Reduction Of CeO2 Research Articles

Electric Field Promoted Complete Oxidation of Benzene over PdCexCoy Catalysts at Low Temperature

The application of electric field promotes benzene oxidation significantly over Pd/CoxCey catalysts. For 1% Pd loading catalysts, the complete oxidation of benzene can be realized at 175 °C with an electric field under an input current of 3 mA, 79 °C lower than the temperature demanded for complete benzene conversion without electric field. The introduction of electric field can save Pd loading in the catalysts while maintaining high benzene conversion. The characterization experiments showed that CeO2 reduction was accelerated with electric field and created more active oxygen, promoting the formation of active sites on the catalyst surface. The OH removal ability of PdO was enhanced by forming CoO(OH) species, which can easily dehydroxylate since the reduction of Co3+ was promoted by the electric field. The optimized Ce/Co ratio is a balance between oxygen availability and OH removal ability.

Effect of EDTA in preparation of Ni catalysts toward a carbon-resistant ethanol reforming

Ni supported catalysts were prepared by an EDTA complexing method with different EDTA/Ni molar ratios: 0, 0.5, 1 and 2. The catalysts were characterised by X-ray diffraction, adsorption-desorption isotherms of N2, infrared spectroscopy, thermogravimetric analysis, temperature programmed reduction and X-ray photoelectron spectroscopy. All the catalysts prepared with EDTA presented a decrease in the NiO particle size of around 60 % with respect to the sample obtained in the absence of a chelating agent. Besides, the EDTA/Ni = 1 catalyst corresponding to the complete Ni chelation showed a Ce3+/Ce4+ high surface ratio. The greater extent of surface reduction of CeO2 suggests changes in the NiO-CeO2 interaction induced by the presence of the chelating agent during preparation. The NiEDTA(1) sample showed the best performance in the ethanol steam reforming reaction at 650 °C. An ethanol conversion of 100 % was reached after seven hours in reaction with a high yield of hydrogen (5.4 mol H2/molC2H5OH) and a low carbon deposition rate (1.4 mgC/gcat h).

Charge transfer and spillover phenomena in ceria-supported iridium catalysts: A model study.

Iridium-based materials are among the most active bifunctional catalysts in heterogeneous catalysis and electrocatalysis. We have investigated the properties of atomically defined Ir/CeO2(111) model systems supported on Cu(111) and Ru(0001) by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, near ambient pressure X-ray photoelectron spectroscopy (NAP XPS), scanning tunneling microscopy, and temperature programmed desorption. Electronic metal-support interactions in the Ir/CeO2(111) system are accompanied by charge transfer and partial reduction of CeO2(111). The magnitude of the charge transfer depends strongly on the Ir coverage. The Ir/CeO2(111) system is stable against sintering upon annealing to 600 K in ultrahigh vacuum (UHV). Annealing of Ir/CeO2(111) in UHV triggers the reverse oxygen spillover above 450 K. The interaction of hydrogen with Ir/CeO2(111) involves hydrogen spillover and reversible spillover between 100 and 400 K accompanied by the formation of water above 190 K. Formation of water coupled with the strong reduction of CeO2(111) represents the dominant reaction channel upon annealing in H2 above 450 K. The interaction of Ir/CeO2(111) with oxygen has been investigated at moderate and NAP conditions. Additionally, the formation and stability of iridium oxide prepared by deposition of Ir in oxygen atmosphere was investigated upon annealing in UHV and under exposure to H2. The oxidation of Ir nanoparticles under NAP conditions yields stable IrOx nanoparticles. The stability of Ir and IrOx nanoparticles under oxidizing conditions is hampered, however, by encapsulation by cerium oxide above 450 K and additionally by copper and ruthenium oxides under NAP conditions.

A Review of Solar Thermochemical CO2 Splitting Using Ceria-Based Ceramics With Designed Morphologies and Microstructures.

This review explores the advances in the synthesis of ceria materials with specific morphologies or porous macro- and microstructures for the solar-driven production of carbon monoxide (CO) from carbon dioxide (CO2). As the demand for renewable energy and fuels continues to grow, there is a great deal of interest in solar thermochemical fuel production (STFP), with the use of concentrated solar light to power the splitting of carbon dioxide. This can be achieved in a two-step cycle, involving the reduction of CeO2 at high temperatures, followed by oxidation at lower temperatures with CO2, splitting it to produce CO, driven by concentrated solar radiation obtained with concentrating solar technologies (CST) to provide the high reaction temperatures of typically up to 1,500°C. Since cerium oxide was first explored as a solar-driven redox material in 2006, and to specifically split CO2 in 2010, there has been an increasing interest in this material. The solar-to-fuel conversion efficiency is influenced by the material composition itself, but also by the material morphology that mostly determines the available surface area for solid/gas reactions (the material oxidation mechanism is mainly governed by surface reaction). The diffusion length and specific surface area affect, respectively, the reduction and oxidation steps. They both depend on the reactive material morphology that also substantially affects the reaction kinetics and heat and mass transport in the material. Accordingly, the main relevant options for materials shaping are summarized. We explore the effects of microstructure and porosity, and the exploitation of designed structures such as fibers, 3-DOM (three-dimensionally ordered macroporous) materials, reticulated and replicated foams, and the new area of biomimetic/biomorphous porous ceria redox materials produced from natural and sustainable templates such as wood or cork, also known as ecoceramics.

Exploration of Mn incorporated CeO2 nanoflakes with meso- and macropores for the effective simultaneous catalytic oxidation of carbon monoxide and propane

Highly porous ceria nanoflakes are prepared by a facile citric acid assisted sol–gel method. A Mn incorporated solid solution of CeO2 was also efficiently developed without disturbing the nanoflake morphology of pure CeO2. The formation of fluorite CeO2 with crystallite size ranging from 5–8 nm is evidently revealed from XRD analysis. The introduction of Mn to CeO2 resulted in the formation of smaller crystallites of CeO2 with reduced particle size and lattice parameters. SEM images displayed the flake like nature of CeO2 and Mn/CeO2 with meso and macro pores. TEM images confirm the pore size reduction of CeO2 with Mn doping which is already indicated from BET-BJH-surface area and pore-volume analysis. The presence of a large amount of oxygen vacancies in CeO2 upon Mn introduction is indicated from the drastic reduction in the peak intensities in the Raman spectrum. XPS results revealed that Ce exists in its +4 oxidation state whereas Mn exists in its +3 oxidation state, both of which are favorable for oxidation reactions. The catalytic performance of the prepared nanoflakes is investigated in the simultaneous oxidation of CO and C3H8. Mn incorporated CeO2 showed 100% oxidation of CO from 300 °C onwards and 84% hydrocarbon oxidation is observed even at a high gas flow rate of 500 ml/min at 400 °C over 0.5 g of the catalyst.

Synthesis and Investigations of Structural, Optical and AC Conductivity Properties of PANI/CeO2 Nanocomposites

This paper explains the details on the preparation of polyaniline/CeO2 (10 wt %) nanocomposites using the method of in situ polymerization. The formation of PANI/CeO2 nanocomposites were proposed via oxidation of aniline and reduction of CeO2, respectively. The effect of CeO2 concentration on AC conductivity, morphological, optical and structural properties of the prepared PANI/CeO2 nanocomposites material were examined. The structural studies show the presence of CeO2 nanoparticles were occurred in PANI of nanocomposites. The average grain size of PANI/CeO2 nanocomposites varied with increasing and decreasing concentration due to the process of nucleation during the polymerization. The average particle grain sizes were measured from HRTEM. The morphology analysis was carried out from SEM. The UV absorption spectrum showed that the absorption peak of CeO2 nanoparticle at 341 nm, shifted to lower wavelength side, the nanocomposite absorption peaks at about 324, 368 and 858 nm was confirmed by the formation of PANI/CeO2 nanocomposites. The electronic structure of the band gap of the nanocomposite materials were calculated using ultraviolet visible absorption spectrum. The band gap energy of nanocomposite is 3.36 eV. The FTIR spectrum clearly showed the strong presence of CeO2 nanoparticles in PANI chain. The AC conductivity varied with varying the temperature and depends on the concentration due to the formation of PANI/CeO2 nanocomposite.

In situ Raman study of laser‐induced stabilization of reduced nanoceria (CeO2−x) supported on graphene

AbstractThis work demonstrates a novel approach to application of in situ Raman spectroscopy to study laser‐induced stabilization of reduced nanoceria (CeO2−x) supported on graphene—a promising nanocomposite for future development of nanomaterial‐enabled gas sensors or vital catalysts for hydrogenation of alcohols under anaerobic conditions. Structural stabilization of CeO2−x nanoparticles (NPs) on a graphene surface is evidenced by significant modification of Raman spectra—the appearance, increase in relative intensity, and low‐wavenumber shift of CeO2 F2g band at excitation laser powers higher than ~5 mW. The effect is related to the reduction in a number of oxygen vacancies in CeO2−x NPs. Analysis of the graphene 2D and G band wavenumbers through ω2D(ωG) correlation indicated a decrease in p‐type graphene doping attributed to the charge transfer between the stabilizing CeO2−x NPs and the graphene that occurs due to trapping of graphene mobile holes by oxygen vacancies in CeO2−x. High‐resolution transmission electron microscopy analysis supported this idea by showing the increased lattice constant of fluorite‐type hexagonal‐shaped CeO2−x NPs as compared with bulk CeO2 and which could be related to partial reduction of CeO2−x NPs as a result of the Ce4+ transformation to Ce3+ with formation of corresponding oxygen vacancies. X‐ray photoelectron spectroscopy data confirmed the mixed Ce4+/Ce3+ valence state of the CeO2−x NPs. Thus, by combining the structure conversion of nanoceria induced by electromagnetic radiation and its stabilization by the graphene support, this work provides a foundation for advanced concepts in the development of the next‐generation catalysts and sensors.

Structure-dependent activity of CeO2 supported Ru catalysts for CO2 methanation

In this study, CeO2 rods (CeO2/r), cubes (CeO2/c) and octahedra (CeO2/o) supported catalysts with identical Ru particle size were prepared. Trend in the activity of these catalysts for CO2 methanation was compared with the trend in their oxygen vacancy concentration measured after calcination. Ru/CeO2/r outperforms the other two catalysts with a reaction rate of 11.0 × 10−8 mols-1mRu-2 and selectivity to methane of 99% at 250 °C. Temperature-programmed reduction (TPR), Raman and X-ray photoemission spectroscopy (XPS) results confirms that Ru addition enhances reduction of CeO2. Also, Ru/CeO2/r is more reducible and contains more oxygen vacancies as compared to Ru/CeO2/o and Ru/CeO2/c, both after calcination as well as under reducing conditions. H2 consumption during TPR shows removal of oxygen equivalent to about 3 monolayers, implying diffusion of vacancies into the subsurface or bulk of CeO2. The catalyst with the highest concentration of oxygen vacancies is also the most active catalyst, suggesting that reactive adsorption CO2 at an oxygen vacancy is the rate determining step.

Formation of Pd-Ce intermetallic compounds by reductive metal-support interaction

The intermetallic compound (IC) formation between Pd and Ce via reductive metal-support interaction (RMSI) has been studied using two different Pd/CeO2 materials: A thin film of Pd particles embedded in fluorite-type CeO2, grown on vacuum-cleaved NaCl(001) facets and CeO2 powder impregnated with small Pd particles. The reduction of CeO2 and formation of different Pd-Ce intermetallic compounds has been monitored by diffraction in an electron microscope and synchrotron-based in situ X-ray diffraction, and is complemented by density functional theoretical calculations (DFT) of the formation enthalpies to judge the relative stabilities of the formed intermetallic compounds. On both studied materials, the formation of similar ICs, namely CePd3 and CePd5, has been observed for the first time at temperatures of 1100 K following RMSI and, thus, indicates that any catalytic pre-treatment in hydrogen, provided the reduction temperature is high enough, must also be considered for the Pd-CeO2 system. The studies are complemented by a brief review of the Pd-Ce phase diagram to discuss the potential pathways of Pd-Ce intermetallic compound formation.

Understanding methanol synthesis from CO/H2 feeds over Cu/CeO2 catalysts

Understanding copper-based methanol synthesis catalysts as a function of catalyst type and applied reaction conditions is an area of active industrial and academic research. In this work, results of a methanol synthesis study over a Cu/CeO2 catalyst using CO/H2 feeds are presented, using catalyst performance data combined with catalyst characterisation information (DRIFTS, XPS, TPR, XRD) obtained during start-up and under steady state methanol synthesis conditions. The results indicate that the active site and reaction mechanism for methanol synthesis over Cu/CeO2 are different from the conventional Cu/ZnO/Al2O3 catalyst, with CO, rather than CO2, being the carbon source for methanol. Fixed-bed micro reactors were employed to obtain catalyst performance data in discrete bed sectors using experimental spatial discretisation methods. Methanol synthesis activity over Cu/CeO2 from CO/H2 is preceded by transient CO2 formation, with the onset of methanol synthesis activity observed when the CO2 formation reaches its peak. Considerable differences between reactor bed sectors are observed during start-up, with CO2 formation, CO2 re-adsorption (as surface carbonates and formates), methanol formation and transient methanol decomposition occurring to different degrees and at different time scales.The interfaces of defective CeO2−x in contact with highly dispersed copper particles form the active site of the catalyst under steady-state methanol synthesis conditions, with the copper oxidation state being a combination of 0 and +1 states. The CeO2−x defect sites are formed during start-up upon reaction of CeO2 with CO. The CO2 thus formed leads to catalyst deactivation due to build-up of carbonate and formate species on the catalyst surface. A gradient of progressive poisoning/deactivation is observed going further down the catalyst bed, where the level of deactivation can be controlled by modifying the CO content in the feed. CO2 thus acts as a poison for the Cu/CeO2 catalyst, and addition of even low levels of CO2 to the feed leads to an evenly deactivated catalyst. During start-up in CO/H2, transient CO2 and H2O formation is also observed in the case of Cu/ZnO/Al2O3, ascribed to partial reduction of ZnO, similar to the partial reduction of CeO2 as seen in Cu/CeO2. The key difference between the two systems is the absence of CO2 and H2O re-adsorption on Cu/ZnO/Al2O3, resulting in stable methanol production distributed equally over the catalyst bed. Introduction of CO2 to the CO/H2 feed in case of Cu/ZnO/Al2O3 leads to reduction of Cu+ to Cu0 and a change in methanol synthesis mechanism from CO to CO2 hydrogenation.

Preferential Oxidation of CO in H2-Rich Stream Over Au/CeO2–NiO Catalysts: Effect of the Preparation Method

Two methods, viz., the hydrothermal (HT) and co-precipitation (CP) methods, were used to prepare CeO2–NiO composite oxides; with them as the supports, Au/CeO2–NiO catalysts were prepared by the colloidal deposition method and used in the preferential oxidation (PROX) of CO in H2-rich stream. Various characterization measures such as N2 sorption, XRD, TEM, H2-TPR, Raman spectroscopy and XPS were used to clarify the influence of preparation method on the structure of CeO2–NiO support and the performance of Au/CeO2–NiO catalyst. The XPS and TEM results reveal that the CeO2–NiO(HT) support prepared by hydrothermal method displays a uniform rod-like shape and exposes preferentially the (110) and (100) planes of CeO2, whereas the CeO2–NiO(CP) support prepared by co-precipitation method is composed of nanorods and irregular nanoparticles dominated by (111) facets of CeO2. After deposition of gold, both the Au/CeO2–NiO(HT) and Au/CeO2–NiO(CP) catalysts are alike in the state and size distribution of deposited Au nanoparticles. The H2-TPR results indicate that the presence of Au strongly promotes the reduction of CeO2 in the Au/CeO2–NiO catalyst. Raman spectra illustrate that the incorporation of Ni ions into CeO2 remarkably increases the amount of oxygen vacancies in the CeO2–NiO supports, especially in CeO2–NiO(HT) prepared by hydrothermal method, which is beneficial to the dispersion and stabilization of gold species. The structure of CeO2–NiO support and catalytic activity of Au/CeO2–NiO in CO PROX is strongly related to the preparation method; Au/CeO2–NiO(HT) exhibits much higher activity than Au/CeO2–NiO(CP). The larger fraction of (110) and (100) CeO2 facets in CeO2–NiO(HT) can promote the dispersion of gold species, formation of oxygen vacancies and migration of oxygen species, which are effective to enhance the redox capacity and activity of the obtained Au/CeO2–NiO(HT) catalyst for CO PROX in H2-rich stream. Au supported on CeO2–NiO nanorods prepared by hydrothermal method exhibits much higher catalytic activity for CO PROX in H2-rich stream.

Combined Ceria Reduction and Methane Reforming in a Solar-Driven Particle-Transport Reactor.

We report on the experimental performance of a solar aerosol reactor for carrying out the combined thermochemical reduction of CeO2 and reforming of CH4 using concentrated radiation as the source of process heat. The 2 kWth solar reactor prototype utilizes a cavity receiver enclosing a vertical Al2O3 tube which contains a downward gravity-driven particle flow of ceria particles, either co-current or counter-current to a CH4 flow. Experimentation under a peak radiative flux of 2264 suns yielded methane conversions up to 89% at 1300 °C for residence times under 1 s. The maximum extent of ceria reduction, given by the nonstoichiometry δ (CeO2−δ), was 0.25. The solar-to-fuel energy conversion efficiency reached 12%. The syngas produced had a H2:CO molar ratio of 2, and its calorific value was solar-upgraded by 24% over that of the CH4 reformed.

Effect of sulfur aging and regeneration on low temperature NO adsorption over hydrothermally treated Pd/CeO2 and Pd/Ce0.58Zr0.42O2 catalysts

Various treatments such as hydrothermal aging (HTA), sulfur aging (SA), and regeneration (DeSOx) were applied to Pd/CeO2 and Pd/Ce0.58Zr0.42O2 (Pd/Ce58) catalysts to understand the effect of sulfur aging and regeneration on the changes in NO adsorption ability and physicochemical properties of catalysts. The addition of Pd to Ce-based supports results in the larger amount of NO adsorbed at higher desorption temperature (250–500°C) than CeO2. Also, Pd/CeO2 has larger amount of NO adsorbed than Pd/Ce58, indicating that ceria is the active site for NO adsorption at low temperature. However, SA treatment on both catalysts gives rise to the negligible NO adsorption ability since it leads to form thermodynamically more stable Ce(SO4)2. Structural and textural analysis after regeneration shows that Pd/Ce58 maintains the pore size distribution with the similar crystalline size, whereas Pd/CeO2 does not, despite the similar amount of residual sulfur over two samples. In addition, Pd dispersion is recovered completely over the former sample, although the latter does not. However, the fact that both regenerated catalysts do not recover the NO adsorption ability to the level of HTA ones implies that either textural property or Pd dispersion cannot account for the degradation of NO adsorption at low temperature. Instead, H2-TPR analysis indicates that the intimate interaction Pd and ceria is lost for both samples after regeneration, as evidenced by the disappearance of the simultaneous reduction of CeO2 and PdO below 20°C. Hence, it can be concluded that the exposure of SO2 and the subsequent regeneration significantly affects the interaction between PdO and CeO2, resulting in the irreversible decrease in the NO adsorption ability at low temperature.

Modeling of a tubular solar reactor for continuous reduction of CeO2: The effect of particle size and loading on radiative heat transfer and conversion

A solar multi-tubular reactor for non-stoichiometric reduction of CeO2 is modeled under continuous operation. An aerosol, consisting on CeO2 particles and Argon, flows upwards through the reactor vertical tubes. Heat, mass and radiation transfer phenomena are efficiently implemented in an axisymmetric domain by using multi-mesh, multi-step, Finite-Volume and Monte Carlo methods. Reaction, particle diffusion, conduction, forced convection as well as radiation absorption, emission and anisotropic scattering are considered. The kinetic model for the non-stoichiometric reduction of CeO2 is taken from Ishida et al. (2014). Model results at steady-state focus on the effect of changing particle loading and diameter at different average residence times. For particle diameters of 1–20μm, increasing particle size favors uniform radiation absorption, minimizing temperature gradients. Finally, for an outer tube surface temperature of 2500K, a particle loading of 0.1kg/m3 and average residence time of 30–60s are recommended.

Structure and Reducibility of CeO2 Doped with Trivalent Cations

The doping of CeO2 with trivalent cations is a common technique for enhancing ionic conductivity in electrolytes for solid oxide fuel cell applications. However, the local defect structure in these materials is yet to be fully explored. Furthermore, many studies have overlooked the effect of the dopants on the reducibility of CeO2, which is important as electronic conductivity can short-circuit the fuel cell. Density functional theory (DFT)+U calculations have been performed on a series of CeO2 systems doped with trivalent cations. The most stable configuration and the relative attraction between dopant cations and oxygen vacancies were determined, and it was found that the defect structure is principally dependent on the ionic radius of the dopant cations. The reduction energy was found to be dependent on the structure around the dopants but did not vary significantly between dopants of similar ionic radii. From these results, it is possible to suggest which trivalent cations would be most suitable to en...

Influence of defect distribution on the reducibility of CeO2−x nanoparticles

Ceria nanoparticles (NPs) are fundamental in heterogeneous catalysis because of their ability to store or release oxygen depending on the ambient conditions. Their oxygen storage capacity is strictly related to the exposed planes, crystallinity, density and distribution of defects. In this work a study of ceria NPs produced with a ligand-free, physical synthesis method is presented. The NP films were grown by a magnetron sputtering based gas aggregation source and studied by high resolution- and scanning-transmission electron microscopy and x-ray photoelectron spectroscopy. In particular, the influence of the oxidation procedure on the NP reducibility has been investigated. The different reducibility has been correlated to the exposed planes, crystallinity and density and distribution of structural defects. The results obtained in this work represent a basis to obtain cerium oxide NP with desired oxygen transport properties.

Mechanochemical Synthesis of High Crystalline Cerium Hexaboride Nanoparticles from CeO2‐B2O3‐Mg Ternary System

AbstractHigh crystalline cerium hexaboride (CeB6) nanoparticles (NPs) were synthesized using mixture of mag‐ nesium (Mg), cerium oxide (CeO2) and boron oxide (B2O3) via the mechanochemical process at room tem‐ perature. Based on the results, magnesiothermic reduction of B2O3 occurred after about 2 h of milling in a mechanically induced self‐sustaining reaction (MSR). The significant amount of heat produced by the reduction reaction resulted in CeO2 reduction to elemental Ce which finally reacted with elemental B and formed CeB6 compound. According to XRD analyses, the degree of crystallinity and lattice parameter of the product was calculated about 93 % and 4.1458 Å, respectively. The morphology observations revealed that the synthesized CeB6 had semi‐cubic shape with the range of size 25–60 nm. The synthesis of CeB6 during the thermal treatment was studied by simultaneous thermal analysis (STA) technique. It was found that the reduction of B2O3 took place after melting of Mg meanwhile, no CeB6 phase achieved even up to 1100 °C.

CeO2-La2O3/ZSM-5 sorbents for high-temperature H2S removal

Performance of CeO2-La2O3/ZSM-5 sorbents for sulfur removal was examined at temperature ranging from 500 oC to 700 oC. The sulfur capacity of 5Ce5La/ZSM-5 was much bigger than that of CeO2/ZSM-5. H2 had a negative impact on the sulfidation; however, CO had little influence on sulfur removal. The characterization results showed that CeO2 and La2O3 were well dispersed on ZSM-5 because of the intimate admixing of La2O3 and CeO2, the major sulfidation products were Ce2O2S and La2O2S, the XRD and SEM results revealed that ZSM-5 structure could remain intact during preparation and sulfidation process, the H2-TPR showed that the reducibility of CeO2 can be remarkably enhanced by addition of La.

Nanostructured Mn-doped ceria solid solutions for efficient oxidation of vanillyl alcohol

Ceria is the hub of constant and diverse research interest due to its wide industrial applications. In the present work, we have developed Mn-doped CeO2 catalysts for vanillyl alcohol oxidation. Accordingly, we synthesized nanostructured Mn-doped CeO2 catalysts with various concentration of MnOx by an economical coprecipitation method and characterized using XRD, ICP-OES, BET surface area, Raman spectroscopy, TEM, XPS, and H2-TPR techniques. XRD and TEM results confirmed formation of nanocrystalline solid solutions with the incorporation of Mn into ceria lattice. ICP-OES analysis confirmed actual amount of MnOx loadings in the respective catalysts. The BET surface area of CeO2 significantly enhanced after the incorporation of MnOx dopant. Raman results disclosed that the incorporation of MnOx into ceria lattice greatly enhances the oxygen vacancies, which are responsible for the improved catalytic activity. XPS analysis revealed that Ce and Mn exist in +3, +4 and +2, +3, +4 oxidation states, respectively, on the surface of the samples. H2-TPR results showed pronounced enhancement in the reduction of CeO2 by MnOx doping. Catalytic activity studies confirmed that among all the catalysts, the Ce0.7Mn0.3O2−δ showed superior activity for vanillyl alcohol oxidation with 89% conversion and 95% selectivity, which could be attributed to higher concentration of oxygen vacancies, a high BET surface area, low temperature reducibility, and facile synergetic interaction between CeO2 and MnOx.

Experimental and theoretical study about sulfur deactivation of Ni/ CeO2 and Rh/CeO2 catalysts

Sulfur deactivation of Ni/CeO2 and Rh/CeO2 catalysts were examined through an experimental and theoretical study. These catalysts were characterized by N2 adsorption, X-ray diffraction, temperature programmed reaction, thermogravimetric analysis, Uv–visible spectroscopy and Raman spectroscopy, and tested under the methane dry reforming reaction in the presence of H2S. On the other hand, different possible interactions of sulfur with Rh, Ni or surface sites of the CeO2 support were evaluated by performing energy calculations with the density functional theory (DFT). Overall, the results indicate that tolerance to sulfur of Rh/CeO2 catalyst is higher than that of Ni/CeO2 one. In this sense, TPR measurements show that reduction of CeO2 is promoted by the presence of Rh. This effect, probably caused by hydrogen spillover to CeO2 support during the reduction of RhOx species, could be linked to a high oxygen donation capacity of Rh/CeO2 catalysts. Accordingly, the O2− species existing on Rh/CeO2 catalysts, revealed by Raman spectra of these samples, could favor sulfur oxidation and prevent Rh–S interactions. Likewise, the theoretical calculations show that desorption of S–O species from Rh/CeO2 system is more favorable than that from Ni/CeO2 one. Therefore, our experimental and theoretical study about sulfur deactivation of Ni and Rh supported on CeO2 allow us to postulate that Rh can help to desorb SOx species formed on the support, retarding sulfur poisoning of the Rh/CeO2 catalysts.