The possibility to use ammonia as a hydrogen storage medium has brought increasing attention to the ammonia decomposition reaction. This reaction proceeds first by dehydrogentation of ammonia, followed by recombination of N and H to form N2 and H2, respectively. The binding energy of the nitrogen atom to the surface must be strong enough to allow ammonia decomposition, but sufficiently weak that the nitrogen recombines to desorb from the surface and complete the catalytic cycle. This trade-off leads to a volcano-type relationship between nitrogen binding energy and ammonia decomposition activity, with ruthenium being at the top of the curve. Even though it is recognized as the most active decomposition catalyst among the single metals, it is expensive and limited in supply. Hence, it is important to develop improved catalysts based on less expensive metal or metal alloys.
Monolayer bimetallic catalysts are a specific type of bimetallic catalysts which consist of a layer of a second metal in the top layers of a host metal. The additional metal layer can be on the surface of the host metal, giving rise to the surface configuration, or below the surface layer, forming the subsurface configuration. The properties of such systems depend on the nature and position of the ad and host metals, and are very different from the corresponding parent metals or alloyed systems. Owing to this nonlinear behavior, there is currently no method to rationally design such novel catalysts.
In the present case study, we investigate the adsorption of nitrogen on four different mono- and bimetallic heterogeneous catalysts: Ni(111), Pt(111), Ni-Pt-Pt and Pt-Ni-Pt surfaces by density functional theory (DFT) calculations using the Quantum Espresso plugin in MAPS. Spin-polarized DFT computations were performed using the PBE exchange-correlation functional and PAW pseudopotentials, along with an energy cutoff of 600 eV and a 3 ´ 3 ´ 1 k-point grid for the Brillouin sampling. A Fermi-Dirac smearing was used with a smearing cutoff of 0.2 eV. The Ni(111) and Pt(111) surfaces were represented by 3 ´ 3 ´ 1 supercells cleaved from the corresponding bulk cells, using a 4-metal layer slab and 15 Å vacuum along the z-direction.
Optimized Ni-Pt-Pt and Pt-Ni-Pt monolayer bimetallic catalytic surfaces are shown in Figure 1. The N atom was adsorbed in the hollow sites at the optimized surfaces and the adsorption energy was computed as Eads = E(surface+N) – E(surface) – E(N). The adsorption energy of nitrogen on each of the four model systems and the nitrogen–metal distance are summarized in Table 1. Table 1: Computed N adsorption energy (in kcal mol-1) and N-metal distance, d(N,M), in Å, for the four catalysts. The values from previous computations, and the difference (in %) with this work are also given.
N adsorption energy (kcal mol-1)
|This work||Previous work||This work||Previous work|
|Pt||-102.8||-102.1 (0.7%)||1.97||1.95 (0.7%)|
|Ni||-123.4||-113.8 (8.4%)||1.77||1.77 (0.1%)|
|Ni-Pt-Pt||-136.6||-130.7 (4.5%)||1.77||1.76 (0.1%)|
|Pt-Ni-Pt||-90.5||-87.5 (3.4%)||1.98||1.94 (2.0%)|
The computed adsorption energies and Nitrogen-metal distances are in agreement with the previous work . Ni-Pt-Pt catalyst was identified as the most active catalyst with an adsorption energy very close to the optimal value of ~134 kcal mol-1 of the Ru single metal. This finding agrees with experiments showing that Ni-Pt-Pt bimetallic monolayer surface is more active than the mono-metallic Ru catalyst. Figure 2 shows the most stable structure of N adsorbed on Ni-Pt-Pt surface.
These computational studies of mono- and bimetallic surfaces demonstrate that the nitrogen binding energy can be used as a descriptor to identify catalysts with desirable activity for ammonia decomposition reaction a priori to any experimental efforts. (1) Hansgen, D. A.; Vlachos, D. G.; Chen, J. G. Nat. Chem. 2010, 2 (6), 484–489. https://doi.org/10.1038/nchem.626.