Supported metals adsorbent volume

ES S studies of catalysts are described in Chapter 7. These experiments present a particular set of problems that need to be addressed. Foremost is the fact that INS is not an intrinsically surface sensitive technique. This is overcome by using large samples to maximise the number of surface sites and hydrogenous adsorbates to give the highest possible contrast. Supported metal catalysts, zeolites and oxides often have low densities, while metal powders have high densities but low surface areas. Both situations require a large volume cell to place sufficient sample in the beam. 

Physical adsorption isotherms involve measuring the volume of an inert gas adsorbed on a material s surface as a function of pressure at a constant temperature (an isotherm). Using nitrogen as the inert gas, at a temperature close to its boiling point (near 77K), such isotherms are used to determine the amount of the inert gas needed to form a physisorbed monolayer on a chemically unreactive surface, through use of the Brunauer, Emmett, and Teller equation (BET). If the area occupied by each physisorbed N2 molecule is known (16.2A ), the surface area can then be determined. For reactive clean metals, the area can be determined using chemisorption of H2 at room temperature. Most clean metals adsorb one H atom per surface metal atom at room temperature (except Pd, which forms a bulk hydride), so if the volume of H2 required for chemisorption is measured, the surface area of the metal can be determined if the atomic spacings for the metal is known. The main use of physical adsorption surface area measurement is to determine the surface areas of finely divided solids, such as oxide catalyst supports or carbon black. The main use of chemisorption surface area measurement is to determine the particle sizes of metal powders and supported metals in catalysts. 

In this case, we used the traditional method of impregnation, carried out in conditions leading to the formation of highly dispersed Ag particles on the support surface (1) samples were prepared with a low content of Ag ( 2 wt.%) (2) Ag was supported by adsorption on SiC>2 surface of the ammonia complex of the diluted silver nitrate solutions. In this case, the formation of the supported particles at the later stages of the sample preparation was mainly performed from the adsorbed silver complex. Contribution of this complex being in volume of support pores was practically excluded. (3) samples with supported silver complex were dried by the method of sublimation or by the adsorption-contact method which preserved the uniformity of adsorbed silver complex distribution on the support surface. This contributed to the obtention of a more homogeneous distribution of metal particles after subsequent reduction. The application of the adsorption-contact drying method for the preparation of the supported metal catalysts has not been found in literature. 

Carbon monoxide chemisorption was used to estimate the surface area of metallic iron after reduction. The quantity of CO chemisorbed was determined [6J by taking the difference between the volumes adsorbed in two isotherms at 195 K where there had been an intervening evacuation for at least 30 min to remove the physical adsorption. Whilst aware of its arbitrariness, we have followed earlier workers [6,10,11] in assuming a stoichiometry of Fe CO = 2.1 to estimate and compare the surface areas of metallic iron in our catalysts. As a second index for this comparison we used reactive N2O adsorption, N20(g) N2(g) + O(ads), the method widely applied for supported copper [12]. However, in view of the greater reactivity of iron, measurements were made at ambient temperature and p = 20 Torr, using a static system. 

The coalescence of atoms into clusters may also be restricted by generating the atoms inside confined volumes of microorganized systems [87] or in porous materials [88]. The ionic precursors are included prior to irradiation. The penetration in depth of ionizing radiation permits the ion reduction in situ, even for opaque materials. The surface of solid supports, adsorbing metal ions, is a strong limit to the diffusion of the nascent atoms formed by irradiation at room temperature, so that quite small clusters can survive. 

An important application of metal nanoparticles is their use as catalysts on solid supports or in confined media. When the solution containing metal ions is in contact with a solid support, the ions can diffuse in the pores and adsorb on the surface. Therefore, the penetration of the ionizing radiation enables the in situ reduction of metal ions and then the further coalescence of metal atoms inside the confined volumes of porous materials, such as zeolites, alumino-silica-gels, colloidal oxides such as TiOj or polymeric membranes. 

The washcoat increases the surface of the catalytic structure to 10000-40000m l support volume. This high internal surface area is needed to adequately disperse the precious metals and to act as an adsorbent for the catalyst poisoning elements present in the exhaust gas. 

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