Surface Mass Spectrometry at the Submicrometer Scale

We report here a first application of elemental mass analysis on surface nanostructures by coincidence ion mass spectrometry (CIMS). The nanostructures under investigation are silver particles on aluminum silicate substrates. This system is used as a catalyst and is made by depositing silver metal and promoters, including cesium promoter, on the surface of preformed aluminum silicate pellets. In this study our goal is to determine the relative concentration of cesium on the surface of the silver particles. We compared results obtained from samples of a freshly prepared and of a used catalyst. The selective characterization of the particles via CIMS is compared with conventional secondary ion mass spectrometry (SIMS). Three mass spectra were obtained on both types of samples: a normal one and spectra of secondary ions coemitted with silver ion and cesium ion, respectively. We show that the concentration of cesium in the surface layer of the silver particles is the same for the freshly prepared and for the used catalyst, although due to migration−agglomeration the silver particles in the used catalyst are larger than in the starting material. To estimate the accuracy of our experiment, we assess the contribution of interfacial impacts, i.e., events where cesium ion from the aluminum silicate substrate is coemitted with silver ion from the silver particles due to the finite size of the desorption volume. The magnitude of the interfacial contribution is evaluated with an "interference coefficient" K. Using a simple model of interference, we calculate K for our experimental conditions. In our case, K is smaller (<10%) than the accuracy of the measurements.

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