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，也是酸雨的主要来源。空气中NOx的来源可分为“热源”、“瞬发”和“燃料”三类。其中，热NOx是由空气中的N_2和O_2在高温下反应生成的，因此在有足够催化剂的情况下，很容易在燃烧气氛中生成。由于除某些特殊情况外，NOx主要由NO组成，因此NOx的还原相当于NO的还原。然而，由于缺乏动力学信息，关于NO在催化剂表面生成或分解的化学机理尚不清楚。本工作旨在研究热NOx在氧化物催化剂表面生成/分解的动力学机理。选择氧化铁(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，而在高温燃烧气氛下，反应器中经常会生成Fe_3O_4。The r…本文首次研究了N_2-O_2混合气体生成NO的更多途径。传质速率一直依赖于气体流量，气相传质的影响很难消除。为此，尝试将同位素交换技术应用于该反应。然而，对应于NO的31质量峰的背景很高，这表明精确测量^&15；NO的交换反应速率将是非常困难的。因此，本工作采用Ar-NO混合气体撞击Fe_3O_4的方法测定了NO的分解速率。结果表明，在Fe_30_4催化剂的作用下，NO有一定程度的分解。结果表明，在600~950℃的温度范围内，NO的分解速率与气体流量无关，与NO的分压成正比，说明NO的分解受催化剂表面的化学反应控制，NO分子的解离占主导地位。分解速率与温度的关系很大，其表观活化能约为400kJ/mol。在本实验中，还使用了其他氧化物催化剂，如二氧化钛和三氧化二铬，发现三氧化二铬是更有效的NO分解催化剂。因此，在今后的工作中，需要对Cr2O3型氧化物催化剂表面分解NO的动力学进行详细的研究。由于Cr_2O_3与Fe_2O_3、Al_2O_3等形成宽固溶体，因此研究催化剂组成对Cr_2O_3固溶体上NO分解动力学的影响具有重要意义。为此，我们还测定了FeO-Cr2O_3铁铬铁矿固溶体的高温热力学性质。这些信息将有助于揭示NO在铬铁矿催化剂表面分解的详细反应机理。较少

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