Johnson Matthey Technology Review - Volume 60, Issue 2, 2016

High-surface area γ-alumina is industrially used as a catalyst support. Catalytically active elements are doped onto the support, where they can bind to AlO4, AlO5 or AlO6 sites on the surface. Pretreating the surface with alkaline earth oxides can alter the availability of these surface sites, hence affecting the catalytic activity. The surface binding sites of strontium oxide (SrO) on γ-alumina were previously unknown. Direct ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) could not detect AlO5 sites at 9.4 T, so ¹H–²⁷Al cross-polarisation (CP) MAS NMR was used to preferentially select the surface environment signals. We directly observed the three surface environments on dehydrated γ-alumina as a function of SrO impregnation up to 4 wt% SrO. We found that Sr²⁺ preferentially binds to AlO5 and AlO6 surface sites. ¹H MAS NMR revealed SrO impregnation causes a reduction in the terminal (−0.3 ppm) and bridging (2.2 ppm) hydroxyl environments, as well as the promotion of a new peak in between these sites, at 0.5 ppm. By using ¹H–²⁷Al CP/MAS NMR the relative proportions of surface sites on γ-alumina can be determined, allowing an optimal level of SrO doping that can saturate all the AlO5 sites. Importantly, this provides a method of subsequently depositing catalytically active elements on just the AlO4 or AlO6 sites, which can provide a different catalytic activity or stability compared to the AlO5 binding site.

Small metallic nanoparticles used for polymer exchange membrane fuel cells (PEMFC) represent a characterisation challenge. Electron microscopy would seem the ideal technique to analyse their structure at high resolution. However, their minute size and sensitivity to irradiation damage makes this difficult. In this review, the latest techniques for overcoming these limitations in order to provide quantitative structural and compositional information are presented, focusing specifically on quantitative annular dark-field (ADF) scanning transmission electron microscopy (STEM) and quantitative energy dispersive X-ray (EDX) analysis. The implications for the study of bimetallic fuel cell catalyst materials are also discussed.

Neutron scattering is a severely underused technique for studies of catalysts. In this review we describe how and why neutrons are useful to catalysis. We illustrate the range of systems that have been studied by both elastic and inelastic neutron scattering. These range from structural studies of adsorbates in zeolites to determination of the structure of surface adsorbates, characterisation of nanoparticles, the measurement and mechanism of diffusion and spectroscopic characterisation of adsorbed species. We conclude with how to access neutron facilities and some future prospects for the application of these techniques to industrially useful materials.

The application of three-dimensional electrical capacitance tomography (3D-ECT) for the in situ monitoring of a hard boundary or interface has been investigated using imaged phantoms that simulate real-life processes. A cylinder-in-tube phantom manufactured from polyethylene (PE), a low di-electric and non-conductive material, was imaged using the linear back projection (LBP) algorithm with the larger tube immersed at varying intervals to test the ability of the technique to image interfaces axially through the sensor. The interface between PE and air is clearly imaged and correlates to the known tube penetration within the sensor. The cylinder phantom is imaged in the centre of the sensor; however, the reduction in measurement density towards the centre of the ECT sensor results in reduced accuracy. A thresholding method, previously applied to binary systems to improve the imaged accuracy of a hard boundary between two separate phases, has been applied to the 3D-ECT tomograms that represent the PE phantom. This approach has been shown to improve the accuracy of the acquired image of a cylinder of air within a non-conductive PE tube.