Catalysts dissolution

A vial containing ( )-4-phenyl-3-butyn-2-ol (73.0 mg, 0.5 mmol) and catalyst (3.3 mg, 5.0 pmol) in t-amyl alcohol (1.0 mL) was capped with a septum and sonicated to assist catalyst dissolution. The resulting purple solution was cooled to 0 °C, and Ac20 (35.4 pL, 0.375 mmol) was added via a syringe. After 49 h, the reaction mixture was quenched by the addition of a large excess of MeOH. After concentration in vacuo, the residue was purified by FC on silica gel (EtOAchexanes, 1 9 to 1 1, then EtOAc hexanes EtsN, 9 9 2) to afford the (R)-acetate (68.6% ee by chiral-GC) and the (S)-alcohol (96.0% ee by chiral-GC on the acetate obtained following esterification). The calculated selectivity value at 58.3% conversion was s = 20.2. 

Finally, the chemical stability of the catalysts employed in this study was tested by means of XRD and EDXS analyses. The examination of fresh and used catalysts shows that during the reaction course metal ions are slowly leached into the aqueous solution, which can be attributed either to the temperature of operation or the presence of complexing carboxylic acids and benzoquinones in the liquid-phase. Contrary to the results obtained in continuous-flow fixed-bed reactors [8, 9], the extent of catalyst dissolution in the slurry reactor was considerable. This is probably due to the higher accumulation of benzoquinones which are known to form stable complexes with metal ions. Examination of the X-ray powder diffraction patterns of the molecular sieves before and after the liquid-phase phenol oxidation... 

Spent Catalyst, Dissolution, Disproportionation, Reverse Precipitataion, Ferrosic Oxide. 

The analysis of the reaction mixture indieated that no catalyst dissolution/leaching occurred during the reaction, i.e. the liquid phase sample from the reactor did not contain any ionic liquid [NB4MPy [BF4 ] within the detection limit of ca. 2 10 g/ml (8.4 10 mol/dm ). According to ICP-MS measurements the reaction mixture contained 45 pg dm (58 ppb) Pd proving that the metal leaching was very low. 

The mechanism of catalyst dissolution occurs to dilfering degrees, depending upon the degree to which platinum is alloyed and what other elements are included in the alloy. Work by Piela et al. [33] suggests that PtRu alloys, as employed in direct methanol fuel cell and reformate anodes to reduce sensitivity to CO, are extremely unstable and that operation leads to ruthenium... 

Another of the most detrimental processes for DMFC lifetime is the catalyst dissolution in the anode side [43]. The present DMFC anode material of choice is an alloy of Pt, Ru and sometimes a third or fourth metal [44—47]. These alloys... 

Three sintering mechanisms have been proposed to explain ECSA loss of fuel cell catalysts catalyst dissolution/reprecipitation, migration of Pt particles, and carbon corrosion [85]. 

In addition, as discussed below, several other factors such as catalyst dissolution and/ or agglomeration " as well as caibon support corrosion are directly related to the loss of electrocatalytic activity of the electrochemical interface. Zhai et al. observed that, after a constant current operation for over 500h, the mean particle size of Pt increased from 4.02 to 8.88 ran, indicating that this... 

For the simulation of the degradation, the model accounts for the numerical feedback between the sub-models describing the non-aging mechanisms (e.g., water transport across the porous electrode) and the sub-models describing the aging mechanisms (e.g., carbon corrosion or catalyst dissolution in the case of PEMFCs) (Fig. 14). At each time step of the simulation, the performance part of the model calculates the... 

For the case of the electrocatalysis, more efforts should be devoted to perform ab initio calculations with solvent and electric field, in relation to the catalyst dissolution and oxidation (how the activity is affected by the catalyst degradation, and conversely, how the catalyst degradation kinetics is affected by the intrinsic catalytic activity ) [42]. 

Pre-macro copic Gelation Monomer Delivery and Catalyst Dissolution... 

Figure 10.7 Fracture surface of a self-healed polymer. Formation of a shell" of cured healing monomer around catalyst particles indicates a mismatch of catalyst dissolution kinetics and healing monomer gelation. Reprinted with permission from Ref. [29].

A nttmber of degradation mechanisms have been proposed for PBI-based MEAs, such as phosphoric acid loss from the membrane, faster catalyst dissolution in the hot acid meditrm, Pt catalyst sintering, thermal stress on fuel cell parts, thermal degradation of the catalyst support and carbon support corrosion. In particttlar, phosphoric acid loss has been specrrlated as a major degradation... 

Two main phenomena have been identified to infiuence the long-term stability of electrocatalysts catalyst dissolution and Ostwald ripening. 

At the nanoscale describes the stmctuial changes of the electrochemical double layer surrounding the catalyst and caibon nanoparticles during the degradation process. These nanoscale models consist of a non-equilibrium compact layer sub-model describing the competitive adsorption of the intermediate reaction species, of the parasitic water molecules and oxide formation on both catalyst and caibon, and of a non-equilibrium diffuse layer sub-model in the electrolyte, describing the transport (electro-migration and diffusion) of protons and metallic ions (produced by the catalyst dissolution) close to the nanoparticles. 

The instantaneous MEA materials structural evolutions (e.g. ECSA, carbon surface area, PEM porosity) in the simulated PEMFC operation, induced by catalyst dissolution and ripening, carbon corrosion and PEM chemical degradation, are determined at each simulation time-step as functions of the elementary degradation chemistry kinetic equations as described. ... 

By employing advanced analytical techniques, more detailed quantitative information can be gained. For example, using a conventional electrochemical cell in combination with a quartz crystal microbalance, Dam and de Bruijn (2007) studied the influence of temperature and potentials on Pt thin-film dissolution in 1 M HCIO4 solution. They detected that the Pt catalyst dissolution rate was accelerated by increasing the temperature and the dissolution potential, which agrees with the Nernst equation. When temperature was increased from 60°C to 80°C, the Pt dissolution rate increased from 0.87 ng h cm" to 1.58 igh cm 2 when exposed to a potential of 1.15 V. However, the amount of Pt dissolution was too small to measure at 40°C. 

The continuous flow oxidation of propylene with O2 to propylene glycol in SCCO2 at 138 bar on a Cu/Cu20/Mn02 catalyst showed strong pressure dependence with a maximum selectivity of 95%. Catalyst dissolution and deactivation did not occur over a run time of 50 hours. However, the space-time yields were still one order of magnitude too low for scale-up. ... 

Load cycling Cathode catalyst surface area loss Membrane pinhole formation Catalyst dissolution by potential cycle Mechanical stress by hydration, pressure and thermal cycle... 

Electrolyte and Matrix Maintenance The hquid electrolyte interface between the electrodes is maintained by a complex force balance involving gas-phase and electrolyte hquid capillary pressure between the anode and cathode and refractory electrolyte matrix. Significant spillage of the electrolyte into the cathode can lead to catalyst dissolution, and the vapor pressure of the electrolyte is nonnegligible, leading to loss of electrolyte through reactant flows. 

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