Nakatani Tetsu, Watanabe Tsunenori, Takahashi Masaru, Miyahara Yuya, Deguchi Hiroshi, Iwamoto Shinji, Kanai Hiroyoshi, Inoue Masashi
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Kyoto 615-8510, Japan.
J Phys Chem A. 2009 Jun 25;113(25):7021-9. doi: 10.1021/jp901569s.
The gamma-Ga(2)O(3)-Al(2)O(3) mixed oxides with a spinel structure were prepared by the solvothermal reaction of gallium acetylacetonate and aluminum isopropoxide in diethylenetriamine. In the crystal structures of the catalysts obtained by the calcination of these mixed oxides, Ga(3+) and Al(3+) ions preferentially occupied tetrahedral and octahedral sites, respectively. The catalysts with low Ga contents had a unique structure with high surface areas and a concentration gradient of decreasing Ga content from the surface to the bulk. In methane-selective catalytic reduction (SCR) of NO, higher NO conversion to N(2) was attained on the catalyst with high occupation of Ga(3+) ions at tetrahedral sites and Al(3+) ions at octahedral sites. For the gamma-Ga(2)O(3)-Al(2)O(3) mixed oxide with a charged Ga molar content of 0.3 (ST(0.3)), tetrahedral and octahedral sites were solely occupied by Ga(3+) and Al(3+) ions, respectively, and the catalyst exhibited the highest NO conversion to N(2). Therefore, it was concluded that the active site for methane-SCR of NO is tetrahedral Ga(3+) ion and octahedral Al(3+) ion, which are linked to each other. Nitrogen monoxide is adsorbed on the isolated hydroxyl group attached to Al(3+) ions and then oxidized by O(2) yielding surface nitrate species. Tetrahedral Ga(3+) ions work as Lewis acid sites for the activation of methane because of their coordinative unsaturation. The Ga(3+) ions in the gamma-Ga(2)O(3)-Al(2)O(3) catalyst have a redox property, which plays important roles in both the oxidation of NO to surface nitrate species and the activation of methane. The most important factor for this catalyst is that the sites for the formation of surface nitrate species reside next to the methane activation sites, which facilitates the reaction between surface nitrate species and the activated species derived from methane, thus mitigating the consumption of methane by simple combustion with O(2). Therefore, ST(0.3), which has the largest number of ensembles of the tetrahedral Ga(3+) ions and octahedral Al(3+) ions, shows the highest activity for methane-SCR of NO.
通过乙酰丙酮镓和异丙醇铝在二乙烯三胺中的溶剂热反应制备了具有尖晶石结构的γ-Ga₂O₃-Al₂O₃混合氧化物。在这些混合氧化物煅烧得到的催化剂晶体结构中，Ga³⁺和Al³⁺离子分别优先占据四面体和八面体位置。低Ga含量的催化剂具有独特的结构，具有高比表面积以及从表面到体相Ga含量降低的浓度梯度。在甲烷选择性催化还原（SCR）NO反应中，四面体位置高占有率的Ga³⁺离子和八面体位置高占有率的Al³⁺离子的催化剂上实现了更高的NO转化为N₂。对于带电Ga摩尔含量为0.3的γ-Ga₂O₃-Al₂O₃混合氧化物（ST(0.3)），四面体和八面体位置分别仅被Ga³⁺和Al³⁺离子占据，并且该催化剂表现出最高的NO转化为N₂。因此，得出结论，NO的甲烷-SCR活性位点是相互连接的四面体Ga³⁺离子和八面体Al³⁺离子。一氧化氮吸附在与Al³⁺离子相连的孤立羟基上，然后被O₂氧化生成表面硝酸盐物种。四面体Ga³⁺离子因其配位不饱和而作为甲烷活化的路易斯酸位点。γ-Ga₂O₃-Al₂O₃催化剂中的Ga³⁺离子具有氧化还原性质，这在NO氧化为表面硝酸盐物种和甲烷活化中都起重要作用。该催化剂最重要的因素是表面硝酸盐物种形成位点位于甲烷活化位点旁边，这促进了表面硝酸盐物种与甲烷衍生的活化物种之间的反应，从而减少了甲烷与O₂简单燃烧的消耗。因此具有最多四面体Ga³⁺离子和八面体Al³⁺离子集合的ST(0.3)对NO的甲烷-SCR表现出最高活性。
