Multi-scale modeling approach to the nature of active sites for a vanadium-based SCR catalyst: NH3 Adsorption
Andres Felipe Suarez Corredor, Scania CV AB / Chalmers University of Technology
A. Suarez[1,2], L. Olsson, M. Babler, M. Skoglundh, B. Westerberg
Scania CV AB, Södertälje, Sweden.
Chalmers University of Technology, Gothenburg, Sweden.
KTH Royal Institute of Technology, Stockholm, Sweden.
Selective Catalytic Reduction (SCR) catalysts are key components in modern after-treatment systems for emission control of internal combustion engines. In a vanadium-based SCR (V-SCR) catalyst, NH3 adsorption affects the catalyst performance under transient conditions depending on temperature, gas concentration and vanadium oxidation state [1, 2]. The adsorption mechanism for NH3 on this dynamic surface is not well understood and calls for further research. Thus, the modelling of these phenomena for industrial applications is a challenge as demands on precision, accuracy and robustness are higher in a context where legislation is moving towards tighter limits and stricter protocols, encouraging vehicle manufacturers for innovation and research to control, design and optimize after-treatment systems [2,3].
In this work, NH3 adsorption and oxidation states of a vanadium-based SCR catalyst are studied experimentally and described by a mechanistic model which relates the microscopic structure behavior of the catalyst with macro-scale measurements. Experimental data were obtained from steady-state measurements in a synthetic gas test bench, where a monolith sample, (V2O5/WO3 on TiO2) was subjected to controlled temperatures and gas compositions considering different washcoat loadings and vanadium oxidation states. Several adsorption models were evaluated in a modeling framework, where computational strategies such as parallelization, clustering and pattern recognition were applied.
From the experimental results, NH3 adsorption occurs on different sites, with different adsorption energies and molecular configurations (Fig. 1). Therefore, a model considering three sites for NH3 adsorption is proposed with adequate performance over a range of operating conditions relevant for industrial applications. The model can describe two phenomena impacting NH3 storage: surface water dynamics: adsorption or dissociation, and vanadium oxidation states.
Water dynamics over the catalyst surface can be observed through the NH3 capacity enhancement in the oxidized catalyst, due to the formation of new Brønsted sites. Alternatively, on the reduced catalyst, water reduces the NH3 storage capacity due to competitive adsorption over the sites (Fig.2). Moreover, an equilibrium between Brønsted and Lewis acid sites was found, being affected by temperature and water concentration. The proposed model can be improved by further mechanism integration with different adsorption sites based on the transformations that vanadium adsorption sites can undergo due to water dynamics, hydrogen mobility, support-site interactions and oxidation states.
Fig. 1. Vanadium sites over a TiO2 support.
Fig. 2. Water effect on NH3 adsorption for the oxidized and reduced catalyst.
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