3 research outputs found
Molybdenum(VI) Oxosulfato Complexes in MoO<sub>3</sub>–K<sub>2</sub>S<sub>2</sub>O<sub>7</sub>–K<sub>2</sub>SO<sub>4</sub> Molten Mixtures: Stoichiometry, Vibrational Properties, and Molecular Structures
The structural and vibrational properties of molybdenum(VI)
oxosulfato
complexes formed in MoO<sub>3</sub>–K<sub>2</sub>S<sub>2</sub>O<sub>7</sub> and MoO<sub>3</sub>–K<sub>2</sub>S<sub>2</sub>O<sub>7</sub>–K<sub>2</sub>SO<sub>4</sub> molten mixtures
under an O<sub>2</sub> atmosphere and static equilibrium conditions
were studied by Raman spectroscopy at temperatures of 400–640
°C. The corresponding composition effects were explored in the <i>X</i><sub>MoO<sub>3</sub></sub><sup>0</sup> = 0–0.5 range. MoO<sub>3</sub> undergoes a dissolution
reaction in molten K<sub>2</sub>S<sub>2</sub>O<sub>7</sub>, and the
Raman spectra point to the formation of molybdenum(VI) oxosulfato
complexes. The MoO stretching region of the Raman spectrum
provides sound evidence for the occurrence of a dioxo Mo(O)<sub>2</sub> configuration as a core. The stoichiometry of the dissolution
reaction MoO<sub>3</sub> + <i>n</i>S<sub>2</sub>O<sub>7</sub><sup>2–</sup> → C<sup>2<i>n</i>–</sup> was inferred by exploiting the Raman band intensities, and it was
found that <i>n</i> = 1. Therefore, depending on the MoO<sub>3</sub> content, monomeric MoO<sub>2</sub>(SO<sub>4</sub>)<sub>2</sub><sup>2–</sup> and/or associated [MoO<sub>2</sub>(SO<sub>4</sub>)<sub>2</sub>]<sub><i>m</i></sub><sup>2<i>m</i>–</sup> complexes are formed in the binary MoO<sub>3</sub>–K<sub>2</sub>S<sub>2</sub>O<sub>7</sub> molten system, and
pertinent structural models are proposed in full consistency with
the Raman data. A 6-fold coordination around Mo is inferred. Adjacent
MoO<sub>2</sub><sup>2+</sup> cores are linked by bidentate bridging
sulfates. With increasing temperature at concentrated melts (i.e.,
high <i>X</i><sub>MoO<sub>3</sub></sub><sup>0</sup>), the observed spectral changes can be explained
by partial dissociation of [MoO<sub>2</sub>(SO<sub>4</sub>)<sub>2</sub>]<sub><i>m</i></sub><sup>2<i>m</i>–</sup> by detachment of S<sub>2</sub>O<sub>7</sub><sup>2–</sup> and
formation of a MoOMo bridge. Addition of K<sub>2</sub>SO<sub>4</sub> in MoO<sub>3</sub>–K<sub>2</sub>S<sub>2</sub>O<sub>7</sub> results in a “follow-up” reaction and
formation of MoO<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub><sup>4–</sup> and/or associated [MoO<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>]<sub><i>m</i></sub><sup>4<i>m</i>–</sup> complexes
in the ternary MoO<sub>3</sub>–K<sub>2</sub>S<sub>2</sub>O<sub>7</sub>–K<sub>2</sub>SO<sub>4</sub> molten system. The 6-fold
Mo coordination comprises two oxide ligands and four O atoms linking
to coordinated sulfate groups in various environments of reduced symmetry.
The most characteristic Raman bands for the molybdenum(VI) oxosulfato
complexes pertain to the Mo(O)<sub>2</sub> stretching modes:
(1) at 957 (polarized) and 918 (depolarized) cm<sup>–1</sup> for the ν<sub>s</sub> and ν<sub>as</sub> Mo(O)<sub>2</sub> modes of MoO<sub>2</sub>(SO<sub>4</sub>)<sub>2</sub><sup>2–</sup> and [MoO<sub>2</sub>(SO<sub>4</sub>)<sub>2</sub>]<sub><i>m</i></sub><sup>2<i>m</i>–</sup> and
(2) at 935 (polarized) and 895 (depolarized) cm<sup>–1</sup> for the respective modes of MoO<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub><sup>4–</sup> and [MoO<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>]<sub><i>m</i></sub><sup>4<i>m</i>–</sup>. The results were tested and found to be in accordance with ab initio
quantum chemical calculations carried out on [MoO<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>]<sup>4–</sup> and [{MoO<sub>2</sub>}<sub>2</sub>(SO<sub>4</sub>)<sub>4</sub>(μ-SO<sub>4</sub>)<sub>2</sub>]<sup>8–</sup> ions, in assumed isolated gaseous free states,
at the DFT/B3LYP (HF) level and with the 3-21G basis set. The calculations
included determination of vibrational infrared and Raman spectra,
by use of force constants in the Gaussian 03W program
Water–Gas Shift Reaction on Pt/Ce<sub>1–<i>x</i></sub>Ti<sub><i>x</i></sub>O<sub>2−δ</sub>: The Effect of Ce/Ti Ratio
Pt nanoparticles (1.2–2.0
nm size) supported on Ce<sub>1–<i>x</i></sub>Ti<sub><i>x</i></sub>O<sub>2−δ</sub> (<i>x</i> = 0, 0.2, 0.5, 0.8, and 1.0) carriers synthesized by the citrate
sol–gel method were tested toward the water–gas shift
(WGS) reaction in the 200–350 °C range. A deep insight
into the effect of two structural parameters, the chemical composition
of support (Ce/Ti atom ratio), and the Pt particle size on the catalytic
performance of Pt-loaded catalysts was realized after employing in
situ X-ray diffraction (XRD), high-resolution transmission electron
microscopy (HR-TEM) and HAADF/STEM, scanning electron microscopy (SEM),
in situ Raman and diffuse reflectance infrared Fourier transform (DRIFT)
spectroscopies under different gas atmospheres, H<sub>2</sub> temperature-programmed
reduction (H<sub>2</sub>-TPR), and temperature-programmed desorption
(NH<sub>3</sub>-TPD and CO<sub>2</sub>-TPD) techniques. The 0.5 wt
% Pt/Ce<sub>0.8</sub>Ti<sub>0.2</sub>O<sub>2−δ</sub> solid
(<i>d</i><sub>Pt</sub> = 1.7 nm) was found to be by far
the best catalyst among all the other solids investigated. In particular,
at 250 °C the CO conversion over Pt/Ce<sub>0.8</sub>Ti<sub>0.2</sub>O<sub>2−δ</sub> was increased by a factor of 2.5 and
1.9 compared to Pt/TiO<sub>2</sub> and Pt/CeO<sub>2</sub>, respectively.
The catalytic superiority of the Pt/Ce<sub>0.8</sub>Ti<sub>0.2</sub>O<sub>2−δ</sub> solid is the result of the support’s
(i) robust morphology preserved during the WGS reaction, (ii) moderate
acidity and basicity, and (iii) better reducibility at lower temperatures
and the significant reduction of “coking” on the Pt
surface and of carbonate accumulation on the Ce<sub>0.8</sub>Ti<sub>0.2</sub>O<sub>2−δ</sub> support. Several of these properties
largely influenced the reactivity of sites (<i>k</i>, s<sup>–1</sup>) at the Pt–support interface. In particular,
the specific WGS reaction rate at 200 °C expressed per length
of the Pt–support interface (μmol CO cm<sup>–1</sup> s<sup>–1</sup>) was found to be 2.2 and 4.6 times larger
on Pt supported on Ce<sub>0.8</sub>Ti<sub>0.2</sub>O<sub>2−δ</sub> (Ti<sup>4+</sup>-doped CeO<sub>2</sub>) compared to TiO<sub>2</sub> and CeO<sub>2</sub> alone, respectively
Structural and Redox Properties of Ce<sub>1–<i>x</i></sub>Zr<sub><i>x</i></sub>O<sub>2−δ</sub> and Ce<sub>0.8</sub>Zr<sub>0.15</sub>RE<sub>0.05</sub>O<sub>2−δ</sub> (RE: La, Nd, Pr, Y) Solids Studied by High Temperature <i>in Situ</i> Raman Spectroscopy
<i>In situ</i> Raman spectroscopy at temperatures up
to 450 °C is used to probe the structural and redox properties
of Ce<sub>1–<i>x</i></sub>Zr<sub><i>x</i></sub>O<sub>2−δ</sub> solids (<i>x</i> = 0–0.8)
prepared by the citrate sol–gel and coprecipitation with urea
methods. The anionic sublattice structure of the solids is dependent
on the preparation route. The composition effects exhibited by the
Raman spectra are adequate for characterizing the phases present and/or
eventual phase segregations. For <i>x</i> = 0.5 the pseudocubic <i>t</i>″ phase occurs for the solid prepared by the citrate
sol–gel method, while phase segregation (cubic, tetragonal)
is evidenced for the corresponding material prepared by the coprecipitation
with urea method. A larger extent of defects and interstitial O atoms
is evidenced for the materials prepared by the citrate sol–gel
method. The well-known “defect” (“D”)
band around 600 cm<sup>–1</sup> for CeO<sub>2</sub> as well
as for Ce<sub>1–<i>x</i></sub>Zr<sub><i>x</i></sub>O<sub>2−δ</sub> consists of at least two components:
“D1” above 600 cm<sup>–1</sup> and “D2”
below 600 cm<sup>–1</sup>. Doping of Ce<sub>0.8</sub>Zr<sub>0.2</sub>O<sub>2−δ</sub> with rare earth cations (La<sup>3+</sup>, Nd<sup>3+</sup>, Y<sup>3+</sup>, Pr<sup>3+</sup>) results
in strengthening of the “D2” band that, however, is
found to be insensitive under reducing conditions of flowing 5% H<sub>2</sub>/He at 450 °C. A novel approach based on sequential <i>in situ</i> Raman spectra under alternating oxidizing (20% O<sub>2</sub>/He) and reducing (5% H<sub>2</sub>/He) gas atmospheres showed
that the “D1” band is selectively attenuated under reducing
conditions at 450 °C and is therefore assigned to a metal–oxygen
vibrational mode involving interstitial oxygen atoms that can be delivered
under suitable conditions. A reversible temperature-dependent evolution
of the anionic sublattice structures of Ce<sub>1–<i>x</i></sub>Zr<sub><i>x</i></sub>O<sub>2−δ</sub> solids is evidenced by <i>in situ</i> Raman spectroscopy.
The results are corroborated by powder XRD and oxygen storage capacity
measurements, and observed structure/function relationships are discussed.
It is shown that at low temperatures (e.g., 450 °C) the function
of oxygen release and refill is based on a mechanism involving oxygen
atoms in interstitial sites rather than on defects induced by hetrovalent
M<sup>4+</sup>→ RE<sup>3+</sup> doping, the latter improving
the pertinent function at high (e.g., >600 °C) temperatures