Preparation on nanostructured TiO2 supported platinum catalysts by photochemical deposition
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abstract
Nowadays, thanks to a growing environmental awareness, selectivity is seen as the major driving
force in catalyst research and development. In order to design the best possible catalyst there are
several parameters that can be fine tuned depending on the type of reaction. Type of metal, support
or thermal surface treatments are variables known to influence selectivity, as in the hydrogenation
of organic substrates containing unsaturated functional groups. Selectivity towards the normally
unfavoured hydrogenation of the carbonyl group instead of the C=C olefinic bond can be improved
in presence of metal particles with sizes above 3 nm1. To a certain extent the metal deposition step
allows particle size control, and innovative techniques, some more simple other more complex, are
currently being developed. In this field, photochemical deposition of noble metals in different
supports is gaining importance due to its simplicity and advantages. The main advantage is the
ability of spreading very effectively the metal throughout the support, leading to high metal
dispersions with particle size in the desired range. Additionally, if the support is a semiconductor,
like TiO2, the deposited metal ions are also automatically reduced mainly through the action of
conduction band electrons.
Titanium dioxide supported platinum catalysts (1 and 3 wt.%) were prepared by liquid phase
photodeposition of H2PtCl6⋅6H2O precursor. Thermal treatments under N2 and H2 were performed
at 773K (1Pt/TiO2-773 and 3Pt/TiO2-773) providing materials with variable particle sizes at the
nanometer scale. The prepared catalysts were tested on the liquid phase selective hydrogenation of
cinnamaldehyde to cinnamyl alcohol (100mL stainless steel autoclave at 363K and 10bar). The
preferred unsaturated alcohol is obtained by the reduction of the carbonyl group as opposed to
hydrogenation of C=C olefinic bond to yield the saturated aldehyde. Allylic alcohols are valuable
intermediates in the production of fragrances, flavoring additives, pharmaceuticals and
agrochemicals.
Under laboratory conditions thermally treated 3Pt/TiO2-773 catalyst revealed a selectivity towards
cinnamyl alcohol of 55% at 87% conversion, against 28% selectivity at 27% conversion under
similar conditions for the untreated catalyst. These results were explained by a combined effect of
SMSI and platinum particle size increase. The catalysts were also tested in oxo-steroid selective
hydrogenation and extremely promising results were obtained.
