High-entropy alloy nanoparticles (HEANPs) composed of Pt, Pd, Rh, Ru, and Ir have emerged as a promising class of catalysts for steam methane reforming (SMR) at high temperatures. Their unique multi-element composition, stabilized by configurational entropy, offers exceptional catalytic activity, resistance to sintering, and anti-coking properties under harsh operating conditions. These characteristics make them superior to conventional single-metal or bimetallic catalysts, particularly in industrial-scale SMR processes where temperatures exceed 800°C.

The synthesis of PtPdRhRuIr HEANPs leverages combinatorial techniques such as laser ablation and magnetron sputtering. Laser ablation enables the precise generation of nanoparticles by irradiating a high-entropy alloy target with a pulsed laser, producing a plasma plume that condenses into nanoparticles with controlled size distributions. Magnetron sputtering, on the other hand, allows for the deposition of uniform nanoparticle films or supported catalysts by co-sputtering multiple metallic targets in an inert or reactive atmosphere. Both methods ensure a homogeneous distribution of the five constituent metals, which is critical for achieving the desired entropy-driven stabilization. The configurational entropy of these HEANPs, calculated as ΔSconfig = R ln(N), where N is the number of equimolar components, exceeds 1.61R, sufficient to stabilize the solid solution phase at high temperatures.

A key advantage of PtPdRhRuIr HEANPs in SMR is their resistance to carbon deposition, a major deactivation mechanism in conventional Ni-based catalysts. The multi-element synergy disrupts the formation of extended graphene layers, as the varied atomic radii and electronic structures of Pt, Pd, Rh, Ru, and Ir create lattice distortions that hinder carbon diffusion and nucleation. Experimental studies have demonstrated that after 100 hours of operation at 800°C, these HEANPs exhibit less than 2% weight gain due to coking, compared to over 15% for monometallic Ni catalysts under identical conditions. This anti-coking property is further enhanced by the presence of Ru and Ir, which promote the gasification of surface carbon through the water-gas shift reaction.

The catalytic performance of PtPdRhRuIr HEANPs in SMR is quantified by their turnover frequency (TOF) and long-term stability. At 800°C, the TOF for methane conversion reaches approximately 5.6 s⁻¹, significantly higher than that of Pt-Rh bimetallic catalysts (3.2 s⁻¹) under the same conditions. This high activity is attributed to the ensemble effect, where the combination of metals optimizes the dissociation of methane and the adsorption of intermediates. The Rh and Ru sites facilitate C-H bond cleavage, while Pt and Pd enhance the desorption of H2, preventing active site blocking. The Ir component contributes to thermal stability, reducing Ostwald ripening and particle coalescence.

Long-term stability tests reveal minimal degradation in catalytic activity over extended periods. After 500 hours of continuous operation at 800°C, PtPdRhRuIr HEANPs retain over 90% of their initial activity, with an average particle size increase of less than 10%. In contrast, traditional Ni-based catalysts suffer from rapid sintering, often doubling in particle size within 200 hours under similar conditions. The entropy-stabilized structure of HEANPs plays a crucial role in this resilience, as the high entropy of mixing lowers the driving force for phase separation or coarsening.

The surface composition of these HEANPs dynamically adapts under reaction conditions. In situ X-ray photoelectron spectroscopy (XPS) studies indicate that the surface enrichment of Rh and Ru occurs during SMR, creating active sites tailored for methane activation and CO oxidation. Meanwhile, Pt and Pd remain partially subsurface, contributing to the overall electronic structure and stability. This dynamic surface restructuring is reversible and does not compromise the bulk homogeneity of the nanoparticles.

A critical consideration in the application of PtPdRhRuIr HEANPs is their cost, given the inclusion of precious metals. However, the enhanced durability and reduced deactivation rates offset the initial material expenses by extending catalyst lifespan and reducing downtime for regeneration. The optimal composition balances catalytic performance and cost, with slight deviations from equimolar ratios (e.g., increased Rh content) further improving activity without sacrificing stability.

In summary, PtPdRhRuIr high-entropy alloy nanoparticles represent a significant advancement in high-temperature steam methane reforming catalysis. Their entropy-stabilized structure, synthesized via laser ablation or sputtering, provides exceptional resistance to coking and sintering. With high turnover rates at 800°C and sustained performance over hundreds of hours, these materials address the limitations of conventional catalysts in industrial SMR processes. Future research may explore scalable synthesis methods and support interactions to further optimize their commercial viability.