Stability of Surface and Bulk Oxides on Pd(111) Revisited by in Situ X-ray Diffraction

Kasper, N. V.; Nolte, P.; Stierle, Andreas

doi:10.1021/jp307434g

Cited by 23 publications

(32 citation statements)

References 22 publications

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“…22 Furthermore, an epitaxial PdO growing with the [101] crystallographic direction perpendicular to the surface of the substrate has also been reported for Pd(111) 8,26 although it was observed that PdO(001) may coexist with PdO(101) by SXRD. 11 The crystallographic orientation is of interest because of the presence of the oxygen undercoordinated Pd atoms on the PdO(101) surface, which The Journal of Physical Chemistry C Article have been assigned to be the sites responsible for catalytic activity. 6,21,27,28 In an attempt to analyze the diffraction pattern further, we calculated the positions of the Bragg reflections for three different epitaxial growth directions.…”

Section: ■ Discussionmentioning

confidence: 99%

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: ■ Discussionmentioning

confidence: 99%

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

et al. 2015

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“…The binding energies of the above-mentioned surface structures are calculated with eq 5 as summarized in Table 2 Table 2. Experimentally, it is found that the phase boundary can be well-described by a line of constant chemical potential of μ O = −1.24 eV above 650 K. 19 The value of μ O determined from our DFT calculations for the transition between the oxygen p(2×2) chemisorption phase and the Pd 5 O 4 surface oxide phase is −1.1 eV with PW91 and −0.91 eV with RPBE, which are systematically above the experimental value. This discrepancy between the experimental and theoretical chemical potential gives rise to a 50 K (100 K for RPBE) error bar in temperature and a factor of 10 2 (10 4 for RPBE) error bar in pressure.…”

Section: Methodsmentioning

confidence: 95%

CO Oxidation on the Pd(111) Surface

Duan

Henkelman

2014

ACS Catal.

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Under technologically relevant oxygen-rich conditions, the reaction mechanism of CO oxidation over transition metals can be complicated by the formation of oxides. Questions of whether the active surface for CO oxidation is a pristine metal, a surface oxide, or a bulk oxide is still under active debate. In this study, density functional theory calculations are used to model CO oxidation on the Pd(111) surface. Our results show that a thin layer of Pd 5 O 4 surface oxide is stable under catalytically relevant gas-phase conditions. Three-fold oxygen atoms in the surface are found to react with gas-phase CO molecules following an Eley−Rideal reaction mechanism. Such CO oxidation reduces the surface oxide, but the oxide can be replenished by O 2 dissociation. Kinetic analysis shows that experimentally observed reaction conditions, that are uninhibited by CO and limited only by mass transfer, correspond to a surface oxide phase with CO oxidation occurring though the Eley−Rideal mechanism. Under steadystate operating conditions, the continuous formation and decomposition of the surface oxide is expected and is key to the high CO oxidation rate on Pd(111).

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“…Interestingly, PdO(101) on an extended (111) surface was also observed in this orientation. 147 Several other facets were observed, some of which were needed to keep the total inclination parallel to the (553) surface. These were (110), basically bunched (111) steps, and (221), 3(111) Â (111).…”

Section: à5mentioning

confidence: 99%

Surface science under reaction conditions: CO oxidation on Pt and Pd model catalysts

2017

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Platinum and palladium are frequently used as catalytic materials, for example for the oxidation of CO. This is one of the most widely studied reactions in the field of surface science. Although seemingly uncomplicated, it remains an active and interesting topic, which is partially explained by the push to conduct experiments on model systems under relevant reaction conditions. Recent developments in the surface-science methodology have allowed obtaining chemical and structural information on the active phase of model catalysts. Tools of the trade include near-ambient-pressure X-ray photoelectron spectroscopy, high-pressure scanning tunneling microscopy, high-pressure surface X-ray diffraction, and high-pressure vibrational spectroscopy. Interpretation is often aided by density functional theory in combination with thermodynamic and kinetic modeling. In this review, results for the catalytic oxidation of CO obtained by these techniques are compared. On several of the Pt and Pd surfaces, new structures develop in excess O 2 . For Pt, this requires a much larger excess of O 2 than for Pd. Most of these structures also develop in pure O 2 and are identified as (surface) oxides. A large body of evidence supports the conjecture that these oxides are more reactive than the corresponding O-covered metallic surfaces under similar conditions, although still debated in the literature. An outlook on this developing field, including directions that move away from CO oxidation towards more complex chemistry, concludes this review.

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Stability of Surface and Bulk Oxides on Pd(111) Revisited by in Situ X-ray Diffraction

Cited by 23 publications

References 22 publications

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

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

CO Oxidation on the Pd(111) Surface

Surface science under reaction conditions: CO oxidation on Pt and Pd model catalysts

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