Surface and Reduction Energetics of the CeO2−ZrO2 Catalysts

A proxy for oxygen storage capacity from high-throughput screening and automated data analysis

Oxygen storage and release is a foundational part of many key pathways in heterogeneous catalysis, such as the Mars-van Krevelen mechanism. However, direct measurement of oxygen storage capacity (OSC) is time-consuming and difficult to parallelise. To accelerate the discovery of stable high OSC rare-earth doped ceria-zirconia oxygen storage catalysts, a high-throughput robotic-based co-precipitation synthesis route was coupled with sequentially automated powder X-ray diffraction (PXRD), Raman and thermogravimetric analysis (TGA) characterisation of the resulting materials libraries. Automated extraction of data enabled rapid trend identification and provided a data set for the development of an OSC prediction model, investigating the significance of each extracted quantity towards OSC. The optimal OSC prediction model produced incorporated variables from only fast-to-measure analytical techniques and gave predicted values of OSC that agreed with experimental observations across an independent validation set. Those measured quantities that feature in the model emerge as proxies for OSC performance. The ability to predict the OSC of the materials accelerates the discovery of high-capacity oxygen storage materials and motivates the development of similar high-throughput workflows to identify candidate catalysts for other heterogeneous transformations.

Effect of Ce/Zr Composition on Structure and Properties of Ce1-xZrxO2 Oxides and Related Ni/Ce1-xZrxO2 Catalysts for CO2 Methanation

Ce1-xZrxO2 oxides (x = 0.1, 0.25, 0.5) prepared via the Pechini route were investigated using XRD analysis, N2 physisorption, TEM, and TPR in combination with density functional theory calculations. The Ni/Ce1-xZrxO2 catalysts were characterized via XRD analysis, SEM-EDX, TEM-EDX, and CO chemisorption and tested in carbon dioxide methanation. The obtained Ce1-xZrxO2 materials were single-phase solid solutions. The increase in Zr content intensified crystal structure strains and favored the reducibility of the Ce1-xZrxO2 oxides but strongly affected their microstructure. The catalytic activity of the Ni/Ce1-xZrxO2 catalysts was found to depend on the composition of the Ce1-xZrxO2 supports. The detected negative effect of Zr content on the catalytic activity was attributed to the decrease in the dispersion of the Ni0 nanoparticles and the length of metal-support contacts due to the worsening microstructure of Ce1-xZrxO2 oxides. The improvement of the redox properties of the Ce1-xZrxO2 oxide supports through cation modification can be negated by changes in their microstructure and textural characteristics.

Enhanced Adhesion Energy at Oxide/Ag Interfaces for Low-Emissivity Glasses: Theoretical Insight into Doping and Vacancy Effects

Low-emissivity glasses rely on multistacked architectures with a thin silver layer sandwiched between oxide layers. The mechanical stability of the silver/oxide interfaces is a critical parameter that must be maximized. Here, we demonstrate by means of quantum-chemical calculations that a low work of adhesion at interfaces can be significantly increased via doping and by introducing vacancies in the oxide layer. For the sake of illustration, we focus on the ZrO₂(111)/Ag(111) interface exhibiting a poor adhesion in the pristine state and quantify the impact of introducing n-type dopants or p-type dopants in ZrO₂ and vacancies in oxygen atoms (nV_O; with n = 1, 2, 4, 8, 10, 16), zirconium atoms (mV_Zr; with m = 1, 2, 4, 8), or both (nV_O + mV_Zr; with m/n = 1:2, 1:4, 2:2, 2:4). In the case of doping, interfacial electron transfer promotes an increase in the work of adhesion, from initially 0.16 to ∼0.8 J m^-2 (n-type) and ∼2.0 J m^-2 (p-type) at 10% doping. A similar increase in the work of adhesion is obtained by introducing vacancies, e.g., V_O [V_Zr] in the oxide layer yields a work of adhesion of ∼1.5-2.0 J m^-2 at 10% vacancies. An increase is also observed when mixing V_O and V_Zr vacancies in a nonstoichiometric ratio (nV_O + mV_Zr; with 2n ≠ m), while a stoichiometric ratio of V_O and V_Zr has no impact on the interfacial properties.

The formation of methanol from glycerol bio-waste over doped ceria-based catalysts

A series of ceria-based solid solution metal oxides were prepared by co-precipitation and evaluated as catalysts for glycerol cleavage, principally to methanol. The catalyst activity and selectivity to methanol were investigated with respect to the reducibility of the catalysts. Oxides comprising Ce-Pr and Ce-Zr were prepared, calcined and compared to CeO₂, Pr₆O₁₁ and ZrO₂. The oxygen storage capacity of the catalysts was examined with analysis of Raman spectroscopic measurements and a temperature programmed reduction, oxidation and reduction cycle. The incorporation of Pr resulted in significant defects, as evidenced by Raman spectroscopy. The materials were evaluated as catalysts for the glycerol to methanol reaction, and it was found that an increased defect density or reducibility was beneficial. The space-time yield of methanol normalized to surface area over CeO₂ was found to be 0.052 mmol_MeOHm^-2h^-1, and over CeZrO₂ and CePrO₂, this was to 0.029 and 0.076 mmol_MeOHm^-2h^-1, respectively. The inclusion of Pr reduced the surface area; however, the carbon mole selectivity to methanol and ethylene glycol remained relatively high, suggesting a shift in the reaction pathway compared to that over ceria. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

Surface reduction properties of ceria-zirconia solid solutions: a first-principles study

Based on the density functional theory (DFT), the reduction properties of Ce_1-x Zr _x O₂ (110) surfaces were systematically calculated using CO as a probe for thermodynamic study, and a large supercell was applied to build the whole composition range (x = 0.125, 0.250, 0.375, 0.500, 0.625, 0.750, 0.875). From the calculated energy barriers of CO oxidation by lattice oxygen, we found that composition Ce_0.875Zr_0.125O₂ exhibited the most promising catalytic effectiveness with the lowest activation energy of 0.899 eV. Moreover, the active surface O_3c ions coordinated by two Zr ions and one Ce ion were facilely released from their bulk positions than the O_3c ions surrounded by two Ce ions and one Zr ion on Ce_0.625Zr_0.375O₂, Ce_0.500Zr_0.500O₂, and Ce_0.375Zr_0.625O₂ (110) surfaces. This difference could be explained by the binding strength of O_3c with different neighboring cations.

Effect of gold addition by the recharge method on silver supported catalysts in the catalytic wet air oxidation (CWAO) of phenol

Catalysts Ag/ZrO2-CeO2 and Au/ZrO2-CeO2 were synthesized by a deposition-precipitation method and Ag-Au/ZrO2-CeO2 was prepared using a recharge method for the second metal (Au). The materials were characterized by physisorption of N2, XRD, ICP, UV-vis RDS, H2-TPR, XPS and TEM. The results obtained show that the specific areas for monometallic materials were 29-37 m2 g-1 and 27-74 m2 g-1 for bimetallics. The tetragonal crystal phase of ZrO2 stabilizes when CeO2 quantity increases. Using XPS an increment in Ce3+ species abundance was determined for bimetallic catalysts in contrast to the monometallic ones; according to the Ag 3d region, this metal oxidation was observed when augmenting the content of CeO2 in the materials, and with Au the opposite effect was produced. It was determined by TEM, that the average size of the metallic particles was smaller at bimetallic catalysts due the preparation method. Catalytic activity was evaluated by CWAO of phenol, the Ag-Au/ZrO2-CeO2 catalyst with 20% wt of cerium reached a degradation of 100% within an hour, being the most active catalyst. Maleic, formic and oxalic acid were identified as reaction intermediates; and at the end of the reaction acetic acid was identified as the main by-product, because it is the most refractory and the conditions for oxidation must be more severe.

Catalytic Behaviour of Flame-Made CuO-CeO2 Nanocatalysts in Efficient CO Oxidation

CuO-CeO2 nanocatalysts with varying CuO contents (1, 5, 9, 14 and 17 wt %) were prepared by one-step flame spray pyrolysis (FSP) and applied to CO oxidation. The influences of CuO content on the as-prepared catalysts were systematically characterized by X-ray diffraction (XRD), N2 adsorption-desorption at −196 °C, field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and hydrogen-temperature programmed reduction (H2-TPR). A superior CO oxidation activity was observed for the 14 wt % CuO-CeO2 catalyst, with 90% CO conversion at 98 °C at space velocity (60,000 mL × g−1 × h−1), which was attributed to abundant surface defects (lattice distortion, Ce3+, and oxygen vacancies) and high reducibility supported by strong synergistic interaction. In addition, the catalyst also displayed excellent stability and resistance to water vapor. Significantly, in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) showed that in the CO catalytic oxidation process, the strong synergistic interaction led readily to dehydroxylation and CO adsorption on Cu+ at low temperature. Furthermore, in the feed of water vapor, although there was an adverse effect on the access of CO adsorption, there was also a positive effect on the formation of fewer carbon intermediates. All these results showed the potential of highly active and water vapor-resistive CuO-CeO2 catalysts prepared by FSP.

Humidity-Independent Oxide Semiconductor Chemiresistors Using Terbium-Doped SnO2 Yolk-Shell Spheres for Real-Time Breath Analysis

The chemiresistive sensing characteristics of metal oxide gas sensors depend closely on ambient humidity. Herein, we report that gas sensors using Tb-doped SnO₂ yolk-shell spheres can be used for reliable acetone detection, regardless of the variations in humidity. Pure SnO₂ and Tb-doped SnO₂ yolk-shell spheres were prepared via ultrasonic spray pyrolysis and their chemiresistive sensing characteristics were studied. The sensor resistance and gas response of the pure SnO₂ yolk-shell spheres significantly changed and deteriorated upon exposure to moisture. In stark contrast, the Tb-doped SnO₂ yolk-shell spheres exhibited similar gas responses and sensor resistances in both dry and humid [relative humidity (RH) 80%] atmospheres. In addition, the Tb-doped SnO₂ yolk-shell sensors showed a high gas response (resistance ratio) of 1.21 to the sub-ppm-levels (50 ppb) of acetone with low responses to the other interference gases. The effects of Tb oxide and the chemical interactions among the Tb oxide, SnO₂, and water vapor on this humidity-independent gas sensing behavior of the Tb-doped SnO₂ yolk-shell sensors were investigated. This strategy can provide a new road to achieve highly sensitive, selective, and humidity-independent sensing of acetone, which will facilitate miniaturized and real-time exhaled breath analysis for diagnosing diabetes.

The atomistic structure of yttria stabilised zirconia at 6.7 mol%: an ab initio study

Yttria stabilized zirconia (YSZ) is an important oxide ion conductor used in solid oxide fuel cells, oxygen sensing devices, and for oxygen separation. Doping pure zirconia (ZrO₂) with yttria (Y₂O₃) stabilizes the cubic structure against phonon induced distortions and this facilitates high oxide ion conductivity. The local atomic structure of the dopant is, however, not fully understood. X-ray and neutron diffraction experiments have established that, for dopant concentrations below 40 mol% Y₂O₃, no long range order is established. A variety of local structures have been suggested on the basis of theoretical and computational models of dopant energetics. These studies have been restricted by the difficulty of establishing force field models with predictive accuracy or exploring the large space of dopant configurations with first principles theory. In the current study a comprehensive search for all symmetry independent configurations (2857 candidates) is performed for 6.7 mol% YSZ modelled in a 2 × 2 × 2 periodic supercell using gradient corrected density functional theory. The lowest energy dopant structures are found to have oxygen vacancy pairs preferentially aligned along the 〈210〉 crystallographic direction in contrast to previous results which have suggested that orientation along the 〈111〉 orientation is favourable. Analysis of the defect structures suggests that the Y³⁺-O_vac interatomic separation is an important parameter for determining the relative configurational energies. Current force field models are found to be poor predictors of the lowest energy structures. It is suggested that the energies from a simple point charge model evaluated at unrelaxed geometries is actually a better descriptor of the energy ordering of dopant structures. Using these observations a pragmatic procedure for identifying low energy structures in more complicated material models is suggested. Calculation of the oxygen vacancy migration activation energies within the lowest energy 〈210〉 oriented structures gives results consistent with experimental observations.

Bi doped CeO2 oxide supported gold nanoparticle catalysts for the aerobic oxidation of alcohols

Gold nanoparticles supported on Bi–CeO₂ with four different bismuth loadings (2 to 8 mol%) were prepared to determine the role of oxide vacancies in doped ceria in the benzyl alcohol oxidation reaction.

Molecular dynamics simulation of the structural, elastic, and thermal properties of pyrochlores

We present a comprehensive simulation study of the effect of composition on the structural, elastic and thermal properties of 25 different compounds from the pyrochlore family.

Total oxidation of ethanol and toluene over ceria—zirconia supported platinum catalysts

The effect of platinum loading (0.09–1.00 mass %) on the performance of ceria–zirconia supported catalysts in the total oxidation of ethanol and toluene in air was investigated. The introduction of platinum promoted the reduction of surface cerium and decreased the acidity of the catalysts. In ethanol oxidation, the temperature of 50 % conversion decreased with increasing platinum content. This increase in catalytic performance was more pronounced for the catalysts with 0.59 mass % and 1.00 mass % Pt. On the other hand, higher amount of by-products (mainly acetaldehyde) was observed at increased platinum loadings. For all catalysts, a correlation between their H

Oxidative methane activation over yttrium stabilised zirconia

The methane C-H bond is extremely stable, requiring significant energy input in reforming processes. We present a novel mechanism for energetically favourable methane C-H bond breaking over yttrium stabilised zirconia in the presence of oxygen, based on results of Density Functional Theory (DFT) and HSE06 hybrid functional calculations. We argue that this mechanism will be relevant to C-H activation over many metal oxide catalyst materials.

Redox properties and catalytic performance of ceria–zirconia nanorods

The shape effect of Ce_1−xZr_xO₂ nanomaterials is associated with the amount of zirconia that is incorporated into the ceria lattice.

Reduction of NO by CO using Pd-CeTb and Pd-CeZr catalysts supported on SiO2 and La2O3-Al2O3

The catalytic activity of Pd catalysts supported on Ce0.73Tb0.27Ox/SiO2, Ce0.6Zr0.4Ox/SiO2, Ce0.73Tb0.27Ox/La2O3-Al2O3 and Ce0.6Zr0.4Ox/La2O3-Al2O3 was studied using the reduction of NO by CO. The catalysts were characterized by X-ray fluorescence, surface area, X-ray diffraction, temperature-programmed reduction, CO chemisorption and oxygen storage capacity. Temperature-programmed reduction results indicated that Tb or Zr incorporation improves the reducibility and oxygen storage capacity. CO chemisorption data suggested the presence of large PdO particles due to the low CO/Pd ratio. No significant differences were obtained in light off temperatures (TLight off) for all Pd catalysts and the most active was 1.5%Pd/Ce0.6Zr0.4Ox/SiO2.

Significant reduction in hydration energy for yttria stabilized zirconia grain boundaries and the consequences for proton conduction

Using well-established atomistic techniques, we investigate the defect chemistry, structural effects, and energetics of proton incorporation at the Σ5(310)/[001] and Σ5(210)/[001] symmetrical tilt grain boundaries of yttria-stabilized zirconia. Building upon past work, we consistently show a dramatic decrease (∼4-5 eV) in the proton incorporation and hydration energies in and around the grain boundary structures compared to values obtained for the bulk material and undoped ZrO2 grain boundaries. This decrease is prevalent in both Y segregated grain boundaries and grain boundaries where the distribution of Y is completely random. The results presented here strongly support the argument that proton conduction in this system is primarily interfacially driven, as reported by numerous experimental studies. Redox properties are also presented for grain boundaries structures both with and without defect segregation. The methodology and results presented here can also be applied to a wide range of proton conductors and will prove essential in any future assessment of the effects of grain boundaries on the defect chemistry of protons in these systems.

Comparison of select polarizable and non-polarizable water models in predicting solvation dynamics of water confined between MgO slabs

We present a molecular dynamics simulation study in which we compare and contrast the performance of a polarizable shell water potential model and non-polarizable water force field-extended simple point charge (SPC/EF) model in predicting the solvation dynamics of confined water molecules sandwiched between MgO(100) slabs. Structural features based on radial distribution functions, atomic density profiles, adsorption patterns, orientational ordering and dynamical correlations such as diffusional characteristics, hydrogen bonding lifetimes and residence probabilities are used as metrics for comparison. The simulations yield significant ordering of water molecules in the two layers adjacent to the oxide interface and the extent of ordering decreases with increasing distance from the oxide-water interface. These results elucidate that the dependence of local ordering and solvation dynamics on the molecular geometry and charge distribution, observed for typical three- and four-site water models, is generally lost for confined water if polarization is explicitly included. While the interfacial water structure predicted by the polarizable and non-polarizable models are similar, the confinement and interface proximity effects on the solvation dynamics are seen to be more pronounced for polarizable water models in comparison to non-polarizable ones. The study also shows that the polarizable water model over predicts the orientational order and under predicts the transport properties of confined water. In addition, analysis of the orientational preferences and hydrogen bonding characteristics of water near oxide interfaces suggests a higher degree of tetrahedral disorder in the polarizable shell compared to the non-polarizable SPC/E flexible model. The origin of the differences in solvation behavior of confined water between oxide slabs is analyzed based on the energetic contributions of the dispersive and electrostatic terms in the two force fields. Our findings suggest some new considerations regarding the role of polarization terms in predicting confinement and interface proximity effects that may guide future development of reliable polarizable water models for confined liquids.

Computational insights into the nature of increased ionic conductivity in concentrated samarium-doped ceria: a genetic algorithm study

Classical force field simulations and genetic algorithms are used to navigate low-energy configurations of samarium-doped ceria (SDC) at a number of concentrations, up to 20% SDC, such that the experimentally observed peak in ionic conductivity is mapped out in its entirety and fresh insight into samarium's role is reported.

Pt/CeO(2)-ZrO(2) present in the mesopores of SBA-15--a better catalyst for CO oxidation

Platinum loaded CeO(2)-ZrO(2)/SBA-15 composite material catalyzes CO oxidation at lower temperatures (<100 degrees C) than conventionally prepared Pt/CeO(2)-ZrO(2) (>150 degrees C). A sequence of reduction and reoxidation of CeO(2)- and ZrO(2)-loaded samples promotes the formation of CeO(2)-ZrO(2) solid solution on the siliceous surface as indicated by wide-angle X-ray diffraction (XRD) patterns. Furthermore, initial coating of ZrO(2) on the silica walls is quite important before CeO(2)-ZrO(2) loading in order to obtain the CeO(2)-ZrO(2) solid solution. The presence of CeO(2)-ZrO(2) particles inside the mesopores is confirmed by N(2) adsorption, low-angle X-ray diffraction, and scanning transmission electron microscopy (STEM) images. The solid solution of CeO(2)-ZrO(2) in the confined mesoporous space forms more Ce(3+) centers associated with oxygen vacancies on the surface due to the smaller size of the dispersed particles and these Ce(3+) centers are believed to be responsible for the higher catalytic activity.