Study On Morphology, Crystal Surface Control And Catalytic Performance Of Cobalt - Based Catalysts

Recent years, with the rapid development of nanotechnology, the composition, morphology and size of solid catalysts can be controlled using buttom-up chemistry technology at nanometer level. Extensive studies over the past few decades have demonstrated that the particle size and morphology of catalyst on the nanometer scale profoundly affects its performance. Further, the nature of the morphology-dependent catalysis attributes to the active facet of the selective exposure material. Consequently, according to the controllable synthesis of morphology of heterogeneous catalysis nanomaterials, the active facet of the materials can be exposed selectively, which can substantially improves the catalytic activity, selectivity, and stability and became a research hotspot of the heterogeneous catalysis nanomaterials. Co3O4 nanomaterials is widely used in heterogeneous catalytic reaction, it is often used for catalytic oxidation CO and the catalytic combustion of hydrocarbons, and its catalytic properties depend on their size and morphology directly. This paper focuses on controllable synthesis of the particle size and morphology of Co3O4 nano materials and its composite materials, the influence morphology of catalytic materials make on CO catalytic activity is also investigated.Three kinds of morphologies Co3O4 nanomaterials were successfully synthesized by novel hydrothermal synthesis process. On this basis, Ce02/Co304 composite oxide catalysts are prepared through secondary hydrothermal method and the composite catalysts were characterized by XRD, BET, SEM, TEM, H2-TPR, the catalytic activity of catalysts with CO oxidation was investigated on fixed-bed reactor. Through experiment, some conclusions are acquired.(1)The effect of cobalt source, reation temperature, reaction time, reaction temperature, surfactant(CTAB), heating rate, calcination temperature for the morphology of catalyst materials are investigated in detail. The results shown that optimal conditions for preparing rod morphology Co3O4 precursor is that using CoCl2 as cobalt source with adding 1 g/100ml CTAB and react at 120 ℃ for 16h, the optimal conditions for preparing plate morphology Co3O4 precursor is that using Co(N03)2 as cobalt source with adding 1 g/100ml CTAB and react at 140 ℃ for 16h, the optimal conditions for preparing rod morphology Co3O4 precursor is that using CoCl2/Co(NO3)2 as cobalt source with adding 1 g/100ml CTAB and react at 200 ℃ for 16 h. By analyzing the impact of cobalt sources for Co3O4 precursor morphology, we proposed the anion effect to explain, namely, when using CoCl2 as cobalt source, the CO32- may act as an inhibitor that selectively decreases the rates of crystal growth along both{001} and{100} directions, resulting in the {010}-elongated nanorods. when using Co(NO3)2 as cobalt source, the CO32- still act as the inhibitors, but it is affected by the steric effect of NO3-, so it turned to selective decreases the rates of crystal growth along{010} directions, which makes the growth speed of {012} directions relatively faster, resulting in the final precursor to form a sheet-like. However, when the reaction is carrying out at high temperatures, no matter what cobalt sources is used for this reaction, the final precursor morphology is cubic nanometer, it may be due to thermal motion of the particles at high temperatures are too intense. Then we combined effects of all experimental factors on the morphology of the precursor,a "synthetic map" is proposed for guiding the synthesis of Co3O4 precursor’morphology.Through high-temperature calcination, the optimum reaction conditions of heating rate and calcination temperature are 2 ℃/min and react at 400 ℃ for 3 h. The obtained Co3O4 morphology nanomaterials greatly inherit its morphology of their precursor and are porous. All of these three Co3O4 morphologies nanomaterials possess the{110} facet.Finally, the catalytic activity of Co3O4 nanomaterials with CO oxidation was investigated, the results shown that the Co3O4 nanomaterials which with a mature morphologies exhibit a better catalytic performance for oxidation CO than those morphologies are immature,and these three morphologies Co3O4 nanomaterials’ CO oxidation performence relationship is:plate morphology Co3O4>cubic Co3O4>rod morphology Co3O4.(2) On the basis of a hydrothermal obtained Co3O4 precursor, we obtained three morphologies CeO2/Co3O4, which CeO2 dispersed highly on Co3O4, by the method of adding cerium source to secondary hydrothermal then calcined. The catalytic activity of composite catalysts CeO2/Co3O4 with CO oxidation was investigated, it showed that catalytic activity of composite catalysts substantially depend on the morphology of carrier and CeO2 loadings. On one hand, the results showed that the catalytic activity of these three kinds morphologies composite catalysts CeO2/Co3O4 with CO oxidation’ performance relationship is:plate-like morphology Co3O4>cube-like Co304>rod-like morphology Co3O4. On the other hand, the CeO2 loadings of composite catalysts CeO2/Co3O4 have significant impact on its catalytic performance, the samples of composite catalysts CeO2/Co3O4 with higher CeO2 loadings exhibit a better catalytic performance for oxidation CO than any others.