NH3 adsorption on anatase-TiO2(101)

Local hydrogen bonding environment induces the deprotonation of surface hydroxyl for continuing ammonia decomposition

There is still a paucity of fundamental understanding about the reaction of ammonia decomposition over TiO2, especially the role of water. Herein, FPMD and DFT calculations were used to address this concern. The results reveal that ammonia decomposition in pure ammonia causes the hydroxylation of the surfaces and reduction of the proton acceptor sites, making proton transfer (PT) difficult, increasing the distances between the NH3 and Obr sites and changing the adsorption configurations of NH3, which are not favourable for accepting protons from NH3 dissociation. When water is introduced, the local hydrogen bonding environment, consisting of NH3 and H2O with the H2O dynamically close to the ObrH, promotes the increase of the positive charge of H atoms from 0.133 to 1.47 e, which increases the ObrH bond dipole moment from 1.136 to 1.400 Debye, resulting in the shortening of the H-bond distances between NH3 and ObrH (1.858 vs. 1.945 Å of only NH3) and enlarging the ObrH bonds (0.980 vs. 1.120 Å). This reduces the activation energy barriers of ObrH deprotonation and causes the surfaces to have low hydroxyl coverage from 0.425 to 0.382 eV. Our study reveals the role of water and provides new insights into ammonia decomposition on TiO2.

Construction of Oxygen Vacancy-Rich TiO2 Nanocrystals for Boosting the Ammonolysis of Caprolactam to 6-Aminocapronitrile

Hexamethylene diamine, an important chemical intermediate for polyamides, can be synthesized through the two-step route of caprolactam (CPL) ammonolysis to 6-aminocapronitrile (ACN), followed by hydrogenation. This method has received increasing attention from academia and industry. However, studies on the catalyst structure-performance correlation in CPL ammonolysis are still sporadic. In this work, a series of anatase TiO2 with different oxygen vacancy concentrations was prepared by chemical reduction using NaBH4. The oxygen vacancy on TiO2 surface, presented as Ti3+ sites, substantially enhances the adsorption and activation of NH3, which are demonstrated as the key steps in ammonolysis. Owing to the synergistic effect of Ti3+ and Ti4+ species, the CPL conversion rate and ACN selectivity of 85 and 97%, respectively, are achieved within 250 h. Density functional theory calculations showed that the intermediates on oxygen vacancy-rich TiO2 had a more favorable adsorption energy compared to those on intact TiO2, which is in good agreement with the experimental results.

Nano-TiO2 aggravates immunotoxic effects of chronic ammonia stress in zebrafish (Danio rerio) intestine

Ammonia and nano-TiO₂ are commonly found pollutants in aquatic environments around the world. NH₃ has been proved to be absorbed on nano-TiO₂ surface, therefore, the biosafety and environmental effects of ammonia and co-occurring nano-TiO₂ in aquatic environments has increased considerably in recent years. To explore the potential interactive effects and mechanisms of ammonia and nano-TiO₂ on the intestinal immune system, three-month-old female zebrafish were exposed to total ammonia nitrogen (TAN; 0, 3, 30 mg/L) with or without nano-TiO₂ (1 mg/L) for 60 d. The results showed that intestinal ammonia levels increased with the increase of TAN exposure concentration in the presence of nano-TiO₂. Histopathological analysis demonstrated that both TAN and nano-TiO₂ caused cell vacuolation, lymphocyte infiltration and goblet cells hyperplasia in the intestine mucosa. Our study also found that the contents and gene expression levels of lysozyme (lys) and β-defensin (def-β) in the intestine of zebrafish exposed to TAN alone or combined with nano-TiO₂ were significantly reduced, suggesting a decline in the intestinal innate immunity of fish. A broad upregulation of TLRs-related genes indicated that TAN and nano-TiO₂ could activate TLR4/5-mediated MyD88-dependent pathway, and eventually induce intestinal inflammation. It should be noted that TAN combined with nano-TiO₂ had more significant inhibitory effects on the intestinal structure and innate immune responses than TAN alone. Current data suggested that ammonia and nano-TiO₂ had a synergistic inhibitory effect on intestinal mucosal immunity, and their associated health risk to aquatic animals and the water ecosystem should not be underestimated.

Ammonia in the presence of nano titanium dioxide (nano-TiO2) induces greater oxidative damage in the gill and liver of female zebrafish

Water pollution caused by a highly hazardous chemical ammonia and a widespread application nanomaterials-nano titanium dioxide (n-TiO₂) in nature water has attracted extensive concern of the world. However, the potential joint effects of the two factors are unknown. Aim to investigate the potential interactive effects of ammonia and n-TiO₂ and the behind mechanisms, adult female zebrafish (Danio rerio) were co-exposed for 8 weeks by total ammonia nitrogen (TAN; 0, 3, 30 mg/L) and n-TiO₂ (0, 0.1, 1 mg/L) in different combination conditions based on a full-factorial design. The analysis of absorption kinetics confirmed that n-TiO₂ could absorb free ammonia (NH₃) in aqueous solution and the loss rate of free NH₃ increased with the rise of n-TiO₂ concentration. Consistent with this, free NH₃ concentrations in the gill and liver were higher in the presence of n-TiO₂ compared to TAN exposure alone. The increases of MDA and PC concentrations in the gill and liver of fish indicated that TAN and n-TiO₂ alone or in combination caused oxidative stress. Simultaneously, the activity and transcription of antioxidant enzymes (T-SOD, CuZn-SOD, Mn-SOD, CAT, GPx and GST) as well as antioxidant GSH contents were extensively inhibited by TAN and n-TiO₂ via Nrf2-Keap1 signaling. The significant interactive effects of TAN and n-TiO₂ were detected on levels of GSH, GST and gstr1 mRNA in the gill, and on levels of GSH, T-SOD, Mn-SOD, CAT levels as well as gpx1a and keap1 mRNAs in the liver, implying synergistic toxic risk of TAN and n-TiO₂. The more severe histopathological alterations and higher IBR analysis in co-treatment groups further proved that the existence of n-TiO₂ excavated ammonia-induced toxicity in the gill and liver, especially in liver. In conclusion, ammonia and n-TiO₂ have a synergistic toxic risk of fish health because ammonia and n-TiO₂ cause oxidative-antioxidative imbalance by inducing ROS overproduction.

Elasticity of Cross-Linked Titania Nanocrystal Assemblies Probed by AFM-Bulge Tests

In order to enable advanced technological applications of nanocrystal composites, e.g., as functional coatings and layers in flexible optics and electronics, it is necessary to understand and control their mechanical properties. The objective of this study was to show how the elasticity of such composites depends on the nanocrystals’ dimensionality. To this end, thin films of titania nanodots (TNDs; diameter: ~3–7 nm), nanorods (TNRs; diameter: ~3.4 nm; length: ~29 nm), and nanoplates (TNPs; thickness: ~6 nm; edge length: ~34 nm) were assembled via layer-by-layer spin-coating. 1,12-dodecanedioic acid (12DAC) was added to cross-link the nanocrystals and to enable regular film deposition. The optical attenuation coefficients of the films were determined by ultraviolet/visible (UV/vis) absorbance measurements, revealing much lower values than those known for titania films prepared via chemical vapor deposition (CVD). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed a homogeneous coverage of the substrates on the µm-scale but a highly disordered arrangement of nanocrystals on the nm-scale. X-ray photoelectron spectroscopy (XPS) analyses confirmed the presence of the 12DAC cross-linker after film fabrication. After transferring the films onto silicon substrates featuring circular apertures (diameter: 32–111 µm), freestanding membranes (thickness: 20–42 nm) were obtained and subjected to atomic force microscopy bulge tests (AFM-bulge tests). These measurements revealed increasing elastic moduli with increasing dimensionality of the nanocrystals, i.e., 2.57 ± 0.18 GPa for the TND films, 5.22 ± 0.39 GPa for the TNR films, and 7.21 ± 1.04 GPa for the TNP films.