Michela Signoretto

Consecutive hydrogenation of benzaldehyde over Pd catalysts: Influence of supports and sulfur poisoning

Abstract Pd catalysts on different supports (active carbon, silica, alumina) were studied for the selective hydrogenation of benzaldehyde to benzyl alcohol. They were characterized by TPR, CO chemisorption and XRD. Batch hydrogenation tests were performed before and after sulfur poisoning. Strong metal–support interaction was found for Pd/alumina, giving high Pd dispersion. Chemisorption stoichiometry Pd/CO=2 was confirmed again. While Pd/C has the highest activity for benzaldehyde hydrogenation, a satisfactory selectivity to benzyl alcohol requires an oxidic carrier or proper sulfur poisoning of Pd/C.

Isomerization ofn-butane on sulfated zirconia: Evidence for the dominant role of Lewis acidity on the catalytic activity

The catalytic activity of a ZrO2/SO4 catalyst in the isomerization ofn-butane at 423 K is reversibly suppressed by addition of CO. IR analysis of the adsorption of CO indicates that the only σ-coordination of CO onto coordinatively unsaturated surface Zr4+ cations occurs in the 300–473 K interval.

On the role of the calcination step in the preparation of active (superacid) sulfated zirconia catalysts

In the preparation of active SO4-ZrO2 catalysts, several steps involving various chemical and/or physical processes are necessary. In particular it has been reported that, after sulfation of amorphous Zr hydrates, a calcination atTcalc>773 K is needed to guarantee the crystallization of ZiO2 in the tetragonal phase. By the use of a stabilized tetragonal ZrO2, it is here demonstrated that a calcination atTcalc>773 K is indeed necessary for all SO4-ZrO2 systems, and that its actual role is the selective elimination of sulfates from highly energetic crystallographic defects. The calcination step atTcalc>773 K so creates the conditions for the formation of strong Lewis acid centres, that are necessary in the catalytic process, and the presence of which is here monitored spectroscopically by the reversible adsorption of carbon monoxide.

Structural investigation on the stoichiometry of β-PdHx in Pd/SiO2 catalysts as a function of metal dispersion

Structural investigations of dispersed Pd/SiO2 catalysts were carried out with XRD and SAXS techniques, supported by TPR and CO chemisorption measurements. The stoichiometry of Pd hydride, β-PdHx, was determined by measuring the shift of Pd 111 XRD reflection in the presence of hydrogen. It was confirmed thatx decreases when the metal dispersion increases. This behaviour could be quantified up to about 0.45 of Pd dispersion since the angular position of XRD peaks cannot be determined with the necessary precision at higher dispersions. TPR data, obtained up to dispersions of about 0.6, confirms such behaviour. The β-PdHx stoichiometry versus Pd dispersion relationship substantially agrees with that found with other techniques by Boudart and Hwang.

Platinum promoted zirconia-sulfate catalysts: one-pot preparation, physical properties and catalytic activity

A new innovative methodology for the single-stage preparation of ZrO2-SO4 and Pt/ZrO2-SO4 catalysts is reported, based on the sol-gel technique. Catalysts are characterized by analysis, XRPD, BET and IR methods and are tested in the isomerization of butane.

Nickel Catalysts Supported Over TiO2, SiO2 and ZrO2 for the Steam Reforming of Glycerol

Ni‐based catalysts supported on TiO2, ZrO2 and SiO2 (in the form of mesoporous Santa Barbara Amorphous 15 (SBA‐15) and amorphous dense nanoparticles), were employed in the steam reforming of glycerol. Each sample was prepared by liquid phase synthesis of the support followed by impregnation with the active phase and calcination at 800 °C or by direct synthesis through flame pyrolysis. Many techniques have been used to assess the physical chemical properties of both the fresh and spent catalysts, such as atomic absorption, N2 adsorption/desorption, XRD, SEM, TEM, temperature‐programmed reduction (TPR), X‐ray photoelectron spectroscopy (XPS), Micro‐Raman and FTIR spectroscopy. The samples showed different textural, structural and morphological properties, as well as different reducibility and thermal resistance depending on the preparation method and support. Some of these properties were tightly bound to catalyst performance, in terms of H2 productivity and stability towards coking and sintering. A key parameter was the metal–support interaction, which strongly depended on the preparation procedure. In particular, the stronger the interaction, the more stable the metallic Ni clusters, which in turn lead to a higher catalytic activity and stability. Surface acidity was also taken into account, in which the nature of the acid sites was differentiated (silanols, titanols or Lewis acid sites). The characterisation of the spent catalysts also allowed us to interpret the deactivation process. The formation of multi‐walled nanotubes was observed for every sample, though it was only in some cases that this led to severe deactivation.

On the strength of Lewis- and Brønsted-acid sites at the surface of sulfated zirconia catalysts

The surface acidity (of both Lewis and Bronsted type) of an anion-free t-ZrO 2 , stabilized in the tetragonal form by forming a solid solution with ca. 3 mol% Y 2 O 3 , of the corresponding sulfated t-ZrO 2 (SZ) system, and of another SZ t-ZrO 2 system prepared through a conventional method have been compared; the two SZ systems turned out to be essentially equivalent. By FTIR and microcalorimetry it was observed that the presence of sulfate modifies to a very limited extent both the concentration an strength of the types of strong (aprotic) Lewis sites that can be revealed by CO adsorption at 300 K. In absolute terms, the acidity of these Lewis sites turns out to be definitely lower than that of typical acidic oxides (e.g., catalytic aluminas). IR spectroscopic data indicate that surface sulfates induce in SZ systems the presence of (protonic) Bronsted acidity, otherwise absent in (anion-free) t-ZrO 2 . On SZ, Bronsted-acid centres are present over the whole vacuum-activation temperature range explored (300–823 K), though in decreasing amounts with increasing temperature. The acid strength of Bronsted sites is confirmed to be definitely lower than that of typical protonic oxide systems (such as, for instance, H-exchanged zeolites). As the Lewis-acid sites present on SZ systems are not particularly strong, and the Bronsted-acid sites present on SZ systems in catalytic conditions are not particularly strong (nor very abundant), it is concluded that the unique catalytic properties of SZ systems, sometimes termed superacid catalysts, must have another origin, possibly connected with the simultaneous presence at the surface of SZ of acid sites of both types.

Wustite as a new precursor of industrial ammonia synthesis catalysts

Contradictory results about the best oxidic precursor of Fe ammonia synthesis catalyst prompted the present comparative investigation on wustite- and magnetite-based catalysts. Many physical (density, porous texture, crystalline phases, reduction rate, metal surface, abrasion loss) and catalytic (kinetic constants, thermoresistancy) properties have been determined on both catalysts. The wustite-based catalyst proved to be much more active, especially at lower temperatures, approaching the performances of Ru/C catalyst, except at high conversion. Possible reasons for such a behavior of the wustite-based catalyst are discussed, suggesting that a reconsideration of the present consolidated knowledge on Fe ammonia synthesis catalyst might be convenient.

Highly Dispersed Gold on Zirconia: Characterization and Activity in Low‐Temperature Water Gas Shift Tests

Gold-loaded zirconia and sulfated zirconia catalysts were tested in the low-temperature water gas shift reaction. The samples were characterized by N2 adsorption analysis, temperature-programmed reduction, X-ray diffraction, pulse-flow CO chemisorption, FTIR spectroscopy, and high-resolution transmission electron microscopy. A reference catalyst, Au/TiO2, provided by the World Gold Council was investigated for comparison. CO chemisorption and FTIR data indicate the presence of only highly dispersed gold clusters on the sulfated sample and both small clusters and small particles on the non-sulfated sample. Both gold-zirconia catalysts are much more active than the Au/TiO2 reference sample over all the temperature range investigated. The sample prepared on sulfated zirconia exhibits higher stability than the catalyst on unmodified zirconia. The prominent role in the water gas shift reaction of gold clusters in close contact with the support was deduced.

Characterization of Fe‐promoted sulfated zirconia catalysts used for the n‐butane isomerization by XPS and Mössbauer spectroscopies

Iron‐promoted sulfated zirconia catalysts have been characterized by Mössbauer spectroscopy and XPS before and after catalytic test for the isomerization of n‐butane. The data obtained by Mössbauer spectroscopy show that iron is present in the fresh catalysts at the surface of zirconia both as isolated cations and small ferric oxide particles. The diameters of these particles do not exceed 4 nm. The characterization of the catalysts by Mössbauer spectroscopy after 20 and 120 min of reaction shows no apparent reduction of the iron cations. However, the analysis of the peak of Fe 3d3/2 in the XPS spectra shows that iron has been partially reduced in the used catalysts. These apparently contradictory results could be explained by the study of a pre‐reduced catalyst by both techniques. This study shows first that the Fe3+ cations in the particles can be reduced into Fe2+ and reoxidized at room temperature, and second that the reduction observed by XPS corresponds to the departure of the O2 re‐adsorbed at room temperature under air, occurring when the catalysts are placed under vacuum. All the data obtained seemed to confirm that the loss of the promoting effect of iron during the catalytic reaction may partially be due to the irreversible reduction of the iron species susceptible to undergo such reduction.