Formation mechanism of thermal NOx on the surface of oxide catalysts

氮氧化物是典型的大气污染物之一，是酸雨的主要来源。空气中NOx的来源可分为“热NOx”、“提示NOx”和“燃料NOx”三大类。其中热态NOx是由空气中N_2与O_2在高温下反应形成的，因此在催化剂充足的情况下，在燃烧气氛中容易形成。由于NOx主要由NO组成，除了一些特殊情况外，NOx的减少相当于NO的减少。然而，由于缺乏动力学信息，NO在催化剂表面形成或分解的化学机理尚不清楚。本工作旨在研究热态NOx在氧化物催化剂表面形成/分解机理的动力学。选用氧化铁（Fe_3O_4）作为氧化物催化剂，是因为钢是燃烧反应器的主要成分，而Fe_3O_4通常在高温燃烧气氛下在反应器中形成。本文首次研究了N_2-O_2混合气体生成NO的速率。反应速率总是取决于气体流速，很难消除气相传质的影响。因此，尝试将同位素交换技术应用于该反应。然而，^<15>NO对应的质量峰31的本底非常高，表明^<15>NO交换反应速率的精确测量将是非常困难的。因此，在本工作中，通过将Ar-NO气体混合物撞击Fe_3O_4来测量NO的分解速率。结果表明，Fe_30_4催化剂对NO有一定的分解作用。在600 ~ 950℃的温度范围内，分解速率与气体流速无关，且与NO的分压成正比，说明分解受催化剂表面化学反应控制，以NO分子解离为主。发现其分解速率的温度依赖性很大，其表观活化能约为400kJ/mol。本工作还采用了TiO_2和Cr_2O_3等氧化物催化剂，发现Cr_2O_3对NO的分解效果更好。因此，对Cr_2O_3基氧化物催化剂表面NO的分解进行详细的动力学研究是我们今后的工作重点。已知Cr_2O_3与氧化铁、Al_2O_3等形成广泛的固溶体，因此研究Cr_2O_3固溶体上NO分解动力学与催化剂组成的关系是非常有趣的。为此，我们还测定了FeO-Cr_2O_3铁铬铁矿固溶体在高温下的热力学性质。这些信息将有助于揭示铬铁矿催化剂表面NO分解的详细反应机理。少

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