Particle dispersion, alumina-supported

A comment regarding the dispersion of the Ru-Rh/Si02 and the Ru-Ir/Si02 is in order. For the case of the supported Pt-Ru catalysts. Increases in dispersion as a result of clustering were very large ( ). This effect was particularly noticeable for bimetallic particles which conform to the cherry model. Evidently, the formation of an inner core enriched in one of the two metals, followed by an outer layer enriched in the other metal, inhibits further crystal growth. For the alumina-supported Pt-Ru bimetallic clusters, the effect, although present, is considerably smaller. 

The present study was initiated to understand the causes of large differences in perfonnance of various catalyst formulations after accelerated thermal aging on an engine dynamometer. In particular, we wished to determine whether performance charaderistics were related to noble metal dispersion (i.e. noble metal surface area), as previous studies have suggested that the thermal durability of alumina-supported Pd catalysts is due to high-temperature spreading or re-dispersion of Pd particles [20-25]. 

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

Inserted into the exhaust system of vehicles, catalytic converters can reduce emissions of carbon monoxide and hydrocarbons by up to 90 per cent. The first catalytic converters used mainly platinum, but now palladium is the predominant catalytic metal. The metals are dispersed as tiny particles on a supporting framework of porous aluminium oxide (alumina) (Fig. 18). 

Figure 4.13 A porous alumina support particle with well-dispersed active sites (left), and zoom-in view of a single active site (right).

Li and Klabunde (72) utilized a pulse reactor (normal pressure) to carry out n-heptane conversions. Pt and Sn were evaporated into a solvent at low temperature following evaporation the solvent was allowed to warm to room temperature where agglomeration of atoms took place to produce a dispersion of colloidal particles that were then added to an alumina support. These catalysts were compared to conventional Pt-Sn-alumina catalysts for n-heptane conversion. The authors proposed that the presence of small amounts of Sn° on the surface of Pt can cause both an increase in catalytic activity and a decrease in hydrogenolysis. 

The description of the molybdenum disulfide particles as regular hexagonal single slabs well dispersed over the alumina support surface permits calculations of the number of molybdenum ion per M0S2 slab and the M0S2 slab surface... 

Ionically conducting polymers and their relevance to lithium batteries were mentioned in a previous section. However, there are several developments which contain both ionically conducting materials and other supporting agents which improve both the bulk conductivity of these materials and the properties of the anode (Li)/electrolyte interface in terms of resistivity, passivity, reversibility, and corrosion protection. A typical example is a composite electrolyte system comprised of polyethylene oxide, lithium salt, and A1203 particles dispersed in the polymeric matrices, as demonstrated by Peled et al. [182], By adding alumina particles, a new conduction mechanism is available, which involved surface conductivity of ions on and among the particles. This enhances considerably the overall conductivity of the composite electrolyte system. There are also a number of other reports that demonstrate the potential of these solid electrolyte systems [183],... 

Figure 8 Typical design of a three-way catalyst for automobile exhaust control. Highlighted here are the honeycomb support and the mounting can used. So-called three-way catalysts, consisting of a combination of Pt, Rh, and Pd particles dispersed on high surface area alumina, are spread on the honeycomb structure to oxidize the carbon monoxide and unbumed hydrocarbons and to reduce the nitrogen oxides released by the engine of the car. (Reprinted from Ref 52, 1998, with permission from Elsevier)...

Another way to change concentration of active material is to modify the catalyst loading on an inert support. For example, the number of supported transition metal particles on a microporous support like alumina or silica can easily be varied during catalyst preparation. As discussed in the previous chapter, selective chemisorption of small molecules like dihydrogen, dioxygen, or carbon monoxide can be used to measure the fraction of exposed metal atoms, or dispersion. If the turnover frequency is independent of metal loading on catalysts with identical metal dispersion, then the observed rate is free of artifacts from transport limitations. The metal particles on the support need to be the same size on the different catalysts to ensure that any observed differences in rate are attributable to transport phenomena instead of structure sensitivity of the reaction. 

The Pt NMR of small platinum particles on classic oxide supports show s that the clean-surface LDOS is largely independent of the support (sihca, alumina, and titania) and of the method of preparation (impregnation, ion exchange, and deposition of colloids). At a given resonance position, one always finds the same relaxation rate, independent of particle size or support. The shape of the spectrum is related to the sample dispersion. The same is true lor particles protected in fihiis of PVP. [However, samples prepared under conditions giving strong SMSIs behave differently 171)]... 

---