Preparation Of Manganese-cerium Composite Oxide And Their Catalytic Performance For SCR Denitrification At Low Temperature

Nitrogen oxides（NO_x）being a kind of major air pollutant,which emitted from industrial production and automobile exhaust,could pose a great threat to the ecological environment and human health.At present,selective catalytic reduction of NO with NH₃（NH₃-SCR）becomes the most widely used technology in industrial denitrification,the main reason of which is that NH₃-SCR can selectively reduce NO_x into non-toxic N₂ and H₂O and shows high efficiency,stable operation,low NH₃ escape and no secondary pollution.Catalyst is the core of the LT-SCR technology.V₂O₅-WO₃（Mo O₃）/Ti O₂is the efficient and typical catalyst and has been widely commercialized for NH₃-SCR at high temperature,but the drawback of toxic inactivation easily,poor activity at low temperature,narrow working temperature window and leaching toxicity made it impossible to apply universally for NH₃-SCR.Recently,NH₃-SCR of NO at low temperatures（LT-SCR）has been extensively investigated because of the remarkable energy conservation and potential industrial commercial value.Therefore,the preparation of highly active catalysts for the reaction become the key to get breakthrough of LT-SCR.In-depth study of the denitrification mechanism of LT-SCR is a prerequisite for the development of efficient and stable catalysts.In this paper,we take Mn O_x-CeO₂as the research object because CeO₂ has the excellent ability of oxygen storage and release,unique redox properties and Mn O_x has variable valence and excellent oxidation reduction ability.We prepared two types of Mn O_x-CeO₂ composite nanomaterials by hydrothermal and impregnation method:CeO₂nanorods coated with Mn O_x nanosheets（Mn O_x@CeO₂）and Mn O_x nanoflowers loaded CeO₂ nanoparticles（Mn-Ce-O_x）.Subsequently,we have characterized the structure properties,catalytic performance and denitrification mechanism of the catalysts by using common methods.The results show that pure CeO₂ nanorods distributes uniformly and has great heat stability,which has the largest specific surface area（73 m²/g）and pore volume（0.3 cm³/g）for the Mn O_x@CeO₂ and Ce/Mn-1-0.25（300℃）has the second largest specific surface area and pore volume and is enriched in the ratio of Ce³⁺,Mn⁴⁺,O_ads on the surface.For Mn-Ce-O_x,we also found that the Mn O₂ phase is attributed toδ-Mn O₂ and the spherical flowers are uniformly distributed and become great heat stability.Ce/Mn-0.17（300℃）has the maximum specific surface area（342 m²/g）,pore volume（0.74 cm³/g）and the minimum average pore diameter（7.63 nm）,which is slightly better than Ce/Mn-0.17（400℃）.But Ce/Mn-0.17（400℃）has the highest proportion of Ce³⁺,Mn⁴⁺and O_ads.We also investigate the effects of Ce/Mn ratio,gas hourly space velocity（GHSV）and pretreatment temperature on catalytic activity.It can be concluded that:GHSV=10000 h^-1,the NO conversion of nanorod-shaped Ce/Mn-1:0.25（400℃）was 73%at 200℃and the nanometer flower-shaped Ce/Mn-0.17（400℃）was 100%under the same conditions.Obviously,the catalytic performance of Mn-Ce-O_x is superior to Mn O_x@CeO₂,which result from the structural characteristics and surface element state of the material.The analysis of In situ DRIFTS shows that the SCR reaction on the surface of Mn O_x@CeO₂ and Mn-Ce-O_x roughly followed two pathways of Langmuir-Hinshelwood（L-H）mechanism and Eley-Rideal（E-R）mechanism at 200℃.The results lay a foundation for the preparation of nano catalyst,the study of the catalytic activity at low temperature and the mechanism analysis.Besides,they have a great significant reference for the application of NH₃-SCR at low temperature of binary catalyst.