RICERCA ITALIANA     

Research program

INNOVATIVE CATALYTIC PROCESSES FOR THE SELECTIVE OXIDATION AND REDUCTION OF GLYCEROL IN WATER: STUDIES OF REACTION MECHANISMS AND KINETICS FOR THE PROCESS OPTIMISATION

University Co-ordinator

Università degli Studi di CAGLIARI - INGEGNERIA CHIMICA E MATERIALI - ()

Research Unit Leader

Simonetta Palmas

Description

The Cagliari Unit will focus the research activity on the preparation and characterization of electrodes based on nanometer-sized powders of semiconductor oxides for the electrophotocatalytic oxidation of the glycerol.

The most important stages of the work will be:

* STAGE 1. Photoelectrodes preparation: scheduled from the starting time of the project (1st month) to the 6th month
* STAGE 2. Sample electrochemical and photoelectrochemical characterization: scheduled from the 4th to the 20th month of the project
* STAGE 3. Electrode performance evaluation in the glycerol oxidation reaction: schedulaed from the 10th to the end of the project (24th month)

STAGE 1. – Photoelectrode preparation –

In this stage the study will be focused to the comparison of the behavior of photocatalytic materials obtained with different techniques. Mixtures with different TiO2 phases and alloyed systems based on Ti dioxide, Sn dioxide and Zn oxide (in collaboration with the Palermo University research unit, which can provide sol-gel prepared powders; contract with the Sassari University for the preparation of milled materials).
Powders will be characterized by immobilizing the thin film samples with single or mixed phases on a conducting support. Particular attention will be paid to select the support, a crucial point for obtaining stable films. Ti will be initially used as support, paying attention to its pre-treatment, fundamental to guarantee electrode stability [14]. Should it be necessary, the stabilization of the thin film on the support will be made by means of suitable polyelectrolytes, which will assure the film adhesion to the support without the loss of catalyst optical properties.
Boron-doped diamond (BDD) will be also used as support, which can be considered a good alternative to favour high efficiencies in terms of photoinduced charge carrier separation. So prepared hybrid systems have indeed shown a good photocatalytic activity in organics oxidation processes [15].

STAGE 2. Sample electrochemical and photoelectrochemical
characterization –

The samples will be then subjected to electrochemical characterization. Electrochemical techniques will be used to evaluate both the catalytic and photocatalytic activity of prepared electrodes and the electrode material electronic structure as well as its variation with the preparation techniques.
Classical electrochemical techniques such as cyclic voltammetries and steady state polarization curves will be in particular used. The trials, carried out in presence and absence of light source, will be a useful tool to monitor the electrode surface. In particular, they can provide information on partially oxidized intermediates and/or secondary products which could be adsorbed at the electrode surface due to the light scattering.
Photocurrent measurements will be also performed in different electrolytes, changing the applied potential and the light intensity: the measurement will be repeated with light sources at different wavelength to verify possible modifications of the sample adsorption interval.
The description of the semiconductor/electrolyte junction will be based on the Gartner model [16] to allow quantitative comparison of the adsorption process at different wavelength of the scattered radiation in terms of quantum efficiency, diffusion path of holes and width of the charge space.
Electrochemical impedance spectrocopy trials – The electrochemical impedance spectroscopy (EIS) can be used to obtain useful information on electrode processes taking place under light. It is an in situ investigation allowing small perturbation measurements (the sinusoidal signals of very low amplitude employed perturb only slightly the steady state system characteristics) and represents a good alternative to the use of classical methods to investigate the kinetics of even complex processes such as the photoelectrochemical one.
The technique will be used to obtain information on both the electronic structure of the semiconductor and the electrode catalytic activity. The Mott-Schottky analysis will be employed to evaluate the interfacial capacity as a function of the applied potential and then obtain information on both the energetic position of the valence and conduction bands and the charge carrier concentration.
In addition, the data, registered in terms of real parts versus imaginary (Nyquist) and impedance modulus and phase respect to the frequency (Bode) will be quantitatively interpreted by means suitable equivalent electrical circuits. The procedure will permit an evaluation of the film characteristics in conditions of both obscurity and illumination: in particular the resistance to the transfer of electrons in the TiO2 layer will be evaluated in order to have a quantitative indication of the different distribution and density of electrons under illumination and not. The analysis of EIS data will permit also to monitor the stability of the photocatalyst allowing the identification of possible phase transformations during the process.

STAGE 3. – Evaluation of the electrode performances in the glycerol oxidation reaction –

On the bases of the results obtained in the previous step the electrodes will be arranged in a laboratory scale reactor in which the fotoelectro-oxidation process of glicerole will be studied.
The photoelectrochemical technology with external electric field applied to the photochemical reactor appears to be an efficient approach to improve the photocatalytic efficiencies. Different reactor configuration can be proposed, based on three-dimensional or planar geometry of the electrode [11]. In this research work a planar configuration will be adopted, in which the reactor will be an impinging jet cell, successfully tested in our laboratory to study the kinetics of different electrochemical processes. The photoelectrode will be placed at the bottom as working electrode, whereas a platinum grid will be used as counter electrode. The photoanode will be illuminated with a light source placed at the top of the cell. Depending on the runs, compressed air will be bubbled through a distributor equipped at the bottom of the reactor. This configuration will allow to study the effect of the experimental parameters on the kinetics of the photochemical oxidation in a wide range of values. Different experimental conditions will be adopted in order to quantify the effect of the operative parameters on the conversion of the reactant and selectivity of the process towards the desired compounds, as well as on energetic and quantum yields. The effect of the applied potential, which is the main parameter in determining the energetic yield, will be investigated: as can be found in the literature, in most of the photoelectrocatalytic systems, the applied anodic potential is always lower than that of direct electrochemical oxidation, since this reaction may interfere with the photocatalytic mechanism [9]. However researches appeared in the literature which discuss the hybrid photoelectrocatalytic technology under high electric field [10].
The performance of our electrodes at different wavelengths will be investigated in order to evaluate the quantum yield and have information on the fraction of visible light which can be useful in our process.
The effect of reactor hydrodynamics will be also investigated: although the photocurrent is not limited by the mass transfer and is insensitive toward hydrodynamics, the mass transfer of the reactant towards the catalytic surface may be important, depending on the concentration of the reactant itself.