In-Situ DRIFTS for Reaction Mechanism and SO2 Poisoning Mechanism of NO Oxidation Over γ-MnO2 with Good Low-Temperature Activity

Abstract

In this study, the γ-MnO 2 catalyst modified with PEG exhibits outstanding low-temperature performances for NO oxidation, and in-situ DRIFTS experiments were used to systematically investigate the low-temperature NO oxidation mechanisms over γ-MnO 2 . These results demonstrated that NO was first adsorbed on the surface of γ-MnO 2 to form the nitrosyls, which could be further oxidized to nitrates under the action of the chemisorbed oxygen or lattice oxygen, and afterwards the formed nitrates were decomposed into nitrogen dioxide. Moreover, the inhibitory mechanism of SO 2 on γ-MnO 2 was also studied, and SO 2 severely inhibit the NO oxidation performance of γ-MnO 2 through forming stable sulfates that could easily consume the active sites of the catalyst to hinder the formation of nitrates, resulting in the termination of oxidation of NO to NO 2 . Clarifying the mechanisms of NO oxidation and SO 2 poison is very essential for developing better NO oxidation catalysts.

A Review on the Impact of SO2 on the Oxidation of NO, Hydrocarbons, and CO in Diesel Emission Control Catalysis

“…Once in contact with SO 2 , sulfation of Mn can poison the catalyst. 194,195,198,201,206 However, the strong interaction existing between some of those couplings (e.g., Mn−Co−Ce-O x ) renders formation of unstable sulfate/ sulfite specie that can be decomposed at relatively low temperatures (e.g., 190 °C) or that dopants help to protect active metal sites or to retain parts of undestroyed nitrates intermediate for conversion into NO 2 . 192,202,203,205,210,215−218 Those features allow for a partial or full recovery of catalyst performance when SO 2 is removed from the gas feed but cannot resist sulfation of metals in the duration of SO 2 .…”

“…209 In some cases, the copresence of H 2 O eases SO 2 poisoning possibly by competitive adsorption effect, but it may also inhibit the transformation of nitro to NO 2 . 195,198,211 It is notable that CrO x /mesoporous TiO 2 -pillared clay (TiO 2 − PILC) or Ce-doped Cr−Ce/TiO 2 −PILC or Cr/Ce x Zr 1−x O 2 catalysts exhibit high sulfur resistance because of the absence of Cr sulfation during the oxidation process. 219−221 Little research attention has been paid to the intrinsic properties of SO 2 influence on non-noble metals.…”

“…As for the other group of non-noble metal materials, MnO x , CeO 2 , CoO x , CuO, or their variations usually excel in the oxidation of NO and CO, especially in NO oxidation. − Catalyst formulation in this regard mainly centers on Mn-based and Ce-based catalysts, in the form as single MnO x oxide of different shapes (e.g., α-, β-, γ-, hollow tunnel structure, etc.) and valences, − single CeO 2 , binary or trimetallic Mn–Ce, − Mn–Co, , Ce–Co, , MnCeM (M = Co, Fe, Sn etc.) mixtures, ,− as well as supported Mn/TiO 2 (or ZrO 2 , Al 2 O 3 ). ,− For example, the unique hollow or tunnel structure and redox cycles between Mn and its partners (e.g., Ce 4+ + Mn 3+ ↔ Ce 3+ + Mn 4+ for Mn–Ce, or Co 2+ +Mn 4+ ↔Mn 3+ +Co 3+ ; Co 3+ +Mn 2+ ↔Mn 3+ +Co 2+ for Mn–Co etc.)…”

“…contribute to the NO oxidation activity by facilitating mass/electron exchange and enriching chemisorbed oxygen species at the interface. Once in contact with SO 2 , sulfation of Mn can poison the catalyst. ,,,, However, the strong interaction existing between some of those couplings (e.g., Mn–Co–Ce-O x ) renders formation of unstable sulfate/sulfite specie that can be decomposed at relatively low temperatures (e.g., 190 °C) or that dopants help to protect active metal sites or to retain parts of undestroyed nitrates intermediate for conversion into NO 2 . ,,,,,− Those features allow for a partial or full recovery of catalyst performance when SO 2 is removed from the gas feed but cannot resist sulfation of metals in the duration of SO 2 . In some cases, the copresence of H 2 O eases SO 2 poisoning possibly by competitive adsorption effect, but it may also inhibit the transformation of nitro to NO 2 . ,, It is notable that CrO x /mesoporous TiO 2 -pillared clay (TiO 2 –PILC) or Ce-doped Cr–Ce/TiO 2 –PILC or Cr/Ce x Zr 1– x O 2 catalysts exhibit high sulfur resistance because of the absence of Cr sulfation during the oxidation process. − …”

Facile H2O-Contributed O2Activation Strategy over Mn-Based SCR Catalysts to Counteract SO2Poisoning

“…As displayed in Figure a, original γ-MnO 2 exhibited a series of crystal phases attributed to (100), (001), and (102), indicating that γ-MnO 2 underwent a random intergrowth of pyrolusite lamellar (1 × 1 channels) and ramsdellite matrix (2 × 1 channels) . After poisoning and regeneration treatment, negligible effects on the structure of γ-MnO 2 were exerted . No diffraction peaks of MnO x with other phases or sulfate species were detected except for the lower crystallinity, which might be attributed to the overlay of amorphous or small-grained sulfates and the effect of secondary roasting.…”