Simone Morpurgo

Preparation, characterisation and catalytic activity of CuZn-based manganites obtained from carbonate precursors

Abstract Copper-zinc manganites with general formula CuxZn1−xMn2O4 (x=0, 0.01, 0.05, 0.10) were prepared by thermal decomposition of carbonate precursors obtained by coprecipitation at constant pH. Precursors were characterised by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), thermogravimetric analysis (TGA-DTA) and FT-IR spectroscopy. For all samples a single rhodochrosite-like phase, CuxZnyMn1−x−yCO3, with Cu2+, Zn2+ and Mn2+ in solid solution was detected by XRD. Precursors were decomposed in air at 723 and 973 K giving spinel-like solid solutions, as evidenced by XRD and measurements of magnetic susceptibility. X-ray photoelectron spectroscopy (XPS) showed that Cu2+ and Mn3+ are present at the surface of the spinel structure and that the spinel surface is enriched in copper at higher calcination temperature. As evidenced by temperature programmed reduction (TPR), the presence of copper markedly enhances the reducibility of CuxZn1−xMn2O4 spinels with respect to ZnMn2O4. Preliminary results of the catalytic activity of copper-zinc-based manganites for the reduction of NO by hydrocarbons are presented.

AB INITIO STUDY OF INTRAMOLECULAR PROTON TRANSFER REACTIONS IN CYTOSINE

Abstract The activation energies of intramolecular proton transfer reactions of cytosine in the gas phase were computed through ab initio calculations. MP2/6-31G ** and MP2/D95 ** energy calculations were performed over geometries obtained at the HF/3-21G, HF/6-31G(N8 * ) and HF/D95(N8 * ) level (d-type polarisation functions were added on the N8 atom). Corrections for the zero-point vibrational energy were included. It is suggested that not only thermodynamic but also kinetic factors may be responsible for the distribution of the tautomeric species in the gas phase. Results are discussed with reference to spectroscopic data in the literature.

Critical test of PM3-calculated proton transfer activation energies: a comparison with ab initio and AM1 calculations

Abstract Intra- and intermolecular proton transfer activation energies are calculated by means of the semiempirical PM3 method and compared with ab initio results obtained at MP2/6–31G ∗ or MP2/6–31G ∗∗ level. It is shown that corrections for zero-point vibrational energy (ZPVE) and thermal contributions do not affect the correlation between semiempirical and ab initio results. The linear relationship obtained suggests that PM3, in spite of some weaknesses, can be employed with a certain confidence to predict proton transfer activation barriers of large molecules or to obtain semi-quantitative trends for large numbers of interrelated systems. It is also shown that PM3 and AM1 results are of comparable quality for intramolecular proton transfers, whereas PM3 is to be preferred for intermolecular reactions, in particular when water molecules are involved.

Cu/Zn/Co/Al/Cr-containing hydrotalcite-type anionic clays

Abstract Compounds with the formula [M62+M23+(OH)16CO3 · 4H2O [M3+ = Cu, Zn, Co; M3+ = Al, Cr] were prepared by precipitation of the metal nitrates with excess of NaHCO3 in controlled conditions of temperature, stirring and pH. The characterization has been performed by X-ray powder diffraction (XRD), diffuse reflectance spectroscopy in the visible-UV region (DRS) and thermal analysis (TG-DTA). Cu/Zn/Co/Al and Cu/Zn/Co/Cr materials contain crystalline hydrotalcite as the predominant phase. Cu/Zn/Co/Cr samples show, in addition to hydrotalcite, the presence of some malachite whose amount decreases with increasing Co content. Cu/Zn/Co/Al/Cr precursors contain malachite as the only crystalline phase whereas hydrotalcite appears as a secondary amorphous phase. The thermal decomposition occurs, independently of cation composition, in four steps: the first three consisting essentially of dehydration and the fourth one of decarboxylation of the samples. The thermal treatment first produces a topotactic decomposition of the brucite-like layers giving some kind of oxycarbonates where the metal-oxygen framework remains essentially intact and the interlayered carbonate anions are substantially retained, and then, by release of carbon dioxide, the formation of mixed oxides. Dehydration temperature of samples is independent of both Co and Cr content. Decarboxylation for Cr-containing hydrotalcite-like, HT-Cr, precursors occurs at much lower temperature than for Al-containing, HT-Al, ones. The increasing amount of cobalt in HT-Al and HT-Cr compounds progressively lowers the crystallinity and the decarboxylation temperature of the samples. The effect of different cation composition on the properties, stability and phase composition of the hydroxycarbonate precursors is discussed.

Pillared hydroxycarbonates and mixed oxides. Part 1.—Copper–zinc–cobalt–aluminium system

Cu–Zn–Co–Al mixed oxides with different Cu : Zn : Co : Al atomic ratios have been obtained by thermal treatment of hydroxycarbonate precursors at different temperatures (623, 723, 823 and 973 K in air). The characterization has been performed by X-ray powder diffraction (XRPD), diffuse reflectance spectroscopy (DRS), thermal analysis (TG/DTA), BET surface-area determination and measurements of magnetic susceptibility. The XRPD patterns showed that the precursors were almost pure pillared-layered (PLS) hydrotalcite-like materials with general stoichiometric formula M6II M2III(OH)16CO3.4H2O (MII= Cu, Zn, Co, MIII= Al). The thermal decomposition of the precursors occurred in four steps, the first three (up to T= 673 K) consisting of complete dehydration of the sample, the fourth (843 ⩽T/K ⩽ 923) due to the release of CO2. The precursor structure collapsed at T⩽ 623 K, giving rise to poorly crystalline materials, in which the Cu2+ ions retained a distorted octahedral symmetry. Crystalline oxide mixtures were formed only after complete release of CO2. XRPD showed that the oxides obtained by calcination at 973 K consisted of a mixture of CuO, ZnO and spinels such as CuAl2O4, ZnAl2O4, ZnCo2O4, and/or spinel solid solutions. According to DRS and magnetic susceptibility, the formation of ZnCo2O4 seems to be privileged with respect to that of Co3O4 and ZnO.

A theoretical study on proton transfer in the mutarotation of sugars

The epimerisation process of the model sugar 2-tetrahydropyranol was studied by means of ab initio calculations. The results suggest that the rate limiting step of sugar ring opening involves a high-energy intramolecular proton transfer reaction or a low-energy process in which the proton transfer is mediated by a catalyst molecule, formic acid in the case investigated. The catalysed process is an asynchronous concerted double proton transfer reaction, where both protons are transferred within the same elementary step but one of them is transferred much earlier than the other one along the reaction coordinate. The motion of both protons in the transition state of the catalysed process is strongly coupled with the breaking of the C–O bond of the sugar ring. Geometry optimisation at the B3LYP/6-31G* level, with additional p polarisation functions located on the hydrogen atoms involved in proton transfer, appears to be suitable for further MP2/6-31G** single point energy calculations, as it provides hydrogen bond and activation energies in good agreement with those obtained from geometry optimisation at the full MP2 level of theory.

A computational study on N2 adsorption in Cu-ZSM-5

The present computational study investigates the adsorption of N(2) by Cu-ZSM-5, with particular regard to the interaction with pairs of Cu(+) ions, employing simple cluster models in the calculations. It shows that several interaction patterns between N(2) and couples of Cu(+) sites are possible within the Cu-ZSM-5 structure. In particular, when pairs of Cu(+) ions are located at opposite sides of ten-membered rings, in the region where linear and sinusoidal channels intersect each other, a quasi-linear Cu-N-N-Cu adsorption occurs. Although lattice restraints cause small deviations from linearity, such interaction turned out to be more favourable than other adsorption patterns within the Cu-ZSM-5 structure. The linearity of the Cu-N-N-Cu fragment and the relatively low concentration of the related sites cause a low extinction coefficient for the N-N IR stretching mode, which is usually detected with very low intensity or not detected at all. The results of the present calculations may explain the experimental evidence for a nearly IR-silent fraction of nitrogen strongly adsorbed in the Cu-ZSM-5 catalyst which, as shown in a previous work, is linearly related to the number of active sites for NO decomposition.

The epimerisation of 2-tetrahydropyranol catalysed by the tautomeric couples 2-pyridone/2-hydroxypyridine and formamide/formamidic acid as a model for the sugar's mutarotation: a theoretical study

In the present computational study a complete reaction mechanism for the sugars mutarotation promoted by tautomeric catalysts in vacuum is outlined. The rate determining step of the process consists in the breaking of the C–O bond of the sugar ring which occurs by a concerted double proton transfer between the substrate and the catalyst. After ring opening, the –CHO group of the substrate rotates around the C2–C3 bond without dissociation of the substrate–catalyst adduct. The activation energy of this step is generally less than 8–9 kcal mol−1. Alternative reaction paths which require the dissociation of the substrate–catalyst complex are less favourable for both probability and energetic reasons. When the mutarotation is catalysed by the 2-pyridone/2-hydroxypyridine (PD/HP) tautomeric couple, the process may be promoted by either PD or HP derived from the dissociation of the (PD)2, (HP)2 or PD–HP dimers. According to B3LYP and MP4 (SDQ) calculations the HP-promoted reaction path should be faster than the PD-promoted one. When the process is catalysed by the formamide/formamidic acid (F/FA) tautomeric couple, the reaction path promoted by FA, which is derived from the dissociation of the F–FA dimer, should be the most favourable, according to B3LYP, MP2 and MP4(SDQ) calculations. Solvent effects in benzene, calculated by the PCM method, did not affect to a relevant extent the mechanism outlined in vacuum. The present study suggests, differing from the hypothesis formulated in many experimental studies, that the sugars mutarotation process is more efficiently catalysed by the less stable tautomer or catalyst dimer rather than by the more stable one of both the PD/HP and F/FA couples.

Copper—zinc—cobalt—chromium hydroxycarbonates and oxides

Abstract Hydroxycarbonate precursors with different Cu/Zn/Co/Cr atomic ratios and CuZnCoCr oxides obtained by thermal treatment of the precursors at different temperatures (623, 723, and 973 K in air) have been examined. Characterization has been performed by X-ray powder diffraction, diffuse reflectance spectroscopy, thermal analysis, surface area determination, and measurement of magnetic susceptibility. X-ray diffraction patterns show that the precursors are essentially hydrotalcite-like materials with the general formula (M2+)6(M3+)2(OH)16CO3 · 4H2O (M2+ = Cu, Zn, Co; M3+ = Cr). Thermal decomposition of the precursors occurs in four steps; the first three (up to T = 623 K) consist of complete dehydration of the sample, and the fourth (623 ≤ T ≤ 773 K) is due to the release of CO2. The precursor structure collapses at T ≤ 623 K, giving rise to nearly amorphous materials. Crystalline oxide mixtures are formed only after complete release of CO2. X-ray diffraction patterns show that the oxides obtained by calcination of 973 K consist of a mixture of CuO, ZnO, and ZnCr2O4ZnCo2O4Co3O4 spinel solid solutions. According to reflectance spectroscopy and magnetic susceptibility measurements, the inclusion of Zn2+ and Co3+ ions in the spinel-like solid solution seems to be privileged with respect to the formation of separate Co3O4 and ZnO phases.

A DFT study on Cu(I) coordination in Cu-ZSM-5: Effects of the functional choice and tuning of the ONIOM approach.

The coordination of Cu+ at the T1 and T7 positions of the M7 ring of Cu‐ZSM‐5, and the interaction of NO with coordinated Cu+ were investigated by means of DFT/ONIOM calculations. The B3LYP, BLYP, PBE1PBE, PBE, M06, and M062X functionals with the def2‐TZVP (def2‐QZVP for Cu) basis set were used in the high‐level part of ONIOM calculations, with the HF/3‐21G, B3LYP/LANL2DZ, M06/LANL2DZ, and M062X/LANL2DZ methods in the low‐level part. The ability of suitable combinations of the above methods to reproduce (i) the crystallographic structure of purely siliceous ZSM‐5, (ii) the tendency of Cu+ to be twofold or fourfold coordinated by framework oxygen atoms of Cu‐ZSM‐5, and (iii) the interaction energy and the NO stretching frequency of adsorbed nitrogen oxide are discussed, showing that different results are obtained depending on the adopted computational approach. With reference to above properties, some considerations about the employment of the ONIOM approximations are also included.