High-entropy alloys (HEAs), composed of multiple principal elements in near-equimolar ratios, have attracted significant attention due to their exceptional properties stemming from high mixing entropy, lattice distortion, sluggish diffusion, and cocktail effects. These properties lead to enhanced mechanical strength, thermal stability, and corrosion resistance. While bulk HEAs are readily synthesized, the fabrication of nanometer-sized HEAs, particularly on conventional catalyst supports, remains challenging. This limitation hinders the exploration of their potential applications, especially in catalysis where nanoscale dimensions offer significant advantages due to increased surface area-to-volume ratio and nanoscale-size effects. Bottom-up approaches, promising uniform chemical composition and size distribution, are preferable but often require high temperatures and specialized equipment. This study addresses this challenge by exploring the use of hydrogen spillover on reducible metal oxides as a facile, low-temperature synthetic route for supported HEA nanoparticles. Hydrogen spillover, the migration of dissociated hydrogen atoms across a surface driven by a concentration gradient, particularly on reducible oxides like TiO₂, WO₃, and MoO₃, involves dissociative chemisorption of H₂, formation of protons (H⁺) and electrons (e⁻), and diffusion of protons to lattice O²⁻ anions, accompanied by partial metal reduction. The authors' previous work demonstrated TiO₂'s effectiveness in synthesizing binary alloy NPs of immiscible metals, leveraging the strong spillover effect for low-temperature synthesis. This research expands this strategy to the synthesis of TiO₂-supported HEA NPs, specifically focusing on CoNiCuRuPd HEA NPs on TiO₂ for CO₂ hydrogenation, a reaction of vital importance for mitigating climate change and reducing reliance on fossil fuels.

The literature review extensively covers existing HEA synthesis methods, highlighting the challenges in creating HEA nanoparticles (NPs) with diameters under 10 nm. Various techniques, including bulk melting, solid-state processing, additive manufacturing, carbothermal shock methods, ultrasonication, solvothermal synthesis, polyol solutions, and fast moving bed pyrolysis, are discussed. However, these methods typically require high temperatures or specialized equipment. The research emphasizes the need for simpler, low-temperature techniques, especially for supporting HEA NPs on conventional materials, to broaden their industrial applications. The phenomenon of hydrogen spillover and its role in catalysis is also reviewed, focusing on the mechanism and the enhanced efficiency seen on reducible metal oxides like TiO₂, compared to non-reducible supports. The authors highlight their prior work on using TiO₂ to synthesize binary alloy NPs of normally immiscible metals, setting the stage for the present study's extension to quinary HEA NPs.

CoNiCuRuPd HEA NPs supported on TiO₂, Al₂O₃, and MgO were synthesized using a simple impregnation method. Metal precursors were dissolved in water, added to the support, the water was evaporated, and the resulting material was reduced under a hydrogen atmosphere at 400 °C (for TiO₂ supported sample) without prior calcination. The choice of CoNiCuRuPd was based on their varying reduction potentials and the fulfillment of criteria for solid-solution HEA formation (atomic size difference <6.6% and enthalpy of mixing between -11.6 and 3.2 kJ/mol). The synthesized catalysts were characterized using various techniques: H₂ temperature-programmed reduction (H₂-TPR) to study the reduction behavior of the precursors; in situ X-ray absorption fine structure (XAFS) spectroscopy and extended XAFS (EXAFS) to analyze the reduction sequence and structural changes during the reduction process; X-ray diffraction (XRD) to determine the crystal structure and phase composition; high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray (EDX) mapping to determine the morphology, particle size, and elemental distribution of the NPs; temperature-programmed desorption (TPD) with adsorbed CO coupled with Fourier transform infrared spectroscopy (FTIR) to investigate CO adsorption characteristics; and environmental transmission electron microscopy (ETEM) to observe the stability of NPs under electron beam irradiation. Density functional theory (DFT) calculations were performed to investigate the mechanism of HEA NP formation via hydrogen spillover on TiO₂(101) and Al₂O₃, and to understand CO and H adsorption on the HEA and Pd surfaces. Molecular dynamics (MD) simulations were conducted to analyze the diffusion coefficients of various elements in both the HEA and pure metal clusters. Catalytic CO₂ hydrogenation was evaluated using a fixed-bed reactor system at atmospheric pressure and temperatures ranging from 300 to 400 °C. Reaction products were analyzed using gas chromatography.

The H₂-TPR results showed that the quinary-component precursors on TiO₂ exhibited a single reduction peak at ~170 °C, indicating simultaneous reduction of all metal ions and the formation of HEA NPs. In contrast, precursors on Al₂O₃ and MgO showed broad reduction peaks, suggesting non-uniform reduction and potential segregation. In situ XAFS analysis confirmed the reduction of all elements to their metallic state at 400 °C, with slight changes in the Pd K-edge suggesting slight disorder in the fcc structure. FT-EXAFS indicated significant differences in interatomic M-M bond lengths compared to bulk references, confirming the formation of the HEA. XRD confirmed the formation of a single-phase fcc HEA structure with an intermediate lattice parameter. HAADF-STEM and EDX mapping showed the formation of uniformly distributed HEA NPs with an average diameter of 1.9 nm on TiO₂, while larger, partially segregated NPs were observed on Al₂O₃ and MgO. The proposed formation mechanism, supported by DFT calculations, involves the initial reduction of Pd to form nuclei, followed by H₂ dissociation, H atom transfer to TiO₂, proton and electron transfer, and subsequent reduction of the other metal ions via hydrogen spillover. DFT calculations showed that H atom transfer on TiO₂ was energetically favorable compared to Al₂O₃. The CoNiCuRuPd/TiO₂ catalyst exhibited significantly higher activity and CH₄ selectivity (68.3% at 400 °C) compared to catalysts on Al₂O₃ and MgO. TPD-FTIR showed stronger CO adsorption on CoNiCuRuPd/TiO₂ compared to Pd/TiO₂, explaining the higher CH₄ selectivity. ETEM and MD simulations revealed the superior stability and sluggish diffusion in CoNiCuRuPd HEA NPs, resulting in high durability during CO₂ hydrogenation (96% activity retention after 72 h) compared to Pd/TiO₂ (76% activity retention after 72 h). DFT calculations showed higher cohesive energy and lower diffusion coefficients for the HEA cluster compared to pure metal clusters.

The findings demonstrate the successful synthesis of highly active and durable CoNiCuRuPd HEA NPs on TiO₂ via a low-temperature method leveraging hydrogen spillover. The superior catalytic performance and stability compared to monometallic Pd and HEAs on other supports are attributed to the synergistic effects of the combined metals (cocktail effect) and the sluggish diffusion effect inherent to HEAs. The proposed hydrogen spillover mechanism, supported by DFT calculations, explains the simultaneous reduction of the metal precursors at low temperatures. The higher CH₄ selectivity observed with CoNiCuRuPd/TiO₂ compared to Pd/TiO₂ is attributed to stronger CO adsorption on the HEA surface, leading to increased hydrogenation to methane. The remarkable durability of the CoNiCuRuPd/TiO₂ catalyst is linked to the suppressed particle growth and structural robustness arising from the sluggish diffusion in HEAs, validated by both experimental observations and DFT/MD calculations. This work represents a significant advance in HEA catalyst design and synthesis, paving the way for further exploration of their catalytic potential.

This study successfully demonstrates a facile low-temperature synthesis of highly active and durable CoNiCuRuPd HEA nanoparticles on TiO₂ using hydrogen spillover. The enhanced catalytic performance and stability are attributed to the cocktail effect and sluggish diffusion within the HEA structure. This approach significantly advances HEA catalyst design and opens new possibilities for exploring diverse compositions and catalytic applications. Future research could explore other HEA compositions and support materials, optimize reaction conditions, and investigate the catalytic performance in different reactions.

The study focuses on a specific HEA composition (CoNiCuRuPd) and support material (TiO₂). The generalizability of the hydrogen spillover-driven synthesis method to other HEA compositions and support materials needs further investigation. While DFT calculations provide insights into the reaction mechanisms, they involve simplified models, and the complexity of the real catalytic system might not be fully captured. The study primarily focuses on atmospheric pressure CO₂ hydrogenation, and the performance at different pressures and reaction conditions should be evaluated.