J. Phys. Chem. C 2007, 111, 12335-12339
12335
Detailed Mechanism for CO Oxidation on AuNi3(111) Extended Surface: A Density Functional Theory Study Gui-Chang Wang,* Jiao Jiao, and Xian-He Bu* Department of Chemistry and the Center of Theoretical Chemistry Study, Nankai UniVersity, Tianjin 300071, People’s Republic of China ReceiVed: February 8, 2007; In Final Form: June 3, 2007
The present density functional theory (DFT) calculations have clearly reproduced the experimental phenomena for low-temperature CO oxidation on the AuNi3(111) alloy surface. Our results indicate that the AuNi3(111) flat surface is also an effective candidate for the catalysis of CO oxidation due to its low activation energy (0.13 eV) compared to Au nanoparticles (0.40 eV), stepped Au(211) (0.46 eV), and supported Au catalysis (0.36∼0.40 eV). Our results also indicate that the catalytic reactivity of CO oxidation is strongly related to the adsorption energy of atomic oxygen, and a possible guide for the design of catalysts is that the metal should have modest adsorption ability for atomic oxygen, like Ni and Pd, rather than too weak, like Pt, or too strong, like Mo.
1. Introduction There is currently great interest in the pioneering work of Haruta et al. concerning the high catalytic reactivity of gold nanoparticles supported by TiO2 for CO oxidation.1 In particular, the oxidation of CO has attracted much interest as the prototype reaction in heterogeneous catalysis because of its simplicity and industrial importance.2-6 Tremendous efforts have been devoted to Au-based catalytic CO oxidation. And recently, the high catalytic activity of gold was considered to be highly correlated to the size of the gold nanoparticles, the nature of the support, and its preparation methods.7 Unfortunately, most of the investigations were aimed at improving the activity of the Aubased nanoparticles, small clusters,8 and defective surfaces due to the poor activity of Au on adsorbing or activating oxygen molecules,2,9-12 while the detailed mechanism of CO oxidation on Au-based catalysts with flat surfaces remains largely elusive. Most recently, Lahr and Ceyer demonstrated that an extended Au alloy surface, AuNi3(111), catalyzes low-temperature (70 K) CO oxidation through the high-resolution electron energy loss spectroscopy (HREELS) method,13 which indicates the need for a theoretical investigation of the CO oxidation mechanism over the AuNi3(111) alloy surface. Thus, aiming to shed some light on the catalytic reactivity of flat surface Au-based catalysts, we have performed extensive density functional theory (DFT) calculations to study the oxidation behavior of CO on the extended AuNi3(111) alloy surface. The DFT results are in general agreement with the experimental phenomena, and a detailed reaction mechanism is proposed in the present work. 2. Calculation Method and Models The oxidation pathways for CO with molecular O2 or atomic O on AuNi3(111) have been performed with the STATE (simulation tool for atom technology) package, which is based on density functional theory (DFT) and has been successfully applied to some surface reaction studies.14-17 The AuNi3(111) * Corresponding authors: tel +86-22-23503824; fax +86-2223502458; e-mail
[email protected] (G.-C.W.), Buxh@ nankai.edu.cn (X.-H.B.).
Figure 1. Top and side views of the adsorption structures for the possible species during the CO oxidation reactions.
surface was modeled by a periodic array of four-layered slabs separated by ∼10 Å of vacuum region. A p(2 × 2) unit cell was chosen, which means a monolayer of adsorbate with coverage of 1/4 ML. The Perdew-Burke-Ernzerhof exchangecorrelation functional18 was used to calculate the total energy. The ion cores were represented by Vanderbilt’s ultrasoft pseudopotentials method19 and the energy cutoffs of the plane wave basis sets are 25 Ry for wavefunctions. In the calculation, a Monkhorst-Pack mesh with 4 × 4 × 1 k-points was used. The adsorbates and the first two layers were allowed to relax. The nudged elastic band (NEB) method20 was implemented to locate the transition-state (TS) structures. In fact, to locate the TS more efficiently, we applied the adaptive nudged elastic band approach (ANEBA) method.21 In this 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 point for the next level NEB calculation. Through three or four such levels of NEB calculation, at the
10.1021/jp071091q CCC: $37.00 © 2007 American Chemical Society Published on Web 07/28/2007
12336 J. Phys. Chem. C, Vol. 111, No. 33, 2007
Wang et al.
TABLE 1: Calculated Adsorption Properties of the Possible Species in the Oxidation Reaction on the AuNi3(111) Extended Surface CO CO O2 (phys) O2 (chem) O O a
adsorption sitea
dC-O, Å
bri Ni3-hol
1.20 1.21
dO-O, Å
dC-Ni, Å
dC-Au, Å
1.90 1.92
2.32
1.15 1.34
di-σ bri Ni3-hol
dO-Ni, Å
4.00 2.33 1.82 1.82
dO-Au, Å
2.73 2.21
Eads, eV -1.61 -2.15 -0.14 -1.31 -3.99 -5.02
bri, near bridge site of two Ni atoms; Ni3-hol, 3-fold hollow site of Ni.
TABLE 2: Energetics of Possible Reactions in the CO Oxidation on AuNi3(111) Surface reactionsa
Ea, eV
∆H, eV
CO(bri) + O2(chem) f CO2(g) + O(Ni3-hol) CO(bri) + O2(chem) f CO2(g) + O(bri) CO(bri) + O2(phys) f CO2(g) + O(Ni3-hol) CO(Ni3-hol) + O2(phys) f CO2(g) + O(Ni3-hol) CO(Ni3-hol) + O2(phys) f CO2(g) + O(bri) O2(chem) f O(bri) + O(Ni3-hol) CO(bri) + O(Ni3-hol) f CO2(g) CO(Ni3-hol) + O(bri) f CO2(g)
0.13 0.81 0.03 1.22 1.11 0.01 1.24 0.41
-3.54 -2.54 -5.10 -4.56 -3.60 -2.46 -0.29 -0.58
a bri, near bridge site of two Ni atoms; Ni3-hol, 3-fold hollow site of Ni; chem, chemisorbed; phys, physisorbed.; g, gas.
last level, the ANEBA calculation will locate three images in which the distance between two images is smaller than 0.3 Å, and the total energy of each image is almost the same (energy difference smaller than 0.1 eV) when the force acting on each atom is smaller than 0.05 eV/Å. Then the point with the highest energy is considered as the TS. The activation energy was estimated by the energy difference between transition state and adsorbed reactant state. 3. Results and Discussion It is obvious that there may be two possible channels for CO oxidation on the alloy surface: (1) CO + O2 f CO2 + O and (2) CO + O f CO2. In the first channel, the adsorbed CO reacts directly with the physisorbed or chemisorbed O2 molecule. In the second channel, molecular O2 has to dissociate to provide active O atoms before the reaction can occur. In the following section, we first give the adsorption properties of CO, O2, and O atoms as well as the possible configurations of the reactant related to the above reactions, then we discuss the reaction kinetics related to the formation of CO2 on the AuNi3(111) surface. 3.1. Possible Adsorption Properties of Species on AuNi3(111) Surface. According to the properties of the alloy surface, we can clearly see that there are two sites that can stabilize the adsorbates: one is the 3-fold hollow site of Ni (Ni3-hol), which is referred to as the “stable” site, the other one is the near bridge site of two Ni atoms (bri), referred to as the “hot” site due to its relatively smaller adsorbing strength for the adsorbates. For the adsorption of species on the AuNi3(111) surface, the optimized structural parameters and calculated adsorption energies are displayed in Table 1. It is found that, the chemisorbed O2 molecule takes a near di-σ configuration over Ni and Au atoms on the surface (Figure 1). The adsorption energies of CO and atomic O are strongly dependent on the local surface structure, and the energy difference of atomic O on the different adsorption sites (bri and Ni3-hol) is found to be relatively larger than that of CO (Table 1). There are possible two channels for CO oxidation: first with molecular O2, and second with atomic O. According to the
properties of the alloy surface, we construct three possible coadsorption configurations for CO and O2 to satisfy the reaction type of CO + O2 f CO2 + O: that is, CO(bri) + O2(chem), CO(bri) + O2(phys), and CO(Ni3-hol) + O2(phys) (Figure 1). CO chemisorption is necessary for the reaction to occur. To the best of our knowledge, there is no report suggesting physically adsorbed CO oxidation on metal surfaces. However, as the other reactant, O2 is able to take two adsorption phases on the alloy surface, that is, the physisorbed and chemisorbed states. For the first coadsorption structure of CO (bri) + O2 (chem), molecular O2 takes a near di-σ configuration over two Ni atoms, while the CO adsorbed on the bri site of two Ni atoms (see Figure 1). When O2 is physisorbed, there are two cases for the reaction to occur: one is with CO adsorbed on the bri site, which is referred as the “hot” site on the surface, and the other is with CO adsorbed on the Ni3-hol site, which proved to have higher adsorption energy (Table 1). Considering the reaction type CO + O f CO2, it may be assumed that there are many coadsorption configurations such as CO(bri) + O(bri), CO(bri) + O(Ni3-hol), CO(Ni3-hol) + O(bri), CO(bri) + O(bri), and CO(Ni3-hol) + O(Ni3-hol). But during the calculation, under the coverage of 1/4 ML, only two stable forms were found: CO(bri) + O(Ni3-hol) and CO(Ni3-hol) + O(bri). It should be pointed here that for the molecular adsorption like H2, O2, and N2, the DFT calculations usually give smaller adsorption energy than that of experimental results due to the neglect of van der Waals (VDW) interaction energy. For example, Xu and Mavrikakis22 reported that the adsorption energy of molecular O2 on Au(111) is very small (even though a positive value), and it is believed that molecular O2 cannot adsorb on Au(111). On Cu(111), the calculated O2 adsorption energy was found to be 0.18 eV reported by Neurock et al.23 In addition, the DFT calculation found the VDW interaction at a distance of 3.4 Å between O2 or H2 molecules and graphite,24,25 which is quite close to our result of 4.0 Å (Table 1) for the O2 molecule in the physisorbed state. In the case of CO adsorption, its adsorption energy obtained by the DFT calculation with the gradient-corrected Perdew-Wang exchange-correlation functional is usually larger than that of experimental data and is lower (i.e., more realistic) with the RPBE.26 In the present calculation, the calculated CO adsorption energy seems a little large, and one of the reasons is that the spin effects are not included in our study. In fact, our previous DFT results showed the CO adsorption energy reduced by 33 kJ/mol after spin correction.15 3.2. Possible Reaction Mechanism for CO Oxidation on AuNi3(111) Surface. We first study the reaction of CO + O2 f CO2 + O. In the first route of CO(bri) + O2(chem) f CO2(g) + O(Ni3-hol), in which both CO and O2 are chemisorbed on the alloy surface in the initial state, the reaction barrier is calculated to be 0.13 eV, and the total energy change is exothermic by 3.54 eV (Table 2). This is in good agreement with the experimental results where CO reacts with molecular
CO Oxidation on AuNi3(111) Extended Surface
J. Phys. Chem. C, Vol. 111, No. 33, 2007 12337
Figure 2. Top and side views of the TS structures for the possible reactions toward the formation of CO2. All distances are given in angstroms; all angles are given in degrees.
O2 preadsorbed on the alloy surface at a temperature as low as 70 K.13 In this case, the molecular O2 is expected to be adsorbed as the chemisorbed phase because of the preadsorption of the O2 molecule in the experimental process.13 The calculated TS structures are displayed in Figure 2. At the TS, the adsorbed CO moves toward O2, whereas the molecular O2 dissociates with one oxygen slipping toward CO to interact with it. After the TS, a CO2 molecule is formed, which then desorbs from the alloy surface. The remaining oxygen atom flips to the most stable Ni3-hol, which is an inactive site for the next oxidation reaction. In the second route, when the O2 is physically adsorbed on the alloy surface, there are two possible oxidation cases: one case involves the reaction of physically adsorbed O2 with CO adsorbed on the metastable bridge site in the initial state, and
the other one involves the reaction of physically adsorbed O2 with CO adsorbed on the most stable site of Ni3-hol. In the first reaction case, CO(bri) + O2(phys) f CO2(g) + O(Ni3hol), the DFT result gives a nearly zero energy barrier (0.03 eV), and the total energy change is 5.10 eV below the reactants. However, the second case, CO(Ni3-hol) + O2(phys) f CO2(g) + O(Ni3-hol), is different due to the different CO adsorption site. Owing to the more stable adsorption of CO at the Ni3-hol site compared to the bri site, the DFT calculations indicate that such a reaction has a much higher energy barrier (1.22 eV) (Table 2), and the total energy change is -4.56 eV. This is consistent with the experimental observations of Lahr and Ceyer.13 In their experiment, no oxidation reaction was detected when the CO-covered alloy surface was exposed to an O2 gas beam. In such a reaction situation, the CO will be adsorbed in
12338 J. Phys. Chem. C, Vol. 111, No. 33, 2007 a stable state like the Ni3-hol site due to the stronger adsorption ability of the Ni3-hol to CO rather than the bri site. We now turn to the reaction of CO + O f CO2. In order to study the oxidation of CO with O atom, one question should be answered: that is, whether the atomic O comes from the reaction of CO + O2 f CO2 + O or from O2 f 2O on the AuNi3(111) alloy surface. Eichler and Hafner27 have determined that O2 has the highest adsorption energy on Pt(111) when adsorbed in a di-σ configuration at a height of 1.92 Å from the surface. Furthermore, the molecular O2 dissociation was calculated to have a barrier of 2.23 eV on the pure Au(111) surface, while on the Ni(111), it will be dissociatively adsorbed at a temperature of 8 K.2,28,29 Here, we take the chemisorbed O2 molecule as the initial state in studying the dissociation of O2. The DFT calculations demonstrate that the O2 molecule is adsorbed on the AuNi3(111) surface with adsorption energy of -1.31 eV (Table 1) and then dissociates to produce atomic oxygen. In our calculation, one of the yielded O atoms was adsorbed on the most stable site of a Ni3-hol, and the other was adsorbed on the “hot” site of bri, which will prove to be an active site for CO oxidation later. The energy barrier for the reaction of O2 (chem) f O (Ni3-hol) + O (bri) on AuNi3(111) is calculated to be very small (0.01 eV), indicating the high activity for dissociation of the O2 molecule. The total energy change of the dissociation of chemisorbed O2 is exothermic by 2.46 eV. On the other hand, the reaction CO(hot) + O2(chem) f CO2(g) + O(hot) will give a higher energy barrier than the reaction of CO(hot) + O2(chem) f CO2(g) + O(stable) owing to the relatively weak adsorption of atomic oxygen at the hot site. So these results indicate that the dissociation of chemisorbed O2 is the main provider for the production of atomic oxygen. As regards the reaction channel of CO with atomic O, there should be two possibilities for the reactant adsorption forms: (i) CO is adsorbed on the bri site and O on the Ni3-hol site (Figure 1), or (ii) CO is adsorbed on the Ni3-hol site while an O atom is adsorbed on the bri site (Figure 1). The DFT calculations show that the energy barrier of i, which is found to be 0.41 eV, is much lower than that of ii (1.24 eV), indicating that reaction i is more favorable than ii from a kinetic point of view (Table 2). This agrees well with the experimental results that CO will react with the “hot” O atom between 105 and 125 K, but there is no reaction between the O atoms adsorbed on the most stable Ni3-hol site and CO.13 Thus, one may conclude that the bri-adsorbed O atom (or “hot” O atom) is the key factor for the CO oxidation reaction to take place. Therefore, our results suggest that the AuNi3(111) flat surface is also an effective candidate for the catalysis of CO oxidation due to its low activation energy (0.13 eV) compared to Au nanoparticles (0.40 eV),30 stepped Au(211) (0.46 eV),2 and supported Au catalysis (0.36∼0.40 eV).31 4. Conclusions In summary, the present DFT calculations have clearly reproduced the experimental phenomena for low-temperature (about 70 K) CO oxidation on the AuNi3(111) alloy surface. Possible reaction mechanisms on the extended alloy surface have been proposed on the basis of the theoretical calculations. First, when both molecular O2 and CO are chemisorbed on the alloy surface, the reaction pathway has the lowest energy barrier of 0.13 eV, suggesting that this is the favored oxidation channel (Scheme 1). In the other oxidation channel, the chemisorbed molecular O2 dissociates with a very low energy barrier to provide “hot” atomic O, which then reacts with the chemisorbed
Wang et al. SCHEME 1: Feasible Oxidation Pathways for CO2 Formation on the AuNi3(111) Extended Alloy Surface
CO to form CO2 with a low barrier of 0.41 eV. Hence one can conclude that in the first oxidation channel, when molecular O2 is one of the reactants, the adsorption properties of CO play a key role in the reaction barrier. On the other hand, in the second channel of CO oxidation with atomic O, the metastable adsorption properties of O are a more important factor for the oxidation reaction to proceed. The detailed reaction mechanism for CO oxidation on the extended AuNi3(111) surface may be summarized as shown in Scheme 1. Our results indicate that the reactive ability of the catalyst of CO oxidation is strongly related to the adsorption energy of atomic oxygen, and a possible guide for the design of catalysts is that the metal should have a modest adsorption ability for atomic oxygen like Ni and Pd rather than too weak an adsorption ability, like Pt, or too strong an adsorption ability, like Mo (in fact, our recent DFT result showed that the reaction of CO + O f CO2 on Mo(100) is endothermic by 1.46 eV). Acknowledgment. We thank Dr. Y. Morikawa for supplying the STATE program. G.-C.W. thanks Professor Junji Nakamura of Tsukuba University for valuable discussion. This work was supported by the National Natural Science Foundation of China (Grant 20273034). This work was also partially supported by the NKStar HPC program of Nankai University as well as the Large Scale Numerical Simulation Project of the Science Information Center, University of Tsukuba, Japan. References and Notes (1) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301. (2) Liu, Z. P.; Hu, P. J. Am. Chem. Soc. 2002, 124, 14770. (3) Langmuir, I. Trans. Faraday Soc. 1921, 17, 612. (4) Ertl, G. Surf. Sci. 1994, 299, 742. (5) Stampfl, C.; Scheffler, M. Phys. ReV. Lett. 1997, 78, 1500. (6) Alavi, A.; Hu, P.; Deutsch, T.; Silvestrelli, P. L.; Hutter, J. Phys. ReV. Lett. 1998, 80, 3650. (7) Bond, G. C.; Thompson, D. T. Catal. ReV.sSci. Eng. 1999, 41, 319. (8) Joshi, A. M.; Tucker, M. H.; Delgass, W. N.; Thomson, K. J. Chem. Phys. 2006, 125, 194707. (9) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (10) Guzman, J.; Gates, B. C. J. Am. Chem. Soc. 2004, 126, 2672. (11) Yoon, B.; Hakkinen, J.; Landman, U.; Worz, A. S.; Antoniette, J. M.; Abbet, S.; Judai, K.; Heiz, U. Science 2005, 307, 403. (12) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252. (13) Lahr, D. L.; Ceyer, S. T. J. Am. Chem. Soc. 2006, 128, 1800. (14) Morikawa, Y.; Iwata, K.; Nakamura, J.; Funjitani, F.; Terabura, K. Chem. Phys. Lett. 1999, 304, 91. (15) Wang, G. C.; Zhou, Y. H.; Morikawa, Y.; Nakamura, J.; Cai, Z. S.; Zhao, X. Z. J. Phys. Chem. B 2005, 109, 25. (16) Wang, G.-C.; Morikawa, Y.; Matsuoto, T.; Nakamura, J. J. Phys. Chem. B 2006, 110, 9. (17) Jun, L.; Li, R. F.; Wang, G. C. J. Phys. Chem. B 2006, 110, 14300. (18) 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. (19) Vanderbilt, D. Phys. ReV. B 1990, 41, 7892. (20) (a) Mills, G.; Jo´nsson, H.; Schenter, G. K. Surf. Sci. 1995, 324, 305. (b) Schenter, G.; Mills, G.; Jo´nsson, H. J. Chem. Phys. 1994, 101, 8964. (c) Henkelman, G.; J. Jo´nsson, H. Chem. Phys. 2000, 113, 9978. (21) Maragakis, P.; Andreev, S. A.; Brumer, K.; Reichman, D. R.; Kaxiras, E. J. Chem. Phys. 2002, 117, 4651. (22) Xu, Y.; Mavrikakis, M. J. Phys. Chem. B 2003, 107, 9298
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