Deactivation of Pt/Al2O3 during propane oxidation at low temperatures: Kinetic regimes and platinum oxide formation

Abstract

The kinetics of propane oxidation over Pt/Al2O3 are investigated in this work as a function of O2/C3H8 ratio in the 150–300°C temperature range. At high O2/C3H8 ratios, the platinum nanoparticles are saturated with oxygen and the reaction rate is zero-order with respect to the oxygen partial pressures in this regime. As the oxygen coverage decreases with decreasing O2/C3H8 ratio, the reaction rate increases and the reaction order changes from zero-order to negative-order in the oxygen partial pressure. The reaction rate is controlled to a large extent by the oxygen coverage on the platinum nanoparticles. However, at lower temperatures and higher oxygen pressures there is a slow deactivation of the catalyst that cannot be explained by a slow change in the oxygen coverage. Diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) of adsorbed CO was performed to track the evolution of the nanoparticle structure over the course of the propane oxidation reaction and to determine whether the slow deactivation was caused by reconstruction of the platinum nanoparticles. We found that the platinum nanoparticles are significantly reconstructed during the course of the reaction, including the formation of a platinum oxide (PtO) which has a characteristic CO-DRIFTS band at 2123cm−1. The extent of PtO formation decreases with increasing temperature and, as a result, deactivation of the catalyst is less severe at higher temperatures. Unexpectedly, increasing the oxygen partial pressure resulted in less PtO formation. We believe that a different platinum oxide phase (e.g. PtO2 or Pt3O4) is formed at higher oxygen pressures, which is reduced to metallic platinum during CO exposure at 25°C, and therefore is not detectable by CO-DRIFTS. These results are unique because they show how the nanoparticle structure evolves over many hours of propane oxidation, and how the temperature and oxygen pressure influence the reconstruction of the nanoparticles, which has implications for a wide range of reactions not limited to propane oxidation.