Search

A La-Sr/CaO catalyst was studied operando during the oxidative coupling of methane (OCM) reaction using the X-ray diffraction computed tomography technique. Full-pattern Rietveld analysis was performed in order to track the evolving solid-state chemistry during the temperature ramp, OCM reaction, as well as after cooling to room temperature. We observed a uniform distribution of the catalyst main components: La2O3, CaO-SrO mixed oxide, and the high-temperature rhombohedral polymorph of SrCO3. These were stable initially in the reaction; however, doubling the gas hourly space velocity resulted in the decomposition of SrCO3 to SrO, which subsequently led to the formation of a second CaO-SrO mixed oxide. These two mixed CaO-SrO oxides differed in terms of the extent of Sr incorporation into their unit cell. By applying Vegard's law during the Rietveld refinement, it was possible to create maps showing the spatial variation of Sr occupancy in the mixed CaO-SrO oxides. The formation of the Sr-doped CaO species is expected to have an important role in this system through the enhancement of the lattice oxygen diffusion as well as increased catalyst basicity.

An X-ray diffraction computed tomography data-collection strategy that allows, post experiment, a choice between temporal and spatial resolution is reported. This strategy enables time-resolved studies on comparatively short timescales, or alternatively allows for improved spatial resolution if the system under study, or components within it, appear to be unchanging. The application of the method for studying an Mn-Na-W/SiO2 fixed-bed reactor in situ is demonstrated. Additionally, the opportunities to improve the data-collection strategy further, enabling post-collection tuning between statistical, temporal and spatial resolutions, are discussed. In principle, the interlaced scanning approach can also be applied to other pencil-beam tomographic techniques, like X-ray fluorescence computed tomography, X-ray absorption fine structure computed tomography, pair distribution function computed tomography and tomographic scanning transmission X-ray microscopy.

A combination of X-ray absorption microcomputed tomography (μ-CT) and diagonal offset raman spectroscopy (DORS) have been used to follow in real time the 2-D and 3-D evolution of Mo species within 3 mm γ-Al2O3 extrudates during catalyst impregnation and drying processes. In a first set of experiments, we have followed the real-time incipient wetness impregnation process using an aqueous solution of ammonium heptamolybdate (AHM). We observed that during the equilibration period, singly impregnated samples formed Al(OH)6Mo6O18 3− (Al−Mo) hot spots distributed over the entire sample volume and that these heterogeneities grow in number and size as a function of time. A second set of measurements focused on the coimpregnation of AHM with H3PO4 and the subsequent equilibration and drying stages. It was found that the presence of phosphorus in the impregnating solution prevented the formation of the hot spots via the formation of weakly bound HxP2Mo5O23 (6−x)− species that were uniformly distributed over the sample after 70 min of equilibration. During drying, however, these species migrated to the periphery of the sample, resulting in an egg shell distribution of HxP2Mo5O23 (6−x)−. We show that by performing these studies noninvasively with a sufficiently high time resolution, the behavior and evolution of the Mo species were reproduced more faithfully than by using more conventional and invasive cut-and-measure approaches.

X-ray diffraction at shallow angles of incidence has been used to examine three silica:titania sol-gel thin films. Comparison with transmission x-ray diffraction measurements of similar materials in the bulk shows a distinct increase in disorder in the silica network. An increase in porosity of the network in thin films is also likely, suggested by an increase in Si-O-H bonds. No differences in structure between samples with differing titania contents were observed using this technique.

X-ray diffraction has been used to study a series of powdered silica:titania sol-gel glasses with titania contents ranging from a 'pure' silica sample through to high titania levels where phase separation may have occurred. Analysis of the data reveals the change in second and third neighbour coordination numbers with increasing Ti content and confirms that for low titanium contents the sol-gels are atomically mixed.