Surface characterization of two Ce0.62Zr0.38O2 mixed oxides with different reducibility

Journal Pre-proofs Full Length Article Surface characterization of two Ce0.62Zr0.38O2 Mixed Oxides with different Reducibility M.P. Yeste, P.A. Primus, R. Alcantara, M.A. Cauqui, J.J. Calvino, J.M. Pintado, G. Blanco PII: DOI: Reference:

S0169-4332(19)33071-5 https://doi.org/10.1016/j.apsusc.2019.144255 APSUSC 144255

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

14 July 2019 23 September 2019 30 September 2019

Please cite this article as: M.P. Yeste, P.A. Primus, R. Alcantara, M.A. Cauqui, J.J. Calvino, J.M. Pintado, G. Blanco, Surface characterization of two Ce0.62Zr0.38O2 Mixed Oxides with different Reducibility, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144255

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Surface characterization of two Ce0.62Zr0.38O2 Mixed Oxides with different Reducibility M.P. Yeste a,*, P.A. Primus b, R. Alcantara c, M.A. Cauqui a, J.J. Calvino a, J.M. Pintado a, G. Blanco a

a Departamento

de Ciencia de los Materiales e Ingeniería Metalúrgica y Química

Inorgánica. University of Cádiz. Apdo. 40. Puerto Real (Cádiz). 11510 SPAIN b

Physical Chemistry, University of Potsdam, Karl-Liebknecht-Str. 24-25, 14476 Potsdam-

Golm GERMANY c Departamento

de Química Física. University of Cádiz. Apdo. 40. Puerto Real (Cádiz).

11510 SPAIN

*Corresponding author, e-mail: [email protected]

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Abstract This paper presents a study of the surface properties of two Ce/Zr mixed oxides with different reducibility, obtained by applying distinct thermal ageing treatments to an oxide with the composition Ce0.62Zr0.38O2. The surface composition was investigated by XPS. Chemical reactivity of the surface was studied by adsorption of the probe molecules CO2, D2 and methanol. Nanostructural characterization was carried out by XRD, Raman and high-resolution Eu3+ spectroscopy (FLNS). The characterization showed only slight variations in surface composition and bulk Ce-Zr distribution, but hardy differences concerning the type and strength of acidic surface centres, as well as strong differences in the ability to dissociate hydrogen. Structural variations between both samples were identified by comparing the optical spectra of Eu3+ in surface doped samples. Keywords: Ce/Zr; Surface properties; Reactive adsorption; Hydrogen activation.

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1. Introduction Cerium-zirconium mixed oxides are widely used in catalysis in a variety of applications. They are key components of catalytic systems used in: low temperature water-gas-shift reaction [1], exhaust gas post-treatment in automobiles [2,3], solid oxide fuel cells [4], oxidation reactions [5], etc. The excellent redox properties exhibited by ceria-zirconia mixed oxides are generally the main argument used to explain their good catalytic performance. Although the combination of these two components generally leads to a synergic effect in most of their physicochemical properties, the reactivity and interaction degree between them can vary greatly depending on the synthesis routine, thermal treatment and other factors influencing the morphology and structure of the resulting mixed oxides. The observations by Otsuka-Yao et al [6,7] and Kaŝpar et al [8,9] reveal a peculiar behaviour for mixed cerium zirconium oxides: after a reduction treatment at a high temperature and subsequent reoxidation at a moderate temperature, the yielded sample is more easily reducible than the untreated sample. Subsequently, the reversibility of this process was shown when the reoxidation treatments were carried out at a higher temperature [10]. This peculiar behaviour is of paramount interest for the application and design of these materials, where the redox exchange with the reactant plays a crucial role. This particular behaviour of mixed cerium/zirconium oxides has been studied by a large number of research groups using a variety of experimental techniques identifying structural parameters [8,11–19]. The central objective of this research has been to establish a correlation of macroscopic redox properties with the structure of these oxides. A great deal of work has been done to propose an interpretation for this extraordinary behaviour, generally suggesting that there is a close relationship between structural changes induced by thermal ageing conditions, and the observed redox properties [11,20,21]. When considering the H2 reduction mechanism for this class of oxides, the rate-determining step is the dissociative adsorption of

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H2 on the oxide surface. A model accounting for structural modifications in relation to thermal ageing conditions has previously been reported [22]. In spite of all these facts, most of the studies have focused their attention on the bulk structure of the solid, possibly because techniques sensitive only to the surface structures present many experimental difficulties. In recent years there have been only few references where the surface of ceria-zirconia mixed oxides has been studied in depth. For example, in [23] the CeZr surface is studied by FTIR of CO at different pressures. In [24] the surface of Pt/CeZr/Al2O3 is studied by FTIR of CO and NO. In [25] the surface is studied by XPS or FTIR of CO. In order to expand the model of structure reactivity relationships, a more detailed study of the surface chemistry of ceria-zirconia mixed oxides with variations in reducibility, as the ones presented in this paper, appears to be an interesting task. In this study, two oxides with different reducibilities have been prepared: one is obtained after applying a high-temperature reduction treatment to the CZ oxide followed by a mild oxidation treatment. The other one is prepared after a high temperature reduction treatment equal to the previous one, but followed by an oxidation treatment also at high temperature. One of the problems with studying these oxides is that the behaviour of these samples is difficult to reproduce, depending on the set of previous treatments applied, and not only those applied immediately before performing a reducibility experiment. Therefore, sufficient quantities of each of the oxides have been prepared so that identical samples are used in all studies. A detailed study of the structural and chemical properties of each of the oxides has been carried out in order to clarify structure reactivity relationships. With respect to the structural characterization of the oxides, XRD, Raman and Fluorescence line-narrowing spectroscopy have been employed. For the chemical properties, both the redox properties of the bulk of the oxides as well as their surface were contributions to the signal in TPR measurements. Additionally, vibrational spectroscopy of adsorbed compounds allows the investigation of

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surface properties, particularly the binding of reactants. In order to determine complementary structural information in almost identical samples (apart from thermal pre-treatment/surface modifications), novel and highly sensitive analytical tools allow correlations between structure, surface morphology, and reactivity. Fluorescence line-narrowing spectroscopy (FLNS) using Eu3+ dopants proved to be a sensitive tool to determine structural parameters such as site symmetry and homogeneity in ceria-based oxide materials [26,27]. The spectroscopic response of Eu3+ is extremely sensitive towards the symmetry of the dopant environment (Judd-Ofelt theory) and reveals even small structural changes in the material. Selective doping of the surface is used in this study as a way to determine and monitor surface-specific structural data to further inform structure reactivity relationships. 2. Materials and Methods Two Ce0.62Zr0.38O2 mixed oxides were prepared as described in reference [21], and will be referred to as CZ-MO (mild oxidation) and CZ-SO (severe oxidation). They were obtained by thermal treatment of a commercial sample (CZ-LS, SBET =19 m2/g) from Grace Davison. Samples were first submitted to a high temperature (950 ºC) reduction treatment under pure H2 in a flow of 500 cm3/min for 5 hours (ramp 5 ºC/min). Finally, the gas flow was switched to He (500 cm3/min), for 1 h, and cooled to 25 ºC under inert gas flow. After completing this severe reduction step, the aging cycles were closed by applying to each of the aliquots a different re-oxidation routine. To prevent the overheating of the reduced mixed oxides, they were first re-oxidized at 25 ºC by pulsing 5% O2/He until there was no evidence of further oxygen consumption. The so-called CZ-MO sample was then obtained by reoxidation of this reduced oxide with 5% O2/He (500 cm3/min) for 1 h at 500 ºC (ramp 5 ºC/min), showing an improved redox behaviour compared to the original sample [21]. CZ-SO was prepared by reoxidation of the heavily reduced oxide under pure O2 (500 cm3/min) at 950ºC for 5 h (ramp

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5 ºC/min), resulting with a lower reducibility than both the CZ-MO and the CZ-LS original sample [21]. A schematic representation of the complete cycle is shown in the Figure 1. BET surface areas were 16 m2/g for CZ-MO and 12 m2/g for CZ-SO. < near Figure 1 > The X-ray diffraction diagrams have been obtained in a Bruker AXS model D8 Advance. The radiation used was Cu K, operating at 40 KV and 40 mA. In the range of 10-18 º, it has been recorded with a counting time of 10 s and a step size of 0.01 º. The Raman spectra were made in an instrument whose excitation source is an Ar laser with emission at 514.5 nm. The dispersing system is a double monochromator of the Jobin-Yvon brand, model U1000. This monochromator has a focal distance of 1m and has adjustable slits at the input and output, with which the final resolution of the spectrum can be controlled. The detector system is a single-channel system consisting of a Hamamatsu photomultiplier, model R-943. For the Fluorescence line-narrowing spectroscopy, samples were impregnated with 0.5 % mol Eu(NO3)3 in water with regard to the cation content of the mixed oxide before the thermal treatment. Fluorescence line-narrowing spectroscopy (FLNS) was carried out by cooling dry samples inside a copper sample holder, placed in a vacuum chamber and attached to the cold plate of a closed cycle liquid Helium cryostat. As excitation light source a narrow bandwidth (15 pm @ 500 nm) OPO (KWU Lasertechnik) in the range between 575 nm – 582 nm was used. The OPO was pumped by a 355 nm Nd:YAG Laser (Quanta Ray, Spectra Physics). The repetition rate was set to 10 Hz with a pulse length of 8 ns. The excitation light was coupled into one of the branches of a Y-shaped fiber bundle while the other branch was connected to a spectrograph (Shamrock SR-303i, Andor Technology) equipped with an intensified CCD camera (iStar DH 720, Andor Technology). The combined excitation and emission bundles are mounted in front of a lens system that allows excitation of the sample and acquisition of the 6

emission light at the same time. The temperature was kept at 4 K for two hours before measurements to ensure an even temperature in the sample. The temperature-programmed reduction mass spectrometry (TPR-MS) studies were performed in an experimental device coupled to a quadrupole mass spectrometer, Pfeiffer, model QSM 200 M2. The amount of sample routinely used in these experiments was 200 mg, the 5% H2/Ar flow rate was 60 cm3/min, and the heating ramp was 10 ºC/min. Prior to all the TPR runs, the samples were cleaned by heating them under flowing 5% O2/He at 60 cm3/min, at a heating rate of 10 ºC/min, up to 500 ºC; then, they were kept for 1 h at this temperature and cooled to 150 ºC under the same flow of diluted oxygen and finally to 25 ºC in a flow of He. In order to calculate the apparent activation energies, TPR-MS studies were performed, but the heating ramp were 3 and 6 ºC/min. In the case of basicity studies, after cleaning the sample with O2/He at 500ºC, 1 atm of CO2 was passed at room temperature through the samples during 1h (flow of 60 cm3/min). Then, the flowing gas was switched to He, and is heated until 900 ºC with a ramp of 10 ºC/min. The Temperature-Programmed Desorption Mass Spectrometry (TPD-MS) studies of pre-adsorbed CO2 were performed in an experimental device coupled to a quadrupole mass spectrometer, Pfeiffer, model QSM 200 M2. Fourier-Transform Infra-Red (FTIR) studies of adsorbed probe molecules were recorded at room temperature in the transmission mode on a Bruker, Vertex 70, instrument. Samples were pressed into self supported disks which were transferred for in situ treatments in a quartz cell. For all FTIR measurements, 200 mg of sample was used. X-ray Photoelectron Spectroscopy (XPS) was recorded using a Kratos Axis Ultra DLD spectrometer. XPS analysis was performed using a monochromatic Al K radiation at 1486.6 eV and a 2 x 1 mm spot size. The analyser was operated in Constant Analyser Energy Transmission (CAE) mode, and a 20 eV pass energy (PE) was used to collect the high

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resolution XP spectra. The powdered samples were pressed into pellets, transferred to the analysis chamber of the instrument, and in situ heated under UHV at 300 ºC for 30 min. To avoid electrostatic charging effects during the experiments, the coaxial charge neutralizer system developed by Kratos was activated. XPS data analysis was performed by using CasaXPS Software, version 2.3.19rev1.1m (Neal Fairley, Casa Software Ltd., UK). ISS analysis was performed on a Qtac100 instrument from ION-TOF. A 5 keV 20Ne+ beam was used to obtain the spectra. Powder samples were slightly pressed to obtain a macroscopically flat surface, and, prior to the experiments, were cleaned by atomic oxygen to remove adsorbed hydrocarbons. ISS spectra were measured in two different locations for each sample, and the average composition measure is given in this paper. 3. Results and discussion 3.1. Structural investigations 3.1.1 XRD studies XRD was employed to find evidence of the postulated pyrochlore phase formation in mixed oxide samples subjected to high-temperature reduction treatments [28]. A powder XRD diagram for such a pyrochlore structure is composed of the typical signals originating from a fluorite-type structure, along with additional reflections [7,15,29]. The latter appear as broadened signals with much lower intensity compared to the reflections originating in the cubic fluorite structure, indicating a small crystallite size and phase content. Re-oxidation of the pyrochlore phase at 500 ºC is not expected to alter the cationic ordering, as would occur at elevated temperatures. The reoxidation at moderate temperature would therefore give rise to a new phase (ĸ phase), in which the cationic ordering of the pyrochlore structure is maintained. The resulting XRD reflections should therefore be similar to the reflections of the pyrochlore structure from which it is derived. [6,7,15,30,31]. Figure 2 shows a comparison of the XRDs corresponding to the CZ-MO and CZ-SO oxides. In order to identify the formation of above

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mentioned superstructure we have chosen the (111) reflection, which typically appears for an angle of 2θ = 140. The diffraction peaks for CZ-MO (a) and CZ-SO (b) oxides indicate an almost identical fluorite-type cubic structures for both treatments. Nevertheless, the expected (111) ĸ phase reflection also appears in in the XRD of the CZ-MO sample: Even though the XRD measurements are close to the limit of detection for this particular phase (low content, small crystallite size), a small peak can be observed indicating a small contribution of the ĸ phase, not observed in the case of CZ-SO oxide. < near Figure 2 >

3.1.2 Raman spectroscopy This technique, like XRD, provides average macroscopic information. However, RAMAN spectroscopy provides information about the anionic subnet, for which the XRD technique is not very sensitive. All the Raman-active mode patterns involve displacement of the oxygen ions and are thus sensitive to the oxygen sublattice occupation. Figure 3 shows the RAMAN spectra corresponding to CZ-MO and CZ-SO oxides. Starting with the CZ-SO sample, we have seen in the XRD diagram that the diffraction peaks correspond to cubic fluorite structure. For this structure, spatial group Fm3m, only the F2g symmetry mode is active in RAMAN. For CeO2, the band corresponding to this F2g symmetry mode appears at a frequency of 465 cm−1. However, in the Raman spectrum of CZ-SO shown in Figure 3, two small bands, at 155 cm−1 and 290 cm−1 accompany the intense band centred at 465 cm−1. According to the bibliography [9,18,32–34] the presence of these bands is indicative of a partial loss of the Fm3m symmetry due to a displacement of the oxygen atoms within their tetrahedral positions, preserving the cubic cation sublattice, or by the tetragonalization of the cation sublattice. In the case of the CZ-MO sample, the RAMAN spectrum is quite different, which implies an oxygen sublattice network different to CZ-SO sample. The RAMAN spectrum is quite similar to the one

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contained by Omata et al [35] for the ĸ phase. The large number of RAMAN active modes originates from the lower symmetry of the kappa phase increases the number of bands in the spectrum corresponding to CZ-MO sample. The bands appear at frequencies of 145, 290, 430, 480 and 590 cm−1. Those bands are according to the active modes for the space group P213. < near Figure 3 > 3.1.3 Structural investigations using FLNS The addition of Eu3+ to Ceria-Zirconia oxides as a structural probe offers a unique opportunity to gain structural information from oxide-based samples by exploring the sensitivity of the Eu3+ luminescence fine structure towards the symmetry and strength of the surrounding crystal field [26,27,36]. To introduce the structural probe, the material was first impregnated with an aqueous solution to introduce 0,5 % mol Eu(NO3)3 in relation to the cations in the nanomaterial, and then heated to 500 °C to convert the nitrates into oxides. Subsequently, the impregnated samples underwent the same redox treatment (SRMO, SRSO) as the samples not previously impregnated with Eu3+. The impregnated samples show the same general changes in TPR measurements with improved reactivity for the SRMO sample over the SRSO samples as the undoped samples. The total luminescence spectra (TLS, representation of spectra with x = em [nm], y = ex [nm], z = intensity [counts]) in Figure 4 show clear differences between the SRMO (left) and the SRSO samples (right), giving a good overview of the spectroscopic response of both samples. The optical emission patterns of Eu3+ generally correspond to specific site symmetries, and therefore the crystal field effective at specific Eu3+ dopant sites. < near Figure 4 > When using FLNS for structural investigations, sharp excitation and emission bands indicate homogenous, crystalline lattice sites with little structural variation (angles and distances

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between anions and cations), whereas amorphous samples produce broad, unstructured excitation and emission bands. Stronger crystal fields lead to a redshift of the 7F0-5D0 excitation band (nephelauxetic effect), which is used to achieve for selective excitation of Eu3+ ions with a narrow bandwidth excitation source. The subsequent emission responses contain selective information on the site symmetry for the probed lattice if there is no excitation overlap. There are three main excitation bands that can be selectively probed, one broad band with a maximum around ex = 579.5 nm, and two sharp excitation bands centered at ex = 580.25 nm and 581.6 nm. The two sharp excitation bands (ex = 580.25 nm and 581.6 nm) structured emission patterns with resolved Stark-splitting of the 5D0-7F1-2 emission bands. The sharp emission and excitation bands indicate an ordered structure. Nevertheless, the TLS suggest a superposition of multiple signals whenever there are more than 3 stark transitions detectable in the 5D07F1 emission region between em ~ 585 - 595 nm. The 7F1 energy level splits into a maximum of 3 (2j+1, j = 1 in this case) levels for any given symmetry. In order to further discriminate between emission signals, selective excitation of the sites with exhibiting the narrow excitation bands was performed using different delay times between excitation and emission acquisition in order to allow the decay of short-lived emission lines (S1). This allows for the species with the longest decay times to dominate the emission spectra. The presented spectroscopic data strongly supports the appearance of a distinct domain with a unique symmetry after the SRMO treatment. The emission pattern excited at lex = 581.6 nm features a strong emission at 611 nm not found in the SRSO sample, especially when introducing a delay between excitation end emission acquisition. We interpret this signal as the spectroscopic response of the ĸ phase.

3.2. Reducibility studies and estimation of the activation energy in the reduction process.

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The CZ-MO sample, obtained by reduction at high temperature and reoxidation at mild temperature, has an improved reducibility compared to the CZ-SO sample (Figure 5). The same behaviour was confirmed in the case of the europium-doped samples, the oxide with an SRMO treatment showing better reducibility than the oxide with an SR-SO treatment (S2). < near Figure 5 > In order to obtain deeper information about the redox behavior of CZ-MO and CZ-SO oxides under reducing atmosphere, an estimation of the apparent activation energy in the reduction process was performed. The stages of the reduction process are the dissociative adsorption of hydrogen to form OH groups on the surface, the recombination of OH, the formation of water, which involves the creation of anionic vacancies on the surface of the sample, and finally the mobilization of oxygen ions from the bulk of the solid to the positions of the surface vacancies. According to [21], the limiting step is thought to be dissociation of hydrogen. The calculation of the energy has been carried out using the Ozawa method [37]. This method is based on the study of a series of TPR curves obtained at different heating rates. The TPR experiments used to determine the activation energy were performed in H2(5%)/Ar, with heating rates of 10, 6 and 3 °C/min. Figure 6 shows the TPR experiments carried out at the different heating rates. In this figure, the TPR diagrams are represented both with respect to temperature and time. As can be expected in this type of experiment, as the heating rate decreases, the temperature at which the different peaks appear also decreases. In addition, different reduction process are better resolved. On the contrary, the measurement of the areas shown in Figure 6 indicates that the total areas do not vary when the heating speed changes, resulting in the same degree of reduction for each of the samples at all heating rates. S3 shows the graphs corresponding to the percentages of reduction versus temperature, obtained from the TPR diagrams. In these curves a 100% conversion has been assigned to the total area enclosed under the m/c:18 curve versus the time in the corresponding TPR experiment. After applying the Ozawa method, the values

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of apparent activation energy for the reduction process are about three times higher in the case of the CZ-SO (129 kJ/mol) than in the case of CZ-MO (39 kJ/mol). The estimation has been performed for a reduction degree of 6%. Under these conditions, these energies are mainly accounting for the activation of the hydrogen molecule. < near Figure 6 > Rhodium oxide samples supported on CZ-MO and CZ-SO oxides were also prepared using the incipient impregnation method. According to the bibliography [38–41], the presence of highly dispersed rhodium increases the rate of generation of atomic hydrogen through the spillover process. As a result, possible kinetic limitations associated with hydrogen activation are eliminated. Curiously, the activation energy is very similar for the MO sample with or without rhodium (43 and 39 kJ/mol). In contrast, the activation energy for the SO sample, initially much higher (129 kJ/mol), decreases in the presence of rhodium (34 kJ/mol) to match the MO samples with or without supported metal. According to this, the dissociation of the hydrogen molecule in CZ-MO is faster than in CZ-SO due to a lower activation energy. 3.3. Surface studies 3.3.1 IR spectroscopic analysis of deuterium adsorption To gain information on the surface reactivity, the interaction of different probe molecules with the surface was investigated. It has been pointed out that the interaction between surface and hydrogen plays a key role in the redox behaviour of ceria-based oxides. For this study, D2 was used instead of H2, because the IR O-D stretching band appears in a spectral range with much less noise (host vibrations). CZ-MO was reduced at 350 ºC under flowing H2/Ar, being the reduction temperature 500 ºC for CZ-SO. These temperatures were chosen according to the OSC results, in order to start with the same reduction degree in both samples [42]. After reduction, adsorbed H2 was removed by heating to 500 ºC under vacuum in both samples. Then,

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samples were cooled to room temperature, and 40 Torr of D2 was introduced into the cell. After heating to 150 ºC, 200 ºC and 250 ºC, and cooling to room temperature, spectra were collected. < near Figure 7 > Figure 7 shows ʋ(O-D) spectra for CZ-MO and CZ-SO oxides. The first point to be noted is that the incorporation of D2 is most pronounced for the CZ-MO sample. For both oxides, the D2 incorporation starts at 150 ºC. D2 activation occurs more rapidly in the CZ-MO oxide compared to the CZ-SO oxide, as can be seen by the greater increase in the intensity of the CZMO bands as the temperature increases. In principle, these bands could be explained by H/D isotopic exchange of hydrogen that would have been retained on the surface of the oxide or by the incorporation of D2 from the gas phase to the reduced oxide. In the case of CZ-MO, there is a sharp band centered at 2688 cm−1, with a shoulder at 2710 cm−1, that, according to the literature [43–46] can be attributed to type II O-D species (Figure 8). The latter band is related to two cations (cerium or zirconium), whereas the former is linked to the presence of vacancies. A small shoulder at 2750 cm−1 can be related to type I O-D species. These types of O-D species are also present in the spectrum corresponding to CZ-SO, although with different relative intensities and slightly shifted towards higher wavenumbers (2752, 2713 and 2696 cm−1). As the temperature increases, for the CZ-MO oxide the most intense bands are shifted to higher frequencies, according to the literature [47,48] this shift is attributed to a greater of reduction degree of the surface. However, the area corresponding to the Ce3+ band is similar for both samples (S4). It must be clarified that the sample quantity and thicknesses of the pellets have been the same. For the CZ-SO oxide, the frequency of the bands does not change. In addition, at 250 ºC, a broad band appears at 2500 cm−1 in the CZ-MO spectrum that is assigned to deuterium bonds. < near Figure 8 >

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To summarize, higher intensity O-D bands in the case of CZ-MO suggest a stronger interaction between D2 and the surface compared to the CZ-SO sample. This, together with the small but measurable displacements of the band frequencies, suggest differences between these oxides regarding their interaction with hydrogen, affecting the dissociation of H2. 3.3.2 IR spectroscopic analysis of methanol Methanol adsorption has been used to characterize the surface state of the oxides. It is known that methanol adsorption is sensitive to the nature and redox state of the cations in the surface of this kind of oxides, allowing to distinguish between Ce3+, Ce4+ and Zr4+ [43,49–52]. Figure 9 shows the C-O stretching region corresponding to the adsorption of methanol at room temperature. Again, there is a strong difference concerning the intensity of the bands, showing much lower intensity in the CZ-SO sample, similar behavior as in the previously shown experiments with adsorption of D2. CZ-MO shows an intense band at 1105 cm−1 that can be assigned to an on-top methoxy species over Ce4+ [43]. The shoulder at 1155 cm−1 corresponds to on-top methoxy species over Zr4+ [43]. The band centered at 1052 cm−1 can be attributed to doubly bridged methoxy groups over Ce4+ or Zr4+. Finally, triply bridged methoxy species are characterized by a band at 1025 cm−1 (Figure 10). In the case of CZ-SO, the same bands are found, but with different relative intensities and shifted towards higher wavenumbers (especially in the case of the doubly and triply bounded species). It should be noted that the ratio of intensities between the band corresponding to the monodentate methoxy group coordinated to Ce4+, and the monodentate methoxy group coordinated at Zr4+ is greater for CZMO oxide than for the CZ-SO oxide, indicating more surface Ce4+ in the case of the MO sample. Figure 7B shows CZ-MO oxide with assignable to C-H tension bands centered at 2915 cm−1 and 2808 cm−1. Other observable bands, centered at 2948, 2888 and 2844 cm−1 correspond to undissociated methanol. For CZ-SO oxide, the frequencies at which the bands appear are the same, changing the relative intensity between the different bands.

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< near Figure 9 > < near Figure 10 > The evolution of the different bands after treating the sample at increasing evacuation temperatures has also been studied. S5 shows this evolution depending on the evacuation temperature for CZ-MO and CZ-SO oxides. For both oxides, the evolution of the IR bands with changing evacuation temperature is similar. According to the assignment of bands reported in the literature [43], the C-O tension band that appears around 1105 cm−1 corresponds to the monodentated methoxy groups coordinated to a single Ce4+ cation. When the evacuation temperature increases this particular band suffers the strongest decrease until it is almost disappearing at a temperature of 350 oC. According to the bibliography, the methoxy groups (CH3O−) are oxidized to formiates (HCOO−), while Ce4+ is reduced to Ce3+. Even though all of the bands in the spectral region of the C-H vibrations are decreasing in intensity, the bands corresponding to undissociated methanol experience the greatest decrease. The appearance of formiate groups can be observed in the range between 1700-1200 cm−1, Figure 11. For both oxides, there is no remarkable evidence for the formation of formiate groups before the evacuation treatment at 200 ºC. < near Figure 11 > For CZ-MO oxide, there are bands at 1585 and 1562 cm−1 (as(OCO)) corresponding to formiate - Ce3+ bonds. However, formiate groups may also migrate to Ce4+ and Zr4+. There are also formiate groups on Ce4+ (band at 1550 cm−1, (as(OCO); 1370 cm−1 (CH) and 1358 cm−1 (s(OCO)). In the case of the CZ-SO oxide, the bands are wider and less resolved. The maximum of the band is about 1580 cm−1 (as(OCO) accompanied by 1383 cm−1 (CH) corresponding to formiate groups on Zr4+. In conclusion, the distribution between formiate species formed and adsorbed either on zirconia or on ceria is different for CZ-MO and CZ-SO.

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3.3.3 Temperature-programmed desorption study of CO2 CO2 adsorption was used to study the surface basicity. In this study, 1 atm of CO2 was passed at room temperature through the samples at room temperature, which were previously cleaned with O2/He at 500 ºC. Then, the gas flow was switched to He, and a TPD-MS was run. Peak positions are related to the strength of the basic centers, whereas the intensity of the signal is linked to their concentration. TPD-MS experiments for CZ-MO and CZ-SO show one main peak centered at 110 ºC, and a shoulder at higher temperature (185ºC) only in the case of CZMO (Figure 12). The total area under the TPD was higher for CZ-MO than for CZ-SO, corresponding to the total number of basic sites. The reported results suggest that most of the surface basic centers have similar strength for both samples, although there is a small population of more basic sites in the case of CZ-MO. The basicity of Ce4+ ions in pure ceria is higher than that of Zr4+ in pure zirconia [53,54]. In mixed oxides, although the Lewis basicity of Ce4+ is decreased and that of Zr4+ ions is increased compared to pure compounds, for CZMO oxide, results indicate higher concentration of surface cerium, due to the greater strength of some of its basic centers. This result would be supported by FTIR of methanol. < near Figure 12 > 3.3.4 Study of the surface chemical composition by XPS Table 1 summarizes quantitative data obtained by XPS. Zirconium atomic percentage was calculated using Zr 3d (BE~180 eV) and Ce 4d (BE~100 eV) core levels, with an analysis depth of around 2.1 nm [55]. Ce 3d/Ce 4d ratio is used to compare the relative composition gradients that could exist in the samples at the surface. Ce 3d data (BE~900 eV) comes from an analysis depth of about 1.2 nm. In case of a homogeneous sample, Ce 3d/Ce 4d ratio should be 1. Values below 1 indicate a surface cerium deficiency. Ce3+ percentage was calculated by decomposing Ce 3d core level as shown in figure S6 and described in reference [56]. < near Table 1 > 17

With respect to Zr surface content of the samples, there is an important disagreement between the bulk composition (38% Zr) and the surface Zr content of both CZ-MO and CZ-SO samples, 25.6% and 27.8%, respectively. When comparing the oxides, there is a small difference between them, CZ-SO showing higher Zr content. Ce 3d/Ce 4d ratio also points in that direction, being close to 1 (0.95 ± 0.05) in the case of CZ-MO sample, and decreasing to 0.87 ± 0.02 in CZ-SO sample, indicating a slight decrease in the surface cerium content, at least for the outermost 2 nm. Additionally, a small difference is observed in Ce3+ percentage, being also higher in the case of CZ-SO. 4. Conclusions The surface properties of two cerium-zirconium oxides with the same composition but different reducibility after SRMO and SRSO treatments were studied. In agreement with previous studies, clear differences in the surface chemical behaviour were observed. However, the specific structural changes in the surface of both oxides (ϰ and fluorite structure) have not been identified until now. In our study, using Raman, DRX and Fluorescence line-narrowing spectroscopy, both oxides show variations in their surface structure and composition. Additionally, IR spectroscopy of different probe molecules adsorbed to the surface allows insights into surface chemistry. Clear differences in reactivity could be detected, which are especially remarkable in the case of hydrogen. The hydrogen activation rate is higher for CZMO than for CZ-SO. A kappa phase on the surface layer with enriched in cerium has been identified, increasing the basicity of the surface layer.

5. Acknowledgments Financial support from the Ministry of Economy and Competitiveness of Spain (Project MINECO/FEDER MAT2017-87579-R), and the Junta de Andalucía (Groups FQM-110 and FQM-334) is acknowledged.

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Figure captions Figure 1. Pictorial description of the samples preparation (CZ-MO and CZ-SO). Figure 2. XRD diagrams corresponding to CZ-MO and CZ-SO oxides. A magnification is included, where a superstructure peak is observed.

Figure 3. Raman spectra corresponding to CZ-MO and CZ-SO oxides.

Figure 4. Total luminescence spectra of CZ-SO (left), CZ-MO (right) after excitation between ex = 578 - 582 nm (7F0-5D0), showing similar emission bands. Excitation between 581.2 581.8 nm shows a clear difference between both samples, the excitation band is predominantly active in the CZ-MO sample. This band is attributed to the appearance of the ĸ phase and can be identified in the excitation spectra (vertical traces).

Figure 5. TPR-MS study in 5% H2/Ar of CZ-MO and CZ-SO mixed oxides. Figure 6. TPR-MS study in 5% H2/Ar at different heating rates, represented versus temperature (A) and time (B).

Figure 7. FTIR spectra corresponding to D2 adsorption at different temperatures for CZ-MO and CZ-SO oxides.

Figure 8. Scheme of the hydroxy species on the surface of CexZr1-xO2 solid solutions. Figure 9. FTIR spectra corresponding to CH3OH adsorption on CZ-MO and CZ-SO. Part A shows C-O stretching bands and part B shows C-H stretching bands for methanol interaction.

Figure 10. Scheme of the methoxy species on the surface of CexZr1-xO2 solid solutions. Figure 11. FTIR spectra corresponding to CH3OH adsorption at 25 ºC and then evacuation at 200 ºC. The formiate groups are shown.

Figure 12. TPD-MS study of CO2 (m/e=44) pre-adsorbed on CZ-MO and CZ-SO mixed oxides.

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Table 1. Surface chemical composition and cerium reduction degree obtained by XPS performed on the mixed oxides previously evacuated at 300 ºC for 30 min in the analysis chamber.

21

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Table 1

Sample

%Zr

Ce 3d/Ce 4d

%Ce3+

CZ-MO

25.6±0.9

0.95±0.05

7

CZ-SO

27.8±0.4

0.87±0.02

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DECLARATION OF INTERESTS We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property. We understand that the Corresponding Author is the sole contact for the Editorial process (including Editorial Manager and direct communications with the office). She is responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs. We confirm that we have provided a current, correct email address which is accessible by the Corresponding Author and which has been configured to accept email from ([email protected])

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HIGHLIGHTS  Different structure and reducibility in Ce/Zr oxides due to different treatment 

Study of surface properties of Ce/Zr mixed oxides with different reducibility



Surface characterization carried out by XPS and adsorption of probe molecules



Strong effect on redox behaviour due to slight differences in surface



FLNS using Eu3+ dopants proved to be a sensitive structural tool

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