An Investigation into the Reactivity, Deactivation, and in Situ Regeneration of Pt-Based Catalysts for the Selective Reduction of NO_x under Lean Burn Conditions

摘要

The activity and deactivation characteristics of Pt-based lean burn De-NO_x catalysts have been investigated and the relationships between temperature, nature of reductant (n-octane) and NO_2 concentrations, and the mechanism(s) of deactivation have been examined. The effects of Pt loading and particle sized on the activity and deactivation have also bee studied. The results show that deactivation of the catalyst is due to site blocking via an unidentified carbonaceous deposit and that the initial surfae state of the Pt is crucial. In all cases clean Pt surfaces were foudn to display an inital period of surprisingly high activity prior to deactivation, the rate f which was inversely related to reaction temperature. Deactivation is proposed t 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 initally on the support in the vicinity of the Pt/support interface. It is possible that these carobn-containing depositis comprise soem 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, whcih, within certain limits, resulted in excellent, stable low-temperature De-NO_x activity. The divergetn activity of SiO_2-, Al_2O_3-, and ZrO_2-based catalysts reflects support effect contributions, with the former displaying poor low temperature activity and rapid deactivation while the latter supports, particularly ZrO_2, exhibited hgih De-NO_x activity at lower temperautres without deactivation. The use of temperature “spiking” and micropulse injection techniques facilitated regeneration of the full De-NO_x activity. In particular, the use of micropulse injections of CH_3OH inot the n-C_8H_18-O_2-NO reaction completely circumvented deactivation and demonstrated the potential for obtaining very high NO_x conversions at low temperatures.