4506
J. Phys. Chem. 1991, 95,4506-4514
of ionization of SiOH groups. This effect is caused by the decrease in the dielectric constant of the solvent as more DMSO is added. Figure 7 presents the 27AIN M R results of the same aluminosilicate solutions for which the 29SiNMR results are presented in Figure 6. As the amount of DMSO in solution increases, the monomer peak shifts upfield slightly and becomes smaller. Due to the higher proportion of AI in the D3R( 1Al) and D4R( 1Al) anions, the Al(3Si) peak at 60.5 ppm in spectrum a increases in intensity as the amount of DMSO in solution increases. The small peak at 67.5 ppm in spectrum a decreases in intensity relative to the other peaks. As discussed above, this peak is tentatively assigned to AI in the linear trimer structure of the type A l U Si-0-Si. The %i NMR resonance a t -27.0 to -27.7 ppm seen in Figure 6 for DMSO concentrations above 20 vol 76 can be assigned to Si atoms in D4R The partial charge on each of the eight Si atoms is calculated to be qsi = 1.65. Reference to Figure 2 indicates that the values of qsi and bsi for Si in D4R anions lie on the correlation line of partial charge and chemical shift. The two additional features appearing at -22.0 to -23.0 ppm and -27.3 to -28.1 ppm in Figure 6 are assigned to the A and B Si atoms in D4R( 1Al) (see Table 11). The calculated partial charge for the A atoms is qSi = 1.63 and that for the B atoms is qsi = 1.67. Assignment of the resonance in the region of -22.0 to -23.0 ppm to A Si atoms and the resonance in the region of -27.3 to -28.1 ppm to B atoms is consistent with the correlation of qSiand Ssi in Figure 2. This assignment is also consistent with (53) Hoebbcl, D.; Garzo, G.; Engelhardt, G.; Vargha, A. Z . Anorg. Allg. Chem. 1982, 494, 31.
the observed 4:3 ratio of peak intensities. AI incorporation into D4R anions has also been observed in TMA (tetramethylammonium) aluminosilicate solutions and methanolic TMA aluminosilicate s o I u t i o n ~ . ~ ~ Conclusions The present study shows that the reaction of aluminate anions with silicate anions in TPA aluminosilicate solutions is a strong function of the silicate ratio R ( R = [Si02]/[(TPA),0]). AS the value of R increases, 29Siand 27AlN M R spectroscopies reveal an increase in the proportion of AI present in aluminosilicate structures. Most of the aluminum is incorporated into the dimer, linear trimer, cyclic trimer, and branched cyclic trimer anions. Evidence is also presented for the incorporation of AI into D3R anions and, with DMSO present, into D4R anions. The partial charges on Si atoms in silicate and aluminosilicate anions, determined from MNDO calculations, are found to correlate with 29Sichemical shifts. The correlation between partial charge and Si chemical shift is used to assign %i NMR peaks associated with D3R( 1AI) and D4R( 1Al) anions.
Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S.Department of Energy under Contract No. DE-AC03-76SF00098, and by a grant from W. R. Grace and Co. Fellowship support for R.F.M. was provided by the Upjohn Company. We thank B. J. Holloway for valuable discussions. (54) Mortlock, R. F.; Bell, A. T.; Radke, C. J. Unpublished work.
Characterization of Ruthenium Specles Generated in H-X Zeolite: Interaction with Carbon Monoxide, Nitrlc Oxide, Oxygen, and Water Guan-Dao Lei and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: October 9, 1990; In Final Form: December 19, 19901
The activation and subsequent interaction with absorbates of the Ru(NH3)d+complex in H-X zeolite have been investigated by electron spin resonance, electron spin echo modulation, infrared, and X-ray photoelectron spectroscopies. Ru(NH3)d+ upon ~ + evacuation at 80 OC. In contrast to M-X incorporated in the a-cage of X zeolite hydrolyzes into R U ( N H ~ ) ~ O H zeolite, where M is an alkali metal, in H-X the formation of the trimeric Ru red complex during activation is suppressed. This cation effect may be due to the availability of more space in H-X zeolite, which provides extra sites for the Ru monomer to be anchored and reduces the mobility of the monomer to decrease the probability of forming the Ru red trimer. Evacuation at 150 OC produces a Ru(II1) complex still coordinated with some NH3, H20 (or OH) and zeolite lattice oxygens. Evacuation at 300 OC produces Ru(II1) ion coordinated only with the zeolite lattice oxygens, probably at a six-ring site. The ESR signal of this Ru(II1) ion is reported for the first time in X zeolite. The interaction of CO, NO, and O2with the Ru(II1) ion produces Ru(III)-(CO)~,Ru(I1)-NO+, and Ru(IV)-O; species in the a-cage. Infrared bands at 2146 and 2088 cm-' are characterized by isotopic substitution as coupled vibrations from a ruthenium(II1) dicarbonyl species. Electron spin echo modulation shows that NO adsorbed on H-X interacts with an aluminum nucleus within less than 0.37 nm.
Introduction Among the various transition metals used in heterogeneous catalysts ruthenium is known to catalyze a variety of reactions.' (1) (a) Dalla Betta, R. A,; Piken, G. A,; Shelef, M. J . Coral. 1974,35,34. (b) Vannice, M. A. Adu. Chem. Ser. 1977, 15, 163. (c) Coughan, B. S.; Narayanan, S.; McCann, W. A,; Carrol, W. M. J . Caral. 1977,97,49. (d) Jacob, P. A.; Nijsand, H. H.; Uytterhoeven, J. B. Prepr.-Am. Chem. Soc. Diu. Per. Chem. 1978, 23,469. (e) Gustafson, B. L.; Lunsford, J. H. J. Coral. 1982, 74, 393. ( f ) Harvey, T. G.; Mathtson, T. W. J . Chem. Soe., Chem. Commun. 1985,188. (g) Moggi. P.; M i e n , 0.;Albanesi, 0 . ; Papadopulsos, S. Appl. Coral. 1989,53. LI. (h) Gao, S.;Schmidt, L. D. J . Carol. 1989,115, 356. (i) Hayashi, T.; Abc, F.; Sakakurai, T.;Tanah, M. J. Mol. Caral. 1990, 58. 165.
0022-365419 1/2095-4506S02.50/0
Alumina- or silica-supported ruthenium selectively reduces nitrogen oxide to molecular nitrogen;2 zeolite-supported ruthenium is an excellent catalyst for the water-gas-shift reaction) and has specific activity for the hydrogenation of carbon monoxide.' The (2) (a) Shelef. M.; Gandhi. H. S. Ind. Eng. Chem., Prod. Res. Dev. 1972,
11, 393. (b) Klimish, R. L.; Taylor, K. C. Enuiron. Sci. Technol. 1973, 7,
127. (c) Clausen, C.; Good, M. L. J. Caral. 1977, 46. 58. (3) (a) Verdonck, J. J.; J a m b , P. A.; Uytterhoeven. J. B. J. Chem. Soc., Chem. Commun. 1979, 191. (b) Jacob. P. A.; Chautillon, R.; De Laet;
Verdouck, J. J. Inrrazeolite Chcmisrry; American Chemical Society: Washington, DC, 1983; p 439. (c) Kellner, C. S.; Bell, A. T.J. Coral. 1981, 71, 288. (d) Chen, Y. W.; Wang, W. J. Coral. Today 1989, 6, 105.
0 1991 American Chemical Society
Ruthenium Species Generated in H-X Zeolite potential utility of supported ruthenium catalysts ranges from automobile emission control to the gasification of coal. To better characterize this catalytic system, the interaction of different valence states of ruthenium in X zeolite with various adsorbates is studied by several spectroscopies. A previous study has shown the existence of alkali-metal ion cocation effects on the formation and adsorbate geometry of ruthenium species from R ~ ( N H ~ ) ~ ~ + - e x c h a nXg ~e de o l i t e . ~In this study the hydrogen form of X zeolite is used as a support, and a unique property of this zeolite to suppress the formation of the trimeric Ru red complex upon activation is observed. Electron spin resonance (ESR) spectroscopy together with infrared (IR) and X-ray photoelectron (XPS) spectroscopies were applied to study ruthenium species generated upon activation and after adsorption of different adsorbates. An ESR signal of Ru(1II) ions coordinated only to the X zeolite lattice is reported for the first time. Experimental Section
Material and Sample Preparation. Linde 13X (Na-X) zeolite was washed with 0.1 M sodium acetate solution and then exchanged with 1 M NH4N03solution four times at 70 OC to obtain NH4-X zeolite. This zeolite was then calcined in air at 450 OC for 8 h to give the H-X zeolite. R u ( N H ~ ) ~(1.5 ~ +wt% Ru) was exchanged into H-X zeolite at room temperature by using [Ru(NH3)6]C13. The Ru-exchanged sample was stirred for about 24 h and filtered, and the zeolite was then washed with distilled water and dried at room temperature. Torr The sample was uactivatedn by evacuation to about followed by slow heating to 300 OC over 5 h. Reduction of this zeolite was done by further evacuation of the activated sample at 350 OC followed by exposure to 500 Torr of hydrogen for 1 h at 350 OC followed by evacuation to about 10-4 Torr. Adsorption of water, carbon monoxide, nitric oxide, and oxygen was studied by exposing the activated and reduced samples to a specific pressure at room temperature. Na-X zeolite was obtained from Linde Co. Ruthenium hexammine trichloride was purchased from Strem Chemicals. Matheson CO (research grade) was purified by passing through a trap at -196 OC before use. Linde NO (98.5% purity) was purified by several vacuum distillations at the melting point of pentane (-129 "C). 13C-enrichedCO (MSD Isotopes, 99.4 atom 8 "C) and hydrogen (Linde, UHP) were used without further purification. Spectroscopic Methods. Electron spin resonance spectra at X-band were recorded at 77 K with a Bruker ESP-300 spectrometer employing a 100-kHz magnetic field modulation. The microwave frequency was measured with a Hewlett-Packard Model 5352B microwave frequency counter, and the magnetic field measurements were made with a Bruker ER-035M nuclear magnetic resonance gaussmeter. Electron spin echo spectra were recorded at 4.2 K with a home-built spectrometer that has been described elsewhereS6 Infrared spectra were obtained with a Nicolet FTIR system 740 spectrometer equipped with a DTGS detector and interfaced with a Nicolet 620 spectroscopy workstation. Zeolite sample disks for IR studies were prepared by pressing the zeolite powder under 1 ton/cm2 into a self-supporting disk having a diameter of 14 mm with weights between 10 and 20 mg. The disks were then inserted into a Pyrex cell equipped with CaF2 windows connected to a vacuum manifold where pretreatment was carried out. IR spectra (4) (a) Nijs, H.; Jacobs, P.A.; Uytterhoeven, J. B. J. Chcm. Soc., Chem. Commun. 1919.180. (b) Nijs, H.;Jacobs, P.A.; Uytterhoeven, J. B.J. Chcm. Soc., Chcm. Commun. 1979. 57, 11. (c) Elliott, D. J.; Lunsford, J. H. J. Coral. 1979, 57. 11. (d) Okuda, 0.; Tatsumi, T.; Fujimoto, K.; Tominagn, H. Chcm. L d t . 1983, 1153. (e) Chen, Y.W.; Wang, H. T.; Goodwin, J. G. J. Coral. 1983,83,415. (f) Chen, Y.W.; Wang, H. T.; Goodwin. J. G. J . Carol. 1 9 8 4 , 6 4 9 9 . (8) Leith, I. R. 1.Coral. 1985, 91, 283. (h) Oukaci, R.; Sayari, A.; Goodwin, J. G. J. Carol. 1987, 106, 318. ( 5 ) Lei, 0. D.; Kevan, L. J . Phys. Chem. 1990, 94,6384. (6) (a) Ichikawa, T.; Kevan. L.; Narayana, P.A. J . Phys. Chcm. 1979, 83.3318. (b) Narayana, P.A.; Kevan. L. Phorochcm. Phorobiol. 1983,37, 105. (c) Narayana, P. A.; Kevan, L. Magn. Rcson. Rev. 1983, 1. 234.
The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 4507 TABLE I: ESR Parameters at 77 K of Paramagnetic Species Formed in RuH-X Zeolite treatment fresh evacuated 120 OC evacuated 220 OC" evacuated 300 OC +H20 +NO
+co +02
gl
probable probable Struct location Ru(NHj)d+ a-cage R u ( N H ~ ) ~ O H ~a-cage + @-cage Ru'+(NHI).(H;O,OH), site I1 Ru3+ or 11' a-cage Ru(H20),'+ a-cage AI'+(NO) a-cage Ru'+(C0)2 a-cage Ru3+(C0)2 a-cage Ru4+02a-cage Ru4*01Ru3+, Ru+ a-cage
2.10 2.58 2.62
2.10 2.24
1.74 1.71
2.17
2.08
1.94
2.075 1.95 2.069 2.054 2.044 2.029
2.002 1.953 1.95 1.84 2.069 1.998 2.054 1.995 2.044 2.006 2.029 2.006 2.07
2.013
2.013 2.009 Ru+(CO),
evacuated 350 "C +H2 350 OC'
+co
g3
g1
a-cage
OESR parameters cannot be determined precisely. RuH-X 91'2.10
A
I FRESH a
b C
EVAC/220° C d EVAC/30O0C e
f
2b7 Figure 1. ESR spectra at 77 K of RuH-X zeolite (a) after ion-exchange and evacuation at (b) 80 O C , (c) 90 OC, (d) 220 OC, (e) 300 OC, and (f) evacuation at 350 OC and reduction with 500 Torr of H2 at 350 OC.
were taken at room temperature with a resolution of 2 cm-'. The X-ray photoelectron spectra were measured in a PerkinElmer PHI Model 500 ESCA/SAM spectrometer using Mg Ka X-rays at 1253.6 eV. Sample pretreatments were carried out in the pretreatment chamber of the XPS instrument. Binding energies were referenced to the Au 4fi1* (84.0 eV) line of a gold spot that had been evaporated onto the samples before pretreatment. The Si 2p (102.4 eV) and A1 2p (74.2 eV) lines of the zeolite were usually employed as secondary, internal standards for binding energies of ruthenium. Data smoothing, subtraction of inelastic scattering, and deconvolution of the spectra were performed with the PHI software available in the spectrometer. All samples were disks as used for IR and were clamped to an aluminum plate grounded to the spectrometer. Diffuse reflectance spectra were obtained with a Hitachi integrating sphere in a Perkin-Elmer 330 spectrophotometer. BaS04 was used as a standard. ReSdtS The zeolite sample after calcination and R u ( N H ~ ) ~exchange ~+ remains highly crystalline to X-rays, giving a characteristic diffraction pattern of H-X, and will be referred to as RuH-X. During the activation process, the sample did not change to a wine-red color typically associated with a Ru red complex. This contrasts with RUM-X and RUM-Y zeolites, where M is an alkali-metal ion, in which a wine-red color is observed upon evacuation at 80 OC of R u ( N H ~ ) ~exchanged C~~ M-X and M-Y
Lei and Kevan
4508 The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 2.069
I gi=2.054
40 G
u
AI
\ 7
EVAC/ 300' C/CO
g;l=2.029
v
f-g3=1.953
y
Figure 3. ESR spectrum at 77 K of RuH-X evacuated at 300 O C and exposed to 20 Torr of water for 1 day. -
RuH-X EVAC/30OoC/NO
EVAC/300°C/0,
G
0
F i y n 2. ESR spectra at 77 K of RuH-X (a) evacuated at 300 O C and then exposed to 100 Torr of CO, (b) evacuated at 350 O C , reduced with H2 at 350 O C , and then exposed to 100 Torr of CO,and (c) evacuated at 300 OC and exposed to 5 Torr of O2 for 20 min.
~eolites.'~Diffuse reflectance spectroscopy was used to monitor the 540-nm range, which is the characteristic adsorption for the Ru red complex, and no significant adsorption was found at this wavelength in RuH-X after the activation process. ESR Results. Table I gives a complete list of the ESR parameters at 77 K for the RuH-X samples after various treatments. Figure 1 shows a series of ESR spectra during activation of RuH-X. For the fresh sample (Figure la), one species with g = 2.10 and gl,= 1.74 is observed. This species has been reported d as R u ( N H & ~ + exchanged into the a-cage of M-X zeolite. Heating in a vacuum to 80 O C produces a species with rhombic symmetry (Figure lb) with gl = 2.58, gz = 2.24, and g3 = 1.75. This species has been assigned as Ru(NH3)5OHZ+in M-X zeolite9 and is destroyed around 200 OC. Another species is observed starting above 150 O C in a vacuum, which reaches its maximum intensity at 220 O C (Figure Id). This species is characterized by an asymmetric ESR signal with a maximum at g = 2.62 and will be designated as species A. Continued heating under vacuum to 300 O C decreases the intensity of species A (Figure le), and an additional species, designated as species B, with gl = 2.17, g2 = 2.08, and g3 = 1.94 is observed. Further heating to 350 'C followed by reduction under flowing hydrogen at the same temperature followed by evacuation at room temperature greatly decreases the intensity of species A and produces a broad ESR signal at g = 2.07 as shown in Figure If. The adsorption of oxygen, carbon monoxide, nitric oxide, and water on activated RuH-X zeolite was studied by ESR. The adsorption of 100 Torr of CO on activated RuH-X removed species B and produced paramagnetic carbonyl adducts as shown in Figure 2a. Two axially symmetric species were assigned to 81 = 2.069, gll = 1.998 and g, = 2.054, gll = 1.995. The assignment of two species was made due to the change of their relative ESR intensities by varying the heating time at 300 OC. Upon adsorption of CO on hydrogen-reduced RuH-X, another species with g, = 2.013, gH= 2.009 is observed (Figure 2b) in addition to the two species described above. Brief evacuation of the HI removes the g, = 2.01 3, gn= 2.009 species, but the other (7) Madhusudhan, C. P.; Patil, M. D.; Good, M. L.Inorg. Chem. 1979, 8, 2384, (81 Verdonck. J. J.: Schoonhevdt. R. A.: Jacobs. P. A. J . Phvs. Chem. 19111, 85, 2393. ( 9 ) Goldwamr, M.; Dutel, J. E; Naccache, C. Zeolites 1989, 9, 55.
I
I
I
1
-EXPT ----CALC
R= a37 nm A. 0.3 MHz 0 I
C I
I
I
I
I
1
2
3
4
5
T, t 4 3 Figure 4. (a) ESR spectrum at 77 K of RuH-X evacuated at 300 OC and exposed to 50 Torr of NO, (b) two-pulse ESEM spectrum at 4 K, and (c) three-pulse ESEM spectrum at 4 K with T = 0.40 ps and simulation neglecting quadrupole interaction. The parameters used for the simulation are indicated. The ESEM experiments were performed at g = 1.95. The vertical marks in (b) and (c) indicate 27Almodulation
periods.
two species remain. They are removed only after evacuation at 200 oc. Exposure of 5 Torr of O2to activated RuH-X removes species B and produces paramagnetic dioxygen adducts as shown in Figure 2c. This spectrum can be interpreted as two axially symmetric species with the same g, = 2.006 and gll = 2.044 and 2.029, respectively. Water adsorption on the activated sample results in the removal of species B and the formation of a rhombically symmetric water adduct with distinctive ESR parameters at gl = 2.075, gz = 2.002, and g3 = 1.953 (Figure 3). Species A was also observed to increase in ESR intensity significantly after water adsorption. The adsorption of nitric oxide on activated RuH-X produces an axially symmetry species with g, = 1.95, gll = 1.84, and species B is removed as shown in Figure 4a. Similar ESR spectra have been observed for nitric oxide adsorbed on a surface site on zeolite.lO*l' ESR studies of nitric oxide adsorbed on zeolite have
The Journal of Physical Chemistry, Vol. 95, No. 11, I991 4509
Ruthenium Species Generated in H-X Zeolite XI
4 & G
SPEFIES A
RuH-X
RuH-X
EVAC/300°C
o
v,
t z 3 EVAC/3OO0C/H20
b
7
r
XI
EVAC/300°C/C0
c
,-1. EVAC/30O0C/O2 d
2 EVAC/3OO0C/NO e
1
,
,
290
RuH-X
of RuH-X (a) evacuated at 300 O C and adsorption of (b) 20 Torr of H 2 0 , (c) 100 Torr of CO, (d) 5 Torr of 02,and (e) 50 Torr of NO.
a FRESH
(1 0) Gardner, C. L.; Weinberger. M. A. Can. J. Chem. 1970,48, 13 17. (1 1) (a) Lunsford, J. H . J . Phys. Chem. 1968, 72, 4163. (b) Lunsford, J. H.J . Phys. Chrm. 1970, 74, 1518. (1 2) Naccache, C.;Ben Taarit, Y. J. Chem. Soc., Faraday Tram. I 1973,
69, 1475. (1 3) Jermyn, J. W.; Johnson, T. J.; Vansant, E. F.;Lunsford. J. H. J . Phys. Chcm. 1973, 77, 2964. (14)Kasai, P. H.;Gaura, R. M. J . Phys. Chem. 1982, 86, 4257. (15) Kevan, L. Acc. Chem. Res. 1987, 20, 1.
(16)Kevan. L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N., Eds.; Wilcy-Interscience: New York. 1979;pp 279-341.
,
1
,
,
1
I
I
285 280 BINDING ENERGY, eV
275
Figure 6. X-ray photoelectron spectra of the C Is and Ru 3d regions in RuH-X after (a) ion-exchange, (b) evacuation at 300 OC, and (c) evacuation at 350 O C and reduction with H, at 350 OC.
Figure 5. ESR spectra at 77 K recorded with a field range of 4000 G
been discussed extensive1y.l*l4 The adsorption sites have been assigned to aluminum in the frameworkll* as well as to sodium in a cation ~ i t e . ~These ~ - ~ two ~ sites can be distinguished by electron spin echo modulation (ESEM) spectroscopy.Is A twopulse ESEM experiment was performed on this sample, and the spectrum (Figure 4b) shows a modulation pattern that corresponds to 27Almodulation. To verify the origin of the modulation, two different zeolites were studied, H-X without Ru and RuNa-X zeolite. After activation and N O adsorption, both the H-X and RuNa-X samples show almost identical ESR and two-pulse ESEM spectra. Therefore, the modulation pattern is assigned to 27Almodulation since the aluminum content in all three samples (RuH-X, H-X, RuNa-X) is the same, whereas the 23Na content differs. The ESR spectrum (Figure 4a) can be interpreted as a nitrosyl molecule interacting with aluminum in the zeolite framework. The threepulse ESEM of the RuH-X sample (Figure 4c) fits a simulation, neglecting quadrupole interaction, with parameters of N = 1, R = 0.37 nm, and Ai, = 0.3 MHz, where N is the number, R is the distance, and Ai, is the isotropic hyperfine constant of the closest interacting nuclei.I6 Since the "AI quadrupole interaction is likely significant, the R value is only approximate. To compare the relative ESR intensity of species A after adsorption of different adsorbates, ESR spectra were also recorded over a 4000-Gscale as shown in Figure 5 . The ESR intensity of species A remains relatively unchanged after adsorption of NO, CO, and 0,; however, adsorption of H 2 0 increases the ESR intensity of species A significantly. XPS Results. X-ray photoelectron spectra of the 3d levels of ruthenium (276-292 eV) were monitored for freshly prepared RuH-X, activated RuH-X, and an activated sample reduced in H2 at 350 'C with subsequent evacuation. Figure 6 shows their photoelectron spectra. Unfortunately, the binding energies for the Ru 3d levels and C 1s level are overlapped. The only peak that can be identified unambiguously is the peak at 280.3 eV formed after hydrogen reduction (Figure 6c). Although the Ru
,
BINDING ENERGY, eV Figure 7. Deconvolutionsof the C Is, Ru 3d5 2, and Ru 3d,,, transitions in the XPS spectra of RuH-X in (a) Figure #a and (b) Figure 7b: (..a) experimental, (- -) components, and (-) sum of components.
-
TABLE II: Summary of Reported Ru 3dblI Binding Energy 3dsp binding Ru energy, eV species/support 279.9 Ruo metal 280.0 Ruo metal 280.0 Ruo/NaY" 280.5 Ru0/TiO2 281.0 Ruo NaYlb Ru' '(NH,)sN212 282.2 Ruii'SnB/A120, 282.3
I
ref 18 17
19 20 19 18 23
3d~p binding Ru energy, eV ref species/support Ru1i'(NH,)5(Me(CN)- 282.9 22 Br, 282.9 21 Ru'ii(OH)3/Ca0 283.0 19 Ru"'(NH,):+/Na-Y 280.7 18 RU'~~, 281.9 19 Ru1V02/Na-Y" 281.0 19 Ru'V02/Na-Yb 282.1 17 Ru'"02
"On exterior surface. bOn interior surface.
3d3/, line is not resolved due to overlap with the C Is line, the binding energy of 280.3 eV agrees quite well with the binding energy of Ru 3dS/, for Ruo Table I1 summarizes some selected XPS binding energies of ruthenium compounds from the l i t e r a t ~ r e . ' ~ The - ~ ~ photoelectron spectra show an unsymmetrical (17) Kim, K. S.;Winograd, N. J . Coral. 1974, 35, 66. (18)Folkesson, B. Acra Chem. Scand. 1973, 27, 287. (19)Pedersen, L. A.; Lunsford, J. H . J . Caral. 1980. 61, 39. (20) Robbins, J. L. J . Caral. 1989, 115, 120. (21) Aika, K.;Ohya, A.; Ozaki, A.; Inwe, Y.; Yasumori, 1. J . Carai. 1985, 92. 305. (22)Battistoni, C.;Furlani, C.; Mattogno, G.;Tom, G.Inorg. Chim. Acra
. --.
1977. 21. L25. -.
~~
(23)Deshpande, V. M.; Patterson, W. R.; Narasimhan. C. S.J . Caral.
1990, 121, 165.
Lei and Kevan
4510 The Journal of Physical Chemistry, Vol. 95, No. 11, 1991
I
RuH-X/EVAC/30@C/CO
RuH-X/CO
o 10torr CO I Omin
blo.02 :I
2qee
c 0.1
f0.02A
-
\\\
c
2100
WAVENUMBER, CM"
WAVENUMBER, CM-'
-
Figure 8. IR spectra at room temperature of IVO adsorbed on RuH-X evacuated at 300 OC as a function of increasing CO pressure.
Figure 9. IR spectra at room temperature of RuH-X evacuated at 300 O C and with (a) adsorption of 99% IVO, (b) coadsorption of a 1:l ratio of I2CO and I3CO, and (c) adsorption of 99% I3CO. around 288 eV is also observed. This peak has too high a binding energy for a Ru 3d level. However, the C 1s level is in this range so for some forms of carbon such as C=O or C-OH we tentatively assign this peak to a carbon 1s transition from a different form of carbon produced by reduction. The photoelectron spectrum of this sample was not deconvoluted due to the complexity of overlapping species. Infrared Results. The infrared spectra between 1900 and 2100 cm-' were recorded at room temperature for the adsorption of carbon monoxide on activated RuH-X as shown in Figure 8. With the CO pressure varied from 0.02 to 100 Torr, three adsorption bands were observed at 2025,2088, and 2146 cm-'. For convenience, these bands are referred to as Iow-, medium-, and high-frequency bands denoted as LF, MF, and HF. From this spectral change while varying the CO pressure, it seems that the MF and H F bands behave similarly. To verify if the MF and H F bands are coupled vibrations from the same species, we carried out isotopically mixed CO adsorption experiments. Since the MF and LF bands are overlapped, only the H F band region can be analyzed, which is shown in Figure 9. Adsorption of a 1:l ratio of 13CO/'zC0 shifted the H F band to lower energy at 2128 cm-' (Figure lob). Adsorption of 99% 13C0 shifted the H F band even more to 2095 cm-I. Figure 10 shows the IR of the adsorption of NO on activated RH-X by varying the NO pressure from 0.02 to 50 Torr. Upon increasing the NO pressure only one band at 1885 cm-I is observed to be associated with adsorbed NO.
peak with a maximum at 284 eV for the fresh sample (Figure 6a) and a much broader peak centered at 283 eV for the activated sample (Figure 6b). Since the C 1s peak so badly overlaps the Ru 3d peaks, XPS of these samples was also monitored at the weak, broad Ru 3p levels in order to verify the XPS observation of Ru. A peak was observed at 463 eV corresponding to a Ru 3p312binding energy; the valence state cannot be distinguished. Auger electron spectroscopy (AES) has also been performed on these samples to monitor the Ru Auger electron line, and a distinct peak was found at 230 eV, which correspond to the Auger electron kinetic energy for a Ru transition. The Ru 3p line and the Auger Ru line confirm that Ru in these samples can be observed by XPS. However, these transitions do not allow determination of the valence state. The peaks in Figure 6a,b are thus assigned to overlapping Ru 3d and C 1s levels. Curve deconvolution of the photoelectron spectra of Figure 6a,b are shown in Figure 7. The C 1s binding energy was referenced to the Au 4f712peak by the relation C 1s-Au 4f7 = 200.8 eV.U3S The deconvoluted C 1s peak area from the Idg K c Y , ,line ~ was adjusted to correspond to the intensity expected based on the Discussion intensity of the resolved C 1s peak at 275 eV from the Mg K c Y ~ ~ After R u ( N H ~ ) ~is~ion-exchanged + into H-X zeolite, the ESR line. The intensity ratio of Mg K q 2 to Mg Ka3,4lines is ?.leia signal indicates that RU(NH~)~'+ is incorporated into the a-cage Also the deconvoluted Ru 3dy2 peak area was adjusted to corof the X zeolite. Since this cation complex is necessary for respond to the intensity expected from the intensity of the resolved balancing the negative zeolite framework charge of the zeolite, Ru 3p3/2 peak. The intensity ratio of Ru 3dS12to Ru 3p312is 3 the chemistry involved in the subsequent heat treatment and based on the Ru metal photoelectron ~pectrum.~"With these associated with adsorption of adsorbates is regarded as taking place constraints, the deconvolution (Figure 7a) shows that the C 1s within the zeolite pores. It has been established that Ru(NH3)2+ line is at 284.8 eV and that Ru(NH3)2+ has a Ru 3dS12binding exchanged into the a-cage of M-X and M-Y zeolites, where M energy of 283.2 eV with a separation of 4.1 eV to the Ru 3dy2 is an alkali-metal ion, hydrolyzes in vacuo at 60 OC with zeolpeak. ite-sorbed water to form [ R u ( N H ~ ) ~ O H ] ~and + The photoelectron spectrum after activation (Figure 6b) shows [(NH3)SRuORu(NHj)40Ru(NH3)s]6+ (Ru red).'+ The fora much broader peak toward the lower energy side. This indicates mation of the trimeric Ru red complex indicates that there is some contribution from species with an oxidation state lower than significant mobility for the Ru complexes within the zeolite pores. 3+ and that some reduction occurs during activation. This is Under the effect of temperature, migration of the Ru complexes expected since the NH3 ligands act as reductants. The deconfacilitates the formation of the trimeric Ru red complex. However, volution of this spectrum was done by adding another Ru species in this study, only [ R U ( N H ~ ) ~ O H ]which ~ + , was characterized with a lower binding energy to the fitting process. The result by its ESR signal, was observed after evacuation at 80 O C . Diffuse (Figure 7b) shows two different Ru species with Ru 3dS binding reflectance spectroscopy confirms that further hydrolysis to Ru energies at 283.0 and 282.0 eV with a separation of 4.i eV from red is suppressed by the absence of its characteristic optical abthe Ru 3d3/2 peaks for both species. sorption. The lack of formation of the Ru red complex in H-X The photoelectron spectrum of the sample after reduction with zeolite suggests that the mobility of the Ru monomer is suppressed. H2 (Figure 6c)shows clearly the formation of Ruo with a binding Further evacuation at 220 OC produces species A, which is energy at 280.3 eV. However, another peak with a binding energy characterized by an asymmetric ESR signal (Figure Id). Species A is suggested to be a Ru(II1) species since Ru(I1) is diamagnetic (24) Kowalczyk, S. P.; Pollock, R.A.; McFeely, F. R.;Ley, L.; Shirley, and reduction to Ru(1) is highly unlikely at such a low temperD.A. Phys. Rev. B 1973,8. 2387. (25) Fuggle, J. C.; Killne, E.;Watson, L. M.; Fabian, D. J. Phys. Reu. B 1977, 16,750. (27) Muilenberg, G. E.Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Norwalk, CT,1978, (a) p 107, (b) p 38. (26) Krause, M.0.;Ferreira, J. G. J . Phys. 1975, B8, 2007.
Ruthenium Species Generated in H-X Zeolite
1rhe Journal of Physical Chemistry, Vol. 95, No. 11, 1991 4511
the reactivity of species A only with H 2 0 among these absorbates. However, this explanation seems weak because similar Ru species have been reported8-31.32to interact with N O and CO to form nitrosyl and carbonyl species. An alternative explanation is that 2 species A is located in a position inaccessible to CO, NO, and 0 hexagonal prism 2.8 a-cage 8.0 but accessible to H 2 0 such as the @-cage a t room temperature. &cage 2.8 Another factor favoring the location of species A in the @-cage is the location of the charge-balancing proton. In H-X zeolite, kinetic kinetic the protons form structural OH groups with framework oxymolecule diam,b A molecule diam,b A g e n ~ . ~ 'The ~ ~ majority of the protons are located at the oxygens 2.65 0 2 3.46 H20 of a six-ring of a hexagonal prism projecting into the hexagonal NO co 3.76 3.17 prism and a t the oxygens of a four-ring of a hexagonal prism 'From Breck, D. W . Zeolite Molecular Sieves; Wiley: N e w York, projecting into the a - ~ a g e .Thus, ~ ~ unlike ~ ~ the M-X or M-Y 1974; pp 65, p 636. *Calculated from the minimum equilibrium zeolite in which most of the site I1 or 11' locations are occupied cross-sectional diameter. by cations,3g H-X zeolite has its six-ring windows more open to the @-cage. This results in a greater possibility for a Ru(II1) atureSk The IR spectrum (not shown) of RuH-X after evacuation complex to migrate into the @-cage to give the ESR signal of a t 220 OC still shows a band at 1335 cm- which is due to NH3 species A, whereas in M-X zeolites this migration is less probable coordinated to R u . ~ Thus, ~ species A is assigned to Ru(II1) and no ESR signal of species A is seen.5 coordinated to NH,, H 2 0 (or OH-), and zeolitic oxygens. To generate species A inside the @-cage,the Ru complex must Evacuation at 300 "C of R u ( N H ~ ) exchanged ~~+ in M-X or be small enough to be able to migrate into the @-cage. A ruM-Y type zeolites has been reported to form Ru(II1) ion coorthenium-amine complex with one or two amine ligands has been dinated to the z e ~ l i t e . ~ *However, ~~ the ESR signal of this shown to form after thermal decomposition of R u ( N H ~ ) ~in~ + paramagnetic ion has not been observed yet. The absence of an Na-X and Na-Y eol lite.^.^',^^ A ruthenium complex with more ESR signal has been suggested to be due to the formation of than one amine ligand seems unlikely to pass through the six-ring Ru(II1) dimer,5 which is diamagnetic. The formation of the window between the a-cage and @-cagedue to its size. However, trimeric Ru red complex during activation supports the possibility a monoamine Ru complex might be able to migrate into the @-cage of dimer formation. In RuH-X zeolite, we did not observe the to form species A. formation of the Ru red complex during activation, and we do The increase of ESR intensity of species A after H20adsorption see a new ESR signal (species B) after evacuation at 300 OC. Thus indicates that the reduced form of species A is oxidized back to we assign this new ESR signal to Ru(II1) coordinated only to species A. Although this mechanism is not understood, similar lattice oxygens in a cation site of X zeolite. This observation behavior has been reported in several other metal zeolite systems supports the previous suggestion of dimer formation in M-X where it has been suggested that a reduced metal species is oxidized zeolites. by water to generate h y d r ~ g e n . ~ ? ~ ~ The XPS results do not provide positive assignment for Ru in Due to the absence of metal cations in the a-cage of H-X the trivalent state since the Ru 3d binding energy heavily overlaps zeolite, Ru complexes in the a-cage will have less steric crowding the carbon 1s line from adventitious hydrocarbon contamination. and electrostatic repulsion from the cations. This factor might Deconvolution of the photoelectron spectrum of fresh samples has enable the Ru complexes to anchor on a specific site and behave shown the existence of Ru in the trivalent state with a 3d5/2 binding differently than in M-X and M-Y zeolites in which Ru(NH,),~+ energy a t 283.2 eV. This binding energy is in agreement with behaves as in an aqueous solution.8 Thus, the mobility of the Ru literature data on pure compounds22and supported c a t a l y s t ~ l ~ - ~ ~complex will be reduced, and the formation of trimeric Ru red of ruthenium in the 111 oxidation state and will be assigned as complex will be suppressed. Ru(NH3)d+ in the H-X zeolite support. The XPS spectrum after Another reaction that may be playing a role in the localization activation does suggest that some reduction occurred since the of the Ru complex in the H-X zeolite is the interaction between line broadens toward the lower binding energy side. Deconvolution a Bronsted acid site and the Ru complex: of this spectrum shows that Ru(I1) and Ru(II1) are the most probable species by comparison with the literature data in Table Ru(NH3), + H+-O(zeolite) 11. This supports the assignment of the ESR signals of the acR~(NH~),~-O(zeolite)+ NH4+ tivated sample to Ru(II1) since Ru(II), which is diamagnetic, will not be observed by ESR. Although the formation of NH4+ ion characterized by an infrared The adsorption of small molecules on a zeolite is important for adsorption at 1445 cm-' (not shown) has been attributed to the the study of chemical reactivity toward the adsorbate and as a hydrolysis of Ru(NH3):+,8 it has also been reported that Bronsted molecular probe. Table 111 shows the maximum cage openings acid sites in the zeolite react with NH3 to form NH4+.7*28Thus in zeolite X and the kinetic diameters of various adsorbate both reactions may contribute, and the reaction of the Ru complex molecules. From the ESR results, the adsorption of CO, NO, with a Bronsted acid site may assist the localization of the Ru 02,and H 2 0at room temperature all remove the ESR signal of complex. This may explain the difference between H-X and M-X species B and lead to the formation of new ESR signals of adducts or M-Y zeolites. of the particular adsorbate. Since CO, NO, and O2are all too big to enter the @-cage of the X zeolite at room temperature, (31) Verdonck. J. J.; Schoonheydt, R. A.; Jacobs, P. A. J . Phys. Chem. species B, which is stable at room temperature, must be accessible 1981, 83, 683. to the a-cage. The most probable location for this species would (32) Pearce, J. R.; Gustafson, B. L.; Lunsford, J. H. Inorg. Chem. 1981, be site I1 in a six-ring between the a- and b-cages or site II*, which 20, 2957. projects from a six-ring into the a - ~ a g e . ~ ~ (33) Olson, D. H.; Dempsey, E. J . Coral. 1969, 13, 221. (34) Stevenson, R. L. J . Carol. 1971, 21, 113. Adsorption of NO, CO, or O2 does not affect species A. (35) Dubsky, J.; Beran, S.; Bosacek, V. J . Mol. Carol. 1979, 6, 321. However adsorption of H 2 0does increase the ESR intensity of (36) Jirak, 2.;Vratislav, S.;Bosacek, V. J . Phys. Chem. Solids 1980.41, species A considerably. This difference may be associated with 1089. TABLE III: Maximum C a p Openings Found in X Zeolite and Diameters of Various MokcuksO max cage max cage w e opening, A cage opening, A ~
-
(28) Ward, J. W. Ado. Chrm. Ser. 1976, 171, 118. (29) Gustafson, B. L.; Lin. M.; Lunsford, J. H. J. Phys. Chem. 1980,84, 2211. (30) The nomenclature for site location follows the convention of: Smith, J. V. In Zeolfre Chemistry and Caralysis; Rabo, J. A.; Ed.; American Chemical Society: Washington, DC, 1976; Chapter 1 .
(37) Jirak, 2.;Vratislav, S.;Zajicek, J.; Bosacek, V. J. Catal. 1977, 49, 112. (38) Bosacek, V.; Beran, S.;Jirak. 2.J . Phys. Chrm. 1981. 85, 3856. (39) Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974; Chapter 2. (40) Kasai. P. H.; Bishop, R. J. J . Phys. Chrm. 1977, 81, 1527. (41) Sass, C. E.; Chen, X.; Kevan, L. J . Chem. Soc., Faraday Trans. 1990, 86. 189.
4512 The Journal of Physical Chemistry, Vol. 95, No. 11, 1991
Ruthenium carbonyl adducts derived from adsorption of CO on RUM-X and RUM-Y zeolites have been characterized by IR and ESR.5,29 Previous studies5 showed that adsorption of C O on RUM-X activated at 300 "C produces two Ru(III)-(CO), species, and the two carbonyl species were suggested to have slightly different locations. In RuH-X, adsorption of CO on the activated sample removes the ESR signal of species B and produces two Ru carbonyl adducts with similar ESR parameters as found in RUM-X.~We attribute these carbonyl adducts to ruthenium(II1) carbonyl species. Pederson and LunsfordI9 have reported for a Ru(NH3)&I3exchanged Na-Y zeolite that the ruthenium remained inside the zeolite pores following reduction in H2 provided that O2is excluded from the system. Thus, we assume the chemistry involved in the adsorption of CO on the hydrogen-reduced sample takes place in the zeolite cavities. Hydrogen reduction of an activated sample a t 350 "C produces a broad ESR signal at g = 2.07. A very similar ESR signal has been reported by Cattania et a1.42 in which Ru,(CO),~impregnated in -pA1203was evacuated between 300 and 500 "C. They suggested that this signal arises from several different Ru species overlapped with each other and that Ru(1) and RU(II1) species were likely, based on their assignments of Ru carbonyl adducts. The interaction of CO with these species produced two sets of ESR signals, which were assigned as ruthenium(1) carbonyl and ruthenium(II1) carbonyl adduct^.'^ In our study, the adsorption of CO on the hydrogen-reduced sample produces a new axially symmetric species with g,, = 2.01 3 and g, = 2.009 (Figure 2b) in addition to the ESR signals that were assigned to ruthenium(II1) carbonyl species after CO adsorption on the activated sample. Since the next lower paramagnetic oxidation state of ruthenium is Ru(I), we suggest that this species is a ruthenium(1) carbonyl adduct. This species is unstable and can be removed by evacuation a t room temperature. This agrees with Cattania et al.'s assignment of a ruthenium(1) carbonyl species that is easily removed by evacuation for a few minutes. As a result we conclude that the broad ESR signal found on the reduced sample is composed of at least two species, which are likely Ru(1) and Ru(II1) ions. The XPS spectrum of the hydrogen-reduced sample shows that a considerable amount of ruthenium has been reduced to Ru(0) characterized by a binding energy at 280.3 eV. However the XPS result is unclear about the formation of Ru(1) due to the interference of the C Is peaks with the Ru 3d5 and 3d3 peaks. ESR shows Ru(II1) and may show Ru(1). Thus both 4PS and ESR are ambiguous about the formation of Ru(1). The adsorption of C O on supported Ru species has been extensively studied by means of infrared ~pectroscopy.4~The results in the literature agree relatively well on the presence of three bands with positions H F at 2140 f 10 cm-l, M F at 2080 f 10 cm-l, and L F at 1990-2040 cm-I. However, opinions differ widely regarding the assignment of these bands. Table IV shows the previous assignments of IR bands associated with CO on supported ruthenium species. ESR evidence that shows the existence of ruthenium( 111) carbonyl species agrees with several IR investig a t i o n ~ ~in- which ' ~ ~ it was concluded that the H F and M F bands are associated with ruthenium(II1). The observation of a constant ratio of the intensities of the HF to M F bands with changing C O coverage (Figure 8 ) indicates t h a t these bands are coupled vi(42) Cattania, M.G.; Gervasini, A.; Marauoni, F.; Scotti, R.; Strumolo, D. J. Chem. Soc., Faraday Trans. 1 1987,83, 3619. (43) (a) Zccchina, A.; Guglielminotti, E. J. Curd. 1982,74, 225,240, 252. (b) Uchiyama, S.;Gates, B. C. J . Curul. 1988,110, 338. (c) Guglielminotti, E.; Bond, G. C. J. Chem. Soc., Faraday Truns. 1990,86,979. (d) Solymosi, F.;Raska, J. J . Cold. 1989.115, 107. (e) Yokomizo, G. H.;Louis, C.; Bell, A. T. J . Curd 1989, 120, 1. (f) Robbins, J. L.J . Coral. 1989,115, 120. (8) Guczi, L.;D o h , S.;Beck, A.; Vizi-orosz, A. Curd Toduy 1989,6,97. (h) Davydov, A. A.; Bell, A. T. J . Coral. 1989, 115, 120. (i) Brown, M. F.; Gonzale. R. D. J. Phys. Chcm. 1976,80. 1731. (j) Kuznetsov, V. L.;Bell, A. T.;Yermakov. Y. J . Coral. 1980,65,374. (k) Yamasaki, H.; Kobori, Y.; Natio, S.;Onishi, T.; Tamaru, K.J. Chem. Soc.,Furuduy Trans, I 1981, 77, 2913. (I) Dalla Betta, R. A. J . Phys. Chem. 1975, 79, 2519. (m) Goodwin, J. G.; Naccache, C. J. Curd. 1980.64.482. (n) Chen, H.;Zhong, 2.;White, J. M.J . Curd 1984, 90,119. ( 0 ) Evans, J.; McNully, G. S.J. Chem. Soc., Dulron Truns. 1984, 1123.
Lei and Kevan TABLE I V Reported IR Assignments of CO Bands Assochted with Ruthenium
species Ru species Ru3+(C0)2
support NaX A1203 A1203 TiO, A1203 Ti02
Ru*(C0)2 RU)+(CO)~ or Ru2+(CO), RU~+(CO)~
Si02 Ti02
vc0, cm-'
2139 2138 2138 2134 2140 2145
2078 2075 2074 2078 2075 2086
A1203
2136 2080 2140 2085 2070 2074 2078 2083 2072
RU+(CO)~ Ru+(C0)2
NaX NaX Si02
2080 2130 2070
RUO(CO)~
A1203 A1203
Ru2+(C0)2
AI203 A1203
Ti02 Ti02
SO2 [Ru0(CO)2In Ruo(CO), ( n L 1) Ru(CO).Xm* Ru(CO)Xmb RuOCO
2130 2070
A1203
SiOz Si02 SO2 Ti02 Si02 Ti02 Si02
RunCO
2140 2060
A1203
-
ref 5 43a 43b 43b 43d 43c
43e 43f 2005 43a 2003 43b 2004 43b 2023 43e 2000 43g 2040 1988" 31 2010 31 43h 2054 1977 43a 2048 1970 43g 43j 2049 1972 43b 2030 4f 2050 4f 4f 2045 43c 2030 43e 43 f 2040 43h 2030 43d
'A peak at 1937 cm-I is also reported. This range is not seen in any other system. b X represents coadsorbed H 2 0 (or OH), CI-, or lattice oxygen. RuH-X/EVAC/300°C/N0
a b
t
0 torr NO 0.03
U
w
W
u
2
m K
am U
I 1900
I600 1700 WAVENUMBER, CM-'
1600
Figure 10. IR spectra at room temperature of N O adsorbed on RuH-X evacuated at 300 OC as a function of increasing N O pressure.
brations. They have generally been attributed to a dicarbonyl species in the literature. However recently, ruthenium tricarbonyl species have been assigned to the HF and M F bands in Si0243c and Ti0243f. For a dicarbonyl species of C , symmetry two bands, AI and BI, are expected, whereas for a tricarbonyl species of local sym, two other bands, AI and E, are also expected. Therefore, metry C adsorption on the adsorption of isotopically mixed 12CO/13C0 activated sample was studied to distinguish the coordination number of CO. Due to the broadness of these IR bands and the overlap of the M F and LF bands after adsorption of a mixture of I2CO and I3CO,a complete analysis of these IR bands is not possible. However, a crude analysis of the H F band allows us to draw some conclusions about the C O coordination number. Although the H F components from the adsorption of a 1:l mixture of IT0 and I3COare not resolved very well, three H F components are deduced from Figure 10 at 2146,2128, and 2095 cm-I. These
Ruthenium Species Generated in H-X Zeolite TABLE V Infrared Bands ud F o m S p d a Adrorkd on RuH-X Zeolite
The Journal of Physical Chemistry, Vol. 95, No. 11, 1991 4513
(K)Constants for IDicarbonyl K,.
specits
vI,,,,, cm-l
vUym, cm-l
2146 (oh) 2088 lobs)
mdyn/A K2 18.08 lcalc) 0.495 Icalc)
0.497 #Not resolved. bvIm and vu,,,, are calculated assuming the force constants determined from RU(WO)~. and y,,,,,, are calculated assuming the force constants determined from RU(”CO)~.
three H F components agree with a dicarbonyl species in which a mixture of R u ( ’ ~ C O ) Ru(I2C0)(l3CO), ~, and R U ( ’ T O ) ~will show three H F components, whereas for a tricarbonyl species four H F components are expected for a mixture of R U ( ’ ~ C O )Ru~, (12CO)2(13CO),R u ( ’ ~ C O ) ( ’ ~ C Oand ) ~ , Ru(13CO),. Additional proof of the existence of the dicarbonyl species comes from an analysis of the normal modes of the isotopically substituted Ru(CO), species. From the observed symmetric and asymmetric bands for the R U ( ’ ~ C Oand ) ~ Ru(”CO)~species, values of the force constants for C-0 stretching (KJ and CO/CO interaction (K2) can be calculated assuming a value for the bond angle.44-45 These are tabulated in Table V for a CRuC bond angle at 89.9’. The agreement between Kl and K2 for these two independent measurements is good. On the basis of these values for KI and Kz, and ulym for Ru(’ZCO)(”CO) were calculated to compare with t e observed bands. The results show good agreement between the predicted and measured usymband based on assignment as a dicarbonyl species. These IR results together with the ESR evidence that the H F and M F bands are due to symmetric and asymmetric vibrations of a Ru(II1)-(CO), species. The assignment of the L F band in the literature has generally been to linearly adsorbed CO on zerovalent Ru. However, in this study, the XPS result shows the possible existence of Ru(I1) rather than Ru(0) on the activated RuH-X sample. This conflicts with the assignment of the L F band at 2025 cm-’ to Ru(0)XO. ESR does not provide any information on ruthenium(I1) or ruthenium(0) carbonyl species due to the diamagnetism of these species. Thus, the identity of the L F band remains unresolved. The adsorption of O2on activated RuH-X removes the ESR signal of species B and produces two axially symmetric dioxygen adducts. The ESR parameters of these adducts agree well with ~ ions those of the superoxide ion on zeolite ~ u p p o r t s . Superoxide have been reported to form with Ru(II1) species in M-X and M-Y ~eolites.~J9 At least two types of superoxide ions are distinguished by the differences of their g,, values. Thus, the two oxygen adducts found in RuH-X zeolite are attributed to superoxide ions. Upon O2adsorption, the Ru(II1) ion, assigned as species B, is oxidized to Ru(1V) according to Ru”’
+ O2
-
RuIV02-
The most convincing assignment of the oxidation state of species B comes from an ESR and IR study of the adsorption of N O on activated RuH-X. Generally nitric oxide may be adsorbed on the surface in three ways.47 (a) donation of the lone-pair from NO to the metal with retention of the unpaired electron in the **-antibonding orbital of the N O molecule; (b) tranfer of an electron from metal to NO followed by lone-pair donation from the NO- ion; (c) transfer of the odd electron from N O to the metal followed by lone-pair donation from the NO+ ion. The ESR results show that at low temperature (177 K) a nitrosyl adduct to Ru is characterized by an axially symmetry ESR signal that results from the retention of the unpaired electron on the N O molecule. ESEM studies show that N O interacts with 27Alin (44) Bratermann, P. S . Metal Carbonyl Spectra, 1st ed.; Academic Press: New York, 1975. (45) Gelin. P.; Coudurier, G.; Ben Taarit, Y.;Naccache, C. J . Catal. 1981, 32,70. (46) Che, M.; Tench, A. J. Ado. Caral. 1983, 31, 1. (47) Davydov, A. A.; Bell, A. T. J . Caral. 1977, 49, 332.
TABLE VI: Reported IR Assignments of an NO Band Associated with Ruthenium Species
Ru species/support Ru(NHIhN03+/Na-Y
vNO,cm-I 1918 1916 1916 191 1 1910 1903 1880 1880 1880 1876 1875 1875, 1870 1873 1870 1862 1860 1853 1847 1845 1840 1829 1592 1517
ref 8 51a 51d 50 51d 51c 51c 50 47 51b 51d 32 43 51c 7 51c 51b 50 50 51d 51b 51b 51b
the lattice. An approximate analysis neglecting the quadrupolar interaction indicates that N O interacts with one aluminum nucleus at a distance of 0.37 nm. However, the actual distance is probably shorter owing to modulation damping by the quadrupolar interaction.48 A more complete analysis including quadrupole interaction involves too many parameters to give a unique fit. IR spectroscopy is helpful in identifying the structure of this Ru nitrosyl complex. The vibrational energy of N O is directly related to the electron density on its **-antibonding orbital.49 Process (b) above results in the addition of an electron to a T*antibonding orbital, which weakens the bond and lowers the vibrational energy. Process (c) above results in no electron in the **-antibonding orbital which strengthens the bond and increases the vibrational energy. Following the classification of Lewis et aLsOvibrational bands between 1980 and 1580 cm-I can be assigned to N O molecules coordinated in the form of -NO+ and bands between 1580 and 1045 cm-I to N O coordinated in the form of either -NO-, -NO, or >NO. Consistent with this classification, the one IR band found at 1885 cm-I after N O adsorption on the activated sample can be regarded as from NO’. This suggests that N O interacts with Ru by process (c) to form diamagnetic Ru(I1)-NO’. This assignment is supported by the vibrational band at 1885 cm-I. By comparing the vibrational band with the literature data shown in Table VI,51it is clear that Ru(I1)-NO+ is the only structure that corresponds to the right vibration band of NO’. Therefore, species B is further confirmed as Ru(III), which forms a Ru(1I)-NO+ complex upon N O adsorption. Similar reduction processes upon NO adsorption have also been reported on CU(II):~ Pd(III),52 and Ni(II)s3 supported on zeolites. Conclusions R U ( N H ~ ) ~can ~ ’be introduced into the supercage of the H-X zeolite by ion exchange. It hydrolyzes into Ru(NH&0H2+ upon evacuation a t 80 OC. Further hydrolysis to the trimeric Ru red (48) Romanelli, M.; Narayana, M.; Kevan, L. J . Chem. Phys. 1984,80, 4044. (49) Davydov, A. A.; Bell, A. T. J. Catal. 1977, 49, 332. (50) Lewis, J.; Irving, R. J.; Wilkinson, G. J . Inorg. Nucl. Chem. 1958, 7, 32, 38. (51) (a) Johson, B. F. G.;McCleverty, J. A. In Progress in Inorganic Chemistry; Cotton, F. A.; Ed.; Wiley: New York, 1966; p 277. (b) ConneHey, N. G.Inorg. Chem. Acra Reo. 1972,6,47. (c) Schreiver, A. F.; Liu, S.W.; Hauser, P. T.; Hopcus, E.A.; Hamm, D. J.; Gunter, J. D. Inorg. Chem. 1972, I I , 880. (d) Murrel, J. N.; Nikolskil, A. B. J . Chem. Soc., Chem. Commun. 1970, 1363. (52) Che, M.; Dutel, J. F.; Primet, M. Proc. 3rd Intern. Conf. Molecular Sieoes; Leuven University Press: Zurich, 1973; p 394. (53) Kasai, P. H.; Bishop, R. J. J . Am. Chem. SOC.1972, 94, 5560.
4514
J . Phys. Chem. 1991, 95, 4514-4521
complex, which occurs in M-X zeolites, is suppressed in H-X. The H-X zeolite is suggested to be able to localize a Ru(II1) species in the @-cageto prevent the formation of the Ru red trimer. Two major paramagnetic Ru(II1) species are formed in activated RuH-X, depending on the activation temperature. Species A, formed at an activation temperature of 150 OC, is suggested to be located in the @-cageand is assigned as a ruthenium(II1) complex probably with NH3, H20 (or OH), and zeolite lattice oxygen in the coordination sphere. Species B, formed a t an activation temperature of 300 O C is assigned to site I1 or II* and is assigned as an uncomplexed Ru(II1) ion coordinated only with the zeolite lattice. The ESR signal of this Ru(II1) ion is observed for the first time in X zeolite.
Ru(III)-(CO)~ species were characterized by ESR and IR upon CO adsorption on activated RuH-X. IR bands at 2146 and 2088 cm-' were characterized as coupled vibrations from a ruthenium(II1) dicarbonyl species. Ru(II1) ions are oxidized by oxygen to form Ru(IV)-O; and are reduced by nitric oxide to form Ru(I1)-NO+. ESEM results show that at low temperature (577 K) nitric oxide interacts with an aluminum nucleus with a distance of less than 0.37 nm.
Acknowledgment. This work was supported by the National Science Foundation, the Robert A. Welch Foundation, and the Texas Advanced Research Program. We thank Dr. D e n e Martin for valuable advice on the XPS experiments.
A Study of Cation Environment and Movement during Dehydration and Reduction of NickeCExchanged Zeolite Y by X-ray Absorption and Diffraction E. Dooryhee, C. R. A. Catlow,* J. W. Couves, P. J. Maddox, J. M. Thomas, Davy Faraday Research Laboratory, The Royal Institution, 21 Albemarle Street, London W l X 4BS, U.K.
G. N. Greaves, S.E.R.C. Daresbury Laboratory, Warrington, WA4 4AD, U.K.. and Department of Chemistry, University of Keele, Keele, Staffordshire ST5 5BG, U.K.
A. T. Steel, and R. P. Townsend Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Merseyside L63 3JW, U.K . (Received: June 20, 1990)
Using synchrotron radiation, we have monitored, by a combination of extended X-ray absorption fine structure (EXAFS) spectroscopy and high-resolution powder X-ray diffraction (XRD), (i) the location of NiZ+ions in the hydrated form of nickel ion exchanged Y zeolite (Si/Al = 2.25) at room temperature, (ii) the environment and evidence of movement of Ni2+ ions upon heating the zeolite to 300 OC in vacuo, and (iii) the changes in local environment of the nickel brought about by subsequent reduction in hydrogen. This combination of techniques gave both local (EXAFS) and long-range (XRD) structural information pertaining to the atomic environments of extra-framework species. We show that during dehydration approximately 70% of the Ni2+,which were present as solvated cations in the supercage, are transferred to the SI,hexagonal prism site, where they are stabilized by inward relaxation of the surrounding oxygen ions. The remaining Ni2+ions which are in a partially solvated state in the sodalite cage are readily reducible by hydrogen to yield small crystallites of metallic nickel.
Introduction Transition-metal-containing zeolites are extensively used in a large number of organic reactions, owing to an advantageous combination of the catalytic activity of the encaged transition metal and to the shape selectivity of the zeolitic host.'-3 The yield of product, as well as the longevity of the catalyst and the precise nature of the selectivity of the metal-exchanged zeolite, depends on the location, the coordination, and the oxidation state of the intrazeolitic active metal. Moreover, although it is difficult to achieve, the formation of uniformly dispersed, small metal particles within the zeolite framework enhances the catalytic activity? The bonding to the framework and the distribution of the exchangeable catalytic species in the nanopores of the zeolite are critically governed by the mode of preparation and subsequent treatment of the catalyst. The preparation of metal-loaded zeolite catalysts generally involves three stages: (i) ion exchange, (ii) calcination, and (iii) reductiorS The quality of the catalyst is particularly affected ( I ) Maxwell, 1. E. Ado. Cutul. 1982, 31, I . (2) Rabo, J. A.; Gjada, G. J. Cuful.Rev. Sei. Eng. 1990, 31, 385. (3) Thomas, J. M. Angew. Chem., Int. Ed. Engl. 1988, 27, 1673. (4) Delafosse, D. J . Chim. Phys. 1986, 83, 761.
0022-3654/91/2095-45 14$02.50/0
by the calcination step, since this process removes the water ligands of exchanged metal cations, thereby allowing the migration of metal ions into smaller The resulting location and bonding of the cations eventually influences the formation of the metal particles during the final stages of reduction and defines the accessibility of the metal species to the reactant molecules.&1o If we are to succeed in improving the performance of such catalyst sites it is necessary to pinpoint the structural changes around the active metal which occur in the course of various pretreatments. Zeolite Y is structurally analogous to the mineral faujasite, as previously determined.l1+I2 The building up of truncated oc(5) Homeyer, S.T.; Sachtler, W. M. H. Zeolires: Fucrs, Figures, Future; Jacobs, P. A., van Santen, R. A., Eds.; Elsevier Science: Amsterdam, 1989. (6) Park, S.H.; Tzou, M. S.; Sachtler, W. M. H. Appl. Curul. 1986.24, 85.
(7) Samart, M. G.; Bergeret, G.; Meitzner, G.; Gallezot, P.; Boudart, M. J . Phys. Chem. 1988, 92, 3547. (8) Tzou, M. S.;Sachtler, W. M. H. Read. Kinet. Curul. Lerr. 1987,35, 207. (9) Mortier, W. J.; Schoonheyat, R. A. Prog. Solid Stare Chem. 1985,16, 1. (IO) Thomas, J. M.;Williams, C.; Rayment, T. J . Chem. Soc., Furuduy Truns. I 1988. 84. 2915. (11) Bergerhoff, G.; Baur, W. H.; Nowacki, W. Neus Juhr Mineral Monrosh 1958, 193.
0 1991 American Chemical Society
