Teschner D, Wild U, Schlögl R, Paál Z
Fritz-Haber-Institut der MPG, Faradayweg 4-6, D-14195 Berlin, Germany.
J Phys Chem B. 2005 Nov 3;109(43):20516-21. doi: 10.1021/jp054012o.
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 C(2)H(4) (also in the presence of H(2)) 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 approximately 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 approximately 284 eV), graphite (approximately 284.6 eV), and ethylene polymer (approximately 286 eV), and also some "atomic" C (BE approximately 283.5 eV). Introduction of H(2) 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 H(2). The C 1s intensity increased, the low-binding-energy C components disappeared, and graphitic carbon (BE approximately equal to 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).
将钯黑单独或与氢气混合后在300K和573K下暴露于乙烯中。通过X射线光电子能谱（XPS）和紫外光电子能谱（UPS）对样品进行了研究。室温下引入C₂H₄（也在H₂存在的情况下）会导致钯3d双峰的结合能（BE）发生位移，并改变其半高宽（fwhm）。UPS特征表明电子从钯d带转移到Pd-H、Pd-C甚至Pd-OH物种。亚乙烯基（BE约为284.1eV）可能是分散钯黑上最丰富的单个表面物种，同时还有处于不同聚合阶段的碳：“无序C”（BE约为284eV）、石墨（约284.6eV）和乙烯聚合物（约286eV），以及一些“原子”C（BE约为283.5eV）。先引入H₂再引入乙烯比按相反顺序暴露导致钯状态的变化更强。这可能表明某些表面碳的存在可能会阻碍块状PdH的分解。当在H₂之前引入乙烯时，氢化钯的形成被阻止。在573K下用乙烯处理后，C 1s强度增加，低结合能C成分消失，石墨碳（BE约等于284.6eV）占主导。在相应的价带和UPS光谱中观察到钯表面态和“PdH”信号的损失。在540K下进行氢气处理无法降低表面碳的浓度并重新建立近表面富氢状态。UPS显示这些样品中存在覆盖层型碳。钯与原料气成分的相互作用改变了其电子结构，这与碳和氢溶解到钯中引起的晶格应变一致，如d带位移和费米能级处电子密度的稀释所示。
