Abstract

Proton transport is an elementary process in the selective catalytic reduction of nitrogen oxides by ammonia (DeNOx by NH3-SCR) using metal-exchanged zeolites as catalysts. This review summarizes recent advancements in the study of proton transport in zeolite catalysts using in situ electrical impedance spectroscopy (IS) under NH3-SCR reaction conditions. Different factors, such as the metal cation type, metal exchange level, zeolite framework type, or formation of intermediates, were found to influence the proton transport properties of zeolite NH3-SCR catalysts. A combination of IS with diffuse reflection infrared Fourier transformation spectroscopy in situ (in situ IS-DRIFTS) allowed to achieve a molecular understanding of the proton transport processes. Several mechanistic aspects, such as the NH3-zeolite interaction, NO-zeolite interaction in the presence of adsorbed NH3, or formation of NH4+ intermediates, have been revealed. These achievements indicate that IS-based in situ methods as complementary tools for conventional techniques (e.g., in situ X-ray absorption spectroscopy) are able to provide new perspectives for the understanding of NH3-SCR on zeolite catalysts.

Highlights

  • Selective catalytic reduction (SCR) is one of the key technologies to reduce nitrogen oxide emissions (NOx ) from “lean-burn” engines and power plants [1,2,3]

  • A further combination of impedance spectroscopy (IS) and diffuse reflection infrared Fourier transformation spectroscopy allowed us to achieve a molecular understanding of the proton transport processes and their impact in NH3 -SCR catalysis [17,27,28,29]

  • Simultaneous IS and DRIFTS measurements were carried out using the same catalyst film, allowing simultaneous monitoring of both the proton conductivity of zeolite catalysts and the vibration modes of the molecules on zeolite catalysts [26,28]

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Summary

Introduction

Selective catalytic reduction (SCR) is one of the key technologies to reduce nitrogen oxide emissions (NOx ) from “lean-burn” engines and power plants [1,2,3]. Because of their superior activity and hydrothermal stability, Cu- or Fe-exchanged zeolites are widely applied as SCR catalysts, especially in diesel-powered automobiles [3,4]. A further combination of IS and diffuse reflection infrared Fourier transformation spectroscopy (in situ IS-DRIFTS) allowed us to achieve a molecular understanding of the proton transport processes and their impact in NH3 -SCR catalysis [17,27,28,29]. The future perspectives, which arise from the achieved understanding, will be discussed at the end (Section 4)

Theory of Impedance Spectroscopy
NH3 -Supported Proton Transport
Schematic illustration the mechanisms mechanisms proton transport occurring
Factors thethe
Metal Cation Type
NH3‐zeolite
Intermediates wtzeolite
H2O Vapor
O vapor only
Zeolite Crystallite
Summary
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