(592c) DFT Study for Developing Novel and Specific B5-Type Step Site Scaling Relations and BEP Analysis for Stepped HCP Catalyst Surfaces

Introduction
In the literature recently there have been reports that show that HCP metal catalyst nanoparticles and their activity may be highly sensitive to the exact nature and number of ‘B5’ step and edge sites present in the faceting and facet-intersections of the nanoparticle.1-3 Unfortunately to date, theory and modeling treatments in the literature for ‘stepped’ HCP surfaces largely rely on using ad-hoc deletion of atoms (or atomic rows) from ideal HCP(0001) surfaces to create representative surface models. Our work in this submission highlights the possible failings of such an approach; specifically we examine for differences in adsorption energies at various B5 step edge types, and any possible trends across the same type of B5 sites on various HCP catalyst species that have not yet been thoroughly characterized. Our work in this manuscript uses the low energy (1016) Miller Index surface of Co, Os, and Ru which exposes 2 distinct and strongly adsorbing step edge sites, the B5-B and B-5A step edge which have been reported as relevant in the literature for Cobalt nanoparticle catalysis applications. Results from this study should be used to help further understand atomistic processes on the stepped surfaces of catalytically active HCP elements.

Materials and Methods
The work described and presented was performed using modern, state-of the art plane-wave DFT calculations as implemented in the VASP 5.4x code deployed in the Materials Design MEDEA environment 2.21. Calculations and supercells for surface-mediate adsorption and catalytic phenomena involved dipole corrections parallel to the surface normal, vacuum spacing of at least 65% of the overall calculation cell height, and slab models of at least 6-8 Å or equivalence to 4 crystalline layers of the material(s) studied. Typical electronic convergence of the SCF was performed to at least 10-6 eV, and atomic force relaxations of the uppermost slab model atoms and adsorbed species was performed to less than 0.035 eV/ Å . Specific functional(s) used for the calculations in this study was rPBE.

Results and Discussion
Our results have determined that there are commonalities and trends in binding for various mono-atomic and diatomic adsorbate molecules on the HCP(1016) surface(s) of Co, Os, and Ru. Though there are some exceptions, we have used these results to examine adsorption energies and transition state energies on this and other stepped HCP surface to determine step-edge dependent scaling and BEP relations related to their potential catalytic activity. Our results show it is very important to understand that not all HCP “steps” are equivalent, and that previous literature attempts to model and develop scaling relations using the simple approach of ad-hoc deletion of atoms from a model HCP(0001) surface slab may be insufficient to truly model atomistic processes on the surface of HCP metal catalyst nanoparticles.

Significance
We present results from DFT studies on the effects local geometric and electronic structure on the relative adsorption and catalytic properties of differing B5-type steps (B5-A, B5-B, etc) of actually stepped HCP model surfaces such as the HCP(10-16) surface. Trends on various metals such as Co,Os, and Ru are discussed. Specific scaling and BEP relations for trends in catalytic activity on these different active sites have not been reported to date; we hope our new work helps fill this gap and leads to enhanced understanding of atomistic processes on HCP metal nanoparticle catalysts.

References
1. Petersen, M. A.; van den Berg, J.-A.; Ciobîcă, I. M.; van Helden, P., Revisiting CO Activation on Co Catalysts: Impact of Step and Kink Sites from DFT. ACS Catalysis 2017, 7 (3), 1984-1992.
2. García-García, F. R.; Guerrero-Ruiz, A.; Rodríguez-Ramos, I., Role of B5-Type Sites in Ru Catalysts used for the NH3 Decomposition Reaction. Topics in Catalysis 2009, 52 (6), 758-764.
3. Agrawal, R.; Phatak, P.; Spanu, L., Effect of phase and size on surface sites in cobalt nanoparticles. Catalysis Today 2018, 312, 174-180.