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1.
Angew Chem Int Ed Engl ; 58(31): 10732-10736, 2019 Jul 29.
Artigo em Inglês | MEDLINE | ID: mdl-31095821

RESUMO

Ru/TiO2 catalysts exhibit an exceptionally high activity in the selective methanation of CO in CO2 - and H2 -rich reformates, but suffer from continuous deactivation during reaction. This limitation can be overcome through the fabrication of highly active and non-deactivating Ru/TiO2 catalysts by engineering the morphology of the TiO2 support. Using anatase TiO2 nanocrystals with mainly {001}, {100}, or {101} facets exposed, we show that after an initial activation period Ru/TiO2 -{100} and Ru/TiO2 -{101} are very stable, while Ru/TiO2 -{001} deactivates continuously. Employing different operando/in situ spectroscopies and ex situ characterizations, we show that differences in the catalytic stability are related to differences in the metal-support interactions (MSIs). The stronger MSIs on the defect-rich TiO2 -{100} and TiO2 -{101} supports stabilize flat Ru nanoparticles, while on TiO2 -{001} hemispherical particles develop. The former MSIs also lead to electronic modifications of Ru surface atoms, reflected by the stronger bonding of adsorbed CO on those catalysts than on Ru/TiO2 -{001}.

2.
J Chem Phys ; 148(19): 193821, 2018 May 21.
Artigo em Inglês | MEDLINE | ID: mdl-30307189

RESUMO

Ionic liquids (ILs) are considered as attractive electrolyte solvents in modern battery concepts such as Li-ion batteries. Here we present a comprehensive review of the results of previous model studies on the interaction of the battery relevant IL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]+[TFSI]-) with a series of structurally and chemically well-defined model electrode surfaces, which are increasingly complex and relevant for battery applications [Ag(111), Au(111), Cu(111), pristine and lithiated highly oriented pyrolytic graphite (HOPG), and rutile TiO2(110)]. Combining surface science techniques such as high resolution scanning tunneling microscopy and X-ray photoelectron spectroscopy for characterizing surface structure and chemical composition in deposited (sub-)monolayer adlayers with dispersion corrected density functional theory based calculations, this work aims at a molecular scale understanding of the fundamental processes at the electrode | electrolyte interface, which are crucial for the development of the so-called solid electrolyte interphase (SEI) layer in batteries. Performed under idealized conditions, in an ultrahigh vacuum environment, these model studies provide detailed insights on the structure formation in the adlayer, the substrate-adsorbate and adsorbate-adsorbate interactions responsible for this, and the tendency for chemically induced decomposition of the IL. To mimic the situation in an electrolyte, we also investigated the interaction of adsorbed IL (sub-)monolayers with coadsorbed lithium. Even at 80 K, postdeposited Li is found to react with the IL, leading to decomposition products such as LiF, Li3N, Li2S, LixSOy, and Li2O. In the absence of a [BMP]+[TFSI]- adlayer, it tends to adsorb, dissolve, or intercalate into the substrate (metals, HOPG) or to react with the substrate (TiO2) above a critical temperature, forming LiOx and Ti3+ species in the latter case. Finally, the formation of stable decomposition products was found to sensitively change the equilibrium between surface Li and Li+ intercalated in the bulk, leading to a deintercalation from lithiated HOPG in the presence of an adsorbed IL adlayer at >230 K. Overall, these results provide detailed insights into the surface chemistry at the solid | electrolyte interface and the initial stages of SEI formation at electrode surfaces in the absence of an applied potential, which is essential for the further improvement of future Li-ion batteries.

3.
J Phys Chem Lett ; 8(23): 5804-5809, 2017 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-29131962

RESUMO

The intercalation and deintercalation of lithium (Li) into / out of graphite(0001), which is a highly important process in Li-ion batteries, was investigated under ultrahigh vacuum conditions as a function of temperature, employing X-ray and ultraviolet photoelectron spectroscopy. Both the up-shifts of the core-level binding energy and the lowering of the work function ΔΦ reveal that heating of a monolayer of the battery-relevant ionic liquid (IL) 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]+[TFSI]-) adsorbed on lithiated graphite at 80 K to >230 K facilitates an accumulation of partially charged Liδ+ atoms at the IL-graphite(0001) interface. This is accompanied by a partial IL decomposition, which is associated with the initial stages of the chemical formation of the solid-electrolyte interphase.

4.
Phys Rev Lett ; 112(10): 107201, 2014 Mar 14.
Artigo em Inglês | MEDLINE | ID: mdl-24679323

RESUMO

X-ray photoemission electron microscopy combined with x-ray magnetic circular dichroism is used to study the magnetic properties of individual iron nanoparticles with sizes ranging from 20 down to 8 nm. While the magnetocrystalline anisotropy of bulk iron suggests superparamagnetic behavior in this size range, ferromagnetically blocked particles are also found at all sizes. Spontaneous transitions from the blocked state to the superparamagnetic state are observed in single particles and suggest that the enhanced magnetic energy barriers in the ferromagnetic particles are due to metastable, structurally excited states with unexpected life times.

5.
Chemphyschem ; 13(15): 3516-25, 2012 Oct 22.
Artigo em Inglês | MEDLINE | ID: mdl-22887474

RESUMO

The dissociative interaction of oxygen with structurally well-defined monolayer Pd(x)Ag(1-x)/Pd(111) surface alloys of different compositions, with well-known distributions of the respective surface atoms (A. K. Engstfeld et al., Phys. Chem. Chem. Phys. 2012, 14, 10754-10761), and the coadsorption of/reaction with CO on oxygen pre-covered surfaces were studied by high-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption/reaction spectroscopy (TPD/TPR). The impact of geometric ensemble effects as well as electronic ligand and strain effects on the adsorption and reaction behaviour of the respective species on the bimetallic surfaces is elucidated and compared with related systems such as CO adsorption on similar surfaces and oxygen adsorption on a Pd(67)Ag(33)(111) bulk alloy surface. The data show a clear dominance of ensemble effects on the oxygen adsorption and CO coadsorption behaviour, with oxygen adsorption limited to threefold-hollow sites on Pd(3) sites, while the combined electronic effects, as evident from modifications in the adsorption and reaction characteristics on the Pd sites, are small.

6.
Beilstein J Nanotechnol ; 2: 47-56, 2011.
Artigo em Inglês | MEDLINE | ID: mdl-21977415

RESUMO

BACKGROUND: Magnetic nanostructures and nanoparticles often show novel magnetic phenomena not known from the respective bulk materials. In the past, several methods to prepare such structures have been developed - ranging from wet chemistry-based to physical-based methods such as self-organization or cluster growth. The preparation method has a significant influence on the resulting properties of the generated nanostructures. Taking chemical approaches, this influence may arise from the chemical environment, reaction kinetics and the preparation route. Taking physical approaches, the thermodynamics and the kinetics of the growth mode or - when depositing preformed clusters/nanoparticles on a surface - the landing kinetics and subsequent relaxation processes have a strong impact and thus need to be considered when attempting to control magnetic and structural properties of supported clusters or nanoparticles. RESULTS: In this contribution we focus on mass-filtered Fe nanoparticles in a size range from 4 nm to 10 nm that are generated in a cluster source and subsequently deposited onto two single crystalline substrates: fcc Ni(111)/W(110) and bcc W(110). We use a combined approach of X-ray magnetic circular dichroism (XMCD), reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) to shed light on the complex and size-dependent relation between magnetic properties, crystallographic structure, orientation and morphology. In particular XMCD reveals that Fe particles on Ni(111)/W(110) have a significantly lower (higher) magnetic spin (orbital) moment compared to bulk iron. The reduced spin moments are attributed to the random particle orientation being confirmed by RHEED together with a competition of magnetic exchange energy at the interface and magnetic anisotropy energy in the particles. The RHEED data also show that the Fe particles on W(110) - despite of the large lattice mismatch between iron and tungsten - are not strained. Thus, strain is most likely not the origin of the enhanced orbital moments as supposed before. Moreover, RHEED uncovers the existence of a spontaneous process for epitaxial alignment of particles below a critical size of about 4 nm. STM basically confirms the shape conservation of the larger particles but shows first indications for an unexpected reshaping occurring at the onset of self-alignment. CONCLUSION: The magnetic and structural properties of nanoparticles are strongly affected by the deposition kinetics even when soft landing conditions are provided. The orientation of the deposited particles and thus their interface with the substrate strongly depend on the particle size with consequences regarding particularly the magnetic behavior. Spontaneous and epitaxial self-alignment can occur below a certain critical size. This may enable the obtainment of samples with controlled, uniform interfaces and crystallographic orientations even in a random deposition process. However, such a reorientation process might be accompanied by a complex reshaping of the particles.

7.
Beilstein J Nanotechnol ; 2: 593-606, 2011.
Artigo em Inglês | MEDLINE | ID: mdl-22003465

RESUMO

Aiming at model systems with close-to-realistic transport properties, we have prepared and studied planar Au/TiO(2) thin-film model catalysts consisting of a thin mesoporous TiO(2) film of 200-400 nm thickness with Au nanoparticles, with a mean particle size of ~2 nm diameter, homogeneously distributed therein. The systems were prepared by spin-coating of a mesoporous TiO(2) film from solutions of ethanolic titanium tetraisopropoxide and Pluronic P123 on planar Si(100) substrates, calcination at 350 °C and subsequent Au loading by a deposition-precipitation procedure, followed by a final calcination step for catalyst activation. The structural and chemical properties of these model systems were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N(2) adsorption, inductively coupled plasma ionization spectroscopy (ICP-OES) and X-ray photoelectron spectroscopy (XPS). The catalytic properties were evaluated through the oxidation of CO as a test reaction, and reactivities were measured directly above the film with a scanning mass spectrometer. We can demonstrate that the thin-film model catalysts closely resemble dispersed Au/TiO(2) supported catalysts in their characteristic structural and catalytic properties, and hence can be considered as suitable for catalytic model studies. The linear increase of the catalytic activity with film thickness indicates that transport limitations inside the Au/TiO(2) film catalyst are negligible, i.e., below the detection limit.

8.
Chemphyschem ; 11(14): 3123-32, 2010 Oct 04.
Artigo em Inglês | MEDLINE | ID: mdl-20715276

RESUMO

The correlation between structural and chemical properties of bimetallic PtRu/Ru(0001) model catalysts and their modification upon stepwise annealing of a submonolayer Pt-covered Ru(0001) surface up to the formation of an equilibrated Pt(x)Ru(1-x)/Ru(0001) monolayer surface alloy was investigated by scanning tunneling microscopy and by the adsorption of CO and D(2) probe molecules. Both temperature-programmed desorption and IR measurements demonstrate the influence of the surface structure on the adsorption properties of the bimetallic surface, which can be explained by changes of the composition of the adsorption ensembles (ensemble effects) for D adsorption and by changes in the electronic interaction (ligand effects, strain effects) of the metallic constituents for CO and D adsorption upon alloy formation.

9.
Phys Chem Chem Phys ; 12(33): 9801-10, 2010 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-20544100

RESUMO

The coadsorption of CO and hydrogen on a structurally well-defined Pt(35)Ru(65)/Ru(0001) monolayer surface alloy and, for comparison, on Ru(0001) were investigated by temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). The data reveal distinct modifications in the hydrogen adsorption behavior and also in the CO adsorption properties compared to adsorption of the individual components both on the mono-metallic and on the bimetallic surface. These modifications are discussed on an atomic scale, in a picture that involves adsorbate-adsorbate interactions and site-specific variations in the (local) adsorption properties of the bimetallic surface, due to electronic ligand and strain effects and geometric ensemble effects.

10.
Phys Chem Chem Phys ; 12(25): 6864-74, 2010 Jul 07.
Artigo em Inglês | MEDLINE | ID: mdl-20461242

RESUMO

The stability of three ultrathin TiO(x)/Pt(111) films with different stoichiometry and defectivity and the corresponding Au/TiO(x)/Pt(111) model catalysts in CO or a CO-O(2) (1 : 1) gas mixture up to a pressure of 100 mbar has been investigated. According to previous studies, the ultrathin films proved to be effective substrates to deposit in UHV Au nanoparticles with specific morphologies and lateral sizes ranging between 1 and 6 nm. The films have been characterized before and after the exposure using X-ray photoemission spectroscopy (XPS), low-energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). Additional in situ measurements of the CO chemisorption behavior were performed using polarization-modulation infrared reflection-absorption spectroscopy (PM-IRAS). A fully oxidized film is stable in CO and CO-O(2) (1 : 1) ambient, while the reduced films undergo an oxidative dewetting process at RT in the latter atmosphere. This process ultimately produces a nano-composite surface, where very tiny (from 0.5 to 3 nm lateral sizes) titania nanograins are mixed with open, uncovered areas of the Pt substrate. IRAS measurements on the corresponding Au/TiO(x)/Pt(111) model catalysts demonstrated that the CO chemisorption strongly depends on the Au nanoparticle size and morphology, while the actual Ti oxidation state of the oxide support does not seem to play a significant role.

11.
Chemphyschem ; 11(7): 1430-7, 2010 May 17.
Artigo em Inglês | MEDLINE | ID: mdl-20379981

RESUMO

Different types of planar Au/TiO(2) model catalysts are produced on TiO(2)(110) single-crystal substrates and thin TiO(2) films on Ru(0001) by physical vapor deposition of gold under ultrahigh-vacuum (UHV) conditions or by micelle-based chemical routes. Both the Au nanoparticles and the support are characterized by surface-science-based methods (such as atomic force microscopy and X-ray photoelectron spectroscopy) as well as by transmission electron microscopy. Finally, the activity of the model catalysts in the CO oxidation reaction is analyzed in a microflow reactor. Au/TiO(2)(110) model catalysts with a stoichiometric TiO(2)(110) support exhibit only a low catalytic activity compared to those with a reduced crystal and Au/TiO(2) model catalysts with thin TiO(2) films on Ru(0001) as a substrate. The possible influence of Ti interstitials in the reduced TiO(2)(110) substrates on the CO oxidation activity is discussed.

12.
Chemphyschem ; 11(7): 1482-90, 2010 May 17.
Artigo em Inglês | MEDLINE | ID: mdl-20043310

RESUMO

The influence of pre-adsorbed hydrogen on the adsorption of CO on well-defined PtRu/Ru(0001) surface alloys and the modification of the adsorption properties of both adsorbate species due to coadsorption was studied by a combination of temperature-programmed desorption and infrared spectroscopic measurements. The hydrogen binding strength is weakened both by surface alloy formation and by CO coadsorption. In the densely packed coadsorbate layers, which can be formed after CO post-adsorption, the adsorption properties of the adsorbed CO molecules are also affected significantly. Depending on the Pt surface concentration, the CO adsorption process on hydrogen pre-covered surfaces is blocked to a different extent with a decreasing blocking tendency for Pt-rich alloys.


Assuntos
Ligas/química , Monóxido de Carbono/química , Hidrogênio/química , Platina/química , Rutênio/química , Adsorção , Propriedades de Superfície
13.
Langmuir ; 22(18): 7873-80, 2006 Aug 29.
Artigo em Inglês | MEDLINE | ID: mdl-16922577

RESUMO

The morphology and stability of well-ordered, nanostructured Au/TiO2(110) surfaces, prepared by deposition of Au loaded micelles on TiO2(110) substrates and subsequent oxidative removal of the polymer shell in an oxygen plasma, was investigated by noncontact AFM, SEM and XPS. The resulting arrays of Au nanoparticles (particle sizes 1-5 nm) form a nearly hexagonal pattern with well-defined interparticle distances and a narrow particle size distribution. Particle size and particle separation can be controlled independently by varying the Au loading and the block-copolymers in the micelle shell. The oxygen plasma treatment does not affect the size and distance of the Au nanoparticles; the latter are fully metallic after subsequent UHV annealing (400 degrees C). The particles are stable under typical CO oxidation reaction conditions, up to at least 200 degrees C, making these surfaces ideally suited as defined model systems for catalytic studies. Significant changes in the height distributions of the Au nanoparticles are found upon 400 degrees C annealing in O2. For adlayers with small interparticle distances, this leads to a bimodal particle size distribution, which together with the preservation of the lateral order points to Ostwald ripening.

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