J. Phys. Chem. B 1999, 103, 3923-3934
3923
Microcalorimetric, Infrared Spectroscopic, and DFT Studies of Ethylene Adsorption on Pt/SiO2 and Pt-Sn/SiO2 Catalysts Jianyi Shen,†,‡ Josephine M. Hill,† Ramchandra M. Watwe,† Brian E. Spiewak,†,‡ and James A. Dumesic*,† Department of Chemical Engineering, UniVersity of WisconsinsMadison, Madison, Wisconsin 53706, and Department of Chemistry, Nanjing UniVersity, Nanjing 210093, China ReceiVed: January 21, 1999; In Final Form: March 22, 1999
Microcalorimetric and infrared spectroscopic measurements for adsorption of ethylene on Pt/SiO2 and PtSn/SiO2 (5-8 wt % Pt) have been carried out at temperatures from 173 to 300 K. Ethylene adsorption on Pt and Pt-Sn at temperatures lower than 233 K leads to the formation of π-bonded and di-σ-bonded ethylene species. Ethylidyne species begin to form at temperatures higher than 263 K. Formation of ethylidyne species is suppressed by the presence of Sn. The main surface species observed during ethylene adsorption at room temperature are ethylidyne species on Pt/SiO2, di-σ-bonded ethylene on 7Pt/Sn/SiO2, and π-bonded ethylene on 3Pt/Sn/SiO2. The heat of formation of ethylidyne species and coadsorbed atomic hydrogen on Pt was found to be 157 kJ/mol, while the average heat for the formation of π-bonded and di-σ-bonded ethylene species on Pt was 125 kJ/mol. The heats for formation of π-bonded and di-σ-bonded ethylene on Pt-Sn surfaces were found to be between 98 and 106 kJ/mol. Results from quantum chemical calculations employing density functional theory (DFT) for Pt19 and Pt16Sn3 clusters indicate that Sn donates electrons to the 6sp and 5d orbitals of platinum. These calculations show that the bonding between ethylidyne species and 3-fold hollow sites composed of adjacent Pt atoms involves electron donation from ethylidyne into 6sp orbitals of Pt and back-donation of electrons from 5d orbitals of Pt to the ethylidyne species. The higher electron density on the Pt atoms caused by Sn leads to more repulsive interactions for the formation of ethylidyne species than for the formation for π-bonded and di-σ-bonded ethylene species, and the higher occupation of the Pt 5d orbitals in the presence of Sn requires more extensive back-donation of electrons from Pt to ethylidyne species. These electronic effects caused by Sn weaken the bonding of ethylidyne species at neighboring 3-fold hollow sites.
Introduction Platinum catalysts are widely used in the chemical and petrochemical industries for hydrocarbon processing reactions such as hydrogenation, dehydrogenation, isomerization, aromatization, and hydrogenolysis.1,2 Tin is added to platinum catalysts to alter the product distribution for hydrocarbon conversion, e.g., to inhibit isomerization and hydrogenolysis reactions and to thereby enhance the selectivity for dehydrogenation processes.3-12 Various studies have been conducted to characterize the properties of Pt and Pt-Sn catalysts in terms of structure, adsorption behavior, and coking characteristics.10,13-29 In the present study, we report results of microcalorimetric measurements, infrared spectroscopic studies, and quantum chemical calculations to probe the role of tin in modifying the surface properties of platinum catalysts. The interactions of light hydrocarbon species with welldefined metal surfaces have been studied extensively in the literature using a variety of experimental techniques (e.g., see refs 30 and 31). For example, vibrational spectroscopies have identified the nature of surface species formed upon adsorption and/or reaction of hydrocarbons with metal single crystals as well as with supported metal catalysts (see refs 32 and 33 and * To whom correspondence should be addressed. † University of WisconsinsMadison. ‡ Nanjing University. § Present address: 3M Center, St. Paul, MN 55144-1000.
references therein). In these studies, it has been observed that the species formed from ethylene adsorption depend on temperature and the structure of the metal surface.24,33 Passos et al.24 have reviewed the species formed on Pt(111), Pt(110), Pt(210), Pt(100), and Pt/Al2O3 catalysts. De La Cruz and Sheppard34 observed π- and di-σ-bonded ethylene species on Pt/SiO2 catalysts at low temperatures ( di-σ bonded ethylene > π-bonded ethylene. The results of the quantum chemical calculations shown in Table 2 indicate that the presence of tin atoms weakens the interactions of π-bonded ethylene, di-σ-bonded ethylene, and ethylidyne species with the platinum ensembles. Importantly, the electronic effect of tin on the adsorptive properties of the neighboring Pt atoms is rather small for the π-bonded and diσ-bonded ethylene species (17 and 29 kJ/mol, respectively), whereas the effect of tin is significantly larger (94 kJ/mol) for the bonding of ethylidyne species to Pt atoms. This effect of Sn for the constrained 19-atom metal clusters is also found for fully optimized Pt6Sn4 clusters. In particular, the heat of formation of ethylidyne species from ethylene is less exothermic by 91 kJ/mol on 3-fold hollow sites of Pt atoms in fully optimized Pt6Sn4 clusters compared to Pt10 clusters. Therefore, these calculations suggest that the formation of ethylidyne species should be significantly inhibited on Pt adsorption sites that are adjacent to Sn atoms. Table 3 shows the Mulliken populations of the 6s, 6p, and 5d orbitals on the three Pt atoms constituting the 3-fold hollow site for the formation of ethylidyne species. These calculations indicate that the populations of 6s, 6p, and 5d orbitals on the three Pt atoms increase by 0.28, 0.57, and 0.62 electrons when Sn is added to the 19-atom cluster. Upon formation of ethylidyne species on the Pt19 cluster, the population of the 6s orbitals on the three Pt atoms of the 3-fold hollow site remain nearly unchanged, the population of the 6p orbitals on the three Pt atoms increases by 0.77 electrons, and the population of the 5d orbitals on the three Pt atoms decreases by 0.48 electrons. These results demonstrate that the surface chemical bonds formed
Figure 14. Differential heat versus adsorbate coverage for adsorption of ethylene at 203 K on Pt/SiO2 (b), 7Pt/Sn/SiO2 (O), and 3Pt/Sn/ SiO2 (9).
between ethylidyne species and Pt atoms involve the donation of electrons from ethylidyne species into Pt 6p orbitals and the back-donation of Pt 5d electrons to the ethylidyne species. Similar results were obtained from calculations using fully optimized 10-atom metal clusters, as shown in Table 3. Our slab calculations show that replacing a Pt atom by a Sn atom weakens the interaction of methylidyne species by 30 kJ/ mol, while replacing two Pt atoms by two Sn atoms weakens the interaction by 130 kJ/mol. This result is consistent with the weakening of ethylidyne species with Pt predicted by the cluster calculations. To summarize our results, IR spectra indicate that π -bonded and di-σ-bonded ethylene species form on our supported Pt/ SiO2 and PtSn/SiO2 catalysts at low temperatures (90% of the ethylidyne formation that occurs on Pt (111). Our experimental results are consistent with this possible geometric effect. In particular, it is well established that the surface of Pt/Sn alloy particles can be enriched with Sn, and so it is possible that our 3Pt/Sn/SiO2 catalyst on which the formation of ethylidyne species is suppressed at room temperature has few adjacent 3-fold Pt sites. However, comparison of the heats of adsorption (see Table 1 and Figure 14) for which the same species are adsorbed in similar ratios according to IR spectroscopy (compare Figures 7d, 9d, and 11f) shows that the heat of adsorption generally decreases as the tin content of the catalyst increases. In addition our quantum calculations indicate that Sn significantly weakens the bonding of ethylidyne species at 3-fold hollow sites (see Table 2). Thus,
Shen et al. while the decrease in the number of 3-fold hollow sites composed of three adjacent Pt surface atoms upon the addition of tin may play an important role in decreasing the amount of ethylidyne species formed on the Pt/Sn/SiO2 samples at room temperature, our quantum chemical calculations also indicate that Sn exerts an electronic effect on the interaction of neighboring Pt atoms with ethylidyne species. Thus, it appears that Sn is a particularly effective agent for suppressing the formation of ethylidyne species on Pt, since the geometric and electronic effects work in the same direction. In closing, we speculate about the origin of the electronic interaction through which tin inhibits the formation of ethylidyne species on 3-fold hollow sites comprised of adjacent Pt surface atoms. As noted earlier, the Mulliken population analyses indicate that the formation of ethylidyne species on Pt involves electron donation from ethylidyne to 6sp orbitals on Pt, accompanied by back-donation from 5d orbitals on Pt to the ethylidyne species. Specifically, nearly 0.9 electrons are transferred to the 6sp orbitals of the three Pt atoms, and about 0.5 electrons are back-donated from Pt to the ethylidyne species on the Pt19 cluster. The addition of Sn to Pt leads to higher populations of the 6sp and 5d orbitals of Pt (by 0.85 and 0.62 electrons, respectively). In general, Pauli repulsion interactions between occupied levels are more important for bonding on high-coordination metal sites (such as 3-fold hollow sites involved in formation of ethylidyne species) than for bonding on lower coordination metal sites (such as atop and bridge sites involved in π-bonded and di-σ-bonded ethylene species, respectively).86 Therefore, the higher electron density on the Pt atoms caused by donation of electrons from Sn should lead to more repulsive interactions for the formation of ethylidyne species than for the formation for π-bonded and di-σ-bonded ethylene species, as observed experimentally. Another result from the quantum chemical calculations is that the formation of ethylidyne species on Pt16Sn3 clusters causes nearly 0.5 electrons to be transferred to the 6sp orbitals of the three Pt atoms and about 0.7 electrons to be back-donated from Pt to the ethylidyne species. It is noteworthy that the extent of back-donation to ethylidyne species is significantly higher for Pt16Sn3 clusters (0.7 electrons) compared to Pt19 clusters (0.5 electrons). This difference is related to the higher population of the Pt 5d orbitals in the presence of Sn. In general, bonding interactions between ethylidyne species and Pt atoms involve the overlap of partially filled orbitals. The 5d orbitals of each Pt atom contain approximately 8.87 and 9.07 electrons in the Pt19 and Pt16Sn3 clusters, respectively. Therefore, electrons are transferred out of these highly filled 5d orbitals for effective overlap with ethylidyne species. Because the 5d orbitals are more fully occupied in Pt16Sn3 clusters compared to Pt19 clusters, the extent of back-donation from Pt to ethylidyne species is greater in Pt16Sn3 clusters. This greater extent of electron promotion to higher energy orbitals in the formation of ethylidyne species on Pt16Sn3 clusters leads to a weaker energy of adsorption. Finally, we note that it may be necessary for the geometric and electronic effects of an additive to work in the same direction for the additive to be most effective in the suppression of ethylidyne formation on Pt. Specifically, an agent that only functions by geometric effects that block 3-fold hollow sites comprised of three adjacent Pt atoms may not prevent the surface from reconstructing in the presence of ethylene to form these 3-fold hollow sites. However, an agent that functions by both geometric and electronic effects may not suffer from effects of possible surface reconstruction, since 3-fold hollow sites
Pt/SiO2 and Pt-Sn/SiO2 containing Pt atoms, which may be formed during reconstruction, would still exhibit weak interactions with ethylidyne species. Conclusions The combination of microcalorimetric and in situ infrared spectroscopic measurements for the adsorption of ethylene on Pt/SiO2 and Pt/Sn/SiO2 catalysts at temperatures from 173 to 300 K provides information about the nature and energetics of adsorbed species on Pt and Pt-Sn surfaces. At temperatures lower than 173 K, the adsorption of ethylene takes place on Pt and silica, resulting in an average adsorption heat for Pt/SiO2 samples. At temperatures higher than 203 K, ethylene molecules preferentially occupy Pt sites, leading to adsorption plateaus versus coverage corresponding to ethylene adsorption on Pt and silica surfaces, respectively. Adsorption of ethylene at temperatures from 173 to 233 K leads to the formation of π-bonded and di-σ-bonded ethylene species on the surface of the Pt/SiO2 catalyst. The heat of adsorption was measured to be about 125 kJ/mol under these conditions. Ethylidyne species began to form on the Pt/SiO2 catalyst at temperatures higher than 263 K. At room temperature, ethylidyne species were essentially the only species observed on the surface of Pt/SiO2, corresponding to an adsorption heat of 157 kJ/mol. The addition of tin to platinum decreases the extent of formation of ethylidyne species and thereby promotes the formation of π-bonded and di-σ-bonded ethylene species on Pt at room temperature. Adsorption of ethylene at 203 and 233 K on 7Pt/Sn/SiO2 and 3Pt/Sn/SiO2 catalysts leads to the formation of π-bonded and di-σ-bonded ethylene species, with the π-bonded species dominating. The heats of adsorption for formation of π-bonded and di-σ-bonded ethylene on Pt-Sn surfaces were found to be between 98 and 106 kJ/mol. At temperatures lower than 233 K, only π-bonded and di-σ bonded ethylene were present on all Pt and Pt-Sn surfaces. In addition, the relative amounts of these two species do not change significantly with different Pt/Sn ratios employed in this study. Hence, the decrease in heat for ethylene adsorption at temperatures lower than 233 K with an increase of tin content of Pt/ Sn/SiO2 catalysts suggests that tin weakens the interaction of molecular ethylene species with platinum. Quantum calculations employing DFT indicate that there is electron transfer from tin to platinum. These calculations show that the interaction of ethylidyne species with 3-fold hollow sites composed of three adjacent Pt atoms is significantly weakened by the presence of neighboring Sn atoms, while the interaction of π-bonded and di-σ-bonded ethylene species with atop sites and bridge sites composed of adjacent Pt atoms are only slightly weakened by the presence of neighboring Sn atoms. Mulliken population analyses indicate that the formation of ethylidyne species on Pt involves electron donation from ethylidyne to 6sp orbitals on Pt, accompanied by back-donation from 5d orbitals on Pt to the ethylidyne species. The higher electron density on the Pt atoms caused by donation of electrons from Sn leads to more repulsive interactions for the formation of ethylidyne species than for the formation for π-bonded and di-σ-bonded ethylene species. In addition, the higher occupation of the Pt 5d orbitals in the presence of Sn requires more extensive back-donation of electrons from Pt to ethylidyne species. These effects cause a significant weakening of the interaction energy between ethylidyne and Pt atoms in the presence of Sn.
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