The effect of chlorine
on mercury oxidation and nitrogen oxides
(NO
x
) reduction over selective catalytic
reduction (SCR) catalysts was investigated in this study. Commercial
SCR catalysts achieved a high Hg
0
oxidation efficiency
when Cl
2
was sprayed into the flue gas. Results indicated
that an appropriate concentration of Cl
2
was found to promote
NO
x
reduction and Hg
0
oxidation
significantly. An optimal concentration of Cl
2
(25 ppm)
was found to significantly promote NO
x
reduction and Hg
0
oxidation. Moreover, we studied the
effects of Cl
2
on NO
x
reduction
and Hg
0
oxidation over SCR catalysts under different concentrations
of SO
2
. The SO
2
poisoning effect was decreased
by Cl
2
when the SO
2
concentration was low (below
1500 ppm). However, sulfate gradually covered the catalyst surface
over time during the reaction, which limited the impact of Cl
2
. Finally, different sulfur-poisoned catalysts were examined
in the presence of Cl
2
. The NO
x
reduction and Hg
0
oxidation performances of sulfate-poisoned
catalysts improved when Cl
2
was added to the flue gas.
Mechanisms for NO
x
reduction and Hg
0
oxidation over fresh catalysts and sulfate-poisoned catalysts
in the presence of Cl
2
were proposed in this study. The
mechanism of Cl
2
-influenced NO
x
reduction was similar to that for the NH
3
-SCR process.
With Cl
2
in the flue gas, the number of Brønsted active
sites increased, which improved catalytic activity. Furthermore, Cl
2
reoxidized V
4+
–OH to V
5+
=O
and caused the NH
3
-SCR process to operate continuously.
The Langmuir–Hinshelwood mechanism was followed for Hg
0
oxidation by SCR catalysts when Cl
2
was in the
flue gas. Cl
2
increased the number of Lewis active sites,
and catalytic activity increased. Hg
0
adsorbed on the surface
of the catalysts and was then oxidized to HgCl
2
. Adding
Cl
2
to the flue gas increased the strength and number of
acid sites on sulfate-poisoned catalysts.