Initial and Final State Effects in the Photoelectron and Auger Spectra

Initial and Final State Effects in the Photoelectron and Auger Spectra of Si and Al Bonded in Zeolites†. Ivan Jirka. J. HeyroVsky´ Institute of Phy...
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J. Phys. Chem. B 1997, 101, 8133-8140

8133

Initial and Final State Effects in the Photoelectron and Auger Spectra of Si and Al Bonded in Zeolites† Ivan Jirka J. HeyroVsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, DolejsˇkoVa 3, 182 23 Prague 8, Czech Republic ReceiVed: April 21, 1997; In Final Form: July 1, 1997X

The dependence of the core level binding energies, Eb, of Si, Al, and O bonded in zeolites on the concentration of skeletal Al is investigated. The line shape changes of Al KLL Auger spectra appearing with decreasing concentration of Al in zeolites are described. Initial and final state effects influencing core level binding energies of zeolites are discussed on a qualitative level. Different screening of the hole states localized on Al and Si is demonstrated. Evidence is shown for the dominant influence of initial state effects on the core level Eb values of Si and Al.

1. Introduction Zeolites are crystalline aluminosilicates with Al and Si atoms in tetrahedral coordination. Their catalytic properties are influenced by the charge distribution in their framework and by the polarizability of their valence electrons. These effects might be related to the local structure of zeolitic aluminum. Information on the electronic structure of zeolites together with a quantitative compositional analysis is available by application of electron spectroscopy for chemical analysis (ESCA). The information is gained mainly by interpreting binding energy shifts ∆Eb of core level photoelectrons occurring in the series of zeolites. An increase of Eb of all zeolite elements (in some cases excluding Al) with an increase in Si/Al ratio was reported in refs 1-7. However, interpretation of the core level binding energy shifts of zeolites is still rather controversial.1,2,5,10,26 The concept of the generalized Auger parameter ζ provides another source of information.9-11 The linear dependence between the change of ζ and the change of extraatomic relaxation energy ∆R of the core hole states was proposed. The variation of relaxation energy of the hole states localized on Al and Si in various compounds was identified with the variation of in-crystal polarizabilities.11,12 Correction of the ∆Eb values with ∆R enables, in principle, the separation of initial and final state effects influencing the binding energy of photoelectrons. Discrepancies in published data, particularly in the binding energies of the Al 2p photoelectron line, may be attributed to the investigation of not well-defined samples. Thus, investigation of zeolites that are characterized by several other methods is essential. Faujasites that were previously characterized in great detail by several methods (FTIR IR, XRD, MAS NMR, chemical analysis, adsorption capacities; see refs 13 and 14) are investigated. The dependence of the Eb of core level (Al 2p, Si 2p, and O 1s) photoelectron lines and kinetic energies Ek of Al and Si KLL Auger transitions as well as Auger line intensities on the Si/Al ratio is discussed for these samples. Data obtained for zeolite A, ferrierite, and ZSM-5 are included to the investigated series of zeolites. Our findings are compared with those known from literature.1-8 The influence of initial and final state effects on the core level Eb values is qualitatively discussed. Chemical effects on energy and intensities of Al (Si) KLL Auger line are demonstrated. 2. Experimental Section Materials. Some characteristics of investigated zeolites are summarized in Table 1. Faujasite (NH4)70Na30-Y (VURUP, † X

This article is dedicated to Dr. Ludmila Kubelkova´. Abstract published in AdVance ACS Abstracts, September 15, 1997.

S1089-5647(97)01367-9 CCC: $14.00

Bratislava, Czechoslovakia) containing 2.1 wt % (wet basis) of Na was investigated as received (sample 1) and after hydrothermal treatment in self-steaming conditions (samples 2 and 3). The same parent zeolite was dehydrated at 300-450 °C and exposed to the SiCl4 vapors (samples 4, 6, 8). The nonskeletal Al species were extracted with Na2H2EDTA (samples 5, 7, 9). Further details on sample preparation may be found elsewhere.13,14 The samples of faujasite were characterized by several methods that confirm their high crystallinity regardless of the modification used. Sorption capacities measured with Ar were decreased only slightly with increasing Si/Al ratio (from 10.9 mmol Ar g-1 in parent zeolite to 9.5 mmol Ar g-1 in zeolite with lowest concentration of Al).13,14 The linear dependence of the unit cell parameter a0 and the wavenumber of asymmetric internal and asymmetric external stretching vibrations of TO4 tetrahedra on concentration of Al in the zeolitic skeleton was found. No nonframework AlOx was found after extraction of the zeolites with Na2H2EDTA.14 The commercial sample of zeolite A (PQ Corporation, sample 10) was measured without any modification. The Na forms ferrierite (sample 11) and ZSM-5 (sample 12, both by Tosoh) were measured after ion exchange into NH4 form with 0.01 M NH4NO3. ESCA Measurements. The photoelectron and Auger spectra were measured on an ESCA III Mk 2 (VG Scientific) spectrometer with an electrostatic hemispherical analyzer and twinning Al/Mg anode, connected on-line with a personal computer for accumulation of the spectra. The nonmonochromatized Al KR1,2 (hν ) 1486.6 eV) line was used to excite the photoelectrons. It was powered at 11 kV; the beam current was 20 mA. The Al 2p, Si 2p, C 1s, and O 1s photoelectron lines were measured using a constant pass energy of 50 eV. Silicon Auger (KLL) lines were measured using the bremsstrahlung component of the Al radiation, while the bremsstrahlung component of Mg radiation was used for measurement of aluminum Auger (KLL) lines. The Al KLL Auger spectrum was measured at a pass energy of 100 eV. Lines used for calibration of Al KLL Auger spectra were measured applying the Mg KR1,2 line (hν ) 1254.4 eV, 12 kV, 20 mA). The slit width used was 4 mm. The pressure of the residual gases during experiment was typically less than ∼10-7 Pa. Samples were measured in the form of self-supported pellets after ∼1 h degassing at room temperature in the preparation chamber of spectrometer. The C 1s photoelectron line (Eb ) 284.4 eV) was used to calibrate the energies of photoelectron and Auger spectra. The intensities of the Al (Si) KLL spectra were normalized relative to the intensity of Al (Si) 2p photoelectron © 1997 American Chemical Society

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Jirka

TABLE 1: Si/Al Ratios Estimated by Wet Analysis and by XPS, Binding Energies Eb of Al 2P, Si 2p, and O 1s Photoelectron Lines (eV) (the Numbers in Brackets Are Full Widths at Half-Maxima of the Pertinent Lines) and Lattice Parameters a0 (A) of the Series of Investigated Zeolites no.

zeolite/modification

a0a

(Si/Al)bulkb

(Si/Al)xps

Eb(Al 2p)

Eb(Si 2p)

Eb(O 1s)

1 2 3 4 5 6 7 8 9 10 11 12

parent faujasite 1, stabilized at 550 °C 1, stabilized at 770 °C 1, SiCl4 4, extractedc 1, SiCl4 6, extracted 1, SiCl4 8, extracted zeolite A ferrierite ZSM-5

24.71 24.57

2.50 2.50 2.90 3.40 5.60 5.70 9.75 6.94 20.00 1

2.76 1.90 1.86 1.62 5.92 3.98 16.55 6.67 12.5 1.22 8.47 11.00

74.2(2.2) 74.3(2.3) 74.5(2.5) 74.0(2.2) 74.2(2.0) 74.0(2.4) 73.7(2.2) 73.8(2.2) 74.4(2.2) 73.7(2.0) 73.7(2.1) 73.9(2.2)

102.7(2.8) 102.4(2.3) 102.6(2.8) 102.7(3.0) 102.8(2.6) 102.9(2.4) 102.9(2.8) 102.6(2.1) 103.0(2.1) 101.1(2.5) 102.4(2.1)

531.8(2.6) 531.8(2.5) 531.8(2.5) 531.6(2.7) 532.2(2.3) 532.0(2.7) 532.0(2.5) 532.0(2.3) 532.2(2.2) 530.8(2.5) 531.6(2.4) 531.6(2.6)

a

b

24.46 24.47 24.34 24.35 24.26 24.30

10.52

c

Reference 13. Reference 12. For extraction Na2H2EDTA is used.

line, measured simultaneously at the same conditions. Normalization of Auger line intensities enables direct comparison of their shapes for zeolites with different skeletal composition. The spectra were fitted by Gaussian functions, and linear background was subtracted. Estimation of the Si/Al ratios by ESCA was done using integral intensities of the Al 2p and Si 2p photoelectron lines, corrected by the probability of photoemission using pertinent values for the photoionization cross sections.26 Results The Si/Al ratios of modified faujasites, estimated by bulk analysis and by XPS together with binding energies Eb and full widths at half-maxima (W) of the Al 2p, Si 2p, and O 1s photoelectron lines are summarized in Table 1. Available data for zeolite A, ZSM-5, and ferrierite are also included. The W and Eb values were estimated by curve fitting. The Si/Al ratios estimated by wet analysis ((Si/Al)bulk) and by XPS ((Si/Al)xps) are summarized in Table 1. Comparing the (Si/Al)bulk and (Si/Al)xps values and assuming the error of estimation of the Si/Al ratio to be ∼5%, the investigated zeolites may be divided into two groups. The surface regions of samples from the first group (2, 3, 4, 6, and 9) are enriched by an AlOx phase. The composition of the surface region of the samples from the second group is the same as in the bulk (sample 1 and 12) or is depleted (samples 5, 7, and 10). Zeolites from the first group might contain both tetrahedrally and octahedrally coordinated Al. The octahedrally coordinated Al in steamed samples was confirmed by 27Al MAS NMR.14 Samples from the second group are in accordance with their previous characterization12,13 assumed not to contain any octahedral Al species. Interaction of faujasite with SiCl4 induces not only skeletal dealumination but also a decrease in the concentration of nonframework AlOx phase (compare the Si/Al ratios of samples 4, 6, and 8 estimated by bulk analysis and by XPS in Table 1). The effect of extraction of dealuminated faujasites by Na2H2EDTA depends on the degree of their dealumination. It may cause quantitative elimination of Al-containing nonframework species (sample 4), Al deficiency of surface layers of zeolite (sample 7) or even enrichment of zeolite surface by Al (sample 9). The latter paradoxical behavior is explainable by the destruction of the zeolite by the treatments used. Simultaneous occurrence of more than one coordination mode of Al in zeolite might influence the W and Eb of Al 2p photoelectron line. The binding energy Eb of the Al 2p photoelectron line of octahedrally coordinated Al might be higher than the Eb of tetrahedrally coordinated Al.21 A small shift to higher Eb and broadening of the Al 2p photoelectron line (relative to the Al 2p line of parent zeolite) is evident for

Figure 1. Al 2p photoelectron lines of the zeolite before and after extraction of nonskeletal Al. (a) Samples 4 and 5; (b) samples 8 and 9.

several samples from the first group. However, the Al 2p photoelectron line of any sample may be successfully fitted by only one Gaussian line (see Figure 1). No attempt was made to fit the Al 2p spectrum by more than one line. The dependencies of the Eb of Al 2p, Si 2p, and O 1s photoelectron lines of zeolites on Si/Al ratio are plotted in Figures 2, 3, and 4. Our data are plotted together with published Eb values. Only the samples with a subsurface region that is not enriched by Al-containing phase are included. Literary data were similarly chosen. The Eb values from literature were recalibrated using the Eb of the C 1s line. According to our measurements, the Eb value of Al 2p photoelectron line first increases from zeolite A (Eb ) 73.7 eV, Si/Al ) 1.22) to parent faujasite (Eb ) 74.2 eV, Si/Al ) 2.5) and then decreases with increasing Si/Al ratio (Eb ) 73.7 eV, Si/Al > 7). A similar dependence was observed for Eb of Si 2p and O 1s lines. The Eb values of Si 2p and O 1s lines of parent faujasite are also increased relative to their values in zeolite A and then decrease; however, another increase of Eb values of these photoelectron lines is evident for samples with Si/Al ∼ 15. The Eb increase at Si/Al ) 2.5 of Si 2p and O 1s lines is about two times greater than for the Al 2p line. General agreement is evident for Eb

Si and Al Bonded in Zeolites

Figure 2. Dependence of the Eb of the Si 2p photoelectron line (eV) on the Si/Al ratio.

values of Si 2p, O 1s, and Al 2p photoelectron lines of aluminum-rich zeolites (Si/Al < 2.5). Discrepancies with part of the literature2,5 are appearing with decreasing of Al content in the zeolites; however, other published data1,3,4,8 support our findings. The KLL Auger spectrum of silicon with final states identified by comparison with standard values14 (1D K L2,3L2,3, 1S KL2,3L2,3, 3P K L1L2,3, 1P K L1L2,3, and 1S K L1L1) is depicted in Figure 5. The KLL final states of Al were analogously identified. A summary of Al KLL Auger lines of the faujasites before and after extraction of nonskeletal Al as well as before and after steaming is given in Figure 6. A comparison of the Si (Al) KLL Auger lines of zeolites A, Y (parent sample), and ZSM-5 is depicted in Figures 7 and 8. The shapes, energies, and intensities of the Al K LL spectra are dependent on the Si/Al ratio. The intensity changes of these spectra due to treatment are not equivalent for the K L2,3L2,3 and K L1L2,3 (K L1L1) Auger lines. The dominant effect is a change of the intensity of the K L1L2,3 (K L1L1) Auger line with decreasing Al concentration in zeolites. The dependence of intensities of Al KLL Auger lines on Si/Al is depicted in Figure 9. Explanation of this phenomenon is not clear at present. The L1 holes produced by K L1L2,3 (K L1L1) Auger transitions might be competitively filled by some other Auger process. Indeed, the Coster Kronig L1L2,3V transition (V stands for valence level) was observed for Al.28 To check this hypothesis, further experiments are essential. The normalized intensities of Si KLL Auger lines are not dependent on Si/Al ratio. The dependencies of the changes of kinetic energies ∆Ek of the 1D K L2,3L2,3, 1P K L1L2,3, and 1S K L1L1 Auger transitions of Al on Si/Al ratio are depicted in Figure 10, and the analogous dependence of Ek of Si KLL Auger spectra is depicted in Figure 11. The ∆Ek value is defined as a difference of kinetic energy of the Auger transition in investigated zeolite and in zeolite A. Dependence of Auger parameters R of Al and Si on Si/Al ratio is displayed in Figure 12. Auger parameters are defined as a sum of the

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Figure 3. Dependence of the Eb of the O 1s photoelectron line (eV) on the Si/Al ratio.

Figure 4. Dependence of the Eb of the Al 2p photoelectron line (eV) on the Si/Al ratio.

kinetic energy of the Al (Si) K L2,3L2,3 Auger line and the binding energy of the Al (Si) 2p photoelectron line. Discussion Increase of the Eb values of O 1s and Si 2p photoelectron lines (∼2 eV) and Al 2p photoelectron line (∼1 eV) from zeolite A (Si/Al ∼ 1) to faujasite (Si/Al ∼ 4) was well established.1,2,5,6 Much less data are available above this concentration region. The observed decrease of Eb of Al 2p, Si 2p, and O 1s

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Figure 5. Auger Si K LL spectrum of faujasite with identified final states.

photoelectron lines from Si/Al ∼ 4 to Si/Al ∼12 is in agreement with data published in refs 3 and 4 (see Figures 2-4). This decrease is ∼0.8 eV for Al 2p photoelectron line and ∼0.5 eV for Si 2p and O 1s photoelectron lines. The Eb shifts are above experimental error, which, in accordance with literature,1,2,15 does not exceed (0.2 eV. The Eb value of faujasite with Si/Al ) 10.4 is taken from ref 3. This sample was prepared by repeated ion exchange with ammonium acetate followed by deep bed calcination. Framework dealumination appeared due to this treatment, and nonframework AlOx species were accumulated at the outer surface of zeolite. The accumulation of Al nonframework species is evident from comparison of Si/Al ratios estimated from the values of the lattice parameter a0 and from the Si 2p/Al 2p photoelectron line intensity ratio (see Table 1 of ref 3). The Eb value of the Al 2p photoelectron line was obtained by curve fitting of the experimental spectrum using two lines. The highenergy Al 2p line at 74.8 eV was previously ascribed to skeletal Al while low energy Al 2p line at 73.6 eV was ascribed to the nonframework Al.3 However, reverse assignment might be correct according to recent literature (see for example ref 21). Moreover, the Si/Al ratio calculated from the lattice parameter a0 is close to the Si/Al ratio estimated using the Al 2p photoelectron line at 73.6 eV. The Si/Al ratio calculated using the Al 2p line at 74.8 eV is substantially higher. Thus, the high-energy line at 74.8 eV is ascribed to extraframework octahedral Al species and the low-energy line at 73.6 eV to Al which is tetrahedrally coordinated in the skeleton of zeolite in this work. The latter value is used in Figure 4. The Eb values of faujasite that was dealuminated by H4EDTA4 decrease with increasing value of Si/Al (up to 9.54, see Figures 2-4). The increase of the values of Eb of the most dealuminated zeolite (Si/Al ) 12.2) is understandable as due to distortion that may occur in heavily dealuminated samples. Discussion of core level binding energy Eb shifts is typically based on interpretation of their changes relative to some standard. The charge potential model17 is generally used:

∆Eb ) Eb(x) - Eb(stand) ) -k∆q + dV - ∆R + ∆χ (1) where q is the charge density on the emitting atom, V is the Madelung potential (∑(qj/Rij, i * j, Rij is the interatomic distance), R is the core hole relaxation energy, and χ is samplespectrometer contact potential. The χ is related to the position of the Fermi edge of the sample relative to the Fermi level of spectrometer. The first two terms of the right-hand side of eq 1 are discussed as the core level chemical shift (initial state effects); the latter

Jirka terms are final state effects. According to some explanations the ∆Eb values are dominantly influenced by chemical shift.1,2,5 Okamoto et al.5 explained the increase of the values of Eb (Si 2p) and Eb (O 1s) with decreasing concentration of Al in zeolites by charge-transfer effects. The positive charge on silicon is reduced with increasing Al content, while the negative charge on oxygen is enhanced. As (AlO2)- tetrahedrons are always surrounded by four (SiO2)- tetrahedrons (Loewenstein rule), the changes in charge density on aluminum atoms with decreasing Al concentration might be rather small according to this explanation. The shifts of the Al 2p photoelectron line were proposed to be induced by change in the Madelung potential. According to Grunnert et al.,2 the latter effect dominantly influences the core level binding energies of zeolites. The dependence of the calculated Madelung potential on the concentration of Al in the zeolite is qualitatively the same as the dependencies of core level binding energies. Assuming the dominant influence of the charge-transfer effects on the Eb values, their positive shifts observed for Al 2p and Si 2p photoelectron lines might be in the framework of charge potential model accompanied by negative shifts of the O 1s photoelectron line. The latter effect was not, however, observed. This fact was discussed by Barr1 who proposed a mere probable influence of group rather than elemental shifts in the zeolite framework. An important feature supporting this interpretation is the absence of any distortion in the Si 2p, Al 2p, and O 1s line shapes, which generally have Gaussian shapes (see Figure 1). Thus, according to this explanation, the Eb shifts of investigated atoms are not related directly to the changes in charge distribution in the first coordination sphere but rather to some average potential. This explanation was used further by Huang et al.,18 who correlated core level binding energies of zeolites and partial charges that were calculated using Sanderson electronegativites and electronegativity equilibration method. The physical meaning of thus calculated charge distribution in the zeolite framework is, however, not clear. The core level shifts of zeolites may be alternatively explained by final state effects, i.e., by a decrease of the relaxation energy ∆R and by the shift of the sample/spectrometer contact potential. Possible influence of the latter effect on the Eb shifts in insulating materials has already been discussed.1,19-21 If this effect is dominant, then the plot of Eb values of cation species (Al, Si) plotted against the Eb of anion (O) should be a straight line with slope equal to 1. These plots are depicted in Figures 13 and 14. The slope equal to 1.032 and correlation coefficient R equal to 0.973 are observed for dependence of Eb(Si 2p) versus Eb(O 1s) (see Figure 13). The slope of the dependence of the Eb(Al 2p) on Eb(O 1s) is ∼0.4, and R is 0.788. The shift of Eb(Si 2p) and Eb(O 1s) with decreasing concentration of Al in the zeolite framework is thus explainable by dominant influence of sample/spectrometer contact potential. The Eb(Al 2p) shifts might be influenced by this potential as well; however, their values are substantially decreased by some other effect(s). The influence of relaxation on ∆Eb values of zeolites may be estimated using concept of generalized Auger parameter ζ,9 in principle. The ζ values are not influenced by position of the reference level of photoelectron (Auger) spectra. Assuming an Auger transition that proceeds on levels 1, 2, and 3, for ζ(2,3) holds:

ζ2,3 ) Eb(1) - Eb(2) - Eb(3) - Ek(1,2,3)

(2)

where Ek(1,2,3) is kinetic energy of (1,2,3) Auger deexcitation and Eb(1), Eb(2), and Eb(3) are binding energies of the levels that participate in this Auger transition. The ζ(2,3) can be expressed as difference between repulsion Coulomb integral

Si and Al Bonded in Zeolites

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Figure 6. Summary of Al K L2,3L2,3 (1), Al K L1L2,3 (2), and Al K L1L1 (3) Auger lines of the faujasites containing nonskeletal Al (line) and after its extraction (open squares). (a) Sample 4 and 5; (b) sample 6 and 7; (c) sample 1 and 3

single hole states on levels 2, and 3, respectively. If the photoelectron one-hole states on levels 1, 2 and 3 and Auger final double hole state are screened in the same way (i.e. if R(1) ) R(2) ) R(3) ) R, Rtot ) 4R) and if the F(2,3) is independent of chemical environment, then the change of ζ(2,3) reflects the change of relaxation energy of the core hole states. For ∆ζ(2,3) in this case

∆ζ(2,3) ) ζ(2,3)zeolite - ζ(2,3)stand ) -2∆R

Figure 7. Silicon Auger K L2,3L2,3 (a), K L1L2,3, and K L1L1 (b) spectra of sample 10 (A), sample 1 (Y), and sample 12 (ZSM-5).

F(2,3) (in the given final Auger state) and Rs:

ζ(2,3) ) F(2,3) - Rs

(3)

The Rs is quantity proportional to Auger final state relaxation energy:

Rs ) Rtot - R(2) - R(3)

(4)

Rtot is the total relaxation energy of the final state after Auger deexcitation and R(2) and R(3) are relaxation energies of the

(5)

where ∆R is the change in extraatomic relaxation energy. The KLL Auger transition is of practical importance for Al (Si). The final state holes of KLL transition of Al (Si) are highly localized, and values of F are thus independent of chemical effects. The values of ∆R calculated by the method described were associated with changes in the polarizability of valence charge on oxygen atoms coordinated around Al (Si).10,11 If eq 5 holds, then the energies of various hole states produced by KLL deexcitation might be shifted from each other only by some constant equal to differences of F of various double hole states. However, an estimate of the Eb of the K (1s) level of Al (Si), which is needed for evaluation of ζ, is impossible using conventional X-ray sources. Fortunately, a linear dependence of ζ(L,L) and modified Auger parameter R (defined as a sum of Ek of the KLL Auger line and Eb of 2p (2s) photoelectron line) was found.10,11 The energy of Al K L2,3L2,3 was proposed to be influenced by the coordination number of aluminum in the zeolite. The three coordination states (Al(III), Al(IV), and Al(VI)) might be distinguishable by curve fitting of the Al K L2,3L2,3 Auger line

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Jirka

Figure 9. Dependence of the intensities of the Al K L2,3L2,3, Al K L1L2,3, and Al K L1L1 Auger lines normalized relative to the intensity of Al 2p photoelectron line on Si/Al ratio.

Figure 10. Dependence of the shift of kinetic energy ∆Ek (relative to the value of Ek in zeolite A) of the Al K L2,3L2,3, Al K L1L2,3, and Al K L1L1 Auger lines on Si/Al ratio.

Figure 8. Aluminum Auger K L2,3L2,3 (a), K L1L2,3 (b) and K L1L1 (c) spectra of sample 10 (A), sample 1 (Y), and sample 12 (ZSM-5).

in the series of dealuminated mordenites.16 If this explanation is correct, the line shape changes observable in the Al K L2,3L2,3 Auger line should be present also in the Al K L1L2,3 and Al K L1L1 Auger spectra. Comparison of these Auger spectra of parent and steamed faujasite as well as faujasites treated with SiCl4 before and after extraction of extraframework aluminum by Na2H2EDTA is shown in Figure 6. Zeolites before extraction might contain Al atoms in at least two coordinations (Al(IV) and Al(VI)). The line shape changes are occurring in the Al K L2,3L2,3 spectra of the samples containing various amount of extraframework aluminum. However, analogous changes in the Al K L1L2,3 and Al K L1L1 spectra were not observed. Interestingly,

Figure 11. Dependence of the shift of kinetic energy ∆Ek (relative to the value of Ek in zeolite A) of the Si K L2,3L2,3, Al K L1L2,3, and Al K L1L1 Auger lines on Si/Al ratio.

the Al K L2,3L2,3 Auger line of the zeolites treated with SiCl4 before extraction of extraframework Al is more narrow than the Al K L2,3L2,3 belonging to the extracted samples. Moreover, the intensities of Al KLL spectra might be the same after normalization; nevertheless, only the intensity of Al K L2,3L2,3 Auger line seems to be roughly independent of chemical treatment (except the steamed sample whose whole Al KLL spectrum is more intense than the Al KLL spectrum of the parent

Si and Al Bonded in Zeolites

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Figure 12. Dependence of the modified Auger parameter R of Si and Al of zeolite A, faujasites, ferrierite, and ZSM-5 on Si/Al ratio.

Figure 14. Correlation of the Eb of Al 2p and O 1s photoelectron lines of investigated zeolites.

Figure 13. Correlation of the Eb of Si 2p and O 1s photoelectron lines of investigated zeolites.

zeolite). A substantial increase of the relative intensities of Al K L1L2,3 and Al K L1L1 spectra of the faujasites after extraction of extraframework aluminum is evident. The observed line shape changes in the Al KLL Auger spectra thus cannot be explained by the simultaneous presence of Al-containing species with different coordination numbers. Similar conclusions follow from discussion of the shapes and intensities of the Al KLL Auger spectra of zeolite A, Y (parent sample), and ZSM-5, which are depicted in Figure 8a-c. The 1S and 1D final states of the K L L 2,3 2,3 Auger line are resolved in the spectrum of zeolites A and Y. Only the 1D K L2,3L2,3 line is observable in the spectra of zeolite ZSM-5. Narrowing of this line is evident for zeolite A in comparison with other zeolites. The asymmetry in the K L2,3L2,3 line of ZSM-5 was also found by others.2 However, the line shapes of the K L1L2,3 Auger lines of zeolites A, Y, and ZSM-5 are close each other; i.e., no substantial broadening or line asymmetry was found. The Auger Al K L1L1 line is well observable for zeolite ZSM-5 only. The trends in intensities of the KLL Auger lines of aluminum are evident in Figure 9, where relative intensities of the Al 1S K L1L1, Al 1P K L1L2,3, and Al K L2,3L2,3 (1S K L2,3L2,3

+ 1D K L2,3L2,3) Auger lines are depicted as dependent on Si/ Al ratio. While the intensity of Al K L2,3L2,3 decreases from zeolite A to faujasites and then is constant, the Al 1S K L1L1 and Al 1P K L1L2,3 line is increasing with increasing Si/Al ratio. From these findings it follow that the line shape changes of the Al KL2,3L2,3 Auger lines of zeolites cannot be simply explained by presence of several nonequivalent coordination states of aluminum, as was proposed by others.7 However, an explanation of this effect is not clear. The change in screening mechanism of the Auger final hole states with decreasing concentration of Al in zeolites might play an essential role. The Coster Kronig (CK) L1L2,3V transition (V stand for valence level) was observed in the spectrum of Al.23 This CK transition may competitively fill the holes on the L1 level that are occurring during deexcitation of the hole state on the 1 s level. However, following arguments in ref 24, the CK L1L2,3V transition might be substantially slower than the K L1L2,3 (K L1L1) Auger transitions. Moreover, the CK L1L2,3V transition was observed also in the spectrum of silicon. However, the intensities of Si K L2,3L2,3, Si K L1L2,3, and Si K L1L1 Auger lines normalized relative to the intensity of the Si 2p photoelectron line do not depend on the Si/Al ratio (see Figure 7). The only one effect that was observed in intensities of the Si KLL Auger spectrum is broadening of the Si K L2,3L2,3 Auger line of faujasite in comparison with these transitions in zeolite A and ZSM-5. Thus, it can be concluded that the hole states localized on Si atoms are screened differently than the hole states on Al. Further experiments are essential to explain this effect. Further information about screening mechanisms of the hole states localized on Al follows from discussion of kinetic energies of Auger electrons emitted by the Al KLL Auger transitions, which were estimated from the positions of maxima of pertinent Auger lines. As is evident from Figures 10 the changes of kinetic energy ∆Ek (relative to the Ek of zeolite A) of Al K L2,3L2,3 and K L1L2,3 lines with increasing Si/Al ratio are within experimental error the same, while the changes of ∆Ek of Al K L1L1 are substantially different. The same effect appears in the ∆Ek values of Si KLL Auger lines (see Figure 11).

8140 J. Phys. Chem. B, Vol. 101, No. 41, 1997 Some information on the screening of the hole states localized on Si and Al atoms is gained from modified Auger parameters R, defined as the sum of the kinetic energy of the Al (Si) KL2,3L2,3 transition and the binding energy of the Al (Si) 2p photoelectron line. From above findings, it follows that R (ζ) cannot be applied to evaluate core level relaxation energy (see conditions simplifying eq 4 to 5. Nevertheless, the Auger parameter is proportional to the relaxation energy of Auger hole states (Rs - eq 3) and thus enables comparison of screening mechanisms of Al and Si Auger hole states. Qualitative trends of this screening with decreasing concentration of Al in the zeolite framework might be the same for screening of the core holes. This information is not influenced by sample/spectrometer contact potential. Published investigations of the dependence of R of zeolites on Si/Al ratio are controversial. The R values of Al are according to ref 11 decrease with increasing Si/Al ratio. According to Remy,16 however, the R values of Al are dependent on the coordination number of Al in zeolites only and not on the concentration of Al. The R of Si is according to ref 11 independent of the Si/Al ratio of zeolite. The dependence of the R of Si and Al on the Si/Al ratio is depicted in Figure 12. Application of Ek of K L1L2,3 and K L1L1 Auger lines for evaluation of R gives qualitatively the same results. While R(Al) decreases with increasing Si/Al ratio, the opposite trend was observed for R(Si). This result is in agreement with the above conclusion on different screening mechanisms of the Al and Si final hole states. Differences in the screening of the hole states localized on Si and Al cannot explain the greater Eb shift observed for the Si 2p photoelectron line in comparison with the shift of Eb of the Al 2p photoelectron line. The relaxation of the hole states results in lowering of the value of Eb; i.e., the observed changes of R with increasing Si/Al ratio exclude the dominant influence of the relaxation effects on the core level binding energy shifts. Conclusions Core level binding energy shifts of the elements bonded in zeolite framework with decreasing concentration of Al in the framework first increase (up to Si/Al ∼ 4) and then decrease (up to Si/Al ∼ 9). Several effects simultaneously influence the Eb shifts of the core level lines. All the lines are systematically shifted by zeolite/spectrometer contact potential. Another effect influencing the Eb shifts is related to the screening of the hole states. As follows from Auger parameter values, the screening of the Auger KLL hole states localized on Si atoms increases with decreasing concentration of Al in the zeolitic skeleton; the opposite trend was observed for the Auger KLL hole states of Al. Screening of the hole states after Auger Al (Si) KLL deexcitation depends on the type of final state involved. The hole states after Al (Si) K L2,3L2,3 and Al (Si) K L1L2,3 Auger deexcitations are screened much less with increasing of the Si/ Al ratio than the hole states after Al (Si) K L1L1 Auger transitions. The relaxation energy of the Auger final states

Jirka estimated from Auger parameter is thus not simply related to the relaxation energy of the core hole states. Screening also influences probabilities of the Al K L1L2,3 and Al K L1L1 Auger transitions; however the origin of this effect is not clear. The intensity of the Al K L1L1 and Al K L1L2,3 Auger lines is increasing in the series of zeolites with increasing Si/Al ratio; i.e., with increasing O-Al-O bond angle. The lower Eb shift of the Al 2p photoelectron line observed in comparison to the shift of Eb of Si 2p photoelectron line cannot be explained by the final state effects discussed above. Acknowledgment. I am grateful to Dr. J. Nova´kova´ and Dr. Z. Bastl for careful reading of the manuscript. This work was support from Grant COST D5/0002/94 and from Ministery of Education of the Czech Republic (OCD.10). References and Notes (1) Barr, T. L.; Lishka, M. A., J. Am. Chem. Soc. 1986, 108, 3178. (2) Grunnert, W.; Muhler, M.; Schroder, K.-P.; Sauer, J.; Schlogl, R. J. Phys. Chem. 1994, 98, 10920. (3) Corma, A.; Fornes, V.; Pallota, O.; Cruz, J. M.; Ayerbe, A. J. Chem. Soc., Chem. Commun. 1986, 333. (4) Shyu, J. Z.; Skopinski, E. T.; Goodwin, J. G.; Sayari, A. Appl. Surf. Sci. 1985, 21, 297. (5) Okamoto, Y.; Ogawa, M.; Maezawa, A.; Imanaka, T. J. Catal. 1988, 112, 427. (6) Wagner, C. D.; Passoja, T. G.; Hillery, H. F.; Kinisky, T. G.; Six, H. A.; Jansen, W. T.; Taylor, J. A. J. Vac. Sci. Technol. 1982, 21, 933. (7) Remy, M. J.; Genet, M. J.; Notte, P. P.; Lardinois, P. F.; Poncelet, G. Microporous Mater. 1993 2, (8) Stoch, J.; Lercher, J.; Ceckiewicz, S. Zeolites 1992, 12, 81. (9) Lang, N. D.; Williams, A. R. Phys. ReV. B 1979, 20, 1369. (10) Riviere, J. C.; Crossley, J. A. A.; Moretti, G. Surf. Interface Anal. 1989, 14, 257. (11) Pellenq, R. J.-M.; Nicholson, D., J. Chem. Soc., Faraday Trans. 1993, 89, 2499. (12) West, R. H.; Castle, J. E. Surf. Interface Anal. 1982, 4, 68. (13) Kubelkova´, L.; V. M., Seidl, V.; Borbe´ly, G.; Beyer, H. K., J. Chem. Soc., Faraday Trans. 1 1988, 84, 1447. (14) Kubelkova´, L.; Beran, S.; Malecka, A.; Mastikhin. Zeolites 1989, 9, 12. (15) Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; John Wiley: New York, 1983. (16) Remy, M. J.; Genet, M. J.; Poncelet, G.; Lardinois, P. F.; Notte´, P. P. J. Phys. Chem. 1992, 96, 2614. (17) Siegbahn, K.; et al. ESCA Applied to Free Molecules; NorthHolland: Amsterdam, 1969. (18) Huang, M.; Adnot, A.; Kaliaguine, S. J. Am. Chem. Soc. 1992, 114, 10005. (19) Casamassima, M.; Darque-Ceretti, E.; Etcheberry, A.; Aucouturier, M. Appl. Surf. Sci. 1991, 52, 205. (20) Mullins, W. M.; Averbach, B. L. Surf. Sci. 1988, 206, 41. (21) Mullins, W. M.; Averbach, B. L. Surf. Sci. 1988, 206, 52. (22) Ebina, T; Iwasaki, T.; Chatterjee, A.; Katagiri, M.; Stucky, G. D. J. Phys. Chem. 1997, 101, 1125. (23) Chen, M. H.; Craseman, B.; Huang, K.; Ayoagi, M.; Mark, H. At. Data Nucl. Data Tables 1977, 19, 97. (24) Citrin, P. H.; Rowe, J. E.; Christman, S. B. Phys. ReV. B 1976, 14, 2642. (25) La´znicˇka, M. Surf. Sci. 1979, 87, L260. (26) Scofield, J. H. J. Electron Spectrosc. 1976, 8, 129. (27) Watts, J. F. Vacuum 1994, 45, 653. (28) Sevier, K. D. Low Energy Electron Spectrometry; Wiley-Inerscience: New York, 1972.