Universiteit Leiden

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Non-adiabatic effects may substantially affect rate of reaction relevant to Haber-Bosch catalysis

Using N2 dissociation on Ru(0001) as a representative showcase (for catalysts employed in the Haber-Bosch process), we have shown for the first time that non-adiabatic effects can substantially reduce a molecule’s dissociation probability on a metal surface. These effects are currently completely unaccounted for in microkinetic models commonly used in heterogeneous catalysis.

Which electronic friction theory for describing electron-hole pair excitation?

Electron−hole pair (ehp) excitation is thought to substantially affect the dynamics of molecules on metal surfaces. However, it is not clear by which version of electronic friction theory this is best addressed. A team from the University of Göttingen and from Leiden University tested orbital-dependent friction (ODF) and the local density friction approximation (LDFA) on a benchmark test case. 

The N2 + Ru(0001) test case

We investigate the effect of ehp excitation on the dissociative chemisorption of N2 on and its inelastic scattering from Ru(0001), which is the benchmark system of highly activated dissociation, with these two different models. Dissociative chemisorption of N2 on a metal surface is also considered to be the rate-limiting step in the Haber-Bosch process, whereby NH3 is produced as a raw material for artificial fertilizer. Ru is a metal used in this process. ODF is in better agreement with the best experimental estimates for the reaction probabilities than LDFA, yields results for vibrational excitation in better agreement with experiment, and only slightly overestimates the translational energy loss during scattering. 

A substantial non-adiabatic effect for a benchmark system

The results of the team also show up N2 on Ru(0001) as the first system for which the ODF and LDFA approaches yield substantially different results for easily accessible experimental observables, including reaction probabilities. The ODF approach yields a substantial non-adiabatic effect, halving the computed reaction probabilities. The team's results will likely provoke new experiments and calculations on both the reaction investigated and other reactions to allow more definitive conclusions on the size of electronically non-adiabatic effects on reaction and to answer the question which theory best predicts this. 

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