Issue 2

The greater area comprising the Attica peninsula and the Saronic Gulf is greatly influenced by various
local circulation types. During winter months, moderately high air pollutants concentrations are
observed when the prevailing condition is either stagnant or a weak southern flow. The case studied in
this paper regards an episode that occurred in mid-December. This episode is characterized by very
weak pressure gradient over Greece, and very little change in wind speed and direction with height,
without the precedence of exceptionally strong warm advection in the lower troposphere during the
days before the episode. This episode was not characterized by very high air pollution values, a rather
common feature during December, that even though it presents one of the highest episode frequencies,
the observed values are not high enough to require the enforcement of restrictive measures.

The activity and selectivity of Pd catalysts supported on YSZ and dosed with different amounts of Na promoter
has been investigated for the reduction of NO by alkenes (C3H6) or alkanes (CH4). It is found that
Na strongly promotes the reduction of NO by C3H6. Rate increases by an order of magnitude are achievable,
while the N2-selectivity is improved from ~75% over the unpromoted Pd catalyst to >95% over the
optimally Na-promoted catalyst. With CH4 as the reductant, a very different behaviour is observed: for all
loadings, Na induces only poisoning. The experimental data indicate that Na increases the strength of NO
chemisorption relative to the hydrocarbons. This is accompanied by weakening of the N-O bond, thus
facilitating NO dissociation, which is proposed as the critical reaction-initiating step. According to this
model the promoting or poisoning effect of Na depends on the interaction strength of the catalyst surface
with the hydrocarbon. The different behaviour of propene and methane reflects the weaker interaction of
alkanes with metal surfaces compared to that of alkenes. XPS and Auger data demonstrate that Na coverage
increases monotonically with promoter loading and that there is no significant tendency for the promoter
to agglomerate with increasing promoter loading. However, a very small but constant tendency to
accumulate subsurface or dissolved Na was observed as the promoter loading is increased.