Modelling the ammonia adsorption–desorption process over an Fe–zeolite catalyst for SCR automotive applications

The accurate description of NH3 adsorption/desorption from the catalyst surface is the basis for the correct description of the NH3–SCR catalytic chemistry. Currently, Temkin-type kinetics are widely and successfully used to describe the NH3 adsorption/desorption process over V-based and metal-promoted zeolite catalysts in the 200–600 °C T-window: in this approach the heterogeneity of the catalytic surface is taken into account considering the activation energy of the desorption process as a linear function of the adsorbed species surface coverage. In the present work, carried out over a commercial Fe–zeolite SCR catalytic system, we show that this modelling approach fails when extended to lower temperatures (T ≤ 150 °C) and in the presence of water, conditions of growing interest for Diesel exhaust after-treatment systems. Based on literature evidence, an improved, physically consistent dual-site modelling approach is implemented, resulting in an accurate description of the ammonia adsorption/desorption process in the extended 50–550 °C temperature window.

Figure optionsDownload high-quality image (72 K)Download as PowerPoint slideHighlights
► Modelling of NH3 adsorption/desorption from Fe–zeolites in the 50–550 °C T-range.
► Inaccurate low-T description with single site model + Temkin-like desorption kinetics.
► Physically consistent dual-site model for NH3 adsorption/desorption developed.
► Temkin-like desorption kinetics for weak adsorption sites.
► Arrhenius desorption kinetics for strong adsorption sites.