The adsorption of NO, species on alpha-alumina and Ag/alpha-alumina is investigated by in situ diffuse reflection infrared Fourier transform (DRIFT) spectroscopy and density functional theory (DFT) calculations. The vibrational spectra obtained by DRIFTS experiments show broad spectral bands In the range between 1650 and 1200 cm(-1). The absence of distinct features is attributed to the heterogeneity of powder samples, i.e, variation in Ag cluster size and alpha-alumina surface termination. DFT calculations are employed to evaluate ground-stale structures and vibrational wavenumbers of different NOx species adsorbed either on alumina or Ag-1-Ag-4 clusters supported on alumina In agreement with experiments, Ag cluster size and surface termination strongly Influence the calculated vibrational properties Although, an unambiguous identification of surface species from the DRIFT spectra IS difficult. the theoretical results provide valuable guidance As such, this approach has the potential to further increase the understanding of the reaction mechanism during hydrocarbon assisted selective catalytic reduction (HC-SCR) of NOx. (C) 2009 Elsevier B.V. All rights reserved.