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Tricyclohexylphosphine Adsorbed on Rhodium Kajsa Uvdal,*,† Hans Kariis,† Gunnar Westermark,‡ Mikael Wirde,§ Ulrik Gelius,§ Ingmar Persson,‡ and Bo Liedberg† Laboratory of Applied Physics, Department of Physics and Measurement Technology, Linko¨ ping University S-581 83 Linko¨ ping, Sweden, and Department of Chemistry, Swedish University of Agricultural Sciences, P.O. Box 7015, S-750 07 Uppsala, Sweden, and Department of Physics, Uppsala University, P.O. Box 530, S-751 21 Uppsala, Sweden Received March 13, 1998. In Final Form: September 15, 1998 Tricyclohexylphosphine (TCHP) adsorbates on rhodium, prepared both from solution and by sublimation in UHV are studied by X-ray photoelectron spectroscopy (XPS), infrared reflection-absorption spectroscopy (IRAS), and temperature-programmed desorption (TPD). XPS and IRAS are used to investigate the molecular orientation of the adsorbates and the coordination to the surface. TPD is used to study the stability of the adsorbates. Molecular surface interactions causing chemical shifts in the core level spectra of the adsorbates on rhodium are investigated using multilayer films and adsorbates on gold as references. In the solution deposition procedure, freshly evaporated rhodium samples are shortly exposed to air, resulting in oxygen covered surfaces. The P(2p) core level XPS spectrum shows only one type of phosphorus with a P(2p3/2) binding energy about 2.7 eV higher than when adsorbed on gold. This is in agreement with molecular adsorption through oxygen to the surface. To study the interaction of TCHP with clean rhodium we also prepared a TCHP adsorbate by an in situ sublimation process in UHV. One dominating state of phosphorus is observed with a P(2p3/2)binding energy about 2.6 eV lower than when adsorbed from solution, but close to what was found when adsorbed on gold. This indicates formation of a soft donor/acceptor bond due to a direct coordination of the molecule through the phosphorus lone pair orbital to the rhodium surface. The soft donor/acceptor and oxygen bridging to rhodium lead to a large electronic reorganization of the phosphorus atom, which manifests itself as substantial infrared peak shifts of the cyclohexyl modes, especially the CH2 stretches. Combined IRAS and TPD measurements show molecular desorption at 465 K for the UHV prepared monolayer, which is consistent with strong chemisorption to the rhodium surface.
Introduction Several articles about different phosphine complexes studied by XPS have been published.1-6 However, there are just a few articles regarding tertiary phosphines adsorbed to metal surfaces. Steiner et al. have studied different types of triphenyl compounds (N, P, As, Sb, Bi) adsorbed from solution onto gold and copper.7 Their results indicate monolayer formation on gold and multilayer formation on copper. We have previously reported IRAS and XPS data for tricyclohexylphosphine (TCHP) adsorbed onto a gold surface.8 Phosphines are easily oxidized in contact with air. When using a partly oxidized starting material in solution adsorption experiments, there is a preferential adsorption of the unoxidized tricyclohexylphosphine.8 The P(2p3/2) binding energy and the results from angle dependent XPS are consistent with a coordination of TCHP through the phosphorus atoms to the gold surface. * To whom correspondence may be addressed. Email: kaj@ ifm.liu.se. † Linko ¨ ping University. ‡ Swedish University of Agricultural Sciences. § Uppsala University. (1) van Attekum, P. M.; van der Velden, T. M.; Trooster, J. M. Inorg. Chem. 1980, 18, 701. (2) Hoste, S. H. W,; van de Vondel, D.; van der Kelen, G. P. J. Electron Spec. Relat. Phenom. 1976, 5, 227. (3) Johnson, O. Chem. Scripta 1975, 8, 166. (4) McNeillie, A.; Brown, D. H.; Smith, W. E. J. Chem. Soc., Dalton Trans. 1980, 5, 767. (5) Morgan, W. E.; Stec, W. J.; Albridge, R. G.; R., V. J. Inorg. Chem. 1971, 10, 926. (6) Leigh, G. J.; Bremser, W. J. J. Chem. Soc., Dalton Trans. 1972, 1216. (7) Steiner, U. B.; Neuenschwander, P.; Caseri, W. R.; Suter, U. W.; Stucki, F. Langmuir 1992, 8, 90. (8) Uvdal, K.; Persson, I.; Liedberg, B. Langmuir 1995, 11, 1252.
In this work we have chosen to study TCHP adsorbed onto a catalytic metal, namely rhodium. The surface chemistry is studied and compared to the results obtained when adsorbing TCHP on gold. Gold is a stable and rather inert metal, while rhodium has extreme catalytic properties. The activity and the selectivity of rhodium catalysis are often found to be strongly dependent on the state of oxidation of the metal. Both initial oxygen chemisorption on rhodium at room temperature and thermal oxidation data are presented in the literature. Several studies on oxygen adsorption on single-crystal rhodium surfaces have been reported.9-14 Comelli et al. have presented a real time XPS study of dissociative oxygen adsorption on Rh(110) with unreconstructive (270 K) and reconstructive (570 K) adsorption of oxygen.12 Low oxygen coverage below 0.35 monolayer (ML) at 270 K results in one O(1s) binding energy peak at 529.65 eV, assigned to oxygen located in long-bridge sites. At coverage above 0.35 ML a second peak appeared at 530.25 eV, assigned to oxygen located in fcc 3-fold sites. At higher coverage (≈0.8 ML) the first peak at 529.65 eV disappeared. Schwegman et al. have recently presented low-energy electron diffraction (LEED) intensity data investigating the atomic geometry of O and CO + O phases on Rh(111).13 The oxygen atoms were found to occupy 3-fold fcc sites. Xu et al. have reported on oxygen adsorption on Rh(111) at room temperature by using scanning tunneling microscopy (9) Castner, D. G.; Somorjai, G. A. Surf. Sci. 1979, 83, 60. (10) Baraldi, A.; Gregoratti, L. Appl. Surf. Sci. 1996, 99, 1. (11) Stokbro, K.; Baroni, S. Surf. Sci. 1997, 370, 2. (12) Comelli, G.; Baraldi, A.; Lizzit, S.; Cocco, D.; Paolucci, G.; Rosei, R. Chem. Phys. Lett. 1996, 261, 253. (13) Schwegmann, S.; Over, H.; De Renzi, V.; Ertl, G. Surf. Sci. 1997, 375, 91. (14) Xu, H.; Ng, K. Y. S. Surf. Sci. 1997, 375, 2.
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(STM) and concluded that attractive interactions between chemisorbed oxygen atoms lead to oxygen island formation.14 Oxygen adsorption on polycrystalline rhodium has been studied showing two different types of oxygen, having different reactivities with carbon monoxide.15 In a study of thermal oxidation of thin rhodium films deposited on gold and polycrystalline rhodium foil surface-enhanced raman spectroscopy (SEARS) and XPS data were reported.16 The sample was initially reduced in H2 at 573 K. Upon heating of the sample (oxidized in O2 for 30 min at 323 K) a broad O(1s) peak appears at 531 eV, which was assigned to at least two distinct species. At the end of the heating cycle (oxygen exposure at 573 K) two distinct O(1s) peaks were evident at 530 and 531.7 eV. The 530 eV peak was assigned to a stoichiometric Rh2O3 structure. The existence of an oxyhydroxide (RhOOH) or dioxide (RhO2) along with adsorbed oxygen was also suggested. In this paper we report XPS, IRAS, and TPD measurements of TCHP adsorbed on rhodium. Freshly evaporated polycrystalline rhodium samples were used in both cases. In the solution adsorption experiments the samples were shortly exposed to air before being immersed into the solution. The rhodium samples used for TCHP adsorption in UHV were sputtered clean and alternately exposed to O2 and H2 at 473 K just before the sublimation process in order to remove oxygen and organic contaminants. Experimental Section The TCHP was obtained from Strem Chemicals Inc. The rhodium substrates were prepared by thermal evaporation of 250 Å onto clean single-crystal Si(100) wafers. Before the rhodium film was made, the silicon wafers had been primed with an adhesion layer of 25 Å titanium. The pressure was
