Kinetic studies of the Pt carbonate-mediated, room-temperature oxidation of carbon monoxide by oxygen
The kinetics involved in a recently revealed ambient-temperature mechanism for the catalytic oxidation of carbon monoxide by oxygen over a 5 wt % Pt/Al2O3 catalyst are evaluated within a periodic, plug flow, redox operation paradigm using combined mass spectrometry (MS), diffuse reflectance infrared spectroscopy (DRIFTS), and time-resolved Pt L3-edge XAFS. The species that are the most active at room temperature are shown to be a high-wavenumber (ca. 1690 cm–1) carbonate that we associate directly with a room-temperature redox process occurring in a fraction of the Pt atoms present in the catalyst. Our results, however, do not exclude the participation of carbonate species native to the Al2O3 support, though these species tend to store CO at ambient temperature and become significant participants in CO oxidation catalysis only at slightly higher temperatures (323–333 K). Pt carbonate formation (1690 cm–1) under CO and the reaction to yield CO2 is shown to be extremely rapid and subject to an average apparent activation energy (Eapp), across the techniques applied, of 8.7 kJ mol–1, within the temperature range investigated (276–343 K). Reoxidation of Pt (XANES) and subsequent CO2 production mediated by Pt carbonates under O2 (MS/IR) displays a first-order dependence upon O2 partial pressure and a negative dependence upon the coverage of CO adsorbed on the Pt nanoparticles also present in this catalyst. This oxidative regeneration/CO2 production step is subject to an apparent activation energy (Eapp) of 56.5 (±5) kJ mol–1, is kinetically limited by the desorption of molecular CO from Pt nanoparticles, and also is shown to be dependent upon the partial pressure of O2 present in the oxidizing half of the cycle that we associate with the direct interaction of O2 with molecular CO adsorbed on the nanoparticles that promotes their desorption. Finally, a minority reactive state producing CO2 in the oxidizing cycle that displays no dependence upon the CO coverage of the nanoparticles can be induced through simple thermal treatment of the catalyst. These results are discussed in terms of the number and types of species present within the reactive system and in terms of the wider possibilities for the development of effective low-temperature CO oxidation using Pt/Al2O3 catalysts.