G. Deganello

Activity of SiO2 supported gold-palladium catalysts in CO oxidation

Abstract Bimetallic Au-Pd catalysts supported on silica with different Au/Pd atomic ratios were prepared by simultaneous reduction of palladium and gold precursors by ethanol in the presence of the polymer, polyvinylpyrrolidone (PVP). Formation of alloyed particles was detected by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and CO chemisorption measurements. The catalysts were tested in the catalytic oxidation of CO using a plug–flow reactor. The CO conversion was determined as a function of temperature. The monometallic palladium and the palladium-rich catalysts behaved quite similarly and were the most active catalysts. A drastic reduction of the CO oxidation activity was observed for the 50/50 Au/Pd catalyst and for samples with increasing amount of gold.

Selective hydrogenation of acetylene in ethylene feedstocks on Pd catalysts

Abstract Pumice supported palladium catalysts were compared with Pd SiO 2 and Pd Al 2 O 3 in the hydrogenation of acetylene using typical industrial ethylene feedstocks: front-end and tail-end cuts. Pd/pumice catalysts exhibit good activity and excellent selectivity and stability in the title reaction. Their activity/selectivity pattern is controlled by the composition of the reaction mixture. The turnover frequency (TOF) increases, and the apparent activation energy (Ea) decreases, with the H 2 C 2 H 2 ratio, but they are not affected by the C 2 H 2 C 2 H 4 ratio. The selectivity to ethane (SE) does not change with acetylene conversion at low H 2 C 2 H 2 ratio (tail-end cut) and increases at high H 2 C 2 H 2 ratio (front-end cut). No dependence of SE with acetylene conversion was found, at any H 2 C 2 H 2 ratio, on varying the metal dispersion. TOF and SE change with metal loading depending on the reaction mixtures; low metal loaded catalysts gave the best activity/selectivity pattern. Pd/pumice catalysts do not show aging phenomena even when used in drastic conditions (high space velocities and front-end cuts). The catalytic behavior of Pd/pumice catalysts, as a function of the composition of reaction mixture, elucidated by using the isokinetic relationship (IKR) approach shows that the relative amount of hydrogen in the feedstocks plays an important role in activation/deactivation processes which occurs on the metal surface. By considering the presence of different centers on the catalyst and by performing data analysis with a suitable mathematical model, the microscopical mechanism and the role of surface deposits can be explained. A similar reaction mechanism is applicable to the other palladium catalysts, at least in the case of tail-end mixtures.

Structural characterisation of silica supported CoMo catalysts by UV Raman spectroscopy, XPS and X-ray diffraction techniques

The structure of Co-Mo catalysts supported on commercial silica, doped with various amounts of sodium ions, was investigated by means of X-ray diffraction (XRD), UV-Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Two series of samples were considered. One series was prepared by classic incipient wet impregnation (WI) and the other series by co-impregnation in the presence of nitrilotriacetic acid (NTA). The effect of sodium on the catalyst structure depended on the preparation procedure: in the case of the WI catalysts, sodium promoted the transformation of the polymolybdate species into monomolybdate Na2MoO4; in the case of the NTA prepared samples, sodium induced transformation from heptamolybdates, Mo2O72− and mixed CoMo oxides into MoO42− units with distorted tetrahedral symmetry without forming Na2MoO4 compound. In addition to purely structural modifications, sodium induced an electronic effect, evidenced by Mo 3d and Co 2p XPS binding energy shifts. The catalytic behaviour of the samples, previously tested in the HDS of thiophene, was explained in terms of structural and electronic changes.

Effect of the Al/Si atomic ratio on surface and structural properties of sol-gel prepared aluminosilicates

Abstract A series of aluminosilicates with an Al/Si ratio ranging from 0 to ∞ (0 for pure silica and ∞ for pure alumina) was prepared by sol–gel process and characterized by surface and structure techniques. Aluminum tri sec butoxide and tetramethylorthosilicate were used as precursors for the sol–gel synthesis. The acidic properties of the oxides were studied by determination of the zero point charges, through mass titration method, and, for selected samples, by FT-IR spectroscopy of adsorbed pyridine used as a probe for both Bronsted and Lewis acidity. A dependence of the acidity on the Al/Si atomic ratio was found. According to the X-ray diffraction patterns, all the oxides have an amorphous structure except pure alumina exhibiting a γ -alumina pattern. The surface areas of the mixed oxides increase with increasing amount of alumina and are higher as compared to the individual oxides. The surface elemental distribution and electronic properties were investigated by X-ray photoelectron spectroscopy. According to the results, good agreement between the surface Al/Si atomic ratio and the analytical ratio is obtained.

Particle size and metal-support interaction effects in pumice supported palladium catalysts

Abstract Pumice supported palladium catalysts with different metal loading and different metal particle sizes have been analyzed with X-ray photoelectron spectroscopy (XPS). The observed negative shift of the Pd 3d doublet binding energies of all the catalysts with respect to the corresponding binding energies of palladium metal powder suggested an increased electron density at the metal centers. Through the combination of the photoelectron and Auger shifts, yielding the Auger parameter shift, relative to the palladium metal, the charge distribution on the palladium atoms was estimated. The variation of that parameter with particle size is attributed to the interaction between the metal and the support rather than to an intrinsic size effect. The XPS intensity ratios of Pd 3d and Si 2p peaks were well reproduced by the Kerkhof and Moulijn model, especially at lower metal loading. Depletion of sodium, naturally present in pumice, has been observed in the catalysts with lower palladium content. An explanation of the catalytic behavior of these samples, in terms of electronic effects and surface elemental composition, is given.

Influence of the preparation method on the thiophene HDS activity of silica supported CoMo catalysts

Abstract The effect of the preparation method of CoMo/SiO2 catalysts with and without sodium ions, on the hydrodesulfurization (HDS) of thiophene was investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and N2 physisorption (BET). The samples were prepared by total sol–gel route, by classic wetness impregnation and by co-impregnation, in the presence of nitrilotriacetic acid (NTA), of commercial and sol–gel prepared silica. The influence of sodium ions added to the sol mixture and to the silica before the impregnation with Co and Mo salt solution, was also considered. The presence of sodium favoured the phase transition of amorphous silica to cristobalite with consequent reduction of the surface area and of the catalytic activity. The catalysts prepared by the sol–gel method exhibited better metal oxide dispersion but lower catalytic activity. On the contrary the catalysts supported on home-made sol–gel silica were quite active in spite of the lower surface areas and the low metal dispersion. Addition of NTA to the impregnation solution yielded better performing catalysts.

Hydrogenation of acetylene in ethylene rich feedstocks: Comparison between palladium catalysts supported on pumice and alumina

Abstract The activity–selectivity patterns of Pd/pumice catalysts are compared with industrial and home prepared Pd/Al 2 O 3 catalysts in the hydrogenation of acetylene in ethylene rich feedstocks (front-end and tail-end cuts). The iso-kinetic relationship (IKR) approach and a new mathematical model, surface site evolution model (SSEM), are employed in this comparison. Pumice and alumina supported Pd catalysts show different metal redox properties. A similar reaction mechanism is adequate to describe the reaction pathway independently by the catalysts and the gas mixtures considered. This mechanism involves the formation of surface polymers during the catalytic reactions and different catalytic sites which are discriminated on the basis of the different steric interactions of the reagents with the surface species. Although the surface mechanism appears to be analogous for all the Pd catalysts considered, industrial Pd/Al 2 O 3 and Pd/pumice catalysts show a different activity–selectivity pattern as a consequence of their different electron density and ensemble size.

Palladium on pumice: new catalysts for the stereoselective semihydrogenation of alkynes to (Z)-alkenes

Abstract High selectivities (93–99%) and excellent stereoselectivities (>99%) in the semihydrogenation of CC triple bonds were achieved using palladium on pumice with a metal loading of 0.5, 1.5 or 3.0% wt as catalyst. The reactions were carried out in ethanol or tetrahydrofuran with only 2.5% of ethylenediamine allowing a self-terminating semihydrogenation independently on the CC triple bond.

Liquid phase hydrogenation of phenylacetylene on pumice supported palladium catalysts

The liquid phase hydrogenation of phenylacetylene on pumice supported palladium catalysts with a large range of metallic dispersions (14% < Dx < 62%) has been carried out in a mechanically stirred glass reactor where any mass transfer effects are eliminated. A five parameters mathematical model has been used to analyze the kinetics. Three basic routes, involving only surface species in the rate determining steps, contributed to the mechanism of the reaction. The hydrogenation of phenylacetylene to styrene, with the pumice supported palladium catalysts, shows good activity and selectivity. The reaction is structure insensitive in the whole range of the considered dispersions, in contrast to that with other Pd catalysts on more traditional supports, which show a marked decrease in activity and selectivity when dispersion of the metallic phase is higher than 20%.

Efficient semihydrogenation of the CC triple bond using palladium on pumice as catalyst

Abstract An efficient semihydrogenation of CC triple bonds was achieved using palladium on pumice as the catalyst with a metal loading of 3 % wt. The results obtained showed better selectivity when compared with Pd/C and better activity when compared with Lindlars catalyst. Hydrogenation of sterically hindered alkynes is slow on Pd/pumice. The selectivity with respect to alkenes is probably due to the presence of sodium ions in the pumice structure that increases the electron density on the supported metal.