Morphological analysis of cerium oxide stabilized nanoporous gold catalysts by soft X-ray ASAXS

Rumancev, Christoph; Gundlach, Andreas R. von; Baier, Sina; Wittstock, Arne; Shi, Junjie; Benzi, Federico; Senkbeil, Tobias; Stuhr, Susan; Garamus, Vasil M.; Grunwaldt, Jan‐Dierk; Rosenhahn, Axel

doi:10.1039/c7ra05396g

Cited by 10 publications

(11 citation statements)

References 45 publications

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“…Whereas bulk gold is chemically inert, ceria nanostructures supported on Au(111) − or on np-Au , exhibit remarkable catalytic activity, as discussed in the Introduction. Therefore, one can expect that ceria NPs play an important role in generating catalytically active sites in the CeO 2 /Au catalytic system.…”

Section: Resultsmentioning

confidence: 99%

“…To reduce thermal and possible reaction-induced coarsening of np-Au, deposition of nanosized oxide particles (TiO 2 or CeO 2 ) offers an opportunity to drastically improve the thermal stability of this material and at the same time to increase its catalytic activity for oxidation reactions − by opening new reaction pathways (e.g., Mars–van Krevelen mechanism discussed below). Furthermore, np-Au functionalized with reducible oxides becomes active for catalyzing further reactions, for which pristine np-Au is not catalytically active, for example, for the water–gas shift reaction , and steam reforming of methanol …”

Section: Introductionmentioning

confidence: 99%

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What Changes on the Inverse Catalyst? Insights from CO Oxidation on Au-Supported Ceria Nanoparticles Using Ab Initio Molecular Dynamics

Bäumer

et al. 2020

ACS Catal.

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Gold-supported ceria nanoparticles (CeO x /Au), constituting an inverse system with respect to the more commonly studied ceria-supported gold nanoparticles, were previously identified as an excellent catalyst for water−gas shift reaction, CO oxidation, and steam reforming of methanol. However, the electronic structure and reactivity of such inverse catalysts have not been well understood. To probe the inherent nanoparticle−support interactions and their mechanistic role for the catalytic CO oxidation over this composite catalyst, ab initio molecular dynamics simulations and static density functional theory computations have been carried out for Au(111)-supported ceria clusters (Ce 10 O 20/19 ), as a realistic model system of an inverse CeO x /Au catalyst. We have identified the perimeter of the supported ceria nanoparticle as the most favorable O vacancy formation site; however, the vacancy further migrates to an inner interface site during the thermalization process, simultaneously triggering electron transfer from ceria to Au. Our study shows that the Au(111) surface always withdraws electron density from ceria, irrespective of the chemical environment, namely, in a reducing (Ce 10 O 19 ) as well as oxidizing (Ce 10 O 20 ) environment. To mimic a realistic catalytic environment, CO and O 2 molecules were preadsorbed on the surface of a composite catalyst. We find a vacancy diffusion-assisted Mars−van Krevelen type of reaction mechanism in which the first CO molecule reacts with a lattice O atom of ceria rather than with an activated O 2 2− species, forming CO 2 and leaving one O vacancy behind. This vacancy becomes subsequently refilled by an O atom diffusing from the site of O 2 reaction with a second CO molecule, recovering the stoichiometry of the Ce 10 O 19 cluster and closing the catalytic cycle. Finally, we discuss differences and similarities between ceria/Au and Au n /ceria with respect to surface dynamics, charge transfer between the gold and the oxide phases, and the mechanism of CO oxidation.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

What Changes on the Inverse Catalyst? Insights from CO Oxidation on Au-Supported Ceria Nanoparticles Using Ab Initio Molecular Dynamics

Bäumer

et al. 2020

ACS Catal.

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“…Experimentally, it was demonstrated that the deposition of nanosized oxide particles (TiO 2 , Al 2 O 3 , or CeO 2 ) on np-Au improves its thermal stability significantly so that coarsening is prevented up to temperatures of 600 °C and above. ,− Besides, this modification leads to an increase in activity for aerobic oxidation reactions already catalyzed by pristine np-Au, such as CO oxidation. , Even more importantly, however, it was reported that the range of potential catalytic applications catalyzed by np-Au can be expanded by this step to reactions, such as NO reduction, the water–gas shift reaction, , and methanol steam reforming . It was suggested that in the case of reducible oxides, where the formation and healing of oxygen vacancies (denoted as O V in the following) are facile, the involvement of lattice oxygen opens new catalytic pathways increasing the activity or even enabling new types of reactions .…”

Section: Introductionmentioning

confidence: 99%

Transient Au–CO Complexes Promote the Activity of an Inverse Ceria/Gold Catalyst: An Insight from Ab Initio Molecular Dynamics

Bäumer

et al. 2021

J. Phys. Chem. C

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To probe particle−support interactions and their mechanistic role for catalytic CO oxidation on nanoporous gold (np-Au) coated with ceria nanoparticles, we carried out ab initio molecular dynamics (AIMD) simulations and standard density functional theory (static DFT) computations. To this end, we studied ceria clusters (Ce 10 O 20/19 ) supported on a Au(321) surface exhibiting a high density of steps and kinks. Our theoretical model represents the structurally inverse situation compared to more commonly studied ceria-supported Au nanoparticle systems. In agreement with previous results for Au (111), we find that reduced (Ce 10 O 19 ) as well as stoichiometric (Ce 10 O 20 ) ceria nanoparticles transfer electrons to the Au(321) support. This charge transfer (particularly strong in the case of Ce 10 O 19 ) reflecting a strong chemical interaction between ceria and Au is probably responsible for the stabilization of np-Au against thermal coarsening experimentally observed upon deposition of oxide nanoparticles. The adsorption energies of the ceria cluster on Au(321) are more negative than on the Au(111) surface by around ∼0.5 eV. AIMD simulations were employed to study the mechanism of catalytic CO oxidation with O 2 for the ceria/Au(321) system. We found that a CO molecule adsorbed near the ceria/gold perimeter interface can extract a Au atom from the surface in the form of a mobile linear Au−CO complex, which results in a very low activation energy when this species reacts with lattice O to CO 2 . The released bare Au adatom subsequently attaches to a step edge of the gold surface, leading to a dynamic restructuring of the Au support. Next, an activated O 2 molecule adsorbed at a perimeter site between ceria and Au reacts with a second CO molecule to CO 2 and an adsorbed O atom, which eventually fills the vacancy site created in the first half of the cycle. As compared to ceria particles supported on Au(111), the reactivity is enhanced as a new low-energy mechanism is enabled, revealing the positive impact of the stepped structure of Au(321).

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“…13 To reduce thermal and possible reaction-induced coarsening of npAu, deposition of nanosized oxide particles (TiO2 or CeO2) has been proven to be an opportunity to drastically improve the thermal stability of the material and at the same time to increase its catalytic activity. 25,[34][35][36][37] The TiO2 and CeO2 nano-particles supported on Au(111) and on np-Au have been found to be effective catalysts for the CO oxidation and NO reduction, 34,36,[38][39] water-gas shift reaction 34,36,[40][41][42] and steam reforming of methanol. 43 To the best of our knowledge, theoretical studies on gold catalysts functionlized with nanometallic-oxides (the inverse systems with respect to oxide-supported Au nanoparticles) are rare (in fact, we are aware of only one work 44 that dealt with O vacancy formation for such ceria/gold system) so that mechanistic details of oxidation reactions on such inverse catalysts still remain unclear.…”

Section: Introductionmentioning

confidence: 99%

What Changes on the Inverse Catalyst? Insight From CO Oxidation on Au-Supported Ceria Nanoparticles Using Ab Initio Molecular Dynamics

Li²,

Bäumer³

et al. 2019

Preprint

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Oxidation reactions catalyzed by Au nanoparticles supported on reducible oxides have been widely studied both experimentally and theoretically, whereas <i>inverse catalysts</i>, in which oxide nanoparticles are supported on metal surfaces, received considerably less attention. In both systems catalytic activity at metal – oxide interfaces can arise not only from each material contributing its functionality, but also from their interactions creating properties beyond the sum of individual components. Inverse catalysts may retain the synergy between the metal and oxide functionalities, while offering further specific advantages, e.g. a possibility to have better control over interfacial sites or to yield improved stability, activity, and selectivity. Our work provides the mechanism of O atom/vacancy diffusion-assisted Mars-van-Krevelen CO oxidation on gold-supported ceria nanoparticle through state-of-the-art ab initio molecular dynamic simulation studies.

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Morphological analysis of cerium oxide stabilized nanoporous gold catalysts by soft X-ray ASAXS

Abstract: Soft X-ray SAXS and ASAXS reveal nanostructural properties and temperature induced morphological changes in catalyst materials. The stabilizing effect of cerium oxide deposits on the gold catalyst and the morphological properties of the cerium oxide were determined.

Cited by 10 publications

References 45 publications

What Changes on the Inverse Catalyst? Insights from CO Oxidation on Au-Supported Ceria Nanoparticles Using Ab Initio Molecular Dynamics

What Changes on the Inverse Catalyst? Insights from CO Oxidation on Au-Supported Ceria Nanoparticles Using Ab Initio Molecular Dynamics

Transient Au–CO Complexes Promote the Activity of an Inverse Ceria/Gold Catalyst: An Insight from Ab Initio Molecular Dynamics

What Changes on the Inverse Catalyst? Insight From CO Oxidation on Au-Supported Ceria Nanoparticles Using Ab Initio Molecular Dynamics

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