Oxidative coupling of methane by pyrochlore oxide A2B2O7 (A = rare earth, B = Ti, Zr, Sn). Relation between C2 selectivity and BO bond energy

et al. 2018

Eur J Inorg Chem

To optimize the reaction performance of La2Ce2O7 for OCM, catalysts with varied La/Ce molar ratios and doped by Ca additive have been synthesized and characterized by different techniques. It is discovered that a La2Ce2O7 compound having a disordered cubic defective fluorite phase is predominantly formed in all the samples. Varying the La/Ce ratio influences the fine crystalline structure of the catalysts. As a result, the abundance of the surface facile oxygen and alkaline sites increase by increasing the La/Ce ratio. Moreover, the Ca additive exists on the surface of Ca0.5La2Ce2O7 and Ca0.5La1.5Ce2O7 as dispersed CaCO3, which covers the active sites and is harmful to the OCM reaction. However, for La2Ce1.5Ca0.5O7, the major part of the Ca additive has entered into the lattice of La2Ce2O7 phase as Ca2+ cations, thus creating more oxygen vacancies and improving the surface alkalinity of the catalyst, which is favorable to the OCM reaction. Both the active surface oxygen and alkaline sites are vital for OCM, and their synergism determines the reactivity. By increasing La/Ce ratio and doping Ca2+ into La2Ce2O7 lattice, the quantities of both kinds of active sites can be significantly improved, thus obtaining catalysts with much improved reaction performance. La2Ce1.5Ca0.5O7 owns the largest amount of both types of active sites, hence depicting the best reaction performance, over which the highest C2 yield of 22.5 % can be achieved even at 750 °C.

Section: Introductionmentioning

confidence: 99%

Optimizing the Reaction Performance of La₂Ce₂O₇‐Based Catalysts for Oxidative Coupling of Methane (OCM) at Lower Temperature by Lattice Doping with Ca Cations

Zhang

et al. 2018

Eur J Inorg Chem

“…Pyrochlore (A 2 B 2 O 7 ) catalysts are promising catalysts for OCM reaction due to their high melting points, thermal stability, tunable M-O bonding, surface alkalinity, and oxygen vacancies [77][78][79][80][81][82]. The crystal structure of A 2 B 2 O 7 compounds can be tuned with the ionic radius ratio of r A and r B sites cations.…”

Section: Pyrochlore Catalystsmentioning

confidence: 99%

Oxidative Coupling of Methane: Perspective for High-Value C2 Chemicals

Murthy

et al. 2021

Crystals

The oxidative coupling of methane (OCM) to C2 hydrocarbons (C2H4 and C2H6) has aroused worldwide interest over the past decade due to the rise of vast new shale gas resources. However, obtaining higher C2 selectivity can be very challenging in a typical OCM process in the presence of easily oxidized products such as C2H4 and C2H6. Regarding this, different types of catalysts have been studied to achieve desirable C2 yields. In this review, we briefly presented three typical types of catalysts such as alkali/alkaline earth metal doped/supported on metal oxide catalysts (mainly for Li doped/supported catalysts), modified transition metal oxide catalysts, and pyrochlore catalysts for OCM and highlighted the features that play key roles in the OCM reactions such as active oxygen species, the mobility of the lattice oxygen and surface alkalinity of the catalysts. In particular, we focused on the pyrochlore (A2B2O7) materials because of their promising properties such as high melting points, thermal stability, surface alkalinity and tunable M-O bonding for OCM reaction.

“…Therefore, in this work, with the purpose of exploring the dispersion behaviour of composite metal oxides on solid support surfaces, and with the expecta-tion of obtaining improved supports for Pd for CO oxidation by combining the special properties of pyrochlore Y 2 Sn 2 O 7 and g-Al 2 O 3 ,m onolayerd ispersion of Y 2 Sn 2 O 7 on g-Al 2 O 3 support was investigated. Y 2 Sn 2 O 7 with pyrochlore structure shows many special properties,s uch as relatively high melting point, [7] abundant oxygen vacancies, [8] hight hermals tability [9] and good catalytic activity. [10][11][12] Moreover,w er ecently demonstrated that La 2 Zr 2 O 7 and La 2 Sn 2 O 7 pyrochlores are superior supports for Ni for methane steam reforming [13] and for Pd for CO oxidation, [14] respectively.H owever, pyrochlore compounds obtained by the conventional coprecipitation method generally have low surface areas, due to the high temperatures required to form the pyrochlore structure, and thust heir use as catalysts or catalyst supports is limited.…”

Section: Introductionmentioning

confidence: 99%

“…Y 2 Sn 2 O 7 with pyrochlore structure shows many special properties, such as relatively high melting point, abundant oxygen vacancies, high thermal stability and good catalytic activity . Moreover, we recently demonstrated that La 2 Zr 2 O 7 and La 2 Sn 2 O 7 pyrochlores are superior supports for Ni for methane steam reforming and for Pd for CO oxidation, respectively.…”

Section: Introductionmentioning

confidence: 99%

Modifying the Surface of γ‐Al₂O₃with Y₂Sn₂O₇Pyrochlore: Monolayer Dispersion Behaviour of Composite Oxides

Tian

et al. 2017

ChemPhysChem

To investigate the dispersion behaviour of composite oxides on supports, and to obtain better supports for Pd for CO oxidation, a series of Y Sn O /Al O composite oxides with different Y Sn O loadings were prepared by a deposition-precipitation method. XRD and X-ray photoelectron spectroscopic extrapolation methods revealed that, similar to single-component metal oxides, composite oxides can also disperse spontaneously on support surfaces to form a monolayer with a certain capacity. The monolayer dispersion capacity/threshold for Y Sn O on the surface of γ-Al O is 0.109 mmol per 100 m γ-Al O , corresponding to 7.2 wt % Y Sn O loading. This is the first work to demonstrate monolayer dispersion of a composite oxide on a support. After combining Y Sn O with γ-Al O , active oxygen species can be introduced onto the catalyst surfaces. Thus, the interaction between Pd and the support is strengthened, the dispersion of Pd is improved in comparison with the single-component Y Sn O support, and a synergistic effect is induced between Pd and the composite support, which is beneficial to catalyst activity. By tuning the γ-Al O surface with different amounts of pyrochlore Y Sn O , CO oxidation activity on 1 % Pd/Y Sn O /Al O was improved. These findings may provide new insights into the design and preparation of effective supported noble metal catalysts with lower contents of noble metals.