REACTIVITY OF ADSORBED CO TOWARD C2H4, H-2, AND NO ON THE SURFACE OF SUPPORTED RHODIUM CATALYSTS

The reactivity of adsorbed CO on supported rhodium catalysts for the i nsertion of CO during ethylene hydroformylation and for CO2 formation during interactions of CO and NO has been studied by in situ infrared spectroscopy. Adsorption of CO on 0.5 wt.% Rh/SiO2 produced Rh carbony l species which are highly selective for CO insertion (i.e. ethylene h ydroformylation) at 323 K. Adsorption of CO on highly dispersed Rh sit es of 0.2 wt% Rh/Al2O3 produced linear CO and gem-dicarbonyl which are inactive for the CO insertion in contrast to linear CO on large Rh cr ystallites and gem-dicarbonyl on thermally decomposed Rh-6(CO)(16)/SiO 2 catalysts. The reactivity and the mode of adsorbed CO on 4 wt.% Rh/S iO2 strongly depends on the partial pressure of gaseous CO and NO. Lin ear and bridged CO on reduced Rh sites and gem-dicarbonyl on Rh+ sites do not show any reactivity toward gaseous NO/CO for the formation of CO2 at 373 K. Prolonged exposure of rhodium catalysts to NO/CO how res ults in modification of catalyst surface, creating the active site for catalyzing CO2 formation at 373 K. Strong dependence of the reactivit y and mode of adsorbates on the partial pressure of reactants and temp erature suggests that the nature of adsorbates should be studied under reaction conditions where both reactants and products are present.