MODIFICATIONS OF SURFACE-PROPERTIES OF NICKEL SILICA CATALYSTS BY NITROGEN-CONTAINING COMPOUNDS .1. AMMONIA

The interactions of ammonia with a reduced Ni/SiO2 sample have been studied as a function of temperature, using magnetic measurements and infrared spectroscopy. The Ni/SiO2 sample contained 20 wt.-% nickel and had a narrow metal particle size distribution, centered around 4 nm. Below room temperature and at low coverage, ammonia is mainly adsorbed in a molecular form (magnetic bond number almost-equal-to 1) onto the nickel surface atoms. Above 373 K and at low coverage, ammonia is fully dissociated (magnetic bond number almost-equal-to 6), with the formation of surface nickel nitride. In the intermediate temperature range, other ammonia fragments, like -NH2 and = NH, are also present at the surface of the metal, the extent of ammonia dissociation increasing with an increase of the adsorption temperature. Whatever the type of detected ammonia fragments, infrared experiments show that the nickel surface atoms are electron enriched. The properties of pure metallic nickel are fully recovered after evacuation at almost-equal-to 673 K of the ammonia treated sample. The effect of ammonia present in the feed, during gas phase hydrogenation of 1,3-butadiene, between 303 and 373 K, was also investigated. A progressive increase of the ammonia partial pressure (up to 500 ppm) improved partial hydrogenation and 1-butene formation. Such selectivity changes agree with an electron enrichment of the nickel particles. In the present reaction conditions, the effect of ammonia addition was fully reversible, i.e., the changes in selectivity were suppressed when ammonia was removed from the feed.