## 1 Introduction
Standard techniques for structural characterization are not always well suited to the characterization of supported MNPs and may fail to achieve this goal. For example, the efficiency of X-ray powder diffraction (XRPD) is definitively reduced in the case of nanometer sized particles dispersed in stabilizing supports because the scattering coming from the matrix overlaps with the weak and broadened diffraction peaks from small MNPs [30]. Even transmission electron microscopy (TEM), employing the most advanced techniques, such as aberration corrected high resolution scanning transmission electron microscopy (HR-STEM), which is sensitive to the atomic numbers and is able to give quantitative information of bimetallic MNPs [31], cannot be fully exploited in the presence of a support except for the few MNPs which are sitting on the external surface of the matrix. So far, based on what is reported in the literature, it is quantitatively difficult to characterize MNPs embedded in a matrix. Resonant (also called anomalous) X-ray diffraction (A-XRD) is a powerful technique often employed to selectively probe micro/nanocrystalline phases embedded in matrices [32; 33; 34]. Anomalous scattering is able to give information similar to the one provided by X-ray absorption measurements, but with the advantage that it is crystallographically site-selective. This technique exploits the ample variation of the anomalous scattering factor (the atomic scattering factor for the atom, j, is given by f\({}_{\text{j}}\) = f\({}_{0}\) + f\({}^{\prime}\) + if\({}^{\prime\prime}\), where f\({}_{0}\) corresponds to the atomic scattering factor for a spherically symmetric collection of free electrons in the atom, and f\({}^{\prime}\) and f\({}^{\prime\prime}\) are the dispersion correction or \"anomalous contributions\") [35] of a resonant atomic species close to its atomic absorption edge as a contrast factor to selectively extract its contribution to the overall scattering intensity from the sample. For this purpose, in the A-XRD experiment, the diffraction pattern is first measured with the X-ray energy tuned near (E\({}_{\text{N}}\)) the absorption edge of one of the metals in the bimetallic sample (the resonant species) and later with the X-ray energy tuned far away (\(\rm E_{F}\)) from the same edge. The intensity of X-rays diffracted by crystals depends on the structure factor and thus on the atomic scattering factors, \(\rm f_{j}\), for each of the atoms, j, so that the intensity difference pattern, \(\rm\Delta I=I_{F}-I_{N}\), contains only the peaks arising from the phases with the resonant species, because only the structure factors of the latter changed from \(\rm I_{N}\) to \(\rm I_{F}\)[36; 37]. Dealing with supported metal catalysts, A-XRD is used as an efficient way to suppress the scattering of the support and to focus on the structure of the MNPs [38; 39]. In the present work, we demonstrate that the A-XRD technique can be employed to highlight fine differences in the crystallographic nanophases composed of two different metals dispersed on a matrix. To our knowledge, this is the first time that this technique has been applied for the characterization and quantification of a very low concentration (less than or equal to 3 weight %) of supported MNPs. Our purpose was to shed some light on the structures of bimetallic MNPs and to correlate them with the catalytic activity. The overall strategy of the investigation is plotted in Scheme 1.