The selective catalytic reduction by ammonia (NH3-SCR) of nitrogen oxides (NOx) is a promising technology that is applied to eliminate NOx pollutants from combustion sources like diesel engines. Mn-based oxides are considered a promising catalyst for this process and many efforts were exerted by scholars to make improvements, including addition of other elements to the catalyst framework. The present study investigates the reaction mechanism and pathways using in-situ DRIFTS FTIR analysis for three Mn-based catalysts: a) mixed metal oxide MnCeTiOx, b) Mn impregnated on mesoporous titanium silicate-1 Mn/MesoTS1, and c) Mn/MesoTS1 after protection by secondary growth of silicalite-1 abbreviated as SG-Mn/MesoTS1. Various experiments were carried out on all the catalysts involving pre adsorbing NH3 then introducing NO+O2 to react with the pre adsorbed species and vice versa. It was found that the mixed metal oxide, MnCeTiOx, exhibited higher activity due to variation of different metals and higher metal content compared to the Mn-zeolite catalysts, approximately 29 Wt% Mn vs 4 Wt %, respectively. However, from comparing the two Mn-zeolite catalysts, each containing roughly 5 Wt% Mn, the catalyst after protection by secondary growth, SG-Mn/MesoTS1, showed improvement in the adsorption capability enhancing the overall performance due to the higher amount of acid sites than Mn/MesoTS1, explained by the presence of additional Brønsted and Lewis acid sites. From DRIFTS experiments, both E-R and L-H mechanism could be coexisting and taking place at 150C for all three catalysts. However, it was concluded that although both mechanisms could take place during the reaction, the acid sites on the catalyst surface for all three samples mostly favor the adsorption of NH3 species over NOx species making the E-R mechanism more assertive at 150C.