Chapter 5 A there-function model reaction for designing DeNOx catalysts

A three-function catalyst model for hydrocarbon SCR of NOx is described, based on experimental evidence for each function, during temperature-programmed surface reactions (TPSR). The release of N2 occurs within function 3. It involves the dissociation of NOx (via a dinitrosyl-adsorbed intermediate), followed by subsequent formation of N2 and scavenging of the adsorbed oxygen species left from NO dissociation. The removal of adsorbed oxygen is due to the total oxidation of an activated reductant (CxHyOz). This reaction corresponds to ‘a supported homogeneous catalytic process’ involving a surface transition metal complex. The corresponding catalytic sequence of elementary steps occurs in the coordinative sphere of the metal cation. A function 2 has to turn over simultaneously to function 3. It has to deliver the active reductant species CxHyOz to function 3, at the temperature where function-3 cycle turns over. Function 2 is the mild oxidation of HC (or any initial oxygenate) by N02, through organic nitrogen-containing intermediates (RNOJ. The very important feature is that these RNOx species are quite thermally instable: they decompose with temperature to CxHyOz + NO (not to N2), according to the following global equation: HC(orCxHyOz)+NO2=Cx'Hy'Oz'+NO It is therefore obvious that functions 2 and 3 have to turn over simultaneously. Nevertheless, at the molecular level, these two functions are not in the same catalytic cycle. A function 1 has also to turn over simultaneously with functions 2 and 3, as it has to provide N02 to function 2. The oxidation of NO to N02 is therefore the first function of any efficient catalyst. The concept of ‘simultaneous turn over’ between catalytic functions in multi-functional catalysis is widely accepted (for instance, in bi-functional metal/acid transformation of alkanes), and this aspect of the proposed mechanism is a ‘normal’ behaviour in steady state.