Structure-Performance Relationships in the Selective Catalytic Reduction of Nitric Oxide with Ammonia over Copper-Based Zeolites

Abstract

Combustion of diesel fuel in automotive engines leads to the formation of nitrogen oxides (NOx) that are harmful to the environment. The Selective Catalytic Reduction (SCR) is a promising process to minimize tailpipe NOx emissions and meet increasingly strict environmental regulations. During the SCR process, NOx is reduced with ammonia producing nitrogen gas and water. Cu-exchanged zeolites are potential SCR catalysts for practical use due to the wide operational temperature window. However, aging and degradation of the catalysts is inevitable in the harsh hydrothermal tailpipe environment. The aim of this thesis is to understand the detrimental effects of hydrothermal aging to the Cu-exchanged zeolites by means of ex situ and operando spectroscopies, therefore to obtain fundamental insights on the reaction mechanism and deactivation pathways. Obervations from novel microscopes including Scanning Transmission X-ray Microscopy (STXM) and Atom Probe Tomography (APT) indicated the heterogeneity of Cu and Al speciations in hydrothermally aged Cu-exchanged zeolites. Octahedral Al, three-fold coordiated Al, Cu+ and CuxOy coexisted, and a complex interplay between the local structure of zeolite framework and active Cu species within the catalysts was observed. Meanwhile, deactivated Cu species and destruction of zeolite framework were found preferentially located at the edge of a single aged catalyst particle at the nanoscale resolution. Operando UV-Vis Diffuse Reflectance Spectroscopy (DRS) and operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) allows to follow the dynamics of active structure and reaction intermediate under NH3-SCR conditions. The hydrothermal aging process was likely casue the aggregation of isolated Cu2+ into [Cux(OH)2x-1]+ oligomers/clusters, which leads to the loss of activity and the production of undesired byproducts. Pseudo-tetrahedral Cu2+ was formed during low-temperature NH3-SCR reaction condition on the steamed catalysts, however, this structure promoted the ammonia oxidation side reaction, which explains the unusal NO conversion profile with a ‘dip’ shape. The results highlight the importance of hydrothermally stable active sites ensuring high catalytic activities during the entire catalysts lifetime. The results described in this thesis are highly relevant for the development of new generations of NH3-SCR catalysts.