PTO

Ethyl 2-nitro-3-(5-benzyloxyindoT3-yl)propanoate (3.7 g, 0.01 mol) was dissolved in abs. ethanol (50 ml) and hydrogenated over PtO catalyst (EOg) until H2 uptake ceased (about 1.75 h). The solution was purged with nitrogen and 20% aq. NaOH solution (4.0 g) w as added. A hydrogen atmosphere was re-established and the hydrolysis was allowed to proceed overnight. The solution was diluted with water (20 ml) and filtered. The pH of the filtrate was adjusted to 6 with HOAc and heated to provide a solid precipitate. The mixture was cooled and filtered to provide 5-benzyloxytryptophan (2.64 g). 

Kaiser and Muchowski (41) reduced A -(r-butoxycarbonyl) pyrroles to the corresponding pyrrolidines over 5% Pt-on-C at room temperature and atmospheric pressure. Under these conditions 0-benzyl groups are retained and 2,5-disubstituted pyrroles are reduced mainly or exclusively to thecis-2,5-disubstituted pyrrolidines. In some cases Pt-on-C proved superior to either Rh-on-C or PtO,. 

Figure 8. SPAIR spectra of the adsorbed intermediates involved in the oxidation of 0.1 M CH3OH in 0.5 M HCIO4 on a Pto 9Ruo i alloy dispersed electrode (p-polar-ized light modulation potential A = 0.3 V reference spectrum taken at 50 mV/RHE averaging of 128 inter-ferograras). Electrode potential (mV/RHE) (1) 100,(2) 150, (3) 200, (4) 250, (5) 300, (6) 350, (7) 400, (8) 450, (9) 500, (10) 550.

For the a-Pt02 system, we find that above an electrode potential of 1.2 V, the (001) surface with bulk composition is most stable and shows only minor relaxation effects (denoted as (OOl)-O in Fig. 5.11a). This surface structure corresponds to experimental UHV measurements of surface oxides on Pt(llO), supported by DFT calculations [Li et al., 2004]. In the case of a very thin surface layer, the layer composition might even be PtO. Increasing the electrode potential above 2.0 V would cause stronger interactions with the surrounding water dipoles and lead to a o -PtO2(011) surface with an enrichment of oxygen (as 0 ) on the surface. 

Figure 9.6 Visual representation of the platinum oxide growth mechanism, (a) Interaction of H2O molecules with the Pt electrode occurring in the 0.27 V < < 0.85 V range, (b) Discharge of 5 ML of H2O molecules and formation of 5 ML of chemisorbed oxygen (Ochem)- (c) Discharge of the second ML of H2O molecules the process is accompanied by the development of repulsive interactions between (Pt-Pt) -Ofi m surface species that stimulate an interfacial place exchange of Ochem and Pt surface atoms, (d) Quasi-3D surface PtO lattice, comprising Pt and moieties, that forms through the place-exchange process. (Reproduced with permission...

Figure 9.19 The Pt and Ir L3 edge in situ XANES spectra for Pto.8lro.2/Pd/C (20 nmol Pd) in 1 M HCIO4 at 0.47 V. The Pd loading in these nanoparticles has been doubled to enhance the Ir signal in the specific experiment. (Reproduced with permission from Vukmirovic et al. [2007].)...

Potential cycling has been found to accelerate Pt dissolution compared with poten-tiostatic conditions. The dissolution mechanisms and dissolved species involved in this process are unclear [Johnson et al., 1970 Kinoshita et al., 1973 Ota et al., 1988 Rand and Woods, 1972]. Darling and Meyers have developed a mathematical model based on (9.5)-(9.7) to smdy Pt dissolution and movement in a PEMFC during potential cycling from 0.87 to 1.2 V [Darling and Meyers, 2003, 2005]. Severe Pt dissolution occurs when the potential switches to the upper limit potential (1.2 V), and then stops once a monolayer of PtO has formed. The charge difference between the anodic and cathodic cycles was found to be consistent with the amount... 

Figure 11.1 Kinetics of adsorption of CO at a Pt catalyst from a 0.01 M methanol solution at different potentials, (a) Pt black catalyst, with Pt loading 0.8 mg cm . (b) Pto.sRuo.s black catalyst, with catalyst loading 0.8 mg cm (0.1 M H2SO4, T = 298K).

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