An experimental and modeling framework for catalysis. NH3 adsorption studies for a vanadium-based SCR catalyst

Licentiate thesis, 2021

Internal combustion engines will have an important role towards sustainable transportation at least in the medium term. New emission legislation is aiming to a further decrease on pollutants and an increase of fuel efficiency. The latter could increase NOx emissions which will require more understanding of the features and reaction mechanisms for Selective catalytic reduction (SCR) catalysts used in the emission after-treatment system for NOx reduction.
In a vanadium-based SCR catalyst, NH3 adsorption is one of the main steps in the SCR reaction mechanism affecting catalyst performance under transient conditions depending on temperature, gas concentration and vanadium oxidation state. Thus, modelling of NH3 adsorption for industrial application is a challenge as it demands precision, accuracy and robustness required by the new legislation. Therefore, new experimental methods and modeling strategies are needed to capture valuable information and maximize data utilisation from experiments.
In this thesis, a new experimental and data pre-processing method was developed using a gas flow reactor for obtaining NH3 adsorption isotherms over a wide experimental region suitable for modeling purposes. Then, an NH3 adsorption model is proposed by a data-drive modeling process. First, a large set of candidate models was generated based on an array of feasible adsorption mechanisms. Then, parameter estimation was performed using different objective functions with increased complexity improving convergence. Finally, a robust model was achieved by a cross-validation and quality assessment step.
As a result, the best selected model for NH3 adsorption over a vanadium-based SCR catalyst involves five adsorption sites: one site with a simple NH3 adsorption mechanism, three sites with a competitive adsorption mechanism and one site with a water activated adsorption mechanism. The model can describe two phenomena impacting NH3 storage: surface water dynamics, adsorption or dissociation, and vanadium oxidation states. Moreover, model parameters show physical significance related to studies at molecular level. The proposed experimental and modeling method will prove useful in developing complex kinetic models with increased validity and extended application.