Etching of the intermetallic compounds PdGa and Pd3Ga7: An effective way to increase catalytic activity?

Asymmetric Molecular Adsorption and Regioselective Bond Cleavage on Chiral PdGa Crystals

Homogenous enantioselective catalysis is nowadays the cornerstone in the manufacturing of enantiopure substances, but its technological implementation suffers from well-known impediments like the lack of endurable catalysts exhibiting long-term stability. The catalytically active intermetallic compound Palladium-Gallium (PdGa), conserving innate bulk chirality on its surfaces, represent a promising system to study asymmetric chemical reactions by heterogeneous catalysis, with prospective relevance for industrial processes. Here, this work investigates the adsorption of 10,10'-dibromo-9,9'-bianthracene (DBBA) on the PdGa:A(1 ¯ 1 ¯ 1 ¯ $\bar{1}\bar{1}\bar{1}$ ) Pd3-terminated surface by means of scanning tunneling microscopy (STM) and spectroscopy (STS). A highly enantioselective adsorption of the molecule evolving into a near 100% enantiomeric excess below room temperature is observed. This exceptionally high enantiomeric excess is attributed to temperature activated conversion of the S to the R chiral conformer. Tip-induced bond cleavage of the R conformer shows a very high regioselectivity of the DBBA debromination. The experimental results are interpreted by density functional theory atomistic simulations. This work extends the knowledge of chirality transfer onto the enantioselective adsorption of non-planar molecules and manifests the ensemble effect of PdGa surfaces resulting in robust regioselective debromination.

Intermetallic Nanocrystals for Fuel-Cells-Based Electrocatalysis

Electrocatalysis underpins the renewable electrochemical conversions for sustainability, which further replies on metallic nanocrystals as vital electrocatalysts. Intermetallic nanocrystals have been known to show distinct properties compared to their disordered counterparts, and been long explored for functional improvements. Tremendous progresses have been made in the past few years, with notable trend of more precise engineering down to an atomic level and the investigation transferring into more practical membrane electrode assembly (MEA), which motivates this timely review. After addressing the basic thermodynamic and kinetic fundamentals, we discuss classic and latest synthetic strategies that enable not only the formation of intermetallic phase but also the rational control of other catalysis-determinant structural parameters, such as size and morphology. We also demonstrate the emerging intermetallic nanomaterials for potentially further advancement in energy electrocatalysis. Then, we discuss the state-of-the-art characterizations and representative intermetallic electrocatalysts with emphasis on oxygen reduction reaction evaluated in a MEA setup. We summarize this review by laying out existing challenges and offering perspective on future research directions toward practicing intermetallic electrocatalysts for energy conversions.

Fundamental understanding of the synthesis of well-defined supported non-noble metal intermetallic compound nanoparticles

Fundamental insights into the synthesis of model-like, supported, non-noble metal intermetallic compound nanoparticle catalysts with phase pure bulk and bulk-like 1st-atomic-layer particle surface composition.

The impact of synthetic method on the catalytic application of intermetallic nanoparticles

Intermetallic alloy nanocrystals have emerged as a promising next generation of nanocatalyst, largely due to their promise of surface tunability. Atomic control of the geometric and electronic structure of the nanoparticle surface offers a precise command of the catalytic surface, with the potential for creating homogeneous active sites that extend over the entire nanoparticle. Realizing this promise, however, has been limited by synthetic difficulties, imparted by differences in parent metal crystal structure, reduction potential, and atomic size. Further, little attention has been paid to the impact of synthetic method on catalytic application. In this review, we seek to connect the two, organizing the current synthesis methods and catalytic scope of intermetallic nanoparticles and suggesting areas where more work is needed. Such analysis should help to guide future intermetallic nanoparticle development, with the ultimate goal of generating precisely controlled nanocatalysts tailored to catalysis.

Manipulation of Neighboring Palladium and Mercury Atoms for Efficient *OH Transformation in Anodic Alcohol Oxidation and Cathodic Oxygen Reduction Reactions

The synergetic effect of neighboring heterogeneous atoms is capable of enabling unexpected catalytic performance, and the design of a well-ordered atomic structure and elaborating the underlying interactions are crucial for the development of superior electrocatalysts in fuel cells. We demonstrate here that an ordered Pd-Hg intermetallic alloy with dimensions of several nanometers can be subtly manipulated using a mild wet-chemical reduction approach. On the basis of combined results of XPS and HAADF-STEM analysis, the adjacent regions of metallic atoms were found to be evenly occupied by heterogeneous elements from the distribution features of the surface structure. Due to charge transfer from Hg to neighboring Pd, the down-shift of the d-band center in PdHg alloys was theoretically beneficial for desorption of crucial intermediates (*OH), both in anodic ethanol oxidation reaction (EOR) and in cathodic oxygen reduction reaction (ORR). In the presence of Hg atoms with lower *OH desorption energy, the rapid dissociation of *OH from Pd facilitated the final H₂O formation, with superior ORR efficiency comparable to Pt/C catalysts. Remarkably, the rapid combination of *OH on Hg atoms with CH₃CO* on neighboring Pd atoms unambiguously favored generation of acetate ions (rate-determining) in the catalytic EOR process, resulting in a high mass activity (7.68 A per mgPd). This well-ordered atomic structure also shows excellent long-term stability in ethylene glycol oxidation reaction and ORR.

Transition Metal-Free Alkyne Hydrogenation Catalysis with BaGa2, a Hydrogen Absorbing Layered Zintl Phase

Inexpensive, transition metal-free intermetallic compounds have received almost no attention as heterogeneous catalysts. Here, we show that BaGa₂, a Zintl-Klemm compound composed of honeycomb sheets of Ga^- anions separated by Ba²⁺ cations and known to react with H₂ under moderate conditions to form a layered polyanionic hydride BaGa₂H₂, effectively catalyzes the hydrogenation of phenylacetylene into styrene and ethylbenzene under modest conditions (1-50 bar H₂, 40-100 °C). Remarkably, the catalytic activity of BaGa₂ (surface specific activities up to 8390 h^-1) is on the same order of magnitude as commercial Pd-based catalysts. In contrast, BaGa₂H₂ shows negligible catalytic activity, thereby indicating that the unsaturated Ga^- framework is necessary for phenylacetylene and styrene adsorption. These findings open up future explorations of utilizing and optimizing the long-term stability of transition metal-free intermetallic hydrogen absorbing compounds for hydrogen-based catalysis.

Catalytic properties of Al13TM4 complex intermetallics: influence of the transition metal and the surface orientation on butadiene hydrogenation

Complex intermetallic compounds such as transition metal (TM) aluminides are promising alternatives to expensive Pd-based catalysts, in particular for the semi-hydrogenation of alkynes or alkadienes. Here, we compare the gas-phase butadiene hydrogenation performances of o-Al₁₃Co₄(100), m-Al₁₃Fe₄(010) and m-Al₁₃Ru₄(010) surfaces, whose bulk terminated structural models exhibit similar cluster-like arrangements. Moreover, the effect of the surface orientation is assessed through a comparison between o-Al₁₃Co₄(100) and o-Al₁₃Co₄(010). As a result, the following room-temperature activity order is determined: Al₁₃Co₄(100) < Al₁₃Co₄(010) < Al₁₃Ru₄(010) < Al₁₃Fe₄(010). Moreover, Al₁₃Co₄(010) is found to be the most active surface at 110°C, and even more selective to butene (100%) than previously investigated Al₁₃Fe₄(010). DFT calculations show that the activity and selectivity results can be rationalized through the determination of butadiene and butene adsorption energies; in contrast, hydrogen adsorption energies do not scale with the catalytic activities. Moreover, the calculation of projected densities of states provides an insight into the Al₁₃TM₄ surface electronic structure. Isolating the TM active centers within the Al matrix induces a narrowing of the TM d-band, which leads to the high catalytic performances of Al₁₃TM₄ compounds.

The shape-controlled synthesis of gallium-palladium (GaPd2) nanomaterials as high-performance electrocatalysts for the hydrogen evolution reaction

Recently, great efforts have been focused on developing more active and stable Pd-based electrocatalysts to partially or completely replace rare and costly Pt. We developed a facile hot injection method and successfully synthesized well-dispersed and shape-controlled GaPd2 nanomaterials including polyhedrons, nanoparticles and nanowires. All the as-synthesized catalysts exhibit superior HER activity compared to commercial pure Pd catalysts and are stable in acidic media. Among them, the GaPd2 nanoparticles required only 24.3 mV overpotential to achieve a 10 mA cm-2 current density, which is outstanding compared to most Pt-based nanomaterials. Also, cycling tests over 10 000 CV sweep cycles (-0.3 to 0.2 vs. RHE) and durability testing for 24 hours were applied, with the GaPd2 catalysts exhibiting similar i-V curves and stable current densities to those obtained in the initial tests. We further evaluated the mass activities of the GaPd2 catalysts, and it is fascinating that the GaPd2 polyhedrons, nanoparticles and nanowires achieved factors of 3.7, 5 and 2.3 enhancement in mass activity at -0.1 V vs. RHE compared with a commercial Pd black catalyst. Meanwhile, with the assistance of a reduced graphene oxide (rGO) support, the GaPd2 nanoparticles/rGO (20 wt%) electrocatalyst presents outstanding HER activity comparable with that of a carbon-supported Pt catalyst (20% Pt/C). This work provides an avenue to develop effective and stable Pd-based catalysts with reduced Pd usage and high HER performance.

Cost-effective promoter-doped Cu-based bimetallic catalysts for the selective hydrogenation of C2H2 to C2H4: the effect of the promoter on selectivity and activity

Different metal promoter M-modified Cu catalysts have been employed to fully investigate the selective hydrogenation of C₂H₂ using density functional theory calculations together with microkinetic modeling.