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

Abstract

Conspectus The 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 ...

New trends in nanoparticle exsolution

“…More complex compositions such as NiFeCoCuPd have been the focus of the work of Gilliard-AbdulAziz and co-authors as recently reviewed in ref. 86.…”

Valorization of carbon dioxide into C1 product via reverse water gas shift reaction using oxide-supported molybdenum carbides

“…Thermal, electro-, and photo-catalysts have all been researched for the reduction of CO 2 , among which thermal heterogeneous catalysis is a promising technology when using a reducing gas such as hydrogen (H 2 ) or methane (CH 4 ) [9][10][11][12][13][14][15] . Compared with noble metal catalysts which are highly active but economically unfavorable, transition metal-based catalysts are more appealing for scalable processes industrially [16][17][18] .…”

Local Structures of Ex-Solved Nanoparticles Identified by Machine-Learned Potentials

“…To verify this hypothesis, we conduct a comparative analysis of the catalytic performances of impregnated nanoparticles and the ex-solved nanoparticles. For the model system, we select the Ni/MgO system due to its status as a benchmark system that has been extensively explored in previous studies. − Figure c shows the optimized structures of both impregnated and ex-solved nanoparticles (see Figure S6 for detailed atomistic model), obtained by DFT calculations with the PBE-D3 functional. , The impregnated nanoparticle model is created by placing the nanoparticle onto the oxide support, reflecting the fact that the support material plays a minimal role in the chemical reactions in the impregnation process. , For the ex-solved nanoparticle, a type II dislocation interface structure is employed because the interface energy of a type II interface is similar to that of a type I interface in the case of MgO (see Table S3). We calculate the kinetic barriers for the reaction *CH + *O → *CHO on bare, impregnated, and ex-solved Ni using the climbing image nudged elastic band (CINEB) method (see Figure d). , Our findings reveal that the activation barrier for ex-solved nanoparticles is significantly lower than the others, underscoring the strong synergy between the metal and support in ex-solved nanoparticles.…”