Identifying the active redox oxygen sites in a mixed Cu and Ce oxide catalyst by in situ X-ray absorption spectroscopy and anaerobic reactions with CO in concentrated H2

Harnessing photocatalytic activity of mesoporous graphitic carbon nitride decorated by copper single-atom catalysts for oxidative dehydrogenation of N-heterocycles

This work describes the application of Cu single-atom catalysts (SACs) for photocatalytic oxidative dehydrogenation of N-heterocyclic amines to the respective N-heteroaromatics through environmentally benign and sustainable pathways. The mesoporous graphitic carbon nitride (mpg-C3N4), prepared by the one-step pyrolysis method, possesses a lightweight material with a high surface area (95 m2 g-1) and an average pore diameter (3.6 nm). A simple microwave-assisted preparation method was employed to decorate Cu single-atom over mpg-C3N4 support. The Cu single-atom decorated on mpg-C3N4 support (Cu@mpg-C3N4) is characterized by various characterization techniques, including XRD, UV-visible spectrophotometry, HRTEM, HAADF-STEM with elemental mapping, AC-STEM, ICP-OES, XANES, EXAFS, and BET surface area. These characterization studies confirmed that the Cu@mpg-C3N4 catalyst exhibited high surface area, mesoporous nature, medium band gap, and low metal loading. The as-synthesized and well-characterized Cu@mpg-C3N4 single-atom photocatalyst is then evaluated for its efficacy in converting N-heterocycles into corresponding N-heteroaromatic compounds with excellent conversion and selectivity (>99 %). This transformation is achieved using water as a green solvent and a 30 W white light as a visible light source, demonstrating the catalyst's potential for sustainable and environmentally benign reactions.

Kinetics of Oxygen Exchange and N2O Decomposition Reaction over MeOx/CeO2 (Me = Fe, Co, Ni) Catalysts

MeO_x/CeO₂ (Me = Fe, Co, Ni) samples were tested in an ¹⁸O₂ temperature-programmed isotope exchange and N₂O decomposition (deN₂O). A decrease in the rate of deN₂O in the presence of oxygen evidences the competitive adsorption of N₂O and O₂ on the same sites. A study of isotope oxygen exchange revealed dissociative oxygen adsorption with the subsequent formation of surface oxygen species. The same species, more probably, result from N₂O adsorption and the following N₂ evolution to the gas phase. We supposed the same mechanism of O₂ formation from surface oxygen species in both reactions, including the stages responsible for its mobility. A detailed analysis of the kinetics of isotope exchange has been performed, and the rates of one-atom (R^I) and two-atom (R^II) types of exchange were evaluated. The rate of the stage characterizing the mobility of surface oxygen was calculated, supposing the same two-step mechanism was relevant for both types of exchange. The effect of oxygen mobility on the kinetics of deN₂O was estimated. An analysis of the possible pathways of isotope transfer from MeO_x to CeO_x showed that direct oxygen exchange on the Me-Ce interface makes a valuable contribution to the rate of this reaction. The principal role of the Me-Ce interface in deN₂O was confirmed with independent experiments on FeO_x/CeO₂ samples with a different iron content.

Investigations of the Effect of H2 in CO Oxidation over Ceria Catalysts

The preferential CO oxidation (so-called CO-PROX) is the selective CO oxidation amid H2-rich atmospheres, a process where ceria-based materials are consolidated catalysts. This article aims to disentangle the potential CO–H2 synergism under CO-PROX conditions on the low-index ceria surfaces (111), (110) and (100). Polycrystalline ceria, nanorods and ceria nanocubes were prepared to assess the physicochemical features of the targeted surfaces. Diffuse reflectance infrared Fourier-transformed spectroscopy (DRIFTS) shows that ceria surfaces are strongly carbonated even at room temperature by the effect of CO, with their depletion related to the CO oxidation onset. Conversely, formate species formed upon OH + CO interaction appear at temperatures around 60 °C and remain adsorbed regardless the reaction degree, indicating that these species do not take part in the CO oxidation. Density functional theory calculations (DFT) reveal that ceria facets exhibit high OH coverages all along the CO-PROX reaction, whilst CO is only chemisorbed on the (110) termination. A CO oxidation mechanism that explains the early formation of carbonates on ceria and the effect of the OH coverage in the overall catalytic cycle is proposed. In short, hydroxyl groups induce surface defects on ceria that increase the COx–catalyst interaction, revealed by the CO adsorption energies and the stabilization of intermediates and readsorbed products. In addition, high OH coverages are shown to facilitate the hydrogen transfer to form less stable HCOx products, which, in the case of the (110) and (100), is key to prevent surface poisoning. Altogether, this work sheds light on the yet unclear CO–H2 interactions on ceria surfaces during CO-PROX reaction, providing valuable insights to guide the design of more efficient reactors and catalysts for this process.

A Photoactivated Cu-CeO2 Catalyst with Cu-[O]-Ce Active Species Designed through MOF Crystal Engineering

Fully utilizing solar energy for catalysis requires the integration of conversion mechanisms and therefore delicate design of catalyst structures and active species. Herein, a MOF crystal engineering method was developed to controllably synthesize a copper-ceria catalyst with well-dispersed photoactive Cu-[O]-Ce species. Using the preferential oxidation of CO as a model reaction, the catalyst showed remarkably efficient and stable photoactivated catalysis, which found practical application in feed gas treatment for fuel cell gas supply. The coexistence of photochemistry and thermochemistry effects contributes to the high efficiency. Our results demonstrate a catalyst design approach with atomic or molecular precision and a combinatorial photoactivation strategy for solar energy conversion.

CoOx-decorated CeO2 heterostructures: effects of morphology on their catalytic properties in diesel soot combustion

The effect of the morphology, which exposes different crystal planes, on the physicochemical properties and catalytic activity in diesel carbon soot oxidation was studied using CoOx-decorated CeO2 (CoCeO2) heterostructured catalysts, such as nanorods (NRs), nanocubes (NCs), and nanoparticles (NPs). The CoOx/CeO2 nanorods (CoCeO2-NR) showed superior carbon soot combustion activity at lower temperatures to CoCeO2-NCs and CoCeO2-NPs under both tight and loose contact modes with soot combustion temperatures (T50) of 321 and 494 °C, respectively. A comprehensive analysis by means of X-ray diffraction, Raman spectroscopy, high-angle annular dark-field scanning transmission electron microscopy, in situ X-ray absorption fine structure, temperature-programmed reduction, oxygen storage/release measurements, and density functional theory calculations revealed that the improved activity of CoCeO2-NRs is mainly ascribed to the high oxygen release rate and strong redox capability of the supported Co species, with complete reversibility. This originates from the high reactivity of oxygen atoms on (110) surfaces, compared to (100) and (111) surfaces over CeO2. Additionally, CoCeO2-NRs displayed durability and recyclability without any significant loss of catalytic activity or structural change. These insights will aid in the rational design of practical catalysts for the purification of diesel exhaust and other important transformations.

Morphology-dependent oxygen vacancies and synergistic effects of Ni/CeO2 catalysts for N2O decomposition

Strong morphology-dependent oxygen vacancies and synergistic effects of Ni/CeO₂ catalysts and their vital effects on N₂O decomposition.

Engineering surface defects and metal–support interactions on Pt/TiO2(B) nanobelts to boost the catalytic oxidation of CO

The thermally reduced Pt/TiO₂(B) catalysts show high catalytic activity and good water resistance for the catalytic oxidation of CO.

Dumbbell-like Au0.5Cu0.5@Fe3O4 Nanocrystals: Synthesis, Characterization, and Catalytic Activity in CO Oxidation

We report the colloidal synthesis of dumbbell-like Au_0.5Cu_0.5@Fe₃O₄ nanocrystals (AuCu@FeOx NCs) and the study of their properties in the CO oxidation reaction. To this aim, the as-prepared NCs were deposited on γ-alumina and pretreated in an oxidizing environment to remove the organic ligands. A comparison of these NCs with bulk Fe₃O₄-supported AuCu NCs showed that the nanosized support was far more effective in preventing the sintering of the metal domains, leading thus to a superior catalytic activity. Nanosizing of the support could be thus an effective, general strategy to improve the thermal stability of metallic NCs. On the other hand, the support size did not affect the chemical transformations experienced by the AuCu NCs during the activation step. Independently from the support size, we observed indeed the segregation of Cu from the alloy phase under oxidative conditions as well as the possible incorporation of the Cu atoms in the iron oxide domain.

MeOx/Al2O3 and MeOx/CeO2 (Me = Fe, Co, Ni) catalysts for high temperature N2O decomposition and NH3 oxidation

Efficient oxygen transfer through Me–CeO₂ interface explains higher activity of MeO_x/CeO₂ (Me = Fe, Co, Ni) samples in deN₂O and NH₃ oxidation compared with MeO_x/Al₂O₃ ones.

Structural features and catalytic performance in CO preferential oxidation of CuO–CeO2 supported on multi-walled carbon nanotubes

MWCNT supported CuO–CeO₂ catalysts show enhanced performance in CO-PROX due to unusual structure features induced by interactions between metal oxides and MWCNT.