Improved Oxidase Mimetic Activity by Praseodymium Incorporation

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Improved Oxidase Mimetic Activity by Praseodymium Incorporation into Ceria Nanocubes Lei Jiang, Susana Fernandez-Garcia, Miguel Tinoco, Zhaoxia Yan, Qi Xue, Ginesa Blanco, José J. Calvino, Ana Belen Hungría, and Xiaowei Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 17 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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ACS Applied Materials & Interfaces

Improved Oxidase Mimetic Activity by Praseodymium Incorporation into Ceria Nanocubes Lei Jiang†, §, *, Susana Fernandez-Garcia‡, §, Miguel Tinoco‡, Zhaoxia Yan†, Qi Xue†, Ginesa Blanco‡, Jose J. Calvino‡, Ana B. Hungria‡, Xiaowei Chen‡, *

†

State Key Laboratory of Heavy Oil Processing and Center for Bioengineering and

Biotechnology, College of Chemical Engineering, China University of Petroleum (East China), Qingdao, 266580, China ‡

Departamento de Ciencia de los Materiales, Ingeniería Metalúrgica y Química

Inorgánica, Facultad de Ciencias, Universidad de Cadiz, Campus Río San Pedro, Puerto Real (Cádiz), E-11510, Spain [§] These two authors contributed equally to this work. [*] Correspondence to: X. Chen, tel: 0034-956-012741, fax: 0034-956-016288, Email: [email protected]. L. Jiang, tel: 0086-532-86981568, fax: 0086-532-86981569, Email: [email protected].

KEYWORDS: Praseodymia, ceria, nanocubes, oxidase, TMB oxidation, redox properties.

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ABSTRACT In this study, ceria nanocubes (NC) modified with increasing concentrations of praseodymium (5, 10, 15 and 20mol.%) have been successfully synthesized by a hydrothermal method. The as-synthesized Pr-modified ceria nanocubes exhibit an enhanced oxidase-like activity on the organic dye TMB within a wide range of concentrations and durations. The oxidase activity increases with increasing Pr amounts in Pr-modified ceria nanocubes within the investigated concentration range. Meanwhile, these Pr-modified ceria nanocubes also show higher reducibility than pure ceria nanocubes. The kinetics of their oxidase mimetic activity is fitted with Michaelis-Menten equation. A mechanism has been proposed on how the Pr incorporation could affect the energy level of the bands in ceria, and hence facilitate the TMB oxidation reaction. The presence of Pr3+ species on the surface also contributes to the increasing activity of the Pr-modified ceria nanocubes present higher oxidase activity than pure ceria nanocubes.

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1. INTRODUCTION Ceria and CeO2-based compounds have attracted great interest due to their extensive applications in heterogeneous catalysis,1,2,3 solid oxide fuel cells4 or biomedicine.5 Because of its unique redox properties, it has been demonstrated that CeO2 possesses excellent catalytic properties in different chemical reactions such as CO oxidation,6,7,8 total oxidation of polycyclic aromatic hydrocarbons,9 NO reduction,10 soot combustion11 and H2 oxidation.12 In CO oxidation reaction process, CO abstracts an oxygen atom from a surface -CeIV-O- linkage, creating an oxygen vacancy and resulting in the oxidation of CO. The oxygen vacancy is refilled in the final step of the catalytic cycle after O2 dissociation on the reduced surface. Therefore, the role of ceria is similar to that of an oxidase.13,14 An oxidase is an enzyme that catalyzes an oxidation-reduction reaction involving molecular oxygen (O2).15 Recently, several studies have been published on the oxidase mimetic activity of nanoceria.16,17 In particular, studies by Perez et al.16 revealed that ceria nanoparticles are able to quickly oxidize several organic substrates without any additional oxidizing reagents. Attempts to enhance the oxidase mimetic activity have since then started, e.g. by coating particles with polymers to increase their dispersion and to control their efficiency. Later Gao et al.18 argued that instead of being postulated as an oxidase mimic, ceria nanoparticles behaved as an oxidant which dissolved completely after being reduced under acidic conditions. The main reason for the oxidase-like activity of ceria above mentioned can be attributed to its high Oxygen Storage Capacity (OSC). Ceria is considered as an 3



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efficient “oxygen buffer” because it is able to release and store oxygen when treated alternatively under reducing and oxidizing atmospheres, by means of a facile Ce3+/Ce4+ redox cycle.1 During the past 10 years, a number of evidences indicate that various structures and morphologies of ceria feature unique properties, different from those of traditional ceria particles.19-21 For example, nanoceria with well-defined shapes, such as, octahedra and cubes, has an apparent advantage in enhancing the catalytic activity because it exposes more active facets which might facilitate the redox cycles.19 Meanwhile modifying conventional nanocrystalline ceria with other elements, such as transition metals (Zr)22 or other lanthanides (La, Pr and Yb),23-27 has been proven to enhance the redox properties of ceria due to an increase in the concentration of oxygen vacancies and oxygen mobility in the oxide. In particular, Pr can greatly improve OSC of ceria due to its Pr4+/Pr3+ redox couple. Praseodymia-ceria solid solutions have attracted much attention due to their applications as nontoxic red ceramic pigments, and also because they have been considered promising ion-conducting electrolytes and oxygen storage materials.28-30 However, only a few articles on Pr-modified ceria with controlled morphology have been reported.23,31 It has been verified that Pr incorporation into a 2-D mesoporous ceria caused higher catalytic activity than bare ceria for degradation of Rhodamine B, which is related to a radical oxidation and photodegradation

process.23

Andana

et

al.

claimed

that

nanostructured

ceria-praseodymia catalysts with different praseodymium contents have been prepared through hydrothermal synthesis to study the effect of Pr as a dopant and the effect of morphology towards soot combustion.31 A Ce50Pr50 catalyst, with atomic percentage 4


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of 50% Ce and 50% Pr with mixed structures of nanorods and nanocubes, attained the best catalytic performances due to the synergistic combination of well-defined nanostructures and the use of praseodymium as a dopant.31 Up to now, Pr-modified ceria nanocubes have not been studied for oxidase-like activities in the literature yet. Based on all these findings, in this contribution well-defined ceria nanocubes and Pr-doped ceria nanocubes with different Pr concentrations have been synthesized using a hydrothermal method. Then, their textural, structural and redox properties have also been investigated. The as-synthesized CeO2 NC and 5, 10, 15 and 20 mol.% Pr-modified CeO2 NC, with high exposure of {100} facets, were tested in terms of their oxidase-like activities on 3,3’,5,5’-tetramethylbenzidine (TMB) oxidation. The kinetics of TMB oxidation was also performed over all the samples. The whole set of data prove that the incorporation of Pr into ceria nanocubes leads to an improvement in their redox properties which is paralleled by a higher oxidase activity. 2. EXPERIMENTAL 2.1.

Synthesis

A hydrothermal method described in previous papers32,33 was used. 125 mL of 11.5 M NaOH (Alfa Aesar, 98%) and 115 mL of 0.1 M Ce(NO3)3·6H2O (Alfa Aesar, 99.5%) were mixed and stirred in a 300 mL Teflon container for 30 min. The Teflon container was then sealed with a Teflon lid and placed into a stainless steel autoclave. The reaction mixture was kept at 180 oC for 24 h in an oven. Afterwards, the mixture was cooled down to room temperature, centrifuged and washed with deionized water 5



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several times and then with ethanol once (Panreac, Absolute Ethanol). Finally, the samples were dried at 80 oC for 24 h in an oven. A yellowish powder was obtained as shown in Figure S1a. The synthesis of the Pr-modified CeO2 NC samples with different Pr concentrations was carried out following the same procedure as CeO2 NC. However, the concentrations of Ce and Pr precursors were adjusted to Pr concentrations of 5, 10, 15 and 20mol.%. For example, 10%Pr-CeO2 NC was synthesized using a mixture of 0.094 M Ce(NO3)3·6H2O and 0.01 M Pr(NO3)3·6H2O (Sigma Aldrich, 99.99%). At the end, a reddish 10%Pr-CeO2 NC powder was obtained, as shown in Supporting Information Figure S1b. 2.2.

Physical and Compositional Characterization

X-Ray diffraction (XRD) patterns were recorded on a D8 ADVANCE diffractometer of Bruker using the Cu Kα radiation, with a range of 5-110o, a step of 0.02o and a step time of 1s. The software DiffracPlus was used to measure the width of the {111} diffraction peak of ceria in order to calculate the particle size using the Scherrer equation. ICP measurements were carried out using an ICP-AES Iris Intrepid equipment of Thermal Elemental to determine the actual Pr content of the Pr-modified nanocubes. Brunauer-Emmett-Teller (BET) surface areas of the samples were determined in a Micromeritics ASAP 2020 via nitrogen adsorption at -196 oC. Prior to the analysis, the samples were degasified at 150 oC for 2 h under vacuum.

The surface chemical composition in Ce and Pr of the samples was characterized by X-ray photoelectron spectroscopy (XPS). The analyses were 6


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performed on a Kratos Axis Ultra DLD instrument. Spectra were recorded using monochromatized Al Kα radiation (1486.6 eV), with an X-ray power of 150 W. The spectrometer was operated in the constant analyzer energy mode, with pass energy of 20 eV. Powder samples were pressed into pellets, which were stuck on a double-sided adhesive conducting polymer tape and analyzed without any further treatment. 10%Pr-CeO2 NC sample was also submitted to a thermal treatment with a 5%O2/He flow at 370 oC for 1 h in a catalytic chamber attached to the spectrometer. In this case, the sample could be analyzed without further exposure to air. Surface charging effects were compensated by making use of the Kratos coaxial neutralization system. The binding energy scale was calibrated with respect to the C 1s signal at 284.8 eV.

The samples were also investigated using a variety of transmission and scanning transmission electron microscopy techniques. Most of the analyses were performed in a JEOL 2010-F scanning transmission electron microscope with 0.19 nm spatial resolution at Scherzer defocus conditions in HREM. Scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) images were obtained by using an electron probe of 0.5 nm of diameter at a camera length of 8 cm. Electron energy loss spectroscopy (EELS) analyses were carried out by using an ENFINA spectrometer (Gatan), using a convergence angle of 8 mrad, a collection angle of 24.36 mrad, an exposure time of 2 seconds with a dispersion of 0.3 eV/channel and an aperture of 3 mm. In order to increase the signal-to-noise ratio of the as-collected EELS spectra, PCA algorithm was employed using hyperspy software.34 Additionally, very high spatial resolution energy dispersive X-ray spectroscopy (XEDS) maps were 7



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acquired using the ChemiSTEM capabilities of a FEI Titan3 Themis 60–300 microscope. In this case, a high brightness, sub-angstrom (0.07 nm) diameter, electron probe was combined with a highly stable stage to record these maps. Element mapping was acquired with a screen current of 200 pA and a pixel dwell time of 170 µs. This dwell time results in a frame acquisition time of approximately 25 s after which the drift was corrected using cross correlation. An averaging filter was used on the images as provided in the Esprit software.

2.3.

Redox Properties

Temperature programmed reduction with H2 (H2-TPR) started with a pretreatment consisting in an oxidation under a 5% O2/He flow (60 mLmin-1) at 500 oC for 1 h. After the oxidation pretreatment, the samples were cooled in the same 5% O2/He flow down to 150 oC, and then, the flow was switched to pure He down to room temperature. The samples were reduced in a 60 mLmin-1 flow of 5% H2/Ar with a heating rate of 10 oCmin-1 and a maximum reduction temperature of 950 oC, keeping the sample at this final temperature for 1 h. The outlet of H2-TPR equipment was connected to a Thermostar GSD301T1 mass spectrometer of Pfeiffer Vacuum. The mass/charge ratio (m/z) values used to monitor H2 consumption were 2 and 18 for the concomitant formation of H2O. OSC measurements were carried out by a thermogravimetric method, using a SDT Q600 horizontal thermobalance. The pretreatment used in these experiments was an oxidation in a 5% O2/He flow at 500 oC for 1 h. Then the sample was cooled down in 8


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the same flow to 150 oC, and kept at this temperature for 30 min. The process of analysis included measuring the weight loss of the sample in a 5% H2/Ar flow, with a flowrate of 60 mLmin-1, at isothermal regime at selected increasing temperatures (200, 350, 500 and 700 oC). To proceed from one temperature to the following, a heating rate of 10 oCmin-1 was used upon reaching the desired temperature, which was held for 1 h.

2.4.

Oxidase Activity

The oxidase mimetic activity of the samples was studied by investigating their capability to catalyze the oxidation of TMB. A 1 mM TMB solution was prepared by diluting 100 mM TMB stock solution (in dimethyl sulfoxide (DMSO)) with 10 mM pH 4.0 of acetate buffer. The solution was then stirred thoroughly to avoid precipitation. The ceria nanocube solution was prepared dispersing CeO2 NCs or (5, 10, 15 and 20 mol.%)Pr-CeO2 NCs in Milli Q water after washing and centrifuging the nanocubes at 12000 rpm for 10 min three times. The nanoceria solution was sonicated for 2 h before each measurement. The UV-VIS absorption spectra of TMB before and after the addition

of

nanocubes

were

obtained

through

a

SHIMADZU

UV-2450

spectrophotometer. Steady kinetic assays were monitored in a time course mode at 652 nm using SpectraMax M2e microplate reader (Molecular Devices, US). To explore and compare kinetic activities, the concentrations of the nanocubes were varied (60 µM, 600 µM and 6 mM) with 200 µL TMB of fixed concentration and vice versa. Similarly, the concentration of nanocubes were kept at 6 mM, and 200 µL TMB at 0.5, 1.0, 1.5, 2.0 and 2.5 mM were applied to calculate the steady kinetics analysis.

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3. RESULTS AND DISCUSSION 3.1.

TEM Results

Figure 1 shows representative TEM (1a-1d) and STEM-HAADF (1e) images of the Pr-CeO2 NC samples with different Pr concentrations. Figures S1c and S1d confirm that CeO2 NC sample is comprised of cubic-shaped nanocrystals. The inset of Figure S1d depicts the analyses of the spatial frequencies observed in the Digital Diffraction Pattern (DDP) of this HREM image, showing the presence of {200} reflections of fluorite-type ceria when the nanocubes are oriented along a [001] direction. When Pr is incorporated into the ceria nanocubes, as shown in Figures 1a-1e, the as-synthesized materials still maintain nanocube morphology up to 15 mol.% Pr. 20 mol.% Pr leads to a change of morphology from nanocubes to a mixture of nanocubes and nanorods. For this reason, this sample is referred as to 20%Pr-CeO2 NC+NR. Figures 1e and 1f present a STEM-HAADF image and two XEDS spectra recorded on a nanorod and a nanocube of the 20%Pr-CeO2 NC+NR sample, respectively. It can be observed that the nanocube contains 81mol.% Ce and 19mol.% Pr (spectrum 1), while most nanorods (spectrum 2) contain about 61mol.% Pr. A very recent study reported that only 10% Pr modification of ceria nanocubes induced a mixture of nanorods and nanocubes.31 It can be concluded that our synthesis method successfully produced homogeneous morphology of nanocubes with a maximum of 15mol.% Pr incorporation. Since 20%Pr-CeO2 NC+NR contains a mixture of nanocubes and nanorods, it is not meaningful providing its particle size distribution. The particle size distributions of 10


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the as-synthesized CeO2 NC and the other Pr-modified CeO2 NC, excluding the 20%Pr-CeO2 NC+NR sample, obtained by (S)TEM studies, are presented in Figure 2. To build these distributions, the edge length of roughly 150 nanocubes were measured for each sample. It can be observed that these four nanocube samples are very similar. The edge lengths of CeO2 NC are mainly comprised in the range from 5 to 45 nm. Nanocubes larger than 50 nm are only observed in the Pr-modified ceria nanocubes, but with very small frequencies (less than 5%). It also should be highlighted that in the 5%Pr-CeO2 NC sample 30% of the nanocubes are smaller than 5 nm, which is not observed on the other two Pr-modified ceria NCs. Element mapping was performed on the 10%Pr-CeO2 sample (Figure 3) in order to investigate the homogeneity of this sample. It can be seen that Ce and Pr homogeneously distribute throughout a nanocube. The quantification of XEDS performed in areas 1 (black line) and 2 (solid grey) shows equivalent composition close to the nominal value 10mol.%Pr. The contribution of different crystallographic planes to the total exposed surface can be calculated by using HREM data. For CeO2 NC, the percentage of {100}, {110}, and {111} facets are 84%, 16% and < 1%, as previously reported.32,33 For 10%Pr-CeO2 NC, the corresponding percentages are 89%, 11% and 0% for the same facets, respectively. Therefore, the surface crystallography of these two samples is very similar. For the other nanocubes, 5%Pr-CeO2 NC and 15%Pr-CeO2 NC, similar results were also obtained. In summary, the maximum Pr concentration which could be incorporated into 11



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ceria maintaining a nanocube morphology was 15mol.%. At Pr concentrations equal or higher than 20mol.%, the hydrothermal synthesis led to a mixture of nanocubes and nanorods. Pr is homogeneously distributed within the nanocubes both at surface and bulk levels. On the other hand, the surface crystallography of the nano-oxide is not changed after the incorporation of Pr, which exposes a similar percentage of {100}, {110} and {111} type facets. Interestingly, when nanorods are formed, these depict Pr-rich compositions whereas the remaining nanocube crystals compensate with Pr contents below 20mol.%. 3.2.

XRD, N2 Physisorption and ICP Results

XRD diagrams of the CeO2 NCs and Pr-modified CeO2 NCs are shown in Figure 4. All the diffraction peaks observed in CeO2 NC and Pr-modified CeO2 NC samples can be attributed to a fluorite-like, face-centered cubic structure (Fm-3m) ceria, in good agreement with TEM results of pure CeO2 NC (Figure 1b). The sharp diffraction peaks clearly indicate a single crystal state for all the samples. The XRD profiles for CeO2 NCs and 10%, 15% and 20%Pr-modified CeO2 NCs are almost identical. No apparent angle shifts or signal asymmetries were observed on the XRD diagrams of these samples, which might be caused by the similarity between the cationic radius of Pr4+ (96 pm) and Ce4+ (97 pm).35 The lattice parameters of these samples were calculated using the main diffraction peak at 28.5o and included in Table 1. The obtained values are the same (0.54 nm) since there is no significant shift of the diffraction peaks after the incorporation of Pr with different concentrations. This result is different from what we 12


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observed in our previous work dealing with 10%La-doped ceria nanocubes.33 The average particle sizes of the nanocubes estimated from the Scherrer equation are listed in Table 1. These data are very close to the average edge length calculated by TEM results, which are also shown in Table 1. N2 physisorption at -196 oC has been performed in order to obtain BET specific surface areas of the ceria-containing samples since most of heterogeneous catalytic reactions occurs on the surface of the catalysts. Catalysts with high BET specific surface may provide more active sites and, therefore, speed up the reaction more efficiently. Table 1 also presents the BET specific surface areas of the samples measured by N2 physisorption, which are in the range from 23 to 38 m2·g-1. The specific surface areas of these samples were estimated using the edge length distributions by assuming a model of nontruncated perfect cubes. The values estimated for the different samples were very similar to those obtained experimentally by N2 physisorption measurements. The chemical composition of the Pr-modified CeO2 NCs was determined at macroscopic level by ICP, as shown in Table 1. The molar concentrations of Pr determined were 4.4, 9.7, 14.4 and 19.2%, which are very close to the expected values. 3.3 XPS and EELS Results Table 1 lists the Pr compositions on the surface of the Pr-modified CeO2 NC samples. It can be seen that the Pr concentration of 10%Pr-CeO2 NC is very similar to the ICP results, which represent the average bulk composition of the sample. For the Pr-modified ceria nanocubes with 5, 15 and 20% Pr samples, Pr concentrations 13



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calculated by XPS are higher than the values obtained by ICP results. Figure 5 shows the XPS results for Pr 3d and Ce 3d core levels, in which a depth around 3.5 nm from the surface could be analyzed. XPS analyses in Figure 5 also provide the oxidation states of Pr and Ce at the outermost surface layers. Figure 5a shows the Pr 3d core level for the 10%Pr-CeO2 NC sample, both as-synthesized and after an in situ oxidation at 370 ºC. The characteristic “a´” Pr4+ signal at 968 eV36-38 cannot be observed in any of the two spectra. This indicates that all the praseodymium that can be analyzed by XPS (i.e. that within the first 3.5 nm from the surface) is 100% Pr3+, even after being submitted to an oxidizing treatment in the catalytic chamber attached to the XPS spectrometer. No other type of Pr containing species, different from Pr3+ incorporated in a mixed Pr-Ce oxide solid solution, was detected by XPS. In the collection of Pr 3d spectra recorded on a pure praseodymia sample, shown in Figure S2, the Pr4+ signal of praseodymia can be seen both in a sample without any pretreatment and the sample which was reoxidized praseodymia at 300 oC after reduction at 500 oC. However, “a” and “a'” peaks did not appear in the reduced praseodymia sample, indicating that only Pr3+ species have been detected on this sample. These results are in good agreement with previous data in the literature.29,30,39,40 Concerning the major lanthanide, cerium, the XPS results (Figure 5b) indicate that this element is mainly present as Ce4+ on the surface of the pure and Pr-modified CeO2 NCs. Thus, the fraction of Ce3+ on the outermost surface layers of CeO2 NC amounts only to 2% but it is below the detection limit on the other Pr-modified CeO2 NC samples with different Pr concentrations. 14


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Spectrum-line EELS analyses of 10%Pr-CeO2 NC sample can provide further information about the composition of this sample. EELS spectra of the 10%Pr-CeO2 NCs were recorded every 0.5 nm point by point, along a line on a nanocube which started at one surface, crossed through the bulk of the nanocube and exited at the opposite surface, as marked on the STEM-HAADF image in Figure 6a. As revealed by the HAADF-STEM image intensity profile along the path (Figure 6b), the thickness of the cube remained fairly constant along the scanned line. Spectrum III in Figure 6c corresponds to the average of the first 4 spectra, taking into account a surface layer just 1 nm thickness. In this spectrum the intensities of Pr peaks are very weak. Nevertheless, the total intensity of the Ce peaks is in this case also very low, due to the fact that the thinnest part of the crystallite is being analyzed. In any case, the intensity ratio between the Pr/Ce M4,5 peaks is roughly 16%. Spectrum II corresponds to the average of all the spectra contained in a surface layer thickness of 3.5 nm, which is same thickness as analyzed by XPS. Here, tiny Pr-M4,5 signals are clearly detected. The Pr-M4,5 signals become stronger in Spectrum I of Figure 6c, which corresponds to the average of the 66 individual spectra recorded on bulk positions of the cube (marked on Figure 6a). Since these data suggest that the ratio between the Ce-M4,5 and Pr-M4,5 signals remains roughly constant along the path, EELS results also confirm an homogeneous distribution of Pr throughout the nanocube volume. The Pr concentration determined by EELS analysis in this case is 12 ± 3 at.%. Similar results were obtained when the measurements were repeated on different nanocubes of the 10%Pr-doped NC sample. Combining the results obtained by element 15



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mapping, ICP, XPS and STEM-EELS, it can be concluded that the composition of 10%Pr-CeO2 NC is uniform and that Pr gets mixed with Ce at atomic level. Somacescu et al.41 stated that uniform Ce1-xPrxO2-δ solid solutions with nanoparticle morphology and mesoporous architectures could be synthesized by hydrothermal methods with surfactants in a wide range of x values (x = 0, 0.1, 0.5 and 0.9). Using this synthesis method we have prepared ceria-praseodymia solid solution nanoparticles which maintain a nanocube morphology with amount of Pr incorporated into the ceria nanocube structure up to 15mol.%. EELS can be also used to probe the oxidation state of cerium. The fine structure details of the EELS spectra of Ce3+ and Ce4+ differ in terms of the exact energy position of the M4 and M5 white lines as well as in their relative intensity (Figure S3).42 Moreover, both the M4 and M5 peaks present a low intensity shoulder on their high-energy side in the EELS spectrum of Ce4+, which are not observed in the case of Ce3+. Concerning these changes in the fine structure of the Ce M4,5 edge of Cerium, Sharma et al.43 proposed the use of the intensity ratio of the M5/M4 peaks at their maxima to detect the presence of both oxidation states. This M5/M4 ratio for a sample containing only Ce4+ can reach 1.18, whereas it decreases to 0.87 in pure Ce3+ samples. Intermediate values would indicate the presence of mixed Ce3+/Ce4+ oxidation states. The closer the M5/M4 ratio to the upper value, the larger the fraction of Ce4+, though a fully quantitative and direct relationship cannot be simply established. Figure S3 plots the evolution of the M5/M4 intensity ratio in the pure CeO2 NC and 10%Pr-CeO2 NC samples, as a function of the distance from the surface of the 16


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cubes. Note that in both cases, the values tend to increase from the surface to the bulk of the crystallites. This indicates that in both samples reduced cerium species locate on the surface. The value of the ratio observed in bulk locations does not reach the value expected for a totally oxidized Ce4+ state, which is consistent with the presence of a thin Ce3+-containing surface layer covering the whole crystallites. It should also be highlighted that the value of this ratio observed within the first 4 nm layer (which is roughly the maximum depth analyzed by XPS) of the 10%Pr-CeO2 NC sample, is higher than that of CeO2 NC. This result points out to a lower concentration of reduced cerium in the former. An apparent discrepancy between XPS results and EELS data can be perceived, as the latter indicated the existence of a significant fraction of Ce3+ on the surface of both catalysts. Nevertheless, this disparity could be explained taking into account the reduction effect of the electron beam in transmission electron microscopy.44 In any case, the concentration of Ce3+ in the surface layer (

Improved Oxidase Mimetic Activity by Praseodymium Incorporation

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