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CeO2 catalysts in crotonaldehyde hydrogenation: Selectivity, metal particle size and SMSI states
Applied Catalysis A: General 297 (2006) 48–59 www.elsevier.com/locate/apcata
Pt/CeO2 catalysts in crotonaldehyde hydrogenation: Selectivity, metal particle size and SMSI states Mohamed Abid a,*, Valerie Paul-Boncour b, Raymonde Touroude a a
LMSPC, UMR 7515, ECPM, Louis Pasteur University, 25 Rue Becquerel, 67087 Strasbourg Cedex 2, France b Rare-Earths Chemical Metallurgy Research Laboratory, 2-8 Rue Henri Durant, 94320 Thiais, France Received 7 February 2005; received in revised form 7 June 2005; accepted 26 August 2005 Available online 28 October 2005
Abstract Pt/CeO2 catalysts with different metal particle sizes were prepared by impregnation of chlorine-free precursor, Pt(NH3)4(NO3)2, with different Pt loadings (1, 3, 5, 10 wt%), on a commercial ceria. These catalysts and a CePt5 powder, which was regarded as a reference catalyst, were tested in the hydrogenation of crotonaldehyde as a function of the reduction treatment (from 473–973 K). A dramatic difference in the influence of the reduction temperature on the reactivity was observed between the high and low loaded metal catalysts: after reduction at 973 K, high selectivity to crotyl alcohol (83%) was observed on the high loaded catalyst while on the 1% Pt/CeO2, crotyl alcohol selectivity was below 35%. Characterization by TEM, XPS and XRD showed differences in the particle size distributions and the presence of various nanostructural modifications during the increase of the reduction temperature. On the other hand, the reactivity of CePt5 powder indicates no ability of this compound for the carbonyl bond hydrogenation. The different SMSI states which could influence the reactivity are discussed: formation of epitaxial Pt (1 1 1) layer on CeO2 instead of Pt–CeOx interfacial sites or CePt5 sites, has been retained to be responsible for the increase in crotyl alcohol selectivity. # 2005 Elsevier B.V. All rights reserved. Keywords: Selective crotonaldehyde hydrogenation; Crotyl alcohol selectivity; Pt (1 1 1) epitaxy on ceria; SMSI states; Metal particle size effects; Structure sensitivity
1. Introduction In catalysis, the discovery in 1978 of the strong metal– support interaction (SMSI) effects by Tauster [1] gave rise to a huge amount of further research work in order to find both an explanation and also new catalytic applications [2,3]. These SMSI effects, firstly characterized by the suppression, after high reduction temperature (773 K), of H2 and CO chemisorption on metal supported on TiO2 catalysts have been ascribed to both structural and electronic changes of the metal particles in contact with the reduced support. A full understanding of these phenomena is not yet achieved although the migration of reduced TiOx species onto the metal particles is now well established. The occurrence of SMSI has also been reported for
* Corresponding author. Present address: Louis Pasteur University (EPCM), Rare-Earths Chemical Metallurgy Research Laboratory, LPMN/IPN, Ecole Polytechnique Fe´de´rale de Lausanne, Station 3, CH-1015 Lausanne, Switzerland. Tel.: +41 21 693 45 23. E-mail address: [email protected] (M. Abid). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.08.048
ceria supported noble metals [4]. In this case it has been shown that the electronic metal–support interactions prevail for reduction temperature equal and lower than 773 K, whereas the metal decoration by partially reduced oxide has been observed by high-resolution transmission microscopy after a reduction at 973 K [5]. However, the most remarkable phenomenon of this material is the epitaxial growth of Pt (1 1 1) on CeO2 [6]. This has been observed on Pt/CeO2 catalysts reduced at high temperature, but also on ultrathin ceria films grown on a Pt (1 1 1) surface [7,8] treated under high vacuum at 900 K. The nanostructural modifications versus reduction temperature, in Pt/CeO2 catalysts, were found to be dependent on the metallic salt precursors [9]. In particular, chlorine atoms remaining on the catalyst, after the impregnation of ceria with a chlorinated metal precursor, were found to prevent any interaction between metal and ceria while on a chlorine-free material the CePt5 alloy was seemingly formed after a reduction treatment at 973 K. On Pt supported catalysts the selective hydrogenation of crotonaldehyde (a,b unsaturated aldehyde) could be consid-
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ered as a probe reaction, sensitive to the metal–support interactions [3,9,10]. In other words, when platinum is not modified by the support, the C C bond is selectively hydrogenated while the hydrogenation of the carbonyl bond occurs when the metal is in an SMSI state. On Pt/TiO2 catalysts, the formation of unsaturated alcohol was attributed to the reactivity of the interfacial Pt–TiOx sites [3]. However, in Pt/ CeO2 catalysts, the origin of the specific catalytic behavior of platinum in SMSI state is still a matter of debate: (1) formation of Pt–Ce alloy [9], (2) electronic effect on platinum particles due to the partial reduction of ceria [11], (3) decoration of platinum particles by patches of partially reduced ceria [5]. This paper investigates the influence of the metal particle size on the SMSI in Pt/CeO2 material by using crotonaldehyde hydrogenation as a probe reaction. The nanostructure of the catalysts will be examined by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Secondly, as the formation of CePt5 alloy in Pt/CeO2 reduced at 973 K was previously suggested to be the active sites for the selective hydrogenation of C O [9], the reactivity of the reference CePt5 powder catalyst will be tested. Finally, by taking into account the results obtained on the size effects and on the CePt5 powder reactivity we will attempt to discriminate between the different propositions of the active sites responsible for the selective hydrogenation of crotonaldehyde in Pt/CeO2 systems. In fact we will be brought to reject the Pt–CeOx interfacial sites [10,11] as well as the CePt5 alloy sites [9] to finally propose that the formation of Pt (1 1 1) in epitaxy on ceria plays the major role for the hydrogenation of crotonaldehyde into crotyl alcohol. In addition, we will stress the performances of the Pt/CeO2 catalysts, prepared by a very simple impregnation method since, after appropriate reduction and calcination pretreatments bringing the catalyst some nanostructural modification. Pt/ CeO2 becomes a very selective catalyst (>80% crotyl alcohol), when the metal particle mean size is higher than 3 nm. On the other hand, Pt/CeO2 catalyst containing 95% of particles lower than 3 nm is much less selective whatever the pretreatment. 2. Experimental The different materials 1, 3, 5, 10 wt% Pt/CeO2 were prepared by wet impregnation of CeO2 (Rhoˆne-Poulenc) using an aqueous solution of Pt(NH3)4(NO3)2 (tetraammineplatinum (II) nitrate, STREM chemical). The water was slowly evaporated on a hot plate. The samples were dried for 12 h at 383 K, annealed for 4 h in air at 673 K. Chemical analysis was performed by inductively coupled plasma atomic emission spectroscopy at the CNRS Center of Chemical Analysis (Vernaison). Before impregnation, CeO2 was annealed at 1073 K for 8 h in an oxygen atmosphere. The specific surface decreased from 240 to 54 m2 g1. CePt5 alloy was prepared by mixing metallic cerium with platinum through an annealing treatment at 1173 K under argon atmospheric pressure. The obtained ingot was ground, sifted to
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obtain grain sizes below 100 mm and finally stored. All these operations were performed under argon atmosphere. Reactivity tests were carried out in a glass reactor, operating at atmospheric pressure. The crotonaldehyde, supplied by Fluka puriss and stored in argon, was used as received. A known quantity of crotonaldehyde (between 30 and 300 ml) was introduced through a vaccine cap into a reservoir, installed online and maintained at 273 K to reach a constant partial crotonaldehyde pressure of 8 Torr. The crotonaldehyde gas was carried over the Pt/CeO2 sample by hydrogen at atmospheric pressure. H2 was purified by passing through a trap, maintained at room temperature, containing Pt/Al2O3 mixed with zeolite to remove oxygen and water. Further purification was made through a MnO trap at 293 K, installed just before the crotonaldehyde reservoir; the gas line was thermostated at about 333 K to avoid any condensation. The stability of the crotonaldehyde pressure and the duration of the experiment were controlled by two katharometers inserted upstream and downstream with respect to the reactor. The reaction products were drawn off the line at different times during the catalytic run and analyzed by gas–liquid chromatography, at 358 K, using a flame ionization detector. Before each experiment, the sample was reduced at the desired temperature for 1 h and cooled down to the reaction temperature (353 K) under H2 flow. The reaction activities were calculated as equal to aF/v where a is the crotonaldehyde conversion; F, the crotonaldehyde flow in mole per second; v is the weight of platinum in grams. The turnover frequency (TOF), number of molecules of crotonaldehyde converted per surface atom of platinum per second was calculated with the assumption that platinum particles were cuboctahedral with a hexagonal face in contact with the ceria surface. The selectivity to the different products was calculated as the molar ratio of the selected product to the total formed products. The sensitivity factors were taken as 1 for crotonaldehyde, crotyl alcohol, butanal, butanol and 1.4 for hydrocarbons according to Dietz Tables [12]. The data used to calculate the activities and selectivities were taken at conversions 10%. Butanol, formed after a second hydrogenation step, was negligible in this range of conversions. X-ray diffraction analyses were carried out on a Siemens D5000 Diffractometer using a Bragg–Brentano diffractometer operating in the 2u Bragg configuration. The voltage was set at 50 kV with a 45 mA flux. For 5% Pt/CeO2, the spectra were recorded in 38.5–42.58 (2u) range with 0.01 in step size and 5.0 s per step. This corresponds to the region of platinum metal. The HRTEM images were recorded on a TOPCON 002B electron microscope, operating at 200 kV, with a point-to-point resolution of 0.18 nm. The samples were prepared by grinding the Pt/CeO2 powder and diluting in an ethanol solution. One drop of this solution, previously dispersed in an ultrasonic bath, was deposited onto a Cu grid coated by a carbon-holed film and dried in air. The size distributions of all the Pt/CeO2 catalysts were determined by observing several areas of the grid. To be able to have representative size distributions, more than 250 particles were measured. The following formula used to P was P calculate the mean surface diameter, ds ¼ ni di3 = ni di2 ,
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where ni is the number P of particles of diameter di. The cumulative curves, 100[ inidi (di dk)]/nt as a function of particle size (dk) are also presented where nt is the total number of metal particles. From this plot we can directly estimate the percentage of particles with size dk. X-ray fluorescence was also used to check the presence of cerium and platinum. X-ray phototoelectron spectroscopy (XPS) analyses were performed using a VG ESCA III spectrometer with Mg Ka
radiation (1253.6 eV) as incident beam without a monochromator. Before analysis, the samples were treated in hydrogen atmospheric pressure at different temperatures in the preparation chamber connected to the analysis chamber. Theoretical curves were adjusted to fit the peaks by means of an in-house computer program. A Doniach–Sunjic Lorentzian asymmetric function [13] was convoluted with an experimental Gaussian curve (G = 0.8) and a Shirley background [14] was subtracted.
Fig. 1. Metal particle size distributions in Pt/CeO2 catalysts with different metal weight loadings: (a) 1%, (b) 3%, (c) 5%, (d) 10%, after annealing treatment under hydrogen atmosphere at 973 K.
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XPS analysis of the rare-earth core levels for studies of the screening effects was restricted to the 3d levels because the lifetime broadening of the other levels masks the fine structure related to the screening effects. 3. Results 3.1. TEM results on the different Pt loaded catalysts reduced at 973 K Fig. 1(a–d) represent the size distributions and the accumulated percentages of the metal particles for the samples prepared with different loadings of platinum, 1%, 3%, 5% and 10% supported on CeO2, respectively, after an annealing treatment under hydrogen atmospheric pressure at 973 K during 1 h. Fig. 1a (1% Pt/CeO2) shows that about 95% of the particles measured are lower than 3 nm and none above 4 nm. The mean surface diameter (Ds) calculated for this sample is 2.7 nm. For 3% Pt/CeO2 (Fig. 1b), the size distribution is
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shifted to higher diameter values with a Gaussian distribution curve centered at 2.5 nm and Ds = 3.9 nm. We can notice that the size distribution of 1% Pt/CeO2 catalyst is narrower compared to the 3% Pt/CeO2. For the 5% and 10% Pt/CeO2 samples, the mean particle sizes are around 4.3 and 4.5 nm, respectively, with nearly no particles below 2 nm. The HRTEM micrographs of the Figs. 1, 3 and 5, 10% Pt/CeO2 catalysts are represented in Fig. 2(a–d). In general for all samples, after reduction at 973 K, metal particles presenting ‘‘moire´’’ fringes are observed. The densities of this peculiar figure depend on the metal loading of the catalyst; only a few ‘‘moire´’’ particles were visible on the low loaded catalyst but, on the 5% and 10%, nearly all the metal particles present ‘‘moire´s’’ figures. Following the general interpretation of TEM images [15], these ‘‘moire´’’ figures reveal an epitaxial growth of platinum (1 1 1) on CeO2 (1 1 1). From a careful measurement of the interval between the fringes, a simple calculation allows us to appreciate with an acceptable accuracy the structural modification of platinum growing epitaxially on ceria. By using the
Fig. 2. HRTEM micrographs of Pt/CeO2 catalysts with different metal weight loadings: (a) 1%, (b) 3%, (c) 5%, (d) 10%, after annealing treatment under hydrogen atmosphere at 973 K.
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following relationship [15], D = d1 d2/(d1 d2) with D (nm), the lattice distance measured on the ‘‘moire´’’ fringes, d2 (nm), the known lattice spacing of CeO2 (1 1 1), 0.312 nm, the lattice spacing of the Pt (1 1 1), d1, can be recalculated. The obtained value 0.234 nm compared to the tabulated value of Pt (1 1 1), 0.2265 nm (in JCPDS-4-0802), shows that, during the epitaxial growth, Pt (1 1 1) plane has to expand its lattice parameter by 3.3%. The ‘‘moire´ figure’’ would result of the coincidence of four expanded planes of Pt (1 1 1) with three planes of CeO2 (1 1 1) (3d2 = 4d1). Finally, the Pt particles, at this reduction temperature, are well faceted and exhibit a truncated cubooctahedron structure. The expansion of the Pt (1 1 1) parameter could also correspond to the formation of CePt5 (1 1 1), 0.2281 nm, grown in epitaxy with CeO2. Both hypotheses, Pt (1 1 1) or CePt5 (1 1 1), will be discussed later. The presence of ‘‘moire´ figures’’ formed of metal deposited on ceria and reduced at 973 K have already been noticed [6,16,17] but was never quantitatively interpreted. 3.2. Catalytic activity of different Pt loaded catalysts reduced at different temperatures Fig. 3a shows the activity versus the reduction temperature for catalysts with different loadings of platinum (1%, 3%, 5% and 10%). Two general remarks emerge from this figure: (i) the 1% Pt/CeO2 sample presents the highest activity (measured by gram of Pt) regardless of the reduction temperature, (ii) the evolution of the activity versus the reduction temperature allows to classify the catalysts in two categories: for 1% and 3% Pt/CeO2 a clear maximum in activity appears at 773 K reduction temperature, followed by a strong decrease at higher reduction temperature. For 5% and 10% Pt/CeO2 the maximum of the activity is shifted to a lower reduction temperature (673 K). Finally, the activities are similar for the 1% and 3% Pt/CeO2 catalysts after a reduction at 973 K and are exactly the same for the 5% and 10% Pt/CeO2 catalysts after reduction at 873 and 973 K. The evolution of the crotyl alcohol selectivity versus the reduction temperature depends also on the content of platinum.
For the samples with high contents (5% and 10% Pt/CeO2), the selectivity is low after reduction at 473, 573, 673 K (13%) but it rises sharply after reduction at 773 K to reach a maxima of 70–83% after 973 K. In the case of the 1% Pt/CeO2 sample, the selectivity is very low at low reduction temperatures (5%) and never exceeds 35%, value obtained at the highest reduction temperature, 973 K. Finally, for the 3% Pt/CeO2 sample, an intermediate evolution is observed. To evaluate better the meaning of the modifications of the catalytic behavior noticed above, we have calculated the turn over frequency of crotyl alcohol and the TOF of butanal, and represented them versus the reduction temperature (Fig. 4a and b, respectively) for all the catalysts. A simple comparison points out that the maxima in butanal TOF, for all the catalysts, are situated at the same temperature but at a lower reduction temperature (673 K) than the maxima in crotyl alcohol TOF (773 K), which is also found at the same temperature for all the catalysts. It means that, increasing the reduction temperature from 473 to 673 K, new sites are created for butanal and crotyl alcohol formation but, after reduction at 773 K the number of sites for the butanal formation decreases while the number of sites for the crotyl alcohol formation keeps increasing. At higher reduction temperatures 873 and 973 K, both types of sites decrease. Looking at the ratio between the crotyl alcohol TOF and the butanal TOF (Fig. 4c), there is a net increase with the reduction temperature from 673 K, except for the 1% Pt/ CeO2 catalyst where this ratio remains lower than 1. 3.3. TEM analysis on 5% Pt/CeO2 reduced at 773 K From our catalytic results, the most important feature is the dramatic change of the TOF in crotyl alcohol and butanal during the reduction from 673 to 773 K. Fig. 5a and b show the TEM micrographs and the size distribution histograms for the 5% Pt/CeO2 reduced at 473 and 773 K. TEM picture show clearly the appearance, during the reduction from 473 to 773 K, of the epitaxial growth of Pt (1 1 1) on ceria. However the image of the sample reduced at 773 K displays two kinds of
Fig. 3. Activity (a) and crotyl alcohol selectivity (b), for 1–10 wt% Pt/CeO2 catalysts after different reduction temperatures.
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Fig. 4. Turn over frequency (TOF) for for 1–10 wt% Pt/CeO2 catalysts after different reduction temperatures: (a) crotyl alcohol, (b) butanal, (c) ratio between crotyl alcohol TOF and butanal TOF.
platinum particles: 3–5 nm particles formed as Pt (1 1 1) in epitaxy on ceria and other particles with no epitaxial feature. The size distribution histograms show a small increase in the particle size from the 473 to 773 K. Moreover, the platinum particles in epitaxy are not well faceted as we observed previously on the sample reduced at 973 K. 3.4. XPS and XRD analyses of 5% Pt/CeO2 reduced at different temperatures To further investigate the correlations between surface structure and reactivity we have studied, by XPS and XRD, the electronic states and the nature of the metallic phases, of the 5% Pt/CeO2 material which exhibits the most striking reactivity evolution as a function of the reduction temperature. Fig. 6a shows the XP spectra of the 5% Pt/CeO2 samples in the Ce 3d region, 870–922 eV binding energy (BE) range, after different treatments. The complexity of the spectra, which were first resolved by Burroughs et al. [18], arises from the hybridation between the Ce 4f levels and the O 2p states. The two sets of spin-orbit multiplets, corresponding to 3d3/2 and 3d5/ 2 levels, are labeled u and v, respectively, up to five contributions in each set exist. The peaks labeled v and v00
are assigned to a mixing of the Ce 3d9 4f2 O 2p4 and the Ce 3d9 4f1 O 2p5 Ce(IV) final states. The peak denoted v000 corresponds to the Ce 3d9 4f0 O 2p6 Ce(IV) final state. On the other hand, v8 and v0 lines are assigned to the Ce 3d9 4f2 O 2p5 and the Ce 3d9 4f1 O 2p6 Ce(III) final states, respectively. The same assignment is applied to the u structure, which corresponds to the Ce 3d3/2 level. The degree of ceria reduction was calculated, after deconvolution of the experimental spectra, from the ratio between the sum of the peak area of the uo, u0 , vo and v0 peaks representing Ce(III) species and the sum of the intensities of all the peak contributions as described by Laachir et al. [19]. For the sample oxidized at 673 K, we notice the presence of v0 and u0 components revealing the presence of 20% Ce3+ species. This apparent reduction of ceria in an oxidized ceria supported metal has been already observed and attributed to numerous effects. Laachir et al. [19] evoked the reduction of ceria by the X-ray beam. Many studies have reported this high percentage of Ce3+ on Rh/CeO2 sample. They explained the presence of Ce3+ by an autocatalysis due to the metal presence. This metal could favor the departure of the labile oxygen atoms from the surface [20]. In our experiments the Ce3+ contribution increased after annealing treatment under hydrogen at 473 K, up to 35%, but did not increase more after a reduction at higher temperatures,
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Fig. 5. HRTEM micrographs and particles size distributions of 5% Pt/CeO2 after annealing treatment under hydrogen atmosphere at 473 K (a) and 773 K (b).
773 and 973 K. This stable reduction state, corresponding to CeO1.82 formula (65% CeO2, 35% Ce2O3) indicates that the fluorite structure of CeO2 (in 5% Pt/CeO2) cannot accommodate, at the surface, a larger concentration of oxygen vacancies than 0.18 per unit cell. Fig. 6b shows the XP spectra in the Pt (4f) region 60–85 eV BE range, after different thermal treatments. For the sample reduced at 473 and 773 K, these spectra are very similar and correspond to the 4f7/2 and 4f5/2 peaks reported for Pt metal, at 70.9 and 74.3 eV, respectively [21]. For the sample reduced at 973 K, the lower BE of the Pt (4f) peaks could be due either to an alloy formation or to strong interactions between Pt and reduced ceria. Fig. 6c represents the atomic ratio (Pt/Ce) after different treatments: this ratio is constant from reduction temperatures of 473 to 673 K and gradually decreases after higher reduction temperature (773–
973 K). This is interpreted partly by a coalescence of the metal particles and also by the migration of Ce3+onto the Pt crystallites or by the burial of platinum crystallites into partially reduced ceria. The XP spectra of O(1s) core line (not represented) show, after hydrogen treatment a shoulder on the high binding energy side of the O(1s) peak at 531.1 eV; the main O(1s) peak being situated at 529.2 eV. This additional O(1s) contribution is attributed to the –OH species [22]. Fig. 6c represents the percentage of these –OH species after different treatments: a maximum is shown on the sample reduced at 773 K, which could indicate that the formation of –OH species at the surface prevents a further ceria reduction. X-ray diffraction has been used to elucidate the degree of platinum support interactions in the materials. Fig. 6d represents the diffractograms in the Pt (1 1 1) region of 5%
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Fig. 6. XPS and XRD analyses of 5% Pt/CeO2 after different treatments: (a) Ce(3d), (b) Pt (4f) (c) atomic ratios of Pt/Ce and OH/O(1s) and (d) XRD diffractograms in Pt (1 1 1) region.
Pt/CeO2 sample reduced at different temperatures 473, 773 and 973 K. After reduction at 473 and 773 K, a peak at 39.818 is observed, corresponding to the Pt (1 1 1) line. The sample reduced at 773 K, shows a smaller FWHM than the sample reduced at 473 K. That reveals an increase of the particle size when increasing the reduction temperature, in accordance with our previous results obtained by TEM [9]. After reduction at 973 K, a broad peak is observed with a maximum at 39.458 and two small humps at 38.838 and 41.248, which correspond to (1 1 1), (2 0 0) and (0 0 2) reflections of CePt5 alloy, respectively. However, to get a good fit with reliable FWHM, it has been necessary to add a Pt (1 1 1) contribution at 39.818. The presence of CePt5 may explain the decrease in the binding energies observed in the Pt (4f) XPS core line after reduction at 973 K. Bernal et al. [23] have reported a similar phase change on platinum supported on CeO2, but for a sample reduced at higher temperature, 1173 K instead of 973 K. However Berner and Schierbaum [24], studying the evolution of CeO2 crystallites supported on Pt (1 1 1) single crystal observed the formation of CePt5 patches and two hexagonal dimensional structure islands of Ce2O3 during an annealing treatment at
900 K under ultra-high vacuum (UHV). These CePt5 patches were found better ordered after annealing at 1000 K. In accordance with this study the formation of CePt5 at 973 K reduction temperature observed in our work, in coexistence with platinum seems reasonable. 3.5. CePt5 powder: X-ray photoelectron spectroscopy and reactivity In order to determine if the formation of CePt5 plays a role in the selectivity, we have tested the reactivity of CePt5 powder. The X-ray diffraction pattern of our sample showed a single phase corresponding to polycrystalline CePt5 (JCPDS 170071). This CePt5 sample was analyzed by XPS. Previous XPS research works [25,26] made on some rare-earth metals (Ce, Sm, Eu, Tm) and alloys (CePd3), show that up to three contributions in each spin orbit set of the Ce 3d multiplets exist. These three component f2, f1, f0 correspond to the transitions from the initial states of three ground electronic states of the cerium metal (3d10 . . . (6s 5d)4 4f0, 3d10 . . . (6s 5d)3 4f1, 3d10 (6s 5d)2 4f2) to the corresponding exciton-like final states (3d9
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Fig. 7. CePt5 alloy: (a) Ce(3d) from XPS, (b) activity and selectivity to crotyl alcohol.
(6s 5d)4 4f0, 3d9 (6s 5d)3 4f1, 3d9 (6s 5d)2 4f2) produced during the photoemission process, respectively. The differences in binding energy observed for these three processes are due to the differences in the screening effects. The latter are more important, in energy [26], for the f2 (well screened) than for the f1 (poorly screened) transitions compared to the f0 transition, which is not affected by the screening effects. Therefore, the f2 transition appears at a lower BE than the f1 followed by f0 transitions. In a material, the probability that the screening level will be occupied depends critically on the coupling to the other occupied levels. This coupling is described in terms of a hopping integral, or degree of hybridization with the delocalized electrons of the system. If this degree of hybridization is small, the probability of transferring an electron to the screening level is small and most of the XPS intensity will be found in the poorly screened peaks (f1 contribution). In conclusion, the peak intensity will give us qualitative information on the valence state of Ce and on the hybridization of the 4f level with the conduction band. In our study the Fermi level has been used as BE reference to take into account the possible charge effects. The XP spectra of CePt5 alloy in the region of 3d peaks are represented in Fig. 7a. The 3d levels have a spin-orbit splitting of 14.7 eV. The low binding energy peaks at 882.4 and 901.2 eV of the 3d5/2 and 3d3/2 components, respectively, are attributed to the well-screened 3d9 . . . 4f2 final state configurations while the most intense peaks at 885.8 and 904.4 eV are attributed to the poorly screened 3d9 . . . 4f1 configurations. Finally, the peaks at 889 and 916.8 eV correspond to the 3d9 4f0 configurations. In CePt5, the 4f2 transitions are high compared to the corresponding ones in pure Ce metal where the well-screened 4f2 peaks contribute for less than 5–10% to the total of 3d XPS peaks. However, the studies on CePd3 have also shown that the intensity was more evenly distributed between the well- and poorly-screened peaks in the alloy than in the pure metal. The same phenomenon has been seen for Ce2O3 materials [27]. In the XP spectra of the sample reduced at 573 and 873 K under
hydrogen, a small decrease in f2 (well-screened) contribution is observed. This decrease can be easily related to the reduction of some CeOx species, which formed during the sample preparation where the sample was in contact with air during few minutes although the sample was sputtered with Ar ion before the analysis. The Pt 4f7/2 and 4f5/2 peaks are observed at 70.4 and 73.9 eV and these values do not change with reduction treatment. They are close to the binding energies observed on the 5% Pt/CeO2 sample reduced at 973 K. The CePt5 sample has a low catalytic activity (Fig. 7b) compared to Pt supported on ceria, which could be partly due to the low surface area, but it is noticeable that the crotyl alcohol selectivity does not exceed 30% whatever the reduction temperature; even it still decreases at high reduction temperature. This result clearly demonstrates that CePt5 has no ability to selectively hydrogenate the C O bond compared to the C C bond in crotonaldehyde. 4. Discussion By using the sensitive crotonaldehyde hydrogenation test we have studied the size effects of Pt particles supported on ceria; this support being well known to induce SMSI at high reduction temperature. On a sample reduced at 973 K, we have observed a correlation between the reactivity and the particle size by varying the loading of platinum on CeO2: the catalyst in which 95% of Pt particles are lower than 2 nm (1% Pt/CeO2) gave a very low crotyl alcohol selectivity (<35%) while the catalyst which contains 80% of Pt particles bigger than 2 nm gave 83% crotyl alcohol selectivity. We have observed by HRTEM that 3– 4 nm metal particles are modified. Epitaxial growth of the metal particles onto the support (CeO2) is observed. At this reduction temperature, the epitaxial relation between the small particles (around 2 nm) and the CeO2 support was not apparent. The TOF in butanal and crotyl alcohol versus the reduction temperature show an inversion of the variation in the 673– 773 K temperature reduction ranges. Effectively, a maximum
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for active sites in hydrogenation of butanal was observed at 673 K while for crotyl alcohol this appears at 773 K. We have observed, by HRTEM, on 5% Pt/CeO2 materials, in the same range of reduction temperature, the appearance of platinum particle restructuring leading to an epitaxial growth of Pt (1 1 1). Sen and Vannice [28], and other groups [29–31] have showed that Pt/TiO2 was more selective to the unsaturated alcohol than Pt/SiO2 and Pt/Al2O3 catalysts. They have pointed out an increase in selectivity and TOF, when the reduction temperature was increased up to 773 K. On the other hand, it is well known that Pt/TiO2 catalysts, reduced at high temperature (773 K), induces strong metal–support interactions, which were ascribed to the migration of TiOx entities onto Pt0 particles. Therefore, Sen and Vannice proposed that the Pt–TiOx interfacial sites are responsible for the adsorption of the C O bond in a,b unsaturated aldehyde and subsequent hydrogenation to the unsaturated alcohol. Later SilvestreAlbero et al. [32] and recently Concepcion et al. [11], used the same explanation on Pt/CeO2 system assuming that Pt–CeOx species will favor the hydrogenation of the carbonyl bond. The results published in [11] are not worth to mention here since the crotyl alcohol selectivity found on their catalysts reached values as high as found on our catalysts. On catalysts made of platinum particles supported on mesostructured CeO2 nanoparticles embedded within ultrathin layers of SiO2 binder (Pt/ CeO2-SiO2) and reduced at 773 K, the selectivity in crotyl alcohol reached up to 80% while for the same catalyst, reduced at 473 K the selectivity was low (<20%). In our case, on Pt/ CeO2 catalysts, similar high crotyl alcohol selectivity has been obtained on samples reduced at higher temperature (973 K). However if our catalyst after reduction at 973 K was reoxidized and re-reduced [9], a reduction temperature of 773 K was sufficient to recover a high level of crotyl alcohol selectivity (87%) with a higher activity than found after the previous 973 K reduction treatment (10:4 mmol s1 g1 Pt instead of 2:1 mmol s1 g1 Pt ). The TEM images of this re-oxidized and re-reduced catalyst showed the same moire´ and faceted particles as the sample reduced at 973 K (Fig. 2b). Otherwise Concepcion et al. [11], in Pt/CeO2-SiO2 catalysts, observed no significant change in crotyl alcohol selectivity when changing the Pt percentages (1%, 3%, 5%). However, it is important to note that the particle size histograms on these catalysts did not exhibit a strong variation as observed in our catalysts. Finally, from their FTIR and XPS results, they concluded that interfacial Pt–Ce3+species were responsible for the hydrogenation of the C O bond. From our catalytic results on 5–10% Pt/CeO2, we could also interpret the peculiar evolution of the catalytic properties occurring at 673 and 773 K as due to the formation of interfacial Pt–Ce3+ sites: the transformation of Pt sites into interfacial Pt–Ce3+ sites after reduction at 773 K leading to a decrease in butanal TOF and an increase in crotyl alcohol TOF. But, the results obtained on the 1% Pt/CeO2 catalyst and the catalytic properties observed on the 3%, 5%, 10% Pt/CeO2 reduced at T > 773 K do not support this hypothesis. First, due to a higher surface to volume ratio in the highly dispersed
57
catalyst a higher number of Pt–Ce3+ sites should exist in the 1% and 3% compared to 5% and 10% Pt/CeO2 catalysts, therefore higher crotyl alcohol selectivity should be expected on the low loaded catalyst; on the contrary, we observed a higher crotyl alcohol selectivity for the 5% and 10% Pt/CeO2 compared to the 1% Pt/CeO2. Secondly, the decoration of Pt particles by CeOx species leading to an increase in the number of Pt–Ce3+ interfacial sites is expected to start at a temperature higher than 773 K. Their number (Pt–Ce3+) is expected to increase with the reduction temperature up to 973 K, according to our XPS results (Fig. 6c) and to the HRTEM observations of Bernal et al. [23]. That should lead to an increase of the TON in crotyl alcohol. On the contrary, a decrease in crotyl alcohol TON was observed when the reduction temperature increased above 773 K. From these both series of results, the interfacial Pt– CeOx sites could be hardly considered as the active sites for the hydrogenation of the carbonyl bond. Instead, the effects of the particle size and the reduction temperature on the crotyl alcohol selectivity in our catalysts appear to be much better correlated with a change of the morphology of the Pt particle involving the formation of Pt (1 1 1) in epitaxy on ceria. On the 5% Pt/CeO2 catalyst, during the reduction process from 673 to 773 K, a restructuration process of platinum particles to form Pt (1 1 1) in epitaxy starts and totally develops after reduction at 973 K, in parallel the crotyl alcohol selectivity rose up to 83%. Unlikely, on 1% Pt/CeO2, containing smaller particles, the restructuration process does not operate at 773 K reduction, only a few particles were found in epitaxy with CeO2 after 973 K reduction and on this catalyst the crotyl alcohol selectivity reached only 35%. Therefore, the Pt (1 1 1) layers in epitaxy on ceria could be there considered to favor the hydrogenation of the C O bond to the detriment to the C C bond. This conclusion is in perfect agreement with the previous work relating a theoretical approach [33] on the adsorption of a,b-unsaturated aldehydes on Pt and Pd. The authors concluded that the dense (1 1 1) metal face is not very favorable to the C C coordination and a greater participation of this face in the catalyst surface, with large faceted particles or by support epitaxy, can favor the unsaturated alcohol. From this theoretical study, a,b unsaturated aldehyde hydrogenation has to be considered as a structure sensitive reaction, sensitive to the arrangement of the surface atoms, in relation with the particle size. In fact, some experimental results perfectly support this theory. When prenal ((CH3)2C CH–CHO) was used as reactant [34–36], a molecule containing a more bulky C C bond compared to crotonaldehyde, Pt (1 1 1) single crystal led to 60% unsaturated alcohol selectivity while, on Pt (1 1 0), saturated alcohol and saturated aldehyde were mainly formed. Raab and Lercher [31], studying the particle size effects in crotonaldehyde hydrogenation on Pt/SiO2 and Pt/TiO2 catalysts found that large Pt particles led to higher selectivity and activity than the small particles and they attributed this behavior to the specific reactivity of the Pt (1 1 1) planes towards the preferential hydrogenation of the C O bond. Claus et al. [37,38] found similar sensitivity structure effects on Au, Ag supported on TiO2 in hydrogenation of acrolein and crotonaldehyde, respectively, where the small particles, around
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1 nm, of Ag and Au gave both a lower activity and selectivity to the unsaturated alcohol than the particles around 3 nm. However, some experimental work does not match with this theory. Beccat et al. [39] have found a poor unsaturated alcohol selectivity on Pt (1 1 1) single crystal when studying crotonaldehyde hydrogenation. Some other experiments [40] were performed on a 5% Pt/a-alumina catalyst with large and well faceted Pt particles (50 nm), containing necessarily a large amount of Pt (1 1 1) planes and no more than 10% crotyl alcohol selectivity was obtained. In our catalysts, if the formation of epitaxied Pt (1 1 1) explains satisfactorily the evolution of the selectivities, the decrease in both butanal and crotyl alcohol TOF after 873 and 973 K reveals the loss of active surface. On the 5% Pt/CeO2 catalysts after reduction at 773 K and above, the XP spectra show a decrease in Pt/Ce atomic ratio, the main explanation from 773 to 873 K is that CeOx species will start to cover preferentially the edges and corners of the particles and strongly decrease the C C adsorption in all the catalysts. However, this phenomenon is not restricted to the edges and the corners and can also reach and cover partially the top of the close-packed structure of (1 1 1) platinum leading to a simultaneous decrease of the C O bond hydrogenation. But, as the formation of active sites for the C O hydrogenation will continue to occur by the restructuration process (Pt (1 1 1)) at high reduction temperature, from 773 to 973 K, the decrease in crotyl alcohol TOF is much less important than the decrease in butanal TOF. Finally, at 973 K, further reduction into CeOx species will start to form CePt5 as shown in the X-ray diffractograms where Pt and CePt5 coexist. The formation of CePt5 is confirmed by X-ray photoelectron spectroscopy revealing a shift of the Pt (4f) BE to 70.4 eV instead of 70.9 eV. However, the Ce 3d core spectra did not exhibit any modification at 973 K compare to lower temperature, due to Ce4+ and Ce3+ signals coming from the support, which are dominant. In general, CePt5 alloy has been observed at 1173 K on this kind of materials [24]. However, Schierbaum et al., observed on CeO2/Pt (1 1 1), the formation of CePt5 patches after reduction at 900 K. At 1000 K they observed only the CePt5 phase. Metallic Ce supported on Pt (1 1 1) annealed at 900 K under UHV leads to the formation of two-dimensional islands of CeO2x with hexagonal shape. They concluded that the CePt5 surface alloy islands were formed from such 2D islands which could act as intermediate phase in surface alloying between Pt and Ce. From these observations we could propose that, for our sample reduced at 973 K, the interface between platinum and CeO2, is formed by some layers of CePt5 (1 1 1) ended by a Pt (1 1 1) with an expanded lattice parameters compared to the crystalline Pt (1 1 1) plane. After a re-oxidation treatment on 5% Pt/CeO2 followed by a reduction at 773 K, the increase in activity can be explained by a reverse migration phenomenon of reduced ceria and the presence of high selectivity at lower reduction temperature by the presence of Pt (1 1 1) with a definite particles size. Finally, let us stress the peculiar behavior of the small particles (<2 nm). In the present work, it is important to highlight that the activity is higher in 1% Pt/CeO2 than in the
samples with larger particles size (3%, 5%, 10%) and the TOF in butanal decreases only slightly when going from 673 to 773 K reduction temperature in agreement with the fact that the modification of the Pt particles into Pt (1 1 1) in epitaxy on CeO2 occurs at a very small extent on the 1% Pt/CeO2 sample. In fact, in sufficiently small particles, changes of the surface coordination are linked to change in electronic properties. Gan et al. [40] have observed a stronger interaction of CO on small platinum particles supported on TiO2 (1 1 0) surface. The scanning tunneling spectra revealed that platinum nanoparticles, below 2 nm in diameter, exhibited non-metallic behavior [41]. Therefore, the unusual electronic properties of nanosized platinum particles should be taken into consideration to understand this non-epitaxial relation of the small particles. It is likely that a marked change in the electronic character of nanosized platinum particles below 2 nm may indicate a transition to the non-metallic phase and that the structure sensitivity in this region originates from a quantum-size effect. Effectively, it is well known that the electronic band structure of metal nanoparticles critically depends on its size. For small particles, the electronic states are not continuous, but discrete due to confinement of the electron wavefunction.
5. Conclusion This work shows that, by controlling the nanostructure, size and morphology, of supported platinum particles it is possible to orientate the selectivity in the hydrogenation of crotonaldehyde, an a,b-unsaturated aldehyde. Studying Pt/CeO2 catalysts, prepared from chlorine-free precursor, with different metal loadings and reduced at different temperatures, we could differentiate between the catalysts highly loaded (5 and 10%), containing larger particles, which develop Pt (1 1 1) in epitaxy on ceria around 773 K reduction treatment. The catalysts with a low metal content (1%) present more dispersed metal particles with a large amount of very small particles (<2 nm) showing no particular morphology. On the Pt particles in epitaxy on ceria a very high selectivity for carbonyl bond hydrogenation was obtained (83% to crotyl alcohol) while on the small particles the crotyl alcohol selectivity never exceeded 35%. We conclude that the adsorption of the C O group of the a,b-unsaturated aldehyde is favored by face atoms arranged as Pt (1 1 1) with an expanded lattice parameter in epitaxy with CeO2. Simultaneously to the restructuration process a covering of the Pt particles by reduced ceria was found to take place at reduction temperature higher than 773 K which leads to decrease the production of both butanal and crotyl alcohol. After 973 K reduction, CePt5 alloy was additionally formed but it does not contribute to the increase in the crotyl alcohol selectivity as checked by a study performed on CePt5 powder. A future development to this work would be to verify, by theoretical calculations, our hypothesis that the expanded Pt (1 1 1) in epitaxy on ceria favors the hydrogenation of C O bond in a,b-unsaturated aldehyde.
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Pt/CeO2 catalysts in crotonaldehyde hydrogenation: Selectivity, metal particle size and SMSI states Mohamed Abid a,*, Valerie Paul-Boncour b, Raymonde Touroude a a
LMSPC, UMR 7515, ECPM, Louis Pasteur University, 25 Rue Becquerel, 67087 Strasbourg Cedex 2, France b Rare-Earths Chemical Metallurgy Research Laboratory, 2-8 Rue Henri Durant, 94320 Thiais, France Received 7 February 2005; received in revised form 7 June 2005; accepted 26 August 2005 Available online 28 October 2005
Abstract Pt/CeO2 catalysts with different metal particle sizes were prepared by impregnation of chlorine-free precursor, Pt(NH3)4(NO3)2, with different Pt loadings (1, 3, 5, 10 wt%), on a commercial ceria. These catalysts and a CePt5 powder, which was regarded as a reference catalyst, were tested in the hydrogenation of crotonaldehyde as a function of the reduction treatment (from 473–973 K). A dramatic difference in the influence of the reduction temperature on the reactivity was observed between the high and low loaded metal catalysts: after reduction at 973 K, high selectivity to crotyl alcohol (83%) was observed on the high loaded catalyst while on the 1% Pt/CeO2, crotyl alcohol selectivity was below 35%. Characterization by TEM, XPS and XRD showed differences in the particle size distributions and the presence of various nanostructural modifications during the increase of the reduction temperature. On the other hand, the reactivity of CePt5 powder indicates no ability of this compound for the carbonyl bond hydrogenation. The different SMSI states which could influence the reactivity are discussed: formation of epitaxial Pt (1 1 1) layer on CeO2 instead of Pt–CeOx interfacial sites or CePt5 sites, has been retained to be responsible for the increase in crotyl alcohol selectivity. # 2005 Elsevier B.V. All rights reserved. Keywords: Selective crotonaldehyde hydrogenation; Crotyl alcohol selectivity; Pt (1 1 1) epitaxy on ceria; SMSI states; Metal particle size effects; Structure sensitivity
1. Introduction In catalysis, the discovery in 1978 of the strong metal– support interaction (SMSI) effects by Tauster [1] gave rise to a huge amount of further research work in order to find both an explanation and also new catalytic applications [2,3]. These SMSI effects, firstly characterized by the suppression, after high reduction temperature (773 K), of H2 and CO chemisorption on metal supported on TiO2 catalysts have been ascribed to both structural and electronic changes of the metal particles in contact with the reduced support. A full understanding of these phenomena is not yet achieved although the migration of reduced TiOx species onto the metal particles is now well established. The occurrence of SMSI has also been reported for
* Corresponding author. Present address: Louis Pasteur University (EPCM), Rare-Earths Chemical Metallurgy Research Laboratory, LPMN/IPN, Ecole Polytechnique Fe´de´rale de Lausanne, Station 3, CH-1015 Lausanne, Switzerland. Tel.: +41 21 693 45 23. E-mail address: [email protected] (M. Abid). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.08.048
ceria supported noble metals [4]. In this case it has been shown that the electronic metal–support interactions prevail for reduction temperature equal and lower than 773 K, whereas the metal decoration by partially reduced oxide has been observed by high-resolution transmission microscopy after a reduction at 973 K [5]. However, the most remarkable phenomenon of this material is the epitaxial growth of Pt (1 1 1) on CeO2 [6]. This has been observed on Pt/CeO2 catalysts reduced at high temperature, but also on ultrathin ceria films grown on a Pt (1 1 1) surface [7,8] treated under high vacuum at 900 K. The nanostructural modifications versus reduction temperature, in Pt/CeO2 catalysts, were found to be dependent on the metallic salt precursors [9]. In particular, chlorine atoms remaining on the catalyst, after the impregnation of ceria with a chlorinated metal precursor, were found to prevent any interaction between metal and ceria while on a chlorine-free material the CePt5 alloy was seemingly formed after a reduction treatment at 973 K. On Pt supported catalysts the selective hydrogenation of crotonaldehyde (a,b unsaturated aldehyde) could be consid-
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ered as a probe reaction, sensitive to the metal–support interactions [3,9,10]. In other words, when platinum is not modified by the support, the C C bond is selectively hydrogenated while the hydrogenation of the carbonyl bond occurs when the metal is in an SMSI state. On Pt/TiO2 catalysts, the formation of unsaturated alcohol was attributed to the reactivity of the interfacial Pt–TiOx sites [3]. However, in Pt/ CeO2 catalysts, the origin of the specific catalytic behavior of platinum in SMSI state is still a matter of debate: (1) formation of Pt–Ce alloy [9], (2) electronic effect on platinum particles due to the partial reduction of ceria [11], (3) decoration of platinum particles by patches of partially reduced ceria [5]. This paper investigates the influence of the metal particle size on the SMSI in Pt/CeO2 material by using crotonaldehyde hydrogenation as a probe reaction. The nanostructure of the catalysts will be examined by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Secondly, as the formation of CePt5 alloy in Pt/CeO2 reduced at 973 K was previously suggested to be the active sites for the selective hydrogenation of C O [9], the reactivity of the reference CePt5 powder catalyst will be tested. Finally, by taking into account the results obtained on the size effects and on the CePt5 powder reactivity we will attempt to discriminate between the different propositions of the active sites responsible for the selective hydrogenation of crotonaldehyde in Pt/CeO2 systems. In fact we will be brought to reject the Pt–CeOx interfacial sites [10,11] as well as the CePt5 alloy sites [9] to finally propose that the formation of Pt (1 1 1) in epitaxy on ceria plays the major role for the hydrogenation of crotonaldehyde into crotyl alcohol. In addition, we will stress the performances of the Pt/CeO2 catalysts, prepared by a very simple impregnation method since, after appropriate reduction and calcination pretreatments bringing the catalyst some nanostructural modification. Pt/ CeO2 becomes a very selective catalyst (>80% crotyl alcohol), when the metal particle mean size is higher than 3 nm. On the other hand, Pt/CeO2 catalyst containing 95% of particles lower than 3 nm is much less selective whatever the pretreatment. 2. Experimental The different materials 1, 3, 5, 10 wt% Pt/CeO2 were prepared by wet impregnation of CeO2 (Rhoˆne-Poulenc) using an aqueous solution of Pt(NH3)4(NO3)2 (tetraammineplatinum (II) nitrate, STREM chemical). The water was slowly evaporated on a hot plate. The samples were dried for 12 h at 383 K, annealed for 4 h in air at 673 K. Chemical analysis was performed by inductively coupled plasma atomic emission spectroscopy at the CNRS Center of Chemical Analysis (Vernaison). Before impregnation, CeO2 was annealed at 1073 K for 8 h in an oxygen atmosphere. The specific surface decreased from 240 to 54 m2 g1. CePt5 alloy was prepared by mixing metallic cerium with platinum through an annealing treatment at 1173 K under argon atmospheric pressure. The obtained ingot was ground, sifted to
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obtain grain sizes below 100 mm and finally stored. All these operations were performed under argon atmosphere. Reactivity tests were carried out in a glass reactor, operating at atmospheric pressure. The crotonaldehyde, supplied by Fluka puriss and stored in argon, was used as received. A known quantity of crotonaldehyde (between 30 and 300 ml) was introduced through a vaccine cap into a reservoir, installed online and maintained at 273 K to reach a constant partial crotonaldehyde pressure of 8 Torr. The crotonaldehyde gas was carried over the Pt/CeO2 sample by hydrogen at atmospheric pressure. H2 was purified by passing through a trap, maintained at room temperature, containing Pt/Al2O3 mixed with zeolite to remove oxygen and water. Further purification was made through a MnO trap at 293 K, installed just before the crotonaldehyde reservoir; the gas line was thermostated at about 333 K to avoid any condensation. The stability of the crotonaldehyde pressure and the duration of the experiment were controlled by two katharometers inserted upstream and downstream with respect to the reactor. The reaction products were drawn off the line at different times during the catalytic run and analyzed by gas–liquid chromatography, at 358 K, using a flame ionization detector. Before each experiment, the sample was reduced at the desired temperature for 1 h and cooled down to the reaction temperature (353 K) under H2 flow. The reaction activities were calculated as equal to aF/v where a is the crotonaldehyde conversion; F, the crotonaldehyde flow in mole per second; v is the weight of platinum in grams. The turnover frequency (TOF), number of molecules of crotonaldehyde converted per surface atom of platinum per second was calculated with the assumption that platinum particles were cuboctahedral with a hexagonal face in contact with the ceria surface. The selectivity to the different products was calculated as the molar ratio of the selected product to the total formed products. The sensitivity factors were taken as 1 for crotonaldehyde, crotyl alcohol, butanal, butanol and 1.4 for hydrocarbons according to Dietz Tables [12]. The data used to calculate the activities and selectivities were taken at conversions 10%. Butanol, formed after a second hydrogenation step, was negligible in this range of conversions. X-ray diffraction analyses were carried out on a Siemens D5000 Diffractometer using a Bragg–Brentano diffractometer operating in the 2u Bragg configuration. The voltage was set at 50 kV with a 45 mA flux. For 5% Pt/CeO2, the spectra were recorded in 38.5–42.58 (2u) range with 0.01 in step size and 5.0 s per step. This corresponds to the region of platinum metal. The HRTEM images were recorded on a TOPCON 002B electron microscope, operating at 200 kV, with a point-to-point resolution of 0.18 nm. The samples were prepared by grinding the Pt/CeO2 powder and diluting in an ethanol solution. One drop of this solution, previously dispersed in an ultrasonic bath, was deposited onto a Cu grid coated by a carbon-holed film and dried in air. The size distributions of all the Pt/CeO2 catalysts were determined by observing several areas of the grid. To be able to have representative size distributions, more than 250 particles were measured. The following formula used to P was P calculate the mean surface diameter, ds ¼ ni di3 = ni di2 ,
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where ni is the number P of particles of diameter di. The cumulative curves, 100[ inidi (di dk)]/nt as a function of particle size (dk) are also presented where nt is the total number of metal particles. From this plot we can directly estimate the percentage of particles with size dk. X-ray fluorescence was also used to check the presence of cerium and platinum. X-ray phototoelectron spectroscopy (XPS) analyses were performed using a VG ESCA III spectrometer with Mg Ka
radiation (1253.6 eV) as incident beam without a monochromator. Before analysis, the samples were treated in hydrogen atmospheric pressure at different temperatures in the preparation chamber connected to the analysis chamber. Theoretical curves were adjusted to fit the peaks by means of an in-house computer program. A Doniach–Sunjic Lorentzian asymmetric function [13] was convoluted with an experimental Gaussian curve (G = 0.8) and a Shirley background [14] was subtracted.
Fig. 1. Metal particle size distributions in Pt/CeO2 catalysts with different metal weight loadings: (a) 1%, (b) 3%, (c) 5%, (d) 10%, after annealing treatment under hydrogen atmosphere at 973 K.
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XPS analysis of the rare-earth core levels for studies of the screening effects was restricted to the 3d levels because the lifetime broadening of the other levels masks the fine structure related to the screening effects. 3. Results 3.1. TEM results on the different Pt loaded catalysts reduced at 973 K Fig. 1(a–d) represent the size distributions and the accumulated percentages of the metal particles for the samples prepared with different loadings of platinum, 1%, 3%, 5% and 10% supported on CeO2, respectively, after an annealing treatment under hydrogen atmospheric pressure at 973 K during 1 h. Fig. 1a (1% Pt/CeO2) shows that about 95% of the particles measured are lower than 3 nm and none above 4 nm. The mean surface diameter (Ds) calculated for this sample is 2.7 nm. For 3% Pt/CeO2 (Fig. 1b), the size distribution is
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shifted to higher diameter values with a Gaussian distribution curve centered at 2.5 nm and Ds = 3.9 nm. We can notice that the size distribution of 1% Pt/CeO2 catalyst is narrower compared to the 3% Pt/CeO2. For the 5% and 10% Pt/CeO2 samples, the mean particle sizes are around 4.3 and 4.5 nm, respectively, with nearly no particles below 2 nm. The HRTEM micrographs of the Figs. 1, 3 and 5, 10% Pt/CeO2 catalysts are represented in Fig. 2(a–d). In general for all samples, after reduction at 973 K, metal particles presenting ‘‘moire´’’ fringes are observed. The densities of this peculiar figure depend on the metal loading of the catalyst; only a few ‘‘moire´’’ particles were visible on the low loaded catalyst but, on the 5% and 10%, nearly all the metal particles present ‘‘moire´s’’ figures. Following the general interpretation of TEM images [15], these ‘‘moire´’’ figures reveal an epitaxial growth of platinum (1 1 1) on CeO2 (1 1 1). From a careful measurement of the interval between the fringes, a simple calculation allows us to appreciate with an acceptable accuracy the structural modification of platinum growing epitaxially on ceria. By using the
Fig. 2. HRTEM micrographs of Pt/CeO2 catalysts with different metal weight loadings: (a) 1%, (b) 3%, (c) 5%, (d) 10%, after annealing treatment under hydrogen atmosphere at 973 K.
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following relationship [15], D = d1 d2/(d1 d2) with D (nm), the lattice distance measured on the ‘‘moire´’’ fringes, d2 (nm), the known lattice spacing of CeO2 (1 1 1), 0.312 nm, the lattice spacing of the Pt (1 1 1), d1, can be recalculated. The obtained value 0.234 nm compared to the tabulated value of Pt (1 1 1), 0.2265 nm (in JCPDS-4-0802), shows that, during the epitaxial growth, Pt (1 1 1) plane has to expand its lattice parameter by 3.3%. The ‘‘moire´ figure’’ would result of the coincidence of four expanded planes of Pt (1 1 1) with three planes of CeO2 (1 1 1) (3d2 = 4d1). Finally, the Pt particles, at this reduction temperature, are well faceted and exhibit a truncated cubooctahedron structure. The expansion of the Pt (1 1 1) parameter could also correspond to the formation of CePt5 (1 1 1), 0.2281 nm, grown in epitaxy with CeO2. Both hypotheses, Pt (1 1 1) or CePt5 (1 1 1), will be discussed later. The presence of ‘‘moire´ figures’’ formed of metal deposited on ceria and reduced at 973 K have already been noticed [6,16,17] but was never quantitatively interpreted. 3.2. Catalytic activity of different Pt loaded catalysts reduced at different temperatures Fig. 3a shows the activity versus the reduction temperature for catalysts with different loadings of platinum (1%, 3%, 5% and 10%). Two general remarks emerge from this figure: (i) the 1% Pt/CeO2 sample presents the highest activity (measured by gram of Pt) regardless of the reduction temperature, (ii) the evolution of the activity versus the reduction temperature allows to classify the catalysts in two categories: for 1% and 3% Pt/CeO2 a clear maximum in activity appears at 773 K reduction temperature, followed by a strong decrease at higher reduction temperature. For 5% and 10% Pt/CeO2 the maximum of the activity is shifted to a lower reduction temperature (673 K). Finally, the activities are similar for the 1% and 3% Pt/CeO2 catalysts after a reduction at 973 K and are exactly the same for the 5% and 10% Pt/CeO2 catalysts after reduction at 873 and 973 K. The evolution of the crotyl alcohol selectivity versus the reduction temperature depends also on the content of platinum.
For the samples with high contents (5% and 10% Pt/CeO2), the selectivity is low after reduction at 473, 573, 673 K (13%) but it rises sharply after reduction at 773 K to reach a maxima of 70–83% after 973 K. In the case of the 1% Pt/CeO2 sample, the selectivity is very low at low reduction temperatures (5%) and never exceeds 35%, value obtained at the highest reduction temperature, 973 K. Finally, for the 3% Pt/CeO2 sample, an intermediate evolution is observed. To evaluate better the meaning of the modifications of the catalytic behavior noticed above, we have calculated the turn over frequency of crotyl alcohol and the TOF of butanal, and represented them versus the reduction temperature (Fig. 4a and b, respectively) for all the catalysts. A simple comparison points out that the maxima in butanal TOF, for all the catalysts, are situated at the same temperature but at a lower reduction temperature (673 K) than the maxima in crotyl alcohol TOF (773 K), which is also found at the same temperature for all the catalysts. It means that, increasing the reduction temperature from 473 to 673 K, new sites are created for butanal and crotyl alcohol formation but, after reduction at 773 K the number of sites for the butanal formation decreases while the number of sites for the crotyl alcohol formation keeps increasing. At higher reduction temperatures 873 and 973 K, both types of sites decrease. Looking at the ratio between the crotyl alcohol TOF and the butanal TOF (Fig. 4c), there is a net increase with the reduction temperature from 673 K, except for the 1% Pt/ CeO2 catalyst where this ratio remains lower than 1. 3.3. TEM analysis on 5% Pt/CeO2 reduced at 773 K From our catalytic results, the most important feature is the dramatic change of the TOF in crotyl alcohol and butanal during the reduction from 673 to 773 K. Fig. 5a and b show the TEM micrographs and the size distribution histograms for the 5% Pt/CeO2 reduced at 473 and 773 K. TEM picture show clearly the appearance, during the reduction from 473 to 773 K, of the epitaxial growth of Pt (1 1 1) on ceria. However the image of the sample reduced at 773 K displays two kinds of
Fig. 3. Activity (a) and crotyl alcohol selectivity (b), for 1–10 wt% Pt/CeO2 catalysts after different reduction temperatures.
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Fig. 4. Turn over frequency (TOF) for for 1–10 wt% Pt/CeO2 catalysts after different reduction temperatures: (a) crotyl alcohol, (b) butanal, (c) ratio between crotyl alcohol TOF and butanal TOF.
platinum particles: 3–5 nm particles formed as Pt (1 1 1) in epitaxy on ceria and other particles with no epitaxial feature. The size distribution histograms show a small increase in the particle size from the 473 to 773 K. Moreover, the platinum particles in epitaxy are not well faceted as we observed previously on the sample reduced at 973 K. 3.4. XPS and XRD analyses of 5% Pt/CeO2 reduced at different temperatures To further investigate the correlations between surface structure and reactivity we have studied, by XPS and XRD, the electronic states and the nature of the metallic phases, of the 5% Pt/CeO2 material which exhibits the most striking reactivity evolution as a function of the reduction temperature. Fig. 6a shows the XP spectra of the 5% Pt/CeO2 samples in the Ce 3d region, 870–922 eV binding energy (BE) range, after different treatments. The complexity of the spectra, which were first resolved by Burroughs et al. [18], arises from the hybridation between the Ce 4f levels and the O 2p states. The two sets of spin-orbit multiplets, corresponding to 3d3/2 and 3d5/ 2 levels, are labeled u and v, respectively, up to five contributions in each set exist. The peaks labeled v and v00
are assigned to a mixing of the Ce 3d9 4f2 O 2p4 and the Ce 3d9 4f1 O 2p5 Ce(IV) final states. The peak denoted v000 corresponds to the Ce 3d9 4f0 O 2p6 Ce(IV) final state. On the other hand, v8 and v0 lines are assigned to the Ce 3d9 4f2 O 2p5 and the Ce 3d9 4f1 O 2p6 Ce(III) final states, respectively. The same assignment is applied to the u structure, which corresponds to the Ce 3d3/2 level. The degree of ceria reduction was calculated, after deconvolution of the experimental spectra, from the ratio between the sum of the peak area of the uo, u0 , vo and v0 peaks representing Ce(III) species and the sum of the intensities of all the peak contributions as described by Laachir et al. [19]. For the sample oxidized at 673 K, we notice the presence of v0 and u0 components revealing the presence of 20% Ce3+ species. This apparent reduction of ceria in an oxidized ceria supported metal has been already observed and attributed to numerous effects. Laachir et al. [19] evoked the reduction of ceria by the X-ray beam. Many studies have reported this high percentage of Ce3+ on Rh/CeO2 sample. They explained the presence of Ce3+ by an autocatalysis due to the metal presence. This metal could favor the departure of the labile oxygen atoms from the surface [20]. In our experiments the Ce3+ contribution increased after annealing treatment under hydrogen at 473 K, up to 35%, but did not increase more after a reduction at higher temperatures,
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Fig. 5. HRTEM micrographs and particles size distributions of 5% Pt/CeO2 after annealing treatment under hydrogen atmosphere at 473 K (a) and 773 K (b).
773 and 973 K. This stable reduction state, corresponding to CeO1.82 formula (65% CeO2, 35% Ce2O3) indicates that the fluorite structure of CeO2 (in 5% Pt/CeO2) cannot accommodate, at the surface, a larger concentration of oxygen vacancies than 0.18 per unit cell. Fig. 6b shows the XP spectra in the Pt (4f) region 60–85 eV BE range, after different thermal treatments. For the sample reduced at 473 and 773 K, these spectra are very similar and correspond to the 4f7/2 and 4f5/2 peaks reported for Pt metal, at 70.9 and 74.3 eV, respectively [21]. For the sample reduced at 973 K, the lower BE of the Pt (4f) peaks could be due either to an alloy formation or to strong interactions between Pt and reduced ceria. Fig. 6c represents the atomic ratio (Pt/Ce) after different treatments: this ratio is constant from reduction temperatures of 473 to 673 K and gradually decreases after higher reduction temperature (773–
973 K). This is interpreted partly by a coalescence of the metal particles and also by the migration of Ce3+onto the Pt crystallites or by the burial of platinum crystallites into partially reduced ceria. The XP spectra of O(1s) core line (not represented) show, after hydrogen treatment a shoulder on the high binding energy side of the O(1s) peak at 531.1 eV; the main O(1s) peak being situated at 529.2 eV. This additional O(1s) contribution is attributed to the –OH species [22]. Fig. 6c represents the percentage of these –OH species after different treatments: a maximum is shown on the sample reduced at 773 K, which could indicate that the formation of –OH species at the surface prevents a further ceria reduction. X-ray diffraction has been used to elucidate the degree of platinum support interactions in the materials. Fig. 6d represents the diffractograms in the Pt (1 1 1) region of 5%
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Fig. 6. XPS and XRD analyses of 5% Pt/CeO2 after different treatments: (a) Ce(3d), (b) Pt (4f) (c) atomic ratios of Pt/Ce and OH/O(1s) and (d) XRD diffractograms in Pt (1 1 1) region.
Pt/CeO2 sample reduced at different temperatures 473, 773 and 973 K. After reduction at 473 and 773 K, a peak at 39.818 is observed, corresponding to the Pt (1 1 1) line. The sample reduced at 773 K, shows a smaller FWHM than the sample reduced at 473 K. That reveals an increase of the particle size when increasing the reduction temperature, in accordance with our previous results obtained by TEM [9]. After reduction at 973 K, a broad peak is observed with a maximum at 39.458 and two small humps at 38.838 and 41.248, which correspond to (1 1 1), (2 0 0) and (0 0 2) reflections of CePt5 alloy, respectively. However, to get a good fit with reliable FWHM, it has been necessary to add a Pt (1 1 1) contribution at 39.818. The presence of CePt5 may explain the decrease in the binding energies observed in the Pt (4f) XPS core line after reduction at 973 K. Bernal et al. [23] have reported a similar phase change on platinum supported on CeO2, but for a sample reduced at higher temperature, 1173 K instead of 973 K. However Berner and Schierbaum [24], studying the evolution of CeO2 crystallites supported on Pt (1 1 1) single crystal observed the formation of CePt5 patches and two hexagonal dimensional structure islands of Ce2O3 during an annealing treatment at
900 K under ultra-high vacuum (UHV). These CePt5 patches were found better ordered after annealing at 1000 K. In accordance with this study the formation of CePt5 at 973 K reduction temperature observed in our work, in coexistence with platinum seems reasonable. 3.5. CePt5 powder: X-ray photoelectron spectroscopy and reactivity In order to determine if the formation of CePt5 plays a role in the selectivity, we have tested the reactivity of CePt5 powder. The X-ray diffraction pattern of our sample showed a single phase corresponding to polycrystalline CePt5 (JCPDS 170071). This CePt5 sample was analyzed by XPS. Previous XPS research works [25,26] made on some rare-earth metals (Ce, Sm, Eu, Tm) and alloys (CePd3), show that up to three contributions in each spin orbit set of the Ce 3d multiplets exist. These three component f2, f1, f0 correspond to the transitions from the initial states of three ground electronic states of the cerium metal (3d10 . . . (6s 5d)4 4f0, 3d10 . . . (6s 5d)3 4f1, 3d10 (6s 5d)2 4f2) to the corresponding exciton-like final states (3d9
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Fig. 7. CePt5 alloy: (a) Ce(3d) from XPS, (b) activity and selectivity to crotyl alcohol.
(6s 5d)4 4f0, 3d9 (6s 5d)3 4f1, 3d9 (6s 5d)2 4f2) produced during the photoemission process, respectively. The differences in binding energy observed for these three processes are due to the differences in the screening effects. The latter are more important, in energy [26], for the f2 (well screened) than for the f1 (poorly screened) transitions compared to the f0 transition, which is not affected by the screening effects. Therefore, the f2 transition appears at a lower BE than the f1 followed by f0 transitions. In a material, the probability that the screening level will be occupied depends critically on the coupling to the other occupied levels. This coupling is described in terms of a hopping integral, or degree of hybridization with the delocalized electrons of the system. If this degree of hybridization is small, the probability of transferring an electron to the screening level is small and most of the XPS intensity will be found in the poorly screened peaks (f1 contribution). In conclusion, the peak intensity will give us qualitative information on the valence state of Ce and on the hybridization of the 4f level with the conduction band. In our study the Fermi level has been used as BE reference to take into account the possible charge effects. The XP spectra of CePt5 alloy in the region of 3d peaks are represented in Fig. 7a. The 3d levels have a spin-orbit splitting of 14.7 eV. The low binding energy peaks at 882.4 and 901.2 eV of the 3d5/2 and 3d3/2 components, respectively, are attributed to the well-screened 3d9 . . . 4f2 final state configurations while the most intense peaks at 885.8 and 904.4 eV are attributed to the poorly screened 3d9 . . . 4f1 configurations. Finally, the peaks at 889 and 916.8 eV correspond to the 3d9 4f0 configurations. In CePt5, the 4f2 transitions are high compared to the corresponding ones in pure Ce metal where the well-screened 4f2 peaks contribute for less than 5–10% to the total of 3d XPS peaks. However, the studies on CePd3 have also shown that the intensity was more evenly distributed between the well- and poorly-screened peaks in the alloy than in the pure metal. The same phenomenon has been seen for Ce2O3 materials [27]. In the XP spectra of the sample reduced at 573 and 873 K under
hydrogen, a small decrease in f2 (well-screened) contribution is observed. This decrease can be easily related to the reduction of some CeOx species, which formed during the sample preparation where the sample was in contact with air during few minutes although the sample was sputtered with Ar ion before the analysis. The Pt 4f7/2 and 4f5/2 peaks are observed at 70.4 and 73.9 eV and these values do not change with reduction treatment. They are close to the binding energies observed on the 5% Pt/CeO2 sample reduced at 973 K. The CePt5 sample has a low catalytic activity (Fig. 7b) compared to Pt supported on ceria, which could be partly due to the low surface area, but it is noticeable that the crotyl alcohol selectivity does not exceed 30% whatever the reduction temperature; even it still decreases at high reduction temperature. This result clearly demonstrates that CePt5 has no ability to selectively hydrogenate the C O bond compared to the C C bond in crotonaldehyde. 4. Discussion By using the sensitive crotonaldehyde hydrogenation test we have studied the size effects of Pt particles supported on ceria; this support being well known to induce SMSI at high reduction temperature. On a sample reduced at 973 K, we have observed a correlation between the reactivity and the particle size by varying the loading of platinum on CeO2: the catalyst in which 95% of Pt particles are lower than 2 nm (1% Pt/CeO2) gave a very low crotyl alcohol selectivity (<35%) while the catalyst which contains 80% of Pt particles bigger than 2 nm gave 83% crotyl alcohol selectivity. We have observed by HRTEM that 3– 4 nm metal particles are modified. Epitaxial growth of the metal particles onto the support (CeO2) is observed. At this reduction temperature, the epitaxial relation between the small particles (around 2 nm) and the CeO2 support was not apparent. The TOF in butanal and crotyl alcohol versus the reduction temperature show an inversion of the variation in the 673– 773 K temperature reduction ranges. Effectively, a maximum
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for active sites in hydrogenation of butanal was observed at 673 K while for crotyl alcohol this appears at 773 K. We have observed, by HRTEM, on 5% Pt/CeO2 materials, in the same range of reduction temperature, the appearance of platinum particle restructuring leading to an epitaxial growth of Pt (1 1 1). Sen and Vannice [28], and other groups [29–31] have showed that Pt/TiO2 was more selective to the unsaturated alcohol than Pt/SiO2 and Pt/Al2O3 catalysts. They have pointed out an increase in selectivity and TOF, when the reduction temperature was increased up to 773 K. On the other hand, it is well known that Pt/TiO2 catalysts, reduced at high temperature (773 K), induces strong metal–support interactions, which were ascribed to the migration of TiOx entities onto Pt0 particles. Therefore, Sen and Vannice proposed that the Pt–TiOx interfacial sites are responsible for the adsorption of the C O bond in a,b unsaturated aldehyde and subsequent hydrogenation to the unsaturated alcohol. Later SilvestreAlbero et al. [32] and recently Concepcion et al. [11], used the same explanation on Pt/CeO2 system assuming that Pt–CeOx species will favor the hydrogenation of the carbonyl bond. The results published in [11] are not worth to mention here since the crotyl alcohol selectivity found on their catalysts reached values as high as found on our catalysts. On catalysts made of platinum particles supported on mesostructured CeO2 nanoparticles embedded within ultrathin layers of SiO2 binder (Pt/ CeO2-SiO2) and reduced at 773 K, the selectivity in crotyl alcohol reached up to 80% while for the same catalyst, reduced at 473 K the selectivity was low (<20%). In our case, on Pt/ CeO2 catalysts, similar high crotyl alcohol selectivity has been obtained on samples reduced at higher temperature (973 K). However if our catalyst after reduction at 973 K was reoxidized and re-reduced [9], a reduction temperature of 773 K was sufficient to recover a high level of crotyl alcohol selectivity (87%) with a higher activity than found after the previous 973 K reduction treatment (10:4 mmol s1 g1 Pt instead of 2:1 mmol s1 g1 Pt ). The TEM images of this re-oxidized and re-reduced catalyst showed the same moire´ and faceted particles as the sample reduced at 973 K (Fig. 2b). Otherwise Concepcion et al. [11], in Pt/CeO2-SiO2 catalysts, observed no significant change in crotyl alcohol selectivity when changing the Pt percentages (1%, 3%, 5%). However, it is important to note that the particle size histograms on these catalysts did not exhibit a strong variation as observed in our catalysts. Finally, from their FTIR and XPS results, they concluded that interfacial Pt–Ce3+species were responsible for the hydrogenation of the C O bond. From our catalytic results on 5–10% Pt/CeO2, we could also interpret the peculiar evolution of the catalytic properties occurring at 673 and 773 K as due to the formation of interfacial Pt–Ce3+ sites: the transformation of Pt sites into interfacial Pt–Ce3+ sites after reduction at 773 K leading to a decrease in butanal TOF and an increase in crotyl alcohol TOF. But, the results obtained on the 1% Pt/CeO2 catalyst and the catalytic properties observed on the 3%, 5%, 10% Pt/CeO2 reduced at T > 773 K do not support this hypothesis. First, due to a higher surface to volume ratio in the highly dispersed
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catalyst a higher number of Pt–Ce3+ sites should exist in the 1% and 3% compared to 5% and 10% Pt/CeO2 catalysts, therefore higher crotyl alcohol selectivity should be expected on the low loaded catalyst; on the contrary, we observed a higher crotyl alcohol selectivity for the 5% and 10% Pt/CeO2 compared to the 1% Pt/CeO2. Secondly, the decoration of Pt particles by CeOx species leading to an increase in the number of Pt–Ce3+ interfacial sites is expected to start at a temperature higher than 773 K. Their number (Pt–Ce3+) is expected to increase with the reduction temperature up to 973 K, according to our XPS results (Fig. 6c) and to the HRTEM observations of Bernal et al. [23]. That should lead to an increase of the TON in crotyl alcohol. On the contrary, a decrease in crotyl alcohol TON was observed when the reduction temperature increased above 773 K. From these both series of results, the interfacial Pt– CeOx sites could be hardly considered as the active sites for the hydrogenation of the carbonyl bond. Instead, the effects of the particle size and the reduction temperature on the crotyl alcohol selectivity in our catalysts appear to be much better correlated with a change of the morphology of the Pt particle involving the formation of Pt (1 1 1) in epitaxy on ceria. On the 5% Pt/CeO2 catalyst, during the reduction process from 673 to 773 K, a restructuration process of platinum particles to form Pt (1 1 1) in epitaxy starts and totally develops after reduction at 973 K, in parallel the crotyl alcohol selectivity rose up to 83%. Unlikely, on 1% Pt/CeO2, containing smaller particles, the restructuration process does not operate at 773 K reduction, only a few particles were found in epitaxy with CeO2 after 973 K reduction and on this catalyst the crotyl alcohol selectivity reached only 35%. Therefore, the Pt (1 1 1) layers in epitaxy on ceria could be there considered to favor the hydrogenation of the C O bond to the detriment to the C C bond. This conclusion is in perfect agreement with the previous work relating a theoretical approach [33] on the adsorption of a,b-unsaturated aldehydes on Pt and Pd. The authors concluded that the dense (1 1 1) metal face is not very favorable to the C C coordination and a greater participation of this face in the catalyst surface, with large faceted particles or by support epitaxy, can favor the unsaturated alcohol. From this theoretical study, a,b unsaturated aldehyde hydrogenation has to be considered as a structure sensitive reaction, sensitive to the arrangement of the surface atoms, in relation with the particle size. In fact, some experimental results perfectly support this theory. When prenal ((CH3)2C CH–CHO) was used as reactant [34–36], a molecule containing a more bulky C C bond compared to crotonaldehyde, Pt (1 1 1) single crystal led to 60% unsaturated alcohol selectivity while, on Pt (1 1 0), saturated alcohol and saturated aldehyde were mainly formed. Raab and Lercher [31], studying the particle size effects in crotonaldehyde hydrogenation on Pt/SiO2 and Pt/TiO2 catalysts found that large Pt particles led to higher selectivity and activity than the small particles and they attributed this behavior to the specific reactivity of the Pt (1 1 1) planes towards the preferential hydrogenation of the C O bond. Claus et al. [37,38] found similar sensitivity structure effects on Au, Ag supported on TiO2 in hydrogenation of acrolein and crotonaldehyde, respectively, where the small particles, around
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1 nm, of Ag and Au gave both a lower activity and selectivity to the unsaturated alcohol than the particles around 3 nm. However, some experimental work does not match with this theory. Beccat et al. [39] have found a poor unsaturated alcohol selectivity on Pt (1 1 1) single crystal when studying crotonaldehyde hydrogenation. Some other experiments [40] were performed on a 5% Pt/a-alumina catalyst with large and well faceted Pt particles (50 nm), containing necessarily a large amount of Pt (1 1 1) planes and no more than 10% crotyl alcohol selectivity was obtained. In our catalysts, if the formation of epitaxied Pt (1 1 1) explains satisfactorily the evolution of the selectivities, the decrease in both butanal and crotyl alcohol TOF after 873 and 973 K reveals the loss of active surface. On the 5% Pt/CeO2 catalysts after reduction at 773 K and above, the XP spectra show a decrease in Pt/Ce atomic ratio, the main explanation from 773 to 873 K is that CeOx species will start to cover preferentially the edges and corners of the particles and strongly decrease the C C adsorption in all the catalysts. However, this phenomenon is not restricted to the edges and the corners and can also reach and cover partially the top of the close-packed structure of (1 1 1) platinum leading to a simultaneous decrease of the C O bond hydrogenation. But, as the formation of active sites for the C O hydrogenation will continue to occur by the restructuration process (Pt (1 1 1)) at high reduction temperature, from 773 to 973 K, the decrease in crotyl alcohol TOF is much less important than the decrease in butanal TOF. Finally, at 973 K, further reduction into CeOx species will start to form CePt5 as shown in the X-ray diffractograms where Pt and CePt5 coexist. The formation of CePt5 is confirmed by X-ray photoelectron spectroscopy revealing a shift of the Pt (4f) BE to 70.4 eV instead of 70.9 eV. However, the Ce 3d core spectra did not exhibit any modification at 973 K compare to lower temperature, due to Ce4+ and Ce3+ signals coming from the support, which are dominant. In general, CePt5 alloy has been observed at 1173 K on this kind of materials [24]. However, Schierbaum et al., observed on CeO2/Pt (1 1 1), the formation of CePt5 patches after reduction at 900 K. At 1000 K they observed only the CePt5 phase. Metallic Ce supported on Pt (1 1 1) annealed at 900 K under UHV leads to the formation of two-dimensional islands of CeO2x with hexagonal shape. They concluded that the CePt5 surface alloy islands were formed from such 2D islands which could act as intermediate phase in surface alloying between Pt and Ce. From these observations we could propose that, for our sample reduced at 973 K, the interface between platinum and CeO2, is formed by some layers of CePt5 (1 1 1) ended by a Pt (1 1 1) with an expanded lattice parameters compared to the crystalline Pt (1 1 1) plane. After a re-oxidation treatment on 5% Pt/CeO2 followed by a reduction at 773 K, the increase in activity can be explained by a reverse migration phenomenon of reduced ceria and the presence of high selectivity at lower reduction temperature by the presence of Pt (1 1 1) with a definite particles size. Finally, let us stress the peculiar behavior of the small particles (<2 nm). In the present work, it is important to highlight that the activity is higher in 1% Pt/CeO2 than in the
samples with larger particles size (3%, 5%, 10%) and the TOF in butanal decreases only slightly when going from 673 to 773 K reduction temperature in agreement with the fact that the modification of the Pt particles into Pt (1 1 1) in epitaxy on CeO2 occurs at a very small extent on the 1% Pt/CeO2 sample. In fact, in sufficiently small particles, changes of the surface coordination are linked to change in electronic properties. Gan et al. [40] have observed a stronger interaction of CO on small platinum particles supported on TiO2 (1 1 0) surface. The scanning tunneling spectra revealed that platinum nanoparticles, below 2 nm in diameter, exhibited non-metallic behavior [41]. Therefore, the unusual electronic properties of nanosized platinum particles should be taken into consideration to understand this non-epitaxial relation of the small particles. It is likely that a marked change in the electronic character of nanosized platinum particles below 2 nm may indicate a transition to the non-metallic phase and that the structure sensitivity in this region originates from a quantum-size effect. Effectively, it is well known that the electronic band structure of metal nanoparticles critically depends on its size. For small particles, the electronic states are not continuous, but discrete due to confinement of the electron wavefunction.
5. Conclusion This work shows that, by controlling the nanostructure, size and morphology, of supported platinum particles it is possible to orientate the selectivity in the hydrogenation of crotonaldehyde, an a,b-unsaturated aldehyde. Studying Pt/CeO2 catalysts, prepared from chlorine-free precursor, with different metal loadings and reduced at different temperatures, we could differentiate between the catalysts highly loaded (5 and 10%), containing larger particles, which develop Pt (1 1 1) in epitaxy on ceria around 773 K reduction treatment. The catalysts with a low metal content (1%) present more dispersed metal particles with a large amount of very small particles (<2 nm) showing no particular morphology. On the Pt particles in epitaxy on ceria a very high selectivity for carbonyl bond hydrogenation was obtained (83% to crotyl alcohol) while on the small particles the crotyl alcohol selectivity never exceeded 35%. We conclude that the adsorption of the C O group of the a,b-unsaturated aldehyde is favored by face atoms arranged as Pt (1 1 1) with an expanded lattice parameter in epitaxy with CeO2. Simultaneously to the restructuration process a covering of the Pt particles by reduced ceria was found to take place at reduction temperature higher than 773 K which leads to decrease the production of both butanal and crotyl alcohol. After 973 K reduction, CePt5 alloy was additionally formed but it does not contribute to the increase in the crotyl alcohol selectivity as checked by a study performed on CePt5 powder. A future development to this work would be to verify, by theoretical calculations, our hypothesis that the expanded Pt (1 1 1) in epitaxy on ceria favors the hydrogenation of C O bond in a,b-unsaturated aldehyde.
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