Oxygen vacancy formation in CeO2 and Ce(1-x)Zr(x)O2 solid solutions: electron localization, electrostatic potential and structural relaxation

Effect of Metal Oxides (CeO2, ZnO, TiO2, and Al2O3) as the Support for Silver-Supported Catalysts on the Catalytic Oxidation of Diesel Particulate Matter

This work presented the influence of metal oxides as the support for silver-supported catalysts on the catalytic oxidation of diesel particulate matter (DPM). The supports selected to be used in this work were CeO2 (reducible), ZnO (semiconductor), TiO2 (reducible and semiconductor), and Al2O3 (acidic). The properties of the synthesized catalysts were investigated using XRD, TEM, H2-TPR, and XPS techniques. The DPM oxidation activity was performed using the TGA method. Different states of silver (e.g., Ag° and Ag+) were formed with different concentrations and affected the performance of the DPM oxidation. Ag2O and lattice oxygen, which were mainly generated by Ag/ZnO and Ag/CeO2, were responsible for combusting the VOCs. The metallic silver (Ag°) formed primarily on Ag/Al2O3 and Ag/TiO2 was the main component promoting soot combustion. Contact between the catalyst and DPM had a minor effect on VOC oxidation but significantly affected the soot oxidation activity.

Abundance of Low-Energy Oxygen Vacancy Pairs Dictates the Catalytic Performance of Cerium-Stabilized Zirconia

Cerium-stabilized zirconia (Ce1-xZrxOy, CZO) is renowned for its superior oxygen storage capacity (OSC), a key property long believed to be beneficial to catalytic oxidation reactions. However, 50% Ce-containing CZO recorded with the highest OSC has disappointingly poor performance in catalytic oxidation reactions compared to those with higher Ce contents but lower OSC ability. Here, we employ global neural network (G-NN)-based potential energy surface exploration methods to establish the first ternary phase diagram for bulk structures of CZO, which identifies three critical compositions of CZO, namely, 50, 60, and 80% Ce-containing CZO that are thermodynamically stable under typical synthetic conditions. 50% Ce-containing CZO, although having the highest OSC, exhibits the lowest O vacancy (Ov) diffusion rate. By contrast, 60% Ce-containing CZO, despite lower OSC (33.3% OSC compared to that of 50% Ce-containing CZO), reaches the highest Ov diffusion ability and thus offers the highest CO oxidation catalytic performance. The physical origin of the high performance of 60% Ce-containing CZO is the abundance of energetically favorable Ov pairs along the ⟨110⟩ direction, which reduces the energy barrier of Ov diffusion in the bulk and promotes O2 activation on the surface. Our results clarify the long-standing puzzles on CZO and point out that 60% Ce-containing CZO is the most desirable composition for typical CZO applications.

Cation-doping strategies for tuning of zirconia acid-base properties

The role of Y-, Ca- and Ce-doping of cubic zirconia (c-ZrO₂) (111) surface on its acidity, basicity and the interplay between surface acid-base pairs is investigated by computational methods. The most stable surface structures for this investigation were initially determined based on previous studies of Y-doped c-ZrO₂ (111) and by a detailed exploration of the most stable configuration for Ca-doped c-ZrO₂ (111) and Ce-doped c-ZrO₂ (111). Next, surface mapping by basic probe molecules (NH₃ and pyridine) revealed a general reduction of the acidity of the surface sites, although a few exceptions were observed for zirconium ions at next nearest neighbour (NNN) positions to the oxygen vacancy and at the nearest neighbour (NN) position to the dopants. Adsorption of CO₂ over basic sites revealed a cooperative interplay between acid-base groups. In this case, the overall effect observed was the decrease of the calculated adsorption energies when compared with the pristine surface. Moreover, spontaneous formation of η ³-CO₂ systems from initial η ²-CO₂ configurations indicates a decrease in the required energy for forming oxygen vacancies in the doped ZrO₂ systems at NNN positions or further away from the existing vacancy site.

Photocatalysis triggered CVD synthesis of graphene at low temperature

Photocatalysis is employed to dissociate CH4 which facilitates low temperature CVD growth of graphene.

Design strategies for ceria nanomaterials: untangling key mechanistic concepts

The morphologies of ceria nanocrystals play an essential role in determining their redox and catalytic performances in many applications, yet the effects of synthesis variables on the formation of ceria nanoparticles of different morphologies and their related growth mechanisms have not been systematised. The design of these morphologies is underpinned by a range of fundamental parameters, including crystallography, optical mineralogy, the stabilities of exposed crystallographic planes, CeO_2-x stoichiometry, phase equilibria, thermodynamics, defect equilibria, and the crystal growth mechanisms. These features are formalised and the key analytical methods used for analysing defects, particularly the critical oxygen vacancies, are surveyed, with the aim of providing a source of design parameters for the synthesis of nanocrystals, specifically CeO_2-x. However, the most important aspect in the design of CeO_2-x nanocrystals is an understanding of the roles of the main variables used for synthesis. While there is a substantial body of data on CeO_2-x morphologies fabricated using low cerium concentrations ([Ce]) under different experimental conditions, the present work fully maps the effects of the relevant variables on the resultant CeO_2-x morphologies in terms of the commonly used raw materials [Ce] (and [NO₃^-] in Ce(NO₃)₃·6H₂O) as feedstock, [NaOH] as precipitating agent, temperature, and time (as well as the complementary vapour pressure). Through the combination of consideration of the published literature and the generation of key experimental data to fill in the gaps, a complete mechanistic description of the development of the main CeO_2-x morphologies is illustrated. Further, the mechanisms of the conversion of nanochains into the two variants of nanorods, square and hexagonal, have been elucidated through crystallographic reasoning. Other key conclusions for the crystal growth process are the critical roles of (1) the formation of Ce(OH)₄ crystallite nanochains as the precursors of nanorods and (2) the disassembly of the nanorods into Ce(OH)₄ crystallites and NO₃^--assisted reassembly into nanocubes (and nanospheres) as an unrecognised intermediate stage of crystal growth.

Heteroatom doping effects on interaction of H2O and CeO2 (111) surfaces studied using density functional theory: Key roles of ionic radius and dispersion

Understanding heteroatom doping effects on the interaction between H₂O and cerium oxide (ceria, CeO₂) surfaces is crucially important for elucidating heterogeneous catalytic reactions of CeO₂-based oxides. Surfaces of CeO₂ (111) doped with quadrivalent (Ti, Zr), trivalent (Al, Ga, Sc, Y, La), or divalent (Ca, Sr, Ba) cations are investigated using density functional theory (DFT) calculations modified for onsite Coulomb interactions (DFT + U). Trivalent (except for Al) and divalent cation doping induces the formation of intrinsic oxygen vacancy (O_vac), which is backfilled easily by H₂O. Partially OH-terminated surfaces are formed. Furthermore, dissociative adsorption of H₂O is simulated on the OH terminated surfaces (for trivalent or divalent cation doped models) and pure surfaces (for Al and quadrivalent cation doped surfaces). The ionic radius is crucially important. In fact, H₂O dissociates spontaneously on the small cations. Although a slight change is induced by doping as for the H₂O adsorption energy at Ce sites, the H₂O dissociative adsorption at Ce sites is well-assisted by dopants with a smaller ionic radius. In terms of the amount of promoted Ce sites, the arrangement of dopant sites is also fundamentally important.

An effective structural descriptor to quantify the reactivity of lattice oxygen in CeO2 subnano-clusters

Subnano-clusters are commonly the important active components on many catalysts in heterogeneous catalysis and finding an effective structural descriptor is key to designing new catalysts. However, the progress of obtaining such a descriptor falls far-short of expectation due to their well-known complicated structures. Herein, we propose a function of sigmoid coordination number (f(SCN)) as a structural descriptor, and develop an explicit equation to predict adsorption energies on Ce_nO_2n (n = 1-10) subnano-clusters using H adsorption as an example because of the importance of (de)hydrogenation in heterogeneous catalysis. We show an excellent linear correlation between H-adsorption energies and f(SCN) with RMSE = 0.05 eV and R² = 0.97. The generality of this equation is also verified using other different sizes Ce_nO_2n (n = 12-14) subnano-clusters with RMSE = 0.02 eV. We demonstrate that the structural descriptor not only provides an excellent quantitative structure-reactivity relationship for metal oxide clusters, but also deepens the understanding of structure-reactivity relationship, which may have far-reaching implications in heterogeneous catalysis.

An effective photothermal dual-responsive Pd1Cu4/CexOy catalyst for Suzuki–Miyaura coupling reactions under mild conditions

A Pd₁Cu₄/Ce_xO_y catalyst can efficiently catalyze Suzuki reactions under both heating and visible light irradiation conditions.

Insight into the active site and reaction mechanism for selective oxidation of methane to methanol using H2O2 on a Rh1/ZrO2 catalyst

Five-coordinated Rh leads to the over-oxidation of CH₄, while four-coordinated Rh stabilizes CH₃ and facilitates methanol formation via the CH₃OOH intermediate.

Clarifying the impacts of surface hydroxyls on CO oxidation on CeO2(100) surfaces: a DFT+U study

In heterogeneous catalysis, surface hydroxylation is well recognized as a common phenomenon under realistic reaction conditions.

Singlet oxygen dominated peroxymonosulfate activation by CuO-CeO2 for organic pollutants degradation: Performance and mechanism

In this study, CuO-CeO₂ was synthesized via an easy hydrothermal-calcination method and innovatively applied to peroxymonosulfate (PMS) activation for pollutants degradation under a non-radical oxidation pathway. Singlet oxygen (¹O₂) was the dominated reactive oxygen species in the CuO-CeO₂/PMS system, leading to a dramatical degradation efficiency with Rhodamine B (RhB) as model compounds. The observed rate constant of the CuO-CeO₂/PMS system was 7-11 times higher than that of only PMS, CeO₂/PMS and CuO/PMS systems. Also, under the reaction conditions of 1.6 mM PMS, 0.4 g/L catalyst and initial pH 7, the degradation efficiencies of RhB, Methylene Blue, Reactive Blue 19 and atrazine were respectively up to 100%, 85.39%, 72.84% and 98.44% in 60 min. X-ray photoelectron microscopy analysis indicated that the electrons transfer between CuO and CeO₂ and the formation of oxygen vacancy in CeO₂ should be responsible for the enhanced ¹O₂ production, which involved a new non-radical oxidation pathway for PMS activation by CuO-CeO₂ catalyst. Moreover, the combination of CuO and CeO₂ increased reusability and stability of catalyst, allowing it remove more than 92% of RhB over a wide pH range (pH = 3-9). This study not only proved that CuO-CeO₂ is an efficient and stable PMS activator but also provided a new insight into PMS activation through a non-radical oxidation pathway for organic contaminants removal from wastewater.

Low-valence or tetravalent cation doping of manganese oxide octahedral molecular sieve (K-OMS-2) materials for nitrogen oxide emission abatement

Substitutionally doped oxide catalysts with abundant oxygen vacancy defects (OVDs) can effectively improve the catalytic activity efficiency.

Decomposition of selected chlorinated volatile organic compounds by ceria (CeO2)

Guided by recent experimental measurements, this study theoretically investigates the initial steps operating in the interactions of ceria surface CeO₂(111) with three CVOC model compounds, namely chloroethene (CE), chloroethane (CA) and chlorobenzene (CB).

CO and Soot Oxidation over Ce-Zr-Pr Oxide Catalysts

A set of ceria, ceria-zirconia (Ce 80 at.%, Zr 20 at.%), ceria-praseodymia (Ce 80 at.%, Pr 20 at.%) and ceria-zirconia-praseodymia (Ce 80 at.%, Zr 10 at.% and Pr 10 at.%) catalysts has been prepared by the solution combustion synthesis (SCS). The effects of Zr and Pr as dopants on ceria have been studied in CO and soot oxidation reactions. All the prepared catalysts have been characterized by complementary techniques, including XRD, FESEM, N2 physisorption at -196 °C, H2-temperature-programmed reduction, and X-ray photoelectron spectroscopy to investigate the relationships between the structure and composition of materials and their catalytic performance. Better results for CO oxidation have been obtained with mixed oxides (performance scale, Ce80Zr10Pr10 > Ce80Zr20 > Ce80Pr20) rather than pure ceria, thus confirming the beneficial role of multicomponent catalysts for this prototypical reaction. Since CO oxidation occurs via a Mars-van Krevelen (MvK)-type mechanism over ceria-based catalysts, it appears that the presence of both Zr and Pr species into the ceria framework improves the oxidation activity, via collective properties, such as electrical conductivity and surface or bulk oxygen anion mobility. On the other hand, this positive effect becomes less prominent in soot oxidation, since the effect of catalyst morphology prevails.

Photon management properties of rare-earth (Nd,Yb,Sm)-doped CeO2 films prepared by pulsed laser deposition

CeO2 is a promising material for applications in optoelectronics and photovoltaics due to its large band gap and values of the refractive index and lattice parameters, which are suitable for silicon-based devices. In this study, we show that trivalent Sm, Nd and Yb ions can be successfully inserted and optically activated in CeO2 films grown at a relatively low deposition temperature (400 °C), which is compatible with inorganic photovoltaics. CeO2 thin films can therefore be efficiently functionalized with photon-management properties by doping with trivalent rare earth (RE) ions. Structural and optical analyses provide details of the electronic level structure of the films and of their energy transfer mechanisms. In particular, we give evidence of the existence of an absorption band centered at 350 nm from which energy transfer to rare earth ions occurs. The transfer mechanisms can be completely explained only by considering the spontaneous migration of Ce(3+) ions in CeO2 at a short distance from the RE(3+) ions. The strong absorption cross section of the f-d transitions in Ce(3+) ions efficiently intercepts the UV photons of the solar spectrum and therefore strongly increases the potential of these layers as downshifters and downconverters.

Theoretical insights into how the first C–C bond forms in the methanol-to-olefin process catalysed by HSAPO-34

A mechanism is proposed for the role of the carbenium ion in the first C–C coupling in the methanol-to-olefin process catalysed by HSAPO-34.

Occupation matrix control of d- and f-electron localisations using DFT + U

The use of a density functional theory methodology with on-site corrections (DFT + U) has been repeatedly shown to give an improved description of localised d and f states over those predicted with a standard DFT approach. However, the localisation of electrons also carries with it the problem of metastability, due to the possible occupation of different orbitals and different locations. This study details the use of an occupation matrix control methodology for simulating localised d and f states with a plane-wave DFT + U approach which allows the user to control both the site and orbital localisation. This approach is tested for orbital occupation using octahedral and tetrahedral Ti(iii) and Ce(iii) carbonyl clusters and for orbital and site location using the periodic systems anatase-TiO2 and CeO2. The periodic cells are tested by the addition of an electron and through the formation of a neutral oxygen vacancy (leaving two electrons to localise). These test systems allow the successful study of orbital degeneracies, the presence of metastable states and the importance of controlling the site of localisation within the cell, and it highlights the use an occupation matrix control methodology can have in electronic structure calculations.

Physicochemical characterization and catalytic CO oxidation performance of nanocrystalline Ce–Fe mixed oxides

Fe-doped CeO₂ nano-oxide exhibited superior CO oxidation activity compared to pristine CeO₂ due to its facile reducible nature, enhanced lattice strain, and ample oxygen vacancies.

The local structure of PdxCe1−xO2−x−δsolid solutions

In this paper, physical methods in combination with quantum chemistry calculations are used to study the local structure of Pd_xCe_1−xO_2−δsolid solutions.