Oxidation/reduction processes occurring in rhodium particles supported on TiO 2 have been studied by a combination of XPS and TPR/TPO. Rh(I) and Rh(III) are formed by calcination in excess of O 2 at 773k, Rh(III) being favoured for samples with very small metal particles. For samples with particles in the range 2.0–10.0 nm, oxidation up to Rh 2 O of the whole particles occurs followed by a kinetically controlled growth of a thin Rh 2 O 3 layer at their surface together with the formation of Rh(III) species at the particle/support interface. H 2 adsorption at 300 K on the highly oxidized particles (up to Rh 2 O 3 ) is an activated process which implies reduction of Rh(III) into Rh(I). Further reduction at 373 K leads to Rh(O) formation although some Rh(I) and Rh(III) species still remain unreduced up to T > 473 K. A model is proposed to decribe the evolution of these rhodium oxide particles as well as to explain their different behaviour against H 2 adsorption at 300 K.
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Muñoz et al. (1988) studied this question.