygenate intermediates on stepped Co(0001). The most favored structures have been identified, and are shown in Figure 2. It can be seen from Figure 2 that CHO and its isomer COH sit on the corner site, while the other intermediates adsorb on the step edge (the definitions of sites at surface steps have been given in our previous work35 in detail). Generally, the geometries on the stepped surface are in line with those on the flat surface (see Figure 1). Selected geometric parameters and binding energies are listed in Table 2. As we can see, the BEs of the intermediates on the stepped surface are greater than those on the flat surface except COH, which has a similar BE at both sites. 3.2. Reactions on Flat and Stepped Co(0001). After obtaining the adsorbed structures of the oxygenate intermediates, we



Figure 3. Top view and side view (insets) of the TSs of oxygenate formation on flat Co(0001): (a) CO + H f CHO; (b) CHO + H f CH2O; (c) CH2O + H f CH3O; (d) CH3O + H f CH3OH; (e) CO + H f COH; (f) COH + H f CHOH; (g) CHOH + H f CH2OH; (h) CH2OH + H f CH3OH; (i) CHO + H f CHOH; (j) CH2O + H f CH2OH; (k) CH2 + O f CH2O; (l) CH3 + OH f CH3OH.

have further calculated the elementary steps related to the formation of formaldehyde and methanol. These processes include two CO hydrogenation pathways to methanol, (i) CO + 4H f CHO + 3H f CH2O + 2H f CH3O + H f CH3OH and (ii) CO + 4H f COH + 3H f CHOH + 2H f CH2OH + H f CH3OH; two reactions intercrossing these two pathways, CHO + H f CHOH and CH2O + H f CH2OH; and two other reactions to produce formaldehyde and methanol, CH2 + O f CH2O and CH3 + OH f CH3OH (also see section 4.1). Figure 3a-d and e-h shows the TSs of the sequential hydrogenation steps for the two pathways of CO hydrogenation to methanol on the flat Co(0001), respectively. It is interesting to note that, along the reaction pathway from CO to CH3OH, the oxygenate intermediates move from the hollow site (Figure 3a,b,e,f), to the bridge site (Figure 3c,g) and to the top site (Figure 3d,h), which is consistent with the work of Michaelides and Hu.64 At the TSs of CHO + H f CHOH and CH2O + H f CH2OH, both intermediates sit on the hollow site, as show in Figure 3i,j. In all these hydrogenation reactions, the H atoms are always in close proximity to the reacting atoms at the TSs. The TSs of the coupling reactions of CH2 + O and CH3 + OH are shown in Figure 3k,l. At the TS of CH2 + O f CH2O, the O atom adsorbs on the hollow site, and CH2 sits on the bridge site nearby. For CH3 + OH f CH3OH, both OH and CH3 sit on the top site of two neighboring Co atoms at the TS. The corresponding TS structures on stepped Co(0001) are illustrated in Figure 4a-l. Also, the TSs of the two pathways to produce methanol are shown in Figure 4a-d and e-h, respectively. Similar to those on flat Co(0001), the oxygenate intermediates move from the corner site (Figure 4a,e,f), to the edge bridge site (Figure 4b,c,g) and to the edge top site (Figure 4d,h) along the reaction pathway at the TSs on stepped Co(0001). At the TSs of CHO + H f CHOH (Figure 4i) and CH2O + H f CH2OH (Figure 4j), CHO sits on the corner site, while CH2O is on the step edge. Regarding the two coupling reactions, at the TS of CH2 + O f CH2O, the O atom sits on the hollow site on the lower terrace with CH2 on the edge bridge site; for CH3 + OH f CH3OH, both OH and CH3 are on the top site of two neighboring edge Co atoms at the TS.

The distances (d) between the reacting atoms at the TSs and rev the activation energies (Efor a and Ea for the forward and reverse reactions, respectively) of these reactions on the flat and stepped Co(0001) surfaces are given in Table 3. It can be found that, for each reaction, the distance on the flat surface is quite similar to that on the stepped surface except for CO + H f CHO. Comparing the barriers on both sites, it is more complicated: some barriers on the flat surface are greater than those on the stepped surface (e.g., Efor a for CO + H f CHO), some are smaller (e.g., Eafor for CHO + H f CHOH), and others are similar (e.g., Efor a for COH + OH f CHOH). The general geometries of the TSs located on the flat Co(0001) in the present work (Figure 3) are consistent with those on Pt(111)47,48 and Ni(111).43 Some of the barriers are similar, while others are quite different. For example, the barrier of CO + H f CHO is 1.49 eV on Ni(111) and 1.20 eV on Pt(111), and the barrier of the reverse reaction is 0.30 eV on Ni(111) and 0.23 eV on Pt(111). These barriers are close to our results (1.31 and 0.11 eV on Co(0001) in Table 3). However, for CH3O + H f CH3OH, the TS of which has a very similar geometry on these three surfaces with CH3O on the off-top site, its barrier varies dramatically: 0.19 eV on Pt(111), 0.61 on Ni(111), and 1.45 eV on Co(0001). The reason could be that the potential energy surfaces of CH3O on these surfaces are very different. 3.3. Reactions for C2 Species. In order to study the feasibility of the CO-insertion mechanism, we have computed CH3 + CO coupling on both flat and stepped Co(0001) surfaces. The located TSs and FSs are shown in Figure 5, and the distances between the reacting atoms at the TSs and the barriers are given in Table 4. As we can see in Figure 5a,c, at the TS on the flat surface, CO sits on the hollow site with CH3 on the off-top site nearby, while both CO and CH3 adsorb on the step edge at the TS on the stepped surface. The coupling product, CH3CO, is on the hollow site on the flat surface as shown in Figure 5b, and the geometry is very similar to its C1 counterpart (CHO in Figure 1a). Interestingly, the most stable site on the stepped surface is the step edge (Figure 5d), quite different from the corner site for CHO (Figure 2a). This may be due to the


Cheng et al.

Figure 4. Top view and side view (insets) of the TSs of oxygenate formation on stepped Co(0001): (a) CO + H f CHO; (b) CHO + H f CH2O; (c) CH2O + H f CH3O; (d) CH3O + H f CH3OH; (e) CO + H f COH; (f) COH + H f CHOH; (g) CHOH + H f CH2OH; (h) CH2OH + H f CH3OH; (i) CHO + H f CHOH; (j) CH2O + H f CH2OH; (k) CH2 + O f CH2O; (l) CH3 + OH f CH3OH.

TABLE 3: Selected Geometric Parameters at the TSs and Activation Energies of Elementary Reactions for Oxygenate Formation on Flat and Stepped Co(0001) flat Co(0001)

stepped Co(0001)

reactions

da(Å)

Eaforb(eV)

Earevb (eV)

da(Å)

Eaforb (eV)

Earevb (eV)

CO + H f CHO CHO + H f CH2O CH2O + H f CH3O CH3O + H f CH3OH CO + H f COH COH + H f CHOH CHOH + H f CH2OH CH2OH + H f CH3OH CHO + H f CHOH CH2O + H f CH2OH CH2 + O f CH2O CH3 + OH f CH3OH

1.180 1.655 1.690 1.500 1.383 1.550 1.650 1.390 1.415 1.395 1.935 1.974

1.31 0.55 0.86 1.45 1.80 0.85 0.82 0.98 1.23 1.27 1.38 2.20

0.11 0.37 1.02 0.80 0.95 0.10 0.40 1.12 0.84 0.64 0.95 1.47

1.550 1.590 1.600 1.515 1.230 1.320 1.662 1.340 1.305 1.332 2.042 1.940

0.77 0.71 0.45 1.24 1.46 0.77 0.43 0.82 1.59 1.34 1.28 2.45

0.12 0.34 0.69 0.42 0.51 0.51 0.19 0.67 1.02 0.90 0.85 1.07

d is the distance between the two reacting atoms at the TS. b Eafor and Earev are the activation energies of the forward and reverse reactions, respectively. a

repulsion between CH3 in CH3CO and the lower terrace if it sits on the corner site. Some structural parameters and binding energies of C2 oxygenate intermediates are listed in Table 5. As we can see, all the structural properties of CH3CO adsorbed on the flat surface are almost the same as those of CHO (Table 1), while those on the stepped surface are different (Table 2). Especially, the BE of CH3CO on the stepped surface is obviously smaller than that of CHO. It is also interesting to find from Table 4 that the barriers of the forward and reverse reactions are almost identical on stepped and flat surfaces. Given the very different structures of the TSs and FSs on the flat and stepped surfaces, this finding is quite surprising. We have also calculated the sequential hydrogenation of CH3CO to CH3CH2OH on stepped Co(0001). Figure 6a-c shows the most stable structures of CH3CHO, CH3CH2O, and CH3CH2OH, respectively, and the TS structures are illustrated in Figure 6d-f. Generally, all the geometries of the TSs and FSs are very similar to those of their C1 counterparts (see Figure 2b-d and Figure 4b-d). This can be further seen from Table 5: The structural parameters and binding energies of CH3CHO, CH3CH2O, and CH3CH2OH are very close to those of CH2O,

Figure 5. Top view and side view (insets) of the TSs and FSs of CH3 + CO f CH3CO on flat and stepped Co(0001) surfaces: (a,b) the TS and FS on the flat surface, respectively; (c,d) the TS and FS on the stepped surface, respectively.

CH3O, and CH3OH in Table 2. The distances between the reacting atoms at the TSs and the barriers are listed in Table 6.



TABLE 4: Selected Geometric Parameters at the TSs and Activation Energies of CH3 + CO f CH3CO on the Flat and Stepped Co(0001) Surfacesa CH3 + CO f CH3CO

d (Å)

Eafor (eV)

Earev (eV)

flat Co(0001) stepped Co(0001)

2.000 1.790

1.49 1.46

0.90 0.93

a

See caption of Table 3 for the definitions of d, Eafor, and Earev.

Considering the structural similarity between C1 and C2 species, it is not surprising that the distances are quite close (see Tables rev 3 and 6). For the same reason, the barriers (both Efor a and Ea ) of CH3CHO + H f CH3CH2O and CH3CH2O + H f CH3CH2OH are also nearly identical to those of CH2O + H f CH3O and CH3O + H f CH3OH. However, the barrier of CH3CO + H f CH3CHO is different from that of CHO + H f CH2O. This can be explained by the different adsorption geometries between CHO and CH3CO, leading to the different total energies of the ISs. 4. Discussions 4.1. Pathways of the Formation of Formaldehyde and Methanol. Figure 7 shows the two pathways to produce methanol via CHO (in red) and via COH (in blue), as well as the two intercrossing reactions (in pink). Calculated energy profiles of these processes on flat and stepped Co(0001) are shown in Figures 8 and 9, respectively. The same set of colors used to distinguish the reactions in Figure 7 is applied to Figures 8 and 9 for clarity. First of all, one can clearly see the following from Figure 8 on the flat surface: (i) The energy profile of the pathway via CHO (in red) is generally lower than that via COH (in blue). For the adsorbed intermediates, CH2O is about 0.2 eV more stable than CHOH, and CH3O is about 0.8 eV more stable than CH2OH. Only CHO is approximately 0.3 eV less stable than COH. Regarding the TSs, only the TS of CHO + H f CH2O is slightly (∼0.06 eV) less stable than that of COH + H f CHOH, and the others are more stable for the pathway via CHO than via COH. Thus, our results indicate that the pathway of methanol formation via CHO is more favorable on flat Co(0001), both thermodynamically and kinetically.35 (ii) The last hydrogenation steps, CH2OH + H f CH3OH and CH3O + H f CH3OH, have the highest TS energies in the two pathways. Both of them are about 0.5 eV higher than the second highest energies of the TSs (the penultimate steps). It should be emphasized that, if the influence of reactant coverages is neglected, this large energy difference in the barriers means that the last steps are around 5 orders of magnitude slower than the penultimate steps under the typical FT reaction temperature (500 K).35 Hence, this strongly suggests that the last hydrogenation steps are the rate-determining steps on the flat Co(0001) in both pathways. (iii) The two reactions intercrossing the two pathways (in purple), CHO + H f CHOH and CH2O + H f CH2OH, are unlikely to contribute to the oxygenate formation on the flat Co(0001). Both the reactions possess very high barriers, 1.23 and 1.27 eV, respectively (see Table 3). (iv) The coupling reactions of CH2 + O f CH2O and CH3 + OH f CH3OH (in green) are less important as the alternative mechanisms to produce formaldehyde and methanol: their barriers are 1.27 and 2.20 eV, respectively, and their TS energies are also very high. Especially for CH3 + OH f CH3OH, their contribution can be largely neglected. The same pattern is seen on the stepped Co(0001) from Figure 9. In this case, all the intermediates and the TSs in the pathway

via CHO are more stable than those in the pathway via COH. Again, the two intercrossing reactions and the two coupling reactions are not important for the formation of formaldehyde and methanol. Hence, the pathway CO + 4H f CHO + 3H f CH2O + 2H f CH3O + H f CH3OH is also the mechanism of CO hydrogenation to methanol on the stepped Co(0001), and the last step is rate-determining. From Figures 8 and 9, one can see that the feasible mechanism for the formation of formaldehyde and methanol is CO + 4H f CHO + 3H f CH2O + 2H f CH3O + H f CH3OH on both the flat and stepped Co(0001) surfaces. To determine the reactive site, we contrast the preferable mechanism on both the flat surface and step sites, and the energy profiles are arranged in Figure 10. It is not surprising from Figure 10 that the total energies of the intermediates and the TSs on the stepped surface are lower than those on the flat surface because of the advantageous electronic and geometric effects at the surface steps.65,66 In our previous work,35 a general equation was derived to express the relationship between the reaction rates on the flat (rf) and stepped (rs) surfaces:

( )

rs θ*s 2 ∆ETS ) e RT rf θ*f

(1)

where θ*f (θ*s) is the free site coverage on the flat (stepped) surface, and ∆ETS is the TS energy difference between the flat and stepped surfaces. Equation 1 indicates that, for an elementary surface reaction, the contribution of the site to the overall reaction is determined by the TS energy at each site and the number of the free sites. The lower TS energies result in the step sites being more preferable than the flat surface, whereas the blockage of the active step sites due to the stronger bonding of intermediates compromises to some extent the preference. In FT synthesis, adsorbed CO is the most abundant surface intermediate under typical reaction conditions.67,68 Thus, the amount of free surface sites on different surfaces is strongly affected by the binding energies of CO on these surfaces. Because the BE of CO at the step sites is only about 0.1 eV larger than that on the flat surface (-2.01 eV at steps and -1.90 eV on the terrace), the blockage effect may be trivial. On the other hand, for the last hydrogenation reaction, CH3O + H f CH3OH, which is the rate-determining step for methanol formation on both surfaces, the TS on the stepped surface is 0.53 eV more stable than that on the flat surface (see Figure 10). Therefore, our results suggest that the step sites are more favored for methanol formation. Regarding the formaldehyde formation, the TS of the slowest step, CHO + H f CH2O, is about 0.3 eV more stable at the step sites. Hence, formaldehyde may also mainly be produced at the step sites. In fact, the other pathway via COH and the coupling reactions of CH2 + O and CH3 + OH are also energetically favored at the step sites (see Figures 8 and 9). It is worth comparing methanol formation and direct CO dissociation. Since CH3 + OH f CH3OH is the rate-determining step, the effective barrier of methanol formation should be about 2.02 eV, which is the energy difference between the TS of CH3 + OH f CH3OH and CO + 4H on the surface at step sites (see Figure 9). For CO dissociation, an early work in our group showed that it is about 1.61 eV at Co step sites.32 Thus, it is clear that CO dissociation is much preferred on the stepped Co surface. This may explain why the dominant products of CO hydrogenation on Co catalysts are hydrocarbons rather than alcohols.


Cheng et al.

TABLE 5: Selected Geometric and Energetic Parameters of C2 Oxygenate Intermediates on the Co(0001) Surfacesa speciesb

BE (eV)

configuration

dC-O (Å)

dO-Co (Å)

dC-Co (Å)

CH3CO-flat CH3CO CH3CHO CH3CH2O CH3CH2OH

-2.11 -2.51 -1.21 -3.02 -0.49

hcp: bound through C and O edge bridge: bound through C and O edge bridge: bound through C and O edge bridge: bound through O edge bridge: bound through O

1.313 1.288 1.365 1.467 1.448

1.990 1.982 1.888 1.958, 1.967 2.160

2.030, 2.039 1.904 2.026

a See the caption of Table 1 for parameter definitions. b CH3CO-flat stands for CH3CO adsorbed on flat Co(0001), and all the others are on stepped Co(0001).

Figure 6. Top view and side view (insets) of the TSs and FSs of oxygenate formation on stepped Co(0001): (a-c) the adsorption of CH3CHO, CH3CH2O, and CH3CH2OH, respectively; (d-f) the TSs of CH3CO + H f CH3CHO, CH3CHO + H f CH3CH2O, and CH3CH2O + H f CH3CH2OH, respectively.

TABLE 6: Selected Geometric Parameters at the TSs and Activation Energies of Elementary Reactions of Oxygenate Formation (C2) on Stepped Co(0001)a reactions

d (Å)

Eafor (eV)

Earev (eV)

CH3CO + H f CH3CHO CH3CHO + H f CH3CH2O CH3CH2O + H f CH3CH2OH

1.590 1.580 1.500

0.35 0.47 1.26

0.19 0.64 0.43

a

Figure 7. Reaction scheme of CO hydrogenation to methanol.

See caption of Table 3 for the definitions of d, Eafor, and Earev.

4.2. Comparison between Chain Growth Mechanisms. The carbene mechanism was extensively studied in our previous work.35,36 The reaction rate of each C1 + C1 coupling pathway was estimated by considering both the reactant coverages (CHx, x ) 0-3) and the coupling barrier. It was found that the coverages of CHx species can be evaluated by using their thermodynamic stabilities because CH3 hydrogenation is the slowest step in the sequence of hydrogenation reactions (C + 4H f CH4), and the preceding steps can be considered in quasiequilibrium:

( )

θCHi ) exp -

( )

Ei Ei θHi θC i ) exp θCti RT RT θ*

i ) 1-3

(2)

where θCHi, θH, and θ* are the coverage of CHi, H, and free surface sites, respectively, t is equal to θH/θ*, and Ei is the relative stability of CHi with reference to C on surfaces. Under typical FT reaction conditions, t is usually around 1 on Co surfaces.35 According to transition state theory (TST), the C1 + C1 coupling reaction rate can be written as

( )

Ei,j θ θ ) RT CHi CHj Ei,j + Ei + Ej i+j 2 A exp t θC RT

rCHi+CHj ) A exp -

(

)

i,j ) 0-3

(3)

where Ei,j is the barrier of the CHi + CHj coupling reaction, and Ais the pre-exponential factor. For surface reactions,69 the

Figure 8. Energy profile along the reaction coordinate of CO hydrogenation to methanol on flat Co(0001). All energies are referenced to CO + 2H2 in the gas phase. The reaction coordinate is set according to the H number in the surface intermediate, CHxO + (4 - x)H (x ) 0-4). In the figure, the adsorbed H atoms are omitted in the notation. The paths in red and blue are CO + 4H f CHO + 3H f CH2O + 2H f CH3O + H f CH3OH and CO + 4H f COH + 3H f CHOH + 2H f CH2OH + H f CH3OH, respectively. The paths in purple are CHO + H f CHOH and CH2O + H f CH2OH. The paths in green are CH2 + O f CH2O and CH3 + OH f CH3OH.

pre-exponential factor, A, is usually about 10.13 Ei and Ei,j were calculated and can be found in our previous work.35,36 The fast C1 + C1 coupling reactions were found to be CH3 + C and CH2 + CH2 on stepped Co(0001), both of which possess very small values of Ei,j + Ei + Ej, at 1.55 and 1.59 eV, respectively. In principle, the total C1 + C1 coupling reaction rate (rC-C) should be equal to the sum of all these coupling channels. Since


Figure 9. Energy profile along the reaction coordinate of CO hydrogenation to methanol on stepped Co(0001). See the caption of Figure 8 for details.


Figure 11. Energy profile of the CH3 + CO coupling on flat (dashed line) and stepped (solid line) Co(0001) and the sequential hydrogenation to ethanol on stepped Co(0001). The total energy of CH3 + CO adsorbed at step sites is chosen as a reference.

have very similar barriers, and both the energy diagrams are given in Figure 11. We can see that the TS energy on the stepped Co(0001) is about 0.4 eV lower than that on the flat Co(0001), suggesting that this reaction may also prefer the step sites. According to TST and eq 2, we can write the CH3 + CO reaction rate as follows:

( )

rCH3+CO ) A exp -

Figure 10. Energy profile of CO hydrogenation via CHO to methanol on flat (dashed line) and stepped (solid line) Co(0001) surfaces. See the caption of Figure 8 for details.

the reaction rates of other coupling channels are several orders of magnitude smaller than the major one,35,36 we only consider the fastest channel to estimate the total C-C coupling reaction rate. Thus, rC-C )

∑r i,j

)

CHi+CHj

∑ A exp(i,j

)

Ei,j + Ei + Ej i+j 2 t θC RT

≈ max

( ( ( (

) ) )

Ei,j + Ei + Ej i+j 2 t θC RT min(Ei,j + Ei + Ej) i+j 2 ) A exp t θC RT Eeff,C-C i+j 2 ) A exp t θC (4) RT Eeff,C-C stands for the effective barrier of the C1 + C1 coupling reaction, which is identical to the minimum of Ei,j + Ei + Ej. Therefore, Eeff,C-C is 1.55 eV, corresponding to the CH3 + C coupling on stepped Co(0001). It should be mentioned that Ei,j + Ei + Ej of the CH2 + CH2 coupling is only slightly higher than that of the CH3 + C coupling. But we believe that choosing Ei,j + Ei + Ej of the CH3 + C coupling (1.55 eV) as the effective barrier is accurate enough to semiquantitatively measure the overall C1 + C1 coupling reactions. Regarding the CO-insertion mechanism, our calculations show that the CH3 + CO reactions on the flat and stepped Co(0001) A exp -

)

(

)

Ea Ea + E3 2 θ θ ) A exp t θCθCO RT CH3 CO RT (5)

where Ea is the barrier of the CH3 + CO reaction, and θCO is the CO coverage. Ea + E3 can be treated as the effective barrier of the CH3 + CO reaction (Eeff,CH3+CO), which is computed to be about 1.89 eV from our results. Comparing the rate of the C1 + C1 coupling and the CH3 + CO reaction (eqs 4 and 5), we can obtain

rC-C rCH3+CO

(

) exp

Eeff,CH3+CO - Eeff,C-C RT

)

θC/θCO

(6)

where t is omitted since it is close to 1. From our DFT calculations, the effective barrier of the C1 + C1 coupling is about 0.34 eV smaller than that of the CH3 + CO reaction, which indicates that the C1 + C1 coupling is about 3 orders of magnitude faster than the CH3 + CO reaction at 500 K, provided the C coverage is comparable to the CO coverage. Although CO is the most abundant surface intermediate on the Co surface under FT reaction conditions, experimental work67,68,70 strongly suggests that adsorbed carbonaceous species crowd at step sites under typical FT reaction conditions. By using in situ polarization modulation reflection absorption infrared spectroscopy (PMRAIRS) Beitel et al.67,68 clearly showed that the peak intensity of the adsorption of CO at step sites is suppressed by increasing H2 partial pressure at 493 K. The authors concluded that carbonaceous species produced from CO hydrogenation greatly block the adsorption of CO at step sites. They also estimated the surface coverage of surface intermediates by ex-situ X-ray photoelectron spectroscopy (XPS) measurement. Their results indicated that the coverage of carbonaceous species has the same order of magnitude as the adsorbed CO, and the carbonaceous species coverage is higher on sputtered Co(0001) (which contains more defects) than that on annealed surface. In addition, Wilson and Groot found by scanning tunnelling microscopy (STM) technique that there are many monatomic steps on the Co surface due to CO-induced surface restructuring.70 Thus, it is expected that the CO coverage is unlikely to be 3 orders of

9472 J. Phys. Chem. C, Vol. 112, No. 25, 2008 magnitude greater than the C atom. Therefore, the contribution of the CO-insertion mechanism to chain growth is smaller than that of the carbene mechanism. In the hydroxycarbene mechanism, chain growth proceeds via a condensation reaction of two hydroxycarbene species (CHOH) with the elimination of a water molecule. This condensation reaction is expected not to be favored both energetically and kinetically on the Co surface. First, the hydroxycarbene species is very unstable on both flat and stepped Co(0001). Figures 8 and 9 show that CHOH is almost the most unstable oxygenate intermediate, being only slightly more stable than CH2OH. Second, this reaction involves a C-O cleavage in one CHOH and an O-H scission in the other CHOH simultaneously. Furthermore, both cleavages occur in the gas phase without the assistance of the metal surface. Thus, the barrier is expected to be high. Therefore, the hydroxycarbene mechanism is less important for chain growth in FT synthesis. 4.3. Mechanism of the Formation of Aldehyde and Alcohol. Although the CO-insertion mechanism is not important to the chain growth to produce long-chain hydrocarbons in FT synthesis, it may be responsible for the formation of long-chain oxygenates. In section 4.1, the pathways of the formation of formaldehyde and methanol were reported, and it was found that the most likely pathway is CO hydrogenation via CHO at step sites. Thus, we propose the mechanism of the formation of aldehyde and alcohol as follows: the alkyl group (or H atom) couples with CO at first to give RCO (R ) alkyl group or H atom); then, it is hydrogenated to form RCHO, RCH2O, and RCH2OH; finally, the adsorbed RCHO (aldehyde) or RCH2OH (alcohol) desorbs from the surface. The energy profile of acetaldehyde and ethanol formation on the stepped Co(0001) is illustrated in Figure 11. Because of the geometric and energetic similarity (see section 3.3), the energy profile of the hydrogenation reactions from CH3CO to CH3CH2OH is very similar to that from CHO to CH3OH (see Figure 10). Another possible mechanism is the coupling of RCH + O or RCH2 + OH to give RCHO or RCH2OH. From our DFT calculations, both the coupling reactions of CH2 + O and CH3 + OH possess quite high barriers on both flat and stepped Co(0001) (in Table 3). Such high barriers eliminate the possibility of the formation of formaldehyde and methanol via these pathways. Similarly, it is expected that the coupling of RCH + O and RCH2 + OH will have very high barriers and not be responsible for the formation of aldehyde and alcohol. 5. Conclusions DFT calculations have been used to investigate the mechanism of the formation of aldehdyes and alcohols on Co surfaces in FT synthesis. Three possible pathways on both flat and stepped Co(0001) have been extensively calculated and compared. The following conclusions are reached: (i) The pathway for the formation of formaldehyde and methanol via CHO is thermodynamically and kinetically more favored than the pathway via COH on both the flat and stepped Co(0001). The coupling reactions of CH2 + O and CH3 + OH are also less important for the production of formaldehyde and methanol due to their high barriers. (ii) The energy profiles of these pathways on the stepped Co(0001) are generally lower than those on the flat surface, suggesting that these reactions prefer to occur on the step sites. In particular, for the rate-determining step (CH3O + H f CH3OH) of the most likely pathway via CHO, its TS energy on the stepped surface is 0.53 eV lower than that on the flat surface. Even taking into account the blocking effect at the step

Cheng et al. sites, this pathway is still favored at these sites because of the very low TS energy. (iii) The chain growth mechanism involving oxygenate intermediates, the CO-insertion mechanism, and the hydroxycarbene mechanism are less important than the carbene mechanism in terms of the reaction rate. (iv) The CO-insertion mechanism, however, may be responsible for the production of long chain oxygenates. The mechanism for the formation of aldehyde and alcohol is suggested to be that the alkyl group (or H atom) couples with CO to give RCO (R ) alkyl group or H atom), followed by the sequential hydrogenation reactions to produce RCHO and RCH2OH. Acknowledgment. We gratefully thank The Queen’s University of Belfast for computing time. J.C. acknowledges Johnson Matthey for financial support. References and Notes (1) Dry, M. E. Appl. Catal., A 1996, 138, 319. (2) Dry, M. E. Catal. Today 2002, 71, 227. (3) Geerlings, J. J. C.; Wilson, J. H.; Kramer, G. J.; Kuipers, H. P. C. E.; Hoek, A.; Huisman, H. M. Appl. Catal., A 1999, 186, 27. (4) Biloen, P.; Sachtler, W. M. H. AdV. Catal. 1981, 30, 165. (5) Rofer-Depoorter, C. K. Chem. ReV. 1981, 81, 447. (6) Iglesia, E. Appl. Catal., A 1997, 161, 59. (7) Jager, B.; Espinoza, R. Catal. Today 1995, 23, 17. (8) Adesina, A. A. Appl. Catal., A: Gen. 1996, 138, 345. (9) Schulz, H. Appl. Catal., A 1999, 186, 3. (10) Khodakov, A. Y.; Chu, W.; Fongarland, P. Chem. ReV. 2007, 107, 1692. (11) Yates, I. C.; Satterfiled, C. N. Energy Fuels 1992, 6, 308. (12) Buess, F.; Caers, R. F. I.; Frennet, A; Ghenne, E.; Hubert, C.; Kruse, N. U.S. Patent 6362239B1, 2002. (13) Van der Riet, M.; Copperthwaite, R. G.; Hutchings, G. J. J. Chem. Soc., Faraday Trans 1 1987, 83, 2963. (14) Morales, F.; de Groot, F. M. F.; Gijzeman, O. L. J.; Mens, A.; Stephan, O.; Weckhuysen, B. M. J. Catal. 2005, 230, 310. (15) Baker, J. E.; Burch, R.; Golunski, S. E. Appl. Catal., A 1989, 53, 279. (16) Sugier, A.; Freund, E. U.S. Patent 4122110, 1978.. (17) Yang, Y.; Xiang, H.-W.; Xu, Y.-Y.; Bai, L.; Li, Y.-W. Appl. Catal., A 2004, 244, 181. (18) Fischer, F.; Tropsch, H. Brennst. Chem 1923, 4, 276. Fischer, F.; Tropsch, H. Brennst. Chem. 1926, 7, 79. Fischer, F; Tropsch, H Chem. Ber. 1926, 59, 830. (19) Scorch, H. H.; Goulombic, N.; Anderson, R. B. In The FischerTropsch and Related Syntheses; Wiley: New York, 1951. Kummer, J. F.; Emmett, P. H. J. Am. Chem. Soc. 1953, 75, 5177. (20) Pichler, H.; Schulz, H. Chem. Ing. Tech. 1970, 12, 1160. (21) Kaminsky, M. P.; Winograd, N.; Geoffroy, G. L.; Vannice, M. A. J. Am. Chem. Soc. 1986, 108, 1315. (22) Wu, M. C.; Goodman, D. W. J. Am. Chem. Soc. 1994, 116, 1364. (23) Geerlings, J. J. C.; Zonnevylle, M. C.; de Groot, C. P. M. Surf. Sci. 1991, 241, 302. (24) Brady, R.; Pettit, R. J. Am. Chem. Soc. 1980, 102, 6181. (25) Brady, R.; Pettit, R. J. Am. Chem. Soc. 1981, 103, 1287. (26) Van Barneveld, W. A. A.; Ponec, V. J. Catal. 1984, 88, 382. (27) Ciobıˆcaˇ, I. M.; Frechard, F.; van Santen, R. A.; Kleyn, A. W.; Hafner, J. Chem. Phys. Lett. 1999, 311, 185. (28) Ciobıˆcaˇ, I. M.; Kramer, G. J.; Ge, Q.; Neurock, M.; van Santen, R. A. J. Catal. 2002, 212, 136. (29) Ciobıˆcaˇ, I. M. The molecular basis of the Fischer-Tropsch reaction. Ph.D. Thesis, Eindhoven University of Technology, Eindhoven, The Netherlands, 2002. (30) Liu, Z.-P.; Hu, P J. Am. Chem. Soc. 2002, 124, 11568. (31) Cheng, J.; Song, T.; Hu, P.; Lok, C. M.; Ellis, P.; French, S. J. Catal. 2008, 255, 20. (32) Gong, X.-Q.; Raval, R.; Hu, P Surf. Sci. 2004, 562, 247. (33) Gong, X.-Q.; Raval, R.; Hu, P. Mol. Phys. 2004, 102, 993. (34) Gong, X.-Q.; Raval, R.; Hu, P. J. Chem. Phys. 2005, 122, 024711. (35) Cheng, J.; Gong, X.-Q.; Hu, P.; Lok, C. M.; Ellis, P.; French, S. J. Catal. 2008, 254, 285. (36) Cheng, J.; Hu, P.; Ellis, P.; French, S.; Kelly, G.; Lok, C. M. J. Phys. Chem. C 2008, 112, 6082. (37) Liu, X.-M.; Lu, G. Q.; Yan, Z.-F.; Beltramini, J. Ind. Eng. Chem. Res. 2003, 42, 6518. (38) Borodko, Y.; Somorjar, G. A. Appl. Catal., A 1999, 186, 355.

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JP802242T

A First-Principles Study of Oxygenates on Co Surfaces in Fischer

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