Structural Requirements of Reforming Reactions

Typically, a mono-metallic reforming catalyst contains 0.3-0.5 wt% Pt, in crystallites of the order of 2 nm, dispersed on a defective transitional gamma-eta alumina whose surface area is of the order of 200m2/g. The purpose of the alumina is to provide a platform for Pt in high dispersion, and to provide acid sites with a narrow range of strengths in order to achieve this, the alumina must be chlorided to between 0.9 and 1.1 wt%. In the S/R reforming process both moisture and a source of chloride, typically propylene dichloride (PDC), are added and controlled continuously in the recycle gas in order to maintain an equilibrium level of acidity.26 

At a more fundamental level, the structure of small metal crystallies and their surface sites appears to control activity and selectivity of metal-catalysed reactions. 

Data in Fig. 6.328 shows the rapid decrease in coordination number for d 2 nm leads to changes in electronic configuration this is superposed on a transition towards discrete electron energy levels which approach those of isolated atoms as d decreases, and these are referred to as primary and secondary electronic effects, respectively, the latter having a geometric origin. 

Length of crystal edge Fraction of atoms on surface Total number of atoms in crystal Average coordination number of surface atoms 

In parallel with these electronic effects, the fractions of face, edge and corners atoms on a range of small metal crystallites have been modelled30,31 as a function of d. For fee octahedral (Fig. 6.4), the fraction of face atoms decreases from approx 1 to 0.1 as d decreases from 10 nm to 0.6, and the corresponding fractions of edge and corner atoms increase from 0.1 to approx. 0.5 each, 

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