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.

Adsorption of oxygenates on alkanethiol-functionalized Pd(111) surfaces: mechanistic insights into the role of self-assembled monolayers on catalysis.

Recent work shows that coating a supported palladium catalyst with a self-assembled monolayer (SAM) of alkanethiols can dramatically improve selectivity in the hydrogenation of 1-epoxy-3-butene (EpB) to 1-epoxybutane. Here, we present the results of surface-level investigations of the adsorption of EpB and related molecules on SAM-coated Pd(111), with an aim of identifying mechanistic explanations for the observed catalytic behavior. Alkanethiol SAM-covered Pd(111) surfaces were prepared by conventional techniques and transferred to ultrahigh vacuum, where they were characterized using Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD) of EpB and other probe molecules. Whereas previous studies have shown that EpB undergoes rapid decomposition via epoxide ring opening on uncoated Pd(111), TPD studies show that EpB does not undergo substantial ring opening on SAM-covered surfaces but rather desorbs intact at temperatures less than 300 K. Systematic comparisons of EpB desorption spectra to spectra for other C(4) oxygenates suggest that the SAM creates a kinetic barrier to epoxide ring-opening reactions that does not exist on the uncoated surface. The EpB desorption spectra as a function of exposure show behavior similar to the desorption of olefins from Pd(111), indicating that the binding of the olefin functionality, in contrast to that of the epoxide ring, is not significantly perturbed. EpB desorption spectra from surfaces with less well-ordered SAMs show the presence of weakly bound states not observed on well-ordered SAM surfaces. The lower activity observed on catalysts covered with less well-ordered SAMs is hypothesized to occur due to partial confinement of adsorbates into these weakly bound, less active states.

Controlled selectivity for palladium catalysts using self-assembled monolayers.

The selective reaction of one part of a bifunctional molecule is a fundamental challenge in heterogeneous catalysis and for many processes including the conversion of biomass-derived intermediates. Selective hydrogenation of unsaturated epoxides to saturated epoxides is particularly difficult given the reactivity of the strained epoxide ring, and traditional platinum group catalysts show low selectivities. We describe the preparation of highly selective Pd catalysts involving the deposition of n-alkanethiol self-assembled monolayer (SAM) coatings. These coatings improve the selectivity of 1-epoxybutane formation from 1-epoxy-3-butene on palladium catalysts from 11 to 94% at equivalent reaction conditions and conversions. Although sulphur species are generally considered to be indiscriminate catalyst poisons, the reaction rate to the desired product on a catalyst coated with a thiol was 40% of the rate on an uncoated catalyst. Interestingly the activity decreased for less-ordered SAMs with shorter chains. The behaviour of SAM-coated catalysts was compared with catalysts where surface sites were modified by carbon monoxide, hydrocarbons or sulphur atoms. The results suggest that the SAMs restrict sulphur coverage to enhance selectivity without significantly poisoning the activity of the desired reaction.