Dynamic Reorganization of Bimetallic Nanoparticles under Reaction

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Dynamic reorganization of bimetallic nanoparticles under reaction depending on the support nanoshape: The case of RhPd over ceria nanocubes and nanorods under ethanol steam reforming Lluís Soler, Albert Casanovas, James Ryan, Inmaculada Angurell, Carlos Escudero, Virginia Perez-Dieste, and Jordi Llorca ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00463 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 12, 2019

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Dynamic reorganization of bimetallic nanoparticles under reaction depending on the support nanoshape: The case of RhPd over ceria nanocubes and nanorods under ethanol steam reforming Lluís Soler,†,‡ Albert Casanovas,† James Ryan,† Inmaculada Angurell,§ Carlos Escudero,⊥ Virginia Pérez-Dieste⊥ and Jordi Llorca†,‡,* †Institute

of Energy Technologies and Barcelona Research Center in Multiscale Science and

Engineering, Universitat Politècnica de Catalunya. Eduard Maristany 16, EEBE, 08019 Barcelona, Spain. ‡Departament

d’Enginyeria Química, Universitat Politècnica de Catalunya. Eduard Maristany 16,

EEBE, 08019 Barcelona, Spain. §Departament

de Química Inorgànica i Orgànica, Secció Química Inorgànica, Universitat de

Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain. ⊥ALBA

Synchrotron Light Source, Carretera de la Llum 2-26, 08290 Cerdanyola del Vallès,

Barcelona, Spain.

ACS Paragon Plus Environment

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ABSTRACT. Bimetallic catalysts exhibit high performance compared to that of their monometallic counterparts in a wide range of catalytic reactions. Both geometric and electronic phenomena yield unique properties that cannot be reproduced by other means. Bimetallic nanoparticles reorganize dynamically under reaction conditions, usually forming complex coreshell distributions of metals and oxidation states. This reorganization is even more complex in the presence of a reducible support. Here we show by operando ambient pressure X-ray photoelectron spectroscopy (AP-XPS) that the shape of the support also plays a crucial role in the reorganization of bimetallic nanoparticles, which has important consequences for catalytic performance. We have monitored the surface composition and oxidation states of preformed Rh0.5Pd0.5 model nanoparticles of 4 nm in size supported over CeO2 nanocubes and CeO2 nanorods during the catalytic steam reforming of ethanol (ESR) at 823 K. Over Rh0.5Pd0.5/CeO2nanocubes, both rhodium and palladium become strongly oxidized and ethanol mainly dehydrogenates into acetaldehyde and H2. In contrast, over Rh0.5Pd0.5/CeO2-nanorods, there is an enrichment of Pd toward the surface, both palladium and rhodium undergo significant reduction, and ethanol is reformed efficiently.

KEYWORDS: nanoshaped ceria, operando AP-XPS, metal-support interaction, heterogeneous catalysis, rhodium, palladium.

1. INTRODUCTION The importance of nanoshaped supports (cubes, octahedrons, rods, tubes, etc.) is gaining interest in heterogeneous catalysis. The ability to fabricate such forms at the nanoscale provides a practical way to prepare novel catalysts with metal particles supported over oxide support


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crystallites exhibiting well-defined {100}-type crystallographic planes (cubes), {111}-type (octahedrons), and {111}- and {110}-type (rods).1,2 There are numerous examples and theoretical studies reporting the influence of bare nanoshaped ceria and nanoshaped ceria loaded with metal nanoparticles on the catalytic performance of several reactions, such as CO oxidation,3,4 water gas shift,5,6 NO reduction,7 soot combustion,8 and reforming reactions aimed at the production of hydrogen.9–12 The presence of a reducible support like ceria plays a fundamental role in the ethanol steam reforming (ESR) reaction: on one hand ceria participates actively in the activation of water and, on the other hand, it prevents from coke deposition on the surface of the catalyst, which is one of the main problems for this reaction.13,14 Usually, the role of the nanoshaped ceria support has been attributed mainly to oxygen vacancy facility formation, activation of reactants, preferential adsorption/desorption and migration of adsorbed species to the metal nanoparticles, among others.15,16 Here we provide, for the first time, with direct evidence that the nanoshape of the support also determines the atomic reorganization and oxidation states of the metal nanoparticles under reaction conditions. To this end, we have used operando ambient pressure X-ray photoelectron spectroscopy (AP-XPS) using synchrotron radiation. Synchrotron-based AP-XPS exhibits particular advantages with regards to signal-tonoise ratio and allows the application of depth-profiling experiments to differentiate between surface and subsurface compositions of the catalyst.17–20 We have studied two types of nanoshaped ceria, nanocubes and nanorods, and loaded them with exactly the same amount of preformed Rh0.5Pd0.5 model alloy nanoparticles with uniform size. In this way, the same number of contact points between ceria and the nanoparticles is guaranteed as well as exposed metal surface area. We have used three different photon energies to obtain a depth-profile analysis of the bimetallic nanoparticles under the operando experiments. The RhPd/CeO2 system prepared


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by conventional impregnation from PdCl2 and RhCl3 solutions has already shown excellent catalytic performance in the ethanol steam reforming reaction to produce hydrogen from a renewable fuel.13,21,22 However, this method proved to yield alloy nanoparticles of different sizes and compositions when using ceria nanocubes and nanorods by operando XRD experiments.12 For XPS studies it is necessary to use identical metal nanoparticles to guarantee that the same volume of the nanoparticle is sampled at different photon energies. For that reason, we have used model Rh0.5Pd0.5 nanoparticles and impregnated them over the ceria nanoshaped supports, in order to study the changes in the atomic distribution in the core-shell structure of Rh0.5Pd0.5 nanoparticles under different reactive environments depending on the support nanoshape.23–25 2. MATERIALS AND METHODS 2.1 Synthesis of nanoshaped catalysts. We prepared ceria nanocubes (CeO2-c) and nanorods (CeO2-r) following a hydrothermal method reported elsewhere.4,26 Briefly, both nanoshaped supports were synthesized adding 35 ml of an aqueous solution of 0.4 M Ce(NO3)3·6H2O by means of an electrospray (Digital Ultrasonic Atomizer, Sonaer Inc.) to create a fine aerosol of liquid droplets into 245 ml of a 6.9 M NaOH solution for CeO2-c and 9.0 M NaOH solution for CeO2-r, under vigorous stirring. The resulting suspension was kept under agitation for 30 minutes. Then, we transferred the suspension into a PTFE-lined cylinder and we heated it for 24 h in a sealed stainless steel autoclave. The suspension of CeO2-c was heated at 453 K and the suspension of CeO2-r was heated at 373 K. After cooling down to room temperature, the resulting mixtures were centrifuged and washed three times with deionized water and three times with ethanol for separation and purification of the powders, which were dried at 333 K overnight. Finally, the samples were calcined at 723 K for 4 h.


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Model Rh0.5Pd0.5 bimetallic nanoparticles were obtained by simultaneous co-complexation of Rh3+ and Pd2+ ions, followed by a single reduction step and a final extraction process, as described in detail elsewhere.25 The Rh0.5Pd0.5/CeO2 nanoshaped catalysts (nominal 3% wt. Rh+Pd) were prepared by incipient wetness impregnation from a toluene solution (the solution was homogenized and divided in two parts) containing the model Rh0.5Pd0.5 nanoparticles (16 mM Rh0.5Pd0.5) over CeO2 nanocubes and nanorods. Samples were calcined at 573 K for 6 h to remove the protecting shell. Chemical analysis by ICP-OES revealed a total metal loading of 2.51±0.07 and 2.53±0.08 % wt. for Rh0.5Pd0.5/CeO2-c and Rh0.5Pd0.5/CeO2-r, respectively, with a Rh:Pd atomic ratio of 50:50 (1.28% wt. Pd and 1.23% wt. Rh for Rh0.5Pd0.5/CeO2-c and 1.29% wt. Pd and 1.25% wt. Rh for Rh0.5Pd0.5/CeO2-r). 2.2 Characterization of catalysts. The morphological characterization of the nanoshaped ceria supports was carried out by scanning electron microscopy (SEM) while the structure and crystallographic planes of Rh0.5Pd0.5/CeO2 catalysts was studied by high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). SEM images were recorded at 5 kV using a Zeiss Neon40 Crossbeam Station instrument equipped with a field emission source. About two hundred particles were considered for particle size distribution estimation. HRTEM was carried out using a JEOL 2010F electron microscope equipped with a field emission source at an accelerating voltage of 200 kV. Samples were deposited on holey carbon-coated grids. The point-to-point resolution achieved was 0.19 nm and the resolution between lines was 0.14 nm. HAADF-STEM was carried out using a FEI Tecnai F20 electron microscope equipped with a field emission electron gun operating at 200 kV. The average particle diameter was calculated from the mean diameter frequency distribution with the formula: d=Σnidi/Σni, where ni is the number of particles


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with particle diameter di in a certain range, by using more than 250 particles for each sample. Nitrogen adsorption isotherms were performed at 77 K using a Micromeritics ASAP2020 gas adsorption instrument. The samples were degassed at 473 K for 10 h prior to the adsorption experiments. The specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) method. Temperature Programmed Reduction (TPR) experiments were performed with a Chemstar-TPX instrument equipped with a Thermal Conductivity Detector (TCD) using 10% H2 and 10 K min-1. Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) was carried out using an Agilent 5100 instrument. 2.3 Catalytic ESR tests. The ethanol steam reforming reaction was performed at 823 K and atmospheric pressure in a lab-scale set up. A tubular reactor was loaded with a mixture of 0.1 g of sample dispersed in granulated SiC to achieve a total volume of 0.35 cm3. Both samples (Rh0.5Pd0.5/CeO2-r and Rh0.5Pd0.5/CeO2-c) were tested following the same procedure, consisting of an initial reduction step by flowing 50 ml·min-1 of a gaseous mixture of N2:H2=9:1 molar applying a temperature ramp of 5 K·min-1 from 298 K to 573 K and holding at 573 K for 1 hour, in order to activate the catalysts. Then, under an inert N2 atmosphere, we applied a second temperature ramp of 5 K·min-1 up to 823 K. At this point the temperature was kept constant at 823 K and a gaseous mixture of ethanol and water diluted in inert gas (C2H5OH:H2O:Ar=1:6:30 molar) was introduced into the reactor. The total flow was 52 ml·min-1, which led to a GHSV value of 104 h-1. The gaseous effluent stream was quantitatively evaluated in terms of volumetric total flowrate (bubble soap meter) and composition. A gas chromatograph (Micro GC Agilent 3000A) equipped with MS 5A, Plot U and Stabilwax capillary columns and TCD detectors was used to measure on-line gas concentrations every 5 min. The products of the reaction were H2, CO2, CO, CH4, acetone, acetaldehyde and ethylene. Outlet molar flowrates were determined


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from the measured composition by GC and the total volumetric flowrate of the gaseous outlet stream, whereas outlet flowrate of C2H5OH was evaluated by closing element balances. The ethanol conversion (𝐸𝑡ℎ𝑎𝑛𝑜𝑙) was calculated using the Eq. (1), where nEthanol in is the inlet molar flowrate of ethanol, nCO2 out, nCO out, nCH4 out, nAcetone out, nAcetaldehyde out and nC2H4 out are the outer molar flowrates of the produced CO2, CO, CH4, acetone, acetaldehyde and C2H4, respectively:

𝐸𝑡ℎ𝑎𝑛𝑜𝑙 =

𝑛𝐶𝑂2𝑜𝑢𝑡 + 𝑛𝐶𝑂𝑜𝑢𝑡 + 𝑛𝐶𝐻4𝑜𝑢𝑡 + 3·𝑛𝑎𝑐𝑒𝑡𝑜𝑛𝑒 𝑜𝑢𝑡 + 2·𝑛𝑎𝑐𝑒𝑡𝑎𝑙𝑑𝑒ℎ𝑦𝑑𝑒 𝑜𝑢𝑡 + 2·𝑛𝐶2𝐻4𝑜𝑢𝑡 2·𝑛𝐸𝑡ℎ𝑎𝑛𝑜𝑙 𝑖𝑛

Eq. (1)

2.4 AP-XPS experiments. Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) experiments were carried out at the CIRCE beamline of the ALBA synchrotron light source27 at a sample pressure of 0.05 mbar. The sequence of experiments was: (i) activation under H2 at 573 K, (ii) ESR at 823 K, and (iii) final reduction step under H2 at 823 K. The AP-XP spectra were measured with a commercial PHOIBOS 150 NAP energy analyzer. The analyzer is equipped with four differentially pumped stages. The CIRCE beamline is an undulator beamline with a photon energy range 100-2000 eV. The beam spot size at the sample (polar angle 80) is 100 x 220 μm2. In order to balance resolution and count rate, the AP-XPS spectra were measured with pass energy 20 eV and exit slit XS=50 μm. The total energy resolution of beamline plus analyzer in the measurement conditions was better than 0.35 eV. Three different photon energies (h) of 1150, 875 and 670 eV were used to acquire AP-XP spectra of O 1s, Rh 3d, Pd 3d and Ce 3d (the latter excited only by a photon energy of 1150 eV), in order to perform a depth profile study of the supported Rh0.5Pd0.5 nanoparticles over ceria, as previously described.25 The chosen photon energies 670, 875 and 1150 eV generate Rh 3d and Pd 3d photoelectrons with average kinetic energies (KE) of 349, 554 and 829 eV, respectively, which account for inelastic mean free paths (IMFPs) of about 0.7, 0.9 and 1.2 nm. The IMFP is defined as the distance travelled by an


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electron where the probability of escape has decayed to 1/e or 37 %. These IMFPs define three different volumes in the NPs that were calculated as the volume enclosed within one IMFP from the exterior surface inside a sphere of diameter 4 nm taking into account that photons only reach the particle from one side (the side reached by the photons). The regions defined from these three depths account for about 25, 34 and 44 % of the total volume of the Rh0.5Pd0.5 NPs for 670, 875 and 1150 eV photon energies, respectively. The ethanol-water mixture was prepared in a bubbler to feed the analysis chamber. Hydrogen gas and the ethanol-water mixture were dosed into the analysis chamber by UHV leak valves. The sample was heated using an infrared laser (λ=808 nm) focused on a stainless steel plate on top of which the sample was mounted. The temperature was monitored with a K-type thermocouple in contact with the sample. Data processing was performed with the CasaXPS program (Casa Software Ltd., UK). Energy calibration was performed with an Au foil. Cerium 3d spectra were deconvoluted using six peaks for Ce4+ (V, V’’, V’’’, U, U’’ and U’’’), which correspond to three pairs of spin-orbit doublets, and four peaks for Ce3+ (V0, V’, U0 and U’), which correspond to two doublets, based on the peak positions reported by Mullins et al.,28 where U and V refer to the 3d3/2 and 3d5/2 spin-orbit components, respectively. Rh 3d and Pd 3d spectra were deconvoluted by taking into account the position of the reduced component and the ratio between 3d5/2 and 3d3/2 peak areas.29 The atomic fractions of Rh and Pd for each photon energy were obtained from the calibrated Rh 3d and Pd 3d peak areas. The atomic fraction was then calculated by dividing the corrected area of each noble metal by total area of both metals. The reduced fraction of Rh and Pd was calculated by dividing the corrected area of the peak ascribed to the metallic state by the total corrected area of the metal for each photon energy. The oxidized fraction corresponds to the corrected area of the


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two oxidized components in the case of the Rh0.5Pd0.5/CeO2 catalyst divided by the total corrected area of the metal for each photon energy.

3. RESULTS AND DISCUSSION 3.1 Morphology and catalytic activity. The BET surface areas measured at 77 K were 33 m2·g1

for CeO2-c and 70 m2·g-1 for CeO2-r. Figures 1A and 1C shows SEM representative images of

the bare nanoshaped ceria supports. Most of the nanocubes were 10-30 nm in length (Figure 1A). CeO2 nanorods were 10-15 nm in diameter and roughly 0.2-0.5 µm in length (Figure 1C). Figure 1B illustrates representative HRTEM images of Rh0.5Pd0.5/CeO2-c and Figure 1D shows Rh0.5Pd0.5/CeO2-r. The average diameter of the model Rh0.5Pd0.5 nanoparticles in both cases was 4±1 nm. Fourier Transform (FT) images reveal the presence of a RhPd alloy with lattice spacing at 2.2 and 2.0 Å, which can be ascribed to (111) and (200) crystallographic planes of RhPd alloy, respectively (see Figure 1B-a). EDX analysis performed on individual nanoparticles indicated a constant Rh:Pd ratio of ca. 1:1. The FT image of a representative CeO2 nanocube in Figure 1B-b indicates an interplanar spacing of 2.7 Å, which matches perfectly with the (200) crystallographic plane of fcc CeO2. Ceria nanorods exhibited both {110} and {111} facets (Figure 1D), in accordance to recent observations.30


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Figure 1. (A) SEM image of CeO2-c. (B) HRTEM images of Rh0.5Pd0.5/CeO2-c. The upper right insets show the FT images of a representative supported Rh0.5Pd0.5 nanoparticle and of a CeO2 nanocube. (C) SEM image of CeO2-r. (D) HRTEM images of Rh0.5Pd0.5/CeO2-r. The lower left insets show the FT images of a representative supported Rh0.5Pd0.5 nanoparticle and of a CeO2 nanorod. (E) Ethanol conversion (Ethanol) and product selectivity obtained over Rh0.5Pd0.5/CeO2 nanoshaped samples for ESR at 823 K, steam to carbon S/C=3, F/W=9·10-3 NLgas gcat-1 s-1 and GHSV=104 h-1. As previously reported, the bare ceria supports show similar catalytic performance under ESR conditions, with no dependency on the nanoshape, and RhPd/CeO2 catalysts show a remarkably higher ethanol conversion than bare ceria supports as a result of the metal function.12 However, significant differences in the distribution of the reaction products are evidenced here when


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comparing Rh0.5Pd0.5/CeO2-c and Rh0.5Pd0.5/CeO2-r catalysts (Figure 1E). In the case of Rh0.5Pd0.5 supported over ceria nanocubes, the dehydrogenation of ethanol into acetaldehyde and H2 is the main reaction observed (Eq. 2), with a minor contribution by the decomposition of ethanol into CH4, CO and H2 (Eq. 3). Other reactions involved in the ESR are the water-gas shift (WGS, Eq. 4) and the steam reforming of methane (MSR, Eq. 5): CH3CH2OH  CH3CHO + H2

H = +68 kJ mol-1 Eq. (2)

CH3CH2OH  CH4 + CO + H2

H = +50 kJ mol-1 Eq. (3)

CO + H2O  CO2 + H2

H = -41 kJ mol-1

CH4 + 2H2O  CO + H2

H = +164 kJ mol-1 Eq. (5)

Eq. (4)

The low production of CO2 and the significant presence of acetaldehyde, CH4 and CO indicate that WGS and MSR were not promoted over Rh0.5Pd0.5/CeO2-c, as illustrated in Figure 1E. A completely new scenario is observed when Rh0.5Pd0.5 nanoparticles were supported over CeO2 nanorods. In this case, the marked increase of CO2 and H2 production and the almost absence of acetaldehyde in the reaction products indicate that Rh0.5Pd0.5/CeO2-r strongly promotes Eq. 4 and Eq. 5, yielding a much more efficient ESR catalyst than Rh0.5Pd0.5/CeO2-c. The use of preformed Rh0.5Pd0.5 nanoparticles allows us to observe the particular role of the ceria nanoshaped support, unambiguously concluding that ceria nanorods are a better support than ceria nanocubes for ESR. As deduced from HAADF-STEM analysis of the post-reacted samples, the preformed PdRh alloy nanoparticles do not experience sintering under ESR conditions at 823 K and the average diameter of the Rh0.5Pd0.5 nanoparticles is well preserved at 4±1 nm after reaction (Figure S1).


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3.2 Operando AP-XPS. Figure 2 shows the Pd 3d, Rh 3d and O 1s spectra recorded for Rh0.5Pd0.5/CeO2-c and Rh0.5Pd0.5/CeO2-r catalysts under the different atmospheres at 670 eV, which correspond to the outer shell of the bimetallic nanoparticles (the corresponding spectra recorded at 875 and 1150 eV are compiled in Figures S2, S3 and S4). Remarkable differences in both the Rh and Pd spectra between the two catalysts exist, which can be directly attributed to the effect of the nanoshape of the ceria support on the Rh0.5Pd0.5 nanoparticles under the different gaseous environments. Initially, under H2 at 573 K, the Pd 3d spectra show both Pd0 and PdII species at 335.0 and 335.5-336.5 eV, respectively,25,31 but Rh0.5Pd0.5/CeO2-c contains a larger fraction of metallic Pd than Rh0.5Pd0.5/CeO2-r, as illustrated in Figures 2A and 2D. Moreover, Rh0.5Pd0.5/CeO2-r presents a minor fraction of PdIV species at 337.4 eV, which is almost inexistent in the Rh0.5Pd0.5/CeO2-c catalyst (Figures 2A and 2D). On the other hand, both catalysts show both reduced and oxidized Rh at 307.2 and 307.8-308.5 eV, respectively,25,31 in similar amounts (Figures 2G and 2J). Outstanding differences between the two catalysts are observed under ESR conditions at 823 K. The Pd 3d spectrum recorded over Rh0.5Pd0.5/CeO2-c shows a dramatic oxidation of Pd under the steam reforming atmosphere, with PdIV species at 337.6 eV much more abundant than PdII and Pd0 species (Figure 2B). In contrast, the Pd 3d spectrum of Rh0.5Pd0.5/CeO2-r under the same conditions shows an opposite behavior, that is, Pd gets more reduced under ESR at 823 K with respect to the activation treatment under H2 at 573 K, with minor fractions of PdII and PdIV species (Figure 2E). Concerning Rh, the Rh0.5Pd0.5/CeO2-r catalyst under-goes a slight oxidation under the ESR atmosphere, showing the presence of bands of oxidized RhI and RhIII species at 307.8 eV and 309.2 eV (Figure 2K). However, in the Rh0.5Pd0.5/CeO2-c catalyst Rh dramatically oxidizes mainly to RhI and RhIII species, whereas Rh0 is almost inexistent (Figure 2H).


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Figure 2. XP Spectra of Pd 3d, Rh 3d and O 1s corresponding to Rh0.5Pd0.5/CeO2-c and Rh0.5Pd0.5/CeO2-r catalysts under different gaseous environments using a photon energy of 670 eV. The difference in catalytic performance for these two catalysts in the ESR reaction can now be attributed to the different surface characteristics of the Rh0.5Pd0.5 nanoparticles, which interact strongly, but remarkably different, with the ceria nanocubes and nanorods supports. It has been reported that oxygen donation is thermodynamically more favorable for the {100} planes of ceria nanocubes compared to the {110} and {111} planes usually found in ceria nanorods and polycrystals.15,32 Therefore, oxygen atoms in the {100} crystallographic planes of ceria


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nanocubes can be easily transferred to the Rh0.5Pd0.5 nanoparticles resulting in metal oxidation. It is known that ethanol can be effectively dehydrogenated into acetaldehyde and H2 over metal oxides (Eq. 2),33 but the metallic function is required for methane steam reforming, which is the last step in the steam reforming of ethanol (Eq. 5), where most of the hydrogen is produced. For that reason ESR does not progress on Rh0.5Pd0.5/CeO2-c, which shows a strongly oxidized Pd and Rh outer shell. The situation is completely different over the Rh0.5Pd0.5/CeO2-r catalyst. The donation of oxygen atoms from the crystallographic planes exposed in ceria nanorods is less favored and, consequently, the metallic character of the Rh0.5Pd0.5 nanoparticles is preserved and the ESR process can be completed. In addition, the extent of the ESR and the concomitant high production of H2 maintain the metallic character of the surface of the Rh0.5Pd0.5 nanoparticles supported over the ceria nanorods. The different reducibility of the Rh0.5Pd0.5 nanoparticles supported on ceria nanocubes and nanorods is also evidenced by TPR; the hydrogen consumption recorded over Rh0.5Pd0.5/CeO2-r (102 µmol g-1) is significantly higher than that recorded over the Rh0.5Pd0.5/CeO2-c catalyst (79 µmol g-1). According to this hypothesis, the O 1s spectrum (h=670 eV) recorded under ESR at 823 K over Rh0.5Pd0.5/CeO2-c shows a prominent band at 531.2 eV (Figure 2N) due to the presence of abundant hydroxyl groups anchored at the surface of the catalyst, which do not participate in the reaction. Nevertheless, the intensity of this band in the spectrum recorded over Rh0.5Pd0.5/CeO2-r is much lower (Figure 2Q) since the surface –OH groups readily react under the same conditions. Also, the relative intensity of the band at 529.9 eV attributed to oxygen interacting with CeIII 34 with respect to the band at 528.8 eV ascribed to lattice oxygen from CeIV

34

is higher in

Rh0.5Pd0.5/CeO2-c with respect to Rh0.5Pd0.5/CeO2-r, in accordance to the higher mobility of oxygen in Rh0.5Pd0.5/CeO2-c. In all cases, the higher the photon energy used to record the O 1s


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spectra the lower the contribution of the bands corresponding to hydroxyl groups and O-CeIII to the spectra, since more signal is collected from the ceria bulk (O-CeIV) (Figure S4). At the end of the ESR experiments the catalysts were exposed to H2 at exactly the same temperature, 823 K. As expected, both Pd and Rh reduced to a large extent in Rh0.5Pd0.5/CeO2-c (Figures 2C and 2I), but a significant fraction of Rh remained oxidized in Rh0.5Pd0.5/CeO2-r (Figure 2L). The exchange of the ESR atmosphere by H2 caused the immediate disappearance of the band corresponding to the unreactive hydroxyl groups in the Rh0.5Pd0.5/CeO2-c catalyst (Figure 2O). In contrast, this species was partly retained at the surface of the Rh0.5Pd0.5/CeO2-r catalyst (Figure 2R). As expected, upon exposure to H2 at 823 K, the Ce 3d spectra (h=1150 eV) showed a sharp increase of CeIII species for both catalysts (Figure S5). The CeIII/CeIV ratio in the Rh0.5Pd0.5/CeO2-c catalyst increased from 0.1 under ESR up to 1.3 under H2 at 823 K, whereas in the Rh0.5Pd0.5/CeO2-r catalyst the CeIII/CeIV ratio increased from 0.3 under ESR up to 1.1 under H2 at 823 K (Table S1). The higher CeIII/CeIV ratio under ESR in Rh0.5Pd0.5/CeO2-r with respect to Rh0.5Pd0.5/CeO2-c is attributed to the higher hydrogen production in the former, and the higher CeIII/CeIV ratio under H2 in Rh0.5Pd0.5/CeO2-c is attributed to the facility of oxygen extraction from the {100} crystallographic planes of the ceria nanocubes and concomitant Ce reduction. In addition to differences in the metal oxidation states in the Rh0.5Pd0.5 nanoparticles supported over the different ceria nanoshapes, there are also interesting differences in the relative distribution of the metals. Figure 3 depicts half pie charts illustrating the atomic fractions of Rh and Pd and their oxidation states calculated from the XP spectra recorded at different photon energies. The three regions in each semi-circular graph correspond to each photon energy used and, therefore, correspond to different sampling depth. During the initial reduction with H2 at


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573 K, atomic ratios of Pd:Rh 67:33 were measured for the Rh0.5Pd0.5 nanoparticles supported on ceria nanorods, while a Pd:Rh ratio of 63:37 was observed for nanoparticles supported over ceria nanocubes (Figures 3A and 3B). A similar surface Pd enrichment in ceria-supported Rh0.5Pd0.5 nanoparticles upon reduction with H2 at low temperature has also been reported in previous works.23,25,35 Moreover, a core-shell structure of oxidation states of Pd was observed for the bimetallic nanoparticles supported on CeO2-r, where Pd was more reduced in the inner region of the bimetallic nanoparticles. Under ESR conditions (Figures 3C and 3D), remarkable differences in the atomic rearrangement of the bimetallic nanoparticles depending on the nanoshape of the ceria support are clearly seen; in particular, an additional segregation of Pd towards the surface occurred for the Rh0.5Pd0.5 nanoparticles supported over ceria nanorods (Pd:Rh75:25), whereas the exact opposite (Rh segregation to the surface, Pd:Rh55:45) was observed for the Rh0.5Pd0.5 nanoparticles supported over ceria nanocubes. Furthermore, a core-shell structure of oxidation states for both Pd and Rh is observed in both catalysts, but in the opposite direction; the metals are more reduced towards the surface in the case of Rh0.5Pd0.5/CeO2-r whereas they are more oxidized in Rh0.5Pd0.5/CeO2-c, as discussed above. The strong differences not only in oxidation state, but also in atomic redistribution of Pd and Rh under ESR conditions reinforce the conclusion that a markedly different metal-support interaction occurs between Rh0.5Pd0.5 nanoparticles and CeO2 depending on the crystal planes exposed by ceria, which in turn determines the catalyst reactivity. Finally, the reduction experiment with H2 at 823 K resulted in the reduction of the metals as discussed above, but did not significantly change the relative distribution of Pd and Rh (Figures 3E and 3F); an atomic ratio of Pd:Rh75:25 was maintained in Rh0.5Pd0.5/CeO2-r whereas a slight Rh enrichment was observed in Rh0.5Pd0.5/CeO2-c,


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Pd:Rh50:50, again pointing to a strong, but different, metal support interaction driven by the ceria nanoshape.

Figure 3. Atomic fractions of Pd and Rh calculated for the nanoshaped Rh0.5Pd0.5/CeO2 catalysts. The concentric semi-circles illustrate the atomic fractions of Rh (depicted in blue) and Pd (depicted in orange), where the dark regions correspond to the reduced fractions of each metal. The outer semi-circle was calculated with the XP spectra recorded at h = 670 eV, the intermediate semi-circle corresponds to h = 875 eV and the inner semi-circle corresponds to h = 1150 eV. A, C and E show the sequence of AP-XPS experiments for Rh0.5Pd0.5/CeO2 nanorods and B, D and F for Rh0.5Pd0.5/CeO2 nanocubes.

4. CONCLUSIONS Here we have provided original information about the determinant role of nanoshaped supports on the dynamics of bimetallic particles under reaction. This work represents a step forward in the


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understanding of multimetallic catalytic systems widely employed in several industrial and environmental applications, where the synergy between the metals themselves and between the metals and the support has been recognized to be the key points of their superior catalytic performance compared to monometallic catalysts. Here we have shown that the crystallographic planes of nanoshaped ceria have a strong influence both on the surface reorganization and oxidation states of Rh-Pd bimetallic nanoparticles, which in turn strongly determine the catalytic behavior in the ethanol steam reforming reaction. The effect of the ceria nanoshape on the characteristics of bimetallic metal nanoparticles represents a new and important element to take into consideration in catalyst design. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Additional figures and table (PDF). AUTHOR INFORMATION Corresponding Author *Email: [email protected]. ORCID Lluís Soler: 0000-0003-1591-3366 Jordi Llorca: 0000-0002-7447-9582 Carlos Escudero: 0000-0001-8716-9391 Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT This work has been funded by projects MINECO/FEDER ENE2015-63969-R and GC 2017 SGR 128. JL is a Serra Húnter Fellow and is grateful to ICREA Academia program. The AP-XPS experiments were performed at the CIRCE beamline at ALBA Synchrotron with the collaboration of ALBA staff. REFERENCES (1)

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TABLE OF CONTENTS


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Dynamic Reorganization of Bimetallic Nanoparticles under Reaction

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