Characterization of Rh, Mo and Rh-Mo particles formed onTiO_2(110) surface: A TDS,AES and RAIRS study

摘要

The structural and chemical characterization of Rh, Mo and Rh-Mo nanosized clusters formed by physical vapor deposition on TiO_2 single crystal was performed by Auger Electron Spectroscopy (AES), Thermal Desorption Spectroscopy (TDS) and Reflection Absorption Infrared Spectroscopy (RAIRS), applying CO as test molecule. On a slightly reduced titania surface 2D-like growth of Rh was revealed at 300 K up to 0.23 ML coverage by AES and CO-desorption experiments. For CO-saturated Rh particles TDS showed molecular CO desorption in a broad temperature range with T_p = 400, 440, 490 and 540 K (α-states), the latter state appearing only on the smallest Rh particles. The population of γ-state (T_p = 780-820 K) originating from the recombination of C and O atoms on the support began at θ_(Rh) = 0.23ML and was maximized at around 1-2 ML Rh coverage, corresponding to 30% dissociation of CO. A possible dissociation precursor on Rh particles is identified as linearly bonded CO on step sites characterized by v(C-O) of 2017 cm~(-1). Deliberation of CO_2 could not be detected between 170 and 900 K, showing the absence of disproportionation reaction. Instead of oxidizing CO molecules, oxygen atoms stemming from the dissociation of CO attached to the reduced centers of titania, indicating the role of adsorption sites at the perimeter of Rh particles in the decomposition process. 2 ML of predeposited Mo enhanced markedly the dispersion of Rh particles as a result of strong Rh-Mo interaction, but it slightly reduced the molecular a-CO desorption possibly due to enhanced dissociation. The formation of γ-CO was suppressed considerably through elimination of adsorption centers by Mo on the TiO_2 substrate. The reactivity of Rh layers deposited on Mo-covered surface towards CO was reduced after repeated annealing to 600 K due to partial encapsulation of Rh by titania, manifesting in the suppression of the more strongly bonded γ-state. Mo-deposits (up to 0.5ML) on Rh particles decreased the saturation coverage of a-CO through a site-blocking mechanism without detectable influence on the binding energy of CO to Rh, indicating Mo island formation. The carbon arising from the decomposition of CO dissolved in the Mo-containing particles formed a solid solution stable even at 900 K, suggesting a possible role of molybdenum carbide regarding the enhanced catalytic activity of Rh clusters.