An infrared study of adsorption of N2O on ZnO

The adsorption of N₂O on finely divided ZnO at room temperature shows two principal infrared absorption bands at 2237 cm⁻¹ (strong) and 1255 cm⁻¹ (weak), corresponding to the reversible adsorption of an N₂O surface species. The N₂O is postulated to be coordinated to Zn²⁺ cations by the oxygen atom. Water pre-treatment of the ZnO surface gives only weak bands from adsorbed N₂O, indicating that the latter's adsorption site is taken up by adsorbed water. Spectroscopic experiments on ‘reduced’ surfaces of ZnO at 200°C show that limited reaction of N₂O with the surface has occurred, presumably through decomposition to nitrogen and adsorbed oxygen. New adsorptions on the ZnO surface itself, and a reduced amount of reversibly adsorbed N₂O, implied a reduction in pressure of the adsorbate. Such effects were not observed appreciably over ‘oxidised’ ZnO.     

Adsorption and conversion of ethylene glycol on the surface of Ag-containing catalyst modified with phosphate

The interaction of ethylene glycol with the surface of Ag-containing catalysts modified with phosphates was studied by IR spectroscopy. It was found that ethylene glycol was adsorbed at the surface phosphate groups; the presence of water vapor increased the stability of adsorbed ethylene glycol to a temperature of 400°C. A study of the kinetics of oxidation of ethylene glycol showed that it was consecutively converted into glycolaldehyde and glyoxal at 400°C. At 500–550°C, glyoxal was formed upon both the consecutive and direct conversion of ethylene glycol. The presence of a stage of the consecutive conversion of ethylene glycol into glycolaldehyde and glyoxal was caused by the participation of surface phosphate groups in the adsorption of alcohol molecules to be oxidized.     

Investigation of mechanochemically modified kaolinite surfaces by thermoanalytical and spectroscopic methods

The effect of mechanochemical activation (dry grinding), formamide intercalation, and thermal deintercalation on high- and low-defect kaolinite surfaces was studied by thermogravimetry and diffuse reflectance Fourier transform infrared spectroscopy. These investigations were completed with specific surface area and pore size distribution measurements. The surface acidity of the ground and the ground-and-intercalated kaolinites was probed with ammonia adsorption. The surface area and the pore volume as well as the amount of adsorbed ammonia increased with the rate of mechanochemical activation. At the same time the thermally deintercalated minerals showed increased surface area but decreased pore volume with the time of grinding. Adsorbed ammonia was detected as ammonium ion in the 1400-1500 cm(-1) spectral range.     

Sol–gel process of ZnO and ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript> coated aluminum borate whiskers

Zinc nitrate and citric acid were used to prepare ZnO sol. ZnO and ZnAl₂O₄ coated aluminum borate whiskers were separately prepared by a sol–gel process. The results show that ZnO forms when ZnO xerogel is calcined at 500 °C and it does not undergo any phase transformation in the range of 500 and 1000 °C during calcinations. In ZnO xerogel coated aluminum borate whiskers system, a large amount of heat, gas and pores are produced during the heating process. When ZnO xerogel coated aluminum borate whiskers are calcined at 500 °C, ZnO can be uniformly coated on the surface of the whikers and the coated whiskers can be easily dispersed in distilled water through an ultrasonic vibration apparatus. During the calcination of ZnO coated whiskers at 1000 °C, ZnO reacts with the whiskers and ZnAl₂O₄ forms on the surface of aluminum borate whiskers.     

Facile synthesis of Pd-decorated ZnO nanoparticles for acetone sensors with enhanced performance

ZnO and Pd nanoparticles (NPs) with average diameter of 38 and 10 nm were prepared in advance through a chemical solution method. Pd-functionalized ZnO nanoparticles (Pd@ZnO) were simply synthesized by adding ethanol solution of Pd NPs into ZnO powder, and annealing in argon atmosphere at 500 °C for 1 h after grinding for 30 min. The morphology and structure of the materials were systemically analyzed using Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) techniques. A weak peak in the XRD pattern of Pd@ZnO belonging to the (111) plane of elemental Pd indicated successfully loading of Pd. EDS and TEM results further confirmed successfully coating of Pd NPs onto the surface of ZnO. Sensors using ZnO NPs decorated with Pd (1 wt%) on the surface of exhibited highly elevated sensitivity of 76 in comparing with the response of 36 when based on pure ZnO NPs. In addition, such modification also resulted in a decrease in the operating temperature from 370 to 340 °C for 100 ppm acetone vapor. The sensing mechanism of the sensor based on Pd@ZnO NPs is discussed. Addition of Pd NPs can play an important role in improving the performance of gas sensors, including high sensitivity, good selectivity, and short response/recovery times.     

The influence of exchangeable cation on thermal behaviour of ground vermiculite

Grinding and contact with water or salt solution increased the specific surface (ssa) but lowered the first dehydration effect (escaping up to 150°C) and increased the second dehydration effect (150 to 500°C). The dehydroxylation was moved to lower temperatures and was only ΔM(500-1100°C)=3.7±0.3 % as compared to 5.5% in the parent vermiculite (V). Except ΔM(20-150°C), the mass losses measured at the remaining T ranges, were consistent in the ground samples, thus the grinding for 2 min caused the homogenization of the crystal structure of vermiculite [ΔM(150-500°C)=7.6±0.7%]. DTA curves after grinding and cation exchange indicate an important exothermal peak at 795-870°C, its temperature depending on exchangeable cation. It indicates the formation of high temperature phases (enstatite, forsterite, spinel). The lowest temperature of the peak (795°C) was observed in V-gr-Li, here lithium silicate was formed. The highest peak temperature (870°C) was found in V-gr-K, where almost only forsterite developed. These exothermal peaks were very weak in unground V with various exchangeable cations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Evolution of water vapor from indium-tin-oxide transparent conducting films fabricated by dip coating process

Tin-doped indium oxide In₂O₃ (indium-tin-oxide) transparent conducting films were fabricated on silicon substrates by a dip coating process. The thermal analysis of the ITO films was executed by temperature-programmed desorption (TPD) or thermal desorption spectroscopy (TDS) in high vacuum. Gas evolution from the ITO film mainly consisted of water vapor. The total amount of evolved water vapor increased on increasing the film thickness from approx. 25 to 250 nm and decreased by increasing the preparation temperature from 365 to 600°C and by annealing at the same temperature for extra 10 h. The evolution occurred via two steps; the peak temperatures for 250 nm thick films were approx. 100-120 and 205-215°C. The 25 nm thick films evolved water vapor at much higher temperatures; a shoulder at approx. 150-165°C and a peak at approx. 242°C were observed. The evolution temperatures increased by increasing the preparation and the annealing temperatures except in case of the second peak of the 25 nm thick films. The evolution of water vapor at high temperature was tentatively attributed to thermal decomposition of indium hydroxide, In(OH)₃, formed on the surface of the nm-sized ITO particles. This revised version was published online in August 2006 with corrections to the Cover Date.     

Adsorption of triallylamine on Si(111) and its coadsorption with triethylgallium – A combined HREELS and XPS study

The adsorption of triallylamine [(C₃H₅)₃N; TAA] on Si(111)-(7 × 7) under UHV conditions was studied by means of surface sensitive electron spectroscopy. The High-Resolution Electron Energy Loss Spectroscopy (HREELS) yields the spectrum of vibration modes of the adsorbed species. X-ray Photoelectron Spectroscopy (XPS) gives insight into the chemical environment and the relative concentrations in the near surface region. The tertiary amine TAA physisorbes at room temperature without dissociation. Successive annealing steps induce the dissociation of the physisorbed phase at temperatures above 400°C. Further annealing leads to partial desorption of the allyl groups from the surface. At temperatures above 600°C the remaining allyl groups are fully dissociated. Hydrogen leaves the surface and nitrogen and carbon start to diffuse into the substrate. The surface chemistry of triallylamine adsorbed on a heated substrate behaves in a very similar way. The coadsorption of TAA with triethylgallium [(C₂H₅)₃Ga; TEG] in the temperature range between 500 and 800°C induces no significant change of the surface reactions. Only a small amount of gallium could be detected at the surface. The nucleation of GaN has not been observed, neither on Si(111) nor on Al₂O₃(0001) substrates.     

IR-study of hydrated surface of oxide photocatalysts

IR spectroscopy combined with thermogravimetry was used to investigate the effect of the pretreatment temperature on the degree of coverage of the surface of oxide photocatalysts, TiO₂, ZnO, CeO₂, and Zn²⁺/TiO₂, with water. At room temperature, the amount of adsorbed water per unit area of photocatalysts in the air decreases in the row: ZnO ≥ CeO₂ > TiO₂, whereas the temperature needed for complete removal of physically adsorbed water from the studied oxides decreases in the reverse order. Water is removed from the ZnO surface by evacuation at room temperature; in the case of CeO₂ and TiO₂, it desorbs at 200 and 300 °С, respectively. The terminal OH groups on the oxide surface are the most strongly bonded with adsorbed water. In the zinc modified TiO₂, the terminal OH groups are firstly replaced by Zn cations, which causes both hydrophobization of the samples under atmospheric conditions and a decrease in the temperature at which physically adsorbed water is released from the surface. Evacuation of ZnO at 350 °C removes the surface oxygen and results in the generation of the surface defect sites. This causes strong absorption in the IR spectra in the region of 1000—4000 cm^–1. The formation of surface defects probably causes the appearance of donor levels in the band gap. The energy of the transition of electrons from these levels to the conduction band corresponds to the energy of the IR radiation. After oxidation of such samples in O₂ at 350 °C, strong absorption in the IR spectra disappears.     

Influence of pyrolysis temperature on the properties of sol–gel derived zinc oxide films

The influence of pyrolysis temperature on the properties of sol–gel derived zinc oxide films has been investigated. As-deposited films were pyrolyzed at 300 °C for 30 min and at 500 °C for 10 min. Final annealing was done at 600 °C for 30 min in air. The as-grown films deposited on soda-lime-silica glass substrates were highly c-axis oriented. Distinct grain structure was present in the film pyrolyzed at 500 °C, while the surface of the film pyrolyzed at 300 °C was smooth and no observed texture. The surface of ZnO pyrolyzed at 300 °C was covered with needle-like grain growth. With increasing pyrolysis temperature at 500 °C, a three-dimensional island formation was appeared.     

The synthesis and adsorption properties of polyvinylchloride nanofilms with ligand groups

Polyvinylchloride (PVC) samples with a fairly developed surface were prepared by the spontaneous removal of the solvent (benzyl alcohol) from a polymer gel. The surface of the product was modified to obtain ethanolocyclame and sodium acetocyclame nanofilms. The structure and composition of the nanofilms were studied by IR Fourier spectroscopy. The adsorption of nitrogen vapor at ?196°C and benzene and water at 20°C on the nanofilms was investigated. Surface modification was shown to increase the stability of PVC in water and benzene and the specific surface area and volume of nanopores. The conclusion was drawn that nanofilm ligand groups could stoichiometrically interact with water and benzene molecules on PVC with a modified surface.     

Preparation and optical–electrical properties of Al-doped ZnO films

Among the various semiconducting metal oxide materials, ZnO thin films are highly attractive in the development of materials area. In this paper, Al-doped ZnO thin films were prepared by sol–gel dipping and drawing technology and their composition, structure and optical–electrical properties were investigated. XRD results shows that the Al-doped ZnO thin film is of polycrystalline hexagonal wurtzite structure, and the (002) face of the thin film has the strongest orientation at the annealing temperature of 550 °C. The surface resistance of Al-doped ZnO thin film firstly drops and then increases with the increase in annealing temperature. Al doping concentration is also an important factor for improving the conductivity of modified ZnO thin films, and the surface resistance has the tendency to drop at first and then to increase when the Al concentration is increasing. The surface resistance of modified ZnO thin films drops to the lowest point of 139 KΩ sq^?1 when the Al concentration is 1.6 at% and the annealing temperature is 500 °C. The light transmission measurements show that the doping concentration has little influence on light transmittance. The transmittance at the visible region of films is all over 80 %, and the highest value is up to 91 %.     

Water adsorption and desorption pretreatment of surfaces increases the maximum amount of adsorbed water molecules by a multiple

The amount of adsorbed water on surfaces in an atmosphere with 100 % relative humidity can be increased by a multiple, if the surfaces are pretreated by cycles of adsorption and desorption of water. This was observed on surfaces of diamond, titanium dioxide and silicon dioxide at temperatures around 22 °C. With a sufficient number of such cycles a faster and stronger adsorption of water molecules was obtained, if compared with untreated surfaces. This also means an increased energy transfer from the atmosphere to the surface. Due to the pretreatment the amount of adsorbed water was more than three times increased. The observed effect is explained by small amounts of specially arranged water molecules, which remain on the surface after the desorption process and which support the adsorption of water. The observed effect can be used to moisten surfaces of small particles very efficiently from the gas phase.     

Adsorption-Desorption Characteristics of Water Vapor on Functional Cattle Bone-Originated Apatites Powders with Mesopores

Microcrystals of cattle bone-originated apatites (r-HAp) were prepared by the dissolution-precipitation and freeze-drying processes. The r-HAp particles obtained gave 128 m 2 ;g m 1 specific surface area and 0.376 cm 3 ;g m 1 total pore volume and strong basic surface with mesopores. All the adsorption isotherms of water vapor at 288-308 K for the r-HAp powders obeyed IV-type in a classification of the BDDT. In the repeated adsorption-desorption operations, an adsorption hysteresis resulting from mesopores was recognized. The amounts of water vapor adsorbed obtained in the first adsorption operation were larger than those in the second adsorption operation, indicating that some portion of water vapor adsorbed to be irreversible adsorption. The water vapor-adsorption heats for the r-HAp were 45-60 kJ;mol m 1 , whose values were higher than natural diatomite. The amounts of water vapor adsorbed for the r-HAp were larger than those for the adsorbents, such as natural diatomite, aerosol silica, and activated carbon, at the relative partial pressures of 0-0.7.     

Adsorption of the diazo dye Direct Red 23 onto a zinc oxide surface: A spectroscopic study

The adsorption of the diazo dye Direct Red 23 onto a zinc oxide surface at 30 °C in the dark was investigated. The color reduction was monitored by spectrophotometry at 503 nm. The FTIR and Raman spectra of the Direct Red 23 adsorption as a function of ZnO concentration were registered. From the PM3 semi-empirical calculations of the atomic charge density and dipole moment of the Direct Red 23 molecule, it was demonstrated that the azo dye molecule may be adsorbed onto the ZnO surface through molecule geometry modifications, enhancing the interfacial area causing a variation in the bonding frequencies.     

Transport properties and microstructure changes of talc characterized by emanation thermal analysis

Thermal behavior of talc samples (from locality Puebla de Lillo, Spain) were characterized by emanation thermal analysis (ETA), DTA and TG. The ETA, based on the measurement of radon release rate from samples, revealed a closing up of surface micro-cracks and annealing of microstructure irregularities of the talc samples on heating in the range 200–500°C. For ground talc sample a crystallization of non-crystalline phase formed by grinding, into orthorhombic enstatite was characterized as a decrease of radon mobility in the range 785–825°C and by a DTA exothermal effect with the maximum at 830°C. ETA results characterized the microstructure development of the talc samples on heating and served to evaluate their radon mobility and transport properties on heating and cooling. Transport properties of the talc samples were evaluated by using ETA experimental data measured during heating to 600 and 1300°C, respectively, and subsequent cooling to room temperature.     

Temperature dependence of methanol and ethanol vapor sorption in polyhedral oligomeric silsesquioxane (POSS) containing methacrylate membranes

This study aimed to determine the solubility and temperature dependence of methanol and ethanol vapor caused by the difference in the substituents of polyhedral oligomeric silsesquioxane (POSS)-containing polymethacrylate membranes and the spacer length between the backbone and POSS backbone. Vapor sorption of methanol and ethanol was measured at 25°C, 35°C, and 45°C for three kinds of POSS-containing polymer membranes, namely, poly(methacryl isobutyl POSS), poly(methacrylate isobutyl POSS), and poly(methacryl phenyl POSS). The primary structures of the three POSS-containing polymer chains were columnar. The solubility of alcohol vapor on the POSS-containing polymer membranes followed the mechanism of solid adsorption and not the general dissolution diffusion. The sorption amount at all three temperatures was related to the surface area of the cylindrical primary structure and the solid adsorption property of the alcohol molecule of the POSS substituent. The sorption amount increased because of the large surface area and adsorption property of alcohol molecules. Although a typical glassy polymer shows exothermic mixing and a rubbery polymer displays endothermic mixing, the sample with the POSS substituent of isobutyl group exhibited an unusual behavior of endothermic mixing despite being a glassy polymer.     

Carbon dioxide adsorption by synthetic mordenite isotherms and differential heat of adsorption

Carbon dioxide adsorption isotherms by synthetic mordenite are determined over the ranges 10^?5 atm<P<50 atm and ?77°C<T<+160°C, differential heats of adsorption calorimetrically measured over 10^?5atm<P<l atm and ?77°C< T< + 120°C.Differential heat curves show two adsorption ranges with different energetic characteristics. At the transition a very marked maximum appears.The integral molar entropy and specific heat of adsorbed phase are calculated.Two methods for adsorbed phase density determination are proposed:(i) The first, straight from maximal adsorbed amount at 50 atm. where adsorption isotherms level out.(ii) The second, based on adsorption potential theory with an elementary graphical determination.When the adsorbed phase density is so determined, Dubinin's equation can be successfully applied.     

Adsorption-desorption behavior of phenols on novel polymers containing the pyrrolidinone moiety

Crosslinked polymers having a pyrrolidinone moiety (CPS, CPES, and CVP) were synthesized by radical copolymerization of 4-(2-oxo-1-pyrrolidinyl)methylstyrene, 4-[2-(2-oxo-1-pyrrolidinyl)ethoxy]methylstyrene, or 2-vinylpyrrolidinone with divinylbenzene in the presence of AIBN as a radical initiator. The adsorption-desorption behavior of phenols on these polymers was investigated. The polymers with spacers between the polymer main chain and pyrrolidinone moiety appeared to have a superior adsorption capability to those without such spacers. The amount of phenol adsorbed on the polymers in a solvent decreased in the following order: water > chloroform > methanol. In methanol, the interaction between the polymers and phenol was suggested to come only from charge-transfer stacking (C–T stacking), whereas in chloroform the interaction was caused mainly by both hydrogen bonding and C–T stacking. The interaction in water was attributed not only to both hydrogen bonding and C–T stacking, but also to a hydrophobic interaction. Characterization of polymers (CVP) containing adsorbed phenol was carried out by thermogravimetric analysis (TGA). The TGA curves indicated a two step weight-decrease, namely the first step in the temperature ranging from ca. 100-200°C was attributed to the desorption of phenol while the second step in the temperature ranging from ca. 350-500°C was based on thermal decomposition of the polymers. The desorption of phenol adsorbed on the polymers in water indicated an inverse tendency to the adsorption; that is, the amount of phenol desorbed from the polymers without a spacer was more than those from the polymers with spacers. © 1994 John Wiley & Sons, Inc.     

Synthesis and Characterizations of Hydroxy-aluminum Cross-Linked Montmorillonite

Cross-linked montmorillonite was prepared by reacting homoionic sodium form of bentonite (Na-M) from Istenmezeje (Hungary) with high molecular weight polyhydroxy-aluminum complex. The complex was prepared by controlled hydrolysis of alumina macrocation. The intercalated clay (Na-Al-M) was thermally treated to convert the hydroxy cations to oxide pillars. The pillared products were characterized by X-ray powder diffraction (XRD), Fourie transform infrared spectroscopy (FTIR), (thermogravimetry (TG), differential thermal analysis (DTA) and thermal analysis-mass spectrometry (TA-MS) methods. The specific surface area as well as pore size and pore structure distribution of samples were measured by nitrogen, water and carbon tetrachloride adsorption, and the heat of immersion was also determined. The pillared products were characterized by d(001) reflections of 19 Å, which is stable even at 500°C. The interaction of polymer alumina caused several changes in the obtained FTIR spectra due to the formation of different new bonds. The rate of dehydroxylation of the pillared product is very moderate, the water release occurred in different temperature ranges according to TA-MS results. Dehydration starts at interfaces and at the wall of pores, occurring nearly with uniform rate at 250-500°C. DTA curve indicates the formation of a new phase at 950°C. The obtained surface area of the pillared product by nitrogen adsorption becomes larger (208 m2 g-1) with respect to the non pillared clay, which decreases less than 10% upto 700°C. The pillared sample has a definite pore structure, the quantity of micropores (0-40 Å) decreased with increasing of macropores (>1000 Å). The obtained domestic pillared montmorillonite possesses a high degree of thermal stability and may be used as adsorbent.