Chemical Insights into the Formation of Colloidal High Entropy Alloy Nanoparticles.

Nanoparticles of high entropy alloys (HEAs) have distinct properties that result from their high surface-to-volume ratios coupled with synergistic interactions among their five or more constituent elements, which are randomly distributed throughout a crystalline lattice. Methods to synthesize HEA nanoparticles are emerging, including solution approaches that yield colloidal products. However, the complex multielement compositions of HEA nanoparticles make it challenging to identify and understand their reaction chemistry and the pathways by which they form, which hinders their rational synthesis. Here, we demonstrate the synthesis and elucidate the reaction pathways of seven colloidal HEA nanoparticle systems that contain various combinations of noble metals (Pd, Pt, Rh, Ir), 3d transition metals (Ni, Fe, Co), and a p-block element (Sn). The nanoparticles were synthesized by slowly injecting a solution containing all five constituent metal salts into oleylamine and octadecene at 275 °C. Using NiPdPtRhIr as a lead system, we confirmed the homogeneous colocalization of all five elements and achieved tunable compositions by varying their ratios. We also observed heterogeneities, including Pd-rich regions, in a subpopulation of the NiPdPtRhIr sample. Halting the reaction at early time points and characterizing the isolated products revealed a time-dependent composition evolution from Pd-rich NiPd seeds to the final NiPdPtRhIr HEA. Similar reactions applied to FePdPtRhIr, CoPdPtRhIr, NiFePdPtIr, and NiFeCoPdPt, with modified conditions to most efficiently incorporate all five elements into each HEA, also revealed similar Pd-rich seeds with system-dependent differences in the rates and sequences of element uptake into the nanoparticles. When moving to SnPdPtRhIr and NiSnPdPtIr, the time-dependent formation pathway was more consistent with simultaneous coreduction rather than through formation of reactive seeds. These studies reveal important similarities and differences among the pathways by which different colloidal HEA nanoparticles form using the same synthetic method, as well as establish generality. The results provide guidelines for incorporating a range of different elements into HEA nanoparticles, ultimately providing fundamental knowledge about how to define and optimize synthetic protocols, expand into different HEA nanoparticle systems, and achieve high phase purity.

Defect-mediated selective hydrogenation of nitroarenes on nanostructured WS2.

Transition metal dichalcogenides (TMDs) are well known catalysts as both bulk and nanoscale materials. Two-dimensional (2-D) TMDs, which contain single- and few-layer nanosheets, are increasingly studied as catalytic materials because of their unique thickness-dependent properties and high surface areas. Here, colloidal 2H-WS2 nanostructures are used as a model 2-D TMD system to understand how high catalytic activity and selectivity can be achieved for useful organic transformations. Free-standing, colloidal 2H-WS2 nanostructures containing few-layer nanosheets are shown to catalyze the selective hydrogenation of a broad scope of substituted nitroarenes to their corresponding aniline derivatives in the presence of other reducible functional groups. Microscopic and computational studies reveal the important roles of sulfur vacancy-rich basal planes and tungsten-terminated edges, which are more abundant in nanostructured 2-D materials than in their bulk counterparts, in enabling the functional group selectivity. At tungsten-terminated edges and on regions of the basal planes having high concentrations of sulfur vacancies, vertical adsorption of the nitroarene is favored, thus facilitating hydrogen transfer exclusively to the nitro group due to geometric effects. At lower sulfur vacancy concentrations on the basal planes, parallel adsorption of the nitroarene is favored, and the nitro group is selectively hydrogenated due to a lower kinetic barrier. These mechanistic insights reveal how the various defect structures and configurations on 2-D TMD nanostructures facilitate functional group selectivity through distinct mechanisms that depend upon the adsorption geometry, which may have important implications for the design of new and enhanced 2-D catalytic materials across a potentially broad scope of reactions.

Bulk iron pyrite as a catalyst for the selective hydrogenation of nitroarenes.

Bulk iron pyrite (FeS2) functions as an inexpensive, Earth-abundant, off-the-shelf catalyst capable of selectively hydrogenating a broad scope of substituted nitroarenes to their corresponding aniline derivatives using molecular hydrogen.