2132
J . Phys. Chem. 1986, 90, 2132-2136
Trivalent and Monovalent PaHadtum Cations in PdNa-X Zeolite: Electron Spin Resonance and Electron Spin Echo Modulation Spectroscopic Studies J. Michalik,t M. Heming, and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: August 19, 1985)
Electron spin resonance and electron spin echo modulation (ESEM) studies have been carried out for paramagnetic palladium species in Na-X zeolite containing various amounts of PdZ+. In activated PdI7Na-X zeolite trivalent palladium cations are stabilized very efficiently in hexagonal prisms (site SI). No paramagnetic species are formed in activated Pd,Na-X. Pd3+ at SI and Pd2+ at SI' are reduced at room temperature by methanol and ethanol faster than by hydrogen. The possible mechanisms of reduction are discussed. As a result of methanol or ethanol adsorption, Pd' cations are formed and migrate toward the a-cages of the zeolite structure where they are coordinated by adsorbate molecules. The ESEM results unambiguously indicate that whereas the Pd3+are located in inaccessible SI sites, the Pd' occupy more accessible sites, probably SII' or SI1 in the sodalite cage of the zeolite structure. The different efficiencies of Pd3+ formation in activated Pd,,Na-X and Pd,,Ca-X are related to different formation and stabilization sites of trivalent palladium in both zeolites: SI in Pd,,Na-X and SI1 in Pd13Ca-X.
Introduction In recent years a number of electron spin resonance (ESR), X-ray photoelectron spectroscopic (XPS), infrared (IR), and X-ray diffraction studies proved that, depending on the pretreatment, the zeolitic framework can stabilize different oxidation states of palladium.'-I However, the mechanism of formation and stabilization of Pd+ and Pd3+ states of palladium is still not well understood because the results of these techniques cannot usually be interpreted to obtain the short-range order around the paramagnetic species. Electron spin echo modulation (ESEM) spectroscopy is a rather unique technique which can detect the weak superhyperfine interactions in a powder between surrounding magnetic nuclei and a paramagnetic s p e c i e ~ . l ~ -It~ ~has been successfully exploited to study the interactions of paramagnetic cations with various adsorbates in activated zeolites.16 Recently, using ESR and ESEM methods, we have been able to characterize various Pd+ complexes in PdCa-X zeolite and to propose their location in the zeolite 1attice.l' In this paper we focus on Pd3+cations in PdNa-X zeolite which are stabilized with much higher yield than in PdCa-X. On the basis of ESEM data we discuss the possible location of Pd3+ cation as well as a mechanism of Pd' formation as a result of adsorption of hydrogen, small alcohols, ethylene, benzene, and other adsorbates. Experimental Section Linde Na-X zeolite (13X), after repeatedly washing with 0.1 M sodium acetate, was ion exchanged with 3 mM or 30 mM Pd(NH,),Cl2 solutions to obtain samples with low (-2 Pd2+per unit cell) or high (- 14-17 Pd2+per unit cell) palladium content. The exchange was carried out at room temperature for 17 h. Palladium zeolites with other cocations than Na+ were prepared by two-step ion exchange, first with a 0.1 M solution of one of the salts KNO,, CsC1, CaCl,, or Mg(N03), and next with a Pd(NH,),CI, solution. The final palladium and associated cocation contents were obtained by commercial atomic absorption analysis. The filtered and washed material was put into 3-mm-0.d. Spectrosil quartz tubes and gradually heated in vacuo over 1 day up to 773 K. Then the samples were heated at 773 K with oxygen for 3 h and evacuated at the same temperature overnight (- 17 h). Some samples were slowly heated in flowing oxygen to 773 K and then were degassed also at 773 K. Samples prepared in both of those ways, referred to further as "activated", were exposed to various gaseous adsorbates at 100 torr. Liquid adsorbates were heated to the temperature at which their vapor pressure reaches 100 torr and exposed to the zeolite. The deuterated compounds On leave from the Institute of Nuclear Chemistry and Technology, Department of Radiation Chemistry and Technology, 03- 195 Warsaw, Poland.
0022-3654/86/2090-2132$01.50/0
DzO, CD30H, CH30D, CzH50D,and C,D6 were obtained from Stohler Isotope Chemicals, and ND, and CzD, were from MSD Isotopes. ESR spectra at 4 and 77 K were measured on a Varian E-9 ESR spectrometer at 9.2 GHz. ESR concentrations were determined by comparison with a weighed crystal of copper sulfate. Electron spin echo spectra were recorded at 4 K with a home-built ~pectrometer.l*~'~ Deuterated adsorbates were used for ESEM measurements in order to analyze deuterium modulation by three-pulse echo methods where T is the time between the first two pulses and T is the time between the second and third pulses. 133Cs( I = 7/2) and 27Al(I = 5 / 2 ) modulations were also recorded in a few cases. To detect deuterium or cesium modulation, the separation of the first two pulses, T , was chosen to correspond to the modulation period of 27A1in order to suppress aluminum m o d ~ l a t i o n . ' ~
Results ESR Characteristics of Palladium-X Zeolite. The ESR spectra of palladium-loaded zeolite X recorded after different pretreatment stages depend very much on the palladium concentration and the
(1) Gallezot, P.; Imelik, B. Adu. Chem. Ser. 1973, No. 121, 66. (2) Naccache, C.; Dutel, J. F.; Che, M. J . Catal. 1973, 29, 179. (3) Naccache, C.; Primet, M.; Mathieu, M. V. Adu. Chem. Ser. 1973, No. 121, 266. (4) Che, M.; Dutel, J. F.; Gallezot, P.; Primet, M. J . Phys. Chem. 1976, 80, 2371. (5) Primet, M.; Ben Taarit, Y. J . Phys. Chem. 1977,81, 1317.
(6) Ben Taarit, Y.; Vedrine, J. C.; Dutel, J. F.; Naccache, C. J . Magn. Reson. 1978, 31, 251. (7) Vedrine, J . C.; Dufaux, M.; Naccache, C.; Imelik, B. J . Chem. SOC., Faraday Trans. 1 1978, 74, 440. (8) Bergeret, G.; Gallezot, P.; Imelik, B. J . Phys. Chem. 1981, 85, 41 1. (9) Bergeret, G.; Tran Manh Tri; Gallezot, P. J . Phys. Chem. 1983, 87, 1160. (10) Narayana, M.; Michalik, J.; Contarini, S.; Kevan, L. J . Phys. Chem. 1985, 89, 3895. (1 1) Michalik, J.; Narayana, M.; Kevan, L. J . Phys. Chem. 1985,89,4553. (12) Mims, W. B. Phys. Rev. B Solid State 1972, 5, 2409. (13) Salikhov, K. M.; Semenov, A. G.; Tsvttkov, Yu. D. Electron Spin Echoes and Their Applications; Science: Novosibirsk, 1976. (14) (a) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N., Eds.; Wiley-Interscience: New York, 1979; Chapter 8. (b) Ichikawa, T.; Kevan, L.; Bowman, M.; Dikanov, S. A,; Tsvetkov, Yu. D. J . Chem. Phys. 1979, 719 1167. (15) Ichikawa, T.; Kevan, L.; Narayana, P. A. J . Phys. Chem. 1979,83, 3378. (16) Kevan, L.; Narayana, J. In Intrazeolite Chemistry; Stuckey, G. D., Dwyer, F. G., Eds.; American Chemical Society: Washington, DC, 1983; ACS Symp. Ser. no. 218, Chapter 17. ( 1 7 ) Narayana, P. A,; Kevan, L. Magn. Reson. Reu. 1983, 1, 234.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. IO, 1986 2133
Palladium Cations in PdNa-X Zeolite B(g
=
3.01)
CaX
Pd,,-
Pd,,
“1
R
n II I/
NaX
Pd,,-
- NaX
/I
b
x1
200 G +-----+ A(giso
=
2.23)
B(g, = 2.105)
Figure 1. ESR spectra of activated Pd,,Ca-X (a) and Pd,,Na-X (b) zeolites at 77 K. TABLE I: Half-Lives of Pd3+ Cations in Activated Pd,,Na-X Zeolite with Various Adsorbates adsorbate hydrogen methanol ethanol 2-propanol 1-butanol ethylene
71/2.
min
0.5 X 10’ CO.1 X 10’ gll is expected. In X zeolite, besides SI sites with octahedral symmetry, the only other location of Pd3+which is in accordance with the isotropic character of signal A is site SI1 with trigonal symmetry. For PdCa-X zeolite X-ray diffraction data are not available. In our previous paper," however, we postulated that the strong affinity of CaZ+for SI' sites forced Pd2+ to occupy more accessible sites, SII' or SII. Thus, it seems reasonable to suggest that, in Pd&a-X, Pd3+ cations arise upon oxidation at SI1 or SII' sites and in the latter case migrate to SI1 sites. The more accessible location of Pd3+ in PdCa-X is consistent with the instability of Pd3+ on sample evacuation. We proposed earlier a reverse disproportionation reaction between Pd3+ at SI1 or SII' and Pdo
Michalik et al. migrating inside the supercage to explain the decay of Pd3+with simultaneous formation of Pd+ cations." The fact that Pd3+cations are not stabilized in Ca-X and Na-X zeolites with low Pd2+content cannot be explained by a mechanism involving the formation of Pd3+at SI' in PdNa-X and at SII' in PdCa-X and later migration to SI or SII, respectively. Both sites, SI' in PdNa-X and SII' in PdCa-X, are preferentially occupied by Pd2+ ions, so one expects formation of Pd3+ in zeolites with a low PdZ+content also. However, sites SI in PdNa-X or SI1 in PdCa-X can be occupied only when all preferential sites are already filled. Thus, we conclude that the Pd3+cations are stabilized in X zeolites only when Pd2+ ions occupy a SI site (PdNa-X) or SI1 site (PdCa-X) before oxidation at 773 K. The formation of Pd+ cations upon sample outgassing at 773 K in Pd2Ca-X zeolite but not in Pd2Na-X once again indicates that in Ca-X zeolite palladium cations are located in more accessible sites. We think that in Ca-X zeolite with low Pd2+ concentration the reverse disproportionation reaction is also responsible for Pd+ formation, in this case a reaction between PdZ+ at SI1 or SII' and PdO. The same reaction can also proceed in Ca-X with higher Pd2+ content as was proposed earlier." Conclusions
Trivalent palladium cations are stabilized much more efficiently in Pd17Na-X than in Pd,,Ca-X. This is directly related to the location of Pd3+cations in both zeolites. In Pd17Na-X Pd3+ are formed in inaccessible S I sites whereas in Pd,,Ca-X they are formed in more accessible SI1 sites. At an SI1 site Pd3+can decay as a result of a reverse disproportionation reaction with Pd atoms migrating in the supercage upon sample evacuation at 773 K, and thus Pd3+is less stable in PdCa-X than in PdNa-X. The different locations found for Pd3+ in PdNa-X vs. PdCa-X imply that the precursor Pd2+ions are located more in SI and SI' sites in PdNa-X but are directed more into SI1 and SII' sites in PdCa-X due to the Ca2+affinity for SI' sites. Pd3+cations are reduced more rapidly by methanol and ethanol than by hydrogen at room temperature in PdNa-X. To explain this result, two alternative mechanisms are proposed: (1) an electron-transfer process from an alcohol molecule at an acidic site via the zeolite framework to a palladium cation or (2) alcohol decomposition on the acidic site with formation of H atoms which can directly reduce Pd3+. The same mechanisms are believed to produce monovalent paladium cations by reduction of Pd2+located at SI' sites. However, when PdZ+formed at SI stays at the same site, monovalent palladium can migrate toward the hexagonal windows between the sodalite and the supercages to be coordinated by adsorbate molecules.
Acknowledgment. This research was supported by the National Science Foundation and the Robert A. Welch Foundation. We are grateful to the Energy Laboratory of the University of Houston for equipment support. M.H. thanks the Deutscher Akademischer Austausch Dienst (DAAD) for a NATO Postdoctoral Fellowship. Registry No. CH,OH, 67-56-1; C2H50H,64-17-5; Pd, 7440-05-3; C6H6,71-43-2; C2H4,74-85-1; H2. 1333-74-0; carbon monoxide, 63008-0; ammonia, 7664-41-7; 1-butanol, 7 1-36-3; 2-propano1, 67-63-0.
