V2O5 fuses with transition metals to create dozens of different metal vanadates, whose acidic/redox traits can be diverse yet optimized for selective catalytic NOX reduction (SCR) by changing the metals used or their metal:vanadium stoichiometry. However, no metal vanadate has been compared with its metal oxide composite analogue as an active phase for SCR, albeit a vanadate occasionally outperforms an oxide composite simulating a commercial catalyst (V2O5–WO3). Herein, Cu3V2O8 and CuO–VO2/V2O5 were rationally selected as model phases of metal vanadates and oxide composites and isolated using pH regulation of their synthetic mixture to ≤∼5 (pH1/pH5) and ∼11 (pH11), respectively. The pH1/pH5/pH11 samples were comparable with regard to morphological, textural, and compositional traits but not for crystallographic features. This thus provided the impetus to simulate the pH1/pH5/pH11 surfaces under a SO2-containing feed-gas stream, by which SOA 2–/HSOA – functionalities (A = 3–4) were anchored on their (defective) Lewis acidic metals and/or labile oxygens (Oα). This could result in the formation of pH1-S/pH5-S/pH11-S, whose major surface species were Brönsted acidic bonds (SOA 2–/HSOA –) and redox sites (Oα; mobile oxygen (OM); oxygen vacancy (OV)). pH1-S/pH5-S/pH11-S were similar in terms of NH3 binding energies and energy barriers in SCR yet escalated collision frequencies among the surface species involved in the sequence of pH11-S < pH5-S < pH1-S (via kinetic assessments), as was the case with the numbers of SOA 2–/HSOA – functionalities of the catalysts (via temperature-resolved Raman spectroscopy). These were coupled to elevate the efficiency of acidic cycling on the order of pH11-S < pH5-S < pH1-S. Meanwhile, the amounts of Oα and OV (or OM) innate to pH1-S/pH5-S were smaller than and comparable to those of pH11-S, respectively. Nonetheless, pH1-S/pH5-S provided greater OM mobility than pH11-S, thereby proceeding better with redox cycling than pH11-S (via 18O-labeling O2-on/off runs). Furthermore, pH1-S/pH5-S outperformed pH11-S in SCR under diffusion-limited domains, while enhancing the resistance to H2O, ammonium (bi)­sulfate poisons, or hydro-thermal aging over pH11-S by diversifying the selective N2 production pathway other than SCR.

Mn oxide is a particular class of metal phase highly active in reducing NO X or oxidizing NH3 at low temperatures yet needs amendment in terms of surface acidic/redox sites to improve selectivities to desired N2 (S N2) along with the promotion of SO2 tolerance. This study reports the use of supercritical CO2 extraction (SC-CO2) as a means to adjust the quantities/strengths of surface sites present in the resulting Mn oxides on TiO2 (Mn-CO2) and validates the advantages of SC-CO2 with regard to mechanistic viewpoints via kinetic evaluation and control reactions. SC-CO2 was demonstrated to promote the activity or diversity of Langmuir–Hinshelwood-type or Eley–Rideal-type NO X reduction pathways to produce N2 only. This was enabled by increasing the area of surface sites accessible to NH3/NO X /O2 at ≤200 °C, as evidenced by a large NO X consumption rate and pre-factor of Mn-CO2 in addition to in situ DRIFT experiments. In addition, SC-CO2 could tailor redox sites in such a way as to circumvent an Eley–Rideal-type NO X reduction pathway to produce undesired NO2/N2O at 220–280 °C while detouring Langmuir–Hinshelwood-typed NO X reduction to yield undesired products. Furthermore, SC-CO2 could attenuate the Lewis acidic strength of surface sites and therefore deterred NH3 oxidation at up to ∼280 °C. Meanwhile, Mn-CO2 regulated the formation of intermediates vital to direct NH3 consumption rates (−r NH3) and N2 selectivities in a desired manner at 280–400 °C. Hence, Mn-CO2 provided higher S N2 values despite exhibiting smaller −r NH3 values in comparison with those of the analogue unsubjected to SC-CO2 (Mn). The benefits provided by SC-CO2 were coupled to enhance NO X reduction performance of Mn-CO2 over Mn at 150–400 °C. Importantly, Mn-CO2 enhanced long-term stability in reducing NO X over Mn in the presence of SO2 at ≤200 °C by encouraging the formation of Brönsted acidic sites and hampering the transition of Lewis acidic Mn species to MnSO3/MnSO4.

Anthropogenic flue gases consist of NOX/SO2/H2O, among which H2O resistance is often underrated in activating acidic/redox cycles of NH3-assisted catalytic NOX reduction (SCR), SO2/H2O-induced evolution of ammonium (bi)sulfate (AS/ABS) poisons,...