Exploring similarities in reactivity of superatom species: a combined theoretical and experimental investigation.

The replacement of group 10-based materials by superatoms has gained great attention due to studies presenting similarities in electronic character and reactive nature between pairs. The current study extends the concept to systems of larger and varied composition as the pairs PdO(+) and ZrO(2)(+) as well as NiO(+) and TiO(2)(+) are interacted with C(2)H(4) and CO through DFT calculations and guided-ion-beam mass spectrometry. It is determined that the pairs readily oxidize C(2)H(4) while oxygen transfer is limited towards CO. Interestingly, within the reaction profiles for oxidation of C(2)H(4) by PdO(+) and NiO(+), a spin crossover is observed which greatly increases the exothermicity of the process. This investigation presents a major step in identifying replacements for expensive group 10 metals in catalytic materials.

Tuning cluster reactivity by charge state and composition: experimental and theoretical investigation of CO binding energies to Ag(n)Au(m)(+/-) (n + m = 3).

Temperature-dependent gas-phase reaction kinetics measurements and equilibrium thermodynamics under multicollision conditions in conjunction with ab initio DFT calculations were employed to determine the binding energies of carbon monoxide to triatomic silver-gold binary cluster cations and anions. The binding energies of the first CO molecule to the trimer clusters increase with increasing gold content and with changing charge from negative to positive. Thus, the reactivity of the binary clusters can be sensitively tuned by varying charge state and composition. Also, multiple CO adsorption on the clusters was investigated. The maximum number of adsorbed CO molecules was found to strongly depend on cluster charge and composition as well. Most interestingly, the cationic carbonyl complex Au(3)(CO)(4)(+) is formed at cryogenic temperature, whereas for the anion, only two CO molecules are adsorbed, leading to Au(3)(CO)(2)(-). All other trimer clusters adsorb three CO molecules in the case of the cations and are completely inert to CO in our experiment in the case of the anions.

Reactivity of stoichiometric titanium oxide cations.

We present the results of a reactivity study of titanium cationic clusters towards CO, C(2)H(2), C(2)H(4) and C(3)H(6) based on guided-ion-beam mass spectrometry and DFT calculations. We identified Ti(2)O(4)(+) and to a lesser extent TiO(2)(+) species which preferentially undergo oxidation reactions. An oxygen centered radical of Ti(2)O(4)(+) is responsible for selective oxidation. Energy profiles and MD simulations reveal the mechanisms of the reactions. Regeneration of the oxygen centered radical was achieved experimentally and theoretically through the reaction of N(2)O with Ti(2)O(3)(+).

Composition dependent adsorption of multiple CO molecules on binary silver-gold clusters Ag(n)Au(m)+ (n + m = 5): theory and experiment.

The binding energies of multiple CO molecules to five-atom silver-gold cluster cations have been obtained employing temperature dependent gas phase ion trap measurements and ab initio calculations. The CO binding energies to Ag(n)Au(m)(+) (n + m = 5) decrease with increasing number of silver atoms. Most strikingly, after the adsorption of the fourth CO to Au(5)(+) and of the third CO to Ag(5)(+), respectively, a pronounced decrease in the binding energies of further CO molecules was observed. This is related to a CO-induced structural transformation yielding more compact metal cluster geometries. First principles calculations revealed that the exact structure of the carbonyl complexes with multiple CO and the nature of the CO-induced structural transformation strongly depend on the composition of the metal cluster as well as on the number of adsorbed CO molecules.

Influence of charge state on catalytic oxidation reactions at metal oxide clusters containing radical oxygen centers.

Evidence obtained by guided-ion-beam mass spectrometry experiments and density functional theory calculations indicates that by adding one oxygen atom with a full octet of valence electrons (O(2-)) to stoichiometric cationic zirconium oxide clusters (ZrO(2))(x)(+) (x = 1-4), a series of anionic clusters (Zr(x)O(2x+1))(-) (x = 1-4) are formed which contain radical oxygen centers with elongated (elongation approximately 0.24 +/- 0.02 A) metal-oxygen bonds. These anionic clusters oxidize carbon monoxide, strongly associate acetylene, and weakly associate ethylene, in contrast to the cationic species which were found previously to be highly active toward the oxidation of all three molecules. Theoretical investigations indicate that a critical hydrogen transfer step necessary for the oxidation of ethylene and acetylene at metal oxide clusters containing radical oxygen centers is energetically favorable for cationic clusters but unfavorable for the corresponding anionic species. The calculated electrostatic potential of the cluster reveals that in the case of cations, a favorable interaction with nucleophilic molecules takes place over the whole surface of the (ZrO(2))(x)(+) (x = 1-4) clusters, compared to a restricted interaction of ethylene and acetylene with the less coordinated zirconium atom in the case of the anionic (Zr(x)O(2x+1))(-) (x = 1-4) species. Therefore, in spite of the common presence of a radical oxygen center in specific anionic and cationic stoichiometries, the extent to which various classes of reactions are promoted is influenced by charge state. Moreover, the (Zr(x)O(2x+1))(-) (x = 1-4) series of anionic clusters may be regenerated by reacting oxygen deficient clusters with a strong oxidizer. This indicates that not only cationic species, as shown previously, but also anionic clusters may promote multiple cycles of carbon monoxide oxidation.