The nonlinear absorption of Ag atomic clusters and nanoparticles dispersed in a transparent oxyfluoride glass host has been studied. The as-prepared glass, containing 0.15 at.% Ag, shows an absorption band in the UV/violet attributed to the presence of amorphous Ag atomic nanoclusters with an average size of 1.2 nm. Upon heat-treatment the Ag nanoclusters coalesce into larger nanoparticles that show a surface plasmon absorption band in the visible.Open aperture z-scan experiments using 480 nm nanosecond laser pulses demonstrated nonsaturated and saturated nonlinear absorption with large nonlinear absorption indices for the Ag nanoclusters and nanoparticles, respectively. These properties are promising, e.g., for applications in optical limiting and object's contrast enhancement.
A major drawback of state-of-the-art proton exchange membrane fuel cells is the CO poisoning of platinum catalysts. It is known that CO poisoning is reduced if platinum alloys are used, but the underlying mechanism therefore is still under debate. We study the influence of dopant atoms on the CO adsorption on small platinum clusters using mass spectrometry experiments and density functional theory calculations. A significant reduction in the reactivity for Nb and Mo doped clusters is attributed to electron transfer from those highly coordinated dopants to the Pt atoms and the concomitant lower CO binding energies. On the other hand Sn and Ag dopants have a lower Pt coordination and have a limited effect on the CO adsorption. Analysis of the density of states demonstrates a correlation of dopant induced changes in the electronic structure with the enhanced tolerance to CO poisoning.The development of efficient fuel cells is a promising strategy to diminish the dependence on fossil fuel by making use of environmentally friendly energy sources. [1] Proton exchange membrane fuel cells (PEMFCs) are highly susceptible to CO poisoning of the platinum catalyst. [2] CO molecules, present as trace components in the fuel, preferentially adsorb on Pt nanoparticles, thereby blocking the active sites and degrading the cell's performance. Several Pt alloys, such as Pt-X (X = Sn, Ru, Mo, Nb, W, Ag, and Ni), are known for an enhanced tolerance to the CO poisoning and thus improve the performance of the fuel cell. [3][4][5][6][7] The physical mechanism responsible for the tolerance has been extensively studied and is ascribed to an alteration of the local electronic structure at the reaction site upon alloying and/or to a bi-functional mechanism, in which OH groups adsorbed on the alloying agent interact with CO and form CO2 and H2, which are released from the catalyst, regenerating the active sites. [3] The interaction of transition metal surfaces with CO is a complex problem which has been described by three qualitative models: the d-band center model, [8] the Blyholder model, [9] and the π-σ model. [10] In the Blyholder model, the Pt-CO interaction is described by donation of electron density from the CO 5σ orbital to empty Pt 5d states and back-donation from occupied Pt 5d states to the CO 2π* antibonding orbital. The bottom-line of local electronic structure modifications as explanation for the CO tolerance in Pt-X alloy nanoparticles is that electron transfer from the alloying agent to the empty Pt 5d states reduces the Pt-CO bonding strength. [11] Although few-atom platinum clusters in the gas phase differ significantly in size and in environmental conditions from the nanoparticles used in PEMFCs, they can provide a better understanding for the enhanced tolerance to CO poisoning. The CO binding is a local event, which poisons a Pt active site. Clusters in molecular beams are ideal model system for complex processes that depend on local chemistry. Conditions (cluster size, composition, and charge state) are well c...
different from those of bulk materials. [6]
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