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Title: Development of enhanced photocatalysts for the degradation of pollutants
Other Titles: Ontwikkeling van verbeterde fotokatalysatoren voor de afbraak van vervuilende stoffen
Authors: Cani, Damiano
Issue Date: 18-Oct-2016
Abstract: The use of electromagnetic radiation as environmental friendly energy source to promote relevant chemical reactions is widely investigated but nowadays, there is still lack of materials able to efficiently perform this process. Those reactions that can be initiated or which reaction rate can be increased by radiation are known as photocatalytic. Among the possible reactions, the degradation of pollutants is one of the most studied applications. This process is important in the scenario of an increasingly polluted environment where there is scarcity of clean water sources. The central topic of this Ph.D. thesis is the study and development of new materials which can efficiently degrade pollutants in water under UV or visible irradiation.
Metal oxides have been chosen as class of materials of interested because of their general stability, insolubility in water and non-toxicity. TiO2 is widely studied as photocatalyst and shows good activity under UV irradiation especially when synthesized in the nano – regime. Nonetheless, nanoparticles also bring about disadvantages: they are difficult to recover at the end of the reaction and suffer of agglomeration problems when dispersed in a solvent. Using TiO2 as model, active photocatalysts in which these two issues are avoided, were produced with innovative approaches. Depending on the pH at which the photocatalytic test is performed, the aggregation properties of nanoparticles can be modified. In particular, in the case of titanium dioxide its point of zero charge is found to be at around 6 meaning that when TiO2 nanoparticles are dispersed in a solution which pH is lower than 6, their surface would be positively charged and negatively charge if the pH is above 6. A charged surface would be beneficial to reduce agglomeration that would be increased without surface charges.
Tackling the issue of recovery and reuse of TiO2 nanoparticles, a series of materials where the TiO2 active phase is dispersed in macroscopic SiO2 spherical beads was developed. The size of the beads was chosen based on the availability of commercial resins (i.e. Amberlyte IRA-900 Cl) and with the aim to produce a macroscopic photocatalyst. The materials consist of nanosized TiO2 regions dispersed into a silica matrix in the shape of macroscopic spherical bead. The main expected advantage of such a material consisted in the synthesis of photocatalysts easy to recover from the solution at the end of the reaction. The materials display the advantages of TiO2 in the nano – regime and the macroscopic size of the photocatalysts allows an easy separation and reuse. The newly synthesized materials show favorable features when compared to the benchmark photocatalyst P25 TiO2: along with the increased reusability, the increased adsorption performances achieved through the high surface area, render the bead photocatalyst more active in terms of turn over number than P25 TiO2 in the degradation of pollutants under UV irradiation.
The agglomeration of TiO2 nanoparticles was avoided producing a series of TiO2-SiO2 composites where the nanoparticles are embedded at the surface of a silicon dioxide support. The materials were produced through an innovative synthetic methodology which involves the use of activated carbon as sacrificial template. The TiO2 nanoparticles and then the silica source are first adsorbed in the template which is then removed by means of calcination to produce the desired porous TiO2-SiO2 materials. TiO2 nanoparticles will be embedded at the surface of the silicon dioxide support; doing so, the photoactive phase is stable, does not suffer of agglomeration and it is easily accessible to the reagents. These features lead to a steep increase in the photocatalytic activity of the composite materials when compared to benchmark P25 TiO2. Moreover, the photocatalysts are easy to recover and can be reused in consecutive runs. Spontaneous incorporation of carbon and nitrogen in TiO2 during the synthesis generated inter-band gap states that allow the exploitation of visible light for the photocatalytic reaction.
Carbon and nitrogen doped TiO2 materials are well known for their activity under visible illumination but they generally suffer from limited efficiency in the utilization of visible light. The insertion of doping elements generates recombination centers detrimental for the photocatalytic performances. Moreover, the low concentration of doping elements doesn’t permit to excite all the TiO2 species present in the material. New materials which can absorb visible light to perform a photocatalytic process are required to overcome the limitations shown by doped TiO2 systems. For this reason, a series of solid solutions with different relative ratios of Fe2O3 and In2O3 was synthesized and tested for the photocatalytic degradation of pollutants. The choice of In2O3 might seem surprising given it scarcity but the idea of the study was to generate a proof of concept for the possibility to develop solid solutions as active photocatalyst. Indium oxide and iron oxide could efficiently mixed in various proportions permitting to synthesize a combination of solid solutions with different optical and crystalline properties. Noble metals like platinum, gold and palladium are widely employed in photocatalysis in water splitting and CO2 reduction and more rarely in the degradation of organic pollutants. Indium oxide is more abundant than the aforementioned metals and it is scarcely employed as photocatalyst. Nonetheless, its ability to mix with iron oxide renders this material an interesting candidate for the study performed. Mixing two oxides with similar crystal structures, one with wide band gap and one with narrow band gap, is expected to produce a new material with optical properties in between the one of the pristine oxides. The band gap energy of the solid solutions will then be tuned by variations in the relative ratios of the pristine oxides. The new photocatalysts show similar activities for the degradation of pollutants both under UV and visible irradiation suggesting that the control of the band gap energy upon the formation of a solid solution was achieved. Moreover, the photocatalytic activity of the solid solutions for the degradation of phenol under visible light is higher than the one shown by benchmark P25 TiO2 and the pristine oxides.
Band gap engineering can be achieved in numerous ways and the formation of solid solutions is only one of the many. Alternatively, the size quantization effect can be exploited. Size quantization consists in the increase of the band gap energy when the particles size is reduced below the Bohr radius of a material. A wider band gap translates in the formation of highly energetic species upon excitation by radiation. Nonetheless, the increased band gap will be detrimental for the absorption of radiation: if the band gap is too wide, the material cannot properly exploit sunlight. Only those materials which band gap is small will show an increased photocatalytic activity once their size is decreased below the Bohr radius in the size quantization range. If the same approach is taken into consideration for TiO2 (for example) its already wide band gap will become even wider rendering the material less efficient for harvesting low energy radiation. The use of size quantization to increase the photocatalytic activity of a material, was tested in the case of WO3. The bulk oxide has a band gap of 2.6 eV that can be excited by visible light but its conduction band edge is not at a suitable energy level to achieve the photocatalytic degradation of pollutants. Because of size quantization, the material will be excited only by UV radiation but is expected to being able to promote the photocatalytic processes needed to degrade pollutants. The photocatalyst was synthesized by the growth of WO3 nanoparticles into the pores of a mesoporous SiO2 support. The ordered array of pores acts as template to control the growth of the nanoparticles ensuring the formation of species small enough to display size quantization. The materials were also tested as heterogeneous catalysts for the epoxidation of cyclooctene with hydrogen peroxide.
In the field of photocatalysis, not only the synthesis of active materials is an important topic but also the development of procedures to test and compare the materials is needed. When the degradation of pollutants with high surface area photocatalysts is chosen as test, the residual amount of probe compound at the end of the illumination period is correlated to the photocatalytic activity. All the parameters which influence this value have to be taken into consideration to properly assess the activity. In particular, the importance of the adsorption step in the dark was highlighted in the case of a high surface area photocatalyst, using the commercial MOF Basolite F300  as model. The model used allowed to develop a systematic experimental approach to evaluate the photocatalytic properties of high surface area photocatalysts. A series of experiments with the MOF Basolite F300  as photocatalyst and different substrates with complementary properties (size and charge) were performed. The MOF was able to degrade the substrates under visible light irradiation but the study of the adsorption properties employing tests in the dark (i.e. without radiation) was found to be of fundamental importance to effectively assess the photocatalytic activity of the MOF. An experimental approach in which the adsorption capacity of the photocatalyst is carefully investigated performing tests in the dark with different duration is propose as systematic methodology to study the photocatalytic activity of high surface area materials. Moreover, the results indicated that the MOF Basolite F300  is an active photocatalyst for the degradation of pollutants under visible light irradiation.
Description: -
Table of Contents: Table of Contents
CHAPTER 1
1.1 ELECTROMAGNETIC SPECTRUM AND SOLAR RADIATION 2
1.2 SEMICONDUCTORS AND THEIR INTERACTION WITH RADIATION 3
1.3 PHOTOCATALYSIS 5
1.4 PHOTOCATALYTIC REACTIONS AND MATERIALS 7
1.5 ADVANTAGES AND DISADVANTAGES IN THE USE OF NANOPARTICLES IN PHOTOCATALYSIS 8
1.6 DEGRADATION OF ORGANIC POLLUTANTS VIA PHOTOCATALYTIC PROCESSES 9
1.7 TIO2 AND FE2O3 AS CASE STUDY 10
1.8 POINTS OF CONCERN 13
1.9 THESIS OUTLINE 13
CHAPTER 2 18
2.1. INTRODUCTION 19
2.2 . EXPERIMENTAL SECTION 20
2.2.1 Synthesis of the TiO2-SiO2 porous beads 20
2.2.2 Characterization 20
2.2.3 Photocatalytic tests 21
2.3. RESULTS AND DISCUSSION 22
2.3.1 Synthesis and characterization of the TiO2-SiO2 porous beads 22
2.3.2 Photocatalytic tests 26
2.4. CONCLUSIONS 32
CHAPTER 3 39
3.1. INTRODUCTION 40
3.2. EXPERIMENTAL SECTION 42
3.2.1 Materials 42
3.2.2 Synthesis procedures 43
3.2.3 Characterization 43
3.2.4 Photocatalytic tests 44
3.3. RESULTS AND DISCUSSION 45
3.3.1 Synthesis and characterization of the TiO2NP@SiO2 composites. 45
3.3.2 Photocatalytic performances of the TiO2NP@SiO2 composites. 53
3.4 CONCLUSIONS 60
CHAPTER 4 68
4.1 INTRODUCTION 69
4.2 EXPERIMENTAL SECTION 71
4.2.1 Synthesis of the solid solutions 71
4.2.2 Characterization 72
4.2.3 Photocatalytic tests 72
4.3 RESULTS AND DISCUSSION 73
4.3.1 Characterization of the solid solutions 73
4.3.2 Photocatalytic activity 77
4.4 CONCLUSION 81
CHAPTER 5 88
5.1 INTRODUCTION 89
5.2 EXPERIMENTAL SECTION 91
5.2.1 Materials 91
5.2.2 Synthesis of the X%WO3-SBA-15 91
5.2.3 Characterization of the X%WO3-SBA-15 92
5.2.4 Catalytic activity 92
5.3 RESULTS AND DISCUSSION 93
5.3.1 Synthesis and characterization of WO3-SBA-15 93
5.3.2 Photocatalytic degradation of phenol 99
5.3.3 Epoxidation of alkenes 102
5.4 CONCLUSIONS 107
CHAPTER 6 113
6.1 INTRODUCTION 114
6.2 EXPERIMENTAL SECTION 117
6.2.1 Materials 117
6.2.2 Characterization of Basolite F300® 117
6.2.3 Photocatalytic tests 117
6.3. RESULTS AND DISCUSSION 118
6.3.1 Characterization of Basolite F300® 119
6.3.2 Photocatalytic tests with BasoliteF300® as photocatalyst 122
6.4. CONCLUSIONS 129
CHAPTER 7 134
7.1 CONCLUSIONS 135
7.2 FUTURE PERSPECTIVES 137
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Centre for Surface Chemistry and Catalysis

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