Chiral Gold and Silver Nanoclusters: Preparation, Size Selection, and Chiroptical Properties

In this work we studied different properties of gold and silver nanoclusters (AuNCs and AgNCs) protected by the chiral ligands l-glutathione (L-GSH), and <i>N</i>-acetyl-l-cysteine (NALC), and we present a thorough characterization of the synthesized clusters. The synthesis was performed by reduction of the corresponding metal salt with NaBH₄. Fractions of gold nanoclusters with different sizes were isolated by methanol-induced precipitation. The ellipticity of the clusters was obtained by circular dichroism (CD) spectroscopy, showing that the chirality of the ligands is transferred to the metal core either in its structure or at least in its electronic states via perturbation of the electronic field of the ligands. The optical properties of gold and silver nanoclusters in water were studied by UV–vis spectroscopy. The absorption signal of the clusters shows characteristic bands, which can be assigned to plasmonic transitions of the metal core. In addition, UV–vis spectroscopy has served as a tool for studying the stability of these clusters in air. In general, gold nanoclusters are highly stable in air, and it was found that the stability of Au_<i>n</i>(NALC)_<i>m</i> clusters even exceeds that of Au_<i>n</i>(SG)_<i>m</i> clusters. In contrast to gold clusters, silver nanoclusters very often tend to decompose upon exposure to air. We found, however, that Ag_<i>n</i>(NALC)_<i>m</i> are surprisingly stable at atmospheric pressures. The average molecular formula of the nanoclusters was determined by thermogravimetric analysis (TGA). The particle sizes of AuNCs and AgNCs were assessed by transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) analysis. For studying the fluorescent properties of the metal nanoparticles, photoluminescence spectroscopy (PL) was performed. In summary, we succeeded to synthesize ligand-protected silver clusters (Ag_<i>n</i>(NALC)_<i>m</i>) with very high stability and rather narrow size distribution; furthermore, we could show the controlled precipitation to be applicable to other systems, such as that Au_<i>n</i>(NALC)_<i>m</i>, yielding two fraction of very narrow size distribution

Preparation and Spectroscopic Properties of Monolayer-Protected Silver Nanoclusters

Silver nanoclusters protected by 2-phenylethanethiol (<b>1</b>), 4-fluorothiophenol (<b>2</b>), and l-glutathione (<b>3</b>) ligands were successfully synthesized. The optical properties of the prepared silver nanoclusters were studied. The absorption signal of Ag@SCH₂CH₂Ph in toluene can be found at 469 nm, and Ag@SPhF in THF shows two absorption bands at 395 and 462 nm. Ag@SG in water absorbs at 478 nm. Mie theory in combination with the Drude model clearly indicates the peaks in the spectra originate from plasmonic transitions. In addition, the damping constant as well as the dielectric constant of the surrounding medium was determined. In addition, the CD spectra of silver nanoclusters protected by the three ligands (<b>1</b>–<b>3</b>) were also studied. As expected, only the clusters of type <b>3</b> gave rise to chiroptical activity across the visible and near-ultraviolet regions. The location and strength of the optical activity suggest an electronic structure of the metal that is highly sensitive to the chiral environment imposed by the glutathione ligand. The morphology and size of the prepared nanoclusters were analyzed by using transmission electron microscopy (TEM). TEM analysis showed that the particles of all three types of silver clusters were small than 5 nm, with an average size of around 2 nm. The analysis of the FTIR spectra elucidated the structural properties of the ligands binding to the nanoclusters. By comparing the IR absorption spectra of pure ligands with those of the protected silver nanoclusters, the disappearance of the S–H vibrational band (2535–2564 cm^–1) in the protected silver nanoclusters confirmed the anchoring of ligands to the cluster surface through the sulfur atom. By elemental analysis and thermogravimetric analysis, the Ag/S ratio and, hence, the number of ligands surrounding a Ag atom could be determined

Catalytic Non-Oxidative Coupling of Methane on Ta₈O₂⁺

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

Thermal Control of Selectivity in Photocatalytic, Water-Free Alcohol Photoreforming

The selective oxidation of alcohols has attracted a great deal of attention. While most photocatalytic studies focus on the generation of hydrogen from alcohols, there is also a great potential to replace inefficient thermal reaction pathways (as e.g. the formox process) by light-driven reactions. In this work we focus on the photoreforming of methanol, ethanol, cyclohexanol, benzyl alcohol, and tert-butanol on well-defined Ptx/TiO2(110) under UHV. It is found that, with the exception of tert-butanol, alcohol oxidation can produce the respective water-free aldehydes and ketones along with the formation of stoichiometric molecular hydrogen with 100% selectivity. While α-H-containing alcohols usually exhibit only a disproportionation reaction with the release of H2, another reaction pathway is detected for methanol (and to a much lower extent benzyl alcohol) to yield the respective ester, methyl formate (or benzyl benzoate, respectively). The formation of this product occurs via a consecutive photoreaction and is strongly influenced by temperature. In general, higher temperatures lead to a higher selectivity toward formaldehyde, as product desorption is favored over the consecutive photoreaction. For tert-butanol two parallel photoreactions occur. In addition to the splitting of a C–C bond yielding a methyl radical, hydrogen, and acetone, dehydration to isobutene is observed. The branching ratios of both reaction pathways can be strongly controlled by temperature, by changing the reaction regime from adsorption to desorption limited. The high selectivities toward aldehydes are attributed to the absence of O2 and water, which inhibits an unwanted overoxidation to acids or CO/CO2. This study shows that photocatalysis under such conditions provides a prospective approach for a highly selective and water-free aldehyde production under mild conditions

Size and Coverage Effects of Ni and Pt Co-Catalysts in the Photocatalytic Hydrogen Evolution from Methanol on TiO₂(110)

In the past decade, hydrogen evolution from photocatalytic alcohol oxidation on metal-loaded TiO2 has emerged as an active research field. While the presence of a metal cluster co-catalyst is crucial as a H2 recombination center, size and coverage effects on the catalyst performance are not yet comprehensively understood. To some extent, this is due to the fact that common deposition methods do not allow for an independent change in size and coverage, which can be overcome by the use of cluster sources and the deposition of size-selected clusters. This study compares size-selected Ni and Pt clusters as co-catalysts on a TiO2(110) single crystal and the resulting size- and coverage-dependent effects in the photocatalytic hydrogen evolution from alcohols in ultrahigh vacuum (UHV). Larger clusters and higher coverages of Ni enhance the product formation rate, although deactivation over time occurs. In contrast, Pt co-catalysts exhibit a stable and higher activity and size-specific effects have to be taken into account. While H2 evolution is improved by a higher concentration of Pt clusters, an increase in the metal content by the deposition of larger particles can even be detrimental to the performance of the photocatalyst. The acquired overall mechanistic picture is corroborated by H2 formation kinetics from mass spectrometric data. Consequently, for some metals, size effects are relevant for improving the catalytic performance, while for other co-catalyst materials, merely the coverage is decisive. The elucidation of different size and coverage dependencies represents an important step toward a rational catalyst design for photocatalytic hydrogen evolution

Isomer-Selective Detection of Aromatic Molecules in Temperature-Programmed Desorption for Model Catalysis

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 <i>p</i>-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

Tuning Strong Metal–Support Interaction Kinetics on Pt-Loaded TiO₂(110) by Choosing the Pressure: A Combined Ultrahigh Vacuum/Near-Ambient Pressure XPS Study

Pt catalyst particles on reducible oxide supports often change their activity significantly at elevated temperatures due to the strong metal–support interaction (SMSI), which induces the formation of an encapsulation layer around the noble metal particles. However, the impact of oxidizing and reducing treatments at elevated pressures on this encapsulation layer remains controversial, partly due to the “pressure gap” between surface science studies and applied catalysis. In the present work, we employ synchrotron-based near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to study the effect of O2 and H2 on the SMSI-state of well-defined Pt/TiO2(110) catalysts at pressures of up to 0.1 Torr. By tuning the O2 pressure, we can either selectively oxidize the TiO2 support or both the support and the Pt particles. Catalyzed by metallic Pt, the encapsulating oxide overlayer grows rapidly in 1 × 10–5 Torr O2, but orders of magnitude less effectively at higher O2 pressures, where Pt is in an oxidic state. While the oxidation/reduction of Pt particles is reversible, they remain embedded in the support once encapsulation has occurred

Cluster Size Effects in the Photocatalytic Hydrogen Evolution Reaction

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

Size-Selected Subnanometer Cluster Catalysts on Semiconductor Nanocrystal Films for Atomic Scale Insight into Photocatalysis

We introduce size-selected subnanometer cluster catalysts deposited on thin films of colloidal semiconductor nanocrystals as a novel platform to obtain atomic scale insight into photocatalytic generation of solar fuels. Using Pt-cluster-decorated CdS nanorod films for photocatalytic hydrogen generation as an example, we determine the minimum amount of catalyst necessary to obtain maximum quantum efficiency of hydrogen generation. Further, we provide evidence for tuning photocatalytic activities by precisely controlling the cluster catalyst size