Structural determination of palladous oxide–ceria nanosystem supported on γ-alumina

ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 603 (2009) 178–181

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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

Structural determination of palladous oxide–ceria nanosystem supported on g-alumina I.E. Beck a,, V.V. Kriventsov a, B.N. Novgorodov a, E.P. Yakimchuk a, D.I. Kochubey a, V.I. Zaikovsky a, I.Yu. Pakharukov a, N.Yu. Kozitsyna b, M.N. Vargaftik b, V.I. Bukhtiyarov a a b

Boreskov Institute of Catalysis SB RAS, Pr. Lavrentieva, 5, Novosibirsk 630090, Russia Kurnakov Institute of General and Inorganic Chemistry RAS, Moscow, Russia

a r t i c l e in f o

a b s t r a c t

Available online 9 March 2009

Recently great efforts are being devoted to develop new methods of preparation of high-disperse Pd–CeO2 containing nanosystems stabilized on an oxide matrix. A new approach of synthesis consists in using the heterometallic PdII2CeIV 2 (m-OOCMe)12(H2O)2 complex as a precursor to anchor Pd nanoparticles on the surface of g-alumina in direct contact with CeO2. The present work is devoted to a structural study of this disperse Pd–CeO2 containing nanosystem after oxidative or reductive pretreatments in comparison with monometallic alumina-supported samples by XAFS and TEM. A strong interaction between Pd and ceria in the catalyst produced in the studied system affects reducibility of both PdO and CeO2, which in turn results in an increased low-temperature activity in CO oxidation along with a dramatic change of the ignition–extinction curve. & 2009 Elsevier B.V. All rights reserved.

Keywords: EXAFS XANES TPR Nanoparticles Catalysts CO oxidation

1. Introduction Pd is the main metal used in current TWCs due to its excellent capability of a low-temperature oxidation of CO and hydrocarbons compared with the Pt- and Rh-supported catalysts [1,2]. Recently, ceria-doped Pd catalysts attracted much great interest in various complete oxidation processes connected with environmental catalysis [3,4] as well as in several hydrogenation processes, primarily in syngas conversion [5–7]. The inclusion of ceria into such oxidation systems increases oxygen mobility and provides oxygen storage capacity, which broaden the conversion efficiency for all pollutants (NOx, CO and hydrocarbons) during rich/lean oscillations associated with the feedback control regulating the air-to-fuel ratio used by the engine. Additionally, ceria favors noble metal dispersion, stabilizes the size and structure of Pdactive centers, increases thermal stability, and promotes water gas shift, steam reforming and CO oxidation reactions [8–11]. Ferna´ndez-Garcı´a et al. [12] investigated the CO/O2 reaction over Pd supported on CeO2 and Al2O3 and found that ceria facilitated the activation of both CO and oxygen. Unfortunately, bulk CeO2 does not seem the best carrier for dispersed Pd because of its rather low specific surface area. Hence most attention is devoted today to ceria-promoted Pd catalysts on highly porous refractory carriers such as g-Al2O3. The main problem in this case is to ensure the contact of the whole Pd with ceria in the case of low  Corresponding author. Tel.: +7 383 326 94 60; fax: +7 383 330 80 56.

E-mail address: [email protected] (I.E. Beck). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.03.006

palladium loading and a rather low ceria content. Application of a heterometallic Pd–Ce precursor in which both metals are included into a polynuclear acetate complex allowed one to support the nanosized bimetallic Pd–Ce oxide system on the g-Al2O3 surface. This method ensures both active metal components, PdO and ceria, to remain in a direct contact on the g-Al2O3 surface, because their interaction already has taken place in the crystalline precursor [13] at the stage of impregnating solutions followed by UV–vis. As a result, under the thermo-treatment at 400 1C, mixed oxide Pd–Ce nanoparticles are formed. The present work is devoted to a structural study of the gAl2O3-supported palladous–ceria nanosystem by XAFS and TEM.

2. Experiment All supported catalysts were prepared by incipient wetness impregnation of g-Al2O3 (Sasol TKA-432) followed by air calcination at 400 1C. A tetranuclear heterodimetallic acetate-bridged complex PdII2CeIV 2 (m-OOCMe)12(H2O)2 [13] was used as a precursor for the bimetallic PdCe/Al2O3 catalyst with 3–4 nm particles of the active component. A trinuclear Pd acetate and Ce(IV) nitrate were precursors for reference monometallic Pd/Al2O3 and Ce/Al2O3 samples while co-impregnation by equimolar quantities of Pd(II) and Ce(IV) acetates gives the reference Pd+Ce/Al2O3 sample with only a part of Pd interacting with ceria. All the samples contained 1 wt% Pd and/or 1.5 wt% Ce. The catalytic activity was measured for 0.35 g catalyst samples using a conventional

ARTICLE IN PRESS I.E. Beck et al. / Nuclear Instruments and Methods in Physics Research A 603 (2009) 178–181

Pd-K

a)

∗

b)

Normalized absorption (a.u.)

flow-circulation reactor at gas flow rate 240 ml/min and temperature ramp 2 1C/min. The initial reactant mixture was 1% CO and 20.8% O2 in N2. The products were analyzed by gas chromatography. TPR for 0.1–0.5 g catalyst samples was carried out in the 10% vol. H2 in Ar mixture (40 ml min1) from 40 1C up to the desired temperature (400 or 900 1C). The outlet H2 concentration was measured by a catharometer with an accuracy of 75%. The peak areas were corrected for H2 adsorbed on the reduced Pd. All TEM measurements were performed on a JEM2010 instrument at 200 kV at a resolution of 0.14 nm. Size distribution was determined by measuring 200–400 particles for each sample. EXAFS and XANES spectra of the Pd–K edges for all the samples studied were recorded (transmission) and treated by the standard procedure [14,15] at SSRC, Novosibirsk. The radial distribution function (RDF) was calculated from the EXAFS spectra in k3w(k) as a modulus of Fourier transform at wave number intervals 3.5–13.5 A˚1. Curve fitting with EXCURV92 [16] was used to determine the distances (R) and coordination numbers (CN) using known XRD data.

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c) d)

e)

f)

g)

3. Results and discussion

24250 By analysis of TEM data, it was established that the heterodimetallic precursor ensures a homogenous narrow size distribution of bimetallic nanoparticles with particle sizes of about 2–4 nm throughout the g-Al2O3 surface. However, according to the HRTEM data, the structure of Pd–Ce nanoparticles (Fig. 1a) was mostly shell-like, with the interplanar spacing observed being d ¼ 2.2245 A˚ for Pd0 core (Fm3m) and d ¼ 3.124 A˚ for CeO2 shell (Fm3m). Some Pd0 particles were also observed on the alumina surface depleted in ceria (Fig. 1b), with ceria presence detectable only by EDX microanalysis. We failed to reveal by EDX microanalysis any pure-phase PdO or CeO2 particles on the catalyst surface, but the Pd/Ce ratio varied for different particles on the gAl2O3 surface. On the other hand, by analysis of the XAFS (Pd–K) spectra, the palladium local structure as Pd2+ in oxygen surroundings was determined. The XANES spectra (Pd–K) of the mono-Pd/Al2O3 and bimetallic Pd–Ce/Al2O3 samples and references studied are presented in Fig. 2. The XANES spectra (Pd–K) of samples a and c after oxidative pretreatment (Fig. 2) were of the same type and similar to the PdO reference, with minor differences in the position of the adsorption edges and (1s–4p) maximum, as well as in the shape of spectra and their amplitudes. Obviously, the main part of palladium in these samples is present in an oxidized form, namely as Pd(2+). However in the XANES spectra of samples d and b after reductive pretreatments (Fig. 2) noticeable differences were observed both from Pd foil and PdO reference, suggesting that simultaneous presence of both the oxidic and metallic phases was very probable, but in a ratio different for the two samples studied. So, the analysis of the XANES spectra indicated the

Fig. 1. HR TEM micrographs for two main species of supported Pd–Ce nanoparticles, major species with 43% Pd and 57% Ce (a) and minor species containing 87% Pd and 13% Ce (b).

24300

24350

24400

24450

E (eV) Fig. 2. XANES spectra (Pd–K) of samples: (a) PdCe/Al2O3; (b) sample a after TPR up to 400 1C; (c) Pd/Al2O3; (d) sample c after TPR up to 400 1C; (e) bulk PdO; (f) Pd foil and (g) simulated PdO/Pd0 ¼ 1/1.

content of oxidic phase in the bimetallic Pd–Ce sample b to be minor, in contrast to the monometallic Pd sample d containing comparable amounts of oxidic and metallic phases. It should be noted that similar results were obtained for the Pd–Co/TiO2 nanosystem prepared from the heterodimetallic acetate complex [17]. To define the phase composition of the active component more precisely, the XANES spectra were modeled based on different ratios of reference compounds, Pd foil and PdO. Spectrum modeling for the Pd/PdO ratio 1/1 is shown in Fig. 2(g). It should be noted that this simulated spectrum was nearly identical to the sample d spectrum (Fig. 2); therefore, the ratio of the metallic and oxidic phases in sample d was about 50% per 50%. A rough estimate of phase composition for the pre-reduced Pd–Ce sample b gives 20% of oxidized palladium at most. The RDFs describing the Pd local arrangements for the studied samples and references are presented in Fig. 3. Several peaks were observed on the RDF curve of the reference PdO sample (Fig. 3g). The first intensive peak corresponds to the Pd–O distance (RPdO2.0 A˚, CN ¼ 4) [18], the next very intensive broad peak may be attributed to several Pd–(O)–Pd distances (RPdPd3.0–4.2 A˚). The contribution of several distances in this peak is additionally confirmed by the shoulder in short distances, located between the peaks maxima for the reference samples: Pd–Pd for Pd foil and Pd–(O)–Pd for PdO (marked by a dotted line in Fig. 3). The same peaks were observed on the RDFs for the bimetallic Pd–Ce sample (Fig. 3a) and monometallic Pd sample (Fig. 3d) after oxidative pretreatments. No metallic palladium was detected in these samples (within the method limitation). It should be noted that the positions and intensities of the first peak (Pd–O) for both the samples above are rather similar to the reference PdO, but the amplitudes and shape of the second peak differed more drastically. So, in the case of the Pd–Ce sample a, the second peak is less intensive in comparison with the PdO reference and the above-mentioned shoulder begins to transform into a separate peak. However, for the Pd sample c, two separate peaks with small intensities were well distinguished on the RDF curve. From the analysis of the trend revealed for the samples under study, one

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_Pd-Pd

Pd-O_

Pd-K

_Pd-O-Pd

Magnitude of |FT| (a.u.)

a)

b)

*

c)

d)

e)

f)

0.0

2.0

4.0

6.0

8.0

10.0

R - δ, ( Å ) Fig. 3. RDFs describing Pd local arrangement for samples: (a) PdCe/Al2O3; (b) sample a after TPR up to 400 1C; (c) Pd/Al2O3; (d) sample c after TPR up to 400 1C; (e) bulk PdO and (f) Pd foil (dots).

can suppose only minor variations in the anionic sub-lattice but drastic changes in the cationic sub-lattice. These changes could be due to the difference in particle sizes of the active component and/ or to some disorder in the structures of the supported oxide particles. Two main peaks were observed on the RDFs for the bimetallic Pd–Ce sample (Fig. 3b) and monometallic Pd sample (Fig. 3d) after reductive pretreatments. The first peak is attributed to Pd–O distance as the similar peak in the reference PdO. It should be mentioned that no peaks were observed in the distance range 3.0–4.2 A˚ corresponding to the Pd–(O)–Pd distances in the bulk PdO reference. However, the position of the second peak observed fits a direct Pd–Pd bond as in the Pd foil (RPdPd2.72–2.78 A˚, CN ¼ 12) [18]. It should be noted that the intensities and ratio of these two peaks differ for samples b and d. Thus after prereduction, in both catalyst samples the mixed Pd–PdO systems seem to be formed differing in the phase ratio, but the oxidic phase in these samples was rather disordered and/or (most likely) a surface oxidic layer was formed due to partial reoxidation of the pre-reduced catalyst samples during storage at room temperature. A rough estimate of phase composition based on analysis of peak intensities in comparison with the RDFs for the reference Pd0 and PdO samples gives at most 20% of highly dispersed oxidic palladium for the pre-reduced Pd–Ce sample b and about 50% for the monometallic sample d while the metallic Pd particles seem to be rather small (p30 A˚). These estimates from the XANES and EXAFS data agree well with each other. So, the palladium local structure was Pd2+ in oxygen surroundings for the fresh catalysts independent of the presence of Ce and mixed Pd0–PdO phase for reduced samples differing in Pd0/PdO ratio. This mixed phase is suggested to be formed through partial reoxidation of the reduced palladium by air, resulting in a metallic palladium core with a surface PdO layer because of the absence of the second Pd–(O)–Pd spacing from the bulk PdO in reduced samples, although the first Pd–O coordination sphere is seen on the RDFs. To adjust TEM data with XAFS, the TPR of the intermetallic sample was fulfilled up to different temperatures, with the partially reduced samples being studied by XAFS as well. The first TPR experiments were performed with fresh monometallic Pd

and bimetallic Pd–Ce catalysts calcinated in air at 400 1C. The results are shown in Fig. 4a. The maximum rate of reduction was observed at 15 1C for monometallic Pd/Al2O3 and it shifted up to 90 1C in the presence of ceria in the case of the bimetallic PdCe/ Al2O3 sample. The adsorbed hydrogen causes the appearance of a negative peak behind the Pd reduction peak or on this peak. Thus the direct interaction with ceria proved to negatively affect the OX-RED lability of PdO [19] even in rather large particles. Purephase PdO particles with similar sizes underwent reduction under rather lower temperatures. Reduction of PdO being in contact with ceria seems to occur in two steps—about half of PdO was reduced at 90 1C but the other part reduced at rather high temperatures of about 300 1C, simultaneously with the commencing of CeO2 reduction. The repeated TPR up to 700 1C of the prereduced Pd–Ce sample indicates that interaction with the metallic palladium in turn facilitates the reduction of the cerium surface species and ceria itself (Fig. 4b). Three reduction peaks are visible in the TPR profile of the reference Ce/Al2O3 sample with the maxima near 420, 580 and 900 1C. In line with literature reports, the reduction of nanostructured CeO2 [20] occurs in two steps, appearing on the TPR curve as a peak at 580 1C with a lowtemperature shoulder due to reduction of the surface-capping oxygen in ceria and a peak at 800 1C appearing due to reduction of bulk CeO2 into Ce2O3. Alternatively, the observed peaks can be associated with reduction of the surface oxygen of CeO2 (lit.-peak at 450 1C for CeO2/Al2O3), with the formation of CeAlO3 (lit.-peak

Fig. 4. Comparative H2 TPR profiles of samples: (a) TPR up to 400 1C for fresh monometallic Pd/A12O3 and bimetallic PdCe/A12O3 samples; (b) TPR up to 900 1C for fresh Ce/A12O3 and pre-reduced PdCe/A12O3 samples (after TPR up to 400 1C). The numbers under the TPR profiles represent the H2/Pd and/or H2/Ce consumption values.

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4. Conclusion A strong interaction between Pd and Ce oxides takes place in the bimetallic PdCe/Al2O3 system if the heterometallic carboxylate complex is a precursor. The direct contact of PdO with ceria results in the following: increased ordering of PdO phase; decreased accessible surface due to core–shell particles; increased redox stability of both PdO and Pd0 particles and coexistence of both phases; increased redox lability of ceria being in contact with Pd0; and different catalytic properties in CO oxidation.

Acknowledgments

Fig. 5. The ignition–extinction curves for total CO oxidation catalyzed by monometallic Pd/A12O3, mixed Pd+Ce/A12O3 and bimetallic PdCe/A12O3 samples (1%CO in air, 240 ml min1 and 2 1C min1).

at 720 1C) and with the reduction of CeO2 to Ce2O3 (lit.-peak at 900 1C) [4]. In contact with the metallic palladium, a new reduction peak appears at about 300 1C, which is possibly attributed to the interface CeO2 adjacent to Pd0, along with three peaks that seem similar to the observed CeO2/Al2O3 system. If the same attribution [4] is applicable, the reduction of the surface oxygen of CeO2 appears to be hardly affected by the presence of Pd (maximum at 388 1C), while the other two peaks are shifted to rather lower temperatures—428 and 495 1C, respectively. The direct PdO–CeO2 contact affects not only the Pd0–PdO and CeO2–Ce2O3 redox transformations but also the catalytic activity, as was demonstrated with CO oxidation as an example. The increased stability of the PdO phase being in contact with CeO2 and coexistence of the Pd0 and PdO phases results in drastic decrease of the CO ignition temperature and disappearance of the hysteresis between ignition and extinction (Fig. 5). It should be noted that the mixed Pd+Ce/Al2O3 sample shows an intermediate catalytic behaviour in comparison with the mono-Pd/Al2O3 and bimetallic PdCe/Al2O3 samples, which seems to be due to imperfect PdO–CeO2 contact.

This research was supported by Russian Science and Innovation Agency (Contract no. 02.513.11.3203), RFBR-06-03-33005a, RFBR-08-03-01150a, RFBR-06-03-32578a, RFBR-CNRS-08-0392502a, RFBR-09-03-01012a and RFBR-06-03-08173a grants. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

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