A Comparative Study of Supported and Bulk Cu–Mn–Ce Composite Oxide Catalysts for Low-Temperature CO Oxidation

A series of supported and bulk Cu–Mn–Ce ternary oxide catalysts was synthesized by wet-impregnation (IM), deposition–precipitation (DP), traditional co-precipitation (CP), co-precipitation with cetyltrimethyl ammonium bromide (CC), and sol–gel (SG) methods. The supported catalysts (CuMn/Ce-IM, CuMn/Ce-DP) exhibited significantly higher activity for CO oxidation than the bulk catalysts (CuMnCe-CP, CuMnCe-CC and CuMnCe-SG). The improved performance could be attributed to the presence of more isolated CuO and MnO_x entities on the surface of supported catalysts, which contributed to the efficient utilization of both lattice oxygen from CeO₂ and spillover oxygen from surface MnO_x. For bulk catalysts, major Cu–Mn species were doped to form \({\text{C}}{{\text{u}}_{\text{x}}}{\text{M}}{{\text{n}}_{\text{y}}}{\text{C}}{{\text{e}}_{1 - {\text{x}} - {\text{y}}}}{{\text{O}}_{2 - {\text{z}}}}\) solid solutions and a part of them were coated by ceria mechanically. Lowest 50% CO conversion temperature were achieved at 76.9 °C for CuMn/Ce-IM catalyst. Low-temperature CO oxidation activities of all catalysts were in the sequence of CuMn/Ce-IM?>?CuMn/Ce-DP?>?CuMnCe-SG?>?CuMnCe-CC?>?CuMnCe-CP.

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The Stability Study of Au/La-Co–O Catalysts for CO Oxidation

Perovskite oxide LaCoO₃ and the mixture oxides of La₂O₃ + Co₃O₄ were prepared by sol–gel method. Then Au/La–Co–O catalysts were prepared by deposition- precipitation (DP) method and characterized by means of XRD, BET, XPS, TEM and IR. The catalytic performance for CO low-temperature oxidation and stability over these catalysts were compared. The results of experiment showed gold catalysts supported on perovskite oxides have higher catalytic activity and stability than that of supported on the simple oxides.     

An Investigation of Catalytic Activity for CO Oxidation of CuO/Ce x Zr1–x O2 Catalysts

A series of CuO/Ce _x Zr_1–x O₂ catalyst powders with different Ce/Zr ratio were prepared via an impregnation method and characterized by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman), H₂-Temperature-programmed reduction (TPR) and X-ray photoelectron spectra techniques. The catalytic properties of the catalysts were evaluated by means of a microreactor-GC system. XRD results showed that the addition of CuO had no effect on the crystalline lattice of the support. The structures of the Ce _x Zr_1–x O₂ samples were confirmed by XRD analyses and FT-Raman results. The H₂-TPR profiles for these catalysts had three peaks, which could be attributed to the reduction of three kinds of CuO species, i.e., the highly dispersed CuO, the larger CuO species and the bulk CuO. The TPR analyses and catalytic property tests indicated that the Ce/Zr ratio of CuO/Ce _x Zr_1–x O₂ had an effect on the dispersion degree of CuO and the catalytic activity of the catalysts.     

Effect of pH on the Catalytic Properties of Mo–V–Te–P–O Catalysts for Selective Oxidation of Isobutane

Mo–V–Te–P mixed oxide catalysts, prepared by a dry-up method at various pHs (in the range of about 1.0–9.0), have been tested in the partial oxidation of isobutane. The best catalytic performance was achieved over a catalyst prepared at a pH about 7.0. In this case, high selectivity to methacrolein (37.0%) at an isobutane conversion of 12.7% has been obtained at 380 °C. The surface V⁴⁺/V⁵⁺ ratios of the calcined samples were strongly influenced by the pH in the synthesized solution, which is one of the key factors in the catalytic performance for selective oxidation of isobutane.     

Comparative Study on Partial Oxidation of Methane over Ni/ZrO2, Ni/CeO2 and Ni/Ce–ZrO2 Catalysts

The partial oxidation of methane has been studied by sequential pulse experiments with CH₄ O₂ CH₄ and simultaneous pulse reaction of CH₄/O₂ (2/1) over Ni/CeO₂, Ni/ZrO₂ and Ni/Ce–ZrO₂ catalysts. Over Ni/CeO₂, CH₄ dissociates on Ni and the resultant carbon species quickly migrate to the interface of Ni–CeO₂, and then react with lattice oxygen of CeO₂ to form CO. A synergistic effect between Ni and CeO₂ support contributes to CH₄ conversion. Over Ni/ZrO₂, CH₄ and O₂ are activated on the surface of metallic Ni, and then adsorbed carbon reacts with adsorbed oxygen to produce CO, which is composed of the main path for the partial oxidation of methane. The addition of ceria to zirconia enhances CH₄ dissociation and improves the carbon storage capacity. Moreover, it increases the storage capacity and mobility of oxygen in the catalyst, thus promoting carbon elimination.     

A Comparison Study on the Structure and Performance of Mo–V–O and Mo–V–Te–O Catalysts Synthesized Hydrothermally with Ultrasonic Pretreatment for Propane Oxidation

The Mo_1.00V_0.80O _n and Mo_1.00V_αTe_0.17O _n (α = 0.40–1.00) catalysts were prepared hydrothermally with ultrasonic pretreatment for propane partial oxidation. The fabricated catalysts were characterized by BET, XRD, Raman, XPS, EPR, SEM, H₂–TPR, and NH₃-calorimetric techniques. It is revealed that the content of vanadium in the catalysts is crucial for the generation of phase structure essential for good performance. The role of Te in the catalytic system and the effect of ultrasonic treatment during catalyst fabrication on catalyst properties are interpreted based on the characterization and reaction results obtained.     

Effect of Acid–Base Properties on the Catalytic Activity of Pt/Al2O3 Based Catalysts for Diesel NO Oxidation

The activity of Pt catalysts supported on Al₂O₃ modified with various acid–base additives has been investigated for oxidation of NO to NO₂. Although Pt dispersion was changed by the additives, there was no clear effect of Pt dispersion on the catalytic activity. The measurement of solid acid–base properties of the modified Pt/Al₂O₃ indicated that the NO oxidation activity increased by the increase of surface density of strong acid sites and decreased by the increase of basic sites. It was suggested that platinum on the acidic supports keeps its highly active metallic state for NO oxidation, while the formation of nitrate/nitrite on the basic supports inhibits the reaction on the Pt surface.     

Co–Mg–Al Hydrotalcite Precursors for Catalytic Total Oxidation of Volatile Organic Compounds

Co–Mg–Al hydrotalcite type solids were synthesized as precursors of catalysts for the total oxidation of toluene. After calcination at 500 °C, different mesoporous mixed oxides were obtained with high specific surfaces. The comparison of the catalytic activities of the calcined hydrotalcites with those of calcined hydroxides evidenced the superiority of the first oxides explained meanly by higher specific surfaces and more easily reducible particles. DRIFT “operando” allowed to follow the oxidation reaction and the formation of light coke and carbonate species.     

Structure-activity Relation of Fe2O3–CeO2 Composite Catalysts in CO Oxidation

A series of Fe₂O₃–CeO₂ composite catalysts were synthesized by coprecipitation and characterized by X-ray diffraction (XRD), BET surface area measurement, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Their catalytic activities in CO oxidation were also tested. The Fe₂O₃–CeO₂ composites with an Fe molar percentage below 0.3 form solid solutions with the CeO₂ cubic fluorite structure, in which the doped Fe³⁺ initially substitutes Ce⁴⁺ in fluorite cubic CeO₂, but then mostly locate in the interstitial sites after a critical concentration of doped Fe³⁺. With an Fe molar percentage between 0.3 and 0.95, the Fe₂O₃–CeO₂ composites are mixed oxides of the cubic fluorite CeO₂ solid solution and the hematite Fe₂O₃. XPS results indicate that CeO₂ is enriched in the surface region of Fe₂O₃–CeO₂ composites. The Fe₂O₃–CeO₂ composites have much higher catalytic activities in CO oxidation than the individual pure CeO₂ and Fe₂O₃, and the Fe_0.1Ce_0.9 composite shows the best catalytic performance. The structure-activity relation of the Fe₂O₃–CeO₂ composites in CO oxidation is discussed in terms of the formation of solid solution and surface oxygen vacancies. Our results demonstrate a proportional relation between the catalytic activity of cubic CeO₂-like solid solutions and their density of oxygen vacancies, which directly proves the formation of oxygen vacancies as the key step in CO oxidation over oxide catalysts.