April Corpuz

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.

Nitrogen: unraveling the secret to stable carbon-supported Pt-alloy electrocatalysts

Nitrogen functionalities significantly improve performance for metal-based carbon-supported catalysts, yet their specific role is not well understood. In this work, a direct observation of the nanoscale spatial relationship between surface nitrogen and metal catalyst nanoparticles on a carbon support is established through principal component analysis (PCA) of electron energy loss spectral (EELS) imaging datasets acquired on an aberration-corrected scanning transmission electron microscope (STEM). Improved catalyst–support interactions correlated to high substrate nitrogen content in immediate proximity to stabilized nanoparticles are first demonstrated using model substrates. These insights are applied in direct methanol fuel cell prototypes to achieve substantial improvements in performance and long-term stability using both in-house and commercial catalysts doped with nitrogen. These results have immediate impact in advanced design and optimization of next generation high performance catalyst materials.

Analysis of lipids: metal oxide laser ionization mass spectrometry.

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has been used for lipid analysis; however, one of the drawbacks of this technique is matrix interference peaks at low masses. Metal oxide surfaces are described here for direct, matrix-free analysis of small (MW < 1000 Da) lipid compounds, without interferences in the resulting spectra from traditional matrix background peaks. Spectra from lipid standards produced protonated and sodiated molecular ions. More complex mixtures including vegetable oil shortening and lipid extracts from bacterial and algal sources provided similar results. Mechanistic insight into the mode of ionization from surface spectroscopy, negative ion mass spectrometry, and stable isotope studies is also presented. The metal oxide system is compared to other reported matrix-free systems.

Nanoscale (111) faceted rock-salt metal oxides in catalysis

Facet-specific growth is highly desirable for catalysts, as conversion and selectivity can be altered with increasing amounts of particular active sites. The (100) surface of a rock-salt structure is comprised of alternating oxygen anions and metal cations, similar to the (110) facet. The (111) surface differs substantially in that an ideal (111) surface would consist solely of oxygen anions or metal cations, and cannot exist as-is. However, wet chemical syntheses of MgO(111) and NiO(111) have recently been reported; theory and experiments show the (111) surface is stabilized as a hydroxylated surface. These (111) faceted metal oxides exhibit catalytic properties that differ significantly from their (100) counterparts. Here, we discuss the theory and performance of the (111) rock-salt metal oxides in catalysis, as supports, and as adsorbents.

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.