Unveiling surface stability and oxygen vacancy segregation of Yb2SiO5 and Lu2SiO5 by first-principles calculations

RE₂SiO₅ (RE = Yb and Lu) are significant environmental barrier coating (EBC) materials, in which surface and oxygen vacancy play crucial roles in their structural stability and functionality. In this work, the structural configuration and thermodynamics of (1 0 0), (0 1 0), and (0 0 1) surfaces of RE₂SiO₅ are investigated by first-principles calculations. The (0 0 1) surface is preferred energetically, which is attributed to the weak bond broken environment and large rare-earth polyhedron distortion on this surface. Moreover, the formation energies of various oxygen vacancies on the stable (0 0 1) surface are estimated and the optimal location for oxygen vacancies is held by the [SiO₄] tetrahedron. The oxygen vacancies are more likely to segregate on the surface because of the lower formation energies on the surfaces compared with those in the bulk. These findings are expected to enable the development of RE₂SiO₅-based EBCs by tuning grain size and/or thin film growth orientation.     

Thermodynamics of surface and oxygen vacancy in rare earth stannates by first-principles calculations

The research of nanocrystalline pyrochlores highlights the importance of the surface structure, composition and segregated point defect in their thermal, electrical, optical, magnetic, and catalytic performances. In order to provide a basic view on the surface-related phenomena, thermodynamic stabilities of three low-index (100), (110), and (111) surfaces for A₂Sn₂O₇ (A = La, Ce, Pr, Nd, Pm, Sm, Eu, or Gd), together with their configurations, electronic structures and related oxygen vacancies are investigated using first-principles calculations. The (111) surfaces with A₃SnO₈ and ASn₃O₆ terminations are predicted to be stable due to their low surface energies. Meanwhile, the (110) surfaces with A₂Sn₂O₈ and A₂Sn₂O₆ terminations are found to may also form. For these surface structures, the amount of broken bonds play the main role in their structural stability, and the local coordination environment variation also has minor contribution to it. Moreover, oxygen vacancies are observed to segregate on the surface layer, owing to lower energy of breaking bonds accompanying with oxygen vacancy formation and the larger relaxation space comparing to the counterpart in bulk. These results are expected to provide guidance on optimizing the performances of these compounds through surface engineering.     

Bulk oxygen promoted water–gas shift reaction activity over Pt/Ce0.6Zr0.4O2 catalyst

The water–gas shift (WGS) reaction of Pt/Ce_0.6Zr_0.4O₂ catalyst was studied, as well as the reference catalysts Pt/CeO₂ and Pt/ZrO₂. In situ electronic conductivity measurements under reactive atmosphere show that surface oxygen vacancies of Pt/Ce_0.6Zr_0.4O₂ diffuse into the bulk materials at the temperature typical for the operation of WGS reaction, i.e., bellow 623 K. Compared with Pt/CeO₂, it was found that the oxygen storage capacity (OSC) was higher for Pt/Ce_0.6Zr_0.4O₂ catalyst. Pt/Ce_0.6Zr_0.4O₂ shows a markedly higher CO conversion rate than Pt/CeO₂ and Pt/ZrO₂ catalysts, which was interpreted by more active oxygen species available and higher coke resistance.     

Effect of calcination treatment of zirconia on W/ZrO2 catalysts for transesterification

In this present study, the nanocrystalline ZrO₂ particles synthesized by the solvothermal method were calcined in reductive (H₂), inert (N₂) and oxidative (O₂ and air) atmospheres prior to impregnation with tungsten (W) in order to produce the W/ZrO₂ (WZ) catalysts. Based on the ESR measurement, it revealed that only the ZrO₂ samples calcined in H₂ and N₂ exhibited the F-center (single charged oxygen vacancy) at g = 2.003. None of Zr³⁺ defect was detected for all calcined ZrO₂ samples. After impregnation with tungsten, the WZ catalysts were also characterized. It was present as the polycrystal, which can be seen by the selected area electron distribution (SAED). However, the presence of Zr³⁺ defect was evident in all WZ catalysts, while the F-center was absent. The highest Zr³⁺ intensity detected in the WZ catalyst using ZrO₂ under H₂ calcination atmosphere can be attributed to the transformation of F-center to Zr³⁺ defect. It revealed that the WZ-H₂ catalyst exhibited the highest conversion under transesterification of triacetin and methanol among other WZ catalysts. This can be attributed to the high surface acidity, which was probably induced by large amounts of Zr³⁺ defect.     

Effect of the concentration of anion vacancies and the radius and charge on the cation of the stabilizing oxide on the conductivity of solid solutions based on ZrO2

Conclusions An increase to 4.5–5.5% in the concentration of anion vacancies in the lattice facilitates the transport processes in solid solutions. With a significant concentration of anion vacancies this process becomes difficult, as is indicated by the increase in the activation energy of conductivity.The stabilization of ZrO₂ by oxides with a larger cation radius leads to a reduction in the level of conductivity of the solid solutions.The addition to the ZrO₂ lattice of cations of stabilizing oxides with a charge close to that of Zr⁴⁺ facilitates the transport process of the oxygen ions.It is shown that, in principle, it is possible to predict the level of conductivity of solid solutions based on ZrO₂ at 1200–1400°C.The use of a combined additive consisting of a mixture of rare-earth oxides to stabilize ZrO₂ makes it possible to obtain solid electrolytes close in their electrophysical properties to the ZrO₂-Sc₂O₃ electrolytes.Translated from Ogneupory, No. 4, pp. 15–18, April, 1985.     

Oxygen-vacancy-mediated microstructure and thermophysical properties in Zr3Ln4O12 for high-temperature applications

Yttria-stabilized zirconia (YSZ) has been considered as state-of-the-art material for high-temperature thermal barrier coatings, which provide thermal insulation to the superalloy components in gas turbines and jet engines. Oxygen vacancies induced by yttria substitutions are believed to be mainly responsible for the low thermal conductivity of YSZ due to their phonon scattering effect. However, high mobility of oxygen vacancies in YSZ leads to a rapid oxygen diffusion at high temperatures, therefore accelerates the failure of coatings by grain coarsening, sintering, and simultaneous oxidation of the underlying metallic bondcoat. In the present research, we further explored in the ZrO₂–Ln₂O₃ binary phase diagram and synthesized a series of ceramic materials with the chemical formula of Zr₃Ln₄O₁₂ (Ln = La, Gd, Y, Er, and Yb), in which more oxygen vacancies were involved and extremely low phonon thermal conductivities (1.3-1.6 W/m·K) were obtained, even approaching to the theoretical minimum. In addition, the mobility of these oxygen vacancies was remarkably suppressed by the lattice ordering with the decrease of Ln³⁺ radius, as confirmed by X-ray diffraction, Raman and transmission electron microscopy. Thus, the oxygen barrier property and sintering resistance were significantly enhanced accordingly, which makes Zr₃Ln₄O₁₂ compounds promising thermal barrier coating materials for next generation gas turbines and jet engines.     

Density functional theory study on adsorption of thiophene on TiO2 anatase (0 0 1) surfaces

In order to develop a fundamental understanding of the adsorption mechanism of thiophenic compounds on TiO₂-based adsorbents for ultra-deep desulfurization of liquid hydrocarbon fuels, a density functional theory (DFT) study was conducted on the adsorption of thiophene over the TiO₂ anatase (0 0 1) surface. The perfect, O-poor (with oxygen vacancies), and O-rich (with activated O₂ on the surface) anatase (0 0 1) surfaces were built, and the interaction of thiophene molecule with these surfaces was examined. The adsorption configuration and adsorption energy on the different surfaces and sites were estimated. The results showed that thiophene may be adsorbed on both the perfect and O-poor surfaces through an interaction between the Ti cations on the surface and the S atom in thiophene, whereas on the O-rich surface through an interaction of the activated O atoms (the dissociatively or associatively adsorbed O₂) on the surface with the S atom in thiophene to form a sulfone-like surface species. The adsorption of thiophene on the O-rich surface is significantly stronger than adsorption on the perfect and O-poor surfaces on the basis of the calculated adsorption energies. The results indicate that the activated O₂ on the TiO₂ anatase (0 0 1) surface may play an important role in the adsorption desulfurization over the TiO₂-based adsorbents, and increased concentration of the activated O₂ on the surface may result in improvement of the adsorption capacity of the adsorbents.     

Observation of yttrium oxide segregation in a ZrO2-SiO2 glass-ceramic at nanometer dimensions

Dopant segregation at grain boundaries (GBs) in ceramics has been widely reported, while whether similar segregation behavior occurs in glass-ceramics remains unknown. The distribution of dopant in glass-ceramics may be totally different due to the existence of glass phase. This study examines the distribution of Y³⁺ ions in a ZrO₂-SiO₂ glass-ceramic. Two samples were prepared by hot pressing, yttrium oxide-doped, and undoped 65 mol% ZrO₂-35 mol% SiO₂ nanocrystalline glass-ceramics (NCGCs). The NCGCs had the same microstructure, that is, ZrO₂ nanoparticles (NPs) embedded in an amorphous SiO₂ matrix. XRD results showed that the undoped NCGC was composed of 20.9 wt% (weight percentage) monoclinic ZrO₂ (m-ZrO₂) and 79.1 wt% tetragonal ZrO₂ (t-ZrO₂), while the yttrium oxide-doped NCGC was composed of 9.6 wt% m-ZrO₂ and 90.4 wt% t-ZrO₂. X-ray energy-dispersive spectrometry (EDS) results in scanning electron transmission microscopy (STEM) mode demonstrated that Y³⁺ ions segregated both on the surface of ZrO₂ NPs and within the thin intergranular glass film (with a thickness of approximately 7 Å) between ZrO₂ NPs in the yttrium oxide-doped NCGC. Interestingly, no obvious Y signals were detected in the amorphous SiO₂ matrix. Density functional theory calculation results showed that Y³⁺ ions had a strong segregation tendency in the GB area and the segregation of Y³⁺ ions increased the work of separation of GB layer. These findings provide new understanding of the segregation behavior of dopant in glass-ceramics, which may offer useful guidance for other researchers to tailor the properties of glass-ceramics through GB engineering.     

Enhancement of oxygen storage capacity by reductive treatment of Al2O3 and CeO2–ZrO2 solid solution nanocomposite

Oxygen storage capacity (OSC) of CeO₂–ZrO₂ solid solution, Ce_xZr_(1−x)O₄, is one of the most contributing factors to control the performance of an automotive catalyst. To improve the OSC, heat treatments were employed on a nanoscaled composite of Al₂O₃ and CeZrO₄ (ACZ). Reductive treatments from 700 to 1000 °C significantly improved the complete oxygen storage capacity (OSC-c) of ACZ. In particular, the OSC-c measured at 300 °C reached the theoretical maximum with a sufficient specific surface area (SSA) (35 m²/g) after reductive treatment at 1000 °C. The introduced Al₂O₃ facilitated the regular rearrangement of Ce and Zr ions in CeZrO₄ as well as helped in maintaining the sufficient SSA. Reductive treatments also enhanced the oxygen release rate (OSC-r); however, the OSC-r variation against the evaluation temperature and the reduction temperature differed from that of OSC-c. OSC-r measured below 200 °C reached its maximum against the reduction temperature at 800 °C, while those evaluated at 300 °C increased with the reduction temperature in the same manner as OSC-c.     

Chlorobenzene total oxidation over palladium supported on ZrO2, TiO2 nanostructured supports

Two series of supported Pd catalysts were synthesized on new mesoporous–macroporous supports (ZrO₂, TiO₂) labelled M (Zr and Ti). The deposition of palladium was carried out by wet impregnation on the calcined TiO₂ and ZrO₂ supports at 400 °C (Pd/Zr₄, Pd/Ti₄) and 600 °C (Pd/Zr₆, Pd/Ti₆) and followed by a calcination at 400 °C for 4 h. The pre-reduced Pd/MX catalysts were investigated for the chlorobenzene total oxidation and their catalytic properties where compared to those of a reference catalyst Pd/Ti-Ref (TiO₂ from Huntsman Tioxide recalcined at 500 °C) and of a palladium supported on the fresh mesoporous–macroporous TiO₂ (Pd/Ti). Based on the activity determined by T₅₀, the Pd/Ti and Pd/Ti₄ catalysts have been found to be more active than the reference one. Moreover activity decreased owing to the sequence: Pd/TiX  Pd/ZrX and in each series when the temperature of calcination of the support was raised. The overall results clearly showed that the activity was dependant on the nature of the support. The better activity of Pd/TiX compared to Pd/ZrX was likely due to a better reducibility of the TiO₂ support (Ti⁴⁺ into Ti³⁺) leading to an enhancement of the oxygen mobility. Production of polychlorinated benzenes PhCl_x (x = 2–6) and of Cl₂ was also observed. Nevertheless at 500 °C the selectivity in HCl was higher than 90% for the best catalysts.     

Surface anion vacancies on ceria: Quantum modelling of mutual interactions and oxygen adsorption

Quantum DFT + U calculations using periodic slab models are made to understand the characteristics of surface defects containing one, two or three anion vacancies at the CeO₂(1 1 1) surface. Several non-equivalent energy minima with different positions of localized Ce³⁺ ions can exist due to polaron-type distortions that influence the relative stability of Ce³⁺ in 6- and 7-fold coordinated sites. The calculated total energies do not confirm the tendency to vacancy association suggested by previous works. In the case of associated vacancies spontaneous outwards movement of one oxygen ion from a subsurface layer is observed. Oxygen adsorption on these defects leads to electron transfer to the O₂ molecule, forming peroxide species, except when only one electron is available in which case paramagnetic O₂⁻ species appear; the adsorption energies involved are computed and discussed. All these species appear asymmetrically coordinated to three surface Ce ions. The presence of excess electrons, as is the case when two or more vacancies associate, does not lead in general in these calculations to a barrierless complete reduction of the O₂ molecule, a process which would produce a pair of oxide ions; this is explained considering the relative energies of the orbitals involved. Thermal activation would be needed to complete this process.     

Catalytic behaviour of thermally aged Ce/Zr mixed oxides for the purification of chlorinated VOC-containing gas streams

In the present paper the thermal deactivation of a series of Ce/Zr mixed oxides (CeO₂, Ce_0.8Zr_0.2O₂, Ce_0.68Zr_0.32O₂, Ce_0.5Zr_0.5O₂, Ce_0.15Zr_0.85O₂ and ZrO₂) was investigated. In order to simulate long-term operation, samples were calcined at three different temperatures, namely 550, 750 and 1000 °C in air for 4 h. Structural, morphological and physico-chemical changes caused by high-temperature treatment were analysed by X-ray diffraction, BET measurements, NH₃-temperature-programmed desorption and temperature-programmed reduction with hydrogen, and the behaviour in the oxidation of chlorinated volatile organic compounds (1,2-dichloroethane and trichloroethylene). The catalytic properties of Ce/Zr mixed oxides could be accounted for on the basis of their promoted redox, as characterised by the percentage of oxygen vacancies, and acidic properties due to the incorporation of zirconium in the ceria lattice. An increase in the calcination temperature led to a progressive decrease in the catalytic activity as a result of the modifications provoked by induced thermal aging (decrease in surface area, larger crystal sizes, reducibility at higher temperatures and loss of acid sites). Ce_0.15Zr_0.85O₂ and Ce_0.5Zr_0.5O₂ showed the best resistance to deactivation with combustion temperatures still notably lower in comparison with the homogeneous reaction even after calcination at 1000 °C. Also slight changes in selectivity were evident resulting in favoured yields of hydrogen chloride, which was environmentally beneficial, and incomplete combustion products such as carbon monoxide and chlorinated intermediates.     

Mesoporous nanocrystalline zirconium oxide: novel preparation and photoluminescence property

A novel strategy involving the combination of soft-templating and solid–liquid method (CSSL) is presented to synthesize mesoporous nanocrystalline zirconia with high specific surface area, that is, the mesostructured zirconia hybrid is firstly synthesized via cooperative assembly between zirconium sulphate as inorganic precursor and 1-hexadecyl-3-methylimidazolium bromide (C₁₆mim⁺Br⁻) as the structure-directing agent, and subsequently ground with solid magnesium nitrate salt followed by heat-treatment in air. The resulting zirconia material after calcination at 600 °C possesses a wormlike arrangement of mesopores surrounded by tetragonal ZrO₂ nanocrystallites of ca. 2.3 nm. The BET surface area is 255 m²/g and the pore size is ca. 4.3 nm. However, no mesoporous structure exists in the obtained zirconia material via the simple soft-templating method at the same calcination temperature. Photoluminescence (PL) spectra of the obtained mesoporous nanocrystalline ZrO₂ show a strong emission peak at ca. 394 nm under UV excitation of 250 nm wavelength.     

Temperature effect on mechanical and thermal properties of multicomponent rare-earth zirconate pyrochlores

In order to explore the temperature-dependent structural and mechanical/thermal property evolution of pyrochlores, multicomponent rare-earth zirconates (4RE_1/4)₂Zr₂O₇ (RE = La, Nd, Sm, Eu, and Gd) and corresponding single-component compounds are investigated by molecular dynamics simulations. The structural parameters and mechanical/thermal properties reported in experiments are well reproduced. With temperature enhancement, the bond lengths become large and polyhedrons tend to deform more, whereas the second-order elastic constants and polycrystalline mechanical moduli gradually reduce. It can be explained by the obvious change of (ZrO₆) polyhedrons originating from the Zr–O bond stretching. Furthermore, thermal conductivities show decreasing tendency owing to a sharp decline of the phonon mean free path related to enhanced phonon scattering. As a big influence of temperature on (ZrO₆) polyhedrons occurs, the multicomponent design at the Zr-site is suggested to be paid more attention. This work is expected to shield light on the design for multicomponent pyrochlores as thermal barrier coatings.     

ZrO2-doped YTaO4 as potential CMAS-resistant materials for thermal barrier coatings application

In this contribution, the ZrO₂-doped YTaO₄ (Zr_xY_0.5−x/2Ta_0.5−x/2O₂ (x = 0, 0.1, 0.2, 0.28)) are proposed as potential CMAS-resistant materials for TBCs. The corrosion behavior of those materials under CMAS attack are investigated from thermodynamics and kinetics. The results show that all compositions have the much better CMAS resistance than the classical Gd₂Zr₂O₇. After 50 h corrosion at 1300℃, the corrosion depth in ZrO₂-doped YTaO₄ bulks is about 50–80 µm (for a 20 mg/cm² CMAS deposition) in contrast with ~140 µm in Gd₂Zr₂O₇ bulk. The CMAS corrosion mechanism of ZrO₂-doped YTaO₄ is elucidated, and the excellent CMAS resistance is attributed to the rapid formation and followed thickening of dense reaction product layer. Furthermore, the effects of ZrO₂ doping content on CMAS resistance of YTaO₄ is discussed. It is elucidated that ZrO₂ doping can inhibit the precipitation of apatite, decrease the consumption of CMAS melt, and change the morphology of dense reaction layer. In summary, minor doping of ZrO₂ can ensure the excellent short- and long-term CMAS resistance, but heavy doping of ZrO₂ will degrade the long-term CMAS resistance.     

Structure characterization and mechanical properties of CeO2–ZrO2 solid solution system

The measured and calculated lattice parameters, microstructures, and mechanical properties (fracture toughness and microhardness) of CeO₂–ZrO₂ system ceramics are investigated, using CeO₂–ZrO₂ solid solution powder prepared by a microwave-induced combustion process. The CeO₂–ZrO₂ solid solution ceramics were sintered at 1500 °C for 6 h in air; the density of all specimens was greater than 94% of the theoretical density. For Ce_1−xZr_xO₂ (0.00  x  0.50), the measured lattice parameter is in accordance with that of Kim's doped CeO₂ model_. On the other hand, for x  0.50, the measured values fit Kim's doped ZrO₂ model. The fracture toughness and microhardness of CeO₂–ZrO₂ system ceramics with various compositions were investigated with Vickers indentation. The results showed that the crack mode of CeO₂–ZrO₂ solid solution was Palmqvist cracks under loads of 1 kg. Generally, the fracture toughness should increase with grain size at the submicron scale. However, larger grains may lead to spontaneous transformation, which should decrease the potential toughening at room temperature. This behavior was observed in the Ce_0.25Zr_0.75O₂ ceramic, which demonstrated a high fracture toughness that may be ascribed to two causes: (1) fine grain size and (2) transformation toughening.     

The effect of the change of the structure on oxidative dehydrogenation of propane over cerium–zirconium catalysts

A series of pure CeO₂, ZrO₂, and CeZrO_x mixed metal oxide catalysts were prepared by a wetness impregnation method and were applied to the dehydrogenation of propane to propylene at 500°C and 0.1 MPa. The prepared catalysts were characterized by thermal gravimetric analysis (TGA), Brunauer, Emmett, and Teller (BET), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopes (TEM), Raman spectroscopy, and H₂-TPR. It was observed that the zirconium content of the solid solution of the mixed metal oxide catalyst was 5%–25%, while the zirconium content of the material with phase segregation was higher (50%). The addition of zirconium was proven to decrease the oxygen vacancy concentration on the catalyst surface and change the intensity of (111) crystal of cerium oxide in the catalysts. Among the prepared catalysts, the Ce_0.90Zr_0.10O_x catalyst with the maximum strength of the (111) crystal plane of cerium oxide exhibited the better catalytic oxidation performance for the dehydrogenation of propane to propylene. Compared with ZrO₂ in the blank experiment, the average propane conversion and propylene selectivity of the Ce_0.90Zr_0.10O_x catalyst were increased by 10.78% and 17.95%, respectively.     

Nonstoichiometry and relaxation kinetics of nanocrystalline mixed praseodymium–Cerium oxide Pr0·7Ce0·3O2−x

The oxygen deficiency and kinetics of oxygen uptake and release of nanocrystalline mixed praseodymium–cerium oxide with composition Pr_0·7Ce_0·3O_2−x were investigated by combining coulometric titration and potentiometric measurements using stabilised zirconia oxygen concentration cells. The P(O₂) versus composition isotherms indicate a two-phase region at high P(O₂) [P(O₂)>0·1 bar at 560°C] and a single-phase region at lower P(O₂). The oxygen pressure dependence in the homogeneous region can be described by a power law with an exponent (−1/6), in accordance with doubly charged oxygen vacancies as majority defects. The enthalpy of reduction amounts to (2·9±0·3) eV. The chemical diffusion coefficients are of the order of 10⁻⁶ cm² s⁻¹ at 640°C with an activation energy of ≈0·3 eV. The low activation energy for diffusion may be related to the high density of interface sites in the nanocrystalline material.     

Ferroelectric,dielectric, and impedance properties of Sm-modified Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics

To investigate the effect of Sm doping on the electrical properties of Ba(Zr_0.2Ti_0.8)O₃-x(Ba_0.7Ca_0.3)TiO₃ (BZT-xBCT) (x = 40, 50, 60) ceramics, three Sm-modified ceramics were prepared using the conventional solid-state reaction method. Related electrical measurements, including ferroelectric and dielectric investigations and impedance spectroscopy, were recorded for these ceramics. It was found that a tilted morphotropic phase boundary resulted from the addition of Sm, which induced the best piezoelectric properties and insulating behaviour in the Sm-BZT-60BCT sample. An abnormal P-E loop shrinkage appeared in the Sm-BZT-50BCT sample but not in the other two samples. This could be attributable to the different electronegativities between Ca²⁺ and Ba²⁺ and between Zr⁴⁺ and Ti⁴⁺, whose contents are different in varied samples and have an effect on defect-dipole alignment as well as spontaneous polarization. The activation energies for the bulk conductivity in the three composites were calculated to be 0.28 ± 0.01, 0.08 ± 0.01, and 0.36 ± 0.01 eV, confirming the existence of oxygen vacancies in our samples. The Sm dopant is responsible for the oxygen vacancies. This also leads to an increased Curie temperature in the three composites.     

Methane combustion over Pd/ZrO2/SiC,Pd/CeO2/SiC,and Pd/Zr0.5Ce0.5O2/SiC catalysts

The performances of different promoters (CeO₂, ZrO₂ and Ce_0.5Zr_0.5O₂ solid solution) modified Pd/SiC catalysts for methane combustion are studied. XRD and XPS results showed that Zr⁴⁺ could be incorporated into the CeO₂ lattice to form Zr_0.5Ce_0.5O₂ solid solution. The catalytic activities of Pd/CeO₂/SiC and Pd/ZrO₂/SiC are lower than that of Pd/Zr_0.5Ce_0.5O₂/SiC. The Pd/Zr_0.5Ce_0.5O₂/SiC catalyst can ignite the reaction at 240 °C and obtain a methane conversion of 100% at 340 °C, and keep 100% methane conversion after 10 reaction cycles. These results indicate that active metallic nanoparticles are well stabilized on the SiC surface while the promoters serve as oxygen reservoir and retain good redox properties.