Quantitative surface structure determination using in situ high-energy SXRD: Surface oxide formation on Pd(100) during catalytic CO oxidation

Shipilin, Mikhail; Hejral, Uta; Lundgren, Edvin; Merte, Lindsay R.; Zhang, Chu; Stierle, Andreas; Ruett, Uta; Gutowski, Olof; Skoglundh, Magnus; Carlsson, Per-Anders; Gustafson, Johan

doi:10.1016/j.susc.2014.08.021

Cited by 33 publications

(54 citation statements)

References 27 publications

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“…19 We also reported on the structure of the surface oxide forming on Pd(100) when it becomes catalytically active in a close to stoichiometric mixture of reactants for the reaction of CO oxidation. 20 Epitaxial growth of PdO(101) on Pd(100) has been reported theoretically 15 as well as experimentally in pure oxygen 21 and in a mixture of CO and oxygen. 22 A disordered polycrystalline PdO film has also been reported using pure oxygen.…”

Section: ■ Introductionmentioning

confidence: 97%

Transient Structures of PdO during CO Oxidation over Pd(100)

et al. 2015

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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...

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Section: ■ Introductionmentioning

confidence: 97%

Transient Structures of PdO during CO Oxidation over Pd(100)

et al. 2015

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“…In the case of Pd, the CO oxidation reaction has been investigated on a variety of high and low symmetry crystal surfaces under reaction conditions. [4][5][6][7][8]10,13,[21][22][23][24][25][26][27][28] This will help to separately understand the catalytic performance at crystal planes that shape technologically relevant nanoparticles. However, given the variety of reaction parameters and experimental constraints, comparative in-situ investigations among different surface orientations would be better suited.…”

Section: Introductionmentioning

confidence: 99%

Catalytic Oxidation of Carbon Monoxide on a Curved Pd Crystal: Spatial Variation of Active and Poisoning Phases in Stationary Conditions

et al. 2018

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Understanding nanoparticle catalysis requires novel approaches in which adjoining crystal orientations can be studied under the same reactive conditions. Here we use a curved palladium crystal and near-ambient pressure X-ray photoemission spectroscopy to characterize chemical species during the catalytic oxidation of CO in a whole set of surfaces vicinal to the (111) direction simultaneously. By stabilizing the reaction at fixed temperatures around the ignition point we observe a strong variation of the catalytic activity across the curved surface. Such spatial modulation of the reaction stage is straightforwardly mapped through the photoemission signal from active oxygen species and poisoning CO, which are shown to coexist in a transient regime that depends on the vicinal angle. Line-shape analysis and direct comparison with ultra-high vacuum experiments help identifying and quantifying all such surface species, allowing us to reveal the presence of surface oxides during reaction ignition and cooling-off.

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“…The beamline setup and in situ chamber used are described in Ref. [26]. A beam energy of 78.9 keV was used with a grazing incident angle of 0.05 • .…”

Section: Methodsmentioning

confidence: 99%

Redox behavior of iron at the surface of an O(100) single crystal studied by ambient-pressure photoelectron spectroscopy

Merte

Gustafson

Shipilin

et al. 2017

Catalysis, Structure & Reactivity

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We have studied the oxidation and reduction of iron in an Fe-doped MgO single crystal by O 2 , H 2 and CO 2 using ambient-pressure XPS and NEXAFS. Surface charging of the crystal was rendered manageable by the elevated temperatures and the gas atmospheres. The oxidation state of iron was found to shift reversibly between the Fe 2+ and Fe 3+ states, with a strong asymmetry in the rates; while oxidation by O 2 or CO 2 was nearly complete at 100 • C, reduction by H 2 began at ∼ 300 • C, and was still incomplete at 600 • C. Grazing-incidence XRD characterization of the crystal indicated the presence of octahedral, nanoscale inclusions assigned to the magnesioferrite spinel (MgFe 2 O 4 ). It is proposed that the redox behavior observed involves interconversion between the rock-salt (Fe x Mg 1−x O) and spinel phases, with the more open lattice containing Fe 3+ enabling more rapid ion diffusion and thus more facile oxidation compared to reduction. ARTICLE HISTORY

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Quantitative surface structure determination using in situ high-energy SXRD: Surface oxide formation on Pd(100) during catalytic CO oxidation

Cited by 33 publications

References 27 publications

Transient Structures of PdO during CO Oxidation over Pd(100)

Transient Structures of PdO during CO Oxidation over Pd(100)

Catalytic Oxidation of Carbon Monoxide on a Curved Pd Crystal: Spatial Variation of Active and Poisoning Phases in Stationary Conditions

Redox behavior of iron at the surface of an O(100) single crystal studied by ambient-pressure photoelectron spectroscopy

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