Federico Cioci

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.

High-performance microbial removal of ethanol from contaminated air

Ethanol vapour from air was removed using a bioreactor (2.7 m 3 ) packed with a novel engineered material consisting of a highly porous inorganic matrix coated with activated carbon. The system was operated over a period of two months varying the inlet ethanol between 90 and 2200 mg m –3 . Removal efficiencies ranging from 80 to 99.9% were obtained. A simplified kinetic model of the bioreactor was developed and used to estimate the maximum degradationrate and to predict thermal effects resulting from ethanol oxidation.

Thermostabilization of erythrocyte carbonic anhydrase by polyhydric additives

An experimental investigation was carried out on the influence of some polyhydric additives on the thermal behavior of erythrocyte carbonic anhydrase. The stability of the enzyme was analyzed by thermal unfolding and kinetic experiments. The results obtained showed a stabilizing effect of glycerol and sugars (fructose, glucose, sucrose, and sorbitol) whereas ethylene glycol was found to favor protein unfolding. An analysis of the thermodynamic behavior of the enzyme allowed us to determine the transition temperature and the changes in enthalpy, entropy, and free energy in pure buffer and in the presence of additives. The thermodynamic and kinetic results supported the hypothesis that thermal stabilization of carbonic anhydrase is a consequence of microenvironmental modifications, probably related to preferential exclusion phenomena, which strengthen the macromolecule and preserve the biologic activity of the enzyme under thermally adverse conditions.

Sorbitol-mediated stabilization of human IgG against thermal inactivation

Sorbitol at 30% (w/w) stabilized human IgG to thermal denaturation from 60 to 85°C, increasing the proteins half life from 25 to 266 min at 70°C. A kinetic model based on the Lumry-Eyring inactivation scheme was developed and used to estimate the apparent rate constant and activation energy.

Urease-Loaded Liposomes as Detoxifying Microreactors: Effect of Ammonia Accumulation on Enzyme Kinetics

AbstractA kinetic model was developed which describes the unsteady behaviour of liposomes entrapping the enzyme urease. The model allows to determine how the reactant concentrations, the internal pH and the catalytically active enzyme fraction vary with time. A numerical simulation carried out to elucidate die effects of die buffer pH and ionic strength on die active enzyme decay provided useful directions for die selection of die operating conditions to be used for die removal of urea by urease-loaded liposomes.

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.

Effect of surface tension on the conformational stability of erythrocyte carbonic anhydrase

An experimental investigation was carried out on the influence of some hydroxylic additives (up to 30% by weight) on the thermal stability of erythrocyte carbonic anhydrase. The melting temperatures of the protein in the different media were determined by spectroscopic measurements. Xylitol and maltose stabilized the enzyme, whereas ethanol, 1-propanol, ethanediol and 1,2-propanediol caused a destabilization of the protein. In all the systems examined, the transition temperature was found to be strictly related to the surface tension of the medium. A molecular thermodynamic model was developed, based on the chemical equilibrium between the native and denatured protein forms, which enabled to interpret the observed behaviour.

PERTURBATION OF SURFACE TENSION OF WATER BY POLYHYDRIC ADDITIVES: EFFECT ON GLUCOSE OXIDASE STABILITY

An experimental investigation is reported on the influence of some polyhydric additives (ethanediol, 1,2-propanediol and glycerol) on the thermal behaviour of Aspergillus niger glucose oxidase. Spectroscopic measurements performed to follow protein unfolding showed that all the cosolvents destabilize the enzyme and caused an increase in the standard enthalpy and entropy of transition. Furthermore the melting temperature was found to be intimately related to the surface tension of the medium. The obtained results were interpreted as the consequence of preferential protein-cosolvent interactions determined by the perturbation of the surface tension of water.

Carrier-Mediated Transport Phenomena in Enzyme-Loaded Liposomes

A mathematical model which describes the kinetic behaviour of enzyme-loaded liposomes containing a substrate transporter in the lipid bilayer is presented. The model accounts for the facilitated diffusion across the membrane and the chemical reaction in the aqueous core. Both steady-state and transient kinetics are analysed. The model allows to quantify the influence of transport phenomena on the catalytic properties of the microencapsulated enzyme and provides some directions for the design of an artificial vesicle in which a selective substrate carrier has been included.

An enzymatic membrane reactor for extracorporeal blood oxygenation

The potential use of the enzyme catalase to oxygenate blood was investigated. A microcalorimetric technique was used to study the kinetics of oxygen generation in human blood serum at 25 and 37°C. Experiments carried out in serum and in phosphate buffer provided the apparent rate constants for the enzymatic and non-enzymatic oxygen-generation reactions. A simplified mathematical model was then developed to analyse the behavior of a hollow-fiber membrane oxygenator with catalase covalently bound to the membrane surface. The model was used to simulate an extracorporeal oxygenation treatment and outline differences with current devices.