Activated carbons modified with aluminium–zirconium polycations as adsorbents for ammonia

Microporous/mesoporous wood-based activated carbon was modified by impregnation with aluminium–zirconium oxycations and calcined at 300 °C, and then used as an adsorbent of ammonia. Water was present either on the carbon or in the gas phase. The initial (before ammonia adsorption) and exhausted materials were characterized using adsorption of nitrogen, potentiometric titration, XRD, FTIR and thermal analysis. Aluminium–zirconium polycations improve ammonia uptake by providing new Brønsted acidic centers that form strong interactions with the adsorbed gas in its protonated form. When calcined, the capacity of the impregnated carbon is reduced in dry air stream but increased in wet air stream. Indeed, for this sample, when exposed to dry gas stream, only Lewis acidic centers (associated with the presence of alumina and zirconia formed during the calcination process) are involved, and they lead to weaker interactions with ammonia. On the contrary, under wet gas stream, Brønsted acidic centers are also present and provide stronger interactions with ammonia. It was also evidenced that impregnation with polycations has a “screening” effect on the carbon surface by protecting it from the oxidative effect of hot air when the sample is calcined.