The Role of Adsorbate–Adsorbate Interactions in the Rate Controlling Step and the Most Abundant Reaction Intermediate of NH3 Decomposition on Ru
A. B. Mhadeshwar, J. R. Kitchin, M. A. Barteau and D. G. Vlachos, “The Role of Adsorbate–Adsorbate Interactions in the Rate Controlling Step and the Most Abundant Reaction Intermediate of NH3 Decomposition on Ru”, Catalysis Letters 96(1-2), 13-22 (2004). https://doi.org/10.1023/B:CATL.0000029523.22277.e1
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Question
Ammonia decomposition on Ru had become interesting as a route to hydrogen for fuel cells, but the literature disagreed on which step controls the rate (N2 desorption or N-H bond cleavage), on the most abundant reaction intermediate (MARI: N*, H*, NH* or empty sites, depending on the study), and on how high the N* coverage can go. Several DFT studies reported strong repulsion between adsorbed N atoms on Ru, yet earlier kinetic models ignored it and some predicted N* coverages near one monolayer. The paper asks whether adding DFT-estimated N-N lateral interactions to a microkinetic model changes the rate determining step (RDS) and the MARI (Mhadeshwar et al. 2004).
Approach
Periodic slab DFT (DACAPO, PW91, Vanderbilt ultrasoft pseudopotentials, 350 eV cutoff, 18 Chadi-Cohen k-points) was run on 2x2 cells of a three-layer, fully relaxed Ru(0001) slab with N in the hcp hollow at coverages of 0.25, 0.5, 0.75 and 1.0 ML, and with H co-adsorbed next to 0.25 ML N. Heats of chemisorption of H and NHx were also computed. These results fed a twelve-step (six reversible pairs) mean-field microkinetic model whose activation energies come from the unity bond index-quadratic exponential potential (UBI-QEP) method, with experimental zero-coverage heats of chemisorption for N and H, temperature-dependent heats from statistical mechanics, and thermodynamic consistency at the free energy level. Only the pre-exponential factors were fit, to atmospheric fixed bed NH3 decomposition data. Four parameter sets were compared against these data and against UHV N2 TPD data: set A (with N-N interactions), set A’ (set A with interactions switched off), set B (no interactions, pre-exponentials refit), and a literature mechanism for Cs-promoted Ru (set H). The RDS was identified by a sensitivity analysis that perturbs forward and reverse pre-exponentials together, so equilibrium constants stay fixed.
Key findings
- DFT gave a linear N heat of chemisorption, QN = 128.2 - 33.3 θN kcal/mol, so the N-N repulsion is about 33 kcal/mol/ML, close to earlier values of 36.3 and 33.9 kcal/mol/ML. The Ru-N bond length was 1.99 Å. The cross N-H interaction at 0.25 ML N was only 6.2 kcal/mol/ML of H.
- The model used QN = 137.0 - 35.0 θN kcal/mol. With this, N2 dissociative adsorption is exothermic at zero coverage (about -48 kcal/mol at 300 K) but becomes endothermic above θN = 0.69, which rationalizes the submonolayer saturation coverages seen experimentally.
- In N2 TPD, only set A reproduced the downward shift of the peak temperature with increasing N coverage: a net shift of 156 °C against 151 °C measured, compared with 27 to 38 °C for sets A’, B and H.
- For fixed bed decomposition, sets A, B and H all fit the conversion data fairly well, but set B needed very different pre-exponentials and set A’ fit poorly. The authors stress that the parameter set is not unique, and that TPD data are what discriminate between models.
- Without interactions N* is the MARI at lower temperatures and N2 desorption is the RDS, matching the prevailing view. With interactions H* is the MARI (followed by N*), N* never exceeds about 0.7 ML, and the RDS becomes NH2* decomposition to NH* + H*, with N2 desorption playing only a minor role. NH2* coverage was only 1e-5 to 1e-8.
- Comparing UBI-QEP energetics with literature DFT showed large terrace versus step differences (for example, N2 dissociation barriers of 44.4 kcal/mol on terraces and 9.2 kcal/mol on steps, against 7.9 kcal/mol from experiment), so the UBI-QEP energetics look more like defect sites. The authors conclude that the RDS and MARI assignments may depend on how the energetics are calculated.
Significance
This was, by the authors’ account, the first microkinetic model of NH3 decomposition on Ru to include N-N adsorbate-adsorbate interactions. It shows that lateral interactions can change the qualitative mechanistic picture (RDS and MARI) even when reactor conversion alone can be fit without them, and that coverage-dependent TPD data provide a sharper test. It is an early example of combining DFT lateral interactions with semi-empirical energetics in a hierarchical, thermodynamically consistent kinetic model. Note on the source: the extracted text interleaves the two journal columns and garbles some symbols (θ appears as “h”, minus signs as “)”), but all sections, tables and figure captions are readable.