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 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.

Deciphering Evolution Pathway of Supported NO3• Enabled via Radical Transfer from •OH to Surface NO3– Functionality for Oxidative Degradation of Aqueous Contaminants

“…As shown in Figure S6 , B / L bands resulting from asymmetric or symmetric stretching (or bending) vibrations of N–H bonds of NH 3 coordinated to B / L sites were developed under a NH 3 atmosphere at 50 °C (near ambient temperature). 18 , 39 , 66 − 68 Nevertheless, the areas under the L bands were greater than those of the B bands across the catalysts. This could indicate that the -Q ST,CO2 values of the catalysts mainly originated from CO 2 ···Mn n+ interactions, although the contributions of CO 2 ··· B interactions to the -Q ST,CO values could not be ruled out entirely.…”

“…The XP spectra of the catalysts in the O 1s regimes showed a broad band, which was deconvoluted into three sub-bands assigned to lattice O (O β ), O chemically susceptible (O α ), and O of H 2 O chemisorbed (O′ α ) with binding energies centered at ∼529.8 eV, ∼530.6 eV, and ∼531.9 eV, respectively ( Figure S16 ). 28 , 39 β-MnO 2 with S BET of ∼10 m 2 g CAT –1 provided the smallest concentration of surface O α species (∼30%) among α-/β-/γ-MnO 2 . Therefore, the β-MnO 2 surface could be barely modified by NO Y – species and showed negligible N contents.…”

“… 28 , 39 Hence, the S CO (S BET,CO divided by N CO ) values of the catalysts were quantified with the use of their CO isotherms, all of which provided CO-accessible BET surface areas (S BET,CO ) along with the amounts of CO adsorbed per gram of the catalysts at 1 bar (N CO ). 28 , 39 It should be noted that CO 2 isotherm experiments could also provide S CO2 values of the catalysts analogous to their S CO counterparts yet were not considered due to CO 2 adsorption on Brönsted acidic sites ( Figure S5 ). We attempted to collect CO isotherms of the catalysts near ambient temperature.…”

“… 52 , 56 All of the analytic results stated above could corroborate that in addition to O α species allowing for an access to NO/O 2 , Mn n + species on α-/γ-MnO 2 could also be grafted by gaseous NO Y – species that were formed in proximity to Mn n + sites during the synthesis of α-/γ-MnO 2 -N, which was also suggested and computed elsewhere. 36 − 39 Of importance, the amounts of Lewis acidic sites were comparable across the catalysts subjected to NO Y – modification, which again posed the need to compare − r H2O2,0 /– r PHENOL,0 values of α-/γ-MnO 2 -N per site basis for clarifying their geometrical effect on the • OH → NO 3 • SUP route.…”

“…36 , 37 In addition, labile oxygens (O α ) bound to surface Mn n + species are also prone to bind with NO/O 2 and generate NO 2 – /NO 3 – functionalities with mono - or bi -dentate configuration along with reductive transition of Mn n + to Mn ( n –1)+ . 38 , 39 (See boxes highlighted with yellow/red/orange in Figure 1 .) It should be noted that NO 2 – functionalities can be radicalized via the • OH → NO 2 • SUP route ( Figure 1 B) yet cannot be desirable in decomposing contaminant due to the demerits of NO 2 • stated above.…”