Reactivity of adsorbed CO toward C2H4, H2, and NO on the surface of supported rhodium catalysts

Abstract

The reactivity of adsorbed CO on supported rhodium catalysts for the insertion of CO during ethylene hydroformylation and for CO 2 formation during interactions of CO and NO has been studied by in situ infrared spectroscopy. Adsorption of CO on 0.5 wt.% Rh/SiO 2 produced Rh carbonyl species which are highly selective for CO insertion (i.e. ethylene hydroformylation) at 323 K. Adsorption of CO on highly dispersed Rh sites of 0.2 wt.% Rh/Al 2 O 3 produced linear CO and gem-dicarbonyl which are inactive for the CO insertion in contrast to linear CO on large Rh crystallites 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/SiO 2 strongly depends on the partial pressure of gaseous CO and NO. Linear 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 CO 2 at 373 K. Prolonged exposure of rhodium catalysts to NO/CO flow results in modification of catalyst surface, creating the active site for catalyzing CO 2 formation at 373 K. Strong dependence of the reactivity and mode of adsorbates on the partial pressure of reactants and temperature suggests that the nature of adsorbates should be studied under reaction conditions where both reactants and products are present.