J. Phys. Chem. 1993.97, 1413-1419
1413
XPS Investigations of Lanthanum in Faujasite-Type Zeolites Wolfgang Criinert,'*t*gUlrike Sauerlandt,#Robert Schlijgl,#and Helmut C. Karget Fritz-Haber-Institut der Max-Planck-Gesellschaft. Faradayweg 4-6, W-1000 Berlin 33, Germany, and Institute of Inorganic Chemistry, Johann- Wolfgang-v.- Goethe University, Niederurseler Hang, W-6000 Frankfurt/Main 50, Germany Received: March 31, 1992; In Final Form: October 9, 1992
Faujasite-type zeolites containing lanthanum introduced by various techniques and in different amounts were studied by X-ray photoelectron spectroscopy to elucidate the influence of the zeolite matrix on the binding energy and the shape of the La(3d) line. The results were discussed with regard to the spectral properties of the La(3d) line in LazO3. Significant differences in the La(3d) line shapes and binding energies between La203 and La zeolites were ascribed to final-state effects arising from the hybridization of La final states with valence bands of different extension. The La(3d) line shape of the La zeolites investigated at elevated temperatures was found to respond to structural changes in the surface region of the samples (dehydration, ion migration, formation of highly dispersed L a 4 structures on extraframework sites). These were also reflected in bulkphase properties of the samples (lattice parameters, determined by X-ray diffraction).
Introduction
The exchange of rare-earth cations into faujasite-type zeolites is known to produce acidic zeolite catalysts of high thermal stability. These properties have been ascribed to the formation of extraframework oxygen linkages between two La3+cations in sodalite cages at elevated t e m p e r a t ~ r e d - ~ 0
2La3'(HzO) + 2AI/
'Si
--.c
H
H
0
0
La3( 'La3+
+
2A:
'Si
'0' H
Support for the existence and stability of such a bridged L a 4 structure was provided by high-temperature X-ray diffraction (XRD) studies,'~~ which included an investigation of the radial electron di~tribution,~ and by indirect evidence obtained from Na re-exchange experiments with samples cooled to room temperature after the heat t r e a t m e ~ ~ t Detailed .~.~ conclusions concerning the migration mechanisms operating at elevated temperatures have been drawn from the latter ~ t u d i e s .On ~ the other hand, the early XRD investigationsof Smith et al.' indicate that once the bridged L a 4 structure has been formed, any further displacement of La ions which may occur at still higher temperatures is reversed upon cooling. X-ray photoelectron spectroscopy(XPS)is not usually expected to contribute to the discussion of a structure problem of this kind since it primarily detects changes in the electronic state and the abundance of elements present in the surface region of a solid. However, the complex line shapes of the photoelectron spectra of rare-earth compounds have recently been shown to contain structural information. These line shapes are due to hybridization effects between the valence band of the solid and localized f-states of the lanthanum ion in the final state of photoioni~ation.6~~ It is well-known that these effects depend on the nature of the ligands contributing to the valence band.6-9 On the other hand, they should respond to a change in symmetry and bond lengths in the coordination sphere of the rare-earth ion as well. In thecaseof lanthanum (4Pground state), the photoionization of the 3d levels results in a situation in which the Coulomb Fritz-Haber-Institut der Max-Planck-Gesellschaft. Institute of Inorganic Chemistry. 1 On leave from Central Institute of Organic Chemistry, Permoserstrasse IS, 0-7050 Leipzig, Germany. Current address: AG Katalyse i.T. KAI e.V., Permoserstrasse 15, 0-7050 Leipzig, Germany. + 3
0022-3654/93/2097-1413S04.00/0
attraction exerted by the core holes lowers the energy of the 4f1 final state relative to the 4P final state to an extent that both energies may become comparable. This leads to a hybridization of the twostates under the participation of the overlappingvalence band giving rise to the typical two-peaked shape of the La( 3d) doublet components? Hence, this shape will be observed in the presence of only one type of lanthanum species as it is due to the electronic structure in the final state of ionization. The intensity distribution between the maxima at high and low binding energies is related to the energy difference of the (uncoupled) P and f1 final states and the strength of hybridizati~n.~?' The former depends on the energy differences of the P and f1 initial states and may therefore reflect differences in the electronegativity and polarizability of the ligands contributing to the valence band. The aim of the present investigation was to establish the effect of the zeolite matrix on the La(3d) binding energy and line shape. It was also intended to explore the diagnostic potential of this effect with respect to modificationsin the La coordination sphere that result from the dehydration and migration processes proposed to occur upon temperature increase and, possibly, decrease. Experimental Section
Materials. A L a x and several LaY samples were prepared by ion exchange with aqueous solutions of Lac13 (Aldrich Inc.) employingdifferent temperatures of intermediate calcination and in the final drying step. One LaY sample was obtained by a solid-stateion exchange technique.lOJ' Detailsof the preparations are given in Table I, where the code used to denote the samples is illustrated as well. Table I also compares the degree of ion exchange obtained by atomic absorption spectroscopic analysis of all constituents except La (Na, AI, Si; H20 determined by a thermogravimetric technique) with the surface Na/La atomic ratio obtained by XPS. All samples were stored in air after preparation. The La203 employed was purchased from JohnsonMatthey (99.99% purity). XPS Analysis. Data Acquisition. The XPS spectra were recorded with a Leybold LH SCD 12 spectrometer at a pressure of 110-8mbar employing A1 Ka excitation (hv = 1486.6 eV,run at 12 kV/20 mA). The zeolites were deposited on the sample holder of the spectrometer from a slurry in dry n-pentane previously agitated in a supersonic bath. The sample temperature was varied stepwise between 300 and 870 K. The heating rate was 5 K/min, and -2 h was required to record the spectra at each temperature on both the heating and the cooling branch of the Q 1993 American Chemical Society
Griinert et al.
1414 The Journal of Physical Chemistry, Vol. 97,No. 7,1993
TABLE I: Description of La Zeolite Samples : Preparation Conditions and Analysis of Exchange Degree procedure
intermediate calcination
dried at
% of Na equiv exchanged
NaY
ion exchange, - 343 K, 5X
room temperature (air dried)
93
0
NaY NaY NaY NaX NHIY
ion exchange," 450 K, 6 h ion exchange, 343 K, 3X ion exchange ion exchange, 353 K, 5X solid-state ion exchangeb
16 h at 773 K, after 3rd and 4th exchange none none none none 723 K
313 K room temperature (air dried) 313 K 393 K 723 K
94 52 34 92 60
0 0.7 3.3 0 0
initial ......zeolite ~
sample code LaY-90/c..a.d. I
.
LaY-90/d LaY-SO/a.d. LaY-35/d LaX/d LaY /SSI 0
Na/La atomic ratio (XPS)
Autoclave reaction, hydrothermal conditions. A mixture of Lac13 and NHdY was heated in vacuo ( 5 K/min) to 723 K and maintained there
for 6 h.
temperature cycle. The binding energies (b.e.) of La(3d), O( Is), A1(2s), and Si(2s) were referred to a C(1s) bee. of 285.0 eV. Considerablevariations of surface charging, which occurred with several samples at elevated measurement temperatures, were properly corrected with this calibration. An internal reference (0(1s) = 523.5 eV for LaY zeolites) was derived from samples exhibiting well-shaped C( 1s) lines and applied to those samples where C( 1s) was weak and ill-shaped. This procedure yielded a consistent b.e. for Si(2s) of 154.15 f 0.1 eV. The correspondingvalues for the L a x zeolite were O( 1s) = 532.3 eV and Si(2s) = 153.9 f 0.1 eV. Independent of the type of zeolite, the b.e. for Al(2s) and Na(1s) were found in the ranges of 119.6 f 0.2 and 1073.5 f 0.2 eV, respectively. The b.e. for the frameworkelements in the LaY samples did not differ significantly from those of the parent NaY zeolite, with the exception of O(ls), which was by 0.3-0.4 eV lower in the Laexchanged samples. The homogeneity of the surface charge on the non-conducting samples was tested with a spectrometer facility allowing the exposure of the sample to a weak electric field during data acquisition ("TUBUS experiment"). This field, which shifts the b.e. in charged samplesby some electronvolts,will not change the shape of a line when the charge is equal on all atoms of the corresponding element. If, however, the charge is distributed inhomogeneously, distortions of the line shape will result. This method has been recently employed to detect differential charging on silica-supported Pt.12 Data Reduction. As stated in the Introduction, the two-peaked shape of the La(3d3p) and La(3d5/2)signals is expected to bear information on the structure of the La coordination sphere. In particular, the relation of intensities between the high-b.e. and the low-b.e. region of a doublet component will be considered. To extract this relation, the signal shapes were decomposed into singlet lines using fitting procedures usually employed to separate signals of different species of an element. In this latter case, additional information on line shapes and positions is often available and can be used to reduce the ambiguities inherent in the signal shape fitting. The La(3d) signal shape, however, reflects the electronic structure of the final state of excitationof a singlespecies,although different species may superimpose providing an additional complication. In this situation, the most rational solution of the signal-shape analysis problem will consist of those component lines that describe the whole spectral material under the most restrictiveset of constraintson the parameters (ratios of component line widths, Gauss-Lorentz mixing ratios'3). In the choice of these constraints, one is guided by the high- and low-energy edges of the signal shape, which are likely to reflect the shapes of one of the components each. The signal shape analysis was performed on data previously smoothed with a least-squares routine with second-order polynomials. The constraints used for the analyses reported will be given together with the results. Background subtraction was performed according to ref 14. The results obtained suggested that the La(3d~p)signal of La zeolites may be represented by a set of two component lines, while a third one covering 8-12% of the total signal area is necessary to fit the shape of La(3d5/2)
in LazO3. As outlined before, the third component is not considered to indicate the presence of a second La species but is rather due to the particular electronic structure of the hybrid orbital involved in the final state. This component may also be regarded as a satellite of the low-b.e. component, which would imply that its counterpart at the high-bee.component has been removed by the background subtraction. For the evaluation of line intensities, unsmoothed data were used. In this study, only the 5/2 component of the La(3d) doublet will be considered as the 3/2 component was superimposed by the LaM4N45N45 Auger line. In the La(3d5/2) b.e. range, the influence of this Auger line is negligible (6% of the total signal height). On the other hand, the La(3d5p) component is superimposed by the Ka3,4 X-ray satellite of the 3/2 component, which can be removed by a standard subtraction procedure provided that the 3/2 component has been recorded. Careful analysisof a variety of spectra with different La(3d) signal shapes revealed that the satellite subtraction does not exert a significant influence on the parameters that will be used in this work to characterize the intensity distribution between the high-b.e. and low-b.e.regions of the La(3d5/2)signal (a,a',vide infra). Hence, in extended series, the La(3d3/2) component was not recorded except for control purposes. X-ray Diffraction. As XPS reflects only the properties of the surface region of a solid sample we performed a brief XRD investigation to obtain complementary evidence for the bulk of the material. XRD patterns of NaY and of a highly exchanged LaY sample (LaY-90/c.,a.d., cf. Table I) were compared in a temperature interval of 298-623 K. The diffractograms were recorded with a Siemens D5000 diffractometer employing monochromatized Cu Ka radiation. The samples were deposited onto the sample holder (a Pt plate) from a slurry in acetone. During the temperature cycle, the samples were in contact with the ambient atmosphere. The reflections of Pt at room temperature were used to calibrate the 28 scale of the diffractometer.
Results XPS Investigations. L a 2 0 3 and La Zeolites-Comparison of Binding Energies and Signal Shapes. Figure 1 shows the La(3d512) signals (satellite-subtracted) of La2O3, of various LaY samples and of L a x (cf. Table I), with the results of the signal shape analysis indicated. The upper part of the figure displays the whole La(3d) signal for La203 and the LaY-90/c.,a.d. sample (seeTable I). From Figure 1 it isevident that therearesignificant differences in the signal shape and in the b.e. between La203 and the zeolite samples. Some variation in the La(3d5/2) b.e. and signal shape may be observed also with different zeolite samples. The La(3d512)b.e. of La203(834.0 eV, taken at the low-b.e. maximum) isclose to that published in ref 15. It should be pointed out, however, that the binding energies reported in the literature scatter in a wide range (834-838.5 eV, corrected for differences in calibrationl5-'*). The b.e. difference between La203 and the La zeolites (836.4-837.2 eV, cf. Figures 3-5) is, however, significant since the spectra were measured on one spectrometer and under identical conditions. Recording the spectrum of a La
The Journal of Physical Chemistry, Vol. 97, No. 7, 1993 1415
Lanthanum in Faujasite-Type Zeolites
M as received
573K 073K 300K
(after 87310
b
535
840 835 b.e..eV
530
5b
bo
m
b.e.,eV
0.w
0.10
I
844 b.e.. eV
,
836
iw 1
828
844
836
I
,
828
b.e.. eV
Figure 1. La(3dsp) spectra of La203 and of La zeolites, satellitesubtracted. All samples in the initial state, after contact with the atmosphere. (a) La(3d) region of La203 and of LaY-90/c.,a.d. (b) La(3dsp) signals of La203 and different La zeolites. The results of the signal shape analysis are indicated. Constraints used in the signal shape analysis: La203: fwhm(La1) = fwhm(Lal1); fwhm(Lall1) = 1.8 f 0.1 eV. Gauss-Lorentz mixing ratio: 30, for all components. La zeolites: fwhm(La1) = fwhm(Lal1) + 0.2 eV. Gauss-Lorentz mixing ratio: 40, for both components.
zeolite sample (LaY-90/d) in an electric field (TUBUS experiment, cf. Experimental Section) did not supply any evidence for differential charging on the La component. Figure 2a shows the La(3dsp) and O(1s) lines of La203 as received, at 573 K, at 873 K, and at room temperature after the heat treatment. The O(1s) spectra indicate that the surface of the Laz03sample contains considerable amounts of hydroxyl groups. These are partly removed at 873 K, which is also reflected in an increase of the La(3dsp)/O(ls) intensity ratio from 3.9 to 5.0. As a result of the dehydroxylation the shape of the La(3dsp) signal approaches the shape reported in the the two components become better resolved, their intensity relation changes slightly: On theother hand, the La b.e. remains constant. An apparent slight shift to lower b.e. upon cooling (Figure 2a, lowest curve) originates from a slight increase of the distance between the two signal components. Figures 2b,c summarizes the results obtained in the analysis of the La(3dslz) and O(1s) signal shapes. The relation between the high-b.e. and the low-b.e. regions of the La(3dsp) signal is expressed as the ratio a’ of the component lines at 838.3 and 834.0 eV. The third component, which is nearly midway between the other two, may be disregarded because of its almost constant contribution. The figure shows that the removal of OH groups from the coordination sphere of La leads to a decrease of the a’ parameter of the La(3dsp) signal. Simultaneously, both the La component lines and the O( 1s) lines exhibit a decrease of their line widths (fwhm). When the sample is cooled, a slight rehydroxylationof the surface, probably from H20 in the residual gas, is observed (Figure 2a,b). It is, however, not reflected in the a’ parameter.
Lo
173
45)
bll
T”~nn[Kl
in
IJ
m
tn Tm”
LK1
Figure 2. La(3dsp) and O(1s) spectra of La203 at different temperatures: (a) raw spectra, smoothed, La spectra background-subtracted. (b) Ratio of the La(3d5p) linecomponents (a‘)and of the OH-and O*-lines. (c) Line widths.
XPS Spectra of La Zeolites at Elevated Temperatures. An example of the modification of the La(3dsp) signal shape upon variation of the measurement temperature is given in Figure 3a (LaY/SSI). Itwaschecked by controlexperimentsthatthesignal shapes are characteristic of the corresponding temperatures (repeated recording of the spectrum after a 2-3-h time interval). The changes of the signal shape with increasing temperature reveal a growing contribution of the high-b.e. component (“Lal”) and a decreasing width of both components. Upon cooling, the changes of the La(3d5/2) signal shapes clearly take a course different from that in the heating period. Hence, the signal shape obtained at room temperature after the temperature cycle differs from that measured for the original sample. Characteristic changes were also found with the O( 1s) line width, which decreased upon heating and increased again in the cooling stage (Figure 3b). From the control measurements of the remaining lines (at the temperature maximum and after completion of the cycle) it can be concluded that the Si(2s) line width and, to a smaller extent, that of AI(2s) exhibit variations as well, which, however, do not always parallel those of the O( 1s) fwhm. These changes are not indicative of a lattice degradation because when the fwhm happened to increase beyond the initial value, the increase occurred upon cooling the sample from the temperature maximum to room temperature (cf. Figure 3b for O( 1s)) In Figure 3b, the changes of the La signal shape are quantified by the ratio between the La(3ds12)component lines (a = La]/ Lall) and by their fwhm. The fwhm of the O(1s) line is given as well. The results obtained with the remaining zeolite samples are summarized in Figures 4 and 5 . The error limit of the a parameter was estimated to be hO.01, while for several noisy
-
Griinert et al.
1416 The Journal of Physical Chemistry, Vol. 97, No. 7. 1993 La(3dwl)
a
b
FWHM [evl ......
a
,
l
836.7 t 0.2 e V
TX (down1
TK (UP)
0.8
I
8h
845 b.e.,eV
-
300
873
373
773
473
673
573
SI3
613
473
773
m
873
300
0.7
1
I
I
+\]-+=
3.0 0.8
........................ ............
Si5
3.6,
836.5 t 0.1 eV 0.9
b.e.,eV
i-
3.0. 2.4.
FWHM t e V l
OI
273
Temperature [KI 3.0
I
.....
.....
La,, La,
2.6
413 613 813 Temperature [KI
Figure 4. Development of the signal shape parameter a (=Lal/Lall,a), of the fwhm of the La components and of O(1s) (b) for La zeolites of different preparation (LaY-35/d, Lay-SO/a.d., LaY-90/c.,a.d.). a
a
b
FWHM[eV]
......
3.0
0 1s I -
l -J
............
213
413
613 813 213 413 613 873 Tempnhue [KI Tmpnhrrs [KI Figure 3. La(3ds/2) spectra of LaY prepared by solid-state ion exchange (LaY/SSI). (a) Development of the signal shape during a temperature cycle between 300 and 873 K. (b) Development of the signal shape parameter a (=Lal/Lall), of the fwhm of the La components and of O( 1s) during the temperature cycle; broken lines: reheating after completed temperature cycle.
'.J
..............f ....................................................................... 3 . 6
+ ............... : w
spectra the error was fO.015. The fwhm for all measurements are given with an error of f0.05 eV for the La components and f0.02eV for O(1s). While the general tendencies mentioned 0.9. 3.0. 0 0 above (growing a,decreasing fwhm with increasing temperature, I 1different changes upon cooling) are present in the La signal shapes o Is 0.8. I 2.4. in almost all cases, the details of the curvesvary in a quite complex I LaY-901d-1100K manner. There are curves with smoothly increasing a (LaYi73 413 6i3 813 90/c.,a.d., LaX/d), but in some cases, distinct minima of a occur, 37 - 2 ' O Temperature [KI Temperature [Kl in particular in thecooling stage. These are, indeed, corroborated Figure 5. Development of the signal shape parameter a (=Lal/Lall,a), by thesignal shapes. Insomecases, the fwhmof both the La(3d5/2) of the fwhm of the La components and of O(1s) (b) for La zeolites of line components and the O( 1s) line exhibit higher values in the different preparation (LaY-90/d, LaX/d, LaY-90/d after 24 h 02/Ar cooling stage than in the previous heating period. The most at 1100 K). prominent example for this is the zeolite prepared by solid-state ion exchange (Figure 3). On the other hand, there are samples higher level attained during the cooling stage. A picture very with no significant differences between heating and cooling stage much reminiscent of this was obtained when a L a y sample (LaYin the line width either of the La components or of O( 1s) (LaY90/d) previously treated in flowing Oz/Ar (20% 02)at 1100 K 50/a.d., Figure 4). While the La(3dsl2) and O(1s) line profiles for 24 h was subjected to the standard heating procedure (Figure exhibit considerable differences with varying temperature, no 5). In this run, the sample was transferred to the n-pentane significant deviation of their b.e. was noted from those of the solvent (cf. Experimental Section) without contact to the ambient initial samples. atmosphere. An air contact of about 1 min. during the sample Figure 3b also shows the development of the a and fwhm insertion into the spectrometer could, however, not be avoided. parameters that was obtained when the LaY/SSI sample was Again, the increasing tendency of a with increasing measurement reheated after completion of the full heating-cooling cycle. An temperature had vanished, but maxima and minima were still increase of the a value with growing measurement temperature observed. The La(3d), Si(2s), and in particular the O(1s) fwhm is no longer observed, but maxima and minima are still found were larger that those found in the fresh sample but still in the although with only small differences between them. The La and ranges typically covered during the temperature cycles (cf. Figures O(1s) fwhm behave as in the first heating period, but on the 3-5). The changes exhibited by the La signal shape and the
Lanthanum in Faujasite-Type Zeolites
860
850
8bO
The Journal of Physical Chemistry, Vol. 97, No. 7, 1993 1417
'IIrLJ- i
840
b.e..eV Figure 6. La(3d) signal of a La zeolite (LaY-90/c.,a.d.) after 75-min treatment at 773 K i n different media: evacuation in UHV or reduction in 20 mbar of H2. Both spectra recorded at room temperature, satellitesubtracted.
TABLE II: Atomic Ratios of Elements Present in the M a e e Region of La Zeolites' 102La/Si Si10 %/A1 Na/AI La/Na
zeolite LaY-35fd initial final after calcination at 1100 K LaY-5Ofa.d. initial final LaY-90/c..a.d. initial final LaY-90/d initial final after calcination at 1100 K LaY/SSI initial final Lax (initial)
5.2 5.4 4.4
0.43 0.43 0.43
2.3 2.4 2.9
0.42 0.43 0.12
0.3 0.3
9.8 8.9
0.37 0.46
2.2 2.2
0.15
0.11
1.4 1.8
8.4 8.4
0.43 0.43
3.0 2.9
9.5 9.2 9.0
0.37 0.37 0.46
2.1 nd 3.0
5.9
0.40 0.46 0.23
2.5 2.5
5.6
53
1.1
1,l
For the evaluation of the atomic ratios, the interaction cross sections of ref 19 were employed.
O( 1s) line width were small. The La, Si, and 0 binding energies of the calcined sample were close to those of the initial one. No indication of an additional La species was found. On the other hand, an additional La species was produced by a 75-min treatment of LaY80/c.,a.d. at 773 K in 20 mbar Hz. A new peak emerged at 830.5 eV (Figure 6). Application of an electric field during data acquisition (TUBUS experiment, cf. Experimental Section) revealed that the surface charge of the new species differed from that of the zeolite surface. Table I1 summarizes the atomic ratios of elements present in the surfaceregions of the samples before and after the temperature cycles. Nosignificaqtchangesareintroducedby these treatments, except for a slight Na depletion in the LaSO-Y/a.d. sample. A much more pronounced ecrease of the sodium content in the surface region was found a r the 1100 K thermal treatment of LaY-35/d. This thermal treatment (but also the intermediate calcinations at 823 K in the preparation of LaY-90/c.,a.d.) led to a significant increase of the Si/Al surface atomic ratio. Table I1 reveals a pronounced tendency of surface enrichment of La. In several samples, the Na(1s) line was not detectable although the degree of ion exchangewas only slightly above 90%. In the Lay-SO/a.d. sample, the surface enrichment of La is extraordinarily high. The reason for this is not known. X-ray Diffrrction. The migration of the La ions is also reflected in the dimensions of the zeolite lattice at elevated temperatures. Figure 7a shows the diffractograms of the NaY and the LaY90/c.,a.d. samples in the initial state. As described in the literature,lS5the characteristic reflectances of NaY are present
dftc
t
25 0
