Redox dynamics of Ni catalysts in CO2 reforming of methane

Plasma-Assisted Reforming of Methane

Methane (CH4 ) is inexpensive, high in heating value, relatively low in carbon footprint compared to coal, and thus a promising energy resource. However, the locations of natural gas production sites are typically far from industrial areas. Therefore, transportation is needed, which could considerably increase the sale price of natural gas. Thus, the development of distributed, clean, affordable processes for the efficient conversion of CH4 has increasingly attracted people's attention. Among them are plasma technology with the advantages of mild operating conditions, low space need, and quick generation of energetic and chemically active species, which allows the reaction to occur far from the thermodynamic equilibrium and at a reasonable cost. Significant progress in plasma-assisted reforming of methane (PARM) is achieved and reviewed in this paper from the perspectives of reactor development, thermal and nonthermal PARM routes, and catalysis. The factors affecting the conversion of reactants and the selectivity of products are studied. The findings from the past works and the insight into the existing challenges in this work should benefit the further development of reactors, high-performance catalysts, and PARM routes.

Does CO2 Oxidize Ni Catalysts? A Quick X-ray Absorption Spectroscopy Answer

MgAl₂O₄-supported Ni materials are highly active and cost-effective CO₂ conversion catalysts, yet their oxidation by CO₂ remains dubious. Herein, NiO/MgAl₂O₄, prepared via colloidal synthesis (10 wt % Ni) to limit size distribution, or wet impregnation (5, 10, 20, and 40 wt % Ni), and bare, i.e., unsupported, NiO are examined in H₂ reduction and CO₂ oxidation, using thermal conductivity detector-based measurements and in situ quick X-ray absorption spectroscopy, analyzed via multivariate curve resolution-alternating least-squares. Ni reoxidation does not occur for bare Ni but is observed solely on supported materials. Only samples with the smallest particle sizes get fully reoxidized. The Ni-MgAl₂O₄ interface, exhibiting metal-support interactions, activates CO₂ and channels oxygen into the reduced lattice. Oxygen diffuses inward, away from the interface, oxidizing Ni entirely or partially, depending on the particle size in the applied oxidation time frame. This work provides evidence for Ni oxidation by CO₂ and explores the conditions of its occurrence and the importance of metal-support effects.

Harnessing Strong Metal-Support Interaction to Proliferate the Dry Reforming of Methane Performance by In Situ Reduction

The strong bonding at the interface between the metal and the support, which can inhibit the undesirable aggregation of metal nanoparticles and carbon deposition from reforming of hydrocarbon, is well known as the classical strong metal-support interaction (SMSI). SMSI of nanocatalysts was significantly affected by heat treatment and reducing conditions during catalyst preparation.the heat treatment and reduction conditions during catalyst preparation. SMSI can be weakened by the decrement of metal-doped sites in the supporting oxide and can often deactivate catalysts by the encapsulation of active sites through these processes. To retain SMSI near the active sites and to enhance the catalytic activity of the nanocatalyst, it is essential to increase the number of surficial metal-doped sites between nanometal and the support. Herein, we propose a mild reduction process using dry methane (CH₄/CO₂) gas that suppresses the aggregation of nanoparticles and increases the exposed interface between the metal and support, Ni and cerium oxide. The effects of mild reduction on the chemical state of Ni-cerium oxide nanocatalysts were specifically investigated in this study. As a result, mild reduction led to form large amounts of the Ni³⁺ phase at the catalyst surface of which SMSI was significantly enhanced. It can be easily fabricated while the dry reforming of methane (DRM) reaction is on stream. The superior performance of the catalyst achieved a considerably high CH₄ conversion rate of approximately 60% and stable operation up to 550 h at a low temperature, 600 °C.

Measuring the Unmeasurable by IR Spectroscopy: Carbon Deposition Kinetics in Dry Reforming of Methane

Dry reforming of methane converts two greenhouse gases to syngas, and Ni catalysts are commonly used for this reaction. A major catalyst deactivation mechanism is carbon deposition. Although numerous kinetic modelling works have been performed on carbon formation, there have been only scarce attempts to measure carbon deposition kinetics under relevant (but not real) conditions, owing to technical difficulties. Here, we report the first successful measurements of the kinetics under real reaction conditions. This was made possible by using a novel algorithm that we have developed. We use IR to measure the molar fractions of unreacted CH4 and CO2 , and reaction products, CO and H2 O, in the effluent from the reactor. By applying the general mass balance principle and the relevant reaction stoichiometries, the carbon deposition rate as well as the flow rates of all these gases in the effluent, including H2 , are calculated. Compared to the dominant GC-based approach for catalyst performance evaluation, this method has much higher time resolution and much smaller measurement errors.

Two-Dimensional Layered Double Hydroxides for Reactions of Methanation and Methane Reforming in C1 Chemistry

CH₄ as the paramount ingredient of natural gas plays an eminent role in C1 chemistry. CH₄ catalytically converted to syngas is a significant route to transmute methane into high value-added chemicals. Moreover, the CO/CO₂ methanation reaction is one of the potent technologies for CO₂ valorization and the coal-derived natural gas production process. Due to the high thermal stability and high extent of dispersion of metallic particles, two-dimensional mixed metal oxides through calcined layered double hydroxides (LDHs) precursors are considered as the suitable supports or catalysts for both the reaction of methanation and methane reforming. The LDHs displayed compositional flexibility, small crystal sizes, high surface area and excellent basic properties. In this paper, we review previous works of LDHs applied in the reaction of both methanation and methane reforming, focus on the LDH-derived catalysts, which exhibit better catalytic performance and thermal stability than conventional catalysts prepared by impregnation method and also discuss the anti-coke ability and anti-sintering ability of LDH-derived catalysts. We believe that LDH-derived catalysts are promising materials in the heterogeneous catalytic field and provide new insight for the design of advance LDH-derived catalysts worthy of future research.

Influence of the presence of ruthenium on the activity and stability of Co-Mg-Al-based catalysts in CO2 reforming of methane for syngas production

Hydrogen production by methane dry reforming is an important yet challenging process. A performing catalyst will favor the thermodynamic equilibrium while ensuring good hydrogen selectivity. We hereby report the synthesis of Co _x Mg_6-x Al₂ (with x = 2 and 6) mixed oxide catalysts synthesized via hydrotalcite precursors and the synthesis of a ruthenium-based catalyst on a cobalt, magnesium, and aluminum mixed oxide supports Ru/Co _x Mg_6-x Al₂ (with x = 2 and 6). The impregnation of ruthenium on the hydrotalcites was performed in two ways: by impregnation on the dried hydrotalcite and by memory effect on hydrotalcite calcined at 500 °C. The deposition of ruthenium by memory effect of the magnesium and cobalt support allows the generation of both metallic and basic sites which provides an active and stable catalyst for the dry reforming reaction of methane.

Carbon Dioxide Reforming of Methane using an Isothermal Redox Membrane Reactor

The continuous production of carbon monoxide (CO) and hydrogen (H₂) by dry reforming of methane (CH₄) is demonstrated isothermally using a ceramic redox membrane in absence of additional catalysts. The reactor technology realizes the continuous splitting of CO₂ to CO on the inner side of a tubular membrane and the partial oxidation of CH₄ with the lattice oxygen to form syngas on the outer side. La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) membranes evaluated at 840-1030 °C yielded up to 1.27 μmol _CO   s^-1 from CO₂, 3.77 μmol_H₂ g^-1 s^-1 from CH₄ , and CO from CH₄ at approximately the same rate as CO from CO₂. We compute the free energy of the oxygen vacancy formation for La_0.5Sr_0.5B_0.5B'_0.5O_3-δ (B, B'=Mn, Fe, Co, Cu) using electronic structure theory to understand how CO₂ reduction limits dry reforming of methane using LSCF and to show how the CO₂ conversion can be increased by using advanced redox materials such as La_0.5Sr_0.5MnO_3-δ and La_0.5Sr_0.5Mn_0.5Co_0.5O_3-δ .