Formaldehyde Decomposition and Coupling on V(100): A First

Apr 24, 2012 - Weldon , M. K.; Friend , C. M. Chem. Rev. 1996, 96, 1391– 1412. [ACS Full Text ACS Full Text ], [CAS]. 16. Probing Surface Reaction ...
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Formaldehyde Decomposition and Coupling on V(100): A FirstPrinciples Study Hui Wang, Chao-zheng He, Li-yuan Huai, and Jing-yao Liu* Institute of Theoretical Chemistry, State Key Laboratory of Theoretical and Computational Chemistry, Jilin University, Changchun 130023, People’s Republic of China S Supporting Information *

ABSTRACT: The decomposition of formaldehyde (HCHO) and possible pathways for the formation of C2H4 and CH4 on clean and oxygen-predosed V(100) surfaces were studied by periodic density functional theory (DFT). It is shown that both C−H and C− O bond scissions of HCHO are thermodynamically and kinetically favorable on clean V(100). Three reaction pathways for the formation of C2H4 and two for the formation of CH4 were determined. Our results suggest that the preferred pathway for C2H4 formation at low temperature is the coupling of two methylenes (CH2) produced by an early CO dissociation step at lower O coverage; while as the increase of the onsurface O coverage, this path is suppressed whereas the direct coupling of HCHO to form intermediate OCH2CH2O is favored at high temperature. For the formation of CH4, different mechanisms are also identified corresponding to the two reaction regions. The low-temperature reaction likely occurs via successive hydrogenation of CH2, while the high-temperature reaction may proceed via the CH3O intermediate formed by hydrogenation of HCHO first. The present calculations show that the oxygen deposited on the V(100) surface contributes to the shifting of the mechanisms in low- and high-temperature regions, in line with the experimental results [Shen, M.; Zaera, F. J. Am. Chem. Soc. 2009, 131, 8708].

1. INTRODUCTION Formaldehyde (HCHO) is the dominating indoor pollutant1 and is very bad for health.2 Acute and chronic inhalation can lead to eye, nose, and throat inflammation and possibly lung and nasopharyngeal cancer.3 HCHO is also an important precursor for many chemical compounds, especially for polymers.4 In view of its toxicity and widespread industrial use, the catalytic reduction of HCHO has received much attention in the past several decades.5−7 A comprehensive understanding of the overall process is particularly important not only for controlling indoor air quality in airtight buildings but also for designing fuel cell and modeling catalytic processes such as CO hydrogenation, methyl format, and alcohol synthesis.8 Adsorption and decomposition of HCHO have been extensively studied on different metal 9−18 and metal oxide19−24 surfaces, and efforts have mainly focused on surfaces involving late transition metals. Decomposition of HCHO to carbon monoxide and hydrogen was found to be the most likely dissociation channel occurring on some surfaces. Recently, Shen and Zaera25 investigated the thermal chemistry of HCHO on V(100) under ultrahigh vacuum (UHV) conditions using temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) in combination with isotope labeling experiments. In their work, except for the above typical products as observed on other surfaces, one interesting observation is the formation of ethylene (C2H4) in two distinct temperature regimes around 290 and 540 K, respectively. They proposed different reaction mechanisms for the two reaction © 2012 American Chemical Society

regimes based on their experimental evidence; i.e., the lowtemperature pathway occurs via the coupling of methylene (CH2) formed upon HCHO decomposition, whereas the hightemperature pathway involves the prior formation of a diolate intermediate (OCH2CH2O) followed by C−O bond scission. Meanwhile, desorption of methane (CH4) was also detected within two temperature regions from TPD experiments. Although the possible reaction mechanisms for the formation of C2H4 were proposed by experimental study, many fundamental issues are still elusive, such as the stability and site selectivity of the available surface species (HCHO, C2H4, CO, OCH2CH2O) on V(100), the detailed reaction paths for the formation of C2H4 and CH4 in two temperature regions, and how the on-surface oxygen affects the reaction processes, etc. In addition, the CH4-formation mechanism is unclear experimentally. Thus, the question arises: are there two different mechanisms responsible for the CH4 production in two different temperature regions, similar to C2H4? Theoretically, the complete dehydrogenation of HCHO on many metal surfaces such as Pt(111),26 Cu(100),27 Ag(111),28 and gold clusters Aun29 has been studied by the density functional theory (DFT) method. To the best of our knowledge, no theoretical study has been performed on V(100). Therefore, a systematic theoretical investigation on the thermal chemistry of HCHO on V(100) is very desirable. Received: February 10, 2012 Revised: April 19, 2012 Published: April 24, 2012 10639

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as the difference between the energies of the coadsorbed configuration and infinite separation state (each adsorbate in a separate unit cell at its most stable position). The reaction energy barriers for bimolecular reactions were calculated as the energy difference between the transition state structures and the reactants in infinite separation state.

In this work, we perform DFT calculations to characterize the behavior of HCHO on the clean V(100) and consider the effect of precovered oxygen on the surface reaction mechanisms. The adsorption stability and site preference of HCHO and the derived species on V(100) are analyzed, and the transition states for all elementary steps involved are searched. The minimum energy paths for the dominant elementary steps are also identified on oxygen-predosed V(100). On the basis of the calculated reaction energy profiles, we present the possible low- and high-temperature mechanisms for the formation of C2H4 and CH4. We aim to provide a deep insight into the interaction nature between HCHO and the surface on the basis of the present calculations.

3. RESULTS AND DISCUSSION Various configurations of HCHO and derived species at three adsorption sites (top, bridge, and hollow) on V(100) were considered. The adsorption energies at stable sites are compiled in Table 1. The most stable adsorption configurations are Table 1. Adsorption Energies in eV of Different Species on V(100) at 0.25 MLa

2. COMPUTATIONAL DETAILS Periodic DFT calculations were carried out with the Vienna ab initio simulation package (VASP).30,31 The one-electron wave function was expanded using a plane-wave basis set with an energy cutoff of 400 eV. Ion−electron interactions were described by the projector augmented wave (PAW)32,33 method. Electron correlation was modeled by the generalized gradient approximation (GGA) with the Perdew−Burke− Ernzerhof (PBE)34 functional. Geometries were relaxed using the conjugate gradient algorithm until the forces on all unconstrained atoms were less than 0.04 eV/Å. Spin polarized was considered for all calculations, and the Methfessel−Paxton technique with a smearing width of 0.2 eV was used. The adequacy of different Monkhorst−Pack35 meshes of N × N × N, N = 11, 13, ..., 19 with PBE function had been tested against the calculation of the bulk constant. The calculated lattice constant was between 2.9796 and 2.9806 Å with all these calculations. The PBE function combined with a Monkhorst− Pack mesh of 13 × 13 × 13 gives a lattice constant (a0 = 2.9806 Å), which is in good agreement with the experimental value (a0 =3.0240 Å)36 and thus was chosen to define the V(100) slab in our calculations. The V(100) surface was modeled by the slab supercell approach using a (2 × 2) unit cell and a five-layer periodic slab separated by a vacuum region of 15 Å. The three lower layers were fixed at DFT-bulk geometry, and the two upper V atom layers were allowed to fully relax. This slab thickness is commonly used and presents a compromise between accuracy and computational efficiency. Brillouin zones were sampled with the (6 × 6 × 1) Monkhorst−Pack k-point for surface calculations. For this system, no dipole correction was used, since the energy difference for adsorbed species caused by dipole correction was found to be negligible. The transition states (TS) were determined using the climbing-image nudge elastic band (CI-NEB) method37,38 with eight intermediate images, and minimum energy paths (MEPs) were constructed accordingly. The vibrational analysis shows that the minima possess all real frequencies and the transition states have only one imaginary frequency. Zero-point energy (ZPE) corrections were included in the barrier and reaction energy calculations. The adsorption energy (Eads) for each possible adsorbate was calculated by the following standard equation:

reaction intermediate H O OH CO CHO HCHO CH3O CH2 CH3 CH4 C2H4 CH2CH2O a

hollow −2.93 −8.40 −4.12 −3.32 −4.59 −2.57 −3.46 −8.43 −2.16 −0.03 −1.18 −3.21

(−2.80) (−8.37) (−4.36) (−3.28) (−4.51) (−2.52) (−8.20)

(−0.57) (−3.24)

bridge

top

−2.75 −7.61 −4.77 (−4.66) −1.79

−2.07 −6.62 −4.40 (−4.34) −1.21

−1.03 −3.88 (−3.71) −7.11 −2.17 (−2.07) −0.04(0) −1.09 −3.13 (−3.15)

−1.17 −3.86 (−3.70) −1.78 −0.04

Values in parentheses include ZPE correction.

shown in Figure 1. The other stable configurations are given in the Supporting Information (in Figure S1). Several reaction paths were considered for all reactions, but only the minimum energy paths are reported. 3.1. Stability and Structure of Surface Species. Adsorption of CHnO Fragments. HCHO prefers to adsorb through the C and O atoms over two adjacent bridge sites with each atom binding to two V atoms. The C−O bond is nearly parallel to the V(100) surface, and the C−H bonds bend slightly upward. The C−O bond distance is 1.41 Å, elongated by as much as 0.20 Å relative to the gas phase structure, which leads to softening of the C−O stretching mode. The adsorption energy for this configuration is −2.57 eV, about 1.40 and 1.54 eV stronger than at the top and bridge sites, respectively. In top and bridge configurations, HCHO are upright with the O atom bound to the surface. In order to understand the charge redistribution upon HCHO adsorption and the interaction between HCHO and V(100), we performed Bader analysis with Bader’s program39,40 and local density of states (LDOS) analysis. The results of free and adsorbed HCHO are shown in Figure 2a and b. Plots of the contour surface of the electron-density difference, Δρdiff = ρ[gas-surface] − ρ[surface] − ρ[gas], are shown in Figure 2b. The Bader analysis reveals that HCHO binds to V(100) via C and O atoms and bond angles are evidently distorted compared with gaseous molecule. 1.28 electrons are transferred from the surface V atoms to HCHO, and most tend to locate on C and O atoms, where C gains much more electrons than O atom, 0.93 vs 0.10 electrons. In addition, as it is known that the electronic structure of HCHO in the gas phase can be designated as (1sO)2(1sC)2(2sO)2(σCH)2-

Eads = Egas − surf − (Esurf + Egas)

where Egas−surf, Esurf, and Egas are the total energies of the adsorbed system, the clean surface, and the corresponding gasphase species, respectively. The interaction energy was defined 10640

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Figure 1. Top view and dide view (inserted) of species involved in HCHO reactions on V(100).

Figure 2. Local density of states (LDOS) and charge-density difference for HCHO and OCH2CH2O on clean V(100). Isosurfaces are calculated at 0.01 e Å3. The V(d) is the sum of d-orbitals of V atoms on the first layer. The Fermi level is at E = 0 eV. (a) HCHO and V(100) before interaction. (b) Adsorbed HCHO. (c and d) Adsorbed OCH2CH2O.

(σCH′)2(σCO)2(πCO)2(nO)2(π*CO)0, the six distinct peaks shown in Figure 2a along the energy scale from −10 to 5 eV represent the σCH, σCH′, σCO, πCO, nO, and π*CO orbitals, respectively. Upon adsorption, the peaks below the Fermi level (see Figure 2b) are with somewhat downward shifts; especially the fifth peak is with a remarkable downward shift and height decreasing, which suggests the donation of electrons from the occupied orbital of HCHO into the empty d-orbital of V. At the same time, the energy of the π*CO orbital decreases below the

Fermi level (see the sixth peak), indicative of the back-donation of electrons from V into the empty π*CO antibonding orbital. As the π*CO orbital is filled with electrons, the interaction between C and O atoms in HCHO is weakened, making the C−O bond longer than that in free HCHO as mentioned above. According to the Bader analysis, the adsorbed HCHO is distributed with net negative charges, meaning that the backdonation leading to population of the π*CO orbital is stronger than the donation. 10641

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in Figure 2c and d. The Bader analysis shows that about 1.48 electrons are transferred from the V(100) surface to the OCH2CH2O molecule and mainly locate on the O atoms. It is found that each O atom gains about 0.60 electrons, about 0.50 electrons more than the amount transferred to the O atom in separated adsorption HCHO. Thus, different from HCHO, OCH2CH2O is bound to the surface only via O atoms. This can also be confirmed by the LDOS result that the O(p) and V(d) in OCH2CH2O (Figure 2c) show stronger hybridization while the hybridization between C(p) and V(d) is less remarkable (see Figure 2d). The hybridization of C1(p) and C2(p) (see Figure 2d) in OCH2CH2O indicates the formation of the C−C bond. CH2CH2O is a proposed intermediate which can be produced from the C−O dissociation of OCH2CH2O or the insertion of CH2 into the C−O bond of HCHO. It is bound through O to the bridge site and through the ending C to the opposite or adjacent bridge site. Different from the stability order of the two configurations of OCH2CH2O mentioned above, the configuration with ending atoms over opposite bridge site is more stable by 0.08 eV, with an adsorption energy of −3.21 eV. In this configuration, the C−C, C−O, and C−V bond lengths are 1.53, 1.46, and 2.27 Å, respectively, and the O atom binds to two neighboring V atoms with the same distance of 1.99 Å. The most stable configuration of ethylene (C2H4) is bound to the surface through C atoms, with an adsorption energy of −1.18 eV. The C−C bond is tilted from the surface by 22° with one C atom at the bridge site and the other at the hollow site. The length of the C−C bond is extended to 1.52 Å from 1.33 Å in the gas phase, and the C−H bonds bend slightly upward. The similar configuration of C2H4 at the bridge site (one C at the top site and the other at the bridge site) is also located but less favored by 0.09 eV. Adsorption of OH and Atomic H and O. OH prefers to adsorb at the bridge site via O and is tilted from the surface normal by 54° with an adsorption energy of −4.77 eV. The configurations at the top and hollow sites are about 0.65 and 0.37 eV less stable, respectively. The most stable adsorption sites for O atom and H atom are both at hollow sites. The adsorption energies of H at the top, bridge, and hollow sites are calculated to be −2.07, −2.75, and −2.93 eV, respectively. The adsorption energy is −8.40 eV for O at hollow site, whereas bridge site and hollow site are less stable by 0.79 and 1.68 eV, respectively. 3.2. Reaction Pathways of HCHO on V(100). The configurations of all transition states involved in all elementary steps on the clean V(100) surface are shown in Figure 3. The constructed potential energy profiles for the formation of three main products CO, C2H4, and CH4 are shown in Figures 4−9. In the diagrams, A*···B* denotes the coadsorbed state of species A and B and A* + B* denotes the infinite separation state with each adsorbate (A or B) in separate unit cells at its most stable position. All energies include ZPE correction. 3.2.1. Decomposition of HCHO. The decomposition of HCHO can occur via C−H or C−O bond scission. The PES for complete dehydrogenation of HCHO is shown in Figure 4. The first dehydrogenation step of HCHO occurs via transition state TS1. At TS1, the breaking C−H bond stretches toward the nearest hollow site with a bond distance of 1.77 Å. In the final state, the nascent CHO rotates upward and the two fragments, CHO and H, reside in their most stable hollow sites. This step has a small barrier of 0.38 eV and is strongly

The configuration of methoxyl (CH3O) at the bridge site is shown in Figure 1. It is seen that CH3O binds to V(100) via O atom, which is consistent with the experimental speculation.41 In this configuration, the O atom coordinates with two neighboring V atoms; the V−O bond length is 2.00 Å, and the C−O axis is inclined 35° from the surface normal. The adsorbed configurations of CH3O at the top and hollow sites are both upright. The adsorption energy for the bridge site is −3.88 eV. The top site is only 0.02 eV less stable than the bridge site, suggesting that occupancies of bridge and top sites are both possible, while the hollow site, which is about 0.4 eV higher than the bridge and top sites. Formyl (CHO) is only stable at the hollow site with a Vshaped HCO bond angle. The C−O bond points toward one bridge site, and the C−H directs toward another bridge site. The C atom is coordinated to five V atoms (four in the first layer and one in the second layer). CHO binds to the surface firmly with an adsorption energy of −4.59 eV. There is a clear preference for CO adsorption at the hollow site with an adsorption energy of −3.32 eV. CO is tilted from the surface normal by 57° with the C atom coordinated to five V atoms (same as the case in CHO) and the O atom coordinated to two neighboring V atoms. The configuration of CO at the bridge site is also tilted, while it is upright at the top site. The adsorption energies of CO at bridge and top sites are −1.17 and −1.03 eV, respectively. Adsorption of CHn Fragments. The CH4 molecule has been placed in various orientations at each of the three sites. It is found that the CH4 molecule is only very weakly physisorbed on the surface, with V−C separations of about 3.8 Å and the adsorption energy at each site of about 0.04 eV, indicating that CH4 desorbs easily from the surface. Methyl (CH3) adsorbs on V(100) like an umbrella with the C atom binding to the V atom. The configurations at the bridge and hollow sites have almost the same adsorption energies (−2.17 and −2.16 eV, respectively), which suggests that occupancies of both sites are possible. The top site is less favored by 0.38 eV. Stable configurations of CH2 are found at the hollow and bridge sites, respectively, in which hollow-site CH2 is energetically more favorable. It is strongly bound to the surface through C atom with the same coordination numbers as CO and CHO. The adsorption energy for the hollow site is of −8.43 eV, about 1.32 eV stronger than the bridge site. This large difference in adsorption energy suggests that the diffusion of CH2 on the surface is very difficult. Adsorption of C2H4On Fragments. The most stable configuration of OCH2CH2O is found with two oxygen atoms over two adjacent bridge sites (B1 and B2), as shown in Figure 1. Each C−O bond length is 1.44 Å, elongated by about 0.03 Å relative to the adsorbed HCHO on the V(100) surface. Each O binds with two V atoms, and the O−V bond lengths are all about 2.02 Å. The C−C bond length is 1.52 Å. The other configuration with the oxygen atoms over two opposite bridge sites (B1 and B3) is slightly less stable by 0.07 eV. The further vibrational frequency analysis shows that both of them are local minima with only real frequencies; the C−C stretching mode corresponds to 899 and 869 cm −1 , respectively, for the two configurations. Since no stable OCH2CH2O can be located in the gas phase, no adsorption energy is provided in Table 1. Bader analysis and LDOS analysis were performed for the former configuration of OCH2CH2O, and the results are shown 10642

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CH2 moves to another adjacent hollow site. This dissociation process needs to overcome a small energy barrier of 0.61 eV and is strongly exothermic by 2.89 eV. It is seen that both direct C−H and C−O bond-breaking processes have small energy barriers and are strongly exothermic, thereby being favored on V(100). The barrier for the C−H dissociation is slightly lower than that for the C−O dissociation by 0.23 eV, indicating that the former is kinetically more favored than the latter, while the latter is thermodynamically more favorable than the former due to larger released energy (2.89 eV vs 0.95 eV). Therefore, large amounts of the dissociation products CO, H, CH2, and O can be produced on the surface. As a result, subsequent reactions to produce C2H4 and CH4 are likely to occur. The possible reaction mechanisms for the formation of C2H4 and CH4 will be discussed in the following sections. Considering that surface oxygen species may be active in dehydrogenation reactions, the reaction HCHO* + O* = CHO* + OH* was studied here. In order to find the possible direct H-abstraction reaction path, we took IS1 (see Figure 5) as the initial state that most favors H-abstraction reaction. However, it is found that no direct H-abstraction channel could be located. Instead, IS1 first transforms to another more stable coadsorption state IS2 via TS1′ and then HCHO takes place C−H bond dissociation on the O-predosed surface to form CHO and atomic H (FS1). The barrier of this step is 0.34 eV, which is 0.04 eV lower than that on the clean surface. From FS1, it needs to overcome a barrier of 1.94 eV to form OH radical. Thus, it is concluded that the surface adsorbed O atom does not directly involve the H-abstraction process, while it will promote the decomposition of HCHO to CHO + H than that on the clean surface. Meanwhile, we also studied the hydrogen abstraction reaction of CHO* + O* = CO* + OH*. The initial coadsorption state involves CHO at hollow site and O coadsorbed at the adjacent hollow site nearby H atom, which is 1.17 eV higher than the infinite separate reactants. Starting from IS, the H-abstraction reaction proceeds via a later transition state TS2′ to form CO and OH, while the reaction barrier for this process is 1.29 eV and the reaction is endothermic, indicating that the H-abstraction reaction of CHO by O is thermodynamically and kinetically unfavorable and cannot be competitive to the direct C−H bond scission of CHO by the presence of O on the surface. 3.2.2. Reaction Pathways for C2H4 Formation from HCHO. Pathways on Clean V(100). Three likely pathways for the

Figure 3. Top view and dide view (inserted) of transition state structures involved in HCHO reactions on clean V(100).

exothermic by 0.95 eV. The second dehydrogenation step from CHO at the hollow site to yield carbon monoxide (CO) also takes place with a small energy barrier (0.27 eV) via TS2 and is exothermic by 0.60 eV. The dissociating C−H bond length in TS2 is stretched from 1.15 to 1.59 Å. The final state involves CO sitting near a bridge site while H atom moving to another hollow site. The reaction path for C−O bond scission of HCHO via TS3 is identified. At TS3, the C−O bond of HCHO stretches toward the nearest hollow site adjacent to the initial adsorption site, with the bond length being enlongated from 1.57 to 1.92 Å. In the final state, the atomic O remains in the hollow site and

Figure 4. Potential energy diagram for dehydrogenation of HCHO on clean V(100) with ZPE correction. 10643

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Figure 5. Potential energy diagram and geometries for H-abstraction of HCHO by O on V(100).

by C−O bond scission (path i-2). For the CH2 + CH2 coupling reaction, two CH2 from their most stable coadsorbed state, i.e., two adjacent hollow sites, move to each other along the C−C connection axis, and ends with forming C2H4 at its most stable site. At the translation state (TS4), the forming C−C bond is aligned parallel with the surface, with a distance of 2.09 Å, which is 37.5% longer than the equilibrium bond length of C2H4. A larger barrier of 2.71 eV relative to the infinite separation state is required for this step, maybe due to the strong bonding of CH2 with V(100). In the latter path, the insertion reaction of CH2 into the C−O bond of HCHO passes through TS5 to form intermediate CH2CH2O. This process needs to overcome a barrier of 1.90 eV and is endothermic by 1.08 eV. Subsequently, the C−O bond of CH2CH2O breaks via TS6 to produce C2H4 at its most stable site and leaves the atomic O at a hollow site. The dissociation reaction is very exothermic by 1.99 eV with a barrier of 0.48 eV. Path ii involves the formation of a diolate intermediate (OCH2CH2O) by prior coupling of two HCHO via TS7. In this process, two adsorbed HCHO at two diagonal hollow sites move to each other simultaneously rotate upward to form OCH2CH2O at its most stable adsorption site (as described above). At TS7, the forming C−C bond length is about 2.12 Å, which is about 40% longer than the equilibrium bond length in diolate. This coupling reaction is slightly endothermic by 0.21 eV and has an energy barrier of 1.99 eV including the interaction energy (0.14 eV). Then, one C−O bond breaks, leaving an O atom at the hollow site nearby. In transition state TS8, the breaking C−O bond length is 1.95 Å and the CH2 group bends closer to the surface. This bond-breaking step is strongly exothermic by 2.02 eV and with an energy barrier of 0.91 eV. The C−O bond breaking of CH2CH2O to form C2H4 occurs, as mentioned above. It is found that, once intermediate

formation of C2H4 from HCHO are shown in Figure 6. Starting from the initial state of HCHO + HCHO, there are two

Figure 6. Potential energy diagram for the formation of C2H4 from HCHO on clean V(100) with ZPE correction.

reaction paths: (i) the C−O bond scission of one HCHO to produce CH2 (path i) and (ii) the C−C bond formation by straight coupling of two HCHO to form a diolate, OCH2CH2O, intermediate (path ii). Then, from the adsorbed state of HCHO and CH2, the reaction could undergo two paths to form C2H4, i.e., the C−O bond cleavage of HCHO followed by the C−C bond formation via coupling of CH2 + CH2 (path i-1) and the C−C bond formation by CH2 insertion followed 10644

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Figure 7. Potential energy diagram and geometries for the reactions of HCHO on O-predosed V(100) leading to product C2H4.

than those without interaction energies on the clean surface, respectively. Formation Mechanism for C2H4. On the basis of the DFTcalculated PES information (Figures 6 and 7), we discuss the likely C2H4-formation mechanisms in the low- and hightemperature regimes. Seen from Figure 6, the C−O dissociation of HCHO is much more favored compared with the direct coupling of HCHO because of the lower activation energy (0.61 eV vs 1.99 eV) and larger exothermicity (−2.89 eV vs 0.21 eV) on the clean V(100) surface. Thus, pathway ii can be ruled out for C2H4 formation at lower temperature because of the high barrier for this path. It is seen that, taking HCHO and CH2 as the initial reactants, the reaction undergoes two two-step paths corresponding to an early C−O dissociation preceding C−C coupling and a prior C−C coupling followed by the C−O bond scission, respectively. As discussed above, the C−O bond scission of HCHO is more favorable than the insertion of CH2 into HCHO because of the lower activation energy (0.61 vs 1.90 eV). The large energy released by this step makes the dissociation product CH2 activated, thus facilitating the subsequent coupling of CH2. Therefore, pathway i-1 is more preferred for the formation of C2H4 at low temperature on the clean surface. Meanwhile, during the reaction process, the other dissociation product, oxygen, remained on the surface, which would affect the barriers of surface reactions. Our calculated results show that, on the O-predosed V(100) surface, the barrier for pathway i-1 is 0.82 eV in the (2 × 2) unit cell, slightly higher than that (0.61 eV) on the clean surface, while by the presence of O atoms, the barrier for pathway i-2 is dramatically increased to be 4.41 eV. These results apparently suggest that the favorable pathway for the C2H4 formation at low temperature is the direct coupling of CH2 formed on the surface upon the C−O bond dissociation of HCHO, in line with the proposed low-temperature mechanism in experiment.25 Applying a very rough rule-of-thumb (around 200 K

OCH2CH2O is formed, the C−O scissions of OCH2CH2O on clean V(100) are activated and favored. Effect of Predosed Oxygen. As can be seen, accompanying the C−O bond scission of HCHO, oxygen atoms are produced, and due to the high desorption energies (8.40 eV) and no alternative route for reduction available, they are deposited on the surface during the reaction, consistent with the experimental evidence that no oxygen-containing products other than HCHO and CO were observed.25 Also, it was found from experiment that the adsorbed oxygen significantly shifts the selectivity of ethylene production. Thus, to estimate the effect of the on-surface oxygen on the reaction mechanism, we also investigated the main elementary steps in the formation of C2H4 on oxygen-predosed V(100). Four steps were considered: the C−O bond scission of HCHO, the coupling of CH2, the insertion of CH2 into HCHO, and the coupling of HCHO. Each step is initiated with the coadsorbed reactants and one O atom in the V(2 × 2) cell. The interaction energy, reaction energy, and energy barrier for each step are presented in Figure 7. The geometries for initial, transition, and final states are also depicted in the figure. It is seen that the coadsorbed systems are less stable with respect to the fragments in infinite separation state due to the repulsive interaction, except for the coadsorption of C2H4 and O. The energy barrier for the C− O bond scission of HCHO is 0.82 eV (0.72 eV relative to the coadsorbed initial state), slightly higher than that (0.61 eV) on the clean V(100) surface. For the insertion of CH2 into HCHO, the barrier is 4.41 eV (3.34 eV relative to the coadsorbed initial state), increased by 2.08 eV compared with that on the clean surface. For the two coupling reactions of CH2 + CH2 and HCHO + HCHO, the repulsive interaction energies of the initial states are 0.50 and 1.22 eV, respectively, while if disregarding the interaction energies, the barriers are reduced to be 2.08 and 1.28 eV, about 0.54 and 0.57 eV lower 10645

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for a barrier of 0.50 eV and 300 K for a barrier of 0.75 eV),42 it is estimated that C2H4 could be formed through pathway i-1 (with a barrier of 0.61 eV on the clean surface and 0.82 eV on the O-predosed surface) around 244−328 K. This result is consistent with the experimental observation of the first C2H4 desorption spectra at 290 K.25 As the reaction proceeds, the coverage of O on the surface is gradually increased and, hence, inhibits the C−O scission of HCHO due to site blocking. Thus, path i-1 via the coupling reaction of CH2 + CH2 to form C2H4 is inhibited and becomes unfavorable at higher O-coverage. On the other hand, seen from Figure 7, when the surface is partially covered with oxygen, the repulsive interaction energy in the initial state of path ii is 1.22 eV, but once the reactants are in their coadsorbed state, the barrier for the HCHO−HCHO coupling to intermediate OCH2CH2O is significantly reduced from 1.85 to 1.31 eV (excluding the interaction energy). As generally known, at low HCHO/O coverage, the overall barrier for this step must include the interaction energy which is required to bring the reactants together, while the case is different at high HCHO/O coverage because the reactants have to be close to each other through crowding effects.42 Thus, it may be reasonable for one to propose that a barrier for pathway ii via coupling of HCHO is just 1.31 eV at high HCHO/O coverage. Similarly, as the same rough rule-of-thumb (500 K for 1.25 eV and 600 K for 1.50 eV),42 this coupling reaction is expected to occur with respect to the formation of C2H4 around 524 K, which is in good agreement with the observed second desorption spectra of C2H4 at 540 K in experiment.25 The present calculations show that, in the high temperature region, the surface oxygen atoms promote the C2H4-formation pathway by the HCHO−HCHO coupling, which requires a prior formation of a diolate, OCH2CH2O, followed by C−O bond scission, in accord with the proposed mechanisms by Shen and Zaera.25 3.2.3. Reaction Pathways for CH4 Formation from HCHO. Reaction pathways for the formation of CH4 on clean V(100) are shown in Figure 8. Seen from the figure, there are two paths

state of HCHO + H, the CH2 group of HCHO rotates upward and the H atom at the adjacent hollow site attacks the C atom via transition state TS10. In the final state, CH3O is formed at its most stable bridge site. This step has a barrier of 1.72 eV and is endothermic by about 0.60 eV. Then, the C−O bond of CH3O stretches toward the hollow site and breaks through TS11 with an energy barrier of 0.79 eV. The nascent CH3 and O are placed at two adjacent hollow sites. This step is strongly exothermic by about 2.12 eV. The hydrogenation of CH3 starts from the coadsorbed state with H at the hollow site and the CH3 at the adjacent bridge site, which is just 0.04 eV less stable than the most stable coadsorbed state with CH3 at the adjacent hollow site. As the reaction proceeds, the H atom moves toward the CH3 via TS12 to form CH4 at the bridge site. The energy barrier for the hydrogenation step is 1.03 eV including interaction energy (0.15 eV) and the reaction energy is 0.54 eV. Similarly, we also considered the effect of oxygen on the CH4-formation mechanisms. Four steps were considered: the hydrogenation of CH2, CH3, and HCHO and the C−O bond scission of CH3O. The PES is shown in Figure 9. It is found

Figure 9. Potential energy diagram for the formation of CH4 from HCHO on O-predosed V(100).

that the initial coadsorption state of CH2 + H is stabilized by 0.21 eV with respect to the noninteracting state. However, the coadsorbed CH3 + H + O, HCHO + H + O and CH3O + O are destabilized by 0.24, 0.54, and 0.20 eV, respectively, versus the noninteracting state. The barriers for the hydrogenation of CH2 and CH3 in path 1 are 1.13 and 1.02 eV relative to the noninteracting states and 1.34 and 0.78 eV relative to the coadsorption states. With respect to path 2, the barrier for HCHO + H + O → CH3O + O is 1.65 eV, and if disregarding the interaction energy, the barrier is 1.01 eV. Starting from the coadsorption state, the barrier for the C−O bond scission of CH3O is 0.90 eV. On the basis of the PES information, as shown in Figures 8 and 9, we discuss the likely CH4-formation mechanisms in the low- and high-temperature regimes. Seen from Figure 8, HCHO prefers the C−O dissociation rather than its hydrogenation on the clean V(100) surface due to the lower energy barrier (0.61 vs 1.72 eV) and larger exothermicity (−2.89 vs 0.60 eV). Thus, the latter pathway is unfeasible for CH4 formation at low temperature due to the high energy barrier. In the former pathway, the large heat released by the dissociation of HCHO again serves as a driving force of the subsequent hydrogenation process of the CH2 process, making this endothermic process feasible. The formed CH3 could undergo further hydrogenation to produce CH4, which desorbs from the surface easily. The energy barrier for the path HCHO

Figure 8. Potential energy diagram for the formation of CH4 from HCHO on clean V(100) with ZPE correction.

to form CH3, followed by the hydrogenation step to yield CH4. One is the hydrogenation of CH2 (path 1) which starts from the coadsorbed state of CH2 and H at two adjacent hollow sites. In this process, H moves toward CH2 through TS9 to form CH3 at the hollow site, which then diffuses to its favorite bridge site. The energy barrier for this step is 1.25 eV relative to the infinite separation state. The other path (path 2) involves the surface intermediate CH3O formed via the hydrogenation reaction of HCHO. From the nearest most stable coadsorption 10646

dx.doi.org/10.1021/jp301364w | J. Phys. Chem. C 2012, 116, 10639−10648

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hydrogenation of HCHO first at the high-temperature region. On the basis of the barriers of the two reaction paths, the roughly estimated CH4 formation temperatures are around 244−328 and 540 K, respectively, in agreement with experimental results that two desorption spectra of CH4 were observed at 355 and 540 K, respectively. The present calculations show the oxygen deposited on the surface contributes to the shifting of the mechanisms for the formation of C2H4 and CH4, in accord with the experimental findings.

→ CH2 → CH3 → CH4 is 0.61 eV, the same as that for the formation of C2H4 via path pathway i-1 at low temperature, but the former is less exothermic. As the rough rule-of-thumb mentioned above, it is estimated that CH4 could be formed through the successive hydrogenation of CH2 also around 244−328 K, which is consistent with the experimental finding that the first desorption spectra of CH4 was observed around 355 K.25 Similarly, as the reaction proceeds, the O-coverage on the surface becomes higher and the formation of CH2 is inhibited due to the site blocking. Thus, the above lowtemperature path for the CH4 formation becomes unfavorable. As analyzed above, considering the crowding effect at high HCHO/O coverage, the relative interaction of the initial state on the O-predosed surface can be negligible; as a result, it is seen that, from Figure 9, the barrier for path 2 involving the CH3O intermediate is 1.35 eV (the energy difference between TS11′ and the coadsorption state of HCHO + H + O), which is 0.26 eV lower than the barrier of 1.61 eV (the energy difference between TS10 and the coadsorption state of HCHO + H) on the clean surface. This indicates path 2 is promoted by preadsorbed oxygen on the surface. As a rough rule-of-thumb, this pathway is likely to occur for the formation of CH4 at about 540 K, in good agreement with the second observed desorption spectra of CH4 around 540 K in experiment.25 Present calculated results show that, in the high temperature region, CH4 is likely to be produced via a prior formation of the CH3O intermediate.

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ASSOCIATED CONTENT

S Supporting Information *

All the other stable configurations of each species not given in Figure 1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (20973077, 20303007) and the Program for New Century Excellent Talents in University (NCET). The authors are grateful to the referees for their valuable comments on improving the manuscript.

4. CONCLUSION In this work, we carried out extensive DFT calculations to investigate the adsorption and decomposition of HCHO and the possible mechanisms for the formation of C2H4 and CH4 on the clean and oxygen-predosed V(100) surface. Our results show that both HCHO and C2H4 prefer to adsorb on V(100) with the C−O or C−C bond nearly parallel to the surface and C−H bond bending slightly upward. CH3O and CH3 favor the bridge site, while other intermediates CHO, CH2, CO, O, H, and C2H4On (n = 1, 2) prefer the hollow site. CH4 is weakly physisorbed with no strong preference for a particular surface site over the V(100) surface. The decomposition of HCHO on the clean surface, i.e., C− H and C−O bond scissions, are thermodynamically and kinetically favorable. Three pathways for the formation of C2H4 from HCHO are identified. Through a combination of DFT calculated potential energy profiles on clean and oxygenpredosed V(100) surface, it is shown that, the formation of C2H4 via the coupling of CH2 produced by an early C−O dissociation step of adsorbed HCHO is favored at low temperature with a barrier of 0.61−0.82 eV, and the path involving the intermediate OCH2CH2O formed by coupling of HCHO is the favorable pathway at the high temperature region with a barrier of 1.31 eV when taking into consideration crowding effects at high HCHO/O coverage. As the rough ruleof-thumb, the low-temperature and high-temperature pathways are expected to occur around 244−328 and 524 K, respectively, consistent with the two observed desorption spectra of C2H4 at 290 and 540 K in experiment. Similarly, two pathways for the formation of CH4 on clean and O-predosed V(100) are obtained, i.e., the consecutive hydrogenation of CH2 and the pathway involving the CH3O formed by an early hydrogenation of adsorbed HCHO. The calculated results show that CH4 formation from HCHO favors hydrogenation of CH2 at low temperature, while it shifts to

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