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

Abstract

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 N-2(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 NOx reduction pathways to produce N-2 only. This was enabled by increasing the area of surface sites accessible to NH3/NOx/O-2 at <= 200 degrees 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 degrees 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 similar to 280 degrees C. Meanwhile, Mn-CO2 regulated the formation of intermediates vital to direct NH3 consumption rates (-r(NH3)) and N-2 selectivities in a desired manner at 280-400 degrees 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 NOx reduction performance of Mn-CO2 over Mn at 150-400 degrees C. Importantly, Mn-CO2 enhanced long-term stability in reducing NOx over Mn in the presence of SO2 at <= 200 degrees C by encouraging the formation of Bronsted acidic sites and hampering the transition of Lewis acidic Mn species to MnSO3/MnSO4.