Critical Surface Parameters for the Oxidative Coupling of Methane over the Mn-Na-W/SiO2 Catalyst

Oxidative Addition of Methane and Reductive Elimination of Ethane and Hydrogen on Surfaces: From Pure Metals to Single Atom Alloys

Oxidative addition of CH₄ to the catalyst surface produces CH₃ and H. If the CH₃ species generated on the surface couple with each other, reductive elimination of C₂H₆ may be achieved. Similarly, H's could couple to form H₂. This is the outline of nonoxidative coupling of methane (NOCM). It is difficult to achieve this reaction on a typical Pt catalyst surface. This is because methane is overoxidized and coking occurs. In this study, the authors approach this problem from a molecular aspect, relying on organometallic or complex chemistry concepts. Diagrams obtained by extending the concepts of the Walsh diagram to surface reactions are used extensively. C-H bond activation, i.e., oxidative addition, and C-C and H-H bond formation, i.e., reductive elimination, on metal catalyst surfaces are thoroughly discussed from the point of view of orbital theory. The density functional theory method for structural optimization and accurate energy calculations and the extended Hückel method for detailed analysis of crystal orbital changes and interactions play complementary roles. Limitations of monometallic catalysts are noted. Therefore, a rational design of single atom alloy (SAA) catalysts is attempted. As a result, the effectiveness of the Pt₁/Au(111) SAA catalyst for NOCM is theoretically proposed. On such an SAA surface, one would expect to find a single Pt monatomic site in a sea of inert Au atoms. This is desirable for both inhibiting overoxidation and promoting reductive elimination.

Oxidative coupling of methane-comparisons of MnTiO3-Na2WO4 and MnOx-TiO2-Na2WO4 catalysts on different silica supports

The oxidative coupling of methane (OCM) converts CH₄ to value-added chemicals (C₂₊), such as olefins and paraffin. For a series of MnTiO₃-Na₂WO₄ (MnTiO₃-NW) and MnO_x-TiO₂-Na₂WO₄ (Mn-Ti-NW), the effect of loading of MnTiO₃ or MnO_x-TiO₂, respectively, on two different supports (sol-gel SiO₂ (SG) and commercial fumed SiO₂ (CS)) was examined. The catalyst with the highest C₂₊ yield (21.6% with 60.8% C₂₊ selectivity and 35.6% CH₄ conversion) was 10 wt% MnTiO₃-NW/SG with an olefins/paraffin ratio of 2.2. The catalyst surfaces with low oxygen-binding energies were associated with high CH₄ conversion. Stability tests conducted for over 24 h revealed that SG-supported catalysts were more durable than those on CS because the active phase (especially Na₂WO₄) was more stable in SG than in CS. With the use of SG, the activity of MnTiO₃-NW was not substantially different from that of Mn-Ti-NW, especially at high metal loading.

Nonoxidative Coupling of Methane over Ceria-Supported Single-Atom Pt Catalysts in DBD Plasma

Plasma-catalytic direct nonoxidative coupling of methane (NCM) into C₂ hydrocarbons was investigated over ceria-supported atomically dispersed Pt (Pt/CeO₂-SAC) and nanoparticle Pt (Pt/CeO₂-NP) catalysts in dielectric barrier discharge (DBD) plasma. Nonthermal plasma facilitated C-H bond dissociation in CH₄ at low temperatures (<150 °C) and atmospheric pressure. The presence of Pt/CeO₂ catalysts in plasma further enhanced CH₄ conversion and C₂ hydrocarbon selectivity by enabling the conversion of vibrationally excited methane species with high internal energy on active Pt sites. Noticeably, the Pt/CeO₂-SAC catalyst displayed a more remarkable performance, with a CH₄ conversion of 39% and a C₂ selectivity of 54% at 54 W. The enhanced CH₄ conversion was attributed to abundant coordinatively unsaturated Pt sites in Pt/CeO₂-SAC, which were more active for C-H bond scission. Meanwhile, isolated Pt atoms in Pt/CeO₂-SAC promoted C₂ hydrocarbon formation by hindering the unselective formation of coke from deep dehydrogenation of CH_x^• intermediates and higher hydrocarbons from oligomerization reactions.

Oxidative Coupling of Methane for Ethylene Production: Reviewing Kinetic Modelling Approaches, Thermodynamics and Catalysts

Ethylene production via oxidative coupling of methane (OCM) represents an interesting route for natural gas upscaling, being the focus of intensive research worldwide. Here, OCM developments are analysed in terms of kinetic mechanisms and respective applications in chemical reactor models, discussing current challenges and directions for further developments. Furthermore, some thermodynamic aspects of the OCM reactions are also revised, providing achievable olefins yields in a wide range of operational reaction conditions. Finally, OCM catalysts are reviewed in terms of respective catalytic performances and thermal stability, providing an executive summary for future studies on OCM economic feasibility.

Effect of thermal treatment on the stability of Na-Mn-W/SiO2 catalyst for the oxidative coupling of methane

In this study, we investigate the effect of thermal treatment/calcination on the stability and activity of a Na-Mn-W/SiO₂ catalyst for the oxidative coupling of methane. The catalyst performance and characterisation measurements suggest that the W species are directly involved in the catalyst active site responsible for CH₄ conversion. Under operating conditions, the active components, present in the form of a Na-W-O-Mn molten state, are highly mobile and volatile. By varying the parameters of the calcination protocol, it was shown that these molten components can be partially stabilised, resulting in a catalyst with lower activity (due to loss of surface area) but higher stability even for long duration OCM reaction experiments.

Synthesis of Value-Added Chemicals via Oxidative Coupling of Methanes over Na2WO4-TiO2-MnO x /SiO2 Catalysts with Alkali or Alkali Earth Oxide Additives

Na₂WO₄-TiO₂-MnO _x /SiO₂ (SM) catalysts with alkali (Li, K, Rb, Cs) or alkali earth (Mg, Ca, Sr, Ba) oxide additives, which were prepared using incipient wetness impregnation, were investigated for oxidative coupling of methane (OCM) to value-added hydrocarbons (C₂₊). A screening test that was performed on the catalysts revealed that SM with Sr (SM-Sr) had the highest yield of C₂₊. X-ray photoelectron spectroscopy analyses indicated that the catalysts with a relatively low binding energy of W 4f_7/2 facilitated a high CH₄ conversion. A combination of crystalline MnTiO₃, Mn₂O₃, α-cristobalite, Na₂WO₄, and TiO₂ phases was identified as an essential component for a remarkable improvement in the activity of the catalysts in the OCM reaction. In attempts to optimize the C₂₊ yield, 0.25 wt % Sr onto SM-Sr achieved the highest C₂₊ yield at 22.9% with a 62.5% C₂₊ selectivity and a 36.6% CH₄ conversion. A stability test of the optimal catalyst showed that after 24 h of testing, its activity decreased by 18.7%.

Effects of a Pd/Pt bimetal supported by a γ-Al2O3 surface on methane activation

The catalytic removal of methane (CH₄) in exhaust emissions of natural gas-fueled vehicles is still a major challenge for automotive manufacturers because of the high CH₃-H bond energy and high concentrations of water (H₂O). Density functional theory (DFT) calculations were employed to investigate the adsorption of CH₄ and H₂O, as well as the activation of CH₄, on the surface of a Pd-Pt bimetal supported by γ-Al₂O₃. These are significant factors for catalytic combustion. Pt addition weakened the bonding of the intermediates and increased the availability of electrons on the surface. Besides this, the γ-Al₂O₃ surface and Pt were both beneficial for preventing the aggregation of clusters. CH₄ and H₂O adsorption, as well as the detailed mechanism of CH₄ activation on the Pd-Pt/γ-Al₂O₃ surfaces were simulated. The results showed that a Pt/Pd ratio of three resulted in the best catalytic activity among the different ratios examined in the presence of H₂O.

Synthesis and molecular structure of model silica-supported tungsten oxide catalysts for oxidative coupling of methane (OCM)

Catalysts with only dispersed phase Na–WO₄ sites where Na/W < 2 are slightly less active but significantly more C₂ selective than the traditional Na₂WO₄/SiO₂ catalysts that contain a crystalline phase where Na/W = 2.