Piero Porta

Study of the reducibility of copper in CuOZnO catalysts by temperature-programmed reduction

Temperature-programmed reduction (TPR) has been used in this work to study the reduction of copper in CuOZnO catalysts with different CuZn atomic ratios using H2 as reducing agent. In all catalysts, CuO was completely reduced to metal. The influence exerted by ZnO on the reduction of copper was evaluated for a wide range of composition and a scale of reducibility was established. ZnO affects the hydrogen reduction of copper, CuOZnO samples showing a different behaviour with respect to the pure copper oxide. The reduction is always promoted and, in particular, catalysts with lower copper loading (Cu:Zn < 30:70 as atomic ratio) showed the highest reactivity and are characterised by the presence of two reducible copper species. Results of a kinetic analysis based on the TPR profiles confirmed the role played by ZnO as promoter of the copper oxide reduction. The effect of the preparation method on the catalyst reducibility was also verified and discussed in a specific case. H2CO2H2 redox cycles were carried out on some representative samples which, after the first reduction in H2, were reoxidised with CO2 and then reduced again by H2. These experiments revealed that a small percent of metal copper formed in the CuOZnO catalysts is oxidised by CO2 regardless of the catalyst composition, whereas metal copper formed by reduction of pure CuO is not reoxidised at a detectable level. Furthermore, it was evidenced that the small fraction of copper reoxidised by CO2 was extremely reactive, being reduced at temperatures much lower than those found for the reduction of the as-prepared catalysts. Both the TPR investigation and the H2CO2H2 redox cycles clearly assessed the presence of a synergistic effect arising by the contact of CuO with the ZnO particles.

Catalytic activity of LaMnO3 and LaCoO3 perovskites towards VOCs combustion

The catalytic combustion of some volatile organic compounds (VOCs) has been investigated over LaMnO3 and LaCoO3 perovskite-type oxides. Redox titration has shown that cobalt is present in LaCoO3 exclusively in the 3+ oxidation state whereas LaMnO3 contains considerable amount of Mn4+ (35%) in addition to Mn3+. VOCs reactivity towards total oxidation follows the trend: acetone > isopropanol > benzene. The oxidation of isopropanol occurs through the formation of acetone in the homogeneous reaction. The increase of the oxygen partial pressure is beneficial for total oxidation of acetone. The adsorptive properties of the catalysts towards the VOCs and H2 have been examined by means of temperature programmed desorption. The LaMnO3 surface is the most reactive to the adsorption of VOCs and H2. The role of the adsorbed oxygen has been studied by examining the variations of the electrical conductivity of the catalysts during the processes of oxygen adsorption–desorption.

TPR and XPS study of cobalt–copper mixed oxide catalysts: evidence of a strong Co–Cu interaction

In this work the results of a TPR and XPS investigation of CoxOy–CuO mixed oxides in the range of composition Co : Cu=100:0–8:92 are reported and compared. The final catalysts were obtained by thermal decomposition in air and N2 at 723 K for 24 h of single‐phase cobalt–copper hydroxycarbonates prepared by coprecipitation at constant pH. The Co : Cu=100 : 0 specimen calcined in air formed the Co2+[Co3]2O4 (Co3O4) spinel phase. The copper‐containing catalysts (Co : Cu=85 : 15–8 : 92) showed mainly two phases: (i) spinels, like Co2+[Co3+]2O4, Co1-x2+Cux2+[Co3+]2O4 and (ii) pure CuO, the relative amount of each phase depending on the Co : Cu atomic ratio. The results of the XPS study are consistent with the bulk findings and revealed the presence of Co2+, Co3+ and Cu2+ species at the catalyst surface. Moreover, the surface quantitative analysis evidenced a cobalt enrichment, in particular for the most diluted cobalt samples. The TPR study showed that the catalyst reduction is affected by a strong mutual influence between cobalt and copper. The reducibility of the mixed oxide catalysts was always promoted with respect to that of the pure Co3O4 and CuO phases and the reduction of cobalt was markedly enhanced by the presence of copper. Cobalt and copper were both reduced to metals regardless of the catalyst composition. On the other hand, the Co : Cu=100 : 0 specimen calcined in N2 formed, as expected, CoO. The initial addition of copper resulted in the formation of the Cu+Co3+O2 compound, besides CoO, up to a Co/Cu=1 atomic ratio at which the CuCoO2 phase was the main component. A further addition of copper led to the formation of CuCoO2 and CuO phases. The XPS results were in good agreement with these findings and the surface quantitative analysis revealed a less enrichment of cobalt with respect to the catalysts calcined in air. The TPR analysis confirmed that the reduction of the N2‐calcined catalysts was also remarkably promoted by the presence of copper. Also in this case cobalt and copper metal were the final products of reduction.

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.

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.

Preparation and characterisation of cobalt–copper hydroxysalts and their oxide products of decomposition

Three cobalt–copper hydroxysalts with chemical formulae Co(CO3)0.5(OH)1.0· 0.11H2O, Co0.67Cu0.33(CO3)0.4(OH)1.2 and Co0.49Cu0.51(CO3)0.43(OH)1.14 have been obtained and characterised by powder X-ray diffraction (XRD), thermal analysis (TG, DTG and DTA), diffuse reflectance spectroscopy (DRS), magnetic susceptibility and X-ray photoelectron spectroscopy (XPS). The above precursors treated at 723 K for 24 h both in N2 and in air give several cobalt and copper oxides which were characterised by magnetic susceptibility, X-ray diffraction, X-ray photoelectron spectroscopy and diffuse reflectance. Depending on the Co : Cu atomic ratios and on the treatments with various gases, the products consist of oxides such as CoO, CuxCo1–xO, CuCoO2 and CuxCo3–xO4 for the samples calcined in N2, and Co3O4, CuxCo3–xO4 and CuO for those calcined in air.

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.