Oxidative Coupling of Methane: Examining the Inactivity of the MnOx -Na2 WO4 /SiO2 Catalyst at Low Temperature

Preparation of La2Zr2O7 composite oxides with fluorite/pyrochlore phases by excluding element influences for catalyzing oxidative coupling of methane

To elucidate the effect of the A2B2O7 phase on the oxidative coupling of methane (OCM) while excluding elemental influences, La2Zr2O7 compounds with a disordered defect fluorite (La2Zr2O7-F) structure and an ordered pyrochlore phase (La2Zr2O7-P) have been synthesized. Irrespective of their element composition, the catalytic performance of La2Zr2O7-F exceeds that of La2Zr2O7-P. Furthermore, the La2Zr2O7-F surface has more oxygen vacancies/defects than the La2Zr2O7 surface because La2Zr2O7-F exhibits a higher lattice disorder degree and lower B-O bond strength, which leads to the formation of more reactive oxygen anions (O2- and O22-) and basic sites for OCM. Isotopic exchange results have testified that surface-active oxygen sites are generated due to the gaseous O2 adsorption/activation occurring on the surface vacancies via both simple and multiple hetero-exchange mechanisms. In conclusion, crystal structure is the primary factor that governs the catalytic performance of A2B2O7 compounds, with the disordered defect fluorite phase being the most optimal structure for OCM.

PdCu nanoalloy decorated photocatalysts for efficient and selective oxidative coupling of methane in flow reactors

Methane activation by photocatalysis is one of the promising sustainable technologies for chemical synthesis. However, the current efficiency and stability of the process are moderate. Herein, a PdCu nanoalloy (~2.3 nm) was decorated on TiO2, which works for the efficient, stable, and selective photocatalytic oxidative coupling of methane at room temperature. A high methane conversion rate of 2480 μmol g-1 h-1 to C2 with an apparent quantum efficiency of ~8.4% has been achieved. More importantly, the photocatalyst exhibits the turnover frequency and turnover number of 116 h-1 and 12,642 with respect to PdCu, representing a record among all the photocatalytic processes (λ > 300 nm) operated at room temperature, together with a long stability of over 112 hours. The nanoalloy works as a hole acceptor, in which Pd softens and weakens C-H bond in methane and Cu decreases the adsorption energy of C2 products, leading to the high efficiency and long-time stability.

Fabrication of Ln2Zr2O7 Fluorite and LnAlO3 Perovskite (Ln = La, Nd, Sm) Compounds to Catalyze the OCM Reaction: On the Temperature-Induced Phase Transformation and Oxygen Vacancy

Through synthesizing Ln2Zr2O7 and LnAlO3 (Ln = La, Nd, Sm) catalysts, the origin of active sites for oxidative coupling of methane (OCM) on A2B2O7 fluorite and ABO3 perovskite compounds has been compared and elucidated. Ln2Zr2O7 catalysts show much better reaction performance than the respective LnAlO3 catalysts at low temperatures (500-600 °C), but the difference will be mitigated significantly above 600 °C. The reaction performance ranks in the order of La2Zr2O7 > Nd2Zr2O7 > Sm2Zr2O7 > LaAlO3 > NdAlO3 > SmAlO3. It is revealed that the unit cell free volume (Vf) plays an important role in affecting the catalytic activity, and the Ln2Zr2O7 catalysts with a disordered defect fluorite phase have inherent oxygen vacancies, which can directly activate gas-phase O2 molecules to generate OCM reactive O2- anions. However, the oxygen vacancies of LnAlO3 with a perovskite structure can only be generated by lattice distortion/transformation above 600 °C. Moreover, Ln2Zr2O7 fluorites have weaker B-O bonds than LnAlO3 perovskites, thus making it easier to generate surface vacancies as well as active O2- sites. The surface alkalinity is intimately relevant to the active oxygen species, which act together to decide the OCM performance on both types of catalysts. Indeed, this explains that LnAlO3 catalysts show much worse performance than Ln2Zr2O7 catalysts below 600 °C, which will be evidently improved at elevated temperatures due to phase transformation.

The adsorption of single Au atom and nucleation on γ-Al2O3 surfaces

Single-atom catalysts (SACs) in heterogeneous catalysts have attracted increasing attention and the adsorption and nucleation of single atom on the surface are closely related to the performance of the catalyst. The present work employed density functional theory calculations to examine the adsorption of single Au atom and nucleation on γ-Al₂O₃ surfaces at the atomic level. The effect of surface hydroxyls group on the adsorption and nucleation of single Au atom on γ-Al₂O₃ surfaces is explored. It was found that the spillover reactions of surface hydroxyls H atoms with the deposited Au_- are not available on the hydroxylated surface. The interaction of Au to the clean surface is the stronger than to the hydroxylated surface. The even-odd alternations of Au_x and weak binding of single Au atoms to γ-Al₂O₃ leads to large even-numbered Au cluster on the surface. Density of states and electron density difference analysis show that the electronic structure of Au/γ-Al₂O₃ is quite different from the reported Cu and Pd on Al₂O₃.

Low-Temperature Light-off MnOx-Na2WO4-Based Catalysts: A Step Forward to OCM Process Industrialization

Oxidative coupling of methane (OCM) catalyzed by MnOx-Na2WO4-based catalysts has great industrial potential to convert CH4 directly to C2-3 products, but the high light-off temperature is a big challenge to OCM commercialization. The reaction mechanism studies disclosed that O2/CH4-activation relevant "Mn2+↔Mn3+" redox cycle is tightly linked with the catalyst light-off. One concept is thus put forward that the OCM light-off temperature could be lowered once a "Mn2+↔Mn3+" redox cycle was established to be triggered at low temperature over MnOx-Na2WO4-based catalysts. The relevant studies in recent years are reviewed, showing that the establishment of low-temperature light-off "Mn2+↔Mn3+" redox cycle over the MnOx-Na2WO4-based catalysts indeed works effectively toward a low-temperature light-off OCM process. Moreover, three perspectives for the OCM industrialization are discussed based on this concept, including monolithic catalyst, fluidized-bed method and chemical-looping process.