Adsorption Equilibria of CO Coverage on Î²-Mo2C Surfaces - The

Article pubs.acs.org/JPCC

Adsorption Equilibria of CO Coverage on β-Mo2C Surfaces Tao Wang,† Shengguang Wang,‡ Yong-Wang Li,†,‡ Jianguo Wang,† and Haijun Jiao*,†,§ †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, PR China ‡ Synfuels China Co. Ltd., Huairou, Beijing 101407, China § Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein Strasse 29a, 18059 Rostock, Germany S Supporting Information *

ABSTRACT: Adsorption and surface coverage of CO on the (001), (101), and (201) surfaces of β-Mo2C were computed at the level of density functional theory under the consideration of the temperature and CO partial pressure by using the ab initio atomistic thermodynamic method. On the basis of the computed Gibbs free energies, the relationship between CO coverage on the surfaces and temperature as well as CO partial pressure has been established, and excellent agreements have been found between the predicated CO desorption temperatures and the experimentally recorded temperature programmed desorption (TPD) spectra. These computed phase diagrams show that a stable CO coverage can be obtained within a range of temperature and partial pressure; different surfaces can have different coverage at the same conditions, and different partial pressure has a different desorption temperature. In addition, these phase diagrams provide useful information for adjusting the balance between temperature and CO partial pressure for a stable CO coverage and for identifying the active surface and the initial states under given conditions. These results should also be very interesting for surface science under ultra high vacuum conditions.

■

INTRODUCTION Transition metal carbides are known as refractory materials with high hardness and melting point as well as good thermal stability.1 They also have attracted great interest as promising catalysts with platinum-like behaviors reported by Levy and Boudart.2 As one of the important transition metal carbides, molybdenum carbide (Mo2C) has been widely studied both in experiment and theory. Mo2C mainly has two crystalline structures, i.e., the orthorhombic phase (α-Mo2C) and the hexagonal phase (β-Mo2C). The α-Mo2C phase has a structure with Mo atoms slightly distorted from their positions in close-packed planes and carbon atoms orderly occupying one-half of the octahedral interstitial sites.3 In β-Mo2C, Mo atoms form a hexagonally close packed structure with carbon atoms randomly filling half of the octahedral interstitial sites.4 Because of its noble metal-like catalytic activities, Mo2C has been considered as a potential substitution to noble metals in many catalytic reactions.5 The activity of the water−gas shift (WGS) reaction6,7 and alcohol synthesis from CO hydrogenation8 on Mo2C has been widely investigated experimentally. Compared with the great experimental efforts, less theoretical reports for this system are known. Liu et al.9 studied the interaction of sulfur-containing molecules with α-Mo2C and the role of carbon in desulfurization using density functional theory (DFT) calculations and found that carbon atoms in Mo2C play a key role in the dissociation of S-containing molecules. The hydrogenolysis mechanism of thiophene10 and indole11 on clean β-Mo2C also was systematically investigated on the basis © 2012 American Chemical Society

of DFT calculations. The chemisorption and decomposition of small molecules12 such as nitrogenous compounds, aromatic hydrocarbons, and CO2 were examined on both α-Mo2C and β-Mo2C phases. Theoretical studies of the chemical properties of methanol,13 methyl iodide,14 and the promoting effect of potassium on β-Mo2C3 (α phase in this paper) were systematically reported by Pistonesi et al. based on their surface experiments, and they found that the incorporation of potassium atoms enhances the dissociation ability of the C−I and C−O bonds. Tominaga and Nagai15 built a schematic potential energy surface for the WGS reaction and concluded that the ratelimiting step is CO2 formation from CO oxidation by surface O. Liu et al.16 also calculated the WGS mechanism and emphasized the importance of oxygen on the Mo2C surface. To study the intrinsic WGS activities of Mo2C, Schweitzer et al. loaded Pt on Mo2C17 and found Mo2C to play the role of both support and catalyst. Shi et al.18 and Han et al.19 calculated the surface energies of low miller index surfaces of β-Mo2C to compare their stabilities and concluded that the (011) facet was the most stable surface. The calculated surface free energies of both low and high miller index surfaces of β-Mo2C using the atomistic thermodynamics method under the consideration of the reaction conditions showed that the (101) surface is the dominant surface,20 in agreement with the experiments.21 Received: January 13, 2012 Revised: February 21, 2012 Published: February 21, 2012 6340

dx.doi.org/10.1021/jp300422g | J. Phys. Chem. C 2012, 116, 6340−6348

The Journal of Physical Chemistry C

Article

Within the scope of DFT calculations, however, few studies discussed the stable coverage of simple molecules (like CO) on Mo2C surfaces. Since adsorption of molecules on the catalyst surfaces is usually the first and the most important step in the catalytic process, it is necessary to know the stable coverage of molecules on the surfaces. Here we performed an atomistic thermodynamics study on the stable CO coverage on the (001), (101), and (201) surfaces of β-Mo2C. Our goal is the understanding into the starting point for the Mo2C-catalyzed reactions, i.e., Fischer−Tropsch synthesis, alcohol synthesis, and WGS reaction.

■

COMPUTATIONAL MODEL AND METHOD (a). Model. In this paper, we used the hexagonal β-Mo2C phase with an eclipsed configuration as the unit cell, which is the same as our previous work.20 The calculated DFT lattice parameters for the β-Mo2C bulk are a = 6.075 Å, b = 6.069 Å, and c = 4.722 Å, in good agreement with the experiment (a = b = 2 × 3.011 Å and c = 4.771 Å).22 The surfaces of (001), (101), and (201) were modeled by periodic slabs with p(2 × 2), p(2 × 1), and p(2 × 2) unit cells, respectively. The vacuum layer between periodically repeated slabs was set as 10 Å to avoid interactions between slabs. (b). Method. All calculations were done using the planewave periodic DFT method implemented in the Vienna ab initio simulation package (VASP).23 The electron ion interaction was described with the projector augmented wave (PAW) method.24 The electron exchange correlation energy was treated within the generalized gradient approximation in the Perdew−Burke−Ernzerhof formalism (GGA-PBE).25 To ensure accurate energies with errors due to smearing of less than 1 meV per unit cell, a cutoff energy of 400 eV and the Gaussian electron smearing method with σ = 0.05 eV were used. For bulk optimization, the lattice parameters of β-Mo2C were obtained by minimizing the total energy of the unit cell using a conjugated-gradient algorithm to relax the ions, and a 5 × 5 × 5 Monkhorst−Pack k-point grid was used for sampling the Brillouin zone. The geometry optimization was done when the convergence criterion on forces becomes smaller than 0.02 eV/Å and the energy difference is lower than 10−4 eV. Adsorption energy (Eads) is calculated by subtracting the energy of gaseous CO and the clean surface from the total energy of the adsorbed system as shown in the equation Eads = E(adsorbate/slab) − E(adsorbate) − E(slab), in which the more negative the Eads, the more stable and stronger the adsorption. (c). Thermodynamics. As a convenient tool to solve problems referring to real reaction conditions, atomistic thermodynamics methods, proposed by Scheffler and Reuter,26 have been widely and successfully applied in many systems.27 In this method, the surface free energy (γ) of a surface can be described as in eq 1, in which G is the Gibbs free energy of a solid surface; A is the total surface area of two equilibrium surfaces (top and bottom sides); μi(T, p) is the chemical potential of the species i; and ni is the number of the ith type species. 1 γ(T , p) = [G − A

Figure 1. Possible CO adsorption sites on (001), (101), and (201) (h for hollow, b for bridge, and t for top).

ads {ngas }) is the Gibbs free energy of the (hkl) with n adsorbed gas molecules and μi(T, p) is the chemical potential of all the species in the system (including the adsorbed gas phase molecules).

γads hkl (T , p , ni) =

∑ niμi(T , p)] i (2)

For Mo2C surfaces with n CO adsorption, eq 2 can be rewritten as in eq 3. γads Mo2C(T , p , nCO) 1 ads = [GMo (T , p , {nCO}) − nMoμMo(T , p) 2C A − nCμC(T , p) − nCOμCO(T , p)]

(3)

As discussed in our previous work,20 the surface free energy of a clean surface is given in eq 4

∑ niμi(T , p)] i

1 ads ads [G (T , p , {ngas }) − A hkl

(1)

γclean Mo C(T , p) =

For describing the adsorption of gas molecules on surfaces, we define the surface free energy of a surface ads with ni gas molecules adsorption as in eq 2, where Ghkl (T, p,

2

1 clean [GMo2C(T , p) − nMoμMo(T , p) A − nCμC(T , p)]

6341

(4)



Article

Figure 2. Stepwise CO adsorption structures and energies (ΔEads) on (001) (surface Mo/blue; in CO, C/black; O/red).

The surface free energy of (hkl) surface with n gaseous molecules by inserting 4 to 3 is given in eq 5.

The change of Gibbs free energy for this adsorption, ΔGhkl(T, p, nCO), can be found in eq 6 (hkl) ΔGMo C(T , p , nCO)

γads Mo2C(T , p , nCO)

2

= G[Mo2C(hkl)/{nCO}] − G[Mo2C(hkl)]

1 ads = γclean Mo2C(T , p) + A [GMo2C(T , p , {nCO}) clean (T , p) − n μ (T , p)] − GMo CO CO 2C

− Ggas(CO)

(6)

In this formula, G[Mo2C(hkl)/{nCO}] is the Gibbs free energy of the Mo2C surface with n CO molecules, while G[Mo2C(hkl)] is the Gibbs free energy of the clean Mo2C surface. Compared with the great contribution of vibration to the gases, this contribution to the surfaces is negligible on the basis of our calculations. Therefore, we apply the DFT

(5)

Considering the adsorption process of CO on the Mo2C surface as Mo2C(hkl) + nCO ↔ Mo2C(hkl)/{nCO} 6342



Article

energy, ΔEads = E[(CO)n+1/slab] − E[(CO)n/slab] − E[CO], where a positive ΔEads indicates the saturated adsorption with n CO molecules. Figure 2 shows the most stable adsorption sites for stepwise CO adsorption and also the stepwise adsorption energies (ΔEads) on (001). It shows clearly that the adsorption configurations change upon an increase of the CO coverage, and different adsorption configurations within a given coverage are possible. At very low coverage, for example, the CO adsorbs at the 3-fold hollow site with the carbon atom interacting with three surface Mo atoms and the oxygen atom with two surface Mo atoms. With coverage increase, bridging adsorption configurations become possible, where the carbon atom interacts with two surface Mo atoms and the oxygen atom interacts with two or one surface Mo atom. At very high coverage, CO atop adsorption becomes possible. Similar changes in adsorption configurations are also found in the cases of (101) and (201) surfaces. Those changes in CO adsorption configurations upon coverage increase are due to the repulsive lateral interaction among the adsorbed CO molecules. At very low coverage, such interaction is weak and negligible, and at high or saturated coverage, such interaction becomes stronger and significant. As a result the adsorbed CO molecules move from the hollow sites to the top sites to lower the repulsive lateral interaction. For n = 1−4, the ΔEads are very close, indicating that there are no lateral repulsive interactions between the adsorbed CO molecules, and a high CO coverage is therefore possible at the first adsorption stage. For n > 5, the ΔEads decreases steadily, and the saturated adsorption has sixteen CO molecules on the surface since positive ΔEads is found for n = 17. On (101) (Figure 3), the ΔEads decreases steadily; the saturated

calculated total energy (Etotal) to substitute the Gibbs free energies of clean surfaces, and eq 6 can be rewritten as (hkl) ΔGMo C(T , p , nCO) 2

= E[Mo2C(hkl)/{nCO}] − E[Mo2C(hkl)] − Ggas(CO)

where E[Mo2C(hkl)/{nCO}] and E[Mo2C(hkl)] are the total energies of the corresponding systems. The Ggas(CO) term is equal to nμCO(T, p), and the chemical potential of CO can be described as total + μ̃ (T , p0 ) + k T ln pCO μCO(T , p) = ECO B CO p0 At 0 K, the chemical potential of CO can be regarded as the total energy of the isolated CO molecule (including zero point energy) which can be calculated directly with VASP. The μ̃ CO(T, p0) term includes the contribution from vibration and rotation of CO molecules, and the detailed description of the association of entropic terms with soft vibrations can be found in our previous work.20 It can be calculated, but in this paper we used the experimental values from thermodynamic tables.28 The last term of this formula is the contribution of temperature and CO partial pressure to the CO chemical potential. Finally, the change in the Gibbs free energy of the Mo2C surfaces after the adsorption of n CO molecules can be expressed as in eq 7. (hkl) ΔGMo C(T , p , nCO) 2

total = E[Mo2C(hkl)/{nCO}] − E[Mo2C(hkl)] − nECO p − nμ̃CO(T , p0 ) − nkBT ln CO p0 (7)

In this respect, we can plot ΔGhkl(T, p, nCO) as a function of T and pCO. The system (surface with n CO adsorption) with the lowest value of ΔGhkl(T, p, nCO) will be most stable under the given condition, and this also provides information about the CO equilibrium coverage on the Mo2C surface under fixed condition. Finally, the ΔGhkl(T, p, nCO) part is equal to the second part of eq 5, and we also can get the value of surface free energy of a surface with n CO adsorption under fixed condition by adding the contribution of CO adsorption.

■

RESULTS AND DISCUSSION (I). Stable Adsorption at Different Coverage. For studying CO adsorption, the three most stable surfaces from both theory20 and experiment21 (001) with Mo termination and (101) as well as (201) with mixed Mo/C terminations were used. As shown in Figure 1, there are nine nonequal (three top sites, three bridge sites, and three hollow sites) possible adsorption sites on (001), ten nonequal sites (four top sites and six bridge sites) on (101), and seventeen nonequal sites (five top sites, seven bridge sites, and five hollow sites) on (201). The structures and energies of one CO adsorption on these surfaces are given in the Supporting Information. For stepwise CO adsorption, it is necessary to find the most stable adsorption sites at individual coverage; i.e., one additional CO was added to the previously most stable one to get the next most stable one by considering all possibilities. For getting the saturated monolayer coverage, we used the stepwise adsorption

Figure 3. Stepwise CO adsorption structures and energies (ΔEads) on (101) (surface Mo/blue; surface C/gray; in CO, C/black; O/red). 6343



Article

Figure 4. Stepwise CO adsorption structures and energies (ΔEads) on (201) (surface Mo/blue; surface C/gray; in CO, C/black; O/red).

adsorption reaches at n = 9; and positive ΔEads is found for n = 10. On (201) (Figure 4), the ΔEads also decreases steadily, and the saturated adsorption is found for n = 16. In contrast to an expected steady decrease of the ΔEads, some disorders have been observed, and this might be associated with the change of adsorption configuration upon further CO adsorption. For example, the change of ΔEads from n = 15 to 16 is 0.11 eV on (001); there are two 3-fold coordinations for n = 16, while none is found for n = 15. Similar phenomena also have been reported.27e Considering the first few CO adsorptions at lower coverage, it is found that the (001) and (201) surfaces have much stronger adsorption energies than (101), and (001) and (201) surfaces can have higher CO saturated coverage than (101). This is also in line with the surface stability in the order of (101) > (201) > (001);20 i.e., the more stable the surface, the weaker the adsorption and the lower the coverage.

(II). CO Coverage and Temperature. As referred to in eq 6, ΔG can be used as a criterion to test the stability of a system with n CO adsorptions under different conditions, and a more negative ΔG indicated a more stable structure. On the basis of the calculated total energies of different stable structures and chemical potentials of CO under different conditions, we plotted ΔG as a function of T and pCO (Figure 5), respectively. As shown in Figure 5a to 5c, there are some cross points among those lines, indicating the coverage changes, and the corresponding temperatures present the CO desorption temperatures. This can model the temperature-programmed desorption (TPD) spectra of the adsorbed CO molecules. At first, we fixed the pressure of CO gas at one atmosphere and discussed the dependence of ΔG on temperature. Since these surfaces have different surface areas, not only the coverage (θ, nCO/nm−2) but also the number of adsorbed CO molecules (in parentheses) are given, and a larger θ indicated a higher CO coverage on the surface. On the left side in Figure 5, it shows that the ΔG decreases as the temperature increases. 6344



Article

Figure 5. Dependence of surface Gibbs free energy (ΔG; left side) and CO coverage (θ, nCO/nm−2 and −dθ/dT; right side) on temperature at pCO/p0 = 1 atm.

loss of the last four CO molecules to become a clean surface at T = 1250 K. On (101), there are four desorption peaks: the first one at T = 180 K for the loss of one CO molecule from n = 9 to 8, the second one at T = 500 K for the loss of two CO molecules from n = 8 to 6, the third one at T = 650 K for the loss of two CO molecules from n = 6 to 4, and the last one at T = 810 K for the loss of the last four CO molecules to become a clean surface. On (201), there are five desorption peaks: the first one at T = 350 K for the loss of four CO molecules from n = 16 to 12, the second one at T = 800 K for the loss of eight CO molecules from n = 12 to 4, the third one at about T = 920 K for the loss of one CO molecule, the fourth one at about 1150 K for the loss of one CO molecule, and the last one at T = 1350 K for the loss of the last two CO molecules to become a clean surface.

The saturated monolayer coverage on (001), (101), and (201) surfaces is 10.85/nm −2 (16 CO), 9.64/nm−2 (8 CO), and 11.74/nm−2 (10 CO), respectively. On the right side in Figure 5, it shows that the CO coverage decreases as the temperature increases; most interestingly, there are several peaks on each surface, and each peak has the coverage changes for more than one CO molecule. This indicates that at a given temperature there is more than one adsorbed CO molecule with close energies, and some coverage can exist in equilibrium. For example, the (001) surface has five desorption peaks: the first peak shows the loss of four CO molecules from n = 16 to 12 at T = 450 K; the second peak shows the loss of four CO molecules from n = 12 to 8 at T = 650 K; the third peak shows the loss of two CO molecules from n = 8 to 6 at T = 820 K; the fourth peak shows the loss of two CO molecules from n = 6 to 4 at T = 1100 K; and finally the 6345



Article

Figure 6. Relationship between surface Gibbs free energy (ΔG) and CO partial pressure ln(pCO/p0) at different CO coverage (nCO/nm−2) at 600 K.

Figure 7. Equilibrium phase diagrams of stable CO coverage (θ, nCO/ nm−2, n in parentheses) as a function of temperature and CO partial pressure (ln(pCO/p0).

At this stage, it is interesting to compare the computed desorption peaks with those from experiments for CO adsorptions on β-Mo2C surfaces.29 Since (101) is most stable, followed by (201) and (001), the first CO adsorption energy is the lowest on (101), followed by on (201) and (001). Therefore, it is expected that the first peak at 180 K should belong to the desorption on (101) at the lowest temperature, and the second peak at 350 K to the desorption on (201) at the second lowest temperature, and the third peak at 450 K to the desorption on (001) at the third lowest temperature. Indeed, our results agree very well with the experimental data, 150−175 K for the first desorption peak, 325∼360 K for the second desorption peak, and 450 K for the third desorption peak. However, it might not be so easy to discuss desorption at higher temperature because of the possible overlaps of the peaks on one hand and also the surface reactions on the other hand; i.e., desorption of the recombined CO from the adsorbed C and O on the surface. This is also why our calculated

temperature dependence of the equilibrium CO coverage can not be rigorously identified with the TPD results. Nevertheless, those agreements at low temperatures can provide some useful information for predicting CO desorption properties on the catalyst surfaces. (III). CO Coverage and Partial Pressure. Apart from temperature, CO partial pressure (pCO) also plays an important role on CO coverage because it respects the CO equilibrium concentration in the gas phase. Since the working temperature of alcohol synthesis from CO and H2 is at around 600 K,30 we took this for considering the effect of CO partial pressure on the coverage. Figure 6 shows the plot of ΔG against ln(pCO/p0) at 600 K in a linear relationship. The more negative the ΔG, the more stable the adsorbed system (a surface with certain coverage of CO molecules); increasing CO partial pressure will stabilize the adsorbed system and increases the coverage. 6346



■

Article

CONCLUSION As an important elementary step in many industrial catalytic reactions, CO equilibrium adsorption on the (001), (101), and (201) surface of β-Mo2C has been computed at the density functional theory (DFT) level under the consideration of the temperature and CO partial pressure by using an ab initio atomistic thermodynamics method. It is found that the most stable (101) surface has lower saturated monolayer adsorption (9.64/nm−2; 8 CO) than the less stable (001) (10.85/nm−2; 16 CO) and (201) (11.74/nm−2; 10 CO) surfaces on the calculated stepwise CO adsorption energies. The calculated surface Gibbs free energies as a function of temperatures as well as CO partial pressure reveal the stability of the CO adsorbed surfaces. At a given CO partial pressure, raising temperature will lower the CO coverage, while at a given temperature, raising CO partial pressure will increase the CO coverage. On the basis of the computed Gibbs free energies, the predicated CO desorption temperatures are in excellent agreement with the experimentally measured temperatureprogrammed desorption (TPD) spectra, i.e., the first desorption at 150−175 K for CO desorption on the (101) surface, the second desorption at 325−360 K for CO desorption on the (201) surface, and the third desorption at 450 K for CO desorption on the (001) surface. These excellent agreements between theory and experiment in desorption temperatures validate our methods and models reasonably. In addition, it also worth mentioning that only the positions of desorption temperatures rather than desorption intensities have been estimated. These computed phase diagrams show that a stable CO coverage can be obtained within a range of temperature and partial pressure; different surfaces can have different coverage at the same conditions, and different partial pressure has different desorption temperature. These equilibrium phase diagrams also show the transition between distinguishable coverage in equilibrium. Hence, it is possible to adjust the balance between temperature and CO partial pressure for practical uses and to identify the initial states and the active sites under given conditions, as well as to use the different coverage for different surfaces under different conditions when considering chemical reactions. In addition, the regions in the equilibrium phase diagrams at very low temperature and CO partial might provide useful information for surface science at ultra high vacuum conditions.

As shown in Figure 6, the stable coverage of CO on these three surfaces increases with the increase of CO partial pressure, and (001) and (201) surfaces can form more stable CO coverage than the (101) surface at very low CO partial pressure. At 600 K and pCO = 40 atm (ln(pCO/p0) = 3.7), for example, the stable CO coverage is about 8.16/nm−2 (12 CO) on the (001) surface (Figure 6a), 8.57/nm−2 (8 CO) on the (101) surface (Figure 6b), and 8.80/nm−2 (12 CO) on the (201) surface (Figure 6c). This indicates that under the typical experimental conditions these surfaces do not have the saturated CO adsorption. (IV). Equilibrium Phase Diagram of Stable CO Coverage. In the last two parts, we discussed the stable CO coverage on β-Mo2C surfaces as a function of either temperature or CO partial pressure. However, experimentally it is possible and also necessary to adjust both parameters at one time for practical goals. Therefore, we build the equilibrium phase diagram for stable CO coverage at given ranges of temperature and CO partial pressure on (001), (101), and (201). As shown in Figure 7, each phase diagram has several regions, and each region represents the possibility for getting stable CO coverage within the range of both temperature and partial pressure. Such equilibrium phase diagrams have the advantage over the proposed desorption peaks in Figure 5, and this is because the proposed desorption temperature is a function of partial pressure; i.e., different partial pressure will have different desorption temperature. Moreover, such equilibrium phase diagrams are more informative than Figure 6, and this is because they show the interface or transition between two distinguishable coverages, i.e., the coexistence or equilibrium of two coverages at a given temperature and partial pressure. These phase diagrams are very informative for practical uses, e.g., for getting a reasonable initial state of the reactant on the surface; i.e., a stable CO coverage can be obtained either at a given partial CO pressure under the change of the temperature or at a given temperature under the variation of the partial pressure. On the (001) surface, the saturated adsorption has the temperature range up to 500 K at pCO = 40 atm but up to 400 K at pCO = 0.01 atm. On the (101) surface, the saturated adsorption has the temperature range up to 350 K at pCO = 40 atm but up to 100 K at pCO = 0.01 atm. On the (201) surface, the saturated adsorption has the temperature range up to 400 K at pCO = 40 atm but up to 150 K at pCO = 0.01 atm. At 600 K, the stable CO coverage (8.62/nm−2, 12 CO) can be obtained in the range of p = 40−0.1 atm on (001); the CO stable (8.57/nm−2, 8 CO) can be obtained at the range of p = 40−10 atm on (101); and the CO stable (8.80/nm−2, 12 CO) can be obtained at the range of p = 40−0.05 atm on (201). These phase diagrams show clearly that a stable CO coverage can be obtained within a range of temperature and partial pressure, and different surfaces can have different coverage at the same conditions. They also show that desorption temperature depends on partial pressure; i.e., different partial pressure has different desorption temperature. Therefore, it is reasonable to use the different coverage to treat different surfaces under different conditions when considering chemical reactions. These equilibrium phase diagrams also can provide useful information for surface science under ultra high vacuum conditions, and the saturated adsorption can have much lower temperature ranges.

■

ASSOCIATED CONTENT

* Supporting Information S

The properties (structures, Eads, and dC−O) of stable CO adsorptions on the (001), (101), and (201) surfaces are included. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No. 21073218), National Basic Research Program of China (No. 2011CB201406), Chinese Academy of Science, and Synfuels CHINA Co., Ltd. 6347



■

Article

Piskorz, W.; Cristol, S.; Paul, J. F.; Kotarba, A.; Sojka, Z. J. Phys. Chem. C 2010, 114, 22245. (28) Stull, D. R.; Prophet, H. JANAF thermochemical Tables, 2nd ed.; U. S. National Bureau of Standards: U. S. EPO, Washington D. C., 1971. (29) (a) Wang, J.; Castonguay, M.; Deng, J.; McBreen, P. H. Surf. Sci. 1997, 374, 197. (b) Solymosi, F.; Bugyi, L. J. Phys. Chem. B 2001, 105, 4337. (30) Xiang, M. L.; Li, D. B.; Li, W.-H.; Zhong, B.; Sun, Y. H. Fuel 2006, 85, 2662.

REFERENCES

(1) (a) Chen, J. G. Chem. Rev. 1996, 96, 1447. (b) Wu, H. H. H.; Chen, J. G. Chem. Rev. 2005, 105, 185. (2) Levy, R. B.; Boudart, M. Science 1973, 181, 547. (3) Pistonesi, C.; Juan, A.; Farkas, A. P.; Solymosi, F. Surf. Sci. 2010, 604, 914. (4) (a) Dubois, J.; Epicier, T.; Esnouf, C.; Fantozzi, G.; Convert, P. Acta. Metall. 1988, 8, 1891. (b) Epicier, T.; Dubois, J.; Esnouf, C.; Fantozzi, G.; Convert, P. Acta Metall. 1988, 8, 1903. (5) (a) Oyama, S. T. Catal. Today 1992, 15, 179. (b) Neylon, M. K.; Choi, S.; Kwon, H.; Curry, K. E.; Thompson, L. T. Appl. Catal., A 1999, 183, 253. (c) Solymosi, F.; Szechenyi, A. J. Catal. 2004, 223, 221. (d) Sundaramurthy, V.; Dalai, A. K.; Adjaye, J. Appl. Catal., B 2006, 68, 38. (e) Barthos, R.; Solymosi, F. J. Catal. 2007, 249, 28. (6) Nagai, M.; Matsuda, K. J. Catal. 2006, 238, 489. (7) Schaidle, J. A.; Lausche, A. C.; Thompson, L. T. J. Catal. 2010, 272, 235. (8) (a) Xiang, M. L.; Li, D. B.; Li, W. H.; Zhong, B.; Sun, Y. H. Catal. Commun. 2007, 8, 503. (b) Xiang, M. L.; Li, D. B.; Xiao, H. C.; Zhang, J. L.; Li, W.-H; Zhong, B.; Sun, Y. H. Catal. Today 2008, 131, 489. (9) (a) Liu, P.; Rodriguez, J. A.; Muckerman, J. T. J. Phys. Chem. B 2004, 108, 15662. (b) Liu, P.; Rodriguez, J. A.; Asakura, T.; Gomes, J.; Nakamura, K. J. Phys. Chem. B 2005, 109, 4575. (c) Liu, P.; Rodriguez, J. A.; Muckerman, J. T. J. Mol. Catal. A 2005, 239, 116. (10) Tominaga, H.; Nagai, M. Appl. Catal., A 2008, 343, 95. (11) (a) Piskorz, W.; Adamski, G.; Kotarba, A.; Sojka, Z.; Sayag, C.; Djega-Mariadassou, G. Catal. Today 2007, 119, 39. (b) Kotarba, A.; Adamski, G.; Piskorz, W.; Sojka, Z.; Sayag, C.; Djega-Mariadassou, G. J. Phys. Chem. B 2004, 108, 2885. (12) (a) Nagai, M.; Tominaga, H.; Omi, S. Langmuir 2000, 16, 10215. (b) Ren, J.; Huo, C. F.; Wang, J. G.; Li, Y. W.; Jiao, H. J. Surf. Sci. 2005, 596, 212. (c) Ren, J.; Huo, C. F.; Wang, J. G.; Cao, Z.; Li, Y. W.; Jiao, H. J. Surf. Sci. 2006, 600, 2329. (d) Ren, J.; Wang, J. G.; Huo, C. F.; Wen, X. D.; Cao, Z.; Yuan, S. P.; Li, Y. W.; Jiao, H. J. Surf. Sci. 2007, 601, 1599. (e) Rocha, A. S.; Rocha, A. B.; Silva, V. T. Appl. Catal., A 2010, 379, 54. (f) Shi, X. R.; Wang, J. G.; Hermann, K. J. Phys. Chem. C 2010, 114, 13630. (13) Pistonesi, C.; Juan, A.; Farkas, A. P.; Solymosi, F. Surf. Sci. 2008, 602, 2206. (14) Pronsato, M. E.; Pistonesi, C.; Juan, A.; Farkas, A. P.; Bugyi, L.; Solymosi, F. J. Phys. Chem. C 2011, 115, 2798. (15) Tominaga, H.; Nagai, M. J. Phys. Chem. B 2005, 109, 20415. (16) Liu, P.; Rodriguez, J. A. J. Phys. Chem. B 2006, 110, 19418. (17) Schweitzer, N. M.; Schaidle, J. A.; Ezekoye, O. K.; Pan, X. Q.; Linic, S.; Thompson, L. T. J. Am. Chem. Soc. 2011, 133, 2378. (18) Shi, X. R.; Wang, S. G.; Wang, H.; Deng, C. M.; Qin, Z. F.; Wang, J. G. Surf. Sci. 2009, 603, 852. (19) Han, J. W.; Li, L.; Sholl, D. S. J. Phys. Chem. C 2011, 115, 6870. (20) Wang, T.; Liu, X. W.; Wang, S. G.; Huo, C. F.; Li, Y. W.; Wang, J. G.; Jiao, H. J. J. Phys. Chem. C 2011, 115, 22360. (21) (a) Hanif, A.; Xiao, T. C.; York, A. P. E; Sloan, J.; Green, M. L. H. Chem. Mater. 2002, 14, 1009. (b) Wang, X. H.; Hao, H. L.; Zhang, M. H.; Li, W.; Tao, K. Y. J. Solid State Chem. 2006, 179, 538. (c) Nagai, M.; Zahidul, A. M.; Matsuda, K. Appl. Catal., A 2006, 313, 137. (22) Rudy, E.; Windisch, S.; Stosick, A. J.; Hoffman, J. R. Trans. Metall. Soc. Aime. 1967, 239, 1247. (23) (a) Kresse, G.; Furthmüller, J. Comput. Mater. Sci. 1996, 6, 15. (b) Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169. (24) (a) Blochl, P. E. Phys. Rev. B 1994, 50, 49. (b) Kresse, G. Phys. Rev. B 1999, 59, 1758. (25) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (26) (a) Reuter, K.; Scheffler, M. Phys. Rev. B 2001, 65, 035406. (b) Reuter, K.; Scheffler, M. Phys. Rev. B 2003, 68, 045407. (27) (a) Li, W.-X.; Stampfl, C.; Scheffler, M. Phys. Rev. B 2003, 68, 165412. (b) Rogal, J.; Reuter, K.; Scheffler, M. Phys. Rev. B 2004, 69, 075421. (c) Grillo, M. E.; Ranke, W.; Finnis, M. W. Phys. Rev. B 2008, 77, 075407. (d) Aray, Y.; Vidal, A. B.; Rodriguez, J.; Grillo, M. E.; Vega, D.; Coll, D. S. J. Phys. Chem. C 2009, 113, 19545. (e) Zasada, F.; 6348


Adsorption Equilibria of CO Coverage on Î²-Mo2C Surfaces - The

Recommend Documents