CO Oxidation over Bimetallic Au–Pd Supported on Ceria–Zirconia Catalysts: Effects of Oxidation Temperature and Au:Pd Molar Ratio

The dynamic behavior of dilute metallic alloy PdxAu1−x/SiO2 raspberry colloid templated catalysts under CO oxidation

Dilute palladium-in-gold alloys have potential as efficient oxidation catalysts; controlling the Pd surface distribution is critical.

Enhancing catalytic performance of dilute metal alloy nanomaterials

Dilute alloys are promising materials for sustainable chemical production; however, their composition and structure affect their performance. Herein, a comprehensive study of the effects of pretreatment conditions on the materials properties of Pd0.04Au0.96 nanoparticles partially embedded in porous silica is related to the activity for catalytic hydrogenation of 1-hexyne to 1-hexene. A combination of in situ characterization and theoretical calculations provide evidence that changes in palladium surface content are induced by treatment in oxygen, hydrogen and carbon monoxide at various temperatures. In turn, there are changes in hydrogenation activity because surface palladium is necessary for H2 dissociation. These Pd0.04Au0.96 nanoparticles in the porous silica remain structurally intact under many cycles of activation and deactivation and are remarkably resistant to sintering, demonstrating that dilute alloy catalysts are highly dynamic systems that can be tuned and maintained in a active state.

Selective Oxidation of Veratryl Alcohol over Au-Pd/Ce0.62Zr0.38O₂ Catalysts Synthesized by Sol-Immobilization: Effect of Au:Pd Molar Ratio

The selective oxidation of veratryl alcohol (VA), a model compound of lignin, with oxygen molecules to produce veratraldehyde (VAld) was studied over monometallic Au, Pd, and bimetallic Au:Pd nanoparticles supported on a Ce_0.62Zr_0.38O₂ mixed oxide for the first time. These bimetallic Au-Pd catalysts with Au:Pd molar ratios from 0.4 to 4.3 were synthesized by the sol-immobilization method. Furthermore, all the catalysts were characterized by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), N₂ physisorption, X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) imaging, energy dispersive X-ray spectroscopy (EDXS), and temperature programmed reduction (TPR) techniques. A synergistic effect between gold and palladium was observed over all the bimetallic catalysts in a wide range of studied Au:Pd ratios. Remarkably, the optimum Au:Pd ratio for this reaction was 1.4 with a turnover frequency of almost six times larger than for the monometallic gold and palladium catalysts. Selectivity to veratraldehyde was higher than 99% for the monometallic Au, Pd, and all the bimetallic Au-Pd catalysts, and stayed constant during the reaction time.

Evolution of surface catalytic sites on thermochemically-tuned gold-palladium nanoalloys

Nanoscale alloying constitutes an increasingly-important pathway for design of catalysts for a wide range of technologically important reactions. A key challenge is the ability to control the surface catalytic sites in terms of the alloying composition, thermochemical treatment and phase in correlation with the catalytic properties. Herein we show novel findings of the nanoscale evolution of surface catalytic sites on thermochemically-tuned gold-palladium nanoalloys by probing CO adsorption and oxidation using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technique. In addition to the bimetallic composition and the support, the surface sites are shown to depend strongly on the thermochemical treatment condition, demonstrating that the ratio of three-fold vs. bridge or atop Pd sites is greatly reduced by thermochemical treatment under hydrogen in comparison with that under oxygen. This type of surface reconstruction is further supported by synchrotron high-energy X-ray diffraction coupled to atomic pair distribution function (HE-XRD/PDF) analysis of the nanoalloy structure, revealing an enhanced degree of random alloying for the catalysts thermochemically treated under hydrogen. The nanoscale alloying and surface site evolution characteristics were found to correlate strongly with the catalytic activity of CO oxidation. These findings have significant implications for the nanoalloy-based design of catalytic synergy.

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

Au–Pd colloidal NPs immobilised on ceria nanorods are highly active catalysts for selective oxidation.

Low-temperature CO oxidation over Cu/Pt co-doped ZrO2 nanoparticles synthesized by solution combustion

Zirconia (ZrO2) nanoparticles co-doped with Cu and Pt were applied as catalysts for carbon monoxide (CO) oxidation. These materials were prepared through solution combustion in order to obtain highly active and stable catalytic nanomaterials. This method allows Pt2+ and Cu2+ ions to dissolve into the ZrO2 lattice and thus creates oxygen vacancies due to lattice distortion and charge imbalance. High-resolution transmission electron microscopy (HRTEM) results showed Cu/Pt co-doped ZrO2 nanoparticles with a size of ca. 10 nm. X-ray diffraction (XRD) and Raman spectra confirmed cubic structure and larger oxygen vacancies. The nanoparticles showed excellent activity for CO oxidation. The temperature T50 (the temperature at which 50% of CO are converted) was lowered by 175 °C in comparison to bare ZrO2. Further, they exhibited very high stability for CO reaction (time-on-stream ≈ 70 h). This is due to combined effect of smaller particle size, large oxygen vacancies, high specific surface area and better thermal stability of the Cu/Pt co-doped ZrO2 nanoparticles. The apparent activation energy for CO oxidation is found to be 45.6 kJ·mol-1. The CO conversion decreases with increase in gas hourly space velocity (GHSV) and initial CO concentration.

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

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.