Langmuir 2000, 16, 2213-2219
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Microcalorimetric, Infrared Spectroscopic, and DFT Studies of Ethylene Adsorption on Pd and Pd/Sn Catalysts Josephine M. Hill,† Jianyi Shen,†,‡ Ramchandra M. Watwe,† and James A. Dumesic*,† Department of Chemical Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, and Department of Chemistry, Nanjing University, Nanjing 210093, China Received August 17, 1999. In Final Form: November 12, 1999 Microcalorimetric and infrared spectroscopic (FTIR) measurements for the adsorption of ethylene on Pd/SiO2 and Pd/Sn/SiO2 catalysts (4 wt % Pd, Pd/Sn ) 3) have been performed at temperatures of 300, 263, and 233 K. In addition, microcalorimetric measurements were made for H2 and CO adsorption and FTIR studies were conducted of CO adsorption at 300 K on these catalysts. Quantum chemical calculations employing density functional theory (DFT) were performed using Pd10 and Pd6Sn4 clusters. Ethylene adsorption on the catalysts results in the formation of ethylidyne species, di-σ-bonded ethylene, and π-bonded ethylene species at 300 K, with initial heats of adsorption of 160 and 110 kJ/mol for the Pd and Pd/Sn catalysts, respectively. Only di-σ-bonded ethylene and π-bonded ethylene species form at 263 and 233 K, with the π-bonded ethylene species dominating. The initial heats of ethylene adsorption are equal to 110 and 102 kJ/mol on Pd/SiO2 at 263 and 233 K, respectively; and these values are equal to 90 and 85 kJ/mol on Pd/Sn/SiO2 at these lower temperatures. In addition to the lower heats of ethylene adsorption caused by the addition of Sn, a new band at 1542 cm-1 is observed in the IR spectra of ethylene on Pd/Sn/SiO2, and this band is representative of a weakly adsorbed, π-bonded ethylene species. Quantum chemical calculations indicate that the electronic effect of Sn addition to Pd is most significant for adsorption at 3-fold sites (e.g., formation of ethylidyne species), the effect of Sn is smaller for adsorption at bridge-bonded sites (e.g., formation of di-σ-adsorbed ethylene), and the effect of Sn is smallest for adsorption at atop sites (e.g., formation of π-adsorbed ethylene).
Introduction Palladium catalysts are widely used for the selective hydrogenation of dienes and alkynes to mono-olefins.1 In addition, Pd/Sn catalysts have been studied for toluene acetoxylation,2 CO oxidation,3,4 catalytic coupling of ethyne,5 hydrocracking and hydroisomerization of nheptane,6 and cyclotrimerization of acetylene to benzene.7 In a previous study, we investigated the adsorptive properties of silica-supported Pt and Pt/Sn catalysts, and we found that the addition of Sn to Pt leads to geometric as well as electronic effects on the properties of surface Pt sites for the adsorption of ethylene.8,9 In the present study, we examine the effects of Sn on the properties of silica-supported Pd catalysts for the adsorption of H2, CO, and C2H4. The surface species that form on palladium after exposure to ethylene depend on temperature and the * To whom all correspondence should be addressed. Email:
[email protected]. Tel.: 608-262-1095. Fax: 608-262-5434. † University of Wisconsin-Madison. ‡ Nanjing University. (1) Encyclopedia of Chemical Processing and Design; Marcel Dekker: New York, 1992; Vol. 41. (2) Tanielyan, S. K.; Augustine, R. L. J. Mol. Catal. 1994, 90, 267. (3) Aran˜a, J.; Ramirez de la Piscina, P.; Llorca, J.; Sales, J.; Homs, N. Chem. Mater. 1998, 10, 1333. (4) Logan, A. D.; Paffett, M. T. J. Catal. 1992, 133, 179. (5) Lee, A. F.; Baddeley, C. J.; Hardacre, C.; Moggridge, G. D.; Ormerod, R. M.; Lambert, R. M.; Candy, J. P.; Basset, J.-M. J. Phys. Chem. B 1997, 101, 2797. (6) Henriques, C.; Dufresne, P.; Marcilly, C.; Ribeiro, F. R. Appl. Catal. 1986, 21, 169. (7) Xu, C.; Peck, J. W.; Koel, B. E. J. Am. Chem. Soc. 1993, 115, 751. (8) Spiewak, B. E.; Cortright, R. D.; Dumesic, J. A. J. Catal. 1998, 176, 405. (9) Shen, J.; Hill, J. M.; Watwe, R. M.; Spiewak, B. E.; Dumesic, J. A. J. Phys. Chem. B 1998, 103, 3923.
structure of the surface. Unlike Pt and Ni that readily form di-σ-adsorbed surface species, Pd has a strong preference for π-adsorbed complexes.10 For example, studies on single crystals indicate that π-adsorbed ethylene species form at 90 K on Pd(110)11 and at 150 K on Pd(111),12,13 whereas π-adsorbed and di-σ-adsorbed ethylene species form at 80 K on Pd(100).14 At room temperature, ethylidyne species exist on Pd(111),13,15,16 but they do not form on the Pd(100) surface even after heating to 500 K.14 Similarly, di-σ-adsorbed ethylene species did not form on Pd(111) on heating to 300 K.13 Binet et al.17 observed π-adsorbed ethylene, di-σadsorbed ethylene, and ethylidyne species at room temperature on Pd/Al2O3. Mohsin et al.,18 however, detected only ethylidyne and π-adsorbed ethylene species on Pd/ Al2O3, while they observed ethylidyne species, di-σadsorbed ethylene, and π-adsorbed ethylene species on Pt/Al2O3. They also found that the nature of the species formed depends on the pretreatment temperature. For example, when a reduced Pt/Al2O3 catalyst was evacuated at a temperature of 563 K or lower, then π-adsorbed ethylene species were not observed after the catalyst was exposed to ethylene; however, π-adsorbed ethylene species were observed when the sample was evacuated at tem(10) Sheppard, N. Annu. Rev. Phys. Chem. 1988, 39, 589. (11) Yoshinobu, J.; Sekitani, T.; Onchi, M.; Nishijima, M. J. Electron Spectrosc. Rel. Phenom. 1990, 54/55, 697. (12) Tysoe, W. T.; Nyberg, G. L.; Lambert, R. M. J. Phys. Chem. 1984, 88, 1960. (13) Gates, J. A.; Kesmodel, L. L. Surf. Sci. 1983, 124, 68. (14) Stuve, E. M.; Madix, R. J. J. Phys. Chem. 1985, 89, 105. (15) Kesmodel, L. L.; Gates, J. A. Surf. Sci. 1981, 111, L747. (16) Kaltchev, M.; Thompson, A. W.; Tysoe, W. T. Surf. Sci. 1997, 391, 145. (17) Binet, C.; Jadi, A.; Lavalley, J.-C. J. Chim. Phys. 1989, 86, 471. (18) Mohsin, S. B.; Trenary, M.; Robota, H. J. J. Phys. Chem. 1991, 95, 6657.
10.1021/la991112a CCC: $19.00 © 2000 American Chemical Society Published on Web 01/06/2000
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Hill et al.
Table 1. Compositions and Initial Heats of Ethylene Adsorption for Catalysts
catalyst
Pd loading (wt %)
Sn loading (wt %)
Pd/SiO2 Pd/Sn/SiO2
4.27 3.81
s 1.39
initial heat for C2H4 adsorption (kJ/mol)a 300 K 263 K 233 K 160 110
110 90
102 85
a This heat of adsorption is defined as the negative of the enthalpy change of adsorption per mole of gas adsorbed.
peratures above 563 K during pretreatment. They attributed this effect to the presence of a small amount of adsorbed hydrogen on the surface when the sample was evacuated at temperatures below 563 K. Similarly, Beebe and Yates19 used an outgassing temperature of 475 K during pretreatment, and they did not observe π-adsorbed ethylene species on their Pt or Pd/Al2O3 catalysts. In the present paper, we report the results of microcalorimetric measurements of the adsorption of H2, CO, and C2H4 on Pd/SiO2 and Pd/Sn/SiO2 catalysts, and we compare these results with those obtained on Pt/SiO2 and Pt/Sn/SiO2 catalysts. In addition, we have employed infrared spectroscopy to study the nature of the surface species formed upon adsorption of CO and C2H4 on Pd/ SiO2 and Pd/Sn/SiO2 catalysts. Our microcalorimetric and infrared spectroscopic measurements of C2H4 adsorption were conducted at temperatures of 233, 263, and 300 K. Finally, to complement our experimental results, we have used quantum chemical calculations employing density functional theory to study the adsorption of carbon monoxide and ethylene on Pd10 and Pd6Sn4 clusters. Experimental Section Catalyst Preparation and Characterization. Silica-supported catalysts were prepared by impregnation of silica (Cab-o-Sil Cabot Co.) using solutions containing tetraamine palladium nitrate (Aldrich) and hydrogen tributyl tin acetate (Aldrich). A Pd/SiO2 catalyst (containing 4.27 wt % Pd) was prepared first. A 10 wt % tetraamine palladium nitrate solution was diluted with water and then added to the silica to form a gel upon stirring. The gel was dried at room temperature for 1 day and further dried at 393 K overnight. The Pd/SiO2 catalyst was calcined in O2 at 573 K for 3 h. A portion of this Pd/SiO2 catalyst was used as the starting material for a Pd/Sn/SiO2 catalyst having a Pd/Sn atomic ratio of 3:1. A solution of tributyl tin acetate in methanol was added to the Pd/SiO2 catalyst under a flow of nitrogen. The Pd/Sn/SiO2 catalyst was dried and then calcined at 573 K for 3 h. The elemental compositions (Galbraith Laboratories Inc.) of the catalysts are given in Table 1. The catalysts were pretreated for microcalorimetric experiments in a down-flow Pyrex treatment cell. A Pyrex NMR tube (Wilmad Glass) with an outer diameter of 5 mm and length of 18 cm was sealed to the side of the glass treatment cell and served as a capsule in which to seal the sample. The sample was heated in flowing (300 cm3/min) ultrahigh purity hydrogen (99.999%, Liquid Carbonic) to 723 K over 8 h, held at 723 K for 8 h, after which time the H2 was evacuated and the sample was purged in ultrahigh purity helium (99.999%, Liquid Carbonic) for 2 h. Finally, the sample was sealed in He in the capsule, and this capsule was subsequently loaded into the calorimetric cells. Transmission electron microscopy (TEM) measurements were performed with a Phillips CM200 microscope equipped with a LaB6 filament to determine metal particle sizes. The specimens were prepared by grinding the catalyst in a mortar and pestle with filtered water. A drop of the slurry was placed on copper grids (1000 mesh, Ted Pella) that had been previously dipped in an adhesive solution produced by dissolving in naphtha the adhesive from double-sided tape. The sticky grid with the drop of catalyst slurry was then placed over a hot plate to evaporate the water. (19) Beebe, T. P., Jr.; Yates, J. T., Jr. J. Phys. Chem. 1987, 91, 254.
X-ray photoelectron spectroscopy (XPS) analysis for the detection of residual chlorine was performed on a Perkin-Elmer Phi 5400 ESCA system equipped with a Phi 5000 spherical capacitor analyzer and using Mg KR radiation (15 kV, 20 mA and 45° TOA). Microcalorimetric Measurements. Microcalorimetric measurements were performed at temperatures between 203 and 300 K using a Setaram BT2.15D heat-flux microcalorimeter. The calorimeter was connected to a gas handling and volumetric system, employing Baratron capacitance manometers for precision pressure measurement (( 0.5 × 10-4 Torr).20,21 Each sample was treated ex situ in ultrapure flowing gases and subsequently sealed in a Pyrex capsule, as described above. This capsule containing the sample was then broken in a special set of calorimetric cells21 after the sample had attained thermal equilibrium with the calorimeter. After the capsule had been broken, the microcalorimetric data were collected by sequentially introducing small doses (1-10 µmol) of probe molecules (H2, CO, or C2H4) onto the sample until it became saturated. The resulting heat response for each dose was recorded as a function of time and integrated to determine the energy released (mJ). The amount of gas adsorbed (µmol) was determined volumetrically from the dose and equilibrium pressures and the system volumes and temperatures. The differential heat (kJ/mol), defined as the negative of the enthalpy change of adsorption per mole of gas adsorbed, was calculated for each dose by dividing the heat released by the amount of gas adsorbed. Infrared Spectroscopic Measurements. The infrared spectroscopic cell used in these studies allows the sample to be pretreated in situ and IR spectra collected at temperatures between 150 and 670 K.9 In this manner, the results from IR spectroscopy can be used to determine what species are formed during microcalorimetric measurements at a specific temperature. Catalyst samples (∼25 mg/cm2) were pressed into selfsupporting pellets (13 mm in diameter) and loaded into the IR cell, where they were reduced in flowing H2 for 8 h at 723 K and then purged in flowing He for 2 h at 723 K. The IR cell was then connected to a high vacuum system and placed in the infrared spectrometer (Mattson Galaxy Series FTIR 5000). The cell was evacuated to ca. 0.2 Torr of He within the cell to facilitate heat conduction during cooling to subambient temperatures. After the sample pellet cooled to the desired temperature, an IR spectrum for the sample was collected. Next, a known amount of CO or ethylene was dosed onto the pellet and a spectrum of the sample plus adsorbate was collected. All results reported herein are difference spectra, corresponding to the IR spectra of the sample plus adsorbate minus the IR spectrum of the clean sample at the same temperature. Infrared spectra were collected in the absorbance mode with a resolution of 2 cm-1. Ethylene (99.9%, Matheson) used for both microcalorimetric adsorption and IR spectroscopic studies was purified by successive freeze/pump/thaw cycles with liquid nitrogen. Quantum Chemical Calculations. In the present paper we have conducted quantum chemical calculations employing density functional theory (DFT) using 10-atom metal clusters to represent the catalyst surface. In another publication, we have employed two-dimensional slabs to represent the (111) surface of Pd and PdSn.22 The slab approach rigorously accounts for the true electronic structure and extended field effects for a well-defined surface resulting in better energetics than the cluster approach. Both computational approaches, however, give similar trends for the effects of Sn on the adsorption of C2H4 on Pd, as will be presented below. Quantum chemical DFT calculations on 10-atom clusters were performed with Jaguar software (Schrodinger, Inc.).23 The chosen DFT method, B3LYP, uses a hybrid method employing Becke’s three-parameters approach.24 This functional combines the exact HF exchange, Slater’s local exchange functional, and Becke’s (20) Spiewak, B. E.; Shen, J.; Dumesic, J. A. J. Phys. Chem. 1995, 99, 17640. (21) Spiewak, B. E.; Dumesic, J. A. Thermochim. Acta 1996, 290, 43. (22) Watwe, R. M.; Cortright, R. D.; Hill, J. M.; Nørskov, J. K.; Dumesic, J. A. to be submitted. (23) Jaguar Software, Ver. 3.5, Schro¨dinger, Inc.: Portland, OR 1998. (24) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
Ethylene Adsorption on Pd and Pd/Sn
Langmuir, Vol. 16, No. 5, 2000 2215
Figure 2. Differential heat versus adsorbate coverage for adsorption of H2 at 300 K on Pd/SiO2 (b), and Pd/Sn/SiO2 (9).
Figure 1. Ball and stick model of the 10-atom cluster used for DFT calculations; (a) initial cluster; (b) cluster with π-bonded ethylene, (c) cluster with di-σ-bonded ethylene and (d) cluster with ethylidyne species. Large open circle ) Pd atom; large shaded circle ) Pd or Sn atom; medium filled circle ) C atom; small open circle ) H atom. 1988 nonlocal gradient correction to the exchange functional, with the correlation functionals of Vosko-Wilk-Nusair (VWN) and Lee-Yang-Parr (LYP). The basis set employed in the calculations (LACVP**) uses an effective core potential on all Pd atoms.25 The electrons treated explicitly on Pd are the outermost core and valence electrons (4s2 4p6 4d10). The C and H atoms were treated with the 6-31G** basis set,26 with all electrons being treated explicitly. The electronic energy change of adsorption, ∆Eads, is defined as
∆Eads ) Ecluster/adsorbate - (Ecluster + Eadsorbate) where Ecluster/adsorbate is the electronic energy of the adsorbate on the cluster, Ecluster is the electronic energy of the bare cluster and Eadsorbate is the electronic energy of the adsorbate. We studied the interaction of ethylene with fully optimized, 10-atom Pd clusters. The Pd10 cluster comprised 3 layers containing 6, 3, and 1 Pd atoms (see Figure 1). These layers correspond to (111) planes stacked in the ABC arrangement of the bulk FCC crystal structure. To study the effects of Sn on the adsorptive properties of Pd, we used a Pd6Sn4 cluster in which Sn atoms were placed at the three corners of the 6-atom top layer and at the 1-atom bottom layer. In this manner, the 3-fold hollow site composed of three adjacent Pd atoms was preserved, and the DFT calculations probe the electronic effect of Sn atoms surrounding the 3-fold hollow site. Calculations were conducted for π-bonded ethylene, di-σ-bonded ethylene, and ethylidyne species on Pd10 clusters and Pd6Sn4 clusters.
Results Microcalorimetry and Infrared Spectroscopy. Figure 2 shows the heat of H2 adsorption at 300 K on the Pd/SiO2 and Pd/Sn/SiO2 catalysts. The initial heats are 102 and 92 kJ/mol on the Pd and Pd/Sn catalysts, respectively. The initial heat of 102 kJ/mol on Pd is similar (25) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (26) Hehre, W. J.; Radom, L.; Schleyer, P. V. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; John Wiley: New York: 1987.
Figure 3. Differential heat versus adsorbate coverage for CO adsorption at 300 K on Pd/SiO2 (b), and Pd/Sn/SiO2 (9).
to values of 104 and 102 kJ/mol reported for 1% Pd/SiO227 and for Pd(110),28 respectively. In addition, Chou and Vannice29 obtained an integral heat of 100 kJ/mol for hydrogen adsorption on a highly dispersed 1.2% Pd/SiO2 catalyst. The TEM analysis indicated that the metal particles on both the Pd/SiO2 and Pd/Sn/SiO2 catalysts were smaller than 50 Å in size. Therefore, the low saturation H2 uptake for the Pd/Sn/SiO2 catalyst is not caused by metal particle agglomeration upon the addition of Sn. Instead, the lower saturation H2 uptake on the Pd/ Sn catalyst may be a result of Sn enrichment at the surface, since it has been shown that enrichment of Sn takes place at the surface of Pd/Sn alloy particles.3,5 XPS analysis indicated that there was no residual chlorine on the catalysts after reduction. Figure 3 shows the heat of CO adsorption on the catalysts. The initial heat of adsorption is 132 kJ/mol on Pd/SiO2 and 120 kJ/mol on Pd/Sn/SiO2. The heat on Pd/ SiO2 is lower than an initial heat of 163 kJ/mol measured on Pd(100).30 However, the adsorption of CO is not equilibrated on the Pd/SiO2 catalyst at 300 K (since the high heat of adsorption leads to irreversible adsorption on Pd), and the initial heat of 132 kJ/mol is an integral heat representative of high CO coverages. Our value of 132 kJ/mol is in agreement with the results of Chou and Vannice,31 who obtained integral heats of between 92 and 146 kJ/mol for CO adsorption on supported Pd, depending on the average Pd crystallite size. Comparing the results in Figures 2 and 3, we find that the saturation H2 and CO uptakes for the Pd and Pd/Sn catalysts correspond to H/CO ratios equal to 1.3 and 0.7, respectively. The decrease in the H/CO ratio with the addition of Sn is consistent with (27) Natal-Santiago, M. A.; Podkolzin, S. G.; Cortright, R. D.; Dumesic, J. A. Catal. Lett. 1997, 45, 155. (28) Conrad, H.; Ertl, G.; Latta, E. E. Surf. Sci. 1974, 41, 435. (29) Chou, P.; Vannice, M. A. J. Catal. 1987, 104, 1. (30) Brown, W. A.; Kose, R.; King, D. A. Chem. Rev. 1998, 98, 797. (31) Chou, P.; Vannice, M. A. J. Catal. 1987, 104, 17.
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Figure 4. Infrared spectra for CO adsorption at 300 K with 3 Torr CO in the gas phase on (a) Pd/SiO2, and (b) Pd/Sn/SiO2. The vertical bar corresponds to 0.2 absorbance units.
Figure 5. Differential heat versus adsorbate coverage for C2H4 adsorption on Pd/SiO2 at 300 K (b), 263 K (0), and 233 K (2).
Hill et al.
Figure 6. Infrared spectra for C2H4 adsorption on Pd/SiO2 at (a) 300 K, (b) 263 K, and (c) 233 K. The vertical bar corresponds to 0.001 absorbance units.
Figure 7. Differential heat versus adsorbate coverage for C2H4 adsorption on Pd/Sn/SiO2 at 300 K (b), 263 K (0), and 233 K (2).
results on Pt/Sn alloys.9 The lower value of the H/CO ratio for the Pd/Sn catalyst may be caused by the presence of Pd surface atoms that are surrounded by Sn atoms, such that CO can adsorb molecularly while H2 cannot adsorb dissociatively. Figure 4 shows IR spectra for the adsorption of CO on the Pd/SiO2 and Pd/Sn/SiO2 catalysts, with 3 Torr CO in the gas phase. The peaks at 2086, 1960 and 1850 cm-1 correspond to linearly-bonded, bridge-bonded, and multibonded CO species.32-34 Figure 4 shows that the addition of Sn to Pd decreases the amount of CO adsorbed as bridgeor multi-bonded species, which suggests that some of the Pd atoms have been surrounded by Sn atoms. Alternatively, the electronic effects caused by the presence of Sn for CO adsorption may be more significant for bridge- and 3-fold sites, compared to atop sites. In addition, the C-O bond lengths for adsorbed CO on Pd(111) and Pd3Sn(111) were similar, which is consistent with the observation in the present paper that the IR bands for adsorbed CO do not shift upon addition of Sn to Pd (see Figure 4). Evacuation of gaseous CO from the IR cell at 300 K causes the intensity of the band from linearly-bonded CO to decrease by 25%, while the bands from bridge-bonded and multi-bonded CO are unchanged. This result indicates that the CO is more weakly bound at atop sites. Figure 5 shows microcalorimetric results for the adsorption of C2H4 on Pd/SiO2 at 300, 263, and 233 K. The initial heats at these temperatures are 160, 110, and 102 kJ/mol, respectively, and these values are summarized in Table 1. The initial heat at 300 K is slightly lower than the value of 170 kJ/mol reported for ethylene on a 1% Pd/SiO2 catalyst.27 At 300 K, the plot of heat versus adsorbate coverage shows an apparent maximum, which is caused by the evolution of gaseous ethane produced
from the reaction of ethylene with surface hydrogen atoms that were formed at lower coverages from the dissociative adsorption of ethylene.8 The plots of heat versus coverage for adsorption at 263 and 233 K do not show pronounced maxima. Infrared spectra are shown in Figure 6 for the hydrocarbon species formed on the Pd/SiO2 catalyst upon exposure of the sample to ethylene at the temperatures of the microcalorimetric measurements (Figure 5). We note that the spectra are quite noisy at higher wavenumbers (i.e., between 3200 and 2600 cm-1); therefore, we present results only for the spectral region from 1600 to 1300 cm-1. The spectra contain bands at 1327, 1412, and 1524 cm-1 which correspond to ethylidyne species, di-σ-bonded ethylene, and π-bonded ethylene species, respectively.35,36 At 300 K all three species are present. At 263 and 233 K, however, only di-σ-bonded and π-bonded ethylene species are present, with the latter species dominating. The lower heats of ethylene adsorption observed at 263 and 233 K are consistent with the spectra in Figure 6, which show the absence of ethylidyne species on Pd at these lower temperatures. For comparison, ethylidyne species have been observed on Pd/Al2O3 catalysts at temperatures as low as 195 K,37 whereas ethylidyne species do not form on Pt at temperatures below about 270 K.36 Microcalorimetric results and corresponding IR spectra for ethylene adsorption on Pd/Sn/SiO2 are shown in Figures 7 and 8, respectively. The initial heats of ethylene adsorption are 110, 90, and 85 kJ/mol at 300, 263, and 233 K, respectively, and these values are summarized in
(32) Palazov, A.; Chang, C. C.; Kokes, R. J. J. Catal. 1975, 36, 338. (33) Ashour, S. S.; Bailie, J. E.; Rochester, C. H.; Thomson, J.; Hutchings, G. J. Catal. Lett. 1997, 46, 181. (34) Soma-Noto, Y.; Sachtler, W. M. H. J. Catal. 1974, 32, 315.
(35) Chesters, M. A.; McDougall, G. S.; Pemble, M. E.; Sheppard, N. Appl. Surf. Sci. 1985, 22/23, 369. (36) Sheppard, N.; De La Cruz, C. Adv. Catal. 1996, 41, 1. (37) Soma, Y. J. Catal. 1979, 59, 239.
Ethylene Adsorption on Pd and Pd/Sn
Langmuir, Vol. 16, No. 5, 2000 2217 Table 2. Results from DFT Calculations for Changes in Electronic Energies (kJ/mol) for Interaction of Ethylene with Pd10 and Pd6Sn4 Clusters to Form π-Bonded Ethylene, Di-σ-Bonded Ethylene, and Ethylidyne Species reactiona
Pd10
Pd6Sn4
difference of heat
C2H4+ * S *C2H4(π) C2H4+ * S *C2H4(di-σ) C2H4+ * S *CCH3 +1/2 H2
-75 -138 -93
-6 27 147
-69 -165 -240
a
The * denotes the Pd surface.
Table 3. Calculated Bond Lengths (Å) for Adsorption of Ethylene on Pd10 and Pd6Sn4 Clusters
Figure 8. Infrared spectra for C2H4 adsorption on Pd/Sn/SiO2 at (a) 300 K, (b) 263 K, and (c) 233 K. The vertical bar corresponds to 0.001 absorbance units.
Table 1. At 300 K, there is a weak apparent maximum in the curve of heat versus coverage, which is caused by the formation of gas-phase ethane. This behavior is consistent with the formation of ethylidyne species at 300 K, as seen in the IR spectrum in Figure 8 (a). The plateaus at 30 kJ/mol in the plots of heat versus coverage for ethylene adsorption at 263 and 233 K are caused by adsorption on the silica support. This behavior is consistent with the IR spectra at 263 and 233 K, which show a band at 1442 cm-1 that corresponds to ethylene adsorption on silica. The IR spectra collected after exposure of the Pd/Sn/SiO2 catalyst to ethylene at 263 and 233 K do not show the presence of ethylidyne species, consistent with the lower heats of ethylene adsorption measured under these conditions compared to the value measured at 300 K. Similar to the behavior of ethylene on the Pd/SiO2 catalyst, the π-bonded ethylene species is more prevalent than the di-σ-bonded ethylene species on the Pd/Sn/SiO2 catalyst at all temperatures studied. It is noteworthy that a new band appears at 1542 cm-1 for ethylene adsorption on the Pd/ Sn/SiO2 catalyst, compared to the Pd/SiO2 catalyst. A peak at 1546 cm-1 has been observed at low coverage (
