7 - Structure and characterization of hydrogen insertion compounds of metal oxides

Summary

Introduction

Most transition-metal oxides are unreactive towards molecular hydrogen below elevated temperatures. However, dissociated hydrogen reacts topotactically with a wide range of binary and ternary transition-metal and uranium oxides at ambient temperature to produce hydrogen insertion compounds of formula H_xMO_n or H_xMM′O_n which are often referred to as ‘hydrogen bronzes'. These compounds are, in general, non-stoichiometric, with biphasic regions containing solids of fixed composition in equilibrium, separated by single-phase regions of variable hydrogen content. Powder X-ray diffraction confirms that the lattice parameters of the parent oxide are little changed on hydrogen insertion, with the implication that the metal-oxygen framework is largely retained. (At high x values, however, amorphous products are sometimes found implying that a structural collapse has occurred.) The maximum hydrogen contents achieved under standard conditions are controlled by both structural factors and the redox characteristics of the metal oxidation states involved (Table 7.1).

The hydrogen insertion compounds H_xMO_n are formally mixed valence and, in marked contrast to the parent oxides, often have deep colours and behave as good electronic conductors, either metallic or semiconducting. The controllable variation in electronic properties has been exploited in electrochromic displays, which utilize the colour changes induced by insertion into e.g. WO₃ or IrO₂ films and in other sensors, which respond to conductivity or optical changes in oxides on hydrogen incorporation. The recovery of hydrogen on heating, H_xMO_n = H_x–yMO_n + y/2H₂ (for favourable systems at high x values), suggests a possible application as hydrogen storage materials.