Abstract
The water–gas shift (WGS) activity of Pt/SiO 2, Pt/CeO 2 and Pt/TiO 2 catalysts was studied by in-situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTS). Samples contained a similar amount of Pt, between 0.34 and 0.50%, and were characterized by employing a variety of physical and spectroscopic techniques. The catalyst activities were evaluated through both CO conversion versus temperature and CO conversion versus time tests. The DRIFTS spectra were obtained on stream during the WGS reaction at increasing temperatures, from 303 to 573 K. Reduced ceria was the only active support and promoted the WGS reaction on surface bridging OH groups that react with CO to form formate intermediates. Pt/SiO 2 was more active than CeO 2 and catalyzed the WGS reaction through a monofunctional redox mechanism on metallic Pt sites. The CO conversion turnover rate was more than one order of magnitude greater on Pt/CeO 2 than on Pt/SiO 2 showing that the reaction proceeds faster via a bifunctional metal-support mechanism. Platinum on Pt/CeO 2 increased the concentration of OH groups by increasing the ceria reduction extent and also provided a faster pathway for the formation of formate intermediates in comparison to CeO 2 support. Pt/TiO 2 catalysts were clearly more active than Pt/CeO 2. The WGS reaction on Pt/TiO 2 was catalyzed via a bifunctional metal-support mechanism, probably involving the activation of CO and water on the metal and the support, respectively. The role of platinum on Pt/TiO 2 was critical for promoting the reduction of Ti 4+ ions to Ti 3+ which creates oxygen vacancies in the support to efficiently activate water.
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