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.

Nickel Phosphide Catalysts as Efficient Systems for CO2 Upgrading via Dry Reforming of Methane

This work establishes the primordial role played by the support’s nature when aimed at the constitution of Ni2P active phases for supported catalysts. Thus, carbon dioxide reforming of methane was studied over three novel Ni2P catalysts supported on Al2O3, CeO2 and SiO2-Al2O3 oxides. The catalytic performance, shown by the catalysts’ series, decreased according to the sequence: Ni2P/Al2O3 > Ni2P/CeO2 > Ni2P/SiO2-Al2O3. The depleted CO2 conversion rates discerned for the Ni2P/SiO2-Al2O3 sample were associated to the high sintering rates, large amounts of coke deposits and lower fractions of Ni2P constituted in the catalyst surface. The strong deactivation issues found for the Ni2P/CeO2 catalyst, which also exhibited small amounts of Ni2P species, were majorly associated to Ni oxidation issues. Along with lower surface areas, oxidation reactions might also affect the catalytic behaviour exhibited by the Ni2P/CeO2 sample. With the highest conversion rate and optimal stabilities, the excellent performance depicted by the Ni2P/Al2O3 catalyst was mostly related to the noticeable larger fractions of Ni2P species established.

Total Oxidation of Methane on Oxide and Mixed Oxide Ceria-Containing Catalysts

Methane, discovered in 1766 by Alessandro Volta, is an attractive energy source because of its high heat of combustion per mole of carbon dioxide. However, methane is the most abundant hydrocarbon in the atmosphere and is an important greenhouse gas, with a 21-fold greater relative radiative effectiveness than CO2 on a per-molecule basis. To avoid or limit the formation of pollutants that are dangerous for both human health and the atmospheric environment, the catalytic combustion of methane appears to be one of the most promising alternatives to thermal combustion. Total oxidation of methane, which is environmentally friendly at much lower temperatures, is believed to be an efficient and economically feasible way to eliminate pollutants. This work presents a literature review, a statu quo, on catalytic methane oxidation on transition metal oxide-modified ceria catalysts (MOx/CeO2). Methane was used for this study since it is of great interest as a model compound for understanding the mechanisms of oxidation and catalytic combustion on metal oxides. The objective was to evaluate the conceptual ideas of oxygen vacancy formation through doping to increase the catalytic activity for methane oxidation over CeO2. Oxygen vacancies were created through the formation of solid solutions, and their catalytic activities were compared to the catalytic activity of an undoped CeO2 sample. The reaction conditions, the type of catalysts, the morphology and crystallographic facets exposing the role of oxygen vacancies, the deactivation mechanism, the stability of the catalysts, the reaction mechanism and kinetic characteristics are summarized.

Coke-Resistant Rh and Ni Catalysts Supported on γ-Al2O3 and CeO2 for Biogas Oxidative Steam Reforming

The depletion of fossil fuels and the growing concerns related to the environmental impact of their processing has progressively switched the interest towards the utilization of biomass-derived materials for a large variety of processes. Among them, biogas, which is a CH4-rich gas deriving from anaerobic digestion of biomass, has acquired a lot of interest as a feedstock for reforming processes. The main issue in employing biogas is related to the carbon deposition and active metal sintering, which are both responsible for the deactivation of the catalyst. In this work, bimetallic and monometallic Rh- and Ni-based formulations were supported on alumina and ceria with the aim of evaluating their activity and stability in biogas oxidative steam reforming. The Rh addition to the monometallic Ni/γ-Al2O3 formulation enhances its catalytic performances; nevertheless, this induces a higher coke deposition, thus suggesting a preferential coke formation on Rh sites. The initial activity of the CeO2-supported catalysts was found to be lower than the Al2O3-supported catalysts, but the 5%Ni/CeO2 sample showed a very good stability during the test and, despite the lower activity, 0.5%Rh-5%Ni/CeO2 did not show coke deposition. The results suggest that the promotion of Ni/CeO2 catalysts with other active metals could lead to the selection of a highly stable and performing formulation for biogas oxidative steam reforming.

Tuning the Integration Rate of Ce(Ln)O2 Nanoclusters into Nanoparticulated ZrO2 Supports: When the Cation Size Matters

Three nanostructured catalysts with low total rare earth elements (REEs) content (i.e., 15 mol.%) were prepared by depositing CeO2 or Ln3+-doped CeO2 (Ln3+ = Y3+ or La3+; Ln/Ce = 0.15) on the surface of ZrO2 nanoparticles, as nanometre-thick, fluorite-type clusters. These samples were subjected to successive reduction treatments at increasing temperatures, from 500 to 900 °C. A characterisation study by XPS was performed to clarify the diffusion process of cerium into the bulk of ZrO2 crystallites upon reduction to yield CexZr1−xO2−δ surface phases, and the influence of the incorporation of non-reducible trivalent REE cations, with sizes smaller (Y3+) and larger (La3+) than Ce4+ and Ce3+. For all nanocatalysts, a reduction treatment at a minimum temperature of 900 °C was required to accomplish a significant cerium diffusion. Notwithstanding, the size of the dopant noticeably affected the extent of this diffusion process. As compared to the undoped ZrO2-CeO2 sample, Y3+ incorporation slightly hindered the cerium diffusion, while the opposite effect was found for the La3+-doped nanocatalyst. Furthermore, such differences in cerium diffusion led to changes in the surface and nanostructural features of the oxides, which were tentatively correlated with the redox response of the thermally aged samples.

High-Performing Au-Ag Bimetallic Catalysts Supported on Macro-Mesoporous CeO2 for Preferential Oxidation of CO in H2-Rich Gases

We report here an investigation on the preferential oxidation of carbon monoxide in an H2-rich stream (CO-PROX reaction) over mono and bimetallic Au-Ag samples supported on macro-mesoporous CeO2. The highly porous structure of ceria and the synergistic effect, which occurs between the bimetallic Au-Ag system and the support, led to promising catalytic performance at low temperature (CO2 yield of 88% and CO2 selectivity of 100% at 60 °C), which is suitable for a possible application in the polymer electrolyte membrane fuel cell (PEMFC). The morphological, structural, textural and surface features of the catalysts were determined by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), N2-adsoprtion-desorption measurements, Temperature Programmed Reduction in hydrogen (H2-TPR), Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS). Furthermore, the catalytic stability of the best active catalyst, i.e., the AuAg/CeO2 sample, was evaluated also in the presence of water vapor and carbon dioxide in the gas stream. The excellent performances of the bimetallic sample, favored by the peculiar porosity of the macro-mesoporous CeO2, are promising for possible scale-up applications in the H2 purification for PEM fuel cells.

Synthesis and characterization of CeLaMO2 (M: Sm, Gd, Dy) compounds for solid ceramic electrolytes

In this study, pure CeO2 and Ce0.85La0.10M0.05O2 (M: Sm3+, Gd3+, Dy3+) solid electrolytes were synthesized using the sol-gel method and sintered at 1350?C for 12 h. X-ray diffraction (XRD) was used for crystal structure characterization of the ceramic solid electrolytes. After sintering, all prepared solid electrolytes were indexed to be cubic crystal lattices. The thermal properties of the prepared samples were investigated by thermogravimetric (TG) and differential thermal analysis (DTA) methods. The surface properties of the grain structure of the ceramic solid electrolytes were evaluated by scanning electron microscopy (FE-SEM) confirming the average grain size of about 1 ?m. The electrochemical impedance spectroscopy technique was used to investigate AC electrical properties of prepared solid electrolytes. The conductivity values at 750?C of the Ce0.85La0.10Sm0.05O2, Ce0.85La0.10Gd0.05O2 and Ce0.85La0.10Dy0.05O2 and pure CeO2 were found to be 1.10 ? 10?3 S/cm, 3.05 ? 10?4 S/cm, 8.85 ? 10?4 S/cm and 8.44 ? 10?10 S/cm, respectively. The characterization results showed that the La-Sm co-doped CeO2 sample can be used as a ceramic electrolyte in intermediate temperature solid oxide fuel cells (IT-SOFC).

Ceria Nanoparticles’ Morphological Effects on the N2O Decomposition Performance of Co3O4/CeO2 Mixed Oxides

Ceria-based oxides have been widely explored recently in the direct decomposition of N2O (deN2O) due to their unique redox/surface properties and lower cost as compared to noble metal-based catalysts. Cobalt oxide dispersed on ceria is among the most active mixed oxides with its efficiency strongly affected by counterpart features, such as particle size and morphology. In this work, the morphological effect of ceria nanostructures (nanorods (ΝR), nanocubes (NC), nanopolyhedra (NP)) on the solid-state properties and the deN2O performance of the Co3O4/CeO2 binary system is investigated. Several characterization methods involving N2 adsorption at −196 °C, X-ray diffraction (XRD), temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (ΤΕΜ) were carried out to disclose structure–property relationships. The results revealed the importance of support morphology on the physicochemical properties and the N2O conversion performance of bare ceria samples, following the order nanorods (NR) > nanopolyhedra (NP) > nanocubes (NC). More importantly, Co3O4 impregnation to different carriers towards the formation of Co3O4/CeO2 mixed oxides greatly enhanced the deN2O performance as compared to bare ceria samples, without, however, affecting the conversion sequence, implying the pivotal role of ceria support. The Co3O4/CeO2 sample with the rod-like morphology exhibited the best deN2O performance (100% N2O conversion at 500 °C) due to its abundance in Co2+ active sites and Ce3+ species in conjunction to its improved reducibility, oxygen kinetics and surface area.