Coke formation geometrical effect

Under dehydrogenation conditions (385 °C ratio H2/HC = 4), an increase in the selectivity for aromatics with PtSn,(/Si02 catalyst has been observed. The increase in aromatic selectivity with tin content seems to be due to a geometric effect, favoring aromatic desorption. When the catalyst contains only small amounts of tin, an important poisoning by coke has been observed. As a consequence, it is possible that coke comes from adsorbed aromatic degradation. If aromatic formation starting from olefins had already and previously been proposed in the literature, their formation mechanism was still unknown. The coexistence of two possible dehydrocycHzation mechanisms has been proposed (Scheme 3.24). 

Clearly geometrical factors relative to coke collection on metal surfaces have an important effect on coke formation. There is obviously a need to obtain more information relative to the flow patterns at or near the surface and surface roughness (and composition). Indirectly these factors would certainly affect the wettability of the surface with coke or coke precursors. 

For ethane dehydrogenation at 600 °C with bimetallic Pt-Sn nanoparticles (Pt/Sn = 3) supported on calcined hydrotalcite, a strong decrease in coke formation was reported in comparison with a monometallic platinum catalyst of the same particle size. Moreover, initial turnover frequency (TOF) and selectivity to ethene increased with tin addition, likely due to both geometric and electronic effects induced by tin on the surface of the platinum nanoparticles. 

The pubhc references report a number of investigation targeted to reduce the coking rate by modifying Pt-based catalysts. Among all the modifications, the effect of Sn is most extensively studied. Several su estions have been proposed to explain the effect of Sn as a promoter (Barias et al., 1994). Increased Pt dispersion due to the spacing of Sn, or formation of ensembles of a favorable size are examples of explanation with geometric effects (Barias... 

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