No abstract

We systematically studied the adsorption of O on Au in the size range of 0-1 nm at low temperatures and determined new active sizes with n = 22, 24, 34, and 36. The kinetic measurements more clearly showed the correlation between the reactivity of Au with O and their electronic properties: the sizes with a closed electron shell are always inert, and the sizes with an unpaired electron can chemically adsorb one O molecule if their adiabatic detachment energies (ADEs) are lower than a threshold around 3.5 eV. This ADE threshold dividing the active and inert Au is independent of the clusters' sizes, global geometries, and local adsorption sites. According to the widely accepted electron transfer mechanism, this threshold could stand for the case in which the total energy of the Au and an O roughly equals that of the spin crossover point of the potential surfaces of Au -O and Au ···O.

The adsorption and activation of NO on microsilver species provide the foundation to understand the mechanism of NO removing reactions on silver based catalysts. However, the diversiform of the geometrical structures and electronic properties of microsilver species in condensed phases has posed considerable challenges for exploring these interactions. We study the reactions of NO with bare silver clusters Agn(±) (7-69) in the gas phase using a continuous flow reactor running at low temperatures. Evidence for NO unit adsorption, the formation of (NO)2 and the reduction of NO is observed on different cluster sizes. The kinetic rates of initial NO unit adsorption are closely related to silver clusters' global electronic properties. The low electron binding energy and the unpaired electron of a silver cluster favor the adsorption and activation of NO. In particular, the clusters with one less electron than those of closing electron shells are generally inert and the sizes having one more electron outside these shells are generally quite reactive. These observations depict a general figure of interactions between NO and microsilver species, in which electron transfer from silver to NO dominates.

Conversion of NO to other nitrogen oxides is an elementary step in its catalytic removal processes. On coinage metal surfaces, two kinds of NO activation mechanisms have been well documented: the unimolecular dissociation of NO generates two adsorbed atoms, and the dissociation of an adsorbed (NO) unit generates an adsorbed O and a free NO. In this work, we observed a disproportionation mechanism involving three NO molecules on Au at a very low temperature (150 K), in which an adsorbed (NO) reacts with a free NO forming an adsorbed NO and a free NO. The density functional theory (DFT) calculations indicated that this disproportionation step is significantly exothermic and has a very low activation barrier. The charge distributions on the involved cluster complexes and the correlation between the activity and the electronic properties of Au indicate the important role of extra negative charge in all reaction steps. The disproportionation mechanism revealed in this work could possibly exist in the NO removal processes on real gold catalysts.