Platinum catalyst stabilization

An alternative approach to increase the oxidation rate is the use of alkaline solutions, because bases enhance the reactivity of L-sorbose and weaken the adsorption strength of 2-KLG. Unfortunately, the rate enhancement at higher pH is accompanied by a drop in selectivity due to the poor stability of 2-KLG in alkaline solutions. To circumvent this problem, we have modified the platinum catalysts by adsorbed tertiary amines and carried out the oxidation in neutral aqueous solution [57], This allowed to enhance the rate without increasing the pH of the bulk liquid, which leads to detrimental product decomposition. Small quantities of amines (molar ratio of amine sorbose = 1 1700, and amine Pts = 0.1) are sufficient for modification. Using amines of pKa a 10 for modification, resulted in a considerable rate enhancement (up to a factor of 4.6) with only a moderate loss of selectivity to 2-KLG. The rate enhancement caused by the adsorbed amines is mainly determined by their basicity (pKa). In contrast, the selectivity of the oxidation was found to depend strongly on the structure of the amine. 

Homogeneous catalysts have been reported, which can oxidize methane to other functionalized products via C-H activation, involving an electrophilic substitution process. The conversion of methane into methyl bisulfate, using a platinum catalyst, in sulfuric acid, has been described. The researchers found that a bipyrimidine-based ligand could both stabilize and solubilize the cationic platinum species under the strong acidic conditions and TONs of >500 were observed (Equation (5)).13... 

Toshima et al. obtained colloidal dispersions of platinum by hydrogen- and photo-reduction of chloroplatinic acid in an aqueous solution in the presence of various types of surfactants such as dodecyltrimethylammonium (DTAC) and sodium dodecylsulfate (SDS) [60]. The nanoparticles produced by hydrogen reduction are bigger and more widely distributed in size than those resulting from the photo-irradiation method. Hydrogenation of vinylacetate was chosen as a catalytic reaction to test the activity of these surfactant-stabilized colloids. The reaction was performed in water under atmospheric pressure of hydrogen at 30 °C. The photo-reduced colloidal platinum catalysts proved to be best in terms of activity, a fact explained by their higher surface area as a consequence of their smaller size. 

In order to evaluate the catalytic characteristics of colloidal platinum, a comparison of the efficiency of Pt nanoparticles in the quasi-homogeneous reaction shown in Equation 3.7, with that of supported colloids of the same charge and of a conventional heterogeneous platinum catalyst was performed. The quasi-homogeneous colloidal system surpassed the conventional catalyst in turnover frequency by a factor of 3 [157], Enantioselectivity of the reaction (Equation 3.7) in the presence of polyvinyl-pyrrolidone as stabilizer has been studied by Bradley et al. [158,159], who observed that the presence of HC1 in as-prepared cinchona alkaloids modified Pt sols had a marked effect on the rate and reproducibility [158], Removal of HC1 by dialysis improved the performance of the catalysts in both rate and reproducibility. These purified colloidal catalysts can serve as reliable... 

The results presented here correspond to a series of tin-modified platinum catalysts prepared by SOMC/M techniques, which have the characteristics shown in entries 1, 6, 7 and 13 in Table 6.4. Figure 6.10 shows the variation of crotonaldehyde conversion as a function of time for two successive reaction cycles. A characteristic of these catalytic systems is their stability-only a slight flattening is observed for the Pt/Si02 catalyst. The presence of tin seems to improve this stability. Completely reproducible behavior is observed for both cycles, which is an important result mainly for the PtSn-OM system, which contains butyl groups anchored to the surface. 

In order to stabilize the effect of the support on the activity and selectivity for hydrogen production, several supported platinum catalysts have been prepared and tested [282]. At 225 °C, Pt supported on Ti02 is the most active catalyst. In addition,... 

Bimetallic catalysts based on platinum and tin, supported on y-alumina have become very important commercially. Platinum-tin catalysts are widely used in the dehydrogenation of alkanes. The structure of the catalyst and the role of tin have received a lot of attention. Recently Davis [1] reviewed the often contradicting literature about characterization of the bimetallic system. For the dehydrogenation reactions the main purposes with adding tin to a platinum catalyst are to increase the selectivity and stability towards coke formation. 

A patent (230) to Atlantic Richfield Co. claims that hydride platinum group metal carbonyl complexes such as ClRh(PPh3)3 supported on zeolites, for example, NaY, are suitable catalysts for the hydroformylation of low molecular weight olefins. However, since the bulky metal complex cannot diffuse into the inner pores of the zeolite it must simply be adsorbed on the external surface of the support. This is consistent with the rather poor catalyst stability which was attributed to leaching of the active species from the support. 

The electron-releasing 4-substituents are considered to stabilize the 3,4 double bond toward hydrogenation. An electron-releasing 7-substituent (OMe, NH2, or OH) may also contribute to the stabilization of the 3,4 double bond. Thus the hydrogenation of 7-methoxy-4-methylquinazoline to the 3,4-dihydro derivative was successful only in aqueous hydrochloride or aqueous acetic acid, using palladium and platinum catalysts (eq. 12.84).158 No reduction took place at room temperature in nonaqueous solvents such as ethanol, ethanol-HCl, and acetic acid. 

The interest in FDA arises from its possible application as a renewable-derived replacement for terephthalic acid in the manufacture of polyesters. A multitude of oxidation techniques has been applied to the conversion of HMF into FDA but, on account of the green aspect, platinum-catalyzed aerobic oxidation (see Fig. 8.35), which is fast and quantitative [191], is to be preferred over all other options. The deactivation of the platinum catalyst by oxygen, which is a major obstacle in large-scale applications, has been remedied by using a mixed catalyst, such as platinum-lead [192]. Integration of the latter reaction with fructose dehydration would seem attractive in view of the very limited stability of HMF, but has not yet resulted in an improved overall yield [193]. 

Antolini, E., Formation, microstructural characteristics and stability of carbon supported platinum catalysts for low temperature fuel cells, J. Mater. Sci., 38, 2995, 2003. 

The heart of a fuel cell is the membrane electrode assembly (MEA). In the simplest form, the electrode component of the MEA would consist of a thin film containing a highly dispersed nanoparticle platinum catalyst. This catalyst layer is in good contact with the ionomeric membrane, which serves as the reactant gas separator and electrolyte in this cell. The membrane is about 25-100 p,m thick. The MEA then consists of an ionomeric membrane with thin catalyst layers bonded on each side. Porous and electrically conducting carbon paper/cloth current collectors act as gas distributors (Figure 27.1). Since ohmic losses occur within the ionomeric membrane, it is important to maximize the proton conductivity of the membrane, without sacrificing the mechanical and chemical stability. 

In some earlier life tests performed by Wilson and co-workers, PEFCs utilizing thin-film platinum catalyst layers typically experienced a gradual performance loss over the first 500 to 1000 h of operation and then stabilized at about 70% of the original performance (here, performance is described in terms of the current density measured at 0.50 V, i.e., close to the maximum power output of the cell). In such life tests. 

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