J. Phys. Chem. 1989, 93, 1515-1521 TABLE 11: Carbon Content (%) of a 160 min Sputtered Catalyst no. of recycles C H N ~~
~
0 1
0.4 0.1
2 3
0.07 0.05
0.25
0.06
0.17 0.19 0.21
0.0 0.0 0.0
parently reacted with H 2 at 300 O C during the process and this is shown in Table 11. In conclusion, this work has presented a novel setup for simple metal deposition on powders, via dc sputtering. Deposition of Pt on Ti02 is quick and uncomplicated and can be carefully controlled. Improvement in the mode of agitation of the powder would render a more homogeneous coverage. Work is under way in our laboratory in this direction. In situ ESCA techniques show that sputtered Pt/Ti02 materials have different oxidation states formed
1515
and stabilized at the catalyst surface. The most active catalyst contained only two oxidation states. For charge transfer this condition is therefore sufficient to affect the fluctuating electron density for catalytic activity.26 The present work has combined hydrogen chemisorption on supported metals, electron microscopy (TEM), and ESCA with kinetic data. Acknowledgment. Financial support for the study was given by the Swiss National Science Foundation. We thank I. Rodicio and P. Ruterana of the EPFL for their help with electron microscopy and chemisorption work. Discussions with J. Highfield during the course of this work are appreciated. Registry No. Pt, 7440-06-4;Ti02, 13463-67-7;C2H6, 74-84-0; H2. 1333-74-0. (26) Falicov, L.; Somorjai, G. Proc. Narl. Acad. Sci. USA 1985,82,2207.
Electron Spin Resonance and Diffuse Reflectance Spectroscopy of the Reduction of Ni2+with H, in Zeolites X and Y, Exchanged with La3+ and NH,' Robert A. Schoonheydt,* Ivan Vaesen, and Hugo Leeman Laboratorium voor Oppervlaktechemie, K. U. Leuuen, K. Mercierlaan, 92, B- 3030 Leuven, Belgium (Received: May 18, 1988)
After an oxidative pretreatment the thermal reduction of Ni2+ with H2 to Ni(0) goes over Ni+. In NiLa-Y two types of ESR-active Ni+ were found. Signal I consists of three g,values at 2.67, 2.51, and 2.33 and one g, value at 2.096. It is due to Ni+ in the cubooctahedra. Up to 11% of the Ni2' can be converted to this type of Ni+, which is thermally stable up to 773 K. Signal I1 with g values of 2.16 and 2.064 is due to a thermally unstable Ni+ species and is present only in trace amounts. The stabilization of ESR-active Ni+ requires a low Ni content, polyvalent cations, such as La3+and Ca2+, and trigonally coordinated Ni2+. It is almost absent in NiLa-X, because of the absence of trigonally coordinated Ni2+. In NiH-Y the mobility of Ni+ is too high to stabilize it as a mononuclear ESR-active species, but Ni clusters can be stabilized. The stabilization of Ni+ and Ni clusters is also much less pronounced in NiH-X, due to a combination of factors such as lattice instability and mobility of Ni'. Ni clusters are formed simultaneously with ESR-active Ni+. They are characterized by DRS bands at 13 500,27 500,34000, and 38 000 cm-'. The first two bands are characteristic for molecularly sized clusters. The latter two may be cluster bands.
Introduction The reduction of Ni2+ in dehydrated zeolites depends on the structure type, the coexchanged cation, and the loading.I4 Some representative data are given in Table I. It is shown that under identical experimental conditions the degree of reduction follows the order zeolite X > zeolite Y and mordenite > zeolite Y.2,4 While the coexchanged cations determine the overall chemical properties of the zeolites through the average ele~tronegativity,~ they also compete with Ni2+ for exchange sites. Thus, under the same experimental conditions the degree of reduction of Ni2+ in alkaline-earth-exchanged Y exceeds that of NiNa-Y and follows the order Mg < Ba < Sr < Ca.4 However, the degree of reduction in NiNaX exceeds that of NiCaX.6 In X-type zeolites Ce3+ is known to promote the reduction of NiZ+,while La3+retards (1) Lausch, H.; Morke, W.; Vogt, F.; Bremer, H. Z . Anorg. Allg. Chem. 1983, 499, 213.
(2) Briend-Faure,M.; Jeanjean, J.; Kermarec, M.; Delafosse, D. J . Chem. SOC.,Faraday Trans. I 1978, 7 4 , 1538. ( 3 ) Briend-Faure, M.; Jeanjean, J.; Delafosse, D.; Gallezot, P . J . Phys. Chem. 1980, 84, 875. (4) Suzuki, M.; Tsutsumi, K.; Takahashi, H. Zeolites 1982, 2, 51. (5) Mortier, W. J.; Schoonheydt, R. A. Prog. Solid Srate Chem. 1985, 16, 1.
TABLE I: Degree of Reduction of NiZ+in Zeolites
reduction degree, % 90 60
zeolite NiNa-Y-10" NiNa-Y-72 NiLa-Y- 14
873 K, 643 K, 873 K, 643 K, 643 K,
70 11
Ni H-Y -3 3
NiNa-X-71 NiNa-M-37* NiH-M-28
reduction conditions 870 K, 2 h 873 K, 25 h
100 100 13
25 h 10 h 20 h 10 h 10 h
ref 4 2 3 4 2 4 4
"Degree of Ni exchange in percent of the CEC. b M stands for mordenite.
In the presence of acidic O H groups the reduction of Ni2+ is difficult, because the equilibrium Ni2+
+ H2
Ni(0)
+ 2H+
(1)
is driven to the left.4
Equation 1 represents a complicated set of reactions, involving several intermediates. One intermediate, Ni', has been isolated in small quantities in NiCa-X and NiCa-Y and studied in great detail by electron spin resonance (ESR).11-'4
( 6 ) Contarini, S.; Michalik, J.; Narayana, M.; Kevan, L. J . Phys. Chem. 1986, 90, 4586.
(7) Guilleux, M. F.; Delafosse, D.; Martin, G. A.; Dalmon, J. A. J . Chem. SOC.,Faraday Trans. I 1979, 75, 165. (8) Djemel, S.;Guilleux, M. F.; Jeanjean, J.; Tempere, J. F.; Delafosse, D. J . Chem. Soc., Faraday Trans. 1 1982, 78, 835.
0022-3654/89/2093-1515$01.50/0
(9) Briend-Faure, M.; Jeanjean, J.; Spector, G.; Delafosse, D.; BozonVerduraz, F. J . Chim. Phys. 1982, 79, 489. (10) Sauvion, G.-N.; Tempere, J. F.; Guilleux, M. F.; Djega-Mariadassou, G.; Delafosse, D. J . Chem. Soc., Faraday Trans. 1 1985, 81, 1357.
0 1989 American Chemical Society
1516 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
Schoonheydt et al.
TABLE 11: Exchangeable Cation Content of the Zeolites (in mmol/g Dry)
symbol NiLa-Y-1 NiLa-Y-2 NiLa-Y-3 NiLa-X- 1 NiLa-X-2 NiLa-X-3 NiH-Y-1 NiH-Y-2 NiH-X-1 NiH-X-2
Ni 0.065 0.52 0.48 0.20 0.42 0.30 0.37 1.09 0.40 1.27
La 0.71 0.75 0.89 1.12 1.14 1.44
Na 1.54 0.60 1.06 0.99 0.42 0.69 0.55 0.40 0.90 0.79
NH4
2.01 0.87 2.60 0.97
Two characteristic ESR signals with g, = 2.096 and g, = 2.065 have been described, but several g,,values correspond with these g, values. This together with the small concentrations of these species makes the assignment very difficult if not impossible. Also in NiNa-X trace quantities of Ni+ have been reported on the basis of ESR15 and X-ray photoelectron spectroscopy (XPS) measurements.6 These studies show that a polyvalent cation, Ca2+,and mild reduction procedures are necessary for the stabilization of Ni+. In these conditions the NiCa zeolites acquire a green color. This has been ascribed to the isolated Ni+ species, detected by ESR, but recently it was proposed that the color could be due to charged Ni cl~sters.l'*'~ It follows from the foregoing discussion that the maximalization of Ni+ and eventually of charged Ni clusters requires a strict control of the reduction reaction to prevent a too fast agglomeration of the reduced Ni species. This should be accomplished in the easiest way in La and H zeolites at low Ni loadings. This is the subject of this paper.
Experimental Section Samples. The synthetic zeolites X and Y (Ventron) were treated with Na2S203six times according to the procedure of Derouane et a1.I6 to remove the paramagnetic Fe impurities prior to any treatment. NiLa-X- 1 and NiLa-Y- 1 were prepared by exchange overnight of 12.5 g of Na-X and Na-Y, respectively, at 343 K in 2 dm3of aqueous solution containing 0.05 mol of NiCI, and 0.05 mol of La(N0J3. NiLa-X-2 and NiLa-Y-2 were prepared by exchange of 25 g of Na-X and Na-Y, respectively, in 5 dm3 of La(NOs)3, the amount of La3+ being 3 times the cation-exchange capacity (CEC). The La samples were washed anion-free, deep-bed (DB) calcined at 823 K in air, and exchanged in 5 dm3 of a 0.02 mol NiC12solution. NiLa-X-3 and NiLa-Y-3 were obtained in the same way as NiLa-X-2 and NiLa-Y-2 except that the La exchange was done under reflux during 24 h and the Ni exchange in 0.01 M solutions. NiNH4-X and NiNH,-Y were obtained by exchange of 20 g of Na-X and Na-Y, respectively, in 10 dm3 of 0.01 M NH&l at room temperature during 24 h. The NH, forms were exchanged at room temperature overnight with 10 dm3 of a NiC12 solution, the amount of Ni2+ being equal to the desired exchange level. In all the exchange reactions the salts were used in their hydrated forms. All the samples were washed anion-free and dried in air at room temperature after each exchange. The exchangeable cation composition of the zeolites was determined by atomic absorption spectrometry (AAS) after dissolution of known quantities of the samples in HF/H2S04. The exchangeable cation composition of the samples and the sample symbols are given in Table 11. Pretreatment and Reduction Procedure. The Ni zeolites were granulated, and the size fraction 0.25-0.5 mm was loaded in a ( I 1) Garbowski, E. D.; Mathieu, M.-V.; Primet, M. ACSSymp. Ser. 1977, 40. 281. -1
(12) Kermarec, M.; Olivier, D.; Richard, M.; Che, M.; Bozon-Verduraz, F. J . Phys. Chem. 1982, 86, 2818. ( 1 3 ) Michaiik, J.; Narayana, M.; Kevan, L. J . Phys. Chem. 1984,88, 5236. (14) Schoonheydt, R. A.; Roodhooft, D. J . Phys. Chem. 1986, 90,6319. ( 1 5 ) Che, M.; Richard, M.; Olivier, D. J . Chem. SOC., Furuduy Trans. 1 1980, 76, 1526. (16) Derouane. E. G.; Mestdagh, M.; Vielvoye, L. J . C a u l . 1974, 33, 169
1
10
15
20
25
30
35
40
45
47
I Figure 1. DRS spectra of NiH-Y-1 (A) and NiH-X-1 (B), pretreated in an O2 flow at 773 K.
combined ESR-DRS quartz flow cell. They were pretreated in an O2flow by slowly raising the temperature to 723 or 773 K and keeping this final temperature overnight. The samples were cooled to 373 K, the O2flow was replaced by a He flow, and the samples were cooled to room temperature in He. Then ESR and DRS spectra were taken. The reduction was performed in a H 2 flow of 0.5 cm3 s-' (approximately), starting at 373 K, to 573 K with intervals of 50 K and to 773 K with intervals of 100 K. At each interval the temperature was kept constant for 1 h. Then the samples were cooled to room temperature in H2or flushed with He prior to the spectroscopic measurements. The H2 and He gases are 99.995% and are delivered by L'Air Liquide. He was passed over a Pd catalyst at room temperature to remove traces of O2prior to passing over the zeolite beds. Techniques. ESR spectra were taken in the X band on a Bruker 200D-SRC instrument at 110 K in a double rectangular T E I w cavity. The g values of the signals were determined from the resonance condition hv = gpB,. The frequency was measured with the H P 5342A microwave frequency counter, and the field was read from the oscilloscope after expansion of the scale of the magnetic field abscissa. In some cases the amount of ESR-active Ni+ was determined by double integration of the signals and comparison of the intensity with that of known quantities of Cu(acac), (acac = acetylacetonato) diluted in KC1. Diffuse reflectance spectra (DRS) were taken at room temperature on a Cary 17, equipped with a digital slidewire and with a type I diffuse reflectance attachment in the range 2200-210 nm. The reference was the Eastman Kodak white reflectance standard. At each nanometer the data were transferred to a H P 9825 desktop computer. When the recording was finished, the spectra were calculated into the Kubelka-Munk function, F(R,), the base line was subtracted, and the result plotted as F(R,) against wavenumber. The base line was recorded with the Eastman Kodak white reflectance standard in the sample and the reference beam. The plotted spectra were stored on tape. The crystallinity of the zeolites after pretreatment and after reduction was checked by X-ray diffraction on a Siemens type F powder diffractometer with Cu KO radiation. Results DRS Spectroscopy of the Pretreated Samples. After the oxidative pretreatment Ni2+ is present in octahedral, tetrahedral, and trigonal coordination. The three types of coordination are present in different relative amounts depending on the type of coexchanged cation and the type of zeolite. The spectra of the N i L a zeolites have been published." The NiLa-X samples all show predominantly the presence of tetrahedral Ni2+ with characteristic d-d bands at 8200 cm-' and a doublet at 16 100 and 17 500 cm-I and octahedral Ni2+with d-d bands around 6250, (17) Schoonheydt, R. A,; Roodhooft, D.; Leeman, H. Zeolites 1987, 7, 412.
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1517
Reduction of Ni2 with H2 in Zeolites
2 33
features become apparent in the ESR spectra at g values equal to 2.51 and 2.33. As there are no additional features around g, = 2.096, we interprete these new features as gll values, also associated with g, = 2.096. All these signals are collectively called signal I. Signal I attains its maximum intensity in the range 473-523 K. At higher reduction temperatures ferromagnetic Ni(0) shows up in the ESR spectra, but the perpendicular component of signal I remains visible up to a reduction temperature of 723 K. For NiLa-Y-3 the same ESR signals are observed with the difference that signal I is thermally more stable. Its maximum intensity is obtained after reduction at 673 K and corresponds to 11% of the Ni content of the sample. At higher temperatures ferromagnetic Ni(0) shows up in ESR, but signal I persists up to 773 K, the highest reduction temperature investigated. When a sample having only signal I in ESR is subjected to a He flow, signal I1 reappears. The DRS spectra of the reduction of Ni2+ in NiLa-Y-3 are shown in Figure 3. The reduction is accompanied by a gradual increase of the intensity of three bands. The band at 13 500 cm-' causes the green color of the samples. After reduction at 573 K three components are visible with maxima approximately at 13 100, 13 300, and 13 600 cm-I. The second band has a maximum at 27 900 cm-l. The third band has an ill-defined maximum around 38 000 cm-l. However, with increasing reduction temperature a band at 34 100 cm-' shows up and becomes the most important band in the UV. The intensities of the 13 500- and 27 900-cm-' bands increase with increasing reduction temperature in a parallel way (Figure 4). Figure 4 also shows that there is a fair correlation between the intensity of ESR signal I, measured from the height of the g, = 2.096 line, and the intensity of the 13 500-cm-I band. The spectral features described for NiLa-Y-2 and -3 are also characteristic for NiLa-Y-1. Because of the extremely low Ni content the intensities of the ESR signals and of the DRS bands are 1-2 orders of magnitude lower. Ni2+ in LiLa-X-1, -2, and -3 is extremely difficult to reduce. This is shown by the ESR and DRS spectra of Figures 5 and 6. ESR signals I and I1 are formed upon reduction at 523 K together with an 02-signal. For NiLa-X-3 the maximum intensity of signal I is 1 order of magnitude lower than for NiLa-Y-3, although the Ni contents differ by only a factor of 1.6. The corresponding DRS spectra are typical for NiZ+at all reduction temperatures. Only after reduction at 773 K is there a weak band at 27 900 cm-', indicative of some reduction of Ni2+. Reduction of Ni2+ in NiH-Y and NiH-X. The most typical results are obtained on the samples with low Ni content. The ESR spectra of NiH-Y-1 show weak impurity signals at low reduction temperatures and a weak signal I after reduction at 573 K and higher. Figure 7 shows that the Fe3+ line around g = 4.3 is considerably broadened and contains in addition two low-field maxima around g = 5.63 and g = 6.52. At present we have no explanation for these features. In the corresponding DRS spectra the bands of the green Ni zeolites appear after reduction at 573 K and acquire their maximum intensity after reduction at 623 K (Figure 8). The band maxima are located at 13 500, 28 000,
1;
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Figure 2. ESR spectra of NiLa-Y-2 after reduction with H2during 1 h followed by flushing with He. The reduction temperatures are (A) 373, (B) 473, and (C) 723 K. The receiver gain is 1250.
4
10
15
20
25
30
35
45
40
47
IO^^,-,,-' Figure 3. DRS spectra of the reduction with H2of NiLa-Y-3 at (A) 573, (B) 723, and (C) 773 K.
10 500, and 19 500 cm-I. NiLa-Y samples have these two coordinations of Ni2+ to a lesser extent, and trigonal Ni2+ as the dominant species with a characteristic band around 23 000 cm-'. The d-d spectra of Ni2+ in NiH-Y show only trigonal and octahedral Ni2+with almost complete absence of tetrahedral NiZ+ (Figure 1). In X-type zeolites tetrahedral Ni2+is clearly present, as evidenced by the doublet at 16 100 and 17 500 cm-'. E S R and DRS of the Reduction of Ni2+ in La3+ Zeolites. In NiLa-Y-2 the reduction starts at 373 K with the appearance in ESR of signals at g = 2.67, 2.16, 2.096, and 2.064 (Figure 2). With increasing temperature the signals at g = 2.16 and 2.064 disappear, and the two others gain intensity. The assignment is then as follows: signal I with gll = 2.67 and g, = 2.096; signal I1 is characterized by g,,= 2.16 and g, = 2.064. These signals have already been described for NiCa zeolites and have been ascribed to Ni+.14 After reduction at 473 K new broad and weak
,oat :80-
-R' .9 40 20 Oo: .' 200
300
400
500
C
0
;;;/ I
04
08
12
16
2 0 FIR,
Figure 4. Left: F(R,) as a function of the reduction temperature of NiLa-Y-3 for the 13 500-cm-' band (B) and the 27 900-cm-' band (e). Right: Correlation between the intensity of the g, = 2.096 line in ESR and F(R,) of the 13 500-cm-I band for NiLa-Y-3.
1518 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
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Figure 6. DRS spectra of NiLaX-3 after reduction with H, at (A) 473 and(B)773 K.
and 38 700 cm-I. Reduction above 623 K leads to a reduction in intensity of the three bands and the appearance of a band at 32 500 cm-I. A consequence of the decrease of the intensity is that the residual Ni2+ bands become resolved again. It is worth noting that the intensity ratios of the DRS bands are different in NiH-Y-1 and in NiLa-Y-3. For the former sample the 13 500-cm-' band is the most intense; the two other bands have about equal intensity. In NiLa-Y-3 the 13 500- and 27900-cm-' bands have almost equal intensity and the 38000-cm-] band remains relatively weak. ESR signal I on the other hand is about 50 times less intense for NiHY-1 than for NiLaY-3. Therefore there is no correlation between ESR signal I and the DRS spectra. Also, the different DRS bands may be due to different species. At higher Ni content, such as for NiH-Y-2, ferromagnetic Ni(0) is the main species seen in ESR. The DRS spectra show Ni2+ bands and the bands of the reduced green zeolites, but the latter are much weaker than in NiH-Y-1 after comparable reduction treatments. When NiH-X-1 is reduced in a stepwise manner, only trace amounts of ESR signal I are detected and after reduction at 623
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Figure 8. DRS spectra of NiH-Y-1 reduced with H2 during 1 h at (A) 423 and (B) 623 K.
K Ni(0) starts to show up. In DRS the Ni2+ bands remain dominant, but two new weak bands appear with maxima around
The Journal of Physical Chemistry, Vol. 93, No, 4, 1989 1519
Reduction of Niz with H2 in Zeolites
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............................................ z 8 3 8 8 3
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Figure 11. Top: DRS spectra of NiLa-Y-3 reduced at 523 K with He during 1 h (A), followed by adsorption of ethylene at room temperature during 30 min (B). Bottom: DRS spectra of NiLa-X-3 reduced with H2 at 723 K during 1 h (A), followed by adsorption of ethylene at room temperature during 30 min (B).
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,
Figure 10. ESR spectra of NiLa-Y-3 reduced with H2at 523 K during 1 h (A), followed by adsorption of ethylene at room temperature during 30 min (B).
15OOO and 31 000 cm-I (Figure 9). The crystallinity of NiH-X-2 is damaged upon reduction and ferromagnetic Ni(0) is directly formed. Adsorption of Ethylene and Oxygen. When NiLa-Y-3 and NiLa-X-3 are reduced at 523 and 723 K, respectively, only ESR signal I is apparent. Upon room-temperature adsorption of ethylene during 1800 s the intensity of signal I decreases somewhat and a new weak signal appears with g , = 2.71, g2 = 2.57, and g3 = 1.96 (Figure 10). Tn DRS the typical bands of the green NiLa-Y-3 are almost completely wiped out and replaced by a new intense band a t 32 500 cm-' (Figure 11). The same band appears after adsorption of ethylene on NiLa-X-3, which originally was not green, and the NiZ+bands decrease in intensity. This is also the case when ethylene is adsorbed on the NiLaY samples prior to reduction. The 32 500-cm-I band shows up but is much less intense than for the reduced samples. When O2 is adsorbed at room temperature on a prereduced NiLa-Y-3, signal I in ESR is transformed to signal I1 and the three typical bands of the green Ni zeolites decrease in intensity (Figure 12).
Discussion Our strategy to maximize the concentration of intermediates during the reduction of NiZ+to Ni(0) in faujasite-type zeolites proved to be successful. The following factors are necessary: (1) a low Ni content; (2) the presence of polyvalent cations, such as La3+ and Ca2+; (3) the presence of acidic OH groups. The first two factors are important for ESR-active, mononuclear Ni+. Thus, in NiLa-Y-3 11% of the total Ni content was transformed to ESR-active, mononuclear Ni+. A combination of 1 and 2, 1 and 3, or all three factors together allow a maximum of green zeolites, which is ascribed to Ni clusters for reasons discussed below. Only in the case of NiLa-Y was a correlation found between the concentration of ESR-active Ni+ and the concentration of Ni clusters. Our strategy did not work for X-type zeolites (La-X and H-X). The reduction of Ni2+ is very difficult in NiLa-X because NiZ+ is octahedrally and tetrahedrally coordinated. These are completely filled coordination spheres. Thus, unsaturated coordination, such as the trigonal coordination to oxygen six-rings of sites 1', II', and I1 and the tetragonal coordination of the sites 111 and 111' are necessary for reduction of Ni2+. In addition, octahedral coordination occurs in the hexagonal prisms and tetrahedral coordination primarily in the cubooctahedra. In both cases the H2 molecules have to overcome a diffusion barrier through the oxygen six-rings to reach the Ni2+. NiH-Y-1 and -2 do not develop ESR-active Ni+ to any appreciable amount but do develop the DRS bands of the green zeolites in the order NiH-Y-1 > NiH-Y-2. In both cases trigonal Ni2+ is predominant before reduction and in principle Ni+ can be formed. Although acidity retards the reduction reaction, it is unable to keep Ni+ in an isolated position, as shown by the absence of appreciable quantities of ESR-active Ni+. This has
1520 The Journal of Physical Chemistry, Vol. 93, No. 4 , 1989
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to do with the mobility of the Ni+. The mobility of Na+ in La-Y is almost 2 orders of magnitude less than in H Y with the same number of Na+.I8 We expect the same for Ni+. It follows that the green colour of the partially reduced Ni zeolites is due to aggregates of Ni+ with Ni2+,Ni+, and/or Ni(0). There are no other cations involved, except maybe Na+, because the green color is common to Ca2+,La3+, and H zeolites. The same arguments apply to NiH-X-1. However, the bands of the Ni clusters are at higher frequencies and very weak. Thus, the concentration and the nature of the Ni clusters are different from those of NiH-Y-I, showing that there is a definite influence of the lattice on the electronic properties of the clusters. In summary, evidence has been advanced for a reduction of Ni2+ according to Ni+ is stabilized in appreciable amounts in Y zeolites with low Ni contents, with trigonal Ni2+in the presence of polyvalent cations such as La3+. Ni clusters are stabilized in appreciable amounts in the same conditions and also in HY. Nature of the Reduced Ni Species. Signal I1 is formed upon reduction at 373 K. It can also be generated from the Ni+ species with signal I, by passing O2 or He over the sample at room temperature. There are then two possible explanations for signal 11: Ni' on sites 11' or I1 and Ni(02)+. The ease of formation of signal I1 (low reduction temperature, change from H 2 atmosphere to O2or He) favors the first hypothesis. This is supported by the work of Kermarec et al., who showed that signal I1 can be generated from signal I by adsorption of CO.lZ However gll is unusually low for a d9 ion such as Ni+ on a six-ring of zeolites. The isoelectronic Cu2+has g,,values in the range 2.33-2.39.19 The value of 2.16 means that more delocalization of unpaired spin density toward the ligands occurs in the case of Ni+ than of Cu2+. On the basis of the charges the reverse behavior is expected. The only alternative explanation is that signal I1 is an anisotropic signal with an as yet undetectable third component. The Ni(02)+ hypothesis means that the zeolite still contains some O2 from the pretreatment after desorption at 373 K or that the He, although (18) Schoonheydt, R. A. Proc. 5th Inr. Zeolite Conf. 1980, 242. (19) Packet, D.; Schoonheydt, R. A. Proc. 7rh Int. Zeolire Con$ 1986,385.
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passed over a Pd catalyst, still contains traces of 02. The O2 complex can be disregarded, because it consists of adjacent paramagnetic species (Ni+ and 02).This would lead either to ESR spectra broadened beyond detection by dipolar interaction or to exchange-narrowed spectra. The observations that signal I is hardly affected by ethylene and that O2transforms it to signal I1 on a time scale of several hundreds of seconds are indications that the Ni+, responsible for the signals I, is mainly located in the sodalite cages. H2 is at most a very weakly coordinating ligand, because the signal is stable toward evacuation but changes upon adsorption of O2 in the supercages. All this has been discussed in the literat~re.l'-~~ What is new is that we have found three g,, values at 2.67, 2.51, and 2.33. The first one has been seen also in NiCa-Y by different The two other components are broader and therefore can be seen clearly only in NiLa-Y with the high Ni+ content of 11%. Ni+ is isoelectronic with Cu2+, but two differences are worth mentioning, which are due to the charge difference. The ligand field strength of Ni+ is much lower than that of Cu2+,thus d-d bands are expected at lower energies than those of Cu2+. The latter occur in the range 10000-15000 cm-' for coordination to oxygen six-rings.19 Second, the electron density will be more localized than for Cu2+. Thus the spin-orbit coupling constant of Ni+ will be closer to the free ion value than that of Cu2+. It is expected then that the g values of Ni+ exceed those of Cu2+, as observed experimentally, except for the gll = 2.33. With these ideas in mind the d-d transitions can be estimated from the g values, assuming axial symmetry, with gll = 2.0023 - 8X/E1 and g, = 2.0023 - 2X/E2. E, and E2 are the energy differences between dX24and d, and d,,,d,,,, respectively. With X = -300 cm-I E2 equals 6250 cm-I and El = 3582,4706, and 7273 cm-I for gll = 2.67, 2.51, and 2.33 respectively. X = -300 cm-I corresponds to 50% reduction with respect to the free ion value.20 This is the same reduction as for C U ~ + . ~The I predicted d-d transitions are, as expected, significantly below those for C U ~ + . ~ ' They should be observable somewhere in the range 3000-7500 (20) Griffith, J. S. The Theory of Transition Metal Ions; Cambridge University Press: Cambridge, 1971; p 437. (21) Packet, D.; Schoonheydt, R. A. ACS Symp. Ser. 1988, 368, 203.
Reduction of Ni2 with H2 in Zeolites cm-I. We found only a shoulder at 4500 cm-I. Garbowski et al." found a weak band at 4700 cm-' and attributed it to the d-d transitions of Ni+. When El = E2 = 4700 cm-I, -394, -300, and -194 cm-l are obtained for A,,, corresponding respectively to gl, = 2.67, 2.51, and 2.33 and A, = -226 cm-I. AI, = -194 cm-' is an extremely small value. Thus, the signal at gll = 2.33 probably cannot be interpreted with this oversimplified model. For the two other gllvalues 4700 cm-' seems to be a reasonable energy for the d-d transitions of Ni+ in zeolites. Confirmation and direct correlation between DRS and ESR spectra on identical samples are however necessary. We did not succeed in our attempts. Garbowski et ale1'interpreted the DRS bands at 13 500 and 29 000 cm-I as d-s transitions of Ni+, because in the free ion the 2F, 2D, and 2P states are at 13 600, 23 000, and 29 500 cm-'. Our experimental results do not support this interpretation, at least when Ni+ is understood to be the ESR-active Ni+ (signal I). Indeed the DRS spectrum can be generated without an appreciable intensity of signal I in NiHY. Moreover the ratio of the band intensities of the DRS bands differs for NiH-Y-1 and for NiLa-Y-2 and -3. This indicates that our previous suggestion that the DRS spectrum is due to charged clusters, Ninxf, is rea~onab1e.l~ Moreover it could be that different clusters are present, one absorbing at 13 500 and 27 500 cm-I, others absorbing at 38 000 and 34 OOO cm-I. The relatively sharp and well-defined bands at 13 500 and 27 500 cm-' are indicative of clusters of molecular size. It may be that Ni+ behaves spectroscopically as an isolated Ni+ species in that cluster. The 13 500- and 27 500-cm-' bands are then interpreted as proposed by Garbowski." The absorption bands at 38 000 and 34000 cm-l are cluster bands. These clusters are certainly not colloidal Ni particles, because the latter absorb around 29 600 cm-l (ref 22) and because of the absence of ferromagnetic Ni(0). Other interpretations are less probable or can be excluded. Thus, when one considers the dimer (Ni+)2,coordinated to the lattice oxygens, a complex is obtained that is isoelectronic with C U ~ ( O A C(Ac ) ~ = acetyl). The latter has d-d bands at 14285 cm-l and p(0) d(Cu) and a(0) d(Cu) charge-transfer bands at 27 030 and 38 000 cm-I, respectively.23 For the Ni dimer the d-d bands are expected at much lower energies because of the charge difference. The 13 500- and 28 OOO-cm-l bands should then be the ligand-to-metal charge-transfer bands of the dimer. The red shift with respect to Cu2+is due to the much easier reducibility of Ni+. Ascribing some of the bands to intervalence absorption Ni+-Ni2+ can be excluded, because Hush's relations are not obeyed.24
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(22) Moskovitz, M.; Hulse, J. E. J . Chem. Phys. 1977, 66, 3988. (23) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier: Amsterdam, 1984; pp 636-639. (24) Wong, K. Y . ;Schatz, P. N. Prog. Inorg. Chem. 1981, 28, 369.
The Journal of Physical Chemistry, Vol. 93, No. 4 , 1989
1521
Location of Ni+ and Ni Clusters. Ethylene and O2adsorb in the supercages. Any change in the spectra is due to direct interaction between Ni+ and Ni clusters in the supercages and the adsorbed molecules or to the migration of Ni+ to other sites without coordination to the adsorbed molecules. In the case of ethylene ESR signal I is almost unaffected, and only traces of an ESR signal with g , = 2.71, g2 = 2.57, and g3 = 1.96, usually ascribed to a Ni+-ethylene complex, are detected.2s*26 The majority of Ni+ is located in the cubooctahedra. The DRS bands of the Ni clusters, on the other hand, disappear almost immediately, showing that they are mainly located in the supercages. Moreover, an intense band appears at 32 500 cm-' both in partially reduced and in the Ni2+zeolites, but less intense in the latter case. Thus, in these partially reduced zeolites there is more Ni2+ in the supercages than in the unreduced samples. The 32 500-cm-I band is interpreted as a d(Ni2+) a*(ethylene) charge transfer by analogy with the interpretation of the 35 500-cm-' band of Ni2+ complexes with the >C=N m ~ i e t y . ~ ' * ~ * When O2is adsorbed in the supercages, Ni+ (signal I) migrates to sites in the cubooctahedra, characterized by signal 11. Similar changes of sites occur upon adsorption of C 0 . l 2 Also the Ni clusters disappear: they are oxidized to Ni2+.
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Conclusions The conditions for the stabilization of ESR-active mononuclear Ni+ in faujasite-type zeolites are (1) low Ni content, (2) the presence of the polyvalent cations La3+ and Ca2+,which reduce the mobility of Ni+, and (3) trigonal coordination of Ni2+, i.e., at oxygen six-rings. The tetrahedral and octahedral Ni2+ in NiLa-X need such high reduction temperatures that Ni+ or Ni cluster intermediates cannot be stabilized to any significant extent. Ni+ cannot be stabilized in N i H zeolites because of the high mobility of Ni+ at least when compared to its mobility in La zeolites. Ni clusters, with very low mobility anyway, can be stabilized both in La-Y and in H-Y. It is remarkable that only traces of clusters are found in H-X. These clusters are located in the supercages. This implies a migration of Ni2+ from the small cavities upon reduction. Thus, in these partially reduced zeolites there is more Ni2+ in the supercages than before reduction. Acknowledgment. This work was financially supported by the National Fund of Scientific Research (Belgium). R.A.S. acknowledges that institution for a research directorship. Registry No. 02,7782-44-7; ethylene, 74-85-1. (25) Bonneviot, L.; Olivier, D.; Che, M. J . Mol. Catal. 1983, 21, 415. (26) Elev, I. V.; Shelimov, B. N.; Kazansky, V. B. J . Catal. 1984,89,470. (27) Kobayashi, H.; Korybut-Daszkiewicz, B. Bull. Chem. Soc. Jpn. 1972, 45, 2485. (28) Kolinski, R. A.; Korybut-Daszkiewicz, B.; Kubaj, 2.;Mrozinski, J . Inorg. Chim.Acta 1982, 57, 269.
