The Effects Of Water On The NH₃-SCR Reaction Over Cu-SAPO-34 Catalysts

Ammonia selective catalytic reduction technology（NH₃-SCR）is considered the most effective technology for removal of NOx under oxygen-rich conditions.Cu-SAPO-34 molecular sieve catalyst with chabazite（CHA）structure has better SCR reaction activity;N₂ selectivity and excellent hydrothermal stability have received widespread attention.In this paper,the effect of water vapor on the NH₃-SCR activity of catalysts with different Si/Al ratios and different Cu loadings was investigated.The mechanism of the effect of water vapor on the Cu species on the catalyst surface was investigated by means of H₂-TPR,NH₃-TPD,XRD,and XPS.A series of Cu-SAPO-34 catalysts with different Cu loadings Si/Al of 1/4 and 1/10were prepared by ion exchange method.The activity testing was carried out under water-containing（5 vol.%H₂O）and non-water-containing SCR atmosphere（O₂ content is 5%or 10%）.Transient experiments were designed to study the transient response of catalyst NH₃-SCR activity to the presence or absence of water vapor.Then,experiments were designed to compare the changes in catalyst activity after treatment with water-containing SCR atmosphere.The results show that for the catalysts with the same Si/Al,the higher the Cu loading,the higher the activity at low temperature,and the activity at high temperature is not much different.The presence of water will promote the activity of each sample in the entire reaction interval.At low temperatures,the presence of water will reduce the energy barrier of the reoxidation reaction,and then promote the reaction activity.At high temperatures,water will promote the transfer of Cu O on the catalyst surface to other species,inhibit NH₃ oxidation,thereby enhancing SCR reaction activity.Transient experiments show that in the SCR reaction atmosphere without water,the addition of water vapor will cause a rapid increase in the activity of the catalyst.After the water vapor is stopped,the SCR of the catalyst will show a rapid-slow decline,but relative to the activity before the water vapor is introduced.It has a smaller permanent activity boost.After 1h of water-containing SCR atmosphere treatment,the activity of NH₃-SCR in the entire temperature range of the catalyst is improved relative to the untreated fresh catalyst,which is caused by the synergistic effect of transient and permanent effects brought about by water treatment.The catalysts were treated under SCR and water-SCR atmospheres for one hour respectively,and the changes of Cu species on the catalyst surface were explored by means of characterization.The results show that after SCR feed treatment,the amount of[Cu^II（OH）]⁺has decreased,the Cu species on the catalyst surface are more dispersed,and the reducibility of Z₂Cu-6MR species is reduced.The Cu O content on the catalyst surface is reduced compared with fresh catalyst,and the weakly bound Cu species on the catalyst surface partially migrate to form a more stable Cu species.After water-SCR feed treatment,the migration of weakly bound Cu species to more stable defect sites was greatly promoted,the amount of Cu O on the catalyst surface was further reduced,and the amount of Cu²⁺and[Cu^II（OH）]⁺increased.That is to say,after the water-containing atmosphere treatment,the unanchored Cu species（such as Cu O）on the catalyst surface migrate and re-disperse and anchor to the defect sites on the catalyst surface in the presence of water.The redispersed anchoring of the unstable Cu species on the catalyst surface creates the effect of water vapor on the permanent activation of the catalyst NH₃-SCR.