Review
Direct decomposition of NO by microwave heating over Fe/NaZSM-5

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Abstract

Catalytic decomposition of NO was studied over Fe/NaZSM-5 catalyst. Novel results were observed with the microwave heating mode. The conversion of NO to N2 increased remarkably with the increasing of Fe loading. The effects of a series of reaction parameters, including reaction temperature, O2 concentration, NO concentration, gas flow rate and H2O addition, on the productivity of N2 have been investigated. It is shown that the catalyst exhibited good endurance to excess O2 in the microwave heating mode. Under all reaction conditions, NO converted predominantly to N2. The highest conversion of NO to N2 was up to 70%.

Introduction

NO is one of air main pollutants since it causes smog and acid rain. Although considerable progress has been achieved in the reduction of NO emission, the release of NO from burning fossil-fuel is still a major environmental problem [1], [2], [3], [4], [5]. Especially, under a high air-fuel ratio condition in which the fuel efficiency is higher, the engines emit comparatively higher levels of NO due to the higher combustion temperature [6]. For the removal of NO emitted from fossil-fuel burning vehicles, there are two commonly used techniques, namely modification of engines and treatment of exhaust gases. However, techniques for diesel engine modification have reached their technology limit for reducing NO emission levels in recent years. Therefore, it is recognized that engine modification is insufficient to meet recent regulations for NO emission levels. Consequently, it is necessary to develop new exhaust gas treatment process for NO removal [7].

In principle, direct decomposition of NO is the most attractive way to reduce NO pollution [7], [8]. For the decomposition of NO, it has been indicated that Cu-ZSM-5 is the most active catalyst [8], [9], [10]. Unfortunately, over-exchanged capacity of Cu-ZSM-5 is active only in a limited interval of temperatures and in the absence of O2 [11]. In addition, other molecules like H2O or SO2 act as inhibitors or cause rapid deactivation of the catalyst [12].

It has been proved that microwave irradiation promotes many chemical reactions [13], [14], [15], [16]. Since the quanta of microwave energy is far insufficient for breaking directly the chemical bonds of molecule [17], microwave heating (MH) action considered to be the most probable way in chemical reactions is the activation of solid catalysts. The approach to microwave catalysis is based the idea of selective heating of an active metal supported on a porous dielectric matrix. A choice of the catalyst for a MH process appears to be of great importance. Evidently, such a catalyst has to be a good receptor of microwave energy, and yet not to change its structure and properties under intense microwave radiation.

In the present work, Fe/NaZSM-5 has been selected as a catalyst for exploring NO decomposition. Its activity was observed in MH and conventional reaction (CR). Furthermore, we have investigated the conversion of NO with varying the Fe loading. The effects of O2 concentration, NO concentration, flow rate of the gas and H2O addition on the productivity of N2 were also investigated.

Section snippets

Experimental apparatus

The experimental diagram is presented in Fig. 1. The MH reaction system consisted mainly of a microwave generator, a rectangular waveguide, a resonant cavity and a tuning plunger. The microwave energy was supplied by a 2.45 GHz microwave generator the power of which could be varied continuously in the range of 0–200 W. For these experiments, a quartz reactor of i.d. 8 mm was designed to carry out the experiments. The reactor was aligned vertically at the center of the single mode resonant cavity,

Results and discussion

A choice of the catalyst for a MH process is of great importance, as it must meet some specific requirements in addition to the common demands of high activity, selectivity, stability, etc. Evidently, such a catalyst should be a good absorber of microwave energy and not to alter its structure and properties under intense microwave radiation. So far, a synthesis of catalysts for microwave chemistry is driven by intuition rather than by exact knowledge. First, the conversions of NO to N2 were

Conclusions

The direct decomposition of NO over the Fe/NaZSM-5 catalyst was investigated in MH and CR modes. The results show that the process of MH is effective for an environmentally friendly catalytic conversion of NO. NO is mainly converted to N2 and the maximum conversion can be up to 70%. The catalyst has a good O2 endurance and stable lifetime in MH. In addition, the addition of H2O affects obviously the conversion of NO to N2 in MH.

Acknowledgements

Ms. Lixin Pang is acknowledged for helpful discussions. We also thank professor Changhai Xu for his contributions to this work.

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