Locating manganese vanadate phase with PO43−-modified Mn2+–O–V5+ motifs optimized for catalytic NOX and poison abatement under oxidative wet conditions

Abstract

Metal oxide crystallites possess characteristic structural motifs, which can act as active centers to direct the activities, selectivities, and sustainabilities of target reactions. In this context, Mn²⁺–O–V⁵⁺ motifs are essential to construct Mn²⁺/V⁵⁺-centered sub-units for manganese vanadate phases (Mn_XV₂O_X+5; X = 1–3). Surface Mn²⁺–O–V⁵⁺ motifs are readily fragmented to bear labile oxygens (O_L) and Lewis acidic (LA) defects adjacent to oxygen vacancies (O_V) functioning as reservoirs of mobile oxygens (O_M). The LA defects possess high affinity for O₂/H₃PO₄ poison included in NO_X/SO₂-containing, wet feed gases. Meanwhile, the LA defects afford PO₄³⁻ functionalities, whose terminal P⁵⁺–O²⁻ bonds act as Brønsted acidic species (BA⁻–H⁺) upon protonation. The resulting BA⁻–H⁺-rich, fragmented Mn²⁺–O–V⁵⁺ motifs are particularly conducive to activate the selective reduction of NO_X (SCR) via the Eley–Rideal (ER) model or the pyrolysis of ammonium (bi-)sulfate (AS/ABS) poisons via the pyrosulfate disintegration model. Marked acceleration of the ER and pyrosulfate fragmentation models hinge on the energy barrier (E_BARRIER) reduction/collision frequency elevation/strong hydrophobicity, whose trends versus X anticipated using the local Mn²⁺–O–V⁵⁺ connectivities in the Mn²⁺-centered sub-units are opposite to those in their V⁵⁺-centered counterparts. Nonetheless, the Mn²⁺/V⁵⁺-centered sub-units were verified to impart Mn²⁺–O–V⁵⁺ motifs that were highly associated and similarly contributed to disclosing the trends of the kinetic parameters , hydrophobicity, or amounts/strengths of the BA⁻–H⁺/redox sites (O_L/O_V/O_M) versus X, throughout which X = 1–2 were more desired than X = 3 except for the values alongside with a higher hydrophobic surface area provided by X = 1 compared to those provided by X = 2–3. Notably, Sb₂O₅-promoted Mn₁V₂O₆ subjected to PO₄³⁻ modification (Mn₁–Sb–P) was superior to WO₃-promoted V₂O₅ (commercial control) or SO_A²⁻-modified analogue (Mn₁–Sb–S) in achieving higher activities and/or maximum-obtainable performance for the SCR or AS/ABS pyrolysis, as substantiated by controlled or ¹⁸O₂-labelled runs under kinetic/diffusion-limited domains.