NO reduction by CO over gold catalysts based on ceria supports prepared by mechanochemical activation modified by Me3+ (Me=Al or lanthanides): Effect of water in the feed gas

Abstract

The reduction of NO by CO was studied over gold catalysts supported on ceria modified by Me3+ ions (Me = Al, La, Sm, Gd and Yb). The ceria supports were prepared by mechanochemical activation. The samples were characterized using XRD, TPR, XPS and Raman spectroscopy. According to the XPS data the concentration of the oxidized gold species was higher than that of metallic gold in the fresh samples modified by lanthanides. On the fresh samples modified by Al only a small part of metallic gold existed in oxidized state. After the catalytic test, only metallic gold was found on the lanthanide-containing catalysts while on the M-modified catalyst a small amount of oxidized Au species in addition to metallic Au was detected. No substantial differences in the average particle sizes of gold, the lattice parameters and the average size of ceria particles were observed. The nature of the modifier and the applied method of ceria supports preparation and gold deposition determined most likely the differences observed in the Raman and TPR data, as well as the catalytic activity results. The catalytic tests were performed under two different conditions: (i) in the presence of H-2 in the gas feed and (ii) adding also water to the gas feed. The lowest activity was observed over the Al-containing catalyst under dry feed, which correlates with the TPR results. The addition of water to the feed led to a significant improvement of the NO and CO conversions over all of the samples studied. At 200 degrees C, Yb-containing gold catalyst exhibited the highest NO and CO conversions. Very promising results for the selectivity toward N-2 were achieved using the lanthanides as dopants. In contrast to the gold supported on Al-doped ceria, no NH3 formation was observed within the whole temperature interval up to 400 degrees C over gold catalysts supported on ceria modified by La, Sm, Gd or Yb.