Molybdenum(VI) Oxosulfato Complexes in MoO₃–K₂S₂O₇–K₂SO₄ Molten Mixtures: Stoichiometry, Vibrational Properties, and Molecular Structures

The structural and vibrational properties of molybdenum­(VI) oxosulfato complexes formed in MoO₃–K₂S₂O₇ and MoO₃–K₂S₂O₇–K₂SO₄ molten mixtures under an O₂ 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>_MoO₃⁰ = 0–0.5 range. MoO₃ undergoes a dissolution reaction in molten K₂S₂O₇, and the Raman spectra point to the formation of molybdenum­(VI) oxosulfato complexes. The MoO stretching region of the Raman spectrum provides sound evidence for the occurrence of a dioxo Mo­(O)₂ configuration as a core. The stoichiometry of the dissolution reaction MoO₃ + <i>n</i>S₂O₇^2– → C^2<i>n</i>– was inferred by exploiting the Raman band intensities, and it was found that <i>n</i> = 1. Therefore, depending on the MoO₃ content, monomeric MoO₂(SO₄)₂^2– and/or associated [MoO₂(SO₄)₂]_<i>m</i>^2<i>m</i>– complexes are formed in the binary MoO₃–K₂S₂O₇ molten system, and pertinent structural models are proposed in full consistency with the Raman data. A 6-fold coordination around Mo is inferred. Adjacent MoO₂²⁺ cores are linked by bidentate bridging sulfates. With increasing temperature at concentrated melts (i.e., high <i>X</i>_MoO₃⁰), the observed spectral changes can be explained by partial dissociation of [MoO₂(SO₄)₂]_<i>m</i>^2<i>m</i>– by detachment of S₂O₇^2– and formation of a MoOMo bridge. Addition of K₂SO₄ in MoO₃–K₂S₂O₇ results in a “follow-up” reaction and formation of MoO₂(SO₄)₃^4– and/or associated [MoO₂(SO₄)₃]_<i>m</i>^4<i>m</i>– complexes in the ternary MoO₃–K₂S₂O₇–K₂SO₄ 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)₂ stretching modes: (1) at 957 (polarized) and 918 (depolarized) cm^–1 for the ν_s and ν_as Mo­(O)₂ modes of MoO₂(SO₄)₂^2– and [MoO₂(SO₄)₂]_<i>m</i>^2<i>m</i>– and (2) at 935 (polarized) and 895 (depolarized) cm^–1 for the respective modes of MoO₂(SO₄)₃^4– and [MoO₂(SO₄)₃]_<i>m</i>^4<i>m</i>–. The results were tested and found to be in accordance with ab initio quantum chemical calculations carried out on [MoO₂(SO₄)₃]^4– and [{MoO₂}₂(SO₄)₄(μ-SO₄)₂]^8– 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_1–<i>x</i>Ti_<i>x</i>O_2−δ: The Effect of Ce/Ti Ratio

Pt nanoparticles (1.2–2.0 nm size) supported on Ce_1–<i>x</i>Ti_<i>x</i>O_2−δ (<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, H₂ temperature-programmed reduction (H₂-TPR), and temperature-programmed desorption (NH₃-TPD and CO₂-TPD) techniques. The 0.5 wt % Pt/Ce_0.8Ti_0.2O_2−δ solid (<i>d</i>_Pt = 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_0.8Ti_0.2O_2−δ was increased by a factor of 2.5 and 1.9 compared to Pt/TiO₂ and Pt/CeO₂, respectively. The catalytic superiority of the Pt/Ce_0.8Ti_0.2O_2−δ 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_0.8Ti_0.2O_2−δ support. Several of these properties largely influenced the reactivity of sites (<i>k</i>, s^–1) 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^–1 s^–1) was found to be 2.2 and 4.6 times larger on Pt supported on Ce_0.8Ti_0.2O_2−δ (Ti⁴⁺-doped CeO₂) compared to TiO₂ and CeO₂ alone, respectively

Structural and Redox Properties of Ce_1–<i>x</i>Zr_<i>x</i>O_2−δ and Ce_0.8Zr_0.15RE_0.05O_2−δ (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_1–<i>x</i>Zr_<i>x</i>O_2−δ 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^–1 for CeO₂ as well as for Ce_1–<i>x</i>Zr_<i>x</i>O_2−δ consists of at least two components: “D1” above 600 cm^–1 and “D2” below 600 cm^–1. Doping of Ce_0.8Zr_0.2O_2−δ with rare earth cations (La³⁺, Nd³⁺, Y³⁺, Pr³⁺) results in strengthening of the “D2” band that, however, is found to be insensitive under reducing conditions of flowing 5% H₂/He at 450 °C. A novel approach based on sequential <i>in situ</i> Raman spectra under alternating oxidizing (20% O₂/He) and reducing (5% H₂/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_1–<i>x</i>Zr_<i>x</i>O_2−δ 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 M⁴⁺→ RE³⁺ doping, the latter improving the pertinent function at high (e.g., >600 °C) temperatures