Charging and Chemical Reactivity of Gold Nanoparticles and Adatoms on the (111) Surface of Single-Crystal Magnetite: A Scanning Tunneling Microscopy/Spectroscopy Study

Rim, Kwang Taeg; Eom, Daejin; Liu, Li; Stolyarova, Elena; Raitano, Joan M.; Chan, Siu‐Wai; Flytzani‐Stephanopoulos, Maria; Flynn, George W.

doi:10.1021/jp8112599

Cited by 77 publications

(107 citation statements)

References 36 publications

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“…18 Such metalÀ oxygenÀsupport structures are also suggested to be the origin of the stability of SACs in various catalytic processes. 30,33,47 In addition, the property of charged single atoms on supports also contributes to its stability. For example, the cationic species coordinated with supports could not be removed whereas metallic species were leached by CN À .…”

Section: The Correlations Between the Properties And Reactivities Of mentioning

confidence: 99%

Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis

et al. 2013

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Supported metal nanostructures are the most widely used type of heterogeneous catalyst in industrial processes. The size of metal particles is a key factor in determining the performance of such catalysts. In particular, because low-coordinated metal atoms often function as the catalytically active sites, the specific activity per metal atom usually increases with decreasing size of the metal particles. However, the surface free energy of metals increases significantly with decreasing particle size, promoting aggregation of small clusters. Using an appropriate support material that strongly interacts with the metal species prevents this aggregation, creating stable, finely dispersed metal clusters with a high catalytic activity, an approach industry has used for a long time. Nevertheless, practical supported metal catalysts are inhomogeneous and usually consist of a mixture of sizes from nanoparticles to subnanometer clusters. Such heterogeneity not only reduces the metal atom efficiency but also frequently leads to undesired side reactions. It also makes it extremely difficult, if not impossible, to uniquely identify and control the active sites of interest.The ultimate small-size limit for metal particles is the single-atom catalyst (SAC), which contains isolated metal atoms singly dispersed on supports. SACs maximize the efficiency of metal atom use, which is particularly important for supported noble metal catalysts. Moreover, with well-defined and uniform single-atom dispersion, SACs offer great potential for achieving high activity and selectivity.In this Account, we highlight recent advances in preparation, characterization, and catalytic performance of SACs, with a focus on single atoms anchored to metal oxides, metal surfaces, and graphene. We discuss experimental and theoretical studies for a variety of reactions, including oxidation, water gas shift, and hydrogenation. We describe advances in understanding the spatial arrangements and electronic properties of single atoms, as well as their interactions with the support. Single metal atoms on support surfaces provide a unique opportunity to tune active sites and optimize the activity, selectivity, and stability of heterogeneous catalysts, offering the potential for applications in a variety of industrial chemical reactions.

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Section: The Correlations Between the Properties And Reactivities Of mentioning

confidence: 99%

Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis

et al. 2013

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“…[20][21][22][23][24][25][26][27][28][29][30][31] In most cases, atomically dispersed metals are found to be anchored in surface cationic positions of the host oxide. 23,28,30,32 Depending on the oxidation state of the anchored metal atom, spontaneous formation of anionic or cationic vacancies occurs to balance the charge. In some cases, anchoring of the metal atom is achieved through the spatial confinement in pores or open channels of the support.…”

Section: Introductionmentioning

confidence: 99%

Oxide-based nanomaterials for fuel cell catalysis: the interplay between supported single Pt atoms and particles

Lykhach

Bruix

Fabris

et al. 2017

Catal. Sci. Technol.

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The concept of single atom catalysis offers maximum noble metal efficiency for the development of lowcost catalytic materials. Among possible applications are catalytic materials for proton exchange membrane fuel cells. In the present review, recent efforts towards the fabrication of single atom catalysts on nanostructured ceria and their reactivity are discussed in the prospect of their employment as anode catalysts.The remarkable performance and the durability of the ceria-based anode catalysts with ultra-low Pt loading result from the interplay between two states associated with supported atomically dispersed Pt and subnanometer Pt particles. The occurrence of these two states is a consequence of strong interactions between Pt and nanostructured ceria that yield atomically dispersed species under oxidizing conditions and sub-nanometer Pt particles under reducing conditions. The square-planar arrangement of four O atoms on {100} nanoterraces has been identified as the key structural element on the surface of the nanostructured ceria where Pt is anchored in the form of Pt 2+ species. The conversion of Pt 2+ species to subnanometer Pt particles is triggered by a redox process involving Ce 3+ centers. The latter emerge due to either oxygen vacancies or adsorption of reducing agents. The unique properties of the sub-nanometer Pt particles arise from metal-support interactions involving charge transfer, structural flexibility, and spillover phenomena. The abundance of specific adsorption sites similar to those on {100} nanoterraces determines the ideal (maximum) Pt loading in Pt-CeO x films that still allows reversible switching between the atomically dispersed Pt and sub-nanometer particles yielding high activity and durability during fuel cell operation.

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“…Also, nonlocal spectroscopic techniques failed to prove the relevance of defects in certain adsorption processes. Photoemission studies of reduced CeO 2 and Fe 3 O 4 sparsely loaded with gold detected mainly cationic Au species [19,20], although binding to O vacancies should result in negatively charged atoms [5,7,9]. Similarly, cationic gold and not Au − species associated with O vacancies were observed on defective MgO [21].…”

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confidence: 99%

Diffusion Barriers Block Defect Occupation on ReducedCeO2(111)

et al. 2016

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Surface defects are believed to govern the adsorption behavior of reducible oxides. We challenge this perception on the basis of a combined scanning-tunneling-microscopy and density-functional-theory study, addressing the Au adsorption on reduced CeO 2−x ð111Þ. Despite a clear thermodynamic preference for oxygen vacancies, individual Au atoms were found to bind mostly to regular surface sites. Even at an elevated temperature, aggregation at step edges and not decoration of defects turned out to be the main consequence of adatom diffusion. Our findings are explained with the polaronic nature of the Au-ceria system, which imprints a strong diabatic character onto the diffusive motion of adatoms. Diabatic barriers are generally higher than those in the adiabatic regime, especially if the hopping step couples to an electron transfer into the ad-gold. As the population of O vacancies always requires a charge exchange, defect decoration by Au atoms becomes kinetically hindered. Our study demonstrates that polaronic effects determine not only electron transport in reducible oxides but also the adsorption characteristics and therewith the surface chemistry. DOI: 10.1103/PhysRevLett.116.236101 Surface defects are of decisive importance for the physics and chemistry of oxides [1]. They represent local perturbations of the saturated metal-oxygen network and often comprise dangling-bond states and uncompensated surface charges. Not surprisingly, density-functional theory (DFT) finds defects to be the preferred adsorption sites for metal atoms and molecules on oxide surfaces [2,3], with oxygen vacancies being the most relevant type due to their relatively low formation energy [4,5]. On nonreducible MgO, for example, Au atoms substantially bind to O defects, while the ideal surface offers weak van der Waals interactions only [6]. The same holds for ceria, as a reducible oxide [7][8][9]. Again, gold hardly interacts with the stoichiometric surface, while binding to surface O vacancies (V O S ) is highly favorable and accompanied by an electron transfer from a nearby Ce 3þ ion [10]. Defects are also involved in stabilizing and activating molecular species and therefore of relevance for oxide chemistry [11].While theory suggests a large impact of defects on various oxide properties, the correlation is not so clear on the experimental side. Predominantly defect-mediated adsorption is found for weakly bound adsorbates, e.g., for water and methanol on TiO 2 ð110Þ [12,13] ð111Þ [18], even the hindered occupation of defects was reported. Also, nonlocal spectroscopic techniques failed to prove the relevance of defects in certain adsorption processes. Photoemission studies of reduced CeO 2 and Fe 3 O 4 sparsely loaded with gold detected mainly cationic Au species [19,20], although binding to O vacancies should result in negatively charged atoms [5,7,9]. Similarly, cationic gold and not Au − species associated with O vacancies were observed on defective MgO [21]. Finally, the yield of ceria-catalyzed water-gas-shift reactions was fou...

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Charging and Chemical Reactivity of Gold Nanoparticles and Adatoms on the (111) Surface of Single-Crystal Magnetite: A Scanning Tunneling Microscopy/Spectroscopy Study

Cited by 77 publications

References 36 publications

Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis

Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis

Oxide-based nanomaterials for fuel cell catalysis: the interplay between supported single Pt atoms and particles

Diffusion Barriers Block Defect Occupation on ReducedCeO2(111)

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