Anna Maria Venezia

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.

Co3O4 nanocrystals and Co3O4–MOx binary oxides for CO, CH4 and VOC oxidation at low temperatures: a review

Among the possible substitutes for noble metals, cobalt-based catalysts represent promising alternative systems. In recent years, many articles have been devoted to the synthesis, characterization and reactivity of cobalt oxides. This article provides a comprehensive review of the state-of-the-art activities that concentrate on the synthesis, structural properties and catalytic applications of Co3O4 nanocrystals and Co3O4–MOx binary oxides in CO, CH4 and VOC oxidation at low temperatures. It begins with the major synthetic approaches and basic properties of Co3O4 nanocrystals and Co3O4–MOx binary oxides and subsequently highlights the relationship between the peculiar structure of Co3O4 nanocrystals and their catalytic activity (or between the redox properties of Co3O4–MOx binary oxides and their catalytic activity). Finally, the active sites and key factors determining the catalytic oxidation over Co3O4 and Co3O4–MOx are discussed. The perspective with respect to future research on Co3O4 nanocrystals and Co3O4–MOx binary oxides is considered.

X-ray photoelectron spectroscopy (XPS) for catalysts characterization

Abstract The general principles of X-ray photoelectron spectroscopy (XPS) as applied in the field of heterogeneous catalysis are reviewed. In particular, the use of this technique in the determination of chemical and physical changes of catalysts upon exposure to gaseous molecules and upon different thermal treatments is examined. Furthermore, examples of methods useful in obtaining the dispersion of supported catalysts are described and, for this purpose, theoretical models of the particle-support distribution are also discussed. The XPS characterization of supported Pd–Ag catalysts is reported, emphasising the advantages of using XPS to investigate surface segregation processes. In the case of supported Pd/Pt bimetallic catalysts it is shown how both, Auger and photoelectron peaks, characterized by different kinetic energies, allow to get depth profile non-destructive analysis. Finally the surface behaviour of CoMo catalysts, used for hydrodesulfurization reactions, is investigated on different supports and under different pre-treatment and reaction conditions.

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.

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.

Co3O4 particles grown over nanocrystalline CeO2: influence of precipitation agents and calcination temperature on the catalytic activity for methane oxidation

Crystalline cobalt oxides were prepared by a precipitation method using three different precipitation agents, (NH4)2CO3, Na2CO3 and CO(NH2)2. Cobalt oxide nanoparticles corresponding to a Co3O4 loading of 30 wt% were also deposited over high-surface area nanocrystalline ceria by the same precipitation agents. The effect of calcination temperature, 350 or 650 °C, on the morphological and structural properties was evaluated. Characterization by BET, XRD, SEM, TEM, Raman spectroscopy, H2-TPR, XPS and NH3-TPD was performed and the catalytic properties were explored in the methane oxidation reaction. The nature of the precipitation agent strongly influenced the textural properties of Co3O4 and the Co3O4–CeO2 interface. The best control of the particle size was achieved by using CO(NH2)2 that produced small and regular crystallites of Co3O4 homogeneously deposited over the CeO2 surface. Such a Co3O4–CeO2 system precipitated by urea showed enhanced low-temperature reducibility and high surface Co3+ concentration, which were identified as the key factors for promoting methane oxidation at low temperature. Moreover, the synergic effect of cobalt oxide and nanocrystalline ceria produced stable full conversion of methane in the entire range of investigated temperature, up to 700–800 °C, at which Co3O4 deactivation usually occurs.

Combined sulfating and non-sulfating support to prevent water and sulfur poisoning of Pd catalysts for methane combustion

The appropriate combination of titania and silica, sulfating and non-sulfating support, respectively, results in Pd catalysts with improved water and sulfur tolerance in methane combustion. For the first time the catalyst recovers the initial activity after one cycle under lean-burn conditions without additional regenerating treatments.

Partial oxidation of methane to synthesis gas over Pt nanoparticles supported on nanocrystalline CeO2 catalyst

Pt-nanoparticles supported on CeO2 have been prepared by a post synthesis method (Pt–CeO2PS). In the post synthesis method, CeO2 nanoparticles were prepared by a hydrothermal method, followed by the deposition of Pt nanoparticles over the CeO2. The prepared catalyst was characterized by XRD, BET-surface area, TPR, SEM, TEM, XPS and XAFS. It was observed that the catalyst prepared by the post synthesis method contained Pt nanoparticles with sizes between 2–5 nm supported on CeO2 nanoparticles with sizes between 20–60 nm. The catalytic performance of the Pt–CeO2PS catalyst was evaluated in the partial oxidation of methane for synthesis gas production. The Pt–CeO2PS catalyst could activate methane at 350 °C. We believe that the nanosized Pt particles and the synergy between the Pt particles, the CeO2 nanoparticles and the presence of a strong metal–support interaction play key roles in the activation of methane at such a low temperature. Different reaction parameters, like Pt-loading, reaction temperature, space velocity, and time on stream, were studied in detail. The Pt–CeO2PS catalyst does not deactivate till 100 h with a constant H2/CO mole ratio of 1.9 at 800 °C.

Supported Co3O4-CeO2 monoliths: effect of preparation method and Pd-Pt promotion on the CO/CH4 oxidation activity

Abstract Two structured composite oxides, Co3O4(30wt%)-CeO2(70wt%), have been prepared by washcoating commercial cordierite monoliths with a CeO2-γAl2O3 layer, on which the active phase Co3O4-CeO2 was added through two different methods: dip-coating from a suspension containing the preformed active oxide or impregnation with a solution of the cobalt and cerium precursors. Morphological characterizations of the monoliths have been performed by BET, and SEM-EDAX analyses. Electronic and reduction properties have been evaluated by XPS and H2-TPR, respectively. The effect of the preparation method has been investigated in the catalytic oxidation of CO, whereas the promotion by a low content of Pd-Pt has been evaluated in combined CO-CH4 oxidation tests. Four successive CO oxidation cycles performed over the structured Co3O4-CeO2 indicate that the best performing sample is the monolith prepared by dip-coating, whereas that one obtained by impregnation method manifests a certain deactivation upon two consecutive cycles. Accordingly, XPS spectra are consistent with the presence of the active phase CO3O4 in the former, while Co2+ has been identified as the main species in the latter sample.