Photocatalytic Degradation of Some Pesticides and Polycyclic Aromatic Hydrocarbons by Metal Doped Titania Nanoparticles

Abstract

Photocatalysis is an area of immense research for finding suitable benign photocatalyst for fast and complete mineralization of environmental pollutants such as pesticides and polycyclic aromatic hydrocarbons. Undeniably, TiO2 specifically P25-TiO2 which is commercially available TiO2 have been widely studied for the same, however suffers from some disadvantages such as crystal phase transformation at high temperature, UV-light sensitivity and low percentage of mineralization of above mentioned molecules, limiting its utilization. This creates a scope to modify TiO2 and/or to find other Ti-O based materials possessing similar physicochemical properties but with improved photocatalytic activity for decomposition of aforementioned molecules. In this context, present work considers the synthesis of titania/titanate nanostructures in different sizes, shapes, crystal structures which were loaded by metals/coated by SiO2 of different shell thickness and studied for the decomposition of some pesticides and polycyclic aromatic hydrocarbons to CO2 in comparison to P25-TiO2. Titania based nanocatalysts such as sodium titanates of different morphology having superior surface properties are getting wide importance in photocatalytic research. Despite having sodium (Na) content and its high temperature synthesis (that generally deteriorate the photoactvity), these Na-titanates often exhibit better photoactivity than P25-TiO2 catalyst. Hence, chapter-3 demonstrated the influence of crystal structure, BET surface area, surface charge, zeta potential and metal loading on the photocatalytic activity of as-prepared sodium titanate nanotubes and titania nanorods. Straw like hollow orthorhombic nanotube (Na2Ti2O5·H2O) particles (diameter = 9-12 nm and length = 82-115 nm) and rice like anatase nanorod particles (diameter = 8–13 nm and length = 81-134 nm) were obtained by the hydrothermal treatment of P25-TiO2 with NaOH, which in fact, altered the net surface charge of these as-prepared nanoparticles. The observed zeta potential = -2.82 (P25-TiO2), -13.5 (nanotubes) and -22.5 mV (nanorods) are significantly altered by the Ag and Cu deposition. It has been found here that nanotube particles displayed best photocatalytic activity for imidacloprid insecticide degradation to CO2 formation under UV irradiation because of its largest surface area 176 m2g-1 among the studied catalysts. Sodium titanate nanotubes having higher photocatalytic activity than P25-TiO2 was photodeposited by Ag-nanoparticles (0.2-1.0 wt%, 3-5 nm) and studied for decomposition of imidacloprid in chapter-4. The enhanced pseudo-first order rate constants for photooxidation of v imidacloprid to CO2 were found to increase with increase in Ag-photodeposition onto nanotubes upto 0.5 wt%. The investigation for the fate of heteroatoms (N, O and Cl) present in imidacloprid showed increase in formation of inorganic ions (nitrate, nitrite and chloride) with time of photooxidation as confirmed by Ion chromatograph. Despite complete degradation of imidacloprid in 180 min, its mineralization to CO2 was 70% due to formation of some persistent heteroatom containing intermediates as revealed by time dependant GC-MS study. Based upon these results, a probable pathway for mineralization of imidacloprid and a chemical mass balanced equation in relation to theoretically expected equation after its complete mineralization is proposed and described. Preparation of crystalline monoclinic stick like sodium titanate nanorods by calciantion (at 800 oC) of sodium titanate nanotubes, having much less surface area (21 m2g-1) but higher average relaxation time of excited charge carriers to that of nanotubes and P25-TiO2, as confirmed by XRD, TEM and time resolved photoluminescence decay studies is presented in chapter-5. These nanostructures were modified by Au-photodeposition (0.5 and 2.0 wt%) and studied for photoreduction of m-dinitrobenzene and photooxidation of sulfosulfuron. The selective formation of m-diaminobenzene by photoreduction of m-dinitrobenzene was comparable to sodium titanate nanorods (89.5 ± 0.5%) and P25-TiO2 (98.2±0.8%), whereas Audeposition (0.5 and 2 wt%) notably altered the products (m-nitroaniline and m-diaminobenzene) distribution after 8 h of UV-light irradiation, confirmed by GC-MS analysis. This high photoactivity of as-prepared nanorods could be credited to better delocalization and longer relaxation lifetime (68 ìs) of photoexcited e-/h+ pairs along the length of crystalline sodium titanate nanorods than P25-TiO2 (45 ìs). The photooxidation of sulfosulfuron herbicide (1000 ppm) and corresponding CO2 formation was found to be highest with sodium titanate nanotubes due to the presence of more hydroxyl groups over the largest surface area which was found to dominates over its least relaxation lifetime (41 ìs). An understanding of the collective influence of Au-loading and calcination for the change in crystal-structure, morphology, phase composition and their photocatalytic-activity of titanatenanostructures is investigated in chapter-6. Bare and Au-loaded (Au+3, Auo and Aunanoparticles) sodium-titanate-nanotubes were calcined in air at 800 oC, where TNT having BET-surface-area 176 m2g-1 transformed into sodium-titanate-nanorods of surface-area of 21 m2g-1 while Au-loaded nanotubes led to a variety of fragmented-particles having different vi crystal-structures, surface-area (21-39 m2g-1) and sizes (50-75 nm), attributed to the strain induced thermal-decomposition of nanotubes after Au-loading. Their comparative photocatalytic- activities were evaluated by the photooxidation of insecticide imidacloprid with identification of intermediates formed during its mineralization to CO2 under UV-light irradiation, where 0.5 wt% Auo-deposited-nanotubes followed by calcination showed highest photocatalytic-activity in-contrast to other catalysts. These results are well explained in correlation with their crystal phase, surface-area, size, shape, and relaxation time of photoexcited electron-hole pairs. An important finding of the present work is delivered by chapter-7, showing complete retention (> 98%) of anatase TiO2 crystalline phase after high temperature (800 oC) treatment of rice-like TiO2 nanorods relative to 100% conversion of the rutile phase after calcination of P25- TiO2 under similar conditions. In addition to XRD, the existence of anatase phase at > 800 oC was confirmed by the presence of its characteristic vibrational bands (144, 395, 513 and 639 cm-1) in the Raman spectra. It was found that TiO2 nanorods undergo fragmentation to a highly crystalline irregular morphology (60-70 nm), nanopolygons (91-110 nm) and smaller rod-shaped particles (length = 60-110 nm and diameter = 7-12 nm), accompanied by a gradual increase in their crystallite size (from 16 to 40 nm) and decrease in surface area (from 79 to 31 m2 g-1) with increased calcination temperatures from 200 to 900 oC. This TiO2 anatase phase displayed enhanced photocatalytic oxidation rate (~2–11 times higher than rutile TiO2) for methyl parathion (a neurotoxic pesticide) degradation to various intermediate products and ultimately to CO2, whereas 1.0 wt% Au–TiO2 significantly improved the photoactivity. Preparation of SiO2 coated TiO2 particles having different thickness of silica shell and their comparative photocatalytic activity, for the photooxidation of naphthalene (20 ppm) and anthracene (20 ppm) is shown in chapter-8. The presence of SiO2 over TiO2 was demonstrated by FT-IR analysis, showing peaks corresponding to Si-O-Si (1081 cm-1) and Si-O-Ti (950 cm-1) bonds. HR-TEM analysis confirmed the presence of SiO2 in as-prepared samples of shell thickness 12-15 nm and > 50 nm, with Si amount (0.8-4.2 at.%) determined by EDS analysis. Increase in thickness of SiO2 increases the surface area of TiO2 (69-235 m2g-1) which in turn improves the adsorption of naphthalene/anthracene. However, the observed trend for the photocatalytic activity is in contrast to the trend of adsorption studies, where, as-prepared samples with highest surface area exhibited the least photocatalytic activity, while catalyst of vii least surface area (among silica coated samples) showed highest activity for degradation of naphthalene and anthracene to CO2. Despite complete degradation of naphthalene and anthracene, an incomplete mineralization occurred, ascribed to the existence of various intermediates identified by GC-MS analysis. Influence of TiO2 morphology and Au-loading (1.0 wt%) for photocatalytic activity under UV- and Sun-light is delivered in chapter-9. The phase composition obtained by XRD study of as-prepared TiO2 nanowires (anatase = 72%, rutile = 28%) was found to be almost similar to commercially available P25-TiO2 (anatase = 70%, rutile = 30%). SEM analysis revealed the presence of elongated nanostructures corresponding to length = 1.9-8.0 ìm and diameter = 80-270 nm for bare sample. The TEM analysis for Au-loaded TNW showed uniform dispersion of Au nanoparticles (3-6 nm) over elongated nanostructures having dimension in agreement with the SEM analysis. Time resolved analysis revealed that by changing the shape from nearly spherical (P25-TiO2) to lengthy nanostructures, the recombination time of e-/h+ pair increases from 45 to 64 ìs and was 79 ìs after Au-loading on the later. The photocatalytic activity of bare and Au-loaded TiO2 nanowires under UV- and Sun-light was assessed by decomposition of propiconazole to CO2, where Au-loaded TNW always exhibited higher activity than P25-TiO2, and various intermediates after its complete degradation were identified by GCMS analysis.