Pt black samples were exposed to 2,4-hexadiene (24HD) at 603 and 693 K, alone or in the presence of excess hydrogen. The 24HD underwent C=C double-bond shift, geometric isomerization, and minor aromatization during its contact with Pt in the absence of H-2 It was hydrogenated to hexenes and hexane when 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. The amount, structure, and possible chemical state of residual carbon were examined by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The sintered metal contained some inherent (likely harmless) carbon impurity, the amount of which increased upon exposure to 24HD to 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 all, of the solid carbon deposit. Difference spectra of hexadiene-treated and regenerated samples showed an excess of graphite in the carbonized samples whereas Pt/C was more abundant after regeneration, in agreement with C 1 s line fitting. Transmission electron microscopic examinations showed mainly pyrolytic carbon. Graphitic layers (not highly ordered ones) perpendicular to the Pt surface were identified after exposure to 24HD/H-2 mixtures of various composition. Graphitic and amorphous C caused a nonselective deactivation. 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 catalytic propensities in skeletal reactions (isomerization and C-5 cyclization) and the difficulty of self-reactivation in the prolonged test runs were consistent with the presence of this type of deposit representing, likely, a disordered nongraphitic hydrocarbon oligomer. (C) 2001 Academic Press.