Marko Melander

CO dissociation on iron nanoparticles: Size and geometry effects

The reactivity of 0.5-1.4 nm iron nanoparticles and corresponding bulk surfaces has been systematically studied using density functional theory. The study includes both ideally symmetric and more realistic rugged nanoparticles. The activation energies for CO dissociation vary between 1.1 and 2.1 eV. An increasing particle size and roughness result in lower activation energies. For a single particle, variations as large as 0.9 eV occur indicating the importance of local particle morphology. Depending on the nanoparticle size, geometry, and CO dissociation pathway the reaction rates span five orders of magnitude at conditions relevant for gas-phase chemical vapor deposition synthesis of carbon nanotubes. Studies on the smallest particles and bulk surfaces show that these systems cannot be used as reliable models for catalysis on larger iron nanoparticles. It has also been demonstrated that predictive d-band and linear-energy relationships cannot be used to explain the reactivity of iron for CO dissociation as reaction mechanisms vary from one particle to another. The changes in reaction mechanisms can be rationalized by the varying Fe-Fe bond lengths in different particles leading to changes in back-bonding between the iron surface and CO. CO dissociation on nano-sized iron seems to be more complex than that seen on more conventional non-magnetic noble metal particles.

Removing External Degrees of Freedom from Transition-State Search Methods using Quaternions.

In finite systems, such as nanoparticles and gas-phase molecules, calculations of minimum energy paths (MEPs) connecting initial and final states of transitions as well as searches for saddle points are complicated by the presence of external degrees of freedom, such as overall translation and rotation. A method based on quaternion algebra for removing the external degrees of freedom is described here and applied in calculations using two commonly used methods: the nudged elastic band (NEB) method for MEPs and the DIMER method for finding the minimum mode in minimum mode following searches of first-order saddle points. With the quaternion approach, fewer images in the NEB are needed to represent MEPs accurately. In both NEB and DIMER calculations of finite systems, the number of iterations required to reach convergence is significantly reduced. The algorithms have been implemented in the Atomic Simulation Environment (ASE) open source software.

DFT simulations and microkinetic modelling of 1-pentyne hydrogenation on Cu20 model catalysts

Adsorption and dissociation of H2 and hydrogenation of 1-pentyne on neutral and anionic Cu20 clusters have been investigated using the density functional theory and microkinetic modelling. Molecular adsorption of H2 is found to occur strictly at atop sites. The H2 dimer is activated upon adsorption, and the dissociation occurs with moderate energy barriers. The dissociated H atoms reside preferentially on 3-fold face and 2-fold edge sites. Based on these results, the reaction paths leading to the partial and total hydrogenation of 1-pentyne have been studied step-by-step. The results suggest that copper clusters can display selective activity on the hydrogenation of alkyne and alkene molecules. The hydrogenated products are more stable than the corresponding initial reactants following an energetic staircase with the number of added H atoms. Stable semi-hydrogenated intermediates are formed before the partial (1-pentene) and total (pentane) hydrogenation stages of 1-pentyne. The microkinetic model analysis shows that C5H10 is the dominant product. Increasing the reactants (C5H8/H2) ratio enhances the formation of products (C5H10 and C5H12).