Giuliano Minelli

Structural characterization of malachite-like coprecipitated precursors of binary CuOZnO catalysts

Copper and zinc hydroxycarbonates with Cu/Zn atomic ratios of 100/0, 92/8, 85/15, 77/23, 67/33, and 50/50 were prepared by the coprecipitation method. The samples with the ratios 100/0, 92/8 and 85/15 were monophasic with the monoclinic P2/sub 1//a malachite structure, whereas the richer zinc samples contained in addition to the malachite phase the orthorhombic B22/sub 1/2 aurichalcite, whose concentration was very low in the 77/23 specimen but increased continuously with zinc content. The amount of aurichalcite in the biphasic powders has been estimated by means of quantitative X-ray diffraction and differential thermal analysis. With only those X-ray reflections belonging to the malachite structure taken into account, the unit cell constants for all the solid solution hydroxycarbonates have been determined, showing that as a whole the cell volume of malachite decreases continuously with increasing zinc content. This result is related, as shown also by DTA measurements and thermochemical literature data, to an increasing stability of compounds richer in zinc, to increasing covalent effects, and/or to less octahedral site distortion around the metal atoms when zinc substitutes for Cu/sup 2 +/ ions in the malachite lattice. Magnetic susceptibility and thermogravimetric measurements together with diffuse reflectance spectra are also presented and discussed.

Studies on chromia/zirconia catalysts I. Preparation and characterization of the system

Abstract The preparation and characterization (chemical, textural, DTA, XRD, XPS) of chromium oxide/zirconia, of interest as hydrogenation catalysts, are reported. The support (obtained via ZrOCl2 hydrolysis) can be tailored in surface area from high (360 m2g−1) to low (about 20 m2g−1) values, and in texture from microporous to mesoporous and macroporous according to the treatment temperature (from 383 to 923 K) of the starting “hydrous zirconia.” By contacting the support with Cr(VI) solution, chromium-loaded specimens, ZC, are prepared. The Cr uptake is roughly constant (1.5 to 1.9 Cr atoms nm−2) for zirconia previously heated at T ≥ 573 to 923 K. Higher loadings can be reached on the hydrous zirconia. Supported Cr oxide is an effective antisintering agent for zirconia, and it also opposes the tetragonal → monoclinic transition. By subjecting the ZC specimens to various heat and redox treatments, the average oxidation number of Cr, n changes. From an initial value of +6, n decreases to +5.5 after oxygen treatment at 773 K, to +2.5 after CO treatment at 623 K, and can be restored to +5.5 if the sample is reoxidized in oxygen at 773 K. Treatment in H2O vapor at 723 K of a reduced ( n = 2.5 ) specimen brings n to 3.0. The existence of Cr(VI), Cr(V), Cr(III), and Cr(II) is inferred and is supported by XPS analysis. A separate paper presents an ESR investigation and discusses the nature of the surface Cr species.

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.

IR spectra of VO2 and V2O3

Abstract IR spectra of the tetragonal modification of VO 2 and of the trigonal form of V 2 O 3 are recorded at room temperature and compared with that of V 2 O 5 . The investigated samples of the two lower-valent vanadium oxides, obtained on temperature-programmed reduction treatment, were also characterized with diffuse reflectance and electron-paramagnetic resonance spectra. The effect of atmospheric oxygen on these materials was revealed with XPS measurements and also studied with IR spectra.

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.

A structural, thermogravimetric, magnetic, electron spin resonance, and optical reflectance study of the NbOxTiO2 system

Abstract Titanium dioxide samples containing niobium (up to 10%), heated at 1223 K in vacuo and subsequently in air, have been investigated by X-ray diffraction, thermogravimetry, magnetic susceptibility measurements, electron spin resonance, and optical reflectance spectroscopy. The results show that for the samples prepared in vacuo, niobium is incorporated as Nb(V) compensated by an equivalent amount of Ti(III). The analysis of the magnetic data reveals that some Ti(III) are paired giving a metal-metal interaction. On subsequent heating in air the Ti(III) ions are oxidized to Ti(IV), with the Nb(V) in solid solution being compensated by cation vacancies. The maximum amount of Nb(V) that may be accommodated in the rutile structure is 6.6 Nb atoms/100 Ti atoms. The niobium in excess reacts with TiO2 giving TiNb2O7. The effect of the various incorporated species on the TiO2 unit-cell volume is discussed.

Lean NOx reduction CuZSM5 catalysts: Evaluation of performance at the spark ignition engine exhaust

Abstract CuZSM5 catalysts, prepared by ion exchange of HZSM5 (Si/Al = 80), enable selective lean NOχ reduction at the spark ignition engine exhaust. All catalysts, even if exchanged to very high copper loading (556% apparent exchange level) give no evidence for segregated copper oxide phases. In the presence of water they are easily reduced in vacuum at 550°C and are stable to redox cycles (O2, vacuum) treatments. Increasing Cu content to about 200% apparent exchange level results in increasing NOχ, CO and HC conversions. From 200 to 550% exchange CuZSM5 catalysts give similar performances. Preliminary investigation shows that after catalytic tests there is evidence for CuO formation only in the copper-richest catalysts.

XPS and EPR study of high and low surface area CoO–MgO solid solutions: surface composition and Co2+ ion dispersion

Solid solutions CoO–MgO (MCo) of low and high surface area (LSA and HSA) have been studied by XPS and EPR analysis to evaluate the surface composition and the dispersion of cobalt ions in the systems. Different preparation routes were adopted: wet impregnation (Iw), dry impregnation (Id) and coprecipitation (C). LSA solid solutions (As= 10–20 m2 g–1) were obtained after treatment in air at 1173 K (1173 LSA) or at 1473 K (1473 LSA), HSA materials (As= 200–300 m2 g–1) after treatment under vacuum at 1173 (1173 HSA). XPS intensity ratios, ICo/IMgvs. xCo/xMg molar fraction showed no appreciable differences between the bulk and surface compositions of LSA and HSA solid solutions from Id and C precursors, calcined at 1173 and at 1473 K. MCo 1173 LSA Iw samples had evident cobalt enrichment, due to Co3O4 segregation, whereas Iw HSA did not. NO2 or O2 treatments led to the formation of CoIII. The Co2+ bulk dispersion in LSA and HSA was studied by EPR.

Structural, surface, and catalytic properties of bismuth molybdovanadates containing foreign atoms: I. X-ray characterization of iron-containing bismuth molybdovanadate catalysts☆

Abstract The selective oxidation of propene to acrolein has been studied on iron-containing bismuth molybdovanadates with the scheelite structure and general formula Bi 1 − x 3 x 3 − y Me y ( V 1 − x Mo x − y Fe y ) O 4 , where Me = Fe or Bi , = cation vacancy, with x = 0.45 and 0.60 and 0 ⩽ y ⩽ x 3 , using the pulse technique in the temperature range 573–673 K. Total conversion, rate constant, and selectivity were found to depend on catalyst composition, in particular cation vacancy concentration, content, and coordination symmetry of the iron ions. The results, discussed in terms of compositional parameter y and surface geometry of BiO and MoO species, confirm the importance of the cation vacancies in the selective oxidation process and suggest a role of the eight-coordinated cations in the allyl formation.