Preparation And Properties Of High Surface Area Composite Oxides For Catalytic NOx Removal

Nitrogen oxides （NO_x） can bring about serious environmental problems, such as aciddeposition, ozone depletion, photochemical smog and so on, which results in huge economiclosses and threatens the life and health of humanity. The cerium-titanium （Ce-Ti） compositeoxide catalysts exhibit excellent catalytic activity and high N₂selectivity in SCR reactions.Because three-dimensional hierarchical porous materials which contain specificmeso-macropores can offer the optimal flow velocity and high efficiency for the diffusion ofreaction gases, preparation of Ce-Ti composite oxides with the meso-macroporous structure,and exploring their properties in SCR reactions are important issues.Ce-Ti composite oxides which contain three-dimensional ordered hierarchicallymeso-macroporous structure were prepared by gaseous infiltration-precipitation method. Inthis method a colloidal crystal which constituted with polymethyl methacrylate （PMMA）microspheres were used as hard templates and P123as the soft template. The voids ofcolloidal crystal template were filled with soluble inorganic salts, and then the filled saltsprecipitated with gaseous NH3implementing solidification to hydroxides. After thecalcination of template removal, three-dimensional ordered hierarchically meso-macroporousstructured catalysts were finally obtained. The prepared catalysts were characterized by X-raydiffraction （XRD）, N₂adsorption-desorption （BET）, scanning electron microscope （SEM） andtransmission electron microscope （TEM） techniques. The characterization results showed that:（1） As the structure and morphology of the catalysts were significantly influenced by thecalcination condition, two-step calcination process was chosen as the optimal calcinationmethod.（2） A series of Ce-Ti composite oxides with different Ce/Ti molar ratio were preparedusing gaseous infiltration-precipitation method under the optimal calcination condition, andall the samples possessed well-ordered three-dimensional macroporous structure withmesopores in macroporous skeletons,（3） Ce（0.2）Ti-500-N₂samples which has the mostamount of mesopores exhibited the highest specific surface area.The composition and the element distribution of the as-prepared samples were alsoinvestigated using SEM and corresponding distribution mapping. The result reveals that Ceand Ti elements were well dispersed, and the compositions were nearly the same as the nominal values. These facts indicated that the composite oxides can be readily produced withuniform composition and a controllable metal molar ratio by the gaseous infiltration-precipitation method.The catalytic activity evaluation results showed that hierarchically orderedmeso-macroporous Ce-Ti composite oxides exhibited excellent catalytic activities. Reactionconditions were as follows: total gas flow300mL/min and mass hourly space velocity（MHSV）500000mL·g^-1·h^-1. Especially, Ce（0.2）Ti-500-N₂showed NO_xconversion above90%from300to450℃and the N₂selectivity was always above90%. After the activityevaluation, the catalysts still maintained their well-ordered hierarchically meso-macroporousstructures, indicating the high thermal stability of the catalysts.Pure Y₂O₃, Cr₂O₃and La₂O₃oxides were also prepared by this gaseousinfiltration-precipitation method. The characterization results showed that the prepared pureoxides possess hierarchically order meso-macroporous structures. These illustrated that thegaseous infiltration-precipitation method is applicable to fabricate both composite and pureoxides with hierarchically meso-macroporous structures.