Preparation, Characterization And Catalytic Activity Of Mixed Oxides Catalysts Towards VOCs Combustion

Volatile Organic Compounds（VOCs） are recognized as major contributors to air pollution besides SO_X and NO_x.Among many techniques for controlling VOCs emissions,catalytic combustion is one of the most important and promising processes for industrial VOCs elimination due to its low cost,low energy requirement and environmental friendly process.In general,supported noble metal catalysts exhibit high catalytic activities at lower temperatures,and have been widely applied in industrialized VOCs elimination.However,these catalysts are are expensive,sensitive to poisons and easily sintered at high temperature.Therefore,it is necessary to develop non-noble metal catalysts,which are cheap,efficient and stable for VOCs catalytic combustion.In this dissertation,the perovskite and Cu-Mn oxide were studied systematically in following three key issues:（1） Preparation methods of monolithic catalysts.（2） Stability of complex oxides.（3） Reaction mechanism of catalytic combustion of VOCs on mixed oxides catalysts.LaBO₃（B=Cr、Mn、Fe、Co、Ni、Cu） perovskite catalysts were prepared by co-precipitation in present dissertion.The present catalysts were characterized by XRD,BET,H₂-TPR,O₂-TPD and their catalytic performances are studied by total oxidation of benzene,toluene,ethyl acetate and acetone.The results show that catalytic performances of LaBO₃ catalysts depend on the nature of oxidation-reduction capacity of B-site ions.LaMnO₃ and LaCoO₃ have the best catalytic activity among all materials studied.LaCoO₃ has anion-deficient perovskites structure which is suitable for catalytic combustion of oxygen-containing organic pollutants such as ethyl acetate and acetone.LaMnO₃ has cation-deficient structure which is suitable for the catalytic combustion of organic pollutants with high C-H bond（e.g.benzene）.LaMnO₃ has better thermal stability and resistance to chloride poisoning than LaCoO₃.The effects of Sr and Au doping on structure and properties of LaMnO₃ were also investigated.The results show that the increase of Mn⁴⁺ content of Sr-doped catalysts lead to better activity in catalytic combustion of toluene.The improvment of the catalytic activity by doping of precious metal Au are due to enhancing redox of LaMnO₃ catalyst.A slurry dip-coating method was used to prepare cordierite honeycomb La_0.8Sr_0.2MnO₃ catalysts coated with Al₂O₃ and hexaaluminate(HA, Sr_0.3Ba_0.5La_0.2MnAl₁₁O₁₉).It is found that HA coating can reduce the interaction of cordierite support and perovskite at high temperature for La_0.8Sr_0.2MnO₃.Thus solve the deactivation problem of the catalyst at high temperature.The catalytic activities of the powder,wire-mesh honeycomb and ceramic honeycomb La_0.8Sr_0.2MnO₃ catalysts in toluene combustion in fixed bed were studied.The results show that the powder, wire-mesh honeycomb and ceramic honeycomb La_0.8Sr_0.2MnO₃ catalysts show similar catalytic performance.The wire-mesh honeycomb catalyst exhibits better mass transfer and higher thermal response rate,and it is more suitable for treating VOCs at the conditions of large blast volume and big temperature fluctuation than other forms.Cu-Mn catalysts with different Cu/Mn ratio were prepared by combustion method and characterized by XRD,BET,H₂-TPR,O₂-TPD,and XPS methods.The catalysts were tested in toluene total oxidation at the temperature range of 100-300℃.It is found that Cu₁Mn₆ catalyst with Cu_1.5Mn_1.5O₄ and Mn₃O₄ mixed crystalline structure has the best activity.The possible catalytic mechanism was propsed as the followings: Cu_1.5Mn_1.5O₄ with strong oxidation capacity acts as the active center,and the Mn₃O₄ acts as oxygen supply center.Ce doping can further improve the surface oxygen mobility and catalytic activity of Cu-Mn oxide.Cordierite ceramic honeycomb-type Cu-Mn and Cu-Mn-Ce catalyst were prepared by in-situ combustion synthesis technology.Results show that the proportion of the surface ion on monolithic catalysts was proportional to the ion ratio in dipping solution.Monolithic Cu-Mn and Cu-Mn-Ce catalysts have similar activity trend with powder catalyst.This suggests that cordierite ceramics can not selective adsorb ions in solution,and cordierite carrier has little effect on the intrinsic activities of metal oxides.The catalytic performances of monolithic commercial Pd catalyst,Cu₁Mn₂Ce₄ and La_0.8Sr_0.2MnO₃ were investigated in catalytic combustion of various types of VOCs.The results show that noble metal Pd catalyst is more suitble for catalytic combustion of aromatic VOCs.Mixed Oxide catalyst is more suitable for catalytic combustion of oxygen,nitrogen-containing VOCs and alkanes.In summary, monolithic Cu₁Mn₂Ce₄ catalyst can be used in catalytic combustion of various types of VOCs,which is superior to commercially noble metal Pd catalyst.