Propene induced reversible deactivation effects in diesel oxidation catalysts

By applying different transient test protocols in a lab reactor, it is demonstrated that in the presence of C3H6 a Pt/Pd based oxidation catalyst experiences a persistent deactivation that can be attributed to the accumulation of propene partial oxidation products on the catalyst surface. The deactivation can be reversed by treating the catalyst at higher temperature in a hydrocarbon free atmosphere. The three most important scenarios giving rise to the activation/deactivation phenomenon are:(1) Consecutive heat-up/cool-down cycles: If several consecutive heat-up/cool-down cycles are performed with a catalyst pretreated in a propene free oxidative atmosphere, a lower ignition temperature is observed during the first heating ramp than during the following cycles. The effect can be explained by a reversible blocking of active sites by hydrocarbon (HC) intermediates, which are formed during the ignition branch, then entirely are removed after the HC ignition is completed and finally are formed again during the extinction branch, providing a deactivated state of the catalyst for the following ignition. The deactivation can be completely reversed either by a conditioning in an oxidizing HC free gas mixture at temperatures above 110 °C or a conditioning in N2 above 140 °C. The deactivation effect is more pronounced under conditions that lead to a low light-off temperature and completely disappears at conditions leading to higher light-off temperatures, i.e. high CO concentrations or high space velocities.(2) Light-off tests with varying temperature ramp rate: For faster ramp rates, CO/propene ignition is observed at lower inlet temperatures. This is the opposite behavior than would be expected based on thermal considerations and can be explained by a higher accumulation of HC intermediates during the slower ramp.(3) Transient observation of the deactivation effect: If an initially clean catalyst is operated at a constant temperature in the light-off range, a transient decrease in CO/HC conversion is observed that can be attributed to the gradual build-up of the blocking HC intermediates.A kinetic model is presented which considers not only the standard CO and hydrocarbon oxidation kinetics, but also the formation of site blocking intermediates by partial oxidation of propene and the removal of these blocking intermediates either by total oxidation with O2 or by thermal desorption. The model nearly quantitatively reproduces the experimental results of the different test protocols.

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