Graciela T. Baronetti

Hydrogen production from steam reforming of bioethanol using Cu/Ni/K/γ-Al2O3 catalysts. Effect of Ni

Hydrogen to be used as a raw material in fuel cells or even as a direct fuel can be obtained from steam reforming of bioethanol. The key aim of this process is to maximize hydrogen production, discouraging at the same time those reactions leading to undesirable products, such as methane, acetaldehyde, diethyl ether or acetic acid, that compete with H2 for the hydrogen atoms. Cu–Ni–K/γ-Al2O3 catalysts are suitable for this reaction since they are able to produce acceptable amounts of hydrogen working at atmospheric pressure and a temperature of 300°C. The effect of nickel content in the catalyst on the steam-reforming reaction was analyzed. Nickel addition enhances ethanol gasification, increasing the gas yield and reducing acetaldehyde and acetic acid production.

Effect of the carbon pre-treatment on the properties and performance for nitrobenzene hydrogenation of Pt/C catalysts

Abstract This paper reports a study of the effect of the purification and functionalization treatments of a peach pit derived carbon on the properties and performance for nitrobenzene hydrogenation reaction of Pt/C catalysts. Results show that the elimination of inorganic impurities, mainly sulphur, enhances the nitrobenzene hydrogenation rate. Moreover, the functionalization treatments of purified carbon with ozone and hydrogen peroxide have a positive effect both on the Pt dispersion and on the hydrogenation capacity of the catalyst, while the HNO3-treatment has a lower effect. The effect of the different oxidants can be related to the nature of the functional groups developed on the carbon surface. Thus, HNO3-treated carbon displays a high density of both strong and weak acid sites, while H2O2- and O3-treated carbons show an important concentration of weak acid sites but a low concentration of strong acid sites, according to the TPD results. Moreover, the H2PtCl6 isotherms in liquid phase at 298 K show a stronger interaction of the metallic precursor with the carbons of low acidity (like those treated with H2O2 or O3) than with the most acidic carbon (treated with HNO3). Carbons functionalized with weak oxidants, which develop acidic sites with moderate strength and show strong interaction with H2PtCl6 during impregnation, would favour the Pt dispersion on the carbon surface and consequently the catalytic behaviour.

State of metallic phase in PtSn/Al2O3 catalysts prepared by different deposition techniques

Abstract The state of the metallic phase after thermal treatments in Pt Sn/Al 2 O 3 catalysts, prepared by different deposition techniques (coimpregnation and successive impregnations), was investigated by temperature programmed reduction, X-ray photoelectron spectroscopy and cyclohexane dehydrogenation and cyclopentane hydrogenolysis reactions. In all catalysts the presence of zerovalent Sn was detected due to a catalytic effect of platinum on tin reduction. Besides, there seems to exist a relationship between the amount of [Pt Cl 2 (Sn Cl 3 )2] 2- complex deposited on the alumina after the impregnation step, tin reducibility and Pt Sn alloy formation.

PtSn/Al2O3 catalysts: Studies of the impregnation step

Abstract The impregnating solution (H2PtCl6 + SnCl2 + 1 M HCl; Sn(II)/Pt(IV) molar ratio = 1.6) used to prepare Pt Sn/Al2O3 catalysts was studied by uv-visible spectroscopy. Three different species were found in the above-mentioned solution: Pt(II), Sn(IV) and a Pt Sn complex with the structure (PtCl2(SnCl3)2)2−. Moreover, solutions with Sn(II)/Pt(IV) molar ratio of 1 and 3 were studied. The results indicate that the complex formation takes place only for solutions with Sn(II)/Pt(IV) molar ratio higher than 1. The Pt(0.3 wt%)-Sn(0.3 wt%)/Al2O3 catalyst precursors prepared by using three different deposition techniques: a) coimpregnation, b) two-step impregnation (Pt deposition, drying, Sn deposition) and c) two-step impregnation as in b) but in an inverse sequence of metal deposition, were characterized by diffuse reflectance spectroscopy and temperature programmed reduction. When the catalyst is prepared using the deposition techniques named a or b, the (PtCl2(SnCl3)2)2− complex is adsorbed on the alumina surface maintaining its structure. But when the catalyst is prepared using the technique named c, the complex formation does not take place, and only Pt(IV) and Sn(IV) species are detected on the alumina. Hence, the use of different impregnation techniques leads to different adsorbed species in the Pt Sn/Al2O3 catalyst precursors.

Platinum—tin/alumina catalyst: Modification of the metallic phase after successive oxidation—reduction cycles

Abstract PtSn/Al2O3 catalysts, prepared by different deposition techniques and submitted to successive oxidation—reduction cycles at 500°C, were characterized by TPR and test reactions (cyclohexane dehydrogenation and cyclopentane hydrogenolysis), and compared with fresh samples. Interpretation of the results is based on the fact that in catalysts obtained by both coimpregnation and successive impregnation, a higher quantity of PtSn alloys was produced as a consequence of the oxidation—reduction cycles. In catalysts prepared by impregnating the support with the [PtCl2(SnCl3)2]2− complex, the cycles produced changes in the metallic phase probably due to either a surface enrichment of tin in alloy particles or a modification of the alloy phase composition.

Efficient Microwave Solvent-Free Synthesis of Flavones, Chromones, Coumarins and Dihydrocoumarins

Simple, clean, environmentally friendly procedures for the solvent-free preparation of coumarins, dihydrocou- marins, flavones and chromones under microwave heating are described. Silica-supported Wells-Dawson heteropolyacid (H6P2W18O62·24 H2O) was employed as catalyst. High selectivity, very good yields and short reaction times were ob- tained. The results are compared with those of the reactions performed under conventional heating.

A theoretical-experimental study of Wells-Dawson acid: An explanation of their catalytic activity

Abstract A theoretical–experimental study about the behaviour of the protons and their interactions with water molecules on the Wells–Dawson acid and their influence on the catalytic activity was done. The Wells–Dawson acid (H6P2W18O62·nH2O) was characterised by TGA analysis. Besides, DRIFTS and 1 H MAS-NMR measurements as a function of acid treatment temperature were done. These experimental results, consistent with prior studies, suggest that the protonic acidity is related to the presence of water molecules in the heteropolyoxoanion (HPA) structure. In particular, the H5O2+ species associated with the last two H2O molecules determined by TGA analysis and the 1 H MAS-NMR measurements show that the presence of H5O2+ species plays an important role in the acidity–catalytic activity relationship. The Wells–Dawson acid structure formation was studied by calculating the relative energy of the system. The possible acid molecular structure (P2W18O62H6·nH2O) was analysed considering different steps by associating to the Wells–Dawson anion (P2W18O62)−6, the protons, water molecules and secondary structures. The theoretical calculus by extended Huckel method (EHMO) was done analysing the most energetically favourable positions for the protons. The theoretical results indicate that three different water species associated to the Wells–Dawson acid structure can exist. Besides, these results show that the H5O2+ species are the most energetically stable ones in the acid structure. These species bonds two secondary structures of Wells–Dawson acid leading to minimum system energy. It can be expected that these species are the last water molecules lost in the acid structure at the end of acid dehydration process. This assumption is in agreement with the results obtained with the three experimental techniques previously mentioned.

Steam reforming of ethanol using Cu-Ni supported catalysts

The effect of calcination temperature (450–800°C) and copper loading (lower and higher than 4wt%) on the catalytic behaviour and structure of Cu/Ni/K//γ-Al 2 O 3 catalysts were analysed. These catalysts were proven to be suitable for the steam reforming of ethanol. XPS, SEM-EDX, XRD and N 2 O chemisorption techniques were employed; catalytic activity is evaluated using the steam reforming of ethanol as the test reaction. Depending of both, copper loading and calcination temperature, a non-stoichiometric surface spinel can be obtained which has catalytic properties. The presence of a layered double hydroxide precursor is essential for the formation of this phase.

Influence of the reduction temperature on the characteristics of the metallic phase of Pt-Ge/Al2O3 catalysts

Abstract The state of the metallic phase of Pt—Ge (0.3-0.3 wt.-%) /Al 2 O 3 catalysts prepared by different deposition sequences of the metallic precursors (H 2 PtCl 6 and GeCl 4 ) was studied by temperature programmed reduction (TPR), hydrogen chemisorption and various test reactions. The characteristics of the metallic phase do not depend on the deposition sequence used for the catalyst preparation. At reduction temperatures lower than 350°C, germanium species placed near platinum atoms provide only a geometric effect, but at higher reduction temperatures, germanium, which is mainly in the zerovalent state, modifies the electronic properties of platinum with probably alloys formation. Studies on the acidic function of Pt/Al 2 O 3 , Ge/Al 2 O 3 and Pt—Ge/Al 2 O 3 catalysts indicate that the acidic strength distribution of γ-Al 2 O 3 is not modified by the addition of platinum, germanium or platinum and germanium.

Vanadium antimonate as a partial oxidation catalyst

Abstract The catalytic selective oxidation on vanadium antimonate oxides partially doped with titanium was studied. Solids with nominal composition VSbO 4 , V 0.8 SbTi 0.2 O 4 and VSb 0.8 Ti 0.2 O 4 were prepared by solid-state reaction. The solids were characterised by sorptometry, X-ray diffraction (XRD), temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). Catalytic tests were carried out in a fixed-bed reactor using toluene and air as reactants. It was found that partial substitution of V or Sb by Ti produces changes in the unit-cell composition and the redox properties of the solids. A possible role for V and Sb in the reaction mechanism of partial oxidation was established. Vanadium sites (V 4+ ) are involved in the incorporation of gaseous oxygen as lattice oxygen and hold the Sb sites in an oxidation state (Sb 5+ ) high enough to insert the lattice oxygen in the hydrocarbon molecule. A relationship between activity and V 4+ /Sb 5+ ratio of the unit-cell was found. The selectivity to benzaldehyde is correlated with the redox properties of the solids while the selectivity to total oxidation products could be interpreted by the site isolation theory.