TiO2 Photocatalysis for the Transformation of Aromatic Water Pollutants into Fuels

The growing world energy consumption, with reliance on conventional energy sources and the associated environmental pollution, are considered the most serious threats faced by man-kind. Heterogeneous photocatalysis has become one of the most frequently investigated technolo-gies, due to its dual functionality, i.e., environmental remediation and converting solar energy into chemical energy, especially molecular hydrogen. H2 burns cleanly and has the highest gravimetric gross calorific value among all fuels. However, the use of a suitable electron donor, in what so-called “photocatalytic reforming”, is required to achieve acceptable efficiency. This oxidation half-reaction can be exploited to oxidize the dissolved organic pollutants, thus, simultaneously improving the water quality. Such pollutants would replace other potentially costly electron donors, achieving the dual-functionality purpose. Since the aromatic compounds are widely spread in the environment, they are considered attractive targets to apply this technology. In this review, different aspects are highlighted, including the employing of different polymorphs of pristine titanium dioxide as pho-tocatalysts in the photocatalytic processes, also improving the photocatalytic activity of TiO2 by loading different types of metal co-catalysts, especially platinum nanoparticles, and comparing the effect of various loading methods of such metal co-catalysts. Finally, the photocatalytic reforming of aromatic compounds employing TiO2-based semiconductors is presented

Boosting the H2 Production Efficiency via Photocatalytic Organic Reforming: The Role of Additional Hole Scavenging System

The simultaneous photocatalytic H2 evolution with environmental remediation over sem-iconducting metal oxides is a fascinating process for sustainable fuel production. However, most of the previously reported photocatalytic reforming showed nonstoichiometric amounts of the evolved H2 when organic substrates were used. To explain the reasons for this phenomenon, a care-ful analysis of the products and intermediates in gas and aqueous phases upon the photocatalytic hydrogen evolution from oxalic acid using Pt/TiO2 was performed. A quadrupole mass spectrome-ter (QMS) was used for the continuous flow monitoring of the evolved gases, while high performance ion chromatography (HPIC), isotopic labeling, and electron paramagnetic resonance (EPR) were employed to understand the reactions in the solution. The entire consumption of oxalic acid led to a ~30% lower H2 amount than theoretically expected. Due to the contribution of the photo-Kolbe reaction mechanism, a tiny amount of formic acid was produced then disappeared shortly after the complete consumption of oxalic acid. Nevertheless, a much lower concentration of formic acid was generated compared to the nonstoichiometric difference between the formed H2 and the consumed oxalic acid. Isotopic labeling measurements showed that the evolved H2, HD, and/or D2 matched those of the solvent; however, using D2O decreased the reaction rate. Interestingly, the presence of KI as an additional hole scavenger with oxalic acid had a considerable impact on the reaction mechanism, and thus the hydrogen yield, as indicated by the QMS and the EPR measure-ments. The added KI promoted H2 evolution to reach the theoretically predictable amount and in-hibited the formation of intermediates without affecting the oxalic acid degradation rate. The pro-posed mechanism, by which KI boosts the photocatalytic performance, is of great importance in enhancing the overall energy efficiency for hydrogen production via photocatalytic organic reform-ing

Polycyclic aromatic hydrocarbons in citrus fruit irrigated with fresh water under arid conditions: Concentrations, sources, and risk assessment

In Jordan, as well as in all the world countries, consumption of citrus fruits is an essential part of the daily diet, so it is important to assess the potential risk of the persistent organic pollutants such as polyaromatic hydrocarbons (PAHs) in these fruits to the human health and identify their sources in order to eliminate or reduce them. This study reports 16 priority PAHs content in four types of peeled citrus fruits grown in Jordan valley. PAHs were detected in all the studied samples in variable quantities depending on the type of citrus fruits. The results showed that the highest PAH level corresponded to acenaphthene (35.018 µg/kg) in grapefruit. Among the carcinogenic PAHs, benzo[a]anthracene (BaA) and benzo(a)pyrene (BaP) were the most representative and found in all the analyzed fruit, soil, and water samples, whereas anthracene (ANT) was not detected at all. The mean ∑16 PAHs for the different fruits were found to be 62.593 µg kg−1 in grapefruit, 24.840 µg kg−1 in lemon, 22.901 µg kg−1 in mandarin, and 5.082 µg kg−1 in orange. The dominance of naphthalene (NAP) and acenaphthene (ACE) in soil under hot climatic conditions indicates the recent and continuous input of these types in the investigated area. The bioconcentration factor (BCF) for ∑16 PAHs was observed in the order of grapefruit > lemon > mandarin > orange. Based on the results of the principal component analysis (PCA), the primary sources of PAHs in agricultural soils mainly originated from biomass burning and vehicular emissions. The incremental lifetime cancer risk (ILCR) indicated that consumption of these four citrus fruits may expose human health to potential cancer risk. The findings of this study call the policymakers and public administrations to the formulation of stringent policies and actions to control biomass burning and vehicular emissions

Application of EPR Spectroscopy in TiO2 and Nb2O5 Photocatalysis

The interaction of light with semiconducting materials becomes the center of a wide range of technologies, such as photocatalysis. This technology has recently attracted increasing attention due to its prospective uses in green energy and environmental remediation. The characterization of the electronic structure of the semiconductors is essential to a deep understanding of the photocatalytic process since they influence and govern the photocatalytic activity by the formation of reactive radical species. Electron paramagnetic resonance (EPR) spectroscopy is a unique analytical tool that can be employed to monitor the photoinduced phenomena occurring in the solid and liquid phases and provides precise insights into the dynamic and reactivity of the photocatalyst under different experimental conditions. This review focus on the application of EPR in the observation of paramagnetic centers formed upon irradiation of titanium dioxide and niobium oxide photocatalysts. TiO2 and Nb2O5 are very well-known semiconductors that have been widely used for photocatalytic applications. A large number of experimental results on both materials offer a reliable platform to illustrate the contribution of the EPR studies on heterogeneous photocatalysis, particularly in monitoring the photogenerated charge carriers, trap states, and surface charge transfer steps. A detailed overview of EPR-spin trapping techniques in mechanistic studies to follow the nature of the photogenerated species in suspension during the photocatalytic process is presented. The role of the electron donors or the electron acceptors and their effect on the photocatalytic process in the solid or the liquid phase are highlighted

Green Synthesis of Silver Nanoparticles Using Hypericum perforatum L. Aqueous Extract with the Evaluation of Its Antibacterial Activity against Clinical and Food Pathogens

The rapid development of nanotechnology and its applications in medicine has provided the perfect solution against a wide range of different microbes, especially antibiotic-resistant ones. In this study, a one-step approach was used in preparing silver nanoparticles (AgNPs) by mixing silver nitrate with hot Hypericum perforatum (St. John’s wort) aqueous extract under high stirring to prevent agglomeration. The formation of silver nanoparticles was monitored by continuous measurement of the surface plasma resonance spectra (UV-VIS). The effect of St. John’s wort aqueous extract on the formation of silver nanoparticles was evaluated and fully characterized by using different physicochemical techniques. The obtained silver nanoparticles were spherical, monodisperse, face-centered cubic (fcc) crystal structures, and the size ranges between 20 to 40 nm. They were covered with a capping layer of organic compounds considered as a nano dimension protective layer that prevents agglomeration and sedimentation. AgNPs revealed antibacterial activity against both tested Gram-positive and Gram-negative bacterial strains causing the formation of 13–32 mm inhibition zones with MIC 6.25–12.5 µg/mL; Escherichia coli strains were resistant to tested AgNPs. The specific growth rate of S. aureus was significantly reduced due to tested AgNPs at concentrations ≥½ MIC. AgNPs did not affect wound migration in fibroblast cell lines compared to control. Our results highlighted the potential use of AgNPs capped with plant extracts in the pharmaceutical and food industries to control bacterial pathogens’ growth; however, further studies are required to confirm their wound healing capability and their health impact must be critically evaluate

Accumulation, Source Identification, and Cancer Risk Assessment of Polycyclic Aromatic Hydrocarbons (PAHs) in Different Jordanian Vegetables

The accumulation of polyaromatic hydrocarbons in plants is considered one of the most serious threats faced by mankind because of their persistence in the environment and their carcinogenic and teratogenic effect on human health. The concentrations of sixteen priority polycyclic aromatic hydrocarbons (16 PAHs) were determined in four types of edible vegetables (tomatoes, zucchini, eggplants, and cucumbers), irrigation water, and agriculture soil, where samples were collected from the Jordan Valley, Jordan. The mean total concentration of 16 PAHs (∑16PAHs) ranged from 10.649 to 21.774 µg kg−1 in vegetables, 28.72 µg kg−1 in soil, and 0.218 µg L−1 in the water samples. The tomato samples posed the highest ∑16PAH concentration level in the vegetables, whereas the zucchini samples had the lowest. Generally, the PAHs with a high molecular weight and four or more benzene rings prevailed among the studied samples. The diagnostic ratios and the principal component analysis (PCA) revealed that the PAH contamination sources in soil and vegetables mainly originated from a pyrogenic origin, traffic emission sources, and biomass combustion. The bioconcentration factors (BCF) for ∑16PAHs have been observed in the order of tomatoes > cucumbers and eggplants > zucchini. A potential cancer risk related to lifetime consumption was revealed based on calculating the incremental lifetime cancer risk of PAHs (ILCR). Therefore, sustainable agricultural practices and avoiding biomass combusting would greatly help in minimizing the potential health risk from dietary exposure to PAHs

Photocatalytic transformation of water pollutants into fuels

The exploitation of solar radiation in the remediation of organic water pollutants has the potential to be a sustainable energy source to replace fossil fuel. Conceptually, coupling H2 evolution and the degradation of organic pollutants through dual-functional TiO2 photocatalysis has been frequently nominated as one of the most sustainable and environmentally friendly technologies. However, the low quantum efficiencies achieved so far by using TiO2 photocatalysts, and the lack of a fundamental understanding of the reaction mechanism represent big challenges that need to be overcome before their potential can be fully realized. Despite the large number of studies every year that utilize the dual functional photocatalysis processes for the H2 production from simple water pollutants such as methanol, formaldehyde, and formic acid, aromatic pollutants, however, have been rarely considered. Thus, several open questions regarding the photocatalytic efficiencies and the reaction mechanism during the photoreforming of aromatic compounds still exist. In the presented work, the photocatalytic reforming of naphthalene in water has been investigated employing TiO2-based photocatalysts under solar irradiation. The effect of different crystalline phases of TiO2, namely Sachtleben Hombikat UV100 (pure anatase) and Evonik Aeroxide P25 (mixed phase of anatase and rutile), were assessed under the same experimental conditions. In order to understand the effect of Pt co-catalysts on H2 production, different fraction ratios were deposited on P25 and UV100. At the optimum loading ratio, 0.5 wt.% Pt-UV100 exhibited the higher photocatalytic activity toward the H2 formation and photooxidation of naphthalene comparing to all Pt-P25 samples. The observed decrease of the photocatalytic activity of the P25 after the platinization indicates that Pt nanoparticles act as a recombination center for the photogenerated charge carriers as revealed by electron paramagnetic spectroscopy techniques. Besides that, it has been found that the platinization method strongly affects the H2 formation during the photoreforming of naphthalene over Pt-UV100. The evolution of H2 is 40 % higher when using Pt-UV100 that was prepared by the photodeposition method than that prepared by the physical mixing of TiO2 with Pt nanoparticles. The analysis of the charge carrier dynamics employing different spectroscopic techniques revealed that the strong metal-semiconductor interaction resulting from the photodeposition process leads to a significant decrease in the charge carrier recombination rates, which increases the rate of the photocatalytic reactions and enhances the H2 formation. The analysis of the reaction intermediates employing a combination of chromatographic, and mass spectroscopic techniques reveals that 1- and 2-naphthalenol, among other hydroxylated compounds, are the main reaction products during the photoreforming of naphthalene. Moreover, the accumulation of these intermediates in the photocatalytic system was found to be the main reason for the inhibition of the H2 formation and the photooxidation of naphthalene, besides, the unequal mass balance between the half reactions. Several possible pathways for the photoreforming mechanism of naphthalene involving different reactive species has been tested. Combining the spin trapping studies with different spectroscopic techniques suggested that the holes play the significant role in naphthalene oxidation. Isotopic labeling studies showed that lattice oxygen atoms were not incorporated into the photooxidation products, and the photogenerated electrons reduced the proton originating mainly from water to molecular hydrogen, whereas, water (but not naphthalene) appears to be involved in the rate-determining step of the reaction

Photocatalytic H2 Production from Naphthalene by Various TiO2 Photocatalysts: Impact of Pt Loading and Formation of Intermediates

This work presents a comparative study of the efficiency of two commercial TiO2 photocatalysts, Aeroxide P25 (ATiO2) and Sachtleben Hombikat UV100 (HTiO2), in H2 production from an aqueous solution of naphthalene. The TiO2 photocatalysts were platinized by the photodeposition method varying the platinum content of the suspension to 0.5, 1.0, and 5.0 wt%. A full physicochemical characterization for these materials was performed, showing no structural effects from the deposition method, and confirming a well dispersion of nanosized-Pt0 particles on the surface of both photocatalysts. Pristine ATiO2 shows around 14% higher photocatalytic fractional conversion of naphthalene than pristine HTiO2 after 240 min of irradiation, while both materials exhibit negligible activity for H2 formation. The 0.5 wt% Pt- HTiO2 increases the photocatalytic fractional conversion of naphthalene from 71% to 82% and produces 6 µmol of H2. However, using a higher Pt content than the optimal platinization ratio of 0.5 wt% dramatically inhibits both processes. On the other hand, regardless of the fractional ratio of Pt, the platinization of ATiO2 results in a decrease in the fractional conversion of naphthalene by 4% to 33% of the pristine value. Although the presence of Pt islands on the surface of the ATiO2 is essential for the H2 evolution, no dependency between the Pt ratio and the H2 formation rate was observed since all the platinized materials show a similar H2 formation of around 3 µmol. Based on the EPR results, the higher photocatalytic activity of the Pt-HTiO2 is attributed to the efficient charge carrier separation and its larger surface area. The recyclability test confirms that the inhibition of the photocatalytic process is related to the deactivation of the photocatalyst surface by the adsorption of the photoformed intermediates. A strong relationship between the photocatalytic activity and the kind of the aromatic compounds was observed. The H2 evolution and the photooxidation of the aromatic hydrocarbons exhibit higher photonic efficiencies than that of their corresponding hydroxylated compounds over the Pt-HTiO2

Photocatalytic H2 Production from Naphthalene by Various TiO2 Photocatalysts : Impact of Pt Loading and Formation of Intermediates

This work presents a comparative study of the efficiency of two commercial TiO2 photocatalysts, Aeroxide P25 (ATiO2) and Sachtleben Hombikat UV100 (HTiO2), in H2 production from an aqueous solution of naphthalene. The TiO2 photocatalysts were platinized by the photodeposition method varying the platinum content of the suspension to 0.5, 1.0, and 5.0 wt%. A full physicochemical characterization for these materials was performed, showing no structural effects from the deposition method, and confirming a well dispersion of nanosized-Pt0 particles on the surface of both photocatalysts. Pristine ATiO2 shows around 14% higher photocatalytic fractional conversion of naphthalene than pristine HTiO2 after 240 min of irradiation, while both materials exhibit negligible activity for H2 formation. The 0.5 wt% Pt- HTiO2 increases the photocatalytic fractional conversion of naphthalene from 71% to 82% and produces 6 µmol of H2. However, using a higher Pt content than the optimal platinization ratio of 0.5 wt% dramatically inhibits both processes. On the other hand, regardless of the fractional ratio of Pt, the platinization of ATiO2 results in a decrease in the fractional conversion of naphthalene by 4% to 33% of the pristine value. Although the presence of Pt islands on the surface of the ATiO2 is essential for the H2 evolution, no dependency between the Pt ratio and the H2 formation rate was observed since all the platinized materials show a similar H2 formation of around 3 µmol. Based on the EPR results, the higher photocatalytic activity of the Pt-HTiO2 is attributed to the efficient charge carrier separation and its larger surface area. The recyclability test confirms that the inhibition of the photocatalytic process is related to the deactivation of the photocatalyst surface by the adsorption of the photoformed intermediates. A strong relationship between the photocatalytic activity and the kind of the aromatic compounds was observed. The H2 evolution and the photooxidation of the aromatic hydrocarbons exhibit higher photonic efficiencies than that of their corresponding hydroxylated compounds over the Pt-HTiO

Photocatalytic H₂ Production from Naphthalene by Various TiO₂ Photocatalysts: Impact of Pt Loading and Formation of Intermediates

This work presents a comparative study of the efficiency of two commercial TiO2 photocatalysts, Aeroxide P25 (ATiO2) and Sachtleben Hombikat UV100 (HTiO2), in H2 production from an aqueous solution of naphthalene. The TiO2 photocatalysts were platinized by the photodeposition method varying the platinum content of the suspension to 0.5, 1.0, and 5.0 wt%. A full physicochemical characterization for these materials was performed, showing no structural effects from the deposition method, and confirming a well dispersion of nanosized-Pt0 particles on the surface of both photocatalysts. Pristine ATiO2 shows around 14% higher photocatalytic fractional conversion of naphthalene than pristine HTiO2 after 240 min of irradiation, while both materials exhibit negligible activity for H2 formation. The 0.5 wt% Pt- HTiO2 increases the photocatalytic fractional conversion of naphthalene from 71% to 82% and produces 6 µmol of H2. However, using a higher Pt content than the optimal platinization ratio of 0.5 wt% dramatically inhibits both processes. On the other hand, regardless of the fractional ratio of Pt, the platinization of ATiO2 results in a decrease in the fractional conversion of naphthalene by 4% to 33% of the pristine value. Although the presence of Pt islands on the surface of the ATiO2 is essential for the H2 evolution, no dependency between the Pt ratio and the H2 formation rate was observed since all the platinized materials show a similar H2 formation of around 3 µmol. Based on the EPR results, the higher photocatalytic activity of the Pt-HTiO2 is attributed to the efficient charge carrier separation and its larger surface area. The recyclability test confirms that the inhibition of the photocatalytic process is related to the deactivation of the photocatalyst surface by the adsorption of the photoformed intermediates. A strong relationship between the photocatalytic activity and the kind of the aromatic compounds was observed. The H2 evolution and the photooxidation of the aromatic hydrocarbons exhibit higher photonic efficiencies than that of their corresponding hydroxylated compounds over the Pt-HTiO2