Long-range chemical sensitivity in the sulfur K-edge X-ray absorption spectra of substituted thiophenes

© 2014 American Chemical Society. Thiophenes are the simplest aromatic sulfur-containing compounds and are stable and widespread in fossil fuels. Regulation of sulfur levels in fuels and emissions has become and continues to be ever more stringent as part of governments' efforts to address negative environmental impacts of sulfur dioxide. In turn, more effective removal methods are continually being sought. In a chemical sense, thiophenes are somewhat obdurate and hence their removal from fossil fuels poses problems for the industrial chemist. Sulfur K-edge X-ray absorption spectroscopy provides key information on thiophenic components in fuels. Here we present a systematic study of the spectroscopic sensitivity to chemical modifications of the thiophene system. We conclude that while the utility of sulfur K-edge X-ray absorption spectra in understanding the chemical composition of sulfur-containing fossil fuels has already been demonstrated, care must be exercised in interpreting these spectra because the assumption of an invariant spectrum for thiophenic forms may not always be valid

CO2 reduction on supported Ru/Al2O3 catalysts: Cluster size dependence of product selectivity

The catalytic performance of a series of Ru/Al2O3 catalysts with Ru content in the 0.1-5% range was examined in the reduction of CO2 with H2. At low Ru loadings (???0.5%) where the active metal phase is highly dispersed (mostly atomically) on the alumina support, CO is formed with high selectivity. With increasing metal loading, the selectivity toward CH4 formation increases, while that for CO production decreases. In the 0.1% Ru/Al2O3 catalyst, Ru is mostly present in atomic dispersion, as scanning transmission electron microscopy (STEM) images obtained from the fresh sample prior to catalytic testing reveal. STEM images recorded from this same sample, following the temperature programmed reaction test, clearly show the agglomeration of small metal particles (and atoms) into 3D clusters. The clustering of the highly dispersed metal phase is responsible for the observed dramatic selectivity change during elevated temperature tests: dramatic decrease in CO and large increase in CH4 selectivity. Apparent activation energies, estimated from the slopes of Arrhenius plots, of 82 and 62 kJ/mol for CO and CH 4 formation were determined, respectively, regardless of Ru loading. These results suggest that the formation of CO and CH4 follow different reaction pathways or proceed on active centers of a different nature. Reactions with CO2/H2 and CO/H2 mixtures (under otherwise identical reaction conditions) reveal that the onset temperature of CO2 reduction is about 150 C lower than of CO reduction.close3