Surface properties acid-base

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. 

The acid-base oxides such as aluminas were used as catalysts, adsorbents or catalyst supports and it was interesting to know the surface acid-base properties of these catalysts. 

It took some time to adopt a similar view of other heterogeneous elimination and substitution reactions. Most efficient experimental tools have been found in stereochemical studies, correlation of structure effects on rates and measurement of deuterium kinetic isotope effects. The usual kinetic studies were not of much help due to the complex nature of catalytic reactions and relatively large experimental error. The progress has been made possible also by the studies of surface acid—base properties of the solids and their meaning for catalysis (for a detailed treatment see ref. 5). 

Shen, J., Kobe, J. M., Chen, Y. and Dumesic, J. A. (1994). Synthesis and surface acid/base properties of magnesium-aluminium mixed oxides obtained from hydrotalcites. Langmuir 10, 3902. 

Shen, J. Y., Tu, M. and Hu, C. (1998). Structural and surface acid/base properties of hydrotalcite-derived MgAlO oxides calcined at varying temperatures. J. Solid State Chem. 137, 295. 

Nagy, N. M., and J. Konya. 2007. Study of pH-dependent charges of soils by surface acid-base properties. J. Coll. Interface Sci. 305 94—100. 

When studying the surface acid-base properties of montmorillonite, it is essential to understand that hydrogen ions and cations of the support electrolyte can also participate in cation-exchange processes. The processes on the internal and external surfaces have to be taken into consideration simultaneously, and they both have to be included into the equilibrium thermodynamical models. 

For the interpretation of the results using the surface-complexation model, reactions 2.47-2.53 have to be taken into account. In addition, the surface acid-base properties and the neutralization reactions of the layer charge have to be included as in Section 2.4.2 the parameters determined there are treated as fixed, input data. In the case of copper- and zinc-montmorillonite, the copper and zinc concentration of the solution and solid also have to be determined, and these data have to be taken into consideration. That is, the quantity of the total sorbed valine and the copper or zinc ion concentrations versus pH function can be fitted, and KH2Valx, KAioH2Vai> and KSi0CuVal stability constants can be computed. The results of the parameter fit for copper- and zinc-montmorillonites as well as the obtained stability constants are shown in Figures 2.17 and 2.18, and in Table 2.12, respectively. 

In this chapter, the relationship of geological origins and interfacial properties of bentonite clay will be reviewed first. Then we will discuss the migration of water-soluble substances in rocks and soil, and the effect of sorption on the migration. A linear model will be derived by which the quantity of ion sorbed on rocks can be estimated when the mineral composition and sorption parameters of the mineral components are known. Surface acid-base properties of soils will be discussed, and the sorption of an anion (cyanide ion) will be shown on different soils and sediments. 

Similar classification can be made on the basis of the surface acid-base properties of bentonite samples (Chapter 1, Section 1.3.2.1.1 and Chapter 2, Section 2.4). The number and ratio of the edge silanol and aluminol sites, as well as the intrinsic stability constants of the protonation and deprotonation constants (Chapter 1, Equations 1.54-1.56 Chapter 2, Equations 2.3-2.5) are very different for sedimentary bentonites (layers B-I.b. and B-II.a.) and for the bentonitic tuff (B-II.b. layer Table 3.3). 

Some characteristic properties of bentonites (CEC, sorption properties) are mainly governed by the montmorillonite content and the layer charge of montmorillonite. Other properties, however, depend on the circumstances under which the rock is formed. These are particle size distribution, external specific surface area, and surface acid-base properties. The quantity of the edge sites mainly depends on the specific surface area. The protonation and deprotonation reactions take place on the edge sites of other silicates and aluminosilicates present beside montmorillonite, so their effects manifest via surface reactions. Consequently, the origin of bentonite determines all properties that are related to external surfaces. 

The surface acid-base properties of bulk oxides can be conveniently investigated by studying the adsorption of suitably chosen basic-acidic probe molecules on the solid. Acidic and basic sites are often present simultaneously on solid surfaces. The two centers may work independently or in a concerted way, and the occurrence of bifunctional reaction pathways requiring a cooperative action of acidic and basic centers has also received considerable attention [39]. The acid-base properties of numerous amorphous metal oxides investigated by mrcrocalorime-try have been summarized in an extensive review by Cardona-Martinez and Dumesic [11]. 

The surface acid-base properties of polycrystalline MgO surfaces have been assessed by means of thermogravimetry and DSC of desorption of pyridine and CO2 in the room temperature to 400 °C temperature range [44]. The endotherms and corresponding AH of desorption were discussed in relation with results determined previously using differential adsorption calorimetry and taking into account the structure, surface area and defects of the studied surfaces. 

Figure 4 shows the variation of surface acid-base properties of various Pt/C catalysts, reflected by the pHzpc, as a function of the activation temperature. The observed behavior is qualitatively similar to that reported in Figure 3. Figure 5 shows that an increase of acidic or basic surface groups (Ca, Cb) produces a corresponding shift of the pHzpc toward acidic or basic regions, respectively. Thus, the pHzpc clearly reflects the concentration of surface functional groups. 

Yang, X. et al.. Surface acid-base properties and hydration/dehydration mechanisms of aluminum (hydr)oxides, J. Colloid Interf. Sci., 308. 395, 2007. 

Yang, X.F. et al., Adsorption of phosphate at the aluminum (hydr)oxides-water interface Role of surface acid-base properties. Colloids Surf. A, 297, 84, 2007. 

Lin, X.Y., Creuzet, E, and Arribart, H., Atomic force microscopy for local characterization of surface acid-base properties, J. Phys. Chem., 97, 7272, 1993. 

Zaki, M.L, Hasan, M.A., and Pasupulety, L., Surface reactions of acetone on AljO, TiOj, ZrO-, and CeOji IR spectroscopic assessment of impacts of the surface acid-base properties, Langmuir, 17, 768, 2001. 

J. Keranen, C. Guimon, A. Auroux, E.I. Iiskola, L. Niinisto, Gas-phase synthesis, structure and surface acid-base properties of highly dispersed vanadia/titania/silica catalysts, Phys. Chem. Chem. Phys. 5 (2003) 5333. 

In the present work the behaviour of zirconia samples doped with oxides of alkali metals and alkaline-earth metals was investigated, in order to better understand the role of both the nature and the amount of the doping cation. Li-, K-, Ca-, and Ba-doped zirconia samples were prepared. Their surface acid-base properties were assessed by means of adsorption microcalorimetry, using ammonia and carbon dioxide as probe molecules. Their catalytic activity for the 4-methylpentan-2-ol dehydration was tested in a flow microreactor. 

Sample composition, surface acid-base properties and catalytic activity... 

Results can be explained by considering that the catalyst surface acid-base properties determine the preferential formation of a given trimeric intermediate which in turn defines both the formation of the final product released to the gas phase (MO, IP or MES) and the nature of the carbon deposit. This interpretation is depicted in Fig. 5. Coke formed from the 4,6-dimethylhepta-3,5-dien-2-one intermediate on acidic catalysts will be... 

The MgyAlOx activity declines in the acetone oligomerization reaction due to a blockage of both basic and acid active sites by a carbonaceous residue formed by secondary aldol condensation reactions. The key intermediate species for coke formation are highly unsaturated linear trimers that are formed by aldol condensation of mesityl oxide with acetone and remain strongly bound to the catalyst surface. The catalyst surface acid-base properties determine the preferential formation of a given trimeric intermediate, which in turn defines the chemical nature of the carbon deposit. Aromatic hydrocarbons are the main component of coke formed on acidic Al-rich MgyAlOx samples whereas heavy a,P-unsaturated ketones preferentially form on basic Mg-rich catalysts. 

The measurement of the acidity of solid acid surfaces has been the focus of a vast number of studies. The most commonly used techniques are Hammett titrations, chemisorption of bases and TPD. Extensive discussions of diese methods and their shortcomings are available in the literature [4], The use of adsorption calorimetry makes it possible to determine quantitatively the surface acidity and the acid-strength distribution of solid acids. Surface acid-base properties of catalytic solids can also be studied by base desorption using TG [71]. 

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