Chemical interaction of Ce-Fe mixed oxides was investigated in methane selective oxidation via methane temperature programmed reduction and methane isothermal reaction tests over Ce-Fe oxygen carriers. In methane temperature programmed reduction test, Ce-Fe oxide behaved complete oxidation at the lower temperature and selective oxidation at higher temperatures. Ce-Fe mixed oxides with the Fe content in the range of 0.1~）.5 was able to produce syngas with high selectivity in high-temperature range （800-900 ~C）, and a higher Fe amount over 0.5 seemed to depress the CO formation. In isothermal reaction, complete oxidation oc- curred at beginning following with selective oxidation later. Ce~_xFexO2~ oxygen carriers （x5_0.5） were proved to be suitable for the selective oxidation of methane. Ce-Fe mixed oxides had the well-pleasing reducibility with high oxygen releasing rate and CO selec- tivity due to the interaction between Ce and Fe species. Strong chemical interaction of Ce-Fe mixed oxides originated from both Fe＊ activated CeO2 and Ce3＋ activated iron oxides （FeOm）, and those chemical interaction greatly enhanced the oxygen mobility and selectivity.
Chemical-looping steam methane reforming （CL-SMR） is a novel process towards the production of pure hydrogen and syngas, consisting ofa syngas production reaction and a hydrogen production reaction. Macroporous CeQ-ZrO2 oxygen carders with different pore sizes prepared by colloidal crystal templating method and characterized by techniques of scalming electron microscopy （SEM）, transmission electron microscopy （TEM）, Brunauer-Emmett-Teller （BET）, X-ray diffraction （XRD） and temperature pro- grammed reduction （H2-TPR） were tested in CL-SMR process. For comparison, nonporous CeO2-ZrO2 oxygen carrier prepared by precipitation method was also investigated. It was found that macroporous CeO2-ZrO2 oxygen carriers owned higher reducibility and reactivity in CL-SMR process than nonporous samples. For the macroporous CeO2-ZrO2 sample, the decline of pore size could im- prove the reducibility and reactivity. The macroporous sample with a pore size of 100 nm （labeled as Ce-Zr-100） showed the highest performance for the co-production of syngas and hydrogen during the successive CL-SMR redox cycles. After 10 redox cycles, it still retained good porous structure and reducibility. It was found that the porous structure could accelerate the oxygen release from bulk to surface, leading to a good mobility of oxygen and higher reducibility. In addition, it was also favorable for diffusion and penetration of methane and water steam into the sample particles to accelerate the reaction rate.
A series of Ceo.sFeo.30Zr0.20O2 catalysts were prepared by different methods （co-precipitations method, citric acid sol-gel method, impregnation method, physical mixed method, and hydrotherrnal method） and characterized by X-ray diffraction （XRD）, Raman spectroscopy, Brunauer-Emmett-Teller （BET） and H2-TPR measurements. Potential of the catalysts in the soot oxidation was evaluated in a temperature-programmed oxidation （TPO） apparatus. The results showed that all the Fe3＋ and Zr4＋ were incor- porated into ceria lattice to form a pure Ce-Fe-Zr-O solid solution for the co-precipitation sample, but two kinds of Fe phases ex- isted in the Ce-Fe-Zr-O catalysts prepared by other methods： Fe3＋ incorporated into CeO2 lattice and dispersed Fe2O3 clusters. The free Fe2O3 clusters could improve the activity of catalysts for soot oxidation comparing with the pure Ce-Fe-Zr-O solid solution owing to the synergetic effect between free Fe2O3 and surface oxygen vacancies. In addition, the activity of catalysts strongly relied on the surface reducibility of free Fe2O3 particles. Holding both abundant free Fe2O3 particles and high oxygen vacancy concentration, the hydrothermal Ce0.5Fe0.3Zr0.202 catalyst presented the lowest Ti （251℃, ignition temperature of soot oxidation） and Tm （310 ℃, maximum oxidation rate temperature） for soot combustion （with tight-contact between soot and catalysts） among the five samples. Even after aging at 800 ℃ for 10 h, the Ti and Tm were still relatively low, at 273 and 361 ℃, respectively, indicating high catalytic stability.
This study focuses on drawing a hydrothermal synthesis process map for Co3O4 nanoparticles with various morphologies and investigating the effects of Co3O4 nanocatalyst morphology on CO oxidation.A series of cobalt-hydroxide-carbonate nanoparticles with various morphologies（i.e.,nanorods,nanosheets,and nanocubes） were successfully synthesized,and Co3O4 nanoparticles were obtained by thermal decomposition of the cobalt-hydroxide-carbonate precursors.The results suggest that the cobalt source is a key factor for controlling the morphology of cobalt-hydroxide-carbonate at relatively low hydrothermal temperatures（≤ 140℃）.Nanorods can be synthesized in CoCl2 solution,while Co（NO3）2 solution promotes the formation of nanosheets.Further increasing the synthesis temperature（higher than 140 ℃） results in the formation of nanocubes in either Co（NO3）2 or CoCl2 solution.The reaction time only affects the size of the obtained nanoparticles.The presence of CTAB could improve the uniformity and dispersion of particles.Co3O4 nanosheets showed much higher catalytic activity for CO oxidation than nanorods and nanocubes because it has more abundant Co^（3＋） on the surface,much higher reducibility,and better oxygen desorption capacity.
Pr0.7Zr0.3O2-δ solid solution was prepared by co-precipitation method and used as an oxygen carrier in the selective oxidation of methane to syngas(methane/air redox process). The evolution on the physicochemical properties of Pr0.7Zr0.3O2-δ during the redox process was studied by means of X-ray diffraction(XRD), H2 temperature-programmed reduction(H2-TPR), O2temperature-programmed desorption(O2-TPD), Brunauer-Emmett-Teller(BET) surface area measurement and X-ray photoelectron spectroscopy(XPS) technologies. The results indicated that Pr0.7Zr0.3O2-δ solid solution showed the high activity for the methane conversion to syngas with a high CO selectivity in the range of 83.5%-88.1%. Though Pr-Zr solid solution possessed high thermal stability, lattice oxygen was obviously reduced for the recycled sample due to decreased surface oxygen which promoted oxygen vacancies. The increased oxygen vacancies seemed to enhance the oxygen transfer ability in the redox process and provided sufficient oxygen for the methane selective oxidation, resulting in a satisfactory activity. The problem of hot pot was avoided by comparing fresh, aged and recycle sample in the reaction.
