Defect-rich 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 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)... 

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]. 

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...

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... 

Fig. 7 (A) MIE spectra of an MgO thin film at 95 K as a function of NO exposure. The Upper-most spectrum corresponds to that of the clean MgO(lOO) surface while the bottommost spectrum is that after a 307 L exposure of NO (B) MIE spectra of defect-rich MgO thin film at 95 K as a function of NO exposure. The uppermost spectrum corresponds to the defect-rich MgO film and the top-most spectrum to the 307 L-thick NO coverage (C) UP spectra of NO adsorbed on a defect-rich MgO film at 95 K. The dotted lines denote changes in the work function.

Fig. 17.8. TPR spectra of the catalytic formation of CeHe, C4H6, and C4H8 for a defect-rich MgO thin film, Pd], Pd4, Pde, Pdg, Pdi3, Pd20r and Pd3o- The relative ion...

Fig. 1.69. Left TPR experiments for the CO oxidation on selected Au clusters on defect-rich MgO(lOO) films. The model catalysts were saturated at 90K with and 02, and the isotopomer was detected with a mass spectrometer as...

Detailed Discussion of the Reactivity on Aug. When deposited on MgO(lOO) films with a vanishing density of F centers, Aug is inert (Fig. 1.70a). Surprisingly, Aug turns active on defect-rich MgO(lOO) films and forms CO2 at 140 and 280 K (Fig. 1.70b). Evaluating the area of the TPR signal yields... 

Fig. 1.100. TPR spectra of CeHe formed on Ag, Pd, and Rh atoms deposited on defect-rich MgO thin films grown on Mo(lOO) surfaces. For comparison, the same experiment was performed on a clean defect-rich MgO film. Shown is also the calculated (C4H4)(C2H2)/Pdi/FBc intermediate of the cyclotrimerization reaction on Pd atoms adsorbed on an F center of the MgO(lOO) surface. For Pd atoms, the formation of benzene was also observed at 220 K...

Fig. 1.104. Reactivity (a) (expressed as the number of product molecules per cluster) and selectivity (b) (expressed as the relative amount in percent) of the polymerization of C2H2 on size-selected Pd (n = 1-30) deposited on defect-rich MgO thin films. Also shown is the relative number of reacted C2H2 as function of cluster...

Electronic and optical properties of structured thin films and heterojunctions are treated in Section 6.4. Since non-optimal electronic properties are to be accommodated in the new cell design, the transport properties to be discussed will naturally be those of defect-rich materials. 

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