Surface OH Groups Research Articles

Cyclohexane selective photocatalytic oxidation by anatase TiO2: influence of particle size and crystallinity

A systematic study is presented on the effect of crystallite size of Anatase (Hombikat, Sachtleben), varied by calcination at different temperatures up to 800 degrees C, on photocatalytic activity in cyclohexane selective oxidation. Two different reactors were used to test the materials: a top illumination reactor and an in situ ATR-FTIR cell. Properties such as crystallinity and associated availability of holes and electrons for surface reactions, as well as the amount of surface OH-groups, are shown to have a significant influence on TiO(2) activity, (surface) selectivity, and stability. Upon increasing the crystallite size, productivity (g(-1)(catalyst)) decreases, while (i) the TOF (moles of cyclohexanone formed per minute per OH-site), (ii) the rate of cyclohexanone desorption, (iii) catalytic site stability, and (iv) the cyclohexanol/cyclohexanone ratio increase. The results are discussed on the basis of a reaction scheme, and a simple reaction rate equation.

In Situ Reaction Mechanism Studies on Atomic Layer Deposition of Sb2Te3 and GeTe from (Et3Si)2Te and Chlorides

Reaction mechanisms in the atomic layer deposition (ALD) of Sb2Te3 from SbCl3 and (Et3Si)2Te at 60 °C and GeTe from GeCl2·C4H8O2 (1,4-dioxane complex of GeCl2) and (Et3Si)2Te at 90 °C were studied in situ with a quadrupole mass spectrometer (QMS) and a quartz crystal microbalance (QCM). Also some experiments were conducted on reactions in ALD of the phase change material GST (germanium antimony telluride, Ge2Sb2Te5). The byproduct in both the binary telluride processes was found to be Et3SiCl, and about 78% (36%) of it was released during the SbCl3 (GeCl2·C4H8O2) pulse. Obviously −Te(SiEt3) surface groups serve as reactive sites for the metal precursors, cf. −OH surface groups in the oxide ALD processes that use water as the oxygen source. The dioxane, on the other hand, was expectedly found to be released entirely during the GeCl2·C4H8O2 pulse. When depositing GST the mechanism of the SbCl3-(Et3Si)2Te reaction was found to change so that only about 50% of the byproduct Et3SiCl was released during the SbC...

Role of Intrasurface Hydrogen Bonding on Silica Dissolution

A recently formulated reactive Monte Carlo (RxMC) algorithm has been applied to determine the differential dissolution probability in topographically distinct crystallographic faces of β-cristobalite. Although the contribution of edge, kink, and step sites to the overall dissolution from a crystal is well-established, the effect of hydrogen bonding on flat terrace sites is less understood. In an earlier study (J. Phys. Chem. 111, 5169, 2007), 40 β-cristobalite surfaces were shown to have distinct intrasurface hydrogen bonding patterns between the hydroxylated surface groups. This study is an extension of the previous work with the aim to identify the role intrasurface hydrogen bonding has toward overall dissolution from these surfaces. Five surfaces have been chosen with varying ratios of Q2 (two surface OH groups per Si site) and Q3 (one surface OH group per Si site) groups. The results successfully show that pure Q2 {100} terrace sites with intrasurface hydrogen are less reactive than non-hydrogen bonde...

Synthesis of Polymer/Silica Hybrid Nanoparticles Using Anionic Polymerization Techniques

We report a new “grafting to” technique for the functionalization of silica particles with anionically produced polymers. It is based on a two-step procedure. In the first step the silica nanoparticles were modified with multifunctional chlorosilanes. This procedure allows replacing the original Si−OH surface groups by Si−Cl groups. In the second step the anionically synthesized polymers were linked to the Si−Cl functionalized nanoparticle surface. Both the chlorosilane functionalization of the nanoparticles and the subsequent reaction with living polymer can be carried out without irreversible particle aggregation. This was proved by examining the reaction products with static and dynamic light scattering as well as small-angle X-ray scattering. The polymer linking event is accompanied by termination reactions, most likely due to residual Si−OH groups. Therefore, the raw products were purified by a simple fractionation procedure. The examination of the products by size exclusion chromatography showed that this procedure allowed removing the free polymer quantitatively. The new anionic based method offers the possibility for grafting densities up to 1 chain per nm2 of particle surface, which is significantly higher than reported in the past for other “grafting to” approaches. The now obtained grafting densities are similar to the ones reported for “grafting from” techniques which are mainly based on controlled radical polymerization. Our new approach offers the possibility to obtain hybrid materials containing polymers which are not accessible via controlled radical techniques like polydienes. In addition, the new technique allows grafting polymers having molecular weights up to 500 000 g/mol and still narrow molecular weight distributions.

Pre-clinical and behavioural toxicity profile of PAMAM dendrimers in mice

A toxicity study of different polyamidoamine (PAMAM) dendrimers in Swiss albino mice was performed with a clinical overview. In the current study, mice were treated with various G4 dendrimers (NH2and OH surface groups) at different dosage levels (low, medium and high) via the intraperitoneal route for 15 consecutive days, followed by a 15 day recovery period. The toxicity profile was investigated in mice for their general behaviour, feed intake, body weight, carbohydrate, lipid and protein metabolism, haematological parameters, histopathology and cell viability. No significant differences were observed in feed intake, body weight and organ weight among the various dendrimer-fed groups and control. In general, dendrimers have no significant effect on the carbohydrate and lipid metabolism. However, declined glucose levels in the high-NH2dose group indicates the interference of this dosage level with glucose metabolism. Although all the test groups showed activity in the normal range, the high-NH2group showed comparatively higher alanine aminotransferase and aspartate aminotransferase activity. Minor, but insignificant (p> 0.05), differences were observed in the red blood cells, haematocrit value and haemoglobin of the treated group when compared to the control group in a dose-dependent manner, but these recovered during the recovery period. There was no effect on other haematological parameters. Histopathological evaluation of dendrimer-treated groups did not reveal any abnormalities in the low- and medium-dose groups, but at a high dose level, toxicity was observed in the liver and kidney. However, after the recovery period, toxicity in the high dose level was not found. A cell viability study of cationic dendrimers has shown reduced cytotoxicity after the complexation with a guest molecule such as indomethacin. This study showed that dendrimers are not toxic, in general. The high dose of G4-NH2displayed some toxicity; however, that too was recovered after some time.

Synthesis of Hydroxyl-Rich Anatase Nanocrystallites, Their Characterization and Performance as Photoanode in Dye-Sensitized Solar Cells

In this work an environmentally-friendly aqueous solution process was elaborated to produce hydroxyl-rich anatase nanocrystallites of very large surface area. The aqueous-synthesized nanocrystalline anatase material was shown to have favorable physical properties (e.g. 160-280 m2/g specific surface area, 5~8 nm crystallite size, high concentration of OH surface groups) for fabricating photoanodes for dye-sensitized solar cells (DSSCs). The fabrication process involved paste preparation, "double-layer" screen printing, annealing and sensitization with the N719 dye [cis - (2,2'- bipyridyl - 4,4'- dicarboxylate)2 (NCS)2 ruthenium(II)]. Photocurrent -voltage measurements of DSSCs based on the aqueous-synthesized titania showed higher photocurrent and conversion efficiency when compared to cells built from commercial benchmark nanotitania materials. The enhanced performance of the aqueous titania-based DSSC is discussed on the basis of extensive characterization work that included XRD, TEM, UV-Vis and Raman/ATR-FTIR spectroscopy.

Kinetics and mechanism of partial oxidation of ethane to ethylene and acetic acid over MoV type catalysts

A kinetics study for the partial oxidation of ethane to ethylene and acetic acid is performed over MoV type catalysts. It is established that ethylene is the primary product and acetic acid and carbon oxides are secondary products. Formation of acetic acid product increases significantly with the co-feeding of water into the reactor. The elementary steps of the reaction network are formulated using a two-site EleyRideal-Redox (ERR) model, which includes the participation of water in the reaction scheme through surface OH � groups. The derived ERR model predicts the experimental data satisfactorily.

HCl Uptake through Films of Pentanoic Acid and Pentanoic Acid/Hexanol Mixtures at the Surface of Sulfuric Acid

Molecular beam scattering experiments are used to investigate collisions and reactions of HCl with deuterated sulfuric acid containing 0-0.2 M pentanoic acid (PA) and mixtures of PA and hexanol. Surface tension measurements at 296 K indicate that PA segregates to the surface of the acid, reaching coverages of 58% and 52% of maximum packing on 60 and 68 wt % D(2)SO(4), respectively. We find that these films increase HCl entry into the acid at low PA surface coverage at 213 K. This enhancement is attributed to the dissociation of HCl molecules that come into contact with surface COOH groups and protonate them. At higher coverages, the PA film becomes more compact and impedes HCl uptake. Comparisons with films of pure hexanol and pentanoic acid/hexanol mixtures indicate that surface OH groups are more effective than COOH groups in catalyzing HCl entry. They also suggest that the PA films consist of patchy regions of tightly packed molecules, which are pushed away from the surface upon addition of the more surface active hexanol. HCl entry into the pure and mixed films can be analyzed quantitatively using a two-step model in which adsorbed HCl molecules penetrate between the alkyl chains and then dissociate at the surfactant-acid interface.

IR spectroscopic study of the basicity of aminated silica gels

The aminated silica gels SiO2/SOCl2/NH3 (I), SiO2/SiCl4/NH3 (II), SiO2/BCl3/NH3 (III), and SiO2/γ-aminopropyltriethoxysilane (SiO2/APTES, IV) have been synthesized. According to DRIFT spectroscopy and chemical analysis data, the surface amino groups of I–III are “free,” while those of IV interact with the surface OH groups of the silica gel and with one another. The strength of basic sites has been measured on the proton affinity (PA) scale as the shift of the ν(CD) band of adsorbed deuterochloroform. The basicity of an aminated silica gel depends on its chemical composition. Silica gel IV (PA = 938 kJ/mol) is a stronger base than I–III (PA = 829 kJ/mol). As the basicity of the NH2 group decreases, the N-H stretching band shifts to higher frequencies.

Analysis of Surface OH Groups on TiO2 Single Crystal with Polarization Modulation Infrared External Reflection Spectroscopy

The surface OH groups on rutile TiO2 single-crystal surfaces under atmospheric conditions were investigated with polarization modulation infrared external reflection spectroscopy (PM-IER). The PM technique is effective for selective detection of the oriented surface OH groups on the TiO2 surface due to the cancellation of strong IR signals from three-dimensional isotropic water vapor and bulk water adsorbed on the surface under atmospheric conditions. The PM-IER spectrum on the TiO2 (110) surface gave only a single peak at 3279 cm−1. The absence of coexisting symmetric and asymmetric peaks of water molecules reveals that the peak is not derived from the water molecules adsorbed on the surface but from the OH groups directly binding to the TiO2 surface. A remarkable decrease of the peak frequency reaching ca. 400 cm−1 compared to that observed by high-resolution electron energy loss spectroscopy (HREELS) under a vacuum condition (3690 cm−1) was observed, indicating that the surface OH groups under atmosphe...

DFT Study of Hydrogen Storage by Spillover on Graphite with Oxygen Surface Groups

DFT modeling was used to understand the role of epoxide (C-O-C) and hydroxyl (C-OH) functional groups on the spillover mechanism for hydrogen storage on graphite oxide and oxygen-modified carbons. A primary spillover model was used, consisting of a Pt(4) cluster, a graphite substrate model, and O and OH functional groups adsorbed on graphite. The spillover mechanism was found to proceed via the migration of dissociated hydrogen atoms from the Pt cluster to epoxide groups adjacent to the cluster (to form OH), followed by H migration by hopping on the adsorbed O atoms. The low energy barriers required for the relevant elementary steps indicate that the spillover process is facile when the carbon substrate is decorated with oxygen functionalities, leading to enhanced hydrogen uptake and faster charge/discharge kinetics. However, a reaction path was also identified, in which surface OH groups can react to form water, which can have adverse consequences for hydrogen storage on oxygenated carbons via spillover.

General Strategy for High-Density Covalent Functionalization of Diamond Nanoparticles Using Fenton Chemistry

Diamond nanoparticles (npD) from detonation do not undergo covalent functionalization to a large extent, as assessed by conventional IR and NMR spectroscopy. This low level of surface modification is due to the low density of surface hydroxyl groups and the presence of soot matter covering npD. We have found that the Fenton reaction of npD remarkably increases the density of surface OH groups, decreases the undesirable amorphous soot matter, and makes possible the subsequent high-density covalent functionalization of these nanoparticles. In this way, the polarity and solubility of npD can be controlled and, besides by IR spectroscopy, we have shown that it is even possible to characterize the resulting diamond nanoparticles by conventional solution 1H NMR spectroscopy in which the signals corresponding to the proton ligands can be monitored.

In situ DRIFTS study of photocatalytic CO2 reduction under UV irradiation

Photocatalytic reduction of CO2 on TiO2 and Cu/TiO2 photocatalysts was studied by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) under UV irradiation. The photocatalysts were prepared by sol-gel method via controlled hydrolysis of titanium (IV) butoxide. Copper precursor was loaded onto TiO2 during sol-gel procedure. A large amount of adsorbed H2O and surface OH groups was detected at 25°C on the TiO2 photocatalyst after being treated at 500°C under air stream. Carbonate and bicarbonate were formed rapidly due to the reaction of CO2 with oxygen-vacancy and OH groups, respectively, on TiO2 surface upon CO2 adsorption. The IR spectra indicated that, under UV irradiation, gas-phase CO2 further combined with oxygen-vacancy and OH groups to produce more carbonate or bicarbonate. The weak signals of reaction intermediates were found on the IR spectra, which were due to the slow photocatalytic CO2 reduction on photocatalysts. Photogenerated electrons merge with H+ ions to form H atoms, which progressively reduce CO2 to form formic acid, dioxymethylene, formaldehyde and methoxy as observed in the IR spectra. The well-dispersed Cu, acting as the active site significantly increases the amount of formaldehyde and dioxymethylene, thus promotes the photoactivity of CO2 reduction on Cu/TiO2. A possible mechanism of the photocatalytic CO2 reduction is proposed based on these intermediates and products on the photocatalysts.

Cobalt ferrite aerogels by epoxide sol–gel addition: Efficient catalysts for the hydrolysis of 4-nitrophenyl phosphate

Porous cobalt ferrite aerogel catalysts were obtained by 1,2-epoxide sol–gel process and investigated in the hydrolysis of 4-nitrophenyl phosphate. These materials were synthesized by reacting cobalt and iron salts with propylene oxide in methanol, dried by supercritical carbon dioxide, and calcined between 200 and 800 °C. The catalysts were characterized using Fourier Transform Infrared (FTIR) spectroscopy, nitrogen adsorption–desorption technique, and powder X-ray diffraction (XRD). The “as-prepared aerogel” surface exhibits M–OH groups that disappear after annealing, which enhances the spinel structure. This was coupled with a better crystallinity revealed by XRD peaks sharpness. The crystallite sizes were found to be between 6.3 and 28.1 nm. In addition, the catalysts revealed high porosities that decrease as the annealing temperature increases. The catalysis showed that the catalytic activity significantly rely on the synthesis procedure and mainly the calcination temperature. Samples calcined at 600 °C and above did not show any catalytic activity, however, the highest catalytic efficiency was for those calcined at 200 °C with 100% selectivity for the 4-nitrophenol. The correlation of the characterization techniques and the catalysis tests revealed that the catalytic properties of these sol–gel materials are due to the existence of residual surface OH groups.

Adsorption and Desorption of NOx on Commercial Ceria-Zirconia (CexZr1−xO2) Mixed Oxides: A Combined TGA, TPD-MS, and DRIFTS study

The adsorption/desorption of NOx on commercial CexZr1−xO2 solid solutions (x = 1, 0.85, 0.69, 0.5, 0.21, 0.1, and 0) is reported in this paper. In the first part, the characterization of the ceria-zirconia is carried out by N2 adsorption at 77 K, TEM, Raman, XRD, and DRIFTS in order to establish relationships between their bulk/surface properties and their NO and NO2 adsorption/desorption behavior. As deduced from thermogravimetric experiments, the adsorption of NO2 is promoted in comparison to that of NO with typical surface densities spreading between 3.1−4.6 NO2/nm2 and 1.5−4.5 NO/nm2 among the CexZr1−xO2 series. DRIFTS experiments reveal that NOx adsorption proceeds mainly via electron transfer and/or participation of surface oxygens and OH groups, inducing the formation of negatively charged surface species, such as hyponitrites, nitroxyl anions, nitrites, and nitrates. The mechanistic pathways promoting the formation of the different kinds of ad-NOx species are discussed in function of the catalyst ...

Self-cleaning modified TiO2–cotton pretreated by UVC-light (185 nm) and RF-plasma in vacuum and also under atmospheric pressure

Two new innovative findings presented in this study are: (a) TiO2–cotton fabrics obtained by pretreatment with UVC-light (185nm) at atmospheric pressure introduced functionalities into the cotton surface enabling the chelation/binding of TiO2. This was possible since the molar absorption coefficient of O2 and N2 is very low at 185nm and (b) the radiofrequency (RF-plasma) pretreatment of cotton surface lead to the formation active binding sites on the cotton at atmospheric pressure. This unexpected RF effect was due to the drastic localized heating of the cotton leading to intermolecular H-bond breaking between the cellulose surface-OH groups of adjacent molecules with the formation of functionalized groups in the cellulose fibers. The discoloration kinetics of the wine stain on the TiO2–cotton pretreated by RF at atmospheric pressure for 10min was the most favorable. The red wine stains discoloration under Suntest simulated light was monitored by diffuse reflectance spectroscopy (DRS) and by the CO2 evolution during the stain mineralization. By X-ray photoelectron spectroscopy (XPS) it was possible to monitor the decrease of the C, N, S-species on the textile topmost layers during the discoloration process. The XPS Ti 2p3/2 peak shifts indicating Ti4+/Ti3+ oxido-reduction taking place during the photocatalysis. X-ray diffraction showed the formation of the anatase phase on the cotton. By X-ray fluorescence the loading of TiO2 before and after the discoloration process was found to be ∼0.8%. High-resolution electron microscopy (HRTEM) shows transparent TiO2 anatase 8–18nm coating the cotton with layers ∼31nm (±10%). These 3–4 TiO2 layers on the cotton did not affect the touch or handling properties of the cotton enabling the potential commercial use of the TiO2–cotton fabrics.

Grafting Strategy to Develop Single Site Titanium on an Amorphous Silica Surface

Titanium/silica systems were prepared by grafting a titanium alkoxide (titanium isopropoxide and titanium (triethanolaminate) isopropoxide) precursor onto amorphous silica. The grafting process, which consisted of the hydrolysis of the Ti precursor by the hydroxyl groups on the silica surface, yielded samples containing Ti-loadings of 1-1.6 wt %. The as synthesized and calcined TiO(2)-SiO(2) samples were characterized by UV-vis, FTIR, XPS, and XANES spectroscopic techniques. These systems were tested in the liquid-phase epoxidation of oct-1-ene with hydrogen peroxide reaction. Spectroscopic data indicated that titanium anchoring takes place by reaction between the alkoxide precursor and surface OH groups of the silica substrate. The nature of surface titanium species generated by chemical grafting depends largely on the titanium precursor employed. Thus, the titanium isopropoxide precursor yields tetrahedrally coordinated polymeric titanium species, which give rise to a low-efficiency catalyst. However, if an atrane precursor (titanium (triethanolaminate) isopropoxide) is employed, isolated titanium species are obtained. The fact that these species remain isolated even after calcination is due to the protective effect of the triethanolaminate ligand that avoids titanium polymerization. These differences in the titanium environment have a pivotal role in the performance of these systems in the epoxidation of alkenes with hydrogen peroxide.

Hydroxide ZnSn(OH)6: A promising new photocatalyst for benzene degradation

Cube-shaped hydroxide ZnSn(OH)6 (ZHS) has been synthesized by a solvothermal process. The obtained sample was characterized by X-ray diffraction (XRD), N2-sorption (BET surface area), UV–vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier transformation infrared spectroscopy (FTIR). Compared with commercial TiO2 (Degussa P25), the as-prepared ZHS showed much higher conversion and mineralization of benzene under 254nm UV irradiation, and no obvious deactivation was observed during the prolonged operation of 50h. The reasons for the high activity and long-term stability of ZHS will be discussed in terms of the population and the regeneration of surface OH groups.

Insights into the Geometry, Stability and Vibrational Properties of OH Groups on γ-Al2O3, TiO2-Anatase and MgO from DFT Calculations

The properties of hydroxyl groups on γ-Al2O3, TiO2-anatase and MgO are rationalized by Density Functional Theory calculations. Oxide ionicity is found to be determinant in the water/oxide interaction strength when dissociation of H2O occurs, and in the bridging ability of monocoordinated surface OH groups. For a hydroxyl, being the hydrogen bond donor, acceptor or having an isolated nature is found to be the most important parameter governing the O–H stretching frequency.

Ultraviolet Irradiation Suppresses Adhesion on TiO2

Environmental concerns have recently spurred a quest for materials that stay clean, such as TiO2, when subjected to the combined action of sunlight irradiation and exposure to rain. However, the fundamental mechanism that governs the self-cleaning properties of TiO2 still needs to be elucidated. TiO2 is known to be photocatalytic as well as to decompose organic adsorbents, but these properties do not explain its capacity to eliminate mineral contaminants. In the present paper, we report that hydrophilic UV-irradiated TiO2 layers are nonadhesive in the presence of water, thus preventing adhesion of mineral particles. Surface force measurements done using atomic force microscopy reveal the presence of an additional short-range repulsive force, which screens the van der Waals attractive forces, while long-range interactions are preserved. This additional short-range force does not originate from UV-induced trapping of surface charges or OH group creation, as we demonstrate by second harmonic generation and ambient pressure X-ray photoelectron spectroscopy investigations. This short-range repulsive force, which appears to be intrinsic to the TiO2 surface, is certainly a key phenomenon for a strong self-cleaning capacity.