Synergy and Anti-Synergy between Palladium and Gold in Nanoparticles Dispersed on a Reducible Support

Abstract

Highly active and stable bimetallic Au–Pd catalysts have been extensively studied for several liquid-phase oxidation reactions in recent years, but there are far fewer reports on the use of these catalysts for low-temperature gas-phase reactions. Here we initially established the presence of a synergistic effect in a range of bimetallic Au–Pd/CeZrO4 catalysts, by measuring their activity for selective oxidation of benzyl alcohol. The catalysts were then evaluated for low-temperature WGS, CO oxidation, and formic acid decomposition, all of which are believed to be mechanistically related. A strong anti-synergy between Au and Pd was observed for these reactions, whereby the introduction of Pd to a monometallic Au catalyst resulted in a significant decrease in catalytic activity. Furthermore, monometallic Pd was more active than Pd-rich bimetallic catalysts. The nature of the anti-synergy was probed by several ex situ techniques, which all indicated a growth in metal nanoparticle size with Pd addition. However, the most definitive information was provided by in situ CO-DRIFTS, in which CO adsorption associated with interfacial sites was found to vary with the molar ratio of the metals and could be correlated with the catalytic activity of each reaction. As a similar correlation was observed between activity and the presence of Au0* (as detected by XPS), it is proposed that peripheral Au0* species form part of the active centers in the most active catalysts for the three gas-phase reactions. In contrast, the active sites for the selective oxidation of benzyl alcohol are generally thought to be electronically modified gold atoms at the surface of the nanoparticles.

Activation and Deactivation of Gold/Ceria–Zirconia in the Low‐Temperature Water–Gas Shift Reaction

“…The Au 4f region (Supporting Information, Figure S8) gives valuable information on the form of Au present in the catalyst and is comprised of three distinct species; namely Au 0 (84.0 eV) and Au 3+ (86.1 eV), and a species at 85.0 eV. The assignment of the latter species, denoted Au 0 *, remains controversial with some studies attributing it to small Au NPs or clusters– whereas others relate it to positively charged Au species . Our microscopy supports an assignment of the Au 0 * species as small Au NPs or clusters.…”

Au–Pd NPs immobilised on nanostructured ceria and titania: impact of support morphology on the catalytic activity for selective oxidation

“…may indicate the presence of either oxidised gold species (Au δ+ ) or very small gold nanoparticles not seen in the HDAAF-STEM images. 43,44 The Pd (3d 5/2 ) spectra clearly indicates that both metallic palladium (Pd 0 ) and Pd δ+ species are present in the analysed catalyst samples.…”

“…can be attributed to the electronic modification of Au species by Pd, and is indicative of the close interaction between the Au and Pd atoms, and the formation of Au-Pd alloys. 43 It can also be observed that the magnitude of the shift in the B.E. varies between the different catalyst samples.…”

“…Indeed, the size and direction of the binding energy shift have been shown to be largely dependent on the catalyst support and the ratio of Au and Pd present in the alloy phase. 43 The difference in the B.…”

Effect of the Chemical Composition of Mesoporous Cerium-Zirconium Oxides on the Modification with Sulfur and Gold Species and Their Application in Glycerol Oxidation

“…2 yield was calculated only for liquid products of glycerol oxidation which were in the solution after reaction. 3 TOF was calculated as the number of glycerol moles reacted on one mole of gold per one hour (after 5 h of the reaction).…”