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

O2-activation

Al-distribution

DFT

NH3-SCR

Cu-CHA

Cu-SSZ-13

PJ-salen, Fysikgården 2B, Chalmers
Opponent: Professor Joachim Sauer, Humboldt University of Berlin, Germany

Författare

Lin Chen

Chalmers, Fysik, Kemisk fysik

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Catalysis Science and Technology,; Vol. 8(2018)p. 2131-2136

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Activation of oxygen on (NH3–Cu–NH3)+ in NH3-SCR over Cu-CHA

Journal of Catalysis,; Vol. 358(2018)p. 179-186

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Mechanism for Solid-State Ion-Exchange of Cu+ into Zeolites

Journal of Physical Chemistry C,; Vol. 120(2016)p. 29182-29189

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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

Have you ever realized that hazardous nitrogen oxides (NOX) are emitted when you are driving your car? NOx is primarily produced during combustion from reaction between nitrogen and oxygen at high temperatures. NOx can interact with water, oxygen and other compounds in the atmosphere to form acid rain. The nitrate species that result from NOx can also cause haze. As the legislations on NOx emission for the diesel engines are getting stricter, there is an urgent need to develop more efficient aftertreatment systems to reduce NOx emissions. 

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.

Drivkrafter

Hållbar utveckling

Styrkeområden

Energi

Materialvetenskap

Fundament

Grundläggande vetenskaper

Infrastruktur

C3SE (Chalmers Centre for Computational Science and Engineering)

Ämneskategorier

Atom- och molekylfysik och optik

Teoretisk kemi

ISBN

978-91-7905-222-5

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4689

Utgivare

Chalmers tekniska högskola

PJ-salen, Fysikgården 2B, Chalmers

Opponent: Professor Joachim Sauer, Humboldt University of Berlin, Germany

Mer information

Senast uppdaterat

2019-11-13