Combined experimental and density functional theory (DFT) studies on the catalyst design for the oxidative coupling of methane

Density Functional Tight-Binding Models for Band Structures of Transition-Metal Alloys and Surfaces across the d-Block

First-principles electronic structure simulations are an invaluable tool for understanding chemical bonding and reactions. While machine-learning models such as interatomic potentials significantly accelerate the exploration of potential energy surfaces, electronic structure information is generally lost. Particularly in the field of heterogeneous catalysis, simulated electron band structures provide fundamental insights into catalytic reactivity. This ab initio knowledge is preserved in semiempirical methods such as density functional tight binding (DFTB), which extend the accessible computational length and time scales beyond first-principles approaches. In this paper we present Shell-Optimized Atomic Confinement (SOAC) DFTB electronic-part-only parametrizations for bulk and surface band structures of all d-block transition metals that enable efficient predictions of electronic descriptors for large structures or high-throughput studies on complex systems outside the computational reach of density functional theory.

Atomistic insights from DFT calculations into the catalytic properties on ceria-lanthanum clusters for methane activation

Improving the catalytic performance of materials based on cerium oxide (CeO2) for the activation of methane (CH4) can be achieved through the following strategies: mixture of CeO2 with different oxides (e.g., CeO2-La2O3) and the use of particles with different sizes. In this study, we present a theoretical investigation of the initial CH4 dehydrogenation on (La2Ce2O7)n clusters, where n = 2, 4, and 6. Our framework relies on density functional theory calculations combined with the unity bond index-quadratic exponential potential approximation. Our results indicate that chemical species arising from the first dehydrogenation of CH4, that is, CH3 and H, bind through the formation of C-O and H-O bonds with the clusters, respectively. The coordination of the adsorption site and the chemical environment plays a crucial role in the magnitude of the adsorption energy; for example, species adsorb more strongly in the low-coordinated topO sites located close to the La atoms. Thus, it affects the activation energy barrier, which tends to be lower in configurations where the adsorption of the chemical species is stronger. During CH4 dehydrogenation, the CH3 radical can be present in a planar or tetrahedral configuration. Its conformation changes as a function of the charge transference between the molecule and the cluster, which depends on the CH3-cluster distance. Finally, we analyze the effects of the Hubbard effective parameter (Ueff) on adsorption properties, as the magnitude of localization of Ce f-states affects the hybridization of the interaction between the molecule and the clusters and hence the magnitude of the adsorption energies. We obtained a linear decrease in the adsorption energies by increasing the Ueff parameter; however, the activation energy is only slightly affected.

Sr2TiO4 Prepared Using Mechanochemical Activation: Influence of the Initial Compounds’ Nature on Formation, Structural and Catalytic Properties in Oxidative Coupling of Methane

Methane oxidative coupling (OCM) is considered a potential direct route to produce C2 hydrocarbons. Layered perovskite-like Sr2TiO4 is a promising OCM catalyst. Mechanochemical activation (MA) is known to be an environmentally friendly method for perovskite synthesis. Sr2TiO4 were synthesized using MA of the mixtures containing SrCO3 or SrO and TiO2 or TiO(OH)2 and annealing at 900 and 1100 °C. XRD and FT-IRS showed that MA leads to the starting component disordering and formation of SrTiO3 only for SrO being pronounced when using TiO(OH)2. After annealing at 900 °C, Sr2TiO4 was mainly produced from the mixtures of SrCO3 or SrO and TiO(OH)2. The single-phase Sr2TiO4 was only obtained from MA products containing SrCO3 after calcination at 1100 °C. The surface enrichment with Sr was observed by XPS for all samples annealed at 1100 °C depending on the MA product composition. The OCM activity of the samples correlated with the surface Sr concentration and the ratio of the surface oxygen amount in SrO and perovskite (Oo/Op). The maximal CH4 conversion and C2 yield (25.6 and 15.5% at 900 °C, respectively), and the high long-term stability were observed for the sample obtained from (SrCO3 + TiO2), showing the specific surface morphology and optimal values of the surface Sr concentration and Oo/Op ratio.

Synthesis of Heterogeneous Catalysts in Catalyst Informatics to Bridge Experiment and High-Throughput Calculation

The coupling of high-throughput calculations with catalyst informatics is proposed as an alternative way to design heterogeneous catalysts. High-throughput first-principles calculations for the oxidative coupling of methane (OCM) reaction are designed and performed where 1972 catalyst surface planes for the CH₄ to CH₃ reaction are calculated. Several catalysts for the OCM reaction are designed based on key elements that are unveiled via data visualization and network analysis. Among the designed catalysts, several active catalysts such as CoAg/TiO₂, Mg/BaO, and Ti/BaO are found to result in high C₂ yield. Results illustrate that designing catalysts using high-throughput calculations is achievable in principle if appropriate trends and patterns within the data generated via high-throughput calculations are identified. Thus, high-throughput calculations in combination with catalyst informatics offer a potential alternative method for catalyst design.

Deep Insight into the Mechanism of Catalytic Combustion of CO and CH4 over SrTi1-xBxO3 (B = Co, Fe, Mn, Ni, and Cu) Perovskite via Flame Spray Pyrolysis

Perovskites have been recognized as affordable substitutes for noble-metal catalysts for their tunable catalytic activity and thermal stability. Nevertheless, the highly demanding synthesis procedure still hinders the application of perovskites in catalytic combustion. In this work, a series of nanostructured SiTiO3 perovskites with B-site partial substitution by Co, Fe, Mn, Ni, and Cu are synthesized via flame spray pyrolysis in one step. The comprehensive characterizations on textural properties of nanostructured perovskites reveal that the flame-made perovskite nanoparticles all exhibit high crystal purity and large specific surface area (∼40 m2/g). Furthermore, the highest catalytic activity is achieved by SrTi0.5Co0.5O3 due to the formation of favorable oxygen vacancies, outstanding reducibility, and oxygen desorption capability. Additionally, the presence of 10 vol % water vapor during long-term testing indicates remarkable durability and water resistance. Finally, the CO oxidation and CH4 dehydrogenation on SrTiO3 incorporating Co atoms are more thermodynamically and kinetically favorable than those on other doped surfaces.

Na2WO4/Mn/SiO2 Catalyst Pellets for Upgrading H2S-Containing Biogas via the Oxidative Coupling of Methane

Biogas is a promising renewable energy source; however, it needs to be upgraded to increase its low calorific value. In this study, oxidative coupling of methane (OCM) was selected to convert it to a higher fuel standard. Prior to establishing the scaled-up OCM process, the effect of organic/inorganic binders on catalytic activity was examined. The selection of the binders and composition of the catalyst pellet influenced the pore structure, fracture strength, and catalytic activity of the catalyst pellets. It was also observed that the O2 supply from the inorganic binder is a key factor in determining catalytic activity, based on which the composition of the catalyst pellets was optimized. The higher heating value increased from 39.9 (CH4, Wobbe index = 53.5 MJ/Nm3) to 41.0 MJ/Nm3 (OCM product mixture, Wobbe index = 54.2 MJ/Nm3), achieving the fuel standard prescribed in many countries (Wobbe index = 45.5–55.0 MJ/Nm3). The reaction parameters (temperature, gas hourly space velocity, size of the reaction system, and the CH4/O2 ratio) were also optimized, followed by a sensitivity analysis. Furthermore, the catalyst was stable for a long-term (100 h) operation under the optimized conditions.

SiO2@MnO x @Na2WO4@SiO2 core-shell-derived catalyst for oxidative coupling of methane

SiO₂@MnO_x@Na₂WO₄@SiO₂ core–shell catalysts were prepared and their fabrication was confirmed using transmission electron microscopy. The formation of Mn-based nanosheets on the silica spheres is important for the deposition of nanoscopic Na₂WO₄. The SiO₂@MnO_x@Na₂WO₄@SiO₂ core–shell catalysts were used for the oxidative coupling of methane at a temperature of 700–800 °C at which the nanostructures were completely destroyed. Although the core–shell structures did not survive the high-temperature oxidative coupling of methane, the selective production of olefins and paraffins can be attributed to highly dispersed Na₂WO₄ derived from confined core–shell structures.

SiO₂@MnO_x@Na₂WO₄@SiO₂ core–shell catalysts were prepared for the oxidative coupling of methane.

Study on the unsteady state oxidative coupling of methane: effects of oxygen species from O2, surface lattice oxygen, and CO2 on the C2+ selectivity

This study examined the effects of oxygen species on the unsteady-state oxidative coupling of methane (OCM) using a lengthy catalyst bed of Na₂WO₄/Mn/SiO₂. The reaction conditions, including the methane-to-oxygen ratio, ratio of feed gas dilution by N₂, quantity of catalyst, and feed flow rate were adjusted for the continuous flow fixed bed reaction system. While the O₂ gas initiated methyl radical formation from methane, the surface lattice oxygen atoms improved the dehydrogenation of paraffins to olefins without significant activation of methane. The addition of CO₂ as a mild oxidizing agent was also tested and slightly improved OCM selectivity with slightly lower methane conversion were observed.

This study examined the effects of oxygen species on the unsteady-state oxidative coupling of methane (OCM) using a lengthy catalyst bed of Na₂WO₄/Mn/SiO₂.

Catalyst screening for the oxidative coupling of methane: from isothermal to adiabatic operation via microkinetic simulations

OCM catalysts underperforming in typical isothermal conditions could result in above average performances in adiabatically-relevant operating conditions.