The effect of metal composition on the performance of Ir–Au/TiO2 catalysts for citral hydrogenation.

Abstract

The effect of gold addition to iridium catalysts and the nature of active sites for citral hydrogenation were investigated over Ir–Au/TiO2 catalysts. All samples (Au/TiO2, Ir/TiO2 and Ir–Au/TiO2) were prepared by deposition-precipitation with urea (DPU). Bimetallic catalysts were synthetized by co-deposition at different Ir/Au atomic ratios (3, 1, 0.3). The catalysts were characterized by ICP, BET, H2–TPR, H2-Chemisorption, TEM, DRIFTS and XPS techniques. A partial coverage of iridium sites by gold atoms takes place as the gold amount in the catalyst increases as shown by chemisorption and XPS measurements. The latter also evidenced a trend consistent with modification of the electronic environment of iridium due to interaction with gold atoms. Also, compared to Ir/TiO2, the amount of Irδ+ species was higher in the bimetallic catalysts reaching a maximum in Ir–Au (1) sample. Characterization of the catalytic surface using DRIFTS of adsorbed CO evidenced a shift towards higher wavenumbers as a function of the gold content indicating, as the XPS results, a modification of the adsorption site. The catalytic activity for citral hydrogenation increased as a function of the gold content. The selectivity to unsaturated alcohol is related to the amount of Irδ+ species which in turns depends on the catalyst composition. For the most active sample Ir–Au (1), a suitable Irδ+/Ir0 ratio is already obtained when the catalyst is reduced at 573K and does not vary with reduction temperature.