Defect-poor films

The reaction is conceived to occur with the adsorption of CO on the cluster and the adsorption of oxygen on the particle periphery as shown in Fig. 16.5 [6]. The production of CO was greatly enhanced for Au clusters supported on defect-rich films as compared to clusters supported on defect-poor films. Density functional theory (DFT) calculations indicated the reaction barrier was lowered from 0.8 to... 

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]. 

The interaction of methanol with the defect-poor and defect-rich films was studied using thermal desorption spectroscopy (TDS) (Fig. Ic). For both films, the desorption of physisorbed methanol at around 180 K is most dominant. On the defect-poor films, small amounts of chemisorbed methanol desorb up to around 350 K. On defect-rich films, the desorption of chemisorbed methanol evolves in three distinct peaks at 200, 260, and 340K. A small reproducible feature is observed at around 500 K. Most important, H2 desorbs at 580 K only on defect-rich films. The corresponding infrared spectra taken at 90 K (insets of Fig. Ic) confirm the presence of mainly physisorbed CH3OH with the typical vibrational band for the OH group at 3285 cm , bands of the symmetric C-H stretch (2930 cm"V2828 cm" ) and... 

Bridging. The separation of a paint film from the substrate at internal corners or other depressions due to shrinkage of the film or the formation of paint film over a depression or crack. Undercoats or primers that do not have adequate filling properties will give rise to this defect. Poor surface preparation is another cause. The remedy is to provide adequate surface preparation, and apply an undercoat with good filling properties. A lower application viscosity may also be helpful. 

The interaction of methanol vith the defect-poor and defect-rich films was studied using thermal desorption spectroscopy (TDS) (Figure 17.1c). For both films the desorption of physisorbed methanol at around 180 K is most dominant. 

CH30 -H+). (c) Thermal desorption spectra of CHjOH and H2 on defect-poor and defect-rich MgO(lOO) films. Note the desorption of H2 at 580 l< for defect-rich films. The insets show FTIR spectra recorded at 90 l< for adsorbed CH3OH on both defect-poor and defect-rich films. 

Fig. 1.60. (a) EEL spectra of thin defect-poor and defect-rich MgO(lOO) films grown on Mo(lOO) at different experimental conditions. A-D are losses which are attribnted theoretically to transitions characteristic of neutral F centers on MgO. (b) Model of an oxygen vacancy at a terrace of an MgO(lOO) surface with chemisorbed CH3-OH (CH3OH). (c) Thermal desorption spectra of CH3OH and H2 on defect-poor and defect-rich MgO(lOO) films. Note the desorption of H2 at 580K for defect-rich films. The insets show FTIR spectra recorded at 90 K for adsorbed CH3OH on both defect-poor (a) and defect-rich films (b)... 

Fig. 1.70. TPR experiments for the CO oxidation on Aus clusters on defect-poor (a) and defect-rich (b) MgO(lOO) films. The model catalysts were saturated at 90 K with CO and 02, and the isotopomer C 0 0 was detected with a mass spectrometer as a function of temperature...

Fig. 1.91. Shown are the evolutions of the absolute turn-over frequencies as a function of temperature for (a) Pd-atoms, (b) Pds, and (c) Pdso deposited on defect-poor and defect-rich films, respectively. In (b) the one-heating cycle experiments for Pds are also shown. Note that for Pds on defect-rich films, the contribution of CO2 formed at high temperatures is increased...

Experiments on size-selected gold clusters, Au (2soft-landed on MgO(lOO) thin film [169] showed that the activity in CO oxidation depends on the number of cluster atoms, and is higher when the clusters are supported on defect-rich MgO rather than on defect-poor MgO. Ah initio calculations performed with Aug showed that the F centers on MgO induce a partial electron transfer to the cluster, which promotes the binding of O2 and CO, and activates the 0—0 bond to a peroxo-Uke adsorbate state that can react with gas-phase CO or adsorbed CO. 

Besides phase identification XRD is also widely used for strain and particle size determination in thin films. Both produce peak broadenings, but they are distinguishable. Compared to TEM, XRD has poor area resolution capability, although by using synchrotron radiation beam diameters of a few pm can be obtained. Defect topography in epitaxial films can be determined at this resolution. 

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