Geometric and electronic aspects of the dissociative chemisorption of hydrogen and nitrogen on metal surfaces

Butler, David Austin (1994) Geometric and electronic aspects of the dissociative chemisorption of hydrogen and nitrogen on metal surfaces. University of Southampton, Doctoral Thesis.

Abstract

The effect of geometric and electronic modifications on the dissociative chemisorption of hydrogen and nitrogen has been studied on single crystal surfaces and through the reactivity of catalysts.

The role of the promoters in iron based ammonia catalysts has been shown to be very sensitive to the presence of surface adsorbates. Alumina is seen to stabilize a surface reconstruction to a more active geometry, producing a hysteresis in the reactivity during heating and cooling experiments. This reconstruction is believed to be induced by adsorbed nitrogen, the coverage of which increases rapidly with the partial pressure of ammonia. Potassium has very little effect on the reaction rate at low nitrogen coverages, but as this increases with the rate of synthesis, significant promotion is observed. This is explained through an increased removal of N_ads as ammonia, freeing sites for further reaction, rather than previous models of directly promoted nitrogen dissociation. The oxidation and reduction of the surface causes a rapid reconstruction, producing a large but transient desorption of ammonia. On the promoted samples, the oxygen blocks all reaction, but on pure iron actually produces a more active surface.

The dynamics of dissociative adsorption of hydrogen on the W(100)-c(2x2)Cu and W(100)-c(2x2)N surfaces has been studied under UHV conditions with a supersonic molecular beam. On both surfaces an indirect channel exists, very similar to that on W(100). This is believed to be due to the dissociation at defects, such as steps, of an essentially tungsten nature.

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