An investigation into the reactivity, deactivation, and in situ regeneration of Pt-based catalysts for the selective reduction of NOx under lean burnconditions
R. Burch et al., An investigation into the reactivity, deactivation, and in situ regeneration of Pt-based catalysts for the selective reduction of NOx under lean burnconditions, J CATALYSIS, 182(1), 1999, pp. 234-243
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 particl
e 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 unid
entified 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 wh
ich was inversely related to reaction temperature. Deactivation is proposed
to arise from a combination of factors which inhibit adsorption and reacti
on of n-octane, due to encroachment onto the Pt surface of hydrocarbonaceou
s species accumulating initially on the support in the vicinity of the Pt/s
upport interface. It is possible that these carbon-containing deposits comp
rise some form of organonitrogen species. The loss of activity due to this
gradual encroachment results in a reduction in the temperature of the Pt pa
rticles, 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 dea
ctivating 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 activi
ty. The divergent activity of SiO2-, Al2O3-, and ZrO2-based catalysts refle
cts support effect contributions, with the former displaying poor low tempe
rature activity and rapid deactivation while the latter supports, particula
rly ZrO2, exhibited high De-NOx activity at lower temperatures without deac
tivation. The use of temperature "spiking" and micropulse injection techniq
ues facilitated regeneration of the full De-NOx activity. In particular, th
e use of micropulse injections of CH3OH into the n-C8H18-O-2-NO reaction co
mpletely circumvented deactivation and demonstrated the potential for obtai
ning very high NOx conversions at low temperatures. (C) 1999 Academic Press
.