Constructing La₂B₂O₇ (B = Ti, Zr, Ce) Compounds with Three Typical Crystalline Phases for the Oxidative Coupling of Methane: The Effect of Phase Structures, Superoxide Anions, and Alkalinity on the Reactivity

Abstract

To probe the phase structure–reactivity relationship of A2B2O7 catalysts for the oxidative coupling of methane (OCM), three model La2B2O7 compounds with Ti4+, Zr4+, or Ce4+ at the B-site have been purposely designed. By decreasing the r A/r B ratios in the order of La2Ti2O7 > La2Zr2O7 > La2Ce2O7, typical monoclinic layered perovskite, cubic ordered pyrochlore, and disordered defective cubic fluorite phase are formed, respectively. The reaction performance of the catalysts based on CH4 conversion and C2 product yield follow the order of La2Ce2O7 > La2Zr2O7 > La2Ti2O7. It has been discovered that superoxide O2 – is the active oxygen species detected on all the catalysts and is responsible for the OCM reaction, whose amount follows also the sequence of La2Ce2O7 > La2Zr2O7 > La2Ti2O7. Moreover, the surface alkalinity related to the superoxide anions observes the same order. This testifies that the amount of surface superoxide O2 – determines the OCM reaction performance over the La2B2O7 compounds. On the basis of the characterization results, the formation of active O2 – species could follow two pathways. For La2Zr2O7 and La2Ce2O7 possessing intrinsic 8a oxygen vacancies, O2 – anions are formed by activating the oxygen species entering into the vacancies in the bulk and then migrating to the catalyst surface. For La2Ti2O7 possessing no oxygen vacancies, they are formed directly by transforming the O2 molecules adsorbed on its surface. Usually, the former pathway generates more abundant O2 – species than the latter one. La2Ce2O7 displays not only promising reaction performance in the low-temperature region, but also potent sulfur and lead poisoning resistance, thus having the potential for application after further optimization.

Study on the Structure–Reactivity Relationship of LnMn2O5 (Ln = La, Pr, Sm, Y) Mullite Catalysts for Soot Combustion

“…Interestingly, starting from PrMn 2 O 5 , shoulder reduction peaks below 300 °C (labeled as α group) become evident, especially for SmMn 2 O 5 and YMn 2 O 5 . The oxygen anions corresponding to this group of peak could be ascribed to surface deficient oxygen species induced by the surface defects/ vacancies, which is widely observed in many metal oxide catalysts [26]. In addition, it is obviously very reactive, and could be critical to soot combustion [27].…”

High Catalytic Activity of Crystalline Lithium Calcium Silicate for Oxidative Coupling of Methane Originated from Crystallographic Joint Effects of Multiple Cations

“…Single‐phase catalysts are believed to provide well‐defined active sites and thereby advantageous to mechanistic studies. Except for a few reports, such an exploring strategy has scarcely been implemented for novel OCM catalysts. Our recent research on various crystalline oxides revealed a significantly high OCM activity of a quasi‐ternary oxide Li 2 CaSiO 4 (Table ).…”

Stable CuO/La2Sn2O7 catalysts for soot combustion: Study on the monolayer dispersion behavior of CuO over a La2Sn2O7 pyrochlore support

“…A2B2O7 pyrochlore compounds are important functional materials that have been applied in gas sensors [30], ferroelectric materials [31], and catalysts [32,33]. In particular, these compounds possess excellent thermal stability and excellent oxygen anion conductivity, which are vital features of redox catalysts for exothermic processes [34,35]. Moreover, in the unit cell of pyrochlore compounds, eight inherent 8a oxygen vacancies are present, which results in excellent lattice oxygen mobility, especially when the radii of the A-and B-site ions are similar [35,36].…”

“…In particular, these compounds possess excellent thermal stability and excellent oxygen anion conductivity, which are vital features of redox catalysts for exothermic processes [34,35]. Moreover, in the unit cell of pyrochlore compounds, eight inherent 8a oxygen vacancies are present, which results in excellent lattice oxygen mobility, especially when the radii of the A-and B-site ions are similar [35,36]. As a consequence, active O2 -and O2 2-anions at the surface are generated, which is favorable for many redox reactions, as demonstrated in our previous publications [35,37].…”

“…Moreover, in the unit cell of pyrochlore compounds, eight inherent 8a oxygen vacancies are present, which results in excellent lattice oxygen mobility, especially when the radii of the A-and B-site ions are similar [35,36]. As a consequence, active O2 -and O2 2-anions at the surface are generated, which is favorable for many redox reactions, as demonstrated in our previous publications [35,37]. In particular, we have shown that supporting Pd onto a high-surface-area La2Sn2O7 pyrochlore results in catalysts with better activities than those of Pd/Al2O3 and Pd/SnO2 [38].…”