Many catalytic reactions under fixed conditions exhibit oscillatory behaviour. The oscillations are often attributed to dynamic changes in the catalyst surface. So far, however, such relationships were difficult to determine for catalysts consisting of supported nanoparticles. Here, we employ a nanoreactor to study the oscillatory CO oxidation catalysed by Pt nanoparticles using time-resolved high-resolution transmission electron microscopy, mass spectrometry and calorimetry. The observations reveal that periodic changes in the CO oxidation are synchronous with a periodic refacetting of the Pt nanoparticles. The oscillatory reaction is modelled using density functional theory and mass transport calculations, considering the CO adsorption energy and the oxidation rate as site-dependent. We find that to successfully explain the oscillations, the model must contain the phenomenon of refacetting. The nanoreactor approach can thus provide atomic-scale information that is specific to surface sites. This will improve the understanding of dynamic properties in catalysis and related fields.

The introduction of scanning/transmission electron microscopes (S/TEM) with sub-Angstrom resolution as well as fast and sensitive detection solutions support direct observation of dynamic phenomena in-situ at the atomic scale. Thereby, in-situ specimen holders play a crucial role: accurate control of the applied in-situ stimulus on the nanostructure combined with the overall system stability to assure atomic resolution are paramount for a successful in-situ S/ TEM experiment. For those reasons, MEMS-based TEM sample holders are becoming one of the preferred choices, also enabling a high precision in measurements of the in-situ parameter for more reproducible data. A newly developed MEMS-based microheater is presented in combination with the new NanoEx TM -i/v TEM sample holder. The concept is built on a four-point probe temperature measurement approach allowing active, accurate local temperature control as well as calorimetry. In this paper, it is shown that it provides high temperature stability up to 1,3008C with a peak temperature of 1,5008C (also working accurately in gaseous environments), high temperature measurement accuracy (<4%) and uniform temperature distribution over the heated specimen area (<1%), enabling not only in-situ S/TEM imaging experiments, but also elemental mapping at elevated temperatures using energy-dispersive X-ray spectroscopy (EDS). Moreover, it has the unique capability to enable simultaneous heating and biasing experiments. Microsc. Res.

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Silicon Drift Detectors (SDD) [1] are rapidly replacing Si(Li) detectors for EDX microanalysis in SEM, but have yet to have an impact in the S/TEM world. Main reason for this difference is the low count rate created by thin S/TEM samples compared to the bulk samples in SEM . These low count rates make EDX mapping a very slow process in S/TEM. However, the recent introduction of higher brightness electron sources [2] and probe Cs-correctors has led to significantly increased beam currents in small electron probes and, potentially, to higher EDX count rates. Since a key advantage of the SDD is the high count rate capability, the throughput improvement compared to the Si(Li) detectors will be considerable in these new instruments. Compared to SEM, the smaller excited volumes obtained with the atomic-scale probes in the new S/TEM instruments can lead to radiation damage of beam-sensitive materials before the analysis is completed. Therefore S/TEM microanalysis needs not only the higher count rate capability, but also higher collection efficiency of the X-rays generated, in order to reduce the dose on the sample. In this paper we present a new prototype EDX detector system for an FEI 200kV TEM/STEM, in which FEI has integrated a detector system consisting of multiple SDDs, placed symmetrically around the electron beam axis in the objective lens chamber without affecting the S/TEM resolution. The SDDs with a total active area of 120 mm 2 were designed by PN Sensor to fit into the FEI design to achieve a quantum leap in solid angle of collection compared to previous designs in S/TEMs. The SDDs are cooled to achieve the optimum energy resolution, typically below 130 eV. The windowless design allows for better sensitivity for light-element detection than conventional thin-window detectors. The specially designed front-end electronics and ultra fast multi-channel pulse processor are provided by Bruker AXS MA in collaboration with FEI. The processor is capable of fast mapping with pixel dwell times down to a few microseconds and >100 kcps count rates per channel. Compared to currently available Si(Li) detectors the anticipated count rates will be an order of magnitude higher with the new detector. Additionally the new high brightness gun of FEI (X-FEG) [2] increases the brightness of the electron source compared to conventional Schottky sources, leading to a further increase in count rate, and an equivalent significant decrease in mapping time at the same spatial resolution. This improvement is illustrated in Fig. 1 where the relative minimum detectable mass MDM ~ (t.P.P/B) -1/2 (t=analysis time, P=elemental peak counts, P/B = peak-to-background ratio) [3] is shown for conventional and new EDX detector count rates at the same spatial resolution. Fig.1 also compares the MDM with EELS and, for the specific case of strontium titanate, shows that the new EDX detector is expected to be more sensitive than EELS. Further results will be reported at the conference.

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