Computational study on the reactions of H2O2 on TiO2 anatase (101) and rutile (110) surfaces

Structure-sensitive epoxidation of dicyclopentadiene over TiO2 catalysts

Epoxidation of dicyclopentadiene (DCPD) is studied on a series of TiO₂ catalysts using hydrogen peroxide as an oxidant. DCPD derivatives have applications in several areas including polymer, pharmaceutical and pesticide products. The control of selectivity leading to the desired product is important for many of these applications. Using experimental and computational studies, we show that the surface crystalline phases of TiO₂ play crucial roles not only in the formation of peroxo species but also in the selective epoxidation of two different CC double bonds in DCPD.

H2O2adsorption and dissociation on various CeO2(111) surface models: a first-principles study

Periodic density functional theory (DFT) calculations using the hybrid PBE0 functional and atom-centered Gaussian functions as basis sets were carried out to investigate the absorption and the first steps involved in the decomposition of hydrogen peroxide (H₂O₂) on three different models of the ceria (111) surface. One of the models is a clean surface, and the others are defective and partially hydroxylated ceria surfaces. On the clean surface, we found that the minimum energy path of hydrogen peroxide decomposition involves a three-step process, i.e., adsorption, deprotonation, and formation of the peroxide anion, stabilized through its interaction with the surface at a Ce (IV) site, with activation barriers of less than about 0.5 eV. The subsequent formation of superoxide anions and molecular oxygen species is attributed to electron transfer from the reactants to the Ce (IV) ions underneath. On the defective surface, H₂O₂dissociation is an energetically downhill reaction thermodynamically driven by the healing of the O vacancies, after the reduction and decomposition of H₂O₂into oxygen and water. On the hydroxylated surface, H₂O₂is first adsorbed by forming a favorable H-bond and then undergoes heterolytic dissociation, forming two hydroxyl groups at two vicinal Ce sites.

Treatment of organic wastewater by a synergic electrocatalysis process with Ti3+ self-doped TiO2 nanotube arrays electrode as both cathode and anode

Electrochemical anodic oxidation (AO) is a promising technology for wastewater treatment due to its strong oxidation property and environmental compatibility. However, it suffers from high energy consumption for pollutants removal due to the side-reactions of hydrogen evolution reaction on cathode and oxygen evolution reaction on anode. Combining electro-Fenton (EF) with AO not only generated •OH for pollutants degradation but also increased current efficiency. This work investigated a synergic electrocatalysis process between EF and AO with Ti³⁺ self-doped TiO₂ nanotube arrays (Ti³⁺/TNTAs) electrode as both cathode and anode for wastewater treatment. The pseudo-first-order kinetic rate constant of phenol degradation by EF+AO (0.107 min^-1) was 9.7 or 6.3 times as much as that of only EF (0.011 min^-1) or AO (0.017 min^-1) process, respectively. Enhanced pollutants removal of EF+AO could be attributed to the coexistence of •OH oxidation and direct oxidation on Ti³⁺/TNTAs surface. The COD of secondary effluent of coking wastewater decreased from 159.3 mg L^-1 to 47.0 mg L^-1 by EF+AO within 120 min with low specific energy consumption (9.5 kWh kg^-1 COD^-1). This work provided a new insight into design of the energy-efficient synergic electrocatalysis process for refractory pollutants degradation.

A permeable electrochemical reactive barrier for underground water remediation using TiO2/graphite composites as heterogeneous electrocatalysts without releasing of chemical substances

Permeable reactive barriers (PRBs) are well-studied and widely-applied technologies in underground water remediation. However, the releasing of chemical substances cannot be avoided during the PRBs operation. In this study, a novel permeable electrochemical reactive barrier (PERB) was fabricated for underground water remediation using a TiO₂/graphite composite (TiO₂/C) as the heterogeneous electrocatalyst. TiO₂/C performed an electro-Fenton-like reaction on cathode and an anodic oxidation on anode respectively, along with the variety of the TiO₂ lattice. The performance of this PERB system was evaluated using tetracycline hydrochloride (TTC) degradation. TTC could be degraded at a low applied potential and a wide range of pH. The degradation rate of about 60% was obtained at the optimized reaction condition: the interelectrode potential difference of 1.2 V, pH 3.0, the anode 10 cm above cathode. The relative position and spacing of the electrodes effected the mass transfer equilibrium of TTC. During the 25-day persistent degradation of TTC, the PERB system shown a perfect stability with rarely leaching of Ti. This work explored the potential for underground water remediation by the electrocatalysis with the goal of establishing a clean and eco-friendly PERB system.

Deployment of MIL-88B(Fe)/TiO2 Nanotube-Supported Ti Wires as Reusable Electrochemiluminescence Microelectrodes for Noninvasive Sensing of H2O2 from Single Cancer Cells

As one of the significant intracellular signaling molecules, hydrogen peroxide (H₂O₂) regulates some vital biological processes. However, it remains a challenge to develop noninvasive electrodes that can be used for sensing trace H₂O₂ at the cellular level. Here, we evaluated a high-performance solid-state electrochemiluminescence (ECL) H₂O₂ sensor based on MIL-88B(Fe) nanocrystal-anchored Ti microwires. Semiconducting TiO₂ nanotubes (TiNTs) vertically grown around a Ti wire via an anodization technique act as an intrinsic ECL luminophore. By integrating with MIL-88B(Fe), the synergistic effect of the TiO₂ luminophore and the remarkable peroxidase-like activity of MIL-88B(Fe) enable the resulting H₂O₂ sensor an ultrahigh sensitivity featuring a minimum detection limit of 0.1 nM (S/N = 3), long-term stability, high durativity, and wide-range linear response to a concentration of up to 10 mM. To demonstrate the concept of a MIL-88B(Fe)@TiO₂ microelectrode for single-cell sensing, the electrode was used to detect intracellular H₂O₂ in a single cell. Moreover, benefiting from the heterojunction of MIL-88B(Fe)/TiO₂, the microelectrode was found to exhibit excellent photocatalytic activity in the visible-light range, that is, the sensor surface can be self-cleaning after a short visible-light treatment. These advanced sensor characteristics involving easy reusability reveal that the MIL-88B(Fe)@TiO₂ microelectrode is a new platform for cytosensing. This study provides a new strategy to design semiconductor materials with arbitrary shape and size, allowing for profound applications in biomedical and clinical analysis.

The effect of Ag atom doped Cu@CuO core-shell structure on its electronic properties and catalytic performance: a first principles study

Density functional theory was used to study the Ag-doped Cu@CuO core-shell structure, electronic properties and catalytic properties. Similar to the undoped Cu@CuO clusters, the Ag doped clusters also retain the core-shell structure. Ag doping increases the charge transfer between surrounding O atoms and Cu atoms and reduces the potential of the core-shell structure, thereby increasing its surface activity. The study of its orbital distribution found that the doping of Ag atoms caused the interaction between the inner Cu core and the outer CuO shell, which changed the electron orbital motion inside the shell. The internal chemical stability of the core-shell material is improved. In addition, Ag atom doping accelerates the decomposition of H₂O₂ on Cu@CuO structure and increases its adsorption of small molecules, which indicates that Ag atom doping improves the catalytic performance of Cu@CuO structure.

Impact of H2O2 on the Lactic and Formic Acid Degradation in Presence of TiO2 Rutile and Anatase Phases under UV and Visible Light

The degradation rates of formic acid and lactic acid in the presence and absence of H2O2 were studied, utilizing several TiO2 catalysts: PC105 (100% anatase), MPT 625 (100% rutile), and P25 (80% anatase/20% rutile), and the results were discussed with regards to the current literature. The impact of hydrogen peroxide on the photocatalytic efficiency of eleven TiO2 samples was then determined, using commercial anatase structures (PC105, PC500, UV100), commercial mixed anatase/rutile (P25 and P90), and six rutile (two commercial samples: MPT 625 and C-R160, and four home-made rutile samples were synthesized by TiCl4 hydrolysis). The effect of catalyst surface area and TiO2 phase on the degradation rate of lactic acid (LA) and the decomposition of H2O2 was studied and discussed in regard to the active species generated. The intermediate products formed in the absence and presence of H2O2 were also an important factor in the comparison. Finally, the efficiency of the degradation of LA and formic acid (FA) in the presence of rutile and H2O2 was determined under visible light, and their reactivity was compared. The intermediate products formed in the degradation of LA were identified and quantified and compared to those obtained under UV (Ultra-Violet).

Direct evidence of the key role of UV-formed peroxide species in photocatalytic gas-solid oxidation in air on anatase TiO2 particles

IR spectroscopy was applied for the in situ investigations of surface intermediates formed on the surface of TiO2 (anatase) and ZnO under UV light illumination in air and their reactivity in the elimination of simple pollutant molecules. UV-irradiation of TiO2 (anatase) in air leads to the generation of peroxo-species with the peaks at 852 and 912 cm-1, but the bands of O2˙-ads were not detected. This is, to our knowledge, the first direct in situ IR spectroscopic detection of O2 photosorption intermediates in moist air. The formation of peroxo-species in these conditions is specific for TiO2 (anatase), whereas on ZnO the predominant species under UV light illumination in air are O2˙-ads and H2O2, desorbing into gas phase. Adsorbed water and surface hydroxyl groups contribute to the formation and stabilization of peroxo-species on TiO2 anatase during UV illumination in an oxygen atmosphere. If UV-irradiation is carried out in the environment of moist argon instead of moist air, the peroxo-species on TiO2 anatase are formed from water in a negligible quantity. Peroxo-species formed after O2 photoadsorption on TiO2 anatase in moist air have band positions similar to peroxo-species formed after photodecomposition of H2O2 (with accompanying color change of this sample from yellow to white). Direct experimental IR-spectroscopic evidence of peroxo-species reactivity as oxidative intermediates on TiO2 (anatase) in CO and ethanol vapor photooxidative processes is firstly obtained. These results confirm our early conclusion that peroxo-species formed under UV-irradiation in O2 on the hydrated surface of TiO2 (anatase) can be responsible for the surprising extreme dependence of the CO photooxidation rate on the adsorbed water coverage with the maximum at ∼0.5 ML. The ZnO sample was not active in the photooxidation of these molecules in air. It is concluded that UV formed peroxo-species are important diamagnetic oxidative intermediates in heterogeneous photochemical gas-solid oxidation processes on TiO2 (anatase).

The elusive photocatalytic water splitting reaction using sunlight on suspended nanoparticles: is there a way forward?

For many decades hydrogen production from water by photocatalytic methods has been pursued over a variety of semiconductor powder catalysts featuring many structures and compositions. The stoichiometric formation of molecular hydrogen and oxygen has stayed largely elusive.

KSCN-induced Interfacial Dipole in Black TiO2 for Enhanced Photocatalytic CO2 Reduction

Tuning the electronic band structure of black titania to improve photocatalytic performance through conventional band engineering methods has been challenging because of the defect-induced charge carrier and trapping sites. In this study, KSCN-modified hydrogenated nickel nanocluster-modified black TiO₂ (SCN-H-Ni-TiO₂) exhibits enhanced photocatalytic CO₂ reduction due to the interfacial dipole effect. Upon combining the experimental and theoretical simulation approach, the presence of an electrostatic interfacial dipole associated with chemisorption of SCN has dramatic effects on the photocatalyst band structure in SCN-H-Ni-TiO₂. An interfacial dipole possesses a more negative zeta potential shift of the isoelectric point from 5.20 to 3.20, which will accelerate the charge carrier separation and electron transfer process. Thiocyanate ion passivation on black TiO₂ demonstrated an increased work function around 0.60 eV, which was induced by the interracial dipole effect. Overall, the SCN-H-Ni-TiO₂ photocatalyst showed an enhanced CO₂ reduction to solar fuel yield by 2.80 times higher than H-Ni-TiO₂ and retained around 88% product formation yield after 40 h.

The fate of O2 in photocatalytic CO2 reduction on TiO2 under conditions of highest purity

Modification of P25-TiO₂ with IrO_x allowed the detection of gas-phase O₂ during photocatalytic CO₂ reduction with H₂O. The effect on the overall CO₂ conversion on P25 is discussed.

EUV-induced oxidation of carbon on TiO2

Previously we reported estimates of the maximum etch rates of C on TiO₂ by oxidizers including NO, O₃ and H₂O₂ when irradiated by a spatially-non-uniform beam of extreme ultraviolet (EUV) radiation at 13.5 nm (Faradzhev et al., 2013). Here we extend that work by presenting temporally and spatially resolved measurements of the C etching by these oxidizers as a function of EUV intensity in the range (0.3 to 3) mW/mm² [(0.2 to 2) × 10¹⁶ photons s^-1 cm^-2]. We find that the rates for NO scale linearly with intensity and are smaller than those for O₃, which exhibit a weak, sub-linear intensity dependence in this range. We demonstrate that these behaviors are consistent with adsorption of the oxidizing precursor on the C surface followed by a photon-stimulated reaction resulting in volatile C-containing products. The kinetics of photon-induced C etching by hydrogen peroxide, however, appear to be more complex. The spatially resolved measurements reveal that C removal by H₂O₂ begins at the edges of the C spot, where the light intensity is the lowest, and proceeds toward the center of the spot. This localization of the reaction may occur because hydroxyl radicals are produced efficiently on the catalytically active TiO₂ surface.

Quantum chemical investigation on the role of Li adsorbed on anatase (101) surface nano-materials on the storage of molecular hydrogen

Lithiation of TiO2 has been shown to enhance the storage of hydrogen up to 5.6 wt% (Hu et al. J Am Chem Soc 128:11740-11741, 2006). The mechanism for the process is still unknown. In this work we have carried out a study on the adsorption and diffusion of Li atoms on the surface and migration into subsurface layers of anatase (101) by periodic density functional theory calculations implementing on-site Coulomb interactions (DFT+U). The model consists of 24 [TiO2] units with 11.097 × 7.655 Å(2) surface area. Adsorption energies have been calculated for different Li atoms (1-14) on the surface. A maximum of 13 Li atoms can be accommodated on the surface at two bridged O, Ti-O, and Ti atom adsorption sites, with 83 kcal mol(-1) adsorption energy for a single Li atom adsorbed between two bridged O atoms from where it can migrate into the subsurface layer with 27 kcal mol(-1) energy barrier. The predicted adsorption energies for H2 on the lithiated TiO2 (101) surface with 1-10 Li atoms revealed that the highest adsorption energies occurred on 1-Li, 5-Li, and 9-Li surfaces with 3.5, 4.4, and 7.6 kcal mol(-1), respectively. The values decrease rapidly with additional H2 co-adsorbed on the lithiated surfaces; the maximum H2 adsorption on the 9Li-TiO2(a) surface was estimated to be only 0.32 wt% under 100 atm H2 pressure at 77 K. The result of Bader charge analysis indicated that the reduction of Ti occurred depending on the Li atoms covered on the TiO2 surface.

Binding mechanism of arsenate on rutile (110) and (001) planes studied using grazing-incidence EXAFS measurement and DFT calculation

Characterization of contaminant molecules on different exposed crystal planes is required to conclusively describe its behavior on mineral surfaces. Here, the structural properties and relative stability of arsenate adsorbed on rutile TiO2 (110) and (001) surfaces were investigated using grazing-incidence extended X-ray absorption fine structure (GI-EXAFS) spectra and periodic density functional theory (DFT) calculation. The combined results indicated that arsenate mainly formed inner-sphere bidentate binuclear (BB) and monodentate mononuclear (MM) complexes on both surfaces, but the orientational polar angles of arsenate on the (110) surface were commonly smaller than that on the (001) surface for the two adsorption modes. The DFT calculation showed that the (110) plane had a higher affinity toward arsenate than the (001) plane, suggesting that, for a given adsorption mode (i.e., MM or BB structure), a small polar angle was more favorable for arsenate stabilized on the rutile surfaces.

Reactivity of metal oxide clusters with hydrogen peroxide and water--a DFT study evaluating the performance of different exchange-correlation functionals

We have performed a density functional theory (DFT) investigation of the interactions of H2O2, H2O and HO radicals with clusters of ZrO2, TiO2 and Y2O3. Different modes of H2O adsorption onto the clusters were studied. In almost all the cases the dissociative adsorption is more exothermic than molecular adsorption. At the surfaces where H2O has undergone dissociative adsorption, the adsorption of H2O2 and the transition state for its decomposition are mediated by hydrogen bonding with the surface HO groups. Using the functionals B3LYP, B3LYP-D and M06 with clusters of 26 and 8 units of ZrO2, the M06 functional performed better than B3LYP in describing the reaction of decomposition of H2O2 and the adsorption of H2O. Additionally, we investigated clusters of the type (ZrO2)2, (TiO2)2 and (Y2O3) and the performance of the functionals B3LYP, B3LYP-D, B3LYP*, M06, M06-L, PBE0, PBE and PWPW91 in describing H2O2, H2O and HO˙ adsorption and the energy barrier for decomposition of H2O2. The trends obtained for HO˙ adsorption onto the clusters are discussed in terms of the ionization energy of the metal cation present in the oxide. In order to correctly account for the existence of an energy barrier for the decomposition of H2O2, the functional used must include Hartree-Fock exchange. Using minimal cluster models, the best performance in describing the energy barrier for H2O2 decomposition was obtained with the M06 and PBE0 functionals - the average absolute deviations from experiments are 6 kJ mol(-1) and 5 kJ mol(-1) respectively. With the M06 functional and a larger monoclinic (ZrO2)8 cluster model, the performance is in excellent agreement with experimental data. For the different oxides, PBE0 was found to be the most effective functional in terms of performance and computational time cost.

Effect of hydrogen on O2 adsorption and dissociation on a TiO2 anatase (001) surface

The effect of hydrogen on the adsorption and dissociation of the oxygen molecule on a TiO2 anatase (001) surface is studied by first-principles calculations coupled with the nudged elastic band (NEB) method. Hydrogen adatoms on the surface can increase the absolute value of the adsorption energy of the oxygen molecule. A single H adatom on an anatase (001) surface can lower dramatically the dissociation barrier of the oxygen molecule. The adsorption energy of an O2 molecule is high enough to break the O=O bond. The system energy is lowered after dissociation. If two H adatoms are together on the surface, an oxygen molecule can be also strongly adsorbed, and the adsorption energy is high enough to break the O=O bond. However, the system energy increases after dissociation. Because dissociation of the oxygen molecule on a hydrogenated anatase (001) surface is more efficient, and the oxygen adatoms on the anatase surface can be used to oxidize other adsorbed toxic small gas molecules, hydrogenated anatase is a promising catalyst candidate.