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EXFAS electron spectroscopy as a new tool of local characterisation of copper in Cu-Beta zeolite
Solid State Sciences 3 (2001) 637–640 www.elsevier.com/locate/ssscie
EXFAS electron spectroscopy as a new tool of local characterisation of copper in Cu-Beta zeolite Francisco Márquez ∗ , Antonio Palomares Instituto de Tecnología Química, Universidad Politecnica de Valencia, CSIC, Av. de los Naranjos s/n, 46022 Valencia, Spain Received 29 November 2000; revised 31 January 2001; accepted 7 February 2001
Abstract EXFAS spectroscopy has been applied for the first time to the study of the local characterisation of reduced copper in zeolites. The oscillating features observed beyond the Cu M2,3 VV Auger transition have been isolated and analysed following the standard EXAFS procedure. 2001 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: EXFAS; Zeolite; Copper
1. Introduction Cu-Beta zeolite has been reported as catalyst having high activity for the selective catalytic reduction (SCR) of NOx [1]. In the last few years special attention has been paid to the study of possible mechanisms involved in the NOx reduction by zeolites. However, the detailed mechanism for this reaction has not been established. The determination of the geometric and electronic structure of the copper species is a very important aspect to understand this process. For this purpose, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, using a synchrotron radiation facility, is a powerful method even though this spectroscopy suffers from some practical limitations. In contrast to this limited accessibility, EXFAS (Extended Fine Auger Structure) spectroscopy [2–4] is a valuable tool for the local structural study of solid surfaces that can be used in a laboratory system. The EXFAS spectroscopy is based on the study of the extended oscillating features which extend for several hundreds of eV beyond the MVV and NVV Auger electron transitions of different elements, particularly in transitions metals. * Correspondence and reprints.
E-mail address: [email protected] (F. Márquez).
EXFAS results can be analysed following the conventional EXAFS procedure with equivalent results to those obtained with synchrotron radiation. The present paper reports, as far as we know, the first EXFAS (Extended Fine Auger Structure) results obtained for a catalyst and their analysis with the aid of the EXAFS procedure. This technique can represent a very useful tool to characterise the local structure of solids due to the high signal intensity with respect to other techniques (such as EELFS) used for this purpose [5,6].
2. EXFAS theoretical considerations The EXFAS signal has been reported for 3d and 4d transition metal surfaces. The origin of the oscillating features was unclear and several authors ascribed this effect to a diffraction process experienced by secondary electrons in their escaping from the solid surface [7]. Afterwards, other results were interpreted in terms of an EXAFS-like origin [8], although contribution from diffraction processes could also be involved. According to this interpretation, the EXFAS signal is based on an autoionisation process in which a core electron is excited by an electron beam into a virtual orbital and after that
1293-2558/01/$ – see front matter 2001 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 1 2 9 3 - 2 5 5 8 ( 0 1 ) 0 1 1 6 0 - 8
638
F. Márquez, A. Palomares / Solid State Sciences 3 (2001) 637–640
it recombines with its core hole. The difference between the excited state and the core level energies is used to promote a valence band electron which is ejected from the sample surface with kinetic energy greater than the characteristic Auger transition: EK = EB + δ − EV − eφ, where EB is the binding energy of the core electron, δ is the energy corresponding to the empty state lying (above the Fermi edge level), EV is the binding energy of the valence band electron and eφ is the work function of the spectrometer. This energy loss process can be generated by exciting the core level electron to a wide range of virtual levels probing the EXAFS-like modulation. This gives rise to oscillating features that are superimposed on the background, which are corresponding neither to Auger electronic transitions nor to plasmons. To obtain the structural information from the EXFAS results it is necessary to isolate the oscillating features and to analyse them by using the EXAFS procedure.
Fig. 1. EXFAS spectra recorded above the Cu MVV Auger transition after reducing in H2 (20% in N2 ) at 473 K (a), 573 K (b), 673 K (c) and 773 K (d).
3. Experimental details
3.3. In situ treatments
3.1. Catalyst preparation
In situ type reductions were conducted in a high pressure gas cell (HPGC) installed into the preparation chamber of the spectrometer. Powdered sample was pressed as a self supporting wafer of 9 mm diameter and ca. 10 mg weight that was fixed on a circular sample holder specially designed for the HPGC [1]. The reduction treatment in H2 (20% in N2 ) was carried out into this cell at atmospheric pressure with a gas flowrate of 100 ml min−1 during 2 h at different temperatures, followed by cooling in vacuum to room temperature.
The Cu-Beta zeolite, with a nominal ion exchange level of 186%, was prepared as follows. The starting zeolite was a commercial PQ (CP 811) sample (Si/Al = 11). 10 g of zeolite was slurried at room temperature for 24 h in 1000 ml of distilled water containing copper (II) acetate in the adequate concentration to achieve the desired ion exchange level. The over-exchanged zeolite was obtained by adding NH4 OH and by adjusting the pH to 6.0. The Cu-Beta zeolite was collected by filtration and subsequently was washed with distilled water, dried at 80◦ C and calcined at 450◦ C for 4 hours. 3.2. In situ EXFAS experimental setup EXFAS experiments were performed on a Vacuum Generators Escalab-210 spectrometer by using a high performance electron gun working in fixed retard ratio mode (crr = 4) along with a hemispherical energy analyser. All spectra were collected with an incident electron energy Ep = 1000 eV and a current of 1 µA on the sample (incidence angle φ = 45◦ ). To improve the signal-to-noise ratio 10 scans were acquired and numerically averaged.
To minimise the effects due to the electron irradiation on the catalyst sample was cooled to 173 K and maintained at this temperature during measurements. The pressure of the analysis chamber was maintained at 5·10–10 mB.
4. Results EXFAS measurements were obtained from the catalyst previously reduced by flowing H2 at different temperatures. In normal conditions this catalyst is a nonconducting material and only when the Cu exchange level is high enough (186%) and the copper is in metallic form these measurements can be obtained [9]. To increase the signalto-background ratio the spectra were detected by recording the first derivative of the electron distribution dN(E). Fig. 1 shows the EXFAS signal measured above the Cu M2,3 VV Auger transition after in situ reduction in flowing H2 at different temperatures. Reduced copper is
F. Márquez, A. Palomares / Solid State Sciences 3 (2001) 637–640
Fig. 2. Fourier transform (modulus) of EXFAS spectrum reported on Fig. 1(b).
clearly forming small particles that can sinterise to form large aggregates under treatment at higher temperatures. The level of reduced copper is depending on the temperature and only when the catalyst is reduced at high temperature (773 K) all copper ions are fully reduced, forming large aggregates mainly located on the surface of the catalyst [1]. The structure at ca. 103 eV corresponds to the Cu M2,3 VV Auger transition. As can be seen there the oscillating features are depending on the reducing treatment of the sample. This behaviour could be connected with the level of reduced copper present in the catalyst after treatment. To analyse these results the continuum features were isolated following the usual EXAFS procedure. Thus, EXFAS raw data from Fig. 1 were background subtracted and normalized (with the Lengeler–Eisenberger method) in wave vector k space χ(k) by using the E0 threshold energy at the Cu M2,3 VV Auger transition. Subsequently, Fourier transformation of χ(k) modulations, by using a Kaiser window, into the real R space provided the radial distribution function F (R) which contains the structural information on the positions of different neighbours around the excited atoms. Fig. 2 shows the Fourier integration F (R) of the χ(k) modulations previously isolated for the catalyst reduced in H2 at 573 K (Fig. 1(b)). The main peak is shown at ca. 2.2 Å and it can be assigned to the first coordination shell of copper neighbours (theoretically at 2.56 Å). The discrepancy between the experimental results and the crystallographic data should be attributed to the phase shift experienced by the excited electron that is involved in the Auger mechanism [10]. Fig. 2 shows also one secondary peak at ca. 4 Å that we have tentatively assigned to the overlap between the EXFAS
639
Fig. 3. Fourier transform (modulus) of EXFAS spectrum reported on Fig. 1(d).
signal of the third coordination shell and the signal due to a diffraction mechanism involving the emitted electrons [11]. This explanation has been previously argued to justify the presence of this peak in polycrystalline copper. Fig. 3 shows the radial distribution function F (R) corresponding to the EXFAS spectrum of Fig. 1(d) (after reducing in H2 at 773 K). As can be seen there this figure is different from Fig. 2 showing two peaks at around 1.4 and 2.54 Å, respectively. The second peak at 2.54 Å has been assigned to the first coordination shell of copper neighbours (observed for metallic copper at ca. 2.56 Å). The first peak (ca. 1.4 Å) was also ascribed to the first coordination shell and in this case, this discrepancy could be justified as previously for the catalyst reduced at 573 K (Fig. 1(b)), as due to the phase shift.
5. Conclusions As far as we know we have obtained the first EXFAS spectra of a real catalyst by using an electron gun designed for conventional Auger spectroscopy. From the analysis of the measured spectra (by using the conventional EXAFS procedure) we have shown that the interatomic distance for the first coordination shell of the aggregates of copper of the catalyst is in agreement with that obtained from crystallographic data, showing a small discrepancy due to a phase shift. To conclude, the results obtained with Cu-Beta zeolite indicate that EXFAS spectroscopy, that has been applied to metals, could be a very useful tool to give us new information on the local geometry around the excited atom, even of some catalysts, by using a very simple laboratory surface spectrometer.
640
F. Márquez, A. Palomares / Solid State Sciences 3 (2001) 637–640
References [1] A. Corma, A. Palomares, F. Marquez, J. Catal. 170 (1997) 132– 139. [2] I. Davoli, R. Bernardini, C. Battistoni, P. Castrucci, R. Gunnella, M. Decrescenzi, Surf. Sci. 306 (1994) 144–154. [3] M. Decrescenzi, L. Lozzi, M. Passacantando, P. Picozzi, S. Santucci, Thin Solid Films 193 (1990) 318–324. [4] M. Decrescenzi, R. Gunnella, I.J. Davoli, Electron Spectrosc. Relat. Phenom. 76 (1995) 29–36. [5] D.V. Surnin, A.N. Deev, D.E. Guy, Y.V.J. Ruts, Electron Spec-
trosc. Relat. Phenom. 95 (1998) 193–202. [6] D.V. Surnin, D.E. Denisov, Y.V. Ruts, P.M. Knjazev, J. Phys. IV 7 (1997) 577–578. [7] D.P. Woodruff, Surf. Sci. 189/190 (1987) 64. [8] L. Lozzi, M. Passacantando, P. Picozzi, S. Santucci, M. De Crescenzi, Surf. Rev. Lett. 2 (1995) 255–268. [9] A. Corma, A.E. Palomares, F. Márquez, in: J.M. Sanz, J.P. Espinós (Eds.), Proc. 8th ECASIA 99, Sevilla, 1999. [10] E.A. Stern, D.E. Sayers, F.W. Lytle, Phys. Rev. B 11 (1975) 4836. [11] M. Crescenzi, A.P. Hitchcock, T. Tyliszczak, Phys. Rev. B 39 (1989) 9839.
EXFAS electron spectroscopy as a new tool of local characterisation of copper in Cu-Beta zeolite Francisco Márquez ∗ , Antonio Palomares Instituto de Tecnología Química, Universidad Politecnica de Valencia, CSIC, Av. de los Naranjos s/n, 46022 Valencia, Spain Received 29 November 2000; revised 31 January 2001; accepted 7 February 2001
Abstract EXFAS spectroscopy has been applied for the first time to the study of the local characterisation of reduced copper in zeolites. The oscillating features observed beyond the Cu M2,3 VV Auger transition have been isolated and analysed following the standard EXAFS procedure. 2001 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: EXFAS; Zeolite; Copper
1. Introduction Cu-Beta zeolite has been reported as catalyst having high activity for the selective catalytic reduction (SCR) of NOx [1]. In the last few years special attention has been paid to the study of possible mechanisms involved in the NOx reduction by zeolites. However, the detailed mechanism for this reaction has not been established. The determination of the geometric and electronic structure of the copper species is a very important aspect to understand this process. For this purpose, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, using a synchrotron radiation facility, is a powerful method even though this spectroscopy suffers from some practical limitations. In contrast to this limited accessibility, EXFAS (Extended Fine Auger Structure) spectroscopy [2–4] is a valuable tool for the local structural study of solid surfaces that can be used in a laboratory system. The EXFAS spectroscopy is based on the study of the extended oscillating features which extend for several hundreds of eV beyond the MVV and NVV Auger electron transitions of different elements, particularly in transitions metals. * Correspondence and reprints.
E-mail address: [email protected] (F. Márquez).
EXFAS results can be analysed following the conventional EXAFS procedure with equivalent results to those obtained with synchrotron radiation. The present paper reports, as far as we know, the first EXFAS (Extended Fine Auger Structure) results obtained for a catalyst and their analysis with the aid of the EXAFS procedure. This technique can represent a very useful tool to characterise the local structure of solids due to the high signal intensity with respect to other techniques (such as EELFS) used for this purpose [5,6].
2. EXFAS theoretical considerations The EXFAS signal has been reported for 3d and 4d transition metal surfaces. The origin of the oscillating features was unclear and several authors ascribed this effect to a diffraction process experienced by secondary electrons in their escaping from the solid surface [7]. Afterwards, other results were interpreted in terms of an EXAFS-like origin [8], although contribution from diffraction processes could also be involved. According to this interpretation, the EXFAS signal is based on an autoionisation process in which a core electron is excited by an electron beam into a virtual orbital and after that
1293-2558/01/$ – see front matter 2001 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 1 2 9 3 - 2 5 5 8 ( 0 1 ) 0 1 1 6 0 - 8
638
F. Márquez, A. Palomares / Solid State Sciences 3 (2001) 637–640
it recombines with its core hole. The difference between the excited state and the core level energies is used to promote a valence band electron which is ejected from the sample surface with kinetic energy greater than the characteristic Auger transition: EK = EB + δ − EV − eφ, where EB is the binding energy of the core electron, δ is the energy corresponding to the empty state lying (above the Fermi edge level), EV is the binding energy of the valence band electron and eφ is the work function of the spectrometer. This energy loss process can be generated by exciting the core level electron to a wide range of virtual levels probing the EXAFS-like modulation. This gives rise to oscillating features that are superimposed on the background, which are corresponding neither to Auger electronic transitions nor to plasmons. To obtain the structural information from the EXFAS results it is necessary to isolate the oscillating features and to analyse them by using the EXAFS procedure.
Fig. 1. EXFAS spectra recorded above the Cu MVV Auger transition after reducing in H2 (20% in N2 ) at 473 K (a), 573 K (b), 673 K (c) and 773 K (d).
3. Experimental details
3.3. In situ treatments
3.1. Catalyst preparation
In situ type reductions were conducted in a high pressure gas cell (HPGC) installed into the preparation chamber of the spectrometer. Powdered sample was pressed as a self supporting wafer of 9 mm diameter and ca. 10 mg weight that was fixed on a circular sample holder specially designed for the HPGC [1]. The reduction treatment in H2 (20% in N2 ) was carried out into this cell at atmospheric pressure with a gas flowrate of 100 ml min−1 during 2 h at different temperatures, followed by cooling in vacuum to room temperature.
The Cu-Beta zeolite, with a nominal ion exchange level of 186%, was prepared as follows. The starting zeolite was a commercial PQ (CP 811) sample (Si/Al = 11). 10 g of zeolite was slurried at room temperature for 24 h in 1000 ml of distilled water containing copper (II) acetate in the adequate concentration to achieve the desired ion exchange level. The over-exchanged zeolite was obtained by adding NH4 OH and by adjusting the pH to 6.0. The Cu-Beta zeolite was collected by filtration and subsequently was washed with distilled water, dried at 80◦ C and calcined at 450◦ C for 4 hours. 3.2. In situ EXFAS experimental setup EXFAS experiments were performed on a Vacuum Generators Escalab-210 spectrometer by using a high performance electron gun working in fixed retard ratio mode (crr = 4) along with a hemispherical energy analyser. All spectra were collected with an incident electron energy Ep = 1000 eV and a current of 1 µA on the sample (incidence angle φ = 45◦ ). To improve the signal-to-noise ratio 10 scans were acquired and numerically averaged.
To minimise the effects due to the electron irradiation on the catalyst sample was cooled to 173 K and maintained at this temperature during measurements. The pressure of the analysis chamber was maintained at 5·10–10 mB.
4. Results EXFAS measurements were obtained from the catalyst previously reduced by flowing H2 at different temperatures. In normal conditions this catalyst is a nonconducting material and only when the Cu exchange level is high enough (186%) and the copper is in metallic form these measurements can be obtained [9]. To increase the signalto-background ratio the spectra were detected by recording the first derivative of the electron distribution dN(E). Fig. 1 shows the EXFAS signal measured above the Cu M2,3 VV Auger transition after in situ reduction in flowing H2 at different temperatures. Reduced copper is
F. Márquez, A. Palomares / Solid State Sciences 3 (2001) 637–640
Fig. 2. Fourier transform (modulus) of EXFAS spectrum reported on Fig. 1(b).
clearly forming small particles that can sinterise to form large aggregates under treatment at higher temperatures. The level of reduced copper is depending on the temperature and only when the catalyst is reduced at high temperature (773 K) all copper ions are fully reduced, forming large aggregates mainly located on the surface of the catalyst [1]. The structure at ca. 103 eV corresponds to the Cu M2,3 VV Auger transition. As can be seen there the oscillating features are depending on the reducing treatment of the sample. This behaviour could be connected with the level of reduced copper present in the catalyst after treatment. To analyse these results the continuum features were isolated following the usual EXAFS procedure. Thus, EXFAS raw data from Fig. 1 were background subtracted and normalized (with the Lengeler–Eisenberger method) in wave vector k space χ(k) by using the E0 threshold energy at the Cu M2,3 VV Auger transition. Subsequently, Fourier transformation of χ(k) modulations, by using a Kaiser window, into the real R space provided the radial distribution function F (R) which contains the structural information on the positions of different neighbours around the excited atoms. Fig. 2 shows the Fourier integration F (R) of the χ(k) modulations previously isolated for the catalyst reduced in H2 at 573 K (Fig. 1(b)). The main peak is shown at ca. 2.2 Å and it can be assigned to the first coordination shell of copper neighbours (theoretically at 2.56 Å). The discrepancy between the experimental results and the crystallographic data should be attributed to the phase shift experienced by the excited electron that is involved in the Auger mechanism [10]. Fig. 2 shows also one secondary peak at ca. 4 Å that we have tentatively assigned to the overlap between the EXFAS
639
Fig. 3. Fourier transform (modulus) of EXFAS spectrum reported on Fig. 1(d).
signal of the third coordination shell and the signal due to a diffraction mechanism involving the emitted electrons [11]. This explanation has been previously argued to justify the presence of this peak in polycrystalline copper. Fig. 3 shows the radial distribution function F (R) corresponding to the EXFAS spectrum of Fig. 1(d) (after reducing in H2 at 773 K). As can be seen there this figure is different from Fig. 2 showing two peaks at around 1.4 and 2.54 Å, respectively. The second peak at 2.54 Å has been assigned to the first coordination shell of copper neighbours (observed for metallic copper at ca. 2.56 Å). The first peak (ca. 1.4 Å) was also ascribed to the first coordination shell and in this case, this discrepancy could be justified as previously for the catalyst reduced at 573 K (Fig. 1(b)), as due to the phase shift.
5. Conclusions As far as we know we have obtained the first EXFAS spectra of a real catalyst by using an electron gun designed for conventional Auger spectroscopy. From the analysis of the measured spectra (by using the conventional EXAFS procedure) we have shown that the interatomic distance for the first coordination shell of the aggregates of copper of the catalyst is in agreement with that obtained from crystallographic data, showing a small discrepancy due to a phase shift. To conclude, the results obtained with Cu-Beta zeolite indicate that EXFAS spectroscopy, that has been applied to metals, could be a very useful tool to give us new information on the local geometry around the excited atom, even of some catalysts, by using a very simple laboratory surface spectrometer.
640
F. Márquez, A. Palomares / Solid State Sciences 3 (2001) 637–640
References [1] A. Corma, A. Palomares, F. Marquez, J. Catal. 170 (1997) 132– 139. [2] I. Davoli, R. Bernardini, C. Battistoni, P. Castrucci, R. Gunnella, M. Decrescenzi, Surf. Sci. 306 (1994) 144–154. [3] M. Decrescenzi, L. Lozzi, M. Passacantando, P. Picozzi, S. Santucci, Thin Solid Films 193 (1990) 318–324. [4] M. Decrescenzi, R. Gunnella, I.J. Davoli, Electron Spectrosc. Relat. Phenom. 76 (1995) 29–36. [5] D.V. Surnin, A.N. Deev, D.E. Guy, Y.V.J. Ruts, Electron Spec-
trosc. Relat. Phenom. 95 (1998) 193–202. [6] D.V. Surnin, D.E. Denisov, Y.V. Ruts, P.M. Knjazev, J. Phys. IV 7 (1997) 577–578. [7] D.P. Woodruff, Surf. Sci. 189/190 (1987) 64. [8] L. Lozzi, M. Passacantando, P. Picozzi, S. Santucci, M. De Crescenzi, Surf. Rev. Lett. 2 (1995) 255–268. [9] A. Corma, A.E. Palomares, F. Márquez, in: J.M. Sanz, J.P. Espinós (Eds.), Proc. 8th ECASIA 99, Sevilla, 1999. [10] E.A. Stern, D.E. Sayers, F.W. Lytle, Phys. Rev. B 11 (1975) 4836. [11] M. Crescenzi, A.P. Hitchcock, T. Tyliszczak, Phys. Rev. B 39 (1989) 9839.