Hydrogen Adsorption on Ordered and Disordered Pt-Ni Alloys

Revolutionizing Ammonia Synthesis: FeCoNiAlSi High-Entropy Alloy Catalyst for Low-Pressure, Low-Temperature Applications

The Haber-Bosch (HB) process, critical for global ammonia production, is hindered by its high energy consumption and operational demands, requiring extreme pressures and temperatures. Developing catalysts that reduce these demands while maintaining practical efficiency is essential for achieving sustainable ammonia synthesis. Here, we investigate the FeCoNi(AlSi)0.76 high-entropy alloy (HEA) as a catalyst for the HB process using quantum mechanics (QM) and kinetic Monte Carlo (kMC) simulations. Mechanistic analysis revealed significantly lower reaction barriers compared to pure Fe, and kMC simulations predict an NH3 turnover frequency (TOF) that is 65 times higher than pure Fe under industrial conditions. Under reduced pressure (21 atm) and moderate temperature condition (673 K), the HEA retained half the NH3 production rate of pure Fe at extreme industrial conditions, revealing its potential to reduce energy and pressure requirements. This study demonstrates the promise of HEAs in enabling more energy-efficient and sustainable ammonia production technologies.

Hydrogen Adsorption on Ordered and Disordered Pt-Fe and Pt-Co Alloys

The bulk properties and surface chemical reactivity of compositionally disordered Pt-Fe and Pt-Co alloys in the fcc A1 phase have been investigated theoretically in comparison to the ordered alloys of the same compositions. The results are analyzed together with our previously reported findings for Pt-Ni. Nonlinear variation is observed in lattice constant, d band center, magnetic moment, and hydrogen adsorption energy across the composition range (0-100 atomic % of Pt, x Pt). The Pt 5d states are strongly perturbed by the 3d states of the base metals, leading to notable density of states above the Fermi level and residual magnetic moments at high x Pt. Surface reactivity in terms of average H adsorption energy varies continuously with composition between the monometallic Fe-Pt and Co-Pt limits, going through a maximum around x Pt = 0.5-0.75. Close inspection reveals a significant variation in site reactivity at x Pt < 0.75, particularly with disordered Pt-Fe alloys due in part to the inherent disparity in chemical reactivity between Fe and Pt. Furthermore, the strong interaction between Fe and Pt causes Pt-rich sites to be less reactive toward H than Pt-rich sites on disordered Pt-Ni alloy surfaces, despite less compressive strain caused. These results provide theoretical underpinnings for conceptualizing and understanding the performance of these Pt-base metal alloys in key catalytic applications and for efforts to tailor Pt-alloys as catalysts.

Applicability of the d-Band Model to Predict the Influence of Elastic Strains on the Adsorption Energy of Different Adsorbates onto Pt and PtO2 Surfaces

The influence of elastic strains on the adsorption processes of seven adsorbates (H, C, N, O, CO, NO, and H) onto the surface of Pt(111) and PtO2 (110) has been investigated using density functional theory (DFT) simulations. The total adsorption energy was decomposed into mechanical and electronic contributions. Our results indicate that elastic strain in metals affects the adsorption energy by modifying the electronic structure of the surface rather than changing the physical space where the atoms reside after adsorption. In fact, the mechanical contribution to the adsorption energy in Pt was negligible compared to the electronic interaction and independent of the deformation. The mechanical contribution in the case of PtO2 was also independent of the applied strain, but its magnitude was slightly higher due to the ionic bonding between Pt and O atoms in the slab. The variation of the electronic contribution to the adsorption energy in Pt and PtO2 followed the predictions of the d-band model for all adsorbates, expanding its applicability to different adsorbates onto the same surface and to oxides.

CoCrFeNi High-Entropy Alloy as an Enhanced Hydrogen Evolution Catalyst in an Acidic Solution

High-entropy alloys (HEAs) have intriguing material properties, but their potential as catalysts has not been widely explored. Based on a concise theoretical model, we predict that the surface of a quaternary HEA of base metals, CoCrFeNi, should go from being nearly fully oxidized except for pure Ni sites when exposed to O2 to being partially oxidized in an acidic solution under cathodic bias, and that such a partially oxidized surface should be more active for the electrochemical hydrogen evolution reaction (HER) in acidic solutions than all the component metals. These predictions are confirmed by electrochemical and surface science experiments: the Ni in the HEA is found to be most resistant to oxidation, and when deployed in 0.5 M H2SO4, the HEA exhibits an overpotential of only 60 mV relative to Pt for the HER at a current density of 1 mA/cm2.

Synthesis and Characterization of Ni-Pt Alloy Thin Films Prepared by Supercritical Fluid Chemical Deposition Technique

Ni-Pt alloy thin films have been successfully synthesized and characterized; the films were prepared by the supercritical fluid chemical deposition (SFCD) technique from Ni(hfac)₂·3H₂O and Pt(hfac)₂ precursors by hydrogen reduction. The results indicated that the deposition rate of the Ni-Pt alloy thin films decreased with increasing Ni content and gradually increased as the precursor concentration was increased. The film peaks determined by X-ray diffraction shifted to lower diffraction angles with decreasing Ni content. The deposited films were single-phase polycrystalline Ni-Pt solid solution and it exhibited smooth, continuous, and uniform distribution on the substrate for all elemental compositions as determined by scanning electron microscopy and scanning transmission electron microscopy analyses. In the X-ray photoelectron spectroscopy (XPS) analysis, the intensity of the Pt 4f peaks of the films decreased as the Ni content increased, and vice versa for the Ni 2p peak intensities. Furthermore, based on the depth profiles determined by XPS, there was no evidence of atomic diffusion between Pt and Ni, which indicated alloy formation in the film. Therefore, Ni-Pt alloy films deposited by the SFCD technique can be used as a suitable model for catalytic reactions due to their high activity and good stability for various reactions.

The Magnetic Band-Structures of Ordered PtxFe1−x, PtxCo1−x, and PtxNi1−x (x = 0.25, 0.50, and 0.75)

The electronic band structures of the ordered L12 and L10 phases of the PtxM1−x (M = Fe, Co and Ni) alloys were investigated using spin-polarized density functional theory (DFT). The relative contributions of both itinerant (Stoner) and localized magnetism at the high-symmetry k-points were determined and discussed qualitatively. Significant directional effects were identified along the A and R directions of the L10 and L12 alloys, respectively, and are discussed in terms of charge channeling effects.