Synthesis Of M_xCe_1-xO_y Mixed Oxides And Their Application In Catalytic N₂O Decomposition

Nitrous oxide (N2O) is a strong greenhouse gas, which shows 310 and 21 times of the global warming potential (GWP) as compared to those of CO2 and CH4, respectively. Furthermore, it contributes to the stratospheric ozone destruction. The concentration of N2O in the atmosphere is increasing by 0.2-0.3% per year, which is mainly caused by anthropogenic activities such as chemical manufacture and energy production. To reduce the anthropogenic N2O emissions, direct catalytic N2O decomposition is an attractive method in the past decades.The development of effective and economic catalysts for decomposition of N2O is urgently required. Although the noble metal catalyst exhibits excellent catalytic performance, but the high price greatly limits its application. Considering economic cost, the non-noble metal oxides are of great potential for the industrial application. In this dissertation, cerium-containing mixed oxides for the catalytic decomposition of N2O are researched. X-ray powder diffraction (XRD), N2 adsorption-desorption isotherms, Scanning electron microscope (SEM),Transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction of hydrogen (H2-TPR) and in situ infrared diffuse reflectance spectroscopy (in situ-DRBFTS) are applied as the research means. The proportion of cerium oxide, additives, synthesis methods, and the presence of inhibitors are studied to analyze the relationship between the catalytic activity of the catalysts and the textural properties.1. Catalytic decomposition of N2O over copper-cerium mixed oxidesThe catalytic N2O decomposition is investigated over a series of CuCe mixed oxides with different Cu/Ce molar ratios. When Cu/Ce ratio reaches 2/1, the catalyst exhibits the best activity among the mixed oxides. It can completely decompose N2O to N2 and O2 at 420℃, and maintain 100% conversion for 80 h at 480℃. Characterizations of XRD, N2 adsorption-desorption isotherms, XPS, and H2-TPR are applied to correlate their properties with the corresponding catalytic performance, and reveal the synergetic effect between CuO and CeO2 for N2O decomposition. In situ DRIFTS investigation confirms the presence of Cu+ species that is closely related with the activity of CuCe catalysts. The synergy of mixed oxides promotes the stability and the ability to regenerate the active Cu+ site.2. Effect of synthesis methods and Cs doped in Cu5OCe5O mixed oxides on the catalytic decomposition of N2OAlkali metal can affect the redox properties of the active sites, which causing a significant impact on the catalytic activity of mixed oxides. Co-precipitation method, solid-state method, citrate acid method are used to prepare Cu50Ce5O catalysts. Although the specific surface area is small, the residual K significantly promotes the catalytic activity of Cu5OCe5O-CP, which could completely decompose N2O at 420℃.Through the electronic effect, Cs could improve the redox properties of the catalysts, promote the generation of Cu+ active site, and enhance the N2O decomposition activity. The best catalyst 2.5Cs-Cu50Ce50 achieves its 100% N2O conversion at 400℃.3. Preparation of NiCe mixed oxides for catalytic decomposition of N2OOxygen atoms formed by the decomposition of nitrous oxide are hardly desorbed from the catalyst surface and accumulated on the surface, finally causing catalyst deactivation. O2 in the industrial tail gas could inhibit the catalytic activity significantly. NiO shows better activity than CuO in presence of O2. So, it is interesting to explore the catalytic behavior of NiCe mixed oxides in presence of O2.Compared to the catalysts prepared by ammonia-evaporation method and co-precipitation method, smaller sizes of particles and larger surface areas are observed in the catalyst prepared by citrate acid method. In the Ni-rich catalysts, CeO2 aggregates on the surface of NiO, prevents the sintering of that, and preserves a high surface area.5-10 mol% CeO2 introduction to NiO leads to a significant increase in the catalytic activity. Ni90Ce10 can completely decompose N2O to N2 and O2 at 360℃. An appropriated amount of CeO2 introduction to NiO leads to a strong interaction in the NiCe mixed oxides catalysts. The strong interaction significantly promotes the oxygen mobility and facilitates the catalytic activity of N2O decomposition in the presence of O2. Ni90Ce10 also exhibits the best activity in the presence of O2, which reaches 100% conversion at 400℃. It is noteworthy that, the Ce-rich catalysts suffer less activity loss when O2 is added. It proves that CeO2 can enhance oxygen mobility, and resist the inhibition of O2.4. Catalytic decomposition of N2O over NiCe mixed oxides prepared by ultrasonic co-precipitation methodThe use of ultrasonic process could provide an efficient way to rapidly synthesize various as-synthesized mixed oxides and rationally control their particle size, distribution, and morphology. During the ultrasonic process, highly strained crystals form as a consequence of the activation. The prominent lattice distortion induced by ultrasonic process is suggested to play a key role in creating and maintaining a high density of surface defects, which leads to highly activity and stability of the mixed oxides. Catalysts synthesized by ultrasonic co-precipitation method show the unique shape and the sphere. Numerous packed nanoparticles (20-30 nm) assemble a sphere (1-3 μm). NiCe-Ac exhibits good catalytic activity and stability, which could maintain 100% N2O conversion up to 48 hours at 330℃.