On the reaction mechanism for selective catalytic reduction of NOx by NH3 over Cu-zeolites
Doctoral thesis, 2019
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.
Chalmers, Physics, Chemical Physics
A comparative test of different density functionals for calculations of NH3-SCR over Cu-Chabazite
Physical Chemistry Chemical Physics,; Vol. 21(2019)p. 10923-10930
Interpretation of NH 3 -TPD Profiles from Cu-CHA Using First-Principles Calculations
Topics in Catalysis,; Vol. 62(2019)p. 93-99
Effect of Al-distribution on oxygen activation over Cu-CHA
Catalysis Science and Technology,; Vol. 8(2018)p. 2131-2136
Activation of oxygen on (NH3–Cu–NH3)+ in NH3-SCR over Cu-CHA
Journal of Catalysis,; Vol. 358(2018)p. 179-186
Monte Carlo Potential Energy Sampling for Molecular Entropy in Zeolites
Journal of Physical Chemistry C,; Vol. 122(2018)p. 20351-20357
Mechanism for Solid-State Ion-Exchange of Cu+ into Zeolites
Journal of Physical Chemistry C,; Vol. 120(2016)p. 29182-29189
L. Chen, T.V.W. Janssens, P.N.R. Vennestrom, J. Jansson, M. Skoglundh and H. Grönbeck; A complete multi-site reaction mechanism for low-temperature NH3-SCR over Cu-CHA
X.T. Wang, L. Chen, P.N.R. Vennestrom, T.V.W. Janssens, J. Jansson, H. Grönbeck and M. Skoglundh; Direct evidence of NH3-promoted O2 activation over Cu-CHA at low temperature
In order to remove NOx, a catalyst is needed. A catalyst speeds up a chemical reaction without being consumed such that it is available for multiple reaction cycles. Currently, NOx emitted in gasoline engine exhausts can be effectively converted to N2 and H2O through a series of reactions over a three-way catalyst. The function of the three-way catalyst is conditional on stoichiometric reaction conditions. However, for heavy duty diesel engines, the excess of oxygen prevents the use of the three-way catalyst technology. Instead, selective catalytic reduction (SCR) using NH3 as a reducing agent (NH3-SCR) is a promising approach for NOx abatement. Metal-exchanged zeolites, particularly Cu-exchanged small-pore zeolites, are presently the catalysts of choice for NH3-SCR, having a NOx conversion efficiency close to 100%. However, the catalysts are not perfect and need to be developed with respect to activity, selectivity and durability.
The developments of computational methods and computers have made it possible to simulate catalytic reactions at the atomic scale. In this kind of work, reaction paths can be explored, providing a detailed knowledge of kinetic bottlenecks.
In this thesis, a complete reaction mechanism for NH3-SCR over Cu-zeolites has been constructed from first principle calculations. The proposed reaction mechanism provides a solid basis for future investigations in the NH3-SCR area and development of catalysts with enhanced properties.
Areas of Advance
C3SE (Chalmers Centre for Computational Science and Engineering)
Atom and Molecular Physics and Optics
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4689
Chalmers University of Technology
PJ-salen, Fysikgården 2B, Chalmers
Opponent: Professor Joachim Sauer, Humboldt University of Berlin, Germany