La(1−x)SrxCo1−yFeyO3 perovskites prepared by sol–gel method: Characterization and relationships with catalytic properties for total oxidation of toluene

La_(1−x)Sr_xCo_(1−y)Fe_yO₃ samples have been prepared by sol–gel method using EDTA and citric acid as complexing agents. For the first time, Raman mappings were achieved on this type of samples especially to look for traces of Co₃O₄ that can be present as additional phase and not detect by XRD. The prepared samples were pure perovskites with good structural homogeneity. All these perovskites were very active for total oxidation of toluene above 200 °C. The ageing procedure used indicated good thermal stability of the samples. A strong improvement of catalytic properties was obtained substituting 30% of La³⁺ by Sr²⁺ cations and a slight additional improvement was observed substituting 20% of cobalt by iron. Hence, the optimized composition was La_0.7Sr_0.3Co_0.8Fe_0.2O₃. The samples were also characterized by BET measurements, SEM and XRD techniques. Iron oxidation states were determined by Mössbauer spectroscopy. Cobalt oxidation states and the amount of O⁻ electrophilic species were analyzed from XPS achieved after treatment without re-exposition to ambient air. Textural characterization revealed a strong increase in the specific surface area and a complete change of the shape of primary particles substituting La³⁺ by Sr²⁺. The strong lowering of the temperature at conversion 20% for the La_0.7Sr_0.3Co_(1−y)Fe_yO₃ samples can be explained by these changes. X photoelectron spectra obtained with our procedure evidenced very high amount of O⁻ electrophilic species for the La_0.7Sr_0.3Co_(1−y)Fe_yO₃ samples. These species able to activate hydrocarbons could be the active sites. The partial substitution of cobalt by iron has only a limited effect on the textural properties and the amount of O⁻ species. However, Raman spectroscopy revealed a strong dynamic structural distortion by Jahn–Teller effect and Mössbauer spectroscopy evidenced the presence of Fe⁴⁺ cations in the iron containing samples. These structural modifications could improve the reactivity of the active sites explaining the better specific activity rate of the La_0.7Sr_0.3Co_0.8Fe_0.2O₃ sample. Finally, an additional improvement of catalytic properties was obtained by the addition of 5% of cobalt cations in the solution of preparation. As evidenced by Raman mappings and TEM images, this method of preparation allowed to well-dispersed small Co₃O₄ particles that are very efficient for total oxidation of toluene with good thermal stability contrary to bulk Co₃O₄.     

The Role of Support Acidity in the Selective Catalytic Reduction of NO by C3H6 Under Lean-Burn Conditions

Mechanical mixtures consisting of a catalyst (Pt/SiO₂ or Pt/SiO₂–Al₂O₃) and supports of varying acidity (hydrotalcite, SiO₂, SiO₂–Al₂O₃, and ZSM-5 zeolite) were tested for the selective reduction of NO by C₃H₆. A certain degree of support acidity appears to favour N₂ selectivity, but if there are too many acid sites, carbon deposition becomes extensive and leads to catalyst deactivation.     

Effect of diesel oxidation catalysts on the diesel particulate filter regeneration process

A Diesel Particulate Filter (DPF) regeneration process was investigated during aftertreatment exhaust of a simulated diesel engine under the influence of a Diesel Oxidation Catalyst (DOC). Aerosol mass spectrometry analysis showed that the presence of the DOC decreases the Organic Carbon (OC) fraction adsorbed to soot particles. The activation energy values determined for soot nanoparticles oxidation were 97 ± 5 and 101 ± 8 kJ mol(-1) with and without the DOC, respectively; suggesting that the DOC does not facilitate elementary carbon oxidation. The minimum temperature necessary for DPF regeneration was strongly affected by the presence of the DOC in the aftertreatment. The conversion of NO to NO(2) inside the DOC induced the DPF regeneration process at a lower temperature than O(2) (ΔT = 30 K). Also, it was verified that the OC fraction, which decreases in the presence of the DOC, plays an important role to ignite soot combustion.     

Catalytic Removal of Toluene over Co3O4–CeO2 Mixed Oxide Catalysts: Comparison with Pt/Al2O3

The catalytic oxidation of toluene, chosen as VOC probe molecule, was investigated over Co₃O₄, CeO₂ and over Co₃O₄–CeO₂ mixed oxides and compared with the catalytic behavior of a conventional Pt(1 wt%)/Al₂O₃ catalyst. Complete toluene oxidation to carbon dioxide and water was achieved over all the investigated systems at temperatures below 500 °C. The most efficient catalyst, Co₃O₄(30 wt%)–CeO₂(70 wt%), showed full toluene conversion at 275 °C, comparing favorably with Pt/Al₂O₃ (100% toluene conversion at 225 °C).