4586
J . Phys. Chem. 1986, 90, 4586-4590
The high density of sugar units in the chloroplast lamella membranes similarly may contribute to the structure and stability of the grana. Apart from the interactive stability/inertness of the sugar groups, the aqueous characteristics imparted by these units a t the membrane interface will undoubtedly influence the movement of solutes across the cell membrane.
Acknowledgment. The fluorescent probe RuLB2+was kindly supplied by Drs. W. H. F. Sasse and D. N. Furlong of the CSIRO
Division of Applied Organic Chemistry. Samples of DM and DC were generous gifts from David Siege1 of Procter and Gamble, Miami Valley Laboratories. G.G.W. and C.J.D. acknowledge the receipt of Commonwealth Postgraduate Research Awards from the Australian Government. We also thank Professor T. W. Healy for his encouragement and interest in this work. This work was funded by the Australian Research Grants Scheme. Registry No. DM, 7451 3-19-2; DC, 69227-93-6; C,*Es, 3055-98-9.
Correlated X-ray Photoelectron and Electron Spin Resonance Spectroscopic Investigations of the Reducibility of Nickel(I1) in Na-X and Ca-X Zeolites S. Contarini, J. Michalik, M. Narayana,+*and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: November 7, 1985; In Final Form: February 20, 1986)
Correlated X-ray photoelectron spectroscopic (XPS) and electron spin resonance (ESR) studies have been carried out on the reducibility of NiZ+in Na-X and Ca-X zeolites. Reduction of Ni2+ by static Hzin NiNa-X readily formed Nio at 600 K, while in NiCa-X reduction was more difficult and at 600 K only formed Ni' as observed by ESR. At 773 K reduction to Nio in NiCa-X was observed. During reduction the Ni(2p) XPS peak broadened toward lower binding energies. Deconvolution required three components that suggest Ni+ at 854 eV as an intermediate state in the reduction. In accordance with this conclusion the intermediate XPS component correlated with the ESR spectrum of Ni+. The greater ease of reduction of NiZ+in Na-X vs. Ca-X contrasts with the opposite trend of NiZ+being reduced easier in Ca-Y vs. Na-Y. Both cocations and zeolite structure can control the reducibility of exchanged Ni2+in zeolites.
Introduction Acidic zeolite catalysts involved in hydrogenation-isomerization type reactions often contain transition-metal species to aid in hydrogenation/dehydrogenation.' The valence state, location, and dispersion of the transition-metal species are of importance and generally the metallic form is optimal. Thus the reducibility of transition-metal species is of interest. The reduction of nickel ions in zeolites has been studied for some time.2 The ease of reduction depends on the type of zeolite, the extent of metal loading, and the temperature. More recently, it has been shown that the major cocation present in the zeolite plays an important role in determining the reducibility as well as the final dispersion of the metallic X-ray photoelectron spectroscopy (XPS) has been used as a characterization tool for nickel compounds6 and to determine the oxidation states formed during oxidation and reduction cycles of nickel and on zeolites.9-'' We recently demonstrated that correlation of XPS data with electron spin resonance (ESR) results often allows a more definitive assignment of specific XPS peaks to metal ion valence states in X and Y zeolites which are p a r a m a g n e t i ~ . ' ~ - 'Recently ~ we studied16 by ESR methods the formation and interaction of Ni+ ions with different ligands in Ca-X zeolites. In these experiments we observed that while the reduction from Ni2+ to Ni+ was possible a t relatively low temperatures (373-423 K), complete reduction to the metallic Ni phase was not possible even at temperatures above 673 K. In this article we present comparative XPS and ESR results for NiNa-X and NiCa-X zeolites and demonstrate how the major cocation affects the reducibility of the nickel ion. Experimental Section Linde Na-X zeolite (1 3X) was used after washing with 0.1 M NaCl solutions. The exchange of calcium was done with 0.05 M calcium chloride solutions at 340 K. Commercial atomic absorption analysis was used to confirm the complete replacement 'Present address: Shell Development Co., Houston, Texas 77001.
of sodium by calcium ions. Then various amounts of Ni2+ ions were exchanged into the Na-X and Ca-X zeolites by ion exchange with 0.01 M nickel nitrate solutions at ambient temperatures. Some samples were also prepared with nickel chloride. The results obtained with two particular nickel concentrations, Niz3Na,,,-X and N i 1 3 C a d ,will be presented in detail in the following sections. The zeolites after ion exchange were dried in air at room temperature (60% relative humidity) and pressed in a hydraulic die at about 200 kg to form a thin smooth round (1-cm-diameter) pellet that was then positioned on a sample holder with a silver clamp for XPS measurements. Photoelectron spectra were obtained with a Perkin-Elmer PHI Model 550 ESCA/SAM spectrometer using Mg Ka X-rays a t 1253.6 eV as the excitation source. All binding energies were referenced to the hydrocarbon
(1) Bager, K. H.; Bogt, F.; Bremer, H. ACS Symp. Ser. 1977, 40, 528. (2) For example, see: (a) Yates, D. J. C. J. Phys. Chem. 1965,69, 1676. (b) Richardson, J. T. J. Catal. 1971,21, 122. (c) Herd, A. C.; Pope, C. G. J . Chem. Soc., Faraday Trans. 1 1973, 69, 833. (3) Briend-Faure, M.; Jeanjean, J.; Kermarec, M.; Delafosse, D. J. Chem. SOC.,Faraday Trans. I 1978, 74, 1538. (4) Guilleux, M. F.;Delafosse, D.; Martin, G. A,; Dalmon, J. A. J . Chem. Soc., Faraday Trans. I 1979, 75, 165. (5) Suzuki, M.; Tsutsumi, K.; Takahashi, H. Zeolites 1982, 2, 51. (6) Matienzo, L. J.; Yin, L. 11; Grim, S. 0.;Swartz, W. E. Inorg. Chem. 1973, 12, 2762. (7) Kim, K. S.; Winograd, N. Surf. Sci. 1974, 43, 625. (8) Barr, T. L. J . Phys. Chem. 1978, 82, 1801. (9) Vedrine, J. C.; Hollinger, G.; Duc, T. M. J . Phys. Chem. 1978, 82, 1515. (10) Badrinarayana, S.; Hegde, R. I.; Balakrishnan, I.; Kulkarni, S. B.; Ratnasamy, P.J. Catal. 1981, 71, 439. (11) Narayanan, S. Zeolites 1984, 4, 231. (12) Narayana, M.; Contarini, S.; Kevan, L. J. Catal. 1985, 94, 370. (13) Narayana, M.; Michalik, J.; Contarini, S.; Kevan, L. J. Phys. Chem. 1985, 89, 3895. (14) Contarini, S.; Kevan, L. J . Phys. Chem. 1986, 90, 1630. (15) Minachev, Kh. M.; Antoshin, G. V.; Yusifov, Yu. A.; Shpiro, E. S. React. Kinet. Catal. Lett. 1976, 4, 137. (16) Michalik, J.; Narayana, M.; Kevan, L. J. Phys. Chem. 1984,88, 5236.
0022-3654/86/2090-4586$01.50/00 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 19, 1986 4581
Reducibility of Nickel(I1) in Zeolites
n
XPS OF NiNaX
ESR OF NiNaX
/
I
Q = 2.006
b
\' Figure 1. ESR spectra at 77 K of Ni,Na*-X dehydrated at 523 K: (a) reduced with H2at 523 K for 2 h and (b) reduced with H2 at 600 K for 2 h. In samples with H2 at temperatures between 423 and 540 K the spectra were identical with that in (a), and in samples reduced at temperatures above 580 K the spectra were identical with that in (b). The lines in (a) occur at g = 2.104 and 2.006.
carbon 1s line, due to adventitious hydrocarbon contamination, at 285.0 eV. The base pressure for analysis was in the Torr range. Data smoothing, subtraction of the contribution of inelastic scattering, and deconvolution of the spectra were performed with the PHI software available in the spectrometer. ESR measurements were made with a Varian E-9 spectrometer at 9.1 GHz at room temperature and 77 K. The zeolite powder was loaded into 3-mm-0.d. Suprasil quartz tubes for this purpose. Reduction of the samples is carried out in two different ways, either as pellets in the pretreatment chamber of the XPS instrument or as a powder in ESR tubes in a greaseless, glass high-vacuum system. In the latter, the powder is evacuated to 1 X lo-, Torr for 4 h at room temperature, slowly heated under vacuum to 523 K over -3 h, and held at 523 K for 4 h until the residual pressure was about Torr. Then the sample is contacted with 200 Torr of H2 (Union Carbide 99.99%) for 2 h at different temperatures ranging from 373 to 673 K. A broken pellet gave the same ESR spectra as the powder, so the compacting used did not affect this. Prior to XPS measurements the pellets were evacuated to about Torr at room temperature in the pretreatment chamber of the XPS instrument for about 4 h and then the sample temperature was raised slowly to 523 K and held at that temperature for 2 h before exposure to 200-800 Torr of static H2. To obtain XPS data the sample was moved into the analysis chamber of the XPS instrument. The contact time with H 2 was varied from 2 to 15 h with interspersed XPS measurements to detect the degree of reduction. It should be noted that all reductions for both ESR and XPS were done with static H,; reduction with flowing H,, which could occur at a different rate, was not investigated.
1
1
1
1
,
875 855 BINDING ENERGY, eV
Figure 2. X-ray photoelectron spectra of the Ni 2p region in Ni23Na40-X dehydrated at 523 K: (a) dehydrated sample, (b) after contacting the dehydrated sample with 600 Torr of H2 at 523 K for 3 h and evacuating prior to measurement, and (c) after contacting the sample in (b) with 800 Torr of Hz at 600 K for 2.5 h and evacuating prior to measurement. XPS OF NiNaX
--- COMPONENTS
-
Results and Discussion ESR spectra were measured both with and without H2 in the sample tubes after cooling them to ambient temperature or to 77 K. ESR spectra of Ni2,Na,-X at 77 K are shown in Figure 1; the line at g = 2.104 is characteristic of Ni+ probably coordinated to H 2 in X zeolites. These were obtained after the zeolite was dehydrated at 523 K for 4 h under vacuum with subsequent reduction in static H2at different temperatures. Dehydration of Torr the zeolite up to 673 K to a residual pressure of about without added Hz did not result in any ESR-active nickel species. Although Ni2+ itself is paramagnetic, it often has a sufficiently short spin-lattice relaxation time that it is not observed by ESR at higher temperatures. Nickel in the monovalent state however is isoelectronic with Cu2+and has a distinct and easily observable ESR spectrum at 77 K and room temperature. On contact with H2or CO the dehydrated zeolite immediately exhibits a Ni+ ESR spectrum, the intensity of which increases as the temperature of reduction is raised. Ni+ is seen until the reduction temperature reaches 573 K; above that a broader, more intense ferromagnetic
.< 1
866
I
L.P
I
I-,
Y.,'
I.* /' . I
-%L
862 858 854 BINDING ENERGY, eV
850
Figure 3. Deconvolutions of the 2p312transition and associated satellite for the XPS spectra of NiNa-X in Figure 2.
resonance line of metallic nickel particles is seen as shown in Figure 1. The maximum amount of Ni+ observed was
