Planar oxide supported rhodium nanoparticles as model catalysts.

C(2)H(4)/CO/H(2) reaction is investigated on Rh/SiO(2) model catalyst surfaces. Kinetic reactivity and infrared spectroscopic measurements are investigated as a function of Rh particle size under near atmospheric reaction conditions. Results show that propionaldehyde turnover frequency (TOF) (CO insertion pathway) exhibits a maximum activity near <d(p)> = 2.5 nm. Polarization modulation infrared reflection absorption spectroscopy under CO and reaction (C(2)H(4)/CO/H(2)) conditions indicate the presence of Rh carbonyl species (Rh(CO)(2), Rh(CO)H) on small Rh particles, whereas larger particles appear resistant to dispersion and carbonyl formation. Combined these observations suggest the observed particle size dependence for propionaldehyde production via CO insertion is driven by two factors: (i) an increase in propionaldehyde formation on undercoordinated Rh sites and (ii) creation of carbonyl hydride species (Rh(CO)H)) on smaller Rh particles, whose presence correlates with the lower activity for propionaldehyde formation for <d(p)> < 2.5 nm.