Pt(100

Article pubs.acs.org/JPCC

Cyclohexene Dehydrogenation to Produce Benzene on nAu/Pt(100) and nPt/Au(100) (n = 0, 1, 2) Surfaces From a First-Principles Study Hong-Yan Ma,†,‡ Zhen-Feng Shang,† Wen-Ge Xu,† and Gui-Chang Wang*,†,§ †

Department of Chemistry and the Tianjin Key Lab of Metal and Molecule-based Material Chemistry, Nankai University, Tianjin 300071, P. R. China ‡ RenAi College of Tianjin University, Tianjin 301636, P. R. China § College of Chemistry and Chemical Engineering, Shanxi Datong University, Datong 037009, Shanxi Province, P. R. China S Supporting Information *

ABSTRACT: Bimetallic alloys such as Au/Pt are known as an efficient catalyst for promoting cyclohexene dehydrogenation to benzene. In this work, we try to understand the unique high reactivity of the Au/Pt surface by the first-principles density functional theory (DFT) calculations. Our DFT results and microkinetic model analysis show that the model modified by carbon and hydrogen was significantly improved in the explanation of the experimental results, and the barriers of rate-determining step (rds) are 1.37, 1.08, and 1.05 eV on Pt(100), 2Pt/Au(100), and Au/Pt (100), respectively. This is in general agreement with the order of the benzene formation rate: Pt(100) < 2Pt/Au(100) < Au/Pt(100). Additionally, the possible reaction mechanism of carbon formation on the Pt surface has been gained in this work for the first time, that is, C6H6 → C6H5 → C6H4 → C6H3 → C + C5H3, and the rate-determining step is the first dehydrogenation step with the energy barrier of ca. 1.80 eV.

1. INTRODUCTION The catalytic properties of thin metal overlayers deposited on a foreign substrate could be enhanced with respect to those of the parent metals.1−7 For example, graphitic carbon could poison Pt catalysis, whereas the Au/Pt bimetallic surfaces could resist coking by avoiding carbon deposition.1 In general, the Au/Pt bimetallic surfaces show unique catalytic properties for a series of reactions such as the oxidation of formic acid,3 methanol,4 and hydrogen5 and the reduction of oxygen.6 In particular, the experimental results showed that the Au/Pt monolayer bimetallic surfaces exhibited unexpected catalyst reactivity in promoting the dehydrogenation of cyclohexene to benzene,2 and the dehydrogenation rate was sensitive to the layer thickness of the deposited Au on Pt. In their experimental study, they found that the rate of benzene formation increased with the increasing Au coverage on Pt(100) and reached the maximum value at the coverage of monolayer (Au/Pt(100)), and then the reaction rate decreased when the Au coverage was further increased until two more layers of gold were on the Pt surface (2Au/Pt(100)). On the other hand, when the platinum was deposited on the Au(100) surface, it was found that the © 2012 American Chemical Society

rate of cyclohexene dehydrogenation to benzene was increased with the increasing platinum coverage and reached a maximum value at the situation of two platinum layers (2Pt/Au(100)). Wang et al.7 have performed electronic structure analysis to understand why Pt/Au or Au/Pt shows high activity toward the formation of benzene, but their calculation results were quite different from the experiment results. This means only the electronic structure study is insufficient, and the detailed discussion based on the reaction kinetic mechanism is required. The study of bimetallic catalysts has aroused much enthusiasm8−12 because an understanding of the principles that govern the activity of bimetallic surfaces could lead to the rational design of catalytic materials. In general, two factors control the reactivity of the bimetallic surfaces: the strain effect caused by the lattice mismatch and the electronic interaction between the substrate and overlayer.9 Periodic density functional theory (DFT) calculations have been proved to be Received: January 13, 2012 Revised: March 30, 2012 Published: April 16, 2012 9996

dx.doi.org/10.1021/jp300424t | J. Phys. Chem. C 2012, 116, 9996−10008

The Journal of Physical Chemistry C

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Figure 1. (A) Pt/Au(100) surface periodic model (the blue balls are for platinum, and the yellow balls are for gold). Calculation models for different adsorption sites (the dark gray balls are for C atom, and the light gray for H, and the blue ones for Pt atom). (C, D, E, F, G, and H) Model of Pt(100) at 4/9 carbon coverage, 2Pt/Au(100) at 4/9 carbon coverage, Pt(100) at 4/9 H coverage, Au/Pt(100) at 1/3 H coverage, Pt(100) at 4/9 H and 4/9 C coverage, and 2Pt/Au(100) at 4/9 H and 4/9 C coverage, respectively.

Table 1. Comparative Adsorption Energies on Clean nAu/Pt(100) (n = 0, 1, 2)20 and nPt/Au (100) (n = 0, 1, 2) Surfacesa adsorption energy (eV) species cyclohexene cyclohexenyl 1,3-cyclohexadiene cyclohexadienyl benzene

adsorption site

Pt(100)

Au/Pt(100)

2Au/Pt(100)

Au(100)

Pt/Au(100)

2Pt/Au(100)

hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45

−1.01 −1.00 −2.12 −2.37 −2.20 −2.15 −3.27 −3.26 −2.01 −2.02

−0.52 −0.63 −1.01 −1.08 −0.39 −0.36 −0.86 −0.69 −0.15 −0.14

−0.56 −0.59 −0.91 −0.88 −0.36 −0.35 −0.77 −0.59 −0.13 −0.13

−0.45 −0.45 −0.70 −0.67 −0.35 −0.31 −0.71 −0.61 −0.16 −0.17

−1.43 −1.35 −2.50 −2.51 −2.48 −2.29 −3.61 −3.61 −2.40 −2.41

−1.29(−1.27) −1.33(−1.32) −2.72(−2.68) −2.65(−2.62) −2.43(−2.39) −2.42(−2.38) −3.53(−3.48) −3.53(−3.49) −2.43(−2.40) −2.46(−2.43)

a Note: The values in the parentheses are calculated with the energy-cut of 400 eV and five-layers model, the discrepency is in the range of 0.02−0.05 eV.

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Figure 2. (A) Adsorption energies of the species (cyclohexene, π-allyl c-C6H9, 1,3-cyclohexadiene, cyclohexadienyl, and benzene) adsorbed at a hollow site as a function of the number of Pt overlayers on Au(100) and the number of Au overlayers on Pt(100). (B) Position of the local center of the d band with respect to the Fermi energy as a function of the number of Pt overlayers on Au(100) and the number of Au overlayers on Pt(100) for the upmost surface layer. The Pt overlayers with the lateral lattice constant of Au (a = 4.18 Å) and Au overlayers with the lateral lattice of Pt (a = 3.99 Å) are labeled by Pt@Au and Au@Pt, respectively. The pure Pt substrates with the lattice of Pt (a = 3.99 Å) and the pure Au substrates with the lattice constant of Au (a = 4.18) are labeled by Pt and Au, respectively.

= EA/M − EA − EM, where EA/M, EA, and EM are the total energy of the adsorbed system, isolated molecule, and substrate, respectively. Lattice mismatch resulted in the contraction or expansion of lattice of the bimetal surface compared with that of bulk pure metal.2 For the nAu/Pt system, the calculations were performed with the DFT optimized clean Pt lattice constant of 3.99 Å, and for the nPt/Au system the clean Au lattice constant of 4.18 Å was used. The nudged elastic band (NEB)16 implemented in the VASP has been systematically applied to find these saddle points along the adiabatic minimal energy path (MEP) connecting each initial state (IS) and final state (FS) of a given elementary step. Then, a quasi-Newton algorithm was utilized to refine the obtained approximate transition states by minimizing residual forces below 0.05 eV/Å. Finally, the transient states (TSs) were identified by verifying the existence of one and only one normal mode associated with a pure imaginary frequency. It should be pointed out that the model of Pt(100) used in this work is not stable under vacuum conditions, and a hexagonal rearrangement of the top monolayer was measured by LEED experiment.17 However, the reflection−absorption infrared spectroscopy (RAIRS) experiment18 as well as the theoretical study19 indicated that the adsorbed species such as CO and oxygen could remove such reconstruction. Considering the strong adsorption of certain species like the carbon atom during the process of cyclohexene dehydrogenation, we utilized the unreconstructed Pt(100) and Au(100) in our calculations.

an efficient and reliable tool to obtain important insights into the structural stability, electronic density of state, and chemical reactivity of bimetallic systems.10,12 The main objective of this paper is to investigate the chemical property of the Au/Pt (and Pt/Au) monolayer bimetallic surfaces produced by depositing Au (Pt) on single Pt (Au) substrate, utilizing a combination of DFT and microkinetic modeling. Specifically, the dehydrogenation of cyclohexene to benzene is used as a probe reaction to determine the reactivity of the bimetallic systems. The DFT calculations focus on optimizing structures, searching for the transitional state (TS), and electronic analysis. Moreover, the rate for cyclohexene dehydrogenation could be used to estimate the reactivity of Au/Pt and Pt/Au systems directly.

2. COMPUTATIONAL DETAILS The calculations were implemented using the Vienna ab initio simulation package (VASP).13 The exchange−correlation energy and electron−ion interaction were described by the generalized gradient approximation (Perdew−Wang 9114) and the projector-augmented wave (PAW) scheme.15 A cutoff of 315 eV has been applied to the plane-wave basis set. The Brillouin-zone integrations were performed using a 4 × 4 × 1 Monkhorst−Pack grid. The Pt/Au and Au/Pt bimetallic surfaces were modeled with a p (3 × 3) super cell (see Figure 1A), and each super cell contained four layers of metal atoms and a vacuum region around 11.5 Å. The uppermost two metal layers as well as the adsorbed molecule were allowed to relax until the atomic forces are smaller than 0.05 eV/Å. We have checked the effects of cutoff energy (with the energy of 400 eV) and thickness of slab (with the five-layer model) on the adsorption energy over the 2Pt/Au(100) surface (Table 1) and found that the discrepancy was in the range of 0.02−0.05 eV, so the cutoff energy of 315 eV and four-layer model could provide enough precision. Three different surface sites were chosen for adsorption: hollow, bridge, and top sites. For each adsorption site, there were two different adsorption configurations according to the different orientations of the molecule on the surface being adsorbed. The calculated models of the adsorption for cyclohexene are shown in Figure 1B. The position of the adsorbed isomers is defined by the center of the orbicular molecule, and the adsorption energy is defined as follows: Eads

3. RESULTS AND DISCUSSION This section has been split into two parts. In the first part, the adsorption energies of the species involved during the dehydrogenation of cyclohexene are discussed. In the second one, we present kinetic studies of the cyclohexene dehydrogenation to discuss the activity of nAu/Pt(100) and nPt/Au(100) (n = 0, 1, 2) surfaces. 3.1. Adsorption of Cyclohexene and Other Intermediates on nAu/Pt(100) and nPt/Au(100). We have studied the adsorption of cyclohexene and stable intermediates involved in cyclohexene dehydrogenation to benzene, such as π-allyl c-C6H9 and 1,3-cyclohexadiene on nAu/Pt(100) (n = 0, 1, 2) surfaces,20 and found that the hollow site (with two orientations) was the most favored adsorption site for these 9998



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Figure 3. Possible pathways of cyclohexene dehydrogenation to benzene on Au/Pt(100) (A), 2Au/Pt(100) (B), Au(100) (C), 2Pt/Au(100) (D), Pt/Au(100) (E), and Pt(100) (F).

observed by Roudgar and Groß10 who studied the local reactivity of Pd overlayers supported by Au and found that depositing a reactive metal on an inert metal with a larger lattice constant resulted in a higher reactivity of the overlayer. Christoffersen et al. found that the Pt should become more reactive when it is deposited onto Ag and Au.21 The electronic structure analysis may help to have deep insight into the reactivity of metal surfaces. The position of the whole d bands (εd) relative to the Fermi level is an important surface factor in determining the reactivity.22 The occupied dband center (or the whole d-band center) (εcd) is figured by the formula:22 εcd = [(∫ Ef−∞Eρd(E)dE)/(∫ Ef−∞ρd(E)dE)] (or εcd = +∞ [(∫ +∞ −∞Eρd(E)dE)/(∫ −∞ρd(E)dE)]), where ρd represents the

species (see Figure S1, Supporting Information). So, we just investigated the hollow site adsorbed species on nPt/Au(100) (n = 0, 1, 2) surfaces in this paper. Table 1 shows the calculated adsorption energy of cyclohexene and other species on nAu/ Pt(100) (n = 0, 1, 2)20 and nPt/Au(100) (n = 0, 1, 2) surfaces. The adsorption energy of these species approximately decreases in the order of Pt/Au(100) > 2Pt/Au(100) > Pt(100) > Au/ Pt(100) > 2Au/Pt(100) > Au(100). Taking the hollow-site adsorbed species as an example, we notice the adsorption energies of cyclohexene, π-allyl c-C6H9, 1,3-cyclohexadiene, cyclohexadienyl, and benzene as a function of the number of Pt overlayers on Au(100) and the number of Au overlayers on Pt(100) (shown in Figure 2A). A similar phenomenon was 9999



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surfaces, and this could explain the experimental results that the Au/Pt(100) surface shows higher reactivity than the other two surfaces. 3.3. Cyclohexene Dehydrogenation on the 2Pt/ Au(100), Pt/Au(100), and Pt(100) Surface. The rate of cyclohexene dehydrogenation to benzene reached a maximum value between one and two Pt overlayers (as shown in Figure 2 of ref 4), which is why the 2Pt/Au(100) surface aroused our interest. We have studied possible pathways of cyclohexene dehydrogenation on the 2Pt/Au(100) surface. To investigate the dehydrogenation process in more detail, both the most stable and the second most stable of the species involved in the dehydrogenation are used to search for TSs. Figure 3D demonstrates possible reaction pathways for cyclohexene dehydrogenation on 2Pt/Au(100). The dominant reaction pathways are signified in bold. The dominant reaction pathway is usually taken as the one with the lower barrier compared to the other parallel reactions. Consequently, the corresponding production is regarded as the dominant production. Only the pathways starting from the dominant production are discussed in this paper. The hollow R45 adsorbed C6H9 (π-allyl c-C6H9) is considered as the dominant production of the first dehydrogenation, for the corresponding barrier is lower than others. There should be two possible dehydrogenation pathways starting from the hollow R45 adsorbed C6H9. The first pathway that produces hollow adsorbed C6H8 (1, 3cyclohexadiene) is the dominant step with the barrier of 0.55 eV. The barrier for the latter pathway with the production of the hollow R45 adsorbed C6H8 (1,4-cyclohexadiene) must be higher than 0.55 eV because this process contains two steps at least: one H atom dissociated from alkyl carbon and another H atom transferring from olefinic carbon to alkyl carbon (see Figure S5, Supporting Information). The pathway that produces the hollow side adsorbed C6H7 is the preferred path for the third dehydrogenation step with the barrier of 0.28 eV. The rds is the fourth dehydrogenation step with the highest barrier of 0.64 eV. Cyclohexene dehydrogenation on Pt/Au(100) and Pt(100) was also studied (shown in Figures 3E, 3F). The dehydrogenation process on Pt/Au(100) and Pt(100) is similar to that on 2Pt/Au(100), but the barrier for each dehydrogenation step on Pt/Au(100) and Pt(100) is higher than on 2Pt/Au(100). The fourth step is the rds on the 2Pt/Au(100), Pt/Au(100), and Pt(100) surfaces, which is in agreement with the literature26,27 that reported the addition of the first hydrogen to benzene (C6H6 + H → C6H7) was the rds at low temperature on Pt(100). The barriers of the rds's are 0.64, 0.84, and 0.84 eV on the 2Pt/Au(100), Pt/Au(100), and Pt(100), respectively, suggesting the 2Pt/Au(100) surface with higher catalytic reactivity, which is consistent with the experimental results.2 By careful examination of the above DFT calculation results, we may conclude that the 2Pt/Au(100) will show the highest reaction rate for the benzene formation due to its lowest energy barrier (0.64 eV) among the studied model catalysts, and the reactivity toward the benzene formation on nPt/Au(100) will generally be higher than on nAu/Pt(100) if the rds was considered as the dominant factor to control the reactivity. However, this is quite different from the experimental observations that the benzene formation rate is Au/Pt(100) > 2Pt/Au(100) > Pt/Au(100) > Pt(100). So, there must be an obvious difference between our calculated model and the real catalysts, and the detailed discussion utilizing the more real models will be given in the following study.

density of states projected onto the d band of the metal atom and Ef is the Fermi energy. Figure 2B shows the position of the d band center with respect to the Fermi energy as a function of the number of Pt overlayers on Au(100) and the number of Au overlayers on Pt(100) for the uppermost surface layer. If the d band of a metal is more than half-filled, an expanded pseudomorphic monolayer may result in an upshift of εdc owing to band narrowing and energy conservation.23 The d band center of Pt/Au(100) is pushed to a higher-energy level by the subsurface Au layers, compared with the pure Pt(100) surface. This could be responsible for the large adsorption energy on Pt/Au(100) because the closer the d-band center to the Fermi level the more active the metal. Similarly, the subsurface Pt layers bring about an upshift of the d-band center of nAu/Pt(100) (n = 1, 2) surfaces, thus the nAu/Pt(100) (n = 1, 2) surfaces are more active than the pure Au(100). The one Pt overlayer (Pt/Au(100)) shows higher adsorption energy than that of pure Pt or two-overlayer Pt, probably owing to the strain effect (ca. 10%) of the overlayer of Pt caused by the substrate Au atom and the weak interaction of the surface Pt with the substrate Au atom. Indeed, the detailed discussion about the strain effect can be found in the pioneering works of Mavrikakis et al.24 and Kitchin et al.25 After getting the most stable adsorption configuration for the possible adsorbed species, we have explored the detailed reaction mechanisms of cyclohexene dehydrogenation. The energetic data for each elemental step are given in Figure 3, and the corresponding transition state structures are displayed in the Supporting Information (Figures S2−S4). 3.2. Cyclohexene Dehydrogenation on the Au/ Pt(100), 2Au/Pt(100), and Au(100) Surface. The dehydrogenation of cyclohexene is a complex process that contains a series of consecutive elementary steps: C6H10 → C6H9 + H → C6H8 + 2H → C6H7 + 3H → C6H6 + 4H. There are also many intermediate involved in the dehydrogenation, e.g., 1,3cyclohexadiene and 1,4-cyclohexadiene. The 1,3-cyclohexadiene is more stable than other C6H8 isomers on the clean nPt/ Au(100) and nAu/Pt(100) (n = 0, 1, 2) surfaces, so it is used to search for the transition state. According to Sachtler et al.’s2 results, the Au/P (100) is more active than 2Au/Pt(100) and Au(100) in catalyzing cyclohexene dehydrogenation. So, we first investigate the process of cyclohexene dehydrogenation on Au/Pt(100), and the barriers are calculatd as 1.18, 0.75, 0.55, and 1.10 eV for the first, second, third, and fourth dehydrogenation steps, respectively. The rate-determining step is the first dehydrogenation step on Au/Pt(100). The reaction pathway and the corresponding structures of the TSs are shown in Figure 3A and Figure S2 (see Supporting Information). We calculate the reaction heat for each dehydrogenation step on 2Au/Pt(100) and Au 100) surfaces because the reaction heat could provide useful clues for the identification of the rate-determining step (rds). The first three dehydrogenation reactions are endothermic, but the fourth one is exothermic on 2Au/Pt(100) and Au(100) surfaces. Moreover, the reaction enthalpy of the first step is 0.2−0.6 eV more endothemic than that of the other steps. So the first dehydrogenation step (C6H10 → C6H9 + H) is more likely to be the rate-determining step on the two surfaces. In fact, the barriers are calculated as 1.46, 0.70, 0.80, and 0.80 eV for the first, second, third, and fourth dehydrogenation steps on 2Au/Pt(100) and 1.49, 0.77, 0.82, and 0.20 eV on the Au(100) surface. The activation energy of the rds of cyclohexene dehydrogenation on Au/Pt(100) is the lowest among the three 10000



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Figure 4. Possible reaction mechanisms for the carbon formation on Pt(100).

Table 2. Comparative Adsorption Energies on 1/9 C 2Pt/Au(100), 2/9 C 2Pt/Au(100), 4/9 C 2Pt/Au(100), 4/9 H Pt(100), 1/ 3 H Au/Pt(100), 4/9 C 4/9 H 2Pt/Au(100), and 4/9 C 4/9 H Pt(100) adsorption energy (eV) species cyclohexene cyclohexenyl 1,3cyclohexadiene cyclohexadienyl benzene

adsorption site

1/9 C 2Pt/ Au(100)

2/9 C 2Pt/ Au(100)

4/9 C 2Pt/ Au(100)

4/9 H 4/9 C 2Pt/ Au(100)

4/9 C Pt(100)

4/9 H Pt(100)

4/9 H 4/9 C Pt(100)

1/3 H Au/ Pt(100)

hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45

−1.06 −1.19 −2.13 −2.37 −2.43 −2.18 −2.88 −2.87 −1.83 −1.67

−0.94 −0.82 −1.73 −2.12 −1.87 −1.78 −2.38 −2.71 −1.20 −1.07

−0.28 −0.34 −1.38 −1.13 −0.41 −0.06 −1.00 −1.05 −0.05 −0.03

0.29 −0.07 −0.35 −0.80 −0.10 −0.15 −0.94 −0.70 −0.07 −0.06

−0.26 −0.46 −1.12 −1.09 −0.27 −0.26 −1.23 −1.07 −0.15 −0.14

−0.84 −0.76 −1.70 −1.96 −2.04 −2.03 −3.03 −3.04 −1.84 −1.84

−0.07 −0.03 −0.50 −0.76 0.01 −0.16 −0.69 −0.66 −0.06 0.02

−0.09 −0.10 −0.62 −0.64 −0.57 −0.30 −0.97 −0.57 −0.42 −0.14

the first scission bond is the C4−C5 bond with the barrier of 0.96 eV. The barriers for the C5−C6 and C6−H bond scission are 1.79 and 1.67 eV. To sum up, the first pathway is the dominant pathway because the required barrier is much lower. The possible reaction mechanism for the carbon formation on Pt is C6H6 → C6H5 → C6H4 → C6H3 → C + C5H3, and the rds is the first dehydrogenation step with the barrier of ca. 1.80 eV. Thus the carbon atoms could be accumulated on the Pt(100) at higher temperature, which is consistent with the experimental finding that the carbon was produced within the temperature rang of ca. 450−800 K.28 3.4.2. Adsorption of Cyclohexene and Other Intermediates on the Carbon-Modified Pt(100) and 2Pt/Au(100) Surface. The effect of carbon deposition on Pt(100) should be taken into account. At first, we calculated the adsorption of the carbon atom on the Pt(100) surface. The estimated adsorption energies of the carbon atom are −7.76, −6.86, and −5.52 eV for the hollow, bridge, and top site, respectively. The hollow site is the most stable adsorption site, and this kind of carbon insertion model is adopted in the following study to simplify the calculation. As the carbon concentration was suggested to be about 0.5 ML (monolayer) in experiments,2 the model of Pt(100) at the 4/9 ML carbon coverage was built to measure the influence of carbon deposition on the reactivity of Pt(100) (see Figure 1C). The corresponding adsorption energy is listed in Table 2. The adsorption energy of each adsorbate is less

3.4. Effect of Carbon Deposition on Pt(100) and 2Pt/ Au(100). 3.4.1. Origin of Carbon on the Pt(100) Catalysts. Experiments28,29 showed that benzene was dehydrogenated to graphitic carbon on the Pt(111) and Pt(110) at high temperature. Gao et al.30 have investigated the process of benzene dehydrogenation to C6H3 on the Pt(111) by ab initio density functional theory calculations. In this section, we will present our results on the process of benzene decomposition to C, H atoms, and C4H2, and we also try to find out the origin of the carbon residue on Pt(100). To the best our knowledge, there is no report on this issue based on the theoretical investigation. The possible pathways are shown in Figure 4. The barriers for the first, second, and third C−H scission of benzene on Pt(100) are calculated as 1.83, 1.20, and 1.70 eV, respectively. Then, the C6 ring is broken, and the barrier for the first C−C scission is 0.99 eV (see TS4). There are three possible pathways starting from the chain C6H3 (A−C6H3). In the first pathway, the carbon atom and C5H3 are the productions with the barrier of 0.88 eV, indicating that carbon atoms could be feasibly formed on the Pt(100) surfaces. Considering the barrier for A−C6H3 transferring to B−C6H3 is less than 0.10 eV, A−C6H3 could isomerize to B−C6H3 easily. As a result, the last two pathways begin from B−C6H3. In the second pathway, the C5−C6 bond is broken first and is closely followed by the C6−H bond dissociation. The corresponding barriers are calculated as 1.44 and 1.49 eV. In the third pathway, 10001



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Figure 5. Possible pathways of cyclohexene dehydrogenation to benzene on 4/9 C Pt(100) (A), 1/9 C 2Pt/Au(100) (B), 2/9 C 2Pt/Au(100) (C), 4/9 C 2Pt/Au(100) (D), 4/9 H Pt(100) (E), 1/3 H Au/Pt(100) (F), 4/9 C 4/9 H Pt(100) (G), and 4/9 C 4/9 H 2Pt/Au(100) (H).

atoms induces not only a larger density of states at a lower energy level but also a smaller density of states at the Fermi level (Ef) compared to the clean surfaces. This could explain the passivity of carbon-deposited surfaces. Besides, the 2 +∞ bandwidth [(∫ +∞ −∞E ρ(E)dE)/(∫ −∞ρ(E)dE)] of clean Pt(100) and 2Pt/Au(100) is 7.75 and 9.79 eV, whereas the bandwidth of 4/9 C Pt(100) and 4/9 C 2Pt/Au(100) is 14.55 and 11.25 eV, indicating the deposited carbon atoms increase the bandwidth and shift the d-band center relevant to the corresponding Ef to lower energy level. The adsorption energy can be decomposed into three terms. The first one is related to the binding interaction to the metal (Einteraction). The second and third one (Edist) reflect the

exothermic on the 4/9 carbon-modified Pt(100) surface when compared to that on the clean Pt(100). Furthermore, different carbon concentrations (1/9, 2/9, and 4/9 ML) were tested to examine the effect of deposited carbon on the 2Pt/Au(100). The adsorption energies on carbon-modified 2Pt/Au(100) surfaces decrease in the order 1/9 > 2/9 > 4/9 ML (see Table 2), and a similar case was found on carbon-modified Mo(100) surfaces.31 The deposit carbon atom changes the electronic structures of the Pt overlayer. Take the 4/9 ML carbon concentration for instance. Figure 5A displays the PDOS plots of the metallic d band for the clean Pt(100) and 2Pt/Au(100) and these two surfaces at 4/9 ML carbon coverage. The inception of carbon 10002



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Table 3. Terms of Energy Decomposition of the Calculated Adsorption Energy (eV) species cyclohexene

metal surface Pt(100) 4/9 C Pt(100) 2Pt/Au(100) 4/9 C 2Pt/Au(100)

cyclohexenyl

Pt(100) 4/9 C Pt(100) 2Pt/Au(100) 4/9 C 2Pt/Au(100)

1,3-cyclohexadiene

Pt(100) 4/9 C Pt(100) 2Pt/Au(100) 4/9 C 2Pt/Au(100)

cyclohexadienyl

Pt(100) 4/9 C Pt(100) 2Pt/Au(100) 4/9 C 2Pt/Au(100)

benzene

Pt(100) 4/9 C Pt(100) 2Pt/Au(100) 4/9 C 2Pt/Au(100)

adsorption site

Eads

Edist(molec)

Edist(surf.)

Einteraction

hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45 hollow hollowR45

−1.01 −1.00 −0.26 −0.46 −1.29 −1.33 −0.28 −0.34 −2.12 −2.37 −1.12 −1.09 −2.72 −2.65 −1.38 −1.13 −2.20 −2.15 −0.27 −0.26 −2.43 −2.42 −0.41 −0.06 −3.27 −3.26 −1.23 −1.07 −3.53 −3.53 −1.00 −1.05 −2.01 −2.02 −0.15 −0.14 −2.43 −2.46 −0.05 −0.03

1.58 0.43 1.51 0.28 1.68 0.44 1.83 0.19 2.00 1.04 1.00 0.73 0.86 0.26 1.54 0.69 3.74 3.73 3.90 3.83 3.98 3.92 3.29 3.52 2.36 2.15 1.64 0.68 2.22 2.32 1.53 1.72 1.34 1.37 0.00 0.00 1.74 1.90 0.00 0.00

0.22 0.35 0.29 0.17 0.15 0.20 0.24 0.16 0.39 0.29 0.33 0.13 0.23 0.24 0.29 0.64 0.24 0.24 0.41 0.37 0.21 0.21 0.32 0.49 0.29 0.29 0.38 0.10 0.23 0.25 0.38 0.31 0.30 0.30 0.01 0.01 0.31 0.29 0.06 0.06

−2.81 −1.78 −2.06 −0.91 −3.12 −1.97 −2.35 −0.69 −4.51 −3.7 −2.45 −1.95 −3.81 −3.15 −3.21 −2.46 −6.18 −6.12 −4.58 −4.46 −6.62 −6.55 −4.02 −4.07 −5.92 −5.7 −3.25 −1.85 −5.98 −6.1 −2.91 −3.08 −3.65 −3.69 −0.16 −0.15 −4.48 −4.65 −0.11 −0.09

decreases when distortion increases.32,33 For example, the HOMO−LUMO gap of hollow R45 adsorbed cyclohexene is 4.33, 4.53, 4.32, and 4.62 eV (see Figure 6 B and C) on the Pt(100), 4/9 C Pt(100), 2Pt/Au(100), and 4/9 C 2Pt/ Au(111) surfaces, respectively, and the corresponding Edist(molec) is 0.43, 0.28, 0.44, and 0.19 eV, respectively. Besides, the distortion of the surface on the carbon modified Pt(100) and 2Pt/Au(100) is more serious than on the clean surfaces under most conditions, when the molecule is chemisorbed. 3.4.3. Dehydrogenation of Cyclohexene on the CarbonModified Pt(100) and 2Pt/Au(100) Surface. The cyclohexene dehydrogenation at the 4/9 coverage of C on the Pt(100) surface had been investigated, and the barrier is 1.13, 1.99, 1.23, and 0.33 eV for the first, second, third, and fourth dehydrogenation steps, respectively (Figure 6A). The second dehydrogenation step turns to be the rds. The barriers of the first three dehydrogenations rise by 0.60−1.30 eV on the 4/9 C Pt surface in comparison with that on clean Pt(100). Conversely, the barrier of the fourth dehydrogenation step

distortion of the molecule and the metal when compared with its gas-phase state.32 These terms take opposing effect on the adsorption energy. The interaction to the metal enhances the stability of the system. In contrast, the distortions of the molecule and metal surface induce a destabilization.33 This can be expressed by an equation Eads = Edist(molec) + Edist(surf.) + E interaction

(1)

These terms are listed for cyclohexene and other dehydrogenation intermediates on the Pt(100), 4/9 C Pt(100), 2Pt/ Au(100), and 4/9 C 2Pt/Au(100) in Table 3. Generally speaking, the absolute value of Eads and Einteraction on the carboncovered Pt(100) and 2Pt/Au(100) is smaller than on the clean Pt(100) and 2Pt/Au(100), implying the deposited carbon atoms weaken the interaction between the molecule and the surface. In fact, Edist(molec) depends on the geometrical distortion and the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of the adsorbates. The HOMO−LUMO gap 10003



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Figure 6. PDOS projected on the surface Pt atom in the case of Pt(100), 4/9 C Pt(100) and 2Pt/Au(111), 4/9 C 2Pt/Au (100). (B) PDOS of the cyclohexene adsorbed in the Hollow R45 configuration on the Pt(100), 4/9 C Pt(100). (C) PDOS of the cyclohexene adsorbed in the Hollow R 45 configuration on the 2Pt/Au(100) and 4/9 C 2Pt/Au(100) surfaces.

bonded to the Pt overlayer, thus it may block the reaction site and increases the dehydrogenation barrier. To examine the influence of different carbon concentrations on the reactivity of the Pt/Au bimetallic surfaces, we have studied the dehydrogenation of cyclohexene at 1/9, 2/9, and 4/ 9 ML carbon coverage on the 2Pt/Au(100) surfaces (Figures 5B, C, D) and find the chemical property of the Pt overlayers is

drops by 0.51 eV, possibly due to the physisorbed benzene production. Besides, we studied the barrier of C 6 H 10 dehydrogenated to C6H9 on the 1/9 C covered Pt(100) surface (see the Figure S6, Supporting Information) and find that the dehydrogenation barrier is higher than that on the clean Pt(100) (0.73 vs 0.60 eV). The carbon atom is firmly 10004



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Table 4. Terms of Energy Decomposition of the Calculated Activation Energy (eV) C6H10 → C6H9 + H (A, C6H9; B, H)

Ea

ΔEsub

ΔEABdef

−EABIS

EATS

EBTS

EATS + EBTS

EA···Bint

Pt(100) 2Pt/Au(100) Pt/Au(100) Au/Pt(100) 2Au/Pt(100) Au(100) 4/9 H Pt(100) 4/9 C Pt(100) 1/3 H Au/Pt(100) 4/9 C 4/9 H Pt(100) 1/9 C 2Pt/Au(100) 2/9 C 2Pt/Au(100) 4/9 C 2Pt/Au(100) 4/9 C 4/9 H 2Pt/Au(100)

0.60 0.37 0.55 1.18 1.46 1.49 0.67 1.13 0.94 1.12 0.45 0.63 1.60 1.02

0.11 0.01 0.02 0.07 0.01 0.15 0.20 0.00 0.07 0.13 0.04 −0.03 0.36 0.46

2.12 1.84 1.99 1.75 2.01 3.96 1.67 4.01 1.81 1.64 0.98 1.60 3.12 1.26

1.00 1.29 1.43 0.52 0.59 0.45 0.76 0.46 0.09 0.07 1.06 0.82 0.34 0.07

−3.28 −3.59 −3.33 −0.86 −0.64 0.20 −0.58 −1.68 −0.79 −1.16 −3.21 −3.09 −2.46 −0.59

−2.78 −2.89 −2.70 −2.11 −1.90 −1.97 −1.87 −2.39 −1.96 −2.27 −2.74 −2.11 −2.42 −1.33

−6.06 −6.48 −6.03 −2.97 −2.54 −1.77 −2.45 −4.07 −2.75 −3.43 −5.95 −5.20 −4.88 −1.92

3.43 3.71 3.14 1.81 1.39 −1.30 0.49 0.73 1.72 2.71 4.32 3.44 2.66 1.15

C6H8 in Figure S7B (Supporting Information), and the dissociated H atoms are near to the C6 ring. In this situation, the barriers for C6H9 dehydrogenated to C6H8 and C6H7 are calculated to be 0.69 and 1.12 eV, respectively, much higher than that on the clean Pt(100) (see Figure 3F; the corresponding barriers are 0.68 and 0.56 eV). We may draw the conclusion that the influence of H atoms depends on its location and concentration. 3.5.2. H-Modified Au/Pt(100) Surface. Here we choose 1/3 ML hydrogen coverage for the Au/Pt(100) surface, considering the Au/Pt(100) surface less active than Pt(100) in the activation of molecular H2 due to its d-band center is far away from the Fermi level (see Figure 2B). The bridge site is the favorite adsorption site for the hydrogen atom, so the three hydrogen atoms are arranged at the bridge site and are far away from the molecule of cyclohexene (see Figure 1F). We calculated the dehydrogenation barrier for cyclohexene on the hydrogen-modified Au/Pt(100) surface (Figure 6F and Figure S4F, Supporting Information). In contrast to the clean Au/ Pt(100) surface, all the dehydrogenation barriers are lower on the H-modified Au/Pt(100) surface, except the barrier of the third dehydrogenation step. The rds moves to the third dehydrogenation step with the barrier of 1.05 eV on the Hmodified Au/Pt(100) surfaces, and this barrier is lower than the barrier of rds on clean Au/Pt(100) (1.18 eV). These preadsorbed hydrogen atoms could accelerate the cyclohexene dehydrogenation on the Au/Pt(100) surface based on the above DFT calculations. 3.6. Coeffect of Surface H and C on Pt(100) and 2Pt/ Au(100). Under the real conditions, both the hydrogen and the carbon graphic could exist on the Pt layer at the same time. Thus, we build the complex model of Pt(100) at 4/9 carbon and 4/9 hydrogen coverage (4/9 C 4/9 H Pt(100) for short) and the model of 2Pt/Au(100) at 4/9 carbon and 4/9 hydrogen coverage (4/9 C 4/9 H 2Pt/Au(100)). The four carbon atoms are still adsorbed at the hollow site. Affected by the coadsorbed carbon atoms, two hydrogen atoms prefer the top site of carbon atoms, and the others prefer the bridge site composed by two Pt atoms (Figure 1G,H). Figure 6G,H and Figure S4G,H (Supporting Information) show the possible pathways and the TS structures. The dehydrogenation barriers for each step on 4/9 H 4/9 C Pt(100) are lower than on 4/9 C Pt(100), and the rds remains the second dehydrogenation step with the barrier of 1.37 eV. A similar case is found on the 4/9 C4/9 H 2Pt/Au(100): all the dehydrogenation barriers on 4/9

sensitive to the coverage of carbons. The barriers for each dehydrogenation step increase with the increasing carbon concentration up to 2/9 C coverage. From 2/9 to 4/9 C coverage, the barriers for the first two dehydrogenation steps still increase, but the barriers for the last two dehydrogenation steps are reduced. Moreover, the rds changes from the fourth dehydrogenation step at 1/9 carbon coverage, to the third step at 2/9 carbon coverage, and to the second step at 4/9 carbon coverage on 2Pt/Au(100). The barriers for the rds are 0.85, 1.35, and 1.66 eV on 2Pt/Au(100) at the carbon coverage of 1/ 9, 2/9, and 4/9. Thus, the high carbon concentration seriously inhibits cyclohexene dehydrogenation to benzene on the Pt catalyst. 3.5. Effect of Surface H Atom on Pt(100) and Au/ Pt(100). 3.5.1. H-Modified Pt(100) Surface. The preadsorbed H atoms influenced not only the adsorption of hydrocarbons,29 but also some hydrogenation/dehydrogenation reactions.34,35 There was an initial hydrogen particle pressure of 1 × 10−6 Torr in the cyclohexene dehydrogenation experiments.2 The hydrogen atoms must be preadsorbed on the Pt(100),35 so we built the model of the Pt(100) with the hydrogen coverage of 4/9 (shown in Figure 1E). The four hydrogen atoms are artificially located at the bridge site in this 4/9 H Pt(100) model. The possible pathways are shown in Figure 6E, and the corresponding TS structures are exhibited in Figure S4E (Supporting Information). By comparison with the clean Pt(100) surface, all the dehydrogenation barriers are reduced on the H-modified Pt(100) surface, except the first dehydrogenation step. The rds is the fourth dehydrogenation step with the barrier of 0.69 eV on the H-modified Pt(100) surface. This barrier is lower than that on the clean Pt(100) surface (0.84 eV). In addition, we find that the barrier of C6H10 dehydrogenated to C6H9 on the 1/9 H Pt(100) surface is lower than that on clean Pt(100) (0.54 vs 0.60 eV, see Figure S7A, Supporting Information). Thus, these artificially fixed four H atoms could promote the dehydrogenation, and it seems the coverage of H has a close relationship with the dehydrogenation steps that are accelerated. However, the H atom could also inhibit the dehydrogenation, when the H atoms are located too near to the C6 ring because they may block the active sites. For example, Figure S7B (Supporting Information) shows the reaction mechanism of C6H9 dehydrogenation to C6H7 on Pt(100) on the assumption that the H atoms produced by dehydrogenation are adsorbed on Pt(100). There is one H atom coadsorbed with C6H9 and two H atoms coadsorbed with 10005



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C 4/9 H 2Pt/Au(100) are lower than on 4/9 C 2Pt/Au(100), except the barrier of the fourth dehydrogenation step; the rds is still the second dehydrogenation step with the barrier of 1.08 eV. Thus, the 2Pt/Au(100) is still more active than the Pt(100) in catalyzing cyclohexene dehydrogenation under the coeffect of carbon and hydrogen. 3.7. Energy Decomposition of the Reaction Barrier. By decomposing the calculated barrier (Eact) with the following formula, we may find some clues about which factors determine the barrier Ea = ΔEsub + ΔEABdef − EAB IS + EA TS + E B TS + + EA···B int

(2)

where ΔEsub = EsubTS − EsubIS and reflects the influence of the structural change of the substrate from the initial state (IS) to the TS on the activation energy. ΔEABdef = EA···Bgas − EABgas and is called the deformation energy, which shows the effect of the structural deformation of AB on the barrier. EABIS is the adsorption energy of AB in the IS configuration. The EATS and EBTS indicate the binding energies of A (without B) and B (without A) with the surface in the TS structure and are calculated as EA(B)TS = EA(B)/M − EA(B) − EM, where subscripts A(B), M, and A(B)/M signify adsorbate, substrate, and the system of A(B) adsorbed on M at the TS, respectively. The sum of EATS and EBTS is determined by the local electronic effect of metals. The EA···Bint reflects the interaction between A and B in the TS configuration, and it contains bonding competition induced by A and B sharing bonding with the same surface atom36,37 and the direct Pauli repulsion between A and B. Thus, EA···Bint is a quantitative measure of the geometrical effect on catalytic reactions. EATS + EBTS and EA···Bint are closely related to the TS structure.38 Each term of the contributions to the barrier is displayed in Table 4. A denotes C6H9, and B denotes H for the first C−H bond breaking. As can be seen: (i) All the ΔEsub values are positive (except the ΔEsub on the 2/9 C 2Pt/Au(100)), which means a rise for the activation barrier. (ii) The ΔEABdef contributes to the increase of the activation energy. It is 3.96 and 4.01 eV on clean Au(100) and 4/9 C Pt(100) surface. Besides, both ΔEABdef and Eact share the same tread on carbonmodified 2Pt/Au(100) surfaces, 4/9 C > 2/9 C >1/9 C, indicating the insert of carbon atoms raises the C−H dissociation barrier by increasing the deformation energy of C6H10. (iii) The absolute value of the local electronic effect (EATS + EBTS) is usually much larger than the other terms, and there is a good linear relationship between the local electronic effect (EATS + EBTS) and the Ea on the clean nAu/Pt(100) and nPt/Au(100) (n = 0, 1, 2) surfaces. Figure 7 shows that Ea is reduced with the increasing absolute value of the local electronic effect, suggesting the local electronic effect plays an important role in reducing the barrier. Furthermore, the surface hydrogen and carbon atoms induce a sharp diminishing of the absolute value of local electronic effect. For instance, the absolute value of (EATS + EBTS) on the 4/9 H Pt(100), 4/9 C Pt(100), and 4/9 C 4/9 H Pt(100) surfaces is reduced by 3.61, 1.99, and 2.63 eV, compared to that on the clean Pt(100). (iv) According to eq 2, the value of the geometric effect (EA···Bint) was calculated. No matter on the clean surfaces or the modified surfaces, the value of geometric effect is less than the absolute value of the local electronic effect, thus the electronic effect is the largest term and may play the dominant role in determining the barrier.

Figure 7. Relationship between the EATS + EBTS (eV) and the activation energy Ea (eV).

3.8. Microkinetic Modeling. Experiment results2 indicated that the rate of benzene formation on two overlayers of Pt deposited on Au(100) (i.e., 2Pt/Au(100)) was TOF = 8.50 × 10−4 molecules·site−1·s−1, and the rate on the one overlayer Au deposited on Pt(100) (i.e., Au/Pt(100)) was TOF = 10.80 × 10−4 molecules·site−1·s−1. To compare to the experimental results more directly, the microkinetic model has been developed to investigate the activity and selectivity of benzene formation on different model catalysts including Au/Pt(100), 2Pt/Au(100), Pt(100), and the related carbon/hydrogen modified model catalysts. The microkinetic modeling details are in the Supporting Information. Here we first give the simulation results based on the clean surface model, i.e., exclude the effect of carbon and hydrogen atom on catalytic surface. Figure 8 shows the temperature dependence of the rate of gas benzene on Pt(100) and Au/Pt(100). The rate for gas benzene on Au/Pt(100) is much faster than that on 2Pt/Au(100) from 375 to 550 K. The benzene formation rate on Au/Pt(100) decreases with increasing temperature, but the trend is opposite on the 2Pt/Au(100). The rate for gas benzene is estimated as TOF = 1.83 × 10−4 molecules·site−1·s−1 on Au/Pt(100) at 375 K under typical experimental conditions (PC6H10 = 8.0 × 10−6 Pa; PH2 = 1.3 × 10−4 Pa). However, the rate for gas benzene on 2Pt/Au(100) is nearly zero, which does not coincide with the experiment results,2 suggesting the clean 2Pt/Au(100) and Au/ Pt(100) models can not reflect the actual condition of the Pt catalyst. So we chose the modified models in the following. We have also roughly estimated the rate for gas benzene on the carbon- and hydrogen-modified surfaces. The benzene forming rate on the three modified surfaces decreases with the increase of temperature (see Figure 8B). The coverage of an empty site is 0.50 ML on 4/9 C 4/9 H Pt(100), 0.49 ML on 4/ 9 C 4/9 H 2Pt/Au(100), and 0.49 ML on 1/3 H Au/Pt(100), and the rate for gas benzene is 3.76 × 10−7 molecules·site−1·s−1 on 4/9 C 4/9 H Pt(100), 4.02 × 10−3 molecules·site−1·s−1 on 4/9 C 4/9 H 2Pt/Au(100), and 1.39 × 10−2 molecules·site −1 ·s −1 on 1/3 H Au/Pt(100), under the typical experimental conditions (PC6H10 = 8.0 × 10−6 Pa; PH2 = 1.3 × 10−4 Pa). The simulated results are generally in agreement with the experimental observations,2 and the 2Pt/Au(100) and 10006



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Figure 8. Temperature dependence of the rate of gas benzene produced by the cyclohexene dehydrogenation on 2Pt/Au(100), Au/Pt(100), and the modified surfaces using the microkinetic modeling technique.

Figure 9. (A) Influence of the adsorption energy of cyclohexene (C6H10 (gas) + * → C6H10*) on the rate of gas benzene. The absolute value of benzene adsorption energy is taken as the barrier of benzene desorption approximately. The adsorption energy of cyclohexene is the only variable factor. (B) The effect of variations of the difference in the barrier of benzene desorption (C6H6* → C6H6 (gas) + *) and the barrier of benzene dehydrogenation (C6H6* + * → C6H5* + H*) on the rate of gas benzene on Au/Pt(100) surfaces.

case that the difference in these two barriers is more than 0.40 eV. Thus, a good catalyst should be the case with the high binding energy of cyclohexene and relative low binding energy of benzene.

Au/Pt(100) surfaces exhibit similar reactivity for catalyzing cyclohexene dehydrogenation to benzene. The simulation results are very sensitive to two factors: one is the adsorption energy of cyclohexene, and the other is the difference in the barrier of benzene desorption and the barrier of benzene dehydrogenation. Take the case of Au/Pt(100) as an example. The benzene formation rate is diminished sharply when the adsorption energy of cyclohexene becomes more and more endothermic (see Figure 9A). In Figure 9B, the X-axis means the difference in the barrier of benzene desorption and the barrier of benzene dehydrogenation (E desorption − Edehydrogenation), so the origin of the X-axis means the value of the benzene desorption barrier is equal to that of the benzene dehydrogenation barrier. The benzene desorption barrier is artificially changed, whereas the dehydrogenation barrier is fixed. It is notable that the benzene forming rate decreases obviously as the desorption barrier is close to the dehydrogenation barrier. In reverse, the rate nearly remains the same in the

4. CONCLUSIONS Adsorption of cyclohexene and its dehydrogenation to benzene on nAu/Pt(100) and nPt/Au(100) (n = 0, 1, 2) surfaces have been studied systematically by the DFT-GGA slab calculation. The orders of the adsorption energies are determined, Pt/ Au(100) > 2Pt/Au(100) > Pt(100) > Au/Pt(100) > 2Au/ Pt(100) > Au(100), from the calculation, and this order is consistent with the position of the d-band center of these surfaces. On clean Pt(100), Pt/Au(100), and 2Pt/Au(100) surfaces, the rate-determining step is the fourth dehydrogenation step, i.e., C6H7 → C6H6 + H, and the barriers for rds are 0.84, 0.84, and 0.64 eV, respectively. However, on the clean Au/Pt(100), 10007



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2Au/Pt(100), and Au(100) surfaces, the rds is the first dehydrogenation step (C6H10 → C6H9 + H) with the barrier of 1.18, 1.46, and 1.49 eV. So, the 2Pt/Au(100) shows the highest reactivity for promoting cyclohexene dehydrogenation to benzene on the clean surfaces. The rds shifts from the fourth step to the second step and the corresponding barriers are increased to 1.99 eV on 4/9 C Pt(100) and 1.66 eV on 4/9 C 2Pt/Au(100), implying carbon atoms inhibit cyclohexene dehydrogenation. Furthermore, the per-covered hydrogen atom is taken into consideration, and the rds of cyclohexene dehydrogenation is the second step on both the Pt(100) and 2Pt/Au(100) surfaces at 4/9 hydrogen and 4/ 9 carbon. Compared to the only carbon-modified Pt(100) and 2Pt/Au(100), the coeffect of precovered hydrogen and carbon accelerates cyclohexene dehydrogenation because the barriers for rds are lower on the surfaces modified by H and C (1.37 and 1.08 eV). The 2Pt/Au(100) surface remains highly reactive for catalyzing cyclohexene dehydrogenation, under the carbon and hydrogen preadsorbed conditions. Besides, the hydrogen atom modified Au/Pt(100) surface exhibits the highest reactivity toward benzene formation among all the investigated model catalysts based on both the DFT calculation and the microkinetic model analysis. We thus suggest that the 2Pt/ Au(100) and Au/Pt(100) surfaces may be a good choice for catalyzing cyclohexene dehydrogenation to benzene.

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ASSOCIATED CONTENT

S Supporting Information *

Full description of the material. 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]. Telephone: +86-2223503824 (O). Fax: +86-22-23502458. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants No. 20273034, 20673063). The work was carried out at National Supercomputer Center in Tianjin, and the calculations were performed on TianHe-1 (A).

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REFERENCES

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