Formation mechanism of thermal NOx on the surface of oxide catalysts

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NOx is one of the typical air pollutants and is the main source of acid rain. The origin of NOx in air can be classified into three groups as "thermal", "prompt" and "fuel NOx". Among them, thermal NOx is formed by the reaction between N_2 and O_2 in air at high temperature, so that it forms easily in the combustion atmosphere under the presence of adequate catalyst. Since NOx consists of maily NO except for some special cases, the reduction of NOx would be equivalent to the reduction of NO. However, chemical mechanism on the formation or decomposition of NO on the surface of the catalyst is not well known yet due to the lack of kinetic information. The present work aims to study the kinetics on the formation/decomposition mechanism of thermal NOx on the surface of oxide catalysts. Iron oxide (Fe_3O_4) is selected as an oxide catalyst because the steel is major component of the combustion reactor and Fe_3O_4 is often forms in the reactor at high temperature combustion atmosphere. The r … More ate of formation of NO from N_2-O_2 gas mixture has first been studied in the present work. The rate was always depends on the gas flow rate and it was hard to eliminate the effect of gas phase mass transfer. Therefore, the isotope exchange technique was tried to apply in this reaction. However, the background of mass peak of 31 which corresponded to ^<15>NO was very high, indicating the precise measurement of the exchange reaction rate of ^<15>NO would be very difficult. Thus, the rate of decomposition of NO was measured in the present work by impinging Ar-NO gas mixture onto Fe_3O_4. As a result, NO was decomposed to a certain extent with Fe_30_4 catalyst. It was found that the rate of decomposition was independent of gas flow rate in the temperature range from 600 to 950℃ and proportional to the partial pressure of NO, indicating that the decomposition would be controlled by the chemical reaction on the surface of catalyst and the dissociation of NO molecule would be predominant. The temperature dependence of the decomposition rate was found to be very large and its apparent activation energy was evaluated as approximately 400kJ/mol. In the present wok, other oxide catalysts such as TiO_2 and Cr_2O_3 were also used and Cr_2O_3 was found to be more effective catalyst for NO decomposition. Therefore, detail kinetic study on the decomposition of NO on the surface of Cr_2O_3 based oxide catalyst in our future work. It is known that Cr_2O_3 forms wide solid solution with iron oxide, Al_2O_3 and so on, so that it is quite interesting to study the kinetics of NO decomposition on Cr_2O_3 solid solution as a function of catalyst composition. For this purpose, we have also determined thermodynamic properties of FeO-Cr_2O_3 iron chromite solid solution at high temperature. Such information would be very useful to reveal detail reaction mechanism of NO decomposition on the surface of chromite catalyst. Less