AsH₃ Removal By Catalytic Oxidation In Industrial Tail Gas By Cerium Modified HZSM-5 Zeolite

The AsH₃ removal is of great significance to human health,industrial production and environmental protection.AsH₃ has strong reducibility.Generally speaking,the removal mechanism of modified adsorbent used in the past is that the active components on the adsorbent carrier can oxidize AsH₃ to arsenic trioxide and arsenic pentoxide,so as to capture and purify the gas from the air stream.Compared with elemental As,the recovery of arsenic oxide is lower,which indicates that it is very important to develop a method for catalytic oxidation of AsH₃ to non-toxic products.In recent years,gas-solid catalytic oxidation has become a trend for the efficient purification of AsH₃.Compared with the method of removing AsH₃ by modified adsorbent,the gas-solid catalytic oxidation method can remove AsH₃ more efficiently through the rich active components and chemical oxygen on the catalyst surface.However,the products produced by AsH₃ after catalytic oxidation process are diverse and uncertain.Under some conditions,the products are mostly arsenic trioxide and arsenic pentoxide.For example,when the catalyst is loaded with strong oxidizing active components,the reaction temperature is too high and the oxygen content is too high,the products of AsH₃ catalytic oxidation are mostly arsenic pentoxide,doped with a small amount of arsenic trioxide.As a rare earth metal,cerium?Ce?can be loaded on the catalyst carrier as an accelerator to improve the catalytic performance because of its high oxygen storage capacity in catalytic reaction,and has a wide range of catalytic prospects.In this study,in order to prevent the excessive oxidation of AsH₃,Ce,a metal with moderate oxidation capacity,was selected as an active component to remove AsH₃and its mechanism was studied.HZSM-5 zeolite is a nano-sized zeolite with a particle size less than 100 nm.It has a larger surface area,a higher mesoporous volume and a smaller pore size,so it is chosen as catalyst carrier.The effects of Cerium Modified HZSM-5 zeolite on AsH₃ removal and its mechanism were studied.Four factors including cerium precursor type,cerium loading content,calcination temperature and reaction temperature were studied.A series of Ce modified catalysts were prepared by equivalent volume ultrasonic impregnation for the removal of AsH₃.The gas-solid catalytic oxidation of hydrogen arsenide was tested under different preparation conditions and reaction conditions.Through screening tests,the optimum preparation conditions were determined as follows:impregnating the catalyst with 10 wt%Ce?CH₃COO?₃ precursor and calcining at 550?.The oxidation efficiency of Ce-HZSM-5 catalyst can be maintained over 90%within 82hours when the reaction temperature reaches 170?.In order to reveal the reaction mechanism,the physicochemical properties,phase composition and surface properties of the catalyst were analyzed by means of scanning electron microscopy?SEM-EDS?,specific surface area?N₂-BET?,X-ray diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?,temperature programmed reduction?H₂-TPR?,Fourier transform infrared spectroscopy?FT-IR?.The results show that Ce⁴⁺and Ce³⁺mainly exist on the surface of the catalyst.In the reaction process,the conversion of chemisorbed oxygen to lattice oxygen plays a key role,and their transfer between Ce⁴⁺and Ce³⁺valence states.After being captured by catalyst,AsH₃ in the gas phase is first oxidized to elemental As,then some elemental As condenses along the tube wall in the latter half of the gas stream,while others remain on the catalyst surface and are further oxidized to As₂O₃.It is proved that Ce-HZSM-5 is a promising catalyst for the recovery of arsenic.The Ce-HZSM-5 catalyst proposed in this study can provide a new idea for the removal of AsH₃.