Synthesis Of Co/Mn Nanosized Metal Oxides, Nitrides And Their Application In NO_x Abatement

Nitrogen oxides (NOx) can induce the formation of acid rain and photochemical fog. Nowadays, it is very urgent to limit the emission of NOx for the protection of environment in China. The key to the development of NOx removal is the synthesis of denitration catalyst with high catalytic activity. It has been known that the catalytic activities of denitration catalysts have close relationship with their surface structures. The efficient catalytic reduction of NOx with NH3 (NH3-SCR reaction) can be achieved by catalysts with high surface areas, abundant surface acid sites, more active surface oxygen, as well as good redox capacity. Owing to the rapid development of synthetic technique for nanocrystals, it is feasible to modulate the size and morphology of nanocrystals and subsequently tune the characteristics of their surface structures. Then it will bring a new synthetic strategy for the creation of denitration catalyst with high catalytic activity, namely synthesis of catalyst with special surface structure which is conducive to accelerate the process of the catalytic removal of NOxIn this thesis, Co3O4 nanocrystals with different shapes (nanorods and nanoparticles) are prepared and used as catalysts in SCR reaction for the removal of NOx. It is revealed that the catalytic performances of the Co3O4 catalysts are well associated with their morphologies. A series of Mn-based mixed metal oxide catalysts (Co-Mn-O, Fe-Mn-O, Ni-Mn-O) with novel fluffy structures are synthesized and their surface structure and catalytic activities for NH3-SCR reaction are investigated. Finally, Co3O4 nanorods, Co3O4 nanoparticles and Co-Mn-O nanocomposites are used as precursors for the preparation of cobalt nitrides. The activities for NO dissociation of the nitrides are studied. The main results presented in the dissertation have been summarized as follows:1. Synthesis of Co3O4 nanorods and nanoparticles and their catalytic performance for NH3-SCR reaction. Cobalt hydroxide carbonates are synthesized through ethylene glycol-mediated precipitation at different temperatures. Co3O4 nanorods are obtained from the calcination at 450 ℃ of cobalt hydroxide carbonates precipitated at 160 ℃, while nanoparticles from hydroxide carbonates precipitated at 20 ℃. Compared with nanoparticles, there are abundant Co3+ species distributed on the surface of Co3O4 nanorods. Since Co3+ ions exposed on Co3O4 nanorods surfaces greatly facilitate NH3 chemisorption and thus improve the catalytic activity of Co3O4 nanorods, during the NH3-SCR reaction (with a feed gas of 400 ppm NO/600 ppm NH3/2% O2/N2,30,000 ml-g-1h-1) in the temperature range from 150 ℃ to 200 ℃,Co3O4 nanorods show nearly 100% conversion of NOx, while the NOx conversion over Co3O4 nanoparticles is only about 65%.2. Synthesis of Mn-based mixed metal oxides with fluffy structures and their catalytic performance for NH3-SCR reaction. Mn-based mixed metal oxide Co-Mn-O, Fe-Mn-O and Ni-Mn-O have been synthesized via low-temperature crystal splitting. The TEM images of Co-Mn-O, Fe-Mn-O and Ni-Mn-O reveal that they all possess fine fluffy structures. During the NH3-SCR reaction (with a feed gas of 400 ppm NO/400 ppm NH3/2% O2/N2,30,000 ml-g-1h-1), the Mn-based mixed metal oxide catalysts exhibit more than 90% NOx conversion in the temperature range from 100 ℃ to 250 ℃. The H2-TPR and NH3-TPD profiles of Mn-based composite metal oxide catalysts demonstrate that there are abundant surface acid sites and large active surface oxygen distributed on their surface, which facilitate the chemisorption and activation of ammonia and eventually enhance their catalytic activities for NH3-SCR reaction. The effect of precipitation temperature of catalyst precursors on the catalytic performance of Mn-based mixed metal oxide is investigated and the results reveal that the ultra-low temperature is crucial for the preparation of Mn-based mixed metal oxide catalysts with high De-NOx activity.3. Synthesis of cobalt nitrides and their catalytic performance for NO dissociation. Co3O4 nanorods and nanoparticles have been used to synthesize Co4N by using NH3 temperature-programmed reaction method. During the NO dissociation reaction, Co4N exhibit superior activities, but heavy accumulation of surface oxygen generated during NO dissociation can cause deactivation of Co4N. While in the presence of CO, a catalytic NO dissociation and reduction cycle can be established on the surface of Co4N. When NO+CO reaction lasts for 500 min at the constant temperature 400 ℃ (500 ppm NO/500 ppm CO/He, 60,000 ml-g-1h-1), Co4N formed from Co3O4 nanorods exhibit 43% NO conversion, while Co4N formed from Co3O4 nanoparticles exhibit 90% NO conversion. Obviously, compared with the former, there are more active sites distributed on the surface of the latter. In addition, Co-Mn-O synthesized at low temperature is used to synthesize nitrides. The XRD and H2-TPR analyses of nitridation products indicate that they are the mixture of Co4N and MnO, their TEM images revealed that the existence of MnO can suppressed the agglomeration of Co4N at high temperature.