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.