Tuning the metal loading of Pt/CeO2 catalysts for the water-gas shift reaction

Abstract

Identifying active platinum species at the surface of Pt/CeO₂ catalysts is still a hot topic in the literature. In this work, an oxidizing pretreatment at 500 °C was applied to generate ultradispersed PtO_x species before the reaction. It is shown that the molar activity of such catalysts for the water-gas shift reaction is strongly dependent on the platinum content, increasing by a factor of 2.5 from 0.1 to 0.6 wt% and stabilizing from 0.6 to 1.4 wt%. The tracking of Pt species present under reaction conditions (230 °C, H₂O/CO=4) was performed using operando DRIFT spectroscopy, CO-TPR and STEM in connection with the catalytic activity. A major structural change was found for Pt loadings above 0.6 wt% through the formation of metallic Pt⁰ nanoparticles of ca 1.4 nm from oxidized Pt single atoms and clusters. Conversely, for Pt contents below 0.6 wt%, Pt species possess a stronger interaction with CeO₂ as well as a lower nuclearity, limiting their activation under reaction conditions. This strongly suggests that metallic Pt nanoparticles, prevalent at high loading, are more active than oxidic Pt single atoms and small clusters, which are predominantly present at low loading. This study highlights the key role of PtO_x reducibility and the importance to optimize the Pt loading to obtain active catalysts for the water-gas shift reaction.