Abstract: LaMnAl₁₁O₁₉ catalysts were prepared by co-precipitation method and characterized with XRD, BET and XPS. The conversion of NH₃ at the conditions of catalytic combustion and homogeneous combustion were studied by combustion of simulated biomass gasification gas and NH₃ oxidation, respectively. Moreover, the NH₃ adsorption and oxidation on the surfaces of the catalyst samples were examined by in-situ DRIFT experiments. It was found that calcination of the percursors at 1200℃ led to the formation of a final monophasic materials with MP structure and high surface area, while the Mn ions were either divalent or trivalent. Under homogeneous combustion condition, NH₃ at simulated biomass gasification gas started to react at 500℃, then NO was formed. Under catalytic combustion condition, the curves of NH₃ oxidation with and without addition of simulated gasification gas showed no obvious differences. NH₃ started to react at 310℃, and NO exhibited higher concentration in the temperature range of 350℃~800℃. However, NO₂ was generated at higher temperature within a narrow temperature range. The concentration of N₂O during the reaction was less than 10 ×10^-6. More than 40% of the NH₃ converted to NO during the experiments. The results of in-situ DRIFT indicated that the reaction of ammonia conversion followed the imide (-NH) mechanism, that is, the ammonia adsorbed on the catalyst surface was decomposed to -NH firstly, then the -NH reacted with atomic oxygen (O) to further form nitroxyl (HNO) and N₂ or nitrous oxide (N₂O), or -NH reacted with molecular O₂ to produce nitric oxide (NO) directly.