Platinum hydrocarbons, structural effects

In some cases, the step sites have different chemistry, i.e., they break chemical bonds, thereby producing new chemical species on the surface. This happens for example during NO adsorption on a stepped platinum surface l In this circumstance the step effect on ordering is through the new types of chemistry introduced by the presence of steps. Hydrocarbons for example dissociate readily at stepped surfaces of platinum or nickel while this occurs much more slowly on the low Miller-Index surfaces in the absence of a large concentration of steps As a result ordered hydrocarbon surface structures cannot be formed on the stepped surfaces of these metals while they can be produced on the low Miller-Index surfaces. 

Effect of Hydrocarbon Molecular Structure on Ionization from Catalytic Oxidation in Air. Figure 1 shows the data for the ion yields obtained for several hydrocarbons during oxidation in air over a platinum filament in the approximate temperature range of 400° to 900°C. The hydrocarbons fall into two broad categories those which produced about 0.04 coulomb per mole of hydrocarbon oxidized, and those which produced about 1.0 coulomb or more per mole. The latter yield is similar to that obtained during combustion of hydrocarbons in a hydrogen flame... 

Complex oxides of the perovskite structure containing rare earths like lanthanum have proved effective for oxidation of CO and hydrocarbons and for the decomposition of nitrogen oxides. These catalysts are cheaper alternatives than noble metals like platinum and rhodium which are used in automotive catalytic converters. The most effective catalysts are systems of the type Lai vSrvM03, where M = cobalt, manganese, iron, chromium, copper. Further, perovskites used as active phases in catalytic converters have to be stabilized on the rare earth containing washcoat layers. This then leads to an increase in rare earth content of a catalytic converter unit by factors up to ten compared to the three way catalyst. 

EXAFS is also well suited for the study of finely divided metal (or metal oxide or metal sulfide) clusters supported within the pore structure (see Chapter 6). These particles are readily observed by X-ray spectroscopy, even if they are disordered throughout the solid. Analysis can even determine the average particle size of such clusters, which is of vital importance in catalytic preparation. Typically, for example, platinum supported on zeolites (and other solid acids) is a highly effective catalyst in the reforming of hydrocarbons. 

---