With a density functional theory method, we studied computationally the size dependence of adsorption properties of metal nanoparticles for CO as a probe on Pd(n) clusters with n = 13-116 atoms. For large particles, the values slowly decrease with cluster size from the asymptotic value for an (ideal) infinite surface. For clusters of 13-25 atoms, starting well above the asymptotic value, the adsorption energies drop quite steeply with increasing cluster size. These opposite trends meet in an intermediate size range, for clusters of 30-50 atoms, yielding the lowest adsorption energies. These computational results help to resolve a controversy on the size-dependent behavior of adsorption energies of metal nanoparticles.

Interatomic distances in metal nanoparticles are reduced from their values in the bulk. We studied computationally how this size-dependent geometry change (from the bulk) relates to the size-dependence of other properties of large metal clusters, including their reactivity. For this purpose, using an all-electron scalar-relativistic density-functional approach, we calculated structures and binding energies for the example of CO adsorption on 3-fold hollow sites at the center of (111) facets of cuboctahedral nanoscale clusters Pd n (n = 55−260). The average nearest-neighbor Pd−Pd distance of optimized structures is 4−7 pm (2−3%) shorter than the extrapolated limit of the lateral distance within an infinite (111) surface. In consequence, the energy of CO adsorption on a cluster of ∼100 atoms is ∼15 kJ mol−1 smaller than the extrapolated limit. On the basis of these results, we suggest a strategy for modeling particles of larger size, e.g. of 1000 atoms and more, with the help of smaller model particles of up to ∼300 atoms where one keeps the core of a model cluster fixed at the bulk structure and restricts the structure optimization to the outermost shell of cluster atoms.

The hydrodeoxygenation (HDO) of aromatic oxygenates over ruthenium was studied computationally on the model system guaiacol (2-methoxyphenol) on Ru(0001) using a DFT method. In addition to the adsorption geometries of the aromatic intermediates, the study focused on the energetics of elementary reaction steps that occur during the HDO of guaiacol. Bond scissions at the aliphatic side group were calculated to have barriers of at most 69 kJ mol −1 . In contrast, barriers for the cleavage of the aromatic bonds C aryl −O were determined at more than 100 kJ mol −1 . On the basis of calculated energetics, a reaction pathway for the HDO of guaiacol is proposed in which first the methyl group of the methoxy moiety is removed to yield catecholate. Subsequently, the oxo groups are replaced by H, yielding first phenolate and, finally, benzene. For the removal of the first oxygen center of catecholate, a substantially lower barrier (106 kJ mol −1 ) than for the C aryl −O cleavage of phenolate (189 kJ mol −1 ) was calculated. This is rationalized by the strained structure of adsorbed catecholate. The high barrier for the second C aryl −O scission step is line with recent experiments that yield phenol as the main product of guaiacol HDO over Ru/C.