Elsevier

Catalysis Today

Volume 151, Issues 3–4, 19 June 2010, Pages 212-222
Catalysis Today

Overview of the practically important behaviors of zeolite-based urea-SCR catalysts, using compact experimental protocol

https://doi.org/10.1016/j.cattod.2010.03.055Get rights and content

Abstract

Selective catalytic reduction with NH3 (NH3-SCR technology), based on V2O5/WO3/TiO2 catalysts, has been previously commercialized for abating NOx emissions from various stationary and mobile lean-burn or diesel engines. However, meeting the uniquely stringent US EPA 2010 regulations for diesel engines required introduction of a new class of SCR catalysts, based on Cu- or Fe-exchanged zeolites. While remarkably active and stable, these new materials proved substantially more difficult than vanadia-based catalysts to operate transiently on the road, due to their much higher NH3 storage. The objective of this work was to develop a concise experimental protocol, elucidating multiple catalytic functions, steady-state and transient, of practical relevance to the mobile SCR applications. This paper provides a comprehensive overview of such functions, using select data from various representative Cu- and Fe-zeolite catalysts. While the bulk of the reported results originated directly from the developed protocol, additional experiments, validating the assumptions or clarifying unexpected experimental observations, are included.

Introduction

Despite rapid developments in the areas of alternative energy, diesel engines remain a preferred source of propulsion, especially in heavy-duty transportation. This is due to their superior fuel efficiency compared to conventional gasoline engines, high power density, and a number of other characteristics beneficial to the customer, such as high torque at low speed. However, their broader market penetration in the US has been hindered by the increasingly stringent environmental regulations, especially for the oxides of nitrogen, NO and NO2, collectively referred to as NOx. Selective catalytic reduction with NH3, based on V2O5/WO3/TiO2 catalysts, has been used for several decades to reduce NOx emissions under net oxidizing, fuel-lean conditions [1]. The same class of materials was later applied to fulfill the requirements of Euro-IV regulations for diesel-powered vehicles, using urea decomposition and hydrolysis as a source of NH3 [2]. The more stringent US EPA 2010 on-road emission regulations, which forced catalyzed, actively regenerated diesel particulate filters (DPF) to be used along with the NOx reduction catalysts, rendered this class of catalysts impractical because they could not withstand high temperatures, in excess of 550–600 °C, associated with active DPF regeneration [3]. In response to this challenge, a number of new materials were developed in the recent years, based on mixed metal oxides, as well as Fe- and Cu-exchanged zeolites [4], [5], [6], [7], [8], [9]. This latter class of catalysts possesses outstanding hydrothermal stability, as illustrated in Fig. 1.

Notwithstanding their advantageous characteristics, these zeolite-based catalysts represent both substantial new challenges and opportunities for practical application due to their much higher NH3 storage compared to V2O5-based materials, as illustrated in Fig. 2. Depending on the conditions, the new zeolite catalysts may take over an hour to attain equilibrium ammonia coverage and reach steady-state performance. Thus, in real-world driving with rapidly changing temperature, flow and gas composition in the exhaust, zeolite-based SCR catalysts are operating far from steady-state most of the time. This limits the usefulness of the steady-state conversion maps such as those reported in Fig. 1. Furthermore, acting as NH3 capacitors, the new materials drastically complicate urea dosing strategy, because at a given set of engine exhaust conditions, they behave differently depending on their ammonia coverage. Therefore, successful application of the new, zeolite-based SCR catalysts, demands detailed understanding of their performance over a broad range of conditions, including transients. In this work, an experimental protocol is defined, which was optimized to yield such information that elucidates both steady-state and transient characteristics critical to practical application.

Section snippets

Bench-reactor systems

A simplified schematic of the bench-reactor system, used to collect the bulk of the data in this study, is shown in Fig. 3. The system was designed to provide rapid response to gas composition transients. To that end, the entire gas stream exiting the catalyst (typically 26 slpm) is fed directly into a fast-response model of the MKS MultiGas 2030 FTIR analyzer. Furthermore, the system can create well-defined steps in the ammonia concentration using a 4-way valve arrangement as shown in Fig. 3. A

Experimental results

The following section contains a step-by-step review of the functions probed by the described protocol.

Summary

The information obtained through the developed protocol can guide the design and development of SCR system as well as optimization of the urea dosing control through function-specific process understanding.

At the stage of the catalyst system design, the choice of the catalyst formulation and catalytic device sizing are both highly dependent on matching catalyst properties to the application. For example, applications with very pronounced temperature transients in the low-temperature range, such

Acknowledgements

The authors express sincere gratitude to Dr. Haiying Chen, Dr. Joseph Fedeyko, and Dr. Mario Castagnola of Johnson Matthey, for providing catalyst samples and, in particular, for very valuable discussions.

We would also like to thank Mr. Randall P. Jines and Mr. Jason L. Ferguson for their help with collecting experimental data.

References (25)

  • P. Forzatti

    Appl. Catal. A

    (2001)
  • M. Schwidder et al.

    J. Catal.

    (2008)
  • H.Y. Chen et al.

    Catal. Today

    (1998)
  • P. Balle et al.

    Appl. Catal. B

    (2009)
  • W.P. Partridge et al.

    Appl. Catal. B

    (2009)
  • A. Grossale et al.

    J. Catal.

    (2008)
  • B.R. Wood et al.

    J. Catal.

    (2004)
  • S.S. Mulla et al.

    J. Catal.

    (2006)
  • M. Iwasaki et al.

    J. Catal.

    (2008)
  • L. Olsson et al.

    Appl. Catal. B

    (2009)
  • M. Iwasaki et al.

    Appl. Catal. A

    (2009)
  • L. Olsson et al.

    Appl. Catal. B

    (2008)
  • Cited by (0)

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