Concerning the isotope effect in the decomposition of ammonia on tungsten surfaces

It has been stated in the literature that NH3 decomposition on W being 1.6 times faster than ND3 precludes N2 desorption being the rate-determining step as has been suggested from kinetic measurements. Having found ND3 is a better electron donor than NH3 we present a simple molecular orbital model of bonding to the surface which suggests the isotope effect can be explained as a β-secondary isotope effect which is compatible with N2 desorption being rate limiting.