Application of a sensitive catalytic reactor to the study of CO oxidation over SrTiO ₃ (100) and BaTiO ₃ /SrTiO ₃ (100) ferroelectric surfaces

The use of ferroelectric materials as adsorbents or catalysts could be a promising way to control surface chemical reactions by tuning the ferroelectric polarization. In this context, the oxidation of carbon monoxide over well-defined SrTiO ₃ (100) and BaTiO ₃ /SrTiO ₃ (100) perovskite surfaces was investigated under stoichiometric gas-phase conditions at a total pressure of 3 Torr, in the 100-300 °C temperature range. To this aim, an ultrahigh vacuum-compatible reactor equipped with external laser heating, and specially designed to detect small yields of gaseous products by mass spectrometry, was used. Prior to the catalytic tests, with the help of Auger electron spectroscopy, a sample cleaning procedure under low oxygen pressure was established in order to remove carbon impurities. The as-prepared surfaces appear poorly active in CO oxidation, but their activity increases if a hightemperature pre-annealing treatment under ultrahigh vacuum is applied in order to produce oxygen vacancies, which act as adsorption and/or reaction sites. The activity of the bare SrTiO ₃ substrates, whether doped with Nb or not, is similar to that of the supported BaTiO ₃ films, but the latter lose less activity upon evacuation/reaction cycles. Isotopic labeling experiments using ¹⁸ O ₂ show that the reaction involves CO (and O ₂ ) dissociation, and that lattice oxygen participates to the reaction via the so-called Mars-van Krevelen mechanism.