The Catalytic Nanodiode:  Gas Phase Catalytic Reaction Generated Electron Flow Using Nanoscale Platinum Titanium Oxide Schottky Diodes

Ji, Xiaozhong; Zuppero, Anthony; Gidwani, Jawahar M.; Somorjai, Gábor A.

doi:10.1021/nl050241a

Cited by 66 publications

(67 citation statements)

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“…Hot electrons have been reported during CO oxidation on Pt nanoparticles on TiO 2 . 42,43 The observed reactioninduced enhancement of coarsening can therefore proceed via an electronic excitation of the cluster substrate system.…”

Section: ■ Resultsmentioning

confidence: 99%

Reaction-Induced Cluster Ripening and Initial Size-Dependent Reaction Rates for CO Oxidation on Pt_n/TiO₂(110)-(1×1)

et al. 2014

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We determined the CO oxidation rates for sizeselected Pt n (n ∈ {3,7,10}) clusters deposited onto TiO 2 (110). In addition, we investigated the cluster morphologies and their mean sizes before and after the reaction. While the clusters are fairly stable upon annealing in ultrahigh vacuum up to 600 K, increasing the temperature while adsorbing either one of the two reactants leads to ripening already from 430 K on. This coarsening is even more pronounced when both reactants are dosed simultaneously, i.e., running the CO oxidation reaction. Since the ripening depends on the size initially deposited, there is nevertheless a size effect; the catalytic activity decreases monotonically with increasing initial cluster size. ■ INTRODUCTIONIt is well established that small metal clusters have physical and chemical properties distinctively different from those of the bulk. While the properties change rather smoothly in the socalled scalable size regime, each additional atom can substantially change the properties of interest in the nonscalable size range. 1 Metal clusters of well-defined size, shape, and composition, supporting on metal oxide surfaces, provide model catalysts that offer a fundamental understanding of the processes taking place in real catalysts via structure−reactivity correlation at the molecular level. 2 This has been shown for the smallest clusters in numerous examples, three decades ago in the famous photographic experiment by Fayet et al., 3 and more recently in the work of Heiz et al. for metal clusters on MgO(100) 4−9 and of Anderson et al., 10,11 Watanabe et al.,12 and our group 13 for metal clusters on rutile TiO 2 (110). In all these examples, size-selected cluster deposition was employed in order to prepare monodispersed supported clusters which are difficult to obtain by conventional preparation methods. We note that experiments with a finite cluster size distribution also make it possible to study size effects in the scalable regime by varying the mean cluster size. 14 However, to firmly associate a catalytic property with a certain size, one has to probe the cluster morphology not only before but also after the reaction. This information is essentially lacking in most of the experiments cited above.It is fairly well accepted that metal catalysts deactivate, in particular under reaction conditions, as a consequence of morphological changes of the supported clusters, and that this change depends on the cluster size and cluster−support interaction. 2,15−24 This argument is even more important when the smallest entities, with only a handful of atoms, are under investigation. Accordingly, not only in model catalysts with well-defined initial conditions but also in real catalysts, this stability under the reaction conditions sets a clear limitation to the minimum cluster size that can be reasonably employed.Here we present a systematic study of the initial sizedependent reactivity and of the thermal as well as chemical stability of very small Pt clusters on rutile TiO 2 (110)-(1×1) in ...

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Section: ■ Resultsmentioning

confidence: 99%

Reaction-Induced Cluster Ripening and Initial Size-Dependent Reaction Rates for CO Oxidation on Pt_n/TiO₂(110)-(1×1)

et al. 2014

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“…Also, when vibrationally excited molecules strike another surface they are rapidly deexcited because of hot electron formation [42][43][44] . We fabricated a catalytic nanodiode where catalytically reactive platinum thin films were deposited on an oxide to form a Schottky barrier and we made contact to both sides of this barrier of Pt or Pd on TiO x or GaN [45][46][47] . Figure 18a shows the typical I-V curves measured on Pd/TiO x diode.…”

Section: The Active Sites At the Oxide Metal Interfacementioning

confidence: 99%

Colloid Science of Metal Nanoparticle Catalysts in 2D and 3D Structures. Challenges of Nucleation, Growth, Composition, Particle Shape, Size Control and Their Influence on Activity and Selectivity

2008

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Recent breakthroughs in synthesis in nanosciences have achieved control of size and shapes of nanoparticles that are relevant for catalyst design. In this article, we review the advance of synthesis of nanoparticles, fabrication of two and three dimensional model catalyst system, characterization, and studies of activity and selectivity. The ability to synthesize monodispersed platinum and rhodium nanoparticles in the 1-10 nm range permitted us to study the influence of composition, structure, and dynamic properties of monodispersed metal nanoparticle on chemical reactivity and selectivity. We review the importance of size and shape of nanoparticles to determine the reaction selectivity in multi-path reactions. The influence of metal-support interaction has been studied by probing the hot electron flows through the metal-oxide interface in catalytic nanodiodes. Novel designs of nanoparticle catalytic systems are discussed.2

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“…3, 8, and 9 and those of refs. [10][11][12] have independently detected electronic excitations during exposure of thin metal film/ semiconductor Schottky diodes to species that exothermically adsorb or chemically react on the metal film. Some of these electronic excitations reach the metal/semiconductor interface where the exit threshold for electrons is significantly lowered from the work function to the Schottky barrier and then enter the conduction band of the semiconductor and give rise to the chemicurrent I c , a measurable electric current that runs through the diode as a result of the surface chemical process.…”

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confidence: 99%

Chemistry of fast electrons

Maximoff

Head‐Gordon

2009

Proc. Natl. Acad. Sci. U.S.A.

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A chemicurrent is a flux of fast (kinetic energy տ 0.5؊1.3 eV) metal electrons caused by moderately exothermic (1؊3 eV) chemical reactions over high work function (4؊6 eV) metal surfaces. In this report, the relation between chemicurrent and surface chemistry is elucidated with a combination of top-down phenomenology and bottom-up atomic-scale modeling. Examination of catalytic CO oxidation, an example which exhibits a chemicurrent, reveals 3 constituents of this relation: The localization of some conduction electrons to the surface via a reduction reaction, 0.5 O2 ؉ ␦e ؊ 3 O ␦ ؊ (Red); the delocalization of some surface electrons into a conduction band in an oxidation reaction, O ␦ ؊ ؉ CO 3 CO 2 ␦ ؊ 3 CO2 ؉ ␦e ؊ (Ox); and relaxation without charge transfer (Rel). Juxtaposition of Red, Ox, and Rel produces a daunting variety of metal electronic excitations, but only those that originate from CO2 reactive desorption are long-range and fast enough to dominate the chemicurrent. The chemicurrent yield depends on the universality class of the desorption process and the distribution of the desorption thresholds. This analysis implies a power-law relation with exponent 2.66 between the chemicurrent and the heat of adsorption, which is consistent with experimental findings for a range of systems. This picture also applies to other oxidationreduction reactions over high work function metal surfaces.heterogeneous catalysis ͉ hot electrons ͉ metal surfaces ͉ surface science ͉ transition metals T he significance of electronic excitations on metal surfaces was probably first recognized within the photoelectric effect (1, 2) and has been reaffirmed ever since. Photoelectrons are emitted into the vacuum from metal surfaces upon exposure to light, the energy quantum of which exceeds an exit thresholdthe work function. Just as light ejects photoelectrons that map the surface, chemical reactions of exothermicity exceeding the work function eject exoelectrons (3) that map the chemistry on low work function (Շ3 eV) metal surfaces. Low work function metal substrates (e.g., groups IA, IIA, and IIIA) are too reactive to be practical for catalysis unlike their noble or late transition d-metal counterparts, which support rich chemistry, often of industrial significance. These more interesting substrates have work functions of 4Ϫ6 eV (4), too high for exoelectrons to emerge. But electronic excitations in metals are omnipresent: Any surface movement invokes, by virtue of Anderson's orthogonality catastrophe, sub-work function electronic excitations (5) that stay inside the metal-unseen unless looked for-and ultimately dissolve in the sea of thermal conduction electrons, phonons, and photons.Experimental evidence of internal metal excitations associated with chemistry on high work function metal surfaces has emerged only recently. A pattern in quenching of NO molecules ro-vibrationally excited by 2.9Ϫ3.8 eV near an Au(111) surface (6) alludes to the involvement of sub-work function electronic excitations. Involvement of the Au(111) surface...

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The Catalytic Nanodiode: Gas Phase Catalytic Reaction Generated Electron Flow Using Nanoscale Platinum Titanium Oxide Schottky Diodes

Cited by 66 publications

References 9 publications

Reaction-Induced Cluster Ripening and Initial Size-Dependent Reaction Rates for CO Oxidation on Pt_n/TiO₂(110)-(1×1)

Reaction-Induced Cluster Ripening and Initial Size-Dependent Reaction Rates for CO Oxidation on Pt_n/TiO₂(110)-(1×1)

Colloid Science of Metal Nanoparticle Catalysts in 2D and 3D Structures. Challenges of Nucleation, Growth, Composition, Particle Shape, Size Control and Their Influence on Activity and Selectivity

Chemistry of fast electrons

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The Catalytic Nanodiode: Gas Phase Catalytic Reaction Generated Electron Flow Using Nanoscale Platinum Titanium Oxide Schottky Diodes

Cited by 66 publications

References 9 publications

Reaction-Induced Cluster Ripening and Initial Size-Dependent Reaction Rates for CO Oxidation on Ptn/TiO2(110)-(1×1)

Reaction-Induced Cluster Ripening and Initial Size-Dependent Reaction Rates for CO Oxidation on Ptn/TiO2(110)-(1×1)

Colloid Science of Metal Nanoparticle Catalysts in 2D and 3D Structures. Challenges of Nucleation, Growth, Composition, Particle Shape, Size Control and Their Influence on Activity and Selectivity

Chemistry of fast electrons

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Reaction-Induced Cluster Ripening and Initial Size-Dependent Reaction Rates for CO Oxidation on Pt_n/TiO₂(110)-(1×1)

Reaction-Induced Cluster Ripening and Initial Size-Dependent Reaction Rates for CO Oxidation on Pt_n/TiO₂(110)-(1×1)