Abstract
Density functional theory is central to the study of catalytic processes, but its accuracy is widely debated, and lack of data complicates accuracy estimates. To address these issues, this work explores a simple eight-step process of iron-catalyzed ammonia synthesis. The modelsâs importance lies in the availability of experimental data and the accessibility of coupled-cluster CCSD(T) calculations, enabling direct assessment of method accuracy for all reaction steps. While many functionals accurately describe the net process N2 + 3H2 â NH3, errors of +100 kJ molâ1 occur in many individual steps for popular functionals such as PBE, RPBE, and B3LYP, which are much worse than commonly assumed. Inclusion of the stoichiometric reaction coefficients reveals major accuracy bottlenecks surprisingly distinct from the NâN dissociation step and dependent on the applied functional. More focus should be directed to these problematic steps in order to improve the accuracy of modeling the catalytic process.
Highlights
Density functional theory is central to the study of catalytic processes, but its accuracy is widely debated, and lack of data complicates accuracy estimates
Many theoretical studies of transition metal catalysis routinely use a single popular functional such as RPBE or B3LYP for all steps as the basis for conclusions, with assumptions of accuracy largely extrapolated from previous onestep benchmarks[23,24,25]
We need to understand (i) the benchmarking of full catalytic processes and how errors arise in all individual catalytic steps, (ii) whether performance is determined by specific steps of the processes, i.e., âaccuracy bottlenecksâ, (iii) to what extent overall accuracy can benefit from cancellation of systematic errors, and (iv) if there are clear performance issues due to the type of density functional theory (DFT) used that should be solved
Summary
Density functional theory is central to the study of catalytic processes, but its accuracy is widely debated, and lack of data complicates accuracy estimates To address these issues, this work explores a simple eight-step process of iron-catalyzed ammonia synthesis. We need to understand (i) the benchmarking of full catalytic processes and how errors arise in all individual catalytic steps, (ii) whether performance is determined by specific steps of the processes, i.e., âaccuracy bottlenecksâ, (iii) to what extent overall accuracy can benefit from cancellation of systematic errors, and (iv) if there are clear performance issues due to the type of DFT used that should be solved Such assessments are difficult because most available data reflect overall rates and adsorption energies of the full systems[26].
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