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Synergy of Contact between ZnO Surface Planes and PdZn Nanostructures: Morphology and Chemical Properties Effects in the Intermetallic Sites for Selective 1,3-Butadiene Hydrogenation Eva Castillejos-Lopez, Giovanni Agostini, Marco Di Michiel, Ana Iglesias-Juez, and Belén Bachiller-Baeza ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03009 • Publication Date (Web): 13 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016
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Synergy of Contact between ZnO Surface Planes and PdZn Nanostructures: Morphology and
Chemical
Properties
Effects
in
the
Intermetallic Sites for Selective 1,3-Butadiene Hydrogenation Eva Castillejos-López1, Giovanni Agostini2, Marco Di Michel2,
Ana
Iglesias-Juez3*, Belén Bachiller-Baeza3* 1
Dpto. Química Inorgánica y Técnica, Fac. de Ciencias, UNED, C/ Senda del Rey nº 9,
28040, Madrid, Spain. 2
ESRF- The European Synchrotron, 71 Avenue des Martyrs, 38000 Grenoble, France.
3
Instituto de Catálisis y Petroleoquímica, CSIC, c/Marie Curie No. 2, Cantoblanco,
28049 Madrid, Spain.
Corresponding Authors: *Belén Bachiller-Baeza:
[email protected] *Ana Iglesias-Juez:
[email protected] 1 ACS Paragon Plus Environment
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Abstract: In this work, different shaped ZnO supports were employed to study their effect on PdZn nanostructures. Our goal was to understand the role of ZnO morphology and consequently the major exposed faces (polar (0001), nonpolar (1120) or nonpolar (1010) facets) on the structural and electronic properties of the formed PdZn alloys and how affects their chemical properties in selective 1,3-butadiene hydrogenation. A multitechnique approach that includes in situ synchrotron X-ray diffraction (XRD) and in situ X-ray absorption spectroscopy (XAS) measurements combined with in situ diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) and mass spectrometry (MS) has been accomplished to understand the formation of the PdZn alloy and the structural and electronic properties. Complementary characterization was obtained by high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy analysis of adsorbed CO (CO-FTIR) and X-ray photoelectron spectroscopy (XPS). The alloying degree and the PdZn nanoparticle shape depend on the contacted ZnO surface. The Zn% in the alloy increases from nonpolar (1010) to nonpolar (1120) and to polar (0001) facets. Moreover, zinc modifies low-coordinated surface sites on Pd nanoparticles, increases Pd electron density and produces elongation of the lattice bonds which play a crucial role in diolefin hydrogenation but also important, the results unravel the sensitivity to the alloy phase structure of this reaction. It seems PdZn(100) surface constitutes an active and selective catalytic phase for partial hydrogenation of 1,3-butadiene. Synchronous DRIFTS has allowed us to analyze the relative concentrations of surface species during 1,3-butadiene hydrogenation and confirmed the different stability of the adsorbed intermediate alkenes depending on the nanoparticle shape and degree of alloying. The differences in selectivities in the 1,3-butadiene hydrogenation reaction have been explain in base of the structural and electronic properties of the formed alloy. 2 ACS Paragon Plus Environment
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Keywords: PdZn alloy, ZnO facet, 1,3-butadiene hydrogenation, in situ synchrotron XRD, in situ XAS, DRIFTS
1. Introduction Supported palladium containing catalysts have been extensively studied in the partial hydrogenation of alkynes and alkadienes due to their excellent catalytic performance that can be influenced by controlling several factors. In most situations the monometallic catalyst needs to be modified by using either promoters (Ag, Au, Ga, or Cu) or additives (CO or sulfur) in order to improve the selectivity to the unsaturated compounds [1-4]. In addition, recent studies have also shown that the hydrocarbon hydrogenation presents some structural sensitivity in terms of both activity and selectivity and thus the size of Pd nanoparticles and the geometry of the Pd exposed surfaces are determinant [5-8]. Combined theoretical calculations and experimental results suggest that Pd(100) surface is more active for total hydrogenation than the Pd(111) surface [9]. Studies on model catalysts have shown that Pd(110) exhibits a higher selectivity for butenes formation than Pd(111) [10,11]. Besides, it is well known that surface and subsurface chemistry (hydride and carbide formation) have an important role on the selective hydrogenation of unsaturated hydrocarbons and depend on the preferred exposed crystal planes [12-14]. As commented, alloying with another metal has being attempted to modify Pd catalytic properties and in particular the selectivity since the creation of specific rearrangements results in both electronic and geometric effects [2]. In the end, these modifications can change the stability of bulk palladium phases (metal, hydride) and/or the availability of surface and subsurface hydrogen species affecting Pd catalysts 3 ACS Paragon Plus Environment
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performance [15,16]. So, the increase in the hydrogenation selectivity with respect to acetylene formation over the production of ethane on Pd(111) and (211) surfaces doped with M = Cu, Ag and Au, was explained theoretically by changes in the adsorption configuration and by the blocking of low coordination sites at the step edges [12]. But, electronic effects seem to play an important role in reducing the C2H4 adsorption energies on Pd–M/Pd(111) compared with those on Pd(111). Also, a strong modification of the CO adsorption properties of Pd(111) is observed when it reacts with a metal as zinc and forms ZnPd near-surface intermetallic phases. PdZn-based catalysts, using a ZnO support or a ZnO-promoted support, have been explored in recent years since they exhibited high performance in terms of selectivity and stability in different reactions including steam reforming of methanol, water–gas shift and hydrogenation of alkenes [17-19]. After reduction of the systems, formation of α- and/or β-PdZn alloys and of intermetallic structures have been reported [18]. The effect that these structures have on the mechanism of the different reactions was explained in terms of geometric modifications changing the number of the ensemble of active atoms after Zn incorporation and/or changes in the Pd electronic properties resulting from a charge transfer between Zn and Pd [19,20]. The catalytic properties of different models for the most stable (111) and (100) surfaces of bulk ZnPd system have been studied [18,21,22]. But these models are far from the actual used systems. More recently, some works have focused on supported nanoparticles showing that differ in their adsorption properties from the bulk compounds. It is also important to recall that ZnO can exhibit distinct morphologies dominated by five low Miller index planes: the mixed terminated nonpolar (1010) and (1120) faces, as well as the polar (0001), (0001) and (1121) faces, which depend on the synthesis method and conditions employed [23]. Experimental and theoretical studies have been 4 ACS Paragon Plus Environment
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focused on the energetically most favorable nonpolar (1010) facet and the less stable (0001) and (0001) facets [24]. Generally, the discussion is stablished about polar and non-polar faces without differentiating between the two possible nonpolar faces. In contrast, the nonpolar (1120) facet remains by far less studied, but the little information about it point to different reactivity than the other nonpolar (1010) facet [25]. The higher instability of the polar facets that makes them more reactive and the possible role of the number of defects and the oxygen vacancies at the surface seem to play a crucial role in the activation of reactant molecules [24-26]. Also, it is well known that structural factors, i.e. the different polar/nonpolar exposed facet ratios, affect the reaction behavior of ZnO in photocatalytic reactions and other catalytic applications [27-32]. But different exposed facets not only show different reactivity, they will also present different interaction with a supported metal and therefore they will affect the dispersion and morphology of the metal nanoparticles. Although the role of ZnO faceting on metal supported catalysts has been barely studied, the data seem to confirm this point [33,34]. In line with these findings, formation of the different PdZn phases and consequently the catalytic performance are influenced by the exposed facets. A recent study of Pdsupported on a ZnO with predominant non-polar facets and on a commercial ZnO without any dominant facets in the methanol steam reforming showed clear differences which also depended on the Pd content and on the ZnO crystallite faceting [33]. Besides, differences in the CO adsorption and in their activity of methanol on Pd/ZnO (0001) and Pd/ZnO (1010) have been described [35]. In general, stronger electronic interaction and easy PdZn alloy formation was obtained on polar facets [33]. Nevertheless, when keeping these differences in mind, studies correlating the structural and electronic properties of PdZn exposed surfaces with those exposed by the ZnO used as support are scarce. 5 ACS Paragon Plus Environment
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To our knowledge, the relationship between the ZnO polarity ratio and selectivity toward selective hydrogenation reactions over Pd/ZnO systems has not been extensively studied. In addition, the mayor attention has been focus on the characteristics of ZnO support and less on the PdZn entities. Therefore in this work, different shaped ZnO supports exposing distinct planar surfaces were employed to study the effect on Zn content, morphology and chemical properties of the supported PdZn nanostructures. A multitechnique approach that includes in situ synchrotron XRD and in situ XAS measurements combined with in situ DRIFTS and MS has been accomplished to understand the formation of the alloy and to correlate the catalytic performance in the selective hydrogenation of 1,3-butadiene with the structural properties. Synchrotron XRD is valuable technique when low metal loadings and small particles are investigated and EXAFS complement valuable information of the local structure. IR techniques and XANES will provide clue electronic information and in situ DRIFTS allows to studying reaction intermediates, the surface and its interaction with reactants. Our goal was to understand the role of ZnO morphology and consequently the exposed faces on the structural and electronic properties of the formed PdZn alloys and how affects their catalytic behavior.
2. Experimental 2.1. Preparation of ZnO supports and catalysts. Three ZnO supports with different shapes and therefore different face exposure were used: needles (ZnO-n), bricks (ZnOb) and tetrapods (ZnO-t). Briefly, materials ZnO-n and ZnO-b were prepared using a microemulsion method employing n-heptane (Scharlau) as organic media, Triton X-100 (C14H22O(C2H4O)n) (Aldrich) as surfactant and hexanol (Aldrich) as cosurfactant. Zn was introduced from the nitrate form (Aldrich). Water/Zn molar ratio was fixed at 110 6 ACS Paragon Plus Environment
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and water/surfactant molar ratio of 18 (ZnO-b), and 36 (ZnO-n). After introducing Zn into the aqueous phase and 30 min of stirring, a double quantity of tetramethyl ammonium-hydroxide was introduced from the aqueous phase of a similar microemulsion. The resulting mixture was stirred for 24 h, centrifuged, and the separated solid precursors rinsed with methanol and dried at 110 ºC for 12 h. See more details at [27]. High purity tetrapods of ZnO (ZnO-t) were prepared by gas phase oxidation of zinc in the presence of air at 1073 K, where the temperature of zinc sublimation and the temperature of oxidation are tuned to achieve shape and size control [36].The synthesis was carried out in a horizontal three zone furnace with a tubular quartz gas flow reactor under atmospheric pressure. The Zn metal powder (325 mesh size) contained in an alumina boat was introduced into the central hot zone, and once the temperature was stable, the air was introduced at the third zone of the reactor. The white flakes of ZnO were collected in cold traps at the end of the reactor. The tetrapod structure was formed by the gas phase self-assembly of nanorods. Pd-supported on ZnO were prepared by incipient wetness impregnation using water solutions of Pd(NO3)2. The catalysts were prepared with a 2 wt% Pd loading. The catalysts were dried in air overnight. A catalysts using TiO2 support (commercial P-25 Degussa) was also studied as a reference for pure Pd systems and was prepared in the same way.
2.2 Instrumentation and catalysts characterization. Powder X-ray diffraction (PXRD) patterns of the samples were obtained using a Polycristal X'Pert Pro PANalytical diffractometer with Ni-filtered Cu Kα1 radiation (λ = 0.15406 nm) operating at 45 kV and 40 mA with a Bragg-Brentano geometry. For each sample,
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Bragg's angles between 4 and 90° were scanned with steps of 0.02° and integration time of 50 s per step. Transmission electron microscopy (TEM) images of the catalysts were measured using a JEOL JEM-2100 field-emission gun electron microscope operated at 200 kV. The samples were ground and ultrasonically suspended in ethanol before TEM images were generated. The samples were prepared by grinding and ultrasonic dispersal in an acetone solution. The mean diameter (d) of the Pd particle size were calculated measuring between 150 and 200 particles, using the following equation where ni is the number of particles with diameter di: =
∑ ∑
CO adsorbed infrared spectra were recorded on a Varian 670 Fourier Transform IR spectrophotometer equipped with an MCT detector with a resolution of 4 cm-1. Selfsupporting wafers of the samples with weight-to-surface ratios of about 10 mg·cm-2 were placed in a vacuum cell assembled with greaseless stopcocks and KBr windows. Pretreatment was carried out with an in-situ furnace. The specimens were reduced in a H2 flow of 30 ml·min-1at 523 K for 1 h and outgassed at the same temperature for 1 h. After cooling to room temperature, CO (80 Torr) was adsorbed and the IR recorded after removing the gas phase. The infrared spectra of the adsorbed species were obtained by subtracting the spectrum of the clean sample from the spectrum obtained after adsorption. All the samples were characterized by X-ray photoelectron spectroscopy (XPS). The spectra were recorded with an ESCA-PROBE P (Omicron) spectrometer by using nonmonochromatized Mg-Kα (1253.6 eV) or Al-Kα radiation (1486.7 eV). Each sample 8 ACS Paragon Plus Environment
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was pressed into a small pellet of 15 mm diameter, placed in the sample holder of a coupled sample treatment chamber and reduced at 250 ºC under a flow of 50 ml/min of a 10% of H2 in Ar mixture. Then, the sample was transferred to the preparation chamber, degassed for 6-8 h to achieve a dynamic vacuum below 10-8 Pa before analysis and finally transferred to the analysis chamber. The spectral data for each sample was analyzed using CASA XPS software. The relative concentrations and atomic ratios were determined from the integrated intensities of photoelectron lines corrected for the corresponding atomic sensitivity factor.
2.3. Synchrotron in situ XRD experiments. High-energy X-ray diffraction (HEXRD) data were collected at the ID15A beamline at the ESRF-The European Synchrotron with a wavelength of 0.17750 Ȧ (69.85 KeV), using a Perkin Elmer area detector and a sample-detector distance of 650 mm. The diffraction patterns were collected with an exposure time of 30 seconds. Diffraction data were presented versus scattering vector, Q=4π(sinθ)/λ. The reactor and heat guns were mounted on a stage capable of translations in the x, y and z directions. The catalysts were pressed into pellets and sieved to a size of 0.075 to 0.150 mm. Aliquots of these pellet samples were loaded in a fixed bed quartz tube of 2 mm internal diameter; glass wool was used to prevent sample movement. A constant reaction flow was feed to the reactor at 20 mL·min-1, with compositions of 15% H2 in helium during temperature programmed reduction experiments and 15% H2 + 3% butadiene (Bd) in helium during hydrogenation reaction conditions. H2 / H2+Bd / H2 alternate exposures at constant temperature were applied with an exposure time of 30 min for each atmosphere. The reactor was heated with one Leister LE mini heat gung fitted with a heat spreader. The temperature inside the catalytic bed was measured using a thermocouple inserted into the carbon supported 9 ACS Paragon Plus Environment
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sample. The range of operation temperatures was from room temperature up to 250 ºC. The products from the reactor were analyzed using a European Spectrometry ecoSyst-P Man-Portable mass spectrometer with capillary inlet and heated inlet tubes.
2.4. Synchrotron in situ XANES-DRIFTS combined experiments. Pd K edge (24.350 eV) X-ray absorption measurements combining simultaneously with diffuse reflectance infrared spectroscopy (DRIFTS) and mass spectrometry (MS) were recorded at beamline BM23 of the European Synchrotron Radiation Facility (ESRF, Grenoble, France) [37]. The catalysts, ca. 30-40 mg of sample, were loaded in a 4 mm cell that allows working with XAS technique synchronously with DRIFTS and MS techniques. The sample was aligned respect to X-Ray with micrometric precision tightening four bolts by a 34 size wrench. XAS measurements were performed in transmission mode using ionization chambers for both I0 and I1. A third ionization chamber was used to acquire the reference Pd foil XANES simultaneously and ensure an appropriate energy calibration. The white beam emitted by the storage ring was monochromatized using a double crystal Si(111) monochromathor. DRIFTS measurements were made employing a Varian 670 (mid range) IR spectrometer coupled with a MCT detector and special designed DRIFTS optics for this application. Evolving gases analysis was carried out using a quadrupole mass spectrometer (Pfeiffer Omnistar). The flow of reaction gases feed to the reactor was constant at 60 mLmin-1, with compositions of 15% H2 in helium during temperature programmed reduction experiments and 15% H2 + 3% Bd in helium during hydrogenation reaction conditions. The temperature was raised by 3ºC min-1 up to 250 ºC during the TPR. During the heating and cooling ramps XANES spectra were collected from 24.450 to 24.270 keV in continuous acquisition mode (q-EXAFS) resulting in a time resolution of 1.5 min. After cooling to the reaction temperature, 10 ACS Paragon Plus Environment
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EXAFS spectra were collected up to 11 Å-1, with a variable sampling step in energy, resulting in: 5 eV in the pre-edge region, 0.5 eV in XANES region and ∆k constant in EXAFS region with 0.03 Å-1 step, and integration time 2 s along all spectra. Then, XANES-DRIFTS time resolved experiments (0.5 s) with H2 / H2+Bd / H2 alternate exposures at constant temperature were applied with an exposure time of 30 min for each atmosphere. The exhaust products from the reactor cell were analyzed using MS.
2.5. Catalytic measurements: hydrogenation of 1,3-butadiene. The catalytic experiments for butadiene (Bd) hydrogenation were carried out in a continuous flow fixed-bed reactor. Before reaction, the catalyst was pretreated in flowing H2 at 250 ºC for 1 h. The reactants, H2 (10% vol) and Bd (2% vol) with the balance of N2, passed through the catalyst bed at a total flow rate of 60 mL/min. The hydrogen amount was in large excess. The reaction temperature was varied between room temperature and 250 ºC at atmospheric pressure. The reactor effluent was on-line analyzed using a gas Varian 3400 gas chromatograph with flame ion detector (FID) and thermal conductivity detector (TDC) with a 20% BMEA Chromosorb P80/100 column. The conversion of Bd was calculated as;
(%) =
− × 100
where CBdi represents the initial Bd molar flow rate (mmol/min) and CBdf the final Bd molar flow rate. The selectivity of each product was calculated as; (%) =
× 100
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where Cprod represents the molar flow rate of the product whose selectivity is being calculated e.g. 1-butene, and Ctotal
prod
represents the total molar flow rate of all the
products e.g. 1-butene, cis-2-butene, trans-2-butene and butane.
3. Results 3.1 Characterization of catalysts PXRD patterns of the Pd-ZnO samples are shown in Figure 1. No diffraction peaks relating to Pd phases were detected for any of the samples due to the low loading here employed. Diffraction peaks associated to ZnO würtzite phase were observed (JCPDS 0)/(0002) diffraction peaks intensity ratio (Table 1) 36-1451). In these systems the (101 is used to evaluate morphology differences, mainly between polar and non-polar face exposure relation [38,39]. A diminish of that ratio indicates a morphology variation in which the piling along the crystallographic c axis is favored, therefore leading to a decrease of the polar vs. nonpolar surface extension. According to the PXRD results ZnO-n sample presents highest value (1.45) evidencing higher extension of polar planes at the surface than ZnO-t and ZnO-b samples that exhibit similar values, 1.14 and 1.10 respectively. In ZnO-n the growth direction is perpendicular to c; in contrast, the other supports grow along c axis. Considering other diffraction peak intensity relations we can better dissociate differences between ZnO-t and ZnO-b supports. The 0)/(1012) and (112 0)/(101 2) ratios can also provide information about the (101 stacking along the c axis. Pd-ZnO-t presents lower values than Pd-ZnO-b which suggests longer entities growing along the c axis for Pd-ZnO-t than Pd-ZnO-b. In these 0)/(1120) ratio provides an idea about the relative proportion two last samples, the (101 between the two non-polar faces exposed. The lower value for Pd-ZnO-t indicates that 12 ACS Paragon Plus Environment
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0) planes than Pd-ZnO-b. In the case of Pd-ZnO-n it exposes higher proportion of (101 suggests that the [110] axis is the growth direction of the ZnO needles thus showing the highest value. According to the present PXRD results Pd-ZnO-n maximize the exposure of polar planes, Pd-ZnO-t the proportion of nonpolar (1010) surfaces and Pd-ZnO-b of nonpolar (1120) surfaces.
(0002)
(1011) ‾
(1010) ‾ (1120) ‾
Intensity (a.u.)
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(1012) ‾
c
b
a 20
30
40
50
60
70
80
2θ (º)
Figure 1. PXRD patterns of a) Pd-ZnO-t, b) Pd-ZnO-b and c) Pd-ZnO-n catalysts. (λ=0.15406 nm) Table 1. Diffraction peaks intensity ratios. Sample ZnO-t ZnO-b ZnO-n
(1010)/(0002) 1.14 1.10 1.45
(1010)/( 1120) 1.45 1.66 2.08
(1010)/( 1012) 2.11 3.86 3.33
(1120)/( 1012) 1.46 2.32 1.60
The PXRD results are further corroborated by additional morphological analysis of the catalyst by TEM. Figure 2 and Figures S1 to S4 (in Supported information) present TEM and HRTEM pictures, and the particle size distribution of the palladium 13 ACS Paragon Plus Environment
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nanoparticles for catalysts prepared on TiO2 and on the three ZnO materials. The Pd mean particle diameters d were calculated from TEM measurements.
Figure 2. Representative TEM images of (A) Pd-TiO2, (B) Pd-ZnO-t, (C) Pd-ZnO-b and (D) Pd-ZnO-n catalysts. As shown in Figure 2 and Figure S1, Pd-TiO2 exhibits a wide particle size distribution. The estimated average particle size was 5.3 nm, which is the biggest among the studied catalyst. The spacing of the lattice fringes from the Pd nanoparticles were found to be 0.23 nm which can be indexed as Pd(111), according to JCPDS (ICDD) card number 046-1043. For the tetrapod-type ZnO morphology like in sample Pd-ZnO-t, an uniform distribution of relatively semi-spherical/half-truncated octahedral palladium particles of about 2-3 nm were observed (Figure S2A). Fig. S2B presents a HRTEM image for this sample and Figure S2D the zoom of the highlighted area and the corresponding Fast Fourier Transform (FFT) diffraction patterns of selected zones of the ZnO rod and of a 14 ACS Paragon Plus Environment
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Pd particle. The ZnO FFT diffraction pattern indicates the presence of well-defined nanostructures displaying a [2110] zone axis. Therefore, the rods are elongated along the c crystallographic orientation and present larger contribution of nonpolar (1010) as above described [36]. The spacing of the lattice fringes from the Pd nanoparticles were found to be 0.23 nm which can be again indexed as Pd(111). For Pd-ZnO-b sample (Figure S3), the support has well-defined nanostructures similar to those described in the literature [27] that correspond to slightly elongated particles along the c crystallographic orientation and presenting larger contribution of nonpolar (1120) plane types as PXRD showed. Here Pd NPs are more flat and present an average particle size of 2.9 nm. Sample Pd-ZnO-n (Figure S4A) showed sharp needle-like structures of the support that grow in a direction perpendicular to the c-axis as already reported [27]. Here Pd particles display a more 3D structure with higher dimensions: 4 nm average size. Finally, the analysis shows that the supports retain the original morphology and are not modified after introducing Pd.
3.2. Alloy Formation: 3.2.1. In situ XRD during H2 treatment To characterize the Pd component, in situ synchrotron XRD patterns were collected during temperature programed reduction of the samples. Figure 3 shows a selected region where the main lines of the metallic Pd FFC phase are expected. In the HEXRD patterns of Pd-TiO2 diffraction features arising for TiO2 and Pd phases would be superimposed. Although no subtraction of the TiO2 phase was performed, some inferences can be obtained viewing the evolution of the patterns with increasing 15 ACS Paragon Plus Environment
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temperature. At room temperature only peaks arising for the anatase and rutile TiO2 structure were observed. As the temperature increased two shoulders at around Q = 2.8 and 3.2 Å-1 started to develop, and they became more apparent at the end of the ramp of temperature. These features correspond to the (111) and (200) diffraction peaks of metallic palladium FFC phase (according to JCPDS (ICDD) card number 046-1043). On the other hand, the samples on ZnO supports were characterized with strong reflections corresponding to the ZnO würtzite structure (JCPDS 36-1451) (Figure S5) The small random peak appearing at Q = 3 Å-1 in the Pd-ZnO-t could be an artefact originated by aluminum scattering from a slight unaligned of the heating gun (misaligned respect to the plug-flow reactor). For Pd-ZnO-n and Pd-ZnO-b the additional peaks that appear at RT in the range Q = 2.7-3.1 Å-1 and that were suppressed when the temperature was increased, are due to residual species coming from the preparation of the samples. No peaks ascribed to pure metallic palladium were discerned in the patterns. However, it can be generally seen for the three catalysts that upon increasing temperature a very broad band centered around Q = 2.87 Å-1 emerged. This feature could be ascribed to the (111) diffraction peak from a PdZn alloy. If a Pd-rich FCC phase is present, the shift in the (111) diffraction peak position toward higher angles compared to metallic Pd is explained by the contraction of the lattice due to the smaller atomic radius of zinc compared to palladium. The corresponding (200) Pd peak overlaps with the intense (1012) reflection from the ZnO würtzite structure and cannot be analyzed. Therefore, under H2 treatment PdZn alloy takes place and after reduction at 250 ºC, Zn is randomly substituting the Pd keeping the fcc structure, probably forming an α-phase type alloy and not a tetragonal 1:1 β-phase, since in such case two peaks at Q = 2.9 and 3.1 Å-1 should have developed [40,41] . This α-phase has been reported for solid solutions with up to 18 at% substitution [42]. 16 ACS Paragon Plus Environment
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The morphology of the ZnO supports or in other word, the different interaction with the preferential exposed planes of each oxide affects the temperature onset of PdZn alloy formation, following the trend Pd-ZnO-n< Pd-ZnO-b < Pd-ZnO-t. And it also defines the final particle size of the PdZn entities as inferred from the differences in the peak width, which is in agreement with TEM observations.
A
C
B
D ‾ ZnO(1012)
‾ ZnO(1012)
‾ ZnO(1012)
PdZn(111)
Pd(111)
Pd(200) PdZn(111)
Intensity (a.u.)
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523 K PdZn(111)
RT TiO2(111) 2.6
2.8
TiO2(210) 3.0 -1
Q (Å )
3.2
2.6
2.8
3.0
3.2
-1
2.6
2.8
3.0
3.2
-1
Q (Å )
Q (Å )
2.6
2.8
3.0
3.2
-1
Q (Å )
Figure 3. In situ HEXRD during TPR for Pd-TiO2 (A), Pd-ZnO-t (B), Pd-ZnO-b (C), Pd-ZnO-n (D).
3.2.2. In situ XAS during H2 treatment X-ray absorption near-edge spectra were registered during in situ H2-TPR for each sample. The initial Pd K-edge XANES spectra for all the studied samples correspond to Pd(II) and are displayed in Figure S6 proving similar initial state after precursor impregnation. For Pd-TiO2 the initial spectrum changed markedly right after the cell was supplied with the H2 mixture (Figure S7) resembling immediately to that for Pd foil 17 ACS Paragon Plus Environment
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reference. During the ramp, the continue resonance (CRs) shape become more define indicating the increasing of the nanoparticle size [43]. Also a shift in the CRs maximum to higher energies is observed (Figure S7). Since the edge position correspond to Pd(0) and there is no modification during the ramp, this change could be related to a contraction of the lattice distances considering the 1/R2-rule [44]. Pd affinity to form hydride species in H2 presence is well known and reported [45-48]. These species are no stable over temperatures of 120-150 ºC. So for Pd-TiO2 sample, reduction of palladium from the initial Pd2+ state occurs through an intermediate species, Pd hydride that is formed as soon as the sample is exposed to H2. This species progress to metallic palladium as the temperature increases. However, in the samples on ZnO supports, the XANES spectra evolve gradually with temperature. Particularly above 50−100 °C, depending on the sample, the absorption edge shifts toward lower energies and the wide and prominent first resonance (white line at 24370 eV) becomes decreased, leading to a spectrum similar to that of Pd(0). The set of XANES spectra were subjected to principal component analysis (PCA) to determine the number of Pd species and its evolution during the H2-TPR experiment (see SI for more details). PCA results indicate that the reduction process occurs in one step for all of them, however the temperature at which reduction starts differs. As shown in Figure 4, the on-set temperature varies with the support and follows the order Pd-ZnO-n < Pd-ZnO-b < Pd-ZnO-t that agrees with the conclusions found by in-situ HEXRD.
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1.0
Fraction of pure species
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0.8
0.6
a)
c)
b)
d)
0.4
0.2
0.0 20
40
60
80
100
120
140
Temperature (ºC)
Figure 4. Fraction of initial (closed symbols) and final species (open symbols) during TPR in H2. a) Pd-TiO2, b) Pd-ZnO-n, c) Pd-ZnO-b and d) Pd-ZnO-t.
The XANES spectra of the final Pd-species obtained for all samples are displayed in Figure 5. For all, XANES shape is readily ascribable to a Pd(0) species. Although the edge position and CR characteristics allow an assignment to a zero-valent Pd(0) species in a fcc-like matrix [49], for Pd systems on ZnO supports there are differences with the typical pure metal XANES spectrum (as that displayed by the Pd-TiO2 sample) concerning the energy position of the edge and the CRs, and their intensity. ZnO supported samples present lower values of the Pd K-edge energy position than Pd-foil reference (expanded view as inset in Figure 5). This suggests higher electronic density of Pd entities than in the foil. There is also a shift to lower energy visible for the 5sp (ca. 24,365 eV) and 4f (ca. 24,395 eV) CRs as well as a significant decrease of their intensity. The intensity of the 5sp and 4f CRs, can be related to a lower density of unoccupied states in agreement with the above indicated higher electronic density. This suggests a contribution from Zn atoms which can transfer electronic density into the 19 ACS Paragon Plus Environment
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empty Pd states. Also, small differences among ZnO samples can be explained on the basis of moderate variations in particle size, which, on the other hand, appears to be lower (considering the lower height and larger width of the CRs [43]) for the Pd on ZnO-t ~ ZnO-b < ZnO-n systems than on TiO2. The energy shift of CRs could be justified on the basis of Zn dilution into Pd structure which increases the Pd-Pd distances reflecting in a decrease in the RC energy position (1/R2-rule). Different morphologies, a 2D planar-like morphology for the Pd particle [43] could also affect. All these facts could be understood by considering the formation of PdZn alloy (see below). Besides, the slight shift observed between the spectra for the three ZnO samples suggests different Pd environments depending on the ZnO morphology.
0.8
0.6
PdTi 250ºC PdTetra PdE2 PdE5 Pd Foil 1.0
Normalized Absorbance (a.u.)
1.0
Normalized Absorbance (a.u.)
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0.4
0.2
0.8
PdTi 250ºC PdTetra PdE2 PdE5
0.6
0.4
0.2
24350
24360
24370
Energy(eV)
0.0 24340
24360
24380
24400
Energy(eV)
Figure 5. Pd K-edge XANES spectra for final species after TPR in H2 for Pd-TiO2, PdZnO-t, Pd-ZnO-b, Pd-ZnO-n and Pd foil reference.
Therefore, in the Pd-TiO2 catalyst, Pd reduction occurs immediately after H2 contact and the largest Pd particle size is obtained which stabilizes as hydride phase in this 20 ACS Paragon Plus Environment
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hydrogen atmosphere when the temperature of the treatment decreases. In systems on ZnO, there is a strong interaction with the support that delays Pd reduction to higher temperatures and leads to the formation of a PdZn alloy. This interaction depends on the specific surface plane on which Pd entities has been deposited (in a majority) observing that the higher interaction the greater alloying degree and this PdZn alloy is obtained at lower temperatures. The temperature formation onset increases from the Pd-ZnO-n sample, followed by Pd-ZnO-b and Pd-ZnO-t, being the mostly exposed ZnO faces in each case: polar (0001), nonpolar (1120) and nonpolar (1010) planes, respectively. Zn is randomly substituting the Pd in the fcc structure probably forming an α-phase alloy. The alloying modifies the electronic and structural properties of the Pd centers and it is detected greater electron density and greater elongation of the lattice bonds when higher degree of alloying. The temperature onsets were slightly higher for HEXRD because this technique needs higher long range ordering for detection. Along H2 treatment, increasing temperature increases the particle growth allowing its detection by HEXRD. It is important to remark that when PdZn alloy is formed there is a suppression of the hydrogen diffusion into the metal particle. After the reduction treatment, EXAFS spectra at the Pd K-edge were acquired for all the samples to investigate the local structure around the Pd atoms. The Fourier transforms of the Pd k3-weighted spectra are shown in Figure 6 and Figure S8, and the structural parameters determined from the curve-fitting are summarized in Tables 2 and 3 (complementary information in Table S1). The EXAFS data of the Pd-TiO2 catalyst show that Pd is present as Pd(0) species. The Pd−Pd atomic distance in the first coordination shell is 2.80 Å that is slightly higher than the 2.750 Å value characterizing bulk Pd metal. This suggests, as mentioned above, that H atoms diffuse inside the FCC 21 ACS Paragon Plus Environment
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Pd structure lengthening the Pd lattice distances. The formation of this hydride metallic nanoparticles phase has been extensively reported for Pd [47,48].
A
B 12
12
Foil Pd-TiO2
8
Pd-ZnO-n Pd-ZnO-b Pd-ZnO-t
8 4
FT(Å-4)
4
FT(Å-4)
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0 -4
0 -4
-8
-8
-12
-12 1
2
3
4
5
1
R(Å)
2
3
4
5
R(Å)
Figure 6. k3-weighted modulus and imaginary part of the Fourier Transform of Pd Kedge EXAFS spectra corresponding to the reduced catalyst at RT. A) Pd-TiO2, B) Pd on ZnO supports. The Fourier Transform of the Pd foil is shown using dot lines as reference.
Table 2. Structural parameters derived from analysis of the EXAFS data in Figure 6 for Pd catalysts at RT after reduction treatment up to 250 ºC. 250 ºC Sample Pd-TiO2
Pd-Zn shell R N1 -
Pd-Pd shell R N2 2.80 8.5
N1/N2 -
Pd-ZnO-t
2.54
0.6
2.90
2.1
0.3
Pd-ZnO-b
2.49 2.51
1.3 1.4
2.86 2.88
1.5 1.4
0.9 1
Pd-ZnO-n
The cases of the samples supported on zinc oxides, clearly differ from the reference Pd foil (Figure 6B). It is necessary to introduce another coordination shell, Pd-Zn, at 22 ACS Paragon Plus Environment
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smaller distances (R = 2.6 Å) to obtain a good fit to the data (Table 2). The presence of this Pd-Zn coordination shell is evidence for the existence of a PdZn alloy in the catalysts. Thus, a second Pd–Pd shell with longer interatomic distance (around 2.9 Å) than in the foil was also included for the fit. This scattering path corresponds to Pd-Pd bond in alloyed Pd [50]. The coordination number of Pd-Zn path increased from 0.6 to 1.4 from ZnO-t < ZnO-b < ZnO-n, with concomitant decrease of the Pd-Pd coordination number. This indicates higher Zn content in the alloy phase for the Pd-ZnO-n sample. An estimation of the Zn content can be obtained by comparing the coordination numbers of Pd–Zn and Pd–Pd shells obtained from the fitting results. In this case, the fit with approximately 1.4 Pd–Zn nearest neighbors at 2.51 Å and close to 1.4 Pd–Pd neighbors at a longer distance of 2.90 Å would be consistent with formation of a PdZn intermetallic structure as proposed above with ~15-20% Zn content. However, for PdZnO-t the exact structure cannot be decisively determined from EXAFS numbers. Therefore it is not clear whether the alloy is random or ordered or if there are multiple phases present. For instance, the small Pd–Zn contribution could be due to alloy formation occurring at the surface of the particles, leaving behind a Pd-rich. The absence of the hydride phase under a H2 atmosphere agrees with the suppression of hydride formation reported on silica or alumina-supported Pd catalyst when added ZnO [50,51], and suggests that if a Pd-rich FCC phase is present, it was not accessible to the H2 atmosphere. Base on HEXRD data the presence of only one phase is suggested. Coordination numbers obtained from the EXAFS analysis also provide an indication of the Pd phase particle size. It can be concluded in good correlation with previous results the highest particle size was obtained over TiO2 support and then on ZnO-n followed by ZnO-b and ZnO-t which present similar ones. PdZn entities differ in the amount of Zn in the alloy and a progressive Zn enrichment is observed from ZnO-t to 23 ACS Paragon Plus Environment
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ZnO-b and ZnO-n. The presence of Zn does not show metal particle size dependence, as the results show similar Pd size for PdZnO-t and PdZnO-b. It seems the degree of PdZn alloying after the reduction treatment at 250 ºC depends on surface properties of the employed ZnO support. Reduction treatment at higher temperature, 350 ºC, produced a greater degree of alloying, evidenced by the Pd-Pd increased distance up to about 2.89 A, while Pd-Zn distance remained constant around 2.56 (Table 3). The ratio between the CNs of both shells changes and presents values around 2 for all samples, indicating the formation of 1:1 PdZn alloy on the three ZnO supports [40,52].
Table 3. Structural parameters derived from analysis of the EXAFS data in Figure S8 for Pd catalysts at RT after reduction treatment up to 350 ºC.
Sample Pd-TiO2
350 ºC Pd-Zn shell R N1 -
Pd-Pd shell R N2 2.80 10.4
N1/N2 -
Pd-ZnO-t
2.53
3.5
2.86
1.6
2.1
Pd-ZnO-b
2.56 2.56
3.6 3.5
2.82 2.87
1.8 1.8
2.0
Pd-ZnO-n
1.9
3.2.3. XPS study The electronic properties of Pd were studied analyzing the XPS spectra for the doublet Pd 3d5/2and 3d3/2 of all the fresh and the in-situ reduced catalysts as shown in Figure 7. The peak Pd 3d5/2 of the spectrum for the Pd-TiO2 fresh sample can be curvefitted with two individual components or states of Pd at 336.8 and 335.3 eV. The first contribution is assigned to Pd (II) species [53] as expected for the metal precursor used, 24 ACS Paragon Plus Environment
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and the second to Pd(0) [54] in lower proportion which reflects the partial reduction of the catalysts under vacuum conditions. After H2 treatment only this last contribution is observed indicative of the Pd reduction. The spectra for the fresh samples supported on the different Zn oxides show the contribution at 336.8 eV due to Pd(II) initial state but in addition a peak at higher binding energy (BE), 339.4 eV, can be also observed. This component has not clear assignation and disappeared after H2 treatment. Then, the spectra can be fitted with only one contribution with BE around 335 eV for Pd 3d5/2, corroborating the reduction of the Pd species. Chemical shifts to higher BE of the Pd signal varying from 0.6 to 1 eV have been reported when PdZn alloy is formed due to electronic modifications [22,55]. However, very slight variations, within the error, of the peak position could be observed for the different catalysts. That suggests that low amounts of Zn are incorporated into the Pd structures given the formation of an intermetallic compound similar to the αphase [42]. The Pd 3d5/2 and 3d3/2 peak intensity decrease from Pd-ZnO-t > Pd-ZnO-n > Pd-ZnO-b implies that the proportion of incorporated Zn atoms varies between the samples changing the number of available Pd sites on the surface.
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Pd-ZnO-n
Pd-ZnO-b
CPS
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Pd-ZnO-t
Pd-TiO2
346
344
342
340
338
336
334
332
BE (eV)
Figure 7. XPS spectra of the Pd 3d core level for Pd-TiO2, Pd-ZnO-t, Pd-ZnO-b and Pd-ZnO-n. Black lines: fresh samples; red lines: after H2 treatment at 250ºC.
For the samples using ZnO supports the O 1s and the Zn LMM Auger peaks were also registered to analyze the differences on exposed surfaces and possible Pd induced modifications. O 1s signals that are shown in Figure S9 can be coherently deconvoluted in three components in agreement with previously reported studies [56-58]. Data are collected in Table S2. The intensity of the Oa peak at 530.3 eV, attributed to oxygens from bulk Zn–O bonds [31,56,57], exceeds those of the Ob (531.8 eV) and Oc (532.8 eV) peaks. The higher binding energy component Oc at 532.8 eV is usually attributed to 26 ACS Paragon Plus Environment
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adsorbed oxygen species, such as −CO3, H2O or O2, but it has been also related to hydroxyl species associated to defect sites on the ZnO surface [57,58]. This component follows the trend Pd-ZnO-b ≤ Pd-ZnO-t
