Ammonia adsorption and dehydrogenation on Fe nanoparticles

OTERO, GUADALUPE SOL; MIOTTO, RONEY; BRANDA, MARÍA MARTA; BELELLI, PATRICIA GABRIELA

Santiago

Congreso; 10th Congress of the World Association of Theoretical and Computational Chemists; 2014

Pontificia Univesidad Católica de Chile

Ammonia decomposition has become an important process in recent years, due to the implementation of hydrogen based proton exchange membrane fuel cell (PEMFCs)[[1]]. The iron (Fe) surfaces are the catalysts most used in ammonia dehydrogenation because of its low cost, although its reactivity is lower than Ni or Ru surfaces [[2]]. The activity could be enhanced by the replacement of metal surfaces by nanoparticles. The sites exposed ?the ones on the corners and edges, have low coordination; and this makes nanoparticles more reactive than surfaces.In the present work, ammonia adsorption process on Fe nanoparticles with different sizes and shapes ?between 16 and 190 atoms, has been studied. The geometries, adsorption sites and adsorption energies of NH3 on nanoparticles have been compared to extended surfaces Fe(111) and Fe(111) with a Fe adatom. Besides, the transition state for the first ammonia dehydrogenation (NH2 and H products) on nanoparticles with the most stable adsorption energies have been studied with the Nudged Elastic Band method (NEB). All the calculations have been performed with the periodic Density Functional Theory (DFT) using the Vienna Ab-Initio Simulation Package (VASP). For all nanoparticles evaluated, the most stable adsorption site of ammonia is on-top (see Figure 1), in agreement with our results on Fe(111) and with previous published results on Fe(110), Co(111) and Ni(111)surfaces[[1]]; while bridge and hollow are not favorable sites. The NH3 adsorption energy on Fe(111) is -0.38eV, but with adatom this energy increases up to -0.51eV. These results indicate that low coordinated Fe adatom is more reactive. The adsorption energies on the nanoparticles are more favorable than on Fe(111) and on Fe(111) adatom. Moreover these values are even bigger when sizes of the nanoparticles increase.  Acknowledgments: We acknowledgethe financial support of Sol G. Otero to work with Prof. Dr. R. Miotto under the project called ?Programa de Associação para Fortalecimento da Pós-Graduação, Sector Educational do Mercosul, CAPES, PFPG 011/2011?. [[1]] X. Duan,J.Ji, J.Molec.Catal. A: Chem.357 (2012) 81.