Observation of Novel Low-Temperature Hydrogenation Activity on Co

Lars Barzen , Matthias Tombers , Christine Merkert , Joachim Hewer , Gereon Niedner-Schatteburg. International Journal of Mass Spectrometry 2012 330-3...
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15748

J. Phys. Chem. B 2004, 108, 15748-15754

Observation of Novel Low-Temperature Hydrogenation Activity on Co/Pt(111) Surfaces Neetha A. Khan,† Luis E. Murillo†, and Jingguang G. Chen*,‡ Departments of Materials Science and Engineering and of Chemical Engineering, Center for Catalytic Science and Technology, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: May 13, 2004; In Final Form: July 28, 2004

The reactions of hydrogen, ethylene, and cyclohexene have been investigated on Co/Pt(111) surfaces using temperature-programmed desorption (TPD). When Co is deposited on Pt(111) at 0.5-1 ML (monolayer) Co coverage, hydrogen desorption occurs in the temperature range of 150-250 K, which is significantly lower in temperature than from pure Pt(111) or a thick Co film on Pt(111). The ethylene decomposition reaction pathway is also significantly decreased on the 0.5-1 ML Co/Pt(111) surfaces. In addition, cyclohexene reaction pathways on 1 ML Co/Pt(111) are different from pure Pt(111) or thick Co films. The only reaction pathways of cyclohexene on Pt(111) or thick Co films are dehydrogenation to benzene and complete decomposition. However, on the ∼1 ML Co/Pt(111) surface, a new hydrogenation reaction pathway to cyclohexane is present at 221 K. These reactions on the Co/Pt(111) surfaces are further compared to our earlier studies of the Ni/ Pt(111) surfaces. The novel activity of the 1 ML Co/Pt(111) surface is explained by the modification of the Pt d-band center by the subsurface Co atoms.

1. Introduction

2. Experimental Section

Bimetallic catalysts are well-known for exhibiting properties that are distinctly different from those of the pure metal catalysts.1,2 Various chemical, electronic, and physical probes have been utilized on model bimetallic surfaces to understand, at an atomic level, the novel reactivity of bimetallic catalysts. Surface science studies have been able to elucidate novel reaction mechanisms and electronic properties that are present on bimetallic surfaces. Previously, we have conducted extensive studies on Ni/Pt(111) bimetallic surfaces.3-5 At a coverage of ∼1 ML Ni, the Ni/Pt(111) surfaces are characterized by a lowtemperature desorption of H2 and by a novel self-hydrogenation pathway of cyclohexene. We have also found that Ni atoms diffuse into the Pt(111) subsurface regions at temperatures above 300 K. We believe that the presence of subsurface Ni is critical in inducing the low-temperature activity observed on the 1 ML Ni/Pt(111) surface.5

The ultrahigh-vacuum (UHV) chamber utilized for TPD and surface characterization is a two-level stainless steel chamber (base pressure of 1 × 10-10 Torr) equipped with Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and a mass spectrometer for temperature-programmed desorption experiments (TPD). For the TPD experiments, the Pt(111) sample was heated with a linear heating rate of 3 K/s. The opening of the random flux shield of the quadrupole mass spectrometer was placed at a distance of ∼5 mm from the sample surface. The experimental setup allowed us to monitor up to 10 masses simultaneously. The Pt single-crystal sample was a (111) oriented, 1.5 mm thick disk (99.999%), 8 mm in diameter. The crystal was spot welded directly to two tantalum posts that served as electrical connections for resistive heating, as well as thermal contacts for cooling with liquid nitrogen. With this mounting scheme, the temperature of the crystal could be varied between 90 and 1200 K. The crystal was cleaned with cycles of Ne+ sputtering and flash annealing to 1100 K, followed by O2 treatment at 900 K then annealing for ∼5 min at 1100 K. Surface cleanliness was monitored using AES. Carbon and oxygen impurities remain below the noise level in the AES spectrum after this cleaning procedure. Cyclohexene (c-C6H10), (Aldrich, 99+% purity) was purified by successive freeze-pump-thaw cycles prior to use. The purity was verified in situ by mass spectrometry. Ethylene, neon, and hydrogen were all of research grade purity (99.999%) and were introduced into the UHV chamber without further purification. Doses are reported in langmuirs (1 langmuir ) 1 × 10-6 Torr‚s) and are uncorrected for ion gauge sensitivities. Cyclohexene dosing was made with a crystal temperature of 100 K or below and by backfilling the chamber through leak valves. Hydrogen and ethylene were dosed through a directional dosing tube, with a diameter of ∼8 mm. The directional dosing tube allowed a small dosage of H2 (0.5 langmuir) to result in ∼4050% saturation coverage.

In supported bimetallic catalysts and single-crystal studies, Co and Ni have shown similar properties.6,7 Englisch et. al have found that adding Co or Ni to Pt/SiO2 increases the hydrogenation activity of the catalyst by a factor of 3.6 Experimental and theoretical studies have suggested that Co and Ni both diffuse into the Pt(111) subsurface regions.8,9 In addition, several studies of the structural properties of Co-Pt single-crystal alloys have shown similar properties to that of Ni-Pt single-crystal alloys.10,11 Therefore, the Co-Pt system is a good system to compare with the Ni/Pt(111) system. In the current study, we have used hydrogen, ethylene, and cyclohexene as probe molecules to compare the reactivities of 1 ML Co/Pt(111) with those of Pt(111) and thick Co films on Pt(111). * Corresponding author. E-mail: [email protected]. Fax: (302) 8311048. † Department of Materials Science and Engineering. ‡ Department of Chemical Engineering.

10.1021/jp0479419 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/14/2004

Low-Temperature Hydrogenation on Co/Pt(111) Surfaces

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Figure 1. (a) AES intensities of Pt(241 eV) and Co(777 eV) as a function of deposition time. (b) Co(777 eV)/Pt(241 eV) AES ratio as a function of time at 300 and 600 K deposition temperatures.

The deposition of the Co/Pt(111) bimetallic surfaces was carried out with crystal temperatures of 300 and 600 K, as noted in the figures. The TPD experiments were carried out on the 600 K deposited surfaces to prevent background CO adsorption during deposition. During the evaporation of Co, the chamber pressure remained at ∼5 × 10-10 Torr or below. The Co deposition was checked for uniformity on the Pt(111) surface using AES, which revealed that the surface contained less than 1% impurities (C or O) during Co deposition. The coverage of the Co overlayer was estimated using the reduction fo the Pt(241 eV) AES intensity after Co deposition, with the assumption that the Co overlayer grows in a layer-by-layer fashion on Pt(111). 3. Results 3.1. Characterization of Co/Pt(111) Surfaces. Numerous groups have studied the physical and electronic properties of the Co/Pt(111) surface using LEED, AES STM, and DFT calculations.8,12-16 STM studies have concluded that Co grows on Pt in a quasi layer-by-layer mode at 300 K,14 similar to the Ni/Pt(111) system at 300 K.5 When approximately 4 ML Co on Pt(111) is annealed to ∼700 K, an alloy surface ∼10 ML deep is produced with ∼20% Co and ∼80% Pt in the topmost layer.17 LEED studies have shown a diffuse (1 × 1) pattern at 1 ML Co coverage and at a deposition temperature of ∼600 K.8 In this work, AES was used to characterize the Co/Pt(111) surfaces. Figure 1a shows the Co(777 eV) and Pt(241 eV) AES peaks as a function of deposition time at 600 K. The Co peak increases and the Pt peak decreases continuously until the Pt(241 eV) signal almost disappears. Additionally, the curves do not exhibit any distinct breaks that would indicate the onset of a monolayer. The absence of any breaks in the curves in Figure 1a suggests that multilayers are formed without well-defined layer-by-layer growth. In addition, Figure 1b shows the evolution of the Co/Pt AES ratios as a function of time at two deposition temperatures, 300 and 600 K. The Co/Pt AES ratio increases at a faster rate at 300 K than at 600 K, indicating that either the sticking coefficient of Co is higher at 300 K or the Co atoms are diffusing into the Pt(111) subsurface regions at 600 K. To estimate the approximate Co coverages, the reduction of the Pt(241 eV) AES peak-to-peak intensity was used to estimate the onset of a monolayer by assuming the formation of a uniform Co monolayer on the Pt(111) surface at 600 K. With an inelastic mean free path of the Pt(241 eV) electrons of

Figure 2. Co(777 eV)/Pt(241 eV) AES ratios after annealing different coverages of Co on Pt(111).

∼6.4 Å and a Co atomic radius of 1.25 Å,18 the formation of a monolayer Co should reduce the Pt(241 eV) intensity to 67% of that of a clean Pt surface. Furthermore, by comparing the intensities of the Pt(241 eV) and Co(777 eV) in Figure 1a, the onset of a monolayer at 600 K (67% reduction of the signal) occurs at an AES ratio of Co(777 eV)/Pt(241 eV) ∼ 2.0. The calculated segregation energies of Co in Pt(111) indicate that Co atoms are more stable in the bulk of Pt(111) than on the surface.19 Figure 2 shows the effect of annealing the Co/ Pt(111) surfaces with different coverages to temperatures up to 1100 K. In all cases, the Co(777 eV)/Pt(241 eV) ratio decreases until it reaches approximately zero at ∼950 K. On Co/Pt(111) surfaces with Co coverages of ∼0.5 and ∼1.0 ML, the ratio begins to decrease at around 500 K. On the surface with a Co coverage >2 ML, the ratio also decreases by a small amount at ∼600 K, but the major decrease in intensity occurs at temperatures between 650 and 900 K. These results show that Co diffuses into Pt(111) at low temperatures. 3.2. Adsorption and Desorption of H2. Figure 3 shows the desorption of H2 after exposing Pt(111) and Co/Pt(111) bimetallic surfaces to a saturation coverage of hydrogen. On a clean Pt(111) surface, hydrogen desorbs from the surface as a broad peak centered at ∼286 K. When 0.4 ML Co is deposited on the Pt(111) surface at 600 K, the desorption temperature of

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Figure 3. TPD spectra of hydrogen desorption from the adsorption of ∼10 langmuirs of H2 on Co/Pt(111) surfaces at 100 K. The Co/Pt(111) surfaces were prepared at a crystal temperature of 600 K.

Figure 4. TPD spectra of hydrogen desorption from the decomposition of 0.3 langmuir of ethylene on Co/Pt(111) surfaces.

hydrogen decreases to 148 K. This peak is narrower and the peak area is reduced as compared to clean Pt(111). Hydrogen desorbs at ∼150 K, with a broad tail that reaches ∼300 K, from a ∼0.8 ML Co/Pt(111) surface. As more Co is deposited on Pt(111), the desorption temperature increases to 270 K at ∼4.5 ML Co/Pt(111). For comparison, previous studies of hydrogen desorption from a Co(0001) surface showed hydrogen desorbing from the surface in the range of 300-450 K.20 A similar effect of the reduction in the desorption temperature has been observed for carbon monoxide from the CoPt3(111) alloy surface. Bardi and co-workers showed that the desorption of CO from a CoPt3(111) surface occurred at a temperature that was 50 K lower than a pure Pt(111) surface.21 3.3. Ethylene Decomposition. The decomposition of ethylene was used to probe the reactivity of the Co/Pt(111) surfaces. Figure 4 shows the desorption of the hydrogen product from the decomposition of ethylene on Co/Pt(111) surfaces. The ethylene decomposition pathway on clean Pt(111) has been studied in detail previously.3,22 Briefly, ethylene decomposes to produce an ethylidyne intermediate on Pt(111), leading to

Khan et al. the first desorption peak at 283 K. This surface species further decomposes, leading to the second hydrogen desorption peak at ∼478 K. Deposition of Co on Pt(111) at ∼0.5 ML Co significantly decreases the activity toward ethylene decomposition, as indicated by the negligible amount of hydrogen desorption. As more Co is deposited on the Pt(111) surface, the decomposition activity increases with two desorption peaks occurring at 283 and 383 K. However, the ∼1.9 ML Co/Pt(111) surface is still significantly less active than a pure Pt(111) surface, as indicated by the relatively small peak areas of the H2 product. It should be pointed out that the reduction in the H2 peak areas on the submonolayer and near monolayer Co/Pt(111) surfaces can also be due to a lower sticking coefficient of ethylene on the Co/Pt(111) surfaces. We could not quantify the amount of molecularly desorbed ethylene because the TPD peaks of ethylene appear as soon as the surface was heated, making it difficult to separate contributions from the surface and from the heating leads. In our previous studies of ethylene on near monolayer Ni/Pt(111) surfaces,3 we employed highresolution electron energy loss spectroscopy (HREELS) to identify the presence of weakly π-bonded ethylene, confirming the reduction in the H2 peak area was due to a weaker interaction of molecularly adsorbed ethylene on the near monolayer Ni/ Pt(111) surface instead of due to a lower sticking coefficient. We will perform similar HREELS studies in the future to further determine the origin of the novel chemistry of the near monolayer Co/Pt(111) surfaces. 3.4. Reaction Pathways of Cyclohexene. 3.4.1. TPD of Cyclohexene Reaction Products. The desorption of cyclohexene, benzene, and cyclohexane from Co/Pt(111) surfaces after the adsorption of 3 langmuirs of cyclohexene at 100 K is shown in Figure 5. On pure Pt(111), the only reaction pathways present are decomposition and dehydrogenation, consistent with previous publications.4,23 As shown in Figure 5a, only a small amount of cyclohexene molecularly desorbs from Pt(111) at 270 K at the 3 langmuir exposure. The Co/Pt(111) surfaces with near monolayer Co coverage desorb molecular cyclohexene at 200220 K. On thick Co films on Pt(111), the desorption of cyclohexene shifts to 179 K. Figure 5b shows the desorption of benzene from the dehydrogenation of cyclohexene on the Co/Pt(111) surfaces. Benzene desorbs in two overlapping peaks on Pt(111), with the first peak appearing at ∼383 K. Previous vibrational studies indicate that cyclohexene forms a c-C6H9 allyl intermediate, and then converts to benzene species on the Pt(111) surface.4,24 On the 0.4 ML Co/Pt(111) surface, benzene desorbs in a sharp peak at 237 K with a long tail that extends to ∼450 K. The 0.9 and 1.4 ML Co surfaces exhibit similar dehydrogenation properties. On both surfaces, benzene desorbs from the surface in a sharp peak at ∼225 K and a broad peak centered around 330 K. The thick Co film exhibits properties different from the Co/Pt(111) surfaces with near monolayer coverage, desorbing benzene at 237 and 376 K. The desorption of the self-hydrogenation product, cyclohexane, occurs only on the Co/Pt(111) surfaces. On Pt(111), a negligible amount of cyclohexane is detected in the TPD spectrum. As Co is deposited on Pt(111), the TPD area of the self-hydrogenation product increases at 221 K, with the cyclohexane desorption reaching a maximum at ∼0.9 ML Co/Pt(111). The thick Co film does not produce cyclohexane. Figure 6 shows the desorption of the H2 product from the cyclohexene reaction on Co/Pt(111) surfaces, further revealing the unique reaction pathways on the 0.5-0.8 ML Co/Pt(111) surfaces. On Pt(111), hydrogen desorbs in three peaks, at 279,

Low-Temperature Hydrogenation on Co/Pt(111) Surfaces

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Figure 5. TPD spectra of cyclohexene (82 amu), benzene (78 amu), and cyclohexane (84 amu) from the reaction of 3 langmuirs of cyclohexene on Co/Pt(111) surfaces.

cyclohexane production. As shown in Figure 7c, the hydrogenation activity, as judged from the cyclohexane peak area, increases by a factor of 2.8 when surface hydrogen is present. A lower temperature state of cyclohexane desorption also appears at 187 K. Last, the hydrogen desorption peak does not change significantly with the presence of surface hydrogen. 4. Discussion 4.1. Cyclohexene Reaction Pathways on Co/Pt(111) Surfaces. The selectivity and activity of the cyclohexene reaction pathways on the Pt(111) and Co/Pt(111) surfaces were estimated to further quantify the different chemical properties of the Pt(111) and Co/Pt(111) surfaces. On Pt(111) and the thick Co film, the only reaction pathways are dehydrogenation and decomposition. On the ∼1 ML Co/Pt(111) surface, the cyclohexene self-hydrogenation reaction is also included. The reaction pathways are summarized below for the three types of surfaces:

Figure 6. TPD spectra of the desorption of the hydrogen (2 amu) product from the reaction of cyclohexene on Co/Pt(111) surfaces.

360, and 507 K. The first two hydrogen peaks represent the formation of an allyl intermediate, and the formation of benzene,4 and the peak at 507 K represents the further decomposition of benzene to surface carbon and H2. A sharp peak of hydrogen desorption is detected at 232 K on Co/Pt(111) surfaces with 0.5-0.8 ML Co coverage. This sharp peak of desorption becomes broad at Co coverages above 1.8 ML. The thick Co film desorbs hydrogen in a broad peak at 310 K followed by a peak at 379 K. 3.4.2. Effect of Preadsorption of Hydrogen. The effect of preadsorbed hydrogen on the reaction pathways of cyclohexene on a 0.8 ML Co/Pt(111) surface is shown in Figure 7. The surface was exposed to 0.5 langmuir of hydrogen (corresponding to ∼40-50% saturation) at 100 K prior to the exposure to 3 langmuirs of cyclohexene. The presence of surface hydrogen induces a low-temperature state of cyclohexene molecular desorption at 181 K, as shown in Figure 7a. In addition, the activity of benzene production at ∼230 K is reduced by a factor of 0.75, on the basis of the reduction in the benzene peak area. The most pronounced effect of preadsorbed hydrogen is on the

Pt(111):

a c-C6H10 f 6a C(a) + 5a H2

(1)

b c-C6H10 f b C6H6 + 2b H2

(2)

∼1 ML Co/Pt(111):

c c-C6H10 f 6c C(a) + 5c H2

(3)

d c-C6H10 f d C6H6 + 2d H2

(4)

e c-C6H10 + e H2 f e c-C6H12 (5) thick Co film:

f c-C6H10 f 6f C(a) + 5f H2

(6)

g c-C6H10 f g C6H6 + 2g H2

(7)

The activities for the Pt(111) surface were calculated in detail by Rodriguez and Campbell.23 Using their calculated activities, and calibrating them to the exposures used in the current work, we calculated the activity for decomposition and dehydrogenation on Pt(111) to be 0.087 and 0.032 cyclohexene per Pt atom, respectively. To calculate the activity toward cyclohexene dehydrogenation on the 1 ML Co/Pt(111) surface, the peak areas of benzene desorption from Figure 5b were compared, as shown in eq 8.

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Figure 7. TPD spectra of cyclohexene (82 amu), benzene (78 amu), cyclohexane (84 amu) and hydrogen (2 amu) from the reaction of 0.5 langmuir of H2 and 3 langmuirs of cyclohexene on 0.8 ML Co/Pt(111).

This calculation leads to a value of 0.034 cyclohexene molecules per Pt atom for d.

TABLE 1: Activity and Selectivity of Cyclohexene Hydrogenation on Different Co/Pt(111) Surfaces activity (per Pt (or Co) atom)

AREA C6H6 from Pt(111) AREA C6H6 from Co/Pt(111)

) 0.92 )

0.032 d

(8)

The activity for cyclohexene self-hydrogenation on 1 ML Co/ Pt(111) was calculated by comparing the peak areas of benzene and cyclohexane desorption from the 1 ML Co/Pt(111) surface, shown in eq 9. The mass spectrometer sensitivity for cyclohexane and benzene was determined experimentally as follows: By taking into consideration of the different ion gauge sensitivities for c-C6H12 (6.4) and C6H6 (5.9), and by exposing the UHV chamber to equal pressures of c-C6H12 (6.4 × 10-8 Torr) and C6H6 (5.9 × 10-8 Torr), we determined the mass spectrometer intensity ratio to be 0.41 c-C6H12/C6H6. This method led to a cyclohexane activity (e) of 0.002 c-C6H10 molecule/Pt atom.

AREA C6H6 from Co/Pt(111) AREA c-C6H12 from Co/Pt(111)

0.41 ) 16.4 )

d e

(9)

The calculation for the decomposition activity on the 1 ML Co/ Pt(111) surface was performed using the peak areas of the hydrogen product in Figure 6. As shown in eq 10 below, the hydrogen desorption is related to the amount of hydrogen produced in the dehydrogenation and decomposition reactions

surface

C(a)

selectivity (%)

C6H6 c-C6H12 total C(a) C6H6 c-C6H12

Pt(111) 0.087 0.032 ∼1 ML Co/Pt(111) 0.068 0.034 thick Co film 0.056 0.035 H/∼1 ML Co/Pt(111) 0.026

0.002 0.006

0.119 73 0.104 65 0.091 62

27 33 38

2 -

(a and b or c and d) and the hydrogen consumed in the selfhydrogenation reaction (e). The value for c can then be calculated to be 0.068 C6H10 molecule/Pt atom using eq 10. The calculated activity and selectivity of the two surfaces are summarized in Table 1.

AREA H2 from Pt(111) AREA H2 from Co/Pt(111)

) 1.22 )

5a + 2b 5c + 2d - e

(10)

The activity for the thick Co film on Pt(111) was also calculated using TPD peak areas. Similar to the 1 ML Co/Pt(111) surface, the peak areas for benzene and hydrogen were compared with that of Pt(111) (eqs 11 and 12) to determine values for f and g to be 0.07 and 0.044 molecules/Pt atom, respectively. Due to the large differences in Co and Pt radii (10-11% lattice mismatch), this activity was normalized on a per Co atom basis. The density of atoms on the Pt(111) surface was compared to that of a Co(0001) surface (densityCo/densityPt ) rPt2/rCo2 )1.24). Using this factor, f and g were reduced to 0.056 and

Low-Temperature Hydrogenation on Co/Pt(111) Surfaces

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Figure 8. Hydrogen desorption from (a) Ni/Pt(111) and (b) Co/Pt(111) surfaces deposited at 600 K (bottom spectrum) and 300 K (middle spectrum) and deposited at 300 K, then annealed to 600 K (top spectrum).

Figure 9. Comparison of TPD spectra of H2, benzene, and cyclohexane from the reaction of cyclohexene on 0.9 ML Ni/Pt(111) and 0.8 ML Co/Pt(111).

0.035 molecules/Co atom. These values are also summarized in Table 1.

AREA C6H6 from Pt(111) 0.032 ) 0.73 ) AREA C6H6 from thick Co g AREA H2 from Pt(111) AREA H2 from Co/Pt(111)

) 1.14 )

5a + 2b 5f + 2g

(11)

(12)

Finally, the effect of preadsorbed hydrogen on the dehydrogenation and hydrogenation is also estimated by comparing the peak areas with and without preadsorbed hydrogen. The activities for the dehydrogenation and hydrogenation pathways are also included in Table 1. The AES and TPD measurements were reproduced at least twice on all the surfaces. Similar to our previous AES and TPD analysis on other surfaces,4,25 the error bars in our estimation of the activity and selectivity are in the range of (10% of the values reported in Table 1. 4.2. Comparison with Ni/Pt(111) Surfaces. The Co/Pt(111) surfaces have exhibited chemical properties that are strikingly similar to those observed in the Ni/Pt(111) system. For example, the molecular desorption of H2 occurs in the temperature range 150-300 K from both Co/Pt(111) and Ni/Pt(111) surfaces.

Figure 8 shows a comparison of hydrogen desorption on the ∼1 ML Ni/Pt(111) and Co/Pt(111) surfaces, with the metal deposited (Ni or Co) at crystal temperatures of 600 and 300 K. Both bimetallic systems show the same effect. When Ni or Co is deposited on Pt(111) at 300 K, hydrogen desorbs in the temperature range 300-400 K. At a deposition temperature of 600 K, hydrogen desorbs from the Ni/Pt(111) surface at ∼220300 K and from the Co/Pt(111) surface at 130-250 K. In both systems, when the 300 K deposited surface is annealed to 600 K (300 K/600 K), the low-temperature hydrogen desorption state reappears. These desorption experiments reveal that the 600 K deposited surface and the 300 K/600 K flash annealed surface of Ni/Pt(111) and Co/Pt(111) have similar properties, but the 300 K deposited surface is significantly different. Surfaces with low Co coverages are inactive to the ethylene decomposition reaction, which is again consistent with TPD studies of ethylene on Ni/Pt(111) surfaces.3,25 A probable explanation of the lack of reactivity on 0.5-1 ML Co/Pt(111) is that the ethylene is so weakly bonded to the surface that it desorbs before any reaction can occur, similar to what is seen on the ∼1 ML Ni/Pt(111) surface. The cyclohexene reaction pathways on Co/Pt(111) surfaces also exhibit reactivities similar to the Ni/Pt(111) system. For comparison, the TPD spectra of the cyclohexane, benzene, and hydrogen products are shown

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TABLE 2: A Comparison of DFT Calculations of the d-Band Center and the Hydrogen Binding Energy, Illustrating the Effect of Subsurface Co and Ni in Pt(111) surface

d-band center (eV)

HBE (eV)

Pt(111) surface Ni on Pt(111) subsurface Ni/Pt(111) surface Co on Pt(111) subsurface Co/Pt(111)

-2.52 -1.14 -2.60 -1.03 -2.74

-2.60 -2.98 -2.42 -3.03 -2.32

in Figure 9 for the 600 K ∼1 ML Co/Pt(111) and Ni/Pt(111) surfaces. Both surfaces produce cyclohexane at ∼220-230 K. In addition, the peak shapes of the hydrogen and benzene TPD spectra appear to be very similar. 4.3. Possible Origins of the Low-Temperature Reactivity. The overall similarities of the Co/Pt(111) and Ni/Pt(111) bimetallic systems are almost certainly related to the similar modification effects of Co and Ni on the Pt(111) surface. Both bimetallic systems form alloy structures where the topmost surface is Pt-enriched and the second layer is Co or Nienriched.5,10,17 Gauthier and co-workers have determined that the atomic composition of the surface is critical in CO adsorption on the Pt-Co(111) surfaces.17 Using a combination of STM and DFT, they found that incorporating Co atoms into the topmost layer increases the calculated CO binding energy, but incorporating the Co atoms into the second layer decreases that binding energy.17 Furthermore, a recent DFT study from our group5,9 showed that the presence of subsurface Co or Ni in a Pt slab shifts the surface Pt d-band away from the Fermi level as compared to clean Pt(111). Using the model proposed by Nørskov and coworkers,26,27 a shift of the d-band center away from the Fermi level should result in a weaker hydrogen binding energy. This is consistent with the observation of a lower desorption temperature of hydrogen from Co/Pt(111) and Ni/Pt(111) than clean Pt(111), as shown in Figure 8 for the 600 K deposited surfaces. For comparison, the surfaces with Ni and Co as topmost layers on Pt(111) have d-band centers that are shifted closer to the Fermi level than Pt(111). These surfaces should consequently bind hydrogen more strongly than clean Pt(111), which is confirmed in the TPD spectra in Figure 8 on the 300 K Co/Pt(111) and Ni/Pt(111) deposited surfaces. Table 2 compares the DFT calculations of the d-band center and hydrogen binding energy (HBE) of Pt(111), Ni/Pt(111), and Co/ Pt(111) surfaces. Details of the calculations and discussion can be found in ref 9. The calculated hydrogen binding energies also exhibit this similar trend, with the surfaces containing subsurface Ni or Co exhibiting HBEs weaker than that of Pt. These calculations clearly show that incorporating subsurface Co or Ni into a Pt(111) substrate changes the electronic properties of the surface Pt d-band and the adsorption properties of hydrogen. As demonstrated experimentally in the current paper, these changes in the electronic properties also have an effect on the ethylene and cyclohexene reaction pathways on the bimetallic surfaces. Finally, in addition to the electronic modification, the d-band center can also be modified by the lattice mismatch between the two metals; this strain effect is discussed in detail in a separate paper for Pt-based bimetallic systems.28

5. Conclusion In this paper we have shown that the Co/Pt(111) surfaces exhibit novel low-temperature hydrogenation properties. At low coverages of Co, hydrogen desorption begins to occur at 150200 K. However, as thicker films of Co are deposited on Pt(111), the hydrogen desorption temperature increases to above 250 K. On the ∼1 ML Co/Pt(111) surfaces, ethylene binds very weakly to the surface, leading to the molecular desorption before any decomposition occurs. In addition, on Co/Pt(111) surfaces with approximately 1 ML Co coverage, cyclohexene selfhydrogenates to form cyclohexane at 221 K. The novel activities of the ∼1 ML Co/Pt(111) surfaces can be attributed to the presence of subsurface Co atoms, which modify the electronic properties of the Pt(111) surface. Acknowledgment. We acknowledge the Department of Energy, Office of Basic Energy Sciences (Grant No. DE-FG0204ER15501) for funding. N.A.K. acknowledges the UD Presidential Fellowship for partial funding. We also acknowledge J. Kitchin for providing the DFT modeling results in Table 2. References and Notes (1) Sinfelt, J. H. Bimetallic Catalysts: DiscoVeries, Concepts and Applications; John Wiley and Sons: New York, 1983. (2) Rodriguez, J. A. Surf. Sci. Rep. 1996, 24, 225. (3) Fru¨hberger, B.; Eng, J.; Chen, J. G. Catal. Lett. 1997, 45, 85. (4) Hwu, H. H.; Eng, J.; Chen, J. G. J. Am. Chem. Soc. 2002, 124, 702. (5) Kitchin, J. R.; Khan, N. A.; Barteau, M. A.; Chen, J. G.; Yakshinskiy, B.; Madey, T. E. Surf. Sci. 2003, 544, 295. (6) Englisch, M.; Ranade, V. S.; Lercher, J. A. J. Mol. Catal. A 1997, 121, 69. (7) Kuhn, M.; Rodriguez, J. A. Surf. Sci. 1996, 355, 85. (8) Tsay, J. S.; Shern, C. S. Surf. Sci. 1998, 396, 313. (9) Kitchin, J. R.; Nørskov, J. K.; Barteau, M. A.; Chen, J. G. J. Chem. Phys. 2004, 120, 10240. (10) Bertolini, J. C.; Massardier, J. Catal. Lett. 1991, 9, 183. (11) Gauthier, Y.; Baudoing-Savois, R.; Bugnard, J. M.; Hebenstreit, W.; Schmid, M.; Varga, P. Surf. Sci. 2000, 466, 155. (12) Lundgren, E.; Stanka, B.; Koprolin, W.; Schmid, M.; Varga, P. Surf. Sci. 1999, 423, 357. (13) Lundgren, E.; Leonardelli, G.; Schmid, M.; Varga, P. Surf. Sci. 2002, 498, 257. (14) Grutter, P.; Durig, U. T. Phys. ReV. B 1994, 49, 2021. (15) Galeotti, M.; Atrei, A.; Bardi, U.; Cortigiani, B.; Rovida, G.; Torrini, M. Surf. ReV. Lett. 1996, 3, 1691. (16) Thiele, J.; Barrett, N. T.; Belkhou, R.; Guillot, C.; Koundi, H. J. Phys. Condens. Matter 1994, 6, 5025. (17) Gauthier, Y.; Schmid, M.; Padovani, S.; Lundgren, E.; Bus, V.; Kresse, G.; Redinger, J.; Varga, P. Phys. ReV. Lett. 2001, 87. (18) http://www.webelements.com (19) Ruban, A.; Skriver, H. L.; Nørskov, J. K. Phys. ReV. B 1999, 59, 15990. (20) Bridge, M. E.; Comrie, C. M.; Lambert, R. M. J. Catal. 1979, 58, 28. (21) Bardi, U.; Beard, B. C.; Ross, P. N. J. Catal. 1990, 124, 22. (22) Bent, B. E. Chem. ReV. 1996, 96, 1361. (23) Rodriguez, J. A.; Campbell, C. T. J. Catal. 1989, 115, 500. (24) Henn, F. C.; Diaz, A. L.; Bussell, M. E.; Hugenschmidt, M. B.; Domagala, M. E.; Campbell, C. T. J. Phys. Chem. 1992, 96, 5965. (25) Khan, N. A.; Zellner, M. B.; Murillo, L. E.; Chen, J. G. Catal. Lett. 2004, 95, 1. (26) Hammer, B.; Nørskov, J. K. Surf. Sci. 1995, 343, 211. (27) Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Nørskov, J. K. J. Mol. Catal. A-Chem. 1997, 115, 421. (28) Kitchin, J. R.; Nørskov, J. K.; Barteau, M. A.; Chen, J. G. Phys. ReV. Lett., in press.