Abstract

Ruthenium was dispersed in SnO₂ by cogelation of tin and ruthenium precursor compounds: tetra(tert-butoxy)- tin(IV), [Sn(OBu^t)₄], and tris(acetylacetonato)ruthenium(III) [Ru(acac)₃]. This resulted in samples with a controlled and reproducible metal/SnO₂ molar ratio (0.8). An electron paramagnetic resonance study on sample reactivity showed that: [Ru(acac)₃] was included in the SnO₂ gel without significant matrix-induced quali-quantitative modifications of the coordination compound; thermal treatment of [Ru(acac)₃]–SnO₂ in air led to oxidation of most of the Ru³⁺ centres to diamagnetic Ru⁴⁺ centres; reduction with CO(0.5%)–argon at 773 K reformed all the original Ru³⁺ centres. In the CO–argon treated samples, the Ru³⁺ centres were in the field of oxide ligands; lower paramagnetic oxidation states, clustering effects and paramagnetic singly ionized oxygen vacancies (V_O⁺) were absent. It was concluded that a great many Ru⁴⁺centres could substitute Sn⁴⁺in samples prepared by cogelation, thus optimizing the intimacy of metal–semiconductor contact.

As an alternative to cogelation, ruthenium was more conventionally dispersed in SnO₂, impregnating the oxide with [Ru(acac)₃]. [Ru(acac)₃]–SnO₂-impregnated samples, thermally decomposed in air and reduced by CO(0.5%)–argon, gave fewer trivalent ruthenium centres and stable singly ionised oxygen vacancies, V_O⁺, were evident. This suggested that metal centre clustering took place and electron transfer from V_O⁺ to the metal was less efficient than in sol–gel prepared samples, due to the less intimate metal–semiconductor contact of the metal clusters.

Both types of ruthenium-dispersed tin oxide samples reacted with oxygen, the resultant amounts of Sn⁴⁺–O₂⁻ being much greater for the sol–gel prepared samples than for the impregnated analogues.

The sol–gel method of metal dispersion seemed much more efficient in preparing samples highly sensitive to both reducing and oxidising gases. The electronic sensitisation induced in SnO₂ by dispersed ruthenium seems to depend on the extent of electron transfer from the oxygen vacancies to ruthenium and then to O₂. The greater the transfer, the larger the electron deficient space-charge layer.