First-Principles Microkinetic Model for Low-Temperature NH3-Assisted Selective Catalytic Reduction of NO over Cu-CHA

Yingxin Feng,Peter N R Vennestrøm,Magnus Skoglundh,Jonas Jansson,Xueting Wang,Ton V W Janssens,Henrik Grönbeck

doi:10.1021/acscatal.1c03973

Yingxin Feng, Peter N R Vennestrøm + Show 5 more

Open Access

https://doi.org/10.1021/acscatal.1c03973

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Journal: ACS Catalysis	Publication Date: Nov 15, 2021
Citations: 28	License type: CC BY 4.0

Affiliation: Chalmers University of Technology, Volvo (Sweden)

Abstract

A first-principles microkinetic model is developed to investigate the low-temperature ammonia-assisted selective catalytic reduction (NH3-SCR) of NO over Cu-chabazite (Cu-CHA). The reaction proceeds over NH3-solvated Cu sites by the formation of H2NNO and HONO, which decompose to N2 and H2O over Brønsted acid sites. Nonselective N2O formation is considered by H2NNO decomposition over the Cu sites. The adsorption of NH3 at oxidized Cu sites is found to inhibit the reaction at low temperatures by hindering NO adsorption. For the reactions, we find positive reaction orders with respect to NO and O2, whereas the reaction order with respect to NH3 is negative. The reaction orders and the obtained apparent activation energy are in good agreement with experimental data. A degree of rate control analysis shows that NH3-SCR over a pair of Cu(NH3)2+ is mainly controlled by NO adsorption below 200 °C, whereas the formation of HONO and H2NNO becomes controlling at higher temperatures. The successful formulation of a first-principles microkinetic model for NH3-SCR rationalizes previous phenomenological models and links the kinetic behavior with materials properties, which results in unprecedented insights into the function of Cu-CHA catalysts for NH3-SCR.

Highlights

Combustion in oxygen excess is energy-efficient but requires catalytic aftertreatment systems to control NOx emissions.[1]
We have developed a first-principles microkinetic model for lowtemperature NH3-selective catalytic reduction (SCR) and N2O formation over Cu-CHA catalysts, based on density functional theory (DFT) calculations and detailed entropy analysis
N2O formation is included via H2NNO decomposition over Cu sites

Summary

Introduction

Combustion in oxygen excess is energy-efficient but requires catalytic aftertreatment systems to control NOx emissions.[1] The current technology for lean-burn NOx reduction is selective catalytic reduction (SCR) with ammonia as the reducing agent (NH3-SCR), which reduces NOx emissions efficiently.[1] Catalysts based on Cu-chabazite (Cu-CHA) are common caantdalygsotsodforhyNdHro3t-hSeCrmR atlhasntakbsiltioty.t1h−e3 low-temperature activity The overall NH3-SCR reaction with only NO (standard SCR) is (R1). This is a redox reaction that requires catalytic sites that can change oxidation state during the reaction, which for Cu means an alternation between CuI and CuII. The reaction order of NH3 depends on the NH3-to-NOx ratio and has recently been measured to be positive at low (0.1−0.5) ratios.[8]

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