Facet-Dependent Reactivity of Ceria Nanoparticles Exemplified by CeO2-Based Transition Metal Catalysts: A Critical Review

N2 as an Efficient IR Probe Molecule for the Investigation of Ceria-Containing Materials

Ceria and ceria-based catalysts are very important in redox and acid-base catalysis. Nanoceria have also been found to be important in biomedical applications. To design efficient materials, it is necessary to thoroughly understand the surface chemistry of ceria, and one of the techniques that provides such information about the surface is the vibrational spectroscopy of probe molecules. Although the most commonly used probe is CO, it has some disadvantages when applied to ceria and ceria-based catalysts. CO can easily reduce the material, forming carbonate-like species, and can be disproportionate, thus modifying the surface. Here, we offer a pioneering study of the adsorption of 15N2 at 100 K, demonstrating that dinitrogen can be more advantageous than CO when studying ceria-based materials. As an inert gas, N2 is not able to oxidize or reduce cerium cations and does not form any surface anionic species able to modify the surface. It is infrared and transparent, and thus there is no need to subtract the gas phase spectrum, something that often increases the noise level. Being a weaker base than CO, N2 has a negligible induction effect. By using stoichiometric nano-shaped ceria samples, we concluded that 15N2 can distinguish between surface Ce4+ sites on different, low index planes; with cations on the {110} facets and on some of the edges, Ce4+-15N2 species with IR bands at 2258-2257 cm-1 are formed. Bridging species, where one of the N atoms from the molecule interacts with two Ce4+ cations, are formed on the {100} facets (2253-2252 cm-1), while the interaction with the {111} facets is very weak and does not lead to the formation of measurable amounts of complexes. All species are formed by electrostatic interaction and disappear during evacuation at 100 K. In addition, N2 provides more accurate information than CO on the acidity of the different OH groups because it does not change the binding mode of the hydroxyls.

CeO2-Supported TiO2-Pt Nanorod Composites as Efficient Catalysts for CO Oxidation

Supported Pt-based catalysts have been identified as highly selective catalysts for CO oxidation, but their potential for applications has been hampered by the high cost and scarcity of Pt metals as well as aggregation problems at relatively high temperatures. In this work, nanorod structured (TiO₂-Pt)/CeO₂ catalysts with the addition of 0.3 at% Pt and different atomic ratios of Ti were prepared through a combined dealloying and calcination method. XRD, XPS, SEM, TEM, and STEM measurements were used to confirm the phase composition, surface morphology, and structure of synthesized samples. After calcination treatment, Pt nanoparticles were semi-inlayed on the surface of the CeO₂ nanorod, and TiO₂ was highly dispersed into the catalyst system, resulting in the formation of (TiO₂-Pt)/CeO₂ with high specific surface area and large pore volume. The unique structure can provide more reaction path and active sites for catalytic CO oxidation, thus contributing to the generation of catalysts with high catalytic activity. The outstanding catalytic performance is ascribed to the stable structure and proper TiO₂ doping as well as the combined effect of Pt, TiO₂, and CeO₂. The research results are of importance for further development of high catalytic performance nanoporous catalytic materials.

MOF-Derived CeO2 and CeZrOx Solid Solutions: Exploring Ce Reduction through FTIR and NEXAFS Spectroscopy

The development of Ce-based materials is directly dependent on the catalyst surface defects, which is caused by the calcination steps required to increase structural stability. At the same time, the evaluation of cerium's redox properties under reaction conditions is of increasing relevant importance. The synthesis of Ce-UiO-66 and CeZr-UiO-66 and their subsequent calcination are presented here as a simple and inexpensive approach for achieving homogeneous and stable CeO₂ and CeZrO_x nanocrystals. The resulting materials constitute an ideal case study to thoroughly understand cerium redox properties. The Ce³⁺/Ce⁴⁺ redox properties are investigated by H₂-TPR experiments exploited by in situ FT-IR and Ce M₅-edge AP-NEXAFS spectroscopy. In the latter case, Ce³⁺ formation is quantified using the MCR-ALS protocol. FT-IR is then presented as a high potential/easily accessible technique for extracting valuable information about the cerium oxidation state under operating conditions. The dependence of the OH stretching vibration frequency on temperature and Ce reduction is described, providing a novel tool for qualitative monitoring of surface oxygen vacancy formation. Based on the reported results, the molecular absorption coefficient of the Ce³⁺ characteristic IR transition is tentatively evaluated, thus providing a basis for future Ce³⁺ quantification through FT-IR spectroscopy. Finally, the FT-IR limitations for Ce³⁺ quantification are discussed.

Commemorative Issue in Honor of Professor Gerhard Ertl on the Occasion of His 85th Birthday

This Special Issue (SI) is dedicated to Professor Gerhard Ertl on his eighty-fifth birthday [...]

Hybrid Biochar/Ceria Nanomaterials: Synthesis, Characterization and Activity Assessment for the Persulfate-Induced Degradation of Antibiotic Sulfamethoxazole

Biochar from spent malt rootlets was employed as the template to synthesize hybrid biochar-ceria materials through a wet impregnation method. The materials were tested for the activation of persulfate (SPS) and subsequent degradation of sulfamethoxazole (SMX), a representative antibiotic, in various matrices. Different calcination temperatures in the range 300-500 °C were employed and the resulting materials were characterized by means of N2 adsorption and potentiometric mass titration as well as TGA, XRD, SEM, FTIR, DRS, and Raman spectroscopy. Calcination temperature affects the biochar content and the physicochemical properties of the hybrid materials, which were tested for the degradation of 500 μg L-1 SMX with SPS (in the range 200-500 mg L-1) in various matrices including ultrapure water (UPW), bottled water, wastewater, and UPW spiked with bicarbonate, chloride, or humic acid. Materials calcined at 300-350 °C, with a surface area of ca. 120 m2 g-1, were the most active, yielding ca. 65% SMX degradation after 120 min of reaction in UPW; materials calcined at higher temperatures as well as bare biochar were less active. Degradation decreased with increasing matrix complexity due to the interactions amongst the surface, the contaminant, and the oxidant. Experiments in the presence of scavengers (i.e., methanol, t-butanol, and sodium azide) revealed that sulfate and hydroxyl radicals as well as singlet oxygen were the main oxidative species.

Assessing the oxygen reduction reaction by a 2-electron mechanism on ceria surfaces

The 2-electron pathway of the oxygen reduction reaction is an unwanted process in the development of fuel cells. In contrast, it has gained the scientific community's attention due to its importance as a promising way of removing emergent pollutants and endocrine disruptors from water bodies and a more sustainable alternative for large-scale commercial hydrogen peroxide production. Cerium oxide has shown remarkable potential and selectivity experimentally for this mechanism, and its possible applications, exceeding the previous reference materials. In this work, we studied the 2-electron pathway for oxygen reduction on different ceria-cleaving directions (100), (110), (221), and (331) by first principles methods based on density functional theory. Our results show that the (100) surface is the most favorable for reduction, with the (331) crystallographic plane also showing potential for good catalytic activity. This fact could be essential for designing new nanostructures, with higher portions of those planes exposed, for higher catalytic activity.

Rational Design of Non-Precious Metal Oxide Catalysts by Means of Advanced Synthetic and Promotional Routes

Catalysis is an indispensable part of our society, involved in numerous energy and environmental applications, such as the production of value-added chemicals/fuels, hydrocarbons processing, fuel cells applications, abatement of hazardous pollutants, among others [...]

Shape Effects of Ceria Nanoparticles on the Water‒Gas Shift Performance of CuOx/CeO2 Catalysts

The copper–ceria (CuOx/CeO2) system has been extensively investigated in several catalytic processes, given its distinctive properties and considerable low cost compared to noble metal-based catalysts. The fine-tuning of key parameters, e.g., the particle size and shape of individual counterparts, can significantly affect the physicochemical properties and subsequently the catalytic performance of the binary oxide. To this end, the present work focuses on the morphology effects of ceria nanoparticles, i.e., nanopolyhedra (P), nanocubes (C), and nanorods (R), on the water–gas shift (WGS) performance of CuOx/CeO2 catalysts. Various characterization techniques were employed to unveil the effect of shape on the structural, redox and surface properties. According to the acquired results, the support morphology affects to a different extent the reducibility and mobility of oxygen species, following the trend: R > P > C. This consequently influences copper–ceria interactions and the stabilization of partially reduced copper species (Cu+) through the Cu2+/Cu+ and Ce4+/Ce3+ redox cycles. Regarding the WGS performance, bare ceria supports exhibit no activity, while the addition of copper to the different ceria nanostructures alters significantly this behaviour. The CuOx/CeO2 sample of rod-like morphology demonstrates the best catalytic activity and stability, approaching the thermodynamic equilibrium conversion at 350 °C. The greater abundance in loosely bound oxygen species, oxygen vacancies and highly dispersed Cu+ species can be mainly accounted for its superior catalytic performance.