Effects of Oxygen Mobility in La–Fe-Based Perovskites on the Catalytic Activity and Selectivity of Methane Oxidation

Abstract

The mechanism and structure requirements of selective and total oxidation of methane in a chemical looping process are both experimentally and theoretically examined on La 1−x Sr x FeO 3−δ (x = 0, 0.2, and 0.5) and La 0.5 Sr 0.5 Fe 1−x Co x O 3−δ (x = 0.5 and 1) perovskites. The oxygen mobility in the perovskites described by the formation energy of oxygen vacancy is found to have a pronounced effect on the catalytic activity and selectivity. In particular, the selectivity is controlled largely by the surface oxygen concentration or the oxygen vacancy concentration on perovskites, which depends strongly on the bulk oxygen concentration and the relative rate of the lattice oxygen diffusion with respect to the surface reaction. The substitution of Sr for La at the A site and the substitution of Co for Fe at the B site of the ABO 3 perovskites dramatically increase the oxygen mobility. A higher oxygen diffusion rate, and hence enrichment of oxygen on the surface, would improve the catalyst selectivity toward total oxidation.

Recent Advances of Oxygen Carriers for Chemical Looping Reforming of Methane

“…examined the reactivity of La 1‐x Sr x FeO 3‐δ (x=0, 0.2, and 0.5) and La 0.5 Sr 0.5 Fe 1‐x Co x O 3‐δ (x=0.5 and 1) perovskites for CLR of CH 4 and found that CO selectivity decreased from 92 % to 9 %, whereas the reducible ability promoted greatly with Sr and Co substitution resulted from Fe 4+ or oxygen vacancies formed. By the combination of experimental results and theoretical calculations, the authors established a quantitative relationship between bulk oxygen (O b ), surface oxygen concentration (O s ) and oxygen mobility which was associated with the relative ratio of the surface reactions rate to the diffusion rate of the lattice oxygen measured by Φ and Φ′values, as shown in Figure 9 [48b] . O s decreased with reduced oxygen mobility (a higher Φ and Φ′values) at the fixed O b thus preferred the partial oxidation and vice versa thus benefited total oxidation of methane.…”

“… Relationship among [O s ], [O b ], Φ and Φ′ at (a) partial oxidation and (b) Total oxidation dominating [48b] . Reproduced with permission from Ref.…”

Promotional Effect of Manganese on Selective Catalytic Reduction of NO by CO in the Presence of Excess O2 over M@La–Fe/AC (M = Mn, Ce) Catalyst

“…The oxygen in the gas phase can be activated by the surface chemical oxygen on the catalyst's surface, it can facilitate the oxidation of NO to NO 2 and thus accelerating the CO-SCR process [16,[44][45][46]. Chang et al [20] studied the surface properties of LaFeO 3 perovskite catalysts. They reported that the peak with the lowest binding energy between 527 and 531 eV was attributed to the lattice O species, whereas the chemisorbed O or OH − or hydroxides species, and particularly adsorbed water species were observed at the highest binding energies between 530 and 536 eV.…”

“…However, the mono iron catalyst shows weak catalytic activity for NO removal at low temperatures [19]. Chang et al [20] and Mihai et al [21] reported that the concentration of oxygen vacancies on the LaFeO 3 perovskite catalyst's surface is strongly related to the bulk oxygen concentration and the relative diffusion rate of the lattice oxygen with respect to the surface reaction. They found that the partial replacement of La with Sr at the A-site, and partial replacement of Fe with Co or Mn at the B-site of the ABO 3 perovskite, caused an increase in the surface oxygen vacancies and oxygen mobility, which can affect the catalytic activity.…”

Effect of dz2 orbital electron-distribution of La-based inorganic perovskites on surface kinetics of a model reaction

“…28,29 The ABO 3 perovskites has been reported to catalyse oxidation reactions due to their unique properties such as flexibility of crystal structure and tuneable electronic properties. 30 These include use as supports for active metal species, substitution of either the A or B site of the ABO 3 perovskites. The La-based inorganic perovskites are inexpensive and readily available.…”