Metal dispersion, calcination conditions

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. 

The effect of metal loading on the reducibility was examined with PdNaY (71). For samples calcined at 500°C, the TPR peak maximum shifts from 190 to 150°C the Pd loading increases from 2.0 to 6.7 wt%. This has been attributed to the formation of ion pairs in sodalite cages. The reduction conditions are important for the resultant metal dispersion. TEM and radial electron distribution (RED) evidence shows that reduction of Ir ions in an H2 flow results in much smaller Ir aggregates than reduction under static H2 (173). 

The use of pillared clays as metal supports has also been reported. The more defined interlamelar spacing available with these supports should give a more predictable shape selectivity to the resulting supported metal catalysts. Further, since the pillars prevent the collapse of the layers on drying and further heating, the pillared clay supported metals salts can be calcined and reduced under conditions that can give the best metal dispersion without any concern for a change in the structure of the support. ... 

Table 2 shows the optimum calcination conditions to prepare active Au/Mg(OH)2 catalysts [14]. When gold is atomically dispersed over Mg(OH)2 having no Au-Au coordination or gold is deposited on MgO as metallic particles having Au-Au coordination numbers close to that of bulk gold, it is inactive [16]. 

The present work deals with sintering of Ni/AbOa catalysts under reducing and steamreforming reaction conditions. The effects of preparation method (impregnation and coprecipitation), lanthanum oxide promoter, oxide phases developed after calcination, sintering temperature and atmosphere were studied in terms of the time evolution of metal dispersion, size distribution properties and kinetic parameters obtained from a GPLE model. 

Because of the cost of noble metal, the inetal loading should be low. and the inetal must be well dispersed to make the exposed metal surface as high as possible. Each type of metal complex, the conditions for ion exchange, the calcination procedure, and the reduction conditions have profound effects on the dispersion of the metal. 

The textural and the structural characterisation performed on the Pd/AbOs catalyst prepared by sol-gel method, shows a BET surface area of 270 m /g, a mesoporous texture and a uniform porous distribution with an average pore diameter of 3.3 nm. The metallic dispersion obtained by hydrogen chemisorption is 45 %. This later result is confirmed by the palladium particles diameter varying between 1 and 10 nm with an average of 3 nm obtained form the MET analysis. The palladium content determined by inductively coupled plasma is closely to 1.9 %. No significant BET surface area decrease nor a metallic dispersion loss were observed when the catalyst is aged under catalytic conditions up to the steady state. Since the thermal stability of the catalyst is needed to minimize the modification of the palladium particles structure, the later result justifies the choice of the sol-gel synthesis method and the calcinations temperature (700°C) selection. 

The importance of treatment conditions (calcination pretreatment and hydrogen reduction) for the control of the final metal dispersion will be illustrated with detailed results of case studies examined in Sect. 3. 

The acidic conditions of standard SBA-15 synthesis [35] cause the precipitation of metal nanoparticles without silica encapsulation, or the formation of amorphous silica due to the presence of the polymer used for nanoparticle synthesis. Therefore, the SBA-15 framework was synthesized under neutral condition using sodium fluoride as a hydrolysis catalyst and tetramethylorthosilicate (TMOS) as the silica precursor. Pt particles with different sizes were dispersed in the aqueous template polymer solution sodium fluoride and TMOS were added to the reaction mixture. The slurry aged at 313 K for a day, followed by an additional day at 373 K. Pt(X)/SBA-15-NE (X = 1.7, 2.9, 3.6, and 7.1nm) catalysts were obtained by ex-situ calcination (see Section 3.2). TEM images of the ordered... 

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