Concentration Of Surface Hydroxyl Groups Research Articles

Surface Characterization of γ‐Al2O3 Powders and Their Co2+ Adsorption Properties

The relationship between structural and surface properties of γ‐alumina prepared by various methods has been studied, and the adsorption ability of the powders for cobalt ions from aqueous solution is presented, based on the results of X‐ray diffraction, specific surface area, scanning electron microscopy, and surface hydroxyl group concentration of the resulting products. Results show that γ‐alumina obtained by solution‐combustion method (specific surface area: 232.9 m2/g; morphology: lamellar and mesoporous; hydroxyl group content: 0.24 meq OH−/g) exhibited the best Co2+ adsorption. Furthermore, the synthesis of γ‐alumina is highly helpful to study various structural and surface properties of this material.

X-ray photoelectron spectroscopic study on the goethites with variations in crystallinity and morphology: their effects on surface hydroxyl concentration

We examined the effects of crystallinity and crystal morphology on surface hydroxyl concentration of goethite using four kinds of goethite samples prepared by aging a hydrous ferric oxide (ferrihydrite) at different temperatures ranging from 4 to 70 °C. These samples were analyzed with the following methods: powder X-ray diffraction, Brunauer–Emmett–Teller surface area analyzer, atomic force microscopy, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and high-resolution X-ray photoelectron spectroscopy (XPS). The first three analyses showed that the goethite aged at lower temperature has lower crystallinity, smaller particle size and lower aspect ratio. The ATR-FTIR analysis revealed that the OH-stretching band of goethite aged at 4 °C moves to slightly higher wavenumber than those of the samples aged at higher temperatures. Their XPS analyses demonstrated that variation in the concentration of surface hydroxyl group ranges from 6.6 (nm−2) for the sample aged at 4 °C to 7.7 (nm−2) for the sample aged at 70 °C. Considering the analytical depth of the XPS (~5 nm), photoelectrons are emitted from the uppermost layers of the analyte. Hence, XPS results are interpreted to reflect variations in the morphology of fine crystals (<100 nm thick). The morphology of the present goethites can be characterized by their aspect ratio, in other words, the degree of development of (110)–(110) intersection edges at which protons preferentially accumulate. This variation in the concentration of surface hydroxyl group revealed by XPS analysis offers experimental evidence for the effects of crystallinity and morphology on the surface chemical properties of goethite.

Low-temperature synthesis of mesoporous TiO2 photocatalyst with self-cleaning strategy to remove organic templates

A novel low-temperature method integrated solvothermal and self-cleaning technique has been developed to prepare mesoporous TiO2 photocatalyst using triblock copolymers P123 as templates. The organic templates could be removed completely by the self-cleaning strategy under simulated sunlight irradiation. The product is characterized by FTIR, TG, XRD, N2 adsorption–desorption isotherms and TEM. The mild condition of the method is advantageous to retain the mesostructure compared with the calcination procedure. Further study indicates that the sample displays a higher photocatalytic activity for MO degradation than that of calcined sample, which could be attributed to its higher surface area, smaller crystal size and larger surface hydroxyl content.

Mesoporous Au/TiO2 composites preparation, characterization, and photocatalytic properties

In this work, mesoporous Au/TiO2 composites have been synthesized and tested on photodegradation of methylene blue dye solution. Mesoporous TiO2 prepared at 450 °C using triblock polymer F127 as structure-directing agent was applied as substrate, while various HAuCl4 concentrations were used for Au loading through deposition-precipitation method using urea as precipitator and hydrogen reducing process. The influences of Au loading on the microstructures of mesoporous TiO2 including degree of dispersion, particle size, surface area, light absorption, and band gap were studied with transmission electron microscopy (TEM), X-ray diffraction (XRD), diffuse reflection infrared Fourier transformed spectroscopy (DRIFT), N2 adsorption–desorption isotherm analysis (BET), and UV–Vis diffuse reflectance spectra. With Au loading, the size of TiO2 nanoparticles in Au/TiO2 composites is similar as that of TiO2 substrate. However, the degree of dispersion was greatly improved. Furthermore, an obvious surface plasmon resonance centered at 570 nm was found in UV–Vis diffuse reflectance spectra for Au/TiO2 composites. Au loading also induced an obvious red shift of light absorption from UV region to visible region and strengthened both UV and visible light absorption in contrast to substrate. Photodegradation results verified that photocatalytic activity of mesoporous TiO2 was improved by Au loading. 0.25%Au/TiO2 composite showed the highest activity, which may be ascribed to its high surface hydroxyl content and the formed Schottky junction after Au loading. These results suggested that noble metal modification is a promising way to synthesize photocatalysts with both high activity and visible light sensitivity.

Carbon nanotube/titania composites prepared by a micro-emulsion method exhibiting improved photocatalytic activity

A micro-emulsion method was used to prepare TiO2-coated carbon nanotubes (ME-TCNTs). The physico-chemical properties of these composites were characterized by modern analytical tools. Crystalline TiO2 within the composites was composed solely of anatase. Carbon nanotubes (CNTs) within the composites were virtually uniformly covered with TiO2. CNT addition increased the surface area and the amount of hydroxyl groups on the composite surface, which suppressed the recombination of photo-generated electron/hole pairs (excitons). The composite photoactivity was greater than that of pure TiO2 and the commercial photocatalyst P25 for methylene blue degradation, predominantly due to the effect of CNT addition. An optimal TiO2 loading of 12% was found to result in the highest photoactivity in comparison with other two loadings (6% and 15%). Too little TiO2 or excessive CNT addition shielded the TiO2 and reduced the UV intensity, due to photon scattering by the bare CNTs. However, a high TiO2 content was found to be ineffective in suppressing exciton recombination because of the large distance between the TiO2 and CNTs. The photoactivity of ME-TCNTs was greater than that of TiO2 coated carbon nanotubes (TiO2-CNTs) prepared by the sol–gel method. The uniform nanometer-scale TiO2 layer on the CNT surface provided a high concentration of surface hydroxyl groups, a low exciton recombination rate and a high surface area.

Detection of surface silanol groups on pristine and functionalized silica mixed oxides and zirconia

The surface hydroxyl content and surface structure of silica and other oxides with and without surface modification were systematically studied by solid state 29Si NMR, thermogravimetric analysis, and the lithium alanate method. Aerosil 90 as a well described reference system and functionalized zirconia–silica particles were used in the validation of the lithium alanate method. 3-Methacryloxypropyltrimethoxysilane and dodecylphosphonic acid were applied as surface modifiers. The determination of silanol content of Aerosil 90 by 29Si NMR and TGA confirms the results obtained by the lithium alanate method, which also allows for the determination of the remaining surface hydroxyl content after surface modification. For both silane coupling agents, the residual hydroxyl content of modified zirconia–silica is decreased by a factor of approximately 2 compared with that of the unmodified mixed oxide, whereas after modification with dodecylphosphonic acid, the hydroxyl content is slightly higher. These results are again in good agreement with those by 29Si NMR confirming that the lithium alanate method is a reliable and easily practicable method for surface hydroxyl determination.

Initiation of atomic layer deposition of metal oxides on polymer substrates by water plasma pretreatment

The role of surface hydroxyl content in atomic layer deposition (ALD) of aluminum oxide (AO) on polymers is demonstrated by performing an atomic layer deposition of AO onto a variety of polymer types, before and after pretreatment in a plasma struck in water vapor. The treatment and deposition reactions are performed in situ in a high vacuum chamber that is interfaced to an x-ray photoelectron spectrometer to prevent adventitious exposure to atmospheric contaminants. X-ray photoelectron spectroscopy is used to follow the surface chemistries of the polymers, including theformation of surface hydroxyls and subsequent growth of AO by ALD. Using dimethyl aluminum isopropoxide and water as reactants, ALD is obtained for water-plasma-treated poly(styrene) (PS), poly(propylene) (PP), poly(vinyl alcohol) (PVA), and poly(ethylene naphthalate) (PEN). For PS, PP, and PEN, initial growth rates of AO on the native (untreated) polymers are at least an order of magnitude lower than on the same polymer surface following the plasma treatment. By contrast, native PVA is shown to initiate ALD of AO as a result of the presence of intrinsic surface hydroxyls that are derived from the repeat unit of this polymer.

Characterization and mechanism analysis of N doped TiO2 with visible light response and its enhanced visible activity

Nitrogen doped TiO2 nanoparticles were synthesized through a hydrolysis–precipitation process using ammonia water as the doping species. The resulting materials were characterized by XRD, DRS, SPS, XPS and FT-IR. Further, the activity enhanced-mechanism was discussed in detail. XRD results showed that doping with nitrogen could effectively retard the phase transformation of TiO2 from anatase to rutile and increase the anatase crystallinity. DRS and SPS results indicated that the light absorbance edge of nitrogen doped TiO2 nanoparticle was obviously red-shifted to visible light region and the separation rates of photogenerated charge carriers were greatly improved, respectively. XPS and FT-IR analysis implied that the contents of surface hydroxyl groups were improved significantly and the VBM (valance bond maximum) of O2p was 2.3eV. Under the visible light irradiation with 120min, a 65.3% degradation rate of phenol could be achieved. The photocatalytic activity of nitrogen doped TiO2 was 2.08 and 1.97 times than that of pure TiO2 and P25 TiO2, respectively. The enhanced visible light activity was attributed to the well anatase crystallinity, small crystallite size, intense light absorbance edge in visible region, more content of surface hydroxyl groups and high separation efficiency of photogenerated charge carriers.

Visible light-induced photocatalysis through surface plasmon excitation of platinum-metallized titania for photocatalytic bleaching of rhodamine B

Nanocomposites consisting of titania nanoparticles and metallic platinum were prepared via a soft chemical reduction method. The detailed structural, compositional, and optical characterization and physicochemical properties of the obtained products were analyzed by X-ray diffraction, nitrogen adsorption, Raman spectroscopy, UV–Vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, and FT-IR spectroscopy techniques. Employing photodegradation of rhodamine B as the model reaction, we found that the as-prepared Pt/TiO2 nanocomposite showed an excellent photocatalytic oxidation activity under visible light irradiation. On the basis of these results, the intrinsic mechanism of visible light-induced photocatalytic oxidation of organic compounds on the platinized titania is proposed and discussed. The superior visible light-driven photocatalytic efficiency of the Pt/TiO2 nanocomposite photocatalyst can be ascribed to the high efficiency of charge-pair separation due to the presence of deposited Pt serving as electron sinks to retard the rapid e−–h+ couple recombination; the good photoabsorption capacity in the visible light region; and the higher concentration of surface hydroxyl groups, which are able to effectively scavenge photogenerated valence band holes. Accumulation of the holes at the catalyst surface increases the probability of the formation of OH· as a reactive species that readily oxidizes the organic dye molecule.

High-Temperature Photocatalytic Ethylene Oxidation over TiO2

Photocatalytic oxidation of ethylene was conducted in a novel two-dimensional packed bed reactor at higher temperatures (60 °C < T < 520 °C) than previously reported. Degussa P25 TiO2 was used as a photocatalyst; samples of this TiO2 were heat treated to different temperatures to obtain different phase compositions, surface areas, and surface hydroxyl concentrations. Maximum photocatalytic ethylene oxidation rates were observed between 100 and 200 °C. Further, oxidation rates greater than 75% of the maximum oxidation rate were observed over a large temperature range: 60 °C < T < 300 °C. At higher temperatures (T > 300 °C), decreasing oxidation rates were attributed to the nonradiative, multiphonon recombination of photogenerated charge carriers. Regression analysis of a nonlinear, multiphonon recombination model and a simplified chemical rate law were used to determine apparent reaction rate law parameters for the photocatalytic oxidation of ethylene and recombination parameters for the TiO2 photocatalyst. Insights for the development and operation of novel high-temperature photocatalysts are also discussed.

Photocatalytic Activity of Combustion Synthesized ZrO2 and ZrO2–TiO2 Mixed Oxides

Tetragonal ZrO2, synthesized by solution combustion technique, was found to be photocatalytically active for the degradation of anionic dyes. The compound was characterized by FT-Raman spectroscopy, X-ray photoelectron spectroscopy, FT-infrared spectroscopy, UV–vis spectroscopy, BET surface area analysis, and zero point charge pH measurement. A high concentration of surface hydroxyl groups was observed over the catalyst, as confirmed by XPS and FTIR. The photocatalytic degradation of orange G, amido black, remazol brilliant blue R, and alizarin cyanine green (ACG) was carried out with this material. The effect of pH, inorganic salts, and H2O2 on the activity of the catalyst was also studied, and it was found that the catalyst maintained its activity at a wide range of pH and in the presence of inorganic salts. Having established that ZrO2 was photocatalytically active, mixed oxide catalysts of TiO2–ZrO2 were also tested for the photocatalytic degradation of ACG, and the 50% ZrO2–TiO2 mixed oxides showed a...

Surface hydroxyl concentration on Zr(OH) 4 quantified by 1H MAS NMR

Zirconium hydroxide (Zr(OH) 4) is a catalytic support material whose reactivity depends on intrinsic surface hydroxyl groups. Here we present a 1H MAS NMR study where we identify bridging and terminal hydroxyl groups along with their concentrations even in completely dehydrated material. The dehydrated material contains 13 mmol/g bridging hydroxyl groups and 3.9 mmol/g terminal hydroxyl groups, yielding a ratio of 3.4:1, and is verified by XPS. Upon hydration, the number of surface hydroxyl groups remains constant while only bulk water is adsorbed on the surface. Knowing both the types and amounts of surface hydroxyl groups on Zr(OH) 4 helps explain the catalytic activity of Zr(OH) 4 in many applications.

Platinum catalysts supported on silicas: effect of silica characteristics on their catalytic activity in carbon monoxide oxidation

Platinum catalysts supported on silicas with various physicochemical properties were prepared in order to investigate the effect of silica characteristics on their platinum dispersion and catalytic activity in the oxidation of carbon monoxide. Although titania-incorporation into silica and further treatment of the impregnated platinum precursor with hydrogen peroxide were effective for improving the dispersion and stability of platinum catalysts supported on silicas, regardless of the characteristic of the silicas, the platinum catalysts supported on fumed silica with a medium level of surface hydroxyl group concentration exhibited the highest catalytic activity among those supported on mesoporous silica, silica gel, and precipitated silica. The required properties of the highly active platinum catalyst seemed to be a high dispersion of platinum, the formation of a stable titania layer, and the generation of strong acid sites. By contrast, the precipitated silica with a small surface area and high concentration of surface hydroxyl groups was not appropriate for a catalytic support for platinum catalysts.

Fabrication and photocatalytic activity of TiO2/SiO2 composite nanotubes

TiO2/SiO2 composite nanotube photocatalysts supported on Al substrates were prepared by the anodic aluminum oxide (AAO) liquid-phase deposition method for degradation of 100 mg/L methyl blue (MB). AAO templates were prepared in H3PO4 acidic electrolytes through the two-step anodic oxidation method. The photocatalysts were characterized by field-emission scanning electron microscopy, thermal analysis (TG-DTA), and Fourier-transform infrared, x-ray photoelectron, and UV–Vis diffuse spectroscopies. The results showed that the presence of SiO2 in TiO2 inhibited the crystal phase transformation temperature effectively, with a greater wall thickness of 80–100 nm. It was found that Ti–O–Si bonds were formed on the surface and the surface hydroxyl concentration was increased, resulting in enhanced photocatalytic activity, being about 20% higher than for pure TiO2 films. The calculated band structure for TiO2/SiO2 was investigated using first-principle calculations. The TiO2/SiO2 composite exhibited a wider conduction band which would effectively prohibit recombination of photogenerated electrons and holes.

X-ray photoelectron spectroscopic analysis of the surface chemistry of silica nanowires

A thorough analysis of the surface chemistry of silica nanowires has been performed by X-ray photoelectron spectroscopy to investigate unexpected surface changes. Examination of the Si 2p, O 1s, and C 1s core level states before and after exposure of the nanowires to various liquid media showed that silica nanowires are capable of much higher surface hydroxyl concentrations than planar native oxides. It is further demonstrated that the nanowire surface hydroxyl concentration corresponds to the pH of the aqueous media to which the nanowires are exposed. Spectral feature changes due to water exposure similar to those observed for fibronectin binding suggests that fibronectin binding is competitively inhibited by slow changes in surface chemistry resulting from water exposure.

Aqueous Solution Synthesis of Crystalline Anatase Nanocolloids for the Fabrication of DSC Photoanodes

The production of anatase nanocolloids via a novel aqueous solution synthesis method is described. The process involves forced hydrolysis of aqueous solution at low temperature leading to fast nucleation of anatase nanoparticles . The isolated nanopowders were found to consist predominantly of anatase as the major phase , possess a very high surface area (up to ), and a relatively elevated surface hydroxyl content in comparison with the P25 reference material. Single-layer ( thick) photoanodes prepared with aqueous-synthesized powders were found to have a specific surface area of and the anatase nanocrystallites to have grown to and assume a higher degree of crystallinity but still carry surface OH groups. The pore size of the film was determined to be . By comparison, the film prepared with P25 nanoparticles had the following properties: , crystallite size, 0.2% surface OH, and pore size. Dye-sensitized solar cell (DSC) devices assembled with the newly fabricated photoanodes showed equivalent photo-electrochemical performance [power conversion efficiency η, short-circuit current voltage , open-circuit voltage , and Fill Factor (FF)] to cells prepared using the benchmark P25 titania powder.

Effect of Thermal Treatment of Silica Gels on Their Amino Functionalization and Subsequent Adsorption Properties for Cu2+ from Aqueous Solution of Copper Sulfate

Hydrothermally prepared mesoporous silica gel (prepared under acidic condition) was calcined at different temperatures to control concentration of surface hydroxyl groups. These materials were functionalized with 3-aminopropyltrimethoxysilane (APTMS) and used for the adsorption of Cu2+ ions. Adsorption isotherm and adsorption capacity of the amine functionalized materials for Cu2+ were followed by inductively coupled plasma optical emission spectrometry (ICP-OES). The adsorption capacity for functionalized silica gel was correlated with population density of hydroxyl groups and hence amine content in the materials, which was determined by CHN analysis. Nitrogen adsorption/desorption measurements at liquid N2 temperature, Fourier transform infrared spectrometry (FTIR), and 29Si MAS NMR spectra were used to follow the changes in the textural parameters and surface properties of the mesoporous silica gel samples. This study indicates that thermal treatment of silica gel determines the population density of s...

Condensation reactions of propanal over Ce xZr 1− xO 2 mixed oxide catalysts

Vapor phase condensation reactions of propanal were investigated over Ce x Zr 1− x O 2 mixed oxides as a model reaction to produce gasoline range molecules from short aldehydes found in bio-oil mixtures. Several operating parameters were investigated. These included the type of carrier gas used (H 2 or He) and the incorporation of acids and water in the feed. Propanal is converted to higher carbon chain oxygenates on Ce x Zr 1− x O 2 by two pathways, aldol condensation and ketonization. The major products of these condensation reactions include 3-pentanone, 2-methyl-2-pentenal, 2-methylpentanal, 3-heptanone and 4-methyl-3-heptanone. It is proposed that the primary intermediate for the ketonization path is a surface carboxylate. The presence of acids in the feed inhibits the aldol condensation pathway by competitive adsorption that reduces the aldehyde conversion. Water also promotes ketonization and inhibits aldol condensation by increasing the concentration of surface hydroxyl groups that enhance the formation of surface carboxylates with the aldehyde. Hydrogen enhances cracking and production of light oxygenates and hydrocarbons. The light oxygenates may in turn be reincorporated into the reaction path, giving secondary products. However, the hydrocarbons do not react further. Analysis of the fresh and spent catalysts by XPS showed varying degrees of reduction of the oxide under different operating conditions that were consistent with the reaction results. Changing the proportion of the parent oxides showed that increased Zr favored formation of aldol products while increased Ce favored ketonization. This occurs by shifting the balance of the acid–base properties of the active sites.

Influence of ZrO 2 properties on catalytic hydrodechlorination of chlorobenzene over Pd/ZrO 2 catalysts

Pd/ZrO 2 catalysts using different ZrO 2 as supports were prepared using the deposition–precipitation method and were characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, N 2 adsorption, temperature programmed reduction, H 2 chemisorption and measurement of surface hydroxyl group. Catalytic hydrodechlorination (HDC) of chlorobenzene was used to evaluate the activity and stability of the catalyst. The results showed that ZrO 2 support calcined at 300 °C was amorphous in nature, whereas ZrO 2 supports calcined at 500 and 600 °C consisted of both monoclinic and tetragonal phases. In addition, increasing calcination temperature led to the decrease of specific surface area and surface hydroxyl group content of the ZrO 2 support. For temperature programmed reduction of PdO/ZrO 2 samples, two H 2 consumption peaks with varied reduction temperature were distinctly observed, implying the existence of different Pd species in Pd/ZrO 2 catalysts. In addition, Pd/ZrO 2 catalyst with ZrO 2 calcined at 500 °C had a relatively higher content of Pd species with strong metal–support interaction than other catalysts. For catalytic HDC of chlorobenzene, Pd/ZrO 2 catalyst with ZrO 2 support calcined at 500 °C exhibited a higher initial activity and stability as compared to other catalysts, indicative of a strong dependence of the catalytic behavior of the Pd/ZrO 2 catalyst on the support properties for catalytic HDC of chlorobenzene.

Effect of surface hydroxyl concentration on the bonding and morphology of aminopropylsilane thin films on austenitic stainless steel

AbstractThe adsorption of 3‐aminopropyltrimethoxysilane thin films on Fe18Cr7Mn3Ni (austenitic stainless steel) was investigated by X‐ray photoelectron spectroscopy (XPS) and inelastic electron background analysis. The bonding and morphology of the films were strongly dependent on the surface hydroxyl concentration, which was controlled by the oxidation pretreatment of the substrate. In particular, an aminopropylsilane (APS) monolayer with high degree of bonding to the substrate was obtained on an electrochemically passivated surface with very high hydroxyl concentration. On the other hand, the deposition of weakly bound APS clusters was observed on substrates having relatively low hydroxyl concentrations. The adsorption occurred initially via hydrogen bonding, whereas heating to 373 K resulted in the formation of covalent SiOM bonds at the silane/metal oxide interface. The results of this study provide insight into the interaction between silanes and stainless steels surfaces, and can be applied for functionalization of stainless steel materials in an extensive range of applications. Copyright © 2010 John Wiley &amp; Sons, Ltd.