Increased thermal stability of activated N₂adsorbed on K-promoted Ni{110}

Abstract

Industrial synthesis of ammonia takes place at high temperatures and pressures via the dissociative adsorption of molecular nitrogen on a transition metal catalyst. In contrast, biological ammonia synthesis occurs under ambient conditions via the hydrogenation of intact molecular nitrogen at the active site of an enzyme. We hypothesise that the latter process may be mimicked within an inorganic system if the intact nitrogen molecule can be polarised, rendering it particularly susceptible to attack by hydrogen. Furthermore, by analogy with the surface chemistry of carbon monoxide at alkali-modified nickel and cobalt surfaces, we consider whether such a polarisation may be achieved by coadsorption with potassium on the same or similar transition metals. Here, we report on reflection absorption infrared spectroscopy results, interpreted with the aid of first-principles density functional calculations, which reveal both similarities and differences between the behaviour of carbon monoxide and nitrogen. Importantly, our calculations suggest that the surface-induced dipole of molecular nitrogen can indeed be enhanced by the coadsorbed alkali metal.

Proton transfer dynamics modified by CH-stretching excitation

“…21, 43 Liu and coworkers refer to these effects as being analogous to an optical lens for approaching reactants steering them towards or away from the preferred transition state geometry. 19 It is surprising that we find a similar effect despite the very different strength of the long range interaction due to the ion-dipole interaction in the title reaction.…”

“…The effect that at lower collision energies translation is more efficient in promoting reactivity has been mentioned previously. 19 It even occurs in 'late' barrier reactions. 16,17 Guo and coworkers propose that this occurs, because a minimum of momentum transfer is required for the reaction to proceed, which leads to a preference for translation at low collision energies.…”

“…E-mail: roland.wester@uibk.ac.at; Tel: +43 512 507 52620 by SVP and QCT calculations. 28 For the proton transfer reaction the situation is different and more similar to neutral halide reactions with CHD 3 , 19 as we excite the bond that is broken during the reaction. This leads to interesting questions: Is vibration more efficient in promoting the reaction compared to collision energy and if so, does the enhancement depend on the collision energy?…”

“…The effect was corroborated using quasi-classical trajectory (QCT) calculations on an ab initio potential energy surface (PES). [19][20][21][22][23] In order to extend the Polanyi rules to poly-atomic systems and predict the influence of vibrations, the sudden vector projection model (SVP) has been developed by Guo and coworkers. 24,25 It projects the different internal modes and translational coordinates onto the intrinsic reaction coordinate at the transition state and calculates the individual overlap.…”