**1. Introduction**

178 Electrochemical Cells – New Advances in Fundamental Researches and Applications

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> Striking progress has recently been made in understanding the central role of nitrogen oxide radicals, NOx, in atmospheric processes (Lerdau et al., 2000). NOx is implicated in the formation of acid rain and a tropospheric ozone (the principal toxic component of smog and a greenhouse gas) (Finlayson-Pitts, B.J. & Pitts, J.N., 1997; Lerdau et al., 2000). The major known source of NOx is fuel combustion and biomass burning. Air pollution by nitrogen oxides (NO*x*) in combustion waste causes serious environmental problems in urban areas. The reduction of nitrogen oxide emissions has become one of the greatest challenges in environment protection (Libby, 1971; Nishihata et al., 2002). This is why the different methods of NOx decomposition are intensely studied by numerous groups from academic as well as industrial research laboratories (Garin, 2001; Parvulescu et al., 1998).

> The main activities of scientific groups working in the field of NO decomposition are concentrated on the reduction of the NOx in the presence of NH3, CO, H2 or hydrocarbons. These scientific groups have been tested a large number of categories of catalysts with a different ways of NO decomposition reactions. The main directions of the research can be described as follows.

> **First,** the selective catalytic reduction of NO with ammonia, typical for chemical industrial plants and stationary power stations (Bosch & Janssen, 1988; Janssen & Meijer, 1993). The main step is the reduction on NO or NO2 to N2 and H2O. Generally, liquid ammonia is injected in the residual gas before the catalytic reaction takes place.

 4NO+4NH3+O2 4N2+6H2O 6NO+4NH3 5N2+6H2O

**Second,** the catalytic reduction of NO in the presence of CO and/or hydrogen. These reactions are typical for the automotive pollution control. The use of CO or H2 for catalytic reduction was one of the first possibilities investigated in view of eliminating NO from automotive exhaust gas (Baker & Doerr, 1964, 1965; Klimisch & Barnes, 1972; Nishihata et al., 2002; Roth & Doerr, 1961).

Electrochemical Cells with Multilayer Functional Electrodes for NO Decomposition 181

As a result, the additional ionic current though the cell associated with the oxygen ions produced due to this unwanted reaction (Eq.(3)) far exceeds the current associated with the desired reaction (Eq.(2)). In 1997 *Hibino et al.* (Hibino et al., 1997) has shown that at first stage the electrochemical oxygen pumping is carried out without NO decomposition, and that NO decomposition began at corresponding currents after the electrochemical oxygen pump is complete. As illustration Fig.2 shows the dependence of NO conversion on the value of the current passing through the two chambers cell at 1000ppm of NO without oxygen (Curve 1) and at 2% of Oxygen (Curve 2) in He (the balance) at gas flow rate 50ml/min. It is seen that in the presence of oxygen the decomposition of NO take place only

Recently, many attempts to improve the properties of electrochemical cells operating in the presence of excess oxygen have been carried out by using different catalysts as the cathode material (Hibino, 2000a, 2000b; Marwood & Vayenas, 1997; Nakatani et al., 1996; Walsh & Fedkiw, 1997). *Walsh* (Walsh & Fedkiw, 1997) proposed substitute dense Pt electrodes to the porous platinum and to use a mixture of ionic (CeO) and electronic (Pt) conductors as a porous cathode. It is well known that substitution of dense electrode to the porous should increase gas penetration to the *tpb* on the surface of the YSZ-disc solid electrolyte and using of the mixture of ionic (CeO) and electronic (Pt) conductors should lead to the increase of the *tpb* surface area inside the cathode. As the result both oxygen and nitrogen oxide decomposition take place in such cells and for effective NO adsorption and decomposition the *tpb* should be free from the adsorbed oxygen. This conclusion agrees well with a fact that

when all oxygen should be pumped away from the near electrode area.

the NO decomposes after the oxygen pumping is completed.

Fig. 1. Conceptual representation of the electrochemical cell for NO decomposition.

To improve the selectivity for NO gas adsorption and decomposition in the presence of the oxygen excess *K. Iwayama* (Iwayama & Wang, 1998; Washman et al., 2000) proposed to coat Pt cathode by different metals or metal oxide. Decomposition activity was measured on metal oxide/Pd(cathode)/YSZ/Pd(anode) at 773–973 K and 3.0V of applied voltage in a flow of 50 ml/min containing 1000 ppm of NO and 6% of O2 in helium. Coating of various metal oxides onto the cathode electrode greatly changed the decomposition activity; the order was RuO2>>Pt>Rh2O3>Ni>none>Ag>WO3. The activity of the system modified by RuO2 has been investigated as a function of the kind of electrode, the applied voltage, and

$$\text{NO} \star \text{CO} \xrightarrow[]{} \text{NO} \text{-} \begin{array}{c} \text{NO} \text{-} \text{1}/2 \text{ N}\_2 \text{O} \end{array}$$

$$\text{NO} \text{+} \text{H}\_2 \xrightarrow[]{} \text{1}/2 \text{ N}\_2 \text{ +} \text{H}\_2\text{O}$$

**Third,** the selective catalytic reduction of NO in the presence of hydrocarbons and more particularly methane, a method which has not yet reached industrial use but can be applied both for automotive pollution control and in various industrial plants (Armor, 1995; Hamada et al., 1991; Iwamoto, 1990; Libby, 1971; Miura et al., 2001; Sato et al., 1992).

**Fourth,** the direct decomposition of NO. The decomposition of NO would represent the most attractive solution in emission control, because the reaction does not require that any reactant be added to NO exhaust gas and could potentially lead to the formation of only N2 and O2 (Garin, 2001; Lindsay et al., 1998; Miura et al., 2001; Rickardsson et al., 1998).

The goal of this paper is to represent a **fifth** direction of an intense research effort focused on electrochemical cells for the reduction of NOx gases due to the need to design an effective method for the purification of the exhaust gases from lean burn and diesel engines.
