Tuning the oxidation of carbon monoxide using nanoassembled model catalysts

While the oxidation of CO has long been considered to be structure insensitive, nano-assembled model catalysts consisting of size-selected metal clusters (Au(n), Pt(n), Pd(n), and Rh(n), n = 1-20), supported on thin MgO(100) films, reveal distinct size-effects. When adding a single Pt atom to Pt₁₄, platinum clusters increase their reactivity by a factor of three. Rh₂₀ shows the highest reactivity of all the investigated clusters, oxidizing about 13 CO molecules per cluster at 350 K. While F-centers on MgO films transform Au₈ from an inert to an active catalyst, the reactivity of Pd₈ is not suppressed when deposited on defect-poor films. These different catalytic properties are rationalized within simple frontier orbital models, whereas for Au₈ the reaction mechanism for the low temperature oxidation of CO is elucidated within first-principle calculations.