Yang Yang

Suzhi Li
email: lisuzhi@xjtu.edu.cn

Xiangdong Ding
email: dingxd@mail.xjtu.edu.cn

Jun Sun

Jerome Weiss
email: jerome.weiss@univ-grenoble-alpes.fr

Ekhard K H Salje
email: ekhard@esc.cam.ac.uk

Twisting of pre-twinned -Fe nanowires: from mild to wild avalanche dynamics

Keywords: twinning, -Fe nanowire, torsion, avalanches, mild wild fluctuations, torsion spectroscopy

We studied the torsion behavior of -Fe nanowires seeded with twin boundaries (TBs) using molecular dynamics simulations. Twisting the wire generates topological defects in the twin walls, namely kinks inside the twin walls for small twist angles, and junctions between kinks for large twist angles. During twisting the kink motion is jerky and uncorrelated at small twist angles. The probability density function (PDF) of jerk energies follows approximately a Gaussian distribution, indicating a mild deformation mode. The kink dynamics transforms from mild to wild at larger twist angles when complex twin patterns with a high density of junctions are generated. The collective motion of kinks now shows avalanche behavior with the energy being power-law distributed. The wildness, which measures the proportion of strain energy relaxed through such avalanches, is correlated with the junction density, and controlled by the external length scale (wire diameter) as well as an internal length scale (twin boundary spacing). Good strain-stress recoverability is achieved when unloading the wire before the formation of complex twin patterns. We correlate the evolution of twin patterns with a statistical analysis of jerk dynamics, which identifies the unique mechanical properties governed by twin boundary motion in nanowires.

Introduction

In metals, plastic deformation is associated with the motion and interactions of structural defects such as dislocations or twin boundaries. The classical paradigm of dislocation-mediated plasticity in bulk crystals assumes that correlations between individual dislocation motions are weak, allowing a homogenized description. However, jerky, intermittent plasticity was already noticed by Orowan and Becker on Zinc rods in 1932 [START_REF] Becker | Sudden expansion of zinc crystals[END_REF]. Twenty years ago, acoustic emission (AE) data in plastically deformed HCP materials showed that plastic jerks are, in these materials, clustered in space [START_REF] Weiss | Three-dimensional mapping of dislocation avalanches: clustering and space/time coupling[END_REF] and time [START_REF] Weiss | Dislocation avalanche correlations[END_REF], as well as power-law distributed in size and energy [START_REF] Miguel | Intermittent dislocation flow in viscoplastic deformation[END_REF], in complete contradiction with the classical paradigm. It was realized more recently that a full spectrum of behavior can be observed: from wild plasticity accommodated essentially through dislocation avalanches, e.g. in HCP materials, to mild plasticity for which plastic jerks are uncorrelated and associated with a characteristic size or energy scale ("Gaussian-like"), e.g. in FCC metals at large systems sizes. Furthermore, mild and wild fluctuations can coexist during plastic deformation [START_REF] Weiss | From mild to wild fluctuations in crystal plasticity[END_REF]. The wildness of dislocation-mediated plasticity is associated with size effects: In materials characterized by a mild plasticity at large scales, such as in FCC metals, deformation becomes wild for sub-µm sample sizes [START_REF] Dimiduk | Scale-free intermittent flow in crystal plasticity[END_REF][START_REF] Brinckmann | Fundamental differences in mechanical behavior between two types of crystals at the nanoscale[END_REF], signing a "smaller is wilder" size effect [START_REF] Zhang | Taming intermittent plasticity at small scales[END_REF] with potential detrimental effects for micro/nanoscale engineering [START_REF] Csikor | Dislocation avalanches, strain bursts, and the problem of plastic forming at the micrometer scale[END_REF][START_REF] Hu | Strategies to approach stabilized plasticity in metals with diminutive volume: a brief review[END_REF]. Wildness also depends on an internal length scale that emerges from dislocations with mutual interactions in pure metals. The length scale corresponds to a dislocation mean free path in some cases, or to added defects in alloys [START_REF] Zhang | Taming intermittent plasticity at small scales[END_REF]. These two size effects combine through a unique dimensionless ratio of scales (external/internal) that controls the wild-to-mild transition. Thus, the wildness of dislocationmediated plasticity is not controlled by the system size only. Pre-straining the material before cutting the micropillar will develop an internal microstructure of dislocation topologies (at least in FCC materials) [START_REF] Csikor | Dislocation avalanches, strain bursts, and the problem of plastic forming at the micrometer scale[END_REF][START_REF] Papanikolaou | The Λ-invariant and topological pathways to influence sub-micron strength and crystal plasticity[END_REF][START_REF] Papanikolaou | Avalanches and plastic flow in crystal plasticity: an overview[END_REF], with a characteristic internal scale. This external/internal ratio controls wildness in dislocation-mediated plasticity [START_REF] Zhang | Taming intermittent plasticity at small scales[END_REF].

In HCP metals like Zinc or Cadmium, plasticity occurs through a combination of dislocation motion and twinning events, with the latter dominating deformation at large strains. It was shown from a statistical analysis of AE data that the two mechanisms interact within an overall wild regime, meaning that dislocation avalanches can trigger twinning events, as well as the inverse effect of twins triggering dislocations [START_REF] Richeton | On the critical character of plasticity in metallic single crystals[END_REF]. However, work has been carried out rarely for twin-mediated deformation in systems where twin boundaries are tailored. Known as domain boundary engineering, the manipulation of twin patterns is an effective way for achieving unique properties in materials [START_REF] Salje | Domain boundary engineering[END_REF]. The fundamental idea is that domain boundaries possess electronic transport [START_REF] Riek | Direct sampling of electric-field vacuum fluctuations[END_REF], polarization [START_REF] Salje | Polar twin boundaries and nonconventional ferroelectric switching[END_REF][START_REF] Lu | Piezoelectricity and electrostriction in ferroelastic materials with polar twin boundaries and domain junctions[END_REF][START_REF] Salje | Flexoelectricity and the polarity of complex ferroelastic twin patterns[END_REF], heat conduction [START_REF] Li | Strain-controlled thermal conductivity in ferroic twinned films[END_REF] and mechanical properties [START_REF] Chen | Grain size dependence of tensile properties in ultrafine-grained Cu with nanoscale twins[END_REF][START_REF] Li | Superelasticity in bcc nanowires by a reversible twinning mechanism[END_REF][START_REF] Yang | Interface Driven Pseudo-Elasticity in a-Fe Nanowires[END_REF][START_REF] Lu | Nano-sized twins induce high rate sensitivity of flow stress in pure copper[END_REF][START_REF] Otsuka | A new type of pseudoelasticity in single variant twinned Martensites[END_REF][START_REF] Lu | Stabilizing nanostructures in metals using grain and twin boundary architectures[END_REF][START_REF] Lu | Ultrahigh strength and high electrical conductivity in copper[END_REF][START_REF] He | The interaction between vacancies and twin walls, junctions, and kinks, and their mechanical properties in ferroelastic materials[END_REF] that do not exist in the bulk. For example, the domain walls are observed to be polar while the bulk counterpart is non-polar in CaTiO3 [START_REF] Van Aert | Direct observation of ferrielectricity at ferroelastic domain boundaries in CaTiO3 by electron microscopy[END_REF]. The domain boundaries become superconducting in insulating WO3-x [START_REF] Aird | Sheet superconductivity in twin walls: experimental evidence of[END_REF]. Twin boundaries (TBs) are found to greatly improve the strength and toughness in nano-twinned materials [START_REF] Lu | Nano-sized twins induce high rate sensitivity of flow stress in pure copper[END_REF][START_REF] Lu | Stabilizing nanostructures in metals using grain and twin boundary architectures[END_REF][START_REF] Lu | Ultrahigh strength and high electrical conductivity in copper[END_REF]. In addition, some functional properties can be achieved with the aid of TB motion, such as the recovery of large deformation [START_REF] Li | Superelasticity in bcc nanowires by a reversible twinning mechanism[END_REF], or the storage of mechanical energy [START_REF] Li | High-efficiency mechanical energy storage and retrieval using interfaces in nanowires[END_REF]. These unique properties are strongly dependent on the complexity of twin patterns, however. Sometimes, the formation of a highly complex twin pattern could even degrade the functional properties [START_REF] Li | Large recovery of six-fold twinned nanowires of α-Fe[END_REF].

Many studies have been carried out to determine the movement of domain walls under a uniform stress field, such as uniaxial tension or compression [START_REF] Sainath | Molecular dynamics simulation studies on the size dependent tensile deformation and fracture behaviour of body centred cubic iron nanowires[END_REF][START_REF] Sainath | Deformation behaviour of body centered cubic iron nanopillars containing coherent twin boundaries[END_REF]. However, little is known how twin boundaries evolve under non-uniform applied stress. Early work has shown that bending deformation could generate nonconventional {110}-type interfaces in α-Fe nanowires [START_REF] Yang | Interface Driven Pseudo-Elasticity in a-Fe Nanowires[END_REF]. Such interfaces provide the primary driving force for shape recovery upon unloading, leading to superelasticity. Besides bending, torsion is another typical deformation mode that generates shear stresses and strain gradients in devices such as nano-springs [START_REF] Da Fonseca | Mechanical properties of nanosprings[END_REF], torsional oscillators [START_REF] Evoy | Nanofabrication and electrostatic operation of single-crystal silicon paddle oscillators[END_REF], torsional actuators [START_REF] Hsieh | A novel microelectrostatic torsional actuator[END_REF]. As an example, six-fold twinned α-Fe nanowires were found to sustain very large twists under torsion [START_REF] Li | Large recovery of six-fold twinned nanowires of α-Fe[END_REF]. Yet, the structural evolution, especially the role that TBs play under twisting, is still not well understood.

In this paper, using molecular dynamics simulations, we study the mechanical performance of twinned -Fe nanowires during torsion and perform a statistical analysis of the evolution of twin walls and deformation jerks during deformation. We found that torsion promotes the deformation of TBs by kink nucleation and motion. The recoverability is directly related to the complexity of the twin patterns. For small-applied twist angles, the nanowires recover completely under unloading and only isolated kinks are generated. However, such recoverability is largely suppressed when a high density of junctions is formed through multikink intersections. Our statistical analysis indicates that the deformation mode changes from a mild regime at small twist angles to a wild regime at higher angles. In addition, the associated wildness is controlled by both an internal length scale (twin boundary spacing) and the external length scale (wire diameter, while keeping the twin boundary spacing constant), much like in dislocation-mediated plasticity [START_REF] Zhang | Taming intermittent plasticity at small scales[END_REF][START_REF] Zhang | Plate-like precipitate effects on plasticity of Al-Cu alloys at micrometer to sub-micrometer scales[END_REF]. This suggests material design strategies to limit these detrimental wild fluctuations and the generation of complex patterns.

Simulation method

The embedded atom method [START_REF] Daw | Embedded-atom method: Derivation and application to impurities, surfaces, and other defects in metals[END_REF] (EAM) developed by Mendelev [39] is used to describe the atomic interactions of -Fe. This potential gives a good fit to the basic properties of α-Fe, such as the elastic constants, interstitial and vacancy formation energy, six-fold core structure of screw dislocation, etc. We constructed [112]-oriented twinned nanowires with preexisting {112}/<111> TBs, as shown in Fig. 1a. All nanowires have circular cross sections and maintain the same aspect ratio. The diameter d varies from 5 nm to 15 nm, with the total length varying from 22.5 nm to 67.5 nm. The twin spacing s ranges from 3.8 nm to 15.0 nm. Before torsion, the nanowires were relaxed at 1 K for 0.5 nanoseconds using a Nosé-Hoover thermostat [START_REF] Nose | A Unified Formulation of the Constant Temperature Molecular-Dynamics Methods[END_REF][START_REF] Hoover | Canonical Dynamics -Equilibrium Phase-Space Distributions[END_REF]. Several atomic layers at both ends of the nanowire are fixed rigidly as the loading grip during torsion. We then rotate one rigid end along the axis of the nanowire while the other end is fixed (see the inset in Fig. 1a). The maximum shear strain γmax is calculated as half of the torsion angle θ divided by the aspect ratio of the sample, γmax = θd/(2L). The maximum shear stress is τmax = Td/(2I), where T is the torque applied and I is the polar moment of inertia. The twist was applied with a rate of 0.05° per picosecond, corresponding to a shear strain-rate around 1×10 7 s -1 . Unloading was performed in a similar way by rotating the grip in the opposite direction. The MD calculations were carried out in a canonical ensemble by using LAMMPS code [START_REF] Plimpton | Fast Parallel Algorithms for Short-Range Molecular-Dynamics[END_REF] and the atomic configurations were displayed by AtomEye [START_REF] Li | AtomEye: an efficient atomistic configuration viewer[END_REF].

Results and discussion

Torsion deformation in twinned nanowires

Figure 1a shows the variation of the applied torque as a function of the torsion angle  during loading/unloading of a twinned nanowire with a diameter of 10 nm and twin spacing of 6.6 nm.

Yielding occurs at a small torsion angle ( = 17 o , γmax = 0.033). The torsion stiffness of the twinned nanowire is 6.4×10 5 nN•nm 3 , almost the same as that of pristine nanowires (Fig. A1). TBs have hence little effect on the shear modulus of the nanowire, a result consistent with a previous study [START_REF] Klöffel | Impact of twin boundaries on bulk elastic constants: Density-functional theory data for Young׳s modulus of Ag[END_REF]. As shown in Fig. 1a, after plastic yield, the nanowire first undergoes a strain hardening as the torque continuously increases with an increasing applied twist angle (17 o  < 57.5°, 0.033 < γmax < 0.112, stage A), while the torque saturates for higher twist angles (57.5 o  < 100°, 0.112 < γmax < 0.194, stage B). Strong fluctuations occur during this stage B. After unloading at the end of the strain-hardening stage A, the twinned nanowire fully recovers. On the contrary, deformation is only partially recovered unloading after entering stage B. As shown below, strain hardening is associated with strain recoverability, mild fluctuations, and homogeneous deformation patterns while the saturation stage is associated with non-recoverable deformation, wild fluctuations, and complex domain patterns. Figures 1b and1c show the atomic configurations at two typical states after torsion of the twinned nanowires (43 o , γmax = 0.084 and 100 o , γmax = 0.194). We notice that the preexisting TBs undergo most of the deformation, while almost no dislocations are generated in the bulk between the TBs. Under weak applied twist (43 o ), the preexisting TBs form stripe patterns, while more complex patterns emerge when the twist angle is large (100 o ). The latter regime corresponds to stage B. Hence, the complexity of the microstructure is associated with different recoverability upon unloading: when unloading a twinned nanowire before the formation of complex patterns, the sample almost entirely recovers back to the original state (blue lines in Fig. 1a). Once complex patterns are formed, the shape recovery is largely suppressed and permanent deformation remains after unloading (red lines in Fig. 1a).

We further checked the evolution of the microstructure inside the twin walls under twisting. Figure 2 shows typical images of the structure of the interfaces at different stages of deformation. The left panel shows schematic illustrations of key features of the twin wall structure, the right panel shows the corresponding atomic images. Starting from an undeformed state with preexisting {112} twin planes (Fig. 2a), we find that the initial twin planes transform to {110} interfaces via the motion of a/6[111 ] partial dislocations under small torsion (Fig. 2b). Specifically, partial dislocations nucleate at the surface and glide in the (112) plane, resulting in a lattice misfit in one (101) atomic layer. The sequential partial dislocations follow a similar path, i.e. they nucleate and propagate in the next (101) layer. A nonconventional {110} interface is gradually formed by piling up a/6<111> partial dislocations on sequential (112) planes. Such {110}-type interfaces were reported in nanowires sustaining bending deformation [START_REF] Yang | Interface Driven Pseudo-Elasticity in a-Fe Nanowires[END_REF]. A closer inspection reveals that the partial dislocations contain kinks. The kinks undergo some rotation under twisting. Considering the twinning partial as a primary "kink", the step in the partial dislocation line appears as a secondary kink and this twisted nanowire forms a nonconventional structure with walls inside walls and kinks inside kinks (Fig. 2b). This domain wall structure has been previously reported in ferroelectric domain walls [START_REF] Salje | Domains within domains and walls within walls: evidence for polar domains in cryogenic SrTiO 3[END_REF][START_REF] Faran | The exploration of the effect of microstructure on crackling noise systems[END_REF] but has never been reported in metallic nanowires. At larger rotation angles, in deformation stage B, the TBs deform by the collective motion of partial dislocations in different orientations, generating wild fluctuations and forming complex twin patterns (Fig. 2c). This movement generates intersections and junctions that are indicated by yellow arrows in Fig. 2c.

Statistical analysis of the deformation process

To characterize the deformation dynamics of TBs under torsion, we define the excess energy of the structural change (such as TB movements). The evolution of torque during deformation in the twinned sample is strongly 'jerky', which means that the stored strain energy is relaxed through discontinuous steps, in particular at large torsion angles. We quantify a 'jerk' in the following way: the potential energy Ep evolves discontinuously with increasing and decreasing torque. We square the first time derivative and define a jerk strength as J ~ (dEp/dt) 2 , where dt is the timestep (5 fs in our statistical analysis). This quantity is known as a slew rate in electrical engineering, plasma physics etc. [START_REF] Liu | Plasma excitation dependence on voltage slew rates in 10-200 Torr argonnitrogen gas mixture DBD[END_REF]. Next, we carried out a statistical analysis of these energy fluctuations and correlate the strength distribution profile with the microstructural changes at each stage of deformation.

Considering the situation depicted in Fig. 1, we first analyze deformation stage A at small torsion angles (17° <  < 57.5°) (Fig. 3a). The evolution of the torsion deformation is not smooth because TB movements are jerky. The probability density function (PDF) of jerks is peaked around a characteristic value of 9.1×10 -8 eV 2 •fs -2 (Fig. 3b). Within this deformation stage, jerks are related to nucleation and propagation of kinks. The peaked PDF signifies that kink nucleation and motion is associated with a characteristic energy scale, which is related to the motion of secondary kinks (Fig. 2b). The associated dynamics is mild [START_REF] Weiss | From mild to wild fluctuations in crystal plasticity[END_REF]. At large torsion angles ( > 57.5°), the twinned nanowire sustains large deformations through the motion of kinks and TBs. We choose the interval 57.5° <  < 100° to determine the jerk spectra (Fig. 3c). Large torsion angles generate kinks in this regime. These kinks propagate along different directions, forming complex patterns including kink intersections and junctions. Consequently, TBs are highly deformed and hardly recognizable in Fig. 1c. Figure 3d shows that the PDF of jerks strength exhibits a power-law distribution, 𝑃 𝐽 ~𝐽 with an exponent εJ = 1.50 ± 0.05. This implies a wild regime of deformation [START_REF] Weiss | From mild to wild fluctuations in crystal plasticity[END_REF]. Various mechanisms can be associated with these wild fluctuations. By tracking the microstructural changes, we observed that many jerks are generated by depinning of jammed/entangled configurations within the TBs, while few large jerks are associated with the initiation of screw dislocations in the bulk (Fig. A2). In summary, stage A is characterized by the prevalence of a characteristic jerk strength, while stage B is associated with scale-free jerk statistics. One may therefore wonder if the transition from stage A to B could be signaled by a growing maximum jerk strength 𝐽 possibly diverging at a critical point. We checked this possibility as follows.

For a power-law distribution with a cutoff, the distribution of probability of the jerks strength would be 𝑃 𝐽 𝐽 𝑓 𝐽/𝐽 , where 𝜀 is the exponent, 𝑓 𝑥 is a scaling function rapidly decaying for x>1, 𝐽 is the cutoff scale. We can use a dimensionless ratio 〈𝐽 〉/〈𝐽〉 to track the possible changes in the distribution 𝑃 𝐽 . The heavier is the tail of the distribution, the larger will be this ratio.

It can be shown that this ratio scales as 𝐽 , hence we can monitor a possible evolution of the maximum event size (~𝐽 ) using 〈𝐽 〉/〈𝐽〉 during the whole torsion deformation. To obtain a statistically stable result, we calculate this ratio over rotation bins 2 o (each bin contains 8000 data points). Figure 4 shows the variation of 〈𝐽 〉/〈𝐽〉 as a function of applied torsion angle . We found that this ratio remains low, signing mild distributions (Fig. 3b) for   , without any detectable long-term increase as approaching the transition to stage B, while this ratio strongly increases after that. The rough boundary separating two regimes at   (the blue dotted line) is some kind of "delayed" (or "overdue") in comparison to that of   determined above (the red dotted line). Thus, there is a crossover (      ) for regimes separated by statistical laws and maximum event size. This analysis indicates that thiscrossover from stage A to stage B is not a critical transition, where the maximum event size would increase as approaching the transition and diverge at this point. We found kinks with different orientations starting to nucleate and moving to the free surface in this crossover regime, where the generated jerky energy signals are still very weak. Before the crossover, only parallel kinks nucleate and propagate with small signals. The strong kinks pinning-depinning events occurs statistically with big signals after that. windows of 8000 data points (corresponding to torsion angle of 2°) in a twinned nanowire (d=10 nm; L = 44.8 nm; s = 6.6 nm). J is the jerks strength and 〈J〉 is the jerks strength averaged over the time interval.

We calculated the degree of wildness, which is defined as 𝑊 ∑ ∑ [START_REF] Weiss | From mild to wild fluctuations in crystal plasticity[END_REF][START_REF] Zhang | Taming intermittent plasticity at small scales[END_REF]. Jt is the jerk strength threshold. We used an open-source package to obtain the value of threshold Jt [START_REF] Alstott | powerlaw: a Python package for analysis of heavy-tailed distributions[END_REF]. Specifically, the optimal threshold is determined by making a power-law fit of the complementary cumulative distribution function (CCDF) from each value in the dataset, and selecting the one that has the minimum Kolmogorov-Smirnov between the data and the fit. Jt is determined to be 9×10 -8 eV 2 ⋅fs -2 in plasticity (see more details in Fig. A3). We checked that imposing a small change of Jt by a factor of ±10% does not alter the exponents and degree of wildness significantly (see Fig. A4). For the Gaussian distribution, Jt is set to filter the long tail. We obtained a value of W = 0.06 for stage A of deformation, and of W = 0.87 for stage B at larger twist angles. This shows that mild and wild fluctuations can coexist during the torsion of twinned nanowires, much like during the dislocation-mediated plasticity of metallic materials, and confirms the transition from an essentially mild regime to an essentially wild regime when the applied torsion angle is increased.

We further explored the dynamics in the time domain. Figures 5a and5b show the PDF of waiting times between detected jerks in the mild and wild regimes, respectively. R, the ratio between the mean value and the standard deviation of waiting times, is used to quantify the intermittency of the process. The value of R is close to 1.0 in the mild regime. Along with an exponential PDF (Fig. 5a), this implies an uncorrelated Poisson process without significant intermittency. Instead, at larger twist angles, the wild regime is associated with R = 0.82 < 1.0 and a power law distribution of waiting times tW (Fig. 5b), P ∆t W ~∆t W -δ W with  W=1.68 ± 0.08, i.e., as expected, with intermittency and time correlations between events. The statistical results above were obtained by using the signals of jerks strength. We further carried out the statistical analysis on the signals of "true" avalanche energies, which were calculated as the integration of -dEp/dt over the duration. Figure 6a shows the complementary cumulative distribution functions (CCDF) of avalanche energies in the wild regime. It follows a good power-law distribution. The exponent is 1.66, larger than that of 1.50 obtained using jerk strength signals in Fig. 3d. The CCDF of waiting times tW and duration times tD also show the power-law distributions, as shown in Fig. 6b and6c. The exponents W (= 1.91) for waiting times and D (= 1.98) for duration times are very close, which keeps agreement with previous study [START_REF] Janićević | Interevent correlations from avalanches hiding below the detection threshold[END_REF].

Size effects

The deformation dynamics of twisted nanowires with d = 10 nm, s = 6.6 nm is given in section 3.2. We now extend this analysis to other configurations in order to investigate the effects of the sample size (wire diameter d) and of an internal length scale (the TB spacing s) on the statistical results. We first analyze the influence of the wire size on the torsion deformation with d = 5 nm, 10 nm and 15 nm while keeping the aspect ratio length/diameter at 4.5 (Fig. 7a). The twin boundary spacing is fixed at 6.6 nm. The total number N of TBs within the nanowire increases linearly with the diameter N ~ d. Corresponding stress-strain curves are shown in Fig. 7b. Two types of size effects are readily apparent: (i) a "smaller is stronger" size effect on the maximum shear stress sustained during deformation, and (ii) a "smaller is wilder" size effect of the stress fluctuations after yield. The corresponding jerk spectra are shown in Fig. A5. When increasing the wire diameter, the onset of the wild regime (equivalent to stage B in Fig. 1a) is postponed to a larger applied shear strain. Consequently, when calculated over the entire deformation history up to a rotation of 100° corresponding to a maximum shear strain of max  0.19, the degree of wildness W decreases with increasing sample size (blue line in Fig. 7c), quantifying the "smaller is wilder" size effect.

Hence, in terms of external size effects, twinned -Fe nanowires deformed under torsion mimic the dislocation-mediated plasticity of metallic nanopillars which exhibit as well a "smaller is stronger" effect [START_REF] Uchic | Sample dimensions influence strength and crystal plasticity[END_REF] as well as a "smaller is wilder" behavior [START_REF] Zhang | Taming intermittent plasticity at small scales[END_REF]. However, as noted above, the configuration shown on Fig. 7a implies that N ~ d. Considering that nearly all the inelastic deformation occurs within the TBs [START_REF] Wang | In situ atomic-scale observation of twinningdominated deformation in nanoscale body-centred cubic tungsten[END_REF], a larger diameter implies that the total strain will be shared among a larger number of TBs. Hence, for a fixed total rotation of 100°, each TB will deform on average by an angle 100°/N (neglecting here possible boundary effects inducing less deformed TBs close to the wire ends). As the wildness increases with increasing deformation, when entering stage B (Fig. 1), this implies a decreasing W under increasing N and increasing d. Fig. 7d shows the wildness calculated over the deformation history, up to a fixed rotation angle of 12.5° per TB The overall size effect on wildness is in this case very weak.

We then carried out another set of simulations by varying the internal TB spacing s, ranging from 3.8 nm to 15 nm, while keeping the wire size at d = 10 nm (Fig. 8a). s is inversely proportional to the TB density, s ~1/N. An increase of number of TBs, i.e. a smaller s-value, implies a smaller maximum shear stress sustained during deformation (see Fig. 8b). The onset of the wild regime (equivalent to stage B in Fig. 1a) is postponed to larger applied shear strains (see the corresponding jerk spectra in Fig. A6). The postponement corresponds to the reduction of the twin spacing for higher TB densities. The plastic strain for a given applied rotation is carried by individual TBs and decreases with the increase of the TB density. Figure 9 shows the typical twin pattern with different s at a fixed twist angle (θ = 75°). The twin patterns now become more complex with increasing s. Simultaneously, the samples with larger s show a wilder plastic deformation. When then calculated the wildness over the entire torsion history up to θ = 100°. The wildness W decreases when decreasing the TB spacing s (Fig. 8c). However, if one calculates the wildness up to a fixed rotation angle of 10.4° per TB, the trend is the opposite: the wildness is largest for the smallest TB spacing (3.8 nm), then rapidly decreases to saturate at W  0.2 for larger spacings (Fig. 8d).

Tuning wildness by internal and external length scales

The wildness of torsional deformation of the twinned nanowires is closely related to the complexity of the twin patterns, which can be characterized by the junction density ρ. Here ρ is calculated as the number of junctions per unit volume over the entire nanowire. The number of junctions is counted for the same torsion angle (θ = 100°) for all samples. The circles in Fig. 10a show the variation of W as a function of ρ by collecting data from samples with different diameters and twin boundary spacings at a fixed twist angle. In addition, we collected the data points for a fixed twist angle per TB in Fig. 7d (triangle symbols in Fig. 10) and in Fig. 8d (square symbols in Fig. 10). A clear correlation between ρ and W is observed, namely an increase of the junction density implies wilder deformations. A similar relationship is obtained when W is shown as function of the junction density per TB (Fig. 10b). This suggests that wild plastic fluctuations within twinned nanowires are associated with unlocking and rearrangements of complex patterns previously pinned by junctions between kinks (see Fig. 2c). Such events involve the collective motion of multiple kinks along different directions, hence are much more energetic event than the individual motion of a/6<111> partial dislocations along a single direction, which characterizes the mild regime (Fig. 2b). An interesting contrast with dislocation-mediated plasticity within untwinned crystals occurs where dislocation junctions in multislip systems (such as FCC metals) are at the origin of dislocation entanglements, the formation of dislocation patterns, and consequently of isotropic strain hardening. Although possibly prone to episodic rearrangements, these strongly locked configurations are considered to frustrate the spreading of dislocation avalanches, hence to reduce wildness [START_REF] Zhang | Taming intermittent plasticity at small scales[END_REF]. An opposite role is played by kink junctions in the present case (Fig. 10). During the pattern evolution, the individual twin walls (the big flat ones) first form kinks in the wall and do not interact. The nucleation and possible movement of these kinks gives some small jerks, corresponding to a mild plastic regime. Further torsion results in the generation of many kinks forming the more or less regular stripe patterns, still associated with a mild regime. Wild jerks arise when these kinks start colliding or interacting dynamically. As the torsion becomes even more severe, kinks, kinks inside kinks, and junctions are forming some very disordered kink configurations. The main interaction stems from the high energy of the junctions. The reorganization of such disordered configurations occurs through sudden events leading to wild plastic behavior. However, if the junctions were even denser, one might even switch back to a milder regime, reminiscent of dislocation tangles, which is not attained in our simulations and would need to be further explored. In this context, the junction density  appears as an order parameter of the wild-to-mild transition. We finally note the absence of strain hardening in the wild regime of nanowires deformation (Figs 1a and7b), which is fully consistent with the proposed scenario. The absence of strain hardening is also associated with wildness [START_REF] Weiss | Ice: the paradigm of wild plasticity[END_REF] in dislocation-mediated plasticity. The wire diameter and twin boundary density affect the junction density. As shown in Fig. 11, the junction density decreases when increasing the ratio d/s. Thus, we may tune the wildness either by controlling the external length scale (d), the internal length scale (s), or, more specifically, the ratio between both. This can be understood from the number N of TBs within the nanowire: increasing d or decreasing s increases the number of TBs that share the total imposed deformation. They reduce the strain accommodated by each TB, hence postpone the onset of stage B and reduce the wildness (Fig. 7c and8c). On the contrary, by taking into account this sharing effect per TB by calculating W up to a fixed rotation angle, no significant sample size effect is observed over the range explored in Fig. 7d. The wildness only grows when the TB spacings fall below 5 nm. This might indicate the onset of significant mechanical interactions between neighboring TBs. For close enough TBs, a kink deformation nucleating within one TB can nucleate other kinks in other TBs through elastic stress redistributions within the matrix, generating avalanches. Owing to the fast decay of these elastic interactions with distance [START_REF] Everhardt | Periodicity-doubling cascades: Direct observation in ferroelastic materials[END_REF][START_REF] Lu | Electrically driven ferroelastic domain walls, domain wall interactions, and moving needle domains[END_REF], such mechanism is expected to vanish for TB spacings above few nanometers (Fig. 8d). Dedicate simulations would be needed to confirm and quantify this point which is left for future work.

Conclusion

(a) Atomic simulations reveal the mechanism of torsion of pre-twinned -Fe nanowires. At small twist angles in deformation stage A, the preexisting {112} twin planes progressively transform into {110} interfaces through the motion of a/6<111> partial dislocations on sequential (112) planes. At larger twist angles at stage B, more complex twin patterns are formed with a high density of junctions when multiple kinks intersect.

(b) The deformed nanowires exhibit a good recoverability upon unloading from a simple twin pattern in stage A. The deformation can only partially recover once complex twin patterns are formed in stage B.

(c) The dynamics of the deformation process shows characteristic statistical properties, depending on the deformation stage. The PDF of jerks strength is approximately Gaussian, associated with a characteristic strength scale, and non-intermittent at small torsion angles. This characterizes mild dynamics as previously observed experimentally by Faran et al [START_REF] Salje | Domains within domains and walls within walls: evidence for polar domains in cryogenic SrTiO 3[END_REF][START_REF] Faran | The exploration of the effect of microstructure on crackling noise systems[END_REF]. For large torsion angles, kinks move along different directions, they form junctions and jammed configurations. The reorganization of such jammed configurations under increasing torsion

Fig. 1 .

 1 Fig. 1. Mechanical behavior of twinned nanowires under torsion in -Fe, for a wire diameter d of 10 nm, a wire length L of 44.8 nm, and a TB spacing s of 6.6 nm. (a) Variation of torque (and τ max ) with the applied torsion angle (and γ max ) for [112]-oriented twinned nanowires. Two stages of plasticity are distinguished according to the recoverability. Atomic configurations of TB at (b)  = 43 o and (c)  = 100.0 o . The colors are coded by the centrosymmetry parameter P [45]. P has the values of 2.0~3.0 for atoms inside {112} twin walls (dark blue), 3.0~4.5 for those around the a/6<111> partials (light blue), 4.5~6.0 for those at the non-conventional {110} interfaces (green), and larger than 6.0 for atoms in heavily deformed regions (red).

Fig. 2 .

 2 Fig. 2. (a) Initial configuration with conventional {112} twin walls. (b) The {112} twin walls transform to {110} interfaces with parallel kinks motion. (c) Complex domain structures in highly twisted nanowires in the wild deformation regime. Kinks nucleate along various directions and form junctions (indicated by yellow arrows) in a highly deformed {110} interfaces. The colors of atomic images are coded by the centrosymmetry parameter P [45]. P has the values of 2.0~3.0 for atoms inside {112} twin walls (dark blue), 3.0~4.5 for those around the a/6<111> partials (light blue), 4.5~6.0 for those at the non-conventional {110} interfaces (green), and larger than 6.0 for atoms in heavily deformed regions (red).

Fig. 3 .

 3 Fig. 3. Statistical analysis of jerks strength for twinned nanowires deformed under torsion (d=10 nm; L = 44.8 nm; s = 6.6 nm). (a) Variation of the potential energy (red lines) with the torsion angle and the corresponding jerk spectrum (black lines) at stage A (17° <  < 57.5°). (b) The corresponding PDF of jerks strength. The red line is a fitted Gaussian function, where the maximum value occurs at 9.06×10 - 8 eV 2 •fs -2 , and the standard deviation is 6.43×10 -8 eV 2 •fs -2 . (c) Variation of potential energy (red line) and the corresponding jerk spectrum (black line) in stage B (57.5° <  < 100°). (d) Log-log plot of the PDF of jerks strength showing a power-law distribution characterizing a wild regime.

Fig. 4 .

 4 Fig. 4. The variation of ratio 〈𝐽 〉/〈𝐽〉 as function of the torsion angle during torsion over successive

Fig. 5 .

 5 Fig. 5. Statistical analysis of waiting times between successive jerks for twinned nanowires deformed under torsion (d = 10 nm; L = 44.8 nm; s = 6.6 nm). The corresponding PDF shows (a) an exponential distribution for stage A (17° <  < 57.5°) and (b) a power-law distribution for stage B (57.5° <  < 100°).

Fig. 6 .

 6 Fig. 6. Statistical analysis of (a) avalanche energies, (b) waiting times and (c) duration times for twinned nanowires in the wild regime (d = 10 nm; L = 44.8 nm; s = 6.6 nm).

Fig. 7 .

 7 Fig. 7. The effect of external size (diameter d) on the torsion deformation of twinned nanowires. (a) Schematic illustration of the twinned structure by varying the wire diameter and keeping the aspect ratio fixed (b) Maximum shear stress  max vs maximum shear strain  max curves. Dependence of the degree of wildness W on the wire diameter d (c) at a fixed twist angle  100° and (d) at a fixed twist angle 12.5 o per TB. W is calculated in the regime from the yielding point to the given twist angle.

Fig. 8 .

 8 Fig. 8. The effect of internal size (TB spacing s) on the torsion deformation of twinned nanowires. (a) Schematic illustration of the twinned structure by varying the TB spacing and keeping the wire diameter fixed (d = 10 nm) (b) Maximum shear stress  max vs maximum shear strain max curves. Dependence of the degree of wildness W on the TB spacing s (c) at a fixed twist angle  100° and (d) at a fixed twist angle of 10.4° per TB. W is calculated in the regime from the yielding point to the given twist angle.

Fig. 9 .

 9 Fig. 9. Typical TB morphology at a given torsion angle (θ = 75°, γ max = 0.145) for twinned α-Fe nanowires with a twin spacing s of (a) 3.8 nm, (b) 6.6 nm, (c) 9.4 nm and (d) 15.0 nm. All samples have the same diameter ~10 nm and length ~44.8 nm. The colors of atomic images are coded by the centrosymmetry parameter P [45]. P has the values of 2.0~3.0 for atoms inside {112}10 twin walls (dark blue), 3.0~4.5 for those around the a/6<111> partials (light blue), 4.5~6.0 for those at the nonconventional {110} interfaces (green), and larger than 6.0 for atoms in heavily deformed regions (red).

Fig. 10 .

 10 Fig. 10. The relationship between the degree of wildness and the junction density. The variation of W as a function of (a) junction density ρ and (b) junction density per TB. The circles indicate samples with different diameters and TB spacings at a fixed twist angle (θ = 100°). The triangles show samples with the same TB spacing but different diameters and a constant twist angle of 12.5 o per TB (Fig.7d). The squares are samples with the same diameter but different TB spacing at a fixed twist angle of 10.4° per TB (Fig.8d). The red dashed lines are guides to the eyes.

Fig. 11 .

 11 Fig. 11. The variation of the junction density as function of d/s, for all samples (all diameters, all TB spacings) at a constant twist angle 100°. The black dashed lines are guides for the eyes.

Acknowledgement

X.D. and J.S. appreciate the support of NSFC (51931004, 51621063) and 111 project (BP2018008). E.K.H.S. is grateful to EPSRC (EP/K009702/1) and the Leverhulme Foundation (RPG-2012-564) for support. S.L. appreciates the support from the NKRDPC (Grant No. 2019YFA0307900).


André L A Marinho

Raimundo C Rabelo-Neto

Florence Epron

Nicolas Bion

Fabio S Toniolo

Fabio Bellot Noronha

Embedded Ni nanoparticles in CeZrO2 as stable catalyst for dry reforming of methane

Keywords: Ni-embedded, Ceria, Ceria-Zirconia, Oxygen Isotopic Exchange, Methane Dry Reforming, Biogas

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1-Introduction

Biogas obtained by anaerobic digestion of biomass is a source of CH4 and CO2 and it can be regarded as an alternative renewable source of methane. Nowadays, the biogas produced from biomass waste in the farms is mostly used to produce thermal or electrical energy.

However, biogas can be used for the production of renewable energy and added-value chemicals [START_REF] Yentekakis | Biogas Management: Advanced Utilization for Production of Renewable Energy and Added-value Chemicals[END_REF]. Hydrogen produced via biogas reforming might be used for energy generation on low-temperature fuel cells or it can be directly fed to high-temperature fuel cells [START_REF] Gao | A review of recent developments in hydrogen production via biogas dry reforming[END_REF][START_REF] Gür | Comprehensive review of methane conversion in solid oxide fuel cells: Prospects for efficient electricity generation from natural gas[END_REF][START_REF] Da Silva | A techno-economic evaluation of the energy generation by proton exchange membrane fuel cell using biogas reforming[END_REF][START_REF] Yang | Progress and perspectives in converting biogas to transportation fuels[END_REF].

Steam reforming of methane is the main industrial process to produce H2. However, the dry reforming of methane (DRM) (Eq. 1.1), using directly the biogas as feedstock, could be an alternative for obtaining hydrogen [START_REF] Stagg-Williams | CO2 reforming of CH4 over Pt/ZrO2 catalysts promoted with La and Ce oxides[END_REF]. The main limitation of DRM technology is catalyst deactivation due to carbon deposition, which occurs under reaction conditions. DRM is an endothermic reaction (ΔH° = 247 kJ/mol) and requires high reaction temperature, which favors several side reactions, especially CH4 decomposition (Eq. 1.2), Boudouard reaction (Eq. 1.3) and reverse water-gas shift (RWGS) (Eq. 1.4) [START_REF] Bradford | CO2 reforming of CH4[END_REF].

CH4 + CO2 ↔ 2 CO + 2H2

(1.1)

CH4 ↔ C + 2H2 (1.2) 2 CO ↔ C + CO2 (1.3) CO2 + H2 ↔ CO + H2O (1.4)
The development of catalysts resistant to deactivation by carbon deposition has been the focus of several studies. Supported metal catalysts have been extensively investigated for the DRM [START_REF] Rostrup-Nielsen | CO2 Reforming of Methane over Transition Metals[END_REF][START_REF] Mortensen | Industrial scale experience on steam reforming of CO2-rich gas[END_REF]. Ni-based catalysts have high activity and low cost, but metal sintering may occur during the DRM and thus favor the production of coke on its surface, leading to catalyst deactivation [START_REF] Rabelo-Neto | CO2 reforming of methane over supported LaNiO3 perovskite-type oxides[END_REF].

The high temperature required to perform the DRM favors the metal sintering process, which motivates the development of novel catalysts able to resist to deactivation. Recently, the literature reported a series of catalysts in which the metal phase is embedded into an oxide, improving the catalyst stability by avoiding the metal sintering [START_REF] Cargnello | Synthesis of dispersible Pd@CeO2 core-shell nanostructures by self-assembly[END_REF][START_REF] Majewski | Tri-reforming of methane over Ni@SiO2 catalyst[END_REF][START_REF] Bonifacio | Thermal Stability of Core-Shell Nanoparticles: A Combined in Situ Study by XPS and TEM[END_REF][START_REF] Pu | Ceria-promoted Ni@Al2O3 coreshell catalyst for steam reforming of acetic acid with enhanced activity and coke resistance[END_REF]. Ni@SiO2 [START_REF] Wang | CO2 reforming with methane over small-sized Ni@SiO2 catalysts with unique features of sintering-free and low carbon[END_REF] and Ni@Al2O3 [START_REF] Huang | Synthesis of a Highly Active and Stable Nickel-Embedded Alumina Catalyst for Methane Dry Reforming: On the Confinement Effects of Alumina Shells for Nickel Nanoparticles[END_REF] catalysts exhibited high resistance to metal sintering during the DRM but carbon formation was not totally inhibited. The development of metallic Ni particles embedded into oxides with high OSC could combine two advantages: to limit the metal sintering and prevent or remove carbon deposition.

The use of supports with redox properties is reported to improve catalyst stability for the DRM, due to the enhancement of the rate of carbon gasification on the catalyst surface. The use of CeO2 has been investigated as a support in many reactions, which required high oxygen storage capacity (OSC) [START_REF] Wang | Ceria-based catalysts for the production of H2 through the water-gas-shift reaction: Time-resolved XRD and XAFS studies[END_REF][START_REF] Faria | Hydrogen production through CO2 reforming of methane over Ni/CeZrO2/Al2O3 catalysts[END_REF]. As a support for the DRM, ceria participates in the dissociative adsorption of CO2 and donates an oxygen atom to the metallic particle containing carbon deposits, promoting the carbon removal mechanism [START_REF] Faria | Hydrogen production through CO2 reforming of methane over Ni/CeZrO2/Al2O3 catalysts[END_REF]. Furthermore, doping ceria structure with appropriate cations can modify the lattice parameters, increasing the amount of oxygen exchangeable species. The literature reports ZrO2 as an excellent dopant to increase the redox properties of ceria [START_REF] Bedrane | Investigation of the oxygen storage process on ceriaand ceriazirconia-supported catalysts[END_REF][START_REF] Silva | Effect of Ce/Zr ratio on the performance of Pt/CeZrO2/Al2O3 catalysts for methane partial oxidation[END_REF][START_REF] Yentekakis | Effect of support oxygen storage capacity on the catalytic performance of Rh nanoparticles for CO2 reforming of methane[END_REF][START_REF] Wolfbeisser | Methane dry reforming over ceria-zirconia supported Ni catalysts[END_REF].

The use of ceria to confine metals has been also investigated [START_REF] Cargnello | Synthesis of dispersible Pd@CeO2 core-shell nanostructures by self-assembly[END_REF][START_REF] Saw | Highly Active and Stable Bimetallic Nickel-Copper Core-Ceria Shell Catalyst for High-Temperature Water-Gas Shift Reaction[END_REF], but the synthesis procedures are quite complex and hardly reproducible with high cost. Recently, Wang et al. [START_REF] Wang | Facile synthesis of Cu@CeO₂ and its catalytic behavior for hydrogenation of methyl acetate to ethanol[END_REF] synthetized Cu nanoparticles embedded in CeO2 by a simple sol-gel method. The catalysts were tested for hydrogenation of methyl acetate reaction at 400 °C and no sintering was observed.

However, to the best of our knowledge, there is still no report in the literature about this sol-gel method for the synthesis of Ni@CeO2 catalyst and its application for the DRM.

In this work, we aim to prepare Ni-embedded CeO2 catalysts using a one-step synthesis method and test them for the DRM. The use of ZrO2 as dopant was also evaluated. A Ni/CeO2 catalyst prepared by traditional impregnation method was also tested for comparison. Important structure properties were investigated by Transmission Electron Microscopy, Raman Spectroscopy, in-situ X-ray Diffraction and in-situ X-ray Absorption. The mobility of oxygen in the material was characterized by Oxygen Isotopic Exchange. The carbon formation was analyzed by Thermogravimetric analysis, Raman Spectroscopy, Scanning and Transmission Electron Microscopy.

Experimental

Catalysts preparation

Catalysts were prepared following an adapted method previously described in the literature [START_REF] Wang | Facile synthesis of Cu@CeO₂ and its catalytic behavior for hydrogenation of methyl acetate to ethanol[END_REF]. A solution of cerium nitrate (1.7 mol.L -1 , 25 mL) was prepared, followed by the addition of an appropriate amount of nickel nitrate to obtain 10 wt% Ni in the catalyst. In another beaker, a solution of citric acid (6.7 mol.L -1 , 25 mL) was prepared, using a citric acid/metals molar ratio equal to 1.0. Both solutions were mixed and stirred for 2 h at room temperature. Then, the solution was heated at 70 °C under vacuum to remove water. The remaining material was dried at 100 °C overnight. The calcination occurred in 2 steps: 2 h at 300 °C and 4 h at 400 °C, using a heating rate of 1 °C/min. This catalyst was denominated Ni@CeO2. The zirconia-doped ceria catalyst (Ni@CeZrO2) was prepared using the same method with the addition of zirconyl nitrate at the metal solution. The Ce/Zr molar ratio was 4.0.

For comparison, a Ni/CeO2 catalyst was prepared by incipient wetness impregnation of CeO2 support with an aqueous solution of nickel nitrate. In this case, CeO2 was synthesized by the method previously described but without the addition of nickel.

X-ray fluorescence (XRF)

The chemical composition of the catalysts was determined by an X-ray fluorescence spectrometer RIGAKU RIX-3100 using samples as pellet.

N2 physisorption

The BET surface area of the samples was measured by N2 adsorption using a Micromeritics Tristar 3000 apparatus at -196 °C. The samples were degassed at 300 °C under vacuum before the analysis.

Transmission electron microscopy (TEM)

TEM analyses of reduced and used catalysts were made in a JEOL 2100 UHR apparatus, with 0.19 and 0.14 nm punctual and linear resolution, respectively, equipped with a LaB6 filament. Energy dispersive X-ray (EDX) analysis was also performed to determine the local chemical composition. The range used to collect the X-rays emitted from the sample upon electron impact was 0 -20 keV. Before the analysis, the samples were reduced at 800 °C for 1h, the same temperature used for the reaction.

In-situ X-ray diffraction (XRD)

In-situ X-ray diffraction was performed at the XPD-10 B beamline at the Brazilian Synchroton Light Laboratory (LNLS). The samples were reduced under a 5 % H2/He mixture at a heating rate of 10 °C/min, from 25 to 800 °C, remaining at the final reduction temperature for 1 h. The XRD patterns were obtained at 2θ range from 23° to 56° and the wavelength used was 1.55002 Å. The average crystallite size of NiO, metallic Ni and CeO2 was calculated using the Scherrer equation. The CeO2 lattice parameter was calculated using the Bragg's Law, assuming an orthorhombic structure and the (111) CeO2 plane.

2.6.

In-situ X-ray absorption near edge structure (XANES)

XANES analysis was performed at the DXAS beamline at LNLS to obtain information about the reduction degree of Ni during the reduction procedure. The samples were diluted with boron nitride, used as inert, and pressed to obtain a pellet. The pellet was placed in a tubular reactor and reduced under a 5 % H2/He mixture up to 750 °C, keeping this temperature for 30 min. The treatment of XANES data was carried out using PrestoPronto software [START_REF] Figueroa | PrestoPronto: a code devoted to handling large data sets[END_REF]. The fraction of metallic Ni phase was calculated as a function of reduction temperature by linear combination using NiO and Ni 0 foil XANES spectra as references.

18 O2/ 16 O2 isotopic exchange

The experiments were carried out in a closed recycling system connected to a Pfeiffer Vacuum quadrupole mass spectrometer and a vacuum pump. Firstly, the sample (c.a. 20 mg)

was pre-treated under 16 O2 flow (50 mL/min, 500 °C, 1 h) and evacuated for 1 h. After the pretreatment, the sample was cooled down to reaction temperature.

For The behavior of the oxygen exchange was studied as a function of the temperature (Temperature-Programmed Oxygen Isotopic Exchange -TPOIE). For this experiment, pure 18 O2 (55 mbar) was injected in the reactor at 200 °C and the sample was heated up to 500 °C (2 °C/min).

The following equations were used to calculate the rate of exchange (RE), the number of oxygen exchanged (NE), where Ng is the total number of oxygen atoms in the gas phase, w is the weight of catalyst, αg t is the atomic fraction of 18 O2 in the gas phase at the time t, αg 0 is the initial atomic fraction of 18 O2 in the gas phase.

∝ 𝑔 𝑡 =

Raman spectroscopy

The calcined samples were analyzed in a Raman Horiba Jobin Yvon spectrometer using a He:Ne laser (632 nm). The post-reaction samples were analyzed in a Raman Witec -Alpha 300 spectrometer. The 532 nm wavelength was used to observe the carbon structure on the samples.

Thermogravimetric analysis (TG)

In order to quantify the amount of coke formed on the catalyst, thermogravimetric analyses of the post-reaction samples were carried out in a TA Instruments equipment (SDT Q600). Approximately 10 mg of the spent catalyst was heated under synthetic air flow (100 mL/min) from room temperature to 1000 °C at a heating rate of 20 °C/min and the weight change was measured.

Scanning electron microscopy (SEM)

SEM analyses of the spent catalysts were carried out using a field emission scanning electron microscope (FE-SEM) Quanta FEG 450 FEI operating with an accelerating voltage of 20 kV.

Catalytic evaluation

Dry reforming of methane was performed in a fixed-bed reactor at atmospheric pressure.

Before the reaction, the catalysts were reduced under pure H2 (30 mL/min) at 800 °C for 1 h and purged under N2 at the same temperature for 30 min. A reactant mixture with CH4:CO2 molar ratio equal to 1.0 under flow rate of 100 mL/min was used for the DRM at 800 ºC.

The samples (20 mg) were diluted with SiC (SiC mass/catalyst mass ratio = 1.5) to avoid hot spot or temperature gradients. The reaction products were analyzed by gas chromatography (Agilent 6890) equipped with a thermal conductivity detector and Carboxen 1010 column (Supelco). The CH4 and CO2 conversion was determined as follow:

X CH 4 = (n CH 4 ) feed -(n CH 4 ) exit (n CH 4 ) feed x 100 X CO 2 = (n CO 2 ) feed -(n CO 2 ) exit (n CO 2 ) feed x 100

Results and discussion

3.1.Catalyst characterization

Table 1 shows the Ni content, Ce/Zr molar ratio and the textural properties of the catalysts. The Ni loading is close to the nominal value (10 wt%) for all samples, as well as the Ce/Zr molar ratio of the Ni@CeZrO2 catalyst (i.e., Ce/Zr = 4). The surface area and pore volume for Ni/CeO2 and Ni@CeO2 catalysts were similar but decreased when zirconia was added to ceria. After the reduction process at 800 °C under H2, the BET surface area dropped to less than 10 m 2 /g to Ni/CeO2 and Ni@CeO2 catalysts. This reduction in the BET surface area can be correlated to sintering of CeO2 at high temperature [START_REF] Li | Tuning the catalytic performance of Ni-catalysed dry reforming of methane and carbon deposition via Ni-CeO2-x interaction[END_REF][START_REF] Ay | Dry reforming of methane over CeO2 supported Ni, Co and Ni-Co catalysts[END_REF]. The decrease in BET surface area of Ni@CeZrO2 sample was lower than that for the other samples. According to the literature [START_REF] Hori | Thermal stability of oxygen storage properties in a mixed CeO2-ZrO2 system[END_REF], the addition of Zr into the ceria structure with the formation of a solid solution improves the thermal stability of the ceria support, inhibiting or minimizing sintering and surface area lost at high temperatures.

The TEM images of the reduced Ni/CeO2, Ni@CeO2 and Ni@CeZrO2 catalysts are shown in Fig. 1. The image of Ni/CeO2 catalyst shows large Ni particle segregated on the surface of the sample, with average particle size around 30 nm. The local chemical analysis showed only the presence of Ni due the large size and CeO2 could not be detected in the area of analysis (white circle). For Ni@CeO2 and Ni@CeZrO2 catalysts, the TEM images reveals the presence of small Ni nanoparticles embedded in the ceria matrix formed during reduction.

Segregated Ni particles could not be observed on the TEM images of Ni@CeO2 and Ni@CeZrO2 catalysts. Ni-embedded catalysts presented smaller metal particle size than Ni/CeO2. For Ni@CeO2 and Ni@CeZrO2, the Ni particles sizes were around 13 and 6 nm, respectively. It was possible to detect the CeO2 signal by EDX in the selected area (white circle) due the small Ni particle size and it seems that the nanoparticles embedded in the ceria matrix are protected.

The Raman spectra of the calcined samples are shown in Fig. 2. The spectrum of CeO2 support is also presented for comparison. The support exhibits only one intense band at 462 cm - 1 assigned to the symmetrical stretching mode between the eight oxygen atoms bound to the cerium atom in the triple degenerate F2g mode. The impregnation of ceria with Ni does not affect the spectrum of Ni/CeO2, but Ni@CeO2 and Ni@CeZrO2 samples show a ceria structure completely different from the impregnated one. Besides the intense band around 462 cm -1 , the spectra display bands at 224, 544 and 632 cm -1 , corresponding to second-order transverse acoustic mode (2TA) and defect-induced mode. These samples exhibited two different defects in the structure, which were reported in the literature as defect space including O 2-vacancy (544 cm -1 ) and defect space without O 2-vacancy (632 cm -1 ) [START_REF] Taniguchi | Identifying defects in ceria-based nanocrystals by UV resonance Raman spectroscopy[END_REF][START_REF] Nakajima | Defect-induced Raman spectra in doped CeO2[END_REF]. The NiO also presents bands in the region of 570 cm -1 corresponding to TO and LO vibration modes [START_REF] Mironova-Ulmane | Raman scattering in nanosized nickel oxide NiO[END_REF]. However, this band is not detected or it has a very weak intensity in the spectrum of Ni/CeO2 sample.

Therefore, the bands observed at 544 cm -1 and 632 cm -1 in the spectra of Ni@CeO2 and Ni@CeZrO2 samples might be attributed to oxygen vacancies. Therefore, the one-step synthesis of the catalyst with Ni embedded in the ceria structure can promote the formation of defect in the ceria, creating oxygen vacancies in its structure.

The Raman spectra did not detect the formation of a Ni-CeO2 solid solution, as described by Barrio et al [START_REF] Barrio | Unusual Physical and Chemical Properties of Ni in Ce1-xNixO2-y Oxides: Structural Characterization and Catalytic Activity for the Water Gas Shift Reaction[END_REF]. Ni@CeZrO2 samples occurs at higher temperatures (700 and 530 °C, respectively). The shift of reduction temperature to higher values is likely due to a higher metal-support interaction [START_REF] Barrio | Unusual Physical and Chemical Properties of Ni in Ce1-xNixO2-y Oxides: Structural Characterization and Catalytic Activity for the Water Gas Shift Reaction[END_REF]. Therefore, these results suggest that nickel oxide particles embedded in the CeO2 or CeZrO2 matrix have stronger interaction with the support than nickel oxide particles impregnated on CeO2. Stronger interaction demands higher temperature of reduction to remove oxygen from the oxide structure. NiO crystallites in this catalyst, indicating that sintering did not occur during the calcination.

For Ni@CeZrO2 sample, the lines characteristic of NiO phase are not detected either whereas the lines corresponding to CeO2 are shifted to higher 2θ positions. The partial substitution of Ce 4+ by Zr 4+ ions in the ceria structure shifted the CeO2 lines to higher Bragg angles that corresponds to a decrease in the lattice parameter (Table 2) due to the difference between the ionic radii of the cations Ce 4+ (0.97 Å) and Zr 4+ (0.84 Å) [START_REF] Bonk | Low-Temperature Reducibility of MxCe1-xO2 (M = Zr, Hf) under Hydrogen Atmosphere[END_REF]. According to the literature, this shift indicates the formation of a CeO2-ZrO2 solid solution for the Ni@CeZrO2 catalyst [START_REF] Faria | Hydrogen production through CO2 reforming of methane over Ni/CeZrO2/Al2O3 catalysts[END_REF][START_REF] Kozlov | Effect of preparation method and redox treatment on the reducibility and structure of supported ceria-zirconia mixed oxide[END_REF].

Ni/CeO2 catalyst exhibited a ceria lattice parameter higher than that one reported in the literature (5.4112 Å), while it further decreased for Ni@CeO2 with Ni embedded in CeO2. The CeO2 crystallite size of the Ni/CeO2 sample is 13.4 nm, while a significant decrease is observed for Ni@CeO2 and Ni@CeZrO2 catalysts (4.4 and 4.1 nm, respectively) (Table 2). Wang et al.

[24] synthesized a Cu@CeO2 catalyst using the same method and they observed the same effect on ceria crystallite size during calcination.

X-ray diffraction patterns obtained during reduction under 5 % H2/He mixture from room temperature to 800 °C for Ni/CeO2, Ni@CeO2 and Ni@CeZrO2 samples are shown in whereas the Ni 0 crystallite size grew from 5 nm (300 °C) to 33 nm (800 °C for 1h).

For Ni@CeO2 and Ni@CeZrO2 catalysts, only the typical lines of CeO2 are observed below 500 °C. Above this temperature, the appearance of the lines corresponding to metallic

Ni is noticed at 525 °C and 600 °C on the diffractograms of Ni@CeO2 and Ni@CeZrO2 catalysts, respectively. These results reveal that the reduction of nickel oxide is more difficult for the samples prepared by the sol-gel method, which is likely due to the strong interaction between nickel oxide and CeO2. These results agree with the TEM images and in situ XANES experiments, which showed that the embedded Ni particles in the ceria matrix are more hardly reduced than the segregated particles characteristic of the impregnated catalyst. The strong interaction between Ni and CeO2 has been reported as an approach to improve the dispersion and avoid the metal sintering at high temperature [START_REF] Zheng | Effects of CeO2 addition on Ni/Al2O3 catalysts for the reaction of ammonia decomposition to hydrogen[END_REF]. Kathiraser at al. [START_REF] Kathiraser | Highly active and coke resistant Ni/SiO2 catalysts for oxidative reforming of model biogas: Effect of low ceria loading[END_REF] studied the influence of Ce addition to Ni/SiO2 catalysts for the oxidative reforming of biogas. They observed the increase in NiO reduction temperature with the presence of small amount of Ce (0.5 wt%) in the catalyst. This result was attributed to a higher interaction Ni-CeO2 than Ni-SiO2 as well as to a smaller NiO crystallite size as revealed by EXAFS. The TPR peaks of Ni@SiO2 core-shell structure were shifted to higher temperatures in comparison to Ni/SiO2 indicating that nickel oxide was hardly reduced in the core-shell structure [START_REF] Wang | Syngas production from CO2 reforming with methane over coreshell Ni@SiO2 catalysts[END_REF]. This result was associated with a stronger metal-support interaction.

For Ni@CeO2, the CeO2 crystallite size increased during the reduction from 5 nm (325 °C) to 68 nm (800 °C for 1h), revealing a strong sintering at high temperature even for this sample prepared by the sol-gel method (Fig. 9B). A significant growth of the Ni 0 crystallite size was also observed, increasing from 5 nm (630 °C) to 19 nm (800 °C for 1h).

In the case of Ni@CeZrO2 catalyst, the CeO2 crystallite size only slightly varied from 5 to 8 nm after reduction at 800 °C for 1h. The same trend was observed for the Ni 0 crystallite size that increased from 5 to 10 nm at the end of the reduction (Fig. 10B).

The values of BET surface area of the samples before and after reduction (Table 1)

showed that the addition of Zr into ceria structure significantly improved the thermal stability of the Ni@CeZrO2 catalyst. The effect of type of dopant on the thermal stability and redox properties of ceria has been studied in the literature [START_REF] Silva | Effect of Ce/Zr ratio on the performance of Pt/CeZrO2/Al2O3 catalysts for methane partial oxidation[END_REF][START_REF] Kašpar | Use of CeO2-based oxides in the three-way catalysis[END_REF][START_REF] Da Silva | Effect of the type of ceria dopant on the performance of Ni/CeO2 SOFC anode for ethanol internal reforming[END_REF]. A higher thermal stability of CeZrO2 in comparison to CeO2 was attributed to the formation of a CeZr solid solution [START_REF] Hori | Thermal stability of oxygen storage properties in a mixed CeO2-ZrO2 system[END_REF].

The samples aged at 1000 °C under reduction condition showed CeO2 crystallite size equal to 73 and 39, for Pt/CeO2 and Pt/Ce0.75Zr0.25O2, respectively. In our work, the higher thermal stability of the ceria structure is likely due to the formation of the CeZrO2 solid solution as revealed by XRD.

The increase in the reduction temperature led to a shift of the ceria lines to lower angles.

The variation of CeO2 lattice parameter during reduction of the samples is shown in Fig. 11.

Increasing the reduction temperature increased the lattice parameter. The increase in the lattice parameter could be due to the thermal expansion as well as the change in the oxidation state of ceria from Ce 4+ to Ce 3+ . In order to evaluate the effect of thermal expansion on the ceria lattice parameter during the reduction of all catalysts, an experiment was carried out with a physical mixture containing NiO and CeO2 (Sigma-Aldrich) under air. The linear increase in the lattice parameter of NiO-CeO2 up to 700 °C (see dotted line in Fig. 11) is attributed to the thermal expansion of the samples. However, the curves obtained for Ni/CeO2, Ni@CeO2 and Ni@CeZrO2 catalysts deviates from this linear increase of the lattice parameter above 200 °C, indicating that the reduction of ceria began. Furthermore, a sharp increase in the lattice parameter was observed for Ni/CeO2 and Ni@CeO2 catalysts around 700 °C. The ceria reduction at low temperature region is generally attributed to the reduction of surface ceria, whereas the reduction at high temperature corresponds to the ceria bulk reduction [START_REF] Perrichon | Reduction of ceria with different textures by hydrogen and their reoxidation by oxygen[END_REF]. The non-linear increase of the lattice parameter with the reduction temperature is likely due to the conversion of Ce 4+ to Ce 3+ [START_REF] Barrio | Unusual Physical and Chemical Properties of Ni in Ce1-xNixO2-y Oxides: Structural Characterization and Catalytic Activity for the Water Gas Shift Reaction[END_REF]. Ce 3+ ions have a higher ionic radius (1.034 Å) as compared to the Ce 4+ ions (0.92 Å). Then, the formation of Ce 3+ ions in ceria structure causes a change in the Ce-O bond length, which produces a lattice expansion and create oxygen vacancies [START_REF] Piumetti | Nanostructured ceria-zirconia catalysts for CO oxidation: Study on surface properties and reactivity[END_REF].

The higher variation of lattice parameter for Ni@CeO2 and Ni@CeZrO2 suggests the formation of a higher number of oxygen vacancies on these catalysts during reduction.

The TPOIE experiments (Fig. 12) were performed to investigate the oxygen exchange capacity of the support as a function of temperature. The CeO2 support only starts to exchange oxygen at 380 °C, reaching the value of 39 x 10 20 at.g -1 . The oxygen exchange begins at around 280 °C for Ni/CeO2 catalyst and it occurs until Ne = 35 x 10 20 at.g -1 at 500 °C. For the embedded Ni@CeO2 catalyst, the exchange started at lower temperature compared to the impregnated one (250 °C). At 500 °C, the number of exchanged oxygen atoms is 39 x 10 20 at.g -1 . In comparison to Ni/CeO2 and Ni@CeO2 catalysts, the oxygen exchange begin at higher temperature for Ni@CeZrO2 (300 °C) and the number of O atoms exchanged at 500 °C was 42 x 10 20 at.g -1 . Da Silva et al. [START_REF] Da Silva | Effect of the type of ceria dopant on the performance of Ni/CeO2 SOFC anode for ethanol internal reforming[END_REF] have reported similar results for CeO2 and CeNb supports. The catalysts showed similar isotopic distribution during the experiment (Fig. 13), indicating the occurrence of simple and multiple heteroexchange mechanisms [START_REF] Madier | Oxygen Mobility in CeO2 and CexZr(1-x)O2 Compounds: Study by CO Transient Oxidation and 18 O/ 16 O Isotopic Exchange[END_REF].

The presence of metal decreases the energy required to start the exchange process compared with the CeO2 support, because the exchange will occur mainly through the metal particle [START_REF] Martin | Mobility of Surface Species on Oxides. 1. Isotopic Exchange of 18 O2 with 16 O of SiO2, Al2O3, ZrO2, MgO, CeO2, and CeO2-Al2O3. Activation by Noble Metals. Correlation with Oxide Basicity[END_REF]. The doping of CeO2 by ZrO2 increases the temperature in which the exchange start. This delay can be associated with a decrease in the surface oxygen concentration due to the lower specific surface area of CeZrO2 compared to CeO2. On the contrary there is a fast increase in the number of atoms exchanged for Ni@CeZrO2, indicating that more oxygen atoms from the bulk phase are participating in the exchange process when Zr 4+ is inserted into the CeO2 structure. Therefore, the Ni@CeZrO2 catalyst exhibited the highest number of oxygen exchanged compared with Ni/CeO2 and Ni@CeO2.

The experiments of isotopic oxygen exchange were carried out at 350, 400 and 450 °C.

Figs. 14 and 15 illustrate the evolution of the number of exchanged atoms (Ne) and the isotopic distribution which occurs during the experiment at 400 °C. Table 3 reports the initial rate of exchange calculated from IOIE for the three temperatures. The exchange for CeO2 is very low compared with the samples containing Ni, because the metal promotes the spillover of oxygen to ceria surface, as observed also in the TPOIE experiment. Comparing the Ni/CeO2 and Ni@CeO2 catalysts, the presence of Ni nanoparticles embedded on ceria structure does not affect significantly the number of O atoms exchanged at the gas/solid equilibrium, but it increases significantly the initial rate of exchange, showing the positive influence of the close contact between Ni nanoparticles and ceria support (Table 3). The lower initial exchange rate and the higher Ne after 20 min reaction that are observed for Ni@CeZrO2 catalyst confirms the results obtained in TPOIE experiment: a lower surface exchange activity compared to ceriabased samples but a higher bulk diffusion.

Indeed, the Ni@CeZrO2 catalyst showed the highest number of Ne and the percentage of O atoms exchanged (Table 4). Furthermore, this catalyst exhibits a different behavior in the beginning of exchange at each temperature, with more formation of 32 O2 than 34 O2. On the opposite, Ni/CeO2 and Ni@CeO2 catalysts had a simultaneous formation of 32 O2 and 34 O2.

Then, the Ni@CeZrO2 catalyst favors the multiple exchange in comparison to the simple exchange at the beginning. Multiple heteroexchange occurs when two oxygen atoms are exchanged at the same time. Duprez et al. [START_REF] Madier | Oxygen Mobility in CeO2 and CexZr(1-x)O2 Compounds: Study by CO Transient Oxidation and 18 O/ 16 O Isotopic Exchange[END_REF] observed the same behavior for Ce0.76Zr0.32O2 samples, which was attributed to a higher concentration of superoxides species (O2 -) at the surface of the samples after oxidizing pre-treatment. Rossignol et al. [START_REF] Rossignol | Effect of the preparation method on the properties of zirconia-ceria materials[END_REF] also correlated the presence of these species to the higher oxygen storage capacity when the Zr 4+ is inserted in the ceria structure.

DRM reaction

The evolution of the CH4 conversion, CO2 conversion and H2/CO molar ratio as a function of time on stream (TOS) at 800 ºC are presented in Figs. 16A, 16B and 16C. The initial CH4 and CO2 conversions were approximately the same for all catalysts and they remained quite constant during 24 h of TOS. The reaction produces H2 and CO with a H2/CO molar ratio around 0.8. The CO2 conversion was higher than the CH4 conversion due to the occurrence of reverse water gas shift reaction (Eq. 1.4), leading to the production of CO and H2O and consequently, the H2/CO ratio was lower than 1.0.

Characterization of used catalysts after DRM reaction

The main challenge on the design of a catalyst for the DRM reaction concerns the deactivation caused by carbon deposition. In order to investigate the formation of carbon during DRM reaction, the spent catalysts were characterized by SEM, TEM, Raman and TGA analysis.

SEM images of the catalysts after DRM at 800 °C for 24 h of TOS are displayed in Fig. 17. SEM image revealed the presence of carbon filaments on Ni/CeO2 and Ni@CeO2, mainly on the catalyst prepared by impregnation. On the other hand, carbon filaments were hardly detected on Ni@CeZrO2 catalyst.

The TEM images of used catalysts shown in Fig. 18 The Raman spectra of used catalysts exhibit two bands in the range of 1200 to 1800 cm - 1 , which are indicative of carbon formation (Fig. 19). The G-band at 1581 cm -1 is indicative of 20 ordered carbon, whereas the band at 1350 cm -1 (D-band) has been related to disordered carbon structures [START_REF] Alvarez | Characterization of single-walled carbon nanotubes (SWNTs) produced by CO disproportionation on Co-Mo catalysts[END_REF]. The D-band in the range of 1285-1300 cm -1 is ascribed to single-walled carbon nanotubes (SWCNT), while the position around 1330 cm -1 is related to multi-walled carbon nanotubes (MWCNT) or crystalline graphite-like forms [START_REF] Silva | Hydrogen Production Through Steam Reforming of Toluene Over Ni Supported on MgAl Mixed Oxides Derived from Hydrotalcite-Like Compounds[END_REF][START_REF] Belin | Characterization methods of carbon nanotubes: a review[END_REF]. The position of the D-band in 1350 cm -1 and the SEM images suggest the presence of MWCNT. Furthermore, the intensity of these bands was higher for Ni/CeO2, indicating that the highest carbon deposition took place on this catalyst. These bands are barely detected on the spectrum of Ni@CeZrO2, which suggests the presence of very low amount of carbon deposits on this catalyst, in agreement with the SEM image.

The derivative of weight during the TGA analysis is shown in Fig. 20. The DTG profile of Ni/CeO2 sample exhibited an intense peak at 614 °C, whereas a broad peak at 518 °C is observed in the DTG profile of Ni@CeO2 catalyst. No peak was detected on the DTG profile of Ni@CeZrO2, indicating that carbon was not deposited on this catalyst.

The effect of Ni embedded in CeO2 on catalyst deactivation for DRM

The mechanism of carbon formation over Ni-based catalysts during DRM has been extensively discussed in the literature [START_REF] Bradford | CO2 reforming of CH4[END_REF][START_REF] Ferreira-Aparicio | Mechanistic aspects of the dry reforming of methane over ruthenium catalysts[END_REF][START_REF] Wei | Isotopic and kinetic assessment of the mechanism of reactions of CH4 with CO2 or H2O to form synthesis gas and carbon on nickel catalysts[END_REF] and involves the dissociation of methane on the surface of nickel particles, producing hydrogen and carbon (Cα). In the presence of a support with redox properties such as ceria and ceria-zirconia mixed oxides, this highly reactive carbon species (Cα) may reacts with oxygen from the support producing CO [START_REF] Silva | Hydrogen Production Through Steam Reforming of Toluene Over Ni Supported on MgAl Mixed Oxides Derived from Hydrotalcite-Like Compounds[END_REF][START_REF] Marinho | Steam reforming of ethanol over Ni-based catalysts obtained from LaNiO3and LaNiO3/CeSiO2 perovskitetype oxides for the production of hydrogen[END_REF][START_REF] Noronha | Correlation between catalytic activity and support reducibility in the CO2 reforming of methane over Pt/CexZr1-xO2 catalysts[END_REF]. The unbalance between the rate of methane decomposition and the reaction rate of carbon with oxygen from the support will lead to the accumulation of a less active carbon (Cβ) that may also migrate into the nickel particle producing carbon filaments. Furthermore, the oxygen vacancies in the catalysts acts like active sites for CO2 dissociative adsorption, which increase the amount of oxygen species on its surface. Therefore, the plays an important role on the DRM reaction, keeping the metal surface free of carbon deposits. Moreover, the rate of carbon formation is strongly affected by the Ni crystallite size [START_REF] Rabelo-Neto | CO2 reforming of methane over supported LaNiO3 perovskite-type oxides[END_REF]. The initiation step of carbon formation is more difficult over smaller particles. Then, carbon formation is minimized or inhibited on catalysts with small Ni particle sizes. Wang et al. [START_REF] Wang | Syngas production from CO2 reforming with methane over coreshell Ni@SiO2 catalysts[END_REF] showed that the confinement of Ni nanoparticles inside SiO2 spheres on Ni@SiO2 inhibited metal sintering during dry reforming of methane, minimizing carbon formation. Pu et al. [START_REF] Pu | Ceria-promoted Ni@Al2O3 coreshell catalyst for steam reforming of acetic acid with enhanced activity and coke resistance[END_REF] evaluated the influence of Ce on Ni@Al2O3 core-shell catalysts to improve the coke resistance for steam reforming of acetic acid. The Ni@Al2O3 core-shell structure could avoid metal sintering (Ni crystallite size < 5 nm) but carbon formation was observed yet. However, the addition of Ce suppressed carbon deposition, which was attributed to the oxygen mobility of the material. The combination of small Ni size with oxygen mobility was fundamental to design a catalyst resistant to carbon formation.

In our work, the slight decrease in CH4 and CO2 conversions is likely due to the growth of Ni particle size as revealed by TEM. Furthermore, the inclusion of Ni nanoparticles into the CeZrO2 structure for Ni@CeZrO2 catalyst reduced metal sintering, as revealed by in situ XRD and TEM, due to the thermal stability of CeZrO2 support during the reduction pre-treatment. In addition, this catalyst exhibited the highest number of O atoms exchanged during the isothermal oxygen isotopic exchange at 400 °C as well as the highest amount of oxygen vacancies formed as shown by XRD and Raman spectroscopy. Therefore, the control of Ni particle size and the high oxygen mobility of Ni@CeZrO2 catalyst inhibited carbon deposition and favored the mechanism of carbon removal, promoting catalyst stability.

4.Conclusions

The one step preparation of Ni@CeO2 and Ni@CeZrO2 catalysts led to Ni nanoparticles embedded in the oxide, which had a positive effect on the stabilization of the Ni particle size during the reduction at high temperature (800 °C). It also generates more oxygen vacancies and increases the interaction with CeZrO2 and CeO2 in comparison with an impregnated catalyst Ni/CeO2. Furthermore, doping of ceria with Zr could maintain the ceria structure throughout the thermal treatment and increase the mobility of oxygen in the material, compared to ceria alone, favoring the carbon removal mechanism during the reaction. Thus, the protection of the metal particle against sintering as well as the oxidation of carbon species by the support for the catalyst Ni@CeZrO2 resulted in the suppression of carbon deposition during the DRM in our work.

  The formation of Ni-CeO2 solid solution significantly shifted the frequency for the first-order F2g peak to lower values, around 445 cm -1 . Then, the absence of the shift of this band for the Ni@CeO2 catalyst shows that the Ni is embedded in CeO2 structure instead of in a solid solution.XANES experiments at the Ni K-edge were performed to study the reduction of nickel oxide for all samples (Figs.3, 4 and 5). The XANES spectrum of the calcined Ni/CeO2 sample shows the white line at 8352 eV characteristic of NiO. Increasing the reduction temperature under 5 % H2/He mixture led to a continuous decrease in the intensity of this peak. At 450 °C, the spectrum is characteristic of metallic Ni, indicating that nickel oxide was completely reduced to Ni 0 . The same trend was observed for Ni@CeO2 and Ni@CeZrO2 catalysts as shown in Figs.4 and 5.However, the temperature range for the reduction of Ni oxide was different for the catalysts prepared by impregnation and sol-gel methods. Fig.6shows the evolution of the metallic Ni species during reduction calculated by the linear combination of Ni K-edge XANES spectra of references. The reduction of Ni/CeO2 and Ni@CeO2 samples starts at 290 and 277 °C, respectively, but at 190 °C for Ni@CeZrO2 catalyst. The total reduction of nickel oxide on Ni/CeO2 sample is achieved at 450 °C, while the complete reduction of the Ni@CeO2 and

Fig. 7

 7 Fig. 7 shows the diffractograms of the calcined samples. Ni/CeO2 and Ni@CeO2

Figs

  Figs. 8A, 9A and 10A. The evolution of the mean crystallite size of NiO, Ni 0 and CeO2 during

  can provide information about metal sintering as well as the formation of carbon filaments during the reaction. Ni/CeO2 catalyst (Figs.18 A and B) presented Ni nanoparticles without any interaction with CeO2 matrix. Some Ni nanoparticles are located inside the carbon filaments, which avoid their sintering. There are also large nanoparticles outside the carbon nanotubes with particle size around 40 nm. For Ni@CeO2 catalyst (Figs.18 C and D), TEM images reveal the presence of Ni nanoparticles into CeO2 bulk but sintering is observed (Ni particle size around 20 nm). This higher interaction inhibits the detachment of Ni particles from CeO2 and the growth of carbon filament. The TEM images of Ni@CeZrO2 (Figs.18 E and F) show Ni nanoparticles embedded into ceria matrix with very small particle size (around 8 nm) after 24 h of TOS. Furthermore, carbon filaments are not observed, which demonstrate that inhibiting Ni sintering in the presence of a support with oxygen mobility suppress the formation of carbon filaments.

Figure 1

 1 Figure 1. TEM images and EDX of selected areas of Ni/CeO2, Ni@CeO2 and

Figure 2 .

 2 Figure 2. Raman spectra for the calcined samples and a CeO2 reference.

Figure 3 .

 3 Figure 3. XANES spectra obtained during the reduction of Ni/CeO2 catalyst.

Figure 4 .

 4 Figure 4. XANES spectra obtained during the reduction of Ni@CeO2 catalyst.

Figure 5 .

 5 Figure 5. XANES spectra obtained during the reduction of Ni@CeZrO2 catalyst.

Figure 6 .

 6 Figure 6. Fraction of Ni 0 calculated by the linear combination of Ni K-edge XANES spectra of

Figure 7 .

 7 Figure 7. X-ray diffraction patterns of calcined samples: (A) Ni/CeO2; (B) Ni@CeO2; and (C)

Figure 8 .

 8 Figure 8. (A) X-ray diffraction patterns obtained during reduction from room temperature to 800 °C for Ni/CeO2; (B) Crystallite size of NiO, Ni 0 and CeO2 during the reduction process.

Figure 9 .

 9 Figure 9. (A) X-ray diffraction patterns obtained during reduction from room temperature to 800 °C for Ni@CeO2; (B) Crystallite size of Ni 0 and CeO2 during the reduction process.

Figure 10 .

 10 Figure 10. (A) X-ray diffraction patterns obtained during reduction from room temperature to 800 °C for Ni@CeZrO2; (B) Crystallite size of Ni 0 and CeO2 during the reduction process.

Figure 11 .

 11 Figure 11. Variation of CeO2 lattice parameter as a function of reduction temperature for all

Figure 12 .

 12 Figure 12. Evolution of the number of exchanged oxygen atoms during TPOIE over Ni/CeO2,

Figure 13 .

 13 Figure 13. Isotopic distribution during the TPOIE for Ni/CeO2, Ni@CeO2, Ni@CeZrO2 and

Figure 14 .

 14 Figure 14. Evolution of the number of exchanged oxygen atoms during IOIE at

Figure 15 .

 15 Figure 15. Isotopic distribution during the IOIE at 400 °C.

Figure 16 .

 16 Figure 16. (A) CH4 conversion, (B) CO2 conversion and (C) H2/CO molar ratio versus TOS for

Figure 17 .

 17 Figure 17. SEM images of spent catalysts after DRM at 800 °C for 24 h of TOS.

Figure 18 .

 18 Figure 18. TEM images of spent catalysts after DRM at 800 °C for 24 h of TOS; (A) and (B)

Figure 19 .

 19 Figure 19. Raman spectra of post-reaction catalysts.

Figure 20 .

 20 Figure 20. DTG of the post-reaction samples.

Table 5

 5

	reports the average rate of carbon formation for each catalyst during DRM
	reaction at 800 °C for 24 h of TOS. Ni/CeO2 catalyst exhibited the highest amount of carbon
	deposits but the carbon formation rate was significantly reduced for the catalysts containing Ni
	nanoparticles embedded in ceria structure. Ni@CeZrO2 catalyst did not show evidences of
	carbon formation after DRM reaction.

  Yan et al.[START_REF] Yan | Highly efficient and stable Ni/CeO2-SiO2 catalyst for dry reforming of methane: Effect of interfacial structure of Ni/CeO2 on SiO2[END_REF] recently demonstrated the importance of reactive oxygen species in close contact with Ni nanoparticles to oxidize the Cα before its polymerization for Ni/CeO2-SiO2 catalysts. The conversion of Cα species to CO by the reactive oxygen species could enhance the catalytic properties and stability of the Ni/CeO2-SiO2 catalyst. The same result was reported by Marinho et al.[START_REF] Marinho | Steam reforming of ethanol over Ni-based catalysts obtained from LaNiO3and LaNiO3/CeSiO2 perovskitetype oxides for the production of hydrogen[END_REF] for the steam reforming of ethanol over LaNiO3/CeSiO2 precursor catalyst. The strong metal-support

interaction favored the transfer of oxygen from the support to metal through the metal-support interface. Roh et al.

[START_REF] Roh | Combined H2O and CO2 Reforming of Methane Ni-Ce-ZrO2 Catalysts for Gas to Liquids (GTL)[END_REF]

successfully synthetized Ni catalyst supported on CeO2-ZrO2 solid solution. They could observe the enhancement on catalyst stability for Ni/Ce0.8Zr0.2O2, which was due to the higher oxygen storage capacity and Ni dispersion. Yentekakis et al.

[START_REF] Wang | Ceria-based catalysts for the production of H2 through the water-gas-shift reaction: Time-resolved XRD and XAFS studies[END_REF]

also proposed that CeZrO2 support promotes the dissociative adsorption of CO2, producing a labile lattice oxygen that contributes to carbon oxidation.

Acknowledgements

The authors thank CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brazil, 308200/2014-7 and 303667/2018-4), CAPES (Coordenação de Aperfeiçoamento de Pessoal de Ensino Superior, Brazil, Finance code 001) and CAPES -COFECUB program (88881.142911/2017-01) for supporting this research and the scholarship received. The group also thanks the LNLS for the beamtime in DXAS and XPD experimental lines for the XANES and XRD analysis, respectively.