Monoatomic oxygen species is more selective towards C₂-hydrocarbons in the oxidative coupling of methane reaction over MnO_x−Na₂WO₄/support catalysts. They can be formed when using N₂O as an oxidant or by co-fed H₂O. The requirements for support to ensure high selectivity are to have low specific surface area and low mobility of lattice oxygen.

Abstract

The present study of oxidative coupling of methane (OCM) over MnO_x−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 MnO_x−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 MnO_x−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 MnO_x−Na₂WO₄. Among the prepared catalysts, MnO_x−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.

Rising CO2 levels are leading to an increase in atmospheric greenhouse gas effect. Hydroxide salts have previously been shown to be promising reagents for capturing CO2. Utilizing a 5%Ru/Al2O3 catalyst, the carbonates obtained through CO2 capture can then be hydrogenated to methane. This conversion occurs at relatively mild temperatures from 200°C to 250°C under 40 to 70 bar H2 with yields of up to 100%. Natural sources of calcium carbonate, like eggshells and seashells, can also be partially converted to methane. The direct air CO2 capture and conversion of CO2 to methane was achieved as well in quantitative yields.

Silver electrocatalysts offer the possibility to produce CO by converting CO2, enabling the use of a greenhouse gas as chemical building block. Compared to nanoparticles, silver nanowires show an enhanced selectivity towards CO. Recent publications proved that oxide-derived electrocatalysts can exhibit better catalytic performance than the pristine metal phase, but oxide-derived silver nanowires have not been investigated. In this work, we report for the first time the electrocatalytic properties of silver nanowires, synthesized via the polyol method, and pretreated by electrochemical oxidation in basic electrolyte. By increasing the oxidation potential, both the percentage of AgxO and the surface roughness of the catalyst were progressively increased. The most oxidized sample showed a remarkably improved CO selectivity (‑294.2 mA m‑2Ag), producing a 3.3-fold larger CO partial current density than the pristine sample (‑89.4 mA m‑2Ag), normalized by electrochemically active silver surface area. This work demonstrates the beneficial effect of the controlled oxidation treatment even on highly selective nanostructures such as silver nanowires.