Hydrogen, interaction with metals

Casassa S and Pisani C 1995 Atomic-hydrogen interaction with metallic lithium an ab /M/o embedded-cluster study Phys. Rev. B 51 7805... 

A. P. Zakharov (Ed.), in Hydrogen Interaction with Metals, Nauka, Moscow, 1987. 

Greg Kubas of Los Alamos National Laboratory has determined how hydrogen interacts with metals. The important part of his work is that hydrogen, a substrate that is redox inactive substrate and not Brpnsted acidic, transforms upon complexation whereupon the coordinated H2 becomes acidic. The deprotonation of a metal dihydrogen complex generates oxidizable species and in this way, H2 is connected to electrons and heterolytic activation. Rauchfuss explained that Kubas discovery has helped guide his team s effort to connect H2 binding to this redox-active iron metal. 

In this case, the catalyst is usually a metal such as platinum or nickel, and the function of the catalyst is of considerable interest. In order to understand how the catalyst works, it is necessary to know how hydrogen interacts with metals. 

Hydrogen interacts with metals in three principal ways (i) by dissociative chemisorption at the surface (ii) by physical adsorption as molecules at very low temperatures and (iii) by dissolution or occlusion. As we shall see, to these three extreme forms have been added numerous intermediate states of various lifetimes and stabilities, some of which may have importance in catalysis. There is for example clear evidence for a molecular state formed at about 100 K on stepped surfaces saturated with atomic hydrogen (on Ni(510), Pd(510) and (210) ) this is distinct from a molecular precursor stzlt such as that seen with deuterium on Ni( 111) at 100 K. The role of such states will be discussed further below (Sections 3.2.2 and 3.3.3). The small size and electronic simplicity of the hydrogen atom formed by dissociation enable it to bond to metal surfaces in different ways, and simple-minded notions about its forming only a single covalent bond to another atom have to be abandoned. 

In the two previous sections, evidence has been presented concerning the chemisorbed states formed when benzene interacts with metal surfaces. It is not the intention in this Section to discuss benzene hydrogenation in detail, but rather to enquire whether studies of this hydrogen-addition reaction provide information about the chemisorbed state of benzene. 

Salomonsson, A., Eriksson, M. and Dannetun, H., Hydrogen interaction with platinum and palladium metal-insulator-semiconductor devices, Journal of Applied Physics, 98,014505,2005. 

Palladium hydride is not a stoichiometric chemical compound but simply a metal in which hydrogen is dissolved and stored in solid state, in space between Pd atoms of crystal lattice of the host metal. Relatively high solubility and mobility of H in the FCC (face-centered-cubic) Pd lattice made the Pd H system one of the most transparent, and hence most studied from microstructural, thermodynamic, and kinetic points of view. Over the century that followed many metal-hydrogen systems were investigated while those studies were driven mostly by scientific curiosity. Researchers were interested in the interaction of hydrogen molecule with metal surfaces adsorption and diffusion into metals. Many reports on absorption of in Ni, Fe, Ni, Co, Cu, Pd, Pt, Rh, Pd-Pt, Pd-Rh, Mo-Fe, Ag-Cu, Au-Cu, Cu-Ni, Cu-Pt, Cu-Sn, and lack of absorption in Ag, Au, Cd, Pb, Sn, Zn came from Sieverts et al. [30-33]. 

A Model System for Hydrogen Interaction with a Close Packed Metal Lattice... 

To explain the particles that formed in both the ethylene/oxygen and hydrogen/oxygen mixtures, it was postulated that they form in the gas phase and that the overall etching process takes place in three steps. First, free radicals are formed homogeneously in a boundary layer adjacent to the surface. Second, these radicals interact with metal atoms in the surface. This interaction results in the formation of volatile intermediates. Third, the metastable, volatile intermediates interact in the gas phase so that metal particles are formed and stable product molecules released. Individual metastable species presumably interact with each other and also with particles formed from multiple collisions. The larger particles interact with each other as well. 

The mechanism that is commonly considered to operate in the polymerisation of ethylene and a-olefins in the presence of group 4 metallocene-based catalysts is that devised by Cossee [268, 276, 277] for propylene polymerisation with heterogeneous Ziegler-Natta catalysts, though modifications invoking effects such as a-agostic hydrogen interactions with the metal centre have been proposed [343,344]. 

Density functional molecular orbital calculations suggest that a-agostic hydrogen interaction with the metal atom in a Zr-CH3 unit helps to stabilise it and align the methyl group of this unit for interaction with the ethylene n orbital [351,352]. The a-agostic transition state has one less rotational degree of freedom, which stabilises it and reinforces for the C C bond formation. 

Molecular hydrogen interacts with a metal center using its a- and a -orbitals as donor- and acceptor orbitals, as explained in Section 2-15. The approach of H2 leads to the polarization, elongation, and finally cleavage of the H—H bond in a concerted, three-center addition process (21-VI). 

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