THE EFFECT OF IRON ON THE ACTIVITY AND THE SELECTIVITY OF RH/ZRO2 CATALYSTS IN THE CO HYDROGENATION

Rh/ZrO2 catalysts (1% w/w) with different amounts of iron as promoter (Fe/Rh atomic ratio in the range 0-2) were prepared from three precursors: carbonyl clusters, chloride salts, nitrate salts. The catalysts were characterized by H-2 and O-2 chemisorption, titration of chemisorbed oxygen with hydrogen, TPR, X-ray analysis and IR spectroscopy of chemisorbed CO in the range of 300-773 K. The activity has been tested in the CO hydrogenation reaction (H-2/CO = 3/1) at 493 K under atmospheric pressure. The physico-chemical characterization of the catalysts evidenced that iron is present either as oxidized Fe-delta+ or in the reduced Fe-0 form: IR analysis revealed that some reduced iron either decorates or is near the border of Rh particles. The metal precursors do not seem to influence significantly the behaviour of the catalysts. Two opposing effects of iron were evidenced: (i) a decrease of Rh sites available for carbon monoxide and hydrogen chemisorption; (ii) an increase of doubly bonded CO (Rh-CO-Fe) favoured by Fe-x+ sites, contiguous to Rh sites, that are reduced to Fe-0 sites in reaction conditions. Depending on the surface environment, the bridged CO may dissociate to Rh-C and Fe-O or insert a hydrogen or an alkyl group with formation of an acyl species.