Promotion Effects in the Reduction of NO by CO over Zeolite-Supported Rh Catalysts

Abstract

Rh nanoparticles supported on a series of zeolite Y samples containing different monovalent (H+, Na+, Rb+, and Cs+) and divalent (Mg2+, Ca2+, Sr2+, and Ba2+) cations have been used as model systems to investigate the effect of promoter elements in the reduction of NO by CO. Infrared (IR) spectroscopy with NO as a probe molecule allowed monitoring of the electronic changes in the local environment of Rh. The IR bands corresponding to linearly adsorbed NO, i.e., linear and dinitrosyl species, were found to shift to lower wavenumbers with increasing ionic radius-to-charge ratio of the cation. Simultaneously, a lower ignition temperature for NO reduction was observed. In addition, the relative intensity of the bridge-bonded NO band as compared to the total absorbance of Rh-bonded NO species decreased with increasing Lewis acidity of the cation, as expressed by the Kamlet-Taft parameter R. The latter observation matches with similar trends, observed in a previous study (Lepage, M., et al., J. Phys. Chem. C 2008, 112, 9394) for the same catalysts using CO as the IR probe molecule, which could be related to the Rh activity for the CO oxidation reaction. The samples of the present study also showed different catalytic activities, although a straightforward correlation between the results obtained with NO IR spectroscopy and the catalytic reduction of NO by CO could not be established.