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Optical spectra of Cu, Ag, and Au monomers and dimers at regular sites and oxygen vacancies of the MgO(001) surface were studied by time-dependent density functional study. The embedded cluster model was used which provided an accurate description of substrate relaxation and long-range electrostatic interaction.

Single d-metal atoms on oxygen defects F(sub S) and F(sub S)(super +) of the MgO(001) surface are studied using density functional method combined with cluster models, embedded in an elastic polarizable environment. The implication for an experimental detection of metal atoms on oxide supports based on computed core-level energies are discussed.

A novel scheme is presented to calculate electronic g-tensor values of doublet-state systems within a density functional method. The implementation and results of first application of the scheme are discussed. Conceivable improvements of the method performance are proposed and discussed.

The reaction path of C-O bond scission in various intermediates of methanol decomposition at the surface of Pd nanoparticles is studied by using density functional (DF) calculations. The experimental data on catalyst poisoning is rationalized from the computational result that the decomposition products C[H.sub.3] and C[H.sub.2] are adsorbed at edge sites of nanoparticles.

Experimental findings imply that edge sites (and other defects) on Pd nanocrystallites exposing mainly (111) facets in supported model catalysts are crucial for catalyst modification via deposition of CHx (x = 0-3) byproducts of methanol decomposition. To explore this problem computationally, we applied our recently developed approach to model realistically metal catalyst particles as moderately large three- dimensional crystallites. We present here the first results of this advanced approach where we comprehensively quantify the reactivity of a metal catalyst in an important chemical process. In particular, to unravel the mechanism of how CHx species are formed, we carried out density functional calculations of C-O bond scission in methanol and various dehydrogenated intermediates (CH3O, CH2OH, CH2O, CHO, CO), deposited on the cuboctahedron model particle Pd79. We calculated the lowest activation barriers, ~130 kJ mol-1, of C-O bond breaking and the most favorable thermodynamics for the adsorbed species CH3O and CH2OH which feature a C-O single bond. In contrast, dissociation of adsorbed CO was characterized as negligibly slow. From the computational result that the decomposition products CH3 and CH2 preferentially adsorb at edge sites of nanoparticles, we rationalize experimental data on catalyst poisoning. [ABSTRACT FROM AUTHOR]

Single d-metal atoms on oxygen defects Fs and Fs+ of the MgO(001) surface were studied theoretically. We employed an accurate density functional method combined with cluster models, embedded in an elastic polarizable environment, and we applied two gradient-corrected exchange-correlation functionals. In this way, we quantified how 17 metal atoms from groups 6-11 of the periodic table (Cu, Ag, Au; Ni, Pd, Pt; Co, Rh, Ir; Fe, Ru, Os; Mn, Re; and Cr, Mo, W) interact with terrace sites of MgO. We found bonding with Fs and Fs+ defects to be in general stronger than that with O2- sites, except for Mn-, Re-, and Fe/Fs complexes. In M/Fs systems, electron density is accumulated on the metal center in a notable fashion. The binding energy on both kinds of O defects increases from 3d- to 4d- to 5d-atoms of a given group, at variance with the binding energy trend established earlier for the M/O2- complexes, 4d < 3d < 5d. Regarding the evolution of the binding energy along a period, group 7 atoms are slightly destabilized compared to their group 6 congeners in both the Fs and Fs+ complexes; for later transition elements, the binding energy increases gradually up to group 10 and finally decreases again in group 11, most strongly on the Fs site. This trend is governed by the negative charge on the adsorbed atoms. We discuss implications for an experimental detection of metal atoms on oxide supports based on computed core-level energies. [ABSTRACT FROM AUTHOR]

Using various two-step strategies, we examined how to accurately locate transition states (TS) of reactions using the example of eight reactions at metal surfaces with 14-33 moving atoms. These procedures combined four path-finding methods for locating approximate TS structures (nudged elastic band, standard string, climbing image string, and searching string, using a conjugate gradient or a modified steepest-descent method for optimization and two types of coordinate systems) with subsequent local refinement by two dimer methods. The dimer-Lanczos variant designed for this study required on average 20% fewer gradient calls than the standard dimer method. During the path finding phase, using mixed instead of Cartesian coordinates reduced the numbers of gradient calls on average by an additional 21%, while using a modified steepest-descent method improved that key efficiency criterion on average by 13%. For problematic cases we suggest strategies especially adapted to the problem at hand.

Transition state discovery via application of string methods has been researched on two fronts. The first front involves development of a new string method, named the Searching String method, while the second one aims at estimating transition states from a discretized reaction path. The Searching String method has been benchmarked against a number of previously existing string methods and the Nudged Elastic Band method. The developed methods have led to a reduction in the number of gradient calls required to optimize a transition state, as compared to existing methods. The Searching String method reported here places new beads on a reaction pathway at the midpoint between existing beads, such that the resolution of the path discretization in the region containing the transition state grows exponentially with the number of beads. This approach leads to favorable convergence behavior and generates more accurate estimates of transition states from which convergence to the final transition states occurs more readily. Several techniques for generating improved estimates of transition states from a converged string or nudged elastic band have been developed and benchmarked on 13 chemical test cases. Optimization approaches for string methods, and pitfalls therein, are discussed.

Single d-metal atoms on oxygen defects F(s) and F(s+) of the MgO(001) surface were studied theoretically. We employed an accurate density functional method combined with cluster models, embedded in an elastic polarizable environment, and we applied two gradient-corrected exchange-correlation functionals. In this way, we quantified how 17 metal atoms from groups 6-11 of the periodic table (Cu, Ag, Au; Ni, Pd, Pt; Co, Rh, Ir; Fe, Ru, Os; Mn, Re; and Cr, Mo, W) interact with terrace sites of MgO. We found bonding with F(s) and F(s+) defects to be in general stronger than that with O2- sites, except for Mn-, Re-, and Fe/F(s) complexes. In M/F(s) systems, electron density is accumulated on the metal center in a notable fashion. The binding energy on both kinds of O defects increases from 3d- to 4d- to 5d-atoms of a given group, at variance with the binding energy trend established earlier for the M/O2- complexes, 4d < 3d < 5d. Regarding the evolution of the binding energy along a period, group 7 atoms are slightly destabilized compared to their group 6 congeners in both the F(s) and F(s+) complexes; for later transition elements, the binding energy increases gradually up to group 10 and finally decreases again in group 11, most strongly on the F(s) site. This trend is governed by the negative charge on the adsorbed atoms. We discuss implications for an experimental detection of metal atoms on oxide supports based on computed core-level energies.