  The imaging of active nanoparticles represents a milestone in decoding
heterogeneous catalysts dynamics. We report the facet resolved, surface strain
state of a single PtRh alloy nanoparticle on SrTiO3 determined by coherent
x-ray diffraction imaging under catalytic reaction conditions. Density
functional theory calculations allow us to correlate the facet surface strain
state to its reaction environment dependent chemical composition. We find that
the initially Pt terminated nanoparticle surface gets Rh enriched under CO
oxidation reaction conditions. The local composition is facet orientation
dependent and the Rh enrichment is non-reversible under subsequent CO
reduction. Tracking facet resolved strain and composition under operando
conditions is crucial for a rational design of more efficient heterogeneous
catalysts with tailored activity, selectivity and lifetime.
