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

Cluster Size-Dependent Mechanisms of the CO + NO Reaction on Small Pd<i>_n</i> (<i>n</i> ≤ 30) Clusters on Oxide Surfaces

The CO + NO reaction (2CO + 2NO → N2 + 2CO2) on small size-selected palladium clusters supported on thin MgO(100) films reveals distinct size effects in the size range Pdn with n ≤ 30. Clusters up to the tetramer are inert, while larger clusters form CO2 at around 300 K, and this main reaction mechanism involves adsorbed CO and an adsorbed oxygen atom, a reaction product from the dissociation of NO. In addition, clusters consisting of 20−30 atoms reveal a low-temperature mechanism observed at temperatures below 150 K; the corresponding reaction mechanism can be described as a direct reaction of CO with molecularly adsorbed NO. Interestingly, for all reactive cluster sizes, the reaction temperature of the main mechanism is at least 150 K lower than those for palladium single crystals and larger particles. This indicates that the energetics of the reaction on clusters are distinctly different from those on bulklike systems. In the presented one-cycle experiments, the reaction is inhibited when strongly adsorbed NO blocks the CO adsorption sites. In addition, the obtained results reveal the interaction of NO with the clusters to show differences as a function of size; on larger clusters, both molecularly bonded and dissociated NO coexist, while on small clusters, NO is efficiently dissociated, and hardly any molecularly bonded NO is detected. The desorption of N2 occurs on the reactive clusters between 300 and 500 K

Temperature Dependent CO Oxidation Mechanisms on Size-Selected Clusters

Using p-MBRS experiments and TPR as well as FTIR measurements, it could be shown that dissociative oxygen activation occurs in the same temperature range for small clusters in comparison to faceted nanoparticles (NPs) and Pd single crystals. Surprisingly, CO poisoning does not take place on small Pd clusters at temperatures above 300 K. Furthermore, an oxygen activation has been found in this low temperature range which differs from the normal dissociative activation since it occurs only if CO is already adsorbed, before O2 adsorbs. Hence, the reactivity is promoted by CO under these conditions. This is in contrast to the normal Langmuir−Hinshelwood mechanism which has previously been observed for single crystals and facted NPs. Cooperative effects in the adsorption of the reactants are most likely responsible for such unordinary behaviors

Catalytic Dehydration of 2‑Propanol by Size-Selected (WO₃)_<i>n</i> and (MoO₃)_<i>n</i> Metal Oxide Clusters

Here, we report the catalytic dehydration of 2-propanol by metal oxide clusters, (WO₃)_<i>n</i> and (MoO₃)_<i>n</i> (<i>n</i> = 1, 2, 3, 5, 30), prepared by mass selecting and soft-landing metal oxide cluster anions created in the gas phase. Temperature-programmed reaction (TPR) was used to characterize the catalytic activity of the deposited clusters by measuring the production of propene from 2-propanol. The nature of the support, thermal history, size of the cluster, and cluster composition were all found to play important roles in influencing catalytic activity. (WO₃)₃ clusters deposited on HOPG (highly ordered pyrolytic graphite) and oxide supports exhibited catalytic activity, although (WO₃)₁ monomers deposited on HOPG did not catalyze 2-propanol dehydration effectively, an effect ascribed to their coalescence into large aggregates on HOPG. For tungsten oxide clusters deposited on annealed oxide films, catalytic activity was observed for all cluster sizes and was linearly correlated with the size of the deposited clusters. Two different mechanisms, linear-scaling of active sites and cluster ripening, could account for this linear dependence. However, even on oxide supports, deposited tungsten oxide clusters lost catalytic activity after annealing to 400 °C. The effect is consistent with the loss of dioxo groups rather than any cluster aggregation. Compared to tungsten oxide clusters, molybdenum oxide clusters exhibited little or no catalytic activity toward the dehydration of 2-propanol, rationalized by the decrease in Lewis acidity of molybdenum–oxygen bonds

Cluster Chemistry: Size-Dependent Reactivity Induced by Reverse Spill-Over

In the present work, the CO oxidation rate on size-selected Pd clusters supported on thin MgO films is investigated in pulsed molecular beam experiments. By varying the cluster coverage independent of the cluster size, we were able to change the ratio of direct and diffusion flux (reverse spill-over) of CO onto the cluster catalyst and thus probe the influence of reverse spill-over on the reaction rate for different cluster sizes (Pd8 and Pd30). The experimental results show that the change in reaction rate as a function of cluster coverage is different for Pd8 and Pd30. In order to explain these findings, the CO flux onto the clusters is modeled utilizing the collection zone model for the given experimental conditions. The results indicate that, for small clusters (Pd8), the reaction probability of an impinging CO molecule is independent of whether it is supplied by diffusion or direct flux. By contrast, for larger clusters (Pd30) a reduced reaction probability is found for CO supplied by reverse spill-over compared to CO supplied by direct flux

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