Tsunenori Watanabe

Characterization of γ-Ga2O3-Al2O3 Prepared by Solvothermal Method and Its Performance for Methane-SCR of NO

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.

Effect of modification by alkali on the γ-Ga2O3–Al2O3 mixed oxides prepared by the solvothermal method

The solvothermal reaction of the mixtures of aluminium triisopropoxide (AIP) and gallium triacetylacetonate at 300 °C directly yielded γ-Ga2O3–Al2O3 mixed oxides (solid solutions, but Ga3+ ions preferentially occupied the tetrahedral sites of the defect spinel structure). The reaction of AIP alone in 2-(methylamino)ethanol yielded γ-Al2O3, and the reaction of the mixture of AIP and Ga(acac)3 yielded γ-Ga2O3–Al2O3 mixed oxides having an Al ratio identical to the charged ratio. Although the reaction of AIP alone in toluene at 315 °C yielded χ-Al2O3, the reaction at 300 °C gave an amorphous product, while the reaction of Ga(acac)3 alone afforded γ-Ga2O3 at 300 °C. The reaction of the mixtures of the two starting materials yielded the γ-type mixed oxide even with extremely high charged ratio of Al. It was concluded that, in toluene, the initial formation of γ-Ga2O3 nuclei controlled the crystal structure of the mixed oxide. The mixed oxide catalysts with lower acid density showed lower methane consumption by combustion in selective catalytic reduction of NO with methane. By alkali modification of the catalyst, the acid density of the mixed oxide decreased, and the CH4 combustion rate was diminished. Although NO conversion activity was also decreased by the modification, the efficiency of methane used for NO reduction was improved.