Ida Pettiti

The catalytic activity of Cu-ZSM-5 and Cu-Y zeolites in NO decomposition: dependence on copper concentration

NO decomposition was studied on Cu-ZSM-5 (Cu exchange extent from 23 to 210%) and Cu-Y (Cu exchange extent from 5 to 105%) catalysts at 773 K. The results show that the activity (NO molecules decomposed per gram of catalyst per second) increases by roughly 100-fold when the extent of exchange with copper in the ZSM-5 framework increases from 80 to 100%. This behaviour shows that not all Cu sites are equivalent in their decomposition activity. Cu-ZSM-5 samples prepared with either H-ZSM-5 or Na-ZSM-5 show the same activity pattern.

Structure and reactivity of copper-zinc-cadmium chromite catalysts

Abstract Different mixed oxides containing Cu, Zn, Cd and Cr were obtained by heating hydroxycarbonate precursors at various temperatures and in different atmospheres, and characterized by using X-ray diffraction (XRD), infrared, diffuse reflectance spectroscopy, thermogravimetry, and BET and copper surface area analysis. The reduction process and the phase evolution during reduction of the mixed oxides was followed by temperature-programmed reduction and XRD. The mixed oxides were activated in an H 2 /N 2 flow and then tested as catalysts in the vapour-phase hydrogenation of γ-butyrolactone (GBL) and a solution of maleic anhydride (MA) in GBL (60:40 w/w). Depending on composition, heating temperature and atmosphere, the samples contained different phases, such as CuO, CdO, Cr 2 O 3 , CuCrO 2 , CuCrO 4 , α-CdCrO 4 , cubic and tetragonal CuCr 2 O 4 , and cubic Cu Zn and Cu Cd spinel-type phases. The partial substitution of Cu 2+ ions with Zn 2+ or Cd 2+ ions stabilizes the cubic form of the spinel-type phases, which form via intermediate chromate phases. The final product of reduction in all cases is metallic copper. Cubic CuCr 2 O 4 is more reducible than the corresponding tetragonal phase, which reduces to metallic copper through the intermediate formation of CuCrO 2 . The presence of zinc or cadmium promotes or inhibits, respectively, the copper reducibility both in CuO and in the copper chromite spinel. In the hydrogenation of GBL, the Cu/Cr catalysts obtained by calcination show similar behaviours, with a small increase in activity as a function of the copper content. At the lower temperatures investigated the main products are tetrahydrofuran (THF) and n-butanol, while at 548 K significant amounts of ethanol are observed, favoured by increasing copper content. On the other hand, the Cu/Cr catalyst obtained by heating under a reduced oxygen atmosphere, for which only tetragonal CuCr 2 O 4 is detected before reduction, shows a decrease in catalytic activity. Partial substitution of the Cu 2+ ions gives rise to a decrease in GBL conversion, with a considerable deactivation when Cd 2+ ions are present. Decreasing the H 2 /C 4 molar ratio gives rise to a decrease in activity for all samples, without any significant change in the trend observed. With the MA/GBL solution, the partial substitution of Cd 2+ ions for Cu 2+ ions also gives rise to considerable deactivation. For the other catalysts, the main products at the lower temperatures are GBL and succinic anhydride (SA), while at 548 K overhydrogenation and hydrogenolysis reactions predominate. The presence of Zn 2+ ions gives rise to an increase in yield in GBL, due to an increase in the hydrogenation activity toward SA, but the consecutive transformation of GBL at high temperature to low cost by-products is not inhibited. On the other hand, the presence in the Cu/Cr catalyst of only CuCr 2 O 4 increases the yield in GBL, as a consequence of a strong inhibition of the overhydrogenation and/or hydrogenolysis reactions of GBL. With the MA/GBL solution, similar behaviours are observed when the H 2 /C 4 molar ratio decreases. At the same time, however, the lack in the carbon balance increases and a displacement towards high temperature of the maximum yield in GBL is found.

Anderson-type heteropolyoxomolybdates in catalysis: 2. EXAFS study on γ-Al2O3-supported Mo, Co and Ni sulfided phases as HDS catalysts

Abstract The nature of the Co (or Ni) and Mo sulfided species obtained starting from γ-Al 2 O 3 -supported Anderson heteropolyoxomolybdates (CoMo 6 and NiMo 6 ) has been analyzed by means of the extended X-ray absorption fine structure (EXAFS) technique with the aid of temperature-programmed reduction (TPR) and spectroscopic measurements. Results are discussed in relation to the adsorption isotherms and to the catalytic activity for the thiophene hydrodesulfurization (HDS) reaction. A comparison with traditional catalysts has been made.

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.

XAS characterization and CO oxidation on δ-alumina supported La, Mn, Co and Fe oxides

δ-Al2O3 supported La, Mn, Co and Fe containing catalysts were prepared by impregnation of δ-Al2O3 with citrate-type precursors and calcination at 1073 K. The catalysts were characterized by X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), X-ray absorption spectroscopy (XAS), and BET specific surface area determination. XRD revealed the presence of δ-Al2O3 in all cases and, at 30 wt.% of metal loading, of other single (α-Mn2O3 and α-Fe2O3) or mixed (LaAlO3, LaMnO3, CoAl2O4 and LaFeO3) oxide phases. XAS suggested the formation of some oxide phases also at lower loading. In particular, all Mn and 10 wt.% La–Mn containing samples revealed the formation of α-Mn2O3, while the 30 wt.% La–Mn bimetallic sample showed the formation of LaMnO3 perovskite. All Co containing samples revealed the presence of CoAl2O4 spinel. Fe containing samples showed the formation of α-Fe2O3, while La–Fe containing ones, that of LaFeO3 perovskite. Catalytic tests of CO oxidation were performed in the temperature range 300–800 K. The sample containing 30 wt.% of La and Mn in the form of LaMnO3 perovskite dispersed on δ-Al2O3 was found the most active among all the examined catalysts. Most of the Co containing catalysts were found active at RT too, but they deactivated rapidly. None of the Fe-based samples was active at RT and these catalysts were found, on average, to be substantially less active than the Mn- and Co-based ones.

Cell for the in situ study of heterogeneous catalysts by transmission and fluorescence XAS spectroscopy

A simple cell, assembled with commercial parts, suitable for in situ X-ray-absorption spectroscopy measurements of heterogeneous catalysts, has been designed. The cell, light and easy to handle, allows thermal treatments of the sample under investigation up to 823 K in a reducing or oxidizing atmosphere and measurements at both high and liquid-nitrogen temperature. The cell was tested by studying the decomposition, in an oxygen flow, of ammonium metatungstate to WO3. Extended X-ray absorption fine-structure measurements of the ammonium metatungstate before and after a thermal treatment at 773 K are reported.

Structure of the Different Cu Sites in the Corrugated CuO2 Plane in High Tc Superconductors

By using x-ray absorption spectroscopy different configurations of the Cu site structure in the CuO2 plane of the Bi 2:2:1:2 high Tc superconductor are found. The different Cu sites are characterized by short 2.3 A and long 2.45 A Cu-O(apical) distances. The linear arrays of different Cu sites forming domains with a corrugated-iron-foil shape is proposed to be a key feature of superconducting domains in the CuO2 plane. The wavelength λ of the modulation is close to the superconducting coherence length ξ and kFλ~2π. The ordering of the distorted Cu sites is suggested to be evidence for ordering of polarons driven by the pseudo Jahn Teller electron lattice interaction.

Coupling between the charge carriers and lattice distortions via modulation of the orbital angular momentum Mℓ=0 of the 3d holes by polarized xas spectroscopy

Abstract The change of the orbital angular momentum M l of the Cu 3d holes going from the insulating to the metallic phase has been studied in several families of high Tc superconductors. The symmetry of the 3d9 L states in the metallic phase has been studied by quantitative analysis of the variation of polarized Cu L3 x-ray absorption spectra. At a doping level nh∼15% we have found 10% of Zhang-Rice singlets 3d x 2 -y 2 L (b 1 ) and 5% of 3d 3z 2 -r 2 L (a 1 ) states. Therefore the percentage of the 3d3z2−r2 L states on the total number of the 3d9 L states is about 30% i.e. much larger than the probability of single hole states 3d3z2−r2 in the insulating phase. The EXAFS and XANES studies of the Cu site structure and dynamics in Bi2Sr2Ca1−xYxCu2O8 system point toward the coupling of the charge carriers with distortions of the Cu sites driven by the M l=0 character of the Cu 3d holes that can be called a 3dz2−r2 polaron.

The reducibility of highly stable Ni-containing species in catalysts derived from hydrotalcite-type precursors

A study was conducted on the speciation and reducibility of Ni in catalysts derived from hydrotalcite-type (HT) precursors intercalated by silicates. Silicate and nickel contents in Ni/Mg/Al HT precursors varied and the products obtained by thermal decomposition in the 773–1373 K range were characterized. Sample properties were related to the amount of silicates and nickel. The former altered the formation of the spinel-type phase and decreased the ratio between MgO and MgAl2O4 phases in which the active species were stabilized. On the other hand, the Ni distribution depended on the Ni-loading. Ni-containing species in the spinel phase were more abundant for high Ni-loaded catalysts, and were readily reduced by H2 treatment at 1023 K, whereas those in the Ni1−xMgxO solid solution remained partially unreduced.

Anderson phases as precursors of nickel–molybdenum–tungsten oxides

Mixed Ni–Mo–W oxides have been prepared by decomposition at 873 K of Anderson-type [NiMo 6-x W x O 24 H 6 ](NH 4 ) 4 5H 2 O (x=0, 2, 3, 4, 6) heteropolyoxometallates. Thermal analysis of the precursors, X-ray diffraction and Raman spectroscopy on the mixed oxides were performed. X-Ray powder patterns, collected at the end of the thermal treatment, revealed that the final products are NiMoO 4 and MoO 3 for the x=0 composition, whereas for tungsten-containing materials the final products are Mo 1-x W x O 3 solid solutions, NiWO 4 and WO 3 , depending on the Mo/W atomic ratio. Raman spectra have confirmed the XRPD analysis, revealing the presence of a stable WO 3 -type phase at high molybdenum contents and a high structural stability of Mo 1-x W x O 3 solid solutions.