Surface acidity and basicity

Busca, G. (2006) The surface acidity and basicity of solid oxides and zeolites, Chemical Industries (Boca Raton, FL, United States), 108 (Metal Oxides), 247. 

In all above mentioned applications, the surface properties of group IIIA elements based solids are of primary importance in governing the thermodynamics of the adsorption, reaction, and desorption steps, which represent the core of a catalytic process. The method often used to clarify the mechanism of catalytic action is to search for correlations between the catalyst activity and selectivity and some other properties of its surface as, for instance, surface composition and surface acidity and basicity [58-60]. Also, since contact catalysis involves the adsorption of at least one of the reactants as a step of the reaction mechanism, the correlation of quantities related to the reactant chemisorption with the catalytic activity is necessary. The magnitude of the bonds between reactants and catalysts is obviously a relevant parameter. It has been quantitatively confirmed that only a fraction of the surface sites is active during catalysis, the more reactive sites being inhibited by strongly adsorbed species and the less reactive sites not allowing the formation of active species [61]. 

Another study examined the acidity and basicity of bulk Ga203 by NH3 and SO2 adsorptions microcalorimetry performed at 150°C. As alumina, Ga203 is amphoteric, with heats higher than 100 kJ/mol for both NH3 and SO2 adsorption, respectively [186]. The amphoteric character of bulk gallium oxides and strong heterogeneity of the surface acidic and basic sites were proved also by Petre et al. [179] using microcalorimetry of pyridine adsorption at 150°C and CO2 adsorption at 30°C. 

The necessity of cooperation between surface acidic and basic sites for splitting off the elements of water from alcohol molecules was intuitively suggested quite early [6] and used as a working hypothesis by an increasing number of authors. However, it took some time to recognise which type of acidity and basicity is suitable for dehydration. 

The picture has been clarified by surface acidity and basicity distribution measurements for several catalysts using thermometric titrations with... 

The detailed synthesis procedure and textural properties (surface area, Sggy in m2 g-1 pore volume, V in ml g"1 and main pore diameter, d in nm), determined by nitrogen adsorption from 8.E.T. method have been published elsewhere (refs. 13-18) and are summarized in Table 1, where the surface acidity and basicity of supports are also collected. These values were determined by a spectro-photometric method described elsewhere (ref. 19), that allows titration of the amount (in tunol g 1) of irreversibly adsorbed benzoic acid (BA, pKa> 4.19), pyridine (PY, pka= 5.25) or 2,6-diterbutyl-4-methylpyridine (DTMPY, pKa 7.5) employed as titrant agents of basic and acid sites, respectively. Furthermore, the apparent rate constant values of different supports in the gas-phase skeletal isomerization of cyclohexene (CHSI), in Mmol atm"1 g"1 s-1, at 673 K, are also collected in Table 1, because these values are another way of measuring the stronger acid sites of supports (ref. 19). 

At present the dye techniques are very useful and economical but are somewhat approximate. Advances in use of indicator dyes for measuring surface acidity and basicity may be expected to include a two-parameter measure of acid or base strength similar to the E and C equation of Drago, and the use of fluorescent indicators for colored solids. 

Electrophoretic Methods for Detecting Surface Acidity and Basicity... 

For the characterization of the nature of surface sites, probe molecules are needed. In the following, however, we confine ourselves to those probe molecules that can principally be used as poisons under catalytic conditions. Thus, for example, the indicator molecules usually used for the titration of surface acidity and basicity will not be treated. The conditions under which these measurements are carried out differ greatly from those applied during actual catalysis, and the molecular size of the probe molecules is unfortunately usually very large.2 These methods have been reviewed very recently by Tanabe (20) and by Fomi (42). 

Although the catalysts showed high initial activity, rapid deactivation was also observed. For example, when using a Pt/t -alumina catalyst at 250 C, essentially complete TCA conversion was observed initially however, after 15 h TCA conversion had declined to < 25 percent. To understand the deactivation process, surface acidity and basicity, coke content, chlorine content, and platinum content were measured for both the fresh and the used catalysts. These measurements showed that up to 40 wt% coke formed on the supported platinum catalyst and that the acidity changed significantly during the reaction at 350°C. 

Surface acidity and basicity were measured by adsorption of organic bases such as pyridine (PY, pk,=5.3), morpholine (MP, pk, = 8.33), piperidine (PP, pk, = 11.1) and acidic substrates like acrylic acid (AA, pk = 4.2), phenol (PH, pk, = 9.9), respectively by spectrophotometric method[15,16]. Redox properties (one electron donor and one electron acceptor) were determined by the same method using meta dinitrobenzene (DNB, electron affinity, EA = 2.21eV) and phenothiazine (PNTZ, ionisation energy, IE = 7.13eV) as the adsorbates. 

Zecchina. A Lamberti, C Bordiga, S. Surface acidity and basicity. General concepts. Catalysis Today. 1998 41, 169-177. 

The surface acidity and basicity of iron oxide (a-Fe203) catalysts, pure and surface-doped with variable amoimts of sulfate groups, have been characterized by microcalorimetry of CO adsorption at room temperature. An appreciable decrease of Q with surface coverage was detected [108]. The surface basicity of iron oxide, tested by adsorption of CO2 and monitored by FTIR, which revealed the formation of carbonate-like surface species, is gradually decreased by sulfates, but not suppressed. The vacuum reducibility of iron oxide, which can be spectroscopically evidenced both by a colour change (UV-VIS spectra) and by the formation upon CO adsorption of surface carbonyl-like species with a... 

A thermodynamic scale of surface acidity and basicity can be constructed by exploring the acid-base properties of numerous solids and comparing the heats of adsorption and the adsorption uptakes of gas-phase probe molecules (NH3, CO2, SO2). These solids, varying in their physical and chemical properties, have been selected in order to cover a wide range of acid-base behaviours representative of acidic, amphoteric and basic solids. They can be divided into three main groups according to their adsorption properties towards acidic probes (which interact with basic solids) or basic probe molecules (which adsorb on acidic solids). Amphoteric solids display an adsorption edacity towards both acidic and basic probe molecules. 

Without doubt, surface acidity and basicity are decisive for the properties and the catalytic performance of a solid. Consequendy, measuring the acidity and basicity of a given family of OH groups is of great importance. Below, we shall briefly consider the main principles of such measurements, thereby focusing on vibrational spectroscopy. Several monographs and review papers addressing this problem have recendy been pubfished (56,60,88,129-139). 

Silane additives change the surface interaction potential of PU in distinct ways APS accentuates surface basicity, while CS, MS and VS accentuate surface acidity. The addition of ES results in significant increases in both surface acidity and basicity. 

Zirconia is the only pure metal oxide that possesses four different chemical properties on its surface acidic and basic as well as oxidizing and reducing [15-17], The surface concentration of hydroxyls is higher than 20jimol OH/m [18,19]. 

Independent studies have shown that corona treatments increase both the surface acidity and basicity. Ty ncally, add indexes are increased from 8.7 to 18.9, while surface basidty parameters are increased by corona treatment fiom 24.5 to 27.4. A major increase in the acid/base parameters would not necessarily affed interactions with a neutral matrix sudi as PE on the other hand, acid/base interactions bdween filxes are practically doubled as a result of corona treatment... 

H. Kndzinger, Infrared spectroscopy for the characterization of surface acidity and basicity, in G. Ertl, H. Kndzinger and J. Weitkamp (Eds.), Handbook of heterogeneous catalysis, Vol. 2, Wiley-VCH Weinheim, pp. 707—732, 1997. 

We extend our imderstanding of the concepts of chemical bonding and reactivity learned in Chapter 3 on metals and Chapter 4 on zeolites to catalysis over metal oxides and metal sulfides in Chapter 5. The featmes that lead to the generation of surface acidity and basicity are described via simple electrostatic bonding theory concepts that were initially introduced by Pauling. The acidity of the material and its application to heterogeneous catalysis are sensitive to the presence of water or other protic solvents. We explicitly examine the effects of the reaction medium in which the reaction is carried out. In addition, we compare and contrast the differences between liquid and solid acids. We subsequently describe the influence of covalent contributions to the bonding in oxides and transition to a discussion on the factors that control selective oxidation. 

M. Hunger, NMR Spectroscopy for Characterization of Surface Acidity and Basicity , in Handbook of Heterogeneous Catalysis (2nd Edition), ed. G. Ertl, Wiley-VCH Verlag GmbH Co, KGaA, Weinheim, Germany, 2008, vol. 2, p. 1163. 

Abstract We review the use of adsorption microcalorimetry for the determination of the surface acidity and basicity of various types of zeolites and related materials, as well as the relationship between the information gathered by this technique and catalytic activity. 

The concepts and origins of surface acidity and basicity are discussed in Section 4.3.4 and it is to be noted that these concepts parallel closely the approach of HSAB, differences being mainly in the descriptions of the terms used, with acidic and basic behavior of a graphene layer being equated to hard and soft properties. 

Markowitz, M.A., Schoen, RE., Kust, R, and Gaber,B.R, Surface acidity and basicity of functionalized silica particles. Colloids Surfaces A, 1999,150,85-94. 

Michael A. Markovitz, Paul E. Schoen, Paul Kust, and Bruce P. Gaber Surface acidity and basicity cf functionalized silica particles. Colloids Surf. A Physicochem. Eng. Asp., 150 (1999) 85-94... 

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