Acidity surface properties

In order to elucidate the reasons for the dependence of the catalytic properties of these samples on their preparation method, we studied the acid surface properties of cobalt- and chromium-modified Zr02 catalysts by ammonia thermoprogrammed desorption and IR-spectroscopy. Our results again indicated that the activity of these catalysts in the SCR of NOx by hydrocarbons is a function both of the surface acidity and content of the active metal. The acid site concentration of the starting Zr02 samples prepared by various methods is significant (0.13 and 0.23 mmol/g) but these samples are inactive, while 10% CriOilZtOi prepared by the sol-gel method displays considerable activity in the reaction studied with lower surface acidity. The acid site concentration of the sample with the same composition prepared by the precipitation method is reduced by a factor of 2.5 and, thus, this catalyst has much lower activity in the selective catalytic reduction. 

Since the catalytic role of oxide surfaces is often related to the presence of Bronsted and Lewis acidic sites on the surface, it is desirable to characterize the acidic surface properties and the respective concentrations of these species. Information about the Bronsted sites can be obtained from H MAS chemical shifts, which are strongly correlated with acidities 14,29]. Complementary information is available from multinuclear MAS-NMR studies of basic probe molecules adsorbed to the surface. Interaction of the probe molecules with acidic sites is expected to cause characteristic shift effects compared to the resonances of the same molecules in either unperturbed or physisorbed states, and hence quantitative information about site populations should be available. Although a number of such investigations have been carried out, it is often difficult to compare results obtained in different laboratories on the same system because experimental details such as sample history, surface coverage, and impurities (frequently water) have large effects on the spectra. 

At room temperature, ca 60 wt % ethylene oxide is needed to solubilize the fatty acids. Surface activity of the ethoxylates is moderate and less than that of alcohol or alkylphenol ethoxylates (84). The ethoxylates are low foamers, a useful property in certain appHcations. Emulsification is the most important function. Its importance is reflected in the wide range of lipophilic solubiHties available in the commercial products. Like all organic esters, fatty acid ethoxylates are susceptible to acid and alkaline hydrolysis. 

The choice of a suitable immobilization method for a given enzyme and appHcation is based on a number of considerations including previous experience, new experiments, enzyme cost and productivity, process demands, chemical and physical stabiHty of the support, approval and safety issues regarding support, and chemicals used. Enzyme characteristics that greatly influence the approach include intra- or extraceUular location size surface properties, eg, charge/pl, lysine content, polarity, and carbohydrate and active site, eg, amino acids or cofactors. The size, charge, and polarity of the substrate should also be considered. 

In the present work the acid-base surface properties of three Al O samples for a chromatography are investigated acidic (I), basic (II) and neutral (III) ones with the using of heterogeneous titration of their suspensions by HCl and NaOH solutions. To establish the correlations between the acid-base and adsoi ption properties studied Al O the representatives of cationic dyes -diamond green (DG), fuchsine (F) and anionic dyes - eriochrom black T and chromic dark blue have been used. 

The (I)-(III)-samples sorption ability investigation for cationic dyes microamounts has shown that for DG the maximum rate of extraction is within 70-90 % at pH 3. The isotherm of S-type proves the physical character of solution process and a seeming ionic exchange. Maximal rate of F extraction for all samples was 40-60 % at pH 8 due to electrostatic forces. The anionic dyes have more significant affinity to surface researching Al Oj-samples comparatively with cationic. The forms of obtained soi ption isotherms atpH have mixed character of H,F-type chemosorption mechanism of fonuation of a primary monolayer with the further bilayers formation due to H-bonds and hydrophobic interactions. The different values of pH p for sorbents and dyes confirm their multifunctional character and distinctions in the acid-base properties of adsoi ption centers. 

Coatings of tin produced from tin-containing aqueous solutions by chemical replacement may be used to provide special surface properties such as appearance or low friction, but protect from corrosion only in non-aggressive environments. Copper and brass may be tinned in alkaline cyanide solutions or in acid solutions containing organic addition agents such as thiourea. Steel may be first coated with copper and then treated... 

Referring to the ionic effects, measuring of swelling in solutions which closely model real ones can provide reliable estimates. The papers [58, 132] can serve as examples of such an approach. In choosing a type of SAH suitable for some particular soil it is necessary to take into account the acid-base properties of the gel and the soil because otherwise collapse phenomena are likely to result from common counterions and the sorption on solid surfaces. 

A great number of nonionic surfactants have been ethoxylated and subsequently reacted with P4O10. The acid phosphate esters from this reaction possess surface properties and detergency similar to the nonionic surface-active agents employed as reactants. Detergency tests and foam heights from the Ross-Miles method have been reported for a series of compounds. Various formulations for all-purpose cleaners are given as well [37,40,41,44,48]. 

The observed complexity of the Se(IV) electrochemistry due to adsorption layers, formation of surface compounds, coupled chemical reactions, lack of electroactivity of reduction products, and other interrelated factors has been discussed extensively. Zuman and Somer [31] have provided a thorough literature-based review with almost 170 references on the complex polarographic and voltammetric behavior of Se(-i-IV) (selenous acid), including the acid-base properties, salt and complex formation, chemical reduction and reaction with organic and inorganic... 

Acid-base reactivity is an important property of oxide catalysts, and its control is of interest in surface chemistry as well as being of importance in industrial applications. The exposed cations and anions on oxide surfaces have long been described as acid-base pairs. The polar planes of ZnO showed dissociative adsorption and subsequent decomposition of methanol and formic acid related with their surface acid-base properties[3]. Further examples related to the topic of acid-base properties have been accumulated to date[ 1,4-6]. 

Gas-phase methylation of catechol by methanol was studied on y -AI2O3 modified by the basic elements K, Li, Mg and Ca. Addition of 7.5 at.% Mg to y-AljOa was optimal and increased the 3-methyl catechol selectivity from 0.26 to 0.65. X-ray diffraction experiments showed the diffusion of Li and Mg cations into the y -AI2O3 bulk. This induces a change in the surface species (XPS data) and the surface acid-base properties (TPD experiments). Ca and K addition to y-alumina was ineffective due to formation of basic oxide layers on the sur ce. 

For the studied catechol methylation reaction the catalyst structure and surface properties can explain the catalytic behaviour As mentioned above, the reaction at 260-350°C has to be performed over the acid catalysts. Porchet et al. [2] have shown, by FTIR experiments, the strong adsorption of catechol on Lewis acid/basic sites of the Y-AI2O3 surface. These sites control the reaction mechanism. 

From the previous results, it has been proven that the nature of the support, although it has no significant influence on the Pd electronic properties, modifies the catalytic properties of the solids To permit a better understanding of these supports effects, the surface properties of the supports (in the presence of the metal) have been studied, in particular the acidic properties and the oxygen mobilities. The A1203 and Z1O2 supports have been mainly onsidered. 

The field of surface-mediated synthesis of metal carbonyl clusters has developed briskly in recent years [4-6], although many organometallic chemists still seem to be unfamiliar with the methods or consider themselves ill-equipped to carry them out. In a typical synthesis, a metal salt or an organometallic precursor is brought from solution or the gas phase onto a high-area porous metal oxide, and then gas-phase reactants are brought in contact with the sample to cause conversion of the surface species into the desired products. In these syntheses, characteristics such as the acid-base properties of the support influence fhe chemisfry, much as a solvenf or coreactant influences fhe chemisfry in a convenfional synfhesis. An advanfage of... 

Murthi VS, Urian RC, Mukeijee S. 2004. Oxygen reduction kinetics in low and medium temperature acid environment Correlation of water activation and surface properties in supported Pt and Pt alloy electrocatalysts. J Phys Chem B 108 11011-11023. 

Mg/Me (Me=Al, Fe) mixed oxides prepared from hydrotalcite precursors were compared in the gas-phase m-cresol methylation in order to find out a relationship between catalytic activity and physico-chemical properties. It was found that the regio-selectivity in the methylation is considerably affected by the surface acid-basic properties of the catalysts. The co-existence of Lewis acid sites and basic sites leads to an enhancement of the selectivity to the product of ortho-C-alkylation with respect to the sole presence of basic sites. This derives from the combination of two effects, (i) The H+-abstraction properties of the basic site lead to the generation of the phenolate anion, (ii) The coordinative properties of Lewis acid sites, through their interaction with the aromatic ring, make the mesomeric effect less efficient, with predominance of the inductive effect of the -O species in directing the regio-selectivity of the C-methylation into the ortho position. 

Yokoyama, S. and Nakagaki, M. (1993) Effect of double bond on the surface properties of aqueous solutions of eicosapolyenoic acids. Colloid and Polymer Science, 271 (5), 512-518. 

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