Higher Oxygen Adspecies Concentration Research Articles

Preparation of three-dimensionally ordered macroporous La0.6Sr0.4Fe0.8Bi0.2O3−δ and their excellent catalytic performance for the combustion of toluene

Three-dimensionally ordered macroporous (3DOM) La0.6Sr0.4Fe0.8Bi0.2O3−δ (LSFB) were prepared using the surfactant (Pluronic F127, poly(ethylene glycol), l-lysine or xylitol)-assisted polymethyl methacrylate-templating method. Physicochemical properties of the materials were characterized by means of a number of analytical techniques, and their catalytic activities were evaluated for the combustion of toluene. It is shown that the LSFB samples were of 3DOM-architectured single-phase orthorhombic crystal structure. The nature of surfactant and solvent could influence the pore structures and surface areas of the LSFB samples. The Fe4+/Fe3+ or Oads/Olatt molar ratio and low-temperature reducibility correlated with the catalytic performance of the LSFB sample. The porous LSFB samples much outperformed the bulk counterpart, with the LSFB sample derived with xylitol showing the best catalytic activity (the temperatures required for 50 and 90% toluene conversions were 220 and 242°C at 20,000mL/(gh), respectively). Apparent activation energies of the porous LSFB samples were in the range of 46–74kJ/mol. It is concluded that the high oxygen adspecies concentration and good low-temperature reducibility were responsible for the excellent catalytic activity of the porous LSFB sample derived with xylitol.

Three-dimensionally ordered macroporous La0.6Sr0.4FeO3−δ: High-efficiency catalysts for the oxidative removal of toluene

Three-dimensionally (3D) ordered macroporous (3DOM) La0.6Sr0.4FeO3-δ (LSF-F127, LSF–PEG, LSF–PEG–EtOH, and LSF–Lysine) with mesoporous or nanovoid-like skeletons were prepared using the surfactant (Pluronic F127, poly(ethylene glycol) (PEG) or l–lysine)-assisted polymethyl methacrylate (PMMA)-templating method in aqueous or 40% ethanol aqueous solution. It is shown that the LSF samples displayed a 3DOM architecture and were of a single-phase orthorhombic crystal structure. The nature of surfactant and solvent could influence the pore structure and surface area of the final product. Treating the LSF precursor first in N2 at 500°C and then in air at 750°C favored the formation of 3DOM-structured La0.6Sr0.4FeO3−δ. The porous LSF catalysts performed well in toluene combustion, with LSF–PEG showing the best catalytic performance (T10%=54°C, T50%=225°C and T90%=280°C at 20,000mL/(g h)). It is concluded that the excellent catalytic performance of 3DOM-structured LSF is associated with their larger surface areas, higher oxygen adspecies concentrations, better low-temperature reducibility, and high-quality 3DOM structures.

Rod-, flower-, and dumbbell-like MnO2: Highly active catalysts for the combustion of toluene

The rod-like tetragonal α-MnO2, flower-like hexagonal ɛ-MnO2, and dumbbell-like tetragonal β-MnO2 were prepared using the hydrothermal or water-bathing method under different conditions. It is shown that the α-MnO2, ɛ-MnO2, and β-MnO2 catalysts possessed a surface area of ca. 53, 30, and 114m2/g, respectively. The oxygen adspecies concentration and low-temperature reducibility decreased in the order of α-MnO2>ɛ-MnO2>β-MnO2, coinciding with the sequence of their catalytic activities for toluene combustion. The well-defined morphological MnO2 catalysts performed much better than the bulk counterpart. At a space velocity of 20,000mL/(gh), the temperature for 90% toluene conversion was 238, 229, and 241°C over α-MnO2, ɛ-MnO2, and β-MnO2, respectively. The apparent activation energies of α-MnO2, ɛ-MnO2, and β-MnO2 were in the range of 20–26kJ/mol. It is concluded that higher oxygen adspecies concentrations and better low-temperature reducibility were responsible for the good catalytic performance of the α-MnO2, ɛ-MnO2, and β-MnO2 materials.

PMMA-templating preparation and catalytic activities of three-dimensional macroporous strontium ferrites with high surface areas for toluene combustion

Three-dimensional (3D) macroporous perovskite-type oxides SrFeO3−δ (i.e., SFO-xEG, x=1, 3, and 6) with a cubic perovskite structure have been prepared using the citric acid-assisted poly(methyl methacrylate)-templating method in the presence of a certain amount (1, 3 or 6mL) of ethylene glycol (EG). The physicochemical properties of the materials were characterized by means of numerous techniques and their catalytic activities were evaluated for toluene combustion. It is shown that the SFO-3EG sample possessed the best pore quality and highest surface area (ca. 34m2/g). Most of the pores of SFO-xEG (x=1, 3, and 6) were ca. 72, 52, and 64nm, respectively. There was a good relationship of surface area, oxygen adspecies concentration, and low-temperature reducibility with catalytic performance of the sample (i.e., SFO-3EG>SFO-6EG>SFO-1EG>SFO-bulk). The SFO-3EG catalyst performed the best, giving a T50% of ca. 270°C and a T90% of ca. 310°C at space velocity=5000mL/(g h). It is concluded that the good catalytic performance of SFO-3EG was associated with its large surface area, high oxygen adspecies concentration, and good low-temperature reducibility as well as the high-quality 3D macroporous structure.

Manganese Oxides with Rod-, Wire-, Tube-, and Flower-Like Morphologies: Highly Effective Catalysts for the Removal of Toluene

Nanosized rod-like, wire-like, and tubular α-MnO(2) and flower-like spherical Mn(2)O(3) have been prepared via the hydrothermal method and the CCl(4) solution method, respectively. The physicochemical properties of the materials were characterized using numerous analytical techniques. The catalytic activities of the catalysts were evaluated for toluene oxidation. It is shown that α-MnO(2) nanorods, nanowires, and nanotubes with a surface area of 45-83 m(2)/g were tetragonal in crystal structure, whereas flower-like spherical Mn(2)O(3) with a surface area of 162 m(2)/g was of cubic crystal structure. There were the presence of surface Mn ions in multiple oxidation states (e.g., Mn(3+), Mn(4+), or even Mn(2+)) and the formation of surface oxygen vacancies. The oxygen adspecies concentration and low-temperature reducibility decreased in the order of rod-like α-MnO(2) > tube-like α-MnO(2) > flower-like Mn(2)O(3) > wire-like α-MnO(2), in good agreement with the sequence of the catalytic performance of these samples. The best-performing rod-like α-MnO(2) catalyst could effectively catalyze the total oxidation of toluene at lower temperatures (T(50%) = 210 °C and T(90%) = 225 °C at space velocity = 20,000 mL/(g h)). It is concluded that the excellent catalytic performance of α-MnO(2) nanorods might be associated with the high oxygen adspecies concentration and good low-temperature reducibility. We are sure that such one-dimensional well-defined morphological manganese oxides are promising materials for the catalytic elimination of air pollutants.

Three-dimensionally ordered macroporous SrFeO3−δ with high surface area: Active catalysts for the complete oxidation of toluene

Three-dimensionally ordered macroporous (3DOM) cubic perovskite-type oxides SrFeO3−δ were prepared using the citrate acid-assisted poly(methyl methacrylate)-templating method in the presence of ethylene glycol, sucrose or l-lysine (denoted as SFO-0, SFO-Sucrose, and SFO-Lysine, respectively). It is found that the SrFeO3−δ samples possessed a 3DOM architecture and were high in surface area (34–61m2/g). The porous SrFeO3−δ catalysts performed well in toluene combustion, with the SFO-0 sample exhibiting the highest catalytic activity (T50%=292°C and T90%=340°C at 20,000mL/(gh)). The catalytic activity of the samples showed a good relationship with surface area, oxygen adspecies concentration, and low-temperature reducibility, all decrease in the order of SFO-0>SFO-Lysine>SFO-Sucrose≫SFO-bulk. It is deduced that the good catalytic performance of the porous SrFeO3−δ materials is due to the good low-temperature reducibility, large surface area, and high oxygen adspecies concentration.

Controlled preparation and high catalytic performance of three-dimensionally ordered macroporous LaMnO3 with nanovoid skeletons for the combustion of toluene

Three-dimensionally ordered macroporous (3DOM) single-phase rhombohedral perovskite-type oxide LaMnO3 materials with nanovoid skeletons were prepared using the poly(methyl methacrylate)-templating methods with the assistance of surfactant (poly(ethylene glycol) (PEG) or triblock copolymer (Pluronic P123)). The nature of surfactant influenced the pore structure of the LaMnO3 sample. The use of PEG400 alone led to a 3DOM-structured LaMnO3 without nanovoid skeletons; with the addition of PEG400 and P123, however, one could prepare LaMnO3 samples with high-quality 3DOM structures, nanovoid skeletons, and high surface areas (37–39m2/g). Under the conditions of toluene concentration=1000ppm, toluene/O2 molar ratio=1:400, and space velocity=20,000mL/(gh), the porous LaMnO3 samples were superior to the bulk counterpart in catalytic performance, with the nanovoid-containing 3DOM-structured LaMnO3 catalyst performing the best (the temperatures for toluene conversions of 50% and 90% were 222–232 and 243–253°C, respectively). The apparent activation energies (57–62kJ/mol) over the 3DOM-structured LaMnO3 catalysts were much lower than that (97kJ/mol) over the bulk LaMnO3 catalyst. We believe that the excellent performance of the 3D macroporous LaMnO3 materials in catalyzing the combustion of toluene might be due to factors such as large surface area, high oxygen adspecies concentration, good low-temperature reducibility, and unique nanovoid-containing 3DOM structure of the materials.

Three-dimensional ordered mesoporous cobalt oxides: Highly active catalysts for the oxidation of toluene and methanol

Three-dimensional (3D) ordered mesoporous cubic Co 3 O 4 (denoted as Co-KIT6 and Co-SBA16) were fabricated adopting the KIT-6- and SBA-16-templating strategies, respectively. It is shown that Co-KIT6 and Co-SBA16 possessed large surface areas (118–121 m 2 /g), high oxygen adspecies concentrations, and good low-temperature reducibility. Over Co-KIT6 at space velocity = 20,000 mL/(g h), 90% toluene and methanol conversions were achieved at 180 and 139 °C, respectively. The excellent catalytic performance of Co-KIT6 and Co-SBA16 was associated with their larger surface areas, higher oxygen adspecies concentrations, better low-temperature reducibility, and 3D ordered mesoporous structure. High-surface-area 3D ordered mesoporous Co 3 O 4 (denoted as Co-KIT6 and Co-SBA16) can be fabricated with the KIT-6 and SBA-16-templating strategies, respectively. The Co-KIT6 and Co-SBA16 catalysts possess large surface areas, high oxygen adspecies concentrations, good low-temperature reducibility, and 3D ordered mesoporous structures, which are responsible for their excellent performance for toluene and methanol oxidation. ► Vacuum treatment favors the formation of 3D ordered mesoporous structure. ► 3D ordered mesoporous structure possesses high surface area. ► 3D ordered mesoporous structure exhibits good low-temperature reducibility. ► Mesoporous structure, reducibility, and oxygen species determine catalytic activity.