Understanding surface (un)reactive sites of titania supports towards propyl-phosphonic acid surface modification

Abstract

Organophosphonic acids (PAs) surface modification on metal oxides is important for applications, nevertheless, a detailed study evaluating the influence of metal oxides surface properties on PAs modification is lacking. This work presents a method to qualitatively probe surface (un)reactive sites of titania towards propyl-phosphonic acid (3PA) grafting by methanol. It identifies the more or less reactive sites, i.e., hydroxyl groups and Lewis acid sites, allowing to understand relative differences in maximum modification degrees of 3PA among different titania supports. Therefore, three different types of titania were used, while all other 3PA modification conditions were kept constant. A clear difference in the maximum modification degree on the three titania supports was observed. In-situ diffuse reflectance Fourier transform infrared spectroscopy revealed that not all surface OH groups had reacted with 3PA at the highest modification degree, which were similar to most of the OH groups remaining after methanol chemisorption. Also the adsorption capacities of the strongly bonded chemisorbed methanol elucidated differences in the three titania, which was related to the maximum modification degree of 3PA. Methanol chemisorption can thus aid in the understanding of the kinds of reactive surface sites that play a role in the divergence of 3PA coverage on different types of titania supports.