Effects Of Hydrothermal Treatment On Cu-SAPO-34 Catalyst In NH₃-SCR Reaction

With the increase in the number of diesel vehicles,nitrogen oxides emitted by diesel vehicles have gradually become a main pollutant in the urban environment,and the environmental problems caused thereby have become increasingly prominent.Se-lective catalytic reduction with ammonia?NH₃-SCR?is the most effective technology for eliminating NO_x in diesel vehicle exhaust.In recent years,Cu-SAPO-34 molecular sieve catalysts with chabazite?CHA?structure have attracted extensive attention due to their high denitrification activity and perfect N₂ selectivity.In this paper,the effects of low-temperature and high-temperature hydrothermal treatment on the performance of Cu-SAPO-34 catalyst in NH₃-SCR reaction are investigated,and the migration mechanism of Cu species during the hydrothermal process is explored.In our work,Cu-SAPO-34 catalyst is prepared by a one-pot method,and then the catalyst is hydrothermally treated at 70,100,and 150 ^oC for 20 h with air containing30 vol.%moisture,respectively.These samples are characterized subsequently with SEM,H₂-TPR,NH₃-TPD and EPR,combined with the activity experiments,and the effect of low-temperature hydrothermal treatment on the performance of Cu-SAPO-34catalyst at different temperatures is investigated.The results show that the activity of Cu-SAPO-34 catalyst for NH₃-SCR decreases after low-temperature hydrothermal aging,and the deactivation of Cu-SAPO-34 catalyst becomes more obvious with the decrease of the hydrothermal temperature.The decrease of isolated Cu²⁺content is the main reason for the decline of the low-temperature activity of Cu-SAPO-34 catalyst.The decrease of activity in high temperature region is mainly due to the increase of CuO content and the decrease of acidity.It is more difficult for water vapor to adsorb on the molecular sieve when the hydrothermal temperature increases,which weakens the destruction of Si-OH-Al bonds by water vapor,thereby weakening the deactiva-tion of Cu-SAPO-34 catalyst by hydrothermal treatment.Additionally,Cu-SAPO-34 catalysts with the same Cu content are prepared with the one-pot method and ion-exchange method,respectively.The two samples are hy-drothermally aged at 750 ^oC for at least 16 h.A certain amount of CuO exists in the as-prepared ion-exchange sample,whereas copper is mainly atomically dispersed in the one-pot sample in the form of isolated Cu²⁺.After hydrothermal aging for 16 h,the amount of the inner nanosized CuO in the ion-exchanged sample reduces and the copper disperses to form more isolated Cu²⁺.However,surface CuO particles increase at the cost of isolated Cu²⁺with extending the hydrothermal treatment to 24 h.Ag-gregation of Cu²⁺species also happens during the ageing treatment over the one-pot prepared sample where the initial Cu is atomically dispersed as isolated Cu²⁺.The in-crease of the amount of the isolated Cu²⁺enhances the low-temperature?<350 ^oC?performance.However,the increase of the amount of surface CuO particles in the aged samples promotes the non-selective NH₃ oxidation reaction in the high tempera-ture range and results in the decline of the SCR activity by limiting NH₃ supply.