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Improving the selectivity in the oxidative coupling of methane to ethane/ethylene poses a significant challenge for commercialization. The required improvements are hampered by the uncertainties associated with the reaction mechanism due to its complexity. Herein, we report about 90 % selectivity to the target products at 11 % methane conversion over Gd2O3‐based catalysts at 700 °C using N2O as the oxidant. Sophisticated kinetic studies have suggested the nature of adsorbed oxygen species and their binding strength as key parameters for undesired methane oxidation to carbon oxides. These descriptors can be controlled by a metal oxide promoter for Gd2O3. [ABSTRACT FROM AUTHOR]

The present study of oxidative coupling of methane (OCM) over MnOx−Na2WO4/support catalysts demonstrated that the selectivity to C2H6 and C2H4 (C2‐hydrocarbons) is affected by the kind of support, co‐fed water, and the kind of oxidant (O2 vs. N2O). In addition to previous studies with MnOx−Na2WO4/SiO2, an enhancing water effect was obtained using catalysts based on TiO2‐ or ZrO2‐containing supports. However, a negative effect on methane conversion was established for SiO2−Al2O3‐supported catalysts. Temporal analysis of products with isotopic tracers suggests that the ability of MnOx−Na2WO4 to generate diatomic adsorbed oxygen species depends on the kind of support and is the key property for the water effect. The strength of the water effect on the activity decreases with an increase in the surface area of working catalysts. The kind of support also affects products selectivity due to its influence on the mobility/releasability of lattice oxygen in supported MnOx−Na2WO4. Among the prepared catalysts, MnOx−Na2WO4/TiO2 was found to be promising for H2O‐assisted OCM. The use of N2O instead of O2 further increases the selectivity to C2‐hydrocarbons to 84 % at 6.8 % CH4 conversion due to the formation of predominantly monoatomic oxygen species from N2O that selectively convert CH4 into C2H6. [ABSTRACT FROM AUTHOR]

[Display omitted] • Alkali metal in MnO x -M 2 WO 4 /SiO 2 affects activity under anaerobic conditions. • Lattice oxygen of MnO x -M 2 WO 4 /SiO 2 can convert CH 4 to hydrocarbons. • OCM over MnO x -M 2 WO 4 /SiO 2 and M 2 WO 4 /SiO 2 proceeds via adsorbed oxygen species. • Presence of MnO x does not create more active sites but changes their reactivity. This work unveils the fundamentals relevant for activity and product selectivity in the oxidative coupling of methane (OCM) over the (MnO x)-M 2 WO 4 /SiO 2 (M = Na, K, Rb, or Cs) catalysts. The presence of the molten phase of Na 2 WO 4 and the ability of the catalysts to release lattice oxygen were not found to be the performance-governing factors. As proven by oxygen isotopic exchange experiments combined with thorough catalytic tests, the performance of both tri- and bimetallic catalysts is defined by the involvement of adsorbed oxygen species formed from gas-phase O 2. Lattice oxygen of M 2 WO 4 /SiO 2 is not capable of converting CH 4 to C 2 H 6 , while that of MnO x -M 2 WO 4 /SiO 2 is active for this reaction. The OCM activity of M 2 WO 4 /SiO 2 decreases with the atomic weight of alkali metal, whereas there is no such effect in the presence of MnO x , which, however, increases the activity. The enhancing effect was explained by improving the active site turnover. [ABSTRACT FROM AUTHOR]

Alkalimetallpromotoren werden in großem Umfang für die Herstellung heterogener Katalysatoren eingesetzt, die in vielen industriell wichtigen Reaktionen verwendet werden. Die Ursachen für ihre Wirkung sind jedoch in der Regel nur schwer zu ergründen. In dieser Arbeit werden die mechanistischen und kinetischen Aspekte der Wirkung von Alkalimetallen bei der CO2‐Hydrierung an Fe‐haltigen Katalysatoren mithilfe modernster Charakterisierungstechniken sowie räumlich aufgelöster stationärer und instationärer kinetischer Analysen aufgeklärt. Die Promotoren beeinflussen die elektronischen Eigenschaften von Eisen in Eisenkarbiden. Diese Karbideigenschaften bestimmen die Fähigkeit des Katalysators, H2, CO und CO2 zu aktivieren. Die Elektronegativität des Alkalimetallpromoters nach der Allen‐Skala wurde erfolgreich mit der Reaktionsgeschwindigkeit der CO2‐Hydrierung zu höheren Kohlenwasserstoffen und CH4 sowie mit den Geschwindigkeitskonstanten der einzelnen Schritte der CO‐ oder CO2‐Aktivierung korreliert. Die gewonnenen Erkenntnisse können für die Entwicklung und Herstellung von Katalysatoren für andere Reaktionen, bei welchen derartige Promotoren ebenfalls verwendet werden, von Nutzen sein. [ABSTRACT FROM AUTHOR]

The article titled "Elucidating the Role of Oxygen Species in Oxidative Coupling of Methane over Supported MnOx-Na2WO4-containing Catalysts" in the journal ChemCatChem explores the main products of the oxidative coupling of methane reaction over different catalysts. The researchers found that a low catalyst surface area and low reactivity of support lattice oxygen are beneficial for achieving high selectivity to C2H6 and C2H4. The TiO2 support meets these requirements, while Al2O3 and ZrO2 supports predominantly produce CO or CO2. The selectivity to desired products over TiO2-supported catalysts can be improved by replacing O2 with N2O and/or co-feeding H2O to reduce the concentration of adsorbed diatomic oxygen species. [Extracted from the article]

The simple deposition of tailored Rh or Pt nanoparticles (NP) on γ-Al2O3 results in active, selective, and stable catalysts for partial oxidation of methane (POM) to syngas. The NP were prepared by the ethylene glycol method in strong alkaline solution. This approach was found to be a promising way of providing active metallic NP for catalyzing the POM reaction. NP sizes determined by small angle X-ray scattering (SAXS) in solution and by transmission electron microscopy (TEM) on alumina were very close. This result highlights the possibility of easy pre-characterization of NP by the former method. Supported Rh NP are intrinsically more active in the POM reaction than Pt NP and also showed a superior performance compared with a conventionally prepared Rh catalyst, even if the latter had been pre-reduced. Mechanistic investigations in the temporal analysis of products (TAP) reactor indicate that the higher selectivity of well-defined Rh-NP is determined by their lower activity for consecutive CO transformations. [ABSTRACT FROM AUTHOR]

Abstract: In order to identify factors governing selectivity of an oxygen-conducting perovskite BaCo x Fe y Zr z O3−δ (BCFZ) membrane in the partial oxidation of methane and ethane, mechanistic aspects of product formation in these reactions were investigated with a millisecond time resolution using the temporal analysis of products (TAP) reactor. It was found that the selectivity depends on: (i) reduction degree of the perovskite surface; the higher the reduction degree, the higher the ethane and ethylene selectivity in methane and ethane oxidation, respectively, and (ii) contact time; short contact times favor partial selective oxidation. The influence of contact time on the ethylene selectivity in ethane oxidation at degrees of ethane conversion above 85% was experimentally proven in hollow fiber and disk membranes, which differ in the contact times. The low activity and selectivity in methane oxidation in the BCFZ perovskite membrane reactor were significantly increased, when the membrane on the hydrocarbon side was coated by a Ni-based steam reforming catalyst. This catalyst fulfils a double role: (i) it increases the oxygen transport through the perovskite membrane due to the high oxygen consumption, and (ii) it accelerates syngas production via deep methane oxidation followed by dry and steam reforming of methane. The syngas selectivity increases with an increase in the catalyst reduction degree, which is determined by the ratio of the rate of methane oxidation to the rate of oxygen permeation through the membrane. [Copyright &y& Elsevier]