A DRIFTS study of the stability and reactivity of adsorbed CO species on a Rh/γ-Al2O3 catalyst with a very low metal content

The paper deals with the stability and reactivity of adsorbed CO species in various atmospheres (He, O-2, H-2, H2O) on a 0.05 wt.% Rh/gamma-Al2O3 catalyst calcined in oxygen, and uses 'in situ' diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in combination with temperature programmed reaction (TPR). The dicarbonyl Rh species, which is associated with Rh+ ions, is mainly observed on the catalyst oxidized at 473-823 K or reduced by H-2 at 473-573 K. CO does not reduce the Rh completely to the Rh-0 state, even at 823 K, not because of the stabilization of Rh+ ions by oxygen formed in the dissociation of CO, but mainly because of the difficulty of completely reducing Rh oxide that strongly interacts with the support. Calcination favors the stability of the dicarbonyl species. Complete reduction of the Rh proceeds in H-2 at 823 K. The Rh particles thus formed are not disintegrated by a joint action of CO and hydroxyl groups. The linear CO species, associated with Rh-0 particles, shows a higher reactivity and desorbs before the dicarbonyl species. Data on the desorption and reactivity of adsorbed CO species reveal the inhomogeneity of Rh sites. The modification of the Rh morphology during the interaction of CO with the catalyst is schematically represented. The conversion in the CO methanation reaction decreases simultaneously with the intensity of the band corresponding to the dicarbonyl species. The presence of the dicarbonyl species reflects the sintering process of metallic Rh in this reaction. (C) 1999 Elsevier Science B.V. All rights reserved.