© Copyright 2001 by the American Chemical Society
VOLUME 105, NUMBER 20, MAY 24, 2001
LETTERS Formation of Ca2+(CO)3 Complexes during Low-Temperature CO Adsorption on CaNaY Zeolite K. Hadjiivanov*,† and H. Kno1 zinger‡ Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria, and Department Chemie, Physikalische Chemie, LMU Mu¨ nchen, Butenandtstrasse 5-13 (Haus E), 81377 Mu¨ nchen, Germany ReceiVed: NoVember 20, 2000; In Final Form: March 19, 2001
Adsorption of CO on CaNaY at 85 K results in formation of three types of calcium carbonyl complexes. At high CO coverage (equilibrium CO pressures above ca. 100 Pa) Ca2+(CO)3 species, characterized by an IR band at 2185 cm-1, are predominant. These species are decarbonylated stepwise and first converted into Ca2+(CO)2 dicarbonyls at decreasing coverage during evacuation. The latter complexes lose one CO ligand at further evacuation and form linear Ca2+-CO species that are quite stable at 85 K. Adsorption of a 12CO13CO mixture reveals that the CO ligands in the di- and tricarbonyls behave as independent oscillators. The results obtained are explained by the existence of three coordinative vacancies of Ca2+ cations in SII positions in Y zeolites.
1. Introduction Simultaneous adsorption of two or three molecules on one active site is very important for heterogeneous catalysis because it facilitates the reactions between the adsorbed molecules. The geminal surface species could be divided into two main groups: complex-specified and site-specified.1 In the former case geminal complexes are formed whatever the support due to the formation of stable electron configurations. The site-specified geminal species, on the other hand, are produced as a result of a high number of coordinative vacancies of the atom or cation acting as the adsorption site. A typical example is the formation of dicarbonyls on Ag+-ZSM-5 whereas no geminal species are produced when CO is adsorbed on Ag+ cations supported on nonzeolitic oxides.2,3 The site-specified geminal complexes known so far were detected with cations in certain positions in zeolites. The first * Corresponding author. E-mail:
[email protected]. † Bulgarian Academy of Sciences. ‡ LMU Mu ¨ nchen.
well-documented example has been reported by Paukshtis et al.4 These authors demonstrated that CO adsorption on CaNaY between 100 and 150 K results in the formation of Ca2+(CO)2 species characterized by a carbonyl band at 2186 cm-1. With decreasing equilibrium pressure these species lose one CO ligand and are transformed into Ca2+-CO carbonyls. A similar phenomenon was proposed some years later for the coordination of CO to Na+ cations in Na-ZSM-5.5 More recently, stepwise formation of mono-, di-, and tricarbonyls with Cu-ZSM-56-8 and other copper-exchanged zeolites9 has been reported. Conversion between mono and geminal species was observed also in the following cases: CO on Ag+ in Ag-ZSM-5;2,3 CO on Na+ in NaY,10 Na-ETS-1011 and Na-EMT;12 CO on K+, Rb+, and Cs+ in Me-EMT;12 CO on Cu2+ from Cu-ZSM-5;8 N2 on Na+ in NaY,13 Na-ETS-10,11 and Na-EMT;12 N2 on K+, Rb+, and Cs+ from Me-EMT;12 H2 and NO on Na+ from NaETS-10.11 In all these cases the geminal species were observed at low temperature only. Very recently it has been reported that
10.1021/jp004248m CCC: $20.00 © 2001 American Chemical Society Published on Web 05/02/2001
4532 J. Phys. Chem. B, Vol. 105, No. 20, 2001
Letters
two CO2 molecules can be simultaneously attached to alkalimetal cations in a ZSM-5 matrix.14,15 Na+ cations in NaY occupy the so-called SI, SI′, and SII sites.16 The cations in SI and SI′ positions are not accessible at low temperatures for adsorption even of small molecules, like CO. Recent DF studies have indicated that Na+ cations in SII positions are coordinated to the three oxygen anions of the sixring window, which are facing the supercage.17 In this case, each Na+ cation should possess three coordinative vacancies. However, experimentally it was found that only Na+(CO)2 and Na+(N2)2 species can be formed at low temperature on these cations.5,10-13 It appears that the coordination of a third molecule is difficult because the electrophilicity of the Na+ cations decreases after coordination of two molecules and/or because of steric reasons. The majority of the six-rings in Y-zeolites contain two Al atoms each. It is most likely that exchanged divalent cations in SII positions could compensate simultaneously the charge defect of two Al atoms, thus having three effective coordinative vacancies. Since the Ca2+ anions are definitely characterized by a higher electrophilicity than Na+, we decided to reinvestigate the adsorption of CO on CaY looking for simultaneous coordination of three molecules. 2. Experimental Section The starting NaY material was a commercial Grace Davison product (SP No. 6-5257.0101). The CaNaY sample was prepared by conventional ion exchange with 0.1 M solutions of CaCl2. To achieve a higher exchange degree, this procedure was repeated after calcination of the sample. Carbon monoxide (99.997) was supplied by Linde. Labeled carbon monoxide (13CO) was provided by Aldrich Chemical and had an isotopic purity of 99.0 atom %. It contained about 5% of 13C18O. The IR spectra were recorded on a Bruker IFS-66 spectrometer with a spectral resolution of 2 cm-1. Self-supporting wafers were prepared from the sample powder and thermally treated directly in the IR cell. The cell was purpose-made for lowtemperature experiments and connected to a vacuum-adsorption apparatus with a residual pressure lower than 10-3 Pa. Prior to the adsorption measurements, the samples were activated by calcination at 773 K for 1 h and evacuation for 1 h at the same temperature prior to cooling to 85 K. 3. Results and Discussion Adsorption of CO (400 Pa equilibrium pressure) on CaNaY at 85 K results in the formation of three principal bands in the carbonyl stretching region, namely at 2185, 2167, and 2145 cm-1 (Figure 1, spectrum a). The band at 2145 cm-1 is due to physically adsorbed CO and disappears on evacuation. The band at 2167 cm-1 has been found under similar conditions after CO adsorption on NaY and attributed to Na+(CO)2 species.10 Since the adsorption in this case is weak, both CO molecules behave as independent oscillators so that only one CO stretching mode is observed. On evacuation, these species loose one CO ligand and are converted into Na+-CO linear complexes characterized by a band at 2175 cm-1. With our CaNaY sample we observed the same phenomenon (Figure 1, spectrum b). Further evacuation leads to a gradual decrease in intensity and then disappearance of the band at 2175 cm-1. The band at 2185 cm-1 also decreases in intensity and its maximum is slightly blueshifted. Simultaneously, a new band at 2191 cm-1 grows in and then starts to decline on further evacuation. At lower coverages a band at 2201 cm-1 is formed at the expense of the band at 2191 cm-1. This band reaches maximum intensity prior
Figure 1. FTIR spectra of CO adsorbed on CaNaY at 85 K. Equilibrium pressure of 400 Pa CO (a) and gradual decrease of the coverage during evacuation down to ca. 10-2 Pa (b-r).
to the disappearance of the band at 2191 cm-1 (Figure 1, spectrum q) followed by a slight decrease in intensity. However, the band at 2201 cm-1 is highly resistant to evacuation at 85 K. The presented spectra indicate the existence of three carbonyl species of the Ca2+ cation formed stepwise. The band at 2201 cm-1 is, in agreement with literature data,4,18-20 assigned to Ca2+-CO linear complexes, with the ligand CO being bonded by electrostatic forces. We have detected these species even at ambient temperature, which implies a relatively strong interaction between Ca2+ and CO as compared to the system Na+CO. This is in agreement with the higher charge of Ca2+ leading to a stronger polarizing power of the cation. The position of the 2201 cm-1 band is coverage independent, which is seen from the deconvoluted spectra (Figure 2, spectra a, b). This is easily explained by the fact that the Ca2+ ions acting as adsorption sites are isolated cations. The second type of calcium carbonyl species is characterized by a band at 2191 cm-1. As mentioned above, Paukshtis et al.4 have already assigned this band to geminal Ca2+(CO)2 complexes. The formation of the same species has been reported again very recently.18 The lower carbonyl frequency, as compared to that of linear monocarbonyl species, is caused by a “local” chemical shift: the adsorption of the first CO molecule decreases the polarizing power of the Ca2+ cations and the CO molecules in the geminal species are polarized to a smaller extent as compared to the linear complexes. Since the adsorption is weak, no splitting of the CO modes into symmetric and antisymmetric vibrations occurs and both CO molecules behave as independent oscillators. This conclusion was further supported by experiments involving adsorption of 12CO-13CO isotopic mixtures (see below). Here again, the band position is coverage independent (Figure 2, spectra a-d), also in agreement with the fact that the Ca2+ sites are isolated.
Letters
J. Phys. Chem. B, Vol. 105, No. 20, 2001 4533
Figure 3. FTIR spectra of 12CO-13CO isotopic mixture (mole ratio ca. 1:1) adsorbed on CaNaY at 85 K. Equilibrium pressure of 100 Pa CO (a) and gradual decrease of the coverage during evacuation down to ca. 10-2 Pa (b-j).
Figure 2. Computer deconvolution of some spectra presented in Figure 1.
The third type of calcium carbonyl species are characterized by a band at ca. 2185 cm-1. This band has not been reported in the literature so far most probably because of the higher temperature and lower CO pressures used by previous authors when studying the CaNaY system.4,18 The spectra presented in Figure 1 clearly indicate the formation of Ca2+(CO)3 species. Here, a small red-shift of the band position was observed. At low intensities, the band is detected at 2188 cm-1 (Figure 2, spectra b, c). At ca. half of the maximum intensity value, the band maximum is observed at 2187 cm-1 (Figure 2, spectrum d), whereas it is located at 2185 cm-1 at saturation (Figure 2, spectrum d). A careful inspection of the spectra shows that these phenomena are related to the occupation of the Na+ sites. Thus, the red shift of the Ca2+(CO)3 band with the increase of coverage can be explained by lateral interactions mainly with CO molecules forming Na+-CO and Na+(CO)2 species. To obtain information for eventual vibrational coupling between the adsorbed CO molecules, we studied the coadsorption of 12CO-13CO (molar ratio ca. 1:1) isotopic mixture (Figure 3). The spectra in the 12C-O stretching region are essentially identical (although with lower intensity) to those recorded after 12CO adsorption. The carbonyls formed with the participation of 13CO are detected as follows: Ca2+-13CO, at 2151 cm-1; Ca2+(13CO)2 and Ca2+(13CO)(12CO), at 2142 cm-1; Ca2+(13CO)3, Ca2+(13CO)2(12CO), and Ca2+(13CO)(12CO)2, at 2136 cm-1; Na+-13CO, at 2127 cm-1; Na2+(13CO)2 and Na2+(13CO)(12CO), at 2122 cm-1. In addition, bands arising from species containing 13C18O molecules were detected in the 2100-2060 cm-1 region. The observed shifts coincide very well with the theoretically expected ones. The results support the suggestion made above that in the geminal species the CO molecules behave as independent oscillators. The change in the CO stretching frequency when the Ca2+CO linear carbonyls are converted into geminal Ca2+(CO)2 species is -10 cm-1. The local chemical shifts have to be smaller when dicarbonyls are converted into tricarbonyls, which
is in full agreement with the experimentally obtained frequency of 2188 cm-1. An additional proof for the inference that the band at 21882185 cm-1 characterizes tricarbonyls can be obtained from theoretical studies of the process. It has been predicted from both kinetic1,21 and thermodynamic22 considerations that during the formation of ML2 surface species (L ) ligand) the maximum concentration of the linear species is reached at a total coverage of one-half. However, when ML3 species are produced stepwise, the maximum concentration of the linear species will be reached at a total coverage of one-third and the maximum concentration of the dicarbonyls at a total coverage of two-thirds. Precise investigations have indicated that the molar absorption coefficient of polarized CO is almost independent of the CO stretching frequency.23 Similar conclusions have been drawn from DF studies.24 This allows the relative numbers of the different calcium carbonyl species on CaY to be estimated from the intensity of the IR bands. We observed that the maximum concentration of linear Ca2+-CO species indeed occurs under conditions at which the total number of CO molecules coordinated to calcium cations is ca. one-third of their maximum uptake. The maximum concentration of dicarbonyls was observed when the total number of CO molecules coordinated to calcium cations was ca. two-thirds of their maximum uptake. These observations support the proposed assignments of the IR bands. The ability of simultaneous coordination of two small molecules to cations in some positions in zeolites is a phenomenon well documented by IR spectroscopy.1-15 Very recently this ability was confirmed by DF calculations.25 It was reported that two N2 molecules can be attached to one Na+ site in NaY and the calculation confirmed that both molecules behave as independent oscillators. Since Ca2+ cations are more electrophilic than Na+ and CO is a stronger base than N2, one can expect that simultaneous coordination of three CO molecules to Ca2+ cations in CaY will be energetically possible. The accessible Ca2+ cations in Y zeolites occupy SII positions and are coordinated to the three oxygen anions facing the supercage. This is schematically shown in Figure 4. It is seen that the Ca2+ cations have three coordinative vacancies each and that formation of tricarbonyls is sterically possible. The ionic radii of Na+ and Ca2+ (0.97 and 0.99 Å, respectively) are very similar. Na+ cations in the same position also provide three coordinative vacancies, but their electrophilicity appears to be too weak to form Na+(CO)3 species. Finally, it is worth mentioning that we have obtained similar
4534 J. Phys. Chem. B, Vol. 105, No. 20, 2001
Figure 4. Scheme presenting a SII position in faujasite occupied by a Ca2+ cation.
results for N2 adsorption on the same CaNaY sample and for the adsorption of CO on SrNaY and BaNaY zeolites. Acknowledgment. This work was financially supported by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 338). We also thank Dr. L. Dimitrov for preparing the CaNaY sample. K.H. is indebted to the Alexander-von-HumboldtFoundation for a research fellowship. References and Notes (1) Hadjiivanov, K.; Ivanova, E.; Klissurski, D. Catal. Today, in press. (2) Hadjiivanov, K.; Kno¨zinger, H. J. Phys. Chem. B 1998, 102, 10936. (3) Hadjiivanov, K. Micropor. Mesopor. Mater. 1998, 24, 41. (4) Paukshtis, E.; Soltanov, R.; Yurchenko, E. React. Kinet. Catal. Lett. 1983, 22, 147.
Letters (5) Bordiga, S.; Escalona Platero, E.; Otero Arean, O.; Lamberti, C.; Zecchina, A. J. Catal. 1992, 137, 179. (6) Spoto, G.; Zecchina, A.; Bordiga, S.; Ricchiardi, S.; Martra, G.; Leofanti, G.; Petrini, G. Appl. Catal. B: EnViron. 1994, 3, 151. (7) Pieplu, T.; Poignant, F.; Vallet, A.; Saussey, J.; Lavalley, J.-C. Stud. Surf. Sci. Catal. 1995, 96, 619. (8) Hadjiivanov, K.; Kno¨zinger, H. J. Catal. 2000, 191, 480. (9) Borovkov, V.; Karge, H. J. Chem. Soc., Faraday Trans. 1995, 91, 2035. (10) Hadjiivanov, K.; Kno¨zinger, H. Chem. Phys. Lett. 1999, 303, 513. (11) Zecchina, A.; Otero Arean, C.; Turnes Palomino, G.; Geobaldo, F.; Lamberti, C.; Spoto, G.; Bordiga, S. Phys. Chem. Chem. Phys. 1999, 1, 1649. (12) Hadjiivanov, K.; Massiani, P.; Kno¨zinger, H. Phys. Chem. Chem. Phys. 1999, 1, 3831. (13) Hadjiivanov, K.; Kno¨zinger, H. Catal. Lett. 1999, 58, 21. (14) Bonelli, B.; Onida, B.; Fubini, B.; Otero Arean, C.; Garrone, E. Langmuir 2000, 16, 4976. (15) Bonelli, B.; Civalleri, B.; Fubini, B.; Ugliengo, P.; Otero-Arean, C.; Garrone, E. J. Phys. Chem. B 2000, 104, 10978. (16) Kno¨zinger, H.; Huber, S. J. Chem. Soc., Faraday Trans. 1998, 94, 2047. (17) Vayssilov, G.; Staufer, M.; Belling, T.; Neyman, K.; Kno¨zinger, H.; Ro¨sch, N. J. Phys. Chem. B 1999, 103, 7920. (18) Tsyganenko, A.; Otero Arean, C.; Escalona Platero, E. Stud. Surf. Sci. Catal. 2000, 130, 3143. (19) Li, P.; Xiang, Y.; Grassian, V. H.; Karsen, S. C. J. Phys. Chem. B 1999, 103, 5058. (20) Rabo, J. A.; Angell, C. L.; Kasai, P. H.; Schomaker, V. Discuss. Faraday Soc. 1966, 41, 328. (21) Hadjiivanov, K.; Klissurski, D. React. Kinet. Catal. Lett. 1991, 44, 229. (22) Garrone, E.; Ugliengo, P. J. Chem. Soc., Faraday Trans. 1 1989, 85, 585. (23) Smirnov, K.; Tsyganenko, A. A. Opt. Spektrosk. 1986, 60, 667 (in Russian). (24) Ferrari, A. M.; Neyman, K.; Belling, M.; Mayer, M.; Ro¨sch, N. J. Phys. Chem. B 1999, 103, 216. (25) Vayssilov, G. N.; Hu, A.; Birkenheuer, U.; Ro¨sch, N. J. Mol. Catal. B 2000, 162, 135.
