Structural characterization of Pd-Ag and Pd-Cu bimetallic catalysts by means of EXAFS, WAXS and XPS

Structural characterization of Pd-Ag and Pd-Cu bimetallic catalysts by means of EXAFS, WAXS and XPS

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights res...

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Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.

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S t r u c t u r a l c h a r a c t e r i z a t i o n of Pd-Ag a n d P d - C u b i m e t a l l i c c a t a l y s t s b y m e a n s of EXAFS, WAXS a n d XPS A. Longo a, A. Balerna b, F. Deganello a,c, L.F. Liotta a, C. Meneghini b, A. Martorana a,c and A. M.Venezia a a Istituto di Chimica e Tecnologia dei Prodotti Naturali del CNR (ICTPN-CNR), via Ugo La Malfa 153, 90146 Palermo, Italy

b Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, via E. Fermi 40, 00044 Frascati, Italy c Dipartimento di Chimica Inorganica, Universit~ di Palermo, viale delle Scienze, 90128 Palermo, Italy Bimetallic Pd-Ag and Pd-Cu pumice-supported catalysts have been synthesized following different preparation procedures with the aim of improving the selectivity and reactivity of monometallic Pd/pumice systems. The structural characterization, carried out by X-ray Diffraction, X-ray Absorption and X-ray Photoelectron Spectroscopy, allowed to investigate the importance of the preparation procedures in the alloy formation.

1. INTRODUCTION The hydrogenation of highly unsaturated hydrocarbons is of great industrial relevance. Indeed, small concentrations of alkynes and alkadienes must be removed from C2, C3 and C4 olefin feedstocks before they can be processed by polymerization [1]. Palladium is the most active and selective metal for these hydrogenation reactions. Nevertheless its use involves several drawbacks: the selectivity is restricted by competitive reactions such as the complete hydrogenation of the hydrocarbon and the isomerization of the diolefins containing more than 3 carbon atoms. Several attempts have been made to minimize these undesired side reactions by using bimetallic catalysts. Investigations on Pd-Cu have shown that the bimetallic samples can have better selectivity [2,3] and activity [3] than monometallic palladium; Pd-Ag systems are also studied, either for structural characterization [4,5] or reactivity [6]. The synthesis of bimetallic Pd-Cu and Pd-Ag catalysts is believed to modify the chemical environment of the Pd atoms. Different synthetic strategies have been used for the production of multimetallic clusters. It is well known that the synthetic technique used in the catalysts preparation process sometimes favours alloying, sometimes the growth of monometallic clusters. The aim of this work is

3208 to perform a structural characterization of pumice supported bimetallic Pd-Ag and Pd-Cu samples trying to solve the question of what correlation exists between Ag, Cu and Pd under different synthetic routes. For this reason XAFS measurements at the Cu, Ag and Pd K-edges were used to investigate the short range order structural features. The long range ones were studied by WAXS (Wide Angle X-ray Scattering). The results of the X-ray Photoelectron Spectroscopy (XPS) on the Pd-Cu catalysts [4], allowing to get further information on the surface chemical and elemental composition, are also reported and the comparison with the previously quoted bulk techniques is made. 2. EXPERIMENTAL 2.1 S a m p l e s p r e p a r a t i o n The Pd-Cu samples were synthesized by anchoring Pd(C3H5) 2 on pumice, an amorphous alumino silicate characterized by small specific surface area (~ lm2/g) [7], and subsequently, after reduction of the anchored organometallic intermediate in flowing H 2 at room temperature, (E)-2-ethyl-l-hexenyl-copper (I). The final reduction of the intermediates was performed at the two different temperatures of 298 K (sample labelled Lnl) and 623 K (Ln2) respectively. The bimetallic samples have a Cu content of 0.05 wt% and a Pd one of 0.6 wt%. The Pd-Ag sample was prepared by anchoring on the support Ag(1.5COD)2BF 4 and subsequently Pd(C3H5) 2. The anchored organometallic intermediates were then simultaneously reduced by H 2 flux at room temperature. The final metal content was 0.6wt% for Ag and 0.5wt% for Pd. 2.2 S t r u c t u r a l c h a r a c t e r i z a t i o n The pumice supported samples were prepared for the EXAFS experiments by suspending the fine powder of the samples in ethanol 99.99% and depositing it on millipore membranes under N 2 atmosphere to avoid air contamination. The samples have been enveloped with a kapton film. The EXAFS measurements at the Pd, Ag and Cu K edges were performed at the beamline GILDA of the European Synchrotron Radiation Facility (ESRF). The monochromator operated with Si(311) crystals in dynamical sagittal focusing mode. Harmonic rejection was performed by using Pt mirrors as low pass filter (energy cut-off at ~ 27 KeV) at the Pd and Ag K-edges, and Pd mirrors at the Cu K edge. The XAFS spectra were recorded in fluorescence geometry: the fluorescence intensities (If) excited by the primary beam were detected using a 7-elements Ge multidetector (MD), each element covering an area of 100ram 2. The MD energy resolution allows to improve the data quality thanks to the separation between the specific K~ line and background (elastic, Compton and other fluorescence signals). All the XAFS spectra were recorded at 77 K to reduce the thermal effects. The WAXS experiments on Pd-Cu samples were performed with a Philips Xray diffractometer using Nickel-filtered Cu Ka radiation [4]. The X-ray scattering data for the Ag-Pd sample were collected at the diffraction hutch of the GILDA

3209 beamline at ESRF, with an incoming wavelenght of 0.56362/~ and a Fuji image plate 20X40 cm 2. The FIT2D [8] program was used for the data treatment. XPS measurements were performed with a Kratos ES 300 ESCA instrument working in Fixed Analyser Transmission (FAT) mode using a pass energy of 40 eV. Spectra were generated by A1 Ka X-rays (E=1486.6 eV, 150 W). Further details about the samples treatment and the procedures for the analysis of the spectra are given in Ref. [4]. 3. R E S U L T S AND D I S C U S S I O N In Fig.la the Fourier transforms of the two Pd-Cu samples are drawn (dots); for the L n l sample the different distance components from the Cu absorber are indicated by arrows. The patterns drawn with solid lines are Fourier transformed from the x(k) signals, calculated and fitted to the experimental data using the GNXAS package [9]. Cu K edge

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Fig. lb shows the x(k) partial functions relative to the Cu K edge; in particular, the presence of a Cu-Cu single scattering contribution only in the sample reduced at room temperature is clearly visible. The details of the EXAFS analysis on the Cu edge are given in Table 1. The analysis of the Pd K edge does not show any evidence of Pd-Cu alloying for both samples; the apparent discrepancy can be explained taking into account the lattice parameters for Pd-Cu alloys reported in the literature [10], the very low Cu content (<10%) and the error of about 0.02 .~ attributable to the evaluated EXAFS nearest neighbours distance.

3210 Table 1 . Best fit parameters of the GNXAS procedure for the Pd-Cu catalysts reduced at two different temperatures, measured at the Cu Ka edge. .....................................................

Cu-O component N1 R1 (s g21(- 103A~) Cu-Cu component N2 R2 (/k) g21(. 10-3/~2) Cu-Pd component N1 RI(/~) g21(-10-3A2)

l Pd:c u (298 0.8(2) 1.89 (2) 8.4 (6)

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1.0(2) 1.84 (3) 6.4(5)

4.7 (8) 2.53 (3) 15.6 (6) 6.1 (5) 2.65 (3) 12.9 (5)

8.7 (3) 2.67 (2) 8.5 (3)

The line width of the (111) diffraction peak relative to the L n l sample is quite large, showing the presence of small and / or disordered particles. From the Scherrer analysis an estimated particle size of about 70 A is obtained. The cubic cell edge is determined as 3.883/~, in agreement with the 3.889/~ value of bulk Pd. No evidence of Cu-Pd alloying could be found by WAXS. On the contrary, the (111) peak of the Ln2 sample is almost three times narrower, corresponding to Scherrer particle size of 200/k, and shifted towards larger 20 angles. The calculated cell edge of 3.863 /k corresponds to a homogeneous substitutional Pd-Cu alloy with an atomic number ratio of 10:1 [10], in agreement with the EXAFS analysis that does not see Cu-Cu interaction. Due to presence of several phases, it is not possible to obtain the real coordination numbers from the EXAFS analysis [5]; as a consequence, the comparison with the XRD analysis cannot be carried out. According to the XPS results [4], the sample reduced at 298 K contains CuO, metallic copper and palladium, characterised by binding energies of 193.8 eV, 931.7 eV and 335.7 eV, respectively. The slightly shifted value of metallic Cu 2p3/~ towards lower binding energy with respect to bulk Cu may be attributed to alloying with Pd [4]. Upon reduction at 623 K the copper oxide phase, no more present in the XPS spectra, is still detected by XAFS. It is likely that the formation of CuO can be ascribed to an interaction between the copper and the oxygen of the support and that it is not visible at the XPS analysis because it is covered by the well-grown alloy phase. The disappearance of the Cu-Cu distances in the high temperature treated sample points to the complete alloying of copper with palladium. The EXAFS analysis performed on the Pd-Ag sample is summarised in Fig.2, showing the F.T. at both the Pd and Ag edges of the catalyst and of the respective reference samples. It is evident that the first neighbour distance is longer in the bimetallic than in the reference sample when the absorber is Pd, and shorter for Ag. This constitutes an evidence of alloy formation, the first neighbour distance

3211 being 2.78(2) A from the Pd absorber and 2.85(2) /~ from the Ag one. These figures are to be compared with the 2.75(1) and, respectively, 2.88(1) obtained for the reference samples. The XRD analysis, Fig.3, confirms these results: the 111 peak of the Pd-Ag sample is interpreted as the superposition of two components, one at smaller Q corresponding to an Ag-rich Pd-Ag alloy, the other, at larger Q, containing more Pd. The first neighbour distances, calculated from the respective cell edge values, are 2.861(2) and 2.800(1), respectively. The particle size of the two phases, by the Scherrer equation, are 80 A and 50/k, respectively. Pd K Edge

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4. CONCLUSIONS The investigated bimetallic pumice-supported catalysts are all prepared from organometallic precursors but following different routes. In the Pd-Cu catalysts the anchored Pd intermediate is reduced before being contacted with the Cu precursor, and then the system is reduced at the two quoted temperatures. On the contrary, in the Pd-Ag system, the reduction of the anchored intermediates, anchoring Ag first, is performed in a single step at low temperature. Both syntheses reported in this study produce alloying of the involved metals, no matter the details of the preparation route. On the contrary, it has been demonstrated by EXAFS, AWAXS (Anomalous WAXS, [5]) and XPS [4] that the classical impregnation technique is not effective in alloying palladium and silver

3212 on pumice, producing a coating of very small Ag particles on the Pd ones and, consequently, the poisoning of the catalyst. It is likely that, given that the Pd-Ag and Pd-Cu alloys can exist in a wide range of composition ratios, the closest contact between the interacting species is the crucial point. With this respect, the organometallic routes, able to produce very dispersed supported metal phase even on a low-specific area support like pumice, would favour alloying. As a final remark, it is interesting to stress that it was not possible to get evidence of the L n l alloying by X-ray diffraction: the complementary use of several characterization techniques (namely, the EXAFS spectroscopy) is therefore demonstrated to be of outstanding relevance, most of all when structureproperties relationships are to be established. REFERENCES 1. V. Ponec and G.C. Bond (eds), Catalysis by Metals and Alloys, Stud. Surf. Sci Catal., Vol. 95, 1995. 2. B.K.Furlong, J.W. Hightower, T.Y.-L. Chan, A. Sarkany and L. Guczi, Appl. Catal. A l l 7 (1994) 41. 3. L. Guczi, Z. Schay, Gy. Stefler, L. F. Liotta, G. Deganello and A. M. Venezia, J. Catal. 182 (1999), 456. 4. A. M. Venezia, L. F. Liotta, G. Deganello, Z. Schay and L. Guczi, J. Catal. 182 (1999), 449. 5. A. Balerna, L. Liotta, A. Longo, A. Martorana, C. Meneghini, S. Mobilio and G. Pipitone, Eur. Phys. J. D 7 (1999) 89. 6. G.C. Bond and A.F. Rawle, J. Mol. Catal. A109 (1996) 261.

7. G. Deganello, L.F. Liotta, A. Longo, A. Martorana, Y. Yanev, N. Zotov, J. NonCryst. Solids 232 (1998) 547. 8. A. P. Hammersley, ESRF Internal Report, EXP/AH]95-01, 1995. 9. A. Filipponi and A. Di Cicco, Phys. Rev. B 52 (1995) 15135. 10. R. Burch and R.G. Buss, J. Chem. Soc. Faraday Trans. 71 (1979) 913.