Au-Ir/TiO2 Prepared by Deposition Precipitation with Urea: Improved Activity and Stability in CO Oxidation

摘要

A series of Ir and Au-Ir supported on TiO2 catalysts were prepared by deposition-precipitation with urea to study the activity and stability of these materials in the CO oxidation reaction. Bimetallic samples were prepared using two approaches: one by codeposition of the metal precursors and the other by sequential deposition being iridium the first to be incorporated on the support. Samples were submitted to calcination in air or reduction in hydrogen thermal treatments. Nominal gold and iridium loading were 4 wt %. Samples were characterized by EDS, H2-TPR, HRTEM, HAADF, and CO + O2 adsorption followed by DRIFTS. It is shown for the first time that deposition-precipitation with urea is able to deposit Ir and Au-Ir nanoparticles on TiO2. Catalyst pretreatment had an important effect on the structure of the iridium phase. In calcined samples, Ir spreads over the TiO2 mainly as a thin layer of IrO2 particles preferentially deposited on the rutile phase of TiO2. In reduced samples, Ir particles were homogeneously dispersed on all of the TiO2 crystals. It is shown that, even in the samples reduced at 300 °C, IrO2 was present in the catalysts. Ir/TiO2 samples prepared by deposition-precipitation with urea calcined or reduced in H2 were not active at room temperature (light-off temperature above 250 °C). The preparation protocol in bimetallic catalysts (codeposition or sequential deposition of the metals and different pretreatments) had a strong influence on the catalytic performance of the catalysts. The most active sample was the one prepared by sequential deposition and thermally treated in hydrogen at 300 °C. An enhanced activity was observed when compared to Au/TiO2. Besides this synergetic effect, the bimetallic catalyst was more stable in time on stream and more stable against sintering after reaction. DRIFTS experiments showed that the interaction of Au and Ir could modify the adsorption properties of catalyst surface.