Oxygen Species For CO Oxidation Research Articles

Low-Temperature CO Oxidation over CuO-CeO2/Fe2O3 Catalyst: Effect of KMnO4 Modification

In this paper, the effect of KMnO4 modification on the performance of CO oxidation over a CuCeOx/Fe2O3 catalyst in industrial flue gas was studied. X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature-programmed reduction by H2 (H2-TPR), Fourier transform infrared spectroscope (FTIR), and Raman spectra were employed to reveal the relationship between the structural properties and the performance of the catalyst. The results show that the catalytic activity could be increased by KMnO4 modification. CuCeOx/Fe2O3 modified with 0.07 mol/L KMnO4 exhibited high activity, with a CO conversion of 78.2% at 160 °C. The modification of KMnO4 reduced the grain size and promoted the uniform dispersion of active particles. In addition, the number of oxygen-containing functional groups increased, which could anchor metal particles and provide active oxygen species for CO oxidation.

Tailoring catalytic performance of carbon nanotubes confined CuO[sbnd]CeO2 catalysts for CO preferential oxidation

Multi-wall carbon nanotubes confined CuOCeO2 binary oxide catalysts (CuxCe1-xO/CNTs, x = 0, 0.2, 0.4, 0.6 and 1.0) have been prepared by a special ultrasound-aided impregnation. The effects of CuCe molar ratios on the structure and catalytic performance for CO preferential oxidation have been systematically investigated. Judging from XRD patterns and Raman analysis, Cu2+ ions incorporate into CeO2 lattice to form ceria-base solid solution, accompanied by oxygen vacancies and an interaction produces between the catalyst particles and inner surface of carbon nanotubes. TEM microstructural analysis shows that CuO and CeO2 particles are uniformly dispersed in carbon nanotubes for the CuxCe1-xO/CNTs (x = 0.4) catalyst. Meanwhile, the reducibility of active copper species is enhanced markedly. In addition, the Cu0.4Ce0.6O/CNTs sample possesses more surface lattice oxygen and oxygen vacancies according to surface species characterization, which can provide more active oxygen species for CO oxidation and promote the mobility of lattice oxygen. The CuxCe1-xO/CNTs (x = 0.4) catalyst exhibits excellent activity, wide temperature span of full CO conversion (120–160 °C) and high selectivity especially above 110 °C (maintaining high selectivity until 140 °C).

How Temperature Affects the Mechanism of CO Oxidation on Au/TiO2: A Combined EPR and TAP Reactor Study of the Reactive Removal of TiO2 Surface Lattice Oxygen in Au/TiO2 by CO

Despite enormous breakthroughs in our understanding of the reaction mechanism of the low-temperature CO oxidation on gold catalysts, in particular on Au/TiO2 and down to temperatures as low as −150 °C, there are still many contradictory proposals about the dominant reaction pathway. In this work, we will demonstrate that these discrepancies often originate from the rather different reaction conditions applied in numerous studies, most notably from different reaction temperatures. By combining temporal analysis of products reactor measurements with electron paramagnetic resonance spectroscopy, we will show that removal of TiO2 surface lattice oxygen from a Au/TiO2 catalyst upon exposure to CO (i) readily takes place at 120 °C, where it represents the active oxygen species for CO oxidation, (ii) is still possible at −20 °C, although much slower and to a much lower extent, and (iii) is completely inhibited at −90 °C. Consequences of these findings for our understanding of the dominant reaction pathway for the CO oxidation on Au/TiO2 catalysts, in particular its dependency on the reaction temperature, will be discussed.

Activation of Molecular Oxygen and the Nature of the Active Oxygen Species for CO Oxidation on Oxide Supported Au Catalysts

Although highly dispersed Au catalysts with Au nanoparticles (NPs) of a few nanometers in diameter are well-known for their high catalytic activity for several oxidation and reduction reactions already at rather low temperatures for almost 30 years, central aspects of the reaction mechanism are still unresolved. While most studies focused on the active site, the active Au species, and the effect of the support material, the most crucial step during oxidation reactions, the activation of molecular oxygen and the nature of the resulting active oxygen species (Oact), received more attention just recently. This is topic of this Account, which focuses on the formation, location, and nature of the Oact species present on metal oxide supported Au catalysts under typical reaction conditions, at room temperature and above. It is mainly based on quantitative temporal analysis of products (TAP) reactor measurements, which different from most spectroscopic techniques are able to detect and quantify these species even at the extremely low concentrations present under realistic reaction conditions. Different types of pulse experiments were performed, during which the highly dispersed, realistic powder catalysts are exposed to very low amounts of reactants, CO and/or O2, in order to form and reactively remove Oact species and gain information on their formation, nature, and the active site for Oact formation. Our investigations have shown that the active oxygen species for CO oxidation on Au/TiO2 for reaction at 80 °C and higher is a highly stable atomic species, which at 80 °C is formed only at the perimeter of the Au-oxide interface and whose reactive removal by CO is activated, but not its formation. From these findings, it is concluded that surface lattice oxygen represents the Oact species for the CO oxidation. Accordingly, the CO oxidation proceeds via a Au-assisted Mars-van Krevelen mechanism, during which surface lattice oxygen close to the Au NPs is removed by reaction with CO, resulting in a partially reduced TiO2 surface, which is subsequently reoxidized by O2. We demonstrate that this is the dominant reaction pathway for Au catalysts based on reducible metal oxides in general, at typical reaction temperatures, while for less active Au catalysts based on nonreducible metal oxides, this reaction pathway is not possible and the remaining activity must arise from another pathway, most probably a Au-only mechanism. At lower reaction temperature, reactive removal of Oact becomes increasingly inhibited, leading to a change in the dominant reaction pathway.

CO Adsorption-Driven Surface Segregation of Pd on Au/Pd Bimetallic Surfaces: Role of Defects and Effect on CO Oxidation

We use density functional theory (DFT) to study CO-adsorption-induced Pd surface segregation in Au/Pd bimetallic surfaces, dynamics of Pd–Au swapping, effect of defects on the swapping rate, CO-induced Pd clustering, and the reaction mechanism of CO oxidation. The strong CO-philic nature of Pd atoms supplies a driving force for the preferential surface segregation of Pd atoms and Pd cluster formation. Surface vacancies are found to dramatically accelerate the rate of Pd–Au swapping. We find that Pd clusters consisting of at least four Pd atoms prefer to bind O2 rather than CO. These clusters facilitate the rapid dissociation of O2 and supply reactive oxygen species for CO oxidation. Our findings suggest that geometric, electronic, and dynamic effects should be considered in the function of bimetallic alloys or nanoparticles whose components asymmetrically interact with reacting molecules.

Hydrogen Treatment-Induced Surface Reconstruction: Formation of Superoxide Species on Activated Carbon over Ag/Activated Carbon Catalysts for Selective Oxidation of CO in H2-Rich Gases

Superoxide species were detected on activated carbon (AC) after H2 treatment at high temperatures (>300 °C) and subsequent oxygen adsorption at 80 °C. The introduction of Ag onto the carbon surface promoted the formation of superoxide species. For Ag/AC catalysts, the superoxide species may be able to spill over from the carbon surface to the silver particles and react with CO under reaction conditions; while over AC itself, superoxide species have no activity toward CO oxidation. This is another active oxygen species for CO oxidation over carbon-supported silver catalysts.