MORPHOLOGY CHANGES AND DEACTIVATION OF ALKALI-PROMOTED NI/SIO2 CATALYSTS DURING CARBON-MONOXIDE HYDROGENATION

Deep morphological transformations undergone by unpromoted and alkali-promoted Ni/SiO2, occurring during the first hours of CO hydrogenation at atmospheric pressure, are examined using temperature-programmed hydrogenation, magnetic measurements and surface area determination. It is observed, as a first step, that alkali addition results in an increase in the selectivities Of CO2, C2+ hydrocarbons and alcohols. This behaviour is similar to that reported elsewhere when the reaction is performed at 5 MPa. The stability of these systems varies with the sequence K > Na > Li > unpromoted. It is shown that catalyst reduction at 873 K results in sintering of the support when alkali promoters are present, which does not alter the nickel particle size so long as the promoter concentration is not too high. The surface area of the catalysts is unchanged after reaction. The deactivation of unpromoted samples results from nickel sintering, in contrast to that of alkali-promoted catalysts: this behaviour is discussed in terms of the stabilization of nickel subcarbonyl species by alkali promoters. Deactivation of promoted catalysts parallels the deposition of large amounts of carbon atoms chemically interacting with the nickel phase. In reaction conditions, alkali addition gives rise to a deep carbidization of the nickel phase (up to 85%) which probably accounts for the observed change of selectivity. Two types of carbon chemically interacting with nickel are detected when alkali promoters are added. Both of them can be hydrogenated at low temperature and can be related to surface and bulk carbides.