Surface acidity scales: Experimental measurements of Brønsted acidities on anatase TiO2 and comparison with coinage metal surfaces

Abstract The first quantitative surface acidity scale for Bronsted acids on a solid surface is presented through the use of titration-displacement and equilibrium experiments on anatase TiO 2 . Surface acidities of species on TiO 2 correlated with gas phase acidities, as was previously observed in qualitative studies of Bronsted acid displacement on Ag(110), Cu(110) and Au(111). A 90% compression of the surface acidity scale relative to the gas phase was observed due to compensation from the covalent component of the conjugate base – surface bond. Adsorbed conjugate bases need not be completely anionic for correlations with gas phase acidities to hold. Positive and negative substituent effects, such as substituted fluorine and hydrocarbon sidechain dispersion interactions with the surface, may modify the surface acidity scale, in agreement with previous experimental and theoretical work on Au(111).