Catalytic (and Optical) Properties of (Supported) Alloy (Ultra)NanoStructures

First, we will present our results on computational modeling of catalysis by alloy nanostructures. We start from an extensive study of ammonia synthesis via the Haber-Bosch (HB) process over Fe bcc(111) and bcc(211) via a computational protocol combining Quantum-Mechanics in the form of Density-Functional Theory plus dispersion to unveil the mechanistic steps at the atomic level, and kinetic Monte Carlo (kMC) modeling to predict steady-state catalytic reaction rates under realistic conditions, thus allowing us to validate our predictions against experimental kinetic data from literature. We then use the detailed knowledge derived for this system to consider modifications of the catalyst such as alloying the first few surface layers via a hierarchical high-throughput screening (HHTS) approach to catalyst design. The HHTS approach singles out the most promising alloying elements and configurations as a function of catalyst structure and specific alloying site. The approach is validated in several test cases, such as Rh 0.25 ML substitutional top layer, by reconstructing the complete free-energy diagram, conducting a full kinetic analysis, and comparing the results from those estimated on the basis of the HHTS, finding very good agreement [1]. Focusing then on optical properties, we will present a first-principles time-dependent density-functional theory (TDDFT) study of the optical response of (M/M?)6 and (M)3(M?)3 six-atom clusters (M,M?: Cu, Ag, and Au), both in the gas-phase and supported on the MgO(100) surface as a model of a simple oxide substrate. UV-vis spectra are predicted and analysed to rationalize origin and features of optical absorption in these systems. The interaction with the electric field generated by the charge-separated substrate is found to induce a fragmentation and a shift toward lower energies of the absorption peaks, which is in tune with experiment and can be beneficial in applications as photo-enhancers or promoters. The orientation of the transition dipole moment with respect to the support (parallel or perpendicular) is analysed in view of translating these results to optically active semi-conducting supports to tune interaction with substrate excitations [2]. REFERENCES 1. J. Qian, Q. An, A. Fortunelli, R. J. Nielsen, W. A. Goddard III, J. Am. Chem. Soc. 140, 6288-6297 (2018); Q. An, Y. Shen, A. Fortunelli, W. A. Goddard J. Am. Chem. Soc. 140, 17702-17710 (2018). 2. J. C. Luque-Ceballos, L. Sementa, E. Apra?, A. Fortunelli, A. Posada-Amarillas, J. Phys. Chem. C 122, 23143-23152 (2018).