A temperature-programmed desorption study of NO on Rh particles supported on α-Al2O3 {0001}

The adsorption and dissociation of NO on Rhα-Al2O3 {0001} was studied as a function of Rh particle size using temperature-programmed desorption (TPD). Samples were prepared by vapor depositing varying amounts of Rh onto the sapphire crystal at 300 K. Auger electron spectroscopy indicated that the Rh formed three-dimensional particles under these conditions, and an average Rh particle diameter was estimated from the CO adsorption uptake and the amount of metal deposited. On particles larger than 5 nm, the TPD results were essentially identical to those for Rh(111) and Rh(110), with approximately 60 to 80% of the NO dissociating. Molecular NO desorbed in a peak centered at 440 K, and N2 desorbed in peaks at 460, 550, and above 600 K. For smaller Rh particles, the molecular desorption peaks and the fraction of NO dissociating remained unaffected; however, significant changes were observed in the N2 desorption features. The N2 feature at 460 K, which has been assigned to a reaction between NO and N atoms, disappears for very small particles and the N atom recombination peak at 550 K shifts above 600 K. By comparing these results to those obtained for various single crystals, we conclude that changes observed with particle size are related to site geometry; however, the desorption rates from small particles cannot be considered to be a simple sum of the rates for different single crystals. It is shown that these changes in the N2 desorption rates with particle size may be responsible for observed structure sensitivity in NO reduction by CO.