The active phase of Pd during methane oxidation is a long-standing puzzle, which, if solved, could provide routes for design of improved catalysts. Here, density functional theory and in situ surface X-ray diffraction are used to identify and characterize atomic sites yielding high methane conversion. Calculations are performed for methane dissociation over a range of Pd and PdOx surfaces and reveal facile dissociation on either under-coordinated Pd sites in PdO(101) or metallic surfaces. The experiments show unambiguously that high methane conversion requires sufficiently thick PdO(101) films or metallic Pd, in full agreement with the calculations. The established link between high activity and atomic structure enables rational design of improved catalysts.

We present high-pressure x-ray photoelectron spectroscopy (HP-XPS) and first-principles kinetic Monte Carlo study addressing the nature of the active surface in CO oxidation over Pd(100). Simultaneously measuring the chemical composition at the surface and in the near-surface gas phase, we reveal both O-covered pristine Pd(100) and a surface oxide as stable, highly active phases in the near-ambient regime accessible to HP-XPS. Surprisingly, no adsorbed CO can be detected during high CO(2) production rates, which can be explained by a combination of a remarkably short residence time of the CO molecule on the surface and mass-transfer limitations in the present setup.

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.

High-pressure X-ray photoelectron spectroscopy, mass spectrometry, and density functional theory calculations have been combined to study methane oxidation over Pd(100). The measurements reveal a high activity when a two-layer PdO(101) oriented film is formed. Although a one-layer PdO(101) film exhibits a similar surface structure, no or very little activity is observed. The calculations show that the presence of an oxygen atom directly below the coordinatively unsaturated Pd atom in the two-layer PdO(101) film is crucial for efficient methane dissociation, demonstrating a ligand effect that may be broadly important in determining the catalytic properties of oxide thin films.

The adsorption of CO on clean and oxidized Pd(111) surfaces has been investigated using a combination of high-resolution core level spectroscopy (HRCLS), reflection absorption infrared spectroscopy (RAIRS), and density functional theory (DFT) calculations. The HRCLS and RAIRS measurements reveal that CO adsorbs on Pd(111), Pd 5 O 4 and PdO(101) at 100 ± 10 K and that the CO coverage decreases with increasing oxidation state of Pd for the same CO exposures of 10 Langmuirs. Based on the DFT calculations, the CO layer on clean Pd(111) was found to include molecular adsorption in both hollow and bridge sites, whereas CO occupies a combination of bridge and atop sites on the Pd 5 O 4 and PdO(101) surfaces.