Kinetics of Oxygen Exchange and N2O Decomposition Reaction over MeOx/CeO2 (Me = Fe, Co, Ni) Catalysts

MeO_x/CeO₂ (Me = Fe, Co, Ni) samples were tested in an ¹⁸O₂ temperature-programmed isotope exchange and N₂O decomposition (deN₂O). A decrease in the rate of deN₂O in the presence of oxygen evidences the competitive adsorption of N₂O and O₂ on the same sites. A study of isotope oxygen exchange revealed dissociative oxygen adsorption with the subsequent formation of surface oxygen species. The same species, more probably, result from N₂O adsorption and the following N₂ evolution to the gas phase. We supposed the same mechanism of O₂ formation from surface oxygen species in both reactions, including the stages responsible for its mobility. A detailed analysis of the kinetics of isotope exchange has been performed, and the rates of one-atom (R^I) and two-atom (R^II) types of exchange were evaluated. The rate of the stage characterizing the mobility of surface oxygen was calculated, supposing the same two-step mechanism was relevant for both types of exchange. The effect of oxygen mobility on the kinetics of deN₂O was estimated. An analysis of the possible pathways of isotope transfer from MeO_x to CeO_x showed that direct oxygen exchange on the Me-Ce interface makes a valuable contribution to the rate of this reaction. The principal role of the Me-Ce interface in deN₂O was confirmed with independent experiments on FeO_x/CeO₂ samples with a different iron content.

High-Temperature Abatement of N2O over FeOx/CeO2-Al2O3 Catalysts: The Effects of Oxygen Mobility

CeO2-Al2O3 oxides prepared by co-precipitation (Ce+Al) or CeOx precipitation onto Al2O3 (Ce/Al) to obtain dispersed CeO2 and samples with further supported FeOx (2.5–9.9 weight% in terms of Fe) were characterized by XRD, XPS, DDPA and Raman. Fe/Ce/Al samples with lower surface concentrations of Fe3+ were substantially more active in N2O decomposition at 700–900 0C. It was related to higher oxygen mobility, as estimated from 16O/18O exchange experiments and provided by preferential exposing of (Fe-)Ce oxides. Stabilization of some Ce as isolated Ce3+ in Fe-Ce-Al mixed oxides dominating in the bulk and surface layers of Fe/(Ce + Al) samples retards the steps responsible for fast additional oxygen transfer to the sites of O2 desorption.

Selective catalytic oxidation of ammonia to nitric oxide via chemical looping

Selective oxidation of ammonia to nitric oxide over platinum-group metal alloy gauzes is the crucial step for nitric acid production, a century-old yet greenhouse gas and capital intensive process. Therefore, developing alternative ammonia oxidation technologies with low environmental impacts and reduced catalyst cost are of significant importance. Herein, we propose and demonstrate a chemical looping ammonia oxidation catalyst and process to replace the costly noble metal catalysts and to reduce greenhouse gas emission. The proposed process exhibit near complete NH3 conversion and exceptional NO selectivity with negligible N2O production, using nonprecious V2O5 redox catalyst at 650 oC. Operando spectroscopy techniques and density functional theory calculations point towards a modified, temporally separated Mars-van Krevelen mechanism featuring a reversible V5+/V4+ redox cycle. The V = O sites are suggested to be the catalytically active center leading to the formation of the oxidation products. Meanwhile, both V = O and doubly coordinated oxygen participate in the hydrogen transfer process. The outstanding performance originates from the low activation energies for the successive hydrogen abstraction, facile NO formation as well as the easy regeneration of V = O species. Our results highlight a transformational process in extending the chemical looping strategy to producing base chemicals in a sustainable and cost-effective manner.

Toward development of single-atom ceramic catalysts for selective catalytic reduction of NO with NH3

Insertion of transition metal species into crystalline alumina at low temperatures is proposed to achieve the dispersion of these species at atomic level paired with exceptional textural properties. Precisely, MeAl₂O₄/γ-Al₂O₃ (Me = Mn, Fe, Co, Ni, and/or Cu) nanostructured ceramic catalysts were fabricated with ultra large mesopores (16-30 nm), and high specific surface area (180-290 m² g^-1) and pore volume (1.1-1.6 cm³ g^-1). These ceramics were applied as efficient catalysts for the selective catalytic reduction (SCR) of NO with NH₃, and their selectivity was discussed in terms of N₂O formation, an undesirable byproduct. The catalysts containing Fe, Cu, or Mn showed the highest activities, however, within different temperature ranges. Further tuning of the catalytic activity and selectivity was achieved by creating ceramic catalysts with mixed compositions, e.g., CuFe and MnFe. Upon insertion of the transition metal species into crystalline structure of alumina to maximize atom efficiency, the N₂O formation profile did not change significantly for all metal aluminates except MnAl₂O₄, indicating that these catalysts are suitable for SCR and selectively promote the reduction of NO.

Kinetic modeling of nitrous oxide decomposition on Fe-ZSM-5 in the presence of nitric oxide based on parameters obtained from first-principles calculations

A variety of experiments for the N₂O decomposition over Fe-ZSM-5 catalysts have been simulated in the presence and absence of small amounts of nitric oxide and water vapor.

FeOx/Al2O3 catalysts for high-temperature decomposition of N2O under conditions of NH3 oxidation in nitric acid production

The δ and θ Al₂O₃ phases well stabilized Fe(iii) in T_d or O_h coordination, which were identified as the active species in high temperature decomposition of N₂O in a complex gas mixture produced by oxidation of ammonia.