We report a novel Au-Ag alloy catalyst supported on mesoporous aluminosilicate Au-Ag@MCM prepared by a one-pot synthesis procedure, which is very active for low-temperature CO oxidation. The activity was highly dependent on the hydrogen pretreatment conditions. Reduction at 550-650 degrees C led to high activity at room temperature, whereas as-synthesized or calcined samples did not show any activity at the same temperature. Using various characterization techniques, such as XRD, UV-vis, XPS, and EXAFS, we elucidated the structure and surface composition change during calcination and the reduction process. The XRD patterns show that particle size increased only during the calcination process on those Ag-containing samples. XPS and EXAFS data demonstrate that calcination led to complete phase segregation of the Au-Ag alloy and the catalyst surface is greatly enriched with AgBr after the calcination process. However, subsequent reduction treatment removed Br- completely and the Au-Ag alloy was formed again. The surface composition of the reduced Au-Ag@MCM (nominal Au/Ag = 3/1) was more enriched with Ag, with the surface Au/Ag ratio being 0.75. ESR spectra show that superoxides are formed on the surface of the catalyst and its intensity change correlates well with the trend of catalytic activity. A DFT calculation shows that CO and O2 coadsorption on neighboring sites on the Au-Ag alloy was stronger than that on either Au or Ag. The strong synergism in the coadsorption of CO and O2 on the Au-Ag nanoparticle can thus explain the observed synergetic effect in catalysis.

The 13-atom metal clusters of fcc elements (Al, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au) were studied by density functional theory calculations. The global minima were searched for by the ab initio random structure searching method. In addition to some new lowest-energy structures for Pd13 and Au13, we found that the effective coordination numbers of the lowest-energy clusters would increase with the ratio of the dimer-to-bulk bond length. This correlation, together with the electronic structures of the lowest-energy clusters, divides the 13-atom clusters of these fcc elements into two groups (except for Au13, which prefers a two-dimensional structure due to the relativistic effect). Compact-like clusters that are composed exclusively of triangular motifs are preferred for elements without d-electrons (Al) or with (nearly) filled d-band electrons (Ni, Pd, Cu, Ag). Non-compact clusters composed mainly of square motifs connected by some triangular motifs (Rh, Ir, Pt) are favored for elements with unfilled d-band electrons.

Using density functional calculations, we demonstrate a catalytic reaction path with activation barriers of less than 0.5 eV for CO oxidation on the neutral and unsupported icosahedral nanoclusters of Au(55), Ag(55), and Au(25)Ag(30). Both CO and O(2) adsorb more strongly on these clusters than on the corresponding bulk surfaces. The reaction path consists of an intermediate involving OOCO complex through which the coadsorption energy of CO and O(2) on these clusters is expected to play an important role in the reaction. Based on the studies for the Au and Ag nanoclusters, a model alloy nanocluster of Au(25)Ag(30) was designed to provide a larger coadsorption energy for CO and O(2) and was anticipated to be a better catalyst for CO oxidation from energetic analysis.

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We use ab initio density-functional theory supplemented with the embedded-atom method to study the self-diffusion of small clusters on the (111) surface of eight fcc metals. A zigzag motion is found to be important in the dimer and tetramer diffusions. The dimer diffuses by a zigzag and concerted motion. The trimer diffuses by a concerted three-atom motion. The tetramer diffuses through a zigzag motion where only two atoms move simultaneously in each step. Thus, instead of increasing, the migration energy is lowered (or stays constant) for the tetramer as compared to that for the trimer. This novel break of the upwards trend in migration energy is predicted to be a general phenomenon.

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The distribution of Candida species in candidemia was changed. Although C. albicans remained the major species, the isolation of non-C. albicans spp., especially C. glabrata, increased. Patients with candidemia still had high mortalities due to severity of illness and underlying conditions.

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