Influence of metal-support interaction on nitrate hydrogenation over Rh and Rh-Cu nanoparticles dispersed on Al2O3 and TiO2 supports

Well-defined Rh and Rh-Cu nanoparticles (NP's) of 1.6 nm and 1.3 nm, respectively, were synthesized by alkaline polyol method and then dispersed on insulating (Al2O3) and semiconducting (TiO2) supports. Both colloidal NP's and supported NP's were characterized using various experimental methods (TEM, XPS, XRD, etc.) to gather information about their specific morphology, structure and chemical state. The effects of size and support on the catalytic behavior of NP's for nitrate hydrogenation reaction were analyzed. Oxide supports, especially TiO2, were found to have a strong positive effect on the catalytic activity of metallic NP's. The non-supported, colloidal, Rh and Rh-Cu NP's are either inactive or posses very low hydrogenation activity. For supported materials, the intimate contact between two metals (i.e. Rh-Cu) is required to attain good hydrogenation activity. The strong metal-support interaction, induced by hydrogen spillover, is a key point in determining hydrogenation activity. The Rh-Cu NP's dispersed on TiO2 are extremely active for NO3− and NO2− (intermediate) deep hydrogenation, with high selectivity for NH4+. The hydrogenation activity of Rh-Cu NP's supported on Al2O3 is hindered considerably, the main products of NO3− hydrogenation being NO2− intermediate.