Surface State and Composition of a Disperse Pd Catalyst after Its Exposure to Ethylene

Pd black was exposed to ethylene alone or in its mixture with hydrogen at 300 and 573 K. The samples were investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Room temperature introduction of C2H4 (also in the presence of H2) induced a binding-energy (BE) shift in the Pd 3d doublet and changed its full width at half-maximum (fwhm). The UPS features indicate shifting of electrons from the Pd d-band to Pd−H, Pd−C, and even Pd−OH species. Vinylidene (BE ∼ 284.1 eV) may be the most abundant individual surface species on disperse Pd black, along with carbon in various stages of polymerization: “disordered C” (BE ∼ 284 eV), graphite (∼284.6 eV), and ethylene polymer (∼286 eV), and also some “atomic” C (BE ∼ 283.5 eV). Introduction of H2 followed by ethylene brought about stronger changes in the state of Pd than exposure in the reverse sequence. This may indicate that the presence of some surface C may hinder the decomposition of bulk PdH. Formation of Pd hydride was blocked when ethylene was introduced prior to H2. The C 1s intensity increased, the low-binding-energy C components disappeared, and graphitic carbon (BE ≈ 284.6 eV) prevailed after ethylene treatment at 573 K. The loss of the Pd surface state and “PdH” signal were observed in the corresponding valence band and UPS spectra. Hydrogen treatment at 540 K was not able to decrease the concentration of surface carbon and re-establish the near-surface H-rich state. UPS showed overlayer-type C in these samples. The interaction of Pd with components from the feed gas modified its electronic structure that is consistent with lattice strain induced by dissolution of carbon and hydrogen into Pd, as indicated by the d-band shift and the dilution of the electron density at E F.