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Redox Behavior of Gold Species in Zeolite NaY:  Characterization by Infrared Spectroscopy of Adsorbed CO

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posted on 15.02.2007, 00:00 by Mihail Mihaylov, Bruce C. Gates, Juan C. Fierro-Gonzalez, Konstantin Hadjiivanov, Helmut Knözinger
The state of gold on a Au/NaY zeolite sample prepared from Au(CH3)2(C5H7O2) and subjected to various treatments was monitored by infrared spectroscopy of adsorbed CO as a probe molecule. Spectra of CO adsorbed on a sample exposed to air and evacuated at room temperature indicate predominantly zerovalent gold nanoclusters (Auδ+−CO band at 2147 cm-1, associated with positively charged sites on the metal clusters). The spectra also indicate a small number of Au3+ sites in cation positions (2207 cm-1) and other Au3+ sites (2183 cm-1). However, the majority of the deposited gold did not form any carbonyl species. Evacuation at 423 K resulted in autoreduction of the Au3+ ions. At 473 K, the Auδ+ sites were converted into Au0 (Au0−CO band at 2122 cm-1). New bands at 2086 and 2036 cm-1, assigned to negatively charged gold carbonyls, were produced after CO adsorption on samples evacuated at temperatures ≥473 K. Interaction of the autoreduced sample with O2, even at ambient temperature, led to oxidation of some zerovalent gold to Auδ+ (2127 cm-1). With samples reoxidized at higher temperatures, the Auδ+−CO band was observed between 2132 and 2152 cm-1. When gold was reoxidized with an NO + O2 mixture, an Au+−CO band at ca. 2190 cm-1 was dominant in the carbonyl spectra. When CO was adsorbed at 85 K, the 2190 cm-1 band shifted at high coverage to 2185 cm-1. Results of CO−H2O coadsorption experiments indicate a static interaction of the Au+−CO species with CO molecules adsorbed on Na+ ions; in the presence of water, the Au+−CO band maximum was coverage independent because the Na+ ions were blocked by water molecules.

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