A DFT investigation of CO oxidation over neutral and cationic gold clusters

Foreign atom encapsulated Au12 golden cages for catalysis of CO oxidation

Gold clusters are known for their unique catalytic properties, among which, endohedral gold clusters doped with heteroatoms have remarkable stabilities, with electronic structures tunable by both cluster size and doping element. Thus, it is intriguing and imperative to understand the principles for modulating the catalytic behaviors of these novel clusters. Here, we exploit experimentally produced M@Au₁₂ (M = transition metal) cage clusters for catalysis of CO oxidation. The doping effects of 3d, 4d and 5d transition metals (V, Cr, Mn, Nb, Mo, Ta, W and Re) on the catalytic properties were systematically explored by first-principles calculations. Among the considered M@Au₁₂ clusters, Cr@Au₁₂ and Mn@Au₁₂ provide a suitable binding strength with reaction intermediates and are highly active for CO oxidation with reaction barriers of 0.41 eV under the Langmuir-Hinshelwood mechanism. More importantly, we establish a distinct relationship between catalytic activity and the M-Au bond order and the d orbital center of the M@Au₁₂ clusters, which would help tailor their catalytic performance with atomistic precision and enable utilization of these stable gold cages for catalysis of various chemical processes.

Stability of AumAgn (m + n = 1-6) clusters supported on a F-center MgO(100) surface

A theoretical study has been performed for deposited AumAgn (m + n = 1-6) clusters. The combined use of the Mexican Enhanced Genetic Algorithm (MEGA) and Density Functional Theory (DFT) calculations allows us to explore the potential energy surface and therefore, find the global minimum configuration for each composition. We have performed calculations of clusters deposited on defects (oxygen vacancies) known as F centers on MgO (100) surfaces. Our results show interesting differences in the geometries of the clusters upon deposition and as a consequence in their electronic properties. The combination of two metals with different electronegativities creates an inhomogeneous charge distribution on their exposed surface producing good conditions for a catalytic process to take place.

Exploring the low-lying structures of Aun(CO)+ (n = 1–10): adsorption and stretching frequencies of CO on various coordination sites

Adsorption of CO on cationic gold clusters is insensitive to the structural details of the adsorption site.

Catalytic activities of subnanometer gold clusters (Au₁₆-Au₁₈, Au₂₀, and Au₂₇-Au₃₅) for CO oxidation

Using the CO oxidation as a chemical probe, we perform a comprehensive ab initio study of catalytic activities of subnanometer gold clusters. Particular attention is placed on 12 different clusters in the size range of Au(16)-Au(35), whose atomic structures in the anionic state have been resolved from previous experiments. Adsorption energies of a single CO or O(2) molecule as well as coadsorption energies of both CO and O(2) molecules on various distinctive surface sites of each anionic cluster and their neutral counterpart are computed. In general, the anionic clusters can adsorb CO and O(2) more strongly than their neutral counterparts. The coadsorption energies of both CO and O(2) molecules decrease as the size of gold clusters increases with the exception of Au(34) (an electronic "magic-number" cluster). Besides the known factor of low coordination site, we find that a relatively small cone angle (<110°) associated with each surface site is another key geometric factor that can enhance the binding strength of CO and O(2). For the subnanometer clusters, although the size effect can be important to the strength of CO adsorption, it is less important to the activation energy. Using Au(34) as a prototype model, we show that strong CO and O(2) adsorption sites tend to yield a lower reaction barrier for the CO oxidation, but they have little effect on the stability of the reaction intermediate. Our calculations support the notion that CO and O(2) adsorption energies on the gold clusters can be an effective indicator to assess catalytic activities of subnanometer gold clusters. This systematic study of the site- and size-dependent adsorption energies and reaction pathways enables a quantitative assessment of the site-size-activity relationship for the CO oxidation on subnanometer gold clusters.

Density functional theory for transition metals and transition metal chemistry

We introduce density functional theory and review recent progress in its application to transition metal chemistry. Topics covered include local, meta, hybrid, hybrid meta, and range-separated functionals, band theory, software, validation tests, and applications to spin states, magnetic exchange coupling, spectra, structure, reactivity, and catalysis, including molecules, clusters, nanoparticles, surfaces, and solids.