CHARACTERIZATION OF IRON TITANIUM-OXIDE PHOTOCATALYSTS .2. SURFACE STUDIES

A study of the surfaces of particulate solids, formed by the dispersio n of iron(III) ions in various titanium dioxide preparations, has reve aled that significant changes occur during the calcination of these ma terials in air at progressively higher temperatures (T/K less-than-or- equal-to 1273). In the more dilute systems, containing nominal iron(II I) concentrations of 0.5-1.0 atom%, solid solutions in the anatase pha se exhibit good degrees of dispersion which are maintained as the phas e transformation into rutile progresses at more elevated temperatures (T/K greater-than-or-equal-to 923). With the larger nominal iron conce ntrations (2 and 5 atom%) X-ray photoelectron (XPS) and diffuse-reflec tance (DRS) spectroscopic measurements show that the formation of iron -rich surfaces has commenced already at 773 K through there being a lo wer limit of solubility of iron in the rutile phase and through the an atase to rutile phase transition becoming detectable at lower temperat ures in these solids. The Fe:Ti atomic ratios at the surface with the larger nominal concentrations of iron are consistent with the formatio n of iron-rich surface phases, alpha-Fe2O3 and Fe2TiO5. Inhomogeneitie s in the distribution of the surface ion species have also been demons trated by EDX measurements. The structural character of these solids h as been discussed in relation to their photocatalytic activities for t he reductive fixation of dinitrogen by water, in which only single-pha se systems were found to be active. The inactivity of the multi-phasic solids has been considered in terms of the masking of the active phas e by surface layers which create electronic heterojunctions that encou rage hole-electron recombination.