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.

Understanding the interaction between surfaces and their surroundings is crucial in many materials-science fields, such as catalysis, corrosion, and thin-film electronics, but existing characterization methods have not been capable of fully determining the structure of surfaces during dynamic processes, such as catalytic reactions, in a reasonable time frame. We demonstrate an x-ray-diffraction-based characterization method that uses high-energy photons (85 kiloelectron volts) to provide unexpected gains in data acquisition speed by several orders of magnitude and enables structural determinations of surfaces on time scales suitable for in situ studies. We illustrate the potential of high-energy surface x-ray diffraction by determining the structure of a palladium surface in situ during catalytic carbon monoxide oxidation and follow dynamic restructuring of the surface with subsecond time resolution.

No abstract

Catalytic CO oxidation is a seemingly simple reaction between CO and O2 molecules, one of the reactions in automotive catalytic converters, and the fruit-fly reaction in model catalysis. Surprisingly, the phase responsible for the catalytic activity is still under debate, despite decades of investigations. We have performed a simple but yet conclusive study of single crystal Rh and Pd model catalysts, resolving this controversy. For Rh, the oxygen-covered metallic surface is more active than the oxide, while for Pd, thin oxide films are at least as active as the metallic surface, but a thicker oxide is less active. Apart from resolving a long-standing debate, our results pinpoint important design principles for oxidation catalysts as to prevent catalytic extinction at high oxygen exposures.

In situ high-energy surface X-ray diffraction was employed to determine the surface structure dynamics of a Pd(100) single crystal surface acting as a model catalyst to promote CO oxidation. The measurements were performed under semirealistic conditions, i.e., 100 mbar total gas pressure and 600 K sample temperature. The surface structure was studied in detail both in a steady gas flow and in a gradually changing gas composition with a time resolution of 0.5 s. The experimental technique allows for rapid reciprocal space mapping providing the complete information on structural changes of a surface with unprecedented time resolution in harsh conditions. Our results show that the (√5 × √5)R27°-PdO(101) surface oxide forms in a close to stoichiometric O 2 and CO gas mixture as the mass spectrometry indicates a transition to a highly active state with the reaction rate limited by the CO mass transfer to the Pd(100) surface. Using a low excess of O 2 in the gas stoichiometry, islands of bulk oxide grow epitaxially in the same (101) crystallographic orientation of the bulk PdO unit cell according to a Stranski−Krastanov type of growth. The morphology of the islands is analyzed quantitatively. Upon further increase of the O 2 partial pressure a polycrystalline Pd oxide forms on the surface. ■ INTRODUCTIONFor more than a century heterogeneous catalysis has been extensively exploited by the industry, and as a consequence it has been intensively studied. 1 One of the most prominent examples is the CO oxidation reaction, CO + 1 / 2 O 2 → CO 2 . This process transforms highly toxic carbon monoxide, formed e.g. as a byproduct during incomplete combustion of the fuel in internal combustion engines, to less harmful carbon dioxide gas. However, the reaction is very slow under the operational conditions in the gas phase and requires thus the presence of a solid catalysts to proceed at a sufficiently high rate. Because of its importance and relatively simple mechanism, this reaction has become the subject of numerous studies aiming to resolve the atomic-scale processes that occur on the surface of catalysts. 2 Supported nanoparticles of late transition metals represent a well-known and efficient type of oxidation catalyst and are currently widely used in catalytic converters. 3,4 Hence, a deep understanding of the fundamental processes proceeding in such systems is important for improvement of existing catalyst-based solutions and development of new potential approaches. For this purpose, studies of atomic-scale surface structure and determination of the active phase of catalysts under working conditions are essential. However, the complexity of such systems and the inability of many experimental techniques to work under realistic pressuresthe challenges known as material and pressure gapssignificantly narrow the selection of available methods for structural determination and necessitate the use of model systems. One of the commonly used approaches is to study single crystals with different surface crystallographic orientation...

Time-resolved in situ energy-dispersive X-ray absorption spectroscopy and mass spectrometry have been used to correlate changes in chemical state of alumina and ceria supported palladium nanoparticles with changes in activity and selectivity for methane oxidation. Specifically, modulation excitation spectroscopy experiments were

CO2 methanation over Rh/Al2O3, Rh/CeO2 and Ni/CeO2 at 350 °C highlighting the different surface speciation during reaction.

We communicate experimental results for the oxidation of methane by oxygen over alumina supported Pd and Pt monolith catalysts under transient conditions. Temperature programmed reaction (TPReaction) and reactant pulse-response (PR) experiments have been performed, using a continuous gas-flow reactor equipped with a downstream mass spectrometer for gas phase analysis. Special attention was paid to the influence of gas composition changes, i.e., O 2 and H 2 pulsing, respectively, on the methane conversion. For Pt/Al 2 O 3 oxygen pulsing can significantly increase the methane conversion which can be even further improved by pulsing hydrogen instead. Such transient effects are not observed for the Pd/Al 2 O 3 catalyst for which instead constantly lean conditions is beneficial. Our results suggest that under lean conditions Pd and Pt crystallites may undergo bulk-and partial (surface oxide formation) oxidation, respectively, which for Pd results in more active surfaces, while for Pt the activity is reduced. The latter seems to connect to a lowering of the ability to dissociate methane.