Platinum dispersed

Oxidation and chlorination of the catalyst are then performed to ensure complete carbon removal, restore the catalyst chloride to its proper level, and maintain full platinum dispersion on the catalyst surface. Typically, the catalyst is oxidized in sufficient oxygen at about 510°C for a period of six hours or more. Sufficient chloride is added, usually as an organic chloride, to restore the chloride content and acid function of the catalyst and to provide redispersion of any platinum agglomeration that may have occurred. The catalyst is then reduced to return the metal components to their active form. This reduction is accompHshed by using a flow of electrolytic hydrogen or recycle gas from another Platforming unit at 400 to 480°C for a period of one to two hours. 

Bett JAS, Kinoshita K, Stonehart P. 1976. Crystallite growth of platinum dispersed on graphi-tized carbon black n. Effect of liquid environment. J Catal 41 124-133. 

Higuchi E, Uchida H, Watanahe M. 2005. Effect of loading level in platinum-dispersed carbon black electrocatalysts on oxygen reduction activity evaluated by rotating disk electrode. J Electroanal Chem 583 69-76. 

A. Martinez-Arias, J. M. Coronado, R. Cataluna, J. C. Conesa, and J. C. Soria, Influence of mutual platinum-dispersed ceria interactions on the promoting effect of ceria for the CO oxidation reaction in a R/Ce02/Al203 catalyst, J. Phys. Chem. B 102,4357 365 (1998). 

The catalyst is made up of platinum dispersed on a basic zeolite, K-L or Ba-K-L [85], The conversion of linear paraffins from to Cg is much faster over Pt-KL or... 

To prepare noble metal on H-mordenite catalysts the noble metal ammino complex-containing material is normally heated in air using staged heating (21, 22, 23, 24). In Ref. 24 the calcination of Pt(NH3)4-NH4 mordenite is discussed in detail, and it is shown that during calcination in air at about 300° C a strongly exothermic reaction occurs, presumably a result of the oxidation of NH3. Data are presented on the influence of calcination conditions on platinum dispersion. 

Converters now in use contain noble metals on a ceramic substrate (e.g., platinum dispersed on alumina). The convener is typically located in the exhaust system in one of two general locations an underfloor location, or a close-coupled location near (he manifold. The operating temperature range lor noble metal catalyst is from 600 In I200 F (316 lo 649 C). which is similar to the exhaust pipe skin temperature range normally encountered or standard automobile engines. 

Catalyst structure. The investigation of a large series of Pt/Al203 catalysts showed that catalysts with dispersions > 0.4 give lower optical yields and lower turnover numbers. However, the platinum dispersion is not a sufficient parameter to explain the enantioselectivities observed for the different catalysts. Other factors such as texture of the support, morphology and size distribution of the platinum particles may affect the catalyst performance as well [30,58,59, 61]. 

A commercially available 5% Pt/Al203 catalyst (Engelhard Industries 4759) was used in this study. The catalyst sample had a mean particle size of 55 pm as measured by light scattering, a BET surface area of 140 m2/g, a mean pore radius of 50 A and a density of 5.0 g/ml. The platinum loading was 4.65%, and the platinum dispersion was 0.28 as measured by static CO titration (ref. 11). 

Colloidal platinum dispersions, prepated by photoreduction of tetrachloroplatina-te(II) ion in the presence of a copolymer of IV-vinyl-2-pyrrolidone and acrylamide, are treated with polyacrylamide gel having amino groups, resulting in stable immobilization of colloidal particles onto the gel. The immobilized catalysts exhibit high activities for the hydrogenation of olefins at 30 °C and 1 atm 87). 

Although the forward reaction is favored by increase in pressure, this is not employed in practice since 97 to 99% conversion of sulfur dioxide to sulfur trioxide can be accomplished at the temperature specified here, provided suitable catalysts are used. The first catalyst used for this reaction consisted of finely divided platinum dispersed in asbestos, anhydrous magnesium sulfate, or silica gel. Other catalysts were later discovered. Mixtures of ferric and cupric oxides are useful, but these are less efficient than platinum. Certain mixtures containing vanadium pentoxide (V205) and other compounds of vanadium appear to be as good as or better than platinum. There has been much controversy over the relative merits of platinum and vanadium catalysts, and only time will provide the answer as to which is best. 

Encouraging improvements in platinum dispersion and decline in sulfate content were observed- However, it was decided to repeat the sulfate removal step to bring down the sulphate content to less than 0 04%wt> which took another 24 hrs. The maximum H2S content... 

Accessible metal fractions were determined by hydrogen chemisorption. The volumetric adsorption experiments were performed at room temperature in a conventional vacuum apparatus. Hydrogen uptake was determined using the double isotherm method, as previously reported (3). The platinum dispersion (D ) was calculated by assuming a... 

Hydrogen escaping from electrolyzers in a plant that operates properly contains 3—5 % oxygen and 0.5—2 % chlorine. Sometimes this can be further utilized. In this event the gas is first scrubbed in towers with a caustic soda solution to remove the chlorine, then it passes into the catalytic chambers, where, at 500 °C the oxygen is removed on a catalyst consisting of platinum dispersed on a suitable carrier. The hot purified hydrogen is cooled by spraying with water, then dried and delivered to where required. 

The catalytic reduction of Cu2+ by hydrogen is faster on small platinum particles since the turn over number increases from 0.37 Cu2+ ions reduced per accessible platinum atom per hour to 3.0 when the platinum dispersion increases from 11% to 54% [8], Thus the catalytic hydrogenation of Cu2+ ions is a structure-sensitive reaction occurring preferentially on small platinum particles. 

These unusual changes in susceptibility, at different degrees of dispersion of the metal on the gel, are at present difficult to explain quantitatively. One may, however, draw some qualitative conclusions from the data. The steep part of the curve at low coverage has a parallel in some work done by Kobozev and coworkers (6), who reported similar increases in the susceptibility of certain salts and metals when dispersed in very small amounts on alumina, silica gel, and charcoal. They also reported exceptionally high susceptibilities in the case of platinum dispersed on charcoal in amounts varying from 0.006 to 0.0001 part per gram. They were unable to explain this supermagnetic phenomenon. 

Table VI shows the effects of oxygen and hydrocarbons in H2 on the platinum functions of the catalyst B-7 after reduction at 482°C. The oxygen concentration up to 10% in H2 keeps the catalyst still in good dispersion and high activity. It was proved by another experiment that all oxygen completely formed water, when a mixture of 10%O2 and 90% H2 was passed through the catalyst at 482°C. It indicates that some oxygen contained in hydrogen dose not give visible effect on the platinum dispersion and its catalytic functions during the reduction stage.

Other physico-chemical characteristics measured for these samples included platinum dispersion parameters (see Table 2 and Figure 1), acidity distribution and hydroxyl concentration (see Table 3). 

Ahn. P.K. et al.. Effect of alkaline earth added to platinum-supporting oxides on platinum dispersion and dehydrogenation activity, Appl. Catal. A, 101. 207, 1993. Tamiu a. H.. Katayama. N.. and Furuichi. R., The Co+ adsorption properties of ALO,. Fe,O,. Fe,O4. TiO,. and MnOj evaluated by modeling with the Frumkin isotherm., 7 Colloid Interf. Sci.. 195. 192, 1997. 

The lowest temperatures of Pt reduction were recorded in the case of catalysts deposited on titania, whereas the highest for catalysts deposited on magnesia. An interesting phenomenon was observed in the case of catalysts deposited on silica. The consumption of hydrogen by these systems was very low. This was probably due to the high degree of platinum dispersion on it. The elucidation of this phenomenon will be the subject of a forthcoming paper. 

Platinum dispersion was measured by hydrogen chemisorption. A dynamic pulse technique was used similar to that reported by Heck et al. Brie%, lOOmg of catalyst was introduced into a quartz glass vertical reactor and kept in porition by 2 plugs of quartz wool A pulse of... 

Two aspects of these results will be discussed here, namely the low platinum dispersion and the presence of significant amounts of coke on alumina and zeolite catalysts after testing. The latter finding is prohahly due to a combination of test tenq>eratures below 2S0°C and the propensity for toluene to form coke. In any event, coke formation did not seem to inhibit the oxidation reaction to any significant extent. The low platinum diversions, confirmed by XRD measurements, indicate that the active reside outside the zeolite micropores on... 

In the upper half of the figure, the left-hand section compares the resonance for a silica-supported osmium catalyst containing 1 wt% osmium with that for pure metallic osmium. The magnitude of the resonance is higher for the osmium dispersed on the support, the extent of increase being indicated by the difference spectrum in the lower left-hand section of the figure. This effect is similar to the results we reported for iridium and platinum dispersed on an alumina support (39). 

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