Gold Stabilized by Nanostructured Ceria Supports: Nature of the Active Sites and Catalytic Performance

Abstract

The interaction of gold atoms with CeO 2 nanocrystals having rod and cube shapes has been examined by cyanide leaching, TEM, TPR, CO IR and X-ray absorption spectroscopy. After deposition-precipitation and calcination of gold, these surfaces contain gold nanoparticles in the range 2-6 nm. For the ceria nanorods, a substantial amount of gold is present as cations that replace Ce ions in the surface as follows from their first and second coordination shells of oxygen and cerium by EXAFS analysis. These cations are stable against cyanide leaching in contrast to gold nanoparticles. Upon reduction the isolated Au atoms form finely dispersed metal clusters with a high activity in CO oxidation, the WGS reaction and 1,3-butadiene hydrogenation. By analogy with the very low activity of reduced gold nanoparticles on ceria nanocubes exposing the {100} surface plane, it is inferred that the gold nanoparticles on the ceria nanorod surface also have a low activity in such reactions. Although the finely dispersed Au clusters are thermally stable up to quite high temperature in line with earlier findings (Y. Guan and E. J. M. Hensen, Phys Chem Chem Phys 11:9578, 2009), the presence of gold nanoparticles results in their more facile agglomeration, especially in the presence of water (e.g., WGS conditions). For liquid phase alcohol oxidation, metallic gold nanoparticles are the active sites. In the absence of a base, the O-H bond cleavage appears to be rate limiting, while this shifts to C-H bond activation after addition of NaOH. In the latter case, the gold nanoparticles on the surface of ceria nanocubes are much more active than those on the surface of nanorod ceria.

Low-temperature carbon monoxide oxidation catalysed by regenerable atomically dispersed palladium on alumina

“…In previous work with Au catalysts, the authors had to leach out the metallic component before performing catalytic tests 2,20 . Even if the initial catalyst is in the form of atomically dispersed species, the catalyst can evolve during reaction measurement, as shown by Guan et al 21 and, more recently, by Corma et al 22 These authors suggest that atomically dispersed species can serve as precursors for the formation of very small clusters of the metal. Hence, it is necessary to use in situ spectroscopy to confirm the identity of the species responsible for catalytic reactivity.…”

Understanding of the oxygen activation on ceria- and ceria/alumina-supported gold catalysts: a study combining 18O/16O isotopic exchange and EPR spectroscopy

“…The calculation performed for the ceria of low LSA area shows that N L is the same (N L =0. 15) and therefore that the OSC process is only dependent on the surface area for a given oxide. In contrast to what was expected, the N L values were higher when ceria was supported on alumina.…”

“…The morphology of Electronic supplementary material The online version of this article (doi:10.1007/s13404-013-0103-z) contains supplementary material, which is available to authorized users. the ceria crystallites, the size of the gold clusters (particles), and the oxidation state of the active gold atoms are the parameters usually discussed because of their determining importance for the catalytic oxidation activity [15]. The nature of the intermediate species is also studied.…”

Water–gas shift on gold catalysts: catalyst systems and fundamental studies

“…70 The activities in WGSR followed the sequence of Au/CeO 2 nanorods > Au/CeO 2 polyhedra c Au/ CeO 2 nanocubes, probably because of the presence of a much higher fraction of strongly bound gold species on the former two catalysts, as confirmed by others. 81 In addition, a theoretical study indicated that oxygen vacancies are easier to form on CeO 2 {110} than {100} and {111} planes, 82 thus explaining the stabilization of gold species on CeO 2 nanorods. A similar effect was established for the methanol steam reforming on Au/CeO 2 catalysts, 83 and the shape effects on metal oxide supported gold catalysts in WGSR were reported in Au/ZnO and Au/Fe 3 O 4 systems.…”

“…137 Hensen and co-workers showed that the cationic gold in leached Au/CeO 2 could be partially reduced to metallic gold clusters active for hydrogenation of 1,3-butadiene, CO oxidation, and WGSR. 81,138 Rodriguez and co-workers studied Au/Ce(Gd)O 2 and Au/ CeO x /TiO 2 using in situ time-resolved X-ray absorption spectroscopy (TR-XAS). 114,140 They also found the transformation of cationic gold into metallic gold under WGS conditions.…”