Supercritical Carbon Dioxide Extraction-Mediated Amendment of a Manganese Oxide Surface Desired to Selectively Transform Nitrogen Oxides and/or Ammonia

Mn oxide is a particular class of metal phase highly active in reducing NOX or oxidizing NH3 at low temperatures yet needs amendment in terms of surface acidic/redox sites to improve selectivities to desired N2 (SN2) 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 NOX reduction pathways to produce N2 only. This was enabled by increasing the area of surface sites accessible to NH3/NOX/O2 at ≤200 °C, as evidenced by a large NOX 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 NOX reduction pathway to produce undesired NO2/N2O at 220-280 °C while detouring Langmuir-Hinshelwood-typed NOX 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 (-rNH3) and N2 selectivities in a desired manner at 280-400 °C. Hence, Mn-CO2 provided higher SN2 values despite exhibiting smaller -rNH3 values in comparison with those of the analogue unsubjected to SC-CO2 (Mn). The benefits provided by SC-CO2 were coupled to enhance NOX reduction performance of Mn-CO2 over Mn at 150-400 °C. Importantly, Mn-CO2 enhanced long-term stability in reducing NOX 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.