Selective Catalytic Oxidation Of NH₃at Low Temperature

NH3is one of the world’s largest inorganic produces with a wide range of uses, and they are all plagued with ammonia slip problem. Now, more and more attation is focused on the environment problem caused by waste gas which containing ammonia. Industries which use ammonia as raw material or produce as by-produce want to find an effective way to removal NH3urgently. The low concentration ammonia from the out gas produced by catalystic oxidation hydrolysis of yellow phosphorusand also needs removing. The final goal is converting the toxic ammonia to emviromenal friendly N2and H2O at low temperature with high efficiency. So far as knew, it is hard to macth the goal. To make the goal come true, two-step method is designed and implemented.Ammonia is hard to selective catalytic oxidation to N2and H2O at temperature lower than200℃, a seires of work needs to do to reach the goal planed. The catalysts are chosen by diffrernt evaluation criterion:the direct one and the two-step one. In brief, the direct one is a catalyst which can convert NH3to N2and H:O by direct selective catalytic oxidation, uses only one catalyst; while the two-step one is a catalyst converts parts of the NH3to NO, and uses the right NO-SCR catalyst as the second layer to removal left NH3and NO produced in SCO reaction.The results of supports selection experimental show that catalysts use Al2O3as support and use Cu-Mn as the active compounds has a much higher catalytic activity at low temperature than uses TiO2and SBA-15as supports, this is caused by the suitable active mass and the synergistic effect between Cu-Mn and between Cu-Mn and Al2O3. Based on the experimental, Cu-Mn/Al2O3can be a promising catalyst for NH3selective catalytic oxidation after opitimization, the NH3conversion at250℃is over80%, and the NO yield can be ignored. While for SBA-15, it is a bad support for NH3catalyst. Catalysts use TiO2as support has rather good NH3conversion, but unfortunately, the NO yield is high. This is caused by the good dispersity of active phase and more surface oxygen. Cu-Zr/TiO2has high NH3conversion, but a lot of NO is tasted in the outlet, surely it is not good catalyst for NH3selective catalytic oxidation to N2and H2O. In other way, it gives us a chose that we can use this catalyst in two-step method. As the Cu-Zr/TiO2catalyst produce a lot of NO when it catalytic NH3, and this matches the SCO catalyst used in two-step method, In two-step method, the role NH3catalyst play is converting parts of NH3to NO, and the outlet NH3/NO should be1. After a seires work, catalyst made by coprecipitation method, calcined under N2,1%Ce-1%La-5%Cu-5%Zr/TiO2has a wonderful performance, it reach the goal NH3/NO=1at150-175℃. MnOx is used as the NO-SCR catalyst to remove the produced NO and the left NH3, the results show the two-step method can eliminate NH3well, at150℃, the NH3conversion is near90%and no NO is yield, while at175℃, the NH3conversion is over90%and the NO yield can be ignored. And this is the lowest NH3removal temperature as far as we know.For further study, new support carbon nano-tube was used. The pretreatment of carbon nano-tube can effect catalyst effection a lot. The traditional pretreatment can not meet the goal, and NTP treatment is a promising way for the experimental. CNTs is chosen as catalyst support rather than CNTs(OH) become catalyst use CNTs support have better NH3conversion and higher NO yield. The time NTP treatment needs is also considered and10min is the right time. Co/CNTs has good NH3and higher NO yield when compare with other single-compound catalysts, but still can not meet the goal, while for the two-compounds catalyst, Cu-Zr/CNTs show better performance than Cu-Zr/TiO2. After optimizatipn,1%Ce-1%La-5%Cu-5%Zr/CNTs catalyst made by impregnation method and calcined under N2show the best performance. The outlet reach NH3/NO=1lower than150℃, which means NH3can be removing under150℃by two-step method. At150℃, over90%NH3is converted and no NOX yield. The durable property experiment indicated two-step method uses1%Ce-1%La-5%Cu-5%Zr/CNTs as SCO catalyst and MnOx as SCR catalyst as a system is rather stable, this system still work well after36h.