Ceria Surface Research Articles

Single Ru(II) Ions on Ceria as a Highly Active Catalyst for Abatement of NO.

Atom trapping leads to catalysts with atomically dispersed Ru1O5 sites on (100) facets of ceria, as identified by spectroscopy and DFT calculations. This is a new class of ceria-based materials with Ru properties drastically different from the known M/ceria materials. They show excellent activity in catalytic NO oxidation, a critical step that requires use of large loadings of expensive noble metals in diesel aftertreatment systems. Ru1/CeO2 is stable during continuous cycling, ramping, and cooling as well as the presence of moisture. Furthermore, Ru1/CeO2 shows very high NOx storage properties due to formation of stable Ru-NO complexes as well as a high spill-over rate of NOx onto CeO2. Only ∼0.05 wt % of Ru is required for excellent NOx storage. Ru1O5 sites exhibit much higher stability during calcination in air/steam up to 750 °C in contrast to RuO2 nanoparticles. We clarify the location of Ru(II) ions on the ceria surface and experimentally identify the mechanism of NO storage and oxidation using DFT calculations and in situ DRIFTS/mass spectroscopy. Moreover, we show excellent reactivity of Ru1/CeO2 for NO reduction by CO at low temperatures: only 0.1-0.5 wt % of Ru is sufficient to achieve high activity. Modulation-excitation in situ infrared and XPS measurements reveal the individual elementary steps of NO reduction by CO on an atomically dispersed Ru ceria catalyst, highlighting unique properties of Ru1/CeO2 and its propensity to form oxygen vacancies/Ce+3 sites that are critical for NO reduction, even at low Ru loadings. Our study highlights the applicability of novel ceria-based single-atom catalysts to NO and CO abatement.

Improvement of oxide chemical mechanical polishing performance by increasing Ce3+/Ce4+ ratio in ceria slurry via hydrogen reduction

Ceria-based abrasive is widely used in the oxide chemical mechanical polishing (CMP) process due to its high polishing performance. Ce3+ ions on the surface of ceria form a Ce–O–Si chemical bond with surface of the SiO2 wafer, significantly affecting the material removal rate (MRR). In this study, the ceria surface was reduced by contact with hydrogen gas in a high temperature isothermal environment. Hydrogen gas forms surface hydroxyls on the ceria. Then, the generated hydroxyls form H2O and oxygen vacancies occur through the desorption of the H2O. The color of ceria changed to blue due to a change of the crystal structure by the reduction reaction. The reducing ability of hydrogen gas increased as the reduction temperature increased and the Ce3+ ion concentration increased by 12.7% under isothermal conditions at 1000 °C. The MRR of reduced ceria was improved by up to 37.7% compared to the original ceria.

Proton-Coupled Electron Transfer of Cerium Oxide Nanoparticle Thin-Film Electrodes

Metal oxide (MOx) materials are effective catalysts or cocatalysts in a range of electrochemical reactions and energy systems. A key component of their redox chemistry is proton-coupled electron transfer (PCET). Reported here are studies of isolated cerium oxide nanoparticles deposited on fluorine-doped tin oxide (FTO) electrodes with a Langmuir–Blodgett trough and calcined. Cyclic voltammograms of these films showed well-defined, quasi-reversible waves. The E1/2 values moved with pH by −64 ± 4 mV/pH, close to the ideal Nernstian −59 mV/pH for a 1e–/1H+ couple. These results imply that the electroactive CeO–H bonds had an average bond dissociation free energy (BDFE) of 78.6 ± 1.5 kcal mol–1. Integration of the faradaic currents indicates that 0.15 ± 0.04 electrons can be added per cerium atom in the nanoparticle film or ∼20% of the surface cerium atoms. This study shows how electrochemical investigations can elucidate the stoichiometry and thermochemistry of PCET processes of MOx nanoparticle films. Comparison of these results with those for related ceria nanoparticles shows the remarkable range of properties of this useful material.

In situ diffraction monitoring of nanocrystals structure evolving during catalytic reaction at their surface

With decreasing size of crystals the number of their surface atoms becomes comparable to the number of bulk atoms and their powder diffraction pattern becomes sensitive to a changing surface structure. On the example of nanocrystalline gold supported on also nanocrystalline {text{CeO}}_2 we show evolution of (a) the background pattern due to chemisorption phenomena, (b) peak positions due to adsorption on nonstoichiometric {text{CeO}}_{2-x} particles, (c) Au peaks intensity. The results of the measurements, complemented with mass spectrometry gas analysis, point to (1) a multiply twinned structure of gold, (2) high mobility of Au atoms enabling transport phenomena of Au atoms to the surface of ceria while varying the amount of Au in the crystalline form, and (3) reversible {text{CeO}}_2 peaks position shifts on exposure to He–X–He where X is O2, H2, CO or CO oxidation reaction mixture, suggesting solely internal alternations of the {text{CeO}}_2 crystal structure. We found no evidence of ceria lattice oxygen being consumed/supplied at any stage of the process. The work shows possibility of structurally interpreting different contributions to the multi-phase powder diffraction pattern during a complex physico-chemical process, including effects of physi-, chemisorption and surface evolution. It shows a way to structurally interpret heterogeneous catalytic reactions even if no bulk phase transition is involved.

Catalytic Activity of Ni Based Materials Prepared by Different Methods for Hydrogen Production via the Water Gas Shift Reaction

Water gas shift reactions (WGS) were evaluated over Ni/CeO2 and Ni/CeSmO catalysts for hydrogen production. The effects of catalyst preparation method and Sm loading were investigated. The Ni/ceria and Ni/CeSmO catalysts were synthesized by combustion, sol gel and sol gel-combustion method. After WGS tests, the catalysts were determined the carbon deposition by thermogravimetric analysis. The thermogravimetric analysis and temperature programmed NH3 desorption showed that addition of Sm promoter made higher the weak acid sites and lower the amount of carbon deposition than the unpromoted catalyst due to it being easily removed. CO chemisorption result indicated that Ni/Ce5%SmO catalyst prepared by combustion method has the highest Ni metal dispersion and metallic surface area compared to the other catalysts. The enhancement of WGS activity of this catalyst is due to more surface active sites being exposed to reactants. Furthermore, H2-temperature programmed reduction analysis confirmed an easiest reduction of this catalyst. This behavior accelerates the redox process at the ceria surface and enhances the oxygen vacancy concentration. The catalytic activity measurements exhibited that the optimum Sm loading was 5% wt. and the best catalyst preparation was the combustion method. The high surface area and small crystallite size of the 5%Ni/Ce5%SmO (combustion) catalyst resulted in sufficient dispersion, which closely related to the WGS activity of the catalyst.

Temperature dependence of (111) and (110) ceria surface energy

High-temperature properties of ceria surfaces are important for many applications. Here, we report the temperature dependencies of surface energy for (111) and (110) ${\mathrm{CeO}}_{2}$ obtained in the framework of the extended two-stage up-sampled thermodynamic integration using Langevin dynamics. The method was used together with machine-learning potentials called moment tensor potentials (MTPs), which were fitted to the results of the ab initio molecular dynamics calculations for (111) and (110) ${\mathrm{CeO}}_{2}$ at different temperatures. The parameters of MTP training and fitting were tested, and the optimal algorithm for the ceria systems was proposed. We found that the temperature increases from 0 to 2100 K led to the decrease of the Helmholtz free energy of (111) ${\mathrm{CeO}}_{2}$ from 0.78 to 0.64 $\mathrm{J}/{\mathrm{m}}^{2}$. The energy of (110) ${\mathrm{CeO}}_{2}$ dropped from 1.19 $\mathrm{J}/{\mathrm{m}}^{2}$ at 0 K to 0.92 $\mathrm{J}/{\mathrm{m}}^{2}$ at 1800 K. We show that it is important to consider anharmonicity, as simple consideration of volume expansion gives the wrong temperature dependencies of the surface energies.

Unravelling catalytic activity trends in ceria surfaces toward the oxygen reduction and water oxidation reactions

Different facets of ceria exhibit activities for the entire spectrum of oxygen electrochemistry.

Carbon dioxide reduction processes on a samarium doped ceria electrocatalyst with exsolved Fe particles

Fe-SDC formed Fe nanoparticles on the ceria surface through the exsolution reaction, which significantly improved its electrochemical properties.

Nucleotide Interaction with Nanocrystalline Ceria Surface

The adsorption of nucleotides on the surface of nanocrystalline cerium dioxide (pHpzc = 6.3) in NaCl solutions was investigated using multi-batch adsorption experiments over a wide range of pH. The obtained results were interpreted as a formation of outer and inner sphere surface complexes with the participation of phosphate moieties. The Basic Stern surface complexation model was applied to obtain quantitative equilibrium reaction constants.

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.

The Extent of Platinum-Induced Hydrogen Spillover on Cerium Dioxide.

Hydrogen spillover from metal nanoparticles to oxides is an essential process in hydrogenation catalysis and other applications such as hydrogen storage. It is important to understand how far this process is reaching over the surface of the oxide. Here, we present a combination of advanced sample fabrication of a model system and in situ X-ray photoelectron spectroscopy to disentangle local and far-reaching effects of hydrogen spillover in a platinum-ceria catalyst. At low temperatures (25-100 °C and 1 mbar H2) surface O-H formed by hydrogen spillover on the whole ceria surface extending microns away from the platinum, leading to a reduction of Ce4+ to Ce3+. This process and structures were strongly temperature dependent. At temperatures above 150 °C (at 1 mbar H2), O-H partially disappeared from the surface due to its decreasing thermodynamic stability. This resulted in a ceria reoxidation. Higher hydrogen pressures are likely to shift these transition temperatures upward due to the increasing chemical potential. The findings reveal that on a catalyst containing a structure capable to promote spillover, hydrogen can affect the whole catalyst surface and be involved in catalysis and restructuring.

2D/3D- C3N4/CeO2 S-scheme heterojunctions with enhanced photocatalytic performance

• Successfully developed a novel method to obtain g-C 3 N 4 /CeO 2 composite photo-catalysts via simple, ball milled thermal spreading technique. • Facile control of surface morphology of carbon nitride on ceria. The optical absorption is tuned by morphology control. • The optimum loading was found to be 7.5 wt. %. • Morphology with an intermediate thickness between very thin and bulk carbon nitride on ceria surface is important to achieve maximum efficiency. • High efficiency of methylene blue degradation and good recycling performance. A facile, green and easily scalable method has been developed to facilitate the construction of 2D-C 3 N 4 /3D-CeO 2 heterojunction materials. It uses only cheap urea and ceria in a thermal spreading technique that allows the controlled growth of carbon nitride layers on cerium oxide surface. The exfoliation of bulk carbon nitride and subsequent stabilization of layers on ceria takes place during thermal spreading technique in a single step. Simple variation of carbon nitride allowed to control the morphology of carbon nitride from highly dispersed thin layer up to bulk-like structures over ceria surface. The optical absorption was enhanced with 2D-C 3 N 4 /3D-CeO 2 materials. The variation of structure and consequent differences in the solar powered decomposition of methylene blue has been analyzed. Results reveal that, the optimum loading of 7.5 wt.% carbon nitride can lead to the formation of intermediate structure in between very thin layers and bulk-like structure, that achieves best contact between the two phases making effective heterojuctions. This best catalyst exhibits 98% conversion at a timing of 100 minutes. The radical scavenging studies showed that •O 2− and h+ performed an imperative role. In addition, a plausible mechanism of photo catalytic decomposition is also proposed for 2D-C 3 N 4 /3D-CeO 2 materials.

Morphology, oxygen vacancies, and other factors affecting the performance of supported bimetallic ceria catalysts.

Here in we report the development of a Pt-V/CeO2 catalyst performing under mild conditions in amide hydrogenation. Ceria with different morphologies was employed as support in this study. We further developed a glycol-thermal technique that yields thermally stable quantum dot ceria, which can be applied as a support. A systematic investigation revealed the importance of proximity between the small crystalline hydrogenating sites (Pt) and oxophilic sites (V). The study showed that oxygen vacancies on the ceria surface oxidize both Pt and V, poisoning the hydrogenation reaction. In contrast, the absence of oxygen vacancies promoted the hydrogenating ability of Pt sites and also improved their ability to participate in the H2 spillover mechanism and in situ formation of oxophilic V3+. This study demonstrates how the engineering of the oxygen vacancies on the surface of the redox support can manipulate the nature of active sites toward specific reactions.

The role of ionic and cluster active centers of Pt/CeO2 catalysts in CO oxidation. Experimental study and mathematical modeling

In this work, for the first time, the experimental catalytic kinetics of the CO oxidation reaction on platinum–ceria catalysts was simulated in a wide temperature range −50–450 °C. On the basis of the experiments performed, a substantiation of the catalytic action of two types of Pt-containing active centers in the form of isolated [Pt2+-O4] ions and PtOx clusters is presented. The relative content of these platinum forms on the ceria surface is controlled by the loading of platinum. Using the catalysts with a low content of platinum, where the active centers are isolated [Pt2+-O4] ions, a satisfactory simulation of CO conversion depending on temperature was carried out based on the concepts of the Mars-van Krevelen (MvK) mechanism. It was established that these centers provide the catalysts activity in the temperature range above 100 °C. To simulate the reaction kinetics in the temperature range below 100 °C, the Pt-O-Pt active centers based on PtOx cluster forms were considered. The PtOx cluster forms made it possible to consider both the MvK mechanism and temperature-dependent associative Low-T mechanism. The introduction into the kinetic scheme of stages simulating the implementation of both the Low-T and the MvK mechanisms allowed us to perform complete quantitative modeling of the experimental data over the entire temperature range of the reaction from −50 °C to 450 °C.

Interaction of Hydrogen with Cu-Modified Cerium Oxide Surfaces

We investigate the interaction between molecular hydrogen and ultrathin epitaxial CeO2 films modified with a 2% concentration of Cu atoms using X-ray photoemission spectroscopy (XPS) during thermal reduction cycles in H2. The XPS measurements are combined with density functional theory calculations to obtain further insight into the observed modifications of the film surface. Our results show that the presence of Cu atoms decreases the barrier for H2 dissociation in comparison to that on pure ceria surfaces, leading to the formation of surface OH groups after exposure to H2. Moreover, surface oxygen vacancies are generated already at mild temperatures (470 K), most likely via water formation and desorption. The presence of surface oxygen vacancies and hydroxyls contributes to the observed large increase in surface Ce3+ concentration with increasing reduction temperature. In spite of these atomic scale modifications, the surface morphology observed by scanning tunneling microscopy remains substantially unchanged on the length scale of tens of nm.

Green synthesized Ag decorated CeO2 nanoparticles: Efficient photocatalysts and potential antibacterial agents.

Implication of natural resources for manufacturing of nanoparticles is sustainable, economical and contaminant free approach towards ecological and medical applications. Herein, CeO2 and Ag/CeO2 nanoparticles are green synthesized from Morinda tinctoria plant extract. The phase structure, surface morphology, optical identity, Ce(III) and Ce(IV) valency of the synthesized CeO2 and Ag/CeO2 nanoparticles are explored. The X-ray diffraction analysis indicated the formation of cubic phase CeO2 and cubic silver decorated CeO2 nanoparticles. Fourier transform infrared (FTIR) spectroscopy revealed the metal decoration of CeO2 nanoparticles, metal-oxygen stretching, indicating the plant molecules reduction and stabilization. UV–visible spectroscopy shown the decreased band gap owing to silver modification. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs displayed spherical morphology of the nanoparticles. Elemental composition and sample purity is assessed by energy dispersive spectroscopy (EDS). Double oxidation of Ce, double splitting energy of Ag and lattice oxygen are observed from X-ray photoelectron spectroscopy (XPS). Photocatalytic activity of Ag/CeO2 exposed the enhanced photocatalytic activity up to 94% where CeO2 nanoparticles gave 60% degradation of bromophenol blue (BB). The plasmonic decoration of silver on the ceria surface induced the charge separations and free radical reactions. Moreover, Ag/CeO2 nanoparticles are seen as superior antibacterial agents than CeO2 towards both E.coli and S.aureus. Hence, the silver decorated metal oxide photocatalyst successfully degraded the BB dye and inactivated the bacterial strains. This report established a future research in green synthesis of multipurpose metal nanoparticles.

Ni-Cu Alloy Nanoparticles Confined by Physical Encapsulation with SiO2 and Chemical Metal-Support Interaction with CeO2 for Methane Dry Reforming.

Fabrication of sintering- and carbon-free Ni catalysts for methane dry reforming (MDR), which is attractive to upgrade greenhouse gases CH4 and CO2, is challenging. In this work, we innovatively synthesized Ni-Cu alloy nanoparticles confined by physical encapsulation and chemical metal-support interaction (MSI); the synergism of alloy effect, size effect, MSI, and confinement effect in the catalysts gave high rates of CH4 and CO2 of 6.98 and 7.16 mmol/(gNis), respectively, at 1023 K for 50 h. The rates were 2-3 times enhanced compared to those in the literature. XRD, TEM, H2-TPR, and so forth revealed that the alloy effect, size effect, and MSI of Ni-Cu and CeO2 enhanced the MDR activity; MSI promoted the ceria surface lattice oxygen mobility and generated more oxygen vacancies, almost completely gasifying carbon deposits; chemical confinement from MSI and physical confinement from SiO2 nanospheres realized sintering-free alloys and CeO2 nanoparticles. The synergistic approach provides a universal strategy for sintering- and carbon-free Ni catalyst design for MDR reaction.

Desulfurization reactions of methanethiol on defect CeO2 surfaces

Methanethiol (CH3SH) is a volatile organic compound that poses a high health risk and causes corrosion to equipment in petroleum refineries. The removal of the sulfur content in mercaptans using potent catalysts assumes a fundamental importance from industrial and environmental perspectives. Motivated by experimental studies involving stand-alone cerium oxide (CeO2) catalysts, we have deployed a defect CeO2 surface with an oxygen vacancy (CeO2(111)_Vo) to explore the decomposition chemistry of methanethiol based on first principle density functional theory (DFT) calculations. The study presents potential formation pathways for the major experimentally reported compounds, namely, CH4, H2S, CO, CO2, CH3SCH3, and COS. Initial uptake of CH3SH takes place via either C-S or S-H bond fissions through modest activation barriers. In initial decomposition pathways, Vo spots represent strong acidic sites, and thus facilitating rupture of C-S and S-H fissions. Formation of CO takes place through a series of H transfer reactions that commence from the CH2* adduct. Adjacent HS* and HO*sites undergo a hydrogen diffusion reaction to ultimately produce H2S rather than water. Several investigated reactions lead to the filling of the vacant oxygen sites with S atoms leading to the generation of CeO2-xSy phases. The latter exhibits a neat-metallic character. Through the occurrence of both Eley-Rideal and Langmuir-Hinshelwood-type mechanisms, CeO2-xSy phases maintain catalytic activity. Findings herein provide a detail atomistic understanding of the desulfurization capacity of stand-alone ceria surfaces.

Metal-Support Interaction and Charge Distribution in Ceria-Supported Au Particles Exposed to CO.

Understanding how reaction conditions affect metal–support interactions in catalytic materials is one of the most challenging tasks in heterogeneous catalysis research. Metal nanoparticles and their supports often undergo changes in structure and oxidation state when exposed to reactants, hindering a straightforward understanding of the structure–activity relations using only ex situ or ultrahigh vacuum techniques. Overcoming these limitations, we explored the metal–support interaction between gold nanoparticles and ceria supports in ultrahigh vacuum and after exposure to CO. A combination of in situ methods (on powder and model Au/CeO2 samples) and theoretical calculations was applied to investigate the gold/ceria interface and its reactivity toward CO exposure. X-ray photoelectron spectroscopy measurements rationalized by first-principles calculations reveal a distinctly inhomogeneous charge distribution, with Au+ atoms in contact with the ceria substrate and neutral Au0 atoms at the surface of the Au nanoparticles. Exposure to CO partially reduces the ceria substrate, leading to electron transfer to the supported Au nanoparticles. Transferred electrons can delocalize among the neutral Au atoms of the particle or contribute to forming inert Auδ− atoms near oxygen vacancies at the ceria surface. This charge redistribution is consistent with the evolution of the vibrational frequencies of CO adsorbed on Au particles obtained using diffuse reflectance infrared Fourier transform spectroscopy.

Surface modification of carbon fibres with ammonium cerium nitrate for interfacial shear strength enhancement

An ongoing research focus in the carbon fibre (CF) field has been to identify a practical and more effective surface modification method for improving CF's interfacial adhesion and bonding capacity, which is vital for achieving high performance CF composites. In this research, carbon fibres were modified by using a new and facile treatment process in an aqueous solution of ammonium cerium nitrate (ACN) at a low temperature (20 °C–50 °C). The surface analysis results revealed that the ACN treatment imparted a combined effect of CF surface oxidation and cerium (IV)/(III) immobilisation on the oxidised CF surface through coordination bonds, leading to a significantly increased specific surface area, surface energy and surface reactivity. These new surface characteristics directly contributed to an improved CF interfacial adhesion and load transfer between the modified CFs and an epoxy matrix. The study demonstrated that under specified conditions, the new treatment method resulted in 304% increase in interfacial shear strength (IFSS) and 130% increase in interlaminar shear strength (ILSS) without any significant reduction in CF's mechanical properties. In comparison, the ACN modification yielded CFs with a much higher ILSS than the current industrial processes using electrochemical oxidation and sizing.