Stoichiometric CeO2 Research Articles

Hydrogen diffusion in cerium oxide thin films fabricated by pulsed laser deposition

Cerium oxide (CeO2) is well known to be reducible by hydrogen (H2), yet the diffusion and solution properties of hydrogen in ceria at elevated temperatures have remained challenging to evaluate. We therefore fabricated nanometer-thin (∼100 nm) cerium oxide films on Si(111) substrates by pulsed laser deposition (PLD) and quantitatively investigated the H depth distributions therein by means of resonant 1H(15N,αγ)12C nuclear reaction analysis (NRA) before and after annealing in H2 gas at 773–973 K. X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) reveal that the as-deposited films exhibit single phase CeO2 structure and partially reduced stoichiometry (CeO1.69). H2 annealing does not largely change the H content of the as-deposited films; in all conditions several atomic percent of hydroxyl (OH) are found to exist in a thin (∼4 nm) surface layer, whereas stably bound hydrogen in the bulk of the films is almost uniformly distributed and of much smaller quantity (∼0.2 at.%) than the oxygen vacancy concentration in the partially reduced ceria. Its low concentration and high thermal stability identify this bulk H species as likely being strongly bound to defects in the polycrystalline films rather than as a hydride species that interacts weakly with O-vacancies. The H diffusion coefficient and activation energy in the ceria films are determined as > 10−18 m2 s−1 at 773–973 K and <1.69 eV, respectively. The observed diffusion activation energy is somewhat larger than theoretical predictions for thermal diffusion of H in stoichiometric bulk CeO2, suggesting that defects and oxygen vacancies in the PLD-fabricated ceria thin films possibly have an impact on the H mobility.

Bulk and Surface Contributions to Ionisation Potentials of Metal Oxides

AbstractDetermining the absolute band edge positions in solid materials is crucial for optimising their performance in wide‐ranging applications including photocatalysis and electronic devices. However, obtaining absolute energies is challenging, as seen in CeO2, where experimental measurements show substantial discrepancies in the ionisation potential (IP). Here, we have combined several theoretical approaches, from classical electrostatics to quantum mechanics, to elucidate the bulk and surface contributions to the IP of metal oxides. We have determined a theoretical bulk contribution to the IP of stoichiometric CeO2 of only 5.38 eV, while surface orientation results in intrinsic IP variations ranging from 4.2 eV to 8.2 eV. Highly tuneable IPs were also found in TiO2, ZrO2, and HfO2, in which surface polarisation plays a pivotal role in long‐range energy level shifting. Our analysis, in addition to rationalising the observed range of experimental results, provides a firm basis for future interpretations of experimental and computational studies of oxide band structures.

Bulk and Surface Contributions to Ionisation Potentials of Metal Oxides.

Determining the absolute band edge positions in solid materials is crucial for optimising their performance in wide-ranging applications including photocatalysis and electronic devices. However, obtaining absolute energies is challenging, as seen in CeO2 , where experimental measurements show substantial discrepancies in the ionisation potential (IP). Here, we have combined several theoretical approaches, from classical electrostatics to quantum mechanics, to elucidate the bulk and surface contributions to the IP of metal oxides. We have determined a theoretical bulk contribution to the IP of stoichiometric CeO2 of only 5.38 eV, while surface orientation results in intrinsic IP variations ranging from 4.2 eV to 8.2 eV. Highly tuneable IPs were also found in TiO2 , ZrO2 , and HfO2 , in which surface polarisation plays a pivotal role in long-range energy level shifting. Our analysis, in addition to rationalising the observed range of experimental results, provides a firm basis for future interpretations of experimental and computational studies of oxide band structures.

Interaction of Water with Ceria Thin Film

AbstractWater interaction with ceria is a fundamental process underpinning catalytic reactions such as water‐gas‐shift and water electrolysis. This review summarizes the investigations of water interactions with the ceria low‐index surfaces [(111), (100), and (110)]. The first part introduces: (a) The atomic structures of the stoichiometric CeO2 and reduced CeOx (x=1.5∼2) thin films, including the characteristics of oxygen vacancies; (b) The anisotropic migration mechanisms of oxygen vacancies and the locations of the entangled polarons in the reduced ceria. The second part reviews water‘s adsorption, desorption, dissociation, and redox properties on the ceria low‐index surfaces studied by DFT, TPD, XPS, RPES, TEM, and SPM methods. The impacts of water coverage, ceria film thickness, strain effect, dopant effect, face stability, and solid‐liquid interface on the interaction between water and ceria are also discussed. At last, this review sheds some light on the future directions and challenges in this topic.

Raman spectroscopic study of non‐stoichiometry in cerianite from critical zone

AbstractCerianite (CeO2) is one of the key minerals controlling the behavior of rare earth elements (REE) in supergene environment, yet little is known about the composition and structure of the mineral. We present Raman spectroscopy and compositional data on regolith cerianite from carbonatite‐related supergene REE deposits. Cerianite showed a low content of impurities, distinguishing from high‐temperature environments, where the mineral is characterized by elevated concentrations of lanthanides other than Ce. The Raman spectra are characterized by the shift of the dominant peak located at 464–465 cm−1 in stoichiometric CeO2 to the lower wavenumbers of 450–455 cm−1, the widening of this peak with the increasing shift, and the presence of additional peaks at 225–230, 284, and 604–613 cm−1. The independence of the main spectral characteristics from the laser wavelength (tested by three lasers: 488, 532, and 633 nm) suggests that the features are genuine and not produced by REE luminescence. The anhydrous composition of supergene cerianite is indicated by the absence of Raman bands in the water‐bearing region of the spectra (2700–3200 cm−1). The spectral features of supergene cerianite, combined with the compositional data, suggest non‐stoichiometry in the mineral and the presence of Ce3+ with oxygen vacancies in the mineral structure. Our observations demonstrate the efficiency of Raman spectroscopy for the identification of the non‐stoichiometric composition of cerianite and its common occurrence in the critical zone.

Structural dynamics of Schottky and Frenkel defects in CeO2: a density-functional theory study

Cerium dioxide CeO2 (ceria) is an important material in catalysis and energy applications. The intrinsic Frenkel and Schottky defects can impact a wide range of material properties including the oxygen storage capacity, the redox cycle, and the ionic and thermal transport. Here, we study the impact of Frenkel and Schottky defects on the structural dynamics and thermal properties of ceria using density functional theory. The phonon contributions to the free energy are found to reduce the defect formation free energies at elevated temperature. The phonon dispersions of defective CeO2 show significant broadening of the main branches compared to stoichiometric ceria. Phonon modes associated with the defects are identifiable in the infrared spectra through characteristic shoulders on the main features of the stoichiometric fluorite structure. Finally, the presence of Frenkel and Schottky defects are also found to reduce the thermal conductivity by up to 88% compared to stoichiometric CeO2.

A self-circulating cerium-rich CeO2-x/Bi2MoO6 heterojunction catalyst for boosting photo-Fenton degradation of fluoroquinolone antibiotics

Stoichiometric CeO2 as a Fenton-like cocatalyst generally suffers from low H2O2 activation efficiency and multitudinous metal leaching because of its sluggish circulation of Ce3+/Ce4+ redox couple. Herein, a cerium-rich non-stoichiometric CeO2-x cocatalyst was developed to accelerate the circulation of Ce3+/Ce4+, which greatly promoted the H2O2 activation and avoided the production of metal sludge. In this case, cerium-rich CeO2-x nanodots were perfectly anchored onto Bi2MoO6 nanosheets to fabricate a self-circulating CeO2-x/Bi2MoO6 (CEO/BIM) photo-Fenton system. Systematic studies showed that the optimized CEO/BIM-3 (the mole ratio of Ce to Bi was 0.75) sample manifested the highest photo-Fenton degradation rate (0.026 min−1) of ofloxacin (OX) only under 5 W white LED irradiation, which was 8.66 and 6.50 times that of individual Bi2MoO6 and CeO2-x, respectively. The enhanced performance was mainly attributed to the excellent and continuous circulation of Ce3+/Ce4+, which can greatly accelerate the H2O2 activation to produce more OH radicals. This study demonstrates the great potential of the self-circulating photo-Fenton system in degrading antibiotic pollutants and provides an alternative strategy for designing stable and efficient catalysts.

In Situ Spectroscopy and Microscopy Insights into the CO Oxidation Mechanism on Au/CeO2(111)

In this work, we prepared and investigated in ultra-high vacuum (UHV) two stoichiometric CeO2(111) surfaces containing low and high amounts of step edges decorated with 0.05 ML of gold using synchrotron-radiation photoelectron spectroscopy (SRPES) and scanning tunneling microscopy (STM). The UHV study helped to solve the still unresolved puzzle on how the one-monolayer-high ceria step edges affect the metal-substrate interaction between Au and the CeO2(111) surface. It was found that the concentration of ionic Au+ species on the ceria surface increases with increasing number of ceria step edges and is not correlated with the concentration of Ce3+ ions that are supposed to form on the surface after its interaction with gold nanoparticles. We associated this with an additional channel of Au+ formation on the surface of CeO2(111) related to the interaction of Au atoms with various peroxo oxygen species formed at the ceria step edges during the film growth. The study of CO oxidation on the highly stepped Au/CeO2(111) model sample was performed by combining near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS), UHV-STM, and near-ambient-pressure STM (NAP-STM). This powerful combination provided comprehensive information on the processes occurring on the Au/CeO2(111) surface during the interaction with CO, O2, and CO + O2 (1:1) mixture at conditions close to the real working conditions of CO oxidation. It was found that the system demonstrates high stability in CO. However, the surface undergoes substantial chemical and morphological changes as the O2 is added to the reaction cell. Already at 300 K, gold nanoparticles begin to grow using a mechanism that involves the disintegration of small gold nanoparticles in favor of the large ones. With increasing temperature, the model catalyst quickly transforms into a system of primarily large Au particles that contains no ionic gold species.

Annealing free CeO2 electron transport layer for efficient perovskite solar cells

Electron transport material plays a critical role in perovskite solar cells. The most commonly used electron transport material, TiO2, has to be sintered at high temperature to achieve good crystallinity and high carrier mobility, complicating the preparation procedure and limiting the development of flexible devices. In this work, we firstly propose a kind of stoichiometric CeO2 film as annealing free electron transport layer for efficient perovskite solar cells. The CeO2 nanocrystals are prepared by solvothermal method. The obtained CeO2 nanocrystals can be well re-dispersed in cyclohexane to form a stable, homogeneous and transparent solution. As a result, we can prepare the CeO2 electron transport layer by spin-coating method and without any annealing or drying process. The performance of the CeO2 electron transport layer is regulated by optimizing the solvothermal reaction time. It is found that the CeO2 nanocrystals prepared under 48 ​h display the most excellent electron extraction performance. The corresponding device acquires a power conversion efficiency of 13.69%. The efficiency is further improved to 16.53% after introducing C60 interlayer. The work presents that CeO2 can be an efficient low temperature processed electron transport material for perovskite solar cells.

Redox-mediated C–C bond scission in alcohols adsorbed on CeO2− x thin films

The decomposition mechanisms of ethanol and ethylene glycol on well-ordered stoichiometric CeO2(111) and partially reduced CeO2−x (111) films were investigated by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, and temperature programmed desorption. Both alcohols partially deprotonate upon adsorption at 150 K and subsequent annealing yielding stable ethoxy and ethylenedioxy species. The C–C bond scission in both ethoxy and ethylenedioxy species on stoichiometric CeO2(111) involves formation of acetaldehyde-like intermediates and yields CO and CO2 accompanied by desorption of acetaldehyde, H2O, and H2. This decomposition pathway leads to the formation of oxygen vacancies. In the presence of oxygen vacancies, C–O bond scission in ethoxy species yields C2H4. In contrast, C–C bond scission in ethylenedioxy species on the partially reduced CeO2−x (111) is favored with respect to C–O bond scission and yields methanol, formaldehyde, and CO accompanied by the desorption of H2O and H2. Still, scission of C–O bonds on both sides of the ethylenedioxy species yields minor amounts of accompanying C2H4 and C2H2. C–O bond scission is coupled with a partial recovery of the lattice oxygen in competition with its removal in the form of water.

In situ NAP-XPS study of CO2 and H2O adsorption on cerium oxide thin films

Insights of CO2 and H2O activation on cerium oxide based catalysts are of great interest to many important catalytic processes, which are widely studied in model systems under ultrahigh vacuum conditions. Here, in situ near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) was used to study CO2 and H2O adsorption on fully oxidized CeO2 and partially reduced CeO2−x surfaces. It is found that CO2 only weakly interacts with the stoichiometric CeO2 but strongly adsorbs on partially reduced CeO2−x surfaces with surface carbonate formation. H2O dissociatively adsorbs reversely on the CeO2 surface with H2O exposure pressure above 1 × 10−3 mbar, while H2O strongly dissociates on the partially reduced CeO2−x surfaces. The surface dissociated OH and H species are stable on the CeO2−x surface up to 600 K. Both CO2 and H2O adsorption cause clear reoxidation of partially reduced CeO2−x. This study provides new insights into CO2 and H2O interaction with cerium oxide surfaces at near ambient pressure conditions.

Comparative study of single-atom gold and iridium on CeO2{111}.

Oxide-supported single-atom catalysts have shown promise for a variety of heterogeneous processes. In addition to their inherent activity and selectivity, these materials come at much lower financial cost, avoiding the use of full-bodied precious-metal catalysts, but at the conceptual expense that more complex structural and electronic considerations need to be understood if we are to exploit their full potential. Here, we focus on the adsorption of single-atom iridium at both stoichiometric and defective CeO2{111} surfaces, by means of first-principles density functional theory. Reference calculations for the adsorption of single-atom gold, on the same set of substrates, provide a valuable set of benchmarks against which to interpret our iridium results.

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 (Ov) 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 CeO2−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 Ov 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.

Formation of dimethyl carbonate via direct esterification of CO2 with methanol on reduced or stoichiometric CeO2(111) and (110) surfaces.

CeO2-Catalyzed esterification of CO2, a well-known greenhouse gas, with methanol has been widely recognized as a promising alternative method to produce dimethyl carbonate (DMC). Herein, we performed a comprehensive study of catalytic mechanisms underlying the formation of DMC from CO2 and methanol on both stoichiometric and reduced CeO2(111) and (110) surfaces. To this end, the saddle-point searching algorithm is employed. Specifically, using the monomethyl carbonate (MMC) as the key intermediate, a three-step Langmuir-Hinshelwood (LH) mechanism, including the formation and esterification of monomethyl carbonate and removal of water molecule, is identified for the catalytic DMC formation on either the reduced or the stoichiometric CeO2(111) and (110) surfaces. For both CeO2(111) and (110) surfaces, our study indicates that the presence of oxygen vacancies can markedly lower the activation energy barrier. Different rate-limiting steps are identified, however, for the reduced CeO2(111) and (110) surfaces. Successful identification of the rate-limiting step and the associated active CO2 species will provide atomic-level guidance on selection of metal-oxide-based catalysts toward direct synthesis of DMC from the green-house gas CO2 and methanol.

A computational investigation of the adsorption of small copper clusters on the CeO2(110) surface.

We report a detailed density functional theory (DFT) study of the geometrical and electronic properties, and the growth mechanism of a Cun (n = 1-4) cluster on a stoichiometric, and especially on a defective CeO2(110) surface with one surface oxygen vacancy, without using pre-assumed gas-phase Cun cluster shapes. This gives new and valuable theoretical insight into experimental work regarding debatable active sites of promising CuOx/CeO2-nanorod catalysts in many reactions. We demonstrate that CeO2(110) is highly reducible upon Cun adsorption, with electron transfer from Cun clusters, and that a Cun cluster grows along the long bridge sites until Cu3, so that each Cu atom can interact strongly with surface oxygen ions at these sites, forming stable structures on both stoichiometric and defective CeO2(110) surface. Cu-Cu interactions are, however, limited, since Cu atoms are distant from each other, inhibiting the formation of Cu-Cu bonds. This monolayer then begins to grow into a bilayer as seen in the Cu3 to Cu4 transition, with long-bridge site Cu as anchoring sites. Our calculations on Cu4 adsorption reveal a Cu bilayer rich in Cu+ species at the Cu-O interface.

Oxygen vacancies and alkaline metal boost CeO2 catalyst for enhanced soot combustion activity: A first-principles evidence

The molecular-level oxidation mechanism of graphene-type soot was systematically explored at the soot/CeO2(111) interface with and without oxygen vacancies (Ovac) by first-principles calculations. It was found to follow the Mars-van Krevelen mechanism on stoichiometric CeO2(111), but the low reactivity of lattice oxygen (Olat) results in a limited activity for oxidizing the most-inert edge-CH. By contrast, the molecular O2 species trapped by Ovac on reduced CeO2(111) exhibits a much superior reactivity to eliminate edge-CH, which can trigger subsequent inner-C oxidation to proceed easily. These results explicitly evidence the boosting role of Ovac and clarify the reduced CeO2, rather than the stoichiometric one, playing a more critical role for soot combustion in reality. Notably, increasing the initial surface Ovac concentration or decreasing Ovac formation energy to activate Olat are unambiguously proposed as effective optimization strategies for CeO2. Moreover, a long puzzle on the promotor effects of alkaline K+ on CeO2 was rationalized.

Theoretical Investigation of Water Adsorption Chemistry of CeO2(111) Surfaces by Density Functional Theory

Cerium oxide (ceria, CeO2) is one of the most wide-spread oxide supporting materials for the precious metal nanoparticle class of heterogeneous catalysts. Because ceria can store and release oxygen ions, it is an essential catalytic component for various oxidation reactions such as CO oxidation (2CO + O2 2CO2). Moreover, reduced ceria is known to be reactive for water activation, which is a critical step for activation of water-gas shift reaction (CO + H2O → H2 + CO2). Here, we apply van der Waals-corrected density functional theory (DFT) calculations combined with U correction to study the mechanism of water chemisorption on CeO2(111) surfaces. A stoichiometric CeO2(111) and a defected CeO2(111) surface showed different water adsorption chemistry, suggesting that defected CeO2 surfaces with oxygen vacancies are responsible for water binding and activation. An appropriate level of water-ceria chemisorption energy is deduced by vdW-corrected non-local correlation coupled with the optB86b exchange functional, whereas the conventional PBE functional describes weaker water-ceria interactions, which are insufficient to stabilize (chemisorb) water on the ceria surfaces.

H2 Dissociation and Oxygen Vacancy Formation on Ce2O3 Surfaces

H2 dissociation on ceria (CeO2) has attracted much attention in the last years because of its potential application in catalysis for hydrogenation reactions, as well as for the stabilization of hydride bulk and surface species. The ability of ceria to split hydrogen is strongly dependent on the surface morphology. However, to the best of our knowledge, the reactivity of the cerium sesquioxide Ce2O3 A-type (hexagonal structure with space group $$P\bar{3}m1$$ ) has not been previously addressed. In the present study, we investigate (i) the formation of oxygen vacancies in A-Ce2O3 bulk and (ii) the effect of the surface topology in the H2 dissociation and in the oxygen vacancy formation for the four most stable surfaces of A-Ce2O3: (0001), (01− 11), (11− 20) and (11− 21). Our results indicate a significant decrease of the energetic barrier for the hydrogen dissociation compared to stoichiometric CeO2, with an activation energy of ~ 0.1 eV. Interestingly, Ce2O3 surfaces lead to a heterolytic product with hydride species more stable than the homolytic product, which is the opposite behavior found in CeO2. These results suggest a better performance of Ce2O3 than CeO2 for H2 dissociation and provide insight in the nature of hydride-Ce2O3 interfaces that could be important intermediates in the formation of CeHx phases from cerium oxide.

Dopant solubility in ceria: alloy thermodynamics combined with the DFT+U calculations

Tb-doped CeO2 (ceria) is a promising mixed conductor for oxygen permeation membranes and reversible oxygen sorbents. To predict solubility of Tb ions in ceria for a wide range of concentrations, density functional theory (DFT+U) calculations with two different values of Hubbard U-parameter on Tb and Ce ions were combined with alloy thermodynamics and the Concentration Wave approach. It is shown that, to predict properties of disordered solid solutions at finite temperatures, the energy parameters in the mixing energies can be extracted from the DFT+U calculations performed at T = 0 K for two ordered configurations of the dopant in the supercells. The unlimited solubility of Tb4+ in CeO2 in the quasi-binary cross-section CeO2-TbO2 is predicted in the temperature range where both stoichiometric TbO2 and CeO2 reveal fluorite structures (above 700 °C).

Interaction of HCl with a CeO2(111) Layer Supported on Ru(0001): A Theory and Experiment Combined Study

Ultrathin crystalline CeO2(111) films were grown on Ru(0001) in order to study the interaction of HCl with this surface as a first step of the Deacon reaction with experimental techniques, including low energy electron diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy (TDS) in combination with density functional theory calculations (DFT+U). HCl molecules adsorb dissociatively on CeO2(111), forming a well-ordered (3x3)R30° overlayer structure with one Cl and one H per surface unit cell. DFT calculations indicate that HCl adsorption is exothermic by 1.15 eV and proceeds via an acid–base reaction. The mixed overlayer structure is stabilized by Lewis acid–base pairing (∼0.4 eV). Stoichiometric CeO2(111) films are likely to be not very active in the Deacon process since at 800 K the recombination of adsorbed H* and Cl* to form HCl is far more preferred over Cl* + Cl* recombination to form the desired product Cl2.