Effects of adsorbed F, OH, and Cl ions on formaldehyde adsorption performance and mechanism of anatase TiO2 nanosheets with exposed {001} facets

Response surface methodology (RSM) for optimizing ozone-assisted process parameters for formaldehyde removal

Formaldehyde, a volatile organic compound (VOC), is one of the main gaseous pollutants from commercial cooking. The present study evaluated the effectiveness of a laboratory-scale ozone-assisted indirect plasma method for formaldehyde removal using response surface methodology (RSM). A dielectric barrier discharge (DBD) reactor was used for ozone generation. Inlet HCHO concentration, ozone concentration, and residence time were considered the design parameters, and formaldehyde removal efficiency (response 1) and energy yield (response 2) were considered response parameters. The optimized models showed a positive correlation between the predicted and experimental outcomes. Inlet ozone concentration, the most significant parameter in the removal efficiency model, represented a positive correlation with this response in most parts of the operating region. The optimal point with the highest desirability (i.e., D1 point) was obtained at the inlet HCHO concentration of 120 ppm, inlet ozone concentration of 40 ppm, and reaction time of 11.35 s within the parameter ranges studied, resulting in 64% removal efficiency and 2.64 g/kWh energy yield. At the point with the second highest desirability (D2), 100% removal efficiency along with 0.7 g/kWh energy yield was achieved indicating the very good performance of the process. The indirect plasma approach used in this study presented a successful performance in terms of removal efficiency along with acceptable energy yield compared to other plasma-assisted processes reported in the literature. The results suggested that ozone-assisted indirect plasma treatment can be utilized as an efficient alternative method for formaldehyde removal in commercial kitchens, while efficiency or energy yield should be prioritized for optimizing operating conditions.

High Efficiency of Formaldehyde Removal and Anti-bacterial Capability Realized by a Multi-Scale Micro-Nano Channel Structure in Hybrid Hydrogel Coating Cross-Linked on Microfiber-Based Polyurethane

Inspired by the transpiration in the tree stem having a vertical and porous channel structure, high efficiency of formaldehyde removal is realized by the multi-scale micro-nano channel structure in a hybrid P(AAm/DA)-Ag/MgO hydrogel coating cross-linked on microfiber-based polyurethane. The present multi-scale channel structure is formed by a joint effect of directional freezing and redox polymerization as well as nanoparticles-induced porosity. Due to the large number of vertically aligned channels of micrometer size and an embedded porous structure of nanometer size, the specific surface area is significantly increased. Therefore, formaldehyde from solution can be rapidly adsorbed by the amine group in the hydrogels and efficiently degraded by the Ag/MgO nanoparticles. By only immersing in formaldehyde solution (0.2 mg mL^-1) for 12 h, 83.8% formaldehyde is removed by the hybrid hydrogels with a multi-scale channel structure, which is 60.8% faster than that observed in hydrogels without any channel structure. After cross-linking the hybrid hydrogels with a multi-scale channel structure to microfiber-based polyurethane and exposing to the formaldehyde vapor atmosphere, 79.2% formaldehyde is removed in 12 h, which is again 11.2% higher than that observed in hydrogels without any channel structure. Unlike the traditional approaches to remove formaldehyde by the light catalyst, no external conditions are required in our present hybrid hydrogel coating, which is very suitable for indoor use. In addition, due to the formation of free radicals by the Ag/MgO nanoparticles, the cross-linked hybrid hydrogel coating on polyurethane synthetic leather also shows good anti-bacterial capability. 99.99% of Staphylococcus aureus can be killed on the surface. Based on the good ability to remove formaldehyde and to kill bacteria, the obtained microfiber-based polyurethane cross-linked with a hybrid hydrogel coating containing a multi-scale channel structure can be used in a broad field of applications, such as furniture and car interior parts, to simultaneously solve the indoor air pollution and hygiene problems.

Bio-Stimulated Adsorption of Cr(VI) from Aqueous Solution by Groundnut Shell Activated Carbon@Al Embedded Material

In this study, a low-cost bioadsorbent aluminum metal blended with groundnut shell activated carbon material (Al-GNSC) was used for Cr(VI) adsorption from aqueous solutions. Al-GNSC was prepared and characterized using Fourier transform infrared spectrometer (FT-IR), scanning electron microscopic (SEM) and X-ray diffraction (XRD) to determine its surface morphology. Batch studies were performed and the optimum conditions for maximum Cr(VI) removal (of 94.2%) were found at pH 4.0, initial concentration 100 mg/L, adsorbent dosage 8 g/L of Cr(VI) solution, and time of contact 50 min. Moreover, the Langmuir isotherm model (maximum adsorption capacity of 13.458 mg/g) was the best fit and favored the mono-layered Cr(VI) adsorption. The kinetic studies reveal that the pseudo-second-order model was the best fit and favored chemisorption as the rate-limiting step. The desorption study revealed that Cr(VI) leached with sodium hydroxide solution acted as a regenerating agent. It is proved that Al-GNSC removes the Cr(VI) content in groundwater samples. The methodology developed using the Al-GNSC adsorbent as an alternative for the adsorption of Cr(VI) ions is remarkably successful in this study.

Photocatalytic Decomposition of Gaseous HCHO over Ag Modified TiO₂ Nanosheets at Ambient Temperature

Ag nanoparticles loaded onto TiO₂ nanosheets with exposed {001} facets were synthesized by solvothermal hydrolysis and photoreduction deposition methods. The results suggested that Ag NPs were uniformly dispersed on the surface of anatase TiO₂ NSs with a metallic state. The Raman scattering and visible light absorption performances of Ag/TiO₂ NSs were enhanced by Ag NPs due to their surface plasmon resonance effect. Photocatalytic oxidation experiments for HCHO were carried out under visible light, and the enhanced photocatalytic activity of Ag/TiO₂ NSs can be attributed to the synergistic effects of the following factors: (1) the {001} facets, which possessed higher surface energy, showed higher photocatalytic activity; (2) the Ag NPs, the increased oxygen vacancies, and O₂ adsorption on {001} facets can trap photoelectrons, thus inhibiting the recombination of photoelectrons and holes; (3) the Ag NPs can extend the light response range of TiO₂ into visible light. The in situ FTIR results showed that higher mineralization efficiency of HCHO was achieved on Ag/TiO₂ NSs than on Ag/TiO₂ NPs. Additionally, the mechanism for HCHO photocatalytic oxidation was also discussed.

Pt supported on long-rod β-FeOOH as an efficient catalyst for HCHO oxidation at ambient temperature

A novel Pt/FeOOH catalyst containing highly active interface between Pt and β-FeOOH (010) plane have been developed for HCHO oxidation at ambient temperature.

Measurement of the surface hydrophobicity of engineered nanoparticles using an atomic force microscope

A scanning probe method based on atomic force microscopy (AFM) was used to probe the nanoscale hydrophobicity of nanomaterials in liquid environments.

A theoretical study of formaldehyde adsorption and decomposition on a WC (0001) surface

A lot of research attention has been paid to designing and exploring efficient adsorbents for HCHO adsorption and decomposition. Herein, we have demonstrated a highly active material, WC, for HCHO adsorption through first-principles calculations. Due to the exposed three-coordinated W atoms (W_3c) of the WC (0001) surface, HCHO molecules can be settled on the WC (0001) surface through newly formed O_F–W_3c and/or C_F–W_3c bonds, forming different adsorption configurations. When settled on the WC (0001) surface, the molecular configuration of the HCHO molecule and the corresponding C_F–H_F and C_F–O_F bond lengths would be greatly changed. Due to the enlarged C_F–H_F and C_F–O_F bond lengths, the adsorbed HCHO molecules tend to dissociate through two possible pathways; these are the two-step C_F–H_F bond scission or the one-step C_F–O_F bond scission. The former results in two H adatoms and a CO molecule chemisorbed to the surface and the latter produces an O adatom and a CH₂ group on the surface. Further Cl-NEB calculations demonstrate that the pre-adsorbed O atom has little influence on the first C_F–H_F bond scission and the C_F–O_F bond scission, while promoting the second C_F–H_F bond scission. Considering the dissociative products, H and CH₂ have the potential to couple into a CH₃ group (or even a CH₄ molecule) and two CH₂ groups may couple into a C₂H₄ molecule. In the end, we propose that OH ions may couple with the dissociative products of HCHO, so an alkali solution could be used to post-process the WC (0001) surface to restore its surface active sites. These results demonstrated the potential of WC in HCHO sensing and abatement.

WC is a material capable of HCHO adsorption and dissociation, indicating its potential application in HCHO sensing and elimination.

Preparation of a Pt/TiO2/cotton fiber composite catalyst with low air resistance for efficient formaldehyde oxidation at room temperature

Pt/TiO₂/cotton fiber catalyst was successfully prepared with much lower air resistance than powder-like sample. It can catalyze oxidation of HCHO into CO₂ and H₂O with an optimum Pt loading of 0.75 wt%. It also exhibited good catalytic stability.

High-surface area mesoporous Pt/TiO₂ hollow chains for efficient formaldehyde decomposition at ambient temperature

Room-temperature catalytic decomposition of formaldehyde (HCHO) is considered as one of the most main methods for the removal of indoor HCHO due to its facile reaction conditions. Herein, high-surface area mesoporous Pt/TiO2 hollow chains were synthesized in high yield by using a simple microwave-hydrothermal route, followed by a combined NaOH-assisted NaBH4-reduction deposition of Pt nanoparticles on the as-obtained TiO2 surface. The catalytic activity for HCHO decomposition was evaluated at room temperature. The prepared Pt/TiO2 hollow chains with an optimal Pt loading of 0.5 wt.% exhibited high catalytic activity and recyclability. The apparent reaction rate constant of HCHO oxidation over this catalyst was approximately 1.42×10(-3) ppm(-1) min(-1), exceeding that of the commercial Degussa P25 TiO2 with equal Pt content (k=5.36×10(-4) ppm(-1) min(-1)) by a factor of approximately 2.65. The high catalytic activity of the Pt/TiO2 hollow chains could be mainly attributed to the hollow chain-like structure, high specific surface area, numerous mesopores, and high pore volume of TiO2 support. Consequently, the catalysts exhibited high adsorption capacity for HCHO, fast diffusion and transport of gas molecules, and good contact between gases and active sites. These characteristics enhanced the catalytic activity.

Room-temperature catalytic oxidation of formaldehyde on catalysts

Room-temperature catalytic oxidative decomposition of harmful formaldehyde (HCHO) in indoor air is summarized.

Non-covalent functionalization of WS2 monolayer with small fullerenes: tuning electronic properties and photoactivity

Atomically thin 2-D transition metal dichalcogenide (TMDCs) heterostructures have attracted growing interest due to their massive potential in solar energy applications due to their visible band gap and very strong light–matter interactions.

Flexible Mg–Al layered double hydroxide supported Pt on Al foil for use in room-temperature catalytic decomposition of formaldehyde

Flexible and bendable Mg–Al layered double hydroxide supported Pt catalysts fabricated and used in room-temperature catalytic decomposition of formaldehyde.

Electronic properties and photoactivity of monolayer MoS2/fullerene van der Waals heterostructures

van der Waals (vdW) heterostructures have attracted immense interest recently due to their unusual properties and new phenomena.

Effects of the interaction between TiO2 with different percentages of exposed {001} facets and Cu(2+) on biotoxicity in Daphnia magna

Anatase TiO₂ nanosheets (NSs) with exposed {001} facets have been widely used because of their high activity and particular surface atomic configuration. However, investigations on their biotoxicity are rare. In this study, bioaccumulation of five different TiO₂ (with 10%, 61%, 71%, 74% and 78% exposed {001} facets), as well as copper and enzyme activities in Daphnia magna, are systematically investigated and rationalized. The results indicated that the addition of Cu²⁺ enhanced agglomeration–sedimentation of TiO₂, resulting in the reduction of TiO₂ bioaccumulation by 10% to 26%. TiO₂ nanoparticles (NPs) increased copper bioaccumulation by 9.8%, whereas the other four TiO₂ nanosheets (NSs) decreased it by 43% to 53%, which depended on TiO₂ variant adsorption and free Cu²⁺ concentrations in the supernatant. The levels of superoxide dismutase (SOD) enzyme and Na⁺/K⁺-ATPase activities suggested that oxidative stress, instead of membrane damage, was the main toxicity in D. magna. Meanwhile, the SOD enzyme activities increased with decreasing Cu accumulation and increasing Ti accumulation because of the different functions of Cu and Ti in organisms. This research highlighted the important role of the percentage of exposed {001} facets in nanostructured TiO₂ on bioaccumulation and biotoxicity of TiO₂ and Cu²⁺ in Daphnia magna.

Roles of Crystal Surface in Pt-Loaded Titania for Photocatalytic Conversion of Organic Pollutants: A First-Principle Theoretical Calculation

Titania modified with nanosized metallic clusters is found to substantially enhance its photocatalytic capacity for renewable energy generation and environmental purification, but the underlying mechanism, especially the roles of crystal surface in noble-metal-loaded TiO2, remain unclear. In this work, such roles in the Pt-loaded anatase TiO2 for the photocatalytic conversion of nitrobenzene (NB), a model pollutant, are explored by first-principle calculations. The theoretical calculations reveal that the Pt-TiO2 complex has a higher catalytic activity toward NB conversion than pure Pt clusters, and the (001) facets of TiO2 in this complex tend to accumulate more positively charged holes and thus have a higher photocatalytic activity than the (101) facets. Furthermore, the thermodynamic and kinetic results also show that the Pt cluster loaded on the (001) surface of anatase TiO2 is favored for NB conversion in the photooxidation pathway. This work deepens our fundamental understanding on the evolution of molecule-photocatalyst interface and provides implications for designing and preparing photocatalysts.

Highly active mesoporous ferrihydrite supported pt catalyst for formaldehyde removal at room temperature

Ferrihydrite (Fh) supported Pt (Pt/Fh) catalyst was first prepared by combining microemulsion and NaBH4 reduction methods and investigated for room-temperature removal of formaldehyde (HCHO). It was found that the order of addition of Pt precursor and ferrihydrite in the preparation process has an important effect on the microstructure and performance of the catalyst. Pt/Fh was shown to be an efficient catalyst for complete oxidation of HCHO at room temperature, featuring higher activity than magnetite supported Pt (Pt/Fe3O4). Pt/Fh and Pt/Fe3O4 exhibited much higher catalytic activity than Pt supported over calcined Fh and TiO2. The abundance of surface hydroxyls, high Pt dispersion and excellent adsorption performance of Fh are responsible for superior catalytic activity and stability of the Pt/Fh catalyst. This work provides some indications into the design and fabrication of the cost-effective and environmentally benign catalysts with excellent adsorption and catalytic oxidation performances for HCHO removal at room temperature.

Enhanced catalytic activity of hierarchically macro-/mesoporous Pt/TiO2 toward room-temperature decomposition of formaldehyde

Hierarchically macro-/mesoporous Pt/TiO₂ exhibits excellent catalytic activity and recyclability toward catalytic decomposition of formaldehyde in air at room temperature.

Efficient removal of formaldehyde by nanosized gold on well-defined CeO₂ nanorods at room temperature

Gold (Au) nanoparticles (NPs) supported on well-defined ceria (CeO2) nanorods with exposed {110} and {100} facets were prepared by a deposition-precipitation method and characterized by powder X-ray diffraction, micro-Raman spectroscopy, X-ray photoelectron spectroscopy, nitrogen adsorption-desorption, transmission electron microscopy, high-resolution transmission electron microscopy, and high-angle annular dark-field scanning transmission electron microscopy. Both nanometer and subnanometer gold particles were found to coexist on ceria supports with various Au contents (0.01-5.4 wt %). The catalytic performance of Au/CeO2 catalysts was examined for formaldehyde (HCHO) oxidation into CO2 and H2O at room temperature and shown to be Au content dependent, with 1.8 wt % Au/CeO2 displaying the best performance. On the basis of the results from hydrogen temperature-programmed reduction and in situ Fourier transform infrared spectroscopy observations, the high reactivity and stability of Au/CeO2 catalysts is mainly attributed to the well-defined ceria nanorods with {110} and {100} facets which present a relatively low energy for oxygen vacancy formation. Furthermore, gold NPs could induce the weakened Ce-O bond which in turn promotes HCHO oxidation.

Facet-dependent photocatalytic properties of TiO(2) -based composites for energy conversion and environmental remediation

Titanium dioxide (TiO2 ) is one of the most widely investigated metal oxides because of its extraordinary surface, electronic, and photocatalytic properties. However, the large band gap of TiO2 and the considerable recombination of photogenerated electron-hole pairs limit its photocatalytic efficiency. Therefore, research attention is being increasingly directed towards engineering the surface structure of TiO2 on the atomic level (namely morphological control of {001} facets on the micro- and nanoscale) to fine-tune its physicochemical properties; this could ultimately lead to the optimization of selectivity and reactivity. This Review encompasses the fundamental principles to enhance the photocatalytic activity by using highly reactive {001}-faceted TiO2 -based composites. The current progress of such composites, with particular emphasis on the photodegradation of pollutants and photocatalytic water splitting for hydrogen generation, is also discussed. The progresses made are thoroughly examined for achieving remarkable photocatalytic performances, with additional insights with regard to charge transfer. Finally, a summary and some perspectives on the challenges and new research directions for future exploitation in this emerging frontier are provided, which hopefully would allow for harnessing the outstanding structural and electronic properties of {001} facets for various energy- and environmental-related applications.

Highly reactive {001} facets of TiO2-based composites: synthesis, formation mechanism and characterization

Titanium dioxide (TiO2) is one of the most widely investigated metal oxides due to its extraordinary surface, electronic and catalytic properties. However, the large band gap of TiO2 and massive recombination of photogenerated electron-hole pairs limit its photocatalytic and photovoltaic efficiency. Therefore, increasing research attention is now being directed towards engineering the surface structure of TiO2 at the most fundamental and atomic level namely morphological control of {001} facets in the range of microscale and nanoscale to fine-tune its physicochemical properties, which could ultimately lead to the optimization of its selectivity and reactivity. The synthesis of {001}-faceted TiO2 is currently one of the most active interdisciplinary research areas and demonstrations of catalytic enhancement are abundant. Modifications such as metal and non-metal doping have also been extensively studied to extend its band gap to the visible light region. This steady progress has demonstrated that TiO2-based composites with {001} facets are playing and will continue to play an indispensable role in the environmental remediation and in the search for clean and renewable energy technologies. This review encompasses the state-of-the-art research activities and latest advancements in the design of highly reactive {001} facet-dominated TiO2via various strategies, including hydrothermal/solvothermal, high temperature gas phase reactions and non-hydrolytic alcoholysis methods. The stabilization of {001} facets using fluorine-containing species and fluorine-free capping agents is also critically discussed in this review. To overcome the large band gap of TiO2 and rapid recombination of photogenerated charge carriers, modifications are carried out to manipulate its electronic band structure, including transition metal doping, noble metal doping, non-metal doping and incorporating graphene as a two-dimensional (2D) catalyst support. The advancements made in these aspects are thoroughly examined, with additional insights related to the charge transfer events for each strategy of the modified-TiO2 composites. Finally, we offer a summary and some invigorating perspectives on the major challenges and new research directions for future exploitation in this emerging frontier, which we hope will advance us to rationally harness the outstanding structural and electronic properties of {001} facets for various environmental and energy-related applications.

Microemulsion-assisted synthesis of mesoporous aluminum oxyhydroxide nanoflakes for efficient removal of gaseous formaldehyde

Mesoporous aluminum oxyhydroxides composed of nanoflakes were prepared via a water-in-oil microemulsion-assisted hydrothermal process at 50 °C using aluminum salts as precursors and ammonium hydroxide as a precipitating agent. The microstructure, morphology, and textural properties of the as-prepared materials were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption, and X-ray photoelectron spectroscopy (XPS) techniques. It is shown that the aluminum oxyhydroxide nanostructures studied are effective adsorbents for removal of formaldehyde (HCHO) at ambient temperature, due to the abundance of surface hydroxyl groups, large specific surface area, and suitable pore size. Also, the type of aluminum precursor was essential for the microstructure formation and adsorption performance of the resulting materials. Namely, the sample prepared from aluminum sulfate (Al-s) exhibited a relatively high HCHO adsorption capacity in the first run, while the samples obtained from aluminum nitrate (Al-n) and chloride (Al-c) exhibited high adsorption capacity and relatively stable recyclability. A combination of high surface area and strong surface affinity of the prepared aluminum oxyhydroxide make this material a promising HCHO adsorbent for indoor air purification.