Quasielastic neutron scattering hydrogens

Diffusion of atoms from the point at which they dissociate on a metal surface to the edge of the metal crystallite is one of the component steps of hydrogen spillover. Quasielastic neutron scattering experiments have produced direct evidence for the diffusion coefficients of hydrogen on the surface of catalysts. The mean time between diffusional jumps for hydrogen on a Raney Ni surface has been found to be 2.7 0.5 x 10 9s at 150°C.72 For H on the surface of Pt crystals dispersed within a Y type zeolite the mean time between surface jumps was found73 to lie between 3.0 and 8 x 10-9s at 100 °C. 

Furthermore, neutron scattering techniques, viz. quasielastic neutron scattering, are a valuable tool for studying other dynamic processes in solid hydrate research, such as rotatory and translatory diffusion of water molecules and hydrogen atoms, respectively (see, for example. Refs. 10, 32, 33). 

Proton transfer mechanisms rely on several, generally slow, dynamical events. They are much studied at ambient temperatures in the case of biological samples but high temperatures are needed to obtain significant conduction from crystalline solids. These studies often rely on quasielastic neutron scattering techniques that are not discussed in this book, but see [47]. However, at low temperatures INS spectroscopy can provide direct information on the potential surface that controls the fast step, where the hydrogen atom transfers from the donor well to the acceptor well. 

The polymorphism of hydrogen sulfides of alkali metals M+(HS ) with M — Na, K, Rb, Cs was studied with neutron and X-ray diffraction, quasielastic neutron scattering, DSC and proton NMR. These compounds have three polymorphs and two known phase transitions. When M — Na, K, Rb, the LTP is monoclinic, the medium-temperature phase (MTP) is rho-mbohedral and the HTP is cubic with a NaCl-type structure. Owing to the fact that the sequence of phase transitions is the same for the three cations, this series is expected to be a model where coulombic interaction is modulated by the cation radii, which increases in the order Na, K, Rb, Cs. 

HYDROGEN DIFFUSION IN AMORPHOUS P g Si Ql -A QUASIELASTIC NEUTRON SCATTERING STUDY... 

Hydrogen diffusion in amorphous gQ 20 apparently does not involve a continuous distribution of hydrogen jump rates, but consists of two well-separated classes of jump rates. A two state trapping model allows a semi-quantitative description of our quasielastic neutron scattering data. 

For the investigation of the spatial and temporal development of the microscopic hydrogen diffusion mechanism in metals, quasielastic neutron scattering (QNS) is a unique tool. It has been applied first to simple systems 1) 2)... 

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