Formation mechanism of thermal NOx on the surface of oxide catalysts

氧化物催化剂表面热NOx的形成机理

• 批准号: 12450301
• 负责人: NAGASAKA Tetsuya
• 金额: $ 9.66万
• 依托单位: Tohoku University
• 依托单位国家: 日本
• 项目类别: Grant-in-Aid for Scientific Research (B)
• 财政年份: 2000
• 资助国家: 日本
• 起止时间: 2000 至 2001
• 项目状态: 已结题
• 来源: https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-12450301/
• 关键词: thermal NOx; air pollutant; nitrogen oxide; kinetics; catalystic reaction; oxide catalyst; high temperature heterogeneous reaction; solid solution; 物質移動; 界面化学反応; 気固反応; 同位体交換法

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

NOx是典型的大气污染物之一，也是酸雨的主要来源。空气中氮氧化物的来源可分为“热”、“瞬发”和“燃料氮氧化物”三类。其中，热态NOx是由空气中的N_2和O_2在高温下反应生成的，因此在燃烧气氛中，在适当的催化剂存在下，热态NOx很容易生成。由于NOx主要由NO组成，除某些特殊情况外，NOx的还原将等同于NO的还原。然而，由于缺乏动力学信息，NO在催化剂表面上形成或分解的化学机理尚不清楚。本工作旨在研究氧化物催化剂表面热NOx生成/分解机理的动力学。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，并且Fe_3O_4经常在反应器中在高温燃烧气氛下形成。的r ...更多信息 本文首次研究了N_2-O_2混合气体中NO的生成速率。气液两相传质的影响很难消除，而气液两相传质的影响往往取决于气液两相的流速。因此，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<15>很高，这表明精确测量^ NO的交换反应速率<15>是非常困难的。因此，在本工作中，通过将Ar-NO气体混合物撞击到Fe_3O_4上来测量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。结果表明，在600 ~ 950℃温度范围内，NO的分解速率与气体流量无关，与NO分压成正比，表明NO的分解受催化剂表面化学反应控制，以NO分子的解离为主。分解速率对温度的依赖性很大，其表观活化能约为400 kJ/mol。在本工作中，还使用了其他氧化物催化剂，如TiO_2和Cr_2O_3，发现Cr_2O_3是更有效的催化剂用于NO分解。因此，在今后的工作中，我们将对NO在Cr_2O_3基氧化物催化剂上的分解动力学进行详细的研究。众所周知，Cr_2O_3与氧化铁、Al_2O_3等形成广泛的固溶体，因此研究NO在Cr_2O_3固溶体上分解动力学与催化剂组成的关系是一个很有趣的问题。为此，我们还测定了FeO-Cr_2 O_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在铬酸盐催化剂表面分解的详细反应机理。少