Elsevier

Journal of Catalysis

Volume 208, Issue 1, 15 May 2002, Pages 229-237
Journal of Catalysis

Regular Article
Interfacial Properties of an Ir/TiO2 System and Their Relevance in Crotonaldehyde Hydrogenation

https://doi.org/10.1006/jcat.2002.3566Get rights and content

Abstract

Titania-supported iridium catalysts were prereduced in a hydrogen flow at 473 K (LT) and 723 K (HT). Metal particle sizes determined by H2 chemisorption and by direct observation of metal particles by transmission electron microscopy were quite similar for LT treatment, and close to 3.0 nm. For the HT case, a large difference in particle size between both techniques is obtained, as a consequence of the H2 chemisorption suppression (SMSI effect). Photoelectron spectra revealed that iridium is in different oxidation states, with a contribution of 19% Irδ+ and 81% Ir° species for the LT sample and only a slight increase in Irδ+ species (26%) for the HT catalyst. Further insights into the surface structures developed by LT and HT treatments were derived from the catalytic performance in the vapor-phase hydrogenation of crotonaldehyde. Activity, expressed as turnover frequency, was more than one order of magnitude higher for the HT catalyst than for its LT counterpart. The interfacial metal–TiOx moieties, created upon reduction treatment, appeared to be responsible for the increase in activity and in selectivity to crotyl alcohol, via the formation of a [–CO…surface] σ-bonded complex (detected by in situ DRIFTS as a band at 1650 cm−1) stabilized at the metal–TiOx interface. HT treatment enhances the metal–TiOx contact, which results in an improvement in catalyst performance. The catalysts deactivate slowly with the time of reaction. Two reasons are advanced to explain the catalyst deactivation: (i) the formation of a strongly chemisorbed asymmetric carboxylate (band at 1740 cm−1); and (ii) the formation of heavy products with conjugated CO and CC bonds (band at 1720 cm−1). Both complexes are formed at the expense of the σ-complex and progressively block the active sites responsible for crotonaldehyde hydrogenation.

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