Reaction kinetics of NH3-SCR over Cu-CHA from first principles
Doctoral thesis, 2024
Ammonia-assisted selective catalytic reduction (NH3-SCR) is an advanced technology to reduce nitrogen oxide (NOx) emissions from lean-burn engines. NH3 is added to the exhaust gas and reacts selectively with NOx, forming N2 and H2O. Small-pore Cu-exchanged chabazite (Cu-CHA) is a widely used zeolite-based catalyst for NH3-SCR thanks to its high activity, high selectivity, and durability. However, one issue is the formation of small amounts of nitrous oxide (N2O), which is a strong greenhouse gas. Atomic level understanding is valuable to improve the performance of NH3-SCR catalyst and meet the increasingly stringent emission standards.
The state of Cu-CHA during NH3-SCR reaction depends sensitively on the reaction temperature. At low temperatures, Cu-ions are solvated by NH3, forming Cu[(NH3)2]+ complexes, while framework-bound Cu dominates at high temperatures. Experimentally, a non-monotonic behavior in NO conversion is observed with increasing temperature, which reflects a change in the active site.
In this thesis, NH3-SCR over Cu-CHA is investigated using density functional theory (DFT) calculations and kinetic simulations. In the low-temperature range (<250℃), a pair of Cu[(NH3)2]+ is required for O2 adsorption to form a Cu-peroxo complex, which is a key Cu-intermediate. The reaction intermediates HONO and H2NNO can be converted to N2 and H2O over Brønsted acid sites. H2NNO is found to be the origin of N2O formation when decomposing over Cu-peroxo species. At high-temperature (>350℃), the proposed reaction mechanism suggests that the reaction proceeds over single framework Cu sites and starts from the co-adsorption of O2 and NO. The main source of N2O formation at high temperatures is suggested to be ammonium nitrate decomposition. By combining the reaction mechanisms for high and low temperatures, the non-monotonic temperature-dependent reaction activity is successfully reproduced.
The present work deepens the understanding of the reaction mechanism of NH3-SCR on Cu-CHA in a wide temperature range and provides theoretical support for further improvement of the catalyst performance.
The state of Cu-CHA during NH3-SCR reaction depends sensitively on the reaction temperature. At low temperatures, Cu-ions are solvated by NH3, forming Cu[(NH3)2]+ complexes, while framework-bound Cu dominates at high temperatures. Experimentally, a non-monotonic behavior in NO conversion is observed with increasing temperature, which reflects a change in the active site.
In this thesis, NH3-SCR over Cu-CHA is investigated using density functional theory (DFT) calculations and kinetic simulations. In the low-temperature range (<250℃), a pair of Cu[(NH3)2]+ is required for O2 adsorption to form a Cu-peroxo complex, which is a key Cu-intermediate. The reaction intermediates HONO and H2NNO can be converted to N2 and H2O over Brønsted acid sites. H2NNO is found to be the origin of N2O formation when decomposing over Cu-peroxo species. At high-temperature (>350℃), the proposed reaction mechanism suggests that the reaction proceeds over single framework Cu sites and starts from the co-adsorption of O2 and NO. The main source of N2O formation at high temperatures is suggested to be ammonium nitrate decomposition. By combining the reaction mechanisms for high and low temperatures, the non-monotonic temperature-dependent reaction activity is successfully reproduced.
The present work deepens the understanding of the reaction mechanism of NH3-SCR on Cu-CHA in a wide temperature range and provides theoretical support for further improvement of the catalyst performance.
Catalysis
microkinetic modeling
density functional theory
Cu-CHA
ammonia assisted selective catalytic reduction
Kollektorn, MC2
Opponent: Professor Matteo Maestri, Department of Energy, Politecnico di Milano, Italy
Author
Yingxin Feng
Chalmers, Physics, Chemical Physics
The Role of H+- and Cu+-Sites for N2O Formation during NH3-SCR over Cu-CHA
Journal of Physical Chemistry C,;Vol. 125(2021)p. 4595-4601
Journal article
A First-Principles Microkinetic Model for Low-Temperature NH3 Assisted Selective Catalytic Reduction of NO over Cu-CHA
ACS Catalysis,;Vol. 11(2021)p. 14395-14407
Journal article
Simplified Kinetic Model for NH3‑SCR Over Cu‑CHA Based on First‑Principles Calculations
Topics in Catalysis,;Vol. 66(2023)p. 743-749
Journal article
Yingxin Feng, Ton V. W. Janssens, Peter N. R. Vennestrøm, Jonas Jansson, Magnus Skoglundh, and Henrik Grönbeck. igh-Temperature Reaction Mechanism of NH3-SCR over Cu-CHA: One or Two Copper Ions?
Yingxin Feng, Henrik Grönbeck. Kinetic Monte Carlo simulations elucidate the effect of Al-distribution for low-temperature NH3-SCR over Cu-CHA
Carl Frostenson, Yingxin Feng, Per Hyldgaard and Henrik Grönbeck. Range-Separated Hybrid van der Waals Density Functional to Describe Cu2O2-complexes
In situ DRIFT studies on N2O formation over Cu-functionalized zeolites during ammonia-SCR
Catalysis Science and Technology,;Vol. In Press(2022)
Journal article
Up to the present, the combustion of fossil fuels is one of the most important sources of energy. To improve fuel efficiency, lean-burn engines have been invented and are widely used. However, while high oxygen ratios enhance fuel efficiency, it also promotes the reaction between nitrogen and oxygen forming nitrogen oxides (NOx). Control of NOx emissions is needed as NOx contributes to the formation of smog and acid rain, which are harmful to human health and the environment.
Ammonia assisted selective catalytic reduction (NH3-SCR) is the lead technology for NOx abatement by reducing NOx into harmless nitrogen. Catalyst is crucial for the reaction to proceed with high selectivity. Currently, Cu-exchanged small-pore zeolite, mainly Cu-CHA, is extensively applied thanks to their outstanding performance. However, a small amount of N2O, which is a strong greenhouse gas, is produced as a by-product of the NH3-SCR process.
New legislation standards set stringent requirements for N2O emission, which brings a new challenge for catalyst development. To find approaches to fulfill these requirements, there is an urgent need to explore the reaction mechanisms, especially the generation of N2O during the NH3-SCR process.
In this thesis, reaction mechanisms of NH3-SCR and the link between material structure and catalytic performance have been investigated. This work provides a theoretical basis for the rational design of catalysts and helps to accelerate the improvement of catalysts.
Ammonia assisted selective catalytic reduction (NH3-SCR) is the lead technology for NOx abatement by reducing NOx into harmless nitrogen. Catalyst is crucial for the reaction to proceed with high selectivity. Currently, Cu-exchanged small-pore zeolite, mainly Cu-CHA, is extensively applied thanks to their outstanding performance. However, a small amount of N2O, which is a strong greenhouse gas, is produced as a by-product of the NH3-SCR process.
New legislation standards set stringent requirements for N2O emission, which brings a new challenge for catalyst development. To find approaches to fulfill these requirements, there is an urgent need to explore the reaction mechanisms, especially the generation of N2O during the NH3-SCR process.
In this thesis, reaction mechanisms of NH3-SCR and the link between material structure and catalytic performance have been investigated. This work provides a theoretical basis for the rational design of catalysts and helps to accelerate the improvement of catalysts.
Reducing N2O emissions during NH3-SCR over Cu-exchanged zeolites
Swedish Energy Agency (2018-008019), 2019-01-01 -- 2022-12-31.
Subject Categories
Inorganic Chemistry
Materials Chemistry
Theoretical Chemistry
Organic Chemistry
ISBN
978-91-7905-970-5
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5436
Publisher
Chalmers
Kollektorn, MC2
Opponent: Professor Matteo Maestri, Department of Energy, Politecnico di Milano, Italy