Abstract
In this paper, novel multi-functional mixed-oxide catalysts have been rationally designed and developed for the effective abatement of NOx CaxCO3-xAl hydrotalcite-like compounds (where x = 0.0, 0.5, 1.0,1.5, 2.0, 2.5, 3.0) are first synthesized by co-precipitation and calcined at 800 degrees C for 4 h in air to derive the mixed oxides. The resultant mixed oxides are generally of spinel phase, where the CaO phase is segregated when x >= 2.5. It has subsequently been found that the derived oxides are catalytically multi-functional for NOx decomposition, capture, and reduction. For example, the mixed Ca2CO1Al1-oxide can decompose 55% NO at 300 degrees C in 8% oxygen, completely trap NO for 750 s, and capture 12.88 and 18.06 mg g(-1) NO within 30 and 60 min, respectively. The catalytic activities of the Ca2CO(1)Al(1)-oxide catalyst have been further improved by incorporating La to form a quaternary catalyst Ca2CO1 La0.1Al0.9-oxide. This catalyst significantly enhances the NO decomposition to 75%, extends the complete trapping time to 1100 s, and captures more NO at 300 degrees C in 8% O-2 (19.02 mg g(-1) NO within 60 min). The in-situ IR spectra of the catalysts with adsorbed NO indicates that the major nitrogen species formed on the catalysts are various kinds of nitrites and nitrates, which can be readily reduced by H-2 within 6 min at 350 degrees C. Therefore, the excellent catalytic activity of layered double hydroxide (LDH)-based mixed oxides for NO decomposition, storage, and reduction can be achieved by the elegant combination of normal transition metals.