Laura Cornaglia

Catalytic combustion of diesel soot on Co, K supported catalysts

Catalysts containing 12% Co and 4.5% K, supported on MgO and CeO2 have been studied for diesel soot catalytic combustion. It has been found that this reaction occurs by a redox mechanism when Co and K are deposited on any of the above-mentioned supports. On MgO-supported catalysts, CoOx species are responsible for the supply of oxygen by a redox reaction. In this catalyst, K plays different roles, one of them being the stabilization of the CoOx particles. On CeO2-supported catalysts, Co does not significantly improve the activity of the K/CeO2 catalyst, since in this case the support itself displays redox properties. XPS analyses indicate that the oxygen availability on the surface is much higher on CeO2 than on MgO. On both CeO2 and MgO-supported catalysts, K might provide a route for CO2 release through a carbonate intermediate species. The presence of NO in the gas phase improves the catalytic activity for soot elimination. NO is oxidized to NO2 on the Co, K/CeO2 catalyst, and NO2 is a stronger oxidizing agent than O2, therefore decreasing the temperature needed to burn the soot.

XPS studies of the surface oxidation states on vanadium-phosphorus-oxygen (VPO) equilibrated catalysts

Abstract The oxidation states of vanadium on the surface of VPO catalysts which show the characteristic diffraction pattern of vanadyl pyrophosphate have been investigated by X-ray photoelectron spectroscopy (XPS). The (VO)2P2O7 phase is obtained from crystalline precursors which show the X-ray pattern of VOHPO4·1/2H2O. The XPS spectra of 40 solids (precursors, catalysts, and model compounds) hinted the presence of low concentrations of VV centers on the surface of the bulk VIV catalysts. To further investigate this matter, the solids were oxidized or reduced in a pretreatment chamber attached to the spectrometer. These studies confirmed the presence of VV centers on the surface of the catalysts which may be initiated by strong oxygen chemisorption on exposed VIV sites of (VO)2P2O7. It is also shown that the presence of excess phosphorus prevents the oxidation of the surface (and the bulk) to β-VOPO4, a phase which has a deleterious effect on catalyst performance.

Acidity and catalytic behavior of vanadium-phosphorus-oxygen catalysts

Abstract The surface acidity of equilibrated and non-equilibrated vanadium-phosphorus oxide (VPO) catalysts was studied using basic probe molecules (ammonia, pyridine and acetonitrile) via Fourier transform-IR spectroscopy. The stronger bases detected the presence, in varying proportions, of both Lewis and Bronsted acid centers on all the solids assayed. All the catalysts used exhibited the X-ray diffraction pattern of vanadyl pyrophosphate and much higher Lewis/Bronsted ratios than the β-VOPO 4 compound. This indicates that the Lewis acidity observed on the used catalysts is not due to the presence of small amounts of V v species. The Lewis/Bronsted ratio almost invariably decreased with temperature, suggesting that the Bronsted sites are stronger than the Lewis centers. Acetonitrile served to differentiate Lewis centers of varying strength. The equilibrated catalysts which exhibited a better crystallized pyrophosphate phase showed an increase in very strong Lewis acidity whereas the stacking fold disordered non-equilibrated catalyst showed the presence of both medium strong and very strong Lewis centers. The oxidation of n-butane to maleic anhydride was studied using a plug flow reactor. Both the yield of maleic anhydride and the proportion of very strong Lewis sites increased with time-on-stream. These findings together with previous IR studies reported in the literature allow a better understanding of the relationship among structural features, acidity and catalytic behavior of VPO formulations.

Chemistry of vanadium-phosphorus oxide catalyst preparation

Abstract The chemistry of the reduction process of V 2 O 5 with mixtures of aliphatic and benzyl alcohols was studied using several analytical techniques. The aromatic alcohol, which acted as the reducing agent, was invariably oxidized to benzaldehyde while further oxidation to benzoic acid often occurred. The presence of soluble vanadium (V) alkoxides was detected through IR spectroscopy. At the higher reduction temperatures, which varied between 353 and 393 K, V 2 O 4 , appeared in the solid residue. In no case, however, was the vanadium pentoxide completely reduced with the alcohol mixture. The nature and proportion of the aliphatic alcohol defines the reflux temperature and therefore the rate of the kinetically controlled redox process. The reduction of V 2 O 5 is completed upon addition of H 3 PO 4 (100%). The precursors obtained invariably showed the X-ray diffraction pattern of VOHPO 4 ·/2H 2 O. The extent of disorder of the lattice in the direction perpendicular to the (010) plane increased with a higher proportion of benzyl alcohol in the reduction mixture. The oxidation of n-butane to maleic anhydride was studied using a plug flow reactor. The equilibrated catalysts display very similar X-ray diffraction patterns characteristic of (VO) 2 P 2 O 7 . No correlation was found between the extent of disorder of the precursors prepared in organic medium and the catalytic performance of the equilibrated solids.

Fluoroether bonding to carbon overcoats

A set of small fluorocarbon ethers (CF3CF2OCF2CF3 , CF3OCF2OCF3 , and –CF2OCF2OCF2–) and their hydrocarbon analogs have been used in a model study of the bonding of fluoroether lubricants to the amorphous-carbon (a-CH) overcoats used to protect the surfaces of magnetic storage media. These small ethers allow measurement of desorption kinetics and allow study of the heats of adsorption, a property that cannot be measured directly using the perfluoropolyalkylether lubricants themselves. The a-CH films used in this investigation were provided by three commercial manufacturers of magnetic storage disks. In all cases, the heat of adsorption of the hydrocarbon ether was greater than that of the corresponding fluorocarbon ether, suggesting that the ethers are bonded to the films through donation of the electron pairs on the oxygen atom. The a-CH films are heterogeneous, exposing a variety of adsorption sites at which the ethers bond with a range of interaction strengths. The hydrogen content of the a-CH films wa...

The role of cobalt as promoter of equilibrated vanadium–phosphorus–oxygen catalysts

Catalysts were prepared containing between 1% and 6% of Co by weight. The cobalt was either added during preparation or impregnated on the dry VPO precursor. The catalytic evaluation showed that the cobalt-impregnated solids gave the highest yields to maleic anhydride. To understand the promoting effect of cobalt, the catalysts were characterized using XRD, FTIR, SEM, TPR, Raman spectroscopy and XPS. The solids obtained through the addition of Co during the synthesis of the VPO precursor exhibit less disordered crystalline phases in the (001) direction. No separate Co-containing phase could be detected through the techniques used. In the non-equilibrated catalysts, V(V)-containing phases were detected through Raman spectroscopy. After 500 h on stream under reaction conditions (equilibrated catalysts), vanadyl pyrophosphate was the only crystalline phase detected while V(IV) was the only surface species observed using XPS. XRD, FTIR and SEM coincidentally show that depending on the way cobalt is added the solids obtained develop different structural features.

The beneficial effect of cobalt on VPO catalysts

Abstract The addition of Co to VPO formulations improves the yield of n-butane to maleic anhydride. In this work, different modes of impregnation and two different organic cobalt salts were used. The equilibrated catalysts were characterized using XRD, 31 P SEM NMR, FT-IR and acetonitrile adsorption to evaluate Lewis acidity. The best catalyst was obtained using Co acetyl acetonate for impregnation of the VOHPO4·0.5H2O precursor. This catalyst after equilibration had an optimum concentration of very strong Lewis acid sites, very low concentration of isolated V(V) centers, and no V(V) phases.

Catalytic diesel soot elimination on Co-K/La2O3 catalysts: Reaction mechanism and the effect of NO addition

Catalysts containing Co and/or K supported on La2O3 have been studied for diesel soot catalytic combustion. While supported Co provides redox sites for the reaction, potassium and the support itself contribute to create additional sites for soot consumption by forming carbonates intermediates. The formation of a perovskite structure after high temperature treatment leads to the lost of activity. The presence of NO in the gas phase improves the catalytic activity for soot elimination. NO is oxidized to NO2 on the catalyst surface, and NO2 is a stronger oxidizing agent than O2, therefore decreasing the temperature needed to burn the soot.

Formation of a solid solution of vanadium in TiO2(anatase) on vanadium–titanium solids with high vanadium content

Vanadium–titanium solids, with V/Ti atomic ratios ranging from 0.14 to 7.99, have been prepared by coprecipitation followed by calcination in air. These solids were then washed with a basic medium to obtain pure solid solutions. The products were characterized by means of EDAX, XRD, FTIR, XPS, SEM and laser Raman spectroscopy. The results provide evidence for the formation of a substitutional solid solution of vanadium in TiO2(anatase).

Operando Raman spectroscopic studies of lithium zirconates during CO2 capture at high temperature

Li2ZrO3 based sorbents were synthesized for CO2 capture at high temperature between 500 and 700 °C. Monoclinic ZrO2, tetragonal Li2ZrO3, Li2CO3, K2CO3 and LiKCO3 phases were detected by XRD and Raman spectroscopy. The sorbents showed high stability and moderate capture efficiency. The addition of K resulted in the improvement of capture efficiency from 0.052 g CO2 g mat−1 in materials without K to 0.083 g CO2 g mat−1 in the doped solid. Here, we report an experimental setup of Raman spectroscopy coupled with online mass spectrometry and a demonstration of its capabilities using lithium zirconates as sorbents for high temperature CO2 capture. Operando Raman spectroscopy allowed us to follow up the phase evolution during the capture process for the first time. The results obtained confirmed the presence of molten K and Li carbonates when the K-doped zirconates were exposed to CO2 at 500 °C.