Hydrogen exposure effects on Pt/Al₂O₃ catalysts coated with thiolate monolayers

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to examine the effect of thermal treatment of self-assembled monolayers (SAMs) produced from various thiols (1-hexanethiol, 1-decanethiol, 1-octadecanethiol, 1,6-hexanedithiol, 1,10-decanedithiol, 1,2-benzenedithiol) on a 5 wt % Pt/Al2O3 catalyst. Catalysts were characterized during heating and cooling between 290 and 553 K in both the presence and absence of H2. Overall, the behavior of thiols on Pt/Al2O3 in an inert He environment was similar to the behavior reported in other works on Au, although in the case of the Pt catalyst, C-H bond dissociation in the thiols was apparent at high temperatures. Under H2 flow, however, markedly different behavior was observed; in particular, conformational order was observed to increase with increasing temperature, up to temperatures as high as 500 K for octadecanethiol-coated catalysts. The effects of H2 exposure are much less pronounced for alkanedithiol-coated catalysts. 1,2-Benzenedithiol was found to undergo partial hydrogenation under H2, indicating that hydrogenating reaction conditions can also influence the chemical structure of the monolayer on active metals, such as Pt. The differences in thiolate structure caused by high-temperature exposure to hydrogen were found to have a significant effect on the rate and selectivity for hydrogenation of prenal, indicating that such effects may be broadly important in the use of thiolate-promoted catalysts for hydrogenation reactions.