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Langmuir 1999, 15, 2234-2236
Corrosive Chemisorption of [PtRh5(CO)15]- on MgO W. A. Weber, O.-B. Yang, M. Shirai, and B. C. Gates*,† Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616 Received August 6, 1998. In Final Form: December 31, 1998
Introduction Organometallic compounds often react with metal oxide surfaces much as they react in solutions having properties (e.g., basicity) similar to those of the surfaces.1 The nature of the chemisorption may be simple, for example, resembling ion pairing,1 but it may instead lead to substantial restructuring of the adsorbate, e.g., when [Ir(CO)2(acac)] and CO are converted into [Ir6(CO)16] on γ-Al2O3.2 We now report on an even more complex chemisorptionsa corrosive chemisorptionsofanorganometalliccompound,[PtRh5(CO)15]-, whereby the adsorbate reacts with the adsorbent, MgO, incorporating Mg in the desorbed species. Corrosive chemisorption on metals is well-known, as exemplified by CO on supported Ru3 and tartaric acid on Ni.4 Experimental Methods The preparation of the bimetallic clusters on MgO is described elsewhere;5,6 briefly, [PtRh5(CO)15]- was synthesized from RhCl3‚ xH2O, Na2PtCl6, and CO in methanol slurried with MgO powder (surface area approximately 70 m2/g). The adsorbed species that were left on MgO following removal of the methanol solvent were characterized by νCO infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies and X-ray absorption nearedge spectroscopy (XANES), as reported elsewhere.5,6 The solid samples contained 1.5 wt % (Pt + Rh) on MgO. The samples consisting of the adsorbed species on MgO were stored in a N2-filled drybox for periods up to 6 months. Samples (200 mg) were treated with solutions of [PPN][Cl] [bis(triphenylphosphine)nitrogen(1+) chloride; Aldrich] (10 mg) in methanol (20 mL) to extract the surface species (the anions by cation metathesis). The extract solutions and the supernatant solution in the initial synthesis flask with the MgO were characterized by electrospray ionization mass spectrometry with a Quattro-BQ quadrupole mass spectrometer (Fisons Instruments VG Biotech Division, Manchester, U.K.) in the negative-ion mode. The sample solutions, handled as air-sensitive, were withdrawn by syringe from sealed containers and immediately injected into the mass spectrometer. Each sample was scanned 20 times in the mass range 100-2500 amu at a rate of 1 scan/15 s.
Results Mass spectra of the synthesis solution in contact with MgO (Figure 1A) indicate peaks centered at approximately m/z ) 1073, 1046, 1018, 989, 961, and 935, attributed to * To whom correspondence should be addressed. † Present address: Institut fu ¨ r Physikalische Chemie, Ludwig Maximilians Universita¨t Mu¨nchen, Sophienstrasse 11, 80333 Mu¨nchen, Germany. (1) Lamb, H. H.; Gates, B. C.; Kno¨zinger, H. Angew. Chem., Int. Ed. Engl. 1988, 27, 1127. (2) Zhao, A.; Gates, B. C. Langmuir 1997, 13, 4024. (3) Yokomizo, G. H.; Louis, C.; Bell, A. T. J. Catal. 1989, 120, 1. (4) Hoek, A.; Sachtler, W. M. H. J. Catal. 1979, 58, 276. (5) Xu, Z.; Kawi, S.; Rheingold, A. L.; Gates, B. C. Inorg. Chem. 1994, 33, 4415. (6) Yang, O.-B.; Shirai, M.; Weber, W. A.; Gates, B. C. J. Phys. Chem. B 1998, 102, 8771.
Figure 1. Mass spectra of (A) the supernatant solution removed from the slurry containing [PtRh5(CO)15]- and MgO, (B) the extract solution formed by treatment of the solid sample prepared by removal of solvent from the slurry containing MgO and [PtRh5(CO)15]- (extraction was with [PPN][Cl] in methanol), and (C) the extract solution formed from the solid sample referred to in B after storage of the solid sample under N2 for 6 months (extraction was with [PPN][Cl] in methanol).
PtRh5(CO)x (x ) 13, 12, 11, 10, 9, and 8, respectively), consistent with the conclusion5 that [PtRh5(CO)15]- had formed in the slurry, was extracted intact from the surface, and fragmented in the mass spectrometer by loss of CO ligands. Infrared data summarized elsewhere5 confirm that some of the [PtRh5(CO)15]- that formed in the slurry remained intact on the MgO surface after the solvent had been removed by evacuation; EXAFS data are consistent
10.1021/la9809978 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/26/1999
Notes
with this conclusion; and the identification of this cluster as the principal adsorbed species is confirmed by the agreement between the infrared spectrum of the extracted species and that of the authentic cluster in solution.6 Rh K-edge XANES data indicate the formation of surface species in addition to [PtRh5(CO)15]-; the additional species were inferred to be cationic, likely including rhodium subcarbonyls, Rh(CO)y (y ) 2 or 3).6 Consistent with these conclusions, extraction of the initially formed surface species from the dark brownish gray powder with [PPN][Cl] in methanol gave a solution with an infrared spectrum matching that of [PtRh5(CO)15]-, confirming the observation of Xu et al.5 and the presence of this cluster on the MgO surface. Infrared spectra of the extract solution formed from [PPN][Cl] and the MgOsupported sample that had been stored under N2 in the drybox for 2 days also confirmed the presence of [PtRh5(CO)15]- (νCO 2082 (w), 2041 (s), 2015 (m), 1812 (w) cm-1), and the presence of this cluster was confirmed by the mass spectra indicative of PtRh5(CO)x fragments (Figure 1B). However, another weak peak was also observed in the latter infrared spectra (at 1983 cm-1), suggesting the presence of a small amount of another extracted metal carbonyl. Correspondingly, the mass spectrum of the extract solution gave a family of peaks at approximately m/z ) 1041, 1013, 985, and 957 (Figure 1B); these are attributed to clusters that had lost CO ligands and incorporated Mg, namely, PtRh5(CO)zMg (z ) 11, 10, 9, 8). A smaller amount of these species is also indicated by the mass spectrum of Figure 1A; a comparison of parts A and B of Figure 1 draws us to the inference that the surface species leading to the extraction of PtRh5(CO)zMg (z ) 11, 10, 9, 8) fragments had formed during storage of the sample in the drybox. Data obtained with a sample that had been stored in the drybox for 6 months confirmed the presence of both PtRh5(CO)x (x ) 13, 12, 11, 10, 9, and 8) and PtRh5(CO)zMg (z ) 11, 10, 9, 8) (Figure 1C). The mass spectrum indicative of PtRh5(CO)zMg (z ) 11, 10, 9, 8) fragments from the extract solution was predominant, and that indicative of PtRh5(CO)x (x ) 13, 12, 11, 10, 9, and 8) was smaller (Figure 1C), indicating that substantial chemical reaction had occurred. Thus, the infrared spectrum of the extract solution (Figure 2) (νCO 2060 (s), 2047 (s), 1983 (s), 1790 (ms) cm-1) can be resolved into the major peaks of the spectrum of [PtRh5(CO)15]- (2047 and 1790 cm-1)5 and those of a new metal cluster carbonyl (2060 (s), 1983 (s) cm-1); presumably, any peaks indicating face-bridging CO ligands in the new cluster overlap those of [PtRh5(CO)15]-. The infrared spectrum of the supported metal carbonyl that had been stored in the drybox for 6 months showed the strongest terminal peak at 2019 cm-1 and a shoulder at 2082 cm-1. Although the shape of this spectrum is similar to that of [PtRh5(CO)15]-, the peak positions are similar to those of the sample formed by oxidizing supported [PtRh5(CO)15]- in air for 1 min.7 Thus, we infer that the spectrum of the stored sample is consistent with the presence of both [PtRh5(CO)15]- (the extractable, welldefined species) and cationic species with CO ligands. Because the mass spectra of the extract solution from the sample stored for 6 months indicate lower signal-to-noise ratios than those of the sample stored for only 2 days, we infer that less of the metal-containing species was extractable after 6 months than after 2 days. We suggest (7) Weber, W. A. Ph.D. Thesis, University of California, Davis, CA, 1998.
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Figure 2. Infrared spectra of (A) the solid sample prepared by removal of solvent from the slurry containing MgO and [PtRh5(CO)15]- and subsequently stored for 6 months under N2 and (B) the extract solution formed by treatment of the sample referred to in A with [PPN][Cl] in methanol.
then that over the 6 months that the sample was in the drybox reaction occurred that likely required traces of impurity O2, which oxidized the sample to give nonextractable species such as metal subcarbonyls; alternatively, the initial metal carbonyl clusters might have reacted with surface hydroxyl groups. Discussion The data confirm the results of Xu et al.5 and in addition give evidence that a new metal carbonyl cluster formed on the MgO surface. The infrared spectrum of the new species is similar to that of [PtRh5(CO)15]-, suggesting that the new compound is a cluster with a metal frame similar to that of [PtRh5(CO)15]-. The mass spectral data confirm that the noble metal frame of the new cluster was the same as that of [PtRh5(CO)15]-. The mass spectra show that the fragments formed from the cluster incorporated single Mg atoms and various numbers of CO ligands. Inasmuch as the mass spectra of [PtRh5(CO)15]- indicated PtRh5(CO)13 as the largest observable fragment, we speculate that the new cluster also gives a largest observable fragment that had lost 2 CO ligands, leading to the suggestion that the new cluster was [PtRh5(CO)13Mg] (with an unknown charge). This was presumably the species extracted from the MgO surface. The demonstration of clusters incorporating Mg shows that corrosive chemisorption took place on the MgO. The fate of the CO ligands lost from the initial cluster [PtRh5(CO)15]- is not known; perhaps they formed metal subcarbonyls and/or CO2. To our knowledge, this is the first example of corrosive chemisorption involving removal of a metal atom from a metal oxide surface. The only comparable chemisorption that we are aware of involves rhodium allyl on SiO2, whereby O atoms from the SiO2 react with the allyl ligands to give CO ligands on the Rh.8
2236 Langmuir, Vol. 15, No. 6, 1999
Conclusions [PtRh5(CO)15 formed in a slurry from RhCl3‚xH2O, Na2PtCl6, CO, methanol, and MgO. Removal of methanol led to the adsorption of intact [PtRh5(CO)15]- in addition to species suggested to include rhodium subcarbonyls. The bimetallic clusters were extracted into solution with [PPN][Cl] and characterized by mass spectrometry. The mass spectrum indicated not only fragments formed from [PtRh5(CO)15]- but also fragments identified as PtRh5(CO)zMg, which must have been formed by corrosive chemisorption of [PtRh5(CO)15]- on the MgO. ]-
(8) Dufour, P.; Houtman, C.; Santini, C. C.; Nedez, C.; Basset, J.-M.; Hsu, L. Y.; Shore, S. G. J. Am. Chem. Soc. 1992, 114, 4248.
Notes
Acknowledgment. For support, we thank the Petroleum Research Fund, administered by the American Chemical Society; the Department of Energy, Division of Materials Sciences (Contract DE-FG05-89ER45384), for its role in the operation and development of beam line X-11A at the National Synchrotron Light Source, supported by the Department of Energy, Division of Materials Sciences and Division of Chemical Sciences (Contract DEAC02-76CH00016); and the Stanford Synchrotron Radiation Laboratory.
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