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, 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 H2. 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 H2 concentration resulting in more solid carbon. Regeneration by an O2–H2 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/H2 mixtures of various composition. Graphitic and amorphous C caused a nonselective deactivation. Difference C 1 s spectra showed a component with a binding energy at ∼284.1 between graphite and Pt/C. The suppression of the catalytic propensities in skeletal reactions (isomerization and C5 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.