Study On Catalytic Decomposition Of Nitrous Oxide From Nitric Acid Plant Exhaust Gas

Nitrous oxide tends to damage the ozone layer and triggers the greenhouse effect.The tail gas of nitric acid plants is one of its main sources,so emission reduction of nitric acid plant tail gas is of great significance for environmental protection.In this study,molecular sieve catalysts were prepared by the incipient impregnation method,and hexaaluminate catalysts were prepared by the coprecipitation method.The activity of a series of catalysts for the catalytic decomposition of nitrous oxide at a N₂O concentration of 1200 ppm was evaluated.The physical,chemical and structural properties of the catalysts were characterized by XRD,H₂-TPR,BET,etc.The kinetics of the catalytic decomposition of nitrous oxide by molecular sieve catalysts was studied.Ten commonly used molecular sieves Hβ、HZSM-5、β、ZSM-5、Na Y、Na X、4A、SAPO-34、USY、REY were used as carriers for catalytic decomposition of nitrous oxide catalysts with Cu O as the active component.Among the catalysts with a Cu O loading of 6%,the catalysts supported by Hβand HZSM-5 exhibited better catalytic activity.The catalytic activity of Fe,Co,Cu,Al,Ce,La,Ni,Zn,Mn,Cr,In,Y oxides as single component catalysts was investigated using Hβand HZSM-5 as carriers.The experimental results showed that the catalyst loaded with Fe₂O₃ on Hβhad good catalytic activity,and the temperature for complete conversion of nitrous oxide was 315℃.On this basis,the activity of dual component catalysts with Fe₂O₃ as the main component was investigated,and transition metal oxides（Cd,Co,Cr,Cu,Mn,Ni,Zn）,rare earth metal oxides（La,Y,Nd,Ce）,alkaline earth metals,and main group metal oxides（Mg,Sr,Al,Pb）were added as the second active component.It was obtained that only adding cobalt oxide as the second active component can enhance the activity of iron oxide catalysts,and the temperature for complete conversion of nitrous oxide is 300℃.The experiment investigated the effects of doping amount,preparation conditions,and operating conditions of cobalt oxide on the catalytic decomposition of nitrous oxide,and investigated the thermal stability of the catalyst.Under the conditions of N₂O concentration of 1200 ppm and space velocity of 20000 h^-1,high activity could be maintained for more than 50 hours at 420℃.Single oxide doped lanthanide hexaaluminate catalyst La MAl₁₁O_19-δ（M=Cu,Fe,Mn,Ce）were prepared by the coprecipitation method with ammonium carbonate as the precipitator.For samples doped Cu,Fe,Mn oxide,the activity of the catalysts ranked order by La Mn Al₁₁O_19-δ>La Cu Al₁₁O_19-δ=La Fe Al₁₁O_19-δ>La Ce Al₁₁O_19-δ.To examine the effect of doping amount,La M_xAl_12-xO_19-δ（M=Mn,Cu）series catalysts were prepared,in which La Cu₂Al₁₁O_19-δcatalyst expressed the highest activity.The temperature for complete conversion of nitrous oxide was 480℃.Bimetallic doped La Cu₂MAl₉O_19-δ（M=Nd,Y,Pb,Zn）series catalysts were experimentally investigated.The samples activity ranking as follows:La Cu₂Nd Al₉O_19-δ>La Cu₂Zn Al₉O_19-δ>La Cu₂YAl₉O_19-δ>La Cu₂Al₁₀O_19-δ>La Cu₂Pb Al₉O_19-δ.The effects of doping amount,preparation conditions,and operating conditions of neodymium oxide on the catalytic decomposition of nitrous oxide were experimentally studied.Furthermore,the thermal stability of the catalyst was investigated.Under the conditions of N₂O concentration of 1200 ppm and space velocity of 20000 h^-1,high activity could be maintained for more than 50 hours at 550℃.Experiment research of kinetics was conducted for catalytic decomposition of N₂O on sample Fe₁₂Co₆/Hβcatalyst.Experiment conditions were selected based on the results of elimination of internal and external diffusion effection in the of catalytic reactions.Experimental studies have shown that the catalytic decomposition of N₂O on the Fe₁₂Co₆/Hβcatalyst was a first-order for N₂O and zero power for O₂,respectively.The reaction activation energy was 78.92 k J·mol^-1 for decomposition of N₂O on Fe₁₂Co₆/Hβcatalyst.