  MXenes are a family of 2D transition metal carbides and nitrides with
remarkable properties and great potential for energy storage and catalysis
applications. However, their oxidation behavior is not yet fully understood,
and there are still open questions regarding the spatial distribution and
precise quantification of surface terminations, intercalated ions, and possible
uncontrolled impurities incorporated during synthesis and processing. Here,
atom probe tomography analysis of as-synthesized Ti$_3$C$_2$T$_x$ MXenes
reveals the presence of alkali (Li, Na) and halogen (Cl, F) elements as well as
unetched Al. Following oxidation of the colloidal solution of MXenes, it is
observed that the alkalies enriched in TiO$_2$ nanowires. Although these
elements are tolerated through the incorporation by wet chemical synthesis,
they are often overlooked when the activity of these materials is considered,
particularly during catalytic testing. This work demonstrates how the
capability of atom probe tomography to image these elements in 3D at the
near-atomic scale can help to better understand the activity and degradation of
MXenes, in order to guide their synthesis for superior functional properties.
