The effects of stoichiometry on the properties of exsolved Ni‐Fe alloy nanoparticles for dry methane reforming

Self-Regenerative Ni-Doped CaTiO3/CaO for Integrated CO2 Capture and Dry Reforming of Methane

In this work, a new type of multifunctional materials (MFMs) called self-regenerative Ni-doped CaTiO3/CaO is introduced for the integrated CO2 capture and dry reforming of methane (ICCDRM). These materials consist of a catalytically active Ni-doped CaTiO3 and a CO2 sorbent, CaO. The article proposes a concept where the Ni catalyst can be regenerated in situ, which is crucial for ICCDRM. Exsolved Ni nanoparticles are evenly distributed on the surface of CaTiO3 under H2 or CH4, and are re-dispersed back into the CaTiO3 lattice under CO2. The Ni-doped CaTiO3/CaO MFMs show stable CO2 capture capacity and syngas productivity for 30 cycles of ICCDRM. The presence of CaTiO3 between CaO grains prevents CaO/CaCO3 thermal sintering during carbonation and decarbonation. Moreover, the strong interaction of CaTiO3 with exsolved Ni mitigates severe accumulation of coke deposition. This concept can be useful for developing MFMs with improved properties that can advance integrated carbon capture and conversion.

Co-Exsolution of Ni-Based Alloy Catalysts for the Valorization of Carbon Dioxide and Methane

ConspectusThe reversible coexsolution mechanism of perovskite oxides is emerging as an alternative method for synthesizing alloy catalyst nanoparticles. Co-exsolution is a partial decomposition process where multiple B cations diffuse from the bulk of a solid precursor and nucleate on the surface. The unique properties of exsolved alloy catalysts, including improved dispersion, thermal stability, and compositional malleability, make them particularly useful for converting CO2 into chemical commodities and fuels. However, the coexsolution of alloys is still in development, and fundamental insights into the alloying mechanism, formation of nanoparticles, and defect chemistry are needed.This Account examines the solid-state chemistry of perovskite oxide precursors and reaction parameters that can be altered to control the assembly or exsolution of Ni-based alloys. The characteristics of bulk perovskite oxide precursors heavily influence the exsolved alloy catalyst nanoparticle assembly, growth, and composition. Inherent defects, such as oxygen vacancies and grain boundaries, primarily facilitate the transport of catalytic B-cation dopants from the bulk to the surface. An example of how bulk defects can affect the properties of Ni-based alloy catalysts is demonstrated through the formation of NiFe from La(Fe, Ni)O3. The A/B cation ratio plays a significant role in determining the size and composition of NiFe nanoparticles, which directly impacts their catalytic performance. Using in situ X-ray absorption spectroscopy (in situ XAS), the dynamic behavior of exsolved NiFe nanoparticles can be observed in different reaction environments (oxidation, reduction, and dry reforming of methane) by tracking the oxidation state and local environment of the Ni K-edge and Fe K-edge using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), respectively. Time-resolved experiments with in situ XAS showed that NiFe nanoparticle growth starts at ∼280 °C and transforms from predominantly Ni to NiFe at higher reduction times and temperatures.The challenges of exsolution of higher-order Ni-based alloys, such as 3(NiFeCo), 4(NiCoCuPd), and 5(NiFeCoCuPd) element nanoparticles, to improve the catalyst properties are discussed. The size, concentration, and reducibility of the dopant cation can alter the exsolution kinetics, alloy nanoparticle growth dynamics, and catalyst performance. The size and composition of exsolved Ni-based alloys affect the effectiveness of catalysts in the dry reforming of methane. Large NiFeCo nanoparticles separated from Pd and Cu can lead to catalyst deactivation, but using a complex alloy with smaller NiFeCoPdCu nanoparticles results in a stable performance. The use of in situ XANES reveals how the dry reforming of methane reaction conditions can induce changes in the NiFe with the rapid redissolution of Fe back into the lattice.The dynamicity of the exsolved Ni-based alloy nanoparticles and implications for their regeneration after aging or exposure to waste gas contaminants are discussed. Finally, we summarize the Account and provide promising future directions.

Ni/(R2O3,CaO) Nanocomposites Produced by the Exsolution of R1.5Ca0.5NiO4 Nickelates (R = Nd, Sm, Eu): Rare Earth Effect on the Catalytic Performance in the Dry Reforming and Partial Oxidation of Methane

In order to clarify the role of R₂O₃ in the metal-oxide catalysts derived from complex oxide precursors, a series of R_1.5Ca_0.5NiO₄ (R = Nd, Sm, Eu) complex oxides was obtained. A significant systematic increase in the orthorhombic distortion of the R_1.5Ca_0.5NiO₄ structure (K₂NiF₄ type, Cmce) from Nd to Eu correlates with a corresponding decrease in their ionic radii. A reduction of R_1.5Ca_0.5NiO₄ in the Ar/H₂ gas mixture at 800 °C causes a formation of dense agglomerates of CaO and R₂O₃ coated with spherical 25-30 nm particles of Ni metal. The size of metal particles and oxide agglomerates is similar in all Ni/(R₂O₃,CaO) composites in the study. Their morphology is rather similar to the products of redox exsolution obtained by the partial reduction of complex oxides. All obtained composites demonstrated a significant catalytic activity in the dry reforming (DRM) and partial oxidation (POM) of methane at 700-800 °C. A systematic decrease in the DRM catalytic activity of composites from Nd to Eu could be attributed to the basicity reduction of R₂O₃ components of the composite catalysts. The maximum CH₄ conversion in POM reaction was observed for Ni/(Sm₂O₃,CaO), while the maximum selectivity was demonstrated by Nd₂O₃-based composite. The possible reasons for the observed difference are discussed.

Effects of alloying for steam or dry reforming of methane: a review of recent studies

A survey on the catalytic nature of Ni-based alloy catalysts in recent years provides a direction for future catalyst development.

Methane activation on dual-atom catalysts supported on graphene

Dual-atom Fe catalysts supported by three nitrogen atom doped graphene (Fe-TM/GP, where TM = Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are explored for methane adsorption and activation. The addition of the second metal significantly tunes the properties of the catalysts. The main factor influencing methane adsorption is electron transfer. The second metal promotes methane adsorption by altering the electronic properties such as the band structure and charge transfer. A volcano-shaped relationship is found between the absolute value of adsorption energy and energy barrier at the heteroatom Fe-TM/GP. Fe-Ni/GP has the lowest energy barrier. Heteroatom Fe-TM/GP has a lower energy barrier than Fe-Fe/GP.