Bistability and oscillations of the NO/CO reaction on a Pt-Mo supported catalyst

The reaction of nitric oxide with carbon monoxide on a catalyst of 0.5 wt% Pt-3.4 wt% MoO3 supported on alpha-Al2O3 has been investigated in different reactors at atmospheric pressure. A fixed-bed reactor with external recycle loop exhibited bistability and hysteresis as CO concentration or temperature were varied. In a tubular fixed-bed reactor non-isothermal periodic oscillations (Delta T-max=40 K) were found in a specific pressure-concentration-temperature region (p-c-T region). The amplitude and period time of the oscillations changed smoothly inside the p-c-T region and died out suddenly at the borders of the region. The oscillations were of a relaxation type and lasted for more than 48 h. With fresh catalyst, a new quartz reactor and a specific pretreatment they were reproducible. It was shown that oscillations had kinetic origins and that the changes of the temperature synchronised the elementary oscillators. An autocatalytic model is proposed to explain the experimentally observed phenomena.