Characterization of Au-based Catalysts Using Novel Cerium Oxide Supports

Abstract

Gold dispersed on various oxide supports has been shown to exhibit high activity in a variety of important catalytic reactions, the most widely investigated of which remains the low-temperature oxidation of CO to CO 2 [1]. The nature of the oxide support can have a profound effect on the activity of the catalysts, and Au-catalysts utilizing nanocrystalline oxide supports such as CeO 2 and Y 2 O 3 have been found to possess a significantly greater activity than those dispersed on larger support particles [2]. In the present study, a novel synthesis method involving the use of supercritical CO 2 as an anti-solvent was devised to prepare CeO 2 (scCeO 2 ) support particles. The rapidity of precipitation using this method makes possible the formation of structures that are not available with traditional precipitation methods. This process has previously been used to produce vanadium phosphate catalysts with unusual morphologies and improved activity [3].With this in mind, a systematic series of scCeO 2 supports was synthesized using scCO 2 as an antisolvent under varying conditions of temperature and pressure [4]. These were used to create Aubased catalysts via deposition precipitation. Subsequent catalytic testing showed that, regardless of the different synthesis conditions, all the scCeO 2 samples exhibited a marked improvement in CO conversion relative to the untreated Au/CeO 2 (unCeO 2 ) catalysts. The level of the increase in activity of the Au/scCeO 2 samples depended upon the specific supercritical conditions (temperature and pressure) of the scCO 2 anti-solvent. XPS analysis revealed no clear difference in the Au electronic structure of any of the scCeO 2 catalysts. All of the samples were then studied using a spherical aberration corrected VG HB603 STEM in addition to a JEOL 2000FX TEM operating at 300 and 200 keV, respectively. Bright-field (BF) and high-angle annular dark-field (HAADF) imaging indicated that the scCeO 2 supports consisted of spherical, polycrystalline agglomerates (Figure 1). In addition, the contrast at the perimeter of the spheres is much stronger than that at the center, suggesting that the spheres were in fact hollow. This morphology is a result of densification in the absence of particle growth that occurs upon crystallization inwards from the exterior surface of the amorphous precursor during calcination. STEM-XEDS analysis also confirmed the hollow nature of the agglomerates.STEM-XEDS spectrum imaging revealed the origin of the variation in CO oxidation activity with scCO 2 synthesis conditions. As shown in Figure 2, the best catalyst in terms of CO conversion exhibited no distinct gold particles; instead the Au-L α x-ray signal indicates that the gold is highly (perhaps atomically) dispersed over the spherical scCeO 2 support. In contrast, the catalyst with the lowest CO conversion performance was found to contain distinct 5-10 nm gold crystallites. The catalysts that had intermediate CO conversion exhibited dispersions that were a mixture of particulate and highly dispersed Au. I...

Physical and Chemical Synthesis of Au/CeO2 Nanoparticle Catalysts for Room Temperature CO Oxidation: A Comparative Study

“…The nature of the Au/CeO 2 nanoparticle catalyst involves many complex factors such as the reduction and interaction of Au with the underlying CeO 2 at the interface and the formation of a reduced nonstoichiometric phase at the interface between them [40,41]. Pre-treatment conditions can also affect the stability of the catalyst, depending on the support nature, thereby increasing the metal-support interaction during the synthesis step and can significantly influence the oxidation state and the nature of the active site of Au-based catalyst.…”