Performance of NOx capture with Dawson polyoxometalate H6P2W18O62·28H2O

Abstract

Aiming at developing an effective approach to removing NOx, the Dawson type phosphotungstic heteropolyacid (H6P2W18O62·28H2O, HP2W for short) was studied initially as adsorbent for capturing NOx. The HP2W was prepared and characterized with the aid of IR, XRD and TGA. Effects of temperature, gas velocity, crystal water and pretreatment of HP2W on NOx adsorption were examined. The optimum NOx adsorption temperature was 200 °C. With the increase of space velocity, the capacity of NOx adsorption declined, and the maximum adsorption efficiency of 78% was observed at the SV of 5000 h−1. Two types of crystal water were observed on HP2W by TGA measurement, among which the physically adsorbed water would hinder the NOx adsorption, while the chemically adsorbed water would improve it. HP2W was pretreated in the atmosphere of O2, H2 and He respectively before adsorbing NOx. IR and XRD spectra were taken to investigate the adsorption mechanism, and we found that most of the NOx was adsorbed in the forms of NO linkage in the secondary structure of the solid heterpolyacid and a small quantity of NOx was adsorbed as N2O3 and NO2+. Alternative methods for NOx desorption by raising temperature and by cooling with wet air were evaluated, with the latter found to be much more effective in that it enables the reuse of HP2W. Hence an effective and reusable adsorbent HP2W, deserving further study, was obtained.