Highly Active Large Au Clusters and Even More Active Ag Nanoparticles Supported on Ceria-Zirconia: Impact of Particle Size and Potassium Ion Loading on Activity in Catalytic Transfer Hydrogenation

Abstract

Although heterogeneous monometallic gold catalysts are commonly more active when the gold particles are smaller, this study shows that the reverse is true in the case of liquid phase catalytic transfer hydrogenation of acetophenone with 2-pentanol. Higher catalytic activity of larger gold particles, i.e., over 30 nm in diameter, than of smaller particles of average 4 nm in size was observed. Moreover, this effect was contradictory to that observed for supported monometallic silver catalysts in which the interaction with the support and hence particle size was shown to cause drastic changes in the activity in this reaction, with the large particles being completely inactive and tiny ones being the most active system studied. In this reaction, the ceria-zirconia solid solutions were used as the supports for the catalysts and both zirconium doped ceria, as well as cerium doped zirconia carriers were tested. The supports themselves exhibited little activity in this reaction. It was shown that the activity of the supports and catalysts depends on the Ce/Zr ratio and potassium content. Both types of catalysts showed excellent selectivity to 1-phenylethanol and conversion of acetophenone, although it was noted that a high loading of potassium carbonate in the gold catalysts propelled undesired reactions, thereby reducing the selectivity to 1-phenylethanol.

Catalytic Transfer Hydrogenation Performance of Magnesium-Doped ZrO2 Solid Solutions

“…They might be responsible for the fracture of these materials, but more studies are needed to confirm or disprove this hypothesis. The texture of the samples (Figure 5) is very similar to that of the previously published solid solutions prepared by us using a similar procedure, such as zirconium-doped ceria [43] and cerium-doped zirconia [44]. However, unlike for pure zirconia (Figure 5a) or zirconia doped with 5% Ce ions (Figure 5d), which fracture in large, irregular chunks using a mortar and pestle, both Zr 0.97 Mg 0.03 O x (Figure 5b) and Zr 0.95 Mg 0.05 O x (Figure 5c) show elongated structures with unusual fracture.…”

Anomalous behaviour of silver catalyst for soot oxidation explained: state of silver when operating and the influence of potassium ions

“…The effect of surface enrichment in zirconium ions on the surface of the zirconium doped ceria support has been reported by us and other research groups earlier. 26 In the case of cerium-doped zirconia, an enrichment of the surface with cerium ions has been reported, though after a much higher thermal treatment than in the present study. 39 However, in this case, the low cerium content in the top 10 nm of the surface may be attributed to the presence of silver.…”

“…The zirconium-doped ceria and cerium-doped zirconia supports chosen for further testing were selected based on the results of our previous studies, which have shown that the Ce 0.85 Zr 0.15 O 2 and Zr 0.93 Ce 0.07 O 2 supports contain only one phase, the fluoritetype cubic ceria structure and tetragonal zirconia structure, respectively, and have the most beneficial properties. 25,26 As in the case of all of the silver catalysts for soot combustion in the literature cited in the Introduction section, the XRD studies have shown that the silver deposited onto Zr 0.93 Ce 0.07 O 2 is present in the metallic form (Fig. 1), even in the sample which The values of the specific surface area of the cerium-doped zirconia (ZC) and zirconium-doped ceria supports are 63 and 77 m 2 g −1 , respectively (Fig.…”

“…The term "ceria-zirconia" or "ceria-zirconia mixed oxide" is often found in the literature, but is not precise, and the material itself does not contain both oxides as separate phases, but is rather a solid solution of one type of ions incorporated into the matrix of the oxide of the major component. 25,26 When the Ce/(Ce + Zr) ratio is large, the structure of pure ceria is kept intact, i.e., the fluorite-type, cubic structure, but the zirconium ions do influence the lattice parameter of the oxide due to ionic radius mismatch. In this study, such oxides are referred to as "zirconiumdoped ceria".…”

“…In this study, such oxides are referred to as "zirconiumdoped ceria". In contrast, when the Ce/(Ce + Zr) ratio is small, the structure is that of zirconia with cerium cations inserted into the Zr 4+ sites, 26 and those will be called "cerium-doped zirconia". Understanding this distinction is crucial in comprehending why ceria [27][28][29][30][31] and zirconiumdoped ceria 32 are highly reducible oxides with excellent soot oxidation activity, whereas zirconia and cerium-doped zirconia are only used as a support rather than a catalyst for soot oxidation, 19 though both are referred to as "ceriazirconia".…”