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Abstract: Spatially resolved, combined energy dispersive EXAFS (EDE) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements have been performed over a fixed catalyst bed of Pd/γ-Al2O3 during kinetic oscillations of CO oxidation. The kinetic oscillations of CO oxidation over Pd (or for that matter Pt or Rh) catalysts are a complicated phenomenon that require characterisation techniques with high time resolution and spatial resolution in order to make links between catalyst structure and surface reactivity. By measuring the extent of Pd oxidation at the nanoparticle surface, from Pd K-edge EDE, and matching this with the CO coverage, from DRIFTS spectra, at multiple positions of the fixed bed reactor it is found that the majority of the catalyst undergoes a sharp transition from the CO poisoned catalyst to the highly active, oxidised Pd surface. This transition occurs initially at the end of the catalyst bed, nearest the outlet, and propagates upstream with increasing temperature of the reactor. The oscillations in Pd surface oxide formation and CO coverage are observed only in the first ∼1 mm of the bed, which gives rise to oscillations in CO2 and O2 concentrations observed by end-pipe mass spectrometry after the light-off temperature. The catalyst initially exists as less active, CO poisoned metallic Pd nanoparticles before light-off which transition to a highly active state after light-off when the Pd nanoparticle surface becomes dominated by chemisorbed oxygen. Kinetic oscillations only occur at the front of the catalyst bed where there is sufficient concentration of CO in the gas phase to compete with O2 for adsorption sites at the catalyst surface. We demonstrate the complex nature of the evolving catalyst structure and surface reactivity during catalytic operation and the need for spatially resolved operando methods for understanding and optimising catalyst technologies.

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Abstract: Deposition of gold on supports can produce catalytically active forms of gold as well as spectators, but previous understanding of the nature of active immobilized precursors is poor. By using synchrotron X-ray powder diffraction (SXRD) and X-ray absorption spectroscopy (XAS) techniques, we report a novel synthesis and structural elucidation of atomically dispersed gold species anchored to the internal surface of TS-1 as K+Au(OH)2Na+(Of)3 (“f” signifies the framework atoms of TS-1 in the formula.). It is found that the choice of alkali ions plays a crucial role in nucleation and stabilization of the atomic precursor. These anchored single Au upon controlled reduction in H2 can form uniform gold clusters in direct contact with the TS-1 surface containing isolated Ti sites: their interface exhibits excellent specificity and stability towards epoxidation of propylene in H2/O2 due to synergetic effect.

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Abstract: Unsupported, bulk phase palladium sulfide has been studied for the selective hydrogenation of acetylene. The sample underwent significant change during thermal pretreatments, the extent of which depends on temperature. Exposure to hydrogen at temperatures of 150 °C or above results in the loss of sulfur from the sample, primarily as hydrogen sulfide. As sulfur is lost, the sample is progressively transformed from a sulfur rich phase (PdS) to a sulfur lean phase (Pd4S) via an intermediate phase (Pd16S7). Reduction at 250 °C produces a material, which contains Pd4S as the surface phase, whereas reduction at 350 °C results in a largely pure Pd4S phase. Thermal treatments which produce a Pd4S surface display excellent catalytic properties. At complete acetylene conversion at 250 °C, ethylene selectivities of 82.8% and 90.0% were obtained under non-competitive and competitive conditions, respectively. Beneficial catalytic properties arise from the uniformity of Pd sites due to the crystal structure of Pd4S along with electronic influences on the adsorption/desorption processes arising from sulfur neighbours. No indications of deactivation or significant deposition of carbon were observed over Pd4S sample after 50 h on stream.

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Abstract: The activation of [MoX3(L)] (with X = Cl, Br; L = tridentate ligands with S3 and SNS donor sets) by AlMe3, analogous to the industrially important [CrCl3(L)] catalysts for selective oligomerisation of alkenes, has been investigated by Mo K-edge X-ray absorption (XAS) and UV–visible spectroscopies. Time-resolved stopped-flow XAS, in combination with a newly developed anaerobic freeze-quench approach, have established the complete alkylation of the Mo centres and a slower, stepwise sequence for [MoBr3(L)]. No evidence for directly bonded or bridged Mo–Mo dimers was observed at the high Mo:AlMe3 ratios used in this study. Decomposition of the complexes is in competition with the activation and resulted in precipitation of particulate Mo over time and explains the deactivation as observed in catalytic tests. The novel freeze-quench approach, which can trap reaction solutions within 1 s of mixing, opens up a large field of homogeneous catalysis and liquid chemistry to be studied, being able to quench this rapidly, whilst characterisation techniques with long data acquisition can be performed.