Regeneration of Cu/SAPO-34(MO) with H2O only: too good to be true?

Abstract

The performance failure of Cu/SAPO-34 material used as an NH₃-SCR catalyst at low temperature in the presence of water has caused a gradual withdrawal of its usage from the market. There is an urgent need to clearly understand its deactivation mechanism and to find a way to regenerate the Cu/SAPO-34 catalyst material. Interestingly and surprisingly, we have discovered that, under certain conditions, 10% H₂O can regenerate a previously deactivated Cu/SAPO-34(MO), as long as its zeolite structure is maintained. By using experimental observations of NH₃-SCR reaction, solid state NMR, NO-DRIFTS, and in situ H₂-TPR, a mechanism is proposed which can explain both the deactivation and regeneration of Cu/SAPO-34(MO). We propose that the transformation of Si(4OAl) and Si(3OAl) to (2Al)Si(2OH) and (3Al)Si(OH) and the Si condensation in the pores of the framework, which result from H₂O exposure, are responsible for the deactivation of Cu/SAPO-34(MO). We suggest that the formation of condensed Si which results in Si clusters, hinders the mobility of the linear complex, [Cu–(NH₃)₂]⁺, which is the active species during the low temperature NH₃-SCR reaction. Moreover, we propose a regeneration mechanism when Cu/SAPO-34(MO) is exposed to 10% H₂O, where the Si clusters are transformed back to Si–O–H bonds, and thereafter transferred back to the framework. This can explain the regeneration of deactivated Cu/SAPO-34(MO).