Opposite Face Sensitivity of CeO2 in Hydrogenation and Oxidation Catalysis

The determination of structure–performance relationships of ceria in heterogeneous reactions is enabled by the control of the crystal shape and morphology. Whereas the (100) surface, predominantly exposed in nanocubes, is optimal for CO oxidation, the (111) surface, prevalent in conventional polyhedral CeO₂ particles, dominates in C₂H₂ hydrogenation. This result is attributed to the different oxygen vacancy chemistry on these facets. In contrast to oxidations, hydrogenations on CeO₂ are favored over low‐vacancy surfaces owing to the key role of oxygen on the stabilization of reactive intermediates. The catalytic behavior after ageing at high temperature confirms the inverse face sensitivity of the two reaction families.     

Water Adsorption and Dissociation on Ceria‐Supported Single‐Atom Catalysts: A First‐Principles DFT+U Investigation

Single‐atom catalysts have attracted wide attention owing to their extremely high atom efficiency and activities. In this paper, we applied density functional theory with the inclusion of the on‐site Coulomb interaction (DFT+U) to investigate water adsorption and dissociation on clean CeO₂(111) surfaces and single transition metal atoms (STMAs) adsorbed on the CeO₂(111) surface. It is found that the most stable water configuration is molecular adsorption on the clean CeO₂(111) surface and dissociative adsorption on STMA/CeO₂(111) surfaces, respectively. In addition, our results indicate that the more the electrons that transfer from STMA to the ceria substrate, the stronger the binding energies between the STMA and ceria surfaces. A linear relationship is identified between the water dissociation barriers and the d band centers of STMA, known as the generalized Brønsted–Evans–Polanyi principle. By combining the oxygen spillovers, single‐atom dispersion stabilities, and water dissociation barriers, Zn, Cr, and V are identified as potential candidates for the future design of ceria‐supported single‐atom catalysts for reactions in which the dissociation of water plays an important role, such as the water–gas shift reaction.     

Infrared spectroscopic study on the reaction mechanism of CO hydrogenation over Pd/CeO2

In situ infrared spectroscopy was applied to elucidate the reaction mechanism of CO hydrogenation over Pd/CeO₂. Instead of direct dissociation of CO, a new reaction pathway is proposed for methane formation, involving geminal dicarbonyl intermediates and (HCO)₂(a) intermediates, which may be located on the surface of Pd covered with thin layers of reduced ceria (SMSI effect). Transformation of methane formation sites into methanol formation ones by the oxidation with water vapor formed during the CO?H₂ reaction is proposed, which may be located on the Pd (111) planes adjacent to ceria support.     

Surface and structural properties of pt/ceo2catalyst under preferential co oxidation in hydrogen (PROX)

Summary Preferential oxidation of CO in the presence of excess hydrogen was studied on Pt/CeO₂with 5% metal loading. Catalytic data were similar to those observed on 1% Pt/CeO₂earlier [16]. The optimum temperature region is T￡373 K; conversion and selectivity of CO oxidation strongly decreased at higher temperatures. High-pressure XPS indicated CO adsorbed on platinum particles and significant amount of water on the ceria surface. The top-most ceria surface re-oxidized as small amount of oxygen (3%) was introduced into the H₂/CO feed. Despite this surface re-oxidation, high-resolution TEM after reaction indicated oxygen deficient ceria bulk structure, in which the defects formed a super-cell, with CeO_1.695structure. The defective ceria is suggested to play an important role stabilizing the hydrogen bonded surface water, which (i) suppresses further hydrogen oxidation and (ii) reacts at the metal/support interface with linearly adsorbed CO in a low temperature water-gas-shift type reaction to produce CO₂.</o:p>     

Infrared spectroscopic study on the reaction mechanism of CO hydrogenation over Pd/CeO2

In situ infrared spectroscopy was applied to elucidate the reaction mechanism of CO hydrogenation over Pd/CeO₂. Instead of direct dissociation of CO, a new reaction pathway is proposed for methane formation, involving geminal dicarbonyl intermediates and (HCO)₂(a) intermediates, which may be located on the surface of Pd covered with thin layers of reduced ceria (SMST effect). Transformation of methane formation sites into methanol formation ones by the oxidation with water vapor formed during the CO-H₂ reaction is proposed, which may be located on the Pd (111) planes adjacent to ceria support.     

Interaction of Hydrogen with Ceria: Hydroxylation,Reduction, and Hydride Formation on the Surface and in the Bulk

The study reports the first attempt to address the interplay between surface and bulk in hydride formation in ceria (CeO₂) by combining experiment, using surface sensitive and bulk sensitive spectroscopic techniques on the two sample systems, i.e., CeO₂(111) thin films and CeO₂ powders, and theoretical calculations of CeO₂(111) surfaces with oxygen vacancies (O_v) at the surface and in the bulk. We show that, on a stoichiometric CeO₂(111) surface, H₂ dissociates and forms surface hydroxyls (OH). On the pre-reduced CeO_2−x samples, both films and powders, hydroxyls and hydrides (Ce−H) are formed on the surface as well as in the bulk, accompanied by the Ce³⁺ ↔ Ce⁴⁺ redox reaction. As the O_v concentration increases, hydroxyl is destabilized and hydride becomes more stable. Surface hydroxyl is more stable than bulk hydroxyl, whereas bulk hydride is more stable than surface hydride. The surface hydride formation is the kinetically favorable process at relatively low temperatures, and the resulting surface hydride may diffuse into the bulk region and be stabilized therein. At higher temperatures, surface hydroxyls can react to produce water and create additional oxygen vacancies, increasing its concentration, which controls the H₂/CeO₂ interaction. The results demonstrate a large diversity of reaction pathways, which have to be taken into account for better understanding of reactivity of ceria-based catalysts in a hydrogen-rich atmosphere.     

Shaped Ceria Nanocrystals Catalyze Efficient and Selective Para‐Hydrogen‐Enhanced Polarization

Intense para‐hydrogen‐enhanced NMR signals are observed in the hydrogenation of propene and propyne over ceria nanocubes, nano‐octahedra, and nanorods. The well‐defined ceria shapes, synthesized by a hydrothermal method, expose different crystalline facets with various oxygen vacancy densities, which are known to play a role in hydrogenation and oxidation catalysis. While the catalytic activity of the hydrogenation of propene over ceria is strongly facet‐dependent, the pairwise selectivity is low (2.4 % at 375 °C), which is consistent with stepwise H atom transfer, and it is the same for all three nanocrystal shapes. Selective semi‐hydrogenation of propyne over ceria nanocubes yields hyperpolarized propene with a similar pairwise selectivity of (2.7 % at 300 °C), indicating product formation predominantly by a non‐pairwise addition. Ceria is also shown to be an efficient pairwise replacement catalyst for propene.     

Roles of Oxygen Vacancies of CeO2 and Mn-Doped CeO2 with the Same Morphology in Benzene Catalytic Oxidation

Mn-doped CeO₂ and CeO₂ with the same morphology (nanofiber and nanocube) have been synthesized through hydrothermal method. When applied to benzene oxidation, the catalytic performance of Mn-doped CeO₂ is better than that of CeO₂, due to the difference of the concentration of O vacancy. Compared to CeO₂ with the same morphology, more oxygen vacancies were generated on the surface of Mn-doped CeO₂, due to the replacement of Ce ion with Mn ion. The lattice replacement has been analyzed through XRD, Raman, electron energy loss spectroscopy and electron paramagnetic resonance technology. The formation energies of oxygen vacancy on the different exposed crystal planes such as (110) and (100) for Mn-doped CeO₂ were calculated by the density functional theory (DFT). The results show that the oxygen vacancy is easier to be formed on the (110) plane. Other factors influencing catalytic behavior have also been investigated, indicating that the surface oxygen vacancy plays a crucial role in catalytic reaction.     

Growth and Electronic Properties of Ag Nanoparticles on Reduced CeO2-x(111) Films

Ag nanoparticles grown on reduced CeO_2-x thin films have been studied by X-ray photoelec-tron spectroscopy and resonant photoelectron spectroscopy of the valence band to understand the effect of oxygen vacancies in the CeO_2-x thin films on the growth and interfacial elec-tronic properties of Ag. Ag grows as three-dimensional particles on the CeO_2-x(111) surface at 300 K. Compared to the fully oxidized ceria substrate surface, Ag favors the growth of smaller particles with a larger particle density on the reduced ceria substrate surface, which can be attributed to the nucleation of Ag on oxygen vacancies. The binding energy of Ag3d increases when the Ag particle size decreases, which is mainly attributed to the final-state screening. The interfacial interaction between Ag and CeO_2-x(111) is weak. The resonant enhancement of the 4f level of Ce³⁺ species in RPES indicates a partial Ce⁴⁺→Ce³⁺ re-duction after Ag deposited on reduced ceria surface. The sintering temperature of Ag on CeO_1.85(111) surface during annealing is a little higher than that of Ag on CeO₂(111) surface, indicating that Ag nanoparticles are more stable on the reduced ceria surface.     

In Situ and Theoretical Studies for the Dissociation of Water on an Active Ni/CeO2 Catalyst: Importance of Strong Metal–Support Interactions for the Cleavage of O–H Bonds

Water dissociation is crucial in many catalytic reactions on oxide‐supported transition‐metal catalysts. Supported by experimental and density‐functional theory results, the effect of the support on O? H bond cleavage activity is elucidated for nickel/ceria systems. Ambient‐pressure O 1s photoemission spectra at low Ni loadings on CeO₂(111) reveal a substantially larger amount of OH groups as compared to the bare support. Computed activation energy barriers for water dissociation show an enhanced reactivity of Ni adatoms on CeO₂(111) compared with pyramidal Ni₄ particles with one Ni atom not in contact with the support, and extended Ni(111) surfaces. At the origin of this support effect is the ability of ceria to stabilize oxidized Ni²⁺ species by accommodating electrons in localized f‐states. The fast dissociation of water on Ni/CeO₂ has a dramatic effect on the activity and stability of this system as a catalyst for the water‐gas shift and ethanol steam reforming reactions.     

Insight into the Morphology-Dependent Catalytic Performance of CuO/CeO2 Produced by Tannic Acid for Efficient Hydrogenation of 4-Nitrophenol

The construction of a heterogeneous nanocatalyst with outstanding catalytic performance via an environmentally benign and cost-effective synthetic category has long been one of the challenges in nanotechnology. Herein, we synthesized highly efficient and low-cost mesoporous morphology-dependent CuO/CeO₂-Rods and CuO/CeO₂-Cubes catalysts by employing a green and multifunctional polyphenolic compound (tannic acid) as the stabilizer and chelating agent for 4-nitrophenol (4-NP) reduction reaction. The CuO/CeO₂-Rods exhibited excellent performance, of which the activity was 3.2 times higher than that of CuO/CeO₂-Cubes. This can be connected with the higher density of oxygen vacancy on CeO₂-Rods (110) than CeO₂-Cubes (100), the oxygen vacancy favors anchoring CuO species on the CeO₂ support, which promotes the strong interaction between finely dispersed CuO and CeO₂-Rods at the interfacial positions and facilitates the electron transfer from BH₄⁻ to 4-NP. The synergistic catalytic mechanism illustrated that 4-NP molecules preferentially adsorbed on the CeO₂, while H₂ from BH₄⁻ dissociated over CuO to form highly active H* species, contributing to achieving efficient hydrogenation of 4-NP. This study is expected to shed light on designing and synthesizing cost-effective and high-performance nanocatalysts through a greener synthetic method for the areas of catalysis, nanomaterial science and engineering, and chemical synthesis.     

Engineering surface oxygen vacancy of mesoporous CeO2 nanosheets assembled microspheres for boosting solar-driven photocatalytic performance

Surface oxygen vacancy defects of mesoporous CeO₂ nanosheets assembled microspheres（D-CeO₂） are engineered by polymer precipitation, hydrothermal and surface hydrogenation strategies. The resultant D-CeO₂ with a main pore diameter of 9.3 nm has a large specific surface area（～102.3 m²/g） and high thermal stability. The mesoporous nanosheets assembled microsphere structure prevents the nanosheets from aggregation, which is beneficial to effective mass tr...     

Identifying the O2 diffusion and reduction mechanisms on CeO2 electrolyte in solid oxide fuel cells: A DFT + U study

The interactions and reduction mechanisms of O₂ molecule on the fully oxidized and reduced CeO₂ surface were studied using periodic density functional theory calculations implementing on‐site Coulomb interactions (DFT + U) consideration. The adsorbed O₂ species on the oxidized CeO₂ surface were characterized by physisorption. Their adsorption energies and vibrational frequencies are within ?0.05 to 0.02 eV and 1530–1552 cm^?1, respectively. For the reduced CeO₂ surface, the adsorption of O₂ on Ce⁴⁺, one‐electron defects (Ce³⁺ on the CeO₂ surface) and two‐electron defects (neutral oxygen vacancy) can alter geometrical parameters and results in the formation of surface physisorbed O₂, O₂^a? (0 < a < 1), superoxide (O₂^?), and peroxide (O₂^2?) species. Their corresponding adsorption energies are ?0.01 to ?0.09, ?0.20 to ?0.37, ?1.34 and ?1.86 eV, respectively. The predicted vibrational frequencies of the peroxide, superoxide, O₂^a? (0 < a < 1) and physisorbed species are 897, 1234, 1323–1389, and 1462–1545 cm^?1, respectively, which are in good agreement with experimental data. Potential energy profiles for the O₂ reduction on the oxidized and reduced CeO₂ (111) surface were constructed using the nudged elastic band method. Our calculations show that the reduced surface is energetically more favorable than the unreduced surface for oxygen reduction. In addition, we have studied the oxygen ion diffusion process on the surface and in bulk ceria. The small barrier for the oxygen ion diffusion through the subsurface and bulk implies that ceria‐based oxides are high ionic conductivity at relatively low temperatures which can be suitable for IT‐SOFC electrolyte materials. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

Ceria supported Ru~0-Ru~(δ+) clusters as efficient catalyst for arenes hydrogenation

Selective hydrogenation of aromatic amines,especially chemicals such as aniline and bis(4-aminocyclohexyl)methane for non-yellowing polyurethane,is of particular interests due to the extensive applications.To conquer the existing difficulties,in selective hydrogenation,,the Ru~0-Ru~(δ+)/CeO_2 catalyst with solid frustrated Lewis pairs was developed for aromatic amines hydrogenation with excellent activity and selectivity under relative milder conditions.The morphology,electronic and chemical properties,especially the Ru~O-Ru~(δ+) clusters and reducible ceria were characterized by X-ray diffraction(XRD),transmission electron microscopy(TEM),sca nning electronic microscopy(SEM),X-ray photoelectron sp ectroscopy(XPS),CO_2 tempe rature programmed deso rption(CO_2-TPD),H_2 tempe rature programmed reduction(H_2-TPR),H_2 diffuse reflectance Fourier transform infrared spectroscopy(H_2-DRIFT),Raman,etc.The 2% Ru/CeO_2 catalyst exhibited good conversion of 95% and selectivity greater than 99% toward cyclohexylamine.The volcano curve describing the activity and Ru state was found.Owning to the "acidic site isolation" by surrounding alkaline sites,condensation between the neighboring amine molecules could be effectively suppressed.The catalyst also showed good stability and applicability for other aromatic amines and heteroarenes containing different functional groups.     

Design of nanostructured ceria-based solid electrolytes for development of IT-SOFC

A considerable interest has been shown in the application of doped ceria (CeO₂) compounds for “intermediate” (300–500 °C) temperature operation of solid oxide fuel cells. The microdomains with ordered structure of oxygen vacancy were observed in the microstructure of the M-doped CeO₂-sintered bodies (where M: Gd, Y, and Dy). We have previously shown that the conductivity of doped CeO₂-sintered bodies was lower when the sintered body contained large microdomains within grains. As a consequence of this observation, we have examined the grain size dependence and dopant content on conductivity in specimens where we adjust the microdomain size and a degree of oxygen vacancy ordering in the microdomains by controlling the microstructure. The microdomain size control in Dy-doped CeO₂ specimens was obtained by combining pulsed electric current sintering and conventional sintering. Using these techniques, we were able to improve the conductivity in Dy-doped CeO₂ specimens to a point where it became comparable to that of the more conventional Gd-doped CeO₂ specimens. It is concluded that by combining ultimate high-resolution analysis of these nanostructures with the adjusting processing route design, it is possible to further develop these materials in CeO₂-doped fuel cell application.     

Ionic Nickel Embedded in Ceria with High Specific CO2 Methanation Activity

CO₂ hydrogenation to methane is gaining increasing interest as one of the most promising ways to store intermittent renewable energy in the form of chemical fuels. Ni particles supported on CeO₂ represent a highly efficient, stable and inexpensive catalyst for this reaction. Herein, Ni-doped CeO₂ nanoparticles were tested for CO₂ methanation showing an extremely high Ni mass-specific activity and CH₄ selectivity. Operando characterization reveals that this performance is tightly associated with ionic Νi and Ce³⁺ surface sites, while formation of metallic Ni does not seem to considerably promote the reaction. Theoretical calculations confirmed the stability of interstitial ionic Ni sites on ceria surfaces and highlighted the role of Ce-O frustrated Lewis pair (FLP), Ni-O classical Lewis pair (CLP) and Ni-Ce pair sites to the activation of H₂ and CO₂ molecules. To a large extent, the theoretical predictions were validated by in situ spectroscopy under H₂ and CO₂ : H₂ gaseous environments.     

Phase Analysis and Oxygen Storage Capacity of Ceria-Lanthana-Based TWC Promoters Prepared by Sol-Gel Routes

Ceria-lanthana-based promoters of three-way catalysts are synthesized by two different sol-gel routes, involving nitrate precursors. The oxygen uptake ability of these compounds is measured by O₂ chemisorption. The specific surface area is determined by N₂ adsorption (BET). X-ray diffraction data are analyzed by Rietveld refinement, demonstrating that lanthanum forms solid solution with CeO₂; its total amount in ceria depends on the competitive formation of La-Al mixed oxides and on the synthetic method. The O₂ uptake ability is essentially determined by the La content in the ceria-lanthana solid solution, while it is independent on the surface area and on the CeO₂ particle size. The O₂ uptake ability increases with the La:Ce relative amount in the ceria-lanthana solid solution, but decreases beyond a La:Ce molar ratio greater than ?0.18. This behavior is ascribed to the stable association of vacancy-vacancy or vacancy dopant cation.     

Interface-controlled synthesis of CeO2(111) and CeO2(100) and their structural transition on Pt(111)

Ceria-based catalytic materials are known for their crystal-face-dependent catalytic properties. To obtain a molecular-level understanding of their surface chemistry, controlled synthesis of ceria with well-defined surface structures is required. We have thus studied the growth of CeO_x nanostructures (NSs) and thin films on Pt(111). The strong metal-oxide interaction has often been invoked to explain catalytic processes over the Pt/CeO_x catalysts. However, the Pt-CeO_x interaction has not been understood at the atomic level. We show here that the interfacial interaction between Pt and ceria could indeed affect the surface structures of ceria, which could subsequently determine their catalytic chemistry. While ceria on Pt(111) typically exposes the CeO₂(111) surface, we found that the structures of ceria layers with a thickness of three layers or less are highly dynamic and dependent on the annealing temperatures, owing to the electronic interaction between Pt and CeO_x. A two-step kinetically limited growth procedure was used to prepare the ceria film that fully covers the Pt(111) substrate. For a ceria film of ~3–4 monolayer (ML) thickness on Pt(111), annealing in ultrahigh vacuum (UHV) at 1000 K results in a surface of CeO₂ (100), stabilized by a c-Ce₂O₃(100) buffer layer. Further oxidation at 900 K transforms the surface of the CeO₂(100) thin film into a hexagonal CeO₂(111) surface.     

Exceptional susceptibility of ceria-supported rhodium catalyst to inhibitory SMSI effects including acetone hydrogenation

Complementary lines of evidence, as follows, are presented to show that hydrogen pretreatments of a ceria-supported rhodium catalyst (Rh-CeO₂) at rather moderate temperatures (653–573 K) cause this system to progress deeply into a strongly inhibitory SMSI state: (i) by comparative measurements, with a continuous-flow microcatalytic reactor system at a fixed reaction temperature of only 343 K, of the activities of samples of the Rh–CeO₂ material for hydrogenation of acetone vapour to isopropanol, after prereduction at temperatures in the range 413 - 653 K. Complete loss of hydrogenation activity is thus demonstrated after reduction at 653 K, but full restoration of activity is shown to be achieved upon reoxidation in O₂ at 673 K followed by re-reduction at 413 K; (ii) by selective identifications, using proton NMR, of hydrogen chemisorbed on the rhodium of Rh-CeO₂ and its differentiation from hydrogen absorbed by, or adsorbed onto, the ceria support.These results of selective characterizations of H-Rh upon Rh-CeO₂ samples prereduced in H₂ at temperatures in the range 473–573 K confirm a severe inhibition of the ability to produce H-Rh species after prereduction at 573 K, and thus provide complementary evidence for creation of an SMSI-state in Rh/CeO₂ after prereduction at this relatively low temperature. Additional information on rhodium-induced modifications of surface and bulk properties of Rh/CeO₂ samples exposed to vacuum, H₂ or O₂ at various temperatures emerges from comparative measurements of Rh/CeO₂ and CeO₂ using the techniques of electron spin resonance, volumetric evaluation of carbon monoxide chemisorption, and gravimetric monitoring of hydrogen absorption/adsorption.     

Oxidation of Reduced Ceria by Incorporation of Hydrogen

The interaction of hydrogen with reduced ceria (CeO_2?x) powders and CeO_2?x(111) thin films was studied using several characterization techniques including TEM, XRD, LEED, XPS, RPES, EELS, ESR, and TDS. The results clearly indicate that both in reduced ceria powders as well as in reduced single crystal ceria films hydrogen may form hydroxyls at the surface and hydride species below the surface. The formation of hydrides is clearly linked to the presence of oxygen vacancies and is accompanied by the transfer of an electron from a Ce³⁺ species to hydrogen, which results in the formation of Ce⁴⁺, and thus in oxidation of ceria.