136 research outputs found
Intrazeolite metal carbonyl topotaxy. A comprehensive structural and spectroscopic study of intrazeolite Group VI metal hexacarbonyls and subcarbonyls
This paper focuses attention on the intrazeolite anchoring, thermal decarbonylation, ligand exchange, and addition chemistry of M(CO)6-M'56Y, where M = Cr, Mo, W; M' = H, Li, Na, K, Rb, Cs. The key points to emerge from this study include the following. (i) M(CO)6-M'56Y samples have the hexacarbonylmetal(O) molecule associated with two alpha-cage extraframework cations (or Bronsted protons), via the oxygen end of two trans bonded carbonyls with a saturation loading of 2M(CO)6/alpha-cage. (ii) M(CO)6-M'56Y samples have the hexacarbonylmetal(O) guest confined to the internal surface of the zeolite with a homogeneous distribution throughout the zeolite crystals. (iii) A Mo and Rb EXAFS structure analysis of 8{Mo(CO)6}-Rb56Y shows that the alpha-cage encapsulated Mo(CO)6 guest maintains its structural integrity, with some evidence for anchoring via extraframework Rb+ cations. (iv) A rapid C-13O intrazeolite ligand exchange occurs M(12CO)6-M '56Y to yield M(12CO)m(13CO)6-m-M'56Y, the extent of which depends on the 13CO loading. (v) M(CO)3-M'56Y can be cleanly generated via the mild vacuum thermal decarbonylation of M(CO)6-M56Y, the tricarbonyl stoichiometry of which is unequivocally established from its observed and calculated diagnostic M(12CO)n(13CO)3-n-M'56Y vibrational isotope pattern and from EXAFS structural data. (vi) Intrazeolite ractions of M(CO)3-M'56Y with large and small arenes, trienes, and phosphines cleanly yield the respective intrazeolite six-coordinate complexes (shown to be identical with the products of direct impregnation of the latter complexes), thereby supporting the tricarbonylmetal(0) assignment as well as pinpointing the location of the M(CO)3-M'56Y tricarbonylmetal(0) fragment on the internal surface of the zeolite. (vii) Cation effects in the mid/far-IR, EXAFS data, and optical reflectance spectra indicate that the supercage-confined M(CO)3-M'56Y moiety is anchored to an oxygen framework site rather than to an extrawork cation site via the metal or oxygens of the carbonyls. (viii) The tricarbonyl fragments show C(s) and C3-upsilon symmetry depending on the choice of M and M' which can be rationalized in terms of a second-order Jahn-Teller effect. (ix) EXAFS data for the mild thermal decomposition of Mo(CO)3-Rb56Y demonstrates the formation of molybdenum atoms statistically distributed in the zeolite lattice
Intrazeolite phototopotaxy. EXAFS analysis of precursor 8{W(CO)6}-Na56Y and photooxidation products 16(WO3)-Na56Y and 28(WO3)-Na56Y
The intrazeolite photooxidation chemistry of alpha-cage encapsulated hexacarbonyltungsten(0) in Na56Y with O2, denoted n{W(CO)6}-Na56Y/O2/hv, which has previously been shown to provide a novel synthetic pathway to alpha-cage located tungsten(VI) oxide, denoted n(WO3)-Na56Y, is now the subject of an extended X-ray absorption fine structure (EXAFS) analysis. The EXAFS data of a precursor 8{W(CO)6}Na56Y, which contains on average one W(CO)6 per alpha-cage shows that the W(CO)6 guest maintains its structural integrity with only minor observable perturbations of the skeletal WC and ligand CO bonds compared to those found for the same molecule in the free state. The EXAFS analysis results for the photoxidation products 16(WO3)-Na56Y and 28(WO3)-Na56Y are very similar and display the presence of two terminal tungsten-oxygen bonds (1.75-1.77 angstrom) and two bridging tungsten-oxide bonds (1.94-1.95 angstrom), together with a short distance to a second tungsten (3.24-3.30 angstrom). This bond length and coordination number information for n = 16 and 28 samples is best interpreted in terms of the formation of a single kind of tungsten trioxide dimer unit (WO3)2, most likely interacting with extraframework Na+ cations, denoted ZONa...O2W(mu-O)2WO2...NaOZ. In conjunction with earlier chemical and spectroscopic information on this system, the EXAFS data support the contention that 16(WO3)-Na56Y contains a uniform array of single size and shape tungsten (VI) oxide dimers (WO3)2 housed in the 13-angstrom supercages of the zeolite Y host. The sequential addition of WO3 units to the 16(WO3)-Na56Y sample appears to increase the (WO3)2 dimer population, causing a buildup of alpha-cage encapsulated dimers-of-dimers {(WO3)2}2 rather than further cluster growth to trimers (WO3)2 and/or tetramers (WO3)4
Doping and band-gap engineering of an intrazeolite tungsten(VI) oxide supralattice
New results are presented concerning the topotactic self-assembly, n-type
doping and band-gap engineering of an intrazeolite tungsten(VI) oxide supralattice
n(W03)-Na56Y, where 0 < η < 32, built-up of single size and shape (W03)2
dimers. In particular it has been found that the oxygen content of these dimers
can be quantitatively adjusted by means of a thermal vacuum induced reversible
reductive-elimination oxidative-addition of dioxygen. This provides access to new
n(W03.x)-Na56Y materials (0 < χ ^ 1.0) in which the oxygen content, structural
properties and electronic architecture of the dimers are changed. In this way one
can precisely control the oxidation state, degree of η-doping and band-filling of a
tungsten(VI) oxide supralattice through an approach which can be considered akin
to, but distinct in detail to, that found in the Magneli crystallographic shear phases
of non-stoichiometric bulk W03.x . Another discovery concerns the ability to alter
local electrostatic fields experienced by the tungsten(VI) oxide moieties housed in
the 13Ä supercages of 16(W03)-M36Y, by varying the ionic potential of the
constituent supercage M + cations across the alkali metal series. This method
provides the first opportunity to fine-tune the band-gap of a tungsten(VI) oxide
supralattice. Α miniband electronic description is advanced as a qualitative first
attempt to understand the origin of the above effects. The implications of these
discoveries are that cluster size, composition and intrinsic electrostatic field effects
can be used to "chemically manipulate" (engineer) the doping and band
architecture of intrazeolite supralattices of possible interest in quantum electronics
and nonlinear optics
Designing Materials Acceleration Platforms for Heterogeneous CO2 Photo(thermal)catalysis
Materials acceleration platforms (MAPs) combine automation and artificial
intelligence to accelerate the discovery of molecules and materials. They have
potential to play a role in addressing complex societal problems such as
climate change. Solar chemicals and fuels generation via heterogeneous CO2
photo(thermal)catalysis is a relatively unexplored process that holds potential
for contributing towards an environmentally and economically sustainable
future, and therefore a very promising application for MAP science and
engineering. Here, we present a brief overview of how design and innovation in
heterogeneous CO2 photo(thermal)catalysis, from materials discovery to
engineering and scale-up, could benefit from MAPs. We discuss relevant design
and performance descriptors and the level of automation of state-of-the-art
experimental techniques, and we review examples of artificial intelligence in
data analysis. Based on these precedents, we finally propose a MAP outline for
autonomous and accelerated discoveries in the emerging field of solar chemicals
and fuels sourced from CO2 photo(thermal)catalysis
Kinetics versus Charge SeparationSeparation: Improving the Activity of Stoichiometric and Non-Stoichiometric Hematite Photoanodes Using a Molecular Iridium Water Oxidation Catalyst
Oxygen-deficient
iron oxide thin films, which have recently been
shown to be highly active for photoelectrochemical water oxidation,
were surface-functionalized with a monolayer of a molecular iridium
water oxidation cocatalyst. The iridium catalyst was found to dramatically
improve the kinetics of the water oxidation reaction at both stoichiometric
and nonstoichiometric α-Fe<sub>2</sub>O<sub>3‑x</sub> surfaces. This was found to be the case in both the dark and in
the light as evidenced by cyclic voltammetry, Tafel analysis, and
electrochemical impedance spectroscopy (EIS). Oxygen evolution measurements
under working conditions confirmed high Faradaic efficiencies of 69–100%
and good stability over 22 h of operation for the functionalized electrodes.
The resulting ∼200–300 mV shift in onset potential for
the iridium-functionalized sample was attributed to improved interfacial
charge transfer and oxygen evolution kinetics. Mott–Schottky
plots revealed that there was no shift in flat-band potential or change
in donor density following functionalization with the catalyst. The
effect of the catalyst on thermodynamics and Fermi level pinning was
also found to be negligible, as evidenced by open-circuit potential
measurements. Finally, transient photocurrent measurements revealed
that the tethered molecular catalyst did improve charge separation
and increase charge density at the surface of the photoanodes, but
only at high applied biases and only for the nonstoichiometric oxygen-deficient
iron oxide films. These results demonstrate how molecular catalysts
can be integrated with semiconductors to yield cooperative effects
for photoelectrochemical water oxidation
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