Room-temperature interaction of N18O with ultraviolet-illuminated titanium dioxide

Abstract

Preoxidised (OX) or prereduced (RED) TiO₂(anatase) powder was exposed to N¹⁸O at 295 K in the dark. Three phenomena resulted from subsequent ultraviolet illumination: photoadsorption, photoexchange (with a much higher rate than with ¹⁸O₂) and photodecomposition forming N₂O and, to a lesser extent, N₂. The initial rate of exchange was greater on the OX sample and the exchange revealed surface heterogeneity. Preillumination under vacuum of the RED sample did not modify its behaviour, whereas in the case of the OX sample exchange and decomposition occurred in the dark, but with much slower rates than under illumination. This is attributed to hole trapping involving an excess of oxygen species. Addition of butan-2-ol to N¹⁸O suppressed the nitric oxide isotopic exchange, while the butanone formed was labelled principally with ¹⁶O, which means that the same oxygen species intervened in the exchange and in the oxidation and that butan-2-ol retained its ¹⁶O atom. Photodecomposition accounted for ca. 15–20% of the NO pressure decrease, N₂ being formed only at the beginning and the percentage of N₂¹⁶O being initially higher for the OX sample. These results and their implications for photocatalytic oxidation are discussed.