Characterization and Reactivity of Gold Nanoparticles Supported on TiO2 Prepared by Deposition-Precipitation with NaOH and Urea

Gold 2003
Rodolfo Zanella, Suzanne Giorgio, Claude R. Henry, Chae-Ho Shin,
Abstract Introduction
Gold is catalytic provided that gold particles are small (<5 nm) and supported on metal oxides. The most remarkable catalytic properties of oxide-supported gold have been obtained for the reaction of CO oxidation at sub-ambient temperature by Haruta et al. in 1987 [1]. The most studied and efficient catalyst for CO oxidation is gold supported on TiO2, and the optimum gold particle size is 2-3 nm [2]. Such particle sizes can be achieved owing to suitable preparation methods and careful control of the conditions of preparation.
We have recently developed a new method of preparation of Au/TiO2 catalysts by deposition-precipitation with urea (DP Urea) [3]. It gives the same gold particle size as those obtained by Haruta with the method of deposition-precipitation with NaOH (DP NaOH) [4], but all gold of the preparation solution is deposited on TiO2, so higher gold loading can be reached.
The goal of the study was (i) to compare the catalytic behavior of these two types of catalysts in the reaction of CO oxidation so as to determine whether the DP Urea catalysts are as efficient as the DP NaOH ones; (ii) to determine the activation temperature for which these catalysts are most active in this reaction.

Results and discussion
Most often, the Au/TiO2 catalysts are calcined in air at 300 °C after preparation, to form metallic gold. Under these conditions, the DP NaOH (3 wt % Au) and DP Urea (7.6 wt % Au) catalysts exhibit the same activity per mole of gold in CO oxidation (reaction temperature 5 °C, 1% CO and 4% O2 in N2, total flow rate 99.3 cm3.min-1) (see Figures). This is an indication of the catalytic potential of the catalysts prepared by DP Urea.
These two sets of catalysts were calcined in air at various temperatures (100 to 400 °C), and their catalytic performances were evaluated in CO oxidation at 5 °C. For both series, the catalyst calcined at 200 °C exhibit the highest activity (see Figures). Above 200 °C, their activity decreases. Below 200 °C, the two types of catalysts also exhibit lower activities but with different behaviors. To understand these differences, the catalysts were characterized.
UV-Visible spectroscopy shows that the plasmon band around 550 nm and characteristic of metallic gold, appears at lower calcination temperature for DP NaOH (100 °C) than for DP Urea samples (150 °C). XAS analysis confirms these results. Comparison of their XANES spectra with those of reference compounds (AuIII and Au0) shows that about 40 % of the gold is metallic in the DP NaOH sample calcined at 100 °C, and all gold is metallic after calcination at 150 °C whereas in the DP Urea sample, there is no metallic gold at 100 °C and it begins to form at 150 °C (70 % of Au0) (see figures). These results are consistent with the EXAFS analysis.
In consequence, the increase in activity in CO oxidation after catalyst activation between 100 and 200 °C is explained by the increasing extent of reduction of AuIII into Au0. This confirms that the active gold species is Au0 for CO oxidation [5-8]. Above 200°C when all gold is metallic in both cases, the increase in gold particle size (see Figures) is too small to explain the decrease in activity; this point will be discussed.

Acknowledgment
Rodolfo Zanella is indebted to CONACYT and SFERE for his Ph.D. grant and to FESC, UNAM.
References
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Keywords: preparation, deposition-precipitation with NaOH, Particle size, XAFS, Au/TiO2, deposition-precipitation with urea, CO oxidation, UV-visible
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