Solid State Sciences 3 (2001) 637–640www.elsevier.com/locate/ssscie

EXFAS electron spectroscopy as a new tool of local characterisationof copper in Cu-Beta zeolite

Francisco Márquez∗, Antonio PalomaresInstituto 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. Theoscillating features observed beyond the CuM2,3VV Auger transition have been isolated and analysed following the standard EXAFSprocedure. 2001 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Keywords: EXFAS; Zeolite; Copper

1. Introduction

Cu-Beta zeolite has been reported as catalyst havinghigh activity for the selective catalytic reduction (SCR) ofNOx [1]. In the last few years special attention has beenpaid to the study of possible mechanisms involved in theNOx reduction by zeolites. However, the detailed mech-anism for this reaction has not been established. The de-termination of the geometric and electronic structure ofthe copper species is a very important aspect to under-stand this process. For this purpose, near-edge X-ray ab-sorption fine structure (NEXAFS) spectroscopy, using asynchrotron radiation facility, is a powerful method eventhough this spectroscopy suffers from some practical lim-itations. In contrast to this limited accessibility, EXFAS(Extended Fine Auger Structure) spectroscopy [2–4] isa valuable tool for the local structural study of solid sur-faces that can be used in a laboratory system. The EXFASspectroscopy is based on the study of the extended oscil-lating features which extend for several hundreds of eVbeyond the MVV and NVV Auger electron transitionsof different elements, particularly in transitions metals.

* Correspondence and reprints.E-mail address: fmarquez@upvnet.upv.es (F. Márquez).

EXFAS results can be analysed following the conven-tional EXAFS procedure with equivalent results to thoseobtained with synchrotron radiation.

The present paper reports, as far as we know, the firstEXFAS (Extended Fine Auger Structure) results obtainedfor a catalyst and their analysis with the aid of the EXAFSprocedure. This technique can represent a very useful toolto characterise the local structure of solids due to the highsignal intensity with respect to other techniques (such asEELFS) used for this purpose [5,6].

2. EXFAS theoretical considerations

The EXFAS signal has been reported for 3d and 4dtransition metal surfaces. The origin of the oscillatingfeatures was unclear and several authors ascribed thiseffect to a diffraction process experienced by secondaryelectrons in their escaping from the solid surface [7].Afterwards, other results were interpreted in terms ofan EXAFS-like origin [8], although contribution fromdiffraction processes could also be involved. Accordingto this interpretation, the EXFAS signal is based on anautoionisation process in which a core electron is excitedby 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: S1293-2558(01)01160-8

638 F. Márquez, A. Palomares / Solid State Sciences 3 (2001) 637–640

it recombines with its core hole. The difference betweenthe excited state and the core level energies is used topromote a valence band electron which is ejected fromthe sample surface with kinetic energy greater than thecharacteristic Auger transition:

EK = EB + δ − EV − eφ,

whereEB is the binding energy of the core electron,δ isthe energy corresponding to the empty state lying (abovethe Fermi edge level),EV is the binding energy of thevalence band electron andeφ is the work function of thespectrometer.

This energy loss process can be generated by excitingthe core level electron to a wide range of virtual levelsprobing the EXAFS-like modulation. This gives riseto oscillating features that are superimposed on thebackground, which are corresponding neither to Augerelectronic transitions nor to plasmons.

To obtain the structural information from the EXFASresults it is necessary to isolate the oscillating featuresand to analyse them by using the EXAFS procedure.

3. Experimental details

3.1. Catalyst preparation

The Cu-Beta zeolite, with a nominal ion exchangelevel of 186%, was prepared as follows. The startingzeolite was a commercial PQ (CP 811) sample (Si/Al =11). 10 g of zeolite was slurried at room temperature for24 h in 1000 ml of distilled water containing copper (II)acetate in the adequate concentration to achieve thedesired ion exchange level. The over-exchanged zeolitewas obtained by adding NH4OH and by adjusting the pHto 6.0. The Cu-Beta zeolite was collected by filtration andsubsequently was washed with distilled water, dried at80◦C and calcined at 450◦C for 4 hours.

3.2. In situ EXFAS experimental setup

EXFAS experiments were performed on a VacuumGenerators Escalab-210 spectrometer by using a highperformance electron gun working in fixed retard ratiomode (crr = 4) along with a hemispherical energyanalyser. All spectra were collected with an incidentelectron energyEp = 1000 eV and a current of 1 µAon the sample (incidence angleφ = 45◦). To improvethe signal-to-noise ratio 10 scans were acquired andnumerically averaged.

Fig. 1. EXFAS spectra recorded above the CuMVV Auger transitionafter reducing in H2 (20% in N2) at 473 K (a), 573 K (b), 673 K (c) and773 K (d).

To minimise the effects due to the electron irradia-tion on the catalyst sample was cooled to 173 K andmaintained at this temperature during measurements.The pressure of the analysis chamber was maintained at5·10–10 mB.

3.3. In situ treatments

In situ type reductions were conducted in a highpressure gas cell (HPGC) installed into the preparationchamber of the spectrometer. Powdered sample waspressed as a self supporting wafer of 9 mm diameterand ca. 10 mg weight that was fixed on a circular sam-ple holder specially designed for the HPGC [1]. Thereduction treatment in H2 (20% in N2) was carried outinto this cell at atmospheric pressure with a gas flow-rate of 100 ml min−1 during 2 h at different temperatures,followed by cooling in vacuum to room temperature.

4. Results

EXFAS measurements were obtained from the catalystpreviously reduced by flowing H2 at different tempera-tures. In normal conditions this catalyst is a nonconduct-ing material and only when the Cu exchange level is highenough (186%) and the copper is in metallic form thesemeasurements can be obtained [9]. To increase the signal-to-background ratio the spectra were detected by record-ing the first derivative of the electron distribution dN(E).

Fig. 1 shows the EXFAS signal measured above theCu M2,3VV Auger transition afterin situ reduction inflowing H2 at different temperatures. Reduced copper is

F. Márquez, A. Palomares / Solid State Sciences 3 (2001) 637–640 639

Fig. 2. Fourier transform (modulus) of EXFAS spectrum reported onFig. 1(b).

clearly forming small particles that can sinterise to formlarge aggregates under treatment at higher temperatures.The level of reduced copper is depending on the tempera-ture and only when the catalyst is reduced at high temper-ature (773 K) all copper ions are fully reduced, forminglarge aggregates mainly located on the surface of the cat-alyst [1].

The structure at ca. 103 eV corresponds to the CuM2,3VV Auger transition. As can be seen there the oscil-lating features are depending on the reducing treatmentof the sample. This behaviour could be connected withthe level of reduced copper present in the catalyst aftertreatment.

To analyse these results the continuum features wereisolated following the usual EXAFS procedure. Thus,EXFAS raw data from Fig. 1 were background subtractedand normalized (with the Lengeler–Eisenberger method)in wave vectork spaceχ(k) by using theE0 thresholdenergy at the CuM2,3VV Auger transition. Subsequently,Fourier transformation ofχ(k) modulations, by using aKaiser window, into the realR space provided the ra-dial distribution functionF(R) which contains the struc-tural information on the positions of different neighboursaround the excited atoms. Fig. 2 shows the Fourier inte-grationF(R) of theχ(k) modulations previously isolatedfor the catalyst reduced in H2 at 573 K (Fig. 1(b)). Themain peak is shown at ca. 2.2 Å and it can be assignedto the first coordination shell of copper neighbours (theo-retically at 2.56 Å). The discrepancy between the experi-mental results and the crystallographic data should be at-tributed to the phase shift experienced by the excited elec-tron that is involved in the Auger mechanism [10]. Fig. 2shows also one secondary peak at ca. 4 Å that we havetentatively assigned to the overlap between the EXFAS

Fig. 3. Fourier transform (modulus) of EXFAS spectrum reported onFig. 1(d).

signal of the third coordination shell and the signal dueto a diffraction mechanism involving the emitted elec-trons [11]. This explanation has been previously arguedto justify the presence of this peak in polycrystalline cop-per.

Fig. 3 shows the radial distribution functionF(R)

corresponding to the EXFAS spectrum of Fig. 1(d) (afterreducing in H2 at 773 K). As can be seen there this figureis different from Fig. 2 showing two peaks at around 1.4and 2.54 Å, respectively. The second peak at 2.54 Å hasbeen assigned to the first coordination shell of copperneighbours (observed for metallic copper at ca. 2.56 Å).The first peak (ca. 1.4 Å) was also ascribed to the firstcoordination shell and in this case, this discrepancy couldbe 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 EXFASspectra of a real catalyst by using an electron gundesigned for conventional Auger spectroscopy. Fromthe analysis of the measured spectra (by using theconventional EXAFS procedure) we have shown that theinteratomic distance for the first coordination shell of theaggregates of copper of the catalyst is in agreement withthat obtained from crystallographic data, showing a smalldiscrepancy due to a phase shift.

To conclude, the results obtained with Cu-Beta zeoliteindicate that EXFAS spectroscopy, that has been appliedto metals, could be a very useful tool to give us newinformation on the local geometry around the excitedatom, even of some catalysts, by using a very simplelaboratory surface spectrometer.

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