Review of Hydrocarbon Poisoning and Deactivation Effects on Cu-Zeolite, Fe-Zeolite, and Vanadium-Based Selective Catalytic Reduction Catalysts for NOx Removal from Lean Exhausts

Abstract

Low-temperature operation of NH3-SCR (selective catalytic reduction) systems for NOx abatement in lean streams raises major challenges caused by the need to fix different problems related to poor catalyst activity and to urea injection handling. In this respect, an important issue is related to the presence of unburned hydrocarbons (HCs) in the exhausts that may damage irreversibly the activity of the DeNOx catalysts. The purpose of the present perspective is to summarize the most important effects of HCs on NH3-SCR performances of commercial SCR catalysts, as well as to present a comprehensive inventory of available kinetic models. In particular, the following main aspects will be discussed, according to recent literature indications: (i) competitive adsorption between HC and NH3; (ii) effect of the zeolite structure on HC deactivation; (iii) potential formation of surface intermediates and active site blocking; (iv) pore physical blocking due to large HC molecules or coke formation; (v) possible parasitic reactions between HCs and SCR reagents; vi) description of the available kinetic models able to account for these effects; and vii) design of improved catalysts with enhanced hydrocarbon poisoning resistance.