Hydrogen spillover in tungsten oxide bronzes as observed by broadband neutron spectroscopy

Funding: This research was funded by the UK Science and Technology Facilities Council via direct access proposal (RB1920033, VESUVIO).Hydrogen spillover is an elusive process, and its characterization, using experimental probes and ab initio modeling, poses a serious challenge. In this work, the nuclear quantum dynamics of hydrogen in a palladium-decorated cubic polymorph of tungsten oxide, Pd/cWO3, are characterized by the technique of neutron Compton scattering augmented by ab initio harmonic lattice modeling. The deeply penetrating nature of the neutron scattering process, the lack of spectroscopic selection rules, the inherent high sensitivity to hydrogen, the high energy and momentum resolution for hydrogen, and the mass selectivity of the technique render the neutron Compton scattering a very potent and unique tool for investigating the local dynamics of hydrogen species in bulk matrices. The total neutron Compton scattering response of hydrogen is described in terms of the hydrogen momentum distribution. The distribution is deconvoluted under the assumption of three pools of hydrogen with distinctly different nuclear quantum dynamical behavior: (i) hydrogen-terminated beta-palladium hydride, (ii) hydrogen in acid centers (OH+ groups) on the surface of the cubic phase of tungsten oxide, and (iii) quasi-free atomic hydrogen inside the saturated hydrogen bronze resulting from the spillover process. The ab initio modeling of lattice dynamics yields theoretical predictions for the values of the widths of proton momentum distributions in the first two hydrogen pools, which allows for obtaining the contribution and the width of the momentum distribution of the quasi-free atomic hydrogen resulting from the hydrogen spillover process. The analysis reveals that the local binding strength of the quasi-free hydrogen is characterized by the values of nuclear momentum distribution width, nuclear kinetic energy, and force constant of the underlying potential of the mean force close to those of free, unconstrained hydrogen atomic species in a gas of non-interacting particles described by the Maxwell–Boltzmann distribution. Moreover, this picture of the local dynamics of the quasi-free hydrogen is consistent with the proton polaron model of hydrogen-induced coloration of bulk hydrogenated WO3.Publisher PDFPeer reviewe

Reduction kinetics and thermal behaviour of MoO₃ studed 'in situ' with neutron powder diffraction

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Ethanol conversion over cesium-doped mono- and bi-cationic aluminum and gallium H3PW12O40H_3PW_{12}O_{40} salts

The objective of this study was synthesis, characterization, and catalytic testing of mono-cationic (GaPW12O40, AlPW12O40) and bi-cationic (CsGa0.5H0.5PW12O40, CsAl0.5H0.5PW12O40) salts of tungstophosphoric acid. These heteropoly salts are characterized by Brönsted acidity derived from protons generated by dissociation of water molecules coordinated to the metal atoms. Catalytic activity of the studied salts was evaluated based on ethanol conversion and compared with the well-known parent heteropolyacid H3PW12O40. The reaction products, ethylene and diethyl ether, were monitored simultaneously by FTIR spectroscopy and chromatographic analysis. The infra-red studies indicated that ethylene is formed not only from ethanol but also by diethyl ether decomposition. Cs-doped bi-cationic catalysts exhibited higher activity for ethylene and ether formation than the mono-cationic salts. The following order of catalytic activity was established: H3PW12O40 > CsGa0.5H0.5PW12O40 ∼ CsAl0.5H0.5PW12O40 > GaPW12O40 > AlPW12O40. It is in agreement with the order of their acid strength determined by microcalorimetry and specific surface areas of the studied catalysts