Structure-activity correlations on Rh/Al2O3 and Rh/TiO2 thin film model catalysts after oxidation and reduction

A study of the effect of different supports on the stability and catalytic activity of Rh nanoparticles is only meaningful if the metal particles are perfectly identical in the catalysts to be compared. We have applied epitax ial thin film deposition to produce homogeneous distributions of well-facet ed Rh nanocrystals, divided the sample in two, and subsequently supported t he metal particles by Al2O3 and TiO2. Three corresponding pairs of Rh/Al2O3 and Rh/TiO2 model catalysts with mean particle sizes of 7.8, 13.3, and 16. 7 mn were prepared and activated by different oxidation-reduction treatment s at temperatures up to 723 K. The size, morphology, and structure of the m etal particles and of the support and the changes upon activation were dete rmined by transmission electron microscopy. Wetting of the support and coal escence of Rh particles were observed to occur upon high-temperature reduct ion of large (>10 nn) and closely-spaced Rh particles, around 623 K on tita nia and around 723 K on alumina. Catalysts with smaller Rh particles did no t show such pronounced changes. The rate of ring opening of methylcyclobuta ne at 373 K was measured on the three pairs of Rh/Al2O3 and Rh/TiO2 model c atalysts as a function of the reduction temperature after preoxidation at 7 23 K. Rh/alumina catalysts exhibited maximum activity after reduction at 52 3 K due to the formation of low-coordinated sites as shown by electron micr oscopy. The activity of Rh/titania peaked after reduction at 373 K and decr eased almost exponentially with T-red UP to 673 K. In the case of Rh/titani a, the changes in particle size and microstructure as observed in the elect ron microscope can only account for part of the kinetic results. The rapid activity decrease in titania-supported Rh particles with increasing Tred is paralleled to an increase in the number of oxygen vacancies and in low-val ent Ti cations, on the titania surface. The latter may affect the electric field at the metal-support boundary and thereby reduce the catalytic activi ty for hydrocarbon reactions. In addition, the decoration of Rh particles b y migrating Ti suboxide will account for the diminished activity after redu ction at higher temperature. (C) 1999 Academic Press.