Oxidation and reduction of Pd(100) and aerosol-deposited Pd nanoparticles

Abstract

Using in situ high pressure X-ray photoelectron spectroscopy (HPXPS), we have followed the oxidation and the reduction of Pd model catalysts in oxygen and CO pressures in the mbar range. The study includes a Pd(100) single crystal as well as SiOx supported Pd nanoparticles of 15 or 35 nm diameter respectively. We demonstrate that also nanoparticles form ultra-thin surface oxides prior to the onset of the bulk PdO. The Pd nano particles are observed to bulk oxidize at sample temperatures 40 degrees lower than the single crystal surface. In the Pd 3d 5/2 and the O 1s spectrum we identify a component corresponding to under-coordinated atoms at the surface of the PdO oxide. The experimentally observed PdO CLS is supported by density functional theory calculations (DFT). In a CO atmosphere, the Pd 3d 5/2 component corresponding to under-coordinated PdO atoms is shifted by +0.55 eV with respect to PdO bulk, demonstrating that CO molecules preferably adsorbs at these sites. CO coordinated to Pd atoms in the metallic and the oxidized phase can also be distinguished in the C 1s spectrum. The initial reduction by CO is similar for the single crystal and the nanoparticle samples, but after the complete removal of the oxide we detect a significant deviation between the two systems, namely that the nanoparticles incorporate carbon to form a Pd carbide. Our results indicates that CO can dissociate on the nanoparticle samples, whereas no such behavior is observed for the Pd(100) single crystal. These results demonstrate the similarities, as well as the important differences, between the single crystal used as model systems for catalysis and nm sized particles on oxide supports.

Mechanism for reversed photoemission core-level shifts of oxidized Ag

“…These results are in good agreement with previous reports. 20,21 The calculated Pd 3d CLS for the surface component in clean Pd (100) the first surface layer of Pd(100) are calculated to have shifts grouped at −0.2 and + 0.2 eV, respectively.…”

Effects of non-local exchange on core level shifts for gas-phase and adsorbed molecules

“…This CLS is clearly underestimated in PBE where it is calculated to be ∼1 eV. 35,44,45 Here, we investigate the influence of Fock exchange on the Pd 3d CLS between bulk palladium metal and bulk palladium oxide. As inclusion of both types of atoms in the same computational unit cell is advantageous to obtain accurate CLS, the calculations have been performed with a slab model consisting of commensurable Pd and PdO surfaces.…”

“…35,44 The experiments were performed on oxidized Pd nanoparticles, and oxidized Pd single crystal surfaces. In both cases, the difference between the 3d binding energy in the metal and the oxide was measured to be 1.6 eV.…”

Films of the Pd x Ce1−x O2 solid solution as a model object for the XPS study of the surface chemistry of Pd/CeO2 catalysts

“…This is one of the main reasons why Pd/CeO 2 powder catalysts are typically characterized only by XPS in UHV and are not used as an object for the in situ study of the reaction mechanisms. In order to investigate catalytic reactions by ex situ or in situ XPS, model objects such as metal foils and metallic or oxide single crystals are used [18][19][20][21][22][23][24][25]. The data obtained for these objects are certainly of great value, however, with few exceptions [13], we failed to find the ex/in situ XPS studies on catalytic reactions using real Pd/CeO 2 powder catalysts.…”