Active sites and structure–activity relationships
for methanol synthesis from a stoichiometric mixture of CO2 and H2 were investigated for a series of coprecipitated
Cu-based catalysts with temperature-programmed reduction (TPR), X-ray
diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron
spectroscopy (XPS), and N2O decomposition. Experiments
in a reaction chamber attached to an XPS instrument show that metallic
Cu exists on the surface of both reduced and spent catalysts and there
is no evidence of monovalent Cu+ species. This finding
provides reassurance regarding the active oxidation state of Cu in
methanol synthesis catalysts because it is observed with 6 compositions
possessing different metal oxide additives, Cu particle sizes, and
varying degrees of ZnO crystallinity. Smaller Cu particles demonstrate
larger turnover frequencies (TOF) for methanol formation, confirming
the structure sensitivity of this reaction. No correlation between
TOF and lattice strain in Cu crystallites is observed suggesting this
structural parameter is not responsible for the activity. Moreover,
changes in the observed rates may be ascribed to relative distribution
of different Cu facets as more open and low-index surfaces are present
on the catalysts containing small Cu particles and amorphous or well-dispersed
ZnO. In general, the activity of these systems results from large
Cu surface area, high Cu dispersion, and synergistic interactions
between Cu and metal oxide support components, illustrating that these
are key parameters for developing fundamental mechanistic insight
into the performance of Cu-based methanol synthesis catalysts.
Metal membranes play a vital role in hydrogen purification. Defect-free membranes can exhibit effectively infinite selectivity but must also provide high fluxes, resistance to poisoning, long operational lifetimes, and low cost. Alloying offers one route to improve on membranes based on pure metals such as palladium. We show how ab initio calculations and coarse-grained modeling can accurately predict hydrogen fluxes through binary alloy membranes as functions of alloy composition, temperature, and pressure. Our approach, which requires no experimental input apart from knowledge of bulk crystal structures, is demonstrated for palladium-copper alloys, which show nontrivial behavior due to the existence of face-centered cubic and body-centered cubic crystal structures and have the potential to resist sulfur poisoning. The accuracy of our approach is examined by a comparison with extensive experiments using thick foils at elevated temperatures. Our experiments also demonstrate the ability of these membranes to resist poisoning by hydrogen sulfide.
An
amine sorbent, prepared by impregnation of polyethyleneimine on silica,
was tested for steam stability. The stability of the sorbent was investigated
in a fixed bed reactor using multiple steam cycles of 90 vol % H2O/He at 105 °C, and the gas effluent was monitored with
a mass spectrometer. CO2 uptake of sorbent was found to
decrease with repeated exposure to steam. Characterization of the
spent sorbent using N2 physisorption, SEM, and thermogravimetric
analysis (TGA) showed that the decrease in CO2 loading
can possibly be attributed to a reagglomeration of the amine in the
pores of the silica. No support effect was found in this study. The
commercial SiO2 used, Cariact G10, was found to be stable
under the conditions used. While it was found that subjecting the
sorbent to several steam cycles decreased its CO2 uptake,
a continuous exposure of the sorbent to steam did not have a significant
performance impact. A silanated sorbent, consisting of a mixture of
PEI and aminopropyl-triethoxysilane on SiO2 support, was
also investigated for steam stability. Similarly to the nonsilanated
sorbent, the CO2 loading of this sorbent decreased upon
steam exposure, although a mechanism for this change has not been
postulated at this time.
Rare earth elements (REE) are of strategic importance because they find numerous applications in various sectors of the global economy.The concern about the REE supply challenge has led to increasing interest and research in the recovery of REE from end-of-life products and secondary sources such as coal and coal by-products. The work reported here was focused on examining the technical feasibility of physical separation techniques for the enrichment of REE from coal and coal by-products. Particle size, magnetic and density separations were performed on coal, coal ash, clay and shale samples. It was found that the samples responded to particle size separation differently. For all ash samples, higher REE concentrations were found in the finer fractions. For the clay and shalesamples, however, the REE concentrations decrease as the particle size reduces possibly because RE minerals were not effectively released by grinding. Magnetic separation showed that REE are enriched in non-magnetic fractions for all ash samples. All samples responded similarly to density separation. Among the three methods, density separation showed the highest enrichment of REE. A combination of these methods is recommended. Finally, correlations between elements were demonstrated, which leads to the classification of three groups containing mainly Al/Si, Fe and Ca, respectively. REE are strongly associated with the Al/Si group.
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