XPS, EM, and catalytic studies of the accumulation of carbon on Pt black

Pt black samples were exposed to 2,4-hexadiene (24HD) at 603 and 693 K, alo ne or in the presence of excess hydrogen. The 24HD underwent C=C double-bon d shift, geometric isomerization, and minor aromatization during its contac t with Pt in the absence of H-2 It was hydrogenated to hexenes and hexane w hen hydrogen was present. Test runs with an n-hexane-hydrogen mixture after 24HD treatment showed a more pronounced decrease of activity and a loss of isomerization/cyclization selectivities after exposure without hydrogen. T he amount, structure, and possible chemical state of residual carbon were e xamined by transmission electron microscopy (TEM) and X-ray photoelectron s pectroscopy (XPS). The sintered metal contained some inherent (likely harml ess) carbon impurity, the amount of which increased upon exposure to 24HD t o 44-50%, higher temperatures and lower H-2 concentration resulting in more solid carbon. Regeneration by an O-2-H-2 treatment removed much, but not a ll, of the solid carbon deposit. Difference spectra of hexadiene-treated an d regenerated samples showed an excess of graphite in the carbonized sample s whereas Pt/C was more abundant after regeneration, in agreement with C 1 s line fitting. Transmission electron microscopic examinations showed mainl y pyrolytic carbon. Graphitic layers (not highly ordered ones) perpendicula r to the Pt surface were identified after exposure to 24HD/H-2 mixtures of various composition. Graphitic and amorphous C caused a nonselective deacti vation. Difference C 1s spectra showed a component with a binding energy at similar to 284.1 between graphite and Pt/C. The suppression of the catalyt ic propensities in skeletal reactions (isomerization and C-5 cyclization) a nd the difficulty of self-reactivation in the prolonged test runs were cons istent with the presence of this type of deposit representing, likely, a di sordered nongraphitic hydrocarbon oligomer. (C) 2001 Academic Press.