Promotional effect of Ce doping in Cu4Al1Ox – LDO catalyst for low-T practical NH3-SCR: Steady-state and transient kinetics studies

Abstract

There are very few catalysts reported so far to withstand poisoning by the co-presence of SO2, HCl and H2O in the flue gas stream for the NH3-SCR. The purpose of this work was to report for the first time, to the best of our knowledge, the development of a new catalyst, Ce2/Cu4Al1Ox-layered double oxide (LDO) with high low-temperature de-NOx activity and high poisoning resistance in the presence of H2O, HCl and SO2 in the feed gas stream. In particular, Ce2/Cu4Al1Ox-LDO catalyst in the presence of 5% H2O, 100 ppm HCl and 100 ppm SO2 in the NH3-SCR feed gas stream presented after 9 h of continuous reaction at 200 °C a relatively stable NOx conversion (ca. 57.2%), where all other three control catalysts tested, namely: Cu/Al2O3, Cu-Ce/Al2O3 and Cu4Al1Ox showed severe deactivation, where NOx conversion values of ∼0, 0 and 5.7%, respectively, were measured. It should be noted that the Ce2/Cu4Al1Ox catalyst achieved NOx conversion of 95.3% at 200 °C in the absence of HCl and SO2 in the feed gas stream. A suit of experimental techniques such as BET, XPS, ICS, in situ DRIFTS, pyridine- and NH3-FTIR, NH3-TPD, H2-TPR and transient NH3 chemisorption and NH3-SCR kinetics were employed to reveal possible reasons for the high activity and poisoning resistance exhibited by the Ce2/Cu4Al1Ox catalytic system. XRD and XPS analyses showed that Ce2/Cu4Al1Ox had highly dispersed Cu2+ and Ce3+ species, which likely promote the rate of NH3-SCR. Py-FTIR, NH3-TPD and H2-TPR results indicated that Ce2/Cu4Al1Ox has a larger concentration of surface acid sites and stronger redox properties. According to H2-TPR, ICS and in-situ DRIFTS analyses, the redox properties of Ce2/Cu4Al1Ox were significantly less affected by the presence of HCl and SO2 gases, and lower amounts of metal sulfate and metal chloride species were formed, thus proving its exhibited poisoning resistance. Transient kinetics experiments revealed that the larger site reactivity (k, s−1) and NO oxidation rate to NO2 and not the surface coverage of adsorbed NHx-s active intermediates dictates the higher rate of NH3-SCR over Ce2/Cu4Al1Ox compared to Cu/Al2O3 and Cu-Ce/Al2O3 non LDO- materials.