Catalytic Oxidation Of Low Concentration Ammonia At Low Temperature And In Lean Oxygen

Ammonia plays an important role in photochemical smog, water eutrophication and soil alkalization. Our work was a part of High Technology Research and Development Program (863 Program), focused on the treatment of low concentration ammonia from the off-gas of catalytic hydrolysis of yellow phosphorusand by catalytic oxidation method, and the experiment method was also could applied in chemical industry production, which exhausted low concentration ammonia and large gas capacity. The final aim is to convert the toxic ammonia into harmless nitrogen.Our study worked on the development of new catalysts, which is the the key problem of low temperature catalytic oxidation. The systematical work included catalysts support selection, catalysts ative phase and promoter selection, the ratio of ative phase and promoter selection were measured. Some factors that maybe affect the catalysts activity were investigated, such as catalysts preparation method, reaction condition. Catalysts activity characterization and physical-chemical properties analysis were carried out; the mechnisam of selective catalytic oxidation of ammonia was primarily discussed here.Catalyst supports selection show that catalysts made by TiO2 support gave much higher catalytic activity at low temperature than AC and Al2O3 support because of good dispersity of active phase resulted from the interaction between active phase and TiO2 support. Single-component transition metal catalysts selection show that Cu/TiO2,Co/TiO2 and Cr/TiO2 were candidates as low temperature and high activity catalysts, Fe/TiO2 and Ni/TiO2 were with low activity but low NOx yield. Two-components transition metal catalysts based on copper and chromium selection experiments found three intresting catalysts. The first one is Cu-Mn/TiO2 catlyst with ammonia removal efficiency was high and NOx yield was relatively low on; the second one is Cu-Co/TiO2 catalyst with high ammonia removal efficiency and high NO yield; the last one is Cr-Ni/TiO2 catalyst with high activity and high N2O yield. In order to investigate the factors maybe affect the catalytic properties, optimization experiments of Cu-Mn/TiO2 and Cu-Co/TiO2 catalysts were carried out and results revealed that:Cu-Mn/TiO2 catalyst with Cu-Mn ratio of 5%:5%(wt%), prepared by co-precipitation and calcinated at 350℃was beneficial to SCO-NH3 under the condition of 1% oxygen.80% ammonia removal efficiency and 30% NOx yield were obtained over it. For Cu-Co/TiO2 catalyst, which is with Cu-Co ratio of 10%:5%(wt%) and calcinated at 400℃show much higher activity and NO yield in 1% oxygen. Ammonia removal efficiency and NO yield were above 90% and 100% respectively.SCO-NH3 results show that at low temperature two phenomenons occurred:both ammonia removal efficiency and NOx yield were relatively high, or ammonia removal efficiency was high but NO or N2O yield was much higher. As a result, high ammonia removal efficiency and high nitrogen selecticity were not achieaved by the method of SCO-NH3. Then iSCR-NH3 (in situ selective catalytic reduction of ammonia) method was conceived and put into practice based on earlier experiment results and correlative references. iSCR-NH3 experiments suggested that Cu-Co/TiO2 as the first catalyst, which was with the Cu-Co ratio of 5%:10%(wt%) and calcinated at 400℃, combined with MnOx catalyst as the second catalyst, which was prepared by co-precipitaion show good performance at low temperature. Reaction condition optimization experiments indicated that at 100℃90% ammonia removal efficiency and 10% N2O yield were obtained, and negligible NO and NO2 were detected under the condition of 1% oxygen,15,000h-1 gas hourly space volume and without water vapor. Ammonia adsorption-desorption balance experiments measured over Cu-Co/TiO2—MnOx catalysts, the results show that a large amount of ammonia adsorbed on catalysts at temperature lower than 100℃and desorption time was very long, while NOx yield was much low. On the contrary, few ammonia adsorpted above 100℃and the desorption time was short, but N2O yield increaseda little bit. Catalysts durable property experiment indicated that ammonia removal efficiency decresed from 90% to 85% slowly after the reaction time up to 22h, meanwhile N2O yield also decreased to 5%-10%. However, the catalysts of iSCR-NH3 hold stable high activity and low NOx yield at 100℃.