Sorption and reaction of oxides of nitrogen and carbon on heteropoly oxometalates

Abstract

Nitric oxide and nitrogen dioxide can be removed from a gas stream by heteropoly oxemetalates having the Keggin structure (KU). NO2 can be sorbed (physisorbed and/or chemisorbed) into the bulk structure of 12-tungstophosphoric acid (HPW), 12-tungstosilicic acid (HSiW), and 12-molybdophosphoric acid (HPMo), however with the latter, small quantities were sorbed. A portion of the gas phase NO2 reacts with the water on and in the solid acids to produce nitric acid which desorbs into the gas phase. Infrared, Raman, and NMR spectroscopies indicate that the remainder of the NO2 remains bound in the solid acids through association with the protons contained within to form a nitronium salt. NO cannot be directly sorbed by the acids, but can be sorbed if the catalysts were first exposed to NO2. No reduction of NO nor NO2 to N2 is observed with the heteropoly acids.

With ammonium 12-tungstophosphate and ammonium 12-molybdophosphate, N2 (and small quantities of NO) are found to be the principal products in the effluent of the reactor held at temperatures below 200oC, from the surface (and bulk) reaction of nitrogen dioxide contained in the gas stream and ammonia from the catalyst. At temperatures higher than 200oC, nitrous oxide is produced, as well as N2. Nitric oxide can be reduced, to a certain extent, to N2. However, the NO conversion is significantly increased when NO2 or HNO3 are injected first on the ammonium catalysts. The spent ammonium 12-tungstophosphate sorbs quantities of NO2, while ammonium 12-molybdophosphate does not. Both catalysts are regenerated through injection of gaseous ammonia.

With silica supported 12-tungstophosphoric acid, as with the bulk acid, quantities of NO2 are sorbed. Although the surface area is increased, the number of NO2 per anion decreases from the bulk stoichiometry of 3 NO2/KU to smaller values for lower loadings. As with its bulk acid, silica supported 12-molybdophosphoric acid sorbs vanishingly small quantities of NO2. Little or no NO is sorbed or reduced by the supported acids.

As with the bulk ammonium salts, the injection of nitrogen dioxide on silica supported heteropoly acids, pre-exposed to gaseous ammonia, produces mainly nitrogen. The conversion of NO2 to nitrogen reaches a maximum with approximately 20% loading. Results on the conversion of N2O to N2 by silixa supported 12-molybdophosphoric acid, pre-exposed to gaseous ammonia, are also reported.

Since little or no NO or NO2 are sorbed or reduced by the Li, Na, K, Rb, and Cs salts of HPW, protons evidently participate, directly or indirectly in the sorption and reaction of NO and NO2 by the heteropoly oxometalates.

Heteropoly acids, both pure and SiO2-supported, reduce quantities of CO2 to carbon deposits at temperatures as low as room temperature. As with NOx, the protons contained within the catalysts are necessary for the reduction of CO2. The carbon containing deposits are reoxidized and released to the gas stream when the catalysts are heated to temperatures higher than 550oC. Also, the unsupported and supported heteropoly acids oxidize CO to CO2 without added O2. The addition of traces of oxygen enhances the reaction and regenerates the surface of the catalysts.

In view of the present results and the more stringent regulations for the control of emissions, the heteropoly oxometalates offer an interesting alternative for removal of oxides of nitrogen and carbon from a gas stream.