Removal of formaldehyde over Mn_xCe_(1- x)O_2 catalysts: Thermal catalytic oxidation versus ozone catalytic oxidation

MnxCe1-xO2(x:0.3–0.9)prepared by Pechini method was used as a catalyst for the thermal catalytic oxidation of formaldehyde(HCHO).At x=0.3 and 0.5,most of the manganese was incorporated in the fluorite structure of Ce O2to form a solid solution.The catalytic activity was best at x=0.5,at which the temperature of 100%removal rate is the lowest(270°C).The temperature for 100%removal of HCHO oxidation is reduced by approximately 40°C by loading 5 wt.%Cu Oxinto Mn0.5Ce0.5O2.With ozone catalytic oxidation,HCHO(61 ppm)in gas stream was completely oxidized by adding 506 ppm O3over Mn0.5Ce0.5O2catalyst with a GHSV(gas hourly space velocity)of 10,000 hr-1at 25°C.The effect of the molar ratio of O3to HCHO was also investigated.As O3/HCHO ratio was increased from 3 to 8,the removal efficiency of HCHO was increased from 83.3%to 100%.With O3/HCHO ratio of 8,the mineralization efficiency of HCHO to CO2was 86.1%.At 25°C,the p-type oxide semiconductor(Mn0.5Ce0.5O2)exhibited an excellent ozone decomposition efficiency of 99.2%,which significantly exceeded that of n-type oxide semiconductors such as Ti O2,which had a low ozone decomposition efficiency(9.81%).At a GHSV of 10,000 hr-1,[O3]/[HCHO]=3 and temperature of 25°C,a high HCHO removal efficiency(≥81.2%)was maintained throughout the durability test of 80 hr,indicating the long-term stability of the catalyst for HCHO removal.

Removal of VOCs from gas streams with double perovskite-type catalysts

Double perovskite-type catalysts including La2 CoMnO_6 and La_2 CuMnO_6 are first evaluated for the effectiveness in removing volatile organic compounds(VOCs), and single perovskites(La CoO_3, LaMnO_3, and La Cu O3) are also tested for comparison. All perovskites are tested with the gas hourly space velocity(GHSV) of 30,000 hr^(-1), and the temperature range of100–600°C for C_7H_8 removal. Experimental results indicate that double perovskites have better activity if compared with single perovskites. Especially, toluene(C_7H_8) can be completely oxidized to CO_2 at 300°C as La2 Co MnO_6 is applied. Characterization of catalysts indicates that double perovskites own unique surface properties and are of higher amounts of lattice oxygen,leading to higher activity. Additionally, apparent activation energy of 68 k J/mol is calculated using Mars-van Krevelen model for C7 H8 oxidation with La2 Co Mn O6 as catalyst. For durability test, both La2 Co Mn O6 and La_2 CuMnO_6 maintain high C7 H8 removal efficiencies of 100% and98%, respectively, at 300°C and 30,000 hr^(-1), and they also show good resistance to CO_2(5%) and H2 O(g)(5%) of the gas streams tested. For various VOCs including isopropyl alcohol(C_3H_8 O),ethanal(C_2H_4O), and ethylene(C_2H_4) tested, as high as 100% efficiency could be achieved with double perovskite-type catalysts operated at 300–350°C, indicating that double perovskites are promising catalysts for VOCs removal.

Catalytic removal of phenol from gas streams by perovskite-type catalysts

Three perovskite-type catalysts prepared by citric acid method are applied to remove phenol from gas streams with the total flow rate of 300 mL/min, corresponding to a GHSV of10,000/hr. LaMnO_3 catalyst is first prepared and further partially substituted with Sr and Cu to prepare La_(0.8)Sr_(0.2)MnO_3 and La_(0.8)Sr_(0.2)Mn_(0.8)Cu_(0.2)O_3, and catalytic activities and fundamental characteristics of these three catalysts are compared. The results show that phenol removal efficiency achieved with La_(0.8)Sr_(0.2)Mn_(0.8)Cu_(0.2)O_3 reaches 100% with the operating temperature of 200°C and the rate of mineralization at 300°C is up to 100%, while the phenol removal efficiencies achieved with La_(0.8)Sr_(0.2)MnO_3 and LaMnO_3 are up to 100% with the operating temperature of 300°C and 400°C, respectively. X-ray photoelectron spectroscopy(XPS) analysis shows that the addition of Sr and Cu increases the lattice oxygen of La_(0.8)Sr_(0.2)Mn_(0.8)Cu_(0.2)O_3, and further increases mobility or availability of lattice oxygen. The results indicate that La_(0.8)Sr_(0.2)Mn_(0.8)Cu_(0.2)O_3 has the best activity for phenol removal among three catalysts prepared and the catalytic activity of phenol oxidation is enhanced by the introduction of Sr and Cu into LaMnO_3. Apparent activation energy of 48 k J/mol is calculated by Mars–Van Krevelen Model for phenol oxidation with La_(0.8)Sr_(0.2)Mn_(0.8)Cu_(0.2)O_3 as catalyst.