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.

No abstract

Atomic steps at the surface of a catalyst play an important role in heterogeneous catalysis, for example as special sites with increased catalytic activity. Exposure to reactants can cause entirely new structures to form at the catalyst surface, and these may dramatically influence the reaction by 'poisoning' it or by acting as the catalytically active phase. For example, thin metal oxide films have been identified as highly active structures that form spontaneously on metal surfaces during the catalytic oxidation of carbon monoxide. Here, we present operando X-ray diffraction experiments on a palladium surface during this reaction. They reveal that a high density of steps strongly alters the stability of the thin, catalytically active palladium oxide film. We show that stabilization of the metal, caused by the steps and consequent destabilization of the oxide, is at the heart of the well-known reaction rate oscillations exhibited during CO oxidation at atmospheric pressure.

The surface structure of Pd(100) during CO oxidation was measured using a combination of a flow reactor and in situ surface X-ray diffraction coupled to a large-area 2-dimensional detector. The surface structure was measured for P(O(2))/P(CO) ratios between 0.6 and 10 at a fixed total gas pressure of 200 mbar and a fixed CO pressure of 10 ± 1 mbar. In conjunction with the surface structure the reactivity of the surface was also determined. For all P(O(2))/P(CO) ratios the surface was found to oxidize above a certain temperature. Three different types of oxides were observed: the surface oxide, an epitaxial layer of bulk-like PdO, and a non-epitaxial layer of bulk-like PdO. As soon as an oxide was present the reactivity of the surface was found to be mass transfer limited by the flux of CO molecules reaching the surface.

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.