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The first example of an active layered tantalate photocatalyst containing partly filled lanthanide 4f shell is reported. A single phase of layered perovskite tantalates, RbLnTa2O7, could be obtained with Ln = La, Pr, Nd, and Sm; the ionic radii of these trivalent cations are required to be larger than 0.126 nm for constructing a perovskite slab. Under UV irradiation from a high-pressure Hg lamp, these layered tantalates demonstrated the photocatalytic activity for water splitting into stoichiometric H2/O2 mixtures even without loading metal catalysts. The rates of H2 and O2 evolutions were found to be strongly dependent on the lanthanoids, increasing in the sequence of La ≑ Pr ≪ Sm < Nd. The maximum rate of H2 evolution observed over RbNdTa2O7 reached 47.0 μmol/h. The absorption spectrum of RbNdTa2O7 consists of the internal 4f transitions in the visible region and a broad band overlapping the band-gap transition in the UV region. The latter band indicates that a possible excitation process including the partly filled 4f shell plays a key role in photocatalytic reactions. Loading 0.5 wt % Ni onto RbNdTa2O7 by simple impregnation significantly improved H2 evolution (117.2 μmol/h).
Machida et al. (Tue,) studied this question.