On the reaction mechanism for selective catalytic reduction of NOx by NH3 over Cu-zeolites

Doctoral thesis, 2019

Nitrogen oxides (NOx) are major pollutants from combustion processes, being corrosive and hazardous to human health. The main technology for exhaust aftertreatment of NOx emitted from diesel engines is selective catalytic reduction with ammonia as reducing agent (NH3-SCR). Among a range of catalysts for NH3-SCR, copper-exchanged zeolites are efficient with high activity and selectivity combined with good hydrothermal stability. Zeolites are crystalline microporous aluminosilicates constructed by corner-sharing SiO4 and AlO4 tetrahedra. Replacement of a four-valent Si by a three-valent Al gives the framework a negative charge, which is compensated by a cation. The cation in the case of copper exchanged zeolites is Cu(I) or Cu(II).

In this thesis, the reaction mechanism for NH3-SCR over copper-exchanged zeolites with CHA framework (Cu-CHA) has been studied through density functional theory in combination with ab initio thermodynamics and molecular dynamics. Firstly, the character of the active site for NH3-SCR over Cu-CHA under typical reaction conditions has been investigated. It is found that the Cu(I)-ion is preferably solvated by two NH3 ligands forming a linear Cu(NH3)2+ complex under low-temperature operating conditions. The storage of NH3 in the Cu(NH3)2+ complex is consistent with measured features from NH$_3$ temperature-programmed desorption. Moreover, the linear Cu(NH3)2+ complex is found to be important for solid-state ion exchange of Cu(I) into zeolites, which is one strategy for zeolite functionalization.

Secondly, a complete reaction mechanism for low-temperature NH3-SCR over Cu-CHA has been explored. The reaction is found to proceed in a redox manner via alternating Cu(I) and Cu(II) oxidation states. A pair of Cu(NH3)2+ complexes is found to be required for O2 activation in similarity to O2 activation in homogeneous catalysis. The potential energy surface for O2 dissociation is found to depend strongly on the choice of the exchange-correlation functional. The PBE+U approach together with van der Waals corrections is found to provide a reasonable, simultaneous accuracy of the different bonds in the system. Based on the fact that Cu(I) is solvated and the need of complex pairs for O2 activation, two possible reaction cycles for low-temperature NH3-SCR are proposed. The reaction is suggested to proceed in a multi-site fashion over both copper-sites and Bronsted acid sites. 

The proposed mechanism highlights the similarities between low-temperature NH3-SCR over Cu-CHA and homogeneous liquid-phase catalytic reactions and provides a solid basis for future improvements of Cu-exchanged zeolites for NH3-SCR.