Size Dependent Reactivity

From the observed size dependent hydrogen evolution (Fig. 5.30) two major conclusions can be drawn. First, it is direct evidence that differently sized clusters have different catalytic activity. Second, the reactivity is determined by the precise number 

In order to obtain a better understanding on how the reactivity of the system is tuned by cluster size a schematic sketch of the energy levels for photocatalytic water splitting with focus on the studied partial reaction is presented in Fig. 5.31b. 

The kinetics of the two electron transfers kei and kei) determine the trapping probability for the electrons. The overall rate of the electron transfer from the NR to the cluster (kei) is defined by the LUMO states of the clusters with respect to the semiconductor band edge. Clusters with a low LUMO can trap electrons efficiently and reduce the back reaction. However, with a too low LUMO the electrons are bound so strong on the clusters that the electron transfer rate from the cluster to the hydrogen atoms kei) is reduced, resulting in a lower Hi production. 

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Scheme 1. Palladacycles and their ring-size-dependent reactivity (general pictogram).

How can the size dependent reactivity be rationalized and what are the important factors which are responsible for this cluster size reactivity Experiments have been performed with 1-naphthol associated with other molecules (Knochenmuss et al. 1988 Knochenmuss and Leutwyler 1989). Naphthol undergoes proton transfer with two molecules of piperidine. For phenol, one needs three of four ammonia or three monoethylamine (MEA) molecules for the same process. 

Rottgen MA, Abbet S, Judai K, Antonietti JM, Worz AS, Arenz M, Henry CR, Heiz U (2007) Cluster chemistry Size-dependent reactivity induced by reverse spiU-over. J Am Chem Soc 129 9635... 

Size-dependent Reactivity of the Supported Metal Clusters... 

SIZE-DEPENDENT REACTIVITY OF SUPPORTED METAL CLUSTERS... 

Reaction Mechanism. To understand the size-dependent reactivity of palladium clusters on MgO surfaces in more detail, combined Fourier transform infrared (FTIR) and thermal desorption (TDS) studies were performed. The cluster model catalysts were first exposed to 1 Langmuir of CO at 90 K and subsequently to the same amount of NO. Upon linearly heating the model catalysts, the product molecules C02 and were detected by mass spectrometry as a function of the cluster size for Pd with n < 30. While for Pd4, the formation of C02 is negligible, Pdg and Pdso form C02 at 305 K or 145K and 300K, respectively (Fig. 1.97). 

The TPR spectra of the different products of the polymerization of acetylene on small, monodispersed palladium clusters supported on defect-rich MgO thin films are shown in Figure 6. Striking atom-by-atom size-dependent reactivities and selectivities are observed. Only the three reaction products C5H5, C4H8, and C4H6 are detected. Remarkably, no C3H , C5H , and are formed, indicating the absence... 

The specific heat measurements of isolated metal-cluster ions are mentioned, and the phase transitions manifested in the specific heats of the cluster ions are argued. As a representative of the chemical properties, we pick up the size-dependent reactivity of cluster ions in the gas phase and clusters on solid surfaces. The reaction processes change with the cluster size dramatically. 

Fig. 17. Size-dependent reactivity of iron, cobalt and nickel clusters. Solid circles connected by the solid lines show reactivity in hydrogen-molecule adsorption. Open triangles connected by the dashed lines show promotion energies of an electron from the highest-occupied molecular orbital to the lowest-unoccupied molecular orbital. The correlation between the promotion energy and the reactivity was found, where the higher the promotion energy, the lower the reactivity. (Adapted from Ref. 30.)...

Unsupported Au nanoclusters (or those contacting an inert support material such as BN) exhibit strong size-dependent reactivity, with optimal oxidation performance typically reached < 5 nm diameter [59], For example, colloidal gold stabilized by polyvinylpyrrolidone (PVP) shows pronounced size effects in the aerobic oxidation of benzylic alcohols in water under ambient conditions [60]. Figure 2.1 illustrates this phenomenon for p-hydroxybenzyl alcohol oxidation, wherein 1.3 nm Au clusters achieve 80 % conversion, whereas 9.5 nm clusters are catalytically dead. Differential oxygen adsorption onto these gold clusters is believed to play a crucial role in regulating reactivity. 

Summary—the presented data in this section reveals a size dependent reactivity of the hydrogenation of ethene on small size-selected Pt clusters. First, preliminary results from AES and IRRAS measurements are promising towards an understanding of the differences between single crystal and cluster reactivity, however are at an early stage and need a more detailed study. 

All together the results from the /u-reactors as a platform to test the reactivity of size-selected cluster materials under applied conditions has a high potential to shed light onto the open question of size dependent reactivity. The ongoing measurements of the CO oxidation reaction have the potential to contribute to the understanding of the controversial discussion of the structure in/sensitivity of the reaction for small particles. 

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