Low-temperature NO oxidation using lattice oxygen in Fe-site substituted SrFeO3-δ.

Improvement of the low-temperature activity for NO oxidation catalysts is a crucial issue to improve the NOx storage performance in automotive catalysts. We have recently reported that the lattice oxygen species in SrFeO3-δ (SFO) are reactive in the oxidation of NO to NO2 at low temperatures. The oxidation of NO using lattice oxygen species is a powerful means to oxidize NO in such kinetically restricted temperature regions. This paper shows that Fe-site substitution of SFO with Mn or Co improves the properties of lattice oxygen such as the temperature and amount of oxygen release/storage, resulting in the enhancement of the activity for NO oxidation in a low-temperature range. In particular, NO oxidation on SrFe0.8Mn0.2O3-δ is found to proceed even at extremely low temperatures <423 K. From oxygen release/storage profiles obtained by temperature-programmed reactions, Co doping into SFO increases the amount of released oxygen owing to the reducibility of the Co species and promotes the phase transformation to the brownmillerite phase. On the other hand, Mn doping does not increase the oxygen release amount and suppresses the phase transformation. However, it significantly decreases the oxygen migration barrier of SFO. Substitution with Mn renders the structure of SFO more robust and maintains the perovskite structure after the release of oxygen. Thus, the oxygen release properties are strongly dependent on the crystal structure change before and after oxygen release from the perovskite structure, which has a significant effect on NO oxidation and the NOx storage performance.

A detailed insight into the catalytic reduction of NO operated by Cr-Cu nanostructures embedded in a CeO2 surface.

Replacing rare and expensive elements, such as Pt, Pd, and Rh, commonly used in catalytic devices with more abundant and less expensive ones is mandatory to realize efficient, sustainable and economically appealing three-way catalysts. In this context, the surface of a Cr-Cu/CeO2 system represents a versatile catalyst for the conversion of toxic NO into harmless N2. Yet, a clear picture of the underlying mechanism is still missing. We provide here a detailed insight into such a reaction mechanism by means of a combined experimental and theoretical study. Fourier-transform infrared spectroscopy is used to detect all the products resulting from catalytic reactions of NO and CO on the surface of a Cr-Cu/CeO2 nanocatalyst. CO pulsing experiments unveil that reactions of CO with O atoms at the Cr-Cu/CeO2 surface are the major factors responsible for the formation of surface vacancies. On these grounds, a comprehensive picture of the NO reduction and the role of both Cu and Cr dopants and vacancies is rationalized by first-principles modeling. Our findings provide a general route for the realization of ceria-based cost-effective catalysts.

Potential Interference of Oil Vehicles on Genital Tubercle Development during the Fetal Period in ICR Mice.

Corn oil, sesame oil, and 10% ethanol in corn oil are commonly used as dosing vehicles in toxicology studies. Since these vegetable oils contain bioactive compounds, it is important for toxicology studies to characterize the toxicities of the dosing vehicles themselves. It has been recently proposed that the width of the genital tubercle (GT), the dorsal-ventral length (D-V length) of the GT, and urethral tube closure in mouse fetuses can be used as novel markers for monitoring sexual development in mice. However, how these parameters are influenced by the dosing vehicles themselves remains unclear. Therefore, we evaluated the effects of corn oil, sesame oil, and 10% ethanol in corn oil on GT width, D-V length, and GT morphology in ICR mice. Our results showed that all three vehicles influenced GT width and D-V length, but not GT morphology, suggesting that the effects of dosing vehicles themselves might need to be considered when GT width or D-V length is used as a parameter to evaluate the effects of chemicals on GT development.

Oxidation and Storage Mechanisms for Nitrogen Oxides on Variously Terminated (001) Surfaces of SrFeO3-δ and Sr3Fe2O7-δ Perovskites.

The Ruddlesden-Popper (RP)-type layered perovskite is a candidate material for a new nitrogen oxide (NOx) storage catalyst. Here, we investigate the adsorption and oxidation of NOx on the (001) surfaces of RP-type oxide Sr3Fe2O7-δ for all of the terminations by comparing to those of simple perovskite SrFeO3-δ by the density functional theory (DFT) calculations. The possible (001) cleavages of Sr3Fe2O7 generate two FeO2- and three SrO-terminated surfaces, and the calculated surface energies indicated that the SrO-terminated surface generated by the cleavage at the rock salt layer is the most stable one. The oxygen of the FeO2-terminated surfaces could be removed with significantly low energy because the process involves the favorable reduction of the Fe4+ site. Consequently, the surface oxygen at the FeO2 site could easily oxidize adsorbed NO to NO2 by the Mars-van Krevelen mechanism. The resulting oxygen vacancy in the surface would be filled easily with lattice oxygen in bulk. The oxidation of NO with adsorbed molecular O2 was unfavorable by both the Langmuir-Hinshelwood and Eley-Rideal mechanisms because this process does not involve the reduction of the Fe4+ site. The oxygen of the SrO-terminated surfaces was tightly bound and acted as the adsorption site of NO and NO2. An electron transfer strengthened the NOx binding to the surface by forming nitrite (NO2-) or nitrate (NO3-) species. The DFT calculations revealed that the RP-type structure promoted NOx oxidation and storage properties by forming active oxygen due to the Jahn-Teller distortion and by exposing SrO-terminated surfaces due to the cleavage at the rock salt layer.

NOx Storage Performance at Low Temperature over Platinum Group Metal-Free SrTiO3-Based Material.

Pt-based catalysts are commonly employed as NOx-trapping catalysts for automobiles, while perovskite oxides have received attention as Pt-free NOx-trapping catalysts. However, the NOx storage performance of perovskite catalysts is significantly inferior at low temperatures and with coexisting gases such as H2O, CO2, and SO2. This study demonstrates that NOx storage reactions proceed over redox site (Mn, Fe, and Co)-doped SrTiO3 perovskites. Among the examined catalysts, Mn-doped SrTiO3 exhibited the highest NOx storage capacity (NSC) and showed a high NSC even at a low temperature of 323 K. Moreover, the high NOx storage performance of Mn-doped SrTiO3 was retained in the presence of poisoning gases (H2O, CO2, and SO2). NO oxidation experiments revealed that the NSC of Co-doped SrTiO3 was dependent on the NO oxidation activity from NO to NO2 via lattice oxygen, which resulted in an inferior NSC at low temperatures. On the other hand, Mn-doped SrTiO3 successfully adsorbed NO molecules onto its surface at 323 K without the NO oxidation process using lattice oxygens. This unique adsorption behavior of Mn-doped SrTiO3 was concluded to be responsible for the high NSC in the presence of poisoning gases.

NOx Oxidation and Storage Properties of a Ruddlesden-Popper-Type Sr3Fe2O7-δ-Layered Perovskite Catalyst.

The development of NOx-trapping catalysts for automobiles is highly desired to meet the current strict exhaust emission regulations. This study demonstrates that NOx oxidation and storage reactions proceed over Pt-free Sr3Fe2O7-δ with a Ruddlesden-Popper-type layered perovskite structure. Two types of Sr-Fe perovskite with oxygen storage capacity, namely, SrFeO3-δ and Sr3Fe2O7-δ, are studied as NOx-trapping catalysts. Sr3Fe2O7-δ shows higher NOx storage capacity than SrFeO3-δ; its activity is comparable to that of Pt/Ba/Al2O3 calcined at 1273 K. NOx temperature-programmed desorption and diffuse reflectance infrared Fourier transform experiments confirm the superior NOx-trapping ability of Sr3Fe2O7-δ over SrFeO3-δ. In addition, NO temperature-programmed reactions and O2 temperature-programmed desorption experiments reveal that these catalysts operate through a novel NO oxidation mechanism involving the consumption of their lattice oxygens and topotactic structural changes at a temperature of around 350-400 K. The reduction performance of trapped NOx on Pd-modified Sr-Fe perovskites is investigated by lean-rich cycle experiments using H2 as the reductant. Pd/Sr3Fe2O7-δ shows significantly high NOx removal efficiency over the entirety of each lean-rich period. Modifying Sr3Fe2O7-δ with Pd is also effective for NOx storage in the presence of H2O and CO2 and the regeneration of the catalyst following SOx sorption. Sr3Fe2O7-δ, with both NOx adsorption and NO oxidation capabilities, acts as a Pt-free NOx-trapping catalyst, exhibiting both high NOx storage capacity and high thermal tolerance.