ROLE OF ALKALI PROMOTERS ON MN3O4 IN THE SELECTIVE REDUCTION OF NITROBENZENE

The influence of Li+, Na+ and K+ ions on the activity and selectivity of Mn3O4 in the selective reduction of nitrobenzene has been studied. The alkali ions show both a promoting (low concentrations) and a block ing (higher concentrations) effect. Reducibility measurements of the v arious catalysts (thermogravimetry, temperature-programmed reduction), adsorption heat experiments (heat-flow microcalorimetry) and determin ation of the composition of the surface layers (X-ray photoelectron sp ectroscopy) exclude an influence of the promoter on the reducibility o f the catalyst, on the heat of adsorption of nitrobenzene and on the o xidation state of manganese. It can be seen that most of the alkali io ns are present in the surface layers of the catalyst. The most likely mechanism is that by which the promoter influences a certain adsorptio n mode of nitrobenzene existing at the reaction temperature on the sur face of Mn3O4.