Catalysis

Dramatic improvement of NO reduction activity via reversible re-dispersion of CeO2 nanoparticles into Ce+3 atoms on alumina under high temperature reactive treatment

Authors

Abstract

Ceria nanoparticles supported on alumina have low activity for industrially relevant NO reduction by CO. We discovered that treatment of these catalysts at temperatures between 750 and ~1,000 ºC under the flow of CO and NO in the presence of steam, which typically leads to catalyst deterioration and sintering, in fact, leads to dispersion of ceria nanoparticles into high density (up to 10 wt%) of fully exposed Ce atoms mostly in +3 oxidation state. We characterize them with XPS, FTIR and HAADF-STEM imaging. Their presence changes the alumina surface, as evidenced by XPS and FTIR with probe molecules. Ce+3 ions show dramatically enhanced NO reduction ability in the presence of CO and steam. Infra-red studies reveal close interaction of NO molecules on Ce+3/Alumina surfaces with the formation of N2O species, indicating redox activity of Ce+3 structures. Heating these samples in oxygen (in wet or dry streams) at 800 ºC and above leads to coalescence of Ce into CeO2 nanoparticles, resulting in reversible loss of activity. Further, reactive treatment of CeO2/Al2O3 under high temperature reaction conditions restores Ce+3 cations as well as catalytic activity. Our study shows reversible redispersion of ceria into isolated Ce+3 cations under conditions where typical catalyst sintering is generally assumed to occur and suggests a pathway to utilize these materials as supports for more effective catalysis. Indeed, supporting only 0.1-0.5 wt% Rh on these supports containing atomically dispersed Ce+3 cations on the surface of alumina, shows synergies between a noble metal and atomically dispersed Ce ions with greatly improved activity and stability for NO reduction with CO compared with Rh/CeO2nanoparticles/Al2O3. Furthermore, exposed Ce cations dispersed on alumina in contact with metals (Rh) show much higher catalytic CO oxidation activity than Rh on CeO2 nanoparticles/Al2O3 system. XO2 oxides (where X is Ce, Ti and Zr) show improvements of NO reduction activity via high temperature treatments suggesting redispersion of XO2 nanoparticles into X exposed cations (with abundance of X+3) is a general phenomenon that occurs, leads to improved catalysts and allows these catalysts to maintain high activity after exposure to extreme aging conditions.

Version notes

We added data that further corroborate and strengthen our claims: 1) DFT calculations, 2) catalytic data for CO oxidation and 3). catalytic data for NO reduction on Rh/titania-alumina and Rh/zirconia-alumina

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