Thomas H. Upton

Electronic properties of metal clusters (Ni13 to Ni87) and implications for chemisorption

Abstract First principles calculations of the electronic properties of Ni clusters (up to Ni 87 ) are reported. It is found that the ionization potential (IP) converges to bulk values (work function) by Ni 43 , whereas the electron affinity (EA) is off by 2.5 eV, even for Ni 87 . The conduction band of ∼16 eV appears converged by Ni 87 . It is found that the electron density for surface atoms is significantly lower than the bulk value. The significance of these results for chemisorption on small metallic clusters and for modelling of chemisorption on bulk surfaces is discussed.

Methanation of CO over Ni catalyst: A theoretical study*

Theoretical methods (generalized valence‐bond calculations) were used to examine the bond energies and geometries of numerous species chemisorbed onto Ni clusters representing Ni surface. These results were used to obtain thermochemical information and to examine various mechanisms for the methanation of CO over Ni: CO+3H^(→)_(2(Ni)) CH_4+H_2O. It is found that chemisorbed formyl radicals (Ni–CHO) lead to a favorably appearing chain reaction that is consistent with current experimental results. In addition, we find a chemisorbed C_2 species that may be the catalytically active C_(ad) formed from dissociation of CO.

Chemisorption of H, Cl, Na, O, and S atoms on Ni(100) surfaces: A theoretical study using Ni20 clusters

Abstract In the last few years considerable advances have been made in the characterization of adsorbed species, largely as a result of dramatic improvements in experimental probes (for example HRELS,1 XPS,2 UPS,3 LEED,4 and TPDS5). While this has increased our knowledge of specific systems, it has not as yet led to the formation of a conceptual basis which might be used to make semiquantitative predictions about the properties of new systems. Predictions of this type are critical in assessing the relative importance of postulated reactive intermediates, species too short–lived for observation by conventional experimental methods. Without such data it will be difficult to draw unambiguous conclusions about mechanisms in heterogeneous catalysis.

Theoretical studies of nickel clusters and chemisorption of hydrogen

First principles calculations of high‐ and low‐symmetry clusters (up to Ni87) are reported. Macroscopic properties [ionization potential (IP), electron affinity (EA), and bandwidth] are not sensitive to cluster symmetry and, except for EA, are converged to bulk values by Ni87. Even for Ni87 the EA is over 2.5 eV smaller than the IP; the origin of this effect is discussed. The chemisorption of hydrogen is considered on the low‐symmetry clusters, where it is found that threefold and fourfold sites are most favored with bond energies of ∠3 eV for both types of sites. Geometries are not cluster sensitive with R (NiH) values of 1.57, 1.62, and 1.78 A for twofold, threefold, and fourfold sites, respectively. Vibrational frequencies for these sites are 1420, 1212, and 592 cm−1, respectively.

Theoretical studies of the dissociative adsorption of H2 on Ni(001) using ab initio parameterized LEPS calculations

Abstract Semi-empirical London-Eyring-Polanyi-Sato (LEPS) calculations are reported, comparing the energetics of H2 dissociation at linear, twofold, and fourfold sites on Ni(001). Parameters for the LEPS method were obtained from the results of first principles calculations of both atomic and molecular hydrogen adsorption on model Ni(001) surfaces. Several pathways are found to require no activation energy for dissociation, of which the most favorable is dissociation across a twofold site with subsequent atomic adsorption at fourfold sites. The experimentally observed β1 (high coverage) state is found to be consistent with a geometry in which two hydrogen atoms are adsorbed at a single fourfold site.