Does each atom count in the reactivity of vanadia nanoclusters?

Vanadium oxide cluster anions (V₂O₅)_nV_xO_y ^- (n = 1-31; x = 0, 1; and x + y ≤ 5) with different oxygen deficiencies (δ = 2y - 1-5x = 0, ± 1, and ±2) have been prepared by laser ablation and reacted to abstract hydrogen atoms from alkane molecules (n-butane) in a fast flow reactor. When the cluster size n is less than 25, the δ = 1 series [(V₂O₅)_nO^- clusters] that can contain atomic oxygen radical anions (O^•-) generally have much higher reactivity than the other four cluster series (δ = - 2, - 1, 0, and 2), indicating that each atom counts in the hydrogen-atom abstraction (HAA) reactivity. Unexpectedly, all of the five cluster series have similar HAA reactivity when the cluster size is greater than 25. The critical dimension of vanadia particles separating the cluster behavior (each atom counts) from the bulk behavior (each atom contributes a little part) is thus about 1.6 nm (∼V₅₀O₁₂₅). The strong electron - phonon coupling of the vanadia particles has been proposed to create the O^•- radicals (V⁵⁺ = 0^2-+ heat → V⁴⁺ - O^•-) for the n > 25 clusters with δ = - 2, - 1, 0, and 2. Such a mechanism is supported by a comparative study with the scandium system [(Sc₂O₃)_nSc_xO_y ^- (n = 1-29; x = 0, 1; and x + y ≤ 4)] for which the δ = 1 series [(Sc₂O₃)_nO^- clusters] always have much higher HAA reactivity than the other cluster series.