M. Inversi

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.

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.

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.

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.

Copper–cobalt hydroxysalts and oxysalts: bulk and surface characterization

Copper–cobalt oxysalts of various Cu : Co atomic ratios (100 : 0. 92 : 8, 85 : 15, 77 : 23, 67 : 33, 15 : 85, 0 : 100) have been prepared by coprecipitation, at constant pH, from solutions of copper and cobalt nitrates added to a solution of NaHCO3; the structure and chemical nature of the Cu—Co oxysalts have been investigated by several complementary techniques such as X-ray diffraction (XRD), magnetic susceptibility, reflectance spectroscopy and X-ray photoelectron spectroscopy (XPS) to obtain information both on their bulk and on their surface properties.In the range of Cu : Co atomic ratios from 100 : 0 to 67 : 33 the materials consist of Co-containing malachite, Cu2 –xCoxCO3(OH)2, while for Cu : Co equal to 0 : 100 and 15 : 85 the products are CoCO3, spherocobaltite, and Cu-containing spherocobaltite Co0.85Cu0.15CO3, respectively. From both reflectance spectroscopy and magnetic susceptibility measurements the presence of only Cu2+ and Co2+ species has been ascertained in all materials. Antiferromagnetic interactions are strong at high copper content and become weaker at increasing cobalt loading. The observed decrease in volume of the malachite monoclinic unit cell with incorporation of Co2+ ions, in spite of the larger Co2+ ion radius with respect to that of Cu2+, indicates that distortion of the MO6 polyhedra at the cation site of the malachite lattice is less when Cu2+ is replaced by Co2+. The presence of Cu2+ in the spherocobaltite trigonal lattice shows a small cell volume decrease as expected from ionic radii difference. XPS has confirmed the presence of only Cu2+ and Co2+ species at the surface of the materials. It is also shown that the local nature of the chemical bond is essentially determined by the nature of the first-neighbour ligands. At higher copper content, where the materials are cobalt-containing malachite, the surface is enriched in cobalt.

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.

Structural, surface, and catalytic properties of bismuth molybdovanadates containing foreign atoms: IV. Surface characterization and redox behaviour of iron-containing bismuth molybdovanadate catalysts by X-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy has been applied to the study of the surface composition and the redox behaviour of multicomponent oxide catalysts having 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 (Me = Fe or Bi, □ = cation vacancies), before and after exposure to hydrogen, propene, and a propene + oxygen mixture. The results point to a different reactivity toward the various reactants. By interaction with hydrogen, an extensive reduction is obtained with considerable change of the catalyst surface composition. The reduction behaviour depends on the sample composition, the iron-containing catalysts being reduced at low temperature (140 °C). As regards propene treatment, a preferential reactivity of bismuth with propene has been found, regardless of the sample composition. Bismuth is reduced at low temperature (175 °C) with ensuing migration to the surface. Under conditions leading to the catalytic oxidation of propene, a high surface stability is exhibited by these compounds. The catalysts remain oxidized and the surface composition reflects that of the bulk. A correlation with the catalytic properties has also been made on the basis of the different mobility of the oxygen ions among the samples examined.

On the use of the first-order approximation in the kinetic analysis of TPR profiles

Abstract A theoretical investigation was carried out on the analysis of TPR profiles by the power-law kinetic model. Attention was focused on the use of the first-order approximation, with the aim to assess its limits in the procedure for estimating the activation energy of reduction. Numerical simulations performed by solving the non-isothermal mass-balance equations for the gaseous and solid species indicated a high sensitivity of the reduction patterns to the reaction order with respect to the solid. This quantity was found to exert a considerable influence on both the temperature at peak maximum and the peak shape. A large number of TPR profiles was generated, assuming reaction orders other than unity and activation energies ranging from 80 to 100 kJ mol −1 . These profiles were interpreted by means of the first-order power-law model. The results obtained showed that the unjustified assumption of the first-order approximation may introduce significant errors in the estimate of the activation energy. In several cases, real and estimated values differed by more than 30%. In order to provide some guidelines for a correct kinetic analysis, the causes responsible for such misinterpretation were investigated from both a qualitative and quantitative point of view.

Temperature-Programmed Reduction in Catalysis: A Critical Evaluation of the Method

Abstract Temperature-Programmed Reduction (TPR) has been critically tested by carrying out experiments at different operating conditions on both copper-based catalysts and pure copper oxides (CuO, Cu 2 O). In this work we give experimental evidence that, even if interference caused by mass transfer limitations or dispersion is removed, the reduction profiles are markedly affected by artefacts when an inadequate combination of the typical TPR operating variables is used. Furthermore, an attempt of tracing back to the artefacts origin is also effected.