Catalytic decomposition of N2O

doi:10.1016/j.cattod.2004.04.002

Catalysis Today

Volume 90, Issues 1–2, 15 June 2004, Pages 15-19

https://doi.org/10.1016/j.cattod.2004.04.002 Get rights and content

Abstract

Decomposition of N₂O in the temperature range 200–500 °C was studied on catalysts composed of rhodium supported on γ-Al₂O₃ doped with different amounts of Li, Na, K, Cs cations. It has been found that doping with alkali metals influences the dispersion of rhodium which is reflected in the changes of catalytic activity. Dependence of activity on dispersion is linear for potassium and cesium, whereas in the case of sodium a dramatic decrease of activity is observed when the concentration of sodium surpasses 0.07–0.08 mol% due to the modification of the number or specific activity of active sites. The detrimental effect of sodium may be compensated by doping with potassium.

Introduction

Nitrous oxide is a dangerous environmental pollutant because it contributes to the destruction of stratospheric ozone being at the same time a greenhouse gas. The concentration of N₂O in the atmosphere continues to increase (0.2–0.3% yearly) and this increase appears to be caused mainly by anthropogenic activities [1], [2]. Chemical processes associated with the production and use of nitric acid and fluidized bed combustion are the two main nitrous oxide sources, and their contribution to the total nitrous oxide emissions amounts to about 20% [3].

Reduction of N₂O emission can be achieved principally in two ways, either by lowering the formation of N₂O or after treatment where catalysis offers a route for N₂O abatement by the direct decomposition into nitrogen and oxygen.

Transition metal oxides, zeolites and noble metals, especially rhodium, dispersed on oxide supports show catalytic activity in N₂O decomposition [4], [5], [6], [7]. Rhodium supported on γ-alumina is one of the potential commercial catalyst.

The activity of catalysts containing noble metals depends on the properties of γ-alumina used as a support. Commercial aluminas contain different amounts of sodium ions, modifying the local crystal structure on alumina surface which may have some impact on the activity of noble metals [8], [9].

In an attempt to scale-up the process, commercial aluminas, containing different amount of sodium, have been used as supports for rhodium. It has been found that the catalyst performance in N₂O decomposition strongly depends on the sodium content in Al₂O₃. For high concentration of sodium (0.8 mol%) the catalyst activity was poor.

It seemed therefore of interest from both fundamental and applied research point of view to investigate the influence of the presence of alkali metal ions on the activity of catalysts in nitrous oxide decomposition.

Section snippets

Materials

Commercial γ-Al₂O₃ (Procatalyse, Spheralite 531), containing trace amounts of Na₂O was used as a support for further studies.

Alumina was impregnated with different amounts of lithium, sodium, potassium and cesium carbonates followed by drying at 120 °C for 2 h and then calcining at 550 °C for 5 h.

Supports prepared in this way were impregnated with an aqueous solution of rhodium nitrate. The samples, after drying, were activated in reducing atmosphere at 550 °C for 1 h. The resulting catalysts

Results and discussion

Fig. 1 shows the light-off curves of the decomposition of N₂O for a series of samples doped with different amounts of Na₂O. It may be seen that catalytic activity strongly depends on the concentration of the dopant. In case of undoped sample the reaction starts around 300 °C and attains 100% conversion at 425 °C. Doping with small amounts of Na₂O increases the activity, so that, for the sample containing 0.078 mol% Na₂O the reaction starts at 275 °C and 100% conversion is attained at 375 °C. Further

Conclusions

•
It may be concluded that two effects operate:
- ◦
  influence of deposition of alkali metals on the dispersion of rhodium,
- ◦
  modification of the specific activity of active sites which is reflected in the change of the TOF.
•
Doping with alkali metals influences the dispersion of rhodium. In case of lithium and sodium dispersion increases and attains a maximum at surface concentration of 0.07–0.08 mol%, whereas for potassium and cesium it increases and attains the plateau at this concentration.
•
The changes of

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Catalytic decomposition of N2O

Abstract

Introduction

Section snippets

Materials

Results and discussion

Conclusions

Appl. Catal. B

Appl. Catal. B

Appl. Catal. B

Appl. Catal. B

Catalytic decomposition of N₂O