Alberto J. Marchi

Kinetic study of the reverse water-gas shift reaction over CuO/ZnO/Al2O3 catalysts

The kinetics of the reverse water-gas shift (RWGS) reaction over CuO/ZnO/Al2O3 catalysts was studied by use of CO2H2 cycles, hydrogen chemisorption and catalytic tests performed in both differential and integral plug flow reactors. The effect of the reactant composition on the reaction rate was specifically studied by changing the PH20/PCO20 ratio between 9.0 and 0.3. It was found that different reagents become rate limiting depending upon pressure. While in a H2-rich region the rate increases strongly with CO2 partial pressure and is zero order in hydrogen, under low PH20/PCO20 ratios the reaction is less active and is strongly positive order in hydrogen and low order in carbon dioxide. The experimental data were modeled by considering that the reaction proceeds through a surface redox mechanism, copper being the active metal. A good agreement between experimental and calculated data was obtained by assuming that in the redox mechanism either the dissociative CO2 adsorption (H2-rich region) or both the CO2 dissociation and the water formation (H2-lean region) determine the rate of the overall reaction. Based on previous studies performed on copper crystal surfaces, such a change in kinetics may be explained by assuming that under H2-rich atmosphere a surface structural or phase transition occurs involving a change in reactivity with respect to CO2 dissociation.

Dehydrogenation of isopropylic alcohol on a Cu/SiO2 catalyst: a study of the activity evolution and reactivation of the catalyst

The activation-deactivation behaviour of a Cu/SiO2 catalyst during isopropyl alcohol (IPA) dehydrogenation to acetone has been studied. Simultaneous reduction and sintering processes are responsible for the activation-deactivation behaviour observed. The reducing H2 atmosphere impulses the transformation of CuO into Cu0 and at the same time the sintering of the Cu0 phase formed. The reoxidation treatment in air at 573–673 K is able to regenerate a finely dispersed CuO phase from the sintered Cu0 phase. As a result, the active phase is redispersed and the dehydrogenation activity is recovered. The results indicate the existence of two CuO phases on the catalyst, with different properties. The CuO phase with a low reduction temperature, a small particle size and a poor crystallinity, is the one responsible for the appearance of the finely dispersed Cu0, which is the active phase for this reaction.

Methanol synthesis by means of diffuse reflectance infrared Fourier transform and temperature-programmed reaction spectroscopy

Abstract Diffuse reflectance infrared Fourier transform and temperature-programmed reaction spectroscopies were used to investigate the nature of the adsorbed surface species under methanol synthesis conditions on zinc oxide, partially oxidized polycrystalline copper and on an ICI Cu/ZnO/Al2O3 commercial catalyst (Cu 60 wt.-%, ZnO 30 wt.-% and Al2O3 10 wt.-% ). It was found that formate species adsorbed on copper is the pivotal intermediate leading to methanol. Its hydrogenation to methoxy is the rate limiting step. Formate species adsorbed on ZnO is less active than the formate formed on copper, indicating the limited role of ZnO under low pressure-low temperature methanol synthesis. Yet, zinc oxide seems to play an important role in modifying the electronic properties of metallic copper, thereby inducing changes in its catalytic activity. These changes facilitate the hydrogenation of the formate species on copper at lower temperatures, while the decomposition to carbon dioxide and hydrogen is shifted to higher temperatures.

Characterization of surface species on V/SiO2 and V,Na/SiO2 and their role in the partial oxidation of methane to formaldehyde

Silica-supported vanadium (1–8 wt%) and vanadium (5 wt%)-sodium (0.4 wt%) catalysts have been characterized by laser Raman spectroscopy, temperature-programmed reduction, X-ray photoelectron spectroscopy, NO + NH3 rectangular pulses and oxygen chemisorption. The presence of different vanadium species was correlated with activity and selectivity during the methane partial oxidation reaction. The pre-impregnation of the silica support with sodium favors vanadium dispersion, but strongly diminishes V=O concentration due to the formation of orthovanadate-like compounds. As a result of these modifications, methane conversion is strongly inhibited while formaldehyde decomposition is favored.

Laser Raman spectroscopy (LRS) and time differential perturbed angular correlation (TDPAC) study of surface species on Mo/SiO2 and Mo,Na/SiO2. Their role in the partial oxidation of methane

Silica-supported molybdenum (1.6 and 5.0 wt%) and molybdenum (5 wt%)-sodium (0.4 wt%) catalysts have been characterized by laser Raman spectroscopy (LRS), time differential perturbed angular correlation (TDPAC), temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The presence of different molybdenum species was correlated with activity and selectivity to formaldehyde during the methane partial oxidation reaction. The main species identified on the Mo(5.0 wt%) /SiO2 surface were MoO3 and monomeric species with a single Mo=O terminal bond. The pre-impregnation of the silica support with sodium strongly diminishes the Mo=O concentration due to the formation of Na2Mo2O7 species and tetrahedral monomers with a high degree of symmetry. As a result of these modifications, both methane conversion and formaldehyde formation are strongly inhibited. The combination of LRS and TDPAC techniques resulted in a powerful tool for the identification and quantification of the molybdenum species present on the surface of a silica support.

Influence of the surface structure of Co on the selective hydrogenation of crotonaldehyde

Two series of Co/SiO 2 catalysts prepared by impregnation, using water and ethanol as solvents, were tested in hydrogenation of crotonaldehyde in gas phase. Four types of chemisorbed hydrogen species on the cobalt surface were identified by TPD and labeled α, β, γ and σ. It was established that selectivity towards crotyl alcohol depends on preferential generation of “β” sites as opposed to “γ” and “σ” sites on the metal cobalt surface. Increase of “γ” and “σ” sites leads to a butanol selectivity increase. The “β” site generation directly dependens on Co loading and CoO x -SiO 2 interaction in the precursor. Silica and CoO x interaction depends strongly on the solvent utilized for impregnation and on drying and calcination temperature.

Activity, selectivity and coking of bimetallic Ni-Co-spinel catalysts in selective hydrogenation reactions

The influence of the calcination and reduction temperatures of a Ni-Co-ZnAl 2 O 4 catalyst was correlated with its catalytic activity in the hydrogenation of acetylene. A well interdispersed Ni-Co, catalyst supported in a ZnAl 2 O 4 spinel-like structure was obtained by using a coprecipitation method. Cobalt apparently effects a dilution of Ni surface ensembles, increasing the selectivity to ethylene. The influence of the operating temperature on activity, coking rate and selectivity was analyzed using a deactivation kinetic model.

Synthesis of liquid menthol by hydrogenation of dementholized peppermint oil over Ni catalysts

Hydrogenation of (-)-menthone and (+)-isomenthone was studied at 2.7 MPa and 100 oC. The objective was to produce a liquid menthol mixture rich in (-)-menthol from dementholized peppermint oil. Ni-based catalysts were tested and compared for this reaction: a) 6 and 12% Ni dispersed into a nonstoichiometric magnesium aluminate (Ni-Mg-Al) with spinel structure; b) Ni-Raney catalyst. Both types of catalysts were active for (-)-menthone and (+)-isomenthone hydrogenation. Lower conversion but higher selectivity to (-)-menthol was obtained with Ni-Mg-Al catalysts. However, they rapidly lost their activity. Instead Ni-Raney catalysts kept its original activity even after several hydrogenation runs.

Acetylene hydrogenation with a modified Ni-Zn-Al catalyst. Influence of the operating conditions on the coking rate

Abstract The preparation, characterisation and catalytic behaviour of a Ni-Co-Cr-Zn-Al catalyst in the reaction of acetylene hydrogenation have been investigated. The influence of the temperature and feeding composition on the activity, selectivity and coking have also been examined. An increase in the partial pressure of H 2 in the feed causes an increase in the initial conversion, and a diminution in the deactivation rate, ethylene selectivity and in the rate of coke deposition. The opposite effects are observed with respect to the influence of the partial pressure of acetylene. The temperature has a low influence on the activity and selectivity. However, an increase of the operation temperature produces an augmentation in the initial coking rate and a diminution in the final rate of coke formation. A kinetic model of coking growth, that assumes the existence of two types of active sites, has been used in order to analyse the experimental coking data obtained during the acetylene hydrogenation reaction.