Catalysis using gold nanoparticles supported on oxides has been under extensive investigation for many important application processes. However, how to tune the charge state of a given Au species to perform a specific chemical reaction, e.g. CO oxidation, remains elusive. Here, using first-principles calculations, we show clearly that an intrinsically inert Au anion deposited on oxygen-deficient TiO 2 (110) (Au@TiO 2 (110)) can be tuned and optimized into a highly effective single atom catalyst (SAC), due to the depletion of the d-orbital by substrate doping. Particularly, Ni-and Cu-doped Au@TiO 2 complexes undergo a reconstruction driven by one of the two dissociated O atoms upon CO oxidation. The remaining O atom heals the surface oxygen vacancy and results in a stable bow-shaped surface "O-Au-O" species; thereby the highly oxidized Au single atom now exhibits magnetism and dramatically enhanced activity and stability for O 2 activation and CO oxidation, due to the emergence of high density of states near the Fermi level. Based on further extensive calculations, we establish the "charge selection rule" for O 2 activation and CO oxidation on Au: the positively charged Au SAC is more active than its negatively charged counterpart for O 2 activation, and the more positively charged the Au, the more active it is.
Single-atom
catalysts (SACs) are of great scientific and technical
importance due to their low cost, high site density, and high specificity
to enhance chemical reactions. Nevertheless, a major issue that severely
limits the practical exploration of SACs is their instability, i.e.,
the preference of sintering and clustering over a defect-free substrate
during operation. Here, we employ first-principles calculations to
investigate how substrate engineering can stabilize SACs by strain-tuning
the electronic interactions between the metal and the substrate using
two Pd adatoms on a defect-free, single-layer MoS2 as a
typical example. It is identified that the Pd2 dimer is
prone to dissociate and form highly efficient SACs for CO oxidation
due to the enhanced charge transfer and orbital hybridization with
the MoS2 substrate under a suitable tensile strain. The
straining induces a semiconductive-to-metallic phase transition of
the substrate. Moreover, low-cost elements, such as Ag, Ni, Cu, and
Cr, can also be stabilized into high-performance SACs for CO oxidation
with tunable reaction barriers by straining. The present findings
offer a new avenue to inhibit the transition metal atoms from clustering
into nanoclusters/particles and provide a clear guidance for the development
of highly cost-efficient and stable SACs on defect-free substrates.
First-principles calculations have been performed on the electronic structures for Ru n ͑n =2-14͒ clusters. The calculations show that square represents the basic unit in the growth of Ru n clusters. For the stability, odd-even oscillation with respect to the cluster size is observed. Correspondingly, those clusters comprising integer number of square units are magic clusters with high stability. Simple cubic growth mode is detected with cluster size increasing, in line with latest publication. However, for some clusters, such as Ru 13 , more stable structures different from previous calculated results have been found. The calculated magnetic moment for the ground state of Ru 13 ͑0.15 B /atom͒ agrees better with experimental observation ͑Ͻ0.29 B /atom͒ than those obtained before. The current optimized new structure is also found to be favored for some other 13-atom 4d transition metal clusters, such as Tc 13 . Our findings may thoroughly resolve a long standing discrepancy between theories and experiments and contribute basic data for design of novel catalysts.
First-principles calculations are used to systematically investigate the geometric and electronic structures of both pure TM(n) (n=2-4) and Ag-modulated AgTM(n-1) (n=2-4; 3d-transition metal (TM): from Sc to Cu; 4d-TM: from Y to Ag elements) clusters. Some new ground state structures are found for the pure TM(n) clusters, such as a low symmetry configuration for Cr(3), which is found to be about 0.20 eV more stable than the previously reported C(2v) symmetry. In the most cases, Ag-doping can significantly elongate the bond lengths of the clusters and induce geometric distortions of the small clusters from the high dimensional to the low dimensional configurations. Importantly, introduction of Ag significantly changes the electronic structures of the small clusters and modulates the density of states in the proximity of the Fermi levels, which also varies with the size and the type of the cluster. The results contribute to future design of effective bimetallic alloy Ag/TM catalysts.
First-principles total energy calculations within density functional theory have been performed to study the geometric and electronic structures of Ru n nanoclusters of varying size n ͑14Յ n Յ 42͒. Strikingly, for the size range of n = 14 to 38, the clusters always prefer a hexagonal bilayer structure with A-A stacking, rather than some of the more closely packed forms, or bilayer with A-B stacking. Such an intriguing "molecular doublewheel" form is stabilized by substantially enhanced interlayer covalent bonding associated with strong s-d hybridization. Similar A-A stacking is also observed in the ground states or low-lying isomers of the clusters composed of other hcp elements, such as Os, Tc, Re, and Co. Note that these "molecular double-wheels" show enhanced chemical activity toward H 2 O splitting relative to their bulk counterpart, implying its potential applications as nanocatalysts.
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