The per second. The apparent activation energy

The acidity of catalyst is an important
factor in NH3-SCR activity for the activation of NH3 and provide NH4+
adsorbed 5. Therefore, the acidities of both catalyst samples are investigated by
NH3-TPD analysis. NH3-TPD results over catalyst before
and after ion-exchanged are displayed Fig.4. Compared with the zeolite before ion-exchanged, the
intensity of high temperature peak on Cu/zeolite catalysts are low because of
the ion exchange of H+ with Cu2+. Cu/H-SUZ-4 showed
high intensity in broader temperature window. In NH3-TPD spectrum,
two ammonia desorption peaks are often observed with large peak at low
temperature around 100-200°C (L-peak) and desorption at high temperature
(H-peak). The L-peak is attributed to
weakly bound NH3,
whereas the H-peak is attributed to strong acid
sites 14. Desorption peak area of Cu/H-SUZ-4 is greater than Cu/H-ZSM-5,
suggesting that large amount of acid sites is present over the surface of Cu/H-SUZ-4. So, the NH3 adsorption capacity of Cu/H-SUZ-4 is higher than that of Cu/H-ZSM-5. As
a results, the high ability of NH3
adsorption has influence on good SCR performances.

NH3-TPD
analysis

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NH3
oxidation is undesired side reaction in NH3-SCR. Fig. 3. showed NH3
oxidation as function of NO production with temperature from
150-650°C. NH3
oxidation take places at high temperature, above 400°C. Both
of catalysts displayed a decrease of NO conversion due to the ammonia oxidation
reaction started at 400°C (Fig. 1). These observations were in agreement with
the ammonia oxidation results. NH3 oxidation on Cu/H-ZSM-5 more favorable
than Cu/H-SUZ-4. Concentration of NO production at 500°C on Cu/H-SUZ-4 and
Cu/H-ZSM-5 were 1 and 9 ppm, respectively. Therefore, the
differences in SCR activities may be related to catalyst abilities to oxidize ammonia. The probable reason is Cu/H-SUZ-4 having small-pore
beside medium-pore on the structure that inhibits NH3 oxidation
reaction.

NH3 oxidation

NO conversions were assumed by first-order
rate equation. Then the Arrhenius equation was used to calculate
pre-exponential factors and apparent activation energies. Fig.
2. showed Arrhenius plots of the turnover of frequency (TOF) at 100-200°C. TOF
was defined as NO molecules converted per Cu per second. The apparent
activation energy from the Arrhenius plot for Cu/H-SUZ-4 was 39 kJ/mol while
Cu/H-ZSM-5 (as standard catalyst) was 42 kJ/mol, clearly shown that Cu/SUZ-4
was active for NO reduction. The apparent activation energy of the SCR reaction
of Cu/H-SUZ-4 was similar with value for Cu/H-ZSM-5. This indicates at low
temperature, both of catalysts have identical reaction rate-limiting mechanisms.

Kinetic
study

The NO conversions over
SUZ-4 and ZSM-5 as the function of temperature from 150-650°C are presented in
Fig. 1. The activity of pure H-SUZ-4
is also investigated for comparison. The
H–SUZ-4 exhibited a very low activity, the maximum NOx conversion is only 5.8%.
The NO conversion
increases when copper is introduced to the zeolite support. Below
400°C, both Cu/zeolite catalyst showed high activities with maximum NO
conversion more than 90%. In this range, the NO conversion increased with
increasing of copper loading. Compared to Cu/H-ZSM-5, copper exchanged H-SUZ-4
has maintained high activity even at high temperature. For instance, the NO conversion over Cu/H-SUZ-4 at 500 °C is 91.8%, while
the NO conversions over Cu/H-ZSM-5 under the same conditions decrease by 60.8%.
The results of
activity test suggest that the differences may be originated from structural
perspective. The experimental results in the following characterization
sections will supply more information about the structure-activity and also
physicochemical properties.

SCR activity

The copper weight percentage was measured by ICP analysis. The analysis results showed
amount of copper in Cu/H-ZSM-5 and Cu/H-SUZ-4 were 1.68% and 1.49%,
respectively.

Results and discussion