No abstract

Although there are only a few known examples of supported single-atom catalysts, they are unique because they bridge the gap between homogeneous and heterogeneous catalysis. Here, we report the CO oxidation activity of monodisperse single Pt atoms supported on an inert substrate, θ-alumina (Al2O3), in the presence of stoichiometric oxygen. Since CO oxidation on single Pt atoms cannot occur via a conventional Langmuir-Hinshelwood scheme (L-H scheme) which requires at least one Pt-Pt bond, we carried out a first-principles density functional theoretical study of a proposed pathway which is a variation on the conventional L-H scheme and inspired by the organometallic chemistry of platinum. We find that a single supported Pt atom prefers to bond to O2 over CO. CO then bonds with the oxygenated Pt atom and forms a carbonate which dissociates to liberate CO2, leaving an oxygen atom on Pt. Subsequent reaction with another CO molecule regenerates the single-atom catalyst. The energetics of the proposed mechanism suggests that the single Pt atoms will get covered with CO3 unless the temperature is raised to eliminate CO2. We find evidence for CO3 coverage at room temperature supporting the proposed mechanism in an in situ diffuse reflectance infrared study of CO adsorption on the catalyst's supported single atoms. Thus, our results clearly show that supported Pt single atoms are catalytically active and that this catalytic activity can occur without involving the substrate. Characterization by electron microscopy and X-ray absorption studies of the monodisperse Pt/θ-Al2O3 are also presented.

A grand challenge in materials research is to understand complex electronic correlation and non-equilibrium atomic interactions, and how such intrinsic properties and dynamic processes affect energy transfer and defect evolution in irradiated materials. Here we report that chemical disorder, with an increasing number of principal elements and/or altered concentrations of specific elements, in single-phase concentrated solid solution alloys can lead to substantial reduction in electron mean free path and orders of magnitude decrease in electrical and thermal conductivity. The subsequently slow energy dissipation affects defect dynamics at the early stages, and consequentially may result in less deleterious defects. Suppressed damage accumulation with increasing chemical disorder from pure nickel to binary and to more complex quaternary solid solutions is observed. Understanding and controlling energy dissipation and defect dynamics by altering alloy complexity may pave the way for new design principles of radiation-tolerant structural alloys for energy applications.

High-entropy alloys constitute a new class of materials whose very existence poses fundamental questions regarding the physical principles underlying their unusual phase stability. Originally thought to be stabilized by the large entropy of mixing associated with their large number of components (five or more), these alloys have attracted attention for their potential applications. Yet, no model capable of robustly predicting which combinations of elements will form a single phase currently exists. Here, we propose a model that, through the use of high-throughput computation of the enthalpies of formation of binary compounds, predicts specific combinations of elements most likely to form single-phase, highentropy alloys. The model correctly identifies all known single-phase alloys while rejecting similar elemental combinations that are known to form an alloy comprising multiple phases. In addition, we predict numerous potential single-phase alloy compositions and provide three tables with the ten most likely five-, six-, and seven-component single-phase alloys to guide experimental searches. The term high-entropy alloy (HEA) has come to signify nontraditional alloy systems composed of five or more elements at, or near, equiatomic ratio that form random, single-phase solid solutions on simple underlying facecentered-cubic (fcc) and body-centered-cubic (bcc) lattices [1][2][3][4][5][6][7][8][9][10][11]. HEAs stand in sharp contrast to traditional metal alloys that are typically based on one or two primary elements and where addition of further alloying elements often leads to the formation of new phases. Clearly, the existence of HEAs poses important questions regarding the driving mechanism for their unexpected stability and how to identify the specific combinations of elements that are most likely to form a single-phase HEA.Although there are several proposals regarding the stability of HEAs, much of the existing work uses semiempirical approaches based, for example, on HumeRothery rules, thus focusing on the differences of the atomic sizes (δ), electronegativities (Δχ), and electron-toatom ratio (e=a) [12][13][14][15][16][17]. Some approaches utilize calculation of phase diagrams methods [18], while others consider δ, the enthalpy of mixing (ΔH mix ), and the ideal entropy of mixing of the alloys to develop criteria for the phase stability [1,12]. For example, in the work of Guo et al. [14], the use of δ and ΔH mix as independent variables clearly separates solid-solution phases from amorphous phases but does not necessarily isolate intermetallic compounds from either one of these phases. In addition, in the work of Otto et al. [11], there are atomic substitutions to the solid-solution CrMnFeCoNi alloy that are specifically chosen to follow the Hume-Rothery rules in respect of δ, Δχ, and crystal structure yet do not form a single-phase solid solution. Useful as many of these attempts to encapsulate features of the underlying bonding mechanisms have been, it is clear that a model that can robustly predict, out ...