Ανάπτυξη ολοκληρωμένης διεργασίας για τη φωτοκαταλυτική διάσπαση του νερού προς παραγωγή υδρογόνου με χρήση ηλιακής ακτινοβολίας

Abstract

In the present study a detailed investigation has been carried out in an attempt to obtain photocatalytic hydrogen production with simulated the solar irradiation. The employed photocatalysts were either cadmium sulfide (CdS) in the presence of sodium sulfide and sulfide salts (Na₂S-Na₂SO₂) or titanium dioxide (TiO₂) in the presence of glycerol (C₃O₅(OH)₃). Using cadmium sulfide (CdS) as phototcatalyst, the possibility of producing stabilized photocatalysts was investigated with i) various solvents, ii) various precursors and iii) combined with wide band gap photocatalysts, like zinc sulphide (ZnS) or/and titanium dioxide (TiO₂), with different ratio. Results revealed that CdS preparation process significantly influences the photocatalytic effectiveness and increase the hydrogen production. Increase in hydrogen production can be achieved by coupling CdS with ZnS in a ratio of CdS/ZnS (25-75). A further increase in the rate of hydrogen production was observed by dispersion of Pt (0.5 wt %) in the surface of the compined photocatalyst, CdS/ZnS (25-75). Additionally, as experimental results indicated, increase in hydrogen production can be achieved by coupling CdS with TiO2 (0.5% Pt-CdS/ TiO₂) or alternatively by mixing CdS with TiO₂ and ZnS (0.5% Pt-CdS/ZnS/TiO₂). In both cases, the obtained rates of hydrogen production were not much different with that obtained with the use of 0.5% Pt-CdS/ZnS (25-75). Replacing the conventional sacrificial agents (Na2S-Na2SO3) with ethanol (1% v/v), the rates of hydrogen production became zero, in the case of photocatalysts 0.5%Pt-CdS/ZnS (25-70) and 0.5%Pt-CdS or significantly decreased, in the case of photocatalysts 0.5%Pt CdS/TiO₂ (50-50) and 0.5%Pt CdS/ZnS/TiO₂ ((25-75):1). Simultaneous use of sacrificial agents with 1% v/v ethanol (Na₂SO₃-Na₂S and 1% ethanol), results in successive action of the two sacrificial agents, only for the case of 0.5%Pt CdS/ZnS/TiO₂ ((25-75):1). Using titanium dioxide (TiO₂), as phototcatalyst in the presence of glycerol (C₃O₅(OH)₃), resulted both in photocatalytic splitting of water and light-induced oxidation of organic substrates into a single process, that takes place at ambient conditions in the absence of gas-phase oxygen. According to the proposed mechanism, the water is reduced to hydrogen, while the organic compound acts as electron donor and gradually oxidized towards to carbon dioxide (CO₂). Irradiation of aqueous glycerol, results in notably strong evolution of H2, because of the additional production of hydrogen (additional hydrogen, H₂, add.), which comes from the splitting of glycerol. The total amounts of H₂, add. and CO₂ are practically equal to the molar ratio H₂, add..:CO₂ 7:3, which corresponds in the reaction of reforming (C₃H₈O₃ + 3H₂O -> 3CO₂ + 7H₂). The results of experiments were conducted in order to investigate the effects of operating parameters on the rate of hydrogen production, showed that the rate of hydrogen production depends strongly on glycerol concentration and, to a lesser extent, on the kind of metal loading, on solution pH and temperature and the concentration of photocatalyst in solution. Specifically, experiments of the effectiveness of the nature of the dispersed metal on catalytic performance showed that the maximum rate of hydrogen production is obtained with the use of Pt, as efficiency of the metal doped TiO₂ is in the order of Pt > Pd > Ru >Rh >Ag. The effect of metal loading (Pt) on the reaction rate has been investigated in the range of 0.0-5.0 wt.%, and showed that the maximum rate of hydrogen production increased with increasing Pt loading from 0.05 to 0.5 wt% but decreased for load greater than 2% wt. Pt. Increase the catalyst concentration (0,5%Pt/TiO₂) in the solution in the range of 0,6 g/L until 2,66 g/L leads to progressive increase on the reaction rate, which is finally stabilized on the biggest rate of hydrogen production. The rate of hydrogen evolution is higher at neutral or basic solutions, compared to acidic solutions and increased with increasing temperature from 40oC to 60-80oC (Fig. 3). Further increase of solution temperature up to 80°C did not induce significant changes on the rate. The most important parameter, which mainly determines the rate of hydrogen production, is glycerol concentration (C) in solution. In particular, increasing the glycerol concentration (C) in solution, in the range of 10-4 until 100 mol L-1, results in a linear increase of the maximum rate of hydrogen production with logCglyc. The maximum rate increased about two orders of magnitude with increase of Cglyc from zero to 1M. Corresponding experiments of effect of glycerol, photocatalyst concentration in the suspension and % Pt loading in the surface of 0.5%Pt/TiO₂ were conducted in separate reactor. The photo-reactor includes the lamp, which simulates UV irradiation emitting at 365 nm. In all cases, the results were qualitatively similar with those obtained with the use of UV-Vis irradiation. The only difference was that with UV irradiation bigger rates of hydrogen production are obtained. Experiments designed with the purpose to define the formation of reaction’s intermediates and final products (gas or/and liquid), in glycerol solution concentration 17 mM, indicated that methanol is the main reaction intermediate, small amounts of acetic was also detected in the liquid phase. The final products were only H₂ and CO₂, which are in great agreement with those predicted from the stoichiometry of the reforming reaction. The investigation of slurries for the non-stoichiometric oxygen production, in the presence or in the absent of glycerol (concentration 0.0-1.36 mM), indicated that photogenerated oxygen is bound at the catalyst surface as peroxotitanate complexes or titanoxide, or is released in the solution as Η₂Ο₂. In all cases, the quantity of peroxide species at the catalyst’s surface tends to a marginal value (~20 μmol), which corresponds to the possibility of maximum cover of TiO₂. Qualitatively similar results were obtained with the use of glucose slurries in concentration 0.0-0.417mM. The quantum yield (QY) of hydrogen evolution was measured in the photoreactor with the UV lamp emitting at 365 nm. It is observed that quantum yield increased from 1.8%, for pure water to 70%, for glycerol solution concentration 1M. The results of my thesis can be used for waste degradation in ambient conditions with simultaneous production of hydrogen and high efficiency. In the present study a detailed investigation has been carried out in an attempt to obtain photocatalytic hydrogen production with simulated the solar irradiation. The employed photocatalysts were either cadmium sulfide (CdS) in the presence of sodium sulfide and sulfide salts (Na₂S-Na₂SO₃) or titanium dioxide (TiO₂) in the presence of glycerol (C₃O₅(OH)₃). Using cadmium sulfide (CdS) as phototcatalyst, the possibility of producing stabilized photocatalysts was investigated with i) various solvents, ii) various precursors and iii) combined with wide band gap photocatalysts, like zinc sulphide (ZnS) or/and titanium dioxide (TiO₂), with different ratio. Results revealed that CdS preparation process significantly influences the photocatalytic effectiveness and increase the hydrogen production. Increase in hydrogen production can be achieved by coupling CdS with ZnS in a ratio of CdS/ZnS (25-75). A further increase in the rate of hydrogen production was observed by dispersion of Pt (0.5 wt %) in the surface of the compined photocatalyst, CdS/ZnS (25-75). Additionally, as experimental results indicated, increase in hydrogen production can be achieved by coupling CdS with TiO₂ (0.5% Pt-CdS/TiO₂) or alternatively by mixing CdS with TiO₂ and ZnS (0.5% Pt-CdS/ZnS/ TiO₂). In both cases, the obtained rates of hydrogen production were not much different with that obtained with the use of 0.5% Pt-CdS/ZnS (25-75). Replacing the conventional sacrificial agents (Na₂S-Na₂SO₃) with ethanol (1% v/v), the rates of hydrogen production became zero, in the case of photocatalysts 0.5%Pt-CdS/ZnS (25-70) and 0.5%Pt-CdS or significantly decreased, in the case of photocatalysts 0.5%Pt CdS/TiO₂ (50-50) and 0.5%Pt CdS/ZnS/TiO₂ ((25-75):1). Simultaneous use of sacrificial agents with 1% v/v ethanol (Na₂SO₃- Na₂S and 1% ethanol), results in successive action of the two sacrificial agents, only for the case of 0.5%Pt CdS/ZnS/TiO₂ ((25-75):1). Using titanium dioxide (TiO₂), as phototcatalyst in the presence of glycerol (C₃O₅(OH)₃), resulted both in photocatalytic splitting of water and light-induced oxidation of organic substrates into a single process, that takes place at ambient conditions in the absence of gas-phase oxygen. According to the proposed mechanism, the water is reduced to hydrogen, while the organic compound acts as electron donor and gradually oxidized towards to carbon dioxide (CO₂). Irradiation of aqueous glycerol, results in notably strong evolution of H₂, because of the additional production of hydrogen (additional hydrogen, H2, add.), which comes from the splitting of glycerol. The total amounts of H₂, add. and CO₂ are practically equal to the molar ratio H₂ ,add..:CO₂ 7:3, which corresponds in the reaction of reforming (C₃H₈O₃ + 3H₂O 3CO₂ + 7H₂). The results of experiments were conducted in order to investigate the effects of operating parameters on the rate of hydrogen production, showed that the rate of hydrogen production depends strongly on glycerol concentration and, to a lesser extent, on the kind of metal loading, on solution pH and temperature and the concentration of photocatalyst in solution. Specifically, experiments of the effectiveness of the nature of the dispersed metal on catalytic performance showed that the maximum rate of hydrogen production is obtained with the use of Pt, as efficiency of the metal doped TiO₂ is in the order of Pt > Pd > Ru > Rh > Ag. The effect of metal loading (Pt) on the reaction rate has been investigated in the range of 0.0-5.0 wt.%, and showed that the maximum rate of hydrogen production increased with increasing Pt loading from 0.05 to 0.5 wt% but decreased for load greater than 2% wt. Pt. Increase the catalyst concentration (0,5%Pt/TiO₂) in the solution in the range of 0,6 g/L until 2,66 g/L leads to progressive increase on the reaction rate, which is finally stabilized on the biggest rate of hydrogen production. The rate of hydrogen evolution is higher at neutral or basic solutions, compared to acidic solutions and increased with increasing temperature from 40oC to 60-80oC (Fig. 3). Further increase of solution temperature up to 80oC did not induce significant changes on the rate. The most important parameter, which mainly determines the rate of hydrogen production, is glycerol concentration (C) in solution. In particular, increasing the glycerol concentration (C) in solution, in the range of 10-4 until 100 mol L-1, results in a linear increase of the maximum rate of hydrogen production with logCglyc. The maximum rate increased about two orders of magnitude with increase of Cglyc from zero to 1M. Corresponding experiments of effect of glycerol, photocatalyst concentration in the suspension and % Pt loading in the surface of 0.5%Pt/TiO₂ were conducted in separate reactor. The photo-reactor includes the lamp, which simulates UV irradiation emitting at 365 nm. In all cases, the results were qualitatively similar with those obtained with the use of UV-Vis irradiation. The only difference was that with UV irradiation bigger rates of hydrogen production are obtained. Experiments designed with the purpose to define the formation of reaction’s intermediates and final products (gas or/and liquid), in glycerol solution concentration 17 mM, indicated that methanol is the main reaction intermediate, small amounts of acetic was also detected in the liquid phase. The final products were only H₂ and CO₂, which are in great agreement with those predicted from the stoichiometry of the reforming reaction. The investigation of slurries for the non-stoichiometric oxygen production, in the presence or in the absent of glycerol (concentration 0.0-1.36 mM), indicated that photogenerated oxygen is bound at the catalyst surface as peroxotitanate complexes or titanoxide, or is released in the solution as Η₂Ο₂. In all cases, the quantity of peroxide species at the catalyst’s surface tends to a marginal value (~20 μmol), which corresponds to the possibility of maximum cover of TiO₂. Qualitatively similar results were obtained with the use of glucose slurries in concentration 0.0-0.417mM. The quantum yield (QY) of hydrogen evolution was measured in the photoreactor with the UV lamp emitting at 365 nm. It is observed that quantum yield increased from 1.8%, for pure water to 70%, for glycerol solution concentration 1M. The results of my thesis can be used for waste degradation in ambient conditions with simultaneous production of hydrogen and high efficiency.

Στην παρούσα διατριβή μελετάται η διεργασία παραγωγής H₂ μέσω φωτοκαταλυτικής διάσπασης του H₂O με χρήση υδατικών αιωρημάτων είτε Cds σε συνδυασμό με Na₂S-Na₂SO₃ είτε TiO₂ με γλυκερόλη. Οι χρησιμοποιούμενες ενώσεις απατώνται σε απόβλητα Βιομηχανίας και η χρησιμοποιούμενη φωτεινή πηγή είναι λάμπα Xe, η οποία προσομοιώνει την ηλιακή ακτινοβολία. Χρησιμοποιώντας TiO₂ μελετήθηκε η δυνατότητα αύξησης του ρυθμού φωτοκαταλυτικής παραγωγής H₂ από υδατικά διαλύματα γλυκερόλης. Τα αποτελέσματα που ελήφθησαν από σειρά πειραμάτων που πραγματοποιήθηκαν μεταβάλλοντας τις λειτουργικές παραμέτρους της αντίδρασης έδειξαν ότι ο ρυθμός παραγωγής H₂ εξαρτάται κυρίως από τη συγκέντρωση της γλυκερολης και σε μικρότερο βαθμό από το είδος και τη φόρτιση του μετάλλου, το pH στη θερμοκρασία του διαλύματος και τη συγκέντρωση του καταλύτη στο αιώρημα. Ανάλογα με τις πειραματικές συνθήκες ο ρύθμος αυξάνεται έως και 2 τάξεις μεγέθους σε σύγκριση με αυτού του λαμβάνεται από το σκέτο H₂O. Η συνολική αντίδραση μπορεί να περιγράψει η φωτο-επαγόμενη αναμόρφωση της γλυκερόλης σε συνθήκες δωματίου. Η αντίδραση προχωρεί με ενδιάμεση παραγωγή μεθανόλης και οξικού οξέος, ενώ τα τελικά προϊόντα είναι μόνο H₂ και CO₂. Μέρος του φωτοπαραγόμεου O₂ που παράγεται από τη διάσπαση του H₂O παραμένει δεσμευμένο στην επιφάνεια του καταλύτη με τη μορφή H₂O₂. Στο 2ο μέρος χρησιμοποιώντας Cds τα αποτελέσματα φανέρωναν ότι η μέθοδος παρασκευής επηρεάζει σημαντικά τη φωτοκαταλυτική του ενεργότητα. Η προσθήκη ημιαγωγού μεγαλύτερου ενεργειακού χάσματος (ZnS ή/και TiO₂) βελτιώνει περαιτέρω την ενεργότητα με τρόπο που εξαρτάται από τη σύσταση του καταλύτη. Η εναπόθεση Pt οδηγεί σε σημαντική αύξηση του ρυθμού. Απαραίτητη προϋπόθεση για την πραγματοποίηση των παραπάνω πειραμάτων είναι η χρήση Na₂S-Na₂SO₃. Αντικατάσταση αυτών με άλλα οργανικά απόβλητα, μόνο στην περίπτωση του 0.5% Pt-Cds/ZnS/TiO₂, (25-75):1 οδήγησε σε διαδοχική δράση των δύο ενώσεων. Τα αποτελέσματα φανερώνουν ότι είναι δυνατή η ανάπτυξη ενός αποδοτικού φωτοκαταλυτικού συστήματος ικανού να παράγει H₂ με χρήση ηλιακής ακτινοβολίας νερού και βιομηχανικών αποβλήτων.