Physical Review B

Single-atom heterogeneous catalysts (SACs) attached to carefully chosen hosts are attracting considerable interest; principally because they offer maximum utilization per metal atom and are usually readily recyclable. However, diminution of the atomic population of nanoparticles or nanoclusters to single atoms can significantly alter reactivity because of the consequent changes in the active-site structure. By examining various diverse applications, we ascertain whether the performance of SACs is enhanced or suppressed. We also note that SACs generally display unique kinds of catalytic cycles. The choice of host is crucial since it influences both the electronic and steric environment of the metal center. Moreover, it may function as a cocatalyst. All these aspects impact upon the design of new SACs, which exhibit similarities to hetero- and homogeneous predecessors. Additionally, SACs offer a viable replacement of soluble metal complexes in processes that remain difficult to heterogenize.

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The Multifaceted Reactivity of Single-Atom Heterogeneous Catalysts.

The atomic dispersing of metal atoms supported on an optimal substrate exhibits an ideal strategy for maximizing metal utilization for catalysis, which is particularly significant for exploiting new catalysts with low cost and high catalytic efficiency. The dramatic development of atomic metal catalysts, including single atom catalysts (SACs), double atoms catalysts (DACs), and triple atoms catalysts (TACs), has spawned two remarkable platforms: (1) bridging homogeneous catalysts and heterogeneous catalysts; (2) linking theoretical calculations and experimental results. In this review, recent syntheses, characterizations, and applications of SACs, DACs, and TACs are highlighted through a focus on various applied substrates. We extensively discuss the synthetic strategies of successfully achieving SACs, DACs, and TACs. Moreover, the opportunities and challenges in developing SACs, DACs, and TACs are pointed out, together with the prospects for the development of atomic catalysis.

Atomic (single, double, and triple atoms) catalysis: frontiers, opportunities, and challenges

Selective hydrogenolysis of biomass-derived glycerol to propanediol is an important reaction to produce high value-added chemicals but remains a big challenge. Herein we report a PtCu single atom alloy (SAA) catalyst with single Pt atom dispersed on Cu nanoclusters, which exhibits dramatically boosted catalytic performance (yield: 98.8%) towards glycerol hydrogenolysis to 1,2-propanediol. Remarkably, the turnover frequency reaches up to 2.6 × 103 molglycerol·molPtCu–SAA−1·h−1, which is to our knowledge the largest value among reported heterogeneous metal catalysts. Both in situ experimental studies and theoretical calculations verify interface sites of PtCu–SAA serve as intrinsic active sites, in which the single Pt atom facilitates the breakage of central C–H bond whilst the terminal C–O bond undergoes dissociation adsorption on adjacent Cu atom. This interfacial synergistic catalysis based on PtCu–SAA changes the reaction pathway with a decreased activation energy, which can be extended to other noble metal alloy systems. Selective hydrogenolysis of biomass glycerol to propanediol is a promising route for the production of high-value chemicals but remains a challenge. Here, the authors find a PtCu single atom alloy catalyst exhibits remarkably boosted performance with a turnover frequency value of 2.6 × 103 molglycerol·molPtCu–SAA−1·h−1.

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Platinum-copper single atom alloy catalysts with high performance towards glycerol hydrogenolysis.

Single-atom catalysts (SACs), with atomically distributed active metal sites on supports, serve as a newly advanced material in catalysis, and open broad prospects for a wide variety of catalytic processes owing to their unique catalytic behaviors. To construct SACs with precise structures and high density of accessible single-atom sites, while preventing aggregation to large nanoparticles, various strategies for their chemical synthesis have been recently developed by improving the distribution and chemical bonding of active sites on supports, which results in excellent activity and selectivity in a variety of catalytic reactions. Noble-metal-based SACs are discussed, and their structural properties, chemical synthesis, and catalytic applications are highlighted. The structure-activity relationships and the underlying catalytic mechanisms are addressed, including the influences of surface species and reducibility of supports on the activity and stability, impact of the unique structural and electronic properties of single-atom centers modulated by metal/support interactions on catalytic activity and selectivity, and how the modified catalytic mechanism obtained by inhibiting the multiatoms involves catalytic pathways. Finally, the prospects and challenges for development in this field are highlighted.

Supported Noble-Metal Single Atoms for Heterogeneous Catalysis.

Eight years since the discovery of the binary antiferromagnetic (AFM) quasicrystal approximants $\mathrm{C}{\mathrm{d}}_{6}R$ [R. Tamura et al., Phys. Rev. B 82, 220201 (2010)], we report the observation of an AFM transition in quasicrystal approximants Au-Al-R ($R=\mathrm{Gd}$ and Tb), which represent examples of AFM in ternary quasicrystal approximants. Magnetic susceptibility, magnetization, and specific heat coherently show that the Au-Al-R ($R=\mathrm{Gd}$ and Tb) approximants undergo an AFM transition at ${T}_{\mathrm{N}}=9.4$ and 11.8 K, respectively. The AFM transitions are found near the phase boundary of the wide single-phase region where the chemical disorder is substantially low. This finding of ternary AFM approximants has far deeper implications compared with that of the binary AFM approximants: It now indicates the possibility to synthesize new AFM approximants in many other systems by the appropriate replacement of Cd with two other elements, and thus enables exploration of various magnetic orders over AFM spin icosahedra in solids, in particular, their evolution as a function of the electron-per-atom (e/a) ratio. Moreover, the finding provides us a missing route to synthesize an AFM quasicrystal by similar composition tuning.

Antiferromagnetic order is possible in ternary quasicrystal approximants

The atomic structure of a 0.2 atom % Pt-doped complex metallic alloy, monoclinic Al13Fe4, was investigated using a single crystal prepared by the Czochralski method. High-angle annular dark-field scanning transmission electron microscopy showed that the Pt atoms were dispersed as single atoms and substituted at Fe sites in Al13Fe4. Single-crystal X-ray structural analysis revealed that the Pt atoms preferentially substitute at Fe(1). Unlike those that have been reported, Pt single atoms in the surface layers showed lower activity and selectivity than those of Al2Pt and bulk Pt for propyne hydrogenation, indicating that the active state of a given single-atom Pt site is strongly dominated by the bonding to surrounding Al atoms.

Probing Single Pt Atoms in Complex Intermetallic Al13Fe4.

Quasicrystals (QCs), first discovered in 1984, generally do not exhibit long-range magnetic order. Here, we report on long-range magnetic order in the real icosahedral quasicrystals (i QCs) Au-Ga-Gd and Au-Ga-Tb. The Au65Ga20Gd15i QC exhibits a ferromagnetic transition at TC = 23 K, manifested as a sharp anomaly in both magnetic susceptibility and specific heat measurements, along with an appearance of magnetic Bragg peak below TC. This is the first observation of long-range magnetic order in a real quasicrystal, in contrast to the spin-glass-like behaviors observed for the other magnetic quasicrystals found to date. Moreover, when Gd is replaced by Tb, i.e., for the Au65Ga20Tb15i QC, a ferromagnetic behavior is still retained with TC = 16 K. Although the sharp anomaly in the specific heat observed for the Au65Ga20Gd15i QC becomes broadened upon Tb substitution, neutron diffraction experiments clearly show marked development of magnetic Bragg peaks just below TC, indicating long-range magnetic order for the Au65Ga20Tb15i QC also. Our findings can contribute to the further investigation of exotic magnetic orders formed on real quasiperiodic lattices with unprecedented highest global symmetry, i.e., icosahedral symmetry.

Experimental Observation of Long-Range Magnetic Order in Icosahedral Quasicrystals.

Magnetism of tsai-type quasicrystal approximants

We report the observation of antiferromagnetic (AFM) transitions in 1/1 and 2/1 quasicrystal approximants having slightly different electron-per-atom $(e/a)$ ratios, in the same Au-Ga-Eu system, at N\'eel temperatures of 7.0 and 8.5 K, respectively. This clearly demonstrates that an antiferromagnetic order survives when the degree of approximation to the quasicrystals is raised provided that the $e/a$ ratio is kept nearly the same. Since, unlike the 1/1 approximant, the 2/1 approximant has all the building blocks necessary for the construction of the quasicrystal, this finding can be a significant leap toward the long-sought realization of an AFM quasicrystal.

Tsunetomo Yamada

Papers

Antiferromagnetic order is possible in ternary quasicrystal approximants

Probing Single Pt Atoms in Complex Intermetallic Al13Fe4.

Experimental Observation of Long-Range Magnetic Order in Icosahedral Quasicrystals.

Magnetism of tsai-type quasicrystal approximants

Antiferromagnetic order survives in the higher-order quasicrystal approximant