Introduction
In 1954, about 2,900,000 metric tons of ammonia were produced in the
United States, 70% of which was utilized by the agricultural
sector.1 More than 60 years later, in 2018, U.S.
ammonia production reached an estimated 12,500,000 metric tons, with the
world total coming in at 140,000,000 tons.2 This
essential industrial process currently uses over 1% of the
world’s power.3 Ammonia is used in the synthesis of a
variety of products, most notably, fertilizers. The Haber-Bosch process,
the process by which ammonia is produced, is a large source of carbon
emissions primarily because the synthesis reaction is performed at very
high temperatures and pressures (350-550⁰C and 150-350
atm).4 Researchers are looking for alternative
mechanistic pathways to bypass the energy requirements of Haber-Bosch.
Such processes include biological nitrogen fixation,5ultraviolet promotion of N2,6 and
electrochemical nitrogen reduction.7–11 For the
industrial Haber-Bosch process, iron and ruthenium are commonly
studied12–14 because transition metals readily adsorb
and release oxygen.15 Catalysts studied for the
electrochemical process include ruthenium, iron oxides, and nitride
compounds.7,9,10,16 Osmium and platinum have also been
identified as good catalysts.17,18 Experimentally,
Allagui et al. found that a bimetallic PtIr nanoparticle catalyst
decomposed ammonia at a 33% higher rate than platinum
alone.19 Le Vot et al. found that iridium lowers the
overpotential of ammonia oxidation.20 Boggs and Botte
came to the same conclusion in their study of PtIr anodes deposited on
carbon substrates in alkaline solution.21 Lastly,
Estejab and Botte used small cluster calculations to show
Ir3 is more active than Pt3 in nitrogen
oxidation.18 These studies suggest a basis for
exploring mechanistic behavior on larger clusters of iridium for
nitrogen reduction. By better understanding the thermodynamics of the
reaction, new steps can be taken in the lab to increase ammonia yield.
When using clusters to represent catalysts, the computational scheme is
very important. The popular B3LYP Density Functional Theory (DFT) method
does not describe dispersion between molecules well and thus cannot
appropriately describe N2 adsorption on a catalyst
surface. Hopmann et al. computationally studied bond formation and
ligand exchange reactions mediated by iridium in solution. They found
that the free energies of B3LYP systems with dispersion provide moderate
overall accuracy and significant improvement over B3LYP
alone.22 Therefore, this study tests B3LYP against its
dispersion-corrected form, as well as against a less computationally
expensive dispersion-corrected functional, B97-D3.
This paper has two primary objectives. The first is to calculate
adsorption energies, binding sites, and vibrational spectra for the
intermediates involved in the ammonia synthesis reaction on pure
iridium. This data can be used to shed light on the reaction mechanism
of ammonia synthesis on the (111) and (100) surfaces. The activation
energies, which correlate to the speeds of the possible reactions, can
also be calculated, but they are beyond the scope of this study.
Calculations on pure iridium will serve as a direct comparison to
previous calculations performed on pure platinum
clusters.23 Key differences will be analyzed with
respect to adsorption energetics and bonding sites. These calculations
can also serve as a benchmark for future bimetallic calculations.
Additionally, significant energy savings would result from an
electrochemical process that takes place at ambient temperatures and
pressures. 24,25 Thus, all calculations in this study
are run at standard temperature and pressure (STP). The second objective
is to compare the accuracy of three Density Functional Theory methods –
B3LYP, B3LYP-D3 and B97-D3 – in predicting bonding sites, bonding
lengths, bonding energies and vibrational frequencies on the catalyst
surface. These characteristics are well-modeled by cluster
calculations.18,23,26