Results of atomic-level investigations

Investigations of mobility of protonic species have concentrated primarily on tungsten and molybdenum bronzes. In the case of hydrogen oxide bronzes, apparent inconsistencies can arise in comparing studies of materials prepared chemically (Zn/Ha(aq)) and materials prepared by hydrogen spillover (H2(g)/Pt-catalyst). In phases in which H is present 

On substitution of the host (Re03-type) framework metal atoms, NMR studies reveal the dynamical picture above to be retained .  

The structure of the parent lamellar M0O3 host is illustrated in Fig. 29.3. Studies of mobility in derived hydrogen bronzes have been numerous. This is due to the occurrence of rapid H-atom self-diffusion at ambient 

Zn/HCl(aq) reduction is the highest H-content (x 2.0) green monoclinic phase Lower H-content phases are obtainable chemically using coproportionation reactions of M0O3 and H,Mo03 in the presence of HjOO) . 

For the lowest H-content (blue orthorhombic, x 0.2-0.4) phase, slow H-atom self-diffusion (hopping between sites corresponding to attachment as hydroxyl) along the intra-layer 02. .. 02. .. 02. .. ribbon in the c direction (see Fig. 29.3) has been established by NMR relaxation studies Simple treatment of the relaxation data leads to an anomalous prefactor (t° 3 x 10 s, = 11 kJ mol ), interpretable as a consequence of the low-dimensionality of the motion . H NMR M2 and lineshape studies (for the rigid lattice) indicate non-random distribution of H-atoms in sites in the intralayer 02-atom ribbons. Clustering of H-atoms into pairs (neighbouring 02... 02 lines occupied by H) and higher clusters occur. 

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