Effect of Pre-covered Oxygen on the Dehydrogenation Reactions over

Dehydrogenation of five species including CH3OH, CH3O, H2COO, NH3, and H2O over clean and oxygen-modified copper surfaces has been investigated by ...
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J. Phys. Chem. B 2006, 110, 26045-26054

26045

Effect of Pre-covered Oxygen on the Dehydrogenation Reactions over Copper Surface: A Density Functional Theory Study Shu-Xia Tao, Gui-Chang Wang,* and Xian-He Bu Department of Chemistry and the Center of Theoretical Chemistry Study, Nankai UniVersity, Tianjin 300071, People’s Republic of China ReceiVed: August 2, 2006; In Final Form: October 2, 2006

Dehydrogenation of five species including CH3OH, CH3O, H2COO, NH3, and H2O over clean and oxygenmodified copper surfaces has been investigated by the first-principle density functional calculations within the generalized gradient approximation. The reaction enthalpies and the activation energies have been calculated for 10 elementary steps corresponding to the direct and oxygen-assisted cleavage of X-H bonds (X ) O, N, C). The DFT-GGA results showed that the pre-adsorbed oxygen always facilitates the dehydrogenation reaction by decreasing the reaction enthalpies and the activation energies. The obtained results are in general agreement with experimental observations.

1. Introduction The outcome of many catalytic processes can be altered dramatically by introduction of a small amount of either promoters that are capable of speeding up certain reaction steps or poisons that can slow particular reaction steps.1 Both atomic and molecular oxygen surface species have been speculated as governing chemical precursors for several important transition metal-catalyzed reactions. Transition metals are used extensively in heterogeneous catalysis to activate the X-H bond for dehydrogenation reactions, and it is well known that, in the periodic table, the chemical activity for dehydrogenation reactions increases from right to left along a series, and from bottom to top in a group. Accordingly, metals in group IB, as Cu, Ag, and Au, are not so effective in dissociating X-H bonds at room temperature or below.2-4 It is well known, however, that great interests have been devoted to the activity of the oxygenmodified group IB metals for dehydrogenation reactions, and some discrepancies and open questions remain. For example, oxygen has been shown to both strongly activate the oxygen-, carbon-, sulfur-, and nitrogen-hydrogen bonds on transitionmetal surfaces (at low coverage)5-8 and inhibit dissociation (at high coverage).9,10 While it is generally agreed that the formation of strong chemisorbed atomic oxygen poisons the active surface sites and is responsible for inhibition, the nature of the activation process is still unclear. CH3OH, CH3O, H2COO, NH3, and H2O are among the most extensively studied dehydrogenation species on transition-metal surfaces. There are a number of motivations coming up from the heterogeneous catalysis as an interest in the surface chemistry of these molecules: (i) they have been traditionally used as probe reagents in catalysis, due to their simpleness (especially H2O and NH3) and importance; (ii) selective oxidations of methanol and hydrocarbons lead to useful oxygenation chemicals; (iii) the decomposition pathway of methanol on copper, in particular the elementary step in this pathway that involves the dehydrogenation of methoxy to formaldehyde, plays an important role in the methanol steam re-forming process; and (iv) the synthesis of higher alcohols * Corresponding author. E-mail: [email protected].

and the oxygenations from CO and H for fuels continue to be of interest. In short, the dehydrogenation reactions at metal surfaces are relevant to a wide variety of catalytic processes, and a fundamental understanding of these reactions may therefore contribute to catalytic synthesis of a surprising array of products. As we can see from the analysis above, although great efforts were made on the dehydrogenation reactions over pre-adsorbed oxygen copper surfaces from the experimental studies, the fundamental theoretical research for the effect of the pre-covered oxygen is lacking at present. Therefore, there stands a requirement that these fundamental issues should be energetically described and predicted from the first-principle theoretical method. In the present study, we will study the following five dehydrogenation species involved in the former paragraph, that is, CH3OH, CH3O, H2COO, NH3, and H2O, on clean Cu(111) and oxygen pre-covered Cu(111) surfaces, by periodic DFT calculation within the generalized gradient approximation.11 The 10 reaction paths follow:

CH3OH(a) f CH3O(a) + H(a)

(1)

CH3OH(a) + O(a) f CH3O(a) + OH(a)

(2)

CH3O(a) f H2CO(a) + H(a)

(3)

CH3O(a) + O(a) f H2CO(a) + OH(a)

(4)

H2COO(a) f HCOO(a) + H(a)

(5)

H2COO(a) + O(a) f HCOO(a) + OH(a)

(6)

NH3(a) f NH2(A) + H(a)

(7)

NH3(a) + O(a) f NH2(a) + OH(a)

(8)

H2O(a) f OH(a) + H(a)

(9)

H2O(a) + O(a) f 2OH(a)

(10)

In this paper, details of the calculations are discussed in section 2. In section 3, these five groups of reactions are introduced, respectively, in sections 3.1-3.5, and in each section, a brief overview of the geometric structure of reactants,

10.1021/jp0649495 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/01/2006

26046 J. Phys. Chem. B, Vol. 110, No. 51, 2006 transition states, and products in our investigation is presented. Also the energetics of each elementary step in the group have been determined by the nudged elastic band (NEB) method,12-15 and the comparison of two reaction paths on clean and oxygen pre-covered Cu(111) surface is given. In section 3.6, the potential characteristic of the reaction energy of the 10 reaction paths is discussed. In section 4, we summarize the results and draw some conclusions. 2. Theoretical Method and Models To investigate the energy and structural details of water dissociation on clean and oxygen pre-adsorbed surfaces, we have performed periodic, self-consistent, density functional theory (DFT) calculations. The GGA with the Perdew, Burke, and Ernzerhof16 functional was used for the exchange and correlation energy calculation. All of the calculations were done in STATE (Simulation Tool for Atom Technology), which has been successfully applied to study adsorption problems in the case of metal surfaces.17,18 Ion cores are treated by TroullierMartins-type norm-conserving pseudopotential,19 and valence wave functions are expanded by a plane wave basis set with a cutoff energy of 25 Ry. The metal surfaces were modeled by a periodical array of three-layered slabs separated by ∼10 Å of vacuum region. A p (3 × 2) unit cell was chosen, which means a monolayer of adsorbate with coverage of 1/6 ML. In calculations, a Monkhorst-Pack mesh of 4 × 6 × 1 special k-point sampling in the surface Brillouin zone was used. In this work, the adsorption energy (Qads) or binding energy (B.E.) of species A on metal surfaces is calculated according to the formula B.E.(A) ) EA/M - EM - EA, where E is the calculated total energy. For a reaction such as AB ) A + B, the calculated heats of reaction (or total energy change) for those reactions are listed under the definitions: ∆H ) E(A+B)/M - EAB/M (where E(A+B)/M is the total energy for the co-adsorption system of A/B/ M). The reaction paths of water dissociation on metal surfaces are investigated by the NEB method. The transition-state search is initiated by interpolating a series of images of the system between the initial and final states on the potential energy surface. On the potential energy surface, a spring force between the adjacent images is added to keep the spacing between the images constant, and the true force is applied to keep the images sliding toward the MEP (minimum-energy path), thus mimicking an elastic band. Each image is optimized with the NEB algorithm (a constrained molecular dynamics algorithm). This approach helps the images converge to the reaction path being searched, as well as locating the highest point of the MEP. The highest point of the optimized reaction coordinate along the MEP should be the transition state along the chosen reaction path, and this highest energy relative to that of the initial state gives the activation barrier of the reaction. In fact, to increase the density of images near the transition state (TS) and to locate the TS more accurately, the modified NEB method (i.e., ANEBA method20) was used. In the ANEBA method, we choose three movable images connecting two local minima on the potential energy surface and use the NEB method as a starting level. After the calculation converges to some given accuracy, we choose the two images adjacent to the one that has the highest energy as our new starting points for the next level NEB calculation. Through several such levels of NEB calculation, at the last level, the ANEBA calculation will locate three images in which the total energy of each is almost the same, and then the one in the middle is considered the TS. Although this approach does not employ the frequency analyses, it has been shown in many cases to give excellent convergence to saddle points on the analytical potential energy surface.21-24

Tao et al. To test the applicability of the ANEBA method, we compared the results of the conventional NEB method and ANEBA method, taking water dissociation on a clean Cu(111) surface for example. First, the conventional NEB method was used. A large number of images (16 images) were chosen to bracket the saddle point with high accuracy, and a barrier of 1.42 eV was obtained. Also, then the ANEBA method was used, increasing the resolution in the neighborhood of the saddle point, and the final saddle point had a maximal force smaller than 50 kJ/mol/nm. Four-levels recursion of ANEBA was made. The reaction barrier turned out to be nearly the same (1.40 eV). As we can see, as compared to the conventional NEB method, the ANEBA is applicable and more efficient. 3. Results and Discussion The transition state for the reaction such as AB ) A + B is usually searched using two approaches with the same initial state (IS): one is that every adsorbate in the final state (FS) is placed on the most stable adsorption site, and the other is that the final state is optimized from IS in which the two dissociated parts are not in the most stable site. We take methanol dissociation on clean Cu(111), for example, to test which approach is better. Two FSs are optimized, respectively, and the results show that the configuration obtained from the former (i.e., in the most stable site) is more stable. In our previous investigation on the methylamine decomposition mechanism on Pd(111),25 the calculated results show that the activation energy obtained from the former is much lower as compared to the case of products from IS. In addition, we also tested the final state for the H2O dissociation on Cu(111) using several possible configurations, that is, OH(fcc) + H(fcc), OH(bri) + H(fcc), and OH(bridge) + H(hcp), and it was found that configurations of OH(fcc) + H(fcc) had the lowest energy among these three configurations. Therefore, in this work, adsorbates in the IS and FS are optimized at its most stable adsorption site. After being optimized, the adsorbates may have some deflection from their most stable adsorption site due to the interaction of one adsorbate and another. 3.1. Methanol Decomposition. It is well established that the methanol decomposition on Cu is initiated by the O-H bond scission of methanol, because methoxy radical is the most abundant species found from the first step of methanol decomposition,26,27 and the resulting intermediates of O-H scission are CH3O and H. It has been pointed out that the methanol is weakly adsorbed on the top site of the Cu(111) surface with its C-O axis highly bent and the O-H bond is roughly parallel to the surface.28-30 For the products, the coadsorption structure of CH3O and H on Cu(111), the hydrogen atom and methoxy radical are placed above the two closest fcc hollow sites with the C-O bond being quasi perpendicular to the surface.31,32 The optimized structure of reactants and the products in our calculation is shown in Figure 1a and Table 1, respectively, which are in good agreement with the available experimental and theoretical data.28-30 Following the NEB method, the TS for this reaction can be approximated by interpolating a series of images between the adsorbed methanol and the coadsorbed CH3O/H under a full optimization of the coordinates of these adsorbed species. The TS structure obtained is displayed in Figure 1a (TS), the form of which is similar to that of the coadsorbed CH3O/H. In the TS structure as shown in Figure 1a, the O-H bond becomes weaker, because the O-H distance is much longer in the TS, and the distance from the O atom to the metal surface is 1.87 Å, much shorter than its initial value in methanol adsorption, 2.72 Å, which indicates that part

Effect of Oxygen on Dehydrogenation Reactions

J. Phys. Chem. B, Vol. 110, No. 51, 2006 26047

Figure 1. Top and side views of the reactants, possible TSs, and products of the methanol dissociation on clean and oxygen pre-adsorbed Cu(111). (a) Structures on clean Cu(111). (b) Structures on oxygen pre-adsorbed Cu(111).

of the methoxy in the TS approaches closer to the metal surface. The calculated energy barrier and the total energy change for this reaction step are 1.36 and 0.16 eV (is endothermic by 0.16 eV), respectively (Table 2). To facilitate methanol decomposition, single-crystal surfaces are generally predosed with oxygen, and it is proposed that the presence of oxygen on copper strongly promotes the decomposition of methanol.26,27 Methanol is converted to methoxy on copper surface predosed with oxygen, via the formation of surface hydroxyl. The optimized structure of coadsorbed oxygen and methanol is given in Figure 1b. Oxygen is located on the fcc site, and the optimized structure of methanol is similar to

that on clean Cu(111) except for the hydrogen in the H-O bond being more closer to the metal surface. When we determine the final state, the hydroxyl and the methoxy were put on their most stable site (fcc), and the optimized configuration is shown in Figure 1b. The hydrogen essentially moves straight from its initial location in the methanol molecule to the pre-covered oxygen atom to form its final state (hydroxyl). The structure of TS is methoxy-like, suggesting that the TS is final-state-like. The calculated reaction energies are listed in Table 2: the dissociation of methanol on oxygen-assisted Cu(111) surface is hindered by a barrier of about 0.55 eV and is exothermic by 0.30 eV. As compared to that on clean surface, the barrier is

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TABLE 1: Calculated Adsorption Properties of Possible Adsorbed Species for the Dissociation of CH3OH, CH3O, H2COO, NH3, and H2O on Clean and Oxygen Pre-covered Cu(111)a species

site

R(⊥)

R(OH)

CH3OH CH3OH(O) CH3O CH3O(O) H2CO H2COO H2COO(O) HCOO NH3 NH3(O) NH2 H2O H2O(O) OH

top top fcc bri η2 bri bri bri top top bri top top fcc

2.720 2.789 1.435 1.436 3.205 3.480 3.597 2.668 1.815 2.179 1.603 2.745 2.964 1.424

0.983 1.000

a

R(CH)

R(NH)

1.100 1.098

R(CCu)

∠HOC

2.724 2.681

4.037 4.262

110.3 110.6

-0.08

1.961

2.857 111.9 115.2

0.00

0.988 0.985

∠HOH

-0.28 1.965b 2.768 3.037

0.985 0.999

Qads

-2.17

1.080 1.027 104.4 104.6

-0.15

All distances are in angstroms, angles are in degrees, and energies are in eV. b The distance of N-Cu.

TABLE 2: Calculated Reaction Energies of Dehydrogenation Reactions on Clean and Oxygen Pre-covered Cu(111) Surfaces (Units in eV) clean surface

oxygen pre-covered surface

species

Eaa

∆H

Eaa

∆H

CH3OH CH3O H2COO NH3 H2O

1.36 1.53 0.62 1.30 1.40

0.16 1.04 0.53 0.77 0.26

0.55 0.80 0.11 0.82 0.22

-0.30 0.27 -1.32 0.56 -0.14

a

R(OCu)

Ea is the activation energy.

much smaller on oxygen pre-covered surface, and the oxygenassisted O-H bond cleavage is distinctly more exothermic. Our calculated results are in agreement with the prediction of previous researchers. Wachs26 and Sexton27 proposed that the presence of oxygen on copper strongly promotes the decomposition of methanol, but in the work of Russell,33 they pointed out that at low oxygen coverages (0.26) site blocking is dominant. Similar behavior has also been observed on Cu(100)34 and Cu(110).26,27 Our results done at the coverage of 1/6 ML (∼0.17) are in agreement with the experimental result at low coverages (350 K),27 methoxy radical is decomposed to formaldehyde. Also, it is proposed that methoxy radical decomposition to form formaldehyde is the rate-determining step of methanol decomposition on Cu(111).28,35,38 Depending on the experimental conditions, formaldehyde is the major product of methanol oxidation on copper surfaces, and it is desorbed on Cu(111)31 and on Cu(110)28,29 at about 370 K. Less information is available about the interaction of formaldehyde with the Cu(111) surface. Experimental studies are difficult because formaldehyde is easily polymerized on metal surfaces,39 and theoretical studies are also challenging because of the weakly adsorbed state of formaldehyde on Cu(111).29,36 The adsorption structure of formaldehyde is presented in Figure 2a: it seems that formaldehyde is preferentially adsorbed with the oxygen atom almost above one copper atom, and the C-O bond is roughly parallel to the surface. The O-C-H angle is 120.5°, and the H-C-H angle is 119.1°; the oxygen and the carbon atoms are 3.411 and 3.205

Å far from the metal surface. Our results are similar to those concluded by Gomes40 and Greeley28 by DFT calculations. The TS structure is shown in Figure 2a, and it looks like the final state of the product formaldehyde and hydrogen. A barrier of 1.53 eV is found for the methoxy hydrogen abstraction step on clean Cu(111), and the reaction is endothermic by 1.04 eV. The barrier we found is comparable to the results obtained by various theoretical and experimental investigations on copper surfaces. Theoretical calculations predict a barrier of 1.80 eV by Gomes36 and 1.42 eV by Greeley28 on Cu(111). The difference in the reaction barrier may be explained by using different methods and models. On the oxygen preadsorbed Cu(111) surface, the reaction mechanism is different from that on the clean surface: the presence of the oxygen atom leads to the spontaneous breaking of the methanol O-H bond to form hydroxyl. The adsorption of methoxy radical occurs as the reactant moves from its stable site (fcc) to the nearest bridge site by the repulsion from the predosed oxygen atom. The barrier on oxygen pre-covered Cu(111) is 0.80 eV, and the total energy change is 0.27 eV, which is in agreement with the experimental value of 1.06 eV by Yates and co-workers33 and is somewhat smaller than the theoretical results of 1.30 eV on (2 × 1) O/Cu(110) by Groβ.35 As we can see, the presence of adsorbed oxygen has lowered the reaction barrier (1.53 vs 0.80) and the total energy change (1.04 vs 0.27) for methoxy dehydrogenation. Our energetics results of the methanol decomposition to methoxy and the methoxy further dehydrogenation to formaldehyde indicate that the dehydrogenation of methoxy to formaldehyde is the rate-limiting step of methanol dehydrogenation and pre-covered oxygen atom facilitates this reaction by reducing the barriers. 3.3. Formic Acid Decomposition. For the adsorption of HCOOH on Cu(111), we predict a type of nonactivated dissociative adsorption from the gas phase. The experimental literature showed that on Cu surfaces, HCOOH at low coverages dissociates to form HCOO and H at temperatures about 270 K.34,41-43 In our calculation, the formic acid molecule adsorbed on the bridge site with molecular plane perpendicular to the metal surface. As to the product species, formate is bonded to the Cu(111) surface through the two oxygen atoms in a bidenate configuration (i.e., with the two oxygen atoms in two equivalent sites of the surface) with the hydrogen in the nearest fcc site, and the formate molecular plane is essentially perpendicular to the surface. The optimized structures are present in Figure 3a, which are well in agreement with the experimental results reported through techniques such as XPS,44 IRAS,45 NEXAFS, and SEXAFS.46-48 The TS structure obtained is similar to that

Effect of Oxygen on Dehydrogenation Reactions

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Figure 2. Top and side views of the reactants, possible TSs, and products of the methoxyl dissociation on clean and oxygen pre-adsorbed Cu(111). (a) Structures on clean Cu(111). (b) Structures on oxygen pre-adsorbed Cu(111).

of the coadsorbed HCOO/H. The O-H bond does become weaker, because the O-H distance is much longer in the TS (1.590 vs 0.988), and the distance from the C atom to the metal surface is 2.850 Å, much shorter than its initial value in formic acid adsorption, 3.480 Å, which indicates that both part of the formate and the hydrogen atom in the TS approach closer to the metal surface. The energy barrier and the total energy change for this reaction are 0.62 and 0.53 eV, respectively. On the oxygen pre-covered Cu(111) surface, the mechanism of formic acid decomposition is different from that on the clean surface. The optimized structure of reactants and products is shown in Figure 3b, and the structure of formic acid and formate is similar to that on the clean surface, with the oxygen atom

and hydroxyl on the fcc site. Comparing the structure of formic acid on the oxygen pre-covered surface to that on the clean surface, the H-O-C angle increased by 3.3° (from 111.9° to 115.2°), and that is probably due to the abstraction effect of the pre-covered oxygen atom. It can be seen in Table 2 that the calculated reaction barrier of decomposition of HCOOH is 0.11 eV on the oxygen preadsorbed Cu(111), and the corresponding total reaction energy change is -1.32 eV. The pre-covered oxygen abstracts the acid hydrogen of HCOOH more efficiently than does the clean Cu surface, which can be seen from our calculated results in Table 2, and the presence of oxygen dramatically decreases the values of both ∆H and ∆E. The promotion effect obtained here is well

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Figure 3. Top and side views of the reactants, possible TSs, and products of the formic acid dissociation on clean and oxygen pre-adsorbed Cu(111). (a) Structures on clean Cu(111). (b) Structures on oxygen pre-adsorbed Cu(111).

in agreement with the experimental results by Bowker and Madix,44 that the pre-adsorbed oxygen increases the formate by abstracting the acid hydrogen atoms. Also, Henn et al.49 investigated the formic acid decomposition reaction over the clean Cu(110) and O/Cu(110) and found that HCOOH f HCOO + H and HCOOH + O f HCOO + OH occur, respectively, at T ≈ 270 and T ) 110-270 K. This suggests the following trend in H-abstraction ability: copper pre-adsorbed

with oxygen > clean copper, which is well in agreement with our calculation results. 3.4. Ammonia Decomposition. The ammonia molecule is found to occupy an atop bonding site with a Cu-N bond length of 2.09 Å, which is well in agreement with the experimental result of 2.09 Å by Woodruff.50 Yet it is shorter than the calculated result of 2.41 Å on the Cu(8, 3) cluster by Neurock;51 this difference between the two results may come from the

Effect of Oxygen on Dehydrogenation Reactions

J. Phys. Chem. B, Vol. 110, No. 51, 2006 26051

Figure 4. Top and side views of the reactants, possible TSs, and products of the ammonia dissociation on clean and oxygen pre-adsorbed Cu(111). (a) Structures on clean Cu(111). (b) Structures on oxygen pre-adsorbed Cu(111).

different model used (our result is obtained on the Cu(3 × 2) slab model). The ammonia molecule directly decomposes to form the surface amide and hydrogen. The stable structure of amide is also given in Figure 4a; the bridge site is favored with a Cu-N bond length of 1.965 Å. Ammonia decomposition on a model of the Cu(111) surface is hindered by a barrier of 1.30 eV, and the total energy change is 0.77 eV. The role of pre-adsorbed oxygen atom in the dissociation of ammonia on the metal Cu surface has been experimentally52-55 and theoretically51,56,57 investigated extensively. On Cu(111), Roberts and co1leagues54,55 found that ammonia readily dissociates in the presence of oxygen. They suggested that the mechanism for the initial dissociation might be controlled by a “hot” transient atomic oxygen species. It is speculated that these species rapidly diffuse across the surface and abstract hydrogen

from the coadsorbed ammonia before becoming thermally equilibrated with the surface bonding. Oxygen and ammonia are coadsorbed with NH3 on the top site and oxygen on the fcc site. Atomic oxygen abstracts a single hydrogen from neighboring ammonia to form surface amide and hydroxyl. This path is hindered with a barrier of 0.82 eV with a total energy change of 0.56 eV. As we can see, in the presence of coadsorbed oxygen atom, the dissociation of ammonia over copper is an energetically favorable process by lowering the activation energy from 1.30 to 0.82 eV. Dissociation of ammonia in the absence of oxygen is endothermic, and dissociation of ammonia to NH2 in the presence of oxygen is endothermic, but less than the system without oxygen. The promoted effect is consistent with the result of DFT calculations on the model Cu(8, 3) cluster of the Cu(111) surface by Neurock.51 Yet in the same work, it is reported

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Figure 5. Top and side views of the reactants, possible TSs, and products of the water dissociation on clean and oxygen pre-adsorbed Cu(111). (a) Structures on clean Cu(111). (b) Structures on oxygen pre-adsorbed Cu(111).

that the strong atomic oxygen surface bonding, however, leads to poisoning of surface sites, which inhibits the overall dissociation kinetics. 3.5. Water Decomposition. Water dissociation is an excellent probe for studying catalysis due to its simplicity and importance in the surface reactions such as a water-gas shift reaction.58-61 Thiel, Madey,62 and Henderson63 have provided comprehensive reviews on the H2O interaction with the clean and oxygen precovered single-crystal metal surfaces and the real catalyst surfaces. It is generally held that the reaction between water and oxygen involves hydrogen abstraction from water by oxygen to produce hydroxyl, according to H2O + O ) 2OH. We determined the most stable initial state for H2O and H2O + O coadsorbed system on Cu surface. Top site is favored for H2O,

and for the coadsorbed system, H2O is on the top site and O is on the nearest fcc site. Figure 5 gives the structure of reactants, transition state, and products calculated by DFT-GGA over clean Cu(111) and oxygen pre-covered Cu(111), respectively. From Table 2, one may observe that the calculated activation barriers (1.40 eV) for the water dissociation (H2O f OH + H) over the clean Cu(111) surface is close to the experiment results, that is, 1.23 eV on Cu(111),64,65 but it is a little higher than the theoretical value (1.10 eV) from UBI-QEP microkinetic model analysis66 and our previous result (1.02 eV) using cluster models.67 The difference between the barriers is probably due to the variation in the calculation method and models. On the oxygen pre-covered Cu(111) surface, the hydrogen abstraction mechanism for water dissociation on oxygen pre-covered

Effect of Oxygen on Dehydrogenation Reactions surfaces is significantly different from the mechanism by which water is activated on the clean metal surfaces, through dissociation to atomic hydrogen and OH or O. The change in reaction products from OH + H to pure OH due to coadsorbed oxygen is important. The calculated activation barrier of the reaction of H2O + O ) 2OH is 0.22 eV, which is much lower than that on the clean Cu(111). That is to say, pre-adsorbed oxygen favored the water dissociation reaction by lowering the activity barrier of the water dissociation reaction. 3.6. Potential Characteristic of Reaction Energy of the 10 Elemental Reactions. On the clean Cu(111), it is found that the transition states of the five dissociation reactions are all finalstate-like, and this phenomenon is always found in dissociation reactions; on the other hand, on the oxygen-modified Cu(111), the oxygen atom changes the reaction mechanism by abstracting the hydrogen to form hydroxyl. The reaction energies of the five species on clean and oxygen pre-covered Cu(111) are listed in Table 2. As we can see, the barrier and the total energy change of each dehydrogenation reaction on oxygen pre-covered Cu(111) are more or less smaller than those on clean Cu(111). For a certain reaction, the larger is the change of the reaction heat (or total energy change), the larger is the change of the reaction barrier, and this relationship can be explained from the view of thermodynamics. The promotion effect of pre-adsorbed oxygen can be explained as follows: On oxygen pre-adsorbed surface, the hydrogen abstraction mechanism by the preadsorbed oxygen atom is significantly different from the mechanism by which molecules are activated on clean metal surfaces, through hydrogen abstracted from molecules by oxygen to produce hydroxyl. The pre-adsorbed oxygen can interact both with the metal atom and with the breaking hydrogen atom from molecules. There is a competition between the forming of the metal-oxygen bond (O-M) with the formation of the hydrogenoxygen bond (O-H).68 The adsorption energy of oxygen on Cu(111) is 4.33 eV, and the oxygen atoms are not as strongly bound on the surface as compared to those of transition metals such as Ru,68 so it would be expected that the pre-adsorbed oxygen atom has a much greater abstraction effect on the hydrogen of the molecules. One of the classical approaches to developing quantitative reactivity correlations with predictive utility in heterogeneous catalysis is the formation of the Bronsted-Evans-Polanyi (BEP) relationship. For these 10 different reactions investigated here, there is not a good linear correlation between the reaction barriers and the enthalpy change, and this probably is because the five species have different reaction mechanisms. 4. Conclusions In summary, this work represents a systematic theoretical study of some typical dehydrogenation reactions on clean and oxygen pre-covered Cu(111). The reaction enthalpies and the activation energies have been calculated for 10 elementary steps corresponding to the direct and the oxygen-assisted cleavage of X-H bonds (X ) O, N, C). The DFT-GGA results show that the pre-adsorbed oxygen at low coverage (1/6 ML) always facilitates the dehydrogenation reaction by decreasing the reaction enthalpies and the activation energies. The pre-adsorbed oxygen can interact both with the metal atom and with the breaking hydrogen atom from molecules. The oxygen atoms are not so strongly bound on the Cu(111) surface, so the oxygen has a much greater abstraction effect on the hydrogen of the molecules. The obtained results are in general agreement with experimental observations.

J. Phys. Chem. B, Vol. 110, No. 51, 2006 26053 Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant No. 20273034) and the NKStar HPC program, as well as the Large Scale Numerical Simulation Project of the Science Information Center of the University of Tsukuba in Japan. References and Notes (1) Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds. Handbook of Heterogeneous Catalysis; VCH: Weinheim, 1997. (2) Canning, N. D. S.; Madix, R. J. J. Phys. Chem. 1994, 88, 2437. (3) Madix, R. J. Catal. ReV.-Sci. Eng. 1984, 26, 281. (4) Thornburg, D. M.; Madix, R. J. Surf. Sci. 1989, 220, 268. (5) Shustorovich, E.; Bell, A. T. Surf. Sci. 1992, 268, 397. (6) Au, C. T.; Roberts, M. W. Chem. Phys. Lett. 1980, 74, 472. (7) Sexton, B. A.; Hughes, A. E.; Avery, N. R. Surf. Sci. 1985, 155, 366. (8) Lu, J. P.; Albert, M.; Bernasek, S. L.; Dwyer, D. S. Surf. Sci. 1990, 238, 49. (9) Afsin, B.; Davies, P. R.; Pashusky, A.; Roberts, M. W.; Vincent, D. Surf. Sci. 1993, 284, 109. (10) Boronin, A.; Pashusky, A.; Roberts, M. W. Catal. Lett. 1992, 16, 345. (11) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671. (12) Schenter, G.; Mills, G.; Jo´nsson, H. J. Chem. Phys. 1994, 101, 8964. (13) Mills, G.; Jo´nsson, H.; Schenter, G. Surf. Sci. 1995, 324, 305. (14) Henkelman, G.; Jo´nsson, H. J. Chem. Phys. 2000, 113, 9978. (15) Henkelman, G.; Uberuaga, B. P.; Jo´nsson, H. J. Chem. Phys. 2000, 113, 9901. (16) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (17) Wang, G. C.; Morikawa, Y.; Matsumoto, T.; Nakamura, J. J. Phys. Chem. B 2006, 110, 9. (18) Smsuda, S.; Szuki, R.; Aoki, M.; Morikawa, Y.; Kishi, R.; Kawai, K. J. Chem. Phys. 2001, 114, 8546. (19) Schenter, G.; Mills, G.; Jo´nsson, H. J. Chem. Phys. 1994, 101, 8964. (20) Maragakis, P.; Andreev, S. A.; Brumer, K.; Reichman, D. R.; Kaxiras, E. J. Chem. Phys. 2002, 117, 4651. (21) Sanket, K. D.; Matthew, N.; Kourtakis, K. J. Phys. Chem. B 2002, 106, 2559. (22) Hayashi, T.; Morikawa, Y.; Nozoye, H. J. Chem. Phys. 2001, 114, 7615. (23) Greeley, J.; Mavrikakis, M. J. Am. Chem. Soc. 2002, 124, 7193. (24) Wang, G. C.; Zhou, Y. H.; Morikawa, Y.; Nakamura, J.; Cai, Z. S.; Zhao, X. Z. J. Phys. Chem. B 2005, 109, 65. (25) Li, J.; Li, R. F.; Wang, G. C. J. Phys. Chem. B 2006, 110, 14300. (26) Wachs, I. E.; Madix, R. J. J. Catal. 1978, 53, 208. (27) Sexton, B. A.; Hughes, A. E.; Avery, N. R. Surf. Sci. 1985, 155, 366. (28) Greeley, J.; Mavrikakis, M. J. Catal. 2002, 208, 291. (29) Gomes, J. R. B.; Gomes, J. A. N. F. Surf. Sci. 2001, 471, 59. (30) Sun, Q.; Shen, B.; Fan, K.; Deng, J. Chem. Phys. Lett. 2000, 322, 1. (31) Gomes, J. R. B.; Gomes, J. A. N. F. J. Mol. Struct. (THEOCHEM) 1999, 463, 163. (32) Chen, Z. X.; Neyman, K. M.; Lim, K. H.; Roesch, N. Langmuir 2004, 20, 8068. (33) Russell, J. N., Jr.; Gates, S. M.; Yates, J. T., Jr. Surf. Sci. 1985, 163, 516. (34) Sexton, B. A. Surf. Sci. 1979, 88, 299. (35) Sakong, S.; Groβ, A. J. Catal. 2005, 231, 420. (36) Gomes, J. R. B.; Gomes, J. A. N. F.; Illas, F. Surf. Sci. 1999, 443, 165. (37) Witko, M.; Hermann, K. J. Chem. Phys. 1994, 101, 10173. (38) Peppley, B. A.; Amphlett, J. C.; Kearns, L. M.; Mann, R. F. Appl. Catal. 1999, 31, 179. (39) Bryden, T. R.; Garrett, S. J. J. Phys. Chem. B 1999, 103, 10481. (40) Gomes, J. R. B.; Gomes, J. A. N. F. J. Electroanal. Chem. 2000, 180, 483. (41) Bennett, R. A.; Poulston, S.; Bowker, M. J. Chem. Phys. 1998, 108, 6916. (42) Joyner, R. W.; Roberts, M. W. Proc. R. Soc. 1976, A350, 107. (43) Ying, D. H. S.; Madix, R. J. J. Catal. 1980, 61, 48. (44) Bowker, M.; Madix, R. J. Surf. Sci. 1981, 102, 542. (45) Hayden, B. E.; Menzel, D. Surf. Sci. 1987, 191, 353. (46) Puschmann, A.; Haase, J.; Crapper, M. D.; Riley, C. E.; Woodruff, D. P. Phys. ReV. Lett. 1985, 54, 2250. (47) Crapper, M. D.; Riley, C. E; Woodruff, D. P.; Puschmann, A.; Haase, J. Surf. Sci. 1986, 171, 1. (48) Crapper, M. D.; Riley, C. E.; Woodruff, D. P. Surf. Sci. 1987, 184, 121.

26054 J. Phys. Chem. B, Vol. 110, No. 51, 2006 (49) Henn, F. C.; Rodriguez, J. A.; Campbell, C. T. Surf. Sci. 1990, 236, 282. (50) Baumga¨rtel, P.; Lindsay, R.; Giessel, T.; Schaff, O.; Bradshaw, A. M.; Woodruff, D. P. J. Phys. Chem. B 2000, 104, 3044. (51) Neurock, M.; van Santen, A.; Biemolt, W.; Jansen, A. P. J. J. Am. Chem. Soc. 1994, 116, 6860. (52) Afsin, B.; Davies, P. R.; Pashusky, A.; Roberts, M. W. Surf. Sci. Lett. 1991, 259, 724. (53) Afsin, B.; Davies, P. R.; Pashusky, A.; Roberts, M. W.; Vincent, D. Surf. Sci. 1993, 284, 109. (54) Au, C. T.; Roberts, M. W. Chem. Phys. Lett. 1980, 74, 472. (55) Boronin, A.; Pashusky, A.; Roberts, M. W. Catal. Lett. 1992, 16, 345. (56) van de Kerkhof, G. J. C. S.; Biemolt, W.; Jansen, A. P. J.; van Santen, R. A. Surf. Sci. 1993, 284, 361.

Tao et al. (57) Biemolt, W.; Jansen, A. P. J.; Neurock, M.; van de Kerkhof, G. J. C. S.; van Santen, R. A. Surf. Sci. 1993, 287-288, 183. (58) van Herwijnen, T.; de Jong, W. A. J. Catal. 1980, 63, 83. (59) Grenoble, D. C.; Estadt, M. M.; Ollis, D. F. J. Catal. 1981, 67, 90. (60) Salmi, T.; Hakkarainer, R. Appl. Catal. 1989, 49, 285. (61) Fishtik, I.; Datta, R. Surf. Sci. 2002, 512, 229. (62) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (63) Henderson, M. A. Surf. Sci. Rep. 2002, 46, 1. (64) Nakumura, J.; Rodriguez, J. A.; Campbell, C. T. J. Phys.: Condens. Matter 1989, 1, 149. (65) Ernst, K. H.; Campbell, C. T.; Moretti, G. J. Catal. 1992, 134, 66. (66) Fishtik, I.; Datta, I. R. Surf. Sci. 2002, 521, 229. (67) Wang, G. C.; Jiang, L.; Pang, X. Y.; Cai, Z. S.; Pan, Y. M.; Zhao, X. Z.; Morikawa, Y.; Nakamura, J. Surf. Sci. 2003, 543, 118. (68) Wang, G. C.; Tao, S. X.; Bu, X. H. J. Catal. 2006, 244, 10.