(225o) Adsorption of NO, NO2 and CO over Agx Faujasite with Water and Oxygen Gas – DFT, Equilibrium, and Dynamic Experiment Investigations

In the context of preventing harmful releases from thermal engine of non-road vehicles in confined environment, AgX faujasite (Ag₈₄Na₂[(AlO₂)₈₆(SiO₂)₁₀₆], Si/Al = 1.2) has been studied for its ability to selectively trap CO, NO and NO₂ pollutants from water and oxygen. First, DFT calculations were performed to investigate the performance of a series of monovalent cation-exchanged faujasite X zeolites, resulting in AgX being selected as the most selective material. Subsequently, experimental equilibrium results have yielded in an excellent agreement with the predicted isosteric heat of AgX. A continuous fixed-bed system has been used to analyse the breakthrough characteristics of CO, NO and NO₂ with and without water vapour from granular AgX zeolite. Investigations on its regenerability for reuse has been carried out.

DFT calculations are in agreement with experimental results. The adsorption energy of these three pure species at zero coverage has the same behaviour. Different operating conditions have significant effects on the adsorption capacity of AgX. Naturally, the adsorbed amount of all chemical species reduces when temperature increases.

Dynamic Experiments showed that AgX adsorbs NO weakly, but it can be used to trap or separate CO and NO₂ (Figure 1). The adsorption capacities and the breakthrough times increased in the following order: NO<CO<NO₂. Also, water vapour, which assists in the generation of the roll-up, decreases the adsorbed amount and the breakthrough time.

Comparison with literature and other dynamic experiments has been performed especially thanks to experiments carried out with NaX zeolite with the same Si/Al ratio. Considering the adsorption capacity, AgX and NaX are potential adsorbents for the trapping of CO, NO and NO₂; the AgX zeolite presents better adsorption capacities than NaY for NO₂ and CO. However, chemical reaction occurs between both zeolites like NO₂ with water, to generate NO.

This work shows AgX offering great performances and a long-term stability. However, although AgX has proven to be a very effective trap for CO, NO, and NO₂, its affinity for water remains high. The breakthrough model and experiments supported by DFT calculations indicated that this approach can be applied for improving the design, scaling up and optimising the continuous fixed-bed adsorption systems to treat exhaust gases.

Figure 1: Breakthrough curve obtained for a mixture of CO, NO, NO₂, O₂ and water for AgX zeolite –Explanation of chemical reaction between NO and NO2 and the observation of two roll-up- C_CO,in =1 000 ppm, C_NO,in= C_NO2,in 350 ppm, C_O2,in = 8.5%, C_water,in =1% ,Q_v=70 NL h-1, T=30°C, P=101.3 kPa, granular bed length = 50 mm, diameter=10 mm, 0.355<d_p<1mm, m_AgX=4.515 g. AgX had previously been calcined (1°C/min & 500°C during 4h). The outlet concentration was determined by FTIR analysis.