Porous MnO_x for low-temperature NH₃-SCR of NO_x: the intrinsic relationship between surface physicochemical property and catalytic activity

Three kinds of porous MnO_x catalysts consisted of nanoparticles (about 6.5, 8.5, and 21 nm, respectively) were successfully prepared by three different methods, co-precipitation method (CP), citric acid method (CA), and hydrothermal method (HT), respectively. Their physicochemical properties were characterized by TEM, XRD, BET, XPS, H₂-TPR, and NH₃-TPD in detail, and their catalytic activities were evaluated by the selective catalytic reduction (SCR) of NO_x with NH₃ in the temperature range of 60~300 °C. The results showed that their catalytic activities decreased in the order of MnO_x/HT > MnO_x/CA > MnO_x/CP in the region of 60–120 °C due to the dominant factor resulted from the reducibility of MnO_x. In contrast, their catalytic activities declined in the order of MnO_x/CA > MnO_x/HT > MnO_x/CP in the region of 180–300 °C, which can be attributed to the amount of acid sites on the surface of these catalysts. In the region of 120–180 °C, the as-prepared three catalysts exhibited high catalytic activity with 100% NO_x conversion under a high gas hourly space velocity (GHSV) of 36,000 h⁻¹. [Figure not available: see fulltext.].