Gold clusters, supported

Grinding conditions are modified by the nature of supports. Gold clusters with a diameter of 1.5 nm were obtained on an aluminum-containing porous coordination polymer by grinding in an agate mortar in air for 20 min, followed by the reduction in a stream of H2 in N2 at 120°C for 2 h ]50]. In contrast, ball milling is favorable... 

A number of years ago, Lee et al. [117] reported XPS measurements on supported gold clusters, using variable photon energies. From the development of the shape of the valence band as a function of cluster size and measured as a function of energy, they concluded that the average number of gold atoms needed to approach metallic behavior must be about 200. This is not in conflict with our conclusion of metallic binding, since their work is concerned more with the development of the crystalline momentum k as a valid quantum number than with the delocalization of the valence electrons. 

Since Haruta s initial report (Sanchez et al., 1997) of the unexpected activity of supported gold catalysts for low-temperature CO oxidation, there has been a resurgence of research and interest in gold-mediated catalysis. Supported gold clusters have since been found to be active in a... 

HIGH PRESSURE STM STUDIES OF SUPPORTED GOLD CLUSTERS... 

High pressure STM studies of supported gold clusters 399... 

Vajda S, Winans RE, Elam JW, Lee BD, PeUin MJ, Seifert S, Tikhonov GY, Tomczyk NA (2006) Supported gold clusters and cluster-based nanomaterials characterization, stability and growth studies by in situ GISAXS under vacuum conditions and in the presence of hydrogen. Top Catal 39 161... 

Catalytic Turn-Over-Frequency. From the kinetic fit, it is possible to simulate the CO2 formation rate and thus to obtain the turn-over-frequency (TOF) of the catalytic reaction. The simulated C02-yield under the reaction conditions of Fig. 1.64a is displayed in Fig. 1.68. The corresponding TOF amounts to 0.4 CO2 molecules per gold cluster per second. For the conditions of Fig. 1.64c, a TOF of 0.3 CO2 molecules per gold cluster per second was estimated. These values are in the same order of magnitude as the catalytic activity of oxide supported gold cluster particles with a size of a few nanometers, which ranges between 0.2 s per Au atom ( 2 nm diameter particles at 273 K) and 4s per Au atom (3.5 nm particles at 350 K) [238,366,367]. 

The mechanistic details for the combustion of CO on supported gold clusters are discussed next. Small gold clusters, Au (n < 20) were deposited after size-selection from the gas phase onto defect-poor and defect-rich MgO(lOO) films. As described in Sect. 1.5.1, defect-rich films are characterized by a given density ( 5% ML) of extended defects and point defects (F centers), whereas for defect-poor films the density of F-centers is negligible. The CO-oxidation was studied by combined temperature programmed reaction (TPR) and Fourier transform infrared spectroscopy and the obtained results were compared to extensive ab initio calculations [209,368,369]. 

Coadsorption of O2 and H2 O on Free and Supported Gold Clusters. It has been established experimentally that extended clean gold surfaces are hydrophilic [403]. The calculations showed that water adsorbs (relatively weakly) on free and supported Au clusters (Table 1.4), with adsorption energies that... 

The reaction of Au(CH3)2(acac) with the partially dehydroxylated surface of y-AI2O3 (calcined at 673 K) and the subsequent formation of supported gold clusters... 

Liu, Y, Tsunoyama, H., Akita, T., et al. (2010). Size Effect of Sihca-Supported Gold Clusters in the Microwave-Assisted Oxidation of Benzyl Alcohol with H2O2, Chem. Lett., 39, pp. 159-161. 

In addition to the methods described above, some other methods have been developed to prepare supported gold clusters [62-65]. Huang et al. [62] have... 

HAADF-STEM is also widely used to characterize supported gold clusters. Recent research shows that tiny gold clusters (even a single gold atom) can be observed by aberration-corrected HAADF-STEM, when supported on iron oxides... 

The enhancement of catalytic activity of supported gold clusters such as Aug by moisture was also studied by Bongiorno and Landman [98] with first-principles calculations (Figure 11.20). The results indicated that the introduction of H2O in the feed gas would greatly enhance the adsorption of O2 on Aug. Furthermore, Oj would react with H2O to form hydroperoxyl-hke complexes, even if Aug clusters were supported on a defect-free MgO(lOO) film. The formed hydroperoxyl-like complex was responsible for CO oxidation to CO2 with a small activation barrier of 0.5 eV. 

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