An investigation into the reactivity, deactivation, and in situ regeneration of Pt-based catalysts for the selective reduction of NOx under lean burn conditions

The activity and deactivation characteristics of Pt-based lean burn De-NOx catalysts have been investigated and the relationships between temperature, nature of reductant (n-octane) and NO2 concentrations, and the mechanism(s) of deactivation have been examined. The effects of Pt loading and particle size on the activity and deactivation have also been studied. The results show that deactivation of the catalyst is due to site blocking via an unidentified carbonaceous deposit and that the initial surface state of the Pt is crucial. In all cases clean Pt surfaces were found to display an initial period of surprisingly high activity prior to deactivation, the rate of which was inversely related to reaction temperature. Deactivation is proposed to arise from a combination of factors which inhibit adsorption and reaction of n-octane, due to encroachment onto the Pt surface of hydrocarbonaceous species accumulating initially on the support in the vicinity of the Pt/support interface. It is possible that these carbon-containing deposits comprise some form of organonitrogen species. The loss of activity due to this gradual encroachment results in a reduction in the temperature of the Pt particles, leading to a further decrease in reaction and/or desorption rates, and rapid deactivation then ensues. The use of higher Pt loadings leads to enhanced activity at lower temperatures and increased tolerance to the deactivating effects of surface deposition. Catalyst activity and tolerance to deactivation were further enhanced by controlled sintering, which, within certain limits, resulted in excellent, stable low-temperature De-NOx activity. The divergent activity of SiO2-, Al2O3-, and ZrO2-based catalysts reflects support effect contributions, with the former displaying poor low temperature activity and rapid deactivation while the latter supports, particularly ZrO2, exhibited high De-NOx activity at lower temperatures without deactivation. The use of temperature "spiking" and micropulse injection techniques facilitated regeneration of the full De-NOx activity. In particular, the use of micropulse injections of CH3OH into the n-C8H18-O2-NO reaction completely circumvented deactivation and demonstrated the potential for obtaining very high NOx conversions at low temperatures.