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The adsorption isotherms of catechol (1,2-dihydroxybenzene) and gallic acid (3,4,5-trihydroxybenzoic acid) onto titanium dioxide (Degussa P-25) were measured at various pH values and room temperature using attenuated total reflection Fourier transform infrared (FTIR-ATR) data, processed by singular value decomposition. The affinity is largely pH independent, although the deprotonatation of the carboxylic group in gallic acid might produce a slight increase in the affinity. Catechol was shown to form two complexes, with Langmuir stability constants log K of 4.66 (strong mode) and 3.65 (weak mode). Both complexes have the same spectral signature, and mononuclear and binuclear chelate structures are proposed for them. Gallic acid chemisorbs by complexation through two -OH groups and forms one complex only, log K = 4.70. The third -OH and the pendant carboxylate do not influence much the stability of the surface complex. Comparison with literature data demonstrates that the affinity of 4-chlorocatechol is also similar, whereas 2,3-dihidroxynaphthalene and 4-nitrocatechol form more stable complexes, probably because of the solvation contribution to the overall Gibbs adsorption energy. All quoted constants refer to the surface complexation equilibria written as follows: ([triple bond]Ti-OH)2 + H2L = ([triple bond]Ti)2-L + 2H2O, i.e., as electroneutral processes. The FTIR-ATR spectra of the surface complexes are also discussed.

The adsorption and photooxidation of salicylic acid on dispersed TiO2 (Degussa P-25) particles was studied as a function of substrate concentration and pH. Salicylic acid chemisorbs at the particle interface, forming inner-sphere titanium(IV) salicylate surface complexes. The visible differential diffuse reflectance spectra of the surface complexes present a band, with maximum absorption at 420 nm, which is assigned to the internal ligand to metal charge-transfer transition. The surface excess of salicylic acid increases with decreasing pH and levels off around pK a1. At constant pH, the surface excess increases with the concentration of salicylic acid, the isotherm reflecting surface site heterogeneity. Photooxidation rates in air-saturated solutions, on the other hand, are independent of both pH and salicylic acid concentration, in the entire studied range. Chemisorption results are accounted for by a multisite surface complexation model in which two different surface titanium sites and three complexation modes are considered. The mismatch between salicylic acid surface excess values and photooxidation rates is interpreted in terms of the different reactivities of the titanium(IV) salicylate surface complexes and is attributed to the fastest hole capture by bidentate salicylate binding a single surface titanium ion. The advanced rationale illustrates the importance of the basic principles of coordination chemistry in the interpretation of apparent kinetic orders in photolyte concentration.

The adsorption and photoreaction of oxalic acid on the surface of anatase and rutile TiO2 nanoparticles have been studied using a combined experimental and theoretical approach. In the dark, the experimental adsorption reaches an equilibrium state that can be described as a mixture of adsorbed water and oxalic acid molecules, with the latter forming two different surface complexes on anatase and one on rutile particles. When the system is subsequently illuminated with UV(A) light, the surface becomes enriched with absorbed oxalic acid, which replaces photo-desorbed water molecules, and one of the adsorbed oxalic acid structures on anatase is favoured over the other.

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Uniform microcrystalline nickel(II) hydroxide particles have been prepared via the homogeneous alkalinization of nickel(II) nitrate solutions by urea hydrolysis. The nature of the precipitated solids depends on the temperature of synthesis. At ca. 423 K, i.e., under the hydrothermal conditions attained by microwave heating, β-Ni(OH) 2 forms upon the fast ripening of R-Ni(OH) 2 . In the range 363-343 K, R-Ni(OH) 2 is always the final product. The evolution of the systems during the precipitation of R-Ni(OH) 2 is analyzed in terms of the kinetic factors that control nucleation and growth. A precipitation mechanism, based on the principle of minimal structural change, is proposed; it is suggested that the edge-on condensation of Ni 4 (OH) 4 4+ tetramers is a key step in the sequence of events that conduct from hexaaquo Ni 2+ to R-Ni(OH) 2 .

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The adsorption of oxalic acid from the aqueous phase at the surface of rutile nanoparticles has been investigated by attenuated total-reflection Fourier-transformed infrared (ATR-FTIR) measurements. A combination of high resolution transmission electron microscopy (HRTEM) and Wulff-type construction was used to elucidate the typical morphology of the nanocrystals. It is estimated that (110)-type facets present more than 85% of the exposed surface in the powder. The aqueous system was also studied quantum-chemically using the semiempirical method MSINDO. Geometry optimizations have been performed, and the vibration spectra of the most stable surface complexes have been calculated. A sequence of model types has been applied in the quantum-chemical calculations in order to take into account the effect of interaction of water and oxalic acid on the adsorption mechanism and the vibration spectra. It was found that the presence of the aqueous phase significantly changes the stability of the oxalic acid surface complexes compared with the bare TiO(2) surface. The combination of experimental and theoretical information allowed identification of three species as the main contributors to the surface speciation. Two bidentate species were found with the C-C bond parallel to the TiO(2) surface, one monoprotonated and one deprotonated, and a third species being monodentate and monoprotonated.