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