Structure sensitivity of the oxidative activation of methane over MgO model catalysts: I. Kinetic study

Operando Electron Microscopy of Catalysts: The Missing Cornerstone in Heterogeneous Catalysis Research?

Heterogeneous catalysis in thermal gas-phase and electrochemical liquid-phase chemical conversion plays an important role in our modern energy landscape. However, many of the structural features that drive efficient chemical energy conversion are still unknown. These features are, in general, highly distinct on the local scale and lack translational symmetry, and thus, they are difficult to capture without the required spatial and temporal resolution. Correlating these structures to their function will, conversely, allow us to disentangle irrelevant and relevant features, explore the entanglement of different local structures, and provide us with the necessary understanding to tailor novel catalyst systems with improved productivity. This critical review provides a summary of the still immature field of operando electron microscopy for thermal gas-phase and electrochemical liquid-phase reactions. It focuses on the complexity of investigating catalytic reactions and catalysts, progress in the field, and analysis. The forthcoming advances are discussed in view of correlative techniques, artificial intelligence in analysis, and novel reactor designs.

Nanoparticles with cubic symmetry: classification of polyhedral shapes

Structural studies of polyhedral bodies can help to analyze geometric details of observed crystalline nanoparticles (NP) where we consider compact polyhedra of cubic point symmetry as simple models. Their surfaces are described by facets with normal vectors along selected Cartesian directions (a, b, c) together with their symmetry equivalents forming a direction family {abc}. Here we focus on polyhedra with facets of families {100}, {110}, and {111}, suggested for metal and oxide NPs with cubic lattices. Resulting generic polyhedra, cubic, rhombohedral, octahedral, and tetrahexahedral, have been observed as NP shapes by electron microscopy. They can serve for a complete description of non-generic polyhedra as intersections of corresponding generic species, not studied by experiment so far. Their structural properties are shown to be fully determined by only three parameters, facet distancesR100,R110, andR111of the three facet types. This provides a novel phase diagram to systematically classify all corresponding polyhedra. Their structural properties, such as shape, size, and facet geometry, are discussed in analytical and numerical detail with visualization of typical examples. The results may be used for respective NP simulations but also as a repository stimulating the structural interpretation of new NP shapes to be observed by experiment.

Sr2Fe1.575Mo0.5O6-δ Promotes the Conversion of Methane to Ethylene and Ethane

Oxidative coupling of methane can produce various valuable products, such as ethane and ethylene, and solid oxide electrolysis cells (SOECs) can electrolyze CH₄ to produce C₂H₄ and C₂H₆. In this work, Sr₂Fe_1.575Mo_0.5O_6-δ electrode materials were prepared by impregnation and in situ precipitation, and Sr₂Fe_1.5Mo_0.5O_6-δ was taken as a reference to study the role of metal-oxide interfaces in the catalytic process. When the Fe/Sr₂Fe_1.575Mo_0.5O_6-δ interface is well constructed, the selectivity for C₂ can reach 78.18% at 850 °C with a potential of 1.2 V, and the conversion rate of CH₄ is 11.61%. These results further prove that a well-constructed metal-oxide interface significantly improves the catalytic activity and facilitates the reaction.

Transition-Metal-Doping of CaO as Catalyst for the OCM Reaction, a Reality Check

In this study, first-row transition metal-doped calcium oxide materials (Mn, Ni, Cr, Co., and Zn) were synthesized, characterized, and tested for the OCM reaction. Doped carbonate precursors were prepared by a co-precipitation method. The synthesis parameters were optimized to yield materials with a pure calcite phase, which was verified by XRD. EPR measurements on the doped CaO materials indicate a successful substitution of Ca2+ with transition metal ions in the CaO lattice. The materials were tested for their performance in the OCM reaction, where a beneficial effect towards selectivity and activity effect could be observed for Mn, Ni, and Zn-doped samples, where the selectivity of Co- and Cr-doped CaO was strongly reduced. The optimum doping concentration could be identified in the range of 0.04-0.10 atom%, showing the strongest decrease in the apparent activation energy, as well as the maximum increase in selectivity.

Activation of CO2 and CH4 on MgO surfaces: mechanistic insights from first-principles theory

One of the most challenging topics in heterogeneous catalysis is conversion of CH₄ to higher hydrocarbons. Direct conversion of CH₄ to ethylene can be achieved via the oxidative coupling of methane (OCM) reaction. Despite studies which have shown MgO to activate CH₄ and initiate the OCM reaction, its large-scale applications face a significant impediment due to formation of a byproduct, CO₂, and poisoning of the catalyst due to carbonate formation. In the present work, we address two aspects of the OCM reaction on MgO surfaces: carbonate formation on the surface of the catalyst, and (dissociative) adsorption of CH₄. We use first-principles density functional theoretical calculations to determine the energetics and underlying mechanisms of interaction of CO₂ and CH₄ with various surfaces of MgO: (100), (110), and (111) (both Mg- and O-terminations), and the seldom studied, hydroxylated (111) MgO surface with O-termination. We find that the strength of the interaction of CO₂ with MgO surfaces depends on several factors: their surface energies, coordination number of surface O atoms, and ability to donate electrons. However, the O-terminated (111) surface of MgO bucks all aforementioned factors, with only oxygen richness affecting its reactivity towards CO₂. The interaction of CH₄ with MgO surfaces depends primarily on the coordination number of the surface O atoms and the orientation of the CH₄ molecule with respect to the surface. Finally, we provide insights into (a) formation of surface carbonates, which is relevant to CO₂ capture and conversion, and (b) C-H bond activation on MgO surfaces, which is crucial for direct conversion of CH₄ to value-added chemicals.

Metal-oxide interface enhances methane oxidative coupling reaction in solid oxide electrolyzer

At present, converting methane into fuel and more valuable chemicals is highly desired for industrial application. Here, we demonstrate the in-situ electrochemical conversion of methane (C1) into ethane and ethylene...

Oxidative Coupling of Methane: Perspective for High-Value C2 Chemicals

The oxidative coupling of methane (OCM) to C2 hydrocarbons (C2H4 and C2H6) has aroused worldwide interest over the past decade due to the rise of vast new shale gas resources. However, obtaining higher C2 selectivity can be very challenging in a typical OCM process in the presence of easily oxidized products such as C2H4 and C2H6. Regarding this, different types of catalysts have been studied to achieve desirable C2 yields. In this review, we briefly presented three typical types of catalysts such as alkali/alkaline earth metal doped/supported on metal oxide catalysts (mainly for Li doped/supported catalysts), modified transition metal oxide catalysts, and pyrochlore catalysts for OCM and highlighted the features that play key roles in the OCM reactions such as active oxygen species, the mobility of the lattice oxygen and surface alkalinity of the catalysts. In particular, we focused on the pyrochlore (A2B2O7) materials because of their promising properties such as high melting points, thermal stability, surface alkalinity and tunable M-O bonding for OCM reaction.

Isolated Cobalt Ions Embedded in Magnesium Oxide Nanostructures: Spectroscopic Properties and Redox Activity

Atomic dispersion of dopants and control over their defect chemistry are central goals in the development of oxide nanoparticles for functional materials with dedicated electronic, optical or magnetic properties. We produced highly dispersed oxide nanocubes with atomic distribution of cobalt ions in substitutional sites of the MgO host lattice via metal organic chemical vapor synthesis. Vacuum annealing of the nanoparticle powders up to 1173 K has no effect on the shape of the individual particles and only leads to moderate particle coarsening. Such materials processing, however, gives rise to the electronic reduction of particle surfaces, which-upon O2 admission-stabilize anionic oxygen radicals that are accessible to UV/Vis diffuse reflectance and electron paramagnetic resonance (EPR) spectroscopy. Multi-reference quantum chemical calculations show that the optical bands observed mainly originate from transitions into 4 A2g (4 F), 4 T1g (4 P) states with a contribution of transitions into 2 T1g , 2 T2g (2 G) states through spin-orbit coupling and gain intensity through vibrational motion of the MgO lattice or the asymmetric ion field. Related nanostructures are a promising material system for single atomic site catalysis. At the same time, it represents an extremely valuable model system for the study of interfacial electron transfer processes that are key to nanoparticle chemistry and photochemistry at room temperature, and in heterogeneous catalysis.

Promoting Li/MgO Catalyst with Molybdenum Oxide for Oxidative Conversion of n-Hexane

In this work, molybdena-promoted Li/MgO is studied as a catalyst for the oxidative conversion of n-hexane. The structure of the catalysts is investigated with X-ray Diffraction (XRD) and Raman spectroscopy. The MoO3/Li/MgO catalyst contains three types of molybdena-containing species, the presence of which depend on molybdena loading. At low Mo/Li ratios (i) isolated dispersed [MoO4]2− anionic species are observed. At high Mo/Li ratios, the formation of crystalline lithium molybdate phases such as (ii) monomeric Li2MoO4 and tentatively (iii) polymeric Li2Mo4O13 are concluded. The presence of these lithium molybdates diminishes the formation of Li2CO3 in the catalyst. Subsequently, the catalyst maintains high surface area and stability with time-on-stream during oxidative conversion. Molybdena loading as low as 0.5 wt % is sufficient to induce these improvements, maintaining the non-redox characteristics of the catalyst, whereas higher loadings enhance deep oxidation and oxidative dehydrogenation reactions. Promoting a Li/MgO catalyst with 0.5 wt % MoO3 is thus efficient for selective conversion of n-hexane to alkenes, giving alkene yield up to 24% as well as good stability.

Catalytic activity, water formation, and sintering: Methane activation over Co- and Fe-doped MgO nanocrystals

Microstructure, structure, and compositional homogeneity of metal oxide nanoparticles can change dramatically during catalysis. Considering the different stabilities of cobalt and iron ions in the MgO host lattice [M. Niedermaier et al., J. Phys. Chem. C 123, 25991 (2019)], we employed MgO nanocube powders with or without transition metal admixtures for the oxidative coupling of methane (OCM) reaction to analyze characteristic differences in catalytic activity and sintering behavior. Undoped MgO nanocrystals exhibit the highest C₂ selectivity and retain the nanocrystallinity of the starting material after 24 h time on stream. For the Co-Mg-O nanoparticle powder, which exhibits the highest activity and CO_x selectivity and where OCM-induced coarsening is strongest, we found that the Co²⁺ ions remain homogeneously distributed over the MgO lattice. Trivalent Fe ions migrate to the surface of Fe-Mg-O nanoparticles where they form a magnesioferrite phase (MgFe₂O₄) with a characteristic impact on catalytic performance: Fe-Mg-O is initially less selective than MgO despite its lower activity. An increase in C₂ selectivity and a decrease in the CO₂/CO ratio with time on stream are attributed to the increasing fraction of coarsened particles that become depleted in redox active Fe. Surface water is a by-product of the OCM reaction, favors mass transport across the particle surfaces, and serves as a sintering aid during catalysis. The characteristic changes in size and morphology of MgO, Co-doped, and Fe-doped MgO particles can be consistently explained by activity and C₂ selectivity trends. The original morphology of the nanocubes as a starting material for the OCM reaction does not impact the catalytic activity.

How to control selectivity in alkane oxidation?

The bulk crystal structure of an oxidation catalyst as the most popular descriptor in oxidation catalysis is not solely responsible for catalytic performance.

The well-defined particle morphology of crystalline MnWO₄ catalysts investigated in the present study facilitates obtaining insight into the origin of selectivity limitations in alkane oxidation. Hydrothermal synthesis at variable pH values granted access to a series of phase-pure MnWO₄ catalysts with particles ranging from cube-like (aspect ratio 1.5) to rod- or needle-like (aspect ratio 6.8) shapes. Kinetic studies reveal a strong dependence of the propane consumption rate on the particle shape. The true origin of the structure sensitivity was unraveled by comprehensive bulk and surface analysis using nitrogen adsorption, XRD, SEM, ADF-STEM, STEM-EELS, XPS, multi-laser excitation Raman and DRIFT/operando FTIR spectroscopies, temperature-programmed oxidation (TPO), in situ NEXAFS, and DFT calculations. The active phase is composed of a thin manganese oxy-hydroxide layer formed on the surface of crystalline MnWO_4. The differences in catalytic performance within the series clearly illustrate that the structural motif as the most popular descriptor in oxidation catalysis is not essential, since all MnWO₄ catalysts in the series under study exhibit the same bulk crystal structure and bulk chemical composition and are phase pure and homogenous. The variable particle shape serves as a proxy that reflects the formation of varying abundance of redox active Mn²⁺/Mn³⁺ surface sites, which correlates with catalytic activity. Operando FTIR spectroscopy directly confirms the formation of Mn–OH surface species by abstraction of hydrogen atoms from the propane molecule on nucleophilic oxygen atoms and suggests that active site regeneration occurs via oxidative dehydrogenation of Mn–OH species indicating a single-site nature of the active sites that does not allow four-electron reduction of molecular oxygen. Instead, intermediates are created that cause side reactions and lower the selectivity. The findings highlight fundamental design criteria that may be applied to advance the development of new alkane oxidation catalysts with improved selectivity.

Thin water films and particle morphology evolution in nanocrystalline MgO

A key question in the field of ceramics and catalysis is how and to what extent residual water in the reactive environment of a metal oxide particle powder affects particle coarsening and morphology. With X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM), we investigated annealing-induced morphology changes on powders of MgO nanocubes in different gaseous H2O environments. The use of such a model system for particle powders enabled us to describe how adsorbed water that originates from short exposure to air determines the evolution of MgO grain size, morphology, and microstructure. While cubic nanoparticles with a predominant abundance of (100) surface planes retain their shape after annealing to T = 1173 K under continuous pumping with a base pressure of water p(H2O) = 10-5 mbar, higher water partial pressures promote mass transport on the surfaces and across interfaces of such particle systems. This leads to substantial growth and intergrowth of particles and simultaneously favors the formation of step edges and shallow protrusions on terraces. The mass transfer is promoted by thin films of water providing a two-dimensional solvent for Mg2+ ion hydration. In addition, we obtained direct evidence for hydroxylation-induced stabilization of (110) faces and step edges of the grain surfaces.

A Selection of Recent Advances in C1 Chemistry

This review presents a selection of recent publications related to the chemistry and catalysis of C1 molecules, including methane, methanol, carbon monoxide, and carbon dioxide. These molecules play an important role in the current supply of energy and chemicals and will likely become even more relevant because of the need to decarbonize fuels (shift from coal to natural gas) in line with CO2 capture and use to mitigate global warming, as well as a gradual shift on the supply side from crude oil to natural gas. This review includes both recent industrial developments, such as the huge increase in methanol-to-olefins-capacity build in China and the demonstration of oxidative coupling of methane, and scientific developments in these chemistries facilitated by improved capabilities in, for example, analytical tools and computational modeling.

La2O3 catalysts with diverse spatial dimensionality for oxidative coupling of methane to produce ethylene and ethane

The La₂O₃ catalyst with a two-dimensional (2D) structure is more effective than 0D, 1D and 3D La₂O₃ for oxidative coupling of methane.