Acidity Scale for Metal Oxides and Sanderson's Electronegativities of Lanthanide Elements

Abstract

Metal oxides are widely used in industry and academia. [1,2] As their electron-acceptor or acidic strengths play vital roles in their applications, there needs to be a general scale that can quantitatively compare their relative acidic strengths. Conventionally, calorimetric heat measurements during adsorption of probe molecules, [3] infrared spectroscopic analyses of adsorbed bases or acids, [5,6] application of indicator dyes, [4] and temperature-programmed desorption of the pre-adsorbed bases are standard methods for the analyses of their acidic strengths. [6][7][8] However, these methods are not suitable for a quantitative comparison. Thus, unlike metal ions in solution, [9] no such scales have been available for metal oxides.One of the important types of interaction between adsorbates and metal oxides is the formation of coordinate covalent bonding between adsorbates and the surface metal ions. For instance, in the case of TiO 2 , those compounds that have enediol, [10][11][12][13][14] carboxylate, [15][16][17][18] and nitrile [19,20] groups have been shown to form coordinate covalent bonding with the surface Ti 4+ ions. In this type of interaction, the adsorbateto-metal charge-transfer interaction is often the lowestenergy electronic transition. However, in the case of alizarin (Figure 1 a, inset) on TiO 2 , a theoretical study has suggested that the intramolecular charge-transfer (IMCT) band from the catechol moiety to the entire ring system is the lowestenergy transition. [11] Electronegativity (EN) is one of the most important fundamental properties of an atom, which represents "the power of an atom in a compound to attract electrons to itself". [21,22] Among various EN scales that have been developed, [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] Sandersons scale and the associated EN equalization principle [31][32][33][34][35] are successful in calculating the bond energies of various compounds [32][33][34][35][36] , elucidating the acidic and basic properties of zeolites, [37,38] and establishing the relationship between the reactivity and the composition of the zeolite that served as the guideline for the preparation of optimum zeolite catalysts.[39] These methods have also been used for various other purposes. [40][41][42][43][44] However, owing to a lack of experimental data, Sandersons EN scale has not been extended to lanthanides (Ln) during the last five decades, despite the fact that lanthanide-containing compounds are widely used.Herein, we report that the IMCT transition of alizarin is still the lowest-energy transition when it is adsorbed on various metal oxides and sulfides, regardless of the nature of the metal ion. The charge-transfer transition serves as a highly sensitive and accurate probe for the quantitative comparison of the acidic strengths of the metal oxides and sulfides. We also report the factors that govern the surface acidity, which allows us to assign for the first time the important Sandersons EN values of Ln 3+ ions (S Ln 3+) and Ce 4+. To experimentally ver...

Oxygen permeation and stability study of La0.6Sr0.4Co0.8Ga0.2O3− (LSCG) hollow fiber membrane with exposure to CO2, CH4 and He

“…It is possible that presence of the stable Ga 3 þ element in the LSCG perovskite have contributed to an increase in relative acidity of the perovskite structure. This is because based on Sanderson's relative acidity scale, the relative acidity of Ga 3 þ is more than double of La 3 þ and Sr 2 þ [54,55]. Therefore, there is tendency for lower affinity for SrCO 3 formation for the Ga-doped membrane due to the increased resistance of the membrane towards CO 2 adsorption (even though the presence of SrCO 3 is found on the membrane surface as deduced from EDX analysis).…”

Structure and spectroscopy of ground and excited states of LiYb

“…The EN of Li and Yb atom are close to each other in any of various definitions. 45 The ENs of Yb and Li are, respectively, 1.26 and 0.98. 46 Therefore, at large R, the net charge transfer from Li to Yb is small and LiYb has a small negative .…”

Acid–base characterization of heterogeneous catalysts: an up-to-date overview

“…This has allowed a classification of the material versus the alkali metal: Li \ Na \ K \ Rb \ Cs. Note that electronegativity is one of the most important fundamental properties of an atom, as it represents ''the power of an atom in a compound to attract electron in itself'' [45][46][47].…”