13 research outputs found
Controlling catalytic activity and selectivity for partial hydrogenation by tuning the environment around active sites in iridium complexes bonded to supports
The Surface Chemistry of Metal Oxide Clusters : From Metal–Organic Frameworks to Minerals
Many metal–organic frameworks (MOFs) incorporate nodes that are small metal oxide clusters. Some of these MOFs are stable at high temperatures, offering good prospects as catalysts—prospects that focus attention on their defect sites and reactivities—all part of a broader subject: the surface chemistry of metal oxide clusters, illustrated here for MOF nodes and for polyoxocations and polyoxoanions. Ligands on MOF defect sites form during synthesis and are central to the understanding and control of MOF reactivity. Reactions of alcohols are illustrative probes of Zr6O8 node defects in UiO-66, characterized by the interconversions of formate, methoxy, hydroxy, and linker carboxylate ligands and by catalysis of alcohol dehydration reactions. We posit that new reactivities of MOF nodes will emerge from incorporation of a wide range of groups on their surfaces and from targeted substitutions of metals within them
Ultrathin wide band gap kesterites
Kesterite Cu2ZnSnS4 (CZTS), used for thin film solar cells, has a band gap energy around 1.5–1.6 eV with possibilities for further increase through alloying. In some applications for wide band gap solar cells, reduced absorber thickness can be beneficial, to allow partial light transmission. Reduced thickness can also be beneficial to reduce bulk recombination, and so called ultrathin solar cells (<700 nm thick) have been studied for several materials systems. Here, we report performance for CZTS devices down to 250 nm thickness and show that performance loss from thickness reduction is relatively small, partly due to short minority carrier diffusion length. Insertion of thin passivation layers (Al2O3, SiO2 or HfO2) at the Mo/CZTS interface gives improved performance of ultrathin devices, from 4.7% to 5.6% efficiency for best performing cells having 250 nm thick CZTS with Mo as compared to Mo/Al2O3 back contact. The approach of NaF post deposition for making isolating passivation layers conductive is tested for the first time for CZTS and is shown to work. For fabrication of CZTS devices on transparent ITO back contact, the insertion of passivation layers can reduce diffusion of indium into CZTS, but device performance is lower than on Mo back contacts
Thermal Stability Limits of Imidazolium Ionic Liquids Immobilized on Metal-Oxides
Thermal
stability limits of 33 imidazolium ionic liquids (ILs)
immobilized on three of the most commonly used high surface area metal-oxides,
SiO<sub>2</sub>, γ-Al<sub>2</sub>O<sub>3</sub>, and MgO, were
investigated. ILs were chosen from a family of 13 cations and 18 anions.
Results show that the acidity of C2H of an imidazolium ring is one
of the key factors controlling the thermal stability. An increase
in C2H bonding strength of ILs leads to an increase in their stability
limits accompanied by a decrease in interionic energy. Systematic
changes in IL structure, such as changes in electronic structure and
size of anion/cation, methylation on C2 site, and substitution of
alkyl groups on the imidazolium ring with functional groups have significant
effects on thermal stability limits. Furthermore, thermal stability
limits of ILs are influenced strongly by acidic character of the metal-oxide
surface. Generally, as the point of zero charge (PZC) of the metal-oxide
increases from SiO<sub>2</sub> to MgO, the interactions of IL and
metal-oxide dominate over interionic interactions, and metal-oxide
becomes the significant factor controlling the stability limits. However,
thermal stability limits of some ILs show the opposite trend, as the
chemical activities of the cation functional group or the electron
donating properties of the anion alter IL/metal-oxide interactions.
Results presented here can help in choosing the most suitable ILs
for materials involving ILs supported on metal-oxides, such as for
supported ionic liquid membranes (SILM) in separation applications
or for solid catalyst with ionic liquid layer (SCILL) and supported
ionic liquid phase (SILP) catalysts in catalysis
Interactions of [BMIM][BF<sub>4</sub>] with Metal Oxides and Their Consequences on Stability Limits
Interactions
between 1-<i>n</i>-butyl-3-methylimidazolium
tetrafluoroborate, [BMIM]Â[BF<sub>4</sub>], and high-surface-area metal
oxides, SiO<sub>2</sub>, TiO<sub>2</sub>, Fe<sub>2</sub>O<sub>3</sub>, ZnO, γ-Al<sub>2</sub>O<sub>3</sub>, CeO<sub>2</sub>, MgO,
and La<sub>2</sub>O<sub>3</sub>, covering a wide range of point of
zero charges (PZC), from pH = 2 to 11, were investigated by combining
infrared (IR) spectroscopy with density functional theory (DFT) calculations.
The shifts in spectroscopic features of the ionic liquid (IL) upon
coating different metal oxides were evaluated to elucidate the interactions
between IL and metal oxides as a function of surface acidity. Consequences
of these interactions on the short- and long-term thermal stability
limits as well as the apparent activation energy (<i>E</i><sub>a</sub>) and rate constant for thermal decomposition of the
supported IL were evaluated. Results showed that stability limits
and <i>E</i><sub>a</sub> of the IL on each metal oxide significantly
decrease with increasing PZC of the metal oxide. Results presented
here indicate that the surface acidity strongly controls the IL–surface
interactions, which determine the material properties, such as thermal
stability. Elucidation of these effects offers opportunities for rational
design of materials which include direct interactions of ILs with
metal oxides, such as solid catalysts with ionic liquid layer (SCILL),
and supported ionic liquid phase (SILP) catalysts for catalysis applications
or supported ionic liquid membranes (SILM) for separation applications
Atomic Layer Grown Zinc–Tin Oxide as an Alternative Buffer Layer for Cu2ZnSnS4-Based Thin Film Solar Cells: Influence of Absorber Surface Treatment on Buffer Layer Growth
Zn1–xSnxOy (ZTO) deposited by atomic layer deposition has shown promising results as a buffer layer material for kesterite Cu2ZnSnS4 (CZTS) thin film solar cells. Increased performance was observed when a ZTO buffer layer was used as compared to the traditional CdS buffer, and the performance was further increased after an air annealing treatment of the absorber. In this work, we study how CZTS absorber surface treatments may influence the chemical and electronic properties at the ZTO/CZTS interface and the reactions that may occur at the absorber surface prior to atomic layer deposition of the buffer layer. For this, we have used a combination of microscopy and synchrotron-based spectroscopies with variable information depths (X-ray photoelectron spectroscopy, high-energy X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy), allowing for an in-depth analysis of the CZTS near-surface regions and bulk material properties. No significant ZTO buffer thickness variation is observed for the differently treated CZTS absorbers, and no differences are observed when comparing the bulk properties of the samples. However, the formation of SnOx and compositional changes observed toward the CZTS surface upon an air annealing treatment may be linked to the modified buffer layer growth. Further, the results indicate that the initial N2 annealing step integrated in the buffer layer growth by atomic layer deposition, which removes Na–COx species from the CZTS surface, may be useful for the ZTO/CZTS device performance.
Controlling catalytic activity and selectivity for partial hydrogenation by tuning the environment around active sites in iridium complexes bonded to supports
Single-site Ir(CO)(2) complexes bonded to high-surface-area metal oxide supports, SiO2, TiO2, Fe2O3, CeO2, MgO, and La2O3, were synthesized by chemisorption of Ir(CO)(2)(acac) (acac = acetylacetonate) followed by coating with each of the following ionic liquids (ILs): 1-n-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4], 1-n-butyl-3-methylimidazolium acetate, [BMIM][Ac], and 1-(3-cyanopropyl)-3-methylimidazolium dicyanamide, [CPMIM][DCA]. Extended X-ray absorption fine structure spectroscopy showed that site-isolated iridium was bonded to oxygen atoms of the support. Electron densities on the iridium enveloped by each IL sheath/support combination were characterized by carbonyl infrared spectroscopy of the iridium gem-dicarbonyls and by X-ray absorption near-edge structure data. The electron-donor/acceptor tendencies of both the support and IL determine the activity and selectivity of the catalysts for the hydrogenation of 1,3-butadiene, with electron-rich iridium being selective for partial hydrogenation. The results resolve the effects of the IL and support as ligands; for example, the effect of the IL becomes dominant when the support has a weak electron-donor character. The combined effects of supports and ILs as ligands offer broad opportunities for tuning catalytic properties of supported metal catalysts
Effects of interionic interactions in 1,3-dialkylimidazolium ionic liquids on the electronic structure of metal sites in solid catalysts with ionic liquid layer (SCILL)
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Controlling catalytic activity and selectivity for partial hydrogenation by tuning the environment around active sites in iridium complexes bonded to supports.
Single-site Ir(CO)2 complexes bonded to high-surface-area metal oxide supports, SiO2, TiO2, Fe2O3, CeO2, MgO, and La2O3, were synthesized by chemisorption of Ir(CO)2(acac) (acac = acetylacetonate) followed by coating with each of the following ionic liquids (ILs): 1-n-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4], 1-n-butyl-3-methylimidazolium acetate, [BMIM][Ac], and 1-(3-cyanopropyl)-3-methylimidazolium dicyanamide, [CPMIM][DCA]. Extended X-ray absorption fine structure spectroscopy showed that site-isolated iridium was bonded to oxygen atoms of the support. Electron densities on the iridium enveloped by each IL sheath/support combination were characterized by carbonyl infrared spectroscopy of the iridium gem-dicarbonyls and by X-ray absorption near-edge structure data. The electron-donor/acceptor tendencies of both the support and IL determine the activity and selectivity of the catalysts for the hydrogenation of 1,3-butadiene, with electron-rich iridium being selective for partial hydrogenation. The results resolve the effects of the IL and support as ligands; for example, the effect of the IL becomes dominant when the support has a weak electron-donor character. The combined effects of supports and ILs as ligands offer broad opportunities for tuning catalytic properties of supported metal catalysts