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 hy drogenation, magnetic measurements and surface area determination. It is observed, as a first step, that alkali addition results in an incre ase in the selectivities Of CO2, C2+ hydrocarbons and alcohols. This b ehaviour is similar to that reported elsewhere when the reaction is pe rformed at 5 MPa. The stability of these systems varies with the seque nce K > Na > Li > unpromoted. It is shown that catalyst reduction at 8 73 K results in sintering of the support when alkali promoters are pre sent, which does not alter the nickel particle size so long as the pro moter concentration is not too high. The surface area of the catalysts is unchanged after reaction. The deactivation of unpromoted samples r esults from nickel sintering, in contrast to that of alkali-promoted c atalysts: this behaviour is discussed in terms of the stabilization of nickel subcarbonyl species by alkali promoters. Deactivation of promo ted catalysts parallels the deposition of large amounts of carbon atom s chemically interacting with the nickel phase. In reaction conditions , alkali addition gives rise to a deep carbidization of the nickel pha se (up to 85%) which probably accounts for the observed change of sele ctivity. Two types of carbon chemically interacting with nickel are de tected when alkali promoters are added. Both of them can be hydrogenat ed at low temperature and can be related to surface and bulk carbides.