Particle size effects on the reducibility of titanium dioxide and its relation to the water–gas shift activity of Pt/TiO2 catalysts

The effect of primary particle size of titanium dioxide on the reducibility of Pt/TiO2 catalysts by carbon monoxide and hydrogen was investigated using temperature-programmed reduction (TPR) techniques and in situ Raman and Fourier transform infrared spectroscopy (FTIR). Experiments were conducted over Pt catalysts supported on four commercial titanium dioxide carriers of variable structural characteristics, the water–gas shift (WGS) activity of which increases significantly with decreasing primary crystallite size of the support. TPR profiles obtained for the preoxidized catalysts using H2 as a reductant are characterized by a low-temperature consumption peak, attributed to the reduction of image species, and a high-temperature peak, attributed to the reduction of TiO2. The intensity of the latter peak, which is a measure of the reducibility of the support, increases drastically with increasing specific surface area of the catalyst or, conversely, with decreasing primary particle size of TiO2. In situ Raman experiments conducted under hydrogen flow verified that formation of substoichiometric image species begins at lower temperatures and is more facile over Pt/TiO2 catalysts with smaller titania particle sizes. The TPR profiles obtained using CO as a reductant gave qualitatively similar results, exhibiting two CO2 peaks corresponding to reduction of image at low temperatures and of TiO2 at high temperatures. An additional CO2 peak located at intermediate temperatures, which was always accompanied by evolution of gas-phase H2, is attributed to the WGS reaction. FTIR experiments indicate that the reaction occurs via interaction between CO and hydroxyl groups of the support, with intermediate formation of formates. The effect of the titanium dioxide particle size on the reducibility of Pt/TiO2 catalysts and its relation to their WGS activity is discussed with respect to the “regenerative” and “associative” reaction mechanisms.