A study on active sites of A2BO4 catalysts with perovskite-like structures in oxidative coupling of methane

Oxidative coupling of methane to ethane

Selective oxygen species for the oxidative coupling of methane

In this study, we aimed to enhance the catalytic activity of perovskite catalysts and elucidate their catalytic behavior in the oxidative coupling of methane (OCM), using alkali-added LaAlO3 perovskite catalysts. We prepared LaAlO3_XY (X = Li, Na, K, Y = mol %) catalysts and applied them to the OCM reaction. The results showed that the alkali-added catalysts’ activities were promoted compared to the LaAlO3 catalyst. In this reaction, ethane was first synthesized through the dimerization of methyl radicals, which were produced from the reaction of methane and oxygen vacancy in the perovskite catalysts. The high ethylene selectivity of the alkali-added catalysts originated from their abundance of electrophilic lattice oxygen species, facilitating the selective formation of C2 hydrocarbons from ethane. The high COx (carbon monoxide and carbon dioxide) selectivity of the LaAlO3 catalyst originated from its abundance of nucleophilic lattice oxygen species, favoring the selective production of COx from ethane. We concluded that electrophilic lattice oxygen species play a significant role in producing ethylene. We obtained that alkali-adding could be an effective method for improving the catalytic activity of perovskite catalysts in the OCM reaction.

Preparation of LaAlO3 perovskite catalysts by simple solid-state method for oxidative coupling of methane

The catalytic oxidative coupling of methane (OCM), which can be used to obtain ethylene, is a major challenge in heterogeneous catalysis. This chapter mainly discusses the active sites of OCM catalysts, their reaction mechanisms, and their catalytic performance under various oxidative reaction conditions, including the OCM reaction network. In the OCM reaction, CH4 is oxidatively converted to C2H6 and then C2H4. After activation of CH4 on catalysts such as metal oxides, the formation of C2H6 proceeds in a homogenous gas phase via a free-radical mechanism. Thus, C2H6 is produced mainly by the coupling of the surface-generated •CH3 radical (methyl radical) in the gas phase. The C2H4/C2H6 yields are limited by the secondary reaction of •CH3 radicals with the catalyst and reactor surfaces and the further oxidation of C2H4 on the catalyst surface and in the gas phase. The nature of the active sites and the reaction mechanism have been investigated. Reactive oxygen ions, such as O− or O22−, are required for the activation of methane on catalysts. However, no feasible processes have resulted, despite a reasonable understanding of the elementary reactions in the OCM reaction. The non-oxidative coupling of methane (dehydrogenative coupling of methane) gives C2H4 and aromatic hydrocarbons at ~1000 K. Although the dehydrogenative coupling of methane is thermodynamically disadvantageous due to the large positive change in free energy, over-oxidation does not occur, and CO and CO2 are not formed. The catalytic performance of supported Fe catalysts, such as SiO2-supported Fe, are discussed, along with their catalytic properties.

A K2NiF4-type La2Li0.5Al0.5O4 catalyst for the oxidative coupling of methane (OCM)

We examine the influence of precursor functional groups on the formation and electrocatalytic performance of iron ion-chelating ordered mesoporous carbon (Fe-OMC) fuel cell catalysts. First, we study whether the active sites in these catalysts consist of Fe–Nx or Fe–Ox chelates. To verify this, catalysts were prepared from two different molecular precursors (furfurylamine and furfuryl alcohol) and the functional groups’ (−NH2 vs −OH) role in the formation of iron ion-chelating active sites in the catalysts was established. From electrochemical tests and EPR spectroscopy, conspicuously different behaviors were obtained for the catalyst prepared from furfurylamine compared to that prepared from furfuryl alcohol. It was unambiguously established that the amine group is central to the formation of electrocatalytically active sites in Fe-OMC catalysts and that these are of the Fe–Nx-OMC type. Additional Fe-OMC catalysts were prepared with the purpose to determine the influence of the two precursors on the formation of the carbon matrix. By complementing the furfurylamine with the more readily polymerizing furfuryl alcohol and using a mixture of the two as precursor solution in the synthesis, an overall improvement over the pure furfurylamine was achieved. The mixture gave a catalyst with a larger pore volume and surface area, a higher conductivity, and a higher oxygen conversion rate.

The catalytic mechanism and the nature of active sites are revealed for the oxygen reduction reaction (ORR) with new non-noble-metal nitrogen-doped carbon-supported transition-metal catalysts (metal-N-C catalyst). Specifically, new nitrogen-doped carbon-supported cobalt catalysts (Co-N-C catalysts) are made by pyrolyzing various ratios of the nitrogen-atom rich heterocycle compound, 1-ethyl-3-methyl imidazolium dicyanamide (EMIM-dca) and cobalt salt (Co(NO3)2). The ORR activity (JK at 0.8 V vs RHE, in 0.1 M KOH solution) of a typical catalyst in this family, Co15-N-C800, is 8.25 mA/mg, which is much higher than the ORR activity values of N-C catalysts (0.41 mA/mg). The active site in the catalyst is found to be the Co-N species, which is most likely in the form of Co2N. Metallic cobalt (Co) particles, Co3C species, and N-C species are not catalytically active sites, nor do these moieties interact with the Co-N active sites during the catalysis of the ORR. Increasing the Co salt content during the synthesis favors the formation of Co-N active sites in the final catalyst. Higher pyrolysis temperatures (e.g., a temperature higher than 800 °C) do not favor the formation of the Co-N active sites, but cause the formed Co-N active sites to decompose, which, therefore, leads to a lower catalytic activity. This reveals that the control of the parameters that affect the final structure is critical to catalyst performance and, therefore, the effective development of high-performance heteroatom-doped non-noble-metal ORR catalysts.

The formation of active sites in the molybdenum-zeolite catalyst for methane dehydroaromatization was studied by the density functional theory method. The interaction of MoO2(OH)2 particle with the Bronsted site, anionic site, and electron hole of the zeolite was studied. The mechanism governing the formation of mononuclear active sites was proposed. It was shown that the formation of the MoO2 mononuclear active site with participation of electron hole of the zeolite is thermodynamically possible and is accompanied by electron density transfer from zeolite oxygen atom to molybdenum atom.

Study on the active site structure of MgO catalysts for oxidative coupling of methane

Elucidating the effect of barium halide promoters on La2O3/CaO catalyst for oxidative coupling of methane

Alkaline-earth metal oxides with the rocksalt structure, which are simple ionic solids, have attracted attention in attempts to gain fundamental insights into the properties of metal oxides. The surfaces of alkaline-earth metal oxides are considered promising catalysts for the oxidative coupling of methane (OCM); however, the development of such catalysts remains a central research topic. In this paper, we performed first-principles calculations to investigate the ability of four alkaline-earth metal oxides (MgO, CaO, SrO, and BaO) to catalyze the OCM. We adopted five types of surfaces of rocksalt phases as research targets: the (100), (110), stepped (100), oxygen-terminated octopolar (111), and metal-terminated octopolar (111) surfaces. We found that the formation energy of surface O vacancies is a good descriptor for the adsorption energy of a H atom and a methyl radical. The energies related to the OCM mechanism show that, compared with the most stable surface, the minor surfaces better promote the C – H bond cleavage of methane. However, as the trade-off for this advantage, the minor surfaces exhibit increased affinity for the methyl radical. On the basis of this trade-off relationship between properties, we identified several surfaces that we expect to be promising OCM catalysts. Our investigation of the temperature dependence of the Gibbs free energy indicated that, at higher temperatures, the step (100) surface exhibits properties that might benefit the OCM mechanism.

Boosting CO2 Conversion with Terminal Alkynes by Molecular Architecture of Graphene Oxide-Supported Ag Nanoparticles

Surface coupling of methyl radicals for efficient low-temperature oxidative coupling of methane

A new approach to the preparation of systems that exhibit catalytic activity in the oxidative coupling of methane (OCM) is considered. With the use of ferrospheres separated from power-generation ashes from different sources as an example, it was demonstrated that OCM catalysts can be prepared by the crystallization/solidification of oxide melts with the formation of microspherical particles. The dependence of activity and selectivity for the oxidative reforming of methane on the ferrospheres containing from 36.2 to 92.5 wt % Fe2O3 into the products of deep oxidation and OCM was studied. It was found that deep oxidation reactions on ferrospheres with Fe2O3 contents higher than 85% were suppressed, and the main reaction path of CH4 conversion was its oxidative coupling with the formation of C2 products (with selectivity to 60% at 750°C); moreover, the selectivity for C2 formation in this region was proportional to the concentration of Fe2O3. Phases responsible for the catalytic conversion of methane into COx and OCM products were considered, and it was shown that the catalytic activity and selectivity of the oxidative transformation of CH4 on ferrospheres is determined by the position of the point that corresponds to their composition on a phase diagram of CaO-Fe2O3-SiO2.