Nitrogen oxides (NOx) has been a major environmental concern
related to the air quality because these cause the photochemical
smog, acid rain, ozone depletion and greenhouse effects. The
primary nitrogen oxide produced from combustion processes with
fossil fuel is NO and NO2 is less than one tenth of NO in the total
nitrogen oxides emission from stationary sources such as thermoelectric
power plan. Selective catalytic reduction (SCR) of NOx
to N2 using NH3 as a reductant is now considered to be the most
effective process for the treatment of stack gases from stationary
sources, and the well-known industrial catalyst for this process is
based on the vanadia system. The catalysts also contain WO3,
or MoO3, that are supported, together with V2O5 as the catalytic
active phase. The SCR catalysts of vanadia system have good
sulfur-poisoning resistance for de-NOx in SO2-containing gases.
Therefore, vanadia catalytic system has been applied as an industrial
catalyst for many years.