Adsorption of Carbon Monoxide on Pt - American Chemical Society

Mar 5, 2010 - (335) and (112) stepped surfaces, CO molecules adsorb on the step top site initially. With the coverage increasing, CO molecules diffuse...
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J. Phys. Chem. C 2010, 114, 5886–5890

Adsorption of Carbon Monoxide on Pt (335) and (112) Surfaces Hongzhang Wu, Zhongni Wang, Zexin Wang,* and Zhaoyu Diao Chemistry Department, Shandong Normal UniVersity, Jinan, Shandong, 250014, China ReceiVed: June 14, 2009; ReVised Manuscript ReceiVed: December 19, 2009

Adsorption of carbon monoxide on Pt (335) and (112) stepped surfaces has been investigated by the extended LEPS (London-Eryring-Polanyi-Sato) method which is constructed by the 5-parameter Morse Potential. The calculated results show there have common characteristics of CO adsorption on the two surfaces. On both Pt (335) and (112) stepped surfaces, CO molecules adsorb on the step top site initially. With the coverage increasing, CO molecules diffuse from the step top sites to the stable terrace bridge sites and the terrace top sites sequentially. However, the step bridge site only corresponds to a transition state on the two surfaces. The predictions for vibrational frequency are in good agreement with the experimental results. 1. Introduction Studying the adsorption of molecules on the surface of welldefined stepped and kinked surfaces is an important first step toward understanding the role of defects in adsorption and catalysis.1 The comprehensive adsorption characters, such as eigenvalue for vibration, adsorption geometry, and adsorption energy, play an important role in understanding the behavior of adsorbed molecules and helping to clarify the mechanics of adsorption and catalysis. In order to obtain the critical characters, we have investigated the adsorption of CO on the stepped Pt (335) and Pt (112) surfaces by the extended LEPS (LondonEryring-Polanyi-Sato) method which is constructed by 5-MP (the 5-parameter Morse Potential). Earlier studies2-5 found only two types of on-top CO on step and terrace of the stepped Pt (335). Afterward, the characteristic frequency band of the bridge site was detected by electron energy loss spectroscopy (EELS)6-8 and infrared reflection absorption spectroscopy (IRAS).9-13 Finally, four vibrational modes, assigned to atop and bridged CO adsorption on both step and terrace sites, appeared in IRAS9-13 at high CO coverage. Although the CO vibrational modes are detected more and more, the latest and previous results have some common characteristics. Previous investigations shown the on-top CO adsorbed preferentially on the step site compared with the terrace site. The adsorption initially occurred on step plane: step atop followed by step bridge, and then terrace atop followed by terrace bridge. The studies9,13,14 of CO oxidation have shown that the CO molecules on terrace sites reacted with oxygen more easily than those on step sites. Like CO on Pt (335), previous studies12,13,15-19 found the only on-top CO chemisorbed on step and terrace sites of the stepped Pt (112), until recent IRAS studies1,20 observed four vibrational modes, which indicated that the stable bridge site also exists on Pt (112). Theoretical studies about CO on stepped Pt planes are seldom compared with experimental research. The calculations of large-scale density-functional21 and first-principles theory22 shown that the step bridge site is more favorable than the step atop site, in contrast to experiments, where the step atop is preferentially obtained. The calculations of density functional theory (DFT)20,23 obtained consistency of assignment of site preference with experimental results. * Corresponding author. E-mail: [email protected].

Some useful information for CO on stepped Pt (335) and Pt (112) surfaces was obtained through experimental and theoretical methods. Adsorbed CO on similar stepped surfaces Pt (335) (Pt(s)-[4(111) × (100)]) and Pt (112) (Pt(s)-[3(111) × (100)]) displayed some common characteristics, such as vibrational band and adsorption site. However, there remain some ambiguous questions, such as whether the intensity of the frequency band of atop CO on terrace is stronger than that on step and are CO molecules on the terrace site more reactive than on the step while reacting with O atoms. This work attempts to reveal the adsorption dynamics of CO on stepped Pt surfaces and tries to answer the questions above. On the basis of our previous study,24 this work further investigates the adsorption of CO on Pt stepped (335) and (112) surfaces with the extended LEPS. The extended LEPS constructed by 5-MP is the credible potential function based on the experimental information of the interaction between CO and Pt. We have successfully studied CO adsorbed on Ni, Pt, Rh, and Pd and obtained results that agreed well with the experimental data.24-27 2. Extended LEPS Constructed by 5-MP and Cluster Models 2.1. Theoretical Model. The extended LEPS method was first put forward by McCreey and Wolken28,29 and is the technique most often used in investigating the interaction between the diatomic molecule and the metal surface cluster. The extended LEPS potential deduced by valence bond theory is constructed by three pair-potential functions: one is for a gaseous diatom; the other two are for single atom and surface cluster, respectively. The pair-potential functions for a gaseous diatom can be obtained from spectroscopic data, while the other two should be constructed by other means; therefore, the latter is the brilliant part of the extended LEPS method. In this paper, the extended LEPS constructed by the 5-MP is the pair potential for an atom adsorbed on surface clusters. 5-MP has been explained in the literature.24-27 Here, it is stated briefly as follows: on the assumption that the metal cluster b) is the 5-MP between an is frozen, the interaction energy U(R atom, for which the coordinate is specified by b R. So, the whole metal surface cluster can be written in the form of a Morse potential

10.1021/jp905569r  2010 American Chemical Society Published on Web 03/05/2010

Adsorption of CO on Pt (335) and (112) Surfaces

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TABLE 1: Parameters for O-Pt and C-Pt Systems system

D/eV

β/Å-1

R0/Å

Q1/Å

Q2/Å

O-Pt C-Pt

0.75 1.26

2.25 1.85

2.20 1.78

2.53 1.90

0.91 0.11

(

)

b) ) D U(R

cluster

∑ i)1

hi + Q1 {exp[-2β(Ri - R0)] Ri + Q2 2 exp[-β(Ri - R0)]}

(1)

Figure 1. Schematic top view of cluster model and adsorption sites for Pt (335) surface.

In eq 1, there are five adjustable parameters: D, β, R0, Q1,and Q2 have been described in detailed in the literature.24-27 The five adjustable parameters of the C-Pt and O-Pt25 systems obtained from the literature are listed in Table 1. Equation 1 is regarded as the bonded interaction potential; thus, the antibonded interaction potential between the adatom and the metal surface cluster can be expressed as eq 2:

b) ) D+ U+(R

cluster

∑ i)1

(

)

hi + Q1 {exp[-2β(Ri - R0)] + Ri + Q2 2 exp[-β(Ri - R0)]}

Figure 2. Schematic top view of cluster model and adsorption sites for Pt (112) surface.

(2)

Here, D+ ) 0.5D(1 - ∆)/(1 + ∆); ∆ is so-called Sato parameter. Thus, the Coulomb integral J and the exchange integral K for a special point can be obtained from eqs 1 and 2. Finally, the extended LEPS potential energy surface of diatomic molecule (AB) and metal (M) surface can be expressed as

E ) JCO + JCPt + JOPt + 2 + (KCPt + KOPt)2 - KCO(KCPt + KOPt)]1/2 [KCO

(3)

In eq 3, the potential function includes three adjustable sato parameters (∆C-O, ∆C-Pt, and ∆O-Pt). Just like the five parameters mentioned above in the 5-MP, we also adjust them simultaneously according to the experimental data of three lowindex (100), (110), and (111) surfaces.25 In this paper, we study the CO-Pt (335) and (112) systems using the same set of optimized sato parameters(∆C-O, ∆C-Pt, and ∆O-Pt) of CO adsorption and diffusion on low index surfaces.25 2.2. Construction of Cluster Models for Platinum Surfaces. Metal platinum belongs to fcc lattice with the lattice constant a0 ) 3.924 Å. Considering the local geometrical symmetry in a point group and the displacement symmetry for a surface crystal cell as well as the boundary effect of adatoms, we simulate the metal cluster with at least 10 layers of cell atoms, and each layer contains at least 10 (length) × 10 (width) atoms. The size of the cluster is defined by the principle of no boundary effect in our calculation, which means the following: “The cluster simulated must have certain size. For the same adsorption site, the critical characteristics of adsorption atom at the boundary are different slightly from that of in the interior of the cluster surface, which is the so-called boundary effect. In order to ignore the boundary effect, the range of the surface cluster is simulated at least two unit cells from the center to boundary.” Figures 1 and 2 show a schematic diagram of the surface cluster model and surface adsorption sites of Pt (335) and Pt (112) stepped surface, respectively. Here, S and T are abbreviated to step and terrace; the numbers 1, 2, and 3 represent the first, second, and third terrace; t and b represent the top and bridge sites. Generally, the zero point of the coordinate is oriented on the top atom, the x axis and y axis, respectively, corresponding with the crystal axis as shown in Figure 1 and Figure 2, the z axis point to a vacuum, and the distance between the nearest two

lines (or two rows) is regarded as the unit length of x and y axes, which is the so-called lattice coordinate. There are six degrees of freedom in the movement of the diatomic molecule, and there are several definitions of the coordinate. We adopt the lattice coordinates for the centroid of the diatomic molecule to denote the orientation of the molecule on the cluster surface, and adopt the polar coordinates for the diatomic molecule itself. Here, θ shows the angle between C-O bond and surface normal (z-axis), while φ is the angle between the projection of C-O bond and x-axis. E(x, y, z, r, θ, φ) the systemic potential function includes six variables that constitute seven dimensions of energy on the hypersurface. The critical point is the one with the first derivative being zero on the hypersurface, and the second derivatives of the critical points form the Hessian matrix. The number (denoted as λ) of negative eigenvalues of Hessian matrix characterizes the nature of the critical points. The critical point of λ ) 0 corresponds to the stable state of the system, and the one with λ ) 1 corresponds to the trans-state, to which we pay more attention. At high coverage, the transition state could translate into the stable state. In this paper, we aim to study the adsorption of CO molecule on the stepped (335) and (112) surfaces of Pt, so just pay attention to the critical points with λ ) 0 or λ ) 1. 3. Result and Discussion The optimization parameters for C-Pt and O-Pt are listed in Table 1, and the sato parameters are ∆O-Pt ) -0.45, ∆C-Pt ) 1.80, and ∆C-O ) -11 according to our previous work.25 We calculate all critical characteristics (listed in Table 2) of CO-Pt (335) and (112) stepped surfaces using this set of parameters in this work. Eb represents the binding energy and fC-O is the C-O stretch vibration frequency. ZC-Pt is the height of the C atom to the surface. RC-O represents the C-O bond length, and RC-Pt is the distance of the C atom to the nearest coordination Pt atoms. The meanings of λ, θ, and φ are the same as mentioned above. The comparison between our results and the data from literature is listed in Table 3. 3.1. CO-Pt (335) System. The (335) surface consists of fouratom-wide (111) terrace and one-atom-high (100) step. It can be clearly seen from Table 2 that five stable adsorption states with λ ) 0 exist on Pt (335) surface: they are the step top site (St); the first, second, and third terrace top site (T1t, T2t, T3t); and the third terrace bridge site (T3b) (as signed in Figure 1)

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Wu et al.

TABLE 2: Critical Characteristics of CO-Pt Surface System CO-Pt

site

λ

Eb/eV

f/cm-1

RC-O/Å

(112)

St Sb T1t T1b T2t T2b St Sb T1t T1b T2t T2b T3t T3b

0 1 0 1 0 0 0 1 0 1 0 1 0 0

1.58 1.37 1.64 1.50 1.56 1.43 1.58 1.46 1.68 1.57 1.68 1.57 1.60 1.49

2017 1984 2035 1958 2000 1887 2017 1980 2034 1957 2035 1956 1980 1889

1.151 1.155 1.151 1.158 1.155 1.167 1.151 1.156 1.151 1.159 1.151 1.159 1.157 1.166

(335)

RC-Pt/Å

ZC-Pt/Å

θ/°

1.47 1.18 0.72 0.42 -0.07 -0.35 1.46 1.17 0.91 0.61 0.31 -0.02 -0.34 -0.59

90 90 90 90 90 90 90 90 90 90 90 90 90 100

1.47 1.83 × 2 1.52, 2.68 × 2 1.85 1.62 1.87 1.46 1.83 1.51 1.84 1.51 1.84 1.66 1.89

TABLE 3: Comparison between Our Results and Literature CO-Pt RC-O(Å)

system 112

RC-Pt(Å)

112

F (cm-1)

112

335

Eb (eV)

112

site St Sb Tt Tb St Sb Tt Tb St Sb Tt Tb St Sb Tt Tb St Sb Tt Tb

experimental data

theoretical data 1.1622,23 1.1822,23 1.1622,23 1.1823 1.8622,23 2.0122,23 1.85,22 1.8323 2.02,23 2.0323

2070-2080,1 206812 1888,1 185412 2090-20951 19181 2065-2078,2 2066-2075,5 2080,6 2109,7 2072,9 2017-2064,10 207512,13 1850,6 1900,9 1788-1876,10 188012 2086-2097,2 2088,5 1877,7 2092,12 210013 ≈180012

corresponding to the adsorption energy of 1.58, 1.68, 1.68, 1.60, and 1.49 eV, respectively. Three transition states with λ ) 1 are the step bridge site (Sb), the second and third terrace bridge site (T1b and T2b). Using Tt (atop site on terrace) represents T1t, T2t, and T3t, and Tb (bridge site on terrace) represents the T3b, because T1b and T2b are not stable adsorption states. The CO vibrational frequency of the three stable adsorption states St, Tt, and Tb and one transition state Sb is 2017, 1980-2034, 1889, and 1980 cm-1, respectively, which coincides with and reproduces the IRAS9,11-13 values (see Table 3). On the step top site (St), the calculated frequency band of CO is 2017 cm-1, which is close to the IRAS10-13 results of ∼2072 cm-1. At low CO coverage, adsorption energy and the structure of adsorption site are the main factors affecting the occupancy of adsorption site. The discrepancy of adsorption energy among those stable sites is small, and the step is the exposed outer edge on Pt(335), so St will be occupied preferentially compared to the terrace site. The assignment is consistent with the experimental results that there is only a frequency band of step top site appearing in IRAS10-13 at the coverage of θCO < 0.1 ML. The calculations indicate that the terrace bridge site Tb (represented by T3b) is a stable site corresponding to the frequency band of 1889 cm-1 in agreement with EELS6-8 and IRAS9-13 results. However, the assignment of adsorption sites is different than the previous experimental results,6-13 which suggests that the step bridge site is more stable than the terrace

this work 1.151 1.155 1.151 1.158, 1.167 1.47 1.83 1.52 1.85, 1.87 2017 2000, 2035 1887 2017

2.15,22 2.4123 2.38,22 2.3223 1.58,22 1.9723 1.8523

1980, 2034, 2035 1889 1.58 1.37 1.64

bridge site, even more than the terrace top site. The step bridge site is the transition state with λ ) 1 in this work. As mentioned above, the step bridge site, which could become a stable state at high coverage, corresponds to the frequency band of 1980 cm-1 in agreement with IRAS9-13 results. To deal with the bothersome discrepancy between this work and the experimental results, it makes us resort to analyzing the structure of adsorption and the diffusion of CO on Pt (335). In line with the experimental results, the calculations imply that CO molecules prefer adsorbing on the step top site initially; the diffusion of CO is from the step top (St) to the third terrace bridge site (T3b) with the coverage of CO slightly increasing. The step bridge site with λ ) 1 is a transition state, the adsorption on this site is not stable at low coverage, and the diffusion from top site to bridge site is forbidden. Although the interpretation is a presumption, it agrees with the experimental results,9,13,14 which indicate that the CO molecule on the lower terrace site reacts with the O atom on the step site. The experiments9 could not distinguish between CO reactant molecules on upper and lower terrace sites; Yates13 suggests that CO reactant molecules locate on the lower terrace sites. We also think CO reactant molecules should be on the lower terrace sites in terms of the calculations, because the vibrational band 1889 cm-1 of the third terrace bridge site (T3b) appears following the vibrational band of the step top site. CO molecules migrate from the step top site to the third terrace site, while O atoms occupying the step top sites and then CO molecules on the terrace site are reacting with O

Adsorption of CO on Pt (335) and (112) Surfaces

Figure 3. Change of adsorption state on Pt(335) with the CO coverage increasing.

adatoms at the step top sites. This speculation can give the reason the terrace site is the active site for oxidation on Pt (335). There have three terrace top sites (T1t,T2t, and T3t) on Pt (335), and those sites are the stable sites. With the coverage of CO continuously increasing, CO molecules would adsorb on the terrace top sites. The intensity of terrace sites become strong naturally when all the terrace top sites are occupied by adsorbed CO molecules. The assignment is also consistent with the experimental results that the frequency band of the terrace top site appears in electroreflectance vibrational spectroscopy (EVS)5 and IRAS9-13 at the coverage of θCO > 0.23 ML, and becomes a predominant band quickly. The change of intensity is directly proportional to numbers transforming the adsorption sites with the coverage increasing in EELS.6-8 The calculations indicate that the numbers of stable terrace top sites play a key role in transition of intensity of frequency band between step top site and terrace top site. Jazzman et al.9 finds the spectrum intensity of CO on terrace is stronger than on step, but suggests that it should be attributed to the vibrational mode coupling effect of CO between step and terrace. Obviously, this work gives a better explanation for the experimental results. The step bridge site becomes the stable site only at high coverage, so it appeared finally. Figure 3 shows the change of adsorption site on Pt (335) with the CO coverage increasing. From the above analysis, the step top site is occupied initially, and then the terrace bridge site and the terrace top sites are occupied sequentially with the coverage increasing. However, the step bridge site, which corresponds to the transition state with λ ) 1, is occupied finally at saturated coverage of CO. The intensity of CO on the terrace top site is stronger than on the step top site, when the terrace top sites (T1t,T2t, and T3t) are occupied. 3.2. CO-Pt (112) System. The (112) surface consists of three-atom-wide (111) terrace and one-atom-high (100) step. It can be clearly seen from Table 2 that four stable adsorption states with λ ) 0 exist on Pt (112) surface: they are the step top site (St), the first and second terrace top site (T1t and T2t), and the second terrace bridge site (T2b) (as labeled in Figure 2) corresponding to the adsorption energies of 1.58, 1.64, 1.56, and 1.43 eV, respectively. Two transition states with λ ) 1 are the step bridge site (Sb) and the second terrace bridge site (T1b). Like Pt (335), using Tt (atop site on terrace) represents T1t and T2t, and Tb (bridge site on terrace) represents T2b, because T1b is not a stable adsorption state. The CO vibrational frequency of St, Tt, Tb, and Sb sites is 2017, ∼2030, 1887, and 1984 cm-1, respectively, which coincides with and reproduces the IRAS1,20 values (see Table 3). Because of the similar structures of Pt (112) and Pt (335) and the similarity of the calculated results of CO on two stepped surfaces, the explanation of adsorbed CO on Pt (112) will be the same as Pt (335). On the step top site (St), the calculated frequency band of CO is 2017 cm-1, which is close to the IRAS1,20 results of 2070-2080 cm-1 and 2068 cm-1. At low CO coverage, the

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Figure 4. Change of adsorption state on Pt(112) with the CO coverage increasing.

discrepancy of adsorption energy among those stable sites is small and the step is exposed outer edge on Pt (112), so St will be occupied initially compared to the terrace site. Tb is a stable site and gives rise to frequency band at 1887 cm-1. The frequency band at 1888 cm-1 is the second band appearing in IRAS,1,20 so we think Tb is the second adsorption site with the coverage of CO slightly increasing. However, the assignment of adsorption sites is contrary to previous experimental and theoretical results,1,20 because the frequency band at 1888 cm-1 was assigned to the step bridge site (Sb) by previous research. The step bridge site is the transition state in this work, which translates into the stable adsorption state under saturated coverage condition. Figure 4 shows the change of adsorption site on Pt (112) with the CO coverage increasing. From calculations, there are two stable atop adsorption states T1t and T2t on terrace and only one stable atop adsorption state St on step. The intensity of frequency band will become stronger when the terrace top sites are occupied. So, this work indicates that the stronger intensity of terrace site should be attributed to the amounts of terrace top CO. The Mukerji group1,20 indicates that the reason the intensity of the terrace bonded CO is so strong compared to step CO at higher coverage can be attributed to either the reorientation of the step CO molecules or intensity stealing by higher frequency band, other than the amounts of terrace top CO. Furthermore, CO molecules on the terrace site are more reactive than on the step sites obtained by Szabo´,30 which indicates that there are some stable adsorption sites on terrace. As mentioned above, the terrace bridge and top site are the stable adsorption sites and the step bridge site is the transition state. CO adsorbed on terrace can react with oxygen adatoms when the step top site is occupied by oxygen adatom. Although the assignment of the step bridge site as a transition state and the terrace bridge site as a stable site is different from previous studies, the calculations also give a reasonable interpretation of the change of intensity of frequency bands in IRAS1,20 and the reactive activity of the terrace site in oxidation of CO molecules. 4. Conclusions Adsorption of CO molecules on Pt (335) and (112) surfaces has been investigated by the extended LEPS, which is constructed by 5-MP. Pt (335) and (112) step surfaces consist of similar (111) terrace and (100) step. There exist common characteristics of CO molecules adsorption on these two surfaces. CO molecules will occupy the step top site of Pt (335), (112) surfaces at low coverage. The varied stable adsorption states on terrace will appear with the coverage increasing. On Pt (335), the third terrace bridge site is a stable site, CO molecules can diffuse from the step top sites to the third terrace bridge sites, the three terrace top sites are stable sites, and CO molecules will occupy them followed by the third terrace bridge sites. On Pt (112), the second terrace bridge site is a stable site, CO

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molecules can diffuse from the step top sites to the second terrace bridge sites, the two terrace top sites are stable sites, and CO molecules will occupy them sequentially. However, on both Pt (335) and Pt (112), the step bridge site corresponds to a transition state, which can become a stable site only at high coverage. Acknowledgment. The authors thank the Natural Science Foundation of Shandong Province (Y2006B29) for financial support. References and Notes (1) Mukerji, R. J.; Bolina, A. S.; Brown, W. A. Surf. Sci. 2003, 527, 198. (2) Hayden, B. E.; Kretzschmar, K.; Bradshaw, A. M.; Greenler, R. G. Surf. Sci. 1985, 149, 394. (3) Greenler, R. G.; Leibsle, F. M.; Sorbello, R. S. Phys. ReV. B 1985, 32, 8431. (4) Leibsle, F. M.; Sorbello, R. S.; Greenler, R. G. Surf. Sci. 1987, 179, 101. (5) Lambert, D. K.; Tobin, R. G. Surf. Sci. 1990, 232, 149. (6) Luo, J. S.; Tobin, R. G.; Lambert, D. K.; Wagner, F. T.; Moylan, T. E. J. Electron Spectrosc. Relat. Phenom. 1990, 54-55, 469. (7) Luo, J. S.; Tobin, R. G.; Lambert, D. K.; Fisher, G. B.; DiMaggio, C. L. Surf. Sci. 1992, 274, 53. (8) Luo, J. S.; Tobin, R. G.; Lambert, D. K.; Fisher, G. B.; DiMaggio, C. L. J. Chem. Phys. 1993, 99, 1347. (9) Xu, J. Z.; Yates, J. T., Jr. J. Chem. Phys. 1993, 99, 725. (10) Kim, C. S.; Kotzeniewski, C.; Tornquistb, W. J. J. Chem. Phys. 1994, 100, 628.

Wu et al. (11) Xu, J. Z.; Henriksen, P. N.; Yates, J. T., Jr. Langmuir 1994, 10, 3663. (12) Xu, J. Z.; Yates, J. T., Jr. Surf. Sci. 1995, 327, 193. (13) Yates, J. T., Jr. J. Vac. Sci. Technol. A 1995, 13, 1359. (14) Xu, J.; Henriksen, P.; Yates, J. T., Jr. J. Chem. Phys. 1992, 97, 5250. (15) Siddiqui, H. R.; Guo, X.; Chorkendorff, I.; Yates, J. T., Jr. Surf. Sci. 1987, 191, L813. (16) Henderson, M. A.; Szabo´, A.; Yates, J. T., Jr. J. Chem. Phys. 1989, 91, 7245. (17) Henderson, M. A.; Szabo´, A.; Yates, J. T., Jr. J. Chem. Phys. 1989, 91, 7255. (18) Henderson, M. A.; Szabo´, A.; Yates, J. T., Jr. Chem. Phys. Lett. 1990, 168, 51. (19) Henderson, M. A.; Yates, J. T., Jr. Surf. Sci. 1992, 268, 189. (20) Creighan, S. C.; Mukerji, R. J.; Bolina, A. S.; Lewis, D. W.; Brown, W. A. Catal. Lett. 2003, 88, 39. (21) Hammer, B.; Nielsen, O. H.; Nfrskov, J. K. Catal. Lett. 1997, 46, 31. (22) Karmazyn, A. D.; Fiorin, V.; Jenkins, S. J.; King, D. A. Surf. Sci. 2003, 538, 171. (23) Orita, H.; Itoh, N.; Inada, Y. Surf. Sci. 2004, 571, 161. (24) Jia, H. Y.; Wang, Z. X. Acta Phys. -Chim. Sin. 2004, 20 (2), 144. (25) Wang, Z. X.; Qiao, Q. A.; Chen, S. G.; Zhang, W. X. Surf. Sci. 2002, 517, 29. (26) Wang, Z. X.; Pang, X. H.; Wang, R. Chin. Sci. Bull. 2004, 49 (10), 1012. (27) Zhang, J.; Zhang, X. N.; Wang, Z. X.; Diao, Zh. Y. Appl. Surf. Sci. 2008, 254, 6327. (28) McCreey, J. H.; Wolken, G., Jr. J. Chem. Phys. 1975, 63, 2340. (29) McCreey, J. H.; Wolken, G., Jr. J. Chem. Phys. 1977, 66, 2316. (30) Szabo´, A.; Henderson, M. A.; Yates, J. T., Jr. J. Chem. Phys. 1992, 96, 9161.

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