Selective catalytic reduction of NO with NH3 over natural zeolites and its application to stationary diesel engine exhaust

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Abstract

Cation exchanged natural zeolite catalysts for the reduction of NO with NH3 were developed. The synthetic mordenite containing Cu, V, and Cr cations showed high activity for the reduction of NO with NH3. Natural Izukalite containing mainly mordenite exhibits high activity for the selective reduction of NO when they were anchored with Cu, Mn, Co, and V cations. Furthermore, the anchored pellet catalysts prepared from Zeo 3S (clinoptilolite) and Izukalite exchanged with vanadium ions and mixed with TiO2 exhibited high activity for the reduction of NO with NH3. The honeycomb catalysts prepared from V-Izukalite were found to be especially effective for stationary diesel engine exhaust.

Introduction

NOx is the main source of photochemical smog and contributes devastatingly to acid rain. The reduction and/or elimination of NO emission can be considered one of the most important keys to solving environmental problems. The selective catalytic reduction (SCR) of NO has been generally used for stationary exhaust sources. The V2O5/TiO2 catalyst has been commercially used in the presence of NH3. The reaction mechanism has been studied by many researchers 1, 2, 3, 4, 5, 6, 7, 8, 9. More recently, other oxide catalysts 10, 11, 12and zeolites 13, 14, 15, 16, 17for NO removal by NH3 have been studied. A large number of zeolite based catalysts are also being developed 18, 19, 20, 21. Zeolites are able to contain low concentrations of various cations and have micropores for NH3 adsorption as the reductant. However, V2O5/TiO2 catalysts generally contain V2O5 of ca. 10 wt% This may cause environmental problems in catalyst disposal. In addition, synthetic zeolites may be expensive to use as SCR catalysts. In this work, natural zeolite catalysts exchanged with various cations including vanadium ions were developed for the selective reduction of NO. Furthermore, we have attempted the development of catalysts which contain low concentrations of vanadium oxide with a high activity for NO reduction in the presence of NH3 to be used for stationary diesel engine exhaust.

Section snippets

Zeolites and their cation exchange

Synthetic mordenite (MOR, JRC-Z-HM20) provided by the Catalysis Society of Japan was used. The first natural zeolite is Zeo 3S which mainly consists of clinoptilolite (HEU) and small amounts of crystallite and quartz (produced in Akita, Japan). The second is Izukalite which mainly consists of mordenite (MOR) and a small amount of quartz (produced in Shimane, Japan). The elemental composition of these zeolites are shown in Table 1, which were determined by ICP photoemission analysis.

The cation

NO conversion over cation exchanged mordenite (JRC-Z-HM20)

The catalytic reduction of NO in the presence of NH3 was carried out over synthetic mordenite exchanged with various cations. The catalysts used are shown in Table 1. The conversion of NO are found to be higher over Cu, Fe, Cr, V, Ni and Ce exchanged catalysts at space velocity (SV) 30 000 h−1 and at 350–500°C while the catalysts exchanged with H, Zn, Tl, Zr, Ti and Ag exhibited lower activities (Table 4). Fig. 2 shows the results at low SV 3000 h−1 over the catalysts which exhibit high

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