Combined effect of H2O and SO2 on V2O5/AC catalysts for NO reduction with ammonia at lower temperatures

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Abstract

Combined effect of H2O and SO2 on V2O5/AC the activity of catalyst for selective catalytic reduction (SCR) of NO with NH3 at lower temperatures was studied. In the absence of SO2, H2O inhibits the catalytic activity, which may be attributed to competitive adsorption of H2O and reactants (NO and/or NH3). Although SO2 promotes the SCR activity of the V2O5/AC catalyst in the absence of H2O, it speeds the deactivation of the catalyst in the presence of H2O. The dual effect of SO2 is attributed to the SO42− formed on the catalyst surface, which stays as ammonium-sulfate salts on the catalyst surface. In the absence of H2O, a small amount of ammonium-sulfate salts deposits on the surface of the catalyst, which promote the SCR activity; in the presence of H2O, however, the deposition rate of ammonium-sulfate salts is much greater, which results in blocking of the catalyst pores and deactivates the catalyst. Decreasing V2O5 loading decreases the deactivation rate of the catalyst. The catalyst can be used stably at a space velocity of 9000 h−1 and temperature of 250 °C.

Introduction

One of the most proven technologies to remove nitrogen oxides (NOx) from the flue gases is selective catalytic reduction (SCR) with ammonia in the presence of oxygen [1], [2]. Low-temperature (150–250 °C) SCR process is preferable because it avoids pre-heating of flue gas in many cases and matches easily with the existing burner systems. However, removal of SO2 prior to the SCR is necessary to prevent catalyst from deactivation [3]. Recent studies at our laboratory [4], [5], [6], [7], [8] revealed that activated carbon (AC) supported vanadium oxide catalyst (V2O5/AC) shows high activity and good tolerance to SO2 poisoning in the temperature range of 180–250 °C in the absence of H2O. At V2O5 loadings of 1–5 wt.% and reaction temperatures of higher than 180 °C, SO2 promotes the SCR activity [6] by the increasing surface acidity of the catalyst through formation of SO42− on the catalytic surface [4]. The catalytic activity is stable with no deactivation [5]. The high stability and the SO2-promoted activity of the V2O5/AC catalyst result from high reactivity of ammonium-sulfate salts with NO in the temperature range [8].

Since most of the flue gases contain some amounts of H2O (2–18%), H2O tolerance is also an important issue for the catalyst. It was observed that, in the absence of SO2, addition of H2O inhibits the SCR reaction on various other catalysts at lower temperatures [9], [10], [11], [12], [13], likely due to competitive adsorption of H2O and the reactants, such as ammonia and/or nitric oxide, but this inhibition decreases at higher temperatures [14], [15], [16], [17]. In the presence of SO2, H2O also affects a number of SCR catalysts [18], [19], [20], with significant deactivation at low temperatures and promotion at high temperatures. The increased catalytic activity at higher temperatures was attributed to increased surface acidity due to the formation of surface sulfate species. However, the combined effect of H2O and SO2 on the V2O5/AC catalyst was not studied.

In this work, we investigated the effect of H2O and SO2 on activity and stability of the V2O5/AC catalyst at a number of vanadium loadings and space velocities for the SCR reaction.

Section snippets

Catalyst preparation

The AC was prepared from a commercial coal-derived semi-coke through steam activation at 900 °C for 1.5 h. The AC was then oxidized with concentrated HNO3 at 60 °C for 1 h, followed by filtration, washing by de-ionized water and drying at 120 °C for 4 h.

The V2O5/AC catalyst was prepared by using pore volume impregnation as described in detail elsewhere [24]. The principle of pore volume impregnation is that the volume of solution used in the impregnation is equal to the pore volume of the supports,

Effect of H2O and SO2 on the V2O5/AC catalyst

Fig. 1 shows the effect of H2O and SO2 on NO conversion over 1 wt.% V2O5/AC catalyst at 250 °C and space velocity of 90000 h−1. In the absence of SO2, the steady state NO conversion is 61%. When H2O was introduced into the feed, the NO conversion decreased to about 50%. Noticeably, H2O has some effect on the 1 wt.% V2O5/AC catalyst, but the activity is relatively stable in the range of time on stream. The results are similar to many previous reports. The activity of MnOx/Al2O3 catalyst decreased in

Conclusions

H2O and SO2 have large influence on the activity of the V2O5/AC catalyst for NO reduction with NH3 at low temperatures. In the absence of SO2, the inhibition effect of H2O is small and possibly caused by competitive adsorption of H2O and the reactants, such as NH3 and/or NO. The effect of SO2 is two-fold. In the absence of H2O, the activity of V2O5/AC catalyst increases with the addition of SO2; in the presence of H2O, however, SO2 deactivates the V2O5/AC catalyst. The deactivation may be

Acknowledgements

The authors gratefully acknowledge financial support from the Natural Science Foundation, China (29633030, 29876046), Chinese Academy of Sciences and the Shanxi Natural Science Foundation.

References (24)

  • P. Forzatti

    Catal. Today

    (2000)
  • Z.P. Zhu et al.

    J. Catal.

    (2001)
  • Z.P. Zhu et al.

    J. Catal.

    (1999)
  • Z.P. Zhu et al.

    J. Catal.

    (2000)
  • M.D. Amiridis et al.

    J. Catal.

    (1996)
  • M. Mizumoto et al.

    J. Catal.

    (1979)
  • M. Turco et al.

    Appl. Catal. B

    (1994)
  • J. Muniz et al.

    Appl. Catal. B

    (2000)
  • W.S. Kijlstra et al.

    Appl. Catal. B

    (1996)
  • G. Bagnasco et al.

    Appl. Catal. B

    (2000)
  • G. Piehl et al.

    Catal. Today

    (1999)
  • R.Q. Long et al.

    Appl. Catal. B

    (2000)
  • Cited by (0)

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