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Abstract:
A pulsed isotope exchange technique was applied to study the oxygen scrambling activity of polycrystalline calcium oxide under temperatures and pressures relevant for the oxidative coupling of methane (OCM). Oxygen exchange was observed above 400 °C. The onset was attributed to the removal of impurities on the catalyst surface. By trapping impurities in the gas feed, the scrambling could already be observed at room temperature. An activation energy of 80 kJ/mol was determined for the oxygen scrambling of O2 on the surface of polycrystalline CaO powder in absence of other gases. Presence of water and carbon dioxide shift the onset of the reaction to higher temperatures and increase the activation energy significantly to 110 and 150 kJ/mol, respectively. The OCM activity could be directly linked to the oxygen scrambling activity of the material in pulsed OCM operation. It is proposed that the same sites are responsible for oxygen scrambling and OCM reaction and that the rate is dictated by desorption of CO2 and H2O. The high reaction temperatures in OCM in case of CaO are only required to regenerate the active sites, which may apply to basic OCM catalysts in general. In situ Raman and thermogravimetric experiments verified the formation of a bulk calcite phase below 750 °C, which is inactive in OCM and oxygen scrambling. Above 750 °C no surface oxygen species or adsorbates were found by Raman spectroscopy suggesting that only surface defects are responsible for catalytic activity of CaO.
