Supported metals, small particles characteristics, 157

Pt NMR has been used to study platinum particles embedded in a zeolite and to compare their characteristics with the more common oxide-supported platinum catalysts (Tong etal. 1993). Spin-lattice relaxation measurements indicated that a measurable fraction of the platinum in a zeolite-Y sample is not in a metallic environment, but it was not clear whether the loss of metallic signal refiected very small particle sizes or was due to interactions with the framework and its counter-ions. The latter possibility was supported by the observation that part of the metallic signal was restored by chemisorption of hydrogen (Tong et al. 1993). 

Some Characteristics of Small Supported Metal Particles... 

Some characteristic properties of small supported metal particles are summarized in Table XVII. 

The most fundamental characteristics of any industrial catalyst is its chemical composition. The other factors such as surface area, distribution of pore volumes, pore sizes, stability and mechanical properties are also very important. Such catalysts as metals or oxides of various kinds (pure or mixed) are not thermally stable in the high surface area form in which they have to be applied. So, they are prepared as small particles bonded to the support material, usually oxides, such as alumina and silica gels. 

MgO is a basic metal oxide and has the same crystal structure as NiO. As a result, the combination of MgO and NiO results in a solid-solution catalyst with a basic surface (171,172), and both characteristics are helpful in inhibiting carbon deposition (171,172,239). The basic surface increases C02 adsorption, which reduces or inhibits carbon-deposition (Section ALB). The NiO-MgO solid solution can control the nickel particle sizes in the catalyst. This control occurs because in the solid solution NiO has strong interactions with MgO and, as indicated by TPR data (26), the former oxide can no longer be easily reduced. Consequently, only a small amount of NiO is expected to be reduced, and thus small nickel particles are formed on the surface of the solid solution, smaller than the size necessary for coke formation. Indeed, the nickel particles on a reduced 16.7 wt% NiO/MgO solid-solution catalyst were too small to be observed by TEM (171). Furthermore, two additional important qualities stimulated the selection of MgO as a support its high thermal stability and low cost (250,251). 

TEM images recorded on the activated Rh containing catalysts showed the presence of metal particles on the support matrfac either as small and homogeneously dispersed particles or as large aggregates (Fig. 1) and evidenced the polymorph nature of the support, microporous and substantially amorphous in some zones (Fig. 2), more defined in others (Fig. 3). The polymorph nature of niobia was confirmed by XRD analysis. The XRD pattern showed the diffraction peaks characteristic of the monoclinic TT niobia phase together with an amorphous halo (Fig. 4). 

The selection of the carrier is relatively simple. It may be imposed by the type of reaction to be promoted. For instance, if the latter requires a bifunctional catalyst (metal + acid functions), acidic supports such as silica-aluminas, zeolites, or chlorinated aluminas, will be used. On the other hand, if the reaction occurs only on the metal, a more inert support such as silica will be used. In certain cases, other requirements (shock resistance, thermal conductivity, crush resistance, and flow characteristics) may dominate and structural supports (monoliths) have to be used. For the purpose of obtaining small metal particles, the use of zeolites has turned out to be an effective means to control their size. However, the problem of accessibility and acidity appearing on reduction may mask the evidence of the effect of metal particle size on the catalytic properties. 

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