The characterization of activated carbons with oxygen and nitrogen surface groups

Acetoxylation of ethylene to vinyl acetate (VA) was used to investigate the mechanism of the promotional effect of gold (Au) in a palladium (Pd)-Au alloy catalyst. The enhanced rates of VA formation for low Pd coverages relative to high Pd coverages on Au single-crystal surfaces demonstrate that the critical reaction site for VA synthesis consists of two noncontiguous, suitably spaced, Pd monomers. The role of Au is to isolate single Pd sites that facilitate the coupling of critical surface species to product, while inhibiting the formation of undesirable reaction by-products.

https://science.sciencemag.org/content/sci/310/5746/291.full.pdf

The Promotional Effect of Gold in Catalysis by Palladium-Gold

Metal-mediated reductive hydrodehalogenation of organic halides.

Mono- and bimetallic colloidal particles have gained increasing attention in science and application throughout the last several years. In this contribution, we present a synopsis of the wet chemical syntheses of these materials and survey potential applications in catalysis and materials science. Methods for the characterization of these particles and their surfaces are not reviewed here.

Nanoscopic Metal Particles − Synthetic Methods and Potential Applications

Interaction of nanostructured metal overlayers with oxide surfaces

Alumina displays low catalytic activity at the initial stage of the reaction of CCl 2 F 2 with hydrogen, giving mainly halogen exchange products, and this activity quickly decays with time-on-stream. In the case of Pd/Al 2 O 3 catalysts, the contribution of the support is negligible at 180°C. Catalytic activity of Pd/γ-Al 2 O 3 in CCl 2 F 2 hydrodechlorination strongly depends on metal dispersion: poorly dispersed Pd samples exhibit the highest turnover frequencies. The same samples also show the highest selectivities towards the formation of CH 2 F 2 . Time-on-stream behaviour and considerable amounts of carbon found in used catalysts suggest that the catalytic properties of Pd/Al 2 O 3 are regulated by incorporation of carbon into Pd lattice, or the formation of Pd carbide. Poorly dispersed Pd catalysts contain a higher proportion of plane atoms and, therefore, are subjected to a more severe carbiding. Such a transformation generates surfaces which bind freon molecules less strongly, resulting in higher activity and selectivity to partial dehalogenation, i.e formation of CH 2 F 2 . High-temperature reduction at 600°C does not much change the overall activity of Pd/Al 2 O 3 . However, the selectivity to CH 2 F 2 is somewhat increased. It is believed that the Pd Al 2 O 3 interface changes upon high-temperature reduction, leading to a Pd Al compound. At corrosive conditions of hydrodehalogenation of CCl 2 F 2 , the Pd Al would be converted to AlF x species much more easily than the Al 2 O 3 species at the Pd Al 2 O 3 interface of mildly reduced Pd/Al 2 O 3 catalysts.

Hydrodechlorination of CCl2F2 (CFC-12) over γ-alumina supported palladium catalysts

Characterization of Pd–Au/SiO2 Catalysts by X-ray Diffraction, Temperature-Programmed Hydride Decomposition, and Catalytic Probes

Characterization of supported palladium catalysts: III. PdAl2O3

Two series of well-mixed Pd–Au/SiO2 catalysts prepared by direct redox method were tested in the reaction of CCl2F2 (CFC-12) hydrodechlorination A moderate selectivity for CH2F2 exhibited by monometallic Pd/SiO2 (∼40%) is significantly increased, up to ∼95%, with Au addition Proper Pd–Au alloying is essential to obtain such a selectivity enhancement Poorly mixed Pd–Au/SiO2 catalysts show the catalytic behaviour not very much different from that of Pd/SiO2, producing a lot of methane Changes in the apparent activation energy are also associated with the degree of Pd–Au alloying After reaction, the Pd/SiO2 and Pd–Au/SiO2 catalysts contain dissolved carbon Hydrogen treatment of carbonized Pd–Au/SiO2 catalysts removes this carbon XRD study of the regenerated samples shows no change in the structure and composition of Pd–Au phase, compared to freshly prereduced bimetallic catalysts

Wojciech Juszczyk

Papers

Hydrodechlorination of CCl2F2 (CFC-12) over γ-alumina supported palladium catalysts

Characterization of Pd–Au/SiO2 Catalysts by X-ray Diffraction, Temperature-Programmed Hydride Decomposition, and Catalytic Probes

Characterization of supported palladium catalysts: III. PdAl2O3

Hydrodechlorination of CCl2F2 (CFC-12) over silica-supported palladium–gold catalysts

Characterization and catalytic activity of differently pretreated Pd/Al2O3 catalysts: the role of acid sites and of palladium–alumina interactions