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XAFS study of high-disperse Pd-containing nanosystem supported on TiO2 oxide matrix
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 575 (2007) 180–184 www.elsevier.com/locate/nima
XAFS study of high-disperse Pd-containing nanosystem supported on TiO2 oxide matrix V.V. Kriventsova,, B.N. Novgorodova, D.I. Kochubeya, O.V. Bukhtenkob, M.V. Tsodikovb, N.Yu. Kozitsynac, M.N. Vargaftikc, I.I. Moiseevc, G. Colond, M.C. Hidalgod, J.A. Naviod, S.G. Nikitenkoe a Boreskov Institute of Catalysis SB RAS, Novosibirsk, Russia Topchiev Institute of Petrochemical Synthesis RAS, Moscow, Russia c Kurnakov Institute of General and Inorganic Chemistry RAS, Moscow, Russia d Instituto de Ciencia de Materiales, Centro Mixto CSIC-Universidad, Sevilla, Spain e KU Leuven, DUBBLE-CRG & ESRF, Grenoble, France b
Available online 19 January 2007
Abstract Recently, great efforts are devoted to develop new methods of preparation of high-disperse Pd-containing nanosystems (composed of metal and/or oxide substance) stabilized on oxide matrix. New approach of synthesis is usage of PdCo(m-OOCMe)4(NCMe) complex and Ti(OBu)4 as precursors to anchor Pd on the surface of oxide matrix surface in a highly dispersed form. The present work is devoted to the structural study of this high-disperse Pd-containing nanosystem by the XAFS spectroscopy. The strong interaction between Pd and Co cations takes place in the studied system. Seemingly, Co cations incorporate into TiO2 oxide matrix, forming mixed oxides. This allows to anchor Pd cations, with the formation of the palladium oxide structures, modified by interaction with Co and Ti cations. These compounds are non-stochiometric and have typical structural features of mixed oxides. All possible structural models are discussed in detail. r 2007 Elsevier B.V. All rights reserved. PACS: 61.10.Ht; 82.65.Jv; 82.65.My; 82.70.Gg Keywords: Exafs; Nanoparticles; Modified oxide; Mixed oxide; Xanes
1. Introduction Recently in the field of directional formation of metalloxide material and catalysts, great attention is attracted to synthesis of mixed oxides, both bulk and supported mixed oxide. Promising approach to directional formation of metalloxide system are various modifications of the alkoxo-synthesis based on alkoxo-oxide and other metalorganic precursors [1,2]. These methods are characterized by a high sensitivity to the chemical nature of precursors, because their interaction already has taken place at the stage of sol formation in mother solutions which is followed by formation of a heterometallic Corresponding author. Tel: +7 383 339 40 13; fax: +7 383 330 80 56.
E-mail address: [email protected] (V.V. Kriventsov). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.01.062
roentgenamorphous gel [3]. As a result, under the thermo-treatment at low temperatures single phase oxides are formed. As rule these oxides are either solid solution or ionic structures as spinel or perovskite types. Application of heterometallic precursors in which both metals are included into a uniform-structure system is new development of alkoxo-method, that allowed to synthesize nanoclaster polynuclear oxide systems in which the active metal components remain at similar interatomic distances in the structure of single phase oxides. Thus a single phase YSZ ceramic, modified by Fe3+ and having a cubic structure, was prepared [4]. It was shown by Ichikava et al. [5] that use of the Rh–Co binuclear complex as a precursor yields polymetallic clasters on the surface of typical prevalent supports. In the last years, a special interest was devoted to the developing of new methods of
ARTICLE IN PRESS V.V. Kriventsov et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 180–184
preparation of high-disperse Pd–containing nanosystems, composed of metal and/or oxide substances, stabilized on an oxide matrix. These nanosystems are very promising for catalyst used in inner of fuel cell elements, as well as for catalysts applied in reactive reagents medium, namely for the hydrodehalogenation reaction. However, efforts for direct dispersing on support of the palladium complexes do not always yield in formation of highly dispersed nanosystems, because Pd atoms have a high ability of aggregation under calcination. For these reason, an alkoxo-method, based on bimetallic (heterometallic) palladium-containing precursors, is very perspective to produce model oxide systems containing highly dispersed Pd compounds. The carboxylated bimetallic (hetero–metallic) complex PdCo(m-OOCMe)4(NCMe) [6] (containing both Pd and Co cations in its structure) and Ti(OBun) were used as precursors. It is well-known that a number of bivalent and trivalent metals cations, during alkoxo-synthesis, have an ability to easily incorporate into the structure of TiO2 matrix, forming solid solutions [7,8]. The new approach to the synthesis using Pd–Co bimetallic complex as a precursor was following; Co cations form solid solution within structure of TiO2 matrix to anchor Pd on the surface in a highly dispersed form. However, only detail structural information about this nanosystem may provide correct direction of development of a preparation method to construct a highly active system possessing necessary catalytic properties.
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to obtain 0.5 jump at the absorption edges. XANES and EXAFS spectra were recorded with the steps of about 0.3 and 1.5 eV, respectively. Si(1 1 1) monochromator was used. The EXAFS spectra were treated using the standard procedures [9,10]. The radial distribution of the atoms (RDF) function was calculated from the EXAFS spectra in kw(k) as modulus of Fourier transform at the wave number interval of 3.0–12.5 A˚1. Curve fitting procedure with EXCURV92 code [11] was employed to determine the distances and coordination numbers. It was realized for kw(k) in similar wave number intervals after preliminary Fourier filtering using the known XRD data for bulk reference compounds. The values of Debye–Waller factors were fixed (0.005 A˚2). 3. Results and discussion The XANES spectra (Pd-K, Co-K edges) of the Pd–Co/ TiO2 samples studied and the Pd-foil and Co3O4 reference samples are presented in Figs. 1 and 2. Some propositions concerning the charge states of palladium and cobalt of the studied samples may be made by comparing the position of adsorption edges of the XANES spectra with those of the
Pd-K edge
a)
2. Experiment
b) Absorption (a. u.)
Preparation of catalysts samples: The bimetallic complex pdco(m-OOCMe)4(NCMe) [6] and Ti(OBun)4 were used as precursors for preparation of the samples of the Pd–Co/ Tio2 catalysts. All the samples of Pd–Co/TiO2 catalysts were prepared by co-precipitation of toluene solution of both precursors by adding aqueous EtOH, so that for hydrolysis, the amount of water contained was stoichiometric with respect to Ti(OBun)4. The obtained gel was dried at the room temperature and then treated by three different ways: (1) sample 1, air calcination at t ¼ 550 1C during 5 h; (2) sample 2, calcination in argon at t ¼ 550 1C during 5 h; (3) sample 3, SHF—treatment, calcination in argon at t ¼ 5501C during 2 h. It was established by elemental analysis that all organic remains were removed by calcination and all the samples studied contained 1 wt% Co and 2 wt% Pd. According to xrd data, it was found that TiO2 oxide matrix has mainly an anatase structure. XAFS method: All EXAFS and XANES spectra for the samples studied (Co-K, Pd-K edges) were recorded under transmission mode at Siberian Synchrotron Radiation Center (SSRC, Novosibirsk, Russia) and European Synchrotron Radiation Facility (DUBBLE-CRG&ESRF, Grenoble, France). The average stored current was 100 mA. For the EXAFS and XANES measurements, the samples were prepared as pellets with thickness varied
c)
d)
24250
24300
24350 24400 E (eV)
24450
24500
Fig. 1. XANES spectra (Pd-K kpa{) of the samples studied: (a) sample 1; (b) sample 2; (c) sample 3; (d) Pd foil, reference sample.
ARTICLE IN PRESS V.V. Kriventsov et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 180–184
182
Co-K edge
Co-O_
Co-K edge _Co-O-Me
a)
_Co-O-Me
a) *
c)
Magnitude of |FT| (a. u.)
Absorption (a. u.)
b)
d) b)
c)
d)
7650
7700
7750 E (eV)
7800
7850
Fig. 2. XANES spectra (Co-K kpa{) of the samples studied: (a) sample 1; (b) sample 2; (c) sample 3; (d) Co3O4, reference sample.
reference samples. In the case of increase of the effective positive charge of the cation, the adsorption edge position shifts to the direction of greater values of energy. The comparison of XANES spectra of Pd–K edges for the studied samples with that for the Pd foil gives the evidence that main part of palladium is present in an oxide form, seemingly as Pd(2+). For cobalt, a similar situation is observed. The cobalt exists in an oxide form, that follows from analysis of the adsorption edge (Co–K) position of the samples studied. These are slightly shifted to the direction of less values of energy in comparison with that for the reference Co3O4 sample, which contains two thirds of cobalt cations as Co(3+). Perhaps cobalt cations of the studied samples mainly exist as Co(2+). The RDF of atoms describing the Co and Pd local arrangements for the studied samples are presented in Figs. 3 and 4, respectively. From the point of view of the cobalt local arrangement, it should be noted that the RDF curves (Fig. 3.) of the samples studied are very similar. First of all, this fact clearly demonstrates that main strong interaction of the Pd–Co bimetallic complex precursor with
0.0
2.0
4.0
6.0
8.0
10.0
Distance (Å) Fig. 3. Radial distribution functions of atoms (RDF) describing of Co local arrangement for the samples studied: (a) Co3O4, reference sample; (b) sample 1; (c) sample 2; (d) sample 3.
titanium takes place during the co-precipitation and gel formation stage, because the following different treatments of the gel obtained do not influence on the local structure of resulting cobalt compounds. The structural data, distances (R) and effective coordination numbers (CN) for EXAFS method, are determined by spectra simulation (fitting) according to the hypothesis about chemical nature of neighboring atoms. It should be noted that the first peak (Fig. 3) may correspond to the Co–O distance only. The fitting gives the following results: RCoO ¼ 1.96 A˚, CN ¼ 3.9–4.1. According to crystallographic data, both Co–O distances, in the case of Co(2+), for which a tetrahedral arrangement is more typical and in the case of Co(3+), for which an octahedral arrangement is more typical, have a similar value 2 A˚. As a result of the above-mentioned proposition about strong interaction of Pd–Co complex precursors with titanium, it should be expected that cobalt has an octahedral arrangement, but the obtained effective coordination number (4) is lower. This fact may be explained by
ARTICLE IN PRESS V.V. Kriventsov et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 180–184
Pd-K edge Pd-O_
Magnitude of |FT| (a. u.)
_Pd-Pd
*
a)
_Pd-O-Me
b)
c)
d) 0.0
2.0
4.0
6.0
8.0
10.0
Distance (Å) Fig. 4. Radial distribution functions of atoms (RDF) describing of Pd local arrangement for the samples studied: (a) Pd-foil, reference sample; (b) sample 1; (c) sample 2; (d) sample 3.
oxygen octahedron extending, with RCoO distortions up to 0.08 A˚. These distortions of distances results in lowering of the effective coordination numbers obtained by fitting within EXAFS method limitations, and this lowering is correlated with the values of RCoO distortions. Of course, we can exclude the some contribution of the oxygen tetrahedral arrangement as well. The next peaks (Fig. 3) correspond to the Co–Me (Me ¼ Co, Ti, Pd) distances of the cobalt cation sublattice. It is well-known that cobalt may form compounds, up to stochiometric composition, with both titanium and palladium, for examples: CoTiO3 [12] and CoPdO2 [13]. Both possible structural models are used for fitting. It was found that cobalt is mainly Co(2+) and both Co–Co distances and/or Co–Ti, Co–Pd distances are observed for the cobalt local arrangement. It is impossible to unambiguously determine the type of neighboring atoms owing to very small amplitude contribution of the corresponding peaks (Fig. 3). However, the best fitting is achieved in the case of attributing of the peaks observed to the structural model
183
assuming three Co–Me distances (2.85, 3.32, 3.76 A˚) and the corresponding coordination numbers—(0.2, 0.6, 0.3), respectively. By analysis of structural data it was found that the third distance corresponds more to Co–Ti type, but the value 3.76 A˚ is close to the Ti–Ti distance of the anatase structure. This fact gives additional evidence of a strong interaction of Pd–Co complex precursors with titanium of the oxide matrix. The first and second distances may be attributed to the Co–Me (Co, Ti, Pd) distances and/or some distance combinations. But it should be noted that distances are close to those for CoTiO3 (RCoCo ¼ ˚ RCoTi ¼ 3:39 A; ˚ RCoTi ¼ 3:74 A) ˚ 2:99 A; and CoPdO2 ˚ ˚ (RCoCo ¼ 2:83 A; RCoPd ¼ 3:38 A). Some lowering of the coordination numbers, in comparison with those for stochiometric compounds, is explained by small particle sizes and great distortions due to a strong interaction with the TiO2 matrix. It should be noted that the structure of local Pd arrangement (as seen from analysis of RDF curves Fig. 4.) demonstrates the similar trend as that for cobalt. Also, as in the case of cobalt, the first intensive peak corresponds to the Pd–O distance and all next peaks may be attributed to the Pd–Me distances. According to literature data concerning the palladium oxide compounds, Pd(2+) cation is typically surrounded by four oxygen anions with RPdO1.98–06 A˚. The fitting gives results: RPdO ¼ 2.0 A˚, CN ¼ 3.6–3.8. Since the coordination number, obtained by fitting, is very close to 4 within the method limitation 10%, that is an additional argument in favour of the above-mentioned conclusion from XANES data about a complete oxidation of palladium. The next second (low) and the third (intensive) peaks (Fig. 4.) are attributed to the distance from palladium to the cations. Seemingly, the second low peak more likely corresponds to Pd–Co distances (2.8 A˚) [13] or Pd–Pd distances 2.7 A˚ (non-typical for PdO) [14]. According to fitting, one may conclude that the main contribution to the third peak belongs to Pd–Pd distances. In this case the fitting gives the following results: RPdPd ¼ 3.0 A˚, N ¼ 0.9–1.0; RPdPd ¼ 3.4 A˚, N ¼ 4.8–5.1. But this does not exclude some possible contribution to this peaks from Pd–Co distance 3.4– 3.6 A˚ at all. So, it may be proposed that palladium compounds exist in two phases, formed as a result of interaction with cobalt and titanium cations. Perhaps a small amount of the PdCoO2 phase, stabilized by titanium oxide, is also present in which palladium is surrounded by Pd and Co cations with the distances 2.83 and 3.38 A˚, respectively. But the main part of palladium compound exists as palladium oxide, also modified by interaction with titanium oxide. 4. Conclusion The strong interaction between Pd and Co cations takes place in the studied system using Ti(OBun)4 and Pd–Co bimetallic complex as precursors. Seemingly, Co cations
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incorporate into TiO2 oxide matrix, forming mixed oxides. This allows to anchor Pd cations, with the formation of the palladium oxide structures, modified by interaction with Co and Ti cations. These compounds are non-stochiometric and have typical structural features of mixed oxides. Acknowledgments This research was supported by RFBR (06-03-33005), RFBR (05-03-32683), RFBR (06-03-32578), RFBR (06-0308173), RFBR (05-03-32577) and NATO (Ref. EST.CLG 979855) Grants. References [1] N.Y.a. Turova, E.P. Turevskaya, V.G. Kessler, M.I. Yanovskaya, in: The Chemistry of Metal Alkoxides, vol. 34, Kluwer Academic Publishers, Dordrecht, 2001, p. 250. [2] Y.u.V. Maksimov, I.P. Suzdalev, M.V. Tsodikov, V. Ya. Kugel, O.V. Bukhtenko, E.V. Slivinskii, J.A. Navio, J. Mol. Catal. A. Chem. 105 (1996) 167. [3] M.V. Tsodikov, O.G. Ellert, O.V. Bukhtenko, D.I. Kochubey, V.V. Shcherbakov, Mater. Sci. 30 (1995) 1087.
[4] O.G. Ellert, I.A. Petrunenko, M.V. Tsodikov, O.V. Bukhtenko, D.I. Kochubey, Y.u.V. Maksimov, Dominges–Rodrigez, E.L. Cumbrera, J.A. Navio, J. Sol-Gel Sci. Technol. 8 (1997) 213. [5] M. Ichikava, J. Catal. 56 (1979) 127. [6] N.Y.u. Kozitsyna, S.E. Nefedov, N.V. Cherkashina, V.N. Ikorskii, M.N. Vargaftik, I.I. Moiseev, Russian Chem. Bull. 9 (2005) 2215. [7] M.V. Tsodikov, Y.e.A. Trusova, Y.e.V. Slivinskii, G.G. Hernandez, D.I. Kochubey, V.G. Lipovich, J.A. Navio, Stud. Surf. Catal. 118 (1998) 679. [8] M.V. Tsodikov, Y.e.A. Trusova, O.V. Bukhtenko, G.G. Hernandez, B.B. Kriventsov, V.V. Kalinkin, M.S. Hidalgo, J.A. Navio, EuropaCat V, EFCATS-Europen Federation of Catalysts Societies—Irish Catalysis Society 2–7 September 2001, Abstract Book 3, Enviromental Catalysis 2, 8-P-52. [9] D.I. Kochubey, EXAFS Spectroscopy of Catalysts, Nauka, Novosibirsk, 1992. [10] K.V. Klementiev, Code VIPER for Windows, freeware: /http:// www.desy.de/klmn/viper.htmlS; K.V. Klementev, J. Phys. D: Appl. Phys. 34 (2001) 209. [11] N. Binsted, J.V. Campbell, S.J. Gurman, P.C. Stephenson, SERC Daresbury Laboratory EXCURV92 program 1991. [12] K. Kidoh, K. Tanaka, F. Marumo, H. Takei, Acta Crystallogr. 40 (1984) 92. [13] C.T. Prewitt, R.D. Shannon, D.B. Rogers, Inorg. Chem. 10 (1972) 719. [14] H.-J. Meyer, H.K. Mu¨ller-Buschbaum, Z. Naturforsch. Teil B. Anorg. Chem. Org. Chem. 34 (1979) 1661.
Nuclear Instruments and Methods in Physics Research A 575 (2007) 180–184 www.elsevier.com/locate/nima
XAFS study of high-disperse Pd-containing nanosystem supported on TiO2 oxide matrix V.V. Kriventsova,, B.N. Novgorodova, D.I. Kochubeya, O.V. Bukhtenkob, M.V. Tsodikovb, N.Yu. Kozitsynac, M.N. Vargaftikc, I.I. Moiseevc, G. Colond, M.C. Hidalgod, J.A. Naviod, S.G. Nikitenkoe a Boreskov Institute of Catalysis SB RAS, Novosibirsk, Russia Topchiev Institute of Petrochemical Synthesis RAS, Moscow, Russia c Kurnakov Institute of General and Inorganic Chemistry RAS, Moscow, Russia d Instituto de Ciencia de Materiales, Centro Mixto CSIC-Universidad, Sevilla, Spain e KU Leuven, DUBBLE-CRG & ESRF, Grenoble, France b
Available online 19 January 2007
Abstract Recently, great efforts are devoted to develop new methods of preparation of high-disperse Pd-containing nanosystems (composed of metal and/or oxide substance) stabilized on oxide matrix. New approach of synthesis is usage of PdCo(m-OOCMe)4(NCMe) complex and Ti(OBu)4 as precursors to anchor Pd on the surface of oxide matrix surface in a highly dispersed form. The present work is devoted to the structural study of this high-disperse Pd-containing nanosystem by the XAFS spectroscopy. The strong interaction between Pd and Co cations takes place in the studied system. Seemingly, Co cations incorporate into TiO2 oxide matrix, forming mixed oxides. This allows to anchor Pd cations, with the formation of the palladium oxide structures, modified by interaction with Co and Ti cations. These compounds are non-stochiometric and have typical structural features of mixed oxides. All possible structural models are discussed in detail. r 2007 Elsevier B.V. All rights reserved. PACS: 61.10.Ht; 82.65.Jv; 82.65.My; 82.70.Gg Keywords: Exafs; Nanoparticles; Modified oxide; Mixed oxide; Xanes
1. Introduction Recently in the field of directional formation of metalloxide material and catalysts, great attention is attracted to synthesis of mixed oxides, both bulk and supported mixed oxide. Promising approach to directional formation of metalloxide system are various modifications of the alkoxo-synthesis based on alkoxo-oxide and other metalorganic precursors [1,2]. These methods are characterized by a high sensitivity to the chemical nature of precursors, because their interaction already has taken place at the stage of sol formation in mother solutions which is followed by formation of a heterometallic Corresponding author. Tel: +7 383 339 40 13; fax: +7 383 330 80 56.
E-mail address: [email protected] (V.V. Kriventsov). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.01.062
roentgenamorphous gel [3]. As a result, under the thermo-treatment at low temperatures single phase oxides are formed. As rule these oxides are either solid solution or ionic structures as spinel or perovskite types. Application of heterometallic precursors in which both metals are included into a uniform-structure system is new development of alkoxo-method, that allowed to synthesize nanoclaster polynuclear oxide systems in which the active metal components remain at similar interatomic distances in the structure of single phase oxides. Thus a single phase YSZ ceramic, modified by Fe3+ and having a cubic structure, was prepared [4]. It was shown by Ichikava et al. [5] that use of the Rh–Co binuclear complex as a precursor yields polymetallic clasters on the surface of typical prevalent supports. In the last years, a special interest was devoted to the developing of new methods of
ARTICLE IN PRESS V.V. Kriventsov et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 180–184
preparation of high-disperse Pd–containing nanosystems, composed of metal and/or oxide substances, stabilized on an oxide matrix. These nanosystems are very promising for catalyst used in inner of fuel cell elements, as well as for catalysts applied in reactive reagents medium, namely for the hydrodehalogenation reaction. However, efforts for direct dispersing on support of the palladium complexes do not always yield in formation of highly dispersed nanosystems, because Pd atoms have a high ability of aggregation under calcination. For these reason, an alkoxo-method, based on bimetallic (heterometallic) palladium-containing precursors, is very perspective to produce model oxide systems containing highly dispersed Pd compounds. The carboxylated bimetallic (hetero–metallic) complex PdCo(m-OOCMe)4(NCMe) [6] (containing both Pd and Co cations in its structure) and Ti(OBun) were used as precursors. It is well-known that a number of bivalent and trivalent metals cations, during alkoxo-synthesis, have an ability to easily incorporate into the structure of TiO2 matrix, forming solid solutions [7,8]. The new approach to the synthesis using Pd–Co bimetallic complex as a precursor was following; Co cations form solid solution within structure of TiO2 matrix to anchor Pd on the surface in a highly dispersed form. However, only detail structural information about this nanosystem may provide correct direction of development of a preparation method to construct a highly active system possessing necessary catalytic properties.
181
to obtain 0.5 jump at the absorption edges. XANES and EXAFS spectra were recorded with the steps of about 0.3 and 1.5 eV, respectively. Si(1 1 1) monochromator was used. The EXAFS spectra were treated using the standard procedures [9,10]. The radial distribution of the atoms (RDF) function was calculated from the EXAFS spectra in kw(k) as modulus of Fourier transform at the wave number interval of 3.0–12.5 A˚1. Curve fitting procedure with EXCURV92 code [11] was employed to determine the distances and coordination numbers. It was realized for kw(k) in similar wave number intervals after preliminary Fourier filtering using the known XRD data for bulk reference compounds. The values of Debye–Waller factors were fixed (0.005 A˚2). 3. Results and discussion The XANES spectra (Pd-K, Co-K edges) of the Pd–Co/ TiO2 samples studied and the Pd-foil and Co3O4 reference samples are presented in Figs. 1 and 2. Some propositions concerning the charge states of palladium and cobalt of the studied samples may be made by comparing the position of adsorption edges of the XANES spectra with those of the
Pd-K edge
a)
2. Experiment
b) Absorption (a. u.)
Preparation of catalysts samples: The bimetallic complex pdco(m-OOCMe)4(NCMe) [6] and Ti(OBun)4 were used as precursors for preparation of the samples of the Pd–Co/ Tio2 catalysts. All the samples of Pd–Co/TiO2 catalysts were prepared by co-precipitation of toluene solution of both precursors by adding aqueous EtOH, so that for hydrolysis, the amount of water contained was stoichiometric with respect to Ti(OBun)4. The obtained gel was dried at the room temperature and then treated by three different ways: (1) sample 1, air calcination at t ¼ 550 1C during 5 h; (2) sample 2, calcination in argon at t ¼ 550 1C during 5 h; (3) sample 3, SHF—treatment, calcination in argon at t ¼ 5501C during 2 h. It was established by elemental analysis that all organic remains were removed by calcination and all the samples studied contained 1 wt% Co and 2 wt% Pd. According to xrd data, it was found that TiO2 oxide matrix has mainly an anatase structure. XAFS method: All EXAFS and XANES spectra for the samples studied (Co-K, Pd-K edges) were recorded under transmission mode at Siberian Synchrotron Radiation Center (SSRC, Novosibirsk, Russia) and European Synchrotron Radiation Facility (DUBBLE-CRG&ESRF, Grenoble, France). The average stored current was 100 mA. For the EXAFS and XANES measurements, the samples were prepared as pellets with thickness varied
c)
d)
24250
24300
24350 24400 E (eV)
24450
24500
Fig. 1. XANES spectra (Pd-K kpa{) of the samples studied: (a) sample 1; (b) sample 2; (c) sample 3; (d) Pd foil, reference sample.
ARTICLE IN PRESS V.V. Kriventsov et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 180–184
182
Co-K edge
Co-O_
Co-K edge _Co-O-Me
a)
_Co-O-Me
a) *
c)
Magnitude of |FT| (a. u.)
Absorption (a. u.)
b)
d) b)
c)
d)
7650
7700
7750 E (eV)
7800
7850
Fig. 2. XANES spectra (Co-K kpa{) of the samples studied: (a) sample 1; (b) sample 2; (c) sample 3; (d) Co3O4, reference sample.
reference samples. In the case of increase of the effective positive charge of the cation, the adsorption edge position shifts to the direction of greater values of energy. The comparison of XANES spectra of Pd–K edges for the studied samples with that for the Pd foil gives the evidence that main part of palladium is present in an oxide form, seemingly as Pd(2+). For cobalt, a similar situation is observed. The cobalt exists in an oxide form, that follows from analysis of the adsorption edge (Co–K) position of the samples studied. These are slightly shifted to the direction of less values of energy in comparison with that for the reference Co3O4 sample, which contains two thirds of cobalt cations as Co(3+). Perhaps cobalt cations of the studied samples mainly exist as Co(2+). The RDF of atoms describing the Co and Pd local arrangements for the studied samples are presented in Figs. 3 and 4, respectively. From the point of view of the cobalt local arrangement, it should be noted that the RDF curves (Fig. 3.) of the samples studied are very similar. First of all, this fact clearly demonstrates that main strong interaction of the Pd–Co bimetallic complex precursor with
0.0
2.0
4.0
6.0
8.0
10.0
Distance (Å) Fig. 3. Radial distribution functions of atoms (RDF) describing of Co local arrangement for the samples studied: (a) Co3O4, reference sample; (b) sample 1; (c) sample 2; (d) sample 3.
titanium takes place during the co-precipitation and gel formation stage, because the following different treatments of the gel obtained do not influence on the local structure of resulting cobalt compounds. The structural data, distances (R) and effective coordination numbers (CN) for EXAFS method, are determined by spectra simulation (fitting) according to the hypothesis about chemical nature of neighboring atoms. It should be noted that the first peak (Fig. 3) may correspond to the Co–O distance only. The fitting gives the following results: RCoO ¼ 1.96 A˚, CN ¼ 3.9–4.1. According to crystallographic data, both Co–O distances, in the case of Co(2+), for which a tetrahedral arrangement is more typical and in the case of Co(3+), for which an octahedral arrangement is more typical, have a similar value 2 A˚. As a result of the above-mentioned proposition about strong interaction of Pd–Co complex precursors with titanium, it should be expected that cobalt has an octahedral arrangement, but the obtained effective coordination number (4) is lower. This fact may be explained by
ARTICLE IN PRESS V.V. Kriventsov et al. / Nuclear Instruments and Methods in Physics Research A 575 (2007) 180–184
Pd-K edge Pd-O_
Magnitude of |FT| (a. u.)
_Pd-Pd
*
a)
_Pd-O-Me
b)
c)
d) 0.0
2.0
4.0
6.0
8.0
10.0
Distance (Å) Fig. 4. Radial distribution functions of atoms (RDF) describing of Pd local arrangement for the samples studied: (a) Pd-foil, reference sample; (b) sample 1; (c) sample 2; (d) sample 3.
oxygen octahedron extending, with RCoO distortions up to 0.08 A˚. These distortions of distances results in lowering of the effective coordination numbers obtained by fitting within EXAFS method limitations, and this lowering is correlated with the values of RCoO distortions. Of course, we can exclude the some contribution of the oxygen tetrahedral arrangement as well. The next peaks (Fig. 3) correspond to the Co–Me (Me ¼ Co, Ti, Pd) distances of the cobalt cation sublattice. It is well-known that cobalt may form compounds, up to stochiometric composition, with both titanium and palladium, for examples: CoTiO3 [12] and CoPdO2 [13]. Both possible structural models are used for fitting. It was found that cobalt is mainly Co(2+) and both Co–Co distances and/or Co–Ti, Co–Pd distances are observed for the cobalt local arrangement. It is impossible to unambiguously determine the type of neighboring atoms owing to very small amplitude contribution of the corresponding peaks (Fig. 3). However, the best fitting is achieved in the case of attributing of the peaks observed to the structural model
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assuming three Co–Me distances (2.85, 3.32, 3.76 A˚) and the corresponding coordination numbers—(0.2, 0.6, 0.3), respectively. By analysis of structural data it was found that the third distance corresponds more to Co–Ti type, but the value 3.76 A˚ is close to the Ti–Ti distance of the anatase structure. This fact gives additional evidence of a strong interaction of Pd–Co complex precursors with titanium of the oxide matrix. The first and second distances may be attributed to the Co–Me (Co, Ti, Pd) distances and/or some distance combinations. But it should be noted that distances are close to those for CoTiO3 (RCoCo ¼ ˚ RCoTi ¼ 3:39 A; ˚ RCoTi ¼ 3:74 A) ˚ 2:99 A; and CoPdO2 ˚ ˚ (RCoCo ¼ 2:83 A; RCoPd ¼ 3:38 A). Some lowering of the coordination numbers, in comparison with those for stochiometric compounds, is explained by small particle sizes and great distortions due to a strong interaction with the TiO2 matrix. It should be noted that the structure of local Pd arrangement (as seen from analysis of RDF curves Fig. 4.) demonstrates the similar trend as that for cobalt. Also, as in the case of cobalt, the first intensive peak corresponds to the Pd–O distance and all next peaks may be attributed to the Pd–Me distances. According to literature data concerning the palladium oxide compounds, Pd(2+) cation is typically surrounded by four oxygen anions with RPdO1.98–06 A˚. The fitting gives results: RPdO ¼ 2.0 A˚, CN ¼ 3.6–3.8. Since the coordination number, obtained by fitting, is very close to 4 within the method limitation 10%, that is an additional argument in favour of the above-mentioned conclusion from XANES data about a complete oxidation of palladium. The next second (low) and the third (intensive) peaks (Fig. 4.) are attributed to the distance from palladium to the cations. Seemingly, the second low peak more likely corresponds to Pd–Co distances (2.8 A˚) [13] or Pd–Pd distances 2.7 A˚ (non-typical for PdO) [14]. According to fitting, one may conclude that the main contribution to the third peak belongs to Pd–Pd distances. In this case the fitting gives the following results: RPdPd ¼ 3.0 A˚, N ¼ 0.9–1.0; RPdPd ¼ 3.4 A˚, N ¼ 4.8–5.1. But this does not exclude some possible contribution to this peaks from Pd–Co distance 3.4– 3.6 A˚ at all. So, it may be proposed that palladium compounds exist in two phases, formed as a result of interaction with cobalt and titanium cations. Perhaps a small amount of the PdCoO2 phase, stabilized by titanium oxide, is also present in which palladium is surrounded by Pd and Co cations with the distances 2.83 and 3.38 A˚, respectively. But the main part of palladium compound exists as palladium oxide, also modified by interaction with titanium oxide. 4. Conclusion The strong interaction between Pd and Co cations takes place in the studied system using Ti(OBun)4 and Pd–Co bimetallic complex as precursors. Seemingly, Co cations
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incorporate into TiO2 oxide matrix, forming mixed oxides. This allows to anchor Pd cations, with the formation of the palladium oxide structures, modified by interaction with Co and Ti cations. These compounds are non-stochiometric and have typical structural features of mixed oxides. Acknowledgments This research was supported by RFBR (06-03-33005), RFBR (05-03-32683), RFBR (06-03-32578), RFBR (06-0308173), RFBR (05-03-32577) and NATO (Ref. EST.CLG 979855) Grants. References [1] N.Y.a. Turova, E.P. Turevskaya, V.G. Kessler, M.I. Yanovskaya, in: The Chemistry of Metal Alkoxides, vol. 34, Kluwer Academic Publishers, Dordrecht, 2001, p. 250. [2] Y.u.V. Maksimov, I.P. Suzdalev, M.V. Tsodikov, V. Ya. Kugel, O.V. Bukhtenko, E.V. Slivinskii, J.A. Navio, J. Mol. Catal. A. Chem. 105 (1996) 167. [3] M.V. Tsodikov, O.G. Ellert, O.V. Bukhtenko, D.I. Kochubey, V.V. Shcherbakov, Mater. Sci. 30 (1995) 1087.
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