Germanium-based polyoxometalates for the adsorption-decomposition of NO_x

A series of germanium-based Keggin-type polyoxometalates (POMs), including H₄GeW₁₂O₄₀ (HGeW), H₅GeW₁₁VO₄₀ (HGeWV), H₅GeMo₁₁VO₄₀ (HGeMoV) and H₅GeW₉Mo₂VO₄₀ HGeWMoV), were first synthesized and utilized as catalysts for removal of NOx. The fourier transform infrared spectroscopy (FT-IR) and temperature-programmed desorption-mass spectroscopy (TPD-MS) characterizations were carried out to investigate the adsorption-desorption behavior and decomposition mechanism of NOx. The adsorption experiments revealed that HGeW had the highest NOx adsorption capacity up to 16.2 mg NOx per gram catalyst. Two infrared characteristic bands appeared at 2210 and 1851 cm^-1 after NOx adsorption, while the latter firstly discovered over NOx-absorbed POMs could be assigned to nitrosyl radical (NO·). The regeneration of HGeW could be realized via reducing the temperature in wet atmosphere to desorb NOx. TPD-MS was conducted to investigate the decomposition behavior of NOx over HGeW and tungstophosphoric acid (HPW). O₂ was firstly detected among the decomposition products, besides N₂ and N₂O. According to conservation calculation based on N mass, NOx removal rate of 81.5 % and N₂ selectivity of 68.3 % could be achieved for HGeW. While the NOx conversion rate and N₂ selectivity for HPW reached 54.1 % and 53.4 %, respectively.