To - Catalyst Program, for letting me be an official member of the sponsored research on modified TiO2 synthesis and methyl orange and phenol photocatalytic degradation at Hanoi Univers
Necessity of the study
Soil and groundwater pollution is a serious national concern, driven by uncoordinated growth of economic zones that contaminate water sources with heavy metals and persistent organic pollutants (POPs), including phenol and its derivatives The primary sources of these phenolic contaminants are the manufacture of synthetic plastics, insecticides, paints, and petroleum products.
The textile industry releases significant amounts of hazardous chemicals into the atmosphere, including azo dyes such as methyl orange As a result, remediation of environments contaminated with phenol and methyl orange has emerged as a global priority, attracting attention from researchers, policymakers, and industry stakeholders worldwide.
Historically, water remediation has relied mainly on physicochemical and biological treatment approaches Among these, adsorption is widely used for removing chemical contaminants from water because of its ease of operation and the availability of diverse adsorbents Biological treatment can remove up to about 90% of organic pollutants, but it is less effective for recalcitrant compounds such as phenol and methyl orange To tackle these challenging contaminants, extensive research has explored methods including electrochemical approaches, ion exchange, ozonation, and adsorption on activated carbon.
On the other hand, these approaches are rarely used in practice due to inherent constraints, including heavy equipment requirements, complex operating techniques, high capital and operating costs, and potential process abnormalities; moreover, a sludge post-treatment step is essential, or efficiency will remain poor.
Photocatalysis offers an environmentally friendly approach to treating polluted water by using natural solar energy to degrade organic contaminants that are difficult to break down, all without adding extra chemicals or generating sludge in the system Among the semiconductor materials studied as photocatalysts, TiO2, ZnO, and Fe2O3 are prominent, with TiO2 being the most researched due to its outstanding properties TiO2 is environmentally safe, chemically and biologically inert, self-cleaning, and produces minimal byproducts during production.
TiO2 nanoparticles have been central to the photodegradation of organic pollutants, favored as a cheap, photostable, abundant photocatalyst with strong oxidizing power against a wide range of contaminants Yet its use is hampered by a large band gap of about 3.2 eV, difficulty separating TiO2 from solution, and rapid recombination of photogenerated electron–hole pairs, which curtails photocatalytic efficiency While TiO2 can decompose numerous organic, inorganic, and toxic compounds in both liquid and gas phases, the 3.2 eV band gap confines activity to UV light (around 387 nm), representing only about 4% of solar radiation To boost photocatalytic activity, researchers have pursued multiple strategies, including chemical treatments such as doping with non-metals, transition metals, dye sensitization, spatial structuring, and rare earth metal doping, as well as alternative approaches like microwave or ultrasonic irradiation of TiO2 systems Supported on substrates like activated carbon, TiO2 can be evaluated for its drawbacks and potential improvements in photocatalysis.
This discovery holds significant promise due to the synergistic combination of the photocatalytic activity of the catalyst and the adsorption capacity of activated carbon Commercial activated carbon is well established as an effective adsorbent for removing organic pollutants from the liquid phase, yet its use remains limited by prohibitive costs Activated carbon can also be produced from waste materials—agricultural by-products, the wood industry, and non-conventional municipal and industrial waste items—offering a potential route to minimize waste disposal impacts while providing a viable source of adsorption material Utilizing waste-derived activated carbon aligns with environmental and economic benefits by turning waste into value-added adsorbents When TiO2 is used, a major challenge is separating the powder catalyst from high-concentration effluents, which can lead to catalyst coagulation and the formation of aggregates, reducing process efficiency.
Activated carbon-TiO2 composites leverage activated carbon’s high porosity, large surface area, and room-temperature stability to boost photocatalytic performance, while remaining easy to recover from the reaction mixture Although other supports such as clays, zeolite, silica, alumina, and glass have been tested, they offered little additional photocatalytic benefit, underscoring a synergistic advantage of activated carbon with TiO2 Under certain conditions, interaction between TiO2 and specific pollutants can cause coagulation that blocks UV or solar irradiation from reaching the catalyst’s active core and reduces photocatalytic activity due to decreased surface area; however, activated carbon on the catalyst surface serves as an efficient adsorption trap, promoting mass transfer of pollutants to the photoreactive site The higher adsorption of substrates onto activated carbon is a key factor contributing to enhanced photocatalytic pollutant elimination.
To extend the TiO2 photocatalyst’s absorption into the visible-light region, which accounts for about 45 percent of solar energy, metal or nonmetal modifications are incorporated into the TiO2 structure This approach leverages the renewable and abundant nature of solar energy, highlighting the goal of broadening TiO2’s absorption spectrum Recently, graphene oxide has gained attention for enhancing TiO2 photocatalysts under visible light, due to its advantages in improving catalytic performance in visible-light conditions [21–25].
Objectives of the study
This study aims to develop TiO2-based catalysts modified with activated carbon and graphene oxide, coated on diverse substrates to enhance the degradation of organic pollutants in wastewater Target pollutants include methyl orange and phenol, two harmful substances prevalent in textile and other industrial effluents in Vietnam and around the world The TiO2/activated carbon/graphene oxide composites improve adsorption, charge separation, and photocatalytic efficiency under light irradiation, enabling more effective wastewater treatment By evaluating performance across various support materials, the work seeks to enable scalable, sustainable wastewater management for Vietnam and global applications.
Key objectives include investigating process parameters in catalyst synthesis to optimize performance, identifying the optimum catalysts for each synthesis method, fabricating catalyst thin films on diverse substrates for efficient degradation of methyl orange, and modifying the catalysts to deliver strong activity under full-spectrum light conditions.
Content of the thesis
Firstly, literature review on previous studies will be investigated to select the preparation methods of the catalysts, materials to modify catalyst, coating techniques and model pollutants to conduct research
TiO2 photocatalysts were synthesized using sol-gel, co-precipitation, and hydrothermal methods, and subsequently modified with activated carbon, graphene oxide, and silica gel The resulting catalysts were characterized by physical adsorption, scanning electron microscopy (SEM), X-ray diffraction (XRD), and UV-Vis spectroscopy to assess their textural properties, crystalline structure, and optical behavior.
The catalytic activities of these catalysts were conducted for methyl orange and phenol, one stable organic compound, in UV-C and full range light condition
The main process parameters in phenol photodegradation of the optimum catalysts were evaluated and do kinetics study this process.
Methodologies of the study
Literature review summarizes prior research on catalyst composites formed from activated carbon and graphene oxide, detailing preparation and coating methods and their performance in the photodegradation of pollutants such as methyl orange and phenol In the experimental section, catalysts were prepared by sol-gel, co-precipitation, and hydrothermal methods, and then characterized by BET surface area analysis, scanning electron microscopy (SEM), and X-ray diffraction (XRD); their photodegradation performance was evaluated in dedicated reactor systems using UV-Vis spectroscopy and high-performance liquid chromatography (HPLC) to quantify degradation efficiency.
Data analysis and processing: the method is used to gather and determine the concentration of methyl orange and phenol based on the calibration curve of these substances.
Scope of the study
Organic pollutants: Methyl orange and phenol were chosen to evaluate the catalyst performance since they are popular pollutants in wastewater
Catalyst thin films: Catalyst thin films made by dip coating and CVD methods on various substrates as cordierite, glass and aluminum are studied.
Scientific and practical meanings
This thesis provides a scientific basis for synthesizing TiO2-based photocatalysts under laboratory conditions and evaluating their ability to photodegrade methyl orange and phenol, two representative aromatic pollutants Since methyl orange and phenol are popular yet challenging to degrade, a TiO2 catalyst that shows high degradation efficiency against these compounds is likely to be effective for photodegrading other persistent contaminants The work underscores the potential of TiO2 photocatalysis as an environmentally friendly approach to pollutant removal and offers insights for optimizing catalyst performance and extending applications to a broader range of aromatic pollutants.
Thin-film catalysts were synthesized by dip coating and chemical vapor deposition (CVD) The fabrication parameters and methods for these thin films were explored, revealing approaches that can be applied to industrial wastewater treatment to enhance catalytic performance.
Novelty of the study
The main innovations of this research include:
1 Process parameter optimization for catalysts synthesized via co-precipitation, hydrothermal, and sol-gel methods
2 Catalytic film formation optimization on various substrates using CVD (chemical vapor deposition) and dip coating
3 Research on the modification of catalysts synthesized by sol-gel and hydrothermal methods on activated carbon and graphene oxide carriers appliedd in the treatment of methyl orange and phenol
This thesis comprises four chapters Chapter 1 reviews the literature on methyl orange (MO) and phenol contamination and surveys methods for preparing titanium dioxide (TiO2) to enhance its photocatalytic degradation of MO and phenol Chapter 2 details the synthesis methods for preparing the various catalysts, explains the basic physico-chemical principles involved, and describes the experimental setup used in the study Chapter 3 evaluates the properties of the prepared catalysts and analyzes how different synthesis routes affect their performance in the photodegradation of methyl orange and phenol.
Finally, the fourth chapter summarizes the main points of the thesis and gives some recommendations for future works.
LITERATURE REVIEW
Textile industry and Methyl Orange dye
Vietnam's textiles and garment industry has long been a cornerstone of the economy, employing more than 3 million people across over 7,000 factories nationwide The sector's heavy reliance on water and resultant wastewater make it essential for stakeholders to understand the water-related threats it faces, their impacts, and the practical approaches available to address these challenges, as noted in [26].
In the textile and dyeing industry, wastewater is generated across sizing, cooking, bleaching, dyeing, and finishing, with the majority arising from the washing steps after each cycle Water use is high and varies by product, but expert analysis shows that about 72.3% of total water consumption occurs during manufacturing, mainly in the dyeing and finishing stages Rough estimates indicate a water requirement of roughly 12–65 liters per meter of fabric, accompanied by an effluent discharge of about 10–40 liters Water contamination stands as the most significant environmental challenge facing the textile sector, and the dyeing sector is often deemed the most polluting when both wastewater volume and pollutant types are considered.
The primary contaminants in textile dyeing wastewater include persistent organic chemicals, dyes, surfactants, organic halogen compounds, neutral salts that enhance the total solids content, and temperature Due to the high alkalinity, the effluent pH is also high Among these, dyes are the most complex to process, particularly azo dyes, which account for 60-70 percent of the dye industry [29-32] During the dyeing process, the pigments in the dyes do not normally attach themselves to the fibers of the cloth; nonetheless, a certain quantity of the pigments still stays in the wastewater There may be as much as fifty percent of the original quantity of color left in the material after it has been dyed [29-30] Because of this, the wastewater that is produced from the textile dyeing process has a strong color and a significant concentration of contaminants
Methyl orange, often known as MO, is an anionic azo dye that has found widespread use in a variety of different sectors, including those dealing with textiles, printing, paper, pharmaceuticals, food, photography, and leather Methyl orange and the various compounds that come from it are responsible for significant amounts of pollution that are released into the environment It has been shown that this coloring agent may cause cancer as well as genetic mutations [33] In addition to being a dye that is soluble in water, methyl orange is characterized by a high degree of stability as well as unique color qualities This compound has an orange appearance when it is in a basic medium, but it has a red appearance when it is in an acidic media It was discovered that the reductive breakage of the azo bond (–N=N–) by the azo reductase enzyme that is present in liver creates aromatic amines, and that these aromatic amines may potentially contribute to intestinal cancer if they are taken by human humans [34]
Fig 1.1: Chemical structure of MO molecule [33,34]
Methyl orange (C14H14N3SO3Na) served as the model pollutant in this investigation, representing azo-dye contamination common in industry for its color stability About 70% of today's dyes are azo compounds—synthetic aromatic dyes that confer durability in use It is estimated that 10–15% of the dye used in textile production is wasted and discharged as effluent, a form of non-aesthetic pollution because concentrations in water sources are typically below 1 ppm While this discharge is the primary driver of methyl orange–related degradation concerns, dye wastewater can also produce harmful byproducts through oxidation and hydrolysis The stability of azo-dyes, driven by their aromatic structure, makes biological treatment challenging, with some processes only decolorizing effluents rather than achieving full degradation.
Phenol in industry and its impact to the health
Phenol (C6H6OH) was first identified in 1834 during coal distillation and was initially called carbolic acid because coal distillation was the primary source of phenol before the rise of the petrochemical industry Today, a variety of chemical processes can produce phenol, and a substantial amount of phenol finds its way into industrial wastewater from steel plants Pure phenol is colorless or white and forms solid crystals that can persist in air for long periods; partial oxidation may impart a pink hue and it can decompose when exposed to water vapor The odor threshold for phenol is about 0.04 ppm, at which point its smell is mildly to strongly pungent Phenol plays a major role in industry as the raw material for plastics, chemical fibers, agricultural pharmaceuticals, antiseptics, fungicides, pharmaceuticals, dyes, and explosives, and serves as a feedstock for many other plastic-related products.
Phenol exposure can occur through inhalation or contact with the skin, eyes, and mucous membranes, and ingestion of high-phenol-content substances can be fatal, causing seizures, loss of coordination, coma, respiratory failure, and a drop in blood pressure Ingested phenol is highly toxic, with initial liver involvement followed by possible heart damage; some studies report muscle pain and liver enlargement after prolonged exposure Skin contact can cause burns and arrhythmias The permitted level of phenol in the body is limited to 0.6 mg per kg of body weight While data on low-dose exposure are limited, many scientists warn that chronic phenol exposure may retard growth, cause heritable changes in offspring, and raise the risk of premature birth in pregnant women.
Phenolic compounds are among the most widely used chemical substances in manufacturing, yet they are toxic pollutants that can harm both ecosystems and humans if not properly treated before release The wastewater from the Formosa Ha Tinh factory in Vietnam has discharged high levels of phenol into the marine environment, triggering mass fish kills along the central coastal provinces This highlights the urgent need for comprehensive treatment measures to prevent environmental damage Wastewater often contains high concentrations of chemical contaminants, especially long-lasting organic compounds like phenol and its derivatives, making phenol degradation a matter of global significance, not only for Vietnam but for the world.
Titanium Dioxide, also known as TiO2 or titania, is celebrated for the stability of its chemical structure, biocompatibility, and its standout physical, optical, and electrical properties It is a multipurpose material used in products from pigments for paint and sunscreen formulations to electrochemical electrodes, capacitors, solar cells, and even as a food coloring additive in toothpaste Developed and applied over the past decades with the goal of removing harmful compounds from air and water, TiO2 can help reduce pollutants in the air, including volatile organic compounds; under sunlight, its photocatalytic activity can break down hazardous molecules.
Titanium dioxide (TiO2) occurs in three crystal forms: rutile (stable, tetragonal), and the metastable anatase (tetragonal) and brookite (orthorhombic) phases, both capable of transforming into rutile at temperatures outside their normal ranges Compared with anatase, rutile exhibits a higher recombination rate of excess charge carriers and participates in the charge transfer between the catalyst and reactants, which helps explain why rutile is commonly used in paints while anatase—though it recombines more slowly—offers stronger charge-transfer performance Anatase, with a band gap of about 3.2 eV, is often regarded as the most active TiO2 phase due to its favorable band positions and high surface area, whereas rutile has a band gap near 3.0 eV and dominates many pigment applications The wavelengths associated with the anatase and rutile phases are approximately 388 nm and 410 nm, respectively.
TiO2 serves as a photo-activated semiconductor that can generate a reactive redox environment under light, enabling the destruction of both organic and inorganic contaminants Table 1.1 summarizes the overarching steps of the photocatalytic reaction on irradiated TiO2, from light absorption and charge-carrier generation to reactive species formation and pollutant mineralization, highlighting how excited TiO2 drives oxidative and reductive pathways to degrade contaminants.
Photodegradation of pollutants by TiO2 begins when UV light is absorbed by TiO2 particles, which have a band gap of about 3.2 eV for anatase and 3.0 eV for rutile This absorption generates electron-hole pairs: holes in the valence band and electrons in the conduction band of the semiconductor Although both anatase and rutile TiO2 can absorb UV radiation, rutile may also absorb photons closer to the visible light range, whereas anatase exhibits higher photocatalytic activity because its conduction-band position provides stronger reducing power The absorbed energy and the kinetic energy of the recombining electron-hole pairs drive redox processes, as described in the relevant equations and tables Electron donors and acceptors adsorbed on the semiconductor surface, or even species in the surrounding double layer, participate in these redox reactions when an electron or hole with sufficient energy crosses the double layer.
Table 1.1 The General Mechanism of the Photocatalytic Reaction Process on TiO 2 [49]
TiO2 + hv → TiO2 -+ OH - (or TiO2 +)
(semiconductor valence band hole and conduction band electron) Electron removal from the conduction band
TiO2 - + H2O2 + H + → TiO2 + H2O+ OH - TiO2 -+ 2H + → TiO2 - + H2
Nonproductive radical reactions TiO2 - + OH - + H + → TiO2 + H2O
Fig 1.3: The mechanism of photocatalytic activity of TiO 2 [50]
At the semiconductor–liquid interface, the solid side establishes an electric field that promotes spatial separation of photoexcited electron–hole pairs, preventing their recombination As a result, holes migrate to the illuminated regions of TiO2 while electrons move toward the unilluminated areas on the particle surface, achieving charge separation on the TiO2 The avoidance of recombination and the resulting hole trapping lead to the formation of highly reactive, short‑lived hydroxyl radicals (OH•), which is widely regarded as the initial step in photocatalytic degradation OH• can form either from the oxidation of adsorbed pollutant molecules under UV radiation or from a highly hydroxylated semiconductor surface, and both pathways may operate simultaneously in some cases This process follows the reduction of adsorbed oxygen species, which can originate from dissolved O2 or other electron acceptors available in the aqueous system.
During this study, reactive oxygen species generated by TiO2-driven photocatalysis attack the organic constituents in polluted water Methylene orange (MO) and phenol are used as model pollutants to evaluate TiO2 synthesized by sol–gel and other methods for its role in wastewater treatment TiO2 can be employed in photocatalytic processes either as suspended particles in aqueous media or immobilized on support materials Common supports include quartz sand, glass, activated carbon, zeolites, and noble metals Relevant reactor designs include fluidized-bed and fixed-bed configurations Matthews and McEvoy found in 1992 that photocatalytic reactors with immobilized photocatalysts had lower efficiency than those employing dispersed titania particles.
Lower efficiencies observed with immobilized photocatalysts can be attributed to two main factors: a reduced number of activated sites within a fixed photoactivated volume when the catalyst is immobilized compared with the same mass of suspended catalyst, and potential mass transfer limitations that become rate-controlling at low flow rates Under intense illumination, mass transport may struggle to keep up with the surface reactions, making the overall process mass-transfer limited; in such cases, increasing photon intensity does not noticeably increase the reaction rate.
Akpan and Hameed investigated how operational settings influence the photocatalytic degradation of textile colors using TiO2-based photocatalysts The research also notes that a variety of processes are involved in manufacturing TiO2-based photocatalysts Among these, the Sol–Gel process is particularly popular because it enables the production of nanometer-sized crystalline TiO2 powder with high purity at low temperatures.
1.4 Principles of Precipitation, sol-gel and hydrothermal synthesis methods
In this synthesis, precursors are dissolved in water to form a homogeneous solution, and a precipitant is then added to generate the solid precipitate The solids are separated from the solution, washed to remove contaminants, dried in an oven, and calcined at a high temperature to yield the final material This method enables atomic-scale diffusion of reactants, enhancing their contact and potentially boosting reaction efficiency A major drawback is that the desired elemental ratio in the product cannot be guaranteed with this approach.
The sol-gel method is a powerful approach for making catalytic supports, beginning with metal alkoxides that are hydrolyzed in an organic medium and then polymerized by condensation of hydroxyl and/or alkoxy groups As polymerization and cross-linking advance, the system solidifies into a gel, and calcination yields an oxide whose porosity, surface area, pore volume, and pore-size distribution are strongly influenced by the size and branching of the inorganic polymer and the degree of cross-linking Gels with highly branched, cross-linked polymer networks tend to generate materials with extensive voids, high stiffness, and oxides rich in macropores and mesopores, while gels with limited branching or cross-links produce fewer void regions, weaker structures, and calcination-induced shrinkage that favors micropores and smaller surface area Nevertheless, the sol-gel process allows nano-materials to be synthesized by assembling reactants at the atomic level, enabling controlled textural properties essential for catalytic performance.
The following are some of the many benefits that come with using the sol-gel process [53]:
(1) High-purity materials may be created by using synthetic chemicals rather than minerals, which makes this process possible
By using liquid solutions instead of traditional methods to combine raw components, this approach enables faster and more thorough blending The liquids’ low viscosity allows homogenization to occur rapidly, achieving a uniform mix at the molecular level.
Uniformly mixed precursors in the solution create molecular-level homogeneity in the resulting gel, ensuring a straightforward chemical reaction during gel formation This well-distributed composition allows the reaction to proceed efficiently at a low temperature when the gel is heated, yielding consistent gel properties and low-energy processing.
(4) It is possible to change the physical features of the material, such as the pore size distribution and the pore volume
(5) It is possible to include many different components into a single process step
(6) It is possible to produce a variety of various samples in their physical shapes
Principles of Precipitation, sol-gel and hydrothermal synthesis methods 13 1 Preparation of photocatalyst using sol-gel method
Dissolve the precursors in water to form a homogeneous solution, then add the precipitant to precipitate solids, completing the preparation of the homogeneous solution The solids are extracted from the solution, washed to remove contaminants, dried in an oven, and calcined at high temperature to yield the materials This approach enables diffusion of reactants at the atomic scale, increasing contact between reactants and promoting the reaction A major drawback is that the exact desired elemental ratio in the product cannot be guaranteed using this method [51].
The sol-gel method has emerged as a versatile route for making catalytic supports, starting from metal alkoxides that are hydrolyzed in an organic medium by water and then undergo polymerization through condensation of hydroxyl and alkoxy groups As polymerization and cross-linking progress, the system transitions to a solid gel whose porosity, surface area, pore volume, pore-size distribution, and the thermal stability of the calcined oxide are strongly controlled by the size and branching of the inorganic polymer and the degree of cross-linking Gels with highly branched, highly cross-linked polymer networks tend to form extensive voids and become rigid, giving calcined oxides with mostly macropores and mesopores; conversely, gels with limited branching or lower cross-link density exhibit fewer voids, are mechanically weaker, and can collapse during calcination, yielding oxides dominated by micropores and smaller surface areas Nevertheless, the sol-gel process enables nanomaterial synthesis by assembling reactants at the atomic level.
The following are some of the many benefits that come with using the sol-gel process [53]:
(1) High-purity materials may be created by using synthetic chemicals rather than minerals, which makes this process possible
Using liquid solutions replaces traditional methods of combining raw components, enabling rapid homogenization Because the liquids involved have low viscosity, homogenization can be completed in a very short time at the molecular level.
Thoroughly mixed precursors in the solution yield a uniform molecular-level distribution as the gel forms, ensuring the chemical reaction proceeds smoothly This homogeneity enables the reaction to occur at a lower temperature when the gel is heated.
(4) It is possible to change the physical features of the material, such as the pore size distribution and the pore volume
(5) It is possible to include many different components into a single process step
(6) It is possible to produce a variety of various samples in their physical shapes
Hydrothermal processing offers an unconventional route to produce nanocrystalline inorganic materials with tunable properties This approach relies on a direct precursor–product correlation, enabling the synthesis of nearly any material without the need for additional structure-directing agents As a result, crystallinity, particle size, and morphology can be tailored within a streamlined, flexible pathway, broadening the range of materials accessible through hydrothermal methods.
During hydrothermal synthesis, the precursor continuously dissolves in the hydrothermal fluid as the system is held at conditions conducive to the reaction, such as roughly 300°C and about 1 kilobar of water pressure Even when aluminosilicate materials are used, gel formation is not observed at any stage of the process This absence of gels is due to the hydrolysis of larger molecular units driven by the elevated temperature and pressure.
Under autogeneous pressure conditions well below the critical point in an aqueous solution, different dissolution states may exist, including not only the basic structural building units but also colloidal states; this occurs because the critical point is the pressure at which autogeneous pressure becomes critical.
During high-pressure hydrothermal synthesis, units larger than those stable in true solutions are not durable under these conditions, so an initial chemical breakdown targets any present macromolecular species These macromolecular units may appear as a colloidal solution, as precipitated colloids that are crystalline or partially crystalline (gel-like), glassy, or amorphous, or as solid-state precursor materials Consequently, a true solution is expected to emerge, where the smallest feasible structural building blocks, together with their hydration shells around cations, are transported.
1.4.1 Preparation of photocatalyst using sol-gel method
According to O Carp [55], titanium dioxide (TiO2) can be manufactured as powder, crystals, or thin films, a versatility that expands its applications Crystallites can range from a few nanometers to several micrometers in size and may be used to produce powders, films, or both Notably, nanosized crystallites tend to aggregate into larger structures, a tendency that affects material behavior and processing strategies.
Deagglomeration is often required to obtain nanoparticles that are distinctly separate, and many cutting-edge synthesis methods can yield such particles without an additional deagglomeration step Nano-TiO2 is produced via the hydrolysis of titanium precursors followed by steps such as annealing, flame synthesis, hydrothermal processing, and sol-gel methods The sol-gel approach is widely used because it enables nanoparticle synthesis under ambient conditions—room temperature and atmospheric pressure—with a relatively simple setup Sol-gel offers advantages over other preparation techniques in terms of purity, homogeneity, and the flexibility to introduce dopants at high concentrations for precise stoichiometry control, as well as ease of processing and composition control since it is a solution-based process.
Sol-gel synthesis is the formation of solid materials from a liquid medium, typically at low temperatures, where a dispersed colloidal suspension (sol) of particles evolves to form a three-dimensional open gel network (gel) The typical molecular precursors are metallo-organic compounds such as metal alkoxides M(OR)n, where M is a metal like Si, Ti, or another similar element, and R represents an alkyl group (for example CH3, C2H5) In titanium dioxide production, Ti(OiC3H7)4 is used as a precursor in the TiO2 synthesis.
Sol-gel synthesis of TiO2-based materials starts with the hydrolysis of titanium alkoxides, such as Ti(OiPr)4, in an alcohol solvent, with the reaction Ti(OiPr)4 + 2 H2O → TiO2 + 4 iPrOH illustrating the basic chemistry Water addition, additives (for example acetic acid), the water-to-alkoxide ratio, and mixing speed all influence the size, porosity, and phase of the final oxide In this process, a water-soluble precursor hydrolyzes to form a colloidal dispersion (the sol); subsequent condensation creates a three-dimensional network (the gel), which is dried and typically heated to obtain the TiO2 material This sol-gel route can produce very pure materials at relatively low temperatures compared with traditional solid-state methods, and it offers versatility for tuning microstructure for coatings, ceramics, and photocatalysis applications In related terms, Sharpless epoxidation demonstrates how titanium-based reagents can enable selective transformations in organic synthesis, underscoring the broad utility of titanium alkoxides in chemistry.
Homogeneous multi-component systems can be generated by combining precursor solutions, enabling straightforward access to chemical doping of the prepared materials The rheological characteristics of the sol-gel dictate processing options such as dip coating of thin films and fiber spinning A new generic method for manufacturing nanostructures of semiconductors and other inorganic materials emerges from merging sol-gel synthesis with template fabrication for nano-materials, for example by performing sol-gel synthesis inside the pores of microporous and nanoporous membranes to generate monodisperse tubules and fibrils of the desired material In the sol-gel synthesis of nano TiO2, maintaining a high water ratio increases the nucleophilic attack of water on titanium(IV) isopropoxide and prevents rapid condensation of titanium(IV) isopropoxide species, yielding TiO2 nanocrystals.
Xu presented a novel approach to the synthesis of titanium dioxide in the year 1991
Using a sol-gel approach with a cellulose membrane and heat peptization allows separation of by-products from the solvent (alcohol), while sols subjected to thermal peptization form a denser aggregate than those dialyzed, because slow proton removal and diffusion-driven aggregation are restrained as charges are screened by electrolyte ions; TiO2 with fewer particles tends to have a greater surface area than densely packed TiO2 Sol-gel processing is one of the most common ways to generate photocatalyst TiO2 in coatings and powder forms, offering a straightforward method that does not require sophisticated equipment and can produce nanoparticles at room temperature and ambient pressure In studies of N-TiO2 made from Titanium(IV) isopropoxide, Venkatchalam examined how hydrolyzing agents and water content affect particle size, finding that acetic acid as the hydrolyzing agent yields smaller TiO2 particles and promotes rapid hydrolysis of titanium hydroxide and condensation into TiO2 nanoparticles.
Fig 1.4: Nanocrystalline Metal Oxide Preparation using Sol-Gel method
Excess acetate anion adsorption on the surface of TiO2 can inhibit growth and reduce crystallite size, with acetate complexes on anatase TiO2 during sol-gel synthesis blamed for shrinking TiO2 crystallites To promote controlled nanocrystal formation, a high water ratio is used to boost the nucleophilic attack of water on titanium(IV) isopropoxide while slowing the fast condensation of Ti(IV) isopropoxide species, enabling TiO2 nanocrystals to form The presence of residual alkoxy groups further slows crystallization, promoting the production of the less dense anatase phase exclusively In contrast, very low hydrolysis rates due to little water and an excess of titanium alkoxide in the solvent encourage the formation of Ti–O–Ti chains through alkoxolation, leading to three-dimensional polymeric skeletons with tight packing that produce a high rutile fraction, since each titanium atom is coordinated with four oxygen atoms.
Support and thin films
Cordierite is a ceramic material in the MgO–Al2O3–SiO2 system with the nominal formula 2MgO·2Al2O3·5SiO2 Its oxide composition is 13.78% MgO, 34.86% Al2O3, and 51.36% SiO2 This composition places cordierite in the ceramic group with a high mullite content, represented by the mullite-like phase 3Al2O3·2SiO2.
Cordierite crystals confer several advantages to cordierite ceramic, including a very low coefficient of thermal expansion and minimal thermal loss, making it ideal for components that experience rapid temperature changes Cordierite ceramic, whose primary crystal is cordierite, offers exceptional thermal stability and easy porosity, supporting uses in motor-filter production, catalyst carriers, and as lining material in arc welding MIG/MAG technology Because cordierite ceramic has a narrow firing temperature window, it is one of the more challenging ceramics to process Calcination reactions depend on the maximum calcination temperature, heating rate, dwell time at peak temperature, particle size, composition, and impurity content, and achieving full equilibrium can be difficult; therefore, typical preparation before use involves heating to the target temperature according to the intended application Green bodies intended to yield high mechanical strength, porosity, and low water absorption or bulk density must avoid clumping, and to solidify properly under normal conditions often require heating to at least 0.8T (where T is the melting temperature), i.e., above 1200°C.
C for the intended result, which is cordierite [68]
1.5.2 Mesoporous TiO 2 and coating techniques
Porous materials are essential to modern civilization, enabling a wide range of applications—such as catalysis, adsorption, optics, sensing, insulating coatings, and ultralow-density materials They are widely used in catalysis and have a significant impact on the global economy by enabling reactions to occur under lower energy conditions A prime example is petroleum refining, where diverse microporous zeolites play a prominent role in catalytic cracking.
Microporous materials, including zeolites, have restricted pore apertures that prevent their use in demanding applications such as oil refining This pore-size constraint is one of the most significant limitations of these materials, underscoring the challenge of applying them to high-demand industrial processes.
Stucky and colleagues [73] report on the synthesis of large-pore, mesoporous metal oxide powders and films using P123, highlighting the role of metal chloride salts as the inorganic starting materials in these studies.
In order to postpone the crystallization of titanium, a non-hydrolytic approach that involves the breakage of carbon-oxygen bonds was developed This seems to be an essential step when creating the mesostructures in a controlled manner
Using TBT, TET, or TPT as precursors, Sanchez and colleagues conducted an in- depth study on the function that water plays in the process [74] Condensation does not take place before the formation of the mesostructured hybrid stage because the condensation rate is very low when the water content is low On the other hand, condensation reactions, which take place in the presence of considerable amounts of water and contribute to the formation of oxo clusters, come before the formation of hybrid processes In spite of this, the addition of an excessive amount of water resulted in the production of gels that lacked periodicity
In the EISA-based synthesis of mesoporous titanium dioxide, CTAB and TET were used, with NBB self-assembling around the micelles to form the titaniatropic assembly, conceptualized as Ti–OH + X–CTAB interactions where X denotes the CTAB bromide anion and/or HCl chloride ions involved in the synthesis Yan and his team showed that altering solvents and co-solvents enabled the production of distinct titanium phases, such that using TiCl4 with P123 or F127 and changing the solvent from methanol to ethanol, 1-butanol, or 1-octanol yielded combinations of anatase and rutile There is a clear relationship between chlorine release and the length of the carbon chain: as the chain length increases, more chlorine is retained in TiCl4x(OR)x moieties, leading to greater obstruction In these phases, anatase tends to form at low acidity, while rutile forms at high acidity [75].
TiO2 mesostructures were produced by using TiCl4 and TBT in conjunction with P123, as stated in reference [70] Intriguingly, the pore walls exhibit a mixed phase composition of rutile and anatase, and the mesoporous material has a surface area of 244 m2/g, generated at a P123/TBT/TiCl4 mole ratio of 0.2 [76] Citric acid was added to the TPT/F127-containing mixture to produce mesoporous titania [77] The citric acid-functionalized hydrophilic titanium surfaces of the nanoparticles enhance binding to the ethylene oxide units of F127 The production of the titania–P123 mesostructure was followed by the addition of ethylenediamine, yielding a thermally stable mesoporous anatase Calcination at temperatures up to 700 °C causes the ethylenediamine molecules to bond to the titanium surface, preventing pore collapse and hindering anatase-to-rutile conversion [78].
TiO2 catalyst powder dispersed in water enables a wide range of photocatalytic applications due to its large accessible surface area, but recovering the catalyst from suspension is time‑consuming and costly In addition, the suspended TiO2 can hinder ultraviolet light transmission, reducing overall catalytic efficiency Immobilizing TiO2 on a substrate has emerged as a key strategy to address these drawbacks, especially for the photocatalytic treatment of organic pollutants This approach is just one of several potential solutions researchers are pursuing to optimize TiO2‑based photocatalysis.
Immobilized TiO2 photocatalysts are well known for being inexpensive, highly stable, and resistant to photocorrosion, with their favorable surface characteristics making them prime candidates for large-scale wastewater treatment Recently, TiO2 has been immobilized on various substrates using diverse methods such as anodization, sol–gel processing, reactive DC magnetron sputtering, chemical vapor deposition, electrostatic sol-spray deposition, and aerosol pyrolysis Choosing a catalyst immobilization strategy requires considering the catalyst substrate or support, the nature of the pollutant, and the operating environment (liquid or gas) Loading TiO2 onto a support can modify the photocatalytic properties of the system, with the main drawback often being a reduced surface area of the support Within the sol-gel route, sol deposition on the substrate can be achieved via spraying, electrophoresis, inkjet printing, dip-coating, and spin coating.
Spin coating yields a uniform coating, and TiO2 deposition under vacuum produces a hard coating with residual air eliminated This technique can place TiO2 homogeneously on AC by using vacuum and rotation, although dip coating offers advantages due to its simple, low‑equipment requirements A variety of substrates—glass, silicon, stainless steel, and titanium—have been used, with binders added to the TiO2 suspension and post‑deposition annealing performed to improve adhesion When TiO2 is used as the starting material, the binding approach can enhance crystal quality, whereas the direct creation method often yields crystals of lower quality.
TiO 2 /AC Materials
Fujishima and Honda’s 1972 discovery that photocatalytic water splitting can be achieved on TiO2 electrodes launched a new era of heterogeneous photocatalysis with TiO2 as a semiconductor Since then, TiO2 has been shown to function as a photocatalyst for the photodegradation of organic pollutants, transforming them into less hazardous compounds even as researchers continue to enhance its performance Controlling key parameters such as calcination temperature, pH, and aging time can steer the process in the right direction, and incorporating titania-containing supports is an effective strategy to boost photocatalytic efficiency Based on extensive literature, several fundamental criteria for selecting an appropriate catalyst load are identified: a) the composite should be transparent to at least some ultraviolet light and chemically inert to pollutant molecules, their intermediates, and the aquatic environment; b) it should attach to TiO2 in a way that does not inhibit TiO2 reactivity; c) it should offer a large surface area and strong adsorption toward the contaminants to be degraded, reducing intermediates and improving mass transfer; and d) it should enable easy recovery and reuse of the photocatalyst, with or without regeneration.
Activated carbon–TiO2 composites offer enhanced photocatalytic performance by combining activated carbon's superior adsorption properties with TiO2's photocatalytic activity Activated carbon's high surface area, microporous structure, and tunable surface chemistry underpin its wide industrial use for purification, decolorization, deodorization, dechlorination, detoxification, filtration, desalination, separation, and recovery When coupled with TiO2, activated carbon also improves the removal of water contaminants by facilitating diffusion and promoting pollutant breakdown to water and CO2 during photocatalysis, with the composite's photo-activity greater than TiO2 alone In a 2007 study by Wang et al., TiO2 was prepared by sol–gel and combined with activated carbon at 20%, 50%, and 80% carbon by weight, then calcined at several temperatures The composite calcined at 450°C displayed superior properties relative to samples calcined at lower or higher temperatures TG analysis showed the carbon content remained essentially constant during use, though higher calcination reduced carbon content and surface area due to carbon gasification SEM images revealed stronger interfacial contact as TiO2 penetrated the carbon pores with higher calcination temperatures, which contributed to the observed surface-area reduction AC performed best below 450°C (notably 300°C), while 450°C induced a pronounced interphase reaction and enhanced photodegradation of chromotrope 2R; the 80-AC-TiO2-450 catalyst — calcined at 450°C and containing about 80% of the original carbon content converted to TiO2 — showed the maximum performance across pollutant concentrations.
Together with TiO2, activated carbon fibers (ACF) were used in Liu and colleagues' research The TiO2 deposited on the outside surface of ACFs appeared to form a fractured film rather than a thin, compact coating, and the deposition of TiO2 and the subsequent calcination did not harm the micropore structure of ACFs nor affect their high specific surface area The TiO2/ACFs system performed well, especially for degrading low-molecular-weight organic contaminants in wastewater, with the synergistic action of TiO2 photocatalysis and ACFs inhibiting intermediate species formation during photocatalysis The catalyst exhibited high photocatalytic activity and strong regeneration capacity XRD showed anatase with a small amount of rutile, and BET analysis indicated the surface area decreased from 1065 m2/g for pristine ACFs to 845 m2/g for TiO2/ACFs, yet the top layer remained mesoporous SEM images confirmed uniform surface morphology across the composites The TiO2/ACFs catalyst demonstrated a strong capacity for breakdown of MB, achieving 94% degradation.
Degradation was rapid, with the reaction completing in 40 minutes and reaching 100% within three hours Findings show that the TiO2/ACFs catalyst exhibits greater degradation activity than bare ACFs and pure TiO2 This enhanced performance arises because activated carbon fibers concentrate organic contaminants near TiO2, the sites where degradation occurs [103].
Calcination temperature markedly affects the structure and photocatalytic performance of TiO2-mounted activated carbon used for phenol removal from water In a composite prepared by hydrolytic precipitation, the TiO2 coating blocked pore entrances on the activated carbon, reducing its surface area, and as heat treatment continued, TiO2 particle size increased, leading to pore clogging and lower efficiency; adsorption enabled the best phenol removal at 900°C Activated carbon alone served as a comparison, and composites calcined at temperatures above 700°C, notably at 800°C, showed reduced photocatalytic activity because the TiO2 crystalline structure shifted from anatase to rutile, which is less active.
Li et al (2007) investigated TiO2-coated activated carbon composites versus unmodified TiO2 using a sol–gel approach to prepare both catalysts and calcined the samples from 300 to 700 °C XRD analysis determined anatase/rutile ratios and average crystallite size, showing anatase as the dominant phase for both materials after calcination at 300–500 °C, with a transition to rutile at 600–700 °C in both cases The composite catalyst exhibited slower crystallite growth than the pure TiO2, attributed to the high-surface-area activated carbon (435 m2/g) that creates anti-calcination effects at the interface and suppresses TiO2 particle agglomeration BET measurements showed the composite’s surface area increased with calcination temperature, whereas the surface area of pure TiO2 decreased Photocatalytic activity tests with methylene blue revealed the pure TiO2 removed 61% of the dye after 200 minutes, while the TiO2/AC composite nearly completely removed it, due to the accelerated concentration of the organic substrate near TiO2 on the carbon support.
Graphene oxide (GO)
Graphene is a single-layer carbon sheet with a perfect sp2-hybridized two-dimensional structure that has attracted immense research attention since its discovery by Novoselov and colleagues Its exceptional properties include a high specific surface area of about 2630 m2 g-1, excellent optical transparency (approximately 97.7%), and outstanding thermal conductivity, making it a foundational building block for other carbon materials such as C60, graphite, and carbon nanotubes A broad range of synthesis techniques has been developed for graphene, including chemical vapor deposition (CVD), epitaxial growth on silicon carbide, arc-discharge, substrate-free gas-phase synthesis, chemical reduction of graphene oxide, electrochemical synthesis, and unzipping CNTs to form graphene nanoribbons Graphene-based films with tailored thickness and composition have enabled applications across fuel cells, supercapacitors, hydrogen storage, lithium-ion batteries, solar cells, electrochemical sensors, and fluorescent sensors, among others; however, the hydrophobic nature of pristine graphene limits its use in water and wastewater treatment.
Fig 1.5: Structures of graphene, C60, CNT and graphite [109]
Graphene oxide (GO) is a major derivative of graphene formed by the chemical oxidation of natural graphite The Hummers method, a robust oxidation process that combines flake graphite with potassium permanganate and concentrated sulfuric acid, remains the most widely used route for producing graphite oxide (GO) GO’s structure is rich in oxygen-containing functional groups, including hydroxyl and carboxyl groups, which confer hydrophilicity and make GO an excellent support for inorganic nanoparticles.
Significant progress has been made in synthesizing G/GO-based materials, including G/GO-metal composites, G/GO-metal oxide composites, and G/GO-polymer composites These materials are produced through two broad synthetic strategies: ex-situ hybridization and in-situ crystallization The in-situ crystallization approach encompasses chemical reduction, electroless deposition, sol–gel processing, hydrothermal treatment, electrochemical deposition, thermal evaporation, and other related methods.
Graphene/graphene oxide (G/GO) sheets have been combined with a range of metals—such as silver, gold, platinum, palladium, nickel, and copper—to further enhance electrochemical and analytical performance For example, Liu and colleagues constructed a graphene–Pt composite by integrating Pt nanoparticles with graphene, which showed increased oxygen reduction activity due to a larger electrochemical surface area than commercial catalysts Additionally, graphene–gold composites were synthesized via in-situ chemical reduction of chloroauric acid, depositing gold nanoparticles on reduced graphene oxide (RGO) sheets; these composites exhibited excellent photodegradation of RhB dye under visible light, driven by high adsorption capacity for organic dyes, slow charge recombination, and strong interaction with dye chromophores.
TiO 2 /GO Materials
Over the past two decades, advanced oxidation processes (AOPs), including photocatalysis, have attracted substantial attention for a wide range of environmental and non-environmental applications, such as energy storage and processing Photocatalytic processes offer potential in environmental systems, yet the relatively low efficiency of photoactivated catalysts has limited their practical use, prompting extensive research [116–120] to enhance the technology.
Graphene oxide (GO) surfaces host active sites due to available epoxide and hydroxyl groups, with carboxyl, quinoidal, ketone, and lactone groups decorating margins and rim areas around vacancies The oxygen-rich functional groups enable further chemical modification or functionalization, providing an efficient way to tailor GO's physical and chemical properties to the desired level Beyond its outstanding optical and mechanical properties, GO supports a broad range of applications, though residual defects and pores from its reduction can compromise the electrical performance of reduced GO (r-GO) Accordingly, GO and GO-based composites hold great potential for energy storage and conversion as well as environmental protection applications.
Supported photocatalysts are a prominent strategy in industrial catalysis, enabling improved reactant access through strong photocatalyst–support interactions that drive long-term efficiency TiO2 powders in suspension offer a high specific surface area (30–300 m2/g) that enhances mass transfer and sustains robust photocatalytic activity under sunlight, but TiO2 alone suffers from rapid recombination of conduction-band electrons and valence-band holes, yielding poor intrinsic activity Increasing catalyst loading can introduce transport limitations, and recovering nanoscale TiO2 from water after remediation remains challenging To overcome these drawbacks, immobilizing catalyst particles on a surface can aid separation and reuse, yet immobilization introduces mass-transfer constraints, potential light-absorption by substrates, and a reduced surface-to-volume ratio that may diminish oxidation efficiency compared with suspension.
Photocatalysts can be effectively integrated with a range of materials, with graphene oxide (GO) emerging as an ideal substrate due to its highly specialized surface and excellent electron mobility Immobilizing TiO2 photocatalysts on GO and other supports increases the surface-to-volume ratio, enhancing photocatalytic oxidation provided the surface area supports efficient light absorption The preparation of GO-based nanocomposites is therefore crucial, with synthesis achievable through hydrothermal, electrochemical, in-situ polymerization, microwave-assisted, vacuum deposition, and sol-gel techniques In GO-based nanocomposites, GO acts as either a functional component or a supporting matrix for immobilizing other active components, enabling versatile GO-TiO2 photocatalytic systems.
Table 1.2 Summary of TiO 2 and GO composites used as photocatalyst
Composites TiO2 particle size GO content Pollutant Ref
Pt-GO-TiO 2 /GR 30 nm 0.5 wt% Dodecylbenzenesulfonate Neppolian et al
5-15 nm 90 wt% Methylene blue Ismail et al 2013
2013 TiO 2 /GO - 10 mg Methylene blue Min et al 2012
TiO 2 /GO 57 nm 3.3 wt% Diphenhydramine methyl Pastrana et al
10 wt% Methylene blue Zhang et al 2011
TiO 2 /GO 20-40 nm 4.6 wt% Methyl orange Jiang et al 2011
TiO 2 /GO 30 nm 10 wt% Methyl blue Nguyen et al 2011
TiO 2 /GO - GO:TiO2 = 1.5 wt
Methyl orange Pu et al 2013
TiO 2 /GO 10 nm 0.03 mg GO Methyl blue Yoo D-H et al
TiO 2 /GO 4-5 nm 3.3 - 4.0 wt% Diphenhydramine and methyl orange
TiO 2 /GO 15 nm ~10% Rhodamine B Liang et al 2010
TiO 2 /GO 10 nm GO:TiO2 = 3:2 wt
Methyl orange Gao et al 2010
TiO 2 /GO 6-9 nm 1 wt% Methylene blue Kim et al 2014
Fujishima and Honda’s pioneering photocatalysis research sparked modern scientific inquiry into activating photocatalytic processes, beginning with their demonstration that a semiconductor particulate can dissociate water when irradiated with light of the appropriate wavelength Since then, numerous other photocatalysts have become the focus of intensive study, including graphene–metal oxide composites discussed in prior work for water treatment, photocatalysis, adsorption, and disinfection Among photocatalysts, titanium dioxide (TiO2) emerged as the earliest and most influential binary transition metal oxide studied, prized for its chemical stability and insolubility in aqueous media, which simplifies post-reaction separation TiO2’s non-toxicity further reinforces its suitability for environmentally oriented photocatalytic applications.
Researchers have examined graphene oxide (GO) as an electron-accepting molecule in TiO2-based composites, showing that particle size, GO loading, and the specific targeted pollutants determine the performance of various TiO2/GO combinations The examples illustrating these effects are provided in Table 1.2, highlighting how different GO content and TiO2 types influence pollutant removal across a range of contaminants.
MO photocatalytic degradation
Extensive research on methyl orange degradation via photocatalysis shows that Ag+-modified TiO2 suspensions are more effective than other metal ions (Cu2+, Co2+, Fe3+, Ce4+) In Ag+-containing systems, the presence of O2 or N2 bubbling has minimal effect on MO decolorization; the decolorization rate increases with pH and peaks around pH 8.75, but declines at higher pH due to precipitation of Ag+ with OH-, which reduces available Ag+ Increasing Ag+ loading boosts the apparent primary quantum yield up to a point, after which catalytic efficiency declines as the silver layer shades the semiconductor surface Under solar irradiation, Al-Qaradawi and Salman reported that MO breakdown occurs with TiO2 as catalyst, and that the degradation pace is not simply proportional to light intensity: a high rate of MO deterioration can occur with a relatively small photon flux; the highest degradation rate was observed at pH 3 The most effective MO concentration reported was 4 × 10−5 M, while higher MO concentrations slow the degradation, with five hours enough to transform the molecule into a different chemical state Silver-modified TiO2 thin-film catalysts demonstrated threefold faster MO degradation than undoped Degussa P25 films, though further increases in Ag+ can reduce catalytic efficiency due to surface shading; importantly, the activity of these silver-modified films remained stable over six successive pollutant additions Guettai and Amar examined monoazo MO oxidation in TiO2/UV aqueous solutions and found no MO degradation in the dark, while examining how pH, substrate concentration, and catalyst dosage influence the degradation rate, for example testing 0.8 g/L catalyst dosage at a given pH and noting the effects of other parameters. -**Support Pollinations.AI:** -🌸 **Ad** 🌸Powered by Pollinations.AI free text APIs [Support our mission](https://pollinations.ai/redirect/kofi) to keep AI accessible for everyone.
With an initial MO concentration of 50 mg/L, the degradation rate achieved its maximum under the tested conditions Chen et al [128] reported that pelagite from the East Pacific Ocean can serve as a low-cost, highly effective catalyst for the complete breakdown and decolorization of methyl orange within the system.
Under UV irradiation, the catalytic degradation of methyl orange was investigated, with pelagite displaying an adequate level of catalytic effectiveness in degrading organic molecules Li et al studied methyl orange decomposition using a TiO2-coated activated carbon (TiO2/AC) catalyst in an aqueous solution exposed to UV light and found that the degradation followed pseudo-first-order kinetics.
AC presence improved the photoefficiency of the titanium dioxide catalyst, and a modified Langmuir–Hinshelwood model may be used to characterize the kinetic behavior; Sharma et al reported the production of pure and nickel-doped TiO2 thin films on soda-lime glass substrates using a sol–gel dip-coating technique, with catalytic activity under ultraviolet light evaluated by the breakdown of methyl orange in aqueous solution, while Liu and Sun, by performing several impregnations, synthesized Fe2O3-CeO2-TiO2/-Al2O3 catalysts that were characterized by BET, SEM, XRF, XPS and chemical analysis; at ambient temperature and atmospheric pressure a CWAO process using Fe2O3-CeO2-TiO2/-Al2O3 as a catalyst degraded methyl orange, showing remarkable activity for treating synthetic wastewater containing 500 mg/L methyl orange with 98.09% color removal and 96.08% TOC removal in 2.5 hours, and UV-Vis and FT-IR spectra were used to investigate the methyl orange breakdown process; Ma et al reported a CuO-MoO3-P2O5 catalyst with high activity for catalytic wet oxidation at lower temperatures (35°C) and atmospheric pressure, degrading methyl orange and methylene blue, where solid-state synthesis was used and the catalyst exhibited high MB degradation (99.65%) but comparatively lower MO degradation (55%) under the same conditions; Rashed and Al-Amin studied the catalytic oxidation of methyl orange using TiO2 under various irradiation sources (including a 1000-watt halogen lamp, a fluorescent lamp, and sunlight), examining how starting dye concentrations, irradiation time, and light intensity affected degradation, with results showing faster degradation under sunlight than halogen or fluorescent light and attainment of pseudo-first-order kinetics; under visible-light irradiation, Zhang et al reported related observations.
Pt-TiO2–SiO2 and TiO2 catalyze methyl orange degradation, with Pt-TiO2–SiO2 showing a significantly higher rate due to a 16 charge-transfer on the TiO2–SiO2 framework and improved trapping of photo-generated electrons on Pt-derived states on TiO2 surfaces Huang et al synthesized Pt-TiO2–zeolites by loading Pt-modified TiO2 via sol–gel and photo-reductive deposition; under UV irradiation, Pt doping enhances methyl orange decolorization, with about 1.5 wt% Pt yielding 86.2% decolorization in 30 minutes The impact of calcination temperature, catalyst concentration, H2O2 oxidant amount, and pH on activity was explored, and five cycling runs demonstrated satisfactory repeatability Sohn et al studied methyl orange breakdown on TiO2 nanotubes; TiO2 nanotubes prepared in ethylene glycol by ultrasonic treatment and annealed in nitrogen showed much higher dye degradation activity than stirred samples, and applying an external bias further increased degradation, with oxidants such as O2 and H2O2 enhancing the effect.
Fig 1.7: Possible mechanism of MO with TiO 2 [138]
Sun et al [139] reported the synthesis of Sb2S3 as a novel catalyst and examined the visible-light–driven degradation of methyl orange in an aqueous solution, demonstrating that methyl orange can be efficiently degraded under visible irradiation with Sb2S3 and highlighting Sb2S3 as a promising photocatalyst for dye remediation in water.
Thirty minutes of irradiation yielded methyl orange degradation up to 97%, significantly higher than the rates achieved by CdS and TiO2 under the same conditions Liquid chromatography and mass spectrometry were used to investigate the potential mechanism behind the catalytic process Zhao and Zhu reported the synthesis of gold- or platinum-loaded titania nanotubes and showed degradation rates of 96.1% (Au/TiNT-500) and 95.1% (Pt/TiNT-500) for methyl orange using a light-deposition loading method Zhu and colleagues described the manufacture of Polythiophene/TiO2 (PT/TiO2) catalysts via in situ chemical oxidative polymerization, with the resulting composites analyzed by transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-Vis diffuse reflectance spectroscopy.
Degradation of methyl orange (MO) was used to gauge catalytic activity, and Pt/TiO2 composites showed strong MO adsorption due to electrostatic attraction between the positively charged surfaces of the composite particles and MO molecules Moreover, incorporating Pt into TiO2 composites enhances the catalytic degradation of MO under both UV and visible light Guo et al reported the degradation of Rhodamine B (RhB) and MO in aqueous solutions under simulated solar illumination using one-dimensional TiO2 nanostructures—nanotubes and nanowires—synthesized by a hydrothermal method with Degussa P25 TiO2 as a precursor; these TiO2 nanostructures exhibited markedly higher degradation activity than P25.
pH is a key factor that governs the rate of dye photodegradation because changing pH alters the surface charge of TiO2 particles, which shifts the catalytic potentials, changes dye adsorption on the surface, and thus modulates the reaction rate The point of zero charge (PZC) of the catalyst is used to gauge how readily organic contaminants can adsorb, with acidic dye solutions tending to adsorb more on catalysts with higher PZC Since TiO2 has a PZC around pH 6.8, values above 6.8 render the surface negatively charged, influencing adsorption and the degradation kinetics.
TiOH and OH lead to the formation of H2O and TiO on the TiO2 surface, as shown in equation [144] When the environmental pH is below the point of zero charge (PZC), the TiO2 surface becomes positively charged, influencing adsorption and surface reactions This pH‑dependent surface charge behavior explains why TiO2 surfaces are positively charged at low pH and become less positive or negative as pH increases, in line with the referenced equation [144].
Photocatalytic performance hinges on catalyst structure, and TiO2 exists in anatase, rutile, and brookite, with anatase standing out for photocatalysis due to its favorable conduction-band position, hydroxylation, and adsorption properties Morphology also shapes degradation efficiency, while nanostructuring TiO2 yields much higher photocatalytic activity than bulk materials because of a larger surface area and smaller size In water treatment, nanosized TiO2 outperforms bulk TiO2, since reducing particle size increases the surface area-to-volume ratio and boosts charge-carrier transfer, expanding the number of active sites Because the photocatalytic redox reaction largely occurs on the catalyst surface, surface features strongly influence overall effectiveness.
Phenol photocatalysis degradation
To reduce phenol in wastewater streams, researchers have explored a range of degradation strategies Enzymatic polymerization of phenol with hydrogen peroxide can be highly effective, but the required enzymes can make the process costly Biological approaches, such as activated sludge in membrane bioreactors, have proven successful for phenol removal, though fouling can lead to high cleaning and maintenance costs Additional methods investigated include electrocoagulation, extraction, adsorption, ion exchange, and photodecomposition.
Titanium dioxide (TiO2) is a cost-effective, readily available photocatalyst that can stimulate the decomposition and mineralization of a wide range of organic pollutants, making phenol degradation in water via TiO2 photocatalysis a potential approach for environmental cleanup However, TiO2-based phenol degradation methods suffer from low quantum efficiency and a dependence on UV light, limiting their practical applications To overcome these constraints, TiO2 is often coupled with other materials, such as activated carbon, which has been shown to boost photocatalytic activity, while graphene oxide has attracted attention as a potential substitute due to its favorable electrical, adsorption, thermal, and mechanical properties.
However, the use of ultraviolet (UV) radiation to breakdown significant quantities of wastewater is not a feasible option In order to hasten the process of degrading phenol and removing it completely from water, researchers are always looking for environmentally friendly, cost-effective, and chemically or catalytically based solutions to the problem In addition, the purpose of this thesis is to investigate the deterioration of phenol via the process of photocatalysis by making use of titanium dioxide modified catalysts and illuminating the reaction mixture with both visible light and ultraviolet light It has the potential to completely transform the way that industrial wastewater is treated if photodegradation of phenol and other organic compounds can be carried out using just visible light In a heterogeneous photocatalytic system, photo-induced molecular transformations or reactions typically take place at the surface of the catalyst The first thing that has to be done is the system needs to be excited by employing photons of light The absorption of photons by molecules results in the production of a highly reactive and electrically excited state The photon has to have enough energy to push an electron over the band gap, also known as the empty area that may be found between the top of the full valence band and the bottom of the unoccupied band Because TiO2 has a large band gap of 3.2 eV, visible light is insufficient for the reaction In order for the photoreaction to begin, then, TiO2 has to be exposed to light in the ultraviolet spectrum, which has a wavelength that is shorter than 390 nm After this initial excitation, an electron transfer occurs to the solvent or organic species, which then becomes absorbed on the surface of the catalyst In the case of TiO2, an excited electron is responsible for the reduction of an oxygen molecule to the gas O2 During this time, the positively charged electron hole, shown by the symbol h+, interacts with water to form a hydroxyl radical, denoted by the symbol OH Within the framework of the photocatalytic process, the hydroxyl radical serves as the catalyst for the beginning of the chemical processes [147] The following deexcitation happens at a significantly slower pace than the initial absorption of a photon of light, which happens extremely quickly (on the range of tens to hundreds of milliseconds) On a timescale of 10-12 to 10-9 seconds, the photochemical processes take place [148]
The promotion of an electron results in the creation of an electron-hole pair, which may lead to one of two different possible routes It is preferable for the photoinduced electron to either transfer to the organic or inorganic species that have been adsorbed, or it might transfer to the solvent The unfavorable mechanism known as electron-hole recombination is in direct rivalry with the charge transfer process The efficiency of a photocatalytic process is inversely proportional to the sum of the charge transfer rate and the electron-hole recombination rate The efficiency is related to the rate of the charge transfer process, which in turn is proportional to the rate of the charge transfer process As a result, the recombination of the electron-hole pair is something that has to be prevented in order to make the process of charge transfer on the catalyst surface more effective It has been suggested that decreasing the recombination rate may be accomplished by making modifications to the surfaces of semiconductors using metals or combining them with other semiconductors [148]
Two main obstacles limit the efficiency of photocatalysis First, photons reaching the catalyst must have enough energy to excite electrons across the material’s band gap; for TiO2, visible light is insufficient and ultraviolet (UV) light is required However, UV light cannot be exploited at industrial scales for wastewater treatment because UV constitutes less than 5% of the solar spectrum at the Earth’s surface Second, to drive an effective photocatalytic reaction, electron–hole recombination must be significantly suppressed to maintain charge separation and reaction rates.
Fig 1.8: Mechanism of Phenol Decomposition Reaction [152]
TiO 2 has been shown to be an effective catalyst for the photocatalytic degradation of phenol The interaction of phenol with OH- radical ions results in the formation of a number of intermediate products These products include hydroquinone (HQ), pyrocatechol (CC), 1,2,4-benzenetriol (HHQ), pyrogallol (PG), 2-hydroxy-1,4- benzoquinone (HBQ), and 1,4-benzoquinone (BQ) These intermediates are then subjected to further photocatalytic oxidation, which results in the production of highly polar intermediates such as carboxylic acids and aldehydes, and ultimately carbon dioxide and water [150,151]
Nonmetal doping of TiO2 catalysts reduces the band gap energy, boosting photocatalytic activity across a broader spectrum, including visible light This expanded light response enables more efficient utilization of solar energy, the primary energy source driving photocatalytic processes To evaluate this potential, nitrogen doping of TiO2 was chosen to determine whether visible-light irradiation can further enhance photocatalytic performance.
Summary
Environmental health remains one of the most pressing daily challenges, with a broad range of pollutants threatening human life and ecosystems This article focuses on developing a solution to address the prevalence of organic contaminants in the environment, including methyl orange and phenol, which are recognized as harmful to living beings By studying how loading TiO2 with carbon and graphene oxide (GO) supports and adjusting the synthesis conditions, we can boost TiO2’s photocatalytic activity and accelerate the degradation of methyl orange and phenol under UV-C and visible light irradiation Numerous researchers have reported on manufacturing modified TiO2 materials and increasing their activity, with extensive work on modified TiO2 photocatalysts conducted in Vietnam using the sol-gel process.
Fe-modified TiO2 material coated on silica gel (SiO2) beads was successfully synthesized, as reported in [153] The decomposition of a 10 ppm methylene blue (MB) solution indicated that the TiO2/SiO2 system modified with Fe exhibited effective MB degradation, underscoring its photocatalytic potential for dye removal.
Fe had a higher MB decomposition efficiency than the TiO2/SiO2 sample without Fe This was indicated by the fact that the sampled TiO2/SiO2 modified with Fe had a higher
TiO2 was modified with Fe2O3 via the sol–gel process at concentrations of 0.025, 0.05, 0.10, 0.50, 1.00 and 2.00 percent, as described in paper [154] The Fe–TiO2 catalysts showed p-xylene degradation activity about two to three times higher than unmodified TiO2, and the combination of ultraviolet and visible light led to a significant reduction in p-xylene conversion for samples containing 2% Fe2O3 Additional studies [155–158] examined further enhancements of TiO2 photocatalytic activity by doping with metals such as Nd, F, Ag, and N These findings inform strategies for reducing water contaminants and may aid the development of methods to limit other potentially dangerous organic pollutants, while also providing a useful resource for researchers exploring catalyst enhancements or related topics.
The data gathered in this study will inform further research conducted under the Rohan Catalyst Program, guiding upcoming investigations into wastewater treatment and environmental preservation These future studies will explore innovative treatment methods, scalability, and sustainable practices to advance environmental outcomes.