Amount Of Surface Hydroxyl Groups Research Articles

Catalytic and photocatalytic decomposition of ozone at room temperature over titanium(IV) oxide

Dark and photoinduced decomposition of ozone (O3, ca. 80 ppm) in air over various titanium(IV) oxide (TiO2) powders has been studied with a flow-type reactor (1.5 dm3 min–1) at room temperature. The ozone decomposition in the dark was attributed to catalysis by TiO2. The catalytic activity, evaluated from % decrease in O3 concentration, increased with the increase in the specific surface area of TiO2 and also in the amount of surface hydroxyls. The fact that substitution of hydroxyls with fluoride anions reduced the activity of TiO2(Merck, anatase) to less than half, suggested a mechanism including O3 decomposition on the surface hydroxyls. A TiO2 powder, contaminated with sulfate anion, exhibited a catalytic activity lower than expected from the specific surface area, but increased more than three-fold by washing with aqueous alkaline solution to eliminate sulfate anions and recover hydroxyls. The O3 decomposition under irradiation (>200 nm) occurred not only by direct photolysis of O3 adsorbed on the TiO2 surface (mainly by 254 nm irradiation), but also by a photocatalytic reaction initiated by absorption of photons by TiO2 at <ca. 400 nm, as well as the dark catalytic decomposition. The O3 decomposition via the photocatalytic reaction was evaluated separately by cutting off the light of wavelength <290 nm. The photocatalytic activity of TiO2 was independent of the specific surface area, but dependent on its crystal structure. The photocatalytic activity of TiO2 was independent of the specific surface area, but dependent on its crystal structure. The photocatalytic activity increased in the order anatase anatase–rutile mixture < rutile. The mechanism of these O3 decomposition processes has been discussed.

Influence of the Density of Surface Hydroxyl Groups on TiO2 Photocatalytic Activities

AbstractThe photocatalytic activities of titanium dioxide for the reduction of silver ions and the oxidation of isopropanol were evaluated by using titanium dioxide with the various amount of surface hydroxyl groups per unit surface area. The photocatalytic activities increased with increasing concentration of the surface hydroxyl groups on the catalyst and then decreased from an inflection point. The mechanism was discussed from the stand point of the surface hydroxyl radical formation and their interaction.

Estimation of the distribution of surface hydroxyl groups on silica gel, using chemical modification with trichlorosilane

An estimate of the relative distribution of the free and bridged surface hydroxyl groups is made, using a surface modification with trichlorosilane according to the following main reaction Si—OH + HSiCl3→Si—O—SiHCl2+ HCl. The amount of chemisorbed chloride groups on the substrates is compared with data on the total amount of surface hydroxyl groups and with hydroxyl group information derived from infrared spectra. The interpretation of these data gives an indication of the reactivity, distribution and concentration of the different types of hydroxyl groups on the surface of silica gel.Trichlorosilane reacts mainly with bridged hydroxyl groups on silica gel, pretreated at low temperatures and exclusively with free hydroxyls at high pretreatment temperatures.Extrapolation of the reactivity of trichlorosilane towards the surface hydroxyl groups demonstrates that bridged hydroxyl groups are present up to pretretment temperatures of 973 K.

ChemInform Abstract: Hydrolysis of Europium Cations in Zeolite X: A Fourier Transform IR Spectroscopic Study.

AbstractIt is found that the conditions used for the ion exchange influence the distribution of surface hydroxyl species in thermally activated Eu‐exchanged zeolite X. Increasing time of contact between the exchange solution (aqueous EuCl3) and the parent NaX sample results in an increase in the amount of surface hydroxyl groups.

Mechanism of gas sensitivity change with time of SnO 2 gas sensors

The H 2 sensitivity of SnO 2 sensors tends to increase if the sensor is energized for a long time. The mechanism of this tendency was studied by X-ray diffractometry, the TPD method, a test of H 2 oxidation reactivity and Fourier transform infrared spectrometry. The increasing sensitivity was related to the decreased oxidation reactivity of H 2. The results showed that the decrease can be caused by lowering the amount of surface hydroxyl groups at the sensor's energizing temperature of 350 °C and by lowering the amount of surface OH groups and chemisorbed oxygen at 450 °C. When the sensor was left unenergized in humid air for a long time after energization, the sensitivity and oxidation reactivity approached the levels at the beginning of energization. The results showed that this may be caused by increasing the amount of surface OH groups. The added SiO 2 binder enhanced the increase of H 2 sensitivity during energization. The cause of the enhancement was also discussed.

ChemInform Abstract: RELATION BETWEEN AMOUNTS OF CHEMISORBED WATER AND AMMONIA ON ZINC OXIDE

AbstractFür die NH3‐Adsorption an ZnO‐Oberflächen mit definiertem Anteil an Oberflächen‐ OH‐Gruppen werden die 25°C ‐Isothermen aufgenommen.

Relation between Amounts of Chemisorbed Water and Ammonia on Zinc Oxide

Abstract The adsorption isotherm of NH3 was measured at 25 °C on ZnO surfaces with a controlled amount of surface hydroxyls. It was found that NH3 can be chemisorbed at 25 °C on the dehydroxylated sites of ZnO in a 1 : 1 molecular ratio of adsorbed NH3 to desorbed H2O. H2O can be chemisorbed for prechemisorbed NH3 through exchange reaction to form surface hydroxyls and gaseous NH3. The physisorption of NH3 at 25 °C can take place only on a limited part of ZnO surfaces which were fully hydroxylated or ammoniated.

ChemInform Abstract: INTERACTION BETWEEN CARBON DIOXIDE AND THE ZINC OXIDE SURFACE

The adsorption of carbon dioxide has been measured at 25°C on zinc oxide surfaces with different amounts of preadsorbed carbon dioxide. As a result, it has been found that carbon dioxide molecules are chemisorbed more weakly to zinc oxide surfaces than water; the desorbability of carbon dioxide is almost complete upon evacuation at temperatures higher than 300 °C, whereas upon evacuation at 25°C 80% of the carbon dioxide remains on the surface. Also, the adsorption isotherms of carbon dioxide have been obtained at 25°C on differentlyhydroxylated surfaces of zinc oxide. On such surfaces, the adsorption amount of carbon dioxide decreases linearly with an increase in the amount of surface hydroxyl groups. This suggests that most of the carbon dioxide is adsorbed strongly on the vacant sites formed by dehydroxylation, and not on the surface hydroxyl groups.

Interaction between Carbon Dioxide and the Zinc Oxide Surface

Abstract The adsorption of carbon dioxide has been measured at 25°C on zinc oxide surfaces with different amounts of preadsorbed carbon dioxide. As a result, it has been found that carbon dioxide molecules are chemisorbed more weakly to zinc oxide surfaces than water; the desorbability of carbon dioxide is almost complete upon evacuation at temperatures higher than 300 °C, whereas upon evacuation at 25°C 80% of the carbon dioxide remains on the surface. Also, the adsorption isotherms of carbon dioxide have been obtained at 25°C on differentlyhydroxylated surfaces of zinc oxide. On such surfaces, the adsorption amount of carbon dioxide decreases linearly with an increase in the amount of surface hydroxyl groups. This suggests that most of the carbon dioxide is adsorbed strongly on the vacant sites formed by dehydroxylation, and not on the surface hydroxyl groups.

Water Content on Metal Oxides. I. Water Content on Silica Gel, Magnesium Oxide, Zinc Oxide, and Titanium Dioxide

Abstract The surface water content has been determined on silica gel, magnesium oxide, zinc oxide, and titanium dioxide by means of active hydrogen analysis and by the successive-ignition-loss method. Except for magnesium oxide pretreated at 100 °C, it has been found that the water contents obtained from the two methods on every oxide tested are in good agreement with each other. The successive-ignition-loss method gives an extraordinarily high value of water content on magnesium oxide pretreated at 100 °C, probably because of the retention of inner hydroxyl groups. The present results indicate that the two methods are equally reliable for the quantitative determination of the water content on metal oxides. Moreover, the simultaneous application of both methods make it possible to estimate each amount of surface hydroxyl groups and physisorbed water molecules.

Reactivity of the surface of some schistose silicates

1. A study was made of the reactivity of talc, serpentine asbestos, montmorillonite and muscovite by comparing the amount of surface hydroxyl groups and carbon in the starting samples and the same samples after they had been refluxed with methyl alcohol. 2. The investigated silicates differ in the character of their reaction with methyl alcohol at the boiling point of the latter (64.7°): talc and serpentine asbestos exhibit irreversible adsorption, which takes place without a transformation of the hydroxyl groups; the montmorillonite and muscovite are apparently esterified. 3. In the case of muscovite, together with the postulated esterification, there also takes place another process, leading to a high carbon content in the samples that had been treated with the alcohol.