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1.
J Am Chem Soc ; 2020 Jul 15.
Artigo em Inglês | MEDLINE | ID: mdl-32598843

RESUMO

Selective photocatalytic transformations of chemicals derived from biomass, such as isobutanol, have been long envisioned for a sustainable chemical production. A strong temperature dependence in the reaction selectivity is found for isobutanol photo-oxidation on rutile TiO2(110). The strong temperature dependence is attributed to competition between thermal desorption of the primary photoproduct and secondary photochemical steps. The aldehyde, isobutanal, is the primary photoproduct of isobutanol. At room temperature, isobutanal is obtained selectively from photo-oxidation because of rapid thermal desorption. In contrast, secondary photo-oxidation of isobutanal to propane dominates at lower temperature (240 K) due to the persistence of isobutanal on the surface after it is formed. The byproduct of isobutanal photo-oxidation is CO, which is evolved at higher temperature as a consequence of thermal decomposition of an intermediate, such as formate. The photo-oxidation to isobutanal proceeds after thermally induced isobutoxy formation. These results have strong implications for controlling the selectivity of photochemical processes more generally, in that, selectivity is governed by competition of desorption vs secondary photoreaction of products. This competition can be exploited to design photocatalytic processes to favor specific chemical transformations of organic molecules.

2.
J Am Chem Soc ; 142(12): 5862-5869, 2020 Mar 25.
Artigo em Inglês | MEDLINE | ID: mdl-32125833

RESUMO

Mass-selected Ta8O2+ cluster ions catalyze the transformation of methane in a gas-phase ion trap experiment via nonoxidative coupling into ethane and H2, which is a prospective reaction for the generation of valuable chemicals on an industrial scale. Systematic variation of the reaction conditions and the isotopic labeling of methane by deuterium allow for an unambiguous identification of a catalytic cycle. Comparison with the proposed catalytic cycle for tantalum-doped silica catalysts reveals surprising similarities as the mechanism of the C-C coupling step, but also peculiar differences like the mechanism of the eventual formation of molecular hydrogen and ethane. Therefore, this work not only supplies insights into the mechanisms of methane coupling reactions but also illustrates how the study of trapped ionic catalysts can contribute to the understanding of reactions, which are otherwise difficult to study.

3.
J Phys Condens Matter ; 31(47): 473002, 2019 Nov 27.
Artigo em Inglês | MEDLINE | ID: mdl-31342942

RESUMO

Mechanisms in heterogeneous photocatalysis have traditionally been interpreted by the band-structure model and analogously to electrochemistry. This has led to the establishment of 'band-engineering' as a leading principle for the discovery of more efficient photocatalysts. In such a picture, mainly thermodynamic aspects are taken into account, while kinetics are often ignored. This holds in particular for chemical kinetics, which are, other than those for charge carrier dynamics, often not at all considered for the interpretation of the catalysts' photocatalytic performance. However, while being usually neglected in photocatalyis, they are a traditional and powerful tool in thermal catalysis and are still applied with great success in this field. While surface science studies made substantial contributes to thermal catalysis, analogous studies in heterogeneous photocatalysis still play only a minor role. In this review, the authors show that the photo-physics of defined materials in well-defined environments can be correlated with photochemical events on a surface, highlighting the importance of well-characterized semiconductors for the interpretation of mechanisms in heterogeneous photochemistry. The work focuses on contributions from surface science, which were obtained for the model system of a titania single crystal and alcohol photo-reforming. It is demonstrated that only surface science studies have so far enabled the elucidation of molecularly precise reaction mechanisms, the determination of reaction intermediates and assignment of reactive sites. As the identification of these properties remain major prerequisites for a breakthrough in photocatalysis research, the work also discusses the implications of the findings for applied systems. In general, the results from surface science demonstrate that photocatalytic systems shall also be approached by a perspective originating from heterogeneous catalysis rather than solely from an electrochemical point of view.

4.
Phys Chem Chem Phys ; 21(3): 1491-1496, 2019 Jan 17.
Artigo em Inglês | MEDLINE | ID: mdl-30607405

RESUMO

As the conduction band edge of rutile is close to the reduction potential of hydrogen, there is a long-lasting discussion on whether molecular hydrogen can be evolved from this semiconductor. Our study on methanol photoreforming in the ultra-high vacuum reveals that photocatalysts comprising a TiO2(110) single crystal decorated with platinum clusters indeed enable the evolution of H2. This is attributed to a new type of mechanism, in which the co-catalyst acts as a recombination center for hydrogen and not as a reduction site of a photoreaction. This mechanism is an alternative pathway to the commonly used mechanism derived from photoelectrochemistry and must particularly be considered for systems, in which reducible semiconductors enable the surface diffusion of hydrogen species.

5.
Chemphyschem ; 20(1): 62-69, 2019 01 07.
Artigo em Inglês | MEDLINE | ID: mdl-30444574

RESUMO

We have studied the circular dichroism (CD), in the ultraviolet and visible regions, of the transparent, chiral molecule 1,1'-Bi-2-naphtol (BINOL) in 1.5 µm thick films. The initial transparent film shows an additional negative cotton effect in the CD compared to solution. With time under room temperature the film undergoes a structural phase transition. This goes hand in hand with a cotton effect at the low energy absorption band which inverts with opposite propagation direction of light through the film which is revealed as a polarity reversal of ellipticity (PRE). After completion of the phase transition the film exhibits circular differential scattering throughout the visible range which also shows PRE. The structure change was studied with Raman, microscopy under cross polarization conditions and nonlinear second-harmonic generation circular dichroism (SHG-CD). The superposition of the optical activity of individual molecules and isotropy effects makes an interpretation challenging. Yet overcoming this challenge by finding a suitable model structural information can be derived from CD measurements.

6.
Chemphyschem ; 20(1): 134-141, 2019 01 07.
Artigo em Inglês | MEDLINE | ID: mdl-30403318

RESUMO

In this work, we present an experimental setup for the in situ and ex situ study of the optical activity of samples, which can be prepared under ultra-high vacuum (UHV) conditions by second-harmonic generation circular dichroism (SHG-CD) over a broad spectral range. The use of a racemic mixture as a qualified reference for the anisotropy factor is described and, as an example, the chiroptical properties of 1.5 µm thick (multilayers) as well as sub-monolayer thin films of the R- and S-enantiomer of 1,1'-Bi-2-naphthol (BINOL) evaporated onto BK7 substrates were investigated.

7.
J Chem Phys ; 151(24): 244304, 2019 Dec 28.
Artigo em Inglês | MEDLINE | ID: mdl-31893869

RESUMO

The hydrogenation of ethylene and acetylene was studied on a Pdn/MgO/Mo(100) model system containing palladium particles with a narrow size distribution around Pd26 (Pd20 to Pd35). Reactivity measurements were carried out in an ultrahigh vacuum chamber under isothermal conditions in the presence of deuterium. The catalyst system can readily hydrogenate both of these small molecules, and for acetylene, an alternative reaction network exists, in which it is trimerized to benzene. Distinct deactivation behavior was found for the two molecules and ascribed to different adsorption sites formed and influenced by the carbonaceous overlayer formed during the course of the reaction. These findings extend the A-E-model by Borodzinski and Golȩbiowski to extremely small particles and low partial pressures and show that it is possible to study realistic catalytic sites under highly defined conditions.

8.
Phys Chem Chem Phys ; 20(10): 7105-7111, 2018 Mar 07.
Artigo em Inglês | MEDLINE | ID: mdl-29479600

RESUMO

The thermal and photochemical mechanistic pathways for tertiary alcohols on the rutile TiO2(110)-surface are studied with the example of tert-butanol. While the thermal reaction is known to yield isobutene, the photochemical ejection of a methyl radical is observed at 100 K. The C-C scission, which is accompanied by the formation of acetone, is the only photochemical reaction pathway at this temperature and can be attributed to the reaction of photoholes that are created upon UV-light illumination at the surface of the n-type semiconductor. At 293 K the selectivity of the reaction changes, as isobutene is additionally formed photochemically. A comparison of the kinetics of the different reactions reveals further insights. Together with the quantitative evaluation of the reaction products at low temperatures and the comparison of the reaction pathways at different temperatures it is demonstrated how thermal effects can influence the selectivity of the reactions in photocatalysis.

9.
J Chem Phys ; 147(12): 124704, 2017 Sep 28.
Artigo em Inglês | MEDLINE | ID: mdl-28964022

RESUMO

In this work, ethanol is used as a chemical probe to study the passivation of molecular beam epitaxy-grown GaN(0001) by surface oxidation. With a high degree of oxidation, no reaction from ethanol to acetaldehyde in temperature-programmed desorption experiments is observed. The acetaldehyde formation is attributed to a mechanism based on α-H abstraction from the dissociatively bound alcohol molecule. The reactivity is related to negatively charged surface states, which are removed upon oxidation of the GaN(0001) surface. This is compared with the Ga2O3(2¯01) single crystal surface, which is found to be inert for the acetaldehyde production. These results offer a toolbox to explore the surface chemistry of nitrides and oxynitrides on an atomic scale and relate their intrinsic activity to systems under ambient atmosphere.

10.
Angew Chem Int Ed Engl ; 55(31): 8953-7, 2016 07 25.
Artigo em Inglês | MEDLINE | ID: mdl-27356301

RESUMO

Ethylene hydrogenation was investigated on size-selected Pt13 clusters supported on three amorphous silica (a-SiO2 ) thin films with different stoichiometries. Activity measurements of the reaction at 300 K revealed that on a silicon-rich and a stoichiometric film, Pt13 exhibits a similar activity to that of Pt(111), in line with the known structure insensitivity of the reaction. On an oxygen-rich film, a threefold increased rate was measured. Pulsing ethylene at 400 K, then measuring the activity at 300 K, resulted in complete loss of activity on the silicon-rich surface compared to only marginal losses on the other surfaces. The measured reactivity trends correlate with charging characteristics of a Pt13 cluster on the SiO2 films, predicted through first-principle calculations. The results reveal that the stoichiometry-dependent charging by the support can be used to tune the selectivity of reaction pathways during a catalytic hydrogenation reaction.

11.
J Colloid Interface Sci ; 478: 72-80, 2016 Sep 15.
Artigo em Inglês | MEDLINE | ID: mdl-27288572

RESUMO

We report the binding mode of amines and phosphines on platinum nanoparticles. Protective ligands comprising different functional groups are systematically studied for the elucidation of ligand binding at different functionalization conditions. From the functionalization conditions it is concluded that the binding of amines to the nanoparticles occurs via the formation of a PtHN moiety or electrostatic interaction, which is supported by spectroscopic evidences. In particular from complex chemistry such a binding mode is surprising, as amines are expected to bind via their electron pair to the metal. Similar results from functionalization are observed for phosphine-protected nanoparticles, which suggest similar binding modes in these systems. In contrast to the strong covalent bond of the protection with thiols, considerable weakly binding systems result. The characteristics of the binding mode are reflected by the stability of the colloids and their catalytic properties. In the selective hydrogenation of 3-hexyne to 3-hexene thiolate-stabilized Pt particles are highly stable, but exhibit the lowest activity. On the other hand, amine- and phosphine-capped platinum nanoparticles show a significantly higher activity, but rapidly agglomerate.

12.
Anal Chem ; 88(10): 5392-7, 2016 05 17.
Artigo em Inglês | MEDLINE | ID: mdl-27078611

RESUMO

Based on three different molecules dosed on a Pt(111) single crystal the selectivity and sensitivity of REMPI-TPD in UHV is investigated for a potential application in heterogeneous catalysis. It is shown that the two structural isomers ethylbenzene and p-xylene can be discriminated by REMPI in a standard TPD experiment. The latter is not possible for the ionization with electrons in a Q-MS. It is further demonstrated by benzene TPD studies that the sensitivity of the REMPI-TOF-MS is comparable to commercial EI-Q-MS solutions and enables the detection of less than 0.6% molecules of a monolayer.

13.
Nat Commun ; 7: 10700, 2016 Feb 25.
Artigo em Inglês | MEDLINE | ID: mdl-26911248

RESUMO

Nature employs self-assembly to fabricate the most complex molecularly precise machinery known to man. Heteromolecular, two-dimensional self-assembled networks provide a route to spatially organize different building blocks relative to each other, enabling synthetic molecularly precise fabrication. Here we demonstrate optoelectronic function in a near-to-monolayer molecular architecture approaching atomically defined spatial disposition of all components. The active layer consists of a self-assembled terrylene-based dye, forming a bicomponent supramolecular network with melamine. The assembly at the graphene-diamond interface shows an absorption maximum at 740 nm whereby the photoresponse can be measured with a gallium counter electrode. We find photocurrents of 0.5 nA and open-circuit voltages of 270 mV employing 19 mW cm(-2) irradiation intensities at 710 nm. With an ex situ calculated contact area of 9.9 × 10(2) µm(2), an incident photon to current efficiency of 0.6% at 710 nm is estimated, opening up intriguing possibilities in bottom-up optoelectronic device fabrication with molecular resolution.

14.
Nat Commun ; 7: 10389, 2016 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-26817713

RESUMO

The sensitivity, or insensitivity, of catalysed reactions to catalyst structure is a commonly employed fundamental concept. Here we report on the nature of nano-catalysed ethylene hydrogenation, investigated through experiments on size-selected Ptn (n=8-15) clusters soft-landed on magnesia and first-principles simulations, yielding benchmark information about the validity of structure sensitivity/insensitivity at the bottom of the catalyst size range. Both ethylene-hydrogenation-to-ethane and the parallel hydrogenation-dehydrogenation ethylidyne-producing route are considered, uncovering that at the <1 nm size-scale the reaction exhibits characteristics consistent with structure sensitivity, in contrast to structure insensitivity found for larger particles. The onset of catalysed hydrogenation occurs for Ptn (n ≥ 10) clusters at T>150 K, with maximum room temperature reactivity observed for Pt13. Structure insensitivity, inherent for specific cluster sizes, is induced in the more active Pt13 by a temperature increase up to 400 K leading to ethylidyne formation. Control of sub-nanometre particle size may be used for tuning catalysed hydrogenation activity and selectivity.

15.
Phys Chem Chem Phys ; 17(35): 22809-14, 2015 Sep 21.
Artigo em Inglês | MEDLINE | ID: mdl-26264863

RESUMO

In this work we present a stoichiometric reaction mechanism for the photocatalytic ethanol oxidation on TiO2(110). The reaction products are analyzed either under reaction conditions or after irradiation at lower temperatures. Water is identified as a quantitative by-product, which resides in a defect site. These water molecules cause a blocking of the defect sites which results in poisoning of the catalyst. By different preparation techniques of the TiO2(110) surface, the role of surface defects is further elucidated and the role of molecular oxygen is investigated. Based on the investigation, a complete photochemical reaction mechanism is given, which provides insights into general photon driven oxidation mechanisms on TiO2.

16.
Nano Lett ; 14(10): 5803-9, 2014 Oct 08.
Artigo em Inglês | MEDLINE | ID: mdl-25198035

RESUMO

Employing rationally designed model systems with precise atom-by-atom particle size control, we demonstrate by means of combining noninvasive in situ indirect nanoplasmonic sensing and ex situ scanning transmission electron microscopy that monomodal size-selected platinum cluster catalysts on different supports exhibit remarkable intrinsic sintering resistance even under reaction conditions. The observed stability is related to suppression of Ostwald ripening by elimination of its main driving force via size-selection. This study thus constitutes a general blueprint for the rational design of sintering resistant catalyst systems and for efficient experimental strategies to determine sintering mechanisms. Moreover, this is the first systematic experimental investigation of sintering processes in nanoparticle systems with an initially perfectly monomodal size distribution under ambient conditions.

17.
J Colloid Interface Sci ; 426: 264-9, 2014 Jul 15.
Artigo em Inglês | MEDLINE | ID: mdl-24863792

RESUMO

The functionalization of "unprotected" Pt clusters with N-acetyl-cysteine (NAC) at different pH-values is presented that allows for binding NAC either via the thiol or the amide group to the particle. NMR-spectroscopy was used to study the chemical nature of NAC at weakly acidic and alkaline conditions. The formation of a cyclic isomer of NAC was found at high pH-values which occurs through an intramolecular reaction between the thiol and the amide group delivering a cyclic thioether. The absence of the bare thiol groups in aqueous alkaline solutions leads to binding of the cyclic isomer of NAC to the Pt clusters via its nitrogen atom. IR spectroscopy was applied, which confirmed that the cyclic isomer is, however, not stable upon drying, but undergoes ring-opening yielding the "normal" non-cyclic form. This distinctive property of NAC in combination with the use of "unprotected" clusters allows for binding the same ligand to clusters from the same batch, but with different binding modes, while the particle size is preserved. As a consequence, differences in the cluster properties can be related exclusively to the influence of the binding properties of the ligand. Finally, the catalytic hydrogenation of 2-butanone was used as a probe reaction and the resulting differences in the enantioselectivity can thus be related to this particular change in the binding mode.


Assuntos
Acetilcisteína/química , Platina/química , Catálise , Ligantes , Espectroscopia de Ressonância Magnética , Espectrofotometria Infravermelho
18.
Phys Chem Chem Phys ; 16(16): 7299-306, 2014 Apr 28.
Artigo em Inglês | MEDLINE | ID: mdl-24618806

RESUMO

Well defined thin molecular films of 2,2'-dihydroxy-1,1'binaphthyl (binol) molecules at coverages between 5 × 10(15) molecules per cm(2) and 10(17) molecules per cm(2) on thin glass (BK7) substrates were investigated under ultra-high-vacuum (UHV) conditions. Second-Harmonic-Generation Optical-Rotatory-Dispersion measurements (SHG-ORD) were performed using a dedicated spectroscopic setup which allows for the determination of the rotation angle of the SH-signal of two enantiomers. Rotation angles of up to 38 degrees were measured. The chirality of the two enantiomers has been studied at 674 nm (337 nm resonance wavelength) in the transmission mode. Coverage dependent orientation evolution of binol molecular films was revealed by precise monitoring of the surface coverage while performing SHG-ORD experiments. We show that the molecules reach their final orientation at a surface coverage of 5 × 10(16) molecules per cm(2). From the obtained experimental data the ratio of chiral and achiral susceptibility components could be calculated and was observed to change with coverage.

19.
Phys Chem Chem Phys ; 15(44): 19253-61, 2013 Nov 28.
Artigo em Inglês | MEDLINE | ID: mdl-24113576

RESUMO

A proof of principle is presented for the rational design of metal clusters functionalized with hydrophilic, chiral ligands. A colloidal method is used to prepare "unprotected" metal clusters of well-defined size that are subsequently functionalized in a separate step with hydrophilic, chiral ligands. As clusters from the same batch are functionalized with different organic molecules while the cluster size is maintained, the approach allows for systematic investigations and the differences in the observed properties to be related to the influence of the functionalizing ligand. Within this work cysteine and two cysteine derivatives (glutathione and N-acetyl-cysteine) are used as functionalizing ligands for Pt clusters. The materials are characterized using various methods allowing for the determination of ligand coverage, binding mode and chiro-optical properties. Finally, 2-butanone hydrogenation is used as a simple model reaction to demonstrate that these systems exhibit the potential to be used as asymmetric, heterogeneous catalysts. The observed differences in selectivity and reactivity are discussed based on the knowledge gained from the characterization.


Assuntos
Ligantes , Metais/química , Acetilcisteína/química , Butanonas/química , Catálise , Cisteína/química , Glutationa/química , Hidrogenação , Interações Hidrofóbicas e Hidrofílicas , Platina/química , Estereoisomerismo
20.
J Am Chem Soc ; 135(36): 13262-5, 2013 Sep 11.
Artigo em Inglês | MEDLINE | ID: mdl-23961721

RESUMO

The photocatalytic water reduction reaction on CdS nanorods was studied as function of Pt cluster size. Maximum H2 production is found for Pt46. This effect is attributed to the size dependent electronic properties (e.g., LUMO) of the clusters with respect to the band edges of the semiconductor. This observation may be applicable for the study and interpretation of other systems and reactions, e.g. H2O oxidation or CO2 reduction.

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