Pure Pd Catalyst Research Articles

Structure−Reactivity Correlations in the Catalytic Coupling of Ethyne over Novel Bimetallic Pd/Sn Catalysts

The structure, thermal stability, and catalytic behavior of a novel highly dispersed silica-supported Pd/Sn catalyst prepared by an organometallic route have been examined by X-ray photoelectron, X-ray diffraction, and X-ray absorption, fine structure spectroscopies, the latter two measurements being carried out with an in situ reaction cell. Additional reactor measurements were performed on a more Sn-rich catalyst and on a pure Pd catalyst. Varying the temperature of reduction induced large variations in catalytic performance toward ethyne-coupling reactions. These changes are understandable in terms of the destruction of SnO2-like structures surrounding the Pd core, yielding a skin of metallic Sn which subsequently undergoes intermixing with Pd. The overall thermal and catalytic behavior of these highly dispersed materials accords well with the analogous single-crystal model system.

Novel microfabricated Pd-Au/SiO2 bimetallic model catalysts for the hydrogenation of 1,3-butadiene

Model Pd-Au/SiO2 bimetallic catalysts were prepared via a microfabrication method and their catalytic activity for the hydrogenation of 1,3-butadiene was compared with thin film catalysts. The microfabricated catalyst consisted of Pd-Au squares with various surface compositions, approximately 4 μm in size and separated by 4 μm on a silica/silicon support. The main advantage of the microfabricated bimetallic catalysts is that alloy formation is guaranteed and XPS results showed that surface segregation occurred for the Au-rich compositions. The apparent activation energies of the bimetallic catalysts were lower than that of the pure Pd catalyst with a minimum occurring at ~ 65 at% Pd. The bimetallic catalysts, however, have fewer active Pd sites, hence the conversions are comparable. The microfabricated catalysts were more active than the thin film samples, because the thin film samples showed evidence of a high degree of sintering after pretreatment, unlike the microfabricated catalysts in which sintering did not occur.

Novel Microfabricated Pd-Au/SiO2Model Catalysts for the Hydrogenation of 1,3-Butadiene

ABSTRACTModel Pd-Au/SiO2catalysts were prepared via a microfabrication methodology and their catalytic activity was evaluated using the hydrogenation of 1,3-butadiene as a probe reaction. The catalytic particles of the microfabricated catalyst were Pd-Au squares, ranging in composition, each approximately 4 µm in size and separated by 4 µm on a silica/silicon support. The reaction was carried out at atmospheric pressure with a hydrogen to hydrocarbon ratio of 125. The bimetallic catalysts exhibited lower apparent activation energies than the pure Pd catalyst with a minimum occurring at ˜65 at% Pd. Corresponding to the decrease in activation energy, there is an increase in the turnover frequency over the monometallic catalyst at low temperatures, i.e. room temperature. At high temperatures (above 200°C) the pure Pd catalyst has the highest conversion (0.61) and the largest pre-exponential factor (4.71×1016cc/g Pd/s), while the 73 at% Pd catalyst has the lowest conversion (0.45) and the smallest pre-exponential factor (5.59 × 106cc/g Pd/s). The bimetallic catalysts also showed a higher selectivity towards 1-butene than the monometallic catalyst; however, at lower conversions (less than 25%) the pure Pd catalyst was more selectivity towards 1-butene. AFM was used to characterize the surface morphology of each of the samples.

Preparation of colloidal platinum/palladium alloy particles from non-ionic microemulsions: Characterization and catalytic behaviour

Bimetallic particles of platinum and palladium have been prepared by reduction, with hydrazine at arabient temperature, of H 2PtCl 6 and PdCl 2 dissolved in microemulsions consisting of water, hexadecane and pentacthylene glycol dodecyl ether. The particles were deposited onto Al 2O 3. Examination by scanning transmission electron microscopy and extended X-ray adsorption fine structure measurements of particles with atomic ratio Pt:Pd = 35:65 showed that the particles are true Pt/Pd alloys. Indirect evidence was also obtained that particles containing only 4% Pd are alloys. The diameter of the particles ranges from 10 to 100 nm; there is evidence that they are formed by aggregation of particles in the diameter range 2–5 nm. The catalytic properties of the Pt/Pd/Al 2O 3 catalysts prepared from microemulsions were similar to those of catalysts prepared by conventional impregnation. Catalysts with low Pd content had the same selectivity in the isomerization of 2-methylpentane and hydrogenolysis of methylcyclopentane as a highly dispersed Pt catalyst (or a pure Pd catalyst). The catalytic cracking of 2-methylpentane indicates that the particle surfaces behave as Pt surfaces up to a Pd concentration of 60% in the particles. Isomerization of hexanes is dominated by the cyclic mechanism even at Pd concentrations as low as 4%; for these very low Pd concentrations hydrogenolysis of methylcyclopentane is the same as observed for very small Pt particles. It is concluded that Pd is enriched in the particle surfaces in such a way that they consist of Pd and some highly dispersed Pt. even at bulk Pd concentrations as low is 4%.

Supported PdPt alloy catalysts: Synthesis, structure characterization and effects of hydrogen absorption

Pd Pt alloy catalysts were prepared by anchoring Pd(II) and Pt(II) organometallic species to the support surface, followed by H 2 reduction under mild conditions. This preparation method was exploited for the synthesis of Pd Pt catalysts having different Pd Pt ratios and with total metal loading ~2%; pure Pd and pure Pt catalysts were also obtained. Small Angle X-ray Scattering (SAXS) analysis was used to determine metal dispersion: average metal particle diameter was in the range 20–40 Å both for alloy and single Pd and Pt catalysts. The occurrence of Pd Pt alloy formation was demonstrated by Wide Angle X-ray Scattering (WAXS) and Radial Distribution Function (RDF) analyses. Results both from WAXS and RDF are consistent with each other in ascertaining alloy occurrence, also providing a detailed picture of the behavior of Pt, Pd, and Pd Pt systems in the presence of H 2.

Hydrogenation of Aniline with Ruthenium-Palladium and Ruthenium-Platinum Alloy Catalysts Supported on Activated Carbon

Hydrogenation of aniline with Ru-Pd and Ru-Pt alloy catalysts supported on activated carbon was investigated under an atmospheric pressure of hydrogen in the temperature range from 30° to 60° C. Under these conditions the pure Ru and pure Pd catalysts had very low activity for this reaction, but the alloy catalysts of these metals were active and in these alloy series a maximum activity appeared at about 25 atom% of Ru content. At the maximum point the activity was about 15 times as great as that of the pure Pd catalyst which was prepared by the same method as these alloy catalysts. The selectivity for cyclohexylamine increased by 15 mol% as compared with that of pure Pd at the composition of 25% Ru. In order to study the influence of the amino group of aniline on the hydrogenation of the benzen ring, benzene was hydrogenated with these alloy catalysts. For this reaction a maximum activity appeared at about 75 atom% of Ru content. On Ru-Pt alloy series, the hydrogenation of aniline did not show a maximum activity and the activity decreased greately when Ru was added to Pt. On this Ru-Pt series, the selectivity for cyclohexylamine changed almost linearly between the both pure metals. The hydrogenation of benzene with this alloy series was also studied. This alloy catalysts had higher activity than that of the pure metal when the reaction was carried out at 30° C. On both Ru-Pd and Ru-Pt alloy series, the compensation effect for the hydrogenation of aniline was recognised except the pure Ru catalyst. The activity and selec-tivity of the catalysts which were prepared by the deposition of a small amount of Ru on Pd or Pt metal surface, were also investigated.

Hydrogen adsorption on platinum/silica catalysts

An alloy of palladium and silver dispersed on Al2O3 has been used for the selective hydrogenation of acetylene. The activity of Pd–Ag catalyst is lower than that of pure metal Pd catalyst. But the selectivity of Pd–Ag catalyst is higher and less impaired by temperature increase than that of Pd catalyst. X-ray diffraction (XRD) indicates that metal Pd and Ag can form an alloy on the surface of alumina. X-ray photoelectron spectroscopy (XPS) shows that the Pd–Ag alloy is formed with enrichment of Ag on the surface. Results of hydrogen adsorption isotherm, temperature-programmed reduction (TPR) and temperature-programmed desorption (TPD) reveal that the bulk of palladium of Pd–Ag catalyst adsorbed a small amount of hydrogen, which is desorbed at a higher temperature than that in the case of Pd catalyst.