Surfaces, acidity

The term surface acidity comprises acidic sites roughly distinguished as Bronsted and Lewis centers as well as basic sites, the latter attracting growing interest at present [30]. 

This topic has recently been reviewed, and in order that we may be brief we would like to refer the reader to the article of Karge [23] for more information on methods other than IR spectroscopy for detecting and characterizing the various kinds of sites such as titration, MAS-NMR, ESR, TDS, microcalorimetric measurements and application of test reactions. 

Bronsted acid sites (B sites) are proton-donating OH groups, above all bridging s Si-OH-Al s structures. Lewis acid centers (L sites) on the other hand are coordinatively unsaturated sites and therefore capable of accepting electron pairs like trigonal A1 atoms, so-called true Lewis sites (charged extraframework AlxOy species) or cations. 

To determine the density of B sites the intensities of the OH bands themselves may be used. A second possibility to analyze the concentration of both B and L sites is their quantitative determination using various base molecules [31-36]. The selection of the most suitable probe molecule always seems to be difficult. In any case exact knowledge of the absorption coefficient is necessary, which additionally may be coverage-dependent. In the case of pyridine the literary data have been critically evaluated [37]. 

The strength of acid B sites can be determined from the shift of the OH frequency with and without probe molecules [38]. Kubelkova et al. [39] demonstrated that upon CO adsorption on faujasites, ZSM-5, SAPO-5 and A1P04-5 von is shifted downscale by 90-332 cm in correlation with the proton affinity of the hydroxyls. 

In addition to understanding the bulk structure of catalysts and the distribution of bulk nuclei at the surface, an appreciation of the number and nature of acid sites is also of key importance. A number of NMR techniques have been developed in order to probe both Lewis and Br0nsted acidity at surfaces, and extensive reviews on many of them have been produced by various authors [183, 195, 196]. 

Bronsted acid sites can be directly probed through solid-state H NMR spectroscopy, as chemical shifts can be correlated with acid strength [195, 197, 198]. The precise chemical shift observed for any given Bronsted acid site is dependent on the material upon which it is located. For instance, on silica values of 1.6ppm are typically observed zirconia has two distinct OH sites, at 2.4 and 4.8ppm while on alumina a typical range may be -0.2 to 4.3 ppm. Early studies employing H NMR to study Bronsted acid sites focused on the characterization of the surface of amorphous silica-alumina materials [165, 199-201]. Extensive work, however. 

Sn02 catalyst (b). The asterisk denotes spinning sidebands. 

Adapted from ref. [167] reprinted with permission from the Royal Society of Chemistry. 

Different chemical models can be combined with either of the electrostatic models. The chemical models quantify the uptake of protons and of the ions of an inert electrolyte in terms of expressions similar to the mass law. 

The model based on a single reaction (Reaction 2.14) was originally introduced in [735] with r = Vi and =S = =AIOH, and is called the l-p/C model. The equilibrium constant of Reaction 2.14 defined by Equation 2.15 can be calculated directly from the experimentally determined PZC [2], 

More precisely, the PZC calculated from the best-fit p/f i and pT j in n 2-p7if model by means of Equation 2.23 matches the experimental PZC when the acidic and basic branches of the charging curves are nearly symmetrical. Otherwise, the PZC in the best-fit model curve calculated from Equation 2.23 may deviate substantially from the experimental PZC. Several PZC values reported in Chapter 3 were calculated by means of Equation 2.23. Namely, several publications report the best-fit acidity constants rather than the experimental PZC. 

In the classical version of the 2-pX model, sites at three different degrees of protonation coexist in the vicinity of the PZC. Coexistence of solution species at three different degrees of protonation seldom occurs in small molecules, and this has been used as an argument against the 2-pA model and in favor of the l-pA model. Namely, similarity between protonation of surface species and of analogous solution species is expected. To avoid three different degrees of protonation 

The triple-layer model (TLM) [753] considers surface protonation and deprotonation according to Reactions 2.21 and 2.22 (2-pA model) and two additional types of surface species  

---

Spectral studies at low temperatures enable us to broaden the number of test molecules for surface acidic sites and besides ammonia pyridine and nitriles, to use CO, NO and that do not adsorb at 300 K. 

Without the addition of corrosion inhibitors, acid cleaning or pickling processes to remove oxides and scales would result in severe corrosion of exposed metal surfaces. Acid corrosion is an electrochemical or redox process, and raising cleaning temperatures or acid strength (lowering the pH) increases the hydrogen ion concentration and consequently the rate of corrosion. 

Fig. 15-4 Analogy between dissolved ligands and adsorbents (surface-bound ligands) (a) surface acid-base reactions (b) surface complexation of free metals (c) formation of "mixed-ligand" surface complexes.

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 surface acidity thus estimated is shown in Figure 1. The surface acidity decreases at first with the Cs content, but sharply increases when x exceeds 2. The maximum appeared at x =2.5. 

Figure 2. Catalytic activities of CsxH3-xPWi2O40 for decomposition of isopropylacetate as a function of the surface acidity. The reaction was carried out at 373 K in liquid-solid reaction system.

IR spectroscopy of two supports was used for the determination of their surface acidity. The presence of Lewis acid sites on the surface of sepiolite allowed the preparation of a catalyst able to transform citral into menthol in fairly good yield under veiy mild conditions (90°C, 1 barH2). 

As it can be observed in Table 13.1, Ir supported over pure oxides exhibited low acidity, but Ir supported on mixed Nb20s-Si02 displayed an important enhancement in the surface acidity with surface coverage by niobia increases. Binding energies (BE) of core-level electrons and metal surface composition were obtained from XP spectra. The BE values of Si 2p, Ti 2p3/2, Nb 3ds/2 were 103.4, 458.5 and 123 eV respectively, which are exactly the expected values considering the presence of oxides of Si (IV), Ti (IV) and Nb (V). With regard to Ir 4f7/2 core level, a... 

A number of chemical or electrochemical treatments may be applied after the forming of aluminum or aluminum alloy products. Solvent, acid and alkaline solutions, and detergents can be used to clean soils such as oil and grease from the aluminum surface. Acid and alkaline solutions can be used to etch the product or brighten its surface. Acid solutions are also used for deoxidizing and desmutting. 

Busca, G. (1999) The surface acidity of solid oxides and its characterization by IR spectroscopic methods. An attempt at systematisation, Phys. Chem. Chem. Phys., 1, 723. 

Lercher, J.A., Gruendling, C. and Eder-Mirth, G. (1996) Infrared studies of the surface acidity of oxides and zeolites using adsorbed probe molecules, Catal. Today, 27, 353. 

Wakabayashi, F. and Domen, K. (1997) A new method for characterizing solid surface acidity - an infrared spectroscopic method using probe molecules such as N2 and rare gases. Catalysis Surveys from Japan 1 181. 

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

SCR systems at stationary diesel engines profit from the high exhaust gas temperatures of about 350-400 C, caused by the usually constant high load operation conditions of the diesel engine. In this temperature window nearly all known SCR catalysts are very active. Moreover, weight and size of the exhaust gas catalyst are usually not strictly limited, which results in a good NO, reduction efficiency (DeNOJ. However, DeNO, is not the only criterion for an SCR catalyst. Further requirements are excellent selectivities regarding NO and urea/ammonia as well as low ammonia slip, which is an undesired secondary emission of the SCR process. Therefore, all SCR catalysts exhibit surface acidity, which is necessary to store ammonia on the catalyst surface and, thus, to prevent ammonia slip. 

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. 

---