Effects of Zr substitution on soot combustion over cubic fluorite-structured nanoceria: Soot-ceria contact and interfacial oxygen evolution

Electrospun Ce-Mn oxide as an efficient catalyst for soot combustion: Ce-Mn synergy, soot-catalyst contact, and catalytic oxidation mechanism

Increasing the contact efficiency and improving the intrinsic activity are two effective strategies to obtain efficient catalysts for soot combustion. Herein, the electrospinning method is used to synthesize fiber-like Ce-Mn oxide with a strong synergistic effect. The slow combustion of PVP in precursors and highly soluble manganese acetate in spinning solution facilitates the formation of fibrous Ce-Mn oxides. The fluid simulation clearly indicates that the slender and uniform fibers provide more interwoven macropores to capture soot particles than the cubes and spheres do. Accordingly, electrospun Ce-Mn oxide exhibits better catalytic activity than reference catalysts, including Ce-Mn oxides by co-precipitation and sol-gel methods. The characterizations suggest that Mn³⁺ substitution into fluorite-type CeO₂ enhances the reducibility through the acceleration of Mn-Ce electron transfer, improves the lattice oxygen mobility by weakening the Ce-O bonds, and induces oxygen vacancies for the activation of O₂. The theoretical calculation reveals that the release of lattice oxygen becomes easy because of a low formation energy of oxygen vacancy, while the high reduction potential is beneficial for the activation of O₂ on Ce³⁺-Ov (oxygen vacancies). Due to above Ce-Mn synergy, the CeMnO_x-ES shows more active oxygen species and higher oxygen storage capacity than CeO₂-ES and MnO_x-ES. The theoretical calculation and experimental results suggest that the adsorbed O₂ is more active than lattice oxygen and the catalytic oxidation mainly follows the Langmuir-Hinshelwood mechanism. This study indicates that electrospinning is a novel method to obtain efficient Ce-Mn oxide.

Effect of iron substitution in cryptomelane on the heterogeneous reaction with isoprene

Biogenic isoprene is an important pollutant for regional air quality. Being ubiquitously distributed on the earth surface, manganese (hydr)oxides should play a vital role in the transformation of isoprene. Cryptomelane is a typical manganese oxide with isomorphous substitution of Fe for Mn, but less attention has been paid to its heterogeneous reaction with isoprene. When Fe³⁺ replaces Mn³⁺, K⁺ is depleted and Mn³⁺ is oxidized to Mn⁴⁺. In contrast, oxygen vacancies are formed when Fe³⁺ substitutes Mn⁴⁺. Fe substitution creates weak crystallites and abundant mesopores, resulting in the increase of isoprene adsorption. As found by theoretical calculations, the Mn⁴⁺-O^2- bonds at the cross sections of the tunnels is more active than that on the outer wall of the tunnels. After the adsorption of isoprene, bridging carboxylate species and hydrogen-bonding water are produced and the surface octahedra are distorted, i.e., Mn⁴⁺O₆ → Mn³⁺O_6-δ. As the heat facilitates the breakage of Mn⁴⁺-O^2-, the increase of environmental temperature enhances the oxidation of isoprene. The above findings shed light on the effect of Fe substitution in cryptomelane to enhance the oxidation of isoprene, and illustrates that heterogeneous reaction with isoprene impairs the transformation of other environmental substances on cryptomelane.

Revealing the promoting effect of multiple Mn valences on the catalytic activity of CeO2 nanorods toward soot oxidation

Mn-modified CeO₂ nanomaterials have attracted extensive attention as efficient and promising catalysts for soot combustion due to their low cost and high catalytic activity. However, a detailed mechanism of how Mn promotes soot oxidation over CeO₂ is still not clearly elucidated, which is crucial to further optimize the catalyst for achieving its practical applications. We here report a Mn-doped CeO₂ catalyst with tunable surface Mn chemical valence states to study the Mn-promoting mechanism for improving CeO₂ catalyst activity in soot oxidation. Experimental results show that Mn-doped CeO₂ nanorods with surface Mn chemical valence states being optimized (Mn_0.19Ce_0.81O₂) can lower the eliminating temperature of soot to 410 °C (T₉₀) when in a loose contact and exhibit a strong resistance towards water molecules. The catalytic performances of Mn_0.19Ce_0.81O₂ nanorods are comparable with those of other reported oxide catalysts both in the mimetic realistic and ideal reaction environments. Detailed characterization and theoretical calculation results demonstrate that balanced multiple Mn valences can dramatically enhance the catalysts' redox properties and their ability to activate O₂ molecules, as well as improve the dynamic contact efficiency during the oxidation, which synergistically result in superior catalytic performances. This work might provide insight for the future design and preparation of catalysts to efficiently eliminate soot particles.

Preparation of Ce x Zr1-x O2 by Different Methods and Its Catalytic Oxidation Activity for Diesel Soot

Novel Ce _x Zr_1-x O₂ (x = 0.67, 0.8, 0.9, 1.0) catalysts were designed and synthesized by solvothermal, calcination, and sol-gel methods and were used to catalyze oxidation of soot from diesel vehicle exhaust. The influence of catalysts synthesized by different methods and Ce/Zr molar ratios on the performance was investigated. These catalysts were characterized by XRD, N₂ adsorption-desorption, FT-IR, TEM, XPS, H₂-temperature programmed reduction (TPR), and O₂-temperature programmed desorption (TPD) techniques. The results indicated that Ce_0.8Zr_0.2O₂ prepared by the calcination method has excellent activity and stability at low temperature. The soot ignition point is 322 °C, and the ratio of soot conversion reaches 90% at 497 °C, which is lower than that from the solvothermal and sol-gel methods. The XRD, Raman, SEM, XPS and H₂-TPR results reveal that the structure and oxygen adsorption properties are crucial to soot oxidation activity, and Zr⁴⁺ is successfully doped into the CeO₂ lattice and forms a homogeneous solid solution. Nanostructured Ce_0.8Zr_0.2O₂ with 110.2 m²/g surface areas is produced. The proportion of chemical oxygen and surface adsorbed oxygen in the catalyst prepared from the calcination method is the highest at 23.18%. The structure may lead to charge imbalance, unsaturated bonds, and oxygen vacancies, thus increasing the adsorption of oxygen on the catalyst surface.

Catalytic Materials for Gasoline Particulate Filters Soot Oxidation

The energy efficiency of Gasoline Direct Injection (GDI) engines is leading to a continuous increase in GDI engine vehicle population. Consequently, their particulate matter (soot) emissions are also becoming a matter of concern. As required for diesel engines, to meet the limits set by regulations, catalyzed particulate filters are considered as an effective solution through which soot could be trapped and burnt out. However, in contrast to diesel application, the regeneration of gasoline particulate filters (GPF) is critical, as it occurs with almost an absence of NOx and under oxygen deficiency. Therefore, in the recent years it was of scientific interest to develop efficient soot oxidation catalysts that fit such particular gasoline operating conditions. Among them ceria- and perovskite-based formulations are emerging as the most promising materials. This overview summarizes the very recent academic contributions focusing on soot oxidation materials for GDI, in order to point out the most promising directions in this research area.