Promotion Mechanism Of NO Removal By Surface Oxygen Groups With Carbon-Metal-Oxygen Group And Their Interaction

The research and application of carbon-based catalyst was quite extensive. As for nitrogen oxide（NOx） catalysis, carbon supported transition metal catalyst had been widely studied, since it had low reaction temperature and high reactivity. Moreover, there were abundant sources for the production of carbon-based catalyst. Carbon supported transition metal catalyst involved two important problems in the research of NOx-SCR. One was that surface oxygen groups were beneficial to NO decomposition at low temperature; another important issue was that supported metal was beneficial to enhancment of NOx-SCR activity. But so far there were few literatures detailedly discussing the reaction process and mechanism of the two problems. Understanding of surface oxygen groups and supported metal could not only help to reveal the catalytic reaction path and nature of NOx-SCR catalyzed by carbon-based material, but also design highly effective NOx-SCR catalysts for low temperature reaction. In this work, graphene oxide（GO） with abundant surface oxygen groups was used to show the main structural unit in NOx-SCR, which involved GO and GO supported iron catalyst. This article investigated the role of surface oxygen groups and carbon-metal-oxygen group on the catalytic reduction of NOx, by building characterization technology of temperature programming and transient dynamic response. It also explored the promotion mechanism of surface oxygen groups and its interaction with carbon-metal-oxygen group. As a result, the following conclusions were obtained:（1） Determination method for surface chemical properties of carbon based catalysts was established. For the purpose of exploring the catalysis effect of surface functional groups and analyzing the catalytic mechanism of C-NO, the standard characterization method was required. Temperature programmed technology was established to determine the surface oxygen groups of GO, Fe@GO and GO&Fe₂O₃, and the surface adsorption sites for NH₃ and NO. It was found that GO contains a large amount of carboxylic acid groups, about 9.84 mmol/g; C-Fe-O structure would lead to a new path of NO reduction, which located at 738 oC on TPD curve of Fe@GO; Lewis acid sites were generated on Fe@GO, which could desorb NH₃ at 400 oC and give rise to strong adsorption binding energy toward NO. At the same time, the intermediate and oxygen atom catalytic reaction path on Fe@GO was explored. The result showed that carbon surface active sites were produced during the decomposition of functional groups and reacted with O atoms dissociatied from NO. Thus, intermediate C（O） was generated and released in gaseous CO and CO₂. Based on these results, temperature programmed decomposition was established to determine the surface oxygen groups; temperature programmed desorption was established to determine the surface adsorption sites; and transient dynamics experiment was established to measure the intermediate and the reaction path of reactant atoms on the surface of the catalyst.（2） Carboxylic acid group played a main role in the process of NO removal at low temperature. According to the characteristic that GO has plenty of carboxylic acid groups, NO removal by GO activated at different temperature and the amount of various kind of surface oxygen groups were compared. It was found that carboxylic acid groups and the catalytic efficiency of NO had the following order: GO100>GO₂00>GO300. The result showed that carboxylic acid groups seemed to play a decisive role in the process of NO removal at low temperature. However, it also transformed into anhydride and lactones which hindered NO reduction. Carboxylic acid groups on GO100, GO₂00 and GO300 were 6.95, 2.07 and 0 mmol/g, respectively. The amount of anhydride groups was 0.49 mmol/g on GO100. It increased to 1.66 and 2.81 mmol/g on GO₂00 and GO300. The amount of lactone groups also increased with reaction temperature. Transient kinetic experiments were conducted to quantitatively determine the reaction intermediate namely carbon surface active site（Cf）. It was found that NO catalytic efficiency and Cf amount decreased with the increasing reaction time, and O atom adsorbed on the surface activity sites led to weight gain of GO. Comparing XPS results of GO reacted with NO at 100 oC for 10 and 60 min, it was found that O atoms left on GO surface achieved the regeneration of carboxyl and oxidation of functional groups. Carboxyl acid group reacted easily with carbon to form CO₂, generating free active site（-Cn-H）, which had a higher affinity toward NO reduction; NO reacted with the active sites to form N2 and regenerated carboxyl acid group.（3） Anhydride and C-Fe-O group synergistically facilitated NO reduction at medium temperature. In order to determine the high efficient catalysis structure and its reaction mechanism of Fe@GO, the role of different components in Fe@GO, anhydride group and C-Fe-O structure in the process of NO removal were compared. It was found that at 100 oC, NO removal rate on GO was up to 45.7%, while on Fe@GO was 3.5-5.0%. This was due to the fact that carboxylic acid groups on GO resulted in a higher catalytic effect. But at 300 oC, NO removal of Fe@GO was about 30%, while on GO was about 1%. This was attributed to anhydride groups and the unique structure of C-Fe-O on Fe@GO, which made its catalytic effect higher than GO. The existence of anhydride groups significantly increased NO removal, the amount of anhydride group on Fe@GO350 was higher than that on Fe@GO400. NO removal of Fe@GO350 was also higher than Fe@GO400, the removal was 66.1% and 41.5%, respectively. Interestingly, consumption of anhydride groups and amount of NO cumulative removal presented a linear correlation. The loss of C-Fe-O group and the accumulation amount of NO removal were 0.11 and 0.10 mmol/g, indicating that C-Fe-O was the direct structural unit for catalytic decomposition of NO. Comparing TG-MS of GO and Fe@GO, it was found that when NO was swiched on, CO₂ generated from the two samples showed the same order of magnitude. Since there was relatively small number of the carbon content in the Fe@GO system, the presence of Fe can not only decompose NO directly, but also promote the decomposition of anhydride groups. This promotion could produce more surface active sites and enhance the migration and transformation of O atom. Therefore, there was interaction between Fe and anhydride groups, and the interaction would accelerate the catalytic decomposition of NO.