Palladium atom clustering

In the past, d-band theory has been successfully used to explain the reactivity of a wide range of catalysts. It was taken for granted that the participation of the structureless and wide sp band was not relevant. However, we want to emphasize that this approach is an oversimplification that could lead to wrong results. We have included two different pictures of the same system in order to show the behavior of the electron bands during the adsorption process. The electronic properties of a hydrogen atom approaching to the surface of a three palladium atoms cluster on Au(lll) are shown in Figure 1.16. 

FIGURE 1.16 DOS of hydrogen atom interacting with a three-palladium-atom cluster on Au(lll) at (a) the equilibrium distance and (b) 2 A from the equilibrium position. The states are projected onto the d band (blue line) and sp band (red line) of palladium and the s orbital of hydrogen (black line). (From Quaino, P. et al., Catal Today, 111, 55, 2011.)... 

II. The change of the sign between these two models may be also important for clustering and the surface growth mechanism. In the selfconsistent theory, the silver atoms tend to establish bonds with palladium atoms, while in the non-selfconsistent theory, the Ag-Ag bonds are favored. This explains the differences between the concentration profiles calculated within the models I and II and the fact that they do not simply follow the variations of the on-site terms. 

In the case of palladium particles supported on magnesium oxide, Heiz and his colleagues have shown,29 in an elegant study, a correlation between the number of palladium atoms in a cluster and the selectivity for the conversion of acetylene to benzene, butadiene and butane, whereas in the industrially significant area of catalytic hydrodesulfurisation, the Aarhus group,33 with support from theory, have pinpointed by STM metallic edge states as the active sites in the MoS2 catalysts. 

In the second cluster, the two Ru6 octahedra are linked through two palladium atoms. The third cluster contains two additional palladium atoms. The Pd4 skeleton adopts the form of a bent square. The two Ru6 octahedra have local structures similar to those in the second cluster, but their relative orientation is now twisted. Apparently, formation of these heterometallic cluster complexes does not result from a simple combination reaction between cationic and anionic complexes but is accompanied by partial redox reactions.900... 

Palladium metallic clusters have been prepared at room temperature by sonochemical reduction of Pd(OAc)2 and a surfactant, myristyltrimethylammonium bromide, in THE or MeOH [160[. It is noteworthy that nanosized amorphous Pd is obtained in THE, but in a crystalline form in MeOH. In this solvent, and in higher homologous alcohols, sonolysis of tetrachloropalladate(II) leads to Pd nanoclusters in which carbon atoms, formed by complete decomposition of the solvent, can diffuse. This results in an interstitial solid having the formula PdQ (0 < x < 0.15) [161]. Noble metal nanoparticles of Au, Pd, and Ag are obtained by sonicating aqueous solutions of the corresponding salts in the presence of a surfactant, which largely stabilise the naked col-... 

Balch et al. have described the preparation of (18 equation 10).107 This unusual cluster contains a linear array of palladium atoms, each palladium atom having a square planar geometry and Pd—Pd 2.592 A. 

The poisoning of Pt oxidation catalysts by phosphorus was also studied by Hegedus and Gumbleton107 and by Angele and co-workers.108-110 In a work by Dulcey and co-workers for the first time the thermal desorption of the PO radical was observed in the catalytic decomposition of DMMP on a polycrystalline Pt surface.111 Catalytic decomposition of Sarin using a Pt catalyst results initially in stoichiometric amounts of the oxidation products C02, HF, H20, and H3P04.106 The application of quantum-chemical methods to platinum and palladium surfaces has been quite limited because of the difficulty to treat large clusters of platinum and palladium atoms. 

Palladium nanoparticies can be easily prepared by displacement of the dba ligands by CO from Pd(dba)2. An intermediate unstable carbonyl complex forms, which collapses into clusters. By this method, palladium nanoparticies of sizes in the range 2-5 nm could be obtained using PVP or cellulose derivatives as stabilizers. Such particles were recently used to probe the mechanism of the Heck reaction. A correlation between the initial reaction rate and the particle sizes was established. The small particles proved more active in agreement with the presence of more surface palladium atoms with low metal-metal coordination number. ... 

An investigation of the oxidative addition of ChUand CD4[57a], as well as ethane [57b], to a bare palladium atom has demonstrated that quantum tunneling plays a very important role in the process. The barrier of insertion of different transition metal atoms into a C-C bond has been found to be 14-20 kcal mol higher than the barrier for insertion into a C-H bond [57c], Calculations for the activation of the C-H bond in ethylene by second row transition metal atoms showed that the oxidative addition barrier is lowest for the atoms to the right (for rhodium there is no barrier and for palladium the barrier is almost zero) [57d], The activation energy for B2 insertion into methane has been predicted to be 4.1 kcal mol while this value increases to 16.2 kcal mol for insertion of B [58], Two mechanisms have been considered by the SCF CNDO/S method for the oxidative addition of methane to the palladium cluster Pd2 [59a], In the first possible reaction, the C-H bond oxidatively adds to different palladium atoms ... 

The calculations have elucidated the higher catalytic activity of the cluster compared to a bare palladium atom. 

Addition of PPhi to the linear cluster [Pd3(CNMe)8] affords the substitution of CNMe and gives the linear cluster [Pd3(CNMe)6(PPh3)2]" where the three palladium atoms are colinear with the two phosphorous atoms of triphenylphosphine groups, forming a five atoms chain. 

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