Search

The field of intermetallic catalysts, alloying a p-block and a transition metal to form a pM–TM bimetallic alloy, is experiencing robust growth, emerging as a vibrant frontier in catalysis research. Although such materials are increasingly used in the form of nanoparticles, a precise description of their atomic arrangements at the nanoscale remains scarce. Based on the In–Pd binary as a typical pM–TM system, we performed density functional theory calculations to investigate the morphologies, relative stabilities and electronic properties of 24 Å and 36 Å nanoparticles built from the In3Pd2, InPd and InPd3compounds. Wulff equilibrium structures are compared to other ordered and disordered structures. Surface energies are computed to discuss their thermodynamic stability, while work functions are calculated to examine their electronic structures. For any compound, increasing the size leads to the stabilisation of Wulff polyhedra, which are found to offer smaller surface energies than non-crystalline and chemically disordered structures. Disordered In3Pd2and InPd nanoparticles show a tendency towards amorphisation, owing to repulsive short In–In bonds. Tuning nanoparticles’ work functions can be achieved through the control of the surface structure and composition, by virtue of the roughly linear correlation found between the surface composition and the work function which nevertheless includes a certain number of outliers. This work paves the way to rationalisation of both structural and electronic properties of pM–TM nanoparticles.

Steel is the most commonly manufactured material in the world. Its performances can be improved by hot-dip coating with the low weight aluminum metal. The structure of the Al∥Fe interface, which is known to contain a buffer layer made of complex intermetallic compounds such as Al5Fe2and Al13Fe4, is crucial for the properties. On the basis of surface X-ray diffraction, combined with theoretical calculations, we derive in this work a consistent model at the atomic scale for the complex Al13Fe4(010)∥Al5Fe2(001) interface. The epitaxial relationships are found to be [130]Al5Fe2∥[010]Al13Fe4and [1 1̅0]Al5Fe2∥[100]Al13Fe4. Interfacial and constrained energies, as well as works of adhesion, calculated for several structural models based on density functional theory, identify the lattice mismatch and the interfacial chemical composition as main factors for the stability of the interface. Molecular dynamics simulations suggest a mechanism of Al diffusion to explain the formation of the complex Al13Fe4and Al5Fe2phases at the Al∥Fe interface.

A few low-order approximants to decagonal quasicrystals have been shown to provide excellent activity and selectivity for the hydrogenation of alkenes and alkynes. It is the case for the Al13Co4compound, for which the catalytic properties of the pseudo-2-fold orientation have been revealed to be among the best. A combination of surface science studies, including surface X-ray diffraction, and calculations based on density functional theory is used here to derive an atomistic model for the pseudo-2-fold o-Al13Co4surface, whose faceted and columnar structure is found very similar to the one of the 2-fold surface of the d-Al–Ni-Co quasicrystal. Facets substantially stabilize the system, with energies in the range 1.19–1.31 J/m2, i.e., much smaller than the ones of the pseudo-10-fold (1.49–1.68 J/m2) and pseudo-2-fold (1.66 J/m2) surfaces. Faceting is also a main factor at the origin of the Al13Co4catalytic performances, as illustrated by the comparison of the pseudo-10-fold, pseudo-2-fold and facet potential energy maps for hydrogen adsorption. This work gives insights toward the design of complex intermetallic catalysts through surface nanostructuration for optimized catalytic performances.

Good wetting is generally observed for liquid metals on metallic substrates, while poor wetting usually occurs for metals on insulating oxides. In this work, we report unexpected large contact angles for lead on two metallic approximants to decagonal quasicrystals, namely, Al5Co2and Al13Co4. Intrinsic surface wettability is predicted from first principles, using a thermodynamic model based on the Young equation, and validated by the good agreement with experimental measurements performed under ultra-high vacuum by scanning electron microscopy. The atomistic details of the atomic and electronic structures at the Pb-substrate interface, and the comparison with Pb(111)/Al(111), underline the influence of the specific electronic structures of quasicrystalline approximants on wetting. Our work suggests a possible correlation of the contact angles with the density of states at the Fermi energy and paves the way for a better fundamental understanding of wettability on intermetallic substrates, which has potential consequences in several applications such as supported catalysts, protective coatings, or crystal growth.

Replacing noble metal catalysts with inexpensive, environmentally harmless, active, selective, and stable substitutes is a great challenge for the chemical industry. In this paper, the noble metal-free Al5Co2(21̅0) complex intermetallic surface is experimentally identified as active and selective for the semihydrogenation of butadiene. The catalyst surface structure and chemical composition are determined by experimental techniques—surface X-ray diffraction (SXRD) and scanning tunneling microscopy—combined with ab initio calculations. Theoretical investigations of the adsorption properties under reaction conditions demonstrate that the surface Co atomic density drastically impacts the thermodynamic feasibility of the hydrogenation reaction, and they provide information on the reaction mechanism. This work offers insights into the rational design of Al-based catalysts for hydrocarbon hydrogenation reactions.

A new ternary phase with a composition Al1+xV2Sn2–x(x= 0.19) has been found during investigation of the Al–V–Sn ternary system. Single-crystal X-ray diffraction measurements reveal that this ternary phase crystallizes with an orthorhombic structure with a= 5.5931(1) Å, b= 18.8017(5) Å, and c= 6.7005(2) Å (space group Cmce). This compound is thus isostructural to the GaV2Sn2structure type, showing a layered structure composed of vanadium cluster bands formed with pentagonal faces intercalated by Sn atom layers. High-resolution transmission electron microscopy measurements confirm the orthorhombic structure. Regarding lattice perfection, no dislocation could be identified within the probed Al1.19V2Sn1.81single-crystal lamella. Ab initio calculations reveal a reduction of the density of states at the Fermi level, which could be attributed to both a Hume–Rothery effect combined with strong spd hybridization.

Intermetallic compounds are promising materials in numerous fields, especially those involving surface interactions, such as catalysis. A key factor to investigate their surface properties lies in adsorption energy maps, typically built using first-principles approaches. However, exploring the adsorption energy landscapes of intermetallic compounds can be cumbersome, usually requiring huge computational resources. In this work, we propose an efficient method to predict adsorption energies, based on a Machine Learning (ML) scheme fed by a few Density Functional Theory (DFT) estimates performed on nsites selected through the Farthest Point Sampling (FPS) process. We detail its application on the Al13Co4(100) quasicrystalline approximant surface for several atomic adsorbates (H, O, and Pb). On this specific example, our approach is shown to outperform both simple interpolation strategies and the recent ML force field MACE [arXiv.2206.07697], especially when the number nis small, i.e., below 36 sites. The ground-truth DFT adsorption energies are much more correlated with the predicted FPS-ML estimates (Pearson R-factor of 0.71, 0.73, and 0.90 for H, O and Pb, respectively, when n= 36) than with interpolation-based or MACE-ML ones (Pearson R-factors of 0.43, 0.39, and 0.56 for H, O, and Pb, in the former case and 0.22, 0.35, and 0.63 in the latter case). The unbiased root-mean-square error (ubRMSE) is lower for FPS-ML than for interpolation-based and MACE-ML predictions (0.15, 0.17, and 0.17 eV, respectively, for hydrogen and 0.17, 0.25, and 0.22 eV for lead), except for oxygen (0.55, 0.47, and 0.46 eV) due to large surface relaxations in this case. We believe that these findings and the corresponding methodology can be extended to a wide range of systems, which will motivate the discovery of novel functional materials.

The atomic structure of the Al13Ru4(010) approximant surface, investigated for annealing temperatures ranging from 873 to 1143 K, can be described by pentagonal motifs and vacancies. It exhibits an atypical surface reconstruction which manifests itself in real space by well-separated stripes running about 10° off the [001] direction. Their mutual ordering drastically improves at high annealing temperature. This is simultaneously accompanied by a filling of the pentagonal hollow sites present in the stripes with atoms or groups of atoms. We provide evidence that these unidirectional features are of true surface origin. The bulk electronic density of states calculated within the density functional theory framework matches the experimental valence band spectra. In addition to step edges, molecular adsorption reveals that stripes are initially the most reactive sites, leading to anisotropic island growth.

Compared to elemental crystals (Al, Cu, Ag, etc.), the local atomic arrangement within Al-based complex intermetallics is usually best described by highly symmetric clusters decorating the unit cell. With the latter containing tens to several thousand atoms (or an infinite number for the case of quasicrystals), this translates to structurally complex surfaces exhibiting unique potential energy landscapes. This review will focus on the different studies reporting the adsorption of C60molecules on such complex metallic alloy surfaces, aiming to benefit from this complexity to create exotic molecular nanostructures. First, we will recall the main adsorption mechanisms and surface phases that have been identified when fullerene adsorption is carried out on single crystal surfaces. Second, we will discuss how surfaces of increasing structural complexity impact the film properties. The presence of five-fold symmetric adsorption sites is another intrinsic property of these complex intermetallic surfaces. As will be presented in this review, this leads to specific molecular orientations to maximize substrate–adsorbate symmetry matching, hence introducing another degree of freedom to create new 2-D molecular architectures. The local electronic interactions at the adsorption site interface will also be introduced. Furthermore, the different fullerene structures formed upon adsorption on aperiodic surfaces of varying chemical composition and on Bi allotropes will be discussed. Finally, suggestions will be given for future work along with the foreseen area of interests.

Self-organized molecular films with long-range quasiperiodic order have been grown by using the complex potential energy landscape of quasicrystalline surfaces as templates. The long-range order arises from a specific subset of quasilattice sites acting as preferred adsorption sites for the molecules, thus enforcing a quasiperiodic structure in the film. These adsorption sites exhibit a local 5-fold symmetry resulting from the cut by the surface plane through the cluster units identified in the bulk solid. Symmetry matching between the C60fullerene and the substrate leads to a preferred adsorption configuration of the molecules with a pentagonal face down, a feature unique to quasicrystalline surfaces, enabling efficient chemical bonding at the molecule–substrate interface. This finding offers opportunities to investigate the physical properties of model 2D quasiperiodic systems, as the molecules can be functionalized to yield architectures with tailor-made properties.

Recently, transition metal electrides have attracted the attention of scientists as promising catalysts for ammonia synthesis. But very little is known about the atomic processes involved. In this work, extensive DFT calculations have been performed to explore the detailed mechanism of ammonia synthesis on LaRuSi, a typical electride catalyst. Unlike previous studies involving the LaRuSi(001) high energy Ru-terminated surface, we propose here an alternative reaction path at the most stable La termination. Our study addresses the contrasted catalytic properties of the isostructural LaRuSi electride and CaRuSi non-electride compounds. It points to the role of surface La atoms in the catalytic performances of LaRuSi and shows that active sites are not necessarily transition metal atoms. Our findings open up future explorations of transition metal free catalysts for ammonia synthesis, active under mild conditions.

Abstract: The best known cause for colors in insulating minerals is due to transition metal ions as impurities. As an example, Cr3+ is responsible for the red color of ruby (α-Al2O3:Cr3+) and the green color of eskolaite (α-Cr2O3). Using X-ray absorption measurements, we connect the colors of the Cr x Al2−x O3 series with the structural and electronic local environment around Cr. UV–VIS electronic parameters, such as the crystal field and the Racah parameter B, are related to those deduced from the analysis of the isotropic and XMCD spectra at the Cr L2,3-edges in Cr0.07Al1.93O3 and eskolaite. The Cr–O bond lengths are extracted by EXAFS at the Cr K-edge in the whole Cr x Al2−x O3 (0.07≤x< 2) solid solution series. The variation of the mean Cr–O distance between Cr0.07Al1.93O3 and α-Cr2O3 is evaluated to be 0.015 Å (≈1%). The variation of the crystal field in the Cr x Al2−x O3 series is discussed in relation with the variation of the averaged Cr–O distances.