Potential energy surfaces for Rh-CO from DFT calculations

We present potential energy surfaces for Rh-CO obtained from d. functional theory for two electronic states of Rh-CO. We have performed local spin-d. calcns. including relativistic as well as gradient corrections. The construction of a reasonably accurate atom-atom potential for Rh-CO is not possible. We were much more successful in constructing the potential energy surfaces by representing the potential as a spherical expansion. The expansion coeffs., which are functions of the distance between the rhodium atom and the carbon monoxide center of mass, can be represented by Lennard-Jones, Buckingham, or Morse functions, with an error of the fit within 10 kJ/mol. The potential energy surfaces, using Morse functions, predict that the electronic ground state of Rh-CO is 2S+ or 2D. This is a linear structure with an equil. distance of rhodium to the CO center of mass of 0.253 nm. The bonding energy is -184 kJ/mol. Morse functions predict that the first excited state is 3A'. This is a bent structure (?Rh-CO - 14 Deg) with an equil. distance of rhodium to the carbon monoxide center of mass of 0.298 nm. The bonding energy of this state is -60 kJ/mol. Both these predictions are in good agreement with the actual d. functional calcns. We found 0.250 nm with -205 kJ/mol for 2S+ and 0.253 nm with -199 kJ/mol for 2D. For 4A', we found 0.271 nm, ?Rh-CO = 30 Deg, with -63 kJ/mol. The larger deviation for 4A' than for 2S+ or 2D is a consequence of the fact that the min. for 4A' is a very shallow well. [on SciFinder (R)

Chemisorption theory of ammonia on copper

We present local-density-approximation calculations of ammonia adsorption on copper clusters of different sizes (6 to 18 atoms) modelling the (100) and (111) surface. Including for some of the copper atoms only one instead of eleven electrons explicitly in the calculation, did not always work satisfactorily. Comparison of adsorption energies for clusters of related geometry indicates a preference for onefold adsorption. This is due to the Pauli repulsion of the lone-pair orbital of ammonia with the copper 3d electrons. which is minimal for onefold adsorption. as well as an interaction with 4s electrons, which is most attractive in the onefold geometr

In Situ Observations during Chemical Vapor Deposition of Hexagonal Boron Nitride on Polycrystalline Copper.

Using a combination of complementary in situ X-ray photoelectron spectroscopy and X-ray diffraction, we study the fundamental mechanisms underlying the chemical vapor deposition (CVD) of hexagonal boron nitride (h-BN) on polycrystalline Cu. The nucleation and growth of h-BN layers is found to occur isothermally, i.e., at constant elevated temperature, on the Cu surface during exposure to borazine. A Cu lattice expansion during borazine exposure and B precipitation from Cu upon cooling highlight that B is incorporated into the Cu bulk, i.e., that growth is not just surface-mediated. On this basis we suggest that B is taken up in the Cu catalyst while N is not (by relative amounts), indicating element-specific feeding mechanisms including the bulk of the catalyst. We further show that oxygen intercalation readily occurs under as-grown h-BN during ambient air exposure, as is common in further processing, and that this negatively affects the stability of h-BN on the catalyst. For extended air exposure Cu oxidation is observed, and upon re-heating in vacuum an oxygen-mediated disintegration of the h-BN film via volatile boron oxides occurs. Importantly, this disintegration is catalyst mediated, i.e., occurs at the catalyst/h-BN interface and depends on the level of oxygen fed to this interface. In turn, however, deliberate feeding of oxygen during h-BN deposition can positively affect control over film morphology. We discuss the implications of these observations in the context of corrosion protection and relate them to challenges in process integration and heterostructure CVD.P.R.K. acknowledges funding from the Cambridge Commonwealth Trust and the Lindemann Trust Fellowship. R.S.W. acknowledges a research fellowship from St. John’s College, Cambridge. S.H. acknowledges funding from ERC grant InsituNANO (no. 279342), EPSRC under grant GRAPHTED (project reference EP/K016636/1), Grant EP/H047565/1 and EU FP7 Work Programme under grant GRAFOL (project reference 285275). The European Synchrotron Radiation Facility (ESRF) is acknowledged for provision of synchrotron radiation and assistance in using beamline BM20/ROBL. We acknowledge Helmholtz-Zentrum-Berlin Electron storage ring BESSY II for synchrotron radiation at the ISISS beamline and continuous support of our experiments.This is the final version. It was first published by ACS at http://pubs.acs.org/doi/abs/10.1021/cm502603

Density-functional calculations on cation-induced changes in the adsorption of sulfur on iridium tetraatomic clusters

The results are given of d. functional calcns., including relativistic effects, for S adsorption on a tetrahedral Ir4 cluster. A Vosko-Wilk-Nussair exchange-correlation potential was used with Stoll and Becke nonlocal correction. The S atom is found to prefer a twofold coordination. Introducing a Mg2+ ion at the opposite side of the Ir4 cluster lowers the adsorption energy for the onefold geometry, but increases it for the threefold geometry. The twofold geometry is influenced only slightly. The results are analyzed in terms of the steric repulsion and orbital interactions, as a function of the electronic configuration and the distance between S and the cluste

Atomic and molecular oxygen as chemical precursors in the oxidation of ammonia by copper

The role of at. and mol. oxygen precursors in the overall catalytic cycle for ammonia dissocn. is analyzed using first-principle d. functional calcns. Adsorption energies for ammonia, mol. oxygen, NHx, NO, and various intermediates and adatoms were computed from geometry optimized calcns. on the model Cu(8,3) cluster of the Cu(111) surface. Reported values systematically underpredict exptl. adsorption energies by 30 kJ/mol due to the finite cluster size. Attractive and repulsive lateral interactions were important in accessing accurate adsorption energies. At. oxygen enhances N-H bond activation; however, it also acts to poison active surface sites and inhibit ammonia dissocn. kinetics. Transient mol. oxygen adsorbs weakly in both parallel (-17 kJ/mol) and perpendicular orientations (-10 kJ/mol) to the surface. Parallel adsorption appears to be a precursor for oxygen dissocn., whereas perpendicular adsorption is the precursor for ammonia dissocn. The mechanism in which hydrogen atoms are abstracted sequentially to form OOH* intermediate [E* (apparent) = 0 kJ/mol] is favored over that in which two hydrogens are simultaneously transferred to form water directly [E*(apparent) = +67 kJ/mol]. The nonactivated transient mol. path in which hydrogen is abstracted sequentially is the most favored of all of the four paths studied. In light of the exptl. O2 dissocn. energy over Cu(111),transient O2 is more likely than hot at. oxygen as the dominant chem. precursor for ammonia dissocn. Subsequent dissocn. of the NHx fragments lead to N*. While enthalpic considerations favor recombinative desorption ofN2, at reaction conditions the MARI is at. oxygen thus making the recombinative desorption of NO more likely reaction path. [on SciFinder (R)

The adsorption site of ammonia at copper surfaces

The adsorption of ammonia at copper surfaces was studied using Hartree-Fock-Slater LCAO calculations to investigate ammonia interaction with copper clusters. Important factors influencing ammonia adsorption were established from trends in its behaviour at clusters of different size. Adsorption at different coordination sites was examined in light of these results and it was found that a strong electrostatic contribution directs the ammonia towards lower coordination site