The thermal activation of MnOx-Al2O3 catalysts: Effect of gallium doping

In this study, the effect produced by the gallium oxides intercalation into MnOx-Al2O3 catalysts on their catalytic properties and structural aspects of the active component formation was investigated. Three series of the catalysts, Mn-Al, Mn-Ga, and Mn-Al-Ga, having a similar Mn content but different Al/Ga ratio, were synthesized by coprecipitation and subsequent calcination at temperatures from 600 to 1200 degrees C degrees. The catalysts were tested in CO oxidation. For the Mn-Al series of catalysts, a thermal activation effect is observed, defined as an increase in catalytic activity after high-temperature treatment at 900-1000 degrees C. The increase in the activity after calcination is related to the formation of the Mn3-xAlxO4 solid solution at the synthesis temperature and its segregation upon cooling. Segregation of the solid solution leads to the formation of the Mn3O4+delta defect oxide and an amorphous aluminum-containing component. The addition of 5%Ga to the Mn-Al catalyst changes the phase transformation route during thermal activation. The in situ XRD data has showed that at 1000 degrees C the high-temperature solid solution (MnGaAl)3O4 with the cubic spinel structure is formed; however, its further segregation upon cooling is hindered. The introduction of gallium stabilizes the structure of the (MnGaAl)3O4 mixed oxide, the parent oxide is partially decomposed after cooling with Mn3O4 nanoparticles formation. However the amount of produced active Mn3O4 particles and/or weakly bound oxygen is much lower, which in turn adversely affects the catalytic properties.