T Huizinga

A temperature programmed reduction study of Pt on Al2O3 and TiO2

The reducibility of platinum on γ-Al2O3- and TiO2 was studied with the aid of temperature programmed reduction. The reduction peak temperature was found to be dependent on the temperature of the primary oxidation after impregnation and drying. The higher the oxidation temperature the lower the TPR peak temperature and the higher the H2 consumption. During the oxidation small PtO2 particles were formed which were more easily reduced than the original isolated Pt2+ ions. For Pt/Al2O3 no decomposition of PtO2 was observed up to 750 K, while bulk PtO2 decomposed around 600 K. This demonstrated that there is a substantial interaction between PtO2 and Al2O3. For PtO2 supported on TiO2 and SiO2 this interaction is much weaker and on these supports PtO2 decomposed at lower temperatures than on Al2O3. Reduction of passivated catalysts, with H/Pt < 0.8 in the metallic state, took place even at 223 K. After passivation these catalysts consist of a metal core surrounded by a metal oxide skin. Due to the presence of the metallic core, H2 can be dissociatively chemisorbed at low temperatures and induce reduction of the oxide layer. The implications of this for the O2-H2 titration method are discussed. Reduction of Pt/TiO2 led to metal assisted reduction of the support. Below 500 K only a small part of the support is (reversibly) reduced in the near vicinity of the metal particles. Above 500 K further metal assisted reduction of the TiO2 support takes place, probably promoted by increased ion mobility.

The morphology of rhodium supported on TiO2 and Al2O3 as studied by temperature-programmed reduction-oxidation and transmission electron microscopy

Supported RhAl2O3 and RhTiO2 catalysts with varying metal loadings were investigated by chemisorption and temperature-programmed reduction and oxidation. Hydrogen chemisorption showed that all the Rh on Al2O3 was well dispersed (HRh > 1 for loadings below 5 wt% and H>Rh > 0.5 up to 20 wt%), while the dispersion on TiO2 was much lower. TPR/TPO showed that this was due to the growth of two different kinds of RhRh2O3 particles on TiO2; one kind was easily reduced/oxidized, with a high dispersion, and the other kind was harder to reduce/oxidize, with a lower dispersion. TEM showed that the first kind of Rh2O3 consisted of flat, raftlike particles and the second kind of spherical particles.

Reduction and oxidation of Rh/Al2O3 and Rh/TiO2 catalysts as studied by temperature-programmed reduction and oxidation

Abstract Careful preparation of Rh/Al 2 O 3 catalysts leads to ultradisperse systems (H/Rh > 1.0). Temperature-programmed reduction (TPR) shows that these catalysts are almost completely oxidized during passivation. Identical preparation of Rh/TiO 2 catalysts leads to less disperse systems (H/Rh = 0.3), which exhibit two reduction peaks in TPR. These peaks are due to the reduction of small, well-dispersed Rh 2 O 3 particles and of large, bulk-like Rh 2 O 3 particles. In all cases reduction of Rh 2 O 3 is complete above 450 K. TiO 2 is partly reduced by a metal-catalysed process above 500 K.

The catalytic behaviour of rhodium supported on Al2O3 and TiO2 in synthesis gas reactions

Abstract The catalytic acitivity of Rh/TiO2and Rh/A1203 catalysts for the hydrogenation of CO at atmospheric pressure and 523 K was investigated. Normal, non-SMSI state Rh/TiO2 as well as Rh/TiO2 catalysts in the SMSI state with varying dispersion were studied. In all cases only hydrocarbons and no oxygenated products were formed. When measured at equal dispersions the activities of Rh/A1203 and non-SMSI Rh/TiO2 catalysts hardly differed. The specific activity of Rh/TiO2 catalysts increased an order of magnitude when changing the dispersion from 1.10 to 0.12. This increase in specific activity was acompanied by an increase in the olefin-to-paraffin ratio, but neither the selectivity to methane nor the probability for chain growth was affected much. Reduction of the Rh/TiO2 catalysts at 773 K decreased their initial activities substantially compared to reduction at 523 K. whereas the steady-state activities were hardly affected. Apparently the SMSI state is removed to a great extent during CO hydrogenation. The lowering effect of SMSI on activity was a function of dispersion. The effect was much more pronounced for small metal particles, indicating that SMSI might be due to covering.